know your machine tool · manual is to provide more direct insight into important aspects ofmachine...

KNOW YOURMACHINE TOOL

An Organized ApproachUsing Capacitance Sensors

by Tim Sheridan

© LION PRECISION 1991/2008

1.

PREFACE

Preface to the 2008 Electronic Reprint In 1991, Know Your Machine Tool became an instant classic for professionals in the machine tool metrology industry. The book was written before the power of computer data acquisition systems and analytical software were harnessed to measure machine tool spindle performance. The entire book is based on measurements made with Targa single-channel capacitive sensors. The Targa is referred to frequently and shown in the illustrations. That version of the Targa has been out of production for many years. Our current technology, the Elite Series, has considerably higher performance and is designed to connect directly to National Instruments™ data acquisition systems. In addition to better sensors, ten years of development have resulted in an advanced version of the Spindle Error Analyzer software system. This new Spindle Error Analyzer system was used in the creation of a new book on the state-of-the-art of machine tool measurements: Precision Spindle Metrology. The book was authored by Eric Marsh Ph.D. of The Pennsylvania State University Machine Dynamics Lab and is published by DesTech Publishers and is available at Amazon.com or from Lion Precision. More information about the Spindle Error Analyzer is available at: www.spindleanalysis.com More information about the book, Precision Spindle Measurement is available at: www.precisionspindlemetrology.com More information on the Elite Series Capacitive Sensors is available at: www.elitesensors.com

PREFACE

Preface

"When you cannot measure what you arespeaking about, when you cannot express it innumbers, your knowledge is ofa meager andunsatisfactory kind; it may be the beginning ofknowledge, but you have scarcely in yourthoughts advanced to the stage ofscience.whatever the matter may be. "

- Lord Kelvin

Lord Kelvin paraphrased by Allen Sanner ofProfessional Instruments Co.: "Jfyou canmeasure it, you can make it. "

© LION PRECISION 1991

2.

This treatise is about measurement as itapplies to machine tools and the evaluation oftheir performance.· Measurement is a fundamental aspect of the science of machine tooldesign as well as the science of making· thingswith machine tools.

I would like to thank Debra Condit, JaneBechaka and Michael Rioux of Lion Precisionfor their unrelenting cheerfulness and technical competence in the preparation of the manuscript and illustrations. Also, I would like tothank Professional Instruments Company for12 years of opportunities to learn how to makethings right. Finally, I would like to thankMaureen for putting up with me.

Tim SheridanSeptember, 1991

Foreword

FOREWORD

CONTENTS

Know Your Machine Tool:An Organized Approach Using

Capacitance Sensors

SECTION DESCRIPTION PAGE.

0.0 Introduction and Theory of Operation 8

1.0 1.1 Equipment Description 101.2 Equipment Interconnection Diagram 11

2.0 Probe Mounting Considerations 132.1 Probe Mount Materials 14

3.0 Grounding, Interference, and Electrical Noise 163.1 Grounding within the System Components 193.2 Use of the Optional Ground Isolator 19

4.0 Single Targa Applications 214.1 Runout 224.1.1 Definition 224.1.2 Setup 224.1.3 Oscilloscope Plots of Runout Signals 244.1.4 Synchronous & Asynchronous Error Motion 254.1.5 Imbalance 344.2 Axial Error Motion ; 364.3 Thermal Growth and Distortion 394.4 Linear Axis Applications 394.4.1 Straightness 394.4.2 Position Repeatability 424.4.3 Settling Time 444.5 Vibration 50

5.0 Dual Targa Applications 575.1 Axis of Rotation 575.1.1 Rotating Sensitive Direction Error Motion

(Boring, Milling, Drilling Machines) 575.1.2 Fixed Sensitive Direction Error Motion

(Lathes and Turning Machines) 615.1.3 Asynchronous Error Motion 615.2 Quasi-Differential Measurements 635.2.1 Tilt Error Motion of an Axis of Rotation 645.2.2 Straightness 655.3 Multi Axis Applications 665.3.1 Multi Axis Position Repeatability 66


4.

SECTION DESCRIPTION

CONTENTS

PAGE

5.3.2 Multi Axis Settling Time 665.3.3 Multi Axis Vibration 675.3.4. Multi Axis Thermal Growth 67

6.0 Capped Probe and Calibration Tests :..686.1.1 Capped Probe System Noise Test. ,. 696.1.2 Thermal Stability Test. 706.1.3 Calibration Check 70

7.0 Reversal Techniques for Eliminating Artifact Error 737.1 For an Axis of Rotation 737.2 For a Straightedge on a Linear Axis 75

8.0 Epilogue 79

9.0 References, Associated Standards and Sources of Information 81

Addendum A Fixed Sensitive Direction Error Motion 83

List of Figures

Figure 0.0 A Parallel Plate Capacitor 8Figure 1.0 Elements of Machine Tool Evaluation System 10Figure 1.1 Equipment Interconnect Diagram 11Figure 2.1 Probe Mount Considerations 13Figure 3.1 Proper Grounding of Target 16Figure 3.2 Ground Cable Routing 18Figure 4.1 Setup for Radial Runout and Radial Error Motion Measurements 22Figure 4.2 Setup for Axial Error Motion Measurements 36Figure 4.3 Setup for Straightness Error Measurement 40Figure 4.4 Explanation of Angular Error 41Figure 4.5 Rotary Table Angular Position Repeatability 43Figure 4.6 Explanation of Settling Time .44Figure 4.7 Settling Time: Over Damped 45Figure 4.8 Settling Time: Critically Damped .46Figure 4.9 Settling Time: Under Damped 46Figure 4.10 Settling Time: Oscillating .47Figure 4.11 " Settling Time: Quantization Error .48Figure 5.1 Asynchronous Error Motion Value Determination 62Figure 5.2 Straightness Measurement Using Two Probes 65Figure 6.1 Probe Cap Fixture 69Figure 6.2 Probe Calibration Check Fixture 71Figure 7.1 Reversal Method for a Rotary Axis 73Figure 7.2 Reversal Method for a Linear Axis 76


5.Machine Tool Analysis

CONTENTS

List of Plots

SECTION DESCRIPTION PAGE

Plot 3.1 Spindle Rotor Not Grounded 17Plot 3.2 Spindle Rotor Properly Grounded 1,7Plot 4.1 Radial Runout and Error Motion 24Plot 4.1a Radial Runout and Error Motion 35Plot 4.2 Determination of Rotational Speed 25Plot 4.3 Error Motion Slightly Larger Than Workpiece Form 28Plot 4.4a Almost No Error Motion Signal 29Plot 4.4b Screw Tension Induced Workpiece Distortion 30Plot 4.5 Asynchronous Radial Error Motion 31Plot 4.6 High Resolution Display of Asynchronous Radial Erior Motion 32Plot 4.7 "Grease Pencil" Test 33Plot 4.8 The Effect of Introducing an Unbalanced Condition 35Plot 4.9 Axial Error Motion 37Plot 4.10 Asynchronous Axial Error Motion 37Plot 4.11 Individual Ball Error Motion 38Plot 4.12 Ball Frequency 38Plot 4.13 Critically Damped Servo System .47Plot 4.14 Stiction and 'Stair Stepping' 49Plot 4.15 Grinding Machine, Coolant and Hydraulic Pumps On 51Plot 4.16 Grinding Machine, Coolant and Hydraulic Pumps Off 51Plot 4.17 Natural Frequency of Probe Mount 52Plot 4.18 Extended View of Probe Mount Response 53Plot 4.19 The Response of a Good Probe Mount.. 54Plot 4.20 The Response of a Very High Performance Probe Mount 55Plot 5.1 Plot of Two Probes Mount 90 Degrees Apart .59Plot 5.2 Typical Lissajous Pattem 62Plot 8.1 Asynchronous Error Motion of an Air Bearing Spindle 79


6.

PURPOSE

Purpose

The purpose of this manual is several fold.First and foremost, this manual is an introduction to the use of capacitive displacement sensors for the analysis of machine performance.However, an underlying theme of the manualwill be to use these sensors to acquire thegreatest amount of information with the leastamount of effort. Emphasis will be placed onsimple setups that can provide enough information to make the decision whether or not tocontinue analysis using more sophisticatedtechniques, leading to the prediction ofmachine tool performance and the identification of areas of improvement.

Second, this manual serves as an introductionto several ANSI/ASME Standards. Two are


7.

published (ANSI/ASME B89.3.4M-1985Axes of Rotation: Methods for Specifying andTesting~ and ANSI/ASME B89.3.1-1972,Measurement of Out-of-Roundness) while theother (ANSIjASME B5.S4-1991, Methods ForPerformance Evaluation of ComputerNumerically Controlled Machine Centers) is adraft Standard, subject to review and revisionprior to initial publication. A final goal of thismanual is to provide more direct insight intoimportant aspects of machine tool design andperformance such as servo control, vibration,thermal drift and distortion, accuracy of spindles and slides, and the influence of driveforces, nearby machines and other environmental noise sources.

Machine Tool Analysis

THEORY OF OPERATION

THEORY OF OPERATION

A capacitor consists of two conductive surfaces separated by an insulating medium. Theinsulating medium is known as the dielectric.The capacitance (measured in farads, afterMichael Faraday) of a capacitor is a functionof the shape of the conductive surfaces, thephysical properties of the dielectric and thedistance separating the conductive surfaces. Ifthe conductive surface shape and the dielectricare fixed, as in a capacitance probe, only thedistance between the conductive surfaces cancause a change in capacitance. By constructing a probe in such a way that the targetbecomes one of the conductive surfaces andthe probe the other conductive surface, then

changing the distance between them changesthe capacitance. Measuring and displaying thechange in capacitance provides a way to sh0'Ythe change in position of the target withrespect to the probe. Linearizing and calibrating the capacitance change as a function ofprobe to target separation provides a powerfultool for analyzing static and dynamic systems.A properly designed capacitive sensor is characterized by low noise, low drift, high sensitivity, wide bandwidth, and, of course, noncontact sensing. These properties make capacitive sensing ideal for a wide range of positionsensing applications.

D

t

c = KAEoD

[C] = capacitance (farads)[K] = dielectric constant (dimensionless)[A] = area of plate (meters2)

[Eo] = permittitivity of free space (coulombs2/N-m2)

[0] = distance between plates (meters)

FIGURE 0.0 A Parallel Plate Capacitor


8.

SECTION 1

EQUIPMENT DESCRIPTION


9.


SECTION I - Equipment Description

Section 1.1 System Equipment Description

o

r;:::;:::;ITOJ"Q(la "L!al Pre<!S!llll

"====:. • a:: 01·

t::r~I"OIla"i);ft Pre<!S!llll

~. a: 01·D !.,,"000 .o•• !1=000 OOgOII U U ~o@ ~ @ "0

I~.; 0i 0'" UJi

r--- ---1

HelOa -ITEK..

I_________ 1:::o

FIGURE 1.0 Machine Tool Evaluation System

Targa 1, Targa 2 - Targa single channeldimensional gagingsystem. +/-.002 range,.006 inch nominalstandoff, 1 microinchresolution, DC-10kHz,with selectable 2 Hz and 1kHz filters.

Tek 2211 - Tektronix 2211Analog/Digital StorageOscilloscope, 2 channelwith X-Y operation, 20Megasamples/Seconddigitizing rate, 8 bitresolution, 4K recordlength.

Tek HClOO - Tektronix model HClOOpen plotter.

Tek 212 - Tektronix Model K212Scopemobile InstrumentCart with plotter shelf.

This system is a two channel non-contactcapacitive displacement sensing system withreal time displacement display and permanenthard copy capabilities. As such, it is wellsuited for a wide variety of measurement tasksrelated to spindle error motion, slide errormotion, vibration, axis settling time, etc.Depending upon the kind of measurement,either one or both of the Targa units will beused. In keeping with the maximum information/minimum effort theme of this manual,single Targa applications will be covered first,followed by dual Targa applications.


10.

SECTION I - Equipment Description

Section 1.2 Equipment Interconnect Diagram

120VAC 120VAC

LIONPIS

ILIONIPIS

MASTER

O~_~_D_D _

O&f'- •0 0 0 0

/... .. " SLAVE.

// l20VAC ALL 120VAC

1---- TER~~NTAOLSTRIPI 6 RS23

rEK2211

PROBE 'X'

==:11-,GROUND AT~ ___

MI'.ASUREMENT SITE Y-----

----------

FRONT

120VAC

REAR

1-------[[ (i] (i] [Q] [Q] IUN/UFF

TERMINAL STRIP I

FIGURE 1.1 Equipment Interconnect Diagram

© LION PRECISI,ON 1991 Machine Tool Analysis

11.

Section 2

,P,robe Mounting Considerations


12.


SECTION 2 - Probe Mounting Considerations

Section 2 Probe Mounting Considerations

At first it might seem that minimal attention toprobe mounts or holders is required; after all,this is a non.:contact sensor so there is no elastic deformation of the structure between theprobe and the target due to contact forces. Inreality, the mounting of the probe requiressome care in order to obtain repeatable readings. This is especially true for dynamic measurements. Generally, probe holders shouldgrip the precision diameter of the probe nearthe active end. The grip length should be atleast equal to the diameter of the probe. Someprobes have a guard ring (used to shield themeasuring capacitance from static capacitanceshifts due to nearby structures) at the very endof the probe. Do not allow the probe mount tocontact the guard ring. Pinch-type holders

generally work the best as long as the pinchhole is straight, round, and provides a closeslip fit when the pinch screw is loose - seeFigure 2.1.

A straight, round pinch hole provides the bestsupport, allows easy high precision adjustmentof the standoff distance, and minimal shiftwhen the pinch screw is tightened. A barrelshaped hole is acceptable but may damage theprobe due to high local forces when the pinchscrew is tightened. Waisted or tapered holesgrip the probe at only one location, allowingthe probe to pivot easily and should beavoided. The rest of the probe mountshould be as short and stiff as possible tominimize vibrations when excited by the

pinch scr"'ew

probe Mounts here

Best Ok Bod

PROBF MOUNT CONSIDFRATIDNSFIGURE 2.1 Probe Mount Considerations


13.


SECTION 2 - Probe Mount Considerations

system being measured. As a rule of thumb,the natural or resonant frequency of the probeholder should be at least 2 times higher thanthe highest structural vibration frequencycomponent of the system to be measured. Itshould be noted this situation may be difficultto achieve on some roller bearing spindles dueto the high frequency, impact-like nature oftheir motion (See Section 4.5 on vibration fora method of measuring the natural frequency).Additionally, when the probe mount isattached to the structure of interest, the jointshould be hard and hysteresis free. The hysteresis can be directly measured using the Targa.When the probe mount is attached and theprobe is brought into the measuring range,simply note the Targa reading, then push onthe probe mount to deflect it elastically.Release the probe mount and check to see if

Section 2.1 Probe Mount Materials

Nickel plated mild steel makes an excellentmaterial for probe mounts. It is readily available, easy to machine, has a high modulus ofelasticity, good corrosion resistance whenplated, and excellent electrical conductivity.Also, if the nickel plating is electroless nickel,the as-plated hardness of the nickel (RC 52)results in a relatively non-galling interfacebetween the probe and the probe holder. Thisallows easy sliding of the probe in the holderfor standoff adjustment. Aluminum is also anadequate probe holder material provided several precautions are taken. First, mounting andpinch screws should have hardened steelwashers under the heads to eliminate loosen-


14.

the Targa reading returns to the previouslynoted reading. A good mount should return tothe same reading within plus or minus a millionth of an inch.

High grade joints between the mount and thestructure of interest are easiest to achievewhen the mating surfaces are flat, and preferably ground. This type of joint is a must forrepeatable, millionth inch dynamic measurements. Other types of joints (V-blocks, 3-pad,etc.) will be acceptable for lower precision,low frequency measurements. The hysteresistest is a quick, practical method to evaluate aprobe mount. Probe mounting is the greatestsingle cause of erroneous readings. If something doesn't look right, or changes suddenly,look first to the mechanics of the situation, andthen the electronics.

ing or hysteresis due to yielding of the material. Second, hard-coat anodize the aluminumto provide a hard non-galling interfacebetween the probe and the mount to facilitateadjustment. Finally, grind away or machinethrough the anodize to provide a good electrical ground for the probe holder. See Section 3for information on grounding.

A variety of probe holders, hard washers, andother fixturing components are available fromProfessional Instruments Company,4601 Highway 7, Minneapolis, MN 55416.Telephone: 612-933-1222.

Section 3

Grounding, Interference,

and Electrical Noise


15.


SECTION 3 - Grounding, Interference, and Electrical Noise

Section 3 Grounding, Interference, and Electrical Noise

The Targa instrument has an internal noiselevel low enough to allow it to resolve to 1millionth of an"'inch (0.025 micrometer). Inthis system composed of 2 Targas, an oscilloscope and a plotter, the goal has been to retainthat capability in spite of the distributed natureof the system. A distributed system is one thatis not in the same box, or does not share thesame power supplies, or it is physically separated so the "ground" or "+ 15 volts" in one element of the system is not necessarily "ground"or "+ 15 volts" in another element. Since 1millivolt is equal to 1 microinch in the Targa,the difference between "ground" in any of theinstruments (excluding the plotter) must besignificantly less than 1 millivolt for bestresults. A 10-1 rule for signal-to-noise ratio isa good one; if 50 microinches is the desiredresolution, the measurement system noiseshould not exceed 5 millivolts.

Unfortunately, when the probes are mountedon a machine tool, spindle, or other devices tobe analyzed, that device becomes part of thedistributed system and can contaminate thesignals from the probes. Several techniquesmay be employed to minimize these problems.The most common method is to use a separateground lead connected between the Targa case

Probe

Lrr---'-tprobe holder

ground (banana jack on front panel) and thespindle housing or machine tool element beinganalyzed. In some cases, a separate groundlead from the probe to the front panel banana.jack may also be required. For spindle analysis, the ground lead would ideally be connected to the rotating component. For lowspeed ball bearing spindles the electrical connection through the bearing balls is usuallyquite good, and the ground lead can be connected to the spindle housing. For high speedball bearing spindles, and fluid film (oil orgas) spindles, the potential on the rotatingcomponent may be of indeterminate nature. Insuch cases, a small wire, leaf spring, etc. connected for rubbing contact between the spindlehousing or wherever the probe is mounted andthe rotating component should be sufficient toground the rotating component. In high vibration or high speed environments, multiple rubbing contacts might be required to insure therotating components are always grounded.The rubbing contact should touch the rotatingcomponent on as small a diameter as possibleto minimize the tendency to lift off due tohydrodynamic effects. One of the best locations for a rubbing contact is on the comer ofa small diameter shoulder as shown inFigure 3.1:

Hard copper leo. f SpY'ingrulobJng on corner

gY'ound leQd

Conduc tive Mbunt for leo f' spring

Housing

FIGURE 3.1 Proper Grounding Of Target


16.,

---~


Plot 3.1 shows the Targa output of a gas bear··ing spindle driven by a DC motor and an SCRtype controller without a ground to the rotating components. The sharp voltage spike produced when the SCRs turn on induces verylarge noise spikes (of over 0.0015 inches) in

the Targa signal. Plot 3.2 shows the same situation with a small leaf spring grounding thespindle rotor. The controller induced noise isabsent. Always insure the measured element isat the same potential as the Targa front panelbanana jack.

Trig 1. 15V CHl

~ ~

o 5V 10ms SAVE

PLOT 3.1 Spindle Rotor - Not Grounded.Noise Spikes of 0.0015 Inch Equivalent Size

Trig O.OOV CH1

~

O.5V 10ms SAVE

PLOT 3.2 Spindle Rotor - Properly Grounded


17.



In some cases it is not possible to completelyeliminate the noise from SCR or PWM (PulseWidth Modulation) type ~ives. In these cases,two approaches are commonly taken:

1. The spindle is run up to speed and thenthe drive power is turned off.Measurements are taken while the spindle is coasting.

2. Recognize that the spikes are of electrical origin, not mechanical, and can beignored.

When routing the probe and ground cablesminimize the area enclosed by the loop that isformed by the probe and ground cables. Thebest method would be to twist the probe &ground cables together.

The reason for doing this is that the loop

bo.d

becomes an antenna and the area of theantenna loop determines its sensitivity to interference. The Targa capacitive system uses a 1MHz carrier signal that is applied to the measuring circuit; changes in the carrier signal aredetected, filtered, and amplified to produce themeasurement signal. 1 MHz is in the AM"broadcast band and a loop antenna that is theright size and shape formed out of the probeand ground leads could be susceptible to interference from nearby radio stations. Also, thereare a variety of industrial noise sources (welders, switching power supplies, PWM motorcontrols, etc.) that may radiate energy in thecarrier frequency range. A loop of the rightsize and shape may make the Targa susceptible to radiated noise pick-up from thesesources. Twisting the probe and ground leadstogether minimizes the antenna loop area andresults in minimum system sensitivity to suchsignals.

lo.y'ge enclosed loop

bet ter SI"'lO,(l enclosed loop

Best I c.ll"'lost no enclosed loop

FIGURE 3.2 Ground Cable Routing


18.


Section 3.1 Grounding Within the System Components

When the two Targa instruments are used withan oscilloscope, a continuous system ground isnecessary for good system performance. Thetwo Targa instruments should be connected tothe oscilloscope ground on the front panel ofthe oscilloscope by independent ground leads.Do not "daisy-chain" the Targas together byconnecting the ground of one into the otherand then connecting the pair to the oscilloscope ground. Note that these grounds are inaddition to the third wire safety ground supplied in the instrument power cords.

WARNINGUnder no circumstances should the third wiresafety ground be defeated by permanent, passive methods such as "cheaters" or removal ofthe ground prong from any instrument powercord or the system terminal strip power cord.To do so could allow the instrument chassis tofloat to a high voltage potential, resulting in afatal shock to the operator.

The only time the oscilloscope chassis can beallowed to float is when the optional groundisolator is used. See Figure 1.2 SystemInterconnect Diagram, for details of grounded

signal interconnections.

Since the Targa can operate from its internalbattery, it is possible to isolate the Targa,ground from earth/safety ground. To operatethe Targa from its internal battery:

1. Tum Targa power switch to 'OFF' position.

2. Unplug AC power cord from outletstrip.

3. Unplug power adapter cord from rearpanel on Targa.

4. Tum Targa power switch to 'ON'position.

Operation on battery power is useful when theTarga itself must be portable, Le. taken to aremote location where there is no power available. Another instance where operation frombatteries is helpful is in environments wherethere is significant conducted electrical noiseon the AC line. The Targa power supplieshave no line filters to reduce the effect of conducted noise. The oscilloscope does have linefilters and will function properly in moderately noisy environments.

Section 3.2 Use of the Optional Ground Isolator

In some unusual cases, the measurement system ground might be different than themachine tool ground by a significant value(e.g. tens or hundreds of millivolts). This canhappen when the machine tool AC power supply, usually a 4 or 5 wire polyphase source, isseparated from the normal 120VAC singlephase source. The ground for the 120 VACand the polyphase ground might be connectedtogether only at the service transformer, forexample. If there are significant neutral orground leakage currents in one or the other of


19.

the two power sources and the Targa system isfar away from the point where the grounds areconnected together, the potential differencebetween the two grounds can be significant.For example, if the resistance of the groundwire between the point of measurement andthe point where the grounds are connected is.01 ohm and the leakage current in that groundwire is 2 amps, the voltage drop along thatlength of wire is .01 ohm x 2A = .02 V = 20millivolts. Structural grounding systems aredesigned for safety considerations and not the



ultimate in low noise grounding. Incidentally,leakage currents of 2 amps or more in neutralleads (for polyphase) or ground leads (for single phase) are not uncommon. Installationsthat have many personal computers, workstations, fax machines, copiers, etc. are particularly prone to have significant currentsflowing in wires that are supposed to carrycurrent only during fault conditions. The reason for this is that the switching power supplies in such equipment draw current from thesource in short pulses, leading to harmonicdistortion of the power source. The harmonics(at higher frequencies than 60 Hz) are muchmore readily capacitively coupled to theground or neutral wires than is the 60 Hz fundamental. Also, the high switching frequency(hundreds of kilohertz in some cases) allowcurrent to leak through the chassis of thepower supply, which is nearly alwaysgrounded. The harmonic distortion problem issevere enough that legislation is pending concerning allowable limits. In any event, whenthe Targa system is connected and groundedand the machine tool has a different groundand the two grounds are connected, cuuentsflow in the "ground loop" formed by the con·nections and can contaminate measurements.The most common effect is 60 Hz noise on theoscilloscope signal. The amount of 60 Hznoise can be readily measured by getting atrace on the oscilloscope (See Section 4.1.2)and setting the TRIGGER SOURCE to LINE.Line related signals will be synchronized tothe oscilloscope sweep and easily displayed.The usual solution to this problem is to "float"


the measurement system ground by disconnecting it from the 120 VAC ground. This isusually accomplished by using a 3-prong to 2prong adapter, some times called a "cheater".The Targa system ground (oscilloscope chassis, Targa chassis) is then connected to themachine tool ground. This eliminates theground loop problem

If the newly established ground shouldbecome disconnected, the measurement system chassis may charge up to a high voltagehom, for example, the CRT power supply inthe oscilloscope (accelerating voltage is about15 KV!). This could render a fatal shock to theoperator.

Fortunately, there is a solution to this problem.UL allows "indirect" grounding for potentialdifferences between grounds of less than 42volts peak. Tektronix has designed a groundisolator to take advantage of the UL rule.Basically, the ground isolator (Tek modelA6901) contains a relay, a measuring circuit, aground line cord, and several outlets. Therelay disconnects the outlet grounds from theline cord ground. The measuring circuit monitors the potential difference between the twogrounds and trips the relay, connecting theoutlet grounds to the line ground if the potential exceeds 42 volts. This allows groundloops to be broken up while still providingoperator safety.

20.

Section 4

Single Targa Applications


21.


SECTION 4 - Single Targa Applications

Section 4.1 Runout

Section 4.1.1 Definition

Runout is defined as "the total displacementmeasured by an instrument sensing against amoving surface or moved with respect to afixed surface" (ANSI B89.3.4M-1985, page5.5, section 2.17). The term TIR (TotalIndicator Reading) is equivalent to runout. Inessence, all single channel Targa measurements, except thermal drift, are runout measurements. Two cases are portrayed in thedefinition. The first is when the probe is stationary and the surface of interest, or artifact,is moving. The other case is where the artifactis stationary and the probe is moving. Why is

Section 4.1.2 Setup

Runout measurements are equivalent to usinga single dial indicator against the surface of

TO-rget

one interested in doing a runout test? Thereare several reasons. The principle reason is tdestablish the quality of the J?rimary machinetool elements such as spindles and slides.Second, runout is an easy test to set up. Itrequires the least amount of equipment and issimple to understand. Because of this, runoutallows a rapid, preliminary evaluation of agiven system enabling one to either reject thesystem immediately or subject the system tofurther, more exhaustive analysis. Finally, asimple runout test can provide a surprisingamount of information.

interest; therefore, setup is relatively straightforward (see Figure 4.1).

g~Q\)r:d leac!

Pr'obe/Mount

[=:J To.rgo.a =lion PreclSion... f} ... := --

Coo.xio.l co.blefrOM HoutH on reo.rof Targo. to Ch 1on TEK 2211

FIGURE 4.1 Set Up For Radial Runout And Radial Error Motion Measurements


22.

1) Connect the probe to the Targa.

2) Connect a BNC-BNC coaxial cablebetween the "OUT" terminal on the rearof the Targa and the CHI input on thefront of the TEK oscilloscope.

3) Mount the probe and holder on thestructure.

4) Insure that the target is grounded to theTarga front panel banana jack.

5) Connect the Targa front panel ground tothe oscilloscope front panel ground.

6) Insure the Targa and oscilloscope areturned on, then turn on the terminalpower strip.

7) Set the filter switch on the rear panel ofthe Targa to the desired cutoff fre-quency (normally the 1 kHz position).There are 3 settings: 2Hz, 1kHz, and nofiltering (approximately flat response to5 kHz, -6 dB at 10 kHz). Adjust theprobe standoff distance (the distancebetween the sensing end of the probeand the target) so the Targa 'CalibratedRange Display' indicator is centeredand the panel meter reads approximatelyzero. Use the ZERO knob to set the dis-play to exactly zero. The output fromthe Targa to the oscilloscope is nowzero.

8) Make sure the TEK 2211 is in theNON-STORE mode. The STORE/NON-STORE switch is in the upperright hand comer of the TEK frontpanel. Then set the vertical modeswitches to CHI, NORM, and CHOP.The timebase (SEC/DIV) should be set


to about 2 msec/div and the MAGswitch set to Xl. The trigger slopeshould be set to ...r and the triggermode to P-P AUTO. The source shouldbe CHI and the coupling should be setto LF. At this time there should be a single trace across the CRT. Ifnot, adjustthe focus, intensity. and CHI verticalposition to display a trace. Set the CHIinput coupling switch to GND and usethe CHI vertical position to position thesweep on the center horizontal line.Next, set the CHI input coupling switchto DC. The sweep should not move vertically, although it may move slightly ifthe CHI VOLTS/DIY is set to 5 mV orlOmV.

9) If the Targa, probe, and 2211 have beenset up properly, the trace should beabout .1 division wide when the 2211CHI VOLTS/DIY is set to its highestsensitivity. (5 mv/division). The traceshould move vertically when the TargaZERO knob is adjusted, or the probemount is deflected. The system sensitivity is 5 millionths of an inch per verticaldivision since the Targa transfer function is 1 millionth of an inch per millivolt. An additional factor of 10 insystem sensitivity is available by pullingthe CAL knob on the CHI VOLTS/DIYswitch out. The system sensitivity isthen 0.5 millionths per vertical division.The trace may widen to 1 division asthis level of sensitivity is near the system noise level even under the bestcircumstances.

10) The Targa/Osc~lloscopemeasuring system is now ready for use in the NONSTORE MODE.


23.Machine Tool Analysis


Section 4.1.3 Oscilloscope Plots of Runout Signals

"V1 212.0mV Trig HFrej CHl

( ~ \.~

t ./

2120mV If \1\ J,(212 microinches) 'v\

Total Indicator ~ "Reading

h l/i ! ,1.;-..;------------------- ---- 1--- 1--- -~---I=--l-L~.~ -- ---

50mV 2ms SAVE

Plot 4.1 Radial Runout and Error Motion Plot

Radial runout measurement is the sum of the radialerror motion of the spindle at the angle the probe ismounted, the form error of the workpiece and theeccentricity of the workpiece relative to the axis ofrotation. The oscilloscope A.V cursors have been positioned to display the peak to peak voltage (TIR) as212 mV which is equivalent to 212 millionths of aninch (0.000212 inches).


24.

t:. T 16. 680ms


Trig HFrej CHl

II

( ~ '\. r"'"""...I"

II I \..(\ IJ,I

I 'VI,.

~I r'I

h )II

I V IV~I

I

16.68 milliseconds1 cycle equals 1

revolution

50mV 2ms SAVE

Plot 4.2 Determination of Rotational Speed

The AT/ I/A T cursors can be used to measure therotational speed of the spindle. The period is 16.68milliseconds. The equation for conversion to RPM is60/ T. In this case, the computed speed is 3597 RPM.

Section 4.1.4 ~ynchronous and Asynchronous Error Motion

Synchronous error motion is the motion of aspindle that is related to the angular positionof the spindle. Le. the spindle is at the sameposition in space over consecutive revolutions.Asynchronous error motion is the motion of aspindle that is not related to the angular position of the spindle. These motions occursimultaneously while the spindle rotates. Themeasurement of synchronous and asynchronous error motion of spindles is one of theprinciple applications of capacitive sensors.Fundamentally, synchronous error predicts thepotential part geometry the spindle is capableof producing. Asynchronous motion, on theother hand, is useful in predicting the potentialsurface fmish capability of the spindle. Thesestatements are strictly true for single pointmachining operation like turning, facing, and

boring.' While they are not strictly true for processes like grinding, where an averaging effecttakes place due to multiple sparkout passesand lack of rotational syn<~hronization betweenthe wheel and the work, practical experiencehas shown these concepts are useful in grinding applications. Spindles with low synchronous error motion cause less damage to thesurface due to lack of impact while low asynchronous error motion results in less time(fewer sparkout passes) to achieve a certainsurface finish. An ex~mpleof a non-machinetool application where these characteristics areimportant is a disk drive spindle. The synchronous error motion would be used to predict therequired motion of the head to access the samesequence of data bits over consecutive revolutions and the asynchronous error motion


25.



would be used to predict how tightly the datacan be packed on the surface, assuming allother operating parameters such as sensordesign, flying height, magnetic properties,etc., remain fixed.

While a single Targa analysis of these twoimportant aspects of spindle performance doesnot completely characterize a spindle, the single Targa set-up can yield a surprising amountof information. Error motion and asynchronous error motion estimates are made fromthe oscilloscope CRT while the spindle isrotating. The best measurements are madewhen the surface that the probe is measuringagainst is as close to a "perfect workpiece"(ANSI B89.3.4-1985, page 1, Section 2.5) aspossible. Spheres are the best workpiecesfrom a cost and geometry standpoint; Grade 3balls (maximum deviation from sphericity ofless than 3 millionths of an inch) are availablefrom several of the established ball manufacturers (Spheric, Metal Techtonics, etc.) at asurprisingly modest cost. For cylindricalworkpieces, class XXX gage pins (e.g.Deltronics) usually have a cross sectional circularity of less than 5 tTlillionths of an inchand are amazingly straight as well. The use ofsuch high quality workpieces allows errormotion estimates in the 10-15 microinch rangeto be made without resorting to workpiecemapping or reversal techniques (ANSIB89.3.4M-1985, Appendix B and section 7) toremove the errors introduced by an imperfectworkpiece (assuming the centering error ofthe workpiece on the spindle is on the sameorder as the workpiece geometry errors).Asynchronous error motion estimates (ANSIB89.3.4M-1985, page 2, Section 2. 12C) arerelatively unaffected by minor geometry andcentering errors.

To make measurements:

1) Mount the workpiece on the spindle.


26.

2) Center the workpiece on the spindle forminimum runout. The Targa can beused as a dial indicator for this purpose.Rotate the spindle by hand and note theposition of the high and low readings. Ifthe workpiece is far off center, the high,and low will be approximately 180degrees apart. Note the differencebetween the two readings, then rotatethe spindle until the Targa reads thelowest value. At this point, the targetwill be farthest away from the probe.Use whatever adjustment technique isavailable (screw adjustment, tappingwith a small hammer, etc.) to move theworkpiece half of the differencebetween the high and low readings.Rotate the spindle by hand again andnote the difference between the highand low readings. Position the spindle tothe low point and again adjust the workpiece toward the probe an amount equalto one-half of the difference betweenthe high and low readings. Continue torepeat until the difference between thehigh and low values is as close to zeroas possible. At some point, the high andlow values will not be 180 degreesapart. When this happens, the workpieceis close to being as perfectly centered asthe error motion of the spindle willallow. This may also indicate the formerror of the workpiece. At this point,minimization of the TIR is dependent ofthe shape of the TIR signal. Generally,the TIR is minimized by adjusting theworkpiece so the two high spots are1800 apart. This would correspond tothe smallest perfect ring gage that thepart (plug) wouldfit into. The residualerror motion is the sum of the workpiece form error and the spindle errormotion. The workpiece eccentricity isconsidered to be zero. If there are multiple highs and lows, however, the TIRwould be minimized by getting two

highs 180 degrees apart to have thesame value and two lows 180 degreesapart but at some.other angular locationthan the highs, to have the same value.A TIR signal that is essentially round(unchanging) except for a "bump"would be adjusted to the major portion(circular) part of the signal. After finaladjustment, the workpiece is locked inplace and rechecked to insure that thelocking mechanism does not disturb theworkpiece position when activated. Thelocking mechanism should be rigid andnot subject to drift or change due torotational or vibrational influence.

3) Rotate the spindle at the desired testvelocity.

4) The signal on the oscilloscope will generally be roughly sinusoidal in form;adjust the oscilloscope timebase (SEC/DIV) to display approximately 1 fullcycle across the screen (as in Plots 4.1and 4.2).

Example:


5) Adjust the vertical sensitivity (CHIVOLTS/DIY) so the entire waveform isdisplayed on the screen.

6) The total runout is calculated by multi.plying the waveform peak to peakamplitude in divisions by the verticalsensitivity (VOLT$/DIV) and multiplying this number by 1000. This will giverunout in microinches. The use of thecursor feature built into the TEK 2211oscilloscope makes the peak-to-peakmeasurement even easier. See pages 6-7through 6-9 of the TEK 2211 manual.Caution: the cursor readout will give thescope waveform amplitude in millivoltsor volts, depending on the VOLTS/DIYsetting of the oscilloscope. If the reading is in millivolts, the cursor readoutvalue is a direct measure of the runoutin microinches. Remember, the Targa iscalibrated so 1 microinch equals 1 millivolt. If the cursor readout is in volts, thenumber must be multiplied by 1000 toconvert the measurement tomicroinches.

(4.2 DIV) (.05 VOLTS/DIY) (1 Microinch/.OOI Volt)t t t

signal vertical Targa scaleamplitude sensitivity factor

= 210 microinchest

actual runoutvalue

7) There are 3 regimes of interest whenusing this method for error motionestimates:

A: The spindle error motion is "large".In this situation, the error motion isat least 10 times greater than theform error of the workpiece and theability to center the workpiece isalmost completely dependent on thespindle error motion. Most rolling


27.

element machine tool spindles haveerror motions in the 50-100 microinch range and fall into this category. In this area, the form error of aGrade 3 ball used as the workpiececan be essehtially ignored and thesignal on the oscilloscope can beanalyzed by considering it to be thesum of the error motion and the centering error. Plot 4.1 on page 24 is atypical example.



B: The spindle error motion is slightlylarger than the workpiece foon errorand centering error. This regime,where the total signal amplitude isin the 10-30 microinch range, provides the least aniount of infoonation. This is because the foon errormay add to or subtract from theerror motion, causing the sum ofthese two components range fromnear zero (foon error equal to andopposite error motion) to near double (foon error equal to and in phasewith error motion). Centering erroris always additive. See ANSI

B89.3.4M-1985 page 23 SectionA7.5 for more detailed infoonationon centering error and page 28,Section AlO on error motion versusrunout. Note: This regime is particularly troublesome when asynchronous error motion is also on the'same order as the.error motion. Thenon-repetitive nature of asynchronous error motion makes it difficultto center the workpiece as well asget a stable display on the oscilloscope for triggering and measurement. See Plot 4.3.

"",V 1 48.8mV Trig AC CHl

q

Ur ('If

'VV,\ I.~-/ v

I\A ~rvv

~t1(\

W J& '.' 'Vi__ V.- ,- -.- , _.- .- . ,_.- _.- .

I···

20mV lOms

Plot 4.3 Error Motion Slightly Larger ThanWorkpiece Form

The "grease pencil" technique (see step 9, p. 32) isused to produce a sharp spike in the signal, making iteasy for the oscilloscope to trigger properly and produce a stable display.

© LION PRECISION 199128.

c: The spindle error motion is less thanthe form error of the workpiece.This circumstance is characterizedby the ability to adjust the runout ofthe workpiece to near or possiblyless than the workpiece form error.High quality hydrostatic spindles areavailable with error motions of lessthan 1 microinch and reliable measurements in this range are challenging at best (see Plot 4.4a and 4.4b).If this situation should occur, onetechnique is to center the workpieceto the best level possible and use thedigital storage capability of the TEK2211 to SAVE a display (see pages5-6 through 6-7 in the 2211 manual), then reorient the workpiece onthe spindle by rotating approximately 180 degrees and recenter it.Store the new display (at the same


speed) and compare it to theSAVED previously. This procedurewill reduce the probability that theworkpiece form error and the spindle error motion cancelled eachother out in the first test. A more,complicated procedure would be touse the SAVE capability of the TEK2211 in conjunction with theReversal Technique (Section 7.1) toseparate the form error from thespindle error motion. The ReversalTechnique involves moving theprobe and reorienting the workpieceas well as creating some tables fromthe SAVED displays, but will resultin the best possible measurementperformance in this situation. Moreadvanced methods (dual probe) arerequired to fully confirm the spindleperformance.

AVl 5.40mV Trig AC CHl

~- .- - ,-, ,-",- -, .- f-. _. -:" .~

. - ._, .

~ .... ~J.1'\'11'"~....

Ill' VII' ,.... :... ,..''IJI "WIl' "<;1'"""\

5mV 20ms

Plot 4.4a Almost No Error Motion Signal

The workpiece form error dominates the error motionof this spindle. Actually, the workpiece could be centered even better; note the definite once per revolutionchange in the signal. The real form error is smallerand rnostly comes from the distortion of the workpiece surface due to the tension in 12 attachmentscrews. See Plot 4.4b.


29.



AVl 1. 60mV Trig AC CHl

~ ,- .- ,_.- ._.- .-~N ~

._.- .Iil"I.. ,.. ...., ,",,-

IY' "'" V 'lo' ... "I" v''*Ii

5mV 20ms

Plot 4.4b Screw Tension Induced WorkpieceDistortion

The regularly spaced distortion of the workpiece dueto the tension of 12 attachment screws. The distortionmeasures about 1.6 microinches peak-to-peak and thenumber of instances can be readily determined bycounting the cycles in one revolution.

8) Asynchronous error motion is mucheasier to characterize on a qualitativebasis with a single probe than are othererror motions. A spindle that has littleor no asynchronous error motion is onethat does exactly the same thing everyrevolution. This means that the wave·,form on the screen does not change atall. Therefore, to get a qualitativeassessment of asynchronous errormotion, one need only study a singlepoint on the waveform. The best one tostudy is usually the peak point.

A: Set the CHI VOLTS/DIV to a highsensitivity.

B: Use the CHI POSITION knob toadjust the position of the peak point


30.

to the center of the screen. It may benecessary to use the ZERO knob onthe Targa in addition to thePOSITION knob on the 2211.

C: Observe the signal. Spindles withlow asynchronous error motionshow little activity. High quality(See Plot 4.4) hydrostatic spindlesbehave this way. Rolling elementspindles, with their multitude of surfaces in contact and differentialmotion between the inner race, rolling elements, and outer race showlittle spikes, traveling waves andother aberrations on the waveform(See Plot 4.9). Note: If the waveform slowly drifts on the screen, setthe CHI input coupling to AC. This

should eliminate the drift (if it isthennal drift) but unfortunately willmake the ZERO knob on the Targaineffective for adjusting the wavefonn position on the screen. Only


the CHI POSITION knob will beeffective. Plots 4.5 and 4.6 showtypical asynchronous error motionof a high quality ball bearingspindle.

,e,VI 16.0mV Trig HFrej CHI

IA ~- IA- - -111 --r -A- -~-- rr1-- - 7tI ~J I

I~ 1\.1~ ~ W V V ~ V VI W V

50mV 20ms SAVE

Plot 4.5 Asynchronous Radial Error Motionof 16 Microinches

Asynchronous radial error motion measured over 12consecutive revolutions. The measurement resolutionis set low to display the entire waveform.


31.



",VI 15.20mV Trig HFrej CHl

-- -- -----~------ ----

20mslOmV

U-L---.JL...L-..I.J..L-.-LL-l-l...L...LL-LL.L.-ILL....llJ---..l--l--..L.J..L.Ll.-Y

SAVEl

Plot 4.6 High Resolution Display ofAsynchronous Radial Error Motion of 15.2Microinches

The same measurement as in the previous plot exceptthe vertical resolution is 5 times higher. The ZEROknob on the Targa and the vertical position knob onthe oscilloscope are used to move most of the signaloff the oscilloscope screen so only the changing peakheights are shown.

9) The "Grease Pencil" test: occasionally,it will be necessary to provide an arti:'fact on the workpiece that will result ina sharp spike in the oscilloscope signal.Observing only the tip of this spike overseveral consecutive revolutions is animproved technique for measuring theasynchronous error motion. This alsohas the advantage of providing a morestable signal for the oscilloscope trigger. A common technique is to simplydraw a line (approximately .1 inchwide) on the workpiece, parallel to theaxis of rotation. A "magic marker" isfrequently used; the thickness of the inkfilm is about 20 microinches and thedielectric constant of the ink is differentthan air, so the probe can "see" the line.For this reason, this technique has beencolloquially called the "grease pencil"

test. This technique is generally attributed to Bill Bryan.

Rotate the spindle at the test velocity.The roughly sinusoidal waveform willhave a narrow spike in it, occurringonce per revolution (See Plot 4.7). Setthe TRIGGER MODE to NORM andacUust the level so the oscilloscope istriggering on the spike. Offset the waveform as in the previous section, until thepeaks are centered on the oscilloscopescreen. Change the timebase(SEC/DIV)until 10 to 15 peaks are displayed on theoscilloscope. Variations in peak heightcan be considered asynchronous errormotion. Use of the cursors can easilymeasure the total variation from thehighest peak to the lowest peak.


32.


Trig 10 .. OmV CH1

\I. 1J.,1 Ill. k, ~ if.

\ i'j~ ..v \ArJ

V\ \'~" ~' I\i\ ~~rv

'\ Vl 1'\1 r "'\ 11

5mV 50ms SAVE

Plot 4.7 "Grease Pencil" Test

"Grease Pencil" test on a high quality spindle. The asynchronous error motion measures about 2 microinches,however, the probe mounting arrangement used here isnot optimized for measurements at this level, so somecaution must be used in considering these values.

The digital storage capabilities of theTektronix oscilloscope can be quite useful in producing a permanent hardcopyof the spindle error motion. Use the following procedure:

1) Put the oscilloscope in the STOREmode by pushing the STORE/NONSTORE button in.

2) The oscilloscope will begin storingthe waveform displayed on the CRT.

3) Push the SAVE button. The oscilloscope will acquire 1 record length(4 K) of data. The number of peakswill be determined by the SEC/DIVsetting. The oscilloscope will stopacquiring data when the memory isfull.


33.

4) Load a sheet of paper into theplotter.

5) Push the PLOT button (on the righthand side of the TEK 2211 oscilloscope) to plot the information dis-·played on the CRT.

Please be aware that the previous sections onsynchronous error motion should be used forqualitative analysis only. Single probe measurements can be used for weeding out candidate spindles and deciding which should beanalyzed further. Dual and sometimes tripleprobe measurements are required for fullquantitative analysis and comparison.



Section 4.1.5 ImbalanceImbalance of rotating components produces asinusoidal once per revolution force that varies with the sqq.are of the angular velocity. Asingle probe system can qualitatively showwhether or not a significant amount of imbalance exists. It is useful but not necessary to beable to vary the spindle speed in order to dothis procedure.

1. Obtain a stable display at a moderatespeed which is less than normal operating speed.

2. Double the speed so it is above the normal operating speed. If there is littleimbalance, the amplitude of the displaywill not change. If there is a significantimbalance, there are two cases that canbe looked at:

A: Hydrostatic bearings: here the stiffness of the bearing films is usuallyreasonably linear and doubling thespeed would have the effect ofincreasing the imbalance inducedrunout by a factor of 4. Some quantitative data might be drawn fromthis.

B: Hydrodynamic or rolling elementbearings: the stiffness of these typesof spindles is generally very nonlinear. Quantitative conclusions aredifficult if not impossible to drawwithout detailed information aboutthe force versus deflection characteristics of the spindle. See ANSIB89.3.4M-1985, pages 27-28,Section A8).

In both cases, the linear or nonlinear response is a property of thebearings themselves, not of the spindle as a whole. For example, in aspindle with a small diameter shaftand widely spaced bearings, the


34.

shaft between the bearings maydeflect more than the bearings themselves when an extemalload such asimbalance is applied. The shaftdeflection is linear and the system .will appear to be !inear even thoughthe response of the bearings is nonlinear. Fortunately, most modemmachine tool spindles have a greatdeal of attention lavished on theconcept of stiffness and the shaft ismade as robust as possible in orderto make full use of the bearings.

In both cases above there is a smallchance that imbalance may cause areduction in ronout if the imbalanceis located such that it opposes a onceper revolution error motion. Thecancellation will be complete (zeroronout) at one speed only and varying the speed over a wide range canexpose the situation.

3. If the spindle speed cannot be easilyvaried, as in the case of line operatedAC spindles, the effect of imbalance canbe seen by adding a small mass locatedoff the axis of rotation. This procedureis used to produce Plot 4.8 (See alsoPlot 4.1, which is the radial error motionof the same spindle, nominallybalanced).

Interestingly, imbalance does not affect thesurface finish capability of a spindle because itis a synchronous effect. The force vector thatis produced by the asymmetric mass distribution rotates with the spindle and therefore hasa fixed relationship with the angular positionof the spindle. Only effects that can changewith respect to the angular position of thespindle can cause surface finish defects. For amore complete discussion of this phenomenon, see ANSI B89.3.34m-1985, pg. 27.

""VI 212.0mV


Trig HFrgj CHl

( 'V\.,r\ ~;/

If \l/\ I),'v\~ r'

h V,- - - - - ,- - - ,- ,- -_'!. y-- - - - -

50mV 2ms SAVE

Plot 4.1A Radial Runoutand Error Motion ofa Nominally Balanced Spindle

""VI 264 OmV Trig HFrgj CHI

V''''~\ l/""'> .y""'"\

)

1\ , ../

'\ !\ I\ (

I '

50mV 2ms SAVE.

Plot 4.8 Effect of Introducing an UnbalanceCondition

Radial error motion after unbalancing the spindle(adding a 3.5 gram screw at a 1.88 inch radius). Thefundamental amplitude increases from 212 to 264microinches and the higher frequency effects of theballs are reduced. This might indicate that the spindleis improperly preloaded. A rough estimate of the spin-


35.



dIe stiffness can be made from this plot. The forcevector due to the imbalance is F=mrw2 . Using consistent units (SI), the force in newtons is (.0035 Kg)(.048m)(377 rad/sec)2 = 23.7 newtons. This vector rotateswith the spindle and "stretches" the "spring" 1/2 ofthe difference between the p-p value of the runoutwith the screw (Plot 4.8) and the runout without thescrew (Plot 4.1A). The difference in these two plots is .52 mV (52 microinches) so 1/2 of the peak to peakdifference is 26 microinches or .66 micrometers. Thestiffness is calculated by dividing the force (23.7 N)by the deflection (.66 micrometers). This results in astiffness of 35.9 xl06 N/m. This is equal to 205,000lb/in. This test works only if the residual imbalance ofthe spindle is low so that the dominant force in thesystem is due to the unbalancing. By varying theweight of the screw 01 the spindle speed, a force vs.deflection plot can be made.

Section 4.2 Axial Error Motion

Axial error motion can be fully quantifiedwith a single probe system.

1) Mount a workpiece on the spindle andcenter it. A sphere is generally the bestform although this test is relativelyinsensitive to geometry errors of theworkpiece.

2) Mount the probe so the axis of theactive region of the probe coincideswith the axis of rotation.

3) Rotate the spindle at the test velocityand observe the axial error motion.

SPHERE

PROBE: POSITIONING FOR MEASURINGAXIAL. ERROR MOTION

FIGURE 4.2 Set-up For Axial Error Motion Measurements


36.

-------

",V1 302 .. 0mV


Trig HFrej CH1

V\- -- - -- - .- .- - -- - -- - - - -

1\/\f\ f\fJ V rU v

1\ /I

V\ I\~

1("'./'

\I' ri

.~~/U

I -- ..I

I

50mV 2ms SAvE

Plot 4.9 Axial Error Motion

Axial error motion of surface grinder spindle. A 3/8inch diameter ball is glued to the 60 degree machinecenter in the end of the spindle shaft using a fast setting cyanoacrylate adhesive. The error motion has amagnitude of 302 microinches.

",V1 12.00mV Trig HFrej CH1

10mV

-- -- ---- -- ---- -- -

20ms SAVE

Plot 4.10 Asynchronous Axial Error Motion

Plot showing the asynchronous axial error motionover twelve consecutive revolutions to be 12 microinches in magnitude.


37.



-",Vl 52 .. 0mV Trig HFrgj CHl

f\ f\ 1\ \/\V -\.-1' - - - -- - -- ~ _.- - -- - - If JJ.

/I ('

V\ f.\~

'J'

\II rI~ Z\I'J

y V

50mV SAVE

Plot 4.11 Individual Ball Error Motion

The error motion amplitude of the individual balls canbe estimated from the axial error motion plots. Thecursors help delineate the limits. The magnitude is theheight of the smaller wave form which is between 40and 50 mV (40 to 50 microinches).

'/", T 763. 3Hz Trig HFrgJ CHI

f\ f\ r\/\V V ('j VI

t~\I

f. II

\1\ (\/' II

VI r III

~ j I

V\( II

V

50mV 2ms SAVE

Plot 4.12 Ball Frequency

The ball frequency can also be measured from theaxial plot (or the radial plot for that matter). The ballfrequency is determined to be 763 Hz.


38.


Section 4.3 Thermal Drift and Distortion

Thermal Drift (or thermal distortion) can bedivided into two areas. In the first area, thermal growth causes a slow change in the position of the probe with respect to theworkpiece. This is indicated by a slow shift inthe waveform on the screen over time but withno significant change in the waveform itself.The CHI input must be set to DC for thismeasurement. Simply set the probe and workpiece up for radial and/or axial measurements,establish a baseline at time zero, start the spindle and observe the drift of the entire waveform over the time of interest. The otherthermally induced problem is when heat inputto the spindle causes thermal distortion thatdisturbs the geometry or operating characteristics of the spindle. This can occur when heatinput causes the bearing mounting to becomenon-coaxial, changes bearing preload, or evenchanges lubricant characteristics. Thesechanges will be seen as changes in the wave-

form rather than drift of the waveform as awhole. If spindle performance changes due tothermal input must be separated from probelworkpiece position changes, use the CHI,input in the AC mode. This will eliminate theDC or low frequency drift while retaining thecapability to view changes in the ronoutwaveform.

Section 4 of ASME B5.54 titled Environmental Tests, Subsection 4.2 discussesmeasurement of machine tools relative to thermal growth of machining centers due tochanges in the ambient temperature. Section5.7.4 of the same standard titled ThermalSpindle Stability Tests, Fixed and RotatingSensitive Direction, outlines measurementtechniques for quantifying thermal spindlegrowth due to self-heating of the spindle during operation.

Section 4.4 Single Probe Linear Axis Applications

Section 4.4.1 Straightness

A single Targa/probe combination can be usedto measure the straightness of travel of an axisin one plane at a time. A straightedge or someother reference surface is required. The probeis used as a non-contact dial indicator. Theform error of the straightedge must be smallcompared to the expected measurement inorder for qualitative measurements to be madedirectly. Unfortunately, high quality straightedges are considerably more expensive anddifficult to procure than spheres or cylinders.Mitigating this situation considerably is thefact that the frequency content of a linear axis

© LION PRECISION 199139.

signal is usually much lower than a rotaryaxis. This makes workpiece mapping or reversal techniques much more attractive as a wayto provide qualitative measurements. In fact,reversal is a excellent option in this situation.See section 7.2 for a reversal technique toeliminate workpiece form error in a linearapplication.

NOTE: The straightedge must be fabricatedfrom a conductive material. Granite andceramic straight edges will not produce a signal unless the reference surface is plated andproperly grounded.



~--------------------------------l

I II II II I

I ~ II " I: ~ probe . strQight edge :L___ _ ----J

Targo. Lion PrecisionC;i=

e ~ =::ground

FIGURE 4.3 Set-up For Straightness Error Measurement

1) Set the straight edge on the axis to bemeasured.

2) Clamp the probe on some non-movingpart of the machine.

3) Adjust the straightedge so the probereads in the middle of the measurementrange. Use the ZERO knob to adjust thedigital display on the Targa to read zero.The probe should be positioned nearone end of the straightedge.


40.

4) Move the axis so the probe is positionednear the other end of the straightedge.

5) Adjust the straightedge so the digitalmeter on the Targa reads zero.

6) Move the axis so the probe is positionednear the first end of the straightedge andverify that the digital meter reads zero.If not, readjust the straightedge until thedigital meter reads zero.

7) Repeat steps 4-7 until both ends of thestraightedge read zero or as close to it aspossible. Inability to get a zero readingat each end of the axis gives an indication of the lack of repeatability of theaxis in the direction perpendicular to thedirection of travel of the axis.

8) Move the axis in short increments (e.g.1 inch) and record the deviation(+ and -) from zero. These may be plotted and connected to give an indicationof the straightness errors. If a way tosynchronize or otherwise time a strip


chart recorder or plotter to the axis displacement can be arranged, the analogoutput signal on the rear of the Targacan be connected to the recordingdevice to make a trace.

9) This test is only sensitive to translational errors in the direction perpendicular to the direction of travel (See Figure4.4). If the errors are angular rather thantranslational, then two probes used in adifferential configuration should beused. See Section 5.2 for a differentialprobe set-up.

Eo.l9ht~ t perpencHculo.rt tro.nsitionL. __ ._.. . .J

probe is senSitive only to MotionI perpenoilCulo.r to probe o.xls

------..I ----- YQW Qxis

R

- -.-..-L...,_=_=_=_-_-_-_-_-=_=_::::! _

probe notso sensitive

Angulo.r er-ror(Yo.w if o.xls is vertlco.l)

r/FIGURE 4.4 Explanation Of Angular Error


41.


J


Section 4.4.2 Position Repeatability

Position repeatability is the ability of a systemto return to the same position when commanded to move away and then commanded toreturn to the original position. This test is mostapplicable to systems that are under some formof servo control. However, the Targa systemmay also be used to position somethingmanually.

1) Mount a target on the axis to be measured, with a reference (flat) surface perpendicular to the direction of travel.Insure that the target is grounded to theTarga front panel.

2) Mount the probe on some non-movingpart of the machine. (Generally the spindle housing, etc.)

3) Move the axis or the probe until theprobe reads in the middle of the measurement range. Use the ZERO knob toadjust the digital display to read zero.

4) Set the current axis position in the system position controller to zero or"home".

5) Using the system position controller,move the axis away from the probe andthen command the axis to return to zeroor home.


42.

6) The axis position controller will returnto zero or home internally (if it is functioning correctly); however, the mechanical axis mayor may not (probably will,not) return to exactly the same position.The digital meter on the Targa will indicate the error in repeatability. This errormay be positive or negative. This testshould be done several times in order toget a statistical picture of the extent ofthe non-repeatability.

7) For an axis with small position incre··ments (e.g. 0.0001), the 0.006 inchstandoff of the probe allows bidirectional repeatability tests to beaccomplished. This involves moving thetarget toward the probe 30 or 40 posi··tion counts. (0.003 - 0.004 inches) andcommanding the axis to return to zero.This can provide additional informationabout axis performance when directionchanges are required during use as incontouring.

8) This test can also be done on a rotarytable. For best results, the flat targetshould be located so its plane passesthrough the axis of rotation of the rotarytable. This eliminates cosine errors. SeeFigure 4.5.


""--'---

oxis of I'''ototion

~. probe

reference.--/"

----.--_~.l------ 5 U r f Q C e

FIGURE 4.5 Rotary Table Angular Position Repeatability

The repeatability can be expressed as a linearvalue or used with the distance of the probefrom the axis of rotation to calculate an angular repeatability. Simply divide the linearmeasurements from the Targa by the probedistance CD) from the axis of rotation. Theresult is in radians. To convert the radianvalue to arcseconds, multiply by 206,265. To


43.

convert the radian value to decimal degrees,multiply by 57.296. For example, if D = 5.5inches and the Targa measurement is 337microinches, the radian value would be.000061 radians or 61 microradians. Thiswould be equal to 12.6 arcseconds or .00351degrees.



Section 4.4.3 Settling Time

Settling time is the time required for a system(usually under servo control) to approach theftnal or commanded position within a certainerror after a move or slew command.

1) The target, probe, and Targa are all setup as in the section on position repeatability (Section 4.4.2). The additionalpiece of equipment required is the TEK2211 oscilloscope. Connect the outputon the rear of the Targa to the CHIinput on the oscilloscope. Set the CHIinput coupling switch to DC.

2) Make sure the home or zero position ofthe axis corresponds to zero on theTarga digital meter. Ifnot, adjust theZERO knob until the meter reads zero.

3) Turn on the oscilloscope. Set it in NONSTORE. Set the VERTICAL MODEswitch to CHI and the TRIGGERMODE switch to P-P AUTO. The

TRIGGER SOURCE should be CHIand the TRIGGER coupling should beHF REJ. At this time there should be asingle trace across the CRT near themiddle. Use the VERTICALPOSITION knob to move the trace tohalfway between the middle and the topof the CRT.

4) Move the target away from the probe.

5) Set the oscilloscope to STORE and thePRETRIG to 25%. Set the TRIGGERMODE to SGL SWP, the SLOPE TO_r ,and the LEVEL slightly toward'-' from its midrange position.

6) Press the TRIGGER RESET button.The READY LED should light.

7) Command the axis to return to zero orhome.

overshoot \

1-.-

o.l.lowo.ble error ]-

undershoot

position ~ Slop. Iod'co'" st•• ro'. oro.xls velocIty

t-- _

tiMeFIGURE 4.6 Explanation Of Settling Time


44.

8) When the input to CHI (output fromTarga) is greater than the trigger levelset by the LEVEL knob and the slope ispositive, the TRIG'D/READY LED willgo out, indicating the oscilloscope isdigitizing and storing the input signal toCHI. When the oscilloscope is doneacquiring a full record, it will displaythe record on the CRT. The cursors canbe used to measure the settling time.

9) Adjustments will need to be made onthe CHI VOLTS/DIV, the timebase(SEC/DIV) and the TRIGGER LEVELto get a trace similar to Plot 4.13 orFigure 4.6.

10) This test should be done several times,at different axis velocities and with different amounts of weight on the axis (ifvarying weight is an operational mode).The axis performance will change as theoperating conditions change.


11) This test can be applied to rotary tables.The set-up is the same as in Section4.2.2, #8.

12) This test can be used to determine if theaxis position servo is properly compynsated. Compensation involves adjustingthe servo parameters so the axis settlesin a minimum amount of time, usuallywith no overshoot. Such a system is saidto be critically damped. Generally, thereis a region in the operating range of themachine where the axis is criticallydamped i.e. average part weight, average feed rates, etc. Operation outsidethis range, as with a part near the maximum capacity of the machine, willcause the servo response to be nonoptimum. There are several other wellknown responses to be aware of.

flno-l position

,o.xls velocity

tif'Yle

ove("'-'do.Mpea-=) long settling tiMeFIGURE 4.7 Settling Time: Over Damped


45.



finoJ positionsettling tll'ie-

------,-,,---- +----'-------.

o.xis velocity

/tiME?

FIGURE 4.8 Settling Time: Critically Damped

Critically damped ==> shortest settling time with no overshoot

~. se ttling pain t

FIGURE 4.9 Settling Time: Under Damped

under-damped - shorter settling time than criticallydamped is possible, but there is some overshoot.


46.


'-----------------

FIGURE 4.10 Settling Time: Oscillating

oscillatory - gain too high or dampingtoo low, not an operational mode

Trig 0 .. 10V CHl

/!

/

/I

I"O. 2V 0.5s SAVE

Plot 4.13 Critically Damped Servo System

Slew and stop plot of a well-behaved (criticallydamped) servo system.

Gain and damping are usually adjustableservo parameters. In general, the gain isincreased until the system oscillates, then thedamping is adjusted until the oscillation stops.This process is repeated until maximum gainin conjunction with the appropriate amount ofdamping yields the best performance over theextremes of the axis variables. A systemshould never be allowed to operate in theoscillatory mode. Other names for gain and


damping are proportional and derivativeadjustments. Some servos also have integraladjustments designed·to eliminate (or reduce)steady state errors. The integral adjustmentwill interact with the gain and damping adjustments and the methodology for parameteradjustment may differ depending on the servo.In all cases, please consult the operation manual for the servo or machine before makingany changes.


47.


13) Systems with little mechanical damping(usually friction) and digital positionfeedback may oscillate +/- 1 positioncount around a commanded position andnot be considered to be operatingimproperly (See Figure 4.11).

This is due to the digital nature of the positionfeedback in that the controller cannot tell thatthe actual position of the axis is not at thecommanded position until it is at least 1 count

away. In a similar way, if axis position isdetermined by a digital position sensor (e.g.optical encoder) and the axis velocity is alsoderived from the same sensor (this is commonly done in many "all-digital" servos) thenthe axis velocity may appear jerky at veryslow axis velocities. These circumstances rep~resent fundamental limitations to axis performance and should not be viewed asoperational problems.

FINAL POSIT ION -+--------+-t-+-+-+-+-+-+-t--.

POSITION

1 _

TIME

FIGURE 4.11 Settling Time: Quantization Error

Settling time showing velocity and position 'errorsof quantization' at a very low axis velocity.


48.


Trig O.06V CH2..

I.---.-..

\ ("'-

"" '""~~'"

.......-.1-'

O. 2VJ. O. 2s SAVE

Plot 4.14 Stiction and "Stair Stepping"

This is a plain way slide commanded to move at aslow speed. The first few command pulses cause elastic windup of the leadscrew. Slide stiction preventsmotion. When the elastic forces are large enough, theslide breaks away with a short high speed jump (indicated by the steep downward slope in the displacement signal). Once moving, the slide operates muchbetter, but the digital motion of the slide/position feedback system is apparent in the trace. The subtle "staircase" effect in the position is due to the digital natureof the feedback and control system. During the retracemove both the slide and the control system operatebetter because of the higher velocity. The slide operates better because it flies higher on its hydrodynamicoil film, and the position control system operates better because of t4e higher rate of position feedbackinformation. This allows the controller to calculate theactual velocity more accurately, resulting in betterposition loop damping and velocity control.


49.



Section 4.5 Vibration

Vibration is the unwanted (generally) displacement of one machine element withrespect to another. The Targa system with theTEK oscilloscope can give a general view ofthe nature of the vibrations if the signal iscomplex (i.e. made up of many frequencies).The system can give a reasonably good picture if the vibration is a relatively pure frequency such as might be encountered if oneelement in the machine tool were at resonance. The peak hold function on the Targacan indicate the maximum excursion over anytime frame. If complex signals need to be analyzed there are two choices: use the TEKoscilloscope in the digital storage thode alongwith a PC and a PC based waveform analysisprogram and the TEK Grabber II waveformtransfer software. In this system, the oscilloscope acquires a waveform and transfers it tothe PC via the TEK software. Then the PCand a waveform analysis program break downthe waveform into its spectral components anddisplay them. The other choice is to route theoutput of the Targa directly into a real-timespectrum analyzer. The filter switch on therear of the Targa should be set to "unfiltered"to take advantage of the full performance ofthe Targa system. Signals analyzed in thisfashion can accommodate nearly any mechanical system including rolling bearing induced


50.

high frequency impulse signals. The use of aspectrum analyzer can also help to characterize noise (both mechanical and electrical) andvibration as to the source and its magnitude a~

a function of frequency. The ability to see theactual displacement between elements ofinterest as opposed to, for example, measuringthe acceleration of one element with an accelerometer/charge amplifier combination, provides far deeper insight into machine tooldynamics.The setup for vibration is similar to the othersetups discussed in Section 4, the probe ismounted on one part of the machine tool and atarget surface is mounted on another element.Generally, one is interested in the relativemotion between the tool and the part so theprobe might be mounted on the table and a target surface (e.g. a gage pin) would bemounted in the spindle. The oscilloscopeshould be connected and turned on. In thisway, the amplitude of system noise (partiCUlarly electrical noise) can be immediatelyqualitatively assessed. Then simply turn onvarious machine tool subsystems likehydraulic pumps, axis servomotors, etc. one ata time to see their effect on the position of the·tool with respect to the part. All of these subsystems contribute to the background noise orsignature of the machine.

",V1 4.00mV


Trig -18.0mV CH1

- .,- - - - ,-- - ,_.-u..~- - -- - .- ,- .ii" -

~~ I"~N ~ ""M ,N\N.~ '\0-~

5mV 20ms

Plot 4.15 Grinding Machine, Coolant andHydraulic Pumps Off

Wheel to workpiece vibration is 4 microinches. Thesource is ambient structural vibrations and acousticnOIse.

",V 1 10 20mV Trig 21.4mV CH1

Plot 4.16 Grinding Machine, Coolant andHydraulic Pumps On; Wheel to WorkpieceVibration is 10.2 Microinches

Wheel to workpiece displacement on a grinding machine with subsystems on and off, wheel is not rotating.


51.



As a cautionary note, if high frequency signalsare expected, probe mounting and target surface natural frequency are important considerations. See Section 2 for further informationon probe holders. Also, the Targa/Oscilloscope system can help to determine thenatural frequency of the probe mount and thetarget surface. To do this, use the followingsteps:

1) Set up the Targa and the Oscilloscope inthe digital storage mode as described insteps 1-10 for settling time measurements (Section 4.4.3) except set thetrace to the center of the CRT ratherthan in the middle of the upper half.

2) Arm the trigger by pushing the RESETbutton in the TRIGGER section. TheREADY LED should light.

3) With a small soft faced hammer, rap theprobe mount or the target surfacesharply.

4) Make adjustments to the CHI VOLTS/DIV and the SEC/DIV to displayapproximately 5 full cycles on the CRT,pushing the RESET button and strikingthe surface of interest each time anadjustment is made.

5) Ignoring the first cycle, use the cursors(See page 6-15 of the TEK 2211 manual) to measure the frequency ofoscillation.

Ii" T 1262Hz Trig HFrej CH1

II

II

II

- {\ r\ ( \ 11:\ / \I~ I \/ l/ V I V \-I

I

\)/ III

IV 5ms

Plot 4.17 Natural Frequency of Probe Mount

Natural frequency of a probe mount. Note the disturbed initial response due to higher order vibrationfrom the impact process. It is difficult to store energyat high frequencies and such artifacts quickly disappear leaving only the lowest order, or fundamental frequency of the probe holder. The l1T/ 1/11 T cursors areused to measure the actual frequency.


52.

The first cycle may contain high frequency artifacts from the impact process, but these should die quickly. Thefollowing excursions will be structuraloscillations at the "natural frequency" orthe lowest frequency at which the structure will resonate. The spacing betweenany two peaks should be the same. Ifnot, there may be two or more elementsin the structure whose natural frequencies are close together. The rate atwhich the amplitude decays is an indication of the amount of internal dampingavailable. Things that oscillate for along time (a la tuning forks) have eitherthe capability to store a large amount ofenergy or have minimal internal damping, or both. Conversely, things that


oscillate only a few cycles either cannotstore much energy or have a largeamount of internal damping. A goodprobe holder will oscillate with a smallamplitude at a very high frequency foronly a few cycles. Theoretically, me<;lsurements can be made when the structu-·ral vibrations in th€ system are close tothe natural frequency of the probeholder and the workpiece as long as thedamping of those elements is high. Inpractice, however, the probe/target natural frequencies should be at least twiceas high as the highest structural vibration frequency applied to them and atleast 5 times higher if high precisionmeasurements are to be made. It shouldbe noted that operation with a spectrum

.6.T 379,25ms Tiig HFrej CHI

II

II

II

-+

~~WfAI

IVV

I

III

IV 50ms

Plot 4.18 Extended View of Probe MountResponse

This is an expanded view of the probe holder responseto an impact. Even after nearly 400 milliseconds thepeak to peak oscillation of the probe with respect tothe target is still 200 microinches. The low natural frequency (126 Hz) and the long time to damp out indicate this is not a very good probe mount, especiallyfor dynamic use.


53.



1/",T 188. 3Hz Trig AC CH1

I \.1 \

1/ \ V \. ......i. ...\ /: ~~ I

III

5mV 2ms

Plot 4.19 The Response of a Good ProbeMount

This probe mount is slightly better than the previousone from a natural frequency standpoint, but muchbetter in terms of its damping; after only 1 cycle it iswithin 2 microinches of its initial position.

analyzer somewhat mitigates theserequirements due to the analyzer's ability to discriminate exactly these sorts ofsignals; however, operation near resonance should be avoided altogether.Operation above resonance is possible,keeping in mind that the structure thatresonates goes through a 1800 phaseshift and a large amplitude peak at resonance. This complicates measurementssince the distance between the probeand the target may change significantlyif measurements are made near resonance. If the driving frequency is highenough so the 1800 phase shift is complete, the resonating structure begins toact as a mechanical filter. Finally, theprobe holder will oscillate readily atexact integer multiples of the natural


54.

frequency and high frequency operationmay excite vibrational modes other thanthe fundamental mode.

6) Curing probe holder problems such asdescribed above is not a trivial task. Ingeneral, use short robust structures anddesign in some type of asymmetry likean off-axis hole. Symmetric structuresresonate readily since their uniform geometric features facilitate the generationof standing waves. Also, use materialsthat have high internal damping. Castiron is one of the best and some composite materials also exhibit this property. Also, pay attention to the otherconsiderations mentioned in Section 2such as electrical conductivity and goodjoint design.

I~T 1. 515kHz Trig AC


CH1

II

' ,

II

II

\,I

\,

~ I.......,. '-, ,,

5mV 1ms

Plot 4.20 The Response of a Very HighPerformance Probe Mount

A very high performance probe mount, in terms ofboth natural frequency and damping. A natural frequency of over 1500 Hz and very good damping allowthe full performance of the Targa system to be used,even in mechanically noisy environments. Of particular note is the low amplitude of the natural frequencyoscillation compared to the large amplitude of theimpact. This mount is constructed of a short, thickwalled, ring structure with the joint surfaces groundflat to 10 microinches.

Plot 4.19 shows the response of a probemount that is quite suitable for most situations. The mount has a reasonablenatural frequency and very good damp-


55.

ing. Plot 4.20 shows the response of avery high performance probe mount,suitable for the most demanding work.


Section 5.0

Dual Targa Applications

©L10N PRECISION 1991

56.


SECTION 5 - Dual Targa Operations

Section 5.0 Dual Targa Applications

The normalsystem described in Section lofthis manual is a combination of 2 Targacapacitive sensor systems and a dual channelanalog/digital storage oscilloscope. Such acombination can make many measurements asingle channel system cannot make and theability to process the sensor signals in unusual

ways, such as a differential connection, furtherenhances the utility of this configuration.

Note: when using two Targas for combined,measurements, both Targas must have thesame sensor oscillator frequency. This isaccomplished by connecting the "sync" signals together. See Figure 1.1, page 11.

Section 5.1 Axis of Rotation Applications

One of the major applications for this measurement system is the assessment of the quality of an axis of rotation. This is a veryimportant parameter to be able to measure ascivilization is intimately dependent on thingsthat rotate. Some modern technologies such asmachine tools, computer disk drives, jetengines, etc., are performance limited by thequality of their axes of rotation. With theseconsiderations in mind, an American National

Standard (ANSI/ASME B89.3.4m-1985) relating to axes of rotation has been developed andpublished. The standard contains a great dealof information and should be studied carefully. Some, but not all, of the informationwill be repeated here, but the emphasis of thismanual will be on application of the Targa/Oscilloscope system to facilitate the measurements in the standard. The emphasis will beprimarily on machine tool applications.

Section 5.1.1 Rotating Sensitive Direction Error Motion

An axis of rotation with a rotating sensitivedirection is one where the tool rotates with thespindle as in a boring machine, a millingmachine or a drilling machine. The dualTarga/Oscilloscope system can fully characterize both the synchronous and asynchronouserror motions of such a spindle.

1) A probe holder is required that canmount the two probes in the same plane90 degrees apart. All the usual considerations for probe holders still apply, andthe 90 degree angle should be reasonably good, within 1 degree. The probeholder should be mounted right on the


57.

spindle housing if possible, otherwiseone should strive for the shortest structuralloop possible.

2) A "perfect workpiece" (usually a ball)should be mounted in the spindle. Someprovision for adjusting the workpieceeccentricity with respect to the rotational axis should be made and thisadjustment must have a rigid lockingcapability so as not to shift under rotational and vibrational influences. Thegeometry of the workpiece should besuch that its errors are small comparedto the expected measurements.


Fortunately, balls (grade 3) with errorsin sphericity of less than 3 millionths ofan inch are readily available.

3) Insure that the Targas have a commonoscillator frequency. This is achieved byconnecting the 'sync' tenninals at therear of the Targa. See Figure 1.1, page11. Connect the output of one Targa tothe CHI input on the oscilloscope andthe output of the other Targa to the CH2input. Both oscilloscope inputs shouldbe set to DC, the VERTICAL MODEswitches should be set to BOTH,NORM, and CHOP.

4) Tum on the system power by turning onthe power strip switch. Insure that bothTargas and the oscilloscope are turnedon.

5) Using the digital meter on the Targaconnected to CHI, bring the CHI probeinto the middle of the measurementrange and adjust the eccentricity of theworkpiece to a low value (Le. center theworkpiece reasonably well). Theamount of eccentricity depends on theexpected magnitude of the error motionand the system noise level and there is atrade-off between the two. If adjusted toa low value, error motion is magnifiedon the CRT but so is system noise. If setto a high value, the system noise is lessapparent but the measurement sensitivity is low. A general rule of thumbwould be to set the total indicated reading of the workpiece to about 4 timesthe expected measurement amplitude. Inother words, if measurements on theorder of 20 millionths of an inch areexpected, adjust the eccentricity of theworkpiece to about 80 microinches TIR.This allows a reasonable base circle tobe displayed on the CRT while stillretaining sufficient resolution to show


58.


the error motion. Thenset the CHI andCH2 VOLTS/DIV knobs so that 1 division is equal to the expected measurement value, which in this example,would be 20 mV/division (remember,the Targa transfer function is 1 micro- i

inch per millivolt). Also insure theeccentricity is symmetric about zero onthe digital meter of the Targa. If not,reposition the probe so the middle LEDon the linear range scale is lit and usethe ZERO adjustment knob so the digital display indicates +/- 40 millionths atthe extremes of the workpieceeccentricity.

6) Position the second probe so it is in themiddle of the measurement range anduse the ZERO knob to fine tune the output so the output is nominally zero with+/- 40 microinch excursions.

7) Rotate the spindle at the test velocity.The oscilloscope should display twowavefonns on top of each other near thecenter of the CRT. See Plot 5.1. Use theVERTICAL POSITION KNOBS toseparate the two wavefonns. Eachshould be about 4 divisions peak to peakand one should lead the other by a quarter of a cycle. Adjust the SEC/DIV knobto display 1 or 2 cycles across thescreen. Reversing the direction of rotation should cause the other wavefonn tolead by a quarter of a cycle. Ifno display is present, check to make sure theoscilloscope is triggering properly. TheTRIGGER SLOPE should be J- , theMODE should be P-P AUTO, theSOURCE should be CHI, and theCOUPLING should be HF REJ.

8) Assuming the two sensors are producingtraces on the CRT, tum the SEC/DIVknob to the extreme CCW positionmarked X-Y. The timebase sweep is


",Vi 0.200V Trig AC CHI

r\ / Y i\ r.~ 1/';f\.\ I II. \ \ II .f

V\ \ / I 1\ i\ I~KJ \.) J \ XV \ l!C

O. 1V" >0. 1V- 2ms SAVE

Plot 5.1 Plot of Two Probes, Mounted 90Degrees Apart

disabled and the CHI and CH2 inputsignals are routed, after amplificationby the input amplifiers, to the horizontaland vertical deflection amplifiers for theCRT. A positive input to CHI (X) willcause the electron beam that makes theCRT flouresce to move to the right(positive X direction in a normalCartesian coordinate system); likewise anegative input to CH2 (Y) will causethe electron beam to deflect downward.The result of the changing sinusoidalwaveforms being input to the X and Ychannels is that the electron beam willtrace out a roughly circular figure onthe CRT. The degree of circularitydepends on how close to a perfect sineand cosine the waveforms are. The 90degree angle between the waveforms isestablished by the probe holder, and theworkpiece form error is small comparedto signals of interest. The Targa sensingis designed to reproduce these signalsaccurately and hopefully system noise(especially electrical noise) is low. Theonly remaining source to corrupt the

perfect sine and cosine waves is theactual error motion of the workpiece i.e.it is not where it should be to produceperfect waves. The closed figure on theCRT is called a Lissajous pattern andthe spindle error motion can be determined from it.

9) Once a Lissajous pattern is displayed onthe CRT, use the CH2 VERTICALPOSITION and HORIZONTALPOSITION knobs to center it on theCRT. The CHI and CH2 VOLTS/DIVknobs may be used to change the size ofthe Lissajous pattern but they shouldboth be set to the same sensitivity. Thespindle error motion value is equal tothe difference in radii of two concentriccircles that will just enclose theLissajous pattern. The value obtaineddepends on the location of the commoncenter of these two circles. There are 4methods for locating the commoncenter:


59.


A) Minimum Radial Separation(MRS)

B) Least Squares Circle (LSC)C) Maximum Inscribed Circle (MIC)D) Minimum Circumscribed Circle

(MCC)

The Axes of Rotation Standard (ANSI/ASME B89.3.4M-1985) contains acomplete section on each of these methods (Section All, pages 28-31) andthey will not be repeated here.

10) Once the center has been established,the CHI and CH2 VOLTS/DIV settingsdetermine the difference in radii of thetwo circles in millivolts. The error valueis easily arrived at by remembering thatthe Targa transfer function is 1 microinch per millivolt.

11) Unfortunately, the Tektronix 2211Oscilloscope cannot store in the X-Ymode. This means that the center mustbe established by one of the methodslisted above and the error motion valueestimated while the spindle is running.

12) There are many other artifacts of interest buried in the Lissajous pattern:

A) Transmission or gear noise: manymachine tools have gearboxes tovary the speed and torque of thespindle over a wide range. Thesecan affect the Lissajous patterndepending on what speed range thetransmission is in. Gear mesh noisecauses high frequency, low levelsignals to move around the mainform of the pattern. If the numbersof teeth in mesh in the various speedranges are known, a spectrum ana1yzer can look directly at the proper


60.


frequencies and measure their contribution to error motion. Also, gearmesh noise may exacerbated duringacceleration and deceleration.

B) Spindle bearings: the rolling ele- I

ments (balls, tapered, spherical orcylindrical rollers) rotate on theirown axes as well as precess aroundthe spindle axis at an angular velocity that is different than the spindlevelocity. These effects are manifested in both high frequency signals(from rotation about element axes)and low frequency signals (from theelements precessing as a group)superimposed on the Lissajous pattern. These signals are less affectedby spindle acceleration and deceleration than is the gear mesh signal.However, if angular acceleration ishigh enough, the rolling elementsmay skid or slide rather than roll,resulting in anomalous signals.Also, centrifugal loading in the bearings caused by very high speedmight change the effective preloadon the bearings and alter the errormotion Lissajous pattern.

C) Imbalance: imbalance will cause achange in the size of the Lissajouspattern as a function of spindlespeed. The pattern may get larger orsmaller depending on where theimbalance is with respect to the lineconnecting the axis of rotation andthe center of the workpiece. Theamount that the size changes as afunction of sp.eed depends on howlinear the stiffness of the spindle is.Rolling element spindles are notvery linear; hydrostatic spindles aregenerally relatively linear.


Section 5.1.2 Fixed Sensitive Direction Error Motion

A machine tool with a fixed sensitive axiserror motion is one where the tool is fixed ordoes not rotate with the rotational axis. Anexample of a fixed sensitive axis machine is alathe or turning machine. In such a machine,only relative motion between the tool and theworkpiece, in the direction of the tool, contributes significantly to part size or geometryerror. Error motion of the spindle that is perpendicular to the tool has little effect on thesize or geometry of the part. A computer discdrive, with its fixed head positioning mechanism, should be considered a fixed sensitivedirection application. In general, the probeshould replace the tool Le. be mounted in thesame position, for meaningful results to be

obtained. A single probe at the tool positionand some means for generating the base circleon the oscilloscope are required to make polarplots. However, the 2 channel TargajOsciltoscope system cannot make fixed sensitivedirection polar plots without some auxiliaryequipment. Either a third channel, some basecircle generating cams, and some additionalanalog electronics are required, or some sortof spindle position sensor (e.g. encoder orresolver) and a digital computer are requiredto achieve proper results. See ANSIjASMEB89.3.4M-1985 Sections A14 through AI6,pages 32-35 for a more in-depth discussion ofthis situation. Also, see Addendum A(p. 83-89) for additional discussion.

Section 5.1.3 Asynchronous Error Motion

Asynchronous error motion is the deviation ofthe total error motion polar plot from the average error motion polar plot. In other words, itrepresents the revolution to revolution variance of the spindle from its position as determined by an average over many revolutions.To make asynchronous error motion valuemeasurements, operate the spindle at the testvelocity and set the oscilloscope to producethe Lissajous pattern. Then, draw an imaginary line from the center of the Lissajous pattern at any angle through the Lissajouspattern. Over many revolutions record theextreme values at this one angle. Do the same


61.

for several different angles. The maximumradial value minus the minimum radial valuefound at any angle represents the asynchronous error motion value. This value representsthe peak to peak surface finish that themachine is capable of just as the error motionvalue represents the potential part geometry orroundness. See ANSIjASME B89.3.4M-1985,Sections A7.2 and A7.3, pages 20-23, foradditional information on the application ofthis information. Also, see Section AIl.5,page 31 for information on choosing the correct center in the polar error motion plot forhigh accuracy measurements.



SEVERALANGLES -+---.....

-,.--.'/

EXTREMES AT ANYFIXED ANG~E

AVERAGE ERROR

FIGURE 5.1 Asynchronous Motion Value Determination

200150100

Y"'"..,J-~~-~.~~

.~// "

! "\/ )c

\ f/,

~~./'----..~~~~~

5050

150

200

100

PLOT 5.2 Typical Lissajous Pattern


62.


Section 5.2 Quasi-Differential Measurement

The availability of two measurement channelsfrom the Targa instruments combined with theability of the oscilloscope to "subtract" CH2from CHI, allows the possibility of makingdifferential measurements. Differential measurements are measurements where one isinterested in the difference only, and not theactual value of, two signals. The differencebetween the signals is usually small comparedto the signals, or there may be noise or driftpresent. Processing signals this way yieldsmany benefits, mostly in the areas ofimproved sensitivity (usually a factor of 2)and reduced influence of noise and drift. Thedrawbacks are the requirement of two sensorsfor a single measurement (although this is notalways true) and more complicatedelectronics.

This section is titled 'Quasi-' because whilethe oscilloscope can be configured to subtractone signal from another, it does not do a particularly good job of subtraction when compared to systems specifically designed fordifferential measurements. The term "true"differential system will be used for a systemspecifically designed to have excellent performance when making differential measurements. When making differentialmeasurements, one is concerned with measuring only the difference between the signals,regardless of the actual value of the signalsthemselves. Thus, a true differential systemwith two inputs of 2.000 volts and 2.001 voltswould have an output of either -0.001 volts or0.001 volts (depending on which signal is subtracted). The same true differential systemwould have the same output if the inputs were5.237 volts and 5.238 volts or -11.713 voltsand -11.714 volts.

The output is the difference between the signals irrespective of the signals themselves.The (usually large compared to the difference)


63.

signal that accompanies the two signals, iscalled the common-mode signal or value. Atrue differential system has the property ofeliminating or rejecting this common mode,signal. No system is perfect and a smallamount leaks through to corrupt the measurement slightly. A well designed differentialsystem would reject or reduce the effect of thecommon mode signal by a factor of perhaps100,000. This means that, in the originalexample above, the true difference would be0.001 volts and the common-mode errorwould be 2 volts (the common-mode signal)divided by 100,000 (the common-mode rejection ratio) or 0.000020 volts. Therefore, theoutput of the differential system would bebetween 0.000980 volts and 0.001020 voltswhen the exact difference is 0.00100 volts, foran error of about 2%of the difference value.There is not enough range on the oscilloscopescreen when the input signals are in the twovolt range and the sensitivity (VOLTS/DIV) isset high enough to discriminate a 0.001 voltdifference, or to even display the signals.

For all differential systems, the ability tor~ject a common mode signal is best at DCand low frequencies. The common-moderejection ratio degrades as the common-modesignal frequency increases. This means that adifferential system can reduce one of the mostpervasive of system noises: 60 Hz powerlinenoise. This can be done because the 60 Hzsignal generally appears in both sensors withapproximately the same amplitude (if component layout is symmetric and antenna looparea is similar, etc.) and at the same time(because of the single source) so it is a common-mode signal and is rejected. If one wereinterested in a 15 millivolt differential signalwhere the 60 Hz noise on the two channels is105 mV and 107 mV respectively, the 15 mVsignal would have only 2 mV of 60 Hz noisewhen displayed differentially. The same sig-


nal would be unmeasureable if the 60 Hznoise was present during a single channelmeasurement. Any signal that affects bothsensors simultaneously, even if it is not electrical in origin,. such as thermal drift or vibration, is a common-mode signal and thusreduced by the common mode rejection ratioat the frequency of the signal. As was statedpreviously, the higher the frequency of thecommon signal, the lower the common-moden~jection ratio and the less effective the system is at reducing the signals effects.

Unfortunately, general purpose oscilloscopesare not designed to be "true" differentialmeasuring systems. Such instruments have alow common-mode r~jection ratio at DC anddegrades rapidly as the common-mode signalfrequency increases. In addition, if the common-mode voltage is high and the desiredmeasurement sensitivity is high (e.g. measuring 0.001 volt difference with a 5 volt common-mode signal), the oscilloscope inputamplifiers may saturate or operate in a non-


linear fashion, causing erroneous results. Themoral here is to adjust the probes or the offsetadjustment on the front panel of the Targa tokeep the outputs near zero volts. This minimizes the common-mode signal. Also, don'tdepend on significant rejection of commonmode signals with frequencies above 150 Hz:The Tektronix 2211 oscilloscope used in thismanual has a common mode rejection ratio ofabout 100:1 at 150 Hz for a 500 mV p-p sinewave input to both channels. Any significantcommon mode voltage (Le. 100 mV) seriouslycompromises this value. Oscilloscope manufacturers do not generally specify the commonmode rejection ratio of general purpose oscilloscopes. For these reasons, the oscilloscopemay be considered a "quasi-differential"measuring device, unless fitted with specialprovisions for differential measurements.Technically, the oscilloscope can make themeasurements but they are subject to relatively severe restrictions, and should be madewith those restrictions firmly in mind.

Section 5.2.1 Tilt Error Motion of an Axis of Rotation

Tilt error motion of an axis of rotation is oneof the most common measurements madeusing two probes in the differential mode. Themeasurement can be made by placing theprobes so they measure on the face of a perfect workpiece or by placing the probes sothey look at the radial motion at two locationsalong the axis of rotation. See Section A6 andFigure A8 in ANSI/ASME B89.3.4M-1985for additional information on nomenclatureand probe placement. In either case, the difference signal is displayed on the oscilloscope,and the value at any point must be divided bythe distance between the probes. The resultwill be the instantaneous tilt error, given inradians. To convert radians to arcseconds,multiply the radian value by 206,265.


64.

Multiply the radian value by 57.296 to convertto decimal degrees. For example, if a differential signal at a particular spindle angle has avalue of 410 microinches (.000410 inches)and the separation between the probes is 2.5inches, the instantaneous tilt is .000410/2.5 =

164xlO-6 radians or 164 microradians. This isequal to .00940 degrees or 33.8 arcseconds.

Obtaining a stable display when making differential tilt error meaSUf6ments can be tricky.The most reasonable method is to adjust thetime base until one full sweep across the oscilloscope screen is equal to one complete revolution. This can be done by using the SEC/DIV knob and the CAL knob that is coaxialwith the SEC/DIY knob. The SEC/DIY knob


sets discrete sweep rates and the CAL knoballows sweep rates between the discrete rates.Alternatively, an external trigger signal (froman index mark on an encoder or a reflective

Section 5.2.2 Straightness

Two probe differential measurements forstraightness are superior to single probe measurements (See Section 4.2.1) because differential measurements are sensitive to rotation(angular motion), but not to translation, of theinstrumented axis. This allows separation oftranslation (seen with either of the two probesused as a single channel measurement) fromangular error motion. The set-up is the sameas a single channel straightness measurementexcept the second probe is mounted some distance from the first probe. Either probe is used

photosensor, for example) can be input intothe EXT INPUT connector in the trigger section. The second method is superior to the firstbut requires additional hardware.

to measure the translation and the two areused differentially to measure the angularmotion. As in the previous section, a singlevalue at any axial position of the axis will beobtained from the difference between the twoprobes; this value must be divided by the distance separating the probes to yield the angular motion in radians. The conversion factorslisted in the previous section for conversion ofradians to degrees or arc seconds also applywhen measuring straightness.

r---·--·-----IIIIIIIL ..__ . .

--_. ------l

IIIIII______ .J

stro.lght edge

,.--, ,D.rgo. L!o~ rec!slon

L...-oI 1:1=~ ® 1:1:

,.--, Tars.(\. Ion reL...-I 1:1 =~®I:I:

FIGURE 5.2 Straightness Measurement Using 2 Probes


65.



Section 5.3 Multi-Axis Applications

Multi-axis applications of single channelmeasurements are possible because of the twoprobes, signal eonditioning electronics, andoscilloscope with two input channels. The setups are identical to the single channel except

that two probes are used and the measurementresult in one direction is compared to themeasurement result in the other direction. Atypical application and some brief comments,will be given in four different areas.

Section 5.3.1 Multi-Axis Position Repeatability

A typical application would be an X-ymachine tool slide such as a milling machineor a lathe. In these machines, one axis usuallyhas quite a bit more travel than the other axis.The long travel axis has a long ballscrew andquite possibly more friction than the shortaxis. The lower torsional stiffness of the longballscrew combined with the extra frictionconspire to reduce the positioning performance of the long axis. This situation could beevaluated by making subsequent single channel measurements, but it is more time effective to make a single two-channelmeasurement. For the example of a milling

machine, the two probes are mounted on the .table in the directions of the two axes. A target(e.g. precision ball or gage pin) is mounted inthe spindle and the spindle moved into position so the two probes sense the target. Theprobes are arranged so they are coplanar in theZ-axis and their axes intersect the spindle axis.

The spindle is moved away from the setupposition and commanded to return to "home"or zero several times. The variations arerecorded and analyzed as in the single channelmethod.

Section 5.3.2 Multi-Axis Settling Time

For the same reason as the previous section,(torsional stiffness and friction) the settlingtime of the short axis may be more rapid thanthe long axis. Also, since nearly all CNC controls are serial processors (i.e. doing all thecalculations for one axis and updating it, thendoing the same thing for the next axis, etc.) itmight be possible for one axis to actually becommanded to stop before another, eventhough these events are thought to be simultaneous by the user. Knowledge about amachine tool in this regard might cause a userto orient a part in a particular way on themachine tool to take advantage of the higherperformance axis.


The setup is exactly the same as in Section5.3.1. The oscilloscope should be used in thedigital storage mode and the trigger properlyset (see Section 4.2.3). The spindle is movedaway from the probes, the oscilloscope triggeris reset, and the machine tool is commanded toreturn to zero. This test might also be donewith an application program rather than theinstrinsic machine function. Swapping the Xaxis and Y axis command line positions in theapplication program should reveal the axisprocessing sequence.

66.


Section 5.3.3 Multi-Axis Vibration

Multi-Axis vibration analysis is useful whenthere is the possibility of significant vibrationin a particular direction and it is important tocharacterize the effect of this vibration on thesystem in a different direction. A useful example would be a machine with a hydraulic cylinder driving an axis where pressurepulsations from the hydraulic pump causemotion (vibration) along the axis of the cylinder. A user might wish to characterize theeffect of this pulsation in the direction perpendicular to the cylinder, thereby establishingthe effect on, for example, surface finish. Onecan set up two probes, one parallel to the axis,and the other normal to the axis and directlymeasure the motions in the two directions.

The component of the normal signal that is atthe same frequency as the vibration signal onthe other channel is the response in the normaldirection to the parallel direction excitatioll.To get a reliable measurement, be sure theoscilloscope is triggering' on the parallel axisvibration signal, which is roughly sinusoidaland uniform in frequency.

Many other set-ups are possible and of coursethe angle between the probes is not restrictedto 90 degrees. For example, Section 5.7.3.2 ofthe B5.54 standard describes using five probesto characterize five of the six degrees of freedom of an axis of rotation.

Section 5.3.4 Multi-Axis Thermal Growth

Multi-Axis thermal growth or distortion issimply the set-up and use of two probes ratherthan one as in Section 4.3. The advantage ofusing two probes is the additional informationgained from instrumenting two dimensions oraxes simultaneously. Such a set-up can provide deeper insight to machine distortions dur-


67.

ing thermal transients or during warm-up.Combining 2-D thermal distortion data with asystem assembly diagram may help point outasymmetric thermal paths, poor thermaljoints, etc., that can contribute to machine toolperformance variations.


Section 6.0

Capped Probe and Calibration Tests


68.


SECTION 6 - Capped Probe and Calibration Tests

Capped probe tests for noise and thermal stability demonstrate system performance undervery closely controlled conditions. They areuseful for determining the performance suita-

bility of a capacitive position sensing systemfor a given measurement task, and can also beused to measure the influence of externalnoise sources on the measuring system.

Section 6.1.1 Capped Probe System Noise Test

The noise test is used for determining both theminimum noise level the system is capable ofachieving and also to determine the susceptibility of the system to external noise. Theexternal noise is usually electrical in natureand consists of both radiated and conductednoise. The test is quite simple in that the probetip is covered with a cap that simulates a target

but also shields the measurement electric fieldfrom any external sources. The cap must be avery low expansion metallic material likeInvar. The engaged length of the cap on thesensing end of the probe should be as short aspossible to minimize thermal growth. SeeFigure 6.1.

¢,80=====--d:--~

¢,60

relief to eliMino. te thePOSSibility of shortingprobe gUQrol to fixture,

set to VQrious oIistQnces inMeQSUreMent rQnge to checknoise Qt oIifferentstQnoloffs,

MiniMize oils tQnce0..5 Much QS pOSSible

FIGURE 6.1 Probe Cap Fixture

Mo.terio.l: invQr

setscrew lightlyor pinch clQMP

close slip fit

© LION PRECISION 1991 Machine Tool Analysis

69.

Once the measurement electric field is fullycontained by such a structure, the measurement can no longer be affected by any external influence and all remaining signalaberrations are.. either the result of noise in thesystem electronics that is independent of themeasuring process or external noise that thesystem picks up. The oscilloscope sensitivityis then set to maximum by setting the CHIand CH2 VOLTS/DIY knobs to 5 mV/division and pulling out the 'CAL' knob. Thisincreases the vertical sensitivity to 0.5 MV/DIV or 0.5 millionths of an inch per verticaldivision on the oscilloscope CRT. The width

Section 6.1.2 Thermal Stability Test


of the trace indicates the system noise level.For reliable measurements, the actual signalsmust be at least 3 times (preferably 10 times)larger than the system noise level. Also, theinfluence of external electrical noise sources(motor controls, welders, 60 Hz noise, etc.)can now be determined by simply turning onthe offending device and noting the change.60 Hz noise can be examined by setting theTRIGGER SOURCE to LINE.

Note: The specified Targa/oscilloscope systemnoise level with capped probes in an electrically quiet area is 2 mY.

oscilloscope can be subjected to the same test,provided the environmental limits of theinstruments are not exceeded. Maximum operating temperatures are as follows:

The capped probe test can also be used fortesting thermal stability of the probe, theTarga or the TEK 2211, or any to them in anycombination. The probe, for example can betested for thermal stability by capping it andplacing it in an environmental chamber. Thetemperature of the probe is varied and thechange in Targa readings is recorded as afunction of temperature. The Targa or the

Probe:Targa:

TEK2211:

250 degree F110 degree F93 degree F

Section 6.1.3 Probe Calibration Check

Occasionally, it is helpful to rapidly establishwhether or not the Targa system is operatingcorrectly and generally calibrated. A fixture isavailable in which the probe can be quicklymounted and its operating characteristicsdetermined. See Figure 6.2.

The Calibration Check Procedure is as follows(NOTE: THIS IS NOT A CALIBRATION):

1) Set the fixture to zero.


70.

2) Connect a digital voltmeter to the outputcoaxial cable from the back of the Targausing the BNC - double banana connector supplied with the calibration fixture.Insure the ground side of the connector(identified by the GND tab) goes in theLO or COM port on the voltmeter. Setthe voltmeter to read DC volts.

3) Mount the probe and adjust it so theposition indicator on the Targa frontpanel is in the mid range.


4) Adjust the Targa front panel ZEROknob so the Targa display reads 0.000.

5) Insure that the rear panel "scale" switchis set to "THOUS" and move the targettoward the probe 0.002 inches by turning the spindle on the check fixture.

6) The front panel display should readapproximately 2.000 and the digitalvoltmeter should read the same value ason the front panel display, within I mY.

7) Move the target .004 inches away fromthe probe by turning the check fixturespindle in the opposite direction.

8) The front panel display and the digitalvoltmeter should both readapproximately -2.000.

--

9) Reset the check fixture to zero. Both thefront panel display and the voltmetershould read about zero.

10) Successful completion of steps 5-9 indicates that the probe is operating correctly and is generally calibrated. '

Note: The oscilloscope can be used in place ofthe digital voltmeter if the input channelconnected to the system being checkedhas its coupling set to DC. The triggersource should be set to CHI or CH2,corresponding to which Targa/probe isbeing checked, and best results will beobtained by setting the trigger MODE top-p AUTO. As the target is moved withrespect to the probe, the trace on theoscilloscope CRT should move up whenthe front panel display indicates positivevalues and down when the display indicates negative values.

FIGURE 6.2 Probe Calibration Check Fixture


71.


Section 7

Reversal Techniques for Separating

Workpiece Form Error From Spindle or

Slide Error Motion


72.


SECTION 7 - Reversal Techniques

Section 7 Reversal Techniques for Separating Workpiece Form ErrorFrom Spindle or Slide Error Motion

Occasionally, it might be necessary to characterize a spindle or a slide with a less than perfect workpiece. This is especially true forslides when their error motion is measuredusing a straightedge. High quality balls andpins with form errors approaching 1 microinchare readily available at a surprisingly modestcost. High precision straight edges, especiallyones whose lengths are more than 12 inches,are another matter entirely. Fortunately, there

is a technique that is applicable to both spindles and slides that allows the form error ofthe workpiece to be separated from the ern;>rmotion of the spindle or slide. All that isrequired is two sets of data and some simplemathematics. This technique is called theReversal Method and was described by BobDonaldson of Livermore National Laboratoryin 1972. This method is sometimes referred toas the "Donaldson Reversal Method."

Section 7.1 Reversal Method for an Axis of Rotation

The procedure is completely explained inAppendix B of ANSI/ASME B89.3.4M-1985.Please read Appendix B before proceedingwith this section. In that explanation, the useof a polar chart for graphical interpretation ofthe data is used. Since the Targa system can-

not make overlapping polar charts, the bestthat can be done is to collect the data at asmany angular positions as is deemed reasonable for accuracy, do the calculations at eachangular position, and hand plot if required. Anexample follows:

Moster

120/

\240l

270

Reversecl Moster'

Reversecl Probe

,----------------...90

60 I 12\ --------. /

/300

--Housing ---------

Spine/Ie --------"\240

/300 I~270

Setup for'T1

Setup for12 & 13

FIGURE 7.1 Reversal Method for a Rotary Axis


73.



1) Mount the workpiece on the spindle and T2 and T3. Also move the probe 180center it as close as possible. While it is degrees so it still senses against thenot necessary to center the workpiece, it marked point on the workpiece whenis good practice to do so because most the position pointer is at zero. It istransducers (whether they are capacitive important that the probe be at the samein nature or use some other phenome- height on the workpiece in both loca-,non to sense displacement) are not per- tions. The probe must sense the samefecdy linear over large excursions. circumferential track.on the workpieceReducing the transducer excursion to a in order to produce good results.minimum enhances the measurementreliability. 6) Adjust the probe so the Targa front

panel reads as close to zero as possible.2) Mark the spindle, housing, and work- This insures that the same region of the

piece as shown in the setup for Tl. probe sensing range is used.(Different angular steps may be used ifa different number of data points is 7) After adjusting the probe position to asdesired). close to zero as possible, use the front

panel ZERO knob to set the display to3) Position the probe as shown in the set- zero when the position pointer is at

up for Tl and adjust it so the front panel zero.bar graph is mid-range. Use the ZEROknob to set the front panel display to 8) Rotate the spindle and record the Targazero when the spindle position pointer display value at each angular position.is at zero. This is the T2 value. Again, if the spin-

dle exhibits asynchronous error motion4) Record the Targa front panel reading at (readings don't repeat over consecutive

each of the desired angular positions. revolutions at any particular position)Insure that the Targa reads zero or very record values at each position over sev-close to it when the position pointer eral revolutions and average them atcomes around to zero again. If it does each position to arrive at the calculationnot read zero, the spindle probably has value for T2.some degree of asynchronous errormotion. See Section B4, page 41, 9) Construct another column with valuesANSI/ASME B89.3.4M-1985 for fur- identical to T2 but with the oppositether information on this problem. The sign and label the column T3.only solution is to record readings ateach position over several (e.g. 10) rev- 10) The out of roundness (P) of the work-olutions and average them to get the piece at any angular position is equal tocalculation value (Tl) at each position. 1/2(Tl+T2) and the spindle error

motion (S) at any position is 1/25) Rotate the workpiece 180 degrees on (Tl +T3).

the spindle and recenter it. See setup for


74.


Example Data

PQsitiQn T1 T2 T3 £ S

0 .. .000 .000 .000 0 030 .004 -.003 .003 .5 3.560 .006 -.007 .007 -.5 6.590 .007 -.002 .002 2.5 4.5

120 .005 .001 -.001 3.0 2.0150 .008 .005 -.005 6.5 1.5180 .009 .007 -.007 8.0 1.0210 .011 .002 -.002 6.5 4.5

24 .007 -.003 .003 2.0 5.0270 .006 -.004 .004 1.0 5.0300 .004 -.003 .003 .5 3.5330 .003 .002 -.002 2.5 .5

NQte: The data (T1, T2,) are recQrdeddirectly from the Targa display,which displays millivQlts when therear panel scale switch is in the"THOUS" pQsitiQn. In this mQde, 1millivQlt equals 1 microinch, SQ Tland T2 are measurements in milliQnths Qf an inch. P and S are calculated directly in milliQnths with thedecimal pQint and leading zerQS thatappear in T1, T2, and T3, remQved.

11) The P and S data CQuid be graphed inpQlar fQrm similar tQ the diagramsshQwn Qn page 40, Figure B2, ANSI/ASME B89.3.4m-1985. The individualdata pQints from the data reductiQn chartWQuid have tQ be cQnnected by straightlines from cQnsecutive angular pQsitiQnsQn the pQlar chart. If a large number QfPQints (e.g. 50) were taken, the handplQtted pQlar chars WQuid IQQk prettyreasQnable.

Section 7.2 Reversal Method for a Linear Axis

CQnceptually, the Reversal MethQd fQr slidesis identical tQ the methQd fQr spindles.HQwever, the fact that the straightedge dQesnQt clQse Qn itself like a circular wQrkpiecedQes, and the pQssible error invQlved in pQsi-


75.

tiQning the probe when the straightedge isreversed require a slightly different prQceduretQ Qbtain gOQd results. As with the spindle procedure, tWQ set-ups are required.


z

Stro.igh teagesupportpOints (3)

Stro.ighteage

Machine tablezero marks

slide wo.y

z


Straightedge

Mo.chlne tablezero marKs

MachIne slide way

fhe prolae l'1ust sense on exoctlythe sOl'1e region of the stro.ighteolge,Iooth vertico.lly o.nol o.long the o.xis oftro.vel oIur~g loath set-ups

FIGURE 7.2 Reversal Method for a Linear Axis

1) Mount the straightedge on the slide.While the straightedge reference surfacedoes not have to be parallel to the axisof travel, it is good practice to adjust thestraightedge so it is parallel to the axisof travel for the same reasons as statedin the previous section.

2) Mount the probe so the axis of theactive area is at the vertical mid-pointof the reference area on the straightedge. Adjust the probe stand off untilthe range bar graph on the Targa frontpanel is lit at the middle. Use the ZEROknob to set the Targa display to zero.Be sure the slide is at the "zero" position and the probe is at the zero on thestraight edge.

3) Record zero at the zero slide positionand record the Targa front panel readingas often as desired for as long a distanceas desired. This data will be Tl forTrace 1. Several sets of data should betaken at each position to ascertain therepeatability of the slide. This is identical in principle to the problem causedby asynchronous error motion in spindles, although no equivalent term forslides exists. The actual recorded valueat each position should be the averageof all the data at that position. Whenreturning the slide to the zero position tostart a new trial, be sure and move theslide past zero, reverse it, and bring itback to zero, to take out any backlash inthe drive system, unless, of course, theeffect of backlash is being measured.


76.

SECTION 7 - Reversal Technigues

4) Turn the straightedge upside down onthe slide by rotating the straightedgeabout its axis that is parallel to the axisof travel. The reference surface is now180 degrees from where it was in step 3but it is still parallel to the axis of travel.

5) Adjust the straightedge so it is parallelto the axis of travel. Insure that the slideposition is at zero and move the probeto the opposite side of where it wasmounted for step 3, so that the probesenses against the same straightedge reference surface.

6) Adjust the probe mount vertically so theprobe senses the mid-point of thestraightedge, and horizontally in thedirection of travel so the probe lines upwith the straightedge zero mark. It iscritical to the Reversal Method that theprobe sense exactly the same region Inboth set-ups.

7) Adjust the probe standoff to as close tozero as possible and use the ZERO knobon Targa to set the digital display toexactly zero.

8) Record zero at the slide position zero.and then move the slide to the samepositions as in step. 3 and record theTarga display value at each position.Label this data T2.

9) Construct a third column of values foreach position by using the same value asT2 but with opposite sign.

10) The straightedge error (P) and the slideerror (S) are calculated at every positionby the same equations as for spindles,Le. P = 1/2(Tl+T2) and S = 1/2(Tl +T3).

Tl T2 D £ S-

O .000 .000 .000 0 01 .009 .006 -.006 7.5 1.52 .013 .002 -.002 7.5 5.53 .011 -.004 .004 3.5 7.54 .006 .001 -.001 3.5 2.55 .001 -.003 .003 -1.0 2.06 -.004 -.006 .006 -5.0 1.07 -.011 -.010 .010 -10.5 -.58 -.013 .003 -.003 -5.0 -8.0


77.



Section 8

Epilogue

78.


SECTION 8 - Epilogue

Epilogue

With a thorough understanding of the principles of probe mount design, grounding andshielding, electrical and structural noise, andthe concepts and terminology of machine toolperformance assessment, one can push theTarga system to its very limits. Plot 8.1 showsthe asynchronous error motion of a 4 inch airbearing spindle at .16 microinches over 9 consecutive revolutions. The spindle performanceis actually better than this since the signal onthe plot is the sum of all the affects listed

above. It is not possible to characterize theexact asynchronous error motion of the spindle but it can be confidently stated that theasynchronous error motion is less than .16 'microinches. Capacitance sensing approachesthe performance of the highest resolutiondevices available (tunneling and atomic forcemicroscopes) at a fraction of the cost, withmuch greater potential utility (in the machinetool environment), and far less sensitivity toabuse and other environmental factors.

.0.V1 O. 160mV Trig O.60mV CHl

-, ,- ._.- . .- """\ --'-r'- ,- -

I

~ III III,j ! l

O.5mVI\. 50ms

Plot 8.1 Asynchronous Error Motion of an AirBearing Spindle

Asynchronous error motion of a ProfessionalInstruments Model4R Blockhead® Air BearingSpindle. The oscilloscope vertical sensitivity isincreased to .5 millivolts per division by pulling outthe CAL knob on the CHI VOLTS/DIY switch. Thesystem sensitivity is then .5 millionths per division.The asynchronous error motion is .16 microinches.


79.


Section 9

References, Associated Standards

and Sources of Information


80.


SECTION 9 - References, Associated Standards and Sources of Information

Section 9.1 Related StandardsAxes of Rotation Methods for Specifying and Testing B89.3.4M-1985

Measurement of Out of Roundness B89.3.1-1972(RI988)

Temperature and Humidity Environment forDimensional Measurement B89.6.2-1973(Rl988)

Methods for Performance Evaluation of ComputerControlled Machining Centers and Work Centers .....Draft ASME B5 TC52

Section 9.2 Related Books, Articles, and Information

Blaedel, K. and Bryan, 1., "Tutorial on Thermal Effects," Presented at 1990 Annual Meeting ofthe American Society of Precision Engineering.

Bryan, 1., and Carter, D., "HowStraight is Straight," American Machinist, Vol. 133, No. 12,December, 1989.

Bryan, J., Clouser, R, Holland, E., "Spindle Accuracy," American Machinist Spec. Report No.612, 1967.

Bryan, W.J. "Construction of a Diamond Turning Machine," 1978.

Donaldson, R, "A Simple Method for Seperating Spindle Error from Test Ball RoundnessError," CIRP Annals, Vol. 21 21/1, 1972.

Hocken, R J., "Technology Machine Tools: A Survey of the State of the Art by the MachineTool Task Force - Volume 5: Machine Tool Accuracy," October, 1980.

Lion, K. ard Foldvari, T., "Capacitive Transducers," Instruments and Control Systems,November, 1964.

Martin, D., "Application of Precision Capacitive Displacement Sensors," Session 302A-l,Proceedings of the Sensors Expo, 1989.

Martin D. and Wilcox, R, "Spindle Runout Effects on Positional Accuracy," Presented at theUniversity of Wisconsin 'Symposium on Small Hole Technology,' February 22, 1990.

Rolt, F. H., Gauges and Fine Measurements, Macmillan and Co., 1929.

Moore, W.R, Foundations of Mechanical Accuracy, Moore Special Tool Company, 1970.

Schlesinger, G., Testing Machine Tools, Machinery Publishing, 1938.

Tlusty, J., "System and Methods of Testing Machine Tools," Microtecnic, Vol. XIII, 1959.

Wilcox, R., "Dynamic Measurement of High Speed Spindle Runout," Printed CircuitFabrication, Volume 12, No.3, March 1989.

Wilcox, R., "New Developments in Dynamic Spindle Runout Measurement," IPC TP..844,Published by IPC, Institute for Interconnecting and Packaging Electronic Circuits,September, 1989.


81.


SECTION 9 - References, Associated Standards and Sources of Information

Section 9.3 Organizations

American Society of Mechanical Engineers (ASME)United Engineering Center345 47th StreetNew York, NY 10017

American National Standards Institute (ANSI)105 - 111 South State StreetHackensack, NJ 07601

American Society for Precision Engineering (ASPE)401 Oberlin Road, Suite 108P.O. Box 10826Raleigh, NC 27605-0826


82.

Addendum A

Fixed Sensitive Direction

Error Motion


83.


Addendum A - Fixed Sensitive Direction Error Motion

Addendum A Fixed Sensitive Direction Error Motion

Figure A1

As discussed in Section 5.1.2, a machine toolwith a fixed sensitive axis error motion is onewhere the t091 is fixed or does not rotate withthe rotational axis. An example of a fixed sensitive axis machine is a lathe or turningmachine.· In such a machine, only relativemotion between the tool and the workpiece, inthe direction of the tool, contributes significantly to part size or geometry error. Errormotion of the spindle that is perpendicular tothe tool has little effect on the size or geometry of the part. A computer disc drive, with itsfixed head positioning mechanism, should beconsidered a fixed sensitive direction application. In general, the probe should replace thetool Le. be mounted in the same position, formeaningful results to be obtained. A singleprobe at the tool position and some means forgenerating the base circle on the oscilloscopeare required to make polar plots. However, the2 channel Targa/Oscilloscope system cannotmake fixed sensitive direction polar plotswithout some auxiliary equipment. Either athird channel, some base circle generatingcams, and some additional analog electronicsare required, or some sort of spindle positionsensor (e.g. encoder or resolver) and a digitalcomputer are required to achieve properresults. See ANSIjASME B89.3.4M-1985Sections A14 through A16, pages 32-35 for amore in-depth discussion of this situation.Still, the two channel Targa system can provide a fair amount of information about theperformance of a fixed sensitive directionspindle utilizing a "Test of Consequences."Jim Bryan coined this term to describe thewell-known fact that an indicator measuringagainst a machined radial surface will readzero runout when located where the tool was,and twice the actual runout when located 1800

from the tool, provided the workpiece is notdisturbed when the indicator is moved. Thisconcept can be used to get an estimate ofradial error motion and can be extended to tilterror motion measurements when combined


84.

with thy second probe and the ability of theoscilloscope to make differential measurements (see Section 5.2).

This technique involves machining an axialand a radial surface on a ;workpiece and thenmaking several measurements on themachined surfaces without disturbing theworkpiece. In addition, measurements on theaxis must be made, so a reference surfacemust be produced on the axis of the machine.One possible arrangement for a workpiecewith an axial reference is shown in Figure AI.The ball is the reference for axial measurements and should be centered as well as possible. The axial and radial surfaces aremachined as smooth as possible. An aluminum workpiece machined with a large radiusdiamond tool with a slow feed rate will produce excellent results.

1/2" steel ball gluedinto a 60° center

-L....!.~.....JI -Axial surface

Radial surface

A possible workpiece design. The workpiece is mounted on the spindle and theball centered as well as possible. Theaxial and radial surfaces are machinedand the measurements made from thesesurfaces.


Once the ball is centered and the axial andradial surfaces are machined, measurementscan be made according to the following procedure. Be sure to make all measurements atexactly the same speed that the machiningwas done at, as this technique is sensitive toimbalance induced errors. In addition, put agrease pencil mark on the axial and radial surfaces at the same angular location.

1) Replace the tool with a probe and measurethe runout of the radial surface and theasynchronous motion of the radial greasepencil mark. See Figures A2 and A3.

The value obtained in each of these measurements should be essentially the sameand it is equal to the asynchronous radialerror motion of the spindle. This is becausethe synchronous radial error (the motion


steel ball

Figure A2 Probe Location

",VI 14 OOmV Trig HFrej CHi ",VI 14 BOmV Trig HFrej CHI

I.1 ~II

~ .~.~

I~' ~~~ ~Ir,:lflJlI1ij

~ 1~!lIIII.\, 1\ MI~~ WlyI'tW ~

,~ ~ I~ ~- L~ ~,~ I~ -'~ ~,

I

I

Runout of Machined Surface10mV 20m8 5mV 0 58 ROLL

Asynchronous Motion of Radial GreasePencil Work

Figure A3


85.

I,,

- - 1# -'- .- - ,-- ~ ,- ,- - ,-- -- - ,,-

~ .~ ir~ hi~'l rl ~ I.~~ I

j~ ~! 'IV~~ V~ l!

1~1111~

~,~ ~ltlJwl 'ij IfY

~,.'v,II

I

Figure A4 Probe Location


that is the same over many consecutive revolutions) and the fixed tool "machine out"the radial synchronous error motion whenmeasured at the tool position. If these twovalues are not close (Le., less than 10% difference), then something is wrong. Themost likely culprit would be the machiningprocess. A dull tool, insufficient coolant,built up edge, etc., can all conspire toincrease the error in this measurement.

2) Move the probe 1800 from where the radialtool machined the surface and measure therunout. See Figure A4 and AS.

At this location, the probe signal will beequal to the sum of the asynchronous radialerror motion plus twice the synchronousradial error motion. However, the asynchronous radial error motion was already determined in the previous step so the radialsynchronous error motion can be estimatedby subtracting the asynchronous radial errormotion value from the 1800 runout valueand dividing by 2 or:

Radial Synchronous Error Motion

30mV - 14mV . .= 2 = 8mV = 8 mlcfOmches

This number can be used for comparisonbetween spindles; however, it does not provide any information about the shape of theerror, only about the value. Only the polorplot method specified in ANSI B89.3-41985 can fully characterize the value andthe shape of the error motion, and thispoints out the inherent limitation of the testof the consequences.

",Vl

10mV

30. OOmV

steel ball

Tr-ig HFrej

20ms

CHl


86.

Figure ASRadial Rundut 1800 from tool


3) The synchronous and asynchronous tilterror motions can be arrived at in a similarway, although the mea~urements are complicated by the fact that the axial measurements include components from both tiltand axial error motions simultaneously. Thesecond probe is used to measure the axialerror motions and can be subtracted electrically by the TEK 2211.

4) As with the radial measurements, a probesensing against the axial surface where thetool actually machined the surface willmeasure only asynchronous effects. Withthe axial case both the tilt and the axial synchronous error motion are "machined" outsimultaneously at the toollocatibn and theprobe should measure only the sum of theinstantaneous tilt and axial asynchronouserror motions. Therefore, whether the actualrunout is measured or the asynchronous


effects are measured by looking at the axialgrease pencil mark, the values obtainedshould be similar. See Figures A6 and A7.

steel ball

Figure A6 Probe at Tool Location

",V1 8.00mV Trig HFreJ CH1 ",V1 8 40mV Trig HFreJ CH1

5mV 20ms

Axial Runout at Tool Location


10mV O.ls ROLL

Asynchronous Motion of Axial GreasePencil Mark

Figure A7

87.

- - - _._,- ---,--,.- - - ,

.

MI-t

IT


5) The axial effects can be subtracted by setting the second probe on the ball on the axisand configuring the oscilloscope to subtractCH2 from CHI. The'subtraction is done bysetting the input mode switches on the TEK2211 to BOTH, CH2 INV, and ADD. Forsubtraction, or "differential" measurements,measurement integrity is enhanced by having the average value of the signals as closeto zero as possible. Once the spindle isrotating, use the offset knobs on the Targainstruments to set the digital to zero on bothinstruments. Also, record the distance "D"between the probes. See Figures A8 andA9.

I.>. V 10.80mV Trig HFrej CH1

Figure A9 Asynchronous Tilt ErrorMotion Before Correction for ProbeSpacing "0"

7) Similar to the technique for synchronousradial error motion, the synchronous tilterror motion is arrived at by moving theCHI probe to a point 1800 away fromwhere the tool machined the axial surface.The axial reference probe is still used toremove both the synchronous and asynchronous axial error motions electrically. Theresultant signal is the sum of the asynchro-·nous tilt and twice the synchronous tilt errormotions. Since the asynchronous tilt errormotion value was measured in the previousstep, it is subtracted away manually and theresult divided by 2. This gives the synchronous tilt error motion at a specific radius,and the value must be divided by the radiusto get an angular value. See Figures A10and All.

steel ball probe

I ~--

em 1~'D"

1"'"1_~;>:>1~be__J__CHI

Figure AS Probe Arrangement

6) The actual tilt value can be calculated bydividing the measured value by the probeseparation (3.3 inches in this example):

Asynchronous Tilt Error Motion

= 10.8 gin 3 3 d'= . ". lJra lans3.3 in

10mV + 1OmV t o ls ROLL


88.



~~~. .! r,~

i'.1, . If

I",I.,

.ill""ilr ~I ~~w~ '\, .I.ILd

I'''' ~ I "~ {"I'III

-- -- --,~-- _.- - - ,_.- - - .~

: probe'-1--";>:>"ffv\./ ---

CHI r"0"

_I ball probe jI =0/--

CH2

Figure A10 Probe Arrangement

"v

20mV

82.4mV

+ 20mVJ..

Trig HFrej

20ms

CHI

8) The tilt synchronous error motion value iscalculated as follows:

82.4 /lin - 10.8 /lin = 10.8 IJradians(2) (3.3 inches)

Again, this value does not contain anyinformation about the shape of the errorcurve, only a value.

Figure A11 Runout of Axial Surface1800 from Tool Point with Axial EffectsSubtracted

9) For completeness, the axial synchronousand asynchronous error motions, which aremeasured directly with the probe over theball, are shown in Figure A12 and A13.

IOmV 20ms

Figure A12 Axial Synchronous

1"* 11 - _.- - _.- .- - _.- ~M~~ - - --

I\~ IJ~ .r r""1

IA

.~ -++++

~,

1~1

~W \IVA lM,I

~~,

~~I "i/oi h II~ ! iI~

I

AV2 28, 40mV Trig HFrej CHI AV2 5 20mV Trig HFrej CHI

-- - - - - - - - - - - -- -

II Iii I

5mV 0 Is ROLL

Figure A13 Axial Asynchronous


----------~-- ~ --- -~-

89.

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