large machine compensation- improving … machine compensation-improving accuracy, reducing...

Large Machine Compensation-Improving Accuracy, reducing

compensation time

Rob FlynnElectroimpact, Inc

CMSC 2011

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Today’s Scope

• Who is doing Volumetric Compensation?

• What are the Benefits – time and accuracy

• Universal implementation tips– Targeting

– Triggering

– CNC software maintenance tools

• Uncertainty Analysis for better results– Case studies



Large machine benefits

• Volumetric comp works with any size

• Benefits may be more obvious for large machine

• Applicable to standard machines as well as bespoke



Large versus small

• Large machine – may not be able to grind large parts

• May not be able to machine parts in single setup

• Tracker error a much smaller % of volume



Large machines?



Volumetric Comp Developers

• API-Boeing-Cincinnati MAG-Siemens-DOD team VALMT

• Electroimpact – Todd Rudberg

• Renishaw• Precision Technologies (UK) - University of

Huddersfield



Few players but more coming…



VALMT

• US research project, Volumetric Accuracy for Large Machine Tools (VALMT)

• API, Boeing, Siemens, MAG Cincinnati and DOD

• Siemens VCS is the control technology



Electroimpact Volumetric Comp

• Developed by Todd Rudberg

• Successfully Used with up to 7 axis machines

• Method predicates resultant accuracy prior to final check – validates metrology

• Used on 25+ large machines



CNC Suppliers with volumetric comp features

• Siemens

• FANUC

• Fagor Automation

• Others?



Volumetric Compensation – Metrology Software of Interest

• New River Kinematics (started with Robots)

• Metrologic

• Verisurf



What are we compensating?

• Only repeatable errors.



Some machine error sources

• Consider an X axis – rack and pinion drive with encoder feedback: leaves several types of error– Pinion-rack backlash (not corrected with

compensation)

– Gearbox hysteresis

– Rack pitch error (correctable with single axis compensation)

– Transient errors such as structure deflection



Secondary errors

• As the machine moves vertically (Y axis), axis non-perpendicularity causes Z and X error

• As machine moves in X, 2 axis bed height variations cause the toolpoint to move in X, Y and Z.

• Additional X for X, X for Y, X for Z



Mechanical Errors

• Generally accepted 43 mechanical error sources for a 5 axis machine



CAN WE COMP ERRORS?

• SINGLE AXIS ERRORS CAN BE COMPENSATED

• AXIS FOR AXIS ERRORS – MAY OR MAY NOT BE COMPENSATED, DEPENDING ON CNC

• MAY RUN OUT OF OVERLAY TABLES• SOME ERRORS MAY REQUIRE MECHANICAL

COMPENSATION



Why Volumetric Comp?

• Eliminates much or all mechanical compensation

• Potential for improved accuracy

• Provides complete kinematic model of machine behavior

• Much faster process for many machines

• Eliminates axis by axis comp tables in CNC

• Enables more complex machine benefits



OTHER BENEFITS

• ENABLES SHARED COORDINATE FRAME FOR CELL

• ENABLES SHARED PART PROGRAM FOR MULTIPLE MACHINES (E.G. LEFT, RIGHT, OR REDESIGNED MACHINE)



Is Volumetric Method Valid?

• Test using random point check!

• Over 25 machines of similar scale successfully compensated with this process



Not a substitute…

• Volumetric compensation is not a substitute for making a good machine. Still must have accurate parallel surfaces for linear bearing rails, etc.

• Must be stiff

• Must be very highly repeatable



Solver behavior

• If compensation data is bad for any reason (e.g. tracker is bumped during session or machine is not repeatable in an axis), solver cannot resolve a complete solution.

• Bad data = NO SOLUTION



Solver error

• Solver solutions are not perfect, due to tracker error

• More stations get you less tracker uncertainty

• More measurement accuracy gets you better solver solutions



Tools to aid volumetric comp

• FRS – Foundation Reference System

• Improved triggering methods

• CNC Compensation screen (with CNC maintenance tools)

• Uncertainty analysis tool (e.g. SA, Metrologic)



FRS aka Control Network

Foundation Reference System

•Permanent monuments

•Accurately valued

•Validated with independent measurements



WHY A CONTROL NETWORK?



Why a control network?

• Multiple stations can be bundled to improve accuracy for compensation…and a better solver solution!

• FRS is another tool to discover problems– Foundation cracking– Foundation curing shrinkage– Foundation shifting

• Tomorrow’s problem – an FRS may help answer hard questions in the future



CNC Compensation Screen



Triggering methods

• SA-measure stable point

• Swifty – dry contact to PC USB input

• X-Keys-serial tool for dry contact to serial input

• Closed loop method



SWIFTY

• CONVERTS DRY RELAY CONTACT TO A USB INPUT

• COMES WITH INTERFACE SOFTWARE AND DRIVERS



SWIFTY IMPLEMENTATION

CNC

RELAY

TWO WIRE CABLE TO SWIFTY

SWIFTY USB DEVICE

TRACKER PC



Pneumatic trigger?



Locating the tracker-considerations

• VERIFY THAT MACHINE MOTION DOES NOT INDUCE MOVEMENT OF TRACKER DUE TO FOUNDATION DEFLECTIONS

• CHECK LINES OF SIGHT FOR TARGETS/MEASUREMENT LOCATIONS

• MINIMIZE SHOT LENGTHS• FOR MULTIPLE STATIONS, TRY TO ACHIEVE

LARGE ANGLES FROM TRACKER TO MOST OF THE TARGETS



UNCERTAINTY ANALYSIS – WHAT FOR?

• DETERMINE UNCERTAINTY FOR POINTS OF INTEREST

• QUICKLY EVALUATE ALTERNATE TRACKER LOCATIONS

• EVALUATE VALUE OF ADDITIONAL STATIONS FOR REDUCING UNCERTAINTY

IMPROVED ACCURACY FOR LESS WORK



UNCERTAINTY ANALYSIS EXAMPLES

• CASE 1 – AFP MACHINE

• CASE 2 – MYSTERY MACHINE



CASE 1

• MOVING COLUMN AFP MACHINE

• 19.5M x 6.4M x 4.2M ( 64‘by 21' by 14‘) WORK ENVELOPE

• 1000 POINTS USED IN COMPENSATION

• STATIONS USED: 1

• How much would we benefit from the use of additional stations?



SINGLE STATION UNCERTAINTIES

• AVERAGE UNCERTAINTY 0.064mm



CONFIGURATION “A”

• TWO STATIONS

• AVERAGE UNCERTAINTY: 0.051mm



CONFIGURATION “B”

• TWO STATIONS

• AVERAGE UNCERTAINTY: 0.031



CONFIGURATION “C”

• 3 STATIONS

• AVERAGE UNCERTAINTY 0.030



CONFIGURATION “D”

• 3 STATIONS




RESULT OF SHIFTING STATION LOCATION

• 52% REDUCTION IN AVERAGE UNCERTAINTY IF WE ADD A SECOND STATION – IN THE RIGHT PLACE.

• ADDING A THIRD STATION DROPS UNCERTAINTY ONLY TO 59% REDUCTION FROM ORIGINAL.



Uncertainty Results

Point 148 254 375 442 Average*

Single station .100 .083 .064 .075 .064

2 STATIONS “A” .044 .084 .037 .070 .051

2 STATIONS “B” .063 .059 .048 .054 .031

3 STATIONS “C” .029 .035 .023 .027 .030

3 STATIONS “D” .041 .047 .031 .038 .026

AVERAGE UNCERTAINTY IS FOR ALL POINTS IN THE GROUP, NOT JUST THE SAMPLES SHOWN



Case 1 Volumetric Compensation Results

• Max radial error of < 0.2mm (0.008”) within the work envelope of 19.5M x 6.4M x 4.2M ( 64‘ by 21' by 14‘) (excluding measurement error)



CASE 2

• VERTICAL DRILLING/FASTENING MACHINE

• WORK ENVELOPE 34M X 6M X 1M

• 700 POINTS USED IN COMPENSATION

• 2 STATIONS



SINGLE STATION

• AVERAGE UNCERTAINTY: 0.11



TWO STATIONS


• N288 REVEALS WEAKNESS OF COLINEAR POINTS AND STATIONS



Uncertainty Results

Point 288 223 82 287 145 Average*

SINGLE STATION

.26 .13 .13 .04 .04 .11

2 STATIONS .14 .04 .04 .03 .03 .05

. .

. .

. .

*AVERAGE UNCERTAINTY IS FOR ALL POINTS IN THE GROUP, NOT JUST THE SAMPLE POINTS SHOWN

ADDDING A SECOND STATION YIELDS A 55% DROP IN UNCERTAINTY OR DELTA OF 0.06MM (0.002”).



Compensation case studies conclusion:

• Uncertainty analysis illuminates compensation metrology:– Quantifies benefits of adding a particular station

– Enables meaningful comparison of alternate metrology plans

– Allows quantitative comparison of alternate stations



Summary

• A few companies are doing 5+ axis volumetric comp

• Benefits include reduced time to comp, accuracy

• Active Target and smarter triggering can help

• Use uncertainty analysis for better results

• Eyes open –always new developments!



VNC server + VNC Client + wireless router + Ipod Touch = full remote

control of your tracker



Thank you!



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