using glue on strain guages

7/29/2019 Using glue on strain guages

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Using Glue on Strain Gages and other High level inputs

By

John L. Paul

Field System Engineer

Introduction:

This paper is to discuss the information and proceedures needed to connect High level(+- 10VDC) inputs into the 5500 systems with BlueHill software thru the Strain channels

or thru the Versa channel.

Scope:

This will be specific to Glue on strain gages but we will show the steps nessacery to

connect other High level devices to the 5500 system with Blue hill.The proceedure will use 300 ohm and 120 ohm glue on strain gages and a Vishay 2100

signal conditioner.

Wiring conciderations:

To wire the system we will assume that the 5500 and Blue hill are set up and functioning

properly. The versa-channel National Instraments PCI- 6229 should be installed in the

computer and connected. The BNC-2111 connection unit should be connected to theconnector 1 connection on the PCI-6229, with the sync connection (PFI0) on the BNC-

2111 to the sync connection on the digilink board.

The Wiring on the Vishay signal conditioner includes the hook up wires to the gage, the

BNC to BNC conector to the versa-channel or an Instron High level input cable (PN2210-864).

The Hook up wires from the Vishay to the gage is determand by the bridge compleationcircuit desired. This example uses quarter bridge circuit, using the internal compleation

circuit. This can be set up to make the selection of the gage resistance set up by the

wiring. So one connector can be set up for 350 ohm gages and another is set up for 120

ohm gages. See Figure 1.

The calibration can be set up so the 350 ohm gages can be calibrated using the A cal

position and the 120 ohm gages to cal using the B cal position . Wire the jumpers and

calabration resistors per Figure 2. Later we will discuss how to pick the calibrationresisters.


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Figure 1 : Use Quarter Bridge A1.


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Figure 2: Upper and lower Jumper in 120 B middle two jumpers in 350 A -

Upper and lower A calibration resister use the 350 Shunt resisters Upper and

lower B Calibration resister use the 120 Shunt resisters. (Use Presision resisters)

Electrical set-up and calibration: refer to figure 3.

To electricaly set up the Vishay, connect a bonded gage thru the hookup cable to theinput of any channel. On the 2110 power supply make sure the DC setting is at 10.

Change the switch to Channel 1 and adjust the bridge excitation on the front of the first

conditioner to 10 VDC, then continue on to Channel 2 and adjust to 10VDC, continue

thru all the channels.Without any gage atached adjust the Amp Zero until both lights are out. Connect the

gage and adjust the Balance until the lights are again both out. Now is the time you

need to know the range you will be setting up. We will be setting up the gages for a maxoutput of 3% strain. The gages have spicific specifications that are important to set the

system up. The gage resistance, the gage phisical size, and the gage factor which is the

multiplier that is used to correct manufacturing diferances within the gage. The gage wewill be using is a 350 ohm gage with a .25 inch gage leanth and a Gage factor of 2.095.


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Figure 3:

Clip on the Gage Set the Gain Multiplier to X20 Shunt the Gage with an external

presision resistance substitution box (RC201) . Mathmaticaly 55K will give you .3%strain. Or 3000 microstrain. Using the National Instruments Mesurments and

Automation program select the device and test panals connect to the Versachannel box

set for rsa and input to channel 0. on the test panal use the channel 16 with rsa input.

Starting the test panal and adjust the Gain potentometer to on out out on the N I test panalto 1 Vdc this shoud give a 3% output of 10 Vdc by changing the shunt resister to 6000

ohms you should simulate a full scale out put. With out the versachannel or the National

Instraments program you can set the outputs with a Digital voltmeter.

Now connect the Vishay to the Strain input or start Bluehill and select the channel thatyou are connected to. Channel 1 on the versachannel is the National instrments 0 inputon the BNC and channel 16 on the test panal.

The channel has to be enabled and defined in the admin page. (remember that you have to

restart Bluehill if you make any changes.) select the versachannel Icon to start thecalibration. The procedure is the same for the versachannel and the High level strain

input. Make sure that the 10Vdc input is checked. Set the full scale to .075 inches which

is 3% of the .25 inch gage length and the calibration point is .0419 mm or 1.65 mil.

Tell the program to calibrate manualy. It will ask you to put it at the gage length and hit

enter. Then to go to the cal point and hit enter. At this time press the cal switch to A orshunt the gage with a 25K ohm resistor and press enter. Then return to gage length

(remove shunt) and press enter. The system should be calibrated.

The use of the 25 K ohm resistor is to give a signal larger than 2 Vdc. The versachannel

takes a minimum of 2 volts to calibrate. The strain channels take a minimum of 1 vdc.


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Technical therory:

Formulas in attched XL file.

Formula for Full scale in Inches.

Gage Length in inches___________ = Full scale in inches

Desired Full scale strain

Gage Length is the Physical gage size listed in the gage documentation.

Formula for the Calibration point in inches.

Gage Length in Inches = Calibration point in inches

Desired calibration point strain

Equivalent units:

.1% strain = 1000 micro strain = .001 strain1% strain = 10000 micro strain = .01 strain

3% strain = 30000 micro strain = .03 strain

The cal resistor in the Vishay is set for 1000 microstrain, which is only .1% strain if the

system is set up for these numbers the full scale would only be 1% or 10,000 micro

strain.

The formula for equilivant resistance.

Gage Resistance * Shunt resistance = Equivilent resistanceGage Resistance + Shunt Resistance

The formula for delta resistance.

Delta Resistance = Gage Factor * desired Strain * Gage Resistance


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The formula for the readings when the resistance is known.

Strain = Equivilent Resistance ,

Gage Factor*(shunt Resistor + Equivilent Resistance)

The formula to calculate the Calibration resistor.

Shunt Resistor = Equivilent resistance * Gage resistance ,

Gage resistance Equivilent resistance

You need to know the full scale in percent or microstrain.

Calculate shunt resistance.

After the system is set up and checked for accuracy the Gage factor is built into the ratiothat is formed with the calibration system. If the gage factor changes the output of the

Vishay changes and when we calibrate the Bluehill strain or versachannel we use the newfull scale output as the full scale and the software will span the readings acordingly so the

porportionality of the fullscale to output stays the same. So the gage factor is acounted

for in the way we set up the gages as long as we do not change the numbers or the Vishayoutput.

Verifications and checks: See attached XL file.

Atachments: Copys of Strain theory information.

Given that strain results in the deformation of a body, it can be measured by calculatingthe change in length of a line or by the change in angle between two lines (where these

lines are theoretical constructs within the deformed body). The change in length of a line

is termed the stretch, absolute strain, or extension, and may be written as. Then the

(relative) strain, , is given by

where


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is strain in measured direction

is the original length of the material.is the current length of the material.

The extension () is positive if the material has gained length (in tension) and negative if it

has reduced length (in compression). Because is always positive, the sign of the strain isalways the same as the sign of the extension.

Strain is a dimensionless quantity. It has no units of measure because in the formula the

units of length "cancel out".

Strain is often expressed in dimensions of meters /metre or inches/inch anyway, as a

reminder that the number represents a change of length. But the units of length areredundant in such expressions, because they cancel out. When the units of length are left

off, strain is seen to be a pure number, which can be expressed as a decimal fraction, a

percentage or in parts-per notation. In common solid materials, the change in length is

generally a very small fraction of the length, so strain tends to be a very small number. Itis very common to express strain in units of micrometre/metre orm/m. When the units

ofm/m are canceled out, strain is expressed as a number followed by , the SI prefix all

by itself. It is usually clear from the context that is used for its SI prefix meaning,which is interchangeable with "x 10

6" or "ppm" (parts per million), and not one of the

many other possible meanings for. From wikimedia

Overview

This article introduces strain, types of commonly measured of strain, and how to choose

the strain gauge that best meets your needs, so you can take faster, more precise

measurements.

Table of Contents

1. Overview2. The Strain Gauge3. Measuring Strain4. Choosing the Right Type of Strain Gauge

Overview

Strain is the amount of deformation of a body due to an applied force. More specifically,strain (e) is defined as the fractional change in length, as shown in the figure defining

strain gauge below.
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Definition of Strain

Strain can be positive (tensile) or negative (compressive). Although dimensionless, strainis sometimes expressed in units such as in/in or mm/mm. In practice, the magnitude of

measured strain is very small. Therefore, strain is often expressed as microstrain ( ),

which is E x 10-6

.

When you strain a bar with a uniaxial force, as depicted in the figure defining strain

gauge above, a phenomenon known as Poisson strain causes the girth of the bar, D, to

contract in the transverse, or perpendicular, direction. The magnitude of this transverse

contraction is a material property indicated by its Poisson's ratio. The Poisson's ratio (v)of a material is defined as the negative ratio of the strain in the transverse direction

(perpendicular to the force) to the strain in the axial direction (parallel to the force), or. For example, Poisson's ratio for steel ranges from 0.25 to 0.3.

The Strain Gauge

While there are several methods of measuring strain, the most common is with a straingauge. A strain gauge's electrical resistance varies in proportion to the amount of strain

placed on it. The most widely used gauge is the bonded metallic strain gauge.

The metallic strain gauge consists of a very fine wire or, more commonly, metallic foil

arranged in a grid pattern. The grid pattern maximizes the amount of metallic wire or foilsubject to strain in the parallel direction (shown as the "active grid length" in the Bonded

Metallic Strain Gauge figure). The cross sectional area of the grid is minimized to reduce

the effect of shear strain and Poisson strain.


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Bonded Metallic Strain Guage

It is very important that you properly mount the strain gauge onto the test specimen. Thisensures the strain accurately transfers from the test specimen through the adhesive and

strain gauge backing to the foil.

A fundamental parameter of the strain gauge is its sensitivity to strain, expressedquantitatively as the gauge factor (GF). Gauge factor is the ratio of fractional change in

electrical resistance to the fractional change in length (strain):

The gauge factor for metallic strain gauges is typically around two.

Ideally, the resistance of the strain gauge would change only in response to applied strain.

However, strain gauge material, as well as the specimen material to which you apply the

gage, will also respond to changes in temperature. Strain gauge manufacturers attempt tominimize sensitivity to temperature by processing the gauge material to compensate for

the thermal expansion of the specimen material intended for the gauge. While

compensated gauges reduce the thermal sensitivity, they do not remove it completely. Forexample, consider a gauge compensated for aluminum that has a temperature coefficient

of 23 ppm/C. With a nominal resistance of 1000 GF = 2, the equivalent strain error is

still 11.5 /C. Therefore, additional temperature compensation is important.See Also:

How is Temperature Affecting Your Strain Measurement Accuracy?

Measuring Strain

In practice, the strain measurements rarely involve quantities larger than a few millistrain

( x 10-3). Therefore, measuring strain requires accurate measurement of very smallchanges in resistance. For example, suppose a test specimen undergoes a substantial

strain of 500 . A strain gauge with a gauge factor GF = 2 will exhibit a change in
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electrical resistance of only 2(500 x 10-6

) = 0.1%. For a 120 gauge, this is a change of

only 0.12 .

Quarter-Bridge Circut

Alternatively, you can double the sensitivity of the bridge to strain by making bothgauges active, although in different directions. For example, the Half-Bridge Circuit

figure illustrates a bending beam application with one bridge mounted in tension (RG +R) and the other mounted in compression (RG - R). This half-bridge configurati

on, whose circuit diagram is also illustrated in the Half-Bridge Circuit figure, yields an

output voltage that is linear and approximately double that of the quarter-bridge circuit.

Half-Bridge Circuit

Finally, you can further increase the sensitivity of the circuit by making all four of thearms of the bridge active strain gauges and mounting two gauges in tension and two

gauges in compression. The full-bridge circuit is shown in the Full-Bridge Circuit figure

below.


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Full-Bridge Circuit

The equations given here for the Wheatstone bridge circuits assume an initially balanced

bridge that generates zero output when you do not apply strain. In practice however,

resistance tolerances and strain induced by gauge application will generate some initialoffset voltage. This initial offset voltage is typically handled in two ways. First, you can

use a special offset-nulling, or balancing, circuit to adjust the resistance in the bridge to

rebalance the bridge to zero output. Alternatively, you can measure the initial unstrainedoutput of the circuit and compensate in software.

With this in mind, there are several types of commonly measured strain (in order ofrelative popularity):

Bending Strain -- resulting from a linear force (FV) exerted in the vertical direction.


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Axial Strain -- resulting from a linear force (Fa) exerted in the horizontal direction.

Shear Strain -- resulting from a linear force (FS) with components in both the vertical andhorizontal direction.


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Torsional Strain -- resulting from a circular force (FT) with components in both the

vertical and horizontal direction.

Choosing the Right Type of Strain Gauge

The two primary criteria for selecting the right type of strain gauge are sensitivity andprecision. In general, if you use more strain gauges, (a full-bridge circuit rather than a

quarter-bridge) your measurement will respond more quickly and be more precise. On the

other hand, cost will also play a large part in determining the type of strain gauge youselect. Typically, full-bridge strain gauges are significantly more expensive than half-

bridge and quarter-bridge gauges. For a summary of the various types of strain and strain

gauges, please refer to the Strain Gauge Summary table below.


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Strain Gauge Summary

From National Instruments web site.

W hen external forces are applied to a stationary object, stress andstrain are the result. Stress is defined as the object's internal resistingforces, and strain is defined as the displacement and deformation that

occur. For a uniform distribution of internal resisting forces, stress canbe calculated (Figure 2-1) by dividing the force (F) applied by the unit

area (A):


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Strain is defined as the amount of deformation per unit length of anobject when a load is applied. Strain is calculated by dividing the total

deformation of the original length by the original length (L):

Typical values for strain are less than 0.005 inch/inch and are often

expressed in micro-strain units:

Strain may be compressive or tensile and is typically measured bystrain gages. It was Lord Kelvin who first reported in 1856 that

metallic conductors subjected to mechanical strain exhibit a change intheir electrical resistance. This phenomenon was first put to practical

use in the 1930s.

Figure 2-1: Definitions of Stress & Strain

Fundamentally, all strain gages are designed to convert mechanical

motion into an electronic signal. A change in capacitance, inductance,or resistance is proportional to the strain experienced by the sensor.

If a wire is held under tension, it gets slightly longer and its cross-

sectional area is reduced. This changes its resistance (R) in proportion

to the strain sensitivity (S) of the wire's resistance. When a strain is

introduced, the strain sensitivity, which is also called the gage factor(GF), is given by:

The ideal strain gage would change resistance only due to the

deformations of the surface to which the sensor is attached. However,in real applications, temperature, material properties, the adhesive

that bonds the gage to the surface, and the stability of the metal all


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affect the detected resistance. Because most materials do not have

the same properties in all directions, a knowledge of the axial strain

alone is insufficient for a complete analysis. Poisson, bending, andtorsional strains also need to be measured. Each requires a different

strain gage arrangement.Shearing strainconsiders the angular distortion of an object under

stress. Imagine that a horizontal force is acting on the top rightcorner of a thick book on a table, forcing the book to becomesomewhat trapezoidal (Figure 2-2). The shearing strain in this case

can be expressed as the angular change in radians between the

vertical y-axis and the new position. The shearing strain is thetangent of this angle.

Figure 2-2: Shearing Strain

Poisson strainexpresses both the thinning and elongation that

occurs in a strained bar (Figure 2-3). Poisson strain is defined as thenegative ratio of the strain in the traverse direction (caused by the

contraction of the bar's diameter) to the strain in the longitudinal

direction. As the length increases and the cross sectional areadecreases, the electrical resistance of the wire also rises.

Figure 2-3: Poisson Strain

Bending strain, or moment strain, is calculated by determining the

relationship between the force and the amount of bending whichresults from it. Although not as commonly detected as the other types

of strain, torsional strain is measured when the strain produced bytwisting is of interest. Torsional strain is calculated by dividing the

torsional stress by the torsional modulus of elasticity.


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Sensor Des igns

The deformation of an object can be measured by mechanical, optical,

acoustical, pneumatic, and electrical means. The earliest strain gageswere mechanical devices that measured strain by measuring the

change in length and comparing it to the original length of the object.For example, the extension meter (extensiometer) uses a series of

levers to amplify strain to a readable value. In general, however,mechanical devices tend to provide low resolutions, and are bulky and

difficult to use.

Figure 2-4: Strain Gage Designs

Optical sensors are sensitive and accurate, but are delicate and not

very popular in industrial applications. They use interference fringes

produced by optical flats to measure strain. Optical sensors operatebest under laboratory conditions.The most widely used characteristic that varies in proportion to

strain is electrical resistance. Although capacitance and inductance-

based strain gages have been constructed, these devices' sensitivityto vibration, their mounting requirements, and circuit complexity have

limited their application. The photoelectric gage uses a light beam,two fine gratings, and a photocell detector to generate an electrical

current that is proportional to strain. The gage length of these devicescan be as short as 1/16 inch, but they are costly and delicate.

The first bonded, metallic wire-type strain gage was developed in1938. The metallic foil-type strain gage consists of a grid of wire

filament (a resistor) of approximately 0.001 in. (0.025 mm)thickness, bonded directly to the strained surface by a thin layer ofepoxy resin (Figure 2-4A). When a load is applied to the surface, the

resulting change in surface length is communicated to the resistor andthe corresponding strain is measured in terms of the electrical

resistance of the foil wire, which varies linearly with strain. The foildiaphragm and the adhesive bonding agent must work together in

transmitting the strain, while the adhesive must also serve as an


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When selecting a strain gage, one must consider not only the strain

characteristics of the sensor, but also its stability and temperature

sensitivity. Unfortunately, the most desirable strain gage materialsare also sensitive to temperature variations and tend to change

resistance as they age. For tests of short duration, this may not be a

serious concern, but for continuous industrial measurement, one must

include temperature and drift compensation.Each strain gage wire material has its characteristic gage factor,

resistance, temperature coefficient of gage factor, thermal coefficient

of resistivity, and stability. Typical materials include Constantan(copper-nickel alloy), Nichrome V (nickel-chrome alloy), platinum

alloys (usually tungsten), Isoelastic (nickel-iron alloy), or Karma-type

alloy wires (nickel-chrome alloy), foils, or semiconductor materials.The most popular alloys used for strain gages are copper-nickel alloys

and nickel-chromium alloys.In the mid-1950s, scientists at Bell Laboratories discovered the

piezoresistive characteristics of germanium and silicon. Although thematerials exhibited substantial nonlinearity and temperature

sensitivity, they had gage factors more than fifty times, and

sensitivity more than a 100 times, that of metallic wire or foil straingages. Silicon wafers are also more elastic than metallic ones. After

being strained, they return more readily to their original shapes.Around 1970, the first semiconductor (silicon) strain gages were

developed for the automotive industry. As opposed to other types ofstrain gages, semiconductor strain gages depend on the piezoresistive

effects of silicon or germanium and measure the change in resistancewith stress as opposed to strain. The semiconductor bonded strain

gage is a wafer with the resistance element diffused into a substrateof silicon. The wafer element usually is not provided with a backing,

and bonding it to the strained surface requires great care as only a

thin layer of epoxy is used to attach it (Figure 2-4B). The size is much

smaller and the cost much lower than for a metallic foil sensor. Thesame epoxies that are used to attach foil gages also are used to bondsemiconductor gages.

While the higher unit resistance and sensitivity of semiconductorwafer sensors are definite advantages, their greater sensitivity to

temperature variations and tendency to drift are disadvantages in

comparison to metallic foil sensors. Another disadvantage ofsemiconductor strain gages is that the resistance-to-strain

relationship is nonlinear, varying 10-20% from a straight-lineequation. With computer-controlled instrumentation, these limitations

can be overcome through software compensation.A further improvement is the thin-film strain gage that eliminates

the need for adhesive bonding (Figure 2-4C). The gage is produced by

first depositing an electrical insulation (typically a ceramic) onto thestressed metal surface, and then depositing the strain gage onto this

insulation layer. Vacuum deposition or sputtering techniques are usedto bond the materials molecularly.

Because the thin-film gage is molecularly bonded to the specimen,the installation is much more stable and the resistance values

experience less drift. Another advantage is that the stressed forcedetector can be a metallic diaphragm or beam with a deposited layer

of ceramic insulation.


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Diffused semiconductor strain gages represent a further

improvement in strain gage technology because they eliminate the

need for bonding agents. By eliminating bonding agents, errors due tocreep and hysteresis also are eliminated. The diffused semiconductor

strain gage uses photolithography masking techniques and solid-statediffusion of boron to molecularly bond the resistance elements.

Electrical leads are directly attached to the pattern (Figure 2-4D).The diffused gage is limited to moderate-temperature applications

and requires temperature compensation. Diffused semiconductors

often are used as sensing elements in pressure transducers. They aresmall, inexpensive, accurate and repeatable, provide a wide pressure

range, and generate a strong output signal. Their limitations include

sensitivity to ambient temperature variations, which can becompensated for in intelligent transmitter designs.

In summary, the ideal strain gage is small in size and mass, low incost, easily attached, and highly sensitive to strain but insensitive to

ambient or process temperature variations.

Figure 2-5: Bonded ResistanceStrain Gage Construction

Bonded Resis tance Gages

The bonded semiconductor strain gage was schematically described inFigures 2-4A and 2-4B. These devices represent a popular method of

measuring strain. The gage consists of a grid of very fine metallicwire, foil, or semiconductor material bonded to the strained surface or

carrier matrix by a thin insulated layer of epoxy (Figure 2-5). When

the carrier matrix is strained, the strain is transmitted to the gridmaterial through the adhesive. The variations in the electrical

resistance of the grid are measured as an indication of strain. The grid

shape is designed to provide maximum gage resistance while keepingboth the length and width of the gage to a minimum.Bonded resistance strain gages have a good reputation. They are

relatively inexpensive, can achieve overall accuracy of better than +/-0.10%, are available in a short gage length, are only moderately

affected by temperature changes, have small physical size and low

mass, and are highly sensitive. Bonded resistance strain gages can beused to measure both static and dynamic strain.


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Typical metal-foil strain gages.

In bonding strain gage elements to a strained surface, it is importantthat the gage experience the same strain as the object. With an

adhesive material inserted between the sensors and the strainedsurface, the installation is sensitive to creep due to degradation of thebond, temperature influences, and hysteresis caused by thermoelastic

strain. Because many glues and epoxy resins are prone to creep, it isimportant to use resins designed specifically for strain gages.

The bonded resistance strain gage is suitable for a wide variety of

environmental conditions. It can measure strain in jet engine turbinesoperating at very high temperatures and in cryogenic fluid

applications at temperatures as low as -452*F (-269*C). It has lowmass and size, high sensitivity, and is suitable for static and dynamic

applications. Foil elements are available with unit resistances from

120 to 5,000 ohms. Gage lengths from 0.008 in. to 4 in. are availablecommercially. The three primary considerations in gage selection are:

operating temperature, the nature of the strain to be detected, andstability requirements. In addition, selecting the right carrier material,grid alloy, adhesive, and protective coating will guarantee the success

of the application.

Measur ing Ci rcu i t s

In order to measure strain with a bonded

resistance strain gage, it must be connected to anelectric circuit that is capable of measuring the

minute changes in resistance corresponding tostrain. Strain gage transducers usually employ

four strain gage elements electrically connected to

form a Wheatstone bridge circuit (Figure 2-6).A Wheatstone bridge is a divided bridge circuit

used for the measurement of static or dynamicelectrical resistance. The output voltage of the


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Wheatstone bridge is expressed in millivolts output

per volt input. The Wheatstone circuit is also well

suited for temperature compensation.

Figure 2-6: Wheatstone Bridge Circuit Schematic

In Figure 2-6, if R1, R2, R3, and R4 are equal,and a voltage, VIN, is applied between points A

and C, then the output between points B and D will

show no potential difference. However, if R4 ischanged to some value which does not equal R1,

R2, and R3, the bridge will become unbalancedand a voltage will exist at the output terminals. In

a so-called G-bridge configuration, the variablestrain sensor has resistance Rg, while the other

arms are fixed value resistors.The sensor, however, can occupy one, two, or

four arms of the bridge, depending on the

application. The total strain, or output voltage ofthe circuit (VOUT) is equivalent to the difference

between the voltage drop across R1 and R4, or Rg.This can also be written as:

For more detail, see Figure 2-6. The bridge is

considered balanced when R1/R2 = Rg/R3 and,

therefore, VOUT equals zero.Any small change in the resistance of the sensing

grid will throw the bridge out of balance, making itsuitable for the detection of strain. When the

bridge is set up so that Rg is the only active strain


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gage, a small change in Rg will result in an output

voltage from the bridge. If the gage factor is GF,

the strain measurement is related to the change inRg as follows:

The number of active strain gages that should be

connected to the bridge depends on theapplication. For example, it may be useful to

connect gages that are on opposite sides of a

beam, one in compression and the other intension. In this arrangement, one can effectively

double the bridge output for the same strain. In

installations where all of the arms are connectedto strain gages, temperature compensation is

automatic, as resistance change due totemperature variations will be the same for all

arms of the bridge.

In a four-element Wheatstone bridge, usually twogages are wired in compression and two in

tension. For example, if R1 and R3 are in tension

(positive) and R2 and R4 are in compression(negative), then the output will be proportional to

the sum of all the strains measured separately. Forgages located on adjacent legs, the bridge

becomes unbalanced in proportion to the

difference in strain. For gages on opposite legs,

the bridge balances in proportion to the sum of thestrains. Whether bending strain, axial strain, shearstrain, or torsional strain is being measured, the

strain gage arrangement will determine therelationship between the output and the type of

strain being measured. As shown in Figure 2-6, if

a positive tensile strain occurs on gages R2 andR3, and a negative strain is experienced by gages

R1 and R4, the total output, VOUT, would be four

times the resistance of a single gage.

Figure 2-7: Chevron Bridge Circuit Schematic


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The Chevr on Br idg e

The Chevron bridge is illustrated in Figure 2-7. It

is a multiple channel arrangement that serves tocompensate for the changes in bridge-arm

resistances by periodically switching them. Here,

the four channel positions are used to switch thedigital voltmeter (DVM) between G-bridge (one

active gage) and H-bridge (two active gages)configurations. The DVM measurement device

always shares the power supply and an internal H-

bridge. This arrangement is most popular for strain

measurements on rotating machines, where it canreduce the number of slip rings required.

Figure 2-8: Four-Wire Ohm Circuit Schematic

Four -Wi r e Ohm Ci rcu i t

Although the Wheatstone bridge is one of the mostpopular methods of measuring electrical

resistance, other methods can also be used. Themain advantage of a four-wire ohm circuit is that

the lead wires do not affect the measurement

because the voltage is detected directly across thestrain gage element.

A four-wire ohm circuit installation might consistof a voltmeter, a current source, and four lead

resistors, R1, in series with a gage resistor, Rg(Figure 2-8). The voltmeter is connected to the

ohms sense terminals of the DVM, and the currentsource is connected to the ohms source terminals

of the DVM. To measure the value of strain, a low

current flow (typically one milliampere) is supplied

to the circuit. While the voltmeter measures thevoltage drop across Rg, the absolute resistancevalue is computed by the multimeter from the

values of current and voltage.

The measurement is usually done by firstmeasuring the value of gage resistance in an

unstrained condition and then making a second

measurement with strain applied. The difference in

the measured gage resistances divided by the


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unstrained resistance gives a fractional value of

the strain. This value is used with the gage factor

(GF) to calculate strain.The four-wire circuit is also suitable for automatic

voltage offset compensation. The voltage is firstmeasured when there is no current flow. This

measured value is then subtracted from thevoltage reading when current is flowing. Theresulting voltage difference is then used to

compute the gage resistance. Because of theirsensitivity, four-wire strain gages are typically

used to measure low frequency dynamic strains.

When measuring higher frequency strains, thebridge output needs to be amplified. The same

circuit also can be used with a semiconductorstrain-gage sensor and high speed digital

voltmeter. If the DVM sensitivity is 100 microvolts,

the current source is 0.44 milliamperes, the strain-

gage element resistance is 350 ohms and its gage

factor is 100, the resolution of the measurementwill be 6 microstrains.

Figure 2-9: Constant Current Circuit Schematic

Constan t Cur ren t Ci r cu i t Resistance can be measured by exciting the bridge

with either a constant voltage or a constant

current source. Because R = V/I, if either V or I isheld constant, the other will vary with the

resistance. Both methods can be used.While there is no theoretical advantage to using a

constant current source (Figure 2-9) as comparedto a constant voltage, in some cases the bridge

output will be more linear in a constant currentsystem. Also, if a constant current source is used,

it eliminates the need to sense the voltage at the

bridge; therefore, only two wires need to beconnected to the strain gage element.

The constant current circuit is most effectivewhen dynamic strain is being measured. This is

because, if a dynamic force is causing a change in

the resistance of the strain gage (Rg), one would


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measure the time varying component of the output

(VOUT), whereas slowly changing effects such as

changes in lead resistance due to temperaturevariations would be rejected. Using this

configuration, temperature drifts become nearly

negligible.

App l i ca t i on & I ns t a l l a t i on

The output of a strain gage circuit is a very low-level voltage signal requiring a sensitivity of 100

microvolts or better. The low level of the signalmakes it particularly susceptible to unwanted noise

from other electrical devices. Capacitive couplingcaused by the lead wires' running too close to AC

power cables or ground currents are potential

error sources in strain measurement. Other errorsources may include magnetically induced voltages

when the lead wires pass through variablemagnetic fields, parasitic (unwanted) contact

resistances of lead wires, insulation failure, andthermocouple effects at the junction of dissimilar

metals. The sum of such interferences can result insignificant signal degradation.

Shie ld ing

Most electric interference and noise problems canbe solved by shielding and guarding. A shield

around the measurement lead wires will interceptinterferences and may also reduce any errors

caused by insulation degradation. Shielding also

will guard the measurement from capacitive

coupling. If the measurement leads are routednear electromagnetic interference sources such as

transformers, twisting the leads will minimizesignal degradation due to magnetic induction. By

twisting the wire, the flux-induced current isinverted and the areas that the flux crosses cancel

out. For industrial process applications, twisted

and shielded lead wires are used almost withoutexception.

Guard ing

Guarding the instrumentation itself is just as

important as shielding the wires. A guard is asheet-metal box surrounding the analog circuitry

and is connected to the shield. If ground currentsflow through the strain-gage element or its lead

wires, a Wheatstone bridge circuit cannotdistinguish them from the flow generated by the

current source. Guarding guarantees that

terminals of electrical components are at the same

potential, which thereby prevents extraneous


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current flows.

Connecting a guard lead between the test

specimen and the negative terminal of the powersupply provides an additional current path around

the measuring circuit. By placing a guard lead pathin the path of an error-producing current, all of the

elements involved (i.e., floating power supply,strain gage, all other measuring equipment) will

be at the same potential as the test specimen. By

using twisted and shielded lead wires andintegrating DVMs with guarding, common mode

noise error can virtually be eliminated.

Figure 2-10: Alternative Lead-Wire Configurations

Lead-Wire Ef fec ts

Strain gages are sometimes mounted at a distancefrom the measuring equipment. This increases the

possibility of errors due to temperature variations,

lead desensitization, and lead-wire resistancechanges. In a two-wire installation (Figure 2-10A),

the two leads are in series with the strain-gageelement, and any change in the lead-wire

resistance (R1) will be indistinguishable fromchanges in the resistance of the strain gage (Rg).

To correct for lead-wire effects, an additional,third lead can be introduced to the top arm of the

bridge, as shown in Figure 2-10B. In this

configuration, wire C acts as a sense lead with nocurrent flowing in it, and wires A and B are in

opposite legs of the bridge. This is the minimumacceptable method of wiring strain gages to a

bridge to cancel at least part of the effect ofextension wire errors. Theoretically, if the lead

wires to the sensor have the same nominal

resistance, the same temperature coefficient, andare maintained at the same temperature, full


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compensation is obtained. In reality, wires are

manufactured to a tolerance of about 10%, and

three-wire installation does not completelyeliminate two-wire errors, but it does reduce them

by an order of magnitude. If further improvement

is desired, four-wire and offset-compensated

installations (Figures 2-10C and 2-10D) should beconsidered.In two-wire installations, the error introduced by

lead-wire resistance is a function of the resistance

ratio R1/Rg. The lead error is usually notsignificant if the lead-wire resistance (R1) is small

in comparison to the gage resistance (Rg), but ifthe lead-wire resistance exceeds 0.1% of the

nominal gage resistance, this source of errorbecomes significant. Therefore, in industrial

applications, lead-wire lengths should beminimized or eliminated by locating the

transmitter directly at the sensor.

Figure 2-11: Gage-Factor Temperature

Dependence

Tempera t u re and t he Gage Fac tor Strain-sensing materials, such as copper, change

their internal structure at high temperatures.

Temperature can alter not only the properties of astrain gage element, but also can alter the

properties of the base material to which the straingage is attached. Differences in expansion

coefficients between the gage and base materials

may cause dimensional changes in the sensor

element.Expansion or contraction of the strain-gage

element and/or the base material introduces errors

that are difficult to correct. For example, a changein the resistivity or in the temperature coefficient

of resistance of the strain gage element changesthe zero reference used to calibrate the unit.

The gage factor is the strain sensitivity of the

sensor. The manufacturer should always supply


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data on the temperature sensitivity of the gage

factor. Figure 2-11 shows the variation in gage

factors of the various strain gage materials as afunction of operating temperature. Copper-nickel

alloys such as Advance have gage factors that are

relatively sensitive to operating temperature

variations, making them the most popular choicefor strain gage materials.

Figure 2-12: Apparent Strain Variationwith Temperature

Appar en t S t r a in Apparent strain is any change in gage resistance that is not caused by

the strain on the force element. Apparent strain is the result of the

interaction of the thermal coefficient of the strain gage and thedifference in expansion between the gage and the test specimen. The

variation in the apparent strain of various strain-gage materials as afunction of operating temperature is shown in Figure 2-12. In addition

to the temperature effects, apparent strain also can change because ofaging and instability of the metal and the bonding agent.

Compensation for apparent strain is necessary if the temperaturevaries while the strain is being measured. In most applications, the

amount of error depends on the alloy used, the accuracy required, and

the amount of the temperature variation. If the operating temperatureof the gage and the apparent strain characteristics are known,

compensation is possible.

Stab i l i t y Cons idera t ions

It is desirable that the strain-gage measurement system be stable and

not drift with time. In calibrated instruments, the passage of timealways causes some drift and loss of calibration. The stability of bonded

strain-gage transducers is inferior to that of diffused strain-gageelements. Hysteresis and creeping caused by imperfect bonding is one

of the fundamental causes of instability, particularly in high operating


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temperature environments.

Before mounting strain-gage elements, it should be established that

the stressed force detector itself is uniform and homogeneous, becauseany surface deformities will result in instability errors. In order to

remove any residual stresses in the force detectors, they should be

carefully annealed, hardened, and stress-relieved using temperature

aging. A transducer that uses force-detector springs, diaphragms, orbellows should also be provided with mechanical isolation. This willprotect the sensor element from external stresses caused either by the

strain of mounting or by the attaching of electric conduits to the

transducer.If stable sensors are used, such as deposited thin-film element types,

and if the force-detector structure is well designed, balancing andcompensation resistors will be sufficient for periodic recalibration of the

unit. The most stable sensors are made from platinum or other low-temperature coefficient materials. It is also important that the

transducer be operated within its design limits. Otherwise, permanentcalibration shifts can result. Exposing the transducer to temperatures

outside its operating limits can also degrade performance. Similarly, the

transducer should be protected from vibration, acceleration, and shock.

Figure 2-13: Strain Gage Installation Alternatives

Transducer Des ignsStrain gages are used to measure displacement, force, load, pressure,

torque or weight. Modern strain-gage transducers usually employ a gridof four strain elements electrically connected to form a Wheatstone

bridge measuring circuit.The strain-gage sensor is one of the most widely used means of load,weight, and force detection. In Figure 2-13A, a vertical beam is

subjected to a force acting on the vertical axis. As the force is applied,the support column experiences elastic deformation and changes the

electrical resistance of each strain gage. By the use of a Wheatstone

bridge, the value of the load can be measured. Load cells are popularweighing elements for tanks and silos and have proven accurate in

many other weighing applications.


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Strain gages may be bonded to cantilever springs to measure the force

of bending (Figure 2-13B). The strain gages mounted on the top of the

beam experience tension, while the strain gages on the bottomexperience compression. The transducers are wired in a Wheatstone

circuit and are used to determine the amount of force applied to thebeam.

Strain-gage elements also are used widely in the design of industrialpressure transmitters. Figure 2-13C shows a bellows type pressuresensor in which the reference pressure is sealed inside the bellows on

the right, while the other bellows is exposed to the process pressure.

When there is a difference between the two pressures, the straindetector elements bonded to the cantilever beam measure the resulting

compressive or tensile forces.A diaphragm-type pressure transducer is created when four strain

gages are attached to a diaphragm (Figure 2-13D). When the processpressure is applied to the diaphragm, the two central gage elements aresubjected to tension, while the two gages at the edges are subjected to

compression. The corresponding changes in resistance are a measure of

the process pressure. When all of the strain gages are subjected to the

same temperature, such as in this design, errors due to operatingtemperature variations are reduced.

I ns t a l l a t i on D iagnos t i c s

All strain gage installations should be checked using the following steps:

1. Measure the base resistance of the unstrained strain gage

after it is mounted, but before wiring is connected.2. Check for surface contamination by measuring the isolation

resistance between the gage grid and the stressed force detector

specimen using an ohmmeter, if the specimen is conductive. This

should be done before connecting the lead wires to theinstrumentation. If the isolation resistance is under 500megaohms, contamination is likely.

3. Check for extraneous induced voltages in the circuit byreading the voltage when the power supply to the bridge is

disconnected. Bridge output voltage readings for each strain-

gage channel should be nearly zero.4. Connect the excitation power supply to the bridge and

ensure both the correct voltage level and its stability.

5. Check the strain gage bond by applying pressure to thegage. The reading should be unaffected.

References & Fur th er Read ing

Omegadyne Pressure, Force, Load, Torque Databook, OMEGADYNE, Inc., 1996

The Pressure, Strain, and Force Handbook, Omega Press LLC, 1996.

Instrument Engineers' Handbook, Bela Liptak, CRC Press LLC, 1995.

Marks' Standard Handbook for Mechanical Engineers, 10th Edition, Eugene A. Avallone, andTheodore Baumeister, McGraw-Hill, 1996.

McGraw-Hill Concise Encyclopedia of Science and Technology, McGraw-Hill, 1998.


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Process/Industrial Instruments and Controls Handbook, 4th Edition, Douglas M. Considine, McGraw-Hill, 1993.Van Nostrand's Scientific Encyclopedia, Douglas M. Considine and Glenn D. Considine, VanNostrand, 1997.

using glue on strain guages

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