resistance strain gauge

8/7/2019 resistance strain gauge

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Resistance

StrainGauges

:- Surface preparation,

Signal conditioning,Bridge circuits,

applications of Strain

Gauges

Submitted By :-

1. Ankit Garg (08107079)2. Nitish Sharma

(08107078)

3. Karunesh Piwania(08107077)

4. Atulya Aggarwal(08107080)


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INTRODUCTION

When external forces are applied to a stationary object, stress and strainare the result. Stress is defined as the object's internal resisting forces, andstrain is defined as the displacement and deformation that occur. For auniform distribution of internal resisting forces, stress can be calculated bydividing the force (F) applied by the unit area (A):

Strain is defined as the amount of deformation per unit length of anobject when a load is applied. Strain is calculated by dividing the totaldeformation 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 by straingages. It was Lord Kelvin who first reported in 1856 that metallicconductors subjected to mechanical strain exhibit a change in theirelectrical resistance. This phenomenon was first put to practical use in the1930s.


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Figure : Definitions of Stress & Strain

Fundamentally, all strain gages are designed to convert mechanical motion

into an electronic signal. A change in capacitance, inductance, or resistanceis proportional to the strain experienced by the sensor. If a wire is heldunder tension, it gets slightly longer and its cross-sectional area is reduced.This changes its resistance (R) in proportion to the strain sensitivity (S) ofthe wire's resistance.

The ideal strain gage would change resistance only dueto 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 metalall affect the detected resistance. Because most materials do not have thesame properties in all directions, a knowledge of the axial strain alone isinsufficient for a complete analysis. Poisson, bending, and torsional strainsalso need to be measured. Each requires a different strain gagearrangement.


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Gauge factor

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

where

R is the change in resistance caused by strain,RG

is the resistance of the undeformed gauge, and

is strain.

For metallic foil gauges, the gauge factor is usually a little over 2. For asingle active gauge and three dummy resistors, the output v from the

bridge is:

where

BV is the bridge excitation voltage.

Foil gauges typically have active areas of about 210 mm2 in size. With

careful installation, the correct gauge, and the correct adhesive, strains upto at least 10% can be measured.


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Bonded Resistance Gages

Figure : Bonded ResistanceStrain Gage Construction

The bonded semiconductor strain gage is schematically described in Figure.These devices represent a popular method of measuring strain. The gageconsists of a grid of very fine metallic wire, foil, or semiconductor materialbonded to the strained surface or carrier matrix by a thin insulated layer of

epoxy. When the carrier matrix is strained, the strain is transmitted to thegrid material through the adhesive. The variations in the electricalresistance of the grid are measured as an indication of strain. The gridshape is designed to provide maximum gage resistance while keeping boththe length and width of the gage to a minimum.

Bonded resistance strain gages have a good reputation. They are relativelyinexpensive, can achieve overall accuracy of better than +/-0.10%, areavailable in a short gage length, are only moderately affected bytemperature changes, have small physical size and low mass, and are highly

sensitive. Bonded resistance strain gages can be used to measure both staticand dynamic strain.


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Typical metal-foil strain gages

In bonding strain gage elements to a strained surface, it is important that

the gage experience the same strain as the object. With an adhesivematerial inserted between the sensors and the strained surface, theinstallation is sensitive to creep due to degradation of the bond,temperature influences, and hysteresis caused by thermoelastic strain.Because many glues and epoxy resins are prone to creep, it is important touse resins designed specifically for strain gages.

The bonded resistance strain gage is suitable for a wide variety ofenvironmental conditions. It can measure strain in jet engine turbinesoperating at very high temperatures and in cryogenic fluid applications attemperatures as low as -452*F (-269*C). It has low mass and size, highsensitivity, and is suitable for static and dynamic applications. Foil elementsare available with unit resistances from 120 to 5,000 ohms. Gage lengthsfrom 0.008 in. to 4 in. are available commercially. The three primaryconsiderations in gage selection are: operating temperature, the nature ofthe strain to be detected, and stability requirements. In addition, selectingthe right carrier material, grid alloy, adhesive, and protective coating willguarantee the success of the application.


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Temperature and the Gage Factor

Figure: Gage-Factor Temperature Dependence

Strain-sensing materials, such as copper, change their internal structure at

high temperatures. Temperature can alter not only the properties of a

strain gage element, but also can alter the properties of the base material

to which the strain gage 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

change in the resistivity or in the temperature coefficient of resistance of

the strain gage element changes the zero reference used to calibrate the

unit.

The gage factor is the strain sensitivity of the sensor. The manufacturer

should always supply data on the temperature sensitivity of the gage factor.

Figure shows the variation in gage factors of the various strain gage

materials as a function 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 choice for strain

gage materials.

Most strain gauges are made from a constantan alloy. Various constantan

alloys and Karma alloys have been designed so that the temperature effects

on the resistance of the strain gauge itself cancel out the resistance change


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of the gauge due to the thermal expansion of the object under test. Because

different materials have different amounts of thermal expansion, self-

temperature compensation (STC) requires selecting a particular alloy

matched to the material of the object under test.

Even with strain gauges that are not self-temperature-compensated (suchas isoelastic alloy), use of a Wheatstone bridge arrangement allows

compensating for temperature changes in the specimen under test and the

strain gauge. To do this in a Wheatstone bridge made of four gauges, two

gauges are attached to the specimen, and two are left unattached,

unstrained, and at the same temperature as the specimen and the attached

gauges.


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Installation of Strain Gauges

The installation of a strain gage to a specimen is probably the most critical,yet least emphasized step in the strain measurement. An improperinstallation may seriously degrade or even completely mess up the validityof a test.

Certain types of strain gages can be welded on the surface of surface.However, adhesives are still the most commonly used agents for mountingstrain gages. This section describes one of the most popular installationmethod for adhesive mounting the tape-assisted installation method.

Procedures of General Purpose Tape-Assisted Installation Method

The tape-assisted installation method is the most popular method to installmetal-foil strain gages. Its procedures can be summarized as follows.Further detail in each step can be found by provided links.

Surface preparation

- Cleaning the surface

- Abrading the surface

- Marking the layout

Lines

- Conditioning

- Neutralizing

Gage Bonding- Preparation

- Preparing the gage


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- Transferring the gage

- Applying the catalyst

- Applying the adhesive

- Removing the tape

Leadwire Attachment- Stripping the leadwires

- Tinning the leadwires

- Tinning the gage

- Attaching the leadwires

- Removing Rosin

- Anchoring the leadwires

Protective Coating- Preparation

- Applying the coating


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Surface Preparation- Cleaning Surface : Remove grease and oil on the

specimen surface by solvent, e.g., Alcohol, Acetone,or some other degreasing agent.

- Abrading Surface: Use silicon-carbide paperto sand away uneven surface, paint, or rustand smooth the gaging area. Usually 320grit first, follow by 400 or finer grit. Donot over abrade.

- Marking Layout Lines : Use a clean rule and a fine pencil (2H or harder)or ball-point pen to draw the layout lines, usually a dash-cross, a crossskip the targeting strain gage area, for alignment.

- Conditioning: Re-clean the gaging area.

- Neutralizing: This is an optional step. A proper neutralizer will providethe right pH level at the specimen surface for better bonding with

adhesive.


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Gage Bonding- Preparation: Wash hands with soap and water. Clean the working desk

area and all related tools with solvent or degreasing agent.

- Preparing Gage: Carefully open the foldercontaining the gage. Use a tweezer, not barehands, to grasp the gage. Avoid touching thegrid. Place on the clean working area with thebonding side down.

- Transferring Gage: Use a proper length, about 15 cm (6 in), ofcellophane tape to pick up the strain gage and transfer it to the gagingarea of the specimen. Align the gage with the layout lines. Press one endof the tape to the specimen, then smoothly and gently apply the wholetape and gage into position.

- Applying Catalyst : Lift one end of thetape such that the gage does not contactthe gaging area and the bonding site isexposed. Apply catalyst evenly andgently on the gage.

- Applying Adhesive: Apply enough adhesiveto provide sufficient coverage under thegage for proper adhesion. (Determining"sufficient" might require some trial anderror iterations). Place the tape and the gageback to the specimen smoothly and gently.Immediately place thumb over the gage andapply firm and steady pressure on the gage

for at least one minute.


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- Removing Tape : Leave the tape in place atleast two more minutes after the thumb wasremoved. Peel the tape from the specimenslowly and smoothly from one end to theother end.

Note: Some adhesives require mixing two compounds vigorously for asufficient time, usually 5 minutes. Others require longer curing time up to24 hours and/or higher temperature, usually by blowing hot air using aheat gun or placing in an oven. Some applications require higherclamping pressure as high as 350 kPa (50 psi). Please consult with thetechnical notes from the vendor for the right process parameters.


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Leadwire Attachment- Stripping Leadwire : Cut the leadwires to the desired length. Strip off 2 -

3 cm (1 in) insulation for attachment. Twist each bundle of conductorstogether. Do not damage the leadwires by overtwisting or nicking them.

- Tinning Leadwires: Have the soldering station set to the propertemperature. Use the solder pencil / gun to melt a pool of fresh solder.Coat the non-insulated parts of the leadwires with solder.

- Tinning Gage: Place the solder on the copper tabs of the gage. Press theheated solder pencil through the solder to the tabs. A smoothhemispherical "solder pillow" will be formed in this tinning operation. Ifnot, feed in more fresh solder.

- Attaching Leadwire: Positionthe non-insulated conductorsdirectly on top of the solderpillow. Press a piece of tapeto the end of leadwireinsulation to fix the leadwiresat right position of thespecimen. Press the solderpencil on the conductor and

push it into the solderpillow.

- Removing Rosin: Use solvent to clean the gaging area. Remove the tapeattached on the leadwires.

- Anchoring Leadwire: Make sure that no non-insulated conductors contactwith the specimen. Secured the leadwires to the specimen (whenpossible) by a durable tape.

Note: Some strain gages do not have integral copper tabs for solderingleadwires. Instead, they come with built-in leadwires and separated terminalstrips. To install this kind of strain gages, the terminal is attached to thespecimen (using the same gage bonding procedures) near (2 - 10 mm; 1/16 -3/8 in) the strain gage. Both the built-in leadwires and the leadwires forthe electrical circuit are soldered to the terminal. Although the separateterminal may lengthen the installation procedures and require more spacefor placing a strain gage, it is a good practice because it isolates the strainfrom the measurement device.


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Protective CoatingsProtective Coatings are mainly used to protect the strain gage againstmoisture and other contaminants which may affect gage stability. They arerecommended for applications in the field and for long term

measurements.

- Preparation: Clean the gaging area.- Applying Coating: Apply silicon rubber, polyurethane, or acrylic lacquer

to the gaging area. If further protection is needed, PTFE film andneoprene rubber sheets can be used to cover both gages and nearbyleadwires.


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The Strain Gage Measurement

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

millistrain (e x 10-3

). Therefore, to measure the strain requires accuratemeasurement of very small changes in resistance. For example, suppose a

test specimen undergoes a strain of 500 me. A strain gage with a gage

factor of 2 will exhibit a change in electrical resistance of only 2 (500 x 10 -

6) = 0.1%. For a 120 gage, this is a change of only 0.12 .

To measure such small changes in resistance, strain gages are almost

always used in a bridge configuration with a voltage excitation source


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Bridge Circuit

A bridge circuit is a special type of electrical circuit wherein thecurrent from a voltage source splits into two parallel paths. These

parallel paths contain components (such as resistors, capacitors, andinductors), the types and arrangement of which depend on what thepurpose of the bridge circuit is. The parallel paths recombine againto let the current return to the source in a single conductor, therebyclosing the circuit.

The parallel paths are 'bridged' together by another electrical paththat usually contains a load or a measuring device (such as agalvanometer), hence the name 'bridge circuit.' Bridge circuits areprimarily used in measurement applications and power supplies.

The best known bridge circuit is the Wheatstone Bridge, which isshown in Figure below. Here, one can see that the circuit splits intotwo paths; the left path contains R2 and R1 while the right pathcontains R3 and R unknown. The two parallel paths are bridgedtogether by an ammeter or galvanometer connected between nodes Aand B.

The Wheatstone Bridge is used for accurately measuring the value ofunknown, provided that the values of the other resistors are knownand may be adjusted.

Figure 1. The Wheatstone Bridge


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Types of Bridge CircuitsAside from the Wheatstone Bridge, there are many other bridge circuits,the more widely known of which are as follows:

1) Wien Bridge2) Schering Bridge3) Hay Bridge4) Owen Bridge5) Maxwell Bridge6) Resonance Bridge


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Wein BridgeA Wien Bridge is a bridge circuit used for measuring an unknowncapacitance by balancing the loads of its four arms, one of whichcontains the unknown capacitance. Figure below shows a diagram ofthe Wien Bridge.

The Wien Bridge


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Schering BridgeA Schering Bridge is a bridge circuit used for measuring an unknownelectrical capacitance and its dissipation factor. The dissipation factor

of a capacitor is the the ratio of its resistance to its capacitivereactance. The Schering Bridge is basically a four-arm alternating-current (AC) bridge circuit whose measurement depends onbalancing the loads on its arms. Figure below shows a diagram of theSchering Bridge.

The Schering Bridge


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Hay BridgeA Hay Bridge is an AC bridge circuit used for measuring an unknowninductance by balancing the loads of its four arms, one of whichcontains the unknown inductance. One of the arms of a Hay Bridgehas a capacitor of known characteristics, which is the principalcomponent used for determining the unknown inductance value.Figure below shows a diagram of the Hay Bridge.

The Hay Bridge


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Owen BridgeAn Owen Bridge is an AC bridge circuit used for measuring anunknown inductance by balancing the loads of its four arms, one ofwhich contains the unknown inductance. Figure below shows adiagram of the Owen Bridge.

The Owen Bridge


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Maxwell Bridge

A Maxwell Bridge, also known as the Maxwell-Wien Bridge, is an ACbridge circuit used for measuring an unknown inductance bybalancing the loads of its four arms, one of which contains theunknown inductance. Figure below shows a diagram of the MaxwellBridge.

The Maxwell Bridge


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Resonance BridgeA Resonance Bridge is an AC bridge circuit used for measuring anunknown inductance, an unknown capacitance, or an unknownfrequency, by balancing the loads of its four arms. Figure belowshows a diagram of the Resonance Bridge.

The Resonance Bridge


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Sensing Strain in a Member Under Uniaxial Load

It is often necessary to measure the strain in a prismatic member (a bar)

subjected to an axial load. A common example might be a truss element

which is designed to carry axial load. In this case application of a single

strain gage oriented in the axial direction on the bar would appear to be

sufficient. However, several problems arise. First, there is the problem ofwhat to do about the other 3 arms of the Wheatstone Bridge, but second,

it is not always so easy to assume that a single gage will correctly sense the

axial strain in the bar. For example, while the stress stateAE3145

Resistance Strain Gage Circuits may be uniaxial (consist of only a single

nonzero stress), the strain state is not, and there are significant lateral

strains that the gage might sense, especially if it is not accurately aligned.

Moreover, any slight bending in the member (due to initial eccentricities,for example) or other irregularities might cause the axial strain to vary

across the cross section of the bar.


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Sensing Strain in a Member Under Bending Load

Quite the opposite to what was considered in the previous section can also

be true. That is it may be necessary to sense the bending induced strains

in a prismatic member and NOT the strains due to axial loading. This is

usually the case when dealing with beams or other so-called flexural

elements. For these cases, the bridge can be wired so that the equal and

opposite strains that are induced on the upper and lower surfaces of a

simple beam will appear in adjacent arms where the strains will be

combined. Even when the beam cross section is such that the centroid isnot equal distances from the top and bottom surfaces (e.g., a tee section),

the strains will still be of opposite sign and will add constructively in the

bridge equation.


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Sensing Torsion Strains in a Circular Shaft

Torsion presents an interesting challenge because the dominant stress is a

shear stress, x, and therefore it is usually necessary to measure the

resulting shear strain, x. We have seen earlier that while strain gagescan directly sense only an extensional strain, a rosette can be used to

measure the strain state at a point and to thereby infer any particular 2D

strain component. In the general case as treated earlier, it is necessary to

apply at least 3 gages in a rosette to determine the 2D strain state.

However, if some information is already known, it is usually possible to use

fewer than 3 gages. This is the case when sensing the torsion strain in a

circular shaft where the directions of principal strain are known in advance

to be at 45 to the axial direction on the surface of the shaft. In view of

this knowledge, a simple two-element tee or 90 rosette can be mounted

such that the two individual gages are aligned in the principal strain

directions.

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resistance strain gauge