Strain Gauge - Wikipedia, The Free Encyclopedia

Typical foil strain gauge. The gauge is

far more sensitive to strain in the

vertical direction than in the horizontal

direction. The markings outside the

active area help to align the gauge

during installation.

From Wikipedia, the free encyclopedia

A strain gauge (also strain gage) is a device used to measure strain onan object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938,the most common type of strain gauge consists of an insulating flexiblebacking which supports a metallic foil pattern. The gauge is attached to

the object by a suitable adhesive, such as cyanoacrylate.[1] As the objectis deformed, the foil is deformed, causing its electrical resistance tochange. This resistance change, usually measured using a Wheatstonebridge, is related to the strain by the quantity known as the gauge factor.

1 Physical operation2 Gauge factor3 Gauges in practice

3.1 Variations in temperature4 Errors and compensation5 Other gauge types6 Mechanical types7 See also8 References9 External links

A strain gauge takes advantage of the physical property of electrical conductance and its dependence on theconductor's geometry. When an electrical conductor is stretched within the limits of its elasticity such that itdoes not break or permanently deform, it will become narrower and longer, changes that increase its electricalresistance end-to-end. Conversely, when a conductor is compressed such that it does not buckle, it will broadenand shorten, changes that decrease its electrical resistance end-to-end. From the measured electrical resistanceof the strain gauge, the amount of applied stress may be inferred. A typical strain gauge arranges a long, thinconductive strip in a zig-zag pattern of parallel lines such that a small amount of stress in the direction of theorientation of the parallel lines results in a multiplicatively larger strain measurement over the effective length ofthe conductor surfaces in the array of conductive lines—and hence a multiplicatively larger change inresistance—than would be observed with a single straight-line conductive wire.

The gauge factor is defined as:

Strain gauge - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Strain_gauge

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where

is the change in resistance caused by strain, is the resistance of the undeformed gauge, and

is strain.

For metallic foil gauges, the gauge factor is usually a little over 2.[2] For a single active gauge and three dummyresistors, the output from the bridge is:

where

is the bridge excitation voltage.

Foil gauges typically have active areas of about 2–10 mm2 in size. With careful installation, the correct gauge,and the correct adhesive, strains up to at least 10% can be measured.

An excitation voltage is applied to input leads of the gauge network, and a voltage reading is taken from theoutput leads. Typical input voltages are 5 V or 12 V and typical output readings are in millivolts.

Foil strain gauges are used in many situations. Different applications place different requirements on the gauge.In most cases the orientation of the strain gauge is significant.

Gauges attached to a load cell would normally be expected to remain stable over a period of years, if notdecades; while those used to measure response in a dynamic experiment may only need to remain attached tothe object for a few days, be energized for less than an hour, and operate for less than a second.

Strain gauges are attached to the substrate with a special glue. The type of glue depends on the required lifetimeof the measurement system. For short term measurements (up to some weeks) cyanoacrylic glue is appropriate,for long lasting installation epoxy glue is required. Usually epoxy glue requires high temperature curing (at about80-100°C). The preparation of the surface where the strain gauge is to be glued is of the utmost importance. Thesurface must be smoothed (e.g. with very fine sand paper), deoiled with solvents, the solvent traces must then beremoved and the strain gauge must be glued immediately after this to avoid oxidation or pollution of theprepared area. If these steps are not followed the strain gauge binding to the surface may be unreliable andunpredictable measurement errors may be generated.

Strain gauge based technology is utilized commonly in the manufacture of pressure sensors. The gauges used inpressure sensors themselves are commonly made from silicon, polysilicon, metal film, thick film, and bondedfoil.

Variations in temperature

Variations in temperature will cause a multitude of effects. The object will change in size by thermal expansion,which will be detected as a strain by the gauge. Resistance of the gauge will change, and resistance of theconnecting wires will change.


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Visualization of the working concept behind the strain gauge on a beam

under exaggerated bending.

Most strain gauges are made from a

constantan alloy.[3] Various constantanalloys and Karma alloys have been designedso that the temperature effects on theresistance of the strain gauge itself cancelout the resistance change of the gauge due tothe thermal expansion of the object undertest. Because different materials havedifferent amounts of thermal expansion,self-temperature compensation (STC)requires selecting a particular alloy matchedto the material of the object under test.

Strain gauges that are not self-temperature-compensated (such as isoelastic alloy) canbe temperature compensated by use of thedummy gauge technique. A dummy gauge(identical to the active strain gauge) isinstalled on an unstrained sample of thesame material as the test specimen. Thesample with the dummy gauge is placed inthermal contact with the test specimen,adjacent to the active gauge. The dummygauge is wired into a Wheatstone bridge onan adjacent arm to the active gauge so thatthe temperature effects on the active and

dummy gauges cancel each other.[4]

(Murphy's Law was originally coined inresponse to a set of gauges being incorrectly

wired into a Wheatstone bridge.[5])

Temperature effects on the lead wires can becancelled by using a "3-wire bridge" or a

"4-wire ohm circuit"[6] (also called a "4-wireKelvin connection").

In any case it is a good engineering practice to keep the Wheatstone bridge voltage drive low enough to avoidthe self heating of the strain gauge. The self heating of the strain gauge depends on its mechanical characteristic(large strain gauges are less prone to self heating). Low voltage drive levels of the bridge reduce the sensitivityof the overall system.

Zero Offset - If the impedance of the four gauge arms are not exactly the same after bonding the gauge to theforce collector, there will be a zero offset which can be compensated by introducing a parallel resistor to one ormore of the gauge arms.

Temperature coefficient of Gauge Factor (TCGF) - This is the change of sensitivity of the device to strain withchange in temperature. This is generally compensated for by the introduction of a fixed resistance in the input


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leg, whereby the effective supplied voltage will increase with temperature, compensating for the decrease insensitivity with temperature.

Zero Shift with temperature - If the TCGF of each gauge is not the same, there will be a zero shift withtemperature. This is also caused by anomalies in the force collector. This is usually compensated for with one ormore resistors strategically placed in the compensation network.

Linearity - This is an error whereby the sensitivity changes across the pressure range. This is commonly afunction of the force collection thickness selection for the intended pressure and/or the quality of the bonding.

Hysteresis - This is an error of return to zero after pressure excursion.

Repeatability - This error is sometimes tied-in with hysteresis but is across the pressure range.

EMI induced errors - As strain gauges output voltage is in the mV range, even μV if the Wheatstone bridgevoltage drive is kept low to avoid self heating of the element, special care must be taken in output signalamplification to avoid amplifying also the superimposed noise. A solution which is frequently adopted is to use"carrier frequency" amplifiers which convert the voltage variation into a frequency variation (as in VCOs) andhave a narrow bandwidth thus reducing out of band EMI.

Overloading - If a strain gauge is loaded beyond its design limit (measured in microstrain) its performancedegrades and can not be recovered. Normally good engineering practice suggests not to stress strain gaugesbeyond +/-3000 microstrain.

Humidity - If the wires connecting the strain gauge to the signal conditioner are not protected against humidity(bare wire) a parasitic resistance creates between the wires and the substrate to which the strain gauge is glued,or between the two wires themselves. This resistance introduces an error which is proportional to the resistanceof the strain gauge. For this reason low resistance strain gauges (120 ohm) are less prone to this type of error. Toavoid this error it is sufficient to protect the strain gauges wires with insulating enamel (e.g., epoxy orpolyurethanic type). Strain gauges with unprotected wires may be used only in a dry laboratory environment butnot in an industrial one.

In some applications, strain gauges add mass and damping to the vibration profiles of the hardware they areintended to measure. In the turbomachinery industry, one used alternative to strain gauge technology in themeasurement of vibrations on rotating hardware is the Non-Intrusive Stress Measurement System, which allowsmeasurement of blade vibrations without any blade or disc-mounted hardware.

For measurements of small strain, semiconductor strain gauges, so called piezoresistors, are often preferred overfoil gauges. A semiconductor gauge usually has a larger gauge factor than a foil gauge. Semiconductor gaugestend to be more expensive, more sensitive to temperature changes, and are more fragile than foil gauges.

In biological measurements, especially blood flow / tissue swelling, a variant called mercury-in-rubber straingauge is used. This kind of strain gauge consists of a small amount of liquid mercury enclosed in a small rubbertube, which is applied around e.g., a toe or leg. Swelling of the body part results in stretching of the tube, makingit both longer and thinner, which increases electrical resistance.

Fiber optic sensing can be employed to measure strain along an optical fiber. Measurements can be distributedalong the fiber, or taken at predetermined points on the fiber. The 2010 America's Cup boats Alinghi 5 andUSA-17 both employ embedded sensors of this type [1] (http://www.nytimes.com/2010/02/09/science


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Mechanical strain gauge used to

measure the growth of a crack in a

masonry foundation. This one is

installed on the Hudson-Athens

Lighthouse

/09sail.html).

Capacitive strain gauges use a variable capacitor to indicate the level of mechanical deformation.

Vibrating wire strain gauges are used in Geotechnical and Civil Engineering applications. The gauge consists of avibrating, tensioned wire. The strain is calculated by measuring the resonant frequency of the wire (an increasein tension increases the resonant frequency).

Simple mechanical types (such as illustrated to the left) are used in civilengineering to measure movement of buildings, foundations, and otherstructures. In the illustrated example, the two halves of the device arerigidly attached to the foundation wall on opposite sides of the crack.The red reference lines are on the transparent half and the grid is on theopaque white half. Both vertical and horizontal movement can bemonitored over time. In this picture, the crack can be seen to havewidened by approximately 0.3 mm (with no vertical movement) sincethe gauge was installed.

More sophisticated mechanical types incorporate dial indicators andmechanisms to compensate for temperature changes. These types can

measure movements as small as 0.002 mm.[7]

Resistance thermometer

^ Strain Gage: Materials (http://www.efunda.com/designstandards/sensors/strain_gages/strain_gage_selection_matl.cfm)

1.

^ Strain Gage: Sensitivity (http://www.efunda.com/designstandards/sensors/strain_gages/strain_gage_sensitivity.cfm)

2.

^ Constantan Alloy: Strain Gauge Selection (http://www.vishaypg.com/docs/11055/tn505.pdf)3.^ Shull, Larry C., "Basic Circuits", Hannah, R.L. and Reed, S.E. (Eds.) (1992).Strain Gage Users' Manual, p. 122.Society for Experimental Mechanics. ISBN 0-912053-36-4.

4.

^ Spark, N. (2006). A History of Murphy's Law. Periscope Film. ISBN 978-0-9786388-9-45.^ The Strain Gage (http://www.omega.com/literature/transactions/volume3/strain2.html)6.^ Mastrad Quality and Test Systems web site (http://www.mastrad.com/demecsg.htm)7.

British Society for Strain Measurement (http://www.bssm.org)Applying Finite Element Analysis Methods to Strain Gage Design (http://www.loadcelltheory.com/LoadCellSupportTheoryPDF/FEA_StrainGageDesign.pdf)An Introduction to Measurements using Strain Gages by Karl Hoffmann (http://www.hbm.com/fileadmin/mediapool/techarticles/hoffmannbook/Hoffmann-book_EN.pdf)Fiber optic strain gauges - NASA patented technology (http://www.4fos.com)


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