By: Pinank ShahDate : 03/22/2006nWhat is Strain?n What is Strain Gauge?n Operation of Strain Gaugen Grid Patternsn Strain Gauge Installationn Wheatstone bridgen Instrumentation Amplifiern Embedded system and Strain Gaugen Strain Measurement Systemn Applications of a Strain Gauge nStrain 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.nnStrain can be positive (tensile) or negative (compressive). Although dimensionless, strain is sometimes expressed in units such as in./in. or mm/mm.nIn practice, the magnitude of measured strain is very small. Therefore, strain is often expressed as microstrain (me), which is e x 10-6. n Strain Gauge is a device used to measure deformation (strain) of an object. n Strain gauges have been developed for the accurate measurement of strainn Fundamentally, all strain gauges are designed to convert mechanical motion into an electronic signal. !n The gauge shown here is primarily sensitive to strain in the X direction, as the majority of the wire length is parallel to the X axis.Solder Tags -for attachment of wires.Insulated backingGauge, wire / foil approx. 0.025 mm thickXYn The name "bonded gauge" is given to strain gauges that are glued to a larger structure under stress (called the test specimen)."nGage length is an important consideration in strain gage selection nThe gage length is the dimension of the active grid as measured inside the grid end loops. nThe gage length (GGG ) ranges from 0.008 in (0.2 mm) to 4 in (100 mm).nThis schematic shows how the strain gauge resistance varies with strain (deformation).nOn applying a force a change in resistance takes place.nTension causes resistance increase.nCompression causes resistance decrease. ΩΩ+Ω-#(a) (b) (c) (d)nUniaxial Gage with a single grid for measuring strain in the grid direction .nBiaxial Rosettes Gage with two perpendicular gridsused to determine principal strains when their directions are known.nThree-Element Rosettes Gage with three independent grids in three directions for ascertaining the principal strains and their directions.nShear Patterns Gage having two chevron grids used in half-bridge circuits for direct indication of shear strains (difference in normal strains) .$$n The Strain Gauge is bonded to the specimen under test, only after the following:q cleaning the surface using a degreaserq cleaning it again with a conditioner solution (mild acid that accelerates the cleaning process)q neutralizing by applying a base (neutralizes any chemical reaction introduced by the Conditioner)q finally bonding it with a super glue.n The Strain Gauge has 2 leads which exhibit variation in resistance when strain is applied.%## $$nThe metallic strain gauge consists of a very fine wire or metallic foil arranged in a grid pattern. nThe grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction.nThe grid is bonded to a thin backing, called the carrier, which is attached directly to the test specimen. nThe strain experienced by the test specimen is transferred directly to the strain gauge, which responds with a linear change in electrical resistance. nGauge factor is defined as:&'nIn order to measure strain with a bonded resistance strain gauge, it must be connected to an electric circuit that is capable of measuring the minute changes in resistance corresponding to strainnStrain gauge is connected in a Wheatstone bridge circuit nA strain gauge bridge circuit indicates measured strain by the degree of imbalance nIt provides an accurate measurement of that imbalance (#nIn Figure, if R1, R2, R3, and Strain gauge 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. nHowever, if R4 is changed to some value which does not equal R1, R2, and R3, the bridge will become unbalanced and a voltage will exist at the output terminals. nThe variable strain sensor has resistance Rg, while the other arms are fixed value resistors. (#n The sensor, however, can occupy one, two, or four arms of the bridge, depending on the application. n The total strain, or output voltage of the circuit (Vout) is equivalent to the difference between the voltage drop across R1 and R4, or Rg. n It is given by Vout= Vcd– Vcb (# nThe bridge is considered balanced when R1/R2 = Rg/R3 and, therefore, VOUTequals zero. nAny small change in the resistance of the sensing grid will throw the bridge out of balance, making it suitable for the detection of strain. nA small change in Rg will result in an output voltage from the bridge. nIf the gage factor is GF, the strain measurement is related to the change in Rg as follows:%$ ) ""$nThe output of a strain gauge circuit is a very low-level voltage signalnThe low level of the signal makes it particularly susceptible tounwanted noise from other electrical devices.nCapacitive coupling caused by the lead wires' running too close to AC power cables or ground currents are potential error sources in
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