amem 314 labs strain gauges

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Experiment 6:Strain Measurements (Strain Gauges)1. OBJECTIVEAssess the performance of strain gauges for measurement of linear displacements. Oncompletion of this experiment you will: Appreciate the positioning of strain gauges within a system affected by displacement. Understandtheuseof straingauges inapotentialdivider, quarter, half andfull bridgeconfigurations and the relative sensitivities in each case. Assess the sources of error in using strain gauges to measure displacement.2. IT!O"#CTIOStrain gauges are devices designed and constructed so that their resistance changes when theyare strained:thatis theirphysicaldimensionsincreaseor decrease.hisisusually arranged tohappenwhenthebodytowhichtheyarebonded!stuc"# changesandsothestraingaugesresistance may be used to measure the amount of strain the body is experiencing.o maximise this effect there are two main considerations to ta"e into account when using straingauges. he first is to design strain gauges so that their resistance changes appreciably with strainandsecondlythat theyareattachedtoasystemsuchthat theyareaffectedbystrain. Otherconsiderations are made to minimise any changes in resistance caused by any effect other thanstrain: the main one is temperature.$nthefollowingsequenceof experiments, straingaugesincon%unctionwithfixedresistorsindifferent configurations formpotentiometric and bridge circuits.$n using and predicting theperformanceof thesecircuitsit mayhelptoconsider themasformingseries&parallelresistivecircuits and apply the universalelectricalcircuit laws that you willhave encountered in earlierlessons.$nthe S$S'ardware (odulethefour straingauges, nominal resistanceof )*+,, mounton aflexiblebeam.hisprovidesanexaggeratedamount of movement at itsfreeendwhenthe-inear Assembly is moved but relatively little at the clamped end at which the straingauges areaffixed. wo strain gauges mount on each side of the beam, such that when the beam deflects totheleft, twoof thegaugesareintension!increaseintheir resistance#andtheothertwoincompression !decrease in their resistance#. .eflecting the beam to the right reverses this effect.$. %!OCE"#!E%art &: T'e Strain Gauge %(tentia) "i*i+erhis circuit is a simple potential divider with a strain gauge !tension#, Rsg, connected in series witha fixed matchingresistor,R,andapower supplyconnected across both,Vin.his is shownschematically in /igure ).)./igure 1.1:Strain gauge potential dividerhe equation relating the output voltage to the other circuit parameters is:$n other words, the output is governed by the ratio of the strain gauge resistance,Rsg,to the totalresistance, Rsg + R.heob%ect of thisexperiment istodeterminetheperformanceof thestraingaugepotentialdivider. Use the patching leads supplied to connect the equipment as shown in /igure ).*. 0onfirmthat this is the same circuit as that shown in /igure ).).(ove the -inear Assembly to the right by rotating the rotary scale cloc"wise until it reaches theend stop. 0arefully ad%ust the dial until the 1ero aligns with the edge of the moulding. Observe theinitial value of meter reading and record it here.(eter reading 23345asthisvalueexpected6Usethepotential dividerequationgivenpreviouslytosupport youranswer. he equation predicts that a change in the resistance of the strain gauge should producea change in the output voltage. (anually rotate the rotary scale anticloc"wise to move the -inearAssembly over the whole range of movement and observe the change in the indicated voltage.7eturn the -inear Assembly to the start position once you have completed this exercise.8ou should see that the indicated meter reading changes very little, if at all. 5hy could this be ifwe"now that the strain gauge resistance has to have changed with the amount ofdisplacement caused6 he answer is that the change in resistance is very small and the voltagechange resulting is less than the resolution of the meter !+.+) 4#. o increase the sensitivity ofthe measurement system we need to amplify the output signal.Figure 1.2:Potential divider connection diagram(odify the circuit to include the .ifferential Amplifier between the output and the meter, as shown in /igure ).9 and represented schematically in /igure ).:Figure 1.3:Connection diagram of potential divider it! differential amplifierFigure 1.": Strain gauge potential divider it! differential amplifierSet thegainsof theamplifier,")and"*, tomaximumandad%ust thesettingof 7ef)toma"e the indicated meter value as small as possible !less than ; 4 to be within the range of the

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