By: Piak Shah Date : 03/22/2006
What is Strai? What is Strai Gauge? Operatio of Strai Gauge Grid Patters Strai Gauge Istallatio Wheatstoe bridge Istrumetatio Amplifier Embedded system ad Strai Gauge Strai Measuremet System Applicatios of a Strai Gauge
Strai is the amout of deformatio of a body due to a applied force. More specifically, strai (e) is defied as the fractioal chage i legth. Strai ca be positive (tesile) or egative (compressive). Although dimesioless, strai is sometimes expressed i uits such as i./i. or mm/mm. I practice, the magitude of measured strai is very small. Therefore, strai is ofte expressed as microstrai (me), which is e x 10-6.
Strai Gauge is a device used to measure deformatio (strai) of a object. Strai gauges have bee developed for the accurate measuremet of strai Fudametally, all strai gauges are desiged to covert mechaical motio ito a electroic sigal.
! Isulated backig Solder Tags - for attachmet of wires. Gauge, wire / foil approx. 0.025 mm thick Y X The gauge show here is primarily sesitive to strai i the X directio, as the majority of the wire legth is parallel to the X axis.
The ame "boded gauge" is give to strai gauges that are glued to a larger structure uder stress (called the test specime).
" Gage legth is a importat cosideratio i strai gage selectio The gage legth is the dimesio of the active grid as measured iside the grid ed loops. The gage legth (GGG ) rages from 0.008 i (0.2 mm) to 4 i (100 mm).
Ω Ω + - Ω This schematic shows how the strai gauge resistace varies with strai (deformatio). O applyig a force a chage i resistace takes place. Tesio causes resistace icrease. Compressio causes resistace decrease.
# (a) (b) (c) (d) Uiaxial Gage with a sigle grid for measurig strai i the grid directio. Biaxial Rosettes Gage with two perpedicular grids used to determie pricipal strais whe their directios are kow. Three-Elemet Rosettes Gage with three idepedet grids i three directios for ascertaiig the pricipal strais ad their directios. Shear Patters Gage havig two chevro grids used i half-bridge circuits for direct idicatio of shear strais (differece i ormal strais).
$ The Strai Gauge is boded to the specime uder test, oly after the followig: q q q q cleaig the surface usig a degreaser cleaig it agai with a coditioer solutio (mild acid that accelerates the cleaig process) eutralizig by applyig a base (eutralizes ay chemical reactio itroduced by the Coditioer) fially bodig it with a super glue. The Strai Gauge has 2 leads which exhibit variatio i resistace whe strai is applied.
%## $ The metallic strai gauge cosists of a very fie wire or metallic foil arraged i a grid patter. The grid patter maximizes the amout of metallic wire or foil subject to strai i the parallel directio. The grid is boded to a thi backig, called the carrier, which is attached directly to the test specime. The strai experieced by the test specime is trasferred directly to the strai gauge, which respods with a liear chage i electrical resistace. Gauge factor is defied as:
&' I order to measure strai with a boded resistace strai gauge, it must be coected to a electric circuit that is capable of measurig the miute chages i resistace correspodig to strai Strai gauge is coected i a Wheatstoe bridge circuit A strai gauge bridge circuit idicates measured strai by the degree of imbalace It provides a accurate measuremet of that imbalace
(# I Figure, if R1, R2, R3, ad Strai gauge are equal, ad a voltage, VIN, is applied betwee poits A ad C, the the output betwee poits B ad D will show o potetial differece. However, if R4 is chaged to some value which does ot equal R1, R2, ad R3, the bridge will become ubalaced ad a voltage will exist at the output termials. The variable strai sesor has resistace Rg, while the other arms are fixed value resistors.
(# The sesor, however, ca occupy oe, two, or four arms of the bridge, depedig o the applicatio. The total strai, or output voltage of the circuit (V out ) is equivalet to the differece betwee the voltage drop across R1 ad R4, or Rg. It is give by V out = V cd V cb
(# The bridge is cosidered balaced whe R1/R2 = Rg/R3 ad, therefore, V OUT equals zero. Ay small chage i the resistace of the sesig grid will throw the bridge out of balace, makig it suitable for the detectio of strai. A small chage i Rg will result i a output voltage from the bridge. If the gage factor is GF, the strai measuremet is related to the chage i Rg as follows:
%$ )""$ The output of a strai gauge circuit is a very low-level voltage sigal The low level of the sigal makes it particularly susceptible to uwated oise from other electrical devices. Capacitive couplig caused by the lead wires' ruig too close to AC power cables or groud currets are potetial error sources i strai measuremet. Other error sources may iclude magetically iduced voltages whe the lead wires pass through variable magetic fields, parasitic (uwated) cotact resistaces of lead wires, isulatio failure, ad thermocouple effects at the juctio of dissimilar metals. The sum of such iterfereces ca result i sigificat sigal degradatio.
$ Shieldig: Most electric iterferece ad oise problems ca be solved by shieldig. A shield aroud the measuremet lead wires will itercept iterfereces ad may also reduce ay errors caused by isulatio degradatio. Shieldig also will guard the measuremet from capacitive couplig. If the measuremet leads are routed ear electromagetic iterferece sources such as trasformers, twistig the leads will miimize sigal degradatio due to magetic iductio. By twistig the wire, the flux-iduced curret is iverted ad the areas that the flux crosses cacel out. For idustrial process applicatios, twisted ad shielded lead wires are used almost without exceptio.
$ The variatio i voltage at the output of the bridge is i the rage of millivolts. It eeds to be amplified i order to calculate precise value of strai.
* $ Differetial iputs helps i accurate voltage trasfer High CMRR. Low offset voltage: 50uv max. Variable Gai. The gai of the istrumetatio amplifier is give by Av = 1 + (2 * R2) / Rg where R2 is 10 K ohms fixed resistor ad Rg is the gai select resistor Three 741 Op-amps are used to build the circuit for istrumetatio amplifier.
$ & $ $ The output of the istrumetatio amplifier is coected to the M16C/62P microcotroller. The ADC, coverts the o/p ito digital value ad the voltage read i is displayed o the LCD display available o M16CSKP board. A_D coverter iput port o. 10_3 is used The A_D coverter is cofigured to read i the aalog value after every 1 secod ad is set to covert at a resolutio of 10 bits for better precisio. #
& Cosists Of: Test Specime-piece of metal Strai gauge-placemet of the Strai Gauge o the specime is very crucial for precise measuremet of the strai Wheatstoe bridge Istrumetatio Amplifier Microcotroller M16C/62P +
, The software performs the followig calculatio o the digital value:- q The value read i is coverted ito digital ad is available i oe of the registers of the microcotroller. The ADC resolutio is selected as 10 bit. The step size thus becomes 48.8 mv. q Multiply it by 48.8 mv to get the actual aalog voltage q Divide the aalog voltage by 24(gai of the amplifier). q Vo / Vex = (Gauge Factor * E )/2 where E is the strai i micro strai. q E = (Vo *2 / Gauge Factor * Vex ) q = Vo / ((1.03)*(4V)) [Gauge Factor = 2.06] [Vex = 4volts] q = Vo / 4.12 [Vo is the output of the Bridge] q Divide the value by 4.120 q This gives the value of the strai. q Value of the strai displayed o the LCD is refreshed every 1 secod.
# Results: Uder o strai coditio: Vout = 2.20volts (output of the istrumetatio amplifier) E = 0.91 micro strai. Bed the test specime both eds dowwards (Elogatio effect o the Gauge) Vout = 2.28volts (output of the istrumetatio amplifier) E = 0.94 micro strai. Bed the test specime both eds upwards (Compressio effect o the Gauge) Vout = 2.16volts (output of the istrumetatio amplifier) E = 0.89 micro strai.
*& + This system is very compatible ad is cost effective. The microcotroller used has may other features like UART iterface which ca be utilized to trasfer the strai readigs to a PC. Upcomig Project: - A multi strai measuremet system (SMS) - Makig SMS wireless
($- + & +
$"-#&#$ Ultra low power trasmissio 10 mw trasmissio power High ati-iterferece ad low BER (Bit error Rate) Log trasmissio distace Multi-chael-the stadard radio module cofiguratio provides 8 chaels Low power cosumptio ad sleepig fuctio -receivig, curret is <30mA, trasmittig curret is <40mA, ad sleep curret is <20uA. High reliability, small ad light
+& + The advaced strai measuremet system accumulates the data from all the widespread gauges, processes the collected data, eables wireless trasmissio of collected iformatio to the remote Data Acquisitio System. With the combiatio of low power microprocessors, flexible software operatig modes this system is optimized for very low power operatio, while permittig high speed data loggig ad wireless commuicatio capabilities.
$ I load cells for weighbridges, scales, vehicles ad i medical ad educatioal applicatios. For moitorig structures such as bridges ad buildigs. I research ad developmet applicatios, icludig automotive, aerospace, medical, process, oil ad gas, ad power geeratio. Virtually every other sector of idustry.
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