3-Signal conditioning


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Sensors & Actuators
Dr. Hassan SHARABATY
BY
Aleppo – February 2012
Department of Mechatronics Engineering
EEM304 Mechatronics
3- Sensor Signal Conditioning
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
2/18
Introduction
Typicallyasensorcannotbedirectlyconnectedtotheinstrumentsbecausethesignalmaybeincompatibleormaybetooweakand/ornoisy.Thesignalmustbeconditioned-i.e.,cleanedup,amplified,andputintoacompatibleformat.
Resistiveelementsaresomeofthemostcommonsensors.Theyareinexpensivetomanufactureandrelativelyeasytointerfacewithsignalconditioningcircuits.
Resistiveelementscanbemadesensitivetotemperature,strain,light,fluidflow,force,pressure,acceleration,…etc.
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
3/18
Introduction
Sensorelements’resistancescanrangefromlessthan100ΩtoseveralhundredkΩ,dependingonthesensordesignandthephysicalenvironmenttobemeasured
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
4/18
Introduction
-Onetechniqueformeasuringresistanceistoforceaconstantcurrentthroughtheresistivesensorandmeasurethevoltageoutput.
-Thisrequiresbothanaccuratecurrentsourceandanaccuratemeansofmeasuringthevoltage.
-Inaddition,thepowerdissipationintheresistivesensormustbesmall,inaccordancewiththemanufacturer’srecommandations,sothatself-heatingdoesnotproduceerrors,thereforethedrivecurrentmustbesmall.
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
5/18
Bridge Circuits
-WhenR1/R4=R2/R3,theresistancebridgeisatanull,regardlessofthemodeofexcitation(currentorvoltage,ACorDC),themagnitudeofexcitation,themodeofreadout(currentorvoltage),ortheimpedanceofthedetector.
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
6/18
Thevalueofthefixedbridgeresistor,R,ischosentobeequaltothenominalvalueofthevariableresistor(s).
Bridge Circuits
Thedeviationofthevariableresistor(s)aboutthenominalvalueisproportionaltothequantitybeingmeasured,suchasstrain(inthecaseofastraingage)ortemperature(inthecaseofanRTD).
Thesensitivityofabridgeistheratioofthemaximumexpectedchangeintheoutputvoltagetotheexcitationvoltage.Forinstance,ifVB=10V,andthefull-scalebridgeoutputis10mV,thenthesensitivityis1mV/V.
Single-elementvaryingbridge
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
7/18
SincethebridgeoutputisdirectlyproportionaltoVB,themeasurementaccuracycann’tbebetterthanthatoftheaccuracyoftheexcitationvoltage.
Bridge Circuits
Single-elementvaryingbridge
Output voltage
TherelationshipbetweenthebridgeoutputandΔRisnotlinear.
Note:Theabovenonlinearityreferstothenonlinearityofthebridgeitselfandnotthesensor.Inpractice,mostsensorsexhibitacertainamountoftheirownnonlinearitywhichmustbeaccountedforinthefinalmeasurement
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
8/18
Insomeapplications,thebridgenonlinearitymaybeacceptable,buttherearevariousmethodsavailabletolinearizebridges.
Sincethereisafixedrelationshipbetweenthebridgeresistancechangeanditsoutput(shownintheequations),softwarecanbeusedtoremovethelinearityerrorindigitalsystems.
Circuittechniquescanalsobeusedtolinearizethebridgeoutputdirectly,andthesewillbediscussedshortly.
Bridge Circuits
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
9/18
Bridge Circuits
Two-elementvaryingbridge
Possibility1:
both elements change in the same direction
Possibility2:
elements vary in opposite directions
This configuration is linear, and has twice the gainof the single-element configuration.
The nonlinearityis the same as that of the single-element varying bridge,
the gain is twice
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
10/18
Bridge Circuits
All-elementvaryingbridge
Theall-elementvaryingbridgeproducesthemostsignalforagivenresistancechangeandisinherentlylinear.Itisanindustry-standardconfigurationforloadcellswhichareconstructedfromfouridenticalstraingages.
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
11/18
Bridge Circuits
-Bridgesmayalsobedrivenfromconstantcurrentsources.
-Currentdrive,althoughnotaspopularasvoltagedrive,hasanadvantagewhenthebridgeislocatedremotelyfromthesourceofexcitationbecausethewiringresistancedoesnotintroduceerrorsinthemeasurement.
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
12/18
Driving Bridges
Wiringresistanceandnoisepickuparethebiggestproblemsassociatedwithremotelylocatedbridges.
Thetemperaturecoefficientofthecopperwireis0.385%/°C.
a total lead resistance increase of +0.404 Ω
(10.5Ω×0.00385/°C×10°C)ineachlead.
Thetotaladditionalleadresistance(ofthetwoleads)is+0.808Ω.
Error=23%fullscale
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
13/18
Driving Bridges
The three-wiremethod works well for remotely located resistive elements which make up one leg of a single-element varying bridge.
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
14/18
Driving Bridges
However,all-elementvaryingbridgesgenerallyarehousedinacompleteassembly,asinthecaseofaloadcell.Whenthesebridgesareremotelylocatedfromtheconditioningelectronics,specialtechniquesmustbeusedtomaintainaccuracy.
Dr. Hassan SHARABATY
Sensors& Actuators -Sensor Signal Conditioning
15/18
END

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