Basics of Measurement Systems and Classification

Introduction to Measurement Systems

1. AAE 5060: Data Acquisition, Tracking &
Post Flight Analysis
Lecture 1: Introduction to Measurement
Systems
Dr. S. Meenatchisundaram
Email: [email protected]

2. Introduction
•Measurement:
•Processofgatheringinformationfromaphysicalworld
andcomparingthisinformationwithknownstandard
(References)
•Purpose:
•istolinktheobservertotheprocess,asshowninFigure

3. Introduction
•Wecanrefertotheinformationvariableasameasured
variable.
•Theinputtothemeasurementsystemisthetruevalue
ofthevariable;thesystemoutputisthemeasured
valueofthevariable.
•Theaccuracyofthesystemcanbedefinedasthe
closenessofthemeasuredvaluetothetruevalue.
•Aperfectlyaccuratesystemisatheoreticalidealand
theaccuracyofarealsystemisquantifiedusing
measurementsystemerrorE,
E = measured value − true value
E = system output − system input

4. Common Physical Variables
Common Physical variable Typical signal variable
Force Voltage
Length Displacement
Temperature Current
Acceleration Force
Velocity Pressure
Pressure Light
Frequency Frequency
Capacity
Resistance
Time
Common Physical variable Typical signal variable

5. Classification of Measurement
•DirectMethod
•Inthismethod,thevalueofaquantityisobtained
directlybycomparingtheunknownwiththe
standard.
•Directmethodsarecommonforthemeasurementof
physicalquantitiessuchaslength,massandtime.
•Itinvolvesnomathematicalcalculationstoarriveat
theresults,forexample,measurementoflengthbya
graduatedscale.
•Themethodisnotveryaccuratebecauseitdependson
humaninsensitivenessinmakingjudgment.

6. Classification of Measurement
•IndirectMethod
•Inthismethodseveralparameters(towhichthe
quantitytobemeasuredislinkedwith)aremeasured
directlyandthenthevalueisdeterminedby
mathematicalrelationship.
•Forexample,measurementofdensitybymeasuring
massandgeometricaldimensions.

7. Elements of a measurement
system

8. Elements of a measurement
system
•PrimarySensingElement(PSE):Theseelementsarein
directcontactwithquantityundermeasurement.Itisused
tosensethequantitytobemeasured.
•e.g.transducerandothersensingelement.
•DataConversionElement(DCE):Thiselementconverts
oneformofthedatatoanotherform,butthebasic
informationcarriedoverbythedataispreserved.
•e.g.voltagetofrequencyconverter,voltageto
currentconverter,ADC,DAC.

9. Elements of a measurement
system
•DataManipulationElement(DME):Thiselementchanges
thelevelofsignalpreservingitsbasicnature.
•e.g.amplification,modulation,attenuationetc.
•DataTransmissionElement(DTE):Thiselementprovides
transmissionchannel.
•e.g.opticalfibres,coaxialcables,transmissionlines
etc.
•DataPresentationElements(DPE):Thiselementisused
eithertostoreortodisplaythesignalreceiver.

10. References:
•JohnP.Bentley,“PrinciplesofMeasurementSystems”,Fourth
Edition,PearsonEducationLimited,2005.
•ErnestO.Doebelin,“MeasurementSystems:Applicationand
Design”,FourthEdition,McGraw