McGraw-Hill CreateTM ReviewCopyforInstructorNicolescu.Notfordistribution31IntroductiontoMechatronicsandMeasurementSystems,FourthEdition9HHAPTERSensorshis chapterdescribes various sensorsimportantin mechatronic systemdesign.MECHANICAL SYSTEMsystem modeldynamic responseINPUT SIGNALACTUATORSSENSORSCONDITIONINGsolenoids, voice coilsswitchesstrain gageANDINTERFACING-DC motorspotentiometerthermocouple- stepper motorsphotoelectricsaccelerometer- servo motors-discrete circuits-fltersMENSdigital encoder- hydraulics, pneuma-A/D, D/D-amplifiersOUTPUT SIGNALDIGITAL CONTROLGRAPHICALCONDITIONINGARCHITECTURESDISPLAYSANDINTERFACING- sequencing and timinglogic circuits- LEDs-LCDrocontroller- logic and arithmeric-DIA, D/D - power transistors.-SBC-CRT- digital displays- control algorithmsampifiespowerop amps-PLC- communication-PWMCHAPTER OBJECTIVESAfteryouread,discuss,study,andapplyideas inthis chapter,youwill1.Understandthefundamentals of simpleelectromechanical sensors,includingproximity sensors and switches,potentiometers,linear variable differentialtransformers,optical encoders,straingages,loadcells,thermocouples,andaccelerometers2.Beabletodescribehownatural andbinarycodesareusedtoencodelinearandrotationalpositionindigitalencoders3.Be able to apply engineering mechanics principles to interpret data from asinglestraingageorstraingagerosette375
Confirming Pages 375 C H A P T E R 9 Sensors T his chapter describes various sensors important in mechatronic system design. ■ INPUT SIGNAL CONDITIONING AND INTERFACING - discrete circuits - amplifiers - filters - A/D, D/D OUTPUT SIGNAL CONDITIONING AND INTERFACING - D/A, D/D - PWM - power transistors - power op amps GRAPHICAL DISPLAYS - LEDs - digital displays - LCD - CRT SENSORS switches potentiometer photoelectrics digital encoder strain gage thermocouple accelerometer MEMs ACTUATORS - solenoids, voice coils - DC motors - stepper motors - servo motors - hydraulics, pneumatics MECHANICAL SYSTEM DIGITAL CONTROL ARCHITECTURES - logic circuits - microcontroller - SBC - PLC - sequencing and timing - logic and arithmetic - control algorithms - communication - amplifiers system model dynamic response CHAPTER OBJECTIVES After you read, discuss, study, and apply ideas in this chapter, you will: 1. Understand the fundamentals of simple electromechanical sensors, including proximity sensors and switches, potentiometers, linear variable differential transformers, optical encoders, strain gages, load cells, thermocouples, and accelerometers 2. Be able to describe how natural and binary codes are used to encode linear and rotational position in digital encoders 3. Be able to apply engineering mechanics principles to interpret data from a single strain gage or strain gage rosette alc80237_ch09_375-430.indd 375 lc80237_ch09_375-430.indd 375 10/01/11 10:09 PM 0/01/11 10:09 PM Introduction to Mechatronics and Measurement Systems, Fourth Edition 31 McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution
McGraw-Hill CreateTM ReviewCopyforInstructorNicolescu.Notfordistribution.32MeasurementSystems376CHAPTER9Sensors4.Beabletomakeaccuratetemperaturemeasurementsusingthermocouples5.Know howtomeasure acceleration and understand thefrequency response ofaccelerometers6.Understand whata microelectromechanical (MEM)system is9.1INTRODUCTIONA sensor is an element in a mechatronic or measurement system that detects themagnitudeof aphysicalparameterandchanges it intoasignal thatcanbeprocessedby the system.Often the active element of a sensor is referred to as a transducer.Monitoring and control systems require sensors to measure physical quantities suchas position,distance,force,strain,temperature, vibration, and acceleration.Thefollowing sections present devices and techniques for measuring these and otherphysicalquantities.Sensor and transducer design always involves the application of some law orprincipleof physics orchemistry thatrelates thequantityof interestto somemeasur-ableevent.AppendixB summarizesmanyof thephysical lawsandprinciplesthathavepotentialapplicationinsensorandtransducerdesign.Someexamplesofapplications are also provided.This list is useful to a transducer designer who is searchingfor amethodto measure a physical quantity.Practically every transducer applies oneormoreof theseprinciples initsoperation.Internet LinkInternet Link 9.1 provides links to numerous vendors and online resources forawiderangeof commerciallyavailablesensorsandtransducers.TheInternetis9.1Sensoronlinea good resource for finding the latest products in the mechatronics field. This isresourcesandespecially true for sensors,where new technologies and improvements evolvevendorscontinuously.9.2POSITIONANDSPEED MEASUREMENTOther than electrical measurements (e.g.,voltage,current,resistance),themostcommonlymeasured quantityinmechatronic systemsisposition.Weoften need toknowwherevarious parts of a systemare in orderto control the system.Section 9.2.1presents proximity sensors and limit switches that area subset of position sen-sors that detect whether or not something is close or has reached a limit of travel.Section9.2.2presentsthepotentiometer,whichisaninexpensiveanalogdeviceformeasuring rotary or linear position.Section 9.2.3 presents the linear variable differ-ential transformer,which is an analog device capable of accurately measuring lineardisplacement.Finally,Section 9.2.4 presents the digital encoder,which isusefulformeasuring a position with an output in digital form suitable for direct interface to acomputerorotherdigital system.Because most applications involvemeasuring and controlling shaft rotation(e.g., in robot joints, numerically controlled lathe and mill axes,motors,and gen-erators),rotarypositionsensorsaremorecommonthanlinearsensors.Also,linear
Confirming Pages 376 C H A P T E R 9 Sensors 4. Be able to make accurate temperature measurements using thermocouples 5. Know how to measure acceleration and understand the frequency response of accelerometers 6. Understand what a microelectromechanical (MEM) system is 9.1 INTRODUCTION A sensor is an element in a mechatronic or measurement system that detects the magnitude of a physical parameter and changes it into a signal that can be processed by the system. Often the active element of a sensor is referred to as a transducer. Monitoring and control systems require sensors to measure physical quantities such as position, distance, force, strain, temperature, vibration, and acceleration. The following sections present devices and techniques for measuring these and other physical quantities. Sensor and transducer design always involves the application of some law or principle of physics or chemistry that relates the quantity of interest to some measurable event. Appendix B summarizes many of the physical laws and principles that have potential application in sensor and transducer design. Some examples of applications are also provided. This list is useful to a transducer designer who is searching for a method to measure a physical quantity. Practically every transducer applies one or more of these principles in its operation. Internet Link 9.1 provides links to numerous vendors and online resources for a wide range of commercially available sensors and transducers. The Internet is a good resource for finding the latest products in the mechatronics field. This is especially true for sensors, where new technologies and improvements evolve continuously. 9.2 POSITION AND SPEED MEASUREMENT Other than electrical measurements (e.g., voltage, current, resistance), the most commonly measured quantity in mechatronic systems is position. We often need to know where various parts of a system are in order to control the system. Section 9.2.1 presents proximity sensors and limit switches that are a subset of position sensors that detect whether or not something is close or has reached a limit of travel. Section 9.2.2 presents the potentiometer, which is an inexpensive analog device for measuring rotary or linear position. Section 9.2.3 presents the linear variable differential transformer, which is an analog device capable of accurately measuring linear displacement. Finally, Section 9.2.4 presents the digital encoder, which is useful for measuring a position with an output in digital form suitable for direct interface to a computer or other digital system. Because most applications involve measuring and controlling shaft rotation (e.g., in robot joints, numerically controlled lathe and mill axes, motors, and generators), rotary position sensors are more common than linear sensors. Also, linear Internet Link 9.1 Sensor online resources and vendors alc80237_ch09_375-430.indd 376 lc80237_ch09_375-430.indd 376 10/01/11 10:09 PM 0/01/11 10:09 PM 32 Measurement Systems McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution
McGraw-Hill CreateTM Review Copyfor Instructor Nicolescu.Not fordistributionIntroduction to Mechatronics and Measurement Systems,Fourth Edition333779.2PositionandSpeedMeasurementmotion can often be easily converted to rotary motion (e.g., with a belt, gear, orwheel mechanism),allowing the use of rotary position sensors in linear motionapplications.Speedmeasurements canbeobtainedbytakingconsecutivepositionmeasure-mentsatknowntimeintervals andcomputingthetimerateof changeof thepositionvalues.A tachometerisan exampleof a speed sensorthatdoes thisfora rotatingshaft.9.2.1ProximitySensorsandSwitchesAproximitysensorconsists ofanelementthatchangeseitherits stateorananalogsignal when it is close to, but often not actually touching, an object. Magnetic,electrical capacitance,inductance,and eddy current methods areparticularly suitedtothedesign ofaproximity sensor.VideoDemo9.1showsanexampleapplicationforamagneticproximity sensor.Aphotoemitter-detectorpairrepresents anotherapproach, where interruption or reflection of a beam of light is used to detect anobjectin anoncontact manner.The emitter can bea laser or focused LED,andVideo Demothe detector is usually a phototransistor orphotodiode.Various configurations for9.1Magneticphotoemitter-detector pairs are illustrated in Figure 9.1. In the opposed and ret-pickuptachom-roreflective configurations,the objectinterrupts the beam; and in the proximityeterused inaPIDconfiguration,the objectreflectsthebeam.Figure9.2showsa commercial sensorspeedcontrollerthat canbe used in theretroreflective orproximity configurations.Video Demo9.2test-standshows an interesting studentprojectexampleoftheproximityconfiguration.Com-9.2Automatedmon applicationsforproximitysensorsand limitswitchesaredetectingthepres-laboratoryratence of an object (e.g., a man in front of a public urinal), counting moving objectsexercisemachine(e.g.,passingbyon a conveyorbelt),and in limiting thetraverseof a mechanismwithinfrared(e.g., bydetecting the end of travel of a slider or joint).sensorandThere are many designs for limit switches, including pushbutton and leveredsteppermotormicroswitches.All switches are used to open or close connections withincircuitsAs illustrated inFigure9.3, switchesarecharacterized bythenumber of poles(P)Proximity (Diffuse) Mode AlignmentOpposedModeAlignmentRetroreflective Mode AlignmentModeAligr:MoveEminterective Mode AligDiffuse Mode AlignrMove Target Up-Down, Lef-RighRotate Up-Down, Leff-Rightciver Up-Down,Left-Right, and RotateFigure 9.1 Various configurations for photoemitter-detector pairs.(Courtesy of Banner Engineering,Minneapolis,MN)
Confirming Pages Figure 9.1 Various configurations for photoemitter-detector pairs. (Courtesy of Banner Engineering, Minneapolis, MN) 9.2 Position and Speed Measurement 377 motion can often be easily converted to rotary motion (e.g., with a belt, gear, or wheel mechanism), allowing the use of rotary position sensors in linear motion applications. Speed measurements can be obtained by taking consecutive position measurements at known time intervals and computing the time rate of change of the position values. A tachometer is an example of a speed sensor that does this for a rotating shaft. 9.2.1 Proximity Sensors and Switches A proximity sensor consists of an element that changes either its state or an analog signal when it is close to, but often not actually touching, an object. Magnetic, electrical capacitance, inductance, and eddy current methods are particularly suited to the design of a proximity sensor. Video Demo 9.1 shows an example application for a magnetic proximity sensor. A photoemitter-detector pair represents another approach, where interruption or reflection of a beam of light is used to detect an object in a noncontact manner. The emitter can be a laser or focused LED, and the detector is usually a phototransistor or photodiode. Various configurations for photoemitter-detector pairs are illustrated in Figure 9.1 . In the opposed and retroreflective configurations, the object interrupts the beam; and in the proximity configuration, the object reflects the beam. Figure 9.2 shows a commercial sensor that can be used in the retroreflective or proximity configurations. Video Demo 9.2 shows an interesting student project example of the proximity configuration. Common applications for proximity sensors and limit switches are detecting the presence of an object (e.g., a man in front of a public urinal), counting moving objects (e.g., passing by on a conveyor belt), and in limiting the traverse of a mechanism (e.g., by detecting the end of travel of a slider or joint). There are many designs for limit switches, including pushbutton and levered microswitches. All switches are used to open or close connections within circuits. As illustrated in Figure 9.3 , switches are characterized by the number of poles (P) Video Demo 9.1 Magnetic pickup tachometer used in a PID speed controller test-stand 9.2 Automated laboratory rat exercise machine with infrared sensor and stepper motor alc80237_ch09_375-430.indd 377 lc80237_ch09_375-430.indd 377 10/01/11 10:09 PM 0/01/11 10:09 PM Introduction to Mechatronics and Measurement Systems, Fourth Edition 33 McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution
McGraw-Hill CreateTM ReviewCopyforInstructorNicolescu.Notfordistribution.34MeasurementSystems378CHAPTER9SensorsFigure9.2Exampleofaphotoemitter-detectorpair in a single housing. (Courtesy of BannerEngineering,Minneapolis,MN)SPSTNO pushbuttonNCNOSPDTNC pushbuttonFigure9.3Switches.andthrows (T)and whether connectionsarenormallyopen (NO)ornormallyclosed (NC). A pole is a moving element in the switch that makes or breaks con-nections,and a throw isa contactpointfor a pole.The SPST switch is a single-pole(SP),single-throw(ST)devicethatopens or closes a singleconnection.The SPDTswitch changes thepolebetweentwo different throwpositions.There aremanyvari-VideoDemoations on thepole and throw configurations of switches,but theirfunction is easilyunderstoodfromthebasicterminology.Figure9.4andVideoDemo9.3showvari-9.3Switchesous types of switches with the appropriate designations.Video Demo 9.4 shows an9.4 Thermostatinteresting example ofa normally open mercury switch that is used to turn on or offwithbimetallican air conditioning or heating unit when a bimetallic strip coil (see Section 9.4.2)strip and mercuryrotatesacertain amount.switchWhen mechanical switches are opened or closed, they exhibit switch bou-nce,where many break-reconnect transitions occur before a new state is estab-lished.If a switch is connected toa digital circuit that requires a singletransition,the switch outputmustbedebounced using a circuit or softwareas described inSection 6.10.1
Confirming Pages Figure 9.2 Example of a photoemitter-detector pair in a single housing. (Courtesy of Banner Engineering, Minneapolis, MN) Figure 9.3 Switches. SPST NC SPDT NO pushbutton NC pushbutton NO 378 C H A P T E R 9 Sensors and throws (T) and whether connections are normally open (NO) or normally closed (NC). A pole is a moving element in the switch that makes or breaks connections, and a throw is a contact point for a pole. The SPST switch is a single-pole (SP), single-throw (ST) device that opens or closes a single connection. The SPDT switch changes the pole between two different throw positions. There are many variations on the pole and throw configurations of switches, but their function is easily understood from the basic terminology. Figure 9.4 and Video Demo 9.3 show various types of switches with the appropriate designations. Video Demo 9.4 shows an interesting example of a normally open mercury switch that is used to turn on or off an air conditioning or heating unit when a bimetallic strip coil (see Section 9.4.2 ) rotates a certain amount. When mechanical switches are opened or closed, they exhibit switch bounce, where many break-reconnect transitions occur before a new state is established. If a switch is connected to a digital circuit that requires a single transition, the switch output must be debounced using a circuit or software as described in Section 6.10.1. Video Demo 9.3 Switches 9.4 Thermostat with bimetallic strip and mercury switch alc80237_ch09_375-430.indd 378 lc80237_ch09_375-430.indd 378 10/01/11 10:09 PM 0/01/11 10:09 PM 34 Measurement Systems McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution
McGraw-Hill CreateTM Review Copyfor lnstructor Nicolescu.Not fordistribution35IntroductiontoMechatronicsandMeasurementSystems,FourthEdition3799.2PositionandSpeedMeasurementNOSPST SPSTtogglepushbuttonswitchswitchtNODPST(dual SPST)2NOpushbuttonSPSTswitchesresetswitchSPDTmicroswitchesFigure9.4Photographofvarioustypesofswitches.CLASS DISCUSSIONITEM 9.1Household Three-WaySwitchAhouseholdthree-wayswitch isusedtoallowlightstobe controlledfromtwoloca-tions (e.g., at the top and bottom of a stairwell). Note that the term three-way refersto the number of terminals (3)on each switchand not the number of switches (2).Athree-way switchistheSPDTvariety.Drawa schematicof howACpowerandthe two switches can be wired to a light fixture to achieve the desired functionality,whereeither switchcan beused toturn the light on oroff9.2.2PotentiometerThe rotarypotentiometer (akapot)is a variableresistance device that canbeusedto measure angularposition.It consists of awiperthatmakes contactwitha resis-tive element,and as this point of contact moves, theresistance between the wiperand endleadsofthedevicechanges inproportiontothe angulardisplacement.Figure 9.5 illustrates the form and internal schematic for a typical rotary potenti-ometer.Figure 9.6 shows two common typesof potentiometers.The one on the leftis called a trim pot.It has a small screw on the left side that can be turned with aMMUUOwiperFigure9.5Potentiometer
Confirming Pages Figure 9.4 Photograph of various types of switches. NO SPST reset switch SPST toggle switch NO DPST (dual SPST) pushbutton switches SPDT microswitches NO SPST pushbutton switch 9.2 Position and Speed Measurement 379 ■ C L A S S D I S C U S S I O N I T E M 9 . 1 Household Three-Way Switch A household three-way switch is used to allow lights to be controlled from two locations (e.g., at the top and bottom of a stairwell). Note that the term three-way refers to the number of terminals (3) on each switch and not the number of switches (2). A three-way switch is the SPDT variety. Draw a schematic of how AC power and the two switches can be wired to a light fixture to achieve the desired functionality, where either switch can be used to turn the light on or off. 9.2.2 Potentiometer The rotary potentiometer (aka pot ) is a variable resistance device that can be used to measure angular position. It consists of a wiper that makes contact with a resistive element, and as this point of contact moves, the resistance between the wiper and end leads of the device changes in proportion to the angular displacement. Figure 9.5 illustrates the form and internal schematic for a typical rotary potentiometer. Figure 9.6 shows two common types of potentiometers. The one on the left is called a trim pot. It has a small screw on the left side that can be turned with a Figure 9.5 Potentiometer. wiper alc80237_ch09_375-430.indd 379 lc80237_ch09_375-430.indd 379 10/01/11 10:09 PM 0/01/11 10:09 PM Introduction to Mechatronics and Measurement Systems, Fourth Edition 35 McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution