《工程测试与信号处理》课程教学资源（文献资料）FORCE MEASUREMENTS

52512.2SensorsThe angular velocity can be found either from the amplitude or the frequency of the output signal.Thevoltageamplitudesignal is susceptibletonoiseand loadingerrors.Thus,lesserroris introduced ifthefrequencyis usedtodetermine theangularvelocity:typically,somemeansof counting thepulseselectronically is employed. This frequency information can be transmitted digitally for recording,which eliminates the noise and loading error problems associated with voltage signals.Force MeasurementThe measurement of force is most familiar as the process of weighing, ranging from weighingmicrograms of a medicine to weighing trucks on the highway.Force is a quantity derived fromthe fundamental dimensions of mass, length, and time. Standards and units of measure for thesequantitiesare defined in Chapter 1.The mostcommon techniques for force measurement aredescribed in this section.Load Cells"Load cell" is a term used to describe a transducer that generates a voltage signal as a result of anappliedforce,usually along aparticular direction.Suchforcetransducers often consistof an elasticmember and a deflection sensor. These deflection sensors may employ changes in capacitance,resistance, or the piezoelectric effect to sense deflection. Atechnology overview for such devices isprovided elsewhere (5).Consider first load cells that are designed using a linearly elastic memberinstrumented with strain gauges.Strain Gauge Load CellsStrain gauge load cells are most often constructed of a metal, and have ashape such that the range of forces to be measured results in a measurable output voltage overthedesired operating range.The shape of the linearly elastic member is designed to meet the followinggoals: (1) provide an appropriate range of force-measuring capability with necessary accuracy, (2)provide sensitivity to forces in a particular direction, and (3) have low sensitivity to forcecomponents in other directions.A variety of designs of linearly elastic load cells are shown in Figure 12.20. In general, loadcells may be characterized as beam-type load cells,proving rings,or columnar-type designs. Beam-type load cells may be characterized as bending beam load cells or shear beam load cells.Abending beamload cell,as shown inFigure12.21,isconfigured such that the sensingelementof the load cellfunctions as a cantileverbeam. Strain gauges are mounted on the top and bottom ofthebeam tomeasurenormal orbending stresses.Figure 12.21 providesqualitative indication of theshear and normal stress distributions in a cantilever beam. In the linear elastic range of theload cell,the bending stresses are linearly related to the applied load.In a shear beam load cell the beam cross section is that of an I-beam.The resulting shear stress inthe web is nearly constant, allowing placement of a strain gauge essentially anywhere in the web withreasonable accuracy. Such a load cell is illustrated schematically in Figure 12.22, along with the shearstress distribution in the beam.In general, bending beam load cells are less costly due to theirconstruction; however, the shear beam load cells have several advantages, including lower creep andfaster response times. Typical load cells for industrial applications are illustrated in Figure 12.23.Piezoelectric Load CellsPiezoelectric materials are characterized by their ability to develop a chargewhen subject toa mechanical strain.The mostcommon piezoelectric material is single-crystal quartz.The

E1C12 09/14/2010 13:54:11 Page 525 The angular velocity can be found either from the amplitude or the frequency of the output signal. The voltage amplitude signal is susceptible to noise and loading errors. Thus, less error is introduced if the frequency is used to determine the angular velocity; typically, some means of counting the pulses electronically is employed. This frequency information can be transmitted digitally for recording, which eliminates the noise and loading error problems associated with voltage signals. Force Measurement The measurement of force is most familiar as the process of weighing, ranging from weighing micrograms of a medicine to weighing trucks on the highway. Force is a quantity derived from the fundamental dimensions of mass, length, and time. Standards and units of measure for these quantities are defined in Chapter 1. The most common techniques for force measurement are described in this section. Load Cells ‘‘Load cell’’ is a term used to describe a transducer that generates a voltage signal as a result of an applied force, usually along a particular direction. Such force transducers often consist of an elastic member and a deflection sensor. These deflection sensors may employ changes in capacitance, resistance, or the piezoelectric effect to sense deflection. A technology overview for such devices is provided elsewhere (5). Consider first load cells that are designed using a linearly elastic member instrumented with strain gauges. Strain Gauge Load Cells Strain gauge load cells are most often constructed of a metal, and have a shape such that the range of forces to be measured results in a measurable output voltage over the desired operating range. The shape of the linearly elastic member is designed to meet the following goals: (1) provide an appropriate range of force-measuring capability with necessary accuracy, (2) provide sensitivity to forces in a particular direction, and (3) have low sensitivity to force components in other directions. A variety of designs of linearly elastic load cells are shown in Figure 12.20. In general, load cells may be characterized as beam-type load cells, proving rings, or columnar-type designs. Beam￾type load cells may be characterized as bending beam load cells or shear beam load cells. A bending beam load cell, as shown in Figure 12.21, is configured such that the sensing element of the load cell functions as a cantilever beam. Strain gauges are mounted on the top and bottom of the beam to measure normal or bending stresses. Figure 12.21 provides qualitative indication of the shear and normal stress distributions in a cantilever beam. In the linear elastic range of the load cell, the bending stresses are linearly related to the applied load. In a shear beam load cell the beam cross section is that of an I-beam. The resulting shear stress in the web is nearly constant, allowing placement of a strain gauge essentially anywhere in the web with reasonable accuracy. Such a load cell is illustrated schematically in Figure 12.22, along with the shear stress distribution in the beam. In general, bending beam load cells are less costly due to their construction; however, the shear beam load cells have several advantages, including lower creep and faster response times. Typical load cells for industrial applications are illustrated in Figure 12.23. Piezoelectric Load Cells Piezoelectric materials are characterized by their ability to develop a charge when subjectto a mechanical strain. The most common piezoelectric materialis single-crystal quartz. The 12.2 Sensors 525

526Chapter12Mechatronics:Sensors,Actuators,andControlsColumnColumn withHollow columnstress concentrationFigure12.20Elastic load cellFlexuredesigns.Normal stress distributionAppliedloadStrain/gaugelocations Normal stressShear stress distributionBending beam load cellShear stressFigure12.21Bending beamload cell and stress distributions

E1C12 09/14/2010 13:54:11 Page 526 Column Column with stress concentration Hollow column Flexure Figure 12.20 Elastic load cell designs. y y Normal stress distribution Normal stress Shear stress distribution Applied load Strain gauge locations Bending beam load cell Shear stress Figure 12.21 Bending beam load cell and stress distributions. 526 Chapter 12 Mechatronics: Sensors, Actuators, and Controls

52712.2SensorsAppliedloadShearstress inI-beam正7Center line of webStraingauge locationsShear stressFigure12.22 Shear beamload cell and shear stress distributionFigure12.23Typicalloadcells.(CourtesyofTransducerTechniques, Inc.)basic principle of transduction that occurs in a piezoelectric element may best be thought of as a chargegenerator and a capacitor. The frequency response of piezoelectric transducers is very high, since thefrequency response is determined primarily by the size and material properties of the quartz crystal. Themodulus ofelasticity ofquartz is approximately 85 GPa,yieldingload cells withtypical static sensitivitiesrangingfrom0.05to10mV/N,and a frequencyresponse upto15,000Hz.Atypical piezoelectricload cellconstructionisshowninFigure12.24.Proving RingA ring-type load cell can be employed as a local force standard. Such a ring-typeload cell, as shown in Figure 12.25,is often employed in the calibration of materials testingmachines because of the high degree of precision and accuracy possible with this arrangement of

E1C12 09/14/2010 13:54:11 Page 527 basic principle of transduction that occurs in a piezoelectric element may best be thought of as a charge generator and a capacitor. The frequency response of piezoelectric transducers is very high, since the frequency response is determined primarily by the size and material properties of the quartz crystal. The modulus of elasticity of quartz is approximately 85 GPa, yielding load cells with typical static sensitivities ranging from 0.05to 10 mV/N, and a frequency response upto 15,000 Hz. Atypical piezoelectricload cell construction is shown in Figure 12.24. Proving Ring A ring-type load cell can be employed as a local force standard. Such a ring-type load cell, as shown in Figure 12.25, is often employed in the calibration of materials testing machines because of the high degree of precision and accuracy possible with this arrangement of Applied load Strain gauge locations Shear stress Shear stress in I-beam y Center line of web Figure 12.22 Shear beam load cell and shear stress distribution. Figure 12.23 Typical load cells. (Courtesy of Transducer Techniques, Inc.) 12.2 Sensors 527

528Chapter12Mechatronics:Sensors,Actuators,andControlsLoad-bearingsurfaceQuartz crystalsCharge pickupCablemountImpedanceFigure12.24Piezoelectricconverterload cell design.(Courtesy ofMounting threadMounting surfacethe Kistler Instrument Co.)transducer and sensor. If the sensor is approximated as a circular right cylinder, the relationshipbetweenappliedforceanddeflectionisgivenbyFnD3(12.17)Oy16EI1AppliedloadStrainStraingaugesgaugesDisplacementMethodMethod 2Method 2sensor1AppliedloadFigure12.25Ringtype load cell, or provingring

E1C12 09/14/2010 13:54:11 Page 528 transducer and sensor. If the sensor is approximated as a circular right cylinder, the relationship between applied force and deflection is given by dy ¼ p 2 4 p
FnD3 16EI ð12:17Þ Load-bearing surface Quartz crystals Charge pickup Cable mount Impedance converter Mounting thread Mounting surface Figure 12.24 Piezoelectric load cell design. (Courtesy of the Kistler Instrument Co.) Strain gauges Strain gauges 2 dohteM Method 2 dohteM 1 Applied load Applied load Displacement sensor Figure 12.25 Ring type load cell, or proving ring. 528 Chapter 12 Mechatronics: Sensors, Actuators, and Controls

52912.2SensorswhereOy=deflection along the applied forceFn=applied forceD=diameterE= modulus of elasticityI=moment of inertiaThe application of the proving ring involves measuring the deflection of the proving ring inthe direction of the applied force.Typical methods for this displacement measurement includedisplacementtransducers,whichmeasureoveralldisplacement,andstraingauges.Thesemethodsareillustrated inFigure12.25TorqueMeasurementsTorque and mechanical power measurements are often associated with the energy conversionprocesses that serve to provide mechanical and electrical power to our industrial world.Suchenergy conversion processes are largely characterized by the mechanical transmission ofpower produced by prime movers such as internal combustion engines.From automobiles toturbine-generator sets,mechanical power transmission occurs through torque acting through arotating shaft.Themeasurement of torque is important in avariety of applications,including sizing ofload-carrying shafts. This measurement is also a crucial aspect of the measurement of shaftpower, such as in an enginedynamometer.Strain-gauge-based torque cells are constructed inamannersimilartoload cells,inwhichatorsional straininanelasticelementissensedbystraingauges appropriately placed on the elastic element. Figure 12.26 shows a circular shaftinstrumented with strain gauges for the purpose of measuring torque,and a commerciallyavailabletorque sensor.ADirection of principal stressesTorque cellStraingaugesStrain gaugesBridge setupGauge positionGauge positionfront viewrearview(mirror image of front view)Figure12.26Shaftinstrumentedfortorquemeasurement

E1C12 09/14/2010 13:54:11 Page 529 where dy ¼ deflection along the applied force Fn ¼ applied force D ¼ diameter E ¼ modulus of elasticity I ¼ moment of inertia The application of the proving ring involves measuring the deflection of the proving ring in the direction of the applied force. Typical methods for this displacement measurement include displacement transducers, which measure overall displacement, and strain gauges. These methods are illustrated in Figure 12.25. Torque Measurements Torque and mechanical power measurements are often associated with the energy conversion processes that serve to provide mechanical and electrical power to our industrial world. Such energy conversion processes are largely characterized by the mechanical transmission of power produced by prime movers such as internal combustion engines. From automobiles to turbine-generator sets, mechanical power transmission occurs through torque acting through a rotating shaft. The measurement of torque is important in a variety of applications, including sizing of load-carrying shafts. This measurement is also a crucial aspect of the measurement of shaft power, such as in an engine dynamometer. Strain-gauge–based torque cells are constructed in a manner similar to load cells, in which a torsional strain in an elastic element is sensed by strain gauges appropriately placed on the elastic element. Figure 12.26 shows a circular shaft instrumented with strain gauges for the purpose of measuring torque, and a commercially available torque sensor. Gauge position front view Gauge position rear view (mirror image of front view) Direction of principal stresses seguag niartS seguag niartS 45 54 54 45 1 4 4 B' E0 A' B A T 2 4 1 3 3 1 2 2 3 Bridge setup Torque cell – – – – + + + + 1 2 3 4 Figure 12.26 Shaft instrumented for torque measurement. 12.2 Sensors 529

530Chapter12Mechatronics:Sensors,Actuators,and ControlsConsider the stresses created in a shaft of radius Ro subject to a torque T.The maximum shearingstress in a circular shaft occurs on the surface and may be calculated from the torsion formula(12.18)Tmax =TRo/JwhereTmax = maximum shearing stressT = applied torqueJ=polarmoment of inertia (R/2fora solid circular shaft)Fora shaft in pure torsion, there are no normal stresses,ox,y, or o:.The principal stresses lie alonga line that makes a 45-degree angle with the axis of the shaft, as illustrated in Figure 12.26,and havea value equal to Tmax. Strains that occur along the curve labeled A-A'are opposite in sign from thosethat occur along B-B'.These locations allow placement offour active strain gauges in a Wheatstonebridge arrangement, and the direct measurement of torque in terms of bridge output voltage.MechanicalPowerMeasurementsAlmostuniversally,primemovers suchas internal combustion(IC)engines andgasturbines convertchemical energy in a fuel to thermodynamic work transmitted by a shaftto the end use.Inautomotiveapplications,thepistons createatorqueonthecrankshaft,whichisultimatelytrans-mitted to the driving wheels.In each case, the power is transmitted through a mechanical coupling.This section discusses the measurement of such mechanical power transmission.Rotational Speed,Torque,and ShaftPowerShaft power is related to rotational speed and torque asP,=xT(12.19)wherePsistheshaftpower,wistherotational velocityvector,andTisthetorquevector.Ingeneral.the orientation of thetorque and rotational velocityvectors are such that the equation maybe writtenin scalar form as(12.20)P,=wTTable12.3provides a summaryofuseful equationsrelated to shaftpower,torque,and speedasemployed inmechanical measurements.Historically,a devicecalledaPronybrake wasusedtomeasure shaft power. A typical Prony brake arrangement is shown in Figure 12.27. Consider usingthe Pronybrake to measure poweroutput for an ICengine.The Pronybrake servesto provide a well-defined load for the engine, with the power output of the engine dissipated as thermal energy in thebraking material. By adjusting the load, the power output over a range of speeds and throttle settingscan be realized. The power is measured by recording the torque acting on the torque arm and therotational speed of the engine.Clearly,this device is limited in speed and power,but does serve todemonstrate the operating principles of power measurement, and is historically significant as thefirsttechniqueformeasuringpower.4 Coulomb developed the torsion formula in 1775 in connection with electrical instruments

E1C12 09/14/2010 13:54:11 Page 530 Consider the stresses created in a shaft of radius R0 subject to a torque T. The maximum shearing stress in a circular shaft occurs on the surface and may be calculated from the torsion formula4 tmax ¼ TR0=J ð12:18Þ where tmax ¼ maximum shearing stress T ¼ applied torque J ¼ polar moment of inertia (pR2 0=2 for a solid circular shaft) For a shaft in pure torsion, there are no normal stresses, sx; sy; or sz. The principal stresses lie along a line that makes a 45-degree angle with the axis of the shaft, as illustrated in Figure 12.26, and have a value equal to tmax. Strains that occur along the curve labeled A-A0 are opposite in sign from those that occur along B-B0 . These locations allow placement of four active strain gauges in a Wheatstone bridge arrangement, and the direct measurement of torque in terms of bridge output voltage. Mechanical Power Measurements Almost universally, prime movers such as internal combustion (IC) engines and gas turbines convert chemical energy in a fuel to thermodynamic work transmitted by a shaft to the end use. In automotive applications, the pistons create a torque on the crankshaft, which is ultimately trans￾mitted to the driving wheels. In each case, the power is transmitted through a mechanical coupling. This section discusses the measurement of such mechanical power transmission. Rotational Speed, Torque, and Shaft Power Shaft power is related to rotational speed and torque as P * s ¼ v *  T * ð12:19Þ where Ps is the shaft power, v is the rotational velocity vector, and T is the torque vector. In general, the orientation of the torque and rotational velocity vectors are such that the equation may be written in scalar form as Ps ¼ vT ð12:20Þ Table 12.3 provides a summary of useful equations related to shaft power, torque, and speed as employed in mechanical measurements. Historically, a device called a Prony brake was used to measure shaft power. A typical Prony brake arrangement is shown in Figure 12.27. Consider using the Prony brake to measure power output for an IC engine. The Prony brake serves to provide a well￾defined load for the engine, with the power output of the engine dissipated as thermal energy in the braking material. By adjusting the load, the power output over a range of speeds and throttle settings can be realized. The power is measured by recording the torque acting on the torque arm and the rotational speed of the engine. Clearly, this device is limited in speed and power, but does serve to demonstrate the operating principles of power measurement, and is historically significant as the first technique for measuring power. 4Coulomb developed the torsion formula in 1775 in connection with electrical instruments. 530 Chapter 12 Mechatronics: Sensors, Actuators, and Controls

53112.2SensorsTable12.3Shaft Power,Torque,and Speed RelationshipsSIU.S.Customary2mnTP=P=TShaft power, P2550PowerP(W)P (hp)Rotational speedw (rad/s)n (rev/s)TorqueT(Nm)T (ft Ib)Torque armLoadadjustingnutsFarceBrake blockmeasuringdeviceRotating wheelconnectedto driverStrap withcleatsattachedTorquearmradius, RFigure 12.27 Prony brake. (Courtesy of the American Society of Mechanical Engineers, New York, NY.Reprinted fromPTC 19.7-1980 6.)Cradled DynamometersAPronybrake is an example of an absorbingdynamometer.The term"dynamometer"refers to adevice that absorbs and measures the power output of a prime mover. Prime movers are largemechanical power-producing devices such as gasoline or diesel engines or gas turbines. Severalmethods of energy dissipation are utilized in various ranges of power, but the measurementtechniques are governed by the same underlying principles. Thus, we consider first the measurementof power,and then discuss means fordissipating the sometimes large amounts of power generatedbyprimemovers.The cradled dynamometermeasures mechanical power by measuring the rotational speed of theshaft, which transmits the power, and the reaction torque required to prevent movement of thestationary part of the prime mover. This reaction torque is impressively illustrated by so-calledwheelies by motorcycle riders. A cradled dynamometer is supported in bearings, which are calledtrunion bearings, such that the reaction torque is transmitted to a torque or force-measuring device.The state-of-the-art dynamometer shown in Figure 12.28 is designed for emissions testing,withpowerabsorptionratingsabove200HPandatopspeedof 120MPH.In principle, the operation of the dynamometer involves the steady-state measurement of theloadF,createdbythereactiontorqueandthemeasurementofshaft speed.FromEquation12.19thetransmitted shaftpowercanbecalculated directly

E1C12 09/14/2010 13:54:12 Page 531 Cradled Dynamometers A Prony brake is an example of an absorbing dynamometer. The term ‘‘dynamometer’’ refers to a device that absorbs and measures the power output of a prime mover. Prime movers are large mechanical power-producing devices such as gasoline or diesel engines or gas turbines. Several methods of energy dissipation are utilized in various ranges of power, but the measurement techniques are governed by the same underlying principles. Thus, we consider first the measurement of power, and then discuss means for dissipating the sometimes large amounts of power generated by prime movers. The cradled dynamometer measures mechanical power by measuring the rotational speed of the shaft, which transmits the power, and the reaction torque required to prevent movement of the stationary part of the prime mover. This reaction torque is impressively illustrated by so-called wheelies by motorcycle riders. A cradled dynamometer is supported in bearings, which are called trunion bearings, such that the reaction torque is transmitted to a torque or force-measuring device. The state-of-the-art dynamometer shown in Figure 12.28 is designed for emissions testing, with power absorption ratings above 200 HP and a top speed of 120 MPH. In principle, the operation of the dynamometer involves the steady-state measurement of the load Fr created by the reaction torque and the measurement of shaft speed. From Equation 12.19 the transmitted shaft power can be calculated directly. Torque arm Force measuring device Torque arm radius, R Load adjusting nuts Brake block Rotating wheel connected to driver Strap with cleats attached Figure 12.27 Prony brake. (Courtesy of the American Society of Mechanical Engineers, New York, NY. Reprinted from PTC 19.7-1980 6.) Table 12.3 Shaft Power, Torque, and Speed Relationships SI U.S. Customary Shaft power, P P ¼ vT P ¼ 2pnT 550 Power P (W) P (hp) Rotational speed v (rad/s) n (rev/s) Torque T (N m) T (ft lb) 12.2 Sensors 531

532Chapter12Mechatronics:Sensors,ActuatorsandControlsFigure12.28Dynamometer.(Courtesy of BurkeE.Porter MachineryCo.,GrandRapids,MI)TheAmerican Society of Mechanical Engineers (ASME)PerformanceTest Code(PTC)19.7(6)provides guidelinesforthe measurement of shaftpower.AccordingtoPTC19.7,overalluncertaintyinthemeasurementofshaftpowerbyacradleddynamometerresultsfrom(1)trunnion-bearingfriction error uncertainty,(2)force measurement uncertainty (F),(3)moment arm lengthuncertainty (L), (4)static unbalance of dynamometer error uncertainty and, (5)uncertainty inrotational speed measurement.A means of supplying a controllable load to the prime mover, and dissipating the energyabsorbed in the dynamometer, are an integral part of the design of anydynamometer.Severaltechniques are described for providing an appropriate load.Eddy Current Dynamometers:A direct current field coil and a rotor allow shaft power to bedissipated by eddy currents in the stator winding. The resulting conversion to thermal energy byjoulianheatingof theeddycurrentsnecessitates somecoolingbesupplied,typicallyusingcoolingwater.AlternatingCurrrentandDCGenerators:CradledAC and DCmachines areemployed as powerabsorbing elements in dynamometers.The AC applications requirevariable frequency capabilitiesto allow a widerange of power and speed measurements.Thepower produced in such dynamome-ters maybe dissipated as thermal energyusing resistiveloads.Waterbrake Dynamometers:Awaterbrake dynamometer employs fluid friction and momentumtransport to create a means of energy dissipation. Two representative designs are provided inFigure 12.29.The viscous shear type brake is useful for high rotational speeds, and the agitator typeunit is used over a range of speeds and loads.Waterbrakesmaybe employed for applications up to10,000HP (7450kW).Theload absorbed bywaterbrakes can be adjusted using water level and flowrates in the brake

E1C12 09/14/2010 13:54:12 Page 532 The American Society of Mechanical Engineers (ASME) Performance Test Code (PTC) 19.7 (6) provides guidelines for the measurement of shaft power. According to PTC 19.7, overall uncertainty in the measurement of shaft power by a cradled dynamometer results from (1) trunnion￾bearing friction error uncertainty, (2) force measurement uncertainty (Fr), (3) moment arm length uncertainty (Lr), (4) static unbalance of dynamometer error uncertainty and, (5) uncertainty in rotational speed measurement. A means of supplying a controllable load to the prime mover, and dissipating the energy absorbed in the dynamometer, are an integral part of the design of any dynamometer. Several techniques are described for providing an appropriate load. Eddy Current Dynamometers: A direct current field coil and a rotor allow shaft power to be dissipated by eddy currents in the stator winding. The resulting conversion to thermal energy by joulian heating of the eddy currents necessitates some cooling be supplied, typically using cooling water. Alternating Currrent and DC Generators: Cradled AC and DC machines are employed as power absorbing elements in dynamometers. The AC applications require variable frequency capabilities to allow a wide range of power and speed measurements. The power produced in such dynamome￾ters may be dissipated as thermal energy using resistive loads. Waterbrake Dynamometers: A waterbrake dynamometer employs fluid friction and momentum transport to create a means of energy dissipation. Two representative designs are provided in Figure 12.29. The viscous shear type brake is useful for high rotational speeds, and the agitator type unit is used over a range of speeds and loads. Waterbrakes may be employed for applications up to 10,000 HP (7450 kW). The load absorbed by waterbrakes can be adjusted using water level and flow rates in the brake. Figure 12.28 Dynamometer. (Courtesy of Burke E. Porter Machinery Co., Grand Rapids, MI) 532 Chapter 12 Mechatronics: Sensors, Actuators, and Controls

53312.2SensorsinletsSmooth disk rotorTrunnionbearing豆XSea豆Wateroutlet(a) Viscous shear typeWaterinletsRotorpocketTrunnionbearingStatorpocketC豆豆XFaOFigure 12.29 Waterbrakedynamometers. (Courtesy ofthe American Society ofWaterMechanical Engineers, NewfoutletsYork,NY.Reprinted fromPTC(b) Momentum exchange type19.7-1980 6.)

E1C12 09/14/2010 13:54:12 Page 533 Water inlets Water outlets Smooth disk rotor Trunnion bearing Trunnion bearing Seal Water outlet (a) Viscous shear type (b) Momentum exchange type Water inlets Rotor pocket Stator pocket Seal Figure 12.29 Waterbrake dynamometers. (Courtesy of the American Society of Mechanical Engineers, New York, NY. Reprinted from PTC 19.7-1980 6.) 12.2 Sensors 533

534Chapter12Mechatronics:Sensors,Actuators,andControls12.3ACTUATORSLinear ActuatorsThetaskof a linearactuatoristoprovidemotioninastraight line.Wediscussthreewaystoachievelinear motion:1.Conversion of rotarymotion into linear motion.This can be accomplished using a linkage,asin the slider-crank mechanism, or using screw threads coupled to a rotary motion source.2. Use of a fluid pressure to move a piston in a cylinder. When air or another gas is used as theworkingfluid,thesystemiscalledapneumaticsystem.Whenafluidsuchasoilisusedastheworkingfluid, thesystemistermed hydraulic.3.ElectromagneticSlider-CrankMechanismA commonmeansofgenerating areciprocating linearmotion,or converting linear motion to rotarymotion,is the slider-crank mechanism,as illustrated inFigure12.30.Such a mechanismis thebasisof transforming the reciprocating motion of the piston in an internal combustion engine.This orsimilar linkages could also be applied in pick-and-place operations, or in a variety of automationapplications.Screw-drive linear motionAcommonmeansfortranslatingrotarymotionintolinearmotionisalead screw.Alead screwhashelical threads that are designed for minimum backlash to allow precise positioning.Numerousdesigns exist for such actuating threads. The basic principle is illustrated in Figure 12.31.The rotarymotion of thelead screw is translated into linear motion of thenut, with the torque required to drivethe lead screw directly related to the thrust the particular application requires.Figure 12.30 Slider-crank mechanismThrust2output区Nut区XThrustoutputFigure 12.31 Linear actuation using a leadTorqueinputscrew

E1C12 09/14/2010 13:54:12 Page 534 12.3 ACTUATORS Linear Actuators The task of a linear actuator is to provide motion in a straight line. We discuss three ways to achieve linear motion: 1. Conversion of rotary motion into linear motion. This can be accomplished using a linkage, as in the slider-crank mechanism, or using screw threads coupled to a rotary motion source. 2. Use of a fluid pressure to move a piston in a cylinder. When air or another gas is used as the working fluid, the system is called a pneumatic system. When a fluid such as oil is used as the working fluid, the system is termed hydraulic. 3. Electromagnetic Slider–Crank Mechanism A common means of generating a reciprocating linear motion, or converting linear motion to rotary motion, is the slider–crank mechanism, as illustrated in Figure 12.30. Such a mechanism is the basis of transforming the reciprocating motion of the piston in an internal combustion engine. This or similar linkages could also be applied in pick-and-place operations, or in a variety of automation applications. Screw-drive linear motion A common means for translating rotary motion into linear motion is a lead screw. A lead screw has helical threads that are designed for minimum backlash to allow precise positioning. Numerous designs exist for such actuating threads. The basic principle is illustrated in Figure 12.31. The rotary motion of the lead screw is translated into linear motion of the nut, with the torque required to drive the lead screw directly related to the thrust the particular application requires. Figure 12.30 Slider–crank mechanism. Thrust output Torque input Thrust output Nut Figure 12.31 Linear actuation using a lead screw. 534 Chapter 12 Mechatronics: Sensors, Actuators, and Controls