《工程测试与信号处理》课程教学资源（文献资料）Actuators

McGraw-Hill CreateTM ReviewCopy forInstructorNicolescu.Notfor distribution.124MeasurementSystems468CHAPTER10Actuators10.8HYDRAULICSHydraulic systems are designed to move large loads by controlling a high-pressurefluid in distribution lines and pistons with mechanical orelectromechanical valves.A hydraulic system, illustrated in Figure 10.34, consists of a pump to deliver high-pressure fluid, a pressure regulator to limit the pressure in the system, valves to con-trolflowratesandpressures,adistribution systemcomposed ofhosesorpipes,andlinear or rotaryactuators.The infrastructure,which consists of the elements con-tained in the dashed box in the figure, is typicallyused to power many hydraulicvalve-actuatorsubsystems.Ahydraulic pump is usually driven by an electricmotor (e.g.,a large AC induc-tion motor)or an internal combustion engine.Typical fluid pressures generated bypumps used in heavy equipment (e.g., construction equipment and large industrialmachines) are in the 1000 psi (6.89 MPa) to 3000 psi (20.7 MPa) range. The hydrau-lic fluid is selected to have the following characteristics: good lubrication to preventwear in moving components (e.g.,between pistons and cylinders),corrosion resis-tance,and incompressibilitytoproviderapidresponse.Mosthydraulicpumpsactbypositive displacement, which means they deliver a fixed volume of fluid with eachcycle or rotation of the pump.The three main types of positive displacement pumpsusedinhydraulic systemsaregearpumps,vanepumps,andpistonpumps.Anexam-ple of a gear pump, which displaces the fluid around a housing between teeth ofmeshinggears, is shown inFigure10.35.Note that themeshing teeth provide a seal, infrastructurecontroloresstrefilterregulatorvalvePAcylinderFigure10.34Hydraulicsystemcomponentsoutlet (P)shaft rotatiofluidcarriedbetweenteethinlet (7)Figure10.35Gearpump

Confirming Pages 468 C H A P T E R 10 Actuators 10.8 HYDRAULICS Hydraulic systems are designed to move large loads by controlling a high-pressure fluid in distribution lines and pistons with mechanical or electromechanical valves. A hydraulic system, illustrated in Figure 10.34 , consists of a pump to deliver high￾pressure fluid, a pressure regulator to limit the pressure in the system, valves to con￾trol flow rates and pressures, a distribution system composed of hoses or pipes, and linear or rotary actuators. The infrastructure, which consists of the elements con￾tained in the dashed box in the figure, is typically used to power many hydraulic valve-actuator subsystems. A hydraulic pump is usually driven by an electric motor (e.g., a large AC induc￾tion motor) or an internal combustion engine. Typical fluid pressures generated by pumps used in heavy equipment (e.g., construction equipment and large industrial machines) are in the 1000 psi (6.89 MPa) to 3000 psi (20.7 MPa) range. The hydrau￾lic fluid is selected to have the following characteristics: good lubrication to prevent wear in moving components (e.g., between pistons and cylinders), corrosion resis￾tance, and incompressibility to provide rapid response. Most hydraulic pumps act by positive displacement, which means they deliver a fixed volume of fluid with each cycle or rotation of the pump. The three main types of positive displacement pumps used in hydraulic systems are gear pumps, vane pumps, and piston pumps. An exam￾ple of a gear pump, which displaces the fluid around a housing between teeth of meshing gears, is shown in Figure 10.35 . Note that the meshing teeth provide a seal, tank filter motor pump pressure regulator control valve cylinder P A T B infrastructure Figure 10.34 Hydraulic system components. Figure 10.35 Gear pump. shaft rotation outlet (P) inlet (T) fluid carried between teeth alc80237_ch10_431-477_sss.indd 468 lc80237_ch10_431-477_sss.indd 468 10/01/11 10:24 PM 0/01/11 10:24 PM 124 Measurement Systems McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution

McGraw-Hill CreateTM ReviewCopyforInstructorNicolescu.NotfordistributionIntroductiontoMechatronicsandMeasurementSystems,FourthEdition12510.8469Hydraulicsspring orvanehydraulicpressureslotted rotorvane guidoinletoutietmotor shaftFigure 10.36VanepumpcylindersectionviewblockinletRinput shaftinlet/outletmanifoldsadjustableoutleangle, fixedswash platepistonanVeWoutletmanifoldcylindersinletmanifoldFigure10.37Swashplatepistonpumpand thefluid is displaced from the inlet to the outlet along the nonmeshing side of thegears. Video Demos 10.22 and 10.23 show and describe various types of gear pumps.Figure10.36illustratesavanepump,whichdisplacesthefluidbetweenvanesguided in rotor slots riding against the housing and vane guide. The vane guide sup-VideoDemoports the vanes from one side of the housing to the next and is constructed to allow10.22Gear pumpsthefluidtopass.Theoutputdisplacementcanbevaried(withaconstantmotor10.23Hydraulicspeed)bymovingthe shaftverticallyrelativetothehousinggearpumpsFigure 10.37 illustrates a piston pump.The cylinder block is rotated by theinput shaft, and thepiston ends aredriven in and out as theyride in thefixed swashplate slot, which is angled with respectto the axis of the shaft.Apiston drawsfluidfrom an inlet manifold overhalf theswash plate and expelsfluid intothe outletmanifold duringthe otherhalf.The displacement of thepump canbe changed sim-ply by changing the angle of the fixed swash plate.Table 10.5 lists and compares thegeneral characteristics of thedifferentpumptypes.Since positive displacement hydraulic pumps provide a fixed volumetric flowrate, it is necessary to include a pressure relief valve, called a pressure regulator

Confirming Pages Figure 10.36 Vane pump. inlet outlet vane spring or hydraulic pressure slotted rotor vane guide motor shaft and the fluid is displaced from the inlet to the outlet along the nonmeshing side of the gears. Video Demos 10.22 and 10.23 show and describe various types of gear pumps. Figure 10.36 illustrates a vane pump, which displaces the fluid between vanes guided in rotor slots riding against the housing and vane guide. The vane guide sup￾ports the vanes from one side of the housing to the next and is constructed to allow the fluid to pass. The output displacement can be varied (with a constant motor speed) by moving the shaft vertically relative to the housing. Figure 10.37 illustrates a piston pump. The cylinder block is rotated by the input shaft, and the piston ends are driven in and out as they ride in the fixed swash plate slot, which is angled with respect to the axis of the shaft. A piston draws fluid from an inlet manifold over half the swash plate and expels fluid into the outlet manifold during the other half. The displacement of the pump can be changed sim￾ply by changing the angle of the fixed swash plate. Table 10.5 lists and compares the general characteristics of the different pump types. Since positive displacement hydraulic pumps provide a fixed volumetric flow rate, it is necessary to include a pressure relief valve, called a pressure regulator, Figure 10.37 Swash plate piston pump. end views cylinders outlet manifold inlet manifold adjustable angle, fixed swash plate inlet outlet input shaft piston cylinder block inlet/outlet manifolds section view Video Demo 10.22 Gear pumps 10.23 Hydraulic gear pumps 10.8 Hydraulics 469 alc80237_ch10_431-477_sss.indd 469 lc80237_ch10_431-477_sss.indd 469 10/01/11 10:24 PM 0/01/11 10:24 PM Introduction to Mechatronics and Measurement Systems, Fourth Edition 125 McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution

McGraw-HillCreateTM ReviewCopyforlnstructorNicolescu.Notfordistribution.126MeasurementSystems470CHAPTER10ActuatorsTable10.5ComparisonofpumpcharacteristicsTypicalPumptypeCostDisplacementpressure (psi)Fixed2000LowGearVaneVariable3000MediumPistonVariable6000Highto prevent the pressure from exceeding design limits. The simplest pressure regu-lator is the spring-ball arrangement illustrated in Figure 10.38.When the pressureforce exceeds the spring force, fluid is vented back to the tank, preventing a fur-ther increase in pressure. The threshold pressure, or cracking pressure, is usuallyadjusted by changing the spring's compressed length and therefore its resisting force.10.8.1HydraulicValvesThere are two types of hydraulic valves: the infinite position valve that allows anyposition between open and closed tomodulate flow or pressure,and the finite posi-tion valve that has discrete positions, usually just open and closed, each providinga differentpressureandflow condition.Inlet and outlet connections to a valve arecalledports.Finitepositionvalvesarecommonlydescribedbyanx/ydesignation,wherexisthenumberofportsandyisthenumber ofpositions.As an example,a 4/3valve,with4ports and3positions,is illustrated inschematicform inFigure10.39Inposition1,systempressureis vented totank;inposition2,outputportAispres-surized and portBisvented totank; and inposition3,outputportB is pressurizedand port A is vented to tank.As illustrated in Figure 10.40, this particular valve isuseful in controlling a double-acting hydraulic cylinder where ports A and B connect-system pressure (P)ballspringadjustablesupportreturn to tank (7)Figure10.38Pressure regulator.Bposition 1position 2position 3Figure10.394/3valveschematic

Confirming Pages 470 C H A P T E R 10 Actuators to prevent the pressure from exceeding design limits. The simplest pressure regu￾lator is the spring-ball arrangement illustrated in Figure 10.38 . When the pressure force exceeds the spring force, fluid is vented back to the tank, preventing a fur￾ther increase in pressure. The threshold pressure, or cracking pressure, is usually adjusted by changing the spring’s compressed length and therefore its resisting force. 10.8.1 Hydraulic Valves There are two types of hydraulic valves: the infinite position valve that allows any position between open and closed to modulate flow or pressure, and the finite posi￾tion valve that has discrete positions, usually just open and closed, each providing a different pressure and flow condition. Inlet and outlet connections to a valve are called ports. Finite position valves are commonly described by an x / y designation, where x is the number of ports and y is the number of positions. As an example, a 4/3 valve, with 4 ports and 3 positions, is illustrated in schematic form in Figure 10.39 . In position 1, system pressure is vented to tank; in position 2, output port A is pres￾surized and port B is vented to tank; and in position 3, output port B is pressurized and port A is vented to tank. As illustrated in Figure 10.40 , this particular valve is useful in controlling a double-acting hydraulic cylinder where ports A and B connect Table 10.5 Comparison of pump characteristics Pump type Displacement Typical pressure (psi) Cost Gear Fixed 2000 Low Vane Variable 3000 Medium Piston Variable 6000 High Figure 10.38 Pressure regulator. return to tank (T) adjustable support system pressure (P) ball spring Figure 10.39 4/3 valve schematic. P T A B P T A B P T A B position 1 position 3 position 2 alc80237_ch10_431-477_sss.indd 470 lc80237_ch10_431-477_sss.indd 470 10/01/11 10:24 PM 0/01/11 10:24 PM 126 Measurement Systems McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution

McGraw-Hill CreateTM Review Copyfor lnstructor Nicolescu.Not fordistributionIntroduction to Mechatronics and Measurement Systems,Fourth Edition12710.8Hydraulics471to oppositeends of the cylinder,applyingor ventingpressure on opposite sides ofthe piston. In position 1, the cylinder does not move, because the pressure is ventedto the tank.In position 2thecylinder moves to theright, since pressureis applied tothe left side of the piston. In position 3, the cylinder moves to the left, because pres-sureisappliedtotherightsideofthepiston.Commontypesoffixedpositionvalvesarecheckvalves,poppetvalves,spoolvalves, and rotary valves. Figure 10.41 illustrates check and poppet valves. Thecheck valve allows flow in one direction only.The poppet valve is a check valvethat canbeforced open to allowreverseflow.As illustrated in Figure 10.42,a spool valve consists of a cylindrical spool withmultiplelobesmoving within a cylindrical casing containing multiple ports.The spoolcan be moved back and forth to align spaces between the spool lobes with input andoutputportsinthehousingtodirecthigh-pressureflowtodifferentconduits inthesys-tem.Thespool isbalanced ineachpositionbecausethe staticpressureisthesame onopposing internalfaces ofthe lobes.Therefore,noforce is required tohold a position.In theleftposition,portAispressurized, and portBisvented tothetank;and intherightposition,portBispressurized andportAisvented.Tomovethespool betweenpositions,an axial force is required,from an actuatorormanual control lever,to overcomethehydrodynamicforcesassociated with changingthemomentum oftheflow.In the design of a spool valve where large hydrodynamic forces occur, a pilotvalve is added, as shown in Figure 10.43. The pilot valve operates at a lower pres-sure, called pilot pressure, and at much lowerflow rates and thereforerequires lessforce to actuate.Thepilot valve directs pilot pressure to one side of the main spool,and theforce generated by the pressure acting over the main spool lobeface is largeenoughto actuatethemainvalve.Theeffect ofthepilotvalveis toamplifyforceprovided by the solenoid or mechanical lever acting on the pilot spool. In the figure,the pilot spool is in the full left position, causing pilot pressure to be applied to thepistonP4/3valveBFigure10.40Double-actinghydrauliccylinderup-down plunglight springballseano flowunlessplunger isAKVflowflovdown(b) poppet valve(a), check valveFigure 10.41 Check and poppet valves

Confirming Pages to opposite ends of the cylinder, applying or venting pressure on opposite sides of the piston. In position 1, the cylinder does not move, because the pressure is vented to the tank. In position 2, the cylinder moves to the right, since pressure is applied to the left side of the piston. In position 3, the cylinder moves to the left, because pres￾sure is applied to the right side of the piston. Common types of fixed position valves are check valves, poppet valves, spool valves, and rotary valves. Figure 10.41 illustrates check and poppet valves. The check valve allows flow in one direction only. The poppet valve is a check valve that can be forced open to allow reverse flow. As illustrated in Figure 10.42 , a spool valve consists of a cylindrical spool with multiple lobes moving within a cylindrical casing containing multiple ports. The spool can be moved back and forth to align spaces between the spool lobes with input and output ports in the housing to direct high-pressure flow to different conduits in the sys￾tem. The spool is balanced in each position because the static pressure is the same on opposing internal faces of the lobes. Therefore, no force is required to hold a position. In the left position, port A is pressurized, and port B is vented to the tank; and in the right position, port B is pressurized and port A is vented. To move the spool between positions, an axial force is required, from an actuator or manual control lever, to over￾come the hydrodynamic forces associated with changing the momentum of the flow. In the design of a spool valve where large hydrodynamic forces occur, a pilot valve is added, as shown in Figure 10.43 . The pilot valve operates at a lower pres￾sure, called pilot pressure, and at much lower flow rates and therefore requires less force to actuate. The pilot valve directs pilot pressure to one side of the main spool, and the force generated by the pressure acting over the main spool lobe face is large enough to actuate the main valve. The effect of the pilot valve is to amplify force provided by the solenoid or mechanical lever acting on the pilot spool. In the figure, the pilot spool is in the full left position, causing pilot pressure to be applied to the Figure 10.40 Double-acting hydraulic cylinder. P T A B piston 4/3 valve Figure 10.41 Check and poppet valves. (a) check valve (b) poppet valve flow no flow ball seat light spring up-down plunger no flow unless plunger is down flow 10.8 Hydraulics 471 alc80237_ch10_431-477_sss.indd 471 lc80237_ch10_431-477_sss.indd 471 10/01/11 10:24 PM 0/01/11 10:24 PM Introduction to Mechatronics and Measurement Systems, Fourth Edition 127 McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution

McGraw-Hill CreateTM ReviewCopyforInstructorNicolescu.Notfordistribution128Measurement Systems472CHAPTER10ActuatorsspoollobeforoTAPBTITATTPBITleft position (P-A, B-T)right position (A-T, P-B)(a) schematicSPOOLUGROOVESImproves contamination toleranceofspools over conventional V grooves.SEALED WET ARMATURE SOLENOIDSSignificantly reduces spool hang uPMaximum protection againstmoisture,resulting in loss of production. corrosion and dirt.OPERATORPROTECTIONHigh temperature elements arejisolated from direct contact.STANDARDMOUNTINGSConforms to NFPA andANSI/ISO standards.2WIRELEAD50/60 Hz coils standard forincreased application flexibility.INTERCHANGEABLE SPOOLSProvide easy field maintenance.LOW-PRESSURE DROPSNo matching of parts.Reduces heat load andincreases efficiency,(b) actual(CourtesyofContinental Hydraulics, Savage,MN)Figure10.42Spoolvalve.pilot valvesolenoid-operatedpilot spoolmain spool pilotNBAFigure10.43Pilot-operatedspoolvalveleft side of the main spool and venting fluidfrom theright side of the main spool to1tank,thusdrivingthemainspooltothefull rightposition.Thisappliesmainpressureto port B and vents port A.VideoDemo10.24 illustrates how a pilot valve can beVideo Demousedtocreateanamplifiedhydraulicforce.10.24PilotvalveThediscussionof spoolvalvessofarhasbeenlimitedtooperationbetweenhydraulic amplifiertwo positions only: on and off. Continuous operation can be achieved by using acut-awayproportionalvalve,onewhosespool movesadistanceproportional toamechanical

Confirming Pages Figure 10.42 Spool valve. (a) schematic T A P T A P left position (P-A, B-T) right position (A-T, P-B) force spool lobe casing B T T B SEALED WET ARMATURE SOLENOIDS Maximum protection against moisture, corrosion and dirt. OPERATOR PROTECTION High temperature elements are isolated from direct contact. 2 WIRE LEAD 50/60 Hz coils standard for increased application flexibility. INTERCHANGEABLE SPOOLS Provide easy field maintenance. No matching of parts. LOW-PRESSURE DROPS Reduces heat load and increases efficiency. SPOOL U GROOVES Improves contamination tolerance of spools over conventional V grooves. Significantly reduces spool hang up resulting in loss of production. STANDARD MOUNTINGS Conforms to NFPA and ANSI/ISO standards. (b) actual (Courtesy of Continental Hydraulics, Savage, MN) Figure 10.43 Pilot-operated spool valve. T T A P B solenoid￾operated T T pilot spool pilot press. pilot valve main spool 472 C H A P T E R 10 Actuators left side of the main spool and venting fluid from the right side of the main spool to tank, thus driving the main spool to the full right position. This applies main pressure to port B and vents port A. Video Demo 10.24 illustrates how a pilot valve can be used to create an amplified hydraulic force. The discussion of spool valves so far has been limited to operation between two positions only: on and off. Continuous operation can be achieved by using a proportional valve, one whose spool moves a distance proportional to a mechanical Video Demo 10.24 Pilot valve hydraulic amplifier cut-away alc80237_ch10_431-477_sss.indd 472 lc80237_ch10_431-477_sss.indd 472 10/01/11 10:24 PM 0/01/11 10:24 PM 128 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 Edition12910.8Hydraulics473or electrical input (e.g-,a lever or an adjustable-current solenoid), thus changing therate of flow and varying the speed and force of the actuator. When the spool positionis controlled by electrical solenoids,the proportional valve is called an electrohy-draulic valve.These valves may be used in open-loop control situations with nofeedback,buttheyoftenincludesensorstomonitor spoolpositionor actuatoroutput.Proportionalvalves equipped with sensorand control circuitryareoftencalled servovalves.Electrohydraulicvalvesare oftenpilot operated wherethesolenoids drivethepilot spool, which in turn controls the position of the primary spool. The pilot spoolcan be driven by a single solenoid with a spring return or a set ofopposing solenoids.The solenoid currents,and therefore displacements,can be controlled by amplifierslinkedtoanalogordigital controllers.10.8.2HydraulicActuatorsThemost commonhydraulicactuatoris a simplecylinderwithapistondrivenbythe pressurized fluid. As illustrated in Figure 10.44, a cylinder can be single acting,where it is driven to and held in onepositionbypressure and returned tothe otherposition bya springorbythe weight of the load,or doubleacting,wherepressureis used to drive the piston in both directions.As illustrated in Figure 10.45,the linearactuatorcanbeveryversatile inachievingavarietyof motions.Cylindermotion inthe hydraulic elevator drives the elevator directly.The scissor jack converts smalllinear motion in the horizontal direction to larger linear motion in the vertical direc-tion.Linear motion of thecylinder inthecraneresults in rotarymotion of itspivotedboom.Rotary motion can also be achieved with hydraulic systems directly with arotaryactuator.Onetypeof rotaryactuator,called agearmotor,is simplyagearpump (see Figure 10.35)driven in reverse where pressure is applied, resulting inrotation of a shaft.DANsingle-acting cylinderdouble-acting cylinderFigure10.44 Single-actinganddouble-actingcylindershydraulic elevatorscissor jack"cherry picker" craneFigure10.45Examplemechanisms driven bya hydraulic cylinder

Confirming Pages or electrical input (e.g., a lever or an adjustable-current solenoid), thus changing the rate of flow and varying the speed and force of the actuator. When the spool position is controlled by electrical solenoids, the proportional valve is called an electrohy￾draulic valve. These valves may be used in open-loop control situations with no feedback, but they often include sensors to monitor spool position or actuator output. Proportional valves equipped with sensor and control circuitry are often called servo valves. Electrohydraulic valves are often pilot operated where the solenoids drive the pilot spool, which in turn controls the position of the primary spool. The pilot spool can be driven by a single solenoid with a spring return or a set of opposing solenoids. The solenoid currents, and therefore displacements, can be controlled by amplifiers linked to analog or digital controllers. 10.8.2 Hydraulic Actuators The most common hydraulic actuator is a simple cylinder with a piston driven by the pressurized fluid. As illustrated in Figure 10.44 , a cylinder can be single acting, where it is driven to and held in one position by pressure and returned to the other position by a spring or by the weight of the load, or double acting, where pressure is used to drive the piston in both directions. As illustrated in Figure 10.45 , the linear actuator can be very versatile in achieving a variety of motions. Cylinder motion in the hydraulic elevator drives the elevator directly. The scissor jack converts small linear motion in the horizontal direction to larger linear motion in the vertical direc￾tion. Linear motion of the cylinder in the crane results in rotary motion of its pivoted boom. Rotary motion can also be achieved with hydraulic systems directly with a rotary actuator. One type of rotary actuator, called a gear motor, is simply a gear pump (see Figure 10.35 ) driven in reverse where pressure is applied, resulting in rotation of a shaft. Figure 10.44 Single-acting and double-acting cylinders. double-acting cylinder A B single-acting cylinder A hydraulic elevator scissor jack “cherry picker” crane Figure 10.45 Example mechanisms driven by a hydraulic cylinder. 10.8 Hydraulics 473 alc80237_ch10_431-477_sss.indd 473 lc80237_ch10_431-477_sss.indd 473 10/01/11 10:24 PM 0/01/11 10:24 PM Introduction to Mechatronics and Measurement Systems, Fourth Edition 129 McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution

McGraw-Hill CreateTM ReviewCopy forInstructorNicolescu.Notfor distribution.130Measurement Systems474CHAPTER10ActuatorsCLASS DISCUSSIONITEM110.8Force Generated by a Double-Acting CylinderFor a given system pressure, is the forcegenerated by a double-acting cylinder dif-ferent depending on the direction of actuation? How is the force determined foreachdirectionofmotion?Hydraulic systems have the advantage of generating extremely large forces fromvery compact actuators.They also can provide precise control at low speeds andhavebuilt-in travel limitsdefinedbythe cylinder stroke.Thedrawbacks of hydraulicsystems includethe need for a large infrastructure (high-pressurepump,tank,andInternet Linkdistribution lines);potential forfluid leaks, which are undesirable in a clean environ-10.9Hydraulicment; possible hazards associated with high pressures (e.g-,a ruptured line); noisyValves.orgoperation;vibration;and maintenance requirements.Because of these disadvan-(valve, pump,tages,electric motor drives are often the preferred choice.However, in large systems,motor, cylinderwhich require extremely large forces,hydraulics often provide the only alternative.manufactures andFormore information,InternetLink 10.9provides linkstoonlineresourcesandinformation)manufacturers of hydraulic components and systems.10.9PNEUMATICSPneumatic systems are similar to hydraulic systems, but they use compressed air asthe working fluid rather than hydraulic liquid.The components in a pneumatic sys-tem are illustrated inFigure 10.46.A compressor isused toprovidepressurized air.usually on the order of 70 to150 psi (482kPA to 1.03MPa),which ismuch lowerthan hydraulic system pressures. As a result of the lower operating pressures, pneu-matic actuators generate much lower forces than hydraulic actuators.After the inlet air is compressed, excess moisture and heat are removed fromthe air with an air treatment unit (seeFigure10.46). Unlikehydraulic pumps,whichprovidepositivedisplacementoffluidathighpressureondemand,compressorscan-notprovidehigh volumeofpressurizedairresponsively;therefore,a largevolumeofhigh-pressure compressed airis stored ina reservoir ortank.The workingpressuredelivered to the system can be controlled by a pressure regulator to bemuch lowerthan the reservoir pressure.The reservoir is equipped with a pressure-sensitive switchthatactivatesthecompressorwhenthepressure startstofall belowthedesired level.Control valves and actuators act in much the samewayasinhydraulic systems,but instead of returning fluid to a tank, the air is simply returned (exhausted) to theatmosphere.Pneumaticsystems areopen systems,alwaysprocessingnewair,andhydraulic systems are closed systems,always recirculating the sameoil.This elimi-nates the needfor a network of return lines in pneumatic systems.Another advantageof pneumatic systems is that air is"cleaner"than oil, although airdoes not have theself-lubricating features of hydraulic oil

Confirming Pages 474 C H A P T E R 10 Actuators Hydraulic systems have the advantage of generating extremely large forces from very compact actuators. They also can provide precise control at low speeds and have built-in travel limits defined by the cylinder stroke. The drawbacks of hydraulic systems include the need for a large infrastructure (high-pressure pump, tank, and distribution lines); potential for fluid leaks, which are undesirable in a clean environ￾ment; possible hazards associated with high pressures (e.g., a ruptured line); noisy operation; vibration; and maintenance requirements. Because of these disadvan￾tages, electric motor drives are often the preferred choice. However, in large systems, which require extremely large forces, hydraulics often provide the only alternative. For more information, Internet Link 10.9 provides links to online resources and manufacturers of hydraulic components and systems. 10.9 PNEUMATICS Pneumatic systems are similar to hydraulic systems, but they use compressed air as the working fluid rather than hydraulic liquid. The components in a pneumatic sys￾tem are illustrated in Figure 10.46 . A compressor is used to provide pressurized air, usually on the order of 70 to 150 psi (482 kPA to 1.03 MPa), which is much lower than hydraulic system pressures. As a result of the lower operating pressures, pneu￾matic actuators generate much lower forces than hydraulic actuators. After the inlet air is compressed, excess moisture and heat are removed from the air with an air treatment unit (see Figure 10.46 ). Unlike hydraulic pumps, which provide positive displacement of fluid at high pressure on demand, compressors can￾not provide high volume of pressurized air responsively; therefore, a large volume of high-pressure compressed air is stored in a reservoir or tank. The working pressure delivered to the system can be controlled by a pressure regulator to be much lower than the reservoir pressure. The reservoir is equipped with a pressure-sensitive switch that activates the compressor when the pressure starts to fall below the desired level. Control valves and actuators act in much the same way as in hydraulic systems, but instead of returning fluid to a tank, the air is simply returned (exhausted) to the atmosphere. Pneumatic systems are open systems, always processing new air, and hydraulic systems are closed systems, always recirculating the same oil. This elimi￾nates the need for a network of return lines in pneumatic systems. Another advantage of pneumatic systems is that air is “cleaner” than oil, although air does not have the self-lubricating features of hydraulic oil. ■ CLASS DISCUSSION ITEM 10.8 Force Generated by a Double-Acting Cylinder For a given system pressure, is the force generated by a double-acting cylinder dif￾ferent depending on the direction of actuation? How is the force determined for each direction of motion? Internet Link 10.9 Hydraulic Valves.org (valve, pump, motor, cylinder manufactures and information) alc80237_ch10_431-477_sss.indd 474 lc80237_ch10_431-477_sss.indd 474 10/01/11 10:24 PM 0/01/11 10:24 PM 130 Measurement Systems McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution

McGraw-Hill CreateTM Review Copy forInstructor Nicolescu.NotfordistributionIntroduction to Mechatronics and Measurement Systems, Fourth Edition13110.9475Pneumaticscontrolvalve0O0OcylinderFigure10.46Pneumaticsystemcomponents.When sources of compressed air arereadily available,asthey often are inengineering-related facilities,pneumatic actuators maybe a good choice.Double-acting or single-acting pneumatic cylinders are ideal for providing low-force linearmotion between two well-defined endpoints.Since air is compressible,pneumaticcylinders arenottypicallyusedfor applications requiringaccuratemotionbetweenVideo Demothe endpoints, especially in the presence of a varying load. Video Demo 10.25 dem-10.25Pneumaticonstrates various types of pneumatic cylinders, and Video Demo10.26 shows ancylinders ofvariousinterestingexampleof anapparatusdrivenbyapneumatic system.types and sizesAnother advantage of pneumatic systems is the possibilityof replacing the10.26Pneumaticinfrastructurewith a high-pressure storagetank and regulator.Thetank servesabiomechanicsfunction analogous to a battery in an electrical system.This makes possible lightexerciseapparatusmobile pneumatic systems (e.g, a pneumatically actuated walking robot). In theseoverviewapplications,thecapacityofthetanklimitstherangeoroperatingtimeofthesystemQUESTIONSANDEXERCISESSection1o.3SolenoidsandRelays10.1.A solenoid can be modeled as an inductor in series with a resistor. Design a circuitto use a digital output to control a 24 V solenoid.Section1o.5DcMotors10.2.Why can the presence of electric motors or solenoids affect thefunction of nearbyelectronic circuits?10.3.If a manufacturer's specifications for a PM DC motor are as follows,what are themotor's no-load speed, stall current, starting torque, and maximum power for anapplied voltage of 10 V?■Torqueconstant=0.12Nm/AElectricalconstant=12V/1000RPMArmatureresistance=1.50

Confirming Pages When sources of compressed air are readily available, as they often are in engineering-related facilities, pneumatic actuators may be a good choice. Double￾acting or single-acting pneumatic cylinders are ideal for providing low-force linear motion between two well-defined endpoints. Since air is compressible, pneumatic cylinders are not typically used for applications requiring accurate motion between the endpoints, especially in the presence of a varying load. Video Demo 10.25 dem￾onstrates various types of pneumatic cylinders, and Video Demo 10.26 shows an interesting example of an apparatus driven by a pneumatic system. Another advantage of pneumatic systems is the possibility of replacing the infrastructure with a high-pressure storage tank and regulator. The tank serves a function analogous to a battery in an electrical system. This makes possible light, mobile pneumatic systems (e.g., a pneumatically actuated walking robot). In these applications, the capacity of the tank limits the range or operating time of the system. QUESTIONS AND EXERCISES Section 10.3 Solenoids and Relays 10.1. A solenoid can be modeled as an inductor in series with a resistor. Design a circuit to use a digital output to control a 24 V solenoid. Section 10.5 DC Motors 10.2. Why can the presence of electric motors or solenoids affect the function of nearby electronic circuits? 10.3. If a manufacturer’s specifications for a PM DC motor are as follows, what are the motor’s no-load speed, stall current, starting torque, and maximum power for an applied voltage of 10 V? ■ Torque constant 0.12 Nm/A ■ Electrical constant 12 V/1000 RPM ■ Armature resistance 1.5 filter motor compressor control valve cylinder P A R B infrastructure exhaust air inlet air treatment on/off control reservoir pressure regulator Figure 10.46 Pneumatic system components. Video Demo 10.25 Pneumatic cylinders of various types and sizes 10.26 Pneumatic biomechanics exercise apparatus overview 10.9 Pneumatics 475 alc80237_ch10_431-477_sss.indd 475 lc80237_ch10_431-477_sss.indd 475 10/01/11 10:24 PM 0/01/11 10:24 PM Introduction to Mechatronics and Measurement Systems, Fourth Edition 131 McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution.