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
53512.3ActuatorsFigure 12.32 Precision translationtable. (UniSlidefrom Velmex,Inc).Common applications that employalead screw includetheworktableforamill,anda varietyofother precision positioning translation tables, such as the one shown in Figure 12.32.PneumaticandHydraulicActuatorsThe term“pneumatic"implies a component or system that uses compressed air as the energysource. On the other hand, a hydraulic system or component uses incompressible oil as the workingfluid. An example of a hydraulic system is the power steering on an automobile; such a system isillustrated in Figure 12.33. Hydraulic fluid is supplied at an elevated pressure from the powersteering pump.When a steering input is made from the driver, the rotaryvalve allows high-pressurefluid to enter the appropriate side of the piston, and aidin turning the wheels.By maintaining a directconnection between the steering column and the rack and pinion, the car can be steered even if thehydraulic system fails.SteeringcolumnRotaryvalveToFluid linesreservoir-FrompumpWFigure12.33SchematicdiagramofapowerRackPistonPinionsteering system
E1C12 09/14/2010 13:54:12 Page 535 Common applications that employ a lead screw include the worktable for a mill, and a variety of other precision positioning translation tables, such as the one shown in Figure 12.32. Pneumatic and Hydraulic Actuators The term ‘‘pneumatic’’ implies a component or system that uses compressed air as the energy source. On the other hand, a hydraulic system or component uses incompressible oil as the working fluid. An example of a hydraulic system is the power steering on an automobile; such a system is illustrated in Figure 12.33. Hydraulic fluid is supplied at an elevated pressure from the power steering pump. When a steering input is made from the driver, the rotary valve allows high-pressure fluid to enter the appropriate side of the piston, and aid in turning the wheels. By maintaining a direct connection between the steering column and the rack and pinion, the car can be steered even if the hydraulic system fails. Figure 12.32 Precision translation table. (UniSlide1 from Velmex, Inc). Pinion Steering column To reservoir From pump Rotary valve Fluid lines Rack Piston Figure 12.33 Schematic diagram of a power steering system. 12.3 Actuators 535
536Chapter12Mechatronics:Sensors,Actuators,andControlsCompressedairinletsPistonsealsPiston rodRodseals andbearingFigure12.34 Construction of a pneumatic cylinder.(Courtesy of ParkerHannifin,Inc.PneumaticActuatorsWhen compressed air is the energy source of choice, a pneumatic cylinder can create linear motion.Ingeneral, the purpose of a pneumaticcylinder is to provide linear motion between two fixedlocations.Figure12.34shows a pneumatic cylinder and acutawayof sucha cylinder.Byapplyinghigh-pressure compressed air to either side of the piston, linear actuation between two definedpositions can easilybeaccomplished.Soleniods"Solenoid"is a term used to describe an electromagnetic device that is employed to create linearmotion of a plunger,as shown inFigure 12.35.The initial force availablefrom a solenoid can bedeterminedfromSNIA(12.21)F82
E1C12 09/14/2010 13:54:12 Page 536 Pneumatic Actuators When compressed air is the energy source of choice, a pneumatic cylinder can create linear motion. In general, the purpose of a pneumatic cylinder is to provide linear motion between two fixed locations. Figure 12.34 shows a pneumatic cylinder and a cutaway of such a cylinder. By applying high-pressure compressed air to either side of the piston, linear actuation between two defined positions can easily be accomplished. Soleniods ‘‘Solenoid’’ is a term used to describe an electromagnetic device that is employed to create linear motion of a plunger, as shown in Figure 12.35. The initial force available from a solenoid can be determined from F ¼ 1 2 ð Þ NI 2 mA d2 ð12:21Þ Figure 12.34 Construction of a pneumatic cylinder. (Courtesy of Parker Hannifin, Inc.) 536 Chapter 12 Mechatronics: Sensors, Actuators, and Controls
53712.3ActuatorsPlungeC-frameFigure 12.35 Construction of a solenoid linear actuator.whereF=force on plungerN-number of turns of wirein theelectromagneticI=currentμ=magneticpermeability of air (4×10-7H/m)S= size of the air gapA=plungercross-sectionalareaWhen the electromagnetis actuated, theresulting magnetic force pulls the plunger into the C-frame.Because the air gapis largestwhen the electromagnet is actuated,the minimum force occurs atactuation and the force increases as the air gap decreases.Rotary ActuatorsStepper MotorsThere is a class of electric motors that has the primarypurpose of providing power to a process.Anexample wouldbethe electricmotorthatdrives an elevator,anescalator,or a centrifugal blower.Inthese applications the electric motor serves as a prime mover, with clear and specific requirementsfor rotational speed, torque, and power. However, some applications have stringent requirements forpositioning.Rotary positioning presents a significant engineering challenge, but one that is so ubiquitousthat it has been addressed through a variety of design strategies. One design strategy is to employ afree-rotatingDCmotorto supplythe motive power and imposeprecisecontrol on the resultingmotion through gearing and some control scheme.DC motors that are subject to feedback controlare generally described as servo-motors. While this may be appropriate and necessary for someapplications, the stepper motor has found wide-ranging applications in precision rotary motioncontrol, and is a better choice for many applications.Stepperor steppingmotors,astheirnameimplies,arecapableofmovingafraction ofarotationwith a great degree of precision. This is accomplished by the design of a rotor that aligns with themagnetic field generated by energized coils.The step size can range from 90 degrees to as little as0.5 degrees or less.Two common types of steppermotors are variable reluctance and unipolardesigns. The design of a variable reluctance stepping motor is illustrated in Figure 12.36. Let'sconsider the operation of this motor.There are three sets of windings, labeled 1, 2, and 3 in thefigure, and there are two sets of teeth on the rotor, labeled X and Y. With the windings labeled 1energized, the rotor snaps to a position where one set of the teeth are aligned with the windings.Thismotion is a result of the magnetic field generated by the windings. Suppose that winding 1 is
E1C12 09/14/2010 13:54:13 Page 537 where F ¼ force on plunger N ¼ number of turns of wire in the electromagnetic I ¼ current m ¼ magnetic permeability of air (4p 107 H=m) d ¼ size of the air gap A ¼ plunger cross-sectional area When the electromagnet is actuated, the resulting magnetic force pulls the plunger into the C-frame. Because the air gap is largest when the electromagnet is actuated, the minimum force occurs at actuation and the force increases as the air gap decreases. Rotary Actuators Stepper Motors There is a class of electric motors that has the primary purpose of providing power to a process. An example would be the electric motor that drives an elevator, an escalator, or a centrifugal blower. In these applications the electric motor serves as a prime mover, with clear and specific requirements for rotational speed, torque, and power. However, some applications have stringent requirements for positioning. Rotary positioning presents a significant engineering challenge, but one that is so ubiquitous that it has been addressed through a variety of design strategies. One design strategy is to employ a free-rotating DC motor to supply the motive power and impose precise control on the resulting motion through gearing and some control scheme. DC motors that are subject to feedback control are generally described as servo-motors. While this may be appropriate and necessary for some applications, the stepper motor has found wide-ranging applications in precision rotary motion control, and is a better choice for many applications. Stepper or stepping motors, as their name implies, are capable of moving a fraction of a rotation with a great degree of precision. This is accomplished by the design of a rotor that aligns with the magnetic field generated by energized coils. The step size can range from 90 degrees to as little as 0.5 degrees or less. Two common types of stepper motors are variable reluctance and unipolar designs. The design of a variable reluctance stepping motor is illustrated in Figure 12.36. Let’s consider the operation of this motor. There are three sets of windings, labeled 1, 2, and 3 in the figure, and there are two sets of teeth on the rotor, labeled X and Y. With the windings labeled 1 energized, the rotor snaps to a position where one set of the teeth are aligned with the windings. This motion is a result of the magnetic field generated by the windings. Suppose that winding 1 is Coil C-frame Plunger Figure 12.35 Construction of a solenoid linear actuator. 12.3 Actuators 537
538Chapter12Mechatronics:Sensors,Actuators,andControlsNSFigure 12.37 Variable reluctance stepper motorFigure 12.36 Variable reluctance stepper motordesign having six poles and two windings.design.de-energized and winding 2 is energized. The rotor will turn until the teeth marked Y are alignedwith winding 2. This produces a 30-degree step.A useful characteristic of stepper motors is holding torque.As long as one of the windings isenergized, the rotor resists motion, until the torque produced by the winding to rotor interaction isovercome.The motor shown in Figure 12.37 is a variable reluctance design. Unipolar motors incor-porate permanent magnets as the rotor. Figure 12.37 shows a rotor having six magnetic poles andtwo sets of windings. The motor moves in 30-degree increments as the windings are alternatelyenergized.Flow-ControlValvesValves are mechanical devices intended to allow, restrict, throttle, or meter fluid flow through pipesorconduits.Flow-control valves are used to regulate eitherthe flow or thepressure of a fluid bytheirelectronic actuation. They generally function by allowing flow while in their open position, stoppingflow when closed, and metering flow or fluid pressure to a desired value with a position that issomewherebetweenthesesettings,whichiscalledproportional control.Thesevalvescontain avalvepositioning element that is driven by an actuator, such as a solenoid.Any valve type can becontrolled.Thecommon control valvedesignofferseithera singlechamberbody containingapoppet with valve seat or a multichamber body containing a sliding spool with multiple poppets.Flow-control valves are used to transfer gases, liquids, and hydraulic fluids.The application ratingsare as follows: general service, for working with common liquids and gases; cryogenic service, forfluids such as liquid oxygen; vacuum service, for low pressure applications; and oxygen service,forcontamination-freeflowofoxygen.The control valve can respond to signals from any type of process variable transducer. Thesignal determines the position of the actuating solenoid. A specific characteristic of any controlvalve refers to whether its nonenergized operating state is open or closed. This is referred to as its"fail position."The fail position of a control valve is determined by the nonenergized solenoidplunger position.This position is an important consideration for process safety.These valves come in various configurations reflecting their number of ports.A two-way valvehas twoports.Two-way position control takes on one of twovalues:open orclosed.Atwo-wayvalvehastwoconnections: supplyport (P)and serviceport(A).Most common household valves fall
E1C12 09/14/2010 13:54:13 Page 538 de-energized and winding 2 is energized. The rotor will turn until the teeth marked Y are aligned with winding 2. This produces a 30-degree step. A useful characteristic of stepper motors is holding torque. As long as one of the windings is energized, the rotor resists motion, until the torque produced by the winding to rotor interaction is overcome. The motor shown in Figure 12.37 is a variable reluctance design. Unipolar motors incorporate permanent magnets as the rotor. Figure 12.37 shows a rotor having six magnetic poles and two sets of windings. The motor moves in 30-degree increments as the windings are alternately energized. Flow-Control Valves Valves are mechanical devices intended to allow, restrict, throttle, or meter fluid flow through pipes or conduits. Flow-control valves are used to regulate either the flow or the pressure of a fluid by their electronic actuation. They generally function by allowing flow while in their open position, stopping flow when closed, and metering flow or fluid pressure to a desired value with a position that is somewhere between these settings, which is called proportional control. These valves contain a valve positioning element that is driven by an actuator, such as a solenoid. Any valve type can be controlled. The common control valve design offers either a single chamber body containing a poppet with valve seat or a multichamber body containing a sliding spool with multiple poppets. Flow-control valves are used to transfer gases, liquids, and hydraulic fluids. The application ratings are as follows: general service, for working with common liquids and gases; cryogenic service, for fluids such as liquid oxygen; vacuum service, for low pressure applications; and oxygen service, for contamination-free flow of oxygen. The control valve can respond to signals from any type of process variable transducer. The signal determines the position of the actuating solenoid. A specific characteristic of any control valve refers to whether its nonenergized operating state is open or closed. This is referred to as its ‘‘fail position.’’ The fail position of a control valve is determined by the nonenergized solenoid plunger position. This position is an important consideration for process safety. These valves come in various configurations reflecting their number of ports. A two-way valve has two ports. Two-way position control takes on one of two values: open or closed. A two-way valve has two connections: supply port (P) and service port (A). Most common household valves fall 1 1 X X Y Y 2 3 3 2 Figure 12.36 Variable reluctance stepper motor design. 2 2 1 S S S S N N N N 1 Figure 12.37 Variable reluctance stepper motor design having six poles and two windings. 538 Chapter 12 Mechatronics: Sensors, Actuators, and Controls