53912.3ActuatorsSliding spoolPoppetSolenoidactuator.800000Port1 (P)2 (T)3 (A)Figure 12.38 Three-way flow control valve (deactivated position shown).into this category.A three-wayvalve has three portconnections: supply (P),exhaust (T),and service(A).The service port may be switched between the supply and the exhaust.A four-way has fourconnections: supply (P), exhaust (T), and two service ports (A and B). The valve connects either P toA and B to T, or P to B and A to T. In general, an N-way valve has N-ports with N number of flowdirections available.An example ofa three-port sliding spool control valve is shown in Figure 12.38.The solenoid drives the spool, which contains two valve seats. In thefully activated position,portPis open to service port A. When the solenoid is deactivated, port Tis open to service port A (shown).For example,in one application this valve can beused to pressurize a system (open the system toport P)for a period of time and then adjust the systempressure to another value (open the system toport A) for a period of time.All valveports offer some level of flow resistance,and this is specified through a flowcoefficient, C Flow resistance can be adjusted in design by varying the internal dimensions of thevalve chamber and can be set operationallyby varying theelement position within the chamber.Theflow coefficient is found based on the formulation detailed in Chapter10 or simply asQ=CVAp(12.22)where Q is the steady flow rate through the valve and p is the corresponding pressure drop. Thisloss is also expressed in terms of a K-factor based on the average velocity through theportsAp=Kpu/2(12.23)Flow-control valves are classified in a number of ways: thetype ofcontrol, the number of portsin the valve housing, the specific function of the valve, and the type of valve element used in theconstruction of the valve.Directional control valves allow or prevent the flow of fluid throughdesignated ports. Flow can move in either direction. Check valves are a special class of directionalvalve that allowflow in only one direction.Proportional valves can be infinitely positioned tocontrolthe amount,pressure, and direction of fluid flow.In a proportional valve, the valve is opened by anamount proportional to the applied current.Thevalves are termed proportional because their outputflow is not exactly linear in relation to the input signal.These valves provide a way to controlpressure or flow rate with a high response rate.In the simplest application, a solenoid is used to turn a valve either on or off.In a moredemanding application, the solenoid is expected to cycle rapidly to open and close the valve.The
E1C12 09/14/2010 13:54:13 Page 539 into this category. A three-way valve has three port connections: supply (P), exhaust (T), and service (A). The service port may be switched between the supply and the exhaust. A four-way has four connections: supply (P), exhaust (T), and two service ports (A and B). The valve connects either P to A and B to T, or P to B and A to T. In general, an N-way valve has N-ports with N number of flow directions available. An example of a three-port sliding spool control valve is shown in Figure 12.38. The solenoid drives the spool, which contains two valve seats. In the fully activated position, port P is open to service port A. When the solenoid is deactivated, port T is open to service port A (shown). For example, in one application this valve can be used to pressurize a system (open the system to port P) for a period of time and then adjust the system pressure to another value (open the system to port A) for a period of time. All valve ports offer some level of flow resistance, and this is specified through a flow coefficient, Cv. Flow resistance can be adjusted in design by varying the internal dimensions of the valve chamber and can be set operationally by varying the element position within the chamber. The flow coefficient is found based on the formulation detailed in Chapter 10 or simply as Q ¼ Cv ffiffiffiffiffiffi Dp p ð12:22Þ where Q is the steady flow rate through the valve and Dp is the corresponding pressure drop. This loss is also expressed in terms of a K-factor based on the average velocity through the ports, Dp ¼ KrU 2 =2 ð12:23Þ Flow-control valves are classified in a number of ways: the type of control, the number of ports in the valve housing, the specific function of the valve, and the type of valve element used in the construction of the valve. Directional control valves allow or prevent the flow of fluid through designated ports. Flow can move in either direction. Check valves are a special class of directional valve that allow flow in only one direction. Proportional valves can be infinitely positioned to control the amount, pressure, and direction of fluid flow. In a proportional valve, the valve is opened by an amount proportional to the applied current. The valves are termed proportional because their output flow is not exactly linear in relation to the input signal. These valves provide a way to control pressure or flow rate with a high response rate. In the simplest application, a solenoid is used to turn a valve either on or off. In a more demanding application, the solenoid is expected to cycle rapidly to open and close the valve. The Port 3 (A) Sliding spool Poppet Solenoid actuator Port 1 (P) Port 2 (T) Figure 12.38 Three-way flow control valve (deactivated position shown). 12.3 Actuators 539
540Chapter12Mechatronics:Sensors,Actuators,andControlstime between each signal cyclecoupled with the internalflowloss characterofthevalvedeterminesthe averageflow.Valveresponsetimecanbedefined in several waysbut all are consistent withthemethods used in Chapter 3. The 90% response time, tgo, is the time required to eitherfill or exhaust atarget device chamber through a valveport, in effect a step function response.There is a separateresponse time for filling or exhausting. Either way,(12.24)t9o=m+FVwhere m isthevalvelagtimebetween when the signal is applied and steadyflow is established at thedesignated port, F is the reciprocal of the average flow rate through the port, and V is the volume ofthetargetdevicechamber.For example,a valve having anFof 0.54ms/cc and a lagtime of 20msrequires tgo=155ms tofill a 250-cc chamber.Altermatively,the valve frequency response can befoundbycyclingthevalvewithasinewaveelectricalsignalandmeasuringtheflowratethroughthevalve.Thevalvefrequency bandwidth is thus established by its -3 dB point.12.4CONTROLSControl of a process or systemcan be exerted in a wide varietyof ways.Suppose ourgoal is tocreatea healthy lawn by appropriate watering.Each day we could monitor the weather forecast,take intoaccount theprobability of precipitation,and choose whethertowaterand forhowlong.We couldchoose to water all of the lawn or just those parts most subject to stress from heat and lack ofmoisture.If we choose to water,we could place the sprinklers and turn on the faucet (rememberingto shut off the flow at an appropriate later time)!All of thefunctions describedaboveforlawn care arecompletelyreasonablefor aperson toaccomplish,and they represent the functioning of an intelligent controller. Suppose we wish tointroduce some automation into the process.Atthesimplestlevel,a timer-basedcontrol systemcouldbe implemented,as shown inFigure12.39.The functioning of this system would be to open and close the faucet at predetermined times of the day.Atthesimplestlevel,thiscouldbeamechanicaltimerthatwateredthelawnonceeach24-hourperiodforapredetermined length oftime.Thistype of control is called an open loopcontrol.For this controlWater supplyTimeTo sprinklersFigure 12.39Open-loop control of a sprinkler system
E1C12 09/14/2010 13:54:13 Page 540 time between each signal cycle coupled with the internal flow loss character of the valve determines the average flow. Valve response time can be defined in several ways but all are consistent with the methods used in Chapter 3. The 90% response time, t90, is the time required to either fill or exhaust a target device chamber through a valve port, in effect a step function response. There is a separate response time for filling or exhausting. Either way, t90 ¼ m þ F8 ð12:24Þ where m is the valve lag time between when the signal is applied and steady flow is established at the designated port, F is the reciprocal of the average flow rate through the port, and 8 is the volume of the target device chamber. For example, a valve having an F of 0.54 ms/cc and a lag time of 20 ms requires t90 ¼ 155 ms to fill a 250-cc chamber. Alternatively, the valve frequency response can be found by cycling the valve with a sine wave electrical signal and measuring the flow rate through the valve. The valve frequency bandwidth is thus established by its 3 dB point. 12.4 CONTROLS Control of a process or system can be exerted in a wide variety of ways. Suppose our goal is to create a healthy lawn by appropriate watering. Each day we could monitor the weather forecast, take into account the probability of precipitation, and choose whether to water and for how long. We could choose to water all of the lawn or just those parts most subject to stress from heat and lack of moisture. If we choose to water, we could place the sprinklers and turn on the faucet (remembering to shut off the flow at an appropriate later time)! All of the functions described above for lawn care are completely reasonable for a person to accomplish, and they represent the functioning of an intelligent controller. Suppose we wish to introduce some automation into the process. At the simplest level, a timer-based control system could beimplemented, as shown in Figure 12.39. The functioning of this system would be to open and close the faucet at predetermined times of the day. Atthe simplestlevel, this could be a mechanicaltimer that watered the lawn once each 24-hour period for a predetermined length of time. This type of control is called an open loop control. For this control Water supply Timer To sprinklers Figure 12.39 Open-loop control of a sprinkler system. 540 Chapter 12 Mechatronics: Sensors, Actuators, and Controls
54112.4ControlsWater supplyClocktimerCCTotalizingflowmeterTo sprinklersFigure12.40Feedbackcontrolofa sprinklersystemsystem there areno sensorsto monitorthe amount ofwaterappliedtothelawn; infact,all that thecontrolsystem is accomplishing is to open and, later, to close the faucet.More advanced automatic control systems implement a closed-loop control.For the presentexample, it might be desired to apply 100 gallons (379 liters) of water to the lawn. A flow meter thatsensedthetotal waterflowthat hadoccurredcould be usedtoprovidefeedback tothecontrol systemtoallowthefaucettobeclosed when theflowtotaled 100gallons.Suchflowmeters arecommon and serveas water meters. The term closed-loop or feedback control simply means that the variable that is to becontrolledis being measured,and that thecontrol system in some way uses this measurementto exert thecontrol.Figure 12.40 llustrates a control system designed to apply 100 gallons of water to the lawn.Thereare two inputs to the controller: the time of day and the output of the totalizing flow meter. At theappropriate time of day,the controller opens the valve.The totalizing flow meter output is used by thecontrollertoclosethevalveafterthetotal flowreaches100gallons.Thistypeofbinarycontrol schemeis termed on-off control.Thevalve controlling the water flow is eitherfully open or fully closed.Probably the most familiar form of a binary on-off control system is the thermostatfor a homefurnace or air conditioner.Figure 12.41 shows the status of a home furnace and a time trace of theinside temperature during a winter day. A schematic representation of this control system is shownFurnaceOnOnoffoffsuroanceInsideDeadbandtemperatureFigure 12.41 Operation of on-offcontroller with a dead band
E1C12 09/14/2010 13:54:13 Page 541 system there are no sensors to monitor the amount of water applied to the lawn; in fact, all that the control system is accomplishing is to open and, later, to close the faucet. More advanced automatic control systems implement a closed-loop control. For the present example, it might be desired to apply 100 gallons (379 liters) of water to the lawn. A flow meter that sensed the total water flow that had occurred could be used to provide feedback to the control system to allow the faucet to be closed when the flow totaled 100 gallons. Such flow meters are common and serve as water meters. The term closed-loop or feedback control simply means that the variable that is to be controlled is being measured, and that the control system in some way uses this measurement to exert the control. Figure 12.40 illustrates a control system designed to apply 100 gallons of water to the lawn. There are two inputs to the controller: the time of day and the output of the totalizing flow meter. At the appropriate time of day, the controller opens the valve. The totalizing flow meter output is used by the controller to close the valve after the total flow reaches 100 gallons. This type of binary control scheme is termed on–off control. The valve controlling the water flow is either fully open or fully closed. Probably the most familiar form of a binary on–off control system is the thermostat for a home furnace or air conditioner. Figure 12.41 shows the status of a home furnace and a time trace of the inside temperature during a winter day. A schematic representation of this control system is shown Water supply To sprinklers Clock timer Totalizing flow meter Figure 12.40 Feedback control of a sprinkler system. Inside temperature Furnace On Off Off Disturbance Dead band On Figure 12.41 Operation of on–off controller with a dead band. 12.4 Controls 541