ASHRAE FUNDAMENTALS IP CH 7-2017 Fundamentals of Control.pdf

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1、7.1CHAPTER 7FUNDAMENTALS OF CONTROLGENERAL 7.1Terminology . 7.1Types of Control Action . 7.2Classification of Control Components by Energy Source . 7.4CONTROL COMPONENTS 7.4Control Devices . 7.4Sensors and Transmitters. 7.9Controllers . 7.11Auxiliary Control Devices . 7.12COMMUNICATION NETWORKS FOR

2、BUILDING AUTOMATION SYSTEMS . 7.14Communication Protocols 7.15OSI Network Model 7.15Network Structure 7.15Specifying Building Automation System Networks. 7.18Approaches to Interoperability 7.18SPECIFYING BUILDING AUTOMATION SYSTEMS 7.18COMMISSIONING 7.19Tuning 7.19Codes and Standards 7.21UTOMATIC co

3、ntrol systems are designed to maintain tempera-A ture, humidity, pressure, energy use, power, lighting levels, andsafe levels of indoor contaminants. Automatic control primarily mod-ulates actuators; stages modes of action; or sequences the mechanicaland electrical equipment on and off to satisfy lo

4、ad requirements, pro-vide safe equipment operation, and maintain safe building contami-nant levels. Automatic control systems can use digital, pneumatic,mechanical, electrical, and electronic control devices. Human inter-vention often involves scheduling equipment operation and adjustingcontrol set

5、points, but also includes tracking trends and programmingcontrol logic algorithms to fulfill building needs.This chapter focuses on the fundamental concepts and devices nor-mally used by a control system designer. It covers (1) control funda-mentals, including terminology; (2) types of control compo

6、nents; (3)methods of connecting components to form various individual controlloops, subsystems, or networks; and (4) commissioning and opera-tion. Chapter 47 of the 2015 ASHRAE HandbookHVAC Applica-tions discusses the design of controls for specific HVAC applications.1. GENERAL1.1 TERMINOLOGYAn open

7、-loop control does not have a direct feedback linkbetween the value of the controlled variable and the controller.Open-loop control anticipates the effect of an external variable onthe system and adjusts its output to minimize the expected deviationof the controlled variable from its set point. An e

8、xample is an out-door thermostat arranged to control heat to a building in proportionto the calculated load caused by changes in outdoor temperature. Inessence, the designer presumes a fixed relationship between outdoorair temperature. The actual space temperature has no effect on thiscontroller. Be

9、cause there is no feedback on the controlled variable(space temperature), the control is an open loop.A closed-loop or feedback control measures actual changes inthe controlled variable and actuates the controlled device to bringabout a change. The corrective action may continue until the con-trolle

10、d variable is at set point or within a prescribed tolerance. Thisarrangement of having the controller respond to the value of the con-trolled variable is known as feedback.Every closed loop must contain a sensor, a controller, and a con-trolled device that will affect the sensor reading(s). Figure 1

11、 showsthe components of the typical control loop. The sensor measures thecontrolled variable and transmits to the controller a signal (pneu-matic, electric, or electronic) having a pressure, voltage, or currentvalue related by a known function to the value of the variable beingmeasured. The controll

12、er compares this value with the set point andsignals to the controlled device for corrective action. A controllercan be hardware or software. A hardware controller is an analogdevice (e.g., thermostat, humidistat, pressure control) that continu-ously receives and acts on data. A software controller

13、is a digitaldevice (e.g., digital algorithm) that receives and acts on data on asample-rate basis.The controlled variable is the temperature, humidity, pressure,or other condition being controlled.The set point is the desired value of the controlled variable. Thecontroller seeks to maintain this set

14、 point. The controlled devicereacts to signals from the controller to vary the control agent.The controlled device is typically a valve, damper, heating ele-ment, or variable-speed drive.The control agent is the medium manipulated by the controlleddevice. It may be air or gas flowing through a dampe

15、r; gas, steam, orwater flowing through a valve; or an electric current.The process is the HVAC apparatus being controlled, such as acoil, fan, or humidifier. It reacts to the control agents output andeffects the change in the controlled variable.Both open and closed control loops can be represented

16、in theform of a block diagram, in which each component is modeled andrepresented in its own block. Figure 2 is a block diagram of theclosed loop shown in Figure 1. Information flow from one compo-nent to the next is shown by lines between the blocks. The figureshows the set point being compared to t

17、he controlled variable. Thedifference is the error. If the error persists, it may be called offset,The preparation of this chapter is assigned to TC 1.4, Control Theory andApplication.Fig. 1 Example of Feedback Control: Discharge Air Temperature Control7.2 2017 ASHRAE HandbookFundamentals drift, dev

18、iation, droop, or steady-state error. The error is fed into thecontroller, which sends an output signal to the controlled device (inthis case, a valve that can change the amount of steam flow throughthe coil of Figure 1). The amount of steam flow is the input to thenext block, which represents the p

19、rocess. From the process blockcomes the controlled variable, which is temperature. The con-trolled variable is sensed by the sensing element and fed to the con-troller as feedback, completing the loop.Control loop performance is greatly affected by time lags, whichare delay periods associated with s

20、eeing a control agent changereflected in the desired end-point condition. Time lags can causecontrol and modeling problems and should be understood and evalu-ated carefully. There are two types of time lags: first-order lags anddead time.First-order lags involve the time it takes for the change to b

21、eabsorbed by the system. If heat is supplied to a cold room, the roomheats up gradually, even though heat may be applied at the maxi-mum rate. The time constant is the unit of measure used to describefirst-order lags and is defined as the time it takes for the controlledvariable of a first-order, li

22、near system to reach 63.2% of its finalvalue when a step change in the input occurs. Components withsmall time constants alter their output rapidly to reflect changes inthe input; components with a larger time constant are sluggish inresponding to input changes.Dead time (or time lag) is the time fr

23、om when a change in thecontroller output is made to when the controlled variable exhibits ameasurable response. Dead time can occur in the control loop ofFigure 1 because of the transportation time of the air from the coilto the space. After a coil temperature changes, there is dead timewhile the su

24、pply air travels the distribution system and finallyreaches the sensor in the space. The mass of air in the space furtherdelays the coil temperature changes effect on the controlled vari-able (space temperature). Dead time can also be caused by a slowsensor or a time lag in the signal from the contr

25、oller when it firstbegins to affect the output of the process. Dead time is most oftenassociated with the time it takes to transport the media changed by thecontrol agent from one place to another. Dead time may also be inad-vertently added to a control loop by a digital controller with an exces-siv

26、e scan time. If the dead time is small, it may be ignored in thecontrol system model; if it is significant, it must be considered.Figure 1 depicts the mechanisms that create both first-order anddead-time lags, and Figure 3 shows the effect related to time. Deadtime is the time it takes warmer air re

27、sulting from a higher set pointto reach the space, followed by the first-order lag created by the wallon which the thermostat is mounted, and that of the temperaturesensor (all of which warm gradually rather than all at once). Thecontrol loop must be tuned to account for the combined effect ofeach t

28、ime lag. Note that, in most HVAC systems, the first-order lagelement predominates.The gain of a transfer function is the amount the output of thecomponent changes per unit of change of input under steady-stateconditions. If the element (valve, damper, and/or temperature/pres-sure differential) is li

29、near, its gain remains constant. However, manycontrol components are nonlinear and have gains that depend on theoperating conditions. Figure 3 shows the response of the first-order-plus-dead-time process to a step change of the input signal. Notethat the process shows no reaction during dead time, f

30、ollowed by aresponse that resembles a first-order exponential.1.2 TYPES OF CONTROL ACTIONControl loops can be classified by the adjustability of the con-trolled device. A two-position controlled device has two operatingstates (e.g., open and closed), whereas a modulating controlled de-vice has a con

31、tinuous range of operating states (e.g., 0 to 100% open).Two-Position ActionThe control device shown in Figure 4 can be positioned only toa maximum or minimum state (i.e., on or off). Because two-positioncontrol is simple and inexpensive, it is used extensively for bothindustrial and commercial cont

32、rol. A typical home thermostat thatstarts and stops a furnace is an example.Controller differential, as it applies to two-position controlaction, is the difference between a setting at which the controlleroperates to one position and a setting at which it operates to theother. Thermostat ratings usu

33、ally refer to the differential (indegrees) that becomes apparent by raising and lowering the dial set-ting. This differential is known as the manual differential of thethermostat. When the same thermostat is applied to an operatingsystem, the total change in temperature that occurs between a “turn-o

34、n” state and a “turn-off” state is usually different from the mechan-ical differential. The operating differential may be greater becauseof thermostat lag or hysteresis, or less because of heating or coolinganticipators built into the thermostat.Anticipation Applied to Two-Position Action. This comm

35、onvariation of strictly two-position action is often used on room ther-mostats to reduce the operating differential. In heating thermostats,a heater element in the thermostat is energized during on periods,thus shortening the on time because the heater warms the thermostat(heat anticipation). The sa

36、me anticipation action can be obtainedin cooling thermostats by energizing a heater thermostat at off peri-ods. In both cases, the percentage of on time is varied in proportionto the load, and the total cycle time remains relatively constant.Fig. 2 Block Diagram of Discharge Air Temperature ControlF

37、ig. 3 Process Subjected to Step Input Fig. 4 Two-Position ControlFundamentals of Control 7.3Modulating ControlWith modulating control, the controllers output can vary over itsentire range. The following terms are used to describe this type ofcontrol:Throttling range is the amount of change in the co

38、ntrolled vari-able required to cause the controller to move the controlled devicefrom one extreme to the other. It can be adjusted to meet jobrequirements. The throttling range is inversely proportional toproportional gainControl point is the actual value of the controlled variable atwhich the instr

39、ument is controlling. It varies within the controllersthrottling range and changes with changing load on the system andother variables.Offset, or error signal, is the difference between the set point andactual control point under stable conditions. This is sometimescalled drift, deviation, droop, or

40、 steady-state error.In each of the following examples of modulating control, there isa set of parameters that quantifies the controllers response. Thevalues of these parameters affect the control loops speed, stability,and accuracy. In every case, control loop performance depends onmatching (or tuni

41、ng) the parameter values to the characteristics ofthe system under control.Proportional Control. In proportional control, the controlleddevice is positioned proportionally in response to changes in thecontrolled variable (Figure 5). A proportional controller can bedescribed mathematically byVp= Kpe

42、+ Vo(1)whereVp= controller outputKp= proportional gain parameter (inversely proportional to throttling range)e = error signal or offsetVo= offset adjustment parameterThe controller output is proportional to the difference between thesensed value, the controlled variable, and its set point. The con-t

43、rolled device is normally adjusted to be in the middle of its controlrange at set point by using an offset adjustment. This control is sim-ilar to that shown in Figure 5.Proportional plus Integral (PI) Control. PI control improveson simple proportional control by adding another component to thecontr

44、ol action that eliminates the offset typical of proportional con-trol (Figure 6). Reset action may be described byVp= Kpe + Ki+ Vo(2)whereKi= integral gain parameter = timeThe second term in Equation (2) implies that the longer error e exists,the more the controller output changes in attempting to e

45、liminate theerror. Proper selection of proportional and integral gain constantsincreases stability and eliminates offset, giving greater control accu-racy.Proportional-Integral-Derivative (PID) Control. This is PIcontrol with a derivative term added to the controller. It varies withthe value of the

46、derivative of the error. The equation for PID control isVp= Kpe + Ki+ Vo(3)whereKd= derivative gain parameter of controllerde/d = time derivative of errorAdding the derivative term gives some anticipatory action to thecontroller, which results in a faster response and greater stability.However, the

47、derivative term also makes the controller more sensi-tive to noisy signals and harder to tune than a PI controller. MostHVAC control loops perform satisfactorily with PI control alone.Adaptive Control. An adaptive controller adjusts the parametersthat define its response as the dynamic characteristi

48、cs of the processchange. If the controller is PID based, then it adjusts feedback gains.An adaptive controller may be based on other feedback rules. Thekey is that it adjusts its parameters to match the characteristics of theprocess. When the process changes, the tuning parameters change tomatch it.

49、 Adaptive control is applied in HVAC systems because nor-mal variations in operating conditions affect the characteristics rel-evant to tuning. For instance, the extent to which zone dampers areopen or closed in a VAV system affects the way duct pressureresponds to fan speed, and entering fluid temperatures at a coil affectthe way the leaving temperature responds to the valve position. Neu-ral networks and self-learning performance-predictive control-lers are sophisticated adaptive controllers.Fuzzy Logic. This type of control offers an alternative to tradi-tional control algori