1、CHAPTER 1CONTROL FUNDAMENTALS (4)Step Control Step controllers operate switches or relays in sequence to enable or disable multiple outputs,or stages,of two-position devices such as electric heaters or reciprocating refrigeration compressors.Step control uses an analog signal to attempt to obtain pr
2、oportional output from equipment that is typically either on or off.Figures 25 and 26 show that the stages may be arranged to operate with or without overlap of the operating(on/off)differentials.In either case,the typical two-position differentials stillFig.25.Electric Heat Stages.exist but the tot
3、al output is proportioned.Fig.26.Staged Reciprocating Chiller Control.Figure 27 shows step control of sequenced DX coils and electric heat.On a rise in temperature through the throttling range at the thermostat,the heating stages sequence off.On a further rise after a dead band,the cooling stages tu
4、rn on in sequence.A variation of step control used to control electric heat is step plus-proportional control,which provides a smooth transition between stages.This control mode requires one of the stages to be a proportional modulating outputand the others,two-position.For most efficient operation,
5、the proportional modulating stage should have at least the same capacity as one two-position stage.Fig.27.Step Control with Sequenced DX Coils and Electric Heat.Starting from no load,as the load on the equipment increases,the modulating stage proportions its load until it reaches full output.Then,th
6、e first two-position stage comes full on and the modulating stage drops to zero output and begins to proportion its output again to match the increasing load.When the modulating stage again reaches full output,the second two-position stage comes full on,the modulating stage returns to zero,and the s
7、equence repeats until all stages required to meet the load condition are on.On a decrease in load,the process reverses.Floating Control Floating control is a variation of two-position control and is often called three-position control.Floating control is not a common control mode,but is available in
8、 most microprocessor-based control systems.A variation of floating control is proportional-speed-floating control.Floating control requires a slow-moving actuator and a fast responding sensor selected according to the rate of response in the controlled system.If the actuator should move too slowly,t
9、he controlled system would not be able to keep pace with sudden changes;if the actuator should move too quickly,two-position control would result.Floating control keeps the control point near the set point at any load level,and can only be used on systems with minimal lag between the controlled medi
10、um and the control sensor.Floating control is used primarily for discharge control systems where the sensor is immediately downstream from the coil,damper,or device that it controls.An example of floating control is the regulation of staticFig.28.Floating Static Pressure Control.pressure in a duct(F
11、ig.28).In a typical application,the control point moves in and out of the dead band,crossing the switch differential(Fig.29).A drop in static pressure below the controller set point causes the actuator to drive the damper toward open.The narrow differential of the controller stops the actuator after
12、 it has moved a short distance.The damper remains in this position until the static pressure further decreases,causing the actuator to drive the damper further open.Fig.29.Floating Control.On a rise in static pressure above the set point,the reverse occurs.Thus,the control point can float between op
13、en and closed limits and the actuator does not move.When the control point moves out of the dead band,the controller moves the actuator toward open or closed until the control point moves into the dead band again.In proportional-speed floating control,the farther the control point moves beyond the d
14、ead band,the faster the actuator moves to correct the deviation.Proportional Control GENERAL Proportional control proportions the output capacity of the equipment(e.g.,the percent a valve is open or closed)to match the heating or cooling load on the building,unlike two-position control in which the
15、mechanical equipment is either full on or full off.In this way,proportional control achieves the desired heat replacement or displacement rate.In a chilled water cooling system,for example(Fig.30),the sensor is placed in the discharge air.The sensor measures the air temperature and sends a signal to
16、 the controller.If a correction is required,the controller calcula-tes the change and sends a new signal to the valve actuator.The actuator repositions the valve to change the water flow in the coil,and thus the discharge temperature.Fig.30.Proportional Control Loop.In proportional control,the final
17、 control element moves to a position proportional to the deviation of the value of the controlled variable from the set point.The position of the final control element is a linear function of the value of the controlled variable(Fig.31).The final control element is seldom in the middle of its range
18、because of the linear relationship between the position of the final control element and the value of the controlled variable.In proportional control systems,the setpoint is typically the middle of the throttling range,so there is usually an offset between control point and set point.Fig.31.Final Co
19、ntrol Element Position as a Function of the Control Point(Cooling System).An example of offset would be the proportional control of a chilled water coil used to cool a space.When the cooling load is 50 percent,the controller is in the middle of its throttling range,the properly sized coil valve is h
20、alf-open,and there is no offset.As the out-door temperature increases,the room temperature rises and more cooling is required to maintain the space temperature.The coil valve must open wider to deliver the required cooling and remain in that position as long as the increased requirement exists.Becau
21、se theposition of the final control element is proportional to the amo-unt of deviation,the temperature must deviate from the set point and sustain that deviation to open the coil valve as far as required.Figure 32 shows that when proportional control is used in a heating application,as the load con
22、dition increases from 50 percent,offset increases toward cooler.As the load condition decreases,offset increases toward warmer.The opposite occurs in a cooling application.Fig.32.Relationship of Offset to Load(Heating Application).The throttling range is the amount of change in the controlled variab
23、le required for the controller to move the controlled device through its full operating range.The amount of change is expressed in degrees Celsius for temperature,in percentages for relative humidity,and in kilo pascals for pressure.For some controllers,throttling range is referred to as proportiona
24、l band.Proportional band is throttling range expressed as a percentage of the controller sensor span:Gain is a term often used in industrial control systems for the change in the controlled variable.Gain is the reciprocal of proportional band:The output of the controller is proportional to the devia
25、tion of the control point from set point.A proportional controller can be mathematically described by:V=KE+M Where:V=output signal K=proportionality constant(gain)E=deviation(control point-setpoint)M=value of the output when the deviation is zero(Usually the output value at 50 percent or the middle
26、of the output range.The generated control signal correction is added to or subtracted from this value.Also called bias or manual reset.)Although the control point in a proportional control system is rarely at setpoint,the offset may be accept-able.Compensation,which is the resetting of the setpoint
27、to compensate for varying load conditions,reduces the effect of offset for more accurate control.An example of compensation is resetting boiler water temperature based on outdoor air temperature.Compensation is also called reset control or cascade control.Compensation Control GENERAL Compensation is
28、 a control technique available in proportional control in which a secondary,or compen-sation,sensor resets the setpoint of the primary sensor.An example of Compensation would be the outdoor temperature resetting the discharge temperature of a fan system so that the discharge temperature increases as
29、 the outdoor temperature decreases.The following sample reset schedule is shown graphically in Figure 33.Figure 34 shows a control diagram for the sample reset system.Fig.33.Typical Reset Schedule for Discharge Air ControlFig.34.Discharge Air Control Loop with Reset.Compensation can either increase
30、or decrease the set point as the compensation input increases.increasing the set point by adding reset on an increase in the compen-sation variable is often referred to as positive or summer compensation.Increasing the set point by adding reset on a decrease in the compensation variable is often ref
31、erred to as negative or winter compensation.Compensation is most commonly used for temperature control,but can also be used with a humidity or other control system.Some controllers provide compensation start point capability.Compensation start point is the value of the compensation sensor at which i
32、t starts resetting the controller primary sensor set point.Proportional-Integral(PI)Control In the proportional-integral(PI)control mode,reset of the control point is automatic.PI control,also called proportional plus-reset control,virtually eliminates offset and makes the proportional band nearly i
33、nvisible.As soon as the controlled variable deviates above or below the set point and offset develops,the proportional band gradually and automatically shifts,and the variable is brought back to the set point.The major difference between proportional and PI control is that proportional control is li
34、mited to a single final control element position for each value of the controlled variable.PI control changes the final control element position to accommodate load changes while keeping the control point at or very near the setpoint.The reset action of the integral component shifts the proportional
35、 band as necessary around the setpoint as the load on the system changes.The graph in Figure 36 shows the shift of the proportional band of a PI controller controlling a normally open heating valve.The shifting of the proportional band keeps the control point at setpoint by making further correction
36、s in the control signal.Because offset is eliminated,the proportional band is usually set fairly wide to ensure system stability under all operating conditions.Fig.35.Proportional Band Shift Due to Offset.Reset of the control point is not instanta-neous.Whenever the load changes,the controlled varia
37、ble changes,producing an offset.The proportional control makes an immediate correction,which usually still leaves an offset.The integral function of the controller then makes control corrections over time to bring the control point back to setpoint(Fig.36).In addition to a proportional band adjustme
38、nt,the PI controller also has a reset time adjustment that determines the rate at which the proportional band shifts when the controlled variable deviates any given amount from the setpoint.Fig.36.Proportional-Integral Control Response to Load Changes.Reset time is proportional to the deviation of t
39、he controlled variable.For example,a four-percent deviation from the setpoint causes a continuous shift of the proportional band at twice the rate of shift for a two-percent deviation.Reset is also proportional to the duration of the deviation.Reset accumulates as long as there is offset,but ceases
40、as soon as the controlled variable returns to the setpoint.With the PI controller,therefore,the position of the final control element depends not only upon the location of the controlled variable within the proportional band(proportional band adjustment)but also upon the duration and magnitude of th
41、e deviation of the controlled variable from the setpoint(reset time adjustment).Under steady state conditions,the control point and setpoint are the same for any load conditions,as shown in Figure 36.PI control adds a component to the proportional control algorithm and is described mathematically by
42、:Where:V=output signalK=proportionality constant(gain)E=deviation(control point-setpoint)T1=reset timeK/T1=reset gaindt=differential of time(increment in time)M=value of the output when the deviation is zero Integral windup,or an excessive overshoot condition,can occur in PI control.Integral windup
43、is caused by the integral function making a continued correction while waiting for feedback on the effects of its correction.While integral action keeps the control point at setpoint during steady state conditions,large overshoots are possible at start-up or during system upsets(e.g.,setpoint change
44、s or large load changes).On many systems,short reset times also cause overshoot.Integral windup can be avoided and its effects diminished.At start-up,some systems disable integral action until measured variables are within their respective proportional bands.Systems often provide integral limits to
45、reduce windup due to load changes.The integral limits define the extent to which integral action can adjust a device(the percent of full travel).The limit is typically set at 50 percent.Proportional-Integral-Derivative(PID)Control（作业）Fig.37.Proportional ControlFig.38.Proportional-Integral Control.Fi
46、g.39.Proportional-Integral-Derivative Control Adaptive Control Adaptive control is available in some microprocessor-based controllers.Adaptive control algorithms enable a controller to adjust its response for optimum control under all load conditions.A controller that has been tuned to control accur
47、ately under one set of conditions cannot always respond well when the conditions change,such as a significant load change or changeover from heating to cooling.An adaptive control algorithm monitors the performance of a system and attempts to improve the performance by adjusting controller gains or
48、parameters.One measurement of performance is the amount of time the system requires to react to a disturbance:usually the shorter the time,the better the performance.The methods used to modify the gains or parameters are determined by the type of adaptive algorithm.An example of a good application f
49、or adaptive control is discharge temperature control of the central system cooling coil function a VAV system.The time constant of a sensor varies as a function of the velocity of the air(or other fluid).Thus the time constant of the discharge air sensor in a VAV system is constantly changing.The ch
50、ange in sensor response affect the system control so the adaptive control algorithm adjusts system parameters such as the reset and rate settings to maintain optimum system performance.Adaptive control is also used in energy management programs such as optimum start.The optimum start program enables
