楼宇第04次课-楼宇第01次课-CHAPTER-1-CONTROL-FUNDAMENTALS-(4课件.ppt
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- 楼宇 04 01 CHAPTER CONTROL FUNDAMENTALS 课件
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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
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