PID控制器及智能控制英文文献(2)

PID controllers can be stand-alone controllers (also called single loop controllers), controllers in PLCs, embedded controllers, or software in Visual Basic or C# computer programs.

PID controllers are process controllers with the following characteristics:

_ Continuous process control

_ Analog input (also known as “measurement” or “Process Variable” or “PV”)

_ Analog output (referred to simply as “output”)

_ Setpoint (SP)

_ Proportional (P) , Integral (I) , and/or Derivative (D) constants

Examples of “continuous process control” are temperature, pressure, flow, and level control.for example, controlling the heating of a tank. For simple control, you have two temperature limit sensors (one low and one high) and then switch the heater on when the low temperature limit sensor turns on and then turn the heater off when the temperature rises to the high temperature limit sensor. This is similar to most home air conditioning & heating thermostats.

In contrast, the PID controller would receive input as the actual temperature and control a valve that regulates the flow of gas to the heater. The PID controller automatically finds thecorrect (constant) flow of gas to the heater that keeps the temperature steady at the setpoint.Instead of the temperature bouncing back and forth between two points, the temperature is held steady. If the setpoint is lowered, then the PID controller automatically reduces the amount of gas flowing to the heater. If the setpoint is raised, then the PID controller automatically increases the amount of gas flowing to the heater. Likewise the PID controller would automatically for hot, sunny days (when it is hotter outside the heater) and for cold, cloudy days.

The analog input (measurement) is called the “process variable” or “PV”. You want the PVto be a highly accurate indication of the process parameter you are trying to control. For example, if you want to maintain a temperature of ＋ or － one degree then we typically strive for at least ten times that or one-tenth of a degree. If the analog input is a 12 bit analog input and the temperature range for the sensor is 0 to 400 degrees then our “theoretical” accuracy is calculated to be 400 degrees divided by 4,096 (12 bits) ＝0.09765625 degrees. [1] We say “theoretical” because it would assume there was no noise and error in our temperature sensor, wiring, and analog converter. There are other assumptions such as linearity, etc.. The point being－with 1/10 of a degree “theoretical” accuracy－even with the usual amount of noise and other problemsone degree of accuracy should easily be attainable.

The analog output is often simply referred to as “output”. Often this is given as 0~100 percent. In this heating example, it would mean the valve is totally closed (0%) or totally open (100%) .

The setpoint (SP) is simply－what process value do you want. In this example－what temperature do you want the process at?

The PID controller’s job is to maintain the output at a level so that there is no difference (error) between the process variable (PV) and the setpoint (SP) .

In Fig. 16.1, the valve could be controlling the gas going to a heater, the chilling of a cooler, the pressure in a pipe, the flow through a pipe, the level in a tank, or any other process control system.

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