introductionstotemperaturecontrol(溫度控制簡(jiǎn)介和pid控制器外文翻譯-文庫吧資料
2024-11-11 06:13本頁面
【正文】 iable or the setpoint quickly changesthe PID controller has to quickly change the output to get the process variable back equal to the setpoint. If you have a walkin cooler with a PID controller and someone opens the door and walks in, the temperature (process variable) could rise very quickly. Therefore the PID controller has to increase the cooling (output) to pensate for this rise in temperature. Once the PID controller has the process variable equal to the setpoint, a good PID controller will not vary the output. You want the output to be very steady (not changing) . If the valve (motor, or other control element) is constantly changing, instead of maintaining a constant value, this could cause more wear on the control element. So there are these two contradictory goals. Fast response (fast change in output) when there is a process upset, but slow response (steady output) when the PV is close to the setpoint. Note that the output often goes past (over shoots) the steadystate output to get the process back to the setpoint. For example, a cooler may normally have its cooling valve open 34% to maintain zero degrees (after the cooler has been closed up and the temperature settled down). If someone opens the cooler, walks in, walks around to find something, then walks back out, and then closes the cooler doorthe PID controller is freaking out because the temperature may have raised 20 degrees! So it may crank the cooling valve open to 50, 75, or even 100 percentto hurry up and cool the cooler back downbefore slowly closing the cooling valve back down to 34 percent. Let39。 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 the correct (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 PV to 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 onetenth 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) = degrees. [~] 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 beingwith 1/10 of a degree theoretical accuracyeven with the usual amount of noise and other problems one 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 simplywhat process value do you want. In this examplewhat temperature do you want the process at? The PID controller39。Introductions to temperature control
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