【正文】 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。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. 3, 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. What the PID controller is looking at is the difference (or error) between the PV and the SP. SETPOINT P，I，amp。D CONSTANTS Difference error PID control algorithm process variable output Fig .3 PID control
　　It looks at the absolute error and the rate of change of error. Absolute error meansis there a big difference in the PV and SP or a little difference? Rate of change of error meansis the difference between the PV or SP getting smaller or larger as time goes on. When there is a process upset, meaning, when the process variable 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。s think about how to design a PID controller. We focus on the difference (error) between the process variable (PV) and the setpoint (SP). There are three ways we can view the error.The absolute error This means how big is the difference between the PV and SP. If there is a small difference between the PV and the SPthen let39。s make a small change in the output. If there is a large difference in the PV and SPthen let39。s make a large change in the output. Absolute error is the proportional (P) ponent of the PID controller.The sum of errors over time Give us a minute and we will show why simply looking at the absolute error (proportional) only is a problem. The sum of errors over time is important and is called the integral (I) ponent of the PID controller. Every time we run the PID algorithm we add the latest error to the sum of errors. In other words Sum of Errors = Error 1 q Error2 + Error3 + Error4 + .... The dead time Dead time refers to the delay between making a change in the output and seeing the change reflected in the PV. The classical example is getting your oven at the right temperature. When you first mm on the heat, it takes a while for the oven to heat up. This is the dead time. If you set an initial temperature, wait for the oven to reach the initial temperature, and then you determine that you set the wrong temperaturethen it will take a while for the oven to reach the new temperature setpoint. This is also referred to as the derivative (D) ponent of the PID controller. This holds some future changes back because the changes in the outpu