Setting of PID parameters

In PID control, there are three parameters: P, I, and D. Only by understanding the meaning and function of these three parameters can one adjust the PID parameters of the controller to achieve optimal control performance. In this article, Changhui Instruments explains the role of the P, I, and D parameters in PID control. Proportional action: A proportional controller is essentially an amplifier with an adjustable gain factor, namely △P=Kp×e, where Kp represents the proportional gain; this value can be either greater than 1 or less than 1 ； e is the input to the controller, that is, the difference between the measured value and the set value, also known as the error. It should be noted that for most analog controllers, the proportional gain Kp is not used as a scale; instead, a proportionality degree is used for scaling, namely δ=1/Kc×100%. In other words, the proportionality is proportional to the reciprocal of the controller’s gain ； The smaller the proportionality of the controller, the greater its amplification factor and its ability to amplify errors; vice versa. By understanding the relationship mentioned above, it can be seen that the greater the proportionality, the smaller the amplification factor of the controller, and the smoother the curve of the controlled parameter ； The smaller the proportionality, the greater the amplification factor of the controller, and the more volatile the curve of the controlled parameter becomes. A drawback of proportional control is that it results in error; to overcome this error, integral action must be introduced. Integral action: The integral action of a controller is designed to eliminate the residual error in an automatic control system. The so-called integration refers to the accumulation over time; that is, when a deviation input e is present, the integral controller accumulates this deviation over time. The rate of this integration is proportional to the magnitude of the deviation e as well as the integration speed. As long as a deviation e exists, the output of the integral controller must change; in other words, integration is always active, and it only stops when there is no deviation. For a constant deviation, the essence of adjusting the integral action is to change the rate of change of the controller’s output; this rate is measured by the time required for the output from the integral action to equal the output from the proportional action. A small integration time indicates a high integration speed, and thus a stronger integrating effect ； Conversely, the larger the integration time, the weaker the integrating effect. If the integration time is infinite, it means there is no integration effect, and the controller becomes a pure proportional controller. In practice, the integral action is rarely used alone; it is usually combined with the proportional action, thereby giving it both the proportional effect of amplifying (or reducing) the deviation and the integral effect of accumulating the deviation over time, with their effects acting in the same direction. At this point, the output of the controller is: △P = Ke + △Pi, where △P represents the change in the controller’s output value ； Ke is the output caused by proportional action ； △Pi is the output resulting from the integral action. Differential action is primarily used to overcome the lag of the controlled object, and it is commonly applied in temperature control systems. In addition to using differential action, when employing control systems, attention must be paid to the lag in measurements during transmission, such as the selection of temperature sensing elements and their installation location. To help remember the three functions of integration, differentiation, and accumulation, three mnemonic phrases are provided for your reference. Mnemonic for proportional action: The proportional governor is like an amplifier ； A deviation occurs, and it’s amplified and sent out ； Check the knob carefully to see what the magnification is ； Increasing the proportionality setting reduces the magnification factor. Mnemonic for integral action: Reset the regulator; accumulation brings capability ； As long as there is a deviation, the accumulation does not stop ； For cumulative fast and slow, check the knob carefully ； The integration time is long, resulting in a low accumulation rate. A mnemonic for differentiation: When it comes to differentiators, there’s nothing mysterious about them ； With a step input, the output jumps up ； Check the knob carefully to see if the descent is fast or slow ； When the differential time is long, the decline is slower.

First, select the controlled variable, then choose an independent variable to control it. If interference can be overcome through control, using P is sufficient; if control is unstable, add an I element to address the interference. If that still doesn’t work, add a D element. If even that fails, consider other control schemes. First, it’s necessary to test how the circuit operates and whether the equipment is functioning properly.