Seeking advice on control laws

Proportional control is a basic control law that provides rapid regulation; the magnitude of its output is proportional to the magnitude of the error. When the error disappears, the output also disappears, so an error must exist. In situations where high control precision is not required, only proportional control can be used； The output of integral action is the integral of the deviation; as long as there is a deviation, this integral value keeps increasing until the deviation becomes zero, at which point the output remains constant. Therefore, integral action can eliminate the deviation between the measured signal and the desired signal, and it is necessary to use integral action when it is required that the control parameters have no deviation ； The output of the differential action is proportional to the rate of change of the deviation; the faster the deviation changes, the greater its output. It exerts a strong effect when the deviation first starts to change, and thus it provides leading regulation. For those systems whose controlled parameters change slowly and have large time constants, such as temperature or analysis processes, differential action is necessary.

The conventional proportional control value is derived from the theoretical calculations related to the process (or from empirical values based on such theoretical calculations). For example, assuming that 1 mol of nitrogen and 2 mol of hydrogen react to produce 3 mol of ammonia, then the control ratio (P) for the amount of nitrogen to hydrogen entering the reaction tower is 1:2； In terms of control design, it is also necessary to consider the impact of interference channels during operation on the control results ； For example, when the detection channel detects an increase in the nitrogen flow rate (or changes in other parameters), the computer will calculate the degree to which the nitrogen valve should be opened or closed based on the magnitude of this increase in flow rate; here, preset integration times designed to eliminate interference are also taken into account ； In short, the ratio values are empirical values based on theoretical calculations, while the integration time and differentiation time also stem largely from empirical knowledge gained in actual production ； These are my humble opinions for your reference ；

Proportional control law is not ratio control; proportional control refers to the situation where the controller’s output is proportional to the deviation from the controller’s input.

The combinations of control laws include Proportional (P), Proportional-Integral (PI), Proportional-Derivative (PD), and Proportional-Integral-Derivative (PID). Among them, the proportional control law is the fundamental control law and is indispensable. The pure proportional control law is a control law with a residual error; to eliminate this residual error, an integral control law must be added. The derivative control law can overcome the time delay in the response of the controlled system. Principles for selecting control laws: 1. For applications where high control precision is not required (such as intermediate storage tanks, pressure buffer tanks, etc.), pure proportional control can be used ； 2. In situations where high control precision is required, with no residual error and minimal capacity lag, a proportional-plus-integral control scheme can be used ； 3. When high control precision is not required but there is significant capacity lag, a proportional-plus-integral control scheme can be used ； 4. When high control precision is required and there is significant capacity lag, a proportional-plus-integral-plus-derivative control scheme can be used ； 5. For systems with noise (such as flow rates), use differentiation cautiously, as it amplifies noise.

This post was last edited by Bodi Environmental Protection on 2015-8-31 08:45. Control PID can be categorized into mechanical, electrical, chemical, and so on； The PID control I described is based on the PID control used in the chemical industry ； In chemical engineering, the proportional control in PID systems also stems from ratio control based on process calculations. The so-called control input and output deviations are as well determined on the basis of process calculations; they are not arbitrary proportions of deviations that appear out of nowhere ；