Setting of PID parameters for complex systems

Temperature and pressure systems have different characteristics. The temperature lag time is relatively large, requiring lead compensation; differentiation has the function of providing lead compensation. Generally, PID control is used in temperature control systems, while pressure systems experience significant fluctuations, so a differentiation term is usually not employed; therefore, PI control is typically used in pressure control systems.

In cascade control, the secondary loop is usually a rapidly changing element, such as a flow element or a pressure element. Since the setpoint of the secondary loop comes from the output of the primary loop, and this setpoint changes rapidly, the control system for the secondary loop needs to be able to track these changes quickly; therefore, PI control or even pure proportional control is typically used. The controlled objects in the main loop are usually large-time-delay systems such as liquid levels and temperatures, and the control method commonly used is PID. When tuning the parameters, it is advisable to make the control effect somewhat weaker – reduce the proportional component slightly and add an appropriate amount of derivative action – in order to avoid large fluctuations in the output of the main circuit, which could lead to significant variations in the secondary circuit.

For cascade control, 1) the main loop requires strict control; large fluctuations are not allowed, nor is any excess error permitted, while the introduction of a secondary loop is mainly aimed at ensuring the main parameters, allowing for larger fluctuations. In this case, the primary control law should be PI or PID, while the requirements for the secondary parameters are not high; generally, a P control law is sufficient to meet the needs. 2. Higher quality requirements are imposed on the adjustment of main parameters, and certain requirements also exist for the adjustment of secondary parameters. In this case, the main regulator should use a PI control law, and the secondary regulator should also use a PI control law. However, at this time, especially when the main parameters change drastically and the setpoint of the secondary regulator itself is a variable parameter, even if the composite regulator has a PI control law, the secondary parameters still cannot be stabilized at the setpoints specified by the process. Regarding such cascade control systems, due attention should be paid to them when tuning the engineering parameters of the controller. 3. The quality requirements for tuning the main parameters are not high, but the secondary parameters need to adjust themselves in a precise and rapid manner in response to changes in the output of the main controller. In this case, the main regulator should use a P control law, while the secondary regulator should use a PI control law. 4. The regulation law of a cascade system where neither the primary nor the secondary parameters are subject to strict requirements. The secondary regulator can use a P law; to prevent the main parameter from deviating too far from the set value, the main regulator can use a PI law