Process Engineering Division – Instrumentation and Automation Section – Daily Topics – Topic No. 2021-01-06
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Process Engineering Division – Instrumentation and Automation Section – Daily Topics – Topic No. 2021-01-06: Discussion question: 349. When the capacity lag and pure lag of the process variable are not significant, the load often experiences large fluctuations, and there is a requirement for precise control over the process, which two elements from PID control should be used for regulation? Please briefly explain the reason. (5 points are given only for the result; extra points are awarded for reasonable explanations.)
When the capacity lag and pure lag of the process variable are not significant, the load often experiences large fluctuations, and there are requirements regarding process tolerances, then PI control within the PID control strategy should be used for regulation. The capacity lag and pure lag are small, indicating that the effect of the derivative D is minimal ； Large fluctuations indicate that the proportionality constant K is set too low ； The requirement for a residual error indicates the presence of a deviation, with the integral I playing a role in this.
When the capacity lag and pure lag of the process variable are not significant, the load often experiences large fluctuations, and there is no requirement for precision in the process, which two elements from PID should be used for control? Please briefly explain the reason. Answer: PI control should be used. Because the output of proportional (P) control is proportional to the input error signal. A drawback of proportional control is that it results in error; to overcome this error, integral action must be introduced. The output of integral (I) control is proportional to the integral of the input error signal. To eliminate steady-state error, an “integral term” must be introduced into the controller. The integral action is designed to eliminate the residual error in automatic control systems. The integration stops only when the deviation is absent. Therefore, a proportional-plus-integral (PI) controller enables the system to have almost no steady-state error after reaching steady state. The output of differential (D) control is proportional to the derivative of the input error signal (i.e., the rate of change of the error). The differential action is primarily used to overcome the lag of the controlled object. For controlled objects with high inertia or significant lag, a proportional-plus-differential (PD) controller can improve the dynamic characteristics of the system during the regulation process. Therefore, when the capacity lag and pure time lag of the process variable are not significant, the load often experiences large fluctuations, and there is no requirement for precision in the process, it is appropriate to use only the PI component of PID for control.
PI control should be used. Because the output of proportional (P) control is proportional to the input error signal. A drawback of proportional control is that it results in error; to overcome this error, integral action must be introduced. The output of integral (I) control is proportional to the integral of the input error signal. To eliminate steady-state error, an “integral term” must be introduced into the controller. The integral action is designed to eliminate the residual error in automatic control systems. The integration stops only when the deviation is absent. Therefore, a proportional-plus-integral (PI) controller enables the system to have almost no steady-state error after reaching steady state. The output of differential (D) control is proportional to the derivative of the input error signal (i.e., the rate of change of the error). The differential action is primarily used to overcome the lag of the controlled object. For controlled objects with high inertia or significant lag, a proportional-plus-differential (PD) controller can improve the dynamic characteristics of the system during the regulation process. Therefore, when the capacity lag and pure time lag of the process variable are not significant, the load often experiences large fluctuations, and there is no requirement for precision in the process, it is appropriate to use only the PI component of PID for control.
The PI control law is adopted. The P control law is a proportional control law; it is a basic control law that must be present ； The I-control law, which is an integral control law, can eliminate residual errors, but it causes the operating frequency of the control system to slow down slightly ； The D control law is a differential control law that can improve the control performance of systems with large time delays. In this case, since the controlled system does not have a significant time delay, it is not necessary to use the differential control law ； In engineering, pure integral or pure differential control laws are hardly used; instead, proportional + integral + differential combined control laws are predominantly employed. In summary, the PI control law is appropriate for this problem.