This paper presents the performance/robustness trade-off analysis of servo-control and regulatory-control systems. In this analysis, first-order-plus-dead-time (FOPDT) and second-order-plus-dead-time (SOPDT) controlled processes with one-degree-of-freedom (1DoF) and two-degree-of-freedom (2DoF) proportional integral (PI) and proportional integral derivative (PID) controllers are considered. The analysis shows that for the same process, performance-optimized 1DoF PI controllers are more robust to changes in the process characteristics, but the optimal performance of these controllers is inferior to that of the corresponding PID controllers. Regulatory-control performance-optimized PI and PID controllers are less robust than the corresponding servo-control systems, and a greater level of performance degradation is required in the former case to ensure that the robustness level becomes comparable to that in the latter case. On the basis of this analysis, a simply robust tuning method was developed for 2DoF PID controllers; the use of this procedure helps in achieving the desired level of robustness in closed-loop control systems. The proposed tuning method allows the use of a single set of rules for the robust tuning of PID controllers for FOPDT and SOPDT controlled processes © 2012 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society.
- PID control
- two degrees of freedom