Stability margins characterization of a combined servo/regulation tuning for PID controllers

Robustness and performance analysis for a control system with a PID controller tuning from optimal settings are presented. Those analysis are conducted from the point of view of the stability margins characterization and of the operating mode (either servo or regulation mode) of the control loop and tuning mode of the controller, respectively. It is well known that, specially for optimization based settings, the performance of the control loop is defined in terms of the expected operating mode of the control loop. When the control system is not operating in the same operating mode as the controller was tuned, the performance may exhibit very poor results. The performance degradation with respect to the optimal performance is defined and analyzed for settings based on ISE-like optimization criteria. As a consequence, in order to get a minimal overall performance degradation with respect to both operating modes, tradeoff settings are proposed and the stability margins for the transition between both servo and regulation tunings are obtained.