Filtered PID Control Loop for Third Order Plants With Delay: Dominant Pole Placement Approach

The paper deals with tuning the filtered PID controller applied to third order plants with delay. This model option is chosen as a representative case where the loop characteristic quasi-polynomial is of higher order than three. Applying the similarity theory for introducing dimensionless parameterization a comparative model of third order plant dynamics is obtained. Four dominant poles – from the infinite spectrum of the control loop – are assigned by means of tuning three controller gains and a filter time constant where a specific argument increment criterion proves their dominance. The pole prescription coordinates are parameterized via damping, root and natural frequency ratios optimized in the space of the introduced similarity numbers according to the IAE criterion with respect to robustness and filtering constraints. Particularly the natural frequency ratio is a new parameter introduced to tune robustly the PID together with its filter. For the constrained IAE optimization the response of disturbance rejection is used as a representative of control loop behavior. In the space of similarity numbers of the plant it is shown that a limited range of plants is suited to be controlled on the PID control principle and the boundaries of this range are outlined. Survey maps of optimum controller parameters are presented and a comparative study on benchmark application example is added.