Identification and Controller Tuning of Cascade Control Systems Based on Closed-Loop Step Responses

This paper presents a new automatic tuning method for cascade control systems based on a single closed-loop step test. The proposed method identifies the required process information with the help of B-spline series expansions for the step responses. The two PID controllers are then tuned using an internal model control (IMC) approach. The secondary controller is designed for enhanced disturbance rejection, and the primary controller is designed, without requiring an additional test, based on an identified process model that accurately accounts for the inner loop dynamics. The desired levels of system robustness explicitly guide the selection of the IMC tuning parameters. The proposed method is robust to measurement noises because of the filtering property of the B-splines, and can provide satisfactory control performance. Simulation examples confirm the effectiveness of the proposed method.