Joint Monitoring of PID-Controlled Processes

Statistical process control (SPC) monitoring of the special causes of a process, along with engineering feedback control such as proportional-integral-derivative (PID) control, is a major tool for on-line quality improvement. In this paper, a strategy to jointly monitor the PID-controlled outputs and the manipulated inputs using bivariate SPC is proposed, and a specific design procedure that takes into account both the controller and the disturbance model is provided. The run length properties of the joint monitoring schemes, using Hotelling's approach and Bonferroni's approach, are systematically investigated. The robustness of the proposed monitoring schemes with respect to disturbance model uncertainty is studied, and a decision rule to select an appropriate monitoring scheme is proposed.