Three-Parameter Models for Conservative Relay Feedback Autotuning

Abstract

Relay feedback method is one of the standard methods for PID controller autotuning in field. Relay feedback oscillations provide the ultimate gain and period of processes to design PID controllers with Ziegler-Nichols type tuning rules. Since the original relay feedback method uses only two process data of ultimate gain and period, its performances can be limited. For wider applications of the relay feedback autotuning method, methods using additional process data such as the steady state gain have been available. They often use the first order plus time delay (FOPTD) models and the FOPTD model-based tuning rules. Here it is shown that FOPTD model-based methods can provide closed-loop systems with very oscillatory responses for some processes. To relieve such drawbacks, the critically damped second order plus time delay (C2PTD) model that uses the same process data of previous FOPTD model-based methods is proposed. When three process data are extracted from the relay feedback oscillation, the C2PTD model-based method instead of the usual FOPTD model-based methods is shown to be better for wider applications of the relay feedback autotuning.