A new method of direct PID controller design based on the principle of cascaded damping ratios

Abstract

The paper presents a universal method of direct controller design. Similar to second order systems cascaded damping ratios are defined for third order systems, which allow a direct controller design for maximum bandwidth of the control circuit. Using the low frequency parameters of the plant the parameters of the process controller (PI, PID) may be calculated for proportional plants of arbitrary lag order and dead-time. Large dead-time as well as nonminimum-phase characteristic can be handled without problems. A prescribed dynamic limiting of the correcting variable is easily accomplished by introducing a time lag to the controller. Simplified design rules are derived referring only to the average resident time and approximate order of the plant to be controlled. A simple procedure of parameter estimation based on the inflection tangent at the step response of the plant is outlined for parameter tuning using minimal process information.