A simple method of tuning cascade controllers

Abstract

A simple method is proposed to design PID controllers for a series cascade control system. The method is based on matching the coefficients of corresponding powers of q and q2 in the numerator to ? 1 and ? 2 times that in the denominator of the closed loop transfer function model for a servo problem. This method can be used only when the inner and outer loop transfer functions are known. If these functions are not known, then an identification step needs to be carried out. The method is first applied to design a proportional (P) controller for the inner loop and then to a proportional plus integral (PI) controller for the outer loop. The performance of the proposed controllers is evaluated for an FOPTD model of the inner loop and outer loop process transfer function models. The response and robustness due to perturbation in model parameters are evaluated and compared with the methods of Krishnaswamy and Rangaiah (When to use cascade control, Ind. Engng Chem. Res., 29, 2163-2166 (1990)) and Lee et al. (PID controller tuning to obtain closed loop response for cascade control scheme,
Ind. Engng Chem. Res., 37, 1859-1865 (1998)). The proposed method gives better servo and regulatory performance.