The present chapter is concerned with simultaneous relay autotuning of cascade controllers. Traditionally, autotuning procedures for cascade controllers are applied using a sequential, one-loop-at-a-time method. Saraf et al. (2003) proposed a simultaneous method of the relay tuning of series cascade control system. In this method, they assumed the principal harmonic analysis of relay oscillations. This results in error in calculation of the ultimate gains. The higher order harmonics are to be considered in the analysis of simultaneous relay tuning of series and parallel cascade control systems. Using improved ultimate gains, the inner loop (PI) and outer loop (PID) controllers are designed by the Ziegler–Nichols(Z-N) tuning method. The improved performance of the control is compared with the principal harmonic analysis reported by Saraf et al. (2003)

Introduction

Hang et al. (1994) proposed a sequential relay autotuning of series cascade control of stable systems. In this method, the outer loop is open. The on–off relay is used in the inner loop and the value of ku is obtained from 4h/(πa0). The controllers in the inner loop are tuned using the Ziegler–Nichols tuning formulae. With the inner loop under PI control action, the relay test is repeated for the outer loop.

Simultaneous relay tuning method is required whenever the system is to be kept under a closed loop in order to reduce the effect of disturbances or when the open loop system is unstable.