Cascade controller tuning by relay auto tune method

Abstract

The present work extends the method of Srinivasan and Chidambaram (An improved auto tune identification method, Proc. Int. Conf. on Digital Modeling and Simulation (DAMS-2003), Jan 6-8, Coimbatore, India (2003)) to analyze conventional on-off relay oscillations for a single loop feedback controller to relay tuning of cascade controllers. Using the ultimate gain and ultimate crossover frequency of the two loops, the inner loop (PI) and outer loop (PID) controllers are designed by Ziegler-Nichols tuning method. The performance of the controllers is compared with the results based on conventional relay analysis. The improved method of analyzing biased auto tune method proposed for single feedback controller by Srinivasan and Chidambaram (Modified relay feedback metho for improved system identification, Comp. Chem. Engng, 27, 727-732 (2003)) is also extended to relay auto tune of cascade controllers. The proposed methods give an improved performance over that of the conventional on-off relay tune method.