A Modified Repetitive Control Strategy for Nonlinear Systems

In this research work, a new attempt is made to implement a Modified Repetitive Control Strategy
(MRCS) directly in a Liquid Level System (LLS) and indirectly in a Spherical Tank Level System
(STLS). The dynamics of the both level processes are described by the worst case model parameters
which are identified by influencing the step test method. In direct approach, MRCS is incorporated in
the conventional liquid level control loop of proportional (P) mode. Ziegler-Nichols Tuning Rule
(ZNTR) based proportional controller parameter is considered in the loop. A periodic signal of sine
wave in inflow to the level system is generated and real time runs of the LLS are carried out for the
periodic input tracking with MRCS based P mode control loop. In indirect approach, by utilizing relay
feedback technique, the periodic reference signal of MRCS is generated. From the input and output
chattering signals of the MRCS, the optimized PI controller parameters are identified using Recursive
Least Squares (RLS) fitting technique. Real time results are endowed to demonstrate the efficiency of
the proposed tuning method. A similar runs are carried out with the non linear systems having
conventional PI-mode structure in the both Liquid level and Spherical tank control loops. A
comparison in the performance analysis clearly indicates that the incorporation of direct approach of
MRCS in the control loop in LLS and indirect approach of MRCS in the control loop in STLS provides
a better tracking performance than the conventional PI mode.