Multiloop version of the modified Ziegler-Nichols method for two input two output processes

In this paper, a multiloop version of the modified Ziegler-Nichols method(1) is proposed for two input two output (TITO) processes. The key idea is to regard each loop together with its corresponding interactions from all other loops as an equivalent single input, single output (SISO) plant and design for it a SISO controller by shifting a given point on the Nyquist curve of the equivalent SISO plant to a desired position. This is, however, a nonlinear problem because the equivalent plants depend on unknown controllers. A novel approach is thus presented to obtain the controller parameters which achieve the above design objective. Unlike many other methods that emphasize the suppression of interactions, our method is developed to channel the effect of interactions to individual loops to increase the speed of loop responses. Simulation examples are provided to show the effectiveness of the proposed method and comparisons are made with the biggest log modulus tuning (BLT) method.