Stability of discrete override and cascade-limiter single-loop control systems

Nonlinear elements such as internal limits in cascade regulators or override loops are often added to standard linear regulators in order to keep an additional process output within the limits during startup or large load swings. These systems generally are designed intuitively and tuned by experiments on the plant. Stability analysis is not often feasible due to time constraints, but could improve both security and insight. The authors are engaged in the development of analytical tools for the design of such systems. Here, the equivalence of different discrete-time implementations is proved. Stability tests are prepared that are directly applicable in practical design work. The analysis is shown to cover also control saturation with antiwindup in discrete forms and all corresponding cases with continuous regulators. A numerical example is given to illustrate the design procedures.>