NONLINEAR FILTER IN CASCADE CONTROL SCHEMES

The present paper addresses the problem of using a cascade control structure in order to control systems which have primary and secondary processes with comparable dynamics. As classical tuning techniques may be unable to give good performances, since they do not fully take into account the interaction between the two loops, a nonlinear filter is introduced between the two controllers in order to partially separate the dynamics of the loops. Such a filter, which involves the use of a single tuning constant, is meant to damp the action of the primary controller when it tends to amplify the variation of the manipulated variable. The nonlinear filter, besides having a stabilizing effect on the cascade structure, reduces the tuning difficulties. Significant improvements in robustness and performance are obtained in series cascade; moreover, with an appropriate choice of the secondary measurement, the benefits of the nonlinear filter structure can be extended to a parallel cascade scheme.