Robust variable structure control of model reference systems

A variable structure control system (VSCS) yields total or selective invariance to system parameter variations and disturbances when in the sliding mode. In order to prescribe the system performance, a model-following approach to VSCS design is used. The required behaviour is achieved by defining a model with ideal response characteristics, and a VSCS is designed so that the error between the plant and the model is forced to exhibit a sliding mode. A parameter insensitive controller with improved disturbance rejection which achieves desired system performance results. The resulting nonlinear scheme cannot be assessed using conventional frequency domain techniques. A new analysis tool is therefore proposed and used to explore the robustness of such a nonlinear control scheme. The design technique is illustrated for lateral multimode control. The aircraft system requires a parameter insensitive control type with improved disturbance rejection if possible. In addition, the ability to prescribe flying quality requirements directly is important. The problem thus provides a suitable challenge for the technique. The robustness analysis is illustrated using the resulting VSCS applied to a range of realistic perturbation models of the fully nonlinear aircraft model.