Robust output regulation of variable structure systems with multivalued controls

We consider the problem of robust output regulation for a class of passive linear systems. We take a `control by interconnection of systems´ approach, where the controller is defined by means of a multivalued function. The resulting closed-loop system can be cast into the form of a variable structure system with interesting properties: output feedback (perfect knowledge of the plant state is not required) and perfect regulation despite parametric uncertainty and unmatched disturbances. The methodology is illustrated through a physical and an abstract example.