Control problems in underactuated manipulators

We discuss some recent control techniques for underactuated manipulators, a special instance of mechanical systems having fewer input commands than degrees of freedom. This class includes robots with passive joints, elastic joints, or flexible links. Structural system properties are investigated showing that robots with passive joints are the most difficult to control. With reference to these, solutions are proposed for the typical problems of trajectory planning and tracking, and of set-point regulation. The relevance of nonlinear control techniques such as dynamic feedback linearization and iterative state steering is clarified through illustrative examples.