A long-term goal of our work is to develop the basic control theory for me- chanical systems, and Lagrangian systems in particular. There are several reasons why mechanical systems are good candidates for new results in nonlinear control. On the practical end, mechanical systems are often quite well identified, and ac- curate models exist for specific systems, such as robots, airplanes, and spacecraft. Furthermore, instrumentation of mechanical systems is relatively easy to achieve and hence modern nonlinear techniques (which often rely on full state feedback) can be readily applied. We also note that the present setup suggests that some of the traditional concepts such as controllability itself may.
