**6. Conclusions**

The feedback linearization method provides an effective design tool for controlling nonlinear systems. We improved this technique for application to a class of underactuated mechanical systems. We provided two examples to illustrate the proposed method in which PFL was successfully applied to construct nonlinear controllers for a moving inverted double pendulum and a 3D overhead crane. In general, a nonlinear feedback controller for an underactuated mechanical system consists of two components. The first is for canceling the nonlinearities in the system and the second is for stabilizing state variables.
