A PPA robot, which has 3 degrees of freedom(DOF) but only has one actuator, is a novel class of underactuated
mechanical systems. For the motion control of this robot, a zoning control strategy is presented to drive it away from the
straight-down position and balance it at the straight-up position. Firstly, the motion space is divided into swing-up area and
attractive area. Then a swing-up control law based on the Lyapunov function is employed to increase the system energy and
control the posture. A balancing control law adopting optimal control is designed for a stable control in the attractive area.
Finally, simulation results show the effectiveness of the proposed method.