Abstract: The dynamical servo control problem of a vertical planar underactuated manipulator acrobot is studied in this paper. This problem aims to drive acrobot to any position in the construction space. Since the acrobot cannot be stabilized at any position except the equilibrium points, we consider to stabilize the system to a periodic orbit which passes through the target position. The virtual constraint is used to describe these periodic orbits, and then the zero dynamics and the relative integral curve under the selected virtual constraint are given. A cascade form controller is designed. The inner loop controller is based on the improved feedback linearization method and a virtual input which makes the inner loop like a second-order system is introduced. With this virtual input, an outer loop controller is designed based on Lyapunov stabilizing theory. The simulation results show the effectiveness of the designed control law, and that this method is better than the energy based method.