Nonlinear Computed Torque Control of 6-Dof Parallel Manipulators

This paper presents an experimental study on high-precision motion control of heavy-duty parallel manipulators. Conventional independent joint control and computed torque control schemes are modified by the introduction of nonlinear loop gains in order to improve tracking performances. Asymptotic stability of the controllers has been analyzed and proved based on the Lyapunov’s direct method. An extended state observer for feedback compensation against disturbances is designed and implemented in the control loops. The observer error dynamics is shown to be exponentially stable and the error convergence rate can be made arbitrarily high by tuning the gain of the observer. Performances of the proposed nonlinear controllers are experimentally investigated in the control of a 6-DoF Stewart-Gough Platform in roll-pitch motion. Comparisons between the proposed controllers and conventional controllers show that tracking precision of the platform is improved through the nonlinear design of the controller gains in both independent joint control and computed torque control schemes. Although the computed torque control method proves to achieve the best tracking precision in high speed motions of the platform legs, independent joint control with nonlinear gains displays challenging performance as an interesting alternative.