Observer-based cascade control of a 6-DOF parallel hydraulic manipulator in joint space coordinate

This paper addresses the joint space control problem of a 6-DOF (degree of freedom) parallel hydraulic manipulator. High precision motion control of a six-degree parallel manipulator is hardly achieved due to the existence of uncertain payload and other disturbance such as coupling force. A disturbance observer for this parallel manipulator is first constructed to estimate and compensate the unknown disturbance. A cascade control algorithm is then applied to separate the hydraulic dynamics from the mechanical part, which can mask the hydraulic dynamics with an inner loop. With such a control structure, known control design methods within the area of manipulator control can be directly used in the outer loop. In this approach, the complex dynamics and direct kinematics of the parallel manipulator are not required and acceleration feedback is also avoided. Experimental results are presented to show the effectiveness of the proposed scheme.