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Robust nonlinear task space control for a 6 DOF parallel manipulator

This paper presents a robust nonlinear controller equipped with a friction estimator for a 6 degree of freedom (DOF) parallel manipulator in the task space coordinates. The requisite 6 DOF system states for the task space control are acquired by an alpha-beta tracker and a numerical forward kinematic solution. The Friedland-Park friction observer in the joint space coordinates provides friction estimates that help to improve the control performance. The fidelity of the indirect 6 DOF state estimates is confirmed via an inverse kinematic solution and measured gyro signals. The friction estimates are compared to independently measured friction values, which shows reasonable agreement. Finally, the robust nonlinear task space control (RNTC) with the friction estimator is implemented, which shows excellent tracking and regulating control performances. The RNTC turns out to outperform a popularly adopted proportional plus integral plus derivative control with the friction estimator in the joint space coordinates.