Design, development and control of a 2PRP-2PPR planar parallel manipulator for lower limb rehabilitation therapies

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This paper proposes a vertical planar 2PRP-2PPR parallel manipulator along with a serial planar RRR passive orthosis (exoskeleton/supporting system) for performing sitting/lying type (stationary trainer) lower limb rehabilitation therapies in the sagittal plane. Proposed system's kinematic arrangement along with forward and inverse solutions is presented. System dynamics is derived to understand the behavior of the over-constrained manipulator-orthosis system. Further, a robust motion control scheme is proposed and the motion control scheme is based on non-singular fast terminal sliding mode control along with a nonlinear disturbance observer. The effectiveness and usefulness of the proposed manipulator is shown with the implementation of the motion controller through computer based numerical simulations using a clinical gait motion pattern. The proposed motion control scheme is also validated on an in-house fabricated prototype through motion control experiments. The controller parameter sensitivity and controller robustness are further analyzed at different working conditions. In comparison to the conventional controllers, the proposed control scheme possess few advantages namely better robustness, less chattering, high precision and fast finite time convergence, and can work in the presence of parameter uncertainties. From the demonstration, the proposed manipulator has certain advantages over existing stationary lower limb rehabilitation trainers namely, simple design, larger workspace, higher stiffness, modular design and low cost.