Design of a parallel shoulder assistive robot with pneumatic muscle actuators
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Given the increasing stroke incidence and ageing population, robotic assistance for people suffering from physically weak upper limbs in their activities of daily life (ADL) is becoming more promising. However, most of the current upper limb assistive robots (or upper limb exoskeletons) are bulky and heavy when designed to meet the requirements of sufficient degrees of freedom (DoFs), workspace and joint torques. The objective of this thesis is to develop dynamic models of pneumatic actuators and design a new mechanism towards developing a compact and lightweight upper limb exoskeleton, while providing proper kinematic capability to assist a human’s upper limbs in their ADL. This research first focused on parallel mechanisms given their advantages of compactness and high stiffness. Multiple parallel mechanisms are reviewed in terms of their capability in delivering 3D rotational motion and safety concerning the forces transmitted to the shoulder joint when mechanisms are applied as a shoulder joint. Then, a 3UPU wrist mechanism is selected given its superior kinematic capability. An alternative forward kinematics solution for the 3UPU wrist mechanism is presented so that the upper limb’s orientation can be estimated using the universal joint’s rotation angles on the base, rather than measuring the mechanism’s limb length. Pneumatic muscle actuators (PMAs) are then selected for driving the robotic exoskeleton because of their superiority of high strength-to-weight ratio and inherent elasticity. An enhanced dynamic force model is developed to depict the PMA’s nonlinear relationship between its length, pressure and external load. By introducing a model of Coulomb friction element, this dynamic force model overcomes the problems related to the current over-simplified models. The improvement of this enhanced model is evidently witnessed in situations where softer and more elastic PMAs are pressurised to perform large contractions. A 3UPU wrist mechanism test rig that can measure the universal joint angles is developed for verifying the mechanism’s inverse kinematics and the proposed alternative forward kinematics. Experimental results validated the inverse kinematics of this mechanism in most cases and verified the solutions of platform orientation obtained from the alternative forward kinematics. A prototype exoskeleton is developed based on the 3UPU wrist mechanism, and is used to test the performance of the PMAs and the 3UPU wrist mechanism. A proportional–integral (PI) controller is used for the PMA position control. Two basic ADL movements are tested on the prototype. The experimental results and future work are then discussed.
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