Upper Limb & Wrist Exoskeleton

Overview

The human arm and wrist joints play significant roles in people’s daily life, but it is particularly susceptible to musculoskeletal and neurological disorders. Affected joints often show an increased spasticity and stiffness, caused by impairments of the surrounding muscles and tendons. This research explores the use of an exoskeleton robot for physical rehabilitation of the human upper limb. Another goal of this research is to develop and control a robotic orthosis to assist the patient’s wrist to perform rehabilitation exercise in a compliant way.

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A wrist rehabilitation robot was further developed for clinical use. The robot was designed with flexibility in generating various degree of freedom (DOF) training by simply changing configuration of device. Functionally, the wrist robot incorporates sensor-based assessment techniques to facilitate the robotic control for passive training, active training and enhanced safety. With a configurable handle, the wrist robot is able to conduct rehabilitative training for two degrees-of-freedom (flexion-extension, radial deviation-ulnar deviation). Development of different control strategies to conduct passive and active training are completed. Two trajectory tracking control, one adaptive passive control and an active control are completed and their performance and preciseness have been assessed in clinical environment.

Research

The optimized 4R mechanism is used to develop a 5 DOF active exoskeleton system for the shoulder and elbow joints. To maneuver the exoskeleton, an algorithm is developed to generate smooth point-to-point trajectories that are similar to the trajectories in normal human motion. This algorithm is further expanded into a trajectory planner which combines a sequence of point-to-point movements into a single smooth trajectory. To control the exoskeleton, two types of interactive control strategies are developed. Admittance control allows the user’s limb to move the exoskeleton by applying forces at the designated interfaces, during which the exoskeleton can assist or resist user movement. Impedance control involves actuation of the exoskeleton to move the user’s limb through a specified trajectory with an artificial compliance.

A 1-DOF robotic device with parallel mechanism is designed for the wrist joint by utilising pneumatic artificial muscles (PAMs) that are compliant and lightweight. The mechanical hinge is placed coaxial with the biological joint, which requires the connection of the muscles with the hinge pulleys through a redirected steel wire. Ball bearings are used to reduce friction in the hinge. Miniature in-line load cells are mounted on each muscle to measure the axial force. Furthermore, a potentiometer is utilised as an angle sensor, placed on the outside of the hinge’s left pulley. The materials used in the device’s frame are either 3D printed plastic or laser-cut acrylic resin, which leads to a low overall weight of 1086g. Any other electrical and pneumatic components are placed on a remote board. The model-based controller is implemented for the PAMs, with an open loop displacement control with force feedback and an embedded closed-loop pressure control.

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