Design and Development of a Mechanical Safety Stop Mechanism and an Adaptive Variable Impedance Controller for a Soft Hand Exoskeleton

In this paper, the design and performance evaluation of a new safety stop mechanism for a soft-hand exoskeleton are presented. The considered wearable orthosis comprises an under-actuated tendon-driven mechanism to perform flexion and extension on each finger using a separate electrical motor. The flexor and extensor tendons move through different paths connected to the motors with custom-made double-groove rollers to flex and extend the thumb, index, and middle fingers. A series flexible actuator unit is designed, including a mechanical module and a flexible one, fulfilling the hand-wearable robot’s functional and safety requirements. The actuator system’s flexible module uses a novel safety stop mechanism to prevent hyper-flexion, hyper-extension, or large forces applied to the user’s hand. Experiments are conducted on a healthy subject to evaluate the effectiveness of the design exoskeleton for patients with hand movement disorders. The experimental results show that the robot can follow the desired path by preserving its back-drivability and compliant properties. The efficacy of the novel safety stop mechanism is also evaluated on the prototype version of the robot, which shows that it can mechanically restrict the range of motion to a safe range. Finally, an adaptive variable impedance controller is designed to achieve assist-as-needed characteristics for the robot. The proposed controller overcomes the need to direct measurement of the participation of the patient via force sensors, which could ease the rehabilitation process.