Pulley-based McKibben actuator mechanism for adjustable soft hand rehabilitation splint

Abstract

Hand rehabilitation robots were developed to assist in rehabilitation procedures conducted by rehabilitation professionals. However, current hand rehabilitation robots are mostly made from heavy and rigid structures that caused discomfort and fitting issues to the patients. McKibben actuator is a type of soft actuator that could be used in hand rehabilitation robots for its flexibility and light weight. However, it has a limited contraction ratio for the required range of motion for finger flexion. In this thesis, a pulley mechanism is proposed to improve McKibben actuator’s contraction ratio while providing the required contraction force. A double groove pulley made of a hybrid of gear and pulley is proposed to enhance McKibben’s actuator contraction ratio. Various pulley ratio was studied to find optimum contraction ratio and its relation to contraction force. A pulley ratio of 1:4 increases the contraction ratio from 19.85 % to 76.67 % but reduces the contraction force from 42.68 N to 9.69 N. Hence, pulley ratio of 1:2 was implemented to the McKibben linear actuator based on its optimized 39.72 % contraction ratio and 20 N contraction force for the soft splint application. Next, an adjustable finger size soft splint with fixed wrist motion was developed. It consists of three parts, namely pulley-based McKibben actuator, wrist component, and McKibben ring actuators. The wrist component was designed with an adjustable strap buckle while the finger insertion part utilized the elasticity of McKibben ring actuator during contraction to fit a wide range of sizes. The size range for wrist and hand circumference is 12 cm - 21.6 cm and 15.8 cm - 22.3 cm respectively, which fit 90 % of Malaysian young adults. The soft splint was tested on two healthy subjects. At 400 kPa supply pressure, the bending angle of the finger joints achieved was [71.8°, 72.8°, 18.70] for Metacarpophalangeal, Proximal Interphalangeal and Distal Interphalangeal respectively. The range of motion achieved by the soft splint is lower than the functional range of motion, but higher compared to other research works. The subjects were able to grasp and lift objects of different shapes including a box, cylinder, and irregular shape under 250 g while wearing the soft splint. The developed soft splint with adjustable McKibben ring actuators and pulley-based McKibben linear actuator could initiate finger motion and assist object grasping for a possible clinical hand rehabilitation assessment.