Force, Stiffness and Viscous Damping Control of a Stewart-Platform-Type Ankle-Foot Rehabilitation Assist Device with Pneumatic Actuator

Abstract

The number of physically disabled people in need of rehabilitation is increasing. Unfortunately, there is a shortage of physical therapists specializing in such rehabilitation. This has increased the demand for rehabilitation assist devices that can lessen the burden of physical therapists. In this study, the authors develop a device that can assist in the rehabilitation of the ankle joint by employing a Stewart-platform (SP)-type parallel-link mechanism. With the SP-type parallel-link mechanism, it is possible to measure and control six degrees-of-freedom (DOFs) of ankle-foot movement during rehabilitation. Because the device enables the measurements of the ankle and foot, it is possible to calculate the instantaneous center of the ankle joint. In previous studies, the authors proposed methods to calculate and control the posture of the ankle and foot by an SP-type parallel-link mechanism and verified their accuracy. In this paper, the authors propose a method for force control using the device and also verify its accuracy. Using this device, the force acting on the ankle-foot can be controlled by six air cylinders. The force produced by a single air cylinder is determined by controlling the pressures in the extension and retraction directions. The accuracy of the force control method is verified for a single air cylinder and for the assist device when all six air cylinders are engaged. Results show that the accuracy of the single air cylinder has a mean square error of 0.24 N or less, while those for force control of the entire device are 4.2 N or less for parallel translation and 3.2 Nm or less for rotation. This demonstrates a satisfactory accuracy. To incorporate rehabilitation assistance by means of stiffness or viscous damping in the future, the authors also propose methods to control the coefficients of stiffness and viscous damping of the air cylinder and verify their accuracy. The mean square errors for the accuracies in controlling the coefficients of stiffness and viscous damping are 3.4 N/m and 1.4 Ns/m, respectively, thus verifying the validity of the proposed methods.