Development and performance evaluation of parallel link type human ankle rehabilitation assistive device

This paper presents a novel rehabilitation assistive device called a “parallel link type human ankle rehabilitation assistive device (PHARAD).” It can accurately measure foot motions in six degrees of freedom and reproduce the ankle joint motions. By reproducing the input motions of the ankle joint, PHARAD conducts passive exercises for ankle rehabilitation. To measure and reproduce complex foot motions, we adopted a parallel link mechanism using six pneumatic cylinders with displacement sensors. In this research, we define the motions of a foot plate attached to a foot sole as the foot motions. A posture of the foot plate, i.e., the three-dimensional (3D) position (x, y, z) and rotation angle (θ, φ, ψ), is numerically calculated by solving the forward kinematics of the PHARAD. We conducted two kinds of experiments to evaluate the performance of the PHARAD. First, verification experiments for the accuracy were implemented by comparing the measured motions of the foot plate by the PHARAD with those obtained by a motion capture system. The experimental results showed that the maximum root mean square error (RMSE) values of the 3D position and rotation angle were 2.6 mm and 1.5°, respectively. Then, verification experiments for the reproducibility were implemented by comparing the reproduced motions with the input motions. The experimental results showed that the RMSE values of the 3D position and rotation angle were 5.6 mm and 6.1°, respectively. Moreover, after reproducing the input motions ten times, the standard deviations of the 3D position and rotation angle were 1.4 mm and 0.7°, respectively. These experimental results show that the PHARAD can precisely measure and reproduce complex ankle motions, and has the potential to reproduce the exercise therapy presented by physical therapists.