Force, stiffness and viscous damping control in six degrees of freedom of the ankle-foot rehabilitation assistive device

In this research, we developed a novel ankle-foot assistive device for rehabilitation. This device uses a Stewart Platform Mechanism (SPM) to measure and assist the movements of a human ankle joint in six Degrees Of Freedom(DOFs). The SPM adapts to the displacement of the rotation axis of a human ankle joint during the movements of a human foot. In our previous work, we have proposed methods of measuring and controlling the movements of a human foot. In this paper, we propose a method of a force control of the device. The force can be controlled by changing the pneumatic pressures in two chambers of each cylinder without using any force sensors. The force control method is tested in experiments for one cylinder and for the developed six DOFs assistive device. These test results show that the mean squared error of one cylinder is 0. 19[Nm] or less, respectively. The mean squared error about the force and torque control of this device is 0.49[N]and 0.06 Nm or less, respectively. The experimental results showed that desired force can also be realized in a high enough accuracy using the proposed method. For stiffness and water rehabilitation, we have proposed methods of controlling stiffness rate and viscous damping coefficient of one cylinder. The mean squared error of stiffness controlling is 3.4[N/m]or less, and that of viscous damping coefficient controlling is 1.4[Ns/m]or less. These results suggest that the developed six DOFs assistive device could be used during ankle-foot rehabilitation without force sensor.