Shank-foot trajectory control: A forward dynamics approach using computed-torque control

This paper presents an approach to shank-foot trajectory control using computed-torque control method. The aim is to provide rehabilitative measures to help patients with lower limb disorders improve their mobility performance. This measure is achieved by designing a desired trajectory to be followed by the shank-foot model for the rehabilitative purpose. The shank-foot model is designed to depict the knee-ankle orthosis having joints at the knee and ankle with respect to the subject being in a sitting position. The trajectories were defined using inverse kinematic transformations and Matsuoka oscillators. Inverse kinematic transformation is employed to convert the specified circular trajectory into joint space trajectories for the purpose of control; while the Matsuoka oscillator has no explicit desired trajectory since it is being generated internally by its dynamics. The nonlinear forward dynamics of the Shank-Foot model is derived using Lagrange's equation method to describe the two degree of freedom (DoF) system. Computed-Torque Proportional-Integral-Derivative (PID) control method was used for tracking the circular designed trajectory, whereas a simple Proportional-Derivative (PD) control law incorporated into the Computed-Torque was used for the tracking of the internally generated trajectory of the oscillator. Tracking capabilities of both control methods based on each trajectory in the presence of external disturbances (uncertainties) and joint friction were analysed using SCILAB. The results obtained show satisfactory performances of the proposed approach.