Robust tracking/impedance control: Application to prosthetics

Stability and human-like motion are among the main factors that should be considered while designing a prosthetic limb. The prosthetic limb should be capable of accommodating environmental forces. The existence of parametric uncertainties has raised the need for robust stability and performance of the prosthesis. In this paper, a mixed tracking/impedance robust controller is developed based on passivity techniques. The controller is developed for general robotic manipulators and then applied to a powered knee/ankle prosthesis model attached to a robotic testing machine. Tracking control is used for the hip and thigh links of the test robot, while impedance control is used for the knee and ankle joints of the prosthesis. The dynamics resulting from the interaction between robot and environment are stabilized by specifying a suitable target impedance. The robust passivity framework was used to derive a joint space controller. A Lyapunov function is used to show that the tracking errors of the motion-controlled joints approach zero, while the impedance of the remaining joints approaches the designed target. A simulation study shows how impedance parameters can be used to trade off reference tracking of the impedance-controlled joints and interaction forces.