Impedance & Admittance-Based Coordination Control Strategies for Robotic Lower Limb Prostheses

This article presents and compares two different control systems for a powered knee and ankle prosthesis for transfemoral amputees, which were constructed to provide the user a safe, intuitive, and well-coordinated interaction with the prosthesis. The piecewise-passive impedance (PPI) controller utilizes only impedance-like behaviors, while the second – a hybrid impedance-admittance (HIA) controller – utilizes both impedance-like and admittance-like behaviors in a hybrid approach. The HIA approach maintains many of the desirable characteristics of the PPI controller while also reducing the number of selectable control parameters. The HIA approach essentially incorporates the PPI control structure during the early and middle stance phases of gait, and a trajectory tracking control approach in terminal stance and swing. These controllers were implemented on a powered knee and ankle prosthesis and tested in walking trials by a transfemoral amputee. Data from these trials indicate that both controllers achieve comparable performance with respect to healthy subject data, despite some substantial structural differences between the two.