A Control Framework for Accurate Mechanical Impedance Rendering With Series-Elastic Joints in Prosthetic Actuation Applications

In addition to lifting up the body during gait, human legs provide stabilizing torques that can be modeled as a spring-damper mechanical impedance. While powered prosthetic leg actuators can also imitate spring-damper behaviors, the rendered impedance can be quite different from the desired impedance, stemming from unmodeled transmission characteristics (e.g., sliding friction, bearing damping, gear inefficiency, etc.). Moreover, for powered prostheses to mimic human joint impedance, they will need actuators that accurately render a wide range of mechanical impedances in a variety of ground contact conditions, including nearly free-swinging behavior in swing phase and stiff spring-like behavior in stance phase. For series-elastic prosthetic leg actuators, as in the Open-Source Leg (OSL), these sudden output inertia changes present a challenge for traditional cascaded impedance control. In this paper we propose a solution based on disturbance observers (DOBs) and full-state feedback (FSF) impedance control. The DOB serves to mask transmission imperfections, while the FSF controller (via pole-zero placement) specifies the actuator impedance that couples to the uncertain joint inertia. We validate our control framework on an OSL-like two-actuator dynamometry testbed.