Biomimetic virtual constraint control of a transfemoral powered prosthetic leg

This paper presents a novel control strategy for a powered knee-ankle prosthesis based on biomimetic virtual constraints. We begin by deriving kinematic constraints for the “effective shape” of the human leg during locomotion. This shape characterizes ankle and knee motion as a function of the Center of Pressure (COP)-the point on the foot sole where the ground reaction force is imparted. Since the COP moves monotonically from heel to toe during steady walking, we adopt the COP as the phase variable of an autonomous feedback controller. We show that our kinematic constraints can be enforced virtually by an output linearizing controller that uses only feedback available to sensors onboard a prosthetic leg. This controller produces walking gaits with human-like knee flexion in simulations of a 6-link biped with feet. Hence, both knee and ankle control can be coordinated by one simple control objective: maintaining a constant-curvature effective shape.