Powered lower-limb prostheses have shown promise in helping individuals with amputation regain functionality that passive prostheses cannot provide. However, the best method for controlling these devices in coordination with their users is still an open research topic. While powered devices can replicate normative joint kinematics and kinetics, active control also holds the potential to shape system-level characteristics such as the center of mass (CoM) that play an important role in balance. Controlling the prosthesis based on these system-level, or task-space, variables would further represent a new way of coordinating the user and their device.This paper explores the initial implementation of task-space control for a powered ankle prosthesis, characterizing the emergent outcomes of this new coordination strategy. One able-bodied subject walked using a bypass adapter while prosthesis torques were commanded based on reference ground reaction force (GRF) and CoM trajectories. The subject could walk comfortably and continuously at their preferred walking speed, achieving normative ankle torques and joint trajectories despite not tracking explicit joint-level references in stance.

For robotic transtibial prosthesis control, the global tibia kinematics can be used to monitor gait cycle progression and command smooth and continuous actuation. In this work, these global tibia kinematics define a phase variable impedance controller (PVIC), which is implemented as the nonvolitional base controller within a hybrid volitional control framework (PVI-HVC). The gait progression estimation and biomechanic performance of one able-bodied individual walking on a robotic ankle prosthesis via a bypass adapter are compared for three control schemes: benchmark passive controller, PVIC, and PVI-HVC. The different actuation of each had a direct effect on the global tibia kinematics, but the average deviation between the estimated and ground truth gait percentages were 1.6%, 1.8%, and 2.1%, respectively, for each controller. Both PVIC and PVI-HVC produced good agreement with able-bodied kinematic and kinetic references. As designed, PVI-HVC results were similar to those of PVIC when the user used low volitional intent, but yielded higher peak plantarflexion, peak torque, and peak power when the user commanded high volitional input in late stance. This additional torque and power also allowed the user to volitionally and continuously achieve activities beyond level walking, such as ascending ramps, avoiding obstacles, standing on tip-toes, and tapping the foot. In this way, PVI-HVC offers the kinetic and kinematic performance of the PVIC during level ground walking, along with the freedom to volitionally pursue alternative activities.