Hybrid Volitional Control as a Framework for Lower-Limb Prosthetic Control: A Simulation Study

Realizing the potential of active lower-limb prostheses to increase user mobility and efficiency requires safe, reliable, stable, and intuitive control strategies. The two prevailing classes of lower-limb prosthesis control can be categorized as volitional and non-volitional. Volitional control strategies (VCs) directly sense the user's intentions, but this generally intuitive approach can be quite demanding, leading to user fatigue and device misactivation. Non-volitional control strategies (NVCs) sense the state of the system instead, often taking advantage of the gait's cyclic nature to produce robust and reliable outputs. NVCs, however, do not give the user freedom to realize nonstandard movements. This paper introduces a Hybrid Volitional Control (HVC) approach that operates across the entire gait cycle and seeks to balance the reliability, safety, and low demand of NVCs with the freedom and intuitive control of VCs. Through simulations of 2.5 degrees ramp descent, level ground walking, and 5 degrees ramp ascent, HVC shows an opportunity to reduce the torque error from an able-bodied reference in all cases compared to two NVCs (as much as 94%). Similarly, HVC shows an opportunity to reduce volitional demands compared to a pure VC (as much as 91%). Volitional thresholding is considered for users to ambulate reliably with minimal volitional input demands, while maintaining continuous freedom to alter the dynamics of the device. HVC could allow users to participate in a wider range of activities, helping to further erase the distinction between ability and disability from amputation.