Controlling Powered Prosthesis Joint Impedance Over Continuous Stance Transitions Between Walking and Stair Ascent/Descent.

Transition strides between level-ground walking and stairs are important parts of everyday locomotion that help maintain balance while sustaining the momentum of the user between these activities of daily living. However, most individuals with transfemoral amputations are unable to perform these continuous transitions with their conventional passive prostheses, instead being forced to initiate transitions with a specific leg or pause at the threshold of the staircase. Powered prostheses have the potential to allow for continuous transitions due to their ability to provide positive work and active control during level-ground walking and stair locomotion, but modern impedance control approaches switch discretely between steady-state controllers instead of emulating continuous joint biomechanics. This work presents a phase-based hybrid kinematic impedance controller that provides biologically-inspired knee and ankle impedance during continuous stance-phase transitions between level-ground walking and stair ascent/descent, assuming high-level knowledge of the transition stride. In an offline analysis of N=12 participants, we show that our continuous stance transition modeling approach significantly outperforms a typical discrete switching strategy in most cases. To experimentally implement the transition model, we define a common thigh-based phase variable for both steady-state and transition strides, giving the user control over prosthesis stride progression. Pilot experiments with two K4 transfemoral amputee participants using a powered knee-ankle prosthesis demonstrate biomimetic kinematic/kinetic features during stair ascent/descent transitions for two stair incline configurations, without subject-specific tuning of control parameters.