Improving Sit/Stand Loading Symmetry and Timing Through Unified Variable
Impedance Control of a Powered Knee-Ankle Prosthesis
Abstract
Individuals using passive prostheses typically rely heavily on their
biological limb to complete sitting and standing tasks, leading to
slower completion times and increased rates of osteoarthritis and lower
back pain. Powered prostheses can address these challenges, but have
control methods that divide sit-stand transitions into discrete phases,
limiting user synchronization across the motion and requiring long
manual tuning times. This paper extends our preliminary work using a
thigh-based phase variable to parameterize optimized data-driven
impedance parameter trajectories for sitting, standing, and walking,
with only two classification modes. We decouple the stand-to-sit and
sit-to-stand equilibrium angles through a knee velocity-dependent
scaling term, reducing the model fitting error by approximately half
compared to our previous results. We then experimentally validate the
controller with three individuals with above-knee amputation performing
sitting and standing transitions to/from three different chair heights.
We show that our controller implemented on a powered knee-ankle
prosthesis produced biomimetic joint mechanics, resulting in
significantly reduced sit/stand loading asymmetry and time to complete a
5x sit-to-stand task compared to participants’ passive prostheses.
Integration with a previously developed walking controller also allowed
sit/walk transitions between different chair heights. The controller’s
biomimetic assistance may reduce the overreliance on the biological limb
caused by inadequate passive prostheses, helping improve mobility for
people with above-knee amputations.