Energetic Consequences of Series and Parallel Springs in Lower-Extremity Powered Prostheses

Abstract

We present electric energetic consequences for mechanical design trade-offs in lower-extremity powered prostheses. There are four main hardware components commonly implemented in these devices that can be tuned to achieve desired performance: motor, reduction ratio N, series spring stiffness Ks, and parallel spring stiffness Kp. The allowed joint range of motion is a fifth parameter that can also drastically change energy consumption. We apply a kinematically clamped analysis to the system equations to map the electric cost of transport (COT) for knee and ankle level-ground walking, in addition to ankle stair ascent and descent. We also utilize an optimization procedure to identify minimum energy hardware configurations. The energy map provides insight into consequences of variance from optimal parameters. Our results support the contribution of the series elastic element for improved power output. Parallel stiffness can provide up to 8% improvements in walking with minimal negative effect with varied terrain, and a varying ankle transmission ratio can similarly improve COT by 8% from level-ground to stair ascent. Limited dorsiflexion can further improve COT by 30%. These observations can provide the designer clarity to how design decisions modulate hardware performance.