Fuel efficiency of a Portable Powered Ankle-Foot Orthosis

Abstract

A Portable Powered Ankle-Foot Orthosis (PPAFO) has been designed for gait assistance. The PPAFO can supply assistive torque at the ankle joint in plantarflexion and dorsiflexion using a bidirectional pneumatic actuator. Two control schemes have been developed to regulate timings of the assistive torques during different phases in the gait cycle. The Direct Event (DE) controller uses heel and toe force sensors to detect the start and end of key phases using specific events (e.g., heel strike and toe-off). The State Estimation (SE) controller finds the least-square-error between real-time sensor data and a reference model from training data to estimate the gait state and to detect phases based on this estimate. A pneumatic recycling scheme for improved fuel efficiency was also implemented. This scheme regenerates energy from plantarflexion exhaust gas to power dorsiflexion actuation. The objective of this study was to assess the fuel efficiency of these two controllers and pneumatic recycling scheme, as measured by fuel consumption and work output. Data were collected from 3 minute walking trials with the PPAFO by five healthy young control subjects. The SE with recycling (SER) scheme had an average fuel savings of 25% compared to the SE control scheme, and 24% compared to the DE controller. The SER controller allowed for comparable net work output to the SE controller which both did more net work than the DE controller. These observations can be applicable to other portable fluid-powered orthotics, prosthetics, and robotics in terms of potential impact of controller choice and energy regeneration on fuel consumption.