A novel energy storage and return prosthetic ankle using cams and miniature hydraulics

A novel design for an energy-storage-and-return (ESR) prosthetic ankle is described, based on simple miniature hydraulics. The aim is to overcome the weaknesses of prosthetic feet currently on the market and also previous research attempts, which are not able to mimic the energy-recycling behaviour of an intact ankle, penalising lower-limb amputees’ gait in terms of metabolic cost and walking speed. The ankle joint drives two cams, which in turn drive two hydraulic rams. The “stance cam-ram system” captures the eccentric (negative) work done from foot-flat until maximum dorsiflexion, by pumping oil into an accumulator. The “push-off cam-ram system” returns concentric (positive) work during push-off to provide forward propulsion through fluid flowing from the accumulator to the ram. Simulation results show that the new design can mimic intact ankle torque during level walking, storing the negative work done from foot-flat to maximum dorsiflexion and returning it during push-off in a controlled way for forward propulsion. In level walking, of the total eccentric work done by the prosthetic ankle over the gait cycle, which is the maximum amount of energy available to be stored and returned, 78% is returned, mainly during push-off; 14% is carried forward for future gait cycles (e.g., for ascending slopes); and 8% is lost. The estimated prosthesis height from the ground to the distal end of the socket is approximately 28 cm, which would be suitable for over 97.72% of transtibial amputees. The estimated mass is 2.35 kg, which is comparable with the missing anatomy mass.