Performance of optimised prosthetic ankle designs that are based on a hydraulic variable displacement actuator (VDA)

Current energy storage and return (ESR) prostheticfeet only marginally reduce the cost of amputee locomotioncompared to basic solid ankle cushioned heel (SACH) feet,possibly due to their lack of push-off at the end of stance. To our knowledge, a prosthetic ankle that utilises a hydraulic variable displacement actuator (VDA) to improve push-off performance has not previously been proposed. Therefore, here we report a design optimisation and simulation feasibility study for a VDA based prosthetic ankle. The proposed device stores the eccentric ankle work done from heel strike to maximum dorsiflexion in ahydraulic accumulator and then returns the stored energy topower push-off. Optimisation was used to establish the bestspring characteristic and gear ratio between ankle and VDA. The corresponding simulations show that, in level walking, normal push-off is achieved and, per gait cycle, the energy stored in the accumulator increases by 22% of the requirements for normal push-off. Although the results are promising, there are many unanswered questions and, for this approach to be a success, a new miniature, low-losses, lightweight VDA would be required that is half the size of the smallest commercially available device