Optimal spring layout and specifications for rotary active ankle orthoses

Abstract

An active assistive device, unlike passive ones, can be capable of fully providing the torque a disabled user requires in daily tasks such as walking. An active device will however need an external power source, which will result in a heavier, more sizeable device; it is hence essential that power requirements and energy consumption are minimal. The use of relatively light passive elements such as springs can alter these parameters without having negative effects on weight and size. In this investigation the effects of different possible spring layouts on a rotary active ankle orthosis intended for hemiplegic patients are assessed based on power and energy requirements. Different layouts vary in the presence or absence of series and parallel springs and also their preload and wherever possible offset. The effect of these springs, when optimal, and the impact of their layout is evaluated in order to yield an efficient rotary ankle orthosis actuator. It is shown that a series elastic actuator (SEA) reasonably decreases both power and energy consumption but parallel elasticity has greater impact on reducing peak power. A plantarflexion parallel spring effects neither energy nor peak power, whereas a dorsiflexion parallel spring offers either lower peak power or less energy consumption. Combining a SEA or parallel elastic actuator with a parallel or dorsiflexion parallel spring offers acceptable power and energy requirements.