Optimal Spring Layout and Specifications for Linear Active Ankle Orthoses

Ahmad Bagheri and Mohammad J. Sadigh


Biomechanics, Linear active ankle orthosis, Minimum energy


Passive orthoses are incapable of providing the torque disabled users require to walk in a natural manner. An active assistive device contrarily, can fully follow the required torque in each instant of gait; an active device however, raises issues on energy consumption. The use of passive elements such as springs can affect the power required. In this investigation different possible linear actuator geometries and layouts for an orthosis designed for a hemiplegic patient, are evaluated based on motor energy consumption and peak power; different architectures consist of the presence or absence of parallel and series springs. In each case the optimal spring stiffness, preload and offset (if existent) is found. Based on the variations mentioned above, the best actuator layout and the optimal parameters of passive elements in an active orthosis are derived in order to yield an efficient active ankle orthosis actuator. It was found that the presence of elasticity in most cases improves peak power and/or energy consumption of the actuator. Series parallel elasticity has great effects on the reduction of peak power and series elasticity reduces both peak power and energy consumption to some extent. A plantarflexion parallel spring has no effect on peak power and little effect on energy consumption; a dorsiflexion parallel spring can decrease either peak power or energy consumption. The combination of a series elasticity with a dorsiflexion parallel elasticity offers both acceptable power and energy requirements.

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