XPED2: A Passive Exoskeleton with Artificial Tendons

Abstract

Wearable exoskeletons might reduce human effort during walking. However, many of the current exoskeletons rely on heavy actuators and/or external power supplies; this has a negative impact on their efficiency and operation range. As an alternative, (quasi)passive exoskeletons have been developed. One of the proposed passive exoskeleton concepts is the exotendon concept of van den Bogert [1]. In this concept, long elastic cables span multiple joints. The cables can temporarily store and transfer energy between joints. In simulation, the average absolute joint torque can be reduced by 71%. The simulations are based on the hypotheses 1) that the exoskeleton does not influence the joint angles and 2) the total joint torques and a reduction in the human joint torques results in a reduction in the metabolic cost of walking. The goal of this article is to experimentally evaluate the exotendon concept and test the hypotheses underlying it. We implemented the exotendon concept in a lightweight exoskeleton. Experimental results show that the exotendons indeed reduced the average absolute joint torques. However, the exotendons also influenced the joint kinematics, and the metabolic cost of walking did not decrease. Therefore, the underlying assumptions of the exotendon concept are invalid. We also found that, in practice, the amount of support given by the exotendons is limited to about 35% of the theoretical optimal support. For higher levels of support, the motion is hindered and the support is experienced as uncomfortable by the users of the exoskeleton.