Enhancing force controllability by mechanics in exoskeleton design

This paper presents an exoskeleton concept based on low-size and low-cost electromagnetic motors which allows to improve force controllability and reduce costs at the same time. The idea comes from the theoretical analysis of a simple interaction control model which suggests that lowering the motor inertia leads to improved performance robustness. In particular, we show that a lower motor inertia reduces the sensitivity to the wearer impedance which is usually characterized by high uncertainties. In order to reduce the motor size, this paper proposes to lower the motor torque requirements and considers a parallelogram-based architecture with mechanical gravity compensation. A two degree of freedom prototype is realized to implement the concept and to experimentally validate our hypotheses, showing coherence with theoretical expectations and paving the way for a new generation of affordable forced-controlled exoskeletons.