Design of a Lower Extremity Exoskeleton Robot for Balance Control of Paraplegics

This paper proposes a robotic lower extremity exoskeleton for maintaining the balance of a paraplegic patient during walking gait. Stable gait is achieved by actuating five degrees-of-freedom per leg and by adopting a gait pattern based on a variation of the nonlinear inverted pendulum model. The exoskeleton allows the shifting of the center of mass in the horizontal plane to maintain a positive margin of stability during the walking gait cycle. A mathematical model was developed to determine the design specifications of the exoskeleton, i.e., model dimensions and actuator performance, to achieve a desired margin of stability and dynamic balance control. The simulation results indicate that the exoskeleton is successful in maintaining a stable gait with a low positive dynamic margin of stability of 15 mm at a chosen speed of 0.15 m/s. However, to address the limitations of the standalone exoskeleton such as recovering balance following perturbations and the need for actuators with higher bandwidth for meeting an actively balanced gait, a compact support structure is introduced as an accessory to the proposed exoskeleton. The additional structure included increases the margin of stability while partially supporting the weight of the user and exoskeleton, while allowing a natural gait cycle. The structure is comparable to a wheelchair in terms of form factor while offering greater level of health benefits and safety compared to a standalone lower extremity exoskeleton. According to the simulation results, the minimum margin of stability during walking gait can be increased to 230 mm, with a supporting structure. The support structure allows the proposed system to be used for both motion assistance in daily living and rehabilitation of paraplegics without the need for crutches or upper body strength to maintain balance during walking.