Fundamental study of force control method for pelvis-supporting body weight support system

An active body weight support (BWS) system, which unloads body weight with pelvic support, has been developed to assist the walking movement. This system unloads body weight with a motor-actuated device from below with pelvic support, unlike prevailing BWS systems that lift up the subject from above via a harness connected to a wire. The force control method to unload body weight has not been sufficiently studied. As a first step, a comparison study between normal walking and walking with the developed BWS mechanism was conducted to specify the force control method. Since the precise and constant unloading force is believed to be an important prerequisite for BWS gait therapy, different constant unloading forces were set as the target unloading force. The target unloading force was varied from 100 (N) to 300 (N) to observe the difference among forces. The measured unloading forces were not completely constant but fluctuated. The motor introduced a delay at over 200 (N) during lifting. As a result, the floor reaction force was reduced by the target unloading force. However, some differences were found in the bimodal shape of the floor reaction force and the trajectory of the sacrum's vertical position. The results showed the necessity of damping the fluctuation of unloading force by improving following characteristics and adjusting target velocity. Further precise force control will be carried out to realize constant unloading force.