Impedance Control Based on a Position Sensor in a Rehabilitation Robot

We examine the implementation of impedance control based on a position sensor in a wearable robot designed for rehabilitation. The first goal is to achieve the lowest possible impedance for the robot, with only a position sensor and an ON/OFF type pressure sensor, so that it is dynamically transparent to the user. We built a wearable robot for controlling the user’s arm with a brushless DC motor and a two-stage gearing system involving a planetary gearbox and a Capstan drive. We designed and implemented a controller to compensate for the inherent friction and reflected inertia using joint angle feedback from the robot, and for the stiction using the user’s intended direction detected by a pressure sensor. Stability conditions are analyzed first for the robotic system alone and then for the coupled human-robot system. Experiments with the robot show that the apparent impedance was significantly reduced with compensation. Experiments involving free motions driven by a user proved that the user’s physical effort to move the robot is dramatically decreased with compensation, thus making the robot feel lighter to the user.