Friction Compensation in Cable-Conduit Transmission System of Hand Rehabilitation Robot

In cable-conduit transmission system of hand rehabilitation robot, friction between cable and conduit can cause power loss in force control process, sometimes results in dead zone and hysteresis phenomena, even brings other problems such as system instability. Thus, it is needed to compensate the friction. Firstly, the tension in cable is formulated when finger bending in arbitrary shape. Then, the transmission mode of a finger joint is simplified as follows: motor 1 runs in position control mode to simulate the motion of the joint. Motor 2 runs in torque control mode to compensate the friction in cable. Each cable passes through a conduit and the ends of cable are fastened on output pulleys attached to each of the motors. The tension difference between both ends of cable is the friction in cable and conduit. Friction is transformed into equivalent friction torque of motor 2 shaft. The compensation motor runs in the torque control mode, and the direction of its output torque is opposite to the equivalent friction torque. Assuming that time interval is small enough and finger moves slowly, equivalent friction torque at t moment can be calculated by tensions at t-1 moment. Using this iterative method, compensation torque at different moments that motor 2 should output can be obtained to compensate friction in cable and conduit. Finally, an active rehabilitation experiment is carried out on an experiment setup, and the result shows the resistance reduces to 15% after compensation, which means the method is effective.