Force Sensor-less Compliant Control of Robot Based on Position-loop

Force control is needed in many robot operations. Force sensor can be used to precisely and rapidly estimate interaction forces, but may be expensive and affect the system's stiffness. Additional, there are robot resistances caused by friction and gravity making negative effects. A common approach is to establish a model and compensate and/or suppress the resistance in the motor input torque, but it may be not accessible to researchers due to the closure of motor drivers. A force sensor-less compliant control method for robot is proposed in this paper. Without force sensor, the external torques caused by force acting on the robot arm, is estimated through motor current. The resistance is addressed as part of the external torque, and then converted together into movement according to robot dynamic. A dither signal is also added to the position-loop to suppress the static friction at start-up. The control scheme is applied to flexible joint control and drag teaching control, which are simulated in MATLAB and experimentally validated on a mon-axial robot.