Compliance Control for Robot Manipulation in Contact with a Varied Environment Based on a New Joint Torque Controller

Compliance control is required in the applications of robots for assembling, grinding, polishing and human-robot interface, which needs both position control and force control of robots. The impedance control based on joint torque servo is a promising and practical method to realize compliance control in industrial applications. The performance of a joint torque servo controller is thus crucial to the success of compliance control. However, both nonlinear friction torque of the joint motor and varied dynamics of environmental contact during manipulation increase control difficulty greatly. This paper focuses on the compliance control problem and presents a new joint torque servo controller which is a cascading structure including an inner velocity feedback loop and an outer torque control loop. Due to the high signal-to-noise ratio of the motor velocity, an extended state observer is designed to effectively estimate and compensate for the motor friction torque, varied dynamics of environmental contact and other unknown disturbance. And benefiting from the introduction of the efficient and powerful velocity inner loop, the new torque controller performs very well not only in the rigid but also elastic contact environment. Then a standard PD controller is designed in the outer torque loop to produce the control law. Based on the satisfactory joint torque controller, a simplified impedance control algorithm is designed to achieve the force control. Experiments with three other joint torque controllers on a robot manipulator are conducted to demonstrate the proposed joint torque method and the overall force control framework. These results show the proposed joint torque controller can reduce the steady-state error from 3.07 Nm to 0.21 Nm which indicates the joint actuator friction can be eliminated by more than 99%. And a substantial improvement can also be observed in the experiments of force control based on the new joint torque controller.