Task Space Outer-Loop Integrated DOB-Based Admittance Control of an Industrial Robot

Admittance control can improve robot performance and robustness in interactive tasks but is still limited by stability when implemented on low-admittance hardware, such as position-controlled industrial robots. This limits applications that require payload, reach, or positioning accuracy. While the idealized reference admittance behavior would be stable with any passive environment (provided positive damping), real robots can be unstable, especially in high-stiffness environments. Thus, instability comes from deviation from the ideal reference model, due to either inner-loop bandwidth, time delay, or other model error. To improve the accuracy of rendered dynamics and reduce contact forces, a novel integrated disturbance observer (DOB)-based admittance control method is proposed. This method does not require access to the robot's inner-loop position control; instead, it is designed and built around it in task space. The task space multisensor information, i.e., the velocity command, measured output velocity, and the force/torque (F/T) sensor measurement are integrated to estimate and robustly suppress the disturbances. Theoretical analyses and experiments on the actual robot show that the proposed method is able to improve admittance tracking accuracy and reduce contact forces even at higher admittance.