Abstract: The stable control of contact force is generally required for industrial robot to perform the contact work. For example, in the grinding process, the surface quality can be easily affected by the unstable contact force in the normal direction. To solve the overshoot of normal speed and the unstable contact force in the normal direction, which caused by uncertain environment, a force tracking method for industrial robot based on model predictive control is proposed. Firstly, the trajectory of the end tool is calculated based on the geometric information of the workpiece. And the Cartesian velocity of the end tool is then calculated by combining with the current robot position. Secondly, the state-space model of contact status between the end tool and the workpiece is developed, and the damping coefficient in the normal direction is adjusted online based on the attitude of end tool. Thirdly, the normal velocity is corrected by the model predictive control algorithm based on the feedback of the real-time force signal to achieve constant force tracking of the surface. Finally, two surface tracking experiments are conducted under the situations of constant and changeable end tool attitude respectively, by Staubli TX90 industrial robot. The experimental results show that, the contact force in the normal direction fluctuated in the range of 1 N and 2 N with variance of 0.038 1 N² and 0.105 9 N², respectively, which can realize favorable force tracking.

Key words: model predictive control, surface tracking, velocity correction, constant force control