Offsets and Gravity Parameters Estimation for Noncontact Force Compensation With Wrist-Mounted Force/Torque Sensor

Abstract

Automatic maintenance of components within fusion reactors requires precise force perception, typically achieved by installing force/torque (F/T) sensor on the robot’s wrist. However, the force measured by F/T sensor include not only contact force but also noncontact force and associated torques, such as end-effector gravity, inertial force, and sensor zero offsets. Inertial force can be ignored due to the low operational speed and small speed variation during the maintenance. In this article, we propose a novel noncontact force compensation method, which can estimate gravity and centroid of tool and load, sensor zero offsets, as well as robot installation and sensor installation offsets. The Gröbner basis method is used to estimate the gravity of the tool and load in robot base frame and the orientation offsets between the robot end and F/T sensor. The constraints of rotation matrix are naturally satisfied with the quaternion parameterization. All the parameters can be estimated with only three measurements. A nonlinear least squares method is employed to refine estimation results by utilizing all available measurements, thereby enhancing the estimation reliability. A comparative study, incorporating both simulations and experiments, was conducted. The results show that our method outperforms the existing methods in various aspects with fewer than five measurements, and achieves the same compensation accuracy compared to the state-of-the-art method when the number of measurements is greater. Moreover, our method eliminates the need for robot installation angle.