Robotic Force Estimation Using Motor Torques and Modeling of Low Velocity Friction Disturbances

Abstract

For many assembly operations force control is needed, but force sensors may be expensive and add mass to the system. An alternative is to use the motor torques, though friction causes large disturbances. The Coulomb friction can be quite well known when a joint is moving, but has much larger uncertainties for velocities close to zero. This paper presents a method for force estimation that accounts for the velocity dependent uncertainty of the Coulomb friction and combines data from several joints to produce accurate estimates. The estimate is calculated by solving a convex optimization problem in real time. The proposed method is experimentally evaluated on a force-controlled dual-arm assembly operation and validated with data from a force sensor. The estimates are shown to improve with the number of joints used, and the method can even exploit data from an arm that is controlled not to move.