Improving force control performance by computational elimination of non-contact forces/torques

Regarding manipulators with wrist-mounted force/torque sensors a major issue is the high execution time of force-guided and force-guarded motions compared to purely position-controlled tasks. An important factor that aggravates the reduction of the execution time is the influence of non-contact forces, e.g. inertial forces, centrifugal forces, Coriolis forces, and associated torques, which are exerted onto the sensor by a load attached to it. Considering force-guided or force-guarded motions, these non-contact forces may significantly deteriorate contact detection and force control performance when executing dynamic movements. In addition to these disturbance forces/torques, resets of the force/torque sensor consume execution time. This paper presents an approach to eliminating all non-contact forces and associated torques from force/torque sensor measurements thus enabling pure contact force control. Apart from facilitating pure contact force control, the presented approach renders resets of the force/torque sensor unnecessary. To achieve this aim, the ten inertial parameters (mass, coordinates of the center of mass, and the elements of the inertia matrix) of the load attached to the sensor as well as the force/torque sensor offsets are estimated on-line employing a variant of the recursive instrumental variables method. These parameters are used to calculate the non- contact forces/torques acting upon the sensor. The current non-contact forces/torques and sensor offsets are subtracted from the force/torque measurements thus yielding the contact forces/torques. Experimental results show that both contact detection and force control performance are improved significantly by this approach.