Series admittance–impedance controller for more robust and stable extension of force control

To control a robot performing cooperative work between a human and robot, not only the position but also the force must be controlled from the viewpoint of human–robot contact. In addition, when a robot is used for fitting and handling, tasks that are conventionally performed by experienced humans, controlling the grasping force and the force exerted by the joints can produce motions similar to those of humans and contribute to improving the success rate of the work. In the field of force control, in addition to direct force control, admittance control and impedance control are modes based on the relationship between position and force, which are known to be robust and safe. However, admittance control often becomes unstable when the robot comes into contact with a rigid body, and the performance of impedance control is degraded by friction. In this study, we aim to realize safe and accurate force control in cooperative work with humans. As a precursor, we propose admittance and impedance control, which is a series connection of conventional admittance control and impedance control. We show that the proposed force control is more robust, stable, and accurate than impedance control and admittance controls alone, or at least as good as them, when in contact with an unknown environment. Its basic effectiveness and practical usefulness are demonstrated through numerical simulations and experimental results.