Force Control for Active Chassis Balancing

This paper presents the realization of the worldwide first automated walking excavator chassis. To this end, the authors build a new generation of high-performance hydraulic valves with integrated pressure feedback to achieve fast and accurate cylinder force tracking. This allows automatically adapting the legs to uneven terrains and optimally shaping the ground reaction forces in order to change the orientation and height of the cabin. Due to the contact redundancy, automated balancing is implemented as a contact force optimization problem, including constraints on contact forces and joint torques. The corresponding prioritized optimization problem can be simplified by using a quasi-static approximation of the system dynamics and a complexity reduction due to the kinematic structure of the legs. Our approach considers the unknown configuration and load of the cabin, arm, and bucket as system disturbances, whereby gravitational effects are approximated as well as possible. It is tested in a Gazebo simulation and validated in different experiments by using a prototype walking excavator machine. The proposed method revolutionizes operator control of these versatile but complex multipurpose vehicles: Instead of manual and coordinatively very demanding cylinder position adjustment, the operator can command simple high-level commands like a cabin pose. Furthermore, it significantly reduces peak forces in the cylinders and at the contact points, which causes less damage to the mechanics and the ground.