Control of a quadrupedal manipulator using hierarchical inverse dynamics

Abstract

Currently the prevalence of general purpose mobile robots with manipulation capabilities is still low, despite various applications of such systems such as: disaster response, payload delivery, and assistive/service tasks. A suitable design for such a robot would be that of a torque-controllable quadrupedal manipulator. Its capability for legged locomotion enables high mobility, specifically on rough terrain, while its quadrupedal morphology provides a relatively large and stable base of support compared to bipedal robots. Torque-controlled joints allow for safer and more controllable interaction with the environment.

For such a system a challenge lies in the design of a controller that actually achieves the promising capabilities for locomotion and manipulation that the mechanical system offers. One of the currently most promising control frameworks for this purpose is that of hierarchical inverse dynamics. This real-time whole-body control framework allows for dynamic whole-body motions and compliant interaction with the environment while enforcing a strict priority between the desired control tasks. Although promising results have previously been attained with such controllers, it has not yet been applied for the control of a quadrupedal manipulator. The focus of this thesis is on the implementation of a hierarchical inverse dynamics controller for the control of a simulated quadrupedal manipulator, with a particular focus on the design of prioritized sets of control tasks which generate forms of stationary whole-body manipulation.

First a basic set of prioritized control tasks is presented, which is shown to satisfy the robot's most crucial control requirements. Subsequently three extended sets of control tasks are presented, which result in additional desirable emergent behavior of the robot. The first one of these depends on the inclusion of kinematic joint limit tasks to prevent kinematic singularities and self-collision, and to trigger whole-body reaching motion. Secondly a set of tasks is presented which is focused on utilizing motion of some of the torso's degrees of freedom to optimize the arm's posture according to a posture-related cost function. Thirdly a set of tasks is presented which enables contact force control at the end-effector for force-based manipulation. In addition to this final set of tasks, a higher-level controller is presented which detects external forces acting on the robot and computes a desired shift of the position of the robot's center of mass in order to mitigate the balance-disturbing effects of external forces. All of the presented sets of tasks do not exclude each other and allow to be implemented simultaneously in order to combine the individual benefits that they offer.

The results of this research project show that hierarchical inverse dynamics control can be successfully applied for the control of a simulated torque-controllable quadrupedal manipulator. Moreover, it is shown that well-designed sets of prioritized control tasks allow for emergent whole-body behaviors which exploit the advantages that both the robotic system and the control framework offer. Future work will need to investigate the transferability of these results to a physical robot.