A Multi-Robot Hoisting System that Achieves Precision by Constructing and Exploiting Mechanical Constraints through Non-Prehensile Control

Abstract

This paper describes a multi-robot solution for pick-and-place operations, that creates and exploits mechanical constraints to control the payload through interactions. The task requirements are split across a strong, but imprecise, crane and an accurate, but weak, dual-arm manipulator. First, the design of the manipulator is discussed. Its task is to align its endeffectors with features in the environment and create additional mechanical constraints. Second, a coordinated control strategy between agents is described in order to accurately position and align a payload relative to another object in the environment. The crane controller exploits interactions between the payload and the mechanical constraints created by the manipulator to control the payload and reduce its state uncertainty. A box stacking use case was performed to validate the method. The task could be successfully executed, even when tracking errors were superimposed on the position- and velocity setpoints of the crane control. This highlights the robustness of the approach, and the possibility to implement it on existing, imprecise cranes with low control bandwidth.