Cooperative Control Strategy of Multiple Magnetic Millirobots for Automatic Assembly

Cooperative control of multiple small-scale robots significantly increases capability compared to a single one. However, multiple robots driven by the same magnetic field receive the same input signal, causing hardly different responses. In our previous work, multiple robots with heterogeneous magnetization angles could be at the same velocity or only one could move and the others not. However, cooperatively controlling multiple robots in the same magnetic field remains a challenge because of the difficulty of arbitrarily adjusting the velocity relationships of multiple robots. In this work, we propose a novel cooperative control strategy of multiple magnetic millirobots based on their velocity difference for the automatic manipulation and assembly of objects. The velocity difference between the multiple millirobots with different magnetization angles is modeled and controlled by the inclination angle of the external oscillating magnetic field. We developed a differential wheel-inspired cooperative control method of a two- or four-millirobot team for cooperative object transport in closed-loop control. Considering the efficiency and collision avoidance of high-precision assembly of multiple objects, an optimal task allocation method based on the shortest total transport path and an improved sampling-based path planning method in real-time are proposed. Experiments demonstrated two and four-millirobots were able to cooperatively transport objects of different shapes with a maximum mass of 16 times that of a single robot. Furthermore, the multimillirobot team could autonomously assemble three strip objects to the desired shape with a position error of less than 2.16 mm and an orientation angle error of fewer than 5.07$^\circ$.