Cooperative Control of Two Magnetically Driven Microrobots for Automated Assembly

In this work, we propose a novel cooperative control strategy of two magnetic millirobots based on their velocity difference for the automatic manipulation and assembly of objects. The velocity difference between the two millirobots is modeled and controlled by the inclination angle of the external oscillating magnetic field, thus cooperatively transporting and steering arbitrarily shaped objects in contact. Controlling the positions and formations of two millirobots simultaneously in the same magnetic field makes the system underdriven because there are more states than control inputs. A cooperative control method of the two-millirobot team inspired by differential wheels is proposed to address the underdriven problem, thus enabling closed-loop control of two millirobots for collaborative object transport. 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 that the millirobot team could cooperatively transport objects 4 times heavier and 26 times larger than a single robot in closedloop path-following control. Furthermore, the millirobot team could autonomously assemble 3 strip objects to the desired shape with a position error of less than 2.16 mm and the orientation angle error of fewer than 5.07 degrees.