Leader–Follower Formation Control of Magnetically Actuated Millirobots for Automatic Navigation

Multiple magnetically actuated small-scale robots enable cooperative tasks and have great potential for many applications. In general, magnetic robots behave in the same way with the same input magnetic fields. Therefore, cooperative control of magnetic robots is challenging. In this article, we propose a leader–follower formation control of magnetically actuated millirobots with heterogeneous magnetization, including a model-free controller with an expansive state observer for the follower, a fuzzy logic-based controller for the leader, and a cooperative controller. The model-free controller for the follower is developed to robustly transform and maintain the desired geometric formation. The fuzzy logic-based controller for the leader is used to follow the given reference path with rapid responses. Two cooperative control strategies based on the velocity control of the millirobot team are proposed to achieve path-following control of team. Experiments with the two-millirobot team verify that one strategy shows better performance with varying geometric formation constraints, and the other presents high efficiency with invariant geometric formation constraints. The experiment demonstrates the ability of the three-millirobot team to cross a narrow obstacle by changing formation. A novel path planning algorithm called enhanced informed optimal rapidly exploring random tree algorithm is developed to achieve an automatic navigation, which integrally considers the spatial distribution characteristics of the electromagnetic field, search efficiency, and obstacle avoidance. Experiments demonstrate the performance of the method for navigating the leader–follower formation in confined environments.