3-D Visual Servoing of Magnetic Miniature Swimmers Using Parallel Mobile Coils

Untethered magnetic microrobots can be remotely controlled by dynamic magnetic fields. The ability to pass through complex and narrow regions makes them have great potential in biomedical applications. To date, various magnetic actuation systems and control methods have been developed to drive these tiny agents. However, steering their 3D locomotion at the human scale using electromagnetic systems is still challenging. In this regard, this article explores a closed-loop control strategy that guides swimming of magnetic microrobots at low Reynolds numbers in a large workspace. An eye-in-hand stereo-vision mobile-coil system is adopted to provide required dynamic magnetic fields and vision information. Both corkscrew-type and flexible-oar-type swimmers are analyzed, and a unified kinematic model is established. Based on the system and model, a triple-loop visual servoing scheme is proposed to fulfill simultaneous mechanism tracking and swimmer steering. Two different scaled-up untethered microrobots are employed in the path following experiment. Our prototype magnetic actuation system reaches a cylindrical workspace of diameter over 200 mm, and the mean error of path tracking is within 2 mm. These results provide a preliminary study for dexterous magnetic actuation of miniature robots for medical treatment.