Control of untethered magnetically actuated tools with localization uncertainty using a rotating permanent magnet

Magnetically actuated tools (MATs) that utilize rotating magnetic fields for propulsion, such as active capsule endoscopes and magnetic microrobots, have typically been controlled using either arrangements of electromagnets or permanent-magnet systems operated in limited configurations. It was recently shown that a rotating magnetic field for MAT actuation can be generated using a single rotating permanent magnet (RPM) from any position in space with a unique axis of rotation. The method has potential benefits for clinical systems, but it requires knowledge of the MAT position with respect to the RPM. In any application, MAT localization will be subject to uncertainty caused by sensor noise, slow update rates, and/or localization failure. In this paper, we develop and experimentally verify worst-case bounds on properties of the rotating dipole field, given a worst-case bound on localization error, which can be used to design operating procedures that mitigate undesired MAT behavior in the presence of known localization uncertainty. The results are important for the robust operation of rotating MATs actuated using a single rotating permanent magnet in a clinical setting.

I. Introduction

Untethered magnetic devices, such as magnetic micro robots [1] and active capsule endoscopes [2], have become an active area of research because of their potential impact to minimally invasive medicine. These devices typically consist of a rigidly attached magnetic body on which magnetic forces and torques are applied by an external field. Due to the difficulty of scaling electromagnetic systems to clinical sizes, researchers are considering the use of permanent magnets for actuation. Some of these approaches utilize magnetic forces for pulling [3], while others apply torque generated by rotating magnetic fields to roll on a surface [4], [5], or crawl through a lumen via helical propulsion [6]. Because these devices can be viewed as simple end-effectors of a larger robotic system, and they may range in size from the microscale to the mesoscale, we refer to them herein as magnetically actuated tools (MATs) without any implied size.