Towards Recovering a Lost Degree of Freedom in Magnet-Driven Robotic Capsule Endoscopy
Authors: Slawinski Piotr R. , Garcia Collin T. , Taddese Addisu Z. , Obstein Keith L. , Valdastri Pietro 
Published: October 2017

Flexible endoscopy, a procedure during which an operator pushes a semi-rigid endoscope through a patient’s gastrointestinal tract, has been the gold-standard screening method for colon cancer screening (colonoscopy) for over 50 years. Owing to the large amounts of tissue stress that result from the need for transmitting a force to the tip of the endoscope while the device wraps through the bowel, implementing a front-actuated endoscopy system has been a popular area of research [1]. The pursuit of such a concept was accelerated by the advent of ingestible capsule endoscopes, which, since then, have been augmented by researchers to include therapeutic capabilities, modalities for maneuverability, amongst other diagnostic functions [2]. One of the more common approaches investigated has been the use of magnetic fields to apply forces and torques to steer the tip of an endoscope [3]. Recent efforts in magnetic actuation have resulted in the use of robot manipulators with permanent magnets at their end effectors that are used to manipulate endoscopes with embedded permanent magnets.
Recently, we implemented closed loop control of a tethered magnetic capsule by using real-time magnetic localization and the linearization of a magnetic wrench applied to the capsule by the actuating magnet [4]. This control was implemented in 2 degrees-of-freedom (DoF) in position (in the horizontal plane) and 2 DoF in orientation (panning and tilting). One DoF in position is lost owing to the tethered capsule being actuated in air and thus lacking a restoring force to counter the high field gradient. The 3rd orientation DoF is lost owing to the axial symmetry of the permanent magnet in the capsule; this prevents the application of torque in the axial direction and thus controlled roll and introduces a singularity in the capsule’s actuation. Although another dipole could be used to eliminate this singularity, this would complicate both the actuation and localization methods. In this manuscript, we consider the consequences of the embedded magnet (EM) being radially offset from the center of the capsule while being manipulated by an external actuating magnet (AM).
We have developed a tethered capsule endoscope that contains a cylindrical EM (11.11 mm in length and diameter) with a residual flux density of 1.48 T that is offset by 1.85 mm from the center of the capsule; a distance that is less than 10% of the capsule diameter. Our investigation into the topic results from repeated observation of the capsule’s preference to align such that the internal magnet is closest to the actuating magnet (AM).
The AM is a cylindrical magnet (101.6 mm in length and diameter) with a residual flux density of 1.48 T that is mounted at the end effector of a 6 DoF manipulator, as seen in Figure 1. In this manuscript, we evaluate the torqueing effects of the presence of this magnet offset with the goal of determining whether the torque effect is negligible, or impacts capsule motion and thus can potentially be used for the benefit of endoscope manipulation. A concept schematic of this effect is shown in Figure 2. A discussion of how to use this torque is beyond the scope of this manuscript. To the authors’ knowledge, the use of such concept in permanent-magnet based control has not been investigated.