Closed Loop Control of a Tethered Magnetic Capsule Endoscope

Abstract

Magnetic field gradients have repeatedly been shown to be the most feasible mechanism for gastrointestinal capsule endoscope actuation. An inverse quartic magnetic force variation with distance results in large force gradients induced by small movements of a driving magnet; this necessitates robotic actuation of magnets to implement stable control of the device. A typical system consists of a serial robot with a permanent magnet at its end effector that actuates a capsule with an embedded permanent magnet. We present a tethered capsule system where a capsule with an embedded magnet is closed loop controlled in 2 degree-of-freedom in position and 2 degree-of-freedom in orientation. Capitalizing on the magnetic field of the external driving permanent magnet, the capsule is localized in 6-D allowing for both position and orientation feedback to be used in a control scheme. We developed a relationship between the serial robot's joint parameters and the magnetic force and torque that is exerted onto the capsule. Our methodology was validated both in a dynamic simulation environment where a custom plug-in for magnetic interaction was written, as well as on an experimental platform. The tethered capsule was demonstrated to follow desired trajectories in both position and orientation with accuracy that is acceptable for colonoscopy.

Fig. 1

(a) Tethered capsule system during in vivo colonoscopy trial (b) The capsule maintains all functionality of a standard colonoscope.

Fig. 2

(a) Rendering generated in COMSOL showing the sum of the magnetic fields of the external and internal permanent magnets (EPM and IPM). Sensor readings from the capsule are used to search the field map of the EPM to localize the capsule. (b) At the indicated capsule position, the magnetic field of the EPM is along z^EPM.

Fig. 3

Intended robot-patient positioning for tethered capsule colonoscopy. Links 5 and 6 are desired to maintain elbow-up configuration.

Fig. 4

Gazebo simulation environment with built-in physics engine. A custom plug-in allows for simulation of magnetic interaction between the EPM and capsule.

Fig. 5

Experimental setup for trajectory following of the tethered capsule. The tether is constrained near the beginning of the trajectory. The sinusoidal trajectory is shown for visualization purposes only.

Fig. 6

Results of four trials of the capsule being maneuvered through the two types of desired trajectories while maintaining an orientation that is parallel to the vertical barrier surface. The shaded region shows one (only for b, d) and three standard deviations from the mean. Simulation results are shown in (a) and (b) while experimental results are shown in (c) and (d).

Fig. 7

The tethered capsule was commanded to follow this sinusoidal trajectory starting near x = 0.2 m and maintain a heading that was tangential to the sine curve. This heading is parallel to the vertical barrier.