Investigation of the Potential of Low Cost Position Tracking Using Permanent Magnets

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Anderson, Ryan
Localization , Position Tracking , Magnetics
Purpose: In computer assisted surgery accurate navigation tools are a necessity in many applications. The current most widely used navigation systems are expensive compared to the cheaptest imaging modalities such as ultrasound. Permanent magnet based tracking systems potentially offer adequate tracking accuracy and workspace size for limited clinical adoption. This thesis investigates the development of such a tracking system, in order to facilitate future work in developing this technology. Methods: Tracking the position of a permanent magnet involves finding a solution to a field model which matches the measured field to a position around the magnet. The work here first compared common magnetic field models to one another, and to measured magnetic field data to determine their suitability in tracking systems. A repeatable experimental data collection method is presented along with a method of calibration to reduce systematic error. A simulator was also written in MATLAB which simulated the experimental setup allowing systematic and random errors to me controlled. Results: Two simple magnetic field models were compared and found to have nearly identical fields. A more complex model was compared and had measureable different field magnitudes to the dipole and monopole models (on the order of 10% difference), but was unusable in the tracking algorithm due to limitations of the non-linear optimizer and not investigated further. Simulated results showed a high sensitivity to most errors sources, particularly the orientation error in the data collection setup and in the sensors minimum resolution of measurable magnetic field. Tracking accuracy on experimentally collected data in 3-DOF after calibration was less than desired, with position error exceeding 5 mm at distances as low as 10 cm between sensor and magnet. Conclusion: Permanent magnet tracking was found to be less accurate than is clinically useful, and highly sensitive to errors in sensors and experimental setup. The range of the system was found to be particularly limited by the sensor measurement resolution. This work provides solutions to some common error sources and could be used as a starting point for future investigations into permanent magnet tracking systems.
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