DOI

10.17077/etd.cetctkqj

Document Type

Thesis

Date of Degree

Spring 2015

Degree Name

MS (Master of Science)

Degree In

Biomedical Engineering

First Advisor

Rahmatalla, Salam

First Committee Member

Wilder, David

Second Committee Member

Marler, Tim

Third Committee Member

Grosland, Nicole

Fourth Committee Member

Dove, Edwin

Abstract

Magnetic interference in the motion capture environment is caused primarily by ferromagnetic objects and current-carrying devices disturbing the ambient, geomagnetic field. Inertial sensors gather magnetic data to determine and stabilize their global heading estimates, and such magnetic field disturbances alter heading estimates. This decreases orientation accuracy and therefore decreases motion capture accuracy. The often used Kalman Filter approach deals with magnetic interference by ignoring the magnetic data during periods interference is encountered, but this method is only effective when the disturbances are ephemeral, and cannot not retroactively repair data from disturbed time periods.

The objective of this research is to develop a method of magnetic interference mitigation for environments where magnetic interference is the norm rather than the exception. To the knowledge of this author, the ability to use inertial and magnetic sensors to capture accurate, global, and drift-free orientation data in magnetically disturbed areas has yet to be developed. Furthermore there are no methods known to this author that are able to use data from undisturbed time periods to retroactively repair data from disturbed time periods. The investigation begins by exploring the use of magnetic shielding, with the reasoning that application of shielding so as to impede disturbed fields from affecting the inertial sensors would increase orientation accuracy. It was concluded that while shielding can mitigate the effect of magnetic interference, its application requires a tedious trial and error testing that was not guaranteed to improve results. Furthermore, shielding works by redirecting magnetic field lines, increasing field complexity, and thus has a high potential to exacerbate magnetic interference.

Shielding was determined to be an impractical approach, and development of a magnetic inference mitigation algorithm began. The algorithm was constructed such that magnetic data would be filtered before inclusion in the orientation estimate, with the result that exposure in an undisturbed environment would improve estimation, but exposure to a disturbed environment would have no effect. The algorithm was designed for post-processing, rather than real-time use as Kalman Filters are, which enabled magnetic data gathered before and after a time point could affect estimation.

The algorithm was evaluated by comparing it with the Kalman Filter approach of the company XSENS, using the gold standard of optical motion capture as the reference point. Under the tested conditions of stationary periods and smooth planar motion, the developed algorithm was resistant to magnetic interference for the duration of testing, while the Kalman Filter began to degrade after approximately 15 seconds. In a 190 second test, of which 180 were spent in a disturbed environment, the developed algorithm resulted in 0.4 degrees of absolute error, compared to the of the Kalman Filter’s 78.8 degrees.

The developed algorithm shows the potential for inertial systems to be used effectively in situations of consistent magnetic interference. As the benefits of inertial motion capture make it a more attractive option than optical motion capture, immunity to magnetic interference significantly expands the usable range of motion capture environments. Such expansion would be beneficial for motion capture studies as a whole, allowing for the cheaper, more practical inertial approach to motion capture to supplant the more expensive and time consuming optimal option.

Public Abstract

Motion capture is the process of recording motion such that it can be later recreated and analyzed. The benefits of motion capture can be seen across many disciplines, as it allows for investigation into the methodology of human motion. Applications range from improving animated motion to identifying the effect of cerebral palsy on human gait.

Recent advancements in sensor technology have simplified the motion capture process, allowing for direct application of sensors to the subjects body. Previously motion capture was primarily accomplished via cameras, necessitating large initial expense as well as a dedicated motion capture environment. In Inertial motion capture, motion is captured by sensors placed on the subject’s body. It is comparatively cheap, and can be performed in a wide variety of environments. For these reasons inertial motion capture is often preferred to optical motion capture.

For all its benefits, a weakness of inertial motion capture is error accumulation due to magnetic interference. Magnetic interference is caused primarily by metal objects close to the subject. As inertial sensors measure the ambient magnetic field such interference can generate error. The focus of this thesis is the use of inertial sensors in magnetically interfered environments. The developed algorithm operates by filtering the magnetic field data and discarding data compromised by interference. The resulting algorithm allows for motion capture resistant to magnetic interference, and shows the potential for inertial motion capture to be used effectively in a broader range of environments than is currently feasible.

Keywords

Disturbance, Inertial, MagneticInterference, Mitigation, Motion Capture, Sensors

Pages

xix, 111 pages

Bibliography

Includes bibliographical references (pages 109-111).

Copyright

Copyright © 2015 Eric Christopher Frick

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