7. PersonalSensing2 - Personal Sensing Continued Tarek...

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1 Personal Sensing Continued Tarek Abdelzaher Dept. of Computer Science University of Illinois at Urbana Champaign Transportation Mode Detection Based on phone accelerometers Samuli Hemminki, Petteri Nurmi, and Sasu Tarkoma, “Accelerometer-based Transportation Mode Detection on Smartphones,” ACM SenSys, November 2013
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2 Challenge: Gravity Estimation Accelerometers measure the superposition of two forces: Forces due to acceleration along each axis Forces due to gravity along each axis Since accelerometer orientation is unknown, it is hard to separate the two. What is the gravity component of the measurements read along each axis? Solution #1 Insight: Gravity is fixed. Other forces come and go. Solution: Average the acceleration measurements over a long enough time window Transient forces will tend to cancel out Constants (i.e., gravity) will remain Pros/cons?
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3 Solution #2 If there are intervals of relatively low accelerometer variance, it means that the accelerometer is not “moving”.
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