I Sense a Disturbance in the Force: Unobtrusive Detection of Interactions with RFID-tagged Objects Presenter : 葉舜元

Background Context inferencing is a cornerstone of ubiquitous computing. Activity inferencing is a major component of context inferencing. By detecting person-objects interactions, we can infer the activities.

Person-object interaction detection Accelerometer + mem Very high accuracy –False negative nearly impossible –False positive only when obj is jostled Difficult to hide Do not support in situ analysis Passive RFID tag Batteryless Cheap Hidable Robust User need to employ a wearable (reader)

Overcome the disadvantages Enhance a tag –Blended two kind of sensors –incompatible Reader energy analysis –Energy back = function ( The energy emitted by the reader, The distance between reader and tag, The angle between reader and tag ); –When a object is interacted with, typically these last two parameters changes

Response rate (α) α: the ratio of responses to the polls. [0..1] α is analogous to the received signal power with the distance α is a noisy signal

Response rate v.s. distance/angle 0 Distance 300Orientation α

Metal floor effect Wave guide Improved reader distance More sensitive Laying aluminum foil strips down on a wooden floor (cheap) 0 Distance (cm) 450 α

Coupling effect Caused by the proximity of tags or metal Negative coupling –The top/front tag occludes the return signal from other. α decreased. Positive coupling –Two tags are placed at a particular distance and relationship to each other. –Moving tag reflect the extra energy to the others. α increase.

Increasing accuracy By detecting occlusion Using multiple tags –Cross-correlate α in the same object Using multiple readers –Perpendicular to each other

Excluding false positive due to mutual coupling Using correlation analysis

Algorithm Set checks –Jump away/to α=1 (delta >= 0.1) –Jump away/to α=0 (delta >= 0.1) –A large change in α (3 standard deviations) Set other checks –Jump away/to α=1 (delta >= 0.1) –Jump away/to α=0 (delta >= 0.1) –Edge detected Coupling check Occlusion check

Experiments Variables –Flooring –Distance between tag and reader –Num of tags on the object and their placement –Num of readers deployment topology –Num of nearby tags –Num of objects moved simultaneously – Tag orientation –Amount and direction of tag rotation –Tag type –Type of reader

Base condition 1 tag 1 reader Distance = 50 cm x y z reader Z tag

Experimental Results