EchoTrack: Acoustic device-free hand tracking on smart phones

Abstract

This paper explores the limits of acoustic ranging on smart phone in the scenario of device-free hand tracking. Tracking the hand is challenging since it requires continuously locating the moving hand in the air with fine resolution. Existing work on hand tracking relies on special hardware or requires users hold the mobile device. This paper presents EchoTrack, which continuously locates the hand by leveraging mobile audio hardware advances without special infrastructure supported. EchoTrack measures the distance from the hand to the speaker array embedded in smart phone via the chirp's Time of Flight (TOF). The speaker array and hand yield a unique triangle. The hand can be located with this triangular geometry. The trajectory accuracy can be improved with the method of Doppler shift compensation and trajectory correction (i.e., roughness penalty smoothing method). We implement a prototype on smart phone and the evaluation shows that EchoTrack can achieve tracking accuracy within about three centimeters of 76% and two centimeters of 48%.