In-air ultrasonic gesture sensing with MEMS microphones

The use of hand gestures for user interaction with computing devices is gaining popularity. This paper demonstrates that an effective ultrasonic gesture-sensing system can be implemented using commercially-available MEMS microphones. The system works by sending ultrasonic pulses with a center frequency of 40kHz using a piezo-electric transmitter and detecting the resulting echoes using an array of seven microphones. The microphones are mounted at a pitch of 4.3mm to enable beamforming with acceptable grating-lobe levels. The received signals are digitized using successive approximation ADCs, after which a B-mode image is constructed in an FPGA and object locations are estimated using peak detection. Within a range of 5cm to 100cm, object locations can be detected with a resolution of 1.67cm and 1.5°. Based on the detected object locations, a simple gesture-recognition algorithm implemented in software can accurately detect horizontal swiping gestures.