Respiratory Bidirectional Ultrasonic TOF Flow Sensor Resilience to Ambient Temperature Fluctuations

Measuring fluid flow velocity using ultrasonic based sensors has been established for some time, however implementation of this technology in respiratory systems is relatively recent. This paper presents the design of a respiratory flow sensor made possible by the air-coupling ultrasound principle. A prototype has been developed as a proof-of-concept. To enhance the signal to noise ratio (SNR), the modulated ultrasound arrival time is translated to a digital word using Time-to-Digital Conversion (TDC). Hence, the respiration is digitally captured in real-time to offer a synchronous respiratory monitoring. The near-zero deviation is further alleviated by Least Mean Square (LMS) technique accomplished off-line. Down to 0.03 L/s of resolution and over 95% of linearity is achieved. The bidirectional transmit/receive echo signals have been leveraged to provide a system level cancelation of the environmental conditions affecting measurement accuracy. The developed sensor was used on adult volunteers and tested experimentally using an ASL5000 lung simulator. The results validate the system functionality in the presence of real-world conditions.