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Contact-Free Biosignal Acquisition via Capacitive and Ultrasonic Sensors

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Document pages: 13 pages

Abstract: Contact-free detection of human vital signs like heart rate and respirationrate will improve the patients comfort and enables long-term monitoring ofnewborns or bedridden patients. For that, reliable and safe measurementtechniques are indispensable. The aim of this work is the development andcomparison of novel ultrasonic and capacitive measurement setups, sharing acommon hardware platform. Both measurement techniques that are implemented andcompared are based on the detection of minor chest wall vibrations inmillimeter ranges, due to geometrical thorax changes during respiration andheartbeat activities. After examining the physical measurement conditions andsimulating the capacitive sensor, a problem-specific measurement setup isproposed. The system is characterized to be capable of detecting distancechanges below 2 { mu}m via the ultrasonic sensor and below 800 { mu}m via thecapacitive sensor. First subject measurements show that the detection of heartactivities is possible under ideal conditions and exclusively with the proposedultrasonic approach. However, the capacitive sensor works reliably forrespiration monitoring, even when the subject is fully-clothed and covered witha blanket. The chosen ultrasonic approach is sensitive regarding minor changesof the reflecting surface and therefore has high uncertainty. In contrast,capacitive respiration detection is very reliable. It is conceivable thatimprovements in the capacitive sensor circuitry will also enable the detectionof heart activities. The proposed ultrasonic approach presents current problemsof this technique. In contrast to that, the unusual approach of capacitivesensing demonstrates a high potential regarding vital signs acquisition.

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