Contact-Free Biosignal Acquisition via Capacitive and Ultrasonic Sensors

TL;DR

This paper develops and compares ultrasonic and capacitive contact-free sensors for detecting vital signs, finding capacitive sensors more reliable for respiration and ultrasonic sensors capable of detecting heart activity under ideal conditions.

Contribution

It introduces a novel hardware platform for contact-free vital sign detection using ultrasonic and capacitive sensors and compares their effectiveness in different scenarios.

Findings

Capacitive sensors reliably detect respiration even when clothed or covered.

Ultrasonic sensors can detect heart activity under ideal conditions.

Capacitive sensing shows high potential for vital signs acquisition.

Abstract

Contact-free detection of human vital signs like heart rate and respiration rate will improve the patients' comfort and enables long-term monitoring of newborns or bedridden patients. For that, reliable and safe measurement techniques are indispensable. The aim of this work is the development and comparison of novel ultrasonic and capacitive measurement setups, sharing a common hardware platform. Both measurement techniques that are implemented and compared are based on the detection of minor chest wall vibrations in millimeter ranges, due to geometrical thorax changes during respiration and heartbeat activities. After examining the physical measurement conditions and simulating the capacitive sensor, a problem-specific measurement setup is proposed. The system is characterized to be capable of detecting distance changes below 2 {\mu}m via the ultrasonic sensor and below 800 {\mu}m via the capacitive sensor. First subject measurements show that the detection of heart activities is possible under ideal conditions and exclusively with the proposed ultrasonic approach. However, the capacitive sensor works reliably for respiration monitoring, even when the subject is fully-clothed and covered with a blanket. The chosen ultrasonic approach is sensitive regarding minor changes of the reflecting surface and therefore has high uncertainty. In contrast, capacitive respiration detection is very reliable. It is conceivable that improvements in the capacitive sensor circuitry will also enable the detection of heart activities. The proposed ultrasonic approach presents current problems of this technique. In contrast to that, the unusual approach of capacitive sensing demonstrates a high potential regarding vital signs acquisition.