17-11-2012, 05:13 PM
Non-contact Heartbeat Sensor using LC oscillator circuit
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Abstract
This paper proposes non-contact heartbeat sensor
using LC oscillator for personal healthcare. The signals from
chest and wrist are measured using LC oscillator and electrodes.
The proposed sensor can detect human heartbeat signal without
direct contact to skin and is readily a portable heartbeat
monitoring sensor. The capacitive and inductive electrodes are
addressed, which are key elements of the sensor circuit.
INTRODUCTION
uman heartbeat signal is one of the important
information in personal healthcare, especially for
patients at risk like post myocardial infarction. Monitoring
heart and lung activity non-invasively without removing
clothing is desirable in many emergency circumstances and
long-term home monitoring.
A variety of heartbeat measurement devices have been
presented in the literature. For example, a finger-ring
plethsmographic sensor, a watch-type vital-sign sensor, a
bed-type vital-sign sensor and a phonocardiographic sensor
have been proposed [1-4]. However, these sensors need
transducers such as piezo-elements, pressure sensors and
microphone. Non-contact ECG measurement using
capacitively-coupled insulated electrodes [5,6] and magnetic
monitoring of heart and lung activity [7,8] also have been
proposed which uses ultra-high input impedance system and
frequency counter.
INDUCTIVE ELECTRODE
Capacitive electrode is not suitable to measure the
impedance of wrist because it is hard to make small patch
electrodes structure that the electric field at low frequency is
focused in a specific area like wrist. Therefore, we use
inductive electrode for wrist measurement.
Fig. 4 shows inductive electrode configuration of the
sensor. An Inductive electrode is connected to the inductor L
in series. The inductive electrode is designed to be used as a
watch-type heartbeat monitoring sensor, and the inductance
electrode is the shape of spiral inductor whose diameter is
3cm with 24 turns. We designed the f0 to be around 1 MHz
because frequency sensitivity by the inductance variation is
proportional to the f0 and the dielectric constant of blood is
maximized around 100 Hz~1 MHz as shown in (2) and
reference [8].
CONCLUSION
We presented a non contact vital signal detector based on
LC oscillator and electrodes. The signals of respiration and
heartbeat can be obtained with capacitive electrode from
chest, and the signal of heartbeat can be also obtained with
inductive electrodes from wrist. The free running frequency
of oscillator is designed to 760 kHz (capacitive electrode) and
950 kHz (inductive electrode). The heartbeat signal from
wrist using inductive electrode are compared with that of
infrared finger PPG sensor