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Figure 5. Sensing principle of a pCO
2
electrode
2.3. Heart sound sensor
The expansion and shrinkage of heart necessarily lead to the vibration of artery that is formed
by blood turbulence in vein. When the vibration of artery is transported to the surface of
Biomedical Sensor, Device and Measurement Systems
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thoracic cavity, heart sound will take place. Heart sound is very valuable for doctor to diagnose
many kinds of diseases in our body.
The range of heart sound is from 20Hz to 200Hz. Low limit frequency of heart sound could
reach about 4Hz and high frequency limit is greater than 1000Hz. There are many kinds of
medical heart sound sensors that are divided into two classifications: air conduction heart
sound sensor and direct conduction heart sound sensor. Air conduction heart sound sensor
consists of air chamber and common sensor. Such sensor has obvious defects: low sensitivity
and easy disturb by surrounding circumstance. Hence, in clinic application, the most sensors
applied are direct conduction heart sound sensor.
2.3.1. Piezoelectric heart sound sensor
The sensing structure of piezoelectric acceleration sensor is illustrated in figure 6. Such sensor
is used to measure heart sound. Its structure is very simple, which consists of vibration mass
block and piezoelectric crystal. A stress spring is utilized to exert a certain stress on vibration
mass block between top shell and mass block. Such method could timely adjust the linear
characteristic of sensing component. This sensor’s gravity is less than 30g, and is used to detect
heart sound and buffeting from body organisms.
Figure 6. Sensing structure of piezoelectric acceleration sensor
2.3.2. Fetus heart sound sensor
Detecting fetus heart sound is very important in clinic application for doctor sometimes needs
to grasp the present body status of fetus. PVDF piezoelectric thin film sensor is utilized to be
fit for the measurement of fetus’s heart sound as illustrated in figure 7. Its piezoelectric
coefficient is the following:
Advances in Bioengineering
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d
31
=(15~30)×10
−12
C
/
N
d
33
= −(5~8)×10
−12
C
/
N
In this structure, silicon rubber converts the vertical motion of itself into the radial motion of
PVDF piezoelectric thin film and then corresponding dynamic charge produced by PVDF thin
film is proportional to the externally transient force. The voltage along thickness direction is
output. Obviously, its work mode is 
d
31
work mode. For both thin film and silicon rubber are
very soft, they could well touch the skin of body belly. Then fetus heart sound is gained to
judge fetus heart. Design requirement of PVDF heart sound sensor is as follows: