Biomedical Engineering Reference
In-Depth Information
4
m
= 0
m = 0.3
m
= 1
3
2
1
0
0
50
100
150
200
Normal applied pressure (kPa)
Figure 3.17
The response of PVDF film against applied pressure for different friction coefficients
electrical boundary conditions are defined in such a way that the whole PVDF film forms
only one sensing element. Therefore, for each given load, there exists only one voltage
output. The applied load ranges from 1 to 200 kPa and the results for three typical friction
coefficients, μ : 0, 0.3, 1 are plotted. As demonstrated in Figure 3.17, for a constant friction
coefficient, there is a linear relationship between the normal applied load and the sensor
output voltage. This justifies why industrial sensors are performing well and linearly,
even though the frictional forces have not been taken into consideration. These results
also show that the outputs of identical sensors are the same if the friction coefficients of
the sensor surfaces are also exactly the same. This could be one of the main reasons for
the discrepancy in output voltage of sensors that are similar in structure, but have different
contact frictions which can, quite probably, occur during the manufacturing phase. This
figure also shows that if, for any reason, uniformity of manufactured sensors cannot be
attained, it is better to work in the high-friction region. In this way, the dependency of
the sensors on the surface friction will be reduced to a minimum. However, if the contact
surface is too rough, then the projections on the surface might cause unexpected results
and even damage the PVDF film.
3.8.2 Biaxial PVDF Film
As mentioned earlier, there are some differences between biaxial and uniaxial piezoelectric
PVDF films. For instance, biaxial film exhibits isotropic properties in terms of both
mechanical ( E 1 = E 2 ) and electrical ( d 31 = d 32 ) performance. Another important factor is
the opposite polarity of d 33 in uniaxial and biaxial PVDF films. The manufacturer [19],
reports the values of d 33 for the uniaxial and biaxial PVDF films as being
20 and 15
pC N -1 , respectively.
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