Biomedical Engineering Reference
In-Depth Information
1.0
0.8
0.6
V
total
V
d
31
V
d
32
0.4
0.2
0
0 0 0 0 0
Orientation angle (degree)
50
60
70
80
90
Figure 3.12
Variation of output voltage of the uniaxial piezoelectric PVDF film versus force
amplitude for different PVDF deviation angles
3.6 The Anisotropic Property of Biaxial PVDF Film and Its
Influence on Sensor Applications
Since the piezoelectric coefficients of biaxial PVDF are equal in the axis 1 and transverse
axis 2 directions, this provides a unit ratio of
d
31
/
d
32
for biaxial PVDF film. This equality
of piezoelectric coefficients is in compliance with the isotropic mechanical property of
biaxial PVDF film (i.e.,
E
1
=
E
2
). Figure 3.12 shows the result of the output voltage versus
the orientation angle of the biaxial PVDF film. The output voltage is normalized with
respect to the total output voltage,
V
total
. The variations in the contributing components
in the total output voltage, namely
V
d
31
and
V
d
32
due to the piezoelectric coefficients
d
31
and
d
32
respectively, are clearly shown in this figure. It should be noted that, in order to
compute the voltage component associated with each piezoelectric coefficient, the other
coefficient were assumed to be 0. From this, it can be interpolated that the total output
voltage in biaxial PVDF film is constant and is hence independent of any deviation angle.
The symmetry observed in Figure 3.12 is due to the equality of
d
31
and
d
32
in the two
perpendicular directions along with the isotropic material properties of the biaxial PVDF
film. It is observed that in the 0
◦
deviation, the contribution of
d
32
to the total output is
about 7%. It should be noted that the contribution of
d
33
to total output voltage depends
on the boundary conditions and, when there is no constraint on the
z
-axis, neither is there
any stress developed in this direction so there will be no contribution due to
d
33
in the
total output.
3.7 Characterization of Sandwiched Piezoelectric PVDF Films
Despite the dominant applications of PVDF film in the extensional mode, there are some
situations in which PVDF must be placed between two surfaces. For instance, in traditional
piezoelectric force sensors, it is customary to place the piezoelectric sensing element
