Biomedical Engineering Reference
In-Depth Information
3
F
2
F
1
F
1.0
0.8
0.6
0.4
0.2
0
0 0 0 0 0
Orientation angle (degree)
50
60
70
80
90
Figure 3.9
The predicted variations of normalized output voltage of the uniaxial piezoelectric
PVDF film for different applied loads. For this simulation load
F
was 10 N
As mentioned earlier, the behavior of uniaxial PVDF film under similar conditions is
different to that of biaxial PVDF film. Figure 3.9 shows the predicted variations of nor-
malized total output voltages in terms of the deviation angles of uniaxial PVDF subjected
to a set of forces,
F
,2
F
,and3
F
,inwhich
F
is a downward point load of 10 N applied
at the tip of the cantilever. The length, width, and thickness of the cantilever are 24 cm,
2.4 cm, and 2 mm, respectively. In Figure 3.9, the curve corresponding to 3
F
is normal-
ized and is used as a reference for other force values. Even though the results are shown
for a force
F
= 10N, the trend is identical for any force
F
in the linear range. The results
show that for each force state, the variation of the voltage, due to the orientation angle,
is nonlinear and significant (the voltage output at
θ
=90
◦
drops to 4% of its initial value
at 0
◦
).
This significant decrease is due not only to the difference between the piezoelectric
coefficients in directions 1 and 2, but also because of the difference in Young's modulus
of PVDF in these two principal directions. Despite a nonlinear decrease in output voltage
due to the increase in deviation angle, the output voltage increases linearly with increasing
applied force for each specific angle. Figure 3.10 shows the contribution of each piezo-
electric coefficient in the total response when the deviation angle varies between 0
◦
and
90
◦
. These results are shown in Figure 3.11, in which the variation of output voltage of
the uniaxial piezoelectric PVDF film versus force amplitude for different PVDF devia-
tion angles is shown, and the total voltage (
V
total
), as well as its components associated
with coefficients
d
31
(
V
d
31
)and
d
32
(
V
d
32
), are plotted separately. It can be seen that the
total output voltage is mainly comprised of the voltage associated with
d
31
. For example,
when
θ
=0
◦
, only 1.5% of the total voltage is due to
d
32
, whose reduced contribution
to the output voltage can be attributed to the combined effect of mechanical anisotropic
properties of the uniaxial PVDF film, as well as the significant difference between the
values of
d
31
and
d
32
in the uniaxial PVDF.
The lower Young's modulus in the transverse direction yields a lower stress, hence
a lower charge output for the same strain value compared with the biaxial film. It can
