Biomedical Engineering Reference
In-Depth Information
1.5
1.3
1.1
0.9
0.7
0
0.2
0.4
PVDF length
0.6
0.8
1.0
Figure 3.16
Voltage distribution of each sensing elements (for
μ
=1)
the output voltages of 10 equal PVDF sensing elements. While the applied load is kept
constant, the friction coefficient is varied between 0 and 1. The horizontal axis of the
graph is normalized by half the length of the PVDF film. It should be emphasized that
each point in this figure represents a constant voltage over the area of a sensing element.
For further clarification, Figure 3.16 shows the distribution of output voltage over the
uniaxial PVDF film segments compressed between two plates in the presence of friction
and voltage variations from one plate to the other. The horizontal axis of the graph is
normalized by half the length of the PVDF film.
From Figure 3.16, it can be seen that the output voltage of each sensing element is a
constant value over its area. Since half of the PVDF is modeled, zero is related to the
center of the film and one is related to the edge of PVDF. For the frictionless case, that
is,
μ
= 0, the voltage of all the PVDF sensing elements are equal and at their maxi-
mum value. However, with an increase in the friction coefficient, the voltage output of
each sensing elements begins to reduce. For a given friction coefficient,
μ
, the voltage
attenuation of those elements that are closer to the edges is lower, due to the fact that
the outer sensing elements carry less frictional load than those of the middle sensing
elements. Therefore, the frictional voltage component of the outer sensing elements is
lower, and hence the total output is higher. The trend observed in Figure 3.15 is exactly
the same as the trend for the variation of
σ
x
along the length of the PVDF film at the
contact surface. It is now clear that Figure 3.14 represents the average voltage of a charge
distribution over the whole PVDF film. On the other hand, the data shown in Figure 3.15,
which is the result of segmenting the PVDF into 10 separate sensing elements, are espe-
cially helpful to elaborate the study of the distribution of the frictional forces over the
contact area.
Having observed the nonlinear behavior of the output voltage of the PVDF against the
friction coefficient, the relationship between the amplitude of the applied normal forces
and the total output should also be examined. In this simulation, similar to the first test, the
