Biomedical Engineering Reference
In-Depth Information
can be set. The coefficient of friction between the contact surfaces can be introduced as
the material property of the contact element, CONTA171.
3.8.1 Uniaxial PVDF Film
The uniaxial PVDF film exhibits orthotropic properties which originate from the mechan-
ical processing it undergoes. This orientation dependency exists in both the mechanical
and electrical domains such that the piezoelectric coefficients in directions 1 and 2 are
not the only unequal properties, since the Young's moduli of the uniaxial PVDF film in
directions 1 and 2 (
E
1
and
E
2
, respectively) also differ.
In this simulation, the PVDF film was a square 30 mm
30 mm in which the bottom
surface was electrically grounded and the total area of the top surface was electrically
coupled together to create an equipotential electrode area so, in this case, the whole
PVDF film acted as one sensing element. Then a distributed load of 100 kPa was
applied through the top plate and the coefficient of friction between each contact surface
was increased incrementally, as shown in Figure 3.14. For a better understanding, the
negative output voltage is inverted and is shown in the positive
y
-axis. Although the
magnitude of
μ
can, theoretically, reach very large values, a practical range of
μ<
1
was selected for these simulations. It could be seen that the maximum output voltage is
obtained at the frictionless state (
μ
=
0). The output voltage decreases nonlinearly and
drops by more than 70% when the coefficient of friction is equal to 0.4. This significant
attenuation is associated with the friction-induced voltage component so that for a state
in which no friction exists, the PVDF film can freely expand, such that the only nonzero
stress component would be
σ
3
and the other principal stresses would be 0 (
σ
1
=
σ
2
=
0). However, when
μ
=
0, the friction force reduces the lateral expansion of the PVDF
film, which will develop stress in the 1 and 2-directions. The reason that the inclusion
of the
x
and
y
components causes a reduction in the total output voltage is due to
×
1.8
1.6
1.4
1.2
1.0
0.8
0
0.2
0.4 0.6
Friction coefficient
0.8
1.0
Figure 3.14
The total output voltage of a uniaxial PVDF film compressed between two blocks in
the presence of friction
