Biomedical Engineering Reference
In-Depth Information
Fig. 4.
(left) Scheme of the REEH transducer; (right) Acetabulum of high density polyethylene
modulation of the PEH generator. A piezoelectric ceramic diaphragm (ref. 7BB-12-9,
muRata
Corporate, Kyoto, Japan) with
9
mm of diameter and
0
.
22
mm of thickness (
12
mm of plate size,
0
.
1
mm of plate thickness and
9
.
0
1
.
0
kHz of resonant frequency)
was placed on the lower half of ball head of the hip prosthesis.
±
Neural Network Model of the PEH System.
Piezoelectric energy harvesters can be
modelled as mechanical damping systems [29, 43]. Because the piezoelectric element
is attached to the hip prosthesis structure, the mechanical damping ratio and proof mass
are very difficult to find due to the geometry of the prosthesis. An artificial neural net-
work (ANN) model was developed to predict the power and energy generation [44].
A multilayer “feed-forward” ANN was trained to perform the matching between a se-
ries of pairs of the frequency and amplitude of sinusoidal axial forces, and the aver-
age power and peak-to-peak voltage generation. The ANN consists of one input layer,
with two neurons, two hidden layers, with seven neurons each, and one output layer,
with two neurons, as shown in figure 5 and predicted by equation (4). The Levenberg-
Marquardt's algorithm was used as the training algorithm and the mean square error
of
1
.
0
10
−
20
as the convergence criteria for the network training. Sigmoid functions
(Tansig) for the hidden layers and linear function (Purelin) for the output layer were
used as the transfer functions.
y
N
=
f
L
LW
2
f
S
LW
1
f
S
IW
1
i
N
+
b
1
+
b
2
+
b
3
×
(4)
Here,
y
N
is the output
2
×
1
matrix,
i
N
is the
2
×
1
input matrix,
IW
1
is a input weight
7
×
2
matrix,
LW
1
and
LW
2
are respectively layer weight
7
×
7
and
2
×
7
matrices,
and
b
1
,
b
2
and
b
3
are respectively bias
7
×
1
,
7
×
1
and
2
×
1
matrices.
f
L
and
f
S
are
linear and sigmoid functions, respectively.
4
Experimental and Simulation Results
The experimental average and peak power, energy and peak-to-peak voltage genera-
tion were analysed. These experimental results were compared with the linear models
reported in sections 3.2, 3.3 and 3.4.
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