Biomedical Engineering Reference
In-Depth Information
Ta b l e 9 .
PEH generator — modulation error of the average power
Frequency (Hz)
0.5
0.5
1
1
1.5
3
3.5
4
4
Amplitude (N)
100
200
125
200
200
125
200
100
200
Absolute error (W)
(1)
0
.
0004 0
.
003
0
.
0003 0
.
003
0
.
002
0
.
23
0
.
11
0
.
03
0
.
13
(1)
×
10
−
6
.
Table 10.
PEH generator — modulation error of the peak-to-peak voltage
Frequency (Hz)
0.5
0.5
1
1
1.5
3
3.5
4
4
Amplitude (N)
100
200
125
200
200
125
200
100
200
Absolute error (W)
0.32
1.18
1.22
1.18
0.80
10.19
0.98
3.49
0.88
5.2
Choice of the Energy Harvesting Method
There are several methods to harvest electric energy from the surrounding environment.
Biofuel cells, magnetic induction, thermoelectric and vibration are some of the main
sources used to harvest energy. Vibration-based generation is being considered the most
appropriate solution to harvest electrical energy on prostheses [45]. Although there are
no studies supporting this hypothesis, their ease of implementation, their ability of being
fully autonomous and operating without maintenance, ensuring safety throughout the
life span of the implant, were relevant features taken into consideration to validate this
new concept of energy harvesting systems applied to hip prostheses.
5.3
Performance of the Linear Models
Linear models can only ensure accuracy within a narrow window of the systems' op-
erating range. The modulation errors, presented in sections 4.1, 4.2 and 4.3, confirm
the inaccuracy of the linear models of the generators, especially those used to model
the behaviour of the TEEH generator. The real translational and rotational hip displace-
ments, the friction and gravity forces acting on these systems, the non-linearity behavior
of inductors and transduction damping coefficients were jeopardized from linear mod-
els. Because experimental results show a higher performance of the TEEH generator
in this particular application, it is mandatory the development of non-linear models for
high accuracy prediction of electric energy generation considering a broad specter of
the generators' operation.
5.4
Optimization of the Multi-source Harvester System
Each transducer must be optimized in order to maximize electric generation during typ-
ical walking speeds, namely in the range between 0.5 Hz and 2 Hz. The optimization of
the TEEH and PEH generators demands for a “perfect” match between the frequency of
the hip kinematics and their resonant frequencies. The implementation of a continuous-
matching system is very complex [46] because the duration and frequency of every-day
human activities are unpredictable [47]. New methods must be developed to ensure
high performance tracking of the hip kinematics' frequency. Each generator must be
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