![](bg4.png)
Figure 7: Energy storage and power supply circuit.
Only when capacitors’ combined voltage reaches
a usable level (V
H
= 3.2 V) this voltage is connected
to the DC-DC converter and subsequently to the
telemetric system. Once capacitors’ combined
voltage drops to a considered minimum value (V
L
=
1.2 V), the start-up sub-module disconnects
generator’s load.
Considering that each capacitor will be charged
at half this values, using E = C/2 [(V
H
)
2
– (V
L
)
2
]
with C = C
1
= C
2
= 1000 µF an energy of 1.1 mJ is
stored in each capacitor, leaving us with a total 2.2
mJ of usable energy.
Fig. 8 clearly shows that, for a worst case
scenario, more than 40 seconds are needed to
recharge the capacitors.
Figure 8: Average time taken to recharge storage
capacitors, for a range of gait speeds.
Fig. 9 presents actual capacitors’ voltage
evolution, over time, for a set of pace frequencies,
covering the full range of tested walking speeds.
Figure 9: Capacitors’ voltage evolution over time.
Considering that patients will probably not walk
at a steady pace and may even stop to rest, if
repeated data acquisition cycles are needed, 300
seconds between cycles is a secure compromise.
Taking all this preliminary results into account, it
is considered that the proposed electromagnetic
transducer is more than capable of solely powering
the envisioned microelectronic telemetric system.
4 CONCLUSIONS
Since the end application is located inside a hip
prosthesis, where vibrations are expected with
abundance, the proposed electromagnetic generator
follows the vibration to electrical energy conversion.
As demonstrated, using mechanical vibrations
produced by the human gait it is possible to harvest
enough energy, with this generator, to suffice power
needs of the aforementioned telemetric system.
As future work it is intended to further maximize
useful energy storage. This will allow extended
telemetric system’s working cycle and upgrade
system functionalities.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the
Portuguese Foundation of Science and Technology
(FCT) that partially sponsors this research work
through the project reference PTDC/EME-
PME/105465/2008.
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