Biomedical Engineering Reference
In-Depth Information
that it produced up to 90 mW when excited at its natural resonant fre-
quency of 6 Hz with a vibration amplitude of 5.5 mm.
Wang et al. (2005a) have also improved upon the original Seiko
design. Their objective was to increase the power output density from
about 7.5 to 50 mW/cm
3
. This group fabricated a miniature eight-pole
permanent magnet generator using an imbricated-pole stator with a
single wire-wound coil to be driven at high speed. A prototype gener-
ated 15 mW at 6,000 rpm (100 Hz) after rectification by a Schottky-
diode bridge for a volume slightly larger than 1 cm
3
.
In addition to the rotational design from wristwatches, linear dis-
placement generators, similar to the commercial shake-driven flashlights,
have also been investigated for body motion. Duffy and Carroll (2004)
described one such design situated inside a shoe sole. The generator was
composed of two opposing magnets attached together inside a container
with three wrapped coils, about 45 mm long and 13 mm in diameter.
The shoe generator produced 8.5 mW when tested at a frequency of
5 Hz. A second generator design consisting of a set of fixed magnets fac-
ing a moving magnet with a coil in between was also tested. This set
was able to produce up to 230
W of power at a frequency of 5 Hz.
Further work (Duffy and Carroll, 2005) evaluated different rectification
circuits: half- and full-wave designs versus doubler and quadrupler volt-
age multipliers. The doubler was found to produce a higher voltage and
power output. A six-coil design with sliding magnets produced peak-to-
peak voltages of 4 V (400 mVrms) when tested at 2 Hz. The voltage
doubler offered a rectified power output close to 1 mW when using a
0.1 F double-layer capacitor; although up to 2 mW was expected. It was
assumed that this was due to the capacitor taking longer to charge.
μ
Studies made by Niu and Chapman (2006) evaluated arm swinging,
foot movement, and trunk displacement as potential locations for
energy harvesting. Their proposed design used a linear electromagnetic
generator tested on the mentioned body locations. An average power
output of 10 mW and an open-circuit peak voltage of 7 V were
reported for the device placed on the arm. A backpack-situated genera-
tor produced 50
80 mW with a peak voltage of 20 V (open circuit),
while the power output for the harvester worn on a shoe was 80 mW
for an open-circuit peak voltage close to 27 V. Power was measured
after rectification while charging a battery. It was reported that imped-
ance matching would increase the power by a factor of 3.
