Biomedical Engineering Reference
In-Depth Information
supplying over 1 mW/cm
3
from running. The ankle location has an
associated power density as high as 25 mW/cm
3
from walking and as
high as 68 mW/cm
3
from running.
A linear energy harvester could use the forward, vertical, or lateral
motion of a walking or running individual to generate power according
to the analysis provided. Larger amounts of power output can be
achieved if energy is harvested from several axes at the same time. A
generator design employing the three acceleration components for
energy generation would be relatively difficult to accomplish while
remaining small in size (with relatively large dimensions along the
three axes). An energy harvester using two of the acceleration axes
would benefit from a planar design which is more appropriate for sur-
gical implantation or for portable electronics. A planar topology can
be accomplished by using two individual linear generators, one linear
generator aligned to the resultant acceleration vector or a rotational
approach. As the lateral axis has smaller acceleration components,
a resultant vector formed by the forward and vertical axes will have a
larger magnitude.
As a comparison, power can also be estimated using the mechanical
work
W
and the frequency
f
as
P
5
Wf
(2.11)
where the work
W
of a mass
m
alongadistance
d
against the Earth
'
s
gravity
g
is
W
5
mgd
. Considering the average vertical displacement
while walking to be 5 cm (Rome et al., 2005), the work done by a 1 g
mass is
B
0.5 mJ. Then, the power associated at a walking frequency
of 2 Hz is
B
1 mW but only half of that can be converted into elec-
tricity (Stephen, 2006). This result is at the same power level as the
one presented. In summary, a 1 g proof mass generator traveling
5 cm at 2 Hz has an available power content of
B
0.5 mW. Thus, a
larger device could produce more than 0.5 mW, and a smaller energy
harvester would produce only a fraction of that. As it could be
expected, larger energy harvesters can produce a larger power output,
thus generators can be sized according to the power required for
implantable or portable electronics.
In summary, available power density is found to be over
0.5 mW/cm
3
for most of the body locations while walking (ankle and
knee present higher magnitudes). Running gives power density levels
