26-05-2012, 01:30 PM
Parasitic Power Harvesting in Shoes
[attachment=23150]
Introduction
As wearable electronic devices evolve and proliferate,
there will be a growing need for more power delivery to
distributed points around the human body. Today, much of
that storage is provided by batteries and power delivery is
via wires. The current approach to power distribution is
clearly becoming problematic -- as more appliances are
carried, we are forced to either use more small batteries that
require replacement everywhere or run wires through our
clothing to supply appliances from a central power source.
Both are undesirable. A better solution is clearly to
generate power where it is being used, bypassing the storage
and distribution problem altogether. As power requirements
drop for most wearable devices, it is no longer infeasible to
harvest a useful amount of energy "parasitically" from a
normal range of human activity.
Background Information
The context in which we place our generator is that of a
sport sneaker. This type of shoe differs from ordinary shoes
in one important feature—its energy dissipating sole.
While walking in ordinary "hard" shoes, the foot is rapidly
decelerated from its relatively high downward speed to zero
velocity relative to the ground—an action that requires the
application of relatively large and sudden forces to the foot.
Barring shock absorption in the feet, this can be simply
modeled as a sudden step in velocity;
System Descriptions
One obvious means of parasitically tapping energy in
this context is to harness the bending of the sole, which is
attempted in our first system. This is a laminate of
piezoelectric foil, shaped into an elongated hexagon, as
shown in Fig. 3. This “stave” is a bimorph built around a
central 2-mm flexible plastic substrate, atop and below
which are sandwiched 8-layer stacks of 28-micron PVDF
(polyvinylidineflouride) sheets [6], epoxy-bonded as shown.
This stave was designed in collaboration with K. Park and
M. Toda of the Sensor Products Division of Measurement
Specialties (formerly AMP Sensors) [7]; its shape was
chosen to conform to the footprint and bending distribution
of a standard shoe sole.
Performance
In order to evaluate their power-generating capability,
we have installed all three of these systems in sneakers.
The magnetic generator was affixed as shown in Fig. 6 and
the PVDF and PZT elements were mounted between the
removable insole and rubber sole as indicated in Fig. 7.
Although all of these elements could, in principle, be
mounted in the same shoe, the PVDF, PZT, and magnetic
devices were installed in separate shoes during our tests.
The region around the heel of our men's size 111/2 (US)
Nike Air running shoe was too small to accommodate the
TH 7R unimorph, hence we used the smaller TH 6R there
instead. When inserted beneath the insole (as in Fig. 7; see
also Fig. 11), these devices could barely be noticed under
the foot and had no effect on gait. The piezoelectric
generators, being high-impedance devices, were terminated
with 250KW resistors, which approximated their equivalent
source resistance at the excitation frequencies, hence yielded
maximum power transfer [14].
Applications and Discussion
Even though the energy produced by the PVDF stave is
very limited, it is still useful for a variety of low-power,
low duty-cycle applications (see the RF tagging example
below), plus it promises to be the least invasive and most
accommodating solution when laminated directly into the
sole structure.
Various calculations [7,15] have predicted considerably
more power generation from PVDF foil in shoes, ranging
from tens to hundreds of milliwatts. The major source of
this discrepancy is in the assumed efficiency of stretching
the foil. The most efficient mechanical coupling into
PVDF is through the longitudinal (3-1) mode (approaching
25% efficiency), which requires it to be pulled.
[attachment=23150]
Introduction
As wearable electronic devices evolve and proliferate,
there will be a growing need for more power delivery to
distributed points around the human body. Today, much of
that storage is provided by batteries and power delivery is
via wires. The current approach to power distribution is
clearly becoming problematic -- as more appliances are
carried, we are forced to either use more small batteries that
require replacement everywhere or run wires through our
clothing to supply appliances from a central power source.
Both are undesirable. A better solution is clearly to
generate power where it is being used, bypassing the storage
and distribution problem altogether. As power requirements
drop for most wearable devices, it is no longer infeasible to
harvest a useful amount of energy "parasitically" from a
normal range of human activity.
Background Information
The context in which we place our generator is that of a
sport sneaker. This type of shoe differs from ordinary shoes
in one important feature—its energy dissipating sole.
While walking in ordinary "hard" shoes, the foot is rapidly
decelerated from its relatively high downward speed to zero
velocity relative to the ground—an action that requires the
application of relatively large and sudden forces to the foot.
Barring shock absorption in the feet, this can be simply
modeled as a sudden step in velocity;
System Descriptions
One obvious means of parasitically tapping energy in
this context is to harness the bending of the sole, which is
attempted in our first system. This is a laminate of
piezoelectric foil, shaped into an elongated hexagon, as
shown in Fig. 3. This “stave” is a bimorph built around a
central 2-mm flexible plastic substrate, atop and below
which are sandwiched 8-layer stacks of 28-micron PVDF
(polyvinylidineflouride) sheets [6], epoxy-bonded as shown.
This stave was designed in collaboration with K. Park and
M. Toda of the Sensor Products Division of Measurement
Specialties (formerly AMP Sensors) [7]; its shape was
chosen to conform to the footprint and bending distribution
of a standard shoe sole.
Performance
In order to evaluate their power-generating capability,
we have installed all three of these systems in sneakers.
The magnetic generator was affixed as shown in Fig. 6 and
the PVDF and PZT elements were mounted between the
removable insole and rubber sole as indicated in Fig. 7.
Although all of these elements could, in principle, be
mounted in the same shoe, the PVDF, PZT, and magnetic
devices were installed in separate shoes during our tests.
The region around the heel of our men's size 111/2 (US)
Nike Air running shoe was too small to accommodate the
TH 7R unimorph, hence we used the smaller TH 6R there
instead. When inserted beneath the insole (as in Fig. 7; see
also Fig. 11), these devices could barely be noticed under
the foot and had no effect on gait. The piezoelectric
generators, being high-impedance devices, were terminated
with 250KW resistors, which approximated their equivalent
source resistance at the excitation frequencies, hence yielded
maximum power transfer [14].
Applications and Discussion
Even though the energy produced by the PVDF stave is
very limited, it is still useful for a variety of low-power,
low duty-cycle applications (see the RF tagging example
below), plus it promises to be the least invasive and most
accommodating solution when laminated directly into the
sole structure.
Various calculations [7,15] have predicted considerably
more power generation from PVDF foil in shoes, ranging
from tens to hundreds of milliwatts. The major source of
this discrepancy is in the assumed efficiency of stretching
the foil. The most efficient mechanical coupling into
PVDF is through the longitudinal (3-1) mode (approaching
25% efficiency), which requires it to be pulled.