Shoe Power Ville Kaajakari
Louisiana Tech University
2009
Ville Kaajakari, Louisiana Tech University
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Human power
WalkingPout = 30 W
WheelchairPout = 20 W
•We can “parasitically” take 1% of the human power without noticeable effect.
•With 33% conversion efficiency, this translates to 100 mW of electrical power
Ville Kaajakari, Louisiana Tech University
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Nike + iPod Sport Kit: do you want to charge your shoes periodically?
Ville Kaajakari, Louisiana Tech University
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Desired properties for a shoe power generator
- Low cost and ecological (plastic)- Soft for shock absorption
- Simple design. No complex bimorphs. - Piezoelectric (no voltage bias needed)- Light weight (6 g)- Output measured in voltages andmilliwatts (and not millivolts and microwatts).
Ideal transducer is:
Power generator
Ville Kaajakari, Louisiana Tech University
5
Size
reference
Polymer
transducer
Rectifier and regulator.
Efficiency > 70%
demonstrated
Integrated transducer and rectifier/regulator
Factory installed shock absorber.
Power generating
shock absorber.
Ville Kaajakari, Louisiana Tech University
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First shoe prototype with integrated power generator
• DC rectified power output deliver to a storage capacitor.
• Substantial power output demonstrated with average power output being 2 mW per shoe.
• Further optimization possible to obtain higher powers.
• Steps can be detected for zero velocity updating for more accurate IMU.
Power per
step: 3 mJ
Ville Kaajakari, Louisiana Tech University
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LED demonstration
Ville Kaajakari, Louisiana Tech University
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Power conversion challenge
• Piezotransducer is a high impedance source=> high voltages (50-500 V) but low currents (~100 μA) are generated
• Applications require low voltage (~3 V) and modest current (1-10 mA)
• No commercial converters available:– Inductor based “buck” converters work for
transformation ratios up to ten. Efficiency drops quickly for higher transformation ratios.
– Point of reference: MIT demonstrated 17.6% efficiency for their shoe power generator.
Ville Kaajakari, Louisiana Tech University
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Results on power converter
C1
C2
CN
V 3
12
N
VV
Wake up P = 10
µW
Control P = 0.2
mW
V 1001 V
Charge cycle
Load cycle
Input current
Output current
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 1 2 3 4 5 6
Eff
icie
ncy
Load voltage [V}
70% efficiency demonstrated for conversion from 120 V to 3.5 V!
Energy stored in a battery or supercapacitor (Dr. Scott Gold)
Ville Kaajakari, Louisiana Tech University
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Conclusions
• Low cost shoe power generator has been demonstrated.
• Current prototype demonstrates 2 mW of power.• Custom power regulation circuit with high
effieciency.• Future research focused on:
– Non-shoe applications – Film optimization and more efficient rectifier/regulator– Commercialization