shoe power

Shoe Power Ville Kaajakari

Louisiana Tech University

2009

Ville Kaajakari, Louisiana Tech University

2

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


3

Nike + iPod Sport Kit: do you want to charge your shoes periodically?


4

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


5

Size

reference

Polymer

transducer

Rectifier and regulator.

Efficiency > 70%

demonstrated

Integrated transducer and rectifier/regulator

Factory installed shock absorber.

Power generating

shock absorber.


6

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


7

LED demonstration


8

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.


9

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)


10

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

shoe power

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