Shoe Power Ville Kaajakari Louisiana Tech University 2009

Ville Kaajakari, Louisiana Tech University 2 Human power Walking P out = 30 W Wheelchair P out = 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 3 Nike + iPod Sport Kit: do you want to charge your shoes periodically?

Ville Kaajakari, Louisiana Tech University 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 and milliwatts (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 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

Ville Kaajakari, Louisiana Tech University 7 LED demonstration

Ville Kaajakari, Louisiana Tech University 8 Power conversion challenge Piezotransducer is a high impedance source => high voltages ( 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 9 Results on power converter C1 C2 CN Wake up P = 10 µW Control P = 0.2 mW Charge cycle Load cycle Input current Output current Efficiency 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 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