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Shoe Power Ville Kaajakari Louisiana Tech University 2009 2 Human power Walking Pout = 30 W Wheelchair Pout = 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 Ideal transducer is: - 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). Power generator Ville Kaajakari, Louisiana Tech University 5 Integrated transducer and rectifier/regulator Polymer transducer Rectifier and regulator. Efficiency > 70% demonstrated Size reference Factory installed shock absorber. Power generating shock absorber. Ville Kaajakari, Louisiana Tech University 6 First shoe prototype with integrated power generator Power per step: 3 mJ • 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. 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 (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 9 Results on power converter Charge cycle 70% efficiency demonstrated for conversion from 120 V to 3.5 V! Input current Load cycle 0.8 Output current Efficiency 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 C1 0 V1 100 V C2 Wake up P = 10 µW Control P = 0.2 mW CN V1 N 3V V2 1 2 3 4 5 6 Load voltage [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 Ville Kaajakari, Louisiana Tech University