Assistant Professor, Louisiana Tech University
Energy harvesting is an attractive way to power MEMS sensors and locator devices such as GPS; however, the power harvesting technologies often fall short in terms of power output. For example, vibratory MEMS generators might give out only microwatts of electrical power. While this may be sufficient for emerging ultra low power sensors, many current applications require milliwatt power levels. As an example, commercially available running sensors for shoes consume over 100 uW of electrical power and requirements for GPS locators are even higher.
Piezoelectric transducers generate electrical charge when compressed. This makes piezoelectric materials especially advantageous for power harvesting as they do not require bias voltage for operation. In principle, a piezoelectric transducer together with two rectifying diodes is sufficient for generating dc output voltage.
The shoe power generator that our group has developed is based on a low-cost polymer transducer that has metallized surfaces for electrical contact. Unlike the conventional ceramic transducers, the plastic-based generator is soft and robust matching the properties of regular shoe fillings. The transducer can therefore replace the regular heel shock absorber with no loss in user experience.
A significant challenge in harvesting piezoelectric energy is that piezoelectic materials are optimal for generating high voltages but provide only a low current output. The polymer used in the shoe transducer provides over 5 mJ of energy per step but at voltages too large (>50 V) to be directly used in low power sensors. A breakthrough in piezoelectric power generation is the new voltage regulation circuits that we developed at Louisiana Tech University that efficiently converts the piezoelectric charge into a usable voltage. A conversion circuit coverts the high voltage to a regulated 3 V output for charging batteries or for directly powering electronics at better than 70% conversion efficiency. Combined with the polymer transducer, the regulation circuit gives time-averaged power of 2 mW per shoe during a regular walk.
The generated power output can be compared to typical storage capacity of 30 mAh for lithium coin/button cells -- with an average current consumption 0.5 mA, a miniature coin cell is depleted in less than three days whereas the shoe power generator gives power output as long as the user keeps walking. The total energy output can therefore easily surpass conventional batteries. In addition to running sensors and inertial navigation, the show power generator can be used to power RF transponders, GPS receivers, and locator tags that require a milliwatt power source.
Copyright 2010 MEMS Investor Journal