Circuit gets more power from shakes

By Kimberly Patch, Technology Research News

Energy is all around, the trick is finding ways to harvest it to do the work of your choosing.

Researchers at Pennsylvania State University have boosted the efficiency of piezoelectric circuits, which transfer the mechanical energy of vibrations into useful electric power. Piezoelectric circuits are commonly used to convert a watch-wearer's motion into energy to power the watch.

The Penn State piezoelectric circuit harvests four times more power from vibrational energy than the circuits currently in use, said George Lesieutre, a professor of aerospace engineering at Penn State. Key to the increased efficiency is an adaptive control technique that optimizes the actions of a converter that transfers the harvested energy to a battery for storage.

The researchers prototype generates 50 milliwatts of energy, and should be efficient enough to convert the motion of a runner into the power needed to run an electronic music device, according to Lesieutre. Fifty milliwatts is one twentieth of a watt. The circuitry could also be used to power wireless networks of tiny sensors, and to power devices that dampen vibrations, according to Lesieutre.

In principle, energy can be extracted from any place where two areas consistently hold different amounts of energy, such as the temperature difference between a warm body and the outside air or between sun and shadow. Similarly, energy can be extracted from mechanical vibrational fields such as those caused by people walking or tires running over a road. One working example is tiny light-emitting diodes on downhill skis that are powered by the motion of the skis.

Like all piezoelectric circuits, the researchers' device depends on a key property of piezoelectric crystals like quartz. These crystals do not conduct electricity, but when they are exposed to mechanical stress, electrical charges appear on their surfaces.

Piezoelectric crystal also works in reverse. When exposed to an alternating electric current, the crystal vibrates, generating high-frequency sound waves. This effect is widely used in loudspeakers, microphones and devices that control frequencies in radio transmitters.

The Penn State circuit works by capturing the alternating voltage formed by piezoelectric crystal in response to vibrations. The device includes a rectifier, which converts the alternating electrical input to a positive electrical flow, a filter capacitor that smooths the electrical flow, and a converter that allows a battery to store the harvested energy.

The key to making the device more efficient is the engineering involved in the converter, said Lesieutre. The converter includes an adaptive control technique that adjusts to find the optimal power transfer options for the moment, including an efficient duty cycle. A duty cycle dictates when and for how long a device is active. "The smarts is in the... converter, which operates at [a specific] frequency and duty cycle," he said. The adaptive technique allows the converter to transfer power more efficiently in order to maximize the power stored by the battery. "The duty cycle selection is especially critical," he added.

The researchers also sized the circuit with an eye toward maximum efficiency, said Lesieutre.

In the researchers' experiments, the adaptive converter increased power transfer by more than 400 percent, he said.

Devices that require more than 50 milliwatts could be powered using an array of several piezoelectric circuits, or they could be powered periodically, Lesieutre said.

For example, the device could power an environmental-control sensor that periodically wakes up, processes data, then communicates it to a central location via wireless transmitter, said Lesieutre. Even though it may take more than 50 milliwatts to power the sensor and transmitter, they can be operated intermittently. One can store a lot of energy in a battery, even at low charging rates, if one can wait long enough," he said.

The circuit could also be used for control and guidance robotics in manufacturing, patient monitoring and diagnostics, home security systems, and interactive toys, according to Lesieutre. Researchers are also looking to use power-scavenging techniques to power microelectrical mechanical systems (MEMS) and micro-robots.

The work is "another useful piece of analysis and engineering," said Andrew Brown, an electronics professor and head of the electronic systems design group at the University of Southampton in England. The amount of power that can be scavenged by techniques like these is still extremely small, he added.

However, recent technology has enabled the efficient and cheap extraction of these small amounts of power, and the ability to use them. "Two aspects of technology are converging to make this application workable: it's becoming feasible to mine low levels of power, and it's becoming possible to do useful things with that level of power," said Brown. "Very low power design is an enormous and growing field in its own right," he said.

The researchers' next steps are to make the system even more efficient, and to size it for various applications, including very small ones, Lesieutre said. The possibilities include powering wearable electronics, he said.

The device could be used in products in less than two years, he said. Ways to power sensors and wireless transmitters would be useful to pursue first, he said.

Lesieutre's research colleagues were Geffrey K. Ottman, Heath Hoftmann and Archin C. Bhatt. They published the research in the September, 2002 issue of IEEE Transactions on Power Electronics. The research was funded by the Office of Naval Research (ONR).

Timeline:   < 2 years
Funding:   Government
TRN Categories:   Energy; Engineering
Story Type:   News
Related Elements:  Technical paper, "Adaptive Piezoelectric Energy Harvesting Circuit for Wireless, Remote Power Supply," IEEE Transactions on Power Electronics, September, 2002.


November 13/20, 2002

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