New MEMS Device Turns Vibrations into Energy

Researchers at MIT have designed a device the size of a U.S. quarter that harvests energy from low-frequency vibrations, such as those that might be felt along a pipeline or bridge. The tiny energy harvester — known technically as a microelectromechanical system, or MEMS — picks up a wider range of vibrations than current designs, and is able to generate 100 times the power of devices of similar size.

“There are wireless sensors widely available, but there is no supportive power package,” says Sang-Gook Kim, a professor of mechanical engineering at MIT and co-author of the paper. “I think our vibrational-energy harvesters are a solution for that.”

The motivation behind the research is to overcome one limit that wireless sensors, which are ubiquitous across several industries, face: power. Their batteries need regular changes. In order to do away with them, researchers are trying to tap electricity from low-power sources such as vibrations from swaying bridges, humming machinery and rumbling foot traffic. These could replace batteries, MIT says.

How to do that is the question. MIT says the most common way to go about it is to use piezoelectric materials such as quartz and other types of crystals. Piezo means in Greek to squeeze or press and these materials naturally accumulate electric charge in response to mechanical stress. So far these materials have been exploited with tiny MEMS devices that generate small amounts of power.

One approach that has been tried is to use a small microchip with layers of PZT glued to the top of a tiny cantilever beam, which will move up and down like a wobbly diving board when exposed to vibrations. The beam bends and stresses the PZT layers, and the stressed material builds up an electric charge, which can be picked up by arrays of tiny electrodes. The problem with this approach is that the beam has its own resonant frequency and its wobbling response drops off outside of this frequency. As a consequence, so does the amount of power that can be generated.

“In the lab, you can move and shake the devices at the frequencies you want, and it works,” says co-author Arman Hajati, who conducted the work as a PhD student at MIT. “But in reality, the source of vibration is not constant, and you get very little power if the frequency is not what you were expecting.”

Researchers have dealt with the problem by taking a “power in numbers” approach. They simply increased the number of cantilever beams and PZT layers occupying a chip. But it’s expensive and wasteful.

“In order to deploy millions of sensors, if the energy harvesting device is $10, it may be too costly,” says Kim, who is a member of MIT’s Microsystems Technology Laboratories. “But if it is a single-layer MEMS device, then we can fabricate the device for less than $1.”

Kim and Hajati’s solution is a design that increases the device’s frequency range (or bandwidth), while maximizing the power density, or energy generated per square centimeter of the chip. Instead of taking a cantilever-based approach, they engineered a microchip with a small bridge-like structure that’s anchored to the chip at both ends. The researchers deposited a single layer of PZT to the bridge, placing a small weight in the middle of it.

During several tests carried out, the researchers found the device was able to respond not just at one specific frequency, but also at a wide range of other low frequencies. They calculated that the device was able to generate 45 microwatts of power with just a single layer of PZT, which represents an improvement of two orders of magnitude compared to current designs.

The results of the research appeared in the Aug. 23 online edition of Applied Physics Letters.

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