The advantages of wireless sensors to monitor equipment and structures in remote locales are obvious, but are lessened significantly if their batteries need to be regularly changed. We've seen a number of microelectromechanical systems, or MEMS, that harvest energy from the environment, such as ambient light and radio waves and vibrations. Now MIT News is reporting the development of a new piezoelectric device that is about the size of a U.S. quarter and can generate 100 times as much power as similarly sized devices.
One of the most common piezoelectric designs in recent years sees a small microchip with layers of PZT - a material that shows a marked piezoelectric effect - glued to the top of a tiny cantilever beam. When the chip is exposed to vibrations, the beam moves up and down like a wobbly diving board. As the beam bends it stresses the PZT layers, which build up an electric charge that can be picked up by arrays of tiny electrodes.
As with everything, the cantilever beam has a frequency at which it wobbles the most. This is known as the resonant frequency and outside of it, the beam's wobbling response drops off along with the amount of power the device can generate.
"In the lab, you can move and shake the devices at the frequencies you want, and it works," says Arman Hajati, who conducted the work as a PhD student at MIT and co-authored the study. "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."
Some researchers have increased the number of cantilever beams and PZT layers on a chip to overcome this problem, but Hajati and Sang-Gook Kim, a professor of mechanical engineering at MIT and co-author of the paper, say this approach is not only wasteful, but also expensive.
"In order to deploy millions of sensors, if the energy harvesting device is $10, it may be too costly," says Kim, who is also 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."
To create a device with just one layer that is still able to pick up a wider range of vibrations, Kim and Hajati threw out the cantilever design and engineered a microchip with a small bridge-like structure that is anchored to the chip at both ends. They then deposited a single layer of PZT on the bridge and placed a small weight in the middle of it.
After putting the device through a series of vibration tests, the researchers found that it responded to a wide range of low frequencies. They calculated that it was able to generate 45 microwatts of power with a single layer of PZT, which is 100 times more than similarly sized devices of current design.
"If the ambient vibration is always at a single frequency and does not vary, [current designs] work fine," Daniel Inman, professor of aerospace engineering at the University of Michigan told MIT News. "But as soon as the frequency varies or shifts a little, the power decreases drastically. This design allows the bandwidth to be larger, meaning the problem is, in principle, solved."
Inman added that, since few vibrations in nature occur at the relatively high frequency ranges captured by the device, the MIT researchers will have to aim lower in the frequencies they pick up, which they say is just what they intend to do.
"Our target is at least 100 microwatts, and that's what all the electronics guys are asking us to get to," says Hajati. "For monitoring a pipeline, if you generate 100 microwatts, you can power a network of smart sensors that can talk forever with each other, using this system."
The MIT team published its results in the Aug. 23 online edition of Applied Physics Letters.