MIT redesigns MEMS for better energy harvester

September 14, 2011 — MIT researchers have developed a micro electro mechanical system (MEMS) that harvests energy from low-frequency vibrations, and occupies a coin-sized form factor. MIT’s MEMS device picks up a wider range of vibrations than current designs thanks to a bridge, rather than cantilever, MEMS structure.

The aim is to replace batteries on far-flung and ubiquitous wireless sensor networks (WSN). For example, the MEMS energy harvesters could power bridge-structure monitoring sensors by converting the bridge’s swaying and vehicle-traffic vibrations to electricity. This device provides the "supportive power package" to bring WSNs into their full potential application, said Sang-Gook Kim, a professor of mechanical engineering at MIT.

The MIT design increases the MEMS frequency range with maximum power density, able to generate 100 times the power of devices of similar size. Rather than using piezoelectric material (PZT) on a cantilever MEMS device, the researchers created a bridge anchored to the chip at each end. A single layer of piezoelectric material was deposited onto the bridge structure, with a small weight in the middle. In a series of vibration tests, the MEMS device responded over a range of low frequencies. The researchers calculated that the device was able to generate 45 microwatts of power with a single layer of PZT.

PZT accumulates electric charge when acted upon by mechanical forces. MIT’s use of PZT avoids frequency limitations found with cantilever designs, and also prevents the use of too much piezoelectric material, which can be costly, said Arman Hajati, who conducted the work as a PhD student at MIT. The energy harvester is designed to work with real-world variables, like changing frequencies, and real-world purchasing budgets.

If the energy harvester costs $10, a WSN of millions of sensors could be too costly, says Kim, who is a member of MIT’s Microsystems Technology Laboratories. This single-layer MEMS design can be fabricated for less than $1.

Still, few vibrations in nature occur at the relatively high frequency ranges captured by the device, so the MIT team is optimizing the MEMS design to respond to a lower frequency range. They aim for 100 microwatt energy capture, said Hajati, now a MEMS development engineer at FujiFilm Dimatix. 100 microwatts can power a network of smart sensors that can talk forever with each other.

Results are published in the August 23 online edition of Applied Physics Letters. Access it here:

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