Sensors Mag

April R&D Round Up

April 1, 2011 By: Melanie Martella, Sensors

E-mail Melanie Martella

This month we've got an adaptive MEMS-based energy harvester for low-frequency vibrations, a nanotube-based strain sensor, and a chemical sensor that uses three different types of selectivity to boost its sensitivity.

Harvesting Low-Frequency Vibrations
There's a truly excellent article over at EE Times detailing a novel MEMS vibration energy harvester that's being developed at CEA-Leti. Be sure to read the entire article for all the details, but I'll give you a quick summation here. Basically, the researchers have developed an electrostatic MEMS microstructure that reacts to an input vibration by converting it into multiple capacitance variations, which are then converted into electrical energy. The trick is to improve the efficiency of the energy transfer; low-frequency vibrations don't pack much of a wallop and so as much energy as possible needs to be translated from vibration to usable energy.

The device incorporates a specially patterned electrode structure, a novel electret material for the electrodes that can maintain an electrostatic charge for a long time, and a mechanical nonlinear spring that helps to regulate the displacement of the seismic mass. The resulting energy harvester achieves an impressive mechanical-to-electrical conversion efficiency of 60%, and was able to output 3 V from vibrations of <0.2 g at 50 Hz. The article, written by Ghislain Despesse of CEA-Leti is "Self-adaptive MEMS vibration energy harvester targets low frequencies."

Stretchy Nanotubes Sense Strain
Kenji Hata and his colleagues at the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan, have created a very nifty strain sensor by gluing blocks of single-walled carbon nanotubes onto a stretchy silicon rubber substrate. As the substrate stretches, the carbon nanotubes crack to form blocks of nanotubes connected by filamentous bridges. The bridges limit the electron flow through the nanotubes, leading to an uptick in resistance. When the substrate is no longer stretched, the carbon nanotubes snap back to their previous configuration. The sensor can be stretched a whopping 280% beyond its normal size, and it can survive 10,000 repeated stretches to 2.5 times its length. The researchers also report that they can stretch the sensor again after a delay as short as 14 ms. The combination of stretchiness, robustness, and the ability to measure large strains make the device potentially valuable for a wide range of applications.

To quote the Royal Society of Chemistry's Chemistry World article, "Nanotubes make 'exceptional' strain sensor", "Hata and his colleagues have also attached it to bandages, stockings and gloves to test how well it detected the motion of a human body.' Any large motion can be detected, and it can be used for millions of times,' explains Hata."

An Improved Chemical Sensor
When it comes to assessing the components of a chemical stew, a single analysis method is not going to be sufficient. University of Cincinnati researchers have been busily developing a chemical sensor that offers not one, not two, but three different selectivity modes to seriously boost the sensitivity of the device. Described in "UC Research Produces Novel Sensor with Improved Detection Selectivity", the device combines electrochemistry, spectroscopy, and selective partitioning.

What this means in practical terms is that if you're only interested in whether a given compound is present within a mixture, the sensor uses three stages to gradually (and selectively) winnow the components of the mixture before trying to detect the one you're interested in. Specialized coatings are used to limit the chemicals that can pass on to the next stage of the device. In the next stage, electrochemistry comes into play as an applied potential electrolyzes a smaller number of compounds. The remaining compounds are then subjected to spectroscopy with a specific wavelength of light to detect the compound of interest. According to the article, the sensor has already been tested at the U.S. Dept. of Energy's Hanford site.

About the Author: Melanie Martella

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