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BAW gyroscope outdoes MEMS in accuracy

Posted: 01 Dec 2014 ?? ?Print Version ?Bookmark and Share

Keywords:BAW? MEMS? gyroscope?

Qualtre Inc. has developed a bulk acoustic wave (BAW) single-axis gyroscope, which the company claims is the most accurate gyroscope in the world. The BAW, which was originally licensed from the Georgia Institute of Technology, uses a different approach than the traditional "tuning fork" type of architecture used by all other MEMS gyroscopes.

"TFGs [tuning-fork gyroscopes] have hit the wall in terms of performance, but bulk acoustic wave gyroscopes have overcome these obstacles," Sreeni Rao, vice president of Vertical Markets told EE Times. "And we have a headstart by holding the patents on that, which could improve those tuning-fork gyros."

Rao's title is a little deceiving, since he has a PhD in engineeringsystems and signal processingand has worked as a systems designer, processor designer and finally analogue designer, before transitioning to marketing after obtaining an MBA.

Crystal silicon disc

The single crystal silicon disc (centre) moves to track angular motion, which is sensed capacitively by the electrodes around the edge. (Source: Qualtre)

According to Rao, since licensing the basic patents from Georgia Tech, Qualtre has been busy enhancing and perfecting the BAW gyroscope by improving it in ways tuning forks could also be improved, then filing patents to protect its intellectual property. So far, the company has more than 30 granted patents, with more pending. Its first-generation gyroscope, which it began work on four years ago, is a three-axis unit that Qualtre has been sampling to select customers since June 2014.

Since it is more expensive but higher performance than three-axis tuning-fork MEMS gyros, its first customers are manufacturers of heavy industrial machinery that are willing to pay for higher performance, and platforms that must maintain absolutely perfect stabilityor as close to it as they can get. For instance, satellite and other antenna on military ships must maintain near perfect aim at the antennas to which they are communicating. Qualtre claims its BAW-based gyroscopes outperform all other types for stabilising ship antenna platforms as the ship yaws, rolls and pitches.

Its newest single-axis BAW gyroscope, on the other hand, has capitalised on those 30 granted patents since its founding to make it even higher performance than the most expensive single-axis tuning fork gyroscope, as well as lower in cost.

"In the course of sampling our three-axis products and interacting with customers we found significant interest in a single-axis version. Our second-generation single-axis gyroscope beats the very excellent single-axis gyroscopes from Analog Devices, and yet is cheaper than ADI's," Rao told us. "The second generation includes improvements all round. We have improved noise, bias instability, as well as our hallmark vibration rejection."

200nm gap

The tiny 200nm gap between the central disc and the electrodes around the edge changes its capacitance as the disc rolls, yaws and pitches.

Both its first- and second-generation gyros use an improved version of a process many MEMS devices usecalled HARPSS for High Aspect-Ratio Poly and Single-crystalline Silicon. Generation two improves the design and packaging, and has optimised some of the process steps, to result in the performance improvements reported, according to the company.

The HARPSS process qualifies Qualtre's BAW as a MEMS device, it's just that the moving parts only move a few nanometres. A round disc is surrounded by electrodes that today are at about 200nm gaps with the single crystal silicon disc in the middle. To improve performance further, all they have to do is scale that gap to smaller sizes.

Eventually, Qualtre intends to come out with models inexpensive enough for the burgeoning consumer market for gyroscopes, but for now it is satisfied with its industrial and military clientele. The package size is 7mm x 7mm x 2.1mm, but Qualtre's goal is 5mm x 5mm 1mm.

- R. Colin Johnson
??EE Times

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