The ultra-high-precision MEMS gyroscope – called the birdbath resonator gyroscope – is almost perfectly symmetrical, and made of nearly-pure glass. This, say the researchers, enables it to vibrate for long periods, similar to the ringing of a wine glass.
“Our gyroscope is 10,000 times more accurate but only 10 times more expensive than gyroscopes used in your typical cell phones,” says Khalil Najafi, the Schlumberger Professor of Engineering at U-M and a professor of electrical engineering and computer science. “This gyroscope is 1,000 times less expensive than much larger gyroscopes with similar performance.”
Most smartphones contain gyroscopes to detect the orientation of the screen and help figure out which way they are facing, but their accuracy is poor and often incorrectly indicate which direction a user is facing during navigation. Autonomous vehicles on the other hand currently use high-performance gyroscopes inside their backup navigation systems in the event of the loss of a GPS signal. But these devices are larger and much more expensive.
A device that enables navigation without a consistent orienting signal – called an inertial measurement unit (IMU) – is made up of three accelerometers and three gyroscopes, one for each axis in space. However, say the researchers, getting a good read on which way you’re going with existing IMUs is so pricey that it has been out of range, even for equipment as expensive as autonomous vehicles. To address this problem, say the researchers, a low-cost IMU with much higher accuracy is strongly desired.
“High-performance gyroscopes are a bottleneck, and they have been for a long time,” says Jae Yoong Cho, an assistant research scientist in electrical engineering and computer science. “This gyroscope can remove this bottleneck by enabling the use of high-precision and low-cost inertial navigation in most autonomous vehicles.”
Better backup navigation equipment, say the researchers, could also help soldiers find their way in areas where GPS signals have been jammed, or help speed up warehouse robots with more accurate indoor navigation.
The key to making the affordable, small gyroscope is a nearly symmetrical mechanical resonator. “It looks like a Bundt pan crossed with a wine glass,” say the researchers, made one centimeter wide. As with wine glasses, the duration of the ringing tone produced when the glass is struck depends on the quality of the glass — but instead of being an aesthetic feature, the ring is crucial to the gyroscope’s function.
The complete device uses electrodes placed around the glass resonator to push and pull on the glass, making it ring and keeping it going.
“Basically, the glass resonator vibrates in a certain pattern,” says Sajal Singh, a doctoral student in electrical and computer engineering who helped develop the manufacturing process. “If you suddenly rotate it, the vibrating pattern wants to stay in its original orientation. So, by monitoring the vibration pattern it is possible to directly measure rotation rate and angle.”
The way that the vibrating motion moves through the glass reveals when, how fast, and by how much the gyroscope spins in space. To make their resonators as perfect as possible, the researchers start with a nearly perfect sheet of pure glass – known as fused-silica – about a quarter of a millimeter thick. They use a blowtorch to heat the glass and then mold it into a Bundt-like shape – known as a “birdbath” resonator since it also resembles an upside-down birdbath.
Then, a metallic coating is added to the shell and electrodes placed around it initiate and measure vibrations in the glass. The whole assembly is encased in a vacuum package, about the footprint size of a postage stamp and half a centimeter tall, which prevents air from quickly damping out the vibrations.
A paper – “0.00016 deg/vhr angle random walk (ARW) and 0.0014 deg/hr bias instability (BI) from a 5.2M-Q and 1-cm precision shell integrating (PSI) gyroscope” – is being presented at the (now virtual) 7th IEEE international Symposium on Inertial Sensors & Systems March 25. The research was supported by the Defense Advanced Research Projects Agency.
Cho and Najafi are co-founders of a startup company, Enertia Microsystems, based on the technology licensed from U-M.
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