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A primer on BAW gyroscopes for inertial sensing

Posted: 29 May 2014 ?? ?Print Version ?Bookmark and Share

Keywords:MEMS? gyroscopes? bulk acoustic wave? BAW? HARPSS?

Systems and products these days incorporate MEMS sensors, particularly MEMS gyroscopes, as essential components. These applications range from portable and wearable devices to industrial robots and critical automotive safety systems. Their requirements for lower power, smaller form-factor, environmental tolerance and lower cost are growing. To satisfy these needs, today's design engineers are considering new solutions and new partners who can bridge theory and practice, and connect the lab to the production line. They are looking for innovation and scale.

These issues are being addressed by an innovative MEMS technology referred to as bulk acoustic wave (BAW) technology. BAW technology is being used to develop solid-state MEMS gyroscopes that not only meet power, size, cost, and high volume production requirements well, but also add higher performance to the mix.

Existing gyroscope technology limitations
The fundamental principle of all commercial MEMS gyroscopes is the samea Coriolis-induced transfer of energy between two vibration modes of a structure in the presence of rotation. The fundamental kinematic relationship that specifies absolute acceleration arising from rotation is used to formulate coupled differential equations that in turn specify motion along the drive and sense vibration modes. Solving the resulting equation leads to the following expression for gyroscope sensitivity (xSNS/?) with respect to the operating frequencies (DRV, SNS), Q-factor (Q) and drive-mode displacement amplitude (xDRV):

Equation 1

It is evident from this equation, that increasing the drive-mode displacement amplitude offers increased rotation sensitivity. However owing to increasing power constraints, a large drive amplitude is mainly possible via reduction of overall stiffness of the device, i.e. operating frequency. As a result, commercially available gyroscopes have operating frequencies between 5kHz and 50kHz. This range of operating frequency not only restricts the vibration and shock tolerance immunity performance, but makes it difficult to utilise the mode-matching advantage of a MEMS vibratory gyroscope. The advantage refers to the dependence of rotation sensitivity on the mechanical Quality Factor as in the following special case of equation [1] C when the two operating frequencies are made equal (DRV = SNS):

Equation 2

In order to achieve mechanical amplifications approaching 20k to 50k, existing MEMS gyroscopes must operate in high-vacuum to eliminate the impact of air-damping. Upon achieving such typically cost-prohibitive vacuum-levels, open-loop bandwidth constraints (SNS/2QSNS) must be addressed by complex and power-consuming force-feedback operation.

Introducing BAW gyroscope technology
In light of the prevailing limitations, the research team at the Georgia Institute of Technology Integrated MEMS Laboratory (GT-IMEMS) developed a new class of MEMS vibratory gyroscope based on degenerate bulk-acoustic modes of circular disks. A BAW gyroscope relies on the transfer of energy between two degenerate BAW modes typically operating in 1MHz to 10MHz range.

Figure 1a: A scanning electron micrograph (SEM) of a silicon BAW disc gyroscope.

This increased stiffness results in BAW gyroscopes being immune to stiction both in manufacturing and during operation in the field, thus removing a major yield and reliability problem found in existing translation-based vibrating tuning-fork architectures.

Figure 1b: Visual representation of the "n=3" in-plane degenerate BAW modes (TOP-drive and BOTTOM-sense) used to detect rotation normal to the plane.

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