An introduction to BAW gyroscopes for inertial sensing

May 08, 2014 //By Mohammad Zaman and Sreeni Rao, Qualtré Inc.
An introduction to BAW gyroscopes for inertial sensing
Authors from Qualtré Inc.provide an introduction to bulk acoustic wave gyroscopes and explain how their company's MEMS manufacturing process can be used to produce inertial sensors today and further integration in the future.

Engineers now design systems and products that include 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 same - a 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/Ω) with respect to the operating frequencies (ωDRV, ωSNS), 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 utilize 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] – when the two operating frequencies are made equal (ωDRV = ωSNS): 

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 (ωSNS/2QSNS) must be addressed by complex and power-consuming force-feedback operation.

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