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An introduction to BAW gyroscopes for inertial sensing

An introduction to BAW gyroscopes for inertial sensing

Technology News |
By eeNews Europe



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.


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 disk 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.

Operating at such elevated frequency allows for high-Q mode-matched operation which enables superior rotation sensitivity without the need for large drive displacement amplitudes, high-vacuum levels and force-feedback architecture. Figure 1a shows an SEM image of a 600-micron diameter BAW disk gyroscope implemented on 35-micron thick SOI substrate [1]. The device utilizes a degenerate pair of in-plane “n=3” 10MHz BAW modes to sense rotation signal perpendicular to the plane of the disk (as seen in Figure 1b).


 

The HARPSS fabrication platform

One of the most important aspects of the evolution of MEMS-based products is the symbiotic relationship between product design and production design. In the case of BAW MEMS, the performance advantages of BAW sensor designs are being realized using the versatility and scalability of a High Aspect-Ratio combined Poly and Single-­crystalline Silicon (HARPSS) fabrication process. Implementing the BAW disk gyroscope design requires a fabrication platform that allows the capacitive air-gaps in both lateral and vertical directions to scale down to sub-micron range without requiring expensive nanolithography techniques.

Figure 2: An SEM close-up of the capacitive gap in a BAW gyroscope defined by the HARPSS process.

The HARPSS process is capable of producing poly- and monocrystalline silicon microstructures tens of microns thick, that are electrically isolated with self-aligned capacitive air gaps. Such high-aspect-ratio capacitive gaps, as seen in Figure 2, increase the efficiency of capacitive transduction substantially, and enable an effective high-frequency interface for vibrating silicon microstructures. This structure produces the highest signal to noise ratios in capacitive MEMS devices and superior noise density enabling better resolution.



Dynamic range, bias stability and vibration immunity

In motion sensing, there are many applications that have varying requirements for the upper and lower detection range. A golf simulator is such an application, where the sensors have to be able to detect intense motions like a tee-off swing while at the same time be able to pick up delicate motion of putting or a wedge shot. The term dynamic range refers the ratio of the largest detectable signal to the smallest detectable signal.

Figure 3: Measured linearity of 0.05% of a BAW gyroscope across an input rotation range of ±1500 deg./sec (limited by measurement setup).

The operating frequency and construction of the BAW gyroscope enables the widest dynamic range of ±5000 deg/s with excellent linearity (Figure 3) allowing designers to create a broader array of applications from a single sensor design. This attribute is particularly appealing for gaming platforms that include special purpose controllers such as the Wii™ remote or multi-purpose consumer gaming platforms, like mobile handsets and tablets that frequently update their designs.

Figure 4: Measured root Allan deviation plot of a BAW gyroscope indicating a bias instability value of 25 deg./hr.

BAW gyroscopes operate at frequency range outside the range of flicker noise of standard CMOS interface circuits enabling a smaller detection limit. This enhances the overall noise in the system that leads to superior bias drift performance as seen in Figure 4, which shows a measured root Allan deviation plot of a typical BAW gyroscope.



Perhaps the most distinct performance advantage of the BAW gyroscope enabled by HARPSS fabrication process is its immunity to random vibration and shock as detailed in Figure 5 where it is compared to existing tuning-fork based gyroscopes currently popular in the consumer market.

Figure 5: A comparison of gyroscope output offset shift measured during random vibration (top) and shock test (bottom) showing the advantage of BAW technology over existing tuning-fork architectures.



The future: lower power, more integration, more application innovation

Consumers are demanding products that are portable and accessible at all times. These untethered products can only be battery powered forcing designers to constantly prioritize performance demands with battery life, size and weight in mind. Devices that consume less power always offer an advantage in this engineering trade-off. The high frequency sensor design of BAW gyroscope with its high Q mechanical gain and sub 20nm drive-mode displacements yield the lowest power per axis. This lower power consumption means longer battery life and greater consumer acceptance for any type of wearable or handheld battery-powered device.

For industrial and automotive applications where performance and robustness are a higher priority, the BAW gyroscope offers the best combination of vibration immunity, low noise, and linearity. In automotive, critical safety applications such as anti-rollover are becoming mainstream, while new applications such as the use of gyroscopic control for radar positioning in ADAS systems are being increasingly adopted. In industrial manufacturing, for instance, gyroscopes are increasingly forming a critical part of the adaptive position control systems in robots. These applications demand performance and robustness, a combination that BAW is positioned to offer.

More dynamic applications of BAW MEMS technology can benefit when rotation is combined with other forms of inertial sensing. Personal or unmanned vehicular guidance systems call upon inertial measurement units (IMUs) which are a combination of accelerometers,gyroscopes, pressure sensors and magnetometers. The HARPSS processing technology, often referred to as the “CMOS” of MEMS, enables the integration of high-performance tri-axial micro-gyroscopes with tri-axial micro-accelerometers, and even tri-axial magnetometers on a common substrate to allow very favorable performance/size/cost ratios for devices such as an IMU. The high-Q operation of the BAW gyroscope can be maintained in near atmosphere conditions, and thereby does not limit the performance of devices such as pressure sensors and accelerometers built on the same platform.

Beyond navigation, IMUs provide 6 to 9 degrees of freedom sensing capabilities, bringing ultra-fine resolution to applications such as medical imaging equipment, surgical instrumentation, and advanced prosthetics. IMUs are also proving themselves useful in applications where the requirement for precision might not be as obvious and, as of recently, solutions were not available or practical. Among the more demonstrative examples are smart golf clubs, tennis racquets, and baseball bats that track and record every movement of an athletes’ swing so that the user’s technique can be refined. Accelerometers measure the acceleration, vibration, and swing plane while gyros measure pronation, or the twist of the user’s hands, during the swing. Each sport usually comes with its own app, which records up to 1,000 data points per second from the sensors and shows the user precisely how hard, how fast, and from what angle they have hit the ball. There are even 3D models that lay out a swing completely, so it can analyze the user’s mistakes. Each sport has an app that’s completely tailored to its needs and recorded data collected during play or practice can be sent via Bluetooth to a smartphone or PC for analysis.


 

Figure 6: A single-axis BAW gyroscope die set on a US penny illustrates the small-form factor

Conclusion

The BAW gyroscope, in its existing form-factor, is unique in its ability to achieve low-noise performance and wide dynamic range with excellent linearity while displaying superior immunity to the effects of temperature and mechanical shock/vibration and requiring low-power. This and other innovative designs using the HARPSS fabrication process are providing a platform for higher integration, smaller size, lower cost, and less system complexity. BAW gyroscopes will help design engineers innovate and differentiate their product designs by enabling new products that may not have been possible earlier.

References:

[1] https://www.qualtre.com/technology.html

Mohammad Zaman is Staff Systems Engineer at Qualtré and Sreeni Rao is vice president responsible for vertical markets at Qualtré.

Related links and articles:

www.qualtre.com

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