This review surveys micromachined gyroscope structure and circuitry technology The principle of micromachined gyroscopes is first introduced Then, different kinds of MEMS gyroscope structures, materials and fabrication technologies are illustrated Micromachined gyroscopes are mainly categorized into micromachined vibrating gyroscopes (MVGs), piezoelectric vibrating gyroscopes (PVGs), surface acoustic wave (SAW) gyroscopes, bulk acoustic wave (BAW) gyroscopes, micromachined electrostatically suspended gyroscopes (MESGs), magnetically suspended gyroscopes (MSGs), micro fiber optic gyroscopes (MFOGs), micro fluid gyroscopes (MFGs), micro atom gyroscopes (MAGs), and special micromachined gyroscopes Next, the control electronics of micromachined gyroscopes are analyzed The control circuits are categorized into typical circuitry and special circuitry technologies The typical circuitry technologies include typical analog circuitry and digital circuitry, while the special circuitry consists of sigma delta, mode matching, temperature/quadrature compensation and novel special technologies Finally, the characteristics of various typical gyroscopes and their development tendency are discussed and investigated in detail

The Development of Micromachined Gyroscope Structure and Circuitry Technology

In this paper, we review a recent technology development based on coupled MEMS resonators that has the potential of fundamentally transforming MEMS resonant sensors. Conventionally MEMS resonant sensors use only a single resonator as the sensing element, and the output of the sensor is typically a frequency shift caused by the external stimulus altering the mechanical properties, i.e. the mass or stiffness, of the resonator. Recently, transduction techniques utilizing additional coupled resonators have emerged. The mode-localized resonant sensor is one example of such a technique. If the mode localization effect is utilized, the vibrational amplitude pattern of the resonators changes as a function of the quantity to be measured. Compared to using frequency shift as an output signal, the sensitivity can be improved by several orders of magnitude. Another feature of the mode-localized sensors is the common mode rejection abilities due to the differential structure. These advantages have opened doors for new sensors with unprecedented sensitivity.

A review on coupled MEMS resonators for sensing applications utilizing mode localization

This paper demonstrates a miniaturized and high resolution (16 nT/Hz
 1/2
) magnetometer based on a high frequency (168.1 MHz) magnetoelectric Microelectromechanical Systems-Complementary metal-oxidesemiconductor (MEMSCMOS) oscillator. For the first time, a high frequency and high electromechanical performance (quality factor, Q ~ 1084 and electromechanical coupling coefficient, k
 t
 2
 ~ 1.18%) magnetoelectric micromechanical resonator based on a self-biased aluminum nitride/iron-gallium-boron (AlN/FeGaB) bilayer nanoplate (250/250 nm) is implemented and used to synthesize a low noise frequency source (2.7 Hz/Hz
 1/2
) whose output frequency is highly sensitive to external magnetic field (169 Hz/μT at zero magnetic field bias). The angular sensitivity of the magnetometer for electronic compass applications is also investigated showing an ultrahigh angular resolution of 0.34° for a 10-μT conservative estimate of the earth's magnetic field, due to the strongly anisotropic sensitivity of the self-biased AlN/FeGaB magnetoelectric resonator. This paper represents the first demonstration of a high resolution self-biased MEMS magnetoelectric resonant sensor interfaced to a compact and low power self-sustained CMOS oscillator as direct frequency readout for the implementation of miniaturized and low power magnetometers with detection limit pushed in ~10s nT/Hz
 1/2
 range.

High Resolution Magnetometer Based on a High Frequency Magnetoelectric MEMS-CMOS Oscillator

Advances in micro- and nanofabrication technologies have enabled the development of novel micro- and nanomechanical resonators which have attracted significant attention due to their fascinating physical properties and growing potential applications. In this review, we have presented a brief overview of the resonance behavior and frequency tuning principles by varying either the mass or the stiffness of resonators. The progress in micro- and nanomechanical resonators using the tuning electrode, tuning fork, and suspended channel structures and made of graphene have been reviewed. We have also highlighted some major influencing factors such as large-amplitude effect, surface effect and fluid effect on the performances of resonators. More specifically, we have addressed the effects of axial stress/strain, residual surface stress and adsorption-induced surface stress on the sensing and detection applications and discussed the current challenges. We have significantly focused on the active and passive frequency tuning methods and techniques for micro- and nanomechanical resonator applications. On one hand, we have comprehensively evaluated the advantages and disadvantages of each strategy, including active methods such as electrothermal, electrostatic, piezoelectrical, dielectric, magnetomotive, photothermal, mode-coupling as well as tension-based tuning mechanisms, and passive techniques such as post-fabrication and post-packaging tuning processes. On the other hand, the tuning capability and challenges to integrate reliable and customizable frequency tuning methods have been addressed. We have additionally concluded with a discussion of important future directions for further tunable micro- and nanomechanical resonators.

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Tunable Micro- and Nanomechanical Resonators

Since two decades piezoelectric MEMS resonator research has been making a headway by leaps and bounds in multiple fronts such as micro/nanofabrication techniques, device and system design, monolithic integration, packaging, material engineering, etc. Piezoelectricity based frequency selective resonant tanks are an integral part of radio frequency communication, inertial sensors, infrared sensors, environmental monitoring modules, energy harvesters, etc. To improve the functionality of the aforementioned modules it is cardinal to understand the underlying concepts of resonator design, material selection, and performance enhancement techniques. Here in this review paper, a comprehensive overview of various piezoelectric thin-film material properties along with different commercial microfabrication platforms with CMOS integration facility is presented. The device characteristics greatly depend on the resonator mode shape and thus suitable modes and thin-film material configurations can be chosen depending on the target frequency, application, and power budget requirements. In addition to device modeling, a synopsis of the several acoustic and material engineering approaches to enhance the figure of merit of the resonator has been presented. The future avenues of piezoelectric MEMS cover a wide spectrum of possibilities such as 5G communication, quantum processing, frequency combs, photonics integration, gesture sensors, etc.

Piezoelectric MEMS Resonators: A Review

This letter reports, for the first time, on a high-frequency resonant square micro-gyroscope using piezoelectric transduction. Degenerate pairs of orthogonal flexural resonance modes are used to provide energy exchange paths for the Coriolis-based resonant gyroscope in response to z-axis rotation. Aluminum nitride thin films have been used to provide highly efficient electromechanical transduction for drive and sense modes without requiring any dc polarization voltage for operation. A proof-of-concept design consisting of a 300 μm× 300 μm square gyro shows linear rate sensitivity of 20.38 μV/°/s when operating in its first flexural mode at ~ 11 MHz.

High-Frequency AlN-on-Silicon Resonant Square Gyroscopes

This paper presents, for the first time, the design, implementation, and characterization of a dynamically mode-matched high-frequency piezoelectrically transduced silicon disk gyroscope utilizing a unique pair of degenerate orthogonal in-plane flexural gyroscopic modes. To eliminate the need for submicron capacitive gaps and large DC polarization voltages that are needed for electrostatic tuning, a linear bidirectional frequency tuning scheme, compatible with all-piezoelectric transduction, is introduced to achieve dynamic mode-matching via active electromechanical feedback. A fabricated 4.34 MHz AlN-on-Silicon 1-mm-diameter solid disk gyroscope supported by a network of peripheral beams was frequency-tuned by 500 ppm (∼2.2 kHz) and achieved a high measured sensitivity of 410 pA/°/s with an effective in-air quality factor of ∼4000, corresponding to a large open-loop operation bandwidth of ∼550 Hz under mode-matched conditions.

A dynamically mode-matched piezoelectrically transduced high-frequency flexural disk gyroscope

The theory of eigenmode operation of Coriolis vibratory gyroscopes and its implementation on a thin-film piezoelectric gyroscope is presented. It is shown analytically that the modal alignment of resonant gyroscopes can be achieved by applying a rotation transformation to the actuation and sensing directions regardless of the transduction mechanism. This technique is especially suitable for mode matching of piezoelectric gyroscopes, obviating the need for narrow capacitive gaps or DC polarization voltages. It can also be applied for mode matching of devices that require sophisticated electrode arrangements for modal alignment, such as electrostatic pitch and roll gyroscopes with slanted electrodes utilized for out-of-plane quadrature cancellation. Gyroscopic operation of a 3.15 MHz AlN-on-Si annulus resonator that utilizes a pair of high-Q degenerate in-plane vibration modes is demonstrated. Modal alignment of the piezoelectric gyroscope is accomplished through virtual alignment of the excitation and readout electrodes to the natural direction of vibration mode shapes in the presence of fabrication nonidealities. Controlled displacement feedback of the gyroscope drive signal is implemented to achieve frequency matching of the two gyroscopic modes. The piezoelectric gyroscope shows a mode-matched operation bandwidth of ~250 Hz, which is one of the largest open-loop bandwidth values reported for a mode-matched MEMS gyroscope, a small motional resistance of ~1300 Ω owing to efficient piezoelectric transduction, and a scale factor of 1.57 nA/°/s for operation at atmospheric pressure, which greatly relaxes packaging requirements. Eigenmode operation results in an ~35 dB reduction in the quadrature error at the resonance frequency. The measured angle random walk of the device is 0.86°/√h with a bias instability of 125°/h limited by the excess noise of the discrete electronics. An alternative design and operation strategy for resonant gyroscopes delivers faster response while also achieving low levels of noise. These tiny devices measure an object’s rate of rotation, and are commonly used in consumer electronics as well as industrial and automotive applications. Resonant gyroscopes are currently limited by a tradeoff between bandwidth and signal-to-noise ratio, but Mojtaba Hodjat-Shamami and Farrokh Ayazi at the Georgia Institute of Technology have devised a solution to this problem. They employed an eigenmode operation strategy with a thin film piezoelectric-on-silicon resonant gyroscope, and demonstrated that they could routinely achieve mode-matched bandwidths of a few hundred Hertz, with greatly reduced error. This approach could greatly bolster the utility of such piezoelectric devices, but the authors note that a similar eigenmode operation strategy could also broadly optimize performance for other resonant gyroscope designs as well.

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Eigenmode operation of piezoelectric resonant gyroscopes.

This paper introduces an active electrical technique for dynamic tuning of MEMS resonators. The proposed technique is based on using the resonator output current to generate displacement or acceleration signals by integration or differentiation operations, respectively. The resulting signal is then scaled to generate an appropriate tuning signal. When applied to the resonator through additional signal ports, the tuning signal electrically modifies the equivalent mechanical stiffness or mass of the resonator, thereby tuning the resonance frequency in a bidirectional fashion depending on the polarity of the scaling. This tuning scheme has been applied to a piezoelectric AlN-on-Si BAW square resonator to tune its 14.2 MHz resonance frequency by 22 kHz, equivalent to 1550 ppm. The proposed tuning technique can be applied to a wide range of MEMS resonators and resonant sensors, e.g., to compensate for temperature or process-induced variations in their resonance frequencies.

Dynamic tuning of MEMS resonators via electromechanical feedback

This paper presents a quadrature error cancellation technique based on the virtual alignment of the gyroscope excitation and readout electrodes to the actual direction of vibration mode shapes in the presence of fabrication non-idealities. The proposed method which is herein referred to as eigenmode operation, enables modal alignment of all-piezoelectric resonant gyroscopes and along with dynamic frequency matching via displacement feedback obviates the need for submicron capacitive gaps and relatively large DC voltages that are otherwise required for electrostatic mode matching. We demonstrate gyroscopic operation of a 3.1 MHz AlN-on-silicon annulus resonator, which utilizes a pair of high-Q degenerate in-plane vibration modes. The piezoelectric gyroscope shows a large operation bandwidth of ∼150 Hz with a scale factor of 1.6 nA/°/s while operating in air, which greatly relaxes packaging requirements. Eigenmode operation results in ∼35 dB reduction of the quadrature error at resonance.

Mojtaba Hodjat-Shamami

Papers

High-Frequency AlN-on-Silicon Resonant Square Gyroscopes

A dynamically mode-matched piezoelectrically transduced high-frequency flexural disk gyroscope

Eigenmode operation of piezoelectric resonant gyroscopes.

Dynamic tuning of MEMS resonators via electromechanical feedback

Eigenmode operation as a quadrature error cancellation technique for piezoelectric resonant gyroscopes