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 reports an acceleration sensing method based on two weakly coupled resonators (WCRs) using the phenomenon of mode localization. When acceleration acts on the proof masses, differential electrostatic stiffness perturbations will be applied to the WCRs, leading to mode localization, and thus, mode shape changes. Therefore, acceleration can be sensed by measuring the amplitude ratio shift. The proposed mode localization with the differential perturbation method leads to a sensitivity enhancement of a factor of 2 than the common single perturbation method. The theoretical model of the sensitivity, bandwidth, and linearity of the accelerometer is established and verified. The measured relative shift in amplitude ratio ( $\sim 312162$ ppm/g) is 302 times higher than the shift in resonance frequency ( $\sim 1035$ ppm/g) within the measurement range of ±1 g. The measured resolution based on the amplitude ratio is 0.619 mg and the nonlinearity is $\sim 3.5$ % in the open-loop measurement operation. [2015-0247]

An Acceleration Sensing Method Based on the Mode Localization of Weakly Coupled Resonators

A new micromachined uniaxial silicon resonant accelerometer characterized by a high sensitivity and very small dimensions is presented. The device's working principle is based on the frequency variations of two resonating beams coupled to a proof mass. Under an external acceleration, the movement of the proof mass causes an axial load on the beams, generating opposite stiffness variations, which, in turn, result in a differential separation of their resonance frequencies. A high level of sensitivity is obtained, owing to an innovative and optimized geometrical design of the device that guarantees a great amplification of the axial loads. The acceleration measure is obtained, owing to a properly designed oscillating circuit. In agreement with the theoretical prediction, the experimental results show a sensitivity of 455 Hz/ ( g being the gravity acceleration) with a resonant frequency of about 58 kHz and a good linearity in the range of interest.

A Resonant Microaccelerometer With High Sensitivity Operating in an Oscillating Circuit

: For many navigation applications, improved accuracy/performance is not necessarily the most important issue, but meeting performance at reduced cost and size is In particular, small navigation sensor size allows the introduction of guidance, navigation, and control into applications previously considered out of reach (eg, artillery shells, guided bullets) Three major technologies have enabled advances in military and commercial capabilities: Ring Laser Gyros, Fiber Optic Gyros, and Micro-Electro-Mechanical Systems (MEMS) gyros and accelerometers RLGs and FOGs are now mature technologies, although there are still technology advances underway for FOGs MEMS is still a very active development area Technology developments in these fields are described with specific emphasis on MEMS sensor design and performance Some aspects of performance drivers are mentioned as they relate to specific sensors Finally, predictions are made of the future applications of the various sensor technologies

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Inertial Navigation Sensors

Microelectromechanical systems (MEMS) have been applied to many measurement problems in physics, chemistry, biology and medicine. In parallel, cavity optomechanical systems have achieved quantum-limited displacement sensitivity and ground state cooling of nanoscale objects. By integrating a novel cavity optomechanical structure into an actuated MEMS sensing platform, we demonstrate a system with high-quality-factor interferometric readout, electrical tuning of the optomechanical coupling by two orders of magnitude and a mechanical transfer function adjustable via feedback. The platform separates optical and mechanical components, allowing flexible customization for specific scientific and commercial applications. We achieve a displacement sensitivity of 4.6fmHz 1/2 and a force sensitivity of 53aNHz 1/2 with only 250nW optical power launched into the sensor. Cold- damping feedback is used to reduce the thermal mechanical vibration of the sensor by three orders of magnitude and to broaden the sensor bandwidth by approximately the same factor, to above twice the fundamental frequency of 40kHz. The readout sensitivity approaching the standard quantum limit is combined with MEMS actuation in a fully integrated, compact, low-power, stable system compatible with Si batch fabrication and electronics integration.

/pdf/a-microelectromechanically-controlled-cavity-optomechanical-12qi1jm688.pdf

A microelectromechanically controlled cavity optomechanical sensing system

We present the design, fabrication, and testing of a push-pull differential resonant accelerometer with double-ended-tuning-fork (DETF) as the inertial force sensor. The accelerometer is fabricated with the silicon-on-insulator microelectromechanical systems (MEMS) technology that bridges surface micromachining and bulk micromachining by integrating the 50-/spl mu/m-thick high-aspect ratio MEMS structure with the standard circuit foundry process. Two DETF resonators serve as the force sensor measuring the acceleration through a frequency shift caused by the inertial force acting as axial loading. Two-stage microleverage mechanisms with an amplification factor of 80 are designed for force amplification to increase the overall sensitivity to 160 Hz/g, which is confirmed by the experimental value of 158 Hz/g. Trans-resistance amplifiers are designed and integrated on the same chip for output signal amplification and processing. The 50-/spl mu/m thickness of the high-aspect ratio MEMS structure has no effect on the amplification factor of the mechanism but contributes to a greater capacitance force; therefore, the resonator can be actuated by a much lower ac voltage comparing to the 2-/spl mu/m-thick DETF resonators. The testing results agree with the designed sensitivity for static acceleration.

S.X.P. Su

Papers

A resonant accelerometer with two-stage microleverage mechanisms fabricated by SOI-MEMS technology