Assessment of Fiber Bragg Grating Sensors for Monitoring Shaft Vibrations of Hydraulic Turbines.

The structural dynamic response of hydraulic turbines needs to be continuously monitored to predict incipient failures and avoid catastrophic breakdowns. Current methods based on traditional off-board vibration sensors mounted on fixed components do not permit inferring loads induced on rotating parts with enough accuracy. Therefore, the present paper assesses the performance of fiber Bragg grating sensors to measure the vibrations induced on a rotating shaft-disc assembly partially submerged in water resembling a hydraulic turbine rotor. An innovative mounting procedure for installing the sensors is developed and tested, which consists of machining a thin groove along a shaft line to embed a fiber-optic array that can pass through the bearings. At the top of the shaft, a rotary joint is used to extract, in real time, the signals to the interrogator. The shaft strain distribution is measured with high spatial resolution at different rotating speeds in air and water. From this, the natural frequencies, damping ratios, and their associated mode shapes are quantified at different operating conditions. Additionally, the change induced in the modes of vibration by the rotation effects is well captured. All in all, these results validate the suitability of this new fiber-optic technology for such applications and its overall better performance in terms of sensitivity and spatial resolution relative to traditional equipment. The next steps will consist of testing this new sensing technology in actual full-scale hydraulic turbines.

Impact of Fluid Flow on CMOS-MEMS Resonators Oriented to Gas Sensing.

Based on experimental data, this paper thoroughly investigates the impact of a gas fluid flow on the behavior of a MEMS resonator specifically oriented to gas sensing. It is demonstrated that the gas stream action itself modifies the device resonance frequency in a way that depends on the resonator clamp shape with a corresponding non-negligible impact on the gravimetric sensor resolution. Results indicate that such an effect must be accounted when designing MEMS resonators with potential applications in the detection of volatile organic compounds (VOCs). In addition, the impact of thermal perturbations was also investigated. Two types of four-anchored CMOS-MEMS plate resonators were designed and fabricated: one with straight anchors, while the other was sustained through folded flexure clamps. The mechanical structures were monolithically integrated together with an embedded readout amplifier to operate as a self-sustained fully integrated oscillator on a commercial CMOS technology, featuring low-cost batch production and easy integration. The folded flexure anchor resonator provided a flow impact reduction of 5× compared to the straight anchor resonator, while the temperature sensitivity was enhanced to -115 ppm/°C, an outstanding result compared to the -2403 ppm/°C measured for the straight anchored structure.

Thermomechanical Noise Characterization in Fully Monolithic CMOS-MEMS Resonators.

We analyzed experimentally the noise characteristics of fully integrated CMOS-MEMS resonators to determine the overall thermomechanical noise and its impact on the limit of detection at the system level. Measurements from four MEMS resonator geometries designed for ultrasensitive detection operating between 2-MHz and 8-MHz monolithically integrated with a low-noise CMOS capacitive readout circuit were analyzed and used to determine the resolution achieved in terms of displacement and capacitance variation. The CMOS-MEMS system provides unprecedented detection resolution of 11 yF·Hz-1/2 equivalent to a minimum detectable displacement (MDD) of 13 fm·Hz-1/2, enabling noise characterization that is experimentally demonstrated by thermomechanical noise detection and compared to theoretical model values.

A Tunable-Gain Transimpedance Amplifier for CMOS-MEMS Resonators Characterization.

CMOS-MEMS resonators have become a promising solution thanks to their miniaturization and on-chip integration capabilities. However, using a CMOS technology to fabricate microelectromechanical system (MEMS) devices limits the electromechanical performance otherwise achieved by specific technologies, requiring a challenging readout circuitry. This paper presents a transimpedance amplifier (TIA) fabricated using a commercial 0.35-µm CMOS technology specifically oriented to drive and sense monolithically integrated CMOS-MEMS resonators up to 50 MHz with a tunable transimpedance gain ranging from 112 dB to 121 dB. The output voltage noise is as low as 225 nV/Hz1/2-input-referred current noise of 192 fA/Hz1/2-at 10 MHz, and the power consumption is kept below 1-mW. In addition, the TIA amplifier exhibits an open-loop gain independent of the parasitic input capacitance-mostly associated with the MEMS layout-representing an advantage in MEMS testing compared to other alternatives such as Pierce oscillator schemes. The work presented includes the characterization of three types of MEMS resonators that have been fabricated and experimentally characterized both in open-loop and self-sustained configurations using the integrated TIA amplifier. The experimental characterization includes an accurate extraction of the electromechanical parameters for the three fabricated structures that enables an accurate MEMS-CMOS circuitry co-design.

CMOS-MEMS VOC sensors functionalized via inkjet polymer deposition for high-sensitivity acetone detection.

CMOS-MEMS microresonators have become excellent candidates for developing portable chemical VOC sensing systems thanks to their extremely large mass sensitivity, extraordinary miniaturization capabilities, and on-chip integration with CMOS circuitry to operate as a self-sustained oscillator. This paper presents two 4-anchored MEMS plate resonators, with a resonance frequency of 2.2 MHz and 380 kHz, fabricated together with the required circuitry using a commercial 0.35 µm CMOS technology and then coated with poly-4-vinylheduorocumyl alcohol (P4V) via inkjet deposition. Such P4V constitutes a functionalization layer for specific acetone detection as a key step in the development of an integrated device for non-invasive diabetes diagnosis through exhaled human breath. The coated sensor system has been proven to increase the acetone injection response by 6-times compared to the uncoated platform and shows a cross-sensitivity to butane of 1 : 11. Experimental data show an acetone sensitivity of -0.012 ppm Hz-1 in the best case that, together with a measured frequency Allan deviation of 0.32 ppm, provides an expected limit of detection as low as 20 ppb of acetone. Additionally, this work presents an alternative resonator design with folded flexure anchors that provide a drastic reduction of the sensor temperature sensitivity and mitigate the impact of a fluid flow inherent to the calibration system.

A 0.35-µm CMOS-MEMS Oscillator for High-Resolution Distributed Mass Detection.

This paper presents the design, fabrication, and electrical characterization of an electrostatically actuated and capacitive sensed 2-MHz plate resonator structure that exhibits a predicted mass sensitivity of ~250 pg·cm-2·Hz-1. The resonator is embedded in a fully on-chip Pierce oscillator scheme, thus obtaining a quasi-digital output sensor with a short-term frequency stability of 1.2 Hz (0.63 ppm) in air conditions, corresponding to an equivalent mass noise floor as low as 300 pg·cm-2. The monolithic CMOS-MEMS sensor device is fabricated using a commercial 0.35-µm 2-poly-4-metal complementary metal-oxide-semiconductor (CMOS) process, thus featuring low cost, batch production, fast turnaround time, and an easy platform for prototyping distributed mass sensors with unprecedented mass resolution for this kind of devices.