Humidity measurement is one of the most significant issues in various areas of applications such as instrumentation, automated systems, agriculture, climatology and GIS. Numerous sorts of humidity sensors fabricated and developed for industrial and laboratory applications are reviewed and presented in this article. The survey frequently concentrates on the RH sensors based upon their organic and inorganic functional materials, e.g., porous ceramics (semiconductors), polymers, ceramic/polymer and electrolytes, as well as conduction mechanism and fabrication technologies. A significant aim of this review is to provide a distinct categorization pursuant to state of the art humidity sensor types, principles of work, sensing substances, transduction mechanisms, and production technologies. Furthermore, performance characteristics of the different humidity sensors such as electrical and statistical data will be detailed and gives an added value to the report. By comparison of overall prospects of the sensors it was revealed that there are still drawbacks as to efficiency of sensing elements and conduction values. The flexibility offered by thick film and thin film processes either in the preparation of materials or in the choice of shape and size of the sensor structure provides advantages over other technologies. These ceramic sensors show faster response than other types.

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Humidity Sensors Principle, Mechanism, and Fabrication Technologies: A Comprehensive Review

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 is a review of the remarkable progress that has been made during the past few decades in design, modeling, and fabrication of micromachined resonators. Although micro-resonators have come a long way since their early days of development, they are yet to fulfill the rightful vision of their pervasive use across a wide variety of applications. This is partially due to the complexities associated with the physics that limit their performance, the intricacies involved in the processes that are used in their manufacturing, and the trade-offs in using different transduction mechanisms for their implementation. This work is intended to offer a brief introduction to all such details with references to the most influential contributions in the field for those interested in a deeper understanding of the material.

Micromachined Resonators: A Review

Micro and nanoelectromechanical systems M/NEMS have been extensively investigated and exploited in the past few decades for various applications and for probing fundamental physical phenomena. Understanding the linear and nonlinear dynamical behaviors of the movable structures in these systems is crucial for their successful implementation in various novel technologies and to meet the long list of new sophisticated requirements. This paper presents a review for some of the recent topics pertaining to the dynamical behaviors, linear and nonlinear, of M/NEMS resonating structures. First, an overview is presented of the various used dynamical approaches to enhance the sensitivity of resonators for sensing applications. Then a summary is presented of the recent works on the linear and nonlinear mode coupling in M/NEMS resonator. Next, recent research is reviewed on coupled M/NEMS resonators, mechanically and electrically, leading to collective behaviors like mode localization. The final part of the paper discusses analytical approaches that have been developed to better understand and investigate the dynamical behavior of M/NEMS resonators focusing on the perturbation method the multiple time scales.

/pdf/linear-and-nonlinear-dynamics-of-micro-and-nano-resonators-1pznaruyjr.pdf

Linear and nonlinear dynamics of micro and nano-resonators: Review of recent advances

This paper reports a high-sensitivity resonant electrometer based on the mode localization of two degree-of-freedom weakly coupled resonators (WCRs). When charges are applied to the input electrodes, the effective stiffness of a specific resonator of the WCRs will be perturbed, leading to a drastic change in mode shape owing to the mode localization phenomenon. By measuring the shift of the amplitude ratio, the small charge fluctuation can be accurately sensed. The theoretical mode of the electrometer is established based on the transfer functions of the WCRs. In particular, we establish the design rules of the coupling factor according to the −3-dB bandwidth, amplitude ratio measurement errors, and frequency misalignment between the resonators. The experimental results show that the amplitude ratio-based sensitivity is $\sim 2151$ times higher than the frequency-based sensitivity. The amplitude ratio-based resolution of the electrometer is approximately 1.269 fC . [2016-0119]

A High-Sensitivity Micromechanical Electrometer Based on Mode Localization of Two Degree-of-Freedom Weakly Coupled Resonators

This paper introduces a tunable, narrow-band, micromechanical filter whose bandwidth and centre frequency can be adjusted independently. The filter is made of two micro-resonators that are electrostatically coupled using a middle electrode. A low coupling strength results in nearly constant bandwidth while one tunes the centre frequency bi-directionally by applying a DC voltage to the coupling electrode. On the other hand, the bandwidth of the filter is independently modified by applying axial stress to one of the resonators, without affecting the signal attenuation through the filter. Analytic and numerical models for the behaviour of the filter are also presented. Test devices with a centre frequency of about 300 kHz were fabricated in a standard micromachining process. Experimental results support the design principle and validity of the proposed models.

A micromechanical bandpass filter with adjustable bandwidth and bidirectional control of centre frequency

This paper describes a method for processing signals from an array of coupled resonators to detect perturbations due to external stimuli. This method is based on simultaneous monitoring of the relative changes in all eigenvalues of the characteristic equation and processing this information to quantify the amount of perturbations and to determine the resonators whose properties have changed. Since the method is based on the relative movements of eigenvalues, the variations in the absolute values of the eigenfrequencies do not strongly affect its accuracy. It will be shown that this technique is capable of detecting perturbations even if direct signals from some of the coupled resonators are not available. The role of coupling strength on the sensitivity of the system is also discussed. It is shown that it is possible to increase the sensitivity of the sensor system to perturbations by a large factor through proper selection of the coupling coefficient between the resonators. The proposed model is experimentally verified using an array of coupled micro-cantilever resonators fabricated in a standard micro-fabrication process.

Characterization of Disturbances in Systems of Coupled Micro-Resonator Arrays

This paper reports on the design and experimental characterization of a higher-order Lame mode to reduce the effect of squeezed-film damping and thus enhancing the Quality factor for near atmospheric pressures. Three vibration modes of the resonator are examined. It is demonstrated that the employing a higher order Lame modes will result in higher f-Q product compared to the fundamental Lame and extensional modes of the resonator. For the tested resonator, the Quality factor for the second Lame mode is 830,000 at a frequency of 16.54MHz resulting in an f-Q product of 1.37×1013 Hz. This is about an order of magnitude higher than the f-Q product for lower modes.

Higher-order Lamé mode square microresonator with improved quality factor and f-Q product

This paper presents a relative humidity sensor that employs polyimide nanoparticles as the sensing medium. Nanoparticles are deposited through electrospraying process and exhibit higher linearity compared to a uniform film of polyimide that is deposited through a spin-on process. Another advantage of this sensor compared to the existing humidity sensors is its simple structure, which can decrease the fabrication costs. Experimental results were obtained to show different linearity of the humidity sensor under different parameters. Various sensor design parameters were studied in order to design a sensor with the best overall performance.

http://ieeexplore.ieee.org/iel5/6381733/6411021/06411213.pdf

A capacitive relative humidity sensor using polymer nanoparticles

This paper introduces a micromachined electrometer with high resolution and wide dynamic range. The sensor is made of electrically coupled microresonators. The input signal modifies the coupling strength between the microresonators, and hence, results in a change in the location of system poles. Additionally, using opposite polarities for the bias voltages of the resonators leads to moving of the resonance frequencies away from each other. This produces a differential signal with high sensitivity compared to devices made from a single resonator. In addition to the high sensitivity, another advantage of this sensor is its robustness against manufacturing tolerances and interfering parameters such as temperature variations.

M. S. Hajhashemi

Papers

A micromechanical bandpass filter with adjustable bandwidth and bidirectional control of centre frequency

Characterization of Disturbances in Systems of Coupled Micro-Resonator Arrays

Higher-order Lamé mode square microresonator with improved quality factor and f-Q product

A capacitive relative humidity sensor using polymer nanoparticles

A differential electrometer based on coupled microresonators