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

Inductor–capacitor ( $LC$ ) passive wireless sensors use a transformer with loose coupling between an external readout coil and an inductor that receives power through this inductive coupling. Changes in the sensor are wirelessly and remotely detected by the readout coil, which makes them highly useful in applications that require the sensor to be powered remotely and to occupy a small volume, such as harsh and sealed environments, where physical access to the sensor is difficult. Although the sensor to accomplish this function dates from the 1960’s, its rapid extension over the past decades has benefited from microelectromechanical systems. This paper provides an overview of the status and challenges in the $LC$ passive wireless sensor toward a wireless sensing platform. The basic sensing principles are first categorized into detecting changes of the sensor in response to the capacitance, resistance, inductance, or coupling distance due to the parameter of interest through monitoring the impedance magnitude and phase spectrum. The present state of the art in sensor applications for pressure, strain, temperature, humidity, biochemical, gas, and so on is then reviewed and compared. For emerging applications from many Internet of Things scenarios, geometrical constraints, such as small and non-invasive coils, reduce the magnetic coupling between the sensor and the readout coil, resulting in a limited interrogation distance. Furthermore, an increasing number of applications also require the simultaneous measurement of multiple parameters. Recent efforts to increase the interrogation distance and to extend the measurement of single parameter to multiple parameters are finally outlined. [2016-0093]

LC Passive Wireless Sensors Toward a Wireless Sensing Platform: Status, Prospects, and Challenges

Development of 30 Watt high-power LEDs vapor chamber-based plate

In this paper, we present an electronic readout system for wireless passive sensors based on inductively coupled LC resonant circuits. The proposed system consists of a reader coil inductively coupled to the sensor circuit, an analog frontend circuit, and a digital signal processing unit. The analog frontend circuit generates a dc voltage representing the sensor resonance curve. The frequency of the reader coil driving signal is continuously readjusted by the digital signal processing unit. Based on analytical calculation and system simulation, we derive a model for the achievable accuracy of the overall sensor and readout system. The accuracy is limited by noise and systematic errors due to the measurement principle. We show how to design the digital signal processing system for optimal insensitivity to voltage noise. The noise sensitivity of the measurement system is inversely proportional to the square of the quality factor of the LC sensor. This means that minimizing the losses of the sensor is of crucial importance to obtain a wireless measurement system with a high range and a good insensitivity to noise. Subsequently, we outline an approach to calculate the sensor resonance frequency, quality factor, and inductive coupling factor from the available voltage signals in the signal processing unit using linear fitting functions. The accuracy of our approach is exemplified by a system simulation for typical sensor parameters. For the system studied, we show that the relative linearization error of the sensor resonance frequency measurement is below 0.02%. Taking the general models presented for both the noise sensitivity and linearization error into account, it is possible to estimate the maximum distance and accuracy for any wireless sensor system based on an inductively coupled LC resonator.

A Wireless Sensor Readout System—Circuit Concept, Simulation, and Accuracy

This paper proposes a fully embedded resonant pressure sensor operating in the MHz range and realized in the standard low-temperature co-fired ceramics (LTCC) technology. Buried sensor design and usage of LTCC materials enable application of this sensor in high-temperature and chemically aggressive environments. Upgraded sensor and sensor-antenna models residing on an analytical concept are used for prediction of the system performance. Also, simulation results show that an increase of Young's modulus for the LTCC tape diminishes the sensor sensitivity. An experimental setup for wireless data retrieval is designed enabling precise measurement of the influence of pressure variation on the sensors resonant frequency. Experimentally attained results are compared with electrical characteristics determined by analytical calculations as well as those derived from electrical simulations.

A Wireless Embedded Resonant Pressure Sensor Fabricated in the Standard LTCC Technology

With increasing power loss of electrical components, thermal performance of an assembled device becomes one of the most important quality factors in electronic packaging. Due to rapid advances in semiconductor technology, particularly in the field of high-power components, the temperature distribution inside of a component is a critical parameter of long-term reliability and must be carefully considered during the design phase. Two main drivers in the electronics industry are miniaturization and reliability. Whereas there is a continuous improvement concerning miniaturization of conductor tracks (i.e., lines and spaces have been reduced continuously over the past years), miniaturization of the circuit carrier itself, however, has mostly been limited to decreased layer counts and base material thickness. This can lead to significant component temperature increase and thence to accelerated system degradation. Enhancement of the system reliability is directly connected to an efficient thermal management on t...

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Advanced Thermal Management Solutions on PCBs for High Power Applications

A new type of integrated temperature and humidity sensor applying Low Temperature Cofired Ceramics technology (LTCC) has been developed and characterized. The proposed device is based on the detection of the difference of thermal conductivity between water vapor and dry air. In this approach, sensing elements are implemented using heated metal film resistors (Pt-elements), where one is exposed to the humid environment that causes the sensor element to cool down with increased humidity, while the other one is sealed from the environment. LTCC-tapes are used for the formation of caps as well are acting as substrate. Sensor design is based on finite element analyses (FEA) where the critical design parameters have been analyzed with regard to the performance characteristic of the device.

https://hal.archives-ouvertes.fr/hal-00257674/document

Design and characterization of a humidity sensor realized in LTCC-technology

This paper presents resonant force sensor designed for the operation in the MHz range and for 0 to 6 N load. The LTCC technology is implemented for the sensor fabrication and a wireless readout of the measured data is provided. Used LTCC tape is characterised in order to demonstrate its mechanical and electrical properties at room temperature. Also, theoretical model of the sensor is developed to predict its behaviour. Fabricated sensor performance is experimentally characterised and obtained results are in good agreement with the ones derived from the presented theoretical model.

Micro force sensor fabricated in the LTCC technology

Various LTCC-tapes of different suppliers have been evaluated concerning their mechanical properties which are required for finite-element-analyses (FEA) in order to design mechanical operating sensors. Bending tests on fired tape samples have been conducted in order to investigate the influence of processing conditions, applied metallization as well as geometrical parameters on the mechanical characteristics. Additionally piezo-resistive thick-film sensors have been applied onto the LTCC-beam elements to correlate bending stress with the sensor signal.

Michael Unger

Papers

A Wireless Embedded Resonant Pressure Sensor Fabricated in the Standard LTCC Technology

Advanced Thermal Management Solutions on PCBs for High Power Applications

Design and characterization of a humidity sensor realized in LTCC-technology

Micro force sensor fabricated in the LTCC technology

Mechanical characteristics of LTCC (Low Temperature Cofired Ceramics) -tapes for mechanical application