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

Various LTCC-tape materials have been characterized regarding their RF-properties at elevated temperature by means of impedance measurement of square-shaped capacitor samples up to a frequency of 3 MHz. Parameters of an equivalent circuit were fitted to reconstruct the impedance characteristics and the parameters extracted. The activation energies of samples were derived from the impedance model.

Permittivity and conductivity of various LTCC–tapes at high temperature

Application of Low Temperature Co-Fired Ceramic (LTCC) technology is widely spread. Its presence is well established in component packaging, sensor applications, high frequency applications [1, 2], chemical reactors and bio-medical applications [3]. For successful realization and implementation of LTCC structures, characteristics and features of LTCC tapes as well as their behavior have to be known. Various investigations related to features of LTCC tapes have been already reported [4-6]. This investigation is focused on physical and electrical characterization of new Heraeus CT708 and Heraeus CT802 LTCC dielectric tapes. Surface roughness has been investigated and presented. Investigation was conducted on shrinkage and weight loss of LTCC tapes and their dependence of the substrate thickness. Circularly shaped LTCC specimens with various thicknesses have been manufactured. The relevant dimensions as well as weights of the specimens were measured before and after firing, results and conclusion are given. Report on electrical permittivity and its dependence on frequency is also presented in this paper. The capacitance method was used. Simple plate capacitors with circular electrodes were realized, where LTCC tapes are used as a dielectric material. Modulus of elasticity as well as flexural strength was investigated by a three-point bending test.

Investigation on physical and electrical behaviour of LTCC dielectric tapes

The material properties (TCE, dielectric number, etc.) of different LTCC (Low Temperature Co-fired Ceramics) — tape materials were characterized separately and compared to the behavior of combinations of these tapes that were laminated and co-fired. The effects of different values of thermal expansion and change in dielectric number of the combined material stacks have been investigated by finite element analyses and verified using a wireless readout resonant temperature sensor as a demonstration device..

Parameters governing the sensor characteristic of capacitive temperature sensors built up in LTCC-technology

A new type of integrated temperature and humidity sensor applying LTCC-technology has been developed and characterized. In this approach, sensing elements are implemented using heated metal 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. Sensor design is based on FEA (Finite Element Analyses) where the critical design parameters have been analyzed with regard to the performance characteristic of the device. The set-up of sensor element will be shown and the functional capability will be demonstrated by experimental results.

Set-up and characterization of a humidity sensor realized in LTCC-technology

Within the scope of this work, we study the impact of embedded mini heat pipes on the overall thermal performance of PCBs. In a first step, the heat transfer of miniaturized heat pipes is quantified by measurements. The effective thermal conductivity and the maximum admissible heat flow are determined in dependence of the environment temperature and gravitational field orientations. In a second step, the heat spreading properties of miniaturized heat pipes embedded in PCBs are investigated experimentally. Finally, the combination of precise thermal characterization together with reliable FEM simulations establish a basis for the design and the expected performance of solutions for heat spreading based on PCB-Embedded mini heat pipes.

Michael Unger

Papers

Permittivity and conductivity of various LTCC–tapes at high temperature

Investigation on physical and electrical behaviour of LTCC dielectric tapes

Parameters governing the sensor characteristic of capacitive temperature sensors built up in LTCC-technology

Set-up and characterization of a humidity sensor realized in LTCC-technology

Embedded heat pipes as thermal solution for PCBs