Parallel Plate Capacitive Sensor Research Articles

Sensitivity Enhancement of a PPM Level Capacitive Moisture Sensor

Measurement of moisture at ppm or ppb level is very difficult and the fabrication of such sensors at low cost is always challenging. High sensitivity is an important parameter for trace level (ppm) humidity sensors. Anelectronic detection circuit for interfacing the humidity sensor with high sensitivity requires a simple hardware circuit with few active devices. The recent trends for increasing the sensitivity include fabricating nanoporous film with a very large surface area. In the present work, the sensitivity of a parallel plate capacitive type sensor with metal oxide sensing film has been significantly improved with an aim to detect moisture from 3 to 100 ppm in the industrial process gases used to fabricate semiconductors and other sensitive electronic devices. The sensitivity has been increased by (i) fabricating a nanoporous film of aluminum oxide using the sol-gel method and (ii) increasing the cross-sectional area of a parallel plate capacitor. A novel double sided capacitive structure has been proposed where two capacitors have been fabricated—one on the top and one on the bottom side of a flat alumina substrate—and then the capacitors are connected in parallel. The structure has twice the sensitivity of a single sensor in the same ppm range but the size of the structure remains unchanged. The important characteristics of the sensors such as the sensitivity (S = Δ C Δ p p m × 100 ), the response time (tr), and the recovery time (tc) are determined and compared with a commercial SHAW, UKdew point meter. The fabricated double sided sensor has comparable sensitivity (S = 100%, tr (s) = 28, tc (s) = 40) with the commercial meter (S = 100.5%, tr (s) = 258) but has a faster response time. The proposed method of sensitivity enhancement is simple, and mass producible.

Conductivity Effect on the Capacitance Measurement of a Parallel-Plate Capacitive Sensor System

In this article design and operation of a parallel-plate capacitive sensor based on the dielectric capacitance and conductance change of the gap medium is reported. The designed system was used to determine characteristics of different water liquids as a result of the capacitance variations. The air gap capacitance is measured and then by filling the gap with a liquid the capacitance is monitored for different liquids. Proposed sensor is used for the distilled, tap, boiled, and salt water measurements and the capacitance results are compared. A big difference about 45.5µF in capacitance values for the salt and distilled water shows a high sensitivity, which can be used to recognize different water liquids. The experimental results are promising for water liquids and verify the successful operation of such a device as a liquid sensor, a useful method for checking the electrical quality of the water that is required for different applications.

Stochastic resonance in MEMS capacitive sensors

The paper presents a theory for stochastic resonance in MEMS capacitive chemical sensors and lays a framework for the calculation of sensor response. A parallel plate capacitive MEMS sensor biased to a fixed DC voltage and driven by a small amplitude AC voltage is considered in the presence of inherent cantilever noise sources. The stochastic resonance occurs due to correlation between the cantilever oscillations and the noise spectral components in the presence of dynamic nonlinearity. The noise power is transferred resonantly to the cantilever oscillations under certain conditions that are determined by the noise correlation rate, damping loss and AC drive frequency. The traditional strategies for enhancing sensor responses explore the optimization of cantilever geometry, DC bias level and AC drive amplitude and frequency. The present work provides theoretical framework for harnessing the noise power for enhancing the amplitude response of capacitive MEMS chemical sensors. The occurrence and usefulness of stochastic resonance in MEMS chemical sensor has been demonstrated by generating simulation results pertaining to explore the dependencies on various sensor parameters such as noise power and correlation rate, damping loss and polymer thickness.

Modeling and Development of a Flexible Carbon Black-Based Capacitive Strain Sensor

The wide range of applications of strain sensors leads to an increased interest on flexible, cost effective, and low-power devices for strain and bending measurements. A simply structured sensor that meets these requirements is studied using a finite-element analysis. The sensor design focuses on the sensor performance variations for different applications and takes into account the fabrication process that can be used for elastomeric flexible sensors. Both large and small strain performance is considered. The simulation results are also compared with experimental measurements exploiting a flexible parallel plate capacitive strain sensor comprising carbon black-polydimethylsiloxane (PDMS) composite layers as electrodes and PDMS as the dielectric and encapsulation material. A simple and low-cost fabrication process was used, and the sensor was tested in a cantilever beam measurement setup. The device exhibits linear capacitive response ( $R^{2}=0.999$ ) and gauge factor 1.45 for small strain changes.

A sensitive and highly linear capacitive thin film sensor for trace moisture measurement in gases

This paper presents the experimental work on the sensitivity enhancement of porous aluminium oxide based capacitive humidity sensor for trace moisture measurement. The pore morphology of nanostructured Al2O3 sensing film has been tuned by adding the different amount of polyethylene glycol (PEG) in the sol solution. X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Bruner–Emmet–Teller (BET) were performed for pore morphology analysis of thin films prepared by sol–gel technique. Three parallel plate capacitive sensors with identical geometries but different pore morphologies have been fabricated on Fluorine doped Tin Oxide (FTO) substrate and experimented to analyze the effects of PEG addition. The electrical characterization of the sensors has been performed to determine sensitivity, hysteresis, response and recovery time using Agilent 4294A impedance analyzer with frequency range of 1–1000kHz. It has been observed that the addition of suitable amount of PEG improves the electrical characteristics of the sensor significantly in the range of 10–210ppm moisture. Response characteristics of the sensors are highly linear (R2=0.995) and the sensitivity is almost twice in comparison to a sensor without PEG addition.

Characterization of Various Types Coal Using Capacitance Measurement Technique

Each type of coal has different composition and properties, which determine the coal rank. One of the new methods for determining the quality of coal is by measuring its capacitance. It is formerly known that the difference in the moisture content of coal can be determined from the difference in its dielectric properties. The purpose of this paper is to characterize the various types of coal based on capacitance measurement. The samples used are Lignite, Sub-bituminous, Bituminous, and Anthracite. The proximate analysis testing was done to determine the content of moisture, volatile matter, ash, and fixed carbon. Capacitance value is measured using 2-channel data acquisition system (DAS) and parallel plate capacitive sensor at frequency 2.5 MHz. The results shows that the capacitance values of each type of coal are different. The capacitance value is affected by moisture content of coal since the moisture content and capacitance value have a linear correlation. And also, the signal characterization using frequency from 1 kHz to 5 MHz was performed to verify whether the frequency used in the DAS is able to characterize coal types.

Conducting absorbent composite for parallel plate chemicapacitive microsensors with improved selectivity

Conducting absorbent composites were prepared using two organic polymers (polar and nonpolar) mixed with conductive carbon nanoparticles and an ionic liquid (BMIPF6). The mixture was deposited between the plates of chemicapacitive microsensors using ink-jet technology. Different coatings were characterized using SEM and DRIFTS techniques. The response magnitude for each sensor depends on numerous phenomenon but changes in permittivity of the analyte and polymer swelling dominate. The performance of individual chemicapacitive sensors were characterized through exposure to concentrations of various volatile organic compounds with different functional groups in a climate controlled vapor delivery system. Sensitivity, selectivity and limits of detection, of each prepared sensor, were compared and the discrimination power was evaluated using quadratic discriminant analysis. Ionic liquid doped polymers were able to enhance the sensitivity and the selectivity of parallel plate capacitive sensors. Improved analyte classification was achieved with the ionic liquid-doped polymers (97% accuracy) over the pure polymers. This preliminary study describes the first use of polymer composites with carbon black and ionic liquids as the absorbent dielectric layer in parallel plate microcapacitive sensors.

Study on the Detection of Slurry Concentration in the Thickener

The detection of flocculation in the thickener has a significant impact on the realization of the closed-loop control of the thickener production. In this paper, a slurry concentration detector is designed based on a parallel plate capacitive sensor. The relation between the slurry concentration and the measurement of the capacitive sensor is obtained for mass percentage concentration of the slurry between 5% and 25% through a large amount of experiments. The results are compared and verified by a known relation experimentally and the errors are less than 5%. The reliability of the relation and the feasibility of the detector equipment are verified, which provides a reliable detection method for the realization of closed-loop control of the thickener production.

Experimental Study of Capacitance Sensors to Test Seed-Flow

The real-time seed-metering device performance detection is one of key techniques to ensure operation quality of a planter. A crossed E-shape plate capacitance sensor, a parallel-plate capacitance sensor and a differential tri-plate capacitance sensor, are proposed to test corn-seed flow. The detection concept and sensor structure parameters are presented. An adjustable stand is designed to investigate the impact of sensor structure and seed velocity on the outputs of capacitance sensors. The sensitivity and the feasibility of 3 sensors are studied by detecting capacitance variations at different corn-seed speeds. Experimental results show that, with average capacitance variation 0.1245pF and 0.1115 pF, respectively, the crossed E-shape plate sensor and the differential tri-plate sensor are suitable to measure corn-seed metering device performance.

Nondestructive Measurement of Moisture Content of Different Varieties of Wheat Using a Single Calibration with a Parallel-Plate Capacitance Sensor

A simple, low-cost instrument that measures impedance and phase angle was used along with a parallel-plate capacitance system to estimate the moisture content (MC) of six varieties of wheat. Moisture content of grain is important and is measured at various stages of processing and storage. A sample of about 150 g of wheat was placed separately between a set of parallel plate electrodes, and the impedance and phase angle of the system were measured at frequencies 1 and 5 MHz. A semi-empirical equation was developed using the measured impedance and phase angle values, the computed capacitance, and the MC values obtained by standard air-oven method. Multiple linear regression (MLR) was used for the empirical equation development using statistical software. In the present work, a low-cost impedance analyzer, designed and assembled in our laboratory, was used to measure the impedance and phase angles. MC values of wheat samples in the moisture range of 9% to 25%, not used in the calibration, were predicted by the equations and compared with their standard air-oven values. For over 97% of the samples tested from the six varieties of wheat, the predicted MC values were within 1% of the air-oven values. This method, being nondestructive and rapid, will have considerable application in the drying and storage processes of wheat and similar field crops.

Implementation of a New Capacitive Touch Sensor Using the Nanoporous Anodic Aluminum Oxide (np-AAO) Structure

This study reports the implementation of a high-performance capacitive-type touch sensor by using the nanoporous anodic aluminum oxide (np-AAO) layer. The np-AAO layer is batch fabricated as the template to enable the direct formation of nanotexture metal film after deposition. The np-AAO is also exploited as the dielectric layer. Thus, the integration of np-AAO layer and the nanotexture metal film is employed to realize a metal-insulator-metal parallel-plate capacitance sensor. The sensitivity of the capacitive-type touch sensor is enhanced by the nanostructures. In application, the Au and Al metal layers and np-AAO are fabricated on Si substrate to form the parallel-plate capacitor. The testing demonstrates the np-AAO-based touch sensor has higher sensitivity. In addition, the detection of small object such as Drosophila using the fabricated np-AAO-based touch sensor is also demonstrated.

A comparison of mass parameters determination using capacitive and optical sensors

This paper presents a comparison study between the use of 1 mm resolution capacitive and optical sensors for the determination of yarn mass parameters. A parallel plate capacitive sensor to determine yarn mass variations and a yarn diameter and hairiness determination solution using optical sensors and integrating optical signal processing based on Fourier analysis are described. As there is a high correlation between yarn diameter and yarn mass, it is possible to determine yarn mass and infer variations in yarn diameter and vice versa. Moreover, by optically detecting the degree of yarn hairiness, one can quantify its influence on the capacitive sensor mass variation measurements. Here we present the results of a signal processing analysis and statistical description of measurements carried out in two 100% cotton yarns with a linear mass of 59 and 295 g/km performed in yarn system quality (YSQ) machine. We conclude that an accurate yarn characterization can be carried out using optical sensors alone, reducing system's cost and complexity and increasing efficiency.

Capacitive sensor for chopped maize throughput measurement

Instantaneous throughput measurement during the harvest is one of the possibilities of data collecting for precision agriculture purposes. In this research, a parallel plate capacitive throughput sensor for the purpose of chopped maize throughput measurement has been tested. Laboratory measurements were taken to determine the dependence of capacitive throughput sensor output value on chopped maize throughput. This dependence was recognised as non-linear which is in good agreement with our previous findings. Coefficients of determination for power function reached value R 2 = 0.99. Changes in chopped maize volume density influenced the measurements significantly. Changes in plant material moisture content mainly involve changes of material volume density; however, changes in material moisture content itself can also influence capacitive throughput measurements especially for materials with lower values of material moisture content. It must be respected during capacitive throughput sensor operation.

A comparison of mass parameters determination using capacitive and optical sensors

This paper presents a comparison study between the use of 1mm resolution capacitive and optical sensors for the determination of yarn mass parameters. A parallel plate capacitive sensor to determine yarn mass variations and a yarn diameter and hairiness determination solution using optical sensors and integrating optical signal processing based on Fourier analysis are described. As there is a high correlation between yarn diameter and yarn mass, it is possible to determine yarn mass and infer variations in yarn diameter and vice-versa. Moreover, by optically detecting the degree of yarn hairiness, one can quantify its influence on the capacitive sensor mass variation measurements. Here we present the results of a signal processing analysis and statistical description of measurements carried out on a 100% cotton 295 g/km linear mass yarn. We conclude that an accurate yarn characterization can be carried out using optical sensors alone, reducing systems cost, complexity and increasing efficiency.

Automatic Yarn Characterization System: Design of a Prototype

This paper presents Yarn System Quality (YSQ), an innovative, low-cost, portable and high-precision yarn evaluation tester, for quality control of yarn characteristics under laboratory conditions. It presents a modular architecture, simultaneously integrating measurements of yarn hairiness, mass, regularity and diameter. An external module to obtain the yarn production characteristics using analogue optics and image processing is also available. The quantification of yarn hairiness and diameter variation (with a sampling resolution length of 1 mm) is carried out using photodiodes; the diameter characterization, based on 0.5 mm width samples, employs a linear photodiode array; the measurements of mass variation, based on samples of 1 mm, uses a parallel plate capacitive sensor. In the YSQ measurement parameters based on optical sensors a coherent signal processing technique with Fourier analysis is used, to obtain linear output signal variations. A comparison between the results obtained using the YSQ tester and a commercial solution is also presented.

Design, Fabrication, and Impedance Characterization of a Capacitance-Based Salinity Sensor for Marine Applications

We report on the fabrication and electrochemical impedance characterization of a parallel plate capacitance sensor for ocean water salinity monitoring. The standard printed circuit board (PCB) fabricated and assembled sensor shows excellent response to changes in salinity with a sensitivity of . Use of the standard PCB process, necessitated by the requirement of low cost and ease of fabrication, posed challenges such as electrode corrosion and fouling. We investigated two polymeric materials as possible electrode passivation coatings, Novolac resin polymer (Novolac) and a proprietary commercial organic polymer (Accuflo). By performing electrochemical impedance spectroscopy (EIS) and electrical equivalent-circuit-based model fitting, we show that Accuflo has better stability and passivation characteristics in comparison with Novolac. The pore resistance of Accuflo was found to be 3 orders of magnitude higher than that of Novolac, which indicates that Accuflo is relatively less porous than Novolac. Optical observations of the Accuflo- and Novolac-coated sensors after of immersion in ocean water show that Accuflo maintains its structural integrity, whereas Novolac is severely compromised and the underlying electrode has undergone corrosion, in agreement with EIS analysis. This confirms that Accuflo is a better choice as an electrode coating for underwater applications.

Dynamic laboratory measurement with dielectric sensor for forage mass flow determination

A new parallel plate capacitance sensor was built consisting of two metal sheets. The sensor – a capacitor and the whole oscillating circuit was driven at 27 MHz frequency. Dynamic laboratory experiments were performed with grass from a natural meadow in order to evaluate the possibility of the forage mass flow determination by means of this sensor. The results revealed a relatively strong linear relationship between the feed rates of the wet forage crop material passing through the sensor between its plates and the measured capacitance sensor circuit output frequency. The coefficients of determination (R<sup>2</sup>) varied from 0.9 to 0.96. Further improvement of the electronic circuit connection and further investigation of the sensor can be recommended.

AC transfer function of electrostatic capacitive sensors based on the 1-D equivalent model: application to silicon microphones

Parallel plate electrostatic transducers can be described with the one-dimensional (1-D) lumped model. The one-dimensional approximation based on the elastic, the damping and the inertial force is extended with the electrostatic force (due to the electrical biasing) to model the behavior of electrostatic actuators. In case of sensors, the effect of the external excitation has to be also included. The final equation describing the dynamic behavior of the sensor can only be solved numerically avoiding a compact solution. In this paper the perturbation method applied to solve the equations describing parallel plate capacitive sensors is presented. A compact expression is obtained and applied to model silicon microphones. For the sake of comparison, the silicon microphone is also modeled with the well-known analog equivalent electric circuit, which is extended to take into account the resistor used to bias the microphone. It is shown in which conditions both modeling techniques give equivalent results. However, in front of the traditional equivalent electric circuit, the model based on mass, spring constant and damping coefficient allows taking into account the pull-in instability. Assessment of the modeling method is carried out by experimental measurements on a silicon microphone and previous experimental results reported in the literature. A very good agreement between theory and measurements is obtained.

A capacitance sensor for the characterization of microgravity two-phase liquid-gas flows

A concave parallel plate capacitance sensor has been developed to measure the two-phase flow void fraction for the purpose of objectively identifying flow regimes. The sensor has been used in conjunction with a two-phase flow loop on board the NASA Lewis DC-9 microgravity aircraft, and data was recently collected. The void fraction time trace and probability density function, along with high-speed video images of the flow were used to determine a more objective method of flow regime identification than is possible through examination of video images alone.

Flow regime identification in microgravity two-phase flows using void fraction signals

A concave parallel plate capacitance sensor has been developed to measure the two-phase flow void fraction for the purpose of objectively identifying flow regimes. The sensor has been used in conjunction with a two-phase flow loop onboard the NASA Lewis DC-9 microgravity aircraft, and data was recently collected. The void fraction time trace and probability density function, along with high speed video images of the flow were used to determine a more objective method of flow regime identification than what is possible through examination of video images alone. A set of criteria was developed to allow the identification of the flow regimes and their transitions through the examination of the void fraction probability density function. Against these criteria, flow regime transition models in the literature for microgravity gas–liquid two-phase flows were evaluated.