Capacitive Sensor Structure Research Articles

A Novel Wearable Sensor for Measuring Respiration Continuously and in Real Time.

In this work, a flexible textile-based capacitive respiratory sensor, based on a capacitive sensor structure, that does not require direct skin contact is designed, optimised, and evaluated using both computational modelling and empirical measurements. In the computational study, the geometry of the sensor was examined. This analysis involved observing the capacitance and frequency variations using a cylindrical model that mimicked the human body. Four designs were selected which were then manufactured by screen printing multiple functional layers on top of a polyester/cotton fabric. The printed sensors were characterised to detect the performance against phantoms and impacts from artefacts, normally present whilst wearing the device. A sensor that has an electrode ratio of 1:3:1 (sensor, reflector, and ground) was shown to be the most sensitive design, as it exhibits the highest sensitivity of 6.2% frequency change when exposed to phantoms. To ensure the replicability of the sensors, several batches of identical sensors were developed and tested using the same physical parameters, which resulted in the same percentage frequency change. The sensor was further tested on volunteers, showing that the sensor measures respiration with 98.68% accuracy compared to manual breath counting.

Low-hysteresis, pressure-insensitive, and transparent capacitive strain sensor for human activity monitoring

Wearable strain sensors have been widely used for human activity monitoring. Most reported strain sensors have mainly focused on material engineering, high stretchability and large gauge factors. Few works have focused on strain sensor’s robustness and reliability, including low hysteresis, good long-term stability, good electrode material stability, and low coupling effects under multi-input signals, which are the factors that limit practical strain sensor applications. To develop a high-performance strain sensor, we propose a flexible capacitive sensor structure with three-dimensional (3D) interdigital electrodes fabricated by vertically aligned carbon nanotubes. Compared with a traditional resistive strain sensor and a capacitive strain sensor with vertical sandwich electrodes, a strain sensor with horizontal parallel interdigital electrodes can benefit from low cross talk in terms of the normal force and improve substrate transparency. Additionally, embedding 3D electrodes into the substrate improves ultrahigh robustness with a low-pressure coupling effect under normal force. Moreover, compared with other reported works, the electrode variation under strain is small (less than 1.6%), which means that the perturbation of inert properties on device performance is small. Finally, the fabricated strain sensor achieves an ultralow hysteresis (0.35%), excellent pressure-insensitive performance (less than 0.8%), fast response (60 ms), good long-term stability, and good transparency. As an application example, a flexible strain sensor was successfully demonstrated as a wearable device for the precise monitoring of different types of human activities, including bending of the finger, knee, elbow, wrist, and neck with large strain signals and small strain signals generated by a mouth-opening activity. This excellent performance indicates that the flexible strain sensor is a promising candidate for human motion detection, soft robotics, and medical care.

Soft Capacitive Force Sensors With Low Hysteresis Based on Folded and Rolled Structures

Force sensors made of a polymer material with soft characteristics have application potential in the fields of soft robotics, exoskeletons, and human motion measurement. However, the hysteresis of soft force sensors is generally large because their sensing materials are rubbers with large dynamic viscoelasticity. The problem of large hysteresis caused by this dynamic viscoelasticity needs to be solved to improve the dynamic measurement precision of these force sensors. In this article, we have tested and found that the hysteresis of silicone rubber is significantly reduced when it operates in a large strain range. Therefore, we propose a design idea to allow the silicone rubber to work at large strain ranges involving a folded and rolled capacitive sensor structure based on large prestretching. Compared with previously reported silicone rubber force sensors, this sensor has a lower hysteresis error of 4.73% and a lower repeatability error of 3.42%. In addition, the folded and rolled sensor exhibits quick response, long-term stability, and durability under periodic dynamic load.

A Simple Method for the Detection of Biofilms Using a Heatable Capacitive Sensor Structure (CSS): Description, Proof of Concept, and Some Technical Improvements.

This article presents a novel method for the detection of biofilms based on a heatable, capacitive sensor structure (CSS). Biofilms are capable of strongly binding large amounts of water to their extracellular biopolymer matrix, which is detectable via its dielectric properties. A main challenge is to determine the difference between the inherent occurring presence of moisture in the ecosystem, which is necessary to form a biofilm and an actual formed biofilm. Therefore, the CSS is carefully heated to evaporate unbound surface moisture and determine whether there is a remaining residual alternation of the capacitance in comparison to the dry state. As a reproduceable substitute for complex, real biofilms, a hygroscopic, medical hydrogel-based on polysaccharides was used and applied by spray coating. Printed circuit boards (PCB) in different geometries and materials were used as CSS and compared in terms of their performance. A layer-thickness of 20 µm for the hydrogel coating to be sufficiently detected was defined as a realistic condition based on known values for real biofilms cited in literature. For this thickness a double-meander structure proves to be preferable over interdigitating and spiral geometries. It does offer a 30% lower, yet sufficient sensitivity, but shows advantages in manufacturing (one layer instead of two) and conductive heating capability. In the experiments, free water showed virtually no residual change, while the hydrogel-coated CSS still shows an approx. 300% higher value compared to a dry capacity. Yet, the overall small capacities of about 6–30 pF in dry state are difficult to measure and therefore sensitive to interferences and noise, which results in a high deviation. The principle of measurement can be evaluated as proofed by the carried out experiments, though offering room for improvement in the design of the study. The new method might be especially useful for pipes (e.g., hydrodynamically ineffective sensors installed in a pipe wall) if they at least are not permanently flooded with an aqueous medium, but can occasionally dry. If the internal surface is still only moist, it can be dried by initial heating.

Detection of Single Steel Strand Distribution in Grouting Duct Based on Capacitive Sensing Technique.

Grouting ducts (containing steel strands) are widely used to increase the structural strengths of infrastructures. The determination of the steel strand’s integrity inside of ducts and the grouting quality are important for a strength evaluation of the structure. In this study, a capacitive sensing technique was applied to identify the cross-sectional distribution of the steel strands. The distribution was expressed in polar coordinates in an external post-tensioned pre-stressed duct model. An improved capacitive sensor structure was designed, which consisted of four electrodes, and different electrode-pairs were used to determine various locations’ information of the steel strands. Two rounds of measurements were conducted using the designed sensor to detect the angle (θ) and center distance (r) of the steel strand in the duct. The simulated and experimental results are presented and analyzed. In general, it is difficult to locate the angle of a steel strand directly from first-round capacitance measurements by analyzing the experimental results. Our method based on Q-factor analysis was presented for the position detection of a steel bar in an external post-tensioned pre-stressed duct. The center distance of the steel bar could be identified by second-round capacitance measurements. The processed results verified the effectiveness of the proposed capacitive sensor structure. Thus, the capacitive sensing technique exhibited potential for steel strand cross-section distribution detection in external post-tensioned pre-stressed ducts.

Capacitive humidity sensing using a metal–organic framework nanoporous thin film fabricated through electrochemical in situ growth

The preparation of nanoporous thin film of metal–organic framework (MOF), Cu–BTC [1,3,5-benzenetricarboxylate or trimesate (BTC)], on the copper plate electrode as dielectric layer of the capacitive sensor was achieved by electrochemical in situ synthesis and film growth. An ionic liquid (IL), 1-methyl-3-octylimidazolium chloride as conducting salt, was used and aid synthesis in the electrochemical synthesis procedure. The structure and morphology of MOF film were properly characterized using scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction analysis and BET techniques. The fabricated sensor was used for the humidity measurement at ppm level using the parallel plates’ capacitive sensor structure. Capacitance variations in the presence of humidity at concentration range of 20–100 ppm was investigated using the fabricated sensor. The performances of the sensor have been examined by measuring the capacitance changes using a LCR meter [inductance (L), capacitance (C), and resistance (R)]. Variations of capacitance versus concentration were linear in the range of humidity concentrations which was used here. Sensitivity of the fabricated sensor was 1.13 pF/ppm. Limit of detection (LOD) of the fabricated sensor calculated as low as 5.45 ppm. n-Hexane and toluene vapors as nonpolar analytes were used to investigate the selectivity of the sensor.

Device/Circuit Mixed-Mode Simulations for Analysis and Design of Projected-Capacitive Touch Sensors

A device/circuit mixed-mode simulation method is proposed to effectively characterize the physical effects of the structure parameters and external noise signals on the sensing performance of projected-capacitive touch sensor devices for physical explanation and optimal design in touch screen applications. With this method, the electrostatic characteristics of the intrinsic capacitive sensor structure were obtained by numerical simulations, and then embedded into the circuit simulation environment to predict the resulted sensing performance. A single-layer mutual capacitance structure sensor device sample was fabricated to verify the simulation method. The related physical mechanisms during the touching procedure were analyzed with the simulation method, which was in agreement with the experimental measurement results.

A capacitive angular sensor with flexible digitated electrodes

Purpose – This paper aims to present a prototype of a capacitive angular-position sensor which exploits advantages of flexible/printed electronics. The novelty of the sensor is that the capacitor structure is placed at the circumference of the rotor and stator, that it posses two channels (capacitor structures) electrically shifted for p/4 and that the rotor is common for both channels. The electrodes of the sensing capacitor are digitated, providing a triangular transfer function. Design/methodology/approach – This sensor prototype consists of two flexible inkjet-printed silver electrodes forming a cylindrical capacitor structure. One of them is wrapped around the stator and another is wrapped around the rotor part of a simple mechanical platform used to precisely adjust the angular displacement. Findings – The capacitance as a function of angular position was measured using an inductance capacitance impedance (LCZ) Meter, and results are presented for a full-turn measurement range. The experimental results are compared with analytical ones and very good agreement has been achieved. Originality/value – The proposed capacitive sensor structure can be used as an absolute or an incremental encoder with different resolutions, and it can be applied in automotive industry or robotics.

Real-Time System for Liquid Level Measurement

A small level measurement system is designed by installing a compact capacitive level sensor in the small container. By analyzing the capacitive sensor structure, a sensor measurement circuit is designed with high sensitivity, measuring stability and good repeatability. Capacitance measurement circuit makes use of multiple harmonic oscillation principle. The microcomputer measures the oscillation frequency of the multivibrator, and calculates the liquid level height based on the monotonic function about the liquid level height and frequency.

Design study for a capacitive ceramic pressure sensor

Purpose – The purpose of this paper is to consider a capacitive pressure sensor fabricated using low‐temperature cofired ceramic (LTCC) materials and technology as a candidate for an energy‐autonomous sensor application. Designing the 3D capacitive sensor structure, with the cofired thick‐film electrodes inside the narrow air gap in the LTCC substrate, was a challenging task, particularly due to the presence of the parasitic elements influencing the sensor's characteristics.Design/methodology/approach – In this work, different design variants for the thick‐film electrodes of the capacitive sensing structure were studied and compared. The test sensors were designed for the pressure range 0‐10 kPa and manufactured with readout electronics based on a capacitance‐to‐digital conversion.Findings – The typical sensitivity obtained was 4 fF/kPa, and the temperature coefficient of the sensitivity was 0.03%/°C. The design variant with the guard‐ring electrode showed the best rms resolution of 50 Pa. One drawback of...

Capacitive Sensor Design for an Automatic Car-Wiper System

A structure of capacitance sensor used for an automatic car-wiper system is presented. It aims to develop a low-cost sensor with high sensitivity and robustness. Firstly, two kinds of sensing technique based on optical and piezoelectric sensors for an automatic car-wiper system are introduced respectively. Then, three types of capacitance sensors of different shapes are designed. The parameters which affecting the sensor signal strength and measurement sensitivity are discussed. Thirdly, the electric field distributions of the sensors are simulated, and the fringe capacitance of the sensors are measured and analyzed. Finally, the optimum excitation frequency of designed sensor is determined, and the sensor noise is discussed in this paper. The experimental results indicate the validity and importance of the capacitance-based sensing technique which can be used for liquid detection on windshield of vehicles.

Analytical capacitive sensor sensitivity distribution and applications

Robustness, versatility, high level of performance and low cost are some of the characteristics that make capacitive sensors well suited for industrial applications. They consist only in electrodes and measurement circuits but give access to position information and/or material properties. The design of capacitive sensors is however not so obvious so that simple structures are generally used to avoid time-consuming calculations and developments. We propose an analytical method to determine the sensitivity distribution of any capacitive sensor structure. This method makes it possible to rapidly optimize structures in order to increase the sensor sensitivity to one parameter or to render it less sensitive to another. Comparisons between the sensitivity map of known sensors and those obtained with the analytical method proposed in this paper show a great accordance.

Effect of Ground-Wall Structure in Capacitive Fingerprint Sensor on Electrostatic Discharge Tolerance

This paper describes the electrostatic discharge (ESD) tolerance of capacitive fingerprint sensor or sensor/identifier LSIs in which the sensor is stacked on a CMOS LSI. The ground-wall structure that we proposed as a capacitive sensor structure for ESD tolerance was investigated. The contact discharge method was used for the fingerprint sensor ESD test. The dependence of ESD failure voltage on the distance between the sensor surface and the ESD electrode was measured. In the planar-type structure, ESD failure voltage decreased as the electrode approached the sensor surface and reached its minimum when the electrode touched the surface. Results for the ground-wall type show a high ESD tolerance regardless of the distance. Moreover, in the test on the fingerprint sensor and sensor/identifier LSIs, the ground-wall effect was obtained with a high ESD tolerance. In conclusion, it was revealed that fingerprint sensor LSI or the fingerprint sensor/identifier LSI with a ground-wall structure has a high ESD tolerance of more than ±20.0 kV.

A novel semiconductor capacitive sensor for a single-chip fingerprint sensor/identifier LSI

We describe a new semiconductor capacitive sensor structure and the fabrication process for a single-chip fingerprint sensor/identifier LSI in which the sensor is stacked on a 0.5-/spl mu/m CMOS LSI. To ascertain the influence of the fabrication process and normal usage on the underlying LSI, sensor chips were subjected to an electrostatic discharge (ESD) test, mechanical stress test, and unsaturated pressure cooker test (USPCT). ESD tolerance is obtained at the value of /spl plusmn/3.0 kV. To investigate mechanical stress, we carried out a tapping test. The sensor is immune to mechanical stress under the condition of 10/sup 4/ taps with the strength of 1 MPa. A multilayer passivation film consisting SiN under polyimide film provides protection against contamination such as water. Thus, under USPCT conditions of 130/spl deg/C, 80% humidity, and 48 h, the chips were not degraded. The tests confirm that the proposed sensor has sufficient reliability for normal identification usage.