Showing papers on "Capacitive sensing published in 2021"

Dielectric polymers for high-temperature capacitive energy storage.

Abstract: Polymers are the preferred materials for dielectrics in high-energy-density capacitors. The electrification of transport and growing demand for advanced electronics require polymer dielectrics capable of operating efficiently at high temperatures. In this review, we critically analyze the most recent development in the dielectric polymers for high-temperature capacitive energy storage applications. While general design considerations are discussed, emphasis is placed on the elucidation of the structural dependence of the high-field dielectric and electrical properties and the capacitive performance, including discharged energy density, charge-discharge efficiency and cyclability, of dielectric polymers at high temperatures. Advantages and limitations of current approaches to high-temperature dielectric polymers are summarized. Challenges along with future research opportunities are highlighted at the end of this article.

Fabrication of hierarchically porous structured PDMS composites and their application as a flexible capacitive pressure sensor

Abstract: Pressure sensors for wearable electronics are mounted on irregular surfaces and exposed to various external stimuli. Therefore, the sensor should have a flexible structure and wide pressure measurement range along with high sensitivity. In this study, we fabricated hierarchically porous structured polydimethylsiloxane (PDMS) composites with a simple and cost-effective method using sugar particles and a water-in-oil emulsion. Hierarchically porous PDMS composites were employed as the dielectric layers of flexible capacitive pressure sensors. The capacitive pressure sensor presents a sensitivity 22.5 times higher (0.18 kPa−1) than the sensors using bulk PDMS with a wide measurement range (0–400 kPa). The finite element analysis was implemented to analyze the non-linearity of sensors by observing the compressive behavior of the PDMS composites. For the practical applications, finger attached-sensor, respiration monitoring system, and sensor array were tested, and the proposed sensors showed sufficient potential for application in wearable electronics.

Recent Progress on Flexible Capacitive Pressure Sensors: From Design and Materials to Applications

Abstract: The authors acknowledge the generous support of King Abdullah University of Science and Technology (KAUST). The authors thank Kelly Rader for proofreading this manuscript.

Flexible and Stretchable Capacitive Sensors with Different Microstructures

Abstract: Recently, sensors that can imitate human skin have received extensive attention. Capacitive sensors have a simple structure, low loss, no temperature drift, and other excellent properties, and can be applied in the fields of robotics, human-machine interactions, medical care, and health monitoring. Polymer matrices are commonly employed in flexible capacitive sensors because of their high flexibility. However, their volume is almost unchanged when pressure is applied, and they are inherently viscoelastic. These shortcomings severely lead to high hysteresis and limit the improvement in sensitivity. Therefore, considerable efforts have been applied to improve the sensing performance by designing different microstructures of materials. Herein, two types of sensors based on the applied forces are discussed, including pressure sensors and strain sensors. Currently, five types of microstructures are commonly used in pressure sensors, while four are used in strain sensors. The advantages, disadvantages, and practical values of the different structures are systematically elaborated. Finally, future perspectives of microstructures for capacitive sensors are discussed, with the aim of providing a guide for designing advanced flexible and stretchable capacitive sensors via ingenious human-made microstructures.

Research progress of flexible capacitive pressure sensor for sensitivity enhancement approaches

Abstract: Flexible pressure sensors have played a great role in acquiring information from human and automatics because of their wide use in electronic skin, soft robot, human-machine interaction and so on. Among a variety of flexible pressure sensors, capacitive pressure sensor has many advantages like simple structure, insensitive to temperature and humidity, low power consumption, etc. It is easy to fabricate such kind of pressure sensor, nevertheless, how to improve its sensitivity to broaden the high effective application has been a hotspot issue in recent years. In this paper, a large amount of research outputs on sensitivity improvement have been reviewed for flexible capacitive pressure sensor, including the aspects from introduction of performance evaluation indicators, working principle, generally used materials and capacitor structures to the methods of how to improve the sensitivity of capacitive pressure sensors. Then, the effective ways to obtain high sensitivity of pressure sensors have been compared and the development trend of flexible capacitive pressure sensor is prospected. This paper aims to provide references for the further research on the efficient fabrication of flexible capacitive pressure sensors and effective usage of such sensors in high sensitivity requirements of application areas.

Bio-Inspired Hybrid Dielectric for Capacitive and Triboelectric Tactile Sensors with High Sensitivity and Ultrawide Linearity Range

Abstract: The trade-off between sensitivity and linearity is critical for preserving the high pressure-resolution over a broad range and simplifying the signal processing/conversion of flexible tactile sensors. Conventional dielectrics suffer from the difficulty of quantitatively controlling the interacted mechanical and dielectric properties, thus causing the restricted sensitivity and linearity of capacitive sensors. Herein, inspired by human skin, a novel hybrid dielectric composed of a low-permittivity (low-k) micro-cilia array, a high-permittivity (high-k) rough surface, and micro-dome array is developed. The pressure-induced series-parallel conversion between the low-k and high-k components of the hybrid dielectric enables the linear effective dielectric constant and controllable initial/resultant capacitance. The gradient compressibility of the hybrid dielectric enables the linear behavior of elastic modulus with pressures, which derives the capacitance variation determined by the effective dielectric constant. Therefore, an ultrawide linearity range up to 1000 kPa and a high sensitivity of 0.314 kPa-1 are simultaneously achieved by the optimized hybrid dielectric. The design is also applicable for triboelectric tactile sensors, which realizes the similar linear behavior of output voltage and enhanced sensitivity. With the high pressure-resolution across a broad range, potential applications such as healthcare monitoring in diverse scenarios and control command conversion via a single sensor are demonstrated.

Highly Sensitive Capacitive Pressure Sensors over a Wide Pressure Range Enabled by the Hybrid Responses of a Highly Porous Nanocomposite

Abstract: Past research aimed at increasing the sensitivity of capacitive pressure sensors has mostly focused on developing dielectric layers with surface/porous structures or higher dielectric constants. However, such strategies have only been effective in improving sensitivities at low pressure ranges (e.g., up to 3 kPa). To overcome this well-known obstacle, herein, a flexible hybrid-response pressure sensor (HRPS) composed of an electrically conductive porous nanocomposite (PNC) laminated with an ultrathin dielectric layer is devised. Using a nickel foam template, the PNC is fabricated with carbon nanotubes (CNTs)-doped Ecoflex to be 86% porous and electrically conductive. The PNC exhibits hybrid piezoresistive and piezocapacitive responses, resulting in significantly enhanced sensitivities (i.e., more than 400%) over wide pressure ranges, from 3.13 kPa-1 within 0-1 kPa to 0.43 kPa-1 within 30-50 kPa. The effect of the hybrid responses is differentiated from the effect of porosity or high dielectric constants by comparing the HRPS with its purely piezocapacitive counterparts. Fundamental understanding of the HRPS and the prediction of optimal CNT doping are achieved through simplified analytical models. The HRPS is able to measure pressures from as subtle as the temporal arterial pulse to as large as footsteps.

Stretchable and suturable fibre sensors for wireless monitoring of connective tissue strain

Abstract: Implantable sensors can be used to monitor biomechanical strain continuously. However, three key challenges need to be addressed before they can be of use in clinical practice: the structural mismatch between the sensors and tissue or organs should be eliminated; a practical suturing attachment process should be developed; and the sensors should be equipped with wireless readout. Here, we report a wireless and suturable fibre strain-sensing system created by combining a capacitive fibre strain sensor with an inductive coil for wireless readout. The sensor is composed of two stretchable conductive fibres organized in a double helical structure with an empty core, and has a sensitivity of around 12. Mathematical analysis and simulation of the sensor can effectively predict its capacitive response and can be used to modulate performance according to the intended application. To illustrate the capabilities of the system, we use it to perform strain measurements on the Achilles tendon and knee ligament in an ex vivo and in vivo porcine leg. A capacitive, fibre-like stretchable strain sensor, formed of two conductors in a double helical structure, can be combined with an inductive coil to create a wireless strain-sensing system for biomedical applications.

High-Resolution and High-Sensitivity Flexible Capacitive Pressure Sensors Enhanced by a Transferable Electrode Array and a Micropillar-PVDF Film.

Abstract: Flexible pressure sensors have attracted increasing attention because they can mimic human skin to sense external pressure; however, for mimicking human skin, the sensing of a pressure point is far from sufficient. To realize fully biomimetic skins, it is crucial for flexible sensors to have high resolution and high sensitivity. We conducted simulations and experiments to determine the relationship between the sensor sensitivity and physical parameters, such as the effective relative permittivity and air ratio of the dielectric layer. According to the results, a micropillar-poly(vinylidene fluoride) (PVDF) dielectric layer was designed to achieve high sensitivity (0.43 kPa-1) in the low-pressure regime (<1 kPa). An 8 × 8 pixel sensor matrix was prepared based on a micropillar-PVDF (MP) film and electrode array (MPEA) to detect the pressure distribution with high resolution (13 dpi). Each pixel could reflect the point of applied pressure through an obvious change in the relative capacitance; moreover, objects with various geometries could be mapped by the pixels of the flexible sensor. A counterweight, a plastic flag, and pine leaves were placed on the flexible sensor, and the shapes were successfully mapped; in particular, the mapping of the ∼0.005 g ultra-lightweight pine leaves with a length of 7 mm and a width of 0.6 mm shows the high sensitivity and high resolution of our flexible pressure sensor.

Highly Sensitive Phase-Variation Dielectric Constant Sensor Based on a Capacitively-Loaded Slow-Wave Transmission Line

Abstract: This paper presents a highly sensitive phase-variation sensor for dielectric constant measurements based on a capacitively-loaded periodic slow-wave transmission line. Such line is implemented in microstrip technology, and the loading capacitors are formed by closely spaced rectangular patches. The sensing area of the device is delimited by the region occupied by the capacitive patches, where the material under test (MUT) must be located. The presence of the MUT modifies the coupling capacitance between adjacent patches, thereby producing a variation in the electrical length, or phase of the transmission line, which is the output variable. A detailed analysis of the equivalent circuit model of the unit cell, useful for design purposes, is carried out in the paper. Based on such analysis, a prototype device sensor is designed and fabricated. It is demonstrated in the paper that the sensitivity of the proposed sensor is by far superior to the one achieved in an ordinary meandered line with similar sensing area. Thus, the proposed slow-wave structure constitutes a good alternative to meandered lines for the implementation of phase-variation sensors with simultaneously high sensitivity and compact size.

Hollow-porous fibers for intrinsically thermally insulating textiles and wearable electronics with ultrahigh working sensitivity.

Abstract: Wearable smart devices should be flexible and functional to imitate the warmth and sensing functions of human skin or animal fur. Despite the recent great progress in wearable smart devices, it is still challenging to achieve the required multi-functionality. Here, stretchable hollow-porous fibers with self-warming ability are designed, and the properties of electrical heating, strain sensing, temperature sensing and pressure sensing are achieved. The hollow-porous TPU fiber possesses an ultra-high stretchability (1468%), and the textiles woven from the fibers present a splendid thermal insulation property (the absolute value difference in temperature |ΔT| = 68.5 and 44 °C at extreme temperatures of 115 and −40.0 °C). Importantly, after conductive filler decoration, the fiber-based strain sensor exhibits one of the highest reported gauge factor (2.3 × 106) towards 100% strain in 7200 working stretch–release cycles. A low detection limit of 0.5% strain is also achieved. Besides, the fibers can be heated to 40 °C in 18 s at a small voltage of 2 V as an electrical heater. The assembled thermal sensors can monitor the temperature from 30 to 90 °C in real time, and the fiber-based capacitive type pressure sensor exhibits good sensing performance under force from 1 to 25 N. The hollow-porous fiber based all-in-one integrated wearable systems illustrate promising prospects for next generation electronic skins to detect human motions and body temperature with thermal therapy and inherent self-warming ability.

Study on a New Power Frequency Capacitive Voltage Transducer for Gas Insulated Substations Based on Capacitive Voltage Division

Abstract: The IVT for power frequency is an essential facility in the power system. Due to the requirement for high-voltage insulation, the sensing part of the conventional IVTs is bulky and costly. The purpose of this article is to propose a new voltage measurement method based on capacitive voltage divider for GIS. The equivalent circuit of the measurement method is built. The numerical simulation model for the measurement system is built based on the actual size of GIS with rated voltage of 220 kV. And FEM is employed to analyze the transfer relation and the accuracy of the proposed measurement method. The experiments are conducted in the laboratory and at an actual GIS platform, respectively. The transfer relation between the applied voltage and the induced voltage is obtained. Compared with conventional IVTs, the dimension of the measurement system is very small. Due to the low voltage of the sensing part, the insulation problem of the sensing part is avoided. And due to simple structure and linear characteristics, the transfer relation of the proposed measurement system can be obtained with high accuracy.

Ultrafast-response/recovery capacitive humidity sensor based on arc-shaped hollow structure with nanocone arrays for human physiological signals monitoring

Abstract: Flexible humidity sensors have attracted substantial attention due to the promising application in medical health monitoring. Up to now, the realization of such devices with ultrafast response/recovery and controllable morphology via a facile and cost-effective approach still remains a severe challenge. Herein, a high-performance flexible capacitive humidity sensor by creating the arc-shaped hollow structure with the poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)] nanocone arrays is developed. Particularly, a facile method based on combining the hot-pressing method and the anodized aluminum oxide template transfer method is proposed to realize the large-scale preparation of the nanostructure arrays with controllable morphology. Benefiting from the open arc-shaped hollow structure and the large-area P(VDF-TrFE) nanocone arrays, the proposed humidity sensor shows several conspicuous features, such as ultrafast response/recovery time (3.693/3.430 s), long-term stability (25 days), excellent bending stability (10,000 cycles), unaffected capacitance response to humidity in a certain temperature range (20−50℃) and high selectivity toward water vapor. The above salient superiorities enable the proposed humidity sensor to be successfully utilized in extraction of a variety of physiological signals including breathing detection, skin noncontact sensing, and real-time monitoring of diaper wetting process and skin humidity, which reveals great potential value in prevention and diagnosis of many diseases.

Resistive and capacitive strain sensors based on customized compliant electrode: Comparison and their wearable applications

Abstract: The demand for stretchable strain sensors has increased exponentially due to their ideal interaction with the human body. However, developing stretchable strain sensors with balanced properties such as a low Young’s modulus, easy fabrication, low cost, large deformation, and fast response time remains a challenge. In this study, we simultaneously prepared two high-performance types of stretchable strain sensors (resistive and capacitive sensors) based on customized compliant electrode. The electrode was developed by embedding multi-walled carbon nanotubes (MCNTs) into plasticized polyvinyl chloride (PVC) using the solvent casting method. The resistive strain sensor was fabricated by the compliant electrode and 3 M 4905 tapes in a sandwich structure. The capacitive sensor was obtained after simple stacking steps based on the preparation of resistive sensor. Then, the performance of the resistive and capacitive sensors was investigated, compared and discussed. The results show that both resistive and capacitive sensors have good static and dynamic performance. The two types of sensors have maximum tensile strain of more than 100 %, low Young's modulus less than 200 kPa, and fast response time less than 140 ms. The linearity of capacitive sensor is better than that of resistive sensor. The repeatability of capacitive sensor is better than that of resistive sensor during a long period. The resistive sensor has higher sensitivity (1.16) than the capacitive sensor (0.44), and has better signal anti-interference capability. Finally, the developed resistive and capacitive sensor were used to monitor the motion status of human joints and the motion angle of human joint, respectively, resulting in accurate sensing of human movement.

Gradient Architecture-Enabled Capacitive Tactile Sensor with High Sensitivity and Ultrabroad Linearity Range.

Abstract: The sensitivity and linearity are critical parameters that can preserve the high pressure-resolution across a wide range and simplify the signal processing process of flexible tactile sensors. Although extensive micro-structured dielectrics have been explored to improve the sensitivity of capacitive sensors, the attenuation of sensitivity with increasing pressure is yet to be fully resolved. Herein, a novel dielectric layer based on the gradient micro-dome architecture (GDA) is presented to simultaneously realize the high sensitivity and ultrabroad linearity range of capacitive sensors. The gradient micro-dome pixels with rationally collocated amount and height can effectively regulate the contact area and hence enable the linear variation in effective dielectric constant of the GDA dielectric layer under varying pressures. With systematical optimization, the sensor exhibits the high sensitivity of 0.065 kPa-1 in an ultrabroad linearity range up to 1700 kPa, which is first reported. Based on the excellent sensitivity and linearity, the high pressure-resolution can be preserved across the full scale of pressure spectrum. Therefore, potential applications such as all-round physiological signal detection in diverse scenarios, control instruction transmission with combinatorial force inputs, and convenient Morse code communication with non-overlapping capacitance signals are successfully demonstrated through a single sensor device.

Flexible Fringe Effect Capacitive Sensors with Simultaneous High‐Performance Contact and Non‐Contact Sensing Capabilities

Abstract: Humans simultaneously leverage multiple sensory inputs to be able to navigate and interact with their surroundings. While robotics researchers are actively working to mimic this capability, it typically requires the use of multiple sensors, making it more challenging to obtain a high density of modalities on a single robot. Robotics applications typically require both pressure and proximity-sensing capabilities to create systems that can interact autonomously with the environment and safely with humans. Robots equipped with only tactile sensors, which sense objects upon contact, cannot appropriately react to approaching objects by decreasing their velocity. As a result, the movement of these robots must be programmed to be slow enough to avoid unsafe collisions or breaking fragile objects. Further, previous works have demonstrated that the ability to distinguish between materials can help robots more autonomously interact with their environments. Proximity sensing has often been introduced using cameras or ultrasound, although a variety of methods exist. However, the single focal point of a camera limits depth perception, making it difficult to understand the environment very near to a robotic gripper. On the other hand, ultrasonic proximity sensors are effective with wide-range proximity sensing, but are limited by their high power consumption. To provide depth perception, a proximity-sensing mechanism that can detect when an object is approaching, especially very near the sensor, is necessary. In most cases, achieving both pressure and proximity sensing requires the use of either two distinct sensors or at least two distinct readout mechanisms. These approaches increase the amount of space that must be occupied by the sensing architecture, thereby reducing the density of sensors that can be integrated onto a single robotic limb. Recapitulating human skin requires high density sensor arrays: there are as many as 241 mechanoreceptive units per cm in just the fingertip, but few reported sensors have the miniaturization capability to achieve this for combined pressure and proximity sensing. Researchers have recently begun to capitalize on the fringe fields in capacitive sensors to create single devices capable of sensing both pressure in contact mode and proximity in noncontact mode, with capacitance as the single readout signal. Fringe field sensors detect incoming objects through the disturbance in the fringe field as the object approaches. For example, our group has demonstrated a fringe field capacitive pressure sensor that wraps around an artery and is capable of detecting fluctuations even if it is not in full contact. However, in each of these cases, introduction of noncontact mode capability required a significant compromise S. R. A. Ruth, Dr. M. Kim, Dr. Y. Khan, Prof. Z. Bao Department of Chemical Engineering Stanford University Stanford, CA 94305, USA E-mail: zbao@stanford.edu Dr. V. R. Feig, J. K. Phong Department of Materials Science and Engineering Stanford University Stanford, CA 94305, USA

Modeling and Reduction of Radiated EMI in a GaN IC-Based Active Clamp Flyback Adapter

Abstract: This article first develops a radiated electromagnetic interference (EMI) model for a gallium nitride (GaN) integrated circuit (IC)-based active clamp flyback converter. Important capacitive couplings, which play a big role in the radiated EMI, are identified, extracted, and validated in the converter. The radiated EMI model is improved to characterize the impact of capacitive couplings. Based on the improved model, techniques to reduce capacitive couplings and the radiated EMI are proposed and experimentally validated.

A Novel All-Optical Sensor Design Based on a Tunable Resonant Nanocavity in Photonic Crystal Microstructure Applicable in MEMS Accelerometers

Abstract: In view of the large scientific and technical interest in the MEMS accelerometer sensor and the limitations of capacitive, resistive piezo, and piezoelectric methods, we focus on the measurement of the seismic mass displacement using a novel design of the all-optical sensor (AOS). The proposed AOS consists of two waveguides and a ring resonator in a two-dimensional rod-based photonic crystal (PhC) microstructure, and a holder which connects the central rod of a nanocavity to a proof mass. The photonic band structure of the AOS is calculated with the plane-wave expansion approach for TE and TM polarization modes, and the light wave propagation inside the sensor is analyzed by solving Maxwell’s equations using the finite-difference time-domain method. The results of our simulations demonstrate that the fundamental PhC has a free spectral range of about 730 nm covering the optical communication wavelength-bands. Simulations also show that the AOS has the resonant peak of 0.8 at 1.644µm, quality factor of 3288, full width at half maximum of 0.5nm, and figure of merit of 0.97. Furthermore, for the maximum 200nm nanocavity displacements in the x- or y-direction, the resonant wavelengths shift to 1.618µm and 1.547µm, respectively. We also calculate all characteristics of the nanocavity displacement in positive and negative directions of the x-axis and y-axis. The small area of 104.35 µm2 and short propagation time of the AOS make it an interesting sensor for various applications, especially in the vehicle navigation systems and aviation safety tools.

Flexible and wearable capacitive pressure sensor for blood pressure monitoring

Abstract: A flexible pressure sensor based on a capacitive transduction mechanism having a polydimethylsiloxane (PDMS) layer incorporated between indium‐‑tin-oxide (ITO) coated flexible polyethylene terephthalate (PET) electrodes has been developed. The sensor's key parameters have been improved by increasing the dielectric layer's porosity by introducing deionized water (DIW) into it. The sensing device with a porous dielectric layer (PDMS-DIW) exhibits a 0.07%–15% relative difference in capacitance for the applied pressure range from 1 Pa to 100 kPa. The device also demonstrates improved sensitivity compared to a PDMS layer (unstructured) throughout the complete external pressure range. The device with porous PDMS layer shows an extensive operating pressure range (1 Pa to 100 kPa), high working stability, quick response (≈110 ms) and ultra-low detection limit of pressure, i.e., 1 Pa. In addition, the blood pressure monitoring was also studied, and the devices gave a signature of the oscillometric waveform for different blood pressure (BP) values. The fabricated flexible pressure sensor can be used for wearable BP devices and biological applications due to its excellent functional properties.