Capacitive Transducer Research Articles

Analysis of Stretch-Dependent Capacitance and Its Effects on Energy Conversion of a Donut-Shaped Dielectric Elastomer Generator

Dielectric elastomer generator (DEG) comprises of soft and stretchable capacitive transducer. The DEG experiences a change in capacitance and provides a voltage gain ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$G$ </tex-math></inline-formula> ) with an applied mechanical excitation. This article interprets DEG as a machine that involves a cyclic change of finite-state transducer capacitances to transmit or modify energy. In this study, a relative analysis of capacitances associated with the DEG transducer is introduced and analyzed to improve the DEG performance. This article focuses on defining a relationship between the material stretch ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda$ </tex-math></inline-formula> ) and finite-state capacitances associated with the DEG transducer. The estimation of stretch-imposed DEG capacitance under mechanical excitation is well investigated. Eventually, the analysis is experimentally validated with the developed mechanical prototype. For distinct transducer deformation, the associated capacitances are estimated with the capacitive extensometer method and the stretch ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda$ </tex-math></inline-formula> ) is calculated. Moreover, for effective energy conversion, the finite state capacitance variation is maintained large enough to provide a relatively high voltage gain. Experimental results show that for 70% increase in capacitance difference at the DEG makes the specific energy increase from 3.5 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$11.5~\mu \text{J}$ </tex-math></inline-formula> /g. This study could lead the DEG as an alternative power source for low-power electronic gadgets.

A Compact Integrated High-Voltage Pulser Insensitive to Supply Transients for 3-D Miniature Ultrasound Probes

In this paper, a compact high-voltage (HV) transmit circuit for dense 2D transducer arrays used in 3D ultrasonic imaging systems is presented. Stringent area requirements are addressed by a unipolar pulser with embedded transmit/receive switch. Combined with a capacitive HV level shifter, it forms the ultrasonic HV transmit circuit with the lowest reported HV transistor count and area without any static power consumption. The balanced latched-based level shifter implementation makes the design insensitive to transients on the HV supply caused by pulsing, facilitating application in probes with limited local supply decoupling, such as imaging catheters. Favorable scaling through resource sharing benefits massively arrayed architectures while preserving full individual functionality. A prototype of 8 x 9 elements was fabricated in TSMC 0.18 &#x03BC;m HV BCD technology and a 160 &#x03BC;m x 160 &#x03BC;m PZT transducer matrix is manufactured on the chip. The system is designed to drive 65 V peak-to-peak pulses on 2 pF transducer capacitance and hardware sharing of 6 elements allows for an area of only 0.008 mm2 per element. Electrical characterization as well as acoustic results obtained with the 6 MHz central frequency transducer are demonstrated.

A silicon micromechanical resonator with capacitive transduction for enhanced photoacoustic spectroscopy

This paper presents the design of a silicon micromechanical resonator with capacitive transduction for a gas sensor based on photoacoustic spectroscopy. It overcomes problems imposed by opposite physical trends originating from the sensor’s working principles. The capacitive transduction mechanism is considered incompatible with the photoacoustic gas detection as it reduces the mechanical displacement by a damping effect, thus decreasing the sensitivity. This occurs because the same part serves two functions: photoacoustic excitation and capacitive transduction. The suggested approach in this study focuses on the spatial separation of these two functions. We propose the first reported mechanical microresonator for photoacoustic gas sensing, which decouples photoacoustic excitation and capacitive transduction. The design has been modeled, fabricated, characterized, and compared with the on-beam Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) technique, a reference for compact and selective gas sensors. The performed photoacoustic measurement has been realized on calibrated concentrations of methane. For a 1 s integration time in first harmonic detection, we obtained a limit of detection (LOD) of 667 ppmv for silicon microresonator. In comparison to on-beam QEPAS technique, we obtained LOD of 163 ppmv, which constitutes a factor 4 between QEPAS and MEMS. The conditions for both experiments were the same.

Mechanical analysis of a tunable capacitive ultrasound transducer using higher order gradient theory

Clinical studies using a capacitive micromachined ultrasound transducer (CMUT) at a high rang of frequency in ultrasonic imaging have encouraged researchers to delve into all aspects of this new technology. CMUTs are used as energy converters between electrical and mechanical domains. So, in this paper, the equivalent circuit model of a capacitive micromachined ultrasound transducer is analyzed in order to explore the coupling coefficient and frequency of the structure. To produce images with a high resolution, we require a high-frequency ultrasound transducer and that needs a structure like plates in microscale using a microelectromechanical system. Higher order gradient theory by introducing a new term and considering the effects of higher gradients of deformation on strain energy density, explains that the material length scale value to the plate thickness ratio (as in Couple Stress Theory) can play an important role in bending of thin plates, as well as material length scale value to the plate radius (as in nonlocal elasticity theory). However, these effects have not been considered simultaneously in existing classical and non-classical theories that have been used in literature for modeling of CMUTs. Hence, in this study, to predict the mechanical behavior of a microscale system accurately, the static and dynamic properties of a micro circular plate in terms of deflection, pull-in instability, phase plane, frequency response, time history, and mechanical shock wave are studied based on the higher order gradient theory by using Galerkin solution method. A fully clamped micro circular plate which can vibrate and is the main mechanical component of a CMUT is investigated to show the size effect. The developed model is solved numerically using a MATLAB script. The obtained results show that the size effect is indeed important in the mechanical behavior of a CMUT. The investigated model is studied structure is indeed a feasible, simple, general and accurate technique to be applied for devices which vastly are used in medical applications.

Real-Time Obstacle Avoidance Using Dual-Type Proximity Sensor for Safe Human-Robot Interaction

This letter introduces a dual-type proximity sensor and a control strategy for a robot manipulator to realize safe human-robot interactions (HRI) by using the sensor. Safety is an essential condition for HRI in practical scenarios. To achieve this condition, information about the relationship between an external objects and the robot is required. To obtain this information, we employ a dual-type proximity sensor, which consists of capacitive and inductive transducers and can detect the distance between a robot and external objects. Further, we propose a real-time trajectory planning method to deal with obstacles by using admittance control and distance measurements. To update the motion of the manipulator according to our control strategy, a Weight-Prioritized solution based on a QP (quadratic programming) formalism was applied. Further, the problem of self-sensing is solved via machine learning using a training dataset consisting of data corresponding to random joint positions. The proposed method was implemented on a collaborate robot (UR10). Experiments were conducted considering realistic human-robot interactions, and safety improvement was validated.

An Autonomous Environmental Logging Microsystem (ELM) for Harsh Environments

This paper describes the design, implementation, and evaluation of an environmental logging microsystem (ELM) for operation at elevated pressure and in corrosive environments, at temperatures up to 125°C. The ELM units are intended to be deployed in large quantities, allowed to collect data, and then retrieved, interrogated, and re-charged. Powered by a rechargeable battery embedded within the system, each ELM incorporates pressure and temperature sensors, control electronics, optical communication elements, and power management and battery re-charging circuits. The pressure sensor is a customized capacitive transducer chip on a sapphire substrate; details are provided in a companion paper. The electronic components and battery used in ELM are selected on the basis of functionality and form factor; packaged components are selected for ease of assembly and for added protection against the environment. The pressure sensor, electronics and battery are assembled on a flexible circuit board, folded into a stack with dimensions 7.2 mm × 6.6 mm × 6.5 mm, and encapsulated in a steel tube filled with optically transparent silicone caulk. This encapsulation provides mechanical protection against shock and abrasion, as well as chemical protection against high salinity environments, while allowing the ambient pressure and temperature to be transferred to the sensing elements. Results are reported from high-temperature and high-pressure tests reaching 125°C and 7,250 psi in brine and other corrosive environments in laboratory conditions. Field tests that were conducted in a brine well to a maximum depth of 1,235 m are also described. The recorded data were post-processed to interpret the environmental pressure and temperature.

Study of the main parameters of the capacitive converter

Determination of the main parameters of a capacitive humidity transducer with a cylindrical electrode is an urgent task. The article discusses the time characteristics of a capacitive humidity transducer with a cylindrical electrode. Experimental and calculated results show that the reliability of the cylindrical transducer is 0.95. The sensitivity depends on the figure of merit of bulk materials and on the design properties of the cylindrical transducer.

Radar Cross Section-Based Chipless Tag With Built-In Reference for Relative Humidity Monitoring of Packaged Food Commodities

Dry food commodities like grains and pulses can be stored safely for several years under controlled storage conditions. The equilibrium moisture content of the packaged grain is one of the most important parameters required to be monitored and controlled for extended safe storage. This paper presents a single element dual-polarized sensing tag based on an annular slot antenna operating at two closely spaced resonant frequencies for radar cross-section-based monitoring of relative humidity in hermetically packaged food commodities. One of the resonant frequencies is functionalized for sensing while the other acts as a built-in reference, mitigating the effects of environmental loading. A polyvinyl alcohol coated interdigitated capacitor, integrated onto the antenna is used as a capacitive transducer for the sensing element. Laboratory scale measurements were carried out using the saturated salt solution method. Results show that the proposed sensor can be used for monitoring a wide range of humidity conditions.

Kirigami-inspired dual-parameter tactile sensor with ultrahigh sensitivity, multimodal and strain-insensitive features

Soft sensors with strain-insensitive and multimodal features are intriguing due to their high practical relevance. However, incorporating these functionalities into sensors made of soft materials has been challenging. Herein, a Kirigami-inspired dual-parameter tactile sensor was developed with strain-insensitive and multimodal features. The tactile sensor uses piezoresistive and capacitive transduction modes allowing simultaneous detection of dynamic and static tensile strains, proximity and normal pressures. The convenient structural design enables ultrahigh piezoresistive sensitivity ∼23 000 kPa−1 in its resistivity-switching threshold region (in high pressure regimes >50 kPa). It achieves a linear capacitive gauge factor of ∼14.48 for uniaxial elongation up to 80% strain and can accurately measure proximity (⩾0.01 pF mm−1) of objects within distances up to 100 mm. The ultrahigh sensitivity in high pressure regimes allows force adjustable lower limit of detection and sensitivity of the sensor by pre-stress enabling real-time measurement of arterial pulsation. The findings of this work support the design of soft sensors for touch recognition applications in the automotive industry, soft robots or self-adjusting grippers requiring a sense of touch and multimodal and strain-insensitive features.

Flexible and wearable capacitive pressure sensor for blood pressure monitoring

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.

A Non-Linear Lumped Model for the Electro-Mechanical Coupling in Capacitive MEMS Microphones

This work provides an analytical non-linear model for the capacitive transduction in MEMS transducers with perforated counter-electrodes, especially applicable to capacitive MEMS microphones. Starting from an electrostatic description of a perforated unit cell of the transducer, analytical formulations of the variable capacitance and electrostatic forces are derived, accounting for the deflection profile of a clamped circular plate. A lumped implementation into conventional circuit simulations tools is enabled via behavioral modeling based on hardware description languages, such as Verilog-A. Therefore, the analytical model is approximated via Taylor series expansions, allowing for a stable and non-linear behavioral implementation. The resulting model finally enables both a small- and large-signal analysis of capacitive MEMS microphones, precisely accounting for non-linearities in the capacitive transduction. This allows to simulate the harmonic distortion of the microphone's output signal and to account for electrostatic spring-softening in simulations of its bias voltage dependent sensitivity.

A Low Impedance CMOS-MEMS Capacitive Resonator Based on Metal-Insulator-Metal (MIM) Capacitor Structure

This letter presents the first nano-gap microelectromechanical systems (MEMS) resonator based on a Metal-Insulator-Metal (MIM) capacitor structure in standard CMOS. We develop an improved mask-less post-CMOS process to chemically remove the thin aluminum copper (AlCu) of the Capacitor-Top-Metal (CTM) with high selectivity to the adjacent materials, yielding a well-defined capacitive transduction gap of only 160 nm. The proof-of-concept resonator is designed in a standard TSMC 180 nm RF CMOS process, with a center frequency of 5.9 MHz, a quality factor of 1,000, and a low motional impedance of 5.7 $\text{k}\Omega $ under a medium dc-bias of 25V. Upon further optimizations, nano-gap CMOS-MEMS resonators based on the proposed CTM-etching technique are promising candidates for fully integrated oscillator applications.

Gas Damping in Capacitive MEMS Transducers in the Free Molecular Flow Regime.

We present a novel analysis of gas damping in capacitive MEMS transducers that is based on a simple analytical model, assisted by Monte-Carlo simulations performed in Molflow+ to obtain an estimate for the geometry dependent gas diffusion time. This combination provides results with minimal computational expense and through freely available software, as well as insight into how the gas damping depends on the transducer geometry in the molecular flow regime. The results can be used to predict damping for arbitrary gas mixtures. The analysis was verified by experimental results for both air and helium atmospheres and matches these data to within 15% over a wide range of pressures.

Vertical cavity capacitive transducer.

The capacitive micromachined ultrasonic transducer (CMUT) has inherent advantages, such as larger bandwidth and monolithic integration capability with electronics, when compared to the piezoelectric transducer. The most significant shortcoming of the device is the trade-off between input and output sensitivities. Adequate receive sensitivity requires an electric field intensity on the order of 105 V/m, which can be achieved by sub-micron gap heights. However, a small gap limits the device stroke and, consequently, the maximum output pressure. This paper addresses this problem by proposing a CMUT with a vertical cavity. The membrane of the device has a piston part that is surrounded by the sidewalls of a vertical cylinder formed in the substrate. The fringing electric field pulls the piston in the vertical direction; hence, the gap height remains fixed, which alleviates the hard limit on device stroke. The performance of the proposed device is compared to that of the conventional CMUT by theoretical and analytical methods, and a micro-fabrication method is devised. Additionally, a millimeter-scale device has been manufactured and tested as a proof of concept.

CRADLE: Combined RF/Acoustic Detection and Localization of Passive Tags

We introduce CRADLE, or combined RF/acoustic detection and localization, for the remote parametric ranging of passive tags. In this approach, a tag consists of an RF antenna connected to an ultrasonic transducer via a passive electrical network. A reader looks for tags by transmitting both continuous-wave RF and pulsed ultrasound into a region of interest; when a tag is within that region, the ultrasound incident on the tag transducer varies the transducer capacitance, creating a time-varying load for the tag antenna and thus parametrically modulating the RF waves reradiated from the tag. The reader localizes the tag by demodulating the received signal to recover the acoustic time of flight. In this paper, we present a proof-of-concept CRADLE system, describing the design of a custom tag and reader. The results of our localization experiments show that we can achieve sub-decimeter accuracy at reader-tag distances of up to 6 m using the proposed system; future improvements in the design of the tag and reader can result in localization of centimeter-sized tags with sub-centimeter accuracy out to distances of several meters.

Graphene acoustic transducers based on electromagnetic interactions

Graphene acoustic transducers have high sensitivity in receiving mode. However, they are used in transmitting mode with low radiation performance. A graphene acoustic transducer with high sensitivity and radiation performance is proposed in this study. The transducer is composed of graphene diaphragm, an insulating layer embedded in a copper planar coil, and a bottom layer plated with copper. The proposed capacitive transducer is driven by electrostatic and electromagnetic excitation. The sensitivity and radiation performance of the transducer are analyzed by transceiver theory and simulation models. The results demonstrate that the proposed capacitive transducer has excellent acoustic performance with sensitivity of −42 dB and the sound pressure level of 106 dB at 4 kHz with a 20-turn coil that is more than doubled compared without a copper coil. In addition, the radiation performance of the transducer is discussed by the coil parameters including coil turns and coil current, which can provide a theoretical basis for further experiments.

Biosensor based on antifouling PEG/Gold nanoparticles composite for sensitive detection of aflatoxin M1 in milk

Detection of ultra-trace amounts of aflatoxin M1 (AFM1) is an important requirement for food safety since it is a toxic mycotoxin, present in cow milk, with strictly regulated low levels. To achieve detection of low levels of AFM1, we developed a new nanometric aptasensing platform based on the modified SPCE, which was decorated with AuNPs, a tetraethylene glycol ferrocene derivative and an anti-AFM1 aptamer. The ferrocene tethered to AuNPs served as a capacitance transducer contributing to an interfacial charge. The capacitive signal was used to quantify the toxin. This study revealed a good sensitivity toward the toxin with a dynamic range of 20 to 300 pg·mL−1 and a LOD as low as 7.14 pg·mL−1 (S/N = 3). The nanoaptosensor exhibits very good metrological performances. To evaluate the sensing platform, the latter was applied to detect the presence of AFM1 in pasteurized cow milk.

Analytic Optimization of Cantilevers for Photoacoustic Gas Sensor with Capacitive Transduction.

We propose a new concept of photoacoustic gas sensing based on capacitive transduction which allows full integration while conserving the required characteristics of the sensor. For the sensor’s performance optimization, trial and error method is not feasible due to economic and time constrains. Therefore, we focus on a theoretical optimization of the sensor reinforced by computational methods implemented in a Python programming environment. We present an analytic model to optimize the geometry of a cantilever used as a capacitive transducer in photoacoustic spectroscopy. We describe all the physical parameters which have to be considered for this optimization (photoacoustic force, damping, mechanical susceptibility, capacitive transduction, etc.). These parameters are characterized by opposite trends. They are studied and compared to obtain geometric values for which the signal output and signal-to-noise ratio are maximized.

Design and Fabrication of Flexible Capacitive Sensor With Cellular Structured Dielectric Layer via 3D Printing

Because of high sensitivity, mechanical robustness, lightweight and wearability, flexible capacitive pressure transducer has been widely considered one of the most critical soft electronics in wearable consumables and e-skins. The enhancement of the pressure sensitivity of a flexible capacitive sensor relies on the introduction of interfacial microstructure to the dielectric layer. We demonstrate a new methodology to fabricate flexible capacitive sensors with copper-plated polyimide (PI) films as the electrodes and a porous polydimethylsiloxane (PDMS) layer 3D printed via the direct-ink-writing approach. Time-of-flight secondary ion mass spectrometry is developed to optimize the electroless copper plated PI films. What is further examined is the impact of the geometric complexity of the cellular PDMS structure, including filament width, spacing and alignment, on sensitivity, repeatability and reliability of the developed capacitive sensor. A robotic gripper equipped with our flexible pressure sensor showcases its competence to grip a soft target with well-posed force control. It is expected that our proposed sensor design and manufacturing methodology will advance the development of soft electronics and wearable sensors.

New Resistorless Third-Order Quadrature Sinusoidal Oscillators

This paper introduces four new resistorless, third-order, electronically tunable, quadrature sinusoidal oscillators using three operational transconductance amplifiers (OTAs) and three capacitors. The proposed third-order quadrature sinusoidal oscillators (TOQSOs) provide noninteracting control of the oscillation condition (OC) and oscillation frequency (OF) by changing the transconductance of different OTAs, to produce sustained oscillations and offer quadrature output voltages and currents. Two of the proposed quadrature oscillator circuits have capacitor control of OF, a feature, useful in capacitive transducers. The presented TOQSO structures have good frequency stability and exhibit low active and passive sensitivities. PSPICE simulations using CMOS OTAs along with hardware results (using off-the-shelf available OTA IC LM13700) have also been provided to confirm the workability of the presented circuits.