Capacitive Ultrasonic Transducers Research Articles

Investigation on Characteristics of a MXenes-Based Air-Coupled Capacitive Ultrasonic Transducer Array

Investigation on Characteristics of a MXenes-Based Air-Coupled Capacitive Ultrasonic Transducer Array

Non-contact Ultrasonic Sonar-based Ranging Technique for In-motion 3D Railroad Tie Deflection Measurements

This paper presents an ultrasonic technique for non-contact surface deflection mapping of railroad ties using concepts of sonar-based ranging. The technique utilizes an array of capacitive ultrasonic transducers arranged along the length of the railroad tie at a lift-off distance of 3 in. from the rail surface to ensure contactless measurements. The transducer array is used in pulse-echo mode and distances from the transducers to the tie surface are measured by tracking the time-of-flight of the waves reflected from the tie surface. A reference-based cross-correlation operation is introduced to compute the time-of-flight, wherein one of the transducers is used as a reference for the distance measurements. The reference-based cross-correlation ensures accurate peak-detection for time-of-flight based differential distance measurements. An acoustic signal strength-based technique is utilized to differentiate between signals reflected from ties and those reflected from ballast. Laboratory scale tests were first performed as a proof-of-concept on a slender wooden beam to measure deflections in loaded and unloaded conditions. Dynamic tests were also performed on this beam to determine the ability to track time-varying positions. Field tests on a replica test track with wood ties were performed at the Rail Defect Testing Facility at University of California San Diego by mounting the prototype on a test-cart moved at walking speed. A dynamic assessment of the prototype was also performed to determine natural frequencies of the mounting beam that may be relevant for future uses at higher speeds. The results indicate the potential of this non-contact system to measure full-field tie deflections in 3D in-motion. This ability may potentially be used to detect conditions of poor tie support that may cause derailments.

Intelligent defect inspection of flip chip based on vibration signals and improved gcForest

The defect inspection of flip chips has become a meaningful and challenging task with the decrease of size and distance of solder balls. In this paper, we proposed a novel defect diagnostic method for flip chips based on vibration signals and improved multi-grained cascade forest (gcForest). Firstly, the flip chip is excited by an air-coupled capacitive ultrasonic transducer, and the corresponding vibration signals of flip chip are captured by a laser scanning vibrometer. Then, the feature information of original vibration signals is extracted by muti-grained scanning (MGS) automatically. Finally, the extracted feature information is input into the cascaded forest for defect diagnosis. Considering the low data transmission efficiency and feature redundancy between MGS and cascaded forest, a feature extraction channel strategy based on kernel principal component analysis (KPCA) is introduced into the cascaded forest. Besides, in order to improve the generalization ability of the improved gcForest, the classifiers of each layer in cascade forest are upgraded. The results demonstrate the superiority of the improved gcForest over the traditional methods on experimental vibration data of flip chips, especially under small training sizes, which is very beneficial to practical engineering.

Study of transmission characteristics of disk transducer in the radially propagated Rayleigh wave excitation mode

A mathematical model of a capacitive ultrasonic transducer is designed allowing to emit ultrasonic vibrations into an electrically conductive product. The influence of a polarizing electrostatic field on the Coulomb forces formation in the surface layer of a metal sample is determined. A closed solution to the electrostatics problem is obtained for a piecewise homogeneous medium in which a half-space is filled with metal having finite values of electrical conductivity and magnetic permeability. An expression is obtained for calculating the surface density of a static electric charge on the metal sample surface. As part of the mathematical model of a capacitive sensor in the mode of converting electric energy to high-frequency mechanical (ultrasonic) energy in metals, closed solutions for electrostatics and electrodynamics problems are constructed in relation to a piecewise-homogeneous medium in which a half-space is filled with metal having finite values of electrical conductivity and magnetic permeability. It is determined that a capacitive disk transducer excites forces acting normally to the surface of electrically conductive products. A quantitative assessment of the surface density of the Coulomb forces is made. The main factors determining the sensitivity of a capacitive disk transducer are specified. As an example of using the simulation results, the amplitude factor of radially propagating Rayleigh waves is calculated. The concept of the wave characteristics of the transducer in the excitation mode of ultrasonic surface waves is introduced.

NVESTIGATION OF TRANSMISSION CHARACTERISTICS OF DISC CAPACITY CONVERTER IN THE MODE OF EXCITATION OF ULTRASOUND RELAY WAVES

A mathematical model of a capacitive ultrasonic transducer is designed allowing to emit ultrasonic vibrations into an electrically conductive product. The influence of a polarizing electrostatic field on the Coulomb forces formation in the surface layer of a metal sample is determined. A closed solution to the electrostatics problem is obtained for a piecewise homogeneous medium in which a half-space is filled with metal having finite values of electrical conductivity and magnetic permeability. An expression is obtained for calculating the surface density of a static electric charge on the metal sample surface. The main influencing factors that determine the spatial distribution of the charge density on the product surface, which will form the radiation pattern and power of the emitted ultrasound field, are defined. As part of the mathematical model of a capacitive sensor in the mode of converting electric energy to high-frequency mechanical (ultrasonic) energy in metals, closed solutions for electrostatics and electrodynamics problems are constructed in relation to a piecewise-homogeneous medium in which a half-space is filled with metal having finite values of electrical conductivity and magnetic permeability. It is determined that a capacitive disk transducer excites forces acting normally to the surface of electrically conductive products. A quantitative assessment of the surface density of the Coulomb forces is made. The main factors determining the sensitivity of a capacitive disk transducer are specified.

Single-polar power supply of high frequency capacitive transducers for measurement

It is possible to increase the sensitivity of capacitive ultrasonic transducers in three ways: increasing the capacitance of the transducer, increasing the polarizing and pulsed high-frequency voltage applied in the transducer cover, using modern methods of processing information packet pulses excited and received from the product. The increase in the capacity of the transducer is limited by the design capabilities of the transducer itself and the selection of the dielectric separating the plate from the surface of the electrically conductive testing object. Using modern methods of processing the received information significantly complicates and increases the cost of implementation of the device using a capacitive transducer. The most acceptable, at the moment, way to increase the sensitivity is to increase the power of high-frequency voltage supply generators provided that packet pulses are excited. The generator presented in the article allows to create pulsed peak voltages in low-capacitance transducers of more than 3 kV with frequencies up to 5 MHz with a duration of 1 period of the filling frequency and sounding frequencies up to 1 kHz. It was experimentally shown that using the prototype of a new generator, the amplitudes of the pulses transmitted through a steel sample 20 mm thick were obtained, the ratio of which to the noise amplitude received by the miniature piezoelectric transducer is 13 times. Using the developed powerful amplifier of voltage pulses will increase the amplitude of the pulses received from the transducer and repeatedly reflected in the product.

An Ultrasonic Heading Goniometer Intrinsically Robust to Magnetic Interference

An ultrasonic heading measurement method, working under magnetic interference prohibitive for magnetometers, was prototyped, validated, and metrologically characterized. Two capacitive ultrasonic transducers convert the mechanical rotation in two correspondingly time-delayed electrical sine waves. Then, the time delay is estimated using the standard tree parameter sine-fitting algorithm. The prototyped goniometer achieves the same repeatability level (<36 mrad) of a magnetometer-based heading in the range [−437 mrad, 437 mrad]. Simultaneously, a throughput of 505 Sa/s is proven on an STM32F303xC Arm Cortex −M4 32-bit microcontroller. An interference analysis revealed the experimental deterministic error well explained by the combined effect of beacon and receiver directivity, as well as by the relative position of beacon, receiver, and reflective surfaces. Noise robustness was assessed in case of SNR decayed to 9.1 dB from the initial value of 36.6 dB; the maximum deterministic error in the range [−437 mrad, 437 mrad] increased less than 10% (from 21 to 23 mrad in absolute value).

Characterization of a parametric resonance based capacitive ultrasonic transducer in air for acoustic power transfer and sensing

Capacitive transducers typically require a DC bias or a pre-charged electret to operate, rendering them non-ideal for applications such as energy harvesting and wireless power transfer. Parametric resonance based capacitive transducers offer an alternative to traditional capacitive transducers as they can be operated without the need for a bias voltage or a pre-charged electret. In this paper, we experimentally validate a 1D lumped parameter model and characterize a CPUT operating in air at 50 kHz for ultrasonic power transfer and sensing applications. This particular CPUT is able to recover 40.5 μW of power at an efficiency of 0.32% without any DC bias when excited by a 50 kHz piezoelectric transducer. The application of the CPUT as a highly sensitive acoustic sensor is explored by making use of active electrical circuits that reduce the resistance of the system. Finally, the capability of the CPUT as a highly directional acoustic sensor is presented with the electrostatic transducer demonstrating a directivity of ±1° when operated as a CPUT as compared to ±13° when operated as a conventional biased receiver. These results set the stage for the application of CPUT as a multi-functional acoustic transducer.

Micro-electro-mechanical-system based capacitive ultrasonic transducer as an efficient immersion sensor

In this paper we have analyzed the influence of the membrane inherent residual stress along with its Young’s modulus for evaluating the characteristics of a capacitive micromachined ultrasonic transducer (CMUT) for medical imaging and sensor applications. The membrane displacement profile, the frequency response characteristic and the coupling coefficient are evaluated and a close conformity with the PZFlex Finite Element method (FEM) simulations has been confirmed. The variation of the coupling coefficient with the different structural parameters indicates the use of thinner membranes with lower transduction gap as efficient transducers. Moreover, coupling factor increases as bias increases and an investigation of the collapse voltage is essential. Various coupling mediums are analyzed to find the transducer equivalent resistance and quality factor to predict its utility as a sensor for gaseous and liquid coupling mediums.

A parametric resonance based capacitive ultrasonic transducer for wireless power transfer in air

Capacitive ultrasonic transducers have been in use for many years for various applications such as medical imaging, wireless power, sensing, and nondestructive testing. Typically, capacitive transducers require a DC bias or electret in order to operate efficiently and with high sensitivity. This makes them less desirable for applications such as wireless power transfer or energy harvesting, where a passive piezoelectric transducer may be preferred. Recently, it was shown that the requirement of a DC bias can be overcome by driving the capacitive transducer into parametric resonance using ultrasound (Surappa et al., Appl. Phys. Lett. 111(4), 043503). In this work, we present the operation of the first ever biasless, chargeless capacitive parametric ultrasonic transducer (CPUT) in air and demonstrate the utility of such a system for acoustic power transfer and sensing. Experiments performed in air show that the CPUT has an open-circuit sensitivity of more than 100 mV/Pa and is able to recover 31 µW at a distance of 10 cm from a 50 kHz ultrasonic source in the absence of a DC bias.

An Indoor Airborne Ultrasonic Wireless Communication Network.

There is an increasing interest in the use of modulated airborne ultrasound as a means of indoor wireless communication. By using commercially available capacitive ultrasonic transducers at 50 kHz, this paper describes the successful practical implementation of a prototype airborne ultrasonic communication network with ceiling-mounted base stations (BSs) and a mobile transceiver unit. An asynchronous ultrasonic location technique using Gold code-modulated ranging signals was chosen to optimize the modulation schemes and data transfer and offered automatic handover between different cell regions on a switch ON and OFF basis as all BSs used the same frequency bands for data transmission. The 16-quadrature amplitude modulation (QAM) based on orthogonal frequency division multiplexing (OFDM) was used to achieve an uplink data transfer rate of 37.4 kb/s, whereas the range was extended by using quadrature phase-shift keying (QPSK)-OFDM with a data rate of 18.7 kb/s. For the uplink connection, the achieved data rates using 16-QAM-OFDM and QPSK-OFDM were 36.1 and 18.1 kb/s, respectively. A more robust handover technique using received signal strength with hysteresis was also proposed to improve system efficiency when multiple mobile receivers used the service.

Non-contact ultrasonic gas flow metering using air-coupled leaky Lamb waves

This paper describes a completely non-contact ultrasonic method of gas flow metering using air-coupled leaky Lamb waves. To show proof of principle, a simplified representation of gas flow in a duct, comprising two separated thin isotropic plates with a gas flowing between them, has been modelled and investigated experimentally. An airborne compression wave emitted from an air-coupled capacitive ultrasonic transducer excited a leaky Lamb wave in the first plate in a non-contact manner. The leakage of this Lamb wave crossed the gas flow at an angle between the two plates as a compression wave, and excited a leaky Lamb wave in the second plate. An air-coupled capacitive ultrasonic transducer on the opposite side of this second plate then detected the airborne compression wave leakage from the second Lamb wave. As the gas flow shifted the wave field between the two plates, the point of Lamb wave excitation in the second plate was displaced in proportion to the gas flow rate. Two such measurements, in opposite directions, formed a completely non-contact contra-propagating Lamb wave flow meter, allowing measurement of the flow velocity between the plates. A COMSOL Multiphysics® model was used to visualize the wave fields, and accurately predicted the time differences that were then measured experimentally. Experiments using different Lamb wave frequencies and plate materials were also similarly verified. This entirely non-contact airborne approach to Lamb wave flow metering could be applied in place of clamp-on techniques in thin-walled ducts or pipes.

A capacitive ultrasonic transducer based on parametric resonance.

A capacitive ultrasonic transducer based on a parametric resonator structure is described and experimentally demonstrated. The transducer structure, which we call capacitive parametric ultrasonic transducer (CPUT), uses a parallel plate capacitor with a movable membrane as part of a degenerate parametric series RLC resonator circuit with a resonance frequency of fo. When the capacitor plate is driven with an incident harmonic ultrasonic wave at the pump frequency of 2fo with sufficient amplitude, the RLC circuit becomes unstable and ultrasonic energy can be efficiently converted to an electrical signal at fo frequency in the RLC circuit. An important characteristic of the CPUT is that unlike other electrostatic transducers, it does not require DC bias or permanent charging to be used as a receiver. We describe the operation of the CPUT using an analytical model and numerical simulations, which shows drive amplitude dependent operation regimes including parametric resonance when a certain threshold is exceeded. We verify these predictions by experiments with a micromachined membrane based capacitor structure in immersion where ultrasonic waves incident at 4.28 MHz parametrically drive a signal with significant amplitude in the 2.14 MHz RLC circuit. With its unique features, the CPUT can be particularly advantageous for applications such as wireless power transfer for biomedical implants and acoustic sensing.

Indoor Airborne Ultrasonic Wireless Communication Using OFDM Methods.

Concerns still exist over the safety of prolonged exposure to radio frequency (RF) wireless transmissions and there are also potential data security issues due to remote signal interception techniques such as Bluesniping. Airborne ultrasound may be used as an alternative to RF for indoor wireless communication systems for securely transmitting data over short ranges, as signals are difficult to intercept from outside the room. Two types of air-coupled capacitive ultrasonic transducer were used in the implementation of an indoor airborne wireless communication system. One was a commercially available SensComp series 600 ultrasonic transducer with a nominal frequency of 50 kHz, and the other was a prototype transducer with a high- k dielectric layer operating at higher frequencies from 200 to 400 kHz. Binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), and quadrature amplitude modulation (QAM)-based orthogonal frequency division multiplexing modulation methods were successfully implemented using multiple orthogonal subchannels. The modulated ultrasonic signal packets were synchronized using a wireless link, and a least-squares channel estimation algorithm was used to compensate the phase and amplitude distortion introduced by the air channel. By sending and receiving the ultrasonic signals using the SensComp transducers, the achieved maximum system data rate was up to 180 kb/s using 16-QAM with ultrasonic channels from 55 to 99 kHz, over a line-of-sight transmission distance of 6 m with no detectable errors. The transmission range could be extended to 9 and 11 m using QPSK and BPSK modulation schemes, respectively. The achieved data rates for the QPSK and BPSK schemes were 90 and 45 kb/s using the same bandwidth. For the high- k ultrasonic transducers, a maximum data rate up to 800 kb/s with no measurable errors was achieved up to a range of 0.7 m. The attainable transmission ranges were increased to 1.1 and 1.2 m with data rates of 400 and 200 kb/s using QPSK and BPSK, respectively.

Evaluation of multiple-channel OFDM based airborne ultrasonic communications

Orthogonal frequency division multiplexing (OFDM) modulation has been extensively used in both wired and wireless communication systems. The use of OFDM technology allows very high spectral efficiency data transmission without using complex equalizers to correct the effect of a frequency-selective channel. This work investigated OFDM methods in an airborne ultrasonic communication system, using commercially available capacitive ultrasonic transducers operating at 50kHz to transmit information through the air. Conventional modulation schemes such as binary phase shift keying (BPSK) and quadrature amplitude modulation (QAM) were used to modulate sub-carrier signals, and the performances were evaluated in an indoor laboratory environment. Line-of-sight (LOS) transmission range up to 11m with no measurable errors was achieved using BPSK at a data rate of 45kb/s and a spectral efficiency of 1b/s/Hz. By implementing a higher order modulation scheme (16-QAM), the system data transfer rate was increased to 180kb/s with a spectral efficiency of 4b/s/Hz at attainable transmission distances up to 6m. Diffraction effects were incorporated into a model of the ultrasonic channel that also accounted for beam spread and attenuation in air. The simulations were a good match to the measured signals and non-LOS signals could be demodulated successfully. The effects of multipath interference were also studied in this work. By adding cyclic prefix (CP) to the OFDM symbols, the bit error rate (BER) performance was significantly improved in a multipath environment.

Full-Duplex Airborne Ultrasonic Data Communication Using a Pilot-Aided QAM-OFDM Modulation Scheme.

Orthogonal frequency division multiplexing (OFDM) has been extensively used in a variety of broadband digital wireless communications applications because of its high bandwidth utilization efficiency and effective immunity to multipath distortion. This paper has investigated quadrature amplitude modulation and OFDM methods in air-coupled ultrasonic communication, using broadband capacitive ultrasonic transducers with high- k dielectric layers. OFDM phase noise was discussed and corrected using a pilot-aided estimation algorithm. The overall system data rate achieved was up to 400 kb/s with a spectral efficiency of 2 b/s/Hz. An ultrasonic propagation model for signal prediction considered atmospheric absorption of sound in air, beam divergence, and transducer frequency response. The simulations were compared with the experimental results, and good agreement was found between the two. Two-way communication through air was also implemented successfully by applying three-way handshaking initialization and an adaptive modulation scheme with variable data rates depending on the transmission distance, estimated using received signal strength indication measurement. It was shown that the error-free transmission range could be extended up to 2.5 m using different system transfer rates from 400 kb/s down to 100 kb/s. In full-duplex transmission mode, the overall error-free system data rate achieved was 0.8 Mb/s up to 1.5 m.

Multichannel Ultrasonic Data Communications in Air Using Range-Dependent Modulation Schemes.

There are several well-developed technologies of wireless communication such as radio frequency (RF) and infrared (IR), but ultrasonic methods can be a good alternative in some situations. A multichannel airborne ultrasonic data communication system is described in this paper. ON-OFF keying (OOK) and binary phase-shift keying (BPSK) modulation schemes were implemented successfully in the system by using a pair of commercially available capacitive ultrasonic transducers in a relatively low multipath indoor laboratory environment. Six channels were used from 50 to 110 kHz with a channel spacing of 12 kHz, allowing multiple 8-bit data packets to be transmitted simultaneously. The system data transfer rate achieved was up to 60 kb/s and ultrasonic wireless synchronization was implemented instead of using a hard-wired link. A model developed in the work could accurately predict ultrasonic signals through the air channels. Signal root mean square (rms) values and system bit error rates (BERs) were analyzed over different distances. Error-free decoding was achieved over ranges up to 5 m using a multichannel OOK modulation scheme. To obtain the highest data transfer rate and the longest error-free transmission distance, a range-dependent multichannel scheme with variable data rates, channel frequencies, and different modulation schemes, was also studied in the work. Within 2 m, error-free transmission was achieved using a five-channel OOK with a data rate of 63 kb/s. Between 2 and 5 m, six-channel OOK with 60 kb/s data transfer rate was error free. Beyond 5 m, the error-free transmission range could be extended up to 10 m using three-channel BPSK with a reduced data rate of 30 kb/s. The situation when two transducers were misaligned using three-channel OOK and BPSK schemes was also investigated in the work. It was concluded that error-free transmission could still be achieved with a lateral displacement of less than 7% and oblique angles of less than 7°, and three-channel BPSK proved to be more robust than three-channel OOK with transducer misalignment.

Micro‐electro‐mechanical systems capacitive ultrasonic transducer with a higher electromechanical coupling coefficient

Since the proposing of capacitive micromachined ultrasonic transducer by Khuri Yakub group in 1994 that this kind of transducer occupying the advantages of wide bandwidth, impedance matching well with the propagation medium especially in fluid and air and high sensitivity, has shown a great potential for wide ranges of applications. This Letter reports kind of micro-electro-mechanical systems (MEMS) capacitive ultrasonic transducer with the novel cavities embedded in the device layer of silicon on insulator wafer bonded with a glass substrate. The optimum geometric dimensions are confirmed by both mechanical vibrating of the membrane and the electrical characteristics analysis. Finite-element analysis is adopted to determine the operation mode. The safety and reliability of the proposed device is ensured by the obtained deflections and equivalent stress under operation/collapse voltage. The bottom electrodes of the proposed transducer are fabricated on the top surface of the glass substrate. The parallel parasitic capacitance is reduced, thus improving the electromechanical coupling coefficient. The test results show that the electromechanical coupling coefficient is 69.65%, which demonstrates that this proposed MEMS capacitive ultrasonic transducer structure can enhance the performance significantly.

A unified equivalent circuit of electromechanical transducers

It has been interesting to find a unified equivalent circuit of electromechanical transducers for a long time. Hunt provided a method using a space operator to represent all transducer types with a single form of equivalent circuit [Hunt, Electroacoustics, 1954]. But, as indicated by Beranek, the space operator k does not commute with the time operator j, so that one must define jk=−kj [Beranek, JASA 77(4), 1309–1313, 1985]. During the last decades, the research on micromachined electrostatic or capacitive ultrasonic transducers stimulates further the interest in the subject [Ge, Sc. China, A 29(11), 1308–1315, 1999]. In this work, the space operator is not used. A transformation factor is defined as a ratio of blocked electrical-to-mechanical transfer impedance to blocked electrical impedance, thus, the factor is a real number for reciprocal transducers and imaginary for antireciprocal ones, which indicates the essential difference of the two types of transducers. Then, the corresponding driving-point electrical impedances are derived, and the expressions for the two are identical, demonstrating their common properties also. Thus, a unified impedance-type equivalent circuit for electromechanical transducers can be further obtained. [Work supported by NSFC (Grant Nos. 60374044 and 60774053).]

A novel 6 × 6 element MEMS capacitive ultrasonic transducer with multiple moving membranes for high performance imaging applications

A 6×6 element multiple moving membrane capacitive micromachined ultrasonic transducer (M3-CMUT) array has been developed for air-coupled detection purposes in the low megahertz frequency range. Unlike conventional capacitive MEMS transducers, the developed transducer array employs a novel cell configuration where more than one flexible membrane contributes in the transducer signal transmission and detection. In this structure, a stack of two vibrating membranes is suspended over a fixed grounded bottom electrode. The transducer array has been fabricated through employing a MEMS sacrificial technique, with the top and middle membrane radii of 65 and 42μm, respectively. The electrical, optical and acoustic evaluation results of this transducer array are presented in this paper. It is shown that using this configuration, the amplitude of membrane displacement is increased which enhances the transducer power output and sensitivity for a given bias condition. The experimental results demonstrate that this transducer array structure can be highly beneficial in high performance imaging application systems.