Maik Hoffmann

Volumetric characterization of ultrasonic transducers for gas flow metering

The design of ultrasonic gas flowmeters requires a thorough three dimensional characterization of the acoustic sound field. For large pipe flowmeters, such as used for flare gas metering, the transducers are operated at frequencies ranging from 20 kHz up to 150 kHz. Thus, in this work we use a commercially available calibrated 1/8-inch microphone, mounted on a 3D positioning system for performing volumetric measurements in a volume of up to 1×1×1 m. By using proper corrections in terms of angular and free-field response of the microphone, the measurement system is efficient and delivers around 30000 measurements in about only eight hours. The data then is visualized in form of 3D figures or various slices to extract all relevant information. The system has been used to identify non-uniform velocity profiles in capacitive micromachined ultrasonic transducers (CMUTs), operating in permanent contact mode. Further, the system can be used to investigate the effect of various acoustic boundary conditions the transducers are facing when mounted inside transducer port cavities and it can be used for general model validation purpose.

Versatile air-coupled phased array transducer for sensor applications

We introduce a versatile one-dimensional (1D) air-coupled phased array transducer device operating at 40 kHz. It allows both - transmit beam steered ultrasound at high sound pressure level and it can sense beam steered ultrasound. This device opens the door to many new sensory applications. Conventionally, low frequency sensor array design for air-coupled application was not possible due to the inter element spacing (also referred as pitch) criteria that needs to be satisfied for given applications. We overcome this limitation by introducing a smart packaging layer that separates the acoustic aperture from the actual radiating aperture of the ultrasonic transducer. By doing this, we ensure that the half wavelength criteria is fulfilled. The proposed new design leads to a wide operation range without any undesirable grating lobes. A proof of concept prototype device was built and its performance evaluated in the laboratory. An impressive 130 ± 1 dB sound pressure level (SPL) was observed at a distance of 1m. The prototype also was capable of steering approximately 110° in total, in both transmit and receive mode of operation.

Diffraction loss calculation based on boundary element method for an air-coupled phased array

We present an efficient numerical method to analyze the diffraction loss of an ultrasonic air-coupled phased array transducer to an arbitrary located receiving aperture. The objective is the efficient calculation of diffraction loss for applications in which such an array is excited with arbitrary input signals on all channels for different beam steering angles. Available software, such as Field II, provide the pressure field in front of the phased transducer array, i.e. they assume a point source receiver. Our calculation method, however, is focused on directly calculating the diffraction loss to a given receiving aperture, such as a microphone or a receiving transducer, with arbitrary location and orientation. The boundary element method (BEM) is used for an efficient calculation. The entire model is implemented in the commercially available software package Mathematica V10 from Wolfram Research. Excellent agreement between calculation results and microphone measurements validate the approach. For future work, we will use this model for further optimizing the development of high-performance air-coupled phased array transducer designs.

Phased array transducer for emitting 40-kHz air-coupled ultrasound without grating lobes

We introduce an ultrasonic one-dimensional (1D) air-coupled phased array transducer, operating at 40 kHz without any grating lobes. It is well known, that this achievement is only possible when each transducer element is small enough for an element pitch that is equal to or less than half of the wavelength, i.e. ≤ 4.3 mm for 40 kHz operation frequency. As far as we know, conventionally low frequency array designs for air-coupled transducer applications were not possible at 40 kHz due to this inter element spacing requirement. The main idea is to separate the acoustic aperture from the actual radiating aperture of the ultrasonic transducer. We use acoustic waveguides, forming a smart packaging layer, to fulfill the half-wavelength criteria for the pitch and to benefit from concentrating the acoustic energy from several single ultrasonic transducers into a smaller effective aperture. A proof-of-concept prototype array was built and characterized. An impressive 130 ± 1 dB sound pressure level (SPL) is observed at a distance of 1 m. The opening angle of the array is 110° in total, without any grating lobes. For future work, we plan extending the approach to build an air-coupled fully populated 2D phased array transducer as well.

Calculation of diffraction loss between non-co-axial ultrasonic transducer configurations

We derive and validate a quadruple integral expression for calculating the diffraction loss in an isotropic propagation medium between circular ultrasonic transducers with time-harmonic radiation. The distinctive feature is the generic non-coaxial configuration with arbitrary transducer size and orientation. By performing several measurements, we validate the integral expression and we demonstrate that nowadays it is feasible to perform the required numerical integration to obtain the diffraction loss over an entire volumetric sound pressure field on a state-of-the-art personal computer with reasonable computation time on the order of a few minutes only. Excellent agreement between calculated results and experiments prove the approach and the generic nature of our expression ensures its wide and simple applicability for efficient diffraction loss calculations.

Extending the receive performance of phased ultrasonic transducer arrays in air down to 40 kHz and below

We present a versatile ultrasonic one-dimensional (1D) air-coupled phased array transducer with a focus on its receive performance. In addition to its beam steering capability in transmit mode, it can be used to listen to ultrasonic waves at 40 kHz and below. We fulfill the half-wavelength criteria for the element spacing by introducing an additional layer, consisting of many tapered sound tubes (waveguides). These sound tubes separate the effective acoustic aperture from the actual receiving aperture of several ultrasonic transducers. This ensures that the array is not affected by grating lobes over a wide range of steering angles, and, thus, the array is capable of receiving directed ultrasound with only one primary receive direction for an adjustable angle (receive-beam forming). Based on this approach, we built and characterized a proof-of-concept prototype device. Our results prove that the array is able to receive ultrasonic waves over a wide range of 110° in total, with a receive sensitivity of -55.9 dB (0 dB equal 1 V/Pa). Thus, the device has huge potential for new sensory applications and the approach is compatible with micromachined ultrasonic transducers as well.

Ultrasonic transducer characterization in air based on an indirect acoustic radiation pressure measurement

We present proof-of-concept measurement results from a simple indirect method that allows us to determine the acoustic radiation pressure acting on air coupled ultrasonic transducers in form of acoustic thrust. The simple and inexpensive method utilizes this acoustic thrust, acting on the ultrasonic transducer mounted on a clamped aluminum cantilever (850 × 20 × 2mm3). This approach is successful in air, because it exploits resonance amplification in return of a longer measurement time. By using a self-tuning circuit, the beam oscillates at its resonance frequency of approximately 2 Hz with a quality factor of 330. Modeling the system as spring-mass-dashpot system allows using the equations of forced damped oscillation to determine the acoustic thrust of various transducers based on only the measured beam displacement, obtained via e.g. strain gauges or a laser distance sensor. We used two commercially available ultrasonic transducers (MA40B8S and MA40S4S, both from MURATA, Japan) of different size and weight to test the setup. Excited with their specified maximum excitation voltages, acoustic thrust forces of up to 61 μN and 161 μN for the MA40S4S and the MA40B8S, respectively, are measured. Over a wide range these measurements are in good agreement with results from a digital high precision scale. Thus, our results show that the setup is able to measure small values of acoustic thrust in the μN-range. For future work, this approach can be used to compare different types of air-coupled ultrasonic transducers in terms of their efficiency, based on their generated acoustic thrust force.

Effect of transducer port cavities in invasive ultrasonic transit-time gas flowmeters

The design of ultrasonic gas flowmeters (UFMs) requires a thorough characterization of the acoustic sound pressure field. For large pipe flowmeters, such as used for custody or flare gas metering, the transducers are operated at frequencies ranging from 20 kHz up to 150 kHz. Thus, we investigate the effect of various acoustic boundary conditions for a 40 kHz air-coupled ultrasonic transducer by varying the mounting position inside the transducer port cavity. We used a volumetric characterization system to measure the sound pressure field for various boundary conditions. A pipe section with a diameter of ten inch and two representative inclination angles for a single-path UFM are used. We observe a maximum bending of the main sound lobe up to 16° and a significant change of the shape from single to multiple lobes. Our results prove that the transducer port cavity has significant effects on the sound pressure field and that these effects need to be taken into account when a flowmeter is supposed to be designed properly in terms of a symmetric or intentionally non-symmetric sound propagation.

CMUTs in Permanent Contact Operation for High Output Pressure

We present the operation of capacitive micromachined ultrasonic transducers (CMUTs) in permanent contact mode as an efficient transducer. The gap height of our transducers is chosen to be slightly smaller than the static deflection of the plate due to the pressure difference between the ambient and the vacuum cavity. Thus, the plates are in contact with the bottom of the cavities even with no dc bias applied. The devices were fabricated based on the thick box process. High-temperature assisted direct wafer bonding technique was used to fabricate devices with such large cell size (radii ∼ 2000 μm) featuring low frequencies ∼100–150 kHz. Extensive acoustic characterization was performed to demonstrate the behavior of such CMUTs in terms of displacement profile, output pressure and acoustic pitch-catch response. A maximum sound pressure of ∼145 dB (SPL) at the transducer surface is measured at 240 V dc and 10 V ac with 100 cycles of burst signal. This is a great improvement from conventional CMUTs (with deeper gap height, operating at 55 kHz), which requires 350 V dc and 200 V ac in order to achieve an output pressure of 129 dB (SPL) at the transducer surface. The results presented in this paper demonstrate that operating CMUTs in permanent contact mode indeed enhances the device output pressure, and provides a good candidate for efficient ultrasonic transducers.Copyright

Finite element analysis of mechanically amplified CMUTs

We introduce the possibility of improving a single-cell capacitive micromachined ultrasonic transducer (CMUT) for air-coupled ultrasound by simply adding a hollow conical-shaped structure (horn) on top of the CMUT plate. The main objective is to improve both transmit and receive sensitivity by lowering the center-to-average displacement ratio, which for bending plate operated devices inherently is limited. In addition, for receive mode the force generated from the impinging sound pressure wave is concentrated to the center of the plate, resulting in larger signals and, in contrast to piston-shaped plates, the horn has the advantage of only moderately increasing the modal mass of the structure. By using finite element analysis and first sound pressure measurements of our modified CMUT, we demonstrate that this idea is feasible and promising for air-coupled CMUTs operating at frequencies below 150 kHz, as it has been been proven to be successful for commercially available piezoelectric-driven bending plate devices as well.