Joao L. Ealo

Broadband EMFi-based transducers for ultrasonic air applications

In this work, we explore the possibilities of electromechanical film (EMFi) as a new material for developing broadband transducers for ultrasonic air applications. The advantages of the EMFi film are its wide usable frequency range and easiness to use, making it highly suitable for self-made, customizable ultrasonic sensors. This paper presents theoretical and experimental information focused on the needs of the sensors end-user, namely, frequency response, actual dynamic mass and Youngs modulus, bandwidth, sensitivity, electromechanical dynamical model, acoustic response, and directivity. It is found empirically that the behavior of the film as an almost ideal piston-like acoustic source permits accurate prediction of the characteristics of transducers built on a developable surface. The results obtained represent the first step to more complex geometries, and, ultimately, to completely customizable field ultrasonic transducers.

Customizable field airborne ultrasonic transducers based on electromechanical film

In this work we empirically show that the Emfit film can be stuck on a curved surface without influencing its original electromechanical performance. This characteristic along with the previously reported piston-like response of the film at frequencies below 150 kHz, opens up the possibility to fabricate ultrasonic transducers of complex developable substrate and subsequently, customizable acoustic field. As a first step to complex acoustic directivity patterns, a quasi-spherical substrate is proposed in order to customize an omnidirectional radiation field. Numerical simulations were used in order to show that such an omnidirectional radiation directivity pattern can be approximated by an Emfit based transducer stuck on a quasi-spherical substrate. This works shows preliminary efforts directing to build an omnidirectional, spherical ultrasonic transducer, based on Emfit film.

Airborne ultrasonic vortex generation using flexible ferroelectrets

Cellular ferroelectrets exhibit interesting electromechanical- acoustical characteristics. Their recent appearance and remarkable properties open up new possibilities for the design and development of ultrasonic transducers. In particular, the feasibility of fabricating ultrasonic vortex generators using ferroelectret films is demonstrated in this work. To this end, a transducer prototype was built by gluing the material onto a tangential-helical surface (outer diameter: 40 mm, pitch: 3.45 mm). Experimental results agree well with the theoretical estimation of the pressure and phase of the acoustic field in the near field and far field, which corroborates the potential of ferroelectrets to customize special acoustic fields. Furthermore, the proposed fabrication procedure is inexpensive and represents a new alternative for exploring and analyzing the special characteristics of acoustical helical wavefronts.

Applications of airborne ultrasound in human-computer interaction.

Airborne ultrasound is a rapidly developing subfield within human-computer interaction (HCI). Touchless ultrasonic interfaces and pen tracking systems are part of recent trends in HCI and are gaining industry momentum. This paper aims to provide the background and overview necessary to understand the capabilities of ultrasound and its potential future in human-computer interaction. The latest developments on the ultrasound transducer side are presented, focusing on capacitive micro-machined ultrasonic transducers, or CMUTs. Their introduction is an important step toward providing real, low-cost multi-sensor array and beam-forming options. We also provide a unified mathematical framework for understanding and analyzing algorithms used for ultrasound detection and tracking for some of the most relevant applications.

A fabrication procedure for airborne ultrasonic phased arrays based on cellular electromechanical film

In this work, a novel procedure that considerably simplifies the fabrication process of ferro electret-based multi-element array transducers is proposed and evaluated. Also, the potential of ferro electrets as active material for air-coupled ultrasonic transducer design is demonstrated. The new construction method of multi-element transducers introduces two distinctive improvements, first, the active ferro electret material is not discretized into elements and, second, the need of structuring upper and lower electrodes in advance of the permanent polarization of the film is removed. In order to validate the procedure, two linear array prototypes of 32 elements were built and evaluated. A low crosstalk among elements, below -30 dB, was measured by interferometry. Likewise, a homogeneous response of the array elements, with a maximum deviation of plusmn1.8 dB, was obtained. Acoustic beam steering measurements were accomplished at different deflection angles using a calibrated microphone. The ultrasonic beam parameters, namely lateral resolution, side lobes level, grating lobes and focus depth, were congruent with theory. The proposed procedure simplifies the manufacturing of multi-dimensional arrays with arbitrary shape elements and not uniformly distributed. Furthermore, this concept can be extended to non-flat arrays as long as the transducer substrate conforms to a developable surface.

5H-5 Broadband Omnidirectional Ultrasonic Transducer for Air Ultrasound based on EMFi

Narrow-band ultrasonic transducers with highly-directive radiating patterns are commonly used in ranging, obstacle detection and localization applications. Consequently, problems such as mutual-interferences, limited accuracy in ranging estimation and poor coverage of regions of interest are typical. Some important efforts have been done in order to develop non resonant omnidirectional broadband transducers useful in industrial applications, such as PVDF cylindrical film transducers, electrostatic transducers, multi-frequency piezoelectric composites, etc. However, the problem is still open because of the trade-off among directivity, bandwidth and sensitivity. Recently, a new multipurpose polypropylene electret film called EMFi has been developed by VTT (Technical Research Centre of Finland). EMFis advantage over other materials, including polymer electrets, is based on its flexibility due to the voided internal structure combined with strong permanent charge and high sensitivity. These particular features of EMFi make it suitable for fabricating non-resonant transducers with almost arbitrary directivity pattern. In this work, characterization results of flat and cylindrical ultrasonic transmitters based on EMFi are presented, i.e. actual frequency response, radiating emitting pattern and transmitting sensitivity. In the cylindrical case, an omnidirectional polar response has been obtained at frequencies ranging from 30 to 100 kHz and a sound pressure level from 64 to 76 dB in a distance of 30 cm and at 220 Vpp of excitation level. A comparative analysis of both types of developed prototypes responses is carried out. Moreover, obtained results are compared with reported in literature for cylindrical omnidirectional PVDF film transmitters. This paper shows that EMFi film proves to be a well-suited material for developing true omnidirectional broadband ultrasonic transducers

ULTRASONIC AIR‐COUPLED INSPECTION OF TEXTILE MATERIALS USING FERROELECTRET‐BASED PHASED ARRAYS

Most common defects in textile manufacturing processes include weaving errors (such as missing threads), oil spots and material inhomogeneities. In this work, we demonstrate the feasibility of using ferroelectret‐based transducers for the inspection of woven material. A linear array of 32 elements was built for this purpose following an easy fabrication procedure recently proposed. Electronic focusing at the textile sample position allowed us to detect weaving errors and oil spots of up to ∼1 mm of width in through transmission mode, at normal incidence and with a good signal‐to‐noise ratio.

Robust regression applied to ultrasound location systems

Local positioning systems (LPS), specially those using ultrasound, are able to accurately estimate the location of persons or objects indoors. However, under certain circumstances, its accuracy can be strongly deteriorated by outlying noise. This paper analyzes and compares several strategies for robust trilateration, such as high-breakdown-point robust methodologies, as well as the parity space outlier detection procedure, which is commonly used in GPS. This analysis is performed by simulation in a typical ultrasound location system scenario based on the actual location of nodes in the 3D-LOCUS system [1]. It is shown how the traditional parity space outlier detection method overcomes robust methodologies when only one ranging error is present, whereas it is not able to detect two simultaneous faults. It is proposed a modification of the LTS robust estimation methodology that offers a good performance even when several range measurements are erroneous, due to multipath and occlusions effects. The complexity of the robust algorithms studied is low enough for being implemented in the 3D-LOCUS system without affecting its current 10 Hz update rate.

Design of acoustic lenses for ultrasonic human-computer interaction

A novel Twisted Acoustic Lens (TAL) design to spread the radiated acoustic energy of a flat transducer is proposed. The lens is comprised of many perforations deployed uniformly, with trajectories that are circular helices of different radii. We have fabricated and characterized a disc-shaped TAL of thickness 7 mm and twist angle of 45° and demonstrated that the energy radiated by a directive ultrasonic transducer can be significantly dispersed. It is possible to almost triple the -6 dB directivity of a circular transducer of 20 mm of diameter at frequencies between 60 kHz and 80 kHz. The proposed lens design has great potential in certain ultrasonic touchless applications in which a commercially available ultrasonic transducer needs to be turned into field-of-view maximizing acoustic component while maintaining a flat front end.

Generation of multiple vortex beam by means of active diffraction gratings

Acoustic vortices have attracted a great deal of attention in recent years due to their numerous applications. We introduce a highly efficient method for the generation of acoustic Bessel vortices in air, using spiral-shaped active diffraction gratings, which can be operated within a broad spectral range of ultrasonic frequencies. Using a single-arm active spiral source, we achieve the simultaneous generation of vortices of different topological charges, well separated among each other along the propagation axis. With a theoretical analysis, numerical simulations, and experiments, we demonstrate some features about spiral diffraction gratings, such as the equivalence between specific diffraction orders of an m-armed spiral and a single-arm spiral, and the annihilation of prescribed diffraction orders by tuning the width to pitch ratio.