Unger, Alexander (2019)
Air Coupled Ultrasonic Transducers for Industrial Applications.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Air-coupled ultrasonic transducers are widely used in many industrial, medical, or domestic applications, such as range finding, gesture sensing, or gas flow metering, to name only a few. The knowledge of the behavior and properties of the ultrasonic transducers is essential for the development and the improve- ment of these applications. The three most utilized work principles, e.g. piezoelectric, electrostatic, and electromagnetic, of ultrasonic transducers are explained and most common representatives of ul- trasonic transducers are described. Within this thesis, several important methods of characterization in three domains (electrical, mechanical, and acoustical) of ultrasonic transducers are described, investig- ated and modified. The electrical impedance measurement delivers the most important characteristics of ultrasonic transducers in the electrical domain. However, the electrical impedance measurement us- ing a common network analyzer reveals issues considering the measurement uncertainties at higher impedance values, such as the open circuit resonance frequency. In order to characterize the behavior of the ultrasonic transducers with the boundary conditions more closely to the applications, electrical impedance measurements of ultrasonic transducers at their rated excitation voltage are conducted in comparison to the common small signal network analyzer measurements. Further, the ultrasonic trans- ducers are characterized with thermal loads, which occur, for example, in gas flow metering. A wide temperature range of almost 450°C from −190°C using a cryo setup to +250°C using an oven are used to characterize a piezoelectric wide range transducer. In doing so, the influence of the excitation voltage and the thermal boundary conditions are investigated. A complex curve fit algorithm is implemented in MATLAB® and is used to fit the Butterworth-van Dyke model to the electrical impedance data gathered. The Butterworth-van Dyke model is extended by using up to seven parameters in total in order to de- scribe additional physical effects such as contact resistance, parasitic contact capacitance, and parasitic parallel resistance. The parameters are monitored and each of them is analyzed individually to achieve a robust complex curve fit algorithm. A laser doppler vibrometer is used to characterize the surface velo- city and displacement of the ultrasonic transducers in the mechanical domain. This measurement setup is extended by translational linear stages in order to obtain the surface velocity of the entire surface area of ultrasonic transducers. Through post processing of the stored A-scans at any spatial point of the area, characteristics such as the transient oscillation at each point, are visualized in a video sequence. In order to characterize ultrasonic transducers in the acoustical domain, two different volumetric sound pressure level measurement systems are used to characterize and visualize the sound pressure field using a calibrated microphone. First, a 3D linear stage which is capable of measuring the sound pressure level in a volume of 1m³ in front of an ultrasonic transducer in equidistant steps in any direction ( X − Y − Z ). Second, a goniometer which is built in an anechoic chamber and capable of measuring the sound pres- sure level in a volume of 144π m³ in front of an ultrasonic transducer, using spherical coordinates r-ϕ-θ. Further, an air-coupled 40kHz 1D-phased array prototype with a half-wavelength pitch is built and char- acterized in both transmit and receive mode. It features a smart packaging layer utilizing waveguides to separate the acoustic aperture from the vibrating aperture. In doing so the acoustic characteristics are manipulated and an impressive sound pressure level of (130 ± 1)dB at a distance of 1m. The ultrasonic beam can be steered electronically in an angle of 110°without creating any significant grating lobes. IIThe approach, used in this prototype, leads to various improvements of existing applications, such as gestures sensing, gas flow meters, or imaging of entire rooms in air. Further, the mechanical amplific- ation, using an additional horn structure on top of the vibrating aperture, of the efficient piezoelectric Murata MA40S4x ultrasonic transducers is investigated and adopted. A capacitive micromachined ultra- sonic transducer is modified with such a horn structure acting as a mechanical amplifier. The horn with a diameter of 2.3mm and a thickness of 100µm is fabricated of aluminum using non-micromachining techniques. It is glued on top of a 55kHz single cell device and an increased sound pressure level of (3 ± 1)dB is obtained. Within this thesis the described methods of characterizations are essential to gain needed knowledge of the behavior of ultrasonic transducers, which can lead to an improvement of many industrial, medical, or domestic applications. The described waveguide approach and mechan- ically amplification opens the door for various modification of ultrasonic transducers and therefore the usage in applications.

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