Arbitrary variable thickness annular piezoelectric ultrasonic transducer based on transfer matrix method
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The variable thickness annular radial piezoelectric ultrasonic transducer can realize impedance transformation and energy concentration, has the advantages of large radiation area and full directivity, and is widely used in power ultrasound, underwater acoustic and other fields. Because solving complex variable thickness metal ring radial vibration wave equation is more difficult, in this paper, the radial vibration of metal rings with variable thickness is transformed into the superposition of the radial vibrations of N metal rings with equal thickness by using the transfer matrix method. The equivalent circuit diagram, the resonance frequency equation and the expression of the displacement amplification coefficient of the radial vibration of the metal thin ring with arbitrary thickness are obtained. The relationship between the displacement amplification coefficient and the geometric size of the cone, power function, exponential and catenary metal rings is analyzed. On this basis, the equivalent circuit and resonance frequency equation of radial vibration of piezoelectric ultrasonic transducer which is composed of a metal ring with variable thickness and a piezoelectric ring with equal thickness are derived. In order to verify the correctness of the theoretical results, the finite element software is used in simulation, and the numerical solutions of the first and second order resonance frequency and displacement amplification coefficients are in good agreement with the theoretical solutions. In this paper, the universal solution of radial vibration of metal ring with arbitrary variable thickness is given, which provides theoretical guidance for designing and optimizing the radial piezoelectric ultrasonic transducers.Cite
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Ultrasonic Testing (UT) is one of most important methods of Non-destructive Testing (NDT) and ultrasonic waves can be generated and detected by means of numeric methods. This thesis focuses on piezoelectric transducers and Electromagnetic Acoustical Transducer (EMAT).
For testing with piezoelectric transducer (also called a probe), couplant has to be applied between the test material and the probe allowing for ultrasonic waves generated in the probe by an active piezoelectric crystal to propagate into the testing material. The couplant can be water, mineral oil, or gel, dependent on the applications concerned and material compatibility, for example, water for immersion tests or automatated inspections.
No two piezoelectric transducer designs are identical in terms of their frequency range, beam propagation characteristics and directionality. The shape, dimensions, backing and matching of the transducer to the pulse generator together play a major role in the generation of the ultrasonic waves. Furthermore, the quantitative interpretation of pulse echo data obtained when such transducers are used in nondestructive evaluation (NDE) requires a complete knowledge of the ultrasonic field transmitted.
Ultrasonic fields of circular probes have been studied experimentally using a miniature probe and theoretically with models developed to predict ultrasonic field of such a probe. Good agreement has been observed. In a fluid, the ultrasonic field generated by a circular piezoelectric transducer can be described in terms of a combination of locally plane waves that radiate in the geometric region straight ahead of the active transducer element and edge waves radiating from the rim of the element. When a piezoelectric transducer is directly mounted onto a solid material, the ultrasonic field includes locally plane longitudinal waves, edge longitudinal waves and mode-converted edge shear waves. Both cases can be studied using miniature piezoelectric probes.
For electrically conductive materials, EMATs can be used for generation by means of Lorentz force or magnetostriction or both, and detection. EMAT technique is non-contact and couplant free and can work at high temperature. These attributes make it ideal for inspection in extreme conditions, such as high temperature, high speed, rough surface, etc. This thesis focuses on Lorentz force generation. The main disadvantages of an EMAT detector are its lower sensitivity compared to a piezoelectric probe and it is not straightforward to miniaturise the device to operate as a point sensor for the range of wavelengths of interest here. Therefore, optimal EMAT design is extremely important for successful EMAT application. Ultrasound may be generated without presence of external magnetic field as excitation electric current provides magnetic field as it induces eddy currents in the material under test, which creates Lorentz forces for ultrasonic generation. Where external magnetic field is applied, EMATs have to be designed correctly to achieve enhanced efficiency. As an example, Rayleigh wave EMAT generation has been studied. It is found that where external magnetic field is applied, constructive or destructive effects have been observed, which is understood dependent on direction of the external magnetic fields applied relevant to electric current direction. Optical interferometer to measure the true normal displacement of the solid surface with a resolution in the order of nanometres, but it is much more complex than an EMAT and a piezoelectric probe and requires an optically flat surface. The receiving EMAT detector measures particle velocity. By careful design, in-plane or out-of-plane (or both) velocities can be chosen for detection. This capability is very useful for the detection of longitudinal waves, shear waves, Rayleigh waves or Lamb waves efficiently.
The ultrasonic pulse-echo technique has been widely used in ultrasonic NDT. Ultrasonic pulse-echo responses and ultrasonic field signals are not the same. Typically, edge waves are rarely seen in a pulse echo response because the plane waves that are normal to the major face of the active crystal of the same probe are nearly in phase to constructively result in a significant signal whilst edge waves arrive at the active crystal in different directions and different phases cancelling each other and destructively producing only a small signal that is barely observable. As an example of ultrasonic pulse-echo application, weak bond evaluation, has been performed. Weak bond evaluation has always been a challenge. As an example of practical applications, this study has evaluated Integrated Circuit packaging in electronic industry using scanning acoustical microscopy. The relationship among resulting ultrasonic C-scan images, destructive mechanical failure measurement, degradation cycles have been observed. The result is promising indicating the SAM is a very useful tool for weak bond evaluation.
Ultrasonic field measurement using a miniature probe and specially design EMAT is very important to characterize and standardize a probe. Such a technique can also find its applications in defect detection and categorization, which has not been considered in this study.
Electromagnetic acoustic transducer
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Sandwich piezoelectric transducer has been widely used in underwater acoustics and ultrasonic fields. The effect of the position of piezoelectric ceramic on power ultrasonic transducer′s resonant frequency is presented. The relationship curve of the two parameters in different conditions is analyzed. The method to design this transducer is given.
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Electromagnetic acoustic transducer
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An anisotropic ultrasonic transducer is proposed for Lamb wave applications, such as passive damage or impact localization based on ultrasonic guided wave theory. This transducer is made from a PMNPT single crystal, and has different piezoelectric coefficients $d_{31}$ and $d_{32}$ , which are the same for the conventional piezoelectric materials, such as Lead zirconate titanate (PZT). Different piezoelectric coefficients result in directionality of guided wave generated by this transducer, in other words, it is an anisotropic ultrasonic transducer. And thus, it has different sensitivity in comparison with conventional ultrasonic transducer. The anisotropic one can provide more information related to the direction when it is used as sensors. This paper first shows its detailed properties, including analytical formulae and finite elements simulations. Then, its application is described.
Lead zirconate titanate
Electromagnetic acoustic transducer
Lamb waves
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A simple ultrasonic displacement measurement system is described, based on a phase-sensitive detection principle. A low-frequency ultrasonic transducer, operating in continuous-wave mode, is used to transmit a 44.2 kHz acoustic wave to the target body. The reflected signal is detected by a matched transducer and the phase difference between the transmitted and received signal is found in real time. Tests show that the system is capable of detecting displacement with an accuracy of +or-5.3 mu m, with targets situated over 2 m from the transducers.
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Electromagnetic acoustic transducer
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This paper deals with acoustic directivity according to vibration displacement distribution of ultrasonic sensors for distance measurement. It is necessary to determine the vibration displacement distribution improving the directivity because the measurable distance is longer with better directivity, which depends on the vibration of the ultrasonic sensor’s housing plate. Directivity for circular housing plate with uniform vibration displacement distribution was theoretically expressed. Vibration displacement distribution and acoustic characteristics of real ultrasonic sensors were obtained by finite element analysis. Functional distribution of vibration displacement and sensor symmetry were considered in the analysis. As the vibration displacement distribution is gentle and approaches the uniform one, the ultrasonic sound pressure level in the direction of central axis is large and acoustic directivity is good. We have achieved the basis of improving the acoustic directivity for design of long-distance ultrasonic sensors.
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Electromagnetic acoustic transducer
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A theoretical method which can be used to calculate the pressure field patterns of pulsed, focused, ultrasonic radiators in attenuating and nonattenuating media is discussed in this paper. The underlying principle involved is the superposition of continuous wave beams to form pressure pulses. The method is adapted to the circumstances associated with using a hydrophone to measure field patterns. Experimentally obtained hydrophone signals are then compared to theoretical predictions. The field patterns of four transducer-pulser combinations are investigated. A medium with tissue-mimicking acoustical properties is used to attenuate the ultrasonic beam in these studies. The theory compares favorably with experiment whether an attenuating medium is present or not. However, the theory fails at positions very close to the transducer face (∠1 cm) and when significant nonlinear effects occur during the transmission of the pulse through the medium. This work may have significant applications in research devoted to designing ultrasonic transducers for particular studies, determining dose profiles of medical ultrasonic machines, and analyzing the ultrasonic signals backscattered from within patients.
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Nonlinear acoustics
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Electromagnetic acoustic transducer
Ultrasonic motor
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Methods of designing ultrasonic piezoelectric transducers in accord with a specified directivity pattern are analyzed and general formulas are presented for the design problem. As an example, the pressure distribution on the working surface of a circular piezoelectric transducer is determined that is capable of forming a narrow weakly divergent beam. Experimental results are presented with respected to ultrasonic fields that support the theoretical conclusions.
Directivity
Electromagnetic acoustic transducer
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