Naoto Wakatsuki

Finite element simulation of piezoelectric transformers

Piezoelectric transformers are nothing but ultrasonic resonators with two pairs of electrodes provided on the surface of a piezoelectric substrate in which electrical energy is carried in the mechanical form. The input and output electrodes are arranged to provide the impedance transformation, which results in the voltage transformation. As they are operated at a resonance, the electrical equivalent circuit approach has traditionally been developed in a rather empirical way and has been used for analysis and design. The present paper deals with the analysis of the piezoelectric transformers based on the three-dimensional finite element modelling. The PIEZO3D code that we have developed is modified to include the external loading conditions. The finite element approach is now available for a wide variety of the electrical boundary conditions. The equivalent circuit of lumped parameters can also be derived from the finite element method (FEM) solution if required. The simulation of the present transformers is made for the low intensity operation and compared with the experimental results. Demonstration is made for basic Rosen-type transformers in which the longitudinal mode of a plate plays an important role; in which the equivalent circuit of lumped constants has been used. However, there are many modes of vibration associated with the plate, the effect of which cannot always be ignored. In the experiment, the double resonances are sometimes observed in the vicinity of the operating frequency. The simulation demonstrates that this is due to the coupling of the longitudinal mode with the flexural mode. Thus, the simulation provides an invaluable guideline to the transformer design.

Detection of Internal Cracks in Square Billets Using Time of Flight of Longitudinal Waves

We propose a method for the reconstruction of internal cracks using time of flight (TOF) of longitudinal waves. Since the longitudinal wave is the fastest elastic wave, it is possible to reconstruct cracks using a matrix method with apparent sound velocities calculated from the TOF. This method can neglect mode transformation and other waves because the first wave received is always the longitudinal wave. In addition, this method has the potential to be free from a blind sector and to detect the cracks precisely, because it is a transmission method and uses time domain analysis. We simulate crack detection using the transmission-line matrix (TLM) method. This proposed method detects cracks regardless of their location, direction, and number. It was also possible to detect the cracks when their length was smaller than the wavelength or when the billet had some error in its shape. This technique is expected to be useful for ultrasonic testing.

Measurements of Wind Velocity and Direction Using Acoustic Reflection against Wall

The measurements of wind velocity and direction using an acoustic reflection against a wall are described. We aim to measure the spatial mean wind velocity and direction to be used for an air-conditioning system. The proposed anemometer consists of a single wall and two pairs of loudspeakers (SP) and microphones (MIC) that form a triangular shape. Two sound paths of direct and reflected waves are available. One is that of the direct wave and the other is that of the wave reflected on the wall. The times of flights (TOFs) of the direct and reflected waves can be measured using a single MIC because there is a difference in the TOF between direct and reflected waves. By using these TOFs, wind velocity and direction can be calculated. In the experiments, the wind velocities and directions were measured in a wind tunnel by changing the wind velocity. The wind direction was examined by changing the setup of the transducers. The measured values using the proposed and conventional anemometers agreed with each other. By using the wave reflected against a wall, wind velocities and directions can be measured using only two pairs of transducers, while four pairs are required in the case of conventional anemometers.

Design for an Aspherical Acoustic Fresnel Lens with Phase Continuity

A convex acoustic lens using room temperature vulcanizing (RTV) silicone rubber, whose acoustic impedance is similar to that of water, is a typical acoustic lens. A phase-continuous Fresnel lens was proposed to thin the shape of the lens because a convex acoustic lens has large attenuation due to its thickness. However, a Fresnel lens based on a convex spherical acoustic lens could not concentrate sound pressure completely on the focal point because of a spherical aberration. We designed an aspherical Fresnel lens by ray theory to remove the spherical aberration. A two dimensional finite difference time domain (2-D FDTD) method was used to survey the sound pressure field focused by the acoustic lens under condition in which the angle of incidence and the aperture were varied. Results showed that the aspherical Fresnel lens can concentrate greater sound pressure than the spherical Fresnel lens at normal and small angles of incidence.

Temperature Distribution Measurement Using Reflection with Acoustic Computerized Tomography

In this paper, we propose a new acoustic computerized tomography (A-CT) method using reflection. This method is alternate optimization for measuring the temperature distribution in a cylindrical space. The A-CT method using reflection includes two inverse problems, namely, the back projection of Radon transform and the division of times of flight (TOFs) of reflected sounds into TOFs of pre-reflection and post-reflection. The TOF estimation problem requires the temperature distribution. To solve this problem mathematically, alternate optimization is applied to the A-CT method using reflection. Simulation results indicate that our new A-CT method using reflection is useful for measuring the temperature distribution in a cylindrical space.

Interval of Observation Plane in Visualization of Region near Defects in Billets Using Ultrasonic Computerized Tomography Method

We performed defect detection simulation considering billets with a deep-hole or spherical defect. We conducted defect detection in a billet of duralumin with a deep-hole defect and found no discrepancy between our previous and present research results because the images obtained are similar. We also conducted defect detection in a billet of steel with a spherical defect. We obtained visualization images in multiple measurement planes. We also obtained three-dimensional visualization images by binarizing the pseudo sound velocity. From the images, we found that the three-dimensional visualization of spherical defects is possible and that the scanning pitch in the longitudinal direction is about 10 mm at maximum.

Nondestructive Inspection for Steel Billet Using Phase-Modulated Signal by Gold Sequence for Improving Measurement Speed

We have proposed an ultrasonic computerized tomography method using the time-of-flight (TOF) of a longitudinal wave as a defect detection method for a steel billet. However, it took a long time to measure the TOFs because the transmissions were made one by one from the requirement of independent signal transmission. In this study, to speed up the TOF measurement, we proposed a simultaneous measurement method of TOFs using the phase-modulated signals by Gold sequences, and evaluated the ability of simultaneous measurement by an experiment. The reflected wave from the billet surface had a very adverse effect on the measurement of TOF, so a short signal was required as the transmitted signal. To make the transmitted signal short, a half-sine pulse phase-modulated by a Gold sequence was employed. As a result, five simultaneous transmissions were possible to be used for the inspection of the billet. When five simultaneous transmissions are made, the total measurement time can be decreased to 1/5 of the previous one.

A tubular piezoelectric vibrator gyroscope

This paper proposes a vibrator gyroscope made of a piezoelectric tube, which has the same configuration as that used for the tri-axial sensors and actuators developed in our previous paper. The gyroscopic operation is the same as a circular rod, but the polarization (in thickness direction of the shell) and, accordingly, the electrode arrangement are much simpler. Wireless LAN arrangement is devised for remote data access with which the measurement is possible for the gyroscope under rotation. The experimental results are compared with the numerical simulation with three-dimensional finite-element calculation. The discrepancy between the measured and the experimental is found to depend on the asymmetrical deformation of the structure, and the cause is clearly demonstrated via simulation. This shows the usefulness of the numerical modeling to investigate the cause, in which the parameters of dimensions and boundary conditions can easily change.

Application of ultrasonic computerized tomography using time-of-flight measured by transmission method to nondestructive inspection for high-attenuation billets

For the nondestructive inspection of steel billets, the pulse echo method is generally employed in their manufacturing process. However, for high-attenuation billets, the pulse echo method is useless owing to the low echo level, while the transmission method is expected to detect defects inside high-attenuation billets. In this study, we consider the effectiveness of ultrasonic computerized tomography (CT) using time-of-flight (TOF) measured by the transmission method compared with the pulse echo method. From the simulation and experimental results, even though it was difficult for the pulse echo method to detect defects inside high-attenuation billets, the defects could be visualized by ultrasonic CT using TOF measured by the transmission method. Therefore, ultrasonic CT using TOF measured by the transmission method is more effective for detecting defects inside high-attenuation billets than the pulse echo method.

Acoustic communication in air using differential biphase shift keying with influence of impulse response and background noise

We describe short-range acoustic communication in air using differential biphase shift keying (DBPSK). In this system, a carrier recovery circuit is unnecessary, since the differential encoding is adopted, and the system configuration is simple. The acoustic transmission channel can be simply constructed of only a loudspeaker (SP) and a microphone (MIC). Acoustic communication in air has three problems that degrade the quality of communication. There are multipath fading by reflected waves, the influence of the impulse response of the SP and MIC and that of background noise. Even when multipath fading is disregarded, the performance is limited by the influence of the impulse response of the SP and MIC and that of the background noise. In this study, the performance limits are experimentally evaluated, and the design guidelines for the one-symbol time and signal-to-noise ratio are obtained for an acoustic DBPSK system. The period of a particularly intense influence of the impulse response of the acoustic system is 0.3 ms or more. If the one-symbol time is set at 0.8 ms, the communication quality becomes stable.