Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Patent
1991-03-08
1994-09-27
Peckman, Kristine L.
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
29 2535, H04R 3100, H01L 4108, H01L 4122
Patent
active
053509641
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of The Invention
The present invention relates to improvement of an ultrasonic beam in the elevation direction of an ultrasonic transducer and more specifically to the shading of an electromechanical coupling coefficient in the elevation direction of a piezoelectric vibrator of an ultrasonic transducer.
2. Description of The Related Art
In view of improving an ultrasonic beam, namely reducing side lobe level of ultrasonic beam, polarization of arranged vibrators forming an ultrasonic transducer as a piezoelectric material has been lowered or reduced in mass toward the end portion of the transducer from the center in the direction orthogonally crossing the arrangement direction of vibrators (namely, in the elevation direction of ultrasonic transducer, and in the elevation direction of a probe).
FIG. 1(a) indicates an example of such a structure. In this figure, the vertical axis indicates an electro-mechanical coupling coefficient, while the horizontal axis indicates the direction orthogonally crossing the arrangement direction of vibrators forming an ultrasonic transducer as the piezoelectric material (namely, elevation direction of ultrasonic transducer, and elevation direction of probe). In FIG. 1(a), the polarized distribution of coupling coefficient is similar to the Gaussian function. Namely, polarization is carried out so that the distribution of electromechanical coupling coefficient kt (hereinafter referred to as coupling coefficient) of vibrators arranged is gradually reduced as the polarization goes to the end portion of the transducer from the center. An acoustic pressure from the ultrasonic transducer in such polarization is shown in FIGS. 3 (a), (b). FIG. 3(a) indicates the ultrasonic beam irradiating direction on the horizontal axis and elevation direction of arranged vibrators (direction orthogonally crossing the arrangement direction) on the vertical axis. The acoustic beam profiles in the graph respectively show +20 dB, +10 dB, -10 dB, -20 dB. FIG. 3(b) indicates a distribution of an acoustic pressure in the area separated by 140 mm from the arranged vibrators, namley the sectional view of the acoustic pressure at the point corresponding to elevation direction of arranged vibrators separated by 140 mm from the arranged vibrators in FIG. 3(a). The vertical axis of FIG. 3(b) indicates acoustic pressure, while the horizontal axis indicates the elevation direction (direction orthogonally crossing the arrangement) of the arranged vibrators.
FIG. 1(b) indicates an example where the polarization of arranged vibrators is uniform for the elevation direction (without shading). The acoustic pressure graph of acoustic beam profile in this case is shown in FIGS. 4(a), and 4(b). The graphs of FIGS. 4(a), and 4(b) indicate just like FIG. 3(a) and 3(b).
In comparison of these graphs, it is understood that the side lobe level is high when the coupling coefficient is not shaded (comparison in FIG. 3(b) and FIG. 4(b)) and that the-beam is not converged (comparison in FIG. 3(a) and FIG. 4(a)).
As the method (a) for changing polarization of arranged vibrators, a method has been proposed by D.K. Hsu in IEEE shown in FIG. 2 on Oct. 9, 1989 ("IEEE 1989 ULTRASONIC SYMPOSIUM AND SHORT COURSES, PROGRAM AND ABSTRACTS NON-UNIFORMLY POLED GAUSSIAN BESSEL FUNCTION TRANSDUCERS"). First, a piezoceramics 102, which is sufficiently thicker than the desired elevation and has the spherical recessed area at a single side, is manufactured. Next, an Ar/Cr film 105 is evaporated to or placed on both sides of piezoelectric ceramics. A spherical electrode 101 matching with the shape of curvature of the recessed area is provided to the spherically arcuated surface of the ceramic and a flat electrode 104 is provided in the opposite side to the spherically arcuated surface for polarization. The ceramic is polarized. Thereafter, a flat piezoelectric ceramic can be obtained by polishing or cutting the material to the determined elevation t. Thereby, the coupling coefficient can gradually be reduced
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Ultrasonics, vol. 25, No. 2, Mar. 1987, Guildford, GB, "Fabrication of Non-Uniformly Excited Wide-Band Ultrasonic Tranducers", R. H. Brittain et al., pp. 100-106.
ISAF '86 Proceedings of the Sixth IEEE International Symposium on Applications of Ferroelectrics, Jun. 1986, LeHigh University, Bethlehem, Penn., USA, "Composite Piezoelectric Materials for Medical Ultrasonic Imaging Tranducers-A Review", W. A. Smith, pp. 249-255.
Hara Yasushi
Ishikawa Hiroshi
Kawabe Kenji
Matsui Kiyoto
Shimura Takaki
Fujitsu Limited
LaBalle C.
Peckman Kristine L.
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