Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Patent
1998-06-15
2000-08-22
Ramirez, Nestor
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
310326, 310334, H01L 4104
Patent
active
061077224
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an ultrasonic transducer having at least one piezoceramic element for emitting and receiving ultrasonic waves in a liquid or gaseous propagation medium, preferably for use in ultrasonic proximity switches.
BACKGROUND INFORMATION
Ultrasonic transducers of the generic type have been disclosed in numerous publications. In using such ultrasonic transducers in ultrasonic proximity switches and distance sensors, there is a need, inter alia, for a narrow sensing characteristic. Objects can be detected specifically thereby, while, as desired, lateral disturbing reflectors, for example, in cramped surroundings or in narrow passages, exert no influence on the functioning of the sensor. This requires ultrasonic transducers which exhibit a very narrow sound lobe characteristic and at the same time have very small minor lobe amplitudes.
German Patent No. 25 41 492 C2 describes the sharply directional ultrasonic transducer in saturn geometry which is most widespread in ultrasonic sensors in industrial use. It is assembled from a piezoceramic disk, an adapting layer disk and a concentric metal ring. The face of the adapting layer is large owing to a wide projection of the adapting layer disk beyond the diameter of the piezoceramic disk, as a result of which a very narrow aperture angle .PHI. of 5.degree. is achieved for the sound lobe. Owing to the large projection of the adapting layer disk, it is difficult in practice always exactly to maintain the favorable vibration amplitude distribution and phase angle over the entire face, the result being problematic lateral minor lobes.
An ultrasonic transducer having a piezoceramic disk and a silicone adapting layer has furthermore been disclosed (see U.S. Pat. No. 3,928,777). The adapting layer has a diameter which is only slightly larger than the piezoceramic disk, the sound lobe opening angle .PHI. consequently being quite large at 10-12.degree.. The minor lobes are so small, with an attenuation of more than -25 dB, that they no longer play a role in practical use. Because of its low chemical resistance, the adapting layer material of silicone cannot be used in all instances of application, and moreover has a strong temperature drift of the mechanical characteristics.
Published International Patent Application No. WO 94/05004 discloses an ultrasonic transducer having a lateral groove in the adapting layer disk. The adapting layer comprises a robust and resistant syntactic foam as in the case of the transducer using saturn geometry. This design has the same minor lobe freedom as the transducer having silicone. Because of its likewise small projection of the adapting layer disk, it also has an equally large sound lobe aperture angle .PHI. of 10-12.degree..
SUMMARY OF THE INVENTION
It is the object of present invention to provide an ultrasonic transducer which has a very narrow main lobe aperture angle and, if possible, at the same time, has minor lobe amplitudes which are so small that in practical application the latter do not exert a disturbing influence on the actual object detection. To be precise, the object detection can be falsified or rendered difficult by the unintended detection of lateral disturbing objects if the minor lobe amplitudes are not kept adequately small.
According to the present invention, this object is achieved by virtue of the fact that the piezoceramic element is joined, at least on one of the two base faces, to a plate-shaped material element, in which the speed of sound is higher than that in the piezoceramic element to form a composite element in which the resonant frequency in the transverse vibration mode is higher than the resonant frequency of the piezoceramic element. The increase resulting therefrom in the resonant frequency of the transverse modes transverse to the direction of the thickness of the plate-shaped composite element by comparison with the piezoceramic element operated in an isolated fashion leads to a narrower sound lobe aperture angle .PHI. when the transducer is o
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patent: 5583293 (1996-12-01), Flogel
patent: 5659220 (1997-08-01), Thurn et al.
L.W. Anson et al., "On the problem of coupling of ultrasonic receivers to layered structures for process control", Ultrasonic, vol. 27 (Nov. 1989) pp. 331-342.
J. Koch, "Piezoxide (PXE)", Herausgebar: Valvo Unternehmensbereich Bauelemente der Philips GmbH, Hamburg, Jul. 1998, pp. 92-113.
Medley Peter
Ramirez Nestor
Siemens AG
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