Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-05-09
2002-06-25
Ponomarenko, Nicholas (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S324000, C310S369000, C310S800000
Reexamination Certificate
active
06411014
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of transducers, and more particularly to cylindrical PVDF ultrasonic airborne transducers.
BACKGROUND OF THE INVENTION
In the environment of ultrasound transducers, it is known that a curved transducer made of a polymer piezoelectric material such as PVDF and clamped at both ends may be used to form an audio or ultrasonic air transducer. Numerous examples of such may be found in the prior art references, such as M. Tamura, T. Yamaguchi, T. Oyabe and T. Yoshimi ‘ELECTROACOUSTIC TRANSDUCERS WITH PIEZOELECTRIC HIGH POLYMER FILMS”, J. Audio Eng. Soc. Vol. 23, No. 1, pp 21-26, (1975); R. Lerch and G. M. Sesler, ‘MICROPHONES WITH RIGIDLY SUPPORTED PIEZOPOLYMER MEMBRANE”, J. Acoust. Soc. Am. Vol. 67, No. 4, pp 1379-81, (1980); Jeff S. Schoenwald and Jim. F. Martin, “PVF2 TRANSDUCERS FOR ACOUSTIC PANNING AND IMAGING IN AIR.”, 1983 Ultrasonic Symposium (IEEE), pp. 577-580; F. Harnisch, N. Kroemer, and W. Manthey, “ULTRASONIC TRANSDUCERS WITH PIEZOELECTRIC POLYMER FOIL”, Sensors and Actuators A25-27, —549-552 (1991); S. Edelman and A. S. DeReggi “COMMENTS ON ELECTROACOUSTIC TRANSDUCERS WITH PIEZOELECTRIC HIGH POLYMER FILMS”, J. Aoudio Eng. Soc vol. 24, No 7, pp. 577-578, (1976); I. Veit “THE PIEZOELECTRIC PVDF-FILM-ITS PROPERTIES AND APPLICATION IN ELECTROACOUSTIC TRANSDUCERS”, Audio Eng. Soc., 84th Convention March 1988 1-4 paris 2604 (G-1); A. S. Fiorillo “DESIGN AND CHARACTERIZATION OF A PVDF ULTRASONIC RANGE SENSOR” IEEE Trans. Ultrasonics, Ferroelectrics and Frequency Control, vol. 39, No. 6, pp. 688-692(1992); R. Lerch, “ELECTROACOUSTIC TRANSDUCER USING PIEZOELECTRIC POLYVINYLIDENE FLUORIDE FILMS.”, J. Acoust. Soc. Am Vol. 66, No. 4, pp. 952-954 (1979); W. Flugge “statik und Dynamik der Schalen” Springer, Berlin 1962; Hong Wang and Minoru Toda, “Curved PVDF Airborne Transducer”, to be published; Leo L. Beranek “Acoustics”, The American Institute of Physics, p. 119. 1986.
Referring now to
FIG. 1A
, there is shown a cylindrical piezoelectric film
54
having its stretched axis wrapped around a cylinder (not shown). Here, when a AC voltage is applied to electrodes
56
on surfaces of the cylindrical film
54
, a lengthwise strain in the curved direction is converted to a film displacement normal to the surface (or vice-verse), due to the cylindrical film structure. Thus, a lengthwise strain in the curved direction is converted to radial vibration. Such a structure can be used as either a transmitter or a receiver with omni-directional angle performance. Depending on the application, often it is necessary to have transducers with limited angle performance (narrower directivity). In such a case, conventional transducers use two end clamped curved film structures as shown in FIG.
1
B. However, in the known application of a curved film with two clamps, two significant problems are present. First, the resonance of the housing
22
on which the clamp is attached, reduces the stiffness of the clamp. One of many resonance modes of the housing structure are often coincident to the main resonance frequency of the curved film. The resonance of the curved film requires a very stiff clamp structure. Control of the housing resonance is thus very difficult and very sensitive to any minor variation of housing dimension, such that the output or sensitivity of each device is not reproducible and entirely non-uniform. Secondly, the thermal expansion coefficient of PVDF film is very high (approximately 120×10
−6
/C, where the metal has a value of between 10×10
−6
/C and 20×10
−6
/C, 10~20×10
−6
/C). At relatively high temperatures (above approximately 45 C. for example) thermal expansion of the film severely deforms the film shape. This is because the clamp material, which has a much lower expansion causing deformation of the film and thereby creating film buckling around the cylinder. Once such buckling occurs, the film shape can not be restored to its original shape, even after the PVDF film is allowed to return to normal ambient temperature conditions.
In order to overcome these problems with clamped transducer structures, a non-clamp structure is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 09/281,398 entitled OMNI-DIRECTIONAL ULTRASONIC TRANSDUCER APPARATUS HAVING CONTROLLED FREQUENCY RESPONSE and co-pending and commonly assigned U.S. patent application Ser. No. 09/281,247 entitled OMNI-DIRECTIONAL TRANSDUCER APPARATUS AND STAKING METHOD disclose such non-clamped structures, the subject matter of which is incorporated herein by reference in their entireties. These documents disclose a non-clamped omni-directional transducer comprising a cylindrical PVDF film wrapped around a spool, where the film is spaced apart from a body portion of the spool to form a gap sized to enable the resonance frequency of the transducer to be controlled by the resonance frequency of the piezoelectric film.
In the above disclosures, the PVDF film included an electrode layer deposited over substantially the entire front surface of the film and a second electrode layer deposited on substantially the entire back surface of the PVDF film, except for the peripheral edges of the film in order to facilitate bonding. The angular performance of acoustic properties of PVDF transducers disposed onto a spool (either clamped or unclamped) is omni-directional. However, depending on the type of application, it is sometimes desirable to limit directivity of the beam angle to within a certain range. It is further desirable in certain instances, such as in the detection of low SNR signals, to obtain a receiver having increased sensitivity for detecting such signals. Still further, certain applications may make use of a wide band transducer having a relatively low Q factor and wider useful common frequency band between transmitter and receiver. A transducer capable of solving the aforementioned problems is highly desired.
SUMMARY OF THE INVENTION
The present invention modifies the structure of a cylindrical PVDF film material to obtain a desired angle performance.
It is an object of the present invention to provide an air transducer apparatus comprising a curved piezoelectric film having a first surface and a second surface opposite the first surface, a first electrode layer disposed on at least a portion of the first surface of the film; and a second electrode layer disposed on a portion of the second surface of the film defining the second electrode layer being sized less than the first electrode layer for generating a signal in response to acoustic energy incident on the piezoelectric film.
It is a further object of the present invention to provide an acoustic receiver comprising a cylindrical piezoelectric film having a first surface and a second surface opposite the first surface, the film responsive to acoustic energy incident thereon for vibrating at a given frequency; a first electrode layer disposed on the first surface of the film; a second electrode layer disposed on a portion of the second surface of the film; and a housing surrounding the piezoelectric film, the housing formed of a material resistant to the propagation of the acoustic waves, the housing having an aperture in alignment with at least a portion of the second electrode covering the film for influencing a receiving beam angle associated with acoustic radiation incident on the piezoelectric film through the aperture.
It is still a further object of the present invention to provide an acoustic transmitter comprising a cylindrical piezoelectric film having a first surface and a second surface opposite the first surface; a first electrode material disposed on the first surface of the film; a second electrode material disposed on the second surface of the film; means for exciting the film to generate omnidirectional acoustic waves at a resonance frequency; and a housing surrounding the piezoelectric film and formed of a material resistant to the propagation of the acoustic waves, the housing having an aperture exposing a
Addison Karen
Duane Morris LLP
Measurement Specialties Inc.
Ponomarenko Nicholas
LandOfFree
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