Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-09-17
2003-01-07
Budd, Mark O. (Department: 3729)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S322000, C310S324000, C310S800000
Reexamination Certificate
active
06504289
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to devices using as a transducer element a film of piezoelectric polymer provided with electrodes on its surfaces, and more particularly to a piezoelectric transducer having protuberances for effective excitation of acoustic energy by resonance effect and a method of making such a transducer.
BACKGROUND OF THE INVENTION
Piezoelectric film has many advantages in the applications of transducers, sensors and electroacoustic devices. Piezoelectric polymeric materials include PVDF or PVF
2
(polyvinylidene fluoride), and its copolymer PVDF-TrFE (polyvinylidene fluoride-trifluoroethylene). When PVDF film is curved along the unidirectionally stretched direction and driven by an electrical field such as an AC field, displacement in the length direction is converted into the direction normal to the surface, and acoustic radiation results. This operates to generate an acoustic wave in the air. Using this design, piezoelectric material such as PVDF film can be used to make a loudspeaker or a microphone when the frequency of the generated acoustic wave is below 20 kHz. When the frequency is above 20 kHz, an ultrasound transducer may be formed.
A flat transducer of PVDF film material can also be made. U.S. Pat. No. 6,011,855, issued Jan. 4, 2000 to Selfridge et al., for example, discloses a speaker device for emitting subsonic, sonic or ultrasonic compression waves. The device includes a hollow drum, a rigid emitter plate attached to the drum, and a plurality of apertures formed within the plate which are covered by a thin flat layer of piezoelectric film disposed across the emitter plate. By using a negative or positive biasing pressure, a back cavity to create tension in the PVDF film, the device vibrates in substantially uniform fashion. However, such a structure and method has significant limitations, including requiring constant application of pressure differential in order to deform the PVDF material and to operate the transducer.
The beam angle of an ultrasonic transducer is controlled by the dimensions of the active area and the drive frequency of the transducer. The resonance frequency of a curved PVDF film (such as PVDF film curved into a partial cylinder, cylinder or sphere) depends on the curvature as well as the material properties of the PVDF film. For the same film, the frequency is higher when the radius of the curvature is small, and lower when the radius is larger. For a loudspeaker resonant at 1 kHz, the radius is about 20 cm. For an ultrasound transducer of 40 kHz, the radius is about 4-5 mm. Thus, it is important to design and control the curvature radius.
Curved PVDF may be used for a variety of applications, such as audio speakers and microphones and for ultrasound transducers with frequencies up to 180 kHz. For the application of omnidirectional ultrasound transducers, the resonance frequency of conventional transducers is limited by the radius/diameter of the film cylinder. For example, as depicted in
FIG. 4D
, transducer
90
having a frequency f
1
of 40 kHz has a cylinder diameter D
1
of about 1 cm. With a height H
1
of about 1 cm, the active radiating area is about 3 cm
2
. As illustrated in
FIG. 4E
, for a transducer
90
′ with a frequency f
2
of 100 kHz or higher, the diameter D
2
of the cylinder would be significantly smaller than D
1
(approximately 0.4 cm) and the radiating area would be smaller than 1.2 cm
2
. However, it is extremely difficult to fabricate such small PVDF film cylinders and ultrasound pressures resulting from their operation are similarly diminished due to the small radiating area.
Moreover, for a partial cylinder or sphere, the PVDF film needs to be supported by an annular/linear supporter in order to obtain the curved shape. In order to efficiently transmit an acoustic wave into the air, the edges of the film need to be rigidly clamped. Such ultrasonic transducers and methods of manufacture are illustrated in commonly assigned co-pending U.S. patent application Ser. No. 09/281,247 filed on Mar. 30, 1999 entitled “Omni-directional Ultrasonic Transducer Apparatus and Staking Method” and Ser. No. 09/281,398 filed on Mar. 30, 1999 entitled “Omni-directional Ultrasonic Transducer Apparatus Having Controlled Frequency Response”, the contents of which are incorporated herein by reference. The requirement of the support to the film and clamping of the edges adds more complexity to the design and manufacturing cost of these types of transducers. For ultrasound transducers, this problem becomes more severe due to the small radius of the curvature and small dimension of the transducer.
A transducer which overcomes the above problems, including frequency limiting on the whole cylinder design and mechanical clamping in the partial cylinder designs, as well as a method of making such a curved PVDF film airborne transducer which has a resonance frequency which is not influenced by thickness, cylinder diameter, or shape is highly desired.
SUMMARY OF THE INVENTION
A method of forming a transducer having thermoformed protuberances comprises providing a substrate having a plurality of perforations of a given dimension. The perforations or apertures are formed in the substrate and operate to determine the resonance frequency associated with the transducer. A film of polymeric material capable of showing piezoelectric properties is then laminated onto the substrate. The film is heated to a given temperature and a pressure differential is applied between the top and bottom surfaces of the film for a sufficient time to form protuberances in the film through the apertures in the substrate. The film is then cooled to a given ambient temperature while maintaining a pressure differential between the top and bottom surfaces of the film for a sufficient time until the ambient temperature is reached so as to cause the protuberances to maintain a substantially permanent curvature.
An alternative method is to apply pressure by rubber type pliable material instead of air pressure for forming an ultrasound transducer having protuberances comprises providing a substrate having a plurality of perforations of a given dimension. The perforations or apertures are formed in the substrate and operate to determine the resonance frequency associated with the transducer. A film of polymeric material capable of showing piezoelectric properties is then laminated onto the top of the substrate. A top plate layer is disposed at a top surface of the film and a bottom plate layer at a bottom surface of the substrate. A layer of pliable material is disposed between the top plate layer and the top surface of the film. The top and bottom plate layers are then compressed toward each other, via a press for example, to cause the layer of pliable material to forcibly engage the film to cause deformation of portions of the film through the perforations in the substrate, defining the protuberances.
A transducer apparatus such as an ultrasonic transducer comprises a substrate having an array of apertures respectively formed therein at predetermined positions on the substrate such that the array forms a given area within the substrate; a layer of polymeric piezoelectric material is disposed on the substrate, the layer of piezoelectric material has a plurality of protuberances each being defined by a respective portion of the piezoelectric material extending into a corresponding one the apertures, the plurality of protuberances being permanently formed and defining an active area of the transducer corresponding to the given area of the substrate; wherein a resonance frequency of the transducer is a function of a shape of the protuberances as determined by at least one dimension of the apertures, and wherein a vertical and horizontal beam angle associated with the transducer is controllable as a function of the active area of the transducer.
A flat ultrasonic transducer comprises a flat substrate having a plurality of apertures formed therein; a layer of polymeric piezoelectric material disposed on the flat substrate, the layer of pi
Park Kyung-Tae
Swan Jeffrey D.
Toda Minoru
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