WO2005017965A2

WO2005017965A2 - Ultrasonic air transducer arrays using polymer piezoelectric films and impedance matching structures for ultrasonic polymer transducer arrays

Info

Publication number: WO2005017965A2
Application number: PCT/US2004/025189
Authority: WO
Inventors: Minoru Toda
Original assignee: Measurement Specialities, Inc.
Priority date: 2003-08-06
Filing date: 2004-08-05
Publication date: 2005-02-24
Also published as: WO2005017965A3

Abstract

An ultrasonic transducer array structure (100) including: a first lattice structure (120) including a first plurality of arcuated (122) and clamping thin walls (126); a second lattice structure (133) including a second plurality of arcuated (134b) and clamping thin walls (136a); and a polymer piezoelectric film (110) held between the first (120) and second (133) lattice structures, the arcuated (122, 134b) and clamping thin walls (126, 136a) of the first (120) and second (133) lattice structures forming the film (110) into an array of curved transducer elements.

Description

ULTRASONIC AIR TRANSDUCER ARRAYS USING POLYMER PIEZOELECTRIC FILMS AND IMPEDANCE MATCHING STRUCTURES FOR ULTRASONIC POLYMER TRANSDUCER ARRAYS CLAIM FOR PRIORITY This application claims the benefit of U.S. Provisional Application No. 60/493,035, filed August 6, 2003 and is incorporated herein by reference. FIELD OF THE INVENTION

[0001] The present invention relates to ultrasonic transducers. More particularly, the present invention relates to ultrasonic air transducer arrays utilizing polymer piezoelectric films and impedance matching structures for ultrasonic polymer transducer arrays.

BACKGROUND OF THE INVENTION

[0002] When a polyvinylidene fluoride (PNDF) film is formed in cylindrical shape and clamped at two ends, the film can be resonated at an ultrasonic frequency ranging between 20 KHz and 500 KHz depending on the curvature radius, to radiate ultrasonic waves in air.

[0003] A PNDF film-based ultrasonic transducer for high directional speaker applications (ultrasonic wave is modulated by audio signal and audible sound is heard by ear via demodulation due to non-linearity in propagation), requires a large transducer area to obtain sufficient acoustic pressure and broadband resonance performance. Because of material costs, only PVDF film-based transducers serve this purpose. More specifically, ceramic-based transducers have too narrow a bandwidth because of sharp resonance. Moreover, ceramic-based transducers have cost problems and the audio reproduction quality of ceramic-based transducers is poor.

[0004] Large area PVDF-film-based transducers can be formed by multiple sections of curved PVDF film. The most important requirements for curved and clamped PVDF film transducer structures are forming an accurate curvature and forming a good, uniform clamp. Another important requirement is matching the thermal expansion between PVDF film and the holder material.

[0005] Regarding the curvature formation requirement, experimental testing of a single- element, curved, clamped PVDF film transducer, shows that a slight deformation in the curvature of the film, i.e., a curvature which deviates from an ideally curved surface, causes a significant reduction of acoustic output. Therefore, as shown in FIG. 1 A, the curvature of the film 10 has to be formed by supporting the film 10 with cylindrical surfaces 12a, 12b located at two regions on an axis A-A' along a cylindrical surface 12 as shown in FIGS. lA-lC. The film curvature is formed by two supporting surfaces with the correct curvature and clamps.

[0006] The clamp regions each have a certain area for holding the film 10. The clamp regions should each form a cylindrical surface 14a, 14b consistent with film curvature as shown in FIG. IB, or each form a flat surface 14a', 14b'that is tangential to the cylindrical surface 12 at the boundary line L between free film and clamp as shown in FIG. lC.

[0007] The film surface should be cylindrical as close as possible to an ideal one, or smoothly curved. Any deformation, such as a dent, a pleat or a wrinkle, will cause a reduction of acoustic output.

[0008] This principle of an ideal cylindrical curvature has to be applied to transducer arrays which utilize multiple PVDF-film transducer elements.

[0009] Regarding the uniform clamp requirement, in a mass production process, it is desirable to eliminate the use of an adhesive, such as an epoxy or other bonding material, as a clamp. In addition, when two clamp materials are compressed and the PVDF film is sandwiched between them, the clamping is not uniform because the surfaces of the two clamp materials are usually not ideally smooth and do not exactly match one another, therefore, only a certain small region defined therebetween compresses the PVDF film. This problem is particularly severe when the clamp materials are stiff and the film is stiff. To improve uniformity of the clamping pressure, it is desirable to use a rubber clamp material on one side of the clamp.

[00010] PVDF has thermal expansion coefficient of 119 ppm/ °C in the direction perpendicular to the stretched (machined) direction, and 25 ppm/ °C in the stretched direction. The clamp is applied along a line perpendicular to the stretched direction of the curved film, to which direction, the PVDF film has high thermal expansion coefficient. When thermal expansion of the clamp material is much lower than that of the film (119 ppm/ °C), a high temperature causes greater expansion of the film than the clamp material, therefore, causing the film to become pleated or wrinkled. In this condition, the film buckles and permanent deformation is formed along the clamp. When the temperature returns to room temperature, this deformation is visible and looks very ugly. Moreover, the performance of the transducer is degraded.

[00011] Generally ultrasonic transducers have a higher mechanical impedance (i.e., a stiff, heavy, and large force has to be applied to obtain a vibration motion) than that of the propagation medium, which is typically air or some other gas. Accordingly, ultrasonic transducers have a large vibrational force, however, this force is not used and its displacement is small and not enough to create strong acoustic waves. In other words, acoustic wave excitation in air or gas requires a large displacement but, does not require a large force.

[00012] Therefore, ultrasonic transducers consume large amounts of energy to vibrate but, the radiated acoustic wave is not strong, and the efficiency of energy transfer to air is low. Such impedance mis-matching of the transducer in air or gas is a common problem. In the past, this problem has been typically addressed by inserting layers having an impedance of an intermediate value between the transducer and the propagation medium to increase energy transfer.

[00013] A very well known impedance matching concept involves inserting a material having an impedance of a geometrical mean value of the transducer and air, with the thickness of the inserted layer being set a quarter of its wavelength.

[00014] Although, this concept works very well with a water or liquid propagation medium, it does not work well for air or gas propagation mediums. The reason is that the impedance of air or gas is too small and it is difficult to synthesize material with the appropriate impedance.

[00015] Another well known impedance matching structure is a horn. Traditionally, a horn was used for musical instruments or loudspeakers. A horn is basically a hole of a small size communicating with an aperture of a large size from which acoustic radiation takes place. High acoustic pressure existing at the source region (high impedance) is gradually converted to low acoustic pressure towards the exit area (low impedance) and the impedance of the vibration source matches to that of free air. [00016] N.H. Fletcher and S. Thwaites (Ultrasonics, vol. 30, no. 2, pp. 67-75 1992, WO9118486, EP 00528910 and AU 786491) applied a multi-horn structure to a piezoelectric ultrasonic air transducer, where, in front of the transducer, a plate was proximately arranged in which multiple of horns were formed by chemical etching. However, it is very difficult to form multiple holes having diameters gradually changing from the inlet region to the exiting region in controllable fashion.

[00017] Generally, it is widely believed that if any structural material is arranged at the front of a ultrasonic transducer, it causes a scattering or shading effect to wave propagation and reduces the transmitted output. The same is true for a receiving transducer. This concept comes from experience with optics or a light source, wherein light transmission is reduced by placing an obstruction formed by a non-transparent material, in front of the light source. This is true because the wavelength of light is much smaller than the size of obstructing structure, and light waves simply cannot go through the obstructing structure.

[00018] If the size of the obstructing structure is smaller than the wavelength, the transmission reduction does not necessarily take place, because the waves can go around the small obstructing structure. This is the concept of diffraction, which is well known in optics. The size of the impedance matching structure discussed in the present invention is comparable or smaller than the wavelength and it does not simply shade, but reflection from the obstructing structure plays an important role.

[00019] When the radiation source is in a propagation medium, the radiation source sees a certain impedance of the propagation medium. When the distance and size of the obstructing structure is appropriately adjusted, the reflection from the obstruction influences the impedance of the propagation medium so that it is possible to match the source impedance to the propagation medium. This is also true for a detector or receiver. There are many examples of microwave waveguide components where impedance matching component is composed of metallic structures. The microwave or electromagnetic wave does not propagate into metal, but the size of the metallic structure is smaller than or comparable to the wavelength. An example is a stub tuner. A metallic post is inserted into waveguide from the wall (normal to the wall surface), and by adjusting the depth of the insertion and the position of location of the stub, impedance matching is performed and the output from source increases, or receiving signal from a detector increases for an appropriate design.

[00020] In the field of ultrasonics, the design theory of matching by an obstructing structure was not known until recent days and, therefore, this principle itself is not widely known. This principle is now discussed in an article entitled "New Type of Matching Layer for Air-Couples Ultrasonic Transducers", IEEE Transaction of Ultrasonics Ferroelectrics and Frequency control vol. 49 No. 7 July 2002 by Minoru Toda, where experimental data and design theory are presented. Toda describes an impedance matching structure formed by a plate with a plurality of holes and a transducer, which was resonated at 40 KHz and made from PZT, that was bonded to the backside of a metallic plate (e.g., aluminum). The optimum condition involving the position of *be plate from transducer surface, the size and the density of the holes (passage rate) and the thickness of plate are described in Toda, but these are different for different frequencies, and also, the best condition is different for different transducers with different impedances. Therefore, the best condition can be found only by calculations or experimental testing and there are no easy methods for finding the best condition. In the frequency range of 30-200 KHz, the thickness of plate may be in a range of about 0.1 mm to a few mm, the passage rate may be 10% to 50%, and the position of plate may be 0 mm to 0.5 mm from the surface of the transducer. The optimum values become smaller for higher frequencies.

[00021] One reason why prediction of the best condition is difficult is because, for higher frequencies, the mechanical impedance of the transducer becomes higher and, therefore, the optimum values of the position, the thickness of plate and the passage rate, are not only the function of wavelength.

SUMMARY OF THE INVENTION

[00022] An ultrasonic transducer array structure including: a first lattice structure including a first plurality of arcuated and clamping thin walls; a second lattice structure including a second plurality of arcuated and clamping thin walls; and a polymer piezoelectric film held between the first and second lattice structures, the arcuated and clamping thin walls of the first and second lattice structures forming the film into an array of curved transducers elements. BRIEF DESCRIPTION OF THE FIGURES

[00023] Understanding of the present invention will be facilitated by consideration of the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which like numerals refer to like parts, and:

[00024] FIGS. 1 A - 1C illustrate a film with cylindrical surfaces;

[00025] FIGS. 2, 2A and 2B illustrate an exemplary embodiment of a CCUTA structure according to an aspect of the present invention;

[00026] FIG. 2C illustrates an exemplary embodiment of a CCUTA structure according to an aspect of the present invention;

[00027] FIGS. 3 and 3 A illustrates an exemplary embodiment of a CPFUTA structure according to an aspect of the present invention;

[00028] FIGS. 4, 5, and 5A collectively an exemplary embodiment of a CPFUTA structure according to an aspect of the present invention;

[00029] FIG. 6 illustrates a view of a corrugated device according to an aspect of the present invention;

[00030] FIGS. 7 and 7 A illustrate an embodiment of an impedance matching structure according to an aspect of the present invention;

[00031] FIG. 7B illustrates an ultrasonic polymer transducer array according to an aspect of the present invention;

[00032] FIGS. 7C and 7D collectively illustrate an ultrasonic polymer corrugated transducer array according to an aspect of the present invention;

[00033] FIGS. 8 and 8 A illustrate an embodiment of an impedance matching structure according to an aspect of the present invention; [00034] FIG. 8B illustrates an ultrasonic polymer transducer array according to an aspect of the present invention;

[00035] FIGS. 9 and 10 illustrate a film and backing according to aspects of the present invention;

[00036] FIGS. 11 and 12 illustrate systems according to aspects of the present invention

DETAILED DESCRIPTION

[00037] It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for the purpose of clarity, many other elements found in typical ultrasonic air transducer arrays and impedance matching structures, and methods of using the same. Those of ordinary skill in the art may recognize that other elements and/or steps are desirable and/or required in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein.

[00038] An aspect of the present invention is a large area, curved, clamped ultrasonic air transducer array (CCUTA) structure. Referring collectively to FIGS. 2, 2A, and 2B, an exemplary embodiment of the CCUTA structure of the present invention, denoted generally by reference numeral 100, includes a polyvinylidene fluoride (PVDF) piezoelectric film held 110 or sandwiched between first and second lattice structures, 120, 130. First lattice structure 120 includes a first peripheral holding frame 121 with arcuated side walls 122 (only one is shown) and non-arcuated end walls 123 (only one shown). The arcuated side walls 122 have periodically arcuated inner surfaces 122a formed by a plurality of curved surfaces 122b, which each curve in a first direction (concave). The non-arcuated end walls 123 have flat, end wall inner surfaces 123 a. A plurality of laterally-spaced, arcuated thin walls 124 extend parallel to one another between the non-arcuated end walls 123 of the first peripheral holding frame 121, and a plurality of laterally-spaced, clamping thin walls 126, which are perpendicular to arcuated thin walls 124, extend parallel to one another between the arcuated side walls 122 of the first peripheral holding frame 121. The arcuated thin walls 124 have periodically arcuated inner surfaces 124a formed by a plurality of curved surfaces 124b, which each curve in the first direction. The clamping thin walls 126 have flat inner (clamp) surfaces 126a, which extend between the curved surfaces 124b of the arcuated thin wall inner surfaces 124a at junctures 127, thereby forming a plurality of rectangular units 128. It may be noted that the position of the rubber may be at a convex position where the film is clamped. During the assembly process, while the first and second lattices are positioned closer and closer, the film may slipped and be clamped only at the final stage.

[00039] The second lattice structure 130 includes a second peripheral holding frame 131 with arcuated side walls 132 (only one is shown) and non-arcuated end walls 133 (only one shown). The arcuated side walls 132 have periodically arcuated inner surfaces 132a formed by a plurality of curved surfaces 132b, which each curve in a second direction (convex) complementary to the first direction (concave). The non-arcuated end walls 133 have flat, end wall inner surfaces 133a. A plurality of laterally-spaced, arcuated thin walls 134 extend parallel to one another between the non-arcuated end walls 133 of the second peripheral holding frame 131, and a plurality of laterally-spaced, clamping thin walls 136, which are perpendicular to arcuated thin walls 134, extend parallel to one another between the arcuated side walls 132 of the second peripheral holding frame 131. The arcuated thin walls 134 have periodically arcuated inner surfaces 134a formed by a plurality of curved surfaces 134b, which each curve in the second direction. The clamping thin walls 136 have flat inner surfaces 136a, which extend between the curved surfaces 134b of the arcuated thin wall inner surfaces 134a at junctures 137, thereby forming a plurality of rectangular units 138. The bottoms of the rectangular units 138 are closed by backwall 139.

[00040] The PVDF film 110 is clamped between rubber strips 140 running along the flat inner surfaces 136a of the clamping thin walls 136 of the second lattice structure 130 and the flat inner surfaces 126a of the clamping thin walls 126 of the first lattice structure 120. This forms the PVDF film 110 into an array of upwardly curved transducer elements. In an alternate embodiment, as shown in FIG. 2C, the rubber strips 140 run along the flat inner surfaces 126a of the clamping thin walls 126 of the first lattice structure 120, and the PVDF film 110 is formed into an array of downwardly curved transducer elements. Electrodes (not shown) are attached in known manner onto the surfaces of the PVDF film 110.

[00041] The stretched direction of PVDF is parallel to the arcuated walls 122, 124, 132, 134, of the first and second lattice structures 120, 130 and perpendicular to the non-arcuated walls 123, 126, 133, 136 of the first and second lattice structures 120, 130.

[00042] The thickness of the thin walls 124, 126, 134, 136 has to be as thin as possible but not too thin to be fragile, typically, 1-2 millimeters (mm). If the wall thickness is too great, the thin walls 124, 126, 134, 136 occupy too much space and the total device becomes excessively heavy and large, and a greater area of PVDF film is necessary. [00043] The side and end walls 122, 123, 132, 133 of the first and second peripheral frames 121, 131 are heavier and thicker. It is desirable to use thermal expansion matched material for the peripheral frames 121, 131 and thin walls 124, 126, 134, 136, including, without limitation, cellulose acetate, vinylidene chloride, polybutylene, acrylic, polypropylene, epoxy nylon, silicone plastic, etc.

[00044] When the PVDF film 110 is excited by a voltage, ultrasound signals are generated from the front and back surfaces. The backwall 139 of the second lattice structure 130 suppresses the back wave so that it does not propagates to the front side and interfere with the main front wave. The material for the backwall 139 may be a stiff, heavy, or absorptive material such as metal, plastic, wood, or wrinkled tissue paper, when the frequency is high (i.e. greater than 20 KHz).

[00045] Another aspect of the present invention is a large area, corrugated PVDF film ultrasonic transducer array structure (CPFUTA) structure. The purpose of CPFUTA structure is to provide a transducer that can be easily mass-produced with high accuracy and high reproducibility of film curvature. Referring collectively to FIGS. 3 and 3 A, an exemplary embodiment of the CPFUTA structure of the present invention, denoted generally by reference numeral 200, includes a PVDF film held 210 or sandwiched between first and second lattice structures, 220, 230, such that the film 210 is accurately maintained in a corrugated shape. First lattice structure 220 includes a first peripheral holding frame 221 with side walls 222 (only one is shown) and end walls 223 (only one shown). A plurality of laterally-spaced, thin walls 224 extend parallel to one another between the end walls 223 of the first peripheral holding frame 221. The thin walls 224 define wavy inner surfaces 225 formed by alternating concave wave surfaces 226 and convex wave surfaces 227.

[00046] The second lattice structure 230 includes a second peripheral holding frame 231 with side walls 232 (only one is shown) and end walls 233 (only one shown). A plurality of laterally- spaced, thin walls 234 extend between the end walls 233 of the second peripheral holding frame 231. The thin walls 234 define wavy inner surfaces 235 formed by alternating convex wave surfaces 236 and concave wave surfaces 237 which are respectively complementary to the alternating concave wave surfaces 226 and convex wave surfaces 227 of the wavy inner surfaces 225 of the thin walls 224 of the first lattice structure 220. The bottom of the second lattice structure 230 may be closed by a backwall 239.

[00047] The thin walls 224, 234 of the CPFUTA structure 200 extend parallel to the stretched direction of PNDF film 210. Electrodes (not shown) are attached in known manner onto the surfaces of the PVDF film 210. In the case of metallized (e.g., by sputtering) PVDF, the resistivity of the metallic surface electrode is generally high, and the electrical connection is through at least one of the thin walls 224. A narrow strip region of silver ink (very low resistivity) is deposited on the metallized surface of the PVDF film 210, and the thin wall 224 utilized for electrical connection, contacts the silver ink region. Preferably, the silver ink region is underneath the connecting thin wall 224. Since silver ink absorbs vibration, the majority of the surface of the PVDF film 210 should be coated by the thin metallic layer.

[00048] When the PVDF film 210 is excited by a voltage, ultrasound signals are generated from the front and back surfaces. The backwall 239 of the second lattice structure 230 suppresses the back wave so that it does not propagates to the front side and interfere with the main front wave. The material for the backwall 239 may be a stiff, heavy, or absorptive material such as metal, plastic, wood, or wrinkled tissue paper, when the frequency is high (i.e. greater than 20 KHz).

[00049] In contrast to the CCUTA structure 100 described earlier, the CPFUTA structure 200 does not utilize clamping thin walls extending perpendicular to the thin walls 224, 234 with the wavy inner surfaces 225, 235.

[00050] The CPFUTA structure 200 is different from the CCUTA structure 100. The first half cycle and next half cycle are smoothly connected (continuous delivative) and form one cycle. The next cycle repeats exactly the same shape and so on, therefore, forming a shape substantially identical to a wave.

[00051] In the CPFUTA structure 200, it is not necessary for each opposing pair of complementary wavy surfaces 225 and 235 to clamp the film 210 with a strong force. More preferably, it is desirable to hold apart each opposing pair of complementary wavy surfaces 225 and 235 by a small gap G, as shown in FIG. 3A, which, for example, can be about lOOum. The purpose of the gap G is to form an accurate film shape, but not to clamp it. In particular, when gap G is filled by an adhesive, a slight temperature rise (e.g. ΔT = 4 °C) causes expansion of the film 210 in the direction perpendicular to the stretched direction which distorts the film 210 and cause unsightly buckling. This problem can be very critical in high power device applications, where the film is easily heated. Accordingly, it is desirable to provide the small gap G between each opposing pair of complementary wavy surfaces 225 and 235. Gap G may be a little larger than the film thickness, for example.

[00052] At a certain high temperature, a 30um thick PVDF film expands in the gap G and the thermal expansion of peripheral holding frames 221, 231 and movement of the thin walls 224, 234 (by thermal expansion but with a different value) do not stress the film. Therefore, any material can be used for holder frames 221, 231 and the thin walls 224, 234 of the first and second lattice structures 220 and 230.

[00053] As described earlier, the CPFUTA structure of FIGS. 3 and 3 A, preferably includes a small gap G between each opposing pair of complementary wavy surfaces 225 and 235 to maintain an accurate film shape. However, when the film is held with a small gap G, film vibration (e.g. at 40 KHz) may cause an audible sound around 3000-4000 Hz with random or discontinuous timing. This sound is generated by soft touching of the film 210 to some portion of the wavy surfaces 225, 235 because the film 210 is vibrating and the touching receives a repulsion force from the touched solid, which causes a slight deformation of the film. The deformation of the film recovers with a much longer period of vibration and then the film 210 touches some portion of the wavy surfaces 225, 235 again.

[00054] Thus, FIGS. 4, 5, and 5A collectively show an alternative embodiment of the CPFUTA structure of the present invention, denoted generally by reference numeral 200', which solves the film touching problem described immediately above. The CPFUTA structure 200' is substantially identical to the previous CPFUTA structure 200 except, the CPFUTA structure 200' includes a PVDF film 210' having laterally spaced, thin, narrow strips of metal material 215, such as aluminum, bonded to opposing sides of the film 210' as shown in FIG. 4. When the PVDF film 210' is placed between the first and second lattice structures 220' and 230', as shown in FIGS. 5 and 5 A, each aluminum strip pair 215 is formed into a corrugated shape, along with the PVDF film 210'.

[00055] The height and period of the corrugated aluminum strip pairs 215 are made to exactly match the height and period of the wavy surfaces 225', 235' of the thin walls 224', 234', and the aluminum strip enforced corrugated PVDF film 210' is held between the wavy surfaces 225', 235 'as shown in FIG. 5A. The vibration of the aluminum strips 215 is much less than that of the film 210' such that the CPFUTA structure 200' does not make the earlier described touching noise.

[00056] The corrugation shape of the PVDF film 210' can be formed by first annealing the aluminum strips 215 at a temperature of about 600 °C, which is below the aluminum strips melting temperature of about 660 °C. After, annealing, the yield point for elasticity becomes very low and the elastic property of the strips 215 is lost such that the strips 215 act non- elastically, as if they were made from lead. The strips 215 are each made flat by pressing them between two flat plates. The strips 215 are then bonded to the PVDF film 210' as shown in FIG. 4. The bonding material may be for example, epoxy or cyano-acrylic.

[00057] The aluminum strip enforced PVDF film 210' is placed between the wavy surfaces 225', 235' of the first and second lattice structures 220' and 230', which are used as shape formers. The corrugation shape is sequentially formed from one side of the PVDF film 210' to the other so that the shape of the corrugation exactly matches with the shapes of the wavy surfaces 225', 235'.

[00058] Referring to FIG. 6, note that, generally, a corrugated device with a large surface area cannot be ideally flat, i.e., where flat is defined as planes PI and P2 defined by the top and bottom surfaces of the corrugated surface S. Since the wavelength is 8 mm at 40 KHz, if the flatness deviates by 4 mm, the acoustic wave coming out from the deviated region does not effectively add to the acoustic total power, and instead, cancels the power because 4 mm is half of the wavelength. Therefore, the corrugated surface S has to be flat within an error of 1-2 mm (for a 40 kHz device for example), which is a very difficult requirement to achieve. [00059] When the aluminum strip enforced PVDF film 210' of the CPFUTA structure 200' of FIGS. 4, 5, and 5A is held between the first and second lattice structures 220' and 230', the accuracy of the flatness is very good because the aluminum strips 215 deform and follow the shape of the wavy surfaces 225' and 235'.

[00060] Another aspect of the present invention is an impedance matching structure for an ultrasonic polymer transducer or transducer array having a curved piezoelectric polymer film (PVDF) with two ends clamped. Referring now to FIGS. 7 and 7 A, an embodiment of the impedance matching structure of the present invention is shown, denoted by reference numeral 300. The impedance matching structure 300 is shown at the front of a curved film transducer 330 having a dimension H (height) extending in the axial direction AD- AD' of a cylindrical curvature forming member 332. The transducer further includes a PVDF film 334 clamped to the cylindrical curvature forming member 332. The impedance matching structure (IMS) 300 is somewhat similar to an acoustic horn, without actually being a horn. The IMS 300 comprises two block-like members 302 made from a solid plastic or metal material, and may be unitarily formed with the curvature forming member 332 of the transducer 330. The bottom surface 303 of each block member 302 defines a recess 304, such that a narrow space S is formed between the top surface 334a of the curved film 334 and the recess 304 of the block members 302. The two block members 302 define opposing inclined surfaces 305, which together define a V-shaped, elongated slit 306 running parallel to the axial direction AD-AD' of the curvature forming member 332. The slit 306 becomes larger in the acoustic wave propagation direction. According to an aspect of the present invention, the curved plane may be substantially straight, as shown by dotted line 350.

[00061] This IMS 300 is different from conventional multi-horn designs, which have a small inlet with a circular cross-sectional shape and a cross sectional area that becomes gradually larger in the propagation direction while maintaining the generally circular cross-sectional shape. In the IMS 300, the inlet area 306a of the elongated slit 306 has a ratio of length to separation of at least 5 to 1 and typically 20 to 1 or more, and the exiting area 306b has much smaller ratio of length to separation because of the wider separation s. Using the IMS 300 of the present invention, the acoustic pressure output for a 40 KHz transducer 330, measured at a certain distance, improved by 50-100%. The inlet area 306a had a 1-2 mm slit and the space S between the bottom surfaces 303 of the block members 302 and the surface 334a of the curved film 334 was about 0.02 mm to about 0.5 mm.

[00062] FIG. 7B shows an ultrasonic polymer transducer array 330' that utilizes a plurality of IMSs, denoted by reference numeral 300'. Each IMS 300' is basically identical to the IMS 300 of FIGS. 7 and 7A, including two block members 302' with opposing inclined surfaces 305' defining a V-shaped, elongated slit 306' that becomes larger in the acoustic wave propagation direction. The transducer array 330' includes multiple curved PVDF film transducer elements 331'. Such a transducer array would typically be used to produce high acoustic pressures. The multiple curved film transducer elements 331' are connected in parallel and aligned on a flat plane. The plurality of IMSs 300' may be unitarily formed with the film curvature former members 332'. Each IMS 300' provides impedance matching for two or more, curved film transducer elements 331'.

[00063] FIGS. 7C and 7D collectively show an ultrasonic polymer corrugated transducer array 330' ' which utilizes a plurality of EVISs, denoted by reference numeral 300' ' , to increase the acoustic output for certain applications. The transducer array 330' ' includes a backplate 332' ' with corrugation forming members 332a" and a corrugated PVDF film 334" with alternating convex and concave curve portions as described in U.S. Patent 6,411, 015 issued to Minoru Toda and assigned to Measurement Specialties, Inc., the assignee herein. Each IMS 300" is basically identical to the IMS 300 of FIGS. 7 and 7A, including two block members 302" with opposing inclined surfaces 305" defining a V-shaped, elongated slit 306" that becomes larger in the acoustic wave propagation direction. However, the EVISs 300" are combined as a single unitary impedance matching member 333" and include corrugation forming members 332b". The top surface 333a" of the impedance matching member 333" may be wavy as shown in FIG. 7C or flat as shown in FIG. 7D. Again, the plane may be wavier or flat, as is generally designated by reference numeral 350.

[00064] FIGS. 8 and 8 A, show another embodiment of the IMS of the present invention, denoted by reference numeral 400, as utilized with a curved film transducer 430 formed by a curved PVDF film 434 clamped to a cylindrical curvature forming member 432. The IMS 400 includes a curved plate 402 having a constant thickness T. The bottom surface 403 of the curved plate 402 defines a recess 404, such that a narrow space S is formed between the top surface 434a of the curved film 434 and the bottom surface 403 of the plate 402. The plate 402 is curved to complement the curved PVDF film 434 of the transducer 430, so that the space S between the surface of the film 434a and surface 404a of the recess 404 of the curved plate 402 is kept constant.

[00065] The curved plate 402 is provided with a plurality of small openings or slits 406. The openings 406 may be formed in any desired shape. The area of each opening 406 occupies a small percentage of total surface area of plate 402 and should be specified such that the passage rate is through the plate 402 is about 10% to about 50%. The thickness T of plate is typically about 1 mm to about 4 mm and the space S is typically about 0.03 mm to about 0.5 mm for a 40 KHz transducer 430. For other frequency transducers, the values and combinations are different, but generally the thickness T and the space S become smaller with higher frequency transducers.

[00066] FIG. 8B shows an ultrasonic polymer transducer array 430' that utilizes a plurality of EVISs, denoted by reference numeral 400'. Each S 400' is basically identical to the IMS 400 of FIGS. 8 and 8 A, including a curved plate 402' having a bottom surface recess 404' forming a narrow space S. The transducer array 430' includes multiple curved PVDF film transducer elements 431'. The multiple curved film transducer elements 431' are connected in parallel and aligned on a flat plane. The plurality of EVISs 400' may be unitarily formed with the film curvature former members 432'. Each IMS 400' provides impedance matching for two or more curved film transducer elements 431'.

[00067] Referring now also to FIGS. 9 - 11, often the back side of corrugation device has to be closed by a cover plate. If this is a flat plate, a sharp peak may appear on the frequency response curve because of reflection coming back from back plate 910 to the film 920. The path length of the reflections A and B have a difference of — x 2 = λ . At the frequency of this

condition a peak may be formed.

[00068] To eliminate this peak there are two methods. One is to make the reflecting surface 930 of plate 910 non-flat so that reflection has different phase depending on the position. The difference of the height may be ~ quarter of the wavelength (i.e., 2mm for 40KHz). By way of non-limiting example, the shape may take the form of a randomly coarse plane such as a wrinkle of paper or cloth. Another way is to remove back plate so back waves are not reflected.

[00069] When a periodic corrugation PVDF is used for high directivity audible sound generation from ultrasound in air, continuous wave and high voltage drive may be necessary. In such a case, the thickness of PVD has to be thick, for example 110 μm may be used, while, in certain circumstances 28 or 52 μm may not be sufficient. When the thickness is thin, vibration amplitude is deformed from a sinusoidal wave, and the deformed wave has spectrum at a frequency other than main resonance which excites spurious resonance, and output at main resonance is decreased.

[00070] Further, electrode material may influence vibration amplitude. Referring now also to FIGS. 11 and 12, when the electrode 1000 is a thin metal 1100 (500-2000 Angstrom - deposited by sputtering), vibration is higher, and so is acoustic output, as compared to a silver ink electrode (-10 μm). However, thin metal has a high resistivity, such that there may be a problem of too high current density near the lead connection area, where electrode sublimate and thin electrode metal disappears. To solve this problem, a narrow strip region of silver ink 1010 may be deposited, on one or both sides, so that current flows parallel on the surface 1020, and lead wires are connected narrow strips of silver ink electrode. The location of the silver ink 1010 may be chosen to be underneath of wavier plates 1030 forming the film into a wave shape, and thus be less visible.

[00071] Those of ordinary skill in the art may recognize that many modifications and variations of the present invention may be implemented without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is: 1. An ultrasonic transducer array structure comprising: a first lattice structure including a first plurality of arcuated and clamping thin walls; a second lattice structure including a second plurality of arcuated and clamping thin walls; and a polymer piezoelectric film held between the first and second lattice structures, the arcuated and clamping thin walls of the first and second lattice structures forming the film into an array of curved transducers elements.

2. An ultrasonic transducer array structure comprising: a first lattice structure including first plurality of wavy thin walls; a second lattice structure including a second plurality of wavy thin walls; and a polymer piezoelectric film held between the first and second lattice structures, the wavy thin walls of the first and second lattice structures maintaining the film in a corrugated shape.

3. The transducer array structure of claim 2, wherein the film includes a plurality of metallic strips bonded to surfaces of the corrugated film.

4. An impedance matching structure for an ultrasonic transducer, the structure comprising: a first block member having a first surface, the first block member for positioning at a front of the transducer; a second block member having a second surface, the second block member for positioning at the front of the transducer; the first and second surfaces of the block members defining a slit therebetween that becomes larger in an acoustic wave propagation direction.

5. The impedance matching structure of claim 4, wherein the transducer comprises a transducer array.

6. An impedance matching structure for an ultrasonic transducer, the structure comprising: a curved plate for positioning at a front of the transducer; the curved plate having a plurality of openings extending therethrough; and the curved plate having a bottom surface, the bottom surface having a recess, the recess defining a narrow space between a top surface of the transducer and a surface of the recess.

7. The impedance matching structure of claim 6, wherein the transducer comprises a transducer array.