Preparation Process of a Sparse Sensing Array for a 3D Camera Sonar

Abstract

The present invention provides a preparation process for a sparse sensing array of a three-dimensional camera sonar. The method comprises the following steps: forming patterned electrodes on a PVDF or P(VDF-TrFE) piezoelectric film by means of vacuum adsorption and screen printing according to the coordinates of sparse array elements, and arranging virtual transition array elements at the periphery; laser-etching the edge contour and exhaust holes of the piezoelectric film; designing an adhesive positioning auxiliary tool, combining an ultra-thin double-sided adhesive tape and a single-sided mask tape, and realizing precise electrical connection between the array elements and the PCB pads by using conductive silver paste; designing a polytetrafluoroethylene-metal flattening module, and pressing the array in a vacuum environment to cure the adhesion; using a low-impedance sound-transmitting material for the matching layer, filling glue in batches from the back and removing air bubbles in a vacuum, and completing the preparation of the sparse sensing array in combination with a polyimide film and a metal ultra-flat plate. The present invention solves the preparation problem of a large-aperture high-frequency imaging sonar on a sensing array, and can improve the position accuracy of array elements, the consistency of array elements and the imaging resolution.

Description

Technical Field

[0001] The present invention belongs to the technical field of sonar, and particularly relates to a preparation process of a sparse sensing array for a three-dimensional camera sonar. Background Art

[0002] The three-dimensional camera sonar technology has important application values in the fields of underwater detection, robot navigation, etc. Its core principle lies in constructing a three-dimensional acoustic image of a target object through the transmission and reception of multi-beam ultrasonic signals. The camera sonar adopts a dense array design, and the arrangement form of array elements, the number of array elements, the coordinate accuracy of array elements, the size of the array aperture, and the acoustic wave frequency will all significantly affect the imaging resolution. With the complication of marine application scenarios and the increasing requirements for detection efficiency and accuracy, three-dimensional camera sonars with high frequency (≥300 kHz) and large aperture (≥200 mm) will become one of the necessary detection tools. The sparse sensing array technology is an effective method to meet the development requirements of camera sonars. By optimizing the spatial distribution of array elements, it can maintain or even improve the imaging performance while reducing the number of array elements. However, there are the following technical problems in the preparation process of large-size sparse arrays:

[0003] Insufficient element consistency: Uneven bonding between material layers, changes in boundary conditions, element displacement during operation, etc. lead to inconsistent signal responses, thereby reducing the imaging quality.

[0004] Insufficient flatness: Large-size flexible piezoelectric materials (such as PVDF (polyvinylidene fluoride) films) are prone to wrinkles and bubbles during the bonding process, resulting in the failure of some array elements.

[0005] Limited acoustic performance: High-frequency sonars require a relatively thin acoustic matching layer. It is difficult to use the grinding preparation method for large sizes, and large sizes are prone to bubble residues during the curing process, weakening the signal sensitivity and restricting the water depth range of sonar applications.

[0006] In view of the above problems, the present invention proposes a new technical solution, considering the consistency, high-frequency adaptability, and process accuracy repeatability of large-size sparse sensing arrays, an innovative process integrating material optimization, precision manufacturing, and acoustic design, breaking through the preparation bottleneck of high-frequency three-dimensional camera sonar sparse arrays. Summary of the Invention

[0007] The present invention provides a preparation process for a sparse sensing array of a three-dimensional imaging sonar, especially for a receiving array of an imaging sonar with an array aperture ≥ 200 mm, an element center spacing ≥ 1.5 times the target frequency wavelength, and a frequency ≥ 300 kHz. The piezoelectric material is a PVDF or P(VDF-TrFE) (copolymer of vinylidene fluoride and trifluoroethylene) film. The content of the present invention includes the material selection of the imaging sonar receiving array, the preparation process, and the design of related auxiliary tools. The beneficial effects of the present invention are as follows: effectively solving the problem of difficulty in fabricating ultrasonic array transducers from large-sized flexible materials, enabling both whole-piece manufacturing and piece-by-piece processing, having high interlayer bonding strength, array flatness, and element consistency, which is beneficial to improving the resolution of sonar imaging. The key process content of the present invention is as follows:

[0008] First, calculate the element coordinates of the sparse sensing array according to methods such as Archimedes spiral, Fibonacci spiral, simulated annealing algorithm, etc., determine the element diameter, positioning mark coordinates, parameters of peripheral virtual transition elements, and design a patterned electrode. Use the designed and fabricated micro-hole vacuum adsorption platform to adsorb the large-sized PVDF or P(VDF-TrFE) piezoelectric film flat. According to the patterned electrode, make a high-mesh screen, and screen-print the patterned silver electrode of the sparse elements on the side facing the PCB circuit board. Laser-etch the edge contour and exhaust holes of the PVDF or P(VDF-TrFE) piezoelectric film by the method of machine vision four-point positioning. The exhaust holes are distributed at intervals between the elements, and an angular positioning protrusion is cut out at the film edge.

[0009] Meanwhile, design and fabricate a PCB circuit board with glue-filling notches preset on four sides, ground pads located on the four sides or four corners of the PCB circuit board, the connector of the data acquisition card located on the back side of the PCB circuit board, the circuit using vias to introduce the positive electrode to the back connector, and the vias filled with resin. The connector is provided with anti-crosstalk ground wires for each element.

[0010] Then, laser-etch the ultra-thin double-sided tape and the single-sided mask tape according to the sparse array pattern, with the element positions hollowed out. Bond the side with lower adhesiveness of the ultra-thin double-sided tape to the PCB circuit board first, exposing all the electrode pads corresponding to the elements, then stick the single-sided mask tape on the upper protective layer of the ultra-thin double-sided tape. Immediately apply conductive silver paste evenly on the entire surface of the single-sided mask tape, tear off the single-sided mask tape and the upper protective layer of the ultra-thin double-sided tape, retaining the adhesive layer of the ultra-thin double-sided tape and exposing the conductive silver paste at the element positions.

[0011] Next, use the designed bonding and positioning auxiliary tool to accurately attach the cut PVDF or P(VDF-TrFE) piezoelectric film onto the ultra-thin double-sided adhesive tape. The positions of the array elements are electrically connected to the PCB pads through patterned silver electrodes using conductive silver paste, and the silver paste is cured for ≥24 hours. After curing, except for the areas covered by the PVDF or P(VDF-TrFE) piezoelectric film, use low-tack single-sided masking tape to cover each area of the PCB circuit board. Screen-print conductive silver paste above the piezoelectric film as the back electrode, and the silver paste is cured for ≥24 hours;

[0012] Next, coat epoxy resin on the edges of the PVDF or P(VDF-TrFE) piezoelectric film, place it in a vacuum chamber, and place a flattening module on the PVDF or P(VDF-TrFE) piezoelectric film for ≥12 hours. The flattening module is composed of a laminated upper and lower layer of a polytetrafluoroethylene block and a metal block. After curing, apply epoxy resin at the vent hole positions as well and place it in the vacuum chamber for ≥12 hours. Use conductive silver paste for the back electrode to connect the negative electrode above the piezoelectric film to the ground pad of the PCB circuit board, and install a reinforcing rib at the back.

[0013] Finally, for the matching layer, use the method of one-time potting from the back. The outer frame of the matching layer is provided with a 1 / 4 wavelength thickness positioning slot. A layer of perforated double-sided polyimide tape and a layer of non-adhesive polyimide film are pasted on the outer frame of the matching layer. The matching layer is a sound-transmitting material with high fluidity and low impedance, and the entire sample is placed on a metal super flat plate during vacuum degassing and curing of the matching layer. Pour the low-impedance and high-fluidity epoxy resin into the back of the outer frame in multiple times, and tilt the sample to evacuate the air each time during pouring. Then place the whole in an oven and dry at 40°C for ≥24 hours. After bonding the sound-absorbing material in the reinforcing rib, the production is completed. Description of the Drawings

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for description in the embodiments. Obviously, the following-described drawings are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0015] FIG. 1 is a schematic design diagram of a sparse sensing array of PVDF or P(VDF-TrFE) piezoelectric film;

[0016] FIG. 2 is a schematic cross-sectional view of a sparse sensing array of a camera sonar;

[0017] FIG. 3 is a schematic diagram of an auxiliary tool for preparing the sensing array;

[0018] FIG. 4 is a schematic diagram of a polytetrafluoroethylene-aluminum alloy flattening module;

[0019] Figure 5 is a schematic diagram of the encapsulation of the matching layer of a sparse sensing array of PVDF or P(VDF-TrFE) piezoelectric films.

[0020] Description of the reference numerals in the attached drawings:

[0021] 101 Sparse array sensing element, 102 Vacuum exhaust hole, 103 Virtual transition element, 104 Laser-etched positioning cross, 105 Positioning protrusion, 106 PVDF or P(VDF-TrFE) piezoelectric film, 201 Acoustic matching layer, 202 Ground electrode layer, 203 PVDF or P(VDF-TrFE) piezoelectric layer, 204 PCB circuit board, 205 Acquisition card electrical connector, 206 Sound-absorbing material, 207 Circuit board stiffener, 208 Glue filling port, 209 Adhesive layer of ultra-thin double-sided tape, 301 PCB circuit board positioning fixture, 302 PCB circuit board support module, 303 Film bonding positioning module, 304 Film pressing module, 401 Aluminum alloy module, 402 Threaded connection hole, 403 Hoisting ring screw hole, 404 Polytetrafluoroethylene module, 501 Matching layer housing, 502 Double-sided adhesive polyimide film, 503 Non-adhesive polyimide film, 504 Metal ultra-flat plate, 505 Positioning card slot, 506 PCB circuit board with the piezoelectric layer already prepared. Specific implementation manners

[0022] The present invention provides a preparation process for a three-dimensional camera sonar sparse sensing array. Here, the entire design, manufacturing process, principle, and beneficial effects of a three-dimensional camera sonar sparse sensing array of the present invention will be clearly and completely described in conjunction with the accompanying drawings. When the following description involves the accompanying drawings, unless otherwise indicated, the same numerals in different drawings represent the same or similar elements. The embodiments described are only a recommended embodiment in the present invention, and some structural modifications made within the scope of the present invention should fall within the protection scope of the present invention.

[0023] If there are terms related to directional indication or positional relationship in the embodiments of the present application (such as up, down, left, right, front, back, inside, outside, top, bottom, center, vertical, horizontal, longitudinal, transverse, length, width, counterclockwise, clockwise, axial, radial, circumferential, etc.), such terms are only used to explain the relative positional relationship and movement conditions between components in a specific posture (as shown in the accompanying drawings); if this specific posture changes, then the directional indication or positional relationship will also change accordingly. In addition, terms such as "first" and "second" involved in the embodiments of the present application are only for the purpose of convenient description and cannot be understood as indicating or implying relative importance.

[0024] The distribution form of the sparse elements 101 of a camera sonar sparse sensing array is as Figure 1 shown, and the cross-section of the camera sonar sensing array is as Figure 2As shown in the figure, from top to bottom, it includes: an acoustic matching layer 201, a ground electrode layer 202, a PVDF or P(VDF-TrFE) piezoelectric layer 203, a patterned positive electrode, a virtual transition element 103 at the edge, a PCB circuit board 204, an electrical connector 205 of a data acquisition card, an acoustic absorption material 206, and a circuit board stiffener 207. The entire preparation process of this array is as follows:

[0025] According to the array directivity requirements, methods such as Archimedes spiral, Fibonacci spiral, and simulated annealing algorithm are used to calculate the distribution form of the sparse sensing array, that is, the coordinates of the sparse elements 101. To prevent the acoustic characteristics of the elements at the edge from being significantly different from those of the internal elements and to promote better consistency of the overall elements, a virtual transition element 103 is provided in the outermost circle of the sparse elements 101, which can make the boundary conditions of the elements change more uniformly. The virtual transition element 103 has the same size as the sparse element 101 and a similar preparation process, but it is not connected to the electrical connector 205 of the data acquisition card.

[0026] The sparse element 101 is formed on a PVDF or P(VDF-TrFE) piezoelectric film 106 by using a patterned electrode, as Figure 1 shown, high-mesh screen printing is used. Since the shear wave velocity of the piezoelectric film is much smaller than the longitudinal wave velocity, there is no need for a partition between the sparse elements 101 to reduce acoustic crosstalk. Before printing, a large-size PVDF or P(VDF-TrFE) piezoelectric film 106 is adsorbed and flattened by a microporous vacuum adsorption platform. Since the PVDF or P(VDF-TrFE) piezoelectric film 106 is a flexible material, attention should be paid to the micropore size and spacing of the adsorption platform. The present invention recommends that the micropore size of the vacuum adsorption platform ≤1 mm and the center spacing of the micropores ≥15 mm. After adsorption and flattening, silver electrodes are screen-printed on the side of the PVDF or P(VDF-TrFE) piezoelectric film 106 facing the PCB circuit board 204 to form the sparse element 101 and the virtual transition element 103, and at the same time, laser-etched positioning cross marks 104 are printed at the four corners. To ensure the density and uniformity of the printed electrodes, the mesh number of the screen printing stencil should ≥400 meshes. After screen printing is completed, the sample is dried at ≤60 °C for ≥24 hours, and the temperature should be less than half of the Curie temperature to avoid high-temperature depolarization of the piezoelectric film.

[0027] With the help of the positioning cross 104 and machine vision, the edge contour of the PVDF or P(VDF-TrFE) piezoelectric film 106 and the vacuum exhaust holes 102 are laser-etched, and angular positioning protrusions 105 are cut on the edge of the piezoelectric film 106. The purpose of the vacuum exhaust holes 102 is to evacuate the air between the PVDF or P(VDF-TrFE) piezoelectric film 106 and the PCB circuit board 204 in the subsequent process, so that the two fit more closely. If the number of vacuum exhaust holes 102 is too small, it will affect the discharge of interlayer air bubbles. If too many, it will be easy for air bubbles to enter again. In the present invention, the number of vacuum exhaust holes 102 is slightly ≥ 10% of the number of sparse sensing elements 101, and the diameter of the vacuum exhaust holes 102 ≤ 20% of the element diameter. In the subsequent process, the positioning protrusion 105 can cooperate with the film bonding positioning module 303 to enable the piezoelectric film 106 to be more precisely pasted on the PCB circuit board 204.

[0028] When designing and manufacturing the PCB circuit board 204, glue injection ports 208 should be provided on its four sides. The positive electrodes of all sparse elements 101 are located on one side of the PCB circuit board 204, and the electrical connector 205 of the data acquisition card is located on the back side of the PCB circuit board 204. The PCB circuit board 204 uses through holes to electrically connect the sparse elements 101 to the electrical connector 205 on the back, and the through holes are filled with resin to improve the voltage resistance strength of the structure. The connector is provided with anti-crosstalk ground wires for each sparse element 101.

[0029] According to the patterns of the sparse elements 101 and the virtual transition elements 103, a thin double-sided tape and a single-sided mask tape are cut using a laser etching machine. The cutting holes of the thin double-sided tape are ≥ the diameter of the sparse elements 101, and the cutting holes of the single-sided mask tape are < the diameter of the sparse elements 101. The single-sided mask tape uses a tape with less viscosity, such as a heat-release tape. Before bonding the laser-etched PVDF or P(VDF-TrFE) piezoelectric layer 203, first bond the side with less viscosity of the thin double-sided tape to the PCB circuit board 204, and expose the electrode pads corresponding to all sparse elements 101. Then bond the single-sided mask tape to the upper protective layer of the thin double-sided tape, also exposing all the electrode pads. Apply conductive silver paste on the entire surface of the single-sided mask tape, and by repeatedly applying, the conductive silver paste uniformly penetrates into the electrode pads through the holes in the single-sided mask tape. Then, simultaneously tear off the single-sided mask tape and the upper protective layer of the thin double-sided tape, and retain the adhesive layer 209 of the thin double-sided tape. To ensure the flatness of the sensing array, the thickness of the double-sided tape should ≤ 10um, that is, the thickness of the double-sided adhesive layer is of the same order of magnitude as the thickness of the electrode layer of the sparse elements 101.

[0030] The present invention designs a set of special auxiliary tools for bonding and positioning, such as Figure 3As shown, it includes a PCB circuit board positioning fixture 301, a PCB circuit board support module 302, a thin film bonding and positioning module 303, and a thin film lamination module 304. Place the just-completed PCB circuit board 204 coated with conductive silver paste on the PCB circuit board support module 302. The PCB circuit board support module 302 is machined with grooves corresponding to the electrical connectors 205 of the data acquisition card. Then place the whole into the PCB circuit board positioning fixture 301, and its four sides protrude to limit the circuit board. Next, buckle the thin film bonding and positioning module 303 on the circuit board. There are card slots inside the bonding and positioning module, and the sizes of the card slots match those of the piezoelectric thin film positioning protrusions 105.

[0031] Use the PCB circuit board positioning fixture 301 and the PCB circuit board support module 302 to accurately attach the cut PVDF or P(VDF-TrFE) piezoelectric layer 203 onto the adhesive layer of the ultra-thin double-sided tape. The area without patterned electrodes is bonded through the adhesive layer 209 of the ultra-thin double-sided tape, and the sparse array element 101 positions achieve electrical connection between the patterned silver electrodes and the PCB circuit board pads through conductive silver paste. The adhesive layer 209 enhances the interlayer bonding strength. Immediately cover the piezoelectric thin film with a plastic protective film, place the thin film lamination module 304 above the protective film, and then cure the whole sample and auxiliary tools under a certain pressure for ≥24 hours. Note that the applied pressure should not be too high, as excessive application will cause depolarization of the piezoelectric thin film.

[0032] After the conductive silver paste is cured, except for the area covered by the PVDF or P(VDF-TrFE) piezoelectric layer 203, cover each area at the edge of the PCB circuit board 204 with a low-adhesion single-sided masking tape. Fix the PCB circuit board 204 and the PCB circuit board support module 302, and screen-print conductive silver paste above the piezoelectric thin film as the grounding electrode layer 202 of the back grounding electrode. Cure the conductive silver paste for ≥24 hours.

[0033] After the conductive silver paste is cured, apply epoxy resin glue along the edge of the PVDF or P(VDF-TrFE) piezoelectric layer 203 using a syringe, place the whole into a vacuum chamber and place a polytetrafluoroethylene flattening module on the PVDF or P(VDF-TrFE) piezoelectric layer 203 for ≥12 hours. The vacuum degree of the vacuum chamber should be maintained at ≤10 Pa. The structure of the flattening module is as Figure 4As shown, it is composed of an aluminum alloy module 401 and a polytetrafluoroethylene module 404 connected by screws. The threaded connection hole 402 on the aluminum alloy module 401 is a through hole, and the threaded connection hole on the polytetrafluoroethylene module 404 is a buried hole. The polytetrafluoroethylene material and the epoxy resin material have the characteristic of not adhering to each other, which can prevent the flattening module from adhering to the epoxy resin glue during the pressing and exhaust process. The size of the flattening module is slightly smaller than the size of the piezoelectric film. Combining the polytetrafluoroethylene block with the aluminum alloy can prevent the problem of deformation of the polytetrafluoroethylene block after long-term use. To facilitate the picking and placing of the flattening module, a lifting ring screw hole 403 is provided on the aluminum alloy module 401.

[0034] After the epoxy resin glue at the edge of the PVDF or P(VDF-TrFE) piezoelectric layer 203 solidifies, epoxy resin glue is also dropped onto the vacuum exhaust hole 102 on the surface of the PVDF or P(VDF-TrFE) piezoelectric layer 203, and then placed in a vacuum chamber for ≥12 hours until the epoxy resin solidifies, exhausting the air between the PCB board 204 and the PVDF or P(VDF-TrFE) piezoelectric layer 203. After taking out the sample, the flatness after bonding of the sensing array layer can be effectively ensured by using the internal and external pressure difference between the materials and the external environment. After the epoxy resin cures, it can prevent air from flowing back into the interlayer voids of the film. Use conductive silver paste to connect the ground electrode layer 202 above the piezoelectric film to the ground pad of the PCB board, and install a circuit board reinforcing rib 207 on the back. The reinforcing rib 207 can enhance the structural strength of the sparse sensing array and also block the matching layer liquid from flowing into the connector 205.

[0035] Since the acoustic matching layer 201 is large in size, thin in thickness, and soft in material, it is difficult to use the grinding preparation method. Therefore, the acoustic matching layer 201 adopts the method of backfilling. As Figure 5 shown, a positioning card slot 505 with a 1 / 4 wavelength thickness is provided inside the matching layer housing 501. Before pouring the matching layer liquid, a layer of hollow double-sided sticky polyimide film 502 and a layer of non-sticky polyimide film 503 are first pasted on the bottom surface of the matching layer housing 501, and the whole is installed on the metal ultra-flat plate 504. If the flatness does not meet the requirements, after the polyimide film 503 is evenly adhered to the metal ultra-flat plate 504, it can be scraped flat with a scraper, and then the matching layer housing 501 pasted with the double-sided sticky polyimide film 502 is installed on the metal ultra-flat plate 504. The matching layer is a sound-transmitting material with high fluidity and low impedance ≤2.5Mray. The PCB board 506 with the piezoelectric layer already prepared is buckled and fixed in the matching layer housing 501, and the epoxy resin matching layer is poured from above the glue filling port 208 on the back in ≥3 times, and each time the sample is tilted and evacuated for ≥20 minutes to remove the bubbles, and then placed in an oven and dried at a temperature above 40°C for ≥24 hours. After bonding the sound-absorbing material in the circuit board reinforcing rib 207, the production of the entire sparse sensing array sample is completed.

[0036] It should be noted that the technical solutions or technical features described in the above embodiments can be combined or supplemented with each other without conflict. The scope of protection of this application is not limited to the precise structures described in the above embodiments and shown in the drawings; all modifications, equivalent replacements, improvements, etc. made within the spirit and principles of this application shall be included within the scope of protection of this application.

Claims

1. A preparation process of a sparse sensing array for a three-dimensional camera sonar, characterized in that, Including: Calculating the element coordinates in the form of a sparse sensing array of Archimedean spiral, Fibonacci spiral, simulated annealing optimization, particle swarm optimization, and genetic optimization algorithm, designing patterned electrodes, and designing a circle of virtual transition elements at the outermost periphery of the sparse array. Using a micro-hole vacuum adsorption platform to adsorb and flatten a large-size PVDF or P(VDF-TrFE) piezoelectric film, screen-printing the patterned silver electrodes of the sparse elements on the side facing the PCB circuit board, laser-etching the edge contour and exhaust holes of the PVDF or P(VDF-TrFE) piezoelectric film through machine vision four-point positioning, and providing angle positioning protrusions at the film edge. At the same time, designing and fabricating a PCB circuit board and providing potting notches on its four sides; Laser-etching an ultra-thin double-sided tape and a single-sided mask tape according to the sparse array pattern, first bonding one side of the ultra-thin double-sided tape to the PCB circuit board and exposing all the electrode pads corresponding to the elements, then pasting the single-sided mask tape on the upper protective layer of the ultra-thin double-sided tape. Immediately, evenly apply conductive silver paste on the entire surface of the single-sided mask tape, tear off the single-sided mask tape and the upper protective layer of the ultra-thin double-sided tape, and retain the adhesive layer of the ultra-thin double-sided tape and expose the conductive silver paste at the element positions; Using the designed bonding and positioning auxiliary tool to accurately attach the cut PVDF or P(VDF-TrFE) piezoelectric film to the ultra-thin double-sided tape. The electrical connection between the patterned silver electrode and the PCB pad is realized through the conductive silver paste at the element positions, and the silver paste is cured for ≥24 hours. After curing, except for the area covered by the PVDF or P(VDF-TrFE) piezoelectric film, use a low-adhesion single-sided mask tape to cover each area of the PCB circuit board, and screen-print conductive silver paste above the piezoelectric film as the back electrode, and the silver paste is cured for ≥24 hours; Apply epoxy resin to the edge of the PVDF or P(VDF-TrFE) piezoelectric film, put it into a vacuum chamber, and place a flattening module on the PVDF or P(VDF-TrFE) piezoelectric film for ≥12 hours; after curing, also apply epoxy resin at the exhaust hole positions and put it into the vacuum chamber for ≥12 hours. The matching layer adopts a one-time potting method from the back of the outer frame. Pour low-impedance and high-fluidity epoxy resin into the back of the outer frame in ≥3 times, and tilt the sample and evacuate it for ≥20 minutes each time when pouring. Then put all of them into an oven and dry at 40°C for ≥24 hours.

2. The preparation process according to claim 1, characterized in that, The mesh count of the screen-printing screen plate is ≥400 meshes. The bottom plate of the screen-printing platform is a micro-hole vacuum adsorption platform, and its micro-hole size is ≤1 mm, and the center distance of the micro-holes is ≥15 mm.

3. The preparation process according to claim 1, characterized in that, There are laser-etching cross positioning points distributed in a triangle around the screen-printing screen plate ink area in the patterned electrodes.

4. The preparation process according to claim 1, characterized in that, There are exhaust holes distributed at intervals between the PVDF or P(VDF-TrFE) piezoelectric film elements of the sparse sensing array. The number of exhaust holes is ≥10% of the number of elements, and the diameter of the exhaust holes is ≤20% of the element diameter.

5. The preparation process according to claim 1, characterized in that, The adhesive force of the ultra-thin double-sided tape on the side bonding to the PCB circuit board is weak, and the adhesive force on the side bonding to the PVDF or P(VDF-TrFE) piezoelectric film is strong. The thickness of the double-sided tape is ≤10 um.

6. The preparation process according to claim 1, characterized in that, The positive electrodes of all array elements are located on one side of the PCB circuit board, and the connector of the data acquisition card is located on the back side of the PCB circuit board. The circuit uses vias to introduce the positive electrodes into the back connector, and the vias are filled with resin. The back electrodes use conductive silver paste to connect the negative electrode above the piezoelectric film to the ground pad of the PCB circuit board. The connector is provided with anti-crosstalk ground wires for each array element.

7. The preparation process according to claim 1, wherein After applying epoxy resin to the edges of the PVDF or P(VDF-TrFE) piezoelectric film and the exhaust holes between the array elements, defoaming is carried out in an environment with a vacuum degree ≤ 10 Pa.

8. The preparation process according to claim 1, characterized in that, The flattening module is composed of a polytetrafluoroethylene block and a metal block laminated up and down.

9. The preparation process according to claim 1, wherein The outer frame of the matching layer is provided with a 1 / 4 wavelength thickness positioning slot. A layer of hollow double-sided polyimide tape and a layer of non-adhesive polyimide film are pasted on the outer frame of the matching layer. The matching layer is a sound-transmitting material with a low impedance ≤ 2.5 Mray, and the whole sample is placed on a metal ultra-flat plate during vacuum defoaming and curing of the matching layer.

10. The preparation process according to claim 1, characterized in that, The back side of the PCB circuit board is fixed with reinforcing ribs, and a sound-absorbing material is bonded to the back side.

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