Preparation technology of three-dimensional camera shooting sonar sparse sensing array

Abstract

The invention provides a preparation technology of a three-dimensional camera shooting sonar sparse sensing array, and the method comprises the following steps: forming a patterned electrode on a PVDF or P (VDF-TrFE) piezoelectric film through vacuum adsorption and silk-screen printing according to sparse array element coordinates, and arranging a virtual transition array element at the periphery; performing laser etching on the edge contour and the exhaust hole of the piezoelectric film; designing a bonding positioning auxiliary tool, combining an ultrathin double-sided adhesive tape and a single-sided mask adhesive tape, and utilizing conductive silver paste to realize accurate electric connection between the array element and the PCB bonding pad; designing a polytetrafluoroethylene-metal flattening module, and laminating the array in a vacuum environment for curing and bonding; the matching layer is made of a low-impedance sound transmission material, glue filling and vacuum defoaming are performed from the back side in batches, and the sparse sensing array is prepared in combination with a polyimide film and a metal super-flat plate. The preparation problem of the large-aperture high-frequency imaging sonar on the sensing array is solved, and the array element position accuracy, the array element consistency and the imaging resolution can be improved.

Description

Technical Field

[0001] The invention belongs to the technical field of sonar, and in particular relates to a preparation process of a three-dimensional imaging sonar sparse sensor array. Background Art

[0002] Three-dimensional imaging sonar technology has important application value in underwater detection, robot navigation and other fields. Its core principle is to construct a three-dimensional acoustic image of the target object through the transmission and reception of multi-beam ultrasonic signals. Imaging sonar adopts a dense array design. The arrangement of array elements, the number of array elements, the accuracy of array element coordinates, the size of the array aperture and the frequency of sound waves will significantly affect the imaging resolution. With the complexity of marine application scenarios, the requirements for detection efficiency and accuracy have become higher. High-frequency (≥300kHz) and large-aperture (≥200mm) three-dimensional imaging sonar will become one of the necessary detection tools. Sparse sensor array technology is an effective method to meet the development requirements of imaging sonar. It can maintain or even improve imaging performance while reducing the number of array elements by optimizing the spatial distribution of array elements. However, the preparation process of large-size sparse arrays has the following technical problems: Insufficient array element consistency: Uneven bonding between material layers, changes in boundary conditions, and array element shifting during operation lead to inconsistent signal responses, thereby reducing imaging quality.

[0003] Insufficient flatness: Large-sized flexible piezoelectric materials (such as PVDF (polyvinylidene fluoride) films) are prone to wrinkles and bubble introduction during the bonding process, causing failure of some array elements.

[0004] Limited acoustic performance: High-frequency sonar requires a thinner sound-transparent matching layer. Large sizes are difficult to prepare by grinding, and large sizes are prone to residual bubbles during the curing process, which weakens signal sensitivity and limits the water depth range of sonar applications.

[0005] In view of the above problems, the present invention proposes a new technical solution, taking into account the consistency, high-frequency adaptability and repeatability of process precision of large-size sparse sensor arrays, and an innovative process integrating material optimization, precision manufacturing and acoustic design, to break through the preparation bottleneck of high-frequency three-dimensional imaging sonar sparse arrays. Summary of the invention

[0006] The present invention proposes a preparation process for a three-dimensional imaging sonar sparse sensing array, especially for an imaging sonar receiving array with an array aperture ≥ 200 mm, an array element center spacing ≥ 1.5 times the target frequency wavelength, and a frequency ≥ 300 kHz, wherein the piezoelectric material is PVDF or P (VDF-TrFE) (copolymer of vinylidene fluoride and trifluoroethylene) film. The content of the present invention includes the selection of imaging sonar receiving array materials, the preparation process and the design of related auxiliary tools. The beneficial effect of the present invention is that it effectively solves the problem of difficulty in making ultrasonic array transducers with large-size flexible materials, and can be manufactured in whole pieces or processed in pieces, and has high interlayer bonding strength, array flatness, and array element consistency, which is beneficial to improving the resolution of sonar imaging. The key process contents of the present invention are as follows: First, the coordinates of the array elements of the sparse sensing array are calculated according to the Archimedean spiral, Fibonacci spiral, simulated annealing algorithm and other methods, and the array element diameter, positioning mark coordinates, peripheral virtual transition array element parameters are determined and patterned electrodes are designed. The large-size PVDF or P (VDF-TrFE) piezoelectric film is adsorbed and flattened using the designed and manufactured microporous vacuum adsorption platform, and a high-mesh screen is made according to the patterned electrode, and the patterned silver electrode of the sparse array element is screen-printed on the side facing the PCB circuit board. The edge contour and exhaust holes of the PVDF or P (VDF-TrFE) piezoelectric film are laser-etched by the four-point positioning method of machine vision. The exhaust holes are spaced between the array elements, and the edge of the film is cut with an angle positioning protrusion.

[0007] At the same time, the PCB circuit board is designed and manufactured, with preset glue filling notches on the four sides, the ground pads are located on the four sides or four corners of the PCB circuit board, the connector of the data acquisition card is located on the back side of the PCB circuit board, the line uses a through hole to introduce the positive electrode into the back connector, and the through hole is filled with resin. The connector is equipped with an anti-crosstalk ground wire for each array element.

[0008] Then, the ultra-thin double-sided adhesive tape and the single-sided masking tape are laser-etched according to the sparse array pattern, and the array element position is hollowed out. The side of the ultra-thin double-sided adhesive tape with lower viscosity is first bonded to the PCB circuit board, exposing the electrode pads corresponding to all array elements, and then the single-sided masking tape is pasted on the upper protective layer of the ultra-thin double-sided adhesive tape, and then the conductive silver paste is evenly applied on the entire surface of the single-sided masking tape, and the single-sided masking tape and the upper protective layer of the ultra-thin double-sided adhesive tape are torn off, and the adhesive layer of the ultra-thin double-sided adhesive tape is retained and the conductive silver paste at the array element position is exposed.

[0009] Next, use the designed bonding positioning auxiliary tool to accurately stick the cut PVDF or P (VDF-TrFE) piezoelectric film on the ultra-thin double-sided adhesive. The array element position uses conductive silver paste to achieve electrical connection between the patterned silver electrode and the PCB pad. 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 low-viscosity single-sided masking tape to cover all areas of the PCB circuit board, and screen-print conductive silver paste as the back electrode on the piezoelectric film. The silver paste is cured for ≥24 hours; Next, apply epoxy resin to the edge film of the PVDF or P (VDF-TrFE) piezoelectric film, put it into a vacuum box, and place a flattening module on the PVDF or P (VDF-TrFE) piezoelectric film for ≥12 hours. The flattening module is made of a polytetrafluoroethylene block and a metal block laminated up and down. After curing, apply epoxy resin to the exhaust hole position and put it into a vacuum box for ≥12 hours. The back electrode uses conductive silver paste to connect the negative electrode above the piezoelectric film to the ground pad of the PCB circuit board, and install a reinforcing rib on the back.

[0010] Finally, the matching layer is formed by back-filling glue in one step. 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 high-fluidity, low-impedance sound-transmitting material, and the entire sample is placed on a metal super flat plate during vacuum degassing and curing of the matching layer. Low-impedance, high-fluidity epoxy resin is poured from the back of the outer frame several times, and the sample is tilted and vacuumed each time it is poured, and then all are placed in an oven and dried at 40°C for ≥24 hours. After the sound-absorbing material is bonded in the reinforcement, the production is completed. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0012] FIG1 is a schematic diagram of a PVDF or P (VDF-TrFE) piezoelectric film sparse sensing array design; Figure 2 is a schematic cross-sectional view of a sparse sensor array of a video sonar; FIG3 is a schematic diagram of an auxiliary tool for preparing a sensor array; Figure 4 is a schematic diagram of a polytetrafluoroethylene-aluminum alloy flattening module; FIG5 is a schematic diagram of a PVDF or P (VDF-TrFE) piezoelectric film sparse sensing array matching layer packaging.

[0013] Description of Figure Numbers: 101 sparse array sensor element, 102 vacuum exhaust hole, 103 virtual transition element, 104 laser-etched positioning cross mark, 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 reinforcement rib, 208 glue filling port, 209 ultra-thin double-sided adhesive layer, 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 eye screw hole, 404 polytetrafluoroethylene module, 501 matching layer shell, 502 double-sided adhesive polyimide film, 503 non-adhesive polyimide film, 504 metal super flat plate, 505 positioning slot, 506 piezoelectric layer has completed the preparation of the PCB circuit board. DETAILED DESCRIPTION

[0014] The present invention provides a preparation process of a three-dimensional imaging sonar sparse sensor array. Here, the entire design and production process, principle and beneficial effects of a three-dimensional imaging sonar sparse sensor array of the present invention are clearly and completely described in combination with the accompanying drawings. When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiment described is only a suggested embodiment of the present invention, and some structural modifications made within the scope of the present invention should belong to the protection scope of the present invention.

[0015] If there are terms involving directional indications or positional relationships 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, movement, etc. between the components under a certain specific posture (as shown in the accompanying drawings); if the specific posture changes, the directional indication or positional relationship will also change accordingly. In addition, the terms "first" and "second" in the embodiments of the present application are only used for the purpose of convenience of description and cannot be understood as indicating or implying relative importance.

[0016] A sparse array element 101 distribution form of a sparse sensor array of a video sonar is as follows Figure 1 As shown in Figure 2, the cross section of the imaging sonar sensor array is as follows: Figure 2As shown, 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, a sound absorbing material 206, and a circuit board reinforcement rib 207. The entire preparation process of the array is as follows: According to the array directivity requirements, the distribution form of the sparse sensor array, that is, the coordinates of the sparse array element 101, is calculated using methods such as the Archimedean spiral, the Fibonacci spiral, and the simulated annealing algorithm. In order to prevent the acoustic characteristics of the edge array element from being greatly different from those of the internal array element, and to promote the overall array element to have better consistency, the outermost circle of the sparse array element 101 is provided with a virtual transition array element 103, which can make the boundary conditions of the array element change more uniformly. The virtual transition array element 103 is the same size as the sparse array element 101, and the preparation process is similar, but it is not connected to the electrical connector 205 of the data acquisition card.

[0017] The sparse array element 101 is formed by patterning electrodes on a PVDF or P (VDF-TrFE) piezoelectric film 106, such as Figure 1 As shown, high-mesh screen printing is used. Because the transverse wave velocity of the piezoelectric film is much smaller than the longitudinal wave velocity, there is no need to partition between the sparse array elements 101 to reduce acoustic crosstalk. Before printing, a large-size PVDF or P (VDF-TrFE) piezoelectric film 106 is adsorbed and leveled using a microporous vacuum adsorption platform. Since the PVDF or P (VDF-TrFE) piezoelectric film 106 is a flexible material, it is necessary to pay attention to the micropore size and spacing of the adsorption platform. The present invention recommends that the micropore size of the vacuum adsorption platform be ≤1mm and the micropore center spacing be ≥15mm. After adsorption and leveling, 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 sparse array elements 101 and virtual transition array elements 103, and laser-etched positioning cross marks 104 are printed on the four corners at the same time. In order to ensure the density and uniformity of the printed electrode, the mesh count of the screen printing screen should be ≥400 meshes. After screen printing, the sample is dried at ≤60℃ for ≥24 hours. The temperature should be less than half of the Curie temperature to avoid high temperature depolarization of the piezoelectric film.

[0018] With the help of the positioning cross mark 104 and machine vision, the edge contour and vacuum exhaust hole 102 of the PVDF or P (VDF-TrFE) piezoelectric film 106 are laser etched, and the edge of the piezoelectric film 106 is cut with an angle positioning protrusion 105. The purpose of the vacuum exhaust hole 102 is to extract 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. Too few vacuum exhaust holes 102 will affect the discharge of interlayer bubbles, and too many will easily cause bubbles to enter again. The number of vacuum exhaust holes 102 used in the present invention is slightly ≥ 10% of the number of sparse sensing array elements 101, and the diameter of the vacuum exhaust hole 102 is ≤ 20% of the diameter of the array element. In the subsequent process, the positioning protrusion 105 can cooperate with the film bonding positioning module 303 to make the piezoelectric film 106 more accurately attached to the PCB circuit board 204.

[0019] When designing and manufacturing the PCB circuit board 204, it is necessary to provide a glue injection port 208 on its four sides. The positive electrodes of all sparse array elements 101 are located on one side of the PCB circuit board 204. 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 array element 101 with the electrical connector 205 on the back, and the through holes are filled with resin to improve the compressive strength of the structure. The connector is provided with an anti-crosstalk grounding wire for each sparse array element 101.

[0020] According to the pattern of the sparse array element 101 and the virtual transition array element 103, a piece of ultra-thin double-sided tape and a piece of single-sided masking tape are cut by a laser etcher. The cutting hole of the ultra-thin double-sided tape is ≥ the diameter of the sparse array element 101, and the cutting hole of the single-sided masking tape is < the diameter of the sparse array element 101. The single-sided masking tape uses a tape with less viscosity, such as a thermal release tape. Before bonding the laser-etched PVDF or P (VDF-TrFE) piezoelectric layer 203, first use the side of the ultra-thin double-sided tape with less viscosity to bond it to the PCB circuit board 204, and expose all the electrode pads corresponding to the sparse array element 101. Then bond the single-sided masking tape to the upper protective layer of the ultra-thin double-sided tape, and also expose all the electrode pads. Apply conductive silver paste on the entire surface of the single-sided masking tape, and make the conductive silver paste evenly penetrate from the holes of the single-sided masking tape to the electrode pads through repeated application. Then, the single-sided masking tape and the upper protective layer of the ultra-thin double-sided tape are removed simultaneously, leaving the ultra-thin double-sided tape adhesive layer 209. To ensure the flatness of the sensor array, the thickness of the double-sided tape should be ≤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 array element 101.

[0021] 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 film bonding positioning module 303, and a film pressing module 304. The PCB circuit board 204 that has just been coated with conductive silver paste is placed on the PCB circuit board support module 302. The PCB circuit board support module 302 is processed with a groove corresponding to the electrical connector 205 of the data acquisition card. Then put the whole into the PCB circuit board positioning fixture 301, and its four sides protrude to limit the circuit board. Then buckle the film bonding positioning module 303 on the circuit board, and a card slot is provided inside the bonding positioning module, and the card slot matches the size of the piezoelectric film positioning protrusion 105.

[0022] The cut PVDF or P (VDF-TrFE) piezoelectric layer 203 is accurately attached to the sticky layer of the ultra-thin double-sided adhesive by using the PCB circuit board positioning fixture 301 and the PCB circuit board support module 302. The area without patterned electrodes is bonded by the sticky layer 209 of the ultra-thin double-sided adhesive. The sparse array element 101 position is electrically connected between the patterned silver electrode and the PCB circuit board pad by using conductive silver paste. The sticky layer 209 enhances the bonding strength between the layers. Then, a layer of plastic protective film is covered on the piezoelectric film, and the film pressing module 304 is placed above the protective film. Then, the entire sample and auxiliary tools are placed under a certain pressure for curing for ≥24 hours. Note that the applied pressure should not be too high. Excessive application will cause the piezoelectric film to depolarize.

[0023] After the conductive silver paste is cured, except for the area covered by the PVDF or P (VDF-TrFE) piezoelectric layer 203, the remaining areas are covered with low-viscosity single-sided masking tape to cover the areas at the edge of the PCB circuit board 204, fix the PCB circuit board 204 and the PCB circuit board support module 302, and screen-print the conductive silver paste above the piezoelectric film as the grounding electrode layer 202 of the back grounding electrode. The conductive silver paste is cured for ≥24 hours.

[0024] After the conductive silver paste is cured, epoxy resin glue is applied along the edge of the PVDF or P (VDF-TrFE) piezoelectric layer 203 using a syringe, and the whole is placed in a vacuum box and a polytetrafluoroethylene flattening module is placed on the PVDF or P (VDF-TrFE) piezoelectric layer 203 for ≥ 12 hours. The vacuum degree of the vacuum box should be maintained at ≤ 10Pa. The structure of the flattening module is as follows 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 exhausting process. The size of the flattening module is slightly smaller than the size of the piezoelectric film. The polytetrafluoroethylene block and the aluminum alloy can prevent the polytetrafluoroethylene block from deforming after long-term use. In order to facilitate the removal and placement of the flattening module, a lifting eye screw hole 403 is provided on the aluminum alloy module 401.

[0025] After the epoxy resin glue at the edge of the PVDF or P (VDF-TrFE) piezoelectric layer 203 solidifies, epoxy resin glue is also dripped on the vacuum exhaust hole 102 on the surface of the PVDF or P (VDF-TrFE) piezoelectric layer 203, and the vacuum box is placed for ≥12 hours until the epoxy resin solidifies, and the air between the PCB circuit board 204 and the PVDF or P (VDF-TrFE) piezoelectric layer 203 is exhausted. After taking out the sample, the pressure difference between the material and the external environment can be used to effectively ensure the flatness of the sensor array after the interlayer bonding. After the epoxy resin is cured, it can prevent air from flowing back into the interlayer gap 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 circuit board, and install the circuit board reinforcement rib 207 on the back. The reinforcement rib 207 can enhance the structural strength of the sparse sensor array and can also prevent the matching layer liquid from flowing into the connector 205.

[0026] Since the acoustic matching layer 201 is large in size, thin in thickness, and made of soft material, it is difficult to prepare it by grinding. Therefore, the acoustic matching layer 201 is prepared by back injection. Figure 5 As shown, a positioning slot 505 with a thickness of 1 / 4 wavelength is provided in the matching layer housing 501. Before pouring the matching layer liquid, a layer of hollow double-sided adhesive polyimide film 502 and a layer of non-adhesive polyimide film 503 are first pasted on the bottom surface of the matching layer housing 501, and the whole is installed on the metal super flat plate 504. If the flatness is not satisfied, the polyimide film 503 can be flatly adhered to the metal super flat plate 504, and then the matching layer housing 501 pasted with the double-sided adhesive polyimide film 502 can be installed on the metal super flat plate 504 after being scraped flat with a scraper. The matching layer is a sound-transmitting material with high fluidity and low impedance ≤2.5Mray. The PCB circuit board 506 with the piezoelectric layer already prepared is turned upside down in the matching layer housing 501 and fixed, and the epoxy resin matching layer is poured from the top of the glue filling port 208 on the back for ≥3 times, and the sample is tilted and vacuumed for ≥20 minutes each time. After removing the bubbles, put it in an oven and dry it at above 40°C for ≥24 hours. After the sound-absorbing material is bonded inside the circuit board reinforcement rib 207, the entire sparse sensor array sample is completed.

[0027] 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 causing conflicts. 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 substitutions, improvements, etc. made within the spirit and principles of this application should be included in the scope of protection of this application.

Claims

1. A process for preparing a sparse sensing array of a three-dimensional imaging sonar, characterized in that: include: Calculate the coordinates of the array elements in the form of Archimedean spiral, Fibonacci spiral, simulated annealing optimization, particle swarm optimization, and genetic optimization algorithm sparse sensor arrays, design patterned electrodes and design a circle of virtual transition array elements at the outermost edge of the sparse array, use a microporous vacuum adsorption platform to adsorb and flatten large-size PVDF or P (VDF-TrFE) piezoelectric film, screen print the patterned silver electrodes of the sparse array elements on the side facing the PCB circuit board, use machine vision four-point positioning laser etching PVDF or P (VDF-TrFE) piezoelectric film edge contours and exhaust holes, and the edge of the film is provided with an angle positioning protrusion, and at the same time design and manufacture a PCB circuit board and set glue filling gaps on its four sides; The ultra-thin double-sided adhesive tape and the single-sided masking tape are laser-etched according to the sparse array pattern, so that one side of the ultra-thin double-sided adhesive tape is first bonded to the PCB circuit board and the electrode pads corresponding to all array elements are exposed, and then the single-sided masking tape is adhered to the upper protective layer of the ultra-thin double-sided adhesive tape, and then the conductive silver paste is evenly applied on the entire surface of the single-sided masking tape, and the single-sided masking tape and the upper protective layer of the ultra-thin double-sided adhesive tape are torn off, and the adhesive layer of the ultra-thin double-sided adhesive tape is retained and the conductive silver paste at the array element position is exposed; Use the designed bonding positioning auxiliary tool to accurately stick the cut PVDF or P (VDF-TrFE) piezoelectric film on the ultra-thin double-sided adhesive. The conductive silver paste is used to realize the electrical connection between the patterned silver electrode and the PCB pad at the array element position. 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 low-viscosity single-sided masking tape to cover all areas of the PCB circuit board, and screen-print the conductive silver paste as the back electrode on the piezoelectric film. The silver paste is cured for ≥24 hours. Apply epoxy resin on the edge film of PVDF or P (VDF-TrFE) piezoelectric film, put it into a vacuum box and place a flattening module on the PVDF or P (VDF-TrFE) piezoelectric film for ≥12 hours; after curing, apply epoxy resin on the exhaust hole and put it into a vacuum box for ≥12 hours; the matching layer adopts the back-side glue injection one-time molding method, and injects low-impedance and high-fluidity epoxy resin from the back of the outer frame for ≥3 times, and each time the sample is injected, the sample is tilted and vacuumed for ≥20 minutes, and then all are put into an oven and dried at 40°C for ≥24 hours.

2. The preparation process according to claim 1, characterized in that: The mesh number of the screen printing screen is ≥400 meshes, and the bottom plate of the screen printing platform is a microporous vacuum adsorption platform, the microporous size of which is ≤1mm, and the microporous center distance is ≥15mm.

3. The preparation process according to claim 1, characterized in that: The patterned electrode is provided with laser-etched cross positioning points in triangular distribution around the ink area of ​​the silk-screen printing plate.

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

5. The preparation process according to claim 1, characterized in that: The ultra-thin double-sided adhesive has weaker adhesion on the side bonded to the PCB circuit board and stronger adhesion on the side bonded to the PVDF or P (VDF-TrFE) piezoelectric film. The thickness of the double-sided adhesive is ≤10um.

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 line uses a through hole to introduce the positive electrode into the back connector, and the through hole is filled with resin. The back electrode uses conductive silver paste to connect the negative electrode above the piezoelectric film to the ground pad of the PCB circuit board. The connector is equipped with an anti-crosstalk grounding wire for each array element.

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

8. The preparation process according to claim 1, characterized in that: The flattening module is formed by laminating a polytetrafluoroethylene block and a metal block.

9. The preparation process according to claim 1, characterized in that: 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 of ≤2.5Mray, and the entire sample is placed on a metal ultra-flat plate when the matching layer is vacuum defoamed and cured.

10. The preparation process according to claim 1, characterized in that: The back of the PCB circuit board is provided with reinforcing ribs for fixing, and the back is bonded with sound absorbing material.

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