Adaptive Pressure Compensation Type-III Flexural Transducer

Abstract

The present invention provides an adaptive pressure-compensated three-type flextensional transducer in the technical field of underwater detection and underwater acoustic transducers, comprising a three-type flextensional transducer and an adaptive pressure-compensating structure, wherein the adaptive pressure-compensating structure is connected to the three-type flextensional transducer. A lower end cover is provided on the adaptive pressure-compensating structure, and a gas compensation chamber is provided in the adaptive pressure-compensating structure. The lower end cover and the gas compensation chamber are provided in an integrated structure, and the lower end cover is connected to the three-type flextensional transducer. A foam block is provided in the three-type flextensional transducer, and an inflation valve is provided on the adaptive pressure-compensating structure. The present invention significantly increases the working water depth of the three-type flextensional transducer by adding an adaptive pressure-compensating structure to the three-type flextensional transducer. By adopting an integrated structure design for the gas compensation chamber and the lower end cover, the internal pressure of the three-type flextensional transducer is automatically compensated in real time by using external water pressure, so that the internal and external pressures of the three-type flextensional transducer are kept balanced, and the transducer is easy to disassemble and maintain.

Description

Technical Field

[0001] The present invention relates to the technical field of underwater detection and underwater acoustic transducers, and in particular to an adaptive pressure-compensated three-type flextensional transducer. Background Art

[0002] Sound waves are the only form of energy that can propagate long distances underwater. Low-frequency sound waves have smaller propagation losses during propagation, and the key to obtaining low-frequency sound waves is to develop low-frequency transmitting transducers. To achieve a longer effective range, modern underwater acoustic equipment shows a clear trend towards low frequencies. The Type III flextensional transducer has good low-frequency transmission performance and is characterized by small size, light weight, and high power. It has good application prospects and has become a focus of current transducer research. However, due to the slits in the flextensional cylinder of the Type III flextensional transducer, its stiffness is weakened and there are cavities inside, resulting in poor hydrostatic pressure resistance, which greatly limits the application range of the Type III flextensional transducer. Therefore, improving the hydrostatic pressure resistance of the Type III flextensional transducer is a problem that needs to be solved. Summary of the Invention

[0003] In view of the defects in the prior art, the present invention aims to provide an adaptive pressure-compensated type III flextensional transducer.

[0004] According to the present invention, an adaptive pressure-compensated three-type flextensional transducer is provided, comprising a three-type flextensional transducer and an adaptive pressure-compensating structure, wherein the adaptive pressure-compensating structure is connected to the three-type flextensional transducer;

[0005] The adaptive pressure compensation structure is provided with a lower end cover, a gas compensation cavity is provided in the adaptive pressure compensation structure, the lower end cover and the gas compensation cavity are provided in an integrated structure, and the lower end cover is connected to the type III flextensional transducer;

[0006] A foam block is provided inside the three-type flextensional transducer, and an inflation valve is provided on the adaptive pressure compensation structure. The adaptive pressure compensation structure automatically compensates the internal pressure of the three-type flextensional transducer in real time by utilizing external water pressure.

[0007] In some embodiments, the device further comprises an upper end cover, a flextensional cylinder, and a piezoelectric vibrator, wherein the lower end cover is screwed to the flextensional cylinder via a thread, and the upper end cover is connected to the flextensional cylinder via a fastening screw;

[0008] The piezoelectric vibrator is mounted at the center grooves of the upper end cover and the lower end cover and is tightened by the fastening screws. The upper end cover and the lower end cover press the piezoelectric vibrator.

[0009] In some embodiments, the piezoelectric vibrator includes a piezoelectric ceramic, an electrode sheet, an upper mass block, a lower mass block, and a prestressed screw;

[0010] The upper mass block and the lower mass block are respectively arranged at both ends of the piezoelectric vibrator, the prestressed screw is perpendicular to the upper mass block and the lower mass block and is arranged in the piezoelectric vibrator, the piezoelectric ceramics are evenly and symmetrically arranged on both sides of the prestressed screw, and the electrode sheets are evenly distributed on both sides of the piezoelectric ceramics;

[0011] The piezoelectric vibrator is fastened to the upper mass block and the lower mass block through the prestressed screw.

[0012] In some embodiments, the outer sides of the flextensional cylinder and the upper end cover are both encapsulated with a polyurethane watertight layer.

[0013] In some embodiments, the lower end cover extends downward to form the gas compensation chamber, and a piston and a sealing ring are installed in the gas compensation chamber;

[0014] The inflation valve is mounted on one end of the piston, the sealing ring is sleeved on the other end of the piston, and the lower end cover is connected to and fastened to the limiting plate via a limiting screw.

[0015] In some embodiments, a vent hole is provided on the lower end cover, and the gas compensation cavity is connected with the internal air cavity of the type-three flextensional transducer on the flextensional cylinder through the vent hole.

[0016] In some embodiments, the upper mass block is provided with a threading hole, through which multiple core wires pass, and the multiple core wires pass through the center hole on the upper end cover, the center hole is connected to a cable, and the multiple core wires are welded to form the cable arrangement.

[0017] In some embodiments, the foam block includes an upper foam block and a lower foam block, the upper foam block and the lower foam block fill the inner space of the type-III flextensional transducer, and the upper foam block and the lower foam block are arranged around the piezoelectric vibrator.

[0018] In some embodiments, the lower surface of the lower foam block is fitted with the upper surface of the lower end cover, and a circular step is provided on the lower end cover, and the circular step is provided to clamp the two ends of the lower surface of the lower foam block.

[0019] An adaptive pressure-compensated type III flextensional transducer, the specific assembly steps are as follows:

[0020] Step 1: Coat the surface of the piezoelectric ceramic and the electrode sheet with epoxy glue. The prestressed screw passes through the lower mass block, the piezoelectric ceramic, and the electrode sheet in sequence and extends out. The extended part of the prestressed screw is tightened with the upper mass block through a threaded connection. The positive and negative poles of the piezoelectric ceramic are welded through the electrode sheet to lead out a core wire to complete the production of the piezoelectric vibrator.

[0021] Step 2: Install the prepared piezoelectric vibrator coaxially with the lower end cover, and position the lower mass block of the piezoelectric vibrator in the central groove of the lower end cover; insert the lower foam block into the lower end cover along the piezoelectric vibrator, and fit the lower surface of the lower foam block with the upper surface of the lower end cover, and clamp it through the circular step; pass the positive and negative core wires of the piezoelectric vibrator through the threading hole of the upper mass block.

[0022] Step 3: Insert the flextension cylinder into the lower end cover, and tighten the two through the internal thread of the flextension cylinder and the external thread of the lower end cover.

[0023] Step 4: Insert the upper foam block downward along the piezoelectric vibrator, and fit the lower surface of the upper foam block with the upper surface of the lower foam block; pass the core wire passing through the threading hole of the upper mass block of the piezoelectric vibrator through the center hole of the upper end cover, and the center groove of the upper end cover is coaxially installed and positioned with the upper mass block of the piezoelectric vibrator. The outer circular step of the upper end cover is matched with the inner circular step of the upper foam block for positioning. The upper end cover and the flexure cylinder are tightened and fixed with evenly distributed fastening screws to ensure that the piezoelectric vibrator is subjected to sufficient prestress.

[0024] Step 5: Weld the positive and negative core wires of the piezoelectric vibrator to the cable, and fill the outer sides of the flextensional cylinder and the upper end cover with a polyurethane watertight layer to complete the watertight packaging of the type III flextensional transducer.

[0025] Step 6: Screw the inflation valve onto the mounting hole of the piston, put a sealing ring on the piston, and install it into the gas compensation chamber; fix the limit plate to the lower end cover with the limit screw to limit the piston so that it can only move in the gas compensation chamber.

[0026] Step 7: Fill the gas compensation chamber with high-pressure gas through the inflation valve until the gas pressure in the gas compensation chamber reaches the design value.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. This invention significantly increases the operating water depth of the Type III flextensional transducer by adding an adaptive pressure compensation structure to it. The volume of the gas compensation chamber and the pressure of the high-pressure gas filled into it are designed according to the specific size and operating depth of the Type III flextensional transducer to meet the operating requirements of Type III flextensional transducers of different specifications in different water depths.

[0029] 2. The present invention integrates the gas compensation chamber and the lower end cover into an integrated structure, utilizing external water pressure to automatically compensate for the internal pressure of the three-type flextensional transducer in real time, thereby maintaining the internal and external pressure balance of the three-type flextensional transducer. This passive compensation method eliminates the need for additional specialized control equipment, resulting in a small size, simple and compact structure, high reliability, and easy assembly and maintenance.

[0030] 3. The present invention reduces the air volume inside the type-III flextensional transducer by providing a foam block to fill the internal space of the type-III flextensional transducer, increases the ratio of the volume of the gas compensation cavity to the volume of the air cavity of the type-III flextensional transducer, and improves the pressure compensation effect;

[0031] And by setting an inflation valve to fill high-pressure gas into the gas compensation cavity, the initial pressure inside the type III flextensional transducer is increased, thereby improving its underwater working depth. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0033] Figure 1 Schematic diagram of the overall structure of the adaptive pressure-compensated three-type flextensional transducer of the present invention;

[0034] Figure 2 Schematic diagram of the lower end cover structure of the adaptive pressure-compensated three-type flextensional transducer of the present invention;

[0035] Figure 3 Schematic diagram of the piezoelectric vibrator structure of the adaptive pressure-compensated three-type flextensional transducer of the present invention;

[0036] Figure 4 Schematic diagram of piston movement when the external water pressure is greater than the internal air pressure of the three-type flextensional transducer in the present invention Figure 1 ;

[0037] Figure 5 Schematic diagram of piston movement when the external water pressure is less than the internal air pressure of the three-type flextensional transducer in the present invention Figure 2 .

[0038] Reference numerals:

[0039] Upper end cover 1 Air cavity 301 inside the three-type flextensional transducer Piston 6

[0040] Center hole 101 Piezoelectric vibrator 4 Inflatable valve 7

[0041] Lower end cover 2 Upper mass block 401 Limiting plate 8

[0042] Center groove 201 Lower mass block 402 Upper foam block 9

[0043] External thread 202 Prestressed screw 403 Lower foam block 10

[0044] Vent 203 Piezoelectric ceramic 404 Sealing ring 11

[0045] Gas compensation chamber 204 Electrode sheet 405 Fastening screw 12

[0046] Positioning screw hole 205 Threading hole 406 Limit screw 13

[0047] Circular step 206 Core wire 407 Cable 14

[0048] Flextensional cylinder 3 Polyurethane watertight layer 5 DETAILED DESCRIPTION

[0049] The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those skilled in the art, several changes and improvements can be made without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0050] Example 1

[0051] like Figure 1-5 As shown, the adaptive pressure-compensated tri-type flextensional transducer of the present invention comprises an upper end cap 1, a lower end cap 2, a flextensional cylinder 3, and a piezoelectric vibrator 4. The lower end cap 2 and the flextensional cylinder 3 are screwed together, and the upper end cap 1 is connected to the flextensional cylinder 3 by a set screw 12. The piezoelectric vibrator 4 is mounted in the center grooves of the upper and lower end caps 1 and 2. When the set screw 12 is tightened, the upper and lower end caps 1 and 2 compress the piezoelectric vibrator 4, applying prestress to it. After the set screw 12 is tightened, the gap between the upper end cap 1 and the flextensional cylinder 3 is approximately 0.3 mm. A polyurethane watertight layer 5 is cast on the outside of the flextensional cylinder 3 and the upper end cap 1 to complete the watertight packaging of the tri-type flextensional transducer.

[0052] In this embodiment, the bending radius of the curved portion of the flextensional cylinder 3 is 250 mm. Slits are provided in the curved portion to reduce stiffness, making it easier for the Type III flextensional transducer to be excited and produce flexural vibrations. However, excessive slits can reduce the axial stiffness of the flextensional cylinder 3, thereby decreasing the water pressure resistance of the Type III flextensional transducer. In this embodiment, there are eight slits, each 1 mm wide. Without pressure compensation, the Type III flextensional transducer has a compressive strength of 1 MPa.

[0053] In this embodiment, the lower end cap 2 extends downward to form a gas compensation chamber 204. The piston 6 and sealing ring 11 are installed in the gas compensation chamber 204. The inflation valve 7 is mounted on the piston 6. The limit plate 8 is fastened to the lower end cap 2 via a limit screw 13, forming an adaptive pressure compensation structure. A vent hole 203 is provided in the lower end cap 2, connecting the gas compensation chamber 204 with the internal air chamber of the flextensional cylinder 3. This ensures that the gas compensation chamber 204 and the air chamber 301 within the three-type flextensional transducer have the same pressure. By compressing the volume of the gas compensation chamber 204, the air chamber 301 within the three-type flextensional transducer is pressurized for pressure compensation. The volume of the gas compensation chamber 204 is 1 L. The internal space of the three-type flextensional transducer is filled with upper and lower foam blocks 9 and 10. The material is 0.4 g / cm³ hard foam with a compressive strength of 10 MPa. Hard foam has high compressive strength and low density, which helps reduce the overall weight. After filling, the volume of the air cavity 301 inside the type III flextensional transducer is about 0.25 L. According to the formula P×V=C, after pressure compensation, the maximum working water depth of the type III flextensional transducer is 5 MPa.

[0054] In this embodiment, before use, high-pressure gas is filled into the gas compensation chamber 204 through the inflation valve 7 to a pressure of 1 MPa. When the Type III flextensional transducer is operating underwater, when the external hydrostatic pressure is less than 1 MPa, the piston 6 does not move at the limit plate 8. When the external hydrostatic pressure is between 1 MPa and 5 MPa, the external hydrostatic pressure is greater than the pressure in the internal air chamber 301 of the Type III flextensional transducer. The piston 6 moves inward under the action of the external hydrostatic pressure, compressing the gas compensation chamber 204 and transferring the pressure to the internal air chamber 301 of the Type III flextensional transducer. The air pressure in the internal air chamber 301 of the Type III flextensional transducer is then balanced with the external water pressure. When the external hydrostatic pressure exceeds 5 MPa, the piston 6 moves to abut against the lower end cap 2, reaching its maximum compensation effect within the adaptive pressure compensation structure, and the pressure in the internal air chamber 301 of the Type III flextensional transducer no longer increases.

[0055] In this embodiment, the specific assembly process of the adaptive pressure-compensated type III flextensional transducer of the present invention is as follows:

[0056] (1) Epoxy glue is applied to the surface of the piezoelectric ceramic 404 and the electrode sheet 405. The prestressed screw 403 passes through the lower mass block 402, the piezoelectric ceramic 404 and the electrode sheet 405 in sequence and extends out. The extended part of the prestressed screw 403 is screwed together with the upper mass block 401 through a threaded connection. The positive and negative electrodes of the piezoelectric ceramic 404 are welded through the electrode sheet 405 to lead out a core wire 407, thereby completing the production of the piezoelectric vibrator 4.

[0057] (2) Position the lower mass block 402 of the piezoelectric vibrator 4 in the central groove 201 of the lower end cover 2, and insert the lower foam block 10 downward into the lower end cover 2 along the piezoelectric vibrator 4. The lower surface of the lower foam block 10 fits with the upper surface of the lower end cover 2 and is clamped by the circular step 206. At the same time, pass the two core wires 407 of the positive and negative electrodes on the piezoelectric vibrator 4 through the threading hole 406 of the upper mass block 401.

[0058] (3) Insert the flextensional cylinder 3 into the lower end cover 2, and tighten the two together through the internal thread of the flextensional cylinder 3 and the external thread 202 of the lower end cover 2.

[0059] (4) Insert the upper foam block 9 downward along the piezoelectric vibrator 4, and fit the lower surface of the upper foam block 9 with the upper surface of the lower foam block 10. At the same time, pass the two core wires 407 passing through the threading hole 406 of the upper mass block 401 through the central hole 101 of the upper end cover 1. The central groove of the upper end cover 1 is coaxially installed and positioned with the upper mass block 401 of the piezoelectric vibrator 4. The outer circular step of the upper end cover 1 is matched with the inner circular step of the upper foam block 9 for positioning. The upper end cover 1 and the flexural cylinder 3 are tightened and fixed by evenly distributed fastening screws to ensure that the piezoelectric vibrator 4 is subjected to sufficient prestress.

[0060] (5) The positive and negative core wires 407 on the piezoelectric vibrator 4 are welded to the cable 14, and the assembled three-type flextensional transducer is placed in a suitable potting mold. The polyurethane watertight layer 5 is potted on the outside of the flextensional cylinder 3 and the upper end cover 1 to complete the watertight packaging of the three-type flextensional transducer. The designed three-type flextensional transducer can withstand a pressure of 1 MPa, so its maximum working depth is 100 meters.

[0061] (6) Screw the inflation valve 7 onto the mounting hole of the piston 6, put the sealing ring 11 on the piston 6, and install it into the gas compensation chamber 204, and fix the limiting plate 8 to the positioning screw hole 205 of the lower end cover 2 through the limiting screw 13 to limit the piston 6 so that it can only move within the gas compensation chamber 204.

[0062] (7) High-pressure gas is filled into the gas compensation chamber 204 through the inflation valve 7 until the air pressure in the gas compensation chamber 204 reaches 1 MPa. At this time, the pressure of the air cavity 301 inside the type-III flextensional transducer is 1 MPa. According to the volume ratio of the gas compensation chamber 204 to the air cavity 301 inside the type-III flextensional transducer, when the piston 6 is pressed to fit with the lower end cover 2 during underwater operation, the volume of the gas compensation chamber 204 is 0, and the air pressure in the air cavity 301 inside the type-III flextensional transducer is increased to 5 MPa. At this time, the maximum operating depth of the type-III flextensional transducer is increased to 500 meters.

[0063] Working principle:

[0064] When an adaptive pressure-compensating type-three flextensional transducer of the present invention operates underwater, the friction between the piston, the sealing ring, and the gas compensation chamber is ignored. As the water depth increases, when the static water pressure is greater than the air pressure in the air chamber, the water pressure pushes the piston in the gas compensation chamber to move toward the inside of the type-three flextensional transducer. The gas compensation chamber is compressed, and the pressure is transmitted to the air chamber. When the air pressure in the air chamber inside the type-three flextensional transducer increases to the same as the external water pressure, the piston stops moving, and the air pressure in the air chamber of the type-three flextensional transducer reaches a balanced state with the external water pressure.

[0065] The upper and lower foam blocks are installed inside the type-III flextensional transducer and positioned by steps on the upper and lower end covers. Through holes are opened in the middle of the upper and lower foam blocks, avoiding the piezoelectric vibrator. The upper and lower foam blocks occupy most of the space inside the type-III flextensional transducer, making the volume of the air cavity as small as possible to accommodate the pressure compensation device. When the compensation capacity of the pressure compensation device reaches its limit, the air volume in the gas compensation cavity is zero, and the total volume of the gas in the gas compensation cavity and the air cavity inside the type-III flextensional transducer becomes 1 / (n+1) of the original. From P×V=C, it can be seen that the internal pressure of the air cavity inside the type-III flextensional transducer becomes n+1 times the pressure of the gas filled. That is, the maximum operating water depth of the type-III flextensional transducer becomes n+1 times the pressure of the gas filled. Therefore, the maximum hydrostatic pressure P1 that the type-III flextensional transducer can withstand is related to the ratio n of the volume of the gas compensation cavity to the air cavity inside the type-III flextensional transducer and the filling gas pressure P0. The relationship is: maximum working water pressure P1 = (n + 1) × P0.

[0066] The upper foam block and the lower foam block are made of low-density pressure-resistant material to prevent the pressure in the air cavity inside the type III flextensional transducer from increasing and compressing its volume, thereby reducing the pressure compensation effect.

[0067] The inflation valve adopts a one-way inflation sealing valve. High-pressure gas is filled into the gas compensation chamber from the outside through the inflation valve. The pressure of the gas filled should be lower than the pressure that the type III flextensional transducer can withstand to avoid damage to the type III flextensional transducer due to excessive gas pressure.

[0068] In the description of this application, it should be understood that the terms "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on this application.

[0069] The above describes specific embodiments of the present invention. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. The embodiments of this application and the features in the embodiments may be combined with each other in any manner unless there is a conflict.

Claims

1. An adaptive pressure-compensated type III flextensional transducer, characterized in that: It includes a three-type flextensional transducer and an adaptive pressure compensation structure, wherein the adaptive pressure compensation structure is connected to the three-type flextensional transducer; A lower end cover (2) is provided on the adaptive pressure compensation structure, a gas compensation cavity (204) is provided in the adaptive pressure compensation structure, the lower end cover (2) and the gas compensation cavity (204) are provided in an integrated structure, and the lower end cover (2) is connected to the three-type flextensional transducer; A foam block is provided in the three-type flextensional transducer, and an inflation valve (7) is provided on the adaptive pressure compensation structure. The adaptive pressure compensation structure automatically compensates the internal pressure of the three-type flextensional transducer in real time by utilizing the external water pressure. It also includes an upper end cover (1), a flexure cylinder (3) and a piezoelectric vibrator (4), wherein the lower end cover (2) and the flexure cylinder (3) are screwed together by a thread, and the upper end cover (1) is connected to the flexure cylinder (3) by a fastening screw (12); The piezoelectric vibrator (4) is mounted on the central groove (201) of the upper end cover (1) and the lower end cover (2) and is tightened by the fastening screw (12), and the upper end cover (1) and the lower end cover (2) press the piezoelectric vibrator (4); The lower end cover (2) extends downward to form the gas compensation chamber (204), and a piston (6) and a sealing ring (11) are installed in the gas compensation chamber (204); The inflation valve (7) is mounted on one end of the piston (6), the sealing ring (11) is sleeved on the other end of the piston (6), and the lower end cover (2) is connected to the limiting plate (8) via a limiting screw (13).

2. The adaptive pressure-compensated type III flextensional transducer according to claim 1, characterized in that: The piezoelectric vibrator (4) comprises a piezoelectric ceramic (404), an electrode sheet (405), an upper mass block (401), a lower mass block (402) and a prestressed screw (403); The upper mass block (401) and the lower mass block (402) are respectively arranged at two ends of the piezoelectric vibrator (4), and the prestressed screw (403) is arranged perpendicular to the upper mass block (401) and the lower mass block (402) and inside the piezoelectric vibrator (4); The piezoelectric ceramics (404) are evenly and symmetrically arranged on both sides of the prestressed screw (403), and the electrode sheets (405) are evenly distributed on both sides of the piezoelectric ceramics (404); The piezoelectric vibrator (4) is fastened to the upper mass block (401) and the lower mass block (402) through the prestressed screw (403).

3. The adaptive pressure-compensated type III flextensional transducer according to claim 1, characterized in that: The outer sides of the flexure cylinder (3) and the upper end cover (1) are both sealed with a polyurethane watertight layer (5).

4. The adaptive pressure-compensated type III flextensional transducer according to claim 1, characterized in that: A vent hole (203) is provided on the lower end cover (2), and the gas compensation cavity (204) is communicated with the internal air cavity (301) of the three-type flextensional transducer on the flextensional cylinder (3) through the vent hole (203).

5. The adaptive pressure-compensated type III flextensional transducer according to claim 2, characterized in that: The upper mass block (401) is provided with a threading hole (406), and a plurality of core wires (407) pass through the threading hole (406); The plurality of core wires (407) pass through the center hole (101) on the upper end cover (1), a cable (14) is connected to the center hole (101), and the plurality of core wires (407) are welded to the cable (14).

6. The adaptive pressure-compensated type III flextensional transducer according to claim 1, characterized in that: The foam block comprises an upper foam block (9) and a lower foam block (10); the upper foam block (9) and the lower foam block (10) fill the internal space of the three-type flextensional transducer, and the upper foam block (9) and the lower foam block (10) are arranged around the piezoelectric vibrator (4).

7. The adaptive pressure-compensated type III flextensional transducer according to claim 6, characterized in that: The lower surface of the lower foam block (10) is arranged in contact with the upper surface of the lower end cover (2), and a circular step (206) is provided on the lower end cover (2), and the circular step (206) is arranged to clamp the two ends of the lower surface of the lower foam block (10).

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