A platform, a rebound device and an underwater robot

Abstract

The application discloses a carrying platform, a rebound apparatus device and an underwater robot. The carrying platform comprises a mounting module, a waterproof shell, a pressurizing module and a suction module. The waterproof shell is slidably arranged on the mounting module along a first direction, and is used for mounting the rebound apparatus. The pressurizing module is used for driving the waterproof shell to move, so that a detection end of the rebound apparatus can be detected along the first direction. The suction module comprises at least two suction disc components, each of which is arranged on the waterproof shell and is in a first plane, and the first plane is perpendicular to the first direction. Through the movement of the underwater robot and the suction of the suction module, the pressurizing module is used to accurately vertically apply pressure on the surface of the underwater concrete structure by the rebound apparatus, so that the nondestructive detection operation of the structural strength is realized. Meanwhile, the rebound apparatus can still keep a stable and good vertical contact state with the surface of the underwater concrete structure under the dynamic environment when the underwater robot moves or the water flow influences, and the measurement accuracy is ensured.

Description

Technical Field

[0001] This application relates to the field of underwater concrete structure inspection technology, and in particular to a platform, a rebound hammer device, and an underwater robot. Background Technology

[0002] Underwater concrete structures bear significant loads and are constantly subjected to water erosion. This harsh service environment means that the strength of underwater concrete structures is far more likely to decrease than that of above-water structures, posing a significant safety hazard to the overall project. Therefore, strength testing of underwater concrete structures is crucial. Traditional destructive testing methods for underwater concrete structures, such as core drilling, are not only costly but also compromise structural safety.

[0003] With the development of science and technology, underwater robots, increasingly used in underwater structure inspection, are becoming more intelligent and multifunctional, offering advantages such as low operating costs, high efficiency, and high safety. However, research on underwater robots equipped with non-destructive testing equipment for strength measurement is limited, and there is a lack of effective solutions for estimating the strength of underwater concrete structures using underwater robots. Although rebound hammers for strength measurement are widely used on land, their direct application in underwater environments still presents many challenges. If water enters the rebound hammer, the impact of the hammer will be affected by water resistance, compromising the accuracy of the test results and further limiting the effectiveness of non-destructive testing of underwater concrete structure strength.

[0004] In summary, the current lack of suitable non-destructive testing equipment for strength measurement on underwater robots limits the application of existing testing methods in estimating the strength of underwater concrete structures. Rebound hammers require extremely high watertightness in underwater environments, and the rapid currents exacerbate the challenges of maintaining stability on the robot-mounted rebound hammer and ensuring proper contact between the hammer's impact rod and the underwater concrete structure. The contact conditions between the impact rod and the underwater concrete surface are crucial to the quality of the test results. Therefore, it is necessary to ensure that the impact rod maintains stable and good vertical contact with the underwater concrete surface even under dynamic conditions such as water flow or robot movement.

[0005] The related technology CN116429615B proposes a rebound hammer for testing the strength of underwater concrete. Although the sealed overall structure ensures the dryness of the rebound hammer inside during the underwater testing process, it requires modification of the traditional rebound hammer, which is a cumbersome process. It also requires manual operation of the rebound hammer to make it make vertical contact with the surface of the underwater concrete structure to be tested, and it is not equipped with an installation structure suitable for underwater robots. Summary of the Invention

[0006] The purpose of this application is to at least solve one of the technical problems existing in the prior art. To this end, this application proposes a mounting platform that can stably mount the rebound hammer, slowly and evenly apply pressure to the rebound hammer, and effectively withstand the impact force when the hammer rebounds, so as to ensure the normal operation of the rebound hammer in an underwater environment.

[0007] This application also proposes a rebound device including the aforementioned mounting platform.

[0008] This application also proposes an underwater robot that includes the aforementioned rebound device.

[0009] The platform according to the first aspect of this application includes:

[0010] Install modules;

[0011] A waterproof housing is slidably disposed on the mounting module along a first direction, the waterproof housing being used to mount the rebound spring;

[0012] A pressurization module is mounted on the mounting module and is used to drive the waterproof housing to move so that the detection end of the rebound spring can perform detection along a first direction;

[0013] The adsorption module includes at least two suction cup components, each of which is spaced apart on the waterproof housing and located in a first plane, wherein the first plane is perpendicular to the first direction.

[0014] The mounting platform according to the first aspect of this application has at least the following beneficial effects: through the movement of the underwater robot and the adsorption of the adsorption module, the pressurization module enables the rebound hammer to accurately and vertically apply pressure to the surface of the underwater concrete structure. The overall structure can effectively withstand the impact force when the hammer rebounds, realizing non-destructive testing of structural strength in complex underwater environments, avoiding the risks of manual underwater operations. At the same time, through the targeted design of the first plane being perpendicular to the first direction, the rebound hammer can still maintain a stable and good vertical contact with the surface of the underwater concrete structure even under the influence of water flow or the dynamic environment of underwater robot movement, ensuring the measurement accuracy of non-destructive testing of the strength of underwater concrete structures.

[0015] According to the first aspect of the embodiment of this application, the mounting platform includes a fixed base, an extension frame, an extension plate, and a support slide. At least two of the fixed bases are respectively disposed on both sides of the waterproof housing along a first direction. Each fixed base is provided with the extension frame and the extension plate. The pressurization module is disposed on the extension plate. The support slide is disposed on each of the extension frames. The support slide is used to guide the movement of the detection end of the rebound spring.

[0016] According to the mounting platform of the first aspect of this application, the waterproof housing includes a housing, a seal, and a fixed slider. A first end of the housing is connected to the pressurization module, and a second end of the housing is used to extend out of the detection end of the rebound hammer. The seal is disposed at the connection between the housing and the detection end of the rebound hammer, and the fixed slider is disposed on the housing and slidably connected to the support slide.

[0017] According to the mounting platform described in the first aspect of this application, the pressurization module includes a pressurization power component, a pressurization plate, and a mounting block. The pressurization power component is connected to the mounting module through the mounting block. The pressurization plate is mounted on the pressurization power component, and the waterproof shell is mounted on the pressurization plate. The pressurization power component is used to drive the rebound hammer to apply pressure to the surface of the underwater concrete structure.

[0018] According to the mounting platform described in the first aspect of this application, the pressurizing power component includes a push rod motor;

[0019] And / or the rebounder is a digital rebounder.

[0020] According to the mounting platform described in the first aspect of this application, the suction cup component includes a suction cup body and an electric valve. The suction cup body can adhere to the surface of an underwater concrete structure and is used to fix the position of the waterproof shell. The suction cup body is provided with a drainage channel, and the electric valve is disposed in the drainage channel and can control the opening and closing of the drainage channel.

[0021] According to the first aspect of the embodiment of this application, the mounting platform has mounting plates on both sides along a first direction in the waterproof housing. The mounting plates are on the same plane and are used to mount the suction cup body, and the drainage channel extends from the suction cup body to the mounting plates.

[0022] The mounting platform according to the first aspect of the present application further includes a control device configured to control the pressurization module to drive the waterproof shell to move based on the contact information of the detection end of the rebound spring at the point to be tested. Once each of the suction cup components is in place, the rebound spring applies pressure and feeds back the detection data to the control device.

[0023] The rebound device according to a second aspect of this application includes: a mounting platform as described in the first aspect of this application.

[0024] An underwater robot according to a third aspect of this application includes: a rebound device as described in a second aspect of this application.

[0025] It is easy to understand that the rebound device in the second aspect embodiment of this application and the underwater robot in the third aspect embodiment of this application both have the same technical effects as the mounting platform in the first aspect embodiment, and therefore will not be described again.

[0026] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0027] The present application will be further described below with reference to the accompanying drawings and embodiments;

[0028] Figure 1 This is a schematic diagram of the structure of an embodiment of this application;

[0029] Figure 2 This is a side view of the initial state in an embodiment of this application;

[0030] Figure 3 This is a side view of the pressurized state in an embodiment of this application;

[0031] Figure 4 This is a bottom view of the pressurized state in an embodiment of this application;

[0032] Figure 5 This is a schematic diagram of an underwater robot installed in an embodiment of this application.

[0033] Figure label:

[0034] 1. Mounting module; 11. Fixed base; 12. Extended frame; 13. Extended plate; 14. Support slide;

[0035] 2. Waterproof outer shell; 21. Housing; 211. Mounting plate; 22. Seal; 23. Fixing slider;

[0036] 3. Pressurization module; 31. Pressurization power component; 32. Pressurization plate; 33. Mounting block;

[0037] 4. Adsorption module; 41. Suction cup body; 42. Electric valve; 43. Drainage channel;

[0038] 5. Rebound hammer. Detailed Implementation

[0039] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0040] In the description of this application, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They 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, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0041] In the description of this application, "several" means one or more, "more than" means at least two, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is for the purpose of distinguishing technical features only and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0042] In the description of this application, unless otherwise expressly defined, terms such as "setup," "installation," and "connection" should be interpreted broadly. Those skilled in the art can reasonably determine the specific meaning of the above terms in this application after considering the specific content of the technical solution.

[0043] Reference Figures 1 to 5 The mounting platform of the first aspect of this application is used to mount the rebound spring 5. The mounting platform includes an installation module 1, a waterproof shell 2, a pressurization module 3, and an adsorption module 4.

[0044] The waterproof housing 2 is slidably mounted on the mounting module 1 along the first direction and is used to mount the rebound hammer 5. The pressure module 3 is mounted on the mounting module 1 and is used to drive the waterproof housing 2 to move so that the detection end of the rebound hammer 5 can perform detection along the first direction. The adsorption module 4 includes at least two suction cup components, each suction cup component is spaced apart on the waterproof housing 2 and is located in the first plane, wherein the first plane is perpendicular to the first direction.

[0045] Understandably, through the movement of the underwater robot and the adsorption of the adsorption module 4, the pressurization module 3 enables the rebound hammer 5 to accurately and vertically apply pressure to the surface of the underwater concrete structure. The overall structure can effectively withstand the impact force when the hammer rebounds, realizing non-destructive testing of structural strength in complex underwater environments and avoiding the risks of manual underwater operations. At the same time, through the targeted design of the first plane being perpendicular to the first direction, the rebound hammer 5 can still maintain a stable and good vertical contact with the surface of the underwater concrete structure even under the influence of water flow or the dynamic environment of underwater robot movement, ensuring the measurement accuracy of non-destructive testing of the strength of underwater concrete structures.

[0046] In some embodiments of this application, the mounting module 1 includes a fixed base 11, an extension frame 12, an extension plate 13, and a support slide 14. At least two fixed bases 11 are respectively disposed on both sides of the waterproof housing 2 along a first direction. Each fixed base 11 is provided with an extension frame 12 and an extension plate 13. A pressurization module 3 is disposed on the extension plate 13, and a support slide 14 is disposed on each extension frame 12. The support slide 14 is used to guide the movement of the detection end of the rebound spring 5. It is understood that the fixed base 11 is rigidly connected to the extension frame 12 and the extension plate 13, and the support slide 14 is rigidly connected to the extension frame 12. The mounting module 1 is installed at the front end below the underwater robot.

[0047] In some embodiments, each fixed base 11 comprises an arc-shaped plate and an end mounting plate. The arc of the arc-shaped plate is adapted to the shape of the underwater robot's body shell. The mounting plate is provided with platform fixing bolt holes, through which the fixed base 11 is fixed to the lower part of the underwater robot's body.

[0048] In some embodiments, the extension frame 12 consists of several long vertical rods that extend upward from the upper end of the arc-shaped plate of the fixed base 11 to the position of the support slide 14. The extension plate 13 is a rectangular flat plate that is installed above the arc-shaped plate behind the fixed base 11 and is used to support and fix the pressure module 3.

[0049] In some embodiments, the support slide 14 is provided with a slide groove that extends along a first direction to serve as a guide.

[0050] In some embodiments of this application, the waterproof housing 2 includes a housing 21, a seal 22, and a fixing slider 23. The first end of the housing 21 is connected to the pressurizing module 3, and the second end of the housing 21 extends outward from the detection end of the rebound hammer 5. The seal 22 is disposed at the connection between the housing 21 and the detection end of the rebound hammer 5. The fixing slider 23 is disposed on the housing 21 and slidably connected to the support slide 14. It is understood that the rebound hammer 5 is placed inside the waterproof housing 2, the rear end of the housing 21 is mounted on the pressurizing module 3, the seal 22 is disposed at the connection between the housing 21 and the impact rod of the rebound hammer 5, and the fixing slider 23 is mounted above the housing 21, rigidly connected to the housing 21, and placed in the groove of the support slide 14.

[0051] In some embodiments, the housing 21 is transparent, rectangular in cross-section, with a central hole matching the shape of the rebound hammer 5. Rigidly connected rectangular mounting plates 211 are located on both sides of the front end and are bolted to the adsorption component. The seal 22 is a dynamic seal ring, specifically an O-ring, with a certain degree of elasticity, which limits axial leakage of moisture and prevents moisture from entering the rebound hammer 5 during its reciprocating motion. The housing 21 and the dynamic seal provide comprehensive waterproofing for the rebound hammer 5, ensuring its internal dryness and not hindering the extension and retraction of the rebound hammer's rod. The transparent housing facilitates observation of the position of the rebound hammer's rod and its contact with the underwater concrete structure surface.

[0052] In some embodiments, the fixed slider 23 is mounted above the housing 21 and is rigidly connected to the housing 21. The upper end has an I-shaped cross section and is slidably connected to the slide groove of the support slide table 14. The waterproof housing 2 of the rebound spring 5 can move back and forth through the slide groove of the support slide table 14 under the drive of the pressurization module 3.

[0053] In some embodiments of this application, the pressurization module 3 includes a pressurization power component 31, a pressurization plate 32, and a mounting block 33. The pressurization power component 31 is connected to the installation module 1 via the mounting block 33. The pressurization plate 32 is mounted on the pressurization power component 31, and the waterproof housing 2 is mounted on the pressurization plate 32. The pressurization power component 31 is used to drive the rebound hammer 5 to apply pressure to the surface of the underwater concrete structure. It is understood that the waterproof housing 2 is bolted to the pressurization plate 32, the pressurization plate 32 is bolted to the pressurization power component 31, and the pressurization power component 31 is mounted to the installation module 1 via the mounting block 33 and bolts. The pressurization module 3 drives the rebound hammer 5 to apply pressure to the surface of the underwater concrete structure.

[0054] In some embodiments, the rebound device 5, based on the underwater robot, is mounted on a platform at the front end below the underwater robot. This satisfies the stability requirement of the underwater robot's low center of gravity and ensures that the rebound device 5 does not shake, misalign, or shift during the detection process. When the impact rod is compressed to its maximum value, its front end still extends beyond the front end of the underwater robot. The front end of the impact rod of the rebound device 5 must always remain within the underwater robot's observation and imaging area.

[0055] In some embodiments of this application, the pressurizing power component 31 includes a push rod motor; it is understood that the pressurizing power component 31 uses a small push rod motor, resulting in a smaller overall installation size, reducing the weight of the underwater robot-based rebounder 5 mounting platform, thereby reducing the underwater robot's energy consumption. The pressurizing power component 31 is mounted on the extension plate 13 of the mounting module 1 via mounting block 33 and bolts, and the pressurizing plate 32 and the push rod on the pressurizing power component 31 are fixedly connected. In the initial state, the impact rod is in the extended state, and the impact hammer inside the rebounder 5 is hooked onto the hook inside the rebounder 5.

[0056] In some embodiments of this application, the rebound hammer 5 is a digital display rebound hammer 5. It is understood that the rebound hammer 5 is placed in the waterproof housing 2. The digital display rebound hammer 5 can support the internal storage and export of measurement data, and automatically record the rebound value after each measurement, without the need for manual reading of the measurement data by locking the impact rod.

[0057] In some embodiments of this application, the suction cup component includes a suction cup body 41 and an electric valve 42. The suction cup body 41 can adhere to the surface of an underwater concrete structure and is used to fix the position of the waterproof shell 2. The suction cup body 41 is provided with a drainage channel 43, and the electric valve 42 is disposed in the drainage channel 43 and can control the opening and closing of the drainage channel 43. It can be understood that two suction cup bodies 41 and two electric valves 42 are respectively disposed on both sides of the front end of the waterproof shell 2 of the rebound hammer 5. The suction cup body 41 can adhere to the surface of an underwater concrete structure and is used to fix the position of the waterproof shell 2.

[0058] In some embodiments of this application, mounting plates 211 are respectively provided on both sides of the waterproof housing 2 along the first direction. The mounting plates 211 are on the same plane and are used to mount the suction cup body 41, and the drainage channel 43 extends from the suction cup body 41 to the mounting plate 211. It can be understood that the suction cup is horn-shaped with a drainage channel 43 in the center. An electric valve 42 is installed in the drainage channel 43 and the rectangular mounting plate 211, which can control the opening and closing of the drainage channel 43 to drain the water in the suction cup, thereby allowing the suction cup to adhere to the surface of the underwater concrete structure and fix the position of the waterproof housing 2 of the rebound hammer 5.

[0059] In some embodiments of this application, a control device is also included. This control device is configured to control the pressurization module 3 to drive the waterproof housing 2 to move based on the contact information of the detection end of the rebound hammer 5 at the point to be tested. Once all suction cup components are in place, the rebound hammer 5 applies pressure and feeds back the detection data to the control device. It is understood that when the impact rod of the rebound hammer 5 contacts the surface of the underwater concrete structure, the pressurization power component 31 begins to operate. The push rod on the pressurization power component 31 pushes the waterproof housing 2 and the rebound hammer 5 forward. At this time, the fixed slider 23 above the housing 21 moves forward synchronously through the groove of the support slide 14, and the impact rod gradually compresses the elastic spring inside the rebound hammer 5. Under different water depth conditions, the push rod on the pressurization power component 31 can maintain stable extension and contraction performance. When the suction cup body 41 contacts the surface of the underwater concrete structure, the electric valve 42 opens the drainage channel 43, and the pressurization module 3 continues to drive the housing 21 forward. Pressure is applied to the suction cup body 41 through the rectangular mounting plates 211 on both sides of the front end of the housing 21, causing the water inside the suction cup body 41 to drain and adhere to the surface of the underwater concrete structure. Then, the electric valve 42 closes the drainage channel 43. The rectangular mounting plates 211 and the suction cup allow the waterproof housing 2 to be perpendicular to the surface of the underwater concrete structure, fixing the impact rod at a specific measuring point and ensuring that the impact rod of the rebound hammer 5 applies pressure perpendicularly to the surface of the underwater concrete structure. After the measurement is completed, the electric valve 42 opens the drainage channel 43, the waterproof housing 2 detaches from the adsorption state, and the rebound hammer 5 can be moved to the next measuring point for testing.

[0060] In some embodiments, since the pressurizing power component 31 can drive the waterproof housing 2 to move in the first direction, the adsorption state information of the two suction cup bodies 41 can be used to determine whether each suction cup component is in place, thereby enabling the rebound hammer 5 to apply pressure and measure perpendicular to the surface of the underwater concrete structure.

[0061] Reference Figures 1 to 5 The rebound device 5 of the second aspect of this application can be a rebound device 5 based on an underwater robot. The rebound device 5 includes the mounting platform of the first aspect of this application to ensure the normal operation of the rebound device 5 in the underwater environment.

[0062] In some embodiments of this application, the strength of different locations of an underwater concrete structure can be detected by the movement of an underwater robot and the adsorption of the adsorption module 4. The axis of the rebound hammer 5 is always perpendicular to the surface of the underwater concrete structure and applies pressure slowly and evenly. After the hammer inside the rebound hammer 5 disengages and impacts the impact rod, the rebound hammer causes the pointer to move backward to a certain position. When the indicator line on the pointer block shows a certain value on the scale, this is the rebound value. At this time, the mounting platform can effectively withstand the impact force of the hammer's rebound. After the digital display rebound hammer 5 automatically records the rebound value of this measurement, the adsorption module 4 disengages from the adsorption state, the pressurizing power component 31 drives the push rod to retract, causing the waterproof shell 2 to move backward, the impact rod to extend again, and the underwater robot moves to the next measuring point via the mounting platform carrying the rebound hammer 5 for testing.

[0063] Understandably, the rebound hammer 5 device and the underwater robot form an integrated whole. The rebound hammer 5 mounting platform, based on the underwater robot, makes installation convenient and quick, with high adaptability, allowing for flexible cooperation with different models of underwater robots. The mounted rebound hammer 5 can be a commercially available digital display rebound hammer 5, requiring no modification to its internal structure. In practical applications, such as inspecting underwater bridge piers and pile foundations, this mounting platform can operate stably in various water flow environments.

[0064] Reference Figures 1 to 5 The underwater robot of the third aspect of this application can be a robot for underwater concrete structure inspection. The underwater robot includes the rebound hammer 5 device of the second aspect of this application, so as to propose a modular, efficient and high-precision strength testing scheme in the field of underwater concrete structure inspection.

[0065] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0066] The embodiments of this application have been described in detail above with reference to the accompanying drawings. However, this application is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of this application.

Claims

1. A platform for mounting, characterized in that, include: Install the module; A waterproof housing is slidably disposed on the mounting module along a first direction, the waterproof housing being used to mount the rebound spring; The waterproof housing includes a housing and a sealing element; the sealing element is disposed at the connection between the housing and the detection end of the rebound hammer. A pressurization module is mounted on the mounting module and is used to drive the waterproof housing to move so that the detection end of the rebound spring can perform detection along a first direction; An adsorption module includes at least two suction cup components, each of which is spaced apart on the waterproof outer shell and located within a first plane, wherein the first plane is perpendicular to the first direction; each suction cup component includes a suction cup body and an electric valve, the suction cup body is capable of adsorbing onto the surface of an underwater concrete structure and used to fix the position of the waterproof outer shell, the suction cup body is provided with a drainage channel, and the electric valve is located within the drainage channel and is capable of controlling the opening and closing of the drainage channel; A control device is configured to control the pressurization module to drive the waterproof shell to move based on the contact information of the detection end of the rebound hammer at the point to be tested. Once each of the suction cup components is in place, the rebound hammer applies pressure and feeds back the detection data to the control device. The installation module includes a fixed base, an extension frame, an extension plate, and a support slide. At least two fixed bases are respectively disposed on both sides of the waterproof housing along a first direction. Each fixed base is provided with an extension frame and an extension plate. The pressurization module is disposed on the extension plate, and the support slide is disposed on each extension frame. The support slide is used to guide the movement of the detection end of the rebound hammer. The waterproof housing includes a fixed slider. A first end of the housing is connected to the pressurization module, and a second end of the housing is used to extend out of the detection end of the rebound hammer. The fixed slider is disposed on the housing and slidably connected to the support slide. The pressurization module includes a pressurization power component, a pressurization plate, and a mounting block. The pressurization power component is connected to the installation module through the mounting block. The pressurization plate is mounted on the pressurization power component, and the waterproof housing is mounted on the pressurization plate. The pressurization power component is used to drive the rebound hammer to apply pressure to the surface of the underwater concrete structure.

2. The mounting platform according to claim 1, characterized in that: The pressurizing power component includes a push rod motor; And / or the rebounder is a digital rebounder.

3. The mounting platform according to claim 1, characterized in that: The waterproof housing has mounting plates on both sides along the first direction. The mounting plates are on the same plane and are used to mount the suction cup body. The drainage channel extends from the suction cup body to the mounting plates.

4. A rebound hammer device, characterized in that, include: The mounting platform as described in any one of claims 1 to 3.

5. An underwater robot, characterized in that, include: The rebound spring device as described in claim 4.

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