Bridge bottom concrete crack detection method

Abstract

The invention discloses a bridge bottom concrete crack detection method, which belongs to the bridge detection technology, and comprises the following steps: shooting the bridge bottom by using a camera to check the cracking condition of the concrete at the bridge bottom, covering and filling a detected suspected crack area in a camera picture by using a detection agent with good fluidity, the method comprises the following steps: detecting a suspected crack, measuring the usage amount of a detection agent, judging whether the suspected crack really exists or not according to the usage amount, characterizing the size of the crack by using the usage amount of the detection agent, and when the detection agent moves along an extension path of the suspected crack through spray irrigation equipment and is sprayed and covered, performing spray irrigation on the suspected crack. The volume of the dosage of the detection agent is equal to the volume of the crack. On the basis of an existing image recognition detection technology, a detection agent filling technology is newly added, and the distribution condition of the cracks can be more visually, accurately and stereoscopically detected.

Description

Technical Field

[0001] The invention relates to the technical field of bridge detection, in particular to a method for detecting concrete cracks at the bottom of a bridge. Background Art

[0002] As an important transportation infrastructure, bridges are subject to enormous load pressure and environmental factors. Over time, their structures may suffer varying degrees of damage, especially concrete cracks, which is one of the key indicators reflecting the health of bridges.

[0003] Traditional bridge crack detection methods often rely on manual visual inspection by professional technicians. This method is time-consuming and laborious, and the accuracy and comprehensiveness of the detection results are greatly limited for areas such as the bottom of the bridge with limited vision and poor accessibility. To this end, the intelligent detection method of concrete cracks at the bottom of the bridge based on machine vision and deep learning has broken through the limitations of traditional detection technology and achieved efficient and accurate detection of concrete cracks at the bottom of the bridge. It mainly uses the camera carried by the drone to take pictures of the bottom of the bridge along the way, obtain images of cracks from the photos, and automatically identify them as cracks through machine learning. Although this automatic recognition technology is extremely efficient and undoubtedly the best in safety, machine recognition of cracks will still produce errors. Some images of the bottom of the bridge that look like cracks will be identified as cracks. Moreover, this machine plane recognition can only determine the width of the crack, but cannot intuitively express the depth or the overall volume of the crack, resulting in significant limitations in crack detection.

[0004] Therefore, it is particularly important for technical personnel in this field to design a new technology that can overcome the above-mentioned defects and achieve efficient and accurate detection of concrete cracks at the bottom of the bridge. Summary of the invention

[0005] 1. Technical issues to be resolved In view of the deficiencies in the prior art, the present invention provides a method for detecting cracks in concrete at the bottom of a bridge. The method for detecting cracks in concrete at the bottom of a bridge can be used to more accurately and comprehensively detect cracks.

[0006] (II) Technical solution To achieve the above-mentioned purpose, the present invention provides the following technical solutions: a method for detecting concrete cracks at the bottom of a bridge, comprising photographing the bottom of the bridge with a camera to check the concrete cracking conditions at the bottom of the bridge, covering and filling the suspected crack areas within the detected camera image with a detection agent with good fluidity, and measuring the amount of the detection agent used, determining whether the suspected cracks actually exist based on the amount used, and the size of the cracks is characterized by the amount of the detection agent used, and when the detection agent is moved along the extension path of the suspected cracks by a sprinkler equipment and sprayed and covered, the volume of the detection agent used is equal to the volume of the cracks.

[0007] Furthermore, the detection agent sprayed by the sprinkler equipment will only be lost in the area with cracks between its nozzle and the bridge. The nozzle is in dynamic and sealed contact with the bottom surface of the bridge without cracks, avoiding the detection agent being smeared in large quantities on the non-crack area of ​​the bridge and causing detection errors.

[0008] Furthermore, the nozzle used includes a rectangular tube body with a strip-shaped spray hole at the bottom of the tube body, and a pair of piston blocks symmetrically arranged on the left and right are installed in the tube body along its axial direction in a sliding manner, and the piston blocks include two rectangular blocks of different sizes, wherein the large rectangular block is installed in the tube hole of the tube body in a sliding manner, and the small rectangular block is installed in the spray hole in a sliding manner, and the storage space of the tube body between the two piston blocks is used to be continuously filled with detection agent; during detection, according to the camera image, the nozzle is moved along the path direction of the suspected crack, and then the detection agent is injected into the crack to fill the crack.

[0009] Furthermore, a flow meter is installed on the nozzle and the input pipe for supplementing the input detection agent. When the nozzle is moved, if a crack is encountered, the flow meter will display a flow rate value greater than the set value, and the flow value will also increase accordingly.

[0010] Furthermore, the piston blocks are connected to the tube body via hydraulic rods, and when the two piston blocks are driven by their respective hydraulic rods, they can move synchronously toward or away from each other to change the length of the storage space.

[0011] Furthermore, a self-rotating screw rod is horizontally installed in one side wall of the tube body, a threaded block is threadedly installed on the screw rod, and the threaded block is fixedly connected to the mounting arm of a moving device. Before the moving device moves the nozzle along the path of the suspected crack, the driving motor installed at the end of the screw rod is started to rotate according to the distribution position of the suspected crack. Because the moving device does not move horizontally, the screw rod and the tube body where it is located move in the horizontal direction and then arrive at the position facing the suspected crack. Then the moving device moves forward, and in the process of moving forward, according to the extension path of the suspected crack, the position of the tube body is continuously adjusted by the driving motor so that the tube body always faces the current suspected crack segment.

[0012] Furthermore, the camera is linked with the nozzle, and the linkage method of the two is: the camera moves forward along the length direction of the bridge, and in the real-time photo within its field of view, the bridge position actually corresponding to the left edge of the photo is selected as the left limit boundary, and then the distance from the starting end of the suspected crack in the real-time photo to the left edge in the horizontal direction is proportionally enlarged to the corresponding enlarged distance value horizontally to the right of the left limit boundary at the bottom of the bridge, which is the actual starting position of the suspected crack at the bottom of the bridge in the horizontal direction; The nozzle is initially installed opposite to the left limit boundary. When a suspected right crack is captured, the nozzle is horizontally offset to the right by the adjustment displacement of the enlarged spacing value, and then moves forward to start moving forward from the starting end of the suspected crack. In subsequent movements, the adjustment displacement that needs to be moved is calculated in real time based on the calculation method of the enlarged spacing value. When this adjustment displacement is higher than the previous value, the nozzle continues to move horizontally to the right by an amount equal to the difference between the two adjustment displacements. If this adjustment displacement is lower than the previous value, the nozzle needs to move horizontally back to the left by an amount equal to the difference between the two adjustment displacements, so that when the nozzle moves forward, it always automatically follows the path direction of the suspected crack captured by the camera, thereby timely detecting the size of the corresponding crack.

[0013] Furthermore, the calculation program of the linkage method is set on a program controller, and the program controller is fixedly installed on the sprinkler irrigation equipment.

[0014] Furthermore, the mobile device is a drone, and the top of the drone has an elastic telescopic arm, and the top of the elastic telescopic arm has a contact plate that contacts the bottom of the bridge. When the drone is flying, on the one hand, it needs to have a stable forward displacement, and on the other hand, the elastic telescopic arm must be squeezed to the limit so that the distance between the drone and the bottom of the bridge always remains unchanged.

[0015] Furthermore, at least one auxiliary engine is provided on the drone, and the auxiliary engine enables the drone to move vertically upward, so that the elastic telescopic arm is always compressed to the limit during the drone's forward straight flight.

[0016] (III) Beneficial effects The present invention provides a method for detecting concrete cracks at the bottom of a bridge. The method is based on the existing technology of using a camera to shoot and view concrete cracks at the bottom of a bridge, and further adds a technology of spraying and filling a detection agent in a suspected crack area. When a crack actually exists, a detection agent will inevitably quickly enter the crack, thereby causing a significant reduction in the detection agent. This reduction can be considered as an intuitive quantity that characterizes the size of the crack, thereby determining whether the suspected crack is a real crack and to what extent the total volume of the crack has reached, thereby achieving a three-dimensional conclusion detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 An overhead view of the nozzle moving along a suspected crack at the bottom of the bridge; Figure 2 A specific structural diagram of a nozzle; Figure 3 A specific structural diagram of a piston block; Figure 4 Schematic diagram of a UAV's flight contact with the bottom of a bridge.

[0018] In the figure: bridge bottom 1, suspected crack 2, nozzle 3, tube body 4, spray hole 5, piston block 6, large rectangular block 601, small rectangular block 602, hydraulic rod 7, screw 8, threaded block 9, mounting arm 10, drive motor 11, left limit line 12 drone 13, elastic telescopic arm 14, contact plate 15, auxiliary engine 16. DETAILED DESCRIPTION

[0019] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] This embodiment introduces a method for detecting cracks in concrete at the bottom of a bridge. Like the prior art, it includes shooting the bottom 1 of the bridge with a camera, so that the concrete cracking condition at the bottom 1 of the bridge can be checked. Specifically, in this embodiment, the difference is that, first, the suspected crack 2 area detected in the camera image is covered and filled with a detection agent with good fluidity, that is, a detection agent that can solidify quickly is injected into these suspected cracks, such as a viscous plugging agent, a caulking agent or even concrete mortar, and the amount of these detection agents used for detection is measured. It is possible to determine whether the suspected crack 2 actually exists based on the amount used, and the size of the crack is characterized by the amount of detection agent used. Specifically, when the detection agent is moved along the extension path of the suspected crack 2 through a sprinkler equipment, its nozzle 3 will be continuously filled with the detection agent. If the position where the nozzle 3 contacts the bottom 1 of the bridge encounters real and obvious concrete cracks, the detection agent filled in the nozzle 3 will naturally and quickly flow into the cracks, that is, spray and cover the corresponding parts of these cracks. Moreover, the volume of the detection agent used is indirectly equal to the volume of the cracks, or it can be considered that the size of the cracks requires so much volume of detection agent to be filled, thereby indirectly obtaining the volume of these cracks to quantitatively detect the specific conditions of the cracks.

[0021] During use, it can be considered that the detection agent sprayed by the sprinkler equipment through the nozzle 3 will only be lost in the area with cracks between the nozzle 3 and the bridge, that is, it will only flow out of the nozzle 3, while the nozzle 3 is in dynamic sealing contact with the bottom surface of the bridge without cracks, the detection agent will not flow out easily, or will not flow out obviously and will be reduced, which can avoid the detection agent being smeared in large quantities on the non-crack area of ​​the bridge and causing detection errors.

[0022] As one of the specific implementation methods, Figure 1-2 As shown, the nozzle 3 used in the present invention includes a rectangular tube body 4, a strip-shaped spray hole 5 is opened at the bottom of the tube body 4, the width and length of the spray hole 5 are adaptively designed, and a pair of piston blocks 6 arranged symmetrically on the left and right are also slidably mounted in the tube body 4 along its axial direction. When it is specifically made, as shown in FIG. Figure 3The piston block 6 includes two rectangular blocks of different sizes, forming a T-shaped structure, in which the large rectangular block 601 is slidably installed in the tube hole of the tube body 4, and the small rectangular block 602 is slidably installed in the spray hole 5, so that the storage space of the tube body 4 between the two piston blocks 6 can be filled with the detection agent. During the inspection, according to the camera image, follow the path direction of the suspected crack 2, and adaptively move the nozzle 3, and then inject the detection agent into the crack, and fill the passing crack. The amount of detection agent lost by the injection can intuitively grasp the overall performance of the crack area and volume of the bridge bottom 1. When the crack volume per unit area exceeds the allowable size required by the design, repair is required.

[0023] In order to make the detection more accurate, a flow meter (not shown in the figure) can be installed on the nozzle 3 and the input pipe for supplementing the input detection agent. When the nozzle 3 is moved, if a crack is encountered, especially when the crack is large, the detection agent will quickly flow out of the nozzle 3, and then the input pipe will quickly replenish the monitoring toward the nozzle 3, so that the flow meter will obviously rotate, thereby displaying a flow rate value greater than the set value, and the flow value will also increase accordingly. The reason why the flow rate is greater than the set value is that in practice, it is inevitable that the detection agent will be accidentally lost, and the loss of this small part of the detection agent may also cause the flow meter to rotate slightly. Therefore, a set value is required. When this value is exceeded, it can be judged that there are obvious cracks at the bottom 1 of the bridge.

[0024] To adapt to different crack shapes and sizes, Figure 2 As shown, the piston block 6 is connected to the tube body 4 through the hydraulic rod 7. When the two piston blocks 6 are driven by their respective hydraulic rods 7, they can move toward or away from each other synchronously to change the length of the storage space mentioned above, that is, to change the size of the temporary storage area for the detection agent. For example, when the suspected crack 2 captured by the camera is small, that is, narrow, the two piston blocks 6 can be brought closer to shorten the length of the above-mentioned storage space, so as to pass through the suspected crack 2 more concentratedly and minimize the waste of the detection meter due to contact with the remaining non-crack areas of the bridge bottom 1.

[0025] See also Figure 2In a specific implementation, a self-rotating screw rod 8 can be installed horizontally (here and hereinafter, it refers to the width direction of the bridge) in one side wall of the tube body 4, and one end of the screw rod 8 is connected to a driving motor 11. A thread block 9 is installed on the screw rod 8 in a threaded manner. The thread block 9 is also a rectangular block structure and moves linearly in the tube body 4 but cannot rotate. At the same time, the threaded block 9 is fixedly connected to the mounting arm 10 of a moving device. Before the moving device moves with the nozzle 3 along the path of the suspected crack 2, the driving motor 11 installed at the end of the screw rod 8 is started to rotate according to the distribution position of the suspected crack 2. Because the moving device does not move horizontally, or temporarily stops moving, the screw rod 8 and the tube body 4 in which it is located move horizontally relative to the threaded block 9 under the guidance of the threaded block 9, that is, the nozzle 3 moves along the width direction of the bridge, and then arrives at the position facing the suspected crack 2, and then the moving device moves forward and crosses the crack, and in the process of moving forward, according to the extension path of the suspected crack 2, the position of the tube body 4 is continuously adjusted through the driving motor 11, so that the tube body 4 is always facing the current suspected crack 2 section.

[0026] As a preferred implementation scheme, in this embodiment, the camera is linked with the nozzle 3, and the linkage method of the two is: the camera is moved forward along the length direction of the bridge, and in the real-time photos within its field of view, the bridge position actually corresponding to the left edge of the photo is selected as the left limit line, that is, the real scene on the left edge of the photo is Figure 1 Then, the distance between the starting end of the suspected crack 2 in the real-time photo and the left edge in the horizontal direction is proportionally enlarged to the corresponding enlarged distance value on the right side of the left limit line of the bridge bottom 1. That is to say, when the position of the crack in the photo is a distance length K meters away from the left edge of the photo, in reality, in the horizontal direction, that is, in the width direction of the bridge, there must be a linear distance length M meters, that is, M=aK, a is a real number that must be greater than 1. At the bottom of the bridge 1, the position M meters away from the real scene at its left limit line is the actual position of the crack. That is, this position must be the actual starting end position of the suspected crack 2 at the bottom of the bridge 1 in the horizontal direction. Because the nozzle 3 will move forward, the position in the length direction of the bridge does not need to be limited at the beginning, and the nozzle 3 can naturally pass under this starting end during the forward movement.

[0027] In more detail, the nozzle 3 is initially installed at the left limit line, that is, in practice, the center of the tube body 4 is located at Figure 1At point A shown in the figure, when the suspected crack 2 on the right is photographed, the nozzle 3 is horizontally offset to the right by the adjustment displacement of the enlarged spacing value, that is, the center of the tube body 4 is located at point B, and then moves forward to start from the starting end of the suspected crack 2. In the subsequent movement, the adjustment displacement that needs to be moved is calculated in real time based on the calculation method of the enlarged spacing value, that is, the principle that the proportion of the photo is consistent with the actual proportion. When this adjustment displacement is higher than the previous value, the nozzle 3 continues to move horizontally to the right by an amount equal to the difference between the two adjustment displacements. If this adjustment displacement is lower than the previous value, the nozzle 3 needs to move horizontally back to the left by an amount equal to the difference between the two adjustment displacements, so that the nozzle 3 will always automatically follow the path direction of the suspected crack 2 photographed by the camera when it moves forward next time, so as to detect the size of the corresponding crack in time and improve the detection efficiency and detection accuracy. During production, the calculation program of the linkage method can be set on a program controller, and the program controller is fixedly installed on the sprinkler irrigation equipment, so that it is more integrated and easy to install and use.

[0028] As for the moving device of the nozzle 3, i.e. the camera, it can be a drone, which is also the simplest and most direct method for obtaining the driving force in the current detection method, considering the particularity of the location of the bridge. However, the difference is that in this embodiment, Figure 4 , an elastic telescopic arm 14 is fixed on the top of the drone used, the elastic telescopic arm 14 is installed vertically, and a contact plate 15 in contact with the bottom of the bridge, or a plurality of rollers arranged in parallel, is fixed on the top of the elastic telescopic arm 14. When the drone is flying, on the one hand, it needs to have a stable forward displacement, which is the performance of the drone itself, and on the other hand, the elastic telescopic arm 14 must be squeezed to the limit so that the distance between the drone and the bottom 1 of the bridge remains unchanged. This is mainly to keep the drone at a fixed height relative to the bottom 1 of the bridge, so that whether it is taking pictures or moving the nozzle 3, it is more stable. Otherwise, if the drone is The drone only relies on its own performance to move forward in a straight line, and it is very likely that there will be fluctuations in the vertical direction, which will cause the shooting picture to shake, and the contact between the nozzle 3 and the bottom 1 of the bridge will change when it moves. Moreover, when the drone is flying, it is necessary to avoid inconsistent speeds when flying forward, but the height must be consistent. Therefore, extra attention is required to maintain the height of the drone. It cannot rely solely on its own height maintenance performance. Even if a high-precision drone is purchased, it cannot guarantee that the distance between the drone and the bottom 1 of the bridge is consistent, because the bottom 1 of the bridge itself is not necessarily absolutely on a flat surface. Although the drone accurately controls the flight altitude, the distance between it and the bottom 1 of the bridge is still inconsistent. In order to ensure this performance, in this embodiment, if Figure 4As shown, at least one auxiliary engine 16 is also provided on the drone, and the auxiliary engine 16 provides the drone with an upward vertical lifting force, which enables the drone to move vertically upward, so that the drone always compresses the elastic telescopic arm 14 to the limit during the forward straight flight, and then realizes the precise control of the drone relative to the bridge bottom 1 during the flight. During the manufacturing process, of course, the elasticity of the elastic telescopic arm 14 must be controlled within a certain reasonable range to avoid excessive elastic force when the elastic telescopic arm 14 is shortened to the limit, which seriously affects the flight speed and stability of the drone.

[0029] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0030] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for detecting concrete cracks at the bottom of a bridge, comprising photographing the bottom of the bridge (1) with a camera to check the concrete cracking conditions at the bottom of the bridge (1), characterized in that: The suspected crack (2) area detected in the camera image is covered and filled with a detection agent with good fluidity, and the amount of the detection agent used is measured. Whether the suspected crack (2) actually exists is determined based on the amount used, and the size of the crack is characterized by the amount of the detection agent used. When the detection agent is moved along the extension path of the suspected crack (2) by a sprinkler device and sprayed and covered, the volume of the detection agent used is equal to the volume of the crack.

2. A method for detecting concrete cracks at the bottom of a bridge according to claim 1, characterized in that: The detection agent sprayed by the sprinkler irrigation equipment is lost only in the area with cracks between the sprinkler head (3) and the bridge. The sprinkler head (3) is in dynamic sealing contact with the bottom surface of the bridge without cracks, thereby preventing the detection agent from being applied in large quantities to the non-crack area of ​​the bridge, thereby preventing detection errors.

3. A method for detecting cracks in concrete at the bottom of a bridge according to claim 2, characterized in that: The nozzle (3) used comprises a rectangular tube body (4), a strip-shaped spray hole (5) is opened at the bottom of the tube body (4), a pair of piston blocks (6) arranged symmetrically on the left and right are installed in the tube body (4) along its axial direction in a sliding manner, and the piston block (6) comprises two rectangular blocks of different sizes, wherein the large rectangular block (601) is installed in the tube hole of the tube body (4) in a sliding manner, and the small rectangular block (602) is installed in the spray hole (5) in a sliding manner, and the storage space of the tube body (4) between the two piston blocks (6) is used to continuously fill with detection agent; during detection, according to the image of the camera, the nozzle (3) is moved along the path direction of the suspected crack (2), and then the detection agent is injected into the crack to fill the crack.

4. A method for detecting cracks in concrete at the bottom of a bridge according to claim 3, characterized in that: A flow meter is installed on the nozzle (3) and the input pipeline for supplementing the input detection agent. When the nozzle (3) is moved, if a crack is encountered, the flow meter will display a flow rate value greater than the set value, and the flow value will also increase accordingly.

5. A method for detecting cracks in concrete at the bottom of a bridge according to claim 3, characterized in that: The piston block (6) is connected to the tube body (4) via a hydraulic rod (7). When the two piston blocks (6) are driven by their respective hydraulic rods (7), they can move synchronously towards or away from each other to change the length of the storage space.

6. A method for detecting cracks in concrete at the bottom of a bridge according to claim 3, characterized in that: A self-rotating screw rod (8) is horizontally installed in one side wall of the tube body (4), and a threaded block (9) is threadedly installed on the screw rod (8). The threaded block (9) is fixedly connected to a mounting arm (10) of a moving device. Before the moving device moves the nozzle (3) along the path of the suspected crack (2), the driving motor (11) installed at the end of the screw rod (8) is started to rotate according to the distribution position of the suspected crack (2). Because the moving device does not move horizontally, the screw rod (8) and the tube body (4) where it is located move in a horizontal direction and then arrive at a position facing the suspected crack (2). Then the moving device moves forward, and in the process of moving forward, according to the extension path of the suspected crack (2), the driving motor (11) is used to continuously adjust the position of the tube body (4) so ​​that the tube body (4) always faces the current suspected crack (2) section.

7. A method for detecting cracks in concrete at the bottom of a bridge according to claim 1, characterized in that: The camera is linked with the nozzle (3), and the linkage method of the two is: The camera moves forward along the length of the bridge, and in the real-time photo within its field of view, the bridge position actually corresponding to the left edge of the photo is selected as the left limit boundary, and then the distance from the starting end of the suspected crack (2) in the real-time photo to the left edge in the horizontal direction is proportionally enlarged to the corresponding enlarged distance value horizontally to the right of the left limit boundary of the bottom of the bridge (1), which is the actual starting position of the suspected crack (2) at the bottom of the bridge (1) in the horizontal direction; The nozzle (3) is initially installed opposite to the left limit boundary. When the right suspected crack (2) is captured, the nozzle (3) is horizontally offset to the right by the adjustment displacement of the enlarged spacing value, and then moves forward to start from the starting end of the suspected crack (2). In the subsequent movement, the adjustment displacement that needs to be moved is calculated in real time based on the calculation method of the enlarged spacing value. When this adjustment displacement is higher than the previous value, the nozzle (3) continues to move horizontally to the right by an amount equal to the difference between the two adjustment displacements. If this adjustment displacement is lower than the previous value, the nozzle (3) needs to move horizontally back to the left by an amount equal to the difference between the two adjustment displacements, so that when the nozzle (3) moves forward, it always automatically follows the path direction of the suspected crack (2) captured by the camera, so as to detect the size of the corresponding crack in time.

8. A method for detecting concrete cracks at the bottom of a bridge according to claim 7, characterized in that: The calculation program of the linkage method is set on a program controller, and the program controller is fixedly installed on the sprinkler irrigation equipment.

9. A method for detecting cracks in concrete at the bottom of a bridge according to claim 6, characterized in that: The mobile device is a drone, and the top of the drone is provided with an elastic telescopic arm (14), and the top of the elastic telescopic arm (14) is provided with a contact plate (15) in contact with the bottom of the bridge. When the drone is flying, on the one hand, it needs to have a stable forward displacement, and on the other hand, it is also necessary to squeeze the elastic telescopic arm (14) to the limit so that the distance between the drone and the bottom of the bridge (1) always remains unchanged.

10. A method for detecting cracks in concrete at the bottom of a bridge according to claim 9, characterized in that: At least one auxiliary engine (16) is also provided on the drone, and the auxiliary engine (16) enables the drone to move vertically upward, so that the elastic telescopic arm (14) is always compressed to the limit during the drone's forward straight flight.

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