A UAV distance detection system and method for long-span bridges

Abstract

The present invention belongs to the technical field of large-span bridge inspection, and particularly relates to a drone fixed-distance inspection system and method for large-span bridges. The system solves the problem that existing drone systems for bridge inspection have difficulty maintaining close-range fixed-distance flight during bridge inspection, and that drones cannot approach narrow areas on bridges, resulting in unclear images and reduced crack identification accuracy. The inspection system includes a drone body and a drone flight control system; the system is special in that it also includes a liftable gimbal, a camera, a distance-measuring device, and an onboard computer; the liftable gimbal includes a lifting assembly and a drone gimbal; the fixed end of the lifting assembly is fixedly mounted on the upper surface of the drone body, and the free end of the lifting assembly is connected to the drone gimbal; the camera is mounted on the drone gimbal; the distance-measuring device is mounted on the camera; the onboard computer is respectively connected to the lifting assembly, the gimbal servo, the camera, and the distance-measuring device; and a fixed-height controller is integrated inside the onboard computer.

Description

Technical Field

[0001] The present invention belongs to the technical field of long-span bridge detection, and in particular relates to a UAV fixed-distance detection system and method for long-span bridges. Background Art

[0002] Regular, comprehensive bridge inspections are essential. Manual inspections of high-altitude, deep-water, wide, and complex bridges face significant engineering challenges, including difficulty, low efficiency, large blind spots, and reduced safety. Bridge inspections are particularly challenging in inaccessible areas beneath the bridge, leading to the use of drones for bridge inspections, a promising approach.

[0003] However, the design of traditional bridge inspection drone systems lacks specific considerations for the bridge environment. Due to complex environmental factors, including varying shooting angles, significant light fluctuations, and interference from surface attachments, image capture during bridge crack detection often suffers from unstable shooting, resulting in insufficient image clarity and, consequently, reduced crack identification accuracy. To improve the robustness of image detection systems, a preferred approach is to maintain a fixed, close distance between the drone and the bridge surface. This approach significantly mitigates light fluctuations and image jitter during capture, resulting in stable and clear images and providing superior image data for subsequent defect diagnosis. However, the underside of bridges is often complex, making close, fixed-distance flight challenging. This is particularly true for side beams, abutments, and piers, where confined space makes it difficult for drones to approach. Summary of the Invention

[0004] The purpose of the present invention is to provide a drone fixed-distance detection system and method for large-span bridges, so as to solve the technical problem that the existing bridge inspection drone system has certain difficulties in maintaining close-range fixed-distance flight during bridge inspection. For some narrow spaces on the bridge, the drone cannot get close, and the captured images are unclear, resulting in a reduced accuracy rate in crack identification.

[0005] The technical solution adopted by the present invention is a UAV distance detection system for long-span bridges, comprising a UAV body and a UAV flight control system; the UAV flight control system is used to control the flight of the UAV body; the special features of the UAV flight control system are:

[0006] It also includes a liftable gimbal, camera, rangefinder, and onboard computer;

[0007] The liftable gimbal includes a lifting assembly and a drone gimbal;

[0008] The fixed end of the lifting assembly is fixedly mounted on the upper surface of the drone body, and the free end of the lifting assembly is connected to the drone gimbal. The lifting assembly is used to realize the lifting and lowering of the drone gimbal;

[0009] The camera is mounted on the drone gimbal;

[0010] The distance measuring device is installed on the camera and is used to measure the distance from the camera to the surface of the bridge to be inspected in real time, and transmit the distance information to the onboard computer;

[0011] The onboard computer is connected to the lifting component, the gimbal servo of the UAV gimbal, the camera, and the ranging device respectively, and is used to control the lifting and lowering of the lifting component, the posture of the UAV gimbal, the switching of the camera, the shooting and saving of photos, the real-time distance measurement of the ranging device, the reception of distance information transmitted by the ranging device, and the transmission of the distance information to the UAV flight control system; the onboard computer is integrated with a fixed-altitude controller; the onboard computer is provided with a communication interface for communicating with the UAV flight control system; the fixed-altitude controller is used to calculate the control input of the UAV body under the premise of ensuring that a fixed distance is maintained between the camera and the surface to be inspected of the bridge, and transmit the control input to the UAV flight control system through the communication interface, so that the UAV flight control system controls the UAV body to maintain a fixed-distance flight.

[0012] Furthermore, in order to achieve highly stable and smooth control and thus enable the UAV body to maintain stable fixed-distance flight, the fixed-altitude controller is a fixed-altitude controller based on sliding mode control designed under the assumption that the UAV body performs fixed-attitude flight and flies parallel to the surface of the bridge to be inspected, and only considers the position of the UAV body.

[0013] Furthermore, the calculation process of the fixed height controller based on sliding mode control includes the following steps:

[0014] Step A: Establish the position dynamics model of the UAV body;

[0015] The position dynamics model of the drone body is shown in the following formula [1]:

[0016]

[0017] In formula [1]: U represents the lift generated by the drone body; Φ, Ψ, and θ represent the roll angle, yaw angle, and pitch angle of the drone body, respectively; x, y, and z represent the positions of the drone body in the horizontal, longitudinal, and vertical directions; m represents the mass of the drone body; and g represents the acceleration due to gravity.

[0018] Step B: Based on the position dynamics model of the UAV body established in step A, the position state equation of the UAV body in the vertical direction is obtained;

[0019] The vertical position state equation of the UAV body is shown in the following equation [2]:

[0020]

[0021] Let x1 = z, From the above formula [2], the following formula [3] can be obtained:

[0022]

[0023] Step C: Based on the vertical position state equation of the UAV body obtained in step B, define the error between the expected flight altitude of the UAV body and the actual altitude of the UAV body in the vertical direction;

[0024] The error between the expected flight height of the drone and the actual height of the drone in the vertical direction is defined as follows [4]:

[0025]

[0026] In formula [4]: represents the expected flight height of the drone; assuming that the height of the bridge surface to be inspected from the ground is h, the fixed distance to be maintained between the camera and the bridge surface to be inspected is d, and the height of the lifting component extended is l. When the lifting component is not extended, l = 0, then Calculate according to the following formula [5]:

[0027]

[0028] Step D: Design a sliding surface based on the error defined in step C, and derive the designed sliding surface;

[0029] The designed sliding surface is shown in the following formula [6]:

[0030]

[0031] In formula [6]: s represents the sliding surface; c is a constant and c>0;

[0032] Derivative s is obtained, and according to equations [3] and [4], the following equation [7] is obtained:

[0033]

[0034] Step E: Design the reaching law of the sliding surface;

[0035] The reaching law of the designed sliding surface is shown in the following formula [8]:

[0036]

[0037] In formula [8]: ε and r are constants, and ε>0, r>0; sgn(s) represents the sign function of s;

[0038] Step F: Calculate the control input of the UAV body, i.e., the lift generated by the UAV body, based on the derivative result of the sliding surface designed in Step D and the reaching law of the sliding surface designed in Step E;

[0039] Combining the above equations [7] and [8], the lift force generated by the drone body is obtained as shown in the following equation [9]:

[0040]

[0041] Furthermore, in order to ensure that the clarity of the captured image is sufficient to meet the requirements of crack identification and accurately identify cracks, and that the distance between the drone body and the bridge to be inspected is sufficient to ensure flight safety, in step C, the fixed distance d to be maintained between the camera and the surface of the bridge to be inspected should be in the range of 2m to 2.5m.

[0042] Furthermore, to prevent the GPS (Global Navigation System) signal from being disrupted by magnetic field disturbances when the drone is flying under a bridge, which could affect navigation, the drone flight control system uses a visual inertial odometry navigation method, which provides better navigation results.

[0043] Furthermore, the lifting assembly is a computer-controlled electric push rod with a maximum lifting distance of 0.8m to 1m. This makes the lifting assembly simple in structure, easy to purchase and remotely control, and can better meet the inspection needs of the specific bridge inspection scenario.

[0044] Furthermore, the distance measuring device is a laser rangefinder.

[0045] Furthermore, in order to prevent the drone body from being overloaded, causing the drone body to fly unstably and thus causing the captured images to be unclear, assuming that the rated load of the drone body is G, the total weight of the elevating gimbal, camera, ranging equipment and onboard computer should be less than 0.5G.

[0046] At the same time, the present invention also provides a method for detecting the distance between a long-span bridge and a drone. The method is special in that: the method is based on the above-mentioned long-span bridge drone detection system, and the method includes the following steps:

[0047] Step 1: Start the UAV flight control system and navigate the UAV to the surface area of the bridge to be inspected;

[0048] Step 2: Determine whether the surface of the bridge to be inspected is flat; if so, execute step 3 below, then skip step 4, and execute step 5 and subsequent steps; if not, skip step 3, and execute step 4 and subsequent steps below;

[0049] Step 3: The onboard computer activates the ranging device. Simultaneously, the drone's flight control system adjusts the drone's flight altitude based on the distance information from the camera to the bridge surface to be inspected, as measured in real time by the ranging device, until the distance from the camera to the bridge surface to be inspected, as measured in real time by the ranging device, equals the fixed distance that should be maintained between the camera and the bridge surface to be inspected.

[0050] Step 4: The onboard computer activates the ranging device and the lifting assembly. Simultaneously, the drone's flight control system adjusts the drone's flight altitude until the distance from the camera to the bridge surface to be inspected, as measured in real time by the ranging device, equals the fixed distance that should be maintained between the camera and the bridge surface to be inspected. The onboard computer then controls the lifting assembly to stop moving.

[0051] Step 5: The onboard computer starts the altitude controller to control the drone to maintain a fixed distance flight.

[0052] Step 6: The onboard computer turns on the camera, takes pictures of the bridge surface to be inspected, and saves the pictures. After the inspection is completed, the drone flies back.

[0053] Furthermore, the specific judgment process of judging whether the surface of the bridge to be inspected is flat in step 2 is as follows:

[0054] First, the onboard computer activates the distance measuring device, which performs dot-dot distance measurement, setting the distance measurement frequency to 10Hz and performing at least five consecutive distance measurements. The onboard computer then calculates the ratio of the upper and lower floating values of the distances between two consecutive measurements during the five consecutive distance measurements to the distance value measured by the preceding measurement, resulting in four ratios. If all four calculated ratios are less than or equal to 10%, the onboard computer determines that the surface of the bridge to be inspected is flat; otherwise, it determines that the surface of the bridge to be inspected is uneven. This eliminates the need for additional equipment and allows the distance measuring device to be used directly to determine whether the bridge surface to be inspected is flat, simplifying the overall structure of the inspection system.

[0055] The beneficial effects of the present invention are:

[0056] (1) The UAV fixed-distance detection system for large-span bridges of the present invention comprises a distance measuring device and an onboard computer; the distance measuring device can measure the distance from the camera to the surface of the bridge to be detected in real time, and transmit the distance information to the onboard computer; the onboard computer is provided with a communication interface for communicating with the UAV flight control system, through which the distance information can be transmitted to the UAV flight control system, and the UAV flight control system can adjust the flight altitude of the UAV body according to the detection requirements; and, in the present invention, a fixed-height controller is integrated inside the onboard computer, and the fixed-height controller calculates the control input of the UAV body under the premise of ensuring that a fixed distance is maintained between the camera and the surface of the bridge to be detected, and transmits the control input to the UAV flight control system through the communication interface, and the UAV body is controlled by the UAV flight control system, so that the UAV body can maintain a stable fixed-distance flight; therefore, the UAV fixed-distance detection system for large-span bridges of the present invention can make the existing bridge detection system more practical. The problem that it is difficult for a drone system to maintain a close-range and fixed-distance flight when inspecting a bridge is solved. At the same time, the drone fixed-distance inspection system for large-span bridges of the present invention includes a liftable gimbal, and the lifting and lowering of the drone gimbal are driven by the lifting and lowering of the lifting component in the liftable gimbal, thereby driving the lifting and lowering of the camera installed on the drone gimbal. In this way, for some uneven and complex surfaces on the bridge that are too small for drones to approach, the lifting and lowering of the lifting component can be controlled by the onboard computer, so that the camera can approach the surface of the bridge to be inspected and maintain a fixed distance from the surface of the bridge to be inspected, ultimately ensuring that a clear inspection image is obtained and the accuracy of crack identification is increased. Therefore, the present invention solves the technical problem that the existing bridge inspection drone system has a certain difficulty in maintaining a close-range and fixed-distance flight when inspecting bridges, and the drone cannot approach some narrow parts of the bridge, so that the captured image is unclear, resulting in a reduced accuracy rate in crack identification.

[0057] (2) The fixed height controller in the present invention preferably adopts a fixed height controller based on sliding mode control, which can achieve highly stable and smooth control, effectively reduce the camera shake during the shooting process, ensure the continuity of the camera during the shooting process, and improve the accuracy of disease image detection.

[0058] (3) The drone fixed-distance detection system and method for large-span bridges of the present invention can detect not only the flat surface under the bridge, but also the uneven and complex surfaces under the bridge, such as the narrow space areas such as the side beams, abutments, and piers of the bridge, and can meet the overall detection needs of the bridge. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 is a schematic diagram of using an embodiment of the system of the present invention to inspect a bridge;

[0060] Figure 2 It is a flow chart of an embodiment of the method of the present invention.

[0061] The descriptions of the numbers in the figure are as follows:

[0062] 1-UAV body, 2-elevable gimbal, 3-camera, 4-distance measuring device, 5-onboard computer, d-fixed distance to be maintained between the camera and the bridge surface to be inspected, l-height of the extended lifting assembly. DETAILED DESCRIPTION

[0063] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0064] See also Figure 1 The present invention provides a UAV distance detection system for a large-span bridge, including a UAV body 1, a UAV flight control system, a liftable gimbal 2, a camera 3, a distance measuring device 4 and an onboard computer 5.

[0065] The above-mentioned UAV flight control system is used to control the flight of the UAV body 1; the above-mentioned liftable gimbal 2 includes a lifting component and a UAV gimbal; the fixed end of the lifting component is fixedly installed on the upper surface of the UAV body 1, and the free end of the lifting component is connected to the UAV gimbal, and the lifting component is used to realize the lifting and lowering of the UAV gimbal; the above-mentioned camera 3 is installed on the UAV gimbal; the above-mentioned distance measuring device 4 is installed on the camera 3, and is used to measure the distance from the camera 3 to the surface to be detected of the bridge in real time, and transmit the distance information to the onboard computer 5; the above-mentioned onboard computer 5 is connected to the lifting component, the gimbal servo of the UAV gimbal, the camera 3, and the distance measuring device 4 respectively, and is used to control the lifting and lowering of the lifting component respectively. The onboard computer 5 is integrated with a fixed-altitude controller; the onboard computer 5 is provided with a communication interface for communicating with the UAV flight control system; the fixed-altitude controller is used to calculate the control input of the UAV body 1 under the premise of ensuring that a fixed distance is maintained between the camera 3 and the surface to be inspected of the bridge, and transmit the control input to the UAV flight control system through the above-mentioned communication interface, so as to control the UAV body 1 to maintain a fixed distance flight through the UAV flight control system.

[0066] In order to achieve stable and smooth altitude control and thus maintain stable fixed-distance flight of the UAV body, in this embodiment, the altitude control controller is designed based on sliding mode control, assuming that the UAV body 1 is performing attitude-fixed flight and flying parallel to the bridge surface to be inspected, and only considering the position of the UAV body 1. In this embodiment, the calculation process of the altitude control controller based on sliding mode control includes the following steps:

[0067] Step A: Establishing a position dynamics model of the UAV body 1;

[0068] The position dynamics model of the drone body 1 is shown in the following formula [1]:

[0069]

[0070] In formula [1]: U represents the lift generated by the drone body 1; Φ, Ψ, and θ represent the roll angle, yaw angle, and pitch angle of the drone body 1, respectively; y and z represent the positions of the drone body 1 in the horizontal, longitudinal, and vertical directions; m represents the mass of the drone body 1; g represents the acceleration due to gravity;

[0071] Step B: Based on the position dynamics model of the drone body 1 established in step A, the position state equation of the drone body 1 in the vertical direction is obtained;

[0072] The vertical position state equation of the drone body 1 is shown in the following equation [2]:

[0073]

[0074] make From the above formula [2], we can get the following formula [3]:

[0075]

[0076] Step C: Based on the vertical position state equation of the drone body 1 obtained in step B, define the error between the expected flight altitude of the drone body 1 and the actual altitude of the drone body 1 in the vertical direction;

[0077] The error between the expected flight height of the drone body 1 and the actual height of the drone body 1 in the vertical direction is defined as follows [4]:

[0078]

[0079] In formula [4]: represents the expected flight height of the drone 1; assuming that the height of the bridge surface to be inspected from the ground is h, the fixed distance to be maintained between the camera 3 and the bridge surface to be inspected is d, and the height of the lifting component extended is l. When the lifting component is not extended, l = 0, then Calculate according to the following formula [5]:

[0080]

[0081] Step D: Design a sliding surface based on the error defined in step C, and derive the designed sliding surface;

[0082] The designed sliding surface is shown in the following formula [6]:

[0083]

[0084] In formula [6]: s represents the sliding surface; c is a constant and c>0;

[0085] Derivative s, and according to the above formulas [3] and [4], we can get the following formula [7]:

[0086]

[0087] Step E: Design the reaching law of the sliding surface;

[0088] The reaching law of the designed sliding surface is shown in the following formula [8]:

[0089]

[0090] In formula [8]: ε and r are constants, and ε>0, r>0; sgn(s) represents the sign function of s;

[0091] Step F: Calculate the control input of the UAV body 1 , i.e., the lift generated by the UAV body 1 , based on the derivative result of the sliding surface designed in Step D and the reaching law of the sliding surface designed in Step E;

[0092] Combining the above equations [7] and [8], the lift force generated by the drone body 1 is obtained as shown in the following equation [9]:

[0093]

[0094] Next, consider the stability of the sliding surface and take the following Lyapunov function as formula

[10] :

[0095]

[0096] Derivative of the above formula

[10] yields formula

[11] :

[0097]

[0098] From the above formula

[11] , we can see that s is convergent. Since s converges to 0, we can know that e and The signs must be opposite, so there must be e and It converges to 0, that is, the tracking error tends to 0. From this, it can be seen that the expected height control can be achieved, and then the fixed-distance flight of the UAV body can be achieved.

[0099] In order to ensure that the clarity of the captured image is sufficient to meet the requirements of crack identification and accurately identify cracks, and to ensure that the distance between the drone body and the bridge to be inspected is sufficient to ensure flight safety, in this embodiment, preferably, in the above step C, the fixed distance d to be maintained between the camera 3 and the surface of the bridge to be inspected has a value range of 2m to 2.5m.

[0100] In order to prevent the GPS (Global Navigation System) signal from being interfered with and affecting the navigation effect due to magnetic field disturbances when the drone body is flying under the bridge, in this embodiment, the drone flight control system preferably adopts a visual inertial odometer navigation method, which has a good navigation effect.

[0101] In this embodiment, the lifting assembly is preferably a computer-controlled electric push rod with a maximum lifting distance of 0.8m to 1m. This provides a simple lifting assembly structure, facilitates procurement and remote control, and better meets the inspection requirements of the specific bridge inspection scenario.

[0102] In this embodiment, the distance measuring device 4 is a laser rangefinder. In addition to being the laser rangefinder of this embodiment, the distance measuring device 4 may also be other distance measuring devices.

[0103] To prevent the drone from being overloaded, causing unstable flight and unclear images, assuming the rated load of the drone 1 is G, the total weight of the elevating gimbal 2, camera 3, ranging device 4, and onboard computer 5 should be less than 0.5G. In this embodiment, the camera 3 is a high-definition camera.

[0104] See also Figure 2 The present invention also provides a method for detecting the distance between a long-span bridge and an unmanned aerial vehicle. The method is based on the above-mentioned system for detecting the distance between a long-span bridge and an unmanned aerial vehicle. The method comprises the following steps:

[0105] Step 1: The UAV flight control system is started, and the UAV body 1 is navigated to the surface area of the bridge to be inspected;

[0106] Step 2: Determine whether the surface of the bridge to be inspected is flat; if so, execute step 3 below, then skip step 4, and execute step 5 and subsequent steps; if not, skip step 3, and execute step 4 and subsequent steps below;

[0107] Step 3: The onboard computer 5 activates the distance measuring device 4. Simultaneously, the UAV flight control system adjusts the flight altitude of the UAV body 1 based on the distance information from the camera 3 to the bridge surface to be inspected measured in real time by the distance measuring device 4 until the distance from the camera 3 to the bridge surface to be inspected measured in real time by the distance measuring device 4 equals the fixed distance to be maintained between the camera 3 and the bridge surface to be inspected.

[0108] Step 4: The onboard computer 5 activates the distance measuring device 4 and the lifting assembly. Simultaneously, the UAV flight control system adjusts the flight altitude of the UAV body 1 until the distance between the camera 3 and the bridge surface to be inspected, as measured in real time by the distance measuring device 4, equals the fixed distance to be maintained between the camera 3 and the bridge surface to be inspected. The onboard computer 5 then controls the lifting assembly to stop moving upwards or downwards.

[0109] Step 5: The onboard computer 5 starts the altitude control device to control the drone body 1 to maintain a fixed distance flight;

[0110] Step 6: The onboard computer 5 turns on the camera 3, takes pictures of the bridge surface to be inspected, and saves the pictures. After the inspection is completed, the drone flies back.

[0111] In this embodiment, the specific judgment process of judging whether the surface of the bridge to be inspected is flat in step 2 is preferably as follows:

[0112] First, the onboard computer 5 starts the distance measuring device 4, which performs dot distance measurement, sets the distance measurement frequency to 10 Hz, and performs distance measurement continuously for at least 5 times; then, the onboard computer 5 calculates the ratio of the upper and lower floating values of the distances of two adjacent measurements in the 5 consecutive distance measurements to the distance value measured in the previous measurement of the above two adjacent measurements, and calculates four ratios; if the four calculated ratios are all less than or equal to 10%, the onboard computer 5 determines that the surface of the bridge to be inspected is flat; otherwise, it determines that the surface of the bridge to be inspected is uneven.

[0113] This embodiment uses the above method to determine whether the surface of the bridge to be inspected is flat. When making the judgment, no additional equipment is needed. The distance measuring device 4 can be directly used to complete the judgment of whether the surface of the bridge to be inspected is flat, and the overall structure of the detection system is simpler.

[0114] The drone distance detection system and method for large-span bridges of the present invention can not only detect the flat surface under the bridge, but also detect the uneven and complex surfaces under the bridge, such as the side beams, abutments, piers and other narrow spaces of the bridge, and can meet the overall detection needs of the bridge.

Claims

1. A UAV distance detection system for a long-span bridge, comprising a UAV body (1) and a UAV flight control system; the UAV flight control system is used to control the flight of the UAV body (1); and is characterized in that: It also includes a liftable platform (2), a camera (3), a distance measuring device (4) and an onboard computer (5); The liftable platform (2) comprises a lifting assembly and a drone platform; The fixed end of the lifting component is fixedly mounted on the upper surface of the drone body (1), and the free end of the lifting component is connected to the drone platform. The lifting component is used to realize the lifting and lowering of the drone platform. The camera (3) is mounted on the drone platform; The distance measuring device (4) is installed on the camera (3) and is used to measure the distance between the camera (3) and the surface to be inspected of the bridge in real time, and transmit the distance information to the onboard computer (5); The onboard computer (5) is connected to the lifting component, the gimbal servo of the unmanned aerial vehicle (UAV) gimbal, the camera (3), and the distance measuring device (4) respectively, and is used to control the lifting and lowering of the lifting component, the posture of the UAV gimbal, the switch of the camera (3), the shooting and saving of photos, the real-time distance measurement of the distance measuring device (4), the reception of distance information input by the distance measuring device (4), and the transmission of the distance information to the UAV flight control system; the onboard computer (5) is internally integrated with a fixed-height controller; the onboard computer (5) is provided with a communication interface for communicating with the UAV flight control system; the fixed-height controller is used to calculate the control input of the UAV body (1) under the premise of ensuring that a fixed distance is maintained between the camera (3) and the surface to be detected of the bridge, and transmit the control input to the UAV flight control system through the communication interface, so that the UAV body (1) is controlled by the UAV flight control system to maintain a fixed-distance flight; The altitude controller is designed based on sliding mode control, assuming that the drone body (1) performs attitude-fixed flight and maintains parallel flight with the surface of the bridge to be inspected, and only considering the position of the drone body (1); The calculation process of the fixed height controller based on sliding mode control includes the following steps: Step A: Establish the position dynamics model of the UAV body (1); The position dynamics model of the drone body (1) is shown in the following formula [1]: In formula [1]: U represents the lift generated by the drone body (1); Φ, Ψ, and θ represent the roll angle, yaw angle, and pitch angle of the drone body (1), respectively; y and z represent the positions of the drone body (1) in the horizontal, longitudinal and vertical directions; m represents the mass of the drone body (1); g represents the acceleration due to gravity; Step B: Based on the position dynamics model of the drone body (1) established in step A, the position state equation of the drone body (1) in the vertical direction is obtained; The vertical position state equation of the drone body (1) is shown in the following equation [2]: make From the above formula [2], the following formula [3] can be obtained: Step C: Based on the position state equation of the drone body (1) in the vertical direction obtained in step B, define the error between the expected flight altitude of the drone body (1) and the actual altitude of the drone body (1) in the vertical direction; The error between the expected flight height of the drone body (1) and the actual height of the drone body (1) in the vertical direction is defined as follows [4]: In formula [4]: represents the expected flight height of the drone body (1); assuming that the height of the bridge surface to be inspected from the ground is h, the fixed distance to be maintained between the camera (3) and the bridge surface to be inspected is d, and the height of the lifting component extended is l. When the lifting component is not extended, l = 0, then Calculate according to the following formula [5]: Step D: Design a sliding surface based on the error defined in step C, and derive the designed sliding surface; The designed sliding surface is shown in the following formula [6]: In formula [6]: s represents the sliding surface; c is a constant and c>0; Derivative s is obtained, and according to equations [3] and [4], the following equation [7] is obtained: Step E: Design the reaching law of the sliding surface; The reaching law of the designed sliding surface is shown in the following formula [8]: In formula [8]: ε and r are constants, and ε>0, r>0; sgn(s) represents the sign function of s; Step F: Calculate the control input of the UAV body (1), i.e., the lift generated by the UAV body (1), based on the derivative result of the sliding surface designed in step D and the convergence law of the sliding surface designed in step E; Combining the above equations [7] and [8], the lift force generated by the drone body (1) is obtained as shown in the following equation [9]:

2. The UAV distance detection system for long-span bridges according to claim 1 is characterized by: In step C, the fixed distance d to be maintained between the camera (3) and the surface of the bridge to be inspected has a value range of 2m to 2.5m.

3. The UAV distance detection system for long-span bridges according to claim 1 is characterized by: The UAV flight control system adopts a visual inertial odometry navigation method.

4. The UAV distance detection system for long-span bridges according to claim 1 is characterized by: The lifting component is an electric push rod that can be controlled by a computer, and the maximum lifting distance of the electric push rod is 0.8m to 1m.

5. The UAV distance detection system for long-span bridges according to claim 1 is characterized by: The distance measuring device (4) is a laser rangefinder.

6. The UAV distance detection system for long-span bridges according to claim 1 is characterized by: Assuming that the rated load of the drone body (1) is G, the total weight of the elevating gimbal (2), camera (3), ranging equipment (4) and onboard computer (5) should be less than 0.5G.

7. A method for detecting distance between a long-span bridge and an unmanned aerial vehicle, characterized by: The method is based on the UAV distance detection system for a long-span bridge according to any one of claims 1 to 6, and comprises the following steps: Step 1: The UAV flight control system is started, and the UAV body (1) is navigated to the surface area of the bridge to be inspected; Step 2: Determine whether the surface of the bridge to be inspected is flat; if so, execute step 3 below, then skip step 4, and execute step 5 and subsequent steps; if not, skip step 3, and execute step 4 and subsequent steps below; Step 3: The onboard computer (5) starts the distance measuring device (4), and at the same time, the UAV flight control system adjusts the flight altitude of the UAV body (1) according to the distance information from the camera (3) to the bridge surface to be inspected measured in real time by the distance measuring device (4), until the distance from the camera (3) to the bridge surface to be inspected measured in real time by the distance measuring device (4) is equal to the fixed distance that should be maintained between the camera (3) and the bridge surface to be inspected; Step 4: The onboard computer (5) starts the distance measuring device (4) and the lifting assembly, and the UAV flight control system adjusts the flight altitude of the UAV body (1) until the distance between the camera (3) and the bridge surface to be inspected measured in real time by the distance measuring device (4) is equal to the fixed distance that should be maintained between the camera (3) and the bridge surface to be inspected, and the onboard computer (5) controls the lifting assembly to stop lifting; Step 5: The onboard computer (5) starts the altitude control device to control the drone body (1) to maintain a fixed distance flight; Step 6: The onboard computer (5) turns on the camera (3), takes pictures of the bridge surface to be inspected and saves the pictures. After the inspection is completed, the drone flies back.

8. The method for detecting the distance between a long-span bridge and an unmanned aerial vehicle according to claim 7, wherein: The specific judgment process of judging whether the surface of the bridge to be inspected is flat in step 2 is as follows: First, the onboard computer (5) starts the distance measuring device (4), and the distance measuring device (4) performs dot distance measurement, sets the distance measurement frequency to 10 Hz, and continuously measures the distance for at least 5 times; then, the onboard computer (5) calculates the ratio of the upper and lower floating values of the distances of two adjacent measurements in the five consecutive distance measurements to the distance value measured by the previous measurement in the two adjacent measurements, and calculates four ratios; If the four calculated ratios are all less than or equal to 10%, the onboard computer (5) determines that the surface of the bridge to be inspected is flat; Otherwise, it is determined that the surface of the bridge to be inspected is uneven.

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