An unmanned aerial vehicle towed underwater acoustic sonar survey system and method

Abstract

The application discloses an unmanned aerial vehicle (UAV) hoisting type underwater sonar survey system and method, which comprises an UAV, a sonar device, a winch mechanism, a cable and a tension sensor. The UAV is provided with a winch mechanism releasing device. The winch mechanism is connected with the sonar device through the cable. The releasing and recovery of the sonar device is controlled through the winch mechanism, and the releasing length of the cable is collected. The tension sensor is used for collecting the actual towing tension of the cable. A space solution matrix module composed of a plurality of height measuring units is arranged on the UAV. The flight control system carried by the UAV is electrically connected with the space solution matrix module, the winch mechanism and the tension sensor. The UAV, the airborne controllable winch, the composite towing cable, the space solution matrix module and the flight control attitude dynamic compensation are integrated to solve the technical problems of the limited water area being unable to be surveyed, the water surface platform being prone to failure, the near-water surface operation being unsafe and the hoisting flight attitude being unstable.

Description

Technical Field

[0001] This invention relates to the field of underwater exploration technology, and in particular to a UAV-mounted underwater sonar exploration system and method. Background Technology

[0002] In routine operations such as river regulation, underwater obstacle detection, channel siltation monitoring, riverbed topography mapping, and reservoir siltation investigation in water conservancy projects, side-scan sonar is commonly used to survey and detect underwater topography, bank slope structure, siltation layers, and underwater obstacles. Currently, the platforms for carrying traditional sonar are mainly divided into two categories: one is a professional manned survey vessel, and the other is an unmanned surface vessel (USV). These two types of platforms are the main operational carriers for underwater exploration in water conservancy projects.

[0003] However, traditional professional survey vessels have deep drafts and large overall size, making them unable to navigate narrow waterways, shallow areas, reservoir tailraces, and reservoir bays, hindering their ability to conduct riverbed surveys and underwater topographic mapping in these areas. This results in numerous blind spots and prevents comprehensive coverage. Furthermore, existing unmanned surface vessels (USVs) using towed side-scan sonar are susceptible to insufficient power and loss of control in turbulent waters. Additionally, their propellers are easily entangled and jammed by floating debris such as weeds, fishing nets, and household waste, leading to anything from interrupted or lost equipment, resulting in extremely low operational stability and safety.

[0004] With the widespread application of drone technology, using drones as a platform is currently a key research direction. However, traditional drone operation methods have many insurmountable problems: First, traditional drones mostly use single-point downward-looking barometers or ultrasonic sensors for altitude hold. When operating near the water surface, they are easily affected by water surface reflection, water mist, backlight, and water surface fluctuations. The altitude hold accuracy can only reach the decimeter or even meter level, which cannot guarantee the safe distance between the rotor and the water surface, making it very easy for the rotor to get wet or crash into the water. Second, underwater sonar will be subjected to complex nonlinear off-center torque generated by the water flow impact during towing. Traditional drone flight control systems can only adjust the attitude through passive PID feedback, which has a strong response lag and cannot effectively suppress the fuselage shaking and tilting caused by the towing force. This will not only lead to deviations in sonar detection data and a decrease in imaging quality, but in severe cases, it will also cause the drone to become unstable and crash. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of the prior art by providing an unmanned aerial vehicle (UAV) towed underwater sonar reconnaissance system and method.

[0006] The objective of this invention is achieved through the following technical solution: a drone-mounted underwater sonar reconnaissance system, comprising a drone, a sonar device, a winch mechanism, a cable, and a tension sensor. The drone is equipped with a winch mechanism for releasing the sonar device, which is connected to the sonar device via the cable. The winch mechanism controls the release and retrieval of the sonar device and collects the release length of the cable. The tension sensor is used to collect the actual towing force of the cable.

[0007] The UAV is equipped with a spatial calculation matrix module consisting of several altimeter units. The flight control system on the UAV is electrically connected to the spatial calculation matrix module, the winch mechanism, and the tension sensor. The flight control system receives altitude vector data from the spatial calculation matrix module to calculate the UAV's reference altitude and attitude data. Based on the cable length and the actual towing tension of the cable, it calculates the underwater towing off-center load moment. Based on the towing off-center load moment, it adjusts the UAV's power distribution matrix to maintain the UAV's safe lift-off altitude and attitude stability.

[0008] Preferably, the drone is a quadcopter drone, which is equipped with two front rotor motors and two rear rotor motors, with a height measuring unit located below each of the front and rear rotor motors.

[0009] Preferably, the height measurement unit is a millimeter-wave radar.

[0010] As a preferred option, it also includes a ground-based telemetry and data processing terminal, which communicates with the UAV's flight control system for setting operational parameters, planning reconnaissance routes, remotely controlling the UAV, analyzing sonar equipment detection data, and modeling water and terrain features.

[0011] A method for underwater sonar reconnaissance using a drone-mounted system includes the following steps: Step 1: Presetting operational parameters and route planning; Step 2: The UAV flies to the target operating altitude above the target operation starting point. The winch mechanism lowers the sonar equipment to the preset underwater depth. The UAV flies at a fixed altitude along the planned route at a preset speed. The spatial calculation matrix module calculates the current fuselage reference altitude and current attitude data of the UAV in real time. Based on the theoretical drag resistance, the actual drag force of the cable, the cable release length, and the cable force tilt angle, the underwater drag load moment is calculated. The thrust redistribution calculation model adjusts the power distribution of each rotor of the UAV based on the underwater drag load moment to maintain the UAV at a constant safe lift-off altitude and attitude stability. Step 3: After the scanning task is completed, the drone retracts the cable via a winch mechanism while hovering, and finally the drone returns to base.

[0012] Preferably, in step two, the formula for calculating the reference height H of the UAV fuselage is as follows: H = (H1 + H2 + H3 + H4) / 4; The attitude data of the UAV includes pitch angle and roll angle, where the pitch angle θ is calculated using the following formula: ; The formula for calculating the roll angle Ф is as follows: ; H1 is the water surface height data measured by the altimeter unit located below the front rotor motor on the left side of the drone; H2 is the water surface height data measured by the altimeter unit located below the front rotor motor on the right side of the drone; H3 is the water surface height data measured by the altimeter unit located below the rear rotor motor on the left side of the drone; and H4 is the water surface height data measured by the altimeter unit located below the rear rotor motor on the right side of the drone. The lateral distance between the left and right rows of altimeter units on the drone; This refers to the longitudinal distance between the front and rear rows of altimeter units on the UAV.

[0013] Preferably, in step two, when the current reference altitude of the UAV is lower than the safe altitude threshold, the flight control system adjusts the rotation speed of the UAV's rotor motors to climb to above the safe altitude threshold. When the current pitch angle of the UAV exceeds the preset safe pitch angle range, the UAV performs attitude correction to bring the UAV's pitch angle back to the preset safe pitch angle range. When the current roll angle of the UAV exceeds the preset safe roll angle range, the UAV performs attitude correction to bring the UAV's roll angle back to the preset safe roll angle range.

[0014] As a preferred method, the calculation of the underwater towing eccentric load moment in step two is as follows: S1: Obtain the ground velocity vector of the UAV and combine it with the actual water flow velocity in the river to obtain the sonar equipment's drag velocity relative to the river flow; calculate the theoretical drag resistance estimate based on the sonar equipment's drag velocity relative to the river flow, the sonar's water-facing cross-sectional area, and the sonar equipment's fluid resistance coefficient; obtain the actual drag force of the cable through the tension sensor on the winch mechanism. S2: Calculate the force angle of the cable relative to the vertical direction based on the inherent gravity of the sonar equipment, the inherent buoyancy of the sonar equipment in the water, and the actual drag force of the cable; and calculate the component of the actual drag force of the cable in the direction of the water flow resistance of the sonar equipment based on the force angle. S3: The component of the actual drag force of the cable in the direction of the water flow resistance of the sonar equipment is weighted and fused with the theoretical drag resistance estimate to obtain a high-confidence drag resistance value. S4: Obtain the cable release length, calculate the offset lever arm length based on the cable release length and the cable's force tilt angle relative to the vertical direction, and take the product of the offset lever arm length and the high-confidence towing resistance value as the underwater towing offset torque.

[0015] Preferably, the theoretical drag resistance estimate F dt The calculation formula is as follows: ； The angle of inclination of the cable relative to the vertical direction The calculation formula is as follows: ; In the formula, ρ is the density of water, and C d Let be the fluid resistance coefficient of the sonar device, A be the water-facing cross-sectional area of ​​the sonar device, and V be the drift velocity of the sonar device relative to the river flow. The actual drag force of the cable is m. s g is the inherent gravity of the sonar device, F b This refers to the inherent buoyancy of the sonar device in water.

[0016] As a preferred approach, the thrust redistribution calculation model adopts a control structure combining cascaded PID and feedforward compensation. The product of the calculated underwater towed off-center load torque and the feedforward gain coefficient is used as the feedforward compensation amount and superimposed on the PID feedback control amount to obtain the total pitch control output. Then, the total pitch control output is converted into PWM duty cycle commands for the four rotor motors of the UAV through the power distribution matrix. The lift of the front and rear rotor motors of the UAV is adjusted by the PWM duty cycle commands to generate a restoring torque that completely cancels out the underwater towed off-center load torque.

[0017] The beneficial effects of this invention are: 1. This invention addresses industry pain points such as large blind spots in existing hydraulic sonar survey platforms, easy entanglement and failure of surface platforms, high risk of drone crashes near the water surface, and unstable towing flight attitude. It proposes an integrated architecture combining a drone, an airborne controllable winch, a composite towing cable, a spatial calculation matrix module, and dynamic flight control attitude compensation. This solves the technical challenges of restricted hydraulic engineering areas being inaccessible for surveying, easy failure of surface platforms, unsafe near-water operations, and unstable towing flight attitude. Employing an aerial towing operation mode, it completely avoids entanglement with floating debris and grounding in shallow water, covering areas inaccessible to ships and unmanned vessels, such as narrow rivers, shallows, reservoir bays, and reservoir tailraces, achieving full coverage of hydraulic engineering surveys. An altimeter unit is deployed below the drone rotor, providing strong resistance to water surface reflection, water mist, and backlight interference, achieving centimeter-level accurate altitude determination near the water surface. Combined with closed-loop altitude threshold protection, it automatically warns and corrects course during ascent, avoiding the risk of rotor water contact or crashing (falling into the water).

[0018] 2. Unlike traditional single-point ranging, this solution innovatively deploys altimeter units on the UAV to form a spatial calculation matrix module. Combined with the unique spatial geometric model of the flight control system, the system effectively eliminates the error interference caused by water surface fluctuations (such as 0.3m-0.5m) on single-point ranging at the algorithm level by fusing and calculating four sets of independent altitude data. This "multi-node physical distribution + spatial geometric fusion" architecture design breaks through the data limitations of a single sensor and truly provides a highly robust accuracy benchmark for near-water surface towing operations.

[0019] 3. Addressing the industry-wide technical challenge of underwater nonlinear drag easily causing UAVs to tilt or even crash while heavily loaded with towed sonar, this invention breaks through the limitations of traditional radar which only performs "one-dimensional altitude measurement." The system not only maintains stable ranging under harsh conditions such as water mist and strong reflection, but also extracts the roll and pitch attitude distortion trends of the fuselage in real time in dynamic environments. This high-precision multi-dimensional spatial perception data is seamlessly integrated into the system's "multi-source tension fusion and dynamic feedforward control law" as a core benchmark. This enables the UAV to pre-trigger thrust redistribution of the underlying motors before macroscopic displacement occurs due to off-center load disturbance, achieving a deep collaborative closed loop of "physical environment perception - attitude decoupling - feedforward dynamic compensation."

[0020] 4. To address the complex nonlinear off-center loads generated by water flow impacts on sonar, the flight control system integrates dual control of "cascaded PID + feedforward compensation." This invention does not simply rely on passive feedback (PID) after fuselage tilting, but rather integrates in real time the spatial attitude perceived by the spatial calculation matrix, the instantaneous tension captured by the tension sensor, and the cable release length fed back by the motor encoder of the winch mechanism. It directly maps the complex drag torque into the off-center load torque Md at the fuselage mounting point. Before the UAV undergoes macroscopic displacement, it adjusts the power of each rotor by actively redistributing the PWM duty cycle of the UAV rotor motor (i.e., pre-increasing the lift on the force-bearing side). This mechanism fundamentally eliminates the interference of nonlinear drag force on the UAV's fuselage attitude, providing a highly stable aerial suspension platform for sonar reconnaissance and ensuring the accuracy and consistency of the survey data. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of the UAV-mounted underwater sonar reconnaissance system of the present invention.

[0022] Figure 2 This is a schematic diagram of the UAV-mounted underwater sonar reconnaissance system of the present invention during operation.

[0023] Figure 3 This is a schematic diagram of the process of the UAV-mounted underwater sonar reconnaissance method of the present invention.

[0024] In the diagram: 1. Unmanned aerial vehicle (UAV), 2. Cable, 3. Winch mechanism, 4. Sonar equipment, 5. Flight control system, 6. Altitude measurement unit, 7. Ground measurement and control and data processing terminal. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0026] Those skilled in the art should understand that, in the disclosure of this invention, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," 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 invention 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, the above terms should not be construed as limiting this invention.

[0027] It is understood that the term "a" should be understood as "at least one" or "one or more", that is, in one embodiment, the number of an element can be one, while in another embodiment, the number of the element can be multiple, and the term "a" should not be understood as a limitation on the number.

[0028] like Figures 1 to 3 As shown, a UAV-mounted underwater sonar reconnaissance system includes a UAV 1, a sonar device 4, a winch mechanism 3, a cable 2, and a tension sensor. The UAV 1 is equipped with a winch mechanism 3 for releasing the winch mechanism 3. The winch mechanism 3 is connected to the sonar device 4 via the cable 2. The winch mechanism 3 controls the release and retrieval of the sonar device 4 and collects the release length of the cable. The tension sensor is used to collect the actual towing tension of the cable. The UAV 1 is equipped with a spatial calculation matrix module consisting of several altimeter units 6. The flight control system 5 carried by the UAV 1 is electrically connected to the spatial calculation matrix module, the winch mechanism 3, and the tension sensor. The flight control system 5 calculates the reference height and attitude data of the UAV 1 by receiving the altitude vector data from the spatial calculation matrix module. Based on the cable 2 release length and the actual towing tension of the cable, it calculates the underwater towing off-center load moment. Based on the towing off-center load moment, it adjusts the power distribution matrix of the UAV 1 to maintain the safe lift-off altitude and attitude stability of the UAV 1.

[0029] Among them, UAV 1 is a quadcopter UAV 1. UAV 1 is equipped with two front rotor motors and two rear rotor motors. A height measuring unit 6 is installed below each of the front rotor motors and the rear rotor motors.

[0030] The UAV 1 adopts an H-type frame layout structure, including two parallel arms arranged on the left and right. Each arm has a rotor motor at both the front and rear ends. The rotor motors located at the front ends of the two arms are the front rotor motors, and the two rotor motors located at the rear ends of the arms are the rear rotor motors. Compared with conventional cross-shaped or X-shaped frames, the unobstructed front end of this structure effectively avoids interference between the cable 2 and the fuselage when towed forward.

[0031] In this embodiment, the altimeter unit 6 is a millimeter-wave radar. The millimeter-wave radar operates at a wavelength of 1-10mm, giving it a strong anti-interference capability against complex marine environments. It has good resistance to weather interference, surface interference, and electromagnetic interference. It can penetrate fog, rain, snow, and salt spray, and is not affected by lighting conditions. It can still work normally under extreme weather conditions such as night, heavy rain, and dense fog.

[0032] The spatial calculation matrix module consists of a spatial calculation array composed of four high-performance millimeter-wave radars (altitude measurement unit 6), with the probe of altitude measurement unit 6 facing vertically towards the water surface. Altitude measurement unit 6 provides the most crucial underlying physical data support for the subsequent spatial geometric calculation and dynamic feedforward compensation of the UAV 1's roll and pitch attitude distortions.

[0033] The bottom of the UAV 1 has a standardized external expansion platform reserved as the physical support base for the winch mechanism 3. The fuselage has a built-in power battery and drive module, and its four rotor motors serve as the execution terminals for feedforward compensation, controlled by the power distribution matrix (Mixer) of the flight control system 5.

[0034] Sonar device 4 employs a dual-frequency side-scan sonar system, which is connected to the winch mechanism 3 via cable 2. Sonar device 4 integrates a transducer, underwater attitude sensor, and data acquisition and storage unit. After being lowered to a designated underwater depth, sonar device 4 moves at a constant speed along the river channel with UAV 1, continuously emitting ultrasonic waves to complete full-coverage side-scan imaging detection of the riverbed, bank slopes, underwater obstacles, and siltation layers.

[0035] The winch mechanism 3, as an independent external payload, is fixedly mounted on the external extension platform at the center of the bottom of the UAV 1. Internally, it features a clear dynamic mechanical transmission and sensing linkage: the power output shaft of the servo motor built into the winch mechanism 3 is mechanically coupled to the input end of the self-locking reduction gear; the output end of this self-locking reduction gear is coaxially and fixedly connected to the take-up and untake-down reel, directly driving the reel to rotate forward and backward to achieve the take-up and untake-down of the cable 2. The encoder built into the servo motor can transmit pulse signals to the UAV 1's flight hole system in real time for integral calculation of the real-time release length L of the cable 2; a tension sensor is connected in series between the main load-bearing base of the winch mechanism 3 and the external extension platform of the UAV 1 (i.e., at the force transmission node between the winch mechanism 3 and the UAV 1) to monitor and collect the transient drag force (the actual drag force of the cable) borne by the lowered cable 2 in real time. The winch mechanism 3 has power-off self-locking and overload anti-breakage functions, and the physical parameters it collects directly provide underlying hardware data support for the system's "multi-source data fusion and hydrodynamic feedforward control law".

[0036] The cable 2 integrates a power supply line and a high-speed data transmission line, and the outer layer is made of waterproof and wear-resistant material. The upper end of the cable 2 is wound and fixed to the winch mechanism 3, and the lower end is connected to the sonar equipment 4. The cable 2 simultaneously undertakes the triple functions of aerial towing, underwater equipment power supply, and real-time transmission of hydraulic sonar exploration and detection data. It has high tensile strength, is resistant to water flow and aquatic weed friction and corrosion, and is suitable for long-term field hydraulic water operations.

[0037] The flight control system 5 on UAV 1 integrates an RTK module, inertial navigation, a six-axis attitude sensor, and a wireless communication unit; it is responsible for UAV 1's flight path planning, speed adjustment, hovering control, winch linkage control, millimeter-wave radar data reception, and altitude closed-loop protection; it realizes two water conservancy exploration operation modes: fully automatic towing along the river and semi-manual operation. By utilizing millimeter-wave radar altitude closed-loop protection and dynamic compensation algorithms, it effectively reduces the detection deviation caused by towing fluctuations and improves the smoothness of the entire system operation.

[0038] The present invention also includes a ground-based telemetry and data processing terminal 7, which is communicatively connected to the flight control system 5 of the UAV 1. This terminal is used for setting operational parameters, planning reconnaissance routes, remotely controlling the UAV 1, analyzing sonar data, and modeling hydraulic terrain. The ground-based telemetry and data processing terminal 7 includes a remote control unit, real-time image transmission display, a route planning interface, and sonar data analysis and hydraulic terrain modeling software. It can remotely start and stop the UAV 1, control the winch's retrieval and deployment, set safe altitude thresholds and sonar descent depths, and transmit the UAV 1's flight status, radar ranging data, and real-time sonar reconnaissance scan images in real time. After the operation is completed, it automatically performs data stitching, noise reduction, and terrain modeling, generating underwater topographic maps of narrow river channels, channel siltation distribution maps, and hydraulic obstacle distribution maps, providing data support for water conservancy and river regulation and waterway maintenance.

[0039] A method for underwater sonar reconnaissance using a drone-mounted system includes the following steps: Step 1: Presetting operational parameters and route planning; In this step, the target river electronic map is loaded into the ground control and data processing terminal 7, and the reconnaissance flight route is planned autonomously; the target operating altitude of UAV 1, the underwater exploration depth of the sonar equipment, and the towing flight speed of UAV 1 are preset, and the safe altitude threshold of UAV 1 is preset simultaneously to complete the initialization of the operation parameters.

[0040] Step 2: UAV 1 flies to the target operating altitude above the target operation starting point. The winch mechanism 3 lowers the sonar equipment to the preset underwater depth. UAV 1 flies at a fixed altitude along the planned route at a preset speed. The spatial calculation matrix module calculates the current fuselage reference altitude and current attitude data of UAV 1 in real time. Based on the theoretical drag resistance, the actual drag force of the cable, the cable release length, and the force tilt angle of cable 2, the underwater drag load moment is calculated. The thrust redistribution calculation model adjusts the power distribution of each rotor of UAV 1 based on the underwater drag load moment to maintain the constant safe lift-off altitude and attitude stability of UAV 1. At the same time, the sonar equipment 4 continuously emits ultrasonic waves to perform full-area underwater side-scan imaging detection. In this step, UAV 1 takes off vertically autonomously from an open shore. Before the sonar device 4 enters the water, the winch mechanism 3 completely retracts the cable 2, making the sonar device 4 close to the belly of UAV 1. UAV 1 climbs to the preset high-altitude cruising altitude, autonomously avoids obstacles along the planned route, and arrives above the target operation starting point. After UAV 1 arrives above the target operation starting point, UAV 1 slowly descends from the cruising altitude to the target operation altitude near the water surface. When the height of the corresponding water surface during the descent of UAV 1 is less than the preset value (e.g., 3m), the flight control system 5 of UAV 1 automatically exits the conventional barometer altitude hold mode, fully activates the spatial calculation matrix module, and the flight control system 5 enters the high-frequency spatial attitude closed-loop control standby state based on the spatial calculation matrix module, and makes UAV 1 hover in a fixed point.

[0041] The formula for calculating the reference height H of the drone's fuselage is as follows: H = (H1 + H2 + H3 + H4) / 4; The attitude data of the UAV includes pitch angle and roll angle, where the pitch angle θ is calculated using the following formula: ; The formula for calculating the roll angle Ф is as follows: ; H1 is the water surface height data measured by the altimeter unit located below the front rotor motor on the left side of the drone; H2 is the water surface height data measured by the altimeter unit located below the front rotor motor on the right side of the drone; H3 is the water surface height data measured by the altimeter unit located below the rear rotor motor on the left side of the drone; and H4 is the water surface height data measured by the altimeter unit located below the rear rotor motor on the right side of the drone. The lateral distance between the left and right rows of altimeter units on the drone; This refers to the longitudinal distance between the front and rear rows of altimeter units on the UAV.

[0042] During the flight of UAV 1, when the current reference altitude of UAV 1 is lower than the safe altitude threshold, the flight control system 5 adjusts the rotation speed of the rotor motor of UAV 1 to climb to above the safe altitude threshold. When the current pitch angle of UAV 1 exceeds the preset safe pitch angle range, UAV 1 performs attitude correction to bring the pitch angle of UAV 1 back to the preset safe pitch angle range. When the current roll angle of UAV 1 exceeds the preset safe roll angle range, UAV 1 performs attitude correction to bring the roll angle of UAV 1 back to the preset safe roll angle range.

[0043] The calculation method for the underwater towing eccentric load moment is as follows: S1: Obtain the ground velocity vector of the UAV and combine it with the actual water flow velocity in the river to obtain the sonar equipment's drag velocity relative to the river flow; calculate the theoretical drag resistance estimate based on the sonar equipment's drag velocity relative to the river flow, the sonar's water-facing cross-sectional area, and the sonar equipment's fluid resistance coefficient; obtain the actual drag force of the cable through the tension sensor on the winch mechanism.

[0044] The actual water flow velocity in the river channel can be obtained through a flow meter pre-installed in the river channel. The UAV's ground velocity vector can be obtained through the RTK module in the UAV flight control system.

[0045] The theoretical drag resistance estimate F is calculated based on fluid dynamics formulas. dt Theoretical drag resistance estimate F dt The calculation formula is as follows: ； In the formula, ρ is the density of water, and C d Let be the fluid resistance coefficient of the sonar device, A be the water-facing cross-sectional area of ​​the sonar device, and V be the drift velocity of the sonar device relative to the river flow. This represents the actual drag force of the cable.

[0046] S2: Calculate the force angle of the cable relative to the vertical direction based on the inherent gravity of the sonar equipment, the inherent buoyancy of the sonar equipment in the water, and the actual drag force of the cable; and calculate the component of the actual drag force of the cable in the direction of the water flow resistance of the sonar equipment based on the force angle.

[0047] For a cable with a force tilt angle of α (i.e., the angle between the cable and the vertical direction), the system is based on the vertical static equilibrium equation of the sonar in towed level flight. Real-time, seamless calculation is performed, and the final formula for calculating the force inclination angle α is: ; In the formula, m s g is the inherent gravity of the sonar device, F b M represents the inherent buoyancy of the sonar device in water. s g、F b All parameters are known and pre-exist in the drone's flight control system, and can be directly accessed.

[0048] S3: The component of the actual drag force of the cable in the direction of the water flow resistance of the sonar equipment is weighted and fused with the theoretical drag resistance estimate to obtain a high-confidence drag resistance value.

[0049] The component of the actual drag force of the cable in the direction of the water flow resistance of the sonar equipment is:

[0050] High confidence drag resistance value The formula is a weighted fusion of the component of the actual drag force of the cable in the direction of the water flow resistance of the sonar equipment and the estimated theoretical drag resistance. ; In the formula, The confidence weight coefficients preset for the system provide a precise data source for PID feedforward control through the fusion of theoretical foresight and experimental calibration.

[0051] S4: Obtain the cable release length, calculate the offset lever arm length based on the cable release length and the cable's force tilt angle relative to the vertical direction, and take the product of the offset lever arm length and the high-confidence towing resistance value as the underwater towing offset torque.

[0052] In this step, the high-confidence drag resistance value calculated in step S3 is... Mapped to the drag pull-down pitch eccentric load moment at the UAV belly hardpoint, i.e., the underwater drag eccentric load moment M. d The specific calculation formula is as follows: ; In the formula, h cgLet L be the offset lever arm constant. Since the UAV flies near the water surface, the cable is essentially underwater and remains approximately a straight line during towing. This offset lever arm constant is determined by the cable release length L and the angle of inclination of the cable under stress. The specific calculation formula is as follows: .

[0053] The cable release length L can be obtained in real time by integrating the encoder pulse count of the servo motor inside the winch mechanism.

[0054] The thrust redistribution calculation model adopts a control structure that combines cascaded PID and feedforward compensation. The product of the calculated underwater towed off-center load torque and the feedforward gain coefficient is used as the feedforward compensation amount and superimposed on the PID feedback control amount to obtain the total pitch control output. Then, the total pitch control output is converted into PWM duty cycle commands for the four rotor motors of the UAV through the power distribution matrix. The lift of the front and rear rotor motors of the UAV is adjusted by the PWM duty cycle commands to generate a restoring torque that completely cancels out the underwater towed off-center load torque.

[0055] By calculating the angular error and its rate of change between the desired pitch angle and the actual pitch angle (the actual pitch angle of the UAV is calculated by the internally mounted inertial measurement unit (IMU) and altimeter unit) in real time, the corresponding duty cycle adjustment value is output according to the proportional-integral-differential algorithm to maintain the basic horizontal stability of the UAV. The nonlinear off-center torque M is calculated based on physical modeling. d As an independent feedforward compensation quantity, it is directly superimposed on the output of the PID feedback control quantity; the total pitch control output is U. pitch for: ； Where k is the feedforward gain coefficient, U pid The pitch angle feedback control output of the attitude control loop of the flight control system is calculated using a conventional PID algorithm based on the error between the desired pitch angle and the actual pitch angle.

[0056] The power distribution relationship of each rotor of the UAV is as follows: Duty cycle output of the two front rotor motors of the drone for: ; Duty cycle output of the two rear rotor motors of the drone for: ; In the above formula, k m This is the thrust mapping conversion constant from torque to PWM. Based on the basic hovering thrust duty cycle This is the PWM duty cycle adjustment amount calculated and output by the flight control system using a conventional PID algorithm to address the roll angle (Roll) attitude error of the UAV. The flight control system calculates and outputs the PWM duty cycle adjustment amount based on the pitch attitude error of the UAV using a conventional PID algorithm.

[0057] Based on the aforementioned determined power distribution relationship, the system can accurately increase the lift of the rear row and proportionally reduce the lift of the front row before the drone's macroscopic attitude undergoes significant descent or pitch, generating a restoring torque that completely offsets the underwater drag torque, thus achieving dynamic stability.

[0058] Step 3: After the scanning task is completed, UAV 1 will retrieve cable 2 via winch mechanism 3 while hovering, and finally UAV 1 will return to base.

[0059] When retrieving cable 2, the drone 1 hovers in place; the winch mechanism 3 retrieves the composite cable 2 at a constant speed, smoothly pulling the underwater sonar out of the water, and completely retracting and locking it under the belly of the drone.

[0060] When UAV 1 returns, it climbs back to a higher preset cruising altitude and leaves the dangerous airspace near the water. Then, UAV 1 autonomously returns to the starting shore along the preset route and completes a precise landing.

[0061] Ground-based telemetry, control, and data processing terminal 7 synchronously receives detection data from sonar equipment and RTK positioning coordinate data. Ground-based telemetry, control, and data processing terminal 7 adopts sonar image processing algorithms commonly used in the field (such as image feature point-based stitching algorithms and conventional filtering and noise reduction algorithms) to automatically complete data stitching, noise removal, and coordinate correction, generating underwater topographic maps of narrow river channels, siltation distribution maps of waterways, and underwater obstacle distribution maps. The data is archived and stored to provide a basis for water conservancy engineering design, river dredging, and waterway operation and maintenance.

[0062] This invention has the following advantages: 1. This invention addresses industry pain points such as large blind spots in existing hydraulic sonar survey platforms, easy entanglement and failure of surface platforms, high risk of drone crashes near water surfaces, and unstable towing flight attitude. It proposes an integrated architecture combining a drone, an airborne controllable winch, a composite towing cable, a spatial calculation matrix module, and dynamic flight control attitude compensation. This solves the technical challenges of restricted hydraulic engineering areas being inaccessible for surveying, easy failure of surface platforms, unsafe near-water operations, and unstable towing flight attitude. Employing an aerial towing operation mode, it completely avoids entanglement with floating debris and grounding in shallow water, covering areas inaccessible to ships and unmanned vessels, such as narrow rivers, shallows, reservoir bays, and reservoir tailraces, achieving full coverage of hydraulic engineering surveys. An altimeter unit is deployed below the drone rotor, providing strong resistance to water surface reflection, water mist, and backlight interference, achieving centimeter-level accurate altitude determination near the water surface. Combined with closed-loop altitude threshold protection, it automatically warns and corrects course during ascent, avoiding the risk of rotor getting wet or crashing (falling into the water). 2. Unlike traditional single-point ranging, this solution innovatively deploys altimeter units on the UAV to form a spatial calculation matrix module. Combined with the unique spatial geometric model of the flight control system, the system effectively eliminates the error interference caused by water surface fluctuations (such as 0.3m-0.5m) on single-point ranging at the algorithm level by fusing and calculating four sets of independent altitude data. This "multi-node physical distribution + spatial geometric fusion" architecture design breaks through the data limitations of a single sensor and truly provides a highly robust accuracy benchmark for near-water surface towing operations.

[0063] 3. This invention addresses the industry-wide technical challenge of underwater nonlinear drag easily causing UAVs to tilt or even crash while carrying heavy-load towed sonar. This invention breaks through the technical limitations of traditional radar which only performs "one-dimensional altitude measurement". The system can not only maintain stable ranging under harsh conditions such as water mist and strong reflection, but also extract the roll and pitch attitude distortion trends of the fuselage in real time in dynamic environments. This high-precision multi-dimensional spatial perception data is seamlessly integrated into the "multi-source tension fusion and dynamic feedforward control law" of the system as a core benchmark. This allows the UAV to pre-trigger the thrust redistribution of the underlying motors before the macroscopic displacement occurs due to off-center load disturbance, realizing a deep collaborative closed loop of "physical environment perception - attitude decoupling - feedforward dynamic compensation".

[0064] 4. To address the complex nonlinear off-center loads generated by water flow impacts on sonar, the flight control system integrates dual control of "cascaded PID + feedforward compensation." This invention does not simply rely on passive feedback (PID) after fuselage tilting, but rather integrates in real time the spatial attitude perceived by the spatial calculation matrix, the instantaneous tension captured by the tension sensor, and the cable release length fed back by the motor encoder of the winch mechanism. It directly maps the complex drag torque into the off-center load torque Md at the fuselage mounting point. Before the UAV undergoes macroscopic displacement, it adjusts the power of each rotor by actively redistributing the PWM duty cycle of the UAV rotor motor (i.e., pre-increasing the lift on the force-bearing side). This mechanism fundamentally eliminates the interference of nonlinear drag force on the UAV's fuselage attitude, providing a highly stable aerial suspension platform for sonar reconnaissance and ensuring the accuracy and consistency of the survey data.

[0065] This invention is not limited to the preferred embodiments described above. Anyone can derive other products in various forms under the guidance of this invention. However, regardless of any changes in shape or structure, any technical solution that is the same as or similar to this application falls within the protection scope of this invention.

Claims

1. A UAV-mounted underwater sonar reconnaissance system, characterized in that, It includes a drone, sonar equipment, winch mechanism, cable, and tension sensor. The drone is equipped with a winch mechanism for releasing the sonar equipment, which is connected to the sonar equipment via the cable. The winch mechanism controls the release and retrieval of the sonar equipment and collects the release length of the cable. The tension sensor is used to collect the actual drag force of the cable. The UAV is equipped with a spatial calculation matrix module consisting of several altimeter units. The flight control system on the UAV is electrically connected to the spatial calculation matrix module, the winch mechanism, and the tension sensor. The flight control system receives the altitude vector data from the spatial calculation matrix module to calculate the UAV's reference altitude and attitude data. Based on the cable length and the actual towing tension of the cable, it calculates the underwater towing off-center load moment. Based on the towing off-center load moment, it adjusts the UAV's power distribution matrix to maintain the UAV's safe lift-off altitude and attitude stability. The drone is a quadcopter drone, which is equipped with two front rotor motors and two rear rotor motors, and a height measuring unit is installed below each of the front rotor motors and the rear rotor motors. The calculation method for underwater towing eccentric load moment is as follows: S1: Obtain the UAV's ground velocity vector and combine it with the actual river flow velocity to obtain the sonar device's drag velocity relative to the river flow; calculate the theoretical drag resistance estimate based on the sonar device's drag velocity relative to the river flow, the sonar's water-facing cross-sectional area, and the sonar device's fluid resistance coefficient; obtain the actual drag force of the cable through the tension sensor on the winch mechanism; the theoretical drag resistance estimate F... dt The calculation formula is as follows: ； The angle of inclination of the cable relative to the vertical direction The calculation formula is as follows: ； In the formula, ρ is the density of water, and C d Let be the fluid resistance coefficient of the sonar device, A be the water-facing cross-sectional area of ​​the sonar device, and V be the drift velocity of the sonar device relative to the river flow. The actual drag force of the cable is m. s g is the inherent gravity of the sonar device, F b The inherent buoyancy of the sonar equipment in water; S2: Calculate the force angle of the cable relative to the vertical direction based on the inherent gravity of the sonar equipment, the inherent buoyancy of the sonar equipment in the water, and the actual drag force of the cable; and calculate the component of the actual drag force of the cable in the direction of the water flow resistance of the sonar equipment based on the force angle. S3: The component of the actual drag force of the cable in the direction of the water flow resistance of the sonar equipment is weighted and fused with the theoretical drag resistance estimate to obtain a high-confidence drag resistance value. S4: Obtain the cable release length, calculate the offset lever arm length based on the cable release length and the cable's force tilt angle relative to the vertical direction, and take the product of the offset lever arm length and the high-confidence towing resistance value as the underwater towing offset torque.

2. The UAV-mounted underwater sonar reconnaissance system according to claim 1, characterized in that, The altimeter unit is a millimeter-wave radar.

3. The UAV-mounted underwater sonar reconnaissance system according to claim 1, characterized in that, It also includes a ground-based telemetry, control and data processing terminal, which communicates with the UAV's flight control system for setting operational parameters, planning reconnaissance routes, remotely controlling the UAV, analyzing sonar equipment detection data, and modeling water and terrain features.

4. A method for unmanned aerial vehicle (UAV) towed underwater sonar reconnaissance, based on the UAV towed underwater sonar reconnaissance system described in claim 1, characterized in that, Includes the following steps: Step 1: Presetting operational parameters and route planning; Step 2: The drone flies to the target operating height above the target operation starting point, and the winch mechanism lowers the sonar equipment to the preset underwater depth. The drone then flies at a fixed altitude along the planned route at a preset speed. The spatial calculation matrix module calculates the current fuselage reference height and current attitude data of the UAV in real time. It calculates the underwater towing off-center load moment based on theoretical drag resistance, actual cable drag tension, cable release length and cable force tilt angle. The thrust redistribution calculation model adjusts the power distribution of each rotor of the UAV based on the underwater towing off-center load moment to maintain the UAV's constant safe lift-off altitude and attitude stability. Step 3: After the scanning task is completed, the drone retracts the cable via a winch mechanism while hovering, and finally the drone returns to base.

5. The UAV-mounted underwater sonar reconnaissance method according to claim 4, characterized in that, In step two, the formula for calculating the reference height H of the drone's fuselage is as follows: H = (H1 + H2 + H3 + H4) / 4; The attitude data of the UAV includes pitch angle and roll angle, where the pitch angle θ is calculated using the following formula: ； The formula for calculating the roll angle Ф is as follows: ； H1 is the water surface height data measured by the altimeter unit located below the front rotor motor on the left side of the drone; H2 is the water surface height data measured by the altimeter unit located below the front rotor motor on the right side of the drone; H3 is the water surface height data measured by the altimeter unit located below the rear rotor motor on the left side of the drone; and H4 is the water surface height data measured by the altimeter unit located below the rear rotor motor on the right side of the drone. The lateral distance between the left and right rows of altimeter units on the drone; This refers to the longitudinal distance between the front and rear rows of altimeter units on the UAV.

6. The UAV-mounted underwater sonar reconnaissance method according to claim 5, characterized in that, In step two, when the current reference altitude of the UAV is lower than the safe altitude threshold, the flight control system adjusts the rotation speed of the UAV's rotor motors to climb to above the safe altitude threshold. When the current pitch angle of the UAV exceeds the preset safe pitch angle range, the UAV performs attitude correction to bring the pitch angle back to the preset safe pitch angle range. When the current roll angle of the UAV exceeds the preset safe roll angle range, the UAV performs attitude correction to bring the roll angle back to the preset safe roll angle range.

7. The UAV-mounted underwater sonar reconnaissance method according to claim 4, characterized in that, The thrust redistribution calculation model adopts a control structure that combines cascaded PID and feedforward compensation. The product of the calculated underwater towed off-center load torque and the feedforward gain coefficient is used as the feedforward compensation amount and superimposed on the PID feedback control amount to obtain the total pitch control output. Then, the total pitch control output is converted into PWM duty cycle commands for the four rotor motors of the UAV through the power distribution matrix. The lift of the front and rear rotor motors of the UAV is adjusted by the PWM duty cycle commands to generate a restoring torque that completely cancels out the underwater towed off-center load torque.

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