Cable-buoy-unmanned aerial vehicle combination air recovery track design method

Abstract

The invention discloses an aerial recovery track design method for a cable-buoy-unmanned aerial vehicle combination, and aims to realize stable and safe recovery of an unmanned aerial vehicle. The method comprises the following steps: firstly, constructing a mooring rope-buoy-unmanned aerial vehicle combination dynamic model; then, the balance position of the assembly under different cable lengths is determined through flight simulation, and a cubic spline interpolation method is applied to obtain the whole-process expected recovery track of the cable-buoy-unmanned aerial vehicle assembly; secondly, a cable winding length instruction is designed by introducing a Sigmoid function, and continuity and smoothness of the cable winding process are ensured; then, according to the whole-process expected recovery trajectory and the cable winding length instruction, a whole-process trajectory tracking instruction in the three-axis direction of the cable-buoy-unmanned aerial vehicle combination is obtained; and finally, verifying the designed recovery instruction by adopting a backstepping control method based on an extended state observer. According to the cable-buoy-unmanned aerial vehicle combination track design method based on the Sigmoid cable winding instruction, the smoothness and continuity of the cable winding instruction and the recycling track can be guaranteed, meanwhile, impact of instantaneous loads on the cable is avoided, and therefore the stability of unmanned aerial vehicle air recycling is improved; the unmanned aerial vehicle can be safely dragged back to the cabin of the mother aircraft.

Description

Technical Field

[0001] The invention relates to a method for designing an aerial recovery trajectory of a cable-buoy-unmanned aerial vehicle combination, and belongs to the technical field of unmanned aerial vehicle recovery. Background Art

[0002] Due to their significant advantages such as high maneuverability, low operating costs, and zero operator casualties, drones have been widely used in military reconnaissance, environmental testing, ground attack and other tasks. However, the flight range and endurance of drones are still important factors that restrict their further use. Due to limited battery capacity or fast fuel consumption, small drones are usually unable to complete long-term or long-distance continuous tasks and must frequently return to the base to replace batteries or fuel, which seriously reduces the combat and mission capabilities of drones. Therefore, improving the endurance of drones and ensuring that drones can continuously and autonomously perform multiple tasks in the air has become one of the important directions for the development of drone technology.

[0003] As one of the solutions that can effectively improve the endurance of drones, aerial recovery has attracted widespread attention from many researchers. Aerial recovery technology uses a large transport aircraft to carry a towing cable-buoy recovery device to dock and lock the drone after completing the mission, and then tow the drone back to the cabin of the mother aircraft by reeling in the cable to complete the aerial recovery. In this process, the reeling speed and smoothness of the cable are key factors affecting the safety and success rate of recovery. Existing cable reeling instructions usually use a constant speed or trapezoidal speed profile, and there are command mutations or unevenness. Due to the complex dynamic coupling relationship between the cable and the buoy-drone combination, command mutations often cause violent oscillations and a sudden increase in cable tension, resulting in unstable attitude and increased recovery risks. Therefore, there is an urgent need for a cable-buoy-drone combination aerial recovery trajectory command design method to solve the problems of uneven signals and unstable trajectory tracking, thereby effectively improving the safety and reliability of recovery in backlight. Summary of the invention

[0004] In view of the above problems, the present invention obtains the expected recovery trajectory of the cable-buoy-UAV combination throughout the entire process through multiple flight simulation analyses, and utilizes the smooth transition and asymptotic characteristics of the Sigmoid function to provide a cable-buoy-UAV combination aerial recovery trajectory design method, thereby achieving smooth reeling of the cable, avoiding attitude instability and trajectory oscillation caused by sudden changes in the reeling command, and ensuring the load balance of the cable during the recovery process, thereby improving the stability of aerial recovery.

[0005] To achieve the above object, the technical solution provided by the present invention is: a method for designing the aerial recovery trajectory of a cable-buoy-UAV combination, the method specifically comprising the following steps:

[0006] Step 1: According to the mechanical relationship between the flexible cable and the buoy-UAV system, a dynamic model of the cable-buoy-UAV combination is constructed;

[0007] Step 2: Determine the equilibrium position of the cable-buoy-UAV combination under different cable lengths through flight simulation experiments, and use the cubic spline interpolation method to obtain the expected recovery trajectory of the cable-buoy-UAV combination throughout the entire process;

[0008] Step 3: Design the cable reeling length instruction through the Sigmoid function to achieve continuous, smooth and safe reeling of the cable during the recovery process;

[0009] Step 4: Based on the expected recovery trajectory of the cable-buoy-UAV combination and the cable reeling length instruction, calculate and generate the trajectory tracking instructions of the cable-buoy-UAV combination in three-axis directions.

[0010] Furthermore, the specific steps of step 1 are:

[0011] The cable-buoy-UAV assembly includes a flexible cable, a buoy and a UAV to be recovered; the cable is assumed to be composed of N cylindrical connecting rods of equal length, and each connecting rod is connected by a frictionless spherical hinge node;

[0012] The mechanical device built into the buoy is used to fix the drone to form a buoy-drone system. The buoy-drone system is located at the last hinge node at the end of the cable and combined with the cable to form a cable-buoy-drone combination. The force analysis is performed on each hinge node in the cable-buoy-drone combination. At the i-th node, the node will be subjected to gravity G i 、Pneumatic force Q i and the cable tension t between adjacent nodes i ;

[0013] The gravitational acceleration at each node is expressed as:

[0014]

[0015] In the formula, m i Indicates the mass of the connecting rod; m Ndu =m N +m du ; G Ndu =G N +G du ; G du represents the gravity of the cable-buoy-UAV combination; the cable tension is the internal force between the connecting rods, which is derived from the following formula:

[0016]

[0017] Where n i represents the direction vector of the i-th link; l represents the length of the cable link; the aerodynamic force Q i and Q N Calculated by the following formula:

[0018]

[0019] Where D i =(0.5ρ(V ai ·n i ) 2 πd c l i c F ) i -(0.5ρ(V ai -(V ai ·n i ) i )d c l i c D )×(V ai -(V ai ·n i ) ) ; ρ is air density; V ai is the airspeed of the ith connecting rod; d c is the cable diameter; c F and c D are surface friction coefficient and pressure difference resistance coefficient respectively; S du is the reference area of ​​the buoy-UAV system; is the drag coefficient of the buoy-UAV system;

[0020] The dynamic model of the buoy-UAV combination is constructed using Newton's second law, the following state variables are defined, and the six-degree-of-freedom dynamic model of the cable-buoy-UAV combination is converted into an affine nonlinear form:

[0021]

[0022] Where, X i , i = 1, 2, 3, 4 represent the flight status information of trajectory loop, velocity loop, attitude loop and angular velocity loop respectively; V0 represents the constant flight speed of the mother aircraft; x, y, z represent the position of the cable-buoy-UAV combination; V x ,V y ,V z represents the speed of the cable-buoy-UAV combination; φ, β, α represent the roll angle, sideslip angle and angle of attack; p, q, r represent the angular velocity; δ ua ,δ ue ,δ urIndicates the deflection angle of the drone’s rudder surface; δ da ,δ de ,δ dr Indicates the rudder deflection angle of the buoy; F i , i = 1, 2, 3, 4 is used to describe the unknown nonlinear dynamics in the system; B i ,i=1,2,3,4 is the control input matrix.

[0023] Further, in step 2, based on the cable-buoy-UAV combination dynamics model, given the cable length, under the condition that the attitude angle and airflow angle of the cable-buoy-UAV combination remain constant, the cable-buoy-UAV combination will stabilize at a position, which is defined as the equilibrium position under the cable length;

[0024] By setting different cable lengths and conducting multiple flight simulation tests, the equilibrium position of the cable-buoy-UAV combination under different cable lengths was obtained;

[0025] Based on these equilibrium positions, the cubic spline interpolation method is used to generate the expected recovery trajectory of the cable-buoy-UAV combination throughout the entire process, where each cable length corresponds to a unique expected recovery position.

[0026] Furthermore, in step 3, the cable recovery length instruction is designed by a Sigmoid function, and the recovery length instruction is expressed as:

[0027]

[0028] Where, L(t) represents the curve of cable length changing with time; L0 represents the initial length of the cable; L c Indicates the final recovery length of the cable; k L represents the time length of the Sigmoid function, which is used to control the cable reeling speed during the recovery process; b represents the time offset, which is used to adjust the start time of the recovery process.

[0029] Furthermore, the specific method of step 4 is as follows:

[0030] According to step 2, the expected recovery trajectory of the cable-buoy-UAV combination is obtained through flight simulation to determine the corresponding relationship between the cable length and the position of the cable-buoy-UAV combination;

[0031] Combined with the cable winding length instruction L(t) based on the Sigmoid function in step 3, the expected tracking trajectory x at each time point is calculated by the cubic spline interpolation method. c (t), y c (t) and z c (t).

[0032] Furthermore, the method further includes adopting a backstepping control method based on an extended state observer, and verifying the validity of the cable winding length instruction L(t) by the backstepping control method based on the extended state observer.

[0033] Beneficial effects:

[0034] The method for designing the aerial recovery trajectory of a cable-buoy-UAV combination provided by the present invention is simple to operate and highly safe. The equilibrium position under different cable lengths is determined by simulation, and the expected recovery trajectory of the whole process is obtained by using the cubic spline interpolation method to ensure the load balance of the cable during the recovery process, which significantly improves the safety of the recovery process. At the same time, the cable winding instruction is designed using the Sigmoid function, which effectively avoids the sudden change or unevenness of the cable winding instruction during the recovery process, realizes the smooth winding of the cable, and finally ensures the stability and safety of the cable-buoy-UAV combination during the recovery process. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a schematic diagram of the towed cable-buoy-UAV combination of the present invention;

[0036] Figure 2 is the expected recovery trajectory of the cable-buoy-UAV combination in the present invention;

[0037] Figure 3 It is the cable winding length instruction and speed instruction based on Sigmoid function in the present invention;

[0038] Figure 4 A comparison diagram of the actual trajectory and the expected trajectory tracking results of the cable-buoy-UAV combination in the present invention;

[0039] Figure 5 This is a simulation result diagram of cable tension during the recovery process of the present invention. DETAILED DESCRIPTION

[0040] In order to facilitate those skilled in the art to understand and implement the present invention, the present invention is further described in detail below with reference to the accompanying drawings.

[0041] The present invention discloses a method for designing the aerial recovery trajectory of a cable-buoy-UAV combination. First, a six-degree-of-freedom dynamic model of the cable-buoy-UAV combination is constructed; on this basis, the equilibrium position of the cable-buoy-UAV combination under different cable lengths is determined through multiple groups of flight simulation experiments, and the expected recovery trajectory of the whole process is generated by using the cubic spline interpolation method; then, the cable reeling instruction is designed by using the smooth transition and asymptotic characteristics of the Sigmoid function, and then the trajectory tracking instruction of the cable-buoy-UAV combination in the three-axis direction is obtained; finally, an extended state observer is used to estimate the unknown nonlinear dynamic model, and as a feedforward compensation, a backstepping controller is used for closed-loop feedback control, and the designed aerial recovery trajectory of the cable-buoy-UAV combination is simulated and verified, and finally the UAV is safely and smoothly reeled back into the cabin.

[0042] In this example, the cable-buoy-drone combination includes a flexible cable, a buoy, and a drone to be recovered, such as Figure 1 The mother aircraft is constantly flying at an altitude of H0 = 2000m, and the flight speed is constant at V0 = 100m / s. The mother aircraft has a mass of 136000kg and a wingspan of 39.88m. The mass of the buoy-UAV system is 156.6kg. The number of cable nodes is N = 10. The initial length of the cable is L0 = 20m, and the final recovery length is L c =5m.

[0043] Furthermore, the method specifically comprises the following steps:

[0044] Step 1: Based on the mechanical relationship between the flexible cable and the buoy-UAV system, a dynamic model of the cable-buoy-UAV combination is constructed to obtain the equilibrium position under different cable lengths. The specific steps are:

[0045] The cable-buoy-drone assembly includes a flexible cable, a buoy, and a drone to be recovered. Assume that the cable is composed of N cylindrical connecting rods of equal length, and each connecting rod is connected by a frictionless spherical hinge. The built-in mechanical device of the buoy is used to fix the drone, forming a buoy-drone system. The system is located at the last hinge node at the end of the cable and is combined with the cable to form a cable-buoy-drone assembly. By analyzing the force on each hinge node, it can be seen that at the i-th node, the node will be subjected to gravity G i 、Pneumatic force Q i and the cable tension t between adjacent nodes i It is particularly important to note that the last node of the cable needs to additionally consider the influence of the gravity and aerodynamic force of the buoy-UAV combination, so its force condition is different from that of other nodes.

[0046] The gravitational acceleration at each node can be expressed as:

[0047]

[0048] In the formula, m i Indicates the mass of the connecting rod; m Ndu =m N +m du ; G Ndu =G N +G du ; G du represents the gravity of the cable-buoy-UAV combination; the cable tension is the internal force between the connecting rods and cannot be directly measured. It can be derived from the following formula:

[0049]

[0050] Where n i represents the direction vector of the i-th link; l represents the length of the cable link; the aerodynamic force Q i and Q N It can be calculated by the following formula:

[0051]

[0052] Where D i =(0.5ρ(V ai ·n i ) 2 πd c l i c F ) i -(0.5ρ(V ai -(V ai ·n i ) i )d c l i c D )×(V ai -(V ai ·n i ) i );ρ is air density;V ai is the airspeed of the ith connecting rod; d c is the cable diameter; c F and c D are surface friction coefficient and pressure difference resistance coefficient respectively; S du is the reference area of ​​the buoy-UAV system; is the drag coefficient of the buoy-UAV system.

[0053] Next, the dynamic model of the buoy-UAV combination is constructed using Newton's second law. In order to clearly and simply describe the dynamic model of the combination, the following state variables are defined, and the six-degree-of-freedom dynamic model of the cable-buoy-UAV combination is converted into an affine nonlinear form:

[0054]

[0055] Where, X i , i = 1, 2, 3, 4 represent the flight status information of trajectory loop, velocity loop, attitude loop and angular velocity loop respectively; V0 represents the constant flight speed of the mother aircraft; x, y, z represent the position of the cable-buoy-UAV combination; V x ,V y ,V z represents the speed of the cable-buoy-UAV combination; φ, β, α represent the roll angle, sideslip angle and angle of attack; p, q, r represent the angular velocity; δ ua ,δ ue ,δ ur Indicates the deflection angle of the drone’s rudder surface; δ da ,δ de ,δ dr Indicates the rudder deflection angle of the buoy; F i , i = 1, 2, 3, 4 is used to describe the unknown nonlinear dynamics in the system; B i ,i=1,2,3,4 is the control input matrix.

[0056] Step 2: To prevent the tension of the cable from exceeding the safe load range during the recovery process, the expected recovery trajectory of the cable-buoy-UAV combination is obtained in advance through simulation analysis. First, based on the dynamic model of the cable-buoy-UAV combination constructed in step 1, given the cable length, under the condition that the attitude angle and airflow angle of the cable-buoy-UAV combination remain constant, the cable-buoy-UAV combination will stabilize at a position, which is defined as the equilibrium position under the cable length.

[0057] Next, by setting different cable lengths, multiple flight simulation tests are performed to determine the equilibrium position of the cable-buoy-UAV combination at different cable lengths. In this example, through multiple flight simulation experiments, the equilibrium position of the cable at lengths of 20m, 15m, 10m, and 5m can be finally obtained, as shown in Table 1.

[0058] Table 1 Balance position of the buoy-UAV assembly during cable recovery process

[0059]

[0060] Then, based on these equilibrium positions, the cubic spline interpolation method is used to generate the expected recovery trajectory of the cable-buoy-UAV combination throughout the process, where each cable length corresponds to a unique expected recovery position, such as Figure 2 In this position, the cable tension is kept in a reasonable balanced state, which effectively reduces the risk of excessive tension and ensures the safety of the recovery process.

[0061] Step 3: To ensure that the UAV can be safely and smoothly dragged back to the cabin of the mother aircraft by the cable during the recovery process, the cable reeling instruction must be kept smooth and monotonous. The cable recovery length instruction is designed through the Sigmoid function, and the recovery length instruction can be expressed as:

[0062]

[0063] Where, L(t) represents the curve of cable length changing with time; L0 represents the initial length of the cable; L c Indicates the final recovery length of the cable; k L represents the time length of the Sigmoid function, which is used to control the cable reeling speed during the recovery process; b represents the time offset, which is used to adjust the start time of the recovery process.

[0064] In this example, the time length k is selected L = 0.4, time offset b = 8. The cable winding length instruction and speed instruction based on the Sigmoid function are as follows Figure 3 shown.

[0065] Step 4: Combine the expected recovery trajectory of the cable-buoy-UAV combination generated in step 2 and the cable reeling length instruction based on Sigmoid described in step 3 to calculate and generate the trajectory tracking instruction of the cable-buoy-UAV combination in the three-axis direction. The specific method is as follows:

[0066] According to step 2, the expected recovery trajectory of the cable-buoy-UAV combination is obtained through flight simulation to determine the corresponding relationship between the cable length and the position of the cable-buoy-UAV combination;

[0067] Combined with the cable winding length instruction L(t) based on the Sigmoid function in step 3, the expected tracking trajectory x at each time point is calculated by the cubic spline interpolation method. c (t), y c (t) and z c (t).

[0068] According to the expected tracking trajectory and the actual position of the cable-buoy-UAV combination, the backstepping control method based on the extended state observer is used to verify the effectiveness of the designed instructions.

[0069] like Figure 4 As shown in the figure, the actual trajectory of the cable-buoy-UAV combination is compared with the expected trajectory tracking results. It can be seen that the expected trajectory design of the system has good smooth transition characteristics, especially in the recovery stage (70s-100s), avoiding sudden changes or sharp jumps in the trajectory. Although the actual trajectory has a certain lag during the cable reeling process, the system quickly recovers after the reeling is completed, and finally accurately tracks the expected trajectory, indicating that the system has good adjustment capabilities.

[0070] Figure 5 The change of cable tension during the reeling process of the cable-buoy-drone combination is demonstrated, reflecting the effective control of the cable load by the trajectory design. As the reeling begins, the cable tension gradually increases, and drops rapidly and returns to stability after the reeling is completed. Through the smooth trajectory design, the system effectively avoids the violent fluctuation of tension, which not only improves the control accuracy, but also reduces the impact of the instantaneous load on the cable, and improves the safety and durability of the system.

[0071] The above analysis and simulation verification fully demonstrate the effectiveness and feasibility of the aerial recovery trajectory design method of the cable-buoy-UAV combination proposed in the present invention.

[0072] The above is only a preferred embodiment of the present invention, and is not intended to limit the scope of implementation of the present invention. Several improvements and modifications may be made without departing from the principles of the present invention, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for designing the aerial recovery trajectory of a cable-buoy-UAV assembly, characterized in that: The method specifically comprises the following steps: Step 1: According to the mechanical relationship between the flexible cable and the buoy-UAV system, a dynamic model of the cable-buoy-UAV combination is constructed; Step 2: Determine the equilibrium position of the cable-buoy-UAV combination under different cable lengths through flight simulation experiments, and use the cubic spline interpolation method to obtain the expected recovery trajectory of the cable-buoy-UAV combination throughout the entire process; Step 3: Design the cable reeling length instruction through the Sigmoid function to achieve continuous, smooth and safe reeling of the cable during the recovery process; Step 4: Based on the expected recovery trajectory of the cable-buoy-UAV combination and the cable reeling length instruction, calculate and generate the trajectory tracking instructions of the cable-buoy-UAV combination in three-axis directions.

2. According to claim 1, a method for designing a trajectory for aerial recovery of a cable-buoy-UAV assembly is characterized by: The specific steps of step 1 are: The cable-buoy-UAV assembly includes a flexible cable, a buoy and a UAV to be recovered; the cable is assumed to be composed of N cylindrical connecting rods of equal length, and each connecting rod is connected by a frictionless spherical hinge node; The mechanical device built into the buoy is used to fix the drone to form a buoy-drone system. The buoy-drone system is located at the last hinge node at the end of the cable and combined with the cable to form a cable-buoy-drone combination. The force analysis is performed on each hinge node in the cable-buoy-drone combination. At the i-th node, the node will be subjected to gravity G i 、Pneumatic force Q i and the cable tension t between adjacent nodes i ; The gravitational acceleration at each node is expressed as: In the formula, m i Indicates the mass of the connecting rod; m Ndu =m N +m du ; G Ndu =G N +G du ; G du represents the gravity of the cable-buoy-UAV combination; the cable tension is the internal force between the connecting rods, which is derived from the following formula: Where n i represents the direction vector of the i-th link; l represents the length of the cable link; the aerodynamic force Q i and Q N Calculated by the following formula: Where D i =(0.5ρ(V ai ·n i ) 2 πd c l i c F ) i -(0.5ρ(V ai -(V ai ·n i ) i )d c l i c D )×(V ai -(V ai ·n i ) i );ρ is air density;V ai is the airspeed of the ith connecting rod; d c is the cable diameter; c F and c D are surface friction coefficient and pressure difference resistance coefficient respectively; S du is the reference area of ​​the buoy-UAV system; is the drag coefficient of the buoy-UAV system; The dynamic model of the buoy-UAV combination is constructed using Newton's second law, the following state variables are defined, and the six-degree-of-freedom dynamic model of the cable-buoy-UAV combination is converted into an affine nonlinear form: In the formula, X i , i = 1, 2, 3, 4 represent the flight status information of trajectory loop, velocity loop, attitude loop and angular velocity loop respectively; V0 represents the constant flight speed of the mother aircraft; x, y, z represent the position of the cable-buoy-UAV combination; V x ,V y ,V z represents the speed of the cable-buoy-UAV combination; φ, β, α represent the roll angle, sideslip angle and angle of attack; p, q, r represent the angular velocity; δ ua ,δ ue ,δ ur Indicates the deflection angle of the drone’s rudder surface; δ da ,δ de ,δ dr Indicates the rudder deflection angle of the buoy; F i , i = 1, 2, 3, 4 is used to describe the unknown nonlinear dynamics in the system; B i ,i=1,2,3,4 is the control input matrix.

3. According to claim 1, a method for designing a trajectory for aerial recovery of a cable-buoy-UAV assembly is characterized by: In step 2, based on the cable-buoy-UAV combination dynamics model, given the cable length, under the condition that the attitude angle and airflow angle of the cable-buoy-UAV combination remain constant, the cable-buoy-UAV combination will stabilize at a position, which is defined as the equilibrium position under the cable length; By setting different cable lengths and conducting multiple flight simulation tests, the equilibrium position of the cable-buoy-UAV combination under different cable lengths was obtained; Based on these equilibrium positions, the cubic spline interpolation method is used to generate the expected recovery trajectory of the cable-buoy-UAV combination throughout the entire process, where each cable length corresponds to a unique expected recovery position.

4. The method for designing the aerial recovery trajectory of a cable-buoy-UAV assembly according to claim 1, characterized in that: In step 3, the cable recovery length instruction is designed by the Sigmoid function, and the recovery length instruction is expressed as: Where, L(t) represents the curve of cable length changing with time; L0 represents the initial length of the cable; L c Indicates the final recovery length of the cable; k L represents the time length of the Sigmoid function, which is used to control the cable reeling speed during the recovery process; b represents the time offset, which is used to adjust the start time of the recovery process.

5. The method for designing the aerial recovery trajectory of a cable-buoy-UAV assembly according to claim 1, characterized in that: The specific method of step 4 is as follows: According to step 2, the expected recovery trajectory of the cable-buoy-UAV combination is obtained through flight simulation to determine the corresponding relationship between the cable length and the position of the cable-buoy-UAV combination; Combined with the cable winding length instruction L(t) based on the Sigmoid function in step 3, the expected tracking trajectory x at each time point is calculated by the cubic spline interpolation method. c (t), y c (t) and z c (t).

6. The method for designing the aerial recovery trajectory of a cable-buoy-UAV assembly according to claim 1, characterized in that: The method further includes adopting a backstepping control method based on an extended state observer, and verifying the validity of the cable winding length instruction L(t) by using the backstepping control method based on the extended state observer.

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