Method for controlling mooring unmanned aerial vehicle to hover along with wind based on admittance control

Abstract

The invention provides a wind-following hovering control method for a mooring unmanned aerial vehicle based on admittance control. The wind-following hovering control method comprises the steps that 1, a complete mooring unmanned aerial vehicle and winder kinetic model is established; 2, solving an expected hovering position of the mooring unmanned aerial vehicle through an admittance model, and designing a controller of the mooring unmanned aerial vehicle; 3, hovering of the mooring unmanned aerial vehicle at the new expected hovering position is achieved through bottom layer control. The attitude angle of the mooring unmanned aerial vehicle is used as the input of the admittance controller to represent the influence of the external force, and the required hovering position can be obtained through the preset admittance model. Through theoretical simulation verification, the control strategy can effectively prevent the posture of the multiple rotors from being excessively inclined when the rotors hover in the wind. In the process, the pulling force of the mooring rope is utilized to assist in resisting wind power, and therefore the problem that the posture is too low possibly occurring when the mooring unmanned aerial vehicle hovers in the top of the wind is prevented.

Description

Technical Field

[0001] The present invention proposes a tethered drone hovering control method based on admittance control. After being affected by the external wind field, the tethered drone using this method actively hovers with the wind, and uses the tension of the tether rope to assist in resisting wind resistance, thereby preventing the tethered drone from tilting too much due to resisting wind resistance. The method takes the attitude angle of the tethered drone as input, obtains the desired hovering position of the tethered drone through an admittance controller, and finally realizes the tethered drone hovering at the new desired hovering position through the bottom-level control, which belongs to the field of aircraft control. Background Art

[0002] In recent years, drones have been used more and more widely, such as in agriculture, logistics, transportation, inspection, etc. However, in some scenarios where drones need to stay in the air for a long time, such as emergency lighting, emergency communications, etc., conventional multi-rotor drones are not competent due to their limited endurance, so tethered drones are used.

[0003] The tethered drone is connected to the ground through a tether rope. Its structure is as follows Figure 1 As shown in the figure, this tethered rope can be used to supply power from the ground or transmit information between the drone and the ground, so the tethered drone can fly in the air for a long time. From this structural feature, it can be seen that the tethered drone is an aerial robot that works in conjunction with the cable. Usually, tethered drones are required to hover at a fixed point. However, in a strong wind environment, the attitude of the tethered drone may be too tilted to counteract wind resistance. On the one hand, the working angle of the payload on the tethered drone, such as the camera and communication antenna, is blocked; on the other hand, if it is disturbed by external interference, the drone with a more tilted attitude is more likely to crash. However, if the requirement for the tethered drone to hover at a fixed point is abandoned and the drone is allowed to hover with the wind, the tethered rope tension can help counteract wind resistance, thereby preventing the attitude of the multi-rotor aircraft from becoming too tilted. How to adaptively allow the multi-rotor aircraft to actively adjust the hovering position in the wind and autonomously return to the initial hovering point when the wind disappears is a problem worth studying, and the development of flexible control of aerial robots has brought a solution to this problem.

[0004] The present invention designs a position control strategy based on the admittance controller. The attitude angle of the tethered UAV is used as the input of the admittance controller to represent the influence of external forces, and the desired hovering position can be obtained through the preset admittance model. Theoretical simulation verification shows that the above control strategy can effectively prevent the attitude of the multi-rotor from being too tilted when hovering in the wind. Summary of the invention

[0005] The present invention proposes a method for controlling a tethered UAV to hover with the wind based on admittance control. First, a dynamic model of the tethered UAV is established; then, the desired hovering position of the tethered UAV is solved through an admittance model; finally, the tethered UAV is hovered at the new desired hovering position through the underlying control. The overall steps are as follows: Figure 2 The detailed steps are as follows:

[0006] Step 1: Establish a complete dynamic model of the tethered drone and the reel.

[0007] Step 1.1: Establish the dynamic model of the tethered UAV. The forces acting on the tethered UAV in the air include gravity, aerodynamic force, tether rope tension, and rotor thrust, so the dynamic model can be obtained as follows:

[0008]

[0009] Where p = [p x p y p z ] T and v = [v x v y v z ] T Represent the position and speed of the tethered UAV respectively; m is the mass of the tethered UAV; T r , T t and F a are the rotor thrust, the three-dimensional tension of the tether at the end of the UAV and the wind force respectively on the tethered UAV in the geodetic coordinate system; g = [00g] T is the acceleration due to gravity; ω is the rotational angular velocity of the tethered drone; [ω] × is the cross product matrix of the vector ω; J is the moment of inertia; τ r , τ g , τ t and τ a They represent the torque generated by the rotor thrust, the torque generated by gravity, the mooring rope tension torque and the wind torque respectively; is the rotation matrix from the rotor coordinate system of the drone to the earth coordinate system, which can be expressed as:

[0010]

[0011] Among them, φ, θ and ψ represent the roll angle, pitch angle and yaw angle of the UAV respectively.

[0012] Step 1.2: Establish the dynamic model of the reel. The dynamic model of the reel can be expressed by important parameters such as the length of the mooring rope cable, the cable recovery speed, and the rotation angular velocity of the reel as follows:

[0013]

[0014] Where L is the length of the mooring rope, v c is the linear velocity of the mooring rope being retrieved by the reel, J w is the moment of inertia of the reel, ω w is the angular velocity of the reel during operation, r is the radius of the reel, τ w is the torsional moment of the reel, F c It is the tension of the belay rope at the reel end.

[0015] Step 2: Design the controller of the tethered drone. The controller as a whole includes the top-level admittance controller and the bottom-level position and attitude control. The overall control scheme is as follows: Figure 3 As shown in the figure, it is mainly divided into two parts: the control of the drone and the control of the reel. For the drone controller, firstly, the attitude R of the drone is used as the input of the admittance controller to solve the expected hovering position p of the tethered drone in the wind field. d ; Then the desired hover position p d and the actual position p are transmitted to the position controller to obtain the desired rotor thrust T rd Desired drone attitude R d ; The next step is to set the desired drone posture R d The actual attitude R is used as the input of the attitude controller to obtain the desired three-dimensional torque τ provided by the rotor rd For the reel, the length of the tethered rope is used as the control input, and this value can be obtained through the position p of the tethered drone in the simulation. Through the controller of the reel, the control output can be obtained as the control torque τ of the reel. wd ; Then, the control torque of the reel is transferred into the dynamic model of the reel, and the tension T of the mooring rope can be obtained. t Finally, the desired UAV rotor thrust T rd , the three-dimensional torque τ provided by the desired rotor rd and the tension of the mooring rope T t By transmitting it into the dynamic model of the tethered UAV, the attitude R and position p of the UAV can be obtained and fed back to the above-mentioned admittance controller, position controller, attitude controller and reel controller.

[0016] Step 2.1: Design the top-level admittance controller. Through a preset admittance controller, the attitude angle of the tethered drone is used as the control input to obtain the desired hovering position of the tethered drone. In this step, only the horizontal position is considered to change with the wind through the admittance model, but the height is kept constant. It can be noted that the direction of the rotor thrust is closely related to the attitude angle and can intuitively reflect the magnitude of the external wind resistance. Therefore, we consider the direction of the rotor thrust expressed by the attitude angle as the input of the controller to obtain the desired hovering position. The designed controller is:

[0017]

[0018] in, is the desired horizontal hovering position of the tethered drone; D is the damping coefficient, which is a positive definite matrix; K is the stiffness coefficient, which is a positive definite matrix; Γ is the input of the controller, which can be expressed as rotor thrust and attitude angle respectively:

[0019]

[0020] In the above controller, the desired horizontal hovering position of the tethered drone when it is stable is determined by matrices D and K, wherein the smaller the parameter value in matrix D, the faster the adjustment speed of the desired hovering position; the smaller the parameter value in matrix K, the farther the desired hovering position.

[0021] Furthermore, combined with the expected flight altitude of the tethered drone The expected hovering position of the tethered drone can be obtained as:

[0022]

[0023] Step 2.2: Set the desired hover position p d The low-level controllers including the position controller and attitude controller are passed to realize the low-level control of the tethered drone. The low-level control here includes various controllers that can realize low-level control and track the desired hovering position, including but not limited to proportional-integral-derivative controller (PID), model predictive control (MPC), etc.

[0024] Step 3: Design of the controller of the reel. In the present invention, the tension of the mooring rope is controlled to be similar to the effect of a spring, so the controller of the reel is designed as follows:

[0025]

[0026] The input of the controller is the rotational angular velocity ω of the reel. w , cable length L and the initial length of the cable when it starts to enter the hovering state The output is the controlled torque τ of the reel w ;kc is the control feedback factor of the reel.

[0027] After completing the above steps, you can simulate on the MATLAB / SIMULINK platform. You can further use the PX4PSP toolbox and RflySim toolchain to automatically generate code to deploy the tethered drone controller on the flight control and the reel controller on the Ubuntu18 platform, set the weight coefficient and various feedback coefficients, and conduct experiments.

[0028] The advantages and beneficial effects of the present invention are as follows: a wind-following hovering controller based on admittance control is designed for a tethered UAV. By taking the attitude angle of the tethered UAV as the control input, the desired hovering position of the tethered UAV is actively controlled to drift with the wind. In this process, the tension of the tethering rope is used to assist in resisting the wind force, thereby preventing the tethered UAV from having a too low attitude when hovering against the wind. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic diagram of a tethered drone.

[0030] Figure 2 It is the whole implementation process of the present invention.

[0031] Figure 3 The present invention relates to an overall control structure of a tethered UAV system that adopts admittance control to hover with the wind.

[0032] Figure 4a It is a schematic diagram of the horizontal wind field set up for the simulation.

[0033] Figure 4b It is a comparison diagram of 3D position curve and 3D attitude curve of a tethered UAV hovering with the wind using the method of the present invention and hovering at a fixed point without using the method of the present invention under a simulated wind field. DETAILED DESCRIPTION

[0034] The present invention designs a hovering control algorithm for a tethered drone. After being affected by the external wind field, the tethered drone actively adjusts its desired hovering position and hovers, using the rotor thrust and the tether rope tension to cooperate to resist the wind resistance, thereby preventing the problem of too low attitude angle when hovering against the wind. The simulation is carried out on a computer with a main frequency of 3.20Ghz and a memory of 32.00GB equipped with a win10 operating system. The platform is MATLAB R2022b version using the PX4PSP toolbox and the RflySim tool chain. After the simulation is successful, the RflySim tool chain can be used to automatically generate code and conduct actual flight experiments.

[0035] Step 1: Measure the system parameters of the tethered drone and establish the mechanical model of the tethered drone and the reel. The mass of the tethered drone is m = 1.02 kg; the moment of inertia is: J = diag (0.01260.00550.0158) kg·m 2 ; Gravitational acceleration is g = 9.8 m / s 2 ; The radius of the reel is r = 0.05m, and the moment of inertia of the reel is J w =1.25·10 -3 kg·m 2 Refer to equations (1)-(3) to build the dynamic model of the tethered UAV.

[0036] Step 2: Design the top-level admittance controller for the tethered UAV. Set the feedback coefficients D = diag (0.01250.0125) and K = diag (0.06250.0625). Obtain the attitude angle of the UAV, and refer to equation (5) to calculate the control input Γ. Refer to equation (4) to get the desired position control value of the tethered UAV, which is further passed to the bottom-level controller to implement bottom-level control.

[0037] Step 3: Select the cascade PID controller as the underlying control method and build the drone-side underlying controller of the tethered drone.

[0038] Step 4: Design the controller of the cable reel according to equation (6) and select k c =6, the initial length of the mooring rope is L 0 =8.8m.

[0039] Step 5: Carry out simulation control experiment according to steps 1 to 4. Figure 4a As shown, after 40 seconds from the start of the simulation, the wind speed v is applied. w =[-8-50] T , 90 seconds after the simulation starts, the wind speed becomes v w =[-5-30] T , 140 seconds after the simulation starts, the wind speed becomes v w =0. The tethered drone using the hovering control method of the present invention is compared with the tethered drone not using the wind-following hovering control method of the present invention, that is, the expected horizontal hovering position of the tethered drone in the control group in step 2 of the control process is 0. The simulated 3D position curves and attitude curves obtained by the two groups are shown in Figure 4b shown.

[0040] Through simulation, it is found that under the same conditions, the attitude angle of the tethered drone using the control method of the present invention is flatter than that of the control group with the assistance of the tethered rope. In addition, the greater the external wind speed, the more the tethered drone's hovering position changes with the wind. When the wind speed decreases to 0, the tethered drone using the method of the present invention can also return to the air above the reel autonomously.

Claims

1. A method for controlling a tethered drone to hover with the wind based on admittance control, characterized in that: Here are the steps: Step 1: Establish a complete dynamic model of the tethered drone and the reel; Step 2: Solve the expected hovering position of the tethered UAV through an admittance model and design the controller of the tethered UAV; Step 3: Use the underlying control to make the tethered drone hover at the new desired hovering position.

2. The method for controlling a tethered UAV to hover with the wind based on admittance control according to claim 1, characterized in that: In step 1, a dynamic model of a tethered UAV is established; the forces acting on the tethered UAV in the air include gravity, aerodynamic force, tether rope tension and rotor thrust, and the dynamic model is: Where p = [p x p y p z ] T and v = [v x v y v z ] T Represent the position and speed of the tethered UAV respectively; m is the mass of the tethered UAV; T r , T t and F a are the rotor thrust, the three-dimensional tension of the tether at the end of the UAV and the wind force respectively on the tethered UAV in the geodetic coordinate system; g = [00g] T is the acceleration due to gravity; ω is the rotational angular velocity of the tethered drone; [ω] × is the cross product matrix of the vector ω; J is the moment of inertia; τ r , τ g , τ t and τ a They represent the torque generated by rotor thrust, the torque generated by gravity, the mooring rope tension torque and the wind torque respectively.

3. The method for controlling a tethered UAV to hover with the wind based on admittance control according to claim 2, characterized in that: is the rotation matrix from the rotor coordinate system of the UAV to the earth coordinate system, expressed as: Among them, φ, θ and ψ represent the roll angle, pitch angle and yaw angle of the UAV respectively.

4. The method for controlling a tethered UAV to hover with the wind based on admittance control according to claim 1, characterized in that: In step 1, a dynamic model of the cable reel is established; the dynamic model of the cable reel is expressed by the length of the mooring rope cable, the cable recovery speed, and the rotational angular velocity of the cable reel as follows: Where L is the length of the mooring rope, v c is the linear velocity of the mooring rope being retrieved by the reel, J w is the moment of inertia of the reel, ω w is the angular velocity of the reel during operation, r is the radius of the reel, τ w is the torsional moment of the reel, F c It is the tension of the belay rope at the reel end.

5. The method for controlling a tethered UAV to hover with the wind based on admittance control according to claim 1, characterized in that: In step 2, the controller as a whole includes the top-level admittance controller and the bottom-level position and attitude control, which is divided into two parts: the control of the UAV and the control of the reel. For the controller of the UAV, firstly, the attitude R of the UAV is used as the input of the admittance controller to solve the expected hovering position p of the tethered UAV in the wind field. d ; Then the desired hover position p d and the actual position p are transmitted to the position controller to obtain the desired rotor thrust T rd Desired drone attitude R d ; The next step is to set the desired drone posture R d The actual attitude R is used as the input of the attitude controller to obtain the desired three-dimensional torque τ provided by the rotor rd For the cable reel, the length of the tethered rope is used as the control input. This value is obtained by the position p of the tethered drone in the simulation. Through the controller of the cable reel, the control output is the control torque τ of the cable reel. wd ; Then the control torque of the reel is transferred into the dynamic model of the reel to obtain the tension T of the mooring rope t ; Finally, the desired UAV rotor thrust T rd , the three-dimensional torque τ provided by the desired rotor rd and the tension of the mooring rope T t It is transmitted into the dynamic model of the tethered UAV to the attitude R and position p of the UAV, and then fed back to the above-mentioned admittance controller, position controller, attitude controller and reel controller.

6. A method for controlling a tethered UAV to hover with the wind based on admittance control according to claim 1 or 5, characterized in that: Design the top-level admittance controller; through a preset admittance controller, take the attitude angle of the tethered drone as the control input to obtain the desired hovering position of the tethered drone; here only consider the horizontal position changing with the wind through the admittance model, but keep a fixed height in the height direction; take the direction of the rotor thrust represented by the attitude angle as the input of the controller to obtain the desired hovering position, and the designed controller is: in, is the desired horizontal hovering position of the tethered drone; D is the damping coefficient, which is a positive definite matrix; K is the stiffness coefficient, which is a positive definite matrix; Γ is the input of the controller, which is expressed in terms of rotor thrust and attitude angle respectively: In the above controller, the desired horizontal hovering position of the tethered drone when it is stable is determined by matrices D and K, wherein the smaller the parameter value in matrix D, the faster the adjustment speed of the desired hovering position; the smaller the parameter value in matrix K, the farther the desired hovering position.

7. The method for controlling a tethered UAV to hover with the wind based on admittance control according to claim 6, characterized in that: Combined with the desired flight altitude of the tethered drone The expected hovering position of the tethered drone is obtained as:

8. The method for controlling a tethered UAV to hover with the wind based on admittance control according to claim 7, characterized in that: Set the desired hover position p d The low-level controller including the position controller and the attitude controller is transmitted to realize the low-level control of the tethered UAV; the low-level control includes various controllers that can realize the low-level control and track the expected hovering position, including but not limited to the proportional-integral-derivative controller PID and the model predictive control MPC.

9. The method for controlling a tethered UAV to hover with the wind based on admittance control according to claim 1, characterized in that: In step 3, design the controller of the cable reel as: The input of the controller is the rotational angular velocity ω of the reel. w , cable length L and the initial length of the cable when it starts to enter the hovering state The output is the controlled torque τ of the reel w ;k c is the control feedback factor of the reel.

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