Transition flight heterogeneous control allocation method for quad tiltrotor unmanned aerial vehicle

Abstract

The application discloses a transition flight heterogeneous control allocation method for a four-tilt-rotor unmanned aerial vehicle, and aims at solving the problem of low efficiency and safety of fixed-wing and rotor heterogeneous redundant control allocation in the tilt transition flight of the four-tilt-rotor unmanned aerial vehicle. According to the fixed-wing and rotor heterogeneous redundant control model of the combined action of the short-cabin tilt angle, the rotor total pitch and the aerodynamic rudder surface, a control allocation method based on a sequential quadratic programming algorithm (SQP algorithm) is designed, the control allocation problem is converted into a multi-constraint optimization problem with nonlinear constraints, the optimal solution of the multi-constraint optimization problem is solved, the reasonable allocation of forces and torques in the tilt process is realized, the influence of adverse aerodynamic effects is weakened, and the decoupling structure design of the control law and the control allocation is provided with the possibility. The control allocation technology based on the sequential quadratic programming algorithm can accurately map the control force and the control torque to the rotor speed and the deflection rudder surface, so as to improve the unmanned aerial vehicle control efficiency and ensure the transition flight safety of the unmanned aerial vehicle under complex disturbance environment.

Description

Technical Field

[0001] This invention relates to the field of unmanned aerial vehicle (UAV) flight control technology, and mainly to a heterogeneous control allocation method for transitional flight of a four-tilt-rotor UAV. Background Technology

[0002] Currently, increasingly mature UAV technology is widely used in tasks such as logistics transportation, electronic warfare, and tactical reconnaissance. However, traditional mainstream models are limited by their configuration, either suffering from drawbacks such as slow maneuverability, short range, and small payload; or relying on airport runways and being unable to take off and land vertically or hover in the air. To address the shortcomings of traditional configuration aircraft, scholars at home and abroad have conducted in-depth research on tiltrotor configuration aircraft.

[0003] Tiltrotor aircraft combine the advantages of both rotorcraft and fixed-wing aircraft. They possess the vertical takeoff and landing, hovering, and excellent low-altitude flight capabilities of conventional rotorcraft, while also exhibiting the high cruising speed, long range, and strong transport capacity of fixed-wing aircraft. They can be used for rapid intercity air transport, disaster relief, and battlefield support missions. Tiltrotor UAVs have three flight modes: rotor mode, tilt transition mode, and fixed-wing mode. During flight, the rotor and wings work together to provide power to the UAV. When the tilt angle changes from 0° to 90°, the UAV completes the tilt and enters fixed-wing mode. The tilt transition mode is achieved through a tilt mechanism mounted on the wing. This unique structural design requires the UAV to control two control systems simultaneously. The numerous control variables result in severe coupling in the heterogeneous and redundant control process, easily generating force and torque interference that reduces control efficiency and seriously threatens the safety of the transition process. To decouple the heterogeneous control, existing control methods use sine and cosine functions to construct the control allocation matrix. However, in reality, the tilt transition control allocation matrix is ​​difficult to describe accurately and uniquely mathematically. Therefore, there is an urgent need to study a transitional flight heterogeneous control allocation method suitable for quad tiltrotor UAVs, to solve the problems of heterogeneous redundancy and coupling, and to achieve safe flight of quad tiltrotor UAVs in complex disturbance environments. Summary of the Invention

[0004] Purpose of the invention: This invention provides a heterogeneous control allocation method for the transition flight of a four-tilt-rotor UAV. The optimal rotor speed and control surface deflection at each tilt angle are obtained by using the SQP algorithm to meet the transition flight requirements of the UAV, thereby reducing the strong coupling of heterogeneous redundant control and improving control efficiency.

[0005] To achieve the above objectives, the technical solution provided by this invention is as follows: a heterogeneous control allocation method for transitional flight of a four-tilt-rotor UAV. This method considers that, due to the limitations of the rotor and control surfaces themselves, the four-tilt-rotor UAV may not be able to meet the desired force and torque output by the attitude controller at the corresponding tilt angle. To make the actual control force and torque acting on the UAV as close as possible to the desired force and torque output by the attitude controller, rotor speed and control surface deflection are used as state variables. Based on the aerodynamic parameters and flight state of the UAV, a control allocation technique based on the SQP algorithm is designed to solve the cost function that satisfies the constraints, thereby calculating the optimal rotor speed and control surface deflection at the corresponding tilt angle.

[0006] It includes the following steps:

[0007] Step S1: Based on the force analysis of the four-tilt-rotor UAV during flight, the control force and torque of the UAV consist of rotor force and torque, force and torque generated by the aircraft body and control surfaces, and gravity and gravitational torque.

[0008]

[0009] In the formula, [F,M] T Represents resultant force and resultant moment; This represents the rotor force and torque generated by the rotor; Represents the aerodynamic forces and moments generated by the deflection of the aircraft's fuselage and control surfaces; Gravity and gravitational torque represent the forces generated by gravity; rotor force and torque. The mapping relationship between the four rotors and the rotor control variable ω can be expressed as:

[0010] Where τ is the Z-axis in the rotor coordinate system. h Y-axis and body coordinate axis b Z b The included angle on the plane is 0° for rotor mode and 90° for fixed-wing mode; ω i＝1,2,3,4 These represent the rotational speeds of the four rotors; k f ,k m These are the thrust coefficient and anti-torque coefficient, which are related to propeller diameter, air density, etc.; ΔX1 is the distance between the projection of the centers of the front two rotors onto the plane of symmetry of the aircraft and the center of gravity in the X-axis direction; ΔX2 is the distance between the projection of the centers of the rear two rotors onto the plane of symmetry of the aircraft and the center of gravity in the X-axis direction; ΔY1 is the distance from the centers of the front two rotors to the plane of symmetry of the aircraft; ΔY2 is the distance from the centers of the rear two rotors to the plane of symmetry of the aircraft.

[0011] The mapping relationship between the forces and moments generated by the aircraft fuselage and control surfaces and the control surfaces, i.e., the aerodynamic control surface control amount δ, can be expressed as:

[0012]

[0013] Among them, Q,S ref b and c represent the dynamic pressure and wing reference area, respectively; b and c represent the mean aerodynamic span and mean aerodynamic chord length, respectively; [C] x0 C y0 C z0 C mx0 C my0 C mz0 ] T The three-axis rudderless yaw force and moment coefficients of the UAV in the body coordinate system; The increment of the aerodynamic force and moment coefficient of the UAV rudder in the body coordinate system; δ a ,δ e ,δ r These are the aileron deflection, elevator deflection, and rudder deflection, respectively.

[0014] Gravity and gravitational torque can be expressed as:

[0015]

[0016] Where m and g are the mass and gravitational acceleration of the UAV, respectively; θ and φ are the pitch and roll angles of the UAV, respectively, with pitch up being positive and roll to the right being positive.

[0017] Step S2: Considering that at the corresponding tilt angle, the four-tilt-rotor UAV may not be able to meet the desired force and torque output by the attitude controller due to the limitations of the rotor and control surfaces themselves, in order to make the actual control force and torque acting on the UAV as close as possible to the desired force and torque output by the attitude controller, based on the error between the desired control force and torque and the actual control force and torque, a cost function is established with the rotor control command ω and the aerodynamic control surface control command δ as the optimization objectives:

[0018] f(S)=S T QS

[0019]

[0020] Here, S refers to the error between the desired control force and torque and the actual control force and torque. Its significance lies in selecting the most suitable rotor speed and control surface deflection at the tilt angle τ to meet the transition flight requirements of the UAV, while ensuring that the error between the actual control torque and the desired torque is as small as possible. To ensure that the error S approaches 0, the weight Q should be sufficiently large.

[0021] Considering the inherent characteristics of the rotor and control surfaces of a quadcopter UAV, the rotor speed and control surface deflection cannot be increased indefinitely. Therefore, it is necessary to limit the rotor speed and control surface deflection.

[0022] ωmin ≤ω≤ω max

[0023] δ min ≤δ≤δ max

[0024] Where, ω min ,ω max These represent the minimum and maximum rotor speeds, respectively; similarly, δ min ,δ max These represent the minimum and maximum values ​​of aerodynamic control surface deflection, respectively.

[0025] In addition to limiting the rotor speed and control surface deflection, the rate of change of rotor speed also needs to be constrained, i.e.

[0026] |ω-ω0|≤Δω

[0027] Where ω0 is the rotor speed at the previous moment; ω is the rotor speed at the current moment; and Δω is the amplitude of the rotor speed change.

[0028] In summary, the constraint equations for the tilt rotor control allocation problem are as follows:

[0029] min f = S T QS

[0030]

[0031] Step S3: Use the optimoptions function in Matlab to select the SQP algorithm to transform the control assignment problem into a multi-constraint optimization problem with nonlinear constraints, and then use the fmincon function to find the optimal solution.

[0032] Beneficial effects:

[0033] The control allocation technology based on the SQP algorithm designed in this invention can accurately calculate the optimal rotor speed and control surface deflection at the corresponding tilt angle, so that the equivalent force and torque output by the controller can be directly mapped to the source of force and torque generation, which weakens the adverse dynamic effects brought about by the tilt transition process, and provides the possibility of decoupling the control law and control allocation structure design, thereby enabling the safe flight of a four-tilt rotor UAV in complex disturbance environments. Attached Figure Description

[0034] Figure 1 A block diagram of the transitional flight heterogeneous maneuvering control allocation method based on the SQP algorithm provided by this invention;

[0035] Figure 2 This is a schematic diagram of the heterogeneous control structure for transitional flight of a four-tilt-rotor UAV provided by the present invention;

[0036] Figure 3 The comparison diagram of three-axis control forces and torques before and after control allocation provided by this invention;

[0037] Figure 4 The diagram showing the rotational speeds of the four rotors after control allocation based on the SQP algorithm provided by this invention;

[0038] Figure 5 The aerodynamic control deflection diagram of the control allocation based on the SQP algorithm provided by this invention;

[0039] In the diagram, 1-4 are the four rotors of the four tiltrotor UAV; 5, 7, 9, and 11 are the ailerons of the four tiltrotor UAV; 6, 8, 10, and 12 are the elevators of the four tiltrotor UAV; 13 is the rudder; and 14 is the tilt servo. Detailed Implementation

[0040] The invention will now be further described with reference to the accompanying drawings.

[0041] This embodiment provides a heterogeneous control allocation method for transitional flight of a four-tilt-rotor UAV. The relevant physical parameters of the four-tilt-rotor UAV are: UAV mass m = 100 kg, wing reference area S. ref =2.317m 2 The average aerodynamic span is b = 0.55 m, the average aerodynamic chord length is c = 0.55 m, and the upper and lower limits of the rotor speed are ω. min =0,ω max =5500rpm, the upper and lower limits of the control surface deflection are δ jmin =-20°,δ jmax =20° j＝a,e,r The rotor thrust coefficient and torque coefficient are k f =0.00002,k m =0.000000944, the distances between the projections of the centers of the front two rotors and the centers of the rear two rotors onto the plane of symmetry of the aircraft and the center of gravity along the X-axis are ΔX1 = 0.4897m and ΔX2 = 1.2143m, respectively. The distances from the centers of the front two rotors and the centers of the rear two rotors to the plane of symmetry of the aircraft are ΔY1 = 1.315m and ΔY2 = 1.415m, respectively. The four tiltrotor UAV has four rotors: rotors 1 and 3 rotate clockwise, and rotors 2 and 4 rotate counterclockwise. Additionally, there are four control surfaces behind each pair of wings. Therefore, the UAV's control mechanism includes four rotors, four elevators (the two inner control surfaces of each pair of wings deflect simultaneously to act as elevators, i.e., control surfaces 6, 8, 10, and 12 act as elevators), four ailerons (the two outer control surfaces of each pair of wings deflect differentially to act as ailerons, i.e., control surfaces 5, 7, 9, and 11 act as ailerons), one rudder, and one tilt servo. For example... Figure 1As shown, this invention transforms the control allocation problem into a multi-constraint optimization problem. Based on the aerodynamic parameters and flight state of the UAV, and using the upper and lower bounds of the rotor and aerodynamic control surfaces as constraints, a cost function is written with the rotor control command ω and the aerodynamic control surface control command δ as the optimization objectives. The SQP algorithm is then used to calculate the optimal rotor speed and control surface deflection at the corresponding tilt angle. The process includes the following steps:

[0042] Step S1: Based on the force analysis of the four-tilt-rotor UAV during flight, the control force and torque of the UAV consist of rotor force and torque, force and torque generated by the aircraft body and control surfaces, and gravity and gravitational torque.

[0043]

[0044] In the formula, [F,M] T Represents resultant force and resultant moment; This represents the rotor force and torque generated by the rotor; Represents the aerodynamic forces and moments generated by the deflection of the aircraft's fuselage and control surfaces; Gravity and gravitational torque represent the forces generated by gravity; rotor force and torque. The mapping relationship between the four rotors and the rotor control variable ω can be expressed as:

[0045] Where τ is the Z-axis in the rotor coordinate system. h Y-axis and body coordinate axis b Z b The included angle on the plane is 0° for rotor mode and 90° for fixed-wing mode; ω i＝1,2,3,4 These represent the rotational speeds of the four rotors; k f ,k m These are the thrust coefficient and anti-torque coefficient, which are related to propeller diameter, air density, etc.; ΔX1 is the distance between the projection of the centers of the front two rotors onto the plane of symmetry of the aircraft and the center of gravity in the X-axis direction; ΔX2 is the distance between the projection of the centers of the rear two rotors onto the plane of symmetry of the aircraft and the center of gravity in the X-axis direction; ΔY1 is the distance from the centers of the front two rotors to the plane of symmetry of the aircraft; ΔY2 is the distance from the centers of the rear two rotors to the plane of symmetry of the aircraft.

[0046] The mapping relationship between the forces and moments generated by the aircraft fuselage and control surfaces and the control surfaces, i.e., the aerodynamic control surface control amount δ, can be expressed as:

[0047]

[0048] Among them, Q,S ref These are the dynamic pressure and the wing reference area, respectively; b, These are the mean aerodynamic span and the mean aerodynamic chord length, respectively; [C x0 C y0 Cz0 C mx0 C my0 C mz0 ] T The three-axis rudderless yaw force and moment coefficients of the UAV in the body coordinate system; The increment of the aerodynamic force and moment coefficient of the UAV rudder in the body coordinate system; δ a ,δ e ,δ r These are the aileron deflection, elevator deflection, and rudder deflection, respectively.

[0049] Gravity and gravitational torque can be expressed as:

[0050]

[0051] Where m and g are the mass and gravitational acceleration of the UAV, respectively; θ and φ are the pitch and roll angles of the UAV, respectively, with pitch up being positive and roll to the right being positive.

[0052] Step S2: Considering that at the corresponding tilt angle, the four-tilt-rotor UAV may not be able to meet the desired force and torque output by the attitude controller due to the limitations of the rotor and control surfaces themselves, in order to make the actual control force and torque acting on the UAV as close as possible to the desired force and torque output by the attitude controller, based on the error between the desired control force and torque and the actual control force and torque, a cost function is established with the rotor control command ω and the aerodynamic control surface control command δ as the optimization objectives:

[0053] f(S)=S T QS

[0054]

[0055] Here, S refers to the error between the desired control force and torque and the actual control force and torque. Its significance lies in selecting the most suitable rotor speed and control surface deflection at the tilt angle τ to meet the transition flight requirements of the UAV, while ensuring that the error between the actual control torque and the desired torque is as small as possible. To ensure that the error S approaches 0, the weight Q should be sufficiently large.

[0056] Considering the inherent characteristics of the rotor and control surfaces of a quadcopter UAV, the rotor speed and control surface deflection cannot be increased indefinitely. Therefore, it is necessary to limit the rotor speed and control surface deflection.

[0057] ω min ≤ω≤ω max

[0058] δ min ≤δ≤δ max

[0059] Where, ωmin ,ω max These represent the minimum and maximum rotor speeds, respectively; similarly, δ min ,δ max These represent the minimum and maximum values ​​of aerodynamic control surface deflection, respectively.

[0060] In addition to limiting the rotor speed and control surface deflection, the rate of change of rotor speed also needs to be constrained, i.e.

[0061] |ω-ω0|≤Δω

[0062] Where ω0 is the rotor speed at the previous moment; ω is the rotor speed at the current moment; and Δω is the amplitude of the rotor speed change.

[0063] In summary, the constraint equations for the tilt rotor control allocation problem are as follows:

[0064] min f = S T QS

[0065]

[0066] Step S3: Use the optimoptions function in Matlab to select the SQP algorithm to transform the control assignment problem into a multi-constraint optimization problem with nonlinear constraints, and then use the fmincon function to find the optimal solution. Figure 3 A comparison diagram of the control forces and torques of the three axes before and after the control distribution of a four-tilt-rotor UAV is presented. Figure 4 The following diagrams are presented showing the rotational speeds of the four rotors after control allocation based on the SQP algorithm. Figure 5 A diagram showing the deflection control of aerodynamic control surfaces is presented.

[0067] The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A heterogeneous control allocation method for transitional flight of a four-tilt-rotor unmanned aerial vehicle, characterized in that: The method includes: The control force and control torque output by the attitude controller are mapped to the rotor and fixed wing. A cost function is established with the rotor control command and aerodynamic control surface control command as the optimization target. The optimal solution of the cost function within the constraint range is solved by the sequential quadratic programming (SQP) algorithm, thereby obtaining the rotor and aerodynamic control surface control quantities that best match the control force and control torque. The method includes: Step S1: Based on the force analysis of the four tiltrotor UAV during flight, establish the mapping relationship expression between the UAV control force and control torque and the heterogeneous redundant control of fixed wing and rotor. Step S2: Based on the error between the desired control force and torque and the actual control force and torque, establish a rotor control command system. and aerodynamic control commands The cost function is defined as the objective to be optimized, and the upper and lower bounds of the rotor and aerodynamic control surfaces are set as control constraints. Step S3: Transform the control allocation problem into a multi-constraint optimization problem with nonlinear constraints. By solving the multi-constraint optimization problem, the optimal solution for the heterogeneous redundant control of fixed-wing and rotor is obtained. In step S1, the UAV control force and control torque consist of rotor force and torque, forces and torques generated by the aircraft body and control surfaces, and gravity and gravitational torque: ； In the formula, Represents resultant force and resultant moment; This represents the rotor force and torque generated by the rotor; Represents the aerodynamic forces and moments generated by the deflection of the aircraft's fuselage and control surfaces; Gravity and gravitational torque represent the forces generated by gravity; rotor force and torque. Generated by four rotors, and related to rotor control. The mapping relationship between them can be represented as: ； in, In the rotor coordinate axis In the axis and the body coordinate axis The angle between the planes, the rotor mode is Fixed-wing mode is ; These represent the rotational speeds of the four rotors; These are the thrust coefficient and anti-torque coefficient, which are related to the propeller diameter and air density. The projection of the centers of the first two rotors onto the plane of symmetry of the aircraft and the center of gravity. Distance along the axial direction; The projection of the rear two rotor centers onto the plane of symmetry of the aircraft and the center of gravity. Distance along the axial direction; This is the distance from the center of the first two rotors to the plane of symmetry of the aircraft; This is the distance from the center of the rear two rotors to the plane of symmetry of the aircraft; The forces and moments generated by the aircraft fuselage and control surfaces are related to the control surfaces, i.e., the aerodynamic control surface manipulation. The mapping relationship between them is expressed as follows: ； in, These are dynamic pressure and wing reference area, respectively. These are the mean aerodynamic span and the mean aerodynamic chord length, respectively. The three-axis rudderless yaw force and moment coefficients of the UAV in the body coordinate system; This represents the increment of the aerodynamic force and moment coefficient of the UAV rudder in the body coordinate system. These are the aileron deflection, elevator deflection, and rudder deflection, respectively. Gravity and gravitational torque are expressed as: ； in, These are the drone's mass and gravitational acceleration, respectively. These are the pitch and roll angles of the drone, respectively, with pitch up being positive and roll to the right being positive.

2. The heterogeneous control and maneuvering allocation method for transitional flight of a four-tilt-rotor UAV according to claim 1, characterized in that: In step S2, based on the errors between the desired control force and torque and the actual control force and torque, a rotor control command is established. and aerodynamic control commands The cost function for the objective to be optimized is: ； in, This refers to the error between the expected control force and torque and the actual control force and torque, which is significant in terms of the tilt angle. The most suitable rotor speed and control surface deflection are selected to meet the transition flight requirements of the UAV; By limiting the rotor speed and the deflection degree of the control surfaces, we obtain: ； ； in, These represent the minimum and maximum rotor speeds, respectively. These represent the minimum and maximum values ​​of aerodynamic control surface deflection, respectively. By constraining the rate of change of rotor speed, we obtain: ； in, This represents the rotor speed at the previous moment; The rotor speed at the current moment; This represents the amplitude of the rotor speed change; The constraint equations for the tilt rotor control allocation problem are as follows: 。 3. The heterogeneous control and maneuvering allocation method for transitional flight of a four-tilt-rotor UAV according to claim 1, characterized in that: In step S3, the optimoptions function in Matlab is used to select the SQP algorithm to transform the control assignment problem into a multi-constraint optimization problem with nonlinear constraints. The fmincon function is used to find the optimal solution, thereby improving the safety of UAV tilt transition flight.

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