A Longitudinal Altitude-Speed Decoupled Nonlinear Control Method for Fixed-Wing UAVs

Abstract

The present invention discloses a longitudinal height and speed decoupling non-linear control method for a fixed-wing unmanned aerial vehicle, which includes: determining the desired flight airspeed and desired flight altitude of the unmanned aerial vehicle; designing the desired height change rate, and calculating the desired flight path inclination angle through the desired height change rate and the desired flight airspeed; calculating the corresponding trim angle of attack and desired attitude angle through the desired flight airspeed and the desired flight path inclination angle; designing a thrust-angle of attack control law based on the incremental non-linear dynamic inversion method according to the deviation between the trim angle of attack and the actual angle of attack, so as to obtain a thrust control command; designing a pitch angle control law based on the incremental non-linear dynamic inversion method according to the deviation between the desired attitude angle and the actual attitude angle, so as to obtain an elevator deflection command. The present invention is applied to the field of unmanned aerial vehicles, which not only solves the influence of the non-linearity of aerodynamic characteristics in the large angle of attack state, but also has good adaptability to model errors, can be applicable to the flight in the large angle of attack state, effectively reduces the landing speed, and improves the safety of the take-off and landing process.

Description

Technical Field

[0001] The present invention relates to the technical field of UAV control, and specifically to a decoupled nonlinear control method for the longitudinal height and speed of a fixed-wing UAV. Background Technique

[0002] Autonomous takeoff and landing is one of the key technical links in the development of UAVs, and it is particularly important for fixed-wing UAVs with a relatively high cruising speed. Compared with the landing process, the takeoff process has relatively high safety. If a wheeled landing gear is used, the key technology lies in runway alignment correction, and the technical verification can be completed through ground tests. The landing process involves the attitude stability control of the UAV body, the precise control of altitude and sink rate, and the runway deceleration control process, etc., with high complexity and great risks. Among them, the landing speed is one of the important factors affecting safety. At present, landing at a large angle of attack has become an effective technical solution to reduce the landing speed, and in-depth research has been carried out in various countries. In addition, the precise control of altitude and speed before landing is also crucial.

[0003] The key points of flying at a large angle of attack lie in attitude stability and angle-of-attack maintenance. Among them, attitude stability specifically refers to the attitude stability control in the state of a large angle of attack. For conventional fixed-wing aircraft, the aerodynamic parameters change approximately linearly with the angle of attack within a small angle-of-attack range. When the angle of attack increases (less than the stall angle of attack), certain nonlinear characteristics appear; for unconventional aircraft such as tailless layouts, the aerodynamic characteristic parameters change nonlinearly strongly with the angle of attack and control rudder deflection. Therefore, in the design of the attitude control law within the large angle-of-attack range, it is necessary to fully consider the influence of the nonlinear aerodynamic model and design a nonlinear control law. Angle-of-attack maintenance means that in addition to achieving attitude stability, the UAV also needs to maintain a large angle-of-attack state. In the conventional control method, the logic scheme of using the elevator to control the pitch angle and the throttle to control the flight speed is adopted. Although the angle of attack can be indirectly controlled by controlling the speed, on the one hand, this method ignores the constraint relationship between the control target states, and there may be situations where the speed still cannot be reduced when the throttle is zero, or the altitude drops too fast. At the same time, according to the lift-weight balance, there is a unique correspondence between speed and angle of attack. In the state of a large angle of attack, the change of the lift coefficient slows down, resulting in a smaller change range of speed. At this time, the angle of attack is more sensitive to the speed change, and the measurement error and disturbance have a greater impact on the control effect.

[0004] On the other hand, in altitude and speed control, there is a strong coupling effect in traditional control methods. That is, during the process of controlling the attitude with the elevator, the flight altitude will change, which in turn affects the speed change. And during the throttle control process, the speed change will change the angle of attack, which in turn changes the flight path and attitude, further affecting the flight altitude. Although the total energy control method proposes a decoupling strategy of using the throttle to control the total energy of the aircraft and the elevator or pitch angle to control the kinetic energy / potential energy balance relationship, there are still some deficiencies: small-angle simplifications are made in the theoretical derivation process, which cannot adapt to large-angle range changes; proportional-integral control is adopted, and the adaptability to nonlinear models is poor; there are many control parameters, increasing the complexity of parameter optimization.

[0005] In summary, the traditional longitudinal speed and altitude control methods for UAVs have certain defects in precise trajectory control and low-speed large-angle-of-attack application scenarios, and it is difficult to meet the actual requirements. Summary of the Invention

[0006] In view of the deficiencies in the design process of the above-mentioned existing UAV control methods, the present invention provides a decoupled nonlinear control method for the longitudinal altitude and speed of a fixed-wing UAV, which not only has strong adaptability to model parameters, but also has low command calculation complexity, and can effectively achieve altitude and speed decoupled control in a large angle-of-attack range.

[0007] To achieve the above object, the present invention provides a decoupled nonlinear control method for the longitudinal altitude and speed of a fixed-wing UAV, which is characterized by including the following steps:

[0008] Step 1, determine the desired flight airspeed and desired flight altitude of the fixed-wing UAV;

[0009] Step 2, design the desired altitude change rate according to the relationship between the desired flight altitude and the actual altitude, and calculate the desired flight path inclination angle through the desired altitude change rate and the desired flight airspeed;

[0010] Step 3, calculate the corresponding trimmed angle of attack and desired attitude angle by using the general trimming method through the desired flight airspeed and the desired flight path inclination angle;

[0011] Step 4, design a thrust-angle-of-attack control law based on the incremental nonlinear dynamic inversion method according to the deviation between the trimmed angle of attack and the actual angle of attack, obtain the thrust control command, and achieve angle-of-attack-speed control;

[0012] Step 5, design a pitch angle control law based on the incremental nonlinear dynamic inversion method according to the deviation between the desired attitude angle and the actual attitude angle, obtain the elevator deflection command, and achieve attitude-flight path control.

[0013] The present invention has the following beneficial technical effects:

[0014] 1. In the present invention, the direct commands of the desired flight airspeed and the desired flight altitude are indirectly converted into the angle of attack and attitude angle commands by utilizing the state trim constraint relationship, thereby achieving decoupled control. The thrust-angle of attack control law enables the UAV to maintain a large range of angle of attack states. When combined with the attitude angle control law, the UAV can be stably maintained at the current angle of attack state and achieve trajectory control. The logic of directly controlling the angle of attack with thrust, on the one hand, avoids the problem that the angle of attack is relatively sensitive to speed changes in the large angle of attack region and weakens the influence of speed disturbances on the angle of attack; on the other hand, if thrust is used to control speed, and speed is related to the angle of attack, there is an intermediate link of angle of attack change, which will increase the control loop and reduce the control efficiency.

[0015] 2. For the angle of attack and attitude control in the present invention, the control law design is carried out by utilizing the incremental nonlinear dynamic inversion theory, and acceleration feedback is used for control, which avoids the influence of a part of model nonlinearity on the control effect and improves the adaptability and anti-interference ability of the control law.

[0016] 3. The response speed of the attitude angle control loop in the present invention is faster than that of the angle of attack control loop. On the one hand, during the change of the angle of attack, it can be considered that the pitch angle has reached the expected value, reducing the variables in the angle of attack control process and making it meet the incremental nonlinear dynamic inversion simplification conditions; on the other hand, during the track angle control process, the track angle control process can be finely adjusted by adjusting the expected pitch angle to make up for the track angle change in the angle of attack control process and improve the track control accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on the structures shown in these drawings.

[0018] Figure 1 It is the logic structure diagram of the longitudinal height-speed decoupled nonlinear control method for a fixed-wing UAV in the embodiment of the present invention;

[0019] Figure 2 It is the structural block diagram of the angle of attack control law in the embodiment of the present invention;

[0020] Figure 3 It is the structural block diagram of the attitude control law in the embodiment of the present invention.

[0021] The realization, functional characteristics and advantages of the object of the present invention will be further described in combination with the embodiments with reference to the drawings. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0023] In addition, the technical solutions between the various embodiments of the present invention can be combined with each other, but it must be based on the ability of those of ordinary skill in the art to implement. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the protection scope required by the present invention.

[0024] As Figure 1 shown, a longitudinal height-speed decoupled nonlinear control method for a fixed-wing unmanned aerial vehicle disclosed in this embodiment is referred to Figure 1 , and specifically includes the following steps 1 to 5.

[0025] Step 1, determine the desired flight airspeed and the desired flight height according to the actual flight scenario requirements such as takeoff, landing, or large angle-of-attack maneuvers of the fixed-wing unmanned aerial vehicle.

[0026] Step 2, design the desired height change rate according to the relationship between the desired flight height and the actual height of the fixed-wing unmanned aerial vehicle, and calculate the desired flight path inclination angle through the desired height change rate and the desired flight airspeed. The specific implementation process is as follows:

[0027] Step 2.1, design the desired height change rate as a first-order link. Specifically:

[0028]

[0029] In the formula, is the desired height change rate, K h is the reciprocal of the time constant of the first-order link, which can be designed according to the desired dynamic response characteristics. For example, if the desired adjustment time is T, then according to the response characteristics of the first-order link, K h is taken as 3 / T, but it is also limited by the control ability of the unmanned aerial vehicle. Therefore, the desired adjustment time should not be too small, and the specific size depends on the motion characteristics of the unmanned aerial vehicle body; h c is the desired flight height, h t is the current actual height;

[0030] Step 2.2, calculate the desired flight path inclination angle according to the desired height change rate and the desired flight airspeed, which is:

[0031]

[0032] In the formula, γ cDenote the desired track inclination angle, V a,c Denote the desired flight airspeed without considering the influence of wind.

[0033] Step 3, through the desired flight airspeed and the desired track inclination angle, using the general trimming method, calculate the corresponding trim angle of attack and the desired attitude angle. The specific implementation process is as follows:

[0034] Step 3.1, according to the desired flight airspeed and the desired track inclination angle, use the UAV dynamics model and the general trimming calculation method to obtain the trim angle of attack corresponding to the current desired flight airspeed and desired track inclination angle state, which is:

[0035] α trim = trim(V a,c , γ c )

[0036] In the formula, α trim is the trim angle of attack, and trim(·) is the trimming calculation process;

[0037] Step 3.2, according to the desired track inclination angle and the trim angle of attack, calculate the desired attitude angle, which is:

[0038] θ c = γ c + α trim

[0039] In the formula, θ c is the desired attitude angle.

[0040] In the specific application process, for the trimming process in Step 3.1, it is necessary to consider the flight performance and control ability of the aircraft body, set corresponding constraint conditions, judge the rationality of the trimmed state and make corresponding adjustments to ensure that the target state is theoretically achievable. For example: through the minimum flight speed limit of the UAV, the maximum flight angle of attack can be determined; through the elevator deflection limit, the pitch angle range that can maintain the pitch moment balance can be determined.

[0041] Step 4, based on the deviation between the trim angle of attack and the actual angle of attack, design a thrust - angle of attack control law based on the incremental nonlinear dynamic inversion method to obtain a thrust control command and achieve angle of attack - speed control. The specific implementation process is as follows:

[0042] The differential equation of the angle of attack dynamics is:

[0043]

[0044] In the formula, is the rate of change of the angle of attack, m is the mass of the UAV, V a is the flight airspeed, L is the lift, γ is the track inclination angle, T is the engine thrust, and α is the angle of attack;

[0045] Reference Figure 2 , according to the incremental nonlinear dynamic inversion theory, the thrust-angle of attack control law is obtained as follows:

[0046]

[0047] In the formula, the subscript (t - 1) represents the state at the previous time step, and the subscript t represents the state at the current time step. At the initial moment, the state of the previous time step is zero or the initial trim value, and K α is a design parameter for the control process.

[0048] Step 5: Based on the deviation between the desired attitude angle and the actual attitude angle, design the pitch angle control law using the incremental nonlinear dynamic inversion method to obtain the elevator deflection command and achieve attitude-trajectory control. Reference Figure 3 , and its specific implementation process is as follows:

[0049] Step 5.1: According to the longitudinal attitude motion dynamics differential equation and performing a first-order Taylor expansion of the pitch moment coefficient at a certain state, the longitudinal attitude dynamics differential equation in the neighborhood of this state is obtained as follows:

[0050]

[0051] In the formula, θ represents the pitch angle, q represents the pitch angular velocity, Q is the dynamic pressure, I yy is the moment of inertia about the pitch axis, S represents the wetted area, c is the longitudinal reference length (mean chord length), and C m represents the pitch moment coefficient (related to the current angle of attack α and elevator deflection δ e );

[0052] Step 5.2: According to the longitudinal attitude dynamics differential equation in Step 5.1, apply the nonlinear dynamic inversion theory to the pitch angle to pitch angular velocity loop to obtain the angle loop control law as follows:

[0053] q c = K θ (θ c - θ)

[0054] In the formula, q c is the desired pitch angular velocity, which will be controlled through the angular velocity to elevator deflection loop. K θ is the control coefficient, which determines the response speed of the angle loop;

[0055] Step 5.3: According to the angular velocity to elevator deflection loop in Step 5.2, apply the incremental nonlinear dynamic inversion theory to obtain the angular velocity loop control law as follows:

[0056]

[0057] In the formula, δe(t) is the elevator deflection at the current time step, δ e(t-1) is the elevator deflection at the previous time step, θ (t) is the pitch angle at the current time step, q (t) is the pitch angular velocity at the current time step, q (t-1) is the pitch angular velocity at the previous time step, C mδe is the partial derivative of the pitch moment coefficient with respect to the elevator deflection. This value is related to the model parameters and flight state. The incremental nonlinear dynamic inversion method can reduce the model dependence to a certain extent, K q is the control parameter.

[0058] In the specific application process, in the angular velocity loop control law of step 5.3, the pitch angle command is calculated based on the desired flight path angle and the actual angle of attack. It is considered that the pitch angle command tracks faster than the angle of attack command to compensate for the trajectory error caused by the slower tracking speed of the angle of attack. The desired pitch angle command θ c in the angle loop control law can be adjusted to γ c +α, that is, the sum of the desired flight path inclination and the current actual angle of attack, to compensate for the angle of attack tracking error.

[0059] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention. Any equivalent structural transformation made under the inventive concept of the present invention, or direct / indirect application in other related technical fields, is included in the patent protection scope of the present invention.

Claims

1. A longitudinal height - speed decoupling nonlinear control method for a fixed - wing unmanned aerial vehicle, characterized in that, it includes the following steps: Step 1, determine the desired flight airspeed and the desired flight height of the fixed - wing unmanned aerial vehicle; Step 2, design the desired height change rate according to the relationship between the desired flight height and the actual height, and calculate the desired flight path inclination angle through the desired height change rate and the desired flight airspeed; Step 3, calculate the corresponding trim angle of attack and the desired attitude angle by using the general trim method through the desired flight airspeed and the desired flight path inclination angle; Step 4, based on the deviation between the trim angle of attack and the actual angle of attack, design a thrust - angle - of - attack control law by using the incremental nonlinear dynamic inversion method to obtain a thrust control command, and achieve angle - of - attack - speed control; Step 5, based on the deviation between the desired attitude angle and the actual attitude angle, design a pitch - angle control law by using the incremental nonlinear dynamic inversion method to obtain an elevator deflection command, and achieve attitude - flight - path control.

2. The longitudinal height - speed decoupling nonlinear control method for a fixed - wing unmanned aerial vehicle according to claim 1, characterized in that, in Step 2, the desired height change rate is designed as a first - order link, which is: Wherein, is the desired height change rate, and K h is the reciprocal of the time constant of the first-order link, h c is the desired flight height, and h t is the current actual height.

3. The longitudinal height - speed decoupling nonlinear control method for a fixed - wing unmanned aerial vehicle according to claim 2, characterized in that, in Step 2, the desired flight path inclination angle is: where γ c represents the desired track inclination angle, and V a,c represents the desired airspeed in flight without considering the influence of wind.

4. The longitudinal height - speed decoupling nonlinear control method for a fixed - wing unmanned aerial vehicle according to claim 1 or 2 or 3, characterized in that, Step 3 specifically includes: Step 3.1, according to the desired flight airspeed and the desired flight path inclination angle, use the unmanned aerial vehicle dynamics model and the general trim calculation method to obtain the trim angle of attack corresponding to the current desired flight airspeed and desired flight path inclination angle state, which is: α trim = trim(V a,c , γ c ) where α trim is the trim angle of attack, trim(·) represents the trim calculation process, V a,c represents the desired airspeed in flight without considering the influence of wind, and γ c represents the desired flight path inclination angle; Step 3.2, calculate the desired attitude angle according to the desired flight path inclination angle and the trim angle of attack, which is: θ c = γ c + α trim where θ c is the desired attitude angle.

5. The longitudinal height - speed decoupling nonlinear control method for a fixed - wing unmanned aerial vehicle according to claim 4, characterized in that, during the trim process in Step 3.1, set corresponding constraint conditions for the flight performance and control ability of the aircraft body, judge the rationality of the trim state and make corresponding adjustments to ensure that the target state is theoretically achievable.

6. The longitudinal height - speed decoupling nonlinear control method for a fixed - wing unmanned aerial vehicle according to claim 4, characterized in that, Step 4 specifically includes: Establish a differential equation of the angle - of - attack dynamics, which is: In the formula, is the angle of attack change rate, m is the mass of the UAV, V a is the flight airspeed, L is the lift force, γ is the flight path inclination angle, T is the engine thrust, and α is the angle of attack; According to the incremental nonlinear dynamic inversion theory, obtain the thrust - angle - of - attack control law, which is: where the subscript (t - 1) represents the state at the previous time step, the subscript t represents the state at the current time step, and K α is a design parameter for the control process.

7. The longitudinal height - speed decoupling nonlinear control method for a fixed - wing unmanned aerial vehicle according to claim 4, characterized in that, Step 5 specifically includes: Step 5.1, according to the longitudinal attitude motion dynamics differential equation, and perform a first - order Taylor expansion of the pitch moment coefficient at a certain state to obtain the longitudinal attitude dynamics differential equation in the neighborhood of this state, which is: Where, θ represents the pitch angle, q represents the pitch angular velocity, Q is the dynamic pressure, I yy is the moment of inertia about the pitch axis, S represents the wetted area, c is the longitudinal reference length, C m represents the pitch moment coefficient related to the current angle of attack α and elevator deflection δ e ; Step 5.2, according to the longitudinal attitude dynamics differential equation in Step 5.1, apply the nonlinear dynamic inversion theory to the pitch - angle - to - pitch - angular - velocity loop to obtain the angle - loop control law, which is: q c = K θ (θ c - θ) where q c is the desired pitch angular velocity, which will be controlled through the angular velocity to elevator deflection loop, and K θ is the control coefficient, which determines the response speed of the angle loop; Step 5.3, according to the angular - velocity - to - elevator - deflection loop in Step 5.2, apply the incremental nonlinear dynamic inversion theory to obtain the angular - velocity - loop control law, which is: where δ e(t) is the elevator deflection at the current time step, δ e(t-1) is the elevator deflection at the previous time step, θ (t) is the pitch angle at the current time step, q (t) is the pitch angular velocity at the current time step, q (t-1) is the pitch angular velocity at the previous time step, is the partial derivative of the pitch moment coefficient with respect to the elevator deflection, and this value is related to the model parameters and flight state. The incremental nonlinear dynamic inversion method can reduce the model dependence to a certain extent. K q is the control parameter.

8. The longitudinal height and speed decoupled nonlinear control method for a fixed-wing UAV according to claim 7, characterized in that, in the angular velocity loop control law of step 5.3, the pitch angle command tracking speed is faster than the angle of attack command tracking speed to compensate for the trajectory error caused by the slower angle of attack tracking speed.

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