Flight control structure of propeller aircraft mode of electric tilting six-rotor unmanned aerial vehicle

Abstract

The invention discloses a flight control system structure of an electric tilting six-rotor unmanned aerial vehicle in a propeller aircraft mode. The flight control system structure is composed of a layered closed-loop architecture composed of outer ring attitude / height (or climbing rate) control and inner ring attitude angular rate control, and an airspeed single-loop control, trajectory guidance and actuator distribution / constraint management module. The flight control system structure of the electric tilting six-rotor unmanned aerial vehicle in the propeller aircraft mode has the remarkable advantages that under the influence of the control surface efficiency changing along with the air speed / attack angle, external disturbance and coupling, the rolling and pitching channel adjusting time is remarkably shortened, and overshoot and steady-state errors can be neglected; the airspeed is kept without overshoot, and the steady-state error is small; the yaw rate response is fast, and the coupling inhibition effect is good; the system structure calculation load is low, parameter calibration is friendly, and robustness is high; the method is suitable for attitude, airspeed and height / climbing rate cooperative control and route autonomous tracking of the electric tilt six-rotor unmanned aerial vehicle in a propeller aircraft mode.

Description

Technical Field

[0001] This invention belongs to the field of aircraft flight control technology, specifically relating to a multi-loop, parameter self-tuning flight control system structure and its control law implementation method for achieving attitude, speed, and altitude / rate of climb control of an electric tiltrotor unmanned aerial vehicle in propeller aircraft mode. Background Technology

[0002] Tiltrotor aircraft combine the advantages of helicopter vertical takeoff and landing / hovering with propeller aircraft high-speed cruise, encompassing three flight modes: helicopter mode, tilt transition mode, and propeller aircraft mode. In propeller aircraft mode, the rotor primarily provides forward thrust, while attitude and altitude control rely on aerodynamic control surfaces (ailerons, V-tail control surfaces) for effectiveness. Common problems with existing technologies include: sensitivity to multi-loop parameter coupling: strong coupling exists between pitch-altitude (rate of climb) and roll-lateral channels, making it difficult for traditional fixed-gain PID control to balance speed, stability, and error; insufficient robustness to time-varying operating conditions: angle of attack, airspeed, and control surface efficiency vary significantly with flight conditions, making fixed-parameter control laws prone to overshoot, oscillation, steady-state error, and tracking lag. Therefore, there is an urgent need for a flight control system structure with internal and external multi-loops and the ability to self-tune parameters in the outer loop, enabling coordinated attitude / speed / altitude control of electric tilttrotor aircraft in propeller aircraft mode, thereby improving dynamic response, steady-state accuracy, and disturbance rejection performance. Summary of the Invention

[0003] Technical problems to be solved The technical problems this invention aims to solve are the sensitivity issues of multi-loop parameter coupling in propeller aircraft mode, the insufficient robustness caused by time-varying operating conditions, and the performance limitations of traditional fixed-parameter control laws. Due to the strong coupling between the pitch-altitude and roll-lateral channels in propeller aircraft mode, traditional fixed-gain PID control struggles to balance speed, stability, and error. Simultaneously, the aircraft's angle of attack, airspeed, and control surface efficiency vary significantly with flight conditions, making fixed-parameter control laws prone to overshoot, oscillations, steady-state errors, and tracking lag. Therefore, there is an urgent need for a flight control system structure with internal and external multi-loops and the ability to self-tune parameters in the outer loop, enabling coordinated attitude / speed / altitude control of the electric tiltrotor aircraft in propeller aircraft mode, thereby improving dynamic response, steady-state accuracy, and disturbance rejection performance.

[0004] Technical solution To address the aforementioned problems, this invention discloses an electrically powered tilting hexacopter unmanned aerial vehicle (UAV) (such as...). Figure 1The diagram shows the flight control system structure in propeller aircraft mode. The system structure employs a hierarchical closed-loop architecture of "outer-loop fuzzy PID control + inner-loop PID control," superimposed with a single-loop airspeed control and L1 guidance law. Safe actuation is achieved through actuator allocation and constraint management, forming a five-layer collaborative control system of "attitude / altitude (or rate of climb) - angular rate - airspeed - guidance - allocation." Its key points are as follows: 1. Architecture and Functional Decomposition In propeller-driven aircraft mode, the rotors of the electrically tilting hexacopter UAV only provide thrust for forward flight. Similar to traditional propeller aircraft control, roll, pitch, and yaw are achieved by controlling the deflection of the ailerons, elevators, and rudder; flight speed is controlled by changing the rotor speed. Furthermore, in propeller-driven aircraft mode, altitude control is achieved through pitch motion. The desired altitude can be achieved either by setting a reasonable pitch angle to input a control loop signal, or by designing a separate altitude control loop.

[0005] The main function of the flight control system for an electric tilt-rotor hexacopter UAV in propeller-driven aircraft mode is to achieve attitude control such as roll, pitch, and yaw during high-speed flight, as well as control of flight speed and altitude during changes in flight. Based on the dual-loop control structure used in conventional flight control systems, and combined with the operating principles and main functions of the electric tilt-rotor hexacopter UAV in propeller-driven aircraft mode, the flight control system structure is determined as follows: Figure 2 As shown. The control module includes four parts: airspeed control, attitude control, altitude control, and trajectory tracking. The airspeed control uses a single-loop PID to close the loop for airspeed error and outputs throttle / thrust commands to the propulsion device. It is decoupled from the pitch-altitude chain to avoid energy and altitude control competing with each other.

[0006] Inner loop attitude angular rate control, for roll angular rate p Pitch rate q yaw rate r An independent PID control closed loop is employed, with a higher bandwidth configuration than the outer loop, to achieve fast damping, coupling suppression, and disturbance rejection. The outer loop attitude control uses Mamdani-type fuzzy PID control, based on attitude angle error. e With error change rate de Input, output for internal / middle loop PID control ( Kp , Ki , KdThe online correction coefficients enable parameter self-tuning capability. Input linguistic variables are NB, NM, NS, ZO, PS, PM, and PB. The input membership function uses a Gaussian / trigonometric combination, and the output membership function is a trigonometric function. The centroid method is used for defuzzification. The correction coefficients are set with dual limits on amplitude and rate of change to ensure stability margin. The altitude control chain is implemented through the pitch channel, employing a three-loop structure: "climb rate outer loop (fuzzy PID or PID) - pitch angle middle loop (fuzzy PID or PID) - pitch rate inner loop (PID)". Altitude is referenced through the climb rate. Drive pitch angle Then by q The ring system completes rapid tracking, establishing a division of labor: "controlling altitude with pitch and stabilizing airspeed with throttle." The trajectory tracking (guidance layer) employs the L1 guidance law, generating the desired horizontal acceleration based on the L1 distance in both straight and circular paths. and yaw rate Feedforward; for L1 distance, maximum lateral overload and By setting boundaries and smoothers, smooth turns and controllable overshoot can be achieved, improving tracking stability.

[0007] 2. Hierarchical closed-loop architecture of outer-loop fuzzy PID + inner-loop PID In the control block diagram above, the pitch and roll angle control employs a combination of fuzzy PID and traditional PID loop control. Based on the relevant theory of fuzzy PID control, the three parameters in PID control are considered. To determine the correlation, Mamdani's fuzzy inference method was adopted, with the system input selected as the error. and error change rate The system output consists of proportional, integral, and derivative correction coefficients. The linguistic variables for both input and output in their respective domains are negative large (NB), negative medium (NM), negative small (NS), zero (ZO), positive small (PS), positive medium (PM), and positive large (PB); that is, the error is defined. Error change rate With correction factor The fuzzy subsets are all .

[0008] Applying fuzzy control rules to the Mamdani fuzzy inference module yields a fuzzy controller for parameter optimization of a PID controller. Simultaneously, the output term under this fuzzy control rule can be determined. With input items The nonlinear correspondence.

[0009] The output of the fuzzy controller The correction coefficients are input as corresponding elements to the PID controller to obtain a complete fuzzy PID controller, where the correction coefficients are... It optimizes the coefficients of the PID controller in real time through fuzzy logic operations and fuzzy rules to achieve the ideal control effect, enabling the control system to adapt to parameter changes and resist external disturbances.

[0010] 3. Trajectory tracking L1 control The trajectory tracking function is achieved using the L1 guidance law. This guidance law involves selecting a reference point on the desired path and calculating the desired horizontal acceleration based on this reference point and the current horizontal velocity. L1 refers to the distance between the reference point on the desired path and the aircraft's current horizontal position. The principle is as follows: Figure 3 As shown, the expected motion of the aircraft at the current moment is a radius of... Speed ​​is Since the trajectory is a circular motion, a corresponding desired yaw rate feedforward should be provided for heading control to ensure tracking performance. Under these conditions, the desired acceleration and the desired yaw rate used for feedforward are calculated as follows: During flight, the flight path to be tracked is generally a straight line or a circle. For irregular curves, linearization is performed. Therefore, this section will analyze the two cases of straight path and circular path separately.

[0011] A straight path, such as Figure 4 As shown. Assuming angle and Very small, then according to Figure 4 From the acceleration formula (1), we can obtain Circular path, such as Figure 5 As shown. Similar to a straight path, assume an angle , Very small, expected acceleration is Based on the above trajectory tracking principle, relevant functional modules were designed, and a Simulink simulation structure was established to conduct flight simulation in propeller aircraft mode.

[0012] Beneficial effects The beneficial effects of this invention are mainly reflected in the following aspects. 1. Improved dynamic response performance and steady-state accuracy: The system adopts a hierarchical closed-loop architecture of "outer-loop fuzzy PID + inner-loop PID". Utilizing the outer-loop fuzzy PID controller, the PID parameters can be tuned online in real time based on attitude angle or altitude errors. Simulation results show that this design significantly shortens the settling time of roll and pitch channels, achieves almost no overshoot and negligible steady-state error, and solves the problems of overshoot, oscillation, and tracking lag that easily occur in traditional fixed-parameter control laws.

[0013] 2. Enhanced robustness to time-varying conditions and external disturbances: Addressing challenges such as time-varying control surface efficiency and uncertain aerodynamic parameters caused by changes in angle of attack and airspeed in propeller aircraft mode, the system effectively improves its disturbance rejection performance through a parameter self-tuning mechanism. The system maintains control stability under complex flight conditions (such as aerodynamic parameter variations and external disturbances), overcoming the shortcomings of traditional PID control in balancing speed and stability.

[0014] 3. Achieved efficient coupling suppression and precise autonomous trajectory tracking: This structure effectively decouples speed and altitude control through independently designed airspeed single-loop control, and achieves smooth and controllable autonomous trajectory tracking by combining L1 guidance law. Meanwhile, the yaw channel employs angular rate single-loop control, reducing the coupling degree and parameter tuning complexity between multiple loops, and offering advantages such as low computational load and user-friendly engineering parameter calibration. Attached Figure Description

[0015] Figure 1 Top view and rotor steering diagram of an electrically tilting hexacopter UAV Figure 2 Flight control system structure diagram of an electric tiltrotor hexacoach UAV in propeller aircraft mode Figure 3 Schematic diagram of L1 guidance law Figure 4 Linear path diagram Figure 5 Circular path diagram Figure 6 Airspeed curve Figure 7 Roll angle curve for an electrically tilting hexacopter UAV Figure 8 Roll rate curve for an electrically tilting hexacopter UAV Figure 9 Pitch angle curve for an electrically tilting hexacopter UAV Figure 10 Pitch rate curves for an electrically tilting hexacopter UAV Figure 11 Yaw rate curve for an electric tilt-rotor hexacoach UAV Figure 12Trajectory tracking curves for an electrically tilting hexacopter UAV in propeller-driven aircraft mode Detailed Implementation Step 1: First, you need to follow the steps as follows Figure 3 The structure of the electric tilt-rotor hexacopter UAV in propeller aircraft mode is constructed. This structure mainly includes four major modules: flight speed control, attitude control, altitude control, and trajectory control. Each control module is described in detail below.

[0016] Step 2: Establish a flight speed control module for the electric tilt-rotor hexacopter UAV in propeller aircraft mode, employing a single-loop control structure and using PID control. The flight control simulation will be conducted using the electric tilt-rotor hexacopter UAV in propeller aircraft mode with constant speed and no attitude change as the verification target. The initial attitude angles for the flight control simulation will be given. All Initial attitude angular rate All are 0 rad / s. System input for The obtained airspeed simulation results are as follows: Figure 6 As shown.

[0017] Step 3: Establish the attitude control module of the UAV. The pitch and roll channels of the six electric tilt rotors require a dual-loop control structure for attitude angle and attitude angular rate, while the yaw channel adopts a single loop to control the yaw rate.

[0018] The flight control system simulation uses the roll, pitch, and yaw control of an electrically powered tilt-rotor hexacopter UAV in propeller-driven aircraft mode as the verification target. The roll and pitch channels both use PID controllers in their inner loops, while the outer loops employ PID control and fuzzy-PID control, respectively. The yaw channel uses a single-loop control and selects PID control. When calculating the control system performance indicators based on the simulation results for the three attitude channels, [the following parameters are used]. Error band. Given the initial attitude angle of the system. All Initial attitude angular rate All are 0 rad / s.

[0019] To verify the roll motion control loop, a desired airspeed input signal was given to the throttle channel at the start of the simulation to simulate attitude changes during high-speed flight. Right roll was defined as positive and left roll as negative. Roll angle command signals were given at 3s, 9s, and 14s, respectively, simulating the UAV completing continuous roll motions with roll angles of 10°, -10°, and 0° in the air. A ramp function was used for transition when the input signal changed, and the response of the roll channel was observed. The simulated roll angle curve and roll rate curve are shown below. Figure 7 and Figure 8As shown.

[0020] For pitch motion, nose-down (pitch) is defined as negative and nose-up (pitch) as positive. For pitch attitude control, an airspeed command signal is given at the start of the simulation, and climb rate command signals are given at 5s, 12s, and 20s respectively. The simulation simulates the aircraft performing continuous pitch motions in the air, and the response of the pitch channel is observed. The simulated pitch angle and pitch rate curves are shown below. Figure 9 and Figure 10 As shown.

[0021] For the yaw rate control loop, yaw rate command signals of 0.5326, -0.5326, and 0 were given at 4s, 9s, and 14s respectively in the simulation. This simulated the aircraft performing continuous yaw motions in the air with yaw rates of 30deg / s, -30deg / s, and 0deg / s, and the response of the yaw channel was observed. The simulated yaw rate curves are shown below. Figure 11 As shown.

[0022] Step four: Establish the altitude control module for the UAV. Since the altitude change of the UAV is achieved through the pitch motion of the airframe in the propeller aircraft mode, a three-loop structure is selected for the altitude control of this electric tilt-rotor six-rotor UAV in propeller aircraft mode. The outermost layer is the climb rate control loop, which adopts PID control. The inner loops are the pitch angle and pitch rate control loops.

[0023] This part of the flight control simulation uses an electrically powered tilt-rotor hexacopter UAV in propeller-driven aircraft mode to achieve altitude hold through pitch motion as the verification objective. The initial attitude angles for the simulation are given below. All Initial attitude angular rate Both are 0 rad / s. The expected input of the system. for For altitude control, an airspeed command signal is given at the start of the simulation, and climb rate command signals are given at 5s, 12s, and 20s respectively. The simulation is performed to simulate the aircraft completing continuous pitch motion in the air and then maintaining a fixed altitude. The response of the climb rate channel is observed.

[0024] Electric tiltrotor drones spend most of their flight time in propeller-driven mode, meaning they often complete the majority of their flight in this mode. Adding a trajectory tracking function to this propeller-driven mode allows for autonomous flight via the flight control system by setting waypoints, without any human intervention. This significantly reduces the difficulty of operating the drone. Furthermore, due to the high sensitivity of the avionics system and the rapid response of the flight control system, coupled with the inherent errors in human observation and the lag in manual operation, this function also prevents flight accidents caused by human error.

[0025] The trajectory tracking function is achieved by using the L1 guidance law. This guidance law is an algorithm that selects a reference point on the desired path and calculates the desired horizontal acceleration based on the reference point and the current horizontal velocity. L1 refers to the distance between the reference point on the desired path and the current horizontal position of the aircraft.

[0026] Define input parameters The spacecraft's mission waypoint position in space, the spacecraft's initial position. The simulation is set up with four waypoints, namely... , , , And eventually returned to the initial position, the simulation yielded the following results: Figure 12 The trajectory tracking results are shown.

[0027] The trajectory tracking curve shows that the UAV exhibits some overshoot (the circular arc portion of the curve) when completing trajectory tracking autonomously, requiring subsequent automatic correction of the flight curve. Aside from this, the overall trajectory tracking curve is smooth and it reaches the set waypoints as expected, demonstrating that the L1 control law can effectively achieve trajectory tracking for the UAV.

Claims

1. A flight control structure for an electrically tilting hexacopter unmanned aerial vehicle (UAV) in propeller-driven aircraft mode, characterized in that, The system includes: The outer loop attitude / height control module adopts a fuzzy PID controller, which takes attitude angle error and error change rate and / or height (or climb rate) error and error change rate as input, and outputs correction coefficients for online correction of the proportional, integral, and derivative parameters of the corresponding PID controller. The inner-loop attitude angular rate control module controls the roll angular rate. p Pitch rate q yaw rate r PID control closed loops are used respectively; The airspeed control module is a single-loop PID control module, which maintains airspeed by adjusting the throttle of the propulsion device. The altitude control link, through the pitch channel, realizes a three-loop control structure including "climb rate outer loop - pitch angle middle loop - pitch rate inner loop"; The trajectory tracking module uses the L1 guidance law to generate the desired horizontal acceleration and yaw rate feedforward, and is cascaded with the outer / inner loop module. The actuator allocation and constraint management module maps roll, pitch, and yaw control values ​​to aileron and V-tail control surfaces, and airspeed control values ​​to propulsion devices, and implements deflection angle, deflection rate, power limiting, and saturation protection. The parameter correction coefficients output by the outer loop attitude / altitude control module are used to tune the parameters of the inner loop PID control and / or the intermediate loop PID control online, so as to improve the dynamic response and steady-state accuracy of the system under conditions of time-varying control surface efficiency, uncertain aerodynamic parameters and external disturbances.

2. The flight control system structure according to claim 1, characterized in that, The fuzzy PID controller adopts Mamdani-type fuzzy inference. The input linguistic variables include NB, NM, NS, ZO, PS, PM, and PB. The output is the correction coefficients of the proportional, integral, and derivative parameters. The input membership function is a combination of Gaussian and trigonometric functions, and the output membership function is a trigonometric function. The defuzzification method is the centroid method.

3. The flight control system structure according to claim 1 or 2, characterized in that, In the height control link: The outer loop takes the height error or climb rate error as input and uses fuzzy PID control to generate a climb rate reference. The middle loop uses the climb rate error as input and employs PID control to generate a pitch angle reference; The inner loop takes pitch angle error as input, and uses fuzzy PID control or traditional PID control to generate a pitch rate reference and close the loop to... q Channel PID control.

4. The flight control system structure according to claim 1, characterized in that, The roll channel adopts a dual-loop structure of "attitude angle fuzzy PID control outer loop + angular rate PID control inner loop"; the yaw channel adopts yaw angular rate single-loop PID control to reduce coupling and parameter tuning complexity.

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