High-precision unmanned aerial vehicle landing control method based on variable time constant

Abstract

The invention discloses a high-precision unmanned aerial vehicle landing control method based on a variable time constant, and belongs to the field of unmanned aerial vehicle flight control, the method mainly acts on a leveling stage of a second stage of an unmanned aerial vehicle landing process, and flight parameters of the leveling stage are calculated through a kinetic equation of the leveling stage. A time constant is solved through continuous iteration by adopting a dichotomy method, and a real-time lifting rate control instruction is solved through the time constant, so that the track of the aircraft in the leveling process is smooth enough, the landing precision is improved, and the landing control method has higher practicability and efficiency.

Description

Technical Field

[0001] The invention belongs to the field of unmanned aerial vehicle flight control, and in particular relates to a high-precision unmanned aerial vehicle landing control method based on a variable time constant. Background Art

[0002] The automatic flight control system is used to automatically control the aircraft from the glide ramp control phase to the hover and landing phase of the landing maneuver; the ultimate goal of any automatic landing control system is to land safely and smoothly within specific geometric and aircraft attitude dynamic boundaries. This automatic control may be limited by the performance of various flight control subsystems, such as the pitch attitude and speed control subsystems, and by certain uncontrollable environmental conditions, such as errors in measuring absolute altitude, errors in the aircraft's deviation from the glide predetermined trajectory when the level indicator is flat, headwinds and tailwinds, and wind shear. The automatic landing control system requires a landing point spread, that is, the spread of landing points along the runway relative to the predetermined point. This requirement places a heavy burden on the capabilities of the automatic flight control system, such as the tolerances of sensors, electronics, and service systems. Therefore, if the control law parameters that determine the glide path characteristics are changed or adjusted according to the existing and detectable approach conditions, the landing point spread can be significantly reduced, so that the tolerance requirements on the flight control system are reduced, but landing automatic control can still be provided within the required spread range.

[0003] At present, PID control method and gain preset technology are widely used in engineering to design automatic landing control law, but this method cannot solve the problem of nonlinear aircraft aerodynamic parameters well, and does not have strong robustness; it causes the aircraft to have weak wind and interference resistance during landing, and it is difficult to cope with runway environment changes during taxiing. During the landing of the UAV, the flight control cannot give the leveling command in time and accurately, resulting in the UAV deviating too much to the side after landing or even crashing. Summary of the invention

[0004] The present invention provides a high-precision UAV landing control method based on a variable time constant, which can solve the problems existing in the UAV landing control method in the prior art, make the trajectory of the aircraft leveling process smooth enough, and improve the landing accuracy.

[0005] To achieve the above objectives, the present invention adopts the following technical solutions:

[0006] A high-precision UAV landing control method based on variable time constant includes the following steps:

[0007] S1: Give the dynamic equation of the leveling phase of the UAV landing process;

[0008] S2: Calculate the flight parameters during the leveling phase;

[0009] S3: using the dichotomy method to screen and obtain the time constant according to the flight parameters obtained in S2;

[0010] S4: The lift rate command is obtained by solving the time constant. With the continuous iteration of the time constant, the lift rate command is also updated, and the lift rate control command is converted into the control of the rudder surface. The flight of the UAV is controlled according to the rudder surface command. In this process, the leveling curve of the UAV gradually becomes smoother.

[0011] The landing process of the drone is divided into three stages, namely:

[0012] The first stage is the landing phase: the UAV begins to land and the aircraft begins to glide along the predetermined glide path, at which time the angle of attack is negative;

[0013] The second stage of the leveling phase: When the drone descends to a suitable leveling point, in order to adjust the landing attitude, the aircraft begins to pull up, and the angle of attack is positive at this time until the rear two wheels of the landing gear touch the ground first;

[0014] The third stage is the landing stage: after the two wheels touch the ground, reduce the throttle and slowly adjust the attitude to make the front wheel touch the ground. After the three wheels touch the ground, the flight control will transmit the braking command until the drone stops steadily.

[0015] The above method is mainly used in the second stage of the landing process of the UAV, the leveling stage. Assuming that the leveling stage basically maintains a uniform flight speed and the leveling trajectory is exponential, the dynamic equation is:

[0016]

[0017] Where J is the time constant, h(t) is the altitude function of the drone in the leveling phase, is the descent rate of the drone at the end of touchdown. It can be seen that when J is larger, h(t) is smaller and the trajectory is flatter;

[0018] The flight parameters during the flare phase are: distance to be flown, altitude of the flare point, and ascent and descent rate. Specifically:

[0019]

[0020] Among them, x1 is the total distance to be flown, h1 is the height of the leveling point, is the descent rate at the leveling point, is the descent rate of the drone at the end of touchdown, J is the time constant, V g is the ground speed of the aircraft, x2 is the distance to be flown from the touchdown point, t is the time, γ is the downhill slope angle.

[0021] In order to quickly find the accurate time constant J in the range of J = 1 to 8, a binary selection method is used:

[0022]

[0023] By continuously dividing the interval where the time constant J is located into two, the two endpoints of the interval are gradually approached to a certain middle value, and then the approximate value of the time constant J is obtained; after obtaining J, it is brought back to recalculate the waiting flight distance and the pull-up period time.

[0024] Beneficial effects: The present invention provides a high-precision UAV landing control method based on a variable time constant. By continuously iteratively solving the time constant, and then solving the real-time lifting rate control instructions through the time constant, the trajectory of the aircraft leveling process is made smooth enough, the landing accuracy is improved, and the UAV's anti-wind and anti-interference capabilities during the landing process are enhanced. It can cope with runway environment changes during the taxiing process. The UAV landing control method of the present invention is more practical and efficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a control structure diagram of the UAV lift rate command to elevator command in an embodiment of the present invention;

[0026] Figure 2 This is a schematic diagram of the segmented landing trajectory of the UAV in an embodiment of the present invention;

[0027] Figure 3 Schematic diagram of the leveling point during the landing process of the UAV in an embodiment of the present invention;

[0028] Figure 4 It is a schematic diagram of the horizontal length of the flattening curve during the landing process of the UAV in an embodiment of the present invention;

[0029] Figure 5 A schematic diagram of a glide slope during the landing process of a UAV in an embodiment of the present invention;

[0030] Figure 6 Schematic diagram of the distance to be flown at the leveling point and the distance to be flown at the touchdown point during the landing process of the UAV in an embodiment of the present invention. DETAILED DESCRIPTION

[0031] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments:

[0032] like Figure 2 As shown in the figure, the landing process of the drone can be divided into the following three stages (red indicates the landing trajectory):

[0033] Phase 1: The drone begins to land and glides along the predetermined glide path, with a negative angle of attack.

[0034] Stage 2: When the drone glides down to a suitable leveling point, in order to adjust the landing attitude, the aircraft starts to pull up, and the angle of attack is positive at this time until the rear two wheels of the landing gear touch the ground first;

[0035] The third stage: After the two wheels touch the ground, reduce the throttle and slowly adjust the posture to make the front wheel touch the ground. After the three wheels touch the ground, the flight control will transmit the braking command until the drone stops steadily.

[0036] A high-precision landing control method based on a variable time constant is mainly used in the second stage of the UAV landing process, and includes the following steps:

[0037] Assuming that the flight speed is basically uniform during the leveling phase and the leveling trajectory is exponential, the dynamic equation is:

[0038]

[0039] Where J is the time constant, h(t) is the altitude function of the drone in the leveling phase, is the descent rate of the drone at the end of touchdown. It can be seen that when J is larger, h(t) is smaller and the trajectory is flatter;

[0040] The flattening height is obtained according to the dynamic equation. The equation for calculating the height is:

[0041]

[0042] Taking the derivative of formula (1.2), we can get the descent rate of the flattening point:

[0043]

[0044] The descent rate at a known flare point and the rate of rise and fall at the end of touchdown The time t can be obtained from formula (1.3):

[0045]

[0046] Get the horizontal length of the flattened curve. The horizontal length x1 is as follows: Figure 4 As shown, V g is the ground speed of the aircraft, t is the time it takes for the aircraft to move from the leveling point to the touchdown point,

[0047] x1=V g · t (1.5)

[0048] Substituting equation (1.4) into equation (1.5) yields the horizontal length x1

[0049]

[0050] Substituting equation (1.5) into equation (1.2), we can get the height of the flattened point:

[0051]

[0052] Get the distance to fly at the leveling point, such as Figure 4 As shown, the glide slope is tangent to the exponential curve. Assuming the glide slope angle is γ, it can be known from the geometric relationship that:

[0053]

[0054] Where x′1 is the distance to be flown at the leveling point. Combining with equation (1.6), we can know that the distance to be flown at the touchdown point is

[0055]

[0056] In summary, we can get a series of parameters such as the distance to be flown and the height of the leveling point:

[0057]

[0058] In order to quickly find the accurate time constant J, according to engineering experience, J is taken within the range of 1 to 8, and the binary method is used for screening:

[0059]

[0060] By continuously dividing the interval where the time constant J is located into two, the two endpoints of the interval are gradually approached to a certain intermediate value, and then the approximate value of the time constant J is obtained. Since the search interval is halved in each iteration, the convergence speed of the binary search method is relatively fast. Set J1=1 and J2=8 at the beginning. If J calculated by formula (1.11) is substituted into formula (1.2), if h(t)≥h(J1), then J=J1; if h(t)≤h(J1) and h(t)≤h(J2), then J=J2; if h(t)≤h(J1) and h(t)≥h(J2), then the binary search method is iterated 5 times, and J is calculated in each iteration process. i If ht>h(J i ) condition, let J i As the new upper limit, continue the binary iteration, otherwise let J i Continue to iterate for the new lower limit, gradually narrowing the range between J1 and J2, and finally After obtaining J, the lifting and lowering rate instructions are calculated by formula (1.3).

[0061] like Figure 1 As shown, the flight control converts the calculated lift rate command into the control of the rudder surface. The lift rate can be expressed as The vertical rate control can be converted into track angle control. The track angle control adopts proportional integral control to obtain the pitch angle command. The pitch angle command is subjected to angle stabilization control (dynamic inversion) to obtain the pitch acceleration command, and then the elevator control surface command δ is obtained through angular acceleration stabilization control (nonlinear active disturbance rejection control (ADRC) control law).e .

[0062] As the time constant continues to iterate, the flattening curve of the drone gradually becomes smoother. Finally, when the rear two wheels touch the ground, the corresponding ascent and descent rate control instructions are After the rear wheel touches the ground, the drone starts to reduce the throttle and the elevator command is [-5°, 5°]. When the three wheels touch the ground, the brake command is turned on and the elevator command is maintained at [-5°, 5°] until the drone stops completely.

[0063] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A high-precision UAV landing control method based on variable time constant, characterized in that: The following steps are involved: S1: Give the dynamic equation of the leveling phase of the UAV landing process; S2: Calculate the flight parameters during the leveling phase according to the dynamic equations; S3: The time constant is obtained by using the dichotomy method based on the calculated flight parameters; S4: The lift rate command is obtained by solving the time constant. With the continuous iteration of the time constant, the lift rate command is also updated, and the lift rate control command is converted into the control of the rudder surface. The flight of the UAV is controlled according to the rudder surface command. In this process, the leveling curve of the UAV gradually becomes smoother.

2. The high-precision UAV landing control method based on variable time constant according to claim 1 is characterized in that: The control method is mainly used in the leveling phase of the UAV landing process.

3. The high-precision UAV landing control method based on variable time constant according to claim 1 or 2, characterized in that: The kinetic equation of the flattening stage is: Among them, J is the time constant, h(t) is the altitude function of the drone in the leveling phase, is the descent rate of the UAV at the end of touchdown. When J is larger, h(t) is smaller and the trajectory is flatter.

4. The high-precision UAV landing control method based on variable time constant according to claim 3 is characterized in that: The flight parameters during the level-off phase are: Among them, x1 is the total distance to be flown, h1 is the height of the leveling point, is the descent rate at the leveling point, is the descent rate of the drone at the end of touchdown, J is the time constant, V g is the ground speed of the aircraft, x2 is the distance to be flown from the touchdown point, t is the time, γ is the downhill slope angle.

5. The high-precision UAV landing control method based on variable time constant according to claim 3 is characterized in that: According to engineering experience, the time constant J is taken within the range of 1 to 8.

6. The high-precision UAV landing control method based on variable time constant according to claim 5 is characterized in that: Use the binary method to screen the time constants: By continuously dividing the interval where the time constant J is located into two, the two endpoints of the interval are gradually approached to a certain middle value, and then the approximate value of the time constant J is obtained; after obtaining J, it is brought back to recalculate the waiting flight distance and the pull-up period time.

7. The high-precision UAV landing control method based on variable time constant according to claim 6 is characterized in that: The specific process of using the dichotomy method to screen the time constant is as follows: Set J1 = 1, J2 = 8 at the beginning. If we use the formula Substitute the calculated J into the formula Afterwards, if h(t)≥h(J1), then J=J1; if h(t)≤h(J1) and h(t)≤h(J2), then J=J2; if h(t)≤h(J1) and h(t)≥h(J2), then use the binary search method to iterate 5 times, with each iteration process J i If ht>h(J i ) condition, let J i is the new upper limit, and the binary division iteration continues. Otherwise, let J i Continue to iterate for the new lower limit, gradually narrowing the range between J1 and J2, and finally After obtaining J, the formula Calculates the lift rate instructions.

8. The high-precision UAV landing control method based on variable time constant according to claim 7 is characterized in that: The process of converting the calculated vertical rate command into the control of the rudder surface is as follows: The rise and fall rate is expressed as The vertical rate control is converted into track inclination control. The track inclination control adopts proportional integral control to obtain the pitch angle command. The pitch angle command is controlled by angle stabilization to obtain the pitch angle acceleration command, and then the elevator control surface command is obtained by angular acceleration stabilization control.

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