Embedded Files
Line Acquisition
Nur Harell, Matthew Berk
In the following, Team SPAERO presents its approach for line acquisition and present two independent experimental flights to prove its effectiveness. The control system logic has changed since PS2 and is therefore presented in the following.
Control System Architecture:
Successive loop closure has been chosen as the basic control logic. The linearization was made for straight level flight so that the longitudinal and lateral EOM decouple. Both the altitude hold and the lateral control system are presented.
Altitude Hold
The velocity loop is closed by adjusting the throttle. After controlling the velocity with thrust, the pitch angle is controlled via the elevator. The commanded pitch is determined from the difference between the commanded altitude and the actual one. In order to avoid stall, the commanded pitch angle has clipping constraints on it. The maximum commanded pitch angle is set to 5 degrees deviation from the trim pitch angle. Should the commanded pitch be larger, it is reduced to that value. A block diagram for the altitude hold is presented in the following.
Line Acquisition
For line acquisition, several loops are closed in order to minimize y, the deviation from the commanded line. The bank angle is controlled with the aileron. The commanded bank angle is determined from the difference between the commanded heading and the excising one. Clipping has been introduced in order to avoid stall. The maximal commanded bank angle has been set to 45 degrees. The commanded heading is split into the heading along the line and a deviation from this angle which results from y. The maximal deviation from the heading along the line is 90 degrees. That means, that if the airplane is very far away from the line, it flies perpendicular to the line to reach it as fast as possible. The block diagram for Line Acquisition is presented in the following.
Test Flight 1:
During the first experiment, a line pointing directly north is to be followed. The experimental data is presented in the following.
As can be seen from the plot, the airplane goes towards the line as soon as auto mode is switched on. As soon as the plane reaches the line, it turns and stabilizes to follow it. It is to be noted that strong wind from the north-east is blowing. This explains the steady-state error and the the airplane drifting south before reaching the line. While on the line, the plane was crabbing into the wind to stay on the line.
As can be seen from the plot above, the airspeed of 8 m/s is held during the flight. The sharp drop at around 27 seconds into the flight is due to the sharp turn when switching into the auto mode. The maximum desired bank angle is exceeded once during the sharp turn after reaching the line. The plane is automatically stabilized quickly and starts following the line.
The desired altitude was set to 70 m. The auto mode was switched on while the plane was very low which explains why the aircraft is gaining altitude. The altitude Control is currently not aggressive enough, since the desired altitude must be reached more rapidly. This will be fixed for future flights.
The commanded control inputs stay within the desired boundaries at any time. The introduced clipping prevents any of the values to reach unrealistic ones.
The deviation from the line y is plotted versus time. As soon as the auto mode is switched on, y is reduced significantly and quickly. Once the line is reached at about 37 seconds and the airplane starts to follow the line, the y value has damped oscillations around 0. Mind the steady state error due to the strong wind.
Test Flight 2:
During the first experiment, a line pointing directly east is to be followed. The experimental data is presented in the following.
As in the first experiment, the airplane turns quickly and flies towards the line as soon as auto mode is switched on. As soon as the plane reaches the line, it turns and stabilizes to follow it.
In this experiment, an airspeed of 9 m/s is held during the flight. The sharp turns at about 77 second and 87 seconds into the flight explain the velocity increase due to the altitude loss. The maximum desired bank angle is exceeded twice during these sharp turns. As before, the plane is automatically stabilized quickly and starts following the line once reaching it.
The desired altitude was again set to 70 and is held pretty well. As noted before, the altitude hold is currently not aggressive enough, since the desired altitude must be reached more rapidly. This will be fixed for future flights.
As in the last experiment, the commanded control inputs stay within the desired boundaries at any time. The introduced clipping prevents any of the values to reach unrealistic ones.
As in the last experiment, the deviation from the line y is plotted versus time. As soon as the auto mode is switched on, y is reduced significantly and quickly. Once the line is reached at about 82 seconds and the airplane starts to follow the line, the y value has damped oscillations around 0.
Page updated
Google Sites
Report abuse