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Figure 1: Block diagram of line hold control system
The control system is tuned to deliver optimal performance during cruise flight, hence once the aircraft reached an altitude of 400 ft and is receiving waypoints from the mission planning algorithm. However, the tuning of the control system for that particular case results in sub optimal behavior at low altitudes, when losing height is unacceptable, and when flying close to the border (Red Zone), which requires more aggressive turns.
The three flight modes presented (Start, Cruise, Red Zone) have contradicting requirements. In order to deliver optimal performance at any time, the controls team decided to design three control modes. The regular control mode for cruise is the same as the one presented in Problem Set 3. The two additional modes are explained in the following.
Docile Mode
Flight test results showed that exploiting the full 60 degrees of bank allowed during regular mode leads to moderate height losses. While it is acceptable during cruise to lose 1 to 2 meters during a sharp turn, it is unacceptable while at low altitudes. Hence, a docile mode was introduced when flying below 130 ft that reduces the maximal commanded bank angle to 30 degrees. Flight test results showed that SPAERO 2 can handle these kind of turns without losing altitude. It is accepted that during that mode, the plane won't be able to follow the waypoints as well. Reaching sufficient altitude first is prioritized. This has the additional safety guard of reducing the likelihood of the airplane dropping below 100ft during maneuvers, which could occur if a fire were found very early.
Aggressive (Red Zone) Mode
Flying out of bounds leads to the immediate termination of the mission with a score of 0. It is therefore of highest concern to make sure the plane stays within the defined boundaries at all times. While the mission planing team has adjusted the search path algorithm to be well within Lake Lag, unexpected fire fighting maneuvers and wind might still endanger the mission. An aggressive red zone mode has therefore been introduced. The red zone is defined as the 20 m wide stripe near the border. When the plane is in the red zone, the limitations on the maximal bank angle are set to 180 degrees. Numerous flight tests showed that the plane can perform stable, very sharp turns with a radius of 2 to 5 meters by having a very large bank angle, stall, flip over and stabilize again. During this maneuver, the plane loses 4 to 5 meters of altitude. While this is not desired during normal cruise, it is accepted in the red zone for the sake of staying in bounds.
Control Systems
Nur Harell and Matthew Berk
In the following, Team SPAERO presents the final adjustments to the control system of SPAERO 2. The control system philosophy remains the same, for the detailed explanation please refer to Problem Set 3.
Successive Loop Closure is a robust and reliable approach that enables SPAERO 2 to acquire and follow lines to desired way points calculated and commanded by the Mission Strategy Team.
For the sake of explaining the final adjustments, the lateral control system architecture for line acquisition from PS 3 is presented again:
Figure 2: Red zone which is inside of the field boundary
While the plane is in the red zone, it constantly updates the line it follows from its current position to point towards the center of the field. If the plane misses the way point, it instantly turns away from the field boundary as sharply as possible. This is desirable, since missing a way point near the border normally means that the plane is flying at 10 m/s directly towards the border. Once out of the red zone the airplane resets a line from its current position to the waypoint and resumes a normal line following mode.
Combined Mode
If the airplane is both under 130ft and in the red zone (a very unlikely scenario) then normal turn performances, i.e., 60 degree max bank, is used. This means mild altitude loss but is necessary as the 30 degree turn is not nearly aggressive enough to ensure field boundary safety.
All modes have been flight tested and showed significantly increased performance. The red zone mode enabled team SPAERO to fly at higher winds without flying out of bounds.
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