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Howto:Design an autopilot: Difference between revisions
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(Created page with '{{WIP}} This howto will guide you through the process of designing a stable and fully functional autopilot in FlightGear. We won't cover every single…') |
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Once you found the correct sign for Kp, slowly (in steps as little as 0.005) increase the value of Kp to get a fast response. Don't forget to reload the config with Shift-F5 after every edit of your file. There is one pitfall that can drive you crazy: the PID-controller computes delta-values for its correction and it starts with the current setting of your aileron when it is enabled or reloaded. If you have your aileron set to -0.2 when you hit Shift-F5, the controller will apply its corrections to a value of -0.2 which will most certainly fail! Make sure that you have your flight controls neutral (press 5) when (re-)enabling PID-controllers! | Once you found the correct sign for Kp, slowly (in steps as little as 0.005) increase the value of Kp to get a fast response. Don't forget to reload the config with Shift-F5 after every edit of your file. There is one pitfall that can drive you crazy: the PID-controller computes delta-values for its correction and it starts with the current setting of your aileron when it is enabled or reloaded. If you have your aileron set to -0.2 when you hit Shift-F5, the controller will apply its corrections to a value of -0.2 which will most certainly fail! Make sure that you have your flight controls neutral (press 5) when (re-)enabling PID-controllers! | ||
You will achive good results by only setting Kp for a roll-computer as a wing-leveler. Slowly increase your Kp until it is just about to become unstable, than divide the current value of Kp by two and use that value. Try several configurations of your aircraft - slow flight, low altitude, fast cruise at higher altitudes. Fly turns with several bank angles, left and right. Whenever you enable your controller, it should level your wings within a few seconds and without bouncing back and forth. | |||
Once you are satisfied with Kp - and not before! - decrease the value for Ti by dividing by a value of 10 and check the response of your controller. When it tends to become unstable, multiply it's Ti by two. Now you should have a fast and stable wing-leveler. | |||
Finally (for the wing-leveler) you can turn it into a roll-hold by setting your target roll as a property for the reference. If you had <reference>0.0</reference>, bind your reference to a property (<reference>autopilot/settings/target-roll-deg</reference>) and your aircraft should roll to any value you set in target-roll-deg. The complete wing-leveler will look similar to the one below (though, the config values may vary and the reference might be replaced by a property). | |||
<pid-controller> | |||
<name>Heading hold</name> | |||
<debug>false</debug> | |||
<enable> | |||
<prop>/autopilot/locks/heading</prop> | |||
<value>wing-leveler</value> | |||
</enable> | |||
<input> | |||
<prop>/orientation/roll-deg</prop> | |||
</input> | |||
<reference>0</reference> | |||
<output> | |||
<prop>/controls/flight/aileron</prop> | |||
</output> | |||
<config> | |||
<Kp>0.015</Kp> | |||
<Ti>10.0</Ti> | |||
<Td>0.0</Td> | |||
<u_min>-1.0</u_min> | |||
<u_max>1.0</u_max> | |||
</config> | |||
</pid-controller> | |||
This is the base for any lateral hold mode, so try to spend some time on it. It can take some hours if this is your first autopilot. | |||
===Pitch hold=== | |||
So, let's move over to a pitch hold. Our goal is to have a controller that maintains an arbitrary pich. The only difference to the wing leveler is that you are only interested in elevator position changes, not absolute values. To put it in other words: increase elevator for more pitch, decrease for less pitch and maintain elevator position if the pitch is right. This calls for a pid-controller, because it only computes offset values. And you will end up with a relative small value for Kp to avoid rapid imediate movements and a relative small value for Ti to have a greater effect over time. | |||
Start again with a no-op pid-controller (Kp=0, Ti=100.000 and Td=0). Input is your current pitch (<tt>/orientation/pitch-deg</tt>), reference is a target-pitch property (you wouldn't want a constant zero here, <tt>/autopilot/settings/target-pitch-deg</tt>) and output goes to elevator-cmd (<tt>/autopilot/internal/elevator-cmd</tt>). | |||
Again increase you Kp slowly and check each step in FlightGear. I'd expect something like -0.025 resulting in an instanteanous full elevator deflection on a 40 degrees pitch offset. Do not try too large values; it results in instable behaviour very quickly. Now, decrease the value of Ti, probably down to 10 or so. The smaller it gets, the faster the elevator moves and the easier it is to get an unstable loop. Again it is very likely that you won't need Td to get a stable controller. | |||
===Simulating servos=== | |||
Once you have this two axies running, you might want to think about simulating the slow movements of the servo or hydraulic motors that drive the control surfaces. You might have noticed, that the controls react rapidly when enabled which is unrealistic. In a [[Seneca]], the aileron servo takes 12 seconds to move from left to right. The elevator needs 26 seconds and the pitch trim takes 45 seconds for full travel. | |||
There is a filter to simulate that behaviour, it's the noise-spike filter. Here is a sample with 25 seconds transition time from -1 to +1: | |||
<filter> | |||
<name>SERVO-DRIVER:elevator</name> | |||
<debug>false</debug> | |||
<feedback-if-disabled>true</feedback-if-disabled> | |||
<enable> | |||
<prop>/autopilot/locks/altitude</prop> | |||
<value>pitch-hold</value> | |||
</enable> | |||
<input>/autopilot/internal/elevator-cmd</input> | |||
<output>/controls/flight/elevator</output> | |||
<type>noise-spike</type> | |||
<max-rate-of-change>0.08</max-rate-of-change> | |||
</filter> | |||
The formula for max-rate-of-change is (max-out - min-out)/transition-time. In our example (1--1)/25=2/25=0.08. Try it out, disable your pitch-hold controller and enable the servo-driver. Set your some/internal/elevator-cmd property to +1 and see your elevator moving to fully up or down within 12.5 seconds. Now set your property to -1 and it will take 25 seconds for the elevator to move to the other direction. The feedback-if-disabled property is needed, if this stage is driven from a pid-controller. | |||
We said before that PID-controllers compute offsets to the curren value of it's output. If you disable the servo-driver from your A/P master switch, the value of your elevator is fed back to the internal value of the elevator-cmd so the pid-controller has a reasonable value to start from when it is enabled. Create servo drivers for elevator, aileron and the trim channels if you intend to use them. A good location for the internal driver properties is /autopilot/internal - but that is up to you.Now change your pitch and roll PID-controller so that they no longer directly drive the output properties but the internal properties. You might need to adjust the Kp and Ti values of the PID-controller because you introduced some lag with the driver. But you should end up with perfectly smooth operation of the two main channels. | |||
Spend some time optimizing these pitch and roll channels, they will be used by all other modes and '''have''' to be stable in any case! | |||
[[Category:Aircraft enhancement|Design an autopilot]] | [[Category:Aircraft enhancement|Design an autopilot]] | ||
[[Category:Howto|Design an autopilot]] | [[Category:Howto|Design an autopilot]] | ||