Replies to This Discussion

3D RoboticsPermalink Reply by Chris Anderson on August 19, 2012 at 7:43pm

I think you misunderstand what an HIL simulator does. It doesn't use any of the onboard sensors at all. Instead, it uses the AP computing hardware to process fake sensor data from the simulator. The APM's own sensors are completely ignored. 

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Permalink Reply by Steven G on August 20, 2012 at 5:28am

Chris,

Thanks. I still am not understanding how I should interpret the results from the simulator. For instance these:

1. I followed the wiki precisely including calibrating the radio, yet even in Manual Mode, I needed to add significant trim in pitch and roll to fly straight and level.

2. When switching to FBWA or Stabilize I have to add yet more trim BUT RTL seems to work just fine.

3. On RTL mode, I am getting some oscillation in roll and throttle. Since this is not real hardware, should I even attempt to adjust PIDs?

In summary, what is the main purpose of the simulator and what are the "take aways" to the real UAV world?

Regards,

Steven

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Permalink Reply by Paul Marsh on August 21, 2012 at 4:40am

Hi Steven,

For me, using the simulator forced me to learn details about using both ArduPilot and Mission Planner that I probably would have learned the hard way if I went straight into flying the real thing.  For example, you can, and should, play with PID settings and observe the results.  A perfect example is going through the PID tuning process in FLW-A mode as described in the Wiki.  This gave me a lot of confidence before attempting it with a real plane.  I knew what to expect and what to look for.  I also learned a lot about controlling the plane from Mission Planner.  I learned how the plane behaves at waypoints.  Basically, just spend lots of time on the sim, try different things, and I think you'll be surprised at what you learn.  I came away thinking no one should ever attempt to fly for real without doing the sim first, and doing it a lot.

Regarding trim, one contributing factor might be the environmental settings in X-Plane.  Make sure you turn off ALL wind effects at all altitudes.  The little PT-60 was getting blown around a bit by the wind on the ground in my case.  Also, you can actually adjust the trim settings of the virtual plane just like the real thing.  You have to use another utility that comes with X-Plane (Plane Designer or something like that).  To be honest though, it's not really necessary.  I went through the process and it worked fine, but I also fly the virtual plane without any trim adjustments.

The flight characteristics you're seeing could indeed be due to PID settings.  I bumped up elevator and aileron servo control to about 1.5 each and elevator and aileron navigation control to about 0.9 or 1.0 each (just "P").  I'm not at a PC at the moment where I can give you the exact PID names.  You'll find that adjusting PID settings doesn't make as drastic a change as you might have thought.  I had this idea that a change of 0.1 anywhere might crash my plane--it won't.  Also, if you over correct a PID in sim, you will see the plane shake or over-react just like the Wiki says it will in real planes.

One thing I've noted before, but that might have something to do with you apparent need to adjust trim.  If your plane is slowly losing altitude in auto mode, you most likely left your throttle on the transmitter at it's highest setting.  There is a control relationship between throttle and pitch, but I don't fully understand it.  I never changed the associated PIDs, but at high throttle, APM does not keep up with the downward pitch.  Simply moving the throttle to its lowest setting when in auto mode corrects the problem.  I missed that in the Wiki initially, but you need to do it.  I learned about it as a result of the sim.

Hope this helps some.

Thanks,
Paul

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Permalink Reply by Steven G on August 21, 2012 at 5:08am

Paul,

Thank you. I can see using the Simulator as a learning tool.

Just to be clear on my part, any adjustments one makes to PID or trim in Simulator applies to the "plane" one is flying in the Sim, right? In other words, it does not apply to the real world with the exception of learning the cause and effect of actions in the MP. For instance, I fly a Skywalker FPV and not a PT-60.

I will check out those wind settings as I am quite certain they were affecting me on the ground. I could barely taxi straight. :-) I was learning the X-plane as much as the MP Flight Simulator.

Regards,

Steven

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Permalink Reply by Paul Marsh on August 21, 2012 at 9:36am

Hi Steven,

If you make parameter changes in sim mode and then upload the firmware for real flying, those parameter changes will still be in APM.  At least that has been my experience.  I would recommend that you save your parameter file before making any changes.

Paul

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