modifications for ArduPilot to work with the latest JSBSim

[WillemEerland]

This modifications allows using the latest JSBSim code

the tutorial to setup SITL ( http://ardupilot.org/dev/docs/setting-up-sitl-on-linux.html#setting-up-sitl-on-linux ) points to www.github.com/tridge/jsbsim which uses the JSBSim code from 2012

these modifications allow the use of the latest JSBSim code found at https://github.com/JSBSim-Team/jsbsim and involves

• removing the version check "(ArduPilot)"
• replacing "udp" with "UDP", otherwise it defaults to the TCP port

[tridge]

does the lock-step scheduling work?
The way to tell is to change change the speed with SIM_SPEEDUP and it should fly exactly the same. I had special hacks for lock-step in my repo.
Another good test is to attach to ArduPilot with gdb and stop it for a bit, then start it again. It should not affect the physics.

[WillemEerland]

Good point. It seems I've "fixed" the connection, but the functionality is not quite there yet.

I have replaced the "step" with "iterate 1", which (I think) has the same functionality.

see "iterate" at https://github.com/JSBSim-Team/jsbsim/blob/master/src/input_output/FGInputSocket.cpp

The simulation still doesn't really do what is expected - any advice you can give me would be appreciated, seeing as you made the original modifications.

[seanmcleod]

@WillemEerland I noticed the following in JSBSim.cpp:

    // Check if increment then hold is on and take appropriate actions if it is
    // Iterate is not supported in realtime - only in batch and playnice modes
    FDMExec->CheckIncrementalHold();

What mode are you running JSBSim in?

[seanmcleod]

I haven't seen the details of the hacks yet for the previous lock-step scheduling, but coincidentally we are busy discussing a pull request for JSBSim for lock-step scheduling.

Take a look at the discussion so far - JSBSim-Team/jsbsim#72

So maybe this pull request will result in an official lock-step implementation that could be used with Ardupilot?

[tridge]

@seanmcleod the "step" mode which supports the current lock-step scheduling in ArduPilot is here:
tridge/jsbsim@9a68300
I'm glad to hear it is being looked at in mainstream JSBSim now! I didn't submit my patch when I did it as I thought it would be considered a bit of a specialist hack.
Lock-step scheduling is essential for ArduPilot usage of JSBSim though. Without it the EKF gets unhappy very quickly as the sensor data is not kinematically consistent. It is also darn useful to be able to stop the simulation in a debugger and not affect the physics

[WillemEerland]

@tridge @seanmcleod Thank you both your comments - I'm going to close this pull request for now, I'll submit a new request when I have the ArduPilot - JSBSim (latest version) SITL simulation working properly.

(to be continued)