Parameter files in SITL

knudj · September 26, 2021, 2:24pm

Hi,

a major feature of a SITL simulation certainly is the chance to manipulate ardupilot (flight controller) and frame (vehicle) parameters and watch the impact on vehicle behavior.

Having setup SITL via cygwin on my Windows computer I found the related files in the ardupilot directory (e.g. folder “default_params” and “aircraft”). However, starting SITL directly via Mission Planner I found only one file “plane.parm” in folder “default_params” and no frame parameters at all in the related Misison Planner folder on my computer.

The simulation nevertheless works for “Plane”, however I wonder where the frame parameters come from. The same applies to the other frame types, where even no ardupilot parameters are given.

As a control engineer it is quite obvious to me that there need to be controller (ardupilot) parameters and system (frame) parameters. However for users with different background it might be helpful to have a very distinct differentiation betweeen both sets and related files (which to me is a general shortcoming in the SITL documentation).

Helpful comments would be appreciated.

Thanks

knudj · June 19, 2022, 12:30pm

Hi,

since no reply up to now I like to bring up my issue again.

Where do the ardupilot (flight controller) and frame (vehicle) parameters come from in Mission Planner for SITL simulation?

I think the answer is straight forward for someone deeper in the code than me - so still hope for comments

Thanks again

Michael_Oborne · June 20, 2022, 7:54am

they are all in github

knudj · June 20, 2022, 10:04am

So I understand that Mission Planner downloads the relevant ardupilot and frame files from github at the time when I start simulation. However for plane there is only the Rascal110.xml frame file. Thus do I always fly a Racal110 in SITL plane simulation?
In github I also found a plane-jet.parm file, however no corresponding xml frame file for a jet plane?

Michael_Oborne · June 20, 2022, 10:46am

the xml’s are for the simulator, not sitl. i think there needs to be more clarification on what your chasing

knudj · June 20, 2022, 11:54am

I want to change ardupilot parameters (e. g. PID parameters) as well as the frame parameters (e. g. mass, wing span) and see the impact in simulation instead of crashing models in the field. This is why I am so much interested to learn about the location of relevant parm and xml files. But may be I misunderstood the concept of SITL.

Mustafa_Gokce · June 20, 2022, 12:26pm

Those are not parametrically defined, but the source code might help to you.

github.com

ArduPilot/ardupilot/blob/master/libraries/SITL/SIM_Plane.h#L48

      
        
                static Aircraft *create(const char *frame_str) {
                    return new Plane(frame_str);
                }
            
            
protected:
                const float hover_throttle = 0.7f;
                const float air_density = 1.225; // kg/m^3 at sea level, ISA conditions
                float angle_of_attack;
                float beta;
            
            
    struct {
                    // from last_letter skywalker_2013/aerodynamics.yaml
                    // thanks to Georacer!
                    float s = 0.45;
                    float b = 1.88;
                    float c = 0.24;
                    float c_lift_0 = 0.56;
                    float c_lift_deltae = 0;
                    float c_lift_a = 6.9;
                    float c_lift_q = 0;
                    float mcoeff = 50;

github.com

Georacer/last_letter/blob/f472cba673d9d1e10af3d1ea8f83c05bc1e619f1/documentation/source/parameters/aircraftAeroParams.rst

Aircraft Aerodynamic Parameters
===============================

This file contains the parameters of the aerodynamics model.

The aerodynamics model assumes that the aircraft aerodynamic response can be modeled as the some of a number of airfoils. This method allows for various common modelling approaches such as:

* Fusing the aircraft aerodynamics into one lift, drag and moment airfoil model, ie separate wing surfaces are not modeled and calculated separately. This is not uncommon to the aircraft identification bibliography
* Specifying different airfoils for the main wing and the stabilizer, each with its own aerodynamics model and location on the airframe relative to the Center of Gravity. Their contributions are summed up to provide the overall response of the aircraft.

``nWings``: The total number of airfoils that the aircraft contains.

.. note::

	The following parameters are referring to a single airfoil. They are groupped under the prefix ``airfoilX/``, where ``X`` is the number-id of the airfoil, starting from 1 for a single-airfoil aircraft (eg ``airfoil1/CGOffset``).

``CGOffset``: The location of the center of lift in terms of the center of gravity, in X/Y/Z body frame coordinates. Negative X means that the CoL is behind the CoG. Positive Y means that the CoL is to the right of the CoG. Negative Z means that the CoL is above the CoG.

``mountOrientation``: The rotation Roll-Pitch-Yaw (in this sequence) rotation angles which rotate the airfoil from the Body Frame to its actual Airfoil Frame fixture. If the whole aircraft is modelled as a single airfoil, this should be [0.0,0.0,0.0]

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Michael_Oborne · June 20, 2022, 11:48pm

SITL support many physics engines, sitl is just the autopilot part, and talks to the physics engine. the basic sitl does include a very simple physics engine, but can be used with external ones such as realflight etc.

give your problem statement, i would suggest maybe realflight, as its easy to adjust some of those parameters

knudj · June 21, 2022, 1:00pm

Thanks a lot,

your comments brought me to the right track

I didn’t realize the frame physics in the SITL libraries (ardupilot/libraries/SITL) before.

Coming to RealFlight: Does the RealFlight Trainer Edition also allow to connect to ArduPilot?

knudj · June 23, 2022, 7:18am

Again coming to RealFlight: I am stunned

Found a manual (G2) and the chapter “Customize your airplane”. I wouldn’t have expected that from a commercial fs software (would have expected that the fs only allows for models that are available for sale somewhere). The whole thing reminds me somewhat of “Kerbal Space Program” that my sons used to play creating all kind of more or less flying objects without really knowing the physics behind. It’s even possible to introduce a specific airfoil profile!

I understand that after connecting RealFlight to ArduPilot, the RealFlight model and physics is used for simulation?

knudj · June 28, 2022, 12:08pm

Here some answers and findings from my own

RealFlight Trainer Edition does not offer the edit option for models. You need to go to the more expensive full version, which can be done by upgrading the trainer editon on steam.

However I was able to connect my FrSky Horus X12S to the trainer edition by using the FrSky XSR-SIM dongle. For the basic setup I used the Windows Gamecontroller setup and checked out the setup of the Horus X12S (CH1, CH2, and so on need to allocated to appropiate inputs). In Realflight the FrSky Simulator is not pre-set, however I found that Gamepad could be used for the inital setup. After setup I used Horus X12S for the new profil name.

When it comes to documentation I found that Understanding SITL with RealFlight on ArduPilot gives a quite good explanation how ArduPilot is connected to RealFlight (big compliments ). A major feature to do this obviously is the flightaxis interface, which somewhat surprisingly is not documented in the Realflight 9 manual

Next step for me might be the upgrade to the RealFlight full version to check out the edit feature.