JSBSim Simulink Function

[kvgkrish]

Hi all, This is Gokul doing a Master of Aerospace Engineering at TUM Germany. Currently working on master thesis at Chalmers University of Technology. I am working on a Bio-Inspired UAV dynamics system. I am new to JSBSIM and I have set up all the files and worked fine with available flights and scripts. If I use a new flight configuration I have simulated the model. It shows trim failure and sometimes flight scripts are not used. Can you help me to resolve this issue?

[seanmcleod]

Are you sure the aircraft should be trimmable at the initial conditions you're specifying for the trim? Or there could be an issue with the FDM file you've created.

If you share the FDM file and the conditions you're using for trim others could potentially take a look and help.

[kvgkrish]

I have attached the flight data, script, and initial conditon for your reference.

aircraft is StratoSurfer, Scipts/Test_StratoSurfer_Cruise, and the Initial condition is initCruise.

https://github.com/UASLab/OpenFlightSim/blob/master/Simulation/aircraft/StratoSurfer/StratoSurfer.xml

[seanmcleod]

Just had a very quick glance. Is the engine definitely on and running? I see the throttle being advanced, but it's not 100% clear that the engine is running.

What is the output in terms of thrust and power and h-agl-ft below?

    <event name="Trim">
      <condition> simulation/frame gt 1 </condition>
      <set name="simulation/do_simple_trim" value="1"/>
      <notify>
        <property>velocities/vtrue-kts</property>
        <property>position/h-agl-ft</property>
        <property>aero/alpha-deg </property>
        <property>aero/beta-deg </property>
        <property>propulsion/engine[0]/thrust-lbs</property>
        <property>propulsion/engine[0]/power-hp</property>
        <property>attitude/phi-rad</property>
        <property>attitude/theta-rad</property>
        <property>attitude/psi-rad</property>
      </notify>
    </event>

[kvgkrish]

I have attached the engine power and new script for your review. Even after this modification, model has failed du to trim error.

[seanmcleod]

The trim routine will adjust the throttle while trying to calculate a trim solution. So it doesn't matter what you set the throttle position to.

But as I mentioned, I'm not sure that the engine is on/running, in which case it doesn't matter what the trim routine does in terms of setting the throttle command, if the engine isn't on it will produce 0 thrust.

Look at the output you produced just above, it shows the thrust as being 0.

[kvgkrish]

Thanks for the response. How do you make sure the engine is on or not? I faced this issue while editing the aircraft file and getting this error.

[seanmcleod]

Take a look at this example of using the <running> element.

https://github.com/JSBSim-Team/jsbsim/blob/4b3ec67e2fffde5797b7fc168a686f2f08c4971d/aircraft/737/rudder_kick_init.xml#L11

The -1 means all engines that the aircraft has.

[kvgkrish]

Now I am getting Very high thrust and again trim failure.

[seanmcleod]

Well then there is something wrong with your engine, thruster model etc. With that sort of maximum thrust and a vehicle with a mass of only 3lbf, the trim routine probably can't find a small enough throttle command like 0.000001 to us to get a trim solution.

[kvgkrish]

I have reduced the command, but it is still getting very high value.

[seanmcleod]

No, no, no.... As I mentioned, the trim routine will try different throttle commands when trying to calculate a trim solution, so it doesn't matter what you set the throttle command to in the Motor Start event!

The point is it appears the engine power is way too high for this sort of vehicle. And the trim routine has probably some minimum throttle increment that it tries when trying to find a solution which is way too much in terms of thrust.

Think about it in terms of a rough sanity check, should a 3lbf vehicle have a motor of 350W, producing some enormous amount of thrust?

[kvgkrish]

It is a very small motor. I have attached the details of it.

[seanmcleod]

Compare the amount of thrust you're seeing to the thrust measured for this engine for different propellers etc.

https://www.innov8tivedesigns.com/images/specs/Cobra_2217-8_Specs.htm

[kvgkrish]

Yes, it is very high compared to the tabulated value. How to resolve this issue.

[seanmcleod]

Step through the JSBSim code where it calculates the thrust and see if you spot what input is making it so large.

[kvgkrish]

But the 737 and c172x are working fine with the Simulink.

[kvgkrish]

The aircraft file, script, and initial condition we use are created for the FlightGear module. Is there any problem with Simulink? Can we please tell me how to run JSBSim standalone and visualize the results in FG.

[seanmcleod]

So run JSBSim stand-alone with your script, look at the command line arguments for JSBSim and see if there is any difference with the trim stand-alone versus with Simulink.

[kvgkrish]

How to run JSBSim stand-alone I tried but the application now opening. Can you please send me some documentation for that?

[seanmcleod]

Did you look at the Quick Start section of the README? https://github.com/JSBSim-Team/jsbsim?tab=readme-ov-file#quick-start

[kvgkrish]

I am getting this error.

[seanmcleod]

Take a look at - https://www.technewstoday.com/fix-command-prompt-access-denied/

[seanmcleod]

Did you download JSBSim.exe from the web, or did you clone the repo and build it yourself? If downloaded from the web take a look at the Properties and check the Unblock checkbox if it's there, it is by default for any files downloaded from the internet.

[kvgkrish]

I have downloaded from the web

[kvgkrish]

I have corrected the model. Working on the controls, here I want to change the control based on the altitude to demonstrate the dynamic soring "S" patter. Can you help me write control logic for dynamic soaring?

[kvgkrish]

@seanmcleod Hi, I want to input the altitude setpoint as input to the S function. But the S function won't take it when its fed through input. I have modified the input output XML file. It only considers when I fed through script files. Can you please help me resolve this issue.

[seanmcleod]

Do you see any error message when the input properties are processed?

https://github.com/JSBSim-Team/jsbsim/blob/6f0c4d805c499cf18280bcf8d7af358aee494876/matlab/JSBSim_SFunction.cpp#L459-L473

[kvgkrish]

Hi, I haven´t get any errors but I cant use that as a input.

[seanmcleod]

So in what way can't you use it?

[kvgkrish]

The figure below is for the desired altitude and headings function.

I want to feed the autopilot to follow the desired trajectory. I have created a function for the desired altitude and desired heading angle with respect to time. I want to feed as an input to the autopilot. If sending this comment through input, the model doesnt't follow the desired altitude. If I write this desired altitude and heading in the script file as a piecewise input, the model won't take it as an input. The figure below is a script with a piecewise function. If I go with the below script, it crashes the model.

[seanmcleod]

Rather than posting screenshots of your XML etc. rather put them in a triple back tick ` block with xml as the type. Means they're easier for other users to find via a search in future etc.

In terms of running this in Simulink, are you sure you have enabled the auto-pilot? Most auto-pilot implementations have a separate property that you have to set to enable them. So maybe you're updating the altitude and heading setpoints, but if the autopilot isn't enabled then the autopilot will ignore the setpoints.

For your script example, it looks to me like you haven't set the Desired Flight Path event to be executed more than once. Also the same comment applies to the script case, make sure the autopilot is enabled.

[kvgkrish]

  <use aircraft="GBSv02" initialize="ini03"/>
    <run start="0.0" end="1000" dt="0.008333333333333333333">

        <event name="Wind aloft">
            <condition>simulation/sim-time-sec  ge  0.01</condition>
            <set name="atmosphere/psiw-rad" value="1.57" />

            <set name="atmosphere/wind-mag-fps">
                <description>Change_of_wind_magnitute_wrt_altitute</description>
                <function>
                    <table>
                        <independentVar lookup="row">position/h-agl-ft</independentVar>
                        <tableData>
                            5       16.93490625
                            10      18.40375301
                            15      19.32134464
                            20      20
                            25      20.54277915
                            30      20.99717881
                            35      21.38920099
                            40      21.73469725
                            45      22.04407591
                            50      22.32455428
                            55      22.58135087
                            60      22.81836623
                            65      23.03859546
                            70      23.2443904
                            75      23.43763291
                            80      23.61985323
                            85      23.79231315
                            90      23.9560658
                            95      24.11199966
                            100     24.26087142
                            1000.0  20.9000
                            2000.0  19.8000
                            2500.0  18.9000
                            3000.0  20.0000
                            3500.0  20.0000
                        </tableData>
                    </table>
                </function>
            </set>
            <!--set name="atmosphere/psiw-rad" value="0" /-->
            <!--set name="atmosphere/wind-mag-fps" value="0" /-->
            <!--set name="atmosphere/wind-down-fps" action="FG_RAMP" value="1" tc="10.0"/-->
        </event>

        <event name="Desired Flight Path">
            <!-->condition> position/h-agl-ft ge 600 </condition-->
            <condition> simulation/sim-time-sec ge 0.1 </condition>

            <set name="ap/altitude_hold" value="1.0" />
            <set name="ap/heading_hold" value="1.0" />
            </event>

  </run>

</runscript>

I have enabled the autopilot for altitude hold and heading hold.

<s_function_config>
    <input>
        <property> fcs/throttle-cmd-norm[0] </property>
        <property> fcs/aileron-cmd-norm </property>
        <property> fcs/elevator-cmd-norm </property>
        <property> fcs/rudder-cmd-norm </property>
        <property>ap/altitude_setpoint</property>
        <property>ap/heading_setpoint</property>

    </input> 

I have included the autopilot setpoint as an input for the S-Function. The model works well for altitude hold, but if I enable the heading, it oscillates around the x-axis.

[seanmcleod]

The model works well for altitude hold, but if I enable the heading, it oscillates around the x-axis.

Then you'll need to look at improving the heading controller.

[kvgkrish]

In the initial stages of the simulation, a predefined crosswind, characterized by a specified magnitude and direction, is introduced to the model. The simulation accurately captures the model's initial response, manifesting in dynamic changes to its trajectory and behavior. However, as the simulation progresses, a noticeable reduction in the visual impact of the crosswind becomes evident. This observation indicates a diminishing influence over time. To address this, I am looking to incorporate a windshear model into the simulation. I have already introduced values representing wind velocity with respect to altitude. However, the current model generates same responses for all directions. I am seeking guidance on how to effectively implement the windshear phenomenon into the atmospheric conditions of the simulation.

<?xml version="1.0"?>
<initialize name="reset03">

  <vc unit="KTS"> 38.87689112646 </vc>
  <longitude unit="DEG"> 0  </longitude>
  <latitude unit="DEG">   0  </latitude>
  <phi unit="DEG">         0.0  </phi> <!-- Roll -->
  <theta unit="DEG">       0.0  </theta> <!-- Pitch -->
  <psi unit="DEG">       0.0  </psi> <!-- Yaw -->
  <altitude unit="FT">     100  </altitude>
    <winddir unit="DEG">   90.0  </winddir> 
  <hwind unit="FT/SEC"> 20 </hwind>

</initialize>

<event name="Wind aloft">
    <condition>simulation/sim-time-sec  ge  0.01</condition>
    <set name="atmosphere/psiw-rad" value="1.57" />

    <set name="atmosphere/wind-mag-fps">
        <description>Change_of_wind_magnitute_wrt_altitute</description>
        <function>
            <table>
                <independentVar lookup="row">position/h-agl-ft</independentVar>
                <tableData>
                    5       16.93490625
                    10      18.40375301
                    15      19.32134464
                    20      20
                    25      20.54277915
                    30      20.99717881
                    35      21.38920099
                    40      21.73469725
                    45      22.04407591
                    50      22.32455428
                    55      22.58135087
                    60      22.81836623
                    65      23.03859546
                    70      23.2443904
                    75      23.43763291
                    80      23.61985323
                    85      23.79231315
                    90      23.9560658
                    95      24.11199966
                    100     24.26087142
                    1000.0  20.9000
                    2000.0  19.8000
                    2500.0  18.9000
                    3000.0  20.0000
                    3500.0  20.0000
                </tableData>
            </table>
        </function>
    </set>
    <!--set name="atmosphere/psiw-rad" value="0" /-->
    <!--set name="atmosphere/wind-mag-fps" value="0" /-->
    <!--set name="atmosphere/wind-down-fps" action="FG_RAMP" value="1" tc="10.0"/-->
</event>

[agodemar]

@kvgkrish did you progress in your study on this topic? Updates to the community are welcomed

[kvgkrish]

@agodemar yes, still working on this