Adjusting ExternalForces through Matlab scripting interface

[Pavlos Silvestros]

Hello all,

I am transitioning to scripting with Matlab to accees the OpenSim API. I am trying to "rewrite" a forward dynamics pipeline I had running in Matlab that just wrote xml setup files and passsed them to OpenSim.

I have managed to set up the model and adjust it as I like, as well as set up the FDtool. However I am struggling to get my head around a couple of concepts.

1) When we want to apply a time varying external force (eg. grf or impact force) do we add it to the model or the FDtool? From what I understand we add it to the model but I cannot get it to work with PerscribedForce, ExternalLoads or ExternalForce. The force data that I am trying to pass to the script is from an external file so it maybe that I am not entering correctly as a opensim-Vector. The reason I want to get the force data into the script is so that I can change different parrameters (eg. point of application, magnitude and direction...) programmatically. Should I just do this separately outside OpenSim environment?

2)When setting up the FDTool is it necessary to pass a general .xml FD setup file?

Thank you for your help!!

Best regards,
Pavlos

[Thomas Uchida]

When we want to apply a time varying external force (eg. grf or impact force) do we add it to the model or the FDtool?

Have you tried setExternalLoadsFileName() (https://simtk.org/api_docs/opensim/api_ ... 180eb1d0e7)?

When setting up the FDTool is it necessary to pass a general .xml FD setup file?

No, in MATLAB you should be able to do the following:

Code: Select all

import org.opensim.modeling.*;
fwdtool = ForwardTool();

See the doxygen documentation for ForwardTool (https://simtk.org/api_docs/opensim/api_ ... dTool.html) which inherits from AbstractTool (https://simtk.org/api_docs/opensim/api_ ... tTool.html).

[Pavlos Silvestros]

Hello Tom,

Thank you for your help, I managed to get it working without the initial forward dynamics (FD) setup file.

The fdTool now applies the forces to the model but will not apply the muscle controllers' file I have specified when I call the tool through Matlab. Furthermore, the values in the output ..._contols file of the simulation are all zeros. However when I run the simulation through the GUI with the same forward dynamics setup file, that I create in Matlab and pass to the fdTool, it applies the controllers?! I have not been able to figure this out. I have noticed that in .xml FD set up files there is an anable_controller tag which I cannot find in the API.

Code: Select all

      <ControllerSet name="Controllers">
         <objects>
            <ControlSetController name="">
               <enable_controller>ALL</enable_controller>
               <controls_file>C:\Users\User\Documents\Pavlos\UniversityofBath\Year4\FYP\Modelling\Simulation_Inputs\Controllers\controls_FR03A00_FL200_EX200.sto</controls_file>
            </ControlSetController>
         </objects>
      </ControllerSet>

My FD setup in Matlab is below:

Code: Select all

            fdTool = ForwardTool(); %setUpFile
            fdTool.setName(simName);
            fdTool.setModel(model);
            fdTool.setModelFilename(NewModelFile);
            
            fdTool.setStartTime(timeStart);
            fdTool.setFinalTime(timeEnd);
            %fdTool.setUseSpecifiedDt(1);
            
            fdTool.setErrorTolerance(errorTol);
            fdTool.setMaximumNumberOfSteps(maxIntergratorSteps);
            fdTool.setMinDT(minStepSize);
            fdTool.setMaxDT(maxStepSize);
            fdTool.setOutputPrecision(outputPrecision);
            
            fdTool.setSolveForEquilibrium(1);
            fdTool.setControlsFileName(FDControlsFile);
            fdTool.setExternalLoadsFileName(grfFile);
            
            %fdTool.setToolOwnsModel(1);
            %fdTool.setPrintResultFiles(1);
            fdTool.setResultsDir([mainDir 'Outputs\API\Results\'])
            fdTool.print([mainDir 'Outputs\API\FDSetUpFiles\' simName '.xml']);
            fdTool.run();
           

Thank you for your help again!

Best regards,
Pavlos

[Thomas Uchida]

Please post example files so I can investigate.

[Pavlos Silvestros]

Thank you again Tom,

Example force file

Forces.zip

(285.21 KiB) Downloaded 107 times

Controls file

Controls.zip

(102.83 KiB) Downloaded 114 times

Model and Matlab script (Bushings are for future use please ignore parameter values)

Model and Matlab Script.zip

(21.63 KiB) Downloaded 107 times

FD setup file

test1707-MySetup-forEquil_Bushings (2).xml

(4.06 KiB) Downloaded 135 times

Best regards,
Pavlos

[Thomas Uchida]

The fdTool now applies the forces to the model but will not apply the muscle controllers' file I have specified when I call the tool through Matlab.

It isn't clear (to me, at least) why fdTool.setControlsFileName() isn't working; I've opened an issue on GitHub to investigate (https://github.com/opensim-org/opensim-core/issues/1832). In the meantime, you can use the following two-line workaround:

Code: Select all

pc = PrescribedController('controls_FR03A00_FL200_EX200.sto'); %Add these two lines...
model.addController(pc);
state = model.initSystem(); %...before this line in your script.

This workaround will add the controller to the model before it's passed to ForwardTool.

[Pavlos Silvestros]

Thank you Tom,

Works well now! Thank you for your help over this!

One last question regarding this:

Is it preferable to add the control file to the ForwardDynamics tool or Prescribe it to the model as you have done?

Best regards,
Pavlos

[Thomas Uchida]

Is it preferable to add the control file to the ForwardDynamics tool or Prescribe it to the model as you have done?

There should be no difference in performance; setControlsFileName() just ends up adding a controller to the model anyway. Actually, the PrescribedController approach gives you more flexibility because you can specify the interpolation method.

[Pavlos Silvestros]

Thank you for all your help Tom, much appreciated!!