Coordinate driven force application

[Evan Dooley]

Hello,

I was wondering if anyone has any experience (or recommendations) about a way to use coordinate information to drive a force in your model. I want to try and apply a force to the model on a coordinate that is different from the coordinate I am tracking.

Looking around, I found "ExpressionBasedCoordinateForce" but this only allows the force applied be a function of the coordinate the force is applied to. The same thing happens with "CoordinateLimitForce". "ExpressionBasedPointToPointForce" only uses the distance between the points in the expression.

Ultimately, I am trying to simulate different assistance forces in Moco that are patterned with the gait cycle of the person. We can reliably determine where the person is in the gait cycle by different joint angles, and I want to try and only apply force relative to portions of the gait cycle.

Any advice or ideas would be greatly appreciated.

Thanks,
Evan

[Nicholas Bianco]

Hi Evan,

Unfortunately there's no force component to do what you're asking. It sounds like you need some sort of controller, but I don't think that type of controller exists in OpenSim, and Moco doesn't support controllers at the moment anyway.

If you're tracking kinematic data you should know the time point when the model is in a certain configuration where you want to apply a force. Unless I'm not understanding your problem correctly?

-Nick

[Evan Dooley]

Thanks Nick.

I was also thinking about using knowledge of where the kinematics should be to time when forces should be applied, and then write and external loads file to do that. However, I am trying to let the simulation be loose enough that the motion could vary a little bit to react to the applied force, so the timings for the future events may differ from those known at the start.

I appreciate your insight, as always.

Best,
Evan

[Nicholas Bianco]

Ah, gotcha. That would definitely be cool and useful to my research as well. But yeah, not sure when we'll support such a thing.

[Aaron Fox]

Hi Evan,

I'm not exactly sure whether this will fit your needs, but it might provide some guidance on how to get there. In this project - https://simtk.org/projects/gh-caps-sims - I modified the expression based coordinate force to take the input of two coordinates in the calculation of the force it applies. This wasn't too difficult, it was just a matter of having the force connect to the two coordinates, rather than one to write the expression. You could equally just re-write this to only use the secondary coordinate in the force calculations.

Aaron

[Evan Dooley]

Hi Aaron,

This is awesome! I've been working on implementing it and noticed in the function "ShoulderCapsulorrhaphySims_3_CreateModels" you call a model file that has the DualEBCF object already in the force list, then it looks like you use your own method to update the parameters of the object and write it to an XML OSIM file then use that file to generate your model in Matlab.

For other force objects we can call things like newforce = CoordinateLimitForce(); then update the parameters as newforce.setCoordinate('knee_angle') etc. Is there anyway to access the DualEBCF object from Matlab? Or is this further implementation that would need to be done on the C++ level of the plugin?

Thanks for your help!

Best,
Evan

[Aaron Fox]

Hi Evan,

I think the 'shortcut' approach I used is the only way to work with plugins in Matlab unless you rebuild OpenSim from source with your plugins included. That wasn't something I was going to put the effort in for the use of a simple plugin like this, so I figured using the XML functionality to help do the leg-work was the most suitable way to go about it.

Aaron

[Evan Dooley]

Cool! That totally makes sense. For my application, I think that implementing it in the XML also makes the most sense, especially at the start.

Thanks for your help. I've got this for the citation: https://pubmed.ncbi.nlm.nih.gov/33241584/ , let me know if you would prefer something else. The readme has the paper citation left as a todo.

Best,
Evan

[Ross Miller]

To my knowledge, building the application from the source code is the only way to get something like this accessible from Matlab. The XML approach is a great solution, I didn't know that worked without a rebuild.

I do as much as possible in XML but mostly just because I'm not very good with C++.

It would be great if there was "ExpressionBasedCostFunction" that we could enter via XML!

Ross

[Nicholas Bianco]

Hey guys,

To confirm Ross and Aaron's thoughts: yes, you would need to modify the OpenSim "bindings", the code that converts C++ to scripting languages, and then rebuild the source to access the class in Matlab.

Ross, I've been thinking about an ExpressionBasedCostFunction! I don't see any reason why it wouldn't work.

-Nick