How to add my exoskeleton model to gait2354simbody?

[Euisun Kim]

Hello all,

I'd like to bring my exoskeleton cad model in Opensim so that I can simulate how muscle activity changes depending on the exoskeleton.
Using this exoskeleton + gait2354 model I'd like to simulate landing motion and see how muscle activity and joint torque(ankle) change with different exoskeleton condition(joint torque, softness of the material...)

However I don't know where to start with.
Any help will be really appreciated!

Thank you.

[Thomas Uchida]

You may find these examples helpful:
- "Simulation-Based Design to Prevent Ankle Injuries" (http://simtk-confluence.stanford.edu:80 ... e+Injuries)
- "Simulation-Based Design to Reduce Metabolic Cost" (http://simtk-confluence.stanford.edu:80 ... bolic+Cost)

[Marta Carrilho]

Hello,

I want to do something similar to Euisun Kim. I started by implementig the AFO of the example in a model. But now, how should I proceed to study the interaction between the model and the exoskeleton?

I tought of doing inverse kinematics and see how it changes, but I don't have data for possible markers on the AFO. I'm not sure if I'm being clear.

I would really be grateful if soeone has an idea.

Thank you,
Marta

[Euisun Kim]

Hello Marta,

Could you elaborate more how you implement your model(such as exoskeleton) in the gait2354simbody model?
I succeed to import my STL files of exoskeleton into opensim, but I have hard time to put them in the right position.
I'm not sure how to choose right values for <location in parent><orientation in parent><location><orientation>, so that I can locate my exoskeleton parts on the right spot. How do we know xyz coordinate for each part??

Thank you.

[Marta Carrilho]

Hello Euisun,

Could you elaborate more how you implement your model(such as exoskeleton) in the gait2354simbody model?
Actually, I’m still working on it. I also succed in importing the .STL files into OpenSim, and then I tried to adjust the position according to the coordinates of the body parts that I wanna connect.

Marta

[Daniel Pina]

You need to place the geometry STL files into OpenSIM's folder, and then edit the gait2354.osim file to include the new geometry. To do this you should create a new "body" within the "BodySet" group. I advise you to use Notepad++ to edit the file, as it's very convenient to tell you how the things are separated in the osim's xml format.

Observe how the other bodies are represented in the file. You'll need to identify the mass, mass center and inertia parameters from your exoskeleton's components, as well as the parent body and joints.


The problem you'll probably run into when trying to do gait analysis with the additional bodies is that you won't have ground reaction forces for the new model. You'll have to port the gait model you want into using contacts instead of ground reaction forces, which is no easy task (I'm working on a model for that and I'll gladly share it when/if it works and if I get proper authorization from the model's original authors).

There is a webinar by the AnyBody team showing how they've done this:

https://www.youtube.com/watch?v=tIC5eqKiaEc

Keep in mind that in OpenSIM, CMC doesn't work very well with contact forces, so unless you're willing to tinker with the program's code you may have to settle with Satic Optimization.

[Marta Carrilho]

Hi Daniel,

Thank you a lot for you explanation.

Do you think that a Bushing Force would work as a contact force?

Thank you,

Marta

[Daniel Pina]

Hi,

The contact forces in OpenSim use the Hunt Crossley model, which is a contact between a sphere and a plane or between two spheres.

You can see examples of the pre-set Hunt Crossley contacts in the *.osim file of the first example shown the "Dynamic Walking Challenge" tutorial:
http://simtk-confluence.stanford.edu:80 ... Id=5113821

[Marta Carrilho]

Hello again,

I am still trying to do the contact between the exoskeleton and the body.

I think OpenSim has 3 contact forces: Bushing Force, elastic Foundation Model and Hunt Crossley model, am I wrong?

Can anyone enlighten me about which of these options it is the best? Or, what are the big differences between them?

I am trying to use Bushing Force but I am not having success.

Best Regards,
Marta

[Thomas Uchida]

which of these options it is the best?

Whether a particular modeling option is best depends on the application. Here are some relevant resources (the examples will give you an idea of how each component could be used):

BushingForce
- https://simtk.org/api_docs/opensim/api_ ... ml#details
- used in the "Simulation-Based Design to Prevent Ankle Injuries" example (http://simtk-confluence.stanford.edu:80 ... e+Injuries)

ElasticFoundationForce
- https://simtk.org/api_docs/opensim/api_ ... ml#details
- used in the "Dynamic Walking Challenge: Go the Distance!" example (http://simtk-confluence.stanford.edu:80 ... Id=5113821)

HuntCrossleyForce
- https://simtk.org/api_docs/opensim/api_ ... ml#details
- used in the "From the Ground Up: Building a Passive Dynamic Walker Model" example (http://simtk-confluence.stanford.edu:80 ... lker+Model)

Other strategies (e.g., WeldJoint) could also be best, depending on the research question.