[Hugo]

Hi @agniv

Sorry for the delay of my reply, we have been busy with the SOFA Week. You can find out more about this annual community event on SofaFramework’s YouTube account.

The contribution computed in the addForce() function and added in the f vector corresponds to the explicit contribution of the internal forces (due to the mechanical constitutive law). If you are in an implicit scheme, the internal forces contribute to the f vector with both an explicit contribution and an implicit contribution, related to the derivative of the internal force with regards to the degrees of freedom in the addDForce() function. More info can be found in the documentation here.

Therefore you sentence “the inverse of the resultant of all the forces is the external force needed to cause the same displacement” would be true only in an explicit scenario. Because in implicit, the mechanical model also influences the system matrix A.

If I remember properly you are working in Hybrid team in Rennes. Is this correct?
In case you need a deeper introduction to SOFA, please let me know.

Best

Hugo

[Hugo]

Don’t be worried, this forum must and will never make anyone uncomfortable. It is an open place for any technical and scientific questions regarding SOFA.

Discussion papers can be interesting when it helps people to better understand the framework itself.

[Hugo]

The paper describes the Signorini’s law in details. The Coulomb’s law is known for years, the theory and its resolution are given in the paper mentioned in my previous post. Finally, its implementation is in SOFA.
What else would you expect?
What do you actually intend to do with SOFA? Are you stuck any how in your use or implementation?

[Hugo]

Hi @jayyang

I am really sorry, I am not at ease with the STL lib. Does this issue ring a bell to you @damien-marchaluniv-lille1-fr ?
Best wishes,

Hugo

[Hugo]

Hi Charles

Both cited papers rely on the work entitled “A Gauss-Seidel like algorithm to solve frictional contact problems” involving contact with frictions based on Signorini’s and Coulomb’s laws.

Hugo

[Hugo]

Hi @outtt

Alright.
At the occasion of the SOFA Week 2019, a tutorial session is organized on 20th Nov (morning). I will attend the Model Order Reduction session. Your simulation case could be interesting to test.
Would this be of interest for you?

Hugo

[Hugo]

PS: other topics in the forum cover this “cutting” topic
PS2: when addressing a new topic, do not hesitate to close the previous topic and open a new one !

Best wishes

Hugo

[Hugo]

Hi @lucaam86

Cutting belongs to what is called “topological changes”.
The SOFA API permits it: you can have a look at the Carving plugin for instance.

You can also take a look at the example scenes located in the folder TopologicalModifiers.

However, volumetric cutting is still work in progress. Indiedevs in the community are working on it, we should soon have impressive videos for this. Stay tuned.

Best,

Hugo

[Hugo]

Ciao @lucaam86

Thank you for all your videos!! you bringing life to this forum!

Do not hesitate your scene file when sharing such a video. It helps others to understand.

But in fact, you understood the problem. The fact is that your interaction with the mouse is done against the collision model (collision spheres inside your grey mesh). The forces due to your mouse actions are applied on these spheres and mapped back to the mechanical model. If you want to be able to “grasp” the grey with the mouse, you need to have the appropriate collision model: either with spheres (sph file) of using. the surface triangles of your current grey mesh.

Let us know if it helps.
Best

Hugo

[Hugo]

Hey @charles

Understanding the source of an error is physics simulation is often complex due to the different possible cause of errors, as you explained. Therefore, there is no straightforward answer.

Numerical analysis is one the main key : analyzing the system matrix, the force vectors, the convergence of the solver and the input parameters. We are working at providing tools like easy export of system matrix, which is not allowed yet. Python can be a powerful tool as well for this.

Best

Hugo

[Hugo]

Hi Charles,

Yes I can close it.
Could you share your conclusions? what was the prob?

Best wishes,

Hugo

[Hugo]

Great @bobiko

Nice job! Let us know and do not hesitate to open a new topic if you have other question or if you struggle in your implementation.

Best

Hugo

[Hugo]

Hi Vlad @bobiko

I am sorry for this late reply. I prepared few weeks ago a reply, but got not published.
Did you solve your issue ?

Best,

Hugo

[Hugo]

After modifying templates and others in your code, it now works. I am just missing an Omni to fully test your scene.

I don’t understand what you are doing with the ROIBarycentricMapping since you are coupling both the needle and the brain. Is this on purpose ?

What would be now the next step you would like to achieve based on this scene?

Hugo

[Hugo]

I’m looking at it @outtt

[Hugo]

Hi @lucaam86

You can have a look at this topic.
You can easily see in my above-posted image that you also have disconnected tetra: one tetrahedron is falling all alone. Basically, you should consider regenerating a new volumetric mesh.
You can have a look at the CGAL plugin in SOFA.

Regarding the FixedConstraint, here are the steps to follow to change the IDs of the fixed nodes/degrees of freedom:
– you can open the MechanicalObject ruling the physics and activate the Visualization option “showIndices”
– choose the indices you want to fix
– add these indices in the field “indices” of the FixedConstraint.

I just did a short video highlighting this:

Best,

Hugo

[Hugo]

Hi @ma1991

The first feature to consider is the time step. You need to use a sufficiently small time step so that a solution can be found.

With a highly nonlinear dynamic system, convergence can be difficult to reach, even with a small time step. Especially, if you are doing one Newton step per time step which is the case in most SOFA schemes. Only two schemes allow for defining the number of Newton step in one time step:
– the StaticSolver
– the VariationalSymplecticSolver

The latter should work in your dynamic case.

I hope this helps.
Best wishes

Hugo

[Hugo]

Hi @jangirrishabh

Yes you can load STL files using the MeshSTLLoader.
ArticulatedSystem works in SOFA but I have very few experience with it.

Best

Hugo

[Hugo]

Hi @lucaam86

I run your simulation, here is the video:

I identified several problems:
– your mechanical mesh (blue in the video) seems to have issues: non-manifoldness, single tetra separated from the rest of the mesh
– your fixedConstraint is only on the botton of your mesh, not on the top. Therefore everything falls

Could you explain again what your mesh is representing? What are you looking to simulate?

Best wishes,

Hugo

[Hugo]

hi @outtt

I am out of the office for meeting partners of the community. I will give a try next week for sure.
Best

Hugo

[Hugo]

Hi @lucaam86

I would need the same *.obj and *.msh files to run the simulation. Can you share it?
It will allow me to answer your last question regarding parameter values.
Best

Hugo

[Hugo]

Great I will therefore close this topic.
You can open a new one if you have another question.

Hugo

[Hugo]

Hi @ma1991

This is a user question for the Paraview forum 😉
Let me help: you can export as a spreadsheet your selection. You can use the PointID given in the first column. This should correspond to the points you selected and specify these indices into the field “indices” of the FixedConstraint.

Best wishes

Hugo

[Hugo]

Thank you @bennetcole for your valuable input.
And welcome on the SOFA forum!

[Hugo]

Sorry @charles but everything looks fine here.

[Author]

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