[Wong]

Hello max,

After searching from the Sofa source code, a “propagate_lambda” flag is indeed in Compliant. However, I cannot find this flag in other components but I can find a “lambda” flag in some Correction and ForceField components.

As far as I know, a contact constraint is an unilateral constraint. Sofa should handle the collision by constructing such a constraint. And I do not know how and in which component Sofa constructing and solving this constraint.I have tried to read some relevant source code of Sofa, but the internal mechanism of sofa is complex so I cannot understand it.

Sofa can still handle the collision even if not using the Compliant. So could you provide me some documents and some simple examples about how Sofa constructing and solving the contact constraint(involving specific mesh vertexes and pertinent formulas)?

In the component LCPForcefeedback, the method computeForce can compute the contact force. Do you know how it computing the contact force and could you provide me a simple example to demonstrate it?

[Wong]

Hello max,

Thank you. The courses are fantastic and it is actually a nice introduce of dynamics simulation.

And I have also read about the theory of the compliant approach. It converts a linear system of equations into a quadratic program (QP) using KKT conditions. So what the solver does is to solve the QP not the linear system ?

And from the sofa documantion, when a collision occurs, the contact force must be computed. So how to extract the value of the contact force when the simulation is running ?

Wong

[Wong]

Hello @hugo,

I have read the page 21 of the SOFA PAPER literally but it does not mention the violation and the equation trans(H).A.H λ = trans(H).y – d.

The bilateral constraint law is Ф(x1,x2)=0 but what is the concrete expression of Ф ?

Could you give me more information about this?

Thanks,
Wong

[Wong]

Hello Hugo,

I want to know more about BilateralInteractionConstraint and I want to comprehend the concrete meaning of the equation trans(H).A.H λ = trans(H).y – d.

What are the concrete forms of H, A, λ and d?

What kind of reference can I find some content about constraint or constraint solver in?

Who is expert at constraint solvers?

Thanks,

Wong

[Wong]

Hello,

I modified two parameters,alarmDistanceandcontactDistance of the component LocalMinDistance,respectively into 0.08 and 0.04.Then the simulation became better.

[Wong]

Hello Hugo,

My scene is just a test example because I think the .scn scene can be conveniently and quickly constructed. I certainly want to constrain the torus at some moment but it would make me write code.

Of course my scene will lead to instability. But I just want to test if the BilateralInteractionConstraint effects. Otherwise I really do not know how to construct my grasping scene with constraint component. I think even though the scene is unstable, the constraint component should still constrain the object.

And I have read the code of BilateralInteractionConstraint but I cannot understand. So I do not know how to do next.

Wong

[Wong]

Hello Hugo,

My OS is Windows and the version of SOFA is two years ago(0b4eb1a15733b234d5df07d4fa74d50ad686ebbe).

But when I use another SOFA with version v15.12(4ec83a523e81bd7d7a1ce789a7e6045a43c95884), the problem still arises.

video link:
https://www.dropbox.com/s/rj9nti2kkyex6jm/problem.avi?dl=0

Wong

[Wong]

Hello Hugo,

I certainly know how to use the events mechanism in SOFA.
My focus is how to implement the constraint.

In my scene, I use the BilateralInteractionConstraint to fix the position of the torus relative to the lower part of the tool.

But I also have the problem like “the constraint do not fix”. The relative displacement of the torus still will occur.

The package file is as follows:
https://www.dropbox.com/s/hm1pejsow5aqkxf/test3.zip?dl=0

Why could this be?

Wong

[Wong]

Hello Hugo,

I did read the code in optitrackGrasping.scn(from Forest’s reply)，GraspingManager.h and EvalSurfaceDistance.inl. But I did not have the device so I could not test the examples. What’s more, the GraspingManager did not work and its functionality was limited.

I realized that it was hard for me and I estimated that I would spend large amounts of effort. So I have not done relevant work for a long time. Besides, I also had other work to do.

But I plan to proceed with this wok in next month. Of course I need further assistance because I still do not know how to implement the dynamical relative fixity(not fixed when scene start, but when some event is triggered, fixed).

Thanks for helping me.

Wong

[Wong]

Thank you. I have built sofa successfully.

[Wong]

Hello Forest,

I have also considered changing the collision group.

Please allow me to ask one more question.

If I want to set both two parts of the grasper in the same group but the torus can only collide with the lower part, what should I do?

Sincerely,
Wong

[Wong]

Hello Hugo,

Do you know how to judge whether a collision model collides with another specific collision model and how to fix the position of a collision model relatively to a rigid DOF(the collision model is free initially) in animating by writing code?

Could you give me some further information or point out some examples to reference?

Sincerely,
Wong

[Wong]

Hello Forest,

Do you know how to judge whether a collision model collides with another specific collision model and how to fix the position of a collision model relatively to a rigid DOF(the collision model is free initially) in animating by writing code?

Could you give me some further information or point out some examples to reference?

Sincerely,
Wong

[Wong]

Hello Forest,
Thank you for your reply.
According to your advice, I change the collision models into two boxes and try to use a deformable torus. But the problem still remains.You can see the package file.
https://www.dropbox.com/s/7l8na8sf0jniqit/test2.zip?dl=0

And in your non-physically method, how to deactivate the collision? Then do you mean fixing the relative position of the torus? How to do? But I want to obtain the collision force. If the collision is deactivated, will the force become zero?

Sincerely,
Wong

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