[SOLVED] How to extract reaction force for certain nodes

Hi
Recently, I met problem when extracting reaction force. The sense I modeled is a rigid gripper to grasp a deformable object, say, soft ball. I want to extract the reaction force the ball exert on the gripper during grasping. I understand that for rigid body the mechanical object is generally 1 indice with 3 position and 4 quaternion elements. So the problem comes how to get the reaction force on this specific node.

I found two related topics in the forum but none can solve my problem.
https://www.sofa-framework.org/community/forum/topic/tactile-sensing-with-deformable-material-in-sofa/
https://www.sofa-framework.org/community/forum/topic/external-force-on-a-vertex/

I use RestShapeSpringsForceField with a very large stiffness to constrain my gripper, and move the rest position the modify to control the gripper. I’ve tried to extract the force for the gripper mechanicalObject, i.e.

node.getChild(‘finger’).getObject(‘mechObj’).force

However, the value is extremely large and cannot be trusted. I guess that this is because this force value considering the recovering force during applying constraint.

Then I found that there is a constraint value in the

node.getChild(‘finger’).getObject(‘collision’).getObject(‘mechObj’).constraint

And the value seems somewhat reasonable, and can even extract the value at each surface mesh indices of the rigid body (this is exactly what I want). However, the format seems a bit difficult for me to understand, as:

0 1 37 -0.841634 0.148139 0.519333  
1 1 37 0.127235 -0.880181 0.457267  
2 1 37 -0.524846 -0.450929 -0.721942  

I’ve tried to read into the source code to find the exact data structure, but still struggling with understanding the whole program architecture. I have some knowledge on the contact algorithm in commercial software and also implemented contact algorithm in my own in-house FEM code. We generally define the slave and master surface, and detect the penetration of slave surface point into master surface and use penalty method for compensation. However, I’m still quite confused by the TriangleCollisionModel, LineCollisionModel and PointCollisionModel, and not sure why need these much “model”. Since I found different model also output different format for the “constraint” object.

Hence now I would like to seek for your help. Could you kindly help to explain the meaning of the constraint value, or more specifically, how to extract the reaction force during grasping?