Using AttachConstraint to tie deformable and rigid objects together

[adkoessler]

Hi,

I am trying to simulate the behaviour of a deformable object grasped by a robotic arm. The problem is to perform motions of the rigid object (gripper) that induce deformations in the flexible part. The code is attached to this post.
robot_grasp_moving.scn.txt

1. The constraint is not performed. There is a warning that I don't understand: [StateAccessor(unnamed)] Link name 'mechanicalStates' already used as a Link in this class or in a parent class

2. I want to constrain a certain number of nodes based on a ROI on the deformable object. How can this be performed? My idea was to create a MechanicalObject using ROI nodes and pair them one to one in the constraint:

<Node name="Deformable">
     ...
    <BoxROI name="gripperROI" box="-0.1 1.05 -0.06 0.1 0.85 0.06" drawBoxes="1" drawSize="1"/>
</Node>
<Node name="Gripper">
    ...
    <MechanicalObject template="Vec3d" name="gripperMO" position=
[robot_grasp_moving.scn.txt](https://github.com/sofa-framework/sofa/files/8152607/robot_grasp_moving.scn.txt)
"@../Deformable/deformableMO.position[@../Deformable/gripperROI.indices]" showObject="1"/>
</Node>
<AttachConstraint name="AttachConstraint" twoWay="true" object1="@Deformable" object2="@Gripper" indices1="@../Deformable/gripperROI.indices" indices2="@../Gripper/gripperMO[:]" constraintFactor="1"/>

What I don't get is the syntax to pass a list of nodal positions based on indices to the "position" field of another component.

Best

[hugtalbot]

Dear @adkoessler

Welcome on the SOFA!

1. Could you specify which component is emitting this warning? is it the constraint? could you try to remove the constraintFactor (its default value is 1 anyway but its size must be the number of constraint if I recall correctly)
   maybe @damienmarchal you know more about this warning?

2. Since I am not sure to understand the point I will start with a generic reply: here you are looking at creating a projective attachment. You must fulfill the following conditions:

• select as many points of object 1 and as many points of object 2
• object 1 and 2 must have the same type of dofs (same template) : if not, creating mapped subnodes is required

Thus, you will be able to create an AttachConstraint

Hope this helps. Do not hesitate to share updates if any.
If I do not answer point 2, please do not hesitate to elaborate your question.
Best

Hugo

[hugtalbot]

any update @adkoessler ?

[adkoessler]

Sorry for the long wait. I have come to an answer thanks to @hugtalbot and I attach some scripts to exemplify the solutions that we found. It seems that using a projective constraint was not what I looked for, so I did not use an AttachConstraint

Solution based on BilateralInteractionConstraint

The principle is as follows:

• Create a node for the deformable object with a solver, a mass, a forcefield...
• Create a MechanicalObject for the interaction points and define a BarycentricMapping between them and the deformable object
• Create nodes for the grippers with only a mechanical object (no solver or physics, we just want to overwrite their position and orientation at each time step)
• Add BilateralInteractionConstraints linking the MO of the interaction points with the MO of the grippers

The example script robot_2grasp_bilateral_controller_hexa.py is attached, where a Python controller is used to move the grippers. In turn, the soft object deforms accordingly.
(run the script with python3 or open in SOFA GUI as you wish)
robot_2grasp_bilateral_controller_hexa.py.txt

Solution based on MechanicalMatrixMapper

The principle is as follows:

• Instead of imposing constraints, some subsets of the deformable object are now rigidified
• The node where deformable characteristics appear (mass, forcefield) is a subnode of both deformable object node and rigid grippers nodes
• In the node where deformable characteristics appear, the component SubsetMultiMapping is used to distinguish flexible and rigidified parts

The example script robot_2grasp_mmm_controller_hexa.py is attached, where a Python controller is used to move the grippers too. The same results are obtained.
(run the script with python3 mandatorily)
robot_2grasp_mmm_controller_hexa.py.txt

Comparison

• The solution using BilateralInteractionConstraint needs to use a FreeMotionAnimationLoop and a constraint solver. These are not needed when using MechanicalMatrixMapper
• The solution using MechanicalMatrixMapper ensure that all nodes within a given ROI have a rigid motion. When using BilateralInteractionConstraint, deformation can still occur "between" the interaction points
• The solution using MechanicalMatrixMapper creates loops in the scene graph
• The solution using MechanicalMatrixMapper is straightforward when the rigid and deformable regions of the mesh can be defined via a BoxROI. When the regions are not parallelepipedic (eg. curved, concave) however, indices of nodes must be hand-picked. This forces the creation of the scene to be split in two functions, with an intermediate step of computing node indices for both regions. Hence, Python scripting becomes mandatory.
• For this example, no obvious difference in computation performance can be seen.