Connect Multiple Deformable Object with Single Rigid Part

[yu-mei]

Connect Multiple Deformable Object with Single Rigid Part

Hello creative community members (@EulalieCoevoet @damienmarchal @alxbilger @hugtalbot @bakpaul @olivier-goury ),

I'm trying to reproduce a simulation similar to the one in the Soft Robotic Toolkit. However, I'm encountering an issue with the connector part (see the image below):

I found a related discussion on a similar structure here, which suggests a spring formulation to simplify the connection. My goal is to use a BoxROI to select points from three different bellows and connect them to a rigid connector with a strong spiral spring. The challenge is ensuring that two of these maintain a fixed spatial relationship.

Below is the code I'm using (inspired by the soft robots plugin tutorial CableGripper). Currently, the simulation produces an unexpected behavior: the deformable part appears to be fixed in the global frame's fixing box rather than being attached to the connector_cap.

I’d really appreciate any guidance or suggestions, as I've been stuck on this for several days.

Screencast.from.03-14-2025.03.27.08.AM.webm

---

    # Connect with deformable part (questionable)
    for i in range(bellow_num):
        angle_rad = math.radians(i * angle_step)
        x = radius * math.cos(angle_rad)
        z = radius * math.sin(angle_rad)

        # Get the bellow model - assuming it has a "modelNode" as in typical Bellow implementation
        bellow = selfNode.getChild(f"Bellow{i+1}")
        # Create a BoxROI to select the top surface of each bellow
        top_margin = 3.0  # Height from top of bellow to select
        box_half_size = 12.0  # Half size of the box in x and z
        bellow.addObject('BoxROI', 
                         name="topROI", 
                         box=[x - box_half_size, height - 43 + top_margin, z - box_half_size, 
                              x + box_half_size, height - 43, z + box_half_size], 
                         drawBoxes=True)
        bellow.addObject('RestShapeSpringsForceField', 
                         name="attachToConnector",
                         points="@topROI.indices", 
                         external_rest_shape='@../ConnectorCap/dofs', 
                         stiffness=1e12,
                         angularStiffness=1e12)

# Environment: Ubuntu 22.04 + SOFA_24.06.00 binary

[bakpaul]

Hello !

If I understood correctly, what you are trying to do here is to attach both object with springs ? If so, you shouldn't be using the RestShapeSpringForcefield which is only here to attach body in space. What you should use is a SpringForceField.

To do so you need to specify the two objects object1=@path/to/first/mstate and object2, then the indices through indices1 and indices2 then the stiffness stiffness that can either be a scalar applying the same stiffness to all spring or you can specify a stiffness per spring. Same for the rest length lengths and same for damping damping

[bakpaul]

I am sorry but if you have this error it means that you are using a quite old version of SOFA. Can you tell me which version of SOFA you are using ?

Rewatching you video have given me some thoughts. : How are you fixing the deformable body parts that are in the drawn boxes ?

[yu-mei]

Hi Paul, I used Ubuntu 22.04 + SOFA_24.06.00 binary, it should not be that old version.

I have fixed this issue by using CGLinearSolver instead of SparseLDLSolver under rigid part. However, although the spring attachment does exist, the simulation is still not realistic.

From the picture, the right bellow extend, but the orientation of each bellow is not consistent with the rigid connector (direction 1 and 2). I think it because the SpringForceField in my case only connect the point (vec3d). I am trying to make 3 attachment points for each ring (change from 3 red points to 9 yellow points in bottom view), it still does not wrok, the result is same as the previous result.

It seems the rigid part cannot apply the force to the deformable object inversely. I am wondering if there are some improvements or other ways in this case, in specific, I extend the right bellow, the rigid connector has some rotation and then it also applies the kind of force push the left bellow up? Do I need to rigidify the tip part of the bellow so that I can also use spring to align both orientation and position (rigid3f)?

[yu-mei]

It seems this case needs inteaction between all 3 deformable part and rigid part, do you think if I need to consider other realization function? like AttachProjectiveConstraint or Lagrangian based constraint?

[bakpaul]

Hello ! It is really difficult for me mto make any statement regarding this strange behavior without being able to run it on my side. Do you have a stand-alone zip that you could share ? If you prefere to keep it private, you can send it to me on discord.

[yu-mei]

Hi Paul, thank you for following up on my issue. I’ve been using the BilateralLagrangianConstraint to address the problem, but the connection remains somewhat unstable. I’ve uploaded my code to a private repository and invited you to review it. Since this is an ongoing project, I’m sharing it privately for now. I will make it public to SOFA community once the project is completed.

I’d really appreciate it if you could take a look and share any insights you might have.

Thanks again for your support!

[yu-mei]

I temporarily solved this questions by assigning different material parameters to one single part to mimic the rigid part, instead of doing the complicated connection.