Fiber Reinforced soft robot

[Shan]

Hi Everypne,
I am new to SOFA. I am using it to model soft robot, I am trying to model one
part of the robotic arm which should expand and bend depending on the pressure
applied on the inner walls of the cylinder. There are 6 such cylinders.
I am using the code from tutorial on PneunetGripper to modify to my need. I want
to restrict the radial expansion of the cylinders by using stiff wire wounded
around it (similar to “tie” constraint in Abaqu). However the wire expands with the cylinder and at some points the
elastic cylinder penetrates through the wire which is not desirable. I am new
to Python as well so your help will be highly appreciated.

Best regards,
Shan

[Hugo]

Hi @shan

Thank you for your question and welcome on the SOFA forum.
I think you could apply a PartialFixedConstraint (defining in which direction you would like to restrict the motion) in your cylinder node.

Best

Hugo

[Shan]

Thanks. I will update you if I am able to resolve the issue with your suggestion.

[Duriez]

Hi Shan,

An other way (more complex but very elegant) is to have a uniform mesh of your cylinder (typically with hexahedrons / quads on surface) and then to prevent any radial expansion of your mesh, you “rigidify” each circle (each set of points of the mesh that are on the surface of the cavity and belong to the same plane perpendicular to the cylinder).
If each of these circle is now a new rigid object, the cylinder can expand in length and can bend but the radial expansion is impossible.

For the “rigidification”, the procedure is explained in the Tripod tutorial..

Cheers,

Christian

[Shan]

Hi Christian,

Thanks for the suggestion. I will look into the tutorial example of Tripod to see how to implement “rigidification”. Hope to get back to you with some progress.

Cheers.
Shan

[Shan]

Hi Cristian,

I looked at Tripod tutorial and also looked at the following example

https://github.com/SofaDefrost/STLIB/blob/master/python/stlib/physics/mixedmaterial/rigidification.py

However I have questions regarding “frames” and “groupIndices”. Could you please explain what do they mean in terms of the mesh. I looked at few topics on rigidification and it seems to me that it has to do with BoxROI. Since I am an amateur in python so I couldn’t completely understand the code so please pardon my lack of knowledge of python. I am unclear about how to rigidify the surface mesh elements of the the outer boundary of the cylindrical structure.
In short how can I obtain frames and indices from the meshed structure.

Best regards,
Shantanu.

[Shan]

Hi,

I have been trying to work with tripod tutorial example to rigidify outer boundary of the cylindrical actuator I mentioned earlier. However I am stuck at extracting the “groupIndices” and “frame”. I would be highly obliged if you could help me out here. The code I implemented is as follows:

import Sofa
 import os

#from stlib.visuals import ShowGrid
from stlib.scene import MainHeader
from stlib.physics.deformable import ElasticMaterialObject
#from stlib.physics.constraint import FixedBox
from stlib.physics.mixedmaterial import Rigidify
from splib.objectmodel import setData

def createScene(rootNode):
        #Another way to add gravity with MainHeader
        MainHeader(rootNode, gravity=[0.0,0.0,-9810.0])
        #MainHeader(rootNode, plugins=["SofaSparseSolver"])
        rootNode.createObject('VisualStyle', displayFlags='showForceFields')
        rootNode.createObject('RequiredPlugin', name='soft', pluginName='SoftRobots')
        rootNode.createObject('RequiredPlugin', name='SofaPython', pluginName='SofaPython')

        rootNode.createObject('PythonScriptController', filename="pythonControllers/fingerController.py", classname="controller")
        rootNode.createObject('FreeMotionAnimationLoop')
        rootNode.createObject('GenericConstraintSolver', tolerance="1e-12", maxIterations="10000")

        # Create Actuator mechanical model

        finger = rootNode.createChild('finger')
        finger.createObject('MeshVTKLoader', name='loader', filename='data/mesh/TestActuator')
        finger.createObject('Mesh', src='@loader', name='container')
        finger.createObject('MechanicalObject', name='tetras', template='Vec3d',showObject='true', showObjectScale='1')
        #finger.createObject('SubsetMultiMapping', template='Vec3d', indices='1 2 3 100')
        finger.createObject('TetrahedronFEMForceField', template='Vec3d', name='FEM', method='large', poissonRatio='0.4',  youngModulus='1500', drawAsEdges="true")
        finger.createObject('UniformMass', totalMass='0.0008')
        finger.createObject('EulerImplicit', name='odesolver', rayleighStiffness='0.1', rayleighMass='0.1')
        finger.createObject('SparseLDLSolver', name='directSolver')
        #finger.createObject('CGLinearSolver', name='Solver')

        # tracking Mesh Nodes 1 2 5 6 8 10 50 55 60 100
        tracking = finger.createChild('tracking')
        tracking.createObject('MechanicalObject', showObject='true', showObjectScale='5')
        tracking.createObject('SubsetMapping', indices='1 2 5 6 8 10 50 55 60 100')
       
 # FixedBox constraint at the bottom of actuator

        finger.createObject('BoxROI', name='boxROI', box='-10.5 -10.5 -4.5 10.5 10.5 -2', drawBoxes='true', drawSize='0.1')
        finger.createObject('RestShapeSpringsForceField', points='@boxROI.indices', stiffness='1e12', angularStiffness='1e12')
        finger.createObject('LinearSolverConstraintCorrection', solverName='directSolver')

        # FixedBox constraint on top of actuator

        finger.createObject('BoxROI', name='boxROI1', box='-10.5 -10.5 82.5 10.5 10.5 86', drawBoxes='true', drawSize='0.1')
        finger.createObject('RestShapeSpringsForceField', points='@boxROI1.indices', stiffness='1e12', angularStiffness='1e12')
        finger.createObject('LinearSolverConstraintCorrection', solverName='directSolver')

        # Cavity of elastic cylinder (Actuator)

        cavity = rootNode.finger.createChild('cavity')
        cavity.createObject('MeshVTKLoader', name='loader2', filename='data/mesh/TestCavity')
        cavity.createObject('Mesh', src='@loader2', name='cavityMesh')
        cavity.createObject('MechanicalObject', name='cavity', showObject='true', showObjectScale='1')
        cavity.createObject('SurfacePressureConstraint', name="SurfacePressureConstraint", template='Vec3d', value="0.0001", valueType="pressure")
        cavity.createObject('BarycentricMapping', name='mapping', mapForces='false', mapMasses='false')

The meshed geometry is in this link
https://drive.google.com/drive/folders/1r3zk_WkzGLM0yhyyY7_W996jcpUx0akv?usp=sharing

Best regards,
Shan

[Shan]

Hi,

Also I have another question regarding the rigidification, what is the need for creating a simulationNode as given one of the topics in forum.

RigidMapping from Rigid3d to only some particles of FEMbody

simulationNode = rootNode.createChild(“Simulation”)
simulationNode.createObject(“EulerImplicitSolver”)
simulationNode.createObject(“CGLinearSolver”)
simulationNode.addChild(o)

Best regrads,
Shan

[Shan]

Hi,

I would be obliged if someone could advise me on the above mentioned query.

Regards,
Shan

[Hugo]

Hi @shan

I requested access to your mesh to test the scene.

Regarding your last question: the way the scene is created here is a bit contra-intuitive. First, all sub-nodes are created. Finally they are added in the simulation (as child node of the root node). This root node is the starting point of the simulation, and this is where the solvers (integration scheme and linear solver) are defined in this case.

Does this help?
Best wishes,

Hugo

[Hugo]

To understand how a rigidification works, you can also check the example: examples/Components/animationloop/MechanicalMatrixMapper.pyscn

Best

Hugo

[Shan]

Hi Hugo,

Thanks for the reply. I granted you access to google drive, please check if you can access it.
Regarding the query about rigidification, I did go through the example you mentioned. However I am a bit confused regarding the arguments “indices” and “frames” as I am not sure how to extract it from mesh. The part I want to make rigid is the outer wall of the column so that it does not expand radially though it can still expand axially and also can bend.

I understand you guys must be busy with the upcoming events on SOFA training in November, though it would be really helpful if you could explain the functions in Python. I have gone through https://www.sofa-framework.org/api/master/sofa/html/index.html to understand the functions and classes used in SOFA Python, but its not really clear to me.

Looking forward to see you for the training session.

Best regards,
Shan

[Hugo]

Hi @shan

Thank you for granting the access. I am able to run your scene.
The data “indices” means the indices of the degrees of freedom (nodes) of your deformable object which will be affected by the rigidification.
The data “frames” corresponds to a list containing the quaternion (position + orientation) of all rigid frames: [[rx,ry,rz], [qx,qy,qz,w]]

Let me know it this clarifies your point.
Regarding the global understanding of SOFA, do you have some more specific questions so that I can answer accurately?

Best wishes,

Hugo

[Author]

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