Write a read-only position vector

[ISIR-GuillaumeS]

Hi !

I have a SOFA scene based on the Cosserat plugin and SofaPython. The idea is to create a continuum manipulator driven by cables (see below).

Here is the code I use to add cables to the manipulator :

def addcables(ParentNode, geoParams, nb_segment_Tot, nb_segment, nb_section, num_section, nb_cable):
        for i in range(nb_cable):
            cableBaseState, cable3DState = create_cable_points(geoParams=geoParams, nb_segment_Tot=nb_segment_Tot, nb_segment=nb_segment, nb_section=nb_section, num_section=num_section, num_cable=i)

            cableNode = ParentNode.addChild("cable"+str(i+1)+"Node")
            cableNode.addObject('MechanicalObject', name="cable"+str(i+1)+"Pos", template='Rigid3d', position=cableBaseState,
                                showObjectScale=0.8, showObject=False)

            cableNode.addObject('RigidMapping', name="cable"+str(i+1)+"Map", rigidIndexPerPoint=[i for i in range((nb_segment * num_section) - ((num_section // nb_section)*(nb_section - 1)) + 1)],
                                globalToLocalCoords=1
                                )
            cable_mechaNode = cableNode.addChild("cable_mecha"+str(i+1)+"Node")
            cable_Mo = cable_mechaNode.addObject('MechanicalObject', name="cable"+str(i+1)+"_mechaPos", template='Vec3d',
                                                position=cable3DState,
                                                showIndices=True)

            cable_mechaNode.addObject('SkinningMapping', nbRef='1')

            cable = cable_mechaNode.addChild("PullingCable")
            cable.addObject('MechanicalObject', position=cable3DState[0], showIndices="1",
                            rotation=rotation, translation=translation, scale=1.0)

            cable.addObject('CableConstraint', template='Vec3', name='aCableActuator',
                                        hasPullPoint=True,
                                        pullPoint=pullPointLocation,
                                        indices=list(range(len(cable3DState[0]))),
                                        value=0.0,
                                        maxPositiveDisp='70',
                                        maxDispVariation='1',
                                        minForce=0,
                                        valueType=valueType)

            cable.addObject('IdentityMapping', name="Mapping")

And then, in my CreateScene function I have

stemNode = addSolverNode(rootNode, name="stemNode", isConstrained=True)

    cosserat = stemNode.addChild(CosseratBase(parent=stemNode, params=Params))
    createRigidDisk(cosserat.cosseratFrame)

    addcables(cosserat.cosseratFrame, geoParams, nb_segment_Tot=nb_segment_Tot, nb_segment = 13, nb_section = 1, num_section = 1, nb_cable=3)

The thing I am trying to perform is to change dynamically the position of the cables pulling point during my control loop. For exemple, I would like the cables to get closer of 1mm of thebackbone at every step of the contol loop. To do that, I tried to overite the position of the pulling point by writing

self.proto.stemNode.CosseratBase.rigidBase.cosseratInSofaFrameNode.cable1Node.cable_mecha1Node.PullingCable.MechanicalObject.position.value[13][0] = 0.70

Or

self.proto.stemNode.CosseratBase.rigidBase.cosseratInSofaFrameNode.cable1Node.cable1Pos.position.value[13][0] = 0.70

However, it gave me the same error in both cases :

Python exception:  
  ValueError: assignment destination is read-only

Do you have any idea on how to do it ?

Thanks for helping !

[hugtalbot]

Hey @ISIR-GuillaumeS thanks for posting!

The error should be related to the fact that the state data of the Mechanical should be write-accessed with accessors. An example is available here: example-forcefield.py

However, I doubt it's the best way to do it. Is it correct to say that you want your cables to progressively get closer to the thebackbone?

[ISIR-GuillaumeS]

"However, I doubt it's the best way to do it. Is it correct to say that you want your cables to progressively get closer to the thebackbone?"
That's exactly what I want to do ! Do you have a more suitable solution ?

[hugtalbot]

I doubt this is feasible with CableConstraint, @adagolodjo do you confirm?

[ISIR-GuillaumeS]

I tried to add an accessor like that

with self.proto.stemNode.CosseratBase.rigidBase.cosseratInSofaFrameNode.cable1Node.cable1Pos.position.writeableArray() as wa:
            wa[:, 1] = 1

Is that the correct way to do it ?

However, that does not work. For example, here I tried to change the Y value of every cables. Writing the read-only vector worked because I can see that values have changed (see below)

But as you can see on the manipulator, cables did not moved. It feels like, I can change the position of cables but that is not take into account.

[hugtalbot]

Unsure whether the syntax wa[:, 1] = 1 works
But more generally on the approach I would need the feedback from @adagolodjo

[adagolodjo]

Hi @ISIR-GuillaumeS, have you tried this kind of modification with self.nodeName.rest_position.writeable() as posA:. Change 'res_Position' instead of 'position'. Let me know if this helped you. By the way, can you push an example scene of your robot into the actuators branch of the Cosserat plugin? The one we worked on together is still not complete.

[hugtalbot]

have you noticed @adagolodjo the idea would be to move the position of the cables during the simulation. @ISIR-GuillaumeS wants the cables to progressively get closer to the thebackbone.

[ISIR-GuillaumeS]

Hi @ISIR-GuillaumeS, have you tried this kind of modification with self.nodeName.rest_position.writeable() as posA:. Change 'res_Position' instead of 'position'. Let me know if this helped you. By the way, can you push an example scene of your robot into the actuators branch of the Cosserat plugin? The one we worked on together is still not complete.

Thanks for answering ! However, Hugo is right, I want to move dynamically the cable positions. I tried to change 'position' by 'res_Position' but that did not changed anything.

Let me clean the scene I am working on and I will push it as soon as I can.

[adagolodjo]

I believe I may have misunderstood your question earlier. Are you looking to move the cable sideways? Given that your cable is attached to fixed points along which it slides, moving one part may affect the others. Let's schedule a video call tomorrow to discuss this further, and then we can provide an update here. Thanks, @hugtalbot, for the clarification.

[ISIR-GuillaumeS]

Ok for a video call tomorrow thank you !

[hugtalbot]

Could you update us about this topic @ISIR-GuillaumeS please? thanks ;)

[ISIR-GuillaumeS]

Hi Hugo,
I do not have lots of insight regarding this issue. A guess from @adagolodjo would be to dig into "sofalauncher" to parallelize tests. However, we still did not had the time to discussi it so far.

I will update as soon as I find the solution !

[hugtalbot]

Alright thanks @ISIR-GuillaumeS

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@ISIR-GuillaumeS
Feedback has been given to you by the project reviewers, however we have not received a response from you. Without further news in the coming weeks, this discussion will be automatically closed in order to keep this forum clean and fresh 🌱 Thank you for your understanding

[github-actions[bot]]

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@ISIR-GuillaumeS
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