Collision of a MechanicalObject with a shape

[Slykkey]

Hello guys,

I'm a beginner to programming on SOFA. I created a code in XML to make a shape of a specific object using the MechanicalObject and also added MeshTopology of lines. From what I have seen in other codes, using a collision model depending on the topology used seemed to work fine. I have attached the code here. When I run the simulation, the objects don't interact with each other and the particle used keeps falling without colliding. Any help that I can get will be really useful. Replacing this particle with a ball or a cube would also be useful if that is possible.

<?xml version="1.0"?>
<Node name="root" gravity="0 -9.81 0" dt="0.01" bbox="-3 -6 -3 3 3 3">

 	<RequiredPlugin name="BeamAdapter"/> <!-- Needed to use components [WireBeamInterpolation WireRestShape] -->

	<RequiredPlugin name="Sofa.Component.Collision.Detection.Algorithm"/> <!-- Needed to use components [BVHNarrowPhase BruteForceBroadPhase DefaultPipeline] -->
    	<RequiredPlugin name="Sofa.Component.Collision.Detection.Intersection"/> <!-- Needed to use components [LocalMinDistance] -->
    	<RequiredPlugin name="Sofa.Component.Collision.Geometry"/> <!-- Needed to use components [LineCollisionModel PointCollisionModel TriangleCollisionModel] -->
    	<RequiredPlugin name="Sofa.Component.Collision.Response.Contact"/> <!-- Needed to use components [DefaultContactManager] -->
    	<RequiredPlugin name="Sofa.Component.Constraint.Lagrangian.Correction"/> <!-- Needed to use components [LinearSolverConstraintCorrection] -->
    	<RequiredPlugin name="Sofa.Component.Constraint.Lagrangian.Solver"/> <!-- Needed to use components [LCPConstraintSolver] -->
	<RequiredPlugin name="Sofa.Component.Constraint.Projective"/> <!-- Needed to use components [FixedConstraint] -->
 	<RequiredPlugin name="Sofa.Component.LinearSolver.Direct"/> <!-- Needed to use components [BTDLinearSolver] -->
 	<RequiredPlugin name="Sofa.Component.ODESolver.Backward"/> <!-- Needed to use components [EulerImplicitSolver] -->
 	<RequiredPlugin name="Sofa.Component.StateContainer"/> <!-- Needed to use components [MechanicalObject] -->
 	<RequiredPlugin name="Sofa.Component.Topology.Container.Constant"/> <!-- Needed to use components [MeshTopology] -->
 	<RequiredPlugin name="Sofa.Component.Visual"/> <!-- Needed to use components [VisualStyle] -->
 	<RequiredPlugin name="Sofa.Component.LinearSolver.Iterative"/> <!-- Needed to use components [CGLinearSolver] -->  
  	<RequiredPlugin name="Sofa.Component.Mass"/> <!-- Needed to use components [UniformMass] -->  
  	<RequiredPlugin name="Sofa.Component.SceneUtility"/> <!-- Needed to use components [InfoComponent] -->  
  	<RequiredPlugin name="Sofa.Component.Topology.Container.Dynamic"/> <!-- Needed to use components [EdgeSetTopologyContainer, EdgeSetTopologyModifier, PointSetTopologyContainer, PointSetTopologyModifier] -->  
	<VisualStyle displayFlags="showBehaviorModels showCollisionModels hideBoundingCollisionModels showForceFields" />

	<DefaultAnimationLoop />
	<DefaultVisualManagerLoop />
	<LCPConstraintSolver mu='0.1' tolerance='1e-10' maxIt='1000' build_lcp='false' />
    	<DefaultPipeline draw='0' depth='6' verbose='1' />
    	<BruteForceBroadPhase name='N2' />
    	<BVHNarrowPhase />
    	<LocalMinDistance contactDistance='1' alarmDistance='5' name='localmindistance' angleCone='0.02' />
    	<DefaultContactManager name='Response' response='FrictionContactConstraint' />

	<Node name="BeamModel">
		<EulerImplicitSolver rayleighStiffness="1000" rayleighMass="0" printLog="false" />
		<BTDLinearSolver verbose="0"/>
		<MechanicalObject template="Rigid3d" name="DOFs" position="0 0 0 0 0 0 1  2 0 0 0 0 0 1  4 0 0 0 0 0 1  6 0 0 0 0 0 1  8 0 0 0 0 0 1  9 -0.5 0 0 0 -0.385 0.924"/> 
		<MeshTopology name="lines" lines="0 1 1 2 2 3 3 4 4 5" /> 
		<FixedConstraint name="FixedConstraint" indices="0" />
		<BeamInterpolation name="BeamInterpolation" radius="0.1" /> 
		<AdaptiveBeamForceFieldAndMass name="BeamForceField"  computeMass="1" massDensity="10"/>
		<Node name="CollisionModel">
            		<EdgeSetTopologyContainer name='collisEdgeSet' />
            		<EdgeSetTopologyModifier name='colliseEdgeModifier' />
            		<MechanicalObject name='CollisionDOFs' />
            		<LineCollisionModel proximity="0.0" />
        	</Node>
	</Node>  
	<Node name="particle1">
		<EulerImplicitSolver />
		<CGLinearSolver iterations="200" tolerance="1e-09" threshold="1e-09" />
		<MeshTopology name="points"/>
		<PointSetTopologyContainer name='collispointset'/>
		<PointSetTopologyModifier name='pointmodifier'/>
		<MechanicalObject template="Vec3d" name="particlefall" position="9 5 0  0 0 0 1" />
		<UniformMass totalMass="20" />
		<PointCollisionModel moving="0" simulated="0" />
	</Node>
</Node>

[bakpaul]

You want to use Langrangian-based constraints. For this to be computed correctly you need to use a FreeMotionAnimationLoop, because the DefaultAnimationLoop doesn't compute all the steps needed for the constraints to be computed.

Secondly, to solve correctly the constraints you'll also nee a LinearConstraintCorrection inside of every mechanical node (BeamModel1 and particle1).

[Slykkey]

Hello there, first of all, thank you so much for your response and the example you provided. I have to try it tomorrow so I will let you know then. Thanks again!

[Slykkey]

Hello @bakpaul , I tried using the freeanimationloop but when I tried using a lower Rayleighstiffness for the beam, it deforms and falls apart when I use the defaultanimationloop and this does not happen when I use the freeanimationloop. I also designed a spherical collision model which appears on the screen but does not fall with gravity. Do you have any help you can offer with this? I would appreciate any feedback.

[hugtalbot]

Hi @Slykkey

Note that Rayleighstiffness is a numerical damping. It has nothing to do with the mechanics/collision detection/response.
Make sure you checked all point of attention listed here: #4173

[Slykkey]

Hello @hugtalbot , I understand that, I just wanted to mention that in case of lower Rayleighstiffness, it has to deform under gravity and it does when I use the defaultanimationloop, but does not show any behaviour when I use freeanimationloop. I will look at the points listed in the example you mentioned.

[bakpaul]

As @hugtalbot told you, the RayleighStiffness is related to the Rayleigh damping following :
$B = \alpha M + \beta K$, where $\alpha$ is the Rayleigh stiffness and $\beta$ the Rayleigh mass. This is multiplied by the velocity in the differential equation you are solving, which means that it behaves such as a real damping but approximated.
Here by having a high RayleighStiffness you increase the internal damping, between nodes, meaning that you reduced a lot the deformation dynamics by slowing the relative motion between nodes.
Having a higher RayleighMass would more affect the velocity of each point but not the deformation itself, $M$ being mostly diagonal.

This being said, it is not strange for us that you have a less stiff beam when you reduce the Rayleigh stiffness. Again, this is artifical and thus should only be used to stabilize the simulation, and not parametrize it.

For your specific problem, it would be good to have a look at your updated file to understand what happen. Firstly, I havent told you but the LinearConstraintCorrection requires a linear solver to compute the matrix it needs. You've got one int the beam (BTDLinearSolver) but not in the particle. Here you only have a CGLinearSolver which is iterative and is not sufficient to compute the matrix. Because you only have one point in your object you can use a UncoupledConstraintCorrection which should normally behave the same as adding a linear solver and a LinearConstraintCorrection.

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@Slykkey
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