Rotation of child rigid in articulations

[hyin]

Hi,

I have read through tutorials and examples about ArticulatedSystemMapping and am trying to model two articulated links with the child rigid having a different orientation:

<?xml version="1.0" ?>
<Node dt="0.01" gravity="0 -9.81 0" name="root">
    <VisualStyle displayFlags="showBehavior showVisual" />
    <RequiredPlugin name="SofaOpenglVisual"/>
    <BruteForceDetection />
    <DefaultContactManager />
    <DefaultPipeline />
    <MinProximityIntersection alarmDistance="1" contactDistance="0.5"/>
    <Node name="panda_arm">
        <EulerImplicitSolver name="cg odesolver" printLog="false"  rayleighStiffness="0.1" rayleighMass="0.1" />
        <CGLinearSolver iterations="100" name="linear solver" threshold="1e-20" tolerance="1e-20" />

            <MechanicalObject name="Articulations" template="Vec1d" position="0" />
            <Node name="arm_dofs">
                <MechanicalObject template="Rigid3d" name="DOFs" position=" 0 0 0       0 0 0 1  
                                                                            0 0 0.333   -0.7071068 0 0 0.7071068  
                                                                            " />

                <UniformMass template="Rigid3d" name="mass" vertexMass="0.1 0.1 [1 0 0,0 1 0,0 0 1]" />
                <FixedConstraint template="Rigid3d" name="fixOrigin" indices="0" />
                <ArticulatedSystemMapping input1="@../Articulations" output="@DOFs" />

            </Node>
            <ArticulatedHierarchyContainer />
            <Node name="articulationCenters">
                <Node name="articulationCenter1">
                    <ArticulationCenter parentIndex="0" childIndex="1" posOnParent="0 0.333 0" posOnChild="0 0 0" articulationProcess="2" />
                    <Node name="articulations">
                        <Articulation translation="0" rotation="1" rotationAxis="0 1 0" articulationIndex="0" />
                    </Node>
                </Node>
            </Node>

    </Node>
</Node>

My expectation is that the child rigid should be oriented with [-0.7071068 0 0 0.7071068] and the articulation is along its Y-axis. However, the result is that child rigid is only translated with [0 0 0.333] and its orientation is same its parent.

I tried to edit the positions of MechanicalObject in GUI and it indeed updates with the expected arrangement. But the result goes back to [0 0 0 1] by taking one simulation step. So my guess is the modeling of articulation might have some incompatibilities and the child is re-oriented when the solver resolves it. Anyone can provide some insights about this?

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