Hey @vanisri
The FixedConstraint aims at fixing a point by projecting a zero delta of velocity between two time steps:
Therefore, if you specify an initial velocity in your MechanicalObject, like:
<MechanicalObject template="Rigid3d" name="myParticle"
position="0 0 0 0 0 0 1"
velocity="1 0 0 0 0 0"
showObject="1" />
Even if the DOF are fixed, a constant velocity will be kept.
Is this what you aim at doing?
Best wishes,
Hugo