Robotic arm physics setup

I have industrial robotic arm model with hierarchy in which top-most parent contains whole mesh, and children represent controllable joints (i.e. each child should move all of its children down hierarchy). I want to have physics on it, so I added Rigidbody to each object in hierarchy (so they are nested now). I also connected all objects with FixedJoints, so they form a link from top to bottom of hierarchy. Now the problem is if I try to rotate particular joint, it rotates the whole arm (because there's no parenting in FixedJoints), but I need it to apply forces only to objects down hierarchy (i.e. I need FixedJoint to work only in one direction). I've tried setting massScale or connectedMassScale of FixedJoint to 0, but it doesn't help