how to reduce the freedom degrees ?

[starimpact]

The feature, motivation and pitch

1. freejoint
   free joint has 6 degrees(x,y,z,rx,ry,rz), but in real robot, i can only get rx,ry,rz three information, how can i remove xyz?
2. constraint joints
   in my quad-bar linkage system, there is only one joint is actuated, the motion of rest joints are constrained by the linkage system. so practically only one freedom is significant, but mujoco gives 4(or 3 after using connect) freedoms. how to reduce 4 to 1?

Alternatives

No response

Additional context

No response