I am trying to convert a URDF file to an MJCF file but the problem when I use the MuJoCo ./compile program I get the error: XML Error: URDF body has multiple parents: Element 'joint', line 199, column 3 The robot uses a prismatic and revolute joint. So it has to be attached to multiple parents. there is a thread answering a similar problem http://www.mujoco.org/forum/index.php?threads/urdf-with-compile-cpp.3698/ but I didn't understand what is the meaning of equality constraints and how can I add them manually. can anyone please clarify more about the equality constraints and I would be thankful if there any examples of solving the closed-loop kinematic chain problem.
URDF and MJCF can't have multiple parents for the same child. To create a kinematic loop, you need to add the equality constraint between the two branches of the loop. http://mujoco.org/book/XMLreference.html#equality-connect This link addresses how to add equality constraint.