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You cannot have closed chains in a URDF model.

If I understand the model correctly, what I would do in your case is have the pistons as an active joint, and then make the ends of the pistons (top and bottom) mimic joints. You would have to derive the kinematics of the arm, and express the end (mimic) joint states with respect to the piston (active) joint state.

This way you will end up with an open chain which, due to the dependence between the joints, behave as a closed chain.

You cannot have closed chains in a URDF model.

If I understand the model correctly, what I would do in your case is have the pistons as an active joint, and then make the ends of the pistons (top and bottom) mimic joints. You would have to derive the kinematics of the arm, and express the end (mimic) joint states with respect to the piston (active) joint state.

This way you will end up with an open chain which, due to the dependence between the joints, behave behaves as a closed chain.