Question about frames' parent joint

[xiaosuyang]

Hello.
Using the example urdf ur5 robot and Print the parent joint and previous frame of each joint:

Why some joints' parent joint is itself but some joints not:

[xiaosuyang]

It seems that if the joint type is revolute,the parent joint is itself. If the joint type is "fixed",the parent joint is its predecessor

[MegMll]

Hello @xiaosuyang,

In pinocchio, "fixed" joint refers to a frame and not to real joint of the kinematic tree which is why the parent joint of the frame "ee_fixed_joint' is the previous joint that appeared in the tree, ie "wrist_3" and not itself.