I am trying to use KDL to do some inverse dynamics calculations. I am a bit confused about the how the center of mass is defined. When I create the chain object from urdf or DH, I get a segment with joint and tip frame origin at the same location. Which is the offset from the previous joint. Now when I set the RigidBodyInertia object, is the center of mass location relative to the previous joint or the current joint in this segment? If I read the web site introduction, it would be relative to the previous tip frame which is located at the previous joint. But this is very unnatural way to define it.

Also according to the website introduction, the tip frame of a segment is at the next joint. Why they are always at the current joint? Thanks