About URDF model

Course Support URDF for Robot Modeling
1

Hi,
I want to ask a few topics that are not mentioned in detail in the URDF course.
1-Parameters such as joint xyz = 0.0023 0 -0.0005 are given according to what. So if I take xyz as 0 0 0, would I do something wrong? And what does minus mean here?
2-I could not understand the difference between rpy = 0 1.5708 0 and rpy = 0 -1.5705 0. So when I do both +1.5708 it gives the same result. Why is one minus and the other plus? Also, for the pitch joint, the roll and yaw value is zero and only the pitch value is -1.5708, for the yaw joint, the roll and yaw value is zero, and only the pitch value is 1.5708. Why we always make the pitch value in rpy 1.5708.
3-What are the parameters kp, kd, mu1 and mu2? How are these values determined?

Thanks in advance for your helps.

2

Hi,

So by parts:

  1. Those xyz values are given according to the frame axis of the mesh, link that is reffered to. It depends on where you put it. For exmple in joints it states the displacement of the child-link frame vs the parent-link frame.
  2. As for the minus and positiove it does make a difference, although depending on the geometry you wont see it. Butif you have a look at the TF frames in RVIZ you will see that the child frame will not be the same orientation. We change the pitch only because in that case is the one need to change. Is just to place the objects in the correct orientation. There are cases where using the roll would give the same result. Its just a preference of how URDFS are built.
  3. Kp, kd are the parameters that indicate the static (kp) and dynamic ( kd) mechanical deformation. A rubber element will deform more and therefore have a lower value than a stone object. M1 is the static friction and the m2 the dynamic friction. The lower this numbers the less friction there is.