Please help me understand origin in URDF

edit

Hi folks,

I have been following the book Programming Robots with ROS and have been struggling with how the origin tag means in URDF files.

In the chapter 18, there is a snippet of URDF as the following:

  <link name="base_link">
    <visual>
      <geometry>
        <cylinder length="0.05" radius="0.1"/>
      </geometry>
      <material name="silver">
        <color rgba="0.75 0.75 0.75 1"/>
      </material>
      <origin rpy="0 0 0" xyz="0 0 0.025"/>
    </visual>
  </link>

The origin tag xyz = 0 0 0.025 puzzles me. The books says the 0.025 is meant to "... offset in z the point of reference for the base link from its default in the cylinder's center to its bottom." What I don't understand is that if we need to push the origin to the bottom, shouldn't we use -0.25? Why would increase the z value make the origin go down instead of going up?

What's more perplexing is that, later in the joint, it seems using a positive z value in origin actually makes the origin go up:

  <joint name="hip" type="continuous">
    <axis xyz="0 0 1"/>
    <parent link="base_link"/>
    <child link="torso"/>
    <origin rpy="0 0 0" xyz="0.0 0.0 0.05"/>
  </joint>

Here my understanding is that the z value 0.05 is meant to push the origin of the joint to be top of the base link so the next link can start there.

Why does positive value in link's origin push origin downward but push origin upward in joints?