<?xml version="1.0"?>
<robot name="mira">
<link name="base_link">
<visual>
<origin rpy="0 0 0" xyz="0 0 0"/>
<geometry>
<cylinder radius="0.06" length="0.09"/>
</geometry>
</visual>
</link>
<link name="roll_M1_link">
<visual>
<origin rpy="0 0 0" xyz="0 0 0"/>
<geometry>
<cylinder radius="0.06" length="0.09"/>
</geometry>
</visual>
</link>
<joint name="roll_joint" type="revolute">
<parent link="base_link"/>
<child link="roll_M1_link"/>
<origin xyz="0.0023 0 -0.0005" rpy="0 0 0"/>
<limit lower="-0.2" upper="0.2" effort="0.1" velocity="0.005"/>
<axis xyz="1 0 0"/>
</joint>
</robot>
what does rpy,xyz stand for ? how is the number calculated ?