URDF joint axis

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I am working on a project but I am stuck with determining the joint rotation axis with my own built model.

How do I determine the joint axis to let the robot rotate about a point? Just like the human knee joint.

Below shows part of my urdf file,

Please help me, thanks a lot!

Blockquote

name="thigh_r">
<inertial>
  <origin
    xyz="0.43096 -0.040854 0.12418"
    rpy="0 0 0" />
  <mass
    value="0.20" />
  <inertia
    ixx="0.0052338"
    ixy="3.2827E-07"
    ixz="-0.00046547"
    iyy="0.0061653"
    iyz="3.0465E-06"
    izz="0.00096993" />
</inertial>
<visual>
  <origin
    xyz="0 0 0"
    rpy="0 0 0" />
  <geometry>
    <mesh
      filename="package://exo1/meshes/thigh_r.STL" />
  </geometry>
  <material
    name="">
    <color
      rgba="0.79216 0.81961 0.93333 1" />
  </material>
</visual>
<collision>
  <origin
    xyz="0 0 0"
    rpy="0 0 0" />
  <geometry>
    <mesh
      filename="package://exo1/meshes/thigh_r.STL" />
  </geometry>
</collision>
name="hip_r"
type="revolute">
<origin
  xyz="0 0 0"
  rpy="0 0 -1.5708" />
<parent
  link="base_link" />
<child
  link="thigh_r" />
<axis
  xyz="0 0.01 0" />
<limit
  lower="-0.1"
  upper="0.4"
  effort="150"
  velocity="0.5" />

</joint>