mujoco

<mujoco>
  <compiler autolimits="true"/>
  <!--
  Adding a high fluid viscosity and using implicit integration for extra stabillity.
  Extra stabillity is required because the abstract "arm" model is not very realistic.
  -->
  <option viscosity="10" integrator="implicit">
    <flag contact="disable"/>
  </option>

  <statistic meansize=".05"/>

  <default>
    <default class="translation">
      <position kp="100" dampratio="1" ctrlrange="-.25 .25"/>
    </default>
    <default class="rotation">
      <!--
      Note that the rotational control range is purposefully limited to (-pi/2, pi/2) to avoid the
      documented instabillity near pi, which is due to taking quaternion differences.
      Increase this range to pi or bigger in order to see the instabillity.
      See here for more details https://mujoco.readthedocs.io/en/latest/XMLreference.html#actuator
      -->
      <position kp=".5" dampratio="1" ctrlrange="-1.571 1.571"/>
    </default>
    <!--
    Clamping the total actuator torque at the joints means that the motion produced by the Cartesian
    commands is achievable by individual joint actuators with the specified torque limits.
    See https://mujoco.readthedocs.io/en/latest//modeling.html#actuator-force-clamping
    -->
    <joint armature="1e-3" damping="1e-3" stiffness="1e-2" actuatorfrcrange="-1 1" actuatorgravcomp="true"/>
    <site type="box" size=".012 .012 .012" rgba=".7 .7 .8 1"/>
  </default>

  <worldbody>
    <light pos="0 0 2"/>
    <geom type="box" size=".25 .25 .01" pos="0 0 -.01"/>
    <site name="reference" pos="0 0 .25"/>
    <body name="arm" pos="-.25 .25 0" gravcomp="1">
      <joint axis="1 0 0"/>
      <joint axis="0 1 0"/>
      <geom type="box" size=".01" fromto="0 0 0 0 0 .25"/>
      <body pos="0 0 .25" gravcomp="1">
        <joint axis="0 1 0"/>
        <joint axis="0 0 1"/>
        <geom type="box" size=".01" fromto="0 0 0 .25 0 0"/>
        <body pos=".25 0 0" gravcomp="1">
          <joint axis="1 0 0"/>
          <joint axis="0 0 1"/>
          <geom type="box" size=".01" fromto="0 0 0 0 -.2 0"/>
          <body pos="0 -.2 0" gravcomp="1">
            <joint axis="1 0 0"/>
            <joint axis="0 0 1"/>
            <joint axis="0 1 0"/>
            <geom type="box" size=".01" fromto="0 0 0 0 -.05 0"/>
            <site name="end_effector" pos="0 -.05 0"/>
          </body>
        </body>
      </body>
    </body>
  </worldbody>

  <actuator>
    <position name="x"  site="end_effector" refsite="reference" gear="1 0 0 0 0 0" class="translation"/>
    <position name="y"  site="end_effector" refsite="reference" gear="0 1 0 0 0 0" class="translation"/>
    <position name="z"  site="end_effector" refsite="reference" gear="0 0 1 0 0 0" class="translation"/>
    <position name="rx" site="end_effector" refsite="reference" gear="0 0 0 1 0 0" class="rotation"/>
    <position name="ry" site="end_effector" refsite="reference" gear="0 0 0 0 1 0" class="rotation"/>
    <position name="rz" site="end_effector" refsite="reference" gear="0 0 0 0 0 1" class="rotation"/>
  </actuator>
</mujoco>