mujoco

<mujoco>
  <!--  linear dynamical system with fixed, anaylticaly computable derivatives  -->
  <default>
    <joint axis="1 0 0" damping=".01" stiffness=".1"/>
    <geom type="box" size=".05 .05 .05"/>
  </default>

  <option>
    <flag gravity="disable"/>
  </option>

  <worldbody>
    <light pos="0 0 1"/>
    <body>
      <joint name="joint0"/>
      <geom/>
      <body pos=".15 0 0">
        <joint name="joint1"/>
        <site name="accelerometer" pos="0 0 -1"/>
        <geom/>
        <body pos=".15 0 0">
          <joint/>
          <geom/>
        </body>
      </body>
    </body>
  </worldbody>

  <actuator>
    <motor joint="joint0" ctrllimited="true" ctrlrange="-1 1"/>
    <motor joint="joint1" ctrllimited="true" ctrlrange="-1 1"/>
  </actuator>

  <sensor>
    <jointvel joint="joint1"/>
    <jointpos joint="joint0"/>
    <accelerometer site="accelerometer"/>
  </sensor>
</mujoco>