mujoco

<mujoco>
  <visual>
    <global elevation="-20"/>
    <headlight ambient=".3 .3 .3" diffuse=".9 .9 .9"/>
  </visual>

  <default>
    <geom rgba="0.8 0.6 .4 1"/>
  </default>

  <worldbody>
    <body name="link1">
      <!-- inertia of link1 is 1.0 -->
      <joint name="link1" axis="0 -1 0"/>
      <geom type="capsule" size=".02" fromto="0 0 0 1 0 0" mass="0"/>
      <geom type="sphere" size=".1" mass="1" pos="1 0 0"/>
    </body>
    <body name="motor1">
      <!-- inertia of motor1 is 1/12 * m * (0.2^2 + 0.2^2) = 0.2
           see https://en.wikipedia.org/wiki/List_of_moments_of_inertia -->
      <joint name="motor1" axis="0 -1 0"/>
      <geom type="box" size=".1 .1 .1" mass="30" contype="0" conaffinity="0" rgba=".6 .4 .8 1"/>
    </body>

    <body name="link2" pos="1.5 0 0">
      <!-- armature from reflected inertia of motor1 is 0.2 * 3^2 = 1.8 -->
      <joint name="link2" armature="1.8" axis="0 -1 0"/>
      <geom type="capsule" size=".02" fromto="0 0 0 1 0 0" mass="0"/>
      <geom type="sphere" size=".1" mass="1" pos="1 0 0"/>
    </body>
  </worldbody>

  <equality>
    <!-- gear ratio between motor1 and link1 is 3:1 -->
    <joint joint1="motor1" joint2="link1" polycoef="0 3"/>
  </equality>

  <actuator>
    <position name="link1" joint="motor1" kp=".3" ctrlrange="-5 5"/>
    <!-- actuators are made equivalent by setting link2 gear to link1 gear ratio -->
    <position name="link2" joint="link2"  kp=".3" ctrlrange="-5 5" gear="3"/>
  </actuator>
</mujoco>