mujoco

<mujoco model="Active adhesion example">
  <!--
    Adding some fluid viscosity to prevent the hanging sphere from jiggling too much.
  -->
  <option viscosity="1"/>

  <size memory="10M"/>

  <visual>
    <headlight diffuse=".2 .2 .2"/>
  </visual>

  <default>
    <joint damping=".3" axis="0 1 0"/>
    <geom type="box" friction=".5"/>
    <default class="wall">
      <geom rgba=".5 .5 .5 .4"/>
    </default>
    <default class="mechanical">
      <geom rgba=".5 .5 .8 1"/>
      <tendon rgba=".5 .5 .8 1"/>
    </default>
    <default class="active_adhesion">
      <!--
        Geoms in the active_adhesion class have a detection gap of 1cm, leading to a layer of
        inactive contacts at a 1cm distance from the surface. The contacts can be used by the
        adhesion actuator, but no contact forces are generated in the gap zone.
      -->
      <geom rgba=".8 .5 .5 1" gap=".01"/>
    </default>
    <default class="object">
      <geom rgba=".5 .8 .5 1" density="100"/>
    </default>
  </default>

  <worldbody>
    <light pos="0.4 -.15 .6" dir="-1 .5 -1" diffuse=".7 .7 .7"/>
    <light pos="-.4 -.15 .6" dir="1 .5 -1" diffuse=".7 .7 .7"/>

    <body name="two crates">
      <joint name="conveyor" type="slide" damping="100" axis="1 0 0"/>
      <geom size=".2 .1 .01" pos="0 0 -.01"/>
      <geom size=".01 .08 .031" pos="-.19 0 .03" class="wall"/>
      <geom size=".01 .08 .031" pos="0  0 .03" class="wall"/>
      <geom size=".01 .08 .031" pos="0.19 0 .03" class="wall"/>
      <geom size="0.2 .01 .031" pos="0 -.09 .03" class="wall"/>
      <geom size="0.2 .01 .031" pos="0 0.09 .03" class="wall"/>
    </body>

    <body name="arm1" pos="-.1 0 .3" childclass="mechanical">
      <joint name="arm1"/>
      <geom type="cylinder" size=".015 .01" zaxis="0 1 0"/>
      <geom type="capsule" size=".01" fromto="0 0 0 -.12 0 -.07"/>
      <body name="arm2" pos="-.12 0 -.07">
        <joint name="arm2"/>
        <geom type="cylinder" size=".015 .01" zaxis="0 1 0"/>
        <geom type="capsule" size=".01" fromto="0 0 0 .12 0 -.07"/>
        <body name="4boxes" pos=".12 0 -.08" childclass="active_adhesion">
          <site name="force_sensor" group="3"/>
          <joint name="arm3" damping=".01" pos="0 -.03 0"/>
          <geom size=".015 .015 .01" pos="0.015 0.015 0"/>
          <geom size=".015 .015 .01" pos="0.015 -.015 0"/>
          <geom size=".015 .015 .01" pos="-.015 0.015 0"/>
          <geom size=".015 .015 .01" pos="-.015 -.015 0"/>
        </body>
      </body>
    </body>

    <body name="box" pos="-.1 0 .05">
      <freejoint/>
      <geom size=".05 .05 .05" class="object"/>
    </body>

    <body name="winch" pos="-.01 0 .35" childclass="mechanical">
      <joint name="winch" damping="1"/>
      <geom type="cylinder" size=".015 .01" zaxis="0 1 0"/>
      <geom type="capsule" size=".01" fromto="0 0 0 .1 0 .05"/>
      <site name="anchor" pos=".1 0 .04"/>
    </body>
    <site name="pulley" pos=".1 0 .32"/>
    <site name="hook_left" pos=".08 0 .3"/>
    <site name="hook_right" pos=".12 0 .3"/>
    <body name="sphere" pos=".1 0 .2" childclass="active_adhesion">
      <freejoint/>
      <!--
        The composite balls in the crate have only 3 linear DoFs with condim=1, effectively
        frictionless point particles. In order to make them stick to the sphere we give the sphere
        priority 2, to force condim=3.

        Also note the sphere has a gap of 3cm as opposed to the 1cm of the arm box.
      -->
      <geom type="sphere" size=".03" priority="2" gap=".03"/>
      <site name="pin_left" pos="-.025 0 .025"/>
      <site name="pin_right" pos=".025 0 .025"/>
    </body>

    <replicate count="4" offset=".025 0 0">
      <replicate count="4" offset="0 .025 0">
        <replicate count="4" offset="0 0 .025">
          <body pos=".06 -.04 .05">
            <joint type="slide" axis="1 0 0"/>
            <joint type="slide" axis="0 1 0"/>
            <joint type="slide" axis="0 0 1"/>
            <geom type="sphere" size=".012" rgba=".5 .8 .5 1" solref=".005 1" condim="1"/>
          </body>
        </replicate>
      </replicate>
    </replicate>
  </worldbody>

  <equality>
    <joint joint1="arm1" joint2="arm2" polycoef="0 -.5 0 0 0"/>
    <joint joint1="arm3" joint2="arm1"/>
  </equality>

  <!--
    By using divisor=3 in the pullies we increase the distance by which the hanging sphere moves
    relative to the motion of the winch arm. One should imagine a double spindle with two radii that
    creates a ratio of 1.5 between the motion of tendon before the pullies and after the pullies.
    (1.5 rather than 3 because the tendon has 2 branches and the length is split between them)
  -->
  <tendon>
    <spatial range="0 .19" limited="true" solreflimit=".01 2" class="mechanical">
      <site site="anchor"/>
      <site site="pulley"/>
      <pulley divisor="3"/>
      <site site="pulley"/>
      <site site="hook_left"/>
      <site site="pin_left"/>
      <pulley divisor="3"/>
      <site site="pulley"/>
      <site site="hook_right"/>
      <site site="pin_right"/>
    </spatial>
  </tendon>

  <actuator>
    <position name="conveyor" joint="conveyor" ctrlrange="-.2 .2" ctrllimited="true" kp="400"/>
    <position name="arm" joint="arm2" ctrlrange="-.8 1" ctrllimited="true" kp="10"/>
    <adhesion name="adhere_arm" body="4boxes" ctrlrange="0 1" gain="5"/>
    <position name="winch" joint="winch" ctrlrange="-.7 .5" ctrllimited="true" kp="10"/>
    <adhesion name="adhere_winch" body="sphere" ctrlrange="0 1" gain="5"/>
  </actuator>

  <sensor>
    <force site="force_sensor"/>
  </sensor>
</mujoco>