Prismatic joint always goes to minimum position when given a lower position in Z than its current position.

[fnndyl]

Environment

• OS Version:
  Ubuntu 22.04
• Source or binary build?
  Harmonic 8.8.0

Description

• Expected behavior: A prismatic joint drives a platform up and down relative to a fixed link. The base link has a fixed joint with the world.
• Actual behavior: Publishing to the prismatic joint controller plugin works as intended when moving UPWARDS in z, however whenever a command is given that is lower than the current Z height of the platform, the platform descends to the minimum value for the prismatic joint and stays there no matter what further commands are given.

Steps to reproduce

1. Run this .sdf file with gz sim:

<?xml version="1.0" ?>
<sdf version="1.6">
  <world name="default">
    <scene>
      <ambient>0.4 0.4 0.4</ambient>
      <grid>false</grid>
    </scene>

    <!--              -->
    <!-- Illumination -->
    <!--              -->

    <light type="directional" name="sun">
      <cast_shadows>false</cast_shadows>
      <pose>5 5 5 0 0 0</pose>
      <diffuse>0.8 0.8 0.8 1</diffuse>
      <specular>0.2 0.2 0.2 1</specular>
      <attenuation>
        <range>1000</range>
        <constant>0.9</constant>
        <linear>0.01</linear>
        <quadratic>0.001</quadratic>
      </attenuation>
      <direction>-1 -1 -1</direction>
    </light>
 
    <!--        -->
    <!-- Models -->
    <!--        -->

    <model name="barge">
      <pose>0 0 0 0 0 0</pose>

      <!-- Fix To World -->
      <joint name="foundation" type="fixed">
        <parent>world</parent>
        <child>root</child>
      </joint>

      <!-- LINKS  -->

      <link name="root">
        <visual name="root_debug_visual">
          <geometry>
            <sphere>
              <radius>0.025</radius>
            </sphere>
          </geometry>
          <material>
            <ambient>0 0.5 0.5 1</ambient>
            <diffuse>0 0.8 0.8 1</diffuse>
            <specular>0.8 0.8 0.8 1</specular>
          </material>
        </visual>
      </link>

      <link name='base'>
        <pose relative_to='root'>0 0 0 0 0 0</pose>
        <visual name='base_visual'>
          <geometry>
            <box>
              <size>0.5 0.5 0.05</size>
            </box>
          </geometry>
          <material>
            <diffuse>1 1 1 1</diffuse>
            <specular>0.4 0.4 0.4 1</specular>
          </material>
        </visual>
        <collision name='base_collision'>
          <geometry>
            <box>
              <size>0.5 0.5 0.05</size>
            </box>
          </geometry>
          <surface>
            <friction>
              <ode>
                <mu>1.0</mu>
                <mu2>1.0</mu2>
              </ode>
            </friction>
            <bounce>
              <restitution_coefficient>0.1</restitution_coefficient>
              <threshold>1.0</threshold>
            </bounce>
          </surface>
        </collision>
      </link>

      <!-- JOINTS -->
      <joint name="heave" type="prismatic">
        <parent>root</parent>
        <child>base</child>
        <pose>0 0 0 0 0 0</pose>
        <axis>
          <xyz>0 0 1</xyz>
          <limit>
            <lower>-2</lower>
            <upper>2</upper>
            <velocity>1</velocity>
            <effort>500</effort>
          </limit>
          <dynamics>
            <damping>3</damping>
          </dynamics>
        </axis>
      </joint>

      <!-- PLUGINS -->
    
      <plugin
        filename="gz-sim-joint-position-controller-system"
        name="gz::sim::systems::JointPositionController">
        <joint_name>heave</joint_name>
        <use_velocity_commands>true</use_velocity_commands>
        <topic>heave_ctrl</topic>
      </plugin>

      <plugin
        filename="gz-sim-joint-state-publisher-system"
        name="gz::sim::systems::JointStatePublisher">
        <joint_name>heave</joint_name>
      </plugin>

    </model>
  </world>
</sdf>

2. Set the base position to higher than 0m in z:
   gz topic -t "/heave_ctrl" -m gz.msgs.Double -p "data: 0.5"

3. Set the base position to lower than 0.5m:
   gz topic -t "/heave_ctrl" -m gz.msgs.Double -p "data: 0"

4. The platform does not return to zero but sinks to -2m as defined as the minimum in the prismatic joint.

[fnndyl]

Have confirmed this doesnt work in Gazebo Ionic 9.1.0. Would love to know if this is simply something I have configured incorrectly.

[azeey]

I have confirmed this locally. It appears to be a bug only in the DART implementation. The example runs without any issues with the bullet-featherstone physics engine. I've also noticed that the issue seems to be related to the velocity limit. Removing the position limits and setting the effort limit to infinity shows that the low velocity limit is the culprit.

      <joint name="heave" type="prismatic">
        <parent>root</parent>
        <child>base</child>
        <pose>0 0 0 0 0 0</pose>
        <axis>
          <xyz>0 0 1</xyz>
          <limit>
            <velocity>1</velocity>
            <effort>inf</effort>
          </limit>
        </axis>
      </joint>

I suspect this is a bug in DART itself.

[fnndyl]

Thank you so much! Feel free to close this if its out of gz-sim scope, but that workaround will sort me out. Will link to this issue on the various other forums I raised this on.