How to get sensor data over a topic with webots_ros2 plugin?

Question

I've installed webots 2023b and after I saw that a world with and inverted pendulum has been already shipped in the default installation directory, I decided to use that world for my project.

Basically I followed this tutorial and this tutorial to be able to publish sensor data directly from the urdf file itself. That means, that I create this file:

<?xml version="1.0" ?>
<robot name="Inverted Pendulum">
    <webots>  
        <device reference="hip" type="PositionSensor">
            <ros>
                <topicName>/hip</topicName>
                <alwaysOn>true</alwaysOn>
            </ros>
        </device>
        <device reference="horizontal position sensor" type="PositionSensor">
            <ros>
                <topicName>/horizontal_position</topicName>
                <alwaysOn>true</alwaysOn>
            </ros>
        </device>
        <plugin type="inverted_pendulum.device_driver.InvertedPendulum"/>
    </webots>
</robot>

but, even if, everything starts correctly (no errors or warnings in webots), doing a:

    $ros2 topic list

does not show any topic with the name /hip or /horizontal_position. It hangs forever waiting for data to be published.

The simulator is not paused, since I created a driver, which moves the cart forth and back.

Any idea?

The code for the world is the same. I changed only the name. But I add it here anyway, just in case.

#VRML_SIM R2023b utf8
    
EXTERNPROTO "https://raw.githubusercontent.com/cyberbotics/webots/R2023b/projects/objects/backgrounds/protos/TexturedBackground.proto"
EXTERNPROTO "https://raw.githubusercontent.com/cyberbotics/webots/R2023b/projects/objects/floors/protos/Floor.proto"

WorldInfo {
  info [
    "An example of hot to solve the Inverted Pendulum problem using a PID controller"
  ]
  title "Inverted Pendulum"
  basicTimeStep 16
  contactProperties [
    ContactProperties {
      material1 "robot_basis"
      material2 "floor"
      coulombFriction [
        0.2
      ]
    }
  ]
}
Viewpoint {
  orientation -0.0996069518072968 -0.03685053329082472 0.9943442529364971 3.666446119327704
  position 13.193129047100818 10.690115274872808 4.2889817843979205
  follow "robot:solid"
}
TexturedBackground {
}
Floor {
  size 1000 2
  appearance PBRAppearance {
    baseColorMap ImageTexture {
      url [
        "https://raw.githubusercontent.com/cyberbotics/webots/R2023b/projects/default/worlds/textures/checkered_marble.jpg"
      ]
    }
    roughness 1
    metalness 0
  }
}
Robot {
  rotation 0 0 1 3.14159
  children [
    SliderJoint {
      jointParameters JointParameters {
        axis 1 0 0
        dampingConstant 1.5
      }
      device [
        LinearMotor {
          name "horizontal_motor"
          maxForce 40
        }
        PositionSensor {
          name "horizontal position sensor"
        }
      ]
      endPoint Solid {
        translation 0 0 0.06
        children [
          DEF ROBOT_SHAPE Shape {
            appearance PBRAppearance {
              baseColor 0.2443427176317998 0.704051270313573 0.1756923781185626
              roughness 1
              metalness 0
            }
            geometry Box {
              size 0.3 0.1 0.1
            }
          }
          DEF HIP HingeJoint {
            jointParameters HingeJointParameters {
              position 0.000161402
              axis 0 1 0
              anchor 0 0 0.03
            }
            device [
              PositionSensor {
                name "hip"
              }
            ]
            endPoint DEF THIGH_BB Solid {
              translation 0 -0.061 0.33000000000000007
              rotation 0 1 0 0
              children [
                Shape {
                  appearance PBRAppearance {
                    baseColor 0.8496833752956435 0.07072556649118791 0.09393453879606317
                    roughness 1
                    metalness 0
                  }
                  geometry DEF THIGH_BOX Box {
                    size 0.05 0.02 0.6
                  }
                }
              ]
              boundingObject USE THIGH_BOX
              physics Physics {
                density -1
                mass 0.05
                centerOfMass [
                  0 0.061 -0.27
                ]
              }
            }
          }
          PointLight {
            attenuation 0 0 1
            intensity 5
            location 0 0 2
          }
        ]
        contactMaterial "robot_basis"
        boundingObject USE ROBOT_SHAPE
        physics Physics {
          density -1
          mass 1
        }
      }
    }
  ]
  name "Inverted_Pendulum"
  boundingObject Box {
    size 200 0.1 0.01
  }
  physics Physics {
    density -1
    mass 30
  }
  controller "<extern>"
}

Answer 1

There is no position sensor ROS plugin in webots_ros2.

There are two ways to get joint position data published:

• Use ros2_control. There is a ros2_control interface for webots_ros2 called webots_ros2_control (see usage example here). In your case, you will need to spawn joint_state_broadcaster which publishes joints state messages. This is generally a recommended way to interact with joints in ROS as you can achieve hard control loops, much better performance, and simpler sim2real transfer.
• Create a custom webots_ros2 plugin (see the tutorial). You can use it to create a joint state publisher and publish joints state messages.