rostopic shows /imu message, but topic monitor shows "unknown"

edit

Hi everyone,

I have been looking for a solution since days now, going through every topics possible here I think.

TLDR: The terminal shows me the data coming from topic /imu but rqt doesn't, and it's impossible to create subscribers to this topic.

I am using ros2 foxy and Gazebo 11, and trying to set up a simulation environment using the turtlebot3.

I'll be using the example files to demonstrate my issue. I then start a gazebo simulation using:

ros2 launch turtlebot3_gazebo empty_world.launch.py

Then, I launch rqt in another terminal.

When I ask to show the data in the terminal, it works

ros2 topic echo /imu
header:
  stamp:
    sec: 11
    nanosec: 890000000
  frame_id: imu_link
orientation:
  x: 0.00029183817507114263
  y: 0.003497131776253974
  z: 0.0005251101507266362
  w: 0.9999937045597582
orientation_covariance:
- 0.0
- 0.0
- 0.0
- 0.0
- 0.0
- 0.0
- 0.0
- 0.0
- 0.0
angular_velocity:
  x: -0.00018927557211639842
  y: -0.00014382556517471507
  z: -5.107971493553278e-05
angular_velocity_covariance:
- 4.0e-08
- 0.0
- 0.0
- 0.0
- 4.0e-08
- 0.0
- 0.0
- 0.0
- 4.0e-08
linear_acceleration:
  x: -0.06601479616987073
  y: -0.006337202558406336
  z: 10.136475698659266
linear_acceleration_covariance:
- 0.00028900000000000003
- 0.0
- 0.0
- 0.0
- 0.00028900000000000003
- 0.0
- 0.0
- 0.0
- 0.00028900000000000003
---

But on rqt I can see this :

The problem is not about the type sensor_msgs, since for joint_states it's showing data.

I also noticed that when I try to create a simple subscriber node to the topic /imu, the node wasn't receiving any message from the topic. It seems the callback function is never called. That is at least what I think is happening according to my investigations: I ask to print in the terminal when the callback function is called, and it never happens.

The code of the subscriber:

import rclpy
from rclpy.node import Node

from sensor_msgs.msg import Imu


class IMUSubscriber(Node):

    def __init__(self):
        super().__init__('imuSubscriber')
        self.subscription = self.create_subscription(
            Imu,
            'imu',
            self.listener_callback,
            10)
        self.subscription  # prevent unused variable warning

    def listener_callback(self, msg):
        self.get_logger().info('nanosec: %d' %(msg.header.stamp.nanosec))


def main(args=None):
    rclpy.init(args=args)

    imuSubscriber = IMUSubscriber()

    rclpy.spin(imuSubscriber)

    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    imuSubscriber.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()

The debug messages from the terminal:

[DEBUG] [1599130457.222676544] [rcl]: Service initialized
[DEBUG] [1599130457.485559162] [rcl]: Initializing subscription for topic name 'imu'
[DEBUG] [1599130457.485710324] [rcl]: Expanded topic name '/imu'
[DEBUG] [1599130457.487330246] [rcl]: Subscription initialized
[DEBUG] [1599130457.488600191] [rcl]: Initializing wait set with '1' subscriptions, '2' guard conditions, '0' timers, '0' clients, '6' services
[DEBUG] [1599130457.488753062] [rcl]: Waiting without timeout
[DEBUG] [1599130457.488787299] [rcl]: Timeout calculated based on next scheduled timer: false
[DEBUG] [1599130457.488815255] [rcl]: Guard condition in wait set is ready
[DEBUG] [1599130457.489213867] [rcl]: Initializing wait set with '1' subscriptions, '2' guard conditions, '0' timers, '0' clients, '6' services
[DEBUG] [1599130457.489320660] [rcl]: Waiting without ...