Can't read scan topic (LaserScan) in Turtlebot3 using Gazebo, ROS2 (foxy) and rclpy

EingeoelterBolt · April 4, 2021, 10:06pm

Hi guys,

for a university project I started working with Turtlebot3, ROS2 (foxy), rclpy and gazebo. While studying them, I found a lot of basic examples for ROS1 (noetic), e.g. how to subscribe and read joint_states and the scan (LaserScan) topic. I tried to convert these examples to rclpy and it worked well for reading the joint_states but it failed for reading the scan topic. To me, it looks like Turtlebot3 hasn’t fully updated the packages and the dependencies for ROS2 or rclpy (yet).

I created two videos to show you the problem:

With ROS1 (noetic), rospy and Ubuntu 20.04 the readings are working:

With ROS2 (foxy), rclpy and Ubuntu 20.04 the readings for the joint_states are working but fail when I try to read the LaserScan. In rqt for example, I can read data for the joint_states but no data is shown for the LaserScan:

Now, my Question is:
Is there a solution to read the scan topic (LaserScan) with rclpy?

willson · April 5, 2021, 5:10am

Hi,
In ROS2, QoS setting could cause this issue.
Please see the issue thread below.

github.com/ROBOTIS-GIT/turtlebot3_simulations

Unable to read LaserScan message from /scan

opened 06:33AM - 21 Aug 19 UTC

sgawalsh

question in progress

I used [this](http://emanual.robotis.com/docs/en/platform/turtlebot3/ros2/) tuto…rial to set up my workspace and am trying to implement a q-learning environment using ROS2. I have written a simple subscriber to the /scan topic which has publisher /turtlebot3_laserscan, but do not get any results when running the script alongside the `ros2 launch turtlebot3_gazebo turtlebot3_world.launch.py` command. scanSub.py import rclpy from rclpy.qos import QoSProfile from sensor_msgs.msg import LaserScan def chatter_callback(msg): #print('Heard {0}'.format(str(msg))) print('heard it') def main(args=None): rclpy.init() node = rclpy.create_node('scanSubcriber') sub = node.create_subscription(LaserScan, 'scan', chatter_callback, QoSProfile(depth=10)) assert sub # prevent unused warning while rclpy.ok(): rclpy.spin_once(node) if __name__ == '__main__': main() However, when I run my own simple publisher to publish custom scan messages, the messages are recieved. testPub.py from time import sleep from sensor_msgs.msg import LaserScan import rclpy from rclpy.qos import QoSProfile def main(args=None): rclpy.init() node = rclpy.create_node('scanPublisher') chatter_pub = node.create_publisher(LaserScan, 'scan', QoSProfile(depth=10)) msg = LaserScan() i = 1 while True: msg.angle_min = float(i) i += 0.5 print('Publishing: "{0}"'.format(msg.angle_min)) chatter_pub.publish(msg) sleep(1) if __name__ == '__main__': main() The `ros2 node info /turtlebot3_laserscan` command gives the message type of /scan as sensor_msgs/msg/LaserScan which I believe is the type I am using on the custom subscriber and publisher. My environment is - Ubuntu 18.04.3, - ros2 Dashing

EingeoelterBolt · April 5, 2021, 9:53pm

Hey Will,

you helped me a lot and I wish I had asked a couple days earlier which probably would have saved me a lot of time trying to figure out the problem

So, thank you very much!

If someone has the same issue and is interested in a working mvp, here is the working code (using Juypter Notebook):

import rclpy
from rclpy.node import Node
from sensor_msgs.msg import LaserScan
from rclpy.qos import qos_profile_sensor_data

def callback(msg):
    print(msg)

rclpy.init()
# subscriber
node = rclpy.create_node("sub_laser")
# qos_profile_sensor_data allows packet loss
scan_sub = node.create_subscription(LaserScan,"/scan", callback ,qos_profile=qos_profile_sensor_data) 
rclpy.spin(node)

willson · April 8, 2021, 7:26am

@EingeoelterBolt
Thank you very much for sharing your solution with community.
I’ll refer this thread whenever there’re similar inquiries.
Happy coding!