Odometry topic help

[r.malmvig]

Hi there

I’ve been struggling for a while to recieve data from the /odom topic. When I subscribe I get a ton of data, and what I really want is a specific data. How do I retrieve this data?
Also how do I find the files with Odometry so I can learn how more about the topic?

Best, Rasmus MT

[girishkumar.kannan]

Hi @r.malmvig ,

Welcome to the Community!

Do a rostopic info /odom and check if the messages are of type nav_msgs/Odometry.

If they are, then do rosmsg show nav_msgs/Odometry.

If they look like following:

std_msgs/Header header
  uint32 seq
  time stamp
  string frame_id
string child_frame_id
geometry_msgs/PoseWithCovariance pose
  geometry_msgs/Pose pose
    geometry_msgs/Point position
      float64 x
      float64 y
      float64 z
    geometry_msgs/Quaternion orientation
      float64 x
      float64 y
      float64 z
      float64 w
  float64[36] covariance
geometry_msgs/TwistWithCovariance twist
  geometry_msgs/Twist twist
    geometry_msgs/Vector3 linear
      float64 x
      float64 y
      float64 z
    geometry_msgs/Vector3 angular
      float64 x
      float64 y
      float64 z
  float64[36] covariance

If you store /odom message as odom_msg.
You can retrieve poses as odom_msg.pose.pose.position.x, quaternion as odom_msg.pose.pose.orientation.x, instantaneous linear velocity as odom_msg.twist.twist.linear.x and instantaneous angular velocity as odom_msg.twist.twist.angular.x.
If you are using python it is odom_msg.pose.pose.position.x
If you are using c++ it is odom_msg->pose.pose.position.x

Hope I helped. Let me know if I did.

Regards,
Girish

[r.malmvig]

Hi Girish,

Thanks a lot for your reply. Somehow I don’t manage to make it happen. This is my code. Where do I need to implement?

import rclpy
# import the ROS2 python libraries
from rclpy.node import Node
# import the Twist module from geometry_msgs interface
from geometry_msgs.msg import Twist
# import the LaserScan module from sensor_msgs interface
from sensor_msgs.msg import LaserScan
from nav_msgs.msg import Odometry
# import Quality of Service library, to set the correct profile and reliability in order to read sensor data.
from rclpy.qos import ReliabilityPolicy, QoSProfile


class TopicsQuizNode(Node):

    def __init__(self):
        # Initialize the publisher
        super().__init__('topics_quiz_node')
        self.publisher_ = self.create_publisher(Twist, 'cmd_vel', 10)
        self.subscriber1 = self.create_subscription(
            LaserScan,
            '/scan',
            self.listener_callback1,
            QoSProfile(depth=10, reliability=ReliabilityPolicy.BEST_EFFORT))
        self.subscriber2 = self.create_subscription(
            Odometry,
            '/odom',
            self.listener_callback2,
            QoSProfile(depth=10, reliability=ReliabilityPolicy.BEST_EFFORT))
        self.laser_forward = 0
        self.odom_data = 0
        timer_period = 0.5
        self.cmd = Twist()
        self.timer = self.create_timer(timer_period, self.timer_callback)

    def listener_callback1(self, msg1):
        self.laser_forward = msg1.ranges[359]

    def listener_callback2(self, msg2):
        self.odom_data = msg2

    def timer_callback(self):
        self.cmd.linear.x = 0.5
        self.get_logger().info('I receive: "%s"' %
                               str(self.odom_data))
        # Publish the message to the topic
        self.publisher_.publish(self.cmd)
        # Display the message on the console
        self.get_logger().info('Publishing: "%s"' % self.cmd)


def main(args=None):
    # initialize the ROS communication
    rclpy.init(args=args)
    # declare the node constructor
    topics_quiz_node = TopicsQuizNode()
    # pause the program execution, waits for a request to kill the node (ctrl+c)
    rclpy.spin(topics_quiz_node)
    # Explicity destroy the node
    topics_quiz_node.destroy_node()
    # shutdown the ROS communication
    rclpy.shutdown()


if __name__ == '__main__':
    main()

[girishkumar.kannan]

@r.malmvig ,

Sure…
I see from your code that the /odom callback is using listener_callback2 (also use a distinct name please! make it odom_callback or similar name).
Anyways, inside your listener_callback2(self, msg2) replace code like this:

def listener_callback2(self, msg2):
    position = msg2.pose.pose.position
    orientation = msg2.pose.pose.orientation
    (posx, posy, posz) = (position.x, position.y, position.z)
    (qx, qy, qz, qw) = (orientation.x, orientation.y, orientation.z, orientation.w)
    # similarly for twist message if you need
    return None

Regards,
Girish

PS: You never tried to do what I said! Please take time to explore and try out or else learning becomes difficult.