Writing a Broadcaster (Python)

You can refer this tutorial as reference for Introduction to Tf2 which might help you understand basics of transforms. (It's official documentation from ROS)

Writing the broadcaster node in python:

import rclpy
from rclpy.node import Node
from tf2_ros import TransformBroadcaster
from geometry_msgs.msg import TransformStamped

class FixedFrameBroadcaster(Node):

    def __init__(self):

        super().__init__('sample_tf2_broadcaster')                              # node initialisation
        self.tf_broadcaster = TransformBroadcaster(self)                        # initializing transform broadcaster object
        self.timer = self.create_timer(0.1, self.broadcast_timer_callback)      # timer based function which iterates on defined time interval

    def broadcast_timer_callback(self):

        t = TransformStamped()
        t.header.stamp = self.get_clock().now().to_msg()                        # select transform time stamp as current clock time
        # frame IDs
        t.header.frame_id = 'base_link'                                         # parent frame link with whom to send transform
        t.child_frame_id = 'obj_1'                                              # child frame link from where to send transfrom
        # translation
        t.transform.translation.x = 0.0
        t.transform.translation.y = 2.0                                         # distance offset in Y axis of 2 units
        t.transform.translation.z = 0.0
        # rotation
        t.transform.rotation.x = 0.0
        t.transform.rotation.y = 0.0
        t.transform.rotation.z = 0.0
        t.transform.rotation.w = 1.0                                            # rotation 0 degrees

        self.tf_broadcaster.sendTransform(t)                                    # publish transform as defined in 't'


def main():
    rclpy.init()
    node = FixedFrameBroadcaster()
    try:
        rclpy.spin(node)
    except KeyboardInterrupt:
        pass

    rclpy.shutdown()

Examine the code:

  • Here we create a new transform, from the parent base_link to the new child obj_1. The obj_1 frame is 2 meters offset in y axis in terms of the base_link frame.

    t = TransformStamped()

t.header.stamp = self.get_clock().now().to_msg()
t.header.frame_id = 'base_link'
t.child_frame_id = 'obj_1'
t.transform.translation.x = 0.0
t.transform.translation.y = 2.0
t.transform.translation.z = 0.0

  • The orientation for the new child frame is at 0 degrees as we haven't changed the rotation part of code and kept it as-

    t.transform.rotation.x = 0.0
t.transform.rotation.y = 0.0
t.transform.rotation.z = 0.0
t.transform.rotation.w = 1.0

  • Finally broadcasting TF using sendTransform() method-

    self.tf_broadcaster.sendTransform(t)    # where t is defined above

References:

You can overview following example codes and documentation provided officially by ROS. It explains how to broadcast tf using various methods.

There are some other examples for C++ and static transfroms too.