Publish /tf in data ROS2 Dashing with python

asked 2020-04-28 07:05:42 -0500

svintissen
75 ●10 ●15 ●16

Hi.

I have created a simple .urdf file describing my "robot". I can change the position of the model in the "world_coordinate_system" running "ros2 run tf2_ros static_tranform_publisher" from terminal, describing a transform from "world" to "base_link". ("base_link" describes the model body)

Further I want to create a Node using python that updates the position of the model by publishing the tranform data from "world" to "base_link", but I cant find any good resources on how this is done in ROS2 Dashing.

So far I have tried using geometry_msgs.msg.TransformStamped(), then giving the transform as an input to tf2_msgs.msgTFMessage(), but I cant get it to work.

Do any of you have any examples on how this is done, or a better way to do it?

Thanks in advance, any help as well as links to tutorials or documentation is appriciated!

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Have you looked at the tf2 tutorials http://wiki.ros.org/tf2/Tutorials ?

DanielRobotics ( 2020-04-28 07:31:58 -0500 )edit

Yes, Ive used the tutorials as a guide, but Im having trouble adapting it to ROS2. Part of the problem Im having is getting the ROS2 equivalent of rospy.time.now().

svintissen ( 2020-04-28 07:38:57 -0500 )edit

ROS2 does not have rospy.time.now() but on your nodes you can use get_clock().now().

DanielRobotics ( 2020-04-28 08:01:25 -0500 )edit

When I try to use get_clock().now() I get the error "The stamp field must be a sub message of type Time". Why is this? Thanks!

svintissen ( 2020-04-28 08:06:42 -0500 )edit

You can use get_clock().now().to_msg() which will return the type of header.stamp

DanielRobotics ( 2020-04-28 08:58:41 -0500 )edit

Thanks, that worked. Still not able to broadcast the transform as the tf2_ros package is not available for ROS2. Any tips as for what I should do instead?

svintissen ( 2020-04-28 11:24:37 -0500 )edit

Have you installed the package? It's available in all the indexed versions of ROS 2 https://index.ros.org/p/tf2_ros/#eloq...

tfoote ( 2020-04-28 12:11:41 -0500 )edit

So it is not supported in Dashing, but in eloquent?

svintissen ( 2020-04-28 13:17:11 -0500 )edit

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answered 2020-04-30 02:30:18 -0500

svintissen
75 ●10 ●15 ●16

Python bindings for tf2_ros not available in ROS2 Dashing, they are added in Eloquent.

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Thanks for filling in the answer here. I've voted it up so you have enough karma to accept your own answer.

tfoote ( 2020-04-30 02:35:04 -0500 )edit

Hey, for everybody looking this up: It is actually possible in the way you discribe it. Put the "TransformStamped" into a "TFMessage" and publish it under the topic "/tf". You can check via "ros2 topic echo tf".

I was able to get it to work on ROS2 Dashing without the TransformBoradcaster python binding. The TransformBroadcaster is only a really shallow wrapper for publishing a list of transforms with a specific QOS afaik.

Karl ( 2021-01-13 10:36:41 -0500 )edit

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answered 2021-01-27 08:17:49 -0500

Karl
31 ●1 ●6 ●7

updated 2021-11-03 05:29:22 -0500

You have to use a list of TFMessage to create the TFMessage. This works for ROS2 Dashing:

from geometry_msgs.msg import TransformStamped
from tf2_msgs.msg import TFMessage

tf_msg = TransformStamped()
# ... do stuff with this transform

# Instead of this:
# broadcaster = TransformBroadcaster(self)
# broadcaster.sendTransform(tf_msg)

# Use this:
publisher = self.create_publisher(msg_type=TFMessage, topic="/tf")
tf_msg = TFMessage()
tf_msg.transforms = [transform_stamped_msg]
publisher.publish(tf_msg)

This essecially equivalent of the source code of TFBroadcaster in ROS2 Foxy. Tested with ROS2 Dashing and RVIZ. If you want to broadcast a static transform, make sure to use /tf_static and the right QOS TRANSIENT_LOCAL.

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Publish /tf in data ROS2 Dashing with python

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