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Transforming in combined tree

asked 2019-03-08 19:54:17 -0500

sisko gravatar image

updated 2019-03-09 03:54:58 -0500

I have a task of using a static ur5 manipulator to detect and point at a mobile turtlebot. I am a beginner and I'm told transforms is the solution I want. (Please comment if you disagree).

I have a ur5 and a turtlebot3 spawned into a an empty gazebo world.

I have a static transform publisher, shown below, in my launch file which I use to combine the tf trees of both robots.

<node pkg='tf2_ros' type='static_transform_publisher' name='ur5_turtlebot3_transform' args='0 0 0 0 0 0 1 world odom'/>

So the merged tf tree is as follows: image description

The green bordered section on the right of the image is the turtlebot3 tf tree. Everything else is the ur5 tf tree. As my static transform code shows, I attached the turtlebot tree to the ur5 tree at the world node.

Further down the ur5 tree I highlighted in red the wrist_3_link node which is the node of origin for my attempted transform. The destination node is the base_footprint node, the second and last node on the turtlebot3 side of the tree.

However, my can_transform attempts at return 0 (false). So as you would expect lookup_transforms errors:

def __init__(self):

    # declarations
    self.robot = moveit_commander.RobotCommander()
    self.scene = moveit_commander.PlanningSceneInterface()
    self.manipulator = moveit_commander.MoveGroupCommander('ur5_bot')

    self.tf_buffer = tf2_ros.Buffer()
    self.tf_listener = tf2_ros.TransformListener(self.tf_buffer)

        # transform_stamped = tf_buffer.lookup_transform('wrist_3_link', 'base_footprint', rospy.Time())
        transform_stamped = self.tf_buffer.can_transform('wrist_3_link', 'base_footprint', rospy.Time())
    except (tf2_ros.LookupException, tf2_ros.ConnectivityException, tf2_ros.ExtrapolationException) as e:

And, just to test, I tried a can_transform between nodes entirely on the ur5 side of the tree, i:e base_link to world, but still the result was false.

I'm not sure what I'm missing and/or doing wrong. I would appreciate some help.

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1 Answer

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answered 2019-03-09 06:48:27 -0500

You're 99% of the way there, there is just one detail of how transform listeners work you're missing.

You cannot create a transform listener then query it for transforms straight away, you need to give it a second or more otherwise it will fail to find a transform as in your case. To fix this you can add a one second sleep after you create the listener object then this should start working.

The reason is that the tf listeners needs to build up a buffer of tf messages before it is aware of all the transforms and is able to correctly answer tf queries.

Hope this helps.

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Thank you buddy ;-)

sisko gravatar image sisko  ( 2019-03-09 08:30:40 -0500 )edit

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Asked: 2019-03-08 19:54:17 -0500

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Last updated: Mar 09 '19