Question

I have set some fixed position and orientation (pose) values to PR2's headplateframe in Rviz, trying to transform pose from headplateframe to base_link frame and save the values in position and quaternion arrays. But instead of going to try block, the except block is being executed, anyone can help? What is the bug here?

I have edited my code, so here is the new code:

  pose1 = PoseStamped()
  pose1.header.stamp = rospy.Time.now()
  pose1.header.frame_id = "head_plate_frame"
  pose1.pose.position.x = 0.7
  pose1.pose.position.y = -0.05
  pose1.pose.position.z = 1.1
  pose1.pose.orientation.x = 0.3
  pose1.pose.orientation.y = 0.3
  pose1.pose.orientation.z = 0.2
  pose1.pose.orientation.w = 0.2


  listener = tf.TransformListener()



  while not rospy.is_shutdown():
    try:
      now = rospy.Time.now()
      listener.waitForTransform('/base_link','/head_plate_frame',now, rospy.Duration(10.0))
      (position, quaternion) = listener.lookupTransform('/base_link', '/head_plate_frame', now)

    except (tf.LookupException, tf.ConnectivityException, tf.ExtrapolationException):
      traceback.print_exc()


  pose_source = geometry_msgs.msg.Pose()
  pose_source.orientation.x = quaternion[0]
  pose_source.orientation.y = quaternion[1]
  pose_source.orientation.z = quaternion[2]
  pose_source.orientation.w = quaternion[3]
  pose_source.position.x = position[0]
  pose_source.position.y = position[1]
  pose_source.position.z = position[2]

Here is the exception message;

Traceback (most recent call last):
  File "/home/tawfiq/catkin_ws/src/pr2_moveit_tutorials/src/move_group_python_interface_tutorial.py", line 128, in <module>
    move_group_python_interface_tutorial()
  File "/home/tawfiq/catkin_ws/src/pr2_moveit_tutorials/src/move_group_python_interface_tutorial.py", line 71, in move_group_python_interface_tutorial
    listener.waitForTransform('/base_link','/head_plate_frame',now, rospy.Duration(10.0))
  File "/opt/ros/indigo/lib/python2.7/dist-packages/tf/listener.py", line 76, in waitForTransform
    raise tf2_ros.TransformException(error_msg or "no such transformation: \"%s\" -> \"%s\"" % (source_frame, target_frame))
tf2.TransformException: Lookup would require extrapolation into the past.  Requested time 1548633924.249253988 but the earliest data is at time 1548633924.770589113, when looking up transform from frame [head_plate_frame] to frame [base_link]

Asked by Tawfiq Chowdhury on 2019-01-27 00:31:39 UTC

Answer 1

It is true that a more specific error message would help, although I can see the problem from looking at your source code.

You're creating the transform listener object immediately before querying it for a transform, this will never work. The transform listener object needs time to collect a buffer full of transform messages before it will be able to respond to requests. It is recommended that a single tf.TransformListener is created for each node when they start up, and that object is used for all transform requests. A structure like this can be seen in the python tutorial.

Hope this helps.

Asked by PeteBlackerThe3rd on 2019-01-27 06:24:34 UTC

Comments

My point was more general than "we cannot help you". Printing "it doesn't work" in an exception handler isn't exactly helpful or good practice.

re: problem: that is indeed most likely the issue.

Asked by gvdhoorn on 2019-01-27 11:35:49 UTC

There seems to be no buffer in tf, it is tf2, are you referring to rate = rospy.Rate(10.0) ?

Asked by Tawfiq Chowdhury on 2019-01-27 18:12:38 UTC

Transforming pose with tf listener is not working in Rviz

Comments

Answers

Comments