Read a position in a tf message

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Hi all,

1. I am using the universal_robot package and I would like to visualize the end-effector's position error in rqt_plot. I guess I need to compute it in a node of my own and publish it so I can have access to the topic in rqt_plot. Is that correct?

2. Now in order to start with I thought I would visualize component X of the effector in rqt_plot. So I had a look at the topics published and I could find topic /tf of type tf2_msgs/TFMessagewhich contains an array of transforms of type geometry_msgs/TransformStamped[] (itself containing the translation info a few levels below). This boils down to me trying to plot topic /tf/transforms[whatToPutHere?]/transform/translation/x. But I never can get my curve correctly displayed:

   • If whatToPutHere = blank (nothing, empty): ros_plot lets me add the topic to plot, but the curve value is 0
   • If whatToPutHere = 0, 1, 2... : ros_plot lets me add the topic to plot, but the curve value is 0
   • If whatToPutHere = ee_link (name of the effector's link): ros_plot does not let me add the topic to plot

Any idea of what goes wrong here?

Thanks,

Antoine.