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treat tf parent as child

asked 2020-05-26 04:05:40 -0500

Lennart gravatar image

updated 2020-05-27 09:00:53 -0500

Hi everybody, I am using the april_tag_ros package where I get a tf Frame usb_cam -->tag_2. While the camera is detecting a apriltag.

I want to have a tf-tree where the transform world --> camera1 equals tag_2 --> usb_cam.

I tried it with static_transform_publisher that world-->tag2 and usb_cam-->camera1 but than the connection between usb_cam and tag_2 get lost (means when I run rosrun tf tf_view there is no direkt connection between usb_cam and tag_2).

I read that for getting the inverse Frame from a --> b I can use tf tf_echo b a. So I tried that with static_transform_publisher but here I lost another connection.

Can somebody help me?

Greetings

Edit1: Probably I have to write a short skript which listens to the transform usb_cam-->tag_2 and publish the inverse as the transform world-->camera1. Does tf have an inverse-matrix-modul for c++? I read that this exist in tf2. Actually my packages are working with tf. What is easiest way to programm that?

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To start with, I don't understand why you want to connect tag_2 with the world, why the camera would be a child of the tag, and how a connection "gets lost". Surely your camera is still publishing the transform from the camera from the tag?

fvd gravatar image fvd  ( 2020-05-26 06:56:55 -0500 )edit

Is your camera fixed to a mobile base or is it also fixed to the world?

Roberto Z. gravatar image Roberto Z.  ( 2020-05-26 08:43:52 -0500 )edit

@roberto: To clear that. I want to have a moveable camera (means fixed to a mobile base). But the Tag_2 is fixed to the world. This should update every time when the transform usb_cam -->tag_2 is updated.

@fvd: "get lost" means when I run rosrun tf tf_view there is no direkt connection between usb_cam and tag_2. When I stop the static_transform_publisher the connection is there again.

Probably I have to write a short skript which listens to the transform usb_cam-->tag_2 and publishs the inverse as the transform world-->camera1.

Lennart gravatar image Lennart  ( 2020-05-27 08:40:22 -0500 )edit

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answered 2020-05-28 04:42:50 -0500

Roberto Z. gravatar image

To localize a robot base given a known position of an apriltag I would suggest these steps:

  1. Make sure you have a robot model with URDF that uses robot_state_publisher

  2. Launch a static transform broadcaster to publish a transform from map frame to tag frame

  3. Create a ROS node that subscribes to the apriltag detected tags

  4. When a apriltag is detected, get the transform of tag w.r.t. robot base

  5. Now calculate the transform of robot base w.r.t. tag (inverse transform of #4)

  6. Calculate the transform of robot base w.r.t. map (pose of robot base in the map coordinates)

  7. Broadcast the last transform (calculated in step #6)

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Asked: 2020-05-26 04:05:40 -0500

Seen: 196 times

Last updated: May 28 '20