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Hi Shawnysh,

As I think you are aware, the robot_state_publisher is used to publish the state of the robot to tf. It publishes the transforms between the links as a function of the joint states.

The sendTransform is used to publish a transform from odom to axis, which happens to be the root frame of the robot. As you can see in the code, this is done such that axis performs a circular motion around odom. If you then choose odom as your fixed frame (as noted in the last step for the linked tutorial) you will see the robot move around the origin. As robot_state_publisher only publishes the transforms between the links, they are also updated for a moving 'base' frame (in this case: axis).

Alternatively, if you would select axis as your fixed frame, the robot will make the same motions, but not move in a circular pattern.

I hope that clears things up.