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To answer questions, 1 & 2. The reason they have two slightly confusing descriptions in the names is that they are describing both the type of output from the controller to the joint as well as the input. For example the effort_controller/joint_position_controller receives position requests from ROS and outputs the efforts required to reach that position.

3) Whichever node is controlling your joints will be publishing a joint_state topic. These messages (may) include the efforts at each joint, this is an optional parameter so will depend on the node that you use.

4) Joint trajectories are specified as a sequence of positions, so you'll need a controller than accepts these. The output will depend on how you're controlling the actual robot, probably an effort controller.

Hope this helps.