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README.md

ROS examples

The examples in this directory are for ROS robots. To be able to use them you should install the ROS bridge as specified in the LRP page. A separate UI will open giving the option to subscribe to topics and to allow publishing to topics.

Subscribing to topics

When subscribing to a topic, a variable name should be given. For example, you use joint when subscribing to /joint_states. The last message received on that topic will be held in that variable. In the last example, robot joint will give you the last message received. Messages are objects that can be read and written in the same style as in rospy (but without the dots between fields as this is Smalltalk not Python :-) ). For example, robot joint position will return the joint position array for the above example.

Publishing to topics

When publishing to a topic, a variable name should also be given. For example, to move a robot that listens to the /mobile_base/commands/velocity topic (with messages of type Twist) you can use velocity as a name. To publish to the topic, write a block to the variable. This block will be executed once, receiving one argument: an empty message of the correct type. The result of the block evaluation will be published on the topic, but only once. For example, to move the previous robot forward robot velocity: [:msg | msg linear x: 0.15]. Note that this message will be sent only once, so the robot will only move 1/10th of a second.

ActionLib

Fundamentally, Actionlib also uses publish-subscribe, and does this on a collection of topics. An overview of how this works is given in the documentation of actionlib. The used topics are subtopics of the action and are as follows:

  • goal - Used to send new goals to servers
  • cancel - Used to send cancel requests to servers
  • status - Used to notify clients on the current state of every goal in the system.
  • feedback - Used to send clients periodic auxiliary information for a goal.
  • result - Used to send clients one-time auxiliary information upon completion of a goal

For example, to use the navigation stack the root is typically a /move_base topic. Hence, to send navigation goals you should publish on the /move_base/goal topic, and result information is obtained from the /move_base/result topic. We have examples that use actionlib to do just that, for example see the Turtlebot Transporter.