Cosi119a

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<h3 id="introduction-to-nodes-and-topics">Introduction to Nodes and Topics</h3>

<ul>
  <li>ROS is a distributed operating system</li>
  <li>Meaning it controls potentially (and typically) multiple computers</li>
  <li>One of them is in the robot itself</li>
  <li>Almost always there is at least one more which is more powerful</li>
  <li>It could be your laptop or your web workspace which runs on our <code>rover</code> cluster</li>
  <li>The computers have to be on the same network</li>
</ul>

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<h3 id="communication-model">Communication model</h3>

<ul>
  <li>Message based</li>
  <li>Most important model is “publish/subscribe”</li>
  <li>publish a <strong>topic</strong>, subscribe to a <strong>topic</strong></li>
  <li>Many to many (Nodes can publish to many topics and subscribe to many topics)</li>
  <li>Topics have a name (e.g. /cmd_vel)</li>
  <li>Topics have an associated message</li>
  <li>Important to understand the difference between Topics and Messages</li>
  <li>Think of the topic as a function name, and the Message as argument list.</li>
</ul>

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<h3 id="ros-nodes">ROS Nodes</h3>

<ul>
  <li>A ROS application is designed as a collection of separate ROS Nodes that communicate with each other</li>
  <li>ROS Nodes are separate processes and they can run on the different computers</li>
  <li>ROS Nodes are nothing more than programs written in C++ or Python</li>
  <li>What makes them a “node” is that they communicate with other programs in very specific ways</li>
</ul>

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<h3 id="roscore">roscore</h3>

<ul>
  <li>roscore is a special program provided by ROS</li>
  <li>It is the “traffic cop” or “name service”</li>
  <li>It is the center of the ROS universe</li>
  <li>It is run on one of the computers</li>
  <li>Whenever a new ROS node starts, the first thing it does is to announce itself to roscore</li>
  <li>That way roscore always has a picture of the graph of ROS Nodes</li>
</ul>

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<h3 id="ros-topics">ROS topics</h3>

<ul>
  <li>ROS Nodes are separate processes (which can even be running on separate computers)</li>
  <li>A function, method or class created in one Node is invisible and cannot be used or called by another</li>
  <li>They are really really separate</li>
  <li>ROS Nodes talk to each other with ROS topics</li>
</ul>

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<h3 id="topics">Topics</h3>

<ul>
  <li>Topics have a name and a message type</li>
  <li>Two of the most common basic ones: <code>/cmd_vel</code> and <code>/odom</code></li>
  <li>Topics can be published and subscribed</li>
  <li>Which sends a receives messages of a certain message type</li>
  <li><code>cmd_vel</code>: sent to a robot to command it to move (locomotion)</li>
  <li><code>odom</code>:  received from a robot to report where (it thinks) it is</li>
</ul>

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<h3 id="message-types">Message Types</h3>

<ul>
  <li><code>Twist</code> message is used for <code>cmd_vel</code></li>
  <li>Expresses a velocity in linear and angular aspects
    <ol>
      <li>Note our robot can only move forward and backward (x axis)</li>
      <li>And it can only rotate around (z axis)</li>
      <li>Twist doesn’t say anything about where the robot (thinks it) is</li>
      <li>“cmd_vel” is the topic to <em>directly</em> command the base (people often refer to the robot as a whole as the base) to move. The message type for cmd_vel is Twist</li>
    </ol>
  </li>
</ul>

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<h3 id="publishing-and-subscribing">Publishing and Subscribing</h3>

<ul>
  <li>A node can “publish” a ROS topic</li>
  <li>Another node can “subscribe” to a ROS topic
    <ul>
      <li>The two ROS Nodes don’t know about each other</li>
      <li>A node can subscribe to a ROS topic that no one is publishing</li>
      <li>Two ROS Nodes can publish the same ROS topic with no one subscribing</li>
      <li>etc. etc. etc.</li>
    </ul>
  </li>
  <li>What is being published and subscribed?
    <ul>
      <li>messages</li>
      <li>A message is a data structure with named and typed fields</li>
      <li>A collection of messages already exists, e.g. Int32, which is message with one element, a 32 bit integer.</li>
    </ul>
  </li>
</ul>

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<h3 id="ros-topic-cmdvel">ROS Topic /cmd_vel</h3>

<ul>
  <li>A common ROS topic that you will become very familiar with</li>
  <li>When a node (for example one you write) publishes <code>cmd_vel</code> it is asking the robot to move</li>
  <li>The message will include speed and direction</li>
  <li>There is a node (e.g. <code>motor_control</code>) which subscribes to cmd_vel</li>
  <li><code>motor_control</code> comes with the robot and is run automatically</li>
  <li><code>motor_control</code> is connected to the motors.</li>
  <li><code>motor_control</code> exists and runs even if no other node is publishing <code>cmd_vel</code></li>
  <li>Or if more than one node publishes <code>cmd_vel</code></li>
  <li>They are totally decoupled!</li>
</ul>

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<h3 id="ros-topic-odom">ROS Topic /odom</h3>

<ul>
  <li>Another very common ROS topic that you will become very familiar with</li>
  <li>Odometry is the term for data coming from the wheels which allow the robot to estimate its location and speed.</li>
  <li>In this case, ROS is publishing the ROS topic <code>odom</code> constantly (say 30 times per second)</li>
  <li>Any node can subscribe to that ROS topic if it needs to know the current Odometry values</li>
  <li><a href="http://docs.ros.org/en/noetic/api/nav_msgs/html/msg/Odometry.html">Here is the message definition</a></li>
  <li><a href="https://claude.site/artifacts/e55a3b72-181e-4871-b274-e5db03f9c3d4">Here is a diagram</a></li>
</ul>

<pre><code class="language-python">
subs = rospy.Subscriber("/odom", Odometry, odom_callback)

def odom_callback(msg):
    x_position = msg.pose.pose.position.x
</code></pre>

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<h3 id="ros-topic-scan">ROS Topic /scan</h3>

<ul>
  <li>Another very common ROS topic that you will learn and use a lot</li>
  <li>/scan contains information coming from the LIDAR sensor on a robot</li>
  <li>It also is published constantly, say 30 times per second</li>
  <li>It contains data about the surrounding area and obstacles seen by the robot</li>
  <li>Any node can subscribe to that ROS topic if it needs to know the current values</li>
  <li><a href="http://docs.ros.org/en/melodic/api/sensor_msgs/html/msg/LaserScan.html">Here is the message definition</a></li>
  <li><a href="https://claude.site/artifacts/d1c6bc70-cf0a-4795-a0e5-35f2dffaca1d">Here is a diagram</a></li>
</ul>

<pre><code class="language-python">
def scan_callback(self, msg):
  indices = list(range(350, 360)) + list(range(0, 11))
  relevant_ranges = [msg.ranges[i] for i in indices]
  valid_ranges = [r for r in relevant_ranges if msg.range_min < r < msg.range_max]
  if valid_ranges:
    self.average_distance = sum(valid_ranges) / len(valid_ranges)

def main():
    rospy.init_node('scan_subscriber', anonymous=True)
    rospy.Subscriber("/scan", LaserScan, scan_callback)
    rospy.spin()
</code></pre>

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<h3 id="takeaways">Takeaways</h3>

<ul>
  <li>ROS: distributed operating system</li>
  <li>Node: One process which can run on any computer</li>
  <li>roscore: Single process which is the master controller</li>
  <li>Topic: Messages sent between nodes</li>
</ul>

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### Some Further details

* Queues
  * When you define your node as a Publisher you need to supply a queue_size
  * Messages that are published are kept in the queue until all subscribers have received them
  * Therefore if you publish at a rate faster than all the subscribers are processing, you will loose topics
  * A good starting queue_size is 10

* Latched Topics
  * New subscribers to a latched topic automatically get the last published message for that topic
  * Useful for topics which change rarely

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### Publishing to a Topic

* You will find all the book's examples in [cosi119_src]](https://github.com/campusrover/cosi119_src)
* If you just type in the example from the book it will not work
* See `~/catkin_ws/src/cosi119_src/src/topic_publisher.py`
  * Remember:
  * to do the necessary code generation and compilation you must run `catkin_make`
  * to run `roscore` in another terminal
* Run it and examine that it is publishing as expected, as follows:

<pre><code class="language-language-bash">
$ roscore

# open a new tab
$ roslanch cosi119_src pubsubexample.launch

# open a new tab
$ rostopic list
$ rostopic echo counter -n 5
</code></pre>

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### Checking That Everything Works as Expected

* Make sure you can see that topics are being published (see `rostopic`)
* Make sure you can see that topics are being subscribed (see console output)
* Try running `rqt_graph` to see the publisher/subscriber relationships for node and topics

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### Subscribing to a Topic

* See `samples` topic_subscriber.py
* It defines and runs a node whch subscribes to the counter, and prints it

<pre><code class="language-language-bash">
# shell commands
$ roscd samples
$ chmod +x scripts/topic_subscriber.py
$ rosrun samples topic_subscriber.py
</code></pre>