* **Reading: [A wall-following algorithm based on dynamic virtual walls for mobile robots navigation](/content/topics/documents/wallfollower.pdf)** * **Reading: <a href="/content/topics/robotics/robotics_notes/pid_control_intro/">PID Control</a>** *Reminder: Readings are your responsibility. You will be expected to come to class prepared, having read the material, and ready to participate in the discussion* <slide_break></slide_break> ## Logistics * PA Basic Control * <a href="/content/topics/robotics/robotics_pas_2/10_pa_basic_control/">PA Basic Control</a> * Slow review of core program loop * Slow review of my_odom * PA Wall follow * <a href="/content/topics/robotics/robotics_pas_2/20_pa_lidar_wall_follow/">PA Lidar wall follow</a> * Let's look at starter code <slide_break></slide_break> <h2 id="ros-best-practices-and-patterns">ROS Best Practices and Patterns</h2> <ul> <li>These are best practices not ironclad rules</li> <li>Rules of thumb are good when you are starting</li> <li>When you know more you will know when to break them :)</li> </ul> <slide_break></slide_break> <h3 id="python">Python</h3> <ul> <li>We use ROS1 Noetic which requires Python 3.x</li> <li>You can choose to use classes or just top level functions</li> <li>If you find yourself using <code>global</code> often, it’s a hint that you should be using classes</li> <li>Other code smells to watch out for are: Functions of more than 20 or so lines, and classes of more than 100 or so lines,</li> </ul> <slide_break></slide_break> <h3 id="ros-nodes-topics-messages">ROS Nodes, Topics, Messages</h3> <ul> <li>There’s rarely a need to design a custom message (.msg)</li> <li>If you need one it’s ok but you should try and avoid it if possible.</li> <li>As a general rule organize your functionality into multiple separate ROS nodes (python modules)</li> <li>As a general rule you can get nicer ROS code if you break the functionality into python classes and in separate files (modules)</li> <li>This works especially well if you can identify bits functionality that may be reused in other applications</li> <li>Don’t assume that /scan topic’s LaserScan message has 360 items in the ranges array. <ul> <li>Use len(msg.ranges) to figure out how many items there are.</li> </ul> </li> <li>Make sure you know how to work with Quaternions. The x, y, z, w fields in a Quaternion have nothing to do with x, y, z in Euler angles.</li> <li>See <a href="https://www.andre-gaschler.com/rotationconverter/">this</a> for a nice tool to help you visualize them.</li> </ul> <slide_break></slide_break> <h3 id="concurrency-threading-race-conditions-etc">Concurrency, threading, race conditions, etc</h3> <div class="callout callout-small"> <span class="callout-badge">NB</span>ROS is heavily concurrent which affects how you structure your code. Your logic may look right but cause misbehavior because of concurrency. </div> <ul> <li>Each ROS node (i.e. something you run with rosrun or roslaunch) is a single linux Process. (Think of it like any other python program.)</li> <li>Each distinct topic subscription within a node forms a single Thread for the callback.</li> <li>Like any other thread you need to be aware of the Python GIL (Global Interpreter Lock).</li> <li>Don’t create threads yourself! If you feel the need to do so, figure out how to break the algorithm into multiple nodes.</li> </ul> <slide_break></slide_break> <h3 id="callbacks-and-race-conditions">Callbacks and race conditions</h3> <ul> <li>Do not use rate.sleep() inside a callback. This is for many reasons, including race conditions.</li> <li>Be careful using globals for communicating between your callback and your main loop. It may lead to a race condition.</li> <li>Your main program should do a rospy.spin() or rospy.is_shutdown() to make sure that you can ^c out of it.</li> </ul> <slide_break></slide_break> <h3 id="package-structure">Package Structure</h3> <ul> <li>Create properly structured packages so they can easily be added to other workspaces.</li> <li>Use the <code>create_package</code> utility to create a correct package</li> <li>This will create a <code>package.xml</code> and <code>CMakeFile.txt</code></li> <li>You rarely (but not never) need to edit either one of them. Eventually you will have to learn them.</li> <li>Name the directory with the programs <code>src</code> and the directory with launch files <code>launch</code></li> <li>Other directories should be present only if you use them</li> <li>It’s a good idea to include a readme.md and a license.md</li> </ul> <slide_break></slide_break> <h3 id="writing-and-running-a-single-ros-node">Writing and running a single ros node</h3> <ul> <li>Put the code for each node in a file called *.py</li> <li>You can also have regular .py programs which are imported, as usual</li> <li>The first line should be a “shebang”: <code>#!/usr/bin/env python</code></li> <li>Set it to be excecutable (chmod +x filename.py)</li> <li>Put it in a package</li> <li><code>rosrun package-name filename.py</code></li> </ul> <slide_break></slide_break> <h3 id="using-launch-files-to-run-multiple-nodes">Using Launch Files to run multiple nodes</h3> <ul> <li>Put the code for each node in a file called *.py</li> <li>As you know oftena ROS application contains multiple nodes.</li> <li>You can run them all at once with a launch file.</li> <li>A launch file is an .xml formatt file with the extension .xml</li> <li>You can find <a href="http://wiki.ros.org/roslaunch/XML">detailed documentation here</a></li> <li>The most important purpose of a launch file is to launch multiple nodes</li> <li><code>roslaunch package-name filename.launch</code></li> </ul> <slide_break></slide_break> <h3 id="sources-and-references-quality-varies-a-lot">Sources and references (quality varies a lot!)</h3> <ul> <li><a href="https://github.com/leggedrobotics/ros_best_practices/wiki">List of other Best Practices</a></li> <li><a href="http://wiki.ros.org/Tutorials/Best%20Practices">ROS Best Practices</a></li> <li><a href="http://wiki.ros.org/ROS/Patterns">ROS Patterns</a></li> <li><a href="http://wiki.ros.org/BestPractices">More ROS Best Practices</a></li> <li><a href="https://towardsdatascience.com/5-python-code-smells-you-should-be-wary-of-c48cc0eb9d8b">Python Code Smells</a></li> <li><a href="https://yuzhangbit.github.io/tools/several-ways-of-writing-a-ros-node/">Several Ways of Writing a ROS Node</a></li> <li><a href="https://github.com/m-elwin/me495_threads">Threading in ROS</a></li> </ul> <slide_break></slide_break> <h2 class="shadow p-3 bg-white rounded">Thank you. Questions?<img class="img-fluid w-100" src="https://picsum.photos/800/100.jpg" /> <small> (random Image from picsum.photos)</small></h2>