<div class="callout callout-small"> <span class="callout-badge">NOTE!</span> All the book instructions make reference to turtlebot_xxx. We will be using turtlebot3_xxx </div> <slide_break></slide_break> ### Intro * We will modify the instructions from the book so follow along here! * Code will be part of `cosi119_src` package which we have been using: [cosi119_src](https://github.com/campusrover/cosi119_src) <slide_break></slide_break> ### Red Light! Green Light! * Pretty trivial, toe in the water * Drive forward for a bit * Stop. * Repeat * Look at [red_light_green_light.py](https://github.com/campusrover/cosi119_src/blob/main/samples/src/red_light_green_light.py) * Run code: <pre><code class="language-bash"> $ cw $ cd src/cosi119_src/samples/src $ roslaunch turtlebot3_gazebo turtlebot3_world.launch $ ./red_light_green_light.py </code></pre> ## Reading Sensor Data * TB3 has a lidar * Lidar returns and array of 360 doubles (and some other stuff) * 0 degrees is straight ahead * Each element has distance to nearest obstacle (in M) * To compute the distance to the obstacle immediately ahead (0 degrees) * `msg.ranges[0]` * To compute the nearest obstacle: * nearest_obstacle = min(msg.ranges) * But of course those won't be very useful (why?) * Experiment by running laser_scan.py program and understanding it * Experiment by seeing how the scan data changes based on where the simulated robot is <pre><code class="language-language-bash"> $ roslaunch turtlebot3_gazebo turtlebot3_world.launch $ rostopic echo scan </code></pre> ## Sensing and Actuation: Wander-bot! * Program will perform a pretty stupid trick * It will drive in a straight line for some seconds * Then it will spin in place for some more seconds * If it is too near an obstacle then it will spin in place * It turns out this is a pretty terrible algorithm * It gets stuck all the time * Take a look at the annotated code: [Wander Bot Code](https://github.com/campusrover/cosi119_src/blob/main/samples/src/wander.py) * `rospy.Time.now()` * returns a time structure with * fields secs and nsecs * Comparison operator is defined * rospy.Duration(x) is an interval, in seconds * Now run the program <pre><code class="language-language-bash"> $ roslaunch turtlebot3_gazebo turtlebot3_stage_3.launch $ chmod +x wander.py $ ./wander.py </code></pre> ## Summary * Logic in a `.py` program can establish behaviors * It's possible to modify an existing python program to be a ROS node * Nodes can publish and subscribe, and do both at the same Time * Need to know the exact names of the topics * Simulator is used for an idealized context for testing