**Reading: <a href="/content/topics/homeworks/hw_readings_119/140_hw_119_chapter7/">Read PRR Chapter 7</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> ### Welcome * Review: how it works in ROS (5 minutes) * All your code (almost) in ROS is a node * All information (almost) is transferred in the form of messages <slide_break></slide_break> ### Sensors * Are electronic devices * Connected to the Robot's computer * Generatel data very fast * They require real-time processing * Publishing information <slide_break></slide_break> ### What kinds of sensors? * LIDAR: Rapidly rotates a laser, returns distance to nearest obstacle on each bearing * Camera: Measures color and brightness of light in a 2D grid * IMU: Measures accelration in 6 DOF. Sometimes also measures magnetic north * Motor encoders>: Measures position of wheels as they rotate * GPS: Uses satelites to measure position on the earth * And many more [types of sensors](https://www.wevolver.com/article/sensors-in-robotics) <slide_break></slide_break> ### All Sensor Data is Noisy * The data you get from them is generally large arrays of floating point numbers * Even if nothing is changing, you will see it change a lot * Signal vs. Noise * There are many techniques for extracting the signatl <slide_break></slide_break> ### Sensor Data delivered as ROS topics * What is the "topic" that the device publishes to * How frequently does it publish (e.g. 10 times per second, once every second) * And what is the message type * Often 10 to 60 times per second. Real Time <slide_break></slide_break> ### Acting on the sensor data * A node subscribes to that topic * Writes a "handler" which is invoked each time the sensor node publishes <slide_break></slide_break> ### Simulation and IRL are different * Sensor's specifications are different (subtly) * Ranges and what happens when out of range * Degree of "randomness" * Exact dimensions of robots, obstacles, etc. <slide_break></slide_break> <h3 id="lidar-refresher">Lidar Refresher</h3> <ul> <li>Rotating laser beam</li> <li>“Time of Flight” Sensor</li> <li>Key raw data is an array of numbers</li> <li>Indexed by the bearing position of the lidar beam at that instance</li> <li>Each cell is the distance in meters</li> </ul> <slide_break></slide_break> <h3 id="data-from-sensors">Data from Sensors</h3> <ul> <li>Each sensor, each model and brand, has different specifications and characteristics</li> <li>Like all other sensors, the data you get from a Lidar is <code>noisy</code></li> <li>Depending on the software you may have to ‘clean’ the data before you can make sense of it</li> <li>Our robots all use a <a href="https://www.ydlidar.com/products/view/5.html">YDLIDAR X4</a></li> </ul> <slide_break></slide_break> <h3 id="scan-topic"><code>/scan</code> topic</h3> <ul> <li>Lidar data is published as a <code>/scan</code> topic with <code>/LaserScan</code> messages</li> <li>The most important field that you need to understand is the “ranges”.</li> <li>You can access the reading of a specific angle by using “range[index]”, and access a sub-array by using “range[index0:index1]”</li> </ul> <slide_break></slide_break> <h3 id="raw-sensor-data">Raw sensor data</h3> <ul> <li>To study the Lidar data we can dump out the lidar data into a .csv file format</li> </ul> <pre><code class="language-language-bash"> rostopic echo /scan -w 4 -p -n 50 > ~/scan_data </code></pre> <slide_break></slide_break> <h3 id="lidar-data">Lidar data</h3> <ul> <li>As mentioned there are different model Lidars as well as the simulated Lidar in Gazebo</li> <li>Here are some of the facts that are required</li> <li>Some of them are present in the <code>/scan</code> message, others are not</li> <li>Lets look at the data: see cosi119_src/data for a short program and the data it generates</li> <li>A simple way to look at this data in this: <a href="https://docs.google.com/spreadsheets/d/1FmGs7XlWe9YGtRFutOiiqSNj7i2DxvQIfJmmn1yMBvw/edit?usp=sharing">google sheet</a></li> </ul> <slide_break></slide_break> <h3 id="present-in-the-scan-message">Present in the <code>/scan</code> message</h3> <ol> <li>How many “cells” are present in each frame</li> <li>What is the minimum and maximum bearing</li> <li>(From that you can compute how many radians each “cell” corresponds to)</li> </ol> <slide_break></slide_break> <h3 id="not-present-in-the-message">Not present in the message</h3> <ol> <li>Height of the Lidar from the floor</li> <li>Direction it is pointing</li> <li>Clockwise vs. counter clockwise</li> <li>(The last three are captured as the “coordinate system” of the Lidar or the <code>tf</code>)</li> <li>Unit of distance (almost always Meters)</li> <li>Closest and furthest valid distance</li> <li>“Invalid” value (0 or NaN or ?)</li> </ol> <slide_break></slide_break> <h3 id="python">Python</h3> <ul> <li>Subscribe to <code>/scan</code> topic</li> <li>Message type <a href="http://docs.ros.org/melodic/api/sensor_msgs/html/msg/LaserScan.html"><code>LaserScan.msg</code></a></li> <li>In Python it will be delivered to you as an object of class <code>LaserScan</code>.</li> <li>Consider writing a separate filter node which takes “raw” LiDAR data (subscribes to /scan) and publishes “cleaned up” LiDAR data (publishes for example to /scan/clean).</li> <li>A related approach is to subscribe to /scan and publish something like /scan/semantic that instead of just cleaning up the data also computes some mins and maxes in different directions of interest.</li> </ul> <slide_break></slide_break> <h3 id="video-tutorials">Video Tutorials</h3> <ul> <li>Watch and study these for a review from a different perspective. <ul> <li>This <a href="https://youtu.be/tEayzulupxE">lecture from Udemy</a> introduces you to a lasert scanner or LIDAR</li> <li>This <a href="https://youtu.be/kze3Z8rTkZo">lecture from Udemy</a> gives you more detail on how to process the lidar data</li> <li>This <a href="http://www.theconstructsim.com/read-laserscan-data/">Reading Laserscan Data</a> provides almost everything you need to understand to manipulate the LaserScan data</li> </ul> </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>