## Logistics * How to submit homework: * SURPRISE: Do *not* put your name in the files. The names are automatically provided by Moodle. * If there's code, then it should be in a valid ROS package. Zip up that package and submit it. * If there's a document, report, ansewr to a question, then it must be a pdf. * If there's a video, then put the url of the video in a separate file called video.txt * Summary: There will be between one and three files in your submission. * Whoops, next Friday is also no-class, so, no-lab! * Should we present project ideas on Thursday or a week from Friday? <slide_break></slide_break> ## Review * An x,y,z is always within a specific coordinate system * For example, is it relative to the bottom left corner of the room, or the center of the robot? * It is far easier to reason in an appropriate coordinate system * As long as there are easy ways to convert back and forth. <slide_break></slide_break> ### Coordinate systems * Pose = position and orientation * Pose is always with respect of a specific coordinate frame or TF <slide_break></slide_break> ### Natural Coordinate system * Usually a coordinate begins as the "natural" way to think about It * Angle of obstacle, with respect to the LIDAR * Position a corner, with respect to the whole room * Angle to turn, from the point of view of the robot itself * So when you see phrases like "with respect to", and "from the point of view of" you know these refer to coordinate frames. <slide_break></slide_break> ### What is the natural TF hierachy? * It is very common to want to see that same point or angle from another's point of view * How much do I have to turn to point directly away from the wall? * What direction is the nearest door from me? * How do I drive in the direction of the fiducial? * Compare starboard/port and driver vs. passenger side <slide_break></slide_break> ### TF * TF package in ROS is central * It recently was rewritten and shows up as TF2 in many places * It is interesting and important to understand in some more detail <slide_break></slide_break> ### TF framework in ROS * An API to make conversions of coordinates between systems easy and efficient * Coordinate systems are given names, e.g. "world" or "lidar-1" * They are organized into a tree * Every named tf has just one parent, and unlimited number of children <slide_break></slide_break> ### TF broadcasts * Any node can report on the relationship between two coordinate system * Generally this is a node who knows something about one or the other. * For example, the node that moved the arm, can report the new relative position * That is, to get from tf "a" to tf "b", add add 10 to the x or rotate 180 degrees around z. * TF Listeners * If my node needs to interpret coordinates I might listen to tf messages * Allowing to convert coordinates between any two coordinate frames. <slide_break></slide_break> ### TF CheatSheet * <a href="/content/topics/cribsheets/tf_cheatsheet/">TF Cheatsheet</a> <slide_break></slide_break> ### Lets look at some code * If you want to follow along, we are going to look at [Introduction to tf2](http://wiki.ros.org/tf2/Tutorials/Introduction%20to%20tf2) <pre><code class="language-shell"> $ sudo apt-get update $ sudo apt-get install ros-noetic-turtle-tf2 ros-noetic-tf2-tools ros-noetic-tf $ sudo apt install evince $ roslaunch turtle_tf2 turtle_tf2_demo.launch </code></pre> * Notice that as you use the cursor keys to teleop the orange turtle around, the grey one follows right on it's heels! <slide_break></slide_break> ### High Level explanation * Code is using tf2 to create three coordinate frames: one for the "world", and one each for the two turtles * Realize that if we give turtle1's coordinates as x,y, we'd still need to say, in what coordinate frame those were. * In t1's coordinate frame, t1's coordinates are always 0,0! <pre><code class="language-shell"> $ rosrun tf2_tools view_frames.py $ evince frames.pdf </code></pre> <img src="/content/topics/images/tf_view_frames.jpg" class="img-thumbnail d-block mx-auto" style="height:300px;"/> * See the three coordinate frames and their hierarchical relationship * The diagram doesn't display the actual transform just the relationships * To see those we need to echo the transform: <pre><code class="language-shell"> $ rosrun tf tf_echo turtle1 turtle2 At time 1677784974.758 - Translation: [0.000, 0.000, 0.000] - Rotation: in Quaternion [0.000, 0.000, -0.200, 0.980] in RPY (radian) [0.000, 0.000, -0.402] in RPY (degree) [0.000, 0.000, -23.046] $ rosrun tf tf_echo turtle1 world rosrun tf tf_echo turtle1 world At time 1677785078.262 - Translation: [-5.544, -5.544, 0.000] - Rotation: in Quaternion [0.000, 0.000, 0.000, 1.000] in RPY (radian) [0.000, -0.000, 0.000] in RPY (degree) [0.000, -0.000, 0.000] </code></pre> * You can see the transform needed to get from turtle1 to turtle2 is different from the one between turtle1 and world. Why is that? <slide_break></slide_break> ### Visualization <pre><code class="language-shell"> $ rosrun rviz rviz -d `rospack find turtle_tf2`/rviz/turtle_rviz.rviz </code></pre> * You now visualize all three tfs. Why do two move and one stay still? <img src="/content/topics/images/rviz_tf.jpg" class="img-thumbnail d-block mx-auto" style="height:300px;"/> <slide_break></slide_break> ### Lets see how all this works: tf2_broadcaster * Examine the code in `samples`: * tf2_demo_launch * turtle_tf2_broadcaster.py * turtle_tf2_listener.py <pre><code class="language-shell"> $ roslaunch samples tf2_demo.launch $ rosrun tf tf_echo /world /turtle1 </code></pre> * Only one turtle so far! * Notice that it is publishing turtle1's `tf`s relative to world * Drive around and notice that it is updating turtle1's pose and also a transform <pre><code class="language-shell"> $ roslaunch samples tf2_demo.launch $ rosrun tf tf_echo /world /turtle1 $ rosrun tf tf_echo /world /turtle2 </code></pre> <slide_break></slide_break> ### tf2 frames are organized in a tree <pre><code class="language-shell"> $ rosrun tf2_tools view_frames.py $ evince frames.pdf </code></pre> * Notice, there are no cycles, one root * This ensures that there's only one path from one frame to another * Which ensure that we can compute the transform by following that path * New frame would be added as a child of an existing frame * How to decide what the parent frame should be? * Its the one that has the most obvious and simple transform <slide_break></slide_break> ### Defining TFs (transforms) * TFs have names which help you understand what physical component it is tied to * TFs for mobile robots follow a typical hierarchy as follows: map -> odom -> base_link * `base_link` for the “center” of the robot. You get to pick it but it should make some intuitive sense. All other parts of the robot are below base_link hierarchically in the tf tree * Sometimes it’s useful to have another TF which is the center of the footprint of the robot, meaning, a TF projected straight down from base_link and intersecting with the ground plane wherever that is. * `odom` for the odometry created TF * `map `for the TF of a map built by hand or through slam * Conventional names and standards for TFs can be found here: (https://www.ros.org/reps/rep-0105.html) <slide_break></slide_break> ### Transforms (TFs) * TFs are fundamental to understanding anything relating to positions in ROS. * TFs are built on a hierarchical system of relative positions between different parts of a robot or different objects in the world. * The base_link is the “center” of the robot. You get to pick it but it should make some intuitive sense. * TFs allow all other parts of the robot to be described as a relative position to either the base_link directly or to other parts on the robot. * Odometry is simply measuring the transform between the base_link and the origin point of the odom tf (usually the position at which the robot started). * Continuing the example from before, lets say I need to know the relative positions of the hand on our robot and an lever in the world. <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>