WEBVTT 00:00.200 --> 00:06.980 Regardless of whether you are creating a real robot or just a simulation of it inside your PC, the 00:06.980 --> 00:10.520 first step is always to define its structure. 00:10.520 --> 00:16.980 So the model of the robot and define how all the different components are connected together. 00:17.000 --> 00:23.690 Define which components are fixed and which one are movable and also where any potential sensor of the 00:23.690 --> 00:25.160 robot is placed. 00:26.270 --> 00:35.300 In order to model a robot, Rosco uses the Urdf convention to represent the structure and the components 00:35.300 --> 00:37.970 of a robot through XML tags. 00:38.500 --> 00:46.880 Urdf stands for Unified Robot Description Format and is basically a convention used in the robotics 00:46.880 --> 00:50.630 community to describe the structure of a robot. 00:51.970 --> 00:59.500 Each robot component is defined by an XML tag that not only creates the component itself, but can also 00:59.500 --> 01:02.320 associate some properties to this component. 01:02.950 --> 01:11.020 If you've ever had a chance to use or at least to see what an HTML file of a web page looks like, then 01:11.020 --> 01:14.710 the Urdf format will sound familiar to you. 01:15.040 --> 01:21.040 Otherwise, don't worry, because in the following laboratory lesson we will implement from scratch 01:21.070 --> 01:25.270 our XML model of the robot using Urdf. 01:26.060 --> 01:33.320 It all starts with a tag called robot that defines the structure of a robot and will be a container 01:33.320 --> 01:37.490 for all the other robot components and how they are connected. 01:38.030 --> 01:45.200 So let's take a look at the main XML tags that we can find within an RDF model of a robot. 01:46.000 --> 01:50.560 The tag link represents any component of the robot. 01:50.950 --> 01:58.420 Basically, this tag creates a reference frame, so namely a frame with some properties. 01:58.720 --> 02:03.610 For example, a link can have a name that is in this case base link. 02:03.940 --> 02:08.260 And eventually a link can also have some physical dimensions. 02:08.290 --> 02:11.740 A mesh or a 3D model associated with it. 02:12.520 --> 02:13.230 To do so. 02:13.240 --> 02:20.530 The tag visual is used for the visualization of the mesh and then the tag collision and inertial are 02:20.530 --> 02:27.670 the one that are used to assign physical properties to the link and thus to assign volume and inertia. 02:28.520 --> 02:35.330 All these stacks will then be used by the physics engine to simulate the robot and so its interactions 02:35.330 --> 02:36.770 with the environment. 02:37.760 --> 02:44.390 In addition to the tag link, another commonly used tag for the creation of a robot model is the tag 02:44.390 --> 02:45.230 joint. 02:46.150 --> 02:53.170 With this tag, one can connect two links together and so to define the properties of the connection. 02:53.920 --> 03:00.700 Within the tag joint, one can specify the name of the two links that are connected together, and this 03:00.700 --> 03:04.220 connection is made following a tree like structure. 03:04.240 --> 03:12.970 So a joint that connects two links always as a parent that in this case is the base link and also as 03:12.970 --> 03:16.840 a child that in this case is the base plate. 03:17.380 --> 03:22.630 It is useful to remember that each link can only be connected to one parent. 03:22.660 --> 03:27.220 Meanwhile, each parent can have multiple child links connected to it. 03:28.120 --> 03:34.780 In this case, the base link, which is the parent, can also have several other children in addition 03:34.780 --> 03:35.980 to the base plate. 03:36.400 --> 03:43.510 On the other hand, the base plate can only be a child of the base link and cannot have other parents. 03:44.340 --> 03:51.930 Moreover, the base plate, in addition of being a child of the base link, can also be the parent of 03:51.930 --> 03:53.070 some other links. 03:53.910 --> 04:01.650 The resulting structure of the links connected by joints resembles a tree in which from a single initial 04:01.650 --> 04:06.960 link, all the other links branch off connected by the same number of joints. 04:08.220 --> 04:09.770 Within a joint tag. 04:09.780 --> 04:12.640 One can also define the type of the connection. 04:12.660 --> 04:16.200 For example, two links can be fixed to each other. 04:16.200 --> 04:21.450 That is, one does not move with respect to the other or they can also be movable. 04:21.450 --> 04:29.190 And so one link, one component of the robot can rotate or translate with respect to another component. 04:29.890 --> 04:36.520 Also within the joint tag, one can define other properties such as the rotation axis of one of the 04:36.520 --> 04:43.510 link with the respect to the other and also the limits of the relative movements between two links. 04:44.020 --> 04:50.800 By reusing these tags to define each component of the robot and its connection to the other component, 04:50.830 --> 04:53.590 we get the complete model of our robot. 04:53.980 --> 05:00.460 Here it is possible to visualize our different links are connected to each other and how they can move. 05:00.490 --> 05:03.370 They can rotate with the respect to each other. 05:04.060 --> 05:10.540 In the next laboratory lesson, we are going to implement the Erdf module of our robot that we are also 05:10.540 --> 05:16.900 going to use throughout the course for visualization simulation and also for defining the structure 05:16.900 --> 05:18.190 of the real robot.