WEBVTT 00:00.140 --> 00:01.790 In the ETF Q library. 00:01.790 --> 00:09.920 The reference frames that make up our robot or any application can be mainly of two types static or 00:09.920 --> 00:10.760 dynamic. 00:11.500 --> 00:16.300 Let's go back to the RDF model of the robot and the links and the joints. 00:16.300 --> 00:22.540 We used to create the reference frames and connect them to each other using transformation matrices. 00:23.190 --> 00:30.870 Some joints are defined in the RDF model of type fixed and therefore the transformation matrices that 00:30.870 --> 00:35.880 connect the two reference frames are static, so they do not change over time. 00:36.740 --> 00:44.070 Disconnections between static reference frames often refer to actual mechanical connection between components. 00:44.090 --> 00:50.060 For example, when they are bolted together, such as when mounting a sensor like a camera on the robot 00:50.060 --> 00:50.900 structure. 00:51.350 --> 00:58.880 In this case, the camera link is fixed, so is static and does not move with respect to the robot's 00:58.880 --> 00:59.960 base footprint. 01:00.750 --> 01:07.050 Regarding the transformation matrices that connects static reference frames which are only connected 01:07.050 --> 01:08.430 by a fixed joints. 01:08.580 --> 01:14.970 Since the transformation matrix is static, its value do not change over time, and so it is published 01:14.970 --> 01:19.950 only once within the two topic called TF Static. 01:21.050 --> 01:28.250 As for movable joints such as those that actuate the robot's arm, the transformation matrix that express 01:28.280 --> 01:36.380 how the links of the robot rotate can vary over time depending on how the motors are actuated and thus 01:36.380 --> 01:37.340 rotated. 01:38.230 --> 01:46.120 In this case, this transformation is repeatedly published with a certain frequency within the TF topic. 01:46.860 --> 01:53.070 At each time step, each motor can be in a different position relative to the robot's reference frame, 01:53.070 --> 01:59.610 and thus the transformation matrix that connects the motor link to the robot's base is published in 01:59.610 --> 02:02.730 the TF topic for that specific time step. 02:03.640 --> 02:10.030 In our subsequent time step, A new transformation matrix that connects these two frames will be again 02:10.030 --> 02:15.730 published in the TF topic, representing the new position of the joints of the robot. 02:16.790 --> 02:23.090 All this information that is published within the TF and TF static topics can then be used, can then 02:23.120 --> 02:30.380 be read by other nodes such as RVs, for example, to visualize the static model of the robot in real 02:30.380 --> 02:34.370 time and observe the movement of its dynamic parts.