1 00:00:08,140 --> 00:00:10,140 We have learned about ros nodes 2 00:00:10,140 --> 00:00:15,720 so let's see how the ros nodes communicate among each other and we are going to learn about the ros topics 3 00:00:15,720 --> 00:00:23,280 ros topics act as a bus among the node and that can exchange 4 00:00:23,280 --> 00:00:32,700 or transfer the ros message so open up the terminal our terminal is already sourced 5 00:00:34,560 --> 00:00:40,620 here you can see how the nodes are communicating with each other you can see two nodes that are 6 00:00:40,620 --> 00:00:47,160 displayed in circle and they are communicating over a topic and one node is publishing the other 7 00:00:47,160 --> 00:00:55,560 one the bottom one is subscribing and they are communicating a message via topic and here 8 00:00:55,560 --> 00:01:02,760 you can see there are multiple node and they are communicating over one topic 9 00:01:02,760 --> 00:01:10,620 two of them are Publishers and now two of them are subscribers so there can be multiple 10 00:01:10,620 --> 00:01:18,780 node that can communicate over one topic any node can communicate with any other node over a common 11 00:01:18,780 --> 00:01:27,960 topic so open up your terminal this terminal is already sourced and drag it here 12 00:01:27,960 --> 00:01:40,200 for the visualization we are going to run again turtlesim node Ros2 run turtlesim package name and the node name turtlesim 13 00:01:43,260 --> 00:01:55,200 and now open up another terminal ros2 topic list so here we have the first two topics will 14 00:01:55,200 --> 00:02:02,400 always appear uh these topic contains information about the parameters if the parameter changes it 15 00:02:02,400 --> 00:02:09,480 will store the information about it the rosout contains the logging information we do not need to 16 00:02:09,480 --> 00:02:16,260 spend too much time with them these three topics are our main concern alright run another 17 00:02:17,220 --> 00:02:28,680 node ros2 run turtlesim teleop key now we have two Nodes and 18 00:02:29,820 --> 00:02:37,500 but the topics are same because the teleop node will be publishing over turtle one 19 00:02:37,500 --> 00:02:45,180 slash cmd vel topic so let's see how we can monitor a topic we can monitor a data on a 20 00:02:45,180 --> 00:02:58,080 specific topic by using a keyword that is echo so we'll just all right ros2 topic echo 21 00:03:00,720 --> 00:03:01,260 turtle 22 00:03:02,081 --> 00:03:04,310 cmd vel 23 00:03:04,310 --> 00:03:09,060 and currently it is not showing us any data because the publisher hasn't 24 00:03:09,060 --> 00:03:14,280 published any data over this topic so we will just press forward key once 25 00:03:15,480 --> 00:03:20,100 yes we just press it once so it has displayed of this data 26 00:03:20,820 --> 00:03:27,960 as you can see X is 2 and all other values are zero if we press a right button once 27 00:03:30,240 --> 00:03:39,060 the Turtle has turned and it has given us the angular z value so this is how you can monitor 28 00:03:39,060 --> 00:03:46,620 a topic and you can access all the data that is being published over it and let's see rqt graph 29 00:03:46,620 --> 00:03:58,620 representation rqt_graph the enter and here it has displayed we will just refresh 30 00:03:59,340 --> 00:04:07,380 and uh do not communicating with each other so you see echo we can find out all the data being 31 00:04:07,380 --> 00:04:14,160 published over topic but we can also find out how many publisher and subscribers are related 32 00:04:14,160 --> 00:04:23,520 to a topic and we can also find out the uh message type related to that topic so if we write 33 00:04:24,804 --> 00:04:26,660 ros2 34 00:04:27,120 --> 00:04:37,800 topic list minus t so it will display the type of the message that a particular 35 00:04:37,800 --> 00:04:45,360 topic is using so it has displayed all the topics and their selected message type in 36 00:04:45,360 --> 00:04:54,660 the list and the turtle1/cmd_vel has the  geometry massages and twist so 37 00:04:54,660 --> 00:05:00,540 you can also see the turtle one post has the message type Turtlesim message and post we 38 00:05:00,540 --> 00:05:07,620 will see these messages and future videos but currently you just have to keep in mind there 39 00:05:07,620 --> 00:05:15,360 are some message type that a particular topic uses to find out more information about a topic 40 00:05:15,360 --> 00:05:24,180 To know the number of subscribers and publisher on related to a topic we can just use ros2 topic 41 00:05:25,080 --> 00:05:31,620 info and let's say we have to learn about this topic 42 00:05:35,340 --> 00:05:42,420 here so it has currently one subscriber and one publisher so why does it have just one publisher 43 00:05:42,420 --> 00:05:50,065 and one subscriber ? the publisher that is publishing over this topic is the teleop and the subscriber is 44 00:05:50,065 --> 00:05:56,597 turtlrsim window type of this topic is displayed here to geometric messages slash 45 00:05:56,100 --> 00:06:05,700 message and twist to find out what is inside this message we can use a command ros2 interface 46 00:06:07,860 --> 00:06:20,460 show and you can simply paste it here so it has a vector linear graphical 3 47 00:06:20,460 --> 00:06:28,320 linear and vector 3 angular okay now we want to publish over a topic let's relaunch 48 00:06:28,320 --> 00:06:33,720 every terminal from the beginning uh press Ctrl C to kill terminal 49 00:06:34,380 --> 00:06:43,680 and here and you can press like Ctrl+L to clear it in every terminal 50 00:06:48,240 --> 00:06:57,300 it enter a window appear and now we have to know the topic list ros2 topic list 51 00:06:58,680 --> 00:07:03,120 so this is our concerning topic we will ros2 topic 52 00:07:03,720 --> 00:07:12,840 info this topic and we know there is a zero publisher and one subscriber and here is the 53 00:07:12,840 --> 00:07:19,020 message type what we will do we'll just copy ros we have to publish over this topic and we 54 00:07:19,020 --> 00:07:25,860 will also use this message type how this message looks like so what we will do ros2 interface 55 00:07:29,040 --> 00:07:37,860 show and this message and it has displayed us so this message has two vectors the 56 00:07:37,860 --> 00:07:45,780 linear and angular so we want to publish uh over cmd vel so here is the command ros2 topic 57 00:07:46,740 --> 00:07:54,060 pub and -- once so it means it will just publish it once and then it will terminate 58 00:07:54,060 --> 00:08:09,600 and the topic name turtle one cmd vel and the message type then inverted comma track it linear 59 00:08:10,680 --> 00:08:17,280 colon there are three further values in the sector so there will be the X colon space 60 00:08:18,060 --> 00:08:35,520 let's say three and then comma and y colon space zero comma Z colon space zero close this with the 61 00:08:35,520 --> 00:08:44,040 curly bracket and then comma and then there is the angular Vector as well and children 62 00:08:45,540 --> 00:08:49,200 colon phase bracket X 63 00:08:52,140 --> 00:08:59,220 X value 0 and Y value 0 64 00:09:02,160 --> 00:09:12,240 and Z value should be uh this is the angular velocity we can give it to uh let's say we give it 65 00:09:12,240 --> 00:09:20,100 uh three and close the curly bracket close the curly bracket again and close the inverted comma so this 66 00:09:20,100 --> 00:09:28,440 will be the convention to publish over this topic and each topic will have a different message type 67 00:09:28,440 --> 00:09:37,080 and there will be different parameters as well so at enter a loop is beginning and tells completed 68 00:09:37,080 --> 00:09:44,160 a semi-circle what what if we want to publish it for a long time so instead of publishing it 69 00:09:44,160 --> 00:09:51,180 here once we want to publish it at a continuous frequency so what we will change we'll just change 70 00:09:52,860 --> 00:10:01,980 and this keyword I will mention it rate and let's say two so it will be publishing 71 00:10:03,480 --> 00:10:11,100 twice a second let's change the value a little bit preview increase the value of linear X it will 72 00:10:11,820 --> 00:10:14,220 be moving in a bigger Circle hopefully 73 00:10:17,580 --> 00:10:24,540 yes it is moving and if you want to make this circle smaller then what you will do 74 00:10:24,540 --> 00:10:35,760 we will press Ctrl C and give a high angular velocity and publish yes see rqt graph 75 00:10:45,600 --> 00:10:50,160 so here you can see it is showing that a publisher is publishing 76 00:10:50,700 --> 00:11:04,800 so what if we now get the info of this topic ros2 topic info turtle 1 and cmd 77 00:11:06,840 --> 00:11:14,640 yes here you can see there is one publisher and one subscriber as well so we can also check the 78 00:11:14,640 --> 00:11:20,940 frequency of a topic so we will check the frequency of this topic ros2 79 00:11:21,850 --> 00:11:23,350 topic 80 00:11:23,760 --> 00:11:29,700 HZ which is represents frequency topic so we will see 81 00:11:32,760 --> 00:11:39,060 so here is the real-time value of the frequency so we have set the frequency to two now you 82 00:11:39,060 --> 00:11:44,220 have learned all the information that you need to know about the ros topic you have learned 83 00:11:44,220 --> 00:11:49,800 to publish you have learned to Echo and you have you can monitor a topic and you can also see 84 00:11:50,340 --> 00:11:56,520 the publisher and subscribe will uh over related to a topic by just using 85 00:11:56,520 --> 00:12:03,720 a keyword that is input and uh you can also check the frequency of a topic