1 00:00:00,720 --> 00:00:03,510 Welcome to your first project of the course. 2 00:00:03,600 --> 00:00:09,390 Based on what you have learned about publishers, subscribers and interface types, it's time for you 3 00:00:09,390 --> 00:00:11,700 to apply this knowledge in this project. 4 00:00:12,120 --> 00:00:17,850 The project scenario is this You have a mobile robot with wheels of a certain radius. 5 00:00:17,940 --> 00:00:23,700 One of the wheels has a rotary encoder sensor on it, which outputs how many rotations per minute. 6 00:00:23,700 --> 00:00:25,170 The wheel is rotating. 7 00:00:25,350 --> 00:00:30,720 For this project, we will keep it simple and say the robot is going at a constant speed. 8 00:00:31,110 --> 00:00:37,590 The first node I want you to create is a publisher node, which publishes to a topic named RPM with 9 00:00:37,590 --> 00:00:42,630 the sensor value that we are going to be publishing, in this case, a constant value of your choice. 10 00:00:42,930 --> 00:00:46,320 Now, I have no idea how fast the robot is actually going on this. 11 00:00:46,320 --> 00:00:51,810 I take into account the size of its wheels, so I want you to create a second node which subscribes 12 00:00:51,810 --> 00:00:57,780 to the RPM topic calculates the speed the robot is going based on its wheel size and then publishes 13 00:00:57,780 --> 00:01:00,780 a result to the topic called speed. 14 00:01:00,780 --> 00:01:06,210 The speed for most robotic systems are in meters per second, but feel free to make it your own preferred 15 00:01:06,210 --> 00:01:07,350 units of measurement. 16 00:01:07,710 --> 00:01:11,100 So let's review what we are trying to accomplish in this project. 17 00:01:12,640 --> 00:01:18,700 In this project, you will create two nodes, one which publishes a value simulating rotations per minute 18 00:01:18,700 --> 00:01:24,700 of the robot wheel spinning and a second script which subscribes to this value, and then goes ahead 19 00:01:24,700 --> 00:01:27,760 and publishes the calculated speed of the robot. 20 00:01:28,270 --> 00:01:33,220 You should be able to run these two nodes in two separate terminals to get your corresponding output. 21 00:01:35,370 --> 00:01:40,920 Go ahead and pause this video and give yourself about 10 minutes or so to give this project a go. 22 00:01:40,980 --> 00:01:45,180 If you get stuck, you can follow along with me in the remainder of this video lecture. 23 00:01:45,180 --> 00:01:48,600 But I encourage you to try to work through this project on your own. 24 00:01:58,330 --> 00:01:58,730 All right. 25 00:01:58,750 --> 00:01:59,500 How'd it go? 26 00:01:59,650 --> 00:02:02,680 If you got stuck, you can follow along with me to see how I did it. 27 00:02:05,750 --> 00:02:12,650 With my Visual Studio code editor open, I can go ahead and create a new script which I will call RPM 28 00:02:12,650 --> 00:02:14,060 Pub Pie. 29 00:02:15,670 --> 00:02:20,230 This note is just going to publish a constant number simulating the speed of our robot. 30 00:02:20,320 --> 00:02:25,660 So I'll actually go ahead and copy the code from the publisher we made in the previous lectures and 31 00:02:25,660 --> 00:02:26,380 modify it. 32 00:02:29,480 --> 00:02:35,660 You can do this by going to edit and we can go to replace or you can use the control H keyboard shortcut. 33 00:02:35,870 --> 00:02:41,300 And first thing I'm going to do is change the class name from Hello World Publisher to RPM Publisher, 34 00:02:41,300 --> 00:02:42,620 so it makes more sense. 35 00:02:44,860 --> 00:02:46,540 And I'll replace all instances. 36 00:02:46,720 --> 00:02:50,710 Next, I will change the node name to RPM pub node. 37 00:02:53,500 --> 00:02:58,410 And instead of publishing a string message type, I will instead use a float 32. 38 00:02:58,420 --> 00:03:01,810 So I will replace all of our string instances like so. 39 00:03:03,140 --> 00:03:09,590 I will change the topic name we are publishing to to be RPM and change the callback function name to 40 00:03:09,590 --> 00:03:12,200 publish rpm so it makes more sense. 41 00:03:14,960 --> 00:03:20,720 Then within our callback function, rather than setting the float message data to Hello world, I will 42 00:03:20,720 --> 00:03:27,290 instead set it to a constant variable which I will call RPM in all caps, and I will then create this 43 00:03:27,290 --> 00:03:30,020 variable at the top of my file and I'll just set it to ten. 44 00:03:30,410 --> 00:03:34,940 Then to make it a bit clearer, I will print that the RPM publisher node is running. 45 00:03:35,970 --> 00:03:36,330 Great. 46 00:03:36,330 --> 00:03:37,740 That's it for our RPM publisher. 47 00:03:37,740 --> 00:03:40,320 We can save the file and test that It works. 48 00:03:40,320 --> 00:03:46,380 Before we open up a terminal, I'll configure our make list text file to include our new script by going 49 00:03:46,380 --> 00:03:52,260 into the install section and adding the scripts slash rpm underscore pub dot pie node. 50 00:03:54,510 --> 00:03:55,050 With that. 51 00:03:55,050 --> 00:03:59,370 I'll save this file and we can go ahead and run a coal can build in terminal. 52 00:03:59,370 --> 00:04:04,590 Or if you follow along with me in the last lecture, go to terminal and run build task to do it for 53 00:04:04,590 --> 00:04:05,040 you. 54 00:04:05,700 --> 00:04:11,460 So we've gone ahead and successfully built our RPM publisher node so we can go ahead and test it. 55 00:04:11,460 --> 00:04:15,450 So I'll go ahead and open up a terminal source, our install setup dot bash file. 56 00:04:18,120 --> 00:04:20,880 And then I can go ahead and run the node to test it out. 57 00:04:23,150 --> 00:04:25,310 So we see RPM publisher Node running. 58 00:04:25,310 --> 00:04:31,190 And if I go ahead and do Ross two topic list, I can go ahead and echo the contents. 59 00:04:35,760 --> 00:04:39,470 And we see our constant value of ten being published. 60 00:04:39,480 --> 00:04:41,340 So I'll go ahead and stop this. 61 00:04:43,540 --> 00:04:43,970 Great. 62 00:04:43,990 --> 00:04:48,250 The RPM node works, so let's now create our speed publisher node. 63 00:04:48,280 --> 00:04:52,840 The first thing our speed publisher node will have to do is subscribe to the value being published over 64 00:04:52,840 --> 00:04:59,890 the RPM topic so I can go over to my subscriber node so that I can copy it over to my newly created 65 00:04:59,890 --> 00:05:00,420 code. 66 00:05:00,430 --> 00:05:06,520 So I'll go over to scripts, create a new file, and I'll call this one Speed Calc dot pie. 67 00:05:08,750 --> 00:05:11,870 Then I'll go ahead and copy the contents from my subscriber's file. 68 00:05:13,870 --> 00:05:17,980 And I'll go ahead and change the class name to Speed calculator. 69 00:05:24,770 --> 00:05:27,710 And change the node name to a speed calc node. 70 00:05:29,230 --> 00:05:30,010 And then I'll change. 71 00:05:30,010 --> 00:05:34,660 Instead of the string message type, we will be subscribing to a float 32 type. 72 00:05:36,250 --> 00:05:38,170 As we did in our RPM publisher. 73 00:05:39,130 --> 00:05:43,810 And we will also match the RPM publisher's topic name in this case RPM. 74 00:05:43,810 --> 00:05:45,720 And I'll go ahead and leave the slash out of there. 75 00:05:45,730 --> 00:05:50,590 You can leave slashes in the topic names, but for scalability reasons, it's better to keep them without 76 00:05:50,590 --> 00:05:50,950 the slash. 77 00:05:50,950 --> 00:05:57,370 When you define them, you'll see why a bit in future lectures and I will change the callback function 78 00:05:57,370 --> 00:05:59,320 to be called calculate speed. 79 00:06:03,610 --> 00:06:04,450 Down in the main function. 80 00:06:04,450 --> 00:06:07,540 I'll change the print statement to say Speed calculator node started. 81 00:06:10,170 --> 00:06:14,580 And I will change this my sub variable to be something that makes a bit more sense. 82 00:06:14,580 --> 00:06:16,140 I'll just call it Speed Calc. 83 00:06:19,060 --> 00:06:20,890 In our calculate speed callback function. 84 00:06:20,890 --> 00:06:23,260 We currently have it set to just print the data out. 85 00:06:23,380 --> 00:06:27,730 Rather than doing that, we will use this data to calculate the speed our robot is going. 86 00:06:27,970 --> 00:06:31,840 First thing I will do is change our message variable to RPM message. 87 00:06:31,840 --> 00:06:33,550 So it's clear what this message is for. 88 00:06:33,640 --> 00:06:39,610 Then I will create a variable called speed, which I can calculate using the RPM message data multiplied 89 00:06:39,610 --> 00:06:45,640 by the circumference of the wheel, which is the wheel radius multiplied by two multiplied by pi. 90 00:06:48,770 --> 00:06:52,370 Which I will approximate as 3.14159. 91 00:06:52,610 --> 00:06:58,850 Then, since I want my speed in meters per second, I will have to divide by 60 to convert rotations 92 00:06:58,850 --> 00:07:01,100 per minute to rotations per second. 93 00:07:04,740 --> 00:07:10,140 Now I need to create this wheel radius variable, which I will just do near the top of the file. 94 00:07:10,170 --> 00:07:12,660 I will set it equal to 12.5 centimeters. 95 00:07:12,810 --> 00:07:17,040 It is worth noting, though, that I want my speed to be in meters per second, so I'll have to convert 96 00:07:17,040 --> 00:07:22,560 this to meters, which I can do by dividing the 12.5 centimeters by 100. 97 00:07:22,560 --> 00:07:25,050 And I can put a note here explaining why I did this. 98 00:07:27,470 --> 00:07:30,720 OC We calculated speed based off the RPM data. 99 00:07:30,750 --> 00:07:36,120 Now we need to publish this speed data, which means I need to create a publisher. 100 00:07:36,270 --> 00:07:40,110 I can just copy over our publisher code from our RPM pub. 101 00:07:42,920 --> 00:07:47,360 And place it here and we will be publishing to the speed topic for this publisher. 102 00:07:47,360 --> 00:07:53,120 So I'll go ahead and change that and I'll go ahead and copy the usage of the publisher's message data 103 00:07:53,120 --> 00:07:53,690 set up. 104 00:07:56,150 --> 00:07:59,480 And place them in my calculate speed callback function. 105 00:08:01,860 --> 00:08:04,860 And I'll rename this speed message so it makes more sense. 106 00:08:08,900 --> 00:08:12,410 And instead of loading in the RPM, we'll be loading in the calculated speed. 107 00:08:14,480 --> 00:08:14,780 Great. 108 00:08:14,780 --> 00:08:16,830 So let's review what is happening in this code. 109 00:08:16,850 --> 00:08:20,480 I created a subscriber which subscribes to the RPM topic. 110 00:08:20,630 --> 00:08:26,940 I also created a publisher which publishes to the speed topic in my subscriber callback function. 111 00:08:26,960 --> 00:08:31,070 I take the RPM data and calculate the robot's speed. 112 00:08:31,340 --> 00:08:36,710 Then I load the value into the float 32 speed message and publish it. 113 00:08:37,490 --> 00:08:37,790 Great. 114 00:08:37,790 --> 00:08:42,590 I'll save the file and head over to my make list text file to include it. 115 00:08:46,850 --> 00:08:51,110 So I'll go ahead and save this and go ahead and recompile our project. 116 00:08:53,530 --> 00:09:00,160 Now I can go ahead and clear my terminal and rerun my RPM publisher node. 117 00:09:01,430 --> 00:09:04,040 And I'll clear this and I'll go ahead and source it. 118 00:09:08,170 --> 00:09:09,790 And run our speed calc node. 119 00:09:15,110 --> 00:09:17,600 Then we can go ahead and open a third terminal tab. 120 00:09:18,830 --> 00:09:20,390 I'll go ahead and move these a bit. 121 00:09:22,000 --> 00:09:28,690 And we can do Ross to node list and we see our two nodes are indeed running and we do Ross to topic 122 00:09:28,690 --> 00:09:29,380 list. 123 00:09:30,010 --> 00:09:32,490 We can see RPM and speed being published too. 124 00:09:32,500 --> 00:09:34,720 So if I go ahead and echo our RPM one more time. 125 00:09:40,100 --> 00:09:41,660 We get our value of ten. 126 00:09:41,660 --> 00:09:43,760 And if I go ahead and echo our speed. 127 00:09:45,110 --> 00:09:53,540 We get the calculated speed of this particular RPM with the wheel radius size I set in my speed calc 128 00:09:53,540 --> 00:09:54,110 node. 129 00:09:55,790 --> 00:10:02,450 It'll be the speed of this particular robot that has this 12.5 centimeter wheel going at ten rotations 130 00:10:02,450 --> 00:10:03,200 per minute. 131 00:10:05,200 --> 00:10:06,220 Congratulations. 132 00:10:06,220 --> 00:10:11,890 You successfully completed your first Ross to Python project where you are able to take advantage of 133 00:10:11,890 --> 00:10:17,020 Ross publishers and subscribers to process data as you would on a real robotic system. 134 00:10:17,410 --> 00:10:22,900 Now notice we had to hard code the values of the RPM and wheel radius into our code. 135 00:10:22,990 --> 00:10:27,010 So what happens if we wanted to use this code for robots with a different wheel size? 136 00:10:27,250 --> 00:10:32,050 Well, we would need to edit our python script depending on the wheel size and rerun it. 137 00:10:32,080 --> 00:10:33,610 Now that's not very functional. 138 00:10:33,610 --> 00:10:37,900 So in the next lecture we are going to learn how we can use parameters to get around this issue rather 139 00:10:37,900 --> 00:10:41,470 than having to edit our Python scripts every time we want to make changes.