1 00:00:00,870 --> 00:00:06,990 In this lecture, we're going to go over using parameters in ROS to parameters are essentially dynamic 2 00:00:06,990 --> 00:00:09,810 variables which are associated with nodes. 3 00:00:09,990 --> 00:00:15,780 These dynamic variables have information stored in it which can be set, retrieved or changed freely. 4 00:00:16,290 --> 00:00:22,650 Parameters can have a variety of uses like keeping track of the state of a system or have data related 5 00:00:22,650 --> 00:00:24,720 to, say, physical attributes of the robot. 6 00:00:25,810 --> 00:00:31,900 Let's go ahead and open up a terminal and run our RPM pub node we created in the publishers and subscriber 7 00:00:31,900 --> 00:00:32,650 project. 8 00:00:41,440 --> 00:00:47,170 Once it's running, we'll go ahead and open a new terminal tab and launch the speed publisher we created 9 00:00:47,170 --> 00:00:48,430 in the same project. 10 00:00:53,040 --> 00:00:57,750 We'll open another terminal tab and we're going to run Ross to parameter list. 11 00:00:58,350 --> 00:01:02,230 Here we can see a list of parameters sorted by Node. 12 00:01:02,250 --> 00:01:07,600 In this case, both our nodes have one parameter called use SIM time. 13 00:01:07,620 --> 00:01:10,230 This is a default parameter added to nodes. 14 00:01:10,860 --> 00:01:15,700 We can see the value of this node by using the ROS two parameter get command. 15 00:01:15,720 --> 00:01:17,760 This command takes in the name of the package. 16 00:01:17,760 --> 00:01:22,800 So for instance, we can do the RPM publisher node and then the name of the parameter we want to see 17 00:01:22,800 --> 00:01:23,310 the value of. 18 00:01:23,340 --> 00:01:30,690 In this case, use SIM time so we can see the parameter is set to false by default, which means it's 19 00:01:30,690 --> 00:01:33,630 using the time on our computer instead of a time from a simulation. 20 00:01:33,630 --> 00:01:37,920 You can set it to true if you wanted to use the time being generated by a simulation you had running. 21 00:01:38,340 --> 00:01:43,200 If you want to see more information about a parameter, you can use the ROS two parameter type command 22 00:01:43,200 --> 00:01:45,120 with the same arguments. 23 00:01:45,360 --> 00:01:49,380 Here we can see the type and any constraints the parameters may have. 24 00:01:50,250 --> 00:01:50,550 All right. 25 00:01:50,550 --> 00:01:52,710 So I'm going to go ahead and stop all of these nodes. 26 00:01:54,280 --> 00:01:54,670 All right. 27 00:01:54,670 --> 00:01:57,880 So how can parameters be useful for us in our Ross code? 28 00:01:58,030 --> 00:02:04,930 Well, in my RPM pub node, I hardcoded the value of our RPM to simulate a wheel encoder sensor going 29 00:02:04,930 --> 00:02:06,310 at a constant rate. 30 00:02:06,490 --> 00:02:11,590 Instead, let's use a parameter so that if someone wants to change the RPM value, they don't have to 31 00:02:11,590 --> 00:02:12,940 edit my source code. 32 00:02:13,360 --> 00:02:15,400 We also did this in the speed calc notes. 33 00:02:15,410 --> 00:02:19,180 If I go ahead and open that up, we hardcoded in a wheel radius. 34 00:02:19,210 --> 00:02:23,080 Now this is a potential value we would change if we were using it with a different robot, which makes 35 00:02:23,080 --> 00:02:25,070 it prime to use it as a parameter. 36 00:02:25,090 --> 00:02:29,890 So let's go ahead and use a parameter by using the declared parameter function right after I initialize 37 00:02:29,890 --> 00:02:36,580 my node to create a parameter called wheel radius and set the default value to our wheel radius variable. 38 00:02:36,940 --> 00:02:41,740 To make this a bit more clear, I will just state that this is the default wheel radius variable. 39 00:02:51,730 --> 00:02:56,680 So for the time being, I'll just also set this wheel radius variable here to wheel radius default. 40 00:02:57,310 --> 00:02:59,920 But we'll be changing this to use our parameter in a bit. 41 00:02:59,920 --> 00:03:05,710 But before we do that, I want to print out some things about our parameter so you can see how it works. 42 00:03:05,710 --> 00:03:08,460 So we can use the get parameter function of our node. 43 00:03:08,470 --> 00:03:13,210 So this function will return a parameter object in order to get the actual value from the parameter. 44 00:03:13,210 --> 00:03:17,000 We have to use the get parameter value method of this particular class. 45 00:03:17,000 --> 00:03:18,160 So we'll go ahead and print it out. 46 00:03:18,550 --> 00:03:22,330 We will have to include one more thing, but let's just see what this looks like so far. 47 00:03:22,330 --> 00:03:24,700 So I'll go ahead and save the file. 48 00:03:24,820 --> 00:03:30,490 And since we're using can build Simulink install, we don't need to recompile our workspace and instead 49 00:03:30,490 --> 00:03:34,480 I'll just head over to our terminal where we are running our speed calc node. 50 00:03:34,630 --> 00:03:39,160 And if I run it, we get this bound method, get parameter value. 51 00:03:39,160 --> 00:03:43,150 And so that's actually because I forgot to put the parentheses to actually call this function. 52 00:03:43,390 --> 00:03:44,440 Let's try this one more time. 53 00:03:44,440 --> 00:03:45,310 Save that. 54 00:03:49,640 --> 00:03:50,750 And we'll rerun the node. 55 00:03:51,170 --> 00:03:55,030 So here we can see printing through the screen the contents of the parameter value message type. 56 00:03:55,040 --> 00:03:59,930 We can see this parameter is type three, which means it is of a type double. 57 00:03:59,930 --> 00:04:07,040 And in the double value section we see the value is 0.125, which is the value we set the default wheel 58 00:04:07,040 --> 00:04:07,820 radius variable. 59 00:04:07,820 --> 00:04:14,480 As we can also see, this parameter is available if we use the ROS two parameter list command and then 60 00:04:14,480 --> 00:04:15,860 use raw super and get. 61 00:04:21,750 --> 00:04:26,910 If we wanted to, we could set a different value for this parameter using the ROS two parameter set 62 00:04:26,910 --> 00:04:27,630 command. 63 00:04:27,660 --> 00:04:31,440 In this case, I can set the wheel radius to 0.5. 64 00:04:34,210 --> 00:04:41,590 And now if I get this parameter one more time, we see that it is indeed changed to 0.5 and this will 65 00:04:41,590 --> 00:04:43,390 also take effect within our node. 66 00:04:43,420 --> 00:04:49,840 So let's go ahead and wrap this up by adjusting our speed calc node to instead use this parameter value 67 00:04:49,840 --> 00:04:52,780 instead of the default wheel radius variable we used. 68 00:05:09,650 --> 00:05:12,770 And we will call the double value to get. 69 00:05:13,990 --> 00:05:17,800 The particular value stored in this double value part of the object. 70 00:05:18,250 --> 00:05:20,500 And I'll go ahead and stop this node from running. 71 00:05:23,950 --> 00:05:28,390 And we can go ahead and change instead of using this default wheel radius variable to instead use the 72 00:05:28,390 --> 00:05:29,080 parameter. 73 00:05:30,900 --> 00:05:37,050 All right, So now in our calculate speed publisher, we check for what the value of the wheel radius 74 00:05:37,050 --> 00:05:40,500 parameter is and get the double value of it. 75 00:05:41,210 --> 00:05:45,620 Then we calculate the speed using that parameter value. 76 00:05:46,190 --> 00:05:46,520 Great. 77 00:05:46,520 --> 00:05:49,820 So with that, we can go ahead and save this speed calc node. 78 00:05:51,150 --> 00:05:52,290 Open up a terminal. 79 00:05:53,150 --> 00:05:54,140 Now click the screen. 80 00:05:55,690 --> 00:05:59,860 And now if I rerun both my speed calc node and my RPM pub node. 81 00:06:01,990 --> 00:06:07,960 I can go ahead and clear the screen on my third terminal and echo out the speed topic. 82 00:06:10,830 --> 00:06:11,190 All right. 83 00:06:11,190 --> 00:06:14,940 So here we see the information coming in from the speed topic. 84 00:06:16,670 --> 00:06:20,600 And if I open up a fourth terminal, I can go ahead and change the parameter value. 85 00:06:20,600 --> 00:06:27,380 So, Ross two parameter set the name of the node and then the name of the parameter we want to change. 86 00:06:27,380 --> 00:06:31,100 So in this case, I'll set the wheel radius parameter to 0.5. 87 00:06:32,230 --> 00:06:36,760 And now that the parameters change, we see that our speed calculations have also changed with it. 88 00:06:36,760 --> 00:06:42,040 So it is successfully using our parameter to help calculate the given speed of our robot. 89 00:06:42,730 --> 00:06:43,720 Congratulations. 90 00:06:43,720 --> 00:06:47,980 You successfully implemented parameters in your very own ROS to code. 91 00:06:48,190 --> 00:06:52,990 It is worth noting that if you have multiple nodes that you want to share parameter values with, you 92 00:06:52,990 --> 00:06:58,240 can use ROS services to request the parameter variables between nodes. 93 00:06:58,270 --> 00:07:01,870 Now we'll learn about services later on in the course, so don't worry about it for now. 94 00:07:02,570 --> 00:07:05,030 In this lecture, we talked about parameters. 95 00:07:05,030 --> 00:07:11,240 We went over a few different terminal commands such as RAS, Super M list, get and set in order to 96 00:07:11,240 --> 00:07:15,200 look at our parameters and see what their values are as well as set them. 97 00:07:15,710 --> 00:07:21,770 We then learn how to use parameters in Python by utilizing the declare parameter function of our node, 98 00:07:21,770 --> 00:07:25,310 which we pass in a parameter name and a particular value. 99 00:07:25,340 --> 00:07:30,530 We can then utilize our nodes, get parameter command to then get this parameters value. 100 00:07:31,810 --> 00:07:37,060 In the next lecture, we'll be learning about launch files where we can automate the launching of nodes 101 00:07:37,060 --> 00:07:39,880 as well as setting of parameter values.