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In the last two lectures, we started using the ignition gazebo simulation and customizing our own simulation

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by creating PDF files to customize our simulation world and models.

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At the time of this recording, Ignition Gazebo does not have a built in model editor, so we'll be

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using the older Gazebo Classic to make our robot model.

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There are other three D design and CAD software you could use to export into the PDF format, but I'll

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try to keep this section constrained to Gazebo and ROS as much as possible.

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So for the time being, we can head over to the Gazebo Classic installation page.

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The easiest way to go about installing the Zeebo is by running the installation script directly from

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the website like so.

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Down below.

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There are alternative installation methods which you can try out if the insulation script fails for

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some reason.

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But I'm just going to go ahead and install it like so.

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Don't have to put in your password.

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And after a bit, that should have installed.

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So go ahead and clear the screen.

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So now we can go ahead and start Gazebo Classic by simply typing in Gazebo.

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All right.

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So it appeared behind my terminal.

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So this is a gazebo by default.

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There is nothing in our scene to navigate.

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Gazebo.

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Use your left mouse button to pan around the scene.

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The right mouse button or scroll wheel to zoom in or out, and then shift left click or press the scroll

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wheel button to rotate around a point in the scene.

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Gazebo has a series of pre-made models which you can use in your simulations.

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If I select insert mode, we can go ahead and head over to the model's gazebo sim dot org dropdown.

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This might take a while to load if you've just installed gazebo to connect to the database, but essentially

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this dropdown allows me to download these models from Gazebos website.

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I can go ahead and select the ambulance and once that's gone ahead and download it, I'll go ahead and

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zoom out with my scroll wheel.

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We can see I can place it within my scene so I can just go ahead and left click and now it's within

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my simulation.

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So feel free to check out the other models that are available.

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So great.

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With that, we have imported a new model in our simulation.

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Now this model is actually very basic, so the wheels don't even spin.

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It's just it's essentially a cube wrapped with this ambulance skin, so you're not able to drive it

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around.

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But we could create our own model which would allow us to do that.

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So go ahead and click on this ambulance.

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You can either hit the delete key on your keyboard or just right click and delete.

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Then if we head over to the edit toolbar tab, I can select model editor and here we can create a model

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using basic three D shapes.

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Let's make a super basic model that is just a box with two wheels and a sphere to act as a caster.

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So first is to add a cube.

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So if I go over to this left panel here, select box, I can go ahead and place it down in my simulation.

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Now, if I double click on this box, I can edit this properties within the link Inspector tab.

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So within the link tab I can scroll down to edit its posts.

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Now keep in mind there are two post sections which might be a little bit confusing, so there is a post

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subsection within inertial tab.

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So usually I just like to close that so I don't get confused.

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And here we just see the standard post tab which controls where our square is within the simulation

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world coordinates.

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So for example, I can try and center it so I can go ahead and put it at zero zero.

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Then besides the link tab, we have the visual and collision tabs, so go ahead and expand those.

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And basically this is what's going to dictate our properties for how these things look and act within

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the simulation.

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So here within the visual tab, I can scroll down to geometry and I can change the X, y, Z attributes

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of this cube.

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So in this case, I can go ahead and set the Y and Z parameters 2.5 meters to make this look more like

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a rectangle.

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So in the main window we can see our changes, but you notice there's a transparent outline of the original

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cube.

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Well, that's actually the collision model we are looking at.

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So if I switch to the collision tab.

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There is again a geometry subtab in which we'd have to change the attributes again.

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So sometimes you'll have a visual model which has a rap or looks slightly different compared to the

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collision model, which you may simplify or have be slightly modified.

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But for now I'm just going to set it to be the same thing.

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So 0.5 Y and 0.54 Z, and now we see that matches up.

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So this collision model is what's used for the physics engine rendering.

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This way you could create a complicated visual appearance, but keep the collision model which will

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have physics applied to it in a simple manner.

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So moving on, I'll go ahead and hit OC.

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So now I'll go ahead and make my two wheels using cylinders.

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I will double click on it, edit its pose so we can turn it on its side again.

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Remember that there's two poses.

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One is for the inertial physics calculation, so be sure to leave that as is.

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So go ahead and collapse that and we can head down to the pose that's in its own section and I can change

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the roll to 1.570796 radians, which roughly equates to 90 degrees of rotation.

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Now we see the cylinder has rotated.

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Now I will edit the size of the cylinder by changing the length to 0.25 meters in both the visual and

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collisions tab.

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So now I have my wheel and you could go ahead and move it however you want in the simulation to configure

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where you want to place it relative to your other components.

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So I'm actually going to go ahead and double click on this and place it where I want to.

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So if I go back into link and into pose.

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Set this to -0.375.

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-0.4 and 0.5.

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And so this now acts as a type of back wheel for our robot.

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On the right side, if that's the front.

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So I can go ahead and hit OC and now I can duplicate this wheel by clicking on it and right, clicking,

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doing copy and we can right click and paste.

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And now I have a cylinder with the same attributes and I'll go ahead and click on it and edit its position.

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Again.

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Don't edit the inertial posts.

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Close this tab.

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This is the pose we want to edit.

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All righty.

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Now, to complete this model, I'm going to go ahead and add in the sphere caster.

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So I'll go ahead and add a sphere.

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And set the size to 0.25 metres.

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All right.

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So I've gone ahead and done that and I'm actually going to replace the sphere.

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So it's at the front of our robot chassis.

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All right, So now I've gone ahead and place it in my simulation.

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Now, the other thing I'm actually going to do is I'm going to double click on this one more time and

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go into collision.

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And if we scroll down, we'll see there's surface friction for these attributes on how we want this

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to interact within our simulation.

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So we want this to nicely slide against the ground plane.

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So I'm going to have to set these friction variables MU and mu two to be something smaller because the

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value of one is similar to having a very rough surface, which is hard to slide across.

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So I want this to be more of a smooth interaction.

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So I'll go ahead and cut this down to say 0.1 for both of them.

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So go ahead and hit Ock.

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And to make this look a little bit less awkward, I'm actually going to extend my cube out a little

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bit.

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So I'll set the X direction to instead be 1.5 and make sure to change it in the collision tab as well.

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Okay, that looks a little bit better.

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So we have this little rectangular chassis with two wheels and a spherical caster meant for sliding.

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Now we have to create joints in order for these different shapes to be combined together.

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So we can do this by selecting the joint icon in the top.

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Then we'll have to choose our parent link, which in this case is our box.

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And then the child will be the wheel, which in this case is closer to me.

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So the back right wheel.

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So above we can see it selected as a resolute type.

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We could change this to one of the other available joints, but note that fixed, resolute and prismatic

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are the most supported joints.

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If you wanted to control these joints and Ross, I will leave it as resolute and I will select negative

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Z axis for the rotation.

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So you can actually see how this changes in our.

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X, Y, and Z.

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So now we have this yellow axis pointing here in our axis of rotation.

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I'll go ahead and hit, create and do the same thing for our other wheel.

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Then last thing.

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I need to fix this sphere to the box.

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So for joint type we will use type fixed.

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So we don't need to worry about any axes of rotation because it's just going to stay right there.

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And I'll go ahead and hit create.

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All right.

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That about covers it for this simple model so I can go to file and exit model editor, and it's going

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to ask if you want to save changes before exiting.

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So we're going to go ahead and hit, save and exit.

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And here we see we're going to be naming our model where it gets saved by default in our home folder

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in a folder called Model Editor Models.

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So I'm just going to go ahead and call this wheeled model and hit save.

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Once I hit Save, we are now back in our simulation world.

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If I go over to the left panel, we now see that the wheel model is part of our list of models, and

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it shows all of our individual links as well as joint relationships.

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If I wanted to interact with these joints, I can drag out the right panel here that's hidden.

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And if I select my robot, we can actually see we can interact with each joint by either applying a

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force, setting a position or a velocity.

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So before I set everything, I will make sure my simulation is paused, which you can check by looking

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at the bottom left where we see this is still the play button.

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Our timer is not playing.

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So I'll go ahead and set the velocity for the first two joints to one meter per second.

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Then on the right, I can set the values of the PID controller, which will try to apply the force of

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the wheel to get it to maintain that speed.

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In this case, I'll increase the gain to three for both of them.

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Now, if I go ahead and play the simulation, we should see the model begin to drive because we are

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turning the wheels with this particular velocity.

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Great.

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With that we are able to set our wheels to move at a certain rate.

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I'll go ahead and pause this for now.

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If I wanted to spin our robot around, I could change the speed to be negative on one wheel in order

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to make our little wheeled bot turn.

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So now if I hit play.

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We see it is now rotating because one wheel is going forward and the other wheel is going backwards.

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If I wanted to place the robot back to the way it was, I can go to edit and select reset model poses

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and that will place it back at the center of my simulation.

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Anyway, this is a good introduction to Gazebo Classic for those of you who have not used it before,

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and hopefully more of these features that you're seeing here get included into the newer ignition gazebo

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interface.

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For now, we have an SDF of our simple wheeled robot model, so I'll go ahead and close out of Gazebo

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Classic.

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You know, we'll go ahead and close our browser.

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Now I'll open up the file Explorer.

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And here in our home folder, we have a model editor models folder which contains our wheeled modeled

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robot folder that has the SDF and config file.

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Now, go ahead and take this model, copy it over and place it into our ROS package within the models

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folder.

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So here we have our ground plane from earlier and now we have this wheeled model within our package.

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Now let's open up V's code and create a new PDF World file to include this model in our underworld.

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I'll hit new file and I'm just going to call this new file wheeled model world that SDF.

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Then I'll copy over the contents from our test world from earlier.

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And I'll paste it in and I'm going to delete the camera model that we were playing with earlier so that

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our environment only contains this ground plane and a sun model and our normal plug ins.

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And now we'll include our wheeled robot model.

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So now I'll go ahead and save this and open up our terminal and set up our gazebo environment variable.

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And let's go ahead and run our wheeled model world file.

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So here we see our model partly wedged into the ground here.

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So I can just go into translate mode and pull it out of the ground.

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But we indeed see that the model we created in Gazebo Classic is able to transfer over to Ignition Gazebo.

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So in this lecture off, let's take a quick look at how we can use plug ins to allow us to use the joints

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that we made.

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To show an example of this, we can check out the diff drive example PSD file.

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So again, here is the world directory with the example PDF files from earlier.

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So again, go into other locations.

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Computer user share, Ignition, Ignition, gazebo, six and Worlds.

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And the file that we're interested in this time around will be diff drive dot SDF.

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So I'm going to go ahead and open this in a standard text editor.

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Now let's also go ahead and launch this in ignition just so that you can see what this file entails

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within the simulation.

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All right.

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So here we see two wheeled models somewhat similar to what we just made.

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So if I go ahead and hit play, nothing really happens.

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But we can actually go ahead and control these two robots by publishing messages to them.

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And if we can control it through ignition, you already know we can just bridge the topic to work in

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ROS using the parameter bridge.

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But for now we will just try this out by copying the command that it actually shows within the top of

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the file.

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So here it says Differential drive plug in demo.

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Try sending the commands ignition topic dashed and you specify the topic, which in this case we're

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commanding the blue vehicle from the command velocity topic.

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The message type is a twist message, and we're publishing a linear and angular part to twist.

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And in this case we're saying go forward in the x direction 0.5 meters a second about and give it an

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angular velocity in the Z axis direction of 0.05.

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So I'll go ahead and copy this.

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Open a second terminal.

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And I'll go ahead and run this message.

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And now we see this purple robot moving forward and slightly to the left since we had that Z angular

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aspect to it.

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And now if I go ahead and go up and set these back to zero, I.

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We should see that the model stops moving.

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And again, this particular simulation is set up with a blue vehicle and a green vehicle with these

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topics already nicely set up for us so we could go ahead and move the second vehicle if we wanted to

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by simply changing that name.

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And I'll just go forward in this case a little bit and change it from blue to green.

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And now we see the green robot moving directly forward.

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And again, we can stop this by changing all the values to zero and publishing it again.

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All right.

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For now, go ahead and pause the simulation and we're going to head back to the C file so we can see

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how this plugin is implemented.

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So this plugin is called twice in this file one for each robot, but for now I'm just going to scroll

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all the way down to the bottom where the second declaration is, since it's easiest to navigate to.

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So here within this secondary model tags, right, we have a plugin called Ignition Gazebo Diff Drive

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System and it points to the particular driver.

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And here we see a couple of attributes such as the left, joint right, joint wheel separation wheel

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radius and as well as some max and minimums for acceleration and velocity.

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If you want more info on the available parameters for the diff drive plugin, you can check out the

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gazebo Simu GitHub repository and check out the comments in the diff drive header file.

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But for now, let's implement this plugin into our wheeled robot model we created to make it more interactive.

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So I'll copy this code snippet.

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And put it towards the end of the model file of our wheeled model.

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Now, if you ever get errors like this, some of the namespaces for HTML does not match up.

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So all we have to do is set this back to XML and we lose some of that functionality to automatically

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autocomplete some scripting for us, but at least it will get rid of that those red error message lines.

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But I'll go ahead and scroll all the way down.

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And right before the model tag ends, we'll go ahead and paste in our plugin.

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So now I'll go ahead and change the parameters to match our setup.

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So if you follow along with my setup, each wheel was 0.4 meters away from the origin, so my wheel

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separation is 0.8 meters and I'll set the wheel radius to 0.5.

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Then our joint names were automatically created in the gazebo model editor.

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So if we scroll up to the top.

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We can see after our model tag we have a link tag.

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This link is the box we started off with.

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As we can see specified in the geometry tag.

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So let's do some renaming in this file so that it makes a bit more sense.

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So if I hit control H in V's code, this will pull up the find and replace window.

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You could also go into the edit tab and go to replace and let's rename link zero so it makes more sense.

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In this case I will name it to chassis since this box acts like a robot chassis that everything is mounted

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to.

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And the reason we are utilizing this replace all functionality is we want to make sure we keep together

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all of our linked connections.

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So all of our joints which are utilizing these link names, we want to make sure that these all get

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changed correctly.

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So I'll just go ahead and hit replace all.

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And if you're unsure, you could actually go ahead and check this.

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So here we see the joint name link zero to Joint zero.

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So now it'll change it to chassis to join zero and it'll change the parent name link zero to match chassis

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and it does that for each of our corresponding joints.

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So I'll go ahead and hit replace all.

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So now if we go over to the next link, it should be called Link one.

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And this is the first will we put onto our robot.

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So if you consider the cluster as a part of the front of the robot, I believe I started with the right

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wheel so I can change it as such.

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So now notice if we hit this arrow button, we see another link called Link one clone.

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Now, that's because I clone the original link, which was our wheel to create a secondary wheel.

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So actually what I'll go ahead and do is go to link one and set it to underscore clone so that we only

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get two of these matches and set that to our corresponding left wheel link.

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And now the normal link one will be our right, we'll link.

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And again, it's just the name of the link that's changing and the child parameter within our joints.

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Then our last link should be link two, which is our caster, so I can just change it as such.

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Great with that.

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All of our linked names make a bit more sense.

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So now let's take a look back at our joint names from our Diff drive plugin.

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So here it's looking for the left joint with a name left wheel joint so we can change this to match

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or if we go back to our joints.

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We can see here Joint zero, joint one and joint two and what each one is doing.

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So we can see chassis joint zero.

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He's combining the chassis to the right.

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We'll link so we can just simply change the name for simplicity to right wheel joint.

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And we can go down to our second joint, which combines the chassis and the left wheel link and we can

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just simply change it to left wheel joint.

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Now we can make sure that these names match up with what we have here for our ignition gazebo plug in,

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which in this case I actually made them a match.

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So left wheel joint here corresponds to the left wheel joint that we have here within the model PSD

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file, which combines the chassis and left wheel link.

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So ideally, this driver will now take control of that particular wheel whenever it needs to.

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Last thing I want to do here is set a post for our robot.

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Since if you remember when we launched our simulation, our robot was partially in the ground.

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So I'll go ahead and save this model file and head over to our wheeled model world file and I will add

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a post tag right after the include tag to put us half a metre off the ground in the Z direction.

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So again, each number represents X, y, z position followed by roll, pitch, yaw.

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All right.

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We'll go ahead and save everything and relaunch our wheeled model world simulation file.

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So here we see our total ground plane and our wheeled model we created in gazebo.

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So I'll go ahead and clear this second screen here.

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So we can control this robot by using the ignition topic command we had used earlier.

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But change the topic name.

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So if I hit up, we can see we use this ignition topic to model vehicle green command velocity.

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So by default, if you check out the GitHub repo notes, if you don't specify a topic within the parameters,

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this simply gets changed.

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The name of our model, which in this case is just called wheeled model here.

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So let's go ahead and change the X parameter to 0.5 so that we can drive forward and we will change

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vehicle green to wheeled model.

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So go ahead and publish that.

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Now, of course, to make sure the simulation is playing and we see our robot goes down and starts driving

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forward.

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And of course we can always stop it by setting this to zero.

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And there we go.

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The model stops.

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Now, currently we are controlling this robot with ignition commands.

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So all we have to do to port this over to Ros is use the ignition gazebo bridge and specify the corresponding

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topic.

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Basically, we are taking the ignition topic and then the ampersand symbol, then the ROS message type,

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which in this case is the twist.

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Message type is from geometry messages ROS package.

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And then I'll include another symbol and then the ignition message type.

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Then I'm passing in ros args with the dash r flag which is remapping the topic name from the current

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topic name to just command velocity.

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Keep in mind the ampersand symbol between the ROS message and the ignition message means that communication

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can come over and received between Ros and an ignition over this topic.

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Whereas if I only use the left square bracket, that means I can only receive messages from ignition

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to ROS.

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And the right bracket means only messages from Ros get sent to ignition.

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I just do the ampersand symbol for both because it's easier for me to remember.

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But just keep that in mind if you ever want to restrict which way data flows.

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So I'll go ahead and run this, then open a third terminal window.

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And we should be able to see this new command of topic.

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Which we do right here.

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So for simplicity, I'm going to open up our cut and publish this topic.

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So go ahead and close our image plugin from the last lecture.

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Head over to topics message publisher and refresh our topic list.

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Head over to command, fill the addition symbol.

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And now if we hit these dropdowns, we have the linear and angular portions of our topic, so I'll go

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ahead and minimize this a bit.

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And now let's go ahead and set the speed to say 0.5.

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And as soon as I hit the check mark, this will begin publishing and we should see our robot move.

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And I can set this back to zero again.

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And if we wanted to, we could make our robot spin for fun.

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So if I set this Z aspect to one, we should see our robot rotate around.

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And if I hit negative one, we will go in the opposite direction.

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And of course, we can add multiple components at the same time to have our robot both try to drive

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and spin around at the same time.

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Now I guess it's wants to spin around more than it wants to move.

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So there we go.

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We can see it's just slightly reversing and slightly wanting to turn.

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And of course, I'm going to set these both to zero.

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So congratulations on making it through this lecture.

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I know it was quite the stretch from making our model in the Gazebo Classic editor to learning about

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the differential drive plug in to then implementing that plug in into our model we made to then bridging

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the ignition topic that interacts with the plug in over Ros using the ignition parameter bridge.

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Just a reminder that if you want to check out more built in plug ins, check out the gazebo sim and

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gazebo sensors, GitHub repos.