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Make a Mobile Robot

Overview

The tutorial demonstrates Gazebo's basic model management, and exercises familiarity with basic model representation inside the model database by taking the user through the process of creating a two wheeled mobile robot that uses a differential drive mechanism for movement.

Setup your model directory

Read through the Model Database documentation. You will be creating your own model, which must follow the formatting rules for the Gazebo Model Database directory structure. Also, for details on model description formats, please refer to the SDF reference.

Create a model directory:
```
mkdir -p ~/.gazebo/models/my_robot
```

Create a model config file:

gedit ~/.gazebo/models/my_robot/model.config

Paste in the following contents:

<?xml version="1.0"?>
<model>
  <name>My Robot</name>
  <version>1.0</version>
  <sdf version='1.4'>model.sdf</sdf>

  <author>
   <name>My Name</name>
   <email>me@my.email</email>
  </author>

  <description>
    My awesome robot.
  </description>
</model>

Create a ~/.gazebo/models/my_robot/model.sdf file.
```
gedit ~/.gazebo/models/my_robot/model.sdf
```

Paste in the following.

<?xml version='1.0'?>
<sdf version='1.4'>
  <model name="my_robot">
  </model>
</sdf>

At this point we have the basic contents for a model. The model.config file describes the robot with some extra meta data. The model.sdf file contains the necessary tags to instantiate a model named my_robot using Gazebo linked against SDF version 1.4.

Build the Model's Structure

This step will create a rectangular base with two wheels.

It is important to start simple, and build up a model in steps. The first step is to layout the basic shapes of the model. To do this we will make our model static, which means it will be ignored by the physics engine. As a result the model will stay in one place and allow us to properly align all the components.

Make the model static by adding a <static>true</static> element to the ~/.gazebo/models/my_robot/model.sdf file:

    <?xml version='1.0'?>
    <sdf version='1.4'>
      <model name="my_robot">

        <static>true</static>

      </model>
    </sdf>

Add the rectangular base by editing the ~/.gazebo/models/my_robot/model.sdf file:

    <?xml version='1.0'?>
      <sdf version='1.4'>
        <model name="my_robot">
        <static>true</static>

          <link name='chassis'>
            <pose>0 0 .1 0 0 0</pose>

            <collision name='collision'>
              <geometry>
                <box>
                  <size>.4 .2 .1</size>
                </box>
              </geometry>
            </collision>

            <visual name='visual'>
              <geometry>
                <box>
                  <size>.4 .2 .1</size>
                </box>
              </geometry>
            </visual>
          </link>

      </model>
    </sdf>

Here we have created a box with a size of 0.4 x 0.2 x 0.1 meters. The collision element specifies the shape used by the collision detection engine. The visual element specifies the shape used by the rendering engine. For most use cases the collision and visual elements are the same. The most common use for different collision and visual elements is to have a simplified collision element paired with a visual element that uses a complex mesh. This will help improve performance.

Try out your model by running gazebo, and importing your model through the Insert Model interface on the GUI.
```
gazebo
```
You should see a white box floating .1 meters above the ground plane.

Now we can add a caster to the robot. The caster is a sphere with no friction. This kind of caster is better than adding a wheel with a joint since it places fewer constraints on the physics engine.

    <?xml version='1.0'?>
    <sdf version='1.4'>
      <model name="my_robot">
        <static>true</static>
        <link name='chassis'>
          <pose>0 0 .1 0 0 0</pose>
          <collision name='collision'>
            <geometry>
              <box>
                <size>.4 .2 .1</size>
              </box>
            </geometry>
          </collision>

          <visual name='visual'>
            <geometry>
              <box>
                <size>.4 .2 .1</size>
              </box>
            </geometry>
          </visual>

          <collision name='caster_collision'>
            <pose>-0.15 0 -0.05 0 0 0</pose>
            <geometry>
                <sphere>
                <radius>.05</radius>
              </sphere>
            </geometry>

            <surface>
              <friction>
                <ode>
                  <mu>0</mu>
                  <mu2>0</mu2>
                  <slip1>1.0</slip1>
                  <slip2>1.0</slip2>
                </ode>
              </friction>
            </surface>
          </collision>

          <visual name='caster_visual'>
            <pose>-0.15 0 -0.05 0 0 0</pose>
            <geometry>
              <sphere>
                <radius>.05</radius>
              </sphere>
            </geometry>
          </visual>

        </link>
      </model>
    </sdf>

Try out your model to make sure the caster appears at the end of the robot. Spawn it in gazebo to see (you don't need to restart Gazebo; it will reload your modified model from disk each time you insert it):

Now let's add a left wheel. Modify the ~/.gazebo/models/my_robot/model.sdf file to be the following:

    <?xml version='1.0'?>
    <sdf version='1.4'>
      <model name="my_robot">
        <static>true</static>
        <link name='chassis'>
          <pose>0 0 .1 0 0 0</pose>
          <collision name='collision'>
            <geometry>
              <box>
                <size>.4 .2 .1</size>
              </box>
            </geometry>
          </collision>

          <visual name='visual'>
            <geometry>
              <box>
                <size>.4 .2 .1</size>
              </box>
            </geometry>
          </visual>

          <collision name='caster_collision'>
            <pose>-0.15 0 -0.05 0 0 0</pose>
            <geometry>
              <sphere>
              <radius>.05</radius>
            </sphere>
          </geometry>

          <surface>
            <friction>
              <ode>
                <mu>0</mu>
                <mu2>0</mu2>
                <slip1>1.0</slip1>
                <slip2>1.0</slip2>
              </ode>
            </friction>
          </surface>
        </collision>

        <visual name='caster_visual'>
          <pose>-0.15 0 -0.05 0 0 0</pose>
          <geometry>
            <sphere>
              <radius>.05</radius>
            </sphere>
          </geometry>
        </visual>
      </link>

      <link name="left_wheel">
        <pose>0.1 0.13 0.1 0 1.5707 1.5707</pose>
        <collision name="collision">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </visual>
      </link>

      </model>
    </sdf>

Run Gazebo, insert your robot model and make sure the wheel has appeared and is in the correct location.

We can make a right wheel by copying the left wheel, and adjusting the wheel link's pose:

    <?xml version='1.0'?>
    <sdf version='1.4'>
      <model name="my_robot">
        <static>true</static>
        <link name='chassis'>
          <pose>0 0 .1 0 0 0</pose>
          <collision name='collision'>
            <geometry>
              <box>
                <size>.4 .2 .1</size>
              </box>
            </geometry>
          </collision>

          <visual name='visual'>
            <geometry>
              <box>
                <size>.4 .2 .1</size>
              </box>
            </geometry>
          </visual>

          <collision name='caster_collision'>
            <pose>-0.15 0 -0.05 0 0 0</pose>
            <geometry>
              <sphere>
              <radius>.05</radius>
            </sphere>
          </geometry>

          <surface>
            <friction>
              <ode>
                <mu>0</mu>
                <mu2>0</mu2>
                <slip1>1.0</slip1>
                <slip2>1.0</slip2>
              </ode>
            </friction>
          </surface>
        </collision>

        <visual name='caster_visual'>
          <pose>-0.15 0 -0.05 0 0 0</pose>
          <geometry>
            <sphere>
              <radius>.05</radius>
            </sphere>
          </geometry>
        </visual>
      </link>
      <link name="left_wheel">
        <pose>0.1 0.13 0.1 0 1.5707 1.5707</pose>
        <collision name="collision">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </visual>
      </link>

      <link name="right_wheel">
        <pose>0.1 -0.13 0.1 0 1.5707 1.5707</pose>
        <collision name="collision">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </visual>
      </link>

      </model>
    </sdf>

At this point the robot should have a chassis with a caster and two wheels.

Make the model dynamic by setting <static> to false, and add two hinge joints for the left and right wheels.

    <?xml version='1.0'?>
    <sdf version='1.4'>
      <model name="my_robot">

        <static>false</static>

        <link name='chassis'>
          <pose>0 0 .1 0 0 0</pose>
          <collision name='collision'>
            <geometry>
              <box>
                <size>.4 .2 .1</size>
              </box>
            </geometry>
          </collision>

          <visual name='visual'>
            <geometry>
              <box>
                <size>.4 .2 .1</size>
              </box>
            </geometry>
          </visual>

          <collision name='caster_collision'>
            <pose>-0.15 0 -0.05 0 0 0</pose>
            <geometry>
              <sphere>
              <radius>.05</radius>
            </sphere>
          </geometry>

          <surface>
            <friction>
              <ode>
                <mu>0</mu>
                <mu2>0</mu2>
                <slip1>1.0</slip1>
                <slip2>1.0</slip2>
              </ode>
            </friction>
          </surface>
        </collision>

        <visual name='caster_visual'>
          <pose>-0.15 0 -0.05 0 0 0</pose>
          <geometry>
            <sphere>
              <radius>.05</radius>
            </sphere>
          </geometry>
        </visual>
      </link>
      <link name="left_wheel">
        <pose>0.1 0.13 0.1 0 1.5707 1.5707</pose>
        <collision name="collision">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </visual>
      </link>

      <link name="right_wheel">
        <pose>0.1 -0.13 0.1 0 1.5707 1.5707</pose>
        <collision name="collision">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </visual>
      </link>

      <joint type="revolute" name="left_wheel_hinge">
        <pose>0 0 -0.03 0 0 0</pose>
        <child>left_wheel</child>
        <parent>chassis</parent>
        <axis>
          <xyz>0 1 0</xyz>
        </axis>
      </joint>

      <joint type="revolute" name="right_wheel_hinge">
        <pose>0 0 0.03 0 0 0</pose>
        <child>right_wheel</child>
        <parent>chassis</parent>
        <axis>
          <xyz>0 1 0</xyz>
        </axis>
      </joint>

      </model>
    </sdf>

The two joints rotate about the y axis <xyz>0 1 0</xyz>, and connect each wheel to the chassis.

Start gazebo, and insert your model. Click on the dots to the right of the screen and drag them to the left.
A new window should appear that contains various controllers for each joint. (Note Make sure the model you want to control is selected)
Under the Force tab, increase the force applied to each joint to about 0.1N-m. The robot should move around:
Congrats, you now have a basic mobile robot.

Try for yourself

Be creative and make a new robot.

Idea: A quadruped that consists of torso with four cylindrical legs. Each leg is attached to the torso with a revolute joint.

Idea: A six wheeled vehicle with a scoop front loading mechanism.

Next: Attach Meshes

Make a Mobile Robot

Table of Contents

Overview

Setup your model directory

Build the Model's Structure

Try for yourself

Next