Gazebo_BlockDiagram

Source : https://classic.gazebosim.org/tutorials

A simplified version of the diagram above is shown below.

Source : https://www.researchgate.net/figure/Architecture-of-ROS-Gazebo-interaction-Mittal-2018_fig2_352871117

What is Gazebo?

• Gazebo is an open-source 3D robotics simulator.
• It is used to simulate and visualize robot motion.
• Gazebo is integrated with ROS, so it is used in the majority of ROS-based projects and has evolved into a simulator software framework in its own right.
• Its strengths include support for many types of robots, sensors, and environments; users can build their own robots and configure their own environments. Robots can also be controlled and sensed during simulation, which yields a large amount of data.
• It provides realistic environment rendering, including lighting, sunlight, shadows, textures, and underwater environments.
• Ignition appeared in 2017 alongside Gazebo Classic. (I have not used it much, but the framework seems to be somewhat lighter.)
• Gazebo Classic integrates the ODE physics engine, OpenGL rendering, and support code for sensor simulation and actuator control, enabling the use of a high-performance physics engine.

Reference image : aws_warehouse (https://github.com/aws-robotics/aws-robomaker-small-warehouse-world)

What makes up a Gazebo environment

A Gazebo environment consists broadly of the environment (World) and the Models (sdf, urdf, xacro) that exist inside it, such as robots and sensors.

Source : https://www.researchgate.net/figure/Gazebo-Robotic-Operating-System-ROS-interface-block-diagram-28_fig4_350175271

World

A world file is written in XML and describes the simulation environment, including the position and shape of robots and other objects, sensors, light sources, and physical properties.

• Model : Used to describe the robots, sensors, etc. in the environment. Each model is defined by its position, rotation, links, and joints.
• Physics Properties : Physical properties of the simulation, such as gravity, friction, and collisions.
• Lights : Various types of lighting for a visually realistic environment; position, type, intensity, etc. can be configured.
• Ground : Describes the floor of the simulation. Terrain, texture, color, etc. can be defined.

• Fluids : Can be configured if an underwater environment is needed in the simulation.

• Example of a .world file (example.world; the relevant parts are briefly explained in comments)

    <?xml version="1.0"?>
    <sdf version="1.6">
      <world name="example">
        
    <!-- set up the ground -->
        <include>
          <uri>model://ground_plane</uri>  
        </include>
          
    <!-- use the sun as the light source -->
        <include>
          <uri>model://sun</uri>
        </include>
          
    <!-- whether to render shadows -->
        <scene>
          <shadows>false</shadows>
        </scene>
          
    <!-- full-screen setting and the initial camera viewpoint at launch -->
        <gui fullscreen='0'>
          <camera name='user_camera'>
            <pose frame=''>0.319654 -0.235002 9.29441 0 1.5138 0.009599</pose>
            <view_controller>orbit</view_controller>
            <projection_type>perspective</projection_type>
          </camera>
        </gui>
          
    <!-- ODE physics engine settings -->
        <physics type="ode">
          <real_time_update_rate>1000.0</real_time_update_rate>
          <max_step_size>0.001</max_step_size>
          <real_time_factor>1</real_time_factor>
          <ode>
            <solver>
              <type>quick</type>
              <iters>150</iters>
              <precon_iters>0</precon_iters>
              <sor>1.400000</sor>
              <use_dynamic_moi_rescaling>1</use_dynamic_moi_rescaling>
            </solver>
            <constraints>
              <cfm>0.00001</cfm>
              <erp>0.2</erp>
              <contact_max_correcting_vel>2000.000000</contact_max_correcting_vel>
              <contact_surface_layer>0.01000</contact_surface_layer>
            </constraints>
          </ode>
        </physics>
          
    <!-- additional structures or robots can be declared here (optional) -->
    <!-- visual (rendering) and collision settings of the link -->
      
        <link name="wall">
          <visual name="wall">
            <geometry>
              <mesh>
                <uri>model://{world_repository}/meshes/wall.obj</uri>
              </mesh>
            </geometry>
            <transparency>0.0</transparency>
            <material>
              <diffuse>1.0 1.0 1.0</diffuse>
              <specular>0.1 0.1 0.1</specular>
              <pbr>
                <metal>
                  <metalness>0.0</metalness>
                  <albedo_map>model://{world_repository}/meshes/white_brick.png</albedo_map>
                </metal>
              </pbr>
              <script>
                <uri>model://{world_repository}/meshes/</uri>
                <name>wall_Diffuse</name>
              </script>
            </material>
          </visual>
            
          <collision name="collision">
            <geometry>
              <mesh>
                <uri>model://{world_repository}/meshes/wall.obj</uri>
              </mesh>
            </geometry>
            <surface>
              <contact>
                <collide_bitmask>0x01</collide_bitmask>
              </contact>
            </surface>
          </collision>
          <pose>-9.817441860465117 -12.710465116279071 0 0 0 0</pose>
        </link>
          
          
      </world>
    </sdf>
  

Model

The file formats for defining robots and sensors are SDF (Simulation Description Format), URDF (Unified Robot Description Format), and Xacro. Their characteristics and differences are as follows.

1. SDF :
   • A file format for the Gazebo simulator that defines simulation elements such as robots, sensors, and environments.
   • Written in XML; defines the physical properties of models, sensor attributes, environment textures, etc.
2. URDF :
   • URDF describes the geometric and visual properties of a robot.
   • Written in XML; describes the robot’s structure by defining its links and joints, i.e. its geometric shape.
   • Used by robot modeling and visualization tools; primarily a file format for ROS.
3. Xacro :
   • An extension of URDF that makes XML-based URDF files more efficient to write. Likewise a file format for ROS.
   • Provides parameterization and macros, making code easier to reuse and more readable.
   • More concise and easier to maintain than plain URDF files.

In summary: SDF is for Gazebo simulation; URDF defines the geometric structure of a robot and is therefore compatible with ROS for robot modeling and visualization; Xacro is an extension of URDF that makes URDF easier to write.

The next post covers the structure of model files and Gazebo plugins.

Reference : Gazebo : Tutorials