Gazebo Simulation Basics

Gazebo is a powerful 3D robot simulator that provides realistic physics, sensors, and environments for testing robots before deploying them in the real world.

Why Use Simulation?

Simulation offers major advantages during robot development:

1. Safety: Test dangerous scenarios without risking hardware
2. Speed: Iterate quickly without waiting for physical assembly
3. Cost: Reduce wear on expensive robot parts
4. Repeatability: Run the same test scenario hundreds of times
5. Parallel Testing: Simulate multiple robots at once

Key Concepts

1. Worlds: The Simulation Environment

Worlds define the 3D environment where your robot operates. They include:

• Ground plane and gravity settings
• Lighting and sky appearance
• Static objects (walls, furniture, obstacles)
• Dynamic objects (other robots, moving obstacles)

2. Models: Robot and Object Descriptions

Models are URDF or SDF (Simulation Description Format) files that describe:

• Visual appearance (colors, textures, 3D meshes)
• Collision shapes (for physics calculations)
• Inertial properties (mass, center of mass)
• Sensors and actuators

3. Plugins: Adding Behavior

Plugins extend Gazebo's functionality:

• Model plugins: Control individual robots
• World plugins: Modify the entire simulation
• Sensor plugins: Process sensor data (cameras, LiDAR)

Getting Started with Gazebo

Launching a Basic World

# Launch an empty world
ros2 launch gazebo_ros gazebo.launch.py

# Launch with a specific world file
ros2 launch gazebo_ros gazebo.launch.py world:=/path/to/my_world.world

Spawning a Robot

# Spawn a robot from URDF
ros2 run gazebo_ros spawn_entity.py -entity my_robot \
    -file /path/to/robot.urdf \
    -x 0 -y 0 -z 0.5

Example: Simple World File

<?xml version="1.0"?>
<sdf version="1.6">
  <world name="basic_world">
    <!-- Physics engine settings -->
    <physics type="ode">
      <max_step_size>0.001</max_step_size>
      <real_time_factor>1</real_time_factor>
    </physics>

    <!-- Lighting -->
    <light name="sun" type="directional">
      <pose>0 0 10 0 0 0</pose>
      <diffuse>1 1 1 1</diffuse>
      <specular>0.5 0.5 0.5 1</specular>
      <direction>-0.5 -0.5 -1</direction>
    </light>

    <!-- Ground plane -->
    <include>
      <uri>model://ground_plane</uri>
    </include>

    <!-- Simple box obstacle -->
    <model name="box">
      <static>true</static>
      <pose>2 0 0.5 0 0 0</pose>
      <link name="link">
        <collision name="collision">
          <geometry>
            <box>
              <size>1 1 1</size>
            </box>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <box>
              <size>1 1 1</size>
            </box>
          </geometry>
          <material>
            <ambient>1 0 0 1</ambient>
          </material>
        </visual>
      </link>
    </model>
  </world>
</sdf>

Physics Simulation

Gazebo uses the ODE (Open Dynamics Engine) by default:

• Timestep: How often physics is calculated (default: 1ms)
• Real-time factor: Simulation speed (1.0 = real-time, 0.5 = half-speed)
• Gravity: Default is Earth gravity (-9.81 m/s²)

Adjusting Physics Performance

<physics type="ode">
  <max_step_size>0.001</max_step_size>  <!-- 1ms timestep -->
  <real_time_factor>1.0</real_time_factor>  <!-- Run at real-time speed -->
  <real_time_update_rate>1000</real_time_update_rate>
</physics>

Gazebo GUI Controls

Action	Control
Rotate view	Left mouse drag
Pan view	Middle mouse drag
Zoom	Scroll wheel
Select object	Left click
Move object	Drag with translate tool
Pause/Play	Bottom toolbar

Common Simulation Patterns

1. Testing Navigation Algorithms

Spawn obstacles and test path planning without risking real robots.

2. Sensor Calibration

Simulate camera and LiDAR data to tune perception algorithms.

3. Multi-Robot Coordination

Run multiple robot instances to test swarm behaviors.

4. Environmental Testing

Change gravity, friction, and lighting to test edge cases.

Best Practices

1. Start simple: Use basic shapes before adding complex meshes
2. Monitor performance: Check real-time factor stays near 1.0
3. Use appropriate timestep: Smaller = more accurate but slower
4. Simplify collision geometry: Use boxes/cylinders instead of detailed meshes
5. Version control worlds: Track simulation environment changes

Troubleshooting

Issue	Solution
Simulation runs slow	Reduce model complexity or increase timestep
Robot falls through ground	Check collision geometry is defined
Sensors return no data	Verify sensor plugins are loaded
Gravity feels wrong	Check physics settings and inertial properties

Try It Yourself

Ask the chatbot:

• "How do I change the gravity in Gazebo?"
• "What's the difference between SDF and URDF?"
• "How do I pause and resume a simulation?"