B2: Running Your First Simulation

Launch Gazebo and interact with a pre-built humanoid world.

Learning Objectives

By the end of this lesson, you will be able to:

• Install and configure Gazebo Classic with ROS 2
• Launch a simulation world from the command line
• Navigate the Gazebo interface
• Monitor simulation performance with RTF
• Interact with the simulated environment

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Prerequisites

Before starting, ensure you have:

• Ubuntu 22.04 (native or WSL2)
• ROS 2 Humble installed
• Chapter 1 completed (URDF fundamentals)

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Installing Gazebo Classic

Gazebo Classic (version 11) is the stable simulation platform for ROS 2 Humble.

Step 1: Install Gazebo and ROS 2 Packages

# Update package lists
sudo apt update

# Install Gazebo Classic
sudo apt install gazebo

# Install ROS 2 Gazebo packages
sudo apt install ros-humble-gazebo-ros-pkgs ros-humble-gazebo-ros2-control

# Verify installation
gazebo --version

Expected output:

Gazebo multi-robot simulator, version 11.x.x

Step 2: Verify ROS 2 Integration

# Source ROS 2
source /opt/ros/humble/setup.bash

# Check Gazebo ROS packages
ros2 pkg list | grep gazebo

Expected output includes:

gazebo_msgs
gazebo_plugins
gazebo_ros
gazebo_ros2_control

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Your First Launch

Let's launch the pre-built humanoid lab world.

Step 1: Set Up Your Workspace

# Create workspace if it doesn't exist
mkdir -p ~/ros2_ws/src
cd ~/ros2_ws/src

# Copy the humanoid_lab.world from chapter assets
# (Assuming you've cloned the book repository)
cp /path/to/robot-book/chapters/02-digital-twin/beginner/assets/humanoid_lab.world .

Step 2: Launch Gazebo with the World

# Source ROS 2
source /opt/ros/humble/setup.bash

# Launch Gazebo with the humanoid lab world
gazebo --verbose humanoid_lab.world

You should see:

1. Gazebo window opens
2. A laboratory environment loads
3. A simple humanoid model appears
4. Physics simulation starts automatically

Step 3: Explore the Interface

The Gazebo interface has several key areas:

┌─────────────────────────────────────────────────────────────┐
│  File  Edit  Camera  View  Window  Help                     │
├───────────────────────────────────────────────┬─────────────┤
│                                               │  World      │
│                                               │  ├─ Models  │
│           3D VIEWPORT                         │  │  └─ ...  │
│                                               │  ├─ Lights  │
│     [Your humanoid robot here]                │  └─ Physics │
│                                               │             │
│                                               │  Insert     │
│                                               │  [Models]   │
├───────────────────────────────────────────────┴─────────────┤
│  Time: 00:00:15.432  RTF: 0.95  Iterations: 15432           │
└─────────────────────────────────────────────────────────────┘

Key Interface Elements:

Element	Location	Purpose
3D Viewport	Center	View and interact with simulation
World Panel	Right	Hierarchy of models and settings
Insert Panel	Right	Add new models from library
Time Display	Bottom	Current simulation time
RTF Display	Bottom	Real-Time Factor performance

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Navigating the 3D Viewport

Mouse Controls

Action	Control
Rotate view	Left-click + drag
Pan view	Middle-click + drag
Zoom	Scroll wheel
Select model	Left-click on model

Keyboard Shortcuts

Key	Action
R	Reset view to default
T	Toggle translation mode
E	Toggle rotation mode
Esc	Deselect / Cancel
Space	Pause/Resume simulation

Practice Exercise

1. Zoom in on the humanoid model
2. Rotate the view to see it from different angles
3. Pan to explore the laboratory environment
4. Press Space to pause, then resume

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Monitoring Performance

Real-Time Factor (RTF)

The RTF indicator at the bottom shows simulation health:

RTF	Status	Action
>= 0.95	Excellent	No action needed
0.8 - 0.95	Good	Acceptable for digital twin
0.5 - 0.8	Warning	Consider simplifying world
< 0.5	Critical	Reduce complexity or use headless

Command-Line Monitoring

# In a new terminal, check Gazebo stats
gz stats

Output shows real-time performance:

Factor[1.00] SimTime[10.52] RealTime[10.52] Paused[F]

Common Performance Issues

Issue	Symptom	Solution
Complex meshes	Low RTF, GPU usage high	Simplify collision geometries
Many models	Gradual slowdown	Reduce model count
Physics instability	RTF spikes, model jitter	Check URDF inertias

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ROS 2 Integration

While Gazebo is running, ROS 2 topics are automatically available.

List Available Topics

# In a new terminal
source /opt/ros/humble/setup.bash

# List all topics
ros2 topic list

Expected topics include:

/clock
/gazebo/link_states
/gazebo/model_states
/joint_states
/parameter_events
/rosout

Echo a Topic

# Watch joint states
ros2 topic echo /joint_states

Output (simplified):

header:
  stamp:
    sec: 10
    nanosec: 520000000
  frame_id: ''
name:
- left_hip_yaw
- left_hip_roll
- ...
position:
- 0.0
- 0.0
- ...
velocity:
- 0.0
- 0.0
- ...

Publish a Command

# Send a simple velocity command (if robot supports it)
ros2 topic pub --once /cmd_vel geometry_msgs/msg/Twist \
  "{linear: {x: 0.1, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.0}}"

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Launching with ROS 2 Launch Files

For production use, launch Gazebo through ROS 2 launch files:

Basic Launch Command

# Launch Gazebo as a ROS 2 node
ros2 launch gazebo_ros gazebo.launch.py world:=humanoid_lab.world

Benefits of ROS 2 Launch

• Automatic parameter handling
• Respawn on crash
• Integrated with other ROS 2 nodes
• Unified logging

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Troubleshooting

Gazebo Won't Start

# Check if another instance is running
killall gzserver gzclient

# Try with verbose logging
gazebo --verbose

Black Screen / No Graphics

# For WSL2 or headless systems
export LIBGL_ALWAYS_SOFTWARE=1
gazebo --verbose

World File Not Found

# Check Gazebo model path
echo $GAZEBO_MODEL_PATH

# Add custom path
export GAZEBO_MODEL_PATH=$GAZEBO_MODEL_PATH:~/ros2_ws/src

Low RTF Warning

# Try headless mode for better performance
gzserver humanoid_lab.world &

# Visualize with lighter client
gzclient

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What's Next?

You've successfully launched your first simulation! In the Intermediate tier, you'll learn to:

• Create custom world files from scratch
• Spawn your own URDF models
• Control joints programmatically

Next: I1: Building Simulation Worlds

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AI Agent Assisted Prompts

Use these prompts with your AI coding assistant:

Installation Help

I'm getting error "gazebo: command not found" on Ubuntu 22.04 with ROS 2
Humble. Walk me through the complete installation process and verify each step.

Performance Optimization

My Gazebo simulation is running at RTF 0.4 with a 20-DOF humanoid model.
I have an Intel i7 with integrated graphics. Suggest optimizations to reach
RTF >= 0.8 without removing model complexity.

ROS 2 Integration

I want to create a Python node that subscribes to /joint_states from Gazebo
and republishes processed data to /joint_states_filtered. Show me the
complete node implementation using rclpy.

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Summary

• Gazebo Classic (v11) is the primary simulation platform for ROS 2 Humble
• Launch worlds with gazebo --verbose world_name.world
• Navigate using mouse (rotate, pan, zoom) and keyboard shortcuts
• Monitor RTF to ensure performance >= 0.8
• ROS 2 topics automatically bridge simulation to your nodes

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B1: Digital Twin Concepts	Beginner Tier	I1: Building Worlds