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Exercise 01: Launch World

Practice launching Gazebo and exploring a simulation world.

Tier: Beginner Time: 30-45 minutes Prerequisites: Lessons B1 and B2 completed

Objective​

In this exercise, you will:

  1. Launch the humanoid lab world in Gazebo
  2. Navigate the 3D environment
  3. Monitor simulation performance
  4. Interact with ROS 2 topics

Part 1: Launch the Simulation​

Task 1.1: Basic Launch​

Open a terminal and run:

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

# Navigate to chapter assets
cd ~/ros2_ws/src  # or your book repository location

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

Checkpoint: Gazebo window opens with laboratory environment.

Task 1.2: Verify Physics​

In the Gazebo window, check the bottom status bar:

  • Time is incrementing
  • RTF displays a value (ideally >= 0.8)
  • No red error messages in terminal

Part 2: Navigation Challenge​

Task 2.1: View the Robot from All Angles​

Using mouse controls, position the camera to see:

  1. Front view: Face the humanoid head-on

    • Screenshot or describe what you see

  2. Top view: Look down at the robot from above

    • Screenshot or describe what you see

  3. Close-up: Zoom in on the humanoid's head

    • Screenshot or describe what you see

Task 2.2: Explore the Lab​

Navigate to find:

  • The lab table with monitor
  • All four walls
  • The humanoid model

Question: What color is the humanoid's torso?

Part 3: Performance Monitoring​

Task 3.1: Check RTF​

Open a new terminal (keep Gazebo running):

# Check Gazebo statistics
gz stats

Record the values:

  • Real-Time Factor: ______
  • Sim Time: ______
  • Real Time: ______

Task 3.2: Performance Analysis​

RTF ValueStatus
Your RTF:______
Target:>= 0.8
Pass/Fail:______

If RTF < 0.8, try:

# Kill current Gazebo
killall gzserver gzclient

# Launch headless (no graphics)
gzserver humanoid_lab.world &

# Attach lighter client
gzclient

Part 4: ROS 2 Integration​

Task 4.1: List Topics​

Open a new terminal:

source /opt/ros/humble/setup.bash

# List all available topics
ros2 topic list

Record the topics you see:

Task 4.2: Echo Gazebo State​

# Echo model states
ros2 topic echo /gazebo/model_states --once

Question: What models are listed in the output?

Task 4.3: Check Clock​

# Watch simulation clock
ros2 topic echo /clock

Observation: Is the simulation time incrementing? (Ctrl+C to stop)

Part 5: Interactive Exploration​

Task 5.1: Pause and Resume​

In Gazebo:

  1. Press Space to pause
  2. Check the terminal - time should stop
  3. Press Space to resume
  4. Verify time continues
  • Pause/resume works correctly

Task 5.2: Reset the World​

From terminal:

# Reset simulation to initial state
ros2 service call /reset_simulation std_srvs/srv/Empty

Observation: Did the simulation reset?

Reflection Questions​

Answer these questions to solidify your understanding:

  1. What is the purpose of RTF in simulation?

    Your answer: _______________________________________________

  2. Why do we use --verbose when launching Gazebo?

    Your answer: _______________________________________________

  3. What ROS 2 topics did Gazebo automatically create?

    Your answer: _______________________________________________

  4. How would a digital twin use these topics?

    Your answer: _______________________________________________

Troubleshooting Checklist​

If you encountered issues, check these:

  • ROS 2 Humble sourced in every terminal
  • Gazebo installed correctly (gazebo --version shows 11.x)
  • No other Gazebo instances running
  • World file path is correct
  • Graphics drivers working (try software rendering if needed)

Completion Criteria​

You have successfully completed this exercise when:

  • Gazebo launches with humanoid_lab.world
  • RTF is monitored and >= 0.8 (or headless workaround used)
  • All ROS 2 topics listed and explored
  • Pause/resume functionality tested
  • Reflection questions answered

Bonus Challenges​

Challenge A: Custom View​

Create a custom camera view and save it using Gazebo's View menu.

Challenge B: Terminal Monitoring​

Create a script that continuously monitors RTF and prints a warning if it drops below 0.8:

#!/bin/bash
# rtf_monitor.sh
while true; do
    rtf=$(gz stats -p | grep "factor" | awk '{print $2}')
    if (( $(echo "$rtf &lt; 0.8" | bc -l) )); then
        echo "WARNING: RTF dropped to $rtf"
    fi
    sleep 1
done

Challenge C: Screenshot Automation​

Use Gazebo's built-in screenshot feature or create a ROS 2 node that captures the camera view.

Next Steps​

After completing this exercise:

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B2: First SimulationExercisesExercise 02: Create World