Skip to main content

Exercise 02: Create World

Build a custom simulation world from scratch.

Tier: Intermediate Time: 45-60 minutes Prerequisites: Lessons I1 and I2 completed

Objective​

In this exercise, you will:

  1. Create a custom world file with specific requirements
  2. Add physics, lighting, and obstacles
  3. Spawn and test your humanoid in the new world
  4. Verify ROS 2 integration

The Challenge​

Create a Robot Testing Arena with these specifications:

Required Elements​

ElementSpecification
Arena Size8m x 8m floor area
GroundHigh-friction surface (mu=1.2)
WallsAt least 2 walls for boundary
ObstaclesMinimum 3 obstacles of different shapes
LightingSun + at least 1 point light
Spawn PointMarked center area for robot

Performance Target​

  • RTF >= 0.8 when world is loaded
  • ROS 2 topics available

Part 1: Create the World File​

Task 1.1: Set Up File Structure​

Create a new file robot_arena.world in your workspace:

mkdir -p ~/ros2_ws/src/my_worlds
cd ~/ros2_ws/src/my_worlds
touch robot_arena.world

Task 1.2: Add Base Structure​

Start with this template:

<?xml version="1.0" ?>
<sdf version="1.6">
  <world name="robot_arena">

    <!-- TODO: Add physics configuration -->

    <!-- TODO: Add scene settings -->

    <!-- TODO: Add lighting -->

    <!-- TODO: Add ground plane -->

    <!-- TODO: Add walls -->

    <!-- TODO: Add obstacles -->

    <!-- TODO: Add ROS 2 plugin -->

  </world>
</sdf>

Task 1.3: Configure Physics​

Add physics with these parameters:

  • Update rate: 1000 Hz
  • Step size: 0.001 s
  • Solver iterations: 50

Your code here:

<!-- Replace this with your physics configuration -->

Task 1.4: Add Lighting​

Requirements:

  • Directional sun light with shadows
  • One point light in the center of the arena

Your code here:

<!-- Replace this with your lighting configuration -->

Part 2: Build the Arena​

Task 2.1: Ground Plane​

Create a ground plane with:

  • Size: 10m x 10m (larger than arena for margin)
  • High friction: mu=1.2, mu2=1.2
  • Light gray color

Checkpoint: Ground appears when you launch the world.

Task 2.2: Boundary Walls​

Add at least 2 walls:

  • Height: 1.5m
  • Thickness: 0.2m
  • Color: White or light gray

Checkpoint: Walls visible and solid (test by throwing object at them).

Task 2.3: Obstacles​

Add exactly 3 obstacles:

  1. Red Box: 0.5m x 0.5m x 0.5m at position (2, 1, 0.25)
  2. Green Cylinder: radius 0.3m, height 1m at position (-2, -1, 0.5)
  3. Blue Sphere: radius 0.4m at position (0, 2, 0.4) - this should be dynamic

Checkpoint: All three obstacles visible with correct colors.

Task 2.4: Spawn Point Marker​

Add a visual-only marker at the center:

  • Thin cylinder (radius 0.5m, height 0.01m)
  • Blue color
  • Position: (0, 0, 0.005)

Checkpoint: Blue circle visible at world center.

Part 3: Test Your World​

Task 3.1: Launch and Verify​

source /opt/ros/humble/setup.bash
gazebo --verbose robot_arena.world

Verification Checklist:

  • World loads without errors
  • Ground plane visible
  • Walls visible
  • All 3 obstacles present
  • Blue sphere falls (dynamic object)
  • Center marker visible

Task 3.2: Check RTF​

gz stats
MetricYour ValueTarget
RTF_______>= 0.8

If RTF < 0.8, simplify your world (reduce wall complexity, use simpler shapes).

Task 3.3: Verify ROS 2 Topics​

ros2 topic list

Required topics present:

  • /clock
  • /gazebo/model_states

Part 4: Spawn Your Humanoid​

Task 4.1: Spawn at Center​

ros2 run gazebo_ros spawn_entity.py \
  -entity humanoid \
  -file /path/to/your/humanoid.urdf \
  -x 0 -y 0 -z 1.0

Checkpoint: Humanoid appears at center marker.

Task 4.2: Verify Joint States​

ros2 topic echo /joint_states --once

Checkpoint: Joint names and positions displayed.

Part 5: Documentation​

Task 5.1: World File Header​

Add a comment header to your world file:

<!--
  Robot Testing Arena - Exercise 02

  Author: [Your Name]
  Date: [Today's Date]
  Purpose: [Brief description]

  Features:
  - [List features]

  Usage:
    gazebo --verbose robot_arena.world
-->

Task 5.2: Reflection Questions​

  1. What physics parameters affect RTF the most?

    Your answer: _______________________________________________

  2. Why does the blue sphere fall but the red box doesn't?

    Your answer: _______________________________________________

  3. How would you add a second floor/platform?

    Your answer: _______________________________________________

Completion Criteria​

You have successfully completed this exercise when:

  • World file created with all required elements
  • RTF >= 0.8 when loaded
  • Dynamic object (sphere) falls correctly
  • ROS 2 topics available
  • Humanoid spawns at center
  • Documentation header added
  • Reflection questions answered

Bonus Challenges​

Challenge A: Friction Zones​

Create two distinct floor areas:

  • Ice zone (mu=0.1) on the left side
  • Rubber zone (mu=2.0) on the right side

Test by rolling the blue sphere across both.

Challenge B: Inclined Ramp​

Add a ramp leading up to a platform:

  • Ramp angle: 15 degrees
  • Platform height: 0.5m
  • Test if humanoid can walk up it

Challenge C: Sensor Zones​

Add visual markers for sensor testing:

  • Camera FOV cone visualization
  • Lidar scan area circle

Common Mistakes​

MistakeSymptomFix
Missing <static>true</static>Objects fall through floorAdd static tag
Invalid inertiaObject explodesCalculate correct inertia for mass
Wrong pose formatObject at wrong positionCheck x,y,z,roll,pitch,yaw order
Missing collisionObjects pass through each otherAdd collision geometry

Reference Solution​

After completing, compare with the reference in: chapters/02-digital-twin/intermediate/assets/simple_lab.world

Next Steps​

After completing this exercise:

PreviousUpNext
I2: Spawning ModelsExercisesExercise 03: Build Bridge