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Robot Simulation with Gazebo

Why Simulation?​

Building a humanoid robot costs thousands of dollars. Crashing it costs even more. Gazebo lets us crash the robot a million times for free.

What is Gazebo?​

Gazebo World

Gazebo is a physics simulator designed for robotics. It calculates:

  • Rigid Body Dynamics: How masses move and rotate.
  • Collision Detection: What hits what.
  • Friction & Contact: How feet interact with the floor.
  • Sensors: What the camera sees (rendering) and what the laser scans (ray casting).

We use the newer Gazebo Harmonic/Garden (formerly Ignition Gazebo), not the old Gazebo Classic (version 9/11).

Spawning Your Robot​

To bring your URDF into Gazebo, you need to wrap it in an SDF (Simulation Description Format) or use a plugin that converts URDF to SDF on the fly.

The gz_ros2_control Plugin​

ROS 2 needs a driver to talk to motors. real motors use hardware drivers. Gazebo motors use ros_control.

You must add this to your URDF:

<ros2_control name="GazeboSystem" type="system">
  <hardware>
    <plugin>gazebo_ros2_control/GazeboSystem</plugin>
  </hardware>
  <joint name="knee_joint">
    <command_interface name="position"/>
    <state_interface name="position"/>
    <state_interface name="velocity"/>
  </joint>
</ros2_control>

<gazebo>
  <plugin filename="libgazebo_ros2_control.so" name="gazebo_ros2_control">
    <parameters>$(find my_robot)/config/controllers.yaml</parameters>
  </plugin>
</gazebo>

Physics Engines​

Gazebo supports multiple physics engines.

  1. Dart: Good for accurate articulated dynamics (chains of joints). Default in newer Gazebo.
  2. ODE (Open Dynamics Engine): Older, stable, standard in Gazebo Classic.
  3. Bullet: Good general-purpose physics.

For humanoids, stability is key. If the time step is too large, the simulation "explodes".

  • Time Step: Usually 1ms (1000 Hz).
  • Real Time Factor: If your computer is slow, the simulation slows down (0.5x speed) to maintain physics accuracy.

Building Worlds​

You can build environments (Worlds) using the Building Editor or by writing SDF files.

<world name="default">
  <physics name="1ms" type="ignored">
    <max_step_size>0.001</max_step_size>
    <real_time_factor>1.0</real_time_factor>
  </physics>
  
  <include>
    <uri>https://fuel.gazebosim.org/1.0/OpenRobotics/models/Sun</uri>
  </include>
  
  <include>
    <uri>https://fuel.gazebosim.org/1.0/OpenRobotics/models/Ground Plane</uri>
  </include>
</world>

Simulating Sensors​

IMU (Inertial Measurement Unit)​

Crucial for balance. It tells the robot "Down is that way".

<sensor name="imu_sensor" type="imu">
  <always_on>1</always_on>
  <update_rate>100</update_rate>
  <topic>imu</topic>
</sensor>

LiDAR & Cameras​

Gazebo renders the scene from the camera's perspective and publishes the image to a ROS topic (/camera/image_raw). You can verify this using rqt_image_view.

Troubleshooting Simulation​

  • Robot falls through floor: Check collision mesh of the feet and ground plane.
  • Robot vibrates/jitters: Reduce physics time step, increase solver iterations, or check inertia values (too light/small inertia often causes jitter).
  • Slipping: Check friction coefficients (mu1, mu2) on the feet interactions.

In the next section, we look at Unity, which offers better graphics for demonstrating our robot to the public.