Physics Simulation in Gazebo

Gazebo Architecture

Gazebo is a multi-robot simulator with:

• gzserver: Headless physics engine (runs on servers)
• gzclient: 3D visualization (OpenGL rendering)
• Plugins: Extend functionality (sensors, actuators, world dynamics)

Key Advantage: Decouple simulation (gzserver) from visualization (gzclient) for performance.

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SDF: Simulation Description Format

While URDF describes robot kinematics, SDF (Simulation Description Format) describes entire simulation worlds.

simple_world.sdf

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

    <!-- Physics Engine Settings -->
    <physics type="ode">
      <max_step_size>0.001</max_step_size>  <!-- 1ms timestep -->
      <real_time_factor>1.0</real_time_factor>
      <real_time_update_rate>1000</real_time_update_rate>
    </physics>

    <!-- Lighting -->
    <light type="directional" name="sun">
      <pose>0 0 10 0 0 0</pose>
      <diffuse>0.8 0.8 0.8 1</diffuse>
      <specular>0.2 0.2 0.2 1</specular>
      <direction>-0.5 0.1 -0.9</direction>
    </light>

    <!-- Ground Plane -->
    <model name="ground_plane">
      <static>true</static>
      <link name="link">
        <collision name="collision">
          <geometry>
            <plane>
              <normal>0 0 1</normal>
              <size>100 100</size>
            </plane>
          </geometry>
          <surface>
            <friction>
              <ode>
                <mu>100</mu>  <!-- Friction coefficient -->
                <mu2>50</mu2>
              </ode>
            </friction>
          </surface>
        </collision>
        <visual name="visual">
          <geometry>
            <plane>
              <normal>0 0 1</normal>
              <size>100 100</size>
            </plane>
          </geometry>
          <material>
            <ambient>0.8 0.8 0.8 1</ambient>
          </material>
        </visual>
      </link>
    </model>

    <!-- Obstacle (Box) -->
    <model name="obstacle">
      <pose>2 0 0.5 0 0 0</pose>
      <link name="link">
        <inertial>
          <mass>10</mass>
          <inertia>
            <ixx>0.83</ixx> <iyy>0.83</iyy> <izz>0.83</izz>
          </inertia>
        </inertial>
        <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>  <!-- Red -->
          </material>
        </visual>
      </link>
    </model>

  </world>
</sdf>

Launching the World:

gazebo simple_world.sdf

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Physics Engines

Gazebo supports multiple physics backends:

Engine	Pros	Cons	Use Case
ODE	Fast, stable	Less accurate contacts	Wheeled robots, quick iteration
DART	Accurate contacts, multibody	Slower	Humanoids, manipulation
Bullet	Real-time performance	Less mature	Fast prototyping
Simbody	Biomechanical accuracy	Complex setup	Biomimetic robots

Changing Physics Engine:

<physics type="dart">
  <dart>
    <solver>
      <solver_type>dantzig</solver_type>
    </solver>
    <collision_detector>fcl</collision_detector>
  </dart>
</physics>

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Tuning Physics Parameters

Timestep (Critical for Stability)

<physics type="ode">
  <max_step_size>0.001</max_step_size>  <!-- Smaller = more stable -->
  <real_time_update_rate>1000</real_time_update_rate>  <!-- 1 / max_step_size -->
</physics>

Rule of Thumb: Use 1ms (0.001s) for humanoids, 5ms (0.005s) for wheeled robots.

Gravity

<gravity>0 0 -9.81</gravity>  <!-- Earth gravity -->
<gravity>0 0 -3.721</gravity>  <!-- Mars gravity -->

Contact Dynamics

<surface>
  <friction>
    <ode>
      <mu>1.0</mu>   <!-- Static friction -->
      <mu2>1.0</mu2> <!-- Dynamic friction -->
    </ode>
  </friction>
  <contact>
    <ode>
      <kp>1e6</kp>  <!-- Stiffness (higher = harder surfaces) -->
      <kd>100</kd>  <!-- Damping (higher = less bounce) -->
    </ode>
  </contact>
</surface>

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Spawning URDF Robots in Gazebo

# Launch empty Gazebo world
gazebo --verbose

# Spawn robot from URDF
ros2 run gazebo_ros spawn_entity.py \
  -entity my_humanoid \
  -file /path/to/humanoid.urdf \
  -x 0 -y 0 -z 1.0

Programmatic Spawning:

spawn_robot.py

import rclpy
from gazebo_msgs.srv import SpawnEntity

def spawn_robot():
    rclpy.init()
    node = rclpy.create_node('entity_spawner')
    client = node.create_client(SpawnEntity, '/spawn_entity')

    request = SpawnEntity.Request()
    request.name = 'my_humanoid'
    request.xml = open('/path/to/robot.urdf').read()
    request.initial_pose.position.z = 1.0

    future = client.call_async(request)
    rclpy.spin_until_future_complete(node, future)

    if future.result() is not None:
        node.get_logger().info('Robot spawned successfully')
    else:
        node.get_logger().error('Failed to spawn robot')

    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    spawn_robot()

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Gazebo Plugins

1. Contact Sensor

Detect when robot foot touches ground (for bipedal walking).

<gazebo reference="left_foot">
  <sensor name="left_foot_contact" type="contact">
    <contact>
      <collision>left_foot_collision</collision>
    </contact>
    <update_rate>100</update_rate>
  </sensor>
  <plugin name="gazebo_ros_bumper" filename="libgazebo_ros_bumper.so">
    <bumperTopicName>/left_foot_contact</bumperTopicName>
    <frameName>left_foot</frameName>
  </plugin>
</gazebo>

2. Joint State Publisher

Publish current joint positions/velocities.

<gazebo>
  <plugin name="gazebo_ros_joint_state_publisher"
          filename="libgazebo_ros_joint_state_publisher.so">
    <update_rate>100</update_rate>
  </plugin>
</gazebo>

3. Differential Drive Controller

For mobile bases (before adding legs).

<gazebo>
  <plugin name="differential_drive_controller"
          filename="libgazebo_ros_diff_drive.so">
    <left_joint>left_wheel_joint</left_joint>
    <right_joint>right_wheel_joint</right_joint>
    <wheel_separation>0.4</wheel_separation>
    <wheel_diameter>0.2</wheel_diameter>
    <command_topic>/cmd_vel</command_topic>
    <odometry_topic>/odom</odometry_topic>
  </plugin>
</gazebo>

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Building Complex Worlds

warehouse.sdf

<model name="warehouse">
  <static>true</static>
  <link name="link">
    <collision name="wall_north">
      <pose>10 0 1 0 0 0</pose>
      <geometry>
        <box><size>20 0.2 2</size></box>
      </geometry>
    </collision>
    <visual name="visual_north">
      <pose>10 0 1 0 0 0</pose>
      <geometry>
        <box><size>20 0.2 2</size></box>
      </geometry>
      <material><ambient>0.7 0.7 0.7 1</ambient></material>
    </visual>
    <!-- Repeat for other walls -->
  </link>
</model>

<!-- Import external models -->
<include>
  <uri>model://coke_can</uri>
  <pose>5 3 0.5 0 0 0</pose>
</include>

Model Database: Gazebo includes 100s of models (~/.gazebo/models/) or use Ignition Fuel.

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Performance Optimization

1. Reduce Collision Complexity

<!-- Bad: High-poly mesh -->
<collision>
  <geometry>
    <mesh><uri>model://robot/meshes/head.dae</uri></mesh>
  </geometry>
</collision>

<!-- Good: Simplified bounding box -->
<collision>
  <geometry>
    <box><size>0.2 0.15 0.25</size></box>
  </geometry>
</collision>

2. Disable Shadows

gazebo --verbose -g  # GUI-only mode (no physics)

3. Parallelize Worlds

Run multiple Gazebo instances with different GAZEBO_MASTER_URI.

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Best Practices

✅ Start Simple: Test with box primitives before adding complex meshes

✅ Validate URDF: check_urdf robot.urdf before spawning

✅ Monitor Physics Rate: gz stats -p (should match real_time_update_rate)

✅ Use DART for Humanoids: Better contact handling for feet

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Next Steps

Now that you understand Gazebo physics, let's integrate virtual sensors.

Continue to Sensor Simulation →