Chapter 3: Robot Simulation with Gazebo

Why Simulation?

Simulation allows you to:

• ✅ Test algorithms without hardware
• ✅ Rapid prototyping and iteration
• ✅ Safe testing of dangerous scenarios
• ✅ Multi-robot experiments
• ✅ Reproducible results

Gazebo Simulator

Gazebo is a 3D robot simulator with:

• Physics engine (gravity, friction, collisions)
• Sensor simulation (LiDAR, cameras, IMU)
• Plugin system for custom behaviors
• ROS2 integration

Installation

sudo apt install ros-humble-gazebo-ros-pkgs
sudo apt install ros-humble-gazebo-ros2-control

URDF (Unified Robot Description Format)

URDF describes robot structure using XML.

Simple Robot Example

<?xml version="1.0"?>
<robot name="simple_robot">
  <!-- Base Link -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.6 0.4 0.2"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 0.8 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.6 0.4 0.2"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="10"/>
      <inertia ixx="0.4" ixy="0.0" ixz="0.0"
               iyy="0.4" iyz="0.0" izz="0.2"/>
    </inertial>
  </link>

  <!-- Wheel Link -->
  <link name="left_wheel">
    <visual>
      <geometry>
        <cylinder radius="0.1" length="0.05"/>
      </geometry>
      <material name="black">
        <color rgba="0 0 0 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder radius="0.1" length="0.05"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1"/>
      <inertia ixx="0.01" ixy="0.0" ixz="0.0"
               iyy="0.01" iyz="0.0" izz="0.01"/>
    </inertial>
  </link>

  <!-- Joint -->
  <joint name="left_wheel_joint" type="continuous">
    <parent link="base_link"/>
    <child link="left_wheel"/>
    <origin xyz="-0.2 0.25 0" rpy="1.57 0 0"/>
    <axis xyz="0 0 1"/>
  </joint>
</robot>

Launch Robot in Gazebo

from launch import LaunchDescription
from launch_ros.actions import Node
from launch.actions import ExecuteProcess
import os

def generate_launch_description():
    # Path to URDF file
    urdf_file = os.path.join(
        get_package_share_directory('my_robot'),
        'urdf',
        'robot.urdf'
    )
    
    return LaunchDescription([
        # Start Gazebo
        ExecuteProcess(
            cmd=['gazebo', '--verbose', '-s', 'libgazebo_ros_factory.so'],
            output='screen'
        ),
        
        # Spawn robot
        Node(
            package='gazebo_ros',
            executable='spawn_entity.py',
            arguments=['-entity', 'my_robot', '-file', urdf_file],
            output='screen'
        ),
        
        # Robot state publisher
        Node(
            package='robot_state_publisher',
            executable='robot_state_publisher',
            parameters=[{'robot_description': urdf_file}]
        ),
    ])

Adding Sensors

LiDAR Sensor

<gazebo reference="laser_link">
  <sensor name="laser" type="ray">
    <pose>0 0 0 0 0 0</pose>
    <visualize>true</visualize>
    <update_rate>10</update_rate>
    <ray>
      <scan>
        <horizontal>
          <samples>360</samples>
          <resolution>1</resolution>
          <min_angle>-3.14159</min_angle>
          <max_angle>3.14159</max_angle>
        </horizontal>
      </scan>
      <range>
        <min>0.1</min>
        <max>10.0</max>
        <resolution>0.01</resolution>
      </range>
    </ray>
    <plugin name="laser_controller" filename="libgazebo_ros_ray_sensor.so">
      <ros>
        <namespace>/</namespace>
        <remapping>~/out:=scan</remapping>
      </ros>
      <output_type>sensor_msgs/LaserScan</output_type>
      <frame_name>laser_link</frame_name>
    </plugin>
  </sensor>
</gazebo>

Camera Sensor

<gazebo reference="camera_link">
  <sensor name="camera" type="camera">
    <update_rate>30</update_rate>
    <camera>
      <horizontal_fov>1.3962634</horizontal_fov>
      <image>
        <width>640</width>
        <height>480</height>
        <format>R8G8B8</format>
      </image>
      <clip>
        <near>0.02</near>
        <far>300</far>
      </clip>
    </camera>
    <plugin name="camera_controller" filename="libgazebo_ros_camera.so">
      <ros>
        <namespace>/</namespace>
        <remapping>image_raw:=camera/image_raw</remapping>
        <remapping>camera_info:=camera/camera_info</remapping>
      </ros>
      <camera_name>camera</camera_name>
      <frame_name>camera_link</frame_name>
    </plugin>
  </sensor>
</gazebo>

Differential Drive Plugin

<gazebo>
  <plugin name="diff_drive" filename="libgazebo_ros_diff_drive.so">
    <update_rate>50</update_rate>
    <left_joint>left_wheel_joint</left_joint>
    <right_joint>right_wheel_joint</right_joint>
    <wheel_separation>0.5</wheel_separation>
    <wheel_diameter>0.2</wheel_diameter>
    <max_wheel_torque>20</max_wheel_torque>
    <max_wheel_acceleration>1.0</max_wheel_acceleration>
    
    <command_topic>cmd_vel</command_topic>
    <odometry_topic>odom</odometry_topic>
    <odometry_frame>odom</odometry_frame>
    <robot_base_frame>base_link</robot_base_frame>
    
    <publish_odom>true</publish_odom>
    <publish_odom_tf>true</publish_odom_tf>
    <publish_wheel_tf>true</publish_wheel_tf>
  </plugin>
</gazebo>

Teleoperation

import rclpy
from rclpy.node import Node
from geometry_msgs.msg import Twist
import sys, select, termios, tty

class TeleopKeyboard(Node):
    def __init__(self):
        super().__init__('teleop_keyboard')
        self.publisher = self.create_publisher(Twist, 'cmd_vel', 10)
        
        self.linear_speed = 0.5
        self.angular_speed = 1.0
        
    def get_key(self):
        tty.setraw(sys.stdin.fileno())
        select.select([sys.stdin], [], [], 0)
        key = sys.stdin.read(1)
        termios.tcsetattr(sys.stdin, termios.TCSADRAIN, settings)
        return key
    
    def run(self):
        print("Control your robot!")
        print("w/s: forward/backward")
        print("a/d: turn left/right")
        print("q: quit")
        
        while True:
            key = self.get_key()
            
            twist = Twist()
            
            if key == 'w':
                twist.linear.x = self.linear_speed
            elif key == 's':
                twist.linear.x = -self.linear_speed
            elif key == 'a':
                twist.angular.z = self.angular_speed
            elif key == 'd':
                twist.angular.z = -self.angular_speed
            elif key == 'q':
                break
            
            self.publisher.publish(twist)

Visualizing in RViz2

rviz2

Add displays:

• RobotModel
• LaserScan
• Camera
• TF

Key Takeaways

1. ✅ URDF describes robot structure
2. ✅ Gazebo simulates physics and sensors
3. ✅ Plugins add functionality
4. ✅ RViz2 visualizes robot state
5. ✅ Teleoperation for manual control

Next: Chapter 4 - Navigation and SLAM!