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Chapter 5: Simulation Environments

Week 5 | Module 2: Simulation & Perception

Learning Objectives
  • ✅ Set up Gazebo and Unity simulation environments
  • ✅ Create URDF robot models
  • ✅ Integrate ROS 2 with simulators
  • ✅ Spawn and control robots in simulation

5.1 Why Simulation?​

Benefits​

  • ✅ Safe testing: No risk of hardware damage
  • ✅ Fast iteration: Test algorithms in minutes
  • ✅ Reproducibility: Exact same conditions every time
  • ✅ Cost: Free vs $10k-100k for physical robot

5.2 URDF: Robot Description Format​

Basic URDF Structure​

<?xml version="1.0"?>
<robot name="simple_arm">
  <!-- Base Link -->
  <link name="base_link">
    <visual>
      <geometry>
        <cylinder radius="0.1" length="0.05"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 0.8 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder radius="0.1" length="0.05"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1.0"/>
      <inertia ixx="0.001" ixy="0.0" ixz="0.0"
               iyy="0.001" iyz="0.0" izz="0.002"/>
    </inertial>
  </link>

  <!-- Link 1 -->
  <link name="link1">
    <visual>
      <geometry>
        <box size="0.05 0.05 0.5"/>
      </geometry>
      <origin xyz="0 0 0.25"/>
      <material name="red">
        <color rgba="0.8 0 0 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.05 0.05 0.5"/>
      </geometry>
      <origin xyz="0 0 0.25"/>
    </collision>
    <inertial>
      <mass value="0.5"/>
      <inertia ixx="0.01" ixy="0.0" ixz="0.0"
               iyy="0.01" iyz="0.0" izz="0.001"/>
    </inertial>
  </link>

  <!-- Joint 1 -->
  <joint name="joint1" type="revolute">
    <parent link="base_link"/>
    <child link="link1"/>
    <origin xyz="0 0 0.05"/>
    <axis xyz="0 0 1"/>
    <limit lower="-3.14" upper="3.14" effort="10" velocity="1.0"/>
  </joint>
</robot>

Visualize URDF​

# Install tools
sudo apt install ros-humble-joint-state-publisher-gui
sudo apt install ros-humble-robot-state-publisher

# Launch visualization
ros2 launch urdf_tutorial display.launch.py model:=simple_arm.urdf

5.3 Gazebo Classic Integration​

Launch Robot in Gazebo​

Launch File (gazebo_spawn.launch.py):

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

def generate_launch_description():
    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', 'simple_arm',
                '-file', 'simple_arm.urdf',
                '-x', '0', '-y', '0', '-z', '0.5'
            ],
            output='screen'
        ),

        # Robot state publisher
        Node(
            package='robot_state_publisher',
            executable='robot_state_publisher',
            parameters=[{'robot_description': 'simple_arm.urdf'}]
        ),
    ])

Control Joints in Gazebo​

import rclpy
from std_msgs.msg import Float64

class JointController(Node):
    def __init__(self):
        super().__init__('joint_controller')
        self.pub = self.create_publisher(
            Float64, '/simple_arm/joint1_position_controller/command', 10)

        self.timer = self.create_timer(0.1, self.control_loop)
        self.angle = 0.0

    def control_loop(self):
        msg = Float64()
        msg.data = self.angle
        self.pub.publish(msg)

        # Sinusoidal motion
        self.angle += 0.01

def main():
    rclpy.init()
    node = JointController()
    rclpy.spin(node)

5.4 Assignment: Week 5​

Project: Create Custom Robot URDF​

Requirements:

  1. 4-DOF manipulator with gripper
  2. Include visual meshes (STL files)
  3. Proper inertial properties
  4. Spawn in Gazebo
  5. Control via ROS 2

Deliverables:

  • robot.urdf
  • Launch file
  • Control script
  • Demo video

Next: Chapter 6: Isaac Sim →