Assessment: Gazebo Simulation Implementation

Overview

This assessment evaluates your ability to create realistic robot simulations using Gazebo. You'll design a custom environment, implement physics-based simulations, and integrate sensors for a mobile manipulator robot.

Learning Objectives

By completing this assessment, you will demonstrate:

• Proficiency in Gazebo world creation and environment design
• Understanding of URDF/SDF robot modeling
• Ability to simulate sensors (cameras, LiDAR, IMU)
• Knowledge of physics simulation parameters
• Integration of Gazebo with ROS 2

Project Requirements

Task: Warehouse Mobile Manipulator Simulation

Create a complete Gazebo simulation of a mobile manipulator operating in a warehouse environment.

1. World Design

Warehouse Environment

Create a warehouse world with:

• Floor: 20m x 20m area with realistic friction
• Shelving units: At least 4 shelf racks with varying heights
• Objects: 10-15 objects of different shapes (boxes, cylinders, spheres)
• Obstacles: Static obstacles (walls, columns, barriers)
• Lighting: Multiple light sources for realistic rendering
• Textures: Realistic materials for floor, walls, objects

warehouse.world:

<?xml version="1.0"?>
<sdf version="1.7">
  <world name="warehouse">
    <!-- Physics settings -->
    <physics name="default_physics" default="0" type="ode">
      <max_step_size>0.001</max_step_size>
      <real_time_factor>1</real_time_factor>
      <real_time_update_rate>1000</real_time_update_rate>
    </physics>

    <!-- Scene -->
    <scene>
      <ambient>0.4 0.4 0.4 1</ambient>
      <background>0.7 0.7 0.7 1</background>
      <shadows>1</shadows>
    </scene>

    <!-- Lighting -->
    <include>
      <uri>model://sun</uri>
    </include>

    <!-- Ground plane -->
    <include>
      <uri>model://ground_plane</uri>
    </include>

    <!-- Warehouse floor -->
    <model name="warehouse_floor">
      <static>true</static>
      <link name="floor_link">
        <collision name="floor_collision">
          <geometry>
            <box>
              <size>20 20 0.1</size>
            </box>
          </geometry>
          <surface>
            <friction>
              <ode>
                <mu>0.8</mu>
                <mu2>0.8</mu2>
              </ode>
            </friction>
          </surface>
        </collision>
        <visual name="floor_visual">
          <geometry>
            <box>
              <size>20 20 0.1</size>
            </box>
          </geometry>
          <material>
            <script>
              <uri>file://media/materials/scripts/gazebo.material</uri>
              <name>Gazebo/Grey</name>
            </script>
          </material>
        </visual>
      </link>
    </model>

    <!-- Shelving units (repeat for multiple shelves) -->
    <include>
      <uri>model://shelf</uri>
      <pose>3 0 0 0 0 0</pose>
      <name>shelf_1</name>
    </include>

    <!-- Objects on shelves -->
    <!-- TODO: Add multiple objects with different properties -->

  </world>
</sdf>

2. Robot Model

Mobile Manipulator URDF

Create a mobile base with:

• Differential drive: Two driven wheels + two casters
• 6-DOF arm: Mounted on top of mobile base
• 2-finger gripper: End effector
• Sensors: Camera, LiDAR, IMU

Key URDF components:

<?xml version="1.0"?>
<robot name="mobile_manipulator" xmlns:xacro="http://www.ros.org/wiki/xacro">

  <!-- Mobile base -->
  <link name="base_link">
    <inertial>
      <mass value="50.0"/>
      <inertia ixx="1.0" ixy="0.0" ixz="0.0"
               iyy="1.0" iyz="0.0" izz="1.0"/>
    </inertial>
    <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>
  </link>

  <!-- Wheels (differential drive) -->
  <xacro:macro name="wheel" params="prefix reflect">
    <link name="${prefix}_wheel">
      <inertial>
        <mass value="2.0"/>
        <inertia ixx="0.01" ixy="0.0" ixz="0.0"
                 iyy="0.01" iyz="0.0" izz="0.01"/>
      </inertial>
      <visual>
        <geometry>
          <cylinder radius="0.15" length="0.05"/>
        </geometry>
        <material name="black"/>
      </visual>
      <collision>
        <geometry>
          <cylinder radius="0.15" length="0.05"/>
        </geometry>
        <surface>
          <friction>
            <ode>
              <mu>1.0</mu>
              <mu2>1.0</mu2>
            </ode>
          </friction>
        </surface>
      </collision>
    </link>

    <joint name="${prefix}_wheel_joint" type="continuous">
      <parent link="base_link"/>
      <child link="${prefix}_wheel"/>
      <origin xyz="0 ${reflect*0.25} 0" rpy="0 0 0"/>
      <axis xyz="0 1 0"/>
    </joint>
  </xacro:macro>

  <xacro:wheel prefix="left" reflect="1"/>
  <xacro:wheel prefix="right" reflect="-1"/>

  <!-- Arm links and joints -->
  <!-- TODO: Define 6-DOF arm structure -->

  <!-- Gripper -->
  <!-- TODO: Define 2-finger gripper -->

  <!-- Sensors -->
  <!-- TODO: Add camera, LiDAR, IMU -->

  <!-- Gazebo plugins -->
  <gazebo>
    <plugin name="differential_drive_controller"
            filename="libgazebo_ros_diff_drive.so">
      <ros>
        <namespace>/</namespace>
      </ros>
      <left_joint>left_wheel_joint</left_joint>
      <right_joint>right_wheel_joint</right_joint>
      <wheel_separation>0.5</wheel_separation>
      <wheel_diameter>0.3</wheel_diameter>
      <max_wheel_torque>20</max_wheel_torque>
      <publish_odom>true</publish_odom>
      <publish_odom_tf>true</publish_odom_tf>
      <odometry_frame>odom</odometry_frame>
      <robot_base_frame>base_link</robot_base_frame>
    </plugin>
  </gazebo>

</robot>

3. Sensor Simulation

Required Sensors

a) RGB-D Camera

<gazebo reference="camera_link">
  <sensor type="depth" name="camera">
    <update_rate>30.0</update_rate>
    <camera name="head">
      <horizontal_fov>1.3962634</horizontal_fov>
      <image>
        <width>640</width>
        <height>480</height>
        <format>R8G8B8</format>
      </image>
      <clip>
        <near>0.05</near>
        <far>10.0</far>
      </clip>
    </camera>
    <plugin name="camera_controller" filename="libgazebo_ros_camera.so">
      <ros>
        <namespace>camera</namespace>
      </ros>
      <camera_name>depth_camera</camera_name>
      <frame_name>camera_link</frame_name>
      <hack_baseline>0.07</hack_baseline>
      <min_depth>0.05</min_depth>
      <max_depth>10.0</max_depth>
    </plugin>
  </sensor>
</gazebo>

b) 2D LiDAR

<gazebo reference="lidar_link">
  <sensor type="ray" name="lidar">
    <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>0.2</min>
        <max>10.0</max>
        <resolution>0.01</resolution>
      </range>
    </ray>
    <plugin name="lidar_controller" filename="libgazebo_ros_ray_sensor.so">
      <ros>
        <namespace>lidar</namespace>
        <remapping>~/out:=scan</remapping>
      </ros>
      <output_type>sensor_msgs/LaserScan</output_type>
      <frame_name>lidar_link</frame_name>
    </plugin>
  </sensor>
</gazebo>

c) IMU

<gazebo reference="imu_link">
  <sensor type="imu" name="imu">
    <always_on>true</always_on>
    <update_rate>100</update_rate>
    <plugin name="imu_controller" filename="libgazebo_ros_imu_sensor.so">
      <ros>
        <namespace>imu</namespace>
        <remapping>~/out:=data</remapping>
      </ros>
      <frame_name>imu_link</frame_name>
    </plugin>
  </sensor>
</gazebo>

4. Physics Simulation

Configure realistic physics parameters:

<gazebo>
  <plugin name="gazebo_ros_state" filename="libgazebo_ros_state.so">
    <ros>
      <namespace>/gazebo</namespace>
    </ros>
    <update_rate>1.0</update_rate>
  </plugin>
</gazebo>

Contact properties:

<gazebo reference="gripper_finger_link">
  <mu1>1.0</mu1>
  <mu2>1.0</mu2>
  <kp>1000000.0</kp>
  <kd>100.0</kd>
  <minDepth>0.001</minDepth>
  <maxVel>1.0</maxVel>
</gazebo>

5. ROS 2 Integration

Launch File

from launch import LaunchDescription
from launch.actions import IncludeLaunchDescription
from launch.launch_description_sources import PythonLaunchDescriptionSource
from launch_ros.actions import Node
from ament_index_python.packages import get_package_share_directory
import os

def generate_launch_description():
    pkg_dir = get_package_share_directory('mobile_manipulator_sim')

    # Gazebo launch
    gazebo = IncludeLaunchDescription(
        PythonLaunchDescriptionSource([
            os.path.join(get_package_share_directory('gazebo_ros'),
                        'launch', 'gazebo.launch.py')
        ]),
        launch_arguments={
            'world': os.path.join(pkg_dir, 'worlds', 'warehouse.world'),
            'verbose': 'true'
        }.items()
    )

    # Spawn robot
    spawn_robot = Node(
        package='gazebo_ros',
        executable='spawn_entity.py',
        arguments=[
            '-entity', 'mobile_manipulator',
            '-file', os.path.join(pkg_dir, 'urdf', 'robot.urdf'),
            '-x', '0', '-y', '0', '-z', '0.5'
        ],
        output='screen'
    )

    # Robot state publisher
    robot_state_pub = Node(
        package='robot_state_publisher',
        executable='robot_state_publisher',
        arguments=[os.path.join(pkg_dir, 'urdf', 'robot.urdf')]
    )

    return LaunchDescription([
        gazebo,
        spawn_robot,
        robot_state_pub
    ])

Testing Requirements

1. Simulation Stability

• Run simulation for 10 minutes without crashes
• Verify physics stability (no objects falling through floor)
• Check real-time factor (should be close to 1.0)

2. Sensor Validation

• Verify camera publishes images at correct rate
• Check LiDAR scan ranges are reasonable
• Validate IMU data (gravity, accelerations)

3. Motion Tests

• Mobile base drives smoothly
• Arm reaches target poses without collisions
• Gripper grasps and holds objects

4. Performance Metrics

• Measure simulation FPS
• Monitor CPU and memory usage
• Verify sensor update rates

Deliverables

1. Simulation Package

mobile_manipulator_sim/
├── package.xml
├── CMakeLists.txt
├── worlds/
│   └── warehouse.world
├── models/
│   ├── shelves/
│   └── objects/
├── urdf/
│   ├── robot.urdf.xacro
│   └── robot.urdf
├── launch/
│   └── simulation.launch.py
├── config/
│   └── physics_params.yaml
└── README.md

2. Documentation

• System architecture diagram
• URDF structure explanation
• Sensor specifications
• Physics parameter justification
• Usage instructions

3. Demonstration Video

• Show warehouse environment
• Demonstrate mobile base navigation
• Show arm manipulation
• Display sensor data (camera, LiDAR, IMU)
• Demonstrate object grasping

4. Performance Report

• Simulation stability metrics
• Sensor performance (update rates, latency)
• Physics realism evaluation
• Computational requirements

Evaluation Criteria

Criteria	Points	Description
World Design	15	Realistic warehouse with objects
Robot Model	20	Proper URDF structure, physics
Sensor Simulation	20	Correct sensor configuration
Physics	15	Realistic physics parameters
ROS 2 Integration	15	Proper launch files, topics
Stability	10	Simulation runs without crashes
Documentation	5	Clear documentation

Total: 100 points

Tips for Success

1. Start with simple models: Build complexity gradually
2. Test sensors independently: Verify each sensor before integration
3. Tune physics parameters: Adjust step size, friction, damping
4. Use visualization: Enable contact visualization, sensor rays
5. Profile performance: Monitor resource usage

Resources

• Gazebo Tutorials
• URDF Tutorial
• Gazebo ROS 2 Integration

Extension Challenges (Optional)

1. Dynamic objects: Objects that can be pushed/moved
2. Multiple robots: Coordinate two robots
3. Advanced sensors: 3D LiDAR, force/torque sensors
4. Deformable objects: Soft grasping simulation

Good luck with your Gazebo simulation!