Gazebo Simulation Setup for Tekbot

This documentation explains how to set up a basic Gazebo simulation using URDF and XACRO files for a robot called Tekbot, including wheels and a sample environment object (cylinder). It is designed for complete beginners using ROS 2 and Gazebo.

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Table of Contents

1. Overview
2. Project Structure
3. URDF and XACRO Explanation
4. Tekbot URDF
5. Wheel URDF Xacro
6. Cylinder URDF Xacro

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Overview

The goal of this setup is to simulate a mobile robot (Tekbot) in Gazebo using:

• URDF (Unified Robot Description Format) to describe the robot
• XACRO (XML Macros) to simplify repetitive URDF elements like wheels
• Gazebo plugins for simulation behavior

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Project Structure

tekbot_ws/
└── src/
    └── tekbot_description/
        ├── urdf/
        │   ├── tekbot.urdf
        │   ├── wheel.urdf.xacro
        │   └── cylinder.urdf.xacro
        ├── launch/
        │   └── gazebo.launch.py
        └── world/
            └── my_world.world

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URDF and XACRO Explanation

tekbot.urdf

This is the main file that builds the robot by connecting multiple parts together.

Purpose:

• Declares the robot name and base
• Loads the wheel models using macros
• Assembles all parts into one robot

Structure:

1. Robot Declaration:

   <robot name="tekbot" xmlns:xacro="http://ros.org/wiki/xacro">

2. Base Link:

   • base_footprint is the main body of the robot. It serves as the central frame.
   • It is defined as a simple box or cylinder (optional visual/collision not shown in this file).

3. Including the Wheels:

   • Two wheels are loaded using the wheel macro from wheel.urdf.xacro

   • Example:

     <xacro:wheel name="left_wheel" x="0" y="0.1" z="0.03" yaw="0"/>
     <xacro:wheel name="right_wheel" x="0" y="-0.1" z="0.03" yaw="0"/>

4. Joints:

   • Joints are added to connect the wheels to the base.
   • They are continuous joints, allowing the wheels to rotate endlessly.
   • Positioned using the same coordinates defined for the wheels.

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wheel.urdf.xacro

This file defines a reusable macro to generate a wheel. It is used in tekbot.urdf to create the left and right wheels.

Purpose:

• Allows customization of each wheel's position and name
• Reduces code duplication
• Wraps the actual wheel geometry in a macro

Structure:

1. Macro Declaration:

   <xacro:macro name="wheel" params="name x y z yaw">

2. Transform:

   • A <joint> is defined to position the wheel relative to the base
   • Uses <origin> tag with x, y, z, and yaw as input parameters

3. Wheel Link:

   • Loads a cylinder shape by calling the cylinder.urdf.xacro macro
   • The link name is passed along with pose and size info

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cylinder.urdf.xacro

This file defines the geometry of a cylinder (used for the wheel body).

Purpose:

• Reusable shape for cylindrical objects like wheels
• Abstracts out the URDF for a basic cylinder

Structure:

1. Macro Declaration:

   <xacro:macro name="cylinder" params="name radius length">

2. Link Block:

   • Defines the visual, collision, and inertial properties
   • Cylinder is defined along the Z-axis

3. Geometry:

   <geometry>
     <cylinder radius="${radius}" length="${length}"/>
   </geometry>

4. Material and Color:

   • The cylinder is given a gray color using a <material> tag