Building a simple robot arm URDF in ROS

Introduction

A URDF file describes a robot's parts and how they connect. It helps robots understand their shape and move correctly.

When you want to simulate a robot arm in a virtual environment.

When you need to control a robot arm by defining its joints and links.

When you want to visualize your robot design in 3D tools like RViz.

When you are building a robot and want to share its structure with others.

Syntax

ROS

<robot name="robot_name">
  <link name="link_name">
    <!-- Visual, collision, and inertial elements -->
  </link>
  <joint name="joint_name" type="joint_type">
    <parent link="parent_link"/>
    <child link="child_link"/>
    <origin xyz="x y z" rpy="roll pitch yaw"/>
    <axis xyz="x y z"/>
  </joint>
</robot>

The <robot> tag wraps the whole robot description.

Each <link> is a part of the robot, and <joint> connects two links.

Examples

A robot with two links connected by a rotating joint.

ROS

<robot name="simple_arm">
  <link name="base_link"/>
  <link name="arm_link"/>
  <joint name="base_to_arm" type="revolute">
    <parent link="base_link"/>
    <child link="arm_link"/>
    <origin xyz="0 0 0.1" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
  </joint>
</robot>

A simple arm with two joints allowing rotation in different directions.

ROS

<robot name="two_joint_arm">
  <link name="base_link"/>
  <link name="link1"/>
  <link name="link2"/>
  <joint name="joint1" type="revolute">
    <parent link="base_link"/>
    <child link="link1"/>
    <origin xyz="0 0 0.1" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
  </joint>
  <joint name="joint2" type="revolute">
    <parent link="link1"/>
    <child link="link2"/>
    <origin xyz="0 0 0.5" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
  </joint>
</robot>

Sample Program

This URDF defines a simple robot arm with a base and one arm link connected by a rotating joint. The base is a small box, and the arm is a blue cylinder that can rotate around the base.

ROS

<?xml version="1.0"?>
<robot name="simple_robot_arm">
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.2 0.2 0.1"/>
      </geometry>
      <material name="gray">
        <color rgba="0.5 0.5 0.5 1"/>
      </material>
    </visual>
  </link>
  <link name="arm_link">
    <visual>
      <geometry>
        <cylinder length="0.5" radius="0.05"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
  </link>
  <joint name="base_to_arm" type="revolute">
    <parent link="base_link"/>
    <child link="arm_link"/>
    <origin xyz="0 0 0.05" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
  </joint>
</robot>

Important Notes

Always check your URDF in visualization tools like RViz to see if it looks right.

Use clear names for links and joints to avoid confusion.

Define the origin carefully to position parts correctly relative to each other.

Summary

A URDF file describes robot parts (links) and how they connect (joints).

Use <link> for parts and <joint> to connect them.

Visualize your URDF to make sure your robot looks and moves as expected.

Practice

1. What is the main purpose of a URDF file in ROS when building a robot arm?

easy

A. To describe the robot's parts and how they connect

B. To write the robot's control software

C. To store sensor data from the robot

D. To compile the robot's firmware

Solution

1. Step 1: Understand URDF role

   A URDF file defines the robot's physical structure, including parts and joints.

2. Step 2: Differentiate from other files

   Control software, sensor data, and firmware are handled separately, not in URDF.

3. Final Answer:

   To describe the robot's parts and how they connect -> Option A

4. Quick Check:

   URDF = robot structure description [OK]

Hint: URDF = robot parts + connections description [OK]

Common Mistakes:

• Confusing URDF with control code
• Thinking URDF stores sensor data
• Assuming URDF compiles firmware

2. Which of the following is the correct way to define a link in a URDF file?

easy

A.

B.

C.

D.

Solution

1. Step 1: Identify correct URDF tag for a link

   The <link> tag with a name attribute defines a robot part.

2. Step 2: Check syntax correctness

   <link name="arm_link"/> uses correct self-closing tag with name attribute. <link arm_link></link> misses quotes and uses open-close tags unnecessarily. Options A and B use wrong tags.

3. Final Answer:

   <link name="arm_link"/> -> Option A

4. Quick Check:

   Link tag syntax = <link name="arm_link"/> [OK]

Hint: Use to define parts [OK]

Common Mistakes:

• Using <joint> instead of <link> for parts
• Missing quotes around name
• Using non-existent <part> tag

3. Given this URDF snippet, what is the parent link of the joint named elbow_joint?

<joint name="elbow_joint" type="revolute">
  <parent link="upper_arm"/>
  <child link="forearm"/>
</joint>

medium

A. wrist

B. elbow_joint

C. forearm

D. upper_arm

Solution

1. Step 1: Locate parent link in joint tag

   The <parent> tag inside the joint defines the parent link connected by the joint.

2. Step 2: Read the parent link attribute

   Here, parent link="upper_arm" means the joint connects from upper_arm to forearm.

3. Final Answer:

   upper_arm -> Option D

4. Quick Check:

   Parent link = upper_arm [OK]

Hint: Parent link is inside [OK]

Common Mistakes:

• Confusing parent and child links
• Choosing joint name as parent
• Picking child link as parent

4. Identify the error in this URDF joint definition:

<joint name="wrist_joint" type="fixed">
  <parent link="forearm"/>
  <child link="hand">
</joint>

medium

A. Incorrect joint type 'fixed'

B. Parent link name is invalid

C. Missing closing tag for <child>

D. Joint name cannot have underscore

Solution

1. Step 1: Check XML tag completeness

   The <child> tag is opened but not closed properly.

2. Step 2: Verify joint type and names

   Joint type 'fixed' is valid, parent link name looks correct, underscores in names are allowed.

3. Final Answer:

   Missing closing tag for <child> -> Option C

4. Quick Check:

   Unclosed tag error = Missing closing tag for <child> [OK]

Hint: Check all XML tags are properly closed [OK]

Common Mistakes:

• Ignoring unclosed tags
• Thinking 'fixed' is invalid joint type
• Assuming underscores are disallowed

5. You want to add a new joint connecting the 'forearm' link to a new 'wrist' link with a revolute joint rotating around the Z axis. Which of these URDF snippets correctly defines this joint?

hard

A.

B.

C.

D.

Solution

1. Step 1: Confirm joint type and link order

   The joint should be revolute, connecting parent 'forearm' to child 'wrist'.

2. Step 2: Check rotation axis

   The axis must be around Z axis: xyz="0 0 1".

3. Step 3: Validate options

   <joint name="wrist_joint" type="revolute"> <parent link="forearm"/> <child link="wrist"/> <axis xyz="0 0 1"/> </joint> matches all requirements. <joint name="wrist_joint" type="fixed"> <parent link="wrist"/> <child link="forearm"/> <axis xyz="0 0 1"/> </joint> reverses parent and child and uses fixed type. <joint name="wrist_joint" type="revolute"> <parent link="wrist"/> <child link="forearm"/> <axis xyz="1 0 0"/> </joint> reverses links and uses wrong axis. <joint name="wrist_joint" type="continuous"> <parent link="forearm"/> <child link="wrist"/> <axis xyz="0 1 0"/> </joint> uses continuous type and wrong axis.

4. Final Answer:

   <joint name="wrist_joint" type="revolute"> <parent link="forearm"/> <child link="wrist"/> <axis xyz="0 0 1"/> </joint> -> Option B

5. Quick Check:

   Correct joint type + parent-child + axis = <joint name="wrist_joint" type="revolute"> <parent link="forearm"/> <child link="wrist"/> <axis xyz="0 0 1"/> </joint> [OK]

Hint: Parent link first, child second, axis xyz="0 0 1" for Z rotation [OK]

Common Mistakes:

• Swapping parent and child links
• Using wrong joint type
• Setting axis to wrong vector