Building a simple robot arm URDF in ROS - Mini Project: Build & Apply

Building a simple robot arm URDF

📖 Scenario: You are creating a simple robot arm model for a robotics simulation. The robot arm has a base, an upper arm, and a forearm. You will write a URDF (Unified Robot Description Format) file step-by-step to describe this robot arm's structure.

🎯 Goal: Build a basic URDF file that defines a simple robot arm with three links and two joints connecting them. This URDF will be ready to use in ROS simulation tools.

📋 What You'll Learn

Create three links named base_link, upper_arm_link, and forearm_link

Define two joints named shoulder_joint and elbow_joint

Set the shoulder_joint to connect base_link to upper_arm_link

Set the elbow_joint to connect upper_arm_link to forearm_link

Use revolute joint type with axis along 0 0 1 for both joints

💡 Why This Matters

🌍 Real World

Robot arms are used in factories, labs, and homes. URDF files describe their shape and joints for simulation and control.

💼 Career

Understanding URDF is essential for robotics engineers and developers working with ROS to build and simulate robots.

1

Create the base link

Create a URDF link element named base_link with an empty visual tag inside.

ROS

Hint

The link element defines a part of the robot. Start with the base link.

2

Add upper arm and forearm links

Add two more link elements named upper_arm_link and forearm_link, each with an empty visual tag, inside the <robot> element.

ROS

Hint

Each link needs a visual tag even if empty to define appearance later.

3

Add shoulder joint connecting base to upper arm

Add a joint element named shoulder_joint of type revolute connecting base_link as parent and upper_arm_link as child. Set the joint axis to 0 0 1.

ROS

Hint

The joint element connects two links. Use parent and child tags to specify links.

4

Add elbow joint connecting upper arm to forearm

Add a joint element named elbow_joint of type revolute connecting upper_arm_link as parent and forearm_link as child. Set the joint axis to 0 0 1. Place this joint inside the <robot> element after the shoulder_joint.

ROS

Hint

Repeat the joint structure for the elbow joint connecting the upper arm to the forearm.

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