Building a simple robot arm URDF in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the root link of the robot arm in URDF.

ROS

<robot name="simple_arm">
  <link name="[1]"/>
</robot>

Drag options to blanks, or click blank then click option'

Abase_link

Bend_effector

Carm_link

Djoint1

Attempts:

3 left

💡 Hint

Common Mistakes

Using joint names instead of link names.

Naming the root link as the end effector.

The root link of a robot arm is usually called base_link. This is the fixed part where the arm starts.

2fill in blank

medium

Complete the code to define a revolute joint connecting the base_link to arm_link.

ROS

<joint name="joint1" type="[1]">
  <parent link="base_link"/>
  <child link="arm_link"/>
  <origin xyz="0 0 0.1" rpy="0 0 0"/>
</joint>

Drag options to blanks, or click blank then click option'

Acontinuous

Bfixed

Cprismatic

Drevolute

Attempts:

3 left

💡 Hint

Common Mistakes

Using fixed which does not allow movement.

Confusing prismatic with rotational joints.

A revolute joint allows rotation around one axis, which is typical for robot arm joints.

3fill in blank

hard

Fix the error in the joint axis definition to rotate around the Z axis.

ROS

<joint name="joint1" type="revolute">
  <parent link="base_link"/>
  <child link="arm_link"/>
  <axis xyz="[1]"/>
</joint>

Drag options to blanks, or click blank then click option'

A0 0 1

B0 1 0

C1 0 0

D1 1 0

Attempts:

3 left

💡 Hint

Common Mistakes

Using X or Y axis values instead of Z.

Using non-unit vectors like 1 1 0.

The Z axis is represented by 0 0 1 in the axis attribute.

4fill in blank

hard

Fill both blanks to define the visual geometry of the arm_link as a box with size 0.1 0.1 0.3.

ROS

<link name="arm_link">
  <visual>
    <geometry>
      <box size="[1] [2] 0.3"/>
    </geometry>
  </visual>
</link>

Drag options to blanks, or click blank then click option'

A0.1

B0.2

C0.05

D0.3

Attempts:

3 left

💡 Hint

Common Mistakes

Using different values for width and depth.

Confusing height with width or depth.

The box size is given as width, depth, and height. Both width and depth are 0.1 here.

5fill in blank

hard

Fill all three blanks to define the origin position and rotation of the end_effector link relative to arm_link.

ROS

<joint name="joint2" type="fixed">
  <parent link="arm_link"/>
  <child link="end_effector"/>
  <origin xyz="[1] [2] [3]" rpy="0 0 0"/>
</joint>

Drag options to blanks, or click blank then click option'

A0.0

B0.3

C0.1

D0.2

Attempts:

3 left

💡 Hint

Common Mistakes

Setting X or Y offsets to non-zero values.

Using incorrect Z offset values.

The end effector is positioned at the tip of the arm, which is 0.3 meters along the Z axis, with no offset in X or Y.

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