Xacro macros for URDF - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a simple Xacro macro named 'wheel'.

ROS

<macro name="wheel">
  <link name="[1]">
    <visual>
      <geometry>
        <cylinder length="0.1" radius="0.05"/>
      </geometry>
    </visual>
  </link>
</macro>

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

Awheel_link

Bwheel1

Clink_wheel

Dwheel

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different name than the macro name for the link.

Forgetting to add the name attribute.

The macro defines a link named 'wheel' which matches the macro's purpose.

2fill in blank

medium

Complete the code to call the 'wheel' macro with the name 'front_wheel'.

ROS

<xacro:wheel name="[1]"/>

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

Awheel

Bwheel_link

Cfront_wheel

Dwheel1

Attempts:

3 left

💡 Hint

Common Mistakes

Using the macro name instead of the instance name.

Omitting the name attribute.

The macro is called with the name 'front_wheel' to create a wheel link with that name.

3fill in blank

hard

Fix the error in the macro parameter declaration to accept a 'radius' parameter.

ROS

<macro name="wheel" [1]>
  <link name="${name}">
    <visual>
      <geometry>
        <cylinder length="0.1" radius="${radius}"/>
      </geometry>
    </visual>
  </link>
</macro>

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

Aparams="name radius"

Bparam="name radius"

Cparams="name, radius"

Dparam="name, radius"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'param' instead of 'params'.

Separating parameters with commas.

The correct attribute to declare parameters is 'params' with space-separated names without commas.

4fill in blank

hard

Fill both blanks to define a macro 'arm' with parameters 'length' and 'radius' and use them in the cylinder geometry.

ROS

<macro name="arm" [1]>
  <link name="arm_link">
    <visual>
      <geometry>
        <cylinder length="$[2]" radius="${radius}"/>
      </geometry>
    </visual>
  </link>
</macro>

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

Aparams="length radius"

Bparam="length radius"

Clength

Dradius

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'param' instead of 'params'.

Using the wrong parameter name inside ${}.

The macro parameters are declared with 'params="length radius"' and the length parameter is used as '${length}'.

5fill in blank

hard

Fill all three blanks to create a macro 'robot_part' with parameters 'name', 'length', and 'radius', and use them correctly in the link name and cylinder geometry.

ROS

<macro name="robot_part" [1]>
  <link name="$[2]">
    <visual>
      <geometry>
        <cylinder length="$[3]" radius="${radius}"/>
      </geometry>
    </visual>
  </link>
</macro>

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

Aparams="name length radius"

Bname

Clength

Dradius

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to declare all parameters.

Using incorrect parameter names inside ${}.

The macro declares parameters 'name', 'length', and 'radius' with 'params'. The link name uses '${name}' and the cylinder length uses '${length}'.

Practice

1. What is the main purpose of using xacro:macro in a URDF file?

easy

A. To define reusable robot parts with customizable parameters

B. To execute robot movement commands

C. To compile the URDF into machine code

D. To visualize the robot in 3D

Solution

1. Step 1: Understand what xacro:macro does

   xacro:macro lets you define a piece of robot description once and reuse it multiple times with different settings.

2. Step 2: Identify the main purpose

   It helps to avoid repeating code and makes the URDF easier to maintain by allowing parameter customization.

3. Final Answer:

   To define reusable robot parts with customizable parameters -> Option A

4. Quick Check:

   Reusability and customization [OK]

Hint: Macros = reusable parts with parameters [OK]

Common Mistakes:

• Thinking macros run robot commands
• Confusing macros with visualization tools
• Believing macros compile code

2. Which of the following is the correct syntax to define a macro named wheel with a parameter radius in Xacro?

easy

A. <xacro:macro name="wheel" radius="1.0">...</xacro:macro>

B. <xacro:macro name="wheel" params="radius">...</xacro:macro>

C. <macro name="wheel" param="radius">...</macro>

D. <xacro:define name="wheel" radius>...</xacro:define>

Solution

1. Step 1: Recall Xacro macro syntax

   Macros are defined with <xacro:macro> tag and parameters are listed in the params attribute as a space-separated string.

2. Step 2: Match the correct syntax

   <xacro:macro name="wheel" params="radius">...</xacro:macro> correctly uses params="radius" inside <xacro:macro> tag.

3. Final Answer:

   <xacro:macro name="wheel" params="radius">...</xacro:macro> -> Option B

4. Quick Check:

   Params attribute [OK]

Hint: Use params="param1 param2" inside [OK]

Common Mistakes:

• Using attributes other than 'params' for parameters
• Omitting the 'params' attribute
• Using incorrect tag names like <macro> or

3. Given the following Xacro macro and call:

<xacro:macro name="link_with_length" params="length">
  <link name="link_${length}">
    <visual>
      <geometry>
        <box size="${length} 0.1 0.1"/>
      </geometry>
    </visual>
  </link>
</xacro:macro>

<xacro:link_with_length length="2.0"/>

What will be the name of the generated link element?

medium

A. link_

B. link_length

C. link_2.0

D. link_${length}

Solution

1. Step 1: Understand macro parameter substitution

   The macro uses ${length} to insert the parameter value into the link name and box size.

2. Step 2: Substitute the parameter value

   The call passes length="2.0", so link_${length} becomes link_2.0.

3. Final Answer:

   link_2.0 -> Option C

4. Quick Check:

   Parameter substitution = link_2.0 [OK]

Hint: Parameter values replace ${param} in macro body [OK]

Common Mistakes:

• Not substituting parameter, leaving ${length} literal
• Using parameter name instead of value
• Leaving name empty

4. Consider this Xacro macro call:

<xacro:link_with_length length=""/>

What is the most likely problem with this call if the macro expects a numeric length parameter?

medium

A. The parameter should be named 'size' instead of 'length'

B. The macro name is incorrect

C. The macro call is missing closing tag

D. The length parameter is empty, causing invalid geometry size

Solution

1. Step 1: Check parameter value in macro call

   The call passes length="", which is empty and not a valid number.

2. Step 2: Understand impact on geometry

   The macro uses ${length} for box size, so empty string leads to invalid or zero size, causing errors or unexpected behavior.

3. Final Answer:

   The length parameter is empty, causing invalid geometry size -> Option D

4. Quick Check:

   Empty parameter causes invalid size [OK]

Hint: Always provide valid parameter values in macro calls [OK]

Common Mistakes:

• Ignoring empty parameter values
• Assuming macro name or tag is wrong
• Confusing parameter names

5. You want to create a robot arm with multiple identical segments but different lengths using Xacro macros. Which approach best achieves this?

hard

A. Define a macro with a length parameter and call it multiple times with different lengths

B. Write separate link elements manually for each segment with hardcoded lengths

C. Use a single macro call without parameters and change lengths later in the URDF

D. Create a macro without parameters and duplicate it multiple times

Solution

1. Step 1: Identify the need for reusable segments with different lengths

   You want to reuse the same segment design but customize length for each segment.

2. Step 2: Choose the approach that supports reuse and customization

   Defining a macro with a length parameter and calling it multiple times with different lengths allows reuse and easy updates.

3. Final Answer:

   Define a macro with a length parameter and call it multiple times with different lengths -> Option A

4. Quick Check:

   Reusable macro with parameters [OK]

Hint: Use parameterized macros for repeated parts with variations [OK]

Common Mistakes:

• Duplicating code manually instead of using macros
• Using macros without parameters losing flexibility
• Changing parameters outside macro calls