Setting parameters from launch file in ROS - Mini Project: Build & Apply

Setting parameters from launch file

📖 Scenario: You are building a simple ROS (Robot Operating System) application where a node needs to read configuration parameters set from a launch file. This is common when you want to change settings without modifying the node code.

🎯 Goal: Create a ROS launch file that sets parameters, and write a ROS node that reads these parameters at startup.

📋 What You'll Learn

Create a ROS launch file named my_robot.launch that sets parameters robot_name and max_speed.

Write a Python ROS node named robot_node.py that reads robot_name and max_speed parameters from the parameter server.

Print the parameter values inside the node to verify they are read correctly.

💡 Why This Matters

🌍 Real World

Setting parameters from launch files is a common way to configure robot behavior without changing code. It helps in testing different settings quickly.

💼 Career

ROS developers often use launch files to manage parameters and node startup. Understanding this is essential for robotics software engineering roles.

1

Create the ROS launch file with parameters

Create a launch file named my_robot.launch that sets the parameter robot_name to "RoboBuddy" and max_speed to 1.5 using the <param> tag.

ROS

Hint

Use the <param> tag inside the <launch> root element to set parameters.

2

Initialize the ROS node and import necessary modules

In a Python file named robot_node.py, import rospy and initialize a ROS node called robot_node.

ROS

import rospy

# Your code here

Hint

Use rospy.init_node() to initialize the node with the name robot_node.

3

Read parameters from the parameter server

Use rospy.get_param to read the parameters robot_name and max_speed from the parameter server and store them in variables robot_name and max_speed.

ROS

import rospy

rospy.init_node('robot_node')

# Read parameters here

Hint

Use rospy.get_param('parameter_name') to get the parameter value.

4

Print the parameter values to verify

Add rospy.loginfo statements to print the values of robot_name and max_speed.

ROS

import rospy

rospy.init_node('robot_node')

robot_name = rospy.get_param('robot_name')
max_speed = rospy.get_param('max_speed')

# Print parameters here

Hint

Use rospy.loginfo() to print messages to the ROS console.

Practice

1. What is the main purpose of using <param> tags in a ROS launch file?

easy

A. To start multiple nodes at the same time

B. To set parameters that nodes can use when they start

C. To define the order in which nodes run

D. To stop nodes after they finish

Solution

1. Step 1: Understand the role of <param> in launch files

   The <param> tag is used to set parameters that nodes read when they start running.

2. Step 2: Differentiate from other launch file tags

   Tags like <node> start nodes, but <param> specifically sets values nodes use.

3. Final Answer:

   To set parameters that nodes can use when they start -> Option B

4. Quick Check:

   Setting parameters = A [OK]

Hint: Remember: <param> sets values, <node> starts nodes [OK]

Common Mistakes:

• Confusing <param> with <node> tag
• Thinking <param> controls node start/stop
• Assuming <param> sets node execution order

2. Which of the following is the correct syntax to set a string parameter named robot_name to "R2D2" in a ROS launch file?

easy

A.

B.

C.

D.

Solution

1. Step 1: Check the correct attribute names

   The <param> tag requires 'name' for the parameter name and 'value' for its value.

2. Step 2: Verify proper quoting of string values

   String values must be inside quotes, so value="R2D2" is correct.

3. Final Answer:

   <param name="robot_name" value="R2D2" /> -> Option C

4. Quick Check:

   Correct attributes and quotes = B [OK]

Hint: Use name="" and value="" with quotes for strings [OK]

Common Mistakes:

• Missing quotes around string values
• Swapping name and value attributes
• Using incorrect attribute names

3. Given this launch file snippet:

<launch>
  <param name="speed" value="2.5" />
  <node pkg="robot_pkg" type="move_node" name="mover" />
</launch>

What will be the value of the speed parameter inside the move_node when it starts?

medium

A. 2.5 (as a float)

B. 0, the default value

C. Undefined, because parameters must be set inside the node tag

D. "2.5" (as a string)

Solution

1. Step 1: Understand parameter value types in launch files

   Values in <param> tags are treated as strings by default unless explicitly converted in code.

2. Step 2: Check the launch file snippet

   The value is "2.5" in quotes, so the parameter is set as a string "2.5".

3. Final Answer:

   "2.5" (as a string) -> Option D

4. Quick Check:

   Parameter values are strings by default = D [OK]

Hint: Parameter values are strings unless converted in code [OK]

Common Mistakes:

• Assuming numeric values are auto-converted to float
• Thinking parameters must be inside <node> tag
• Believing default values apply if parameter is set

4. Identify the error in this launch file snippet:

<launch>
  <param name="use_sim" value=true />
  <node pkg="sim_pkg" type="sim_node" name="simulator" />
</launch>

medium

A. The value true should be in quotes: value="true"

B. The param tag should be inside the node tag

C. The node tag is missing the required args attribute

D. The launch tag is missing a closing slash

Solution

1. Step 1: Check the value attribute format

   Boolean values in <param> tags must be strings, so true should be "true" in quotes.

2. Step 2: Verify tag placement and syntax

   The <param> tag can be outside the <node> tag, and the <launch> tag is correctly closed.

3. Final Answer:

   The value true should be in quotes: value="true" -> Option A

4. Quick Check:

   Boolean values need quotes in <param> = C [OK]

Hint: Always put boolean values in quotes in <param> tags [OK]

Common Mistakes:

• Leaving boolean values unquoted
• Placing <param> tags only inside <node>
• Misunderstanding launch tag syntax

5. You want to set a parameter max_retries to 5 only if it is not already set by another launch file. Which launch file tag should you use to achieve this?

hard

A.

B. 5

C.

D.

Solution

1. Step 1: Understand conditional parameter setting

   The <param> tag supports the 'unless' attribute to set a parameter only if it is not already set.

2. Step 2: Check the options for correct usage

   <param unless="max_retries" name="max_retries" value="5" /> uses 'unless="max_retries"' with name and value, which is the correct syntax.

3. Final Answer:

   <param unless="max_retries" name="max_retries" value="5" /> -> Option A

4. Quick Check:

   Use 'unless' attribute to avoid overwriting = A [OK]

Hint: Use 'unless' attribute to set param only if unset [OK]

Common Mistakes:

• Omitting the 'name' attribute when using 'unless'
• Using <rosparam> without condition
• Misplacing 'unless' attribute