Custom message definition in ROS - Mini Project: Build & Apply

Custom message definition

📖 Scenario: You are building a simple ROS (Robot Operating System) package that needs to send custom data between nodes. To do this, you must define a custom message type that includes specific fields.

🎯 Goal: Create a custom ROS message definition file with exact fields, then use it in a ROS node to publish messages of that type.

📋 What You'll Learn

Create a ROS message file named MyCustomMsg.msg with specified fields

Add a configuration variable for the message queue size

Write a ROS publisher node that publishes messages of type MyCustomMsg

Complete the ROS node setup with proper initialization and publishing

💡 Why This Matters

🌍 Real World

Custom messages let ROS nodes communicate complex data tailored to your robot's needs, like sensor readings or commands.

💼 Career

Defining and using custom messages is essential for robotics software engineers working with ROS to build modular and scalable robot applications.

1

Create the custom message file

Create a ROS message file named MyCustomMsg.msg with these exact fields and types:
int32 id
string name
float64 value

ROS

# Your message file content here

Hint

This file defines the structure of your custom message. Each line specifies a field type and name.

2

Add a queue size configuration variable

In your ROS publisher node script, create a variable called queue_size and set it to 10.

ROS

int32 id
string name
float64 value

# Define queue_size variable
# Your code here

Hint

This variable controls how many messages can be buffered before sending.

3

Write the ROS publisher node core logic

Write code to create a ROS publisher named pub that publishes MyCustomMsg messages on the topic custom_topic with the queue_size variable. Use the ROS Python client library conventions.

ROS

int32 id
string name
float64 value

queue_size = 10

# Create ROS publisher named pub
# Your code here

Hint

Use rospy.Publisher with the topic name, message type, and queue size.

4

Complete the ROS node with publishing logic

Add code to create a MyCustomMsg message instance named msg, set its fields to id=1, name='test', and value=3.14, then publish it using pub.publish(msg).

ROS

int32 id
string name
float64 value

queue_size = 10

import rospy
from your_package.msg import MyCustomMsg

rospy.init_node('custom_publisher')
pub = rospy.Publisher('custom_topic', MyCustomMsg, queue_size=queue_size)

# Create message instance and publish
# Your code here

Hint

Create the message object, set each field exactly, then call publish.

Practice

1. What is the main purpose of defining a custom message in ROS?

easy

A. To send specific data types tailored to your robot's needs

B. To create new robot hardware components

C. To write Python scripts for robot control

D. To install ROS packages automatically

Solution

1. Step 1: Understand the role of custom messages

   Custom messages allow you to define exactly what data your robot nodes exchange.

2. Step 2: Compare with other options

   Options B, C, and D relate to hardware, scripting, or package management, not message definition.

3. Final Answer:

   To send specific data types tailored to your robot's needs -> Option A

4. Quick Check:

   Custom messages = tailored data [OK]

Hint: Custom messages define data, not hardware or scripts [OK]

Common Mistakes:

• Confusing message definition with hardware setup
• Thinking custom messages are scripts
• Assuming messages install packages

2. Which of the following is the correct syntax to define a field named speed of type float32 in a ROS .msg file?

easy

A. speed float32

B. float32 speed

C. float speed32

D. float speed

Solution

1. Step 1: Recall ROS message field syntax

   In ROS .msg files, the type comes first, then the field name, e.g., float32 speed.

2. Step 2: Check each option

   float32 speed matches the correct syntax. Options B, C, and D have incorrect order or wrong types.

3. Final Answer:

   float32 speed -> Option B

4. Quick Check:

   Type before name in .msg files [OK]

Hint: Type first, then field name in .msg files [OK]

Common Mistakes:

• Swapping field name and type order
• Using incorrect type names
• Omitting the type or name

3. Given this custom message definition in RobotStatus.msg:

bool is_active
float32 battery_level
string status_message

What will be the default value of is_active when a new message is created in a ROS node?

medium

A. false

B. 1

C. null

D. true

Solution

1. Step 1: Understand default values in ROS messages

   Boolean fields default to false when a message is created without explicit initialization.

2. Step 2: Evaluate options

   false is false, which matches the default. Options A and B represent true values, and C is invalid for bool.

3. Final Answer:

   false -> Option A

4. Quick Check:

   Boolean default = false [OK]

Hint: Boolean fields default to false unless set [OK]

Common Mistakes:

• Assuming booleans default to true
• Confusing null with default values
• Using numeric 1 instead of boolean true

4. You wrote this in your SensorData.msg file:

int32 temperature
float64 humidity
string location

But when building your ROS package, you get an error. What is the likely cause?

medium

A. Using float64 instead of float32 is invalid in ROS messages

B. Field names cannot be lowercase

C. Missing blank line at the end of the file

D. No error; the message definition is correct

Solution

1. Step 1: Check ROS message type validity

   ROS supports float64 as a valid type, so Using float64 instead of float32 is invalid in ROS messages is incorrect.

2. Step 2: Verify field naming and formatting

   Lowercase field names are allowed, and no blank line is required at the end.

3. Final Answer:

   No error; the message definition is correct -> Option D

4. Quick Check:

   Valid types and names mean no error [OK]

Hint: ROS supports float64 and lowercase names [OK]

Common Mistakes:

• Thinking float64 is invalid
• Assuming field names must be uppercase
• Expecting blank line at file end

5. You want to define a custom message RobotCommand.msg that includes a nested message Header from the standard ROS messages and a custom field command of type string. Which is the correct way to define this?

hard

A. Header header
string command

B. header Header
command string

C. std_msgs/Header header
string command

D. Header: std_msgs/Header
string command

Solution

1. Step 1: Understand nested message syntax in ROS

   To use a message from another package, you must prefix the type with the package name, e.g., std_msgs/Header.

2. Step 2: Check each option

   std_msgs/Header header string command correctly uses std_msgs/Header header. Header header string command misses the package prefix, C reverses type and name, D uses invalid syntax.

3. Final Answer:

   std_msgs/Header header string command -> Option C

4. Quick Check:

   Use package/type for nested messages [OK]

Hint: Prefix nested types with package name [OK]

Common Mistakes:

• Omitting package name for nested messages
• Swapping field name and type
• Using colon syntax incorrectly