ROS2 Communication & Tools

Master ROS2 topics, services, and essential debugging tools

Tutorial Overview

🎯 Learning Objectives

By the end of this tutorial, you will:

✅ Understand ROS2 communication paradigms (topics, services, actions)
✅ List and inspect topics, services, and nodes
✅ Publish and subscribe to topics from command line
✅ Call services to control robot
✅ Use rqt tools for visualization and debugging
✅ Record and playback data with rosbags
✅ Debug communication issues
✅ Understand message types and interfaces

⏱️ Time Required

Reading & Concepts: 25 minutes
Command Line Tools: 30 minutes
RQT Tools: 25 minutes
Rosbag Practice: 20 minutes
Debugging: 15 minutes
Total: ~115 minutes

📚 Prerequisites

✅ Completed Getting Started
✅ Robot powered on and connected
✅ SSH access to robot
✅ Basic Linux command line knowledge
✅ Understanding of terminal navigation

🛠️ What You'll Need

✅ Beetlebot (powered on, base system running)
✅ Laptop with ROS2 Jazzy installed
✅ Both robot and laptop on same WiFi network
✅ Terminal access (SSH to robot or direct)

Part 1: ROS2 Communication Paradigms

What are topics?

Continuous data streams
Many-to-many communication
Publishers send data, subscribers receive
Asynchronous (fire-and-forget)

Example:

LiDAR sensor → publishes → /scan topic → subscribers (SLAM, Nav2, visualization)

When to use:

Sensor data (LiDAR, camera, IMU)
Status updates (battery, position)
Continuous streams (video, telemetry)

Services (Request-Response)

What are services?

One-time request-response
Synchronous (wait for response)
Client sends request, server responds
Like function calls

Example:

Client: "ARM the robot"
Robot service: "OK, motors armed"

When to use:

Commands (ARM, DISARM, emergency stop)
Queries (get parameter, check status)
One-time actions

Actions (Goal-Based)

What are actions?

Long-running tasks with feedback
Can be cancelled
Provides progress updates
Goal → Feedback → Result

Example:

Goal: "Navigate to (x, y)"
Feedback: "50% complete, 10m remaining"
Result: "Goal reached"

When to use:

Navigation goals
Long tasks (mapping, complex motions)
Need progress updates or cancellation

Part 2: Exploring Your Robot's Topics

List All Topics

ros2 topic list

Your robot has these key topics:

/scan               # LiDAR data
/imu/data           # IMU (filtered)
/imu/data_raw       # IMU (raw)
/wheel/odom         # Wheel-only odometry
/odometry/filtered  # Fused odometry (wheel + IMU via EKF)
/odom               # Same as /odometry/filtered (alias)
/cmd_vel            # Velocity commands (main control)
/cmd_vel_joy        # Velocity from joystick
/cmd_vel_nav        # Velocity from autonomous navigation
/joy                # Raw joystick input
/battery_voltage    # Battery status
/wheel_rpm          # Wheel speeds (RPM)
/wheel_ticks        # Encoder ticks
/pi_camera/image_raw           # Camera images
/pi_camera/image_raw/compressed # Compressed camera
/pi_camera/camera_info         # Camera calibration
/map                # Map from SLAM/map_server
/amcl_pose          # Localization estimate
/particle_cloud     # AMCL particles
/plan               # Global navigation path
/tf                 # Transform tree
/tf_static          # Static transforms
/lyra/armed         # ARM status (bool)

Get Topic Information

# Check topic type
ros2 topic type /scan
# Output: sensor_msgs/msg/LaserScan

ros2 topic type /cmd_vel
# Output: geometry_msgs/msg/Twist

ros2 topic type /battery_voltage
# Output: std_msgs/msg/Float32

# Check publish rate
ros2 topic hz /scan
# Output: average rate: 10.xxx Hz

ros2 topic hz /odometry/filtered
# Output: average rate: 30.xxx Hz

# Check bandwidth
ros2 topic bw /scan
# Output: average: X.XX MB/s

# Get detailed info
ros2 topic info /cmd_vel
# Output: 
# Type: geometry_msgs/msg/Twist
# Publisher count: 3
# Subscription count: 2

Exercise 2.1: Topic Discovery

Task: Identify all sensor topics and their rates

# List all topics, save to file
ros2 topic list > topics.txt

# Check each sensor topic rate:
ros2 topic hz /scan              # LiDAR (should be ~10 Hz)
ros2 topic hz /imu/data          # IMU (should be ~100 Hz)
ros2 topic hz /odometry/filtered # Fused odom (should be ~30 Hz)
ros2 topic hz /wheel/odom        # Wheel odom (should be ~20 Hz)
ros2 topic hz /battery_voltage   # Battery (should be ~1-10 Hz)

# Questions to answer:
# 1. Which sensor has highest rate?
# 2. Which has lowest?
# 3. Why might rates differ?

Expected Answers

Highest rate: IMU (~100 Hz) - fast dynamics, needs high sampling
Lowest rate: Battery (~1-10 Hz) - slow changing, doesn't need frequent updates
Rate differences: Based on sensor physics and application needs
- LiDAR: 10 Hz (motor rotation speed)
- IMU: 100 Hz (capture fast dynamics)
- Odometry: 20-30 Hz (control loop rate)
- Battery: 1-10 Hz (slow changing)

Part 3: Reading Topic Data

Echo Topics (View Live Data)

# View LiDAR data (LOTS of output!)
ros2 topic echo /scan

# Press Ctrl+C to stop

# View single message
ros2 topic echo /scan --once

# View specific field
ros2 topic echo /scan --field ranges --once

# View with limited samples
ros2 topic echo /scan --no-arr  # Don't print arrays (cleaner)

Exercise 2.2: Understanding Sensor Data

Task: Examine different sensor message formats

LiDAR data:

ros2 topic echo /scan --once

# Look for:
# - angle_min, angle_max: Scan range (radians)
# - angle_increment: Angular resolution
# - range_min, range_max: Distance limits
# - ranges[]: Array of distance measurements

# Questions:
# 1. How many range measurements per scan?
# 2. What's the angular resolution?
# 3. What's max range?

IMU data:

ros2 topic echo /imu/data --once

# Look for:
# - orientation: Quaternion (x, y, z, w)
# - angular_velocity: Rotation rates (rad/s)
# - linear_acceleration: Accelerations (m/s²)

# Questions:
# 1. What's current orientation?
# 2. Is robot moving? (check angular_velocity)
# 3. What's gravity reading? (linear_acceleration.z)

Odometry data:

ros2 topic echo /odometry/filtered --once

# Look for:
# - pose.pose.position: (x, y, z) in meters
# - pose.pose.orientation: Quaternion
# - twist.twist.linear: Linear velocity
# - twist.twist.angular: Angular velocity
# - covariance: Uncertainty estimates

# Questions:
# 1. Where is robot? (x, y position)
# 2. Which direction facing? (orientation)
# 3. How fast moving? (linear.x)

Part 4: Publishing to Topics

Understanding Message Structure

First, see what a message looks like:

ros2 interface show geometry_msgs/msg/Twist

# Output:
# Vector3  linear
#   float64 x
#   float64 y
#   float64 z
# Vector3  angular
#   float64 x
#   float64 y
#   float64 z

For differential drive (Beetlebot):

linear.x: Forward/backward speed (m/s)
angular.z: Turn rate (rad/s)
Other fields unused (can't strafe sideways or fly!)

Publishing Commands

⚠️ CRITICAL: Command Timeout

Your robot has a 500ms command timeout. Commands must be published continuously (at least 2 Hz) or the robot will automatically DISARM and stop.

❌ THIS WILL NOT WORK:

ros2 topic pub --once /cmd_vel geometry_msgs/msg/Twist \
  "{linear: {x: 0.3, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.0}}"
# Robot ignores single command due to timeout!

✅ THIS WORKS:

# First, ARM the robot
ros2 service call /lyra/arm std_srvs/srv/Trigger

# Then publish continuously at 10 Hz
ros2 topic pub --rate 10 /cmd_vel geometry_msgs/msg/Twist \
  "{linear: {x: 0.3, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.0}}"

# Robot moves forward at 0.3 m/s
# Press Ctrl+C to stop

Exercise 2.3: Manual Robot Control

Task: Drive robot using command line

Test 1: Forward motion

# ARM robot
ros2 service call /lyra/arm std_srvs/srv/Trigger

# Drive forward 0.3 m/s
ros2 topic pub --rate 10 /cmd_vel geometry_msgs/msg/Twist \
  "{linear: {x: 0.3, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.0}}"

# Let it run for 3 seconds, then Ctrl+C
# Robot should travel ~0.9 meters

Test 2: Rotation

# ARM robot (if needed)
ros2 service call /lyra/arm std_srvs/srv/Trigger

# Rotate in place (1.0 rad/s ≈ 57°/s)
ros2 topic pub --rate 10 /cmd_vel geometry_msgs/msg/Twist \
  "{linear: {x: 0.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.0}}"

# Let it run for 1.57 seconds for 90° turn
# Then Ctrl+C

Test 3: Arc motion

# ARM robot
ros2 service call /lyra/arm std_srvs/srv/Trigger

# Drive in arc (forward + turn)
ros2 topic pub --rate 10 /cmd_vel geometry_msgs/msg/Twist \
  "{linear: {x: 0.3, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.5}}"

# Creates curved path
# Ctrl+C to stop

Test 4: Using timeout command

# ARM robot
ros2 service call /lyra/arm std_srvs/srv/Trigger

# Drive for exactly 2 seconds then auto-stop
timeout 2 ros2 topic pub --rate 10 /cmd_vel geometry_msgs/msg/Twist \
  "{linear: {x: 0.5, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.0}}"

# No need for Ctrl+C, stops automatically

Part 5: Services

List Available Services

ros2 service list | grep lyra

# Output:
# /lyra/arm
# /lyra/disarm
# /lyra/emergency_stop
# /lyra/set_ros_mode

Calling Services

ARM/DISARM robot:

# ARM robot (enable motors)
ros2 service call /lyra/arm std_srvs/srv/Trigger

# Output:
# waiting for service to become available...
# requester: making request: std_srvs.srv.Trigger_Request()
# 
# response:
# std_srvs.srv.Trigger_Response(success=True, message='Motors armed')

# DISARM robot (disable motors)
ros2 service call /lyra/disarm std_srvs/srv/Trigger

# Output:
# response:
# std_srvs.srv.Trigger_Response(success=True, message='Motors disarmed')

# Emergency stop
ros2 service call /lyra/emergency_stop std_srvs/srv/Trigger

# Output:
# response:
# std_srvs.srv.Trigger_Response(success=True, message='Emergency stop activated')

Check Service Type and Interface

# Get service type
ros2 service type /lyra/arm
# Output: std_srvs/srv/Trigger

# See interface definition
ros2 interface show std_srvs/srv/Trigger

# Output:
# ---
# bool success     # indicate successful run of triggered service
# string message   # informational, e.g. for error messages

Exercise 2.4: Service Control Flow

Task: Test ARM → Drive → DISARM sequence

# Step 1: Check ARM status
ros2 topic echo /lyra/armed --once
# Expected: data: false

# Step 2: ARM robot
ros2 service call /lyra/arm std_srvs/srv/Trigger
# Expected: success=True

# Step 3: Verify ARMED
ros2 topic echo /lyra/armed --once
# Expected: data: true

# Step 4: Send velocity command
ros2 topic pub --rate 10 /cmd_vel geometry_msgs/msg/Twist \
  "{linear: {x: 0.2, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.0}}"
# Robot should move

# Step 5: Stop command (Ctrl+C)

# Step 6: Wait 1 second (command timeout)
sleep 1

# Step 7: Check ARM status (should auto-DISARM)
ros2 topic echo /lyra/armed --once
# Expected: data: false (auto-disarmed due to timeout)

# Step 8: Manual DISARM (if still armed)
ros2 service call /lyra/disarm std_srvs/srv/Trigger

What you learned:

Robot automatically DISARMs after 500ms without commands
Must continuously publish commands to maintain motion
Services provide control over robot state

Part 6: Understanding the Communication Graph

Using rqt_graph

Visualize node connections:

# Launch rqt_graph
rqt_graph

# If graph appears empty:
rqt_graph --force-discover

# Or wait 10-15 seconds for discovery

What you'll see:

Nodes (ovals):
- /joy_node (joystick input)
- /teleop_twist_joy_node (joystick → cmd_vel)
- /lyra_bridge (main robot control)
- /wheel_odometry (encoders → odometry)
- /ekf_filter_node (sensor fusion)
- /sllidar_node (LiDAR driver)

Topics (rectangles):
- /joy
- /cmd_vel_joy
- /cmd_vel
- /wheel/odom
- /odometry/filtered
- /scan

Connections (arrows):
- joy_node → /joy → teleop_twist_joy_node
- teleop_twist_joy_node → /cmd_vel_joy → cmd_vel_gate
- cmd_vel_gate → /cmd_vel → lyra_bridge

[PLACEHOLDER: Screenshot of rqt_graph showing Beetlebot nodes]

Exercise 2.5: Trace a Command

Task: Follow /cmd_vel from joystick to motors

# In rqt_graph:
# 1. Find /joy_node (reads USB controller)
# 2. Follow arrow to /joy topic
# 3. Follow to /teleop_twist_joy_node (converts joy → velocity)
# 4. Follow to /cmd_vel_joy topic
# 5. Follow to /cmd_vel_gate (safety checks)
# 6. Follow to /cmd_vel topic
# 7. Follow to /lyra_bridge (sends to STM32)

# Questions:
# 1. How many nodes process joystick input before reaching motors?
# 2. What happens if /cmd_vel_gate node crashes?
# 3. Why separate /cmd_vel_joy and /cmd_vel topics?

Answers

3 nodes: joy_node → teleop_twist_joy_node → cmd_vel_gate → lyra_bridge
Robot stops: cmd_vel_gate provides safety (ARM check, limits), without it, no commands reach motors
Multiple sources: /cmd_vel_joy (joystick), /cmd_vel_nav (autonomous), /cmd_vel (keyboard/manual) all merge into single /cmd_vel topic via multiplexer

Part 7: Recording and Playback (rosbag)

Recording Data

Record specific topics:

# Record LiDAR and odometry
ros2 bag record -o test_data /scan /odometry/filtered

# Output:
# [INFO] [rosbag2_storage]: Opened database 'test_data/test_data_0.db3' for writing.
# [INFO] [rosbag2_transport]: Listening for topics...
# [INFO] [rosbag2_transport]: Subscribed to topic '/scan'
# [INFO] [rosbag2_transport]: Subscribed to topic '/odometry/filtered'
# [INFO] [rosbag2_transport]: All requested topics are subscribed. Stopping discovery...

# Drive robot around for 30 seconds
# Press Ctrl+C to stop recording

Record all topics:

# Record everything (⚠️ large file!)
ros2 bag record -a -o full_session

# Not recommended for long sessions (GBs of data)

Record with compression:

# Compress on-the-fly (smaller files)
ros2 bag record --compression-mode file --compression-format zstd \
  -o compressed_data /scan /odometry/filtered /imu/data

Playback Data

Play recorded bag:

# Info about bag
ros2 bag info test_data

# Output:
# Files:             test_data_0.db3
# Bag size:          15.2 MiB
# Storage id:        sqlite3
# Duration:          30.5s
# Start:             Jan  5 2026 14:30:00.123
# End:               Jan  5 2026 14:30:30.654
# Messages:          3050
# Topic information: Topic: /scan | Type: sensor_msgs/msg/LaserScan | Count: 305 | Serialization Format: cdr
#                    Topic: /odometry/filtered | Type: nav_msgs/msg/Odometry | Count: 915 | Serialization Format: cdr

# Play bag (default speed)
ros2 bag play test_data

# Play at 2x speed
ros2 bag play test_data --rate 2.0

# Play at 0.5x speed (slow motion)
ros2 bag play test_data --rate 0.5

# Play in loop
ros2 bag play test_data --loop

# Play starting from specific time
ros2 bag play test_data --start-offset 5.0  # Skip first 5 seconds

Exercise 2.6: Record and Analyze

Task: Record driving session, analyze offline

Recording:

# Record sensors while driving
ros2 bag record -o driving_session \
  /scan /odometry/filtered /imu/data /cmd_vel_joy /battery_voltage

# Drive robot:
# - Forward 2m
# - Turn 90° left
# - Forward 2m
# - Turn 90° left
# - Continue until back at start (square pattern)

# Stop recording (Ctrl+C)

Analysis:

# Get bag info
ros2 bag info driving_session

# Play back and visualize
# Terminal 1: Play bag
ros2 bag play driving_session

# Terminal 2: Echo odometry
ros2 topic echo /odometry/filtered --field pose.pose.position

# Terminal 3: Echo velocity commands
ros2 topic echo /cmd_vel_joy

# Questions:
# 1. What was max speed achieved?
# 2. How far did robot drift from start?
# 3. What was battery voltage change?

Part 8: RQT Tools

rqt_plot (Time Series)

Plot data over time:

rqt_plot

Add topics to plot:

Topic field 1: /odometry/filtered/pose/pose/position/x
Topic field 2: /odometry/filtered/pose/pose/position/y
Topic field 3: /battery_voltage/data

# See position and battery change over time

[PLACEHOLDER: Screenshot of rqt_plot showing odometry]

rqt_image_view (Camera)

View camera stream:

rqt_image_view

# Dropdown: Select /pi_camera/image_raw/compressed
# Camera feed appears

rqt_console (Logs)

View system logs:

rqt_console

# Shows all ROS2 log messages:
# - DEBUG (gray)
# - INFO (white)
# - WARN (yellow)
# - ERROR (red)
# - FATAL (red, bold)

# Filter by node, severity, message content

rqt_topic (Topic Monitor)

Monitor multiple topics:

rqt_topic

# Shows all topics with:
# - Message type
# - Bandwidth
# - Publish rate
# - Echo current value

Exercise 2.7: Multi-Tool Monitoring

Task: Monitor robot with multiple tools

Setup:

# Terminal 1: rqt_plot (position)
rqt_plot /odometry/filtered/pose/pose/position/x:y

# Terminal 2: rqt_console (logs)
rqt_console

# Terminal 3: rqt_topic (monitor)
rqt_topic

# Terminal 4: Drive robot
ros2 service call /lyra/arm std_srvs/srv/Trigger
ros2 topic pub --rate 10 /cmd_vel geometry_msgs/msg/Twist \
  "{linear: {x: 0.5, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.0}}"

Observe:

rqt_plot shows position changing
rqt_console shows any warnings/errors
rqt_topic shows bandwidth and rates

Part 9: Troubleshooting Communication

Problem: Topic Not Publishing

Symptoms: ros2 topic hz /scan shows nothing

Debug steps:

# 1. Check if topic exists
ros2 topic list | grep scan

# 2. Check which nodes publish it
ros2 topic info /scan
# Should show Publisher count > 0

# 3. Check if node is running
ros2 node list | grep sllidar

# 4. Check for errors in logs
ros2 topic echo /rosout | grep -i error

# 5. Power cycle robot
# ⚡ This fixes 90% of issues

Problem: Can't Send Commands

Symptoms: Robot doesn't respond to /cmd_vel

Debug steps:

# 1. Check if ARMED
ros2 topic echo /lyra/armed --once
# If false, ARM it:
ros2 service call /lyra/arm std_srvs/srv/Trigger

# 2. Verify publishing continuously (not --once)
# ❌ Wrong:
ros2 topic pub --once /cmd_vel ...
# ✅ Right:
ros2 topic pub --rate 10 /cmd_vel ...

# 3. Check command timeout (500ms)
# Must publish at least 2 Hz

# 4. Check if another node controlling
ros2 topic info /cmd_vel
# Multiple publishers? One might override

# 5. Check for errors
ros2 topic echo /rosout | grep cmd_vel

Problem: Services Not Responding

Symptoms: Service call hangs or fails

# 1. Check service exists
ros2 service list | grep lyra/arm

# 2. Check node providing service
ros2 service find std_srvs/srv/Trigger

# 3. Check if node alive
ros2 node list | grep lyra_bridge

# 4. Try with timeout
timeout 5 ros2 service call /lyra/arm std_srvs/srv/Trigger

# 5. Restart node or power cycle

Part 10: Knowledge Check

Concept Quiz

What's the difference between topics and services?
Why must cmd_vel be published continuously?
What does /odometry/filtered provide that /wheel/odom doesn't?
When would you use rqt_graph?
What information does rosbag store?

Hands-On Challenge

Task: Record, analyze, and report driving behavior

Requirements:

Record 60-second driving session with:
- /scan
- /odometry/filtered
- /imu/data
- /cmd_vel
- /battery_voltage
Include in recording:
- Forward motion
- Rotation in place
- Arc motion
- Full stop
Analyze recording:
- Maximum linear velocity achieved
- Maximum angular velocity achieved
- Total distance traveled (from odometry)
- Battery voltage drop
- Any anomalies (gaps, errors)
Create report:
- Bag file statistics
- Plots of key data (position, velocity)
- Summary of findings
- Recommendations

Part 11: What You've Learned

✅ Congratulations!

You now understand:

Communication Basics:

✅ Topics, services, and actions
✅ When to use each paradigm
✅ Publish-subscribe architecture

Command Line Tools:

✅ Listing and inspecting topics/services/nodes
✅ Publishing to topics (with correct --rate!)
✅ Calling services
✅ Reading topic data

Robot Control:

✅ ARM/DISARM mechanism
✅ Velocity command structure
✅ Command timeout (500ms)
✅ Safety systems

Debugging:

✅ rqt_graph visualization
✅ rosbag recording/playback
✅ rqt tools (plot, console, topic)
✅ Common issues and solutions

Next Steps

🎯 You're Now Ready For:

Immediate Next: → Robot Simulation with Gazebo - Test safely in simulation

Hardware Tutorials: → Sensor Data Visualization - Deep dive into sensor topics → Teleoperation Control - Advanced driving techniques

Advanced Topics:

Custom node development (Python/C++)
Message type creation
Action servers/clients
Parameter management

Quick Reference

Essential Commands

# --- Topics ---
ros2 topic list                          # List all topics
ros2 topic type /cmd_vel                 # Get message type
ros2 topic hz /scan                      # Check publish rate
ros2 topic echo /odometry/filtered --once # View single message
ros2 topic pub --rate 10 /cmd_vel ...    # Publish continuously

# --- Services ---
ros2 service list | grep lyra            # List robot services
ros2 service type /lyra/arm              # Get service type
ros2 service call /lyra/arm std_srvs/srv/Trigger  # Call service

# --- Nodes ---
ros2 node list                           # List all nodes
ros2 node info /lyra_bridge              # Node details

# --- Recording ---
ros2 bag record -o name /topic1 /topic2  # Record topics
ros2 bag info name                       # Bag statistics
ros2 bag play name                       # Playback

# --- Visualization ---
rqt_graph --force-discover               # Node graph
rqt_plot                                 # Time series
rqt_console                              # Logs
rqt_image_view                           # Camera

# --- Interfaces ---
ros2 interface show geometry_msgs/msg/Twist  # Message structure
ros2 interface show std_srvs/srv/Trigger     # Service structure

Key Topics Reference

Topic

Type

Rate

Purpose

/cmd_vel

Twist

10 Hz

Main velocity control

/scan

LaserScan

10 Hz

LiDAR data

/odometry/filtered

Odometry

30 Hz

Fused position (EKF)

/wheel/odom

Odometry

20 Hz

Wheel-only position

/imu/data

Imu

100 Hz

IMU filtered

/battery_voltage

Float32

1-10 Hz

Battery status

/joy

Joy

50 Hz

Joystick input

/pi_camera/image_raw/compressed

CompressedImage

30 Hz

Camera stream

Important Services

Service

Type

Purpose

/lyra/arm

Trigger

Enable motors

/lyra/disarm

Trigger

Disable motors

/lyra/emergency_stop

Trigger

Emergency stop

Completed ROS2 Communication & Tools! 🎉

→ Continue to Robot Simulation with Gazebo → Or return to Tutorial Index

Last Updated: January 2026 Tutorial 2 of 11 - Beginner Level Estimated completion time: 115 minutes

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Tutorial Overview

🎯 Learning Objectives

⏱️ Time Required

📚 Prerequisites

🛠️ What You'll Need

Part 1: ROS2 Communication Paradigms

Topics (Publish-Subscribe)

Services (Request-Response)

Actions (Goal-Based)

Part 2: Exploring Your Robot's Topics

List All Topics

Get Topic Information

Exercise 2.1: Topic Discovery

Part 3: Reading Topic Data

Echo Topics (View Live Data)

Exercise 2.2: Understanding Sensor Data

Part 4: Publishing to Topics

Understanding Message Structure

Publishing Commands

Exercise 2.3: Manual Robot Control

Part 5: Services

List Available Services

Calling Services

Check Service Type and Interface

Exercise 2.4: Service Control Flow

Part 6: Understanding the Communication Graph

Using rqt_graph

Exercise 2.5: Trace a Command

Part 7: Recording and Playback (rosbag)

Recording Data

Playback Data

Exercise 2.6: Record and Analyze

Part 8: RQT Tools

rqt_plot (Time Series)

rqt_image_view (Camera)

rqt_console (Logs)

rqt_topic (Topic Monitor)

Exercise 2.7: Multi-Tool Monitoring

Part 9: Troubleshooting Communication

Problem: Topic Not Publishing

Problem: Can't Send Commands

Problem: Services Not Responding

Part 10: Knowledge Check

Concept Quiz

Hands-On Challenge

Part 11: What You've Learned

✅ Congratulations!

Next Steps

🎯 You're Now Ready For:

Quick Reference

Essential Commands

Key Topics Reference

Important Services