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ROS Fundamentals: ROS1 and ROS2

This tutorial covers Robot Operating System (ROS) fundamentals for both ROS1 (Noetic) and ROS2 (Humble), with examples in Python and C++.

Topics Covered

Topic	Description
Nodes	Basic execution units in ROS
Topics	Publish/Subscribe communication
Messages	Data structures for communication
Services	Request/Response pattern
Parameters	Runtime configuration
Launch Files	Multi-node startup

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ROS1 vs ROS2 Comparison

Feature	ROS1 (Noetic)	ROS2 (Humble)
Master	Required (roscore)	Not required (DDS)
Build System	catkin	colcon
Python	rospy	rclpy
C++	roscpp	rclcpp
Launch	XML	Python/XML/YAML
OS Support	Ubuntu 20.04	Ubuntu 22.04

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Directory Structure

1_16/
├── ros1/
│   ├── Dockerfile
│   ├── src/
│   │   └── slam_tutorial/
│   │       ├── CMakeLists.txt
│   │       ├── package.xml
│   │       ├── scripts/
│   │       │   ├── publisher.py
│   │       │   └── subscriber.py
│   │       └── src/
│   │           ├── publisher.cpp
│   │           └── subscriber.cpp
│   └── README.md
│
└── ros2/
    ├── Dockerfile
    ├── src/
    │   └── slam_tutorial/
    │       ├── CMakeLists.txt
    │       ├── package.xml
    │       ├── slam_tutorial/
    │       │   ├── __init__.py
    │       │   ├── publisher.py
    │       │   └── subscriber.py
    │       └── src/
    │           ├── publisher.cpp
    │           └── subscriber.cpp
    └── README.md

---

Quick Start

ROS1 (Noetic)

cd ros1
docker build . -t slam_ros1
docker run -it slam_ros1

# Inside container
source /opt/ros/noetic/setup.bash
cd /catkin_ws
catkin_make
source devel/setup.bash

# Terminal 1: Start roscore
roscore

# Terminal 2: Run publisher
rosrun slam_tutorial publisher.py

# Terminal 3: Run subscriber
rosrun slam_tutorial subscriber.py

ROS2 (Humble)

cd ros2
docker build . -t slam_ros2
docker run -it slam_ros2

# Inside container
source /opt/ros/humble/setup.bash
cd /ros2_ws
colcon build
source install/setup.bash

# Terminal 1: Run publisher
ros2 run slam_tutorial publisher

# Terminal 2: Run subscriber
ros2 run slam_tutorial subscriber

---

Core Concepts

1. Nodes

Nodes are independent processes that perform computation.

ROS1 Python:

#!/usr/bin/env python3
import rospy

rospy.init_node('my_node')
rospy.loginfo('Node started')
rospy.spin()

ROS2 Python:

#!/usr/bin/env python3
import rclpy
from rclpy.node import Node

class MyNode(Node):
    def __init__(self):
        super().__init__('my_node')
        self.get_logger().info('Node started')

def main():
    rclpy.init()
    node = MyNode()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

2. Topics (Pub/Sub)

ROS1 Publisher (Python):

import rospy
from std_msgs.msg import String

rospy.init_node('publisher')
pub = rospy.Publisher('chatter', String, queue_size=10)
rate = rospy.Rate(10)  # 10 Hz

while not rospy.is_shutdown():
    pub.publish('Hello ROS1')
    rate.sleep()

ROS2 Publisher (Python):

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class Publisher(Node):
    def __init__(self):
        super().__init__('publisher')
        self.pub = self.create_publisher(String, 'chatter', 10)
        self.timer = self.create_timer(0.1, self.timer_callback)

    def timer_callback(self):
        msg = String()
        msg.data = 'Hello ROS2'
        self.pub.publish(msg)

3. Common Message Types

Type	ROS Package	Description
String	std_msgs	Text data
Int32	std_msgs	Integer
Float64	std_msgs	Double precision float
Pose	geometry_msgs	Position + Orientation
Twist	geometry_msgs	Linear + Angular velocity
Image	sensor_msgs	Camera image
PointCloud2	sensor_msgs	3D point cloud
Imu	sensor_msgs	IMU data
Odometry	nav_msgs	Robot odometry

4. Useful Commands

ROS1:

roscore                    # Start master
rosnode list              # List active nodes
rostopic list             # List topics
rostopic echo /topic      # View topic data
rostopic hz /topic        # Check publish rate
rosmsg show std_msgs/String  # Show message definition
roslaunch pkg launch.launch  # Launch file

ROS2:

ros2 node list            # List active nodes
ros2 topic list           # List topics
ros2 topic echo /topic    # View topic data
ros2 topic hz /topic      # Check publish rate
ros2 interface show std_msgs/msg/String  # Show message definition
ros2 launch pkg launch.py # Launch file

---

C++ Examples

ROS1 Publisher (C++)

#include <ros/ros.h>
#include <std_msgs/String.h>

int main(int argc, char** argv) {
    ros::init(argc, argv, "publisher");
    ros::NodeHandle nh;

    ros::Publisher pub = nh.advertise<std_msgs::String>("chatter", 10);
    ros::Rate rate(10);

    while (ros::ok()) {
        std_msgs::String msg;
        msg.data = "Hello ROS1";
        pub.publish(msg);
        rate.sleep();
    }
    return 0;
}

ROS2 Publisher (C++)

#include <rclcpp/rclcpp.hpp>
#include <std_msgs/msg/string.hpp>

class Publisher : public rclcpp::Node {
public:
    Publisher() : Node("publisher") {
        pub_ = create_publisher<std_msgs::msg::String>("chatter", 10);
        timer_ = create_wall_timer(
            std::chrono::milliseconds(100),
            std::bind(&Publisher::timer_callback, this)
        );
    }

private:
    void timer_callback() {
        auto msg = std_msgs::msg::String();
        msg.data = "Hello ROS2";
        pub_->publish(msg);
    }

    rclcpp::Publisher<std_msgs::msg::String>::SharedPtr pub_;
    rclcpp::TimerBase::SharedPtr timer_;
};

int main(int argc, char** argv) {
    rclcpp::init(argc, argv);
    rclcpp::spin(std::make_shared<Publisher>());
    rclcpp::shutdown();
    return 0;
}

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SLAM-Relevant Topics

Topic Name	Message Type	Description
/camera/image_raw	sensor_msgs/Image	Camera images
/camera/camera_info	sensor_msgs/CameraInfo	Camera intrinsics
/velodyne_points	sensor_msgs/PointCloud2	LiDAR point cloud
/imu/data	sensor_msgs/Imu	IMU measurements
/odom	nav_msgs/Odometry	Wheel odometry
/tf	tf2_msgs/TFMessage	Transform tree

---

Recommended Learning Path

1. Start with ROS2 if learning fresh (ROS1 is deprecated)
2. Practice with simple pub/sub examples
3. Learn tf2 for coordinate transforms
4. Explore sensor message types
5. Build a simple robot simulation

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References

• ROS1 Wiki
• ROS2 Documentation
• ROS2 Tutorials