## **Lab 5** | Working with ROS Contd.

### Design of Autonomous Systems  
### CSCI 6907/4907 - Section 86  
### Prof. **Sibin Mohan**

---

## **Objectives**

- Implement and understand the Publisher-Subscriber Model in ROS
- Learn how to read sensor data using ROS Modules and Nodes
- Install and configure the Intel RealSense IMU in ROS

---

## **Publisher-Subscriber Model**

### **Understanding the Model**
- The Publisher-Subscriber model is the fundamental communication mechanism in ROS.
- **Publisher:** Sends messages to a specific topic.
- **Subscriber:** Listens to messages from a topic.

**Use Cases:**
- A camera publishes images, while an image-processing node subscribes to them.
- A LIDAR sensor node publishes distance data, while a mapping node subscribes to it.

### **Implementing a Publisher-Subscriber Model**

#### **1. Creating a ROS Package**
```bash
mkdir -p ~/catkin_ws/src
cd ~/catkin_ws/src
catkin_create_pkg pubsub_example std_msgs rospy roscpp
cd ~/catkin_ws
catkin_make
```

#### **2. Writing a Publisher Node**
```bash
cd ~/catkin_ws/src/pubsub_example/src
nano publisher.py
```

**Add the following code:**
```python
#!/usr/bin/env python3
import rospy
from std_msgs.msg import String

def publisher():
    pub = rospy.Publisher('chatter', String, queue_size=10)
    rospy.init_node('talker', anonymous=True)
    rate = rospy.Rate(1)
    while not rospy.is_shutdown():
        msg = "Hello, ROS!"
        rospy.loginfo(msg)
        pub.publish(msg)
        rate.sleep()

if __name__ == '__main__':
    try:
        publisher()
    except rospy.ROSInterruptException:
        pass
```

#### **3. Writing a Subscriber Node**
```bash
nano subscriber.py
```

**Add the following code:**
```python
#!/usr/bin/env python3
import rospy
from std_msgs.msg import String

def callback(data):
    rospy.loginfo("Received: %s", data.data)

def subscriber():
    rospy.init_node('listener', anonymous=True)
    rospy.Subscriber('chatter', String, callback)
    rospy.spin()

if __name__ == '__main__':
    subscriber()
```

#### **4. Running the Publisher-Subscriber Model**
```bash
roscore
rosrun pubsub_example publisher.py
rosrun pubsub_example subscriber.py
```

---

## **Reading Sensors with ROS, ROS Modules, and Nodes**

### **IMU Sensor Integration**
1. Install dependencies:
   ```bash
   sudo apt-get install ros-noetic-imu-tools
   ```
2. Run the IMU Sensor Node:
   ```bash
   roslaunch imu_tools imu_filter_madgwick.launch
   ```
3. View IMU Data:
   ```bash
   rostopic echo /imu/data
   ```

---

## **Installing RealSense in ROS**

### **Prerequisites**
- Ubuntu 18.04 or later
- RealSense IMU-compatible device

### **Installation Steps**
```bash
sudo apt-get install ros-noetic-realsense2-camera
roslaunch realsense2_camera rs_camera.launch
```

### **Verifying Installation**
```bash
rostopic list
rviz
```
Add `/camera/color/image_raw` in rviz to visualize RGB camera data.

---

## **Lab Assignment**

### **Create a ROS workspace and demonstrate a basic hello_world ROS node.**

Follow these steps:
1. **Create the workspace:**
   ```bash
   mkdir -p ~/catkin_ws/src
   cd ~/catkin_ws
   catkin_make
   ```
2. **Create a new package:**
   ```bash
   cd ~/catkin_ws/src
   catkin_create_pkg hello_world std_msgs rospy roscpp
   ```
3. **Create a Python script for the node:**
   ```bash
   nano ~/catkin_ws/src/hello_world/src/hello_world.py
   ```
4. **Add the following code to `hello_world.py`:**
   ```python
   #!/usr/bin/env python3
   import rospy
   def main():
       rospy.init_node('hello_world_node')
       rospy.loginfo("Hello, ROS!")
       rospy.spin()
   if __name__ == "__main__":
       main()
   ```
5. **Make the script executable:**
   ```bash
   chmod +x ~/catkin_ws/src/hello_world/src/hello_world.py
   ```
6. **Run the node:**
   ```bash
   rosrun hello_world hello_world.py
   ```

---

## **Summary**

- Implemented a **Publisher-Subscriber** model in ROS.
- Explored **sensor data acquisition** using ROS nodes.
- Installed and configured **Intel RealSense in ROS**.
- Created a ROS workspace and demonstrated a basic `hello_world` ROS node.