2.3 Controlling LEDs & Servo with ROS 2

2.3.1 Hardware Setup for Servo Control

You'll need:

• Servo motor (e.g., SG90 or MG90)

• Jumper wires

• Arduino Nano (or Uno)

• USB cable to connect Arduino to PC

• External 5V power supply (if required)

• Red LED

• Blue LED

• 2 x 220Ω resistors

• Breadboard

Wiring:

• Connect the Red LED to digital pin 7 (with the resistor in series).

• Connect the Blue LED to digital pin 8 (with the resistor in series).

• Servo Signal (Orange/White wire) → Arduino digital pin 9

• Servo VCC (Red wire) → Arduino 5V pin (or an external 5V power supply)

• Servo GND (Brown/Black wire) → Arduino GND pin

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2.3.2 Writing ROS Node for Serial Communication

The first step is writing a simple ROS node in Python that sends a command over serial to the Arduino.

Create a ROS package if you haven’t already:

source /opt/ros/foxy/setup.bash
cd ~/ros_ws/src
ros2 pkg create --build-type ament_python --node-name ser_com ros_arduino_com

You will now have a new folder within your workspace’s src directory called ros_arduino_com.

cd ~/ros_ws/src/ros_arduino_com/ros_arduino_com
nano ser_com.py

We will modify the Python ROS node to include options for controlling the servo motor. It will now send commands to control both the LEDs and the servo.

ser_com.py:

#!/usr/bin/env python3

import rclpy
from rclpy.node import Node
import serial

class ArduinoComm(Node):
    def __init__(self):
        super().__init__('arduino_comm')
        self.ser = serial.Serial('/dev/ttyUSB0', 9600, timeout=1)
        self.ser.flush()

    def send_command(self, command):
        self.ser.write(command.encode())
        self.get_logger().info(f"Sent command: {command}")

def main(args=None):
    rclpy.init(args=args)
    node = ArduinoComm()
    while True:
        command = input("Enter 1 (Red LED ON), 2 (Blue LED ON), 0 (All LEDs OFF), 3 (Servo 0 deg), 9 (Servo 90 deg): ")
        node.send_command(command)
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Make sure to modify /dev/ttyUSB0 to match the correct serial port for your Arduino.

This ROS node allows real-time control by sending commands from the terminal to the Arduino. The commands include:

• 1: Turn on the red LED.

• 2: Turn on the blue LED.

• 0: Turn off both LEDs.

• 3: Move the servo to 0 degrees.

• 9: Move the servo to 90 degrees.

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2.3.3 Uploading Arduino Code with servo control

Now, upload a simple Arduino sketch to receive data from ROS.

Arduino Sketch:

#include <Servo.h>

#define RED_LED 7
#define BLUE_LED 8
Servo myServo;  // Create a Servo object

void setup() {
  Serial.begin(9600);
  pinMode(RED_LED, OUTPUT);
  pinMode(BLUE_LED, OUTPUT);
  myServo.attach(9);  // Attach the servo to pin 9
}

void loop() {
  if (Serial.available() > 0) {
    char received = Serial.read();
    if (received == '1') {
      digitalWrite(RED_LED, HIGH);
      Serial.println("Red LED ON");
    } else if (received == '2') {
      digitalWrite(BLUE_LED, HIGH);
      Serial.println("Blue LED ON");
    } else if (received == '0') {
      digitalWrite(RED_LED, LOW);
      digitalWrite(BLUE_LED, LOW);
      Serial.println("LEDs OFF");
    } else if (received == '3') {
      myServo.write(0);  // Move servo to 0 degrees
      Serial.println("Servo at 0 degrees");
    } else if (received == '9') {
      myServo.write(90);  // Move servo to 90 degrees
      Serial.println("Servo at 90 degrees");
    }
  }
}

This code allows both LEDs and the servo to be controlled from the same command interface.

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2.3.4 Running the Code

1. Open a terminal on your PC and source ROS 2:

   source /opt/ros/foxy/setup.bash
   cd ~/ros_ws
   colcon build
   source install/setup.bash

2. Run the ROS node:

   ros2 run ros_arduino_com ser_com

Now you can see commands being sent from ROS to Arduino. Check the Arduino Serial Monitor to see the received data.

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