Arduino Robot – Ultrasonic Sensor

The goal is to make the robot aware of obstacles in front of him, so he can change direction and avoid them. In the previous article we made the robot move – now we will give him some autonomy.

https://youtu.be/JhjXJYpjn6E

Watch video directly on YouTube

Ultrasonic sensor

HC-SR04 is a circuit able to measure distance to objects up to 4 meters using ultrasonic waves. It sends a ping (like a submarine) and measures the time (in microseconds) between sending and receiving anything back. This time is then divided by 2 as the wave travels back and forth. And then divide by 29 to get a distance in centimeters (or 74 for inches), because sound travels 29.4µs per centimeter (340 m/s). The sensor is very accurate with ~3 mm tolerance and easy to integrate with Arduino.

Circuit

Code

The code below contains only an “ultrasonic” extension to DC motor control using an H-Bridge from the previous article. When the robot detects an obstacle in front of it, he turns around (random degree) and continues moving forward. This functionality could be easily extended to keep turning and detecting obstacles at the same time – so the robot would not turn randomly, but start moving forward only when no object is detected.

int EchoPin = 2; // connect Pin 2 to Echo at HC-SR04
int TrigPin = 3; // connect Pin 3 to Trig at HC-SR04
unsigned long Time_Echo_us = 0;
unsigned long Len_cm  = 0;

void setup() {
  // Set EchoPin as input, to receive measure result
  pinMode(EchoPin, INPUT);
  // Set TrigPin as output, to send a pulse to start
  pinMode(TrigPin, OUTPUT);     

  delay(5000);
}

void loop() {
  if (motorsInStandby) return;  // robot is in standby mode
  
  // Drive both motors forward
  motorDrive(leftMotor, 1, 128);
  motorDrive(rightMotor, 1, 128);

  // begin to send a high pulse to begin measuring
  digitalWrite(TrigPin, HIGH);              
  // set this high pulse width as 50us
  delayMicroseconds(50);                    
  // end this high pulse
  digitalWrite(TrigPin, LOW);                
  
  // calculate the pulse width at EchoPin
  Time_Echo_us = pulseIn(EchoPin, HIGH);    
  // a valid pulse width should be between (1, 60000)
  if ((Time_Echo_us < 60000) && (Time_Echo_us > 1)) {   
    // calculate the distance in cm
    Len_cm = (Time_Echo_us / 2) / 29;       
    if (Len_cm < 10) {
      // very close object (<10 cm), stop the robot (standby)
      motorStop(leftMotor);
      motorStop(rightMotor);
      motorsInStandby = true;
    } else if (Len_cm < 40) {
      // turn around
      motorDrive(leftMotor, 1, 128);
      motorDrive(rightMotor, 0, 128);
      delay(random(220, 800));
    }
  }
}