Pulsein() function blocks other tasks from running silmultaneously

Question

I am using a zumo bot with a reflectance sensor used to follow a black line. I want to use an arduino to make the zumo bot stop once it gets a certain distance from an obstacle.

I have an ultrasonic sensor (HC-SR04) which ive connected to the bot.

Both of these tasks work independently but once i merge the code together(so it follows the line aswell as stoping when it detects an object using the ultrasonic sensor), it doesn't work properly.. (the zumo bot no longer follows the line)

I THINK it is to do with the pulsein() function blocking any other tasks but not sure.

My code is below. Can anyone help please?

#include <Wire.h>
#include <ZumoShield.h>

ZumoBuzzer buzzer;
ZumoReflectanceSensorArray reflectanceSensors;
ZumoMotors motors;
Pushbutton button(ZUMO_BUTTON);
int lastError = 0;

// This is the maximum speed the motors will be allowed to turn.
// (400 lets the motors go at top speed; decrease to impose a speed limit)
const int MAX_SPEED = 400;
#define echoPin A4 
#define trigPin A5

// defines variables
long duration; // variable for the duration of sound wave travel
int distance; // variable for the distance measurement

void setup()
{
  reflectanceSensors.init();

  pinMode(trigPin, OUTPUT); // Sets the trigPin as an OUTPUT
  pinMode(echoPin, INPUT); // Sets the echoPin as an INPUT

  // Initialize the reflectance sensors module

  // Wait for the user button to be pressed and released
  button.waitForButton();

  // Turn on LED to indicate we are in calibration mode
  pinMode(13, OUTPUT);
  digitalWrite(13, HIGH);

  // Wait 1 second and then begin automatic sensor calibration
  // by rotating in place to sweep the sensors over the line
  delay(1000);
  int i;
  for(i = 0; i < 80; i++)
  {
    if ((i > 10 && i <= 30) || (i > 50 && i <= 70))
      motors.setSpeeds(-200, 200);
    else
      motors.setSpeeds(200, -200);
    reflectanceSensors.calibrate();

    // Since our counter runs to 80, the total delay will be
    // 80*20 = 1600 ms.
    delay(20);
  }
  motors.setSpeeds(0,0);

  // Turn off LED to indicate we are through with calibration
  digitalWrite(13, LOW);

  // Wait for the user button to be pressed and released
  button.waitForButton();


  Serial.begin(9600); // // Serial Communication is starting with 9600 of baudrate speed
  Serial.println("Ultrasonic Sensor HC-SR04 Test"); // print some text in Serial Monitor
  Serial.println("with Arduino UNO R3");
  
  
}

void loop()
{

  unsigned int sensors[6];

  // Get the position of the line.  Note that we *must* provide the "sensors"
  // argument to readLine() here, even though we are not interested in the
  // individual sensor readings
  int position = reflectanceSensors.readLine(sensors);


  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  // Sets the trigPin HIGH (ACTIVE) for 10 microseconds
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  // Reads the echoPin, returns the sound wave travel time in microseconds
  duration = pulseIn(echoPin, HIGH);

  // Our "error" is how far we are away from the center of the line, which
  // corresponds to position 2500.
  int error = position - 2500;

  // Get motor speed difference using proportional and derivative PID terms
  // (the integral term is generally not very useful for line following).
  // Here we are using a proportional constant of 1/4 and a derivative
  // constant of 6, which should work decently for many Zumo motor choices.

  int speedDifference = error / 4 + 6 * (error - lastError);

  lastError = error;

  // Get individual motor speeds.  The sign of speedDifference
  // determines if the robot turns left or right.
  int m1Speed = MAX_SPEED + speedDifference;
  int m2Speed = MAX_SPEED - speedDifference;


  if (m1Speed < 0)
    m1Speed = 0;
  if (m2Speed < 0)
    m2Speed = 0;
  if (m1Speed > MAX_SPEED)
    m1Speed = MAX_SPEED;
  if (m2Speed > MAX_SPEED)
    m2Speed = MAX_SPEED;

  motors.setSpeeds(m1Speed, m2Speed);

  //if (distance <20){
  //  motors.setSpeeds(0,0);
 // }
  

////////////////////////////////////////////


  // Calculating the distance
  distance = duration * 0.034 / 2; // Speed of sound wave divided by 2 (go and back)
  // Displays the distance on the Serial Monitor
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm");
  
} 

Answer 1

It is quite likely due to blocking. The description of the pulseIn() function in the Arduino site's reference section says:

Reads a pulse (either HIGH or LOW) on a pin. For example, if value is HIGH, pulseIn() waits for the pin to go from LOW to HIGH, starts timing, then waits for the pin to go LOW and stops timing. Returns the length of the pulse in microseconds or gives up and returns 0 if no complete pulse was received within the timeout.

If you can limit the range within which you want to stop the vehicle, try setting a timeout on pulseIn() that will return control once the maximum sound travel time has passed. Maybe that will give you enough response to correct the vehicle's steering. Slowing the vehicle's velocity will help, too (it won't be able deviate as far from it's path as quickly).

Otherwise, you'll probably need a non-blocking equivalent to pulseIn().