PID Line follower with only 2 on/off IR sensors

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1

Hi guys,

I'm trying to program a line follower robot using PID. The problem is that the robot only has 2 IR sensors which returns 0 or 1. 0 : The sensor detects a line and 1 : no line.

This is the sensor : Me Line Follower | Makeblock Education

Is it possible to use a PID with such basic sensors?

Here's what I tried, without success.

// Param :
//  cT : Current time, basically millis()

void followLineTask(unsigned long cT) {
  static unsigned long lastTime = 0;
  const int rate = 10;

  const float kp = 2;
  const float ki = 0; // I am aware the values are 0
  const float kd = 0;

  static int lastError = 0;
  static int errorSum = 0;

  static int leftSpeed = speed;
  static int rightSpeed = -speed;

  static int error = 0;

  error = line.readSensors();
  
  switch (error) {
    case S1_IN_S2_IN:
      error = 0;
      leftSpeed = speed;
      rightSpeed = -speed;
      break;
    case S1_IN_S2_OUT:
      error = -1;
      break;
    case S1_OUT_S2_IN:
      error = 1;
      break;
    case S1_OUT_S2_OUT:
      error = lastError;
      break;
  }

  // Don't correct to frequently.
  if (cT - lastTime < rate) return;

  lastTime = cT;

  // integral part
  errorSum += error;  

  float pid = kp * error + ki * errorSum + kd * (error - lastError);
  pid = constrain (pid, -maxVariation, maxVariation );

  leftSpeed = constrain (leftSpeed + pid, -255, 255);
  rightSpeed = constrain (rightSpeed + pid, -255, 255);
  
  // Setting the target PWM (speed).
  encRight.setTarPWM(rightSpeed);
  encLeft.setTarPWM(leftSpeed); 

  // for differential part
  lastError = error;
}

Thanks

2

Not really, because "proportional error" is either 0 or 1.

This ridiculously simple line follower, which just uses two photodiodes and a comparator, rather than a computer, to make the steering decision, works reasonably well.

The idea behind PID is that the correction applied is to first order proportional to the error.

3

Can both sensors be on the line at the same time?

4

Yes, both sensors are on the line at the same time.

Since the proportional part is somewhat not possible (1 or 0). I think only the integral part is logically useful. I'm might also be wrong.

5

While searching the web, I've stumbled on this answer on Reddit.

From the reddit answer, I could simulate a proportional value by checking the time spent outside the line. I will need to sum the error let say for 10 ms and do the PID from this value. The set point is 0.

Is it a possible solution?

6

Try it and let us know if it works.

You can certainly average binary 1/0 values over time to get an intermediate number, but whether that would work for proportional steering is not immediately clear.

7

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