IR Detector help!

Forum 2005-2010 (read only) Software Syntax & Programs
1

Hello,

I am trying to measure RPM of a flywheel attached to a DC motor. The flywheel has four holes evenly spaced near the edge. I am using a IR emitter and IR detector for the measurement. I have tried pulseIn() but the time between holes is to small for the pulseIn() function to measure.

I have also tried using attachInterrupt() to add to a counter variable each time a hole passes by the detector. I tried calculating the difference between the counter at two different times. The interrupt code can be seen below:

int irdetector = 2;           // IR detector connected to digital pin 2 (interrupt 0)
int count; 
int count2;
int count3;
int diff;
int interrupt = 0;


///////////////////////////////////////////////////////////////////////////////
void setup()
{
  count = 0;
  count2 = 0;
  count3 = 0;
  diff = 0;
  Serial.begin(9600);               
  pinMode(irdetector, INPUT);      
  attachInterrupt(interrupt, pulseCounter, FALLING); 
}



////////////////////////////////////////////////////////////////////////////////
void loop()
{
  count2 = count;
  delayMicroseconds(500);
  count3 = count;
  diff = count3 - count2;
  Serial.println(count);
  Serial.println(count2);
  Serial.println(count3);
  Serial.println(diff);
  //Serial.println(int(count));      
  //delay(99);
  
  count2 = 0;
  count3 = 0;
  diff = 0;
  count = 0;
  delay(500);
}



///////////////////////////////////////////////////////////////////////////////
void pulseCounter()
{
 count++;
}

The output from the detector is 0.5V and 5V depending on if the hole is in front of the IR LED. Does anybody have any suggestions on how to read a signal that changes faster than the pulseIn() limit of 10microseconds?

Thanks for any help!!

2

The flywheel has four holes evenly spaced near the edge.

Any possibility of covering 3 of them? What would increase the time between interrupts?

I have also tried using attachInterrupt() to add to a counter variable each time a hole passes by the detector. I tried calculating the difference between the counter at two different times. The interrupt code can be seen below

You also failed to say what the problem is with this method.

3

Use of the 'volatile' qualifier is recommended for "count"

4

Any possibility of covering 3 of them?

I tried taping over three of the holes and now pulseIn works but its not very accurate. The RPM is off by 300-1000rpm and the error is not consistent. The error gets larger as the RPM increases.

You also failed to say what the problem is with this method.

Sorry, I forgot to include the problem! Whoops! The problem I was having was between the following lines of code from my original post:

count2 = count;
  delayMicroseconds(500);
  count3 = count;
  diff = count3 - count2;

The only way I am getting a value other than '0' for 'diff' is to make the delay at least 1sec. 'count2' and 'count3' are the same value when the delay is anything less than 1sec.

I can't afford to have such a large delay in my program. I thought I could make that delay very small because the time between interrupts is very small (10us-10ms). Not sure why its not saving the correct 'count' values in 'count2' and 'count3'.

5

Rather than any delay, look at the blink without delay example. You could simply look at the value of count every so often (currMillis - prevMillis > interval), and compute RPM based on the difference between the oldCount and count. Then, reset count and prevMillis to 0 and currMillis.

6

PaulS,

That worked perfectly! Thank you!!!