Read RPM with interrupt; adjust motor speed.

[shi-nee]

RPM code I've been using, which is great for reading RPM. Every time I attempt to switch an output based on the variable 'rpm' I get no activity on 'motorPin'. But if I manually apply voltage to the motor I get a perfect rpm reading. If I digitalWrite motorPin HIGH in loop it will turn on the pin and still give correct rpm.

Code: Select all

   int rpmcount = 0;
   int rpm = 0;
   int timeold = 0;
   int motorPin = 8;
   int ledPin = 13;

void setup()
{
   pinMode(motorPin, OUTPUT);
   pinMode(ledPin, OUTPUT);
   digitalWrite(motorPin, HIGH);
   digitalWrite(ledPin, HIGH);
   
   Serial.begin(57600);
   attachInterrupt(0, rpm_fun, RISING);
}

void loop()
{
   if (rpmcount >= 20) {
     //Update RPM every 20 counts, increase this for better RPM resolution,
     //decrease for faster update
     rpm = 30*1000/(millis() - timeold)*rpmcount;
     timeold = millis();
     rpmcount = 0;
     Serial.println(rpm,DEC);
     if (rpm < 3400){
     digitalWrite(motorPin, HIGH);
     digitalWrite(ledPin, HIGH);
     } else {
     digitalWrite(motorPin, LOW);
     digitalWrite(ledPin, LOW);
     }
   }
}

void rpm_fun()
{
   rpmcount++;
   //Each rotation, this interrupt function is run twice
}


[pylon]

Given the code you posted I estimate you get an interrupt about every 5-10ms. The code inside your "if" is called once in about 150ms (if the motor is running). I don't know your motor and application but don't you think it is possible that the motor stops completely before that code is called again? If this is the case, it never gets to the point again where it would start your motor.

[adafruit_support_bill]

I think Pylon is right there. The control will not execute if the motor is not turning. Better to execute the control based on time instead of revolutions.

[shi-nee]

Interrupts suck.

I think I'm going to end up with either a real time clock or monostable 555 w/ schottky to change RPM into a varying voltage. So yes, time is the answer, but not in code.

Thanks for your help.

[pylon]

You seem to be the hardware guy

Me being the software guy I would not give up that quickly. It's solved very simple: Just let the RPMs be calculated every time you get an interrupt. Don't take the millis() but the micros() and you get enough sensibility (I guess, depends on your setup). Your interrupt handler does not count the number of times it's called but the time difference between these calls.

BTW: in your sketch rpmcount is not declared volatile although it's changed in a interrupt handler. You should only change global variables in an interrupt service routine declared "volatile".

Snippet:

Code: Select all

volatile unsigned long last_irq_time = 0;
volatile unsigned int rpm = 0;

void rpm_fun() {
  rpm = 30000000L / (micros() - last_irq_time);
  last_irq_time = micros();
}


So you have always an accurate RPM value (given you get two rising edges on pin 2 for every round of your motor) and you can decide in every run of loop() if you wanna stop or start your motor. No additional electronics needed .

[shi-nee]

Ideally I want to do it both ways for the education. I just got a couple L293 to replace those I burned in my motorShield so I'll rewrite for AFmotor and try millis() tonight.

Thanks Pylon.. Ada