I have a very powerful 12V PWM fan that I would like to control with an Arduino Nano. I found some code, that I fail to understand, on Ferederico Dossena's site linked below.
How to properly control PWM fans with Arduino - Federico Dossena (fdossena.com)
I merged the code for the PWM control and the RPM measurement together. The PWM control part works fine, but the RPM readout is a bit erratic. I checked the actual PRMs with a tachometer and it is about 20 to 30 rpm from the readings given by the Arduino and it is stable. However, the readout from the Arduino tends to jump up and down about 50 RPM when at 1400 RPM (just as an example). On occasion it jumps up to 2700 - 3000 which seems to be double what the previous reading was - more or less. When I push the duty cycle to 100%, the reading flickers between 5000 and 6000. The actual RPM is 5580 according to the tachometer. This matches the spec sheet for the fan.
As mentioned above, I don't understand most of the code written by Federico so I can only guess that my issue is related to the handling of interrupts or the timers... just a guess.
Also, I looked at the tach output of the fan on a scope and the frequency readout appears to be double what the RPM readout is. Right now the scope is stating 31Hz pulse train, but I am getting a 909 RPM readout from the Arduino, which matches the tachometer. Shouldn't 31Hz be 1860 RPM? Coincidentally, the Arduino RPM reading occasionally jumps up to 1875 RPM... The square waveform of the tach output is also something possibly unusual... see the images below.
#include <Wire.h>
//#include <TimerOne.h>
#include <LiquidCrystal_I2C.h>
#define PIN_SENSE 2 //where we connected the fan sense pin. Must be an interrupt capable pin (2 or 3 on Arduino Uno)
#define DEBOUNCE 5 //0 is fine for most fans, poor quality fans may require 10 or 20 to filter out noise
#define FANSTUCK_THRESHOLD 500 //if no interrupts were received for 500ms, consider the fan as stuck and report 0 RPM
// LCD on 0x27
LiquidCrystal_I2C lcd(0x27, 20, 4);
//Interrupt handler. Stores the timestamps of the last 2 interrupts and handles debouncing
unsigned long volatile ts1=0,ts2=0;
int potVal;
int potPin = A2;
int fanVal;
float fanSet;
//LCD Buffer
char msg[20];
// Generally, you should use "unsigned long" for variables that hold time
// The value will quickly become too large for an int to store
unsigned long previousMillis = 0; // will store last time RPM was updated
// constants won't change :
const long interval = 1000; // interval at which to update RPM (milliseconds)
//configure Timer 1 (pins 9,10) to output 25kHz PWM
void setupTimer1(){
//Set PWM frequency to about 25khz on pins 9,10 (timer 1 mode 10, no prescale, count to 320)
TCCR1A = (1 << COM1A1) | (1 << COM1B1) | (1 << WGM11);
TCCR1B = (1 << CS10) | (1 << WGM13);
ICR1 = 320;
OCR1A = 0;
OCR1B = 0;
}
/*
//configure Timer 2 (pin 3) to output 25kHz PWM. Pin 11 will be unavailable for output in this mode
void setupTimer2(){
//Set PWM frequency to about 25khz on pin 3 (timer 2 mode 5, prescale 8, count to 79)
TIMSK2 = 0;
TIFR2 = 0;
TCCR2A = (1 << COM2B1) | (1 << WGM21) | (1 << WGM20);
TCCR2B = (1 << WGM22) | (1 << CS21);
OCR2A = 79;
OCR2B = 0;
}
*/
//equivalent of analogWrite on pin 9
void setPWM1A(float f){
f=f<0?0:f>1?1:f;
OCR1A = (uint16_t)(320*f);
}
/*
//equivalent of analogWrite on pin 10
void setPWM1B(float f){
f=f<0?0:f>1?1:f;
OCR1B = (uint16_t)(320*f);
}
//equivalent of analogWrite on pin 3
void setPWM2(float f){
f=f<0?0:f>1?1:f;
OCR2B = (uint8_t)(79*f);
}
*/
void setup(){
//enable outputs for Timer 1
pinMode(9,OUTPUT); //1A
//pinMode(10,OUTPUT); //1B
setupTimer1();
//enable outputs for Timer 2
//pinMode(3,OUTPUT); //2
//setupTimer2();
//note that pin 11 will be unavailable for output in this mode!
//example...
//setPWM1A(0.5f); //set duty to 50% on pin 9
//setPWM1B(0.2f); //set duty to 20% on pin 10
//setPWM2(0.8f); //set duty to 80% on pin 3
pinMode(PIN_SENSE,INPUT_PULLUP); //set the sense pin as input with pullup resistor
attachInterrupt(digitalPinToInterrupt(PIN_SENSE),tachISR,FALLING); //set tachISR to be triggered when the signal on the sense pin goes low
//initialize lcd screen
lcd.init();
// turn on the backlight
lcd.backlight();
lcd.clear();
lcd.setCursor(0,0);
lcd.print("FAN SPEED CONTROLLER");
lcd.setCursor(0,1);
lcd.print("PWM DUTY: ");
lcd.setCursor(0,2);
lcd.print("RPM: ");
}
void loop() {
potVal = analogRead(potPin);
fanVal = map(potVal, 0, 1023, 0, 100);
fanSet = float(fanVal)/100;
setPWM1A(fanSet);
lcd.setCursor(0,1);
sprintf(msg, "PWM DUTY: %3d%%", fanVal);
lcd.print(msg);
// Non blocking delay
// check to see if it's time to read RPM. If the difference between
// the current time and last time you read RPM is bigger than the
// interval, then read again.
unsigned long currentMillis = millis();
if (currentMillis - previousMillis >= interval) {
// save the last time you read the RPM
previousMillis = currentMillis;
lcd.setCursor(0,2);
sprintf(msg, "RPM: %4u", calcRPM());
lcd.print(msg);
}
}
//Calculates the RPM based on the timestamps of the last 2 interrupts. Can be called at any time.
unsigned long calcRPM(){
if(millis()-ts2<FANSTUCK_THRESHOLD&&ts2!=0){
return (60000/(ts2-ts1))/2;
}else return 0;
}
void tachISR() {
unsigned long m=millis();
if((m-ts2)>DEBOUNCE){
ts1=ts2;
ts2=m;
}
}
Edit: Adding pictures of the tach output. Is that normal? If not, could it be part of the issue?
Edit2: The frequency above is shown as ~52Hz and the Arduino is reporting 1500 / 1578 / 1666/ 3000 / 3333 with the 15xx being the most frequent RPM.
