Constant RPM Code Issues

First of all Thanks a BUNCH to all the arduinno forum members for helping me out with this code. It would not have been possible without them.

Thank you again

I am using this code to monitor the rpm of the motor… and when the rpm deviates from what i want… the arduinno adjust the PWM accordingly. Thats what i want my code to ultimately do.

Here is an example:
My motor should be running at 7200 rpm.

When the hall sensor gives me an rpm reading below 7200 it should adjust the PWM accordingly to bring it back up to 7200.

The logic i am using is this:

DutyCycle = [200- (RPM - 7200)*X/60]

200 is my OCR1B value which is required to run the motor at 7200.

RPM is whatever RPM i am measuring.

X is the factor i need to adjust my pwm by to change my frequency by 1Hz i.e 60rpm.

However this doesnt work as it floods the OCR1B.=(

I also tried just using simple if else statements to increment the duty cycle by 1 everytime rpm falls below a threshhold… but the increment happens very very slowly. its not the resolution…since if i increase the increment by 20 … theres still a delay before the duty cycle changes.

Is this a limitation of the arduino??

Is there a better way to keep rpm constant? and adjust the PWM according to the load on it.

Here is my code for your reference It includes the code from WanaGo.

// ArduECU version 0.01 alpha
// Compiled with Arduino v0018
// Thanks to the members of the forum for their assistance

// Many comments removed to fit sketch into forum posting box...

extern volatile unsigned long timer0_overflow_count;  // Record the most recent timer ticks
volatile boolean ticks_valid;                         // Indicates a valid reading
volatile unsigned long ticks_per_rev;                 // Number of ticks counted in the current revolution
unsigned long msSinceRPMReading;                      // Number of mSec since the last rpm_sense (used to spot zero RPM)
float lastRPM, peakRPM;                               // Most recent RPM reading, and highest RPM recorded since last reset

const float __TICKS_TO_RPMS = 15e6;                   // Convert clock ticks to RPM by dividng ticks into this number
                                                      // The number will change if there are more magnets in an rpm
                                                      //   (e.g. 2 magnets = 29296.875)
const unsigned int __WAIT_BEFORE_ZERO_RPM = 2000;     // Number of mS to wait for an rpm_sense before assunming RPM = 0.
const int __REV_GAUGE_PIN = 3;                        // 100uA meter on this pin, with 50K current-limiting resistor

unsigned long msSinceSend;                            // mSec when the last data was sent to the serial port
const unsigned int __WAIT_BETWEEN_SENDS = 1000;       // Number of mS to wait before sending a batch of data.

int outputPsuB = 10;  // Timer1-B
int dutycycle = 4369; //50% duty cycle

void setup()

  Serial.begin(9600);                                 // Initialise serial comms.
  msSinceSend = millis();                             // Data sent counter

  attachInterrupt(0, rpm_sense, RISING);              // RPM sense will cause an interrupt on pin2
  msSinceRPMReading = 0;                              // If more than 2000 (i.e. 2 seconds),
                                                      // then RPMs can be assumed to be zero (< 15rpm
                                                      // at most, with a single magnet, no small IC
                                                      // can run that slowly).
  lastRPM = 0;                                        // Current RPM to zero
  peakRPM = 0;                                        // Max recorded RPM to zero
  pinMode(__REV_GAUGE_PIN,OUTPUT);                    // Set pin 3 to be output

  pinMode(outputPsuB, OUTPUT);  // select Pin as ch-B

  TCCR1A = B00100001; // PWM, Phase and frequency correct - change at OCR1A
  TCCR1B = B10010;  // prescaling by 8 the system clock


// ------------------------------------------------------------------------------------------------
// Called whenever the RPM sense signal rises. In my setup, the hall effect switch is normally
// high, goes low when a south pole is introduced to the sensor, and rises back to high as the
// magnet goes away. Thus, the RPM sense is called on the trailing edge of the magnet.
// Version Date        By  Comment
// -------|-----------|---|------------------------------------------------------------
//   0.01a 26-May-2010 JAV Created (with an assist from BenF from the forum
// ------------------------------------------------------------------------------------------------
void rpm_sense()
  static unsigned long pre_count;               // Last timer0_overflow_count value
  unsigned long ticks_now;                      //

  ticks_now = timer0_overflow_count;            // Read the timer

  byte t = TCNT0;
  if ((TIFR0 & _BV(TOV0)) && (t<255))
  ticks_now = (ticks_now << 8) + t;

  if (pre_count == 0) {                         // First time around the loop?
    pre_count = ticks_now;                      // Yes - set the precount, don't use this number.
  } else {
    ticks_per_rev = ticks_now - pre_count;      // No - calculate the number of ticks...
    pre_count = ticks_now;                      // ...reset the counter...
    ticks_valid = true;                         // ...and flag the change up.

void loop()
  unsigned long thisMillis = millis();          // Read the time once

  //Serial.print("dutycycle: ");
  //Generation of PWM Signal
  OCR1A = 8197; // 122.0Hz at 8197us|140Hz at 7143us
  OCR1B = dutycycle; // 99.9% DC at 8196|40% DC at 7071

  // Calculate RPM
  if (ticks_valid) {                            // Only come here if we have a valid RPM reading
    unsigned long thisTicks;

    thisTicks = ticks_per_rev;
    ticks_valid = false;

    lastRPM = __TICKS_TO_RPMS / thisTicks;      // Convert ticks to RPMs
    ticks_valid = false;                        // Reset the flag.
    msSinceRPMReading = thisMillis;             // Set the time we read the RPM.
    if (lastRPM > peakRPM)
      peakRPM = lastRPM;                        // New peak RPM
  } else {
    // No tick this loop - is it more than X seconds since the last one?
    if (thisMillis - msSinceRPMReading > __WAIT_BEFORE_ZERO_RPM) {
      lastRPM = 0;                              // At least 2s since last RPM-sense, so assume zero RPMs
      msSinceRPMReading = thisMillis;           // Reset counter

  // Calculate temperatures
  // not implemented yet

  // Is it time to send the data?
  if (thisMillis < msSinceSend)                  // If thisMillis has recycled, reset it
    msSinceSend = millis();

  if (thisMillis - msSinceSend > __WAIT_BETWEEN_SENDS)
    // Yes: Build the serial output...

    // For now, send everything plaintext. Maybe compression would be a good thing, later down the line...
    Serial.print("uECUv0.01|CurrentRPM: ");                  // Send ID - ditch this for something smaller
    Serial.print(lastRPM);                       // Current RPMs
    Serial.print("|");                           // Field Separator
    Serial.print(peakRPM);                       // Peak RPMs
    Serial.print("|");                           // Field Separator
    Serial.print(dutycycle);                     // Temperature #1 (water in)
    Serial.print("|");                           // Field Separator
    Serial.print(0);                             // Temperature #2 (water out)
    Serial.print("|");                           // Field Separator
    Serial.print(0);                             // Temperature #3 (exhaust gas)

    //Start of compare of RPM values
    if(lastRPM != 7300 && dutycycle <= 8196 && dutycycle >= 10)
      dutycycle = 4369 - ((lastRPM - 7300)/60)*(0.82);
    else if(dutycycle > 8196 || dutycycle < 10)


    // Debugging
    // Serial.print("|");                           // Field Separator
    // Serial.print(ticks_per_rev);                 // clock ticks in the last rpm

    // That'll do for bnow
    Serial.println();                            // EOL.

    msSinceSend = millis();                      // Reset the timer

    if (Serial.available()) {
      char cmd =;
      if (char('R') == cmd) {


  // Set the RPM gauge value in real-time...
  if (lastRPM < 1000) {
    // Sub-1000 RPMs, map to between 0-255
  } else {
    // If RPMs > 1000, write 255 (=meter FSD); but we've got bigger problems if the RPMs ever get this high...


Once again thank you for all the help.

You may want to go over your code again carefully. Lines like this are suspicious:

    if(lastRPM != 7300 && dutycycle <= 8196 && dutycycle >= 10)
      dutycycle = 4369 - ((lastRPM - 7300)/60)*(0.82);

This is suspicious because it doesn’t match what you wrote in your posting – the formula is somewhat different. Perhaps there has been some “tweaking” going on and you forgot to un-tweak?

In general, however, you are building what is called a digital feedback control system. This is not easy to do right, and you are running up against the kinds of problems that caused an entire field of study to emerge.

Fortunately, there is a simple (relatively) method of working with these systems known as PID, and there is even an Arduino library for it here:

You must also be careful with “heavyweight” code. You are:

  1. Using Serial.println to print to the screen
  2. Doing integer division (by non-power-of-2 factors)
  3. Doing floating-point computation

All of the above take a “long” time and will affect the operation of your algorithm. It is best to have a predictable amount of time between each update of the duty cycle.

Best of luck.

Check out our new shield:

Thanks a bunch Rugged Electronics!!!

Yeh the part u mentioned was from when we were running the PWM signal at 122 Hz. The scaling according to that for the duty cycle we wanted was 4369...while we were calibrating the X.. we thought it was .89. But later on changed it to 1 to see what happens.

Now we are sending the PWM at 2KHz so the scaling falls down to 200.

Im sorry should have updated the code.

I will do that and keep you posted on what we discover.

thank you so much! This is for my senior design project and i really appreciate all the help!