Measuring Wind Speed with an Anemometer and Arduino

In this video we look at how to measure wind speed using an anemometer and Arduino. This approach will work on both ARM and AVR based Arduinos.

//*****************Arduino anemometer sketch******************************
const byte interruptPin = 3; //anemomter input to digital pin
volatile unsigned long sTime = 0; //stores start time for wind speed calculation
unsigned long dataTimer = 0; //used to track how often to communicate data
volatile float pulseTime = 0; //stores time between one anemomter relay closing and the next
volatile float culPulseTime = 0; //stores cumulative pulsetimes for averaging
volatile bool start = true; //tracks when a new anemometer measurement starts
volatile unsigned int avgWindCount = 0; //stores anemometer relay counts for doing average wind speed
float aSetting = 60.0; //wind speed setting to signal alarm

void setup() {
  pinMode(13, OUTPUT); //setup LED pin to signal high wind alarm condition
  pinMode(interruptPin, INPUT_PULLUP); //set interrupt pin to input pullup
  attachInterrupt(interruptPin, anemometerISR, RISING); //setup interrupt on anemometer input pin, interrupt will occur whenever falling edge is detected
  dataTimer = millis(); //reset loop timer
}

void loop() {
 
  unsigned long rTime = millis();
  if((rTime - sTime) > 2500) pulseTime = 0; //if the wind speed has dropped below 1MPH than set it to zero
     
  if((rTime - dataTimer) > 1800){ //See if it is time to transmit
   
    detachInterrupt(interruptPin); //shut off wind speed measurement interrupt until done communication
    float aWSpeed = getAvgWindSpeed(culPulseTime,avgWindCount); //calculate average wind speed
    if(aWSpeed >= aSetting) digitalWrite(13, HIGH);   // high speed wind detected so turn the LED on
    else digitalWrite(13, LOW);   //no alarm so ensure LED is off
    culPulseTime = 0; //reset cumulative pulse counter
    avgWindCount = 0; //reset average wind count

    float aFreq = 0; //set to zero initially
    if(pulseTime > 0.0) aFreq = getAnemometerFreq(pulseTime); //calculate frequency in Hz of anemometer, only if pulsetime is non-zero
    float wSpeedMPH = getWindMPH(aFreq); //calculate wind speed in MPH, note that the 2.5 comes from anemometer data sheet
   
    Serial.begin(57600); //start serial monitor to communicate wind data
    Serial.println();
    Serial.println("...................................");
    Serial.print("Anemometer speed in Hz ");
    Serial.println(aFreq);
    Serial.print("Current wind speed is ");
    Serial.println(wSpeedMPH);
    Serial.print("Current average wind speed is ");
    Serial.println(aWSpeed);
    Serial.end(); //serial uses interrupts so we want to turn it off before we turn the wind measurement interrupts back on
   
    start = true; //reset start variable in case we missed wind data while communicating current data out
    attachInterrupt(digitalPinToInterrupt(interruptPin), anemometerISR, RISING); //turn interrupt back on
    dataTimer = millis(); //reset loop timer
  }
}

//using time between anemometer pulses calculate frequency of anemometer
float getAnemometerFreq(float pTime) { return (1/pTime); }
//Use anemometer frequency to calculate wind speed in MPH, note 2.5 comes from anemometer data sheet
float getWindMPH(float freq) { return (freq*2.5); }
//uses wind MPH value to calculate KPH
float getWindKPH(float wMPH) { return (wMPH*1.61); }
//Calculates average wind speed over given time period
float getAvgWindSpeed(float cPulse,int per) {
  if(per) return getWindMPH(getAnemometerFreq((float)(cPulse/per)));
  else return 0; //average wind speed is zero and we can't divide by zero
  }

//This is the interrupt service routine (ISR) for the anemometer input pin
//it is called whenever a falling edge is detected
void anemometerISR() {
  unsigned long cTime = millis(); //get current time
  if(!start) { //This is not the first pulse and we are not at 0 MPH so calculate time between pulses
   // test = cTime - sTime;
    pulseTime = (float)(cTime - sTime)/1000;
    culPulseTime += pulseTime; //add up pulse time measurements for averaging
    avgWindCount++; //anemomter went around so record for calculating average wind speed
  }
  sTime = cTime; //store current time for next pulse time calculation
  start = false; //we have our starting point for a wind speed measurement
}