ACS 712 calibration

ajit.nayak87

Dear all,

I am trying to measure the current through 2 fans. Here i am attaching my code and I need to know how can i calibrate it.
I am using below 2 fans
http://www.mouser.in/ProductDetail/ADDA/AD0612HX-A71GL-LF/?qs=hTx6WCxFMbgcZc/FmemT9A==

http://www.mouser.in/ProductDetail/ADDA/AD0812XB-A73GL-LF/?qs=PieNiMrGZlqvWjV0xsttzA==

Using arduino UNo board for programming.

[COLOR=#000000][FONT=monospace]void CS_Output()[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]{[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]  float average = 0;[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]  for(int i = 0; i < 1000; i++) {[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]    average = average + (.0264 * analogRead(A0) -13.51) / 1000;[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]    delay(1);[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]  }[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]  Serial.print("before calibration:");[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]  Serial.println(average);[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]  average=average+0.07;[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]    Serial.print("after calibration:");[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]  Serial.println(average);  [/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]}[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]void setup() {[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]Serial.begin(9600);[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]}[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]void loop()[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]{[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]CS_Output();[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]delay(1000);[/FONT][/COLOR]
[COLOR=#000000][FONT=monospace]}[/FONT][/COLOR]

Results

slno fans Actual_dc load Before_conversion after_ conversion
1 AD0612HX 0.15A 0.09a 0.15A
2 ADO812XB 0.40A 0.25A 0.32A
3 WHEN COMBINE 0.52A 0.35A 0.42A

Let know how can offset value to meet exact or close to actual_DC load current showing value

Mark_T

[COLOR=#000000][FONT=monospace]    average = average + (.0264 * analogRead(A0) -13.51) / 1000;[/FONT][/COLOR]

is an obscure way to say

[COLOR=#000000][FONT=monospace]    average += 0.0264 * (analogRead(A0) - 512) / 1000;[/FONT][/COLOR]

To calibrate you would normally take a batch of readings at start up, and store their average, then
subtract from the actual reading:

[COLOR=#000000][FONT=monospace]    average += 0.0264 * (analogRead(A0) - 512 - calibration) / 1000;[/FONT][/COLOR]

ajit.nayak87

But still not calibrating properly.still there is difference of 0.3~0.4A w.r.t actual reading

const int num_readings = 10;
float offSet = 2.44009;
const int mad = 2.24;
int readings[num_readings];
int index = 0;
float sample2SolCrnt = 0.0;
  float solar_crnt = 0.0; // Solar panel current variable
  float solarCrntVal = 0.0; // Current callibration variable
  
  
void setup() {
  // sets the serial port to 9600
  Serial.begin(9600);
}


int average() {
  int total = 0;
  for(int i=0; i<num_readings; i++)
    total = total + readings[i];
  return total/num_readings;
}


float standard_deviation(int avg) {
  int total = 0;
  for(int i=0; i<num_readings; i++)
    total = total + pow((avg - readings[i]), 2);
  return sqrt(total/num_readings);
}


void loop() {
  float sum=0.0;
  // read analog input pin 0
  int reading = analogRead(A0);
  readings[index] = reading;
  // incrementing the index
  index = index + 1;
  // if have already been done 10 readings...
  if (index >= num_readings) {
    // set the index to 0
    index = 0;
    // compute the average
    int avg = average();
    // compute the standard deviation
    float std = standard_deviation(avg);
    
    float madstd = mad * std;
    float lowlimit = avg - madstd;
    float highlimit = avg + madstd;
    
    int count = 0;
    int total = 0;
    for(int i=0; i<num_readings; i++) {
      // Check if the values of the readings are within the limits.
      if(readings[i] >= lowlimit && readings[i] <= highlimit) {
        total = total + readings[i];
        count = count + 1;
      }
      
    }
    // compute the new average
    int newaverage = total/count;
    // send it to the serial port (as ASCII digits)
    Serial.println(newaverage, DEC);
    sum=sum+(.0264 *newaverage -13.51);
    
    
   // float current= 0.0264*(newaverage-512);
    
     
     Serial.print("current is :");
  Serial.println(sum);
  }
  
  
  // wait 1000/num_readings ms for next reading
  delay(int(1000/num_readings));
}

Mark_T

1) You have to ensure no current during calibration
2) A hall sensor is much noisier than a shunt sensor, but is isolated.