Square of float result in negative value why ?

Hardware Nano Family Classic Nano
1

I am trying to read the current with acs712. And when I get close to zero then sometimes I get negative values. Why is that ?

#define AnPin 7

float vall;
float current, vallRMS, vallLowPass;

void setup() {
  pinMode(7, INPUT);
  Serial.begin(115200);
}

void loop() {

  for ( int i = 0; i < 10; i ++) {
    vall += sq((float)(analogRead(AnPin) - 507) * 0.004887586);   
  }
  vallRMS = sqrt(vall * 0.1) ;
  vallLowPass = vallLowPass * 0.95 + vallRMS * 0.05;
  current = vallLowPass * 10 - 0.0288;
  if ( current < 0.0 ) { current = 0; }

  /* vall =  analogRead(AnPin);
    vallRMS = vall * 5 / 1023.0;
    current = (vallRMS - 2.4877)*5.405405405;*/


  Serial.print("vall:");
  Serial.print (vall,12);
  Serial.print("\t");
  Serial.print ("current:");
  Serial.println(current, 4 );
  vallRMS = 0;
  vall = 0;
  current = 0;
}

Here an example of the values. Prud is current. :

|vall:0.000071665496|prud:nan|
|vall:0.000023888497|prud:nan|
|vall:0.000071665496|prud:nan|
|vall:0.000023888497|prud:nan|
|vall:0.000047776994|prud:nan|
|vall:0.000071665496|prud:nan|
|vall:0.000023888497|prud:nan|
|vall:0.000000000000|prud:nan|
|vall:0.000000000000|prud:nan|
|vall:0.000023888497|prud:nan|
|vall:0.000119442486|prud:nan|
|vall:0.000000000000|prud:nan|
|vall:-0.000047776994|prud:nan|
|vall:0.000047776994|prud:nan|
|vall:0.000071665496|prud:nan|
|vall:0.000071665496|prud:nan|

2

Please post code that will compile without errors and demonstrates the problem.

"prud" is not declared or defined in the posted code.

3

Also, from the sq() reference page, "Because of the way the sq() function is implemented, avoid using other functions inside the brackets, it may lead to incorrect results."

2 Likes
4 
#define AnPin 7

void setup() {
  Serial.begin(115200);
}

void loop() {
  int16_t vall = 0;
  static float current = 0;
  
  for ( int i = 0; i < 10; i ++) {
    vall += analogRead(AnPin));
    delay(1);
  }
  vall = abs(vall-5120);
  current = vall / 255.75;

  Serial.print("vall:");
  Serial.print (vall);
  Serial.print("\t");
  Serial.print ("current A:");
  Serial.println(current, 3 );
}
5

Just replace prud with current. I wanted to replace the prud with current so it makes more sense and I forget to do it in some places.

6

Please fix the code as requested, and as pointed out above, DO NOT use sq() with functions like analogRead(). Use the multiplication operator instead.

1 Like
7

Your assignment of vallLowPass depends on the previous value of vallLowPass. But the first time through loop(), vallLowPass doesn't have an assigned value. That's bad.

8

vallLowPass is global and is initialized to zero at program startup.

However, it is always best not to rely on such behavior and explicitly initialize all variables.

9

Printing ALL of (vall, vallRMS, vallLowPas, and current) might get you closer to understanding what's happening.

10

Thank you for your help.

11

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