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Etude de la cellule de mesure de la résistance (pont diviseur de la tension)

 Circuit-mesure-de-la-résistance

  • Vin  : Tension d’entrée égale 5V (valeur maximale)
  • R1, R2 : Résistances du pont diviseur
  • Rm  : La résistance à mesurer par le microcontrôleur
  • RB0, RB1 : Broches du microcontrôleur 16F877

Le principe de mesure est très simple !! On connaissant les paramètres : Vin, Vout, R1, R2 on peut déterminer la valeur de la résistance Rm.

  • R1, R2 : Les valeurs des résistances du pont sont définit par rapport à la dynamique de la tension de sortie et la valeur de la résistance à mesurer.
  • Vin, Vout : On peut déterminer ses tension en utilisant deux entrées analogique du microcontrôleur (RB0 et RB1). Le micro dispose de quatre entrées analogique avec une résolution de 10 bits (pas de quantification q d’environ  4.9 mV pour une alimentation de 5v).

 

Calcul de la résistance Rm : 

Equation mesure résistance

Choix des résistances R1 et R2 : 

Le choix de la résistance R1 et R2 dépend de la valeur de résistance à mesurer [Rm_min, Rm_max]  et de la dynamique de la tension de sortie [Vsmin,Vsmax].

  • La tension Vsmax correspond à Rm = 0 ! Vsmax = Vin * R1/(R1 +R2 +0) . En pratique on choisi R1 et R2 de telle sorte que la tension Vsmax soit plus proche de la valeur maximale de lADC (5V).
  • La tension Vmin correspond à la valeur maximale de Rm (Rm_max) : Vsmin = Vin *R1/(R1 +R2 +Rm_max); en Pratique on choisi la valeur des résistances pour avoir une tension Vsmin plus proche de zero.

L’intérêt d’avoir une tension Vsmax ~ 5V et Vsmin ~ 0V est d’exploiter la dynamique totale de l’ADC du microcontrôleur  afin de gagner en précision sur la mesure de la résistance Rm !!

La tension Vs en fonction de la résistance Rm : 

Paramètres de simulation :

  • Vin = 5v
  • R1 = 1MR , 100KR ou 10KR
  • R2 fixe = 1KR
  • Rm variée entre 0 et 10MR

Graphe mesure de la résistance

On constate que pour  mesurer une résistance faible (<1MR), la résistance R1 doit être faible(<100K) (courbe vert et bleu).

En revanche si on veut calculer une résistance de l’ordre de quelques MR , on doit utilisé R1>1MR

Dans notre projet, la valeur de la résistance Rm variée entre 0 et 1MR  d’ou le choix de R1=100K et R2=1K (Vsmax=4.9505V, Vsmin= 0.0495V).

 

Schéma électronique du projet mesure de la résistance (ISIS):

 

Montage ohmmètre numérique

affichage LCD

Code MikroC :

// Connexions LCD
sbit LCD_RS at RB4_bit;
sbit LCD_EN at RB5_bit;
sbit LCD_D4 at RB0_bit;
sbit LCD_D5 at RB1_bit;
sbit LCD_D6 at RB2_bit;
sbit LCD_D7 at RB3_bit;

sbit LCD_RS_Direction at TRISB4_bit;
sbit LCD_EN_Direction at TRISB5_bit;
sbit LCD_D4_Direction at TRISB0_bit;
sbit LCD_D5_Direction at TRISB1_bit;
sbit LCD_D6_Direction at TRISB2_bit;
sbit LCD_D7_Direction at TRISB3_bit;

const int         Taille_LCD =16 ;

// Variables tensions (vin, vout)
unsigned int      Vout_ADC;
float             Vout_tension;
unsigned int      Vin_ADC;
float             Vin_tension;

// Paramètres de la cellule de mesure
float             Res1 = 100e3;  // R1 =100k
float             Res2= 1e3;     // R2= 1k
float             Rm;
float             Vin = 5.0;
float             ADC_Ref = 5.0;

//Variables de l'affichage (résistance & tension Vin)
char              RmStr[Taille_LCD] ;
char              VinStr[Taille_LCD];
char              VoltStr[]="Vin=";
char              Resohm[]="Rm=";

void main()
{
  
  TRISA = 0xFF;                             // PORTA en entrée
  TRISC = 0x00;                             // PORTC en sortie
  TRISB = 0x00;                             // PORTB en sortie
  TRISD = 0x00;                             // PORTD en sortie


  ADC_Init();                               // Initialisation ADC
  Lcd_Init();                               // Initialisation LCD
  Lcd_Cmd(_LCD_CLEAR);                      // Effacer  LCD
  Lcd_Cmd(_LCD_CURSOR_OFF);                 // Désactiver le curseur
  
 do
 {
         //Lecture ADC (0 et 1)

         Vin_ADC =ADC_read(0);
         Vout_ADC =ADC_read(1);
         
         //Conversion de la valeur en tension
         Vin_tension = (float)Vin_ADC *ADC_Ref/1023.0;
         Vout_tension = (float)Vout_ADC *ADC_Ref/1023.0;
         
         // Calcul de la valeur de la résistance
         Rm = Res1*(Vin_tension/Vout_tension - 1) - Res2;
         if(Rm<0.0) Rm=0.0;
         // Conversion de la valeur de Rm en chaîne de caractère 
         FloatToStr( Rm, RmStr );
         FloatToStr( Vin_tension, VinStr );
         
         //Affichage sur LCD
         Lcd_Cmd(_LCD_TURN_ON);
         delay_ms(10);
         Lcd_Out(1,5,RmStr);
         Lcd_Out(1,1, Resohm);
         Lcd_Out(1,16, "R");
         Lcd_Out(2,6,VinStr);
         Lcd_Out(2,1, VoltStr);
         Lcd_Out(2,16, "V");
         delay_ms(1000);
         Lcd_Cmd(_LCD_CLEAR);

 } while(1);
}

Télécharger gratuitement le fichier du projet Ohmmètre numérique (Schéma ISIS + Code MikroC ) :

Revenir au sommaire des projets électronique

 

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