Use a Speed Sensor to know the speed of your motors

                                         
                                                    SCHEMATIC DESIGN
PARTS USED
1.

                                                                 65 mm WHEEL
2.

                                       DC Motor with double shaft Gearbox 1:120

3.

                                        DC Motor with double shaft Gearbox 1:48

4.

                                   Infrared Speed sensor module with encoder Disc

5.

                                           Jumper Wires male to female 20pcs

6.

                                              Jumper Wires male to male – 65pcs

7.

                                  L9110 Motor Driver H-Bridge board Dual Channel

8.

                                                                             UNO R3

CODE

#include <TimerOne.h>

unsigned int counter=0;

int b1a = 6;  // L9110 B-1A

int b1b = 9;  // L9110 B-1B

void docount()  // counts from the speed sensor

{

  counter++;  // increase +1 the counter value

}

void timerIsr()

{

  Timer1.detachInterrupt();  //stop the timer

  Serial.print("Motor Speed: ");

  int rotation = (counter / 20);  // divide by number of holes in Disc

  Serial.print(rotation,DEC);  

  Serial.println(" Rotation per seconds");

  counter=0;  //  reset counter to zero

  Timer1.attachInterrupt( timerIsr );  //enable the timer

}

void setup()

{

  Serial.begin(9600);

  

pinMode(b1a, OUTPUT);

pinMode(b1b, OUTPUT);

  

  Timer1.initialize(1000000); // set timer for 1sec

  attachInterrupt(0, docount, RISING);  // increase counter when speed sensor pin goes High

  Timer1.attachInterrupt( timerIsr ); // enable the timer

}

void loop()

{

  int potvalue = analogRead(1);  // Potentiometer connected to Pin A1

  int motorspeed = map(potvalue, 0, 680, 255, 0);

  analogWrite(b1a, motorspeed);  // set speed of motor (0-255)

  digitalWrite(b1b, 1);  // set rotation of motor to Clockwise

}

IMPORTANT FILES TO DOWNLOAD

TIMER ONE LIBRARY