Basic code for controlling a DC motor and one quadrature rotary encoder.

main.cpp

// LPD3806 rotary encoder

#include <Arduino.h>
#include "rotaryEncoderLPD3806.h"
#include "motorControllerDrokL298.h"

void setup()
{
  Serial.begin(9400);

  // Motor controller
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
  pinMode(ENA, OUTPUT);

  // Rotary encoder
  // Does have one pin interrupt vs two change how fast communication is?
  pinMode(rotaryEncoderPhaseA, INPUT_PULLUP);
  pinMode(rotaryEncoderPhaseB, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(rotaryEncoderPhaseA), readPhaseA, CHANGE);
  attachInterrupt(digitalPinToInterrupt(rotaryEncoderPhaseB), readPhaseB, CHANGE);
}

void loop()
{
  float angleDegrees;
  float angleRadians;
  float distance;
  float idlerRadius = 0.20;

  if (rotaryEncoderCount != previousCount)
  {
    angleRadians = (rotaryEncoderCount / 2400.0) * (2 * PI);
    distance = idlerRadius * angleRadians;
    Serial.println(distance);
  }

  // I have tried two different ways of controlling the motor to see if it would help with efficiency:
  // 1) Writing the code directly
  // 2) Outsourcing it to a function,
  // but both behave the same.
  // If I have the below code uncommented then the rotary encoder won't print values.

  // Method (1)
  // digitalWrite(IN1, LOW);
  // digitalWrite(IN2, HIGH);
  // analogWrite(ENA, 20);

  // Method (2)
  // moveCart('R', 36);
}

motorControllerDrokL298.h

#define IN1 7
#define IN2 8
#define ENA 9

void moveCart(char direction, int speed);
void brake();

void moveCart(char direction, int speed)
{

    if (speed == 0) {
        brake();
    }
    else
    {
        // Rotate motor clockwise
        if (direction == 'L')
        {
            digitalWrite(IN1, LOW);
            digitalWrite(IN2, HIGH);
        }

        // Rotate motor counter-clockwise
        else if (direction == 'R')
        {
            digitalWrite(IN1, HIGH);
            digitalWrite(IN2, LOW);
        }

        else
        {
            brake();
        }
        
        analogWrite(ENA, speed);
        
    }
}

void brake() {
    digitalWrite(IN1, LOW);
    digitalWrite(IN2, LOW);
    analogWrite(ENA, 0);
    delay(100);
}

rotaryEncoderLPD3806.h

#define rotaryEncoderPhaseA 2
#define rotaryEncoderPhaseB 3

volatile float rotaryEncoderCount;
volatile float previousCount = 0;

void readPhaseA();
void readPhaseB();

void readPhaseA()
{

  // If they're the same, then we're moving clockwise
  if (digitalRead(rotaryEncoderPhaseA) == digitalRead(rotaryEncoderPhaseB))
  {
    rotaryEncoderCount--;
  }

  // If they're not the same, then we're moving counter clockwise
  else
  {
    rotaryEncoderCount++;
  }
  
  previousCount = rotaryEncoderCount;  
}

void readPhaseB()
{

  // If they're the same, then we're moving clockwise
  if (digitalRead(rotaryEncoderPhaseA) == digitalRead(rotaryEncoderPhaseB))
  {
    rotaryEncoderCount++;
  }

  // If they're not the same, then we're moving counter clockwise
  else
  {
    rotaryEncoderCount--;
  }

  previousCount = rotaryEncoderCount;
}