DC Motor with Encoder

Motor RPM Measurement
Controller	Arduino
Motor Driver	L293D
Encoder	LM393
Encoder Disc	20 slots
Encoder Position	Gearbox output shaft
Output	RPM

Arduino Motor RPM Measurement is a simple motor-control and speed-measurement setup using an Arduino, L293D motor driver and LM393 optical encoder.

The 20-slot encoder disc is mounted on the gearbox output shaft. Therefore:

20 pulses = 1 output-shaft revolution

The Arduino controls the motor and counts encoder pulses for one second to calculate the output-shaft RPM.

Connections

Device	Pin	Arduino	Function
LM393	DO	D2	Encoder pulse
LM393	VCC	5V	Power
LM393	GND	GND	Ground
L293D	EN1	D5	Motor PWM
L293D	IN1	D7	Motor direction
L293D	IN2	D8	Motor direction

Motor direction

IN1 = HIGH
IN2 = LOW

Encoder

The LM393 produces a digital pulse whenever a slot on the encoder disc passes the sensor.

20 slots ➡️ 20 pulses ➡️ 1 output-shaft revolution

Working

The motor is driven through the L293D. The encoder disc rotates together with the gearbox output shaft.

The LM393 detects the slots and produces digital pulses. These pulses are connected to Arduino D2.

D2 uses an external interrupt to count the pulses.

Every 1 second, the Arduino:

1. Reads the number of encoder pulses.
2. Resets the pulse counter.
3. Converts the pulse count into RPM.
4. Displays the result on the Serial Monitor.

The RPM calculation is:

RPM = (Pulses × 60) / 20

Arduino Code

/*
  Encoder disc:
      20 slots
  Encoder location:
      Gearbox OUTPUT shaft
  Therefore:
      20 pulses = 1 output-shaft revolution
  Encoder:
      LM393 DO -> Arduino D2
  Motor driver:
      L293D
      EN1 -> D5
      IN1 -> D7
      IN2 -> D8
*/

#define ENCODER_PIN 2

#define EN1 5
#define IN1 7
#define IN2 8

#define SLOTS_PER_REV 20
#define MIN_PULSE_INTERVAL_US 2000
#define SAMPLE_TIME_MS 1000

volatile unsigned long pulseCount = 0;
volatile unsigned long lastPulseTime = 0;

int motorPWM = 150;

// ENCODER INTERRUPT
void encoderISR()
{
  unsigned long currentTime = micros();
  if ((currentTime - lastPulseTime) >= MIN_PULSE_INTERVAL_US)
  {
    pulseCount++;
    lastPulseTime = currentTime;
  }
}

void setup()
{
  Serial.begin(115200);
  pinMode(ENCODER_PIN, INPUT);
  pinMode(EN1, OUTPUT);
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
  digitalWrite(IN1, HIGH);
  digitalWrite(IN2, LOW);
  analogWrite(EN1, motorPWM);
  attachInterrupt(
    digitalPinToInterrupt(ENCODER_PIN),
    encoderISR,
    RISING
  );

  Serial.println("RPM");
}

void loop()
{
  static unsigned long lastMeasurement = 0;
  unsigned long currentTime = millis();
  if (currentTime - lastMeasurement >= SAMPLE_TIME_MS)
  {
    unsigned long pulses;
    noInterrupts();
    pulses = pulseCount;
    pulseCount = 0;
    interrupts();

    float rpm =
      (pulses * 60.0) /
      (SLOTS_PER_REV * (SAMPLE_TIME_MS / 1000.0));
    Serial.print ("Pulses ");
    Serial.print (pulses);
    Serial.print (",RPM ");
    Serial.println (rpm);
    lastMeasurement = currentTime;
  }
}

Observation

The Serial Monitor is set to 115200 baud.

Example observations:

Pulse Count	Calculated RPM
80	240 RPM
82	246 RPM
81	243 RPM
83	249 RPM

Example Serial Monitor output:

<syntaxhighlight> RPM Pulses 80,RPM 240.00 Pulses 82,RPM 246.00 Pulses 81,RPM 243.00 Pulses 83,RPM 249.00 </syntaxhighlight>

Result: The Arduino successfully controls the geared motor and measures the gearbox output-shaft RPM using the 20-slot LM393 encoder.