Servo Motor

What is a servo motor?

A servo motor is an actuator that accepts a commanded position and uses internal feedback to hold that position against external forces. Unlike a DC motor — which spins freely at a speed proportional to voltage — a servo does not move continuously. It moves to a specific angle and stays there.

The word servo comes from the Latin servus (servant) and entered engineering through Norbert Wiener's cybernetics work in the 1940s. The feedback principle — compare actual state to desired state, correct the error — is the foundation of all closed-loop control systems, from thermostats to space telescopes.

Why it matters for AI builders

In robotic manipulation, every joint that must hold a position against gravity uses a servo or its industrial equivalent. The diffusion policy and other modern robot learning algorithms output joint angle targets — a servo is what executes those targets in the physical world.

Key roles in physical AI systems:

• Manipulator joints — each degree of freedom in a robot arm is one servo
• Gripper mechanism — open/close commands map directly to servo angle
• Camera pan/tilt — gaze direction control for visual systems
• Bipedal balance — ankle and hip servos respond to IMU feedback
• Soft robotics — servo-actuated tendons drive compliant finger structures

Working principle

Inside an SG90 (and all hobby servos):

1. Motor + gearbox — a small brushed DC motor drives a plastic gear train that increases torque and reduces speed
2. Potentiometer — mechanically coupled to the output shaft; its resistance encodes current angle (0°–180°)
3. Control IC — compares the pot voltage (actual position) to the PWM pulse width (commanded position), drives the motor to minimise the error

PWM encoding. The control IC reads the pulse width of the incoming PWM signal:

• 1.0 ms pulse → 0°
• 1.5 ms pulse → 90° (centre)
• 2.0 ms pulse → 180°
• Period: 20 ms (50 Hz repetition rate)

The pulse width, not the duty cycle percentage, encodes the angle.

Key parameters (SG90)

Pinout

Wire colour conventions:

• Red or orange = VCC
• Brown or black = GND
• Yellow or orange = Signal (varies — check your specific servo's datasheet)

Wiring

Power note. A single SG90 at light load can run from the Arduino 5V pin. Two or more servos under load will brownout the Arduino. Always use an external power source (4× AA batteries or a bench supply) for multiple servos, and ensure a common ground between the Arduino and the power rail.

Code

Basic position control

#include <Servo.h>

Servo myServo;

void setup() {
  myServo.attach(9);    // attach servo to pin 9
  myServo.write(90);    // move to centre position at startup
  delay(500);           // allow servo to reach position
}

void loop() {
  myServo.write(0);     // go to 0 degrees
  delay(1000);
  myServo.write(90);    // go to 90 degrees (centre)
  delay(1000);
  myServo.write(180);   // go to 180 degrees
  delay(1000);
}

Smooth sweep — avoiding mechanical shock

Commanding a servo from 0° to 180° in one step causes maximum mechanical stress and current spike. Ramp the angle instead:

#include <Servo.h>

Servo myServo;

void smoothMove(Servo &s, int from, int to, int stepDelay = 15) {
  int step = (to > from) ? 1 : -1;
  for (int pos = from; pos != to; pos += step) {
    s.write(pos);
    delay(stepDelay);   // 15ms per degree = ~2.7s for 180°
  }
  s.write(to);
}

void setup() {
  myServo.attach(9);
  myServo.write(90);
  delay(500);
}

void loop() {
  smoothMove(myServo, 90, 0);
  delay(500);
  smoothMove(myServo, 0, 180);
  delay(500);
  smoothMove(myServo, 180, 90);
  delay(500);
}

Joystick to servo mapping

#include <Servo.h>

Servo myServo;
const int JOY_PIN = A0;

void setup() {
  myServo.attach(9);
}

void loop() {
  int raw = analogRead(JOY_PIN);         // 0 – 1023
  int angle = map(raw, 0, 1023, 0, 180); // scale to servo range
  myServo.write(angle);
  delay(20);
}

Test procedure

Common mistakes

Robotics and AI use cases

Beginner project — Potentiometer-controlled servo

Control servo angle by turning a knob. This is the most fundamental human-in-the-loop motor control circuit.

Components: Arduino Uno, SG90 servo, 10kΩ potentiometer, external 5V power, breadboard, jumper wires.

Goal: Turning the pot from 0 to full maps the servo from 0° to 180°. Smooth, proportional response with no jitter.

Key learning: analogRead() → map() → myServo.write() — the canonical sensor-to-actuator pipeline.

Advanced project — 2-DOF pan-tilt face tracker

Mount two servos in a pan-tilt bracket with a camera. Run a face detection model (OpenCV haarcascade_frontalface_default) on a computer, send the face centroid coordinates over USB serial to Arduino, and use PD control to minimise the offset between the face position and the image centre.

Components: Arduino Uno, 2× SG90 servos, pan-tilt bracket, USB camera, laptop running Python/OpenCV.

Key learning: Serial communication bridge, PD control loop, coordinate frame transformation, non-blocking servo updates.

# Python side — send servo commands over serial
import cv2, serial, time

ser = serial.Serial('/dev/ttyUSB0', 9600)
cap = cv2.VideoCapture(0)
face_cascade = cv2.CascadeClassifier(cv2.data.haarcascades + 'haarcascade_frontalface_default.xml')

while True:
    ret, frame = cap.read()
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    faces = face_cascade.detectMultiScale(gray, 1.1, 4)
    if len(faces):
        x, y, w, h = faces[0]
        cx = x + w // 2   # face centre x
        cy = y + h // 2   # face centre y
        pan  = int(cx / frame.shape[1] * 180)
        tilt = int(cy / frame.shape[0] * 180)
        ser.write(f"{pan},{tilt}\n".encode())
    time.sleep(0.05)