SG90 Servo Motor

What it is

The SG90 is a micro servo: a self-contained little actuator that rotates to a commanded angle and holds it there. Where a plain DC motor just spins, a servo aims—tell it “90 degrees” and it drives itself to 90 degrees and resists being pushed away. That makes it the natural choice for anything that needs to move to a known position: a robot arm joint, a gripper that opens and closes, a pan-tilt mount for a camera, or the steering on a small rover.

At nine grams and a couple of dollars it is the servo almost everyone starts with. It is not strong or precise enough for serious work, but it is the fastest way to understand how position control feels.

How it works

Crack one open and you find four things: a small DC motor, a gear train that trades speed for torque, a potentiometer geared to the output shaft, and a control circuit. The potentiometer is the trick—it continuously reports the shaft’s actual angle. The circuit compares that measured angle to the angle you are commanding and drives the motor in whichever direction closes the gap. It is a complete feedback loop in miniature, which is exactly why the inverse-kinematics arm can trust a servo to reach a target pose.

You command the angle with a 50 Hz PWM signal, and the pulse width carries the message: roughly 1 ms for one end of travel, 1.5 ms for centre, 2 ms for the other end. This is PWM used as a position code rather than a speed code. In practice the Arduino Servo library hides the timing and you just write an angle.

When to use it

Use an SG90 whenever a robot part must move to and hold a specific angle under light load: gripper fingers, small arm joints, sensor gimbals, or the steering linkage on a lightweight car. Its ~180° range and instant “go to this angle” control are ideal for learning kinematics and building expressive mechanisms.

Look elsewhere when the joint carries real weight (use a metal-gear or standard servo), when you need continuous rotation for a wheel (use a DC motor and an L298N, or a continuous-rotation servo), or when you need many degrees of precise, high-torque motion (use a stepper).

Wiring and gotchas

• Power it separately. A stalling SG90 can pull 500–700 mA—far more than a microcontroller’s 5 V pin can give. Use a dedicated 5 V supply and tie its ground to the board’s ground, or the board will brown out and reset.
• It is not continuous rotation. A standard SG90 travels ~180° and stops. Commanding “spin forever” does nothing; for a driven wheel you need a different actuator.
• Don’t force it or stall it. The nylon gears strip under overload or a sharp knock. Keep loads light, or move to metal gears.
• Expect a little jitter. Cheap servos hunt slightly around the target. A stable, clean power supply and detaching the signal when the joint doesn’t need holding both reduce it.