Servo motor

A servo motor is a small motor that knows its own angle. You tell it "point to 90 degrees" and it rotates there — and holds — no matter what tries to push it back.

That's the magic. A regular DC motor just spins as long as you apply power. A servo positions itself — and stays. It's the difference between a fan (just spin) and a robot arm joint (move precisely and hold).

What's inside

Crack open a typical hobby servo (like an SG90, which costs less than ₹150) and you'll find:

1. A small DC motor — the part that does the actual spinning.
2. A gearbox — slows the motor down and multiplies its torque, the way bicycle gears do.
3. A potentiometer — a sensor that measures the current angle of the output shaft.
4. A control board — a tiny circuit that compares "where am I" (from the potentiometer) to "where do I want to be" (from your command). If there's a difference, it powers the motor in the right direction. When they match, it stops.

That's a closed-loop control system — the simplest, most common form in robotics. Even billion-dollar humanoid robots use the same idea, just with fancier sensors and motors.

How you tell it where to go

You send a PWM signal — see the entry on PWM for how that works in general.

For a standard hobby servo, the rule is:

• 1.0 ms pulse = 0° (one extreme)
• 1.5 ms pulse = 90° (centre)
• 2.0 ms pulse = 180° (the other extreme)

The pulse repeats every 20 ms. The servo board measures the length of each pulse and adjusts.

Continuous-rotation servos

Some servos are modified to spin continuously instead of holding an angle. In those, the PWM signal controls speed and direction — 1.5 ms means stopped, 1.0 ms is full speed one way, 2.0 ms is full speed the other.

These are popular for small wheeled robots, because you can drive them straight from an Arduino without a motor driver.

What servos can and can't do

Can do: precise positioning, slow movements, holding heavy weights against gravity (within their torque rating).

Can't do: spin really fast (most hobby servos top out at 60° per 0.1-0.2 seconds), handle very large loads without stripping their plastic gears (metal-gear servos help), or run for years under heavy load without wearing out.

For serious robotics — robot arms, drones, humanoid joints — engineers move up from hobby servos to brushless motors with encoders and proper motor drivers. Same closed-loop idea, just much more powerful and precise.

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Curious how an Optimus humanoid uses motors like these (just thousands of times bigger)? Read Optimus.