Motors & Actuators

We've talked a lot about robots moving around using kinematics — calculating joint angles, positions, and trajectories. But how does that actually translate to real life? How does a robot know it's rotated exactly 30 degrees, or moved its arm to just the right spot? That's where actuators and encoders come into play.

Actuators — usually motors — are the muscles of a robot. They make things move by turning joints, spinning wheels, or sliding mechanisms in and out. Encoders are sensors that measure how far and how fast something has moved. (We'll get into those in the next lesson.)

You'll often see the words motor and actuator used interchangeably in robotics (including on this site), but there technically is a difference. An actuator is a broad term for any device that takes some form of energy — like electricity, air pressure (pneumatics), or fluid pressure (hydraulics) — and turns it into mechanical motion. A motor is a specific type of actuator that converts electrical energy into rotational motion. So every motor is an actuator, but not every actuator is a motor. It's a small distinction, but in casual robotics conversation most people use the terms interchangeably.

Common Types of Actuators in Robotics

Brushed DC Motors

Brushed motors spin by flipping the direction of electricity through a coil of wire (kind of like flipping a magnet back and forth really fast). Tiny metal parts called brushes press against the spinning part of the motor and help switch the flow of electricity at just the right time. This constant switching keeps the motor spinning.

It's a bit like giving someone a gentle push every time they come around on a merry-go-round — the brushes handle the "when" and "where" to push, making sure the motion continues smoothly.

Where you'll see them

In toys, hobby robots, small RC cars — basically, anywhere low cost and simplicity matter more than precision.

Brushless DC Motors

Brushless motors do the same job as brushed ones but without any physical contact inside to switch the current. Instead of using brushes, they rely on electronics to control the motor.

A small circuit (called an ESC or motor controller) decides when to send current to the coils inside the motor. These coils are fixed in place, and the magnets spin around them. Sensors tell the controller the exact position of the spinning magnets, so it knows when to fire the coils in the right sequence to keep things turning smoothly.

It's like choreographing a dance — the controller knows exactly when to "step" to keep the motion flowing, and it does it all digitally, with no physical contact.

Where you'll see them

Drones, electric vehicles, high-speed robotics, CNC machines

Stepper Motors

Stepper motors rotate in small, precise steps like a clock ticking forward a fixed amount each time. Inside the motor are multiple coils (electromagnets) arranged in a circle. By turning these coils on and off in a specific order, the motor's center (the rotor) rotates a little bit at a time.

Each "step" is a tiny rotation — often something like 1.8° — and by counting those steps, you can control exactly how far the motor turns. No sensors needed, just careful counting. Think of it like walking up stairs: one step at a time, always the same size, and always in the right direction — as long as you don't trip.

The key difference between stepper motors and brushless DC motors is how they move and how they're controlled. Stepper motors move in fixed, predictable steps and don't need position feedback — you just tell them how many steps to take. Brushless DC motors, on the other hand, spin continuously and use sensors to track their position in real time, making them better for high-speed, smooth motion.

Where you'll see them

3D printers, laser cutters, camera sliders, pick-and-place arms

Servo Motors

A servo motor is basically a small DC motor that's been upgraded with a gear system, a position sensor, and its own built-in control circuit. You just tell it what angle to go to and it figures out how to get there.

The position sensor (usually a potentiometer in hobby servos, or a fancy encoder in industrial ones) constantly checks where the motor is and tells the control circuit if it needs to adjust. This feedback loop lets the servo move to and hold a specific angle, even if something tries to push it off target.

Most people are familiar with hobby servos, the small, rectangular ones you plug into an Arduino and control with a single wire. These are simple, affordable, and perfect for projects like small robot arms or RC car steering. But in industrial systems, servo motors can look more like fancy brushless motors with external controllers and high-resolution feedback. They're designed for much higher torque, speed, and precision, often paired with gearboxes and used in robotic arms, CNC machines, and automation lines.

It's kind of like comparing a toy drone to a commercial quadcopter. They work on the same idea, but one is designed for play, and the other is made to perform.

Where you'll see them

Hobby Applications: DIY robot arms, inexpensive camera rigs, model aircraft control surfaces

Industrial Applications: Manufacturing automation, CNC machines, precision positioning systems, professional camera systems

Linear Actuators

Linear actuators take the spinning motion of a motor and convert it into straight-line movement. This is usually done using a lead screw, belt drive, or sliding rail system. Instead of rotating something like a wheel or joint, they push or pull in a straight line (hence linear), so you can move things in and out, up and down, or back and forth.

Most linear actuators are actually powered by other motors (usually stepper or servo) which provide the precision needed to move to a specific distance and stop reliably. So the linear actuator isn't really a standalone motor type; it's more like a mechanical extension that transforms rotary motion into linear motion.

Where you'll see them

Z-axes in 3D printers/ CNC machines, Sliding camera rigs, automated rails

Harmonic Drive

Full disclosure: harmonic drive isn't a motor — it's a special type of gearbox often used with motors in precision robotics. I added them to this article because they work really well in robots (and I think they're neat). Unlike regular gear systems, harmonic drives can deliver extremely accurate motion, very high gear reduction, and almost zero backlashBacklash is a small gap or looseness between mechanical parts (in this case, the teeth of gears) that causes a delay when changing direction. It’s like turning something and having it not move right away because of a tiny bit of slack..

Instead of using rigid gears, harmonic drives use a flexible metal component (called a flexspline) that deforms slightly to engage with gear teeth in a unique way. This allows for very smooth, precise rotation, even under heavy load. Harmonic drives are most often paired with brushless DC motors, frameless motors, or servo motors, especially in robotic arms, humanoid joints, and surgical or aerospace robotics.

Where you'll see them

Industrial robotic arms, Space and satellite mechanisms, Surgical robots, Humanoid robots

Alright I threw a bunch of motor types at you but don't worry - you're not expected to memorize all of these (I still google them when I need to win an argument). The best way to learn more is to just start building stuff with motors and see how they work in practice. Start with a simple Arduino project with a servo or stepper motor and see how it works!

In the next lesson, we'll dig into encoders (the sensors that tell the robot where it is when and while a motor is moving).