Robot Motors & Actuators
Actuators are what give a robot its physical capability to act on the world. They convert electrical energy into mechanical motion. Getting the right actuator for the job is critical — the wrong choice leads to a robot that's too slow, too weak, too imprecise, or too power-hungry. Let's walk through every type you'll encounter.
1. Servo Motors — Precision Positioning
A servo motor is a motor with a built-in position sensor (usually a potentiometer) and feedback control. You send it a target angle, and it drives itself to that angle and holds it.
How servos work
You control a servo with a PWM (Pulse Width Modulation) signal. A pulse of 1ms means "go to 0°", 1.5ms means "go to 90°", and 2ms means "go to 180°". The servo's built-in controller reads the pulse width, compares the target to the current sensor reading, and drives the motor until they match. Simple and reliable.
Hobby servos vs. digital servos
Hobby servos (like the SG90 or MG996R) are cheap ($2–15), easy to use, and found in every robot arm kit. Digital servos have a faster update rate and hold their position more firmly under load — they cost more but are essential for precision applications.
When to use servos
Simple robotic arms, pan-tilt camera mounts, steering systems, gripper fingers. Any application where you need to hold a specific angle, don't need continuous rotation, and can work within the servo's typical torque range (0.5–20 kg·cm).
2. Stepper Motors — Precise Open-Loop Control
A stepper motor moves in discrete "steps". Send it 200 pulses, and it rotates exactly 360°. No position sensor needed — the steps themselves are the position.
How steppers work
Inside a stepper motor are multiple electromagnetic coils arranged around the rotor. By energizing the coils in sequence, you attract the rotor to rotate a small fixed amount — typically 1.8° per step. A stepper driver (like the A4988 or TMC2209) handles the coil sequencing; your microcontroller just sends step pulses and direction signals.
Microstepping
By partially energizing two coils simultaneously, drivers can achieve subdivisions of the full step — 1/2, 1/4, up to 1/256. A motor with 200 full steps can become 51,200 microsteps per revolution. This dramatically smooths movement and reduces noise, which is why 3D printers and CNC machines sound different from a hard-stepping machine.
When to use steppers
3D printers, CNC routers, camera sliders, pick-and-place machines. Any application needing precise positioning without a feedback sensor. Warning: steppers lose steps if overloaded — they have no idea if the commanded step actually happened.
3. Brushless DC Motors — High Speed & Efficiency
Brushless DC (BLDC) motors have no brushes to wear out, making them more efficient, faster, and longer-lived than brushed motors. They're the default choice for drones, electric vehicles, and high-performance robot joints.
How BLDC motors work
The electromagnets are on the outside (stator), and the permanent magnets are on the inside (rotor). The controller (an ESC — Electronic Speed Controller) rapidly switches which stator coils are energized to chase the rotating magnetic field. With no physical brushes, there's no friction, no sparking, and no wear from contact.
Using BLDC with encoders
BLDC motors alone only control speed, not position. Add a magnetic encoder (like the AS5600) to the shaft, and you can implement closed-loop position control. This is how modern robotic joints (like in Unitree's quadruped robots) achieve the combination of high torque, high speed, and precise positioning.
When to use BLDC
Drone propellers, electric vehicle wheels, high-speed conveyor belts, advanced robot joints. Anywhere you need high speed (1,000–50,000 RPM), high efficiency, or continuous operation. They require more complex control electronics than servos or steppers.
4. Hydraulic & Pneumatic Actuators — Raw Power
When you need forces that electric motors can't produce — lifting heavy loads, driving excavator arms, powering robot legs that can jump — hydraulics and pneumatics step in.
Hydraulic actuators
Use pressurized oil to push pistons. The force output scales with pressure × piston area — hydraulics can generate enormous forces from small actuators. Boston Dynamics' early Atlas robot used hydraulics to achieve its jaw-dropping jumping ability. The downside: heavy pumps, oil leaks, and complex plumbing.
Pneumatic actuators
Use compressed air instead of oil. Lighter and cleaner than hydraulics, but lower force and harder to control precisely (air is compressible). Widely used in industrial grippers — a pneumatic gripper can open and close thousands of times per day without maintenance.
The industry trend
The robotics industry is moving away from hydraulics toward high-torque electric actuators. Boston Dynamics' newer Atlas robot is fully electric. The reason: electric actuators are quieter, cleaner, more efficient, and easier to control precisely with modern motor drivers and AI.
Frequently Asked Questions
What's the best first motor to learn with?
A hobby servo (SG90 or MG996R). They're $3–8, connect directly to Arduino, and you can control them with a single line of Arduino code. They teach the concept of PWM control, position feedback, and torque limits in a very forgiving package.
What's the difference between torque and speed?
Torque is rotational force — the ability to overcome resistance. Speed is how fast the motor spins. There's always a trade-off: a motor with more gear reduction has more torque but less speed. Choosing the right motor + gearbox combination for your load is a core mechanical engineering skill in robotics.
What is back-EMF and why does it matter?
When a DC motor spins, it generates its own voltage (back-EMF) opposing the supply. This is actually useful — at high speeds, back-EMF limits current and prevents the motor from burning out. But when you suddenly cut power or reverse direction, the stored energy needs somewhere to go. Flyback diodes and motor driver circuits handle this safely.
How do I calculate what motor I need?
Calculate the torque your load requires (mass × gravity × arm length for a lifting application), add a 2× safety margin, then find a motor+gearbox combination that meets it. For speed, work backwards from your desired RPM to the motor spec before the gearbox. Many motor vendors have online calculators.
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