actuator

An actuator is the part of a robot that actually moves things. It takes in some form of energy — usually electricity, but sometimes pressurized air or oil — and turns it into controlled mechanical motion: a wheel that spins, an arm that lifts, a gripper that squeezes shut. If sensors are a robot's eyes and the computer is its brain, the actuator is its muscle. Nothing a robot decides to do happens until an actuator carries it out.

The most common kind is the electric motor, which makes a shaft rotate. But "actuator" is a broad word, and the family is large: rotary ones that turn (motors of every flavor — DC, brushless, stepper, servo), and linear ones that push and pull in a straight line (hydraulic and pneumatic cylinders, screw drives, artificial muscles). The key idea is the word "controlled." A firework also turns energy into motion, but you cannot steer it. An actuator is built so a controller can dial its motion up, down, and to a chosen position on demand.

Choosing an actuator is a balancing act among several wants that pull against each other: how much force or twist it can deliver, how fast it moves, how precisely it can be positioned, how much it weighs, how much power it draws, and how much it costs. A heavy industrial arm welding cars needs raw strength; a tiny drone needs lightness and speed; a surgical robot needs delicacy. There is no single best actuator, only the right one for the job.