motor driver / electronic speed controller (ESC)
A motor driver is the power-handling chip-and-circuit board that sits between a robot's brain and its motor. The controller is a delicate thinker that can only whisper tiny signals; a motor is a thirsty muscle that gulps far more current than those signals could ever carry. The driver is the strong middleman that takes the brain's gentle commands and pours the right amount of electrical muscle into the motor to obey them.
It does this with fast electronic switches. To set speed, it uses PWM — pulse-width modulation — flicking the power fully on and off thousands of times a second; the longer each on-pulse lasts, the more power the motor feels on average, like dimming a light by blinking it very fast. To set direction, many drivers use an H-bridge, a clever switch arrangement that can flip which way current flows so the motor can spin forward or backward.
For a brushless motor, the driver has a harder job called commutation: it must constantly figure out where the rotor is and energize the coils in just the right rhythm to keep it spinning — this electronic version of a motor driver is what hobbyists call an ESC, or electronic speed controller, the little board that makes drone propellers howl. In every case the driver also protects itself and the motor, guarding against too much current or heat.
A drone's flight computer just says 'spin faster' to each ESC; the ESC works out the rapid switching needed to feed the brushless motor and the propeller speeds up — the brain never touches the heavy current itself.
The muscle between brain and motor: it turns tiny commands into real current.
PWM controls how much power flows by switching it on and off very fast; an H-bridge flips the current's direction; commutation is the extra timing trick a brushless motor needs.