Actuators and motion

Making something move.

What moves things

Electric motors dominate, with servos for position and steppers for repeatable increments. Hydraulics give large forces and pneumatics give compliance and speed.

Gearing trades speed for torque, as everywhere else in mechanics.
Backlash and flexing mean the joint is never exactly where the motor thinks.

Power

Battery capacity limits mobile robots more than any algorithm. Legged robots in particular spend most of their energy simply standing up.