A
stepper motor is a kind of
electric motor where an internal rotor containing permanent
magnets is controlled by a set of external magnets that are switched electronically. A stepper motor is a cross between a
DC electric motor and a
solenoid. Simple stepper motors "
cog[?]" to a limited number of positions, but proportionally controlled stepper motors can rotate extremely smoothly. Computer controlled stepper motors are one of the most versatile forms of
positioning systems[?], particularly when digitally controlled as part of a
servo system[?].