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A servo motor is controlled by counting light pulses inside the servo. A light inside the servo shines on a disk with slits in it and a photocell recieves the light through the slits as the motor rotates. The photocell counts the flashes of light and sends that signal to the motor controller. The controller counts the pulses and adjusts the speed based on the operators required inputs. A stepper motor is controlled by counting magnetic pulses from a magnet and pickup mounted on a motor. A controller counts the pulses and adjusts the speed based on the operators required inputs.
The basic difference between a traditional stepper and a servo-based system is the type of motor and how it is controlled. Steppers typically use50 to100 pole brushless motors while typical servo motors have only4 to12 poles. A pole is an area of a motor where a North or South magnetic pole is generated either by a permanet magnet or by passing current through the coils of a winding.
Steppers don't require encoders since they can accurately move between their many poles whereas servos, with few poles, require an encoder to keep track of their position. Steppers simply move incrementally using pulses [open loop] while servo's read the difference between the motors encoder and the commanded position [closed loop], and adjust the current required to move.