Hello again, mImoto,
A servo system is a closed loop control system; that is, a system that uses some sort of sensor to correct the actions on a controlled variable in order to maintain it at a preset value. When you talk about a servo system, you usually are talking about a position control system. The most simple servo systems are implemented using a potentiometer as position sensor, that way you could process all the signals, SetPoint, ActualPosition, Error, and CommandOutput in analog form, using op amps for example.
Best regards.