A servo system = servo drive + servo motor + feedback sensor (usually an optical encoder on the motor). The drive forms position and speed loops from encoder feedback and adjusts motor current to track the setpoint.

AC servo: high accuracy via encoder, smooth at low speed, constant torque to rated speed then constant power, strong overload (peak ~3× rated), fast response. DC servo: simple control, good low-speed torque, but commutator/brush limits life and speed.

Control loops: current (torque) loop innermost, then speed loop, then position loop. Closed loop eliminates lost steps and overshoot.

Resolution: a 2500-line encoder with 4× quadrature gives 10000 pulses/rev (0.036°); a 17-bit encoder gives 131072 pulses/rev (0.0027°). Higher resolution improves positioning accuracy.

Overload: AC servo sustains short overloads (e.g. 3× rated torque) to overcome inertial start-up torque.

Response: AC servo accelerates to rated speed in milliseconds — ideal for rapid start/stop.

Educational notes. For design use manufacturer datasheets and the applicable standards above.