EASA / CAR Part-66 · Module 4 · 4.3 Servomechanisms · Cat B1/B2
A servo drives an output to match a commanded position. The controller compares demand with feedback, and the error drives the motor. Raise the gain to respond faster, but too much causes overshoot and oscillation.
Feedback subtracts the actual output from the demand to make the error signal; the controller amplifies it to drive the output the right way. Low gain is slow and lags; high gain is fast but overshoots and rings before settling. This is how flight-control actuators and autopilots hold a position.