“Servo motors don’t just run and stop when they’re told to,” says Kirk Gordon, a servo expert.

“The servo controllers do a constant, non-stop comparison of where the motors are (based on the feedback system), and where they’re supposed to be and are continuously sending and varying (and reversing) power to the motors to keep the results of the comparison within specification.

Among the different kinds of comparisons are drift and tracking error. Among the variables that are used to maintain servo performance characteristics are feedback gain, ‘hunting’ speed and current limit.”

In well designed, well built systems, servos are live and supple, capable of astonishing speed, precision, and reliability, even under an amazing variety of conditions. Good systems are also extremely stable over long periods of time.