James H.M., Nichols N.B., Phillips R.S. (ed.) Theory of Servomechanisms

New York: McGraw-Hill, 1947. — 375 p. — (MIT Radiation Laboratory Series. Volume 25)It is nearly as hard for practitioners in the servo art to agree on the definition of a servo as it is for a group of theologians to agree on sin. It has become generally accepted, however, that a servo system involves the control of power by some means or other involving a comparison of the output of the controlled power and the actuating device. This comparison is sometimes referred to as feedback.
A servo system is a combination of elements for the control of a source of power in which the output of the system or some function of the output is fed back for comparison with the input and the difference between these quantities is used in controlling the power.