Theory of servo systems, with particular reference to stabilization

A servo system may be termed an error-actuated automatic control system, or a closed-cycle control. The basic principle finds wide application: for the automatic close regulation of speed, voltage, mechanical tension, etc., and for the remote-position control of gun mountings and the like. One of the major problems in this field is that of achieving an adequate degree of stability; it is shown that instability is mainly caused by response time-lags in the system, and by the basic control characteristic of the servo. It is already known that the addition of controlling signals proportional to the derivatives of error can theoretically produce stability. Such derivatives are seldom available; hence attention is chiefly directed to: (a) Passive networks, which when placed at the input end of the servo give approximations to derivatives and/or integrals of error, so that the system performance is suitably modified. (b) Feedback methods, which achieve results similar to (a), but often have practical advantages. To facilitate calculations and permit the ready determination of constants of the added stabilizing networks, ¿standard forms¿ are given, corresponding to linear differential equations up to the sixth order. These forms are not claimed to be of universal application, but they have been found to apply to widely different electric servos.