Probabilistic error as a measure of control-system performance

AUTOMATIC FEEDBACK-control system synthesis occupies a rather unique place among the various fields of engineering due to the indirectness of its approach. An antenna will be designed for a certain radiation pattern because this radiation pattern is called for in the intended application. On the other hand, a servomechanism which is destined to handle step inputs exclusively may be synthesized by the techniques of Nyquist, Nichols, or Bode. This means that the system is actually designed for a frequency response: to exhibit certain resonance characteristics when subjected to steady-state sinusoidal inputs of varying frequency. Yet the system will never be subjected to this latter type of input. The justification of this approach is that the experience of the designer provides the basis for expecting that the system will perform properly in its actual input environment if it is built for a specific frequency response. Such examples of indirectness could be enumerated almost endlessly.