Basic Principles of Automatic Control Systems

The importance of automatic control systems in a wide field of industrial and military applications has been accentuated during the past few years. The object of this paper is to review some of the basic principles of the subject, with special reference to automatic regulating systems. A major difficulty at present is the lack of standardization of the terminology associated with automatic control systems; in this paper an attempt has been made to co-ordinate, albeit on a small scale, generally recognized industrial usage with the recommendations of the Ministry of Supply Servo Nomenclature Panel. It is convenient, for example, to consider an automatic regulator system as a special type of servo-system. In order to present the nomenclature in a manner which may be readily assimilated, the operation of a simple automatic speed regulator is described in detail. The performance of an automatic control system is usually assessed by ( a) the speed of response of the system subsequent to a sudden disturbance, ( b) the nature of the response, and ( c) the magnitude of the steady-state errors. In the case of complex control systems, such as fire-control systems, it is sometimes desirable to study, in addition, the frequency response characteristics of each main element, and to determine the overall performance of the system by the application of vector methods. The latter have been used widely in the solution of acoustical problems, and in the design of electronic feed-back amplifiers, and their adaptation to the analogous problems of servo-system design has considerably facilitated progress. The stabilization of automatic control systems, and the elimination of steady-state errors can often be achieved by the incorporation of subsidiary feed-back loops. For example, a comparison of the basic operation of a typical position control servo-system with that of a typical automatic regulator shows that certain lags in the operation of the latter can be short-circuited by introducing “disturbance feed-back”. This approach to the problem of improving the performance of these systems does not appear to have been treated extensively in the literature. Its value is demonstrated in the paper by comparing the responses of certain idealized automatic thermal regulating systems, some incorporating “disturbance feed-back”, and others with straightforward controllers.