Time-lag in a control system

Abstract

It often happens in physical experiments and in technical processes that there is some physical quantity, subject to random disturbances, which it is required to keep as nearly constant as possible by the operation of some controlling gear. For example, it may be required to keep a room, or a reaction vessel, at a constant temperature by controlling the electric current passing through a heating coil, or steam passing through heating pipes, or the operation of a heating or cooling engine. The operation of the controlling gear could be by trial and error, but it may often be desirable that it shall be made to depend in some definite way on the behaviour of the physical quantity to be controlled ; the determination of such behaviour and the consequent controlling action may each be carried out either by an operator or by automatic means. It is convenient to make a distinction between the element (such as rheostat or steam valve) whose setting directly affects the physical quantity concerned, and the apparatus employed (if any) to make appropriate adjustments of this setting. We will call the former the “ controlling gear ” and the latter the “ control apparatus ” and the two together the “ control system ”.