Analysis of Control Systems Involving Digital Computers

Digital computers have recently been used as elements in control systems employing otherwise conventional elements. The fact that data passing through the digital computer of the usual type must be sampled is an important property of such mixed systems. The description of the processes of sampling and desampling, as well as operations performed by the digital computer, in the frequency domain permits the extension of conventional servo-mechanisms analysis to mixed systems. The purpose of this article is an introduction to dealing with the unconventional parts of the system. Sampling is shown to be analogous to amplitude modulation by the continuous data of a carrier of unit impulses, while desampling is analogous to ripple filtering. Linear operations of the digital computer on sampled data are described by transfer functions. An illustrative example of computer program design in the frequency domain is given, and the applicability of these methods is indicated.