Analog or digital computer for process control?

Although much has been written about the pushbutton or computer-controlled factory, there has been little discussion of the relative merits of analog and digital computers for control applications. Engineers familiar with computers are aware of the advantages and disadvantages of analog and digital computers as tools for scientific investigations. They have probably not considered their suitability for process control applications. In this paper, we will: 1) Outline some of the tasks that will probably be assigned to a process control computer; 2) State some of the questions that must be answered before any computer is selected; 3) Review some of the characteristics of analog and digital computers with special reference to process control requirements; 4) Describe some systems already installed or about to be installed, in which computers are used. Following this discussion, it will be evident, as might be anticipated in advance, that each type of computer has characteristics which make it preferable for particular kinds of applications. Analog computers are fast, simple, and inexpensive for relatively small control jobs, while digital computers have accuracy, versatility, and flexibility which adapt them to more complex control jobs.