Overview

The word control comes from the Old French word contrerolle, which has to do with auditing a bookkeeping account (contre meaning counter plus rolle meaning account). Indeed, we still have the Office of Controller (or Comptroller from the same root), which directs audits. A current general meaning for control is "to exercise restraining or directing influence over: Regulate." Turning to regulate, we find that the Latin meaning was to rule, but a modern meaning is "to bring order, method or uniformity to" (Webster's New Collegiate Dictionary, 1973).

Any process requires equipment. Discrete processes require equipment that is quite different from the fluid processes of continuous and batch, so discrete processes will not be discussed further. The equipment definitely needs to have order, method, and uniformity brought to it so that it can produce repeatable, predictable, and consistent results. Automated equipment also must be able to take direction so that it can be commanded to perform its processing properly and be restrained from doing something unwanted. In short, process equipment needs control to be useful.

About 70% of continuous process control affects fluid flow. Batch process control is about 70% control of block valve position, but it does use flow control. Batch processing was still done with bucket and paddle operations when continuous processes began demanding better flow control. An early example comes from electric utility boiler control eighty years ago, as Charlie Smoot tells in his story of the founding of Smoot Controls...