Paper: Principles of Automatic Control

Automatic control instruments have a great deal in common with economic principles; they deal with supply and demand. Considering that an upset in a process is a change in demand, an automatic controller must be capable of changing the supply to re-establish process balance; therefore, automatic control may be defined as balancing supply against demand over some period of time. The period of time involved in making the supply equal to the demand may vary widely, and is primarily a function of process conditions.