TABLE OF CONTENTS The two basic modes of process control are sequential control, or On/Off action, and continuous control action. This chapter deals with process control, discussing both sequential and continuous control. Sequential control is an event-based process, in which one event follows another until a process sequence is complete. Many process control functions are sequential.
Continuous control requires continuous monitoring of the process variables, so that they can be held at set levels. Process control systems vary extensively in complexity and industrial application. For example, controllers in the petrochemical industry, automotive industry, and soda processing industry have completely different types of control functions. The control loops can be very complex, requiring microprocessor supervision. Some of the functions need to be very tightly controlled, with tight tolerance on the variables and a quick response time, while in other areas, the tolerances and response times are not so critical. These systems are closed loop systems. The output variable level is monitored against a set reference level, and any difference detected between the two is amplified and used to control an input variable that will maintain the output at the set reference level.
There are many processes in which a variable does not have to be controlled, but a sequence of events has to be controlled. The sequential process, or batch process, can be event-based, time-based, or a combination of both. An event-based system is one in which the occurrence of an event causes an action to take place. For...
