Introduction to Process Control ------------------------------------- Introduction To study the subject of industrial process control effectively, you must first gain a general understanding of its basic principles. To present these control principles clearly and concisely, an intuitive approach to process control is used. First, however, some basic definitions and concepts of pro- cess control are presented. Definition of Process Control The operations that are associated with process control have always existed in nature. Such “natural” process control can be defined as any operation that regulates some internal physical characteristic that is important to a living organism. Examples of natural regulation in humans include body temperature, blood pressure, and heart rate. Early humans found it necessary to regulate some of their external envi- ronmental parameters to maintain life. This regulation could be defined as “artificial process control” or more simply as “process control,” as we will refer to it in this book. This type of process control is accomplished by observing a parameter, comparing it to some desired value, and initiating a control action to bring the parameter as close as possible to the desired value. One of the first examples of such control was early man’s use of fire to maintain the temperature of his environment. The term automatic process control came into wide use when people learned to adapt automatic regulatory procedures to manufacture products or pro-cess material more efficiently. Such procedures are called automatic because no human (manual) intervention is required to regulate them. All process systems consist of three main factors or terms: the manipu- lated variables, disturbances, and the controlled variables (Figure 1-1). Typical manipulated variables are valve position, motor speed, damper position, or blade pitch. The controlled variables are those conditions— such as temperature, level, position, pressure, pH, density, moisture con- tent, weight, and speed—that must be maintained at some desired value. For each controlled variable there is an associated manipulated variable. The control system must adjust the manipulated variables so the desired value or “set point” of the controlled variable is maintained despite any disturbances.
Posted on: Sat, 15 Mar 2014 04:59:40 +0000
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