TABLE OF CONTENTS The word loop is often used in discussion about process control. A loop, the basic unit of process control, typically consists of one process measurement, one set point, and one output.
A process usually has more than one loop, and many processes used in industry have thousands of loops. Some of these loops are independent of other loops; some loops are interconnected with other loops.
A variable that is measured by the process instrumentation is usually known as the measured variable or process variable. Examples of measured variables are liquid and gas flows, levels, temperatures, and pressures.
Some variables are not directly measured from the process but are calculated by the control system (usually using measured variables). Examples of calculated variables are differential temperature (calculated by subtracting two measured temperatures), and a flow ratio (calculated by dividing two measured flows).
Each loop has a measured (or calculated) variable. These variables are usually measurements made from the process using sensors or transmitters. Variables can be classified based on how they are used in the control system.
A controlled variable is held by the control system at either a target or set point. The primary function of a control system is to control these variables.
For every controlled variable, at least one must be manipulated, that is, directly adjusted by the controller or the operator. The manipulated variable is usually a flow that passes through a control valve or some other final...
