Manual on Installation of Refinery Instruments and Control Systems - Part I - Process Instrumentation and Control Section 5 - Automatic controllers (Third Edition)
This section discusses the installation and application of automatic controls. Because installation details will vary with the application@ much of this section will be general in scope. Installation details@ although discussed in general@ should rightfully be referenced to RP 550@ Manual on Installation of Refinery Instruments and Control Systems@ Part I-Process Instrumentation and Control@ Section 7-Transmission Systems. The material presented applies to both pneumatic and electronic controllers. It should be noted that although there are some physical differences between pneumatic and electronic instruments@ these differences do not change the basic requirements for a good installation. Users should install all control instruments in accordance with the manufacturer's recommendations and such additional requirements as are needed@ in the judgment of the design engineer@ to satisfy the particular conditions of the individual installation. The function of the automatic controller in the process control loop is to compare the measured process variable with the desired value of the variable@ called the set point@ and generate an error signal. This error (deviation) signal produces a change in controller output which repositions the final control element to move the process variable to the set point. The error signal is thereby minimized or reduced to zero.