TABLE OF CONTENTS Modern industrial control systems are microprocessor-based programmable systems containing hardware and software for direct digital control, distributed control, programmable control, and PID action. The systems are designed not only for continuous monitoring and adjustment of process variables, but also for sequential control, which is an event-based process, and alarm functions. This chapter discusses the Programmable Logic Controller (PLC), its operation, and the use of the PLC for sequential, continuous control, and alarm functions.
Prior to the PLC, standalone devices, such as indicators, controllers, and recorders, were used for monitoring and control. These devices are still in use in small operations, but are not cost-effective, and are unsuitable for modern control requirements [1].
The processor in a PLC system has software that is easily programmable and flexible, making the initial program, updates, modifications, and changes easy to implement. Because of the complexity and large number of variables in many process control systems, microprocessor-based PLCs are used for decision making. The PLC can be configured to receive a small number of inputs (both analog and digital), and control a small number of outputs. The system also can be expanded with plug-in modules to receive a large number of signals, and simultaneously control a large number of actuators, displays or other types of devices. PLCs are categorized into low-end, midrange, and high-end, where low-end is from 64 expandable up to 256 I/Os, midrange is expandable up to 2,048 I/Os, and high-end is expandable up to 8,192...
