1 - Programmable logic controllers

Abstract

This chapter presents an introduction to the programmable logic controller, its general function, hardware forms, and internal architecture. A programmable logic controller (PLC) is a special form of microprocessor-based controller that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting, and arithmetic to control machines and processes. It is designed to be operated by engineers with perhaps a limited knowledge of computers and computing languages. Input devices and output devices in a system being controlled are connected to the PLC. The operator enters a sequence of instructions, that is, a program, into the memory of the PLC. The controller then monitors the inputs and outputs according to this program and carries out the control rules for which it has been programmed. PLCs are now widely used and extend from small self-contained units for use with perhaps 20 digital inputs/outputs to modular systems, which can be used for large numbers of inputs/outputs, handle digital or analogue inputs/outputs, and also carry out proportional-integral-derivative control modes. A PLC system has the basic functional components of a processor unit, memory, power supply unit, input/output interface section, communications interface, and programming device. It consists of a central processing unit (CPU) containing the system microprocessor, memory, and input/output circuitry. The CPU controls and processes all the operations within the PLC.