7 - PLC systems

Abstract

A PLC (programmable logic controller) is a microprocessor-based system that uses a programmable memory to store instructions and implement function such as logic, sequencing, timing, counting, and arithmetic in order to control machines and processes. The designers of the PLC have pre-programmed it so that the control program can be entered using a simple and intuitive form of language. Input devices, for example sensors such as switches, and output devices in the system being controlled, like motors, valves, etc., are connected to the PLC. The operator then enters a sequence of instructions, that is, a program, into the memory of the PLC. PLCs have the great advantage that the same basic controller can be used with a wide range of control systems. The five basic components of PLC are the processor unit, memory, the power supply unit, input/output interface section, and the programming device. The basic form of programming used with PLCs is ladder programming. Different PLC manufacturers deal with counters in slightly different ways. The case studies are intended to illustrate the application of the PLC programming techniques given in this chapter.