Programmable Logic Controllers

Programmable Logic Controllers (PLC)

Programmable Logic Controllers (PLC)

Programmable logic controllers are the most widely used electronic devices in the control of production and assembly process in most automated factories due to its simplicity and versatility.

A programmable logic controller (PLC) is a user-friendly, microprocessor-based, specialized computer carrying out control functions of many types and levels of complexity in industrial applications.

Programmable Logic Controllers (PLC)

It would be appropriate to think of the PLC as a special-purpose computer. Nevertheless, a programmable logical controller has many characteristics similar to those of a general-purpose computer.

It has a power supply, main processor, memory, inputs and outputs, possibly some peripheral equipment.

Programmable Logic Controllers (PLC)

According to National Electrical Manufacturers Association a PLC is:

A digital electronic device using a programmable memory to store

instructions and to implement specific functions such as logic, sequence,

counting, timing and arithmetic to control machines and processes.

Brief Historical Background

Before the PLCs, automated manufacturing processes had to be controlled using hardware devices such as rotating cams gears, levers and other basic mechanical devices.

As the complexity grew, so did the need for a more sophisticatedcontrol system. This system contained wired relay and switch control elements. These elements were wired as required to provide the control logic necessary for the particular type of machine operation.

This was acceptable for a machine that never needed to be changed or modified, but as manufacturing techniques improved and plant changeover to new products became more desirable and necessary, a more versatile means of controlling this equipment had to be developed.

Brief Historical Background

Hardwired relay and switch logic was cumbersome and time consuming to modify. Wiring had to be removed and replaced to provide for the new control scheme required. This modification was difficult and time consuming to design and install and any small "bug" in the design could be a major problem to correct since that also required rewiring of the system.

However in mid 1970s advances in micro-electronics have made a revolution in industrial control. Electronic programmable controller has replaced relay-based, hard-wired electrical systems.

The first PLC systems evolved from conventional computers in the mid 1970s and were mostly used in automotive industry

Brief Historical Background

Communications abilities began to appear in approximately 1973. The PLC could now talk to other PLCs and they could be far away from the actual machine they were controlling.

Unfortunately, the lack of standardization coupled with continually changing technology has made PLC communications a nightmare of incompatible protocols and physical networks.

The 80's saw an attempt to standardize communications with General Motor's manufacturing automation protocol(MAP). It was also a time for reducing the size of the PLC and making them software programmable through symbolic programming on personal computers instead of dedicated programming terminals or handheldprogrammers. Today the world's smallest PLC is about the size of a single control relay!

In the late 1980s, PCs have been begun to be used in automatic industrial control, and this led a competition between PLCs and PCs. What distinguishes a PLC from a PC are majorly: PLCs are constructed for a specific purpose with

a smaller program, whereas PCs perform general tasks.

PLCs are sequential-type controllers which can process individual steps in a program in order.

PC vs. PLC

Both PLCs and PCs have their particular strengths and weaknesses indeed.

In general, what is seen and done in industrial control of today is that:

PLCs do majority of the control jobs.

PCs handle most of the data and math functions.

PC vs. PLC

PLCs can be programmed, controlled and operated by a person unskilled in operating computers.

The PLC can operate any system that has input/output devices that go on and off ass well as any system with variable input/outputs.

PC vs. PLC

Advantages & Disadvantages of PLCs

Advantages of PLCs: Increase in flexibility, Faster implementation of changes and correction, Lower cost, Easy visualization of process running, Increased visual observations, Increased operation speed, Increased reliability and maintainability, Increased security, Reprogramming capability, Elimination of wiring.

Advantages & Disadvantages of PLCs

On the contrary the disadvantages are:

Fixed program applications, High initial investment cost.

A trade off should be done between the advantages and disadvantages of the PLCs before installing them to the production system.

As level of automation and computer programming in manufacturing increases, using PLC control instead of mechanical systems becomes more feasible.

Advantages & Disadvantages of PLCs

PLC ConfigurationBasic PLCs are available on a single printed circuit board. They are sometimes called single board PLCs or open frame PLCs.

PLCs are also available housed in a single case (sometimes referred to as a shoe box) with all input and output, power and control connection points located on the single unit.

PLC ConfigurationMore sophisticated units, with a wider array of options, are modularized.

The modularized PLC consists of:

CPU module Input and Output

modules(I/O modules) A power supply module Peripheral equipments Programming Unit

Parts of the PLCs

CPU Module:This module consists of a central processing unit (CPU) which acts as the brain of the system and its memory

Parts of the PLCs

Input/Output Modules(I/O Modules):

I/O modules allow the PLC to read sensors and control actuators. There exists a wide variety of I/O module types offered by the PLC manufacturers.

Parts of the PLCs

Power Supply Module:

Power supply module provides power to the CPU and often provides power to drive sensors and low power actuators connected to I/O modules.

Parts of the PLCs

Peripheral Equipments:They are used for purposes such as preparing,

storing and loading control programs,system monitoring and even can be used for communicating with other computers with which the PLC may be networked. Some examples can be;

Racks and chassis for mounting the other three parts.

Programmer/Monitor(PM) which is used toprogram instructions and monitor them,

Parts of the PLCs

Programming UnitThe programming unit allows the engineer or technician to enter and edit the program to be executed. In it's simplest form it can be a hand helddevice with a keypad for program entry and a display device (LED or LCD) for viewing program steps or functions, as shown in Figure. More advanced systems employ a separate personal computer which allows theprogrammer to write, view, edit and download the program to the PLC.

System Block Diagram

Programmable Logic ControllersProgrammable Logic Controllers (PLC)Programmable Logic Controllers (PLC)Programmable Logic Controllers (PLC)Programmable Logic Controllers (PLC)Brief Historical BackgroundBrief Historical BackgroundBrief Historical BackgroundIn the late 1980s, PCs have been begun to be used in automatic industrial control, and this led a competition between PLCs anBoth PLCs and PCs have their particular strengths and weaknesses indeed.In general, what is seen and done in industrial contPLCs can be programmed, controlled and operated by a person unskilled in operating computers.Advantages & Disadvantages of PLCsAdvantages of PLCs:Advantages & Disadvantages of PLCs On the contrary the disadvantages are:A trade off should be done between the advantages and disadvantages of the PLCs before installing them to the production systePLC ConfigurationPLC ConfigurationParts of the PLCs CPU Module:Parts of the PLCs Input/Output Modules(I/O Modules):Parts of the PLCs Power Supply Module:Parts of the PLCs Peripheral Equipments:Parts of the PLCs Programming UnitSystem Block Diagram