PL C

1

A programmable logic controller (PLC) is a solid- state device designed to perform logic functions previously accomplished by electromechanical relays .

The PLC is an assembly of solid-state digital logic elements designed to make logical decisions and provide outputs.

It used for control and operation of manufacturing process equipment and machinery

2

It has been designed to operate in the industrial environment and is equipped with special input/output interfaces and a control programming language

It is capable not only performing relay switching tasks, but also counting, calculating, comparing and the processing the analog signals.

3

It’s eliminated much of the hand wiring associated with conventional relay control circuit.

It’s small and inexpensive compared to equivalent relay-based process control systems.

Capable of simulating a hundred relay, timers and counters.

Easy to program and install Hardware features such as keylocks can restrict

the access to PLC, and software features such as passwords.

It does can be designed with communications capabilities that allow them to converse with other computer systems or to provide human interfaces.

4

5

PLC Diagram

PLC can be divided into parts: The central processing unit (CPU) The input/out (I/O) sectionThe power supply The programming device.

6

CPU - is the ‘brain’ of the PLC. It consists of a microprocessor for

implementing the logic and controlling the communications among the modules.

Required memory for storing the results of the logical operation performed by the microprocessor.

The CPU is designed so that the user can enter the desired circuit in ladder logic.

Power Supply – used to supplies dc power to other modules that plug in rack and other field devices

7

Memory Module – used to control input and output of the process.

Programming Device - are used to enter the desired program into the memory of the processor. This program is entered using relay ladder

logic. The program determines the sequence of operation and ultimate control of the equipment or machinery.

8

9

Hand-held with LED display

Industrial terminal

Software written and run on PC has changed how people work with PLCs. PLC software run on a PLC falls into the following two categories:

PLC software that allows the user to program and gives the user the tools to write a PLC program using ladder logic or other programming language and document or explain the program is as much detail as is necessary.

PLC software that allows the user to monitor and control the process is also called man-machine, or operator, interface. It enables the user to view a process or graphical representation of a process on a CRT, determine how the system is running, trend values, and receives alarm conditions.10

PLC programming language refers to the method by which user communicates information to the PLC.

There are three most common languages: Ladder diagram language

The most common used by PLC language. Boolean language

The statements refers to the basic AND, OR and NOT logic gate function.

Function chart systemIt is a method of programming a control system that uses a more structured approach.

11

PLCs are divided into three major size categories: Small: It covers units up to 128 I/Os and

memories up to 2Kb. It capable of providing simple to advanced levels of machine control.

Medium: It have up to 2048 I/Os and memories up to 32Kb. Special I/Os modules make medium PLCs adaptable to temperature, pressure, flow, weight, position and any type of analog function encountered in process control applications.

Large: The most sophisticated units of PLCs family. They have up to 16,000 I/Os and memories up to 2Mb. It has almost unlimited applications and can control individual production processes or entire plants.

12

There are three major types of PLC applications: Single-ended: It involves one PLC controlling one

process. This would be a stand-alone unit and would not be used for communicating with other computers or PLCs.

Multitask: It usually calls for a medium size PLC and involves one PLC controlling several processes. It can be a subsystem for larger processes and communicating with a central PLC.

Control management: It involves one PLC controlling several others. It requires a large PLC processor designed to communicate with other PLCs and possibly with a computer. The control management PLC supervises several PLCs by downloading programs that tell the other PLCs what has to be done.

13

Some important characteristics distinguish PLCs from general-purpose computers.

The PLC is designed to operate in the industrial environment with the wide range of ambient temperature and humidity.

A well designed PLC is not affected by the electrical noise inherent in most industrial locations

Hardware and software of PLCs are designed for easy use by plant electricians and technicians

Unlike the computer, the PLC is programmed in relay ladder logic or other easily learned languages

The PLC comes with its program language built into permanent memory, whereas a personal computer requires a disk operating system (DOS). PLC is limited by the language it comes with, unless it is a modular type 14