Page 307 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

MODULE 19 : PLC & Electronic pneumatic

LESSON 180 - 187 : PLC

Objectives

At the end of this exercise you shall be able to
• state about PLC
• state the design, working principle & application of PLC.

Introduction
PLC is a digital computer used for automation of electromechanical processes in plants. The PLC is designed for
multiple inputs and outputs arrangements, so we can get the data from the sensors, work with it and command
the actuators.
Evolution of control technology
The first Programmable Logic Controller (PLC) was developed by a group of engineers at General Motors in 1968,
when the company were looking for an alternative to replace complex relay control systems. The new control
system had to meet the following requirements:
• Simple programming

• Program changes without system intervention (no internal rewiring)
• Smaller, cheaper and more reliable than corresponding relay control systems
• Simple, low cost maintenance
The function of an input module is to convert incoming signals into signals that can be processed by the PLC, and
to pass those signals to the central control unit. The reverse task is performed by an output module. This converts
the PLC signal into signals suitable to operate and invoke the plant actuators. The actual processing of the signals
is undertaken in the central control unit and with respect to the program stored in the memory.
Advantages of PLCs
PLC features can be concluded in the following:
They are rugged, withstand industrial environment, such as heat, humidity,
mechanical shocks and vibrations
In a PLC based system we can easily identify the exact location of the fault by simply seeing on the system moni-
tor but where as in conventional relay systems it is difficult to identify the fault location.
If we want to modify any changes in circuit , in PLC based systems we can easily modify the circuit by just chang-
ing the ladder diagram circuit and upload it to the system, but in an over relay based systems it is very difficult to
modify the circuit we had to replace it with new one.
Power consumption by a PLC based system is very less compared to relay based systems.
A multiple devices like (alarms, motor, sensors, pressure gauge , etc..) can be install in a single PLC system but
whereas in relay based system it is not possible to get all the devices all in one system we need to combine them
separately.
Some PLC can be connected to HMI (Human Machine Interfaces) where we can simply operate the device from
the motor terminal by a small display unit it is not possible in relay based systems.
In PLC any complex logics can be built easily by having some system knowledge but in conventional relay system
it is difficult to have a very complex logics.
The fault clearance in PLC is very easy and it is maintenance free.
It can work at any temperatures with ease.
The basic procedure for programming PLC.
Designing the solution to the automation task
Creating a project

292