PLC (Lesson 1 -Introduction)

Introduction

              A programmable logic controller, PLC, or programmable controller is a industrial control system used for Automation of typically industrial processes, 

PLC has been applied in many sectors. Industrial Automation, Building automation, Office automation, Household automation, Transport automation, etc. 

The first Programmable Logic Controller (PLC) was developed by a group of engineers at General Motors in 1968, when the company was 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.

Major PLC manufacturer

• Allen Bradley

   

•         Siemence
•         Honeywell
•         Modicon
•        Omron
•        General Electric
•         Mitsubishi
• 
  

Functions by a PLC

A programmable logic controller can be used to perform the following functions:

Control Type	Functions
1. Sequential Control	a)      Conventional Relay Logic Replacer b)      Timers / counter c)   Auto/ Semi-Auto/Manual control of machines/Processes
2. Sophisticated Control	a)      Arithmetic Operations (+ ,-, ´, ¸ ) b)     Information Handling c)      Analogue Control (temperature., pressure) d)     Stepper-motor Control e)      Servo-motor Control
3. Supervisory Control	a)      Process Monitoring and Alarm b)      Fault Diagnostics c)      Interface with Computer (RS 232/ RS-422/Ethernet) d)     Printer/ ASCII interfacing e)      Local Area Network

 Hardware Configuration of a Programmable Logic Controller

A programmable logic controller system can be divided into the following blocks as shown below.

PLC Configuration

There are two PLC basic configurations that commercial manufacturers offer. that commercial manufacturers offer. 

„Fixed Configuration 

• Modular Configuration

Input/Output Modules

PLC Operation

The PLC operates internally in a way very similar to computers repetitively. 

1. The states of the input devices are continuously monitored and copied from input module into input RAM memory area. 
2. The CPU then executes the control program stored in the program memory area.
3. Depending on the program and the states of inputs, the output states of the relevant output devices are then written into the output RAM memory area.
4. At the end of each execution cycle, data in output RAM area are then copied to the output module.
5. After each execution new cycle begins. (1)

The time required by the PLC for a single execution of a program including the actualisation and output of the process image, is termed the cycle time. The longer the program is and the longer the respective PLC requires to process an individual program line, the longer the cycle. Realistic time periods for this are between approximately 1 and 100 milliseconds.