Advantages of PLCs

1
CHAPTER - INTRODUCTION TO PLCS
Advantages of PLCs
Cost effective for controlling complex systems.
Flexible and can be reapplied to control other
systems quickly and easily. Computational
abilities allow more sophisticated control.
Trouble shooting aids make programming easier and
reduce downtime. Reliable components make these
likely to operate for years before failure.
2
PLC Origin
- Developed to replace relays in the late 1960s -
Costs dropped and became popular by 1980s - Now
used in many industrial designs
3
Relays and Schematic Symbols
4
Logical Control with Relays
5
Relay Logic In a PLC
6
Reading Ladder Logic
7
A Ladder Logic Example
8
Mnemonics
9
SFCs
10
Structured Text
11
PLC in a Control Loop
12
The Four Stages of a PLC Scan
13
Ladder Logic Inputs
14
Ladder Logic Outputs
15
Sample Problem
Try to develop (without looking at the solution)
a relay based controller that will allow three
switches in a room to control a single light.