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page 203

17. PROGRAMMABLE LOGIC CONTROLLERS

• Considering their popularity, PLCs will be used as the basis for exploring discrete event control systems.

17.1 BASIC PLCs

A Programmable Controller (PC) is an input/output processing computer.

Advantages of PLCs,

-cost effective for complex systems

-flexible

-computational abilities

-trouble shooting aids

-reliable

Ladder logic was originally introduced to mimic relay logic.

A thorough review of the research literature is available in Sobh et. al. [1994].

The PLC can be programmed like other computers using specialized “languages”.

-Ladder Logic - A programming technique using a ladder like structure. This was originally adopted because of its similarity to relay logic diagrams to ease its acceptance in manufacturing facilities. The ladder approach is somewhat limited by the lack of loops, etc. (Uses JIC standards)

page 204

 

OUTPUTS

HOT

NEUTRAL

 

INPUTS

POWER NEEDS TO FLOW THROUGH

THE INPUTS TO THE OUTPUTS

- Mnemonic - instructions and opcodes, similar to assembly language. More involved to program, but also more flexible than ladder logic.

e.g. for an Allen-Bradley PLC

00000

XIO

00001

 

00001

XIC

00002

the mnemonic code is equivalent to

00002

AND

 

 

the ladder logic below

00003

XIC

00003

00004

XIC

00004

 

00005

AND

 

 

00006

OR

 

 

00007

OTE

00107

 

 

 

 

00001

00002

00107

 

 

00003

00004

 

 

 

 

 

END

page 205

- Sequential Function Charts (SFC) - an alternative to a flowchart method for concurrent operations. Currently this is not widely used, but it (or a similar version) should become more prevalent in the upcoming years.

Start

function 1

function 3

function 2

End

• Structured text allows programs that are similar to high level languages such as BASIC.

N7:0 := 0

REPEAT

N7:0 := N7:0 + 1;

UNTIL N7:0 >= 10

END_REPEAT;

17.1.1 PLC Connections

page 206

 

PROCESS

 

Feedback from

 

Connections to

sensors/switches

 

Actuators

PLC

The PLC continuously scans the inputs, and changes the outputs.

The Process can be anything - a large press, a car, a security door, a blast furnace.

As inputs change (e.g. a start button) the outputs will be changed. This will cause the process to change, and new inputs to the PLC will be received.

PLC program changes outputs

 

by examining inputs

Set new outputs

 

THE

 

CONTROL

 

LOOP

 

 

process changes and PLC pauses

read inputs

while it checks its own operation

17.1.2 Ladder Logic

As mentioned before, ladder logic has been developed to mimic relay logic. This was done originally to make the computer more acceptable to companies and employees. Original efforts resisted the use of computers because they required new skills and approaches, but the use of ladder logic allowed a much smaller paradigm shift, therefore making them more acceptable.

Original Relay Ladder Logic diagrams show how to hook-up inputs to run outputs.

-Relay - An input coil uses a voltage/current to create a magnetic field. As the coil becomes magnetic it pulls a metal switch (or reed) towards it, and makes an electrical contact. The contact that closes when the coil is energized is normally open.

page 207

There is a contact that the reed touches without the coil energized is called the normally closed contact. Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated.

input coil

normally closed normally

open

Or more simply

OR

- A Circuit - A mix of inputs and outputs allows logical selection of a device.

page 208

115VAC wall plug

relay logic

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

input

 

 

 

 

 

 

 

 

 

 

 

output

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

input

 

 

 

 

(normally

 

 

open)

(normally closed)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(normally open)

ladder logic

page 209

push buttons

power

supply

+24V

com.

PLC

inputs

ladder

logic

outputs

115Vac

AC power

neut.

• many relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. The circuit shown below is an example of this. It is called a seal in circuit.

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