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Joel Castillo Gómez

Course Structure

I. Theoretical basis

II. Digital Circuits

• II.1 Combinational Circuits.

• II.2 Sequential Circuits.

III. Industrial logic.

Thematic Content

4. Industrial Logic

4.1 Pneumatic Logic Circuits4.2 Electrical Logic Circuits4.3 Ladder Diagram.4.4 The Programmable Logic Controller (PLC)4.5 IEC 1131 standard for programming PLC's.4.6 Timers and counters, set of instructions, functions and data blocks.4.7 PLC programming techniques.4.8 Grafcet and Petri Nets.

Symbols

Example

Create a pneumatic circuit that:

Starts with a button

There is a double acting cylinder that moves

forward when button is pushed.

The cylinder returns to its starting position

when the end of the cylinder is reached.

Thematic Content

4. Industrial Logic


Symbols

The symbology in electrical circuits is very important because it standardizes the way we represent the circuits in any system or facility.

There are different standards world wide, however, in México we use the NOM-001-SEMP-1994 norm.

Part of this norm defines the symbologyto use in the representation of electrical circuits.

Symbols

Symbols (comparison)

Thematic Content

4. Industrial Logic


Ladder Diagram

Relay control circuits are drawn using

ladder diagrams.

Basic logic gates in LAD

AND

OR

NOT

SR Memory (Reset dominant)

M=(S + M) R’

Sequential circuits (Cascade)

Another way to represent

sequential circuits is:

with circles which represent “states”.

Arrows which represent transitions.

The function is:

Each state represent an action or

“step” of the system.

It is recommended only one step

active at a time.

The transitions are the conditions

associated to “jump” to the next

step/state.

E0

E1

E2

T0

T1

T2


Solution:

You will use a SR memory with reset

dominant.

The SETs of the memories (states)

will be the following:

○ Start condition (only for initial state)

○ The transition associated to the

previous state along with the state

itself.

The RESETs of the memories:

○ The next state has been activated

Finally, the outputs are activated

with the associated states status.

E0

E1

E2

T0

T1

T2


M=(S+M) R’

For E0:

S=E2 T2 + Start

R=E1, so:

E0=(E2 T2 + Start + E0) E1’

For E1:

S=E0 T0

R=E2, so:

E1=(E0 T0 + E1) E2’

E0

E1

E2

T0

T1

T2


General solution

E_curr=(E_prev T_prev + E_curr) E_next’

Example

Design an electric ladder diagram to control the previous diagram as follows. When a start button is pressed cylinder A moves, once it reaches the + position, B moves, once it reaches the + position both cylinders return.

Thematic Content

4. Industrial Logic


PLC

Intro

Basic components

Memory

I/O Modules

Addressing

Programming

PLC Intro

PLCs = Programmable logic controllers

PLC’s are microcomputer-like

instruments, used to control industrial

equipment.

PLC’s is a software-based relay control

circuit.

Advantages: Programs can be easily

changed for new jobs (flexibility)

PLC Intro

PLC’s Microcomputers

Designed especially for industrial

environments.

Are not built to operate reliably

under industrial conditions

Come with input or output (I/O)

modules available for different

types of signals

Require special circuit cards to be

interfaced with external

equipments

Diagnostic features made

available to programmers. Easy

maintenance.

-

Usually programmed using relay-

type ladder diagrams.

-

Can perform simple arithmetical

operations.

They can achieve complicated

mathematical routines.

PLC’s scan the program line by

line. (Slow)

Fast than PLC’s.

PLC Basic components

PLC Memory

Memory is used to store programs, use

“fictional” “relays” or variables. Some

PLC’s even map or store the information

of I/O modules in its memory.

Basic units are: BIT, BYTE, WORD,

DWORD.

PLC I/O Modules

The function is to provide the necessary

interface between PLC and the

controlled system (ex. Limit switches,

manual switches, thermostats, sensors

in general). They can be divided into:

Binary (on-off)

Analog

Communication

PLC I/O Modules

PLC Addressing

Addressing is the way PLC’s name the

components connected to it.

It can be divided into I/O addressing and

memory addressing.

Each manufacturer defines the naming

or addressing convention.

PLC Addressing

PLC Programming

Programmable controllers are primarly

programmed in ladder logic.

Ladder logic is a symbolic

representation of an electrical circuit.

The main function of a PLC program is

to control outputs based on the condition

of inputs.

The symbols can be divided into two

broad categories: contacts and coils.

PLC Programming

Contacts

Used for “reading”. (Not necessarily inputs)

Coils

Used for “writing”

PLC Programming

Exercise: What’s the status of the

outputs?

PLC Programming

Scan cycle

Thematic Content

4. Industrial Logic


IEC 1131/61131

It is a standard programming for PLC’s. The standard consist of 8 parts:

Part 1: General information

Part 2: Equipment requirements and tests

Part 3: Programming languages

Part 4: User guidelines

Part 5: Messaging service specification

Part 6: Communications via fieldbus (Awaiting completion of fieldbus standards.)

Part 7: Fuzzy control programming

Part 8: Guidelines for the application and implementation of programming languages

IEC 1131/61131

IEC is the International ElectrotechnicalCommission.

It is a worldwide standardization body, meaning nearly all countries have agreed to accept IEC standards.

The most important part of the standard are programming languages: ladder diagram, instruction list, function block diagram, structured text, and sequential function chart.

Structured Text

Function Block Diagram.

Ladder Diagram

Instruction List

Sequential Function Chart

Thematic Content

4. Industrial Logic


Timers

Counters

Exercise

Program the following task in

www.plcs.net.

The task begins with a start button,

turning on a light, after 5 seconds, the

next light is lit. If the same start button is

then pressed 5 times, another light turns

on, pressing the start button begins the

cycle again.

http://www.plcs.net/

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