Programmable Logic Controller PLC

(1)

UPCO ICAI Departamento de Electrónica y Automática 1 Prof. José A. Rodríguez Mondéjar

Programmable Logic Controller

PLC

Industrial Automation

PLC Architecture

PLC Definition

• PLC is a

• user friendly,

• microprocessor

based,

• specialized computer

• that carries out

control functions

• of many types

• and levels of

(2)

More PLC characteristics (I)

•

Flexibility

– Same PLC model in

different machines

– Implementing changes and

correct errors

• Change program

• Versus rewiring

connections

Typical wired relay control

Industrial Automation

More PLC characteristics (II)

•

Lower cost

– Increased technology makes it possible to condense more

functions into smaller and less expensive package.

•

Pilot running

– A PLC based control can be prerun and evaluated in the office or

lab.

•

Reliability and Maintainability

– Solid-states devices are more reliable, in general, than mechanical

systems or relays and timers.

– Consequently, the control system maintenance costs are low and

downtime is minimal.

•

Ladder or Boolean Programming Method

– The PLC programming can be accomplished in the ladder mode

(similar to wired relay) by an electrician or technician (not complex

automation)

(3)

Basic Module Architecture

•

Central processing unit (CPU)

– The heart of the system: processes data, carries out the logic

operations

•

I/O Modules

– Input modules

• Takes input front the outside world (plant)

• Protects the CPU from the outside world (Optical Isolation)

– Output modules

• Provides output voltages to energize motor starters, lights, coils, valves

CPU

I/O modules

Power supply

Industrial Automation

Connection example (motor control)

•

Integrated CPU

VCC E124.0 E124.1 E124.2 GND VCC A124.0 A124.1 A124.2 GND General Run

Temper BTemper Motor relay

(4)

Modular architecture

•

The PLCs are expandable. Expansion modules contain

additional inputs and outputs. These are connected to the

base unit using a ribbon connector or a rack.

Industrial Automation

PLC Digital Input Module layout

•

The input module perform four tasks electronically:

– Senses the presence or absence of an input signal at each of its

input terminals.

• The input signal tells what switch, sensor or other signal is on or off in

the process being controlled

• Standard levels: 24VDC, 110AC, 220AC, 4-20mA

– Converts the signal for on, or high to a DC level usable

– Carries out electronic isolation

– Produces an output to be sensed by the PLC CPU

Converter

Terminal

Optoisolator

Filter (50 Khz)

(5)

PLC Digital Output Module Layout

•

A DC signal from the CPU is converted through each

module section (terminal) to a usable output voltage, either

AC or DC (voltage or current)

– Standard levels: 24VDC, 110AC, 220AC, 4-20mA

Logic

Input

Optoisolator

Converter

Industrial Automation

Digital Output module Classification

•

Relay

(6)

Other modules

•

Analog Input/Output modules

– ± 10 volts

– 0...10 volts

– 0...20 mA

– 4...20 mA

– Important point: speed acquisition / precision

•

PID module

•

Network module

•

ASCII module

Industrial Automation

Internal Block Diagram of a PLC (I)

OUTPUT MODULES INPUT

MODULES

Processor

ROM

RAM

NETWORK TIMERS

COUNTERS

(7)

Internal Block Diagram of a PLC (II)

•

Processor

– The computer center that carries out the logic operations (also

arithmetic)

•

Memory

– The area of the CPU in which information is stored and

manipulated

– ROM (Read Only Memory)

• Operating System

• System Data

– RAM (Random Access Memory)

• User data

• Copy of input signal states

• Copy of output signal states

• Counters

• Timers

• User program

Industrial Automation

Internal Block Diagram of a PLC (III)

•

Clock

– Clock speed determines how quickly microprocessor executes

instructions.

•

Communications

– RS232

• Programming equipment: Personal computer

• Short distance

– RS485 or similar

• Connection with other equipment

• Long distance/noise immunity

– PROFIBUS

– FIPBUS

– MODBUS

– MPI

(8)

How does PLC work?

•

Like a computer:

– Operating System (Real Time Operating System RTOS)

– Executes User Control Program

VCC E124.0 E124.1 E124.2 GND VCC A124.0 A124.1 A124.2 GND Input1 Input2

PLC

main() {

int i1, i2, f;

while (1) {

Read Inputs;

Execute user program; Write Outputs;

}

Industrial Automation

PLC Example

•

Like a computer:

– Keyboard is the input signals for plant

– Screen is the output signals to the plant

PLC

main() {

int i1, i2, f;

while (1) {

i1=Read(Input1); i2=Read(Input2); f=i1&&i2; %AND Write(f,Output1)

}

User Control Program

(9)

Example running

Input1

Output1

i1

i2

f

Input2

Read Inputs

Write Outputs

SO

USER

CONTROL

PROGRAM

Null Task

CPU

Event Loss

Scan time

Industrial Automation

SCAN CYCLE

INPUT

TERMINALS

INPUT

STATUS

TABLE

USER

PROGRAM

OUTPUT

STATUS

TABLE

OUTPUT

TERMINALS

INPUT SCAN:

Input terminals

are read and

inputs status

table is updated

accordingly

PROGRAM SCAN:

During program scan,

data in input table is

applied to user program,

program is executed and

output table is updated

OUTPUT SCAN:

Data associated

with output

status table is

transferred to

output terminals

(10)

IMPORTANT

•

ANY changes in the status of input devices during the

program or output scan are not recognized until the next

input scan

•

Data changes in the output table are not transferred to the

output terminal during the input and program scans. The

transfer affecting the output devices takes place only

during the output scan.

Industrial Automation

Do you understand?

•

Output?

– 0 permanent

– 1 permanent

– 0101010…

PLC

main() {

int i1, i2, f;

while (1) {

i1=Read(Input1); i2=Read(Input2); f=i1&&i2; %AND f=i1||i2; %OR Write(f,Output1)

}

(11)

HOW TO PROGRAM A PLC?

• High level language?

– C Language

– Difficult to program

• Pointers!!

• Language near technicians

background

– Easy to program

– Standard

– Flexible

• Easy and complex languages in the same PLC

PLC

Process

Serial

communication

Control System

Industrial Automation

IEC 1131-3 Standard

•

The IEC standard specifies the following programming

languages

Instruction List

C:= A AND NOT B

A B C

-| |--|/|---( )

AND

A

ANDN

B

=

C

AND

A C

B

Structured Text

Ladder Diagram

Function Block Diagram

Sequential Function Chart

(GRAFCET)

0 Init 2 Fill 3 Heat Empty out Run Full Hot 1 Stopped Empty 4

(12)

Memory model

• Components of the PLC

program:

– Instructions

– Data: Memory

• Matrix

– 8 bits x Address number

– 16 bits x Address number

• Accessing methods

– Bit M0.3

• Work with logical variables

– Byte: MB0

– Word: MW0, MW2.

– Long word: MD0, MD4

• Telemecanique uses

– %MW0

8 bit

0 1 2 3 4 5 6 7 8 9 10 11 12 13 MB0 MW0 MD0 MW8 14 15 MD12 MB5

8x16 Memory

7 6 5 4 3 2 1 0

M3.2

Industrial Automation

Variable Types or Table Types

• Memory

– M0.1, MB3, MW4, MD0

– User variables

• Digital Input Table

– E124.0, EB124, EW124 (Siemens)

– I124.0, IB124, IW124 (IEC)

– Remember the difference between real

input and input variable in the program.

• Digital Ouput Table

– A124.0, AB124, AW124 (Siemens)

– Q124.0, QB124, QW124 (IEC)

• Timer Table

– T1 (16 bits timer

– The value increases by clock

• Counter Table

– Z5 (16 bits counter

– Event count

M0

M255

User

Table

···

··

E0

E127

Input

Table

···

··

A0

A127

Output

Table

···

··

T0

T127

Timers

···

··

Z0

Z63

Counters

···

··

M1

E1

A1

T1

Z1

(13)

Ladder Diagram Example

General Run Térmico Motor

Térmico BTérmico VCC E124.0 E124.1 E124.2 GND VCC A124.0 A124.1 A124.2 GND General Run