introduction a plc

7/26/2019 Introduction a PLC

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Introduction to PLCs 1

Introduction to PLCs

Industrial Control Systems

This is the first part of the Industrial Controls course. In this part we will discuss

the architecture and use of Prtogrammable Logic Controllers (PLCs).

Allen Bradley has donated several PLC-5 and SLC-500 systems to the University ofAkron. Allen Bradley is a very popular architecture and widely used in industry.

For these two reasons we will center all of the lectures and laboratories around the

Allen Bradley architecture and instruction set.


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Overview

Boolean Logic Logic in C

Meaning of Programmable

Alternate Forms used by PLC

History

Modern PLC

As electrical engineering or computer engineering students you have had exposure

to programmable logic in Switching and Logic and in Programming for Engineers

or Introduction to Computer Science. We will discuss both of these approach in

today's lecture.

For field programming with industrial strength interfaces the PLC is unrivaled. We

will discuss why this is so in today's lecture.

A historical perspective helps in understanding why the PLC have their architecture

and instruction set.

A quick look at the systems to be used in this course will make that first laboratoryexercise a little more relaxing.


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Example

There are two pumps referred to as pump1 and pump2 in a controlsystem. Between the two pumps is a tank. The tank has a

normally closed float switch at the top of the tank called below1.

The tank has a normally open switch at the bottom of the tank

called below2.

Control the level in the tank so that is remains between the

bottom and the top by controlling the two pumps

Consider the example below as a starting point to investigate different styles of

programmable logic.

There are two pumps referred to as pump1 and pump2 in a controlsystem. Between the two pumps is a tank. The tank has a normally closed float

switch at the top of the tank called below1. The tank has a normally open

switch at the bottom of the tank called below2.

Control the level in the tank so that is remains between the bottom

and the top by controlling the two pumps

This problem is referred to as the twin-pump problem or twin-pump example.


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Boolean Logic

was = between

between = below1 AND NOT below2

turn1 = ( NOT turn1 AND was AND NOT between ) OR

( turn1 AND ( NOT was OR ( was AND between) ) )

pump1 = NOT was OR ( between AND turn1 )

pump2 = NOT was OR ( between AND NOT turn1 )

One way to represent control logic isvia Boolean logic or equations.

One why to represent the logic that solves the twin-pump problem is in the form of

Boolean equations. These equations can then be used to construct a dedicated

circuit in discreet gates, programmable logic device (PEEL), or a VLSI circuit.

This logic can be translated to configuration files using WinPLACE or VHDL if this

device is to be implement in a programmable logic device such as a PEEL or a Field

programmable Gate Array.

There is a history implied in this set of statements. After the second statement is

executed, was is the previous value of between and between is the present value. In

a hardware device this would be accomplished using some form of memory element

such as a flip flop. In a PLC this would be accomplished with an internal contact

and internal coil. The coil would only change after each scan of the logic iscompleted. More about this in a future lecture.


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Meaning of Programmable

the logic can be readily altered, the size of the logic subsystem can vary,

the size of the logic component can belarge, and is not subject to "hard" limits,

the logic can be defined in the field, in away appropriate to the user of theembedded system.

The P in PLC stands for programmable. What does this imply?

The logic can be readily altered. In a PLD using WinPLACE or VHDL we can

change the equations. We can add additional equations or combine existingequations. The software then produces a configuration file which changes how the

outputs are changed relative to the inputs. In a PLC the programming is

accomplished via a set of data structures and how they are interpreted.

The size of the logic subsystem can change. With PLDs the complexity of the

equations can be increased. This may lead to a higher percentage of the system

resources being used. When the utilization exceeds 100%, a larger device must be

used. In a PLC when the utilization exceeds the existing capacity, the data

structures must be repartitioned. If this still does now provided enough space morememory is added to the system. Often by purchasing a larger more costly module.

The size of the component in theory is not subject to a hard limit. The practicality

of the matter is that cost may force a hard limit.

The device must be programmable by the end user in the field. In a PLD this is

done in a programmer or insuti. In a PLC the program is downloaded from a

computer via a serial cable or over the internet.


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C Language

readfloats( int *bw1, *bw2 ) {readfloats( int *bw1, *bw2 ) {

. . . . . .. . . . . .

}}

main() {main() {

int below1, below2, between, was, turn1,int below1, below2, between, was, turn1,

pump1, pump2;pump1, pump2;

while (1) {while (1) {

was = betweenwas = between

between = below1 && ! below2;between = below1 && ! below2;

readfloats( &below1, &below2 );readfloats( &below1, &below2 );

turn1 = ( ! turn1 && was && ! between ) ||turn1 = ( ! turn1 && was && ! between ) ||

( turn1 && ( ! was || ( was &&( turn1 && ( ! was || ( was &&

between) ) );between) ) );

pump1 = ! was || ( between && turn1 );pump1 = ! was || ( between && turn1 );

pump2 = ! was || ( between && ! turn1 );pump2 = ! was || ( between && ! turn1 );

}}

}}

Another way to represent programmablelogic is in a high level language

The twin-pump problem could also be solved using a C program on an embedded

microcomputer. The example on the above slide solves the problem. Notice that

history is again used. This is accomplished by the fact that statements are executes

in sequence.

With C, a function must be used to read the inputs and write the outputs at two

different locations in the code. With the PLD, the inputs are always present on the

input pins. The equations are evaluated concurrently and then update the outputs.

The inputs and outputs are often held in a stable condition by flip-flops.

With a PLC all of the inputs are read and placed in a table. The logic is evaluated

and the output of each equation is written to an output table. After all the logic has

been evaluated, the end of the ladder, the output table is written to the outputmodules. This read, evaluate, write sequence then allows the PLC to mimic the

PLD and the C program.


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Logic In C

BitwiseBitwise OperationOperation LogicalLogical

&& ANDAND &&&&

|| OROR ||||

~~ NOTNOT !!

^ XORXOR NoneNone

C supports both bitwise and logicalBoolean operators.

C and hardware description languages allow the equations to be expressed as either

bitwise operators or logical operators.

PLCs allow the same thing and they can be expressed in:

Relay Ladder Logic

Sequential Function Charts

Structured Text

We will discuss these three language or representation in lectures latter on in the

course.


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Alternate Forms

Relay Ladder Logic Sequential Function

Charts

Special PLC Languagesuch as StructuredText

Graphical representations of these alternate forms are shown in the above slide. The

meaning of these forms will be discussed latter on in the course. We will also

concern ourselves with how to enter these forms using the software tool RSLogix.


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IEC 1131-3 Standard

Standard specifies five programminglanguages

Structured Text (supported by AB)

Function Block Diagram

Ladder Diagrams (supported by AB)

Instruction List Programming

Sequential Function Charts (supported byAB)

At one time each vendor of PLCs would develop their own set of languages that

could be used to program the device. To allow some uniformity between platforms

the International Electrotechnical Commission (IEC) developed and published the

IEC 1131-3 standard. Most PLCs adhere to this standard in that they support one or

more of the representations. As an example Allen Bradley supports three out of the

five languages. We will learn a little about all three of these languages. The

laboratory exercises will use two of the languages (Ladder Diagram and Sequential

Function Charts).


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Structured Textwas := between;was := between;

between := below1 AND NOT below2;between := below1 AND NOT below2;

turn1 := ( NOT turn1 AND was AND NOT between ) ORturn1 := ( NOT turn1 AND was AND NOT between ) OR

( turn1 AND ( NOT was OR ( was AND between)))( turn1 AND ( NOT was OR ( was AND between)));

pump1 := NOT was OR ( between AND turn1 );pump1 := NOT was OR ( between AND turn1 );

pump2 = NOT was OR ( between AND NOT turn1 );pump2 = NOT was OR ( between AND NOT turn1 );

The above is an example of structured text. Structured text comes the closes to the

PLDs hardware description language and the C language example. This form is the

most natural for students without ladder diagram experience. This is because they

have had experience with PLDs and C language.


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Sequential Function Chart

Graphical representation of logicstructure in design

Must have ladder diagram orstructured text inside each step

Step

Step

Transition

The above example is sequential function charts. This form is easy for those that

like flowcharts and Algorithmic State Machine (ASM) charts. You most likely have

had experience with both of these.


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Ladder Logic

below1 below2 between

between was

turn1 wasbetween turn1

turn1 was

was between

Ladder diagrams are the most alien to electrical engineering and computer

engineering students. Because we have yet to use ladder diagrams to represent

programmable logic. But ladder diagrams are the most popular among practicing

engineers for historic reasons. The lectures will present ladder diagrams first and

then structured text and sequential function charts will be discussed.

It is my contention that most of the problems with PLC programs are because of the

limited capabilities of ladder diagrams. I will try to make you a user of the more

advanced languages of structured text and sequential function charts.


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History

Electromechanical Relay Racks High Documentation Cost

High Fabrication Cost

High Debugging and Modification Costs

Solid State Relays (too unreliable)

Programmable Controllers (PC) ->

Programmable Logic Controller (PLC)

Originally all logic was implemented using electromechanical relays. The relays

predate electronics. Once solid state electronics was developed many vendors tried

unsuccessfully to migrate systems from relays to solid state relays. The problem

with solid state relays were:

The technology was unproven

Not enough of the high in the slide above were reduced to low with the

use of solid state relays

The industrial control industry is a mature field that does not except change

without over whelming benefit

Following solid state relays came the PC or programmable controllers.When IBM introduced the IBM PC the PC term for programmable

controllers was often confused with personal computer. Programmable

Controllers were changed in name only to Programmable Logic Controllers

or PLCs. PLC is just the modern name for PC.


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Modern PLC

Power supply (for electronics only) Processor (embedded controller)

I/O Rack

I/O Modules

Digital (AC, DC)

Analog (ADC, DAC)

Special (PID, BCD, )

The modern PLC has several components in the system. The power supply only

supplies power to the processor and the electronics in the I/O modules. The

external components such as heaters, position sensors, and the like need their own

external supply

The processor module contains the brains or the embedded controller. The

processor contains the logic and the ability to evaluate the logic.

The I/O rack holds the power supply, processor, and I/O modules. The rack is a

chassis with a backplane.

There are a large number of I/O modules available for any given PLC. We will useseveral different type in the laboratory exercises. The I/O modules translate the real

world voltages and currents in the voltages and current suitable for the VLSI

processor chips in the processor module


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I/O Modules

N.O.

N.C.

Input Output

Device

The power supply for the PLC only supplies power to the electronics. It is the

responsibility of the user to provide power from an external source for the input and

output modules. Lets assume that the input module is for AC power and the output

is for AC power. The normally opern (N.O.) and normally closed (N.C.) switches

would be connected between the HOT bus and the input module node. The input

module would then have a common node connected to the NEUTRAL: bus. If

the input module senses a voltage relative to the neutral that is above a specified

threshold the input is sensed as a logic 1 or is on. Else the input voltage is below

the threshold voltage the input is a logic 0 or is off.

For the AC output module the node acts as an AC switch. If a logic 1 is written to

the node, the switch is closed and hot bus is connected to one side of the device, i.e.

lamp. The other side of the device is connected to the neutral bus. This applied hotto neutral voltage across the device.

The hot bus could be replaced with a positive DC voltage and the neural with DC

ground, if the modules are DC input and DC output modules. If both AC and DC

modules are used, two external supplies at a minimum are needed.


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AB PLC-5 & SLC 500

Data File Type O or Output Image (File #0)

I or Input Image (File #1)

S2 or Status (File #2)

B3 or Bit Data (16 bits per word) (File #3)

T4 or Timer (File #4)

C5 or Counter (File #5)

R6 or Control (File #6)

The memory in an AB PLC are grouped in to block of memory called files. The

first 8 files (file 0 through file 7) are assigned by default. You will be exposed to

some of these file types in the laboratory exercises. Others you will only read

about.

O is for the outputs

I is for the inputs

S is for the processor status bits

B is for internal inputs and outputs, those not associated with an Input or

Output module

T is for timers

C is for counters

R is for control registers used with some instructions


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AB PLC-5 & SLC 500

Data File Type N7 or Integer (File #7)

F8 or Floating-Point (File #8)

Dn or BCD (None)

An or ASCII (None)

Additional Files (n = #9 to #999)

Size and number of types limited by

memory size

N is for numbers of integers

F is for floating point numbers

D is for Binary Coded Decimal numbers

A is for ASCII (American Standard Code for Information Interchange)

The size and number of files is limited by the memory size in the processor.

These may be changed from there default size using programming tools. In

fact the file types may be associated with other file numbers if you so desire.


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AB PLC-5 & SLC 500

Program Files System File 0

System File 1

Program File 2

Program Files 3 to 255


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AB PLC-5 & SLC 500

I/O Address O:rg/00 to 17

r = rack number

g=group number (position in rack)

Node number in octal (PLC-5)

Terminal number in decimal (SLC 500)

I:rg/00 to 17

S:e/b

e element number

b bit number 0 to 15 in decimal

Each node of each input or output module must be numbered or have an address.

For the PLC-5 family the numbers are expressed in octal. For the SLC 500 family

the numbers are expressed in decimal. The rack number is the chassis number. The

group number relates to the modules position in the rack (chassis). The node

number related to the relative position of a terminal within a module.


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AB PLC-5 & SLC 500

Xf:n X is other file types

f is file number do not need to use default

if default used; then can be omitted

n element number in file

The address for the other files is of the form Xf:n. X is a letter that is unique to the

file type such as F for floating-point numbers. The f is the file type which is 8 for

floating-point numbers bt default. The n is a unique number starting at 0. Each

n uses a given number of bytes in memory. If n numbers are skipped, that

block of memory is wasted, that is not used.


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Micrologix & Pico Controllers

Allen Bradley has several different families of PLC. For small dedicated

application there are the Picologix and Micrologix family of devices. With these

devices the power supply, processor and input output modules are all integrated into

one systems. If you run out of inputs or outputs you must buy another Pico

Controller or Micrologix device. The devices themselves are not expandable.


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SLC 500 Chassis

For larger applications, the SLC 500 family can be used. The SLC 500 chassis

allows one to place a processor in the first slot and various input and output modules

in the other slots. You can pick and choose which module goes into which of the

remaining slots. This flexibility comes at the higher cost per node over the

Micrologix of Picologix cost per node.


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SLC-500 Modules

The above slide shows a typical SLC 500 system with the power supply to the far

left. The processor is to the right of the power supply. Five modules of either the

input type or the output type are to the processors right. There are larger chassis

for the SLC 500 family that allow more modules.


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PLC-5

The PLC-5 family is older and more mature then the SLC-500 family. It also uses a

power supply, chassis, processor and I/O module architecture.


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PLC-5 Processors

As the processor number increases several things change in the processor. The

memory size increases. The communication capabilities increase. The processor

speed increases.

introduction a plc

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