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Programmable Logic Controllers Definition. Programmable Logic Controllers History

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Programmable Logic Controllers

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Definition

A digitally operated electronic apparatus which uses a programmable memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting, and arithmetic to control, through digital or analog input/output modules, various types of machines or processes.

- National Electrical Manufacturers Association (NEMA)

Components

lInput and output module

lProcessor

lMemory

lPower supply

lDetachable programming device

Programmable Logic Controllers

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History

1969 specifications (General Motors)

lMust be programmable (and reprogrammable)

lDesigned to operate in an industrial environment

lMust accept 120 V ac signals from pushbuttons, switches

lOutputs must switch and continuously operate loads such as motors and relays of two amps rating

Before the 1980’s, programmable controllers were called PCs.

Programmable Logic Controllers

PLC Functions

A PLC performs the functions of conventional relays, timers, counters

Advantages (over conventional devices)

l More flexibility

l Less space, more compact

l Higher reliability (less mechanical parts)

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Programmable Logic Controllers

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Conventional devices

Relays

Programmable Logic Controllers

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Conventional devices

Relays (continued)

Programmable Logic Controllers

Conventional devices

Relays (continued)

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Programmable Logic Controllers

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Conventional devices (continued)

Timers

Programmable Logic Controllers

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Operation

l

Inputs are sampled and stored in memory

l

Control program is executed

l

Calculated values are used to update the outputs

l

Scan time is generally of the order of milliseconds

Programmable Logic Controllers

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PLCs versus Computers

Conceptually very similar, though there are practical differences.

lPLCs are based on solid state technology and generally use

microprocessors as their processing unit.

lPLCs are used to simulate electromechanical devices that were typical of

transfer lines and mass production.

PLC Computer

executes simple program in milliseconds, scan time is much smaller than 1/60 seconds

possibly complicated program, often timing is not critical

objective is to make yes/no, 0/1 decisions tasks may include floating point arithmetic, manipulation of data

special purpose device, same program is repeated continuously

general purpose, possibly different programs no peripherals need peripherals

programmed using ladder logic programmed using high-level languages

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Why PLCs?

Advantages Disadvantages

+ many inputs and outputs, excellent for controlling and monitoring many processes

+ designed for industrial environments, robust and reliable + reprogrammable

+ modular

+ ideally suited to supervisory control

+ easy to set up, good for FMS environment

+ inexpensive

- do not use the full capabilities of the microprocessor

- ladder logic programming style is cumbersome and prone to mistakes - only good for yes/no decisions - do not handle continuous inputs

and outputs, not good for “direct control”

- inflexible (compared to microprocessors)

Programmable Logic Controllers

Logic Control Systems

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Logic Control Systems (continued)

2. Logic Gates

lX is on if and only if A and B are both on (true) or both off (false).

lY is on (true) if and only if A and B are both off (false).

3. PLCs

B

A

X

Y

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Logic Control Systems - Representation

Computer program

if ((A=0) and (B=0)) or ((A=1) and (B=1)), X:=1 else X:=0; if ((A=0) and (B=0)) Y:= 1 else Y:=0;

Truth table

A B X Y 0 0 1 1 1 0 0 0 0 1 0 0 1 1 1 0

Programmable Logic Controllers

Logic Control Systems - Representation

Boolean Logic

X:= A.B

Y:= ~(A+B)

Ladder Logic

X

A

B

A

B

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Logic Control Systems - Representation

PLC Program

STR SHF 00 ENT AND SHF 01 ENT OR SHF 11 ENT OUT SHF 10 ENT STR NOT SHF 00 ENT AND NOT SHF 01 ENT OUT SHF 11 ENT

10

00

01

11

00

01

00

01

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Ladder Logic Diagrams

Input contacts (switches, relays,

photoelectric sensors, limit

switches)

Output loads (actuators, alarms,

lights)

Logical inverse (NOT)

Logical AND operation

Logical OR operation

Programmable Logic Controllers

Ladder Logic Diagrams (continued)

Example

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Ladder Logic Diagrams

Push-button switch

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Ladder Logic Diagrams

Automatic weigh station on a conveyor

Ladder logic diagram

Response from sensors

Sensor X

Sensor Y

Programmable Logic Controllers

20 Rotate base CCW 21 Rotate base CW 22 Lift arm 23 Lower arm 24 Extend arm

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Programmable Logic Controllers

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Application (continued)

Sequence of operations

Programmable Logic Controllers

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Application (continued)

PLC Ladder Logic Diagram

(not all rungs shown)

Output Limit switch Necessary PLC inputs

Programmable Logic Controllers

Application (continued)

PLC Code

References

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