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What is a PLC?
• PLC is a digitally operating 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
machine or processes.
PLC History
• More recently electricity has been used for control and early electrical control was based on
relays.
• These relays allow power to be switched on and off without a mechanical switch.
• It is common to use relays to make simple logical control decision.
• The development of low cost computer has brought the most recent revolution the
Programmable Logic Controller (PLC).
• PLC was developed to replace relays in 1960s. Cost dropped and became popular by
1980s.
• The advent of PLC has become the most common choice for manufacturing controls.
How does PLC differs from a personal computer?
• A Programmable Logic Controller (PLC) is a specialized computer, designed to be
used for industrial control.
• Programmable Logic Controllers are sometime called programmable controllers (PC)
but are more commonly called PLCs.
• A PLC looks different from a standard personal computer.
• For a start, a PLC doesn’t have a keyboard or a monitor.
• The following pictures show several PLCs.
Advantages of Using PLCs in Automated Food Industrial
• PLCs have been gaining popularity on the factory floor and will probably remain predominant.
Most of this is because of the advantages they offer.
• Cost effective for controlling complex system.
• Flexible and can be reapplied to control other systems quickly and easily.
• Reliable components make these likely to operate for years before failure.
• Troubleshooting aids make programming easier and reduce downtime.
• Ease to setup and program.
• PLC are tough enough to keep working in even the dirtiest production environment.
Advantages of Using PLCs in Automated Food Industrial
• Programming the PLC is easier than wiring the relay control panel.
• The PLC can be reprogrammed (conventional controls must be rewired).
• Computational abilities allow more sophisticated control.
• PLCs take less floor space than relay control panels.
• Maintenance of the PLC is easier, and reliability is greater.
• The PLC can be connected to the plant computer systems more easily than relays can.
The use of PLC in manufacturing
PLCPLC
Timer and CounterTimer and Counter
SoftwareSoftwareMotor ControlMotor Control
Switches and SensorsSwitches and Sensors
Field IField I//OO
Operator InterfaceOperator Interface CommunicationCommunication
ConnectorConnector
ProcessesProcesses
PLCs
KeyenceKeyence,, JapanJapan
OmronOmron,, JapanJapanMitsubishiMitsubishi,, JapanJapan
Allen BradleyAllen Bradley,, USAUSASiemens AGSiemens AG,, GermanyGermany
SchneiderSchneider,, GermanyGermany
PLC CU PRELUCRARE PE BIT
PLC îndeplinesc sarcini de conducere proprii echipamentelor
de comandă discretă a proceselor industriale, şi au o
funcţionare binară permiţând detectarea schimbărilor de stare
a unor semnale furnizate de elemente de tip:
1. butoane cu menţinere, cu revenire
2. comutatoare basculante
3. limitatoare de cursă
4. detectoare de proximitate
şi prelucrează, în principal logic, informaţiile furnizate de aceste
elemente în vederea emisiei de semnale de ieşire ce comandă
elemente de tip:
1. contactoare
2. relee
3. electroventile
4. ambreiaje
5. elemente de semnalizare
PLC’s Component
• Many PLC configurations are available, even from a single vendor.
• But, in each of these are common components and concepts.
• The most essential components are:
• CPU Module:
• Input Modules:
• Output Modules
ANSI/ISA 95 standard
Source: ANSI/ISA–95.00.01–2000
the ANS/ISA standard 95 defines terminology and good practices
Enterprise Resource Planning
Manufacturing Execution System
Control & Command System
Business Planning & LogisticsPlant Production Scheduling
Operational Management, etc.
ManufacturingOperations & Control
Dispatching Production, Detailed ProductScheduling, Reliability Assurance,...
Level 4
Level 3
Levels2,1,0
BatchControl
ContinuousControl
DiscreteControl
Location of the PLC in the control architecture
Entreprise Network
directly connectedI/O
Control Bus(e.g. Ethernet)
Engineerstation
I/O I/O I/O I/OCP
U
Sensor Bus (e.g. ASI)
Field Bus (proprietary)
gateway
Field Stations
Control Station with Field Bus
direct I/O
I/O
Field DevicesFB
gateway
gateway
I/OI/OI/OI/OCP
UC
OM
I/OI/OI/OCO
M
CP
U
CO
M
CO
M
CO
M
I/O
Field Bus (standard)
CP
U
CO
M 2
I/O I/O I/OCP
UC
OM
1
CO
M 2
I/OCP
U
Operatorstation
PLC
small PLC local intelligence
PLCPLC
CO
M1
CO
M1
SupervisorStation
General PLC architecture
CPUReal-Time
Clockflash
EPROMROM
buffers
signal conditioning
power amplifiers
relayssignal
conditioning
serial portcontroller
Ethernet
parallel bus
ethernetcontroller
RS 232
analog-digital
converters
digitalanalog
convertersDigital Output
DigitalInput
fieldbuscontroller
externalI/Os
extensionbus
field bus direct Inputs and Outputs
CONSOLA DEPROGRAMARE
MEMORIAPROGRAM
UNITATEA CNTRALĂ
MODULESPECIALE
MODULEDE
IEŞŞŞŞIRE
MODULE DE INTRARE
Trad. MAŞŞŞŞINA ELECTRICĂ Contactoare
Busses and processors in industrial plants
PLC nodes(multi-processors)
fieldbus
Operator panelMimic board
plant (Werk, usine)
P
disk
pool
transducers
controlstations
process bus (500m .. 3 km)
valve thermo-couple motor
Process pictures
Process Data Base
Logging
position
station bus
(0,5.. 30 m)
node bus
workstation bus
instrument bus(mimic board)
sensor bus
directly coupled input/output
open network:TCP/IP, ...
station
P P C
I/O MEM I/O
P P C P
MEM BC
station
M
sensor bus
(30m..2 km)
Example: Production management system
transportationcell control
manufacturingcell control
scheduling maintenance quality control
plant network
floor network
productionplanning
robotcontroll
er
enterprise network
millingmachine
rail-guidedvehicle
cell
Example: ABB Industrial IT (redundant system)
3rd partycontrollers,servers etc
Serial, OPCor fieldbus
engineeringworkplace
Field Bus
Firewall
Plant Network / Intranet
Field Bus
Client/server Network
3rd party application server
applicationserver
aspectserver
Workplaces(clients)
Enterprise Optimization
(clients)
MobileOperator
connectivityserver
Control Network
ProgrammableLogic ControllerAC 800C
RedundantAC 800M
touch-screen
plant
Decentralized Control System (DCS)
engineeringworkstation
operatorworkstation
data logger
controller controller controller controller
field bus
control bus
all controllers can communicate as peers (without going through a central master), restricted only by throughput and modularity considerations.
Note: Honeywell's "DCS™" stands for "Distributed Control System", it is not a decentralized control system, but a control system for the process industry.