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8/8/2019 Ch 4 Plc Lecure
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Programmable Logic Controllers
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Objectives
Define PLC.
Explain where the PLC came from.
Explain why their use is valuable.
Explain where they are used.
Detail what PLCs can do.
Explain how PLCs know what they aresupposed to do.
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What Is
aProgrammable Controller?
Aprogrammable logic controller, usually
called a PLC orprogrammable controller,is a solid-state, digital, industrial computer.
Simply, a programmable controller is acomputer, much like a desktoppersonalcomputer.
A PLC is an industrially hardenedcomputer.
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Where Did thePLC Come From?
In the 1960s, electromechanical relays,
timers, counters, and sequencers were thestandard.
Many control panels contained hundredsof these devices and a mile or more ofwire.
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Where Did thePLC Come From?
Reliability was low and maintenance costs
were high. Cost was high to modify or upgrade control
panels.
In 1968 the General Motors Hydramaticdivision specified a device that wouldbecome what we know today as theprogrammable logic controller.
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Early PLCs
Only relay replacers
Did not have timers or counters
No sequencer instructions
No math instructions
No data manipulation instructions
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Why a PLC?
Easily changeable
Programmable
Reliable
Smaller
Fast switching
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Why a PLC?
Able to withstand harsh factory
environment Consumes less power
Easier to troubleshoot
Easy to install
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Why Use a PLC?
The question why use a PLC? should
really be rephrased to why automate? The PLC is the tool that provides the control
for the automated process.
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Automating Helps aManufacturing Facility
Gain complete control of the
manufacturing process Achieve consistency
Improve quality and accuracy
Work in difficult or hazardousenvironments
Increase productivity
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Automating Helps aManufacturing Facility
Shorten lead time to market
Lower cost of quality, scrap, and rework Offer greaterproduct variety
Allow a quick changeover from one
product to another
Control inventory
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A PLC Upon First Glance
A black box with wires bringing signals in
and other wires sending signals out Some sort of magic being done inside that
somehow decides when field devicesshould be turned on or off
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What Makes a PLC Work?
The heart of any computer is the
microprocessor. The microprocessor, also called the processor
or central processing unit (CPU), supervisessystem control through the userprogram.
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What Makes a PLC Work?
The processor reads input signals and
follows the instructions that theprogrammer has stored in the PLCsmemory.
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What Makes a PLC Work?
As a result of the solved program, the PLC
writes information to outputs, or field-controlled devices, to turn them on or off.
When the PLC is running and following theprograms instructions, this is called
solving the userprogram.
The PLC is running or in RUNMODE.
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What Makes a PLC Work?
The userprogram (ladderprogram) is the
list of instructions that tells the PLC what todo.
The library of instructions available to thePLC is called the instruction set.
The instruction set determines how muchflexibility the programmer has.
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Common PLC Inputs
Pushbuttons
Selector switches Limit switches and level switches
Proximity sensors
Photo switches
Relay contacts
Motor starter contacts
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An Overview ofa PLC System
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An Overview ofaPLC System
Incoming signals, or inputs, interact with
instructions in the userprogram to help thePLC determine when an input instruction iseither true or false.
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vantages o L : Use of s/w programming instead of hardware & wires
to control the i/o devices. Amount of wiring is cut-
down by 80%.
Faster scan time: Operational time of PLC is very fastas compare to Relays. The speed of PLC logic
operation is determine by scan time, which is a matterof millisecond.
Intelligent I/Os & high speed counters.
Supervisory control & ASCII message handlingcapability.
Reliability & Maintainability: PLC has very high
reliability rate & hence control system maintenancecost is low and downtime is minimal.
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Flexibility :- Flexibility in programming &reprogramming in plant. Also flexibility in control
technique. i.e. the control equipment can be easilyreconfigured to accommodate required change inprocess online.
Ability to communicate with computer system in
plant.
COST :- PLC can scan digital & analog inputsthrough relevant sensors. It can execute the logic
w.r.t. the scanned inputs, take necessary decision andsend it to digital / analog outputs. It can also performtimer, counter, sequencer, PID , and other controlfunctions. The cost of all this is much less than a
conventional DATA Logger !!
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Versatility :- The ability to combine discrete (Digital)& Analog logic is a powerful tool for the ControlEngineers. Control of critical start-up parameters, such
as temperature and pressure, can be precisely pre-programmed for each start-up step.
As one common device (PLC) performs multiple
functions in a Plant, fewer spare parts are needed.
Programmable troubleshooting aids reduces downtime.
The software consist of LAD, which is similar to relay
logic, so even technician can maintain the plant.
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Expandability :- As a process changes, it is inevitablethat changes will be needed or any expansion, theseusually require more outputs.
A PLC can easily change the sequence of processthrough software and also accommodates theadditional I/Os without requiring changes in the
existing wiring ( due to modular design).
If a PID loop is to be added, no panel rework isnecessary; only the wiring of new points and some re-
programming to incorporate them is required. Rugged Construction - Can operate in Extremely harsh
field conditions.
Small in physical Size.
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Limitations OfPLC People reluctance to change( New technology): It is
difficult to change mind set of people to newtechnology.
Environmental Considerations: Certain processenvironments, such as high heat, vibrations, noise level,interference with electronic devices limit the use ofPLC.
Fixed circuit operation: If circuit in operation is never
altered, a fixed control system might be less costly thanPLC. The PLC is most effective when periodic changesin operation are made.
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COMPARISION BETWEEN A PLC AND PCCOMPARISION BETWEEN A PLC AND PC
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APPLICATIONSAPPLICATIONS