3. y A programmable logic controller (PLC) is a digital
computer used for automation of electromechanical processes,
such as control of machinery on factory assembly lines,
control of amusement rides, or control of lighting fixtures.
y Programmable logic controllers are the most widely used
electronic devices in the control of production and assembly
process in most automated factories due to its simplicity and
versatility.
y A programmable logic controller (PLC) is a user-friendly,
microprocessor-based, specialized computer carrying out
control functions of many types and levels of complexity in
industrial applications.
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Introduction
6. ‡ CPU: is the unit containing the microprocessor
. Power supply unit: is needed to convert the mains A.C.
voltage to low D.C. Voltage.
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7. ‡ Programming device: is used to
entered the required program
into the memory of the
processor.
‡ Memory unit: is where the
program is stored that is to be
used for control actions.
;
8. ‡ Input-output sections: are where the processor receives
information from external devices and communicates
information to external devices.
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9. The S7-200 PLCs are expandable. Expansion modules
contain additional inputs and outputs. These are connected
to the base unit using a ribbon connector.
‡ Expansion Modules:
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10. PLC Operation
The PLC program is executed as
part of a repetitive process
referred to as a scan. A PLC scan
starts with the CPU reading the
status of inputs. The application
program is executed using the
status of the inputs. Once the
program is completed, the CPU
performs internal diagnostics and
communication tasks. The scan
cycle ends by updating the
outputs, then starts over. The cycle
time depends on the size of the
program, the number of I/Os, and
the amount of communication
required. 10
11. Programming a PLC :Programming a PLC :
y In order to create or change a program, the following items are
needed:
y PLC
y Programming Device
y Programming Software
y Connector Cable
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12. ‡ You can use a personal computer as a programming
device
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13. Advantages & Disadvantages of PLCsAdvantages & Disadvantages of PLCs
Advantages of PLCs:
y Increase in flexibility,
y Faster implementation of changes and correction,
y Lower cost,
y Easy visualization of process running,
y Increased operation speed,
y Increased reliability and maintainability,
y Increased security,
y Reprogramming capability,
y Elimination of wiring.
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14. Disadvantages of PLCsDisadvantages of PLCs
y Too much work required in connecting wires.
y Difficulty with changes or replacements.
y Requiring skillful work force in finding errors.
y Fixed program applications.
y High initial investment cost.
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