6.3.3 List the
advantages of PLC
• Increased
Reliability
• More
Flexibility
• Lower
Cost
• Faster
Response
• Easier
to troubleshoot
• Remote
control capability
• Communication
Capability
VS COMPUTER CONTROLS
• The
architecture of a PLC’s CPU is basically the same as that of a general purpose
computer; however, some important characteristics set them apart.
• First,
unlike computers, PLCs are specifically designed to survive the harsh
conditions of the industrial environment.
• A
well-designed PLC can be placed in an area with substantial amounts of
electrical noise, electromagnetic interference, mechanical vibration, and
noncondensing humidity.
• A
second distinction of PLCs is that their hardware and software are designed
for easy use by plant electricians and technicians.
• The
hardware interfaces for connecting field devices are actually part of the PLC
itself and are easily connected. The modular and self-diagnosing interface
circuits are able to pinpoint malfunctions and, moreover, are easily removed
and replaced.
• Also,
the software programming uses conventional relay ladder symbols, or other
easily learned languages, which are familiar to plant personnel.
• Whereas
computers are complex computing machines capable of executing several programs
or tasks simultaneously and in any order, the standard PLC executes a single
program in an orderly, sequential fashion from first to last instruction.
• Bear
in mind, however, that PLCs as a system continue to become more intelligent.
• Complex
PLC systems now provide multiprocessor and multitasking capabilities, where one
PLC may control several programs in a single CPU enclosure with several
processors
VS PERSONAL COMPUTER
• The
personal computer is not a direct competitor of the PLC in control
applications.
• Rather, it is an ally in the
implementation of the control solution.
• The
personal computer and the PLC possess similar CPU architecture; however, they
distinctively differ in the way they connect field devices.
• While
new, rugged, industrial personal computers can sometimes sustain midrange
industrial environments, their interconnection to field devices still presents
difficulties.
• These
computers must communicate with I/O interfaces not necessarily designed for
them, and their programming languages may not meet the standards of ladder
diagram programming.
• This
presents a problem to people familiar with the ladder diagram standard when
troubleshooting and making changes to the system.
• The
personal computer is, however, being used as the programming device of
choice for PLCs in the market, where PLC manufacturers and third-party PLC
support developers come up with programming and documentation systems for their
PLC product lines.
• Personal
computers are also being employed to gather process data from PLCs and to display
information about the process or machine (i.e., they are being used as
graphic user interfaces, or GUIs).
• Because
of their number-crunching capabilities, personal computers are also well suited
to complement programmable controllers and to bridge the communication gap,
through a network, between a PLC system and other mainframe computers
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