plc英文资料毕业设计
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2010-01-17
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Introduction of Programmable controllers
From a simple heritage, these remarkable systems have evolved to not
only replace electromechanical devices, but to solve an ever-increasing array
of control problems in both process and reprocessed industries. By all
indications, these microprocessor powered giants will continue to break new
ground in the automated factory in the 1990s.
HISTORY
In the 1960s, electromechanical devices were the order of the day
ass far as control was concerned. These devices, commonly known as
relays, were being used by the thousands to control many sequential-type
manufacturing processes and stand-along machines. Many of these relays
were in use in the transportation industry, more specifically, the automotive
industry. These relays used hundreds of wires and their interconnections to
effect a control solution. The performance of a relay was basically reliable - at
least as a single device. But the common applications for relay panels called
for 300 to 500 or more relays, and the reliability and maintenance issues
associated with supporting these panels became a very great challenge. Cost
became another issue, for in spite of the low cost of the relay itself, the
installed cost of the panel could be quite high. The total cost including
purchased parts, wiring, and installation labor, could range from $30~$50 per
relay. To make matters worse, the constantly changing needs of a process
called for recurring modifications of a control panel. With relays, this was a
costly prospect, as it was accomplished by a major rewiring effort on the
panel. In addition these changes were sometimes poorly documented,
causing a second-shift maintenance nightmare months later. In light of this, it
was not uncommon to discard an entire control panel in favor of a new one
with the appropriate components wired in a manner suited for the new
process. Add to this the unpredictable, and potentially high, cost of
maintaining these systems as on high-volume motor vehicle production lines,
and it became clear that something was needed to improve the control
process – to make it more reliable, easier to troubleshoot, and more adaptable
to changing control needs.
That something, in the late 1960s, was the first programmable controller.
This first revolutionary system developed as a specific response to the needs
of the major automotive manufacturers in the United States. These early
controllers, or programmable logic controllers (PLC), represented the first
systems that 1 could be used on the factory floor, 2 could have there ‘logic’
changed without extensive rewiring or component changes, and 3 were easy
to diagnose and repair when problems occurred.
It is interesting to observe the progress that has been made in the past 15
years in the programmable controller area. The pioneer products of the late
1960s must have been confusing and frightening to a great number of people.
For example, what happened to the hardwired and electromechanical devices
that maintenance personnel were used to repairing with hand tools? They
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