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 nonprocess industries. By all
indications, these microprocessor powered giants will continue to break new
ground in the automated factory into 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 wan 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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