(Programmable Logic Controller)
PLC has come a long way from its development in the seventies, whilst
their speed will never match that of the PC but who wants a 3GHz
PLC, its overall reliability, customization and ability to work
under virtually any conditions around the world make the PLC the
only choice when process control is required.
While the PLC is without doubt the number one choice in process
control, this doesn't mean it is without problems there are two
areas that have caused some problems for engineers. The first problem
was that the original concept back in the seventies was to develop
a device that could be easily and quickly programmed, so Ladder
logic was developed to make it familiar to the electrical engineer,
however over the years PLC's were being supplied by several major
names such as: Allen Bradley - Omron - Siemens - Mitsubishi - Square
D, and here the problem was created, each of them developed their
own style of Ladder logic.
developers this meant learning several different styles of programming
and also how to use the differing development tools, while the developer
could cope with this, a large number of maintenance engineers found
it very difficult. I personally know one international company that
has 3 different PLC's and the maintenance engineers only know one
and they struggle with that one, the lack of control within the
company means the company downtime and efficiency is far below what
it could be.
The second problem is new, PLC's have to be robust to survive the
industrial environment, the designs up till 2001 were very well
suited and also lent themselves to rapid replacement of faulty modules,
however some PLC designers have now developed PLC's where the surface
area of plastic is only 50%, this is easily damaged, and during
assembly into racks small metal objects such as screws can fall
into the sensitive electronics.
Also gone is the backplane that allowed rapid replacement, now
modules have to be slotted together, so if the middle one fails
you have to strip the PLC down. Finally the miniature size causes
wiring problems, the engineers I know tend to have fingers the size
of bananas, so you can imagine the fun they have in rack assembly.
Despite these little drawbacks, the PLC is still far better than
the PC, which has more the two failings one of which tends to be
the Operating System.
- a US product. Their Logix Platforms supply the end user with everything
necessary to build a reliable application in control integration
from Control to Drive to HMI Logix, operating over both Open and
· Omron - extensively
used throughout Europe and Far East. They have extended their range
of PLC's with the new CJ1 series for total machine control. The
CJ series is set to replace the very popular and reliable C200H
alpha series. Program development is via Omron's Cx Programmer,
the latest version is v6.0, replacing the dated Syswin ver3.4.
product. For many years Siemens PLC's have been amongst those at
the top, with safety, reliability and flexibility as their selling
points.The Simatic range is very extensive and can provide an overall
solution to process and machine control. Cost is expensive but then
you are paying for a good product, however licensing and their custom
programming laptop I consider too much, paying several thousand
pounds against Omron and Mitsi few hundred pounds. Support is vital
and if you are unfamiliar with the product then support is a priority.
Mitsubishi now have
series PLC, a more powerful and higher performance PLC.
It is very flexible and modular, allowing it to be expanded up to
384 IO, with instructions being processed in 0.065usec. Communicating
via Profibus and Ethernet. Further details on this can be found
via LC Automation and their INFORMATION
Mitsubishi provide an extensive range of products from PLC's to
Drives, Robots and HMI's and most importantly their overall customer
support is good. The PLC development tool is GxDeveloper, this superceded
Medoc, GX Developer is a bit dated, however they now have available
GXiec, which is a very good development package, allowing you to
create programs based upon IEC61131.
GXiec in our opinion is very good.
Keyence The world's
smallest PLC with AC power supply built-in. Easy-to-use Access Window,
Compact operator interface panel available. Fast processing with
10-µs interrupt and 30-kHz high-speed counters. Complete with
User-friendly Windows® ladder logic software.