New Danfoss VLT® Decentral Drive E cient, reliable and powerful

Time to
put HMIs
in control?
PLCs AND HMIs
The boundary between HMIs and Scada systems is breaking down,
with modern CE-based HMIs offering many of the functions previously
restricted to PC-based systems. Graham Isherwood (left), sales manager
for ABB Open Control Systems, examines this trend and
considers what it means for the automation market.
The distinction between HMI and
Scada is becoming increasingly
blurred. Traditionally, an HMI was a
simple interface to a PLC and was in the
domain of the machine-builder. Scada, by
contrast, was the “line controller” which
ran on a PC and had better functions for
visualisation, trending and alarm reporting
– but was usually not cost-effective at the
machine level.
Both technologies have evolved. Scada
vendors now offer Windows CE-based
products that run on HMI hardware, while
HMI vendors are adding features to boost
functionality.
Does this mean that there is now a
simple choice between “large” and
“small” Scada, or is there an application
niche for high-performance HMI panels,
that are both feature-rich and costeffective?
The automation industry has no
accurate definitions for HMI or Scada. The
two terms are often freely
interchangeable – or are simply seen as
being different names for the same thing.
You therefore need to set up a framework
to allow comparisons to be made.
In general, there are two generic
approaches to providing an operator
display – those based on Microsoft’s
Windows CE, and those based on fullscale
PCs. Windows CE displays are
probably less obvious to users because
vendors implement their own flash
screens on boot-up.
Microsoft’s CE developments (driven
largely by the requirements of the mobile
phone market and by CPU developments)
have resulted in CE becoming a powerful
platform for HMI applications.
On the PC-based side, you find Scada
and DCS. Scada vendors often add (and
sometimes remove) functions to provide a
CE-based offer, usually offering the same
development tools as for their PC-based
systems. DCS vendors tend to use CEbased
panels for peripheral operator
stations rather than for those found in
control rooms. The boundaries in the
operator display market are therefore
blurred, but this perhaps ignores other
important attributes in the hardware.
What’s missing?
CE devices tend to be more robust than
PCs, and thus more reliable in the field,
needing no mains power supply (they
operate from 24V DC supplies), no hard
drive (they use flash memory) and no fans.
These factors make CE-based HMIs
functionally attractive – even more so
when relative prices are considered, along
with easier installation and swap-out.
Another attraction is that the
combination of a CE panel and its
associated software is cheaper than the
PC-based Scada alternative. This has a
direct bearing when a system needs
multiple operator stations, and the budget
is a maximum of £1,000 per station.
But you might wonder what is
“missing” in a CE-based HMI that you
might regret later on. If the price per
station is lower, then what are you doing
without?
In the past, CE-based systems posed
some development problems, with an
over-dependence on processor types and
other limitations. In addition, the software
was clumsy and required a knowledge of
electronics as well as of automation.
Thankfully this is over. There are now
two sub-divisions within the CE operator
station market – those from vendors that
supply both hardware and software, and
those that supply these elements
separately.
Both approaches have attractions in
terms of squeezing more out of the CE
format. Holistic hardware with software
combinations offer ease-of-use and
extended, dedicated functions.
If a CE-based panel is going to be costeffective,
it has to offer more for your
money than an equivalent PC-based
system (bearing in mind the software
licencing cost). The front has to look good
(offering a choice of a touchscreen or
function keys), but the detail is at the rear.
Turning an HMI panel around shows its
connections and how thoughtfully and
practically it has been designed. For
example, can you see an RJ-45 Ethernet
connector, a USB socket, a memory card
slot, an old-style D-Sub serial port, a
printer port, a 24V DC terminal, and so
on? You should also check the mounting
www.drives.co.uk November/December 2010 39