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PLCs AND HMIs
method and the sealing arrangements
for dust and fluids.
From this you can gauge the likely
functionality. A quick check of the
headline software features does the rest.
For example, does it offer scalable
graphics (you don’t want to have to
develop separately for each size of
panel)? Also, how does the alarm
handling interface with external systems,
and can the panel generate SMS alerts
directly without needing another
program (as would be the case for a PCbased
alternative)? Is the HMI program
backed-up to non-volatile memory and
can the memory be expanded via
memory card slots? The ability to use
one HMI panel to display another’s
content can also be an advantage. The
list goes on, but attractive and functional
features are exposed in this way.
The remaining item on the checklist is
connectivity to controllers. Extensive
connections are needed for a standard
hardware and software HMI system that
can be deployed across a whole site and
for machine-builders who may have
different controllers on different
machines, but want to benefit from a
standard operator display.
Compact Flash-slot
Can – or should – you abandon PCbased
Scada and move to CE-based
HMI? Well, you would save a lot of
money but, as ever, you need to take a
balanced view. Modern CE-based HMIs
do not have the same compromises and
difficulties that you might remember
from earlier generations. Combined
hardware and software systems will
probably offer useful extra functions
(such as SMS alerting) compared to
RS422/485 Expansion port Internal Compact Flash-slot
USB Host
Ethernet
A peek around the back of an HMI panel reveals its connections and other design features
RS232
piecemeal supplies.
The control room and the line
controller are the domains of Scada and
DCS solutions. But “out there” and
“down there” you need the functions,
robustness and attractive pricing of an
all-in-one CE-based HMI panel. This
could be the right time for you to reevaluate
your view of CE-based HMIs –
there is much to be gained, and nothing
D&C
to be lost.
Dual PLCs ensure a smooth lift for aircraft passengers
An Italian company called Oxley specialises in manufacturing aerial
platform systems, including vehicles used to transfer people with
restricted mobility to and from passenger aircraft.
Traditionally these vehicles have been based on diesel-engined trucks
which have been costly to buy and maintain. Since 2005, Oxley has
been working on an alternative electrically-powered design which is
smaller and cheaper to operate and has been designed to comply with
new European airport and environmental regulations.
A key requirement was the ability to operate the lifting system both
from the ground and from the platform in the raised position, and to
provide the operator with full information on this process.
Initially, Oxley produced a platform using relay-based electronics but
the authorities rejected this approach, saying that it did not comply
with stringent airport standards. Oxley therefore decided to fit the
mechanical components, fully tested, to the vehicle and to adopt
programmable industrial controls.
The latest vehicle – known as the NewOx906Air – is controlled by two
Rockwell Automation Micrologix 1100 PLCs. One PLC is installed on the
platform which carries the passenger and operator. The second is
located within the electric motor bay in the lower section of the
vehicle. The PLCs communicate with each other using the DH485
protocol, allowing communication via three coiled cables which avoid
problems of cable movement and wear.
The use of two PLCs guarantees redundancy of the control systems
responsible for maintaining the platform in the correct horizontal
attitude, and prevents any unexpected movement and the risk of
collision or crushing when docking with the aircraft. Operators can
check the operating parameters via a Rockwell PanelView C300 control
panel which also provides first-level diagnostics for detecting faults.
Maintenance personnel can monitor the control system remotely via
Uplifting: Oxley’s air passenger transport vehicle uses a pair of PLCs to
provide redundant control and to allow operation from two locations
Ethernet, thus reducing maintenance times and costs in the event of
any problems.
The electrically-propelled platform is said to be quick and simple to
operate. It is fitted with all of the safety systems needed to embark and
disembark passengers without risk to their safety or damage to the
aircraft.
40 November/December 2010 www.drives.co.uk