PC vs. PLC Airfi eld Lighting Control and Monitoring Systems: Tower

PC vs. PLC Airfi eld Lighting Control and Monitoring Systems: Tower
PC vs. PLC Airfield Lighting
Control and Monitoring Systems:
Which One Is Best for My Airport?
By Ed Runyon
Advanced Technology Manager
Siemens Airfield Solutions
In the early 1990s, computerized
control systems first started seeing wide
use in airfield lighting control and monitoring systems. Prior to that time, FAA
L-821 control panels were typically used
to interface with Air Traffic Controllers
(ATC). The L-821 typically consisted of
an array of ON/OFF switches, rotary
switches and pushbuttons. The L-821
was hardwired to the various airfield
lighting devices using customized relay
or contactor configurations.
Most major airports today use some
sort of computerized control system to
interface with airfield lighting equipment
using fiber optic, hardwired (copper
cables), or wireless communication
links. Touchscreens are commonly used
as the Human Machine Interface (HMI)
for ATC. In addition, the use of computers for airfield monitoring has allowed
the development of many sophisticated
software tools for airfield maintenance
troubleshooting. This includes
automated series circuit insulation
resistance measurement systems,
detailed Constant Current Regulator
(CCR) input and output monitoring, and
sophisticated graphical user interfaces
(GUI)–allowing detailed event/alarm
data to be graphically displayed in an
easy-to-read format.
A committee of industry experts formed
in 2003 to develop an FAA Advisory
Circular (AC) for these systems. The resulting Advisory Circular, 150/5345-56,
is titled “Specification for L-890 Airport
Lighting Control and Monitoring System
(ALCMS)” and was issued September 30,
2004. This AC allows either PC
(Personal Computer) or PLC (Programmable Logic Controller) based systems
to be used. Questions often come up
about which system should be used for
a particular situation. This article aims
to answer many of those questions.
In general terms, choice of a particular
L-890 architecture depends on what
you need to accomplish. This can be a
complex decision based on multiple
factors or a relatively easy one
depending on your airport’s specific
requirements.
Airfield lighting control and monitoring
systems can be generally placed into
three categories:
PLC-based Architecture
Tower
PC-based Architecture
Tower Touchscreen/
Panel Computer
Tower
Touchscreen
Control Station A
Touchscreen
Control Station B
Remote
Monitoring
Station(s)
Maintenance
Terminal
Following are a series
of questions to help you
decide which system is
right for your airport
Laser printer
• PLC-based architecture
• PC-based architecture
• Hybrid PC/PLC-based architecture
What applications are better suited to
a PLC-based system?
In general, PLC systems are ideal for
small to medium ALCMS applications
where basic monitoring, such as CCR
status and ON/OFF monitoring, is
needed, but more advanced monitoring is not required. And, a PLC system
uses various modules that are easy to
integrate and easy to replace.
A PLC architecture is optimized to
run sequential processes, increasing
reliability. PLC architectures can be
implemented in a number of different
ways. A block diagram of a typical PLC
system for a small to medium ALCMS
application is shown to right.
It is also possible to scale up the
functional capabilities of a PLC-based
system with the appropriate external
devices via a simple contact closure.
Separate L-827/L-829 monitors, for
example, can be connected to PLC
inputs to provide overall fault monitoring. Also, separate megging systems
can be connected to provide overall
caution/fault status indications.
What applications are better suited to
a PC-based architecture?
In general, a PC-based control system is
ideal for medium to large applications
that require detailed FAA L-827/L-829
CCR monitoring or that have Surface
Movement Guidance and Control
System (SMGCS) - monitored Runway
Guard Lights or Stop Bars as part of
the requirements. High level ALCMS
applications require additional data
manipulation, storage and reporting
characteristics that are difficult to
implement in a standard PLC.
What are my operations going to be?
Tower-Vault Ethernet
Communication
Vault
• VFR/IFR – If either no monitoring or
basic monitoring (on/off current
sensing or alarm contact closure
monitoring), a hybrid PC/PLC may
be better.
Communication Network
Vault Touchscreen
Panel Computer
Controls and
Monitoring Terminal
Airfield Lighting
Electical Center A
Controls and
Monitoring Terminal
Airfield Lighting
Electical Center B
Serial Communication
Siemens
Vault PLC
Redundant
communication
links
Redundant
communication
links
Controls and Monitoring Network
Controls and Monitoring Network
• CAT I – Depending on the application
requirements, a PLC may be better,
especially if basic advanced
monitoring (L-827/L-829 contact
closure) is acceptable.
• CAT II/CAT III or (possibly) CAT I –
A PC-based system is usually better
because advanced monitoring is
typically required.
CCR
Current Sensing
Relay (CSR)
CSR
CSR
What does the airport currently use
for control and/or what are their
control expectations?
CSR
PC-based ALCMS architectures are
configured to maximize reliability.
These systems typically use industrial
computers and redundant hardware
and communication links. Also, a
hot-standby computer architecture
(dual-redundant computers) is much
easier to realize in PC-based systems
(It is possible to implement a PLC hotstandby system, but it is more costly and
software intensive). PCs have standard
development platforms, available
high-end application software,
well-known programming languages
and familiar graphical interfaces
(Windows®). Also, megging systems
can be merged into a fully redundant
control and monitoring architecture.
This provides detailed megging data,
which can be viewed at any node in the
system. A block diagram of a typical PC
system is shown to the right.
Can I use a hybrid PC/PLC-based
system to get the best of
both worlds?
Hybrid PC/PLC-based architecture
For larger applications, the need for a
database, the elimination of multiple
hardwired control points and the
benefit of only having to be concerned
with troubleshooting a single type of
device—PCs—indicates that an all
PC-based system is the best choice.
For smaller applications, an integrated
Touchscreen/PC can be used to operate Touchscreen graphics and can also
connect to a standard PLC for remote
control and simple contact monitoring.
This is exactly what we implement
in our Navigator™ system, which is
intended for smaller applications with
up to 18 controllable items. A block
diagram of the Navigator system is
shown to right.
Tower
Touchscreen/
Embedded PC
Does airport operations perform a visual
lighting check every day to generate a
lamp-out report? If so, the airport may
not require reports on the total number
of lamps-out to be generated automatically. So, if either no monitoring or basic
status monitoring (i.e. current sensing;
relay contact feedback) is all that is required, a hybrid PC/PLC or PLC is better.
PLC
Photocell Control
Electrical Interface
Remote Dial-In
Control Electrical
Interface
Vault
Runway and
Taxiway Control/
Monitoring for any
manufacturers’ CCR
• Is it a busy airport that wants to
simplify operations? If yes, then a
PC-based system would be best.
Can circuit status monitoring be
reported manually or are frequent
updates needed?
Communication Link
Radio Control
Electrical Interface
• L-821: If all their existing functionality
can be included in the PLC, then a
PLC-based system would be best.
Control/
Monitoring
for Wind Cone,
Beacon, REILs,
PAPls, etc.
A PC-based system is better if the
airport requires advanced monitoring,
such as the detailed circuit status
information provided by a FAA L-827/
L-829 CCR electronic monitor. This type
of information includes: number of
lamps-out; CCR input and/or output
current, voltage, VA, wattage; CCR
output drop in VA (Low VA); and CCR
output insulation resistance value. If the
airport needs this type of information
quickly, or it needs individual airfield
fixture (i.e. Stop Bar or Runway Guard
Light) monitoring or complex analog
(current/voltage) monitoring, a PC-based
system is better. A PLC-based system cannot inherently perform FAA L-827/L-829
monitoring functions unless a separate
subsystem is connected into the system.
Is a series circuit Insulation Resistance
Monitoring System (IRMS) required?
Either PLC- or PC-based systems can have
IRMS functionality. Because PLC systems
do not have standard modules for airfield
series circuit IRMS monitoring, this would
need to be tied into the PLC as a separate
system. An IRMS is commonly integrated
with PC-based systems, allowing airfield
series circuit insulation resistance trends
to be easily viewed and analyzed on a
monitor. Also, in a distributed PC-based
architecture, there is no single point of
failure in the IRMS system.
Is latching remote control required?
If it is required that, in case of a complete
control system failure, all controlled
items (such as CCRs) stay in their last
state (latched) or go to a preconfigured
state, a PC-based system is best. In a
PC-based system, the local control/monitoring device—in our case, the Advanced
Control Equipment (ACETM) unit—has a
hardwired output to the controlled item
from latching relays.
Standard PLC output modules do not
have latching outputs. Per FAA AC
150/5345-56, par. 7.1, latching outputs
are required for CAT II/III operations. Hybrid PC/PLC or PLC systems do, however,
have the ability to set their output points
to a preset state if the microprocessor
fails (assuming internal power is present
in the PLC cabinet) or last commanded
state if the communication link fails and
assuming internal power is present in the
PLC cabinet. Also note that it is possible
to add latching functionality to a PLC,
but separate latching relay modules
must be added to each of the required
output points.
Do you want to minimize installation
cost/time at a larger airport?
If the application is complex, with many
CCRs and control points, then a PC is
the most cost-effective system with
the quickest installation turnaround,
because a redundant communication
cable is daisy-chained between each
local control/monitoring device—in our
case, the ACE unit. An equivalent PLC
system using PLC I/O modules would
require hundreds to thousands of individual
wires (using many additional conduits) to
implement equivalent control/monitoring
functionality. This would not be a
significant issue, however, if there is only
a small number of CCRs at the airport.
What are the future applications?
You also will want to determine what
type of changes may occur in the future.
A hybrid PC/PLC or a PLC may be better
if basic monitoring is needed today
and future upgrades will have the same
requirements. Equipment interface
upgrades may consist of simply adding
I/O modules for control/monitoring.
A PC-based system is best for a larger
airport that wants to simplify its operations
while retaining the ability to easily make
modifications or upgrades later on.
A PC will be easier to upgrade to a more
complex system or if significant design
changes are needed after installation.
What is the project budget?
• <$35K — Hybrid PC/PLC with
basic monitoring is the best.
• <$50K — PLC with basic monitoring
is best.
• $50K to 100K — PLC or PC. The choice
depends on the application
requirements.
• >$100K — PC with advanced
monitoring is best.
collection of data has pushed PCs
to become even more industrial and
robust. With innovative technologies
such as redundant power supplies and
flash drives, the PC has become a very
reliable choice for control applications.
Also note that PLC-based systems that
use a Touchscreen or need a database for
alarm/event reporting typically require
some sort of PC/microprocessor in the
tower or maintenance center, requiring
maintenance personnel to know how
to troubleshoot both PLC and PC
subsystems. So, in this case, it may
make sense to use only PCs throughout
the system.
Author Profile: Ed Runyon has worked
in airfield lighting for more than 25 years
and has served on various FAA
committees, including the ones that
developed the Runway Guard Light and
L-890 ALCMS specifications and also the
LED Engineering Brief.
©2007, Siemens Airfield Solutions.
All Rights Reserved.
Siemens Airfield Solutions
ALCMS Architecture Design Comparison Table
Design Parameter
Hybrid PC/PLC (Navigator™)
PLC
PC
Small to medium ALCMS
applications with only basic
control and monitoring
Best
OK, but not as cost-effective
as Hybrid PC/PLC
OK, but not as cost-effective as
PLC or Hybrid PC/PLC
Medium to large applications
that require advanced (i.e.,
FAA-L-827/L-829) monitoring
Full L-827/L-829 monitoring
not available
Full L-827/L-829 monitoring
not available unless a separate
subsystem is connected into
the system
Best
Small number (<_18) of
controllable elements
(such as CCRs) with only
basic monitoring desired
Best
OK, but not as cost-effective
as Hybrid PC/PLC
OK, but not as cost-effective as
PLC or Hybrid PC/PLC
Operations are VFR/IFR
Best
OK, but not as cost-effective
as Hybrid PC/PLC
OK, but not as cost-effective as
PLC or Hybrid PC/PLC
Operations are Cat II/III
Not available
Not the best solution because
advanced monitoring required
Best
Control expectations
Best if only L-821 ON/OFF
switch contact control desired
Best if only L-821 ON/OFF
switch contact control desired
and alarm database is required
Best for a busy airport that
wants to simplify its operations
Basic monitoring only
Best
Best
OK, but not as cost-effective as
PLC or Hybrid PC/PLC
Advanced monitoring required
Not available
May not be possible
Best
Airport has SMGCS operations
Not available
Possible, but not the
optimum solution
Best
Hot-standby architecture
Not available
Can be done, but implementation is more costly and
software intensive
Best
IRMS required
Not available
Possible, but tied into the PLC
as a separate system
Can be easily integrated into
the system
Latching outputs required
Not available in PLC modules.
Not available in PLC modules.
Requires separate latching
relay modification kits
Best
Only Preset output required
OK
OK
OK
Minimal installation cost/time
OK, if there is only a small
number of CCRs at the airport
OK, if there is only a small
number of CCRs at the airport
Best if application is complex
with many CCRs and control
points
Upgradability for future
applications
Easily upgraded for basic
control/monitoring applications,
assuming resulting total
number of controllable items
are small
Easily upgraded for
basic control/monitoring
applications
Can be upgraded for any
application
Reliability
Highly reliable, especially with
the use of flash hard drives
Highly reliable
Highly reliable with redundant
architecture and industrial PC
Purchase Cost
Low for a small to medium
size airport with only basic
monitoring requirements
Low for a small to medium
size airport with only basic
monitoring requirements
Low for a medium to large
airport with more advanced
monitoring requirements
Maintenance staff technical
experience
Best if the application is not
complex and someone on staff
has PLC experience
Best if the application is not
complex and someone on
staff has PLC experience
Best if the application has
advanced monitoring and
someone on staff has PC
experience
Ease of maintenance
Easier to troubleshoot if airport
maintenance staff familiar with
PCs and PLCs
Easier to troubleshoot for
basic control/monitoring
applications if airport
maintenance staff familiar
with the specific PLC type
Easier to troubleshoot for
advanced control/monitoring
applications, if airport maintenance staff familiar with PCs
Interactive maintenance/
troubleshooting tools
Not available
Standard PLC has some limited
maintenance screens. Possible if
a computer is separately added
into system to generate graphics.
Has fully interactive
Windows-based Graphical
User Interface (GUI)
Which system is easier to maintain?
It depends. A PLC-based system is easier to
troubleshoot for simple applications if the
airport maintenance staff is familiar with
the specific PLC type. PLC modules are very
easy to replace and mechanically rugged.
And, replacement parts are often
available locally.
For complex ALCMS applications,
a PC-based system is often easy to
troubleshoot, because detailed alarm
and event views are provided that
pinpoint the source of the problem.
PC subsystems and components are
typically available locally or are readily
available from the manufacturer.
For example, SAS maximizes the use of
commercial off-the-shelf parts to reduce
cost and increase spare parts availability.
The entire ACE unit (the L-827/L-829
control/monitoring part of the system)
can be easily replaced. Industries are
becoming more distributed and the
requirement for the reporting and
©2007 Siemens Airfield Solutions. All Rights Reserved.
Rev. B
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