Proceedings 2002/2003 - IRSE

90
INTERNATIONAL CONVENTION
Typical ‘Cerberus’ installation – modem, monitor
and interface devices – (l to r) input optoisolator,
output optoisolator and current sensors
VITAL INPUT OPTOISOLATOR (VIO)
The integrity of railway signalling circuits is
ensured, in part, by maintaining a high degree of
isolation from earth and between circuits.
The VIO provides a safe means of detecting ‘on’
and ‘off’ logic states in vital relay circuits. It gives at
least 10,000 volts isolation between input (signalling
circuits) and output (monitor). The input circuitry is
also designed to protect the signalling circuits from
unreliable operation due to single component faults
in the interface.
8-CHANNEL A-TO-D CONVERTER
As the development proceeded, it became evident
that there was only one ‘of-the-shelf’ analogue to
digital (A-to-D) converter board available to suit the
data acquisition board which formed the basis of the
monitor. Since this A-to-D board was prohibitively
expensive, the team elected to custom-design a
device for ‘Cerberus’.
Mounted piggy-back on the main data acquisition
board, the unit selects from one of eight analogue
inputs, and converts it to an equivalent 10-bit digital
value for logging or processing by the monitor.
The 10 bit digitisation provides measurements of a
20-volt input signal accurate to the nearest 20
millivolts.
MARKET COMPARISON
Throughout the development period of the
‘Cerberus’ system, other level crossing monitoring
systems have been available. Apart from one system
from Sweden, most of these have come from level
crossing equipment manufacturers in the USA. More
recently, a monitoring system has been
developed for Westrail by Motherwell Automation.
Some of these were evaluated for use in New South
Wales, but were found to fall short of local
requirements.
COMPETING SYSTEMS NOT ADEQUATE FOR
REQUIREMENTS
Most competing systems function as simple event
recorders. Few systems provide on-site evaluation
of crossing operation with immediate notification of
fault conditions to a central office. Most either
retained data for interrogation at site, or transmitted
all event data to the central office for logging and
evaluation. At the time of its introduction, the
‘Cerberus’ system was unique in its use of
algorithms to check the correct logic and timing of
crossing protection operation.
The lack of central reporting severely reduces the
ability of the monitor to improve the overall safety of
the crossing protection system, while 100 percent
reporting involves heavy overheads in communication
time and costs.
Lamp checking functions are generally analogue
electronic, and of limited effectiveness. The
‘Harmon’ system from USA has a lamp current
monitor based on similar Hall-effect sensors to
those in the ‘Cerberus’ system, but this is a discrete
unit with a simple ‘OK/Not OK’ input to the data
recorder.
Logger memory capacity is generally limited,
which limits the range of parameters which can be
logged. In addition, the recorded data is frequently
not presented in an easily readable format, making
interpretation difficult.
None of the other systems seen have offered
remote testing of the crossing. In New South Wales,
this feature alone is sufficient to provide economic
justification for installing the system. It must be
noted however, that many other railway administrations
do not apply the same regime of daily
testing that has long been standard in New South
Wales. We have noted also, with some surprise, that
some railways have ordered crossing monitor
systems which included no lamp checking functions!
OUTCOMES & BENEFITS
The ‘Cerberus’ systems delivers a wide range of
benefits to the railway operator.
INCREASED LEVEL CROSSING SAFETY
First and foremost, the ‘Cerberus’ level crossing
monitor delivers an immediate improvement in the
safety of the level crossing protection.
In common with the design of other safety critical
systems, level crossing protection is designed so
that it takes a number of concurrent failures to result
in a loss of warning to road traffic. In an unmonitored
level crossing, an initial fault will not be detected
until, at best, the first daily test after it occurs. In
many cases, the fault will be undetected until the
next monthly visit by maintenance staff. Further,
intermittent faults, unless they are evident at the time
the crossing is being tested or maintained, may go
undetected for a very long time.
Where the ‘Cerberus’ monitor is in operation, and
depending on the precise nature of the fault, it will be
identified immediately, or during the passage of the
first train. This applies equally for intermittent and
persistent faults.