Evolution or revolution?
26 Nov 2009
The DfT states that the European Railway Traffic Management System (ERTMS) will progressively replace conventional signalling on major routes over the next 15 to 35 years. Other estimates place a longer timescale on it. In the meantime, conventional multi-aspect signalling schemes will continue to roll out. One thing that's definitely outmoded though is the incandescent light bulb.
Last month's issue of the rail engineer provided one example of how LED technology can be used to provide a more reliable and cost-effective alternative to filament lamps. Just as importantly, the higher visibility of LED signals and their relative immunity from phantom aspects could provide significant safety improvements.
The adoption of LED technology for signalling purposes has not been without some technical challenges. Stabilisation of LED luminance over time and ensuring that aspect colour - particularly the yellow aspects - is maintained within specification have been the main issues. As so often in life, a problem can have more than one solution and so it has been here. It's interesting to compare the different approaches so let's take a look at how the signal manufacturers are putting LEDs to work on the lineside.
Unipart Rail Ltd, Dorman - Dorman pioneered the technology used in LED signals and remains the market leader in rail. As Dave Tilmouth, Marketing Services Manager for Unipart Rail/Dorman explains, "Commencing in 2004 and completed in 2007, a massive change-out programme saw the elimination of the filament-type position light signal (PLS) in favour of the new Dorman LED units. Being the first widespread use of LED signalling technology in the UK, this programme was closely monitored. The signals performed excellently and were welcomed by drivers who recognised that they offered a significant improvement over the previous filament version."
To date, over 12,600 Dorman PLSs have been installed across the network. Dave Tilmouth continues, "We've gone on to develop and manufacture a full range of LED signals including 16,500 barrier boom lamps, 1,000 junction indicators, 5,000 colour light signals (CLS) - both medium and long range - as well as our miniature and standard indicators, and banner repeaters. A national fitment programme of Dorman LED level crossing wigwags is currently underway, totalling some 3,500 units."
The first Dorman LED multi-aspect colour light signal was fully approved in 2003 and has since become a common sight nationally. Clive Porter, Dorman's Senior Technical Sales Manager - Rail, outlines the design concept. "Unlike any other LED colour light signal, the Dorman CLS uses a searchlight configuration with the lamp assembly displaying the red, yellow and green aspects through a single aperture. Where a double-yellow aspect is required, another lamp unit is used in a second aperture. Each signal aspect is therefore brought closely in line with the driver's eye. Another advantage of the searchlight unit is that the aspect is constantly illuminated so sunlight phantoms are eliminated."
Dorman does not use coloured filters so the output from the LEDs is closely controlled, as Clive Porter is keen to explain. "Within the signals, a separate LED for each chain has its output monitored for light output and colour. We use this system to automatically adjust the drive current of the LEDs via an optical feedback mechanism and so maintain the beam intensity over the voltage range and throughout the signal's life. This system also detects the LEDs' condition and maintains the lamp-proving element required for the interlocking by mimicking the characteristics of a filament lamp. The colour of the light emitted from the LEDs is kept within the Group Standard limits by carefully controlling the thermal environment within the signal unit. This even applies to the second yellow aspect which is always ready in a condition to illuminate at the correct colour when required."
The signal is fitted with an easily replaceable plain convex polycarbonate outer cover which has been developed to reduce the effects of sunlight reflections and provide resistance to vandalism. For close range viewing, an ‘eyebrow' of ten LEDs is positioned around one-quarter of the aspect circumference. The entire signal assembly is very robust with the lamp units being contained within diecast aluminium housings. The lamps are also hinged rearward to allow lens cleaning and light measurement from a position of safety.
In May 2009, Dorman announced its successful bid for a three-year framework contract to supply LED signal heads and LED banner repeater signals to Network Rail.
Howells - Howells Railway Products is a long-established manufacturer with products including conventional filament signals, route indicators and AWS equipment.
For the upgrade of the Jubilee and Northern lines (JNUP), it has provided a wide variety of LED-based signals and indicators including a short-range (200-metre) LED three-aspect colour light signal. This uses an LED module contained within existing signal head components and a conventional spread-light lens. For JNUP, the proceed indication is given by green or blue aspects - blue being displayed when trains are running under automatic control. Development of this signal took place during October 2007 with readability tests being undertaken the following January.
The range of LED indicators for JNUP is growing steadily and includes products covering direction, ‘route secure' and ‘rail gap' along with a point position indicator for the Docklands Light Railway.
Despite patenting technology for LED long-range colour light applications many years ago, the company has instead concentrated on other markets and is yet to produce an approved signal for use on Network Rail infrastructure. Whilst the company will not be drawn on a reason for this, it would appear that the situation might shortly change as it has recently developed several new LED products which specifically target Network Rail applications.
Invensys Rail - Invensys Rail has produced three types of LED signal - two for metro use and one for main line applications.
The WESTLED 100mm LED signal is a short-range colour light metro signal. The casing is just 150mm wide to permit installation in tunnels. During 2004, 47 of these signals were installed on the Jubilee Line extension. They are available with one to five aspects, the options including blue. Each LED light unit comprises a sealed module containing an LED array and a power controller circuit that maintains a constant light output from the LEDs. Lamp proving variants are available and incorporate an integral ballast resistor to emulate the current drawn by a filament lamp. A new version has been developed for use on the Victoria Line upgrade and readability tests will be undertaken shortly. Additionally, a platform repeater variant has been on trial at Gloucester Road since 2006.
The WESTLED Series-3000 colour light signal is intended to provide a direct replacement for filament-type colour light signals on Network Rail. It was trialled from 2003 until 2005 on the Chiltern main line.
As Richard Ward, Customer Account Manager at Invensys Rail, explains, "The WESTLED Series-3000 has been designed as a direct like-for-like replacement for the Series-2000 filament CLS. The mechanical and electrical interfaces are directly interchangeable which simplifies and reduces installation time. There is no requirement to modify drawings or signal sighting records upon a new signal installation. WESTLED is also lighter than all filament signals installed on the network so there's no need for any structural alterations to existing signalling gantries."
Each signal aspect has a patented pattern of 85 LEDs arranged in two groups fed by a double power controller system. The failure of one power controller or associated LED array results in 50% of the LEDs becoming extinguished and a non-urgent alarm output being indicated. Failure of both power controllers - indicated by the aspect drawing <50mA - will result in all the LEDs becoming extinguished and an urgent alarm being triggered. A ballast resistor ensures that each aspect draws the same current as the equivalent SL35 filament lamp.
Directly in front of the LED array is a clear acrylic multi-faceted lens and a plain polycarbonate outer cover. This lens system is designed to minimise reflections from low sun or other light sources. A close viewing sector is configurable on site to allow right or left-hand signal sighting.
Signal House Ltd - In November we examined how one company, SHL, overcame the challenges of applying LED technology to signalling. To eliminate the effects of time and temperature on LED luminance and wavelength, the SHL solution makes use of a closed-loop servo circuit to monitor the LED output and adjust the drive. In addition, light from each aspect is filtered to ensure compliance with the BS1376 standard for signal colours. If the LED peak wavelength were to move out of the filter pass-band, the signal beam intensity would fall. The servo circuit senses this and automatically adjusts the LED drive current to provide a stable signal output.
This new signal has been trialled with great success in Scotland and it has recently gained full approval from Network Rail. SHL has also secured a three-year Network Rail framework supply contract for LED colour light signal heads.
SHL has been working on a new project, as David Eades, Group Sales & Marketing Manager, reveals. "We've developed something called the ‘LED illuminator'. It's quite a grand title for what essentially is the replacement for the quartz halogen lamp found in fibre-optic route indicators. Fibre optics are great - they give exceptional flexibility in display, allow you to keep stock of certain fast-moving items and ensure that you can't get a corrupted aspect. The only problem was the light source.
"The illuminator - essentially an LED light bulb that lasts about five years - can be retrofitted to more modern existing units. It's currently on trial in Scotland in a banner repeater signal (BRS) application and it's our intention to use the technology in stencil and theatre indicators once it's approved. By keeping the flexibility associated with fibre-optics, we can produce eight, nine or ten aspects in a single theatre indicator using the LED illuminator."
One of the fibres is looped back to a sensor that monitors the output and ensures that the fibre bundle is plugged into the LED lamp box. As David Eades explains, "Essentially a unit could be lit but there could be no output. The loopback ensures that the maintainer has remembered to plug the fibre bundle back in!"
The temperature of the LEDs is maintained by a lightweight heatsink and fan system, similar to that developed for SHL's new colour light signal.
VMSL - With an eight-year track record in the rail industry, Variable Message Signs Ltd (VMSL) is a relative newcomer although it has been producing road messaging signage for over 25 years. Its LED colour light signal is now fully approved, without restrictions, for use on Network Rail infrastructure.
Tony Isaacs, Business Manager - Rail for VMSL, says "The signal design utilises the VMSL-patented ‘Rigel' LED system, used previously in Highways Agency-compliant messaging signs and Safewatch speed and vehicle activated warning signs (this was featured in issue 16 of the rail engineer). Requiring no thermal management to maintain colour compliance, each LED sits inside a specially shaped clear lens structure that directs more than 85% of the emitted light forwards and with the application-correct beam distribution. The LEDs are driven at typically 30% of their rating so there's little heat dissipation problem and the LED output is consistent over the longest achievable lifetime."
Forward of the LED array is a unique ‘internal visor' - a deep matt-black honeycomb plastic moulding with cells to allow the LED's light to exit. Its purpose is to absorb incident sunlight, preventing it being reflected back from the LEDs which might otherwise cause a phantom aspect to be displayed. This system is so effective that the signal doesn't need any external hoods. In fact, as Tony Isaacs puts it, "To obtain a similar effect by using an external hood, it would need to be 1.8 metres long!"
Each aspect has a non-magnifying polycarbonate outer lens that is angled downwards to prevent unwanted reflections. This lens also incorporates an internally adjusted wide-angle prismatic viewing sector for close range visibility, which is user adjustable. Exclusively, the VMSL optics ensure compliance for both long-range (800m) usage with 6° narrow-angle beam and medium range (400m) 16° wide-angle beam from a single type without adjustment or additional lenses.
The new signal design was trialled on the West Coast Main Line from 2006 at signals CE177 and CE178 at Clifton, just south of Penrith. Network Rail approval, without restrictions, came in 2008 and VMSL has since won a three-year supply framework deal with Network Rail.
The company has gone on to use similar innovative LED design features in its new banner repeater signal for Network Rail. Offered in two versions (two-state white or three-state white/green), use of the internal visor concept means that no hood is required and the whole BRS is only 70mm thick, weighing just 15kg. The first VMSL BRS was installed for trial purposes at Watford Tunnel in September 2007 - just 12 months after conception - and the design has been fully approved since 2008.
Tony Isaacs points out the importance of this project. "Additional signal types based on the successful lightweight ‘BRS concept' are completing development. These include 12-character Standard Alphanumeric Route Indicators (SARI) and MARI, and a new family of super-lightweight CLS, PLJI and PLS."
Standard revisions - With better understanding of LED technology and operational experience, it's becoming clear that signal specifications may need to be updated, perhaps including the human perception of the information provided by lineside signals. With filament signals, a prescriptive specification (BRS-SE81) was adequate. Now, with the introduction of diverse LED technologies, it could make more sense to identify the functional requirement for signal readability, taking into account LED cluster conspicuity, viewing angles and sun phantom characteristics.
With this in mind, independent consultant OptiConsulting UK has been working closely with RSSB, Network Rail and their suppliers. The 1999 Standard NR/SP/SIG/10062 specifies signal optical performance requirements whilst Group Standard GK/RT0031 defines the readability requirements. The current knowledge of LED technology and ongoing research is likely to be used in preparing revisions of these documents.
The use of LEDs in railway signalling has brought benefits in terms of performance enhancements, reliability and whole-life cost. As we have seen, the LED signal solutions are diverse in design but aim to deliver consistent readability performance to the train driver. These innovations directly support Network Rail's challenges to the industry to create a safer railway signalling system that's reliable, easy to maintain and energy efficient.