Developing a new wireless-operated distant signal
Network Rail’s modular signalling programme was set up some five years ago to develop ways of renewing the signalling on secondary routes at a substantially lower cost than could be achieved by the usual techniques.
Secondary routes are characterised by lower linespeeds, simpler layouts and a lower-frequency train service. This allows the functionality provided by the signalling system to be simplified by comparison with that required for a busier and more complex primary route. It is this simplification, together with an approach to design, construction and testing based on standardisation and increased offsite preparation, which delivers the required cost saving.
With two pilot schemes commissioned and further schemes in development, attention is now turning to further ways to reduce cost. One initiative is to reduce the amount of lineside cable needed by using a wireless controlled Distant signal, with the power required to operate it generated at the signal. The Distant signal is the first signal seen by a driver and is located at braking distance, typically 1600 metres, from the Stop signal which controls access to the next section of track or protects an infrastructure feature such as a level crossing.
A development programme was initiated to investigate the possibilities of controlling such a signal by radio. Westermo took charge of the radio element, Rockwell Automation worked on the PLCs (programmable logic controllers) and Firstco acted as system integrators working with Network Rail.
The resulting signal uses four key elements. Firstly, lightweight, low power consumption LED signals have recently been developed which don’t need to be backwards compatible with traditional filament lamp signals. With a filament lamp, proving is done by measuring the current drawn. First-generation LED signals retained this method, using a ballast resistor to mimic the current draw of a filament lamp. The new LED signals provide feedback that the signal is functioning correctly by means of voltage-free contacts and hence consume far less power.
60MHz wireless systems, which again have a low power consumption, have only recently been made available for licensed use by Ofcom. This frequency band is licensed, giving a degree of control over usage and has a typical maximum range of about five miles which means that the range required for this application, around 2000 metres from the local equipment room to the Distant signal, can be achieved reliably. The wireless system which Westermo selected has modern addressing and encoding facilities to help ensure the integrity of the data carried.
Fuel cells have been around for over a century, but their first commercial use came in NASA space programmes. Over the past 40 or so years typical applications have been to provide high power for relatively short durations but small capacity units, delivering around 100W of power, have become commercially available over the last few years.
A fuel cell works by passing a fuel, in this case methanol, and air across a membrane impregnated with a catalyst. A chemical reaction occurs generating a DC voltage which can be used to charge a battery. Water and carbon dioxide are the only waste products.
There are a number of different types of fuel cells. The methanol type was chosen for this project, partly because it presents no specific hazards to the user or maintainer. The fuel is supplied in robust polyethylene canisters, easily installed without special precautions and safe to transport.
Industrial control equipment, rated at Safety Integrity Level (SIL) 3, is used to control and monitor the signal. A desire to see industrial Programmable Logic Controllers (PLC) used within a signalling system is part of Network Rail’s technical strategy. PLCs offer many advantages – as a result of their wide customer base and large scale manufacture they are very reliable, competitively priced and well supported with a proven history of forwards and backwards compatibility.
From this mix of components it has been possible to design a Distant signal which can be operated over a typical distance of around two kilometres from the local equipment room. Power management is key to the design, so the signal is lit in time to be viewed from the approaching train and then extinguished after the train has gone past. A self test is carried out before a train is allowed to approach the signal to ensure it will function correctly when called upon. By this means overall power consumption is low enough for the methanol fuel canisters to need replacing only once a year.
The concept has been proved by means of a year-long trial away from the railway, with a simulated train service typical of a secondary route. Work is now underway to demonstrate that the system can meet safety targets, particularly with regard to the radio communications, and also cost targets.
Development of the wireless signal matches several of the themes in the Network Rail technical strategy, covering cost effective procurement, energy efficiency and improvements to staff safety. This is achieved by reducing the time required on track both to install and maintain it. Removal of the need to run cable to the signal saves both cost and avoids the risk of theft and the hazards that result whilst train operations are disrupted.