Improved performance with reduced costs: Improved performance with reduced cost is a welcome trend in the braking industry that is as true for bogie and friction brakes as it is for distributed brake control - Braking Technology

International Railway Journal, August, 2001 by Peter Johnson

TWO words sum up one of the most interesting current developments in passenger vehicle braking systems: distributed control. Distributed control is the new system architecture which people at the cutting edge of the brakes systems market are discussing.

"The fundamental concept behind distributed control is to position low cost control valves with integrated electronics and software, local to the point of control, and connect these together using a Local Area Network," explained Mr Ross Bradley, head of research and development at Westinghouse Brakes. "In the context of a pneumatic brake control system on a typical vehicle, this would provide individual bogie control of service and emergency brakes with individual axle control of wheel slide protection. Individual valves can be networked on a per car, or group of cars (segment), or a whole train basis."

With this in mind, the advantages of distributed control quickly and clearly become apparent. First, there is the matter of size reduction. Combining mechanics, electronics and software into a single unit enables functions previously implemented mechanically, to be performed by electronics and/or software. In addition, this also permits unnecessary interfaces to be removed. The second key benefit to the end user is the lower total installed cost. Final assembly underframe pipework is a further significant cost when installing pneumatic brake systems. Localising pneumatic valves through distributed control architecture close to the point of control reduces pipe-work and thus reduces vehicle builder costs.

Lower Installed Cost

Lower total installed cost becomes still more attractive for the users of distributed control systems when it is looked at in the light of an even more important third key benefit--improved failure modes. Grouping control on a per bogie basis means that the number of autonomous cells is increased and the effect of a single-point failure is proportionally reduced.

Enhanced redundancy through duplication of key elements, such as databus links and brake management control units, ensures minimum brake loss following a single-point failure.

There are new trends in technology that have allowed Westinghouse Brakes to develop brakes systems such as EP2002. For Ross Bradley there are five key technological trends in the development of rail vehicle braking systems--mechatronics, databus control, safety traceability, safety integrity levels, and materials for extended life.

Traditionally, mechanical assemblies and electronics software have been supplied as physically separate units. The recent trend has been to embed the electronics and software within the mechanical part to reduce packaging and interface costs. This is what Bradley means by mechatronics, and which is best exemplified by Westinghouse Brakes' use of the RBX4 valve in the EP2002 system.

Databus control provides the ability to distribute "smart valves" around a vehicle using a data communication network to pass control and diagnostic information. Reliable and relatively low cost systems have been available for some time. Alternative structures for brake management systems have been developed and are now in service incorporating features such as brake demand, friction/dynamic blending and cross-car blending, controlled via a databus.

Safety traceability is, for very obvious reasons, an important trend. The increasing reliance on software within brake control systems and in particular for safety-related functions has caused concerns within the industry. Recent changes to the European Norm (EN) standards reflect this concern and specifically target the demonstration of safety during the design process. Experience has shown that it is problematic if not impossible to retrofit the EN safety process onto an existing design and, claiming service experience in lieu of traceable safety within the design process, will not meet the EN standards.

A further technological trend has been improved definitions in the EN standards of safety integrity levels (SIL). Improved SIL definitions and the process of allocation will have a significant effect on the cost of introducing software into safety related functions. It will also change the way systems are partitioned such that flexibility can be maintained within SILO systems whilst SIL3/SIL4 proofs can be achieved in high integrity systems.

So far as extending the life of materials is concerned, improved materials technology is being used to extend the service and overhaul periods for equipment. This is achieved though careful selection and extensive testing. Maintaining the required performance over the full service life requires detailed knowledge of the materials involved and their long term characteristics.

Incompatible materials can cause premature failures. As an example, the chemical reaction between certain synthetic oils and some rubber components can cause loss of performance or failure of the interface. Thus, using tested and approved OEM products for servicing and overhaul is important, if maximum service life and high reliability are to be achieved.