Bernd Riedel and Michael Studer look at the potential for cost savings in the electrical equipment currently installed on railways and public transport systems.

Operators and manufacturers of railway and other public transport systems are facing great challenges. Faster, more comfortable and less expensive are just a few of the key demands. There is great potential for cost savings in the electrical equipment of vehicles. The innovative guiding principle of this concept is to take the technical standards which have become established in automotive and industrial automation and transfer them to the railways.
One aspect of particular importance concerns serial data networking with its many advantages such as the reduced wiring complexity, improved diagnostics, greater comfort, simplified commissioning and maintenance, etc.
The potential for using communication and control (TCC) systems already exists. The systems, their technology and processes are already utilised in large volumes, eg in the engineering or automotive markets, and only need to be converted. The applications include vehicle control, traction control, heating, ventilation, climate control and air-conditioning, data memory, skid prevention, communication, lighting and many more besides. The functions are often connected via a train bus.

Train TCC systems

A train communications network includes the function of converting data and signals from a train bus to a vehicle bus via a train bus coupler. This entire concept is part of the train communications network, TCN for short.
There are three different levels of communication in the train communication network:
* The WTB (wire train bus) is the train bus at the train control level.
* The MVB (multifunction vehicle bus), IBIS and CAN are at the vehicle control level
* The standard serial interfaces RS-422 and RS-485 are used as subsystems for recording and outputting data
The train communication network TCN represents the basis for all train TCC considerations. The standard also describes the use of the CAN bus. Parts of the TCN have also been adopted in the UICCodex556 since the end of 1998. We have classified TCC systems into three groups:
* Closed train unit. The vehicles function as a unit and are fix coupled.
* Open train unit. These are vehicles running on a general rail network irrespective of operator, eg DBAG, …BB, SBB, etc.
* Connectable train unit. These are vehicles running on a closed railway network, eg trams, private rail networks.
Let us consider the classification of a closed train unit. There are various ways of configuring the TCC system in a unit of this type. These are: central control unit and central wiring; central control unit with decentralised peripherals; decentralised control unit with decentralised intelligence. The concept of central control and central wiring will not be dealt with in any more detail here. For central control and decentralised peripherals - CAN vehicle bus - the processing unit is a central controller installed in the driver's cab, for example.
A controller area network (CAN) is used as the serial data bus (vehicle bus) in the rail vehicle. This serial data networking concept is taken from the automotive industry and represents the backbone of technical vehicle control. CAN guarantees the highest levels of serviceability of the connected technical units such as door control, fault recording, diagnosis and air-conditioning, etc.
The system contains other decentralised, non-programmable input/output modules. All relevant signal conditioning systems such as digital and analogue inputs/outputs, counter inputs and pulse width modulation (PWM) signals are available. The system complies with the requirements of EN50155. A train bus connection is not relevant in this application.
The advantage of this type of configuration lies in the minimum wiring complexity because the decentralised, non-intelligent units can be placed locally. However, it is a disadvantage that failure of the central computer also causes the connected units to stop functioning.
In distributed, intelligent control units with two networks - CAN vehicle bus - we have a control unit (CPU) as the processing unit and two different CAN networks. The first is used for connecting all non-intelligent, decentralised modules via the CAN bus. The second connects additional independent processing units which perform functions autonomously. The CAN bus is only used for carrying relevant data, eg as a skid prevention unit. If a control unit fails, the functions contained in the independent control unit are unaffected.

Redundancy

A control unit is used as the central processing unit, as before. A second controller is used as a backup to provide partial redundancy. Its only task is to check the main control unit. If the main control unit fails, it is disconnected and ignored. The second controller then takes over all its functions and continues to operate all the units on the CAN. The range of hardware includes intelligent CPUs with one or two CAN interfaces, decentralised station modules, digital I/Os, analogue I/Os, HMI terminals as well as counter modules and pulse width modulation modules. The system supports 64 I/Os per station. This can be expanded via the CAN network to give 64 stations with 64 I/Os each.
As a result, this is a very open system which provides specific solutions for the individual requirements. In addition, every other external system which has a CAN interface implemented can be addressed and controlled.
Often, only part of the TCC system is replaced, in particular where retrofitting is concerned. Consequently, it is necessary to connect the modern TCC systems to the existing ones in this case, as for example with the MVB (multifunction vehicle bus). This is done using the MVB CAN gateway which converts the protocols.
In closed rail networks, eg private railways, urban railways, trams, company railways, etc, there is no demand for the train bus to be compatible with other systems. It makes sense to have applications in which the costs of the TCC system do not unnecessarily inflate the costs of manufacturing the vehicle. As a result, a relatively over-specified train bus system, eg wire train bus, (WTB) is not used in these applications.
However, serial data networking does offer advantages here all the same. Frequently, it is enough to employ a practical train bus tailored to the requirements, and this also reduces costs. The TBC (train bus coupler) system presented here is based on the CAN bus and offers the following possibilities: train setup, redundancy, reliable bonding and electrical isolation.
The train bus coupler TBC carries out tasks such as reading in data from the CAN Powerline and converting/outputting the data on the vehicle bus CAN. Furthermore, it ensures that the input and output buses are electrically isolated. Like a processing unit, it can be programmed according to the industrial standard IEC61131-3. The programming tools are especially designed for efficient program development and commissioning times. The train setup can be simply called up from a library and installed. The advantages of such a conversion are: train bus tailored to requirements of simple connectable train units; redundancy; straightforward parameter settings; attractive price.
The third and last train unit to be mentioned is the open train unit. In this case, it must be possible for rolling stock from different operating companies to be coupled together, for example in the combined carriages operated by DBAG and the SBB. In this case, the train bus has a set specification, eg via the UIC cable or the wire train bus (WTB). It is essential to have the train bus standardised in detail. The MVB as a vehicle bus is not a decisive factor for the operating company.
The UIC cables are standardised. The signals are wired up in parallel on the I/O level. A standardised processing unit makes it possible to display numerous functions such as ventilation, door control system, etc. The advantage is that carriages in service can be very quickly equipped with new technology, ie in a classic retrofit. One disadvantage concerns the I/O channels which are not always present in adequate numbers.
The current standard in open trains is the UIC cable. It appears that the WTB will become established in future.

Enquiry No 35

Bernd Riedel and Michael Studer are with Selectron Systems AG, Lyss, Switzerland. www.selectron.ch