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PLCs and relays – A logical approach

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Now, here at The Rail Engineer we do try to be technical. After all, this is an engineering magazine and not some sort of Fisher Price catalogue explaining kit in terms of large red buttons. But there are times when a little of ‘back to the beginning’ is called for – and more especially so when it comes to railway signalling. We’ve a wide range of engineers who pore avidly through this magazine and I suspect that some of them, maybe the civil engineers like me, may struggle with and, indeed, switch off when it comes to the intricacies of the grey boxes of twinkling lights that are food and drink to signalling engineers.

So the following couple of paragraphs come with no apology – except perhaps to those who know all about relays and PLCs and who might find the words a little too simple. Civils engineers can have their revenge at another time with esoteric discussions on Euler’s critical loads but, for now, here are some simple words which might help with the rest of the article.

“If this, then that”

Firstly, what do relays do? This is really simple. Pass a (smallish) current through the coil of a relay and (hopefully) there will be a satisfying clunk and a set of contacts – not connected to the circuit that fed the original current – will either make or break. With the types of relays often encountered by general engineers this allows a much larger current to flow through the contacts to a hefty bit of kit – a level crossing barrier motor for example.

But a relay can also act as a logic gate where the second current is not there to drive something but is used to establish that the first current has occurred. This is going right back to the original logics of early computers. “If this, then that” in terms of programming language – the 1s and 0s of the binary system.

So, detecting an input from a treadle fixed to the rail and caused by an approaching train can then set off a range of downstream switching, all generated through the use of relays.

F_A_Electrical_cabinet_1760_cold_2012_08 [online]Life of Riley

This sounds a little, well, like an old technology – tried, tested and thoroughly railway.

It’s pretty reliable, but of course susceptible to failure modes. It uses kit that is mechanical, bulky, delicate – in fact everything you’d expect in the 1960s. Relays live the life of Riley in their own, cosy, temperature-controlled relay rooms. They are very shy with only very few qualified technicians allowed to come anywhere near them and, of course, the relay rooms have to cater for these valued visitors as well. The result is that the kit for a level crossing has to include quite large lumps of infrastructure that largely house (warm) fresh air.

Just for a moment, let us pause and question why level crossings have been mentioned so many times in the first few paragraphs of this article. It’s because Network Rail is seeking to change the way that level crossings are controlled and, with the assistance of the Signalling Innovation Group under Tahir Ayub, senior design authority engineer, is aiming to move from the old relay technology to PLCs.

Inputs and outputs aplenty

And at this point we have to go right back to another beginning and explain in more simple words the meaning of PLC and what a PLC does. PLC – Programmable Logic Controller – they’ve been around now for many years and can be found looking after the operation of machines, factories and processes that potentially have multiple inputs and outputs. They are the modern(ish) equivalent to the logic gate provided by the relay mechanism.

They are, of course, much smaller, far more anonymous and silent. They will be found in some  of the inevitable, enigmatic grey boxes with twinkling lights. They are able to handle a large number of inputs and outputs all within the space of a modest circuit board. They are robust and so there’s no need for a cosseted lifestyle, no special rooms and no warm bath every ten years.

So why have relays hung on for so long? It’s not for a lack of will by generations of signalling engineers. “It’s something we tried twenty years ago” can be heard. But why now and not then? Bluntly, it was very difficult. Not difficult to design a system, but difficult for any system to make it through all the validation and acceptance processes. It would have been scheme by scheme, design by design, component by component. Easier to bang in a relay set-up as that’s what’s acceptable.

All this was not helped by there being no SIL3/4 PLCs around in the commercial market. (That’s kit that has a SIL (Safety Integrity Level) of 3 or 4. SIL? – that’s another story!)

Safety PLCs

But, over the past ten years or so, devices have started to emerge from a number of suppliers driven by the need to use PLCs in more and more demanding applications. It has to be remembered that ‘out there’ are many industries that are just as safety critical as the railways – oil & gas, pharmaceutical and nuclear – and they are the ones that have pushed the development forward. Now there are devices that satisfy the requirements of the CENELEC standard IEC 61508-3, which sets the requirements for this new generation – this new category – of Safety PLCs. So far as the hardware is concerned this is all about the principle of composite fail safety through redundancy and diversity along with formal verification of software construction.

Procuring PLCs that conform to IEC 61508-3 effectively takes out a whole level of angst when pitted against any railway validation process.

Some suppliers have satisfied the requirements of EN 501208 and EN 51209 (which are railway standards for signalling control systems). There are now many opportunities that never existed only a few years ago. Since some suppliers have made kit to these CENELEC standards, some countries have used Safety PLCs for level crossing applications in a large scale in Southern Ireland, France, Germany, Switzerland, Austria, Portugal, and Japan. They’re even being used as communications interfaces between remote computer- based interlockings and relay interlockings as an alternative to TDMs (time division multiplexers).

Communications interfaces

Tahir and his team see that there is a real way forward to achieve a number of tricky targets through CP5, to improve safety at level crossings and drive down  costs. The safety issues can be helped by incorporating new inputs – remember that PLCs can handle a large number. One such input can come from obstacle detectors (OD) on the crossing. Financially, there will be benefits because the PLC can talk to itself and find out how it’s feeling through internal self-monitoring circuits and firmware.

PLCs have communications interfaces which, in turn, give the opportunity for diagnostics, remote asset monitoring and pre-emptive maintenance. Development of applications using pre-certified software function blocks can dramatically reduce design time, enabling factory acceptance testing to reduce site acceptance tests and hence possession times during installation and commissioning.

PSS400_PLC_Pilz [online]Environmental considerations get a tick in the box through the use of modest (unheated) location boxes to house the devices, as well as extremely low power consumption. This, in turn, reduces the quantity of civils work and energy-expensive materials and may reduce land- take because of a much smaller footprint.

Opening up the market

But we’re not quite there yet, despite all these positive aspects. There’s still the matter of commonality of design, modular  construction and openness of source code – in fact all the issues that, unless addressed, might lead to the delicate matter of vendor lock. Perish the thought, but it could be possible for a supplier to corner the market and have a monopoly. Of course, this has never happened in the past, but you never know and Tahir is committed to a model that ensures that it is Network Rail that pulls the strings rather than a monopoly supplier.

By opening up the various elements involved in the supply chain it is intended that more and more of the suppliers ‘out there’ who already make Safety PLCs or deal in PLC systems integration will be able to penetrate the railway market. And, as has been shown in the areas of Class II electrical equipment and aluminium cabling (that’s where you’ve heard of Tahir before!), by opening up the market there will be a spontaneous burgeoning of ideas and innovation further driving down costs or widening applications.

464 level crossings

So, the project underway by the Signalling Innovation Group is less about the gadgets – they’ve been around for years – it’s more about a process and methods, new system design, new testing processes, training and development, and configuration and control. This ensures that the supply chain is less restrained by railway processes and is able to come up with its own ideas within the framework that Network Rail would put in place.

The group has already engaged with a number of Safety PLC vendors, even going as far as collaborating with and developing level crossing simulators using PLCs to prove the technology is not a barrier but that new ways of working are required.

The target is to convert/install 464 level crossings during CP5. This is a process that also involves going back to the beginning when it comes to examining the way that level crossings work – a matter of getting down to the basic logic steps unhindered by the limitations of the relay technology.

Going back to the beginning is something that we non- signalling engineers have to do on occasions.

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