Intelligent design

Necessity is the mother of all invention, says the old proverb. It’s also been said that the basis of invention is science. We could include talent, insight, inspiration and innovation within the mix too. And let’s not forget design and development.

We’re playing with words here perhaps, but all of these elements become important when the railway division of the Institution of Mechanical Engineers (IMechE) holds its annual competition of technical presentations aimed at young engineers. Entitled ‘The Future of Rail’, this event provides a great opportunity for aspiring engineers within the rail industry to showcase their ideas.

This year’s worthy winner was Tara Parandeh, a thirty-year-old development engineer for Transport for London. Her presentation, entitled ‘Development of an Intelligent Sensitive Edge (iSE) Safety System’, focussed on her work with London Underground to produce a directionally sensitive system for the detection of objects trapped in rolling stock doors. This is certainly an invention of necessity, designed to provide a more refined response to the ongoing safety risks and delays caused by garments and other objects becoming trapped in train doors.

Alarm

On the busy London Underground system, significant delays can result from passengers’ clothing getting caught in train doors. This is by no means a problem that’s unique to London Underground of course, with similar problems occurring on the national network and other metro systems. There have been sporadic instances of passengers being dragged along platforms by items of clothing, so the doors of new Victoria line trains are fitted with a sensitive edge system to detect small objects such as clothing or bag straps if they become caught.

Entrapment of a thin object like a belt or scarf may not initially trigger the system, but pulling on the item to free it, either from inside the train or outside it, will trigger a response. An immediate emergency brake application occurs and the train driver receives a sensitive edge warning light. The driver should then reopen and close the train doors so that the trapped object can be released.

Objects becoming caught from inside the train do not represent a significant risk, but the existing sensitive edge system can be triggered when such objects are pulled. On the Victoria line this has led to unnecessary delays. At peak times these hold ups have reached unacceptable levels.

Challenge

Tara Parandeh graduated from Sheffield University in 2008 with a degree in mechanical engineering. Working initially for Interfleet Technology, she was seconded to London Underground in 2011 and transferred there permanently in 2014. She began work to devise a new form of sensitive edge upon her initial secondment, but she has worked on other engineering projects, notably during the build up to the London Olympics. The existing sensitive edge system has an ‘active rubber’ strip along the edge of one door and the other door carries an ‘inactive rubber’ as shown in figure 1.

The sensor system has two conductors surrounded by conductive rubber (natural rubber with a high content of carbon black) that are kept apart within a symmetrical non-conductive rubber extrusion. An object caught in the doors, including any thin object such as a belt that is pulled taught, will deform the active edge profile and create a low resistance path between the conductors.

The system works well and it’s immune from tampering – pressing on the door seal from inside the train for example. Its Achilles’ heel is that there is no directional discrimination for an object pulled through the door seal. The development of a directional alarm activation system was the challenge that Tara took on.

The new design had to be compatible with the existing train door wiring and alarm systems. Tara has therefore continued with the concept of two parallel conductors surrounded by conductive rubber, but cleverly she has staggered them diagonally within an entirely new flexible rubber extrusion. Deformation of this new form of ‘active rubber’ occurs in such a manner that the conductors are brought together when a belt, or other trapped item, is pulled outwards. On the other hand, pulling on similar objects from within the train carriage causes the conductors to be drawn further apart and the alarm will not be triggered.

Importantly, this design also prevents false activation of the alarm by a thumb press on the door seal.

Improved safety

The main requirements for iSE were specified from the outset of the design process. For instance, an external pull on a trapped belt should trigger the alarm when a force of less than 100N is applied. The corresponding threshold for a trapped rope is 60N. Another important safety requirement is that a buggy bar should trigger the alarm system when trapped in the door seal at any angle.

The system needs to comply with these requirements at all door set up tolerances and rubber manufacturing tolerances over the life of the seals (1,350,000km, approximately eight years) and in all LU environmental conditions. Furthermore, the seal material had to satisfy strict requirements on durability. Fire performance and the resistance to damage and tear needed to be equal to or better than the existing fleet design.

The iSE safety system is essentially simple in principle, but a lengthy process of development and testing was involved in perfecting it. For the system to function correctly, the seals need to deform in a precise pre- determined manner. Factors crucial to the success of the seal design are the profile shape and the internal structure of the extrusions.

It was clear from the outset that many design variations would have to be tested, but the manufacture of rubber extrusions requires the production of expensive hand- crafted extrusion dies. Tara’s elegant solution was to utilise water jet cutting to create sample profiles from 20mm rubber sheet. Using a flexible adhesive that performs similarly to the material itself, the cut sections were stacked to form full-height door seal profiles.

New material

The material used in existing LU door seals is EPDM (ethylene propylene diene terpolymer) which is an extremely durable synthetic rubber. For iSE, Tara chose to use a silicone rubber material that has a very high tear resistance. There were several reasons for this. EPDM suffers an ageing process that results in its Shore Hardness increasing. Cold temperatures too can cause the material to become less resilient, resulting in higher activation forces that are already close to the limit. A fire performance concession is also in place for it.

High tear silicone, on the other hand, does not age significantly and its greater flexibility gives activation forces half that of the limit. Its properties do not change significantly with temperature and its tear strength is twice that of EPDM.

Fire testing was conducted, which revealed that the High Tear Silicone is compliant in terms of oxygen index (the amount of oxygen the material requires to burn) and toxic fume production. In the Cone Calorimeter Test (heat transfer rate) the Silicone performed 30% better than EPDM. However, in the Small Scale Smoke Density Test (visibility through smoke) the Silicone material was inferior to EPDM. This appeared to be a stumbling block until the actual door profiles were tested. In reality the Silicone iSE profiles are four times better in terms of smoke Density value than the original iSE EPDM profiles!

Extrusion

With the new door seal profile design now established, the next stage involved the manufacture of extrusion tooling that would produce the rubber profiles within acceptable tolerances. Extrusion tool making is a skilled manual process rather than an exact science. The design loop therefore involved the on-train testing of trial extrusions with the results being used to tweak the tool profile.

It was a lengthy process to create the definitive tool design. Static on-train testing was undertaken at Northumberland Park Depot. With good results being obtained there, the seals could then be subjected to dynamic testing out on the line, albeit not in passenger service. At the same time, the effects of ageing were examined by using heat and humidity to artificially simulate an 8-10 year service life. The effect is to increase the Shore Hardness of the material and thereby reduce its flexibility, after which the testing is repeated.

Into service

Future planned activities include environmental testing and life cycle testing. Making use of a Horiba MIRA Ltd (formerly the Motor Industry Research Association) test facility, the environmental testing will involve extensive trials at a range of temperatures between -15°C and +35°C. Hand-in-hand with this is the repetitive life cycle testing, undertaken using a jig.

Assuming no problems are encountered, the iSE system will then be fitted to a train in passenger service and trialled for two months.

The conclusion of the project will be a Victoria line fleet fit. A total of 47 eight-car units built by Bombardier between 2009 and 2011 operate these services, and the total cost of equipping the entire fleet with the new iSE door seals is estimated at £3 million. There is also, of course, the possibility of the design being incorporated into future ‘Safer and More Reliable’ trains.

It could be said that Tara and the other six finalists of the IMechE’s ‘The Future of Rail’ award are inspirational role models. Tara herself went on to become a finalist in the 2015 FTA Everywoman in Transport and Logistics Awards. Through her highly successful work with London Underground she has demonstrated how women can make a career in rail transport a successful and fulfilling one.

It’s too early to say whether her intelligent door seal system will be adopted system wide, but the 200 million passengers who use the Victoria line each year should soon benefit from reduced delays.