Something to BRAGG about

In 1915, Sir William Lawrence Bragg and his father, Sir William Henry Bragg, were joint winners of the Nobel Prize in Physics: “For their services in the analysis of crystal structure by means of X-ray”.

The Braggs are the only father-son team ever to have won a Nobel Prize in any field. At the time, Lawrence Bragg was also the youngest ever Nobel Laureate in physics. Their work was based on Bragg’s Law which was developed by William Lawrence in 1912 and concerns the constructive interference of X-rays reflecting off various crystal planes.

As optical fibres were developed for telecommunications in the 1970s, physicists such as Ken Hill and Gerald Meltz realised that they could also be used for measurement. This could be achieved by altering the fibre to reflect light back under certain conditions, in accordance with Bragg’s law. This led to the development of Fibre Bragg Grating (FBG) sensors which became commercially available in the 1990s.

These sensors are made by illuminating the core of an optical fibre with intense ultra-violet (UV) laser light. The short wavelength (< 300 nm) UV photons have sufficient energy to break the fibre’s highly-stable silicon/oxygen bonds. This affects the structure of the fibre and slightly increases its refractive index. Interference between two beams of UV laser light results in a periodic variation in its intensity which is then used to create a corresponding periodic variation in the refractive index of the fibre.

All done with mirrors

The modified fibre serves as a wavelength- selective mirror as, at one particular narrow range of wavelengths, constructive interference occurs and light is returned down the fibre.

The reflected wavelength is affected by any variation in the physical or mechanical properties of the fibre either due to strain or variation in temperature. The is given by the equation:

Where Pe is the fibre’s photoelastic constant, ε is the strain in the fibre, α is the fibre thermal expansion coefficient and ζ is the fibre thermal-optic coefficient. The first term in this equation represents the longitudinal strain effect on the FBG and the second concerns the thermal effect, which is a combination of thermal expansion and thermal-optic effect. For a typical free grating with λB = 1550nm, sensitivities to strain and temperature is typically 1.2pm/με and 10pm/°C. This shows how temperature has a significant impact for which compensation is required if FBGs are to be used for accurate strain measurement.

As sensors, FBGs have a number of advantages. They are immune from electromagnetic interference, are quite small and are virtually the same size and strength as the original fibre. As their output is the wavelength of reflected light on-site calibration is not required. A single fibre can have multiple gratings with different characteristics along it whose outputs can be read using multiplexing techniques developed by the telecommunications industry.

Complex harsh interface

A pantograph head operates in a particularly harsh environment. It is exposed to all weathers as its carbon strip rubs along the overhead contact wire at speeds of up to 125 mph. Although pantographs are set with a static upward force of typically 90 Newtons, they have to contend with varying dynamic loads not the least of which are aerodynamic forces for which the pantograph has to be balanced in both directions. Up to 300 amps may be transmitted at 25,000 volts between pantograph and contract wire with occasional arcing, which can be severe. To see this, take a seat under the pantograph of the first train of the day on a frosty winter’s morning for a spectacular light show.

The interface between contact wire and pantograph head is complex and, in truth, not fully understood as measurement of contact forces is not straightforward. When this interaction goes badly wrong the resulting dewirement will cause widespread disruption, typically causing tens of thousands of minutes delay and the cancellation of hundreds of trains.

Whilst there are systems available to measure pantograph force, these use multiple strain gauges and accelerometers which require a low-voltage power supply at the pantograph head, adding to its mass. They also require a method of transmitting data across high voltage insulation and could be subject to electromagnetic interference from traction current collection. The FBG system adds only the fibre to the pantograph head, is robust and transmits its data through the optical fibre which is an insulator. Hence it is an ideal method of measuring force and its location on the pantograph head.

FBG pantograph control

The idea of using FBGs to measure pantograph forces was initiated through a 2011 Collaborative Fund grant from the Engineering and Physical Sciences Research Council (EPSRC) which led to the joint development of an active instrumented pantograph by City University London and Brecknell Willis, now part of the Wabtec group. This initiative was awarded a further £300,000 last year when, at the Railway Industry Association’s Technology and Innovation Conference, it won the best innovation award sponsored by RSSB.

This award is being used to develop the FBG pantograph monitoring and control system to prove the concept in laboratory conditions. Following work by City University London to compensate for temperature effects through the development of new temperature compensation algorithms, the system has now been developed to the stage where it can accurately detect force and its location on the pantograph carbon. This was demonstrated at a recent workshop held at the University at which Lee Brun of Brecknell Willis and Professor Tong Sun of City University gave a joint presentation on this initiative.

The next stage is to develop a control mechanism that uses the sensor output to vary the air pressure in the cylinder used to raise the pantograph. When this is done, the FBG sensor system will change contact force to ensure a reliable contact with the overhead wire. With the pantograph travelling at up to 56 metres per second, it is likely to have travelled a good few metres before the body mounted air cylinder will have adjusted the force on the contact wire through the pantograph mechanism.

Hence the system is unlikely to provide instant force adjustment, but it will respond to changing environmental conditions and transient effects. It also offers the ability to fine- tune pantograph aerodynamic design for which there is currently no comprehensive test facility. Thus it offers the possibility of higher speeds and multiple pantograph operation as well as a reduction in dewirements.

Continuous condition monitoring 

Network Rail’s measurement train provides valuable data about overhead wire condition data by regularly and accurately measuring contact wire position and wire wear. However, force measurement is subject to the practical constraints of current technology. Current detection arrangements add to the mass of the pantograph and may affect its aerodynamics. They are also susceptible to electrical interface. Moreover the overhead line measuring coach MENTOR (Mobile Electrical Network Testing, Observation and Recording) is limited to 100 mph.

This limitation could be overcome with a new 125 mph MENTOR coach but this would require a significant investment. A better option would be fitting pantograph force sensors to a service train. However the train operator concerned would need to be certain that this would not affect pantograph performance. The recent London City University workshop has shown that the use of FBG pantograph sensors would give this assurance and go some way to meeting the requirement of Network Rail’s Technical Strategy to “develop, test and trial new pantograph technologies to manage and optimise the contact force between the pantograph and conductor”.

When the control system on the laboratory demonstration pantograph has been proven and the required data obtained from it, the next stage is trial service use. This requires the FBG sensors and control system to be fitted to a service train, or perhaps one of Network Rail’s overhead line monitoring vehicles.

The trial use of a novel pantograph control system will require safety approval subject to a suitable and sufficient risk assessment. It would be hoped that there should not be any unnecessary delays in arranging its first trial use as the system offers improved risk mitigation for the contact wire interface. Moreover it has significant potential benefits for both Network Rail and train operators. These include higher speed running, multiple pantograph operation and increased pantograph carbon life.

In the long term, the FBG sensor system offers the ability to detect contact forces from the entire service fleet if combined with GPS and suitable telemetry. This offers the potential of continuous real-time monitoring of the entire overhead line network. Then the Braggs’ work on X-ray diffraction of crystals a hundred years ago could well have made overhead line dewirements also a thing of the past.