GB2482347A

GB2482347A - Railway vibration security device for trains

Info

Publication number: GB2482347A
Application number: GB1012880.9A
Authority: GB
Inventors: Dhiraj Sinha
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-07-30
Filing date: 2010-07-30
Publication date: 2012-02-01
Also published as: GB201012880D0

Abstract

Disclosed is a railway network security device 23, 24 that detects the vibration of trains travelling on the tracks and determines the distance and position of the trains, relative to the train 22 housing the device. The device warns the driver when the detected velocity of an incoming train exceeds a safe limit, an unsafe track eg broken or overloaded track is detected. The device may have a unit that extracts data relating to the amplitude and frequency of the track vibrations. The track vibrations may be analysed using a laser Doppler vibrometer using a laser interferometer which sends a laser beam onto the railway track. The vibrations may be analysed using a quadrature homodyne interferometer.

Description

TITLE

A Rail Security Device

FIELD OF THE [NVENTION

The present invention relates to a railway security device.

BACKGROUND OF THE INVENTION

The greatest threat to railway security is collision between two trains which results in immense loss of life and property. Collisions between trains happen when the railway signalling system fails because of serious technical or human error.

Recent innovations in the field of railway security have drastically reduced the accident due to collisions. Balise based railway tracking system is an interesting technology which has been widely adopted in Europe. Balises constitute an integral part of the European Train Control System, where they serve as beacons' giving the exact location of a train. An electronic chip is fitted to the track at certain distances and as the trains pass, the balise records it and transfers it to other trains. In response to radio frequency energy broadcast by a Balise Transmission Module mounted under a passing train, the balise either transmits information to the train or receives information from the train. Balises are deployed in pairs so that the train can determine its direction of travel.

Balises operate with equipment on the train to provide a system that enhances the safety of train operation. The biggest problem associated with it is the cost of its implementation on a large railway network which spans thousands of kilometres.

The Train Protection & Warning System (TPWS) is a train protection system deployed across the entire UK passenger railway network. It automatically activates brakes on any train that has passed a signal at danger or is overspeeding. It aims to stop the train before the point at which a collision with another train could occur.

A standard installation of TPWS consists of an on-track transmitter placed adjacent to a signal and activated when the signal is at danger'. Any train that tries to pass the signal will have its emergency brakes activated. The extent to which TPWS is effective at high speeds will depend largely on the position of the beacon relative to the signal, and the speed at which a train will be tripped' by the beacon. TPWS is unlikely to be totally effective if a train approaches a red signal at high speeds greater than 120 knVh.

As the system does not monitor the train continuously, it does not give complete protection. The system is complex in implementation as it requires installation of electronic devices along the tracks which can be a complex thing in a large network of trains.

Another implementations of a train protection system called Automatic Train Protection (ATP) is also in use in the UK. These systems generally monitor the speed of the train, and compare the train's speed with a safe speed which the system calculates on the basis of the train's distance from red signals, the braking characteristics of the train and other factors. If this calculated speed, which is indicated on the cab display, is exceeded by more than a set margin, automatic braking is applied until the train slows down sufficiently. ATPs prevent the, great majority of signals passed at danger'.

However if a train does pass a red signal, the system will initiate an emergency stop.

ATP is so costly that it has been implemented only on some selected routes in the UK.

Global Positioning System based Anti Collision Device (ACD) is a related technology having immense potential. The ACD is a form of Automatic Train Protection invented by and used on Indian Railways. The ACDs take inputs from GPS satellite system for position updates and network among themselves for exchanging information using their data radio modems to take decisions for timely auto-application of brakes to prevent dangerous collisions', thus fonning a Safety shield.

ACDs fitted (both in Locomotive and Guard's Van of a train) act as a watchdog in the dark as they constantly remain in lookout for other train bound ACDs, within the braking distance required for their relative speeds. They communicate through their radios and identify each other. If they happen to find themselves on the same track and coming closer to each other, they automatically restrain and stop each other, thereby preventing dangerous head-on and rear-end collisions.

The problem with ACDs is its implementation on a wide and exensive network which can run into billions of dollars.

The present invention is on train detection system using Doppler laser vibrometry which is relatively cheaper and mush easier in implementation.

SUMMARY OF INVENTION

An important objective of the present invention is to provide a railway security device aimed at train detection by monitoring the vibration in the tracks generated by the movement of trains.

The first embodiment of the invention is on laser Doppler vibrometer fitted to a moving train. The device comprises of a laser interferometer which sends a beam of laser light to a reflector and another beam of laser light directed on the railway track and allows the two beams to interfere. The beats formed by the interference of signals contains the information related to track vibrations. In this way the vibrations of the track can be monitored. The track vibrations are a result of the superposition of all the moving trains on it. When two trains are moving towards each other, the amplitude of vibrations in track will be much higher and it will go on increasing if the trains continue to move towards each other. We can define a safe distance based on the vibrations in the track.

Another embodiment of the invention is based on laser-photodector system where a pulsed or continuous beam of light falls on the track and the reflected beam is detected by a photodetector which helps in the analysis of the vibrations of the railway track.

BRIEF DESCRIPTION OF THE DRAW[NGS

Embodiments of the invention will now be more fully described, by way of example, with reference to the drawings, of which: Figure 1 shows a simple laser Doppler vibrometer; Figure 2 shows a heterodyne laser photodetector system.

Figure 3 shows a modified heterodyne laser Doppler Vibrometer.

Figure 4 shows a quadrature heterodyne interferometer.

Figure 5 shows a laser-photodetector system of measurement of vibration.

Figure 6 shows a train integrated with laser Doppler vibrometer.

DETAILED DESCRIPTION

The present invention relates to a railway security device based on laser based vibration monitoring system.

Movement of trains along the railway track results in mechanical vibrations which propagate along both the directions of the track. These vibrations get superimposed with other vibrations generated by other trains. The vibrations are also transferred to other railway tracks which are close to each other. A real time monitoring of the vibrations of the railway track can give a clear indication of the position, velocity and direction of other trains which are on the track and on adjacent tracks. The control system of the train dynamics can be linked to the vibrations of the track to alert it of the prospects of trains which may result in accidental collision. When the safe limits are exceeded, the emergency brakes could be automatically applied in response to the situation.

The vibration measurement system should be extremely sensitive and this can only be achieved by laser interferometer based vibration detection systems. This has been described in detail in the following section.

Figure 1 shows a laser Doppler vibrometer used to make non-contact vibration measurements of the vibrations of a surface. The source of laser light could be a laser diode, Nd:YAG or Helium Neon laser. The measurement beam is directed to the target, and scattered light from the target is collected and interfered with the reference beam on a photodetector, typically a photodiode. Most commercial vibrometers work in a heterodyne regime by adding a known frequency shift (typically 30-40 MHz) to one of the beams. This frequency shift is usually generated by a Bragg cell, or acousto-optic modulator.

The laser source 1 in Figure 1 generates a beam 2 which is passed through a beam splitter 4. The beam splitter is made of a partially silvered transparent material.

The laser beam 2 is split into beams 3 and 5. The beam 3 acts as a reference beam which is reflected off the reflector 7 and finally reaches the photodetector 9. The beam 5 acts as measurement beam. It hits the target 6 which is a railway track in the present context.

The beam 5 is reflected from the track 6 falls on the beam splitter 4 and is reflected and eventually falls on the photodetector 9. A convex lens may be included between 6 and 4 to converge the beam. The beams 3 and 5 follow different paths and finally join together resulting in interference which causes beats formation. The phase difference between the beams is dependent on the vibration of the railway track and is detected by the photodetector.

The phases of the two beams correspond to the distance covered by them across the two different paths. The phase of the beam reflected off the target 6 is time dependent while the phase of the reference beam is fixed. The phase difference between the beams is given by the following equation (1) where LL is the vibrational displacement of the railway track and 2 is the wavelength of laser light.

The current at the photodetector is given by 1(t).-131.R + 2K.[I3I5R cos(2Dt + (2) where 13 is the intensity of beam 3 and 15 is the intensity of beam 5, K is mixing efficiency, R is the effective reflectivity of the surface.

The motion of the target 6 adds a Doppler shift to the beam given by fD=2v(t)cos(a)/A (3) where v(t) is the velocity of the target as a function of time, a is the angle between the laser beam and the velocity vector.

The embodiment shown in Figure 1 has got one major drawback, it cannot measure the direction of the mechanical vibration. In order to find the direction of the vibration, an optical frequency shift into one arm of the interferometer is done to obtain a virtual velocity offset. This is explained in detail in Figure 2 where an acousto-optic modulator or Bragg cell is present in one arm of the interferometer.

In Figure 2, the beam 2 from the laser 1, which has a frequency fo is passed through the Bragg cell 10 which is excited by Quartz 11 resulting in a frequency shiftfB.

The frequency shift causes a modulation frequency of the fringe pattern when the object is at rest. This way the target's zero velocity position is transposed and directional sensibility is introduced, If the target moves towards the interferometer, the modulation frequency is reduced and if it moves away the frequency is raised. This means that it is now possible not only to detect the amplitude of movement but also to clearly define its direction.

The frequency shifted beam then passes through beam splitter 4 and is divided into a reference beam 3 and a measurement beam 5. The reference beam 3 falls on the reflector 7 and is again passed through the beam splitter 4 and reaches the photodetector 9. The measurement beam 5 is eventually directed towards the target 6 which is the railway track. Light scatters from the target in all directions, but some portion of the light is reflected by the beamsplitter 4 to the photodetector 9. This light has a frequency equal to fo + fB + fD where fD is the doppler frequency as given by equation 2. This scattered light is combined with the reference beam at the photo-detector where phase difference arises because of interference between the beams.

The initial frequency of the laser light is very high (> i0' Hz), which is higher than the response of the photodetector. The photodetector does respond, however, to the beat frequency between the two beams, which is typically in the tens of MHz range.

The output of the photodetector is a standard frequency modulated (FM) signal, with the Bragg cell frequency as the carrier frequency, and the Doppler shift as the modulation frequency. This signal can be demodulated to derive the velocity vs. time of the vibrating target.

The intensity of the photodetector signal is given by the following equation: 1(t) = 1315R + 2K[iiI5R cos(22z{IB -fD} + (4) The embodiment of Figure 2 can have some modifications in order to give some advantages to the device. For example, as shown in Figure 3, the beam 2 is passed through another beam splitter 4 before being fed to the Bragg cell 10. Similarly, before hitting the target, 9 the beam is passed through another beam splitter 13 after hitting the reflector 9.

The vibration can also be measured by an alternative method which is called quadrature homodyne interferometer. It is explained in Figure 4 in detail. It has got a polarizing beamsplitter 19 and a wave retardation plate 16 besides two photodetectors 9 and 18. The passage of beam through the polariser 19 results in a 450 polarization state.

The beam is divided into a reference beam 3 and a measurement beam 5 by the beam splitter 14. The measurement beam reaches the target 6, is reflected off and eventually falls on the beam smplitter 15. The reference beam passes twice through the eighth wave retardation plate 16 and the light coming back to the beam splitter 14 is circularly polarized. This can be described as the vector sum of two orthogonal polarization states.

A polarizing beamsplitter 15 placed in front of the detectors 9 and 18 separates the two orthogonal components. The result is a quadrature relationship at the detectors (sine and cosine output). The frequency and vibration of the target can be inferred from the resultant signal.

A quadrature homodyne interferometer is much easier to design as simple low-frequency photo detectors and amplifiers can be used. The non-linear behavior of these elements on the other hand causes harmonic distortions of the measurement signal.

To decode signals from homodyne interferometers the baseb and signals of both detector chains are fed into a modulator block which generates a modulated RF carrier with the help of an oscillator at the frequencyfB.

The vibrations of the track can also be monitored by using another embodiment of the invention described in Figure 5. A beam of light from a laser diode I falling on the track 6 is reflected and falls on a quadrant photodetector 20. The quadrant photodetector consists of four photodiodes arranged in four quadrants of a circular structure. These photodiodes are electronically connected in quadrature to compare the intensity in each half of the beam, both horizontally and vertically. There is null signal from the quadrant photodiode when all the four quadrants receive same amount of light.

If the beam is displaced from the center, we get the output from the quadrant photodiode. This output is the measure of the displacement of the beam and the sign of the output indicates the direction of displacement.

The quadrant photodetector is connected to an oscilloscope or a spectrum analyser or related computational device 21. The vibrations of the track are transformed into the vibrations of the. laser light which generates a time varying current in the quadrant photodetector. The arrangement permits the mechanical vibration of the track to be converted into an electrical signal.

Figure 6 shows a railway system with the laser Doppler vibrometer based track monitoring system integrated at the front 23 and the back 24 of the railway coaches 22.

The system can be customised to some set threshold levels of vibrations which will give indication about the incoming trains.

The devices described earlier measure the vertical and lateral displacement of the railway tracks and give an indication about the presence of other trains on the track and on tracks adjacent to it. The data can be connected to some well established software to analyse the state of the track, Warnings can be given when certain levels of irregularity are exceeded and simple algorithms can be used to detect scenarios such as cyclic top", which is known to contribute significantly to derailment risk. Similarly, when the track is broken at certain distance, the track vibrations will have some additional signals which can give an indication of the prospective danger. When the railway track has got some obstruction like a large rock, the mechanical vibrations reflected off from the point will be different and a close analysis will caution the control system of the train dynamics about such dangers.

Claims

CLAIMS1. A railway network security device for use in trains, the device comprising a vibration measurement and monitoring system attached to the train which analyses the vibration of the railway track caused by the motion of the trains along the track and determines the distance and position of other trains moving on the track and on tracks adjacent to it.
2. The railway network security device of claim I comprising a computational unit with a microcomputer which extracts data related to amplitude and frequency of track vibrations and alerts the control system of the train when the position and velocity of an incoming train on the train exceeds its safe limit and there are prospects of collision with another train.
3. The railway network security device of claim 1 comprising a computational unit with a microcomputer which extracts data related to amplitude and frequency of track vibrations in response to the moving train and alerts the control system of the train when the track vibrations show anomalous behaviour having an indication of broken tracks or tracks having a large mechanical loading.
4. The device of claim 1 where the vibration of the railway track is analysed using Laser Doppler vibrometer comprising a laser interferometer device, wherein when a laser beam is sent to the railway track, the Doppler shifi of the laser beam frequency due to the motion of the surface is analysed using a photodetector to extract frequency and amplitude of vibrations of the track.
5. The device of claim 4 where the Laser Doppler vibrometer comprises of a, laser beam from a source of laser passed through a beam splitter to create a reference beam and a measurement beam; where the measurement beam is reflected off a mirror and reaches a photodetector; the reference beam is reflected off the target resulting in a Doppler shift of its frequency and interferes with the measurement beam and the photodetector generates a current corresponding to the interference pattern.
6. The device of claim 4 where the Laser Doppler Vibrometer comprises of a, laser beam from a source of laser passed through a Bragg Cell to create a frequency shift and is passed through a beam splitter to create a reference beam and a measurement beam; where the measurement beam is reflected off a mirror and reaches the photodetector and the reference beam is reflected off the target resulting in Doppler frequency shift and the final beam merges with the measurement beam resulting in interference pattern which results in a current generation by the photodetector.
7. The device of claim 4 where the Laser Doppler Vibrometer comprises of a, laser beam from a source of laser where the laser pulse is modulated by changing the excitation current to create a frequency shift and is passed through a beam splitter to create a reference beam and a measurement beam; where the measurement beam is reflected off a mirror and reaches the photodetector and the reference beam is reflected off the target resulting in Doppler frequency shift and the final beam merges with the measurement beam resulting in interference pattern which results in a current generation by the photodetector.
8. The device in accordance with claim 7 where the laser beam is modulated by changing the injection current with sawtooth or triangular shape.
9. A railway security device in accordance with claim 1 where the vibration of the railway track is analysed using quadrature homodyne interferometer where a laser beam is passed through a polarizing beamsplitter before being passed to a beam splitter which sends a reference beam to an eighth wave retardation plate and a reflector and the reference beam is passed through the retardation plate twice resulting in circular polarisation of light by the beam splitter; where the measurement beam reflected off the target passes through the beam splitter; where the measurement beam and reference beam finally pass through another polainsing beam splitter which separates the orthogonal components and feeds the signals to two different photodetectors resulting in sine and cosine output;
10. The photodetector of Laser Doppler Vibrometer of previous claims connected to a phase locked loop or a demodulator in order to extract the amplitude and frequency information.
11. The method of detection in accordance with claim 1 where track vibrations are measured by shining a laser beam of light on the track and allowing the reflected light to fall on a quadrant photodetector with coupled current to voltage converters.
12. The quadrant photodetector of claimil consisting of four photodiodes arranged in four quadrants of a circular structure, electronically connected in quadrature to compare the intensity in each half of the beam, both horizontally and vertically such that when the beam is displaced from the centre, we get an electrical signal from the device.
13. The devices of preceding claims integrated to the front and/or back of a train and the output linked to the control circuit of the train dynamics.
14. A mechanism of application of emergency brakes when the output of the security device of previous claims detects an incoming train's position beyond safe limits.