Condition Monitoring of Audio Frequency Track Circuits

Transcription

1 Condton Montorng of Audo Frequency Track Crcuts 1 P. F. Weston, J. Chen, E. Stewart and C. Roberts The Brmngham Centre for Ral Research and Educaton, Department of Electronc, Electrcal and Computer Engneerng, The Unversty of Brmngham, Brmngham, UK 1 Abstract Ralways are expected to operate wth ever ncreasng levels of avalablty, relablty, safety and securty. One way n whch hgh levels of dependablty can be ensured s through the use of condton montorng systems. Ths paper presents the ntal results from operatonal trals of track crcut condton montorng equpment nstalled at two TI1 audo frequency track crcuts n the UK. Introducton A track crcut s a safety-crtcal asset. It s desgned to be fal safe. A falure resultng n a secton beng ndcated as occuped when t s not causes delays ether whle the fault s dentfed and corrected, or whlst t s worked around by talkng drvers through the faulty secton. There s lttle publshed work on montorng of track crcuts n the feld. One system avalable n the UK s descrbed n [1, ]. There s a move to ntroduce self-dagnostc features nto track crcut systems n order to dentfy fault condtons or potentally ncpent falures (falures that grow over tme and can be detected before a fault occurs). There are nearly 50,000 track crcuts n operaton on the manlne ralway network wthn the UK. A number of dfferent types of track crcut exst, ncludng DC, AC and audo frequency jontless track crcuts, such as TI1 and FS550. Table 1 shows the number of track crcuts nstalled n Network Ral s Southern Zone together wth typcal falure statstcs. From the table the average falure rate s around 45% per year per nstallaton. Any track crcut falure can cause sgnfcant dsrupton to ral servces and be a safety rsk. Over 1,000 track crcut falures were reported n the UK durng , resultng n 1.5 mllon mnutes of delay. Typcally, the nfrastructure operators are penalsed by 0 60 per delay mnute arsng from nfrastructure falure. Therefore, the ablty to detect and dagnose track crcut falures n order to provde a fast response to falures/ncdents has sgnfcant economc benefts. Track Crcut Type Number Falure reports per year Falure Rate per year Impact Mnutes AC-TC 3, ,000 TI1 1, ,500 FS ,15 HVI TC ,750 Reed TC 1, ,000 Overall 8, ,375 Table 1. Typcal track crcut falure statstcs Audo track crcut operaton The prncple of a track crcut s to detect the presence of a tran by ts wheelsets shortng out a sgnal transmtted along the rals to a dstant recever [7]. In TI1 jontless audo frequency track crcuts, a carrer frequency f 0 s coupled nto the track and detected at the recever. In fact, the carrer frequency s modulated between f 0 17 Hz and f Hz at a rate of 4.8 Hz as detaled n [4]. The recever only accepts the receved sgnal when the receved current s hgh enough and the correct modulaton pattern s observed. The frequences along a track are assgned n pars that are used alternately along the ralway track. Transmtters and recevers are connected to the track va tunng unts that ncorporate an solatng transformer and other passve components desgned to acheve two objectves. Frstly, to form a resonant crcut n combnaton wth around 0 m of track, at the track

2 crcut frequency, and secondly, to appear as a short crcut at the alternate frequency. The recever observes the sgnal at ts set frequency and only holds n sgnallng relay when the receved current exceeds a certan level (and also subject to other condtons lnked to the modulaton pattern). The wheelsets of a passng vehcle dvert the current from the recever, causng the sgnallng relay to drop out. Dfferent track crcut lengths are accommodated by changng the recever gan. Instrumentaton Track crcut voltages and currents can be drectly measured usng portable equpment, as used for nstallaton and mantenance. The currents flowng n the tuned zones can be determned usng equpment separated from the track by a small dstance, ether from the lnesde [6, ], or from a probe attached to a movng vehcle [5]. However, to understand the behavour of real track crcuts n the feld, and to obtan long term data on two partcular track crcut a more comprehensve fxed system s requred. Based on the fault-detecton requrements descrbed n [3], two sequental track crcuts have been nstrumented n the UK, operatng on two frequences labelled A and B. The transmtter voltages are measured as are the currents flowng between each tunng unt and the track, usng clamp-on current probes. In addton, the state of the assocated track crcut relays s also montored, as well as the outsde temperature. Ranfall s also montored usng local meteorologcal data. The montored currents and voltage are spread out over a dstance approachng 1 km, so some means of collectng and transmttng data s requred. A CAN (controlled area network) bus wth a 50 kbt/s data rate was used to network the measurement nodes. A cable carryng both power and dfferental data pars was lad n the exstng cable troughng along one sde of the track. Fg. 1 shows the layout of the CAN nodes. Fgure 1: CAN node layout. Several CAN node varants were constructed to form the montorng system; all featurng a common mcroprocessor and CAN nterface secton. The smplest node measures temperature, consstng of a dgtal temperature measurng semconductor lnked drectly to a mcrocontroller. Another type of node measures the CAN system supply voltage and current, usng smple analogue to dgtal converson on the mcrocontroller. More complcated nodes were developed to collect data over a range of

3 frequences, as shown n fg.. In ths type of node, the sensor sgnal s passed through a swtchedcapactor bandpass flter wth a Q of around 30. The actual centre frequency of the bandpass flter s under mcrocontroller control. The clockng frequency s fed back to the mcrocontroller for frequency measurement. When the centre frequency s changed, the mcrocontroller measures the clockng frequency whle the sgnals n the bandpass flter, rms converter, and analogue to dgtal converter are allowed to reach steady state. The analogue sgnal path could be mplemented usng a dgtal sgnal processor, but the analogue electroncs soluton allows a smple mcrocontroller to be used n each node. Sensor rms a/d CAN Clock gen μp Fgure : Varable frequency nstrumentaton node block dagram. A current CAN node takes n a voltage proportonal to the current measured by a non-contact current sensor. The current sensor s clamped around a cable connectng a tunng unt to one of the rals. A voltage node sensor s made up of a galvancally solated sensor connected to the transmtter termnals wthn a lnesde cabnet. The output of ths sensor s connected to a frequency-varable CAN node. The relay controlled by the recever s montored va spare relay contacts that are connected to a CAN node va an opto-solaton stage. Two sequental track crcuts have been nstrumented, one operatng at frequency A and the other at frequency B. The A and B channel transmtter voltages are measured at ther nomnal frequences, plus or mnus 100 Hz. All currents are measured around both the A and B frequences to obtan a pcture of the current flows for both A and B channels at all locatons. The outdoor temperature and ranfall are measured. Data captured by the nodes are transmtted on the CAN bus to a host CAN nterface attached to a PC located n a lnesde cabnet. For the ntal nstrumentaton exercse, all the raw data collected by the nodes s saved. Ths raw data s beng used to develop processng algorthms that wll, at some later date, be ncorporated nto the nodes so that the amount of data generated wll be much reduced. Data collecton Voltage and current measurements are collected n two dfferent ways dependng on whether a ralway vehcle s present on ether of the two consecutve montored track crcuts. For the majorty of tme, the two track crcuts are unoccuped (the two track crcut relays are held closed) and the centre frequency of the band pass flter s swept over the A and B frequency ranges for current nodes. For voltage nodes, ether the A or B frequency range s covered, dependng on the transmtter frequency. Whenever one or both of the track crcut relays s released, ndcatng the presence of a tran n one or both track crcut sectons, data are collected at one spot frequency for voltage nodes and at two spot frequences for current nodes. Ths provdes a more rapd sequence of data as a ralway vehcle moves over the two track crcut sectons. Data processng Data at each frequency are taken n approxmately 40 ms. Wth approxmately 150 readngs beng taken, coverng a frequency range of the nomnal centre frequency plus or mnus 100 Hz, takng around sx seconds and the sgnal beng modulated at 4.8 Hz, the source frequency actually changes around 0 tmes durng the frequency sweep. Some readngs are taken whle the transmtted frequency s constant, whle others are taken as the sgnal frequency s changng. Ths results n the

4 rms sgnal measurement changng between two envelopes. Ths s shown n fg. 3, where there are planly two bell shaped curves correspondng to the frequency response of the band-pass flter. The measurement data fall ether on one curve or the other when the source sgnal remans on one frequency for the duraton of the readng. Some ponts le n between the two curves, ndcatng that the source sgnal changed frequency durng the readng Current [A] Frequency [Hz ] Fgure 3: Typcal set of frequency/current data ponts. The shape of the bell curve s determned by the band pass flter bult nto the CAN nodes. The peaks of the two curves correspond to the centre frequency plus or mnus 17 Hz. Hence the only parameters are the centre frequency and the heghts of the two curves. The heghts of the two bell curves are proportonal to the rms level of the source sgnal at each of the two frequences. Whle t s easy for the human eye to see two curves n fg. 3 and gnore the ntermedate ponts, an algorthm s requred to automate ths process. The way that curves were ntally ftted s to assume that they have the form a1 y1 = 1+ α f1 a y = 1+ α f f ( f ) ( ) where y 1 and y represent the two curves as functons of frequency. The two frequences f 1 and f are assumed to be plus or mnus 17 Hz from a central frequency f 0, whch s to be estmated. Parameter α determnes the wdth of the bell curves and s determned by the Q-factor (qualty factor) of the bandpass flter on the measurng node. Ths parameter s well known, the qualty factor beng set to Q=30. Hence, there are essentally 3 parameters to be estmated: a 1, a and f 0. These 3 parameters are estmated by mnmsng a cost functon that s the weghted sum of two parts. The frst part s a measure of how well each pont fts the two curves; ether on the curves or between the curves s acceptable. No cost s assocated wth a pont between the two curves, but a quadratc cost s assocated wth ponts lyng outsde of the curves. Ths s not, however, a suffcent defnton of the (1) ()

5 cost. If t were, then t would be possble for one curve to have a very large ampltude and the other to have a small ampltude such that all data ponts le between the two curves and the cost s zero. The second part of the cost functon s proportonal to the square of the area between the two curves, the dea beng to fnd a par of curves that tghtly contan the data ponts. A sutable weghtng between the two components of the cost functon has been found by tral and error. The cost functon s gven by: ( y y j, ) + w ( y1, y, ) J = w1 d (3) Quantty d s set to zero when the data pont y les between the two curves and s set to 1 when the pont les outsde of the two curves, the square of the dstance to the nearest curve (j=1 or j=) beng used n the cost. The second part of the cost functon s based on the dstance between the two curves. The obtaned curves for the data of fg. 3 are shown together wth the orgnal data n fg Current [A] Frequency [Hz ] Fgure 4: Data ponts from fg. 3 wth ftted curves. In ths example, the ampltudes of the two curves are nearly equal. However, f the two carrer frequences are somewhat offset from the resonant frequency at the transmtter or recever ends, then the ampltude of one curve wll be ncreased whle the other s decreased. The rato of the two ampltudes s used later to ndcate the extent of any msmatch between the transmtter frequency and the resonant parts of a track crcut. The data ponts lyng between the two curves arse when the transmtter frequency jumps by 34 Hz durng the rms measurement, resultng n an rms level somewhere between the two curves dependng on when the frequency swtch occurred. The remander of the paper shows the results from data collected n the frst few months of operaton.

6 Intal results The system has been returnng data snce 19 October 007. At the tme of wrtng (Jan 008), only some of the data have been examned to ensure that the system s workng, and to determne f any changes have occurred snce the system was nstalled. The system was nstalled on track crcuts that are well behaved and where nstallaton was relatvely smple. It s antcpated that another system wll be nstalled at a locaton where problems are observed wth the track crcuts. The results shown below are taken from node 34 (0x), the current sensor located on the lead connectng the track to one recever tunng unt. Ths tunng unt acts as a short crcut to the A frequency and as a recever for the B frequency. The effects of ranfall and temperature are of partcular nterest, as well as any long-term changes. The frst result s how ranfall affects the receved B frequency current. Ths s the current that falls dramatcally when a ralway vehcle enters a track crcut, dvertng the current from the recever. Fg. 5 shows the receved current aganst tme and also the hourly ranfall (dvded by to ft more convenently on the dagram). Note that the ranfall nformaton comes from a weather staton located some mles from the test ste. It s ntended that local ranfall wll be measured usng a tppng ran gauge at a future date. The data are only shown when no vehcle s n the vcnty of the track crcut and 6 outlers (out of 680 data) have been edted out Current [A] / Ranfall n 0.5 mm Mean recever current Hourly ranfall 0 1 Oct 8 Oct 4 Nov 11 Nov 18 Nov 5 Nov Dec Date n 007 Fgure 5: Average current ampltude aganst tme and hourly ranfall aganst tme. The current falls by about 10% when t rans, except when only 0. mm of ran falls n one hour. The quantty of ran does not seem to affect the current reducton. Followng ranfall, the current recovers to the normal level over a perod of between a few hours to nearly one day. It s just possble to make out a durnal varaton (7 cycles n each week) assumed to be related to temperature fluctuatons. The result s perhaps expected. Wet condtons could reduce the ballast resstance (the resstance between the rals), allowng more of the transmtted current to leak from one ral to the other and not reach the recever end of the track crcut. The magntude of the reducton on a dfferent ste, for example where concrete sleepers are n poor condton, may be dfferent. The recovery to dry operaton presumably occurs as the sleepers and ballast dry out. The nvarance to the quantty of ran

7 may be because the ste dranage s good and only water soakng nto the surface of sleepers and on the surface of the ballast s mportant. However, a more careful examnaton of the data shows that explanaton n terms of a reducton n ballast shunt resstance s nsuffcent. Fg. 6 shows the receved current and ranfall as n fg. 5, but wth the transmtted current added Transmtted current Current [A] / Ranfall [mm/] Receved current 0 Ranfall / 10/5 11/04 11/14 11/4 1/04 Date (007) Fgure 6: Transmtted current added to fg. 5. The fgure shows that the transmtted current decreases durng wet perods n a smlar way to the receved current. Whle the receved current falls by approxmately 10% durng and followng ranfall, the transmtted current falls only by 5%. The mechansm for the reducton n transmtted current has not yet been dentfed, but t s possble that the tuned zone has a more damped response caused by a reducton n shunt resstance when t s wet, causng a reducton n Q of the tuned crcut. A smlar reducton n Q s possble n the tuned zone at the recever end of the track crcut, further reducng the receved current. Hence, a reducton n shunt resstance along the length of the track crcut may not be requred to explan the observed current changes. An examnaton of an electrcal model of the track crcut s needed to test ths hypothess. The transmtted frequency contnually swtches between 17 Hz above and below the nomnal centre frequency. The current measurng equpment measures the ampltudes of both of these components. If the centre frequency algns wth the tuned zone frequency, and the transmtter generates equal levels of each frequency, then the ampltude rato should be 1. The rato between the ampltudes as seen at the transmtter and recever tunng unts are plotted aganst tme n fg. 7, wth ranfall (dvded by 10) also shown where avalable. There are mnor daly varatons proportonal to the temperature (determned by a close nspecton on a dfferent scale) and small varatons wth ranfall. However, the transmtter and recever ratos show dfferent long term behavours. Whle the transmtter rato remans close to 1.0 over two months, the recever current rato drfts from 0.96 to 0.93 n the same tme. Ths cannot be explaned n terms of temperature or ranfall. Observatons are ongong to establsh f ths trend contnues.

8 1. Rato / ranfall [mm/10] Upper and lower frequency current rato at TX Upper and lower frequency current rato at RX Ranfall / /15 10/5 11/04 11/14 11/4 1/04 1/14 1/4 01/03 Date (007) Fgure 7: Transmtter and recever frequency ampltude ratos, and ranfall. Fg. 8 shows the measured (lower) B frequency aganst tme and also the temperature (n Celsus, dvded by -10 and offset by 90) f B (1) Frequency [Hz] T/ /10 04/11 14/11 4/11 04/1 14/1 4/1 03/01 Date n 007/008 Fgure 8: Receved B (lower) frequency and scaled and shfted temperature (dotted).

9 At the begnnng of the data the frequency varaton matches the temperature varaton. In the rest of the data, the change n frequency s sometmes larger than the correspondng change n temperature. Note that the temperature s measured outsde, whle the transmtter unt s nsde a lnesde cabnet and may see somewhat dfferent temperature changes. The frequency shows some long-term drft unrelated to temperature. Ths changng frequency, although modest, may partly explan the changng ampltude rato observed n fg. 7. As the frequency drfts upwards, assumng that the tunng unt remans unaffected, the rato of the ampltudes should drft downwards, as seen n fg. 7. The propertes of the tunng unt may also be temperature dependent. Conclusons A system has been developed to allow the long-term study of n-servce audo-frequency track crcuts. Frequency-dependent currents and voltages are beng measured contnually, as well as the sgnallng relay states. The data allow the rms ampltude of both modulaton frequences to be determned, from whch long-term changes n track crcut behavour can be montored. At the tme of wrtng, the montorng system has not been operatng for long enough to show long-term trends. However, some ntal results have been shown. Algorthms have been developed to reduce the collected data to sets of three parameters that provde the mportant nformaton requred for long-term montorng. Changes n the receved current have been shown to be affected by ranfall, but also found to be largely ndependent of the amount of ranfall. The stuaton wth other track crcuts may be dfferent, or more change may occur n the event of floodng, for example. The contnual swtchng between two carrer frequences allows nformaton to be obtaned about msmatch n the tunng of the track crcut by comparng the ampltudes of the two frequences. Some varaton wth temperature has been observed. The operatng frequency vares slghtly wth temperature and ths affects the receved ampltudes very slghtly and s suffcent to explan most of the daly varaton observed. However, there appears to be a gradual drft n the track crcut operatng frequency that wll contnue to be montored to assess whether t s a trend. There remans a great deal of data from other nodes that has yet to be analysed. Ths analyss s ongong. Data collecton wll contnue for at least one calendar year to allow analyss of data from all seasons. In addton, t s ntended that the equpment, havng been proved to be relable, wll be duplcated on a track crcut that s more susceptble to falures. Acknowledgements The authors acknowledge the support of Network Ral (UK) and Bombarder (UK). References [1] C. Bell, Pont, track crcut and level crossng condton montorng, IEE Semnar on Ralway Condton Montorng, 3 Feb, (005). [] CDSRal Ltd, Condton montorng apparatus for track crcuts and method, Internatonal patent applcaton, WO/004/07656 (PCT/GB004/000739), (004). [3] J. Chen, C. Roberts, and P. Weston, Fault detecton and dagnoss for ralway track crcuts usng neuro-fuzzy systems, Control Engneerng Practce, In Press, (007). [4] P. J. Cross, D. Kesley and R. H. Perce, Safety cases for electromagnetc compatblty of TI1 track crcuts and Eurostar trans, IET Internatonal Conference on System Safety, London, UK, 6 8 June, pp. 11 9, (006). [5] A. Debolles, L. Oukhellou, P. Aknn, and T. Denoeux, Track crcut automatc dagnoss based on a local electrcal modellng, 7 th World Congress on Ralway Research, Montreal, Canada, 4 8 June, (006).

10 [6] M. C. Falvo, E. Fedel, and R. Lamedca, A measurement campagn on audo frequency track crcuts of Italan hgh speed ralway systems, Internatonal Symposum on Power Electroncs, Electrcal Drves, Automaton and Moton (SPEEDAM), May 3 6, pp , (006). [7] R. T. Sewell, Evoluton of electroncs n ralway sgnalng from the track crcut standpont, IEEE Vehcular Technology Conference, 1 3 May, pp. 9 98, (1985).