WO2014023699A1

WO2014023699A1 - Reflectometry system including an information transmission mechanism

Info

Publication number: WO2014023699A1
Application number: PCT/EP2013/066410
Authority: WO
Inventors: Luca INCARBONE; Laurent Sommervogel
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2012-08-07
Filing date: 2013-08-05
Publication date: 2014-02-13
Also published as: FR2994484A1; FR2994484B1

Abstract

The invention relates to a reflectometry system for detecting defects in a network consisting of at least one cable, said system consisting of at least one reflectometer (400) and at least one reception device connected to said network. The reflectometer (400) feeds a test signal into the network at an feeding point and receives an observation signal at an observation point, the observation signal being analyzed (408) so as to establish a bit sequence that is representative of the state of the network, said sequence being transmitted to the reception device while weighting the test signal prior to the feeding of same using at least one vector, the components of which are used as weighting coefficients, wherein at least one vector is chosen from at least two orthogonal vectors, the weighting vector is chosen depending on the bits of the sequence, and the reception device includes means (422, 423, 424) for detecting which vectors were used to acquire the bits of the emitted sequence.

Description

REFLECTOMETRY SYSTEM COMPRISING AN INFORMATION TRANSMISSION MECHANISM

The present invention relates to a reflectometry system comprising an information transmission mechanism in the cable network to be tested. It applies in particular to the field of distributed reflectometry.

Cables are ubiquitous in all electrical systems, for powering or transmitting information. These cables are subject to constraints and may be subject to failures. It is therefore necessary to be able to test their status and provide information on the detection of defects, but also their location and type, to help with maintenance and prevention. For this, so-called reflectometry methods are implemented. These can also be used to detect faults in an optical fiber network.

The principle of OTDR is based on the injection of a test signal. The shape of this signal changes significantly during its propagation back and forth in a cable, these changes being the consequence of physical phenomena of attenuation and dispersion.

OTDR methods use a principle similar to radar. An electrical signal usually called a test signal, often of high frequency or broadband, is injected at one or more points of the cable or cable network to be tested. Said signal propagates in the cable or network and returns a portion of its energy when it encounters an electrical discontinuity. An electrical discontinuity may result, for example, from a connection, the end of the cable or a fault. The analysis of the signals returned to the injection point makes it possible to deduce information on the presence and the location of these discontinuities, thus possible defects. Several methods of analysis exist such as the TDR (Time Domain Reflectometry) method or the Frequency Domain Reflectometry (FDR) method. The characteristics of the test signal used depend on the reflectometry method used. The result of the analysis of the signal received at a given observation point is, for example, an echogram, that is to say a diagram constituting a measurement of the reflections of the test signal in the cable network.

In order to inject the test signal at a given point and to measure the signal received at a point of observation of the network of at least one cable, a device called a reflectometer is usually used. The point of injection and the point of observation may be the same, but not necessarily.

In a network of cables having multiple branches, it may be useful to use a plurality of simultaneously operating reflectometers. This technique is usually called distributed reflectometry.

Distributed reflectometry is particularly suitable for locating faults or failures impacting cables of very long length, such as signaling cables of a railway network. It also allows, in case of appearance of two faults, to locate the position of the two defects and to determine the distance between them. This type of problem can occur if the cable is, for example, cut into two separate points in order to remove an entire segment of the cable. With a single reflectometer, it is possible to detect only a point defect and one can obtain the distance which separates it from the injection point of the signal but one can not determine the position of a second defect, therefore the length of a cable segment taken can not be determined. Distributed reflectometry in particular makes it possible to obtain this type of information. In addition, distributed reflectometry makes it possible to remove ambiguities of location. For example, in a Y network, a single reflectometer does not tell which branch is located a fault beyond the junction.

In the example mentioned, a second reflectometer is positioned at a second point of the cable, for example at an end opposite to the first injection point. If a portion of the cable is cut off and removed, the information produced by the two reflectometry devices allows to deduce the position of the two defects and the length of the cable portion removed.

Distributed reflectometry also makes it possible to acquire measurements over greater distances. Indeed, if each reflectometer can monitor a certain length of cable, two reflectometers positioned at two distant points can monitor a double length.

However, when several reflectometers are used simultaneously, it is necessary to collect the measurements made by each one of them to analyze them. For this, a processing unit is usually used. This processing unit can be a separate equipment from the reflectometers of the system. Thus, it is required to implement means of communication between the processing unit and the reflectometers. These means of communication can implement radio links, Bluetooth technology being an example allowing it.

However, this approach has several disadvantages. If one of the reflectometers is not in radio range of the processing unit, it will not be able to collect the reflectometry measurements it generates. This is particularly the case when a reflectometer is buried underground. On the other hand, when several reflectometers emit their measurement report to the processing unit, it is possible that they interfere with each other and that part of the measurement reports are not correctly received.

In addition, the reflectometers and the processing unit must implement these communications links to transmit their measurement reports, and this translates into implementation complexity. Indeed, it is necessary to implement additional circuits such as signal processing processors and radiofrequency communication channels. An object of the invention is in particular to overcome the aforementioned drawbacks. For this purpose, the object of the invention is a reflectometry system for detecting defects present in a network of at least one cable, said system being composed of at least one reflectometer and at least one connected reception device. network audit.

The reflectometer injects a test signal into the network at an injection point and receives an observation signal at an observation point, the observation signal being analyzed to establish a sequence of bits representative of the state. network. Said sequence is transmitted to the receiving device by weighting the test signal with at least one vector whose components are used as weighting coefficients, the choice of at least one vector being made from at least two orthogonal vectors, the choice of the bitwise weighting vector of the sequence. The receiving device includes means for detecting which vectors have been used to acquire the bits of the transmitted sequence.

In one embodiment, one of the vectors is chosen when one bit of the sequence is equal to one and the other vector is chosen when a bit of the sequence is equal to zero.

According to one aspect of the invention, the system comprises a plurality of refiectom beings, the vectors used for the emission of binary sequences being different for each reflectometer.

The reception device stores, for example, the different vector pairs associated with the reflectometers connected to the network, the transmitted bit sequences being detected by scalar product of the signal received with said stored vectors.

According to another aspect of the invention, the receiving device is a reflectometer.

In one embodiment, a reflectometer used as a reception device is adapted to receive all the binary sequences transmitted by the reflectometers connected to the network. The receiving device may be a processing unit whose purpose is to collect and analyze measurements transmitted by the at least one reflectometer connected to the network.

The components of these vectors are for example equal to +1 or -1. The components of the weighting vectors can be Hadamard sequences.

The set of weighting vectors can be chosen to remain orthogonal when a circular permutation of any number of samples is applied to them.

In an alternative, the components of the weighting vectors are determined using Rademacher functions.

The reflectometers of the system can be adapted to transmit the bit sequences step by step to a master reflectometer previously chosen.

According to one aspect of the invention, at least one reflectometer transmits binary sequences that it has collected to a remote processing unit that will perform their joint analysis.

The binary sequence corresponds for example to an echogram.

The invention also relates to a reflectometer injecting into the network a test signal at an injection point and receiving an observation signal at a point of observation, the observation signal being analyzed in order to establish a sequence of bits representative of the state of the network, said sequence being transmitted by weighting the test signal before its injection with at least one vector whose components are used as weighting coefficients, the choice of at least one vector being made from two orthogonal vectors, one of the vectors being chosen when one bit of the sequence is equal to one and the other vector is chosen when a bit of the sequence is equal to zero.

The invention also relates to a receiving device adapted to be connected to a network of cables and comprising means for detecting vectors that have been used by at least one reflectometer as previously described and connected to the same network.

Other features and advantages of the invention will become apparent with the aid of the description which follows, given by way of illustration and without limitation, with reference to the appended drawings in which: FIG. 1 gives an example of a network of Y cables. ;

FIG. 2 is a simplified representation of a conventional reflectometer that can be used in a distributed reflectometry system;

FIG. 3 schematically illustrates an example of a system enabling a reflectometer to detect that another reflectometer is or is not connected to the network;

FIG. 4 gives a simplified example of a distributed reflectometry system enabling the different reflectometers to communicate with one another;

FIG. 5 represents a reflectometry network according to the invention composed of three reflectometers and a network of cables comprising three branches;

FIG. 6 gives an example of a reflectometer implementing the invention.

Figure 1 gives an example of a network of Y cables. This example has the advantage of being representative of any network, that is to say point-to-point networks, 1 to 2 , 1-to-N, or arborescent. In a Y-array, at least two reflectometers 100, 101 are required to remove any ambiguity in locating a defect at any position.

The exemplary cable network comprises three branches 102, 103, 104. If a fault is present on one of the branches 103 or 104 and only the reflectometer 100 is used, then there remains an ambiguity after the processing of the return signal and it is difficult to determine if the fault is on the branch 103 or the branch 04. The use of a second reflectometer 101 removes this ambiguity.

When designing a distributed reflectometry system, it is necessary to implement techniques enabling a given reflectometer 101, 102 to analyze the return signal resulting from the test signal that it has emitted without necessarily being disturbed by the signals emitted by the other reflectom beings of the system.

For this, different techniques can be used. By way of example, mention may be made of the weighted average technique disclosed in Publication No. 2937146 of a French patent application or the so-called self-selective average technique presented in French patent application FR1253054. Figure 2 is a simplified representation of a conventional reflectometer for use in a distributed reflectometry system.

A reflectometry device 201, also called a reflectometer, comprises at least one electronic component 21 1 of the integrated circuit type, such as a programmable logic circuit, for example of the FPGA or microcontroller type, a digital-to-analog converter 212 for injecting a test signal into the cable network to be tested 203, an analog-digital converter 213 for receiving the signal reflected on the impedance discontinuities or singularities of the network of at least one cable, a coupling device 214 between the analog-digital converter 213 and the digital-to-analog converter 212 and coupling means not shown between an input / output of the device 201 and the test cable network 203. The coupling means is adapted to inject the output signal of the digital-to-analog converter. It appears in this example that the injection point and the observation point are identical. The reflectometer 201 may be implemented by an electronic card on which are arranged the various elements 21 1, 212, 213, 214 which compose it.

In addition, a processing unit 202, such as a computer, personal digital assistant or the like, may be used to control the reflectometry device 201, display the measurement results on a human-machine interface and jointly analyze the results coming from several reflectometers. for example to remove measurement ambiguities or improve the accuracy of said measurements. This processing unit comprises communication means for receiving messages from at least one reflectometer.

The electronic component 21 1 implements on the one hand the processing steps necessary for the generation 215 of the test signal, on the other hand an analysis 217 of the reflected signal in order to deduce an echogram that can be transmitted to the unit 202. The electronic component 21 1 further comprises a communication module 216 for managing the communication link between the reflectometer 201 and the processing unit 202.

Figure 3 schematically illustrates an example of a system allowing a reflectometer to detect that another reflectometer is or is not connected to the network.

For this, a given reflectometer determines whether the test signals injected by the other reflectometer are present or not in the signal it receives.

In Figure 3 is shown a cable network composed of three branches 302, 303, 304. Two reflectometers 300, 301 are also shown. The first reflectometer 300 comprises a first filter 307. In the remainder of the description, the term "filter" designates means for selecting a given signal and for eliminating the influences of other signals present in the received signal. Thus, the filter 307 is adapted to favor the component of the received signal corresponding to the reflection of the signal from the reflectometer 300 and to reject the component of the received signal corresponding to the test signal emitted by the reflectometer 30. Following this filtering operation, processing means are implemented to perform an analysis, for example of the TDR type, on the filtered signal.

The received signal is also routed to a second filter 309. In contrast to the first filter 307, the purpose of this filter is to favor the component of the received signal corresponding to the test signal emitted by the reflectometer 301 and to reject the component of the signal received. corresponding to the test signal emitted by the reflectometer 300. This filtering operation then makes it possible to determine whether the reflectometer 301 is connected to the network of cables. The result of the filtering 309 can thus be used to generate 310 a flag of Boolean type representative of this connection or this lack of connection.

The reflectometer 301 operates according to the same principle. Thus, it comprises a filter 31 1 adapted to the test signal emitted by the reflectometer 301. The result of this filtering operation is used to implement a TDR type analysis 312. In addition, the signal received by the reflectometer 301 is processed by a filter 313 adapted to the test signal emitted by the reflectometer 300 so as to generate a boolean indicating whether the reflectometer 300 is connected or disconnected to the cable network.

Thus, if a fault 306 exists on the branch 304, the reflectometer 300 can detect that the reflectometer 301 is still connected to the cable network.

On the contrary, if a fault 305 exists on the branch 303, the reflectometer 300 can not detect the presence of the reflectometer 301 because the test signal emitted by it is not received at the reflectometer 300.

Thus, the presence of two reflectometers removes the ambiguity of fault location in a Y-array. Figure 4 gives a simplified example of a distributed OTDR system allowing different reflectometers to communicate with each other.

In this figure, a cable network consisting of three branches 402, 403, 404 is shown. Two reflectometers 400, 401 are also shown.

Reflectometers 400, 401 perform OTDR measurements to locate the fault (s) present in the cable network. The result of these measurements is then communicated to the other reflectometers by weighting the test signal transmitted on said network by coefficients. These weighting coefficients are selected according to the data to be transmitted. This set of coefficients is called in the following filtering vector. A given reflectometer further comprises means for detecting the test signals emitted by the other reflectometers of the system and extracting the message transmitted with it. In this system, the network of cables is used by the different reflectometers as a communication network allowing them to exchange data, such as the result of measurements made by each of them. Advantageously, this system makes it possible to maintain a communication link between the different reflectometers composing the system, if one of the reflectometers is buried or located in a place that can not be within radio range of the other reflectometers.

In a preferred embodiment, at least one reflectometer of the system is associated with a pair of filter vectors of its own. The components of these vectors are for example equal to +1 or -1 and are used to weight the test signal emitted by this reflectometer.

In the example of FIG. 4, the reflectometer 400 is associated with a pair of vectors called "A" and "a". As for the reflectometer 401, it is associated with another pair of vectors called "B" and "b". The four vectors A, a, B and b are orthogonal to each other. When the reflectometer 400 emits its test signal, it is weighted with the coefficients of one of the two vectors associated with it. The reflectometer 400 will impose A or a depending on the bit that it wants to transmit to the other reflectometers. It is possible to choose to associate the vector A with the value of bit Ύ and the vector with the value of bit Ό '. It is therefore possible to send a frame of numerical values successively choosing either A or a to weight the test signal.

In the same way, the reflectometer 401 can use the vectors B and B respectively associated with the bit values 1 and 0 to adapt its test signal and thus transmit binary data.

In order to recover the binary data thus transmitted, a reflectometer must perform several treatments. The received signal must be filtered in parallel by different modules 407, 409, 410. A first module 407 is used for the filtering of the received signal before the reflectometry analysis 408. The same vector A or a as that which is currently to be imposed by the communication block will be used so as to maximize the energy of the received signal component corresponds to the reflection of the test signal emitted by the reflectometer 400.

In addition, two other modules 409, 410 filter the received signal. These two modules 409, 410 are respectively adapted to the vectors B and b used by the reflectometer 401.

The module 409 produces a dot product between the received signal and the vector B associated with the bit Ί '. As for the module 410, it performs a correlation between the received signal and the vector b associated with the bit Ό '.

The reconstitution of the binary message transmitted by the reflectometer 401 can be carried out by a module 41 1 of detection and control. For example, if the result of the correlation performed by the filter module 409 exceeds a predefined threshold value, a bit of value 1 is detected. In the same way, if the result of the correlation performed by the filtering module 410 exceeds another predefined threshold value, a bit of value 0 is detected. The second reflectometer 401 also comprises means 422, 423, 424 for decoding the binary message transmitted by the first reflectometer. For that, he must know the vectors A and a. It also comprises means 420, 421 for receiving and analyzing the reflected signal.

This reflectometry system has the decisive advantage of using pre-existing reflectometry signals. Furthermore, since the vectors A or a and B and b are orthogonal two by two, the information carrying test signals emitted by these two distinct reflectometers do not interfere with each other because the contribution of each of the signals can be identified within the signal received by a given reflectometer.

With regard to the selection of the vectors associated with each of the reflectometers, several choices are possible. If the reflectometry system is synchronous, i.e. the reflectometers are synchronized with each other, the weighting vectors should just be orthogonal two by two, as in the examples presented above. To satisfy this condition, one can use the functions of Hadamard, but other choices are also possible.

If the reflectometry system is asynchronous, the weighting vectors must remain orthogonal when a circular permutation of any number of samples is applied to them. In this case, two alternatives may be considered in particular for the choice of coefficients. A first alternative is to use sinusoids. In a second alternative, Rademacher functions are used which are binary functions. This solution has the particular advantage of avoiding the use of a multiplier in comparison with the use of sinusoids.

FIG. 5 represents a reflectometry network composed of three reflectometers 500, 501, 502 and a network of cables comprising three branches 503, 504, 505. In this example, the reflectometers are adapted to transmit the measurement data step by step. For example, the reflectometer 502 transmits its measurement results to the reflectometer 501. As for the reflectometer 501, it transmits its own measurement results as well as the measurement results that it has acquired by analyzing the test signal of the reflectometer 502. The reflectometer 500 acquires the results of the measurements made by the reflectometers 501 and 502. is obviously in possession of the results of measurements that he has himself made.

In one embodiment, the reflectometer 500 also acts as a processing unit and implements a joint analysis of all the measurements collected. This reflectometer is then called master reflectometer. Alternatively, the reflectometer 500 transmits the collected results to a remote processing unit which will perform their joint analysis.

Figure 6 gives an example of a reflectometer implementing the invention. As the example of the general in FIG. 2, the functions are divided into three main modules: a module 615 responsible for generating the test signal, a module 616 responsible for the communication of binary messages outside the reflectometer and a module 617 responsible for analyzing the received signal to generate a test result. In this example, the reflectometer 601 comprises means for decoding the binary message transmitted by another reflectometer of the system, this other reflectometer being associated with two weighting vectors V1 and V2. Thus, the received analog signal r is converted to a digital signal using an ADC 612 analog-to-digital converter. The coefficients of the weighting vectors V1 and V2 are used to weight the digitized received signal. For this, conventional multipliers 620, 621 may be used. If we use coefficients 1 or -1, we can just use a simple inverter block to perform the multiplication by -1 if the data are coded in complement to 2. This is less heavy than a conventional multiplier for implementation in an FPGA (Field Programmable Gate Array).

The result of these weightings can be temporarily stored in buffers 622, 623. The results of these two weightings are then compared 624 so as to detect which are the data bits transmitted by the remote reflectometer. For this, a threshold detection can be used, as previously described. The binary message thus obtained can then be transmitted outside the reflectometer, for example to a processing unit. For this, an interface module 625 can be used to set up the protocols and circuits necessary for data exchange with said unit.

The reflectometer 601 also includes a module 615 responsible for generating and shaping the test signal. This module generates 626 at first the test signal as such. The latter is then weighted by a series of coefficients CS, and this with the aid of a multiplier 627. The choice of the coefficients CS, that is to say the choice between the coefficients of the vectors V3 or V4, allows to transmit binary information, for example representative of the analysis performed on the signal received by the reflectometer 601.

The analysis module 617 uses the signal received after digitization.

The received signal is weighted by the CS coefficient sequence which is also used for the weighting of the test signal. For this purpose a multiplier 628 can be used. The result of this weighting can then be temporarily stored in a buffer memory 629. This weighting performed on the received signal has the function of canceling the effect of the weighting by the reflectometer 601 on its test signal before transmission. The received signal, after digitization 612 and weighting 628 can then be analyzed 630. A TDR or FDR type method can be used for this, in order to generate an echogram. A binary message corresponding either directly to this echogram or to the result of a pretreatment performed on this echogram can then be transmitted directly over the network of cables. For this purpose, the weighting coefficients CS are chosen to be equal to either the coefficients of the weighting vector V3 or the coefficients of the weighting vector V4. The vectors V3 and V4 are unambiguously associated with the reflectometer 601, that is, other reflectometers positioned in the system will not be able to use them. They must, however, be known from a remote meter of 601 so that it can read the binary message transmitted with the test signal. The choice between the coefficients of the vectors V3 and V4 can be made for example by means of a multiplexer 631, this multiplexer being controlled by the bits 632 of the binary message to be transmitted. The result of the analysis can also be transferred 635 to the interface module 625.

In a preferred embodiment, the coefficients of the vectors V1, V2, V3 and V4 may take as values -1 or +1. The reflectometry system according to the invention comprises for example a plurality of reflectometers as well as a processing unit.

In one embodiment, a reflectometer acts as a single interface with the processing unit. For this, it must be adapted so that all the measurement results made by the other reflectometers in the system can be collected. It is called master reflectometer.

In another embodiment, the master reflectometer and the processing unit are implemented in the same device.

In another embodiment, the reflectometry system comprises a single reflectometer connected to the network of cables to be tested and a processing unit also connected to the network but at another location. In this case, the remote processing unit must include means for decoding the binary message transmitted with the test signal. In other words, the processing unit must know the weighting vectors used by the reflectometer and weight the received signal with its two vectors so as to acquire the binary message thus transmitted.

Claims

OTDR system for detecting faults present in a network of at least one cable, said system being composed of at least one reflectometer (400) and at least one receiving device connected to said network, the reflectometer (400) injecting into the network a test signal at an injection point and receiving an observation signal at an observation point, the observation signal being analyzed (408) to establish a representative bit sequence of the state of the network, said sequence being transmitted to the receiving device by weighting the test signal with its at least one vector whose components are used as weighting coefficients, the choice of at least one vector being made from at least two orthogonal vectors, the choice of the sequence-dependent weighting vector of the sequence, the receiving device comprising means (422, 423, 424) for detecting which vectors are used so as to acquire the bits in the transmitted sequence.

The system of claim 1 wherein one of the vectors is selected when one bit of the sequence is equal to one and the other vector is selected when one bit of the sequence is zero.

System according to one of the preceding claims comprising a plurality of reflectom beings, the vectors used for the transmission of binary sequences being different for each reflectometer.

System according to one of the preceding claims wherein the receiving device stores the different vector pairs associated with the reflectometers connected to the network, the transmitted binary sequences being detected by scalar product of the signal received with said stored vectors. 5. System according to one of the preceding claims wherein the receiving device is a reflectometer (401). 6. The system of claim 5 wherein a reflectometer used as a receiving device is adapted to receive all the bit sequences emitted by the reflectometers connected to the network. 7- System according to one of the preceding claims wherein the receiving device is a processing unit whose purpose is to collect and analyze measurements transmitted by the at least one reflectometer connected to the network. 8- System according to one of the preceding claims wherein the components of these vectors are equal to +1 or -1.

9. System according to one of the preceding claims wherein the components of the weighting vectors are Hadamard sequences.

10- System according to one of claims 1 to 8 wherein the set of weighting vectors are chosen to remain orthogonal when they are applied a circular permutation of any number of samples.

The system of claim 10 wherein the components of the weighting vectors are determined using Rademacher functions.

12- System according to one of the preceding claims wherein the reflectometers of the system are adapted to transmit the bit sequences step by step to a master reflectometer previously chosen. The system of claim 12 wherein at least one reflectometer (500) transmits binary sequences that it has collected to a remote processing unit (202) which will perform their conjoint analysis.

14. System according to one of the preceding claims wherein the binary sequence corresponds to an echogram.

15- Reflectometer (400) injecting into the network a test signal at an injection point and receiving an observation signal at a point of observation, the observation signal being analyzed (408) in order to establish a sequence of bits representative of the state of the network, said sequence being transmitted by weighting the test signal before its injection with at least one vector whose components are used as weighting coefficients, the choice of at least one vector being made from two orthogonal vectors, one of the vectors being chosen when one bit of the sequence is equal to one and the other vector is chosen when a bit of the sequence is equal to zero.

Receiving device adapted to be connected to a network of cables and comprising means (422, 423, 424) for detecting the vectors that have been used by at least one reflectometer according to claim 15 connected to the same network.