Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
  • G01R31/08—Locating faults in cables, transmission lines, or networks
  • G01R31/11—Locating faults in cables, transmission lines, or networks using pulse reflection methods

Definitions

• a method of characterizing a section of a transmission line in particular a section corresponding to a connector or a series of connectors connecting a measurement equipment to a cable.
• the present invention relates to the field of systems and diagnostic methods for cable and in particular the field of methods for detecting and / or locating defects impacting a cable.
• the invention relates more specifically to a method for characterizing a section of a transmission line by modeling in the form of a succession of characteristic impedance sections and clean lengths.
• the invention is advantageously applied to characterize a connector or a connector or a succession of connectors connecting a measuring equipment to a cable.
• the measurement equipment is configured to perform a reflectometry measurement by injecting a reference signal into the cable and by measuring the reflection of this signal on the impedance discontinuities encountered on its path.
• the invention finds application in all areas where it is necessary to characterize the mechanical and / or electrical connections between a cable and a measuring device.
• the invention applies to any type of electrical cable, in particular power transmission cables or communication cables, in fixed or mobile installations.
• the cables concerned may be coaxial, two-wire, parallel lines, twisted pairs, cable strand or other.
• the invention can also be applied to mechanical cables, for example infrastructure support cables such as an elevator or a bridge.
• so-called OTDR methods are used to detect and / or locate electrical or mechanical defects which generate discontinuities or impedance breaks in a cable.
• an electrical signal often of high frequency or broad band, is injected in one or more places of the cable to be tested.
• the signal propagates in the cable or network and returns some 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 more generally a break in the conditions of propagation of the signal in the cable. It most often results from a fault that locally modifies the characteristic impedance of the cable by causing a discontinuity in its linear parameters.
• 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.
• An analysis in the time or frequency domain is usually performed. These methods are designated by the acronyms TDR from the English expression “Time Domain Reflectometry” and FDR from the Anglo-Saxon term “Frequency Domain Reflectometry”.
• the measuring device used to inject the signal into the cable and measure the reflected signal can take various forms, it can be a network analyzer or a device embedded on an electronic card.
• the variety of possible measuring devices as well as the variety of cable types to which the characterization of faults by reflectometry can be applied lead to the need to provide many types of connectors or connectors making it possible to connect the different measuring devices to different types of measurement equipment. of cables.
• a connection is indeed necessary to achieve the mechanical and electrical connection between the cable under test and the measuring device.
• connection itself has a characteristic impedance of its own, but this information is not always accessible.
• standard connectors do not exist, especially for network analyzers available at the current date.
• connection of the connector to the measuring device and to the cable creates an impedance break because of the different values of the characteristic impedances of the different elements.
• the signal injected by the measuring apparatus is thus reflected, even before it enters the cable, on the impedance discontinuity generated by the connector.
• This phenomenon generates the presence of blind zones in the temporal reflectogram obtained from the measurement of the reflected signal.
• a blind zone is located in particular at the connector. If a low amplitude defect is present in this zone, its signature on the reflectogram will be masked by the echo of the signal on the impedance discontinuity linked to the connector.
• the present invention aims to characterize the connector or connectors connecting the measuring equipment to the cable by modeling the portion of the transmission line corresponding to the connector (s) in the form of a succession of characteristic impedance sections. constants separated by interfaces corresponding to impedance discontinuities on which the signal is reflected.
• the invention makes it possible to characterize any connector or more generally any portion of a transmission line.
• the characteristic impedance values obtained by the method, as well as the positions of the impedance discontinuities, are saved in a database that allows not only the characterization of a large number of connections but also a follow-up of the evolution of this connection. characterization as a function of time or as a function of physical parameters such as temperature or humidity.
• the subject of the invention is a method of characterizing a section of a transmission line, a reference signal being injected into the line and a temporal measurement of the reflection of said reference signal in the line being carried out, said method comprising the following steps:
• ⁇ Deduce, from the amplitude of each peak, an estimate of the real part of the reflection coefficient of a reflected wave on each identified impedance discontinuity.
• said section corresponds to a connector or series of connectors connecting a measurement equipment to a cable, the reflection time measurement being taken in a time zone corresponding to the presence zone of the connector or the series of connectors.
• the deconvolution step further comprises a linear interpolation and can be executed using a CLEAN type algorithm.
• the step of eliminating the contribution of at least one secondary reflection comprises the following sub-steps:
• the step of eliminating the contribution of at least one secondary rebound comprises the following sub-steps:
• the first peak selected is the first peak in the time order of appearance
• the second peak selected is the second peak in the temporal order of appearance
• the second peak selected is the peak successive to the two peaks selected at the previous iteration and the first peak selected is one of the peaks selected at one of the previous iterations.
• the estimate of the real part of the reflection coefficient associated with each impedance discontinuity is determined using the following relation:
• the method according to the invention further comprises a step of determining an estimate of the real part of the characteristic impedance associated with each transmission line section delimited by two successive impedance discontinuities from the corresponding estimates of the real parts of the reflection coefficients associated with said two discontinuities of impedances.
• the method according to the invention comprises in a step of determining an estimate of the imaginary part of the characteristic impedance associated with each impedance discontinuity from the real part of said characteristic impedance.
• the method according to the invention further comprises the following steps:
• the method according to the invention further comprises the following steps:
• ⁇ Determine information that is characteristic of the degree of similarity between the convolved sequence and the time measurement of signal reflection.
• the characteristic information of the similarity is taken equal to the coefficient of determination R 2 or to the likelihood coefficient V 2 .
• said method is iterated by modifying at each iteration the number of amplitude peaks extracted during the deconvolution step, the selected solution being that which has the highest degree of similarity.
• the method according to the invention also comprises a step of supplying a database with the positions and reflection coefficients and / or impedances characteristic of the calculated impedance discontinuities, each input of the base. associated with a connector or a set of connectors connected in series for interconnecting a measuring device with a cable.
• the subject of the invention is also a database comprising a plurality of pairs of values of positions and reflection coefficients and / or impedances characteristic of impedance discontinuities determined by performing the method of characterizing a section of a line of transmission according to the invention, each value pair in the database being indexed by an input associated with a connector or a set of connectors connected in series for interconnecting a measuring device with a cable.
• the subject of the invention is also a computer program comprising instructions for executing the method of characterizing a section of a transmission line according to the invention, when the program is executed by a processor as well as a processor.
• processor-readable recording medium on which is recorded a program comprising instructions for performing the method of characterizing a section of a transmission line according to the invention, when the program is executed by a processor.
• FIGS. 1a and 1b a block diagram of a transmission line and an associated time reflectogram
• FIG. 2 a reflectogram representing the temporal signatures of two types of connectors
• FIG. 3 is a flowchart detailing the steps for implementing the method according to the invention.
• FIG. 1 a represents, in a simplified diagram, a cable 101 to be analyzed connected to a measuring device 103 able to generate a signal to inject it into the cable 101 and to measure the reflection of the signal on the impedance discontinuities of the cable.
• the measuring apparatus 103 may be split into two separate apparatuses, a first apparatus for generating and injecting the test signal and a second apparatus for measuring the reflected signal.
• the meter 103 is connected to the cable 101 through a connector or connector 102.
• the connector 102 may consist of a plurality of connectors connected in series.
• the connector 102 can be composed of solder connections, dominoes, coaxial connection cables or any other connection means for connecting a measuring equipment to a cable.
• FIG. 1b represents a temporal reflectogram obtained by a reflectometry measurement made by the measurement apparatus 103.
• This reflectogram there is a first peak 1 10 which corresponds to the reflection of the signal on the impedance break associated with the connection. 102 and a second peak 1 1 1 which corresponds to the reflection of the signal on the impedance break due to the termination of the cable.
• These two reflection peaks are related to impedance discontinuities corresponding to controlled physical characteristics of the system under test.
• the return pulses located in the zones 1 10 and 11 1 may be more or less wide.
• the primary purpose of a reflectometry test is to detect and locate the presence of defects on the cable to be analyzed. If a fault exists on the cable near the zones 1 10 and 1 1 1, the echo associated with this fault may be masked by the echoes associated with the connector and the termination of the cable. This is why we are talking about blind zones to designate zones 1 10 and 1 1 1.
• An object of the invention is in particular to characterize the line portion corresponding to the connector 102 and the beginning of the cable 101 in order to correctly model the contribution of a reflection of the signal on this zone. This characterization may make it possible to compensate for the echo associated with the connector 102 during a reflectometry test for detecting unprepared faults in the zone located just at the beginning of the cable, after the connector 102. In this respect, the invention makes it possible to improve the characterization of defects located in or near blind zones. The invention also makes it possible to monitor the evolution over time of the characteristics of a connector 102 to identify the influence of aging or of certain physical parameters such as temperature, pressure or humidity.
• Z c is the characteristic impedance of the cable. This quantity, expressed in ohms, is complex and depends on the frequency. However, this magnitude is decisive in the energy exchanges between the cable and the systems connected to its ends. Indeed, to ensure the transfer of the maximum energy from one system to another, it is necessary to minimize the differences between the input or output impedances of the cable and its characteristic impedance Z c . When this is not the case, we observe what is called an impedance mismatch as well as a reflected wave at the interface between the two systems. This principle is valid regardless of the position of the impedance break in the cable, may be due to cable faults and is the founding principle of reflectometry.
• an impedance mismatch between a cable and a measuring device results from the difference between the geometric shapes connected to each other as well as the nature of the materials used.
• the associated impedance variation is progressive and not localized and therefore complex to describe.
• the connection between a network analyzer with an output of the SMA type and a twisted pair whose gap between the strands is not constant can not be described by the interplay of a single intermediate impedance .
• it is possible, given the size of the connection elements and the working frequency, to provide a discrete model with more or fewer interfaces. At each of these interfaces is associated a reflection coefficient p £ voltage (or current if current work) which is defined as follows: ⁇ ( ⁇ ) ⁇ + ⁇ ( ⁇ ) ⁇ ⁇ ⁇ ( 0)
• mismatch is a function of the physical nature of the connection between the cable and the systems. connected on it. It can therefore, for the same system and the same cable be different depending on the connection mode.
• FIG. 2 illustrates this phenomenon on a reflectogram which comprises two curves 201, 202.
• the first curve 201 corresponds to a mismatch between a coaxial cable and a twisted pair interconnected by a domino.
• the second curve 202 always corresponds to a connection between a coaxial cable and a twisted pair but this time connected by a weld. Note that the nature of the connector influences very clearly the pace of the mismatch as represented by the echoes 201, 202.
• the invention is based on modeling each equivalent mismatch in a succession of characteristic impedance and clean length sections.
• the attenuation of the signal is neglected because we consider weak lengths, which are of the order of the average length of a connector.
• N is the number of interfaces between two sections on which the signal can be reflected
• t is the length of the i th section
• p £ is the real part of the reflection coefficient of the signal on the interface between the i th section and the (i + 1) th section.
• Xrebonds is a term that depends on the multiple bounces of the signal on different interfaces.
• the invention applies to a measurement 301 of reflectometry, preferably a time measurement, performed by a measuring device 103 on a cable 101.
• the measuring portion 301 corresponding to the blind area or a mismatch that one wishes to characterize.
• the selected measurement portion must have a maximum duration.
• a measurement portion of duration corresponding to four times the width at half the height of the pulse of the signal injected into the cable makes it possible, for example, to ensure a good compromise between sufficient measurement time and low attenuation of the signal over this period.
• a deconvolution 302 is applied to the measuring or measuring portion 301.
• the purpose of the deconvolution step 302 is to remove, from the measurement 301, the contribution of the injected signal in order to obtain a sequence comprising a set of amplitude peaks which correspond to the response of the propagation channel in which the transmission propagates. signal. Each peak is identified by its temporal position and amplitude.
• the deconvolution step may, for example, be performed by a deconvolution algorithm known as the CLEAN algorithm or any other equivalent algorithm.
• the deconvolution algorithm can be coupled to an additional linear interpolation step in order to improve the accuracy of the temporal positioning of the peaks which is limited by the sampling resolution of the apparatus. measurement.
• the peak sequence obtained at the end of the deconvolution step 302 is intended to provide a representation of the impedance mismatch in the measurement zone. More precisely and as explained upstream of the present text, the equivalent impedance mismatch can be modeled by a succession of sections each having a constant characteristic impedance along the length of the section, two consecutive sections being separated by an interface corresponding to a break or an impedance discontinuity on which a part of the injected signal can be reflected.
• the sequence of peaks obtained at the output of step 302 thus gives a first approximation of the equivalent mismatch according to the aforementioned modeling.
• the amplitude A of each peak also consists, in whole or in part, of the contributions of secondary reflections or rebounds of the signal on the interfaces. Secondary rebound of the signal is used to designate the multiple reflections of the signal on an interface, that is to say reflections occurring after the first reflection.
• a second step 303 of the method is therefore applied to the sequence of peaks resulting from the deconvolution step 302 to eliminate, on each amplitude A ,, the contribution of the secondary rebounds of the signal.
• step 303 may consist of the following substeps. First, it is to identify the peaks located at a time position that potentially corresponds to an echo resulting from a multiple reflection of the signal on an interface. Then, for these identified peaks, the amplitude contribution of a multiple reflection is calculated, and then this contribution is subtracted from the amplitude of said identified peak. If the resulting amplitude after the subtraction is substantially zero, this peak is eliminated which corresponds entirely to a secondary reflection of the signal.
• step 303 of the method according to the invention can be carried out using the following algorithm.
• the first two peaks of the deconvolved sequence are selected. It is known that these first two peaks correspond to interfaces on which no multiple bounce of the signal has occurred. The temporal spacing between these two peaks is measured, and then, in the sequence, the peaks located at multiple temporal positions of the temporal distance between the first two peaks are searched. In other words, if we note the temporal distance between the first two selected peaks, we look for the peaks located at distances nd of the first peak, where n is a positive integer. Indeed, it is known that secondary reflections only appear at these positions.
• P j is the real part of the reflection coefficient on the interface j
• R nij is the amplitude of the nth rebound between interface i and j
• T k is the real part of the transmission coefficient on the interface k
• R n value ⁇ i ⁇ j in the amplitude of the n 'th rebound is then cut off the peak amplitudes of the detected temporal positions nd.
• the value R nj is subtracted from its amplitude.
• the result of the subtraction is substantially zero, the associated peak is eliminated from the sequence because it corresponds entirely to a multiple reflection of the signal.
• This test can be performed by comparing the compensated amplitude to a threshold below which the amplitude is considered zero.
• the R nj value of the amplitude of the n 'th rebound is subtracted to the samples of the sequence located at all temporal positions nd, even those for which no peak is detected.
• the parameter n equal to the number of reflections of the signal on an interface is an adjustable parameter of the process.
• the process described above may be repeated for all peaks of the sequence by changing each time the first two peaks selected for which the value R n ⁇ i ⁇ j in the amplitude of the n 'th rebound is calculated.
• the following pairs of peaks are selected: ⁇ 1; 2 ⁇ , ⁇ 1; 3 ⁇ , ⁇ 2; 3 ⁇ , ⁇ 1; 4 ⁇ , ⁇ 2; 4 ⁇ , ⁇ 3; 4 ⁇ and so on.
• the peak at the two peaks selected at the previous iteration is selected as the second peak and as the first peak, one of the peaks selected at one of the preceding iterations by varying the first peak on all peaks selected at previous iterations.
• a corrected sequence is obtained which contains only peaks whose amplitude is characteristic of a single reflection of the signal on the corresponding interface.
• the actual portions of the reflection coefficients can then be calculated from the amplitudes of each peak in accordance with relation (2).
• the temporal positions of the peaks give the real positions of the interfaces according to the modeling considered and according to a principle well known in the field of OTDR which makes it possible to convert time measurements on a reflectogram into distances.
• an additional step 306 is performed consisting in determining a piece of information characteristic of the degree of similarity between an estimate of the reflected signal reconstructed from the amplitude peak sequence obtained at the end of step 303 and the measurement 301.
• one possibility is to convolve the sequence composed of the amplitude peaks retained in step 303 with the reference signal initially injected into the cable.
• the reconstructed estimate is then compared with the measure 301, for example a point-to-point difference between the two signals is performed or any other calculation to represent the difference between the two signals.
• the signal reflected in the cable can be reconstructed for example using a known method of the ABCD method type as described, for example, in the article " System of Network Analysis for a Lossy Cable System, Q.Shi, O.Kanoun, 2012-9th International Multi-Conference on Systems, Signals and Devices »
• FIG. 4 illustrates an example of results obtained by applying the method according to the invention.
• FIG. 4 At the top of Figure 4 is shown a temporal reflectogram 401 obtained for the study of a two-wire cable connected to a network analyzer having a 50 ohms output by a 50-ohm coaxial cable of 25 cm and a "domino".
• the injected signal is a pulse width equal to 0.22 ns.
• the reconstruction 402 of the measurement 401 obtained by the method according to the invention.
• FIG. 4 On the bottom of Figure 4 is shown a diagram which illustrates the cutting into sections of lengths and characteristic impedances identified in the figure. We find well the detail of the connection with the first 28 centimeters corresponding to the coaxial cable 50 ohms then the details of the domino and the configuration change of the two-wire cable.
• the latter furthermore comprises a step of supplying a database with the positions and reflection coefficients and / or impedances characteristic of the calculated impedance discontinuities, each input of the database being associated with a connector or a set of connectors connected in series for interconnecting a measuring device with a cable.
• Such a database makes it possible to gather information on a large number of different connectors and to allow a follow-up of the evolution of the characteristics of these connectors in time but also a follow-up of their evolution according to certain physical parameters such as the temperature, humidity or any other parameter that could impact the characteristic impedance of equivalent mismatch.
• the method according to the invention can be implemented from hardware and / or software elements.
• the method according to the invention can be implemented directly by a processor embedded in the measurement equipment 103 or in a specific device.
• the processor may be a generic processor, a specific processor, an application-specific integrated circuit (also known as the ASIC for "Application-Specific Integrated Circuit") or a network of programmable gates in situ (also known as the English name of FPGA for "Field-Programmable Gate Array”).
• the device according to the invention can use one or more dedicated electronic circuits or a general purpose circuit.
• the technique of the invention can be realized on a reprogrammable calculation machine (a processor or a microcontroller for example) executing a program comprising a sequence of instructions, or on a dedicated calculation machine (for example a set of logic gates). as an FPGA or an ASIC, or any other hardware module).
• the invention can also be implemented exclusively as a computer program, the method then being applied to a previously acquired reflectometry measurement using a conventional reflectometry device.
• the invention can be implemented as a computer program including instructions for its execution.
• the computer program can be recorded on a processor-readable recording medium.
• the support can be electronic, magnetic, optical or electromagnetic.
• the reference to a computer program that, when executed, performs any of the functions described above, is not limited to an application program running on a single host computer.
• the terms computer program and software are used herein in a general sense to refer to any type of computer code (for example, application software, firmware, microcode, or any other form of computer code).
• computer instruction that can be used to program one or more processors to implement aspects of the techniques described herein.
• the means or computer resources can be distributed (“Cloud Computing"), possibly using peer-to-peer technologies.
• the software code may be executed on any suitable processor (for example, a microprocessor) or a processor core or set of processors, whether provided in a single computing device or distributed among a plurality of computing devices (eg example as possibly accessible in the environment of the device).
• the executable code of each program enabling the programmable device to implement the processes according to the invention can be stored, for example, in the hard disk or in read-only memory.
• the program or programs may be loaded into one of the storage means of the device before being executed.
• the central unit can control and direct the execution of instructions or portions of software code of the program or programs according to the invention, instructions that are stored in the hard disk or in the ROM or in the other storage elements mentioned above.
• the invention may also comprise a database fed by the data obtained by the execution of the method described in FIG.

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• 2015
  • 2015-03-27 FR FR1552628A patent/FR3034203B1/fr active Active
• 2016
  • 2016-03-25 EP EP16714816.2A patent/EP3274731A1/de active Pending
  • 2016-03-25 US US15/560,477 patent/US10598719B2/en active Active
  • 2016-03-25 WO PCT/EP2016/056692 patent/WO2016156259A1/fr active Application Filing

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