Temperature variation detection system using reflectometry

Abstract

A temperature increase detection system (300) for an environment consisting of several distinct zones (Z1-Z10), the system comprising a branched cable network with several branches (CS1-CS10), each intended to be deployed in one of the zones, and at least one reflectometry device (DR1) connected to one end of at least one branch of the network and configured to: Acquire an initial reflectometry measurement corresponding to an initial state of said branched cable network; Acquire a current reflectometry measurement corresponding to a current state of said branched cable network; Compare the current reflectometry measurement to the initial reflectometry measurement to infer a temperature increase on one of the branches of said network. Figure 3

Description

Title of the invention: System for detecting temperature variation by reflectometry

[0001] The invention relates to the field of detecting and locating temperature variations in a room in order to prevent the start of a fire. It also relates to methods and systems for analyzing the condition of a cable by reflectometry. The invention is particularly applicable to fire detection in environments such as buildings, means of transport (airplanes, trains), or nuclear power plants.

[0002] The invention aims to solve the general problem of detecting and locating an increase in temperature in a room in order to prevent a possible fire and raise an alarm.

[0003] European patent EP3227651, relating to a method and device for detecting hot spots in an installation, particularly for detecting leaks in air ducts, describes a method based on a reflectometry test applied to a point-to-point cable of a given length. This solution is described in the context of its application to an aircraft for detecting leaks in air ducts.

[0004] One drawback of this solution is its limited range, as it relies on the deployment of a single cable. It is well known that reflectometry methods are susceptible to signal attenuation problems with long cables. This problem is even more significant when the cable used is temperature-sensitive, as this type of cable is less suitable for transmitting reflectometry signals than some standard cables.

[0005] The publication “OMTDR-Based embedded cable diagnosis for multiple fire zones detection and location in aircraft engines,” Wafa Ben Hassen et al., 2017, IEEE Sensors, describes another solution based on an OMTDR (Orthogonal Multi-Tone Time Domain Reflectometry) multi-carrier signal for fire detection in an aircraft engine. This solution is also based on a standard point-to-point cable, which results in a lack of sensitivity and incompatibility for deployment in more complex infrastructures such as buildings.

[0006] The invention aims to provide a fire detection solution based on a reflectometry system that can be deployed in a complex environment composed of different independent zones such as a building, a nuclear power plant, a train composed of several wagons or a tunnel.

[0007] The invention relates to a system for detecting a temperature increase in an environment consisting of several distinct zones, the system comprising a branched cable network comprising several branches, each intended to be deployed in one of the zones, and at least one reflectometry device connected to one end of at least one branch of the network and configured to: - Acquire an initial reflectometry measurement corresponding to an initial state of said branched cable network, - Acquire a current reflectometry measurement corresponding to a current state of said branched cable network, - Compare the current reflectometry measurement to the initial reflectometry measurement to deduce a temperature increase on one of the branches of said network.

[0008] According to a particular aspect of the invention, at least one branch of the branched cable network is a heat-sensitive cable whose insulation melts when the temperature exceeds a given threshold so as to create a short circuit.

[0009] According to a particular aspect of the invention, at least one branch of the branched cable network is a temperature-sensitive cable in which at least one characteristic parameter among permittivity or conductivity varies with temperature.

[0010] According to a particular aspect of the invention, at least one reflectometry device is configured to detect an increase in temperature in the environment when the difference between the current reflectometry measurement and the initial reflectometry measurement is greater, in absolute value, than a predefined threshold.

[0011] According to a particular aspect of the invention, at least one reflectometry device is configured to acquire several successive current reflectometry measurements, identify, in the successive measurements, the same amplitude peak whose value exceeds a first predefined threshold, calculate the rate of change of the amplitude, the time position of said peak or the shape of said peak during the successive measurements and detect an increase in temperature in the environment when the rate of change exceeds a second predefined threshold.

[0012] According to a particular aspect of the invention, at least one reflectometry device is configured to: - When a temperature increase is detected, reconstruct the cable network topology from the initial reflectometry measurement and the current reflectometry measurement. - Identify the branch of the cable network for which a fault corresponding to a temperature increase has been detected,

[0013] According to a particular aspect of the invention, at least one reflectometry device is further configured to: - Detect a temperature increase in the area where the identified branch is located.

[0014] In one embodiment, the system according to the invention comprises several reflectometry devices respectively connected to different ends of the cable network and cooperating together to perform a distributed reflectometry test so as to detect and locate a fault corresponding to a temperature increase.

[0015] According to a particular aspect of the invention, the branched cable network comprises a main cable, one end of which is connected to the reflectometry device, and a plurality of secondary cables connected to the main cable.

[0016] According to a particular aspect of the invention, the deployment environment of the system is a building or a means of transport.

[0017] The invention also relates to a method for detecting a temperature increase in an environment consisting of several distinct zones, the method comprising the steps of: - Deploy a branched cable network throughout the environment so that at least one network cable is present in each area, - Connect at least one reflectometry device to one end of the network, - Acquire an initial reference reflectometry measurement corresponding to an initial state of said branched cable network in a healthy state, - Acquire a current reflectometry measurement corresponding to a current state of said branched cable network, - Compare the current reflectometry measurement to the initial reflectometry measurement to deduce a temperature increase on one of the branches of said network.

[0018] In one embodiment, the method according to the invention comprises: - several successive stages of acquiring a reflectometry measurement, - the identification, in the successive measurements, of at least one identical amplitude peak whose value exceeds a first predefined threshold, - the calculation of the rate of change of the amplitude, the temporal position of said peak or the shape of said peak during successive measurements and - the detection of an increase in temperature in the environment when the rate of change exceeds a second predefined threshold.

[0019] Other features and advantages of the present invention will become more apparent from the following description in relation to the following accompanying drawings.

[0020] [Fig. 1] represents a diagram illustrating the principle of reflectometry applied to the fault detection on a cable according to the prior art,

[0021] [Fig.2] represents a diagram illustrating the principle of reflectometry applied to the detection of a temperature rise in a zone of a cable,

[0022] [Fig.3] represents a diagram illustrating a system for detecting an increase of the temperature according to one embodiment of the invention,

[0023] [Fig.4] represents a flowchart detailing the steps for implementing a method of detecting a temperature rise performed by the system of [Fig.3],

[0024] [Fig. 5] represents an example of a heating detection system according to the invention comprising a network of three cables according to a first heating scenario,

[0025] [Fig.6] represents a reflectogram measured for the grating of [Fig.5] in a healthy state and in a state corresponding to the first warm-up scenario illustrated in [Fig.5],

[0026] [Fig.7] represents the difference between the two reflectograms of [Fig.6],

[0027] [Fig.8] represents the system of [Fig.5] in a second scenario warm-up,

[0028] [Fig.9] represents a differential measurement between the reflectogram measured for the network of [Fig.5] in a healthy state and in a state corresponding to the second heating scenario illustrated in [Fig.8],

[0029] [Fig. 10] represents the system of [Fig. 5] in a third scenario warm-up,

[0030] [Fig. 11] represents a differential measurement between the reflectogram measured for the network of [Fig. 5] in a healthy state and in a state corresponding to the second heating scenario illustrated in [Fig. 10],

[0031] [Fig.12] represents a comparative example of a heating detection system according to the prior art in the same environment as the system shown in [Fig. 5],

[0032] [Fig. 1] schematically illustrates the principle of reflectometry applied to state analysis of a cable, according to the prior art. The principle consists of injecting a signal with a controlled waveform at an injection point INJ in the cable. The incident wave OI propagates along the cable until it encounters an impedance discontinuity AZ caused by an electrical fault. Part of the wave is transmitted OT to the end of the cable FC, and part of the wave is reflected back to the injection point INJ. Using measuring equipment, the reflected signal is measured. Analysis of the measured signal allows the fault to be detected and located.

[0033] A correlator performs the cross-correlation between the generated signal and the received signal in order to produce a time-domain reflectogram R(t). If the reflectometry signal used is a simple time pulse, the COR correlator can be made optional.

[0034] The time reflectogram R(t) has a characteristic amplitude peak of the DF fault at a time abscissa which is related to the distance between the signal measurement point and the DF fault and to the speed of propagation of the signal in the cable.

[0035] As is known in the field of reflectometry diagnostic methods, the position dDNF of the non-obvious fault on the cable, in other words its distance to the point of signal injection, can be directly obtained from the measurement, on the time reflectogram, of the duration tDNF between the first amplitude peak recorded on the reflectogram and the amplitude peak corresponding to the signature of the fault.

[0036] Several known methods can be used to determine the position dDNF. One method involves applying the relationship between distance and time: dDNF = V.tDNF / 2, where V is the signal propagation speed in the cable. Another possible method involves applying a proportionality relationship of the type dDNF / tDNF = L / t0, where L is the cable length and t0 is the time, measured on the reflectogram, between the amplitude peak corresponding to the impedance discontinuity at the injection point and the amplitude peak corresponding to the signal reflection at the cable end.

[0037] Fig. 2 illustrates the principle of applying a reflectometry diagnostic to detect and locate a temperature variation on a cable.

[0038] The heated zone CH comprises an input interface h and an output interface I2, both of which correspond to impedance discontinuities at which the incident wave OI can be reflected. The heated zone CH of the cable has a length Ld. The cable, with characteristic impedance Zcl, is thus divided into three zones as illustrated in [Fig. 2]. The two unheated zones have a characteristic impedance Zcl equal to that of the unheated cable. The heated zone has a characteristic impedance Zc2.

[0039] The temperature increase in the heated zone CH will therefore modify certain physical parameters of the cable, such as the relative permittivity or conductivity, and will vary the characteristic impedance in this zone. Each interface of the heated zone CH introduces two reflection coefficients in opposite phase: Zc1 - Zc2 and Zcj + Zcj.

[0040] The detection capability and localization accuracy of the heated area CH depends on the time width or bandwidth of the signal injected into the cable as well as the electrical characteristics of the cable such as resistance and temperature rise.

[0041] Figure 3 represents a diagram of a temperature increase detection device 300 according to an embodiment of the invention.

[0042] The device 300 comprises at least one DRi reflectometry device including at least one signal generator, one digital-to-analog converter, one coupler for injecting the analog signal into a cable network, and one measuring device for measuring a reflected signal and digitizing it via an analog-to-digital converter. The DRi reflectometry device further includes a processing unit for calculating and analyzing a reflectogram obtained from a measurement of the reflected signal. The calculation of the reflectogram may involve a correlation calculation between the measured signal and the signal injected into the cable, particularly when the signal used differs by more than a simple time pulse.

[0043] The DRi reflectometry device is connected to a branching cable network comprising a main CP cable and several secondary CSi-CSio cables, each connected to the main CP cable. Each secondary cable is deployed in a respective zone ZrZ10 of a building or means of transport. Each secondary cable may be a standard cable or a temperature-sensitive cable.

[0044] According to a first embodiment, a heat-sensitive cable consists of an insulator whose material melts when the temperature exceeds a given threshold, thereby creating a short circuit. According to a second embodiment, a heat-sensitive cable has at least one characteristic parameter, such as permittivity or conductivity, that varies with temperature.

[0045] The topology of the cable network is not limited to that described in [Fig. 3] and is more generally chosen according to the building to be instrumented. In particular, the number of secondary cables is equal to the number of zones in the building to be instrumented.

[0046] In one embodiment, several DRB reflectometry devices DR2, DR3 are connected to different ends of the cable network and cooperate together to implement a distributed reflectometry test as described in Applicant's patent applications FR3012616 or FR3012617

[0047] By way of illustration, [Fig. 3] shows a point-to-point line L connected to the DRI reflectometry device and deployed in the environment to be covered so as to cross all areas of the environment. The line L necessarily has a much greater length than each secondary cable.

[0048] However, as explained in the preamble, reflectometry devices have a lower sensitivity for long cables due to signal attenuation during their propagation along the cable.

[0049] The invention therefore has the advantage of better detection sensitivity by using a network of branched cables.

[0050] Fig. 4 details the steps performed by the system 300 to detect and locate a temperature increase in one of the ZrZi0 zones in which the cable network is deployed.

[0051] In step 401, an initial reflectometry measurement is performed on the cable network in a healthy state.

[0052] In step 402, a reflectometry measurement is then carried out at a later time.

[0053] In step 403, a comparison is made between the two measurements to detect a characteristic change of an increase in heat in one of the ZrZio zones.

[0054] If a temperature increase is detected in step 404, a reconstruction of the cable network topology is determined in step 405 and then in step 406, the area impacted by the heat increase is located using the reconstructed topology.

[0055] Step 405 can be performed using a topology reconstruction algorithm that takes as input the initial reflectometry measurement and / or the current reflectometry measurement. The algorithm used is, for example, that described in one of the patent applications FR3070075, FR3070211, FR3082947.

[0056] Figures 5 to 12 illustrate an implementation of the invention for a simple example of a cable network composed of three branches L1, L2, L3 arranged in three zones Z1, Z2, Z3 of a given environment. In this example, the two cables L2, L3 are terminated by an open circuit, but the load at the end of the cable can be of any value.

[0057] A TDR reflectometry device is connected to one end of the first LL cable. The signal injected into the cable network is, for example, a Gaussian-shaped time pulse, but other more complex signals can be considered.

[0058] In the example of [Fig. 5], an increase in heat (start of fire) occurs in the ZL zone

[0059] Figure 6 shows a first reflectogram 601 measured for the cable network of Figure 5 in a healthy state. This reflectogram corresponds to the reference measured in step 401 of the method. The reflectogram 601 has a first peak corresponding to the injection signal, a second negative peak corresponding to the reflection of the signal at the junction of the network between the three branches and which is located at a time of flight corresponding to the length L1 of the first cable, a third positive peak corresponding to the reflection of the signal at the end of the cable L3 and a fourth positive peak corresponding to the reflection of the signal at the end of the cable L2.

[0060] The second reflectogram 602 is that measured at step 402 after the fire started in the ZL zone

[0061] On this second reflectogram, a negative peak followed by a positive peak is observed in a zone 603 of the reflectogram corresponding to the part of the cable L1 which undergoes the temperature increase in the zone Z1. The spacing between the two peaks in zone 603 depends on the width of the heated area of ​​the cable.

[0062] Figure 7 shows the difference between the two reflectograms 601 and 602 as calculated in step 403 of the method. This difference clearly identifies the double peak 700 corresponding to the heated area of ​​the LL cable.

[0063] Thus, a first possible test to carry out step 404 of detecting an increase in temperature consists of looking for the presence of a double peak composed of two peaks of opposite signs in the difference 700 calculated in step 403. If the amplitude of one or both of the peaks exceeds a predefined threshold, we can conclude that there is heating of the cable.

[0064] In one embodiment, a more elaborate detection test consists of performing several successive reflectometry measurements over time and evaluating the rate of change of the amplitude of the characteristic peak of the heated zone. Indeed, an increase in temperature leads to a change in the characteristics related to the impedance discontinuity caused by heating, and therefore a change over time in one of the characteristics of the peaks (at the input and output of the defect), such as the amplitude, the waveform, and the time position corresponding to this discontinuity. One possible test consists of measuring the rate of change of the peak amplitude, equal to the peak amplitude divided by the time of measurement, comparing this rate to a predefined threshold, and concluding that heating has occurred if the rate exceeds the threshold.

[0065] A similar line of reasoning can be applied to the evolution over time of the peak waveform and the temporal position of the peak. In particular, the peak characteristic of a temperature increase may exhibit a double-peak shape. It may also be distorted and may generally evolve according to the thermal characteristics of the cable in response to thermal stress.

[0066] In other words, another possible test consists of measuring the rate of change of the position of the peak equal to the time abscissa of the peak in the reflectogram divided by the time of measurement and comparing this rate to the predefined threshold.

[0067] More generally, the shape of the signal can be compared between several successive measurements. For example, the signal is taken within a time window around the maximum value of the peak, and the evolution of the signal shape within this time window can be evaluated over time. For example, the root mean square error is calculated between two signals corresponding to two successive measurements, and the rate of change of this error is calculated by dividing this error by the time difference between the times of the two measurements.

[0068] Step 405 allows the network topology to be reconstructed from the initial reflectogram 601 and thus a correspondence to be made between each portion of the reflectogram and the associated cable of the cable network.

[0069] Once this correspondence is established, it is possible to conclude that the double peak 603 characteristic of a temperature increase is located on the cable L1 in the Zl area.

[0070] More generally, the search for a double peak in the reflectogram is replaced by the search for a signature characteristic of a heating zone, this signature being dependent on the waveform of the signal injected into the cable, in particular on the width of the pulse (when the signal is impulsive) but also on the size of the heating zone and its intensity.

[0071] Figure 8 illustrates another scenario in which warming is located in the area Z2.

[0072] Figure 9 shows the differential measurement obtained by calculating the difference between a reflectogram measured for the state of the network represented in [Fig.8] and the initial reflectogram 601.

[0073] On this differential measurement, we observe a double peak 900 located in the area of ​​the reflectogram corresponding to cable L2 and therefore to a heating located in the area Z2.

[0074] Figure 10 illustrates yet another scenario in which warming is located in zone Z3.

[0075] Figure 11 shows the differential measurement obtained by calculating the difference between a reflectogram measured for the state of the network shown in [Fig. 10] and the initial reflectogram 601.

[0076] On this differential measurement, we observe a double peak 1000 located in the area of ​​the reflectogram corresponding to cable L3 and therefore to a heating located in the area Z3.

[0077] The invention has the advantage of better detection sensitivity because the path traveled by the reflectometry signal from the injection point to the different inspection zones is minimized due to the branched network topology.

[0078] By way of comparison, [Fig.12] illustrates an example of a heating detection system based on a simple 1200 point-to-point cable. To cover the three zones Z1, Z2, Z3 to be monitored, the total cable length required in this example is 48 m, which results in significant signal attenuation during its journey from the TDR injection equipment to the end of the cable E.

[0079] Conversely, by using a branched cable network according to the invention as shown in [Fig. 5], the total distance traveled by the signal to the ends of the network is equal to the maximum between L1+L2 and L1+L3, which allows for better sensitivity of detecting the characteristic signature of a heating zone in the reflectogram.

[0080] The device according to the invention has the advantage of allowing flexibility with respect to the environment and the areas to be monitored, particularly their criticality. In particular, the cable network can be heterogeneous and include standard, coaxial, twisted-pair, multi-conductor, or even heat-sensitive cables. The cables exhibiting the highest sensitivity, that is, those for which the amplitude of the signature on the reflectogram of the impedance mismatch associated with the heating zone is the most pronounced, are positioned in the most critical areas. A critical area is understood, for example, to be an area of ​​a sensitive building where the start of a fire must be detected with greater reliability than in other areas.

[0081] In particular, heat-sensitive cables exhibit greater sensitivity, especially cables whose insulation melts to create a short circuit that will be detected more reliably than a low-intensity temperature increase for a standard cable.

[0082] The DRi reflectometry device can be a simple vector network analyzer or an electronic card or more generally any device implemented by means of software and / or hardware elements.

[0083] The invention also has the advantage of allowing the use of existing cable infrastructure in the environment to be covered. For example, in some buildings, heat-sensitive cables or communication or power cables are already deployed on site and can be used alone or in conjunction with other cables to instrument the entire building using the device according to the invention.

[0084] When a temperature increase is detected in an area, the DRi reflectometry device may include a visual and / or audible alert system. It may include a display interface to indicate the area in which the temperature increase was detected.

Claims

Demands

1. A system for detecting (300) an increase in temperature in an environment consisting of several distinct zones (Z1-Z10), the system comprising a branched cable network comprising several branches (CSi-CSio), each intended to be deployed in one of the zones, at least one branch of the branched cable network being a temperature-sensitive cable whose permittivity varies with temperature, the system further comprising at least one reflectometry device (DRi) connected to one end of at least one branch of the network and configured to: - Acquire (401) an initial reflectometry measurement corresponding to an initial state of said branched cable network, - Acquire (402) a current reflectometry measurement corresponding to a current state of said branched cable network,- Compare (403) the current reflectometry measurement to the initial reflectometry measurement in order to deduce (404) a temperature increase on one of the branches of said network.

2. A temperature rise detection system according to claim 1 in which at least one branch of the branched cable network is a heat-sensitive cable whose insulation melts when the temperature exceeds a given threshold so as to create a short circuit.

3. A temperature rise detection system according to any one of the preceding claims wherein at least one reflectometry device (DRi) is configured to detect (404) a temperature rise in the environment when the difference between the current reflectometry measurement and the initial reflectometry measurement is greater, in absolute value, than a predefined threshold.

4. A temperature rise detection system according to any one of claims 1 to 3, wherein at least one reflectometry device (DRi) is configured to acquire (402) several successive current reflectometry measurements, and to identify, in the successive measurements, the same amplitude peak whose value exceeds a first predefined threshold, calculate the rate of change of the amplitude, time position of said peak or shape of said peak during successive measurements and detect (404) an increase in temperature in the environment when the rate of change exceeds a second predefined threshold.

5. A temperature rise detection system according to any one of the preceding claims, wherein at least one reflectometry device (DRi) is configured to: - When a temperature rise is detected, reconstruct (405) the cable network topology from the initial reflectometry measurement and the current reflectometry measurement, - Identify (406) the cable network branch for which a fault corresponding to a temperature rise has been detected,

6. A temperature rise detection system according to claim 5 wherein at least one reflectometry device (DRi) is further configured to: - Detect (406) a temperature rise in the area in which the identified branch is positioned.

7. A temperature rise detection system according to any one of the preceding claims comprising several reflectometry devices (DRi, DR2, DR3) respectively connected to different ends of the cable network and cooperating together to perform a distributed reflectometry test so as to detect and locate a fault corresponding to a temperature rise.

8. A temperature rise detection system according to any one of the preceding claims, wherein the branched cable network comprises a main cable (CP) one end of which is connected to the reflectometry device and a plurality of secondary cables (CSi-CSio) connected to the main cable.

9. A temperature increase detection system according to any one of the preceding claims wherein the deployment environment of the system is a building or a means of transport.

10. A method for detecting a temperature increase in an environment consisting of several distinct zones, the method comprising the steps of: - Deploy a branched cable network in the environment such that at least one cable from the network is laid out in each zone, with at least one branch of the branched cable network being a temperature-sensitive cable whose permittivity varies with temperature, - Connect at least one reflectometry device to one end of the network, - Acquire (401) an initial reference reflectometry measurement corresponding to an initial state of said branched cable network in a healthy state, - Acquire (402) a current reflectometry measurement corresponding to a current state of said branched cable network, - Compare (403) the current reflectometry measurement to the initial reflectometry measurement to deduce (404) a temperature increase on one of the branches of said network.

11. A method for detecting a temperature increase in an environment according to claim 10, comprising: - several successive acquisition steps (402) of a reflectometry measurement, - the identification, in successive measurements, of at least one identical peak in amplitude whose value exceeds a first predefined threshold, - the calculation of the rate of change of the amplitude, the temporal position of said peak or the shape of said peak during successive measurements and - the detection (404) of an increase in temperature in the environment when the rate of change exceeds a second predefined threshold.

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