Device for tomography measurement using a ratiometric electrical process for a solid substrate

Abstract

A device for tomography measurement using a ratiometric electrical process for a solid substrate, comprising sensors (18, 18r), the inputs of which are connected to each other, so that a tomography measurement using a ratiometric electrical process is performed at the output.

Description

[0001] Description

[0002] Title: Ratiometric electrical process tomography measuring device for solid substrate

[0003] The invention relates to the field of monitoring solid substrates, and more generally to monitoring the integrity of structures.

[0004] To meet the needs of industry, the quality and robustness of the mechanical elements of a system are evaluated and characterized by placing the mechanical parts under stress and measuring the behavior of the part with respect to its structural integrity. Stressing can be done in real conditions, while the part is integrated into the final system, or via one or more testing machines, which will exert different successive forces at different points of the part.

[0005] The mechanical behavior of parts, particularly in terms of extensometry, is measured by a dedicated system comprising one or more sensors. Although several technologies exist, the structural monitoring of mechanical elements is still carried out by strain gauges. These have the advantage of being highly sensitive and extremely precise. Their implementation and use are now well-established. Measuring systems dedicated to gauges exist, and different assemblies, mainly gauge bridges, quarter, double or full bridges, allow compensation for the parasitic effects of temperature. Gauges can come in different forms, either single or rosettes integrating several gauges.

[0006] Strain gauges have at least two major drawbacks: their directivity and the small active area measured. Indeed, only the extension in the direction of the gauge is measured, and only under the surface covered by the gauge. This means that when the stress test aims to measure all the deformations along several axes of the surface, or worse, when the aim is to measure a deformation orthogonal to the surface of the part, it is necessary to integrate a considerable number of gauges and gauge configurations on all the surfaces of the mechanical part. This multiplication of gauges results in a large number of connections and an excessively long implementation time.

[0007] The Applicant is developing alternatives using a disruptive technology in the field of solid substrates: electrical impedance tomography (EIT). Indeed, its work has enabled this method to be extended beyond its medical applications and to soft or liquid substrates, to be used in monitoring structural integrity.

[0008] In application FR2114482, the Applicant demonstrated the applicability of electrical process tomography measurements to solid substrates, in the non-limiting list of the following applications:

[0009] - human-machine interface (HMI) intended to stimulate the physical interface and to measure in return the correlated signals resulting from the stimulation of the physical interface in order to measure any variation in local impedance inside the interface, and comprising a source of continuous or variable electrical stimulation and an analog-digital converter,

[0010] - HMI based on electrical impedance tomography and aimed at stimulating the physical interface and measuring the correlated signals resulting from the stimulation of the physical interface in order to measure any local impedance variation within the interface, and comprising a continuous or variable electrical stimulation source and an analog-to-digital converter,

[0011] - HMI based on electrical resistance tomography and aimed at stimulating the physical interface and measuring the correlated signals in return resulting from the stimulation of the physical interface in order to measure any variation of local resistance in the interface, and comprising a source of continuous or variable electrical stimulation and an analog-to-digital converter,

[0012] - HMI based on electrical capacitance tomography and aimed at stimulating the physical interface and measuring the correlated signals in return resulting from the stimulation of the physical interface in order to measure any variation of local capacitance within the interface, and comprising a variable electrical stimulation source and an analog-to-digital converter, - structural health monitoring (SHM) system aimed at stimulating a physical specimen and measuring in return the correlated signals resulting from the stimulation of the physical component in order to measure any variation of local impedance within the sample, and comprising a continuous or variable electrical stimulation source and an analog-to-digital converter,

[0013] - SHM system based on electrical impedance tomography and aimed at stimulating a physical specimen and measuring in return the correlated signals resulting from the stimulation of the physical component in order to measure any local impedance variation inside the specimen, and comprising a continuous or variable electrical stimulation source and an analog-to-digital converter,

[0014] - SHM system based on electrical resistance tomography and aimed at stimulating a physical specimen and measuring in return the correlated signals resulting from the stimulation of the physical element in order to measure any variation in local resistance inside the specimen, and comprising a source of continuous or variable electrical stimulation and an analog-to-digital converter,

[0015] - SHM system based on electrical capacitance tomography and aimed at stimulating a physical specimen and measuring in return the correlated signals resulting from the stimulation of the physical element in order to measure any variation in local capacitance inside the specimen, and comprising a variable electrical stimulation source and an analog-to-digital converter,

[0016] - non-destructive testing (NDT) system intended to stimulate a physical specimen and to measure in return the correlated signals resulting from the stimulation of the physical component in order to measure any variation in local impedance within the sample, and comprising a continuous or variable electrical stimulation source and an analog-to-digital converter,

[0017] - NDT system based on electrical impedance tomography and aimed at stimulating a physical specimen and measuring in return the correlated signals resulting from the stimulation of the physical component in order to measure any variation in local impedance inside the specimen, and comprising a continuous or variable electrical stimulation source and an analog-to-digital converter,

[0018] - NDT system based on electrical resistance tomography and aimed at stimulating a physical specimen and measuring in return the correlated signals resulting from the stimulation of the physical element in order to measure any variation of local resistance inside the specimen, and comprising a source of continuous or variable electrical stimulation and an analog-to-digital converter, and

[0019] - NDT system based on electrical capacitance tomography and aimed at stimulating a physical specimen and measuring in return the correlated signals resulting from the stimulation of the physical element in order to measure any variation in local capacitance inside the specimen, and comprising a variable electrical stimulation source and an analog-to-digital converter.

[0020] Electrical impedance tomography reduces the number of connections required for monitoring. Indeed, measurements made on a limited number of electrodes placed on the periphery of an active capture surface and algorithms for reconstructing the structural characteristics of the capture surface allow EIT-based processes to monitor a complete surface, much larger than that captured by a gauge. They also potentially allow the determination of extension and deformation components along all axes of space by visualizing localized constrained zones.

[0021] On the other hand, EIT-based processes do not inherently allow for the assembly of one or more active surfaces in a bridge, as is the case with gauges. Measurements may therefore be sensitive to certain variations, such as temperature. On the other hand, EIT will be more accurate if the measurements are precise and free from noise and variations external to those of the active surface.

[0022] Existing solutions, such as those in application FR2114482, allow quality measurements to be carried out on an EIT-based sensor, but not on several sensors simultaneously. There is currently no solution for pooling such sensors.

[0023] The invention improves the situation. To this end, it proposes a device for measuring tomography by electrical process for a solid substrate, comprising a current or voltage source, a reference sensor, at least one other sensor, and at least one comparator arranged to emit a signal from the reference sensor or the at least one other sensor, each at least one other sensor comprising a current input, a current output, and a plurality of electrodes suitable for being connected to a solid substrate to be measured, and two inputs for measuring the voltage difference across said measurement inputs, each at least one other sensor being suitable for carrying out a sequence of measurements in which the current input and the current output are each connected to a respective electrode of a pair of excitation electrodes of the plurality of electrodes,and the two measurement inputs are each connected to a respective electrode of a pair of measurement electrodes of the plurality of electrodes, the pair of excitation electrodes and the pair of measurement electrodes being defined by the sequence of measurements at each measurement and the pair of measurement electrodes being connected to the at least one comparator for measuring the propagation in the solid substrate of the current introduced via the pair of excitation electrodes. The current source is connected to the reference sensor,which comprises a current output connected to the current input of the at least one other sensor and is connected to the at least one comparator to emit a signal corresponding to a voltage difference as a function of the measurement sequence. The device is further arranged to determine the ratio between the voltage difference across the measurement inputs of each at least one other sensor for each measurement sequence and the voltage difference across the measurement inputs of the reference sensor.,

[0024] This device is particularly advantageous because it allows the implementation of architectures based on the pooling of stimulation of active surfaces and on the principle of ratiometric measurement between multiple surfaces, for structural monitoring systems based on comparative multi-element electrical tomography.

[0025] According to various embodiments, the invention may have one or more of the following characteristics:

[0026] - each sensor comprises a multiplexer connected on the one hand to its current input and to its current output and on the other hand to its plurality of electrodes,

[0027] - each sensor comprises two multiplexers connected on the one hand to its plurality of electrodes, and on the other hand to the two measurement inputs of the at least one comparator, - each at least one other sensor comprises a comparator connected to the two measurement inputs,

[0028] - the reference sensor comprises a current input, a current output, a plurality of electrodes suitable for being connected to a solid substrate to be measured and a comparator comprising two measurement inputs arranged to emit a signal corresponding to the voltage difference across said measurement inputs, the reference sensor being suitable for carrying out a sequence of measurements within which the current input and the current output are each connected to a respective electrode of a pair of excitation electrodes of the plurality of electrodes, and the two measurement inputs of the comparator are each connected to a respective electrode of a pair of measurement electrodes of the plurality of electrodes,the pair of excitation electrodes and the pair of measurement electrodes being defined by the sequence of measurements at each measurement and the propagation in the solid substrate to be measured of the current introduced via the pair of excitation electrodes being measured at the terminals of the pair of measurement electrodes,

[0029] - the comparator of the reference sensor is a differential amplifier, and in which the comparator of the sensors is an analog-digital converter further comprising a reference input connected to the output of said differential amplifier and arranged to emit a digital signal corresponding to the difference between the voltage of the two measurement inputs proportionally to a voltage level designated by the reference input,

[0030] - wherein each comparator is an analog-to-digital converter further comprising a reference input arranged to output a digital signal corresponding to the difference between the voltage of the two measurement inputs proportionally to a voltage level designated by the reference input, the device further comprising a divider arranged to calculate the ratio between the digital signal of each at least one other sensor and the digital signal of the reference sensor,

[0031] - the reference sensor is a witness sensor, and

[0032] - the reference sensor is a calibration specimen reproducing the voltage differences expected for a solid substrate to be measured.

[0033] The invention also relates to a solid substrate, characterized in that it comprises a device as described above, added to at least one of the surfaces or integrated. Other characteristics and advantages of the invention will appear better on reading the description which follows, taken from examples given for illustrative and non-limiting purposes, taken from the drawings in which:

[0034] - figure 1 represents a diagram of a first embodiment of a device according to the invention,

[0035] - figure 2 represents a diagram of a second embodiment of a device according to the invention,

[0036] - figure 3 represents a diagram of a third embodiment of a device according to the invention,

[0037] - figure 4 represents a particular implementation of the embodiment of figure 3, and

[0038] - Figure 5 represents a diagram of a fourth embodiment of a device according to the invention.

[0039] The drawings and the description below contain, for the most part, elements of a certain character. They may therefore not only serve to better understand the present invention, but also contribute to its definition, if necessary.

[0040] The EIT sensor architectures as described in application FR2114482 have the following general structure: a current or voltage source supplies a first multiplexer which is connected to a plurality of electrodes. This assembly makes it possible to carry out an excitation sequence by which a current is introduced successively between pairs of electrodes, neighboring or distant, according to a chosen excitation pattern. These excitations are measured by pairs of electrodes via a second multiplexer which connects the electrodes to the input of an analog-digital converter which makes it possible to recover the measurement of the voltage which has propagated in the substrate.

[0041] All the measurements can then be processed to reconstruct the electric field of a conductive part of the substrate and to extract information on its state or evolution. When we want to use several EIT sensors to qualify a structure, it is possible to create an architecture where the sensor measurements are carried out in series, or in parallel.

[0042] In the case of serial architecture, the stimulation source and the acquisition set are common to all measurements made on all specimens. The switching of these on the different specimens is done through additional multiplexers upstream and downstream of the multiplexers specific to each specimen. In this case, each measurement is made at a given time and no other is made at the same time. All measurements are thus decorrelated. Noise and variations, particularly due to the stimulation source, cannot be easily filtered. These fluctuations will be detrimental to the performance of the reconstruction algorithms.

[0043] In the case of parallel architecture, a plurality of current sources and acquisition sets are implemented. This system is faster but more expensive. Similarly, even if the parallel measurements are carried out at the same time on the different specimens, the different current sources and acquisition sets are not correlated, nor will their respective noise and variations be.

[0044] The system in Figure 1 shows a device 2 that allows measurement speeds of the parallel architecture type to be obtained with a cost structure approaching that of the serial architecture. In addition, it offers much better performance in terms of accuracy and noise than the two previous architectures thanks to the pooling of stimulation for all the sensors. The effects of noise and variations in stimulation are then correlated across all the measurements and therefore more easily filtered.

[0045] For this, the device of Figure 1 comprises a plurality of specimens 4 on each of which is arranged a plurality of electrodes 6. The device 2 also comprises pairs of multiplexers 8 and 10 associated with each plurality of electrodes 6 in order to carry out the current injection. The multiplexer 8 is used more particularly for the injection of the current, while the multiplexer 10 is used to recover the injected current. Thus, the multiplexer 8 can be seen as a current input and the multiplexer 10 as a current output.

[0046] The device 2 also comprises pairs of multiplexers 12 and 14 associated with each plurality of electrodes 6 in order to carry out the measurement. Each pair of multiplexers 12 and 14 is connected to respective measurement inputs of a comparator 16 which carries out the voltage measurement for the EIT for the specimen 4 concerned. The multiplexers 12 and 14 can be seen as the measurement inputs of the comparators 16.

[0047] The plurality of electrodes 6, the multiplexers 8, 10, 12, 14 and the comparator 16 together form a respective sensor 18 for each specimen 4.

[0048] The device further comprises a current source 20 which is connected to the multiplexers 12 and 14 of one of the sensors 18. This sensor will subsequently be called the reference sensor and referenced 18r in the following because it serves as the basis for the ratiometric measurement. In the rest of the device 2, the multiplexer 10 is connected to the multiplexer 8 of another sensor 18, except the multiplexer 10 of the last sensor 18 which is connected to ground.

[0049] The measurements in this EIT architecture are all dependent on the same source, and this at each instant. Device 2 therefore makes it possible to overcome the noise and relative variations of the sources of the conventional parallel multi-element structure. The connection of the specimens to each other is relatively simple thanks to the connection of the input of specimen #n to the stimulation output of specimen #nl.

[0050] In the case of voltage stimulation, the multiplexer inputs must be interconnected in parallel. Thus, if a variation or noise appears on the stimulation source, all the measurements will be affected by this variation. Filtering this variation in the data will then be facilitated downstream.

[0051] In the embodiment of Figure 1, all comparators 16 are analog-to-digital converters, the outputs of which are connected to a divider 22 in such a way that the latter divides each sensor output 18 by the output of the reference sensor 18r. Thus, the device 2 performs a ratiometric measurement of all sensors 18 with respect to the reference sensor 18r. The divider 22 can be implemented in any way that allows a ratio to be achieved between two digital inputs.

[0052] The information measured by the device 2 is therefore not found in the absolute measurements for each specimen 4 but in the ratio between the measurements obtained on each specimen 4 compared to the specimen of the reference sensor 18r. This is why we speak of ratiometric measurement.

[0053] Considering for example a noisy stimulation current such as l st t m = l re f + I e, then the equivalent measurements on specimen #1 and #2 will be respectively V1= a i a2l es conversion ratios between the measured values ​​and the stimulation current. The corresponding ratiometric measurement with, for example, specimen #1 as reference, will then be — = — . Therefore, the noise from the source disappears from the equation. The same would apply to absolute variations in the current source, to the noise captured across all specimens, or to the effects of temperature on the specimens.

[0054] Figure 2 shows an alternative embodiment in which the ratiometric measurement is carried out without a divider. Thus, the comparator 16 of the reference sensor 18r is here a differential amplifier 24 whose output is an analog signal which is a multiple of the voltage difference of the measurement inputs. This analog signal is used as a reference voltage transmitted to the reference voltage input r of all the comparators 16 of the other sensors 18 which are analog-to-digital converters. Thus, the output of each comparator 16 is inherently a ratiometric measurement between each sensor 18 and the reference sensor 18r.

[0055] The device of Figures 1 and 2 can be used to replace strain gauges in SHM. As mentioned above, strain gauges are often mounted in pairs or larger combinations of gauges, in order to compensate for potential variabilities external to the measurement such as temperature. These arrangements can be reproduced by using capture surfaces (patch) measured using EIT measurement sensors. These capture surfaces are then glued to the structure to be analyzed and coupled together via device 2, whose ratiometric measurement makes it possible to obtain a measurement independent of external variabilities. This makes it possible to optimize the ratiometric measurement by removing variabilities due to stimulation from the measurements.

[0056] Figure 3 shows yet another alternative embodiment of Figure 2. In this embodiment, there are only two sensors in all, and the reference sensor 18r is a witness or standard 26. This witness may be a model of the desired measurement or even a "phantom" (see for example the article by H Griffiths, (1988), “A phantom for electrical impedance tomography”, Clin Phys Physiol Meas., 9 Suppl A: 15-20), i.e. an electronic circuit arranged to react as a witness specimen.

[0057] The 18r sensor has been previously characterized, potentially by other more accurate but also more expensive and / or slower test means. Once made available, the data from this specimen are compared to the measurements from a specimen at the end of production. This principle implies that the measurements are carried out by the same device 2 and that any variability other than that specific to the specimen under test is minimized. In the context of the comparison with a control specimen on a production monitoring system based on EIT, the invention offers the advantage of canceling the variability of the measurement parameters between the two specimens.

[0058] Figure 4 shows an example of implementation of the embodiment of Figure 3 in the context of an HMI, in which the reference sensor 18r and another sensor 18 for ratiometric measurement are placed on either side of a surface 28 of a substrate. Thus, the device 2 makes it possible to detect the pressure on a surface of the substrate since the sensors 18r and 18 will deform differently.

[0059] Figure 5 shows yet another embodiment. In this embodiment, not only has the current source 20 been shared, but also the comparator 16 has been shared, so that there is a single analog-to-digital converter for the entire device 2. In this embodiment, it is therefore necessary to multiplex the measurements, i.e. one measurement of each measurement sequence is carried out sequentially on the sensor 18r and on each sensor 18 before moving on to the next measurement in the measurement sequence.

[0060] This embodiment has the advantage of being the most economical in hardware, and has the disadvantage of having to multiplex the operations, which can lead to a problem of temporal coherence in the presence of numerous sensors 18.

[0061] In the above, all or part of the device 2 may be added to at least one of the surfaces by bonding, deposition or any other attachment or integrated into the solid substrate. By integrated into the solid substrate, it should be understood that the solid substrate may be composed of several layers, such as sandwich composites or others, and the sensors may be integrated into these, in particular when one or more of them are conductive. In addition, by solid substrate, it should be understood any substrate in the solid state, whether it is rigid or has a certain elasticity.

Claims

Claims

1. Electrical process tomography measuring device for solid substrate, comprising a current or voltage source (20), a reference sensor (18r), at least one other sensor (18), and at least one comparator (16) arranged to emit a signal from the reference sensor (18r) or the at least one other sensor (18), each at least one other sensor (18) comprising a current input (8), a current output (10), and a plurality of electrodes (6) suitable for being connected to a solid substrate to be measured (4), and two measurement inputs (12, 14) of the voltage difference across said measurement inputs, each at least one other sensor (18) being suitable for carrying out a sequence of measurements in which the current input (8) and the current output (10) are each connected to a respective electrode of a pair of excitation electrodes of the plurality of electrodes (6), and the two inputs of measurement (12,14) are each connected to a respective electrode of a pair of measuring electrodes of the plurality of electrodes (6), the pair of excitation electrodes and the pair of measuring electrodes being defined by the measurement sequence at each measurement and the pair of measuring electrodes being connected to the at least one comparator (16) for measuring the propagation in the solid substrate of the current introduced via the pair of excitation electrodes, the current source (20) being connected to the reference sensor (18r), which comprises a current output (10) connected to the current input (8) of the at least one other sensor (18) and is connected to the at least one comparator (16) for emitting a signal corresponding to a voltage difference as a function of the measurement sequence,the device (2) being further arranged to determine the ratio between the voltage difference across the measurement inputs of each at least one other sensor (18) for each measurement sequence and the voltage difference across the measurement inputs of the reference sensor (18r).,

2. Device according to claim 1, in which each sensor (18) comprises a multiplexer (8, 10) connected on the one hand to its current input (8) and to its current output (10) and on the other hand to its plurality of electrodes (6).

3. Device according to one of the preceding claims, in which each sensor (18) comprises two multiplexers (12, 14) connected on the one hand to its plurality of electrodes (6), and on the other hand to the two measurement inputs (12, 14) of the at least one comparator (16).

4. Device according to claim 3, wherein each at least one other sensor (18) comprises a comparator (16) connected to the two measurement inputs (12, 14).

5. Device according to one of the preceding claims, in which the reference sensor (18r) comprises a current input (8), a current output (10), a plurality of electrodes (6) suitable for being connected to a solid substrate to be measured (4) and a comparator (16) comprising two measurement inputs (12, 14) arranged to emit a signal corresponding to the voltage difference across said measurement inputs (12, 14), the reference sensor being suitable for carrying out a sequence of measurements within which the current input (8) and the current output (10) are each connected to a respective electrode of a pair of excitation electrodes of the plurality of electrodes (6), and the two measurement inputs (12, 14) of the comparator (16) are each connected to a respective electrode of a pair of measurement electrodes of the plurality of electrodes (6),the pair of excitation electrodes and the pair of measurement electrodes being defined by the sequence of measurements at each measurement and the propagation in the solid substrate to be measured of the current introduced via the pair of excitation electrodes being measured at the terminals of the pair of measurement electrodes.,

6. Device according to claim 5, wherein the comparator (16) of the reference sensor (18r) is a differential amplifier (24), and wherein the comparator (16) of the sensors (18) is an analog-digital converter (16) further comprising a reference input (r) connected to the output of said differential amplifier (24) and arranged to emit a digital signal corresponding to the difference between the voltage of the two measurement inputs (12, 14) proportionally to a voltage level designated by the reference input (r).

7. Device according to one of claims 1 to 5, in which each comparator (16) is an analog-digital converter further comprising a reference input arranged to emit a digital signal corresponding to the difference between the voltage of the two measurement inputs (12, 14) proportionally to a voltage level designated by the reference input, the device further comprising a divider (22) arranged to calculate the ratio between the digital signal of each at least one other sensor (18) and the digital signal of the reference sensor (18r).

8. Device according to one of the preceding claims, in which the reference sensor (18r) is a witness sensor.

9. Device according to one of claims 1 to 7, in which the reference sensor (18r) is a calibration specimen reproducing the voltage differences expected for a solid substrate to be measured.

10. Solid substrate, characterized in that it comprises a device according to one of the preceding claims, added to at least one of the surfaces or integrated.

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