METHOD FOR STANDARDIZING AN ACOUSTIC SIGNAL

Abstract

A method for characterizing a liquid metal, the method employing a probe (1) comprising: a measuring transmitter (11) configured to emit an ultrasonic wave incident in the liquid metal; a receiver (21) configured to detect an ultrasonic wave reflected or diffracted by the liquid metal following the emission of the incident ultrasonic wave; the method comprising normalizing the ultrasonic wave detected by the receiver by a normalization amplitude. The normalization amplitude is determined experimentally from ultrasonic waves detected when the metal is considered to be free of detectable inclusions. Figure 1B.

Description

Title of the invention: METHOD FOR STANDARDIZING AN ACOUSTIC SIGNAL Technical field

[0001] The technical field of the invention is the characterization, by ultrasound, of a casting of liquid metal, for example a casting of an aluminum alloy. Prior art

[0002] In the field of metal casting, it is necessary to control the quality of the liquid metal, in order to detect inclusions. This is particularly the case for aluminum. Some aluminum alloy products, for example strip for casing applications, have high requirements in terms of inclusion health. These inclusions are generated during the melting, production and casting of these alloys when they are in the liquid phase. The quality of the molten metal is determined by the quantity and size of inclusions present in the metal. These may be oxides, nitrides, carbides, fluorides or borides. Inclusions commonly encountered in molten aluminum are, for example, particles of Al2O3, MgAl2O4, A14C3, MgO, CaO, CaF2, TiB2, TiVB2, SiO2. Their size ranges from a few microns to a few hundred microns.

[0003] It is possible to monitor, online, the inclusion quality of a casting by ultrasonic measurement techniques. According to this type of measurement, an ultrasonic acoustic wave propagates through the liquid metal. In the presence of inclusions, a part of the emitted acoustic wave is reflected or diffracted and can be detected. The amplitude of the reflected wave makes it possible to trace the size of inclusions in the metal. The ultrasonic technique allows characterization of a part of the liquid metal, typically 2% when the control is carried out in the chute transporting the liquid metal from the furnace to the casting pit.

[0004] Probes for carrying out measurements, by ultrasound, on castings, have been described in patents EP3204764B1, EP3204763B1.

[0005] Document EPI 194772B1 describes a method for calibrating a probe, in which reflectors are placed at the end of rods, the latter being arranged in liquid metal. The reflectors can be of different sizes. It is thus possible to establish a correlation between the size of each reflector and the amplitude of the detected acoustic background. Such a method needs to be applied for each probe. In addition, it assumes the arrangement of calibrated reflectors in a liquid metal, which can be complex.

[0006] Another possibility to estimate the size of inclusions is to use a curve of correlation between measurements resulting from an ultrasonic probe and measurements resulting from a method considered to be a reference method. The reference method may, for example, be a “LIMCA” (Liquid Metal Cleanliness Analysis) type analysis method, based on an analysis of the electrical resistance of a sample of liquid metal. It may also be the method described in EPI 194772B1, using calibrated reflectors. A correlation function can thus be established between the size of inclusions, measured by the reference method, and the amplitude of the ultrasonic signal.

[0007] A difficulty linked to the use of ultrasonic probes is that their response can vary from one probe to another. However, it is difficult to envisage calibrating each probe with a reference method, such as LIMCA or as described in EP1194772B1. Such calibration requires time.

[0008] The variability of the response between two probes, of identical structures, is due in particular to differences which may affect the electronic circuits, or the materials used. Faced with identical inclusions, two different probes, of identical design, can generate detection signals of different amplitudes, due to the variability of the response of the probes.

[0009] It is preferable, even necessary, to take such variability into account, for example if one wishes to apply the same correlation curve to measurements resulting from different ultrasonic probes.

[0010] The inventors propose a method allowing for the variability of responses of two different probes to be taken into account. The objective is that two probes, of identical design, generate identical signals, or signals that can be considered as such, in the presence of identical inclusions. Statement of the invention

[0011] A first object of the invention is a method for characterizing a liquid metal, the method implementing a probe, comprising: - a measuring transmitter, configured to emit an incident ultrasonic wave into the liquid metal; - a receiver, configured to detect an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal, following the emission of the incident ultrasonic wave;

[0012] the method comprising the following steps: a. arrangement of the probe, such that the measuring transmitter and the receiver are arranged in the liquid metal or facing the liquid metal; b. power supply to the measuring transmitter, so that the measuring transmitter emits an incident ultrasonic wave propagating in the liquid metal; c. detection, by the receiver, of an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal under the effect of the incident wave; d. repeating steps b) to c) during different measurement times, the measurement times forming a measurement time range, and measuring an amplitude of the ultrasonic wave detected by the receiver at each measurement time; e. selection, within the measurement time range, of a normalization period, the normalization period grouping together consecutive measurement times; f. calculation of a normalization amplitude based on the amplitudes measured at the times of the normalization period; g. normalization of the amplitudes measured during the measurement time range, during each step c), by the normalization amplitude calculated during step f).

[0013] According to one possibility, step f) comprises: - assignment of a normalization value to the normalization amplitude; - multiplication of the normalized amplitudes by the normalization value.

[0014] The normalization period may correspond to a predefined part of the measurement time range.

[0015] The normalization period may correspond to the last moments of the measurement time range.

[0016] The normalization period may extend during the last minutes of the measurement time range.

[0017] Step e) may comprise: - ei) analysis of the amplitudes respectively measured during the measurement time range; - eii) identification of a period of minimum amplitude, comprising different successive measurement times, during which the measured amplitudes are minimal, compared to the intensities measured outside the period of minimum intensity; so that the normalization period includes all or part of the minimum amplitude period.

[0018] According to one possibility, - sub-step ei) comprises a calculation of a moving average or a moving median of the amplitudes respectively measured during the measurement time range, the average or the median being calculated according to different successive time windows; - during sub-step eii), the period of minimum amplitude corresponds to a period in which the moving average or median is minimal.

[0019] According to one possibility, - the liquid metal flows along a chute; - the transmitter and receiver are positioned facing the chute.

[0020] According to one possibility, - the measuring transmitter is coupled to a first waveguide, immersed in the liquid metal; - the receiver is coupled to a second waveguide, immersed in the liquid metal.

[0021] A second object of the invention is an ultrasonic probe, comprising: - a measuring transmitter (11) configured to emit an incident ultrasonic wave into the liquid metal; - a receiver (21) configured to detect an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal, following the emission of the incident ultrasonic wave; - a processing unit (32), configured to: - measure an amplitude of the ultrasonic wave detected by the receiver in different measurement times, the different measurement times forming a measurement time range; - implementing steps e) to g) of a method according to the first subject of the invention from the amplitudes measured during the measurement time range.

[0022] The invention will be better understood upon reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0023] [Fig. 1A] represents a first embodiment of a probe allowing implementation of the invention.

[0024] [Fig.1B] represents another embodiment of a probe allowing an implementation of the invention.

[0025] [Fig.2] shows a diagram of acoustic waves detected at a measurement time.

[0026] [Fig.3A] shows schematically the main steps of a method according to the invention.

[0027] [Fig.3B] shows a diagram of a configuration in which the normalization period is located at the end of a measurement time range.

[0028] [Fig.3C] shows a configuration in which the normalization period is located in the middle of a measurement time range.

[0029] [Fig.4] represents a correlation curve between detected wave amplitudes and an inclusion dimension.

[0030] [Fig.5A] shows amplitudes measured at different measurement times.

[0031] [Fig.5B] shows the amplitudes represented in [Fig.5A] after applying a normalization function. Presentation of specific embodiments

[0032] A first example of an ultrasonic probe is shown, in connection with [Fig. 1A], allowing an implementation of the invention. The probe comprises a measurement transmitter 11 and a receiver 21. In this example, the measurement transmitter 11 and the receiver 21 are formed by the same acoustic transducer. The acoustic transducer is for example a piezoelectric transducer, known to those skilled in the art.

[0033] The transducer is for example a pellet of piezoelectric material emitting an acoustic wave at a frequency of 15 MHz. Generally, the frequency of the emitted acoustic wave is preferably between 1 MHz and 20 MHz.

[0034] The probe is intended for detecting inclusions in a liquid medium 2 to be analyzed, and more precisely in a liquid metal, the metal being for example an aluminum alloy. In Figures 1A and 1B, the dotted horizontal line represents an interface between the liquid metal 2 and an ambient medium 3, the latter being for example air.

[0035] The probe 1 comprises a waveguide 13, intended to be immersed in the liquid metal 2. The waveguide is configured to facilitate propagation of an acoustic wave between the transducer, acting as a transmitter-receiver, and the liquid metal 2. The waveguide 13 is for example formed from a material of the silicon nitride or silicon oxynitride (SiAlON) type, or more generally a material, of the refractory metal or ceramic type, considered to be inert to the liquid metal 2. The use of such a waveguide has been described in EP3204763B1 or in EP1194772B1.

[0036] In Figures 1A and 1B, an ultrasonic field 5 is shown diagrammatically, which corresponds to a part of the liquid metal inspected by the probe 1. When an inclusion 4 passes through the ultrasonic field, a part of the acoustic wave emitted by the measuring transmitter 11 is reflected or diffracted towards the receiver 21. The characteristics of the reflected field, in particular its amplitude, allow the detection of an inclusion and its characterization, for example an estimation of its size.

[0037] Preferably, the probe may comprise a power emitter 12, configured to emit an acoustic wave of power, i.e. greater than 10W, and preferably greater than 100W, for example 120W, as described in EP3204764. Indeed, the application of a high-power acoustic wave makes it possible to obtain the wetting of the first waveguide 13 by the liquid metal 2. The power emitter 12 is activated at regular intervals, so as to allow good wetting of the waveguide 13 by the liquid metal 2 to be maintained.

[0038] [Fig.lB] shows a preferred configuration, in which the probe comprises a measuring transmitter 11 and a receiver 21. The measuring transmitter 11 is connected to a first waveguide 13. The receiver 21 is connected to a second waveguide 23, preferably identical to the first waveguide 13 (same material, same dimensions), but distinct from the latter. The length of each waveguide is for example between 10 cm and 50 cm. The first and second waveguides are preferably rectilinear, and extend respectively around a first central axis A1 and a second central axis A2. The central axes A1 and A2 are intersecting, and inclined relative to each other at an angle a. The angle is for example between 25° and 35°. The angle a is for example equal to 28°. Such a configuration has been described in EP3204764B1.

[0039] In this embodiment, the probe may comprise a first power emitter 12, and a second power emitter 22, as described in connection with the first embodiment. The first and second power emitters are activated periodically, so as to improve the wetting of the first waveguide 13 and the second waveguide 23, respectively, by the liquid metal 2.

[0040] The probe comprises a control unit 31, configured to allow control of the emission of an acoustic wave by the measurement transmitter 11, and to allow control of the emission of a power acoustic wave by the power transmitter 12 when the probe comprises such a transmitter.

[0041] Regarding the acoustic waves emitted by the measurement transmitter 11, they are generally emitted in series. In practice, each series of acoustic waves is emitted by the measurement transmitter 11 at a regular frequency, for example 10 Hz. A series comprises at least one acoustic wave, and preferably several successive acoustic waves. Within the same series, the emission frequency of the acoustic waves can be between 100 Hz and 300 Hz, which corresponds to the excitation frequency of the piezoelectric transducer of the measurement transmitter. In Figure 2, three waves detected by the receiver 21 are shown diagrammatically, following the emission of a series of three waves by the measurement transmitter 11. The bracket indicates that the three waves belong to the same series. In Figure 2, the ordinate axis corresponds to the signal detected S(t) by the receiver 21 while the abscissa axis corresponds to time t.

[0042] The probe comprises a processing unit 32, configured to calculate an amplitude of the waves detected by the receiver 21 in response to a series of acoustic waves emitted by the measurement transmitter 11. The calculated amplitude, or measured amplitude, may be an average of the maximum amplitudes of each detected acoustic wave, or the maximum amplitude of all the acoustic waves detected following the emission of a series of waves. Subsequently, the calculated amplitude is noted tm denotes a measurement instant, which corresponds to the detection of a series of acoustic waves by the receiver 21.

[0043] The processing unit 32 may comprise one or more microprocessors or electronic circuits, configured to determine the measured amplitude and implement the steps described below.

[0044] The presence of an inclusion 4, of significant size, in the liquid metal, causes the measured amplitude A(tm) to vary. The measured amplitude A(t^ can make it possible to estimate an inclusion size, from a correlation curve established experimentally, using a reference method, for example the LIMCA method or the method based on the use of calibrated reflectors, these methods being described in connection with the prior art.

[0045] The same correlation curve can be used on different ultrasound probes of identical design, provided that the amplitudes resulting from said ultrasound probes are comparable. However, as previously indicated, it is necessary to take into account the variability of the probes, even if the latter are similar to each other: same dimensions and same materials used.

[0046] The processing unit 32 comprises an amplifier, intended to amplify the signal resulting from the receiver 21, and whose amplification gain Go is generally fixed in the factory, following the manufacture of the probe. The gain is generally not modified during the procedure. It can be subject to an adjustment depending on the conditions of use of the probe.

[0047] The probe 1 is intended to be immersed in a liquid metal 2 flowing from a furnace. The liquid metal may, for example, flow from a furnace, or be present in a ladle or a crucible. For example, the liquid metal flows into a chute. The waveguide(s) of the probe are immersed in the chute. The probe may be placed directly at the outlet of the furnace, or further downstream, at the outlet of a filter or a degasser. This may be a deep bed filter or a CFF (Ceramic Foam Filter) type filter or any other type of filter. When the probe is immersed in a casting downstream of a CFF type filter, the presence of bubbles in the liquid metal 2 may require adjustment of the amplification gain Gode in order to take into account the attenuation effect induced by the bubbles. The probe can also be placed downstream of a degasser and in a crucible.

[0048] For reasons of cost and time, it is not realistic to produce, for each probe, a correlation curve with a reference method. However, it is necessary to take into account the variability of the response of each probe. The inventor proposes a method making it possible to obtain a comparable response for each probe, so as to be able to use a correlation curve as described below, in connection with [Fig.4].

[0049] The main steps of the process are illustrated in [Fig.3A].

[0050] Step 100: carrying out measurements at different measurement times.

[0051] During casting, measurements are regularly carried out, for example at measurement times tm repeated at a frequency for example between 0.1 Hz and 20 Hz. All of the measurement times form a measurement time range Tm. Each measurement time is associated with a measured amplitude A(tm). As described in connection with [Fig. 2], the measured amplitude is calculated by the processing unit 32 from one or a series of waves detected by the receiver 21 following the emission of one or a series of acoustic waves by the measurement transmitter 11.

[0052] Step 110: selection of a normalization period.

[0053] An important aspect of the invention is that during the measurement time range Tm, a normalization period TN can be selected, comprising different consecutive measurement times. The normalization period TN corresponds to measurement times at which the measured amplitude A(tm) is the lowest. Indeed, during a casting, it is considered that at a given position, occupied by the probe 1, along the casting, a time period can be defined, called the minimum amplitude period Tmith during which the measured amplitude A(tm) is minimal. The minimum amplitude period Tmin corresponds to a period during which the casting is considered to be “clean”, in the sense that the metal contains a minimum proportion of inclusions. The normalization period TN corresponds to all or part of the minimum amplitude period Tmin.During the normalization period the measured amplitude A(tm) is then minimal, compared to the amplitude measured during the measurement times before tm and / or after the normalization period. During the normalization period the average amplitude is lower than the average amplitude calculated during the measurement time range Tm.

[0054] The normalization period TN can be defined a priori, depending on the position occupied by the probe 1 with respect to the casting, in particular when the probe 1 is arranged downstream of a filter. It is considered that it is at this position that the liquid metal has a minimum proportion of inclusions compared to the other positions of the casting. At this position, the normalization period TN can correspond to the last moments of the casting, for example to the last minute or minutes of the casting, knowing that the duration of a casting is generally of the order of 1 to 3 hours or even more. The normalization period TN then corresponds to the last measurement moments of the measurement time range Tm. This scenario is illustrated in [Fig.3B].

[0055] According to another possibility, the normalization period is identified a posteriori, following an analysis of the amplitudes A(tm) measured during the measurement times. On the basis of the measurements, the minimum amplitude period Tmin is defined. The normalization period is selected in the minimum amplitude period Tmin. This corresponds to the case illustrated in Figure 3C. The minimum amplitude period Tmin can be identified by calculating a moving average or a moving median of the amplitudes respectively measured during the measurement time range, the average or median being calculated according to different successive time windows, offset from each other by approximately 1 minute. The minimum amplitude period Tmin corresponds to a period in which the moving average or median is minimal.

[0056] When the probe is placed at the outlet of the furnace, depending on the processes implemented, it can be considered that the liquid metal contains a high quantity of inclusions at the start and end of the casting.

[0057] Step 120: Determination of a normalization amplitude

[0058] After the normalization period TN has been selected, a normalization amplitude representative of the normalization period is determined. This may be an average or a median of the amplitudes A(tm) measured during the normalization period TN.

[0059] Step 130: normalization

[0060] During this step, the amplitudes respectively measured at each measurement instant are normalized by the normalization amplitude AN. Preferably, a normalization value, denoted N, is assigned to the normalization amplitude An. Thus, the normalization is carried out in such a way that: A(tm) — f(A(tri^)— Or : - / corresponds to the normalization function. - A ( tm ) is the normalized amplitude - N can be for example 20. In this case, the normalized amplitude f ( A(lm)) is expressed in %. The value of 20% corresponds to a casting considered as clean, that is to say one in which any possible inclusions do not produce significant ultrasonic signals. - An is the previously defined normalization amplitude.

[0061] An advantage of the method is that the value of the normalized amplitude A ( tm) corresponding to the normalization value N, for example 20%, represents a clean casting, regardless of the probe used. The normalization makes it possible to overcome the variability affecting the probes: the normalized amplitudes A ( tm} of the different probes are comparable with each other, because the normalization amplitude is established, for each probe, under comparable conditions, in this case in the presence of a liquid metal considered to be clean.

[0062] Standardization assumes that it is accepted that the liquid metal, considered to be clean, that is to say free from detectable inclusion, generates acoustic waves of which the amplitude is assimilated to noise. This amplitude corresponds to the normalization amplitude A,v.

[0063] Normalization makes it possible to use a correlation curve as shown diagrammatically in Figure 4. The correlation curve is established experimentally with a probe, after applying the normalization function determined for said probe. Thus, the ordinate axis corresponds to f (Ac), where Ac corresponds to measured amplitudes correlated with known inclusion dimensions. The abscissa axis corresponds to a dimension, for example the diameter, measured by the reference method.

[0064] In [Fig.4], points are shown, each point corresponding to an experimental measurement. A calibration curve, in dotted lines, obtained by adjusting the experimental points, is also drawn.

[0065] The correlation function thus established can be applied to normalized amplitudes from different probes. The correlation curve can be implemented using, as ordinates, normalized values ​​A{tm\

[0066] A probe as described in connection with FIG. 1B was implemented in an aluminum casting, the probe being arranged at an output of a CFF filter. The duration of the measurement time range was 60.5 minutes. FIG. 5A corresponds to measured amplitudes A(fm) (y-axis) at different measurement times An (x-axis - acquisition frequency 10 Hz). FIG. 5B shows the amplitudes represented in FIG. 5A after normalization A ( tm ), the normalization value N being 16%. In FIGS. 5A and 5B, the zero amplitudes correspond to times during which the power transmitter 12 was activated. When the power transmitter is activated, the signal detected by the receiver is not taken into account.

[0067] The invention can be applied by implementing ultrasonic probes of the same type, that is to say having the same structure, the variability being due to the differences affecting the components of the probe.

[0068] If it is desired that the measurements be used for the purpose of characterizing the dimensions of the inclusions, it is preferable, or even necessary, that for at least one probe, a correlation has been made between the measured amplitudes and measurements of the inclusion dimensions. The latter are obtained by a reference measurement method. It is preferable that the different probes, which are implemented, are then arranged at the same position, or at positions considered to be equivalent, relative to the casting.

Claims

Claims

1. Method for characterizing a liquid metal, the method using a probe (1), comprising: - a measuring transmitter (11) configured to emit an incident ultrasonic wave into the liquid metal; - a receiver (21) configured to detect an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal, following the emission of the incident ultrasonic wave; the method comprising the following steps: a. arrangement of the probe (1), such that the measuring transmitter and the receiver are arranged in the liquid metal or facing the liquid metal; b. power supply to the measuring transmitter, so that the measuring transmitter emits an incident ultrasonic wave propagating in the liquid metal; c. detection, by the receiver, of an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal under the effect of the incident wave; d. repeating steps b) to c) during different measurement times, the measurement times forming a measurement time range (Tni), and measuring an amplitude of the ultrasonic wave detected by the receiver at each measurement time; e. selection, in the measurement time range of a standardization period (TN\ the standardization period comprising consecutive measurement times; f. calculation of a normalization amplitude (AjV) as a function of the amplitudes measured at the times of the normalization period; g. normalization of the amplitudes measured during the measurement time range, during each step c), by the normalization amplitude calculated during step f).

2. Method according to claim 1, in which the normalization period corresponds to measurement times at which the amplitude measured is the lowest.

3. Method according to any one of the preceding claims, in which step f) comprises - assigning a normalization value (Ai) to the normalization amplitude; - multiplying the normalized amplitudes by the normalization value.

4. A method according to any preceding claim, wherein the normalization period corresponds to a predefined portion of the measurement time range.

5. Method according to claim 4, in which the normalization period corresponds to the last instants of the measurement time range.

6. The method of claim 4, wherein the normalization period extends during the last minutes of the measurement time range.

7. Method according to any one of claims 1 to 3, in which step e) comprises - ei) analysis of the amplitudes respectively measured during the measurement time range; - eii) identification of a period of minimum amplitude, comprising different successive measurement times, during which the measured amplitudes are minimal, compared to the intensities measured outside the period of minimum intensity; so that the normalization period comprises all or part of the period of minimum amplitude.

8. Method according to claim 7, in which: - sub-step ei) comprises a calculation of a sliding average or a sliding median of the amplitudes respectively measured during the measurement time range, the average or the median being calculated according to different successive time windows; - during sub-step eii), the minimum amplitude period corresponds to a period in which the moving average or median is minimal.

9. Method according to any one of the preceding claims, in which - the liquid metal flows along a chute; - the transmitter and the receiver are arranged facing the chute.

10. Method according to any one of the preceding claims, wherein - the measuring transmitter is coupled to a first waveguide (13), immersed in the liquid metal; - the receiver is coupled to a second waveguide (23), immersed in the liquid metal.

11. Ultrasonic probe, comprising: - a measuring transmitter (11) configured to emit an incident ultrasonic wave into the liquid metal; - a receiver (21) configured to detect an ultrasonic wave reflected or diffracted by an inclusion in the liquid metal, following the emission of the incident ultrasonic wave; - a processing unit (32), configured to - measure an amplitude of the ultrasonic wave detected by the receiver at different measurement times (^), the different measurement times forming a measurement time range (Tm); - implement steps e) to g) of a method according to any one of the preceding claims from the measured amplitudes (A(tm)) during the measurement time range (Tm); measure.

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