FR2752939A1 - Determining composition of liquid on surface of structure - Google Patents

Abstract

A block (16) made from a material with an acoustic impedance intermediate between ice and antifreeze liquids is fitted flush with a structure (14) such as an aircraft wing and constitutes a delay line. The block supports emitting (20) and receiving (22) ultrasonic transductors and also a temperature sensor (25). The emitting transductor (20) is supplied at high voltage (30) by an electronic switch (28) controlled by an impulse generator (26). The receiving transductor (22) is connected to an amplifier (36), analogue to digital convertor (38) and memory (40). The operation of the equipment is controlled by a microcomputer (50) such that the composition of the liquid on the wing surface is determined from comparisons between incident and reflected energy.

Description

COMPOSITION DETERMINATION METHOD
FROM A LIQUID TO THE SURFACE OF A STRUCTURE
The present invention relates to the determination, by ultrasound, of the composition of a layer of liquid covering an aircraft structure and the components of which belong to a known family, using the difference in acoustic impedance between these components and between these components and a reference material.

 The invention finds a particularly important application in aeronautics where, during cold periods, it is important to check, before takeoff of an aircraft, the absence of risk of icing.

 At present, the regulations allow an airplane to take off only with "clean" wings. However, the currently known methods of measuring frost do not make it possible to detect the presence of a very thin layer, of less than a tenth of a millimeter approximately. Consequently, the current procedure is as follows in icing conditions: takeoff is authorized, after spreading anti-icing liquid on the wings, within a period of time which depends on the external parameters. If this time is exceeded, it is necessary to spray anti-frost liquid on the wings again.

 The invention aims to provide a simple solution to the problem by using a detection which is not that of the presence of frost. It consists in checking, after spreading anti-icing liquid and immediately before takeoff, that the wings are covered with liquid and in evaluating the composition of the liquid and, more precisely, what is the relative proportion of water and anti-liquid - icing in the liquid layer. There are graphs indicating, for each temperature, what is the minimum content of deicing fluid which guarantees the absence of risk of ice formation and therefore the certainty of taking off "clean wings".

 We already know (EP-A-0619 488) a method for identifying the nature of a contaminant on the surface of a structure and in particular to determine whether it is water, a liquid d anti-icing, a mixture of the two, slush or ice. The present invention aims to adapt this method to the measurement of the composition of the layer on the surface of a wing, when this layer is liquid.

 To this end, the invention proposes a method for measuring the composition of a liquid layer on the surface of an aircraft structure, in particular of a wing, according to which an ultrasonic pulse coming from a transducer is directed. the surface through a block of material, constituting a delay line, having an acoustic impedance intermediate between that of liquids and that of ice, the amplitude of the echo obtained on said surface is compared, under oblique incidence, to that of the incident pulse and the relative content of the layer of liquid in anti-icing liquid and in water is deduced from the ratio of the amplitudes.

 The acoustic impedances are - 1.4 x 106 kg / m2.s for water, - 1.7 x 106 kg / m2.s approximately for most de-icing and anti-icing liquids.

 Consequently, a material with an acoustic impedance of between 2.7 and 3 x 106 kg / m2.s will give good results. In particular, the polysulfone, whose impedance is approximately 2.85 x 10 6, has an impedance well suited to anti-icing liquids than to water.

 In this case the reflection coefficient is maximum, for example of the order of 0.25, for pure water and minimum for pure anti-icing liquid, for example of the order of 0.08 for l 'ethylene glycol. However, this measurement practically requires a minimum thickness of 0.1 mm for it to be precise.

 The measurement requires a reference. This reference can be obtained by sampling the electrical excitation signal from the transmitting transducer; a more precise solution consists in taking a fraction of the acoustic energy using a reflector, which makes it possible to overcome variations in the characteristics of the transmitter due to aging, temperature or other factors. It is obviously still possible to measure the thickness of the layer of liquid and even to detect if there is, above, a film of ice which will be torn off at takeoff because it does not adhere, as a result of the presence of liquid. The thickness measurement is carried out by measuring the time interval which separates the echo on the interface between the structure and the liquid of the echo, the contact surface of the liquid with the air; however, this measurement practically requires a thickness greater than 0.1 mm.

 To improve the accuracy of the thickness measurement, the echo is advantageously subjected, before comparison, to a transformation in the frequency domain, to a division of the spectrum, to filtering, and to a return to the time domain.

 For this, the echo can be sampled and memorized, before transformation, over a time interval starting immediately before the first echo peak and ending after complete arrangement of the echoes on the surface of the block and at the interface between the layer of liquid and air.

The invention also relates to other arrangements advantageously usable in conjunction with the previous ones, but which can be used independently. All these provisions will appear better on reading the following description of a particular embodiment, given by way of non-limiting example. The description refers to the accompanying drawings, in which
- Figure 1 is a block diagram of a device according to a particular embodiment of the invention;
FIG. 2 is a timing diagram showing the shape of the original pulse and of an echo signal obtained with a clean surface,
- Figures 3 and 4, similar to Figure 2, respectively show, in solid lines, an echo signal obtained on a surface carrying a film of anti-icing liquid (Figure 3); and a mixture of water and anti-icing fluid (Figure 4).

 The device, the principle of which is shown in FIG. 1, has a general constitution similar to that described in document EP 619 488. It comprises a probe 10 and at least one excitation and analysis unit 12.

 The probe 10 is mounted in a structure, such as a wing, so that its interface with the ambient medium is level with the external surface of the structure 14. It comprises a block 16 made of a material whose acoustic impedance is close to that of a mixture of liquid phases.

 One can in particular use the polysulfone or PSU whose acoustic impedance is about 2.85 x 106 kg / m2.s. The block 16 generally has a flat interface 18 and has two symmetrical oblique faces intended respectively to receive a transmission transducer 20 and a reception transducer 22.

 The transducers 20 and 22 will generally be constituted by highly damped piezoelectric transducers (often with a coefficient of around 60 t) intended to provide, when excited by a single pulse, an ultrasonic burst constituted by a train of 'damped waves. In particular, transducers having a resonant frequency of about 2 MHz can be used.

 The measurement of composition (and possibly thickness) are improved by carrying out a temperature correction, as in the case of the document already mentioned; for this a temperature probe may be provided.

 The excitation and analysis unit 12 is designed to apply a pulse of determined amplitude and duration to the emission transducer 20 and to analyze the ultrasonic echo received by the transducer 22.

 The measurement of the excitation amplitude can be done from the measurement of the electric excitation pulse, on an output 24. It is however preferable to measure the ultrasonic energy emitted, for example using the transducer 22 placed so as to receive a fraction of the ultrasonic energy emitted, reflected at 27 on a surface provided for this purpose in block 16.

 The excitation unit comprises an emission channel connected to the transducer 20, having a generator 26 of short pulses (200 nanoseconds for example) controlling the closing of an electronic switch 28 connected to a high voltage source 30. The generator may be provided with a clock 32 making it possible to distribute, from an output 34, a clock signal, for example at a frequency of 64 MHz, intended for the sampling of the echo.

 A resonant frequency of the transducers 20 and 22 of approximately 1 to 3 MHz generally gives satisfactory results.

 The unit also includes a reception channel having an amplifier 36, an analog-digital converter (ADC) 38 and a memory 40 organized in a queue and connected to a data bus 42.

 The device can be provided to allow a scale correction and a correction as a function of the temperature.

 The operation of the unit 12 is controlled by a calculation unit 50, comprising a microprocessor and memories, which also performs the analysis of the signals. This calculation unit makes it possible to launch each measurement by sending a start signal to the generator 26.

 The implementation of the device is preceded by a calibration step, on a "clean" structure, during which a pulse is emitted and the echo signal is analyzed, until the first alternation of the echo signal on the free surface of the contaminant layer of maximum thickness to be measured (in the case of a device allowing the measurement. FIG. 2 shows the temporal variation of the signal.

The instants R1 of the start of the echo signal are noted after an emission of amplitude 10 and R2 of the first zero crossing.

The amplitude Ag of at least the first peak is stored. It can be seen that it is relatively high, due to the large difference in acoustic impedance between the block 16 and the air.

 During each actual detection, the emission transducer 20 is excited by pulse. Although the most important time slice is that going from R1 to tl (figure 1) with regard to the composition measurement, we will memorize samples taken over a much longer period T when we want to make a thickness measurement with passage in the frequency domain, which makes it possible to obtain much more precise results. The sampling frequency should be much higher than the resonant frequency of the transducer, for example 64 MHz.

 The curve in solid lines in FIG. 3 shows, by way of example, a signal obtained in the case of a film of pure anti-icing liquid 0.655 mm thick on the surface of the block. To allow a comparison, the signal obtained with a "clean" surface is indicated in dashes.

 There is an extremely significant attenuation of the first echo, because the reflection coefficient at the interface is much lower than with air, of about 0.08: the amplitude A1 is much less than Ag.

 Insofar as the amplitude 10 of the ultrasonic pulse emitted remains the same, the Al / Ao ratio would be significant. However, the amplitude 10 can vary over time and also depending on the temperature. It is therefore preferable to have 10 for each measurement. In the case illustrated in FIG. 1, it suffices for this to provide the microprocessor 50 so that it takes account of the echo on the reflector 27 towards the amplifier 36 during the time interval necessary for the pulse measurement. initial.

 To allow a correction of scale and a correction as a function of the temperature, the device can comprise a pre-amplifier 44 with controlled gain, interposed between the transducer 22 and the amplifier 36 with fixed gain. A switch 46 makes it possible to orient, at will, towards the reception channel, either the output signal of the amplifier 44, or the output signal of a fixed gain amplifier 48 connected to the temperature sensor 25.

 The case illustrated in FIG. 4 is that of a 0.21 mm thick film of a mixture of water and antifreeze liquid. The curve in solid lines shows that the first echo has an amplitude greater than that of FIG. 3, because the coefficient of reflection is higher. This is due to the fact that the difference in acoustic impedance between the mixture and the block is greater than the difference between the pure antifreeze liquid and the block. The amplitude of the echo would be maximum, with a reflection coefficient of the order of 0.25, in the case of pure water.

 In principle, the composition of the mixture could be deduced, using a table, from a simple comparison between the amplitude of the echo and the amplitude Io.

 However, it is much better, to obtain a high precision, to take into account the energies.

 For that, one carries out the relationship between two surfaces of the curve, representative one of the echo, the other of the initial impulse. For that, it is enough to accumulate the samples, that is to say to make an integration during two periods of time. The first period can go from a threshold value to another threshold value of the initial pulse, for example from 1o / 4 to Io / 2 (the second value being used to eliminate the influence of the connecting cables on the shape of the memorized pulse). The second period can go from R1 to R2 (Figure 3). A comparison can also be made between the two reports obtained, one with the clean wing, the other with the presumed contaminated wing.

 The method also makes it possible to measure the thickness of the liquid layer.

 For this, the signal variations on a clean wing and on the contaminated wing over the time interval T are sampled. The samples are memorized and the signal is subjected to a fast Fourier transformation, or TFR over a time interval covering several successive echoes, then a division of the spectra is carried out on the contaminated wing and on the clean wing, the latter previously memorized, during a calibration, and a filtering is carried out intended to spread the frequencies outside the useful zone (for example a bandpass filtering from 1 to 5 MHz for transducers having a resonance frequency of 2 MHz).

Then a TFR 1 is carried out and makes it possible to obtain a representative variation, such as that shown in dashes in FIG. 4.

Claims (6)

 1. Method for determining the composition of a liquid layer, the components of which belong to a known family, on the surface of an aircraft structure, in particular of a wing, according to which an ultrasonic pulse coming from a transducer towards the surface through a block of material, constituting delay line, having an acoustic impedance intermediate between that of liquids and that of ice, the amplitude of the echo obtained on said surface is compared, under oblique incidence, to that of the incident pulse and the relative content of the layer of liquid in anti-icing liquid and in water is deduced from the ratio of the amplitudes.  2. Method according to claim 1, characterized in that the material is polysulfone.  3. Method according to claim 1 or 2, characterized by a prior step of memorizing a reference by sampling from the electrical excitation signal of the transmitter transducer; or a fraction of the acoustic energy emitted.  4. Method according to claim 3, characterized in that the amplitude comparison is carried out by integrating on the one hand the samples of the signal preceding the duration of the first echo, on the other hand the samples of the incident pulse for at least a specified fraction of the latter before making the connection between the two.  5. Method according to any one of claims 1 to 4, allowing thickness measurement, characterized in that the echoes are subjected to a transformation in the frequency domain, to a division of the spectrum, to filtering, and to a return in the domain temporal.  6. Method according to claim 5, characterized in that the echo is sampled and memorized, before transformation, over a time interval starting immediately before the first echo peak and ending after complete disappearance of the echoes on the surface of the block and at the interface between the liquid layer and the air.