WO1993020405A1

WO1993020405A1 - Ultrasonic thickness measuring device and method, and use thereof

Info

Publication number: WO1993020405A1
Application number: PCT/FR1993/000302
Authority: WO
Inventors: Jean-Marie Muller; Christophe Renard; Claude Rowdo
Original assignee: Pont-A-Mousson S.A.
Priority date: 1992-04-03
Filing date: 1993-03-25
Publication date: 1993-10-14
Also published as: FR2689625A1; BR9306180A; FR2689625B1; HU9402687D0; CA2133473A1; AU3892893A

Abstract

Ultrasound is transmitted through an element (4, 41) by local immersion coupling, whereafter a signal (10) containing various data (echoes) on the ultrasonic wave path through the element (4, 41) is acquired, and the time lag (ti) between two echoes and the speed of the ultrasonic wave in the element are deduced therefrom. The thickness of pipes (22) may thereby be measured.

Description

Method and device for thickness measurement by ultrasound and use of such a device

The present invention relates to the measurement of thickness by analysis of ultrasonic waves. It relates firstly to a method for determining the thickness between two surfaces of an element made of a material with a heterogeneous internal structure.

Document US Pat. No. 4,398,421 discloses a method of ultrasonic measurement of the thickness between opposite surfaces of a worked element.

In this known method, the speed of the ultrasonic waves in the material of the element being worked is measured, by means of two translators, a translator working in transmission and a translator working in reception. The ultrasounds are emitted so as to enter the element at an angle of incidence relative to the external surface of the element. The ultrasonic wave is reflected according to the angle of refraction on the surface of the bottom of the element, the reflected wave returns to the external surface of the element at a known point. The receiving translator has been placed at this point. The distance between the point of entry and the point of exit of the ultrasonic wave is known dar determined according to the angle of incidence, the measurement carried out provides the time taken by the ultrasonic wave to go from one translator to another.

With these data, the speed of propagation of the wave in the material of the element is deduced.

Since the speed is known, ultrasonic waves are emitted and received using two straight translators. The measurement of the time elapsed between transmission and reception makes it possible to determine the thickness of the element. This process requires the use of four translators, a special fluid used to couple the ultrasound to the workpiece and numerous operations and adjustments.

This makes automation very difficult.

The main object of the invention is to remedy these drawbacks by a method which makes it possible to measure, without modification, or adjustment, with a single translator per measurement point, numerous ranges of thicknesses of planar elements. or of variable diameter with great precision. The method according to the invention allows the automation of the measurements, without slowing down the cycle times during manufacture by a sufficiently short measurement time per point. To this end, the method according to the invention consists of: a) a time measurement between echoes comprising:

- a test noting the presence of echoes

- an acquisition of the representation of echoes in the form of a signal - a determination of the signal to noise ratio

- detection of the position and amplitude of the echoes

- an analysis of echoes and the elimination of parasitic echoes - a deduction of the time taken by the ultrasonic wave to cross the element as a function of the signal collected b) a measurement of the speed of the ultrasonic wave corresponding to each time measurement including:

- a weighting of the first thickness echo

- a determination of the coefficients of a filter whose input is white noise and the output the first weighted thickness echo - a determination of the response of the filter as a function of the requested frequencies

- a determination of the maximum peak position and the bandwidth of the filter

- a correlation between the characteristics of said filter and the speed of the ultrasonic wave in

Element c) an analysis of the signal as a function of time and speed which determines the thickness between two surfaces delimiting the element of material with heterogeneous internal structure.

The invention secondly relates to a device allowing the implementation of the method, designed on the one hand so as to emit ultrasonic waves entering the material along an axis normal to the surface of the element made of material with a heterogeneous internal structure and on the other hand to ensure good coupling of the ultrasound. The perpendicularity allows the use of a single translator per point of measurement working in emission and reception for the implementation of the method according to the invention, whatever the shape of said element, planar or cylindrical. The coupling is carried out by local immersion, which allows the automation of the measurements. These results are achieved because the device according to the invention is of the type comprising a fluid supply means coupling the ultrasonic waves to the element made of material with a heterogeneous internal structure. It also includes a means for angular geometric positioning of the translator making it possible to maintain said translator normal to the external surface of the element with a heterogeneous structure.

According to other characteristics:

- The angular geometric positioning means of the measuring device according to the invention is a compass comprising at least two arms respectively connected to the element with a heterogeneous internal structure by means of a shoe and, between them, by a means of displacement by ratio to the surface of the element with a heterogeneous internal structure at an identical speed for each arm,

- The means of displacement relative to the surface of the element with a heterogeneous internal structure at an identical speed for each arm, consists of at least one toothed sector mounted on each arm. The toothed sectors are of identical diameter and pitch,

the means of displacement relative to the surface of the element with a heterogeneous internal structure, at an identical speed for each arm, consists of a threaded rod with equal pitch and reversed at each of its ends, each of said ends cooperating with an arm of the compass,

- A pressure and / or traction means is applied to at least one arm between the displacement means and the pad of said arm.

- The pressure means is a spring or a jack. To automate the measurement, it is necessary that the liquid coupling the ultrasonic waves is of a very low viscosity, of a low cost and that the quality of the coupling is constant, for this purpose said liquid is water ( local immersion).

The device according to the invention comprises a stop mounted on the angular positioning means, said stop being able to avoid contact between the element made of a material with a heterogeneous internal structure and the fluid supply means or the transducer.

The invention also relates to the use of a device as defined above for measuring the thickness of cast iron parts. A nonlimiting example of implementation of the invention will now be described with reference to the appended drawings in which: FIG. 1 is a schematic view of a device for generating ultrasound with a representation in the form of a simplified oscillogram of the ultrasound signal;

- Fig. 2 represents in the form of a flowchart the steps of the method according to the invention for measuring the time of movement of the ultrasonic waves in the element with a heterogeneous internal structure;

- Fig. 3 is a flow diagram of the steps of the method making it possible to deduce the speed of the ultrasonic waves in the element with a heterogeneous structure;

- Fig. 4 represents in the form of an oscillogram an example of an ultrasonic signal obtained on an element with a heterogeneous internal structure; Fig. 5 represents the filter with which the first thickness echo is associated; Fig. 6 is the curve illustrating the spectral response of the filter for the requested frequencies; Fig. 7 shows the device ensuring the perpendicularity of the translator and the part and the coupling of the ultrasound;

According to the embodiment of FIG. 1 a transmitter-receiver generator 1 ultrasonic emits an electrical pulse transformed into ultrasonic wave 2 by the piezoelectric element of a translator 3. This ultrasonic wave, carried by a coupling fluid passes through an element 4, here a part to be measured and is reflected on the surface of the bottom 5 of the element 4 (element-air interface). Part of the reflected ultrasonic wave 2 6 is transmitted to the translator 3 and is transformed back into an electrical signal by the translator 3, a signal which is amplified by the receiver stage of the ultrasonic generator 1. At the room interface - coupling fluid 7, only part 8 of the reflected ultrasonic wave 6 is transmitted and returns to the translator 3. Another part 9 is reflected to pass through the material and again be reflected on the surface of the bottom 5 of the element 4. And so on until total absorption of the waves in the material. The path of the ultrasonic wave 2, 6, 8, 9 is represented in the form of a signal 10 on a screen 16 of oscilloscope type 17 having echoes, an emission echo 11, an interface echo 12 7 resulting from the contact of the ultrasonic wave 2 with the external surface of the part 4, echoes of thicknesses 13-15 originating from multiple reflections on the external surfaces and from the bottom 5 of the element 4. The element 4 consists of a solid material or a liquid. The method includes measuring the time taken by the ultrasonic wave to move through the material, which is carried out according to different stages shown in FIG. 2, first a test 20 verifying the presence of an interface echo and positioning this echo. If the echo is present interface the method includes a step 21 of determining the signal to noise ratio. The signal is the representation of the path of the ultrasonic wave in the element. The noise results on the one hand from multiple diffusions of the wave in the material with heterogeneous structure and on the other hand from electrical parasites.

In this step we consider the noise level and the signal amplitude. This can only be detected if it has an amplitude greater than that of the noise. Then the method comprises a step 22 of detection of all the echoes defined by their amplitudes and their positions. This detection is carried out above a limit threshold determined by the amplitude of the noise.

Then the echoes are analyzed in a step 23 with elimination of any parasitic echoes. These parasitic echoes can be produced by the presence of heterogeneities or micro-defects in the material of the part.

This step 23 is followed by a step 24 of determining the amplitude and the position of the thickness echoes, that is to say all the echoes of step 22 minus the echoes of step 23.

The method comprises a step 25 of measuring the time intervals between the echoes which determines the time taken by the ultrasonic wave to pass through the piece of cast iron as a function of the signal collected.

The method comprises a determination of the speed of the ultrasonic wave in the room by steps which may correspond to the embodiment shown in FIG. 3. The speed is determined in seven steps (26-32). The first, step 26 is the determination of N points, corresponding to a digital representation of a KAISER weighting window: f (n) = I _Q (β 1 - (2n / N-ir) / Io (β ) with I ₀ (x): modified BESSEL function of order 0 as defined by Gérard PETIAU (Researcher at CNRS) in "Bessel's theory of functions" CNRS publications service. In the Kaiser function x is represented in the form β V ~ ^~ 1 - (2n / N-1) ^* . β: constant coefficient, n: varies from 0 to (N-1) / ₂ The method then comprises step 27 of weighting the signal of the first thickness echo by the Kaiser window.

The next step 28 consists in determining a digital filter from the data of the signal weighted by the maximum entropy method. The data is considered as leaving the filter, the input being white noise.

The filter is defined by coefficients or poles which are determined by the maximum entropy method as defined in the brochure "Digital signal processing: techniques and applications" Course 412 from the training organization called "Learning Tree".

The following step 29 is the determination of the response of the filter to the frequencies requested as a function of the translator used. Step 30 consists in determining the position of the maximum peak and the bandwidth of the filter determined from the data of the first weighted thickness echo.

Step 30 is followed by step 31 of estimating the speed of the ultrasonic wave in the material from the position of the maximum peak and the bandwidth of the filter by a relation which has been determined at the continuation of tests on samples, for example in cast iron. These tests consisted on the one hand of metallographic and dimensional analyzes of various samples with different graphite forms therefore at different ultrasonic velocities in the cast iron and on the other hand of a determination of parameters of the ultrasonic signal crossing these samples, by estimating the parameters of the corresponding H filter. The conclusion of these tests led to the establishment of a correlation between graphite forms or ultrasonic velocities in the font and associated H filter parameter. What leads to step 32 of determining the speed of the ultrasonic wave in the element.

The time and speed taken by the ultrasonic wave to cross the room in a round trip, being determined, we deduce the thickness of the room by the following relation e = v ^ e being the thickness of the room , v the speed of the wave in the room and t the round trip time of the room.

Fig. 4 represents the electric image 33 (signal) of the path of the ultrasonic wave in a room with a heterogeneous internal structure.

This signal 33 represents the amplitude of the voltages V at the terminals of the piezoelectric element of the translator 3 amplified by the receiver stage of the ultrasonic generator 1 (see FIG. 1).

The signal 33 has large variations in amplitude 34 which correspond to the interface echoes 11 and of thickness 12, 13. These large variations 34 are repeated at substantially constant times t1. In addition, the signal 33 has small amplitude variations 35, this is noise.

From time to time variations in amplitude 36 appear. These are parasitic echoes generated by the heterogeneity of the structure or by micro-defects in the material.

These echoes of variation of average amplitude 36, not repeated at constant time interval, are random parasitic echoes which are eliminated during step 23 of the method according to the invention. The noise prevents echoes of variation in amplitude less than its variation in amplitude 35. The ratio of the signal to the noise is defined by the ratio of the extreme amplitude S of the signal to the amplitude of the noise B.

Fig. 5 represents the digital filter H of step 29 of the method.

The data 37, that is to say the signal of the first echo of thickness 12 weighted by the Kaiser window are considered to be the result obtained at the output of the digital filter H, the input of which is white noise 38. The white noise 38 is a signal of zero mean value whose average power spectral density is a positive constant.

So the H filter transforms the white noise signal. 38

P and a signal 37 according to the relation H (Z) = 1 / (1-Σ a _n Z ⁿ ), n = 1 with p = number of poles of the filter, an = pole of the filter. H (Z) is the Z transform of the H filter. The Z transform is the transformation equivalent to the Laplace transformation of continuous time systems, applied to the study of discrete systems and in particular to digital signal processing. With any signal f (t) sampled at times nTe (n integer. Te sampling period), we can associate

. ώo a transform in Z: F (Z) = Σ f (nTe) Z ^{~ n} , Z corresponds to e - ⁿ P ^Te (p = σ + jw complex variable): it is a delay operator - the delay is nTe: a multiplication by Z ~ ⁿ corresponds to a time delay nTe i.e.

+ _* + offset of n sample (Z is equivalent to the offset of two samples etc). A spectral response 39 of the filter H is shown graphically in FIG. 6.

The ordinate P represents the power or spectral density of the filter. The abscissa represents the frequencies. In step 31 of the method, the frequency position of the peak 40 of the maximum power amplitude of the filter response is determined. The bandwidth of the filter H defined is determined by an attenuation by V2 of the maximum amplitude of the peak. These values are characteristic of the spectral response of the filter H and by the same representative of the first weighted thickness echo 37.

Fig. 7 shows a device for implementing the method according to the invention. The device comprises a translator 3, linked to a means 42 for supplying fluid. It includes a compass 43 to maintain the angular positioning α of the translator 3. The compass 43 comprises two arms 44a and 44b connected together by two toothed sectors 45, each toothed sector is fixed free to rotate along the axis of an arm 44a, 44b.

The two toothed sectors 45 are of identical diameter and pitch and mesh with each other. Each arm 44a, 44b comprises at its respective end a shoe 46a, 46b allowing contact with an element 4a. The arm 44b comprises perpendicularly to its axis a protuberance 48 at the level of the toothed sector 45 and on the side opposite to the arm 44b.

The element 4a, the distance of which is measured between the outer surface 7a and the bottom surface 5a, is a cast iron pipe of variable diameter. The above device is designed in such a way that whatever the diameter, the perpendicularity of the emitted surface / wave is ensured. This distance represents the thickness of the part and the measurement is made using longitudinal ultrasonic waves passing through the cast iron. Cast iron is not a homogeneous material, so the speed of ultrasonic waves varies. Tests have shown the different speed ranges that can be obtained depending on the structure of the graphite of the cast iron parts, four kinds of which are shown in FIG. 7.

The velocities measured in a lamellar graphite cast iron 50 are in a range from 4700 to

5000 m / s. The velocities measured in the vermicular graphite cast irons 51 are between 5000 and 5300 m / s. The speeds recorded inside the compact graphite fonts 52 are in the range from 5300 to 5600 m / s and the speed of propagation of the ultrasonic waves in the spheroidal graphite fonts 53 varies from 5600 to 5700 m / s . These graphite structures are obtained by inoculation of the cast iron (see Applicant's patent No. FR-2,546,783). As can be seen, the speed varies within each family of cast iron with a different graphite structure. It is possible to manufacture cast iron elements each having different graphite structures. This makes necessary to determine the speed of the ultrasonic waves for each thickness measurement. In addition, to determine the thickness of the part with a single translator, the ultrasonic waves must move along a normal path to the outer surface 7a, hence the choice of longitudinal ultrasonic waves and the device according to the invention. .

The operation of the device is as follows: the pipe 4a is approached without coming into contact with the supply means 42 therefore of the translator 3. It separates the pads 46a and 46b which each remain at the same distance from an axis 54 extending the translator 3. In fact, the pads 46a and 46b are linked via the arms 44a and 44b to the toothed sectors 45. The toothed sectors 45 having the same pitch and the same diameter cause each arm to rotate at an identical angle α. The rotation of the arm 44b causes the rotation of the protuberance 48 which abuts on an element 55 linked to the supply means 42 which blocks the rotational movement of the arm 44b, therefore of the arm 44a also, the angles α remaining identical . Thus the compass 43 is at its maximum opening and the pipe 4a is kept centered. The two arms are held close to each other by a spring 56, fixed on the arm near the pads (46a, 46b). The translator 3 emits an ultrasonic wave. The water supply means 42 pours a column of water 57 to the pipe 4a. The ultrasonic wave travels along the axis 54 normal to the external surface 7a of the element which is a pipe 4a.

After passing through the water column, the ultrasonic wave crosses the external surface 7a of the pipe 4a, forming an interface echo. It is reflected by the bottom surface 5a by forming a thickness or bottom echo propagating to the translator 3. The measuring device (Fig. 1) acquires the ultrasonic signal via a device rapid scanning, which allows after processing by the method according to the invention to extract the information indicating the time interval between different thickness echoes, the speed of the ultrasonic waves in the cast iron and the thickness of the pipe 4a.

The device according to the invention makes it possible to determine the type of graphitization of a cast iron and the thickness of a cast iron part.

Claims

1.- Ultrasonic measurement method of the thickness between two surfaces of an element (4, 4a) of material with heterogeneous internal structure, characterized in that it consists of: a) a measurement of time between echoes including:

- a test (20) noting the presence of echoes

- an acquisition of the representation of echoes in the form of a signal

- a determination (21) of the signal to noise ratio - a detection (22) of the position and

The amplitude of the echoes

- a possible step of echo analysis followed by elimination of parasitic echoes (36)

- a deduction (24) of the time taken by the ultrasonic wave to cross the element, as a function of the signal collected

a step (25) for measuring the time intervals between echoes b) a measurement of the speed of the ultrasonic wave corresponding to each time measurement comprising:

- a weighting (26) of the first thickness echo

- a determination (28) of the coefficients of a filter (H) whose input is white noise and the output the first weighted thickness echo - a determination (29) of the response of the filter (H) as a function of frequencies requested

- a determination (30) of the position of the maximum peak (40) and of the pass band (41) of the filter - a correlation between the characteristics of said filter (H) and the speed of the ultrasonic wave in the element . c) an analysis of the signal as a function of time and speed so as to determine the distance between two surfaces (5, 5a, 7, 7a) delimiting the element (4, 4a) made of a material with a heterogeneous internal structure.

2. A device for measuring by ultrasound of an element (47) made of a material with a heterogeneous internal structure allowing the implementation of the method according to claim 1, said device of the type comprising a means (1) for emitting pulses. electrics connected to at least one translator (3) transforming the electrical pulses into ultrasonic waves, said device comprising a means (42) for supplying fluid coupling the ultrasonic waves to the element (47) of structure material heterogeneous internal, characterized in that it comprises a means of angular geometrical positioning of the translator making it possible to maintain said translator (3) normal to the external surface (48) of the element with a heterogeneous structure.

3.- Measuring device according to claim 2, characterized in that the angular geometric positioning means is a compass (43) comprising at least two arms (44a, 44b) respectively connected to the element (47) with internal structure heterogeneous by means of a pad (46a, 46b) and between them by means of displacement relative to normal of the surface of the element with variable internal structure, at identical speed for each arm.

4.- Device according to claim 3, characterized in that the means of displacement relative to normal of the surface of the element with variable internal structure, at identical speed for each arm, consists of at least one toothed sector ( 45) mounted on each arm, the toothed sectors being of identical diameter and pitch.

5.- Device according to claim 3, characterized in that the means of displacement relative to normal of the surface of the element with variable internal structure, at identical speed for each arm, consists of a threaded rod with equal pitch and inverted at each of its ends, each of said ends cooperating with an arm of the compass.

6.- Device according to one of claims 3 to 5, characterized in that a means of pressure and / or traction is applied to at least one arm between the displacement means and the pad of said arm.

7.- Device according to claim 6, characterized in that the pressure means is a spring (56) or a jack.

8.- Device according to one of claims 2 to 7, characterized in that the fluid (57) coupling the ultrasonic waves is water.

9.- Device according to one of claims 2 to 8, characterized in that a stop element (48) is mounted on the means (43) for angular positioning, said stop being able to avoid contact between 1 element of variable structure material and the fluid supply means or the transducer.

10.- Use of a device according to one of claims 2 to 9 for measuring the thickness of cast iron parts and / or determining the structure of the graphite of said cast iron.