WO2006046859A1

WO2006046859A1 - Device for non-destructively examining an object

Info

Publication number: WO2006046859A1
Application number: PCT/NL2005/000760
Authority: WO
Inventors: Pieter Bestebreurtje
Original assignee: Sonimex Bv
Priority date: 2004-10-29
Filing date: 2005-10-24
Publication date: 2006-05-04
Also published as: EP1943477A1; NL1027373C2

Abstract

A device (100) is described for generating a measuring signal (M) being indicative for surface properties of an object (3) which is substantially manufactured of a ferromagnetic material (1). The device comprises: at least one magnet (20) with a magnet axis (21); two magnetic field sensors (30; 40) arranged at mutually different axial positions with respect to the said magnet axis (21); a control member (10) provided with inputs (12; 14) coupled with the magnetic field sensors (30; 40) for receiving the sensor signals (S1; S2) generated by these magnetic field sensors (30; 40), and provided with an output (19) for issuing the measuring signal (M). The control member (10) is adapted for generating the measuring signal (M) based on the sensor signals (Sl; S2) received from both magnetic field sensors (30; 40), according to the formula M = αSl - αβS2, wherein α and β are multiplication factors.

Description

DEVICE FOR NON-DESTRUCTIVELY EXAMINING AN OBJECT

The present invention relates in general to measuring layer thicknesses, although the present invention is also usable in other fields, for example in examining whether a material contains cracks. In particular, the present invention relates to non- destructively measuring the thickness of a non-ferromagnetic layer on a ferromagnetic material, for example iron, steel, etc. For example, the layer may be an oxide skin, a paint layer, a metal layer such as chrome, gold, etc. The invention will hereinafter be explained specifically for this exemplary application, but it is stated with emphasis that this should not be explained as a limitation of the scope of the invention.

For such layer thickness measurement, it is known to make use of an electromagnetic induction coil with which a magnetic field is induced in the object to be examined. With a magnetic field sensor, the strength of the magnetic reaction field is measured, which depends on the magnetic permeability of the material examined and of the possibly caused eddy currents. An example of the base principle of this measuring method is described in US patent 3.359.495, so that a thorough explanation of this measuring method can be omitted here. For as far as necessary, the content of said US patent is deemed to be incorporated here by reference. The measuring principle is based on the fact that the reaction field depends on the material properties of the object to be examined, and on the distance of the coil/sensor combination to the object to be examined. When the material properties of the object to be examined are considered constant, a change of a measuring signal corresponds to a change in distance, which, when the coil/sensor combination is pressed against the surface of the object to be examined, corresponds to a change in the thickness of the layer.

When it is desired to measure the thickness of the layer at different positions on the object to be examined, the problem arises that the material properties of the object to be examined are not the same all over: this causes measuring errors, because variations of those properties cause variations of the measuring signal that do not correspond to variations of the layer thickness. Further, the sensitivity of the sensor used may drift as function of the time and function of the temperature. Further, the measurement is sensitive to external magnetic fields, and variations therein (as function of location, time, temperature, etc.) again cause measuring errors. The German patent 101.45.657 describes a method to reduce such measuring errors. To that end, two measurements are performed, at two different current magnitudes of the coil current, and the two measuring results obtained therein are compared with each other. Temperature influences are compensated by measuring the electric resistance of the sensor, as measure for the temperature.

An important disadvantage of this measuring method, wherein the coil current is switched in order to obtain two measuring results that must be compared with each other, is that it is not possible to do a continuous measurement.

Performing a continuous measurement is desired in particular if one wants to quickly examine a large part of the surface of the object to be examined, for example if one wants to examine a rail for the presence of cracks. It is an object of the present invention to overcome or at least reduce said disadvantages.

In particular, the present invention aims at providing a measuring device that can be built in a compact way, can measure continuously, and provides precise and reproducible measuring results .

According to an important aspect of the present invention, a measuring device comprises a constant magnetic field and two magnetic field sensors, arranged at different positions, wherein the measuring signals obtained from both sensors are compared in order to eliminate variations in environmental factors.

These and other aspects, features and advantages of the present invention will be further explained by the following description with reference to the drawings, in which same reference numbers indicate same or similar parts, and in which: figure 1 schematically shows a measuring device according to the present invention; figure 2 is a block diagram schematically illustrating a preferred embodiment of a control member; figure 3 illustrates several construction details of a probe; figure 4 schematically illustrates a probe with pole shoes; figure 5 is a graph illustrating a measurement signal obtained, as function of the time.

Hereinafter, phrases like "left", "right", "bottom", "top" will be used taking into account the orientation given in the figures, wherein this should not be explained in a limiting way for the invention.

Figure 1 schematically shows a measuring device 100 according to the present invention for non-destructively examining an object 3. The object 3 comprises a ferromagnetic body 1 with a layer 2 of a non-ferromagnetic material on it.

By way of example, the body is a steel beam, the layer 2 is an oxide layer with a thickness D in the order of 0-10 μm, and the measuring device 100 is used to measure the thickness D of the layer 2. The measuring device 100 comprises a magnet 20, with an axis 21 which is directed substantially perpendicular to the surface to be examined. The magnet 20 may be implemented as a permanent magnet, or, as illustrated, as a magnet coil energized with a direct current; a combination is also possible. The axial length of the magnet 20 may be in the order of several millimetres. The magnet 20 generates a primary magnetic field that, inside the magnet, is substantially directed according to the axis 21. Outside the magnet, the exact shape of the magnetic field lines is influenced by the ferromagnetic body 1, in particular by the permeability thereof. The influence is strongest in a space near the magnet axis 21 and near the surface of the layer 2; in this space, the field lines are concentrated more due to the presence of the ferromagnetic body 1. The amount of influence depends inter alia on the distance between the magnet 20 and the ferromagnetic body 1: as this distance increases, the amount of influence decreases.

The measuring device 100 further comprises a first magnetic field sensor 30 and a second magnetic field sensor 40. Each magnetic field sensor 30, 40 may be implemented as a Hall-sensor, but other embodiments of the magnetic field sensor are also possible. In the present preferred example, the two magnetic field sensors 30, 40 are mutually identical Hall-sensors, but it is not essential that the sensors are mutually identical. Hereinafter, the magnetic field sensors will be indicated as Hall-sensors for the sake of convenience.

The position of each Hall-sensor 30, 40 relative to the magnet 20 is fixed. Preferably, each Hall-sensor 30, 40 is fastened to the magnet 20, for example by gluing, clamping, or the like, or for example because these parts are cast together in a synthetic material, a resin or the like (see also figure 3) . The combination of magnet 20 and both sensors 30, 40 will be indicated as probe 50. Preferably, the first Hall-sensor 30 is arranged at a position where the influence of the ferromagnetic body 1 is as large as possible. Preferably, the first Hall-sensor 30 is therefore arranged near the magnet axis 21, at the side of the magnet 20 directed toward the ferromagnetic body 1. Depending on the specific design, the first Hall-sensor 30 may be located at an axial distance below the magnet 20, closer to the object 3, as shown in figure 1. It is also possible that the magnet 20 is provided with an accommodation space for the first Hall-sensor 30 at its bottom end directed toward the object 3. In the case of a permanent magnet 20, however, it is preferred that the first Hall-sensor 30 is attached to the head end of the magnet 20. In the case of a magnet coil 20, the first Hall-sensor 30 may be arranged inside the interior space 23 enclosed by the coil windings 22, but preferably just outside the bottom end of the coil. In the case of a magnet coil 20, it is however preferred that the coil is provided with a coil core, as is known per se and not shown for the sake of simplicity; in that case, it is preferred that the first Hall-sensor 30 is attached to the head end of the coil core.

In principle, it is possible that the bottom end of the magnet 20 or the sensor 30 touches the top surface of the layer 2. However, it is preferred that between the layer 2 on the one hand and the magnet 20 (and the first sensor 30) on the other hand a wear plate or slide plate is arranged, which is also fastened to the magnet 20. Since such wear plates are known per se, this one is not shown for the sake of simplicity. The second Hall-sensor 40 is preferably arranged at the opposite side of the magnet 20, near the magnet axis 21. Depending on the specific design, the second Hall-sensor 40 may be located at an axial distance above the magnet 20, further away from the object 3, as shown in figure 1. In a similar way as mentioned regarding the first sensor 30, the second sensor may be arranged inside the magnet 20, in a special accommodation space or inside the interior space 23 enclosed by the coil windings 22, near the top end of the coil, preferably aligned with the first Hall-sensor 30. In the case of a permanent magnet 20, it is preferred that the second Hall-sensor 40 is attached to the upper head end of the magnet 20. In the case of a magnet coil 20, it is preferred that the coil is provided with a coil core, and that the second Hall- sensor 40 is attached to the head end of the coil core. The device 100 further comprises a control member 10 that may for example be implemented as a suitably programmed microprocessor. The control member 10 has inputs and outputs coupled with the probe 50. More particularly, the control member 10 has a first control output 11 for supplying a first operating current Cl to the first Hall-sensor 30, and a first measuring input 12 for receiving a first measuring voltage Sl from the first Hall-sensor 30. Further, the control member 10 has a second control output 13 for supplying a second operating current C2 to the second Hall-sensor 40, and a second measuring input 14 for receiving a second measuring voltage Sl from the second Hall-sensor 40. In the case of a magnet coil 20, the control member 10 may have a third control output 15 for supplying a third operating current C3 for the coil 20.

Further, the control member 10 has a measuring output 19, and is adapted, based on the two received sensor signals Sl and S2, to generate a measuring signal M at this measuring output 19, which is representative for the thickness D of the layer 2 in a reliable way.

The functioning is as follows.

In a stationary situation, each sensor 30 and 40 issues a sensor signal, the precise magnitude of which depends on the circumstances. If the permeability of the ferromagnetic body 1 changes, this has an almost equally large influence on both sensors. By a suitable processing of both sensor signals, it is possible to obtain two processed signals that are sensitive substantially to the same extent to variations in permeability; the difference between both processed signals is then substantially insensitive to variations in permeability. If the distance between the probe 50 and the object 3 changes, this has a fairly large influence on the shape of the magnetic field lines at the location of the first sensor 30 but a considerably less large influence on the shape of the magnetic field lines at the location of the second sensor 40; the difference between both processed signals is thus sensitive to such distance variations, which may for example arise as a result of variations in the thickness of the layer 2 when the probe 50 is in contact with (or is kept at constant distance to) the layer 2. The relation between this difference signal and the distance variations can be examined and stored in a calibration table or represented by a calibration line.

Preferably, the probe 50 is implemented in such a way that the two sensors 30 and 40 are thermally coupled well with each other. Possible temperature changes will then cause a change in the sensitivity in both sensors in a similar way. This means that the sensitivity for fluctuations in permeability remains mutually equal. Figure 2 is a block diagram schematically illustrating a preferred embodiment of the control member 10. In this preferred embodiment, the control member 10 comprises a first analogue amplifier 61 and a second analogue amplifier 62. The first sensor signal Sl is fed to an input of the first amplifier 61, and the second sensor signal S2 is fed to an input of the second amplifier 62. Both amplifiers 61 and 62 are mutually substantially identical (in particular, they have mutually substantially equal amplification factors α) , and are thermally coupled well to each other. Preferably, both amplifiers are part of a same semiconductor body.

The analogue output signal of the first amplifier 61 is converted into a digital signal Sl' by a first analogue/ digital converter 71. In the same way, the analogue output signal of the second amplifier 62 is converted into a digital signal by a second analogue/digital converter 72, which signal is subsequently multiplied by a factor β by a digital multiplier 73. The resulting digital signal S2'=αβS2 is subtracted from the first digital signal Sl'=αSl (or the other way around) in a digital subtracter 74, in order to issue the output signal M.

If desired, the control member 10 may be provided with a digital/analogue converter (not shown) in order to convert the output signal M to an analogue signal, but this is not shown for the sake of simplicity.

The value of the multiplication factor β is chosen such that the reactions of the two digital signals Sl' and S2' are mutually equal at changes in the permeability of the material 1; in formula form: δSl'/δP = δS2'/δP, in which P represents the permeability. Then, to a great extent, the output signal M is independent of the permeability P, of temperature variations, etc.

Figure 3 is a schematic cross section of the probe 50 illustrating a construction detail. The sensors 30 and 40 are pressed and/or glued on both sides against the magnet 20. The combination of magnet 20 with the sensors 30 and 40 is, at least partly, embedded in a moulding material 80, for example an epoxy or a paste, which material is electrically insulating and thermally conducting. Since such material is known per se, a further explanation thereof is not necessary. The moulding material has as a result that the parts of the probe 50 are held together, are protected, and that there is a good thermal coupling between the two sensors 30 and 40.

Figure 4 is a schematic cross section of another embodiment of the probe 50. A possible moulding material is not shown in figure 4. In this embodiment, a first pole shoe 91 is arranged between the magnet 20 and the first sensor 30, and a second pole shoe 92 is arranged between the magnet 20 and the second sensor 40. The pole shoes 91 and 92 are made of a well magnetizable material, for example soft iron, as known per se. The pole shoes 91 and 92 have a contour adapted to the contour of the magnet 20. In a suitable embodiment, the magnet 20 has a cylinder shape with a circular cross section, and the pole shoes 91 and 92 have the shape of circular discs with preferably mutually equal diameters. The pole shoes may have an axial dimension of several millimetres. Thanks to such pole shoes, there is a better coupling between the magnetic field of the magnet 20 and the ferromagnetic material 1.

Further, it is preferred that the second sensor 40 is provided with a closing plate 94 at its top surface directed away from the magnet 20, as is also illustrated in figure 4. The closing plate 94 is also made of a well magnetizable material, for example soft iron, and may be identical to the pole shoes 91 and 92. By the combination of the second pole shoe 92 and the closing plate 94, it is achieved that the field lines of the magnetic field of the magnet 20 concentrate themselves more in the second sensor 40, resulting in the second sensor 40 being sensitive to an even smaller extent to variations in the thickness D of the layer 2 and being sensitive to a larger extent to factors influencing the magnetic field. Further, the second sensor 40 is less sensitive to stray fields because of this. Over all, the value of the second sensor 40 as reference improves hereby because of this. The apparatus 100 can be calibrated by placing the probe 50 on an object being identical to the closing plate 94, so that the whole arrangement is symmetrical. The then resulting measuring signal M is indicated as zero-signal.

The present invention also provides a method for detecting cracks in the material 1. As explained in the said US patent 3.359.495, an alternating current can be passed through a magnet coil, generating an alternating magnetic field causing an eddy current to be generated in the material 1. This eddy current, in turn, generates a secondary magnetic field. The strength of the eddy current, and thus the strength of the secondary magnetic field, and thus the magnitude of the measuring signal, depends on the electrical conducting properties of the material 1. If a crack is present in the material, perpendicular to the material surface, the eddy current is broken up into two (or more) separate eddy currents, resulting in a much lower measuring signal. For generating such eddy currents, however, an alternating magnetic field with a fairly high frequency is needed, typically in the order of 0,5 MHz. The eddy currents typically occur close to the material surface, so that a measuring signal contains little information about the extent (depth) of the crack. Further, the known method is sensitive to surface defects such as local burnings, fluctuations in permeability, etc.

The present invention provides a crack detection method based on another measuring principle. In this context, use is made of a probe 50 as discussed in the preceding, which is displaced over the surface of the object 3, wherein the distance to the object 3 is kept constant. To that end, the probe 50 can be displaced over the surface of the object 3 in a sliding or rolling way. In a preferred embodiment, the probe 50 is mounted to a measuring train, at small distance above a rail, in order to trace cracks in that rail.

Figure 5 is a graph illustrating the obtained measuring signal M (vertical axis) as function of the time (horizontal axis) . When there are no cracks, the measuring signal M is almost constant in time, as the line part 5.1 illustrates. When the probe 50 approaches a crack, an edge effect occurs: the magnetic field lines have preference to remain in the ferromagnetic body 1, and can thus cross the crack only with difficulty or not at all. Because of this, the symmetry of the magnetic field lines is disturbed, and the number of magnetic field lines that the lower sensor 30 "feels" will reduce. This effect does not, or at best to a reduced extent, occur at the upper sensor 40. As a result, the measuring signal M decreases, as the line part 5.2 illustrates. When the probe 50 is precisely aligned with the crack, the magnetic field lines can "spread" themselves on both sides of the crack, so that the disturbance at the position of the lower sensor 30 is less and the measuring signal M is again somewhat larger, as the line part 5.6 illustrates. When the probe 50 moves away from the crack, the disturbance decreases and the measuring signal increases again, as the line part 5.3 illustrates. When the probe 50 is located outside the sphere of influence of the crack, the measuring signal M again remains almost constant in time, as the line part 5.5. illustrates. Thus, the measuring signal M shows a minimum 5.7, wherein the distance X of this minimum to the normal level 5.1, 5.5 depends on the depth of the crack concerned and the mutual velocity between probe and crack: in case of a small and/or shallow crack, the magnetic field can virtually bend around the crack, and the disturbance is less than in the case of a large and deep crack.

It will be clear to a person skilled in the art that the invention is not limited to the exemplary embodiments discussed in the preceding, but that several variations and modifications are possible within the protective scope of the invention as defined in the attached claims.

For example, it is possible that the magnet 20 comprises two or more magnet segments, arranged axially in each others extension. Further, it is possible that the closing plate 94 is a magnet.

In the preceding, with reference to figure 5, the reaction of the measuring signal M to the presence of a crack is explained. In this context, it is assumed that the measuring signal M is obtained by subtracting the second sensor signal from the first sensor signal. The other way around, it is also possible that the first sensor signal is subtracted from the second sensor signal, in which case the behaviour of the measuring signal M when approaching a crack is mirrored relative to the behaviour illustrated in figure 5, as will be clear to a person skilled in the art.

In the preceding, the present invention has been explained with reference to block diagrams illustrating functional blocks of the device according to the present invention. It should be clear that one or more of these functional blocks may be implemented in hardware, in which case the function of such functional blocks is performed by individual hardware components, but it is also possible that one or more of these functional block are implemented in software, so that the function of such a functional block is performed by one or more program lines of a computer program or by a programmable device such as a microprocessor, micro¬ controller, digital signal processor, etc.

Claims

1. Device (100) for generating a measuring signal (M) being indicative for surface properties of an object (3) which is substantially manufactured of a ferromagnetic material (1) , said device comprising: at least one constant magnet (20) with a magnet axis (21); two magnetic field sensors (30; 40), arranged at mutually different axial positions with respect to the said magnet axis

(21); a control member (10) provided with inputs (12; 14) coupled with the magnetic field sensors (30; 40) for receiving the sensor signals (Sl; S2) generated by these magnetic field sensors (30; 40), and provided with an output (19) for issuing the measuring signal (M) ; and wherein the control member 10 is adapted for generating the measuring signal (M) based on the sensor signals (Sl; S2) received from both magnetic field sensors (30; 40) .

2. Device according to claim 1, wherein the control member (10) is adapted to calculate the measuring signal (M) according to the formula

M = αSl - αβS2 wherein Sl and S2 are the sensor signals from both magnetic field sensors (30; 40) , and wherein α and β are multiplication factors.

3. Device according to claim 2, wherein the control member (10) comprises: a first analogue amplifier (61) with an amplification factor (α) which is coupled for receiving the first sensor signal (Sl) from the first magnetic field sensor (30) ; a second analogue amplifier (62) with an amplification factor (α) which is coupled for receiving the second sensor signal (S2) from the second magnetic field sensor (40) ; a first analogue/digital converter (71) adapted to convert the analogue output signal from the first amplifier (61) into a first digital signal (Sl'); a second analogue/digital converter (72) adapted to convert the analogue output signal from the second amplifier (62) into a second digital signal; a digital multiplier (73) adapted to digitally multiply the digital output signal from the second analogue/digital converter (72) by a factor (β) ; a digital subtracter (74) adapted to digitally subtract from each other the digital output signal (S2¹) from the second analogue/digital converter (72) and the digital output signal (Sl') from the first analogue/digital converter (71) in order to issue the output signal (M) .

4. Device according to claim 3, wherein the value of the multiplication factor (β) is chosen such that the reactions of the two digital signals (Sl') and (S2') are mutually substantially equal in case of changes in the permeability of the material (1) .

5. Device according to claim 3 or 4, wherein both analogue amplifiers (61 and 62) are mutually substantially identical.

6. Device according to any of the claims 3-5, wherein both analogue amplifiers (61 and 62) are thermally coupled well to each other.

7. Device according to any of the claims 3-6, wherein both analogue amplifiers (61 and 62) are part of a same semiconductor body.

8. Device according to any of the preceding claims, wherein the two magnetic field sensors (30; 40) are thermally coupled well to each other.

9. Device according to claim 8, wherein the combination of magnet (20) with the sensors (30 and 40) is, at least partly, embedded in a moulding material (80), for example an epoxy or a paste, which material is electrically insulating and thermally conducting.

10. Device according to any of the preceding claims, further comprising: a first pole shoe (91) arranged between the magnet (20) and the first sensor (30), and a second pole shoe (92) arranged between the magnet (20) and the second sensor (40); wherein the two pole shoes (91 and 92) are made of a well magnetizable material.

11. Device according to any of the preceding claims, wherein the second sensor (40), at its surface directed away from the magnet (20) , is provided with a closing plate (94) made of a well magnetizable material.

12. Device according to any of the preceding claims, wherein the measuring signal M is indicative for the thickness (D) of a non-ferromagnetic layer (2) on the ferromagnetic material (D •

13. Device according to any of the preceding claims, wherein the measuring signal (M) is indicative for the presence of a crack in the ferromagnetic material (1) .

14. Device according to any of the preceding claims, wherein the magnet (20) comprises a permanent magnet.

15. Device according to any of the preceding claims, wherein the magnet (20) comprises a magnet coil, with coil windings (22), and wherein the control member (10) has a control output (15) for supplying a direct current control signal (C3) for the coil (20) .

16. Device according to any of the preceding claims, wherein a magnetic field sensor (30; 40) comprises a Hall-sensor.

17. Device according to any of the preceding claims, wherein the two magnetic field sensors (30; 40) are mutually identical.

18. Device according to any of the preceding claims, wherein the two magnetic field sensors (30; 40) are arranged at the axial ends of the magnet (20) .

19. Device according to any of the preceding claims, wherein the magnet (20) is a permanent magnet, and wherein the two magnetic field sensors (30; 40) are attached to the axial ends of the magnet (20) .

20. Device according to any of the preceding claims, wherein the magnet (20) is a magnet coil with a core body, and wherein the two magnetic field sensors (30; 40) are attached to the axial ends of the core body.