Device for non-destructively examining an object

Method and device for non-destructive examination of an object

The present invention generally relates to the measurement of layer thicknesses, although the present invention is also useful in other fields, for example, in investigating whether a material contains cracks.

In particular, the present invention relates to the non-destructive measurement of the thickness of a non-ferromagnetic layer on a ferromagnetic material, for example, iron, steel, etc. The layer can be, for example, an oxide skin, a paint layer, a metal layer. such as chrome, gold, etc. The invention will be explained below specifically for this application example, but it is emphatically stated that this should not be construed as a limitation of the scope of the invention.

It is known to use an electromagnetic induction coil for such a layer thickness measurement with which a magnetic field is induced in the object to be examined. A magnetic field sensor measures the strength of the magnetic reaction field, which depends on the magnetic permeability of the material under investigation and on any eddy currents caused. An example of the basic principle of this measurement method is described in US patent 3,359,495, so that an in-depth explanation of this measurement method can be omitted here. To the extent necessary, the contents of said US patent are hereby incorporated 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 from the coil / sensor combination to the object to be examined. If the material properties of the object to be examined are assumed to be constant, a change in a measurement 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 everywhere: this causes measurement errors because variations of those properties cause variations of the measurement signal that do not correspond to variations of the layer thickness. Furthermore, the sensitivity of the sensor used may drift as a function of time and as a function of temperature. Furthermore, the measurement is sensitive to external magnetic fields, and variations in it (as a function of location, time, temperature, etc.) cause measurement errors.

The German patent 101.45.657 describes a method for reducing such measurement errors. To this end, two measurements are taken at two different current intensities of the coil current, and the two measurement results thereby obtained are compared with each other. Temperature influences are compensated by measuring the electrical resistance of the sensor, as a measure of the temperature.

A major drawback of this measurement method, in which the coil current is switched to obtain two measurement results that must be compared with each other, is that it is not possible to make a continuous measurement. A continuous measurement is particularly desirable 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 contemplates providing a measuring device that can be built compactly, can measure continuously, and provides accurate and reproducible measurement 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, the measurement signals obtained from both sensors being compared to eliminate variations in environmental factors.

These and other aspects, features and advantages of the present invention will be further elucidated by the following description with reference to the drawings, in which like reference numerals indicate like 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 controller; Figure 3 illustrates some 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 a function of time.

In the following, terms such as left, right, bottom, top will be used taking into account the orientation given in the figures, without this being to be construed as limiting the invention.

Figure 1 shows schematically a measuring device 100 according to the present invention for the non-destructive examination of an object 3. The object 3 comprises a ferromagnetic body 1 with a layer 2 of a non-ferromagnetic material thereon. By way of example, the body is a steel beam, the layer 2 is an oxide layer with a thickness D of the order of 0-10 μπι, and the measuring device

100 used to measure the thickness D of the layer 2.

The measuring device 100 comprises a magnet 20 with a shaft 21 which is directed substantially perpendicular to the surface to be examined. The magnet 20 can be in the form of a permanent magnet, or, as illustrated, as a direct-current magnet coil; a combination is also possible. The axial length of the magnet 20 can be in the order of a few millimeters. The magnet 20 generates a primary magnetic field which, within the magnet, is substantially oriented along the axis 21. Outside the magnet, the exact shape of the magnetic field lines is influenced by the ferromagnetic body 1, in particular by its permeability. The influence is strongest in a space near the magnet axis 21 and near the surface of the layer 2; due to the presence of the ferromagnetic body 1, the field lines in that space become more concentrated. The degree of influence is dependent on, among other things, the distance between the magnet 20 and the ferromagnetic body 1: as that distance becomes larger, the degree 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 can be designed 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 be mutually identical. In the following, the magnetic field sensors will be referred to for convenience as Hall sensors.

The position of each Hall sensor 30, 40 relative to the magnet 20 is fixed. Preferably, each Hall sensor 30, 40 is attached to the magnet 20, for example by gluing, clamping, or the like, or for example because these parts are jointly cast in a plastic, a resin or the like (see also figure 3). The combination of magnet 20 and both sensors 30, 40 will be referred to as probe 50.

The first Hall sensor 30 is preferably arranged at a position where the influence of the ferromagnetic body 1 is as great as possible. Preferably, the first Hall sensor 30 is therefore arranged near the magnet axis 21, on the side of the magnet 20 facing the ferromagnetic body 1. Depending on the specific design, the first Hall sensor 30 can 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 at its lower end facing the object 3 with a receiving space for the first Hall sensor 30. In the case of a permanent magnet 20, however, it is preferred that the first Hall1027373 sensor 30 is disposed against the head end of the magnet 20. In the case of a magnetic coil 20, the first Hall sensor 30 can be arranged within the inner space 23 enclosed by the coil windings 22, but more preferably just outside the lower end of the coil. In the case of a magnetic coil 20, however, it is preferred that the coil be provided with a coil core, as known per se and not shown for the sake of simplicity; in that case, it is preferable that the first Hall sensor 30 is mounted against the head end of the coil core.

In principle, it is possible that the lower end of the magnet 20 or the sensor 30 touches the upper 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 sliding plate is arranged, which is also fixed to the magnet 20. Since such wear plates are known per se, this is not shown for the sake of simplicity.

The second Hall sensor 40 is preferably arranged on the opposite side of the magnet 20, near the magnet axis 21. Depending on the specific design, the second Hall sensor 40 can be located at an axial distance above the magnet 20, further away from the object 3, as shown in figure 1.

In a similar manner as mentioned with respect to the first sensor 30, the second sensor can be arranged within the magnet 20, in a special receiving space or within the inner space 23 enclosed by the coil windings 22, near the upper 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 disposed against the upper end end of the magnet 20. In the case of a magnet coil 20, it is preferred that the coil be provided with a coil core, and that the first Hall sensor 30 is mounted against the end face of the coil core.

The device 100 further comprises a control member 10, which may, for example, be designed as a suitably programmed microprocessor. The controller 10 has inputs and outputs coupled to the probe 50. More specifically, the controller 10 has a first control output 11 for supplying a first operating current C1 to the first Hall sensor 30, and a first meter input 12 for receiving a first measuring voltage S1 of the first Hall5 sensor 30. Furthermore, the controller 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 S1 from the second Hall sensor 40. In the case of a magnetic coil 20, the control member 10 may have a third control output 15 for supplying a third operating current C3 for the coil 20.

The control member 10 also has a measuring output 19, and is adapted to generate, on the basis of the two received sensor signals S1 and S2, a measuring signal M at that measuring output 19 which is reliably representative of the thickness D of the layer 2.

The operation is as follows.

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

The probe 50 is preferably embodied such that the two sensors 30 and 40 are thermally well coupled to each other. Any temperature changes will then cause similar changes in sensitivity in both sensors. This means that the sensitivity to fluctuations in permeability remains the same.

Fig. 2 is a block diagram schematically illustrating a preferred embodiment of the controller 10. In this preferred embodiment, the controller 10 comprises a first analog amplifier 61 and a second analog amplifier 62. The first sensor signal S1 is applied to an input of the first amplifier 61, and the second sensor signal S2 is applied to an input of the second amplifier 62. Both amplifiers 61 and 62 are substantially identical to each other (in particular they have mutually substantially the same gain factors), and are coupled to each other thermally well. Both amplifiers are preferably part of the same semiconductor body.

The analog output signal of the first amplifier 61 is converted by a first analog-to-digital converter 71 into a digital signal S1 '. Similarly, the analogue output signal of the second amplifier 62 is converted by a second analog / digital converter 72 into a digital signal, which is then multiplied by a digital multiplier 73 by a factor β. The resulting digital signal S2 '= a / 3S2 is subtracted in a digital subtractor 74 from the first digital signal S1' = aS1 (or vice versa), to provide the output signal M.

If desired, the controller 10 may be provided with a digital / analog converter (not shown) to convert the output signal M to an analog 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 S1 'and S2' are mutually equal with changes in the permeability of the material 1; in formula form: ÖSI '/ ÖP = ÖS2' / ÖP, where P represents the permeability. The output signal is M

1027373δ then to a good extent independent of the permeability P, of temperature variations, etc.

Figure 3 is a schematic cross section of the probe

50 which illustrates a construction detail. The sensors 30 and are pressed and / or glued against the magnet 20 on both sides. The combination of magnet 20 with sensors 30 and 40 is, at least partially, embedded in a casting mass 80, for example an epoxy or a paste, which mass is electrically insulating and thermally conductive. Since such mass is known per se, a further explanation thereof is not necessary. The casting mass has the consequence that the parts of the probe 50 are held together, 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 casting mass 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 magnetizable material, for example soft iron, as is known per se. The pole shoes 91 and 92 have a contour that is adapted to the contour of the magnet 20. In a suitable embodiment, the magnet 20 has a cylindrical shape with a circular transverse contour, and the pole shoes 91 and 92 have the shape of circular-round discs with preferably mutually equal diameters. The pole shoes can have an axial dimension of a few millimeters. Thanks to such pole shoes, there is a better coupling between the magnetic field of the magnet 20 and the ferro-magnetic material 1.

Furthermore, it is preferable that the second sensor 40 is provided on its upper surface facing away from the magnet 20 with a closing plate 94, as also illustrated in Figure 4. The closing plate 94 is also made of a good magnetizable material, for example soft iron, and can

10273739 are identical to the pole shoes 91 and 92. The combination of the second pole shoe 92 and the closing plate 94 ensures that the field lines of the magnetic field of the magnet 20 concentrate more in the second sensor 40, so that the second sensor 40 is still is less sensitive to variations in the thickness D of the layer 2 and is more sensitive to factors that influence the magnetic field. Furthermore, the second sensor 40 is therefore less sensitive to stray fields. All in all, this improves the value of the second sensor 40 as a reference.

The device 100 can be calibrated by placing the probe 50 on an object identical to the closure plate 94 so that the entire arrangement is symmetrical. The resulting measuring signal M is referred to as a 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 magnetic coil, which generates an alternating magnetic field, whereby an eddy current is generated in the material 1 cheerful. This eddy current in turn generates a secondary magnetic field. The strength of the eddy current, and therefore the strength of the secondary magnetic field, and therefore the magnitude of the measurement signal, depends on the electrical conductivity properties of the material 1. If there is a crack 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 measurement signal.

However, the generation of such eddy currents requires an alternating magnetic field with a fairly high frequency, typically in the order of 0.5 MHz. The eddy currents typically occur close to the material surface, so that a measurement signal contains little information about the extent (depth) of the crack. Furthermore, the known method is sensitive to surface defects such as local burns, fluctuations in permeability, etc.

102737310

The present invention provides a crack detection method based on a different measuring principle. Use is herein made of a probe 50 as discussed above, which is moved over the surface of the object 3, the distance to the object 3 being kept constant. To this end, the probe 50 can be moved slidingly over the surface of the object 3, or rolling. In a preferred embodiment, the probe 50 is mounted on a measurement train, a small distance above a rail, to detect cracks in that rail.

Figure 5 is a graph illustrating the obtained measurement signal M (vertical axis) as a function of time (horizontal axis). When there are no cracks, the measurement signal M is almost constant over time, such as the line portion

5.1 illustrates. When the probe 50 approaches a crack, an edge effect occurs: the magnetic field lines preferentially remain in the ferromagnetic body 1, and can therefore hardly or not cross the crack. This disrupts the symmetry of the magnetic field lines, and the number of magnetic field lines that the lower sensor 30 feels will decrease. This effect does not occur, or at most to a lesser extent, at the upper sensor 40. Consequently, the measurement signal M decreases, as the line section 5.2 illustrates. When the probe 50 is precisely aligned with the crack, the magnetic field lines can spread on both sides of the crack, so that the disturbance at the lower sensor 30 is less and the measurement signal M is again somewhat larger, as the line portion 5.6 illustrates. . As the probe 50 moves away from the crack, the disturbance decreases and the measurement signal increases again, as the line portion 5.3 illustrates. When the probe 50 is outside the sphere of influence of the crack, the measurement signal M again remains virtually constant over time, as the line portion 5.5 illustrates. The measuring signal M thus has a minimum 5.7, the distance X from this minimum to the normal level

5.1, 5.5 depends on the depth of the relevant crack and the mutual speed between probe and crack: with a small and / or shallow crack, the magnetic field can, as it were,

102737j35 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 above, but that various variants and modifications are possible within the scope of the invention as defined in the appended claims.

For example, it is possible for the magnet 20 to comprise two or more magnet segments arranged axially in line with each other. Furthermore, it is possible that the closing plate 94 is a magnet.

In the foregoing, with reference to Figure 5, the response of the measurement signal M to the presence of a crack has been explained. It is assumed that the measurement signal M is obtained by subtracting the second sensor signal from the first sensor signal. Conversely, it is also possible for the first sensor signal to be subtracted from the second sensor signal, in which case the behavior of the measurement signal M upon approaching a crack is mirrored with respect to the behavior illustrated in Figure 5, as for a person skilled in the art will be clear.

In the foregoing, the present invention has been explained with reference to block diagrams illustrating functional blocks of the device according to the present invention. It will be clear that one or more of these functional blocks can be implemented in hardware, the function of such functional blocks being performed by individual hardware components, but it is also possible to implement one or more of these functional blocks in software, such 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, microcontroller, digital signal processor, etc.