1. A method for non destructive evaluation of defects in a metallic object by eddy currents, the method comprising the steps of emitting at least one alternating electromagnetic field at at least one first frequency fi in the neighbourhood of the metallic object and detecting through at least one magnetoresistive sensor a response signal constituted by a return electromagnetic field which is re-emitted by eddy currents induced by the alternating electromagnetic field in said metallic object,
characterized in that it comprises the further steps of driving said at least one magnetoresistive sensor by a current at a second frequency fc which is different from said first frequency fi, so that said at least one magnetoresistive sensor acts as an in situ modulator and filtering the response signal detected by said at least one magnetoresistive sensor to keep either the frequency sum (fi+fc) of said first and second frequencies or the frequency difference (fi\u2212fc) of said first and second frequencies before processing said response signal to extract eddy current information on defects in said metallic object (2).
2. A method according to claim 1, characterized in that said filtering step comprises filtering the response signal detected by said at least one magnetoresistive sensor to keep the frequency difference (fi\u2212fc) of said first and second frequencies before processing said response signal to extract eddy current information on defects in said metallic object.
3. A method according to claim 2, characterized in that the alternating electromagnetic field is emitted at a single first frequency (f1) which is higher than 100 kHz.
4. A method according to claim 3, characterized in that it comprises the steps of amplifying a voltage between first and second terminals of said magnetoresistive sensor, to obtain an amplified voltage and sending said amplified voltage to a signal input of a mixing and filtering system and mixing a frequency reference signal at said second frequency fc with a frequency reference signal at said first frequency f1 in a multiplier to obtain a product reference signal f1\u2212fc which is applied to a reference input of the same mixing and filtering system whose output is processed as an ordinary output signal of an eddy current testing method.
5. A method according to claim 1, characterized in that the at least one magnetoresistive sensor has a sensing axis which is placed orthogonally to the emitted alternating electromagnetic field.
6. A method according to claim 1, characterized in that the at least one magnetoresistive sensor has a sensing axis which is placed parallel to the emitted alternating electromagnetic field.
7. A method according to claim 1, characterized in that the response signal is detected through an array of sensors which are used as in-situ demodulators and are able to detect the different components of the return electromagnetic field which are due to the modification of the eddy currents by a defect.
8. A method according to claim 1, characterized in that said at least one alternating electromagnetic field is emitted in the neighbourhood of the metallic object at a set of different first frequencies (f1, f2) which are all different from said second frequency fc.
9. A method according to claim 8, characterized in that said filtering step comprises filtering the response signal detected by said at least one magnetoresistive sensor to keep the frequency differences (f1\u2212fc, f2\u2212fc) of said first and second frequencies before processing said response signal as a simple demodulated signal giving the useful signal created by the modification of the eddy currents by a defect.
10. A method according to claim 1, characterized in that said at least one magnetoresistive sensor has a non linear behaviour and the response signal detected by said at least one magnetoresistive sensor is filtered to keep either the frequency sum (fi+nfc) of said first frequency and n times the second frequency or the frequency difference (fi\u2212nfc) of said first frequency and n times the second frequency, where n is an integer, before processing said response signal to extract eddy current information or defects in said metallic object.
11. A device for non destructive evaluation of defects in a metallic object by eddy currents, comprising at least one field emitter for emitting at least one alternating electromagnetic field at at least one first frequency fi in the neighbourhood of the metallic object, and at least one magnetoresistive sensor for detecting a response signal constituted by a return electromagnetic field which is re-emitted by eddy currents induced by the alternating electromagnetic field in said metallic object, characterized in that it further comprises:
driving means for driving said at least one magnetoresistive sensor by a current at a second frequency fc which is different from said first frequency fi, so that said at least one magnetoresistive sensor acts as an in situ modulator,
detecting means for detecting between the terminals of the magnetoresistive sensor a response signal,
filtering means for filtering the response signal detected by said at least one magnetoresistive sensor to keep either the frequency sum (fi+fc) of said first and second frequencies or the frequency difference (fi\u2212fc) of said first and second frequencies, and
processing means for processing said filtered response signal and extract eddy current information on defects in said metallic object.
12. A device according to claim 11, characterized in that said detecting means comprises amplification means, means for detecting reference signals at said at least one first frequency fi and at said second frequency fc, multiplying means for mixing said at least one first frequency fi and said second frequency fc and at least a lock-in amplifier for detecting the frequency sum (fi+fc) of said first and second frequencies or the frequency difference (fi\u2212fc) of said first and second frequencies.
13. A device according to claim 11, characterized in that the magnetoresistive sensor comprises multiple contact points for voltage measurements.
14. A device according to claim 11, characterized in that the magnetoresistive sensor comprises an array of sensors.
15. A device according to claim 11, characterized in that the magnetoresistive sensor is a Hall effect sensor.
16. A device according to claim 11, characterized in that the magnetoresistive sensor is an anisotropic magnetoresistive sensor (AMR), a giant magnetoresistive sensor (GMR), a giant magnetoimpedance sensor (GMI) or a tunnel magnetoresistive sensor (TMR).
17. A device according to claim 11, characterized in that the said at least one field emitter is a planar coil.
18. A device according to claim 11, characterized in that the magnetoresistive sensor is built on a very thin silicon substrate.
19. A device according to claim 11, characterized in that the magnetoresistive sensor is built on a bevelled substrate.
20. A device according to claim 11, characterized in that the magnetoresistive sensor is built on a flexible substrate.
21. A device according to claim 16, characterized in that the magnetoresistive sensor has a yoke shape, in that the length (L) of the yoke and the length (l) of the lateral arms of the yoke each are at least three times the width (w) of the yoke, and in that the width (w) of the yoke is comprised between 2 \u03bcm and 12 \u03bcm.
22. A method according to claim 4, characterized in that the at least one magnetoresistive sensor has a sensing axis which is placed orthogonally to the emitted alternating electromagnetic field.
23. A method according to claim 4, characterized in that the at least one magnetoresistive sensor has a sensing axis which is placed parallel to the emitted alternating electromagnetic field.
24. A method according to claim 4, characterized in that the response signal is detected through an array of sensors which are used as in-situ demodulators and are able to detect the different components of the return electromagnetic field which are due to the modification of the eddy currents by a defect.
25. A method according to claim 4, characterized in that said at least one magnetoresistive sensor has a non linear behaviour and the response signal detected by said at least one magnetoresistive sensor is filtered to keep either the frequency sum (fi+nfc) of said first frequency and n times the second frequency or the frequency difference (fi\u2212nfc) of said first frequency and n times the second frequency, where n is an integer, before processing said response signal to extract eddy current information or defects in said metallic object.
26. A device according to claim 12, characterized in that the magnetoresistive sensor comprises multiple contact points for voltage measurements.
27. A device according to claim 12, characterized in that the magnetoresistive sensor comprises an array of sensors.
28. A device according to claim 12, characterized in that the magnetoresistive sensor is a Hall effect sensor.
29. A device according to claim 12, characterized in that the magnetoresistive sensor is an anisotropic magnetoresistive sensor (AMR), a giant magnetoresistive sensor (GMR), a giant magnetoimpedance sensor (GMI) or a tunnel magnetoresistive sensor (TMR).
30. A device according to claim 16, characterized in that the said at least one field emitter is a planar coil.
31. A device according to claim 17, characterized in that the magnetoresistive sensor is built on a very thin silicon substrate.
32. A device according to claim 17, characterized in that the magnetoresistive sensor is built on a bevelled substrate.
33. A device according to claim 17, characterized in that the magnetoresistive sensor is built on a flexible substrate.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1-9. (canceled)
10. A method for equalizing a butting structure on a flat-plate x-ray detector when taking an x-ray image with the flat-plate x-ray detector, comprising:
measuring the butting structure at the flat-plate x-ray detector;
determining a pixel quantity parameter of the butting structure at the flat-plate x-ray detector for an interpolation of a pixel data value; and
performing the interpolation of the pixel data value in the x-ray image taken with the flat-plate x-ray detector based on the pixel quantity parameter.
11. The method as claimed in claim 10, wherein the pixel quantity parameter is determined as a function of a row or a column of the x-ray image and the interpolation is performed as a function of the row or the column based on the pixel quantity parameter.
12. The method as claimed in claim 10, wherein the butting structure is measured using a backlight recording based on the x-ray image recorded on the flat-plate x-ray detector.
13. The method as claimed in claim 12, wherein the backlight recording is preprocessed and is subjected to a threshold value criterion to determine the pixel quantity parameter.
14. The method as claimed in claim 13, wherein the preprocessing includes an offset correction.
15. The method as claimed in claim 13, wherein the preprocessing includes an lowpass filtering.
16. The method as claimed in claim 12,
wherein a plurality of backlight recording are taken at a plurality of different backlight intensities which are assigned to a plurality of different x-ray light doses,
wherein the pixel quantity parameter is determined as a function of the x-ray light doses and the interpolation is performed in the x-ray image based on a selected x-ray light dose used for taking the x-ray image.
17. The method as claimed in claim 12,
wherein a plurality of backlight recordings are taken for a plurality of different mode settings of the flat-plate x-ray detector,
wherein the pixel quantity parameter is determined as a function of the mode settings and the interpolation is performed in the x-ray image based on a selected mode setting of the flat-plate x-ray detector when taking the x-ray image.
18. A method for examining a quality of a butting structure on a flat-plate x-ray detector when manufacturing the flat-plate x-ray detector, comprising:
butting a plurality of plates to the flat-plate x-ray detector;
making the flat-plate x-ray detector ready for taking a backlight recording of the flat-plate x-ray detector prior to link a scintillator to the flat-plate x-ray detector;
measuring the butting structure based on the backlight recording; and
examining the quality of the butting structure on the flat-plate x-ray detector.
19. An x-ray system, comprising:
an x-ray emitter;
a flat-plate x-ray detector consisting of a plurality of plates connected by a butting structure; and
a data processing unit for performing an interpolation in an x-ray image taken by the flat-plate x-ray detector,
wherein a pixel quantity parameter of the butting structure at the flat-plate x-ray detector is determined and the interpolation is performed with a variable number of pixels as a function of a row or a column of the x-ray image based on the determined pixel quantity parameter.