EDDY CURRENT ARRAY PROBE AND METHOD FOR LIFT-OFF COMPENSATION DURING OPERATION WITHOUT KNOWN LIFT REFERENCES

Abstract

The invention provides a method for compensating the sensitivity variations induced by lift-off variations for an eddy current array probe. The invention uses the eddy current array probe coils in two separate ways to produce a first set of detection channels and a second set of lift-off measurement channels without the need to add coils dedicated to the lift-off measurement operation. Another aspect of the invention provides an improved calibration process which combines the detection and lift-off measurement channel calibration on a simple calibration block including a reference defect without the need of a pre-defined lift-off condition.

Claims

1. An EC (Eddy Current) system for detecting flaws in a test object, the system comprising: (a) an EC array probe configured with a sensor arrangement including: (i) a plurality of first type EC sensors arranged in channels and configured to induce eddy currents in the test object and to sense and output first signals representative of flaws in the test object; (ii) a plurality of second type EC sensors configured to produce from the test object second signals indicative of a lift-off distance of said orthogonal and absolute EC sensors relative to said test object, said EC coil arrangement being configured so that a pre-determined ratio is established between said second signals and said first signals, at different lift-off distances; (b) a setup table comprising calibration values for said orthogonal EC sensors with corresponding lift-off compensation values for said channels based on said second signals; and (c) an acquisition unit responsive to said calibration and lift-off compensation values in said setup table and to said second signals and configured to convert said first signals obtained from said orthogonal EC sensors during actual testing of said test object, so as to obtain third signals which are representative of said flows in said test object, said third signals being substantially independent of actual lift-off distances prevailing between said EC sensors and said test object at the time of obtaining said first signals doing said actual testing.

2. The system of claim 1, wherein the EC array probe is provided on a printed circuit board.

3. The system of claim 1, wherein said EC array probe comprises overlapping coils configured as driver and receiver coils.

4. The system of claim 2, wherein said plurality of first type EC sensors are configured to generate a first set of orthogonal channels that extend along a first line.

5. The system of claim 4, wherein said plurality of second type EC sensors are arranged so that absolute channels are arranged along at least one line, that extends parallel to said first line.

6. The system of claim 5, wherein at least one pair of said absolute channels physically sandwich said orthogonal channels.

7. The system of claim 6, wherein an average of two absolute, channel sensitive areas are used to obtain a lift-off value for a corresponding orthogonal channel.

8. The system of claim 1, wherein said absolute channels are located not to be in line with longitudinal or transversal test object cracks when said cracks are located on a sensitive area of an orthogonal channel.

9. The system of claim 1, wherein said acquisition unit is effective to drive the first type and second type channel sensors simultaneously.

10. The system of claim 1, wherein corresponding first type and second type channels use same sets of drive coils to enable faster acquisition and more stable signals.

11. The system of claim 3, wherein said driver coils are connected as part of an impedance bridge to realize the absolute channels.

12. The system of claim 1, wherein said first type sensors are orthogonal sensors and said second type sensors are absolute sensors.

13. The system of claim 1, wherein said system is configured as a differential Eddy Current array probe.

14. The system of claim 12, wherein said absolute EC sensors are formed of physical coils which also form said orthogonal EC sensors.

15. The system of claim 2, wherein said probes are arranged in at least four layers on said circuit board.

16. The system of claim 2, wherein said EC array probe comprises GMR sensors.

17. The system of claim 2, wherein said EC array probe comprises AMR sensors.

18. The system of claim 2, wherein said EC array probe comprises Hall Effect sensors.

19. A method for testing an object using an EC (Eddy Current) system, the method comprising the steps of: providing an EC array probe including: (i) a plurality of first type EC sensors arranged in a plurality of channels and configured to induce eddy currents in a test object and to output first signals representative of flaws in the test object; and (ii) a plurality of second type EC sensors arranged in channels and configured to produce from the test object second signals indicative of a lift-off distance of said first type and second type EC sensors relative to said test object, said EC coil arrangement being configured so that a substantially constant ratio is established between said second signals and said first signals, at different lift-off distances; performing a probe array system setup including storing at least a gain value on each orthogonal channel relative to a known calibration notch using the first type EC sensors; obtaining relative to each orthogonal channel an amplitude vector value by using said second type EC sensors; and storing said gain and amplitude vector values in a setup table.

20. The method of claim 19, further including performing a data acquisition procedure comprising: acquiring actual Eddy Current data for said first type channels and second type channels relative to said object, said data including raw orthogonal data and raw absolute data for each channel; calculating amplitude vector lengths; and compensating said raw orthogonal data for lift-off effects utilizing said absolute vector lengths and applying said calibration gain values to obtain third signal which are representative of flaws in said object and independent of said lift off distances.

21. The method of claim 20, wherein said system setup step includes setting a phase rotation value and a gain value at each orthogonal channel and calculating both gain and phase values at each position.

22. The method of claim 20, including acquiring said orthogonal and absolute data simultaneously during actual testing of said test object.

23. The method of claim 20, including driving said orthogonal and absolute EC sensor simultaneously.

24. The method of claim 20, wherein said amplitude vector values are between air and a calibration base line.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 illustrates a simplified representation of the prior art flat shaped orthogonal eddy current array probe built on a two layer printed circuit board

[0032] FIG. 2 is the four layer printed circuit board extension of the FIG. 1 representation and the illustration of the possible shape of the orthogonal channels

[0033] FIG. 3 illustrates the absolute channels created for the probe structure of FIG. 2

[0034] FIG. 4 shows that cracks affecting the orthogonal channels will not affect the corresponding absolute channel.

[0035] FIG. 5 shows the absolute and orthogonal sensitive area on the four layer probe of FIG. 2.

[0036] FIG. 6 illustrates the scan of a calibration block including a reference notch and a probe lift-off.

[0037] FIG. 7 is the impedance plane results obtain on the orthogonal channels from the scan of FIG. 6 block with four defined lift off condition.

[0038] FIG. 8 is the impedance plane results obtain on the absolute channels from the scan of FIG. 6 block with four defined lift off condition.

[0039] FIG. 9 proposes an amplitude based analysis of the test signals shown on FIG. 7 and FIG. 8.

[0040] FIG. 10 is a flowchart describing the proposed calibration method.

[0041] FIG. 11 is a flowchart describing the proposed signal processing method.

[0042] FIG. 12 shows a hardware configuration for a system of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0043] Related patent application Ser. No. 12/832,620 describes how to build an ECA probe on a printed circuit board. The contents of patent application Ser. Nos. 12/832,620 and of 12/847,074 are incorporated by reference herein. The structure presented in application Ser. No. 12/832,620 is disposed on two PCB layers. A simplified representation of such structure is shown on FIG. 1 for a four sensor orthogonal ECA probe 1 including five driver coils (2a to 2e) and five receiver coils (3a to 3e) resulting in four sensitive spots (4a to 4d) with orthogonal sensor response. The coil arrangement (such as 5 and 6) that generates orthogonal sensor response will be referenced to as orthogonal channels in this document.

[0044] As first stated in paragraph [0035] of the mentioned application, it is also possible to use the multi-layer capabilities of the printed circuit boards to increase the resolution of the orthogonal ECA probe. FIG. 2 illustrates a four layer version of the ECA probe 10 built using this principle. The two bottom layers (11a to 11e and 12a to 12e) are connected to driver signals while the two upper layers (13a to 13e and 14a to 14e) are connected to receiver signals in order to generate orthogonal channels such as 16, 17, 18 and 19. The bottom layer coils 11a to 11e operate with the top layer coils 14a to 14e to generate a first set of orthogonal channels while coils 12a to 12e operate with the coils 13a to 13e to generate a second set of orthogonal channels.

[0045] From the probe structure shown on FIG. 2, it is an object of the current invention to teach how to obtain absolute channels for monitoring the lift-off. FIG. 3 illustrates how these channels are built from the structure of probe 10. Some coils, partially overlapping the driver coils, are connected as receivers. For example, pitch-catch sensor configuration 36 uses two fourth layer coils (11b and 11c) as a driver and one second layer coil (13d) as receiver. In another example, pitch-catch configuration 33 uses two third layer coils (12b and 12c) as a driver and one fourth layer coil (11c) as a receiver. As a matter of fact, the same coil can be used as a driver and a receiver through the sequence (as already disclosed in prior art Pat. No. 6,344,739). Using coil combinations similar to 31, 32, 33, 34, 35 and 36 for the whole probe 10, we obtain a set of nine sensitive areas (30a to 30i) with absolute sensor response extending over the whole probe length. The coil arrangements (such as 31 to 36) that generate absolute sensor response will be referred to herein as absolute channels in this disclosure.

[0046] The newly created absolute channels are inherently very sensitive to lift off, because the proximity of the inspected part will directly impact the magnetic field flux in the shared area of the driver and receiver coils (11c and 13d for example) defining the sensitive area (30f for example) of the absolute channel (36 for example). FIG. 4 also demonstrates that the new absolute channel will not be sensitive to a longitudinal 41 or transversal 40 crack to be detected by the orthogonal channel because the absolute channel sensitive area (30f for example) is not in line with longitudinal or transverse crack when this crack is located on the orthogonal channel sensitive area (15d for example). So, the described structure makes it possible to substantially completely decouple the lift-off and crack measurements for this probe.

[0047] As further demonstrated on FIG. 5, each orthogonal channel sensitive area is located exactly in between two absolute channels sensitive areas on the index axis. So, we can use the average value between these two absolute channels to produce an approximation of the lift-off conditions for the corresponding orthogonal channel.

[0048] It must be understood that the selection of coils to be used in the absolute channel construction was made in order to acquire the orthogonal and absolute channels simultaneously and with a pitch-catch type configuration which is naturally more stable than an impedance bridge. For example, orthogonal channel 16 and absolute channel 36 use the same set of two driver coils 11b and 11c. So, these two channels can be acquired simultaneously by the acquisition electronics. This configuration is advantageous because it allows a faster acquisition (through simultaneous operation) and a stable signal, but it is not a mandatory requirement so there will be other possible arrangements respecting the essence of the invention.

[0049] Connecting the driver coils as part of an impedance bridge to build the absolute channels, for example, is another method to obtain a valid set of absolute channels for lift-off monitoring without adding new coils in the probe structure. It is also possible to envision other ECA probe types respecting the scope of this invention. For example, in U.S. Pat. No. 5,371,461 FIG. 3, one could dispose of the compensation coil 52 of U.S. Pat. No. 5,371,461 by connecting driver coil 42 of said patent through an impedance bridge.

[0050] Now that we have described means for building channels for detection (orthogonal channels in the preferred embodiment) and lift-off monitoring (absolute channels made out from a pitch-catch sensor arrangement in the preferred embodiment), we describe how these signals are processed in order to obtain a lift-off compensated eddy current probe array without the use of a lift-off reference.

[0051] As shown on FIG. 6, a reference block 50 comprising a long transversal reference notch 51 is scanned in direction 53 with a given lift-off 52. The probe is first nulled in AIR to generate a reference point for an infinite lift off condition, which will become important later in this discussion. The block 50 is then scanned four times with increasing lift off (in the example ; Lift-off A=0 mm ; Lift-off B=0.63 mm ; Lift-off C=1.27 mm ; Lift-off D=1.9 mm) to provide the required background information needed to describe the invention.

[0052] FIG. 7 shows the impact of lift-off on the reference defect detection amplitude on orthogonal channel 16 impedance plane display. In this case, defect amplitude 55 is obtained with lift-off A, defect amplitude 56 is obtained with lift-Off B, defect amplitude 57 is obtained with lift-off C and defect amplitude 58 is obtained with lift-off D.

[0053] FIG. 8 shows the impact of lift-off on absolute channel 36 impedance plane display. In this case, the total signal amplitude vector 60 results from lift-off A , the total signal amplitude vector 61 results from lift-off B, the total signal amplitude vector 62 results from lift-off C and the total signal amplitude vector 63 results from lift-off D. It is interesting to note that reference notch 51 generates very weak signals on FIG. 7 compared to the strong lift-off signal. For example, with lift A, defect amplitude 64 is orders of magnitude lower than the corresponding total signal amplitude vector 60 resulting from lift A. This is a desirable behavior since we want to use the absolute channels for lift-off monitoring only.

[0054] FIG. 9 shows a graph representing a combined view of the defect amplitude readings 55, 56, 57, 58 on the orthogonal channels and the total signal amplitude vector readings 60, 61, 62 and 63 on the absolute channels relative to the lift-off conditions. As seen in the figure, both data series can be fitted by exponential curves 70 and 71. Moreover, the shape of curves 70 and 71 (which is defined by the exponent) is almost the same (e.sup.0.6322*Lift vs. e.sup.0.6557*Lift in this example). This observation is very important because it means the ratio Ortho_Amplitude(Lift)/Abs_Vector(Lift) is almost independent of the lift. For example: 0.3834*e.sup.0.6557*Lift/2.3521 e.sup.0.6322*Lift=0.163*e.sup.0.0235*Lift. . . which is about 0.2 dB/mm variation compared to about 5.7 dB/mm for the orthogonal channel. This later observation forms the foundation of the signal processing method of the invention. For the following discussion we will approximate Ortho_Amplitude(Lift)/Abs_Vector(Lift) as being a constant, pre-determined value totally independent of lift-off. It must be understood that the use of the same coil set for defect detection and lift-off monitoring contributes to having similar shaped curves, since the shape of the curve is provided by the magnetic coupling between coils and the inspected part. Thus, by dynamically comparing the orthogonal and absolute amplitudes at each measured point (channel), the orthogonal amplitude can be connected for the actual prevailing lift-off during each measurement, without specific knowledge of the lift-off amount, per se.

[0055] We now turn our attention to FIG. 10, which describes how the probe is to be calibrated with a reference notch such as 51 but without a known reference lift-off. We first NULL the probe in AIR, (Step 102) start the acquisition (Step 104), scan the reference notch (Step 106) and define the notch position by manually or automatically indicating where the notch signal begins and ends (Step 108). At this point, the information we have is equivalent to the signals presented in FIG. 8 and FIG. 7 but for a single unknown lift-off. In other words, if notch 51 is scanned in the calibration process with lift-off B, the system should be able to read defect amplitude 56 and total amplitude vector 61 but the actual value of lift-off B will be unknown to both the acquisition system and the user.

[0056] The information available at this point is first used to calibrate the orthogonal channels by applying a calibration GAIN and ROTATION on the raw signal (Step 1010), in order to reach a pre-defined value for the reference defect 51. This pre-defined value (which typically includes both an angular and amplitude target) is common to all orthogonal channels and thus makes it possible to obtain a uniform detection of the reference defect 51 for all orthogonal channels. The calibration GAIN and ROTATION for each orthogonal channel is saved in the setup (Step 1012).

[0057] Simultaneously, we use the information generated in [0046] on the absolute channels to calculate the vector length between AIR and the signal's baseline obtained on the calibration block 50 (Step 1014). A single absolute vector length value (which could in fact be the average between two absolute channels or other absolute channel combinations adapted to the probe and application) is saved in the setup and associated with its corresponding orthogonal channel. For example, in probe 10, if we use absolute channels at position 30a and 30b to compensate the lift-off for the orthogonal channel at position 15a we could average absolute channels at position 30a and 30b and save this pre-determined value in the setup with reference to the channel at position 15a. This value will be referenced here as Absolute_RefLenght(n,Cal_Lift) where n is the orthogonal channel# identifier and Cal_Lift is the lift-off condition present during calibration (Step 1016).

[0058] Now looking at FIG. 11, we will use the information now included in the calibration file and the properties of the Ortho_Amplitude(Lift)/Abs_Vector(Lift) ratio to generate a lift compensated orthogonal channel. The process described on FIG. 11 is applied dynamically (during the acquisition) but could easily be applied in post-processing as well (after the acquisition). The first step of the process is, again, to NULL the probe in AIR (Step 112) in order to have an infinite lift-off reference. After starting the acquisition (Step 114), each new data set corresponding to the impedance plane results (x,y) for one given orthogonal channel at one given scan position is processed separately (Step 116). Such data set will be referenced here as Ortho_raw(n, Lift) where n is the orthogonal channel# identifier and Lift is the lift-off condition at the time of measuring the data set. The first step in processing is to find the absolute channel total vector length, at the current scan position, corresponding to the orthogonal channel currently being processed (Step 118). The relationship between the orthogonal and absolute channel must be the same as previously defined in calibration. This value will be referenced here as Absolute_VLenght(n, Lift) where n is the orthogonal channel# identifier and Lift is the lift-off condition at the time of measuring the data set.

[0059] Ortho_raw(n,Lift) is then processed with the following relationship to generate a lift-off compensated orthogonal channel reading; Ortho_compensated(n, Cal_Lift)=(Ortho_raw(n,Lift)/Absolute_Vlenght(n,Lift))*Absolute_RefLenght(n) (Step 1110). The generated Ortho_compensated(n,Cal_Lift) channel is then relatively independent of the current lift-off but is then dependent on the lift-off present during the system calibration. To remove this dependency and thus provide a completely lift off independent reading, the calibration GAIN and PHASE are applied to Ortho_compensated(n,Cal_Lift) (Step 1112), until all channels are so processed (Steps 1114, 1116 and 1118). As an end result, for a given flaw size, the system should generate a uniform defect signal amplitude no matter which orthogonal channel detects the flaw and without regard to the calibration and inspection lift-off

[0060] FIG. 12 shows a hardware configuration of a typical system that can implement the foregoing method. The subject EC probe array system comprises a processor 122 or acquisition unit which is operable and controlled through a user interface 124 and which can display test results, commands and the like, display 126. Orthogonal sensors 128, as well as absolute sensors or coils 129 interact, electromagnetically, with the test object 50 to obtain the various signals and to implement the methods described above via software program instructions stored or loaded onto the processor 122, in a manner well known in the art.

[0061] It is important to point out that the described lift compensation method can easily be adapted to operate a multi-frequency inspection. This can be done either by generating absolute and orthogonal channels for each frequency or by using a unique set of absolute channels to compensate the multi-frequency orthogonal channels.

[0062] It is also important to mention that while the figures and description describes an ECA probe with eight orthogonal sensors, the method proposed in this invention is applicable as long as the coil configuration makes it possible to build at least one sensor for defect detection and one sensor for lift-off measurement.

[0063] In the foregoing embodiments, the EC sensors have been described and depicted as being coil windings. However, as will be recognized by one of skill in the art, other types of magnetic field sensors can be used, such as, for example, GMR (Giant Magneto Resistance), AMR (Anisotropic Magneto Resistance), or Hall Effect sensors.

[0064] Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.