System and method for normalizing and calibrating a sensor array

The invention provides a system and method for normalizing and calibrating a sensor array. The sensor array can comprise differential element sensors, such as for example eddy current sensors, or absolute sensors. A single test specimen is used to normalize and calibrate the sensor array using one or more scans of the test specimen. Notably, only one alignment of the sensor array to the test specimen is required. The test specimen is preferably made of the same or similar type of material as the part to be tested and is of a similar geometric shape that can have a simple flat surface or a more complex surface. A linear feature and several notches are machined into the surface of the specimen by using, for example, electro-discharge machining methods, to provide the necessary signals when scanned by the sensor array. Signals from the linear feature on the test specimen are used to remove any bias and to normalize the dynamic ranges of all of the sensors in the array. Signals from the notches are used to establish the gain settings for the sensors in the array.

BACKGROUND OF THE INVENTION
 The present invention relates generally to sensor arrays and more
 particularly to normalizing and calibrating a sensor array.
 An array of sensors can be used to detect and/or measure the dimension of
 defects in metal components and parts. In this regard, each sensor in the
 array is capable of producing an electrical signal indicative of a defect
 in a metal component or part. Multiple sensors can be used together as an
 array to scan an area of the component or part that is larger than if a
 single sensor was used. However, in order to accurately detect and/or
 measure the defect, it is important that all of the sensors in the array
 react in the same way (i.e., produce the same electrical signal) to the
 same defect. For this to occur, the sensors in the array should all have
 the same dynamic range and respond identically in signal amplitude to the
 same defect.
 To obtain uniform reaction from sensors, the sensors are normalized and
 calibrated. For example, flexible eddy current array sensors used to
 inspect aircraft engine components have been previously normalized and
 calibrated using two separate specimens, one to normalize the signal from
 all sensor elements and the other to calibrate the element signal level.
 For normalization, all elements are scanned across a linear feature. A
 correction factor and offset is calculated for each element from its
 signal level and saved for use during later testing of a part. For
 calibration, one of the sensor elements is used to scan a single notch on
 a test specimen in two dimensions. This process can be time consuming
 because a significant amount of time is required to properly align the
 sensor array to each test specimen. In this regard, the sensor needs to be
 aligned with the test specimen at minimum two times. Furthermore, the
 two-dimensional scan is done at a high spatial resolution. Moreover, the
 result's accuracy is limited because the data are obtained using a test
 specimen that may not match the geometric shape of the part to be
 inspected so the part test conditions may not match the calibration
 conditions. Thus, the data may not sufficiently represent the scanning
 conditions of a sensor array used to scan components or parts having
 complex surfaces.
 SUMMARY OF THE INVENTION
 Thus, there is a need for a faster, simpler, and more accurate system and
 method to normalize and calibrate sensor arrays. The present invention is
 a system, test specimen and method for normalizing and calibrating an
 array of sensors using one or more scans of a single test specimen.
 Notably, only one alignment of the sensor to the test specimen is
 required. The test specimen is preferably made of the same or similar type
 of material and is of a similar geometric shape (e.g., a simple flat
 surface or a more complex surface) as the metal component or part to be
 scanned for defects. The test specimen further comprises at least one
 linear feature and multiple notches that are machined into the surface of
 the test specimen. Both the linear feature and the notches cause the
 sensors in the array to produce electrical signals that are used to
 normalize and calibrate the sensor arrays. The method uses the electrical
 signal trace produced by each sensor in the array and caused by the
 respective sensor detecting the linear feature on the test specimen during
 scanning to remove any bias and to normalize the dynamic range of each
 sensor in the array. The method further uses the electrical signal trace
 produced by particular sensors in the array and caused by the respective
 sensor detecting one or more of the multiple notches of the test specimen
 during scanning to establish the gain settings for and calibrate each
 sensor in the sensor array.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 shows a flow chart that sets forth the steps for calibrating and
 normalizing an array 50 of eddy current sensors 60 shown in FIGS. 2 and 4
 according to one embodiment of the present invention. Each of the eddy
 current sensors 60 comprise a pair of differentially-connected elements
 58, 59, i.e., dual coils. It should be noted, however, that the present
 invention is equally applicable to sensors comprising absolute-type
 elements, i.e., single coils.
 Initially, a test specimen is selected. The test specimen will be scanned
 by the sensor array 50 that is to be normalized and calibrated. The test
 specimen preferably comprises the same or similar material and is of a
 similar geometric surface shape as the metal component or part to be
 scanned for defects. In some instances, to simplify fabrication, the test
 specimen can comprise multiple pieces of material adhered together. FIG. 2
 shows a top view of a test specimen 10 having a flat surface. FIG. 3 shows
 a perspective view of a test specimen 18 having a complex surface. FIG. 4
 shows a top view of the test specimen 18 with the interior surface of the
 test specimen 18 flattened out in two dimensions.
 The test specimens comprise at least one linear feature and multiple
 notches 14 that are machined into the surface of the test specimen, using
 for example known electro-discharge machining methods. For some
 applications, the linear feature has a narrow width of about 0.003 inches
 (three mils) and is sufficiently deep to be considered infinitely deep by
 the differential elements of the sensors 60 (i.e., the signal amplitude
 does not increase if the notch 14 is even deeper), although these
 measurements may vary according to the application. For all applications,
 the length of the linear feature or combination of linear features should
 cover the scanning area of the sensor array 50 such that all of the
 sensors 60 produce a signal as a result of detecting the linear feature.
 For some applications, the notches 14 are about fifteen mils deep by about
 thirty mils long by about three mils (0.015".times.0.030".times.0.003"),
 although these measurements may vary according to the application. For all
 applications, the notches 14 should be positioned in the direction of the
 scanning in such a way that an element can only pass over one notch at the
 same time. The notches should be separated by about 3/4 of the element's
 width in the horizontal direction, though other separations may also be
 used. Referring to FIG. 2, the test specimen 10 comprises a linear feature
 12 and notches 14. Referring to FIG. 4, the test specimen 18 comprises
 multiple linear feature s 20, 21 and 22 and notches 14.
 Next, the eddy current sensor array 50, which is shown in FIGS. 2 and 4
 having two staggered rows 56 of sensors 60, is aligned to the previously
 selected test specimen at 30. Referring to FIG. 2, the sensor array 50 is
 aligned to the test specimen such that (i) the linear feature covers the
 scanning area of the sensor array 50; and (ii) only one sensor 54 from the
 first row of the sensor array 50 and only one sensor 52 from the second
 row of the sensor array 50 pass over the notches 14. Notably, only one
 alignment of the sensor array 50 to the test specimen is necessary.
 Next, an initial system phase angle and gain setting are selected, and a
 scan of the test specimen is made at 31. The scan of the test specimen is
 taken in the direction 57. The phase angle and gain setting are selected
 such that the electrical signals produced by the sensors 60 are not
 saturated when the sensor array 50 is used to scan the test specimen. For
 example, the phase angle and gain setting can be selected using data
 obtained from prior experience using the sensor array 50. Alternatively,
 the phase angle may be selected to optimize electrical signal amplitude in
 only one channel. The selected phase angle and gain setting produce a set
 of traces, one from each sensor 60 in the array 50 when a scan of the test
 specimen is taken. By way of example, a trace 51 for sensor 52 and trace
 53 for sensor 54 are shown in FIG. 2. The scanning path 62 for sensor 52
 and the scanning path 64 for sensor 54 are also shown in FIG. 2.
 Next, a dynamic range for each sensor in the array 50 is determined at 32
 from the maximum and minimum amplitude of the electrical signals produced
 by each sensor 60 in the array 50 as a result of detecting the linear
 feature. The dynamic range is a reflection of sensor to surface fit and of
 sensor element fabrication accuracy. The DC bias for each sensor 60 is
 also determined at 32 from the offset of the electrical signals produced
 by each sensor 60 in the array 50. Referring to FIG. 2, the test specimen
 10 comprises the linear feature 12 which spans an area wider than the
 inspection width of the array of sensors 50, and is slanted relative to
 the scanning direction 56 of the sensor array 50. The linear feature 12
 enables the differential components of each sensor 60 to be normalized
 using the electrical signals produced by each sensor in the array 50. As a
 result, the electrical signals produced comprise both maximum and minimum
 amplitude, as shown by the traces in FIG. 2. The width 70 of the signal
 produced relates to the width of the differential element (coils 58 and
 59) and the angle of the linear feature with respect to the element. It
 should be noted that the linear feature 12 is not perpendicular to the
 scanning direction of the sensor array 50. If the linear feature 12 were
 positioned perpendicular to the scanning direction of the sensor array 50,
 it would not be possible to normalize the array of sensors 50 because the
 electrical signals produced by the differential elements 58, 59 of each
 sensor 60 would cancel each other as the sensor 60 passed over the linear
 feature 12. If the array of sensors comprise sensors having absolute
 elements, the linear feature may be positioned perpendicular to the
 scanning direction of the sensor array.
 FIG. 4 shows a test specimen 18 comprising a first linear feature 20, a
 second linear feature 21, and a third linear feature 22, to allow for a
 more accurate measurement of the dynamic range and DC bias in metal
 components and parts having complex surfaces. In this regard, it may be
 difficult to fabricate a single linear feature into a part with a complex
 surface geometry. Consequently, multiple overlapping segments of linear
 features may be fabricated to provide the same function of a single linear
 feature. The multiple linear features cover the complete inspection width
 of the sensor array 50. Referencing FIG. 3, the first linear feature 20 is
 positioned such that the end 23 of the first linear feature 20 overlaps
 with the end 25 of the second linear feature 21, and the end 26 of the
 second linear feature 21 overlaps with the end 27 of the third linear
 feature 22. The overlap compensates for spatial differences between the
 sensor elements 58, 59 and the portion of the surface of the test specimen
 18 being scanned. Next, the DC bias is removed from each signal trace and
 all of the trace signals are normalized to the same dynamic range at 33.
 Next, the maximum signal produced by a sensor 60 as a result of detecting a
 notch 14 is calculated from the notch signals at 34 by choosing either the
 maximum signal or by using some signal combination from sensors 60 that
 are adjacent overlapping elements in the two staggered rows that both pass
 over the notch 14 to sensors 52, 54 to approximate the maximum response.
 Referring to FIGS. 2 and 4, the test specimens 10, 18 comprise multiple
 notches 14, which are perpendicularly oriented to the array of sensors 50
 and offset from each other such that the ends of the notches do not
 overlap. The number of notches 14 and the horizontal offsets between the
 notches 14 are determined based on the geometry and spatial response
 function of a typical sensor 60 to ensure that a peak response can be
 calculated from the acquired data. The more uniform the sensor's 60
 response along the width of the sensor 60, the fewer notches that are
 needed. As shown in FIGS. 2 and 4, the array of sensors 50 comprises two
 staggered rows of eddy current sensors 60. As specifically shown in FIG.
 2, the notches 14 are positioned on the test specimen 10 such that the
 notches 14 fall into the scanning path of only one sensor from the first
 row of sensors and only one sensor from the second row of sensors. By way
 of example, a trace 51 for sensor 52 and trace 53 for sensor 54 are shown
 in FIG. 2. The scanning path 62 for sensor 52 and the scanning path 64 for
 sensor 54 are also shown in FIG. 2. Similarly, in FIG. 4, the notches 14
 are positioned on the test specimen 18 such that the notches 14 fall into
 the scanning path of only one sensor from the first row of sensors and
 only one sensor from the second row of sensors. As a consequence, the
 electrical signal produced by the sensors 52, 54 are used to establish the
 system gain.
 The electrical signals produced by the sensors 52, 54 as a result of
 detecting the notches 14 are next used to improve the maximum signal
 estimate, and the required system gain is then established from the
 maximum signal estimate at 35. If the gain for the sensors 60 that was
 used to normalize is not high enough to give satisfactory signals from the
 sensors 60 for the notches 14, then a second scan may be taken at a higher
 system gain (which now may result in saturated signals from the linear
 feature) to better evaluate the signals produced as result of sensors 52,
 54 detecting the notches 14. Lower notch signal amplitudes using a single
 scan may result in the need for enhancement type signal processing to
 analyze the notch responses, but with two scans this should not be
 required. The normalization/calibration process is then completed and the
 sensor array 50 can be used for testing parts with appropriate settings,
 as determined by the normalization/calibration process, for reliable
 defect detection or other measurements.
 Additional scans may be taken to determine optimum phase angles for one or
 both of the linear feature and the notches. Alternatively, the phase angle
 may be known a priori for a given system/sensor combination and used
 directly. The order of the normalization and calibration steps may be
 interchanged if the test specimen is a simple, flat surface. However, if
 the surface is complex, the sensor array 50 may not conform identically
 across the surface of the test specimen and the normalization step should
 preferably be done first to produce best results.
 The foregoing has been described for eddy current array sensors, but a
 similar normalization/calibration method can also be applied to other
 array sensors (e.g., thermocouple, ultrasound or digital x-ray arrays). In
 addition, different algorithms for analyzing and/or combining the data can
 be developed within the scope of this invention. Variations on the test
 specimen design for dual-use is also within the scope of this invention.
 It is therefore apparent that there has been provided in accordance with
 the present invention, a system, test specimen and method for normalizing
 and calibrating an eddy current sensor array that fully satisfy the aims
 and advantages and objectives set forth herein. The invention has been
 described with reference to several embodiments, however, it will be
 appreciated that variations and modifications can be effected by a person
 of ordinary skill in the art without departing from the scope of the
 invention.