Apparatus and method for automatic inspection of through- holes of a component

The present invention relates to inspection of through-holes of a component, and in particular, to a method and apparatus for automatic inspection of through-holes of a component using thermography.

In several industrial applications, components having through-holes need to be inspected for proper functioning of the component. One such example is a turbomachine blade or vane. Turbomachines , particularly gas turbines, have vanes and blades that contain channels or through-holes which lead from an exterior surface of the blade or vane to a hollow interior (plenum) . In use, pressurized air is applied to the plenum causing cooling airstreams to flow through the through-holes, thereby absorbing heat from the walls of the through-holes, as well as from the surface of the blade or vane, thereby cooling the blade or vane.

In order to function properly, these through-holes, called cooling holes, must be made to a known configuration because the distribution of airflow must be controlled in order to achieve proper cooling. Accordingly, the cooling holes must not be blocked, not even partially. This requires a reliable technique for inspection of the cooling holes to detect blockages from the external surface. If a cooling hole is detected to be blocked, even partially, it may require the component (i.e., the blade or vane) to be discarded and replaced . Currently checking and inspection of cooling holes of blades or vanes of a gas turbine is done by using infrared thermo ^¬ graphy. Herein, a medium, generally a hot gas, is forced through the cooling holes and an infra-red camera is used to capture infra-red radiation emitted by the component while the medium is flowing through the cooling holes, to form a thermographic image. Blocked cooling holes obstruct the flow of heat, which is visible in the thermographic image.

Thus, to decide whether or not the component can continue to be used, an evaluation of the thermographic image is re ^¬ quired. Currently, the evaluation of thermographic images to detect blocked cooling holes is done manually by trained per- sonnel. This requires individual contrast adjustments for the individual thermographic images, recognition of irregulari ^¬ ties in pattern of the cooling holes in the component, count ^¬ ing the rows of hot spots (which correspond to cooling holes) in the thermographic image, and a comparison with the cooling hole pattern in the actual component. Often, it is also necessary to insert a diameter measurement gage or a pin into a cooling hole to inspect irregularities.

Further, evaluation of the thermographic images with standard image processing procedures is not possible because of high noise components in these images and also due to inhomogene- ous illumination in the thermographic images due to curva ^¬ tures in the component. The object of the present invention is to provide a method and an apparatus for automatic inspection of through-holes of a component based on an evaluation of thermographic images which obviates the need for a subjective evaluation as done in the state of the art.

The above object is achieved by the method according to claim 1 and the apparatus according to claim 11.

The underlying idea of the present invention is to provide an automatic evaluation of a thermographic image by correlating the thermographic to a digital image obtained from geometri ^¬ cal data of the component. Using the digital image of the component, a digital image mask is computed based on a deter- mination of the positions of through-holes in the digital image. The thermographic image is fitted on to the digital image and the regions corresponding to through-holes in the thermographic image are extracted using the computed digital image mask. A blockage or irregularity in the through-hole is determined by evaluating this masked thermographic image. One of problems with the existing state of the art is that ther ^¬ mographic images have high noise component and hence it is difficult to identify and analyze through-holes using stan ^¬ dard image processing techniques. This problem is solved herein by the use of the above-mentioned digital image mask, to extract the through-holes in the thermographic image. The above method is completely automatic and is considered to be advantageous over the subjective evaluation of thermographic images in the mentioned state of the art.

In one embodiment, said digital image is obtained from a CAD model of the component. In this embodiment, the expected positions of the through-holes can be directly obtained from position data in the CAD model.

In an alternate embodiment, said digital image is obtained from a three-dimensional geometrical measurement of the com ^¬ ponent. This obviates the need for three-dimensional CAD models and is further not sensitive to deviations in the geometry. The above embodiment thus provides a universal test procedure with very little effort for setting up.

In a further advantageous embodiment, the three-dimensional geometrical measurement of the component is made in-situ with the capturing of the thermographic image and at a viewing position that corresponds to a viewing position of the thermographic image.

In one embodiment, the step of fitting comprises fitting the thermographic image directly on a three-dimensional digital image of the component. This eliminates the additional step of creating a two-dimensional image for fit with the thermo- graphic image. This embodiment is particularly advantageous if the clamping or positioning of the component is reproduci ^¬ ble, which allows the test set up to be calibrated or meas ^¬ ured just once.

In an alternate embodiment, the step of fitting comprises fitting the thermographic image on a two-dimensional digital image of the component corresponding to an viewing position of the thermographic image view. This embodiment is used when a fixed calibration of the test set-up is not possible.

In a further embodiment, a blockage or irregularity in a par ^¬ ticular through-hole is determined by comparing a temperature of that through-hole as indicated in the masked thermographic image to a reference temperature value. In principle, during thermographic imaging when a heated medium is forced into the through-holes, the through-holes act as black-body cavity radiators and as such, they approach behaving as idealized sources of radiation. A blockage in the through hole thus reduces the temperature of the through-hole, which is visible in the thermographic image.

In a further embodiment, for increased contrast, the thermo ^¬ graphic image is generated based on a combination of multiple thermographic images including at least a first and a second thermographic image, wherein:

- the first thermographic image is generated by passing a heated medium through the through-holes during a heat-up cycle and capturing infra-red radiation emitted from the com- ponent during said heat-up cycle, and

- the second thermographic image is generated by passing a cooled medium through the through-holes during a cool-down cycle and capturing infra-red radiation emitted from the com ^¬ ponent during said cool-down cycle.

When both a heat-up cycle and a cool-down cycle are analyzed in tandem, is possible to obtain increased contrast or sensi ^¬ tivity in the thermographic image, which can be used to detect partial blockages or other minor irregularities in the through-holes .

In an exemplary embodiment, said component is a turbomachine blade or vane, and wherein said through-holes are cooling holes. Cooling holes in turbine blades or vanes have ex ^¬ tremely small dimensions, which makes a manual evaluation of thermographic images rather time consuming and labor inten ^¬ sive. The use of the present invention herein reduces the testing time and eliminates the need of trained personnel for this purpose.

The present invention may also be embodied as a program prod ^¬ uct comprising computer readable media having computer read- able program code stored therein, which, when executed on one or more processors, carries out the method according to any of the above-mentioned embodiments.

The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompany ^¬ ing drawings, in which:

FIG 1 is a schematic diagram of an apparatus for inspec- tion of through-holes of a component in accordance with one embodiment of the present invention,

FIG 2A illustrates a digital image of the component ob ^¬ tained from a 3D CAD model,

FIG 2B illustrates a digital image of the component ob ^¬ tained from a 3D measurement of the component,

FIG 3 illustrates a digital image mask for the regions corresponding to through-holes from CAD model or

3D measurement,

FIG 4 illustrates a thermographic image of the component showing hot-spots that correspond to through-holes. FIG 5 illustrates a masked thermographic image wherein the regions corresponding to through-holes in the thermographic image of FIG 4 are extracted, and

FIG 6 is a schematic diagram of an apparatus for inspec ^¬ tion of through-holes of a component, further in ^¬ cluding 3D imaging means in accordance with a further embodiment of the present invention.

The embodiments described below illustrate the use of the present invention for automatic inspection of cooling holes of a turbomachine blade or vane of the type mentioned above. However, the present invention may be used for several other industrial applications that involve automatic inspection of through-holes of a component.

Referring to FIG 1 is illustrated an apparatus 1 for inspect- ing through-holes 6 formed in a component 5. In this example, the component 5 is a turbine blade or vane, and the through- holes 6 include cooling holes. The apparatus 1 broadly in ^¬ cludes an imaging module 2 for generating a thermographic image of the blade or vane 5, an image processing module 3 for extracting regions corresponding to cooling holes from the thermographic image, and an analysis module 4 for evalu ^¬ ating this extracted portion of the thermographic image to determine the presence of a blockage or irregularity in one or more of the cooling holes.

In the illustrated embodiment, the set-up of the imaging module 2 includes a fixture 7, including a table having suit ^¬ able clamping means for securely holding the blade or vane 5 in position. For the purpose of imaging, the table preferably allows movement with all six degrees of freedom, including rotation along the direction 8 and swivel along the direction 9 as shown. A thermographic image of the blade or vane 5 is generated by passing a medium through the cooling holes 6 and capturing infra-red radiation emitted from the blade or vane 5 while the medium is flowing through the cooling holes 6. To that end, the set-up includes a first flow line 10 arranged for conducting a heated medium, such as hot air, into the hollow interior of the blade or vane 5. A suitable heating element 12 may be provided on the flow line 10. Furthermore, a second flow line 11 may be arranged for conducting a cooled medium into interior of the blade or vane 5. In this example, the cooled medium is cooled air provided by a chiller 15 pro- vided on the flow line 11. Alternately, the cooled medium may comprise ambient air, which would not necessitate the chiller 15. The flow lines 10 and 11 are each connected to a flow control valve 13, such as a solenoid valve, which can be switched to alternately allow either the heated medium or the cooled medium to pass through the blade or vane 5. The heated or cooled medium would then traverse through the hollow inte ^¬ rior of the blade or vane 5 and exit through the cooling holes 6 as shown in FIG 1. The set-up further includes an infra-red imaging device 14 such as an infra-red radiometer, an infra-red camera, or the like, to capture infra-red radia ^¬ tion emitted from the blade or vane 5 while the heated or medium is flowing through the cooling holes 6.

In operation, the inspection of the cooling holes 6 of the blade or vane 5 begins with a heat-up cycle by switching solenoid valve 13 to a position to pass heated medium to the interior of blade or vane 5. In this case, air is heated by the heating element 12 to a temperature greater than ambient temperature to provide sufficient contrast in the infrared signature of the cooling holes 6 received by the infra-red imaging device 14 when the heated medium exits cooling holes 6. The heated medium is preferably forced into the hollow interior of blade or vane 5 for a predetermined time period to sufficiently heat the interior walls of blade or vane 5 and the walls surrounding cooling holes 6. This time period will vary depending upon the size of the blade or vane 5 but can be of a sufficient duration so that blade or vane 5 is heated to a steady state condition or so that the blade or vane material is heated to a sufficiently high temperature to cause a contrast in the infrared signature of the holes detected by the infra-red imaging device 14 when solenoid valve 13 is switched to provide the cooled medium to the interior of blade or vane 5 for the cool-down cycle.

A first infrared thermographic image (typically a first series of infra-red thermographic images) of the blade or vane 5 is generated by the infra-red imaging device 14 during the heat-up cycle.

After the heat-up cycle, solenoid valve 13 is switched to pass the cooled medium to the interior of blade or vane to start a cool-down cycle. The cooled medium used herein is cooled air preferably below ambient to improve the contrast or sensitivity of the infrared signature received by the infrared imaging device 14. A second infra-red thermographic image (typically a second series of infra-red thermographic images) of the blade or vane 5 is generated by infra-red imaging device 14 during the cool-down cycle.

The air streams 16 exiting cooling holes 6 during both the heat-up and cool-down cycles cause the cooling holes to act as black-body cavity radiators and as such, they approach behaving as idealized sources of radiation in accordance with Planck's well-known, empirically ascertained law.

The thermographic images, thus obtained, are used for analyz ^¬ ing temperature distribution of the blade or vane 5, to in- spect blockages or irregularities in the cooling holes 6.

Each thermographic image is generated by assigning a differ ^¬ ent color to different temperatures of infra-red radiation captured by the imaging device 14. The temperature of a par ^¬ ticular region in a thermographic image may thus be deter- mined based on the color and/or the intensity (brightness) of that region. FIG 4 shows an exemplary thermographic image 40 of a turbine blade. Herein, as shown, the regions 41 of maxi ^¬ mum temperature (referred to as hot-spots) correspond in gen- eral to cooling holes. However, as can be seen, such a thermographic image has a high noise component, which, in combi ^¬ nation with other reasons such as the small size and close spacing of the cooling holes, and inhomogeneous illumination in the thermographic images due to curvatures in the blade or vane 5, makes it difficult to isolate and analyze the tem ^¬ peratures of these hot spots 41 using standard image process ^¬ ing techniques. The technique described below provides an automatic evalua ^¬ tion of a thermographic image to inspect blockages or irregu ^¬ larities in one or more of the cooling holes.

Referring back to FIG 1, the first and second thermographic images (or first and second series of thermographic images) generated by the imaging module 2 are provided to the image processing module 3. Additionally, these images may be dis ^¬ played on a display monitor 17. The image processing module 3 combines the first and second thermographic images (or first and second series of thermographic images) to generate a thermographic image of desired contrast. It should be noted herein that the purpose of thermographic imaging during a cool-down cycle following a heat-up cycle is to provide improved contrast or sensitivity of the infrared signature received by the infrared imaging device 14. However in an alternate embodiment, a thermographic image may be generated by passing a single medium, preferably a heated medium through the cooling holes, thus omitting the cool-down cycle completely .

In accordance with the present invention, the thermographic image as obtained above is evaluated based on a correlation of the thermographic image to a digital image obtained from geometrical data of the blade or vane 5. Such a digital image may be obtained, as illustrated in this embodiment, from an already existing 3D CAD model of the blade or vane. FIG 2A illustrates a three-dimensional (3D) digital image 20 ob ^¬ tained from a CAD model of the concerned turbine blade or vane. The image processing module correlates the thermo ^¬ graphic image and the digital image from the CAD model to extract the regions corresponding to cooling holes from the thermographic image, as described below.

The first step involves the computation of a digital image mask for the positions 21 of the cooling holes on the digital image 20 (See FIG 2A) . The positions 21 of the cooling holes are determined from position data in the 3D CAD model. Fur- thermore, compensation of positional tolerances may be deter ^¬ mined using best fit method. Using the position data of the cooling holes thus determined, a digital image mask is com ^¬ puted for the regions 21 corresponding to the positions of cooling holes in the digital image 20. Computation of the digital image mask involves setting the pixel values of all bits to ^λ 1' for the regions 21 in the digital image 20, and setting the pixel values of all bits to ^λ 0' in the surround ^¬ ing background region 24, i.e., the remaining portion of the digital image 20. FIG 3 shows a digital image mask 30 ob- tained as a result of the above operation.

The next step involves fitting the thermographic image on to the digital image. In the presently contemplated embodiment, the thermographic image 40 (see FIG 4) is directly fitted on to the 3D image 20 (see FIG 2A) , which corresponds to the viewing position in the thermographic image. This embodiment is particularly advantageous if the clamping or positioning of the blade or vane 5 in the imaging module 2 (see FIG 1) is reproducible, which allows the test set up to be calibrated or measured just once. In an alternate embodiment, to obviate difficulties in calibrating the test set up, the thermo ^¬ graphic image may be fitted on to a two dimensional (2D) image calculated from the CAD model of the blade or vane that corresponds to the viewing position in the thermographic image.

From the fitted thermographic image, the hot spots or regions 41 corresponding to the cooling holes (see FIG 4) are ex- tracted by masking the fitted thermographic image using the computed digital image mask 30 (see FIG 3) . The process of masking the thermographic image involves a logical combina ^¬ tion of the computed digital mask 30 (see FIG 3) with the fitted thermographic image 40 (see FIG 4) using a bitwise AND operation. The result, as shown in FIG 5, is a masked thermo ^¬ graphic image 50 isolating the regions 41 corresponding to cooling holes. Referring back to FIG 1, the masked thermographic image is provided to the analysis module 4 for inspection of blockages and irregularities in the cooling holes. Since the regions corresponding to the cooling holes (hot spots) are isolated and clearly distinguished from the background, it is possible to determine a blockage or irregularity, for example, by com ^¬ paring the temperature values (determined based on color and/or brightness) for each of the isolated hot spots to a threshold or reference value. The above technique is completely automatic and is considered to be advantageous over the subjective evaluation of thermo ^¬ graphic images in the mentioned state of the art. The above- described embodiments of the present invention reduce the testing time and eliminate the need of trained staff for this purpose.

In an alternate embodiment, instead of using a CAD model, the digital image is obtained from a 3D geometrical measurement of the blade or vane. Such a 3D image may be retrieved from an existing image database. However, in a preferred embodi ^¬ ment, the 3D image of the blade or vane is generated in-situ with the thermographic imaging set up. FIG 6 illustrates an apparatus for achieving this. Herein, the imaging module 2 additionally includes 3D imaging means, comprising a light source, such as a structured light projector 60 and an image capturing device, such as a CCD (or CMOS) camera 61, arranged as proximate to the infra-red imaging device 14 shown. The shown arrangement advantageously allows the 3D measurement of the blade or vane to be carried out in the same viewing position as that of the thermographic image, for a given clamping position of the blade or vane 5. FIG 2B shows an exemplary 3D digital image 22 of the turbine blade or vane 5 captured by the CCD (CMOS) camera 61, corresponding to the viewing position of the thermographic image 40 (see FIG 4)

Referring back to FIG 6, the 3D image captured by the CCD (or CMOS) camera 61 is provided to the image processing module 3, along with the thermographic image from the infra-red imaging device 14, for further processing.

The first step involves the computation of a digital image mask for the positions 23 of the cooling holes on the digital image 22 (see FIG 2B) . The positions 23 of the cooling holes are determined, for example by determining the positions depressions on the surface of the blade or vane in the 3D image 22 using standard image processing techniques. Using the position data of the cooling holes thus determined, a digital image mask is computed for the regions 23 correspond ^¬ ing to cooling holes in the digital image 22 as determined using said image processing techniques. Similar to the previously illustrated embodiment, the computation of a digital image mask herein involves setting the pixel values of all bits to ^λ 1' for the regions 23 in the digital image 22, and setting the pixel values of all bits to ^λ 0' in the surround ^¬ ing background region 25, i.e., the remaining portion of the digital image 22. The resultant digital image mask is similar to that illustrated in FIG 3.

The next step involves fitting the thermographic image on to the 3D digital image. In a preferred embodiment, the thermo ^¬ graphic image 40 (see FIG 4) is directly fitted on to the 3D image 22 (see FIG 2B) , which corresponds to the viewing posi ^¬ tion in the thermographic image. In an alternate embodiment, the thermographic image may be fitted on to a 2D image calcu ^¬ lated from the 3D image of the blade or vane captured by the CCD (or CMOS) camera, that corresponds to the viewing posi ^¬ tion in the thermographic image.

From the fitted thermographic image, the hot spots or regions corresponding to the cooling holes are extracted by masking the fitted thermographic image using the computed digital image mask. Again, this step involves a logical combination of the computed digital mask with the fitted thermographic image using a bitwise AND operation. The result is a masked thermographic image isolating the regions corresponding to cooling holes similar to the one shown in FIG 5.

Referring back to FIG 6, the masked thermographic image is provided to the analysis module 4 for inspection of blockages and irregularities in the cooling holes, for example, by com ^¬ paring the temperature values (determined based on color and/or brightness) for each of the isolated hot spots to a threshold or reference value, as mentioned above.

The inventive step involved in the above-described embodiment is to provide a method and apparatus for combining thermo ^¬ graphic image data to 3D geometrical measurements. Advanta ^¬ geously, the above embodiment obviates the need for 3D models. Further, since the 3D geometric measurements are made in situ with thermal imaging, the above technique is not sen ^¬ sitive to deviations in the geometry of the component, i.e., the blade or vane. Furthermore, the above embodiment provides a universal test procedure with very little effort for set ^¬ ting up.

Referring to FIGS 1 and 6, the image processing module 3 and the analysis module 4 may comprise one or more processors having hardware suitable for executing appropriate algorithms to implement the above-described methods. The functionalities of the image processing module 3 and the analysis module 4 are thus rendered by the algorithms executed by them. Thus, the present invention may also be embodied as a computer pro ^¬ gram product comprising computer readable media having com- puter readable program code stored therein, which, when exe ^¬ cuted on one or more processors, carries out the method according to any of the above-mentioned embodiments.

While this invention has been described in detail with refer ^¬ ence to certain preferred embodiments, it should be appreci ^¬ ated that the present invention is not limited to those pre ^¬ cise embodiments. Rather, in view of the present disclosure which describes the current best mode for practicing the invention, many modifications and variations would present themselves, to those of skill in the art without departing from the scope and spirit of this invention. The scope of the invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope.