Patent Description:
Conventionally borescopes are used to view internal components within an assembled gas turbine engine to determine if the components within the engine are damaged and need repair or if they are undamaged and do not require repair. The use of borescopes enables the components to be viewed without having to disassemble the gas turbine engine into modules or sub modules.

The current approach for on-wing assessment of turbine blade creep is to use a borescope to visually estimate the radial growth of a blade with a borescope by observing the size of the gap between a shroud of the blade and a liner forming the outer wall of the working annulus of the engine.

This provides a qualitative measure of creep in the blade, but it is non-quantifiable, and results vary between operators. Metrical (i.e. quantitative) measurement is currently performed by stripping down the engine and precisely measuring the dimensions of the top of the blade using a coordinate measurement machine (CMM). This is time-consuming and expensive, and can only be performed when the engine is in a maintenance facility with the necessary equipment.

Accordingly, current approaches to creep measurement lack consistency, are inefficient, and do not generally provide enough quantitative data to understand creep growth in different cycling stages and on different parts of the blade. As a result, turbine blades may be replaced unnecessarily early in their life cycle, adding substantially to engine running costs. European patent application <CIT> describes systems and methods for evaluating component strain, and monitoring creep in turbine blades by measurement of a reference (passive strain) indicator and comparison with a replicate of the reference indicator. <CIT> discloses a horoscope for optical inspection of gas turbines.

According to a first aspect of the invention there is provided a method according to claim <NUM>.

Advantageously, by using stereo images and triangulation therefrom in this way, it is possible to obtain distance measurements which are sufficiently accurate and reproducible to monitor turbine blade creep, and which do not require the determination of individual image distance conversion scales for converting from pixels to physical distance. Moreover, the method can be performed on-wing and without stripping down the engine. Thus, it facilitates relatively frequent measurements from which creep growth in different cycling stages and on different parts of the blade can be understood. Turbine blades are currently used for a conservative number of operation cycles. More accurate determination of turbine blade creep allows for the turbine blades to be used for a longer duration, reducing operating costs.

Optional features of the method of the first aspect will now be set out. These are applicable singly or in any combination.

The landmarks are respectively on a platform and a shroud of the turbine blade. A measured distance between such landmarks is highly sensitive to creep-induced lengthening of the blade.

The reference distance is the distance between the radially inner and radially outer landmarks for a turbine blade which has not experienced creep. For example, this reference distance may be determined by measuring an actual blade or by extracting the distance information from a 3D model (e.g. a CAD model or a scan data model) of the blade.

The receiving, identifying, mapping, measuring and comparing, may be performed for each of successive turbine blades of the row of turbine blades. In particular the method of the first aspect can be used to monitor all the turbine blades of the row for creep.

The method may further include: calibrating the stereo borescope to determine imaging distortions produced thereby; and using the calibration to adjust the images to remove or reduce imaging distortions before the mapping onto the 3D space.

The stereo borescope may be used to obtain a stereo video of the turbine blade as the row of turbine blades rotates, the stereo images being stills extracted from the stereo video.

The identifying may include performing automated image analysis to extract feature lines of each of the stereo images. For example, the extracted lines can be the trailing edge line, one or more platform edge lines, one or more shroud edge lines from each of the images and/or one or more seal segment edge lines. This can help to remove a source of operator variation. The image analysis may perform image filtering as a precursor to extracting the edge lines of the blade.

The method may further include, preliminary to receiving the image of a turbine blade: locating the stereo borescope in the engine adjacent the row of turbine blades; and using the stereo borescope to obtain the stereo images of the turbine blade of the row of turbine blades.

Locating the borescope in the engine may comprise inserting the borescope into a port on an accessible part of the engine. Guiding the borescope through a guide tunnel until the end of the borescope is at the end of the guide tunnel.

According to a second aspect of the invention there is provided a system for monitoring turbine blade creep in a gas turbine engine according to claim <NUM>.

The system of the second aspect corresponds to the method of the first aspect.

Optional features of the method of the first aspect pertain also to the system of the second aspect.

Thus, the landmarks are respectively on a platform and a shroud of the turbine blade.

The reference distance is the nominal distance between the radially inner and radially outer landmarks for a turbine blade which has not experienced creep.

Optional features of the system of the second aspect will now be set out. These are applicable singly or in any combination.

The processor-based sub-system may be further adapted to: calibrate the stereo borescope to determine imaging distortions produced thereby; and use the calibration to adjust the images to remove or reduce imaging distortions before the mapping onto the 3D space. The system may further include a stereo borescope adapted to be located in the engine adjacent the row of turbine blades for obtaining the stereo images of the turbine blade of the row of turbine blades, the computer readable medium being operatively connectable to the borescope to receive therefrom the images of the turbine blade.

The stereo borescope may be adapted to obtain a stereo video of the turbine blade as the row of turbine blades rotates, the stereo images being stills extracted from the stereo video. In order to identify same features of the blade in each of the stereo images the processor-based sub-system by further adapted to: perform automated image analysis to extract feature lines of each of the stereo images. For example, the extracted lines can be the trailing edge line, one or more platform edge lines, one or more shroud edge lines from each of the images and/or one or more seal segment edge lines. The image analysis may perform image filtering as a precursor to extracting the edge lines of the blade.

The method of the first or second aspect is typically computer-implemented. Accordingly, further aspects of the disclosure provide: a computer program comprising code which, when the code is executed on a computer, causes the computer to perform the method of the first or second aspect; and a computer readable medium storing a computer program comprising code which, when the code is executed on a computer, causes the computer to perform the method of the first or second aspect.

All of the aspects may map identified features into a 3D space. They may then compare the mapped features to a reference model to determine the amount of creep or distortion of a turbine blade.

With reference to <FIG>, a ducted fan gas turbine engine for an aircraft is generally indicated at <NUM> and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake <NUM>, a propulsive fan <NUM>, an intermediate pressure compressor <NUM>, a high-pressure compressor <NUM>, combustion equipment <NUM>, a high-pressure turbine <NUM>, an intermediate pressure turbine <NUM>, a low-pressure turbine <NUM> and a core engine exhaust nozzle <NUM>. A nacelle <NUM> generally surrounds the engine <NUM> and defines the intake <NUM>, a bypass duct <NUM> and a bypass exhaust nozzle <NUM>.

During operation, air entering the intake <NUM> is accelerated by the fan <NUM> to produce two air flows: a first air flow A into the intermediate-pressure compressor <NUM> and a second air flow B which passes through the bypass duct <NUM> to provide propulsive thrust. The intermediate-pressure compressor <NUM> compresses the air flow A directed into it before delivering that air to the high-pressure compressor <NUM> where further compression takes place.

The compressed air exhausted from the high-pressure compressor <NUM> is directed into the combustion equipment <NUM> where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines <NUM>, <NUM>, <NUM> before being exhausted through the nozzle <NUM> to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate-pressure compressors <NUM>, <NUM> and the fan <NUM> by suitable interconnecting shafts.

Other aircraft gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. two) and/or an alternative number of compressors and/or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan.

The turbine blades of the turbines <NUM>, <NUM>, <NUM>, which are exposed to high centrifugal forces and high temperatures from the working gas expanding through the turbines, are vulnerable to creep deformation. Accordingly, regular inspection of the blades is performed using a stereo borescope.

Preliminary to the inspection, the borescope can be calibrated to determine any imaging distortions which it produces. Various calibration procedures are known to the skilled person, such as described for example by <NPL>. The calibration can then be used to adjust images obtained by the borescope to remove or reduce imaging distortions.

The stereo borescope is located adjacent a row of blades to obtain stereo images of part of the row. The row is then rotated so that each blade in turn is moved into position relative to the borescope. This can be achieved by indexing the rotational position of the row, or more conveniently by using the borescope to obtain a stereo video of the row as it continuously rotates. Respective stereo stills can then be extracted from the video for each of the blades, each still corresponding to its blade being in a given position relative to the borescope. For example, <FIG> shows a pair of left and right stereo stills for a blade.

Having extracted the relevant stills and saved them into suitable memory, a processor-based image analyser performs edge detection on each image. For example, the image analyser may perform image filtering (e.g. noising filtering, texture filtering, compression-less filtering etc.) to enhance the images. On each of the left and right images, edges corresponding to the trailing edge of the blade <NUM>, an edge <NUM> of the platform of the blade, and an edge <NUM> of the shroud of the blade are detected by the image analyser (e.g. using template matching, edge detection, textural analysis etc.) and the lines of these edges extracted.

The image analyser may ensure that the trailing edge <NUM> is in a defined region of interest (rectangles R in <FIG>), whereby the image analyser can confirm that the blade is appropriately positioned relative to the stereo borescope. This may further improve the identification of features.

The same features (e.g. trailing edge <NUM>, platform edge <NUM> and shroud edge <NUM>) are identified in both images of the stereo pair. For example, the same identification algorithm may be applied to each image of the stereopair. Alternatively, features may be identified in one image and epipolar triangulation used to roughly locate a reduced region where an identification algorithm can be applied on the second image. The latter may reduce computation requirements. The image analyser then moves on to a 3D analysis. In particular, it takes the stereo images, and maps the features by triangulation onto a 3D space to produce a 3D depth map of the blade. Suitable triangulation techniques are known to the skilled person. See for example:
https://users. au/~hartley/Papers/CVPR99-tutorial/tutorial.

Next, landmarks are identified in the 3D space map, such as a radially inner landmark <NUM> which is the corner of the platform edge <NUM> closest to the trailing edge <NUM>, and a radially outer landmark <NUM> which is the corner of the shroud edge <NUM> closest to the trailing edge <NUM>. The distance D between these two landmarks in the 3D space map is then determined.

The image analyser compares the measured distance D with a reference distance to determine an amount of creep-induced lengthening of the blade. The reference distance is the corresponding distance for a turbine blade which has not experienced creep. This can be obtained by measuring an actual blade before service, or by extracting the distance information from a 3D model of the blade. By using two landmarks on the turbine blade, only distortion of the turbine blade is measured, and not for example, relative movement between the turbine blade and an external reference point.

<FIG> summarises stages of this creep monitoring procedure.

Advantageously, because the measurement of distance can be over the whole radial length of the blade, the accuracy of the measurement is improved. That is, any measurement of change in length due to creep is increased relative to approaches which do not use the whole length.

Using the method, the distance measurement can be obtained with high reproducibility and accuracy. In particular, using features identified in a 2D analysis as the basis for the 3D measurements simplifies and saves computational power. Also, the 2D analysis helps to filter out erroneous features before the 3D measurement, and requiring that the same features are identified in the two stereo images enforces consistency.

The method can also be repeated for further stereo images with the blade changing position slightly (due to rotation) between each image capture to further improve accuracy. An average of the measurements may be determined for comparison with the reference distance.

However, an advantage of building a full or partial 3D depth map of the blade is that more complex shape deviations of the blade can be measured. For example, comparing such a depth map to a 3D reference model (e.g. a CAD model in the form of a 3D point cloud) can allow both elongation and twisting of the blade to be measured. Twisting in particular is difficult to measure using conventional approaches to borescope measurement. Elongation and twisting of a turbine blade both deformations which can limit blade service life.

One example of a partial 3D depth map is a map of just the platform and shroud of the blade. Deviation of the shroud position (e.g. due to blade elongation and/or twisting) relative to the platform position can then be used to monitor for creep. This approach requires less computation than building up an entire 3D model of the blade for comparison with the reference model. Instead, only parts of the shroud and platform observable by the stereo borescope need to be mapped onto 3D space to produce a depth map. <FIG> summarises stages of this more elaborate creep monitoring procedure.

An example of the comparison of a 3D depth map with a 3D reference model may comprise registering both models at the platform, then performing a Procrustes analysis, iterative closest points algorithm, normal closest points transform or similar to return the best fit or average deviation of one model from the other.

In a variant of this procedure, the stereo borescope is used to image a seal segment located radially outwardly of the blades. <FIG> shows a borescope image of a seal segment adjacent a blade shroud. Similarly, to determining an amount of creep-induced distortion by measuring relative twisting between a shroud and a platform, the amount of distortion can also be determined by measuring relative twisting between a shroud and a seal segment when the blade is at a predetermined rotational position relative to the segment. In this case the stereo images capture the shroud and the seal segment, the image analyser identifies features of the shroud and the seal segment, and maps them onto the 3D space. The 3D reference model also has to include the seal segment. Elongation of a turbine blade can also be determined by measuring the distance between the seal segment and the shroud of the turbine blade. In a further embodiment, elongation and/or rotation of the shroud may be identified by first identifying in 3D space a plane associated with the seal segment and a plane associated with the shroud. The distance between and/or relative angles of these planes may then be compared to the same of a 3D reference model. The seal segment and the shroud are close together, which means a smaller field of view can be used; this may allow the stereo borescope to be positioned closer to the turbine blade for further improved accuracy.

An unclaimed example of landmarks on the blade that can be monitored using the above approach are cooling holes on the blade (in particular at the leading edge). Measuring their positions allows a surface strain map of the blade to be produced, i.e. in a manner similar to strain extensometry. For example, the distance between each of the cooling holes may be compared to a 3D reference model and regions on the turbine blade where deformation has occurred may be identified.

In embodiments, any of the preceding comparison methods may be used in combination.

<FIG> shows a system <NUM> according to aspects. The system <NUM> comprises a computer readable storage medium <NUM> for storing I. stereo images or video received from a stereo borescope <NUM>. The system <NUM> also comprises a processor-based sub-system <NUM>. The processor-based sub-system <NUM> is operationally connected to the computer readable storage medium. The operational connection between the processor-based sub-system <NUM> and the computer readable storage medium <NUM> may enable the processor-based sub-system to access stereo images stored on the computer readable storage medium <NUM> and optionally a 3D reference model stored on the computer readable storage medium. The processor-based sub-system <NUM> is adapted to perform the method of the first aspect.

The processor-based sub-system <NUM> may is adapted to identify the trailing edge <NUM>, platform edge <NUM> and shroud edge <NUM> of the blade in each of the stereo images; map each of the trailing edge <NUM>, platform edge <NUM> and shroud edge 32by triangulation onto a 3D space; measure in the 3D space a distance (D) between a radially inner landmark <NUM> at the corner of the platform edge <NUM> closest to the trailing edge <NUM> on the blade, and a radially outer landmark(<NUM>) at the corner of the shroud edge <NUM> closest to the trailing edge (<NUM>) on the blade; and compare the distance (D) with the distance between the radially inner and radially outer landmarks for a turbine blade which has not experienced creep to determine an amount of creep-induced distortion of the blade.

In embodiments, the system may comprise a stereo borescope <NUM> shown in <FIG> in a dashed line. The computer readable medium may be operatively connected to the stereo borescope to receive the stereo images and/or video of the turbine blade from the stereo borescope <NUM>. In some embodiments control of the stereo borescope <NUM> may be performed by the processor-based sub-system <NUM>. The stereo borescope <NUM> may be adapted to be located in the engine adjacent the row of turbine blades for obtaining the stereo images and/or video of the turbine blade of the row of turbine blades. In embodiments, the stereo video of the turbine blade may be captured as a row of turbine blades rotates. The processor-based sub-system <NUM> may be adapted to extract and analyse still stereo images from the video stored on the computer readable storage medium <NUM>.

Claim 1:
A method of monitoring turbine blade creep in a gas turbine engine (<NUM>), the method including:
receiving stereo images of a turbine blade of a row of turbine blades, the stereo images being over the whole radial length of the blade, and having been obtained using a stereo borescope located in the engine adjacent the row of turbine blades;
identifying a trailing edge (<NUM>), platform edge (<NUM>), and shroud edge (<NUM>) of the blade in each of the stereo images;
mapping each of the trailing edge (<NUM>), platform edge (<NUM>), and shroud edge (<NUM>) by triangulation onto a 3D space;
measuring in the 3D space a distance (D) between a radially inner landmark (<NUM>) at the corner of the platform edge (<NUM>) closest to the trailing edge (<NUM>) on the blade, and a radially outer landmark (<NUM>) at the corner of the shroud edge (<NUM>) closest to the trailing edge (<NUM>) on the blade; and
comparing the measured distance (D) with the distance between the radially inner (<NUM>) and radially outer (<NUM>) landmarks for a turbine blade which has not experienced creep to determine an amount of creep-induced distortion of the blade.