Patent ID: 12223640

DETAILED DESCRIPTION

With reference toFIG.1, a ducted fan gas turbine engine for an aircraft is generally indicated at10and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake11, a propulsive fan12, an intermediate pressure compressor13, a high-pressure compressor14, combustion equipment15, a high-pressure turbine16, an intermediate pressure turbine17, a low-pressure turbine18and a core engine exhaust nozzle19. A nacelle21generally surrounds the engine10and defines the intake11, a bypass duct22and a bypass exhaust nozzle23.

During operation, air entering the intake11is accelerated by the fan12to produce two air flows: a first air flow A into the intermediate-pressure compressor13and a second air flow B which passes through the bypass duct22to provide propulsive thrust. The intermediate-pressure compressor13compresses the air flow A directed into it before delivering that air to the high-pressure compressor14where further compression takes place.

The compressed air exhausted from the high-pressure compressor14is directed into the combustion equipment15where 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 turbines16,17,18before being exhausted through the nozzle19to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate-pressure compressors14,13and the fan12by 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 turbines16,17,18, 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 Zhengyou Zhang,A Flexible New Technique for Camera Calibration, Technical Report MSR-TR-98-71, https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/tr98-71.pdf. 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.2shows 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. For example, on each of the left and right images, edges corresponding to the trailing edge of the blade34, an edge30of the platform of the blade, and an edge32of the shroud of the blade may be 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 edge34is in a defined region of interest (rectangles R inFIG.2), 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 edge34, platform edge30and shroud edge32) 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.cecs.anu.edu.au/˜hartley/Papers/CVPR99-tutorial/tutorial.pdf

Next, landmarks are identified in the 3D space map, such as a radially inner landmark36which is the corner of the platform edge30closest to the trailing edge34, and a radially outer landmark38which is the corner of the shroud edge32closest to the trailing edge34. 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 typically 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.3summarises 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.4summarises 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.5shows 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.

Another 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.6shows a system100according to aspects. The system100comprises a computer readable storage medium104for storing I.A. stereo images or video received from a stereo borescope106. The system100also comprises a processor-based sub-system102. The processor-based sub-system102is operationally connected to the computer readable storage medium. The operational connection between the processor-based sub-system102and the computer readable storage medium104may enable the processor-based sub-system to access stereo images stored on the computer readable storage medium104and optionally a 3D reference model stored on the computer readable storage medium. The processor-based sub-system102may be adapted to perform either of the methods of the first or second aspect. The processor-based sub-system102may be adapted to identify same features of the blade in each of the stereo images; map each of the identified features by triangulation onto a 3D space to produce a 3D depth map of at least part of the blade; provide a 3D reference model of the blade; and compare the 3D reference model with the 3D depth map to measure one or more deviations in shape of the blade to determine an amount of creep-induced distortion of the blade.

Alternatively the processor-based sub-system102may be adapted to identify same features of the blade in each of the stereo images; map each of the identified features by triangulation onto a 3D space; measure in the 3D space a distance (D) between radially inner and radially outer landmarks (36,38) on the blade, the positions of the landmarks being determined by reference to the mapped positions of the identified features; and compare the measured distance with a reference distance to determine an amount of creep-induced distortion of the blade.

In embodiments, the system may comprise a stereo borescope106shown inFIG.6in 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 borescope106. In some embodiments control of the stereo borescope106may be performed by the processor-based sub-system102. The stereo borescope106may 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-system102may be adapted to extract and analyse still stereo images from the video stored on the computer readable storage medium104.

Overall, the approaches described above enable automated, reliable, reproducible, quantitative and accurate in-situ creep monitoring that enables effects of creep on different parts of a turbine blade to be understood.

Embodiments may be described as a process which is depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed, but could have additional steps not included in the figure. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function.

The term “computer readable medium” may represent one or more devices for storing data, including read only memory (ROM), random access memory (RAM), magnetic RAM, core memory, magnetic disk storage mediums, optical storage mediums, flash memory devices and/or other machine readable mediums for storing information. The term “computer-readable medium” includes, but is not limited to portable or fixed storage devices, optical storage devices, wireless channels and various other mediums capable of storing, containing or carrying instruction(s) and/or data.

Furthermore, embodiments may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a computer readable medium. One or more processors may perform the necessary tasks. A code segment may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

All references cited herein are incorporated by reference.