Patent Application
20110307220
The present invention relates to the field of monitoring an aeroengine. In particular, the invention relates to identifying failures and to detecting faulty components in an aeroengine.
In numerous industries, such as the aviation and space industries, it is important to be able to identify failures of an aeroengine from measurements that describe the instantaneous state of the engine so as to deduce which physical component is faulty, if any.
Nevertheless, such time-series measurements are expressed in a variety of different physical units and they may vary from flight to flight in arbitrary manner, thereby complicating any analysis and processing of such measurements.
So-called “scoring” tools exist that seek to substitute time-series measurements that may be expressed in various different units, with quality scores. Such tools are based on likelihood calculations leading to quality control systems. Nevertheless, scoring tools are difficult to apply in a multivariate field such as monitoring an aeroengine. Furthermore, those scores correspond to relative values that are not easily transformed into a real environment that is understandable to an engine expert.
There are also classifying or labeling tools that in general accompany solutions for providing statistical control over industrial methods. Nevertheless, such classification tools need to be calibrated on fault databases that are large in size and very difficult to obtain, very expensive, and that require a large amount of calculation time. In particular, such classification tools are extremely difficult to apply in the field of aeroengines. This is because, fortunately, there are very few genuine breakdowns of aeroengines, and it is therefore very difficult to construct a large database of failures.
The present invention provides a method of identifying failures in an aeroengine, the method comprising the following steps:
This method makes it easy to interpret anomaly vectors and reference vectors that correspond to signatures that are represented in a physical frame of reference that is understandable for engine experts. Furthermore, it is possible to rely on knowledge collected from experts rather than on a database of failures or faults that would be very expensive and difficult to construct. This makes it possible to identify failures in a manner that is understandable and fast, and at lower cost.
In an aspect of the present invention, selecting said subset of reference vectors comprises the following steps:
This makes it easy to select the failure signatures that are the closest, even in a space of large dimension, by limiting the dimension of the problem to a subspace that is generated by the selected reference vectors.
According to a feature of the present invention, said sphere is of radius 1.
According to an another aspect of the present invention, the identification of failures comprises the following steps:
This makes it easy to identify the most probable failures.
According to a feature of the present invention, said set of standardized indicators {tilde over ({tilde over (y)}1, . . . {tilde over ({tilde over (y)}n comprises indicators {tilde over (y)}1, . . . {tilde over (y)}m identified using criteria established by experts.
Thus, an engine expert is capable at all times of interacting with and interpreting the anomaly signatures.
According to another feature of the present invention, said set of standardized indicators {tilde over ({tilde over (y)}1, . . . {tilde over ({tilde over (y)}n further comprises dynamic indicators that are constructed as a function of the indicators at present and past instants {tilde over ({tilde over (y)}(t)=f({tilde over (y)}(s); s≦t) representative of the behavior over time of said aeroengine.
It is thus possible to pick up the dynamic behavior of the aeroengine and the way in which it varies.
Advantageously, said anomaly vector is constructed using the following steps:
z=Σ
−1/2({tilde over ({tilde over (y)}−μ),
where μ is the mean of the indicator vectors and Σ is a covariance matrix from which a pseudo-inverse Σ−1 is calculated and also a root Σ−1/2 by decomposition into singular values.
This makes interpretation easier and facilitates calculation performed on the anomaly vectors.
In addition, the method includes the following steps:
d
2
=∥z∥
2=({tilde over ({tilde over (y)}−μ)TΣ−1({tilde over ({tilde over (y)}−μ); and
Thus, the norm of the anomaly vector corresponds to an overall score representative of an abnormal behavior that is easy to detect in a known statistical distribution that may be approximated by a χ2.
Advantageously, said set of reference vectors is constructed in accordance with caricatural behaviors of the indicators in the event of an anomaly.
Thus, the reference vectors may easily be constructed while keeping meaning that is understandable for experts.
The method of the invention also includes the following steps:
This facilitates finding the equipment that is faulty, thus enabling maintenance of the aeroengine to be performed quickly and effectively.
Said decision grid may be formed by a matrix of conditional probabilities that a component is faulty, knowing that a failure has been observed and from a series of coefficients corresponding to a priori probabilities of each component failing.
Thus, the decision grid may easily be constructed from the knowledge of experts.
Advantageously, said decision grid is corroborated by machine learning.
This enables a decision grid to be constructed that is more accurate and more robust.
The invention also provides a system for identifying failures in an aeroengine, the system comprising:
The invention also provides a computer program including instructions for implementing the method of identifying failures using the above steps when executed by processor means.
Other features and advantages of the device and the method of the invention appear better on reading the following description given by way of non-limiting indication with reference to the accompanying drawings, in which:
The system comprises a plurality of sensors 3a-3f for measuring time-series data relating to the engine 1 and its environment. The system also includes processor means 5 for processing information such as a calculator or computer suitable for being used to execute a computer program designed to implement the method of the invention. The processor means 5 comprise the hardware means that are conventionally found in a computer. More particularly, these processor means 5 comprise a central unit 7 that executes the instruction sequences of the program of the method of the invention, a central memory 9 that stores the data and programs being executed, storage means or media 11 for storing digital conserving the data, input peripherals (sensors 3a-3f, keyboard, mouse, . . . ) and output peripherals (screen 13, printer 15, . . . ) for delivering the result of identifying failures.
In a step E1, the processor means 5 are configured to act over time to collect and digitize time-series measurements that are acquired by the sensors 3a-3f from the aeroengine 1 and its environment.
In a step E2, the processor means 5 are configured to define standardized indicators.
On the basis of time-series measurements, it is possible to calculate indicators y1, . . . , yj, . . . , ym that are specific to elements of the engine 1. For example, one indicator may correspond to the delay needed for an engine shaft to reach maximum acceleration after each start of the engine, another indicator may be the temperature gradient of the exhaust gas from the engine, etc.
It should be observed that the indicators may be specific to physical elements, indicating a particular element of the engine 1 or to logical elements, indicating a specific task for an entire set of elements of the engine 1.
These indicators y1, . . . , yj, . . . , ym may be calculated using criteria provided by experts, e.g. on the basis of a document that is drawn up by engine experts and known as failure modes, effects, and criticality analysis (FMECA). That document lists the failures, the pieces of equipment involved, the causes, the consequences, and also the indicators that are calculated from the above measurements that enable a phenomenon to be identified, each of them being associated with a description of the effects that are observed.
Thereafter, the indicators y1, . . . , yj, . . . , ym can be standardized, e.g. using a conventional technique of normalization as a function of an average and of a standard deviation that are calculated a priori on the basis of a previously-digitized data series.
In a variant, it is possible to define standardized indicators {tilde over (y)}1, . . . , {tilde over (y)}j, . . . , {tilde over (y)}m that are independent of the external context and that also take account of stochastic interdependency relationships between the indicators themselves.
Each measurement collected during a flight is taken in specific external or internal conditions. These conditions, which may have an impact on how indicators should be interpreted, may themselves be measured and recorded as exogenous data. The external conditions may comprise outside temperatures and pressures, the attitude and relative speed of the airplane, where the flight is taking place (over an ocean, a desert, land, etc.), weather conditions (rain, snow, ice, etc.), relative humidity, etc. Internal conditions may relate to specific utilization of the engine (shaft speed, exhaust gas temperature, type of fuel, etc.). As an example of exogenous data, the oil temperature immediately before starting the engine may be considered as context data that distinguishes between two types of start (cold start or hot start).
Thus, from the time-series measurements made by the sensors 3a-3f, it is possible to identify an exogenous data set X=(x1, . . . , xh) representative of the external context acting on the indicators y1, . . . , yj, . . . , ym. This may be performed using the criteria of experts performing dependency analysis making it possible to list the contextual data that is associated with the indicators.
Thereafter, for each indicator, a regression of observations is constructed on a space that is generated by the other indicators, the context data, expressions obtained from the analysis of experts, and other functions that are implemented, e.g. in the form of a model with nodes. The space that is constructed and onto which the observations are projected is of much greater dimension than the number of initial indicators.
In other words, for each given indicator Yj, a projection space E(j)=σ(Y(j),X) is constructed. This projection space is generated by the exogenous data set X=(x1, . . . , xh) and by analytic transformations of a subset of indicators Y(j)=(y1, . . . , yj−1, yj+1, . . . ym) that comprises all of the initial indicators other than the given indicator Yj. The analytic transformations expressed physical relationships between the indicators and they may be defined by experts. These analytic transformations may also include an identity transformation, and linear or non-linear transformations or functions providing information about correlations between different indicators. Thereafter, for each given indicator Yj, a corresponding estimator ŷj is calculated by using a regression technique to project the given indicator Yj onto the projection space E(j)=σ(Y(j),X), thereby forming a set of estimators Ŷ=(ŷ1, . . . , ŷj, . . . , ŷm).
Finally, each estimator ŷj may be normalized as a function of a reference value for the corresponding indicator Yj and a residue or difference between each given estimator ŷj and the corresponding indicator Yj so as to form standardized indicators {tilde over (y)}1, . . . , {tilde over (y)}j, . . . , {tilde over (y)}m representative of the operation of the engine 1.
On the basis of these standardized indicators {tilde over (y)}1, . . . , {tilde over (y)}j, . . . , {tilde over (y)}m constructed using the above methods, or any other method, the objective is to diagnose an anomaly and then to deduce a specific failure and possibly the physical components concerned.
Nevertheless, prior to diagnosing anomalies, it is possible, in addition to the indicators {tilde over (y)}1, . . . {tilde over (y)}m as identified by experts, to add indicators concerning the immediate past (trend, curvature, acceleration, shape, . . . ) so as to pick up also the dynamic behavior of the engine and thus how it is varying.
Successive observation of indicators may provide dynamic information about the indicators. Given that the standardized indicators are suitable for being compared (which is not true of the initial indicators), it is possible for the standardized indicators as identified by experts to be combined in dynamic manner.
Thus, it is possible to define a set of standardized indicators {tilde over ({tilde over (y)}1, . . . {tilde over ({tilde over (y)}n that are representative of time-varying behaviors of the aeroengine 1 and that comprise both indicators {tilde over (y)}1, . . . {tilde over (y)}m as identified by experts and dynamic indicators constructed as a function of the indicators {tilde over (y)}1, . . . {tilde over (y)}m as identified by experts as they are at present and as they have been in the past {tilde over ({tilde over (y)}(t)=f({tilde over (y)}(s); s≦t).
A step E3 relates to constructing an anomaly signature that is representative of the behavior of the engine 1. More particularly, the processor means 5 are configured to construct an anomaly vector (or anomaly signature) as a function of the set of standardized indicators {tilde over ({tilde over (y)}1, . . . {tilde over ({tilde over (y)}n.
The construction of the anomaly vector may be performed initially by forming an indicator vector {tilde over ({tilde over (y)} of dimension n from the set of standardized indicators {tilde over ({tilde over (y)}1, . . . {tilde over ({tilde over (y)}n. Thereafter, it is possible to construct the standardized anomaly vector z by renormalizing the indicator vector {tilde over ({tilde over (y)}.
It should be observed that for standardized indicators calculated by the residues obtained using a least-squares minimization technique, the indicator vector {tilde over ({tilde over (y)} may reasonably be normalized with a multivariate Gaussian distribution.
More particularly, the mean μ of the standardized indicator vectors {tilde over ({tilde over (y)}1, . . . {tilde over ({tilde over (y)}n is subtracted so that the vector is centered, the covariance matrix Σ is calculated, and then the anomaly vector is formed by rectifying the indicator vector {tilde over ({tilde over (y)} by the covariance matrix Σ using the following formula:
z=Σ
−1/2({tilde over ({tilde over (y)}−μ)
using the root of a pseudo-inverse of Σ calculated by decomposing into singular values Σ=USUT with UTU=1 and S=diag(σ12, . . . σn-k2, {tilde over (0)}, . . . ). Thus, the standardized anomaly vector z may approximately follow a normal Gaussian distribution on the complement of the core of Σ of dimension k≧0 as identified by the singular values considered as being approximately zero.
A step E4 is an abnormality test. The processor means 5 are configured to diagnose whether the anomaly signature or vector reveals an anomaly.
Normal signatures are fairly flat, whereas abnormalities are represented by large variations and are easily interpretable.
Thus, an anomaly may be detected by calculating the norm of the anomaly vector, e.g. using the Mahalanobis distance expressed using the following formula: d2=∥z∥2=({tilde over ({tilde over (y)}−μ)TΣ−1({tilde over ({tilde over (y)}−μ)
where μ is the mean of the standardized indicator vectors {tilde over ({tilde over (y)}1, . . . {tilde over ({tilde over (y)}n, and Σ is the covariance matrix.
Advantageously, the statistical distribution of the Mahalanobis distance is known and may be approximated by a χ(n−k). Furthermore, the 3σ and 6σ levels (where σ is the standard deviation) relative to the mean value may be obtained directly by analytic calculation. Consequently, it is easy to detect an abnormality of the aeroengine on the basis of a trigger threshold defined as a function of the statistical distribution of the norm of the anomaly vector.
Thus, the norm of the anomaly vector may be considered as a global score representative of abnormal behavior and that is easy to detect.
In the event of an abnormality, it is also possible to visualize the type of failure by performing two-dimensional projection of the anomaly vectors.
At the end of the test in step E4, the method naturally moves on to a following step E5 only in the event of the anomaly vector revealing an abnormality.
Step E5 relates to selecting reference signatures corresponding to listed failures of the aeroengine.
More particularly, in the event of an abnormality being revealed by the anomaly vector, the processor means 5 are configured to select a subset of reference signatures or vectors having directions belonging to a determined neighborhood of the direction of the anomaly vector. The subset of reference vectors is selected from a predetermined set of reference vectors (or signatures) associated with failures of the aeroengine and determined using criteria established by experts.
The set of reference vectors may be constructed in accordance with caricatural behaviors of the indicators in the event of an anomaly.
When devising the FMECA, the experts may list all kinds of possible failures, with each failure being allocated an a priori probability of occurring, and with sufficient elements being provided to define the caricatural behavior of the indicators in the event of anomalies. The caricatural behavior is generally described informally on the lines: “this value is very high”, “this other value is increasing very slowly”, “that may be small when the last value is high”, and so on.
The caricatural behaviors may be converted in known manner in the form of scores leading to a list of known failures being constructed. Furthermore, on the assumption of a context that is standard, this list enables examples of classified failures to be constructed. These examples may be in the form of vectors that are normalized so as to construct a standardized matrix including a standardized reference vector on each row. As a result, the FMECA serves to define reference vectors that describe in caricatural manner the listed failures in a frame of reference that is real and understandable for engine experts. In addition, the FMECA makes it possible to define an a priori probability of occurrence that is associated with each reference vector.
It should be observed that given that the definitions of failures are caricatural, account can be taken only of the directions of the reference vectors. Thus, the subset of reference vectors may be classified or selected by comparing the anomaly vector with the reference vectors on an (n−k−1)-sphere in a vector space of dimension n−k equal to the number of indicators in the set of standardized indicators {tilde over ({tilde over (y)}1, . . . {tilde over ({tilde over (y)}n, minus the number of linear relationships k between these indicators.
This may be performed by calculating geodesic distances between the projection of the anomaly vector and the projections of the reference vectors on the sphere. Calculating distances between vectors becomes meaningless in a space of dimension greater than five.
More particularly, the geodesic distances are calculated between the direction of the anomaly vector and the directions of the reference vectors on the sphere. Thus, the direction of the anomaly vector may be compared with the directions of the reference vectors by calculating geodesic distance on a sphere of radius 1.
The geodesic distance θ2 between the anomaly vector z and a standardized reference vector t (a standard “template”) may be approximated as a normalized scalar product using the following formula:
Naturally, it is possible to make no use of the notion of distance as a distribution parameter in a space of high dimension.
Nevertheless, it is possible to compare these geodesic distances in pairs which makes it possible to classify the reference vectors, e.g. in increasing order of their geodesic distances relative to the anomaly vector. It is then possible to form the subset of reference vectors from the first reference vectors of classification order lower than a determined rank. For example, it is possible to select the first three, four, or five reference vectors to form a subset of a few reference vectors that are representative of the more probable failures.
In a step E6, the processor means 5 are configured to identify the failures associated with the previously-selected subset of reference vectors.
More particularly, the geodesic distances are used for identifying the more probable failures. Since it is always possible to select the main reference vectors closest to the anomaly vector, it is possible to limit the probability model on the corresponding subsphere of smaller dimension and to use the geodesic distances to calculate an a posteriori local probability of occurrence. To do this, a model is used of mixed Gaussians on the sphere. The radii of the Gaussians depend on a priori criteria established by experts.
Thus, for each reference vector, it is possible to calculate an a posteriori probability P(f) of a failure f occurring as a function of an a priori probability of each failure f occurring as defined by experts when devising the FMECA and the geodesic distances used for classifying the subset of reference vectors.
The a posteriori probability of occurrence P(f) may be calculated using a probability model having as a parameter a weighting coefficient λt defined by experts for each reference vector t, using the following formula:
Σtλtexp(−θt2/2σt2)
where θt is the geodesic distance between the anomaly vector z and the reference vector t; and where σt2 is calculated using the a priori probability of occurrence associated with the reference vector t.
Thus, the norm d2 of the anomaly vector gives the level of abnormality and the geodesic distance θ2 serves to identify the more probable failures. This is performed by relying on knowledge collected from experts, and not by relying on a failure database.
After identifying failures by calculating the probability of occurrence for each of them, it is possible to use the probability to detect faulty components. To do this, use is made of a decision grid defined by experts and giving for each physical component under analysis a probability of being faulty when a precise failure is observed.
Thus, in a step E7, the processor means 5 are configured to establish a decision grid on the basis of criteria established by experts. The decision grid may be formed by a matrix Q=(qf,c) of conditional probabilities qf,c=P(c/f) that a component c is faulty, given that a failure f has been observed and a series of coefficients corresponding to a priori probabilities of failures of each component c. The matrix Q=(qf,c) is a symmetrical positive matrix.
Furthermore, it may be observed that the decision grid may be corroborated by launching machine learning. The main role of such learning is merely to verify the information provided by the experts, thus avoiding any need to construct a database.
In a step E8, the processor means 5 are configured to use Bayesian rules to deduce the per component failure probabilities P(c) for each component c on the basis of the a posteriori probabilities of occurrence P(f) and of the decision grid Q=(qf,c).
Thus, for each component c, it is possible to estimate a probability of failure P(c) that is given by the following formula:
P(c)=[βcΣf(2qf,c−1)P(f)]01
where βc is a normalization coefficient that corresponds to an a priori occurrence of the faulty component. Furthermore, the result of the formula is truncated between 0 and 1.
Finally, in a step E9, the processor means 5 are configured to detect the faulty physical components that are responsible for the failures by using the per component failure probabilities calculated in the preceding step.
It should be observed that when an anomaly is detected, step E6 of calculating a posteriori probabilities P(f) of failures occurring makes it easy to represent the probability of each failure diagrammatically on a table or in an image. Furthermore, detecting faulty components in step E9 makes it possible to construct another image in which each failure is replaced by the real name of the component. These images may then easily be consulted by experts.
Thus, the present invention serves firstly to diagnose an anomaly and then to classify failures associated with that anomaly by using a method that is open and suitable for interpretation by engine experts.
Furthermore, decoupling the detection of an abnormality from the classification of failures enables new types of failure to be detected that are not listed by experts, enables them to be analyzed, and then enables them to be included in turn in the list of potential failures.
Furthermore, in a preferred implementation, the various steps of the method of the invention are executed by means of program code instructions.
Consequently, the invention also provides a computer program product, the program being capable of being implemented in the processor means or a computer system, the program including code instructions adapted to implementing a method of the invention as described above.
The program may use any programming language, and be in the form of source code, object code, or code intermediate between source code and object code, such as in a partially compiled form, or in any other desirable form.
The invention also provides a data medium readable by a computer and including computer program instructions as mentioned above.
The data medium may be any entity or device capable of storing the program. For example, the medium may comprise storage means such as a read-only memory (ROM), e.g. a compact disk (CD) ROM, or a microelectronic circuit ROM, or any other recording means.
Furthermore, the data medium may be a transmissible medium such as an electrical or optical signal, that may be conveyed via electrical or optical cable, by radio, or by other means.
Alternatively, the data medium may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the method in question.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0858609 | Dec 2008 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2009/052511 | 12/14/2009 | WO | 00 | 8/29/2011 |
