PROCESS FOR ADAPTING MEASUREMENT SUITE CONFIGURATION FOR GAS
TURBINE PERFORMANCE DIAGNOSTICS
BACKGROUND OF THE INVENTION
[0001] The invention relates generally to the field of gas
turbine engine modeling. More specifically, the invention
relates to methods and systems for adapting the measurement
suite configuration of a gas turbine engine to provide robust
performance tracking in light of sensor failures or data
dropouts.
[0002] Gas turbine performance diagnostics concerns itself with
tracking changes in engine module performance measures
(typically efficiency and flow parameters) as the engine
deteriorates over time. The primary sources of information
driving this methodology are measurements taking along the
engine's gas path, such as temperatures, pressures, speeds, etc.
Tracking fleets of engines across a wide customer/aircraft base
offers the added complexity that the measured parameters differ
in instrumentation and recording fidelity and non-repeatability
across installations.
[0003] Traditional performance diagnostic estimation methods
employ some form of predictor/corrector estimation schemes.
These procedures use the past performance estimation as a priori
information for the current performance estimate calculation.
Many of these approaches use linear estimation methods or
derivatives of them to infer the performance changes from
previous estimates and current data. The successful deployment
of such diagnostic methods depends on many factors, one of which
is its ability to adapt to different measurement suites without
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the need for employing complicated exception logic to cover all
possible measurement scenarios.
[0004] A provision to provide some form of measurement
configuration that will adapt itself to the measurement suite
currently available and adapt to changes in this measurement set
over the life of the'engine monitoring program without requiring
changes in the diagnostic software is a step to providing needed
robustness in the performance tracking process. There are
several factors that drive the need for such a measurement
configuration process.
[0005] One factor is the fact that the number and types of gas
path measurements available for conducting performance health
trending, vary with the gas turbine model and type under
consideration. For example, engines employing one or two spools,
turbojet versus turbofan engines, mixed versus non-mixed flow, a
new generation or a mature model engine, and others are factors
that dictate what is and what is not available in the form of
gas path instrumentation that provide the input parameter stream
for the performance estimation process.
[0006] Many methods known in the art for performing engine
module performance health tracking are generic in the sense that
they may be applied to any type of gas turbine. Engine model
specifics are the numeric model constants and the measurement
suite available for the application. The former is typically a
database issue whereas the latter may affect the actual software
implementation of the process. A process that adapts to any
specific measurement configuration would provide a greater
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degree of robustness and negate the need for software changes to
implement a specific measurement set.
[0007] Another factor that drives the need for measurement
configuration is that data dropouts are commonplace in aircraft
engine monitoring. Parameters may, for whatever reason,
disappear from the recorded input stream either intermittently
for periods of time, or altogether. This may occur because of
instrumentation problems, maintenance actions, recording
anomalies, etc. Whatever the cause, the effective measurement
suite changes as a result. If the performance estimation
processing is dependent on a (pre-selected) measurement suite,
the intermittent (or persistent) loss of one or more input
parameters will cause a gap in the analysis to occur.
[0008] What is needed is a more robust engine performance
tracking process that identifies the current time point
measurement suite and adapts the measurement suite to changes to
allow the performance estimation process to proceed.
SUMMARY OF THE INVENTION
[000 9] Although there are various methods and systems that
perform engine performance tracking, such methods and systems
are not completely satisfactory. The inventor has discovered
that it would be desirable to have methods and systems that
adapt the measurement suite configuration of a gas turbine
engine to provide robust performance tracking in light of sensor
failures or data dropouts.
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[0010] One aspect of the invention provides a method for
adapting measurements from a gas curbine engine for use in
performance tracking. Methods according to this aspect of the
invention preferably start wich selecting a default engine
configuration for the gas turbine engine and its application,
acquiring gas path data samples corresponding to a predetermined
number of measured parameters, deriving performanceparameters
for the gas turbine engine, comparing the measured parameters
and performance parameters at time k with predefined thresholds
to determine the quality and/or availability of each parameter,
wherein parameters determined to be questionable are defined as
configuration matrix Mc based on the default engine
configuration and present measured parameters Z , and a fault
configuration matrix Fc based on the default engine
configuration and present performance parameters .v , and adapting
performance tracking methods using the measurement configuration
matrix Mc and fault configuration matrix Fc .
[0011] Another aspect of the method is where the measurement
configuration matrix Mc is an identity matrix of size m where
m - number of measured parameters .
[0012] Another aspect of the method is where the fault
configuration matrix Fc is an identity matrix of size n where
n -the number of performance faults being estimated .
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[0013] Another aspect of the method is where a zero entry in the
measurement configuration matrix Mc main diagonal represents a
questionable or missing parameter measurement.
[0014] Another aspect of the method is where a zero entry in the
fault configuration matrix Fc main diagonal represents those
faults whose estimation is affected by the loss of a .
corresponding measurement parameter(s) that is used to calculate
the performance estimate.
[0015] The details of one or more embodiments of the invention
are set forth in the accompanying drawings and the description
below. Other features, objects, and advantages of the invention
will be apparent from the description and drawings, and from the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a block diagram of an exemplary method.
[0017] FIG. 2 is an exemplary application framework of the
individual modules of the invention.
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DETAILED DESCRIPTION
[0018] Embodiments of the invention will be described with
reference to the accompanying drawing figures wherein like
numbers represent like elements throughout. Further, it is to be
understood that the phraseology and terminology used herein is
for the purpose of description and should not be regarded as
limiting. The use of "including," "comprising," or "having" and
variations thereof herein is meant to encompass the items listed
thereafter and equivalents thereof as well as additional items.
The terms "mounted," "connected," and "coupled" are used broadly
and encompass both direct and indirect mounting, connecting, and
to physical or mechanical connections or couplings.
[0019] The invention is not limited to any particular software
language described or implied in the figures. A variety of
alternative software languages may be used for implementation of
the invention. Some components and items are illustrated and
described as if they were hardware elements, as is common
practice within the art. However, various components in the
method and system may be implemented in software or hardware.
[0020] The invention is a modular framework and may be deployed
as software as an application program tangibly embodied on a
program storage device. The application code for execution can
reside on a plurality of different types of computer readable
media known to those skilled in the art.
[0021] FIG. 1 shows an embodiment of a method for adapting the
measurement suite configuration of a gas turbine engine to
provide robust performance tracking in light of sensor failures
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or data dropouts. The method begins with acquiring engine gas
path data samples at discrete times k (step 102). The gas path
data comprises a plurality of gas path parameter measurements
such as spool speeds, temperatures, pressures and flow rates.
For a particular gas turbine engine model and its particular
application, there may be a default set of parameters measured
in the engine's gas path. This conforms to a standard "bill-of-
material" sensor configuration fcr that engine model and
application.
[0022] The default set of parameters creates a measurement suite
and is known a priori (step 103). A measurement matrix Mc is
assembled based on the default set of engine measurement
parameters and is maintained. The measurement matrix Mc is an
identity matrix of size m \rnxm) where m is the number of
measured parameters and is defined by

[0024] where i and j are row and column indices.
[0025] The vector of engine gas path parameters at a time k are
typically compared to predefined thresholds in order to
determine the quality and/or availability of the measurement
parameters (step 104). An out-of-range parameter, or a missing
parameter, would fail these tests and the vector of measurement
parameters Z would be flagged as not having the default
configuration. Likewise, if all of the parameters were within
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threshold, then the vector of measurement parameters would be
flagged as having the default configuration and would
immediately be available for performance diagnostic tracking
(step 109).
[0026] If the vector of measurement parameters Z was flagged as
not having the default configuration indicating that at least -
one measurement parameter was either outside of the predefined
threshold or was missing altogether, i.e. a data dropout. In
either case, the questionable, or missing parameter measurement
is determined not present for subsequent performance diagnostic
tracking (step 105) and the measurement configuration matrix Mc
is altered (step 106)

[0028] By way of background, engine performance diagnostic
tracking typically estimates sensor performance as well as
engine module performance. For the engine modules, the
performance parameters usually take the form of changes, or
performance deltas (from a nominal or installed state) in module
efficiency and flow parameters. For the sensors themselves, a
sensor error parameter x is part of the gas path analysis and is
estimated and tracked.
[0029] Performance deltas are calculated from a reference
(nominal). If all of the parameter deltas are close to zero and
one parameter has a large delta value, it may indicate a
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"sensor" error. Sensor error is not limited to the instrument,
but may include an error in the data recording, data reduction,
normalization, etc.
[003 0] The sensor error is a measure of the accuracy of the
sensor and is included in the performance estimation process to
absorb the effects of sensor bias and drift and to a lesser
degree, sensor nonrepeatability. Performance estimation helps in
preventing corruption of the module performance estimates from
sensor errors. Therefore, if a measurement parameter x is not
present, any subsequent analysis should effectively drop the
associated sensor error estimate from the performance estimation
process. This process is controlled by a fault configuration
matrix Fc which is assembled and tracks the measurement
configuration matrix Mc .
[0031] When the measurement configuration matrix Mc is based on
a default engine configuration, the fault configuration matrix
Fc is also an identity matrix. If Mc is altered, Fc will also
be altered. A set of rules may be developed to control how Fc is
changed as a function of the change in Mc , and may vary from
application to application. A fault configuration matrix Fc is
assembled based on the default set of fault parameters and is
maintained. The fault configuration matrix Fc is an identity
matrix of size n {nxn) where n is the number of performance
faults being estimated x by the subsequent performance
diagnostic tracking process and n>m. The fault configuration
matrix Fc is defined by
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[0033] The fault configuration matrix Fc includes the module
estimated performance faults x (efficiency and flow parameter
changes) as well as sensor faults and any other engine system
fault that may be included in the performance diagnostic
tracking process. The list of potential faults (JI) to estimate
for the default measurement configuration is predefined and
specified by the analyst performing the diagnostic tracking.
[0034] If the measurement configuration matrix Mc has been
updated (step 106) from the last time sample k-l, the fault
configuration matrix Fc is similarly updated (step 107). The
fault configuration matrix Fc is altered to exclude those faults
whose estimation is affected by the loss of a corresponding
measurement parameter(s) that is used to calculate the
performance estimate (performed in step 106) .
[0035] For example, if the pressure measurement sensor coupled
to the inlet of the High Pressure Compressor (HPC) (referred to
as P2 5) failed, or if its output signal suffered a dropout, the
measurement configuration matrix Mc is adjusted to reflect this.
If the qth measured parameter is P25, Mc{q,q) would be set to 0 .
The fault configuration matrix Fc is similarly updated,
Fc(p,p) = 0 , where the p'h parameter in the fault set represents
the P2 5 sensor fault. In general, Fc is updated by
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[0037] In the performance tracking calculations that follow, the
measurement configuration matrix Mr and fault configuration
matrix Fc effectively removes the P25 measurement from the
performance fault calculations and the P2 5 sensor error estimate
from the active fault list by adaptation. Performance
calculations are typically performed using a least squares, or
filter is one approach that is used and is a generalized least
squares approach that is used to teach the method.
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[0038] Performance tracking estimates are changes in various
performance parameters such as efficiencies, flow capacities,
sensor faults, etc., from perceived changes in the measured gas
path parameters. The changes, or performance deltas, in the
measured gas path parameters are calculated as percent changes
from a reference that is typically the nominal engine values
expected for the given flight and power condition. The percent
deltas are calculated for each measured parameter m and stored
as wxl vectors of percent deltas Z . The percent delta changes
in the performance parameters n are computed as an nxl vector x
as


[0040] where x0 is an nx1 vector of a priori estimations for the
performance changes, D is an nxm Kalman gain matrix, and H is
an mxn set of influence coefficients in percent which
represents the physics model for how measurement parameters
change for 1% changes in the performance parameters. H
represents a linear approximation model of the
interrelationships between the performance parameter deltas x
and the measured gas path parameter deltas Z .
[0041] The influence coefficient matrix H is typically
fault submatrix H„ and a sensor performance fault submatrix H .
He is an mxnc submatrix and Hs is an mxns submatrix where ne
represents the number of engine performance faults being tracked
and //. is the number of measurement sensor faults being tracked
where ne^ns and ns>m. The total number of performance faults is
n = ne+ns and H is the concatenation of the two submatrices, i.e.
H = [He:Hs]. This linear model takes the form

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[0043] where 6 is an mx\ vector of random noise representing
measurement nonrepeatability. (5) provides a means of
calculating an estimate x for the true (and unknown) vector of
performance changes x . The Kalman gain matrix D is defined as


[0045] where R represents an mxm measurement covariance matrix
and P0 represents an nxn state covariance matrix. In the above
ecuations, a -1 superscript indicates matrix inversion and a T
superscript indicates matrix transposition.
[0046] The measurement covariance matrix R is calculated from
Knowledge of the known measurement sensor nonrepeatability
standard deviations cr ,i = 1,2,...,ns and the sensor performance fault
influence coefficient submatrix H5 as

[0048] where S is an nsx\ vector of the ai , i = 1,2,..., ns values and
diag{S) is an >isx-ns diagonal matrix with 5 entries on the main
diagonal and entries outside of the main diagonal are all zero.
(5), (6), (7) and (8) represent Kalman filter equations.
[0049] Once the measurement configuration Mc and fault
configuration Fc matrices have been updated (steps 106, 107),
they may be used to adapt the performance estimation process
described above to fit the measurement configuration of the
current data point being processed (step 109) as follows.
[0050] The measurement configuration Mc and fault configuration
Fc matrices adapt the influence coefficient matrix H, creating
a modified influence coefficient matrix H
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' [0052] The measurement configuration Mc in conjunction with the
modified influence coefficient matrix H adapt the measurement
sensor nonrepeatability standard deviations S , creating a
modified measurement covariance matrix R

[0054] where / is an mxm identity matrix.
[0055] The modified influence coefficient matrix H and modified
measurement covariance matrix R adapt the Kalman gain matrix D

[0057] The performance delta x is calculated using the modified
values and is output

[0059] The performance changes for x{i) where Fc(i,i) = l, and
i = 1,2,...,n , may be reported to the end-user for performance
tracking purposes.
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[0060] Shown in FIG. 2 is a framework 201 embodiment of the
various modules that comprise and implement the invention. The
framework 2 01 receives data from the monitoring sensors located
on the gas turbine engine.
[0061] The framework 201 comprises a data normalizer 203 coupled
to a measurement adaptation module 205. which is coupled to an
adaptive Kalman filter 207 for estimating gas turbine engine
module performance changes. The data normalizer 203 compares
acquired engine measurement data against an engine model to
t-~\ci-.--i ~~^ the sroins o,Cirf ^rmar ce measurement deltas The
measurement adaptation module 205 comprises a Z vector assembler
209, an Fc matrix assembler 211 and an Mc matrix assembler 213.
The measurement deltas are assembled into a vector Z 209 and
appropriate changes are made to the Mc matrix 213, as previously
described. The Fc matrix assembler 211 modifies the performance
faults that will be estimated from the current configuration of
measurement deltas. The adaptive Kalman filter 207 comprises an
adaptive influence H matrix engine 215, an adaptive measurement
covariance R matrix engine 217, an adaptive Kalman gain D
matrix engine 219 and an adaptive state estimation x engine 221.
The measurement configuration Mc 213 and fault configuration Fc
211 matrices adapt the Kalman filter 207 to a given measurement
suite. This process is performed at every time step k for which
data is available for processing.
[0062] The effect of updating the Kalman filter estimation
equations as outlined above for a specific measurement
configuration Mc and associated fault configuration Fc is to
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produce identically what would be tracked for performance
changes given that measurement configuration suite from the
onset. Since this process may be implemented in software or
hardware, either on-board or off-board, the advantage in
employing the invention is that there is no need for dynamic
reallocation of memory to accommodate the changes in matrix
sizes dictated by different measurement configurations. In the
process described above, the matrix dimensions are fixed to the
default measurement size in and performance fault size n and do
not change.
[0063] The use of the configuration matrices Mc and Fc zero-out
the appropriate rows and columns in the associated Kalman filter
estimation process such that the remaining non-zero portions
provide the same results one would obtain using the lower
dimensional quantifies dictated by the measurement suite. In the
previous example cited where the P2 5 pressure measurement drops
out, the measurement delta vector has essentially been reduced
in size by one, to an (m-l)xl vector. This reduction in
dimensionality would normally require associated reductions in
the model numerical elements, i.e. H, R ,and D . The use of the Mc
and Fc configuration matrices allow the original dimensions to
be maintained. This supports an autonomous process that may
reconfigure itself in accordance to the present measurement
configuration without need for software changes or manual
intervention.
[0064] One or more embodiments of the present invention have
been described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
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scope of the invention. Accordingly, other embodiments are
within the scope of the following claims.
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CLAIMS
What is claimed is:
1. A method for adapting measurements from a gas turbine
engine for use in performance tracking comprising:
selecting a default engine configuration for the gas
turbine engine and its application;
acquiring gas path data samples corresponding to a
predetermined number of measured parameters;
deriving performance parameters for the gas turbine engine;
comparing the measured parameters and performance
parameters at time k with predefined thresholds to determine the
parameters determined to be questionable are defined as not
present for performance tracking;
determining a measurement configuration matrix Mc based on
the default engine configuration and present measured parameters
Z , and a fault configuration matrix Fc based on the default
engine configuration and present performance parameters x0 ; and
adapting performance tracking methods using the measurement
configuration matrix Mc and fault configuration matrix Fc .
2. The method according to claim 1 wherein the measurement
configuration matrix Mc is an identity matrix of size m where
m = number of measured parameters .
3. The method according to claim 2 wherein the fault
configuration matrix Fc is an identity matrix of size n where
n - the number of performance faults being estimated .
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4. The method according to claim 3 further comprising
performing performance tracking calculations using a least
squares or generalized least squares estimation calculation.
5. The method according to claim 4 wherein the least squares
estimation.is a Kalman filter.
6. The method according to claim 5 wherein the Kalman filter
is an adaptive Kalman filter.
7. The method according to claim 6 wherein a zero entry in the
measurement comf ig-aration raaere:-: M.. main diagonal represents a
questionable or missing parameter measurement.
8. The method according to claim 7 wherein a zero entry in the
fault configuration matrix Fc main diagonal represents those
faults whose estimation is affected by the loss of a
corresponding measurement parameter(s) that is used to calculate
the performance estimate.
9. The method according to claim 8 further comprising adapting
an influence coefficient matrix H using the measurement
configuration Mc and fault configuration Fc matrices, creating
a modified influence coefficient matrix H .
10. The method according to claim 9 further comprising adapting
a measurement sensor nonrepeatability standard deviation S using
the measurement configuration Mc matrix and adapted influence
coefficient matrix H, creating a modified measurement covariance
matrix R .
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11. The method according to claim 10 further comprising
adapting a Kalman gain matrix D using the modified influence
coefficient matrix H and modified measurement covariance matrix
R .
12. The method according to claim 11 further comprising
calculating a performance delta x using the adapted Kalman gain
matrix D , the adapted influence coefficient matrix H , present
measured parameters Z , and present performance parameters x0 .
13. A system for adapting measurements from a gas turbine
engine for use in performance tracking comprising:
means for selecting a default engine configuration for the
gas turbine engine and its application;
means for acquiring gas path data samples corresponding to
a predetermined number of measured parameters;
means for deriving performance parameters for the gas
turbine engine;
means for comparing the measured parameters and performance
parameters at time k with predefined thresholds to determine the
quality and/or availability of each parameter, wherein
parameters determined to be questionable are defined as not
present for performance tracking;
means for determining a measurement configuration matrix Mc
based on the default engine configuration and present measured
parameters Z , and a fault configuration matrix Fc based on the
default engine configuration and present performance parameters
x ; and
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means for adapting performance tracking methods using the
measurement configuration matrix Mc and fault configuration
matrix Fc .
14. The system according to claim 13 wherein the measurement
configuration matrix Mc is an identity matrix of size m where
/;; = number of measured parameters .
15. The system according to claim 14 wherein the fault
configuration matrix Fc is an identity matrix of size n where
i' =rhc number of >?er!o'~"uince faults being estimated .
16. The system according to claim 15 further comprising means
•for performing performance tracking calculations using a least
squares or generalized least squares estimation calculation.
17. The system according to claim 16 wherein the least squares
estimation is a Kalman filter.

18. The system according to claim 17 wherein the Kalman filter
is an adaptive Kalman filter.
19. The system according to claim 18 wherein a zero entry in
the measurement configuration matrix Mc main diagonal represents
a questionable or missing parameter measurement.
20. The system according to claim 19 wherein a zero entry in
the fault configuration matrix Fc main diagonal represents those
faults whose estimation is affected by the loss of a
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corresponding measurement parameter(s) that is used to calculate
the performance estimate.
21. The system according to claim 20 further comprising means
for adapting an influence coefficient matrix H using the
measurement configuration Mc and fault configuration Fc .
matrices, creating a modified influence coefficient matrix H .
22. The system according to claim 21 further comprising means
for adapting a measurement sensor nonrepeatability standard
deviation S using the measurement configuration M.. matrix and
adapted influence coefficient matrix H , creating a modified
measurement covariance matrix R .
23. The system according to claim 22 further comprising means
for adapting a Kalman gain matrix D using the modified influence
coefficient matrix H and modified measurement covariance matrix
R .
24. The system according to claim 23 further comprising means
for calculating a performance delta x using the adapted Kalman
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gain matrix D , the adapted influence coefficient matrix H,
present measured parameters Z , and present performance
parameters x0 .

A method and system is described that estimates gas turbine
engine module performance changes from gas path measurement data
using a linear predictor-corrector estimation process. A
measurement configuration matrix Mc and a fault configuration
Fc matrix are assembled. The configuration matrices Mc and Fc
are used to adapt a typical linear predictor-corrector type of
performance estimation process to a given measurement suite and
are ideally suited for linear estimation systems such as those
using a Kalman filter.