Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CLEARANCE MEASUREMENT SYSTEM AND METHOD OF OPERATION
BACKGROUND
The invention relates generally to clearance measurement systems, and more
particularly to a clearance measurement system for measuring a clearance
between a
stationary component and a continuous rotary component of a rotating machine.
Various types of sensors have been used to measure the distance between two
objects.
In addition, these sensors have been used in various applications. For
example, a
steam turbine has a rotating bucket that is disposed adjacent a carrier. The
clearance
between the rotating bucket and the carrier varies due to various operating
conditions,
such as changes in temperature, oxidation of the bucket tip, and so forth. It
is
desirable that a gap or clearance between the rotating bucket and the carrier
be
maintained during operation of the steam turbine.
One existing sensor is a capacitance probe, which measures a capacitance for
estimating the clearance between two components. Unfortunately,
existing
capacitance-based measurement techniques are limited in that they yield a
direct
current voltage based measurements for measuring clearances between stationary
and
rotating structures that are continuous in the direction of rotation. The
measurements
yield a static output in time, such as a direct current voltage level
proportional to the
clearance As a result, the measurements do not account for changes in the
clearance
due to changes in temperature of the components, electronic drifts in the
gain, offset
of the electronics, oxidation of the bucket tip, and other factors.
Moreover, these clearance measurement systems are typically employed to
measure
clearances between components during design and offline testing.
Unfortunately,
these existing systems are ineffective for in-service measurements due to the
noise
and drift generated by changes in the geometry of the components, among other
factors. Instead, in-service clearance control is based on the clearance
measurements
previously taken during design and offline testing of components. As the
components
become worn during service, the offline measurements become ineffective for in-
service clearance control.
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Accordingly, a need exists for providing a clearance measurement system that
provides an accurate measurement of clearance between two components by
minimizing the effect of calibration drift and noise in the system. It would
also be
advantageous to provide a self-calibrating clearance measurement system that
could
be employed for accurate clearance measurement for parts in operation.
BRIEF DESCRIPTION
In accordance with certain embodiments, the present technique has a clearance
measurement system. The clearance measurement system includes a reference
geometry disposed on a first object having an otherwise continuous surface
geometry
or non-continuous surface geometry and a sensor disposed on a second object,
wherein the sensor is configured to generate a first signal representative of
a first
sensed parameter from the first object and a second signal representative of a
second
sensed parameter from the reference geometry. The clearance measurement system
also includes a processing unit configured to process the first and second
signals to
estimate a clearance between the first and second objects based upon a
measurement
difference between the first and second sensed parameters.
In accordance with certain embodiments, the present technique has a rotating
machine. The rotating machine includes a rotating component spaced apart from
a
stationary component, wherein the rotating component comprises a continuous
surface in the direction of rotation of the rotating component and a reference
geometry
disposed on the continuous surface of the rotating component. The rotating
machine
also includes a sensor configured to generate first and second signals
representative of
first and second sensed parameters corresponding to the rotating component and
the
reference geometry, respectively and a processing unit configured to process
the first
and second signals to estimate a clearance between the rotating and stationary
components based upon a measurement difference between the first and second
sensed parameters.
In accordance with certain embodiments, the present technique provides a
method of
measuring a clearance between a first object and a second object. The method
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includes generating a first signal indicative of a first sensed parameter
corresponding
to the first object via a sensor disposed on the second object and generating
a second
signal indicative of a second sensed parameter corresponding to a reference
geometry
disposed on a continuous surface geometry of the first object via the sensor
disposed
on the second object. The method also includes processing the first and second
signals to estimate the clearance between the first and second objects based
upon a
measurement difference between the first and second sensed parameters.
DRAWINGS
These and other features, aspects, and advantages of the present invention
will
become better understood when the following detailed description is read with
reference to the accompanying drawings in which like characters represent like
parts
throughout the drawings, wherein:
FIG. 1 is a diagrammatical perspective illustration of a steam turbine having
a
clearance measurement system in accordance with embodiments of the present
technique;
FIG. 2 is a partial diagrammatical perspective illustration of rotating
buckets of the
steam turbine of FIG. 1 in accordance with embodiments of the present
technique;
FIG. 3 is a diagrammatical perspective illustration of a generator having the
clearance
measurement system in accordance with embodiments of the present technique;
FIG. 4 illustrates a perspective view of the steam turbine of FIG. 1 having
the
clearance measurement system for measuring the clearance between the rotating
buckets and the carrier in accordance with embodiments of the present
technique;
FIG. 5 is a cross-sectional view of a portion of the steam turbine of FIG. 4
wherein
the present clearance control technique can be used in accordance with
embodiments
of the present technique;
FIG. 6 is a detailed cross-section view of FIG. 5 in accordance with
embodiments of
the present technique;
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FIG. 7 illustrates a rotating component with a notch for the steam turbine of
FIGS. 4
and 5 in accordance with embodiments of the present technique;
FIG. 8 is a graphical representation of capacitance measured by the clearance
measurement system of FIG. 4 from the rotor of FIG. 7 in accordance with
embodiments of the present technique;
FIG. 9 illustrates a rotating component with a multi-level notch for the steam
turbine
of FIGS. 4 and 5 in accordance with embodiments of the present technique;
FIG. 10 is a graphical representation of capacitance measured by the clearance
measurement system of FIG. 3 from the rotor of FIG. 9 in accordance with
embodiments of the present technique;
FIG. 11 illustrates a rotating component with multiple notches for the steam
turbine of
FIGS. 4 and 5 in accordance with embodiments of the present technique;
FIG. 12 illustrates an exemplary configuration of sensors employed for the
clearance
measurement system of FIG. 1 in accordance with embodiments of the present
technique;
FIG. 13 is a diagrammatical illustration of the clearance measurement system
of
FIGS. 1 and 4 in accordance with an embodiment of the present technique; and
FIG. 14 is a graphical representation of clearance measured by the clearance
measurement system of FIG. 13 in accordance with embodiments of the present
technique.
DETAILED DESCRIPTION
As discussed in detail below, embodiments of the present technique function to
provide an accurate measurement of clearance between two objects in various
systems
such as a steam turbine, a generator, a turbine engine (e.g., airplane turbine
engine), a
machine having rotating components and so forth. Referring now to the
drawings,
FIG. 1 illustrates a steam turbine 10 having a clearance measurement system 12
for
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measuring a clearance between two objects in the steam turbine 10. In the
illustrated
embodiment, the clearance measurement system 12 is configured for measuring
the
clearance between a rotating component 14 and a stationary component 16 in the
steam turbine 10 that will be described in detail below.
FIG. 2 is a partial diagrammatical perspective illustration of a rotating
component
such as rotating buckets 20 of the steam turbine of FIG. 1 in accordance with
embodiments of the present technique. In the illustrated embodiment, the
rotating
buckets 20 are arranged in a configuration having different stages 22 within
the
stationary component i.e., carrier 16. It should be noted that the carrier 16,
which is
disposed about the stages 22, is not being shown for the ease of illustration.
The
stages 22 within the carrier 16 include a plurality of rotating buckets 24
longitudinally
spaced apart from one another along the length (and axis of rotation) of the
steam
turbine 10 of FIG. 1. In addition, the rotating buckets 24 are radially spaced
apart
from the carrier 16. In other words, the outer diameter of the rotating
buckets 24 is
smaller than the inner diameter of the carrier 16 as illustrated with
reference to FIGS.
1 and 2. Therefore, a
relatively small clearance exists between the outer
circumference of the rotating buckets 24 and the inner surface of the carrier
16.
Further, with the exception of the clearance control features discussed in
detail below,
the rotating buckets 24 form a continuously circular structure about an axis
of rotation
26 of the rotating buckets 24. In this embodiment, the clearance measurement
system
12 (see FIG. 1) is configured to measure the clearance between the stationary
component (i.e., carrier) 16 and the rotating component (i.e., rotating
buckets) 24
having the continuous surface geometry (i.e., a continuously circular
geometry). In
certain embodiments, the clearance measurement system 12 may be employed to
measure the clearance between the stationary and rotating components in a
generator
as will be described below with reference to FIG. 3. However, measurement of
clearance in other rotating machinery having a rotating component with
continuous
surface geometry is within the scope of this application.
FIG. 3 is a diagrammatical perspective illustration of an electrical machine,
such as a
generator 30, having the clearance measurement system 12 in accordance with
embodiments of the present technique. In the illustrated embodiment, the
generator
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30 includes a frame assembly 32 that surrounds and supports various components
of
the generator 30. The generator also includes a rotor assembly 34, which
includes a
rotor shaft 36 extending through a rotor core 38. Further, the rotor assembly
34 also
includes magnetic assemblies 40 that are supported by the outer peripheral
surface of
the rotor core 38 and that generate a magnetic flux. The rotor assembly 34
along with
the shaft 36 can rotate inside the stator assembly 42 in a clockwise or
counter-
clockwise direction as indicated by the directional arrow 44. Such rotation
may be
facilitated by bearing assemblies that surround the rotor shaft 36. As will be
appreciated by those skilled in the art, various kinds of bearing assemblies
may be
utilized to support the rotor shaft 36.
In the illustrated embodiment, the rotor assembly 34 is located in a chamber
of the
stator assembly 42, which is in turn enclosed inside the frame 32. The stator
assembly 42 includes a plurality of stator windings 46 that extend
circumferentially
around and axially along the rotor shaft 36 through the stator assembly 42.
During
operation, rotation of the rotor assembly 34 having the magnetic assemblies 40
causes
a changing magnetic field to occur within the generator 30. This changing
magnetic
field induces voltage in the stator windings 46. Thus, the kinetic energy of
the rotor
assembly 34 is converted into electrical energy in the form of electric
current and
voltage in the stator windings 46. It should be noted that a clearance between
the
rotor and stator assemblies 34 and 42 is maintained within a pre-determined
range. In
a present embodiment, the clearance measurement system 12 is coupled to the
stator
assembly 42 for measuring the clearance between the rotor and stator
assemblies 34
and 42. In this embodiment, the clearance measurement system 12 includes a
capacitive probe and the clearance between the rotor and stator assemblies 34
and 42
is estimated based upon a capacitance sensed via the capacitive probe.
The clearance measurement system 12 employed for measuring the clearance
between
stationary and rotating components in the steam turbine and generator of FIGS.
1 and
2 is configured to convert direct current based capacitive measurements
between the
stationary and rotating components to time-varying capacitive measurements.
The
clearance measurement system 12 performs this conversion based on at least one
reference geometry (e.g., a notch, groove, slot, etc.) interrupting the
continuity of the
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continuous surface geometry (e.g., continuously circular geometry) of the
rotating
component about the axis of rotation 44. Such time-varying capacitive
measurements
are used for estimating the clearance between the rotor and stator assemblies
34 and
42 as will be described in a greater detail below.
FIG. 4 illustrates a rotating machine, such as a steam turbine 50 as
illustrated in FIG.
1, wherein aspects of the present technique can be incorporated to measure
clearance
between rotating and stationary components. The steam turbine 50 includes a
rotor 52
mounted on a shaft 54. A plurality of turbine blades 56, which may also be
referred
to as buckets, are affixed to the rotor 52. In operation, the blades 56 are
subject to
steam 58 at a high temperature and pressure, which causes the blades 56 to
rotate
about an axis 60. The blades 56 rotate within a stationary housing or shroud
62 that is
positioned radially and circumferentially around the blades 16. A relatively
small
clearance exists between the blades 56 and the shroud 62 to facilitate
rotation of the
blades 56 within the shroud 62, while also preventing excessive leakage of the
working fluid, i.e. steam, between the blades 56 and the shroud 62. In
accordance
with the present technique, one or more clearance sensors 64 are disposed
within and
circumferentially around the stationary shroud 62. In the illustrated
embodiment, the
clearance sensors 64 include capacitive probes. In certain embodiments, the
clearance
sensors 64 may include microwave-based sensors, or optical sensors, or eddy
current
sensors, and the sensed parameters may include impedance, or a phase delay, or
an
induced current, respectively. As explained in detail below, each of the
sensors 64 is
configured to generate a signal indicative of a radial and an axial position
of the
blades 56 with respect to the shroud 62 at their respective circumferential
locations.
Referring now to FIG. 5, a cross-sectional view is shown for a bottom or lower
portion 70 of the steam turbine 10 of FIG. 4, illustrating exemplary radial
and axial
clearance that may be measured by the present technique. In the illustrated
embodiment, the tip of the blade 56 includes packing teeth or seal teeth 72
that mesh
into a groove 74 provided on the inner circumference of the shroud 62. In a
present
embodiment, the clearance measurement system 12 (see FIG. 1) may be coupled to
the shroud 62 for measuring the radial and axial clearances between the tip of
the
blade 56 and the shroud 62.
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FIG. 6 is a detailed cross-section view of a portion 76 of the shroud and
blade of the
steam turbine of FIG. 5. As illustrated, the radial clearance between the seal
teeth 72
and the shroud 62 is represented by reference numeral 78 and the axial
clearance
between the teeth 72 and the shroud 62 is represented by reference numeral 80.
In a
present embodiment, the radial and axial clearances 78 and 80 represent the
clearances between the center tooth and the shroud. As will be appreciated by
one
skilled in the art, clearances between the other seal teeth and the shroud 62
may be
similarly estimated through the present technique.
In certain embodiments, due to differences in the rate of thermal expansion of
the
shroud 62 and the rotor 56, there is a potential that the radial clearance 78
may be
reduced to zero, leading to interference between the seal teeth 72 and the
groove 74.
Further, due to this differential rate of expansion, the rotor 56 may grow
axially with
respect to the shroud 62, leading to axial rubbing of the teeth 72 within the
groove 74,
thus increasing the rate of wear on the components. These undesirable
interferences
also can lead to damage of the components. The present technique provides an
on-
line measurement of radial and axial clearances 78 and 80, which may be
incorporated
into a closed-loop control strategy to maintain these clearances at values
within
acceptable limits. The control strategy may include, for example, thermal
actuation of
the shroud 62, causing it to appropriately expand when the clearance between
the
shroud 62 and the seal teeth 72 decreases. In this embodiment, thermal
actuators
utilize the property of thermal expansion to produce movement of the shroud
62. In
certain other embodiments, mechanical actuators may be used to compensate for
axial
growth of the blade 56 within the shroud 62.
As will be appreciated by those of ordinary skill in the art, the capacitance
between
two objects is a function of the overlap surface area and the separation
between the
two objects. In the present embodiment, the capacitance between the rotor 56
and the
shroud 62 is a function of the radial clearance 78 and the overlap area, which
in turn is
directly proportional to the axial clearance of the seal teeth 72 with respect
to the
shroud 62. As the rotor 56 expands radially, the radial clearance between the
seal
teeth 72 and the shroud 62 changes. Similarly, as the seal teeth 72 move
axially
across the groove 74, the area of the sensor head covered by the seal teeth 72
will
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change. These changes will result in a change in measured capacitance. In
accordance with aspects of the present technique discussed below, the change
in
capacitance can be correlated to axial and radial displacements and hence a
composite
clearance measurement may be obtained. The measurement of radial and axial
clearances 78 and 80 via the clearance measurement system 12 will be further
described below with reference to FIGS. 7-13.
FIG. 7 illustrates a rotating component 82 with a notch for the steam turbine
of FIGS.
4-5. In the illustrated embodiment, the rotating component 82 includes a
plurality of
blades or buckets 56 forming a continuously circular structure 84 about the
axis of
rotation. Further, the rotating component 82 also includes a reference
geometry 86
interrupting the continuity of the continuous circular structure 84. Examples
of such
reference geometry 86 include recesses, such as indents, notches, grooves,
slots, and
so forth.
In operation, the sensor 64 (see FIG. 4) disposed on the stationary component
62
generates a first signal representative of a first sensed parameter from the
rotating
component 82 (e.g., the continuously circular geometry). In addition, the
sensor 64
generates a second signal representative of a second sensed parameter from the
reference geometry 86. In this embodiment, the sensor 64 includes a capacitive
probe
and the first and second sensed parameters include a capacitance. Further, the
first
and second signals from the sensor 64 are processed based upon a measurement
difference between the first and second sensed parameters to estimate the
radial and
axial clearance between the stationary and rotating components 62 and 82. In
certain
embodiments, the sensor 64 may include at least two probe tips for measuring
the
axial and radial clearances between the stationary and rotating components 62
and 82.
In the illustrated embodiment, the direct current based capacitive
measurements
between the stationary and rotating components 62 and 82 are converted to time-
varying capacitive measurements based on the reference geometry 86 having a
pre-
determined depth. In certain embodiments, the reference geometry 86 may
include a
material other than the material of the rotating component 82. For example,
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the reference geometry 86 may include a notch on the rotating component 82
that is
filled with a dielectric material. FIG. 8 is a graphical representation of
capacitance 88
measured by the clearance measurement system of FIG. 4 from the rotor of FIG.
7.
The ordinate axis 90 of the capacitive measurements 88 represents the
capacitance
value sensed by the sensor 64 from the rotating component 82 and the abscissa
axis
92 represents the time period. In a present embodiment, the first signal
generated by
the sensor 64 is representative of a first capacitance sensed from the
rotating
component 82 and is represented by reference numeral 94. The first capacitance
is
representative of the clearance between the sensor 64 and the rotating
component 82
(e.g., the continuously circular geometry). Further, the sensor 64 also
generates a
second signal representative of the second capacitance sensed from the
reference
geometry 86 (e.g., recess or break in the continuously circular geometry) that
is
represented by the reference numeral 96. In this embodiment, the second
capacitance
corresponds to the depth 98 of the reference geometry 86. The difference in
the first
and second capacitances sensed by the sensor 64 and the pre-determined depth
98 of
the reference geometry 86 are utilized to determine the clearance between the
stationary and rotating components 62 and 82.
As will be appreciated by those skilled in the art, as the clearance increases
the
difference between the measurements from the rotating component 82 and the
reference geometry 86 will decrease. Similarly, as the clearance decreases
such
difference between the two measurements will increase. Typically, the sensed
capacitances are inversely proportional to the clearance between the
stationary and
rotating components 62 and 82. Therefore, if in an exemplary embodiment, if
the
clearance between the stationary and rotating components 62 and 82 doubles,
the
difference between the sensed capacitances between the stationary and rotating
components 62 and 82 will be reduced by a factor of 0.5. The following example
illustrates the effect of the change in the clearance between the stationary
and rotating
components 62 and 82 on the measured differences between the sensed
capacitances.
Example 1:
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In an exemplary rotating machinery, the sensor output from the sensor 64
corresponding to the rotating component 82 at a distance "a" from the sensor
64 is
represented by "x". Further, the sensor output corresponding to the bottom of
the
reference geometry 86 (having a depth "b") at a distance "a+b" is represented
by "y".
Assuming that the clearance between the stationary and rotating components 62
and
82 changes to "2a" then the measurement from the sensor 64 corresponding to
such
clearance will be "x/2". In this embodiment, the bottom of the reference
geometry 86
will be at a distance "2a+b"from the sensor 64. Therefore, the difference in
signal
corresponding to the rotating component 82 and the reference geometry 86 in
the first
case (at a distance a) will be "x-y". Similarly, the difference in the signal
for the
second case (at a distance 2a) will be "x/2-y". Therefore, the difference
between the
two measurements is approximately x/2 that corresponds to the clearance change
from
"a" to
Thus, in the illustrated embodiment, the clearance is determined by utilizing
the
measurement difference between sensed capacitance values in the vicinity of
the
reference geometry 86 and ones far away from the reference geometry 86.
FIG. 9 illustrates another exemplary embodiment of the rotating component 100
of
the steam turbine of FIGS. 4 and 5. In the illustrated embodiment, the
rotating
component 100 includes a multi-level reference geometry, such as a stepped
notch
102, disposed along and interrupting the continuity of the continuous circular
structure 84. In operation, the sensor 64 generates signals representative of
sensed
capacitance corresponding to the rotating component 100 (e.g., the
continuously
circular geometry) and different levels of the multi-level reference geometry
102.
Subsequently, such measurements may be utilized to estimate the clearance
between
the stationary and rotating components 72 and 100 based upon the measurement
difference between the sensed capacitances.
FIG. 10 is a graphical representation of capacitance 104 measured by the
clearance
measurement system of FIG. 3 from the rotor of FIG. 9 in accordance with
embodiments of the present technique. In the illustrated embodiment, the
sensor 64
generates a signal representative of capacitance corresponding to the
continuously
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circular surface of rotating component 100 (away from the multi-level
structure 102)
as represented by reference numeral 106. Additionally, the sensor 64 generates
signals representative of capacitances corresponding to levels of the multi-
level
structure 102 as represented by reference numerals 108 and 110. Again, the
sensed
capacitance values correspond to pre-determined depths 112 and 114 of the
reference
geometry 102. The difference in the sensed capacitances corresponding to the
levels
of the multi-level structure 102 and the surface of the rotating component 100
is
utilized to determine the clearance between the stationary and rotating
components 62
and 100. In the illustrated embodiment, multiple differences in the sensed
capacitances between the surface of the rotating component 100 and the
different
levels of the multi-level structure 102 are obtained for every rotation of the
rotating
component 100. Further, such measurements are processed and appropriate lookup
tables may be utilized to determine the clearance between the stationary and
rotating
components 62 and 100 based upon the measurement differences and the pre-
determined depths 112 and 114 of the reference geometry 102. Advantageously,
such
multiple difference measurements obtained for every rotation of the rotating
component substantially increases the speed of the clearance measurement
system.
Further, by utilizing multiple measurements (e.g. corresponding to different
levels of
the stepped notch 102) any noise components in the measurement due to factors
such
as drifts in the electronics, changes in the material properties of the
stationary and
rotating components 62 and 100 and so forth may manifest equally among all the
measurements and will be subsequently nullified while estimating the
difference in
the measurements. Thus, employing a reference geometry such as multi-level
structure 102 enables a substantially robust and drift insensitive measurement
through
the clearance measurement system.
FIG. 11 illustrates another exemplary configuration 120 of the rotating
component
having multiple notches disposed on, and interrupting the continuity of, the
continuous surface geometry 84 for the steam turbine of FIG. 1. In a presently
contemplated configuration, the rotating component 120 includes a plurality of
reference geometries or notches, such as represented by reference numerals
122, 124,
126, 128 and 130. For example, the rotating component 120 may include a multi-
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level notch 122 along with semi circular notches 124, 126, 128 and 130 having
different depths for interrupting the continuity of the continuous circular
structure 84.
In the illustrated embodiment, the sensor 64 generates signals representative
of
capacitances corresponding to each of these notches 122, 124, 126, 128 and
130.
Advantageously, the speed of the measurement system increases by employing the
plurality of notches 122, 124, 126, 128 and 130 as multiple differences
between the
sensed parameters are obtained for every rotation of the rotating component
120. In
certain embodiments, such multiple differences may be employed as a means for
self-
calibrating the clearance measurement system.
Further, such sensed parameters (i.e. capacitances) are subsequently processed
to
determine the clearance between the stationary and rotating components 62 and
120
based upon the measurement differences and the pre-determined depths of the
plurality of notches 122, 124, 126, 128 and 130. In the illustrated
embodiment,
multiple measurements based upon the plurality of notches 122, 124, 126, 128
and
130 having pre-determined geometry substantially reduces the effect of any
noise
components in the measurement. For example, noise in the measurement due to
factors such as drifts in the electronics, changes in the material properties
of the
stationary and rotating components 62 and 120 and so forth may be
substantially
reduced by employing the plurality of notches 122, 124, 126, 128 and 130.
Specifically, the noise components may manifest equally among all the
measurements
and are subsequently nullified while estimating the difference in the
measurements.
Thus, employing a plurality of notches on the rotating component 120 enables a
substantially robust and drift insensitive measurement through the clearance
measurement system.
Typically, the size of each of the plurality of notches 122, 124, 126, 128 and
130 is of
the same order as the probe tip size to facilitate receiving signals from the
bottom of
the notches 122, 124, 126, 128 and 130 without interference from the side
walls of the
respective notches. In addition, the size of each of these notches 122, 124,
126, 128
and 130 is selected such that these notches do not affect the dynamics or
performance
of the rotating machinery such as the steam turbine. In general, the probe tip
size is
typically of the same order as the clearance that is being measured. For
example, for
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a steam turbine application, the probe tip may be about 200 mils in diameter,
and the
size of the notch may be about 125-mil radius half circle. That is, the notch
may be
about 250-mil wide and about 125 mil deep half circle. In certain embodiments,
where multiple notches are employed, or where a multi-level notch is employed,
the
size of the notch step may be selected so that the signals corresponding to
the
different levels may be resolved accurately. For example, if the usable range
of the
sensor is about 150mils, and the expected range of clearances is about 100
mils, then
the size of the steps in the notch may be selected to be within 50 mils of
each other, so
that over substantial operating range of the sensor, the various levels of the
notch will
be discernible.
As illustrated above, the sensor 64 may be employed to sense capacitances
corresponding to the rotating component 120 and a plurality of reference
geometries
such as 122, 124, 126, 128 and 130. In the illustrated embodiment, the sensor
64 is a
capacitance probe. In certain embodiments, the capacitive probe 64 includes at
least
two probe tips for measuring an axial and a radial clearance between the
stationary
and rotating component of a rotating machinery. As discussed earlier,
capacitance
between the rotating component 120 and the sensor 64 is a function of two
variables,
namely the radial clearance and the axial clearance. Hence by measuring the
capacitance of the two probes, it is possible to obtain the actual values for
the
variables radial clearance and axial clearance.
FIG. 12 illustrates a plan view of an exemplary configuration 132 of the
sensor 64
employed for measuring the clearance between stationary and rotating
components in
the steam turbine of FIGS. 1 and 4. In the illustrated embodiment, the sensor
132
includes a plurality of capacitive probe tips 134, 136, 138 and 140, which may
include, for example, electrically conductive shafts. The illustrated geometry
and
relative locations of the probes 134, 136, 138 and 140 facilitate measurement
of a
large axial displacement range, for example, in excess of 0.5 inches, while
providing a
desirable resolution for radial measurements, for example, for measuring
displacements on the order of 0.01 inches. The above feature is advantageous
in
applications where the axial displacements of the rotating component 14 are
substantially larger than the radial displacements with respect to the shroud
16.
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In the illustrated embodiment, the probes 134, 136, 138 and 140 are positioned
in a
staggered manner, having a diamond shaped configuration, on the sensor head to
maximize sensitivity to change in overlap area. Other staggered configurations
may
be conceived in embodiments having greater or lesser number of probes. The
diameters of the probes 134, 136, 138 and 140 at the head or tip portion are
suitably
large enough to provide adequate overlap surface area between them and the tip
of the
blade 14. In the illustrated embodiment, i.e. for a steam turbine application,
the
probes 134, 136, 138 and 140 may be formed from a material comprising nickel,
aluminum, cobalt, or combinations thereof, such as Kovar. However, in
applications
involving higher temperatures (e.g., temperatures in excess of 1000 degrees
centigrade), a material comprising platinum, rhodium, or combinations thereof
may
be used for the probes 134, 136, 138, and 140.
FIG. 13 illustrates an exemplary configuration 142 of the clearance
measurement
system of FIGS. 1-4 in accordance with an embodiment of the present technique.
The
clearance measurement system 142 includes a sensor 144 having four probe tips
146,
148, 150 and 152 arranged in a diamond shaped configuration as illustrated
above
with reference to FIG. 12. Further, a signal generator 154 is coupled to the
probe tips
146, 148, 150 and 152 to provide input excitation signals to the sensors 146,
148, 150
and 152. In the illustrated embodiment, the signal generator 154 includes a
voltage-
controlled oscillator (VCO). The excitation signals from the signal generator
154
may be switched between the probe tips 146, 148, 150 and 152 through switches
156,
158, 160 and 162. In certain embodiments, the probe tips 146, 148, 150 and 152
are
simultaneously excited via the signal generator 154. Alternatively, the probe
tips 146,
148, 150 and 152 may be excited at different points in time to reduce the
cross talk
between the probe tips 146, 148, 150 and 152.
Moreover, amplifiers 164, 166, 168 and 170 may be coupled to the signal
generator
154 to amplify input signals received by the probe tips 146, 148, 150 and 152,
respectively. In the illustrated embodiment, a capacitor 172 and a phase
detector 174
are coupled to the probe tip 146 for measuring the capacitance through the
probe tip
146. Similarly, capacitors 176, 178, 180 and phase detectors 182, 184 and 186
may
be coupled to the probe tips 148, 150 and 152, respectively, for measuring the
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capacitance through each of these probe tips. Further, directional couplers
190, 192,
194 and 196 may be coupled to the probe tips 146, 148, 150 and 152 for
separating
incident and reflected signals from the respective probe tips.
In operation, the probe tips 146, 148, 150 and 152 are excited by the signal
generator
154 at an excitation frequency. The excitation frequency may be selected based
upon
a wire length, capacitance, geometry of probe tips 146, 148, 150 and 152, a
static
measurement capacitance, and other factors. In a present embodiment, the phase
detectors 174, 182, 184 and 186 are configured to detect reflected signals
from the
probe tips 146, 148, 150 and 152 based upon the excitation frequency to
generate a
first signal representative of a first sensed parameter, i.e., capacitance,
from the first
object such as the surface of the rotating component 14. The capacitance
through the
probe tips 146, 148, 150 and 152 is measured by measuring a phase difference
between the excitation signals and the corresponding reflected signals by the
capacitors 172, 176, 178, 180 and the phase detectors 174, 182, 184 and 186.
Similarly, the second signal representative of the second sensed parameter,
i.e.,
capacitance, is generated from the reference geometry disposed on the rotating
component 14 by measuring the phase difference between the excitation signal
and
the corresponding reflected signal from the reference geometry. In certain
embodiments, multiple signals may be generated corresponding to multi levels
of the
reference geometry disposed on the rotating component 14 such as illustrated
above
with reference to FIGS. 9 and 10. In certain other embodiments, multiple
signals may
be generated from the sensor 144 corresponding to a plurality of reference
geometries
disposed on the surface of the rotating component 14 such as discussed above
with
reference to FIG. 11.
The first and second signals generated from the sensor 144 may be then
processed via
a processing unit 198. Further, the frequency of the excitation signals from
the signal
generator may be tracked and controlled via a frequency tracking unit 200. In
operation, the processing unit 198 receives signals representative of sensed
capacitances corresponding to the rotating component 14 and the reference
geometries
disposed on the rotating component 14. Further, the processing unit 198
estimates the
clearance between the rotating and stationary components 14 and 16 based upon
the
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measurement difference between the sensed capacitances from the rotating
component 14 and the reference geometries. More specifically, such sensed
capacitances are processed to determine the clearance between the stationary
and
rotating components 14 and 16 based upon the measurement differences and the
pre-
determined sizes of the reference geometries.
The measurements based upon the referenced geometries having pre-determined
size
substantially reduces the effect of any noise components in the measurement by
factors such as drifts in the electronics, changes in the material properties
of the
stationary and rotating components 14 and 16 and so forth. In the illustrated
embodiment, the noise components may manifest equally among all the
measurements and are subsequently nullified while estimating the difference in
the
measurements. Thus, in this embodiment the time varying signals received by
the
processing unit 198 are processed and features of the signal are extracted. In
this
embodiment, the features of the signal include the baseline level and the
notch height.
Further, the extracted notch height is compared against the pre-determined
size of the
notch. As the measured notch height will be scaled depending on the clearance,
the
clearance can be determined using one of several methods. The methods include
a
lookup table, an analytical/physics based model, or a curve fit function. As
described
above, a plurality of such reference geometries may be employed and through
the pre-
determined size of such reference geometries the processing unit 198
determines the
clearance necessary to provide the measured scaling of said reference
geometry.
Therefore, any measurement error that for example introduces a fixed offset
over a
relatively long time (non-time varying, or slowly varying error) will be
eliminated
since the processing is done using the difference in measurements and not the
absolute value of the measurements. Similarly, any gain error can also be
eliminated
when a multi-level geometry is used since the processing is done on multiple
differences of the feature depths. In general, offset (slow varying) errors
can be
eliminated by employing a simple notch and gain/scaling errors can be
eliminated by
using a multi-level geometry.
Thus, by interrupting the continuity of the continuous surface geometry of the
rotating
component 14 by the reference geometry, the clearance measurement system 142
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converts direct current based capacitive measurements between the rotating and
stationary components 14 and 16 to time-varying capacitive measurements. More
particularly, interruption of the continuity of the continuous surface
geometry through
a reference geometry introduces a spike in the signal produced by the
capacitive
probes that can be used for self calibrating the sensing system and ensure
that
measurements are not affected by signal drifts.
As discussed earlier, such time varying capacitive measurements are utilized
to
estimate the clearance between the rotating and stationary components 14 and
16. In
certain embodiments, the processing unit 198 may employ a look-up table, or a
calibration curve, or other techniques for estimating the clearance based upon
the
measurement difference between the sensed capacitances and pre-determined
sizes of
the reference geometries disposed on the rotating component 14. Further, a
clearance
control unit 202 may be coupled to the processing unit 198 for controlling the
clearance between the rotating and stationary components based upon the
clearance
estimated by the processing unit 198.
FIG. 14 is a graphical representation of sensor output 204 measured by the
clearance
measurement system of FIG. 13 in accordance with embodiments of the present
technique. The ordinate axis of the output 204 represents measured notch
height 206
from the probe tips and the abscissa axis represents the axial clearance 208
measured
in mils between the rotating and stationary components 14 and 16. In the
illustrated
embodiment, graphs 210 and 212 represent the notch height measured from two
probe
tips 136 and 140 that are located at same radial position as illustrated in
FIG. 12.
Further, curves 214 and 216 represent the notch height measured from probe
tips 138
and 134 that are located on left and right side of the probe tips 136 and 140.
In a
present embodiment, the measured notch height 206 is a measure of the
clearance
between the stationary and rotating objects.
As illustrated, the electrical signature of the reference geometry such as a
notch that is
represented by the notch height 206 varies according to the notch height,
which, in
turn, is a function of the axial and radial displacement. For example, when
the probe
tip 136 is substantially close to the notch the signal received from the probe
tip is
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=
represented by curve 218. Further, as the probe tip 136 moves away from the
notch
the signals are represented by curves 220 and 224. Thus, the signal received
from the
notch changes according to the notch height and the distance of the notch from
the
probe tips 134, 136, 138 and 140. As the radial clearance between the rotating
and
stationary components increases the sensed output from the probe tips 134,
136, 138
and 138 is reduced. For example, an increase in the radial displacement
represented
by reference numeral 226 is reflected in the signals 218, 220, 222 and 224
from the
probe tip 136. Similarly, the signals from other probe tips 134, 138 and 140
changes
in response to the notch height and the distance from the probe tips as
represented by
curves 210, 212 and 216.
The various aspects of the method described hereinabove have utility in
different
applications. For example, the technique illustrated above may be used for
measuring
the clearance between rotating and static components in a steam turbine. The
technique may also be used in certain other applications, for example, for
measuring
clearance between stationary and rotating components in generators. As noted
above,
even more generally, the method described herein may be advantageous for
providing
accurate measurement of clearance between objects through sensors by
converting
direct current based capacitive measurements between the stationary and
rotating
components to time-varying capacitive measurements based on at least one
reference
geometry interrupting the continuous surface geometry of the rotating
component.
Further, the technique is particularly advantageous to provide a self-
calibrating sensor
system for accurate clearance measurement of parts, even in operation and over
extended periods of time, enabling better clearance control in parts while in
operation.
While only certain features of the invention have been illustrated and
described
herein, many modifications and changes will occur to those skilled in the art.
It is,
therefore, to be understood that the appended claims are intended to cover all
such
modifications and changes as fall within the invention.
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