Clearance detection system and method using frequency identification

Various embodiments of the present disclosure provide a clearance detection system and method using frequency identification. Generally, the clearance detection system of the present disclosure uses a fiber optic sensor to determine a distance between a target traversing through a target area and a wall of a housing or casing within which the target is rotating. In various embodiments, the clearance detection system does so by projecting a light field including multiple alternating and diverging illuminated and non-illuminated regions into the target area, collecting light reflected off of the target as the target traverses through the light field, generating an oscillatory signal based on the collected reflected light, identifying the dominant frequency of the signal, and using the dominant frequency to determine the distance.

This application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 62/054,646, filed on Sep. 24, 2014, the entire contents of which are incorporated herein by reference.

BACKGROUND

Turbines, such as those used in gas turbine engines, generally include a casing or shroud that houses a rotor configured to rotate therein. The rotor includes a hub and a plurality of circumferentially-spaced rotor blades extending outward from the hub. The components of the rotor are positioned, configured, and sized such that a relatively small gap or clearance exists between the tips of the rotor blades and the interior surface of the casing. The size of the clearance is directly related to the efficiency at which the turbine operates. During operation, however, the size of the clearance may vary due to a number of factors, such as the expansion and/or contraction of the components of the turbine due to temperature fluctuations, movement of the components of the turbine, and/or degradation of the tips of the rotor blades and/or the interior surface of the casing.

Since the size of the clearance between the tips of the rotor blades and the interior surface of the casing directly affects the efficiency of the turbine, and since the size of the clearance may vary during operation of the turbine and thus vary the efficiency of the turbine, it is desirable to monitor the size of the clearance to ensure optimal turbine operation. There are a variety of known clearance detection systems configured to determine the size of the clearance.

One known clearance detection system is described in U.S. Pat. No. 4,049,349. This known clearance detection system includes a skewed arrangement of optical fibers connected to a remote light-source and a light detector. The skewed arrangement of the optical fibers generates two diverging light beams. This known clearance detection system measures the time-delay between a target reflecting light from the first and the second light beams. The time-of-flight between the two beams is calibrated to the clearance between the target and the sensor head.

Another known clearance detection system is described in “Time-Of-Flight Tip-Clearance Measurements,” authored by H. S. Dhawal, A. P. Kurkov, and D. C. Janetzke and published in the 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Jun. 20-24, 1999. This known clearance detection system includes a time of flight probe that incorporates two separate optical probes into a sensor head. The two probes are tilted equally with respect to the axis of the sensor head. The signal generated by a target that traverses across the skewed light beams is timed to extract the time of flight between the two beams. The measured time of flight is calibrated to the clearance between a target and the sensor.

Another known clearance detection system is described in U.S. Pat. No. 8,009,939. This known clearance detection system includes a plurality of optical fibers that project light of different wavelengths at a target. This known clearance detection system exploits the relative amplitudes of the wavelengths of the reflected light to calculate the clearance between the target and the sensor face.

Another known clearance detection system is described in U.S. Pat. No. 7,400,418. This known clearance detection system relies on the detection of the power spectrum due to interference between a beam of light reflected from a target and a reference beam into which a deliberate delay was introduced using an acousto-optical module. The interference pattern produced by the two beams depends on the clearance between the moving target and the collimator and the delay introduced into the reference beam by the frequency modulated acousto-optical module. In this clearance detection system, the intensity of the reflected light has to be matched with the delayed reference beam through an attenuator.

Another known clearance detection system is described in U.S. Pat. No. 8,624,604. This known clearance detection system includes a main waveguide and a reference element that is provided at a position intermediate the proximal and distal ends, or at the distal end, of the waveguide. The transmitter/receiver is arranged to transmit an electromagnetic signal through the main waveguide and receive a reflection of the transmitted electromagnetic signal from the reference element, the casing surface, and from a target, enabling the relative positioning of the reference element, the casing surface, and the target to be simultaneously determined.

Another known clearance detection system is described in U.S. Pat. No. 7,333,912. This known clearance detection system includes a reference geometry disposed on a first object having an otherwise 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. This known clearance detection 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.

Another known clearance detection system is described in U.S. Pat. No. 8,164,761. This known clearance detection system uses a reference beam and a signal beam that have different focal lengths or that diverge/converge at different rates. The beams are fixed to the stationary member and proximate to each other. The beams are projected across a clearance between the stationary member and a target toward the target. The reference and signal beams are reflected by the target when the target intersects the reference and signal beam, and the reflected reference and signal pulses are obtained. One or more features of the reflected reference pulse and the reflected signal pulse, such as a rise time of the pulses, a fall time of the pulses, a width of the pulses, and a delay between the reflected reference pulse and the reflected signal pulse (among other factors) are obtained. The width of the clearance is obtained using at least one of these factors.

There is a continuing need for new and improved clearance detection systems and methods for determining the clearance between the blade tip and the casing.

SUMMARY

Various embodiments of the present disclosure provide a clearance detection system and method using frequency identification. Generally, the clearance detection system of the present disclosure uses a fiber optic sensor to determine a distance between a target traversing through a target area and a wall of a housing or casing within which the target is rotating. The fiber optic sensor is mounted to and supported by the wall of the housing. For instance, in an example embodiment in which a turbine rotor including a plurality of turbine blades is rotating within a casing, the clearance detection system uses a fiber optic sensor mounted to and supported by a wall of the casing to determine the distance between a tip of one of the rotor blades of the turbine rotor and the wall of the casing. In various embodiments, the clearance detection system does so by projecting a light field including multiple alternating and diverging illuminated and non-illuminated regions into the target area, collecting light reflected off of the target as the target traverses through the light field, generating an oscillatory signal based on the collected reflected light, identifying the dominant frequency of the signal, and using the dominant frequency to determine the distance.

DETAILED DESCRIPTION

Components

Referring now to the drawings,FIGS. 1, 2, and 3Aillustrate one example embodiment of the clearance detection system of the present disclosure, which is generally indicated by numeral100. In this example embodiment, the clearance detection system100includes a fiber optic sensor200and a processor300.

The fiber optic sensor200includes a housing210; a plurality of optical send fibers220a,220b,220c,220d, and220eeach configured to transmit light and each including a first end (not labeled) and an opposing second end (not labeled); a plurality of optical receive fibers230a,230b,230c, and230deach configured to transmit light and each including a first end (not labeled) and an opposing second end (not labeled); a fiber holder240; a collimator250; a light source260; and a photodetector270.

The fiber holder240is disposed within the housing210and is configured to hold portions of the send fibers220and portions of the receive fibers230in a particular arrangement relative to one another. In this example embodiment, the fiber holder240holds a portion of each send fiber220extending from the first end of that send fiber220and a portion of each receive fiber230extending from the first end of that receive fiber230. In this example embodiment, as best shown inFIGS. 1 and 3A, the portions of the send fibers220and the portions of the receive fibers230held by the fiber holder240are arranged adjacent to one another in an alternating manner such that the longitudinal axes of the portions of the send fibers220and the longitudinal axes of the portions of the receive fibers230disposed within the housing210are substantially coplanar and substantially parallel to one another.

The collimator250is at least partially disposed within the housing210adjacent to and spaced apart from the first ends of the send fibers220and the first ends of the receive fibers230. In this example embodiment, the position of the collimator250relative to the send fibers220and the receive fibers230and, more specifically, the distance between (a) the first ends of the send fibers220and the first ends of the receive fibers230and (b) the collimator250, is optimized to maximize the amount of light received by the receive fibers230. It should be appreciated that the position of the collimator relative to the send and receive fibers may be different when the clearance detection system includes a different collimator.

The light source260is connected to the second end of each send fiber220such that each send fiber220is configured to transmit light received from the light source260at the second end of that send fiber220through that send fiber220and to the first end of that send fiber220.

The photodetector270is connected to the second end of each receive fiber230such that each receive fiber230is configured to transmit light received at the first end of that receive fiber230through that receive fiber230and to the photodetector270at the second end of that receive fiber230. The photodetector270is sensitive to—i.e., is configured to detect—light of a designated wavelength, and is configured to generate an oscillatory signal (such as a voltage signal) based on the detected light of the designated wavelength. Thus, in this example embodiment, when the photodetector270receives light having a variety of different wavelengths from the receive fibers230, the photodetector270detects light of the designated wavelengths received from the receive fibers230—not light of any non-designated wavelength received from the receive fibers230—and and generates the oscillatory signal based on the detected light of the designated wavelength.

The processor300is connected (such as by a wired or a wireless connection) to the photodetector270such that the processor can receive a signal transmitted from the photodetector270.

In this example embodiment, the housing is made of stainless steel, though it should be appreciated that the housing may be made of any other suitably rigid material. The housing may take any suitable shape and have any suitable size.

In this example embodiment, the fiber holder is made of stainless steel, though it should be appreciated that the fiber holder may be made of any other suitably rigid material. The fiber holder may take any suitable shape and have any suitable size.

The send and receive fibers may be any suitable optical fibers such as, but not limited to: optical fibers having silica cores, silica clad optical fibers, optical fibers having polymer coatings, optical fibers having sapphire cores, optical fibers having gold coatings, or optical fibers having aluminum coatings. The send and receive fibers may take any suitable shape and have any suitable size.

In this example embodiment, the collimator is a spherical lens made of sapphire, though it should be appreciated the collimator may be any other suitable transparent lensing element configured to converge light beams. The collimator may take any suitable shape (such as a plano-convex shape or a double-convex shape) and have any suitable size.

In this example embodiment, the light source is a coaxially-packaged, vbg-stabilized, single-emitter, laser-emitting 685 nm wavelength red laser, though it should be appreciated that the light source may be any other suitable light source having a designated wavelength that the photodetector is sensitive to (i.e., is configured to detect).

In this example embodiment, the photodetector is a photodiode, though it should be appreciated that the photodetector may be any other suitable photodetector configured to convert incident light into an oscillating signal (such as a voltage signal) and that is sensitive to the designated wavelength of the light emitted by the light source.

The processor may be any suitable processing device or set of processing devices, such as a microprocessor, a microcontroller-based platform, a suitable integrated circuit, one or more application-specific integrated circuits (ASICs), or one or more field programmable gate arrays (FPGAs).

Operation

In this example embodiment, the clearance detection system100is used to determine the distance DTCbetween a target400a, which is a tip of a rotor blade400of a turbine rotor that rotates within a casing410, and an inner wall410aof the casing410(though it should be appreciated that the clearance detection system of the present disclosure may be used in conjunction with any suitable target). Specifically, the clearance detection system100determines the distance DTCbetween the target400aand the inner wall410aof the casing410by: (1) receiving a measured distance DCHbetween the inner wall410aof the casing410and a reference location on the fiber optic sensor200, which in this example embodiment is a surface210aof the housing210of the fiber optic sensor200; (2) determining the distance DTHbetween the target400aand the surface210aof the housing210of the fiber optic sensor200; and (3) subtracting DCHfrom DTHto determine DTC.

The housing210of the fiber optic sensor200is inserted into and secured within an opening in the casing410such that the surface210aof the housing210is recessed a distance DCHfrom the inner wall410aof the casing410a. It should be appreciated that the radial position of the opening in the casing410within which the fiber optic sensor200is inserted and secured is determined such that the range of expected DTCfall within the working, calibrated range of the fiber optic sensor200. The distance DCHis determined by the axial location of the target400asuch that the target400awill not contact the collimator250during operation.

In operation, the light source260is activated while the target400arotates within the casing410. Once the light source260is activated, the light source260emits light of the designated wavelength, and each send fiber220transmits light of the designated wavelength received from the light source260at the second end of that send fiber220through that send fiber220and to the first end of that send fiber220. Each send fiber220emits a light beam225of the designated wavelength from its first end. Specifically, and as best shown inFIGS. 1 and 2, in this example embodiment the send fiber220aemits a light beam225a, the send fiber220bemits a light beam225b, the send fiber220cemits a light beam225c, the send fiber220demits a light beam225d, and the send fiber220eemits a light beam225e.

The light beams225emitted from the first ends of the send fibers220travel to the collimator250. Before reaching the collimator250, the longitudinal axes of the light beams225are substantially parallel to one another. The collimator250diverts the light beams225such that, after traveling through the collimator250, the longitudinal axes of the light beams225diverge from one another such that the longitudinal axes of the light beams225are not parallel and extend from the collimator250in different directions. The send fibers220, the receive fibers230, and the collimator250are thus arranged relative to one another such that, when the light source260is activated, a light field including a plurality of diverging light beams225of the designated wavelength is projected into the target area through which the target400atraverses.

Put differently, in this example embodiment, the projected light field includes multiple alternating and diverging regions that are illuminated by light of the designated wavelength emitted from the light source260and not illuminated by light of the designated wavelength emitted from the light source260. The regions illuminated by light of the designated wavelength emitted from the light source are referred to herein as “illuminated regions” and the regions not illuminated by light of the designated wavelength emitted from the light source are referred to herein as “non-illuminated regions” for brevity. It should be appreciated that, in certain instances, the non-illuminated regions of the light field (i.e., the regions of the light field that are not illuminated by light of the designated wavelength emitted from the light source) may be at least partially illuminated by ambient light having a wavelength other than the designated wavelength. Such ambient light does not affect the operation of the clearance detection system of the present disclosure because the photodetector is not sensitive to the non-designated wavelength(s) of this ambient light.

As the target400atraverses through the light field (and, particularly, through the multiple alternating and diverging illuminated and non-illuminated regions), the target400areflects light of the designated wavelength back toward the collimator250. The collimator250collimates the reflected light, and each of one or more of the receive fibers230transmits the collimated reflected light received at the first end of that receive fiber230through that receive fiber230and to the photodetector270at the second end of that receive fiber230. When the photodetector270receives the reflected light from the receive fibers230, the photodetector270detects light of the designated wavelength received from the receive fibers230and generates a signal based on the detected reflected light of the designated wavelength. In this example embodiment, the signal is a voltage signal.FIG. 4Ais a graph of voltage versus time for a first instance and a second instance of the target400atraversing through the light field.FIG. 4Bis a graph of voltage versus time for the first instance of the target400atraversing through the light field. After generating the voltage signal, the photodetector270transmits the voltage signal to the processor300in a suitable manner.

The voltage signal has an oscillatory component that corresponds to the frequency at which the target400aencounters the multiple alternating and diverging illuminated and non-illuminated regions of the light field. Because the multiple alternating illuminated and non-illuminated regions of the light field diverge from one another, an inverse relationship exists between the frequency content of the voltage signal and the distance DTHbetween the target400aand the surface210aof the fiber optic sensor200(assuming a constant transverse velocity of the target400aacross the light field). Specifically, the higher the dominant frequency of the voltage signal for a particular instance of the target400atraversing through the light field, the smaller the distance DTHbetween the target400aand the surface210aof the fiber optic sensor200, and the lower the dominant frequency of the voltage signal for a particular instance of the target400atraversing through the light field, the greater the distance DTHbetween the target400aand the surface210aof the fiber optic sensor200. The processor300is configured to use this inverse relationship along with the voltage signal and the transverse velocity of the target400aas the target400amoves through the light field to determine the distance DTHbetween the target400aand the surface210aof the fiber optic sensor200(and, in turn, the distance DTCbetween the target400aand the inner wall410aof the casing410).

Specifically, after receiving the voltage signal in this example embodiment, the processor300uses a Fast Fourier Transform to resolve a portion of the voltage signal corresponding to an instance of the target400atraversing through the light field into that portion of the voltage signal's characteristic frequencies. The processor300then determines the dominant frequency, which is the frequency at which the target400aencountered the multiple alternating and diverging illuminated and non-illuminated regions of the projected light field during this particular instance of the target400atraversing through the light field.FIG. 5is a graph of the characteristic frequencies of the voltage signal shown inFIG. 4B, which corresponds to the first instance of the target400traversing through the light field, generated by performing a Fast Fourier Transform on the voltage signal inFIG. 4B. The dominant frequency for this instance of the target400atraversing through the light field (i.e., the frequency corresponding to the highest amplitude) is 3906 Hz.

After determining the dominant frequency, the processor300non-dimensionalizes the dominant frequency using the transverse velocity of the target400aduring this instance of the target400atraversing through the light field. In this embodiment, the transverse velocity of the target400ais independently calculated by: (1) acquiring an analog signal generated by a sensor (such as an optical probe, an active or passive eddy current probe, and the like) based on the rotation of the turbine shaft; and (2) multiplying the angular velocity of the turbine shaft by the radius of the rotor blade400, though it should be appreciated that the transverse velocity of the target may be determined in any suitable manner. The processor300uses the non-dimensionalized frequency to determine the distance DTHbetween the target400aand the surface210aof the fiber-optic sensor200based on a predetermined linear relationship between the non-dimensionalized frequency and the distance DTH. Equation (1), included below, represents the linear relationship in this example embodiment.
Non-dimensionalized frequency=−0.07 *DTH(in mils)+170   (1)

Using this linear relationship, the processor300determines the distance DTHbetween the target400aand the surface210aof the housing210of the fiber optic sensor200using Equation (2), included below.

After determining the distance DTH, the processor300determines the distance DTCbetween the target400aand the inner wall410aof the casing410using Equation (3), included below.
DTC=DTH−DCH(3)

In certain embodiments, the distance DCHbetween the inner wall410aof the casing410and the surface210aof the housing210of the fiber optic sensor200is measured by a user, and the clearance detection system100receives the distance DCHvia a suitable input device of the clearance detection system100(not shown) and stores the distance DCHin a suitable memory device of the clearance detection system100(not shown). The processor may retrieve the stored distance DCHwhen determining the distance DTC.

In one embodiment, the linear relationship between the non-dimensionalized frequency and the distance is determined by determining non-dimensionalized frequencies corresponding to known distances between the target and the fiber-optic sensor. More specifically, to determine the linear relationship in this embodiment, the target is traversed through the light field at a known transverse velocity for a plurality of different known distances between the fiber-optic sensor and the target. For each instance of the target traversing through the light field, the system generates a voltage signal and determines the dominant frequency of that voltage signal. The system non-dimensionalizes each dominant frequency using the corresponding known transverse velocity of the target. The system then uses the known distances between the fiber-optic sensor and the target and the corresponding determined non-dimensionalized frequencies to determine the linear relationship between the sets of known distances and corresponding determined non-dimensionalized frequencies.FIG. 6is a graph of distance versus non-dimensionalized frequency, and shows various data points representing pairs of known distances and corresponding determined non-dimensionalized frequencies and the resulting linear relationship determined using those data points.

Variations

The fiber optic sensor may include any suitable quantity of send fibers and any suitable quantity of receive fibers.

The send fibers and the receive fibers may be arranged in any suitable manner as long as the projected light field includes at least three alternating and diverging illuminated and non-illuminated regions (i.e., at least two diverging illuminated regions separated by a non-illuminated region). It should be appreciated, however, that the use of more alternating and diverging illuminated and non-illuminated regions results in a more accurate determination of the distance between the target and the fiber-optic sensor. Particularly, more alternating and diverging illuminated and non-illuminated regions results in a more robust voltage signal, which in turn enables more accurate determination of the dominant frequency of that voltage signal, which in turn enables more accurate determination of the distance between the target and the fiber-optic sensor.

The arrangement of the send and receive fibers shown inFIGS. 1 and 3Ais one example arrangement, and the present disclosure contemplates a variety of other suitable arrangements of the send and receive fibers that create a light field including multiple alternating and diverging illuminated and non-illuminated regions. Two example alternative send and receive fiber arrangements are shown inFIGS. 3B and 3C.

In certain embodiments, the fiber-optic sensor may be used as a redundant timing sensor in a conventional tip timing system, such as that described in U.S. Pat. No. 4,049,349.

In various embodiments, the clearance detection system of the present disclosure determines the distance between the fiber-optic sensor and the target in the manner described in “Time-Of-Flight Tip-Clearance Measurements,” authored by H. S. Dhawal, A. P. Kurkov, and D. C. Janetzke and published in the 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Jun. 20-24, 1999. Generally, in these embodiments, for each pair of diverging illuminated regions in the light field separated by a non-illuminated region, the system determines the time it takes the target to traverse the distance between those two diverging light regions. Assuming the light field includes N light regions, the system makesNC2of these determinations. These periods of time can be used to determine the distance between the fiber-optic sensor and the target. In one such embodiment, if multiple determinations are made and multiple corresponding distances are determines, the system averages those distances.

In certain embodiments the fiber-optic sensor includes one or more cooling tubes (such as within the housing) configured to cool certain components of the fiber-optic sensor during operation, such as via circulation of a suitable coolant through the cooling tubes.

The fiber-optic sensor may be attached to the casing in any suitable manner. In one embodiment, the housing of the fiber-optic sensor and the opening in the casing are threaded, which enables the fiber-optic sensor to be screwed into the casing. In another embodiment, the fiber-optic sensor is press-fit into the opening in the casing. In another embodiments, the fiber-optic sensor is attached to the casing using a suitable fastener or fasteners, such as screws. In another embodiment, the fiber-optic sensor is attached to the casing via an adhesive.

The present disclosure also contemplates a rotor casing having the fiber-optic sensor of the present disclosure attached thereto. In one particular example, the present disclosure contemplates a turbine including a turbine casing having the fiber-optic sensor of the present disclosure attached thereto.

It should be appreciated that the embodiment of the clearance detection system illustrated in the accompanying Figures is but one example configuration of components and sizes and shapes of such components. Other embodiments of the clearance detection system may employ different configurations of components and/or components of different sizes or shapes.

The present disclosure contemplates the use of a reference location on the fiber optic sensor that is different than the particular surface of the fiber optic sensor described above with respect toFIG. 1.

In various embodiments, once the processor determines the distance between the target and the casing, the processor causes a display of the determined distance DTC, such as by causing a display device to display the determined distance DTCand/or causing a printer to print the determined distance DTC. In other embodiments, the process causes a message indicating the determined distance DTC, such as an email message or a text message indicating the determined distance DTC, to be sent.

In certain embodiments, if the determined distance DTCis greater than a first predetermined threshold, the processor causes an alert to be sent or displayed indicating that the determined distance DTChas exceeded the first predetermined threshold. In other embodiments, if the determined distance DTCis less than a second predetermined threshold, the processor causes an alert to be sent or displayed indicating that the determined distance DTChas fallen below the second predetermined threshold.

In certain embodiments, the processor automatically causes one or more first remedial processes to be performed if the determined distance DTCexceeds the first predetermined threshold. In other embodiments, the processor automatically causes one or more second remedial processes to be performed if the determined distance DTCfalls below the second predetermined threshold. For instance, in one example embodiment, when the determined distance DTCfalls below the second predetermined threshold (e.g., when the distance between the target and the inner wall of the casing is too small), the processor automatically initiates a cooling process to cool the target, which causes the target to shrink and increases the distance DTCbetween the target and the inner wall of the casing. In another example embodiment, when the determined distance DTCfalls below the second predetermined threshold (e.g., when the distance between the target and the inner wall of the casing is too small), the processor automatically stops the target from rotating.