Patent Description:
Tip clearance sensors are known e.g. in <CIT> where an elongated conductive casing is filled with a dielectric material having a plate member and four capacitors; or <CIT> that describes a method involving an ultrasonic sensor and a radio frequency sensor; or <CIT> where a metal rod is used to provide a tuned circuit. In <CIT> a sensor regulates the moving means of a bearing. <CIT> describes a composite capacitive sensor device for use in a high temperature environment.

The gap between turbine blade tips and the turbine engine case is known to vary over the dissimilar temperature of the blades and the engine case. This gap allows either compressor air or turbine exhaust to "leak" past the blades with a resultant loss of energy generated by combustion. It is an object of the present disclosure to seek to provide an improved blade tip sensor.

In one aspect, there is provided a tip clearance sensor system in accordance with claims <NUM> to <NUM>.

In some embodiments, the composite body includes a metallic mesh separating the capacitor and a resistor of the resistor-capacitor circuit.

In some embodiments, a layer of the composite body includes a metallic mesh that is an electrode in the resistor-capacitor circuit.

In some embodiments, the composite body includes a metallic mesh on opposite sides of a section of the ceramic fibers of one of the layers that is located between two other of the layers. The metallic mesh forms electrodes of the capacitor. The section of the ceramic fibers is part of a dielectric of the capacitor.

In some embodiments, the composite body includes a metallic mesh on opposite sides of a section of electrically conductive fibers of one of the layers that is located between two other of the layers. The metallic mesh form electrodes of a resistor of the resistor-capacitor circuit. Further, the electrically conductive fibers form a resistive element of the resistor.

In some embodiments, the composite body includes electrically conductive fibers.

According to the invention the composite body includes a ceramic matrix composite body.

In another aspect, there is provided a method to form a blade tip sensor according to claim <NUM> to <NUM>.

The blade tip sensor may provide one or more improvements in real-time tip control.

The blade tip sensor may include a transceiver configured to generate and transmit an excitation signal at a plurality of frequencies to the bridge-network circuit.

Determining tip clearance may enable maintaining concentricity. Maintaining concentricity may improve fuel efficiency and decrease maintenance issues.

The blade tip sensor may include a processor configured to detect wear on the blade from a measurement signal received from the bridge-network circuit.

The wear can be implemented into an equipment health monitory system to indicate when maintenance is required.

The blade tip sensor may further include a processor configured to determine engine speed from a measurement signal received from the bridge-network circuit.

The processor may send a request to disable fuel delivery in response to detecting an over-speed condition.

In some embodiments, the blade tip sensor further includes a processor configured to detect shaft break based on a measurement signal received from the bridge-network circuit.

The blade tip sensor may have a lower cost than more complicated electronic systems such as microwave or optical speed measurement systems.

The embodiments may be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale. Moreover, in the figures, like-referenced numerals designate corresponding parts throughout the different views.

According to the invention, a blade tip sensor is provided comprising a bridge-network circuit included in a composite body. The bridge-network circuit comprises a first resistor-capacitor circuit on a first branch and a second resistor-capacitor circuit on a second branch. Each of the first resistor-capacitor circuit and the second resistor-capacitor circuit includes a corresponding capacitor having a capacitance that depends on a distance between the corresponding capacitor and a blade of a rotor.

According to the invention, a method to form a blade tip sensor is provided. Ceramic fibers are arranged in in layers to form a porous ceramic preform. The ceramic preform is formed into a ceramic matrix composite body by melt and/or vapor infiltration. At least one of the ceramic fibers is part of a capacitor of a bridge-network circuit embedded in the ceramic matrix composite body. The bridge-network circuit comprises a first resistor-capacitor circuit on a first branch and a second resistor-capacitor circuit on a second branch. The capacitor is included in the first resistor-capacitor circuit or the second resistor-capacitor circuit. The capacitor has a capacitance that depends on a distance between the capacitor and a blade of a rotor of gas turbine engine.

<FIG> is a cross-sectional view of a gas turbine engine that includes radio frequency (RF) sensors <NUM> integrated into a ceramic matrix composite (CMC) engine shroud <NUM>. The RF sensors <NUM> are arranged at various locations around the CMC engine shroud <NUM>. During operation of the gas turbine engine, blades <NUM> of a rotor <NUM> may rotate so that tips <NUM> of the blades <NUM> pass the RF sensors <NUM>. To help improve accuracy, <NUM> to <NUM> pairs of the RF sensors <NUM> may be located circumferentially around the CMC engine shroud <NUM>. Such an arrangement enables detection of engine casing deflection and, together with a tip clearance control system, enables maintaining concentricity. Maintaining concentricity may improve fuel efficiency and decrease maintenance issues.

The cross-sectional view of the gas turbine engine shown in <FIG> is of a cross-section taken in plane perpendicular to a flow path of a fluid that flows through the rotor <NUM> and past the blades <NUM>. In contrast, <FIG> is a cross-sectional view of the gas turbine engine taken in a plane parallel to the flow path. In <FIG>, the blade tip <NUM> of one of the blades <NUM> is shown in proximity to the CMC engine shroud <NUM> and, in particular, in proximity to one of the RF sensors <NUM>. In the illustrated example, the CMC engine shroud <NUM> includes an engine shroud substrate <NUM> and an abradable substrate <NUM> located in the engine shroud substrate <NUM>. The abradable substrate <NUM> is positioned radially outward from the blade tip <NUM> and has an exposed surface facing the blade tip <NUM>. A distance between the blade tip <NUM> and the abradable substrate <NUM> is referred to as tip clearance <NUM> or blade tip clearance. One of the RF sensors <NUM> is shown embedded in the abradable substrate <NUM> in <FIG>.

<FIG> illustrates a circuit diagram of a blade tip sensor <NUM> or blade tip sensor system, which includes two of the RF sensors <NUM> arranged in a bridge-network circuit <NUM> and an RF transceiver <NUM>. Although not shown in <FIG>, the bridge-network circuit <NUM> is embedded in a ceramic matric composite body, such as the CMC engine shroud <NUM>.

The RF transceiver <NUM> may be located in a different location than the two RF sensors <NUM>. For example, the RF transceiver <NUM> may be located in a cooler location than the RF sensors <NUM>. The RF transceiver <NUM> may be any device configured to generate and transmit an excitation signal to nodes Vexc+ and Vexc- of the bridge-network circuit <NUM>, and to receive measurement signals from nodes VD1 and VD2 of the bridge-network circuit <NUM>. The excitation signal may have a predetermined frequency. The excitation signal may be sine wave, a sawtooth wave, a square wave, or any other shaped periodic signal.

In the example shown in <FIG>, the RF transceiver <NUM> includes an RF transmitter-receiver integrated circuit <NUM> ("RF transceiver IC") and a Class C Amplifier <NUM>. The RF transceiver IC <NUM> is configured to generate and transmit the excitation signal over lines Tx+ and Tx- to the bridge-network circuit <NUM>. The excitation signal is fed through the Class C amplifier <NUM> to drive the bridge-network circuit <NUM> at a relatively high power and relatively high frequency, and to naturally modulate a carrier frequency, such as <NUM>. In other examples, the RF transceiver IC <NUM> includes a built-in Class C amplifier and the Class C amplifier <NUM> that is discrete from the RF transceiver IC <NUM> is not included in the RF transceiver <NUM>. In still other examples, the RF transceiver <NUM> generates and transmits a non-truncated excitation signal and no Class C amplifier is included in the RF transceiver <NUM>. The RF transceiver IC <NUM> is configured to receive the measurement signals from the bridge-network circuit <NUM> over lines Rx+ and Rx-. The RF transceiver IC <NUM> may include a DAC (digital to analog converter) that converts the received measurement signals into digital signals. An example of the RF transceiver IC <NUM> is a product from Analog Devices called the Integrated Dual RF Tx, Rx, and Observation Rx, model ADRV9009. Model ADRV9009 has a relatively wide frequency range, relatively wide bandwidth as a single chip radio.

In the illustrated example, a first one of the RF sensors <NUM> includes a first resistor-capacitor circuit, and a second one of the RF sensors <NUM> includes a second resistor-capacitor circuit. The bridge-network circuit <NUM> includes the first resistor-capacitor circuit on a first branch and the second resistor-capacitor circuit on a second branch. The first resistor-capacitor circuit on the first branch includes a resistor R1 and a capacitor C1 connected in series. The second resistor-capacitor circuit on the second branch includes a resistor R2 and a capacitor C2 connected in series. Current I1 may flow through the first branch, and current I2 may flow through the second branch. The bridge-network circuit <NUM> in the illustrated example is in a half bridge configuration. The bridge-network circuit <NUM> may include any number of active and/or passive RF impedances.

In other words, the bridge-network circuit <NUM> in <FIG> includes two active components that act as variable capacitance sensors (C1 and C2) and two fixed resistors (R1 and R2). The relationship between dielectric permittivity (ε), area (A), and separation (d) associated with capacitance is described by the equation: <MAT>.

As indicated by this equation, the value of the capacitance sensor - in other words, the effective capacitance of the capacitor C1 or C2 - varies as the distance between the capacitor C1 or C2 and the blade tip <NUM> increases or decreases. As a result, the capacitor C1 or C2 has a capacitance that depends on a distance between the capacitor C1 or C2 and the blade tip <NUM>. Furthermore, the capacitance of the capacitor C1 or C2 is an indicator of the size of the tip clearance <NUM>. To improve the performance of the capacitor C1 or C2 acting as a capacitance sensor, the capacitor C1 and C2 may be screened to attenuate effects of parasitic capacitance in integration with the engine.

Based on the high temperatures in the compressor and turbine sections of the gas turbine engine, each of the RF sensors <NUM> may preferably include a capacitive sensor as compared to an inductive sensor. This is due to the material limitations associated with the Curie Temperatures of magnetic materials. In addition, because the blades <NUM> may comprise a non-metallic or a composite material, the capacitive effects may be easier to process with more accuracy. By integrating the RF sensors <NUM> into the engine shroud <NUM>, the RF sensors <NUM> may be located strategically to capture passings of the blade <NUM> with a local maxima and minima of measured capacitances and to operate the bridge-network circuit <NUM> at a higher level of sensitivity than may be possible otherwise.

The RF transceiver <NUM> may operate in a <NUM> to <NUM> frequency band or any other suitable frequency band. For example, the driving signal may have a frequency of <NUM>, <NUM>, and/or any other frequency within the operable frequency band. The fixed resistances, R1 and R2 in the RF sensors <NUM> may be on the order of <NUM> ohms. The capacitor C1 or C2 in each of the RF sensors <NUM> may have a capacitance in a range of <NUM> to <NUM> pF. With such configuration, the bridge-network circuit <NUM> may be able generate differential voltages on the order of <NUM> to <NUM> Volts. With this level of sensitivity, the blade tip sensor <NUM> may detect relatively high levels of tip clearance and tip clearances down to a fraction of <NUM>. As the tip clearance <NUM> gets tighter during the tip clearance control operation, the variable capacitance associated with the blade tip <NUM> passing increases linearly in a predictable pattern. Because of this relationship, a processor <NUM>, such as a digital signal processor, may correlate differential output voltage vs variable capacitance of the capacitors C1 and C2, and determine the tip clearance <NUM> from the measured capacitance. The capacitance associated with the tip clearance variation also may provide an indication of the shape and wear of the blades <NUM> of the compressor or the turbine. Accordingly, the processor <NUM> may determine the shape and/or wear of the blades <NUM> and/or the tip <NUM> of the blades <NUM>.

Examples of the processor <NUM> may include a general processor, a central processing unit, a microcontroller, an engine controller, an application specific integrated circuit (ASIC), a digital signal processor, a field programmable gate array (FPGA), a digital circuit, and/or an analog circuit. The processor <NUM> may be one or more devices operable to execute logic. In some examples, the logic may include computer executable instructions or computer code embodied in memory that when executed by the processor <NUM>, cause the processor to perform the features implemented by the logic. The computer code may include instructions executable with the processor <NUM>.

In some examples, the Class C Amplifier <NUM> may be compatible with a <NUM> to <NUM> carrier frequency of the Analog Devices ADRV9009 when the Analog Devices ADRV9009 is selected as the RF Transceiver IC <NUM>. The Class C Amplifier <NUM> may also comply with power limits imposed by the Federal Communications Commission (FCC), which limit radiated emissions to a <NUM> to <NUM> milliwatt limit. Selecting <NUM> as the carrier frequency may provide sufficient resolution of the blade tip <NUM> and sufficient over-sampling capability in some configurations. In some examples, the excitation signal may be injected at multiple frequencies, such as <NUM> and <NUM>. Signals at the multiple frequencies may be superimposed on the same input nodes, or time division multiplexing techniques may be implemented. Any combination of frequencies for the excitation signal may be selected as long as the as the RF Transceiver IC <NUM> has a high enough bandwidth to accurately decode the measurement signals. Using two frequencies that are close together, such as <NUM> and <NUM>, for the excitation signal may be helpful.

The processor <NUM> may perform signal processing on the measured signals VD1 and VD2. In one example, the processor <NUM> may include an Analog Devices Tiger-Shark <NUM>-bit floating point Digital Signal Processing (DSP) integrated circuit.

Communication between the processor <NUM> and other systems (not shown) may be performed over any communications network. Examples of the communications network may include ARINC-<NUM> (AFDX, based on Ethernet), Bosch CAN bus, ARINC-<NUM>, and/or MIL-STD-1553B. The communications may be used for reporting blade clearance, speed, wear conditions, and/or any other information. The communications standard selected may be application specific.

<FIG> illustrates an example of layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> of a ceramic matrix composite (CMC) body that form one of the RF sensors <NUM>. In the illustrated example, the CMC body includes seven layers.

CMCs comprise ceramic fibers embedded in a ceramic matrix. The matrix and fibers may comprise any ceramic material. Carbon, carbon fibers, and/or silicon carbide fibers may also be considered ceramic materials. Each of the fibers may be a bundle or a tow of ceramic tiles. The fibers in each bundle or tow may be braided or otherwise arranged. The fibers may comprise a material that is stable at temperatures above, for example, <NUM> degrees Celsius. Examples of the fibers include fibers of alumina, mullite, silicon carbide, zirconia, and carbon. Examples of the ceramic matrix material include alumina, mullite, silicon carbide, zirconia, and carbon. Examples of the CMC include C/C, C/SiC, SiC/SiC, Al<NUM>O<NUM>/Al<NUM>O<NUM>, and Ox-Ox.

The RF sensor <NUM> is described herein as being integral to the CMC body and comprising layers of CMC. However, depending the conditions that the RF sensor <NUM> will ultimately be subjected to, the RF sensor <NUM> may instead be integral to and include a type of composite that includes organic material such as a carbon fiber composite. For example, the RF sensor <NUM> may be included in a compressor section of the gas turbine engine, which will not be subjected to temperatures as hot as the turbine section of the gas turbine engine.

A first layer <NUM> includes a layer of ceramic matrix composite. Ceramic matrix composites (CMCs) are a subgroup of composite materials as well as a subgroup of ceramics.

A second layer <NUM> includes a section <NUM> of CMC and a metallic mesh <NUM> located on opposite sides of section <NUM> of CMC. Because the section <NUM> of CMC is a CMC, the section <NUM> includes at least one ceramic fiber and a ceramic matrix material in which the at least one ceramic fiber is embedded. The section <NUM> of CMC, as well as the CMC in the first layer <NUM>, form a dielectric of the capacitor C1 or C2 of the resistor-capacitor circuit of the RF sensor <NUM>. The metallic mesh <NUM> located on opposite sides of section <NUM> of CMC forms electrodes of the capacitor C1 or C2. The section <NUM> of CMC and the CMC in the first layer <NUM> may have a relative permittivity (also known as dialectic constant) εr of, for example, <NUM> to <NUM>.

A third layer <NUM> includes a layer of ceramic matrix composite. A fourth layer <NUM> includes a metallic mesh for electrically shielding the capacitor C1 or C2 from the subsequent layers. A fifth layer <NUM> includes a layer of ceramic matrix composite.

A sixth layer <NUM> includes a section <NUM> of CMC and a metallic mesh <NUM> located on opposite sides of the section <NUM> of CMC. The section <NUM> of CMC includes electrically conductive fibers, such as silicon carbide fibers. The electrically conductive fibers in the section <NUM> of CMC form a resistive element of the resistor of the resistor-capacitor circuit of the RF sensor <NUM>. The metallic mesh <NUM> forms electrodes of the resistor R1 or R2.

A seventh layer <NUM> includes a layer of ceramic matrix composite. The seven layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> may be ordered as shown in <FIG>. Alternatively, the CMC body that forms the RF sensor <NUM> may include fewer, additional, or different layers than illustrated in <FIG>. The layers may be in any order suitable for the electrical elements of the RF sensor <NUM>.

Two or more of the layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> may be fully or partially created and then joined together. Alternatively, the layers may be formed during the formation of the CMC included in the CMC body. For example, the layers may be formed by assembling a porous ceramic preform having layers, and then forming the porous ceramic preform into the ceramic matrix composite body by melt and/or vapor infiltration. Metal components, such as the metallic meshes <NUM>, <NUM>, and <NUM>, may be included in the porous ceramic preform prior to infiltration and/or added after the CMC body (or one or more portions thereof) is formed.

<FIG> is a cross-section of an example of a single RF sensor <NUM> which does not form part of the present invention unless interconnected with a further RF sensor to form a bridge-network circuit;. The capacitor end of the RF sensor <NUM> faces toward the rotor so that the tip <NUM> of the blade <NUM> passes by the capacitor end of the RF sensor <NUM> during operation of the gas turbine engine. A schematic element <NUM> representing a capacitor is shown in <FIG> merely to illustrate that there is a capacitance between the electrodes included in the second layer <NUM> and that the capacitance is influenced by the blade <NUM>.

<FIG> illustrates a cross-section of two of the RF sensors <NUM> that are interconnected to form the bridge-network circuit <NUM>. Electrodes <NUM> are machined into a CMC body <NUM>, which is the engine shroud in the illustrated example. The electrodes <NUM> may couple to cabling via one or more couplers (not shown). The RF sensors <NUM> are integral to the CMC body <NUM>. In other examples, the CMC body <NUM> may be a sensor block, which may be fastened to the engine shroud or any other component where the RF sensors <NUM> may sense the blades <NUM>. The sensor block may be in the shape of a cylinder, a rectangular or square block, or any other three-dimensional shape.

<FIG> illustrate examples of the voltage of various signals in the blade tip sensor <NUM> over a common time period. <FIG> illustrates the excitation signal.

<FIG> illustrates a corresponding measurement signal Rx on the line Rx+ or Rx-. The voltage of the Rx may increase as the blade <NUM> passes by the RF sensor <NUM>. In the illustrated example, the two periods of time in which the magnitude of the Rx signal substantially increases indicates that two adjacent blades <NUM> passed by the RF sensor <NUM>. The largest amplitude of the measurement signal indicates a value from which the tip clearance <NUM> may be calculated. Alternatively or in addition, the tip clearance <NUM> may be calculated from an averaged value of the amplitude over a period of time or from any other value derived from the measurement signal Rx.

<FIG> illustrates a binary signal derived from the measurement signal Rx. The time between the trailing edges, for example, of the binary signal indicates an amount of time that passed between when the first blade <NUM> passed the RF sensor <NUM> and when the second blade <NUM> passed the RF sensor <NUM>. The rotational speed of the rotor may be calculated from the amount time that passed and the angle between the two blades.

<FIG> illustrate differences in the measurement signal Rx. depending on how worn the blade <NUM> is. <FIG> illustrates an example of a shape of the measurement signal Rx if the blade <NUM> passes completely by the RF sensor <NUM> and the blade <NUM> is in its original shape. In contrast, <FIG> illustrates an example of a shape of the measurement signal Rx if the blade <NUM> passes completely by the RF sensor <NUM> and is worn. The blade <NUM> may be considered worn if the shape of the blade <NUM> is substantially different than the original shape of the blade <NUM> and/or if the difference in the shape of the blade <NUM> from the original exceeds a threshold. The processor <NUM> may detect the differences in the shapes by using digital signal processing techniques and/or artificial intelligence. For example, the processor <NUM> may use a neural network trained on past measurement signals and corresponding known wear conditions. Once trained, the neural network may predict whether the blade <NUM> is worn when provided the measurement signal Rx.

The blade tip sensor <NUM> may provide one or more improvements in real-time tip control. The gap between turbine blade tips and the turbine engine case is known to vary over the dissimilar temperature of the blades and the engine case. This gap allows either compressor air or turbine exhaust to "leak" past the blades with a resultant loss of energy generated by combustion. In some examples, the blade tip sensor <NUM> has a resolution of one millimeter and an update rate of <NUM> milliseconds. In other examples, the blade tip sensor <NUM> may have a resolution of <NUM> inches. Alternatively or in addition, the blade tip sensor <NUM> may have any other resolution and/or update rate.

As indicated above, the blade tip sensor <NUM> may detect wear in the blades or blade creep. Blade tips may leave the factory with a substantially rectangular geometry. After about <NUM> hours of engine operation, the geometry may resemble a rounded butter-knife. Engine temperature, and abrasive or corrosive material in engine inlet air, may vary greatly depending on the engine operating environment and engine load. The blade tip sensor <NUM> may detect the loss of material from the blade tip and two centimeters inward, to a resolution of <NUM> percent (assuming <NUM> percent for a factory blade and zero percent for a missing blade). If one blade has a distinctive notch, maintenance software may identify which blade(s) have worn to the limit of a maintenance action (blade replacement). The wear can be implemented into the equipment health monitory system to indicate when maintenance is required.

Alternatively or in addition, engine speed may be measured by, for example, timing the detection of the number of blades for one shaft resolution. The processor <NUM> may invert the time to report frequency and Revolutions per Minute (RPM).

The use of two blade tip sensors or a combination of the blade tip sensor and a different type of rotational sensor, one at the front and one at the rear of a shaft in the gas turbine engine, enable detection of speed signal phase changes. As explained above, phase may be measured at the trailing edge of blades since less wear may be expected at the trailing edge of the blades. This phase may be linearly proportional to Torque (at stresses less than yield strength). Including temperature compensation may improve torque accuracy from <NUM> percent to <NUM> percent.

The blade tip sensor <NUM> may detect a break in the shaft of the gas turbine engine. Detecting the speed and torque may be the basis of detecting different speeds at the front and rear of a shaft. The shaft break detection may be detected within <NUM> to <NUM> milliseconds of the break with <NUM> percent or better confidence.

The blade tip sensor <NUM> may detect over-speed conditions. Detection of speeds exceeding <NUM> to <NUM> percent (with <NUM> percent resolution) enables fuel cut-off before the gas turbine engine self-destructs. The over-speed detection may be detected within <NUM> to <NUM> milliseconds of the over-speed condition, with a <NUM> percent or better confidence. The processor <NUM> may send a request to disable fuel delivery in response to detecting an over-speed condition.

If the RF sensors <NUM> are located circumferentially around the engine shroud, bearing wear may be measured as the differential blade gap opens up due to aging.

The blade tip sensor <NUM> may have a lower cost than more complicated electronic systems such as microwave or optical speed measurement systems. The bridge-network circuit <NUM> may be integrated into the engine shroud <NUM> due to SiC-SiC, CMC-CMC, and other high temperature materials.

The blade tip sensor <NUM> may eliminate and/or reduce inherent parasitic effects (capacitive and inductive) by sampling with multiple excitation frequencies. Arranging the RF sensors <NUM> in the bridge-network circuit <NUM> aids in accuracy and may provide reduced noise stability.

The gas turbine engine may take a variety of forms in various embodiments. For example, the gas turbine engine may be an axial flow engine. The gas turbine engine may have multiple spools and/or may be a centrifugal or mixed centrifugal/axial flow engine. In some forms, the gas turbine engine may be a turboprop, a turbofan, or a turboshaft engine. Furthermore, the gas turbine engine may be an adaptive cycle and/or variable cycle engine. Other variations are also contemplated.

The gas turbine engine may supply power to and/or provide propulsion of an aircraft. Examples of the aircraft may include a helicopter, an airplane, an unmanned space vehicle, a fixed wing vehicle, a variable wing vehicle, a rotary wing vehicle, an unmanned combat aerial vehicle, a tailless aircraft, a hover craft, and any other airborne and/or extraterrestrial (spacecraft) vehicle. Alternatively, the gas turbine engine may be utilized in a configuration unrelated to an aircraft such as, for example, an industrial application, an energy application, a power plant, a pumping set, a marine application (for example, for naval propulsion), a weapon system, a security system, a perimeter defense or security system.

To clarify the use of and to hereby provide notice to the public, the phrases "at least one of <A>, <B>,. and <N>" or "at least one of <A>, <B>,. <N>, or combinations thereof" or "<A>, <B>,. and/or <N>" are defined by the Applicant in the broadest sense, superseding any other implied definitions hereinbefore or hereinafter unless expressly asserted by the Applicant to the contrary, to mean one or more elements selected from the group comprising A, B,. In other words, the phrases mean any combination of one or more of the elements A, B,. or N including any one element alone or the one element in combination with one or more of the other elements which may also include, in combination, additional elements not listed. Unless otherwise indicated or the context suggests otherwise, as used herein, "a" or "an" means "at least one" or "one or more.

Claim 1:
A blade tip clearance sensor system (<NUM>) comprising:
a blade (<NUM>) of a rotor (<NUM>) of a gas turbine engine;
a ceramic matrix composite body (<NUM>) comprising a plurality of ceramic fibers in a plurality of layers (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>); characterised by
a bridge-network circuit (<NUM>) embedded in the ceramic matrix composite body (<NUM>), the bridge-network circuit (<NUM>) comprises a first resistor-capacitor circuit on a first branch and a second resistor-capacitor circuit on a second branch, and wherein a capacitor (C1, C2) is included in the first resistor-capacitor circuit or the second resistor-capacitor circuit, wherein a portion of the ceramic matrix composite body (<NUM>) forms a dielectric of the capacitor (C1, C2) and the capacitor (C1, C2) has a capacitance that depends on a distance (<NUM>) between the capacitor (C1, C2) and the blade (<NUM>) of the rotor (<NUM>) of the gas turbine engine, wherein each of the first and second resistor-capacitor circuits includes a capacitor (C1, C2) and a resistor (R1, R2) connected in series, the first resistor-capacitor circuit and the second resistor-capacitor circuit connecter in parallel, and wherein the bridge-network circuit (<NUM>) is located radially outward from, and in proximity to, a tip (<NUM>) of the blade (<NUM>) as the blade (<NUM>) passes the bridge-network circuit (<NUM>); and
a processor (<NUM>) configured to determine a blade tip clearance from the capacitance of the capacitor (C1, C2) derived from a measurement signal received from the bridge-network circuit (<NUM>).