SENSING SYSTEM FOR DETECTING RUBS EVENTS OR WEAR OF AN ABRADABLE COATING IN TURBO MACHINERY

A sensing system and method for detecting wear of an abradable layer on a stationary engine casing is provided. The system is capable of measuring the abradable thickness of the abradable layer by embedding abradable sensor in the abradable layer and measuring the changing electrical properties as the abradable sensor wears.

FIELD OF INVENTION/BACKGROUND

The subject matter described in this specification relates to methods and systems for detecting rubs events or the wear of an abradable layer on a stationary engine casing in a rotating machine such as a gas turbine engine.

BACKGROUND

Blade tip clearance is defined as the distance or gap between the blade tip and the engine casing. This blade tip clearance is important since it affects the efficiency, stability, and safety of the turbine engine. It is known to those having skill in the art that detecting and managing the clearance between the blade tip and the engine casing is important in providing efficient and safe operation of the engine.

Various approaches to sensing blade tip clearance have been proposed such as eddy current, microwave, capacitance, and optical sensors. Each technology has its merits and disadvantages. However, to date there is no widely accepted in-service blade tip clearance monitoring technology.

Additionally, many engine systems have an abradable layer on the stationary engine casing to avoid damaging the blades or blade tips when a rub event occurs. A major complication with current blade tip clearance monitoring sensors is that they ignore wear of the abradable layer. Wear on the abradable layer is due to rub events or erosion that happens over the life of the engine. Since the gap of interest is between the blade tip and the outer surface of the abradable layer, quantifying the thickness of the abradable layer is a previously unsolved, but important aspect in determining the blade tip clearance.

The present invention provides a means to directly sense abradable wear continuously or in discrete steps with adequate resolution. The present invention may also be used to detect rub events that may occur. Such rub detection may be used to determine a known gap condition. In the case where the sensor is flush with the engine case, zero gap condition would be obtained. Such gap information may be beneficial in calibrating other engine sensing or control systems. The present invention also describes temperature compensation and redundancy. Other benefits such as robustness and manufacturability are also described within the detailed description of this invention.

SUMMARY OF THE INVENTION

In one aspect, a sensing system for detecting blade rubs or rubs of other nature and wear of an abradable layer on a stationary engine casing is provided. The sensing system comprises an abradable sensor. The abradable rub sensor is mounted in the engine case, wherein in the sensor abrades when contacted by a blade tip or erodes due to at least one environmental condition. The abradable sensor further comprises at least one electrically conductive abradable layer, at least one non-electrically conductive abradable layer, and at least one pair of electrical leads capable of providing an electrical signal to and from the abradable sensor.

In another aspect, a sensing system for detecting blade rubs or wear of an abradable layer on a stationary engine casing is provided. The sensing system comprises an abradable sensor and a sensor conditioning unit. The abradable sensor is mounted in the engine case, wherein in the sensor abrades when contacted by a blade tip or erodes due to at least one environmental condition. The abradable sensor further comprises at least one electrically conductive abradable layer, at least one non-electrically conductive abradable layer, and at least one pair of electrical leads capable of providing an electrical signal to and from the abradable sensor. The sensor conditioning unit is in electrical communication with the abradable sensor.

In still another aspect, a method of detecting blade rubs or wear of an abradable layer on a stationary engine casing is provided. The method comprises providing a sensing system, transmitting an electrical signal between the sensor conditioning unit and the sensing abradable sensor, measuring one of a resistance, a capacitance, or a round-trip time of flight for a reflected electrical signal, and correlating the measured resistance, capacitance, or time of flight for the reflected electrical signal against a known initial measurement. The sensing system further comprises an abradable sensor and a sensor conditioning unit. The abradable sensor is embedded in the abradable layer, wherein in the sensor abrades when contacted by a blade tip or erodes due to at least one environmental condition. The abradable sensor further comprises at least one electrically conductive abradable layer, at least one non-electrically conductive abradable layer, and at least one pair of electrical leads capable of providing an electrical signal to and from the abradable sensor. The sensor conditioning unit is in electrical communication with the abradable sensor.

DETAILED DESCRIPTION

Embodiments of this invention use one or more abradable sensors that are embedded within the engine case or the abradable layer of a stationary engine casing to measure blade rubs or the abradable thickness of the abradable layer. As the abradable layer wears through contact by the blades or erosion, the sensor also will wear. The wear of the sensor alters specific electrical properties of the sensor which is detectable by a sensor conditioning unit. The sensor conditioning unit senses changes in the electrical properties of the sensors that are directly related to the wear of an abradable sensor. The sensor elements are constructed of materials that allow them to survive the environment in which they are placed.

The abradable sensor has four basic components: a non-electrically conductive abradable substrate, one or more electrically conductive abradable layers or patterns that form the sensor, a non-conductive abradable encapsulant for protecting the sensor, and electrical contacts for communication with the sensor conditioning unit.

As disclosed herein, the embodiments of this invention measure different electrical properties such as resistance, capacitance, or time of flight of an electrical signal.

FIG.1shows an exemplary high-pressure turbine section10with a turbine blade from a gas turbine engine (not shown). The turbine section10includes a stationary engine casing12and a rotating portion. In this example, the rotating portion is a turbine blade14or compressor blade14, which is part of the rotating portion. To simplify this description, the turbine blade14or compressor blade14may be referred as blade14. As hot compressed air flows through the engine (not shown), it causes the engine (now shown) to spin. As the blades14spin, they grow under load. Additionally, thermal growth and vibration affect the blade tip16clearance depicted inFIG.1. If the blade tip16contacts the stationary engine casing12, which may be referred to as engine casing12, it will rub the abradable layer18. The distance between the blade tip16and the engine casing12or abradable layer18is the blade tip clearance20.

The abradable layer18is a thermally insulating material with unique material properties that allow it to withstand the corrosive high temperature environment and provide good abradable properties. As used herein, high temperature means temperatures in excess of about 600° F. (about 315° C.), in some cases in excess of about 2000° F. (about 1100° C.), and in other cases between about 2000° F. (about 1100° C.) and about 3000° F. (about 1650° C.). The abradable layer18must be soft enough to abrade when the blade tip16makes contact but not too soft to erode excessively under normal operation. Thus, the abradable sensor26is capable of operating in temperatures greater than or equal to at least about 600° F. (at least about 315° C.), and in some cases capable of operating in temperatures greater than or equal to at least about 2000° F. (at least about 1100° C.). Additionally, the abradable sensor is constructed to have similar abradable and wear properties as the abradable layer. Abradable sensor26provides the capability to determine blade rubs and wear of abradable layer18anytime the engine is operating independent of current temperature conditions.

FIG.2shows a simplified sectional view of a single stage of a compressor or turbine section of a gas turbine engine. In this view, the blade14is proximate to the engine casing12with an abradable layer18. Embedded in the engine casing12and within the abradable layer18is an abradable sensor26. The abradable sensor26has at least one or more electrically conductive abradable layers22and at least one or more non-electrically conductive abradable substrates24or layers24. The abradable sensor26has electrical contacts28(not illustrated inFIG.2), which provide a connection to a sensor conditioning unit30with at least one pair of electrical leads40. The at least one pair of electrical leads40can be two or more electrically conductive wires that are bundled together or run separately and are capable of providing electrical signals between the sensor conditioning unit30and the abradable sensor26through the electrical contacts28. As will be demonstrated, there are several embodiments for the abradable sensor26. The blade14is rotating in direction32.

The sensing system33at least includes the abradable sensor26, the electrical leads40, and the sensor condition unit30.

The sensor conditioning unit30is capable of detecting at least one change in electrical properties such as a change in resistance or capacitance in abradable sensor26. Additionally, sensor conditioning unit30could be capable of detecting a time of flight for a reflected electrical signal within the abradable sensor26. All of these are discussed in further detail below. Although not illustrated, the sensor conditioning unit30may also be in electronic communication with an engine controller or a blade clearance control unit provided by an engine manufacturer.

The electrical signals between the abradable sensor26and the sensor condition unit30provide for the abradable sensor26to communicate an electric signal to the sensor conditioning unit30that indicates a measurable change in resistance or capacitance within the abradable sensor26. Similarly, the electrical signals between the abradable sensor26and the sensor condition unit30provide for the abradable sensor26to communicate an electric signal to the sensor conditioning unit30that indicates a time of flight for a reflected electrical signal within the abradable sensor26.

While bothFIG.2show the abradable sensor26flush with the abradable layer18, in some applications the abradable sensor26may stick out past the abradable layer18. In cases where an abradable layer18is not present, the abradable sensor26may stick out just past the engine casing12.

Referring now toFIG.3, a cross-sectional view of the abradable sensor26is illustrated. In this view, a single electrically conductive abradable layer22is depicted. The electrically conductive abradable layer22is deposited onto a single non-electrically conductive abradable substrate24or layer24with a protective non-electrically conductive abradable material encapsulating it.

As depicted in the image on the right-hand side ofFIG.3, the electrical properties of the abradable sensor26change as the abradable sensor26wears. Namely, the electrical resistance of the abradable sensor26increases as the thickness (y) of the electrically conductive abradable layer22decreases. Consequently, a change in resistance ΔR can be correlated to a change Δy in the abradable sensor thickness42.

FIG.3shows the electrically conductive abradable layer22as a flat rectangular area. However, such an electrically conductive abradable layer22could be formed with many different shapes: trapezoidal, triangular, or parabolic. Furthermore, the electrically conductive abradable layer22also does not need to be flat. It could be cylindrical or a cube shape. The shape of the of the electrically conductive layer22should be selected based on sensor integration, manufacturability, and sensor performance. For example, a shape that is wider with less thickness (y), will have a higher sensitivity (ΔR/Δy).

FIGS.2,4, and6-9, illustrate that the electrically conductive abradable layer22has one pair of electrical leads40that are in communication with the sensor conditioning unit30. As illustrated, the sensor conditioning unit30is positioned away from the abradable sensor26to provide improved environmental conditions and temperature regulation. However, having sensor conditioning unit30positioned near or proximate abradable sensor26is an option as long as it is thermally protected to operate in the environment.

FIG.4illustrates a similar embodiment toFIG.3with a single electrically conductive abradable layer22. In this case, the electrically conductive abradable layer22has a distinct pattern44. Such a pattern44can be created by applying a layer of conductive abradable material to a non-electrically conductive substrate24or layer24and etching or laser trimming the pattern44. Alternatively, the pattern44could be printed directly onto the substrate. As used herein, pattern44includes any geometric shape with at least one electrically conductive layer22on a non-electrically conductive substrate24or layer24, and pattern44may include resistive loops46.

The pattern44of electrically conductive abradable layer22material is formed such that it has a plurality of resistive loops46that are electrically conductive. The plurality of resistive loop46are illustrated as being side-by-side. These resistive loops46can form a plurality of parallel resistive loops46that combine to produce an equivalent resistance (Req) as described in the equation below. Where R1, R2, . . . Rnrepresent the resistance of each respective parallel resistive loop46.

As the abradable sensor26wears with the abradable layer18, the parallel resistive loops46are removed from the circuit and the resistance increases. Each parallel resistive loop46is positioned an incremental distance away from the end49of the abradable sensor26. In the non-limiting exemplary configuration ofFIG.4, the distance is about ±1 mils (about ±25.4 micrometers) to ±about 5 mils (about ±0.127 millimeters) in a range between about 5 mils (about 0.127 millimeters) to about 50 mils (about 1.27 millimeters) of the abradable sensor26. In another exemplary configuration, each parallel resistive loop46is positioned an incremental distance away from the end49of the abradable sensor26, wherein the distance is about ±1 mils (about ±25.4 micrometers) to ±about 5 mils (about ±0.127 millimeters) in a range between about 10 mils (about 0.254 millimeters) to about 40 mils (about 1.016 millimeters) of the abradable sensor26. In yet another exemplary configuration, each parallel resistive loop46positioned an incremental distance away from the end49of the abradable sensor26by a distance of about ±0.25 mils (about ±6.35 micrometers) to ±about 1 mil (about ±25.4 micrometers). In all these configurations, a relationship between equivalent resistance and abradable thickness42is created. It may be desirable to position the plurality of parallel resistive loops46closer together in at least one region of the abradable sensor26than at least one other region of the abradable sensor26.

FIG.5shows this relationship between equivalent resistance and abradable sensor thickness42. In this plot, it shows that each parallel resistive loop46was positioned in increments of about 0.001 inches (about 0.0254 millimeters) from the end of the abradable sensor26. As the abradable sensor26wears, parallel resistive loops46are disconnected. The change in resistance is larger as fewer resistive loops46remain. This can be observed mathematically from equation 1 of parallel resistors above.

In some embodiments, the incremental changes (Δy) in abradable thickness42of the position of the resistive loops46can vary according to the need for detection. For example, if a certain region of the abradable thickness42is determined to be more important than the others, this region may have a finer spacing between the parallel resistive loops46than is used in other regions.

Referring toFIG.6, a single electrically conductive abradable layer22is illustrated in a pattern44. It is noted that various patterns can be incorporated to make up the abradable sensor26. For example, concentric squares or triangles as shown inFIG.6could also be used. Additionally, the pattern44does not have to be flat, it could also be three dimensional such as a pattern44wrapped around a cylinder or cube. In this embodiment, each triangle forms a parallel resistive loop that is combined to form an equivalent resistance as shown inFIG.5. However, pattern44may be in any shape that allows for electrically conductive layer(s)22to be divided into multiple parallel resistive loops or elements.

As the abradable sensor26must withstand very harsh environmental conditions and temperature extremes, the materials and manufacturing of the abradable sensor26becomes important. Some exemplary materials and manufacturing techniques are listed here.

In one non-limiting example, the abradable sensor26is made of one or more layers of electrically conductive abradable layers22on a non-electrically conductive abradable substrate24or layer24. The electrically conductive abradable layers22could be thin or thick films of nickel, platinum, nichrome, tantalum nitride, or platinum-tungsten for example. Thin film layers typically have layer thickness on the order of about 0.1 microns and thick film layers typically have thickness of about 100 microns thick. Each of these materials have adequate melting points above an operating temperature where the abradable sensor26is to operate and these materials have good high temperature corrosion/oxidation resistance. In a thin layer, these materials can also be abradable by the blade tip16.

In another non-limiting example, thicker electrically conductive abradable layer(s)22may be made of a traditional metal matrix abradable coating material such as MCrAlY (Where M can be Ni, Co, Fe or a combination thereof). In this case the thickness of the layer may be on the order of millimeter. These electrically conductive abradable materials are typically porous (about 20-60% porosity) and have at least one lubricating agent such as, but not limited to, boron nitride to give them desirable abradable properties.

As described above, the electrically conductive abradable layers22may be comprised of a thin or thick film material selected from the group consisting of nickel, platinum, nichrome, platinum-tungsten, metal matrix abradable coating (MCrAlY), and combinations thereof.

The electrically conductive abradable layers22can be added through various coating methods known in the art such as cold spray or gel, thermal spraying (plasma, combustion wire, electric arc, HVOF, etc.), chemical vapor deposition (sputtering), or electroplating. In some applications, methods such as sintering, or heat treating may also be required. Typically, a protective passivation layer, not shown, over electrically conductive abradable layer22will protect abradable layer from environmental impact of high temperature corrosion and evaporation due to high temperatures. Materials suitable for forming passivation layers are commercially available from multiple suppliers.

Once a layer of material is applied to the substrate, it can be laser trimmed or etched to the appropriate geometry and pattern. Alternatively, the electrically conductive layers abradable22could be printed onto the substrate. Yet another method is to create trenches in the substrate using laser engraving and then apply a layer of electrically conductive abradable material. Where needed, the excess material can be machined, etched, or laser trimmed away.

In a non-limiting example, the non-electrically conductive abradable substrate or layer24may be a ceramic matrix-based material. The abradable ceramic matrix substrate is consistent with abradable layers commonly found on gas turbine engines. The substrate is porous and typically has one or more lubricating agents such as boron nitride to provide good abradability properties. For example, the ceramic matrix-based material may be selected from the group consisting of a mullite, silicon carbide, alumina, zirconium-based ceramic, or combinations thereof. These materials typically have a polyester filler that burn off leaving a 25-40% porosity material in some cases. Such materials are commonly used as abradable layers18in the hot sections of gas turbine engines.

It is beneficial to have an abradable sensor26made from materials with similar environmental and wear properties as the abradable layer18. As such, the abradable layer18and abradable sensor26will abrade and/or wear in the same way preventing a potential mismatch between the two that could result in early failure or measurement error. The materials for the abradable sensor26may be the same or similar to the materials that are used for the abradable coatings found in gas turbine engines where such coatings are porous and have lubricating agents, as discussed above.

Referring now toFIG.7, another embodiment of the invention is shown. Similar in construction and function to the embodiments shownFIGS.3-6,FIG.7shows an abradable sensor26that is made of two or more layers of electrically conductive abradable layers22and two or more electrically non-conductive abradable substrates24or layers24. These electrically conductive abradable layers22are linked together to form parallel resistors as described above. In this case, as the abradable sensor26wears, layers of the abradable sensor26will wear away and cause the resistance of the abradable sensor26to increase similarly to what is shown in FIG.6. Within the abradable sensor26, the spacing of the abradable layers22,24is set at deterministic positions to correlate changes in resistance with changes Δy in abradable sensor thickness42.

FIG.8shows another embodiment of the invention which is similar in construction to the embodiment illustrated inFIG.7. However, instead of detecting changes in resistance, changes in capacitance are detected. In this case a capacitive element50is contained within the abradable sensor26instead of resistive layer(s) or loops46. The capacitive element50is formed with two or more abradable electrodes52made of an electrically conductive abradable layer22similar to those previously described. Between the two or more abradable electrodes52is an abradable dielectric material54. The abradable dielectric material54can be a non-electrically conductive abradable substrate24or layer24such as the substrate previously described. The capacitive element50is embedded in non-electrically conductive abradable material which could be the same material as the substrate previously described. Additionally, the two abradable electrodes52have electrical contacts28which are in electrical communication with the sensor conditioning unit30.

The two abradable electrodes52can be various shapes such as concentric cylinders, polygon planes, or rectangular planes. The shape factor of the capacitive element50can be made to amplify the change in capacitance as the capacitive element50abrades. The shape factor of the at least two abradable electrodes52is related to the gap between electrodes52, the length vs height, or diameter vs height of the electrodes52.

Equation 2 below shows the basic equation for capacitance. The capacitance is proportional to the dielectric permittivity (ε0εr), the height (h), and length (l) of the capacitive element50and inversely proportional to the distance (t) between the two abradable electrodes52.

Since capacitance (C) is proportional to the electrode height, if all the other parameters remain roughly constant, the capacitance will decrease as the height (h) of the abradable sensor26decreases due to wear.

FIG.9shows an alternate embodiment with a capacitive element50. In this case the capacitive element50has multiple layers of alternating electrically conductive abradable layers22and non-electrically conductive abradable substrates24or layers24. In this embodiment, as the layers of the abradable sensor26wear, this will change the capacitance in discrete steps similar to the layered parallel resistor embodiment. In this case, the equivalent capacitance Ceqis the sum of the capacitance of each remaining layer22,24.

The capacitance is measured with the sensor conditioning unit30which can provide alternating current to the capacitive element50. The sensor conditioning unit30reads the voltage across the abradable sensor26which is inversely proportional to the capacitance. Various techniques such as an AC bridge, capacitance to frequency conversion, or capacitance to phase angle conversion can be used to identify small changes in capacitance, such as resolution in the nanofarad capacitance range.

As the operating temperature of the abradable sensor26can change significantly over the operation cycle of the gas turbine engine, this can have an effect on the electrical properties that are being measured. Therefore, temperature should be accounted for. This can be done by measured temperature locally with an embedded temperature sensor or having a temperature estimate based on the operating condition or provided from the engine control computer (not shown). SeeFIGS.14A and18.

The temperature sensor can be positioned to measure or approximate the temperature of the abradable sensor26which is positioned in the abradable layer18. The temperature sensor may also be built into the at least one electrically conductive layers of the sensor26. For example, two different high temperature metallic materials such as tungsten/tungsten-rhenium or platinum/platinum rhodium could be joined at desired location(s) to form thermocouple junction(s) in the abradable sensor.

An alternate approach for temperature compensation is shown inFIGS.10-12.FIG.10shows at least one optional electrically conductive non-abrading reference sensor element56and at least one electrically conductive abradable sensor element27. The sensing element is formed by at least one electrically conductive abradable layer22and at least one non-electrically conductive abradable substrate24or layer24. The reference sensor element56has similar electrical properties as the abradable sensor element27. The reference sensor element56would be positioned proximate to the abradable sensor element27so that it is in or proximate the same thermal environmental. However, the reference sensor element56is typically outside the wear path and its electrical properties should remain the same other than temperature effects. The electrically conductive reference sensor element56is capable of compensating for temperature effects and variations in a cable impedance, the electrically conductive reference sensor being outside of an abradable wear path of the abradable sensor.

An alternate approach is to incorporate the temperature compensating element56in the same layer as the electrically conductive abradable layer22. This is shown inFIG.18. Sensor element27consists of a non-electrically conductive abradable substrate24, and an electrically conductive resistive pattern44. Resistive pattern44consists of abradable resistive loops46and reference sensor elements56. This embodiment shows two versions of reference sensor element56. Reference sensor element56, on the left, between electrical contacts28aand28bis a resistive temperature detector (RTD) element. This type of compensating element is shown schematically inFIG.12. Reference sensor element56, on the right, between electrical contacts28cand28d, is a temperature compensating resistor that can be connected in a wheat stone bridge configuration shown schematically inFIG.11. Either or both forms of temperature compensation can be used and is described in more detail below.

FIG.11shows a half bridge sensor configuration where the abradable sensor element27(Zs+Z(T)) and the temperature compensating reference sensor element56(Zc+Z(T)) form half of a Wheatstone bridge58. These elements are located at the sensor. The other two bridge completion elements are in the sensor conditioning unit30(Z1and Z2). The symbol Z in the diagram refers to the electrical impedance of the sensor element and corresponding cables40. As illustrated, Zs is the nominal impedance of the sensing element and cabling, Zc is the nominal impedance of the compensating or reference sensor element and cabling, Z(T) is the temperature effects on the impedance, and Z1and Z2are the electrical impedances of the completion components in the sensor conditioning unit.

To measure the electrical impedance of the Wheatstone bridge, a constant DC or AC current source60is applied to the node between the sensor element27(Zs+Z(T)) and the temperature compensating element56(Zc+Z(T)). A differential voltage measurement is taken at the two nodes shown after the sensor element27(Zs+Z(T)) and reference element56(Zc+Z(T)). By doing this, the temperature effects and variation in cable impedance (resistance or capacitance) mostly cancel out and only the changes in the sensor element27(Zs+Z(T)) are measured. Alternatively, a constant voltage source could be applied to the top and bottom nodes of the bridge and the differential voltage could be measured between the left and right sides of the bridge. Another alternative is to use a full bridge configuration at the sensor, where Z2would be a redundant sensor element27(Zs+Z(T)), and Z1would be a redundant compensating element56(Zc+Z(T)). These methods listed above are non-limiting examples for measuring the electrical impedance changes of the sensing element using Wheatstone bridge configurations.

FIG.12shows an alternate embodiment, where the sensor element27(Zs+Z(T)) and the reference element56(Zc+Z(T)) are read in with separate Wheatstone bridges58. In this case, each measurement is read into a processing unit62contained within the sensor conditioning unit30. The processing unit62is able to digitally calibrate out the temperature effects in the sensor element27(Zs+Z(T)) by observing the temperature effects of the temperature compensating element56(Zc+Z(T)).

For both embodiments shown inFIGS.11and12, if a resistance measurement is required, this will typically be made by filtering and averaging the DC differential voltage to help improve the signal to noise ratio and reduce unwanted effects of electromagnetic interference

For a capacitance measurement, the differential AC voltage will typically be demodulated, filtered, and averaged to help improve the signal to noise ratio and reduce unwanted effects of electromagnetic interference.

FIG.13shows a cross sectional view of an embodiment where redundancy can be incorporated for higher availability or for safety considerations. This can be done by putting multiple abradable sensor elements27within a single abradable sensor26body. Alternatively, multiple abradable sensors26could be installed to monitor the abradable wear of a given section of the engine casing12and abradable layer18.

Referring toFIGS.14A and14B, an exemplary sensor design where there is an abradable sensor element27encased in a protective layer. Where the protective layer is non-electrically conductive and abradable. The sensor element27is formed from a non-electrically conductive abradable substrate or layer24with an electrically conductive pattern44on the front and back side.

The exemplary pattern44shown inFIGS.14A and14Bprovides parallel resistive loops46. As the sensor element27abrades during the operation of the engine, the parallel resistive loops46will open causing the resistance to increase as previously described. The unique features of this embodiment lie in pattern44of the sensor element27.

Referring toFIG.15, a close-up view of16A, a non-limiting example of sixteen (16) resistive loops46made from a thin film metal deposit are illustrated. Each resistive loop46consists of a conductive abradable resistive loop46with varying width. By varying the width of the conductive abradable resistive loop46, the effective resistance can be tailored. As can be seen inFIG.15, the left most conductive abradable resistive loop46is wide providing a low resistance. The conductive resistive loops46to the right of that one is narrower providing higher resistance. Moving further to the right the conductive abradable resistive loops46widen again providing lower resistance. Such a pattern44can provide a nice linear relationship between abradable thickness and equivalent resistance of conductive parallel resistive loops46. This is shown in theFIGS.16A and16B.

The conductive resistive loops46are connected together and to the electrical contacts28through a portion of the pattern44. This portion of the pattern44may have a thin layer of high electrically conductive material. Additional layers may also be placed to aid in soldering or wire bonding.

InFIGS.14A and15, the left side of each conductive resistive loop46is connected via a pattern44with high electrical conductivity on the front side of the substrate24or layer24. The right side of the resistive loops46is electrically connected via the pattern44with high electrical conductivity on the back side of the substrate. The metalized holes68form an electrical connection between the front and back side of the substrate. Using this type of construction, both sides of the substrate can be used to form the resistive pattern44.

The controlling of the resistance of the resistive loops46illustrated in theFIG.15is accomplished by controlling the pattern44geometry, thickness of the conductive layers, and material properties.

In this embodiment, the conductive resistive loops46form the resistive elements and the high electrically conductive material forms the interconnecting pattern44. Although not shown here, in other embodiments, this could be switched so that the conductive resistive loops46are formed from a higher electrically conductive material and the interconnecting pattern44is formed with a lower electrically conductive material to create the parallel resistive elements. In this alternate embodiment, the thickness and shape factor of the interconnecting pattern44can be tailored to provide a similarly operating abradable sensor element27as what is shown inFIG.15.

Although not shown here, another alternate approach would be to use discrete wire loops. For example, the wires can either act to complete the parallel resistive network or function as resistive elements. One may choose the wire diameter, length, placement, and/or material properties to design a similarly operating abradable sensor element27as shown inFIG.15.

FIG.16Aillustrates a typical equivalent resistance with respect to the abradable thickness for a sensor element27that has mostly uniform parallel resistive loops46and some series resistance.FIG.16Ais representative of some of the other resistive embodiments shown in this invention.

FIG.16Billustrates the equivalent resistance versus the abradable thickness for a sensor element27that has variable resistor pattern44as shownFIG.15. In that configuration, each resistive loop46can be tailored by varying the width of each resistive loop46, the length each resistive loop46, or both the length and width each resistive loop46.FIG.16Billustrates the nice linear relationship that can be achieved. Such a relationship can be expressed with simple gain and offset calibration values.

FIG.17illustrates a zoomed in view ofFIG.14B. InFIG.17, the back side of the non-electrically conductive abradable substrate or layer24has a pattern44with high electrical conductivity is connecting all the metalized through holes68. This approach allows for the completion of the electrical circuit on the front.

Additionally, the back side of the non-electrically conductive abradable substrate or layer24has an optional compensating resistor reference element56. The resistor reference element56is configured to complete one-half of a Wheatstone bridge, as the Wheatstone bridges are described above for compensating effects of temperature and cable resistance.

The abradable sensing systems33described above using one or more of the abradable sensors26described above may be used to detect a blade rub or wear of an abradable layer on a stationary engine casing. Where the sensor conditioning unit30is also used, and electrical signals are transmitted between the sensor conditioning unit30and the abradable sensor26. The sensor conditioning unit30measures one or more electrical characteristic of the abradable sensor26which could be the resistance, the capacitance, and/or the round-trip time of flight for a reflected electrical signal. The sensor conditioning unit30correlates one or more of the measured resistances, capacitance, or time of flight for the reflected electrical signal against a known relationship to abradable sensor thickness. The result provides the current thickness of the abradable sensor. Using this information, the sensor conditioning unit is able to detect a rub event or a zero clearance between the blade tip and the stationary engine casing when the resistance, the capacitance, or the round-trip time of flight for a reflected electrical signal significantly changes in a short period of time.

As an example, a rub event could be detected by losing more than one parallel resistive or capacitive element in less than 10 seconds.

For all the described sensor embodiments, the sensor conditioning unit can be capable of periodic or continuous built-in-test of the sensor health. For example, a check can be made that the change in electrical properties does not exceed a pre-determined threshold within a pre-determined amount of time.

Other embodiments of the present invention will be apparent to one skilled in the art. As such, the foregoing description merely enables and describes the general uses and methods of the present invention. Accordingly, the following claims define the true scope of the present invention.