Patent Description:
Air data probes such as, for example, Pitot probes and Total Air Temperature (TAT) probes measure important aircraft flight parameters. Such flight parameter measurements are used to facilitate safe operation of an aircraft in flight. The quality of such flight parameter measurements can be compromised when ice accretes on exposed surfaces of the air data probes. Such ice accretion can plug the probe pneumatic tubing and deleteriously affect the measurement quality of the measured flight parameters. Air data probes have been equipped with electrical heaters so as to provide deicing capability to the air data probes thus equipped.

The abrupt failure of such electrical heaters can result in a sudden loss of the air data probe's functionality, which might lead to undesirable flight control consequences. Gradual degradation in the quality of measurement can also lead to suboptimal flight consequences. Such electric heater can fail or degrade for various reasons. For example, electrical heaters can fail suddenly due a resistive heating element cracking open, or the resistive heating element short circuiting to a nearby conductive member. Such short circuiting can occur when insulation between the resistive heating element and the conductive member degrades, which can compromise the heater function.

If such insulation cracks, oxygen, dust, carbon, oils, and other contaminants can traverse the insulation so as to be present at the surface of the resistive heating element. Such contaminants, especially when combined with humidity and high temperature can cause the resistive heating element and surrounding insulation to oxidize. Oxidation of the resistive heating element and the surrounding insulation can increase the electrical resistance thereof, which can lead to open circuit failure. Oxidation of the surrounding insulation can cause the insulation to become brittle and/or crack, which can reduce the electrical resistance thereof leading short circuit failure. Both short circuit failure and open circuit failure can result in loss of heater function, thereby leaving the air data probe susceptible to ice accretion and blockage of probe pneumatic tubing. <CIT> relates to a system and method for monitoring a critical component on an aircraft.

Further embodiment are shown in the dependent claims <NUM>-<NUM>.

The claimed apparatus relates to determining health of an electrical heater of an air data probe based on a comparison between a calculated expected value and a measured value of an electrical property of the electrical heater. The expected value of the electrical property is calculated based in part on the electrical power provided to the electrical heater and further based in part on the aircraft flight parameters and/or environmental conditions. Such aircraft flight parameters and/or environmental conditions can include at least one of: electric power source status, airspeed, air pressure, altitude, air temperature, humidity, liquid water content, ice water content, droplet/particle size distribution, angle of attack, and angle of sideslip. These aircraft flight parameters and/or environmental conditions are received via an aircraft interface.

<FIG> is a diagram illustrating an aircraft equipped with air data probes and a probe control/monitor system. In <FIG>, aircraft <NUM> includes air data probes 12a-12d and probe control/monitor system <NUM>. Air data probes 12a-12d can be any of a variety of probes that measure various parameters used during flight operations, such as, for example, airspeed, static air pressure, ram pressure, static air temperature, altitude, etc. As such, air data probes <NUM>-12d are often externally affixed to aircraft <NUM> so as to be exposed to the external atmospheric environment, thus making them susceptible to ice accretion thereon. Each of air data probes 12a-12d is in communication with a probe control/monitor system, such as probe control/monitor system <NUM>. In some embodiments, probe control/monitor system <NUM> is in communication with a plurality of air data sensors, such as, for example, air data sensors 12a-12d. In some embodiments, probe control/monitor system <NUM> can be located in an avionics bay of aircraft <NUM>.

In the depicted embodiment, air data probe 12a is a pitot tube, which is configured to measure ram pressure. Air data probe 12a is equipped with electrical heater <NUM> so as to prevent ice accretion on surfaces of air data probe 12a. Electrical heater <NUM> can have a resistive heating element that is thermally coupled to but electrically isolated from a conductive member of air data probe 12a. For example, electrical heater <NUM> can be coaxial, such that an insulative member is proximate to and coaxial with a resistive heating element, and a conductive shield can be proximate to and coaxial with the insulative member. Health of electrical heater <NUM> can be determined based on electrical resistance of the resistive heating element and/or leakage current between the resistive heating element and the conductive member of air data probe 12a, for example. Both electrical resistance of the resistive heating element and leakage current between the resistive heating element and the conductive member of air data probe 12a, however, can change in response to the thermal load of electrical heater <NUM>. The thermal load of electrical heater <NUM>, in turn, can depend upon aircraft flight parameters and/or environmental conditions.

Probe control/monitor system <NUM> is configured to provide electrical power to electrical heater <NUM> and to measure electrical parameters of electrical heater <NUM>. In the depicted embodiment, probe control/monitor system <NUM> is shown in electrical communication with electrical heater <NUM> of air data probe 12a. Probe control/monitor system <NUM> includes aircraft interface <NUM>, electrical-property calculator <NUM>, electrical power source <NUM>, measurement circuit <NUM>, and health monitor <NUM>. Power source <NUM> can provide electrical power to electrical heater <NUM> in various ways. For example, power source <NUM> can provide a constant voltage to electrical heater <NUM>. In some embodiments, power source <NUM> can maintain constant power provided to electrical heater <NUM>. In some embodiments, power source <NUM> can control the electrical power provided to electrical heater <NUM> based, at least in part, on aircraft flight parameters and/or environmental conditions, which are transmitted by aircraft <NUM> and received by probe control/monitor system <NUM> via aircraft interface <NUM>. Such power control based on aircraft flight parameters and/or environmental conditions can be performed so as to provide power that will result in heating of air data probe 12a to a target temperature, for example.

The electrical-property calculator <NUM> can estimate one or more local flowfield property, based on the received aircraft flight parameters and/or environmental conditions. The local flowfield property can be different, sometimes substantially different, from the corresponding freestream property. For example, the local (e.g., in the immediate vicinity of an air data probe) airspeed can be substantially different from the freestream airspeed. The estimated local flowfield property or properties can then be used in subsequent calculations, such as those described below.

Various electrical properties of electrical heater <NUM> can vary in response to either the electrical power provided to electrical heater <NUM> or the thermal load thereupon. Thus, electrical-property calculator <NUM> can be configured to calculate an expected value of an electrical property of the electrical heater based in part on the electrical power that is or will be provided to electrical heater <NUM> and further based in part on the received aircraft flight parameters and/or environmental conditions, which can be indicative of the thermal load upon electrical heater <NUM>. In some embodiments, the calculated expected value of an electrical parameter is based on one or more estimated local flowfield properties, such as airspeed, air pressure, water content, air temperature, etc. If electrical heater <NUM> behaves as expected, the actual electrical property of electrical heater <NUM> should be approximately equal (e.g., within <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>% etc.) to the expected value calculated by electrical-property calculator <NUM>.

The electrical-parameter calculator <NUM> is further configured to calculate the thermal load of electrical heater <NUM> based on the received flight parameters and/or environmental conditions. Such a thermal load calculation is used in the calculation of the expected value of the electrical property of electrical heater <NUM> and/or used in a calculation of a target value of electrical power to be provided to electrical heater <NUM>. Electrical-property calculator can calculate the target value of electrical power so as to ensure that air data probe 12a is heated to a target temperature, for example. Such a target temperature may be selected so as to prevent ice accretion on exposed surfaces of air data probe 12a.

Measurement circuit <NUM> is configured to measure electrical parameters indicative of the electrical property of electrical heater <NUM> corresponding to the expected value calculated by electrical-property calculator <NUM>. If the calculated value corresponds to a measure of a resistance of the resistive heating element, then measurement circuit <NUM> would be configured to measure electrical parameters indicative of the actual resistance of the resistive heating element (e.g., current through and voltage across the resistive heating element). If the calculated value corresponds to a leakage current between the resistive heating element and the conductive member of air data probe 12a, then measurement circuit <NUM> would be configured to measure electrical parameters indicative of the actual leakage current between the resistive heating element and the conductive member of air data probe 12a (e.g., first and second currents through the resistive heating element at first and second ends, respectively, of the resistive heating element so as to obtain a current difference).

Health monitor <NUM> can then assess the health of electrical heater <NUM>. In some embodiments, health monitor <NUM> is configured to determine health of electrical heater <NUM> based on a comparison between the calculated expected value and the actual electrical property as indicated by the measured electrical parameters. For example, health monitor <NUM> can compare electrical resistance of the resistive heating element as determined from electrical parameters measured by probe control/monitor system <NUM> with an expected value as determined by electrical-property calculator <NUM>. In some embodiments, electrical-property calculator <NUM> can determine an electrical-resistance threshold based on the calculated expected value. For example, the electrical-resistance threshold can be a factor (e.g., <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, etc.) times the calculated expected value for the particular flight parameters and/or environmental conditions.

In some embodiments, electrical-property calculator <NUM> can determine the electrical-resistance threshold based on a historical trend of measured electrical resistance values. For example, the electrical-resistance threshold can be a factor times the measured electrical resistance of the resistive heating element at the time of installation or at a time of calibration. In some embodiments, electrical-property calculator <NUM> can determine the electrical-resistance threshold based on various combinations of such methods. For example, the electrical-resistance threshold can be a factor times a calculated normal value for the particular flight parameters and/or environmental conditions, based on the measured electrical resistance of the resistive heating element at the time of installation or calibration.

Health monitor <NUM> can then assess the health of electrical heater <NUM> based on the comparison of the electrical-resistance threshold and the actual electrical resistance as determined based on the measured electrical parameters. If the electrical resistance exceeds the electrical-resistance threshold, for example, health monitor <NUM> can generate an alert signal and transmit the generated alert signal to a cockpit interface so as to alert the pilot. Such an alert signal can be transmitted to the cockpit interface via the aircraft interface, for example. The data can also be sent to a ground station so as to provide an alert for the need to repair/replace air data probe <NUM> based on the prognostics and health monitoring algorithms. This condition based maintenance system can facilitate the proactive replacement and/or repair of the compromise probe so as to avoid sudden unpredictable failures that lead to flight delays and operations interruptions. Such proactive maintenance can result is significant cost savings to an airline company.

In some embodiments, health monitor <NUM> can compare leakage current between the resistive heating element and the conductive member of air data probe 12a as determined from electrical parameters measured by probe control/monitor system <NUM> with a leakage-current threshold as determined by electrical-property calculator <NUM>. In some embodiments, electrical-property calculator <NUM> can determine the leakage-current threshold based on the calculated normal value. For example, the leakage-current threshold can be a factor (e.g., <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, etc.) times the calculated normal value for the particular flight parameters and/or environmental conditions.

In some embodiments, electrical-property calculator <NUM> can determine the leakage-current threshold based on a historical trend of measured electrical resistance values. For example, the leakage-current threshold can be a factor times the measured leakage current between the resistive heating element and the conductive member of air data probe 12a at the time of installation. In some embodiments, electrical-property calculator <NUM> can determine the leakage-current threshold based on various combinations of such methods. For example, the leakage-current threshold can be a factor times a calculated normal value for the particular flight parameters and/or environmental conditions, based on the measured leakage current between the resistive heating element and the conductive member of air data probe 12a at the time of installation.

Health monitor <NUM> can then assess the health of electrical heater <NUM> based on the comparison of the leakage-current threshold and the actual leakage current as determined based on the measured electrical parameters. If the actual leakage current exceeds the leakage-current threshold, for example, health monitor <NUM> can generate an alert signal and transmit the generated alert signal to a cockpit interface so as to alert the pilot. Such an alert signal can be transmitted to the cockpit interface via the aircraft interface, for example.

Next, with reference to <FIG> various failure mechanisms and failure detection methods of electrical heater <NUM> will be shown. <FIG> is a diagram of air data probe 12a that includes electrical heater <NUM> and probe control/monitor system <NUM> that is electrically connected to electrical heater <NUM>. While illustrated in <FIG> as a TAT probe, air data probe 12b may be any other type of air data probe 12a-12d or sensing element. <FIG> is a cross-sectional view of electrical heater <NUM> of air data probe 12b taken along line B-B of <FIG>. <FIG> is a partial cross-sectional view illustrating open circuit O in electrical heater <NUM>. <FIG> is a partial cross-sectional view illustrating short circuit S in electrical heater <NUM>. <FIG>, <FIG>, <FIG>, and <FIG> will be discussed together.

Air data probe 12b is an aircraft component and includes electrical heater <NUM>. Air data probe 12b is electrically connected to probe control/monitor system <NUM>, which includes aircraft interface <NUM>, electrical power source <NUM>, electrical measurement circuit <NUM>, and processor <NUM>. Aircraft interface <NUM> is in communication with the aircraft <NUM> (depicted in <FIG>) so as to receive flight parameters and/or environmental conditions therefrom. Electrical measurement circuit <NUM> includes first sensing resistor 28A, second sensing resistor 28B, and leakage sensing resistor 28C. Electrical heater <NUM> includes resistive heating element <NUM>, insulative member <NUM>, and conductive shield <NUM>.

Electrical heater <NUM> is depicted as routed through air data probe 12b along a path and is electrically connected to probe control/monitor system <NUM> within aircraft <NUM>. Electrical heater <NUM> receives power (e.g., in the form of a controlled voltage Vs) from voltage source <NUM> of probe control/monitor system <NUM> so as to provide heating of air data probe 12b. Voltage source <NUM> can provide direct current (DC) power or alternating current (AC) power to electrical heater <NUM> depending on the type of air data probe 12b to which electrical heater <NUM> is thermally coupled. First electrical current I<NUM> is electrical current that flows through resistive heating element <NUM> at a first end of electrical heater <NUM>. Second electrical current I<NUM> is electrical current that flows through resistive heating element <NUM> at a second end opposite the first end of electrical heater <NUM>. For example, as seen in <FIG>, first electrical current I<NUM> (which can be DC or AC current) flows into electrical heater <NUM>, and second electrical current I<NUM> (which can be DC or AC current) flows out of electrical heater <NUM>. First current I<NUM> flows through first sensing resistor 28A to produce a sensed voltage, or first sensor signal Vi. In the depicted embodiment, first sensing resistor 28A has a first resistance R<NUM>. Second electrical current I<NUM> flows through second sensing resistor 28B to produce a sensed voltage, or second sensor signal V<NUM>. In the depicted embodiment, second sensing resistor 28B has a second resistance R<NUM>. Leakage current IL is electrical current that flows between resistive heating element <NUM> and conductive shield <NUM>. Conductive shield <NUM> has been biased to electrical ground via resistive leakage sensing resistor 28C. Leakage current IL flows through leakage sensing resistor 28C so as to produce a sensed voltage, or leakage sensor signal VL. In the depicted embodiment, leakage sensing resistor 28C has a leakage resistance RL. Because first resistance R<NUM>, second resistance R<NUM>, and leakage resistance RL can have known values. First sensor voltage Vi, second sensor voltage V<NUM>, and leakage sensor voltage VL are indicative of first current I<NUM>, second current I<NUM>, and leakage current IL, respectively. First sensor voltage Vi, second sensor voltage V<NUM>, and leakage sensor voltage VL, which are indicative of first current I<NUM>, second current I<NUM>, and leakage current IL, respectively, can vary as a function of aging of air data probe 12b, as well as of flight parameters and/or environmental conditions.

In alternate embodiments, the functions performed by first sensor 28A, second sensor 28B, and leakage sensor 28C can be performed in various other manners. For example, first and second currents I<NUM> and I<NUM> can be sensed via a current transformer. In some embodiments a current transformer can be configured to measure a difference between the first and second current I<NUM>-I<NUM> so as to provide a signal that is indicative of the leakage current through the heater insulation.

Processor <NUM> is electrically connected to both electrical measurement circuit <NUM> and electrical power source <NUM>. First sensor voltage Vi, second sensor voltage V<NUM>, and leakage sensor voltage VL are provided to processor <NUM> by electrical measurement circuit <NUM>. Based on first sensor voltage Vi, second sensor voltage V<NUM>, and/or leakage sensor voltage VL, in addition to the heater supply voltage VS, processor <NUM> can determine both electrical resistance of resistive heating element <NUM> and leakage current between resistive heating element <NUM> and conductive shield <NUM> of air data probe 12b. For example, processor can determine electrical resistance of resistive heating element <NUM> by dividing heater supply voltage Vs by either first current I<NUM> second current I<NUM>, or average of first and second currents (I<NUM>+I<NUM>)/<NUM>, among other ways known to persons skilled in the art. Processor can determine leakage current between resistive heating element <NUM> and conductive shield <NUM> by dividing leakage sensor voltage VL by leakage resistance RL, or by measuring the difference between first current I<NUM> and second current I<NUM>, for example.

In the depicted embodiment, processor <NUM> is also electrically connected to aircraft interface <NUM>, from which flight parameters and/or environmental conditions are transmitted. Processor <NUM> can also transmit signals indicative of health of electrical heater <NUM> to aircraft <NUM> via aircraft interface <NUM>. Examples of such signals include signals indicative of status of electrical heater <NUM>, such as signals indicating: OK, ANTICIPATED OPEN, ANTICIPATED SHORT, and/or FUTURE FAILURE/REMAINING USEFUL LIFE.

As seen in <FIG>, electrical heater <NUM> has resistive heating element <NUM>, which conducts first current I<NUM> at the first end and conducts second I<NUM> at the second end. Resistive heating element <NUM> can be made of oxidation resistant material such as Nichrome, or any other suitable material. Insulative member <NUM> surrounds resistive heating element <NUM> or can be interposed between resistive heating element <NUM> and conductive member <NUM> of air data sensor 12b. Insulative member <NUM> can be made of silica, ceramic, or any other suitable insulating material. Conductive member <NUM> can be metallic and can surround insulative member <NUM> such that insulative member <NUM> is captured between resistive heating element <NUM> and conductive shield <NUM>. Conductive shield <NUM> may be made of nickel alloy, copper alloy, or any other suitable oxidation resistant material.

Electrical heater <NUM> is configured to prevent ice from accreting onto surfaces of air data probe 12b when air data probe 12b is exposed to atmospheric conditions conducive to such ice accretion, such as, for example, cold temperatures when flying at high altitudes. Voltage source Vs is configured to supply electrical power to resistive heating element <NUM> such that first current I<NUM> is provided to and driven through resistive heating element <NUM>, producing the required heat for air data probe 12b, and second current I<NUM> flows out of resistive heating element <NUM>.

Insulative member <NUM> protects resistive heating element <NUM> and electrically insulates resistive heating element <NUM>. For example, resistive heating element <NUM> can be electrically insulated from metallic conductive shield <NUM> by insulative member <NUM>. Conductive shield <NUM> is configured to provide electrostatic shielding of resistive heating element <NUM>. Conductive shield <NUM> is further configured to provide mechanical protection of both insulative member <NUM> and resistive heating element <NUM>, such as by keeping moisture and contaminants from compromising electrical heater <NUM>.

If conductive shield <NUM> were to crack, oxygen, moisture, dust, carbon, oils, and other contaminants could leak through conductive shield <NUM> to insulative member <NUM>, and then to resistive heating element <NUM>, thereby causing the material of insulative member <NUM> and resistive heating element <NUM> to oxidize, change properties, and/or otherwise break down. Loss of function of insulative member <NUM> can lead to resistive heating element <NUM> shorting to conductive shield <NUM>, as indicated by short circuit S. Cracking and deterioration of resistive heating element <NUM> can lead to open circuit O. For example, oxidation or cracking of insulative member <NUM> can lead to a gap in insulative member <NUM> and resistive heating element <NUM>, or open circuit O, and loss of function of electrical heater <NUM>, as shown in <FIG>. Additionally, loss of function of insulative member <NUM> can cause resistive heating element <NUM> to contact conductive shield <NUM>, or short circuit S, and loss of function of electrical heater <NUM>, as shown in <FIG>. Open circuit O and short circuit S both represent failures of electrical heater <NUM> as current is no longer able to flow through electrical heater <NUM>. In early stages of open circuit O and short circuit S, intermittent electric arcing can occur as a result of electric discharge through small conductive air gaps in insulative member <NUM>. The ionization of air in the gaps in insulative member <NUM> can permit electric charge to cross the gap, producing plasma that conducts electrical current through the gap and can produce visible light and/or high local temperatures (i.e., electrical arcing). As resistive heating element <NUM> deteriorates, such as in open circuit O, electric arcing can also occur within resistive heating element <NUM>. In cases of short circuit S, electric arcing can occur between resistive heating element <NUM> and conductive shield <NUM>. Electric arcing can lead to either temporary restoration of function of electrical heater <NUM> due to temporarily closing a gap in the case of open circuit O or opening a gap in the case of short circuit S. Electric arcing can also lead to complete open circuit of resistive heating element <NUM> or to more shorting in the insulation, depending on the mechanics or arcing and resulting conditions. Electric arcing can manifest itself as high-frequency noise in sensor signal Vi, second sensor signal V<NUM>, and leakage sensor signal VL at a much higher frequency range than the operating frequencies. For example, electric arcing may be indicated at a range of about <NUM> to about <NUM>.

Processor <NUM> can exploit the frequency signature of arcing by sampling first sensor signal Vi, second sensor signal V<NUM>, and leakage sensor signal VL, which are indicative of first current I<NUM>, second current I<NUM>, and leakage current IL, respectively, at a high-frequency sampling rate. The high-frequency sampling rate can be greater than two times the highest frequency of the electrical noise produced by electric arcing (which can be limited by an anti-aliasing filter), such as from about <NUM> to about <NUM>.

Because electric arcing is a precursor to open circuit O or short circuit S failure of electrical heater <NUM>, prediction processor <NUM> determines status <NUM> of electrical heater <NUM> based on the presence of electric arcing. Electric arcing can manifest itself as a high-frequency noise in first sensor signal Vi, second sensor signal V<NUM>, and leakage sensor signal VL. Such high-frequency noise can be identified in Fourier transformed data of first sensor signal Vi, second sensor signal V<NUM>, and leakage sensor signal VL. Processor <NUM> can then identify the presence of electric arcing in electrical heater <NUM> based on detection of high-frequency noise to determine future failure of electrical heater <NUM>.

Processor <NUM> can be configured to output a status of OK, indicating electrical heater <NUM> is functioning properly, when there is no high-frequency noise in Fourier transformed data representing first sensor signal Vi, second sensor signal V<NUM>, leakage sensor signal VL, and difference voltage VD. Processor <NUM> can be configured to output a status of ANTICIPATED OPEN, indicating an imminent future open circuit O, when there is high-frequency noise in Fourier transformed data representing first sensor signal Vi and second sensor signal V<NUM> and no increase or elevation in signal levels of leakage sensor signal VL and difference voltage VD. Processor <NUM> can be configured to output a status of ANTICIPATED SHORT, indicating an imminent future short circuit S, when there is high-frequency noise in Fourier transformed data representing first sensor signal Vi and second sensor signal V<NUM> as well as high-frequency noise in Fourier transformed data representing leakage sensor signal VL, in addition to a noticeable increase or elevation in signal levels of leakage sensor signal VL. Processor <NUM> can be configured to output a status of FUTURE FAILURE/REMAINING USEFUL LIFE, indicating the remaining useful life of electrical heater <NUM>, based on signatures (specific shapes) and magnitudes of Fourier transformed data representing first sensor signal Vi, second sensor signal V<NUM>, and leakage sensor signal VL.

Electrical heater <NUM> can prevent ice accretion on surface of air data probe 12b, thereby facilitating proper functioning thereof. Electrical heater <NUM> can abruptly fail as a result of open circuit O or short circuit S, which causes a sudden loss of functionality of air data probe 12b. Because proper functioning of air data probe 12b is important for safe operation of aircraft <NUM>, prognostics of electrical heater <NUM> can improve the reliability of air data probe 12b. Predicting future failure of electrical heater <NUM> can permit a user to replace electrical heater <NUM> when necessary (such as between flights or at another convenient time) and prevents unpredictable failures of electrical heater <NUM>, which can reduce flight delays, improve flight safety, and lower aircraft maintenance and flight operation costs.

In the <FIG> embodiment, processor <NUM> is configured to perform the functions of both electrical-property calculator <NUM> and health monitor <NUM> of the <FIG> embodiment. Thus, processor <NUM> is configured to calculate a thermal load of electrical heater <NUM> based on the received aircraft flight parameters and/or environmental conditions. Such aircraft flight parameters and/or environmental conditions include electric power source status, airspeed, air pressure, altitude, air temperature, humidity, ice detection, ice protection status, angle of attack, and angle of sideslip.

Processor <NUM> can be configured to control the electrical power provided to electrical heater <NUM> based on the calculated thermal load and on a target operating temperature of air data probe 12b. For example, processor <NUM> can calculate a target operating power level based on the calculated thermal load and on the target operating temperature. Processor <NUM> can be configured to receive, via electrical measurement circuit <NUM>, signals indicative of the measured electrical parameters and to use such signals in combination with determine health of electrical heater <NUM> based on the measured electrical parameters. For example, in some embodiments, health of electrical heater <NUM> can be determined, at least in part, based on the measured electrical parameters indicating that the deviation of electrical resistance of resistive heating element <NUM> from the predicted value exceeds a calculated threshold. In some embodiments, health of electrical heater <NUM> can be determined, at least in part, based on the measured electrical parameters indicating that the leakage current between resistive heating element <NUM> and the conductive member <NUM> of air data probe 12b exceeds the calculated leakage threshold.

Processor <NUM>, in one example, is configured to implement functionality and/or process instructions for execution within probe control/monitor system <NUM>. For instance, processor <NUM> can be capable of processing instructions stored in program memory. Examples of processor <NUM> can include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other equivalent discrete or integrated logic circuitry.

Aircraft flight parameters and/or environmental conditions are received by probe control/monitor system <NUM>, via aircraft interface <NUM>. Aircraft interface <NUM> can be used to communicate information between probe control/monitor system <NUM> and an aircraft. In some embodiments, probe control/monitor system <NUM> can transmit data processed by probe control/monitor system <NUM>, such as, for example, alert signals. Aircraft interface <NUM> can also include a communications module. Aircraft interface <NUM>, in one example, utilizes the communications module to communicate with external devices via one or more networks, such as one or more wireless or wired networks or both. The communications module can be a network interface card, such as an Ethernet card, an optical transceiver, a radio frequency transceiver, or any other type of device that can send and receive information. Other examples of such network interfaces can include Bluetooth, <NUM>, <NUM>, and Wi-Fi radio computing devices as well as Universal Serial Bus (USB). In some embodiments, communication with the aircraft can be performed via a communications bus, such as, for example, an Aeronautical Radio, Incorporated (ARINC) standard communications protocol. In an exemplary embodiment, aircraft communication with the aircraft can be performed via a communications bus, such as, for example, a Controller Area Network (CAN) bus.

<FIG> is a schematic diagram of a differential leakage current health monitoring system. Shown in <FIG> are electric power source <NUM>, electric heater <NUM>, and probe control/monitor system <NUM>. Electric heater includes resistive heating element <NUM>, insulative member <NUM>, and conductive shield <NUM>. Probe control/monitor system <NUM> includes differential current inductive sensor <NUM>, amplifier <NUM>, rectifier <NUM>, filter <NUM>, analog-to-digital converter <NUM>, and processor <NUM>. Also labeled in <FIG> are inlet current Iin, outlet current Iout, and leakage current IL. Inlet current Iin is the current conducted by resistive heating element <NUM> at first end <NUM>, and outlet current Iout is the current conducted by resistive heating element <NUM> at second end <NUM>. Leakage current IL is equal to the difference between Inlet current Iin and Outlet current Iout.

Power cable <NUM> is depicted schematically, representing an unspecified length of a two-conductor cable is conductively coupled to first <NUM> and second <NUM> of resistive heating element <NUM> so as to provide electrical power thereto. When electrical heater <NUM> is operating normally, inlet current Iin flows into resistive heating element <NUM> at first end <NUM> of electric heater <NUM>, and outlet current Iout flows out of resistive heating element <NUM> at second end <NUM> of electric heater <NUM>. Inlet current Iin will be approximately equal to outlet current Iout when leakage current IL is small with respect to inlet and outlet currents Iin and Iout. A typical value of current flow (i.e., Iin, Iout) can range from about <NUM> - <NUM> amps (A) for 115V AC power supply probes. Different air data probes can present different thermal loads and can have different electrical heating requirements thereby resulting in various different electrical requirements (e.g., different electrical voltage and/or current ranges). A small amount of leakage current IL flows through leakage current path <NUM>, schematically represented as flowing from conductive shield <NUM> to ground (i.e., chassis ground). The relationship between inlet current Iin, outlet current Iout, and leakage current IL can be calculated as follows: <MAT>.

A properly functioning heater <NUM> can experience a nominal value of leakage current IL by virtue of the nature of insulative member <NUM>. When a newly-manufactured electric heater <NUM> and associated air data probe is installed, a baseline value of leakage current IL can be measured and recorded. Such a measured value of leakage current IL can be referred to as the baseline leakage current IL-BASELINE, or as the leakage current IL0 at inception. A typical value of baseline leakage current IL-BASELINE can range from about <NUM> - <NUM> microamps (µA), but this value can vary over a wide range depending on the particular embodiment of electrical heater <NUM>. For example, in some embodiments, baseline leakage current IL-BASELINE can range up to about <NUM> milliamps (mA), or higher. In other embodiments, baseline leakage current IL-BASELINE can be less than <NUM>µA.

During operation, leakage current IL of electric heater <NUM> can gradually increase as a result of minor degradation of insulative member <NUM>. The normal migration of environmental impurities into insulative member <NUM> is a non-limiting example of a normal degradation of insulative member <NUM> during its lifetime. Because operation of electrical heater <NUM> can be cotemporaneous with flight operation of an aircraft, an expected heater lifetime can be expressed as a measure of flight hours. Several factors (e.g., size of heater <NUM>, physical location of electrical heater <NUM>) can affect the expected lifetime of electrical heater <NUM> in a particular embodiment, with typical values ranging from about <NUM> - <NUM> flight hours. Heater end-of-life (EOL) can be associated with a particular threshold value IL-THRESHOLD, which can vary depending on the particular embodiment of electrical heater <NUM>. Exemplary values of threshold value IL-THRESHOLD can range from about <NUM> - <NUM> mA.

A relationship between leakage current IL, service life, and expected lifetime can be determined for a particular embodiment of electrical heater <NUM>. Accordingly, the remaining useful life (RUL) can be estimated from a measured value of leakage current IL. Probe control/monitor system <NUM> can be configured to measure the leakage current IL throughout the service life of electrical heater <NUM>, thereby providing an indication of RUL while also identifying an abnormal condition that could be indicative of a premature failure of electrical heater <NUM>. Air data probe (i.e., and associated electrical heater <NUM>) can be repaired and/or replaced prior to the End of Life (EOL) or prior to the point of failure so as to avoid an operational delay and interruption (ODI) that could result following a failure. Because replacing an air data probe (i.e., and associated electrical heater <NUM>) can be expensive in terms of time and cost, especially if doing so removes the associated aircraft from operation, it can be desirable to extract the maximum useful service life from electrical heater <NUM> before repair and/or replacement is performed.

Referring again to <FIG> and Equation <NUM>, the value of leakage current IL can be expressed as being the difference between inlet current Iin and outlet current Iout, as follows: <MAT>.

Differential current inductive sensor <NUM> produces an electrical signal representing the value of leakage current IL, the detail of which will be shown and described later in <FIG>. Differential current inductive sensor <NUM> can also be called an inductive leakage current sensor or a differential leakage current inductive sensor. The electrical signal representing the value of leakage current IL is amplified by amplifier <NUM>, rectified by rectifier <NUM>, and filtered by filter <NUM>, thereby producing a DC voltage level that is representative of the value of leakage current IL. Analog-to-digital converter <NUM> produces a digital signal representing the DC voltage level provided by filter <NUM> (i.e., the value of leakage current IL). This can be referred to as a digitized leakage current value. In the illustrated embodiment, amplifier <NUM> can be an operational amplifier. In other embodiments, other circuit components that perform similar functions to the ones depicted can be used. For example, amplifier <NUM> can be any electronic circuit that provides amplification, rectifier <NUM> can be any nonlinear component that provides rectification, and filter <NUM> can be any component that filters the rectified value. In yet other embodiments, AC-to-DC conversion can be omitted, with an AC voltage being provided from amplifier <NUM> directly to processor <NUM>.

Referring again to <FIG>, processor <NUM> is a digital processor that receives, stores, scales, and processes the digitized leakage current value that is received throughout the lifecycle of electrical heater <NUM>. Processor <NUM> can receive and process the digitized leakage current value continuously or periodically. In some embodiments, processor <NUM> can receive multiple inputs corresponding to digitized leakage current values from multiple associated electrical heaters (e.g., associated with multiple air data probes). In other embodiments, processor <NUM> can receive other inputs associated with electrical heater <NUM>, with non-limiting examples including inlet current Iin and/or outlet current Iout, and/or the voltage level (not labeled in <FIG>) of electric power source <NUM>. In yet other embodiments, processor <NUM> can also receive and process data from sources other than leakage current IL associated with one or more electrical heaters. In an exemplary embodiment, processor <NUM> can receive data from other aircraft data sources.

In some embodiments, processor <NUM> can utilize data and signal analysis processing techniques on digitized leakage current values. In these or other embodiments, processor <NUM> can be a neural network. In some embodiments, prognostic processor <NUM> can provide information regarding one or more electrical heaters such as, for example, the current value of leakage current IL, the history of leakage current IL over time (e.g., operating time or calendar time), the service life (i.e., operating time), the expected EOL, and the calculated RUL. The aforementioned data can be provided to other systems (e.g., avionics system) for use by crew members. In these or other embodiments, processor <NUM> can provide data that can be transmitted and/or downloaded to engineering teams at an airline's operator, maintenance facility, and/or the various component suppliers whereby the data can be reviewed, analyzed, and/or archived.

When installed on a system that includes one or more of the electrical heaters, probe control/monitor system <NUM> can monitor and/or log metrics of health of each of the electric heaters associated therewith. Such health monitoring can facilitate maintenance personnel to predict when failure is likely to occur so that maintenance can be scheduled prior to the point of expected failure for any particular electric heater <NUM>. Such proactive maintenance scheduling can avoid flight delays that could ground an aircraft for emergent maintenance requirements, as well as preventing inflight failure of a particular electric heater <NUM>, which can disrupt the performance of an associated air data probe <NUM>.

<FIG> is a schematic diagram of differential current inductive sensor <NUM> shown in <FIG>. Shown in <FIG> are differential current inductive sensor <NUM>, toroid core <NUM>, toroid center region <NUM>, toroid split <NUM>, secondary winding <NUM>, resistor <NUM>, and secondary voltage terminals <NUM>. Power cable <NUM> provides an electrical connection between electric power source <NUM> and electric heater <NUM>, as shown and described above in regard to <FIG>. Power cable <NUM> has first conductor <NUM>' and second conductor <NUM>'. First conductor <NUM>' is in conductive communication with first end <NUM> of electric heater <NUM> and second conductor <NUM>' is in conductive communication with second end <NUM> of electric heater <NUM>. Each of first and second conductors <NUM>' and <NUM>' can include a central conductive core that is surrounded by an insulating material, and first and second conductors <NUM>' and <NUM>' can be held together by an outer conductive shield (not labeled) to form power cable <NUM>. The insulating material and the outer cable sheath can both be nonmetallic in the region near differential current inductive sensor <NUM>, thereby providing negligible electromagnetic shielding.

In some embodiments, the outer cable sheath of power cable <NUM> can be omitted. In these or other embodiments, first conductor <NUM>' and second conductor <NUM>' of power cable <NUM> can be twisted together, or they can be untwisted. Toroid core <NUM> defines toroid center region <NUM>, thereby providing for the passage therethrough of wires, cables, and the like. In the illustrated embodiment, toroid core <NUM> has toroid split <NUM>, thereby allowing toroid core <NUM> to be opened and/or separated into two halves (not labeled in <FIG>). In some embodiments, toroid split <NUM> can be omitted from toroid core <NUM>. Power cable <NUM> can be described as passing through toroid center region <NUM>, as shown in <FIG>. Power cable <NUM> can also be described as traversing toroid center region <NUM>.

Referring again to <FIG>, toroid core <NUM> is an iron core transformer. In an exemplary embodiment, toroid core <NUM> is a ferrite core, made from a material that has a relatively high value of magnetic permeability, as may be commonly used in the electrical art as a transformer core. Toroid core <NUM> can be referred to as a circular transformer core. In some embodiments, toroid core can be made from other materials that are capable of providing electromagnetic coupling, as will be described. The number of turns of the primary (NP) and secondary (Ns) winding on toroid core <NUM>, and the electrical wire thickness and insulation, are designed according to the current transformer known design principles.

An alternating current flowing in a conductor passing through a ferrite core induces an alternating magnetic flux Φ (not labeled), thereby creating an alternating magnetic field B, which induces an alternating current in secondary winding <NUM>. The alternating magnetic field B can be annotated with a vector symbol, as shown in <FIG>. As electrical power is delivered to heater <NUM> by power cable <NUM> (e.g., as shown in <FIG>), inlet current Iin flows through power cable <NUM> in a direction that is opposite to that of outlet current Iout, with both inlet current Iin and outlet current Iout flowing through toroid center region <NUM>. Accordingly, the component of alternating magnetic field B associated with inlet current Iin is opposite in direction to the component of alternating magnetic field B associated with outlet current Iin. The difference between inlet current Iin and outlet current Iout is measured through toroid core <NUM> primary winding (i.e., power cable <NUM>) and transformed by toroid core <NUM> to secondary winding <NUM>. This can result in a secondary voltage (Vs) value at secondary voltage terminals <NUM> that is representative of the differential current (i.e., Iin - Iout) flowing through power cable <NUM> (i.e., primary winding).

If inlet current Iin were equal to outlet current Iout (i.e., Iin = Iout), then the resulting alternating magnetic field B would be zero because the respective components of alternating magnetic fields B from inlet current Iin and outlet current Iout are equal in magnitude but opposite in direction. Because leakage current IL is non-zero as a result of the properties of electric heater <NUM>, as described above in regard to <FIG>, the resulting alternating magnetic field B that is induced in differential current inductive sensor <NUM> is proportional to the value of leakage current IL, as shown by Equation <NUM> above. Accordingly, a secondary voltage (VSECONDARY) is induced in secondary winding <NUM> that is proportional in magnitude to both leakage current IL, and to the number of primary turns NP and the number of secondary turns NS. It is to be appreciated that in the embodiment, about twelve secondary turns Ns are shown for simplicity. In some embodiments, a greater number of secondary turns Ns can be used to induce a greater secondary voltage (VSECONDARY) in secondary winding <NUM>. In an exemplary embodiment, the number of secondary turns Ns can range from about <NUM> - <NUM>. In other embodiments, the number of secondary turns Ns can be fewer than <NUM> or greater than <NUM>.

Referring again to <FIG>, the induced secondary voltage Vs results in current flowing through resistor <NUM>, thereby developing a voltage potential that can be measured at secondary voltage terminals <NUM>. Resistor <NUM> can be referred to as a burden resistor or output resistor. Accordingly, the secondary voltage at secondary voltage terminals <NUM> provides an indication that is proportional to the value of leakage current IL. The present embodiment, as shown in <FIG>, includes amplifier <NUM>, rectifier <NUM>, filter <NUM>, and analog-to-digital converter <NUM> which together provide a digital signal that is representative of the value of leakage current IL. Accordingly, the secondary voltage at secondary voltage terminals <NUM> is provided as an input to amplifier <NUM> shown in <FIG>.

In a particular embodiment, differential current inductive sensor <NUM> can be installed while air date probe <NUM> and associated electric heater <NUM> are installed on aircraft <NUM> by passing power cable <NUM> through toroid center region <NUM> prior to completing the electrical connections to power cable <NUM>. In the illustrated embodiment, toroid core <NUM> includes toroid split <NUM> which can permit differential current inductive sensor <NUM> to be installed on an existing power cable <NUM> by opening toroid core <NUM> at toroid split <NUM> so that toroid core <NUM> can be placed around an existing power cable <NUM> and then secured by rejoining toroid core <NUM>.

Various means of holding together toroid core <NUM> having toroid split <NUM> can be used. The resulting configuration in which differential current inductive sensor <NUM> is installed over an existing power cable <NUM> can be used on an aircraft (e.g., aircraft <NUM>, as shown in <FIG>) having installed air data probes <NUM>. The aforementioned method of placing toroid core <NUM> around an existing power cable <NUM> can also be used on newly-installed air data probes <NUM>, for example, where power cable <NUM> is installed in place. Accordingly, the scope of the present disclosure applies to both new installations and retrofits on installed equipment. In a particular embodiment, whereby differential current inductive sensor <NUM> and probe control/monitor system <NUM> is installed on an existing (i.e., already in-service) air data probe <NUM>, leakage current IL that is first measured by probe control/monitor system <NUM> will be indicative of a value of RUL corresponding to an in-service electric heater <NUM>.

<FIG> is a schematic diagram of a second embodiment of the differential current inductive sensor. Shown in <FIG> are power cable <NUM>, first conductor <NUM>', second conductor <NUM>', differential current inductive sensor <NUM>, toroid core <NUM>, toroid center region <NUM>, secondary winding <NUM>, resistor <NUM>, and secondary voltage terminals <NUM>. The descriptions of power cable <NUM>, first conductor <NUM>', second conductor <NUM>', differential current inductive sensor <NUM>, toroid core <NUM>, toroid center region <NUM>, secondary winding <NUM>, resistor <NUM>, and secondary voltage terminals <NUM> are substantially as provided above in regard to <FIG>. A torrid split is not shown in <FIG>, but can be provided in some embodiments, for example, as described above in regard to <FIG>.

In the illustrated embodiment, power cable <NUM> is looped around toroid core <NUM> three times while passing through toroid center region <NUM> three times. The number of primary turns NP can be said to be three, and the resulting alternating magnetic field B for a particular value of leakage current IL will be approximately three times the value of that produced by a single pass through toroid center region <NUM> (e.g., as shown in <FIG>). Accordingly, a greater value of induced secondary voltage Vs can result for a given number of secondary turns Ns. The illustrated embodiment shown in <FIG> can be beneficial in providing a greater sensitivity in measuring leakage current IL, thereby allowing smaller values of leakage current IL to be measured and processed by probe control/monitor system <NUM>. This can improve the sensitivity of differential current inductive sensor <NUM> to smaller values of leakage current IL, and/or improve the measurement resolution of differential current inductive sensor <NUM>. In some embodiments, differential current inductive sensor <NUM> can include two primary turns NP. In other embodiments, differential current inductive sensor <NUM> can include four or more primary turns NP. For example, in a particular embodiment, the number of primary turns NP can range from about <NUM>- <NUM>.

The embodiment shown in <FIG> is exemplary, and in some embodiments, practically any number of primary turns NP can be used, given various factors including, for example, the physical sizes of power cable <NUM> and the physical size of toroid core <NUM>. In an exemplary embodiments shown in <FIG>, first and second conductors <NUM>' and <NUM>' of electrical heater <NUM> can each have a wire size of <NUM> AWG (<NUM><NUM> cross-sectional area), power cable <NUM> can have an outside diameter (not labeled) of about <NUM> inch (<NUM>), and toroid core <NUM> can have an inside diameter (not labeled) of about <NUM> inch (<NUM>). All sizes of power cable <NUM> and toroid core <NUM> are within the scope of the present disclosure. Moreover, any size of wire can be used for secondary winding <NUM>, <NUM>. In some embodiments, power cable <NUM> can include more than two conductors (i.e., first and second conductors <NUM>', <NUM>'). In these or other embodiments, power cable <NUM> can be sheathed (e.g., braided metallic sheath) in regions other than in the vicinity of toroid core <NUM>, <NUM>. Sheathed power cables <NUM> can be generally used for connecting a particular electrical heater to electric power source <NUM> for various reasons (e.g., physical protection, electromagnetic shielding).

Claim 1:
A system for monitoring health of an electrical heater (<NUM>) of an air data probe (<NUM>), the system comprising:
an electrical power source (<NUM>) configured to electrically couple to the electrical heater (<NUM>) so as to provide electrical power thereto, the electrical heater (<NUM>) having an electrical property that changes in response to a thermal load of the electrical heater (<NUM>), wherein the thermal load of the electrical heater (<NUM>), in turn, depends upon aircraft flight parameters and/or environmental conditions;
wherein the electrical property that changes in response to the thermal load of the electrical heater is an electrical resistance of the electrical heater or a leakage current between a resistive heating element (<NUM>) and a conductive member (<NUM>) of the air data probe;
an aircraft interface (<NUM>) configured to communicate with an aircraft so as to receive the aircraft flight parameters and/or the environmental conditions therefrom;
an electrical-property calculator (<NUM>) configured to calculate a thermal load of the electrical heater (<NUM>) based on the received aircraft flight parameters and/or the environmental conditions, the electrical-property calculator further configured to calculate an expected value of the electrical property of the electrical heater (<NUM>) based on the provided electrical power and further based on the aircraft flight parameters and/or environmental conditions;
an electrical measurement circuit (<NUM>) configured to electrically couple to the electrical heater (<NUM>) so as to measure electrical parameters indicative of the electrical property of the electrical heater; and
a health monitor (<NUM>) configured to determine health of the electrical heater (<NUM>) based on a comparison between the calculated expected value and the electrical property as indicated by the measured electrical parameters.