Patent Description:
Unexpected failures of air data probes, such as Pitot probes and total-air-temperature probes can lead to flight delays and related costs for aircraft operators. Air data probes typically include resistive heater elements to prevent operational issues relating to in-flight ice buildup. One of the most common types of failures in air data probes is failure of the resistive heating element. The ability to predict a future failure of the resistive heating element in air data probes would permit and facilitate pre-emptive maintenance to be performed without causing flight delays.

The heater element in an air data probe often consists of a resistive heater wire surrounded by an insulator and encapsulated within a metallic sheath. The insulator creates a very high electrical resistance between the metallic sheath (often electrically connected to the body of the probe) and the heater wire itself. The presence of the heater wire, insulator, and metallic sheath also creates a capacitance between the sheath and the heater wire. The combination of the insulation resistance and the wire-sheath capacitance creates a path for leakage current to flow from the heater wire to the sheath when a voltage is applied to the heating element. <CIT> discloses a system for predicting remaining useful life of a resistive heating element. <CIT> discloses a system for monitoring health of an electrical heater of an air data probe. <CIT> discloses a system for determining remaining useful life of a probe system.

According to a first aspect, a system for predicting failure of a resistive heating element of an air data probe is provided according to claim <NUM>.

According to a second aspect, a method for predicting failure of a resistive heating element of an air data probe is provided according to claim <NUM>.

Apparatus and associated methods relate to predicting failure and estimating remaining useful life of a resistive heating element of an air-data-probe heater by sensing electrical current provided to the resistive heating element.

<FIG> is a schematic diagram of an aircraft equipped with a system for predicting failure and/or estimating remaining useful life of an air-data-probe heater. In <FIG>, aircraft <NUM> includes engine <NUM>, which drives electrical generator <NUM> so as to provide electrical operating power to various powered electrical systems aboard aircraft <NUM>. Aircraft <NUM> is also equipped with various air-data-probe sensors, including air-data-probe sensor <NUM>. Air-data-probe sensor <NUM> is an electrical system powered by aircraft power distribution system <NUM>. Air-data-probe sensor <NUM> receives operating power from aircraft power source <NUM> via electrical operating power cable <NUM>. Aircraft <NUM> is also equipped with health monitoring system <NUM>, which predicts failure and/or estimates remaining useful life of resistive heating element <NUM> of air-data-probe sensor <NUM>. In the depicted embodiment, health monitoring system <NUM> is configured to predict failure and/or estimate remaining useful life of resistive heating element <NUM> based on an electrical metric of electrical operating power provided to resistive heating element <NUM>.

Although health monitoring system <NUM> predicts failure and/or estimates remaining useful life of resistive heating element <NUM> for any of a variety of air-data-probe sensors, in the depicted embodiment, air-data-probe sensor <NUM> senses air pressure. Various air-data-probe sensors sense air pressure for the purpose of determining various air data metrics, such as airspeed, altitude, angle-of-attack, etc. In the depicted embodiment, air-data-probe sensor <NUM> is a Pitot tube airspeed detector that includes resistive heating element <NUM>, and ram pressure air-data-probe sensor <NUM>. Resistive heating element <NUM> is configured to prevent icing of air-data-probe sensor <NUM> when aircraft <NUM> is operating in an atmosphere in which ice accretion can occur.

Health monitoring system <NUM> includes electrical power source <NUM>, electrical sensor <NUM>, and remaining life prediction engine <NUM>. Electrical power source <NUM> converts electrical power received from aircraft power source <NUM> into one or more different power configurations for use by air-data-probe sensor <NUM> and/or resistive heating element <NUM>. Electrical sensor <NUM> is configured to sense an electrical metric of the operating power provided to resistive heating element <NUM>. The electrical metric sensed by electrical sensor <NUM> is at least one of: i) phase relation between electrical current conducted from the resistive heating element to its surrounding conductive sheath and voltage across the resistive heating element; ii) a time-domain profile during a full power cycle of the leakage current conducted from the resistive heating element to its surrounding conductive sheath or the voltage across the resistive heating element; and/or iii) high-frequency components of the electrical current conducted by the resistive heating element and/or of the voltage across the resistive heating element.

Remaining life prediction engine <NUM> predicts failure and/or estimates remaining useful life of resistive heating element <NUM> based on the electrical metric sensed by electrical sensor <NUM>. Each of the various electrical metrics described above can be indicative of health of resistive heating element <NUM>. Each of these described electrical metrics with be further described below along with the relation these electrical metrics have with the health of resistive heating element <NUM>, with reference to <FIG>.

<FIG> is a graph of a cross-sectional diagram of a resistive heating element along with coaxial insulative material and a coaxial conductive sheath. Resistive heating element <NUM> is designed to have a particular resistance-temperature relation. The temperature of resistive heating element <NUM> can be determined based on a determination of electrical resistance of resistive heating element <NUM>. For example, electrical resistance of resistive heating element <NUM> can be monotonically increasing with increasing temperature. For such a resistance-temperature relation, temperature of resistive heating element <NUM> is a function of resistance of resistive heating element <NUM>. Coaxial insulative material <NUM> provides electrical insulation between resistive heating element <NUM> and surrounding coaxial conductive sheath <NUM>. Such electrical insulation permits resistive heating element <NUM> to be electrically biased independently of coaxial conductive sheath <NUM>, which is typically grounded.

<FIG> is simplified schematic circuit demonstrating leakage behavior of a resistive heating element biased by an electrical power source. In <FIG>, electrical power source <NUM> provides operating power to resistive heating element <NUM>. Resistive heating element <NUM> has leakage path <NUM> at a particular location along a length of resistive heating element <NUM>. Simplified lumped parameter model of resistive heating element <NUM> has portions on either side of the particular location along its length, as represented by first heating resistor RH1 and second heating resistor RH2. Although only one leakage path is depicted, every section of resistive heating element <NUM> can be modeled with such a leakage path. Leakage path <NUM> is modeled as leakage resistance RLKG in parallel with leakage capacitance CLKG.

Values of leakage resistance RLKG and leakage capacitance CLKG can change over time for a variety of reasons. For example, coaxial insulative material <NUM> (depicted in <FIG>) can degrade, providing increased conductivity and/or changed capacitive coupling between resistive heating element <NUM> and surrounding coaxial conductive sheath <NUM>. These two mechanisms - increased conductivity and/or increased capacitive coupling - are represented as leakage resistor RLKG and leakage capacitor CLKG, respectively. The primary mechanism in response to degradation of insulative material <NUM> is increased conductivity (i.e., reduced leakage resistance RLKG). As leakage resistance RLKG decreases (e.g., due to degradation of insulative material <NUM>), the character of the leakage path changes from being mostly capacitive to more resistive.

Changes to leakage resistance RLKG and/or leakage capacitance CLKG of leakage path <NUM> changes the electrical behavior of resistive heating element <NUM>. When the leakage path is mostly capacitive (e.g., when insulative material is undegraded), resistive heating element <NUM> and coaxial conductive sheath <NUM> are mostly capacitively coupled to one another (i.e., RLKG is very large). Thus, the phase relation between the leakage current and the voltage across resistive heating element <NUM> will be mostly out of phase (e.g., about <NUM> degrees). For capacitive coupling, the electrical current leads the voltage. But when a short develops between resistive heating element <NUM> and coaxial conductive sheath <NUM>, the phase angle will become less out of phase, as the coupling between resistive heating element <NUM> and coaxial conductive sheath <NUM>, becomes more conductive (i.e., leakage resistance RLKG become smaller) and less capacitive.

<FIG>, which are not according to the present invention, are graphs of AC electrical voltage of operating power provided to a resistive heating element and leakage current, for various health conditions of a resistive heating element. In <FIG>, graph <NUM> includes horizontal axis <NUM>, vertical axis <NUM>, voltage-time relation <NUM>, and current-time relation <NUM>. Horizontal axis <NUM> is indicative of time. Vertical axis <NUM> is indicative of voltage and leakage current. Voltage-time relation <NUM> depicts temporal behavior of voltage across resistive heating element <NUM> (as depicted in <FIG>). In the depicted voltage-time relation, electrical power source <NUM> provides AC operating power to resistive heating element <NUM>. In <FIG>, leakage current-time relation <NUM> is mostly out of phase with voltage-time relation <NUM>. Leakage current-time relation <NUM> leads voltage-time relation <NUM> by about <NUM> degrees. Such a relation occurs when leakage capacitor CLKG is quite large as compared to the resistive path (e.g., the impedance associated with leakage capacitor CLKG is a dominant contributor to the leakage current at the frequency of the AC operating power - the insulation resistance in this case is quite large and has much smaller contribution to leakage current). Such a phase relation is indicative of little or no degradation of insulative material <NUM>.

In <FIG>, graph <NUM> includes horizontal axis <NUM>, vertical axis <NUM>, voltage-time relation <NUM>, and leakage current-time relation <NUM>. Horizontal axis <NUM> is again indicative of time. Vertical axis <NUM> is again indicative of voltage and leakage current. Voltage-time relation <NUM> depicts temporal behavior of voltage across resistive heating element <NUM> (as depicted in <FIG>). In the depicted voltage-time relation, electrical power source <NUM> provides AC operating power to resistive heating element <NUM>. In <FIG>, leakage current-time relation <NUM> is not in phase with voltage-time relation <NUM>. Leakage current-time relation <NUM> leads voltage-time relation <NUM> by about <NUM> degrees. Such a relation can occur when the impedance of leakage capacitor CLKG is comparable to impedance of the leakage resistor RLKG at frequency of the AC operating power. Such a phase relation is indicative of some degradation of insulative material <NUM>.

In <FIG>, graph <NUM> includes horizontal axis <NUM>, vertical axis <NUM>, voltage-time relation <NUM>, and leakage current-time relation <NUM>. Horizontal axis <NUM> is again indicative of time. Vertical axis <NUM> is again indicative of voltage and leakage current. Voltage-time relation <NUM> depicts temporal behavior of voltage across resistive heating element <NUM> (as depicted in <FIG>). In the depicted voltage-time relation, electrical power source <NUM> provides AC operating power to resistive heating element <NUM>. In <FIG>, leakage current-time relation <NUM> is mostly in phase with voltage-time relation <NUM>. Such a relation occurs when insulative material <NUM> becomes compromised (e.g., very thin or non-existent in places).

Temperature also affects the phase relation of the electrical current and the voltage for a degraded resistive heating element. Such a temperature dependency can occur for a variety of reasons. For example, as resistive heating element <NUM> heats up, leakage conductance (i.e., inverse of RLKG) can decrease as moisture is driven away out of coaxial insulative material <NUM>. This decrease in leakage conductance <NUM>/RLKG can cause the phase relation between leakage current and voltage across resistive heating element <NUM> to become somewhat more out of phase (e.g., between a <NUM> and <NUM> degree change from its initial cold temperature phase relation). Also, a temperature dependency can occur because the insulative quality of insulative material <NUM> can change as a function of temperature. Thus, understanding the normal temperature dependency of the leakage current-voltage phase relation can help one to identify when such a phase relation is abnormal.

<FIG>, which is not according to the present invention, is a graph of leakage current to voltage phase relationship at the operating power provided to a resistive heating element as a function of temperature. In <FIG>, graph <NUM> includes horizontal axis <NUM>, vertical axis <NUM>, and leakage current-voltage phase relation <NUM>. Horizontal axis <NUM> is indicative of temperature. Vertical axis <NUM> is indicative of phase delay angle of the leakage current with regard to the voltage of the operating power provided to resistive heating element <NUM> (depicted in <FIG>). Leakage current-voltage phase relation <NUM> indicates that at low temperatures, the electrical current is delayed by about -<NUM> degrees (i.e., the electrical current leads the voltage by about <NUM> degrees). As temperature increases, the phase delay of the leakage current increases (i.e., decreases in negative magnitude). Such a large change in phase relation is indicative of a probe, in which insulative material <NUM> has been degraded. For a healthy probe, changes in the phase relation due to heating of resistive heating element is typically modest (e.g., between <NUM> and <NUM> degrees). At very hot temperatures, the phase delay of the leakage current crosses zero degrees for the resistive heating element with degraded insulative material, as depicted in the figure. Because heating of the resistive heating element can cause some phase relation change, some embodiments of health monitoring system <NUM> use leakage current-voltage phase data acquired at a predetermined standard temperature. Other embodiments of health monitoring system <NUM> compare measurements of leakage current-voltage phase data with a known predetermined leakage current-voltage phase relation, such as, for example, leakage current-voltage phase relation <NUM>.

<FIG>, which is not according to the present invention, is a graph of leakage current phase delay data acquired over a lifetime of a resistive heating element. In <FIG>, graph <NUM> includes horizontal axis <NUM>, vertical axis <NUM>, leakage current data <NUM> and leakage current projection <NUM>. Horizontal axis <NUM> is indicative of heater power cycle number. Vertical axis <NUM> is indicative of leakage current phase delay. Every time that the resistive heating element <NUM> is power cycled (i.e., turned on and operated), data is collected at a predetermined operating temperature, as the temperature of resistive heating element <NUM> crosses that predetermined operating temperature. This acquired data is represented on graph <NUM> as leakage current data <NUM>. A trend line is fit to leakage current data <NUM> and projected as leakage current phase delay projection <NUM> on graph <NUM>. The number of remaining power cycles before leakage current phase delay projection <NUM> crosses a predetermined threshold can indicate a remaining useful life of resistive heating element <NUM>. In the depicted embodiment, predetermined threshold is zero degrees, but such a threshold need not be zero degrees, depending on the lifetime testing margin of the particular embodiment.

Instead of predicting failure and/or estimating remaining useful life of a resistive heating element based on leakage current-voltage phase of the operating power provided thereto, some embodiments predict failure and/or estimate remaining useful life of a resistive heating element based on temporal startup behavior of electrical metrics of the operating power. <FIG>, which is not according to the present invention, is a graph of various start up behaviors for leakage current from a resistive heating element. In <FIG>, graph <NUM> includes horizontal axis <NUM>, vertical axis <NUM>, and leakage current-time relations 96A-96C. Horizontal axis <NUM> is indicative of time measured from initial provision of power to resistive heating element <NUM>. Vertical axis <NUM> is indicative of leakage current measured between resistive heating element <NUM> and coaxial conductive sheath <NUM>. In the depicted voltage-time relation, electrical power source <NUM> provides AC operating power to resistive heating element <NUM>. In such AC embodiments, RMS values of current and power can be monitored and start up behavior of these monitored electrical metrics are indicative of health of coaxial conductive sheath <NUM> and/or coaxial insulative material <NUM>. An increase in leakage current exhibitions of noise fluctuations can indicate a presence of moisture, thus indicating damage to either coaxial conductive sheath <NUM> and/or coaxial insulative material <NUM>. In some embodiments, DC operating power is provided to resistive heating element <NUM>. Such leakage current can have start-up behavior that varies in response to health condition and in response to presence or absence of moisture. Current-time relations 96A-96C represent leakage current of resistive heating element <NUM> as measured at start-up time for three consequent start-up cycles representing different moisture content conditions: i) before moisture ingress; ii) with moisture ingress; and iii) after moisture ingress.

Coaxial conductive sheath <NUM> is configured to provide a physical barrier between resistive heating element <NUM> and coaxial insulative material <NUM> from the atmospheric environment. Moisture in the atmospheric environment can cause degradation to each of coaxial insulative material <NUM> and resistive heating element <NUM>, should such elements be exposed to moisture in the atmosphere. The integrity of the barrier presented by coaxial conductive sheath <NUM> can become compromised with age, though. And should coaxial conductive sheath <NUM> become compromised, leakage currents can increase as a result of degradation (e.g., thinning, cracking, etc.) of coaxial insulative material <NUM>. Such degradation can be due to a compromised coaxial conductive sheath <NUM>, which, when compromised, can permit the ingress of moisture into coaxial insulative material <NUM> and resistive heating element <NUM>. Current-time relation 96A represents the power-cycle leakage current profile for resistive heating element <NUM> before moisture ingress into coaxial insulative material <NUM> and/or resistive heating element <NUM>. Current-time relation 96C represents the start-up leakage current profile for resistive heating element <NUM> after moisture ingress into coaxial insulative material <NUM> and/or resistive heating element <NUM>. Such moisture ingress assists the conduction of electrical currents, thereby increasing the level of leakage current until thermal heating element <NUM> heats up enough to drive away the moisture present. The steady-state current asymptote is higher for current-time relation 96C than the steady-state current asymptote for current-time relation 96A. Current-time relation 96B represents the power-cycle leakage current profile for resistive heating element <NUM> after moisture ingress into coaxial insulative material <NUM> and/or resistive heating element <NUM> (e.g., after the moisture has been driven out of coaxial insulative material <NUM> by temperatures generated by resistive heating element <NUM>). The steady-state current asymptote remains higher for current-time relation 96B than the steady-state current asymptote for current-time relation 96A. Furthermore, there are some high-frequency spikes superimposed upon current-time relation 96B, which are not exhibited in current-time relation 96A.

Another way to estimate remaining useful life of a resistive heating element is to detect high-frequency signal components that are indicative of compromised integrity of coaxial insulative material <NUM> and/or resistive heating element <NUM>. <FIG> is a graph of high-frequency noise in AC electrical currents provided to a resistive heating element showing degradation. In <FIG>, graph <NUM> includes horizontal axis <NUM>, vertical axis <NUM>, and electrical current-time relations <NUM> and <NUM>. Horizontal axis <NUM> is indicative of time. Vertical axis <NUM> is indicative of amplitude of electrical current of operating power provided to resistive heating element <NUM>. Electrical current-time relation <NUM> depicts temporal behavior of electrical current of the operating power provided to resistive heating element <NUM> (as depicted in <FIG>), for a resistive heating element with healthy insulative material <NUM>. In the depicted electrical current-time relation, electrical power source <NUM> provides AC operating power to resistive heating element <NUM>. Electrical current-time relation <NUM> depicts temporal behavior of electrical current of the operating power provided to resistive heating element <NUM> (as depicted in <FIG>), for a resistive heating element with degraded insulative material <NUM>. The main difference between electrical current-time relation <NUM> and electrical current-time relation <NUM> is the high-frequency behavior (i.e., at frequencies high in comparison with the frequency of the AC operating power). Electrical current-time relation <NUM> has discernable high-frequency noise superimposed on the AC signal, whereas electrical current-time relation <NUM> has no discernable high-frequency noise. Such high-frequency noise can be indicative of a corroded resistive heating element <NUM>. Current-time relation <NUM> has such high-frequency components indicative of degradation (e.g., micro cracks) of resistive heating element <NUM> superimposed on the AC current waveform.

High-frequency components in current-time relation <NUM> can be caused by other factors as well. For example, any high-frequency components in voltage of the operating power provided to resistive heating element <NUM> will be replicated in current-time relation <NUM> for resistive heating element <NUM> that are not degraded. Therefore, sensing high-frequency components of both voltage-time relation <NUM> and current-time relation <NUM> can be used to determine if such components are present in but one of the two electrical metrics. For example, a ratio can be made of the high-frequency components of current-time relation <NUM> to the high-frequency components of voltage of operating power provided to resistive heating element <NUM>. Then, such a ratio can be compared with a predetermined threshold value. If the ratio is greater than the predetermined threshold value, remaining life prediction engine <NUM> can generate a signal indicative of the event. A log of such incidences of such a comparison can be maintained. Remaining useful life can be determined based on such a log.

Claim 1:
A system (<NUM>) for predicting failure of a resistive heating element of an air data probe, the system comprising:
an AC electrical power source (<NUM>) configured to provide AC electrical operating power to the resistive heating element (<NUM>);
an electrical sensor (<NUM>) configured to sense electrical current provided to the resistive heating element;
a high-frequency signal detector configured to detect high-frequency components of the electrical current sensed by taking a derivative of the electrical current sensed, wherein high-frequency components have frequencies that are high in comparison with a frequency of the AC electrical operating power; and
a remaining-life prediction engine (<NUM>) configured to estimate a remaining life of the resistive heating element based on high-frequency components of the electrical current detected.