Patent Grant
10895592
The present invention relates generally to probes, and in particular to a system and method for determining a remaining useful life of aircraft sensor probes.
Probes are utilized to determine characteristics of an environment. In aircraft systems, for example, probes may be implemented on the external portions of the aircraft to aide in determination of conditions such as airspeed, Mach number and flight direction, among others. Due to the harsh conditions of flight, ice may build-up on portions of the probe. To combat this, heaters are implemented within the probe to prevent the formation of ice that may impact proper functionality of the probe.
When probes break down, they need to be replaced, often prior to a subsequent takeoff. The heating element of a probe is often the most life-limited part. Therefore, probes need to be replaced as soon as the heating element breaks down. It is desirable to predict a remaining useful life of the probe heating element in order to better predict maintenance needs of the probe itself.
A system for an aircraft includes a probe, an antenna and a control circuit. The probe includes a heater comprising a resistive heating element routed through the probe. An operational voltage is provided to the resistive heating element to provide heating for the probe. The control circuit is configured to determine an antenna interaction between the resistive heating element and the antenna and determine a remaining useful life of the probe based on the antenna interaction over time.
A method for determining a remaining useful life of a probe includes providing an operational voltage to a resistive heating element of the aircraft probe during a plurality of flights of an aircraft to provide heat for the probe; providing, by a radio-frequency (RF) antenna, a radio-frequency (RF) signal to the probe; determining, by a control circuit, an antenna response of the resistive heating element of the aircraft probe during the plurality of flights; and determining the remaining useful life based upon the antenna response over time.
A probe system configured to receive a radio-frequency (RF) signal from a radio-frequency (RF) antenna includes a heater and a control circuit. The heater includes a resistive heating element routed through the probe system. An operational voltage is provided to the resistive heating element to provide heating for the probe system and the resistive heating element has an element capacitance. The control circuit is configured to determine an antenna response of the resistive heating element and determine a remaining useful life of the probe system based on the antenna response over time.
A system and method for determining the remaining useful life of a probe is disclosed herein that includes monitoring characteristics of the probe over time. The probe, which may be an aircraft total-air-temperature (TAT) probe or any other probe, includes a resistive heating element, such as a heater wire, routed through the probe. Over time, as the heater wire ages, the heater wire may degrade, causing characteristics of the heater wire to change. These changing characteristics may be monitored and plotted over time, for example. The remaining useful life of the probe may then be determined and reported based upon the monitored characteristics.
Resistive heating element 14, which may be a heater wire, for example, may receive power directly, or through control and interface circuit 16, from aircraft power bus 18 to provide heating for probe 12a. Power bus 18 may be any direct current (DC) or alternating current (AC) aircraft power bus. For example, resistive heating element 14 may receive power from a 28 Volt DC power bus. An operational current, based on the power received from power bus 18, flows through resistive heating element 14, which provides heating for probe 12a. Control and interface circuit 16 may also be connected to aircraft avionics 20. Alternatively, control and interface circuit 16 may be implemented integral to aircraft avionics 20. Control and interface circuit 16 may be configured to provide data to, and receive data from, aircraft avionics 20.
Current sensors 22a and 22b may sense current flowing into, and out of, resistive heating element 14 at heating element input 26a and heating element output 26b, respectively. Current sensors 22a and 22b may provide a signal indicative of the sensed current at the respective locations to control and interface circuit 16. Temperature sensor 24 may be positioned to sense a temperature of probe 12a and provide the sensed temperature to control and interface circuit 16. In other embodiments, a temperature may be estimated, for example, based upon sensed aircraft conditions and provided to control and interface circuit 16 from avionics 20. For example, avionics 20 may determine a present altitude and/or airspeed of aircraft 10 and may estimate the temperature of probe 12a based upon, among other items, the present altitude and/or airspeed. Current sensors 22a and 22b may be any devices that sense current. In an embodiment, current sensors 22a and 22b may be non-contact current sensors such as, for example, a current transformer or Hall effect sensor.
Thermal imager 28 may be located integral to, or separate from, aircraft 10. Thermal imager 28 may be any device capable of receiving infrared radiation, for example, and providing an electrical output indicative of the received infrared radiation. While not illustrated as such, thermal imager 28 may be connected to communicate with control and interface circuit 16 and/or avionics 20 through a wired or wireless connection. This way, thermal images of probe 12a may be obtained, analyzed and stored over time.
Radio-frequency (RF) antenna 30 is any antenna structure capable of emitting and/or receiving an RF signal. RF antenna 30 may be configured to receive power and emit RF radiation that may be received by heating element 14, for example. Heating element 14, which may include a large loop of heater wire having dielectric properties, may act as an antenna, capable of emitting and/or receiving RF energy. RF antenna 30 may be located integral to, or separate from, aircraft 10. RF antenna 30 may be connected to communicate with control and interface circuit 16 and/or avionics 20 through a wired or wireless connection (not shown). This way, control and interface circuit 16 may control RF antenna 30 to emit a plurality of RF signal frequencies, and/or may analyze a response of RF antenna 30 to an RF signal emitted by heater wire 14, for example.
Over time, resistive heating element 14 may degrade, and eventually break down such that current may no longer flow through resistive heating element 14 to provide heating for probe 12a. Once resistive heating element 14 has broken down, aircraft probe 12a must be repaired or replaced. A TAT probe, for example, may be utilized, among other things, to determine a Mach number for the aircraft. The Mach number may be needed for takeoff and thus, the TAT probe may be required to be functional prior to taking off. If the TAT probe is malfunctioning, it must be replaced, which may cause undesirable flight delays. If the remaining useful life of the TAT probe is known, the TAT probe can be replaced between flights or at another convenient time for repair, preventing delays or other costs incurred due to an unexpected failure of the probe.
During operation of probe 12a, changes may occur, for example, due to hot and cold cycles as well as other varying environmental conditions such as temperature, pressure, humidity, environmental gases and aerosols, among others. These environmental conditions, in addition to the temperature cycling of probe 12a, may cause a sealing of heating element 14 to become compromised, leading to foreign material leaking into insulation 44 and heater element 42. Heating element 14 may be oxidized and the dielectric material properties of heating element 14 may change. This may lead to certain characteristics of heating element 14 such as resistance and the capacitance between wire 40 and metallic sheath 46, among other characteristics, to change over time.
Functions 60 shown in
In an example embodiment, current may be sampled using current sensor 22a or 22b at any time during flight and provided to control and interface circuit 16 several times each flight. This sampled current may be stored in a memory of control and interface circuit 16, for example, or some other memory or storage device. Because current is directly affected by temperature, a temperature may also be sensed using temperature sensor 24 and stored along with a respective sensed current so that the current may be normalized with respect to temperature. Alternatively, a temperature may be estimated using data from avionics 20, for example. Control and interface circuit 16 may utilize the sensed or estimated temperature to directly normalize the current prior to storage, or may store the two values for later normalization.
Function 60, determined during testing, may be used in conjunction with the stored normalized current to determine the half-life estimate of heating element 14. For example, the stored normalized current may be plotted by control and interface circuit 16 such that a present slope of the plot may be determined. This present slope may be utilized, for example, to detect point 64 by detecting that the slope of the plot has transitioned from greater than 1 to less than 1, or any other point on function 60. Once point 64 is detected, for example, the half-life of heating element 14 may be determined.
At the end of the life of heating element 14, thermal fatigue causes heater wire 40 to fracture and gradually become electrically open, which increases the resistance of heating element 14. At low voltage, electron tunneling is the main mechanism for electrical current conduction. Arcing that occurs at 0.1 Volts may indicate a micro fracture with a gap on the order of 1 nanometer (nm), while arcing that occurs at 28 Volts may correspond to an approximately 0.3 micrometer (um) gap. Thus, micro fractures may be detected by monitoring a current response using a low voltage prior to failure of heating element 14.
As seen in
The amount of remaining useful life of probe 12a following detection of a micro fracture may be determined, for example, through testing of probes 12a-12n. Current may be cycled, for example, until heating element 14 of a respective test probe 12a-12n breaks down. Between test cycles, a low voltage may be provided to test probes 12a-12n. While the low voltage is supplied, a current may be sensed and stored. This way, the cycle at which a micro fracture on the order of 1 nm is detected may be determined. Following detection of the micro fracture, once the respective test probe 12a-12n breaks down, a percentage of life after detection of the micro fracture may be determined.
During normal operation of probes 12a-12n, a low voltage may be provided, for example, by control and interface circuit 16 between flights of aircraft 10, or at any other time during which heating element 14 is not receiving an operational voltage. Current sensor 22a or 22b may be utilized by control and interface circuit 16 to obtain a sensed current while providing the low voltage. Upon detection of the sensed current going to zero at low voltage, a remaining useful life may be determined based upon the percentage of life determined during testing of the probes 12a-12n.
In an embodiment, to obtain the resonant frequency, the capacitance between metallic sheath 46 and lead wire 40 may be swept with frequencies ranging from 1 kilohertz (KHz) to 100 megahertz (MHz) by control and interface circuit 16, for example, through capacitive measurement circuit 50. The peak frequency response and bandwidth may be identified by control and interface circuit 16. This peak frequency response may be monitored and stored over time and plotted as shown in
In another embodiment, algorithms that separate data indicative of a healthy probe and data indicative of an increasingly unhealthy probe may be executed utilizing various signal processing techniques to determine the remaining useful life of heating element 14. For example, time-frequency analysis, machine learning, index theory and other signal processing techniques may be utilized to determine remaining useful life of heating element 14 based upon the monitored resonant frequency.
In another embodiment, ring oscillator 48 may be utilized with probe 12a as the driving element of the output frequency of ring oscillator 48. Ring oscillator 48, or other oscillator circuit, has an output frequency dependent upon the structure of the circuit and the input voltage to the circuit. By controlling the input voltage, the output frequency of ring oscillator 48 can be made dependent solely upon the capacitance between metallic sheath 46 and lead wire 40. As this capacitance changes as heating element 14 degrades, so does the output frequency of ring oscillator 48. The changing output frequency may be stored and plotted over time to determine a remaining useful life of probe 12a. For example, testing of probes 12a-12n may result in a function of the output frequency similar to that shown in
In another embodiment, RF antenna 30 may be controlled to sweep a wide range of frequencies. For example, control and interface circuit 16 may provide AC power to antenna 30 at varying frequencies to facilitate emission of RF radiation at a plurality of frequencies from antenna 30. The S12 parameter for heating element 14, which is a measure of the power received at heating element 14 from the RF emission of antenna 30, may be determined and monitored by control and interface circuit 16.
This antenna response of heating element 14 may be analyzed by control and interface circuit 16 to determine, for example, a resonant frequency response of heating element 14. The antenna properties of heating element 14 may be dependent upon, among other things, length and shape of heating element 14, and dielectric properties of heating element 14. These dielectric properties may include, for example, permittivity, permeability, homogeneity and thickness, among others. As the wire ages and degrades, the dielectric properties of heating element 14 may change, which may lead to a change in the resonant frequency of heating element 14.
The determined resonant frequency may be monitored and plotted over time by control and interface circuit 16. Testing of probes 12a-12n may be utilized to determine a function at which the resonant frequency changes over time. This function may substantially follow those shown in
In another embodiment, control and interface circuit 16 may provide AC power to heating element 14 at varying frequencies to facilitate emission of RF radiation at a plurality of frequencies from heating element 14. Control and interface circuit 16 may then monitor the S12 parameter of RF antenna 30 to determine a resonant frequency, which may be monitored and analyzed over time to determine a half-life of heating element 14.
As seen in
A step 104, while the low voltage is being supplied to heating element 14, current is sensed by one of current sensors 22a and 22b. The sensed current may be provided to, and stored by, control and interface circuit 16. The process in steps 102 and 104 is repeated over time, and the sensed current is monitored by control and interface circuit 16 at step 106. At step 108, micro fractures in heating element 14 are detected based upon the monitored current. For example, if the monitored current has dropped to zero, as illustrated in
Following detection of an approximately 1 nm micro fracture, the remaining useful life of heating element 14 may be estimated. This estimation may be based upon testing of heating elements 14, for example. Probes 12a-12n may be tested to determine an average remaining useful life of heating element 14 following detection of a micro fracture of approximately 1 nm.
Once the remaining useful life of resistive heating element 14 is determined by control and interface circuit 16, the remaining useful life may be reported at step 110. This report may be made to the cockpit through avionics 20, or to some other computer system. By reporting the remaining useful life, probes 12a-12n may be replaced prior to breaking down and thus, unnecessary flight delays may be avoided.
During normal operation of probes 12a-12n, current through resistive heating element 14 may fluctuate based upon the operating point of probe 12a-12n. Therefore, it may be desirable at step 124 to sense the operational current at a similar point of operation of heating element 14. For example, upon initial power-on of resistive heating element 14, the current may rise to a peak current. As resistive heating element 14 increases in temperature, the current through resistive heating element 14 will decrease to a lower, steady-state current. Thus, current may be sampled and stored consistently at the peak value, at the steady-state value, or at some other expected value, for example.
Temperature may also be sensed and provided to control and interface circuit 16 using temperature sensor 24. At step 126, control and interface circuit 16 may normalize the sensed current using the sensed temperature. Steps 124 and 126 may be repeated and the normalized sensed current may be plotted over several flights, for example, to establish a curve fit. The curve fit may substantially follow the exponential function determined, for example, at step 122. At step 128, the present slope of the sensed current may be determined based upon the exponential function. This slope may then be utilized to determine the half-life of resistive heating element 14. By knowing the half-life of resistive heating element 14, the remaining useful life of resistive heating element 14 may be determined by control and interface circuit 16, for example.
Once the remaining useful life of resistive heating element 14 is determined by control and interface circuit 16, the remaining useful life may be reported at step 130. This report may be made to the cockpit through avionics 20, or to some other computer system. By reporting the remaining useful life, probes 12a-12n may be replaced prior to breaking down and thus, unnecessary flight delays may be avoided.
At step 144, during normal operation of probes 12a-12n, capacitive measurement circuit 50 may be utilized to determine a capacitance between metallic sheath 46 and lead wire 40. The capacitance and capacitance measurement circuit 50 may be directly affected by temperature. At step 146, temperature may be sensed and provided to control and interface circuit 16 using temperature sensor 24. Steps 144 and 146 may be repeated throughout the life of probe 12a and control and interface circuit 16 may normalize the determined capacitance using the sensed temperature and may plot the normalized determined capacitance over several flights, for example, to establish a curve fit. The curve fit may substantially follow the exponential function determined, for example, at step 142. At step 148, the present slope of the determined capacitance may be determined based upon the exponential function. This slope may then be utilized to determine the half-life of resistive heating element 14. By knowing the half-life of resistive heating element 14, the remaining useful life of resistive heating element 14 may be determined by control and interface circuit 16, for example.
Once the remaining useful life of resistive heating element 14 is determined by control and interface circuit 16, the remaining useful life may be reported at step 150. This report may be made to the cockpit through avionics 20, or to some other computer system. By reporting the remaining useful life, probes 12a-12n may be replaced prior to breaking down and thus, unnecessary flight delays may be avoided.
At step 164, during normal operation of probes 12a-12n, leakage current from resistive heating element 14 may be determined based upon sensed current from both current sensors 22a and 22b. Current through resistive heating element 14 may fluctuate based upon the operating point of probe 12a-12n. Therefore, it may be desirable to determine the leakage current at a similar point of operation of heating element 14 each time the current is sensed and stored. For example, upon initial power-on of resistive heating element 14, the current may rise to a peak current. As resistive heating element 14 increases in temperature, the current through resistive heating element 14 will decrease to a lower, steady-state current. Thus, current may be sampled and stored consistently at the peak value, at the steady-state value, or at some other expected value, for example.
Temperature may also be sensed and provided to control and interface circuit 16 using temperature sensor 24. At step 166, control and interface circuit 16 may normalize the determined leakage current using the sensed temperature. Steps 164 and 166 may be repeated and control and interface circuit 16 may plot the normalized leakage current over several flights, for example, to establish a curve fit. The curve fit may substantially follow the exponential function determined, for example, at step 162. At step 168, the present slope of the determined leakage current may be determined based upon the exponential function. This slope may then be utilized to determine the half-life of resistive heating element 14. By knowing the half-life of resistive heating element 14, the remaining useful life of resistive heating element 14 may be determined by control and interface circuit 16, for example.
Once the remaining useful life of resistive heating element 14 is determined by control and interface circuit 16, the remaining useful life may be reported at step 170. This report may be made to the cockpit through avionics 20, or to some other computer system. By reporting the remaining useful life, probes 12a-12n may be replaced prior to breaking down and thus, unnecessary flight delays may be avoided.
At step 184, during normal operation of probes 12a-12n, capacitive measurement circuit 50 may be used to determine a resonant frequency of heating element 14 or ring oscillator circuit 48 may be utilized to determine an output frequency of ring oscillator 48 driven by the capacitance of heating element 14. At step 186, temperature may also be sensed and provided to control and interface circuit 16 using temperature sensor 24. Control and interface circuit 16 may normalize the determined resonant frequency using the sensed temperature. Steps 184 and 186 may be repeated and control and interface circuit 16 may plot the normalized resonant or output frequency over several flights, for example, to establish a curve fit. While theoretical capacitance is not directly affected by temperature, in practice both the capacitance and ring oscillator circuit 48 may be directly affected by temperature. The curve fit may substantially follow the exponential function determined, for example, at step 182. At step 188, the present slope of the determined resonance may be determined based upon the exponential function. This slope may then be utilized to determine the half-life of resistive heating element 14. By knowing the half-life of resistive heating element 14, the remaining useful life of resistive heating element 14 may be determined by control and interface circuit 16, for example.
Once the remaining useful life of resistive heating element 14 is determined by control and interface circuit 16, the remaining useful life may be reported at step 190. This report may be made to the cockpit through avionics 20, or to some other computer system. By reporting the remaining useful life, probes 12a-12n may be replaced prior to breaking down and thus, unnecessary flight delays may be avoided.
At step 204, during normal operation of probes 12a-12n, thermal imager 28 may be utilized to obtain thermal images of heating element 14. At step 206, temperature may also be sensed and provided to control and interface circuit 16 using temperature sensor 24. Control and interface circuit 16 may normalize the thermal data using the sensed temperature. Control and interface circuit 16 may also normalize the thermal image data to compensate for probe shape and surface emissivity, for example. Steps 204 and 206 may be repeated and control and interface circuit 16 may plot the normalized thermal data over several flights, for example, to establish a curve fit. The curve fit may substantially follow the exponential function determined, for example, at step 202. At step 208, the present slope of the determined thermal data be determined based upon the exponential function. This slope may then be utilized to determine the half-life of resistive heating element 14. By knowing the half-life of resistive heating element 14, the remaining useful life of resistive heating element 14 may be determined by control and interface circuit 16, for example.
Once the remaining useful life of resistive heating element 14 is determined by control and interface circuit 16, the remaining useful life may be reported at step 210. This report may be made to the cockpit through avionics 20, or to some other computer system. By reporting the remaining useful life, probes 12a-12n may be replaced prior to breaking down and thus, unnecessary flight delays may be avoided.
At step 224, during normal operation of probes 12a-12n, RF antenna 30 may be utilized to provide RF radiation to heating element 14. At step 226, control and interface circuit 16 may monitor the S12 parameter of heating element 14 to determine a resonant frequency response of heating element 14 to the RF radiation. In an alternative embodiment, heating element 14 may be energized to provide RF radiation to antenna 30 and control and interface circuit 16 may monitor the S12 parameter of antenna 30 to determine a resonant frequency response of antenna 30.
Steps 224 and 226 may be repeated and control and interface circuit 16 may plot the resonant frequency response over several flights, for example, to establish a curve fit. The curve fit may substantially follow the exponential function determined, for example, at step 222. At step 228, the present slope of the determined RF response may be determined based upon the exponential function. This slope may then be utilized to determine the half-life of resistive heating element 14. By knowing the half-life of resistive heating element 14, the remaining useful life of resistive heating element 14 may be determined by control and interface circuit 16, for example. During steps 224 and 226, the RF response may be normalized to account for, among other things, temperature. This may be accomplished using data from temperature sensor 24, or an estimate temperature based upon data from avionics 20, for example.
Once the remaining useful life of resistive heating element 14 is determined by control and interface circuit 16, the remaining useful life may be reported at step 230. This report may be made to the cockpit through avionics 20, or to some other computer system. By reporting the remaining useful life, probes 12a-12n may be replaced prior to breaking down and thus, unnecessary flight delays may be avoided.
The following are non-exclusive descriptions of possible embodiments of the present invention.
A system for an aircraft includes a probe, an antenna and a control circuit. The probe includes a heater comprising a resistive heating element routed through the probe. An operational voltage is provided to the resistive heating element to provide heating for the probe. The control circuit is configured to determine an antenna interaction between the resistive heating element and the antenna and determine a remaining useful life of the probe based on the antenna interaction over time.
The system of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:
A further embodiment of the foregoing system, wherein the antenna is a radio-frequency (RF) antenna.
A further embodiment of any of the foregoing systems, wherein the RF antenna is configured to provide a radio-frequency (RF) signal to the probe, and wherein the control circuit is configured to determine the response of the resistive heating element to the RF signal as the antenna interaction.
A further embodiment of any of the foregoing systems, wherein the RF signal comprises a plurality of signal frequencies, and wherein the control circuit is configured to determine a resonant frequency from the resistive heating element in response to the plurality of signal frequencies.
A further embodiment of any of the foregoing systems, wherein the control circuit is configured to determine the resonant frequency from the resistive heating element over a plurality of flights of the aircraft.
A further embodiment of any of the foregoing systems, wherein the control circuit is configured to plot the resonant frequency over the plurality of flights and determine a half-life estimate of the resistive heating element based on the plot.
A further embodiment of any of the foregoing systems, wherein the control circuit is configured to provide a heater signal having a plurality of frequencies to the resistive heating element, and wherein the resistive heating element generates a radio-frequency (RF) signal based on the heater signal, and wherein the control circuit is configured to determine the response of the RF antenna to the RF signal.
A further embodiment of any of the foregoing systems, further comprising a temperature sensor configured to sense a temperature of the probe and provide the sensed temperature to the control circuit, wherein the control circuit is configured to normalize the antenna interaction based on the sensed temperature.
A further embodiment of any of the foregoing systems, wherein the control circuit is configured to normalize the antenna interaction based on an estimate temperature of the heater.
A method for determining a remaining useful life of a probe includes providing an operational voltage to a resistive heating element of the aircraft probe during a plurality of flights of an aircraft to provide heat for the probe; providing, by a radio-frequency (RF) antenna, a radio-frequency (RF) signal to the probe; determining, by a control circuit, an antenna response of the resistive heating element of the aircraft probe during the plurality of flights; and determining the remaining useful life based upon the antenna response over time.
The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:
A further embodiment of the foregoing method, wherein providing, by the RF antenna, the RF signal to the probe includes providing the RF signal at plurality of signal frequencies.
A further embodiment of any of the foregoing methods, wherein determining, by the control circuit, the antenna response of the resistive heating element of the aircraft probe during the plurality of flights includes determining a resonant frequency from the resistive heating element in response to the plurality of signal frequencies.
A further embodiment of any of the foregoing methods, wherein determining the remaining useful life based upon the antenna response over time includes plotting the resonant frequency over the plurality of flights; and determining a half-life estimate of the resistive heating element based on the plot.
A probe system configured to receive a radio-frequency (RF) signal from a radio-frequency (RF) antenna includes a heater and a control circuit. The heater includes a resistive heating element routed through the probe system. An operational voltage is provided to the resistive heating element to provide heating for the probe system and the resistive heating element has an element capacitance. The control circuit is configured to determine an antenna response of the resistive heating element and determine a remaining useful life of the probe system based on the antenna response over time.
The probe system of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:
A further embodiment of the foregoing probe system, wherein the RF signal comprises a plurality of signal frequencies, and wherein the control circuit is configured to determine a resonant frequency from the resistive heating element in response to the plurality of signal frequencies.
A further embodiment of any of the foregoing probe systems, wherein the control circuit is configured to determine the resonant frequency from the resistive heating element over a plurality of flights of the aircraft.
A further embodiment of any of the foregoing probe systems, wherein the control circuit is configured to plot the resonant frequency over the plurality of flights and determine a half-life estimate of the resistive heating element based on the plot.
A further embodiment of any of the foregoing probe systems, further comprising a temperature sensor configured to sense a temperature of the probe system and provide the sensed temperature to the control circuit, wherein the control circuit is configured to normalize the antenna response based on the sensed temperature.
A further embodiment of any of the foregoing probe systems, wherein the control circuit is configured to normalize the antenna response based on an estimated temperature of the heater.
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
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