An aircraft fuel tank minimum fuel level high-reliability measuring circuit and detection method

Abstract

The application discloses a kind of high-reliability measurement circuit and detection method of minimum fuel level of aircraft fuel tank, including measurement branch, reference branch, constant voltage source and comparison circuit, measurement branch and reference branch are connected in parallel, comparison circuit is electrically connected with measurement branch and reference branch, for comparing the voltage at both ends of measurement branch and reference branch;Measurement branch is used to measure the corresponding self-heating temperature generated according to the heat dissipation condition of the medium, and the constant voltage source provides the quasi-constant current operating voltage for measurement branch and reference branch.The application has no moving parts, can automatically compensate environmental temperature variation, has high reliability, low power consumption and intrinsic safety, meets the requirements of aviation fuel tank explosion prevention and long-term operation.

Description

Technical Field

[0001] This invention belongs to the field of aviation fuel monitoring technology, and specifically relates to a highly reliable measurement circuit and detection method for the minimum fuel level in an aircraft fuel tank. Background Technology

[0002] In the aviation industry, fuel level monitoring is one of the key technologies for ensuring flight safety. Traditional fuel level detection methods mainly include mechanical float sensors, capacitive sensors, ultrasonic sensors, and intelligent detection systems based on microprocessors or FPGAs (Field Programmable Gate Arrays).

[0003] Mechanical float-type sensors rely on the physical displacement of moving parts to detect liquid levels, which is susceptible to mechanical wear and jamming, and their reliability is limited in aviation vibration environments. Capacitive sensors, while having no moving parts, are sensitive to changes in fuel characteristics and have complex circuitry. Liquid level detection systems based on FPGAs or complex embedded software can achieve high-precision detection and nonlinear correction, but require time-consuming and expensive software development processes, resulting in high hardware costs and complex software certification processes, making it difficult to meet the aviation industry's requirements for simplicity in safety-critical systems. Furthermore, in some application scenarios, power consumption must be strictly kept to a minimum, while complex digital computing systems often have high power consumption and potential electromagnetic compatibility issues.

[0004] More importantly, the interior of an aviation fuel tank is a potentially explosive environment, subject to extremely strict regulations regarding heat generation, current control, and ignition protection. Currently, there is a lack of a fuel level detection solution that meets aviation safety regulations, operates reliably across a wide temperature range of -55°C to +70°C, requires no complex software support, and has an extremely simple structure. Summary of the Invention

[0005] The purpose of this invention is to provide a highly reliable measurement circuit and detection method for the minimum fuel level in an aircraft fuel tank, so as to solve the problems of high software development cost, high power consumption, complex system and poor reliability of mechanical sensors in the prior art.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a high-reliability measurement circuit for the minimum fuel level in an aircraft fuel tank, characterized in that it includes a measurement branch, a reference branch, a constant voltage source, and a comparison circuit. The measurement branch and the reference branch are connected in parallel, and the comparison circuit is electrically connected to the measurement branch and the reference branch for comparing the voltages across the measurement branch and the reference branch. The measurement branch is used to measure the self-heating temperature generated according to the heat dissipation conditions of the medium in which it is located, and the constant voltage source provides a quasi-constant current operating voltage for the measurement branch and the reference branch.

[0007] Furthermore, the measurement branch includes a first low temperature coefficient precision resistor R2 and a first positive temperature coefficient thermistor R_PTC2. The first low temperature coefficient precision resistor R2 and the first positive temperature coefficient thermistor R_PTC2 are connected in series. The first positive temperature coefficient thermistor R_PTC2 is set to the critical height of the fuel level to be detected, and the measurement is based on the self-heating temperature generated according to the heat dissipation conditions of the medium. The reference branch includes a second low temperature coefficient precision resistor R1 and a second positive temperature coefficient thermistor R_PTC1, which are connected in series. The second positive temperature coefficient thermistor R_PTC1 has the same electrical and structural characteristics as the first positive temperature coefficient thermistor R_PTC2.

[0008] Furthermore, the comparison circuit adopts a Wheatstone bridge topology, with the first positive temperature coefficient thermistor R_PTC2 and the second positive temperature coefficient thermistor R_PTC1 forming two opposite arms of the bridge, and the first low temperature coefficient precision resistor R2 and the second low temperature coefficient precision resistor R1 forming the other two opposite arms of the bridge.

[0009] Furthermore, the resistance of the second low temperature coefficient precision resistor R2 is ten times that of the first low temperature coefficient precision resistor R1.

[0010] Furthermore, the comparison circuit includes a signal amplification unit, which amplifies the voltage across the second positive temperature coefficient thermistor R_PTC1 by 10 times and then performs a linear comparison with the voltage across the first positive temperature coefficient thermistor R_PTC2.

[0011] Furthermore, the Curie temperatures of the first positive temperature coefficient thermistor R_PTC2 and the second positive temperature coefficient thermistor R_PTC1 are higher than the maximum operating ambient temperature of the aviation fuel tank and lower than the limit temperature of the internal hot surface of the fuel tank as specified by aviation safety regulations; the temperature coefficients of the first low temperature coefficient precision resistor R2 and the second low temperature coefficient precision resistor R1 are both less than 50ppm / ℃.

[0012] Furthermore, the constant voltage source is connected to a current-limiting protection circuit and a transient voltage suppression diode to prevent accidental ignition in potentially explosive environments.

[0013] Furthermore, the distance between the surface of the first positive temperature coefficient thermistor R_PTC2 and the tank wall is greater than the minimum safe distance specified by aviation safety standards, and the measurement branch, reference branch, constant voltage source and comparison circuit are all housed in the explosion-proof enclosure.

[0014] Furthermore, the explosion-proof housing is made of stainless steel and is sealed.

[0015] Secondly, the present invention provides a method for measuring the minimum fuel level in an aircraft fuel tank using a highly reliable measurement circuit, comprising the following steps: Install the measuring PTC thermistor at the critical height of the lowest fuel level in the fuel tank, and install the reference PTC thermistor in a fixed position inside the fuel tank that is not affected by changes in fuel level; connect the measuring PTC in series with a precision resistor to form the measuring branch; connect the reference PTC in series with another precision resistor with a resistance ten times that of the measuring PTC to form the reference branch. The voltage across the PTC and the voltage across the reference PTC are simultaneously acquired and measured.

[0016] The acquired reference PTC voltage signal is amplified 10 times; the amplified reference voltage and the measured PTC voltage are input together into a voltage comparator for comparison; if the measured voltage is greater than the amplified reference voltage, the comparator outputs a high-level signal, indicating that the fuel level is below the minimum threshold; otherwise, it outputs a low-level signal, indicating that the fuel level is above the minimum threshold.

[0017] Compared with the prior art, the present invention has the following technical effects: This invention utilizes the self-heating effect of a PTC thermistor for liquid level detection, providing a non-contact measurement method. The thermistor does not need to directly contact the fuel; it determines the liquid level simply by sensing changes in the heat dissipation conditions of the surrounding medium, avoiding the problem of sensor contamination or corrosion by fuel. Furthermore, since the entire detection process does not involve any moving mechanical parts, it completely eliminates common mechanical failure modes in traditional float-type sensors, such as jamming, linkage wear, and spring fatigue, significantly improving the sensor's lifespan and reliability.

[0018] This invention achieves automatic compensation for ambient temperature through a measuring branch and a reference branch. The PTC thermistor in the reference branch has a negligible self-heating effect due to its extremely small operating current, and its temperature is essentially equal to the ambient temperature, thus providing a temperature reference that adapts to real-time environmental changes. The PTC thermistor in the measuring branch experiences corresponding temperature changes depending on the medium it is in. The voltage comparison between the two branches effectively eliminates the influence of ambient temperature on the detection results. Regardless of changes in external temperature, the sensor can accurately identify whether the fuel level is below the critical height. This hardware-level automatic compensation mechanism is more reliable, has a faster response speed, and is unaffected by program errors than software compensation.

[0019] The Curie temperature of the PTC thermistor in this invention is limited to between the maximum operating temperature of the fuel tank and the hot surface limit temperature specified by aviation safety regulations. Even in the event of a malfunction, the thermistor's temperature will not reach a dangerous level sufficient to ignite fuel vapors. Secondly, the constant voltage source is equipped with a current-limiting protection circuit and a transient voltage suppression diode, electrically limiting the energy that may be released and preventing the formation of an ignition source. Thirdly, the explosion-proof housing is made of stainless steel and is sealed, meeting the requirements of an explosion-proof structure and preventing any potential internal explosion from propagating to the external environment. Furthermore, a sufficient safety distance is maintained between the thermistor and the fuel tank wall, avoiding the risk of localized overheating igniting the fuel film on the tank wall.

[0020] This invention employs a purely analog circuit design, completely eliminating complex digital electronic components such as microprocessors, FPGAs, and programmable logic devices. It eliminates risks such as software failures, program malfunctions, and electromagnetic interference causing digital circuit errors, making it particularly important in aerospace applications where reliability requirements are extremely high.

[0021] This invention supports a design where multiple measurement branches share the same reference branch. A multiplexer enables simultaneous monitoring of multiple liquid levels within the fuel tank. This architecture maintains system simplicity and economy while providing sensors with flexible configuration capabilities. Monitoring points can be customized according to the fuel tank structure of different aircraft models, enabling functional expansion from simple minimum liquid level alarms to continuous liquid level measurement. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of a circuit for detecting the minimum fuel level in an aircraft fuel tank based on a positive temperature coefficient thermistor (PTC). Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings: Example 1, please refer to Figure 1 This invention provides a highly reliable measurement circuit for the minimum fuel level in an aircraft fuel tank, comprising a measurement branch, a reference branch, a constant voltage source, and a comparison circuit. The measurement branch and the reference branch are connected in parallel, and the comparison circuit is electrically connected to the measurement branch and the reference branch to compare the voltages across the measurement branch and the reference branch. The measurement branch is used to measure the self-heating temperature generated according to the heat dissipation conditions of the medium, and the constant voltage source provides a quasi-constant current operating voltage to the measurement branch and the reference branch.

[0024] This invention provides a technical solution for the specific application scenario of detecting the minimum liquid level in aviation fuel tanks. It is simple in principle, reliable in structure, inherently safe, and easy to certify, and is particularly suitable for the aviation field where safety and reliability requirements are extremely stringent.

[0025] Example 2: The present invention provides a sensor for measuring the minimum fuel level in an aircraft fuel tank based on a positive temperature coefficient thermistor (PTC), including a sensing element and a signal processing circuit housed in an explosion-proof housing. The measuring branch consists of a first low temperature coefficient precision resistor R2 and a first positive temperature coefficient thermistor R_PTC2 connected in series. The first positive temperature coefficient thermistor R_PTC2 is set to the critical height of the fuel level to be detected, and the measurement is based on the corresponding self-heating temperature generated according to the heat dissipation conditions of the medium. The reference branch is composed of a second low temperature coefficient precision resistor R1 and a second positive temperature coefficient thermistor R_PTC1 connected in series. The second positive temperature coefficient thermistor R_PTC1 has the same electrical and structural characteristics as the first positive temperature coefficient thermistor R_PTC2. The operating current of the reference branch is one-tenth of the operating current of the measurement branch, so that the self-heating effect of the second positive temperature coefficient thermistor R_PTC1 can be ignored and its temperature is basically equal to the ambient temperature. A constant voltage source provides quasi-constant current operating conditions for the measurement branch and the reference branch, respectively. The comparison circuit, electrically connected to the measurement branch and the reference branch, is used to compare the voltage across the first positive temperature coefficient thermistor R_PTC2 with the voltage across the second positive temperature coefficient thermistor R_PTC1. When the voltage across the first positive temperature coefficient thermistor R_PTC2 is greater than the voltage across the second positive temperature coefficient thermistor R_PTC1, a detection signal indicating that the fuel level is below the minimum threshold is output.

[0026] The comparison circuit adopts a Wheatstone bridge topology. The first positive temperature coefficient thermistor R_PTC2 and the second positive temperature coefficient thermistor R_PTC1 respectively form two opposite bridge arms of the bridge, and the first low temperature coefficient precision resistor R2 and the second low temperature coefficient precision resistor R1 respectively form the other two opposite bridge arms of the bridge.

[0027] The bridge structure has high sensitivity and strong anti-interference ability; a tiny temperature difference can generate a measurable output voltage.

[0028] The resistance of the second low-temperature coefficient precision resistor R2 is ten times that of the first low-temperature coefficient precision resistor R1, ensuring that the reference branch current is one-tenth of the measurement branch current. According to Ohm's law I=U / R, under the same voltage, a larger resistance results in a smaller current, thus achieving current proportional control simply and reliably.

[0029] The comparison circuit includes a signal amplification unit, used to amplify the voltage across the second positive temperature coefficient thermistor R_PTC1 by a factor of 10, and then perform a linear comparison with the voltage across the first positive temperature coefficient thermistor R_PTC2. Instead of directly comparing the two branch voltages because the difference might be very small, the reference branch voltage is amplified by a factor of 10 before being compared with the measured branch voltage. This achieves the same equivalent comparison effect as in claim 3, but with a different circuit topology.

[0030] The Curie temperature points of the first positive temperature coefficient thermistor R_PTC2 and the second positive temperature coefficient thermistor R_PTC1 are higher than the maximum operating ambient temperature of the aviation fuel tank, but lower than the limit temperature of the internal hot surface of the fuel tank as specified by aviation safety regulations.

[0031] Setting a dual constraint on the Curie temperature point: Lower limit: Higher than the maximum operating temperature of the fuel tank to ensure that the system does not enter the abrupt change zone during normal operation. Upper limit: Below the aviation safety regulations' limit temperature for the hot surface of the fuel tank to prevent it from becoming an ignition source. These are key parameters for aviation explosion-proof safety.

[0032] The temperature coefficients of both the first low temperature coefficient precision resistor R2 and the second low temperature coefficient precision resistor R1 are less than 50 ppm / ℃.

[0033] The constant voltage source is connected to a current limiting protection circuit and a transient voltage suppression diode to prevent accidental ignition in potentially explosive environments.

[0034] The distance between the surface of the first positive temperature coefficient thermistor R_PTC2 and the fuel tank wall is greater than the minimum safe distance specified by aviation safety standards. The explosion-proof housing is made of stainless steel and is sealed. The purpose of ensuring that the distance between the thermistor and the fuel tank wall is greater than the minimum safe distance is to prevent the hot surface from igniting fuel vapor on the fuel tank wall.

[0035] The purpose of using a stainless steel explosion-proof enclosure and sealing is to create an explosion-proof structure that prevents internal explosions from propagating outwards.

[0036] The sensor has no moving mechanical parts, and the comparison circuit is a purely analog circuit, containing no microprocessor, FPGA, or programmable logic device. This invention employs a purely analog circuit design, explicitly excluding digital devices. Firstly, the absence of moving mechanical parts eliminates faults such as wear and jamming. Secondly, the purely analog circuit, without MCU / FPGA / PLD, avoids electromagnetic interference from digital circuits, simplifies explosion-proof certification, makes intrinsically safe / flameproof circuits easier to evaluate, and improves reliability. The reliability of complex electronic equipment in the aerospace field is often questionable.

[0037] The reference branch is composed of a second low temperature coefficient precision resistor R1 and a second positive temperature coefficient thermistor R_PTC1 connected in series. The second positive temperature coefficient thermistor R_PTC1 has the same electrical and structural characteristics as the first positive temperature coefficient thermistor R_PTC2. The reference current Iref of the reference branch is strictly limited to one-tenth of Imeas, so that the self-heating effect of the second positive temperature coefficient thermistor can be ignored and its temperature is basically equal to the ambient temperature. A constant voltage source provides quasi-constant current operating conditions for the measurement branch and the reference branch, respectively. The comparison circuit, electrically connected to the measurement branch and the reference branch, is used to compare the voltage Vmeas across the first positive temperature coefficient thermistor with the voltage Vref across the second positive temperature coefficient thermistor. When Vmeas is greater than 10 times Vref, a detection signal indicating that the fuel level is below the minimum threshold is output.

[0038] Example 2: This invention provides a method for measuring the minimum fuel level in an aircraft fuel tank using a highly reliable measurement circuit. The measurement steps are as follows: Two positive temperature coefficient thermistors (PTCs) with identical structure and electrical characteristics are installed in the aircraft fuel tank. The first PTC is installed at the critical height of the lowest fuel level to be detected; the second PTC is installed in a fixed position in the fuel tank unaffected by the fuel level.

[0039] The first PTC is connected in series with a low-temperature-coefficient precision resistor (R2) to form the measurement branch; the second PTC is connected in series with another low-temperature-coefficient precision resistor (R1) to form the reference branch. The resistance of the precision resistor R1 is set to be ten times the resistance of R2, thereby ensuring that under the same constant voltage source power supply, the operating current of the reference branch is one-tenth of the operating current of the measurement branch.

[0040] A constant voltage source is connected to both branches. This constant voltage source is equipped with current limiting protection and transient voltage suppression to ensure safe operation in potentially explosive environments. A stable operating voltage is provided to both branches, enabling them to operate in a quasi-constant current state.

[0041] When the circuit is connected, the first PTC in the measurement branch exhibits a significant self-heating effect under a large operating current. Its actual temperature depends on the heat dissipation capacity of the surrounding medium. When the liquid level is above the critical height, the first PTC is immersed in the fuel. The good thermal conductivity of the fuel causes its heat to be carried away quickly, and its temperature is basically the same as the fuel temperature (close to the ambient temperature), resulting in a low resistance value.

[0042] When the liquid level is below the critical height, the first PTC is exposed to the air. The poor thermal conductivity of the air prevents the heat from being dissipated effectively, causing its temperature to be significantly higher than the ambient temperature and its resistance to increase sharply.

[0043] The second PTC in the reference branch has a working current that is only one-tenth that of the measurement branch, so its self-heating effect is negligible. Its temperature is always basically the same as the ambient temperature inside the oil tank, and its resistance only changes with the ambient temperature.

[0044] The voltage signals (V) across the first PTC are collected respectively. meas The voltage signal (V) across the second PTC and the voltage signal across the second PTC. ref V meas This reflects the overall temperature of the first PTC (ambient temperature + self-heating temperature rise), V ref This only reflects the ambient temperature.

[0045] The voltage signal V collected by the reference branch ref Inputting a gain unit of 10 yields an amplified signal of 10×V. ref This operation is electrically equivalent to measuring the branch voltage V. meas It is compared with a value ten times the reference voltage to offset the difference in background caused by the different operating currents of the two branches.

[0046] Measuring the branch voltage V meas Compared with the amplified reference voltage 10×V ref Input a voltage comparator for linear comparison.

[0047] The comparator outputs the final liquid level status signal based on the magnitude of the input voltage: When V meas <10×V ref When the temperature of the first PTC is low and the heat dissipation is good, it is determined that the fuel level is higher than the minimum threshold, and the comparator outputs a low-level (e.g., logic "0") alarm signal.

[0048] When V meas >10×V ref When the temperature of the first PTC is high and heat dissipation is poor, the comparator is determined to be below the minimum threshold, and the comparator flips to output a high-level (e.g., logic "1") alarm signal.

[0049] Example 3: Dual-branch PTC level detection like Figure 1 As shown, the minimum fuel level sensor of the present invention adopts a dual-branch symmetrical structure, is powered by a constant voltage source, and mainly includes a measurement branch, a reference branch, a signal conditioning circuit, and a comparator.

[0050] The left side is the reference branch, consisting of a first precision resistor R1 connected in series with a positive temperature coefficient PTC thermistor "PTC-Reference"; the right side is the measurement branch, consisting of a second precision resistor R2 connected in series with a positive temperature coefficient PTC thermistor "PTC-Measurement". Both PTC thermistors have identical electrical characteristics: Curie temperature, B value, resistance at 25°C, and structural dimensions, ensuring consistent temperature characteristics.

[0051] Reference current I in the reference branch ref Designed to measure branch current I meas one-tenth of I ref =I meas / 10. To achieve this proportional relationship, the resistance value of precision resistor R1 is more than ten times that of R2. For example, when R2 is 100Ω, I meas When the design is 25mA, R1 is taken as 1000Ω, then I ref Approximately 2.5mA. In this configuration, the PTC in the reference branch has extremely low power consumption (P=I²R), and its self-heating temperature rise is negligible (<0.1℃). Therefore, the temperature of the "PTC-Reference" is strictly equal to the ambient temperature, and its voltage V... ref It only reflects changes in ambient temperature.

[0052] The "PTC-measurement" in the measurement branch uses a significantly higher current I. meas It generates a self-heating effect, and its surface temperature depends on the heat dissipation capacity of the surrounding medium: Due to the high thermal conductivity of aviation kerosene (approximately 0.15 W / (m·K), the surface temperature of the PTC is close to the fuel temperature and ambient temperature, resulting in a low resistance value. The voltage across R2 is V. meas Relatively small; When the liquid level drops, exposing the PTC to air, the low thermal conductivity of air (approximately 0.026 W / (m·K)) worsens heat dissipation, causing the PTC temperature to rise rapidly and its resistance to increase significantly. meas It then rises.

[0053] To balance the current difference between the two branches, an operational amplifier with a gain of 10, G=10, is used to balance the reference voltage V. ref Magnify. The magnified V ref That is, 10×V ref With V meas Connect the two input terminals V of the voltage comparator meas Connect to the negative terminal, and the amplified V ref Connect the positive end.

[0054] Output logic: When V meas >10×V refWhen the PTC is in the air, the temperature is high and the resistance is large. The comparator outputs a high-level logic "1", indicating that the fuel level is below the minimum threshold. When V meas <10×V ref When the PTC is immersed in fuel, the temperature is low and the resistance is small. The comparator outputs a low-level logic "0", indicating that the fuel level is above the minimum threshold.

[0055] When the ambient temperature rises, the initial resistance of both PTCs decreases simultaneously, causing V to... meas and V ref Year-on-year decline; but due to V ref After being magnified 10 times, and compared with V meas Comparison, differential signal V meas -10×V ref The switching threshold is determined solely by the self-heating effect of the PTC's temperature rise difference, and is independent of the absolute value of the ambient temperature. This ensures stable switching thresholds within the aviation operating temperature range of -55℃ to +70℃, avoiding false alarms.

[0056] All electronic components are encapsulated in an explosion-proof metal housing conforming to DO-160 standards. The PTC thermistor surface maintains a distance greater than the minimum distance specified by aviation safety standards between itself and the fuel tank wall and structural components. R1 and R2 are precision metal film resistors with a temperature coefficient of <50ppm / ℃ to ensure long-term stability. The comparator output is either dry contact or open collector, allowing direct connection to the aircraft fuel warning system.

[0057] Through the above embodiments, the present invention achieves a fuel level detection function that requires no software calibration, has automatic temperature compensation, and is intrinsically safe.

[0058] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A highly reliable measurement circuit for the minimum fuel level in an aircraft fuel tank, characterized in that, It includes a measurement branch, a reference branch, a constant voltage source, and a comparator circuit. The measurement branch and the reference branch are connected in parallel. The comparator circuit is electrically connected to the measurement branch and the reference branch and is used to compare the voltages across the measurement branch and the reference branch. The measurement branch is used to measure the self-heating temperature generated according to the heat dissipation conditions of the medium. The constant voltage source provides a quasi-constant current operating voltage for the measurement branch and the reference branch.

2. The high-reliability measurement circuit for minimum fuel level in an aircraft fuel tank according to claim 1, characterized in that, The measuring branch includes a first low temperature coefficient precision resistor R2 and a first positive temperature coefficient thermistor R_PTC2. The first low temperature coefficient precision resistor R2 and the first positive temperature coefficient thermistor R_PTC2 are connected in series. The first positive temperature coefficient thermistor R_PTC2 is set to the critical height of the fuel level to be detected. The measurement is based on the self-heating temperature generated according to the heat dissipation conditions of the medium. The reference branch includes a second low temperature coefficient precision resistor R1 and a second positive temperature coefficient thermistor R_PTC1, which are connected in series. The second positive temperature coefficient thermistor R_PTC1 has the same electrical and structural characteristics as the first positive temperature coefficient thermistor R_PTC2.

3. The high-reliability measurement circuit for minimum fuel level in an aircraft fuel tank according to claim 1, characterized in that, The comparison circuit adopts a Wheatstone bridge topology. The first positive temperature coefficient thermistor R_PTC2 and the second positive temperature coefficient thermistor R_PTC1 respectively form two opposite arms of the bridge, and the first low temperature coefficient precision resistor R2 and the second low temperature coefficient precision resistor R1 respectively form the other two opposite arms of the bridge.

4. The high-reliability measurement circuit for minimum fuel level in an aircraft fuel tank according to claim 1, characterized in that, The resistance of the second low temperature coefficient precision resistor R2 is ten times that of the first low temperature coefficient precision resistor R1.

5. The high-reliability measurement circuit for minimum fuel level in an aircraft fuel tank according to claim 1, characterized in that, The comparison circuit includes a signal amplification unit, which amplifies the voltage across the second positive temperature coefficient thermistor R_PTC1 by 10 times and then performs a linear comparison with the voltage across the first positive temperature coefficient thermistor R_PTC2.

6. The high-reliability measurement circuit for minimum fuel level in an aircraft fuel tank according to claim 1, characterized in that, The Curie temperatures of the first positive temperature coefficient thermistor R_PTC2 and the second positive temperature coefficient thermistor R_PTC1 are higher than the maximum operating ambient temperature of the aviation fuel tank and lower than the limit temperature of the internal hot surface of the fuel tank as specified by aviation safety regulations; the temperature coefficients of the first low temperature coefficient precision resistor R2 and the second low temperature coefficient precision resistor R1 are both less than 50ppm / ℃.

7. The high-reliability measurement circuit for minimum fuel level in an aircraft fuel tank according to claim 1, characterized in that, The constant voltage source is connected to a current-limiting protection circuit and a transient voltage suppression diode to prevent accidental ignition in potentially explosive environments.

8. The high-reliability measurement circuit for minimum fuel level in an aircraft fuel tank according to claim 1, characterized in that, The distance between the surface of the first positive temperature coefficient thermistor R_PTC2 and the tank wall is greater than the minimum safe distance specified by aviation safety standards. The measuring branch, reference branch, constant voltage source and comparator circuit are all housed in the explosion-proof enclosure.

9. A high-reliability measurement circuit for minimum fuel level in an aircraft fuel tank according to claim 8, characterized in that, The explosion-proof housing is made of stainless steel and is sealed.

10. A method for measuring the minimum fuel level in an aircraft fuel tank using a highly reliable measurement circuit, characterized in that, The aircraft fuel tank minimum fuel level measuring sensor according to any one of claims 1 to 9 Includes the following steps: Install the measuring PTC thermistor at the critical height of the lowest fuel level in the fuel tank, and install the reference PTC thermistor in a fixed position inside the fuel tank that is not affected by changes in fuel level; connect the measuring PTC in series with a precision resistor to form the measuring branch; connect the reference PTC in series with another precision resistor with a resistance ten times that of the measuring PTC to form the reference branch. Simultaneously acquire and measure the voltage across the PTC and the voltage across the reference PTC; The acquired reference PTC voltage signal is amplified 10 times; the amplified reference voltage and the measured PTC voltage are input together into a voltage comparator for comparison; if the measured voltage is greater than the amplified reference voltage, the comparator outputs a high-level signal, indicating that the fuel level is below the minimum threshold; otherwise, it outputs a low-level signal, indicating that the fuel level is above the minimum threshold.

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