CN101709988B - Method for sensing automotive oil level and sensor - Google Patents
Method for sensing automotive oil level and sensor Download PDFInfo
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- CN101709988B CN101709988B CN2009101918490A CN200910191849A CN101709988B CN 101709988 B CN101709988 B CN 101709988B CN 2009101918490 A CN2009101918490 A CN 2009101918490A CN 200910191849 A CN200910191849 A CN 200910191849A CN 101709988 B CN101709988 B CN 101709988B
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- 238000012360 testing method Methods 0.000 claims 4
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- 229910001030 Iron–nickel alloy Inorganic materials 0.000 description 1
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Abstract
The invention relates to a method for sensing an oil level of an automobile fuel tank, which adopts two parallel plate capacitors as a measurement capacitor and a reference compensation capacitor respectively; the height of the measurement capacitor is close to or equal to that of the fuel tank, and the measurement capacitor is vertically arranged in the fuel tank; and the reference compensation capacitor is horizontally arranged at the bottom of the fuel tank and immersed in oil liquid. The two capacitors are of an open type; when the oil-level height in the fuel tank changes, the height of oil liquid entering the measurement capacitor also changes and a capacitance value of the measurement capacitor changes correspondingly, but a capacitance value of the reference compensation capacitor does not change with the change of a liquid level. The measurement capacitor is compensated by the reference compensation capacitor, so an accurate change value of the measurement capacitor can be obtained; and an exact value of the oil-level height in the fuel tank can be acquired by the linear relation of the change value of the measurement capacitor and the oil liquid height through circuit processing so as to realize oil-level measurement. Due to the adoption of the reference compensation capacitor, the oil-level measurement which is irrelevant to media under different environments is realized; and the used sensor has corrosion resistance, no mechanical moving parts, long service life, simple structure, low process requirements and low cost.
Description
Technical Field
The invention relates to a liquid level height sensing technology, in particular to a technology for sensing the oil level of an automobile oil tank.
Background
The automobile oil level sensor can be used for detecting the oil quantity in a fuel oil tank, the engine oil quantity and the brake oil tank. When the engine oil of the engine is insufficient, the sensor senses the condition in time and immediately sends an electric signal to the ECU, the ECU starts a warning system, and a warning lamp is lightened or a warning sound is given out, so that the engine is prevented from continuously working under the condition of poor lubrication, and the engine is effectively prevented from being damaged; when the fuel quantity is insufficient, a driver is reminded to refuel, and the phenomenon that the automobile is anchored halfway due to insufficient fuel is avoided; when the braking oil quantity is insufficient, the ECU starts a warning system, lights a warning lamp or gives out a warning sound, and emergency accidents such as the automobile is not braked flexibly are avoided.
The oil level position sensors commonly used at present can be classified, according to the principle, as: capacitive fuel level sensors, ultrasonic fuel level sensors, resistive fuel level sensors, optical fuel level sensors, magnetostrictive fuel level sensors, and the like. The advantages and disadvantages of each of these types of sensors are as follows:
capacitance type oil level sensor
The liquid level sensor has simple structure and high reliability; the probe resists high temperature and high pressure, and the field adaptation surface is wide; the power consumption is extremely low, and the method is suitable for measuring flammable and explosive dangerous media. In the case of current capacitive level sensors, the relative permittivity of the liquid being measured must be known when making a level measurement. In addition, when the measured liquid is not uniform, the relative dielectric constant is changed or the measured liquid is changed, the calibration is needed again, and the application range of the liquid is greatly limited.
Ultrasonic oil level sensor
According to the ultrasonic pulse signal sent by the ultrasonic probe, the ultrasonic pulse signal is transmitted in the liquid, and is reflected after meeting the interface of the air and the liquid, and the ultrasonic wave transmission time can be obtained after receiving the echo signal. And calculating the propagation distance by using the propagation speed and the propagation time to obtain the liquid level height. Such sensors are susceptible to temperature and are relatively expensive.
Resistance type oil level sensor
The method uses a resistor made of a material with high resistivity and a high temperature coefficient in combination with a level sensor float and a voltage applied across the resistor to detect changes in the oil level. The higher the temperature, the higher the resistance. When the resistor is immersed in the oil liquid, the temperature is reduced, the resistance is reduced, and the change of the resistance is detected, so that the change of the fuel can be obtained. The sensor is an electrothermal fuel level sensor made by attaching a Fe-Ni thin film resistor to a metal foil.
The measurement accuracy is greatly influenced by the liquid pollution condition, errors are easy to generate, and the response speed is low. In addition, the carbon film resistor used at present is easy to corrode in gasoline to cause failure.
Optical oil level sensor
The method utilizes the principle of reflection or transmission of light to measure the oil level according to the difference of the reflected or transmitted light intensity or luminous flux. Because of high price, the product is not widely used at present.
Magnetostrictive oil level sensor
The magnetostrictive liquid level sensor senses the change of a liquid level floater by using the magnetostrictive effect of materials, thereby achieving the purpose of measuring the oil level in a non-contact way. The oil level sensor has high measurement precision, strong environmental adaptability, good safety and convenient installation, can be applied to the liquid level measurement field of petroleum, chemical industry and the like, and is mostly applied to the measurement of the oil level of the airplane at present due to high price.
Disclosure of Invention
Aiming at the respective advantages and disadvantages of oil level sensors of oil tanks with different principles common in the market at present, the invention provides an oil level height position sensing method and sensor based on the parallel flat-plate capacitance principle, which have no mechanical moving part inside.
The method adopts a parallel plate capacitor, and the principle is as follows, referring to fig. 1:
the capacitance value C of two parallel flat capacitors with the distance D and the effective area S is as follows:
wherein: dielectric constant of epsilon
k-insulating constant
As can be seen from equation (1), if the distance D between the two plates is constant, the capacitance value is proportional to the effective area S when the dielectric constant ∈ is fixed (i.e., the electrolyte material between the two plates is not changed).
Therefore, the height of the oil level is corresponded to the change of the effective area S of the parallel plate capacitor, and the oil level height can be obtained by measuring the relation between the capacitance value of the capacitor and the oil level height at the same time of the change of S.
Based on this, the oil level position sensing method proposed by the present invention is as follows:
a parallel plate capacitor is used as a measuring capacitor, and the measuring capacitor is a plate capacitor with a fixed structure; the length of the measuring capacitor is close to or equal to the height of the oil tank, and the measuring capacitor is vertically arranged in the oil tank. The capacitor is of a non-sealed structure and is communicated with an oil liquid space in an oil tank, oil can enter between polar plates, the oil level is changed, the oil liquid height between the polar plates of the capacitor is changed simultaneously, and the capacitance value of the capacitor is in a linear relation with the oil liquid height;
the method comprises the steps that another parallel flat capacitor with unchanged capacitance value in normal work is used as a reference compensation capacitor, the capacitance value of the reference compensation capacitor is equal to or in a direct proportion relation with the maximum capacitance value which can be reached by the measuring capacitor, the reference compensation capacitor is also in a non-closed structure, is horizontally arranged at the bottom of an oil tank and is soaked in oil, and the change of the capacitance value of the environment (such as temperature or oil with different components) is obtained by measuring the change of the capacitance value of the reference compensation capacitor;
the capacitance value measured by the measuring capacitor and the change amount of the environment to the capacitance value obtained by the reference compensation capacitor are processed by a circuit at the rear end, the influence of other environmental factors such as temperature and different oil components on the measured capacitance value is compensated, an accurate measured capacitance change value is obtained, the oil height value can be obtained by measuring the linear relation between the capacitance change value and the oil height and processing the linear relation by the circuit, and the oil level measurement of an automobile is realized.
As can be seen from the above-mentioned description of the present invention, the parallel plate capacitor with an open structure is adopted, and when the oil level changes, the oil level height in the parallel plate capacitor changes accordingly, as shown in FIG. 2, the distance between the two plates D, the plate width W, the plate height L, and the oil height L1Dielectric constant epsilon of oil2Dielectric constant of air ∈1Then, this parallel plate capacitor can be regarded as the parallel connection of two parallel plate capacitors, and one is formed by fluid, and another is formed by the air, then total capacitance value is:
as can be seen from equation (2), the capacitance value C of the capacitor:
C＝A[B+(ε2-ε1)L1] (3)
wherein, <math>
<mrow>
<mi>A</mi>
<mo>=</mo>
<mfrac>
<mi>W</mi>
<mrow>
<mn>4</mn>
<mi>πkD</mi>
</mrow>
</mfrac>
<mo>-</mo>
<mo>-</mo>
<mo>-</mo>
<mrow>
<mo>(</mo>
<mn>4</mn>
<mo>)</mo>
</mrow>
</mrow>
</math>
B＝ε1L (5)
when the structure is fixed (namely the space D, the width W and the height L of the polar plates are constant), the dielectric constant epsilon of air1Is constant, dielectric constant epsilon of oil2When the capacitance is constant, A, B in the equations 4 and 5 is constant, namely the capacitance and the oil height L1And has a linear relationship. As shown by the solid line in fig. 3. That is, the capacitance value of the fixed-structure plate capacitor manufactured by using the capacitor structure shown in fig. 2 is linearly related to the height of the oil, so that the height of the oil can be measured by measuring the capacitance value of the capacitor.
Because the capacitor is susceptible to the influence of temperature, the relative dielectric constants of the oil with different components are different, and the power-saving constant of the same oil is changed under different temperature conditions. The linear relationship according to equations (2) to (5) changes as shown by the broken line in fig. 3. It is clear that an accurate level signal cannot be obtained by means of this measuring capacitance alone. In view of this, in order to minimize the influence of other factors such as temperature and environment on the measured capacitor, the inventor considers that the capacitor made of the same material and process is used as the reference compensation capacitor, when the measurement device works, the reference compensation capacitor is completely soaked in the oil, the capacitance value of the reference compensation capacitor is equal to or in a certain proportional relation with the maximum capacitance value of the measured capacitor (namely when the oil fills the whole pole plate), the reference compensation capacitor and the measured capacitor are in the same environment, and the change of the capacitance value of the environment to the capacitance value is obtained by measuring the change of the capacitance value of the reference compensation capacitor, that is, the change of the capacitance value of the environment to the capacitance value is obtained, thatOilWhile the dielectric constant of air is not changed, i.e. CAir conditionerKnowing, the slope of the line under this condition can be calculated; the capacitance value measured by the measuring capacitor and the change amount of the capacitance value obtained by the reference compensation capacitor are processed by the back-end circuit, that is, the change amount of the capacitance value is obtainedAnd obtaining the accurate value of the oil liquid level through the slope of the straight line, namely the measured capacitance value at the moment. As shown in equation (6).
In the formula (6), n is a proportionality coefficient between the reference capacitance value and the maximum value of the measured capacitance during operation. When n is 1, it represents: the reference capacitance value is equal to the maximum value of the measurement capacitance during operation.
On the basis of the method, the applicant proposes a specific implementation structure of the sensor for implementing the method on the automobile oil level sensing, which is composed of a measuring capacitor, a reference compensation capacitor and a processing circuit.
The measuring capacitor is a parallel flat capacitor with a non-closed structure, the polar plates of the measuring capacitor are not sealed and can be communicated with an oil liquid space in an oil tank, the measuring capacitor is vertically arranged in the oil tank, oil liquid can enter between the polar plates, and when the oil level changes, the capacitance value of the measuring capacitor correspondingly changes.
The reference compensation capacitor is also a parallel flat capacitor with a non-closed structure, the capacitance value of the reference compensation capacitor is equal to or in direct proportion to the maximum capacitance value of the measurement capacitor when the reference compensation capacitor works normally, and the reference compensation capacitor is horizontally arranged at the bottom of the oil tank and is soaked in oil;
the measuring capacitor and the reference compensation capacitor are respectively connected with the processing circuit through signal output lines, the processing circuit obtains accurate values of the measuring capacitor and the reference compensation capacitor, the rear end of the processing circuit is connected with the calculating circuit, and the oil height value is obtained through calculation by utilizing the linear relation between the variation value of the measuring capacitor and the oil height.
It can be seen that the advantages of the present invention are quite evident:
firstly, the oil level sensing is realized by adopting a plate capacitance principle, and high-resolution and high-precision measurement in a large range is realized;
secondly, the sensor is measured without a mechanical movable part inside, so that the characteristic that a sensitive part of the sensor is measured without mechanical contact is realized, and the service life of the sensor is prolonged;
thirdly, the reference compensation capacitor is adopted to compensate the influence of temperature and other environmental factors on the capacitance value, thereby eliminating the influence of factors such as environment and the like, improving the measurement accuracy and realizing the oil level measurement irrelevant to a medium;
fourthly, the structure is simple, the process requirement is not high, and the price is low.
Drawings
FIG. 1 is a schematic diagram of a parallel plate tunable capacitor;
FIG. 2 is an implementation of a fuel level sensor;
FIG. 3 is a linear relationship diagram of the fuel level sensor;
FIG. 4 is a schematic view of the mounting structure of the present sensor;
fig. 5 is a circuit diagram of capacitance measurement.
Detailed Description
The invention is further described below with reference to the accompanying drawings:
referring to fig. 4, the vehicle fuel level sensor is composed of a measuring capacitor 1, a reference compensation capacitor 2 and a processing circuit. The measuring capacitor 1 is a parallel flat capacitor, the polar plates of the measuring capacitor are not sealed, the measuring capacitor is communicated with an oil liquid space in the oil tank 3, the length of the measuring capacitor is close to or equal to the height of the oil tank 3, the measuring capacitor is vertically arranged on the inner side surface of the oil tank 3, and oil liquid 4 can enter between the two polar plates. The reference compensation capacitor 2 is a parallel flat capacitor with unchanged capacitance value in normal operation, the capacitance value of the reference compensation capacitor is equal to or in a certain proportion relation with the maximum capacitance value of the measurement capacitor 1, the reference compensation capacitor 2 and the measurement capacitor 1 are installed in the same oil tank 3, the reference compensation capacitor 2 is horizontally installed at the bottommost part of the oil tank 3 and is soaked in oil liquid 4, and the polar plates of the reference compensation capacitor 2 are also unsealed. The measuring capacitor 1 and the reference compensation capacitor 2 are respectively connected with a processing circuit through signal output lines to process the measuring signals, and the processing circuit outputs the measuring signals to a computing circuit to obtain the specific height of the oil.
The processing circuit used in the present sensor can be implemented by the prior art, and a processing circuit structure is listed below, see fig. 5. CAV424 in FIG. 5 is a dedicated chip for measuring capacitance, Cx1To be measured capacitance, Cx2Is a fixed capacitance. The working principle of the CAV424 is that one is composed of a capacitor COSCThe reference oscillator determining the frequency controls two symmetrically constructed integrators with constant phase and the same period. The amplitudes of the two integrators are passed through a capacitor Cx1And Cx2To be determined. Comparing the voltage amplitude difference of the two integrators to obtain the capacitor Cx1And Cx2The differential signal is converted into a direct current voltage signal through a two-stage low-pass filter and is output through a differential signal output stage with adjustable output, namely V between the fifth stage and the sixth stageDIFFI.e. to obtain an accurate capacitance value.
Two ways VDIFFThe capacitance values of the measurement capacitor and the reference compensation capacitor are respectively connected to the input end of an A/D acquisition card, and as shown in figure 5, the output end of the A/D is connected with a computer. The computer controls the A/D card to collect the liquid level, and the actual liquid level height can be calculated according to the collected measured capacitance value, the reference compensation capacitance value and the relation between the reference compensation capacitance value and the liquid level. The A/D card and the computer can be composed of a multi-channel A/D conversion chip and a single chip microcomputer which can achieve similar functions.
Because the invention relates to two capacitors, one is a measuring capacitor and the other is a reference compensation capacitor, the two capacitors can be respectively measured by using 2 CAVs 424, and after an accurate capacitance value is obtained, an accurate liquid level value can be obtained according to a formula (6).
Wherein: cMeasuringFor measuring capacitance, CGinseng radix (Panax ginseng C.A. Meyer)For the purpose of reference to the capacitance value of the capacitor,
l, W, D is divided into measuring capacitor plate length, width and plate spacing, L1Is the oil height, k is the dielectric constant of the dielectric of the measuring capacitor, epsilon1And n is a proportionality coefficient between the reference capacitance value and the maximum value of the measured capacitance in operation.
Claims (1)
1. An automobile oil level sensor, characterized in that: the device consists of a measuring capacitor, a reference compensation capacitor and a processing circuit;
the measuring capacitor is a parallel flat plate capacitor, the length of the measuring capacitor is close to or equal to the height of the oil tank, the measuring capacitor is vertically arranged in the oil tank, a non-sealing structure is arranged between polar plates of the measuring capacitor and is communicated with an oil liquid space in the oil tank, when the oil level of the oil tank changes, the height of the oil level between the polar plates entering the measuring capacitor changes, and the capacitance value of the measuring capacitor changes correspondingly;
the reference compensation capacitor is a parallel flat plate capacitor and is not in a sealing structure, the capacitance value of the reference compensation capacitor is equal to or in a direct proportion relation with the maximum capacitance value of the measurement capacitor when the reference compensation capacitor works normally, and the reference compensation capacitor is horizontally arranged at the bottom of the oil tank and is soaked in oil liquid;
the measuring capacitor and the reference compensation capacitor are respectively connected with the processing circuit through signal output lines, the processing circuit obtains accurate values of the measuring capacitor and the reference compensation capacitor, the rear end of the processing circuit is connected with the calculating circuit, and the oil height value is obtained through calculation by utilizing the linear relation between the variation value of the measuring capacitor and the oil height;
the processing circuit adopts two special capacitor testing chips CAV424 to test the capacitance value; the initial capacitance values of the two parallel plate capacitors are respectively replaced by a fixed capacitor with similar capacitance value, and the fixed capacitor C is connected to the CAV424x1End, at C of two CAV424x2The ends of the two special capacitor test chips CAV424 are respectively connected with the measurement capacitor and the reference compensation capacitor, and the output of the special capacitor test chips CAV424 represents the accurate values of the measurement capacitor and the reference compensation capacitor;
the rear end of the processing circuit is connected with a calculating circuit for calculating the height of the oil liquid; the computing circuit consists of an A/D acquisition card and a computer or an A/D conversion chip and a singlechip; the outputs of two CAV424 in the processing circuit are respectively connected to an A/D acquisition card or an A/D conversion chip, and the computer or the singlechip calculates the actual oil height according to the measured capacitance value and the reference compensation capacitance value acquired by the A/D acquisition card or the A/D conversion chip and the relation between the reference compensation capacitance value and the liquid level.
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| CN2009101918490A CN101709988B (en) | 2009-12-10 | 2009-12-10 | Method for sensing automotive oil level and sensor |
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| CN201697691U (en) * | 2009-12-10 | 2011-01-05 | 昝昕武 | Automobile oil level sensor |
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