CN105486378B - A method of carrying out level gauging using plate capacitor formula sensor - Google Patents

Abstract

The invention provides a method for measuring liquid level by using a plate-type capacitive sensor. The method includes: installing the plate-type capacitive sensor in the inner cavity of an LNG vehicle bottle in advance; The capacitance values of the right-angled trapezoidal plate and the second right-angled trapezoidal plate are measured simultaneously in real time; according to the capacitance values of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate The height of the plate surface of the plate is used to obtain the liquid level height of the LNG vehicle bottle. The above technology of the present invention can eliminate the measurement error caused by the edge effect of plate capacitance.

Description

A method of liquid level measurement using a plate-type capacitive sensor

technical field

The invention relates to liquefied natural gas technology, in particular to a method for measuring liquid level by using a plate-type capacitive sensor.

Background technique

Liquefied natural gas (LNG for short) vehicles are developing rapidly. In order to ensure the safe and reliable driving of vehicles, drivers need to grasp the usage status of LNG in real time to avoid some accidents. In order to display the liquid level in the LNG vehicle bottle in real time, it is usually equipped with a liquid level detector. The currently used liquid level detector includes: a sensor device installed in the LNG vehicle bottle, the sensor device is externally connected to a signal transmitter, and the signal transmitter is connected to the liquid level display, wherein the sensor used in the LNG vehicle bottle is used as the sensor in the detector. The main components are installed in the car bottle. Now there is a new type of liquid level sensor, which adopts a rectangular plate type. One of the capacitor plates is cut diagonally to form two triangular plates with equal effective areas and symmetrical arrangements. The capacitance value is measured synchronously, and there is an insulating layer on both capacitor plates.

With this new type of liquid level sensor, when the liquid height in the LNG vehicle bottle changes, the two triangular sensors simultaneously convert the LNG liquid level signal into a capacitance signal and transmit it to the signal transmitter through the two tested probes. The results show the opposite trend. In the signal transmitter, through the differential processing of the capacitance signal, the height of the liquid level is only related to the ratio of the micro-changes of the two capacitances, and has nothing to do with the dielectric constant of LNG, thereby realizing liquid level measurement independent of LNG.

However, since this sensor is improved on the basis of parallel-plate capacitors, the biggest defect of parallel-plate capacitors is the influence of edge effects. The existence of the edge effect of the parallel plate capacitor makes the actual capacitance larger than the ideal capacitance when the edge effect is ignored, which is caused by the difference in the added value of the capacitance caused by the edge effect of the parallel plate capacitor. The additional capacitance generated due to the edge effect makes the actual capacitance value greater than the ideal capacitance value. If you directly use the ideal parallel plate capacitance formula when the edge effect is ignored, there will be errors in the result, so the influence of the edge effect cannot be ignored. The edge effect of the diagonal parallel plate will have a greater impact on the liquid level measurement, especially when the liquid level is at the highest or lowest position, the edge effect of the parallel plate will have a greater impact on the measurement accuracy.

Contents of the invention

A brief overview of the invention is given below in order to provide a basic understanding of some aspects of the invention. It should be understood that this summary is not an exhaustive overview of the invention. It is not intended to identify key or critical parts of the invention nor to delineate the scope of the invention. Its purpose is merely to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.

In view of this, the present invention provides a liquid level measurement method using a plate capacitance sensor to eliminate the measurement error caused by the edge effect of the plate capacitance.

According to one aspect of the present invention, a method for measuring liquid level using a plate-type capacitive sensor is provided, wherein the plate-type capacitive sensor includes: a first right-angle trapezoidal plate, a second right-angled trapezoidal plate, and a rectangular Pole plate, the shape of the first right-angle trapezoidal plate and the second right-angled trapezoidal plate are the same and arranged symmetrically; the method for using a plate-type capacitive sensor for liquid level measurement includes: pre-installing the plate-type capacitive sensor It is installed in the inner cavity of the LNG vehicle-mounted bottle as follows: the plates of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate are located in the same plane, and the first right-angled trapezoidal plate and the second The respective hypotenuses of the right-angled trapezoidal pole plates are opposite and arranged in parallel, and the upper and lower bottom edges of the first right-angled trapezoidal pole plate and the second right-angled trapezoidal pole plate are respectively parallel to the liquid level in the inner cavity of the LNG vehicle-mounted bottle The upper base of the first right-angled trapezoidal pole plate is located on the same line as the lower base of the second right-angled trapezoidal pole plate, and the lower base of the first right-angled trapezoidal pole plate is in line with the second right-angled trapezoidal pole plate. The upper base of the pole plate is located on the same straight line; the plate surface of the rectangular pole plate is arranged parallel to the plate surfaces of the first right-angled trapezoidal pole plate and the second right-angled trapezoidal pole plate, and the surface of the rectangular pole plate The plate surface is opposite to the plate surfaces of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate; two test probes are used to test the first right-angled trapezoidal plate and the second right-angled trapezoidal plate respectively Real-time synchronous measurement of the capacitance value; according to the capacitance value of the first right-angle trapezoidal plate and the second right-angled trapezoidal plate and the plate plate of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate The height of the surface is used to obtain the liquid level height of the LNG vehicle bottle.

Further, the distance between the plate surface of the first right-angled trapezoidal plate and the plate surface of the rectangular plate is d; the upper bottom of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate The length of the sides is d/2, the heights of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate are H, and the bottoms of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate are Both sides are D+d/2; the length of the long side of the rectangular pole plate is D+d, the length of the short side of the rectangular pole plate is H, and the short side of the rectangular pole plate and the LNG The liquid level in the inner cavity of the vehicle bottle is vertical; wherein, D, H and d are all positive numbers.

Further, the liquid level of the LNG vehicle bottle is obtained according to the following formula:

in, D is the difference between the lower bottom and the upper bottom of the first right-angled trapezoidal plate, an insulating layer is arranged on the outer walls of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate, and _d0 is the first The distance between the surface of the insulating layer on the outer wall of the right-angled trapezoidal plate and the face of the rectangular plate, d ₂ is the thickness of the insulating layer, ε ₀ is the dielectric constant of air, and ε ₂ is the dielectric constant of the insulating layer; The length of the long side of the pole plate is D+d, the length of the short side of the rectangular pole plate is H, and the short side of the rectangular pole plate is perpendicular to the liquid level in the inner cavity of the LNG vehicle-mounted bottle.

Further, the range of the distance between the respective hypotenuses of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate is (0mm, 5mm].

Further, H is equal to the diameter of the circular section of the LNG vehicle-mounted bottle liner.

Further, D is equal to 80mm; the value of H is one of the following: 500mm; 600mm; 650mm.

Thus, applying the method for measuring liquid level using a plate-type capacitive sensor of the present invention, the two trapezoidal pole plates (that is, the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220) can be tested respectively by two test probes. The capacitance value is measured synchronously, which can eliminate the measurement error caused by the edge effect, thereby improving the measurement accuracy of the LNG vehicle-mounted bottle level detector.

In addition, by setting an insulating layer such as polytetrafluoroethylene on the outer wall of the trapezoidal plate, it can effectively avoid the wall-hanging effect caused by impurities in the LNG, thereby improving the measurement accuracy of the plate-type capacitive sensor and reducing the The probability of occurrence of sensor failure is reduced.

These and other advantages of the present invention will be more apparent through the following detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings.

Description of drawings

The present invention can be better understood by referring to the following description given in conjunction with the accompanying drawings, wherein the same or similar reference numerals are used throughout to designate the same or similar parts. The accompanying drawings, together with the following detailed description, are incorporated in and form a part of this specification, and serve to further illustrate preferred embodiments of the invention and explain the principles and advantages of the invention. In the attached picture:

Fig. 1 is a schematic process flow chart showing the method for liquid level measurement using a plate-type capacitive sensor according to the present invention.

Fig. 2A is a structural schematic diagram showing a plate-type capacitive sensor and a liquid level measurement system including the plate-type capacitive sensor;

Fig. 2B is the view along A-A ' direction of plate capacitive sensor shown in Fig. 1;

Fig. 2C is a schematic diagram showing a structure of the plate capacitive sensor shown in Fig. 1;

Fig. 3 is the schematic diagram of the electrostatic field of the plate capacitive sensor;

Fig. 4 is a schematic diagram of an extended edge of a parallel plate.

It will be appreciated by those skilled in the art that elements in the figures are illustrated for simplicity and clarity only and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of the embodiments of the present invention.

Detailed ways

Exemplary embodiments of the present invention will be described below with reference to the accompanying drawings. In the interest of clarity and conciseness, not all features of an actual implementation are described in this specification. It should be understood, however, that in developing any such practical embodiment, many implementation-specific decisions must be made in order to achieve the developer's specific goals, such as meeting those constraints related to the system and business, and those Restrictions may vary from implementation to implementation. Moreover, it should also be understood that development work, while potentially complex and time-consuming, would at least be a routine undertaking for those skilled in the art having the benefit of this disclosure.

Here, it should also be noted that, in order to avoid obscuring the present invention due to unnecessary details, only the device structure and/or processing steps closely related to the solution according to the present invention are shown in the drawings, and the Other details not relevant to the present invention are described.

An embodiment of the present invention provides a method for measuring liquid level using a plate-type capacitive sensor, wherein the plate-type capacitive sensor includes: a first right-angled trapezoidal plate, a second right-angled trapezoidal plate, and a rectangular plate , the first right-angled trapezoidal plate and the second right-angled trapezoidal plate have the same shape and are arranged symmetrically; the method for using a plate-type capacitive sensor for liquid level measurement includes: pre-setting the plate-type capacitive sensor as follows Installed in the inner cavity of the LNG vehicle-mounted bottle: the plates of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate are located in the same plane, and the first right-angled trapezoidal plate and the second right-angled trapezoidal plate are The respective hypotenuses of the pole plates are opposite and arranged in parallel, and the upper and lower bottom edges of the first right-angled trapezoidal pole plate and the second right-angled trapezoidal pole plate are respectively parallel to the liquid level in the inner cavity of the LNG vehicle-mounted bottle; The upper base of the first right-angled trapezoidal plate and the lower base of the second right-angled trapezoidal plate are on the same straight line, and the lower base of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate The upper base of the upper edge is located on the same straight line; the plate surface of the rectangular pole plate is arranged parallel to the plate surfaces of the first right-angled trapezoidal pole plate and the second right-angled trapezoidal pole plate, and the plate surface of the rectangular pole plate Opposite to the plate surfaces of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate; using two test probes to measure the capacitance values of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate respectively Carry out real-time synchronous measurement; According to the capacitance value of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate and the difference between the plate surface of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate High, to obtain the liquid level height of the LNG vehicle bottle.

In the following, an exemplary processing flow of a method for measuring liquid level by using a plate-type capacitive sensor (hereinafter referred to as "liquid level measuring method") of the present invention will be described with reference to FIG. 1 .

The plate-type capacitive sensor utilized in the above liquid level measurement method may have the structure shown in FIGS. 2A-2C .

As shown in Figures 2A-2C, the above-mentioned plate capacitive sensor 2 may include a first right-angle trapezoidal plate 210, a second right-angled trapezoidal plate 220 and a rectangular plate 230, the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate The effective areas of the plates 220 are equal and arranged symmetrically. The first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 have the same shape.

As shown in FIG. 1 , the liquid level measurement method firstly executes a preprocessing step as shown in step S110 .

In step S110, the plate-type capacitive sensor 2 is preinstalled in the inner chamber 1 of the LNG vehicle bottle shown in FIG. , the respective hypotenuses of the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220 are opposite and arranged in parallel, and make the upper and lower bases of the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220 respectively and The liquid level in the LNG vehicle-mounted bottle inner chamber 1 is parallel; Make the upper base of the first right-angled trapezoidal pole plate 210 and the lower base of the second right-angled trapezoidal pole plate 220 be located on the same straight line, the bottom of the first right-angled trapezoidal pole plate 210 The upper base of the bottom edge and the second right-angled trapezoidal pole plate 220 is located on the same straight line; The plate surface of the rectangular pole plate 230 is arranged in parallel with the plate surfaces of the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220, and The plate surface of the rectangular plate 230 is opposite to the plate surfaces of the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 .

It should be noted that the liquid level mentioned below all refers to the liquid level in the inner chamber 1 of the LNG vehicle bottle.

In step S110, the plate surfaces of the two pole plates of the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 are arranged in the same plane _S0 , the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 There is no covering between the two plate surfaces of the right-angled trapezoidal plate 220 .

According to an implementation manner, in step S110 , the plate surfaces of the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 are arranged vertically to the liquid surface. In this way, in the subsequent measurement process, the height of the plate used when calculating the liquid level height is the height of the plate surface, as shown in Figure 2C H, to avoid inaccurate height calculation due to the inclined placement of the plate surface The resulting inaccurate final measurement improves the measurement accuracy and calculation efficiency.

According to one implementation, in step S110, the distance between the respective hypotenuses of the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 can be set within the range of (0mm, 5mm], for example, the distance Can be set to 1mm.

Next, in step S120, the capacitance values of the first right-angle trapezoidal plate 210 and the second right-angled trapezoidal plate 220 are measured synchronously in real time by using two test probes (not shown in the figure).

Then, in step S130, according to the capacitance values of the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 calculated in step S120 and the poles of the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 The height of the plate surface is used to obtain the liquid level height of the LNG vehicle bottle. Processing flow technical end. Here, "the liquid level of the LNG vehicle-mounted bottle" is also the liquid level in the inner chamber 1 of the LNG vehicle-mounted bottle.

In this way, the capacitance values of the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 are respectively measured synchronously by two test probes. When the liquid level height in the inner chamber 1 of the LNG vehicle-mounted bottle changes, the two capacitance signals measured simultaneously by the first right-angle trapezoidal plate 210 and the second right-angled trapezoidal plate 220 are passed to as shown in Figure 2A through the above-mentioned two test probes. In the signal transmitter 3 shown, the change trends of the measurement results of the two capacitance signals are opposite. The change of the liquid level makes the capacitance of the two triangular plates change accordingly, that is to say, the two plates respectively convert the liquid level signal of LNG into a capacitance signal. In the signal transmitter 3, the processing of step S130 is executed to calculate the liquid level of the LNG vehicle-mounted bottle.

It should be noted that "the capacitance value of the first right-angled trapezoidal plate 210" refers to the capacitance value of the capacitor formed by the first right-angled trapezoidal plate 210 and the rectangular plate 230, and "the capacitance of the second right-angled trapezoidal plate 220 "value" refers to the capacitance value of the capacitor formed by the second right-angled trapezoidal plate 220 and the rectangular plate 230 .

According to one implementation, the distance between the plate surface of the first rectangular trapezoidal plate 210 and the plate surface of the rectangular plate 220 is d; the upper base of the first rectangular trapezoidal plate 210 and the second rectangular trapezoidal plate 220 The lengths are all d/2, the heights of the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220 are both H, and the lower bases of the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220 are D+d/2; the length of the long side of the rectangular plate is D+d, the length of the short side of the rectangular plate is H, and the short side of the rectangular plate is perpendicular to the liquid level in the inner cavity of the LNG vehicle bottle; where , D, H and d are all positive numbers.

In practical applications, for example, a capacitor composed of two parallel rectangular plates (the length of the rectangle is D' and the width is H) can be used to cut one of the two parallel rectangular plates into two Exactly the same, symmetrical right-angled trapezoid, the other rectangular plate remains unchanged. Assuming that the distance d between the plate surface of the first right-angled trapezoidal plate 210 and the plate surface of the rectangular plate 230 is known in use, the upper base of each right-angled trapezoid in the cut two right-angled trapezoids The length is d/2, the bottom is D'-d/2, where D'=D+d, then the difference between the length of the bottom and the bottom is (D'-d/2)-d/2=(D+ d-d/2)-d/2=D.

The cut plate is divided into two symmetrical right-angled trapezoidal plates, the hypotenuses of the two right-angled trapezoidal plates are opposite, and there is a small distance (for example, 1mm) between the two hypotenuses, so that the two right-angled The trapezoidal plate is used as the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 , and the other rectangular plate is used as the rectangular plate 230 in the plate-type capacitive sensor 2 .

Wherein, the actual lengths of H and D are determined according to the actual size of the inner cavity 1 of the LNG vehicle-mounted bottle. For example, H can be equal to the cavity height of the inner chamber 1 of the LNG vehicle-mounted bottle (that is, the diameter of the circular section of the LNG vehicle-mounted bottle liner), and D is smaller than H, which can save material costs while ensuring accurate measurement.

According to an implementation manner, in a container with a volume of 450 L of an LNG vehicle-mounted bottle, H is 650 mm in diameter of the inner liner of the LNG vehicle-mounted bottle; D is 80 mm. H is the diameter of the inner liner of the vehicle bottle, which can accurately measure the height of the liquid level. For example, for a 240-liter gas cylinder, H=500mm; for a 275-liter gas cylinder, H=500mm; for a 340-liter gas cylinder, H=600mm.

Wherein, the distance between the first right-angled trapezoidal plate 210 and the rectangular plate 230 and the distance between the second right-angled trapezoidal plate 220 and the rectangular plate 230 are equal, for example, both are 5mm or 10mm.

If D is set too large, the plate will take up the volume of LNG; if D is set too small, the plate will not be stable and difficult to process. Therefore, when D is set to 80mm, it is beneficial to the stability of the pole plate and the processing of the pole plate, and will not occupy too much volume of liquefied natural gas.

In addition, according to an implementation manner, an insulating layer (such as a polytetrafluoroethylene insulating layer) can be provided on the outer walls of the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 in advance, so as to improve the plate shape of the present invention. Measurement accuracy and success rate of capacitive sensor 2.

In practical applications, when the LNG vehicle bottle is filled with liquid, there will always be impurities in the LNG, and the impurities are easily attached to the plate, which will make the measurement results inaccurate. Thus, by arranging an insulating layer on the outer walls of the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220, the influence caused by the hanging wall can be avoided, and the accuracy and success of the measurement of the plate-type capacitive sensor can be improved. Rate.

For example, polytetrafluoroethylene insulation layer can be used as the above insulation layer. Compared with other materials, this insulation layer has the following advantages: high temperature resistance - the working temperature can reach 250 ° C; low temperature resistance - good mechanical toughness, even The temperature drops to -196°C, and only maintains 5% elongation; corrosion resistance - insoluble in most chemical liquids, such as strong acids and alkalis; weather resistance - extremely long aging life High lubrication - extremely high friction coefficient Low; Non-adhesive - does not stick to anything; Electrical Insulation - Excellent insulation.

According to an implementation manner, the liquid level in step S130 can be obtained according to the following formula:

Among them, A ₁ can be obtained by formula 2:

Here, h _x represents the liquid level height of the LNG vehicle bottle, and D is the difference between the lower bottom and the upper bottom of the first rectangular trapezoidal pole plate 210, the length of the upper bottom is d/2, and the length of the lower bottom is D+d/ 2. D, H and d are all positive numbers. The length of the upper base of the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220 is d/2, and the height of the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220 is H, the first The lower bases of the right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 are both D+d/2. The outer walls of the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220 are all provided with an insulating layer, and _{d is} the distance between the surface of the insulating layer on the outer wall of the first right-angled trapezoidal pole plate to the surface of the rectangular pole plate, d ₂ is the thickness of the insulating layer, _ε0 is the dielectric constant of air, and _ε2 is the dielectric constant of the insulating layer, and the distance between the plate surface of the first right-angled trapezoidal plate 210 and the plate surface of the rectangular plate 230 is d . The length of the long side of the rectangular plate is D+d, the length of the short side of the rectangular plate is H, and the short side of the rectangular plate is perpendicular to the liquid level in the inner cavity of the LNG vehicle bottle.

In addition, FIG. 2A also shows a liquid level measurement system including a plate-type capacitive sensor 2 . As shown in Figure 2A, the liquid level measurement system includes: an inner cavity 1 of the LNG vehicle-mounted bottle, a plate-type capacitive sensor 2 arranged in the inner cavity 1 of the LNG vehicle-mounted bottle, and a signal transmitter 3 connected to the plate-type capacitive sensor 2 And the liquid level indicator 4 connected with the signal transmitter 3. Wherein, the plate-type capacitive sensor 2 includes a first right-angled trapezoidal plate 210 and a second right-angled trapezoidal plate 220 , the structures of which are the same as those described above.

The first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220 in the plate-type capacitive sensor are connected to the signal transmitter 3 through two test probes, and the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 210 are measured by the two test probes. The respective capacitance signals of the two right-angled trapezoidal plates 220, the signal transmitter 3 converts the capacitance signals transmitted by the two test probes into a DC voltage signal, and calculates the liquid level height through the signal processing module arranged in the signal transmitter 3, And the calculated liquid level height is displayed in real time through the liquid level display 4 .

Wherein, the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220 measure the liquid level height in the inner cavity 1 of the LNG vehicle-mounted bottle simultaneously through two test probes, and ensure that when the liquid level changes, the two measured values into a relative trend change.

The signal transmitter 3 can perform the processing of calculating the ratio ΔC of the slight change of the capacitance value of the first right-angled trapezoidal pole plate 210 and the second right-angled trapezoidal pole plate 220 in step S120, and can perform the processing of step S130. Through the above processing, The height of the liquid level can only be related to the conversion of the two values, and finally the liquid level of the LNG vehicle bottle can be displayed in real time through the liquid level display 4 .

Next, an application example of the present invention is described.

Assume that the plate height of the two trapezoidal plates (ie, the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 ) in the plate-type capacitive sensor 2 is H. Set up the plate capacitive sensor 2 according to step S110.

In step S120, two test probes are used to carry out real-time synchronous measurement of the capacitance values of the first right-angled trapezoidal plate 210 and the second right-angled trapezoidal plate 220 respectively, and the capacitance value of the first right-angled trapezoidal plate 210 is denoted as C ₁ , the capacitance value of the second right-angled trapezoidal plate 220 is denoted as C ₂ .

Then, in step S130, the liquid level height h _x of the LNG vehicle-mounted bottle is calculated according to formula 1 and formula 2 mentioned above. Among them, A ₁ can be regarded as a constant.

The derivation and demonstration of the above measurement formula are given below.

For a parallel plate capacitor, it is assumed that the electrode plate of the parallel plate capacitor is a rectangle with a finite width and an infinite length, and there are two upper and lower edge plates. For the convenience of calculation, suppose the edge of the electrode plate is located at x=0, the center of the electrode plate is located at x→-∞; the upper plate is located at y=d/2, the lower plate is located at y=-d/2, the upper, The potentials of the lower plate are V0, -V0 respectively. Then the space occupied by the electrostatic field of the half plate can be regarded as a "quadrangle" ABCD as shown by the dotted line in Fig. 4 . Among them, AB and BC are coincident, which is the upper plate of the capacitor; CD and DA are also coincident, and are the lower plate of the capacitor. The deflection angles at vertices B, C, and D are respectively -π, +π, -π, and the relationship between the charge surface density and the electric field intensity can be obtained by the Schwartz-Christophe transformation. The surface charge density of is:

Formula two:

Formula three:

In formula 2 and formula 3: σ ₀ is the charge surface density on the parallel plate capacitor plate when the edge effect is ignored; σ _inside is the charge surface density on the inner surface of the parallel plate capacitor plate; σ _outside is the parallel plate capacitor plate Surface charge density on the outer surface; x is the distance from the desired point to the edge; d is the distance between the two plates.

It can be seen from formula 2 and formula 3 that when the capacitor plate is far away from the origin, that is, when |x|→∞, σ _inside → σ ₀ , σ _outside → σ ₀ ; and near point B, x→0, Both σ _inside and σ _outside tend to infinity.

It can be seen from formula two and formula three that when x/d=0.5, the edge effect is no longer obvious.

The above results are also applicable to finite square parallel plates. Therefore, both sides of the original diagonal capacitor plate are widened by 0.5d to form two trapezoidal plates, namely the first trapezoidal plate and the second trapezoidal plate. Such a design can reduce the impact of the edge effect on the measurement accuracy, especially when the liquid level is at the highest or lowest position, the effect is more obvious, thus improving the measurement accuracy of the liquid level gauge. In order to avoid the edge effect at the cutting line of the two half-plates (that is, the two trapezoidal plates), the gap at the cutting line should be as small as possible, but it should be controlled within the limit of the breakdown. At the same time, in conjunction with the structural changes of the capacitor plate, adding a correction coefficient of edge effect to the calculation formula can greatly improve the accuracy of liquid level measurement and eliminate the influence of edge effect in liquid level measurement.

From the above analysis, it can be seen that when x/d>0.5, the influence of the edge effect can be ignored, and the capacitance values are C ₁ and C ₂ respectively; when x/d<0.5, the influence of the edge effect cannot be ignored . Assume that the capacitance values of x/d<0.5 on both sides of the polar plate are C _b1 and C _b2 respectively. Since the two sides of the polar plate are completely symmetrical, C _b1 =C _b2 . Let the new capacitances of the two trapezoidal plates be C ₁ ' and C ₂ ' respectively, then:

Formula 4: C ₁ '＝C ₁ +C _b1

Formula 5: C ₂ '＝C ₂ +C _b2

At the same time, the conclusion can be obtained from formula 4 and formula 5 as follows:

Formula 6: C ₁ '+C ₂ '＝C ₁ +C ₂ +2C _b1

Formula 7: C ₁ '-C ₂ '＝C ₁ -C ₂

Since the strength of the edge effect is linearly related to the capacitance of the plates when the distance between the plates is constant, it can be set as follows:

Formula 8: C ₁ '+C ₂ '=α(C ₁ +C ₂ ) (7)

Neglecting the edge effect again, the capacitance measurement parameters of the trapezoidal plate are processed as follows: Let the height of the first trapezoidal plate be H, the upper base be d/2, the lower base be D+d/2, the outer wall of the first right-angled trapezoidal plate be The distance between the surface of the insulating layer and the surface of the rectangular plate is d ₀ , the thickness of the insulating layer is d ₂ , the dielectric constants of air, LNG and the insulating layer are ε ₀ , ε ₁ and ε ₂ respectively, the two trapezoidal plates The capacitors are C ₁ and C ₂ respectively, and the liquid level of the LNG vehicle bottle is h _x . When the edge effect of the electric field between the parallel plates (that is, the electric field between the first or second trapezoidal plate and the rectangular plate) is neglected, according to the capacitance calculation formula of the parallel plate capacitor, it can be deduced:

In the LNG car bottle, the influence of temperature and pressure on the dielectric constant of the gas in the bottle can be ignored, that is, the dielectric constant ε ₀ of the gas in the bottle can be regarded as a constant. Similarly, the dielectric constant of the insulating layer does not change with the environment and can also be regarded as a constant. Assume Then formula 9 and formula 10 can be simplified as:

Formula 11: C ₁ +C ₂ ＝A ₁ +2A ₂ ·Hh _x

Formula 12: C ₁ -C ₂ =2A ₂ ·(Hh _x )h _x

Then it can be deduced from formula 11 and formula 12:

From this, it can be obtained that the required liquid level height is

Since the change of the dielectric constant of the gas in the vehicle bottle can be ignored, A ₁ can be regarded as a constant quantity that has nothing to do with the change of environmental factors. Therefore, it can be seen that the change of the liquid level is only related to C ₁ and C ₂ , and is related to the The dielectric constant of the measured liquid is irrelevant. Substituting formulas 6, 7, and 8 into formula 14, the calculation formula for the liquid level in the LNG vehicle bottle can be obtained when considering the influence of the edge effect:

Among them, the coefficient α can be determined by the ratio of the additional edge to the total width of the plate, and can also be measured by experiments. In this way, the influence of edge effects on the measurement results can be effectively avoided.

While the invention has been described in terms of a limited number of embodiments, it will be apparent to a person skilled in the art having the benefit of the above description that other embodiments are conceivable within the scope of the invention thus described. In addition, it should be noted that the language used in the specification has been chosen primarily for the purpose of readability and instruction rather than to explain or define the inventive subject matter. Accordingly, many modifications and alterations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the appended claims. With respect to the scope of the present invention, the disclosure of the present invention is intended to be illustrative rather than restrictive, and the scope of the present invention is defined by the appended claims.

Claims (5)

1. A method for measuring liquid level using a plate-type capacitive sensor, characterized in that, the plate-type capacitive sensor comprises: a first right-angled trapezoidal plate, a second right-angled trapezoidal plate and a rectangular plate, the first The right-angled trapezoidal plate and the second right-angled trapezoidal plate have the same shape and are arranged symmetrically; The method for measuring liquid level using a plate-type capacitive sensor includes: The plate-type capacitive sensor is installed in the inner cavity of the LNG vehicle-mounted bottle in the following manner in advance: the plates of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate are located in the same plane, and the first right-angled trapezoidal plate is located in the same plane. The respective hypotenuses of the trapezoidal pole plate and the second right-angled trapezoidal pole plate are opposite and arranged in parallel, and the upper and lower bottom edges of the first right-angled trapezoidal pole plate and the second right-angled trapezoidal pole plate are respectively connected with the LNG vehicle-mounted The liquid surface in the bottle cavity is parallel; the upper base of the first right-angled trapezoidal plate is on the same line as the lower base of the second right-angled trapezoidal plate, and the lower base of the first right-angled trapezoidal plate is The edge is on the same straight line as the upper base of the second right-angled trapezoidal plate; the plate surface of the rectangular plate is arranged parallel to the plate surfaces of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate , and the plate surface of the rectangular plate is opposite to the plate surfaces of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate; Using two test probes to measure the capacitance values of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate in real time and synchronously; According to the capacitance value of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate and the height of the plate surface of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate, the The liquid level of the LNG vehicle bottle; The liquid level of the LNG vehicle bottle is obtained according to the following formula: in, h _x represents the liquid level height of the LNG vehicle bottle, the capacitance value of the first right-angled trapezoidal plate is C ₁ , the capacitance value of the second right-angled trapezoidal plate is C ₂ , and D is the capacitance value of the first right-angled trapezoidal plate The difference between the lower bottom and the upper bottom of the plate, the outer walls of the first right-angled trapezoidal pole plate and the second right-angled trapezoidal pole plate are all provided with an insulating layer, d ₀ is the insulating layer on the outer wall of the first right-angled trapezoidal pole plate The distance between the surface and the rectangular plate surface, d ₂ is the thickness of the insulating layer, ε ₀ is the dielectric constant of air, and ε ₂ is the dielectric constant of the insulating layer; the length of the long side of the rectangular plate is D +d, the length of the short side of the rectangular plate is H, and the short side of the rectangular plate is perpendicular to the liquid level in the inner cavity of the LNG vehicle-mounted bottle. 2. the method for utilizing plate capacitive sensor to carry out liquid level measurement according to claim 1, is characterized in that, The distance between the plate surface of the first rectangular trapezoidal plate and the plate surface of the rectangular plate is d; The lengths of the upper bases of the first right-angled trapezoidal pole plate and the second right-angled trapezoidal pole plate are both d/2, and the heights of the first right-angled trapezoidal pole plate and the second right-angled trapezoidal pole plate are both H, the lower bases of the first right-angled trapezoidal plate and the second right-angled trapezoidal plate are both D+d/2; The length of the long side of the rectangular pole plate is D+d, the length of the short side of the rectangular pole plate is H, and the short side of the rectangular pole plate is perpendicular to the liquid level in the inner cavity of the LNG vehicle-mounted bottle ; Wherein, D, H and d are all positive numbers. 3. the method for utilizing plate capacitive sensor to carry out liquid level measurement according to claim 2, is characterized in that, the distance between the respective hypotenuses of the first right-angled trapezoidal pole plate and the second right-angled trapezoidal pole plate is The value range is (0mm, 5mm]. 4. the method utilizing plate capacitive sensor to carry out liquid level measurement according to claim 2, is characterized in that, H equals the diameter of the circular section of LNG vehicle-mounted bottle liner. 5. The method for measuring liquid level utilizing a plate-type capacitive sensor according to claim 2, wherein D is equal to 80mm; the value of H is one of the following: 500mm; 600mm; 650mm.