RU2567018C9 - Method for measuring level of dielectric substance - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: in the method for measuring level of dielectric substance a capacitance level sensor and compensating capacitor are used and sinusoidal voltage of two frequencies is supplied in sequence to them. Currents of the capacitance level sensor and compensating capacitor are measured at the above frequencies. Against current value capacitance growth of the capacitance level sensor is assessed as well as relative value of level in dielectric fluid filling interelectrode space of the sensor.

EFFECT: improved accuracy in measurement of dielectric substance level, increased degree of the measurement process automation and maintainability due to recording of current value of dielectric permeability for the controlled substance as determined directly in process of measurement.

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Description

The present invention relates to electrical engineering and can be used in monitoring and measuring the liquid level in monitoring systems and diagnostics of technical objects, as well as in systems for controlling the level of refueling of space rocket equipment with fuel components.

There is a method of determining the level of dielectric substance described in the work "Measuring instrument-analyzer of the parameters of complex resistance based on a personal computer" by Agamalov Yu.R., Bobyleva D.A., Kneller V.Yu. (Measuring equipment. - 1996. No. 6. P. 56-60), which uses the scheme of indirect measurement of the parameters of a bipolar element during the formation of a sinusoidal voltage on it. The method is invariant to the form of a two-terminal network and its equivalent circuit. In the measurements, the magnitude of the complex currents flowing through the two-terminal network and the resistive resistance (reference), which is affected by the reference voltage with the corresponding phase change, are determined in turn. The currents are converted into voltages that correspond to the projection of the measured voltage vector onto the phase-shifting reference voltage vector. Codes that carry information about the projections of the measured voltage vector onto the reference voltage vector are sent to a computer, where the real and imaginary components of the voltage are calculated at the measurement object and resistive standard.

The disadvantages of the known method of measurement should include its large error, since it requires phase measurements and does not take into account the resistance of the external electrical circuit. Due to the remoteness of the measuring instruments from the capacitive sensor, such a circuit generally contains a communication line, the resistance of key devices in the open state, the input resistance of the capacitor-voltage converter, and the output resistance of a sinusoidal voltage source. The length of the communication line can reach 500 m.

In addition, the measurements in this way do not take into account the relative permittivity and temperature of the vapor-gas mixture and the liquid filling the interelectrode space of the capacitive sensor. And since the electric capacitance of the capacitor is proportional to the relative dielectric constant of its interelectrode space, this leads to significant measurement errors.

A disadvantage of the method can also be considered the fact that it is oriented to work with only one measurement object, while in on-board systems for measuring the level of refueling of rocket and space technology it is advisable to serve several level sensors with one system.

Closest to the claimed method in terms of technical nature and the achieved positive effect is a method for determining the level of dielectric substance described in RF patent No. 2262669 C2, IPC G01F 23/26, G01R 17/00 of 01/10/2003, and focused on measuring the level of gas station missiles fuel components. It is selected as a prototype.

The level measurement in the prototype method is as follows. The equivalent circuit of the capacitive level sensor, consisting of an electric capacitance and active resistance, is set. Choose a standard - the active resistance of a known value. A sinusoidal voltage is generated on the capacitive sensor and on the reference at two frequencies. The complex current is measured and fixed sequentially through a “dry” sensor and a reference at each of two frequencies. Determine and fix the electric capacitance of the "dry" sensor. Using reference data or any other information, the value of the relative dielectric constant of the controlled fluid at a given temperature is determined and recorded. The increment of the electric capacitance of the sensor is calculated and recorded when it is completely immersed in a dielectric substance. From time to time, sequential measurement and fixing of the complex current is carried out through a capacitive level sensor and a reference at each of two frequencies. For each periodic measurement, the electrical capacitance of the capacitive level sensor is determined and fixed. The relative filling of it with a dielectric substance is determined as the difference between the values of the electric capacitance of the filled capacitive level sensor and the electric capacity of the “dry” sensor, related to the increment of the capacitor completely immersed in the dielectric substance of the sensor. Moreover, the prototype method is focused on working with several capacitive sensors in the time sharing mode.

In accordance with the functioning algorithm of the method under consideration, the resistance value of the reference resistor, the electric capacitance of a capacitive level sensor filled with a controlled fluid, the temperature and relative dielectric constant of the controlled fluid should be known a priori and introduced into the system at the stage of preparing it for operation. A large number of preparatory operations leads to low manufacturability and the degree of automation of determining the level. At the same time, atmospheric pressure, impurity content of the controlled liquid, its technological features that affect the relative dielectric constant of the substance, as well as the temperature of the gas mixture above the controlled liquid, are not taken into account.

In addition, the actual (current) values of these parameters during the preparation of the system for operation and during the measurement process can undergo significant changes. It is not possible to take this into account in the system under consideration, which naturally leads to an additional measurement error.

The considered method does not allow to take into account the parameters of the external electric circuit connected to the capacitive level sensor during the measurement. This leads to an additional error in determining the level of the controlled fluid.

The most frequently repeated operation to determine and fix the complex value of the measured currents also seems to be technologically complex and costly, since it involves the determination of two components of the current: reactive and active.

In addition, according to the measurement method under consideration, it is necessary to know the value of the sensor capacity when it is completely immersed in a controlled liquid. Such an operation requires the preliminary filling of the capacities of the launch vehicle tanks with a controlled fluid, which is technologically either very difficult or impossible. Especially if the level of chemically aggressive or cryogenic liquid is controlled. An analytical calculation of this parameter does not provide the necessary accuracy.

All this taken together determines an insufficiently perfect technology for determining the level of rocket refueling with fuel components.

The aim of the invention is to increase the accuracy of measuring the level of dielectric substance, the implementation of the measurement process on a more advanced technological basis.

The goal is achieved by the fact that in the method of measuring the level of the dielectric substance, in which the equivalent circuit of the capacitive level sensor with the known geometric dimensions of its electrodes is set, a sinusoidal voltage is formed on the capacitive sensor at frequencies ω ₁ , ω ₂ , currents and electric capacitance are measured at these frequencies a “dry” level sensor, periodically measure currents through a capacitive level sensor filled with a controlled liquid, determine the electric capacitance and the value of its leakage resistance, determine the capacitance of the level sensor is incremented and fixed due to the partial filling of the interelectrode space with a controlled fluid, in contrast to the prototype, a compensation capacitor with known geometric dimensions of its electrodes is used. Place the compensation capacitor below the capacitive level sensor. Moreover, on it, as well as on a capacitive level sensor, a sinusoidal voltage is alternately formed at frequencies ω ₁ , ω ₂ . At these frequencies, the amplitude values of the currents flowing through the compensation capacitor, the electrical capacitance of the “dry” compensation capacitor, and the resistance of the external circuit with respect to it are measured. After filling the interelectrode space of the compensation capacitor with a controlled liquid, the amplitudes of the currents flowing through it at two frequencies ω ₁ , ω ₂ are measured, its electric capacitance and leakage resistance are determined and fixed, the increment of the capacitance of the compensation capacitor is determined, and the relative value of the level of the controlled liquid is calculated in accordance with the mathematical expression:

where µ _AND = L _AND N _AND / d _AND - structural constant of a capacitive level sensor;

L _AND , N _AND - the width and height of the electrodes of the capacitive level sensor, respectively;

d _AND is the distance between the electrodes of the capacitive level sensor;

ΔС _And - increment of the electric capacity of the capacitive level sensor due to the partial filling of its interelectrode space with a controlled fluid;

µ _K = L _K N _K / d _K - construction constant of the compensation capacitor;

L _K , N _K - width and height of the electrodes of the compensation capacitor, respectively;

d _K is the distance between the electrodes of the compensation capacitor;

ΔС _K is the increment of the electric capacitance of the compensation capacitor by filling its interelectrode space with a controlled fluid.

In this case, the determination of the capacitance of a “dry” and capacitive level sensor filled with a liquid controlled by a liquid is performed by the magnitude of the amplitude values of sinusoidal currents passed through it at frequencies ω ₁ , ω ₂ . The capacity of a “dry”, partially or completely filled with liquid capacitive level sensor and a compensation capacitor is determined by taking into account the resistances of the external electrical circuit with respect to each of them and the current value of the relative dielectric constant of the controlled fluid.

In FIG. 1 shows an equivalent circuit of a “dry” capacitive level sensor with an external electrical circuit connected to it. The structure of the circuit includes: С _И - electric capacitance of a “dry” capacitive level sensor, r ₁ - active resistance of open key devices of an external electric circuit, r ₂ , r ₃ - respectively, the active resistance of the connecting wires and the active input resistance of the current-voltage converter of this circuit , U _IG - the output voltage of the sinusoidal voltage generator in the capacitive level sensor circuit. It is taken into account that the leakage resistance of a “dry” capacitive level sensor in the atmosphere of a vapor-gas mixture is quite large and its effect can be neglected.

The active resistance R _{VI of} the circuit in question is determined by the sum of the resistances:

Then the complex resistance of the electric circuit Ż _AND and its module Z _AND will be determined respectively by the relations:

, $$I_{2ИГ}^2$$

в рассматриваемой электрической цепи на частотах ω₁, ω₂ соответственно можно записать:According to the measurement procedure, it is believed that the source U of the _IG in the electric circuit of FIG. 1 generates alternately sinusoidal voltages at frequencies ω ₁ , ω ₂ . Therefore, for the squares of the amplitude values of the currents $$I_{\mathrm{one}\mathrm{AND}R}^2$$

, $$I_{2\mathrm{AND}R}^2$$

in the considered electric circuit at frequencies ω ₁ , ω _2, respectively, can be written:

Solving together (4), (5), for the active component of the resistance of the external electric circuit R _VI and the capacitive component of the "dry" level sensor C _IG, you can get the ratio:

Where

γ = ω ₂ / ω ₁ .

, $$I_{2КГ}^2$$

на частотах ω₁, ω₂, соответственно:Similarly, for the electrical circuit for connecting a “dry” compensation capacitor C _KG to an external electrical circuit, one can write the equations for the squares of the amplitude values of the currents $$I_{\mathrm{one}\mathrm{TO}R}^2$$

, $$I_{2\mathrm{TO}R}^2$$

at frequencies ω ₁ , ω ₂ , respectively:

where R _VK is the active component of the resistance of the external electrical circuit of the compensation capacitor;

U _KG is the output voltage of the sinusoidal voltage generator in the circuit of the "dry" compensation capacitor.

Solving equations (8), (9), together determine the values of R _BK and C _KG :

Here

Under conditions of partial or complete filling of the interelectrode space of the capacitive level sensor with a controlled liquid, its leakage resistance begins to appear. Therefore, in the calculations, an equivalent circuit of the level sensor is used in the form of a parallel connection of the sensor capacitance C _AND and its leakage resistance R _AND . In FIG. 2 shows an equivalent circuit of a capacitive level sensor partially or completely filled with a controlled fluid. The following notation is used in the diagram: U _AND is the voltage of the sinusoidal voltage generator in the level sensor circuit, R _VI is the internal resistance of the external electrical circuit, C _AND and R _AND are the capacitance and leakage resistance of the capacitive level sensor, respectively.

According to the claimed method, the amplitude values of the currents I _1I , I _2I , flowing in the circuit at frequencies ω ₁ , ω _{2 are} determined. For this, in accordance with FIG. 2, determine the complex conductivity of the level sensor Ÿ:

its complex resistance Ż _X :

complex resistance of the entire electrical circuit:

and the square of the modulus of resistance Z ^{2 of} this circuit:

, $$I_{2И}^2$$

на частотах ω₁, ω₂ можно записать соответственно:As in the previous case, for the squares of the amplitude values of the currents $$I_{\mathrm{one}\mathrm{AND}}^2$$

, $$I_{2\mathrm{AND}}^2$$

at frequencies ω ₁ , ω ₂ can be written, respectively:

Solving together (16), (17), the parameters of the capacitive level sensor С _И , R _И are determined:

Here

;

;

γ = ω ₂ / ω ₁ .

If the value of the leakage resistance R _AND is large enough, and its influence can be neglected, then the expression for C _{AND is} significantly simplified:

Relations (18) - (20) allow us to determine the value of the capacitance and leakage resistance of a capacitive level sensor in the process of filling its interelectrode space with a controlled fluid.

In a similar way, we can obtain the relations for the capacitance C _K and the leakage resistance R _{K of the} compensation capacitor when filling its interelectrode space with a controlled fluid:

Here

U _K is the voltage at the output of the sinusoidal voltage generator in the compensation capacitor circuit;

R _BK - internal resistance of the external electrical circuit of the compensation capacitor.

If the leakage resistance R _{K of the} compensation capacitor is large enough, and its influence can be neglected, then the expression for C _{K is} significantly simplified:

Relations (18) - (24) make it possible to determine by hardware the current values of the parameters of the capacitive level sensor and the compensation capacitor taking into account the current values of the relative permittivity and temperature of the controlled fluid, as well as taking into account the resistance values of external electrical circuits.

The capacity of the “dry” compensation capacitor C _KG , determined by hardware (by measuring the currents flowing in its circuit), includes the capacitance of the interelectrode space C _KM filled with the vapor-gas mixture, and the stray capacitance C _KP :

Spurious capacitance C _KP takes into account the capacitance of the supply wires, the passage capacity of the insulators and fastening elements of the compensation capacitor. Its value is determined at the place of attachment of the compensation capacitor a priori and is entered into the product passport as a constant.

Electric capacitance C _CM compensation capacitor interelectrode space filled with the steam-gas mixture, dependent on the width and height L _K H _K interacting its electrode spacing d _K, the relative permittivity ε _r-vapor medium and an electric constant ε _0:

Hardware this capacity can be determined only in conjunction with its parasitic capacity With _KP :

Here ε _Ж is the relative dielectric constant of the controlled fluid.

The increment of the capacitance of the compensation capacitor ΔС _K due to the filling of its interelectrode space with a controlled liquid will be determined by the difference:

The last relation allows us to determine the difference (ε _W- ε _G ) of the relative permittivities of two media (controlled liquid and combined-cycle gas):

and relative dielectric constant of the controlled fluid:

The capacity of the level sensor C _And , with the gradual filling of its interelectrode space with liquid, is continuously changing. The current value of C _AND can be controlled by hardware in accordance with relation (18). In the General case, the value _And includes the parasitic capacitance With _IP , the capacity of the part of the interelectrode space of the capacitive level sensor With _IZH filled with liquid, and the capacity of the other part of its interelectrode space With _IHF filled with vapor-gas mixture:

As in the case of a compensation capacitor, the parasitic capacitance C _IP takes into account the capacity of the supply wires, the passage capacity of the insulators and fastening elements of the capacitive level sensor. Its value is determined a priori at the place of attachment of the capacitive level sensor and is entered into the product passport as a constant.

The value of component C _IL is determined by the ratio:

where h is the current value of the level of the controlled fluid;

L _And - the width of the interelectrode space of the capacitive level sensor;

d _And is the distance between the electrodes of the capacitive level sensor.

The value of component C of the _HHI will accordingly be equal to:

Here N _AND is the height of the electrodes of the capacitive level sensor (measuring range of the capacitive level sensor).

Substituting (32), (33) into (31) and transforming, we obtain:

In the last ratio, the sum of the parasitic component of the capacitance C _PI and the capacitance of the interelectrode space of the level sensor filled with the vapor-gas mixture:

represents the capacitance C _IG "dry" capacitive level sensor:

the value of which can be determined by hardware in accordance with relation (7). Therefore, according to (34), the increment of the capacitance of the capacitive level sensor ΔС _И :

due to filling part of its interelectrode space with liquid, it can be defined as the difference between the two values of C _AND , C _IG , determined by hardware:

Then relation (33) can be written in the following form:

Substituting (29), (37) in (38), for the relative value of the level of the controlled fluid h / Н _And taking into account the current value of the relative permittivity, we can write:

The process of periodically determining the level of the controlled fluid is continued until the tank of the product is filled to the required level.

In expression (39), the ratio L _K H _K / d _K is determined by the overall dimensions of the compensation capacitor and can be determined a priori as its construction constant μ _K. Similarly, as the ratio L _AND N _AND / d _AND use the structural constant of the capacitive level sensor µ _And . The expression for the relative level of the controlled fluid in this case is:

The procedure for measuring the relative magnitude of the liquid level in the control process according to the claimed method is reduced to a sequence of such operations:

- set the equivalent circuit of the capacitive level sensor in the form of a parallel connection of its electric capacitance C _AND and leakage resistance R _And ;

- install below the capacitive level sensor compensation capacitor with a known length, width of the electrodes and the distance between them;

- determine and fix in a permanent memory the compensation capacitor structural μ _K and the capacitive sensor _and μ level according to the relations _K μ = L _K H _K / d _{K, and} μ = L _and H _and / d _and;

- at the initial stage of filling, when the level of the controlled liquid is lower than the electrodes of the compensation capacitor, stabilized sinusoidal voltage is supplied through external electric circuits at frequencies ω ₁ , ω ₂ alternately, first to the capacitive level sensor, then to the compensation capacitor;

- measure the amplitude values of sinusoidal currents flowing through a compensation capacitor and a capacitive level sensor at frequencies ω ₁ , ω ₂ alternately;

- determine and record in memory the value of the electric capacitance of the "dry" compensation capacitor, taking into account the resistance of the external external circuit to it;

- determine and record in memory the value of the electric capacitance of the "dry" capacitive level sensor, taking into account the resistance of the external external circuit to it;

- when the interelectrode space filled with the compensation capacitor controlled liquid measure amplitude values of the sinusoidal currents flowing through the compensating capacitor at the two frequencies ω _1, ω ₂ alternately;

- taking into account the resistance of the external electric circuit, the leakage resistance, the electric capacitance of the compensation capacitor and the increment of its capacitance ΔС _{K are} determined and recorded in the memory by filling the interelectrode space with a controlled liquid;

- apply sinusoidal voltage to the capacitive level sensor at frequencies ω ₁ , ω ₂ alternately and periodically measure the amplitude values of the sinusoidal currents flowing through the capacitive level sensor at these frequencies;

- after each measurement of currents, taking into account the resistance of the external electric circuit, the current values of leakage resistance, electric capacitance and capacitance increment ΔС _{AND of the} capacitive level sensor are determined by partially filling its interelectrode space with a controlled fluid;

- referring to the memory, periodically determine the current value of the relative magnitude of the level of the controlled fluid according to the expression h / H _AND = µ _K ΔC _AND / µ _AND ΔC _K ;

- periodic measurements of the relative magnitude of the level of the controlled fluid continue until its value reaches the required value.

The application of the sequence of operations of the proposed method for measuring the level of dielectric substance allows you to determine and take into account in the automatic mode the current value of the relative dielectric constant of the controlled fluid directly in the process of filling its tanks with fuel components, taking into account the current value of atmospheric pressure, ambient temperature, controlled fluid and gas mixture above it , taking into account the impurity content of the controlled fluid, its possible heterogeneity and other parameters affecting the relative permittivity.

Therefore, the relative dielectric constant, and atmospheric pressure, and temperature, and the impurity content of the controlled fluid, and its possible heterogeneity are taken into account both in measuring currents and in determining the relative level of the controlled fluid and makes the claimed method technologically more advanced, with a higher degree of automation.

At the same time, the accuracy of level measurement also increases, since when determining the electric capacitance of the level sensor, the current value of the relative dielectric constant of the controlled liquid is taken into account, and not reference data, which cannot be fully taken into account at the time of measurement, either atmospheric pressure, or liquid temperature, or impurity its content, nor possible heterogeneity.

Improving the accuracy of measurements also ensures that the parameters of the external electric circuit of the capacitive level sensor and the compensation capacitor are taken into account. This makes it possible to more accurately determine the amplitude of the sinusoidal currents, and therefore the increment of the capacitance of the capacitive level sensor and compensation capacitor.

In addition, the introduction of a compensation capacitor made it possible to avoid a technologically very complicated operation for determining a priori the capacitance of a level sensor filled with a controlled liquid. It should be borne in mind that the introduction of a compensation capacitor does not entail an increase in the number of operations, since with the claimed method there is no need to measure the voltage and currents of the reference resistance. This also makes the claimed method technologically more advanced, with a higher degree of automation.

Unlike the prototype, the claimed method uses the amplitude value of the sinusoidal current (or its effective value) as the measured value, which makes the measurement procedure more technological, operational and informatively less expensive.

Thus, the inventive method of measuring the level of dielectric substance ensures the achievement of the goal.

A comparative analysis of the proposed technical solution with the prototype allows us to conclude that it meets the criterion of "novelty."

When implementing the proposed method uses typical, widespread methods for the formation, processing and conversion of analog signals: generating a sinusoidal voltage, switching analog signals, converting electric current to voltage, scaling, analog-to-digital signal conversion. Modern means of processing and converting information allow you to perform the above operations in the form of devices with small overall dimensions. The storage and transmission of information, the performance of arithmetic operations in digital form also does not cause significant difficulties at present and can be performed, for example, on Xilinx microcircuits.

The inventive method was tested by the authors on the layout. Currently, based on it, with the participation of the authors of the method, a system is being created for measuring the level of filling of carrier rockets with fuel components for the Vostochny spaceport.

Claims (1)

A method for measuring the level of a dielectric substance, in which an equivalent circuit is set for a capacitive level sensor with a known distance d _AND between its electrodes, with a known height N _AND and a width L _{AND of} electrodes, alternately generate a sinusoidal voltage on it with frequencies ω ₁ , ω ₂ , on these frequencies measure electric currents through a "dry" capacitive level sensor and determine its electrical capacitance at frequencies ω ₁ , ω ₂ , periodically measure currents through a capacitive level sensor filled with a controlled liquid, determine the electric The capacitive capacitance and the value of its leakage resistance are determined and fixed by the increment in the capacitance of the capacitive level sensor due to the partial filling of the interelectrode space with a controlled fluid, characterized in that, in the measurements, an expansion capacitor with a known distance between its electrodes d _K , with a known height H _K and a width L _{K of} electrodes, a compensation capacitor is placed below the capacitive level sensor, on this capacitor, as well as on a capacitive level sensor, sinusoidal voltage is formed uniformly at frequencies ω ₁ , ω ₂ , at these frequencies the amplitude values of the currents flowing through the compensation capacitor are measured, the electric capacitance of the “dry” compensation capacitor and the resistance of the external circuit relative to it are determined, after filling the interelectrode space of the compensation capacitor controlled the liquid measures the amplitudes of the currents flowing through it at two frequencies ω ₁ , ω ₂ , determine and fix its electrical capacitance and resistance leakage, determine the increment of the capacitance of the compensation capacitor and calculate the relative value of the level of the controlled fluid in accordance with the mathematical expression:

where µ _AND = L _AND H _AND / d _AND is the structural constant of the capacitive level sensor,
µ _K = L _K H _K / d _K - construction constant of the compensation capacitor,
ΔС _And - increment of the electric capacity of the capacitive level sensor due to the partial filling of its interelectrode space with a controlled fluid,
ΔС _K is the increment of the electric capacitance of the compensation capacitor by filling its interelectrode space with a controlled liquid, while the capacitance of a dry and capacitive level sensor filled with a controlled liquid is determined by the magnitude of the amplitude values of sinusoidal currents passed through it at frequencies ω ₁ , ω ₂ , determination of the capacity of a “dry”, partially or completely filled with liquid capacitive level sensor and a compensation capacitor is carried out taking into account the resistance outside it with respect to each of these electric circuits and the current value of the relative permittivity of the controlled fluid.

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