RU2601283C2 - Contactless radiowave method of measuring liquid level in reservoir - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention relates to measuring equipment and can be used for high-accuracy determination of liquid level in a reservoir, in particular it can be used for measuring the level of oil products, liquefied gases, a coolant in nuclear reactors, etc. Proposed is a non-contact radio-wave method of measuring liquid level in a reservoir meaning that the fluid surface side perpendicular to it is radiated with FM by linear law electromagnetic waves, reflected electromagnetic waves are received, then the differential frequency signal is separated at the output of the mixer between the falling and the reflected electromagnetic waves, these data are stored in the form of an array of samples for the time period of modulation, the obtained data are approximated with a sinusoid by selection of amplitude, frequency and phase to maximum match with the obtained data, the frequency of the produced sinusoid is used to determine the level of liquid in the reservoir.

EFFECT: technical result is higher measurement accuracy.

1 cl, 2 dwg

Description

The invention relates to measuring equipment and can be used for high-precision determination of the level of a liquid in a container. In particular, it can be used to measure the level of oil products, liquefied gases, coolant in nuclear reactors, etc.

Known radio wave measurement methods that are used for non-contact level measurement of liquid media in containers for storing petroleum products, chemically active, aggressive and viscous liquids (Viktorov V.A., Lunkin B.V., Sovlukov A.S. Radio wave measurements of process parameters. M .: Energoatomizdat, 1989, 208 p.). At the same time, the level gauges implemented on the basis of these methods should provide a sufficiently high identical accuracy (up to 2 mm) in the measuring range from 0.3 to 20 meters and at the same time be reliable, convenient in operation, and inexpensive devices. In problems associated with non-contact radio wave level measurement of liquids, methods with frequency modulation of electromagnetic waves are used.

We will consider the implementation of the method using an example of a non-contact radio wave level meter that uses linear frequency modulation of a carrier wave (LFM) in its work. These frequency-modulated electromagnetic waves are radiated toward the surface of the liquid normal to it. The temporary delay of the wave reflected from the controlled surface relative to the incident wave leads to a frequency shift between the emitted and reflected waves. This differential frequency signal (RMS) is allocated on a special element - a mixer, which is part of the measuring device. In this case, the frequency of the signal reflected from the surface of the controlled medium differs from the frequency of the probing signal by the value of the frequency of the RF system signal: f _p = 2Δf _M L / cT _M , where L is the distance to the surface of the controlled medium, Δf _M is the maximum frequency tuning range, T _M - period of linear modulation, s - speed of light. From this formula follows

Like all frequency rangefinders, there is a methodological discrete error in determining the range δ, due to a finite number of periods of the signal of the difference frequency during the modulation period, which may differ from the whole:

The presence of this error is determined by the frequency measurement method, which is based on counting the number of signal zeros for a certain time. Since, with a slight change in the distance, the phase changes, and therefore the waveform at the output of the mixer, the counting result changes discretely. In this regard, various technical solutions are used to reduce this error (Kagalenko B.I., Marfin V.P., Meshcheryakov V.P. Rangefinder of increased accuracy // Measuring equipment. 1981, No. 12. P. 68-69 .).

A technical solution is also known - measuring the distance by the maximum or weighted average value of the spectrum of the differential frequency signal in a method using frequency modulation, which is the closest to the proposed method in technical essence and adopted as a prototype (Theoretical fundamentals of radar / Ed. By Y.D. Shirman . - M .: Sov. Radio, 1970, 560 p.). This prototype method consists in sensing the surface of the liquid normal to it with frequency-modulated electromagnetic waves, receiving the reflected electromagnetic waves, extracting the RF signal at the mixer output between the incident and reflected electromagnetic waves, and calculating the distance from the difference frequency of the RF signal, determined by its maximum value frequency spectrum.

However, in this case, the methodological discrete error (2) is retained, since the spectral analysis is based on the decomposition of the signal over an integer number of harmonics, while the real maximum when measuring the distance can also be located between harmonics. In order to measure the frequency of the RMS at a minimum distance of 0.3 m, it is necessary to have Δf _M such that at least one period of the RMS signal can be observed. Then it will be the first harmonic in the RMS spectrum. From (1) it follows that Δf _M in this case is 500 MHz, and the error δ is 0.15 m at a measuring range above 0.3 m Therefore, to ensure acceptable accuracy, it is necessary to increase Δf _M.; usually this value for industrial level gauges is 1 ÷ 2 GHz, which corresponds to δ = 7.5 ÷ 3.75 cm. A further increase in accuracy is achieved by using smoothing procedures (Ezersky V.V., Davydochkin V.M. Optimization of the spectral processing of a precision signal distance sensor based on a frequency range finder // Measuring equipment. 2005, No. 2, P. 21-25). However, the use of large values of Δf _M leads to an increase of additional errors because of the chirp from the influence of other elements in the containers and the walls of uneven frequency response paths nonlinearity of oscillator modulation, etc. All this, together with an increase in the cost of a broadband device, leads to a decrease in the functional characteristics of the level gauge.

The technical result of the present invention is to improve the accuracy of measurement.

The technical result in the proposed method for measuring the liquid level in the tank is achieved by the fact that electromagnetic waves are frequency-modulated linearly to the surface of the liquid along the normal surface of the liquid, receive reflected electromagnetic waves, then a difference frequency signal is emitted at the mixer output between incident and reflected electromagnetic waves waves, save this data in the form of an array of samples during the modulation period, approximate the data by a sine wave by selecting the amplitude, often You and phase to a maximum coincidence with the received data obtained by frequency sinusoid judging the liquid level in the vessel.

In FIG. 1 shows a structural diagram of a device for implementing the method. In FIG. 2 is a timing diagram explaining the operation of the method.

In FIG. 1 shows a modulator 1, a generator 2, a directional coupler 3, a transmitting antenna 4, a receiving antenna 5, a mixer 6, a computing device 7.

The method is implemented as follows. The ramp generator 1 modulates the frequency of the microwave generator 2, the output of which electromagnetic waves pass through a directional coupler 3 to the antenna 4 and radiate towards the surface 8. The reflected electromagnetic wave is received by the antenna 5 and fed to the mixer 6, which also receives part of the power the incident wave from the directional coupler 3. At the output of the mixer 6, a difference frequency signal is generated, which enters the computing device 7, where it is recorded in ma Siv data for the period of frequency modulation and determination of the difference signal frequency using the recorded data approximating sinusoid selection of its amplitude, frequency and phase to the maximum coincidence. From the found frequency, the distance L to the controlled surface 8 is determined, according to which the liquid level in the tank is judged.

In FIG. Figures 2a, 2b and c show the simulated frequency response signal with Δf _M = 150 MHz in the presence of a noise component (dots) and the approximation results by sinusoids (solid line) at distances of 0.3, 1.5, and 8.5 m at 256 samples for a linear modulation period of -1 s. The approximating sinusoid is presented in the form of the formula:

S = a 1 * sin (b1x + c1),

where a 1 is the amplitude, b1 is the frequency, c1 is the phase, x is the index of the array of samples of 256 points.

With the indicated data, according to formula (1), the distance to the controlled medium in meters is numerically equal to the frequency of the RMS in hertz. The frequency of the RF signal is determined from the relation:

f _p = b1N / 2πT _M ,

and the distance is according to the formula (1). According to the results of the optimization procedure, we have for these examples, respectively:

a) for a distance of 0.3 m - a 1 = 1.048, b1 = 0.007128, c1 = 0.8092, with a coincidence coefficient R = 0.09633 or f _p = 0.29 Hz, the error is 10 mm;

b) for a distance of 1.5 m - a 1 = 1.013, b1 = 0.03677, c1 = 0.7376, at R = 0.9936 or f _p = 1.4981 Hz, the error is 1.9 mm;

c) for a distance of 8.5 m - a 1 = 1.003, b1 = 0.2086, c1 = 0.5689, at R = 0.9967 or f _p = 8.4991 Hz, the error is 0.9 mm.

The above examples clearly demonstrate the advantage of the proposed method. At the indicated Δf _M = 150 MHz, according to formula (2), the discrete error is δ = 0.5 m. In this case, a narrow frequency band provides a reduction in the cost of the device, reduces the influence of spurious modulation, non-linear modulation of the generator and its stability.

Claims (1)

A non-contact radio wave method for measuring the level of a liquid in a vessel, characterized in that electromagnetic waves frequency-modulated according to the linear law are emitted towards the liquid surface normal to it, receive reflected electromagnetic waves, then a difference frequency signal is emitted at the mixer output between incident and reflected electromagnetic waves , save this data in the form of an array of samples during the modulation period, characterized in that approximate the data by a sinusoid by pa amplitude, frequency and phase to a maximum coincidence with the received data obtained by frequency sinusoid determined liquid level in the vessel.

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