RU2410650C2 - Method to measure level of material in reservoir - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to the field of measurement equipment and may be used to measure level of liquid or loose materials, and also to measure distance. The invention concept is as follows: method to measure level of material in reservoir consists in the fact that inside zone with higher error of measurement caused by availability of interference reflector, formation of likelihood function logarithm (LFL) values is carried out, having multiextremal nature, and local extremum of LFL is found, corresponding to actual value of level, by variation of delay time of reference signal within the limits that correspond to two limits of measured level, which differ from value of level predicted by previous measured values, by not more than half of wave length of carrying oscillation towards both sides. In case of such formation of borders, one and the same extremum of LFL is continuously monitored, corresponding to actual status of medium level. Phase of reference signal for calculation of LFL is determined using previously measured phase-frequency characteristic of metering device, or is accepted as equal to phase of maximum spectral component of differential frequency signal calculated at the border of zone with higher error of measurement. Using produced value of delay time of reflected signal corresponding to maximum of LFL, distance to material surface is calculated. To determine borders of zones with higher error of measurement, preliminary practice is arranged, when position of all interference reflectors is determined on empty reservoir, and two borders of zones with higher error are specified in neighbourhood of each one, differing from distance to according interference reflector by tripled value of element of frequency modulated signal resolution with specified deviation of frequency.

EFFECT: increased accuracy of level measurement with availability of interference signals caused by reflections from elements of reservoir design.

11 cl, 6 dwg

Description

The invention relates to the field of measurement technology and can be used for precision level measurement of liquid materials.

Known radar method of measuring the level [1], including measuring the propagation time of radio waves emitted in the direction to the surface of the medium and reflected from it, and calculating the measured propagation time of radio waves of the distance to the surface of the medium. The specified method does not allow to measure the level with sufficient accuracy in the presence of interfering reflections caused by the design features of the tank with liquid material, since interfering reflections distort the waveform and thereby lead to a large error in measuring the delay time.

A known method of measuring distance, implemented in the device [2], which consists in the fact that they emit a frequency-modulated signal in the direction of the contents of the tank, receive, after the propagation time, the reflected signal and mix it with part of the emitted signal to obtain a differential frequency signal (RFM) . The phase of this signal is used to measure the distance to the surface of the controlled medium, provided that the difference frequency itself is kept constant by controlling the modulation period. In this case, the phase of the signal of the difference frequency when measuring the distance will continuously change within 2 πN + φ in proportion to the change in distance. Here N is the integer number of periods of the RHF contained in the modulation period, φ is the number corresponding to the remainder of the period, that is, the initial phase of the RHF. Thus, the determination of the distance is reduced to counting the number N, measuring the phase φ and calculating the distance.

The disadvantage of this method is the inability to measure the level with a given accuracy in the presence of interfering reflections caused by structural elements of the tank, since the presence of interference greatly changes the phase of the signal and leads to a large error.

Also known is a method of measuring distance [3], including: generating and emitting a probe signal with periodic frequency modulation in the initial frequency modulation range; receiving an echo signal, isolating part of the probe signal and mixing it with a received echo signal; calculation of the spectrum according to the RMS obtained for half the modulation period and determination of its center frequency; calculating the distance from the measured center frequency of the RMS and the number of minimum distance intervals that fit into the measured distance, corresponding to the number of periods of the RMS at the half modulation period; reducing the initial range of frequency modulation to obtain an integer number of distance intervals corresponding to an integer number of periods of the RF system at the half-period of modulation and the maximum spectral component corresponding to the measured distance and calculating the frequency distance of this maximum spectral component of the RF system.

The disadvantage of this method is the impossibility of measuring the level with high accuracy near the interfering reflectors, since the main and side lobes of the interference spectrum distort the shape of the main lobe of the signal spectrum and thereby cause a large error in determining the true position of the extremum of the spectrum.

The closest solution to the set of essential features (prototype) is a level measurement method [4] with a radar oriented to the maximum of the echo signal from the measured surface and including radiation from a sequence of microwave signals in which discrete angular frequencies are uniformly distributed over the scanned frequency range. The signal reflected from the measured surface is mixed with a part of the emitted signal to obtain two differential frequency signals phase-shifted relative to each other by an angle π / 2 (quadrature signals), which, after analog-to-digital conversion, are fed to the microprocessor.

Processing the sequence of quadrature samples includes the inverse discrete Fourier transform, the use of the MUZIC high resolution method for localizing sources of the most powerful radiation, the selection of echo signals in the time domain, and ultimately obtaining the distance to the measured level.

The disadvantage of this method is the insufficiently high accuracy of level measurement in the presence of interfering reflections caused by reservoir structural elements and inhomogeneities of the antenna-waveguide path due to their mutual influence on the useful signal. An analysis of the description and patent claims allows us to conclude that the high resolution method of the MUZIC type or other methods that improve the frequency resolution can increase the measurement accuracy at a distance near the interfering reflector compared to methods based on determining the position of the maximum spectral component, for example [ 3]. However, the main reason for insufficient accuracy in the presence of interfering reflections remains - this is the mutual influence of the spectra of the useful and interfering signals, leading to a distortion of the ratios of the intensities of the spectral components of the useful and interfering reflectors and, as a result, to confusing the useful and false reflectors when they are localized, which causes a large level measurement error , manifested in an abrupt change in the measurement results.

The purpose of the invention is to reduce the error of level measurement in the presence of interfering reflections from reservoir structural elements, provided that the reflection from the useful reflector exceeds the amplitude of the reflection from the interfering reflector and the position of the material level changes from one measurement cycle to another by no more than half the carrier wavelength fluctuations.

This goal is achieved by the fact that in the method of measuring the level of material in the tank, including forming a measuring device of a probing signal of the radio frequency range with a linear stepwise periodic frequency modulation with known values of the carrier frequency and frequency deviation, the radiation of the generated probing signal in the direction of the probed material, receiving a reflected signal, after its propagation time, mixing the reflected signal with part of the probing signal, isolating the signal p value of the difference frequency, obtaining samples of the difference frequency signal using an analog-to-digital converter, calculating by the processor using the fast Fourier transform of the spectrum the product of the samples of the difference frequency signal and the samples of the weight function, for example, the Blackman function, rough determination of the distance to the material from a preliminary estimate of the frequency of the maximum spectral component, refinement of the indicated distance by estimating the difference frequency corresponding to the maximum spectral component found by the continuous-discrete Fourier transform and varying the difference frequency within one frequency discrete spectrum in the vicinity of the preliminary estimate of the frequency of the maximum spectral component in both directions, determine the boundaries of the zones with increased measurement error in an empty tank, an additional processor generates a reference signal in the form of digital readings according to known values of the carrier frequency of the probing frequency modulated signal and its frequency deviation, as well as according to the specified values of time change the delay of the reference signal, its amplitude and phase, determine the logarithm of the likelihood function by finding a negative value of the average square of the difference of the received samples of the difference frequency signal and the samples of the generated reference signal, find the exact value of the delay time of the reference signal by maximizing the obtained value of the logarithm of the likelihood function by converting the shape of the reference signal by using different values of the delay time of the reference signal in the specified predicted limit x and calculating the measured level of the material found on the exact value of the delay time corresponding to a local maximum of the logarithm of the likelihood function.

Preferably, the predetermined predicted limits of the delay time of the reference signal are set to be equal to the delay times of the reflected signal corresponding to two boundaries of the measured level positions, differing from the level predicted from the previous measured values, by no more than half the wavelength of the carrier oscillation of the probe signal in both directions.

Preferably, the level value is predicted from at least two previous measurement results taking into account the rate of change.

It is advisable, when conducting the first measurement, to take the predicted value of the level position equal to the current value obtained on the basis of a continuously discrete Fourier transform.

In the method, the amplitude of the reference signal must be equal to the amplitude of the difference frequency signal.

Preferably, during the first measurement, the phase of the reference signal is determined from the previously measured phase characteristic of the measuring device, taking into account the distance to the material found using the continuously discrete Fourier transform in the current measurement period.

It is advisable to determine the phase of the reference signal from the previously measured phase characteristic of the measuring device, taking into account the previous measured distance to the material.

It is advisable, during the first measurement, to determine the phase of the reference signal at the boundary of the zone with an increased measurement error near (in the vicinity) of the interfering reflector of the tank, by the phase of the maximum spectral component of the spectrum of the signal of the difference frequency, found when refining the distance to the material using a continuously discrete Fourier transform, reduced by an amount equal to the difference in the total phase incursion of the reflected signal at the carrier frequency during the propagation of the specified signal and an integer periods of the reflected signal that fit into the full phase incursion, and the full phase incursion is determined by the known carrier frequency of the probe signal and the reflected signal delay calculated by the position of the material level obtained using the continuously discrete Fourier transform for the current measurement period.

It is preferable to determine the phase of the reference signal at the boundary of the zone with an increased measurement error near (in the vicinity) of the interfering reflector of the tank, by the phase of the maximum spectral component of the spectrum of the difference frequency signal found at the stage of accurate frequency estimation using the continuously discrete Fourier transform, reduced by an amount equal to the difference in the total phase incursion of the reflected signal at the carrier frequency during the propagation of the signal and an integer number of periods of the reflected signal that fit into SG phase shift, the full phase shift determined by a known carrier frequency signal emitted by the probe and the echo delay calculated according to the position of the material level measured at the previous measurement period.

To determine the boundaries of the zone with increased error, it is advisable to record the difference frequency signal on an empty tank, calculate the spectrum of the difference frequency signal using the fast Fourier transform, find the frequencies corresponding to all local maximums of the spectrum exceeding the specified threshold value, and refine the frequencies of the found local extrema using continuously discrete Fourier transform, calculate the distance to the interfering reflectors by the found frequencies and calculate the near and far boundaries of the zones with yshennoy error as a sum or difference respectively calculated distances to interfering reflectors and three times the value of the signal used resolution element with frequency modulation.

It is preferable to obtain the threshold value of the spectrum of the difference frequency signal as a given fraction, for example, -20 dB, from the local maximum of the specified spectrum corresponding to the reflection from the edge of the antenna of the measuring device.

Thus, when forming boundaries in the claimed method, there is a constant tracking of the same local maximum LFP corresponding to the true value of the delay time of the reference signal. Based on the obtained value of the delay time of the reflected signal corresponding to the maximum of the LFP, the distance to the surface of the medium is calculated. The local LFP maximum to be monitored is selected at the boundary of the zone with increased measurement error when this maximum is global and corresponds to the true value of the delay time. Two options are applicable to determine the phase of the reference signal. In one embodiment, the phase of the reference signal is determined by the pre-measured phase characteristic of the measuring device, taking into account the position of the material level measured in the previous measurement cycle. In another embodiment, the phase of the reference signal is determined by the phase of the reflected signal at the zone boundary with an increased measurement error near the interfering reflector.

The inventive method of measuring the level of material in the tank has a combination of features unknown from the prior art for methods of this purpose, which allows us to conclude that the criterion of "novelty".

To prove the inventive step, it is necessary to take into account that the known level measurement method [4], based on the high resolution method of the MUZIC type, allows to increase the resolution and, accordingly, the measurement accuracy in a certain area near the interfering reflection in comparison with methods based on determining the maximum position spectral component, for example [3]. However, the main reason for the lack of accuracy in the presence of interfering reflections remains - this is the mutual influence of the spectra of the useful and interfering signals, causing incorrect transmission of the ratios of the readings of the spectrum of the useful and interfering reflectors, and the presence of spurious amplitude modulation and non-linear distortions of the RMS. Due to the incorrect transmission of the amplitude ratios in the known measurement method, the useful and interfering reflectors are mixed up when the useful reflector is localized by the maximum spectral component, as a result, a large error in the measurement of distance in the form of individual error spikes occurs. Moreover, the greater the noise level, the more this phenomenon manifests itself.

The claimed method does not have this drawback, since when it changes the level inside the zone with an increased measurement error, starting from any of its boundaries, a constant tracking of the same local maximum LFP corresponding to the true position of the level is performed. The interfering reflector distorts the shape of the envelope of the local maxima of the LFP, but does not change their position on the axis of the delay time, if it does not exceed the useful reflector in amplitude. Therefore, the measurement error is significantly reduced at all points of the zone with a high measurement error near the interfering reflector, including directly near the interfering reflector, where the known method leads to increased error. Moreover, the measurement results are slightly affected by the noise level.

These differences do not follow explicitly from available scientific and technical sources, which allows us to conclude that the claimed technical solution meets the criteria of the invention - "Inventive step".

These differences lead to the appearance of a qualitatively new property of the claimed method - the ability to measure the level of the material in the presence of interfering reflections. This new property improves the accuracy of measurements.

The implementation of the claimed method is illustrated using the drawings shown in figures 1-6.

Figure 1 shows a device for measuring the level in the presence of interfering reflectors in the tank.

Figure 2 shows a block diagram of a signal processing program in the level measurement mode.

Figure 3 shows a block diagram of a signal processing program when performing training.

Figure 4 shows the dependence of the error of distance measurement based on the Fourier transform and the proposed method, obtained by modeling the measurement process on a computer.

Figure 5 shows the dependence of the error of distance measurement based on the Fourier transform and a known method based on the high resolution method MUSIK obtained by simulating the measurement process on a computer.

Figure 6 shows the errors of distance measurement based on the Fourier transform and the proposed method, obtained experimentally.

The device for measuring the level contains a driver (Ф) 1 of the signal, the output of which is connected to the input of the microwave amplifier (UHF) 2, a directional coupler (HO) 3, the output of the microwave amplifier 2 is connected to the input of HO 3, the circulator (C) 4, the input of which connected to the first output of the directional coupler 3, the antenna (A) 5 connected to the first output of the circulator 4, a mixer (Cm) 6, the inputs of which are connected to the second outputs of the directional coupler 3 and the circulator 4, and the output is connected through series-connected amplifier (Y) 7 and analog-to-digital the forming (ADC) 8 with the first input of the processor (PR) 9. The second output of generator 1 is connected to a second input of the processor 9, the first output of the processor 9 is connected to a second input of the ADC 8 and the second output of the processor is the output device.

The method of measuring the level of material in the tank is as follows. Shaper 1 generates a sounding radio frequency signal with a linear stepwise periodic frequency modulation. This signal, after amplification in the microwave amplifier 2, enters through the directional coupler 3 and circulator 4 into the antenna 5 and is emitted in the direction of the material being monitored. The proposed method of level measurement uses frequency modulation according to a triangular law. The level measurement occurs when using the MFR obtained in the interval 0 ÷ T _mod / 2, where T _mod is the period of frequency modulation. After the propagation time, the reflected signal is received by the antenna 5 and from the second output of the circulator 4 is fed to the first input of the mixer 6. The second input of the mixer 6 receives a part of the emitted signal from the second output of the directional coupler 3. The RF from the output of the mixer through the amplifier 7 is fed to the input of the ADC 8. The second input of the ADC 8 receives control pulses from the first output of the processor 9. Using the ADC, digital samples of the RMS are obtained u (k) k = 0, ..., N-1. The RMS samples are digitally fed to the first input of processor 9. A second packet of processor 9 receives a packet of synchronizing pulses corresponding to half the modulation period from the second output of former 1. The result of measuring the level of material in the tank is fed to the second output of processor 9.

, n=0, …, N-1 этого сигнала, умноженного на отсчеты весовой функции W(k), k=0, …, N-1 (например, весовой функции Блэкмана):A block diagram of the processor in the mode of measuring the level of material shown in figure 2. In block 10, the necessary variables are initialized. This is zeroing the sign of the first measurement Perv, entering from the reprogrammable ROM the number K of zones with increased measurement error and their boundaries R _{1, k} , R _{2, k} , k = 1, ..., K, which were found in the measurement (training) mode on an empty tank and a table of values φ (R _n ), n = 1, ..., L of the phase characteristic of the measuring device. In block 11, the digital samples u (k) k = 0, ..., N-1 RMS are input during one half-period of modulation. The moments of input of the RMS samples are set by the synchronizing signal supplied to the second input of the processor 9 from the shaper 1. Next, in block 12, a rough estimate of the RMS frequency is performed, for which a discrete spectrum is calculated using the fast Fourier transform

, n = 0, ..., N-1 of this signal multiplied by the samples of the weight function W (k), k = 0, ..., N-1 (for example, the Blackman weight function):

, n=0, …, N-1,

, n = 0, ..., N-1,

и соответствующее значение дискретной частоты ω_nмакс=4πn_макс/T_мод.find the reference number n _max spectral component with maximum amplitude

and the corresponding value of the discrete frequency ω _nmax = 4πn _max / T _mod .

Then make an accurate estimate of the difference frequency. To do this, calculate the continuously-discrete Fourier transform of the RMS, multiplied by the samples of the weight function:

. По полученному значению разностной частоты ω_макс вычисляют текущую i-ю оценку расстояния

до материала:vary the value of the frequency ω in the range from (ω _imax - 4π / T _modes ) to (ω _imax + 4π / T _modes ) in the vicinity of a rough estimate of the frequency ω _imax with a step Δω setting the error in the frequency estimate and find the position ω _{max of} the spectrum modulus

. Based on the obtained value of the differential frequency ω _max calculate the current i-th distance estimate

before material:

,

,

, соответствующую максимуму спектра.where c is the propagation velocity of the electromagnetic wave inside the tank; ΔF is the frequency deviation at FM, and the phase is calculated

corresponding to the maximum of the spectrum.

с границами зон с повышенной погрешностью измерения R_1,k, R_2,k, k=1, …, K и проверяют, находится ли уровень материала в одной из зон с повышенной погрешностью измерения вблизи соответствующего мешающего отражателя:In block 13, the obtained distance estimate is compared.

with the boundaries of zones with increased measurement error R _{1, k} , R _{2, k} , k = 1, ..., K and check whether the material level is in one of the zones with increased measurement error near the corresponding interfering reflector:

R _{1, k} ≤R _i ≤R _{2, k} .

, m=1, …, М, хранящимся в буфере последних результатов измерения, производится предсказание расстояния для установки границ поиска точного положения уровня материала:If the material level is within one of the zones with increased measurement error, the transition to block 14, where the sign Perv_izm of the first measurement in the zone with increased measurement error is checked. If Perv_izm = 0, which corresponds to the first measurement in the zone with increased measurement error, then in block 15 the inverse of the sign of the first measurement is Perv_izm = 1, zeroing the buffer of the last measurement results and storing the phase φ (ω _max ) corresponding to the maximum of the spectrum modulus. If the measurement in the zone of increased error is not the first, then in block 16 according to M≥2 values

, m = 1, ..., M, stored in the buffer of the last measurement results, the distance is predicted to set the search boundaries for the exact position of the material level:

,

,

- предыдущее измеренное значение расстояния, хранящееся в буфере последних измеренных результатов;

- средняя скорость изменения измеряемого расстояния; t_изм - интервал времени между двумя последовательными измерениями уровня.where R _i is the current ie the predicted distance value;

- previous measured value of the distance stored in the buffer of the last measured results;

- the average rate of change of the measured distance; t _ISM is the time interval between two consecutive level measurements.

Then, in block 17, the phase of the reference signal is set. In the first variant of the formation of the reference signal, the phase is set by interpolating the values of the phase characteristic of the measuring device taking into account the predicted distance R _i :

where R _n-1 and R _n are the argument values from the table for the phase characteristic such that R _n-1 ≤R _i ≤R _n .

In the second variant of the formation of the reference signal, the phase of the reference signal is determined by the phase of the maximum readout of the RMS spectrum φ (ω _max ) stored in block 15 at the boundary of the zone with an increased measurement error near (in the vicinity) of the interfering reflector of the tank:

ω₀=2πF₀, F₀ - известная несущая частота СВЧ-сигнала с ЧМ,Where

ω ₀ = 2πF ₀ , F ₀ is the known carrier frequency of the microwave signal with FM,

Int (·) is the operation of calculating the integer part of a number.

When conducting the first measurement when finding the phase, the distance R _{i is used} , which is found by estimating the difference frequency using the continuously discrete Fourier transform in the current measurement period.

In block 18, the amplitude of the reference signal is set approximately equal to the amplitude of the received MFR:

In block 19, the boundaries t _z1 = 2R _m1 / s, t _z2 = 2R _m2 / s of the range of variation of the delay time t _{z of the} reference signal are determined. The boundaries correspond to the delay times corresponding to two positions of the material level R _m1 , R _m2 , different from the predicted value of R _i by half the wavelength of the carrier oscillation λ ₀ :

Then, in this block, N samples of the reference signal are generated with the known frequency modulation parameters specified by the amplitude and phase and delay time t _s within [t _s1 , t _s2 ]:

k=0, …, N-1,

k = 0, ..., N-1,

the formation of LFP Λ [5]:

and varying the delay time of the reference signal t _s within the specified limits with a step δt setting the measurement error, in order to find the delay time t _zmax corresponding to the maximum of the LFP.

и запись его в буфер последних результатов измерения.In block 20, based on the found value of the delay time of the reference signal corresponding to the maximum LFP, the exact distance is calculated

and writing it to the buffer of the last measurement results.

Then, the calculation result is output in block 21 and returned to block 11 to enter a new array of RMS samples, etc. the level measurement procedure is repeated cyclically.

If the level is not in the zone with increased measurement error or it has left it, then in block 22 the characteristic of the first measurement Perv_izm = 0 is zeroed and the phase of the reference signal is set equal to the phase of the maximum spectral component φ _et = φ (ω _max ) defined in block 12, proceeding to block 18 and performing further calculations in the manner described above.

, n=0, …, N-1 сигнала СРЧ с помощью БПФ. В блоке 25 в области первых четырех дискретных спектральных составляющих спектра СРЧ осуществляется поиск локального максимума S_ант, соответствующего отражению от кромки антенны и вычисление порогового значения:The measurement process (training) on an empty tank is performed according to the block diagram of the program shown in figure 3. In block 23, the RMS is recorded on an empty tank; in block 24, the spectrum is calculated

, n = 0, ..., N-1 of the RF system using the FFT. In block 25, in the region of the first four discrete spectral components of the RMS spectrum, a local maximum S _ant corresponding to the reflection from the antenna edge is searched and a threshold value is calculated:

S _then = a S _ant ,

where a is the level of the threshold value with respect to the level of reflection from the edge of the antenna (for example, a = -20 dB).

, n=5, …, N-1, превысивших пороговое значение S_пор, определение их количества K, номеров отсчетов максимумов n_макс,k k=1, …, K и расчет дискретных частот ω_nмакс,k=4πn_мaкc,k/T_мoд, k=1, …, K, соответствующих этим максимумам.In block 26, the maximums of the spectrum are searched

, n = 5, ..., N-1, which exceeded the threshold value of S _then , determining their number K, numbers of maximum samples n _{max, k} k = 1, ..., K and calculating discrete frequencies ω _{nmax, k} = 4πn _{max, k} / T _mod , k = 1, ..., K, corresponding to these maxima.

In block 27, the discrete frequencies found are refined using the continuously discrete Fourier transform, similar to the processor’s operation in the level measurement mode, and the distances R _{mesh, k} , k = 1, ..., K, corresponding to interfering reflectors, are calculated.

In block 28, K pairs of boundaries R _{1, k} , R _{2, k} , k = 1, ..., K of zones with an increased measurement error are determined as the difference and the sum of the distances to the interfering reflectors and the triple value of the resolution element for the used FM signal:

k=1, …, K.

k = 1, ..., K.

In block 29, the obtained values of the boundaries of the zones with an increased measurement error are recorded in the reprogrammable ROM and exit the learning mode.

In the proposed method, when the level inside the zone changes with an increased measurement error near the interfering reflector, the position of the same local maximum of the LFP is constantly monitored. The capture of this maximum occurs at the boundary of the zone with an increased measurement error, when the influence of the interfering reflector is practically not significant. Therefore, at this point, the indicated maximum is global, corresponding to the true distance to the level of the material, i.e. measurement is based on the maximum likelihood method, which is optimal when measured against a background of white normal noise [5]. When a material level moves along a zone with an increased measurement error due to the influence of an interfering reflector, the LFP envelope is distorted, as a result, the captured maximum is no longer global, but its position still corresponds to the true material level position, provided that the interfering reflection does not exceed the useful amplitude signal. This fact can significantly reduce the error in measuring the level of the material.

Modeling the level measurement process showed the high efficiency of the proposed method for measuring the level of material in the tank. So, figure 4 shows the dependence of the measurement error of the distance inside the zone with increased measurement error near the interfering reflector for two measurement methods. In figure 4, the arrow indicates the position of the interfering reflector. A thin solid line shows the error for a method based on the Fourier transform. The thick solid line shows the error for the proposed method. All graphs were obtained under the same measurement conditions for a carrier frequency of 10 GHz, a frequency deviation of 1 GHz, a noise level of 50 dB, and the formation of 1024 samples of a modeled RPS.

In figure 5, the arrow still indicates the position of the interfering reflector. A thin solid line shows the error for a method based on the Fourier transform. The thick solid line shows the error for the known method. At a certain distance from the interference, the known method really allows to significantly reduce the measurement error. However, there is a zone near the interfering reflector in which localization of the useful reflector by the maximum spectral component in the MUSIK method leads to a confusion of the useful and interfering reflectors due to incorrect transmission of the ratio of the intensities of the spectral components of these reflectors in the calculation of the spectrum. As a result, large sharp throws of the distance measurement error are observed, significantly exceeding the error provided by the measurement method based on the Fourier transform.

A comparison of FIGS. 4 and 5 shows that the proposed measurement method provides a significant reduction in the measurement error near the interfering reflector in comparison with known methods.

Figure 6 shows the error of distance measurement obtained experimentally on a real, commercially available level gauge with the above FM parameters. The thin line shows the error for the method based on the Fourier transform, and the thick solid line shows the error for the proposed method. Figure 6 also shows the undoubted advantage of the proposed method of level measurement.

Information sources

1. Theoretical foundations of radar. / Ed. Y.D. Shirman M., Sov. Radio, 1970.

2. Marfin V.P., Kuznetsov F.V. Microwave level gauge. // Devices and control systems. 1979, No. 11. S.28-29.

3. RF patent No. 2234717, G01S 13/34, 03/04/2003.

4. US patent 5504490. MKI G01S 13/08.

5. Tikhonov V.I., Kharisov V.N. Statistical analysis and synthesis of radio engineering devices and systems. - M .: Radio and communications, 1991 .-- 608 p.

Claims (11)

1. The method of measuring the level of material in the tank, including the formation of a probing signal of the radio frequency range with a linear stepwise periodic frequency modulation with known values of the carrier frequency and frequency deviation, the radiation of the generated probing signal in the direction of the probed material, receiving the reflected signal after its propagation time, mixing the reflected signal with part of the probing signal, extracting the difference frequency signal, obtaining the samples drove the difference frequency using an analog-to-digital converter, calculating by the processor using the fast Fourier transform of the spectrum of the product of the samples of the difference frequency signal by the samples of the weight function, for example, the Blackman function, rough determination of the distance to the material from a preliminary estimate of the frequency of the maximum spectral component, refinement of the specified distance by estimation of the difference frequency corresponding to the maximum spectral component found by calculating the continuously discrete transform Fourier transform and variation of the difference frequency within the same frequency discrete spectrum in the vicinity of a preliminary estimate of the frequency of the maximum spectral component in both directions, characterized in that they determine the boundaries of the zones with increased measurement error in an empty tank, the processor additionally generates a reference signal in the form of digital samples, according to the known values of the carrier frequency of the probing frequency-modulated signal and its frequency deviation, as well as the specified values of the delay time e the coupon signal, its amplitudes and phases determine the logarithm of the likelihood function, finding the negative mean square of the difference between the received samples of the differential frequency signal and the samples of the generated reference signal, find the exact value of the delay time of the reference signal by maximizing the obtained value of the logarithm of the likelihood function by converting the shape of the reference signal by using different time values delays of the reference signal within predetermined predicted limits and calculate and measure the material level found by the exact value of the delay time corresponding to a local maximum of the logarithm of the likelihood function. 2. The method according to claim 1, characterized in that the predetermined predicted limits of the delay time of the reference signal are equal to the delay times of the reflected signal, corresponding to two boundaries of the measured level, differing from the level predicted from the previous measured values, by no more than half wavelength of the carrier oscillation of the probe signal in both directions. 3. The method according to claim 2, characterized in that the level value is predicted by at least two previous measurement results, taking into account the rate of change. 4. The method according to claim 2, characterized in that during the first measurement, the predicted value of the level position is taken equal to the current value obtained based on the Fourier transform. 5. The method according to claim 1, characterized in that the amplitude of the reference signal is equal to the amplitude of the difference frequency signal. 6. The method according to claim 1, characterized in that during the first measurement, the phase of the reference signal is determined by the pre-measured phase characteristic of the measuring device, taking into account the distance to the material found using the continuous-discrete Fourier transform in the current measurement period. 7. The method according to claim 1, characterized in that the phase of the reference signal is determined by the previously measured phase characteristic of the measuring device, taking into account the previous measured distance to the material. 8. The method according to claim 1, characterized in that during the first measurement, the phase of the reference signal is determined at the boundary of the zone with increased measurement error near the interfering reflector of the tank, by the phase of the maximum spectral component of the spectrum of the difference frequency signal, found when refining the distance to the material using continuous-discrete Fourier transform, reduced by an amount equal to the difference of the total phase incursion of the reflected signal at the carrier frequency during the propagation of the specified signal and the whole the number of periods of the reflected signal that fit into the full phase incursion, and the full phase incursion is determined by the known carrier frequency of the probe signal and the reflected signal delay calculated by the position of the material level obtained using the continuously discrete Fourier transform for the current measurement period. 9. The method according to claim 1, characterized in that the phase of the reference signal is determined at the boundary of the zone with increased measurement error near the interfering reflector, by the phase of the maximum spectral component of the spectrum of the difference frequency signal, found at the stage of accurate frequency estimation using a continuously discrete Fourier transform , reduced by an amount equal to the difference between the total phase incursion of the reflected signal at the carrier frequency during the propagation of the signal and an integer number of periods of the reflected signal, falling in full on phase ege, the full phase shift is determined from the known carrier frequency of the emitted signal and the reflected signal delay calculated according to the position of the level of the material, measured at the previous measurement period. 10. The method according to claim 1, characterized in that to determine the boundaries of the zone with increased error, record the difference frequency signal on an empty tank, calculate the spectrum of the difference frequency signal using a fast Fourier transform, find the frequencies corresponding to all local spectrum maxima that exceed a predetermined threshold value, the frequencies of the local extrema found are refined using the continuously discrete Fourier transform, the distance to the interfering reflectors is calculated from the found frequencies, and the near and the far boundary of zones with increased error as, respectively, the difference or the sum of the calculated distances to the interfering reflectors and the triple value of the resolution element of the used signal with frequency modulation. 11. The method according to claim 10, characterized in that the threshold value of the spectrum of the differential frequency signal is obtained as a given fraction, for example, 20 dB, from the local maximum of the specified signal spectrum corresponding to reflection from the edge of the antenna of the measuring device.

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