RU2658558C1 - Method for measuring a distance to a controlled environment with a waveguide lfm radar - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention relates to the equipment of industrial level gages using the principle of radiation into a waveguide of a frequency-modulated signal by symmetric triangular law, obtaining a difference frequency signal by mixing reflected and emitted signals. Processing the difference frequency signal includes determining the number of zero values of the difference frequency signal in the half-period of the modulation, determining the radiation frequency corresponding to each zero of the difference frequency signal, for which digital modulation is used. Based on the obtained data, taking into account the size of the waveguide and the modulation parameters according to the above formula, the distance is calculated. In the simplest case, to calculate the distance, is enough to know the number of zeros, radiation frequencies of the first and last zero, and a carrier frequency.

EFFECT: absence of the effect of dispersion of the waveguide on the measured distance and the accompanying simplification of the functional scheme of a meter, reduction of errors arising when applying pre-distortions and potentially other possible methods to counter dispersion.

1 cl, 3 dwg

Description

The invention relates to techniques for industrial level gauges using the location principle of measuring the distance to a controlled environment. The waveguide version of the level gauge is used to measure the level of a liquid medium. The medium enters a vertically mounted waveguide through appropriately made perforations. The frequency modulation of the probe signal (chirp modulation) is used, the measured distance is determined by the difference frequency between the probe and reflected signals [1].

The waveguide version of the level gauge has several advantages:

- the emitted and reflected signal propagate in the enclosed space of the waveguide, which eliminates interference due to reflections from the elements of the capacitance containing the controlled medium;

- the absence of scattering into the space of electromagnetic waves can reduce the power level of the transmitting module, which also improves the noise environment. There are other advantages.

The main disadvantages of waveguide level gauges include:

- the dependence of the signal delay on the frequency (the presence of dispersion), which leads to a change in the difference frequency, and therefore, to significant errors in the measurement of the level of the controlled medium;

- for many media, sticking to the walls of the waveguide is observed, which also leads to errors in measuring the level of the controlled medium.

This invention only considers the effect of dispersion under the assumption that there is no sticking. The weakening of the dispersion effect is possible by introducing a pre-emphasis into the law of frequency modulation (FM) of a microwave signal [2]. This method is an analogue of the invention, however, the necessary pre-emphasis in [2] depends on the distance along the waveguide to the product, the cross section of the waveguide, and a number of other factors. This leads to a complication of the procedure of the emitted signal, and, in addition, it will be accompanied by additional errors, since in real conditions it is impossible to perform ideal predistortion.

Therefore, the problem arises of creating a method for determining the distance to a controlled medium in a waveguide free from the dispersion problem.

This method, the prototype of which is [3], is proposed in this invention and involves calculating the distance according to the following formula:

- критическая частота волновода;

- несущая частота;

- частоты в моменты k-го и первого нулей СРЧ; с - скорость света в вакууме.where k is the number of zeros of the differential frequency signal (RMS) in the observation interval equal to the half-period of modulation;

- critical waveguide frequency;

- carrier frequency;

- the frequencies at the moments of the kth and first zeros of the RMS; c is the speed of light in vacuum.

(свободное пространство) формула (1) переходит в расчетную формулу в [3].It should be noted that with

(free space) formula (1) goes into the calculation formula in [3].

Expression (1) is obtained based on the following analysis.

The signal emitted into the waveguide has the form:

u (t) = U ₀ cosω (t) t.

The signal at the mixer when reflected from a medium located at a distance R by the waveguide is determined by the formula:

u _c (t) = U _c0 cos (ω (t-τ _З ) t + ϕ),

- время задержки.where ϕ is a certain phase shift;

- delay time.

After the mixer, the RF system takes the following form:

u _P (t) = U _P0 cos ((ω (t) -ω (t-τ _З )) t-ϕ).

In this case, the difference frequency

- период симметричной треугольной модуляции; ΔF - диапазон перестройки частоты.Where

- period of symmetric triangular modulation; ΔF - frequency tuning range.

Differential frequency phase

At those points where the RMS is zero,

where n = 1, 2, ..., k, k is the number of zeros of the RMS at the half-period of modulation.

. При линейной ЧМ связь между ними задается формулойEach n value in (3) corresponds to its own pair of values

. In linear FM, the relationship between them is given by the formula

In view of (4) and (3), expression (2) is written as

We take n = 1, then

Assuming n = k in (5) and subtracting (6), we arrive at (1).

Obviously, when determining R, the calculation can be carried out not by the extreme zeros of the RMS - (1, k), but by some of their intermediate values - (l, n), where 1≤l <n, n≤k. The need for this option may be caused by interference.

In this more general case, the calculation of the distance to the controlled environment should be carried out according to the formula

Obviously, formula (1) provides greater accuracy than (7) if there is no concentrated interference.

, где

- измеренное расстояние, для круглого волновода диаметром 25 мм с волной Н₁₁ и симметричной треугольной модуляционной характеристики с периодом модуляции

при различных значениях параметров измерителя.In accordance with (1), the relative distance measurement error was calculated

where

- measured distance, for a circular waveguide with a diameter of 25 mm with a wave of H ₁₁ and a symmetrical triangular modulation characteristic with a modulation period

at different values of the parameters of the meter.

In the calculations, the RMS was set in the form

- время задержки отраженного сигнала; ϕ - произвольная фаза; n(t_j) - аддитивный белый гауссовский шум.Where

- delay time of the reflected signal; ϕ is an arbitrary phase; n (t _j ) is the additive white Gaussian noise.

N=2000; 4000, несущая частота

, критическая частота

, девиация частоты ΔF=300; 500; 800 МГц, величина ϕ варьировалась в пределах [0÷π), усреднение в точках осуществлялось по 10⁶ значениям.The calculations were carried out by the method of statistical tests with the following parameters of the probing signal: the number of samples of the RMS in the analysis interval

N = 2000; 4000 carrier frequency

critical frequency

, frequency deviation ΔF = 300; 500; 800 MHz, ϕ varied within [0 ÷ π), averaging at points was carried out over 10 ⁶ values.

The calculation results are shown in FIG. 1 and FIG. 2. The dependencies in FIG. 1 indicate that there is no effect of dispersion on determining the distance in accordance with formula (1), since the value of ER is in the range of (10 ^-2 ÷ 3⋅10 ^-3 )% and has a clearly methodological nature, decreasing with increasing N. The dependences given by FIG. 2, do not contradict the conclusions made above.

The proposed method for measuring distance in accordance with (1) and (7) for a waveguide FM level gauge is not known for methods and devices, which implies compliance with its “novelty” criterion.

The inventive step is determined by the main property of the proposed method - it is free from the influence of dispersion on the error in determining the distance to a controlled environment, which in the presence of dispersion can be tens of percent, which is unacceptable. The absence of the dispersion effect eliminates the need to neutralize it with the help of special methods, for example, by introducing predistortions into the probe signal, which complicates the meter’s functional circuitry and leads to additional errors. The absence of dispersion effect persists when the frequency deviation and the waveguide length change (Fig. 1). This indicates the generality of the proposed method.

Based on the foregoing, it can be argued that the inventive method meets the criterion of "inventive step".

A possible structural diagram of the implementation of the proposed method is shown in FIG. 3. The designated blocks perform the following functions: 1 - waveguide; 2 - circulator; 3 - digital frequency synthesizer; 4 - microwave transceiver module; 5 - master oscillator; 6 - microprocessor; 7 - block analog processing RMS (amplification, filtering, limitation); 8 - output unit.

Bibliographic data

1. B.A. Atayants, V.M. Davydochkin et al. Precision short-range frequency radar systems for industrial applications. M .: Radio engineering. 2012.

2. B.A. Atayants, V.M. Davydochkin, V.V. Jezersky. Accuracy of level measurement by a waveguide frequency-modulated level gauge. // Radio engineering. 2015. No. 5, S. 73-78.

3. RF patent No. 2436117, MKI G01S 13/34. Publ. 12/10/2011. Bull. Number 34.

Claims (3)

A method of measuring the distance to a controlled medium, including radiation in a hollow waveguide immersed in a liquid medium, an FM signal modulated in frequency according to a periodic symmetric triangular law implemented by a digital frequency synthesizer, receiving a signal reflected from the medium, receiving a difference frequency signal by mixing the received and emitted signals, measurement of the number of zero values of k signal of the differential frequency at each half-period of modulation, calculation of frequencies

corresponding to the selected zero values of the differential frequency signal, characterized in that the measured distance is determined by the formula:

Where

- critical waveguide frequency;

- carrier frequency;

- frequencies corresponding to the selected zero points, 1≤l <n, n≤k; c is the speed of light in vacuum.

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