RU2518373C1 - Radar level gauge - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: radar level gauge comprises a highly stable generator 1, frequency dividers 2 and 3, a controller 4, a sawtooth-voltage generator 5, a modulator 6, a transceiving module 7, a directional coupler 8, an antenna 9, narrow-band filters 10, 11 and 12, shaping amplifiers 13 and 14, mixers 15 and 16 and a differential frequency filter 17.

EFFECT: high measurement accuracy.

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Description

A radar level gauge refers to radio engineering and can be used to build high-precision level meters for liquids or bulk solids in tanks and altimeters of small heights.

Known radar range finder described in the book [G.B.Belotserkovsky. Basics of radar and radar devices. - M .: Owls. Radio. - 1975, p.77] and a 24 GHz radar level transmitter, given in the Istok GNIP informational and advertising collection [Microwave Technology News - No. 12, 2000], as well as a radar station given in Japanese application 30-1591 MKI ⁵ G01S, 13/34 [Ref. Magazine "Inventions of the World". - 1985, No. 115, p.29].

Each of these devices contains a serially connected sawtooth voltage generator, a linear frequency modulated microwave generator, a fixed frequency generator, mixers, an antenna and a controller. These elements are also part of the inventive device.

The work of all three of these analogues is based on radiation in the direction of the reflecting surface of the microwave signal with linear frequency modulation, receiving the reflected signal, mixing it with the emitted signal, generating a difference frequency signal (beat frequency), and determining the frequency of the latter distance from the radiation point to the reflecting surface .

The reason that prevents the achievement of the technical result provided by the invention in these analogs is the relatively low measurement accuracy due to the discreteness of measurement of the beat frequency in the first analogue and the instability of the frequency deviation, leading to a change in the slope of the modulation characteristic in the other two.

Also known is a radar range finder, protected by RF patent No. 2151408, G01S 13/34, 2000, containing serially connected control circuit, sawtooth voltage generator, analog multiplier, modulator, transmitter-receiver module, directional coupler and antenna, as well as frequency label generator connected to the second output of the directional coupler, the outputs of which are connected to the inputs of the control circuit and the discriminator of the time intervals, and the output of the latter through the integrator is connected to the second input of the multiplier I am. In this case, the second output of the transmitting and receiving module through the difference frequency amplifier is connected to the inputs of the frequency meter and the extremum extraction circuit, the output of which is connected to the third input of the control circuit.

Signs common with the claimed level gauge in this analog are a sawtooth voltage generator, a modulator, a receiving and transmitting module, a directional coupler, an antenna, a control circuit (in the claimed level gauge the controller performs its functions) and a difference frequency amplifier.

In this analogue, with the help of a frequency label generator, a control circuit, and a time interval discriminator, the change in the slope of the modulation characteristic is somewhat reduced, and the error is somewhat reduced due to the instability of frequency deviation. The time interval discriminator, integrator and multiplier eliminate phase jumps of the difference frequency signal. This makes it possible to measure the beat frequency not in one but in several cycles of changing the frequency of the probe signal, which reduces the discreteness error.

The reasons that impede the achievement in this analogue of the technical result provided by the invention are as follows.

A certain increase in accuracy was ensured by the use of an automatic control system, which required the introduction of a frequency label generator, a time interval discriminator, a multiplier, an integrator, and an extremum extraction circuit, which complicated the range finder.

In addition, the filters included in the frequency label driver negatively affect the accuracy of stabilization of the frequency deviation, and therefore the accuracy of the range measurement.

The closest in technical essence to the claimed (prototype) is a radar level gauge, protected by the RF patent for utility model No. 32287 G01S 13/34, 2003 and described in the article [Chekrygina IM, Bailov VV, Chepelev V. AND. A way to improve the accuracy of radar liquid level meters. - Questions of special radio electronics. Ser. OVR. - Moscow-Taganrog. - 2001, issue 2. - p. 153-157].

It contains a highly stable generator, a controller, a sawtooth voltage generator, a modulator, a receiving and transmitting module, a directional coupler and an antenna, a differential frequency signal driver, the input of which is connected to the output of the receiving and transmitting module, and the output to the second input of the controller, and a frequency label driver, the first input of which is connected to the output of the directional coupler, the second input to the second output of the highly stable generator, and the output to the third input of the controller pa. In this case, the frequency label generator comprises a mixer, a harmonic generator and a harmonic filter. A narrow-band filter and an amplitude detector, the first input of the mixer is the input of the shaper, the second is connected through series-connected harmonics and the first narrow-band filter to the second input of the shaper, and the output is connected through a series-connected second narrow-band filter and amplitude detector connected to the output of the shaper.

All elements included in the prototype device. With the exception of the harmonic generator and amplitude detector, which are part of the shaper frequency labels, are included in the inventive level gauge.

The operation of the prototype device, like the work of all the other analogues mentioned, is based on radiation in the direction of the reflecting surface of the microwave signal with linear frequency modulation, receiving the reflected signal, mixing it with the emitted signal, generating a difference frequency signal (beat frequency) and determining the beat frequency the distance from the point of emission and reception of the reflected signal to the reflective surface. A feature of the prototype device is the fact that it uses the frequency label generator to determine not only the theoretical, but also the actual time shift between the moments of formation of the minimum and maximum linearly varying frequencies, which is then converted into the actual slope of the modulation characteristic of the device. This allows to a certain extent to reduce the measurement error of the tuning time of the frequency-modulated signal, and therefore, to increase the accuracy of measuring the distance from the antenna to the controlled surface.

The reason that impedes the achievement in the prototype device of the technical result provided by the invention is the fact that the time shift between the beginning and the end of the frequency tuning is not determined accurately enough. The fact is that the moments of the formation of pulses corresponding to the beginning and end of frequency tuning are determined with a significant error equal to the passband of the filter connected in series with the amplitude detector. In this case, this band should be of the order of 1 MHz, otherwise the signal of the amplitude detector will not have time to form at all. This substantially reduces the final measurement accuracy. Thus, the measurement accuracy in the prototype device remains relatively low.

The technical problem to which the invention is directed is to increase the accuracy of measurement.

The technical result is achieved by the fact that a second frequency divider, a second mixer, a differential frequency filter, a second driver amplifier and a third narrow-band filter are introduced into the known radar level gauge, the first narrow-band filter is connected by its input to the output of the highly stable generator and to the input of the second frequency divider, and the output - with the second input of the first mixer, the input of the third narrow-band filter is connected with its input to the output of the second frequency divider, and the output is with the second input of the second mixer, per the output of which is connected to the output of the second narrow-band filter, and the output with the input of the differential frequency filter, the second amplifier-driver is connected between the output of the differential frequency filter and the third input of the controller.

To achieve a technical result, in a known radar level gauge comprising a highly stable generator, a first frequency divider, a controller, a sawtooth voltage generator, a modulator and a transmitting and receiving module, a directional coupler, the first input-output of which is connected to the input-output of the transmitting-transmitting module, the antenna, the input-output of which is connected to the second input-output of the directional coupler, the first narrow-band filter, the first amplifier-driver, the input of which is connected n with the output of the transmitter-receiver module, the first mixer, the first input of which is connected to the output of the directional coupler, and the second narrow-band filter, the input of which is connected to the output of the first mixer, a second frequency divider, a second mixer, a difference frequency filter, a second driver amplifier and the third narrow-band filter, the first narrow-band filter is connected by its input to the output of the highly stable generator and the input of the second frequency divider, and by the output - from the second input of the first mixer, the input of the third narrow-band of the filter is connected by its input to the output of the second frequency divider, and by the output from the second input of the second mixer, the first input of which is connected to the output of the second narrow-band filter, and the output is connected to the input of the filter of the differential frequency, the second amplifier-driver is connected between the output of the filter of the differential frequency and the third controller input.

The totality of the newly introduced frequency divider, narrow-band filter, difference frequency filter, mixer and driver amplifier and new connections is unknown from the information sources available to the applicant. Therefore, the inventive radar level gauge should be considered new and consistent with the inventive step.

The invention is illustrated in the drawing, which shows:

- figure 1 is a structural diagram of the inventive level gauge;

- figure 2 is a graph of the dependence of the control signal on time at the input of the modulator.

The radar level gauge contains a highly stable generator 1, the first and second frequency dividers 2 and 3, a controller 4, a sawtooth voltage generator 5, a modulator 6, a transmit-receive module 7, a directional coupler 8, an antenna 9, narrow-band filters 10, 11 and 12, amplifiers shapers 13 and 14, mixers 15 and 16, and a differential frequency filter 17.

Generator 1, divider 2, controller 4, generator 5, modulator 6 and module 7 are connected in series. The first input-output of the coupler 8 is connected to the input-output of the module 7, the second input-output to the antenna 9, and the output to the first input of the mixer 15. The amplifier-driver 13 is connected between the output of the module 7 and the second input of the controller 4, the third input of which through the series-connected filter 17 and the amplifier-driver 14 are connected to the output of the mixer 16. The output of the generator 1 through the filter 10 is connected to the second input of the mixer 15, and through the series-connected divider 3 and filter 12 to the second input of the mixer 16. The filter 11 is connected between the output mix I'm 15 and the second input of the mixer 16.

The work of the proposed radar level gauge is as follows.

The generator 1 is a highly stable pulse generator of a fixed microwave frequency f _g . The frequency f _{g is} exactly equal to the frequency corresponding to the middle of the tuning range formed in the level meter of the microwave signal with linear frequency modulation (LFM). It amounts to about 850 MHz. From the output of the generator 1, the pulses generated by it are fed to the inputs of the frequency dividers 2 and 3 and the filter 10.

In divider 2, the pulses received at its input are divided by a frequency of approximately 8–12 times, and the result of dividing by a frequency of the order of 10 ⁸ Hz is transmitted to the first input of controller 4 as clock pulses of the reference frequency.

The controller 4 together with the generator 5 form from the clock pulses a control sawtooth signal to control the linear frequency modulation (figure 2). In addition, the controller 4 performs the functions of a calculator when measuring the beat frequency and calculating a controlled level, which will be discussed below.

The controller 4 generates a control signal in digital form according to a given program based on the current count of clock pulses and the corresponding change in direction, speed or suspension of the count. Generator 5 is essentially a high-speed digital-to-analog converter and converts the control signal from digital to analog (FIG. 2).

The control cycle includes two main sections. The first is smoothly linearly increasing, lasting about 2 ms, the second is sharply falling. Each of these sections additionally contains a section of a constant level, lasting about 10 μs.

Under the action of the control signal from the output of the generator 5 and the modulator 6, the frequency of the microwave generator, which is part of module 7, is linearly tuned.

From the input-output of the chirp module 7, the signal is fed to the coupler 8, from the second input-output of which the main part of its power is transmitted through the antenna 9 in the direction of the reflecting surface. From the output of the coupler 8, a small part of this power is supplied to the first input of the mixer 15.

The signal reflected from the liquid surface, which is late for a time, proportional to the distance from the antenna 9 to the reflecting surface, through the antenna 9, the coupler 8 is fed to the microwave input-output of module 7. In the module 7, a difference frequency signal is generated that does not exceed tens of kilohertz (signal the beat frequency F _b ), which is filtered by the filter included in module 7 and from the output of module 7 through the amplifier-driver 13 is fed to the second input of the controller 4. In the amplifier-driver 13, the frequency signal F _b It is limited and in amplitude, turning essentially into a rectangular pulse of the meander type.

Upon receipt of a beat frequency signal F _{b of the} meander wavelength at the second input of controller 4, the last one measures the period of this signal by counting the number of clock pulses that fit in the total number of beat signal periods. Given the high frequency of clock pulses, we can conclude that the discreteness error in measuring the beat frequency is extremely small.

The described process of generating a sounding microwave LFM signal, receiving a reflected signal, generating a difference frequency signal and measuring it is similar to the corresponding process in the prototype device. A significant difference is only in the form of a control signal (figure 2). Since the frequency of the clock pulses in the control of the inventive device is an order of magnitude higher than in the prototype device, the accuracy of measuring the beat frequency in the proposed level meter is not lower than in the prototype.

The distance h from the radiation point of the probe signal to the reflective surface is calculated in the proposed device, as in the prototype device according to the formulas:

$$h=\frac{C\cdot T\cdot F_b}{2\Delta f};\left(\mathrm{one}\right)$$

$$\Delta f=f_{\mathrm{at}}-f_n,\left(2\right)$$

where C = 3 · 10 ⁸ m / s is the speed of light.

f _n and f _in - respectively, the smallest and largest values of the frequency in the process of its linear change (figure 2);

T is the time of a linear increase in frequency during one cycle in the range from f _n to f _in ;

Δf is the frequency deviation.

The time interval T has a value of the order of 2 ms. It is formed using a controller 4 of clock pulses having a high frequency and stability. Since the frequency of these pulses is higher than in the prototype device, the accuracy of determining the parameter T in the inventive level gauge is not lower than in the prototype device.

The main source of error in the level-based altimeters use probing chirp signal is the presence of differences actual parameters f _n and f _in from the calculated. To minimize these differences, the proposed level gauge used suspension linear frequency variation over short intervals of duration τ of 10 microseconds (Figure 2) and the order of the actual measurement values of frequencies f _n and f _in these intervals by the divider 3, the filters 10, 11, 12 and 17, mixers 15, 16, amplifier-driver 14 and controller 4.

This is as follows.

The frequency of the generator 1 is chosen exactly equal to the middle of the range of linear changes in the frequency of the probing signal. We take for definiteness f _g = 850 MHz; f _n = 700 MHz; f _in = 1000 MHz; Δf = 300 MHz. The selected values are calculated. In fact, they may differ from the calculated (theoretical) ones. For definiteness, we assume that these differences δ _e are within ± 2 MHz, that is, | δ _f | ≤2 MHz.

The change in each of the actual frequencies f _n and f _c is controlled by the controller 4. In the time interval τ, when the carrier frequency remains at the level f _n with an error not exceeding | δ _f |, the actual frequency f _n is measured. From the output of the coupler 8, a signal with a frequency of f _n = (700 ± 2) MHz that does not change over time τ is supplied to the first input of the mixer 15. Using the mixer 15 and filters 10 and 11, this signal is transferred to an intermediate frequency of 150 MHz, exactly equal to half deviations Δf.

Filter 10 is tuned to a frequency of 850 MHz and is quite narrowband. It passes to its output and the second input of the mixer 15 only the first harmonic of the pulse signal of the generator 1 supplied to its input. The signal filtered by the filter 10 is the local oscillator in the mixer 15. In the mixer 15, signals are formed equal to the sum and frequency difference of the input signals.

Filter 11, like filter 10, is narrow-band. It is tuned to an intermediate frequency f _pr ≈150 MHz. The signal of the total frequency of the order of 1550 MHz is suppressed by this filter, and the signal of the difference frequency equal to (150 ± 2) MHz is amplified and fed to the first input of the mixer 16.

Divider 3 has such a division coefficient - an integer at which the division result differs from the intermediate frequency of 150 MHz by a value of the order of 10 MHz. For example, with the adopted oscillator frequency of 1850 MHz and the division ratio of divider 3 equal to 6, the division result is 141.667 MHz, which differs from the value of the intermediate frequency f _pr by 8.333 MHz.

This signal subsequently serves as a reference signal. Denote its frequency f _op = 141.667 MHz.

Filter 12 is narrow-band and tuned to this frequency. It passes to its output and to the second input of the mixer 16 only the first harmonic of the pulse signal with a frequency f _op . The signal filtered by the filter 12 is the local oscillator in the mixer 16.

In the mixer 16, signals are formed with frequencies equal to the sum and frequency difference of the input signals.

, ориентировочно находящийся в пределах (6÷12) МГц, усиливается и поступает с выхода фильтра 17 на вход усилителя-формирователя 14. Этот сигнал в дальнейшем будем называть сигналом высокочастотных биений.The filter 17 is tuned to a center frequency equal to the difference between the intermediate f _pr and the reference f _op frequencies, which is approximately 8 MHz. Its bandwidth is about 6 MHz. The signal of the total frequency of the order of 300 MHz is suppressed by this filter, and the signal of the difference frequency $$F_{b_n}$$

, approximately within the range (6 ÷ 12) MHz, is amplified and fed from the output of the filter 17 to the input of the amplifier-former 14. This signal will be referred to as a high-frequency beat signal.

сигнал высокочастотных биений с помощью усилителя-формирователя 14 усиливается и ограничивается по амплитуде, превращаясь по существу в сигнал типа "меандр", который поступает с выхода усилителя-формирователя 14 на третий вход контроллера 4. В последующем осуществляется измерение частоты $$F_{{б}_{н}}$$

путем подсчета числа импульсов тактовой частоты, умещающихся в целом числе полупериодов сигнала высокочастотных биений. Это число легко пересчитывается в частоту $$F_{{б}_{н}}$$

. Учитывая частоту тактовых импульсов 10⁸ Гц, можно сделать вывод, что точность измерения частоты $$F_{{б}_{н}}$$

достаточно высока.To measure the frequency $$F_{b_n}$$

the signal of high-frequency beats with the help of the amplifier-driver 14 is amplified and limited in amplitude, turning essentially into a signal of the "meander" type, which is supplied from the output of the amplifier-driver 14 to the third input of the controller 4. Subsequently, the frequency is measured $$F_{b_n}$$

by counting the number of clock pulses that fit in the whole number of half-periods of the high-frequency beat signal. This number is easily converted to frequency $$F_{b_n}$$

. Given the frequency of clock pulses of 10 ⁸ Hz, we can conclude that the accuracy of the frequency measurement $$F_{b_n}$$

high enough.

, то есть значение частоты f_н с учетом погрешности δ_f связано с частотой $$F_{{б}_{н}}$$

соотношением:Actual Frequency Values $$f_{n_f}$$

, that is, the value of the frequency f _n taking into account the error δ _f associated with the frequency $$F_{b_n}$$

ratio:

$$f_{n_f}=f_g-f_{\mathrm{about}P}-F_{b_n}.\left(3\right)$$

определяется по существу точностью измерения частоты $$F_{{б}_{н}}$$

, поэтому является достаточно высокой.Frequencies f _g and f _op - fixed values. The signals of these frequencies are formed by a highly stable generator 1 and divider 3 and practically do not change during the operation of the level gauge. Therefore, the accuracy of frequency measurement $$f_{n_f}$$

determined essentially by the accuracy of the frequency measurement $$F_{b_n}$$

, therefore, is quite high.

, то есть значения частоты f_в с учетом погрешности δ_f осуществляется аналогично определению значения $$f_{{н}_{ф}}$$

. Разница лишь в том, что частота $$F_{{б}_{в}}$$

сигнала высокочастотных биений измеряется в короткий промежуток времени τ, когда частота зондирующего сигнала остается на уровне f_в с погрешностью, не превышающей по модулю 2 МГц, а частота $$f_{{в}_{ф}}$$

определяется из соотношения:Determination of the actual frequency value $$f_{{\mathrm{at}}_f}$$

, that is, the value of the frequency f _in taking into account the error δ _{f is} carried out similarly to determining the value $$f_{n_f}$$

. The only difference is that the frequency $$F_{b_{\mathrm{at}}}$$

signal of high-frequency beats is measured in a short period of time τ, when the frequency of the probing signal remains at the level f _in with an error not exceeding 2 MHz modulo, and the frequency $$f_{{\mathrm{at}}_f}$$

determined from the ratio:

$$f_{{\mathrm{at}}_f}=f_g-+f_{\mathrm{about}P}+F_{b_{\mathrm{at}}}.\left(\mathrm{four}\right)$$

Thus, in the proposed level gauge, the actual values of the frequencies f _n and f _{c are} determined with sufficiently high accuracy, as well as the frequency F _{b of the} beats, which means that their difference f _in -f _n used in formula (1) when calculating the distance is also determined with high accuracy h from the point of emission of the probe signal to the reflective surface.

The radiation point of the probe signal is located on the edge of the tank with liquid or granular matter, the level of which is subject to control.

The level of the controlled substance is determined from the ratio:

$$\mathrm{At}={\mathrm{At}}_{\max }-h,\left(5\right)$$

where U _max - the maximum possible level of the controlled substance, that is, the height of the tank.

и $$F_{{б}_{в}}$$

, которая при частоте тактовых импульсов в контроле 4 порядка 10⁸ Гц крайне незначительна.From the expressions (1) ÷ (5) it is seen that the accuracy of measuring the level V is determined by the accuracy of measuring the frequency F _b beats, and the latter is determined by the accuracy of measuring the actual frequency difference f _in -f _n . It is shown above that the accuracy of determining the frequency F _b beats in the proposed level meter is not lower than in the prototype device. The same can be said about determining the level of U. In the proposed device, the only source of error in determining the difference in frequencies f _in and f _n is the sampling error in determining frequencies $$F_{b_n}$$

and $$F_{b_{\mathrm{at}}}$$

, which at a frequency of clock pulses in control 4 of the order of 10 ⁸ Hz is extremely insignificant.

In the prototype device, the actual frequencies f _in and f _{n can} either not be determined at all and in formulas (1) and (2) the theoretical values of frequencies f _in and f _n are used to calculate the distance h, or they are determined with a large error. It is due to the fact that, in this device, in addition to the sampling error when measuring the time shift between the moments of the beginning and end of frequency tuning, as noted above, there is a large error due to the fact that the moment of pulse formation corresponding to the moment of establishment of the frequencies f _n and f _in are determined with a large error. This allows us to conclude that the final measurement result, the controlled level of the substance in the tank in the proposed level gauge, is determined with greater accuracy than in the prototype.

The proposed level gauge is quite easy to implement. As a highly stable generator 1, frequency dividers 2 and 3, controller 4, sawtooth generator 5, modulator 6, transmit-receive module 7, directional coupler 8, antenna 9, narrow-band filters 10, 11 and 12, amplifier shapers 13 and 14 Mixers 15 and 16 can be used elements corresponding to the elements of the device of the prototype.

Claims (1)

A radar level gauge containing a highly stable generator, a first frequency divider, a controller, a sawtooth generator, a modulator and a transmitting and receiving module, a directional coupler, the first input-output of which is connected to the input-output of the transmitting and transmitting module, an antenna, the input-output of which connected to the second input-output of the directional coupler, the first narrow-band filter, the first driver amplifier, the input of which is connected to the output of the transmitter-receiver module, and the output is connected inen with the controller input, the first mixer, the first input of which is connected to the output of the directional coupler, and the second narrow-band filter, the input of which is connected to the output of the first mixer, characterized in that a second frequency divider, a second mixer, a difference frequency filter, and a second amplifier are introduced the former and the third narrow-band filter, the first narrow-band filter is connected by its input to the output of the highly stable generator and the input of the second frequency divider, and by the output to the second input of the first mixer, the input is the fifth narrow-band filter is connected to the output of the second frequency divider, and the output to the second input of the second mixer, the first input of which is connected to the output of the second narrow-band filter, and the output to the input of the difference frequency filter, the second driver amplifier is connected between the output of the difference frequency filter and the third controller input.