RU2234716C1 - Method for generating sounding frequency -modulated signal for range finer with periodic frequency modulation - Google Patents

Abstract

FIELD: measuring technique, namely range finding.

SUBSTANCE: method comprises steps of generating periodic modulating voltage; generation and excitement of sounding waves; reception of reflected waves, propagation time interval later; Mixing received waves with part of power of sounding waves; separation of difference frequency signal; correction of periodic modulating voltage according to fluctuation of instant periods of difference frequency signal from their mean value in order to reduce difference of mean and instant values of period duration until predetermined level. It is possible to realize two variants of correction. In first variant range finding and correction are realized by the same difference frequency signal received with use of echo-waves reflected from sounded surface. In second variant correction is realized with use echo-waves received in portion of reference feeder line and range finding is realized with use of echo-waves reflected from sounded surface. Correction of modulating voltage may be performed by calculating correcting voltage summed with modulating voltage or by displacing initial readings of modulating voltage along time axis. In both variants correction is based upon mean value of difference frequency signal.

EFFECT: elimination of error at range finding.

6 cl, 2 dwg

Description

The invention relates to the field of measuring equipment, in particular to measuring distance.

A known method of forming a linearly modulated signal of a frequency-modulated (FM) distance meter [1, 2, 3], for the implementation of which frequency synthesizers with phase-locked loop are used. The disadvantage of this method of signal generation is the relatively high cost of devices for its implementation, inertia and lack of consideration of the dispersion properties of the propagation space of the sounding waves.

A known method of measuring distance and a device for its implementation [4], which implements a method of forming a linearly modulated signal. In the specified method and device, the generated probing microwave signal is converted to a lower intermediate frequency, followed by dividing the frequency, measuring the frequency deviation from the set value at the discrete points of the modulation characteristic and modulating voltage correction to reduce the difference between the set and current frequency values to the specified control level.

This method of generating a sounding signal is implemented in a distance meter, comprising: a controlled generator of the transmitted signal, the output of which is connected to the first input of the directional coupler (BUT), the output of which is connected to the antenna, the second input of the directional coupler is connected to the antenna, and the output is connected to the first input of the mixer the second input of which is connected to a reference signal generator, and the output is connected to a filter and an intermediate frequency amplifier connected in series. The amplifier output is connected to the inputs of two channels: a measuring channel and a correction channel. The measuring channel contains a series-connected mixer of intermediate frequencies, a low-pass filter (low-pass filter), an analog-to-digital converter (ADC), the output of which is connected to the first input of the microcomputer (MK). The correction channel contains a series-connected frequency divider and a frequency counter, the output of which is connected to the second input of the MK. The first MK output is connected to a digital-to-analog converter (DAC) in series and a frequency modulator, the output of which is connected to the control input of the transmitted signal generator.

Said distance measuring method, including a FM signal measuring method of a distance meter, and a distance meter have three disadvantages.

Firstly, the influence of the instability of the frequency and phase of the reference signal generator on the error in the formation of the modulating voltage due to the fact that this formation is performed in the static mode of the generator and the measurement is performed in the dynamic mode.

Secondly, as indicated in the description of the patent, the method and device function normally if the signal level in the measuring channel is much lower than the signal level in the reference channel. Such conditions can be fulfilled only at large distances from the antenna to the probed material. At short distances, a strong reflected signal passes into the correction channel and distorts its operation, which leads to correction errors and even complete inoperability of the method and device.

Thirdly, in the formation of the probe signal, the dispersion properties of the propagation space of the probe waves are not taken into account. In this case, the use of a distance meter in conjunction with a guiding system (for example, a perforated tube) leads to a measurement error during signal processing both in the time and in the spectral region. Due to the significant and asymmetric expansion of the main lobe of the spectrum, it is impossible to accurately determine its center frequency and, accordingly, it is impossible to accurately determine the distance.

The considered factors affect the accuracy of the formation of the modulating function, therefore, lead to the appearance of an additional error in measuring the distance.

The closest set of essential features is the method of generating a probing modulated FM signal of a distance meter with periodic frequency modulation [5], which consists in generating a periodic voltage at the control input of a controlled energy source of probing waves (UIEZV) with a modulation half-cycle duration proportional to the distance to the sensed surface . In this case, the average difference frequency is kept constant by generating a correction signal when the average difference frequency deviates from the set value, averaging it over several periods of modulation and controlling the period of the modulating voltage at the control input of the UEES obtained by the averaged correction signal. The correction signal is obtained in the feedback circuit, which contains a frequency discriminator and an integrator, averaging the signal from the output of the frequency discriminator.

This method of generating a probing modulated FM signal of the distance meter also has the disadvantage that the inertial feedback loop maintains a constant average rather than instantaneous difference frequency of the signal. The inertial feedback loop cannot completely exclude the error of the adjustable parameter (non-linearity of the change in the frequency of the probing signal) due to the non-linearity of the modulation characteristic of the UEISV, especially in areas near the discontinuity of the derivative of the adjustable parameter. As a result, the instantaneous difference frequency or periods of the difference frequency signal (RMS) are not constant, which leads to an error in measuring the distance when processing the RMS both in the time and in the spectral regions.

The purpose of the invention is to reduce the unevenness of the RF periods during the modulation period and, as a result, eliminate the additional error of distance measurement.

This goal is achieved by the fact that in the method of generating a probe adaptive frequency-modulated signal for a rangefinder with periodic frequency modulation, including the formation of periodic modulated waves and excitation of the probe waves, receiving after the propagation time of the reflected echo waves, mixing them with part of the energy of the probe waves, the allocation RMS and correction of the law of modulation of modulated waves, additionally measure the instant periods of RMS, calculate the average value of the periods of the differential frequency for nterval processing signal, which is part of the modulation period, and changing the correction voltage to reduce the difference between the average and the current value to the control level periods.

To measure distance and to obtain corrective voltage, the same difference frequency signal is used.

In the presence of interfering reflectors, an additional reduction in the error in the formation of the probing signal is achieved by using an additional reference channel with a delay line in the form of a segment of the feeder line. In this case, part of the power of the sounding waves is emitted through the measuring channel towards the probed surface, and part of the power of the sounding waves excites the reference channel, made in the form of a piece of the feeder line, and the echo waves reflected from the probed surface are used to measure the distance to it, and the reflected from the end of the feeder line, echo waves are used to obtain corrective voltage at the control input of the controlled energy source of the probe waves.

In this case, to generate a periodic modulation voltage U _mod (t), digital samples of U _mod (t _j ) are generated at fixed times t _j and converted into discrete analog samples using low-pass filtering. The formation of digital samples of the modulating voltage is performed using one of two possible procedures.

The first is that U _{mod, k} (t _j ) at the k-th modulation period are produced recursively in accordance with the voltage of U _{mod, k-1} (t _j ) at the previous (k-1) th period and the correction voltage ΔU _k ( t _j ) obtained taking into account the non-uniformity of the instantaneous periods of the RMS:

- корректирующее напряжение;Where

- corrective voltage;

- постоянный коэффициент, равный средней крутизне нарастания модулирующего напряжения;

- a constant coefficient equal to the average steepness of the rise of the modulating voltage;

U _M is the amplitude of the modulating voltage;

T _M - modulation period;

- относительное изменение периода СРЧ;

- the relative change in the period of the RF system;

- отклонение периода сигнала разностной частоты от среднего значения;

- the deviation of the period of the signal of the differential frequency from the average value;

- средний период сигнала разностной частоты;

- the average period of the differential frequency signal;

N is the number of intersections of the zero level of the RMS during the processing interval, and the values η (t) at the intermediate points t _j between the moments of intersection of the RMS of the zero level t _{i are} calculated using interpolation formulas.

The second procedure consists in the fact that the formation of digital samples of the modulating voltage at the intersection points of the zero level of the RMS in the kth modulation period U _{mod, k} (t _i ) is performed by rearranging on the time axis the similar samples of the (k-1) th period according to the formulas :

and at the intermediate points of the modulation period using interpolation.

The boundaries of the signal processing interval in time and the duration of this interval T, which is part of the modulation period, are determined by the moments of coincidence of the frequency of the emitted signal with the lower F ₁ and upper F ₂ frequencies of the frequency tuning range for frequency modulation.

The claimed method has a combination of features unknown from the prior art for methods of this purpose, which allows us to conclude that the criteria of the invention of "novelty".

To prove the inventive step, it is necessary to take into account that it is known to use a feeder of a given length as a reference channel in distance meters [6, 7]. However, in these methods of measuring distance, the length of the feeder serves as a highly stable, known measure of length with which the measured distance is compared. In the proposed method, the role of the feeder line and its parameters are different. Firstly, its length can have arbitrary long-term instability, because it does not affect the results of a short-term assessment of the unevenness of the periods of the signal of the difference frequency, and the short-term instability of any feeder line is negligible. Secondly, the feeder line is used only to assess the unevenness of the periods of the differential frequency signal and is not used to calculate the measured distance, therefore, its long-term instability does not affect the measurement accuracy. And finally, if the level of interference from parasitic reflectors is small, then the same echo waves reflected by the sensed surface are used to form a law of variation of the frequency of the sounding waves and to measure the distance (for example, when measuring the liquid level in containers with a perforated level gauge waveguide).

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".

The essence of the proposed method and the possibility of its practical implementation are explained using the drawings shown in figures 1 and 2.

Figure 1 shows the time dependences of the frequency of the sounding waves, periods of the RMS and corrective voltage.

Figure 2 shows the structural diagram of a device that implements the proposed method.

The method is as follows. At the initial stage, sounding waves periodically modulated in frequency are generated, the frequency variation graph 1 of which is shown in Fig. 1a. After radiation (or excitation of a waveguide perforated tube), reception of echo waves and mixing with a part of the power of the emitted waves, an RMS is obtained at the output of the mixer, the duration of the instant periods of which is generally variable. Graph 2 of this dependence is shown in figb. Next, measure the duration of each instant period of the RWF and calculate its average value within the processing interval, which is part of the modulation period (in the particular case, this is half the modulation period). Then, the deviation of the instantaneous duration of the RHF periods from the average value is calculated and the correction modulation voltage is generated in accordance with the calculation formula indicated in the description of the method. Graph 3 of this relationship is shown in FIG. This corrective voltage is used in subsequent periods of modulation to form probing waves with a new law of frequency change. The indicated procedure is constantly repeated until the unevenness of the RF periods is reduced below the control level, which ensures the required accuracy of distance measurement. The constant use of this procedure allows you to always maintain such a form of corrective voltage at which there is no excess of the uneven periods of the RHF control level.

The proposed method of generating a probing adaptive frequency-modulated signal for a rangefinder with periodic frequency modulation can be implemented in a distance meter, the structural diagram of which is shown in Fig.2. A distance meter with adaptive frequency modulation comprises a calculator 4, a controlled source of energy of the sounding waves (UIEZV) 5, a directional coupler (BUT) 6, a measuring channel (IR) 7, a pathogen of sounding waves (VZV) 8, a reference channel (OK) 9 and a feeder line (FL) 10. The control output of the calculator 4 is connected to the control input of the UIEZV 5, the microwave output of which is connected to the input of the HO 6. The first output of the NO 6 is connected to the input of the IR 7, the first output of which is connected to the input of the HRA 8, and the second output is connected to measuring input of the calculator 4. Second output HO 6 soy is dined with an input OK 9, the first output of which is connected to the input of the PL 10. The second output OK 9 is connected to the reference input of the calculator 4. The second and third outputs UIEZV 5 are connected to the third and fourth inputs of the calculator 4. The second output of the calculator 4 is the output of the device.

The distance meter works as follows. At the initial stage of operation, the calculator 4 transfers to the control input UIEZV 5 periodic monotonically changing voltage of a triangular shape. Due to the nonlinearity of the modulation characteristic of the UEIWS 5, the frequency of the generated waves at its output will change nonlinearly, as shown by graph 1 in Fig. 1a. Through HO 6, these waves arrive at IR 7 and OK 9. These channels are identical and have the standard structure of a microwave receiver of a frequency-modulated signal. Each of them includes two directional couplers, a mixer and a differential frequency signal amplifier. From the first outputs of these IR 7 and OK 9, the signals are transmitted respectively to VZV 8 and FL 10. The echo signals from the outputs of these blocks are fed back to IR 7 and OK 9, respectively. Two RMS received in IR 7 and OK 9 are fed to the corresponding inputs of the calculator 4. In addition to them, the third and fourth inputs of the calculator 4 receive control signals from the second and third outputs of the UIEZV 5 at the moments of coincidence of the frequency of the probe waves with the lower F ₁ and upper F ₂ frequencies of the frequency tuning range for frequency modulation.

The calculator 4 may include one or two microprocessors. According to the obtained reference RMS, the calculator 4 measures the dependence of 2 on the time of instantaneous periods of the reference RMS, shown in Fig. 1b, calculates the correction voltage represented by curve 3 in Fig. 1c, and the new modulation voltage at its control output, adding the correction voltage to the modulation voltage, used at the current stage of work. Next, the process is repeated in each modulation period. As a result, the dependence of the frequency of the probe waves on time assumes a linear character and the unevenness of the periods of the reference MFR (and hence the measurement signal) disappears. After reducing the unevenness of the periods of the RF system below the control level, the calculator 4 determines the distance from the measuring RF system. Thus, in the process, the change in the non-linearity of the modulation characteristic of the UEIWS 5 is continuously monitored and corrected. In this case, the dispersion properties of the propagation medium of sounding waves are automatically taken into account. The shape of the modulating voltage will always be such that the unevenness of the RF periods is not manifested, and the absence of such unevenness is a prerequisite for achieving high accuracy of distance measurement.

In the absence of external interfering reflections, the reference channel may not be used, i.e. blocks 9 and 10 can be removed from the circuit. In this case, the reference and measuring inputs of the calculator are connected together, and a difference frequency signal from the output of the measuring channel is supplied to them.

Computer simulation and verification on the mock of the claimed method of generating a probing adaptive frequency-modulated signal for a range finder with periodic frequency modulation show that it allows to reduce the unevenness of the periods of the difference frequency signal from the original ones (50-70%), when it is almost impossible to provide an acceptable error of distance measurement, up to tenths of a percent, which eliminates the additional error of distance measurement.

Sources of information

1. US patent No. 5504490 of April 2. 1996.

2. US Patent No. 5546088 of Aug. 13. 1996.

3. US Patent No. 6107957 of Aug. 22. 2000.

4. US patent No. 5387918 from February 7. 1995.

5. RF patent No. 2151408 dated 06/30/1999 G 01 S 13/34.

6. US patent No. 4044355 of August 23. 1977.

7. US patent No. 4665403 of May 12, 1987.

Claims (6)

1. A method of generating a probing adaptive frequency-modulated signal for a rangefinder with periodic frequency modulation, including generating a periodic modulating voltage, generating and exciting sound waves, receiving, after the propagation time, the reflected echo waves, mixing them with part of the power of the sounding waves, isolating the signal the difference frequency and the correction of the periodic modulation voltage, characterized in that they further measure the duration of the instantaneous periods of the signal difference th frequency interval for signal processing, which is part of the modulation period, and generating a correction voltage for reducing the average difference value and the instantaneous duration periods to a predetermined reference level. 2. The method of generating a probing adaptive frequency-modulating signal for a rangefinder with periodic frequency modulation according to claim 1, characterized in that the probing waves emit in the direction to the probed surface and use echo waves reflected from the probed surface to measure the distance to the probed surface and for obtaining a corrective modulation voltage. 3. The method of generating a probe adaptive frequency-modulated signal for a rangefinder with periodic frequency modulation according to claim 1, characterized in that one part of the power of the probe waves is emitted through the measuring channel in the direction of the probe surface and echo waves from the probe surface are used to measure the distance to her, and the second part of the power of the probe waves excite the reference channel, made in the form of a piece of feeder line, and use the echo reflected from the end of the feeder line to obtain corrective voltage modulation. 4. The method of generating a probing adaptive frequency-modulated signal for a rangefinder with periodic frequency modulation according to claim 1, characterized in that the boundaries of the signal processing interval in time and the duration of this interval T, which is part of the modulation period, are determined by the coincidence of the frequency of the emitted signal with lower F ₁ and upper F ₂ frequencies of the frequency tuning range for frequency modulation. 5. The method of generating a probing adaptive frequency-modulated signal for a rangefinder with periodic frequency modulation according to claim 1, characterized in that the formation of a periodic modulation voltage U _mode (t) is performed by generating digital samples U _mode (t _j ) at fixed times t _j , converting them to discrete analog samples and low-pass filtering, while the formation of digital samples of the modulating voltage U _{mod, k} (t _j ) at the k-th modulation period is performed recursively by the voltage U _{mod , k-1} (t _j ) on the previous (k-1) -th period and the corrective voltage ΔU _k (t _j ) obtained taking into account the unevenness of the instantaneous periods of the difference frequency signal U _{mod, k} (t _j ) = U _{mod, k-1} (t _j ) + ΔU _k (t _j ), Where

- a constant coefficient equal to the average steepness of the rise of the modulating voltage; U _m - the amplitude of the modulating voltage; T _m - modulation period;

- the relative change in the period of the signal of the differential frequency; ΔT _p (t _i ) = T _pi -T _cf - the deviation of the period of the signal of the differential frequency from the average value;

- the average period of the differential frequency signal; N is the number of crossings of the zero level signal of the differential frequency for the processing interval, moreover, the values of η (t) at intermediate points t _j between the moments when the signal crosses the differential frequency of the zero level t _{i are} calculated using interpolation formulas. 6. The method of generating a probing adaptive frequency-modulated signal for a rangefinder with periodic frequency modulation according to claim 1, characterized in that the formation of digital samples of the modulating voltage at the zero level crossing points by the difference frequency signal in the kth modulation period U _{mod, k} (t _i ) is performed by rearrangement on the time axis of similar samples of the (k-1) -th period according to the formulas U _{mod, k} (t ₁ ) = U _{mod, k-1} (t ₁ ), U _{mod, k} ((i-1) T _cp ) = U _{mod, k-1} (t _i ), i = 2,3, ... N, and at the intermediate points of the modulation period using interpolation.

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