RU2334953C2 - Method of radio wave level measurement - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention is related to instrumentation for measurement of different substances level. In method of radio wave level measurement, based on linear frequency modulation, which includes generation of microwave signal of alternating frequency, radiated in direction of measured level surface, execution of fast Fourier transform of difference frequency signal of radiated and surface-reflected measured level of signals, detection of distance to the measured level, compared to the known one, signal of fast Fourier transform filter is singled out with maximum amplitude and filter signals lateral from it, by values of measured signals signal of interpolated cubic spline is built with pitch that is less than width of filter of fast Fourier transform of initial signal. Then signal point is detected with frequency that corresponds to maximum frequency of spline filter, based on which distance to measured level is calculated.

EFFECT: reduction of level measurement inaccuracy with preservation of fast action.

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Description

The invention relates to techniques for monitoring and measuring the level of various substances.

Known pulsed radio wave methods of level measurement [1, p. 94], in which the probing signal is a short radio pulse, by the time of passage of which to the surface of the medium and vice versa is estimated range.

To measure ranges not exceeding several tens of meters (the range characteristic of level measurement at technological objects), probing pulses with a duration of not more than tenths of a nanosecond are required. At the same time, the requirements for low-frequency components of the device sharply increase, and the use of these meters does not lead to satisfactory accuracy results.

A known method of radio wave level measurement using linear frequency modulation (LFM) of a microwave signal [2], selected as a prototype, comprising generating a microwave signal of a changing frequency radiated in the direction of the surface of the measured level, performing fast Fourier transform of the signal of the difference frequency of the emitted and reflected from the surface of the measured level of signals, determining the distance to the measured level.

When measuring the level, the surface under control is probed with a linear frequency-modulated electromagnetic wave (EM), at the same time the law of modulation of the EM is recorded, the reference EM and the reflected EM are compared and the difference frequency signal is extracted, the maximum difference frequency of the signal is determined, and the delay time of the reflected EM is determined from the magnitude and modulation law relative to the reference EV. The distance to the controlled surface is determined by the formula: α = (сΔt) / 2, where c is the speed of light. To increase accuracy, determine the distance to the surface to be monitored as the average according to the results of 2N measurements for N = 10 periods of modulation of the electromagnetic field.

When using the fast Fourier transform (FFT), the accuracy of determining the frequency depends on the width of the FFT filter, which in turn is inversely proportional to the observation time (the duration of the LFM scan in this case). To increase the accuracy, a decrease in the FFT filter and an increase in the observation time are necessary. However, the duration of the chirp scan in this case becomes too long, which leads to poor performance.

The technical result of the invention is to reduce the error of level measurement while maintaining performance by reducing the equivalent width of the FFT filter.

The technical result is achieved by the fact that in the method of radio wave level measurement based on linear frequency modulation, which includes generating a microwave signal of varying frequency radiated in the direction of the surface of the measured level, performing fast Fourier transform of the signal of the difference frequency of the signal level emitted and reflected from the surface, determining the range to the measured level, in contrast to the known one, they emit a fast Fourier transform filter signal with a maximum amplitude side away filter the signals from the values of the measured signals produce signal construction cubic spline interpolation with a pitch less than the width of the filter fast Fourier transform of the original signal, then a signal point determined with a frequency corresponding to the maximum frequency filter spline at which calculated distance to the measured level.

The essence of the invention is illustrated in figure 1, which shows:

On figa - FFT results with the choice of the maximum amplitude filter and several side in relation to it;

on figb - FFT results with the choice of the maximum amplitude of the filter and several side in relation to it;

on figv - the results of constructing an interpolation cubic spline;

in Fig.1d - the choice of the maximum value of the spline and the corresponding frequency;

Figure 2 - implementation system of the proposed method of radio wave level measurement.

The system contains a microwave frequency generator 1 with linear frequency modulation, a receiver 2 of the reflected signal associated with a mixer 3, which in turn is connected to a series-connected unit 4 fast Fourier transform, block 5 separation of the harmonic signal with maximum amplitude and side harmonics, block 6 build cubic spline interpolation for the main and selected side harmonics, block 7 extraction from the signal of the spline harmonic with maximum amplitude, block 8 determine the distance to the measured ur vnya.

Microwave generator 1 is a source of microwave oscillations, the frequency of which varies linearly, and can be performed on HMC385LP4 Hittite Microwave Corp. In the method of radio wave level measurement using LFM, in which the microwave signal is radiated through the antenna-feeder path, the reflected signal is transmitted to the receiver 2, in which the frequency of the reflected signal differs from the frequency of the signal generator due to the time delay. Range measurement is reduced to measuring the difference frequency of two signals using FFT (see figa) by the number of the maximum filter. In this case, the error in determining the frequency in this case is up to half the width of the FFT filter.

The reflected signal enters the receiver 2 of the reflected signal, which can be performed on NLB-300 RF Micro Devices, is mixed with a part of the signal of the microwave generator 1, and a difference frequency is allocated in the mixer of the receiver of the reflected signal 3, which is proportional to the delay time of the reflected signal, which carries information about the range to the measured level of the substance. The difference frequency signal from mixer 3 enters block 4, in which a fast Fourier transform is performed and which can be performed on C8051F124 Silicon Lab microcircuits. According to the FFT results, in block 5, made, for example, in the form of an amplitude detector, a filter with a maximum amplitude and several side filters from it are selected (see Fig. 1b), the values of which in block 6, made, for example, in the form of a nonlinear amplifier , an interpolation cubic spline is constructed with a step smaller than the width of the FFT filter (see Fig. 1c), after which, in block 7, made, for example, in the form of an amplitude detector, a harmonic is extracted from the signal with a frequency corresponding to the maximum am litude spline (see. 1D). The number of FFT side filters for constructing an interpolation cubic spline from below is limited by the accuracy of spectrum reproduction and cannot be less than one on each side of the filter with maximum amplitude. The number of FFT side filters for constructing an interpolation cubic spline from above is limited by the presence of nearby spectral components with a frequency different from the main one. When these spectral components are included in the construction of the interpolation cubic spline, the latter is distorted.

In contrast to the known level in the proposed method of radio wave level measurement, the equivalent filter width is determined by the interpolation cubic spline step, which can be significantly less than the filter width of the original FFT, since the original FFT filter is divided into several parts. When divided into two parts, the accuracy is improved twice. The step of the interpolation cubic spline is determined by the necessary measurement accuracy and the computational capabilities of the device that implements the method.

In block 8 determining the range, made, for example, in the form of a multiplier, the range to the measured level is calculated by multiplying the harmonic frequency of the signal corresponding to the maximum frequency of the spline filter by a constant, the value of which is determined by the slope of the linear frequency modulation and the speed of light.

By reducing the equivalent magnitude of the FFT filter width, it becomes possible to reduce the level measurement error while maintaining performance.

Literature

1. V.A. Viktorov, B.V. Lunkin, A.S. Savlukov. Radio wave measurements of process parameters. - M .: Energoatomizdat, 1989, p. 99.

2. Application No. 93026872, 10.20.1995, IPC: G01F 23/28 - prototype.

Claims (1)

A method of radio wave level measurement based on linear frequency modulation, including generating a microwave signal of varying frequency radiated in the direction of the surface of the measured level, performing fast Fourier transform of the signal of the difference frequency of the signal level emitted and reflected from the surface of the measured level, determining the range to the measured level, characterized in that emit the signal of the filter of the fast Fourier transform with maximum amplitude and the side signal filters from it, in terms of and Merenii signal filters produce building signal cubic spline interpolation with a pitch less than the width of the filter fast Fourier transform of the original signal, then determining the signal with a frequency corresponding to the maximum frequency filter spline at which calculated distance to the measured level.

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