B.A. Atajants - Ph.D. (Eng.), General Director, LLC «Kontakt-1», Ryazan. E-mail: market@kontakt-1.ru V.М. Davidochkin - Ph.D. (Eng.), Head of Department, LLC «Kontakt-1», Ryazan. E-mail: skb@kontakt-1.ru V.V. Ezerskiy - Dr.Sc. (Eng.), Professor, Ryazan state radio engineering University. E-mail: ezerski@yandex.ru

The problem of measurement of level of filling of industrial tanks by liquid substances by means of measurement of distance from the measuring device to liquid level in the punched wave guide lowered in the tank is considered. As a level measuring instrument the frequency-modulated (FM) range finder is used. It is shown that owing to presence in a dispersion wave guide in the reflected signal there is a dependence of time of a delay on frequency. It results in nonlinear dependence difference frequencies from time. The formula connecting the law of change of frequency with critical length of a wave of a wave guide, so with its cross-section sizes is received. By means of modelling it is received that the signal spectrum difference frequencies (SDF) at distance increase is strongly deformed. The distortion size is proportional to length of a wave guide, inversely proportional to diameter of a wave guide and with difficulty increases at increase in a range of reorganisation of frequency. The specified distortions of a spectrum lead to increase in an error of measurement of distance. The error is analysed by a method of numerical modelling. At modelling it was used SDF, 9 GHz received with bearing frequency, a range of reorganisation of frequency of 1 GHz and diameter of a round wave guide of 25 mm. Processing SDF was made widely known two stages by algorithm of search of a maximum of a spectrum. Fast transformation of Fure for a rough estimate difference frequencies and a method of the maximum credibility with an unknown initial phase for estimation specification is used. It is shown that if not to take any measures on decrease in influence of a dispersion dependence of an error of measurement has step character with a margin error measurements of the reaching size in some tens centimetres on a wave guide in length more than 12 m at comprehensible size not errors no more than 1 - 5 mm. Three ways of decrease in influence of a dispersion are offered. The first consists in use at spectral processing of weight function (WF). Influence of form WF the Kaiser-Besselja (KB) on size of an error is investigated and shown that there is a range of values of parametre of form WF KB in which the error does not exceed 1 mm. However the error size is influenced strongly by parasitic peak modulation (PPM), essentially increasing error of measurement. As size PPM strongly depends on conditions of application of a level gauge and from time, it is difficult to provide its account. Therefore it is possible to recommend the given method of easing of influence of a dispersion for practical applications at the length of a wave guide exceeding 5 m. However this method demands any additional transformations of the scheme and the level gauge program. More radical is entering of predistortions into law FM, providing a constancy difference frequencies. Expression for the demanded law of frequency modulation is received and the quantitative estimation of size of deviations of law ЧМ from linear dependence is given. It is the most universal method, however, he demands difficult management of frequency at frequency modulation by means of a frequency synthesizer. Perspective for application in the devices working in a wave guide it is possible to consider use of processing SDF in time area. Are considered a method of weight averaging разностной the frequencies, based on linear dependence of average frequency SDF on the measured distance and a method of parabolic approximation of dependence of frequency of an accepted signal from time, allowing to define phase additives SDF on a processing interval. Modelling of these methods shows that they allow to receive an error of measurement less than 1 mm for almost important lengths of wave guides. However their application demands change of the software of a level gauge.

 

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