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Adaptive indemnification in frequency level gauges of parasitic frequency modulation caused by presence of a dispersion

Keywords:

B.A. Atayants – Ph. D. (Eng.), General Director Open Company enterprise «KONTAKT-1» (Ryazan). E-mail: skb@kontakt-1.ru V.M. Davydochkin – Ph. D. (Eng.), Head of Department of Microwave Oven of Devices and Aerials, Open Company enterprise «KONTAKT-1» (Ryazan). E-mail: skb@kontakt-1.ru V.V. Ezersky – Dr. Sc. (Eng.), Professor, Department of Radio Management and Communication, Ryazan State Radio Engineering University. E-mail: ezerski@yandex.ru


In many cases at measurement of level of filling of industrial tanks by means of level gauges with it is frequency modulated (FM) a signal in a measuring path there is a wave guide. Wave guide presence brings features in the processed signal, consisting in occurrence of parasitic frequency modulation (PFM) a signal difference frequencies (SDF) at linear law FM of the transmitter. PFM arises owing to dispersion presence in a wave guide. PFM deforms spectrum SDF and causes occur-rence of the raised error of measurement of level. Often the wave guide is used as the auxiliary directing system used for reference of the measuring block of a level gauge from the aerial, located in a zone with operation severe conditions. Measurement of level of a material is made in free space behind a wave guide. In work it is shown that the size of distortions in this case depends on the cross section sizes of a wave guide, a parity of length of a wave guide and free space, size of a range of reorganisation of frequency at FM and bearing frequency. Besides, on size of distortions in this case renders presence of parasitic peak modulation (PPM) which does not influence an error of measurement in the absence of PFM. Spectrum distortions lead to occurrence of constant displacement of indications of a level gauge and occurrence of jumps of indications at change of size PPM which strongly depends on temperature. Quantitative estimations of possible additional errors are resulted and is shown that their size essentially exceeds requirements of practical applications of level gauges. Besides, dispersion influence reduces a noise stability of a method of measurement of frequency. The estimation of possible size of an error in the presence of stirring reflexion from the aerial is executed. The error has the big constant displacement and an oscillatory component with the amplitude much more exceeding practical requirements and having considerable extent on an axis of frequencies. For elimination of influence of a dispersion on result of measurement of level it is offered to enter predistortions into law FM of the transmitter. The demanded law of change of frequency is received at FM. Complexity consists that the demanded law depends on a parity of the measured distance and length of a wave guide. Therefore for elimination of influence of a dispersion the iterative algorithm consisting in consecutive specification of the law of brought predistortions on the basis of the measured distance from the aerial to a surface of a controllable material is offered. Such specification is made until difference of current result of measurement from the previous decreases less some in advance set size. The estimation of possibilities of the offered algorithm on decrease in an error of measurement of the frequency caused by presence of a dispersion, PPM and stirring reflexion is executed. It is received that at adaptive indemnification of parasitic frequency modulation in both cases constant displacement disappears, the amplitude of an oscillatory making error decreases in nondisturbance to conditions in 4000 times and in 20 times in the presence of stirring reflexion.
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