Development, research and metrological analysis of the measuring channel of a fiber-optic sensor for fluid flow parameters used in information-measuring systems

The article proposes a device for measuring parameters of liquid flows (volume and velocity) (FOSLFP). A physico-mathematical model of optical signal conversion in a fiber-optic flux transducer of the proposed sensor is developed, in which the perceiving element is a truncated cylinder mounted on the inner surface of the bellows, experiencing angular bending during measurements. The conversion of optical signals is carried out using a two-way mirror plate mounted on the outer surface of the bellows in an open fiber optical channel. A circuitanalytical model of a new variant of a differential circuit for converting optical signals to a FOSLFP was developed, in accordance with which a metrological model of the sensor was developed to determine the possible sources of measurement errors. The nominal and real functions of the FOSLFP transformation were derived, the additive and multiplicative components of the errors of the two measuring channels of the sensor and the ways to reduce them were determined.


Introduction
The safety of operation of technical systems and objects created in high-tech branches of science and technology, to which the decree of the President of the Russian Federation dated 07.07.2011 referred as rocket and space and aviation industries, and nuclear power and many other industries, today largely depends on the accuracy of measurements of fluid flows [1].
The problem of creating and improving methods and means of measuring the parameters of substances with specific properties (aggressiveness, unsteadiness of physical and chemical characteristics, high viscosity) and functioning in various difficult operating conditions, despite some progress, remains very relevant [2]. The electrical signal UP, input to the input of the radiation source of the RS, is converted by means of an electro-optical converter of the RS into an optical signal Ф, part of which Ф'0 is fed to the input of the fiber optic cable of the FOC through the alignment unit of the AN1. According to the supply optical fiber of the LFO, the light flux is transferred to the measuring zone of the fiber optic converter FOC, where its intensity Ф0 changes under the action of the measured physical quantity α. The part of the light flux ФME(α), modulated by the modulating element ME in the function of the measured physical quantity α of the flux force F, enters the diverter optical fiber AFO, is transmitted through them through the AN2 to the radiation receiver RR, where the photoelectric conversion takes place. The electrical signal I (α) is removed from the output optoelectronic unit [3].
Under the influence of the FPot force produced by the liquid flow, the SE element will deviate by some angle α, and the reflecting plate of the RP will deviate by the same angle.
The light fluxes Ф01 and Ф02 from the RS according to the LFO1 and LFO2 are sent to the measuring zone to the first and second reflecting surfaces of the plate, respectively. As the angle α is varied, the intensities of the reflected light fluxes Φ1(α) and Φ2(α), respectively, change [4][5][6].
To determine the possible sources of measurement errors and to find ways to reduce them, a metrological analysis of the FOSLFP channel was carried out. For this, a metrological structural scheme of the sensor was developed [7][8].
As shown above, in the differential FOSLFP, the measured flow strength F is converted into an reflecting plate RP offset at an angle α, so the schematic-analytical model of the new version of the differential transformation scheme will have the form shown in Figure 2.  Mathematically, the conversion function is written: where Y -output value (for example, voltage); F -measured fluid flow strength. In the absence of a flow force at F = 0, Ф1 (F) = Ф2 (F). If the RP is shifted, then the light fluxes are redistributed between the receiving fibers: the optical signal Ф1-∆Ф is fed into the first ALF, and the optical signal Ф2 + Ф is fed into the second ALF. If these signals are fed to the RR, and then to the subtractor, a signal proportional to the difference in the radiation fluxes is formed: That network is a doubling of the sensitivity of the conversion. If we form the ratio of the difference of the signals to their sum, then: Consider the structural metrological model of the sensor ( Figure 5).
(5) Absolute errors in the transformation of each of the infrared are determined as follows: where IR1, IR2 -real conversion function.
The multiplicative component of the error is: 1) For the first MC 2) For the second IR Additive component of error: 1) For the first MC 2) For the second MC Non-linear component: 1) For the first MC 2) For the second MC The errors ∆8, ∆9 of the position of the ALF relative to the radiation receivers are practically equal to zero. Errors ∆10 and ∆11 of the spectral matching of RS and RR will be negligible if the RS -RR pairs are chosen correctly [9,10].
Technological errors due to inaccuracy in the manufacture of RP ∆RP1 and ∆RP2 are comparable with the above-mentioned errors in significance. To reduce the error data up to permissible values, it is possible to constructively use the advanced manufacturing technology of FOSLFP [14][15][16].
The errors in the conversion of each of the channels without taking into account errors that can be neglected will be determined by the expressions: and the real conversion function is written: It is obvious that in the differential scheme LSE1LSE2, 23, SE1SE2, 4 5, 6 7, LRP1LRP2, then According to the literature, the error of the KVU is about 0.25% [17]. The technological errors ∆RP1, ∆RP2, caused by inaccuracy in the manufacture of RS, can be excluded or reduced by constructive and technological means [18,19].
Differential transformation of signals in FOSLFP allows you to reduce most of its errors [20].

Results
On the example of the metrological analysis of FOSLFP it was shown that the differential transformation of optical signals with identical manufacturing of two measuring channels allows to improve the metrological characteristics of FOSLFP 2-3 times.