RU2096768C1 - Shf method of measurement of moisture content and sensitive element in the form of open waveguide resonator for its implementation - Google Patents

Abstract

FIELD: indirect method of test of physical properties and composition of substances. SUBSTANCE: SHF method of measurement of moisture content consists in action on examined object by electromagnetic field of sensitive element in the form of open waveguide resonator, in measurement of reflection factor and in determination of dependence between moisture constant and reflection factor. Measurement is carried out on fixed frequencies. Frequency should match resonance frequency, coupling is set critical for resonator loaded on object with minimal or maximal values of moisture constant. Sensitive element of resonator for implementation of method can also be made irregular by decrease of dimension of narrow wall of rectangular waveguide along direction of propagation or of its broad wall. EFFECT: method can be used in SHF humidity meters of discrete and continuous action under conditions of one-side access to object. 4 cl, 7 dwg

Description

 The invention relates to indirect methods for measuring the physical properties and composition of substances, such as humidity, using electromagnetic fields in the microwave range and can be used to determine the moisture content of solid, liquid, pasty, bulk products and products from them. The proposed measurement method can be implemented in resonant microwave moisture meters of both discrete and continuous operation for monitoring the moisture content and regulating technological processes in marine and land engineering geology, chemical, food, dairy industries, and the production of building materials. The proposed sensitive elements in the form of open waveguide resonators are especially convenient for creating resonant humidity sensors in cases where it is undesirable or impossible to place the test object inside the measuring cell, and one-way access to the test object is preferable. This ensures non-destructive, and in some cases, contactless control that does not require preliminary sample preparation.

 It can be considered established that for those areas of technology to which the invention relates, the problem of measuring humidity is most successfully solved using open resonators (OR) as sensitive elements (SE) of microwave resonance sensors [1] An open resonator is a resonant system, from which part of the energy is irreversibly radiated into the environment in the form of an electromagnetic field. A sensitive element is the part of the resonance sensor that directly interacts with the object under study and determines the choice and size of the informative parameter in a decisive way. A waveguide open resonator (BOP) is called an OR made of a segment of a waveguide transmission line. Depending on the choice of the base waveguide, BOPs can be coaxial, cylindrical, rectangular, etc. The resonant properties of VOR are ensured by the presence of inhomogeneities at the ends of the line segment. In a reflective VOR, the input heterogeneity is formed by a communication element (loop, pin, diaphragm) through which the VOR is excited by a microwave generator and energy is transferred to the microwave detector, where an informative signal is generated. The heterogeneity at the other end of the VOR is due to a jump in wave resistances at the boundary of the waveguide of the medium under study.

 Depending on the shape and size of the SE, the selected oscillation mode, the magnitude and type of coupling, the main parameters of the VOR resonant frequency and quality factor show different sensitivity to the humidity of the object under study. The aim of the invention is the selection of the optimal method of measurement and design of sensitive elements, providing maximum sensitivity over a wide range of changes in humidity.

 Known methods for determining humidity in the microwave, involving the use of VOR and measurement of transmission or reflection coefficients [2 and 3] For example, the natural frequency of a VOR located at some distance from the surface of the measured object is tuned by the reciprocating movement of the resonator [2] There are also known tuning methods resonator by changing over time one of the dimensions or using mechanically movable dielectric control elements. At the same time, bell-shaped voltage pulses are emitted at the microwave detector, the amplitude of which is proportional to the value of the reflection coefficient from the resonator, which depends on the humidity of the object. The main disadvantage of these solutions is the presence of an electromechanical assembly that complicates the design of the moisture meter and the measurement method.

 In analogue [3], the VOR is tuned by a varactor diode, and the reflection coefficient is recorded at the moment the resonator eigenfrequency coincides with the frequency of the exciting generator. The disadvantage of this method is the inability to carry out frequency tuning in a wide range of frequencies, which narrows the range of humidity values available for measurement.

 There is a method of measuring the properties of materials on a microwave, such as humidity, which consists in exposing the investigated object to an electromagnetic field of VOR and measuring the reflection coefficient in the frequency range of the excitation [4] Using a measuring line with a movable probe complicates the automation of this method, but even if we consider simplified options used in automatic microwave moisture meters, its significant drawback is the need for tuning the frequency of the exciting generator over a wide range in order to fix the resonance values of the reflection coefficient at different frequencies corresponding to different humidity. In addition, a common drawback of all the aforementioned analogues and prototype is the generally ambiguous dependence of the value of the reflection coefficient at resonance on the humidity of the studied object, which leads to a narrowing of the range of measured humidity, since the conversion function of the moisture meter is monotonous in a limited range.

SEs in the form of BOP are known from the above sources of information. The closest to the proposed technical solutions in terms of features is a rectangular BOP excited in the H _{10n mode} [5]. The CE is a segment of a rectangular waveguide with the dimensions of a wide wall a, narrow b and length c, separated by a diaphragm from the exciting waveguide, and a protective shield from the object under study dielectric plate. When using this SE to implement the proposed method for measuring humidity, the maximum possible sensitivity is not achieved in a wide range of humidity values.

 The proposed method for measuring humidity in the microwave, which consists in exposing the object under study to the electromagnetic field of an open waveguide resonator, is characterized in that the value of the reflection coefficient from the resonator is chosen not at the moment of passage of the resonant frequency, but at a fixed frequency, which is not necessarily resonant, as an information parameter. This frequency is determined in advance by examining the frequency dependences of the reflection coefficient for several samples of different humidity. There must be at least two of these samples, in which case samples are selected that have limit moisture values that correspond to the specific task. The moisture content of the samples used to select the operating frequency, as well as for the subsequent calibration of the moisture meter, is determined by an independent method, for example, thermogravimetric, i.e., weighing before and after drying of the sample. The obtained frequency dependences of the reflection coefficient are the resonance curves of the sensor with the selected SE, which differ from each other by the resonant frequency and half-width.

 The operating frequency of the microwave moisture meter is selected according to the results of measurements of the resonance characteristics so as to provide the greatest increment of the reflection coefficient ΔГ when humidity changes by a value of DW. The technical result provided by the invention is an increase in the sensitivity of DГ / ΔW ,, the value of which is maximum in a wide range of changes in humidity.

ΔГ = Г _max - Г _min represents the difference between the maximum Г _max and the minimum Г _min values of the reflection coefficient. G _min 0 corresponds to the full coordination of the resonant sensor with the exciting waveguide, when all the energy coming from the microwave generator to the sensor input is absorbed into the BOP. Therefore, the operating frequency of the hygrometer and the connection of the BOP with the generator are chosen such that at one of the boundaries of the humidity range (on the driest or wettest sample) agreement is reached (Г≈0). At the same time, conditions are provided for the most accurate measurement of G. G _{max is} reached at another boundary of the humidity range. The value ΔГ, as a rule, exceeds the corresponding increment observed in the selected analogues and prototype, therefore, the proposed measurement method provides a summation of the effects of changes in the quality factor and the shift of the resonant frequency.

 In addition, as experience shows, with a monotonic change in humidity, both the resonant frequency and the resonance value of the reflection coefficient for VOR can not vary monotonously, reaching extreme values within the range of measured humidity. The proposed measurement method provides a monotonic dependence of G (W) in the entire range 0≅W≅1.

 The proposed options for CE are made irregular, which is ensured, for example, by a monotonic change in either the narrow or wide walls of a rectangular waveguide. At the same time, the measurement sensitivity ΔГ / ΔW increases, the electrodynamic contact of the working surface of the CE with the test object improves, the likelihood of sticking of the test material to the working surface of the CE decreases, which improves its operational characteristics.

 Figure 1 shows a typical circuit of a microwave moisture meter, in which the proposed method can be implemented and the proposed SE used. Figure 2 shows the construction of the BOP: 9 prototype, 10 proposed according to claim 3, 11 - proposed according to claim 4 of the claims; figure 3 shows the frequency dependence of the reflection coefficients for two samples of wet material, and the humidity of one of the samples coincides with the limit value of W 0 or W 1; in FIG. 4-6 show the resonance curves in the coordinates of the SWR-frequency obtained experimentally using CE 9, 10, 11, respectively; figure 7 shows the dependence of G (W) for the prototype and the proposed SE.

 The proposed method for measuring humidity can be implemented in microwave moisture meters, the General structural diagram of which is presented in figure 1. The moisture meter consists of a generator 1, an isolation element 2, a splitter 3, a communication element 4, a CE 5 that interacts with the test object 6, and a data processing unit 8. The generator 1 generates a power-stabilized continuous or pulsed microwave signal at a fixed operating frequency. Decoupling element 2, for example, a valve, an attenuator, or a directional coupler, ensures stable operation of the generator 1 at any values of Г that may appear during operation of the moisture meter. Splitter 3, for example a circulator or directional coupler, is designed to transfer energy from the generator 1 to the SE 5 and from the SE to the detector 7. The communication element 4 may have any design (loop, pin, reactive diaphragm); when performing a preset, it should provide the ability to change the coupling coefficient. SE 5 is a VOR, protected at the output end by a dielectric plate to prevent particles of the test object 6 from entering the VOR cavity, elements 1-5, 7 encircled by a dotted line form a microwave sensor that generates an electrical signal for measuring information (voltage proportional to the value of the reflection coefficient) received to the input of data processing 8, which converts it into a signal that displays the value of humidity.

The possibility of carrying out the invention is illustrated by measurement data performed on solutions of isopropyl alcohol C ₃ H ₇ OH in water. The moisture content of the samples was changed by varying the volumetric content of the components:
WV _in / (V _in + V _s ),
where V _in , V _{with the} volume of water and alcohol, respectively.

 The value of W 0 was attributed to isopropyl alcohol in the delivery state, W 1 corresponds to pure water. Experience shows that solutions of isopropyl alcohol in water are a convenient model material for tuning and calibrating microwave moisture meters, because the range of values of the permittivity components ε ′, ε ″ in these solutions is close to the values characteristic of many objects of the moisture metering of solutions, mixtures , soil, food.

As examples of the implementation of the CE, VOR, made on the basis of a standard rectangular waveguide of a 3-cm range with a cross section of ahb 23x10 mm, were chosen. Figure 2 shows the CE 9 in the form of a regular BOP and two irregular BOR CE 10 with a section monotonically changing in size of the narrow wall b__ → b ^' , and CE 11 in a section monotonously changing in size of the wide wall a__ → a' (b '3 mm , a '13 mm). The longitudinal size of all SEs is the same (C 30 mm). A communication element in the form of a flat diaphragm was installed in front of the input flange of the SE. Two types of diaphragms were used: inductive (I) and capacitive (E). The diaphragm sizes were varied to provide different values of the coupling coefficient. Further, abbreviated designations of the diaphragms are used, for example, I8 denotes an inductive diaphragm with a hole size of 10x8 mm; E2.5 denotes a capacitive diaphragm with a hole size of 23x2.5 mm.

The resonant characteristics of the CEs were constructed using a P2-61 type serial panoramic standing wave coefficient (SWR) meter. The communication diaphragm was selected so that at one of the limits of the range of measured humidity (W = 0 or W = 1) the test VOR was matched. The quality of coordination was determined by the smallest value of the coefficient of the standing wave SWR _min 1.0. The reflection coefficient in this case also reaches its minimum value: Г _pmin 0. Experience has shown that with an increase in the humidity of an object, the Q factor of the VOR increases and the resonant frequency tends to decrease, but these changes occur with a violation of monotony. Therefore, the method for determining humidity in a wide range of W values cannot be based on measuring only the resonant frequency or only the resonant value of the reflection coefficient Г _p .

 The proposed method of measuring humidity in the microwave differs in that the measurement of G is performed at a fixed frequency, which is resonant only for an object with a limit value of humidity. Almost any VOR interacting with an object of arbitrary humidity can be tuned to resonance in matching mode. This is achieved by selecting the appropriate communication element.

 In FIG. Figure 3 shows two typical situations: a) agreement is achieved on a sample with maximum humidity, for example, W 1; b) agreement is achieved on a dry sample, for example W 0.

где b = Q_o/Q_вн коэффициент связи;
α = 2Δf Q_н/f обобщенная расстройка;
1/Q_н (1/Q_о)+(1/Q_вн);
Q_о, Q_вн, Q_н собственная, внешняя и нагруженная добротности соответственно;
f частота возбуждения;
Δf = f - f_p разность между частотой возбуждения и резонансной частотой резонатора f_р.As is known, the reflection coefficient from the resonator at an arbitrary frequency f can be calculated by the formula (for a quadratic detector):

where b = Q _o / Q _int coupling coefficient;
α = 2Δf Q _n / f generalized detuning;
1 / Q _n (1 / Q _o ) + (1 / Q _ext );
Q _about , Q _ext , Q _n own, external and loaded Q factors, respectively;
f excitation frequency;
Δf = f - f _{p the} difference between the excitation frequency and the resonant frequency of the resonator f _p .

 The minimum value of the reflection coefficient, Г 0, is achieved at the resonant frequency when the resonator is matched with the exciting generator, i.e. for β = 1, α = 0.

The reflection coefficient at resonance G _p , i.e. in the prototype method, when either the generator or the resonator is adjusted to compensate for the shift of the resonant frequency caused by a change in the humidity of the object, it can also be calculated by formula (1) at α = 0, because 4β (1 = β) ² ≅ 1, and the value of α reaches several units for BOP, then for any humidity Г _р <Г and DГ _p <ΔГ as can be seen in Fig.3.

На фиг.7 показаны зависимости Г(W) для каждого ЧЭ при двух настройках на резонанс при минимальной и максимальной влажности. Видно, что получаемые по предлагаемому способу функции преобразования СВЧ-влагомера в координатах Г(W) имеют вид монотонных возрастающих или падающих кривых, причем вторые более линейны.In FIG. Figures 4-6 show the experimentally obtained resonance curves for SE 9, 10, and 11, respectively. Each SE by adjusting the communication diaphragms was adjusted for matching at W 1 (pure water, solid curves) and W 0 (isopropyl alcohol, dashed curves). The numbers near the resonance curves show the humidity of the solutions (W 0, 0.25, 0.5, 0.75, 1.00). The numbers near the vertical lines indicate the type of aperture and the value of the operating frequency. selected as a result of the tests of SE. By measuring the SWR at the operating frequency for each solution, we calculated the value of G by the formula

Figure 7 shows the dependences G (W) for each SE with two settings for resonance at the minimum and maximum humidity. It can be seen that the conversion functions of the microwave moisture meter obtained in the proposed method in the coordinates Г (W) have the form of monotonous increasing or falling curves, the latter being more linear.

 The settings and tests of the proposed method and SE show that the sensitivity of the measurement of humidity by the proposed method is two times higher than the sensitivity provided by the prototype method; the sensitivity of the measurements with SE, proposed by PP. 3, 4 of the claims, exceeds the sensitivity provided by the SE prototype; the use of the SE proposed according to claim 4 of the claims in comparison with the SE prototype, together with an increase in sensitivity, reduces the frequency stability requirements of the microwave generator by an order of magnitude.

 The proposed method for measuring humidity is implemented in two experimental samples of a microwave moisture meter for marine bottom sediments with decreasing and increasing conversion functions (SVR-6 and SVR-6M) that have passed tests and metrological certification and showed a measurement error of not more than 0.5% in the humidity range objects from 0.15 to 0.80.

Claims (4)

 1. A method of measuring humidity in a microwave oven, which consists in exposing the object under study to the electromagnetic field of a sensitive element in the form of an open waveguide resonator, measuring the reflection coefficient and determining the moisture value from a pre-constructed relationship between humidity and reflection coefficient, characterized in that the measurement is carried out at a fixed frequency and the coupling coefficient with the generator, and the frequency is chosen coinciding with the resonant and the connection is set critical for the resonator, the load applied to an object with a minimum or maximum humidity value.  2. The sensitive element in the form of an open waveguide resonator for implementing the method according to claim 1, characterized in that it is made irregular.  3. The element according to claim 2, characterized in that the irregularity of the resonator is provided by reducing the size of the narrow wall of the rectangular waveguide along the propagation direction.  4. The element according to claim 2, characterized in that the irregularity of the waveguide is provided by reducing the size of the wide wall of the rectangular waveguide along the propagation direction.

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