SU1749810A1 - Method of determining moisture content - Google Patents

Abstract

Use: measurement of moisture solids. SUMMARY OF THE INVENTION: A method for measuring humidity using a three-electrode surface-mounted capacitive sensor, in which electrodes are placed in a line connected in series with a reference resistor, including applying to the first and second electrodes a voltage sensor of the same frequency and phase and adjusting the voltage amplitude on the second electrode until reaching voltage on the first electrode maximum. The voltage measurement between the first and third electrodes is carried out at two frequencies, the humidity value is determined by the calculation formula. 3 or

Description

The invention relates to physico-technical measurements and can be used to measure the moisture content of materials of non-uniform thickness.

A method is known for measuring the moisture content of solids using a capacitive three-electrode sensor, which means that the electrodes are mounted on the object under investigation in the same plane and on the same axis, the second electrode being located between the first and third electrodes so that the distance between the first and second electrodes is shorter the distances between the second and third electrodes, alternating voltage is applied to the electrodes, the capacitance between the electrode pairs is measured, and the moisture content is calculated from the capacitance measurement results a.

The disadvantage of this method is the impossibility of eliminating the influence on the results of measurements of the surface layer of variable thickness, which is caused by the high nonlinearity of the sensitivity of the sensor with coplanar electrodes to the dielectric properties of layers of material located at different depths.

The closest to the invention in its technical essence is a method for measuring the moisture content of solids using a capacitive three-electrode sensor, which consists in the fact that the electrodes are mounted on the object under study. in the same plane and on the same axis, the second electrode being placed between the first and third electrodes so that the distance between the first and second electrodes is less than the distance between the second and third

4 O 00

ABOUT

using electrodes, applying alternating voltage to the electrodes, measuring the capacitance between electrode pairs and measuring the capacitance of the material based on the capacitance measurement results, applying the same frequency and phase voltage to the three electrodes, changing the amplitude of the voltage between the first and second electrodes capacitance values between them measure capacitances between electrode pairs.

The disadvantage of this method is the impossibility of eliminating the effect on the measurement results of the loss resistance of a capacitive three-electrode sensor.

The purpose of the invention is to improve the measurement accuracy by eliminating the effect of loss resistance of a capacitive sensor.

The goal is achieved by the method of measuring the moisture content of solids using a three-electrode spaced capacitance sensor connected in series with a reference resistance consisting in applying the same frequency and phase voltage to the first and second electrodes and adjusting the voltage amplitude on the second electrode , the voltage amplitude control is conducted between the second and third electrodes until the voltage amplitude between the first and second electrodes reaches its maximum, the voltage measurement between the first and third electrodes is carried out at two frequencies, and the humidity value determined by the formula

P AiJL

P AluS iG

where a is the proportionality coefficient;

Ui is the voltage between the first and third electrodes at the first frequency;

U2 is the voltage between the first and third electrodes at the second frequency.

Figure 1 shows the spatial distribution of electric fields in a three-electrode sensor and the test material; figure 2 - block diagram of the device that implements the method; in FIG. 3, an equivalent circuit for replacing a voltage divider consisting of a three-electrode sensor and a reference resistance.

The three-electrode sensor (figure 1) consists of three electrodes 1-3 placed on the object under test 4. This creates two electric fields - the main one created between electrodes 1 and 3 under the action of a potential difference (p, and an additional one created between electrodes 1 and 2 under the action of the potential difference pi, dg er, p2, pz is the potential of the corresponding electrode.

the main field is relatively shallow. If (pi pi, then the power lines of the additional field are directed towards the power lines of the main field, therefore in the surface layer of the material of the object

due to the superposition of the fields, their mutual compensation occurs. By adjusting the potential value (pi on electrode 2 at a constant potential p on electrode 1, the intensity of the additional voltage is changed

the field, while changing its penetration depth, which allows to compensate the main field to a predetermined depth. At this level, the capacitance between the electrodes 1 and 3 changes and reaches a minimum value, i.e. influence is excluded

surface layer of variable thickness.

The device implementing the method (Fig. 2) contains a three-electrode sensor consisting of three electrodes 1-3, an amplifier 5 with a variable gain from 1 to 1.5, a voltage follower 6, two meters 7 and 8 of alternating voltage, reference impedance 9 R3, switch 10, two alternating voltage generators 11 and 12 with output signal frequencies a and. respectively, and the same amplitude of the output signal. Total capacitance Cx sensor

Cx-Sat + Sc + C13-C12,

where Ce is the ballast capacitance of the sensor (substrate, wires, etc.);

Cu is the own partial capacitance of the electrode 1;

Ci3 is a partial capacitance between electrodes 1 and 3;

Ci2 is a partial capacitance between electrodes 1 and 2.

The intensity of the electric field created by the coplanar system of electrodes is maximal on the surface of the material and non-linearly decreases in its thickness. In capacitance C and Ci3 significant contribution

introduces a surface layer of the material of the object, the dielectric properties of which may differ markedly from the rest of the material mass. The capacity of the Cia is entirely determined by the properties of the surface layer.

material. By regulating the voltage on the electrode 2, it is possible to achieve elimination of the influence on the measurement results of the surface layer.

In connection with the presence of loss of total capacitance between electrodes 1 and 3, an equivalent capacitive sensor replacement circuit can be represented as a two-pole impedance with parallel connection of a capacitor Cx - total sensor capacitance and sensor loss resistance (FIG. 3).

Complex voltage in the circuit (Fig. 3)

one

i0 willow + willow (pr- + j w Cx) Re

where Uo and Oviyh are the complex voltages on the voltage divider — the reference resistance and the capacitive sensor, and on the capacitive sensor, respectively;

RX - material loss resistance in the sensor.

The relationship of complex stresses

-rr O + jrHi Ra- (2)

ivykh

The modules of complex voltages are the observed amplitudes of the voltages, respectively.

Square module (2)

ten

15

20

25

1 Iftl -

-k

Cx 2 2 C7)

(O2 - JP

The measurement method is as follows.

A constant voltage amplitude Uo is applied at frequencies and ftfc to a voltage divider consisting of a series-connected reference resistance and a capacitive three-electrode sensor, separately in time. The amplitudes of the voltages Ui and Da are measured on a capacitive three-electrode sensor between the first and third electrodes. When adjusting the voltage amplitude between the first and second electrodes, the elimination of the influence of the surface layer occurs when the total capacitance Cx reaches the minimum value, and the voltage amplitude between the first and third electrodes reaches the maximum value. The voltage amplitude Uo on the voltage divider is kept constant at both frequencies w and an.

Then (7) will take the form

Rach2

+ p2 + CxRi.

(3)

Let us conventionally denote the ratios of the amplitudes of the voltages on the voltage divider to the voltage on the capacitive sensor through k:

kUo

Ufiblx

(four)

Then (3) in view of (4) k2 (1 + feVCxRi.

R

(five)

Observing identical amplitudes of voltages Uo at two frequencies UT and ftfc of input signals on a voltage divider — a reference resistance and a capacitive sensor, and amplitudes of voltages at a capacitive sensor at the same frequencies Ui and U2, we obtain a system of two equations with two unknowns Cx and Rx:

R

(1+) 2 + u 2c2R2 k2. Kh

(1 + j |) 2+ y2: CxRi k2,

Uc

Uc

where ki is ttu and amplitude ratios

voltages at frequencies ((& 1, respectively.

Solution (6)

.. Xg

(eight)

where Uo is the amplitude of the input voltage at both frequencies (nd and wz.

Similarly, the capacitance Cx of a three-electrode sensor with a dry reference material can be measured at the same amplitude of the voltage Uo at the sensor input:

 Ke

and b. and b 2-

ABOUT)

(9)

where Uci and Uc2 are the amplitudes of the voltages at the output of the three-electrode sensor with the dry reference material at two frequencies (d) 1 and I2 of the input signals, respectively.

The ratio of containers (8) and (9) will characterize the moisture level of the matter (6) la:

55

(ten)

The denominator in (10) is a constant value, which can be denoted as A, the proportionality coefficient:

one

1 1 U b U & 1

(eleven)

Then the moisture level of the material is non-uniform in size

RA

Yj1

1and§

(12)

Thus, in the moisture measurement method, the influence of the surface layer on the measurement result is excluded by adjusting the voltage amplitude between the first and second electrodes to the maximum value of the voltage amplitude between the first and third electrodes, and the loss resistance of a three-electrode sensor is excluded by measuring voltage amplitudes at two frequencies of input signals with the subsequent calculation of humidity.

... you i & wy

0

five

Claims (1)

DETAILED DESCRIPTION OF THE INVENTION A method for measuring the moisture content of solids using a three-electrode surface-mounted capacitive sensor, connected in series with a reference resistance, consists in supplying the first and second electrodes with a voltage sensor of the same frequency and phase and controlling the voltage amplitude on the second electrode In order to increase accuracy, the voltage amplitude regulation is carried out between the second and third electrodes until the voltage amplitude between the first and second electrodes is reached. maximum, the voltage measurement between the first and third electrodes is carried out at two frequencies, and the humidity value is determined by the formula RA J1 U iG where A is the proportionality coefficient1,1 Ui is the voltage between the first and third electrodes at the first frequency; 1) 2 - voltage between the first and third electrodes at the second frequency.