SU1434349A1 - Self-excited dielectricity meter - Google Patents

Abstract

The invention relates to the field of measurement technology and can be used to measure the moisture content of bulk materials. The aim of the invention is to reduce the measurement error caused by the influence of the conductivity of the material. The device contains an auto-oscillator, in the measuring circuit of which a capacitive sensor is included, and an additional winding, rectifier-, source pi 1; Accuracy increase is achieved due to the dynamic change of the mode of operation of the autogenerator transistor. 4 il.

Description

u 4b

Od

eleven

The invention relates to a measuring technique, in particular to devices for measuring humidity using a dielectric (capacitive) method, and can be used to measure the moisture content of grain / mineral fertilizers, products of mining and chemical raw materials.

The aim of the invention is to reduce the measurement error caused by the effect of the active conductivity of the material.

FIG. 1 shows a functional diagram of an auto-generator dielectric meter; in fig. 2 - the generalized dependence of the steepness of field-effect transistors on the current; in fig. 3 is a relative change in the oscillation frequency from the sensor conductivity for an auto-oscillator, performed on an insulated-gate field-effect transistor in FIG. And - the same for the auto-generator, performed on the field; transistor with pn-junction.

The dielectric meter consists of an auto-oscillator sensor 1 assembled on a field-effect transistor 2, an oscillating circuit formed by a coil 3 and a capacitor 4, a feedback coil 5, an additional coil 6, a rectifier 7, a bias source 8, and a frequency change converter 9 proportional to this change.

FIG. Figure 2 shows the generalized current versus current slope for field-effect transistors (curve 10); FIG. Figures 3 and 4 show the relative frequency variation of the autogenerator versus the sensor conductivity for an FET with an insulated gate without a rectifier 7 (output is shorted, the auxiliary coil is disconnected) - curve 11 and if present - curve 12 and for a p-junction field-effect transistor curve 13.

FIG. 1-4 marked; Sd and g are the capacitance and conductivity of the sensor, S Motkc is the slope at an arbitrary value of the drain current and the maximum value of the slope for a given type

I and I

- drain current

transistor; and the drain current at which the transistor's slope is maximum, tgi is the loss tangent, f and & f are the frequency of the unloaded circuit and its change.

0

five

0

The change in the capacitance of sensor 1 caused by the dielectric constant of the material placed in it changes the frequency of the self-oscillator. With small changes in the capacitance of the circuit, which is achieved by switching on a capacitor 4j whose capacitance is much greater than Cd, the reading of the recording device is proportional to the dielectric constant of the material.

In the device, the autogenerator is outputted on a field-effect transistor in the variable-slope mode.

To clarify the principle of compensating the conductivity of a sensor, consider the well-known expression for a relative change in the frequency of an oscillator in steady state when its oscillating circuit is shunted by conductivity.

Q f / fo 4l-0,25p (kS-g), (1)

where fp (21-4s) is the natural frequency of the circuit} p 2 irf «L ()

k L / Loc

L, L

wasp

0

- characteristic resistance of the circuit

- feedback ratio}

- inductance of the contour coil 3 i and the feedback coil-5;

5 total capacity

loop capacitor (capacitor 4) and sensor.

For soft excitation in mode A, when the operating point is located in the middle of the linear part of the characteristic, for each value of g you can find a value kS at which Q.- 1. Any change in conductivity 5 will either result in a breakdown of the oscillations or in violation of the condition

kS-g 0. (2) - Analysis of the characteristics of field-effect transistors j, in particular, the dependence of steepness on current, showed that with the correct choice of the mode of their operation, it is possible to self-regulate the steepness of transistors in such a way that condition (2). The characteristic of field-effect transistors is such that their steepness in the initial section depends on the drain current (Fig. 2). The plots of FIG. 2 obtained by processing the characteristics of the known field-effect transistors. For KP902A and KP903B values

Max

And s / wakc are respectively equal

550 and 200 mA and 22 and 100 mA / V. The current of the oscillator at Q 1, which is the purpose of this mode, cannot exceed

 (3)

where E is the drain-source voltage, approximately equal to the voltage of the power supply.

The average value of the current, taking into account the residual voltage, is (0.22-0.28) 1.

Select the feedback factor from the condition

k i3j, 6-4i5UM2Kc

otES,

(four)

and Movc 2.5

where oi is the integral steepness of the dimensionless characteristic, is equal to A () / Л (, ke) determined according to the diagram of fig. 2, g (yy kc maximum conductivity of the sensor.

The graph shown in FIG. 2 is averaged, real values of ot, taking into account the variation of the parameters of field-effect transistors and variations in the steepness of the current corresponding to the linear section, 1.2-6.

If the value of 1d is greater than indicated in (4), the change in S from I will be so much different from linear that condition (2) does not hold.

However, the fulfillment of (4) is not sufficient, since the output of the transistor from mode A will lead to a change in the effective slope, and condition (2) will be violated. To satisfy condition (2), with increasing g, the operating point is shifted linearly to high currents. This is achieved by introducing rectifier 7, which for the purpose of galvanic isolation from bias source 8 is powered by additional coil 5. Voltage of bias source is selected so as to ensure operation with maximum sensor conductivity. Eg 0

five

The radio frequency increases with decreasing conductivity, and the voltage at the output of the rectifier increases, the polarity of which is opposite to the voltage of the bias source. The total bias voltage decreases and the operating point moves to the middle of the new operating characteristic. In this way, a second conductivity compensation condition is provided.

Since in real conditions oL, the max-mchx of the transistor is different from the instance of the instance, the voltage of the bias source and the rectifier must be adjusted within small limits. For this purpose, the rectifier 7 is equipped with a potentiometer.

FIG. Figure 3 shows the characteristic of a device made on a KP902A transistor with the following circuit data: a bias voltage of 5 V, L 25 µH, C 2000 pF. f 894.6-kHz in FIG. 4 for KP903B with the data: a bias voltage of 1.3 V, L 40 µH, C 2000 pF, f 676 kHz. Sensitivity to changes in capacity in all models varied by no more than 3.5% RH. The use of the device allows

 measure the dielectric constant at a loss angle tangent in the circuit is greater than one, while in the whole range of conductivity variation the error does not exceed 0.25 0.3% rel.

Claims (1)

Invention Formula An automatic oscillator including an oscillating circuit with a capacitive sensor included in an oscillator made on a field-effect transistor, an additional coil inductively coupled to the coil of the oscillating circuit, a rectifier, a bias source and a frequency converter to a signal proportional to this change, - characterized by the fact that In order to reduce the measurement error caused by the influence of the active conductivity of the material, the rectifier, the bias source, the coupling coil and the gate of the autogenous field-effect transistor are connected in series. eight w P; ..four X Fue. R (35 "d. 2