SU1302213A1 - Device for measuring dielectric parameters of materials - Google Patents

Abstract

The invention relates to the measurement of dielectric parameters of materials and can be used to create sensitive capacitive sensors, active, reactive and impedance meters, measuring capacitor parameters, the purpose of the invention is to increase the sensitivity of the dielectric parameters of materials. To achieve this goal, an amplitude modulator 11 and a block 9 of pulse commands are additionally introduced into the device. The device also contains a measuring bridge 1, the two arms of which are formed by elements 2 and 3, the third shoulder by elements 4 and 5. The fourth arm includes either the measuring capacitor 6 or the terminal capacitor 7. The device has a switch 8 ,. high-frequency generator 10, high-frequency amplifier 12, phase detectors 13 and 14, phase shifters, 15 and 16, low-frequency amplifiers 17 and 18, synchronous detectors 19 and 20, recorder 21 and 22. 1 c.p. f-ly, 5 ill. (L

Description

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The invention relates to the measurement of dielectric parameters of materials and can be used to create sensitive capacitive sensors, active, reactive and full-resistance meters, and capacitor parameters meters.

The purpose of the invention is to increase the sensitivity of the dielectric parameters of materials. FIG. 1 shows the functional electrical circuit of the proposed device; in fig. 2 - diagrams explaining the sequence of the processes occurring in the device during the time; in fig. 3 - diagrams for elimination of interference arising during the switch operation; in fig. 4 is a block diagram of a pulse command block; in fig. 5 - diagrams explaining the operation of the pulse command block.

The main unit of the device is the measuring bridge 1. The two arms of this bridge are formed by elements 2 and 3 having the same complex resistances. For example, two identical capacitors or two identical inductors can be used as such elements. The third bridge arm consists of a variable resistor 4 and a variable capacitor 5. In the fourth bridge arm, using a switch, connects either a measuring capacitor 6 or a reference capacitor 7. In addition to the measuring bridge 1, the device contains a switch 8, a block 9 of pulse commands, a generator 10 high frequency amplitude modulator, 11, high frequency amplifier 12, phase detectors 13 and 14, phase shifters 15 and 16, low frequency amplifiers 17 and 18, synchronous detectors 19 and 20 and recorders 2 to 22.

The device works as follows.

Using the switch 8, the measuring bridge 6 or the reference capacitor 7 are alternately connected to the same shoulder of the bridge for equal periods of time. The switch 8 is controlled by voltage pulses from the pulsed command unit 9. The shape of the voltage pulses controlling the operation of the switch is shown in plot I (Fig 2). Each plot (Fig. 2 and 5) showed 15

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There is a dependence of the voltage from time. These pulses have a rectangular shape and a duty cycle of two. During the action of the control pulse, the switch connects the measuring capacitor to the bridge, during the pause between the pulses, the switch connects the reference capacitor to the measuring bridge. The high-frequency voltage supplying the measuring bridge comes from the high-frequency generator 10 through the amplitude modulator 11, the latter is controlled by the pulses from the pulse command block 9, and performs 100% amplitude modulation.

The shape of the pulses controlling the modulator is shown in plot II (Fig. 2). The frequency of the pulses controlling the modulator is twice as high as the frequency of the pulses controlling the operation of the switch. Relative. The position in time of the pulses controlling the switch and the modulator is such that the switching process of the measuring and reference capacitors takes place during the pause between the pulses controlling the modulator, i.e. at those time intervals when high frequency voltage is not applied to the measuring bridge. The duration of this pause is sufficient for the switch process of the yatopa to be fully completed during the pause time.

Thus, by using the amplitude modulator and matching the operation of the amplitude modulator and switch, the high-frequency voltage is applied to the measuring bridge only at those times when no switching in the bridge circuit occurs. When the measuring and reference capacitors switch, the high-frequency voltage is not applied to the measuring bridge. This makes it possible to get rid of the powerful noise impulse that occurs in known devices during the switch switching process.

The plots (Fig. 3) show the time dependence of the oscillation amplitude. Plots I and II (Fig. 3) show, respectively, graphs of the time variation of the amplitude of the high-frequency voltage feeding the bridge and the voltage at the output of the bridge in out 31

known devices that do not use an amplitude modulator. In the absence of a modulator, the high-frequency spreader that feeds the measuring bridge is supplied to it continuously. Therefore, in plot I (Fig. 3), the time dependence of the amplitude of the voltage supplying the bridge is a straight line.

As noted, the switching time of the switch has a finite value. During this time, depending on the switch design, either the measuring and standard capacitors are not connected to the measuring bridge, or both are connected. In both cases, a strong imbalance of the bridge occurs during the switch, which causes the voltage amplitude at the output of the bridge to increase dramatically. This creates a powerful noise pulse. These interference pulses are indicated on plot II (Fig. 3) by the letters Q, Interference pulses cause short-term overload of the measuring path and intense transients both in the measuring bridge and in the device nodes following the measuring bridge. Transients are marked on plot II (Fig. 3) by the letters S. Only at the end of the transients, the oscillation amplitude at the output of the bridge takes on the value corresponding to the parameters of the capacitor connected to the bridge. Since the interference pulses have a large value, the transients following the interference pulses have a high intensity and considerable duration.

A different position when using the modulator. Plots III and IV (Fig. 3) show, respectively, a group of variations in time of the amplitude of the high-frequency voltage supplying the bridge and the voltage at the output of the bridge when using an amplitude modulator. The time dependence of the amplitude of the voltage feeding the measuring bridge when using a modulator has the same appearance as the pulses controlling the operation of the modulator shown in plot II. (Fig. 2). When using a modulator, the high frequency voltage is disconnected from the measuring bridge during the entire switching time

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switch. Therefore, a short-term though and strong imbalance of the bridge during switch switching does not cause voltage to appear at the output of the bridge. As a consequence, a powerful interference pulse, which without a torus controller is generated during switch switching, is now absent, and the voltage at the output of the bridge during switch switching is zero.

Decreasing the output voltage of the measuring bridge to zero before switching the switch, and increasing the output voltage of the measuring bridge to a stationary value after switching the switch, are accompanied by transients indicated in diagram IV (Fig. 3) with letters b. However, the intensity of transients when using a modulator is substantially less than the intensity of transients in the absence of a modulator, indicated by the letters S on plot II (Fig. 3).

The decrease in the intensity of transients when using a modulator is due to the fact that in this case the limits of variation of the amplitude of oscillations at the output of the bridge during the transition process are much smaller than the limits of variation of the amplitude Dy during switch switching without using a modulator. This is due to the fact that when using a modulator, the oscillation amplitude at the output of the bridge during the transition process increases from zero to a stationary value corresponding to the balance of the bridge, provided that any one U-sensor is connected to it: either reference or measuring. In both cases, when measuring, the bridge is approximately balanced and the amplitude or output of the bridge is much smaller than when switching the switch in the absence of a modulator, when the unbalance of the bridge is very large, since neither the measuring nor the reference is connected to it. capacitors, or both capacitors are connected at the same time.

Due to the lower intensity of transients when using a modulator, the decay time of the transient process to a negligibly small level should also be shorter.

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Thus, the use of a modulator gives two advantages: firstly, a powerful impulse of interference is eliminated, due to the brief but strong imbalance of bridge 5 during switch switching, and, secondly, the time interval during which transients accompanying the increase and decrease in the amplitude of oscillations on the output of the bridge, have significant intensity. The latter circumstance facilitates the elimination of transient disturbances. This interference is eliminated by synchronous detection.

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The voltage from the output of the measuring bridge is fed to the high-frequency amplifier 12, and then supplied to two phase detectors 13 and 14. 20 The reference voltage to the control inputs of the phase detectors is supplied from generator 10 through phase shifters 15 and 16. Phase shifts introduced by phase shifters 15 and 16 are selected so that the voltage at the output of one phase detector, for example 13, is proportional to the difference in the reactances of the reference and measuring capacitors, and the output voltage of the second phase detector 14 is The difference between active resistances.

The output voltages of the phase detectors are amplified by low-frequency amplifiers 17 and 18, and are fed to synchronous detectors 19 and 20. With synchronous detection, the removal of 0 ts interference from transients, accompanied by their rise and fall in amplitude of oscillations at the output of the measuring bridge. For this purpose, the block of mpul.ssny teams produces a special. 45 voltage pulses controlling the operation of the keys of synchronous detectors. The shape of these pulses is shown in plots III and IV (Fig. 2). These pulses follow with the frequency of the combi- nation of the measuring and reference capacitors. The pulses controlling the synchronous detector begin at the same time as the pulses controlling the modulator and end earlier than the last. Thanks to this, the synchronous detectors are connected to the low-frequency amplifiers after the jump processes caused by the increased

The amplitude of the high-frequency voltage from zero to a stationary value is attenuated and disconnected from the low-frequency amplifiers before the transients caused by a decrease in the amplitude of the high-frequency voltage before the next switching of the measuring and reference capacitors begin. This can be done due to the fact that the use of an amplitude modulator makes it possible to greatly reduce the intensity of transients and thereby reduce their decay time to a negligible amount.

Connecting synchronous detectors to low-frequency amplifiers only at those time intervals when transients are attenuated and cannot affect the measurement result, allows you to eliminate interference from transients and thereby increase the sensitivity and accuracy of measurements. The voltage from the output of synchronous detectors is applied to recorders 21 and 22.

Block 9 of pulse commands (Fig. 4) consists of multivibrator 23, trigger 24, output terminal 25, integrator 26, comparator 27, input terminal 28, elements 2И 29 and 30, output terminals 31 and 32, comparator 33 of the comparator input terminal 34 and output terminal 35.

Block pulse commands works as follows. The multivibrator 23 generates rectangular pulses, the shape of which is shown in plot I (Fig. 5). These pulses arrive at trigger 24. The voltage pulse shape at the trigger outputs is shown in plots II and III (Fig. 5). The voltage from the first output of the trigger 24 is supplied to the output terminal 25 and is used to control the switching of the measuring and reference capacitors.

The voltage from the output of the multivibrator 23 is fed to the input of the integrator 26. The voltage form at the output of the integrator is shown in plot IV (Fig. 5). This voltage is applied to one of the inputs of the comparator 27. The other input (terminal 28) of the comparator 27 is applied to a constant voltage E. The voltage at the output of the comparator 27 has the form of square impulses. The fronts of the latter correspond to the points in time at which the instantaneous values of the sawtooth voltage at the integrator output are compared with the constant voltage E ,. The voltage shape at the output of the comparator 27 is shown in plot V (Fig. 5). The duration of the pulses at the output of the comparator 27 can be adjusted by changing the value of the constant voltage E,. The voltage pulses from the output of the comparator 27 are applied to one of the inputs of each of the two logic elements 2and 29 and 30, which perform the logical operation 2and. The other inputs of these elements are pulsed from the outputs of the trigger 24, the voltage pulse shape at the output of logic elements 2and 29 and 30 is shown in plots VI and VII (Fig. 5). These pulses are applied to terminals 31 and 32 and are used as a reference voltage controlling the synchronous detectors.

The sawtooth voltage from the output of the integrator .26 is also applied to one of the inputs of the comparator 33. A constant voltage E "is applied to the other input of the comparator. The shape of the voltage pulses at the output of the comparator 33 is shown in plot VIII (Fig. 5). The pulses from the output of the comparator 33 are applied to terminal 35 and serve to control the amplitude modulator. The required duration of the pulses controlling the operation of the synchronous detectors and the amplitude modulator is set by varying the values of the constant voltages E and Ej at the input terminals 28 and 34 of the comparators 27 and 33.

Measurements with the device are performed as follows. The measuring capacitor is filled with the test substance and connected to the instrument. In the general case, at the initial time, the parameters of the measuring and reference capacitors are not equal, and the bridge is unbalanced. Due to the imbalance of the bridge and the large difference in the values of the parameters of the measuring and reference capacitors, the output voltage of the bridge can be so large that any elements of the measuring path can be saturated. In order to avoid this, you can, for example, reduce either amp022138

a high frequency generator 10, or a gain of the high frequency amplifier 12.

5 Suppose the second option is used. In this case, the gain of the high frequency amplifier is reduced until the readings of recorders 21 and 22

10 will begin to change. A change in the readings of the recorders means that the signals at the output of synchronous detectors depend on the gain of the measuring path, and, consequently, the latter is not saturated. Further adjustment of the device is carried out by -. with the method of successive approximations. The parameters of the reference capacitor are selected in such a way

20 so that the voltage indicated by the recorders becomes small, but not equal to zero, for example, so that it is one tenth of the maximum value that the recorder can indicate. The presence of voltage at the output of the measuring channels indicates that the parameters of the measured and reference capacitors are not equal, and therefore the sequential replacement of the measured and reference capacitors, carried out by the switch, is accompanied by modulation of the voltage at the output of the bridge.

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Then, change the capacitance of the variable capacitor 5 and the resistance of the variable resistor 4, forming one of the arms of the measuring bridge,

40 achieve maximum readings of the recorders at the outputs of the measuring channels. It is known from the theory of bridge circuits that a change in the parameters of parts of a bridge circuit causes the greatest change in voltage at the output of the bridge if the bridge is approximately balanced. Therefore, if you change the parameters of variable capacitor 5 and variable

0 resistors 4, forming one of the measuring bridge arms, the deepest modulation of the voltage at the bridge output, and hence the highest voltage indicated by the recorder 14,

55 will correspond to the values of the capacitance of the variable capacitor 5 and the resistance of the variable resistor 4, at which the bridge is approximately balanced.

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This completes the first approximation to the optimal setting of the instrument. In this case, an approximate equality of the parameters of the measuring and reference capacitors and an approximate balancing of the bridge are ensured. After tuning in the first approximation, the gain of the high-frequency amplifier 12 slightly increases and the entire tuning process repeats in the second approximation with an increased value of the gain of the amplifier 12. Thus, gradually increasing the gain-/ PIR of the high-frequency amplifier, bring it to such a value as is required for measurement. Then the parameters of the reference capacitor are chosen in such a way that the voltage indicated by the recorders turns to zero. This OZ starts that both the capacitances and the active resistances of the reference and measuring capacitors coincide.

The equality of the parameters of the reference and measuring capacitors is fixed with the greatest accuracy at

provided that the phase shifts introduced by phase shifters 15 and 16 are correctly set.

The right setting of phase shifts introduced by phase shifters can be performed, for example, in the following way. Instead of a measuring capacitor, a capacitor with known parameters is connected to the device. The parameters of the reference capacitor 7 are set equal to the parameters of the connected capacitor, and the bridge is balanced by the method of successive approximations, however now the balancing is performed only by changing the variable resistor 4 and the capacitor 5. The parameters of the reference capacitor are not changed. After the bridge is balanced, it is necessary to slightly change the capacity of the reference capacitor 7. Because of this, the readings of the recorders will differ from zero. After that, the phase shifts of the phase shifters 15 and 16 should be changed, to achieve that the readings of one recorder become maximum, and the second - equal to zero. With this setting phase shifters

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one recorder is proportional to the difference in capacitances of the reference and measured capacitors, and the reading of the second recorder is proportional to the difference in active resistances of the reference and measured capacitors. It is this setting of phase shifters that is optimal.

Thus, reducing to zero the high-frequency voltage supplying the measuring bridge during the switching of the measuring and reference capacitors eliminates the disturbances in the operation of the switch. Connecting synchronous detectors to the output of low-frequency amplifiers only at those intervals when the amplitude of the high-frequency voltage at the output of the measuring bridge does not change, excludes noise caused by transients accompanying the rise and fall of the amplitude of the high-frequency voltage at the output of the bridge.

Claims (2)

1. A device for measuring the dielectric parameters of materials, comprising a high-frequency generator, a measuring bridge, a switch, a high-frequency amplifier, and two measuring channels, each of which consists of a series-connected phase detector, a low-frequency amplifier and a recorder, with the control input of each phase detector through a corresponding phase shifter connected to a high frequency generator, characterized in that, in order to increase sensitivity, it is equipped with an amplitude modulus a pulse and a pulse command block, with the output of the amplitude modulator connected to the measuring bridge, one input to a high-frequency generator, and the other to one of the outputs of a block of pulse commands, the other three outputs of which are connected respectively to the control inputs of the switch and two synchronous detectors .. 2. The device according to claim 1, about tl and - the fact that the block of pulse commands contains a series-connected multivibrator and a trigger, the outputs of which are connected to one of the inputs of two elements 2I, II the other inputs of which are connected to the output of the first comparator, and the output of the multivibrator is connected to the input of the integrator, the output of which is connected t /. : 3 and one VI (/four ff BUT : 302213 12 with one of the inputs of the first and second comparators, the other inputs of which are connected to the outputs of a constant voltage source. VlLZ.t E 9 ug Y: r V Pli2.J ate FIG. ft and I and w and t II I 1 I 1 I I lp p p p p "8.5