RU2399039C1 - Moisture measurement device - Google Patents

Abstract

FIELD: physics. ^ SUBSTANCE: moisture measurement device has a high-frequency controlled pulse generator 1, a voltage-to-current converter 2, a measuring two-terminal element on a LC-circuit 3, a capacitance sensor 4, a first pulse former 5, a phase comparator 6, a second pulse former 7, a data processing unit, a bidirectional counter 9, a clock pulse generator 10, a digital-to-analogue converter 11, an amplitude detector 12, a differential amplifier 13, a reference voltage source 14 and a display unit 15. ^ EFFECT: more accurate measurement of moisture and wider control range. ^ 2 dwg

Description

The invention relates to measuring equipment, and in particular to devices for measuring the moisture content of various bulk materials, including soil moisture.

A device for measuring humidity, comprising a capacitive sensor included in the measuring bipolar generator, the frequency of which varies inversely with the humidity of the material [A.S. USSR No. 529407, cl. G01N 27/22. Automatic electronic moisture meter / V.S. Roife. Publ. 09/25/1976].

The disadvantage of this hygrometer is the dependence of the measurement results on the electrical conductivity of the controlled material, which shunts the capacitance of the sensor, which leads to an increase in measurement error, and sometimes to failure of oscillations of the generator.

The closest in technical essence to the proposed device is a hygrometer containing a measuring two-terminal device with a capacitive sensor, a controlled high-frequency generator, a phase comparator and a display of results based on the measurement frequency counter [A.S. USSR No. 1774244, cl. G01N 27/22. Moisture meter / B.C. Roife, V.I. Shkutov, L.M.Portugalsky, P.A. Maksimtsov. Publ. 11/7/1992].

The introduction of phase-locked loop in a controlled oscillator can reduce the dependence of measurement accuracy on the electrical conductivity of the material being monitored, but the measurement result is affected by the error from the insensitivity of the frequency-locked loop system near the zero phase shift of the monitored signals. An increase in the conductivity of the material leads to a change in the resonant frequency of the measuring two-terminal network and a decrease in the amplitude of the output signal, which increases the phase error of the self-tuning of the generator frequency and, as a result, increases the error in measuring moisture.

The technical result, the achievement of which the present invention is directed, is to increase the accuracy of measurements and expand the range of control of moisture content of bulk materials.

To achieve such a technical result, a humidity sensor, comprising a capacitive sensor connected to a measuring two-terminal device on the LC circuit, a high-frequency controlled pulse generator, a phase comparator and an indication unit, additionally includes an amplitude detector, two pulse shapers, a clock pulse generator, a differential amplifier, reference voltage source, reversible counter, digital-to-analog converter, voltage-to-current converter, and data processing unit. In this case, the first output of a high-frequency controlled pulse generator through a voltage to current converter is connected to the input of the measuring two-terminal on the LC circuit, the output of which is connected through an amplitude detector to the first input of the differential amplifier and connected to the first input of the phase comparator on the “Exclusive OR” element through the first pulse shaper ", The second input of which is connected to the first input of the data processing unit and through the second pulse shaper is connected to the second output of the high-frequency th controlled pulse generator. The output of the phase comparator on the “Exclusive OR” element is connected to the second input of the data processing unit, the third input of which is connected to the output of the clock pulse generator and to the counting input of the reverse counter, the inputs of “Resolution of the account” and “Counting direction” of which are connected respectively to the first and second the outputs of the data processing unit, the third output of which is connected to the display unit. In addition, the fourth input of the data processing unit is connected to the control input of the voltage-to-current converter and to the output of the differential amplifier, the second input of which is connected to a reference voltage source and one input of the digital-to-analog converter, the other input of which is connected to the output of the reversible counter, and the output of the digital-to-analog converter is connected with the input of a high-frequency controlled pulse generator.

Thanks to a new set of essential features, due to the use of a two-phase high-frequency controlled pulse generator and the use of a phase comparator on the “Exclusive OR” element, the deadband of the device near the zero phase shift is eliminated. This achieves a significant increase in the accuracy of measuring the moisture content of controlled materials and the expansion of the control range.

The analysis of the prior art made it possible to establish that analogues that are characterized by a combination of features identical to all the features of the claimed technical solution are absent, which indicates the compliance of the claimed device with the patentability condition of "novelty". Search results for known solutions in this and related fields of technology in order to identify features that match the distinctive features of the claimed device from the prototype have shown that they do not follow explicitly from the prior art. The prior art also did not reveal the popularity of the impact provided by the essential features of the claimed invention, the transformations on the achievement of the specified technical result. Therefore, the claimed invention meets the condition of patentability "inventive step".

Figure 1 shows a structural diagram of the proposed device for measuring humidity, and figure 2 shows the structure of the high-frequency controlled pulse generator.

The device includes a high-frequency controlled pulse generator 1, at the two outputs of which rectangular pulses are formed, shifted by a phase angle of 90 ° relative to each other. The pulses from the first output of the high-frequency controlled generator 1 through a voltage to current converter 2 are fed to a measuring two-terminal 3 containing an LC circuit with a variable capacitor parallel to which a capacitive sensor is connected 4. The signal from the output of the measuring two-terminal 3 is converted by the first driver 5 into rectangular pulses, which are compared by a phase comparator 6 on the element “Exclusive OR” with the phase of the reference pulses coming from the second output of the high-frequency controlled oscillator torus 1 through the second shaper 7. The pulses from the output of the phase comparator 6 and the shaper 7 are fed to the first and second inputs of the data processing unit 8, which controls the operation of the reverse counter 9, the counting input of which is connected to the output of the clock generator 10 and connected to the third input of the block data processing 8. To the output of the reversing counter 9 is connected the code input of the digital-to-analog converter 11, the output voltage of which is used to control the frequency of the pulse generator 1. The amplitude of the output signal measuring two-terminal 3 is converted into direct voltage by an amplitude detector 12 and compared by a differential amplifier 13 with a threshold voltage that is generated by a reference voltage source 14 and is also fed to the reference input of a digital-to-analog converter 11. The output voltage of the differential amplifier 13 is used to adjust the amplitude of the current pulses at the output of the voltage converter in current 2 and fed to the fourth input of data processing unit 8, information from the third output of which displays display unit 15.

To obtain rectangular pulses of different frequencies at the two outputs of the controlled generator 1, shifted by a phase angle of 90 ° relative to each other, a voltage to frequency 16 converter is used as part of the controlled generator, the output of which is connected to the first inputs of the logical inverter 17 and the logical repeater 18 assembled on Exclusive OR elements, the second inputs of the inverter 17 and the repeater 18 are connected to a +5 V power supply and ground, respectively, and two D-flip-flops 19 and 20, operating in counting mode, the inputs of which are connected to the outputs of the inverter 17 and the repeater 18, respectively (figure 2).

A device for measuring humidity operates as follows.

A high-frequency controlled pulse generator 1 generates two sequences of rectangular pulses with an adjustable repetition rate, phase-shifted by an angle of 90 °. The pulses from the first output of the controlled generator 1 through a voltage to current converter 2 having a large output impedance are fed to the LC circuit of the measuring two-terminal 3, to which a capacitive sensor 4 is connected. Due to the selective properties of the LC circuit, a harmonic signal U is generated at the output of the measuring two-terminal 3 ₃ with a frequency equal to the pulse repetition rate of the controlled generator 1. This harmonic signal is converted into a sequence of rectangular pulses by the first pulse shaper owls 5, which are fed to the first input of the phase comparator 6, which performs the function of measuring the phase shifts of the pulse sequences. From the second output of the controlled generator 1, the 90-phase-shifted pulses arrive through the second pulse shaper 7 to the second input of the phase comparator 6 and pass to the first input of the data processing unit 8, the second input of which receives a signal from the output of the phase comparator 6.

Before the start of the measurement cycle, the capacitive sensor 4 is placed in bulk material with a reference value of humidity (permittivity). Then, by adjusting the variable capacitor used in the measuring two-terminal device 3, the resonance frequency of the LC circuit is set approximately equal to the average frequency of the operating humidity control range f _P ≈ f _CP . This ensures high phase sensitivity and linearity of the phase-frequency conversion characteristics when controlling the moisture content of the material. The data processing unit 8 calculates the initial phase difference between the pulses from the output of the second shaper 7 and the output signal U _{3 of the} measuring two-terminal 3, which is converted into pulses by the first shaper 5.

When the humidity of the controlled material changes, the equivalent capacitance of the sensor 4 changes, proportionally to which the resonant frequency f _{P of the} LC circuit of the measuring two-terminal 3 decreases or increases. Therefore, a phase shift Δφ appears between the exciting pulses of the controlled generator 1 and the output signal of the measuring two-pole 3. If the frequency of the exciting pulses f _B is less than the resonant frequency of LC-circuit (f _B <f _P), the phase shift angle is positive (Δφ> 0), and at high frequency (f _B> f _P) the phase goal becomes negative (Δφ <0).

Depending on the sign of the phase shift ± Δφ, the data processing unit 8 supplies a high or low level signal from the first output to the control input "Counting direction" (± 1) of the reverse pulse counter 9, and from the second output, a command to its P-input "Resolution accounts. " In this case, the reversible counter 9 starts to work in the mode of summing or subtracting clock pulses f _T >> f _B coming from the clock generator 10. By changing the code at the output of the reversing counter 9, the output voltage of the digital-to-analog converter 11, which adjusts the frequency f _{B of the} controlled high-frequency generator 1.

The frequency adjustment speed f _{B of the} controlled oscillator varies depending on the phase difference between the output pulses of the controlled oscillator 1 and the output signal of the measuring two-terminal 3. With a large phase difference (Δφ> 5 °), the data processing unit 8 sends a continuous command to the P-input “Resolution »Reverse counter 9, which sums or subtracts all pulses of the clock frequency f _T. With a small phase difference (Δφ <5 °) between the pulses of the controlled generator 1 and the output signal of the measuring two-terminal device 3, the control unit reduces the time for issuing commands to the P-input of the reverse counter 9, which only works when every tenth pulse of the clock frequency f _T which ensures accurate adjustment of the resonance frequency of LC-circuit two-terminal measuring f _P 3 is determined by the onset of resonance the zero value of the phase shift Δφ≈0, then by the data processing unit 8 to the reversing input P Meters withstand 9 is commanded to "ban the account." Then, the data processing unit 8 measures the resonant frequency f _P and calculates the humidity value according to a predetermined algorithm.

When this device is operating (Fig. 1), the amplitude of the exciting current pulses I _B automatically received from the voltage to current converter 2 is automatically adjusted and proceeds as follows.

The amplitude detector 12 and the differential amplifier 13 isolates and amplifies the voltage difference U _P = K _U (U _POP -U _m ) between the threshold value U _POP ≥2 V generated by the reference voltage source 14 and the amplitude U _{m of the} output signal U _{3 of the} measuring two-terminal 3 This unbalance voltage U _P controls the amplitude of the exciting current pulses I _B at the output of the voltage-to-current converter 2, which are supplied to the LC circuit of the measuring two-terminal 3.

With an increase in the active conductivity G or a decrease in the active resistance R of the controlled material, the voltage U _P at the output of the differential amplifier 13 proportionally increases and the amplitude of the current pulses at the output of the voltage-to-current converter 2 increases simultaneously. The unbalance voltage U _P is encoded in the data processing unit 8 and is used as corrections to the result of measuring the resonant frequency of the LC circuit of the measuring two-terminal 3. In this case, after measuring the resonance frequency f _P at zero phase shift between the exciting pulses of the controlled generator and the output signal of the measuring two-terminal device 3 in the data processing unit 8, the corrected value of the resonant frequency f _{P.core of the} LC circuit is calculated. This frequency is calculated taking into account the influence of the active conductivity of the material being monitored or the equivalent Q factor of the LC LC circuit of the measuring two-terminal network with respect to the measured UP and initial U _{P. The} initial unbalance voltages are expressed as

.

.

To reduce the instrumental error of phase separation Δφ in the proposed device, two identical pulse shapers 5 and 7 are used, which makes it possible to compensate for the phase delay of the signals in the reference and measurement channels of the conversion in the high-frequency region.

In real devices, the function of the phase comparator is usually performed by D-flip-flops or reversible counters, which, with a phase angle difference Δφ≈0 °, have a dead band (hysteresis). According to the technical conditions for the use of digital microcircuits, it is impossible to simultaneously supply two signals to the D- and C-inputs of the triggers or to the C-input and the input “Counting direction” (± 1) of reversible counters. When the signals are fed simultaneously to both inputs, the D-flip-flops and the reverse counters do not work, i.e. remain in the previous logical state [Petrovsky II. Logic ICs of the KR1533, KR1554 series. Directory. - M .: BINOM LLP, 1993].

To eliminate this technical contradiction in this device it is proposed to "exchange speed for accuracy." Considering that the maximum switching frequency of digital microcircuits is significantly higher than the frequency of exciting pulses supplied to the LC circuit of the measuring two-terminal network 3 from the controlled generator 1, in the circuit (Fig. 2) a voltage to frequency 16 converter is used that generates doubled frequency pulses f ₁₆ = 2f _B . Then, using the inverter 17 and the repeater 18, the trigger 19 is cut along the slice (trailing edge), and the trigger 20 is provided along the leading edge of the output pulses of the voltage-to-frequency converter 16. This allows the phase of one of the signals at the inputs of the phase comparator 6 to be rotated 90 ° to get out of his hysteresis zone. The phase rotation at start angle ± 90 ° is provided by dividing the frequency f ₁₆ at twice the D-flip-flops 19 and 20.

To compensate for the delay in the elements, which affects the error of the separation of the phase difference, the same functional units (shapers and triggers) are used in each measuring channel of the device.

The use of a two-phase high-frequency controlled generator 1 in the proposed device eliminates the influence of the hysteresis of the phase comparator on the accuracy of the phase shift of high-frequency signals. In this case, the measurement range of the phase difference is limited to small limits

-90 ° <φ <+ 90 °. Therefore, as the phase comparator 6, you can use the exclusive OR gate, which does not have a hysteresis zone. The result of the proposed circuit solutions is a twofold increase in the accuracy of phase shift detection, which also allows to increase the accuracy of measuring the resonant frequency and to ensure reliability of humidity control over a wide range.

Additional advantages of the proposed device are increased phase sensitivity to the measured parameter and the elimination of the effect of "drift" of the resonance frequency f _P characteristic of resonant systems with a linear frequency sweep, as well as increasing the reliability of humidity control by averaging the measurement results.

To increase the phase sensitivity to the measured parameter and increase the quality factor of the LC circuit (Q = 10 ... 200) in this device, it is necessary to use a detector 12 and a pulse shaper 5 with high input resistances (on microcircuits with field-effect transistors). In addition, the exciting pulse signal from the controlled generator 1 should be fed to the measuring two-terminal 3 through a voltage to current converter 2 with a large output resistance, which also improves the quality factor of the LC circuit of the measuring two-terminal 3.

The elimination of the effect of the "drift" of the resonant frequency of the LC circuit when adjusting the frequency of the controlled oscillator 1 is due to a change in the frequency sweep rate provided by the data processing unit 8 with a decrease in the phase shift between the reference pulses and the output signal of the measuring two-terminal 3.

Improving the reliability of humidity control in the proposed device is ensured by averaging the result of measuring the frequency f _B over the time interval T _ISM , a multiple of 20 ms, which makes it possible to sharply reduce the influence of industrial frequency noise (50 Hz) on the conversion result.

In addition, when automatically adjusting the amplitude of the exciting current pulses in the converter 2, the amplitude of the output signal of the measuring two-terminal device 3 is little dependent on the frequency of the exciting pulses and is determined only by the threshold voltage: U _m ≈ U _pores ≥2 V. Therefore, the instrumental errors of the amplitude detector 12 and pulse shaper 5 significantly decrease and practically do not affect the reliability of the results of humidity control.

In the device for measuring humidity can be used capacitive humidity sensors submersible and overhead type. The moisture sensor is a submersible type in the form of a probe mounted on a rod, which is introduced into bulk materials or into loose soils. To measure the moisture content of dense soil, a well is made with a hand drill into which a probe is inserted. An overhead type humidity sensor is applied to flat surfaces of controlled materials.

In the proposed device, the controlled high-frequency generator 1 can be implemented on the chip of the voltage-to-frequency converter КР1108ПП1 and use K561TM2 type triggers and K561LP2 type Exclusive OR elements (Gutnikov BC Integrated electronics in measuring devices. - L.: Energoatomizdat, 1988. - P. 269-272). The pulse shapers 5, 7 and the clock generator can be implemented on K561TL1 type microcircuits, a 9 reverse counter on a K561IE14 microcircuit, a differential amplifier 12 on a KP140UD1208 type microcircuit, and a digital-to-analog converter 11 on a K1108PA1 microcircuit (Gutnikov’s integrated electronics. - L.: Energoatomizdat, 1988 .-- P.231-232). The voltage-to-current converter can be built on a KTZ102 type transistor with a common base and an open collector and with a resistor in the emitter circuit. In the source of reference voltages, you can apply a voltage stabilizer on the K142EN5A chip. In the data processing unit, an AT90S1200 type microcontroller may be used, which contains a built-in ADC for coding the unbalance voltage U _P and an electrically reprogrammable memory device. The obtained digital codes of the values of f _P and U _P can be used as EPROM addresses, in which you can write the corresponding humidity values of the material being monitored, which are displayed on the display unit at the end of each conversion cycle.

The use of a controlled high-frequency generator that generates two sequences of pulses with a phase shift of 90 ° at a variable frequency self-tuning rate eliminates the dead band of the device near the zero phase shift, and in combination with automatic correction of the results, which weaken the effect of active conductivity, it can significantly increase the measurement accuracy and expand humidity control range of various materials.

Claims (1)

A device for measuring humidity, which contains a capacitive sensor connected to a measuring two-terminal device on the LC circuit, a high-frequency controlled pulse generator, a phase comparator and an indication unit, an amplitude detector, two pulse shapers, a clock pulse generator, a differential amplifier, a reference voltage source are additionally introduced into the device , a reversible counter, a digital-to-analog converter, a voltage-to-current converter, and a data processing unit, the first output of a high-frequency the impulse generator through the voltage to current converter is connected to the input of the measuring two-terminal on the LC circuit, the output of which is connected through the amplitude detector to the first input of the differential amplifier and through the first pulse former is connected to the first input of the phase comparator on the “Exclusive OR” element, the second input of which connected to the first input of the data processing unit and through the second pulse shaper connected to the second output of the high-frequency controlled pulse generator, and in the output of the “Exclusive OR” element is connected to the second input of the data processing unit, the third input of which is connected to the output of the clock pulse generator and to the counting input of the reverse counter, the inputs of “Resolution of the account” and “Counting direction” of which are connected respectively to the first and second outputs of the processing unit data, the third output of which is connected to the display unit, and the fourth input of the data processing unit is connected to the control input of the voltage-to-current converter and to the output of the differential amplifier, the second input of which ohm is connected to the output of the reference voltage source and one input of the digital-to-analog converter, the other input of which is connected to the output of the reversible counter, and the output of the digital-to-analog converter is connected to the input of the high-frequency controlled pulse generator.

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