RU2094798C1 - Electrostatic method of measuring the moisture content of loose substances and device intended for its realization - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: method consists in measuring the potential difference between measuring electrode immersed in substance under check and supporting electrode. Measurement is performed in pulse mode. Every measurement cycle includes pulse charging of electrodes, measurement of potential difference between electrodes and pulsed removal of charge from electrodes and from substance being checked. In every subsequent cycle charge of electrodes is increased by fixed value. Measurement cycles are repeated till constant value of potential difference is obtained. Moisture content of substance under check is determined by the above-indicated potential difference with due regard for calibration curve. Device for measuring the moisture content has two electrodes 2,3, clock-pulse generator 15, high-frequency oscillation generator 17, switching pulse forming unit 16 three outputs of which are connected to control inputs of electronic switches connected to two channels of direct current amplification. At input, amplification channels are connected to electrodes 2,3, respectively, and at output they are connected to input of differential amplifier 12 which is in its turn connected to analog-to-digital converter 13 and to indicator 14. Every channel has first electronic switch 20-23, first follower 4, 5, second electronic switch 6, 7, capacitor 10, 11, second follower 8, 9, and third electronic switch 18, 19. EFFECT: higher measurement accuracy. 3 cl, 2 dwg

Description

 The invention relates to measuring equipment and is intended to measure the moisture content of bulk substances, such as sugar, grain, sawdust, washing powder, etc. and can be used in food, chemical, woodworking industries, agriculture, as well as trade and purchasing firms.

 Known methods and devices for measuring the moisture content of bulk solids, based on the determination of static electricity, in particular, the electric capacitance of a capacitor filled with a controlled substance (E. Krichevsky and others. Automatic control of the drying process of phosphate concentrate. Mechanization and automation of production of N 2, 1971, p. 38).

 However, with high accuracy in measuring the capacitance of a capacitor, the reliability of determining moisture depends on the level and packing density of a granular substance in a capacitor.

 Closest to the invention in technical essence is a method for continuously determining the moisture content of wood chips in a process stream, which consists in measuring the potential difference between two rotating electrodes (disks) of a sensor during the passage of a controlled substance (ed. St. USSR N 1165960, class G 01 N 25 / 56, 1985, bull. N 25).

 The implementation of the method, in addition to two electrodes, involves the presence of a means for measuring and indicating the potential difference between the electrodes connected to it.

 During rotation, the electrodes in accordance with their triboelectric properties are charged due to friction against the ambient air. In the absence of a controlled substance, the potential difference is maximum. Upon contact with the controlled substance, leakage of the discharge current leads to a decrease in the potentials of the electrodes and, accordingly, their difference. The degree of reduction of the potential difference is determined only by the humidity of the controlled substance and practically does not depend on the size of the particles and their compaction. The movement of the controlled substance relative to the electrodes ensures the removal of charged particles from the control zone and the supply of uncharged ones.

 The disadvantage of this method is the lack of reliability of the results with irregular measurement, especially during prolonged breaks and rapid control of humidity, for example, upon receipt of bulk food products, industrial raw materials. The triboelectric properties of metal electrodes are determined by the cleanliness of their surface. During interruptions in operation, even short-term ones, the surfaces of the electrodes are oxidized and, accordingly, their triboelectric properties change, therefore, for the reliability of the measurement, preliminary cleaning of the electrodes from oxides is required, for example, by their preliminary run in contact with the controlled substance. Naturally, the longer the break, the longer the sweep time. Artificial correction of the measurement results in this case is not effective, since the degree of purity of the electrodes is uncertain. In addition, this method does not take into account the measurement and humidity of the environment.

 In the present invention, the noted drawbacks are eliminated by cyclic pulse charging of the sensor electrodes from an external current source with an increase in the potential of the electrodes in the next cycle by a predetermined constant value compared to the potential of the electrodes in the previous cycle.

 According to the electrostatic method for measuring the moisture content of solids by the potential difference between the two electrodes in the present invention, the electrodes, measuring and reference, are charged in air to a fixed value, and then the measuring electrode is immersed in a controlled substance, and the measurements are performed in a pulsed mode, with each measurement cycle includes pulse charging of electrodes, measurement of potential difference between electrodes and pulse removal of charge from electrodes and controlled material tva, and in each subsequent cycle, when charging, the potential of the electrodes is increased by a predetermined constant value compared to the potential of the electrodes in the previous cycle, and the cycles are repeated until at least in two consecutive cycles the same difference between the electrodes is obtained, which determines the humidity of the controlled substances taking into account the calibration curve.

 The removal of charge from the electrodes and the controlled substance in the measuring cycle is performed, for example, by applying high-frequency oscillations to the electrodes, while the amplitude of high-frequency oscillations in the packet is first monotonically increased, and then monotonically reduced.

 In a device for measuring the moisture content of bulk solids containing two electrodes connected to measuring and indicating potential differences between the electrodes, the potential difference measuring device includes two direct current amplification channels and a differential amplifier connected to an analog-to-digital converter, and a generator is introduced into the device clock pulses connected to a switching pulse forming unit and a high-frequency oscillation generator, and each constant amplification channel then As it includes the first electronic key, the first repeater, the second electronic key and the second repeater connected in series, a capacitor is connected in parallel between the second key and the input of the second repeater, the output of the second repeater through the third electronic key connected to the input of the first repeater, and the output of the first repeater connected to its input and output of the generator of high-frequency oscillations through the first electronic key, while the control inputs of the first, second and third electronic keys are connected to the corresponding outputs of the formation of the switching pulses, and the outputs of the second repeaters are connected to the inputs of the differential amplifier.

 Improving the reliability of measurements in the proposed method is achieved by the fact that the charging of the electrodes is carried out not by friction against air, but by applying pulses from an external current source, and therefore does not depend on the state of their surfaces. The presence of oxides, given the high input resistance of the amplification channels, practically does not introduce measurement errors, since the oxides of metals or alloys that can be used to make electrodes, for example, nickel, stainless steel, brass, bronze, etc. have semiconductor conductivity.

 Cyclic pulsed charging of the sensor electrodes from an external current source with an increase in the potential of the electrodes in the next cycle by a predetermined constant value compared to the potential of the electrodes in the previous cycle, without changing the general principle of measuring relative humidity by potential difference, allows you to combine the processes of charging from an external current source with a discharge to a controlled substance. Together with the solution of the problem of removing the charge of a controlled substance by applying high-frequency oscillations in the same cycle, the need for both rotation of the electrodes and passage of the controlled substance between the electrodes is eliminated. At the same time, the design of the sensor is greatly simplified, reliability is increased, and power consumption is reduced, which is especially important in a design with autonomous power supply. The monotonic increase and decrease in the amplitude of high-frequency oscillations contributes to the almost complete removal of the residual charge and, accordingly, to increase the measurement accuracy. Without forced charge removal, an increase in the degree of charging of a controlled substance, regardless of humidity, leads to a decrease in current leakage from the electrodes and, ultimately, to charging the measuring electrode to a level determined by the reference electrode.

 The installation of only one electrode in a controlled substance and the use of another as a reference makes it possible to compensate for the influence of environmental humidity on the value of static charge, as well as expand the range of measurement of relative humidity both to lower and higher values.

 In the device for implementing the method, the use of voltage followers allows to increase the potential of the electrodes by a predetermined constant value by shifting their output voltages relative to the inputs, as well as in combination with capacitors connected in parallel with their inputs, storing the potential before removing the charge from the controlled substance. At the same time, the influence on the input resistance of the low output resistance channels of the second repeaters and the generator of high-frequency oscillation packets when charging pulses from the output of the last repeaters through third electronic keys and high-frequency oscillation packets from the generator to the output of the first repeaters and from them through the first electronic keys to their input, since the introduced resistance in K (gain of the current repeater) is times greater than the resistance of the switch in the open state and.

 In FIG. 1 shows a functional diagram of a device that implements an electrostatic method; in FIG. 2 plots of control pulses at the outputs of the switching pulse forming unit.

 The device (Fig. 1) contains a sensor 1 consisting of a measuring electrode 2 and a reference electrode 3. The electrode 2 is connected to the input of the repeater 4, and the electrode 3 is connected to the output of the repeater 5. The output of the repeater 4 is through key 6, and the output of the repeater 5 is through key 7 connected, respectively, to repeaters 8 and 9. Repeaters 4 and 8 comprise a measuring channel for direct current amplification, and repeaters 5 and 9 are a reference channel. The inputs of the last repeaters 8 and 9 in the measurement channels are shunted, respectively, by capacitors 10 and 11, and their outputs are connected to the inputs of a differential amplifier 12 loaded onto an analog-to-digital converter 13 with a digital display unit 14. A clock 15 is connected to the inputs of the forming unit switching pulses 16 and a generator of high-frequency oscillations 17. To the other input of the generator 17 is connected the output of the satellite of block 16. Keys 6 and 7 comprise a group controlled from the input of the AI of block 16. Keys 18 and 19 of the group controlled by the outputs of block 16 are connected between the output and input of each channel for amplifying DC current: a key 18 between the output of the repeater 8 and the input of the repeater 4, and a key 19 between the output of the repeater 9 and the input of the repeater 5. The keys 20, 21, 22, 23 constitute a group, controlled from the output of the satellite of block 16. The keys 20, 21 are connected between the output of the generator 17 and the outputs of the input repeaters 4 and 5, respectively, and the keys 22 and 23 between the inputs and outputs of the repeaters 4 and 5. In the DC amplification channels, voltage repeaters can operational gain be used amplifiers (op amps), characterized by certain values of bias voltage (constant voltage at the op amp output in the absence of a signal at the input). As a rule, to adjust the bias voltage, conclusions are provided to which the control resistors are connected.

 A device for implementing the method works as follows.

 The potentials of the electrodes 2 and 3 of the sensor 1 in the initial state are in air (environment) and are close to zero. The generator 15 generates a continuous sequence of clock pulses with a given repetition rate, which are fed to the inputs of the BFPI block 16 and the high-frequency generator 17. The BFPI block 16 generates three shift-shifted switching pulses (Fig. 2, a, b, c). When passing from the output of the BFPI block 16 of the charge pulse (Fig. 2a), the keys 18 and 19 are closed. The keys are maintained in a closed state for the duration of the passage of the charging pulse. This time is sufficient for the electrodes 2 and 3 of the sensor 1 to be charged to potentials determined by the bias voltages at the outputs of the last repeaters 8 and 9. When the charging pulse ceases, an impulse appears at the output of the AI of the BFPI 16 (Fig. 2 b), which translates the keys 6 and 7 in a closed state. Both DC amplification channels and the instrument as a whole go into measurement mode. The capacitors 10 and 11 are charged to the output voltage of the input repeaters 4 and 5, equal to the potential of the electrodes 2 and 3, and the voltage at the outputs of the last repeaters 8 and 9 exceeds the voltage at the capacitors 10 and 11 by the amount of the bias voltage. The output voltage of the repeaters 8 and 9 is supplied to the inputs of the differential amplifier 12, and from it a signal proportional to the potential difference is transmitted to the ADC 13 and further for visualization to the DI unit 14. Upon the termination of the pulse, when potential measurement occurs, the pulse is generated at the output of the satellite 16 of the unit 16 , which goes to the control inputs of the key group from 20 to 23 and to the second input of the generator 17. If there are clock pulses at the first input, a packet of high-frequency oscillations is formed with a monotonic increase and decrease in amplitude and the frequency specified by the period of repetition of clock pulses (Fig. 2, g). Simultaneously with the formation of the pulse packet, the keys 20 23 are closed. The pulse packet from the output of the HHF generator 17 is fed directly to the electrodes 2 and 3 through the outputs of the repeaters 4 and 5, ensuring the removal of electrostatic charge from them. Open keys 6 and 7 ensure the preservation of the value of the previous potential of the electrodes. After the completion of the pulse, when the charge is removed, the cycle repeats, allowing each time the charge pulse arrives, a predetermined constant value is added to the previous potential. As potentials rise to equilibrium, the leakage of current from the electrodes into the environment increases. In equilibrium, the amount of charge entering the electrodes due to a given constant value is equal to the loss of charge due to leakage of current during the action of the measurement pulse. Upon reaching equilibrium, varying the magnitude of the bias voltage of the last repeaters 8 and 9, regardless of the relative humidity of the environment, a constant value of the potentials on the electrodes is established to ensure the balance of the channels (equal potentials), and then the humidity is measured by releasing the measuring electrode 2 into the controlled substance. The leakage of current from the measuring electrode 2 leads to a decrease in its potential relative to the potential at the reference electrode 3. The steady-state value of the potential difference, taking into account the calibration curve, corresponds to a certain value of the relative humidity of the controlled substance.

 Conducted prototype studies of the proposed method showed that the relative error in measuring moisture does not exceed 5% in the range from 3 to 20% for the measuring range from 1 to 30%. The measurement time does not exceed 5 s at a pulse repetition rate of 20 Hz.

Claims (3)

 1. An electrostatic method for measuring the moisture content of bulk solids, including measuring the potential difference between two electrodes, characterized in that the electrodes, measuring and reference, are charged in air to a fixed value, and then the measuring electrode is immersed in a controlled substance, and the measurements are performed in a pulsed mode, in addition, each measuring cycle includes pulsed charging of the electrodes, measuring the potential difference between the electrodes and pulsed charge removal from the electrodes and controlled in thereafter, in each subsequent cycle, when charging, the potential of the electrodes is increased by a predetermined constant value compared to the potential of the electrodes in the previous cycle, and the cycles are repeated until at least two successive cycles obtain the same potential difference between the electrodes, which determine the humidity of the controlled substance taking into account the calibration curve.  2. The method according to p. 1, characterized in that the removal of charge from the electrodes and the controlled substance is performed by applying to the electrodes a packet of high-frequency oscillations, while the amplitude of the high-frequency oscillations in the packet is first monotonically increased, and then monotonously reduced.  3. A device for measuring the moisture content of bulk solids, containing two electrodes connected to measuring and indicating potential differences between the electrodes, characterized in that the potential difference measuring means includes two direct current amplification channels and a differential amplifier connected to an analog-to-digital converter, thus, a clock pulse generator connected to a switching pulse generation unit and a high-frequency oscillation generator is introduced into the device, with each channel The DC coupler includes a first electronic key, a first repeater, a second electronic key and a second repeater connected in series, a capacitor is connected in parallel between the second key and the input of the second repeater, while the output of the second repeater is connected through the third electronic key to the input of the first repeater, and the output of the first repeater connected to its input and output of the generator of high-frequency oscillations through the first electronic key, while the control inputs of the first, second and third electronic x keys are connected to the corresponding outputs of the switching pulse forming unit, and the outputs of the second repeaters are connected to the inputs of the differential amplifier.

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