JP3018310B2 - Electromagnetic flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting emptyness of an electromagnetic flowmeter, and more particularly to an electromagnetic flowmeter for performing empty detection for detecting whether or not the inside of a pipe for measuring a flow rate with an electromagnetic flowmeter is filled with a fluid. It is about a total.

[0002]

2. Description of the Related Art Conventionally, in an electromagnetic flowmeter for measuring a flow rate based on a signal electromotive force obtained from a fluid in a magnetic field, empty detection of the fluid has been performed by a configuration as shown in FIG. In FIG. 1, reference numeral 1 denotes a detection unit for generating a predetermined magnetic field and detecting a signal electromotive force from a fluid, and 10 performs signal processing on the signal electromotive force from the detector 1 to calculate a flow rate. This is a converter for converting and outputting. In the detector 1, reference numeral 2 denotes a coil for generating a predetermined magnetic field, 3 denotes a tube through which a fluid to be measured flows, and 4a and 4b are detection members disposed opposite to the inner surface of the tube 3 so as to be in contact with the fluid. The electrodes 5 are common electrodes (earth rings) for applying the same potential (ground potential) to the fluid in the tube 3 as the tube 3.

In the converter 10, reference numeral 11 denotes a power supply unit for generating a predetermined power supply voltage from a loop current supplied from a process control device (not shown), and 12 denotes an excitation unit for outputting an excitation current of a predetermined frequency to the coil 2. , 13 are buffers, 14
Is an AC amplifier that differentially amplifies the signal electromotive force obtained from the detection electrodes 4a and 4b, 15 is a high-pass filter that attenuates low-frequency components such as fluid noise in the output of the AC amplifier 14, and 16 is a high-pass filter 15 A which samples the output at a predetermined interval and converts it into digital information.
A / D conversion unit 17 calculates a flow rate from the output of the A / D conversion unit 16 and controls each unit in the converter 10. A control unit 18 generates a predetermined process control signal based on flow rate information from the control unit 17. This is an output interface unit for outputting, and 14 to 17 constitute a flow rate detection system.

Ra and Rb are resistors for supplying a small current to the detection electrodes 4a and 4b for empty detection, and 19 is the potential of the detection electrodes 4a and 4b obtained via the buffer 13 and the respective detection electrodes 4a. , 4b provided with reference voltage V
a, Vb, and comparators 19a, 19b. When the potentials of the detection electrodes 4a, 4b exceed the corresponding reference voltages Va, Vb, the control unit 1 outputs an empty detection signal.
7 is an empty detector. Note that + V is a positive power supply voltage higher than the ground potential, -V is a negative power supply voltage lower than the ground potential, and reference voltages Va and Vb are predetermined voltages between the power supply voltages + V and -V and the ground potential, respectively. Is set to a value.

Next, a conventional empty detection method in an electromagnetic flowmeter will be described. 5A and 5B are explanatory diagrams illustrating a circuit portion related to empty detection, FIG. 5A is an explanatory diagram illustrating a circuit portion related to empty detection in the electromagnetic flowmeter illustrated in FIG. 4, and FIG. 5B is an explanatory diagram illustrating a detection electrode 4a side. FIG. In FIG.
a and Zb are fluid resistances generated between the detection electrodes 4a and 4b and the common electrode 5 by a fluid, respectively, and Ia is a ground potential from the power supply voltage + V via the resistance Ra, the detection electrode 4a, the fluid resistance Za and the common electrode 5. The current Ib flows from the ground potential to the common electrode 5, the fluid resistance Rb, and the detection electrode 4b.
And a current flowing to the power supply voltage −V via the resistor Rb.

When a fluid flows through the tube 3, the detection electrode 4
a, 4b and the common electrode 5 between the fluid resistance Za, Zb,
For example, in the case of water, it is known that a fluid resistance of about several KΩ to about ten and several KΩ is generated, and it is possible to detect the presence / absence of a fluid, that is, to detect an empty space by the change. In practice, as shown in FIG. 5B, a small current I is always applied from the power supply voltage + V to the fluid resistance Za via the resistance Ra.
is supplied, and the comparator 19a compares the midpoint potential between the resistance Ra and the fluid resistance Za, that is, the potential of the detection electrode 4a, with the reference voltage Va.
The change of the current Ia accompanying the change of a is detected.

In this case, the fluid does not come into contact with the detection electrode 4a due to the decrease in the flow rate, the fluid resistance Za increases or becomes infinite, and the potential of the detection electrode 4a becomes the reference voltage V.
a, the output of the comparator 19a is inverted and the fluid in the tube 3, especially the detection electrode 4a and the common electrode 5a,
It is detected and output that the fluid on the side is empty, and is used to determine the validity / invalidity of the flow rate detected immediately before and to output an empty detection alarm. The detection electrode 4b to which the power supply voltage -V is constantly supplied.
In the above, the empty state is detected and output when the midpoint potential, that is, the potential of the detection electrode 4b drops below the reference voltage Vb, contrary to the above.

[0008]

Therefore, in such a conventional electromagnetic flowmeter, current is constantly supplied to the detection electrodes 4a and 4b from the power supply voltages + V and -V, respectively. The movement of electric charge occurs in one direction via Za and Zb, and the electric charge causes an electrochemical reaction at the interface between the detection electrodes 4a and 4b and the fluid due to the relationship between the material of the detection electrodes 4a and 4b and the fluid. An insulator adheres to the surface of the liquid contacting the detection electrode 4a or 4b, and the insulation increases the contact resistance with the fluid, so that the detection electrodes 4a and 4b accurately detect the signal electromotive force indicating the original flow rate. There was a problem that it became impossible to do.

There is also a method of using a metal which does not easily cause an electrochemical reaction, for example, platinum, titanium, tantalum or the like as the detection electrodes 4a and 4b. However, these rare metals are expensive and considerably expensive as a device. There was a problem that it would. The present invention is intended to solve such a problem, and it is an object of the present invention to provide an electromagnetic flowmeter capable of suppressing the corrosion rate of a detection electrode and stably detecting an accurate flow rate over a long period of time. I have.

[0010]

In order to achieve the above object, an electromagnetic flowmeter according to the present invention comprises a current control means for inverting and switching the polarity of a current supplied to each detection electrode; An insulator detection means for detecting the degree of adhesion of the insulator attached to the detection electrode due to an electrochemical reaction occurring at the interface with the detection electrode; and Is configured to reverse the polarity of the current supplied to each detection electrode by the current control means. The insulator detecting means detects the degree of adhesion of the insulator based on a change in contact resistance between each detection electrode and the fluid to be measured. Also, the insulator detecting means detects the degree of adhesion of the insulator based on the difference between the DC potentials generated at the respective detection electrodes according to the current supplied to each of the detection electrodes and the contact resistance. When the voltage falls outside a predetermined threshold voltage range, the adhesion of an insulating material of a predetermined degree or more is detected.

[0011]

Therefore, when the insulation detecting means detects the adhesion of the insulating material of a predetermined degree or more, the polarity of the current supplied to each detecting electrode is reversed by the current control means. Further, the degree of adhesion of the insulator is detected by a change in the contact resistance between each detection electrode and the fluid to be measured.
In addition, the degree of adhesion of the insulator is detected based on the difference between the DC potentials generated at the respective detection electrodes according to the current supplied to the respective detection electrodes and the contact resistance, and the difference between the DC potentials is determined by a predetermined threshold voltage. When the value is out of the range, the adhesion of the insulating material of a predetermined degree or more is detected.

[0012]

Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an electromagnetic flow meter according to an embodiment of the present invention, in which the same reference numerals are given to the same or equivalent parts as described above. In FIG.
Reference numeral 21 denotes a current control unit for inverting and switching the polarity of a minute DC current supplied to each of the detection electrodes 4a and 4b for the purpose of empty detection in accordance with a control signal from the control unit 17. Reference numeral 22 denotes a low-pass filter 22a and 22b and a voltage addition circuit. A voltage difference detection unit 23, which is composed of 22c and detects and outputs the difference between the DC potentials of the two detection electrodes 4a and 4b, samples an output voltage from the voltage difference detection unit 22, performs A / D conversion, and outputs it to the control unit 17. The A / D converter 24 is provided at the input stage of the empty detector 19, and receives a control signal to the current controller 21 so that the comparator 1
It is a switching section for switching and connecting the inputs of 9a and 19b, and constitutes an insulator detecting means for detecting the degree of adhesion of the insulator attached to the detecting electrodes 4a and 4b by the voltage difference detecting section 22.

Usually, when an electrochemical reaction occurs at the interface between the detection electrodes 4a and 4b and the fluid and an insulator starts to adhere to the surface of the detection electrode 4a or 4b in contact with the fluid, the detection electrode 4a or 4b and the fluid , The current supplied from the current control unit 21 to the detection electrode 4a or 4b decreases, and the DC potential of the detection electrode 4a or 4b gradually decreases from the normal value toward the power supply voltage + V or -V side. Changes to
The voltage difference detection unit 22 detects one of the detection electrodes 4a, 4b by detecting the voltage difference between the two detection electrodes 4a, 4b.
b, the control unit 17 compares the voltage difference between the two detection electrodes 4a and 4b input via the A / D conversion unit 23 with a predetermined threshold voltage, and determines the voltage. When the difference exceeds the range of the threshold voltage, the polarity of the DC current is inverted by the control signal for the detection of the sky supplied from the current control unit 21, and further adhesion of the insulator is suppressed.

Next, the operation of the present invention will be described with reference to FIGS. FIG. 2 is an explanatory diagram showing a circuit portion related to the sky detection.
1b is controlled by a control signal from the control unit 17 and is controlled by a resistor R
The switches 24a and 24b are controlled by a control signal from the control unit 17 and both inputs of the comparators 19a and 19b in the empty detection unit 19 are switched. Detection electrodes 4a, 4
b (output of the buffer 13).

In the voltage difference detecting section 22, 22a and 22
b is a low-pass filter that removes AC power noise mixed in phase with the potentials of the detection electrodes 4a and 4b (buffer 13). 22c is the output voltage Ea and Eb of the low-pass filters 22a and 22b and the ground potential of the common electrode 5. This is a voltage addition circuit that adds the voltage as a middle point and outputs the voltage difference Ed. FIG. 3 is a waveform diagram showing signals of various parts in the circuit diagram of FIG. 2. Reference numerals 31 and 32 denote power supply voltages supplied from the current control unit 21 to the resistors Ra and Rb.
The film thicknesses La, Lb, and 34 of the insulator attached to the a and 4b are the output voltages Ea and Eb of the low-pass filters 22a and 22b of the voltage difference detection unit 22 and the output voltages Ea and Eb output from the voltage addition circuit 22c. The difference voltage Ed is shown.

At time T0, both detection electrodes 4a, 4b
, The supply of the DC currents Ia and Ib for the sky detection is started. In this case, the switches 21a,
Reference numeral 21b is switched to the power supply voltage + V side and -V side by a control signal from the control unit 17, respectively. The power supply voltage + V is connected to the resistor Ra, and the resistor Ra passes through the detection electrode 4a and the fluid resistor Za. The DC current Ia flows through the common electrode 5, while the power supply voltage −V is connected to the resistor Rb, and the fluid resistance Zb and the detection electrode 4 are connected from the common electrode 5.
b, DC current Ib flows to power supply voltage -V via resistor Rb.

The switches 24a, 24 of the switching unit 24
b is switched between the detection electrode 4a side and the 4b side by the same signal as the control signal to the current detection unit 21. The comparator 19a compares the potential of the detection electrode 4a with the reference voltage Va, 19b, the potential of the detection electrode 4b is compared with the reference voltage Vb. Therefore, when the fluid in the tube 3 becomes empty and, for example, no longer contacts the detection electrode 4a,
The fluid resistance Za increases or becomes infinite, and the detection electrode 4a
Becomes higher than the reference voltage Va to the power supply voltage + V side (positive side), and this is detected and output by the comparator 19a, and the control unit 17 determines that the empty state is detected.

Here, due to the direct currents Ia and Ib started to be supplied from the time T0, an electrochemical reaction occurs at the interface between the detection electrodes 4a and 4b and the fluid, so that the surface of the liquid contacting the detection electrode 4a or 4b is insulated. Things start to stick. Now, when an electrochemical reaction occurs between the detection electrode on the positive side compared to the ground potential of the common electrode 5 due to the component of the fluid flowing in the tube 3, the DC current Ia is supplied from the power supply voltage + V. An electrochemical reaction occurs only at the detection electrode 4a, and the thickness La of the insulator attached to the detection electrode 4a increases with time. Note that no electrochemical reaction occurs between the common electrode 5 and the detection electrode 4b which is on the negative side as compared with the ground potential, and the thickness of the insulator remains zero.

The degree of adhesion of the insulator increases over a long period depending on the component and concentration of the fluid, for example, several hours to several months, and the contact resistance between the detection electrode 4a and the fluid gradually increases accordingly. However, the potential is not as large as the change in the potential due to the empty state of the fluid, and the potential slightly changes to the power supply voltage + V side. The voltage difference detection unit 22 removes AC noise and the like from the potentials of the detection electrodes 4a and 4b,
2b to generate output voltages Ea and Eb, and output these output voltages Ea and Eb by a voltage adding circuit 22c.
b, and the potential of the detection electrode 4a, which changes to the power supply voltage + V side in accordance with the increase of the insulator, appears as an increase in the output voltage Ea, and the difference from the output voltage Eb which does not change. That is, the change appears in the voltage difference Ed, and this change is detected by the control unit 17 via the A / D conversion unit 23.

At time T1, when the control unit 17 detects that the voltage difference Ed has exceeded the predetermined threshold voltage VTH, the control unit 17 causes the detection electrode 4a or 4b to perform accurate flow rate detection. It is determined that an insulator is adhered to such an extent that it interferes with the current control unit 21 based on the control signal.
Of the detection electrodes 4a, 21b by controlling the switches 21a, 21b of the
The polarity of the current supplied to 4b is inverted. As a result, the switches 21a and 21b are connected to the power supply voltage −
The voltage is switched between the V side and the + V side, and the power supply voltage -V is connected to the resistor Ra. Contrary to the above, the power supply voltage -V is supplied from the common electrode 5 via the fluid resistance Za, the detection electrode 4a, and the resistance Ra. A DC current Ia flows through the resistor Rb, while a power supply voltage + V is connected to the resistor Rb.
b, a DC current Ib is applied to the common electrode 5 through the fluid resistance Zb.
Flows.

At the same time, the switches 24a and 24b of the switching unit 24 are switched by the control signal to the current control unit 21, respectively.
The potential of the detection electrode 4b connected to the power supply voltage + V via the resistor Rb is input to 9a, and the detection electrode 4a connected to the power supply voltage -V via the resistor Ra is input to the comparator 19b. Of each detection electrode 4 according to the change of the fluid resistance Za, Zb.
a and 4b are changed by comparators 19b and 19a, respectively.
When the potentials of the detection electrodes 4a and 4b exceed the reference values Vb and Va, the controller 17 determines that the detection electrodes 4a and 4b are empty.
Will be notified.

Accordingly, in response to the switching of the current control unit 21, the potential of the detection electrode 4a becomes negative as compared with the ground potential of the common electrode 5, and the electric potential generated at the interface between the detection electrode 4a and the fluid. As the chemical reaction stops, the attached insulator begins to dissolve, the thickness La of the insulator attached to the detection electrode 4a decreases, the contact resistance at the detection electrode 4a decreases, and the The inverted output voltage Ea gradually increases. On the other hand, since the potential of the detection electrode 4b is on the positive side as compared with the ground potential of the common electrode 5,
At the interface between b and the fluid, an electrochemical reaction occurs and an insulator adheres, and the thickness Lb gradually increases as described above.

Due to the increase in the thickness of the insulator at the detection electrode 4b, the potential of the detection electrode 4b changes to the power supply voltage + V side, whereby the output voltage Eb gradually increases, and the voltages of these output voltages Ea and Eb are increased. The difference Ed also increases to the positive side. The control unit 17 monitors the voltage difference Ed via the A / D conversion unit 23. If the voltage difference Ed exceeds the threshold voltage VTH at time T2, the control electrode 17 detects the voltage of the detection electrode 4a in the same manner as described above.
Or 4b, it is determined that an insulator is attached to such an extent as to hinder accurate flow rate detection, and the switches 21a and 21b of the current control unit 21 are controlled by a control signal,
The polarity of the current supplied to the detection electrodes 4a and 4b is inverted.

Hereinafter, the control unit 17 is controlled according to the adhesion of the insulator.
As a result, the polarity of the current supplied to the detection electrodes 4a and 4b is repeatedly inverted and controlled.
Adhesion of the insulator to the a and 4b is always suppressed to a thickness having a contact resistance of such an extent that a voltage difference Vd equal to or less than the threshold value VTH is detected. Thus, the detection electrode 4
The detection electrodes 4a, 4b are formed by an electrochemical reaction between
The insulator adhering to the electrode 4b is monitored, and if it is detected that the degree of adhesion of the insulator is greater than a predetermined value, the DC current supplied to each of the detection electrodes 4a and 4b for empty detection is detected. Since the polarity is reversed, an electrochemical reaction occurs alternately at the detection electrodes 4a and 4b, and the insulator adheres, and the insulator once adhered dissolves in the fluid according to the reversal of the current polarity. And the detection electrodes 4a,
The insulating material adhering to 4b is suppressed to a predetermined degree, and accurate flow rate detection is maintained for a long period of time.

As means for monitoring the degree of adhesion of the insulator, the detection electrode 4 which increases or decreases according to the degree of adhesion of the insulator is used.
Since the detection is performed based on the change in the contact resistance between the fluids a and 4b and the fluid, the degree of adhesion of the insulator can be directly and accurately detected. Further, each detection electrode 4 is used for detecting the sky.
A change in the contact resistance between the detection electrodes 4a, 4b and the fluid is detected based on the difference between the DC potentials supplied to the detection electrodes 4a, 4b. 4a and 4b can be detected by the same output signal even when an insulator adheres,
A new signal is applied to the detection electrodes 4a and 4
It is not necessary to apply the voltage to b, and it can be realized with a small circuit configuration.

In the above description, the voltage difference Ed output from the voltage difference detection unit 22 is taken into the control unit 17 via the A / D conversion unit 23, where the threshold voltage VTH,
The case where the polarity inversion control of the DC current supplied to each of the detection electrodes 4a and 4b is performed by comparing with the -VTH, but the A / D converter 23
And instead of the processing of the control unit 17, the voltage difference detection unit 22
Difference Ed output from the inverter and the threshold voltages VTH, -VTH
For example, threshold voltages VTH to -V
A window comparator or the like that detects and outputs whether or not the voltage difference Ed exists in the range of TH may be provided, and the current control unit 21 and the switching unit 24 may be controlled according to the comparison output. The processing in the unit 17 is simplified, and the A / D converter 23 can be omitted.

The switching section 24 is connected to the input stage of the empty detecting section 19.
And switching the input to each of the comparators 19a and 19b in the sky detection unit 19 in accordance with the polarity inversion of the DC current supplied to the detection electrodes 4a and 4b for the sky detection by the current control unit 21. Accordingly, the comparator 19a detects an empty state of the detection electrode to which the potential higher (positive side) than the ground potential of the common electrode 5 is applied, and detects that the potential lower than the ground potential (negative side) is applied. Although the case where the empty state is detected by the comparator 19b with respect to the electrodes has been described, a pair of comparators 19a and 19b may be provided on both of the detection electrodes 4a and 4b. Both detection electrodes 4 are always independent of the polarity of the power supply voltage to be switched and connected.
a, 4b can be monitored to detect an empty state,
The switching control of the input signal by the switching unit 24 can be omitted.

[0028]

As described above, the present invention is characterized in that the current control means for inverting and switching the polarity of the current supplied to each detection electrode, respectively, is provided by the electrochemical reaction occurring at the interface between the fluid to be measured and the detection electrode. And an insulator detecting means for detecting the degree of adhesion of the insulator attached to the detection electrode, and when the insulation detection means detects a predetermined degree or more of adhesion, the current control means controls each detection electrode. Since the polarity of the current supplied to the electrodes is switched, the electrochemical reaction occurs alternately at each detection electrode and the insulator adheres, and the insulator once adhered is measured according to the reversal of the current polarity. Since it becomes soluble in the fluid, the amount of the insulating material adhering to the detection electrode is suppressed to a predetermined degree, and accurate flow rate detection can be maintained for a long period of time.

Further, as the insulator detecting means, the degree of adhesion of the insulator is detected by a change in the contact resistance between each detection electrode and the fluid to be measured. The degree can be directly and accurately detected. In addition, as an insulator detecting means, the degree of adhesion of the insulator is detected based on a difference between a current supplied to each detecting electrode and a DC potential generated at each detecting electrode by a contact resistance and a fluid resistance of a fluid to be measured. When the difference between the DC potentials is outside the range of the predetermined threshold voltage, the adhesion of the insulating material of a predetermined degree or more is detected. In addition to being able to detect directly and accurately, even if an insulator adheres to any of the detection electrodes, it can be detected by the same output, that is, the difference in DC potential, and a new signal is used to detect contact resistance. Need not be applied to the detection electrodes, and can be realized with a small circuit configuration.

[Brief description of the drawings]

FIG. 1 is a block diagram of an electromagnetic flow meter according to one embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a circuit portion related to sky detection.

FIG. 3 is a waveform chart showing signals of respective units in the circuit diagram of FIG. 2;

FIG. 4 is a block diagram of a conventional electromagnetic flow meter.

FIG. 5 is an explanatory diagram showing a circuit portion related to the conventional sky detection.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Detector, 2 ... Coil, 3 ... Tube, 4a, 4b ... Detection electrode, 5 ... Common electrode, 10 ... Converter, 11 ... Power supply part, 1
2 Excitation unit, 13 Buffer, 14 AC amplification unit, 15
... High-pass filter, 16 ... A / D converter, 17 ... Controller, 18 ... Output interface, 19 ... Empty detector,
19a, 19b: comparator, 21: current control unit, 2
2. Voltage difference detection unit, 23 A / D conversion unit, 24 switching unit, Va, Vb reference voltage, Ra, Rb resistance.

Claims (3)

(57) [Claims] An electric current having opposite polarities is supplied to a pair of detection electrodes for detecting a signal electromotive force according to a flow rate, and a potential of each of the detection electrodes is compared with a reference voltage. In an electromagnetic flowmeter that detects an empty state of a fluid to be measured in response to the current, a current control unit that inverts and switches the polarity of a current supplied to each detection electrode, and an electrochemical reaction that occurs at an interface between the fluid to be measured and the detection electrode. An insulation detecting means for detecting the degree of adhesion of the insulating material adhering to the detection electrode, and the current controlling means when the insulating detecting means detects the adhesion of a predetermined amount or more of the insulating material. Wherein the polarity of the current supplied to each of the detection electrodes is reversed. 2. The electromagnetic flowmeter according to claim 1, wherein the insulator detecting means detects the degree of adhesion of the insulator based on a change in contact resistance between each detection electrode and the fluid to be measured. And an electromagnetic flow meter. 3. The electromagnetic flowmeter according to claim 1, wherein the insulator detecting means detects a difference between a DC potential generated at each of the detection electrodes according to the current supplied to each of the detection electrodes and the contact resistance. Detecting the degree of adhesion of the insulator based on the difference between the DC potential and the predetermined threshold voltage, and detecting the adhesion of the insulator at a predetermined level or more. Electromagnetic flow meter.