JPH0829223A - Electromagnetic flowmeter - Google Patents

Abstract

PURPOSE:To provide an electromagnetic flowmeter capable of measuring accurately the flow rate stably for a long time by suppressing the corrosion speed of the detection electrode therein. CONSTITUTION:An alternating current having a prescribed frequency is supplied to detection electrodes 4a, 4b relative to respective ground levels of a common electrode 5 via resisters Ra, Rb from an alternating current signal-generating section 20. Comparators 19a, 19b of a vacancy-detection section 19 informs a control section 17 of information of vacancy-detection when the electric levels of the detection electrodes 4a, 4b exceed respective reference voltage levels Va, Vb.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Conventionally, in an electromagnetic flowmeter which measures a flow rate based on a signal electromotive force obtained from a fluid in a magnetic field, empty detection of the fluid has been carried out by the configuration shown in FIG. In the figure, 1 is a detector for generating a predetermined magnetic field and detecting a signal electromotive force from a fluid, 10 is a signal electromotive force from the detector 1 for signal processing to calculate a flow rate, and a predetermined process control signal is obtained. It is a converter for converting and outputting. In the detector 1, 2 is a coil for generating a predetermined magnetic field, 3 is a pipe through which a fluid to be measured flows, and 4a and 4b are arranged so as to face the inner surface of the pipe 3 and come into contact with the fluid. The electrodes 5 are common electrodes (earth rings) for applying the same potential (ground potential) as the pipe 3 to the fluid in the pipe 3.

In the converter 10, 11 is a power supply section for generating a predetermined power supply voltage from a loop current supplied from a process control device (not shown), and 12 is an exciting section for outputting an exciting current of a predetermined frequency to the coil 2. , 13 is a buffer, 14
Is an AC amplification unit 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 amplification unit 14, and 16 is a high-pass filter 15 The output of A is sampled at predetermined intervals and converted into digital information A
An A / D conversion unit, 17 is a control unit that calculates a flow rate from the output of the A / D conversion unit 16 and controls each unit in the converter 10, and 18 is a predetermined process control signal based on the 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 minute currents to the detection electrodes 4a and 4b for sky detection, and 19 is the potential of the detection electrodes 4a and 4b obtained via the buffer 13 and the detection electrodes 4a. , 4b provided for the reference voltage V
It has comparators 19a and 19b for comparing a and Vb, and when the potentials of the detection electrodes 4a and 4b exceed the corresponding reference voltages Va and Vb, respectively, the control unit 1 outputs an empty detection signal.
It is an empty detection unit that outputs to 7. 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 the reference voltages Va and Vb are predetermined voltages between the power supply voltages + V and -V and the ground potential, respectively. It is set to a value.

Next, a conventional sky detecting method in the electromagnetic flow meter will be described. 5A and 5B are explanatory diagrams showing a circuit portion relating to sky detection, FIG. 5A is an explanatory diagram showing a circuit portion relating to sky detection in the electromagnetic flowmeter shown in FIG. 4, and FIG. 5B is a description showing a detection electrode 4a side. It is a figure. In FIG. 5, Z
a and Zb are fluid resistances generated between the detection electrodes 4a and 4b by the fluid and the common electrode 5, respectively, and Ia is a ground potential via the resistance Ra from the power supply voltage + V, the detection electrode 4a, the fluid resistance Za and the common electrode 5. Current, Ib is 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 the fluid flows in 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 several KΩ to several tens of KΩ is generated, and the change can detect the presence or absence of the fluid, that is, the sky. In practice, as shown in FIG. 5B, a small current I is constantly applied from the power supply voltage + V to the fluid resistance Za via the resistor Ra.
a is supplied and the midpoint potential of the resistance Ra and the fluid resistance Za, that is, the potential of the detection electrode 4a and the reference voltage Va are compared by the comparator 19a to obtain the fluid resistance Z.
The change in the current Ia associated with the change in 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.
When the voltage rises above a, the output of the comparator 19a is inverted, and the fluid inside the tube 3, especially the detection electrode 4a and the common electrode 5
When the fluid on the side is empty, it is detected and output, and is used for determining whether the flow rate detected immediately before is valid or invalid and for outputting an empty detection alarm. The detection electrode 4b to which the power supply voltage -V is constantly supplied
Contrary to 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.

[0008]

Therefore, in such a conventional electromagnetic flowmeter, since the current is constantly supplied to the detection electrodes 4a and 4b from the power supply voltages + V and -V, respectively, the fluid resistance is increased. Electric charges move in one direction via Za and Zb, and due to the electric charges, an electrochemical reaction occurs 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 liquid contact surface of the detection electrode 4a or 4b, and the insulation resistance increases the contact resistance with the fluid, and the signal electromotive force indicating the original flow rate is accurately detected by the detection electrodes 4a and 4b. There was a problem that it was impossible to do.

There is also a method of using a metal in which an electrochemical reaction does not easily occur as the detection electrodes 4a and 4b, for example, platinum, titanium, tantalum, etc., but these rare metals are expensive and considerably expensive as an apparatus. There was a problem that it would end up. The present invention is for solving such a problem, and an object thereof is to provide an electromagnetic flow meter capable of suppressing the corrosion rate of the detection electrode and stably detecting an accurate flow rate over a long period of time. There is.

[0010]

In order to achieve such an object, an electromagnetic flowmeter according to the present invention supplies a predetermined current to a pair of detection electrodes for detecting a signal electromotive force corresponding to a flow rate. At the same time, in the electromagnetic flow meter that compares the potential of each detection electrode with the reference voltage and detects the empty state of the fluid to be measured according to the result of the comparison, an alternating current that supplies alternating current at a predetermined frequency It is provided with a signal generating means. Further, the AC signal generating means supplies AC currents of the same phase to the respective detection electrodes. Further, the AC signal generating means supplies an AC current having a frequency that is sufficiently attenuated by a high-pass filter provided in the flow rate detection system.

[0011]

Therefore, the alternating current signal generating means supplies alternating current having a predetermined frequency to each detection electrode. Further, the alternating-current signal generating means supplies currents of the same phase to the respective detection electrodes. Further, the AC signal generating means supplies an AC current having a frequency that is sufficiently attenuated by the high-pass filter provided in the flow rate detection system.

[0012]

Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an electromagnetic flowmeter which is an embodiment of the present invention. In the figure, the same or equivalent parts as those described above are designated by the same reference numerals. In FIG.
Reference numeral 20 denotes an AC signal generator that generates an AC signal (AC current) alternating at a predetermined frequency with the potential (ground potential) of the common electrode 5 as a middle point, and is composed of, for example, a positive feedback circuit such as a Wien bridge circuit. This AC signal is applied to the resistor R composed of various resistive elements.
It is supplied to the detection electrodes 4a and 4b via a and Rb.

FIG. 2 is a waveform diagram showing the signal waveform of each part in FIG. 1, in which 21 is an AC signal output from the AC signal generator 20, 22 is each detection electrode 4a,
4b is a DC potential, and 23a and 23b are the sky detector 1 respectively.
Output signals from the comparators 19a and 19b of 9
Shows another example of the waveform of the AC signal 21. in this case,
The AC signal 21 is applied to each detection electrode 4 via the resistors Ra and Rb.
a and 4b are commonly supplied to the detection electrodes 4a and 4b.
The potential of indicates the potential obtained by dividing the output voltage of the AC signal 21 by the resistors Ra and Rb and the fluid resistors Za and Zb. In addition, the signal electromotive force generated between the detection electrodes 4 a and 4 b according to the flow rate is superimposed on the signal 22.

The reference voltage Va of the empty detector 19 is a potential divided by the resistance Ra (Rb) and the fluid resistance Za (Zb) when the output voltage of the AC signal 21 becomes the highest value when the water is full. The reference voltage Vb of the empty detection unit 19 is set to a higher voltage to some extent, and the resistance Ra (Rb) and the fluid resistance Za (Zb) are set when the output voltage of the AC signal 21 becomes the lowest value when the water is full. ) Is set to a voltage somewhat lower than the potential divided by and.

If the output voltage of the AC signal 21 increases to the positive side from the ground potential of the common electrode 5 from time T0, the potentials 22 of the detection electrodes 4a and 4b are resistances Ra and Rb and fluid resistance Za, It gradually increases while showing the potential divided by Zb, and the potential 22 of the detection electrodes 4a and 4b also decreases as the output voltage of the AC signal 21 decreases. Time T
1, the output voltage of the AC signal 21 and the detection electrode 4
The potentials 22 of a and 4b become equal to the ground potential of the common electrode 5 respectively, and thereafter, as the output voltage of the AC signal 21 increases to the negative side compared to the ground potential of the common electrode 5, that is, decreases, the detection electrode The potential 22 of 4a and 4b is also the resistance R
The potential divided by a, Rb and the fluid resistances Za, Zb is gradually decreased while showing the potential.

At time T2, when the fluid in the tube 3 becomes empty and does not come into contact with the detection electrodes 4a and 4b,
The fluid resistances Za and Zb increase or become infinite, and the potential 22 of the detection electrodes 4a and 4b changes to the reference voltage V of the empty detection unit 19.
The voltage exceeds b to the negative side and becomes substantially equal to the output voltage of the AC signal 21. The comparator 19b of the sky detection section 19 detects the change in the potential 22 of the detection electrode 4b and outputs the output 23.
By inverting b, the control unit 17 is notified of the sky detection.

At time T3 thereafter, since the output voltage of the AC signal 21 rises above the reference voltage Vb, the output 23b of the comparator 19b of the sky detecting section 19 is inverted again and the notification of sky detection is stopped. However, after the time T4, the output voltage of the AC signal 21 rises above the reference voltage Va, and the potential 22 of the detection electrodes 4a and 4b becomes substantially equal to the output voltage of the AC signal 21 depending on the empty state.
The comparator 19a of the sky detector 19 detects the detected voltage 4
It is detected that the potential 22 of a exceeds the reference voltage Va,
By inverting the output 23a, the sky detection is notified to the control unit 17, and thereafter, the sky detection unit 19 notifies the control unit 17 of the sky detection according to the occurrence of the sky state.

As described above, the AC signal generator 20 for outputting the AC signal 21 having a predetermined frequency is provided to detect the detection electrodes 4a,
Since a current for sky detection is alternately supplied to the ground potential of the common electrode 5 with respect to 4b, the potential of either polarity is fixed to the detection electrodes 4a and 4b as in the conventional case. Detection electrode 4 compared to the
The electrochemical reaction at the interface between a and 4b and the fluid does not continue to proceed in one direction, and it is possible to suppress the attachment rate of the insulator attached to the liquid contact surface of the detection electrodes 4a and 4b, that is, the corrosion rate. This makes it possible to stably detect an accurate flow rate over a longer period. Moreover, since the alternating-current signal 21 having the same phase is supplied to each of the detection electrodes 4a and 4b, the signal generated between the detection electrodes 4a and 4b according to the flow rate due to the differential amplification in the alternating-current amplification unit 14. The effect on the electromotive force is canceled and accurate flow rate detection is maintained.

In the above description, the case where a sine wave is used as the waveform of the AC signal 21 output from the AC signal generator 20 has been described, but the detection electrodes 4a, 4
Other waveforms may be used as long as they do not cause abrupt changes in the potentials of the detection electrodes 4a and 4b that interfere with the detection of the signal electromotive force generated between points b, for example, the signal 24 in FIG. A trapezoidal waveform that alternately alternates with respect to the ground potential may be used. Furthermore, each detection electrode 4a,
Although the case where the AC signal 21 of the same phase is supplied to 4b has been described, the influence on the differential amplification in the AC amplification unit 14 can be mitigated by selecting the frequency and waveform of the AC signal 21. , Each detection electrode 4a, 4b
Even if the AC signal 21 having the opposite phase or the shifted phase is supplied, the same effect as described above can be obtained.

Further, in general, in the flow rate detection system (14 to 17) of this type of electromagnetic flow meter, as shown in FIG. 1, among the output signals from the AC amplification section 14, a signal electromotive force having an excitation frequency is generated. A high-pass filter 15 is provided in order to remove signals such as low-frequency fluid noise that pass and are mixed in the output signal. By selecting a frequency that can be removed by the high-pass filter 15 as the AC signal 21, The influence of the AC signal 21 on the flow rate detection system can be further suppressed. FIG. 3 is an explanatory diagram showing the frequency characteristics of the high-pass filter. Generally, an excitation frequency of 10 Hz or higher is used in consideration of fluid noise and the like.
The first-order high-pass filter having a corner frequency of 10 Hz as described above.

Here, the AC signal 2 for the flow rate detection accuracy
To suppress the effect of 1 to about 0.1%, -60d
Since the attenuation of B is necessary, it can be seen from the high-pass filter of FIG. 3 that a frequency near 0.01 Hz may be used as the AC signal 21. In this way, the AC signal 21
By using a frequency that is sufficiently attenuated by the high-pass filter provided in the flow rate detection system,
It is possible to further suppress the influence of the AC signal 21 on the flow rate detection system.

In the above description, the sky detecting section 19
The comparator 19a is provided on the detection electrode 4a side and the comparator 19b is provided on the detection electrode 4b side, and the AC signal 21 is higher than the ground potential of the common electrode 5 (on the positive side).
In the case, the detection electrode 4a side, that is, the comparator 19a detects the empty state, and when the AC signal 21 is lower than the ground potential (negative side), the detection electrode 4b side, that is, the comparator 19b.
Although the case where the empty state is detected is described above, a pair of comparators 19a and 19b may be provided on both of the detection electrodes 4a and 4b, respectively, so that the detection signal is always independent of the polarity of the AC signal 21. Both detection electrodes 4a,
It becomes possible to detect the empty state by the potential of 4b,
It is possible to improve the detection delay and the detection accuracy.

That is, by providing the comparators 19a and 19b on both the detection electrodes 4a and 4b, respectively, when the AC signal 21 becomes positive and the potential of the detection electrode 4b becomes higher than the reference voltage Va, the detection electrode 4b is detected. It is possible to detect this by the comparator 19a provided on the side and judge that it is empty, and conversely, when the AC signal 21 becomes negative and the potential of the detection electrode 4a becomes lower than the reference voltage Vb, It is possible to detect this by the comparator 19b provided on the side of 4a and judge it as an empty state, and the idle period of empty detection is shortened, and one of the detection electrodes 4a and 4b becomes empty. Even if it is detected correctly.

[0024]

As described above, according to the present invention, a predetermined current is supplied to a pair of detection electrodes for detecting a signal electromotive force according to the flow rate, and the potential of each detection electrode and the reference voltage are set. In comparison, in the electromagnetic flow meter for detecting the empty state of the fluid to be measured according to the comparison result, an alternating current signal generating means for supplying alternating current at a predetermined frequency is provided,
Since an alternating current is supplied to each detection electrode as a current for sky detection, the detection electrode is compared with a conventional one in which a current of either polarity is fixedly supplied to each detection electrode. The electrochemical reaction at the interface between the liquid and the fluid does not continue to proceed in one direction, and it becomes possible to suppress the adhesion rate of the insulator that adheres to the liquid contact surface of the detection electrode, that is, the corrosion rate, and for a longer period. An accurate flow rate can be stably detected over the range.

Further, since the alternating current signal generating means supplies the alternating currents of the same phase to the respective detection electrodes, by the differential amplification in the alternating current amplification part of the flow rate detection system,
The influence of the alternating current on the signal electromotive force generated between the detection electrodes according to the flow rate is canceled, and accurate flow rate detection is maintained. Further, since the AC signal generating means is configured to supply the AC current having a frequency that is sufficiently attenuated by the high-pass filter provided in the flow rate detection system, the influence of the AC current on the flow rate detection system can be achieved without providing new means. Can be further suppressed.

[Brief description of drawings]

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

FIG. 2 is a signal waveform diagram in each part of the embodiment shown in FIG.

FIG. 3 is an explanatory diagram showing frequency characteristics of a high pass filter.

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

FIG. 5 is an explanatory diagram showing a circuit portion related to 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 ... sky detector,
19a, 19b ... Comparator, 20 ... AC signal generator, Va, Vb ... Reference voltage, Ra, Rb ... Resistor.

Claims (3)

[Claims] 1. A predetermined current is supplied to a pair of detection electrodes for detecting a signal electromotive force according to a flow rate, and the potential of each of the detection electrodes is compared with a reference voltage. An electromagnetic flowmeter for detecting an empty state of a fluid to be measured, comprising an AC signal generating means for supplying an alternating current alternating at a predetermined frequency as the current. 2. The electromagnetic flowmeter according to claim 1, wherein the alternating-current signal generating means supplies alternating currents of the same phase to the respective detection electrodes. 3. The electromagnetic flowmeter according to claim 1, wherein the alternating-current signal generating means supplies an alternating-current signal having a frequency that is sufficiently attenuated by a high-pass filter provided in a flow rate detection system. Total.