JP2014109529A - Electromagnetic flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electromagnetic flowmeter capable of suppressing an increase in power consumption and accurately detecting an empty state.SOLUTION: A band attenuation filter 21 outputs an AC flow signal E2 by attenuating a noise component out of an AC flow signal E1. A sample hold circuit 22 outputs a DC flow signal V2 by sampling the AC flow signal E2. A control circuit 14 determines whether or not it is an empty state that a fluid is not contacted with electrodes TA and TB by comparing a voltage ratio of a DC flow signal V1 to the DC flow signal V2 with a predetermined threshold Rth.

Description

  The present invention relates to an empty detection technique for an electromagnetic flow meter that measures the flow rate of a fluid having conductivity in various process systems.

  In general, in an electromagnetic flow meter that measures the flow rate of a fluid having electrical conductivity, an excitation current whose polarity is alternately switched to an excitation coil arranged so that the direction of magnetic field generation is perpendicular to the flow direction of the fluid flowing in the measurement tube And detects an electromotive force generated between a pair of electrodes arranged in the measurement tube perpendicular to the magnetic field generated from the excitation coil, and based on the electromotive force generated between the electrodes, the flow rate of the fluid flowing in the measurement tube Is measuring.

FIG. 7 is a block diagram showing a configuration of a general electromagnetic flow meter. FIG. 8 is a signal waveform diagram showing the operation (normal state) of a general electromagnetic flow meter.
The electromagnetic flow meter 5 includes a detector 5A and a converter 5B.
The detector 5A is provided with a measuring tube P, electrodes TA and TB, and an exciting coil L as main components.
The converter 5B includes an excitation circuit 50, an AC amplifier circuit 51, a sample hold circuit (SH) 52, an A / D converter 53, a control circuit 54, a communication circuit 55, a power supply circuit 56, and a display as main circuit portions. A container 57 is provided.

  When an AC excitation current is supplied from the excitation circuit 50 to the excitation coil L of the detector 5A, an electromotive force E0 having a magnitude corresponding to the fluid flow rate is generated at the electrodes TA and TB of the detector 5A. The electromotive force E0 is input to the AC amplifying circuit 51 through the capacitive element C and is AC amplified, and then output as an AC flow rate signal E1.

The AC flow rate signal E1 is sampled by a sample hold circuit 52 based on the sampling control signals SH1 and SH2 from the control circuit 54. The positive waveform and the negative waveform are sampled during a specific period in which the waveform is stable. It is held and becomes a DC flow rate signal V1. The DC flow rate signal V 1 is A / D converted by the A / D converter 53 and input to the control circuit 54.
The control circuit 54 obtains the voltage value of the DC flow rate signal V1, calculates a flow rate measurement value from this voltage value, converts it to an analog DC current of 4 to 20 mA by the communication circuit 15, and then via the communication line W. Notification is made to a host device (not shown) such as a controller.

In such an electromagnetic flow meter 50, when there is little fluid in the measuring tube P of the detector 5A and it is not in contact with the electrodes TA and TB, so-called empty state, the electrodes TA and TB are independent of the flow rate. The obtained electromotive force E0 becomes unstable, and the flow rate measurement value is not stable.
FIG. 9 is a signal waveform diagram showing the operation (empty state) of a general electromagnetic flow meter. In the normal state where the fluid is in contact with the electrodes TA and TB, as shown in FIG. 8, an electromotive force E0 having a magnitude corresponding to the flow rate is detected, but the fluid is not in contact with the electrodes TA and TB. Then, as shown in FIG. 9, for example, noise having a power supply frequency is mixed in the electromotive force E0.

This is because the impedance between the electrodes TA and TB and the input impedance of the AC amplifier circuit 51 are both high in order to detect an extremely weak electromotive force E0. It is considered to be affected by the electromagnetic wave.
For this reason, it is necessary to detect whether the measured flow rate value is unstable because the actual flow rate is unstable or because the fluid is empty.

  Conventionally, a technique for measuring and determining the impedance between the electrodes TA and TB of the detector 5A has been proposed as an empty detection technique in such an electromagnetic flow meter (see, for example, Patent Document 1). More specifically, a current is passed between the electrodes TA and TB of the detector 5A and the ground potential, the potential generated at the electrodes TA and TB is detected, and this potential is compared with a threshold value so as to be in an empty state. It is determined whether or not.

JP2011-209231A

  However, in such a conventional technique, it is necessary to detect a potential generated at the electrodes TA and TB by flowing a current between the electrodes TA and TB of the detector 5A and the ground potential. However, there is a problem that extra power is always consumed. This is not a problem with a 4-wire electromagnetic flow meter that can use abundant power, but it is extremely serious in a 2-wire electromagnetic flow meter with limited power consumption.

  The present invention is intended to solve such a problem, and an object of the present invention is to provide a sky detection technique that can accurately detect an empty state while suppressing an increase in power consumption.

  In order to achieve such an object, the electromagnetic flow meter according to the present invention detects an electromotive force generated in a fluid in response to excitation of the fluid flowing in the measurement tube by an electrode, and amplifies the electromotive force by alternating current. An electromagnetic flow meter that samples the first AC flow signal obtained by the first sample and hold circuit, calculates the flow value of the fluid by the control circuit based on the obtained first DC flow signal, and outputs it. A band for outputting a second AC flow rate signal in which a frequency component of noise mixed in the electromotive force is attenuated from the first AC flow rate signal when the fluid is not in contact with the electrode. An attenuation filter; and a second sample-and-hold circuit that outputs a second DC flow rate signal by sampling the second AC flow rate signal, and the control circuit includes the first DC flow rate signal and the first DC flow rate signal. 2 straight The voltage ratio between the flow rate signal, by comparing with a predetermined threshold, in which so as to determine whether the empty state.

  In one configuration example of the electromagnetic flowmeter according to the present invention, the band attenuation filter attenuates a commercial power supply frequency component as the noise frequency component.

  According to the present invention, it is possible to detect an empty state only by adding a band attenuation filter and a sample hold circuit, and it is possible to accurately detect the empty state while suppressing an increase in power consumption.

It is a block diagram which shows the structure of the electromagnetic flowmeter of this invention. It is an example of a circuit structure of a band attenuation filter. It is an example of the frequency characteristic of a band attenuation filter. It is a signal waveform diagram which shows operation | movement (normal state) of the electromagnetic flowmeter of this invention. It is a signal waveform diagram which shows operation | movement (empty state) of the electromagnetic flowmeter of this invention. It is a flowchart which shows the sky detection process of this invention. It is a block diagram which shows the structure of a general electromagnetic flowmeter. It is a signal waveform diagram which shows the operation | movement (normal state) of a general electromagnetic flowmeter. It is a signal waveform diagram which shows the operation | movement (empty state) of a general electromagnetic flowmeter.

Next, an embodiment of the present invention will be described with reference to the drawings.
[Configuration of the embodiment]
First, with reference to FIG. 1, the electromagnetic flowmeter 1 concerning this Embodiment is demonstrated. FIG. 1 is a block diagram showing the configuration of the electromagnetic flow meter of the present invention.

  The electromagnetic flow meter 1 is composed of a detector 1A and a converter 1B as a whole, and has a function of measuring the flow rate of a fluid having conductivity in various process systems. In the present embodiment, the electromagnetic flow meter 1 performs transmission using an analog DC current of 4 to 20 mA with a host device (not shown) such as a controller via a two-wire communication line W. A case of a two-wire electromagnetic flow meter will be described as an example. The same applies to a 4-wire ammeter.

The detector 1A is provided with a measuring tube P, electrodes TA and TB, and an exciting coil L as main components.
The measuring tube P is made of a nonmagnetic metal cylinder such as stainless as a whole, and constitutes a flow path through which a fluid to be measured flows.
The exciting coil L is composed of a pair of coils arranged to face the outside of the measuring tube P, and is orthogonal to the flow direction of the fluid flowing through the flow path in accordance with the AC exciting current supplied from the converter 1B. It has a function of generating a magnetic field in the direction.

The electrodes TA and TB are composed of a pair of electrodes arranged in contact with the fluid flowing in the flow path on the inner wall of the measuring tube P, facing the direction perpendicular to the direction of the magnetic field generated by the exciting coil L, It has a function of detecting an electromotive force E0 generated in the fluid in response to excitation of the fluid by a magnetic field and outputting it to the converter 1B.
Note that the potential of the measuring tube P is connected to the ground potential of the converter 1B via the ground terminal TG.

  The converter 1B includes, as main circuit units, an excitation circuit 10, an AC amplifier circuit 11, a sample hold circuit (SH) 12, an A / D converter 13, a control circuit 14, a communication circuit 15, a power supply circuit 16, and a display. In addition to the unit 17, a band attenuation filter (BEF) 21, a sample hold circuit (SH) 22, and an A / D converter 23 are provided.

The excitation circuit 10 has a function of supplying an AC excitation current having a predetermined excitation frequency to the detector 1A.
The AC amplifier circuit 11 has a function of inputting the electromotive force E0 obtained by the detector 1A through the capacitive element C, amplifying the electromotive force E0, and outputting it as an AC flow rate signal E1.

The sample hold circuit (first sample hold circuit) 12 has a stable waveform among the positive side waveform and the negative side waveform of the AC flow rate signal E1 based on the sampling control signals SH1 and SH2 from the control circuit 14. The voltage of a specific period is sampled and held and output as a DC flow rate signal (first DC flow rate signal) V1.
The A / D converter 13 has a function of A / D converting the voltage value of the DC flow rate signal V <b> 1 and outputting it to the control circuit 14.

The band attenuation filter 21 has a function of attenuating a frequency component of noise mixed in the electromotive force E0 in an empty state, such as a component of the commercial power supply frequency, in the AC flow signal E1, and outputting it as an AC flow signal E2. doing.
FIG. 2 is a circuit configuration example of the band attenuation filter. Here, a circuit example is shown in which a twin T-type notch filter is used and an operational amplifier is used in the feedback system.
FIG. 3 shows an example of frequency characteristics of the band attenuation filter. Here, frequency characteristics relating to the gain of the circuit configuration example of FIG. 2 are shown, and it can be seen that attenuation of about 40 dB is obtained at the commercial power supply frequency of 50 Hz.

The sample hold circuit (second sample hold circuit) 22 has a stable waveform among the positive side waveform and the negative side waveform of the AC flow rate signal E1 based on the sampling control signals SH1 and SH2 from the control circuit 14. The voltage of a specific period is sampled and held and output as a DC flow rate signal (second DC flow rate signal) V2.
The A / D converter 23 has a function of A / D converting the voltage value of the DC flow rate signal V2 and outputting it to the control circuit 14.

  The control circuit 14 includes a CPU and its peripheral circuits as a whole, and has a function of executing various processes by reading a program from a memory (not shown) and executing it. The main processing executed by the control circuit 14 includes excitation control processing, sampling control processing, flow rate measurement value calculation processing, communication control processing, and sky detection processing.

  The excitation control process is a process for driving the excitation circuit 10 based on a predetermined excitation frequency. The sampling control process is a process for generating sampling control signals SH1 and SH2 based on the excitation frequency and outputting them to the sample hold circuits 12 and 22. The flow rate measurement value calculation process is a function that calculates a flow rate measurement value based on the voltage value of the DC flow rate signal V1. The communication control process is a process for notifying the calculated flow rate measurement value from the communication circuit 15 to the host device via the communication line W.

  In the sky detection process, the voltage ratio between the DC flow rate signal V1 from the A / D converter 13 and the DC flow rate signal V2 from the A / D converter 14 is compared with a threshold value, thereby measuring the detector 1A. It is a process of determining whether or not the pipe P is in an empty state, and in a case where it is determined that the pipe P is in an empty state, a process of notifying an alarm from the communication circuit 15 via the communication line W.

The communication circuit 15 has a function of notifying information such as a flow rate measurement value and an alarm by communicating with a host device via the communication line W.
The power supply circuit 16 has a function of receiving the power supplied from the host device via the communication line W from the communication circuit 15 and distributing it to each circuit unit in the converter 1B.
The display device 17 includes a display element such as an LCD or an LED, and has a function of displaying various information such as a measured current value and an empty detection result in accordance with an instruction from the control circuit 14.

[Operation of First Embodiment]
Next, with reference to FIG. 4 and FIG. 5, operation | movement of the electromagnetic flowmeter 1 concerning this Embodiment is demonstrated. FIG. 4 is a signal waveform diagram showing the operation (normal state) of the electromagnetic flowmeter of the present invention. FIG. 5 is a signal waveform diagram showing the operation (empty state) of the electromagnetic flowmeter of the present invention. Here, a case where a component of the commercial power supply frequency is mixed as noise in the electromotive force E0 in the empty state will be described as an example.

The excitation circuit 10 supplies an AC excitation current having a predetermined excitation frequency to the excitation coil L of the detector 1A based on the excitation control process of the control circuit 14.
The exciting coil L generates a magnetic field in a direction orthogonal to the flow direction of the fluid flowing through the flow path in accordance with the AC exciting current. As a result, an electromotive force E0 having a magnitude corresponding to the flow rate is generated in the fluid flowing through the measurement pipe P.
In a normal state where the fluid is in contact with the electrodes TA and TB, the electrodes TA and TB detect the electromotive force E0 generated in the fluid and output it to the converter 1B.

The AC amplifying circuit 11 of the converter 1B AC amplifies the electromotive force E0 from the detector 1A input via the capacitive coupling by the capacitive element C, and outputs it as an AC flow signal E1.
The sample hold circuit 12 is based on the sampling control signals SH1 and SH2 output by the sampling control processing of the control circuit 14, and is a specific period in which the waveform is stable among the positive side waveform and the negative side waveform of the AC flow rate signal E1. The A / D converter 13 A / D converts the voltage value of the DC flow rate signal V1 and outputs it to the control circuit 14.

On the other hand, the band attenuation filter 21 attenuates the component of the commercial power supply frequency in the AC flow rate signal E1 from the AC amplification circuit 11, and outputs it as an AC flow rate signal E2.
Based on the sampling control signals SH1 and SH2 from the control circuit 14, the sample and hold circuit 22 samples and holds a voltage in a specific period in which the waveform is stable among the positive side waveform and the negative side waveform of the AC flow signal E2. The A / D converter 23 outputs the voltage value of the DC flow rate signal V2 to the control circuit 14 after A / D conversion.

Therefore, in the normal state, as shown in FIG. 4, since an electromotive force E0 synchronized with the excitation frequency is obtained, both the DC flow rate signal V1 and the DC flow rate signal V2 indicate voltage values corresponding to the flow rate. Become.
On the other hand, in the empty state, as shown in FIG. 5, the electromotive force E0 synchronized with the excitation frequency cannot be obtained, and the commercial power supply frequency component is mixed in the electromotive force E0. For this reason, V1 which has not attenuated the commercial power supply frequency component shows a certain voltage value while being unstable, but V2 which has attenuated the commercial power supply frequency component shows a voltage value significantly smaller than V1. .

Thereafter, the control circuit 14 performs an empty detection process based on the voltage value of the DC flow rate signal V1 acquired from the A / D converter 13 and the voltage value of the DC flow rate signal V2 acquired from the A / D converter 23. Execute.
FIG. 6 is a flowchart showing the sky detection process of the present invention.

In the sky detection process, the control circuit 14 first acquires the DC flow rate signal V1 from the A / D converter 13 (step 100) and the DC flow rate signal V2 from the A / D converter 23 (step 101). .
Next, the control circuit 14 calculates a voltage ratio between V1 and V2 by dividing the DC flow rate signal V2 by the DC flow rate signal V1, and compares this voltage ratio with a preset threshold value Rth. (Step 102).

  Here, when the voltage ratio is equal to or greater than Rth (step 102: YES), V2 which attenuates the commercial frequency component has a certain amount of magnitude compared to V1 where the commercial frequency component is not attenuated. Therefore, it is determined that the state is normal (step 103), and a series of sky detection processing is terminated.

  Therefore, when it is determined that the normal state is obtained by the sky detection process, the control circuit 14 calculates the flow rate measurement value based on the voltage value of the DC flow rate signal V1 acquired from the A / D converter 13 by the flow rate measurement value calculation process. By the communication control process, the communication circuit 15 converts the analog DC current to 4 to 20 mA, and then notifies the host device via the communication line W. In addition, if it is empty at that time, the control circuit 14 stops alarm notification from the communication circuit 15 to the host device via the communication line W.

  On the other hand, when the voltage ratio is less than Rth (step 102: NO), V2 indicates a very small value compared to V1, and therefore, it is determined that the state is empty (step 104), and a series of empty The detection process ends.

  Therefore, when it is determined that there is an empty state by the sky detection process, the control circuit 14 converts the flow rate value (fixed) indicating zero flow rate into an analog DC current of 4 to 20 mA by the communication circuit 15, and then passes through the communication line W. To the host device. In addition, the control circuit 14 notifies an alarm occurrence from the communication circuit 15 to the host device via the communication line W.

[Effect of the first embodiment]
Thus, according to the present embodiment, the band attenuation filter 21 attenuates the noise component from the AC flow signal (first AC flow signal) E1, thereby causing the AC flow signal (second AC flow signal) E2 to attenuate. The sample and hold circuit (second sample and hold circuit) 22 outputs the DC flow signal (second DC flow signal) V2 by sampling the AC flow signal E2, and the control circuit 14 outputs the DC flow rate. By comparing the voltage ratio between the signal (first DC flow rate signal) V1 and the DC flow rate signal V2 with a predetermined threshold value Rth, it is determined whether or not the fluid is in an empty state not in contact with the electrodes TA and TB. Is determined.

As a result, in the normal state, the AC flow signals E1 and E2 exhibit substantially the same voltage value, whereas in the empty state, the electromotive force E0 includes noise components such as a commercial power supply frequency component. Is mixed, and the voltage value of the AC flow signal E2 is lower than that of the AC flow signal E1, so that it is detected as being empty.
Therefore, it is possible to detect the empty state by simply adding the band attenuation filter 21 and the sample hold circuit 22, and it is possible to accurately detect the empty state while suppressing an increase in power consumption.

  In the present embodiment, the threshold value can be arbitrarily set. Therefore, when the electromagnetic flow meter 1 is in an empty state due to factors such as the environment where the electromagnetic flow meter 1 is installed and the type of fluid whose flow rate is measured. Even when the magnitude of the noise component mixed in the electromotive force E changes, the empty state can be appropriately detected by adjusting the threshold value.

  In the present embodiment, the case where the frequency attenuated by the band attenuation filter 21 is a commercial power supply frequency of 50 Hz has been described as an example. However, the present invention is not limited to this, and in an area where the commercial power supply frequency is 60 Hz, 60 Hz What is necessary is just to attenuate a component. Further, if the frequency attenuated by the bandpass filter is 55 Hz, the determination accuracy is slightly deteriorated, but it is possible to stably detect the empty state regardless of the use region regardless of whether the commercial power supply frequency is 50 Hz or 60 Hz. Become.

  Further, when noise having a frequency other than the commercial power supply frequency is mixed, the frequency component of the noise may be attenuated. Note that the band attenuation filter 21 may be realized by software instead of a circuit, whereby a frequency to be attenuated can be arbitrarily selected, and a commercial power supply frequency is changed, and further noise of a frequency other than the commercial power supply frequency is generated. Even when doing so, it can respond flexibly.

[Extended embodiment]
The present invention has been described above with reference to the embodiments, but the present invention is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

  DESCRIPTION OF SYMBOLS 1 ... Electromagnetic flowmeter, 1A ... Detector, 1B ... Converter, 10 ... Excitation circuit, 11 ... AC amplifier circuit, 12 ... Sample hold circuit (1st sample hold circuit), 13 ... A / D converter, 14 ... Control circuit, 15 ... Communication circuit, 16 ... Power supply circuit, 17 ... Display, 21 ... Band attenuation filter, 22 ... Sample hold circuit (second sample hold circuit), 23 ... A / D converter, P ... Measurement Tube, TA, TB ... electrode, L ... excitation coil, E0 ... electromotive force, E1 ... AC flow rate signal (first AC flow rate signal), E2 ... AC flow rate signal (second AC flow rate signal), V1 ... DC flow rate Signal (first DC flow signal), V2... DC flow signal (second DC flow signal), SH1, SH2,... Sampling control signal.

Claims (2)

An electromotive force generated in the fluid in response to excitation of the fluid flowing in the measuring tube is detected by an electrode, and a first AC flow rate signal obtained by AC amplification of the electromotive force is sampled by a first sample hold circuit. A flow rate value of the fluid is calculated by a control circuit based on the obtained first DC flow rate signal and is output,
A band attenuating filter that outputs a second AC flow rate signal in which a frequency component of noise mixed in the electromotive force is attenuated when the fluid is in an empty state where the fluid is not in contact with the electrode, from the first AC flow rate signal; ,
A second sample and hold circuit that outputs a second DC flow signal by sampling the second AC flow signal;
The control circuit determines whether or not the vehicle is in an empty state by comparing a voltage ratio between the first DC flow rate signal and the second DC flow rate signal with a predetermined threshold value. Characteristic electromagnetic flow meter. The electromagnetic flow meter according to claim 1,
The band attenuation filter attenuates a commercial power supply frequency component as the noise frequency component.

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