JPH06307905A - Electromagnetic flowmeter detector - Google Patents

Abstract

PURPOSE:To reduce noise and to improve SN ratio at high exciting frequency while ensuring the sampling interval without modifying the exciting coil by switching the connection of exciting coil in series or parallel. CONSTITUTION:A switch 12 switches the connection of a coil 4 in series and parallel while generating a magnetic field in same direction. When the coil 4 is excited with higher frequency, the impedance of the coil 4 increases in proportion to the exciting frequency but when the connection of the coil 4 is switched from series to parallel by means of the switch 12, the exciting current of the coil 4 can be decreased to about one half of normal level. Consequently, the magnetic field generated by the coil 4 is restrained from decreasing and a relatively stable signal electromotive force is produced thus improving the SN ratio relevant to the fluid noise by an amount corresponding to the increase of exciting frequency. Furthermore, the rising time of signal electromotive force is restricted even in case of high exciting frequency and a sufficient sampling interval is ensured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flow meter for measuring the flow rate of electrically conductive fluid in various process systems.

[0002]

2. Description of the Related Art A conventional electromagnetic flowmeter has a structure as shown in the block diagram of FIG. In the figure, 1 is
A detector for detecting a signal electromotive force according to the flow rate of a fluid, a measuring tube 2 for flowing a fluid for measuring a flow rate, an electrode 3 provided on an inner wall surface of the measuring tube 2, a measuring tube 2 for generating a magnetic field. The coils 4 are provided above and below and are connected in series.

Reference numeral 5 is a converter for converting a signal electromotive force from the detector 1 to send a predetermined flow rate signal, and an exciting circuit 6 for generating a rectangular wave exciting current to be supplied to the coil 4 and an electrode 3. Circuit 7 for amplifying a signal from the control circuit 8, a control circuit 8 for comparing and converting the amplified signal, and an interface circuit for generating an internal power supply from a current supplied from the transmission line 7 and sending a flow rate signal from the control circuit 8. 9
It consists of

Now, an exciting current of a predetermined frequency is supplied from the exciting circuit 6 to the coil 4, and a magnetic field is generated in the measuring tube 2. A fluid flows at a certain flow rate in the measurement tube 2, and a signal electromotive force e is generated in the electrode 3 according to the Faraday electromagnetic induction law. The signal electromotive force e is detected and amplified by the amplifier circuit 7, and then the control circuit 8 samples and A / D converts the waveform in a stable section to generate a signal electromotive force at a reference flow rate measured in advance. It is output from the interface circuit 9 as a flow rate signal indicating a ratio with electric power.

The signal detected by the detector 1 is mixed with fluid noise such as AC noise caused by a commercial power source and EC noise generated by electrochemical interaction between fluid ions and the electrode 3. However, this deteriorates the S / N ratio of the signal electromotive force e that accurately indicates only the flow rate, which is a cause of hindering accurate measurement of the flow rate.

Conventionally, as a means for solving such a problem, a method has been adopted in which the frequency of the exciting current supplied to the coil 4, that is, the exciting frequency is increased to improve the S / N ratio. This is because EC noise, which is relatively large as fluid noise, has a frequency characteristic (1 / f characteristic) that is inversely proportional to the frequency, and is commonly used as an excitation frequency.
By using a frequency higher than around 0 Hz, the fluid noise for the signal electromotive force e indicating the flow rate is reduced.
That is, the S / N ratio is improved.

FIG. 5 shows the coil 4 in the conventional detector 1.
FIG. 4 is a circuit diagram showing the connection of the above, in which coils 4 provided above and below the measuring tube 2 are connected in series, and an exciting current I1 (exciting voltage E) of a predetermined frequency is given from an exciting circuit 6. . Further, FIG. 6 shows the exciting current supplied to this circuit,
It is a wave form diagram which shows the waveform of the signal electromotive force detected from the electrode 3.

In FIG. 6, I1 is an exciting current having a half cycle of T1, e1 is a signal electromotive force detected by the electrode 3 when the exciting current is I1, t1 is an exciting current, that is, a rising time of the signal electromotive force, and s1 is The sampling period in which the signal electromotive force e1 is sampled by the control circuit 8 is shown. I2 and e2 are waveforms of the exciting current and the signal electromotive force in the case of the high-frequency excitation (high-speed excitation) method using a frequency higher than the normal excitation frequency, and T2 is a half cycle, t
2 and s2 indicate the rise time and the sampling period, respectively.

Now, when the exciting current I1 of FIG. 6 is applied to the circuit of FIG. 5, the inductance component of each coil 4 causes a waveform having a distortion indicated by a dotted line at the rising time, which is proportional to the exciting current I1. Also in the signal electromotive force e1 detected as described above, the same waveform distortion occurs. The rising time t1 is calculated by the equation 1. In Equation 1, L is the inductance per coil 4, R is the pure resistance per coil 4, E is the excitation voltage, and ln is the natural logarithmic function.

[0010]

[Equation 1]

Generally, the distortion of the signal electromotive force e1, that is, the rising time t1 is set to 1/2 of the half cycle T1,
Further, in consideration of safety, the sampling period is set to the half period of the latter half of the flat part where there is no distortion at the time of rising.

Further, conventionally, a high-frequency excitation (high-speed excitation) method has been proposed in which the excitation frequency is increased in order to improve the S / N ratio. In this case, the inductance of the coil 3 increases the impedance of the circuit, and the exciting current decreases. The waveforms of I2 and e2 in FIG. 6 are obtained when the excitation frequency is approximately twice the normal frequency (I1), and the excitation voltage E
When is constant, the exciting current I2 is almost 1/2 of I1. The rising time t2 in this case is calculated by the equation 2.

[0013]

[Equation 2]

For example, when the exciting voltage E is 20 (V), the exciting current I1 (= 2 · I2) is 200 (mA), and the pure resistance R per coil 4 is 10 (Ω), the high-speed exciting method (several) In the case of 2), the rise time t2 is about 1/2 of t1 in the case of using the normal excitation frequency (Equation 1). This is because the rising time t2 of the signal electromotive force e2 is about half of t1 because the exciting current I2 is halved in the half cycle T2 which is about 1/2.
It is shown that a sufficient sampling period s2 can be secured as in the case of using a normal excitation frequency.

[0015]

Therefore, in such a conventional electromagnetic flowmeter detector, when a higher excitation frequency is used in the high frequency excitation method, it is necessary to further reduce the excitation current. As a result, the signal electromotive force indicating the flow rate also decreases, so that there is a problem that a significant improvement in the S / N ratio cannot be expected. When a higher excitation frequency is used, a coil with high magnetic field generation efficiency, such as a high-frequency excitation coil having a special structure that suppresses inductance, is used to compensate for the decrease in signal electromotive force due to the decrease in excitation current. There is a problem in that it has to be used and the coil or detector equipment has to be changed.

The present invention is intended to solve such a problem, and it is possible to further improve the S / N ratio with fluid noise by using a higher excitation frequency without changing the excitation coil. It is an object of the present invention to provide a detector for an electromagnetic flow meter that can secure a sufficient sampling period.

[0017]

In order to achieve such an object, the detector of the electromagnetic flow meter according to the present invention has a second frequency higher than the first frequency used when the coils are connected in series as an exciting current. When the exciting current is used, a switching means for switching the connection of the coils to the parallel connection is provided.

[0018]

Therefore, when exciting with the exciting current of the first frequency, the coils provided above and below the measuring tube are connected in series by the switching means, and the exciting current of the second frequency higher than the first frequency is used. When exciting, the switching means connects the coils in parallel.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of a detector of an electromagnetic flowmeter which is an embodiment of the present invention. The parts having the same functions as those in the above figures are designated by the same reference numerals. In FIG. 1, 11 is a detector capable of high-frequency excitation, and 12 is a changeover switch for switching the connection state of the coils 4 provided above and below the measuring tube 2 between series and parallel. However, each coil 4 is connected so as to generate a magnetic field in the same direction in either connection state.

Next, the operation of the present invention will be described with reference to FIGS. FIG. 2 shows a changeover switch 1 for high frequency excitation.
2 is a circuit diagram when the coils 4 provided above and below the measuring tube 2 are connected in parallel by S.
In order to further improve the N ratio, an exciting current I11 (exciting voltage E) having an exciting frequency higher than the above-mentioned exciting frequency is applied from the exciting circuit 6 (see FIG. 4).

FIG. 3 is a waveform diagram showing the waveforms of the exciting current supplied to the circuit of FIG. 2 and the signal electromotive force detected from the electrode 3. In FIG. 3, I11 and e11 are the waveforms of the exciting current and the signal electromotive force in the case of the high-frequency excitation (high-speed excitation) method using a frequency higher than the above-mentioned excitation frequency, and T11 is a half cycle, t11 and s11 indicates the rising time and the sampling period, respectively.

The waveforms of I11 and e11 in FIG. 3 are obtained when the excitation frequency is set to about four times the above-mentioned (I1), and the impedance component is about 4 due to the inductance component of the coil 4.
Although it increases twice, by connecting the coil 4 in parallel,
Since the combined impedance of the circuit (see FIG. 2) decreases, the exciting current I11 is almost the same as I1 when the exciting voltage E is constant.

Now, when the exciting current I11 (exciting voltage E) is applied from the exciting circuit 6 to the circuit of FIG. 2, the inductance component of each coil 4 causes a distorted waveform at the rise of the signal electromotive force e11. Is output. The rising time t11 is obtained by the following equation 3 as in the above equations 1 and 2.

[0024]

[Equation 3]

This is smaller than t2 of the equation (2), and the rising distortion of the signal electromotive force is suppressed. For example, the exciting voltage E is 20 (V) and the exciting current I1 is the same as described above.
Assuming that (= 2 · I2) is 200 (mA) and the pure resistance R per coil 4 is 10 (Ω), the excitation frequency is about 4
The rising time t11 in the case of the doubled high-speed excitation method (Equation 2) is t when the normal excitation frequency is used (Equation 1).
It is about 1/4 of 1.

In the half cycle T11, which is about 1/4, the exciting current I11 of the entire circuit does not change, but the exciting current flowing through the coils 4 connected in parallel is halved, so that the signal electromotive force e2. Since the rising time t11 of the above is also about 1/4 as compared with t1, it indicates that a sufficient sampling period s11 can be secured as in the case of using the normal excitation frequency.

Therefore, when high frequency excitation is performed using a higher frequency, the impedance of the coil 4 increases according to the excitation frequency.
By switching from the series connection to the parallel connection, the exciting current I11 flowing through the coil 4 becomes about 1/2 of that when the normal exciting frequency is used (I1), so that the decrease of the magnetic field generated by the coil 4 is alleviated. As a result, a relatively stable signal electromotive force is output, and as a result, the S / N ratio for fluid noise is further improved due to the higher excitation frequency.
Further, even if the excitation frequency is high, the rise time of the signal electromotive force is suppressed, and a sufficient sampling period can be obtained.

[0028]

As described above, according to the present invention, the changeover switch for switching the exciting coils to the series connection or the parallel connection is provided, and when the high frequency excitation is performed, the changeover switch is switched to the parallel connection. The decrease in the exciting current due to the increase in the impedance is alleviated, stable signal electromotive force is obtained, the rise time of the signal electromotive force is suppressed, and a sufficient sampling period is obtained. Therefore, in the detector of the electromagnetic flow meter, a higher excitation frequency can be used without using a high-frequency excitation coil, and the S / N ratio with respect to fluid noise can be further improved.

[Brief description of drawings]

FIG. 1 is a block diagram of a detector according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing connection of excitation coils in the case of high frequency excitation.

FIG. 3 is a signal waveform diagram showing an exciting current and a signal electromotive force in the case of high frequency excitation.

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

FIG. 5 is a circuit diagram showing a connection of a conventional excitation coil.

FIG. 6 is a signal waveform diagram showing a conventional exciting current and signal electromotive force.

[Explanation of symbols]

 1, 11 Detector 2 Measuring tube 3 Electrode 4 Coil 5 Converter 6 Excitation circuit 7 Amplification circuit 8 Control circuit 9 Interface circuit 12 Changeover switch

Claims (1)

[Claims] 1. A measuring tube in which a fluid flows is connected in series to upper and lower coils, an exciting current of a first frequency is supplied to the coil to generate a magnetic field, and the magnetic field is orthogonal to the magnetic field. An electromagnetic flowmeter detector that detects and outputs a signal electromotive force according to the flow rate of the fluid by means of an electrode that is disposed in the electromagnetic flowmeter detector, wherein an exciting current having a second frequency higher than the first frequency is used as the exciting current. In this case, the electromagnetic flowmeter detector is provided with switching means for switching the connection of the coils to parallel connection so that the magnetic field generation directions are the same.