JPH06102072A - Electromagnetic flowmeter - Google Patents

Abstract

PURPOSE:To enable two-wire operation or the excitation of a permanent magnet as a result of enabling DC excitation and driving due to a small current, to enable miniaturization, wt. reduction, low power consumption and cost reduction and to eliminate the effect of slurry noise or flow noise. CONSTITUTION:A magnetic circuit 1 generates a DC magnetic field and the electromotive force generated in a fluid is detected by main electrodes 4 but auxiliary electrodes 5 are provided to the peripheries of the electrodes 4 and the potentials thereof are changed over between earth potentials and main electrode potentials by a changeover circuit 10. These modulation signals are amplified by an amplifying circuit 6 and the difference between the modulation signals is taken out by a synchronous rectifier circuit 7 and rectified to be transmitted to a conversion circuit 8 as an output signal. A changeover control circuit 9 controls the timing of the changeover circuit 10 to simultaneously send a synchronizing signal to the synchronous rectifier circuit 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an industrial electromagnetic flowmeter, and its excitation method.

[0002]

2. Description of the Related Art Conventionally, for example, the principle shown in FIG. 7 has been used (flow rate measuring method using an electromagnetic flow meter (JIS Z87).
64)).

In FIG. 7, a magnetic field (magnetic flux density B) is applied vertically to a conductive fluid having an average flow velocity V. At this time, if the signal electromotive force generated between a pair of electrodes provided facing each other in the direction perpendicular to both the fluid flow direction and the magnetic field application direction is E (V), E is expressed by the following equation. E = kBdV where: k: constant B: magnetic flux density (T) d: inner diameter (m) V: average flow velocity (m / s) Actually, a DC electromotive force generated by a DC magnetic field occurs between the fluid and the electrode. It is difficult to perform stable measurement because there is an interfacial potential, and the influence of the interfacial potential is generally canceled by applying an alternating magnetic field (rectangular wave or sine wave) to obtain the difference.

[0004]

The above-mentioned prior art has the following problems.

1) In order to obtain a high electromotive force, it is necessary to increase the magnetic flux density B, but for that purpose the magnetomotive force must be increased. However, this increases the inductance of the magnetic circuit, and when a rectangular wave exciting current is passed, it takes some time for the magnetic flux density B and thus the electromotive force E to settle. In addition, the eddy current generated in the magnetic circuit also becomes a factor that delays the response of the magnetic field. Therefore, there is a limit to increase the excitation frequency of the rectangular wave,
Usually, it is used between 3 Hz and 25 Hz. However, when the fluid contains slurry or sludge, low frequency noise having a frequency component of about DC to 100 Hz called slurry noise is generated, and the stability of the flow meter is deteriorated. Also, when measuring a low-conductivity fluid, noise called flow noise is generated, and the stability is reduced as in the above case. In order to remove this, it is necessary to apply an AC magnetic field having a frequency higher than 100 Hz, but it is difficult to realize for the above reason.

2) As described in the section 1), the number of turns of the coil is limited due to the restrictions of the resistance and the inductance and the mounting space, and in order to compensate the decrease in the magnetomotive force (magnetic flux density) and the electromotive force, 0 A large exciting current of about 1 A to 1 A is applied. Therefore, the excitation circuit becomes large in size and power consumption also increases. This is a factor that causes the flowmeter itself to become larger and reduce reliability due to heat generation.

[0007]

In the present invention, in order to achieve the above object, the applied magnetic field is a DC magnetic field, and an auxiliary magnetic pole is provided around the main electrode in order to cancel the influence of the interface potential generated thereby. A modulating means for periodically switching the potential between the ground potential and the main electrode potential is provided, and an accurate flow electromotive force is obtained by the synchronous rectifying means for extracting and rectifying only the difference.

[0008]

With this structure, the switching operation of the magnetic circuit is not required, and only the electrode signal is switched, so that the switching can be performed up to a high speed close to the response speed of the electronic circuit. Also,
Since a DC magnetic field is sufficient, a very large magnetomotive force can be obtained by increasing the number of coil turns even if the exciting current is small.
Furthermore, it is possible to use permanent magnets, which makes it possible to reduce the size of the magnetic circuit and excitation circuit and significantly reduce power consumption.

[0009]

EXAMPLE An example of the present invention will be described below with reference to FIG.

The measurement tube 2 is filled with the fluid 3 to be measured, and a DC magnetic field generated by the magnetic circuit 1 is applied between the main electrodes 4 at both ends to the electromotive force corresponding to the formula 1 by Faraday's law. Occurs. The signal is amplified by the amplifier circuit 6, then passes through the synchronous rectification circuit 7 and the conversion circuit 8, and finally a signal proportional to the flow rate is obtained. on the other hand,
An auxiliary electrode 5 is arranged around the main electrode 4, and the potential of the auxiliary electrode 5 is periodically switched between a ground potential and the main electrode potential by a switching circuit 10 according to a command from a switching control unit 9. As a result, the input / output signal of the amplifier circuit 6 becomes a rectangular wave signal,
Only the difference is rectified by the synchronous rectification circuit 7, and a signal proportional to the flow rate is obtained and becomes the input signal of the conversion circuit 8. The operation will be described with reference to FIG.

When the switching circuit 10 is not grounded (the auxiliary electrode is connected to the main electrode), the electrodes are (Fig. 2)-
The potential shown in (a) is generated. It is a direct current potential that is a combination of the flow electromotive force and the interfacial potential according to Faraday's law shown in Equation 1. The interface potential has a large potential of several hundred mV to several V and varies depending on the interface state, but its frequency component is mainly a relatively low frequency component of several Hz or less.

Next, when the switching circuit 10 is grounded (the auxiliary electrode is grounded), the fluid potential around the main electrode 4 becomes the ground potential, so that an electromotive force is generated in the measuring tube. Also, the flow rate electromotive force is canceled and only the interface potential is detected by the main electrode 4.

The signal obtained at the electrodes when the switching circuit 10 continuously operates to switch between the two modes is (FIG. 2)-
It becomes (b). When only the modulated component is taken out by the synchronous rectification circuit 7, it becomes (Fig. 2)-(c), and the direct current component of the interface potential is removed. After rectification, the flow rate signal of (Fig. 2)-(d) is obtained through the conversion circuit.

In this case, the background of the interfacial potential can be removed by applying the modulation at a frequency higher than the fluctuation frequency component of the interfacial potential. Moreover, since the main electrode and the auxiliary electrode are arranged close to each other, the influence of the interface potential on both the main electrode and the auxiliary electrode is almost the same as for the slurry noise generated in the electrode due to the slurry, and the influence is canceled out. I do not receive it.

FIG. 3 shows the structure of a two-wire type DC excitation type electromagnetic flow meter. The current supplied from the DC power supply 23 is supplied to the electromagnetic flow meter side via the load resistance 24, and a part of the current flows into the exciting coil 19 via the constant current circuit 22 to excite the magnetic circuit 1 with the DC constant current. Another current is supplied to the low voltage circuit 21, and then returns to the DC power supply 23 together with the exciting current. The constant voltage circuit 21 stabilizes the voltage and uses it as a driving power source for the signal processing unit 20. The constant current circuit 22 supplies a constant current equal to or less than the minimum value of the signal current to the exciting coil 19. DC 4-20
In the case of a mA signal transmission system, the exciting current is set to a small value of 4 mA or less, but since it is a direct current, the exciting coil 19
Can reduce the wire diameter and increase the number of turns,
Therefore, it is possible to obtain a magnetomotive force equal to or higher than that of a conventional electromagnetic flowmeter and a sensitivity as a flowmeter. In the above description, the case where the exciting coil is excited by a part of the signal current has been described, but a permanent magnet may be used as long as a DC magnetic field can be obtained. In that case, a magnetic sensor is installed in order to correct the change over time of the magnetic field and the change due to temperature, and the magnetic field is measured.
Therefore, if the signal processing unit is corrected, a more stable and highly accurate electromagnetic flow meter of permanent magnet excitation can be realized.

FIG. 4 shows a concrete structure example. A lining is formed on the surface of the pipe 11 by using, for example, PFA of tetrafluoroethylene resin, and an electrode is inserted into the lining so as to be in contact with the fluid 3 to be measured. As for the structure of the electrode, as shown in FIG. 5, the auxiliary electrode 5 is formed around the main electrode 4 via the insulating portion 13. These electrodes are pressed against the lining surface by the coil spring 16 via the insulating sheets 17 and 18 to keep the airtightness. The other end surface of the coil spring 16 is a lid 14 screwed to the pipe 11.
By pressing the inner surface of the, the necessary surface pressure is secured in an airtight manner. A signal is taken out from the main electrode 4 and the auxiliary electrode 5 by an electrode lead wire 15. Such an electrode can be manufactured by partially metallizing the ceramic with platinum or the like or forming it into a cermet and integrally firing it, or can be realized by partially making the conductive ceramic an insulator. Further, the main electrode 4 and the auxiliary electrode 5 may be made of metal, and an O-ring may be used to seal between them, or an insulator may be inserted. In the above description, the pipe 11
Although the lining 12 and the lining 12 are described as separate parts, they may be integrally formed of ceramic or other insulating material.

FIG. 6 shows another embodiment in which the main electrode 4 and the auxiliary electrode 5 are directly formed on the insulating measuring tube 2 as in the case described above, which is suitable for integral molding of ceramics or resin molding. It is a structure.

Although the basic operation of the signal processing unit has been described as an analog circuit, it is of course possible to perform the same processing by software using a microprocessor or the like.

[0019]

As described above, according to the present invention, since the electromagnetic flowmeter can be constructed by the direct current excitation, the size and weight can be greatly reduced.
Low power consumption and cost reduction are possible, and 2-wire operation and permanent magnet excitation are also possible. Moreover, since high-speed modulation, which is difficult with the conventional method, is possible, it is possible to deal with various problems such as slurry noise and flow noise, which have been difficult to perform stable measurement in the past, except for signal processing. Furthermore, as a result of low power consumption, internal heat generation is reduced, the reliability of the electronic circuit is improved, and at the same time, the environmental resistance temperature can be selected in a wide range.

As described above, as a result of enabling DC excitation without using AC excitation, which has been a major limitation of conventional electromagnetic flowmeters, a large degree of freedom is obtained and its effect is great.

[Brief description of drawings]

FIG. 1 is a diagram showing a specific example of the present invention.

FIG. 2 is a diagram for explaining the operation of FIG.

FIG. 3 is a diagram illustrating another embodiment of the present invention.

FIG. 4 is a diagram showing a specific example of the present invention.

FIG. 5 is a diagram showing another embodiment of the present invention.

FIG. 6 is a diagram showing another embodiment of the present invention.

FIG. 7 is a diagram illustrating a conventional technique.

[Explanation of symbols]

1 ... Magnetic circuit, 2 ... Measuring tube, 3 ... Fluid to be measured, 4 ... Main electrode, 5 ... Auxiliary electrode, 6 ... Amplifying circuit, 7 ... Synchronous rectifying circuit,
8 ... conversion circuit, 9 ... switching control unit, 10 ... switching circuit, 11
... pipe, 12 ... lining, 13 ... insulating part, 14 ... lid, 15 ... electrode lead wire, 16 ... coil spring, 17, 1
8 ... Insulating sheet, 19 ... Excitation coil, 20 ... Signal processing part, 21 ... Low voltage circuit, 22 ... Constant current circuit, 23 ... DC current, 24 ... Load resistance.

Claims (8)

[Claims] 1. A conduit whose inner surface through which a fluid to be measured flows is covered with an insulator, a magnetic circuit for applying a DC magnetic field perpendicular to the tube axis direction thereof, and a direction orthogonal to both the flow direction and the magnetic field of the fluid. In the electromagnetic flowmeter that has a pair of electrodes arranged opposite to each other and converts into a signal proportional to the flow velocity of the fluid to be measured, the main electrode and an auxiliary electrode provided in the vicinity thereof are provided as the electrodes. A switching circuit for periodically switching the potential of the auxiliary electrode to the ground potential, a switching control unit for controlling the switching circuit, and a synchronous rectification circuit that operates in synchronization with the switching control unit,
An electromagnetic flowmeter, comprising: a conversion circuit that converts the synchronous rectified signal into a transmission signal. 2. An electromagnetic flowmeter according to claim 1, wherein a permanent magnet is used as the magnetic circuit. 3. The electromagnetic flowmeter according to claim 1, wherein a part or the whole of a DC constant current transmission signal as the magnetic circuit is caused to flow through an exciting coil to generate a DC magnetic field. 4. An electromagnetic flowmeter according to claim 1, wherein the switching circuit, the switching control circuit, the synchronous rectification circuit, and the conversion circuit are supplied with drive power from a current transmission line to operate. 5. The electromagnetic flowmeter according to claim 1, wherein the main electrode and the auxiliary electrode have a coaxial structure, and the main electrode is arranged outside the center of the auxiliary electrode and concentric with the auxiliary electrode. 6. The electromagnetic flowmeter according to claim 5, wherein the main electrode and the auxiliary electrode are formed of an integral ceramic, and the main electrode and the auxiliary electrode are formed of a conductive metal or a conductive ceramic. 7. The electromagnetic flowmeter according to claim 1, wherein the switching speed of the switching circuit is selected to be 1 Hz to 10 kHz. 8. The electromagnetic flowmeter according to claim 1, wherein the conduit is formed of ceramic or an insulating resin material, and a main electrode and an auxiliary electrode are formed by making a part of the conduit conductive.