JPH04290920A - electromagnetic flow meter - Google Patents

Abstract

PURPOSE:To provide an improved electromagnetic flow meter of small size and simple construction, which measured the rate of flow of a liquid of service water/sewerage, food, pharmaceutical and chemicals substance, etc., as an electroconductive fluid, or the rate of flow of a slurry consisting of such a liquid and fine solids. CONSTITUTION:Permanent magnets 10 of rare earth are installed sandwiching the electrode 4 of an electromagnetic flow meter, with which the magnetic field maximizes in the neighborhood of the electrode, and the rate-of-flow signal 10 times approximately. An inverse voltage is impressed on this rate-of-flow signal electrode 4 externally by an inverse voltage generator controlled by a CPU 7 periodically. The magnitude of the inverse voltage is expressed by the following formula: (inverse voltage)=(rate-of-flow signal sampling time)X(inverse voltage impressing time/inverse voltage impressing time)x(rate-of- flow signal voltage). This permits accomplishing a small-sized sensor and converter without provision of any AC power supply nor any shield cable for energization, and also anti-noise property can be enhanced because of the rate- of-flow signal which is 10 times as great approximately.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is for measuring the flow rate of conductive fluid, and is used to measure the flow rate of liquids such as water supply/sewage systems, medicines, foods, and chemical substances, or of slurry that is a mixture of liquid and fine solids. Regarding electromagnetic flowmeters.

0002

[Prior art] Conventional electromagnetic flowmeters comply with JIS Z 8.
As shown in JIS B 764 and JIS B 7554, according to Fleming's law, a flow rate signal is generated as a voltage signal of several tens of μV to several mV in a direction perpendicular to the fluid flow direction and the magnetic field direction. A flow signal is generated proportional to the fluid velocity flowing within the tube.

As shown in FIG. 2, the currently used electromagnetic flowmeter uses an AC power supply 3 to pass current through a pair of excitation coils 2 disposed above and below a pipe 1 as a means for generating a magnetic field. In recent years, zero point drift and changes over time have been reduced by using square waves as alternating current. The reason for using alternating current is that in DC magnetic field excitation, the flow rate signal generated in the fluid is direct current, so electrochemical reactions occur continuously at the fluid and electrode interface due to the weak current flowing in a fixed direction between the fluid and the electrode. However, the interfacial state changes, producing a weak electromotive force or increasing and fluctuating the interfacial electrical resistance. All of these values are not constant values, but change over time or with the flow velocity. Both become noise to the flow rate signal.

[0004] In order to solve this problem, when AC excitation is used, the flow rate signal becomes AC, and the electrochemical reaction does not proceed because forward and reverse reactions occur alternately, making stable measurement possible.

In the conventional device, the detector and the converter containing the excitation power source that excites the excitation coil, the amplifier 5 that amplifies the flow signal, and the arithmetic circuit 6 that converts it into a standardized flow signal are installed at separate locations. Therefore, the two are connected by a two-core excitation shielded cable 16 for power and a two-core shielded cable 15 for flow signal.

Furthermore, since the lead wire 20 of the electrode that extracts the flow signal to the outside is led out through an alternating current magnetic field, if it is wired carelessly, a weak voltage will be induced in the lead wire and will be added to the flow signal. This occurred regardless of the presence or absence of a flow rate, which caused zero point deviation. Therefore, some know-how was required for how to draw out the lead wires. In the figure, 7 is a CPU that controls the flow rate signal, the excitation circuit, and the output circuit, and 8 is the output circuit.

[0007]

[Problems to be Solved by the Invention] Conventional electromagnetic flowmeters require a large amount of power for the AC excitation power supply that generates the magnetic field, so the converter is large and expensive due to heat dissipation, and the excitation cable is required. was necessary. Furthermore, the structure inside the detector was complicated.

[0008] The present invention solves the above problems by downsizing and simplifying the device, and also improves zero point drift and noise resistance by increasing the flow rate signal.

[0009]

[Means for Solving the Problems] In the present invention, a permanent magnet is used as the magnetic field generating means. The magnetic poles of the permanent magnet are arranged so as to sandwich the electrodes that detect the flow rate signal. In addition, permanent magnets are thermally demagnetized before use in order to stabilize them.

Another aspect of the invention is to include means within the converter for applying a voltage of opposite polarity to the flow rate signal voltage to the electrodes.

[0011]

[Operation] By arranging the magnetic poles of a permanent magnet to sandwich the electrodes and generating a magnetic field perpendicular to both the flow direction and the axis connecting both electrodes, as stated in the explanation of JIS Z 8764, the highest weighting factor can be achieved. Since the maximum value of the magnetic field can be generated at a large position, it is possible to
5 times the flow rate signal is obtained. Furthermore, the permanent magnet used is a rare earth magnet that has an energy product 3 to 5 times that of conventional ferrite magnets or alnico magnets. Rare earth magnets include samarium-cobalt (SmCo series), cerium-cobalt (CeCo series), and neodymium-iron (N
dFe system), and the magnetic field generated by coil excitation is
~3 times the magnetic field strength can be obtained. Therefore, the final flow rate signal can be obtained about 10 times as much as the conventional one.

Furthermore, permanent magnets are affected by temperature, such that the strength of the magnetic field changes depending on the ambient temperature. As a countermeasure for this, after magnetizing the permanent magnet to the saturation point, the permanent magnet is heated to 100 to 200°C and 2 to 10% of the magnetized amount is thermally demagnetized.
It is possible to generate a magnetic field that is stable both thermally and over time.

[0013] Next, during direct current excitation of a permanent magnet, an electrochemical reaction occurs at the electrode portion. In order to eliminate this, a voltage of opposite polarity to the flow rate signal is applied to the electrode, so that the electrochemical reaction caused by the flow rate signal voltage E1 and the electrochemical reaction caused by the reverse voltage E2 applied to the electrode are equalized. The chemical reaction is canceled out and disappears. Equation 1 shows the conditions under which both electrochemical reactions are canceled out. However, T1 is the flow rate signal sampling time, and T2 is the reverse voltage application time.

[0014]

[Math 1]

In practice, the longer the time for sampling the flow rate signal, the better the stability of the measuring instrument; however, by selecting both times equally, the chemical reaction can be carried out under the same conditions in forward and reverse directions, and the time structure of the converter can also be changed. It's easy to do. Therefore, T1=T2
The condition of E2=2E1 is desirable.

[0016]

[Embodiment] An embodiment will be described below with reference to the drawings. FIG. 1 shows the present invention, in which a is a cross-sectional view and b is a partial cross-section of a side view. In a, there is a lining 9 made of rubber, Teflon, etc. inside the steel pipe 1, which also serves as insulation for the metal electrode 4, and a fluid flows inside this lining 9. There is a pair of magnets 10 on the outside of the pipe, and the poles of the magnets are arranged to sandwich the electrodes, creating a unidirectional magnetic field inside the pipe. The N and S poles of a pair of magnets are each connected to a connecting plate 11 made of ferromagnetic material.
This is used to secure the magnet and prevent the magnetic field from leaking to the outside. There is a case 12 on the outermost periphery, which houses the entire device. There are connecting flanges 13 on both sides of the piping in the side view shown in FIG. A permanent magnet has an arcuate cross section and a rectangular shape in side view. At the center of the magnet is a hole 14 for drawing out the electrode. By placing a permanent magnet with this shape in the electrode part, when measuring the magnetic field distribution along the axis connecting the electrodes in the pipe, as shown in Figure 3, the magnetic field is maximum near the magnetic pole, and the magnetic field is at the center of the pipe. is getting smaller. The magnetic field distribution when the pipe diameter is changed to d1<d2<d3 is shown by a dashed line, a solid line, and a dotted line. The size of the permanent magnets used is the same. As can be seen from this figure, even if the diameter changes, there is almost no change in the magnetic field in the portion where the weighting coefficient is large, so if the flow velocity is the same, there is no difference in the magnitude of the flow rate signal. FIG. 4 shows a block diagram of the detector and circuit of the present invention. The flow rate signal from the detector is input to two changeover switches S1 17 in the converter through a shielded cable for flow rate signals. When the S1 switch is connected to the a side, the flow rate signal is amplified by the amplifier, inputted to the arithmetic circuit via the signal changeover switch S2 18, adjusted the zero point and span, and then sent to the CPU.
Enter.

Normally, the flow rate signal for permanent magnet excitation is a DC voltage corresponding to the flow rate as shown in FIG. 5, and this signal voltage causes a weak current to flow at the interface between the fluid and the electrode, causing an electrochemical reaction. In the second invention, in order to prevent an electrochemical reaction from occurring, the changeover switch S1 is switched to the b side, and a voltage with a polarity opposite to that of the flow rate signal is applied to the electrode from a reverse voltage generator 21 that is voltage-controlled by the CPU.

While the reverse voltage is being applied, the selector switch S2 is grounded, and the inputs to the arithmetic circuit and CPU become 0. When the input to the CPU becomes 0, a signal is sent from the CPU to the reverse voltage generator so that the average voltage before the changeover switch S1 switches from a to b is generated. FIG. 6 shows signal voltages at various parts over time. A shows the change in the flow rate signal over time when the selector switch S1 is always set to the a side. T1 to T6 are obtained by time-dividing the flow rate signal, and the average flow rate signal voltage at each time is indicated by E1 to E9. b
indicates the voltage generated from the reverse voltage generator controlled by the CPU. c indicates the input voltage of the arithmetic circuit, and E
1, E4, and E7 indicate the voltages amplified by the amplifiers. d indicates the voltage signal of the electrode, which is a combined signal of a and b. The voltage of the reverse voltage generator shown in b is CP
The signal E1 at time T1 is measured at U, and since there is no input signal during the next time T2, the signal E2 of the reverse voltage generator is measured during this time.
A signal is sent from the CPU to the reverse voltage generator so that a constant voltage such that E2=2E1 is generated. The switching of the above signals is performed by two changeover switches S1 and S2 controlled by the CPU, T1 and T2, T3 and T4, T5 and T6.
are each processed as a set of signals, and in the above example T
1=T2=T3...=T6. Also, T
The value of 1 is 50H so that the time is synchronized with the commercial power supply.
In the z region, 5, 10, 25, 50Hz etc. are selected, and 60
In the Hz region, 3, 6, 12, 30, 60 Hz, etc. are selected. FIG. 7 shows another embodiment, which is an embodiment of the reverse voltage generator section. In the figure, the digital reverse voltage signal from the CPU is converted into a DC signal E by the D/A converter 22, and is further connected to the reduction amplifier 23, the dividing resistor R124, and the dividing resistor R2.
25, the signal is reduced as shown in Equation 2. Therefore, the voltage E2 applied to the electrode is expressed by Equation 2.

[0019]

[Math 2]

Since an actual amplifier has an offset voltage e, the detailed expression of Equation 2 becomes Equation 3.

[0021]

[Math 3]

[0022] The value of e is several mV, which is equivalent to the flow signal of an electromagnetic flowmeter, so if a dividing resistor is not used,

[0023]

[Math 4]

##EQU1## Therefore, it is no longer possible to set the value of the reverse voltage to the relationship E2=2E1.

By using the dividing resistor, the offset voltage is also attenuated to R1/(R1+R2), and R1/
If (R1+R2)≦1/100 is selected, the error in the offset voltage with respect to E2 can be suppressed to 1% or less.

[0026] Even if the method of applying a reverse voltage is applied to a conventional electromagnetic flowmeter using a permanent magnet, due to the nature of the action,
The effect remains unchanged.

[0027]

[Effects of the Invention] According to the present invention, by using a rare earth magnet for excitation of an electromagnetic flowmeter, it is possible to eliminate an excitation power supply and an excitation shield cable connecting a converter and a detector, resulting in miniaturization and It can be simplified.

Furthermore, a large magnetic field can be generated in the vicinity of the electrode having a large weighting coefficient, a flow rate signal that is about 10 times larger can be obtained, and noise resistance can also be achieved.

Furthermore, instability caused by electrochemical reactions caused by direct current excitation using permanent magnets can be removed, making it possible to perform stable measurements without drift. The effect is industrially beneficial.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view and a side view of a detector illustrating the present invention.

FIG. 2 is a diagram showing a block diagram of a conventional device.

FIG. 3 is a diagram showing the magnetic field distribution of the present invention.

FIG. 4 shows a block diagram of the present invention.

FIG. 5 is a diagram showing a flow rate signal.

FIG. 6 is a diagram showing a flow rate signal, a reverse voltage, and a flow rate sampling signal.

FIG. 7 is a block diagram illustrating one embodiment of the present invention.

[Explanation of symbols]

1... Piping, 2... Excitation coil, 3... AC power supply, 4... Electrode,
5... Amplifier, 6... Arithmetic circuit, 7... CPU, 8... Output circuit, 9... Lining, 10... Permanent magnet, 11... Connection plate, 1
2... Case, 13... Flange, 14... Hole, 15... Flow rate signal shield cable, 16... Excitation shield cable, 17... Changeover switch S1, 18... Signal changeover switch S2, 20... Lead wire, 21... Reverse voltage generation Vessel, 21...
D/A converter, 23... reduction amplifier, 24... dividing resistor R1, 25... dividing resistor R2.

Claims (7)

[Claims] Claim 1: The inner surface of the pipe is lined with an insulator, and a pair of electrodes for detecting a flow rate signal isolated from the pipe by the lining, and a pair of magnetic poles of a permanent magnet are placed between the electrodes. An electromagnetic flowmeter characterized by generating a magnetic field perpendicular to both axes connecting electrodes. 2. The electromagnetic flowmeter according to claim 1, wherein the material of the permanent magnet is a rare earth magnet. 3. The electromagnetic flowmeter according to claim 1, wherein the permanent magnet is thermally demagnetized. 4. The electromagnetic flowmeter according to claim 1, wherein the same poles of the permanent magnets are connected by a ferromagnetic connecting plate. 5. The inner surface of the pipe is lined with an insulator, and a pair of electrodes for detecting a flow rate signal isolated from the pipe by the lining and a pair of magnetic poles of a permanent magnet are placed between the electrodes, and An electromagnetic flowmeter characterized in that it is equipped with a reverse voltage generator that generates a magnetic field perpendicular to any of the axes connecting the electrodes, and that periodically applies a voltage of opposite polarity to the flow rate signal generated in the electrodes from the outside. 6. In claim 5, the reverse voltage applied to the electrode is reverse voltage=flow signal sampling time+reverse voltage application time/
An electromagnetic flowmeter characterized by the expression: reverse voltage application time x flow rate signal voltage. 7. The electromagnetic flowmeter according to claim 6, wherein the flow rate signal sampling time and the reverse voltage application time are equal.