JPH04220525A - Electromagnetic flow meter - Google Patents

Abstract

PURPOSE:To obtain an electromagnetic flow meter which is not influenced by differential noise and not influenced by a residual magnetic field of a core. CONSTITUTION:A cycle comprises a first pause period, a first excitation period, a second pause period, a second excitation period wherein excitation is made in the same polarity as that in the first excitation period and a third pause period, and the next cycle performs excitation with the above third pause period as a first pause period and the above first excitation period, the second pause period and the third pause period repeated, wherein during the first pause period, the second pause period, the second excitation period and the third pause period, a first signal twice negative signal voltage, a second signal three times the voltage, a third signal twice the voltage and a fourth signal three times the negative voltage are detected and they are summed up.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flowmeter that performs rectangular waveform three-value excitation, and more particularly to an improvement in a signal processing circuit that eliminates the influence of a residual magnetic field remaining in the core.

0002

2. Description of the Related Art FIG. 3 is a block diagram showing the structure of a conventional electromagnetic flowmeter disclosed in Japanese Patent Application Laid-open No. 195418/1988 entitled "Electromagnetic Flowmeter". FIG. 4 is a waveform diagram explaining the operation. The outline of this invention will be explained below.

[0003] Reference numeral 1 denotes an excitation section, which is composed of an excitation coil 1a, a core 1b, and the like. 2 is a conduit, into which the fluid to be measured flows. Electrodes 3a and 3b are arranged in the conduit 2 in a direction perpendicular to the magnetic field B. 4 and 5 are high impedance buffer amplifiers whose input terminals are connected to the electrode 3a.
, 3b. Buffer amplifier 6 is a differential amplifier equipped with external resistors R1 to R4 to remove common mode noise.
, 5 is received.

7 is the output of the differential amplifier, 8 is the differential amplifier 7
This is an analog/digital converter (hereinafter abbreviated as an A/D converter) that also serves as a sampling of the output of the . 9 is a microprocessor (hereinafter abbreviated as CPU), 10 is memory (ROM/RAM), and 11 is an input/output port (I/O).
, 12 are digital/analog converters (hereinafter abbreviated as D/A converters), and these constitute a microcomputer 13. 14 is an address bus, 15 is a data bus, and 16 is a final output.

[0005] The excitation coil 1a is connected to a DC constant current source 17 by switches SW1-a, SW1-b, SW2-a, SW2-b.
These switches are connected to the excitation coil 1a as shown in Fig. 2(a), with a rectangular wave excitation current having one cycle in five periods T0 to T5 of zero, positive, zero, negative, and zero. It is opened and closed by a timing signal 18 from the CPU 9 via the input/output port 11 so that the current flow occurs. These periods are all the same, and the excitation frequency is, for example, 50/4H.
Let it be z.

The output 7 of the differential amplifier 6 is output at the timing specified by the CPU 9, in this example, as shown in FIG. 4(b), immediately before the excitation current changes once each during excitation periods T1 and T3 and rest periods T0 and T4. The data is sampled by the A/D converter 8 at timings t0, t1, t3, and t4, and the digitized data is stored in the memory 10.

[0007] The CPU 9 updates these data chronologically in the memory 10 one by one and sets four data at each timing t0 to t4 as one set. Performs calculations sequentially and outputs the calculation output.

[0008] The electrodes 3a and 3b of the transmitter of the electromagnetic flowmeter have an excitation current waveform as shown in FIG. 4(c) (FIG. 4(a)).
Signal voltages Vs and -Vs corresponding to are generated. Furthermore, as shown in FIG. 4(d), during the idle period in FIG. 4(a) when the excitation current is zero, for example, T0, T2, T4, and T6, the residual magnetic flux Br, -Br due to the core 1b of the oscillator and the measured A noise voltage generated by the fluid flow rate appears between the electrodes 3a and 3b. Such a noise voltage is caused by the magnetic hysteresis curve (B-H curve) of the core 1b, and when the residual magnetic flux is -Br during the rest period T0, -kVs (k is a constant), a noise voltage is generated, leading to positive excitation during the excitation period T1, and then a positive residual magnetic flux +Br during the rest period T2, which is zero excitation, and +kVs proportional to the flow velocity of the fluid to be measured, and then a negative Excitation period T3
This is because, after passing through, it reaches the rest period T4, returns to its original position on the B-H curve, and generates a noise voltage -kVs. In other words, the pause period T0, which is the signal sampling period, to the pause period T
It makes one round on the B-H curve with a period of 4, and repeats this process thereafter.

FIG. 4(e) shows the electrochemical noise generated between the electrode and the fluid to be measured, which increases at a constant rate over time. Let E be the initial value of electrochemical noise at sampling timing t0,
On the other hand, at each sampling time point (t1 to t6), it increases at a constant rate ΔE.

In FIG. 4, data V00 to V06 sampled via the A/D converter 8 under the control of the CPU 9 at timings t0 to t6 correspond to the flow rate of the fluid to be measured shown in FIG. 4(c). and a signal voltage proportional to the excitation current,
The noise voltage proportional to the residual magnetism of the core and the flow rate of the fluid to be measured, shown in (d) of the same figure, and the electrochemical noise shown in (e) of the same figure are superimposed, and if the values in the figure are used, The components included in each data are as follows. V00=-kVs+E

(1) V01=Vs
+E+ △E
(2) V02=
kVs+E+2△E
(3) V03=-V
s +E+3△E
(4) V0
4=-kVs+E+4△E
(5)
V05= Vs +E+5△E

(6) V06= kVs+E+6
△E
(7) These sampling values are stored in memory 10.
are stored sequentially. The arithmetic expression for calculating only the signal voltage Vs from these stored sampling values differs depending on the following cases.

(a) When starting from the timing before positive excitation (t0 to t4) The arithmetic expression in this case is as follows, assuming that the arithmetic result is Vp. Vp=-V00+2・V01-2・V03+V04
(8) Substituting the above equations (1), (2), (3) and (5) into this equation, Vp=4・Vs

(9), and only the signal voltage from which noise has been removed is obtained.

(b) When starting from the timing before negative excitation (t2 to t6) The calculation formula in this case is as follows, assuming that the calculation result is Vn. Vn=V02-2・V03+2・V05-V06
(10) Substituting the above equations (3), (4), (6) and (7) into this equation, Vn=-4・Vs

(11), and only the signal voltage from which noise has been removed is obtained. Here, if we compare (a) and (b), we can see that the sign of the calculation output simply changes depending on whether it starts from the timing before positive excitation or from the timing before negative excitation. I understand that there is something.

These calculation formulas are stored in the ROM of the memory 10, and these calculations are executed one by one under the control of the CPU 9 to control the I/O port 11 and D/A converter 12, respectively. The output V0 is sequentially outputted through the output terminals.

[0014]

By the way, differential noise is generated when the excitation current is switched to an area perpendicular to the magnetic field formed by the fluid to be measured and the signal leads from the electrodes 3a and 3b. In the above-mentioned electromagnetic flowmeters, no particular consideration has been given to this point. Considering this point, assuming that the differential noise is d, V00'= -kVs+E
+d (
1)'V01'=Vs +E+
△E+d
(2)'V02'=kVs+E
+2△E-d
(3)'V03'=-Vs +E
+3△E-d
(4)'V04'=-kVs+E+
4△E+d
(5)'V05'=Vs+E
+5△E+d
(6)'V06'=kVs+E
+6△E-d
(7)'. This corresponds to equation (8) Vp'=-V00'+2・V01'-2・V03'
+V04' When substituted into (8)', Vp'=4・Vs+4d, which corresponds to equation (9)

(9) Obtain '. This point is also the same for Vn, and the differential noise d basically cannot be removed.

[0015] If we want to remove this differential noise, Fig. 4
As can be seen from the diagram in (f), it is possible to increase each period T0 to T6 and sample the signal after the influence of differential noise disappears, but if this is done, the excitation current will inevitably be driven at a low frequency. I have no choice but to do so. However, when low-frequency excitation is performed, if slurry or the like is present in the fluid in this frequency band, this slurry hits the electrodes, causing flow noise and this effect appears. Therefore, we would like to use high-frequency excitation, but high-frequency excitation generates large differential noise. It is an object of the present invention to be able to eliminate the influence of residual magnetism while also eliminating this.

[0016]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention provides an electromagnetic flow rate that performs rectangular wave excitation through a core, processes a signal voltage generated between electrodes, and outputs it as a flow signal. Regarding the total, the first rest period, first excitation period,
The second rest period and the second excitation period are excited with the same polarity as the first excitation period.
An excitation means that repeats the first excitation period, the second rest period, and the third rest period with the excitation period and the third rest period as one cycle, and in the next cycle, the previous third rest period is the first rest period; a first arithmetic means for amplifying the signal voltage by two times and outputting it as a first signal;
a second calculation means for amplifying the signal twice and outputting it as a second signal;
a third arithmetic means for amplifying the signal voltage twice with a polarity opposite to that of the first signal and outputting the amplified signal voltage as a third signal; and a third calculating means for amplifying the signal voltage three times with the same polarity as the first signal and outputting the amplified signal voltage as a third signal. A fourth calculation means for outputting, and timings for detecting the first signal, second signal, third signal, and fourth signal in the first rest period, second rest period, second excitation period, and third rest period, respectively. and an addition calculation means that performs an addition calculation using each signal from the first signal to the fourth signal detected at this timing and outputs it as a flow signal, and the residual magnetic field caused by the core and the addition calculation means are provided. This is designed to avoid the influence of both differential noise and differential noise generated by rectangular wave excitation.

[0017]

[Operation] Timing means allows the first rest period and the second
During the rest period, second excitation period, and third rest period, the first
The signal, second signal, third signal, and fourth signal are detected. Each of the detected signals is subjected to an addition operation by the addition operation means, and is output as a flow rate signal that is not affected by both the residual magnetic field caused by the core and the differential noise generated by rectangular wave excitation.

[0018]

[Embodiments] Hereinafter, embodiments of the present invention will be explained with reference to the drawings. FIG. 1 is a block diagram showing the configuration of one embodiment of the present invention. Note that parts having the same functions as those of the conventional current/pressure converter shown in FIGS. 3 and 4 are denoted by the same reference numerals, and the explanation thereof will be omitted as appropriate.

The excitation section 1 is composed of an excitation coil 1a, a core 1b, and the like. A fluid to be measured is flowed into the conduit 2 . Electrodes 3a and 3b are arranged in the conduit 2 in a direction perpendicular to the magnetic field B. 4 and 5 are high impedance buffer amplifiers whose input terminals are connected to electrodes 3a and 3b, and whose output terminals are connected to a differential amplifier 2 for removing common mode noise.
It is connected to each input terminal of 0.

The output terminal of the differential amplifier 20 is connected to an amplifier 21 that triples the output voltage V1 appearing at this output terminal, an amplifier 22 that doubles the output voltage V1, and an amplifier 22 that amplifies the output voltage V1 twice, and an amplifier 22 that amplifies the output voltage V1 twice, and an amplifier 22 that inverts the polarity of the output voltage V1. The input terminals of the inverter 23 are connected to the respective input terminals of the inverter 23. Further, the output terminal of the inverter 23 is connected to an amplifier 24 that triples the inverted voltage <V1> appearing at this output terminal, and an amplifier 25 that amplifies the inverted voltage <V1> twice as much.
is connected to each input terminal of the .

Each output terminal of the amplifiers 21, 22, 24, and 25 is connected to a switch SW3 for switching the voltage appearing at the output terminal.
It is connected to the input end of each switch of the multiplexer 26 including SW4, SW5, and SW6. The output terminals of the respective switches of this multiplexer 26 are connected in common and connected to the input terminal of a hold circuit 27 composed of a resistor, a capacitor, and a switch. The voltage appearing at the output end is output to the microprocessor 28, which includes parts necessary for signal processing such as an A/D converter and memory, where the necessary signal processing is performed and a flow rate signal is output to the output end 29. .

The exciting coil 1a is connected to the DC constant current source 17 by switches SW1-a, SW1-b, SW2-a, SW2-b.
These switches are connected to the excitation coil 1a in five periods T0' to T4', zero, positive, zero, positive, zero, as shown in Figure 2(a), for the first cycle and for the next cycle. The cycle has five periods T4' to T8: zero, negative, zero, negative, zero.
It is opened and closed by the timing signal 31 outputted from the timing circuit 30 so that a rectangular-wave excitation current that repeats one cycle at '' flows.

In addition, the timing circuit 30 also outputs a timing signal 32 for switching the switches SW3 to SW6 of the multiplexer 26 in synchronization with the timing signal 31. In the first cycle, these switches have a period T0'
Then switch SW6 is turned on, all switches are turned off during period T1', and switch SW3 is turned on during period T2'.
During the period T3', the switch SW4 is turned on, and during the period T4', the switch SW5 is turned on. In the next cycle, the period T
4', switch SW4 is turned on, period T5', both switches are turned off, period T6', switch SW5 is turned on, period T7', switch SW6 is turned on, period T8'.
Now turn on switch SW3.

By opening and closing each of the switches described above, the hold circuit 27 holds a signal that is the sum of the signals obtained in each period. The capacitor of the hold circuit 27 is once turned on for a short period by the timing signal 33 from the microprocessor 28 after the sum signal is obtained, and the charge is discharged. Further, the timing circuit 30 is controlled by a control signal 34 from the microprocessor 28 and outputs timing signals 31 and 32.

Next, the operation of the electromagnetic flowmeter constructed in this manner will be explained using the waveform diagram shown in FIG. Figure 2 (
2(a) shows the excitation waveform, FIG. 2(b) shows the voltage waveform generated between the electrodes, FIG. 2(c) shows the electrochemical noise, and FIG. 2(d) shows the signal sampling period.

When the excitation is performed as shown in FIG. 2(a), the electrodes 3a and 3b have differential noise D and excitation polarity generated as the excitation current is switched, as shown in FIG. 2(b). DC noise voltages kVs, -kVs based on the residual magnetism of the B-H curve of the core 1b determined by the state of the DC noise voltages kVs and -kVs and a voltage in which the signal voltage Vs is superimposed thereon are generated. Therefore, in the first cycle, in period T0',
D-kVs+E
+△E (1
2) During period T2', -D+ kV
s+E+3△E
(13) In period T3', D+Vs
+E+4△E
(14) During period T4', -D+ kV
s+E+5△E
(15).

Then, the switch SW6 is turned on during the period T0', the switch SW3 is turned on during the period T2', the switch SW4 is turned on during the period T3', and the switch SW5 is turned on during the period T4'. =-2
x formula (12) + 3x formula (13) + 2x formula (14) - 3x
A voltage expressed by equation (15) is obtained. When you calculate this formula,
S1=2Vs+2kVs

(16). Here, the second item is the voltage generated by residual magnetism, but normally this residual magnetism does not show sudden changes and is constant, and this S1 is the differential noise D
It can be seen that it is not affected by

[0028] Also, in the next cycle, in period T4',
−D+ kVs
+E+5△E (
17) In period T6', D-k
Vs+E+7△E
(18) In period T7', −D−Vs+
E+8△E
(19) In period T8', D-k
Vs+E+9△E
(20).

Then, the switch SW4 is turned on during the period T4', the switch SW5 is turned on during the period T6', the switch SW6 is turned on during the period T7', and the switch SW3 is turned on during the period T8'. =
2x formula (17) - 3x formula (18) - 2x formula (19) + 3
A voltage expressed by x equation (21) is obtained. When calculating this formula, S2=2Vs+2kVs

(22) and obtains the same result as equation (16). By repeating the above calculation, a flow rate signal that is not affected by the differential noise D can be continuously obtained.

[0030]

As described above in detail with the embodiments, according to the present invention, the first rest period, the first excitation period, the second rest period, and the first excitation period are excited with the same polarity as the first excitation period. The second excitation period and the third rest period are set as one cycle, and in the next cycle, the previous third rest period is set as the first rest period, and excitation is repeated by repeating the previous first excitation period, second rest period, and third rest period. , first rest period, second rest period, second excitation period, third
The first signal 3 is twice the negative signal voltage during the pause period.
The second signal multiplied by two, the third signal multiplied by two, and the fourth signal multiplied by negative three are detected and added together, so that they are not affected by the residual magnetic field and do not contain differential noise. can do. Furthermore, since the excitation frequency can be extended to a high frequency region, it is not affected by flow noise and the response can be made faster.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of one embodiment of the present invention.

FIG. 2 is a waveform diagram illustrating the operation of the embodiment shown in FIG. 1;

FIG. 3 is a block diagram showing the configuration of a conventional electromagnetic flowmeter.

4 is a waveform diagram illustrating the operation of the electromagnetic flowmeter shown in FIG. 3. FIG.

[Explanation of symbols]

1 Excitation part, 1a Excitation coil 1b core 2 Conduit 3a, 3b electrode 13. Microcomputer 17 DC constant current source 18 Timing signal 21, 22, 24, 25 Amplifier 23 Inverter 26 Multiplexer 27 Hold circuit 28 Microprocessor 30 Timing circuit

Claims (1)

[Claims] Claim 1: In an electromagnetic flowmeter that performs rectangular wave excitation through a core, processes a signal voltage generated between electrodes, and outputs it as a flow signal, the electromagnetic flowmeter includes a first rest period, a first excitation period, and a second rest period. - The second excitation period and the third rest period, which are excited with the same polarity as the first excitation period, are one cycle, and the next cycle is the first excitation period and the third rest period, with the third rest period being the first rest period. excitation means that repeats a second rest period and a third rest period; a first calculation means that doubles the signal voltage and outputs it as a first signal; a second calculation means for amplifying the signal voltage twice and outputting it as a second signal; and a third calculation means for amplifying the signal voltage twice and outputting it as a third signal with a polarity opposite to that of the first signal.
a calculation means; a fourth calculation means for amplifying the signal voltage three times with the same polarity as the first signal and outputting the amplified signal as a third signal; timing means for outputting timings for detecting the first signal, the second signal, the third signal, and the fourth signal in the excitation period and the third rest period, respectively; and the first signal detected at this timing. and an addition calculation means for performing an addition calculation using each signal from to a fourth signal and outputting it as a flow rate signal, the residual magnetic field caused by the core and the differential noise generated by the rectangular wave excitation are both suppressed. An electromagnetic flowmeter characterized by being unaffected by.