JP2884840B2 - Electromagnetic flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flowmeter for measuring a flow rate from an electromotive force generated by applying a magnetic field to a fluid to be measured, and more particularly to an electromagnetic flowmeter having improved response to noise.

[0002]

2. Description of the Related Art Conventionally, an electromagnetic flowmeter for industrial use employs a commercial frequency excitation system which is excited by using a commercial power supply. The commercial frequency excitation method (a) has a high response speed and can be manufactured at low cost. (B) There is an advantage that it is hardly affected by low-frequency random noise (hereinafter referred to as flow noise) that increases with the flow velocity generated by a slurry fluid or low-conductivity fluid. Long term, eg 1
There is a disadvantage that the zero point fluctuates if left unattended for about a day.

[0003] For this reason, a low-frequency excitation system has been adopted in which excitation is performed at a low frequency of 1/2 or less than the commercial frequency. The use of the low-frequency excitation method has an advantage that a stable electromagnetic flowmeter having a zero point can be obtained, as is well known. However, since the excitation frequency is low, it is close to the frequency band of the flow noise, and thus is easily affected by the flow noise. In particular, this effect becomes remarkable when the flow velocity is large. Also, the flow
When damping is applied to reduce the influence of noise, the response is slow.

To solve this problem, there has been proposed a composite excitation system in which an excitation current component having a commercial frequency and an excitation current component having a lower frequency are simultaneously supplied to an excitation coil to form a composite magnetic field. Hereinafter, this type of electromagnetic flowmeter will be described with reference to FIGS.

FIG. 5 is a block diagram showing a configuration of a conventional electromagnetic flow meter. Numeral 10 denotes a conduit for the detector of the electromagnetic flow meter, which is provided with an insulating lining on its inner surface. 1
Reference numerals 1a and 11b are electrodes for detecting a signal voltage.
Reference numeral 12 denotes an excitation coil, and a magnetic field generated by the excitation coil is applied to the fluid to be measured. An excitation current _If is supplied from an excitation circuit 13 to the excitation coil 12.

The excitation circuit 13 is configured as follows.
You. Reference voltage E₁Is the amplifier Q₁To the non-inverting input terminal (+) of
Is applied and its output is_TwoContact the base
Has been continued. Transistor Q_TwoOf the resistor R_f
Connected to the common COM through the amplifier Q₁of
It is connected to the inverting input terminal (-). Common COM and G
Transistor Q_TwoExcitation voltage E between the collector_sBut
Switch_TwoAnd SW_ThreeConnected in parallel with the series circuit
Switch SW_FourAnd SW_FiveApplied through a series circuit
It is. The exciting coil 12 is a switch SW_Two, SW_ThreeConnection
Point and switch SW _Four, SW_FiveConnected to
It is. Timing signal S_Two, S_Three, S_Four, S_FiveEach
Switch SW_Two, SW_Three, SW_Four, SW_FiveControl opening and closing
I will.

On the other hand, the signal voltage is detected by the electrodes 11 a and 11 b and output to the preamplifier 14. Preamplifier 14
Then, the common mode voltage is removed and the impedance is converted, and is output to the output terminal 15 thereof. The output of the preamplifier 14 at the output 15 is an analog / digital converter (A /
D _L ) 16 and analog / digital converter (A / D _H ) 1
At 7, the signals are converted into digital signals and stored in a random access memory (RAM) 19 via a bus 18. A read only memory (ROM) 20 stores a predetermined calculation program and initial data, and is calculated according to a calculation procedure stored in the ROM 20 under the control of a processor (CPU) 21. Is RAM
19 is stored. 22 is a clock generator, wherein the generated clock is supplied to the CPU21 and the analog / digital converter 17 as a frequency in the frequency divider 23 is divided to 1 / n system clocks S _h.

The CPU 21 outputs the timing for determining the waveform of the exciting current _If to the timing signal output port (TO) 24 via the bus 18 in accordance with the arithmetic program stored in the ROM 20. The timing signal output port 24 outputs timing signals S ₂ , S ₃ , S ₄ , S ₅ for switching the exciting current according to this timing. The timing signal output port - DOO 24 samples the output of the preamplifier 14 output a timing signal S _L to the analog / digital converter 16 according to the timing specified by the CPU 21.

On the other hand, an arithmetic program stored in the ROM 20 uses the data stored in the RAM 19 to control the CP.
A predetermined operation is executed by U21, and the result of the operation is stored in the RAM 19 and output as a flow rate output to the output terminal 26 via the digital / analog converter 25 via the bus 18.

Next, the operation of the electromagnetic flow meter shown in FIG. 5 will be described with reference to a timing chart shown in FIG. 6, a flowchart shown in FIG. 7, and a calculation chart shown in FIG. Frequency divider 2 shown in FIG.
3 system clock S _h obtained at the output of FIGS. 6 (a)
Are supplied to the CPU 21.

[0011] In Step 1 of Figure 7, CPU 21 may interrupt timing of the system clock S _h (Fig. 6
In synchronization with (g)), a timing signal indicating the switching timing of the excitation waveform is output to the timing signal output port 24 via the bus 18 by a predetermined arithmetic program stored in the ROM 20.

In step 2, the timing signal output port 24 receives this switching timing, and receives timing signals S ₅ (FIG. 6 (b)), S ₄ (FIG. 6 (c)), and S ₃ (FIG. 6 (d)). )) And S ₂ (FIG. 6 (e))
3 to the switches SW ₅ , SW ₄ , SW ₃ , and SW ₂ . Alternatively, the timing signal S ₄ is supplied to the switches SW ₃ and SW
Outputs simultaneously _4, and a timing signal S ₂ may be output simultaneously to the switch SW _2, SW _5. Excitation circuit 13
Receives these timing signals and outputs an exciting current _If having a waveform shown in FIG. This excitation waveform has a timing number i as shown in FIGS.
Is a waveform that constitutes one cycle from 0 to 15 and repeats this. In FIG. 6, the waveform is shown focusing on the n-cycle portion. The excitation waveform is a multiplication waveform obtained by multiplying a low-frequency waveform and a high-frequency waveform.

Next, the process proceeds to step 3. Step 3
Steps 6 to 6 include the analog / digital converters 16 and 1
7 shows a procedure for reading data from the memory 7. In step 3, data input from the analog / digital converter 17 in each cycle in synchronism with the system clock _Sh (FIG. 6A) is transferred to the bus 18 as shown in FIG.
Predetermined data of RAM19 under the control of the CPU21 through the - stored in a data region H _i.

Next, the operation proceeds to step 4, where it is determined whether or not the read timing number i is 0. If it is not 0, the operation proceeds to step 6, and if it is 0, the operation proceeds to step 5. In step 6, it is determined whether or not the read timing number i is 8, and if not, the process proceeds to step 8, and if it is 8, the process proceeds to step 7.

In step 5, the timing signal output port
The timing signal S _L output from the
(K)), the analog /
The data input from the digital converter 16 is shown in FIG.
.., L ₀ (n-1), L ₀ (n-1) in the RAM 19 under the control of the CPU 21 via the bus 18 as shown in FIG.
₀ (n), L ₀ (n + 1),...

Next, in step 7, the timing signal output port - the timing output from preparative 24 signal S _L (FIG. 6
(K)), the analog /
The data input from the digital converter 16 is shown in FIG.
.., L ₁ (n−1), L ₁ (n−1) in the RAM 19 under the control of the CPU 21 via the bus 18.
₁ (n), L ₁ (n + 1),...

In step 8, it is determined whether or not the timing number i is an odd number. If the timing number i is an odd number, the process proceeds to step 9, and if not, the process proceeds to step 11. Step 9 performs a high-frequency demodulation operation. For demodulation operation,
Stored de in RAM 19 - using data H _i, FIG. 6 (m)
At the timing shown in FIG.
, And the result is stored in the RAM 19 using the formula shown in the column of the high-frequency demodulation calculation e _Hi shown in FIG.
The result of this operation is the relay point CT1 of the operation flow,
It is sent to step 10 via CT2. This relay point C
Between T1 and CT2, an operation step in which a limit operation process described later is performed intervenes. By this demodulation operation, the electrochemical DC voltage generated at the electrodes 11a and 11b is removed, and the differential noise is kept at a constant value and does not become an error factor. In FIG. 8, the constant A is represented by A = T _c / (T _c + ΔT _c ), where T _c is a differentiation or integration constant, and ΔT _c is a calculation cycle shown in FIG.

Next, the routine proceeds to step 10. Here, the high-pass filtering operation F _Hi on the high frequency side is executed. At the time of the filtering operation, the data e _Hi stored in the RAM 19 and the result of the previous filtering operation are used, and the ROM 20 is controlled under the control of the CPU 21.
, And the result is stored in the RAM 19 using the formula shown in the column of the high-pass filtering calculation F _Hi shown in FIG. Next, the process proceeds to step 11. In step 11, it is determined whether or not the timing number i is 0 or 8, and if it is 0 or 8, it proceeds to step 12, and if it is not 0 or 8, it proceeds to step 14.

In step 12, a low frequency demodulation operation is performed.
You. At the time of the demodulation operation, the data stored in the RAM 19
Ta ... L₀(N-1), L₀(N), L₀(N + 1),
... L ₁(N-1), L₁(N), L₁(N + 1), ...
Control by the CPU 21 at the timing shown in FIG.
The low-frequency demodulation shown in FIG.
Calculation e_LiAnd calculate the result with RAM
19 is stored. The result of this operation is the operation flow
Via the relay points CT3 and CT4
You. Between the relay points CT3 and CT4, a limit
An operation step for performing the operation operation is interposed. In addition,
In FIG. 8, the constant B is represented by B = ΔT / (ΔT + T).
Is done.

In step 13, a low-pass filtering operation F _Li on the low frequency side is executed. At the time of the filtering operation, the data e _L0 and e _L8 stored in the RAM 19 and the result of the previous filtering operation are used, and under the control of the CPU 21, the low-pass filtering operation F _Li shown in FIG. The calculation is performed using the calculation formulas shown in the columns, and the result is stored in the RAM 19.

In step 14, it is determined whether or not the timing number i is an odd number. If the timing number i is an odd number, the process proceeds to step 15, and if not, the process proceeds to step 16. Step 15 performs an addition operation. The result F _Hi of the high-pass filtering operation stored in the RAM 19 and the result F _{Li of the} low-pass filtering operation
The calculation is performed by the arithmetic expression shown in the column of the addition operation e _A shown in FIG. 8 stored in the ROM 20 under the control of the CPU 21, the result is stored in the RAM 19, and the process proceeds to step 17. In step 17, the process waits until the timing of the next interrupt, and when the timing of the next interrupt comes, the flow from step 1 to step 17 is executed again.

As described above, the signal voltage including two frequencies of the low frequency and the high frequency detected by the electrodes 11a and 11b is divided into the low frequency side and the high frequency side using a microcomputer, and is read. The low-frequency side demodulates at low frequency and outputs its output through a low-pass filter, and the high-frequency side demodulates at high frequency and outputs its output through a high-pass filter. By adding and synthesizing the outputs of the filter and outputting, the zero point is stable, strong against flow noise, and a flow output with good response can be obtained.

However, if the flow signal changes abruptly due to noise or the like only by the above-described arithmetic processing, it takes time to return to a normal flow signal even if the noise disappears, so the limit shown in FIGS. An operation for processing is inserted. FIG. 9 shows a calculation process for performing the limit process on the high frequency side, and FIG. 10 shows a calculation flow for performing the limit process on the low frequency side.

Referring to FIG. The high-frequency demodulated data obtained in step 9 is introduced to step L1 via the relay point CT1. Step L1 In backed up this read de with respect to a predetermined frequency limit value LMT _H stored in RAM 19 - data e _H (n) and de previously read - the difference between the data e _H (n-1) [E _H (n)
−e _H (n−1)] to determine whether or not the upper limit of the limit value is exceeded. , The process proceeds to step L2 when it exceeds the limit value LMT _H, where de previously read - to step 10 via the data e _H (n-1) to the limit value LMT _H adds to relay point CT2 Transition.

Limit value LMT_HStep if below
Move to L3 and limit value LMT_HLast read for
Data e_H(N-1) and data e read this time_H
Difference from (n) [e_H(N-1) -e_H(N)]
Then, it is determined whether or not the lower limit of the limit value is exceeded.
Limit value LMT_HIf it exceeds, go to step L4
Move to the limit value LMT_HWas loaded last time
Data e_H(N-1) and via the relay point CT2
To step 10. Limit value LMT _HBelow
If there is, the process proceeds to step L5, and the data e read this time
_H(N) as it is via the relay point CT2 to step 10
To be transmitted. Limit value LMT in this case_HIs, for example,
It is fixedly set to about 30% of the span.

Referring to FIG. The demodulated data of the low frequency obtained in step 12 is introduced to step L6 via the relay point CT3. This read de for a given low frequency limit value LMT _L stored in step L6 RAM 19 which is backed up in - data e _L (n)
And the difference between the last read data e _L (n-1) [e
_{L (n) -e L (n} -1)] by comparing the determining whether exceeds the upper limit value. Limit value LMT
_If it exceeds _L , the process proceeds to step L7, where the limit value LM is added to the previously read data e _L (n-1).
_TL is added, and the routine proceeds to step 13 via the relay point CT4.

Limit value LMT_LStep if below
Move to L8 and limit value LMT_LLast read for
Data e_L(N-1) and data e read this time_L
Difference from (n) [e_L(N-1) -e_L(N)]
Then, it is determined whether or not the lower limit of the limit value is exceeded.
Limit value LMT_LIf it exceeds, go to step L9
Move to the limit value LMT_LWas loaded last time
Data e_L(N-1) is subtracted via the relay point CT4.
To step 13. Limit value LMT _LBelow
If there is, move to step L10 and read the data read this time.
e_LStep (n) is directly performed via the relay point CT4 in step 1.
Transmit to 3. Limit value LMT in this case_LIs like
For example, it is fixedly set to about 100% of the span.

[0028]

However, in the above-described noise limiting means, for example, when a signal such that the flow rate output returns to 0% is input in a step-like manner, the high frequency side and the low frequency side are switched. However, there is a problem that the time required to reach 0% is prolonged due to the difference in the sampling timing of the signal, and the response is reduced.

[0029]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides exciting means for supplying exciting current having two different frequencies, a first frequency and a lower second frequency. Sampling means for sampling and outputting a signal voltage which is excited by the exciting means and is generated in accordance with the flow rate in synchronization with rising / falling of the exciting current, and discriminating and outputting the output of the sampling means based on the first frequency First demodulating means, a first limit means for limiting a change in the output of the first demodulating means by a limit value of x% with respect to the span A, and a high-pass filter for high-pass filtering the output of the first limit means. A filtering means, a second demodulating means for discriminating and demodulating the output of the sampling means on the basis of the second frequency, and a change in the output of the second demodulating means with respect to span A by N * x% (N A second limiter for limiting the output of the second limiter with a limit value (a ratio of the number of times of sampling the signal of the first frequency component to the second frequency component), a low-pass filter for low-pass filtering the output of the second limiter, and a high-pass filter. And a synthesizing means for synthesizing each output of the low-pass filtering means and the low-pass filtering means.

[0030]

The excitation means supplies an excitation current having two different frequencies, a first frequency and a lower second frequency. The sampling means samples and outputs a signal voltage which is excited by the exciting means and is generated in accordance with the flow rate in synchronization with the rise / fall of the excitation current. The first demodulation means discriminates and outputs the output of the sampling means based on the first frequency, and the first limit means limits a change in the output of the first demodulation means with respect to span A by a limit value of x%. . The high-pass filtering means performs high-pass filtering on the output of the first limit means.

The second demodulating means discriminates and demodulates the output of the sampling means based on the second frequency. The second limit means limits the change in the output of the second demodulation means with respect to the span A by a limit value of N * x% (N: the ratio of the number of times of sampling of the first frequency component to the second frequency component). The low-pass filtering means low-pass filters the output of the second limit means. The combining means combines the outputs of the high-pass filtering means and the low-pass filtering means and outputs the combined output. This makes it possible to associate the calculation cycle on the high frequency side and the low frequency side with the limit value for the flow rate change amount on the high frequency side and the low frequency side, and reduce the influence on the noise with the same change rate of the signal. It also enables high-speed response.

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 and FIG. 2 are flowcharts for explaining the main part of one embodiment of the present invention. FIG. 3 is a block diagram for executing the flow operation shown in FIG. Parts having the same functions as those of the conventional electromagnetic flowmeter shown in FIGS. 5 to 10 are denoted by the same reference numerals, and the description thereof will be appropriately omitted.

FIG. 3 is a block diagram showing the configuration of this embodiment. The operation program stored in the RAM 30 or ROM 31 and the parameters set are shown in FIG.
Is different from the RAM 19 or the ROM 20 shown in FIG.

The other structure is almost the same as the structure shown in FIG. The basic procedure of the arithmetic processing is almost the same as that shown in FIG. 7, but the arithmetic program inserted into the relay points CT1 and CT2, which is a feature of this embodiment, is the arithmetic program shown in FIG. , Relay point CT3, CT
4 is different in that the operation program is inserted into the operation program shown in FIG.

Referring to FIG. The operation result of the high-frequency demodulation operation shown in step 9 shifts to the setting operation shown in step L11 via the relay point CT1. Step L
As limit LMT _H of the high-frequency side 'At 11, when the span and A (m / s), x with respect to the span A
Make settings to apply the (%) limit.

In step L12, the limit value LMT
It reads de against _H '- data e _H (n) the de read the last time - the difference between the data _{e H (n-1) [} e H (n) -e
_H (n-1)] to determine whether or not the upper limit of the limit value is exceeded. ', The process proceeds to step L13 when beyond, where de previously read - data e _H (n-1) to the limit value LMT _H' limit value LMT _H Step through the relay point CT2 by adding Move to 10.

Limit value LMT_H´ If below
To L14, and the limit value LMT_HLast time for ´
The read data e_H(N-1) and the data read this time
Ta e _HDifference from (n) [e_H(N-1) -e_H(N)] and
To determine if the value is below the lower limit.
I do. Limit value LMT_HIf you are over ´
To L15, where the limit value LMT_HIn front of ´
Data e read multiple times _HIntermediate point subtracted from (n-1)
The process proceeds to step 10 via CT2. Limit value L
MT_HIf it is less than or equal to, read the current
Data e_H(N) via relay point CT2 as it is
Then, it is transmitted to step 10.

Referring to FIG. The low-frequency demodulated data obtained in step 12 is introduced to step L17 via the relay point CT3. In step L17 to set the limit value LMT _L of the low-frequency side '. For example, ROM31
The number of times N of signal sampling on the high frequency side for one cycle at the low frequency stored in
Is executed to calculate the A * N (X / 100) stored in the lower limit value _{LM L} ′.
Is set as a parameter of the arithmetic program stored in the RAM 30.

In step L18, the set low frequency
Limit value LMT_LData e read this time for '
_L(N) and the previously read data e_LDifference from (n-1)
[E _L(N) -e_L(N-1)] and limit
It is determined whether the value exceeds the upper limit. Limit value L
MT_LIf it exceeds ′, the process proceeds to step L19.
Here, the data e read last time_LLimi on (n-1)
Set value LMT_L′ Is added and the status is set via the relay point CT4.
The process proceeds to Step 13.

Limit value LMT_L´ If below
To L20, and the limit value LMT_LLast time for ´
The read data e_L(N-1) and the data read this time
Ta e _LDifference from (n) [e_L(N-1) -e_L(N)] and
To determine if the value is below the lower limit.
I do. Limit value LMT_LIf you are over ´
The process proceeds to L21, where the limit value LMT_LIn front of ´
Data e read multiple times _LIntermediate point subtracted from (n-1)
The process proceeds to step 13 via CT4. Limit value L
MT_L'If it is less than or equal to, read the current reading
Data e_L(N) via relay point CT4
Then, it transmits to step 13.

Next, a description will be given of a point that the response is improved by executing the calculation for performing the limit processing as described above. Now, the response time when the input signal changes from B (m / s) to 0 (m / s) stepwise will be described. Assuming that the low frequency is f _L (Hz), the values at the low frequency and the high frequency are as follows.

First, at low frequency, frequency = f _L (Hz),
Cycle per sampling = 1 / 2f _L (s), limit value = NX (%), actual limit value = B * (NX /
100) (m / s), the number of times of the limit = A [1 /
B (NX / 100)] (times). Therefore, the response time T _L on the low frequency side is: T _L = (period per sampling) * (number of times of the limit) = [1 / 2f _L ] * A [1 / B (NX / 100)] = 50A / (BNXf _L ).

On the other hand, on the high frequency side, the frequency = 2N
f _L (Hz), cycle per sampling = 1 / 2Nf
_L (s), limit value = X (%), actual limit value =
B * (X / 100) (m / s), and the number of times of the limit = A [1 / B (X / 100)] (times). Therefore,
Response time T _H of the high-frequency side, _{similarly, T H = [1 / 2Nf} L] * A [1 / B (X / 100)] = 50A / in the (BNXf _L) above, the low-frequency side response time T _L and the response time T _H of the high-frequency side is can be seen equally.

This will be described with reference to FIG. The abscissa indicates the passage of time, and the ordinate indicates the output.
α _L0 and α _H0 are the low-frequency side and high-frequency side outputs after the limit in the conventional case has been executed, and the output after low-pass filtering calculated based on these outputs is α _L1 and the high-frequency side. The output after the filtering operation is α _H1 . Since the sampling period is high on the high frequency side, it reaches 0% faster than on the low frequency side.

As described above, since the response of the low-frequency side output α _L0 and the high-frequency side output α _H0 are different from each other, the combined output δ is output for a long time as shown in FIG.
Takes time to reach. On the other hand, in the case of the present embodiment, since the outputs β _L0 and β _H0 on the low frequency side and the high frequency side after the limit is executed are changing at the same time, the combined output γ obtained by combining them is It quickly reaches 0% as shown. Here, β _L1 is an output after low-pass filtering operation filtered based on the output β _L0 , and β _H1 is an output after high-pass filtering operation, respectively.

[0046]

As described above, according to the present invention, the ratio of the sampling number of the high-frequency signal to one sampling of the low-frequency signal in relation to each limit value is described. Since the setting is made, there is no deviation between the output on the low frequency side and the output on the high frequency side, and a high-speed response can be obtained as a composite output.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first main configuration of an embodiment of the present invention.

FIG. 2 is a block diagram showing a second essential configuration of one embodiment of the present invention.

FIG. 3 is a block diagram showing the overall configuration of one embodiment of the present invention.

FIG. 4 is a characteristic diagram illustrating characteristics of the example of the present invention.

FIG. 5 is a block diagram showing a configuration of a conventional electromagnetic flow meter.

FIG. 6 is a waveform chart for explaining the operation of the electromagnetic flow meter shown in FIG.

FIG. 7 is a flowchart showing a procedure of signal processing of the electromagnetic flow meter shown in FIG. 5;

FIG. 8 is a calculation diagram showing a calculation procedure in the flow of FIG. 7;

FIG. 9 is a flowchart showing a part of the procedure of the signal processing shown in FIG. 7;
It is a chart.

FIG. 10 is a flowchart showing another partial procedure of the signal processing shown in FIG. 7;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Conduit 12 Excitation coil 13 Excitation circuit 16 and 17 Analog / digital converter 18 Bus 19 and 30 Random access memory 20 and 31 Lead-only memory 21 Microprocessor 22 Clock generator 24 Timing signal output port

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-61-161526 (JP, A) JP-A-62-132118 (JP, A) JP-A-63-289420 (JP, A) JP-A 63-289420 217228 (JP, A) JP-A-63-217227 (JP, A) JP-A-63-139524 (JP, U) JP-A-63-161327 (JP, U) JP-A-63-73624 (JP, U) (58) Field surveyed (Int.Cl. ⁶ , DB name) G01F 1/60

Claims (1)

(57) [Claims] A first frequency and a second frequency lower than the first frequency;
Exciting means for supplying exciting currents having two different frequencies, and sampling means for sampling and outputting a signal voltage excited by the exciting means and corresponding to a flow rate in synchronization with rising / falling of the exciting current. A first demodulation means for discriminating and outputting the output of the sampling means based on the first frequency, and a first demodulation means for limiting a change in the output of the first demodulation means with respect to the span A by a limit value of x%. Limiting means; high-pass filtering means for high-pass filtering the output of the first limiting means; second demodulating means for discriminating and demodulating the output of the sampling means based on the second frequency; and second demodulating means. Is limited by a limit value of N * x% (N: ratio of the number of times of sampling of the first frequency component to the second frequency component) with respect to span A. Second limit means, low-pass filtering means for low-pass filtering the output of the second limit means, and combining means for combining the outputs of the high-pass filtering means and the low-pass filtering means. Characteristic electromagnetic flow meter.