JP3075319B2 - 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 a two-circle waveform electromagnetic flowmeter for measuring a flow rate from an electromotive force generated by applying a magnetic field having two different frequencies to a fluid to be measured. The present invention relates to an electromagnetic flowmeter improved so as to reduce the influence on the output due to the displacement of the high-frequency side span.

[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. 4 is a block diagram showing the configuration of a conventional electromagnetic flow meter disclosed in, for example, Japanese Patent Application No. 3-218959. Numeral 10 denotes a conduit for the detector of the electromagnetic flow meter, which is provided with an insulating lining on its inner surface. 11a,
11b is an electrode 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. The excitation coil 12 includes an excitation circuit 13
Supplies an exciting current _If .

The excitation circuit 13 is configured as follows.
You. Reference voltage E₁Is the amplifier Q₁To the non-inverting input terminal (+) of
And its output is connected to transistor Q_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. 4 will be described with reference to a timing chart shown in FIG. 5, a flowchart shown in FIG. 6, and a calculation chart shown in FIG. Frequency divider 2 shown in FIG.
System clock S _h obtained at the output of 3 FIGS. 5 (a)
Are supplied to the CPU 21.

[0011] In Step 1 of Figure 6, CPU 21 is an interrupt timing of the system clock S _h (Fig. 5
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. 5 (b)), S ₄ (FIG. 5 (c)), S ₃ (FIG. 5 (d) )) And S ₂ (FIG. 5E)
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.
Are the waveforms which constitute one cycle from 0 to 15 and repeat this. In FIG. 5, the waveform is shown centering on the part of n cycles. 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 synchronization with the system clock _Sh (FIG. 5 (a)) is transferred to the bus 18 as shown in FIG. 5 (j).
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 signal output from preparative 24 S _L (FIG. 5
(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. 5 (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.
Then, the result of this operation is sent to step 10. 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. Incidentally, FIG. 5 A becomes constant, constant differential or integrated T _c, [Delta] T _c in FIG. 7
If the calculation cycle shown in (f) is used, A = T _c / (T _c + ΔT)
_c ).

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 in accordance with 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. Then, the result of this operation is
Sent to 3. In FIG. 7, the constant B is B = Δ
It is indicated by T / (ΔT + T).

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 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 number is odd, the flow proceeds to step 15, and if not, a damping operation (time constant T _D ) (not shown) is executed to execute the operation progress point T _1. , step 1 via the T ₂
It is determined to shift to 6. Step 15 performs an addition operation. Result F of high-pass filtering operation stored in RAM 19
_{Using Hi} and the result F _Li of the low-pass filtering operation, the addition operation e shown in FIG.
_A calculation is performed using the calculation formula shown in column _A , and the result is stored in the RAM 19.
, And a not-shown damping operation (time constant T _D ) is executed, and the process proceeds to step 16 via the operation progress points T ₁ and T ₂ . In step 16, 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 16 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 combining the outputs of the filter and outputting (two-frequency output), a zero point is stable, strong against flow noise, and a flow rate output with good response can be obtained.

On the low frequency side, the zero point is stable, but the excitation frequency is low, so that it is close to the frequency band of the flow noise and easily subject to fluctuations of this noise. Therefore, a low-pass filter having a large time constant is used. This has been reduced. Therefore, the response is slow. Therefore, this response is improved by adding a high frequency side.

However, since it is necessary to maintain a smooth response on the high frequency side, the signal is output through the high-pass filter having the same time constant as that of the low-pass filter having a large time constant, so that it is easily affected by differential noise.

Therefore, when the fluctuation of the noise is reduced to a predetermined value or less, it is better to output at a low frequency and to avoid outputting at two frequencies. Therefore, the low frequency output and the dual frequency output are automatically switched in consideration of the magnitude of the noise.

The switching criterion is as follows: T _S is an operation cycle (low frequency half cycle), T _D is an output damping time constant (s), and e _L (n) is an instantaneous value (m /
If s) and e _A (n) are the output (m / s) after the addition operation, and A is an allowable noise value (m / s) in terms of flow velocity, T _S [e _L (n) −e _A (N-1)] / T _D <A (1)

[0027]

However, the above-mentioned conventional electromagnetic flowmeter has the following problems. This problem will be described with reference to FIG.
The high-frequency side is susceptible to differential noise, so it is susceptible to the zero point, and this differential noise varies depending on fluid conditions such as adhesion by the measurement fluid and temperature, so the high-frequency side is not accompanied by span variation. easy.

On the other hand, the span is stable because the low frequency side is hardly affected by differential noise. For this reason, the spans on the high frequency side and the low frequency side do not match, and as a result, the two-frequency output is affected.

FIG. 8 shows the response when the flow rate is reduced stepwise. FIG. 8A shows the flow rate change,
FIG. 8B shows a state in which the two-frequency output is switched to an output on the low-frequency side when the spans on the high-frequency side and the low-frequency side match, and FIG. 2 shows the state when switching from the two-frequency output to the output on the low-frequency side when the spans do not match. However, for simplicity, the addition operation e _A in step 15 is performed.
The damping operation (time constant T _D ) after the step is omitted.

When there is no span shift, as shown in FIG. 8B, the two-frequency output is immediately switched from the two-frequency output to the low-frequency output based on the equation (1) in response to the flow rate change (FIG. 8A). Can be

However, when there is a span shift, FIG.
(C), the in response to the flow rate change (FIG. 8 (a)), without being switched to the low-frequency output from the immediately two-frequency output based on Equation (1), given from the time t ₀ of the flow rate variation time t ₁ is switched from the elapsed. During the time from t ₀ to t ₁ , the output is output as indicated by oblique lines even though the flow rate is actually zero, resulting in an output error.

This is because, if there is a shift between the low-frequency side and the high-frequency side, a component caused by the span shift is included in the value of e _A (n-1) with respect to e _L (n). Since the deviation of this span is a relative value, the value of [e _L (n) −e _A (n−1)] increases as the set span increases, and the expression (1) is no longer satisfied. growing.

[0033]

The present invention solves the above problems.
To solve, a first frequency and a lower second frequency
Magnetic field with two different frequencies applied to the measurement fluid
The resulting signal voltage is passed through a high-pass filter.
High-frequency signal obtained through a low-pass filter and low-frequency signal obtained through a low-pass filter
In an electromagnetic flow meter that adds
Half cycle T of second frequency_SLow frequency signal obtained for each
Instantaneous value e of_L(N) low-frequency calculating means,
Half cycle T_SPrevious flow signal sampled just before
Instantaneous value e of_AFlow rate calculating means for calculating (n-1);
Damping time constant T for flow signal _DAnd high frequency
(%) Of the span on the low frequency side and the set span V_SWhen
Using the allowable noise flow velocity conversion absolute value C and the low frequency
T for every half cycle of the wave_S[E_L(N) -e_A(N-1)] /
T_D<BV_SA determining means for determining according to a determination formula of + C;
If this criterion is satisfied, select a low-frequency signal and satisfy it.
Selection means for selecting the previous flow signal when there is no flow signal
It is made to be.

[0034]

[Operation] The low-frequency calculating means generates an instantaneous value e _{L of a} low-frequency signal obtained every half cycle T _S of a second frequency lower than the first frequency.
(N) is calculated. The flow rate calculation means calculates the value of the half cycle T _S 1
Instantaneous value e _A (n−
Find 1).

The judgment means uses the damping time constant T _D for the flow rate signal, the deviation B (%) of the span on the low frequency side with respect to the high frequency, the set span V _S, and the absolute value C of the allowable noise flow velocity conversion. T _S [e _{L
(N) -e A (n-} 1)] / T D < determines according to the determination expression BV _S + C.

The selection means selects the low-frequency signal when this determination formula is satisfied, and selects the previous flow signal when it is not satisfied. As described above, the switching between the low frequency signal and the flow signal is determined in consideration of the set span V _S , so that even when the set span is large, the output error based on the displacement of the span can be reduced. .

[0037]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a flowchart showing the configuration of the main part of one embodiment of the present invention, and FIG. 2 is a block diagram showing the configuration of an embodiment for executing the procedure of the flowchart shown in FIG. FIG.
FIG. 4 is a waveform diagram illustrating the effect of the embodiment shown in FIG. Parts having the same functions as those of the conventional electromagnetic flow meter shown in FIGS. 4 to 7 are denoted by the same reference numerals, and the description thereof will be omitted as appropriate.

FIG. 2 is a block diagram showing the configuration of the present 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 calculation procedure of the main part of the present embodiment shown in FIG. 1 is realized by adding a part of selection procedure to the basic flow rate calculation procedure shown in FIG. The same step numbers (numbers) are used for the same operation procedure as the operation procedure shown in FIG.
, The newly added arithmetic programs have alphabetic step numbers.

The flow rate calculation procedure shown in FIG. 6 is a basic calculation necessary for calculating the flow rate. In this flow rate calculation procedure, the calculation procedure shown in FIG. 1 is between the calculation progress points T ₁ and T ₂ . By joining, the entire operation procedure is configured. Here, in order to avoid redundant description, the calculation procedure shown in FIG. 1 which is a main part of the present embodiment will be described.

The CPU 21 executes the steps A to D after the calculation up to the step 15, and these steps include a determination means for switching between the low frequency and the two frequencies,
4 shows an operation procedure as selection means.

In step A, a criterion value COMP for determining whether to switch between the two-frequency signal and the low-frequency signal based on the magnitude of the noise is set. Specifically, the determination reference value COMP (m / s) is represented by B (%), the deviation of the span on the low frequency side with respect to the high frequency, V _S (m / s), the conversion of the allowable noise flow rate. Is set as (BV _S + C), assuming that the absolute value of C is (m / s).

The calculation procedure is stored in the ROM 31 and the set span V _S is stored in the RAM 30.
U21 uses these to calculate the criterion value COMP and store it in a predetermined area of the RAM 30.

Next, the processing shifts to step B, and step A
, The determination reference value COMP stored in the RAM 30 and the ROM
The determination formula shown by the formula (2) stored in the RAM 31 and the RAM 3
Using the data e _L (n), e _A (n−1), and the like stored in 0, the CPU 21 determines T _S [e _L (n) −e _A (n−1)] / T _D <COMP (2 The magnitude of the noise is determined by the determination formula of (2).

Where T _D is the damping time constant for the flow signal, T _S is the low frequency half cycle, e _L (n) is the instantaneous value of the low frequency signal, and e _A (n−1) is the half cycle. The instantaneous value e _A (n−) of the flow signal sampled one time before T _S
1) are shown.

As a result of the determination by the equation (2), the determination reference value C
If the noise exceeds OMP, use the following calculation procedure.
The flow then proceeds to step C, where the flow rate signal (addition
Result of arithmetic operation) e_A(N) is output and the calculation progress point T_TwoThrough
Then, the process proceeds to step S16. Judgment reference value COMP or less
In the case of the noise, the process goes to step D and the low-frequency signal e _L
(N) is output and the calculation progress point T_TwoThrough step 16
Move to

Next, the effect when the above-described calculation procedure is executed will be described with reference to the waveform diagram shown in FIG. FIG. 3 is a waveform diagram illustrating the effect when switching between the two-frequency output and the low-frequency output.

FIG. 3A shows that the flow rate is 0 at time t = 0.
3 (b) shows the output when switching based on the conventional fixed criterion A, and FIG. 3 (c) shows the output when switching according to the signal processing procedure shown in FIG. Is shown.

Assuming that the criterion is a fixed criterion such as A as in the prior art, as shown in FIG. 3B, an error occurs due to a two-frequency output including a shift in the span indicated by oblique lines, and the time t = It switched when shifted by t _e, during which the output even though the flow rate is zero is generated errors.

On the other hand, according to the processing procedure shown in FIG. 1, when the criterion COMP is set as a linear function of the set span in consideration of the shift of the span as shown in the equation (2), the set span is large. Even in this case, as shown in FIG. 3C, the two-frequency output can be switched to the low-frequency output without an output error in response to the flow rate change (FIG. 3A). However, for simplicity, the damping operation (time constant T _D ) after the addition operation e _A in step 15 is omitted.

[0051]

According to the present invention, as described above in detail with the embodiments, the frequency is switched from the two frequencies to the low frequency by using the criterion COMP which is a linear function of the set span. Even when the span is large, it is possible to switch from the two-frequency output to the low-frequency output without an output error in response to a change in the flow rate.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a processing procedure of a main part of an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration for realizing the processing procedure shown in FIG.

FIG. 3 is a waveform chart for explaining an effect obtained by the processing procedure shown in FIG. 1;

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

FIG. 5 is a timing chart illustrating the operation of the electromagnetic flow meter shown in FIG.

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

FIG. 7 is a calculation diagram showing a calculation procedure in the flow of FIG. 4;

FIG. 8 is a waveform diagram illustrating a problem of the electromagnetic flow meter shown in FIG.

[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

Claims (1)

(57) [Claims] A first frequency and a second frequency lower than the first frequency;
A magnetic field having two different frequencies is applied to the measurement fluid, and a signal voltage generated by the magnetic field is added to a high-frequency signal obtained through a high-pass filter and a low-frequency signal obtained through a low-pass filter, thereby obtaining a flow rate. A low-frequency calculating means for calculating an instantaneous value e _L (n) of the low-frequency signal obtained every half cycle T _S of the second frequency;
_A flow rate calculating means for obtaining an instantaneous value e _A (n-1) of the flow rate signal sampled one time before the half cycle T _S ;
The damping time constant T _D for the flow rate signal, the shift B (%) of the span on the low frequency side with respect to the high frequency, and the set span V
_{Using S} and the absolute value C of the permissible noise flow velocity conversion, T _S [e _L (n) −e _A (n−
1)] A determination means for determining according to a determination formula of / T _D <BV _S + C, and a selection means for selecting a low-frequency signal when the determination formula is satisfied and selecting the flow rate signal when the determination formula is not satisfied. An electromagnetic flowmeter, characterized in that: