JPH08247813A - Electromagnetic flow meter - Google Patents

Abstract

(57) [Abstract] [Purpose] To eliminate the influence of the magnetic flux rising waveform when switching the excitation period and reduce the influence of noise other than magnetic induction noise while suppressing the number of samplings. [Structure] In an electromagnetic flow meter which applies a magnetic field from an exciting coil 2 to a fluid to be measured flowing through a measuring tube 1 and detects an electromotive force at that time as a flow signal, a square wave current having at least two different exciting periods is generated. Exciting means 12 and 23 that are supplied to the exciting coil 2 to excite them, and sampling means 1 that samples the flow rate signal in each exciting cycle in synchronization with the excitation.
4, 24, and a flow rate value calculating means 27 for obtaining a flow rate value signal when the excitation period is infinite based on the flow rate signals fetched from the sampling means 14, 24.
The excitation means 12 and 23 are excited by each excitation cycle.
A square wave current that is continuous for a period or more is supplied to the exciting coil 2. The flow rate value calculation means 27 is 2 in each excitation cycle.
The flow rate value signal is obtained by using the flow rate signals after the first cycle.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flow meter for measuring the flow rate of a conductive fluid, and particularly, it eliminates the measurement error due to the induced noise without adjusting the zero point accompanying the stationary of the fluid to improve the measurement accuracy. Electromagnetic flowmeter that can be used.

[0002]

2. Description of the Related Art Generally, in the field of measuring the flow rate of a slurry fluid such as pulp liquid, there is no obstacle to the fluid in the measuring pipe, and highly accurate measurement is possible without being affected by the flow velocity distribution in the pipe. Electromagnetic flow meters are widely used.

In a general electromagnetic flowmeter, a magnetic field perpendicular to the tube axis direction is applied to a fluid flowing in a measuring tube from an exciting coil, and an electromotive force generated by electromagnetic induction is generated from a pair of electrodes provided in the measuring tube. It is configured to detect and convert to a flow rate value. In this electromagnetic flow meter, an alternating constant current with a constant cycle (hereinafter referred to as square wave current) is supplied to the exciting coil to give an alternating magnetic flux to the measuring tube, and an electromotive force proportional to the magnetic flux density and the average velocity of the fluid is flown. By converting the value into a value, even if a direct-current polarization voltage is generated in the electrode due to an electrochemical action with the slurry fluid, noise generated in the flow rate signal due to the polarization voltage can be removed.

However, even in the case of the square wave excitation method described above, since the induced noise due to the change in the magnetic flux at the time of switching the magnetic flux is used in the state of being superposed on the flow rate signal, the induced noise is removed to obtain the true flow rate. In order to obtain a signal, it was necessary to make the fluid stand still and adjust the zero point during installation and periodic inspection.

The applicant of the present invention has filed a patent application for a magnetic flowmeter capable of eliminating a measurement error due to inductive noise without performing zero point adjustment accompanied by static fluid, as Japanese Patent Application No. 5-148976.

FIG. 5 is a block diagram of an electromagnetic flowmeter disclosed in Japanese Patent Application No. 5-148976. This electromagnetic flowmeter excites an exciting coil 2 arranged in the vicinity of a measuring tube 1 at an arbitrary plurality of exciting periods, and at the same time, transmits an electromotive force generated between electrodes 3 and 3 from a detector 4 to a converter 11. And is configured to process data.

The converter 11 includes an exciting circuit 12, a preamplifier 13, a sampling switch 14, a capacitor 15, and A.
The D / D converter 16, the signal processing timing control circuit 17, and the output circuit 18 are provided. The exciting circuit 12 includes constant current sources 12a and 12b having polarities different from each other and the constant current sources 12
an excitation changeover switch 12c for connecting either of a and 12b to the excitation coil 2;
2c is switched and controlled based on an excitation switching signal received from the signal processing timing control circuit 17 to excite the excitation coil 2 at a plurality of excitation cycles. A sampling and holding circuit is composed of the sampling switch 14 and the capacitor 15,
The sampling switch 14 is switch-controlled based on the timing signal received from the signal processing timing control circuit 17 to extract the output of the preamplifier 13 at an arbitrary timing and send it to the A / D converter 16. The A / D converter 16 sends the sampling signal obtained by A / D converting the output of the sample hold circuit to the signal processing timing control circuit 17. The signal processing timing control circuit 17 sends an excitation control signal to the excitation changeover switch 12c at a predetermined excitation cycle to control the excitation circuit 12, and sends a timing signal to the sampling switch 14 at a predetermined timing to sample and hold. Control the circuit. Further, the signal processing timing control circuit 17 has a function of performing arithmetic processing on the sampling signal and transmitting the arithmetic result to the output circuit 18. Then, the output circuit 18 outputs the calculation result received from the signal processing timing control circuit 17 as a flow meter output.

In the electromagnetic flowmeter configured as described above, the signal processing timing control circuit 17 gives the excitation switching signal of the long cycle and the short cycle to the excitation switching switch 12c. As a result, a long-wave and short-cycle square wave current is applied from the exciting circuit 12 to the exciting coil 2 in accordance with the excitation switching signal. An exciting current flows through the exciting coil 2 so as to become constant after a predetermined time has passed since the polarity was switched, as shown in FIG. 6A, and a magnetic flux corresponding to the exciting current waveform as shown in FIG. 6B. And the electromotive force shown in FIG. 6C is generated in the electrode 3.

At this time, before the magnetic flux becomes stable, DC noise and magnetic flux change occur in the conductor loop formed by the two inputs of the preamplifier 13, each electrode 3 and the fluid, as shown in FIG.
The induced noise shown in (1) is generated and superposed on the electromotive force.

On the other hand, the signal processing timing control circuit 17
Is the sampling switch 1 when a predetermined time in which the electromotive force of the electrode 3 becomes constant after the excitation switching signal is transmitted.
4, a sampling timing signal is sent at the timing shown in FIG. Upon receipt of this timing signal, the sampling switch 14 is closed for a predetermined period immediately before the polarity switching in each excitation period, and the capacitor 15 is charged with the electromotive force amplified signal. That is, two samplings are performed to extract the positive stable region and the negative stable region in the electromotive force amplified signal in each of the two excitation periods of the long period and the short period. A / D this signal
As shown in FIG. 6 (f), the converter 16 fetches the signal processing timing control circuit 17 as a sampling signal for each excitation period and its polarity.

When the signal processing timing control circuit 17 receives the positive and negative polarity sampling signals for each long cycle and short cycle, the signal processing timing control circuit 17 performs the primary processing for subtracting the negative polarity from the positive polarity for each cycle, and the primary processing is performed. As a result, the primary processing flow rate signal is obtained as shown in FIG.

The primary processing flow rate signals corresponding to the long cycle and the short cycle include noise of a low DC component due to the primary processing, while the induced noise Nc is included as shown in black. Further, of the primary processing flow rate signals corresponding to the long and short cycles, the primary processing flow rate signal corresponding to the long cycle has a smaller fixed time of the magnetic flux until sampling, and thus the induced noise Nc is smaller. That is, the magnitude of the induced noise Nc is inversely proportional to the excitation cycle and is proportional to the excitation frequency.

The signal processing timing control circuit 17 executes an extrapolation operation by linear approximation as shown in FIG. 7 on the basis of the primary processing flow rate signals of the long and short periods to set the flow rate when the excitation period is infinite. A value, in other words, a flow rate value signal when the excitation frequency becomes zero is obtained and sent to the output circuit 18.

Therefore, according to the above-mentioned electromagnetic flowmeter, the extrapolation is performed based on the two temporarily processed flow rate signals including the induction noise in proportion to the excitation frequency, and theoretically the excitation noise does not include the induction noise. Since the flow rate value signal when the frequency is zero is obtained, the measurement error due to the induced noise Nc can be removed and the measurement accuracy can be improved without performing the zero point adjustment accompanied by the stationary of the fluid.

[0015]

However, the above-mentioned electromagnetic flowmeter also has the following technical problems to be improved. In the square wave excitation method, switching of the excitation magnetic pole property is accompanied by a delay in rising due to the influence of the residual magnetic flux of the excitation coil. As described above, in order to correctly measure the flow rate, the flow rate signal is sampled in a stable state in which the magnetic flux has sufficiently risen, but this stable state is also an extension of the rise of the magnetic flux, so the change amount is accurate. Exists.

The waveform of the rising of the magnetic flux changes depending on the strength of the magnetic flux immediately before the polarity switching, which is the starting point of the rising,
In the case of single-cycle excitation, since the polarity switching timing is always constant, the waveform of the magnetic flux becomes symmetrical in both polarities, and a slight change in magnetic flux during sampling does not affect the measurement. However, when the excitation cycle and the polarity are switched by the above-described two or more excitation cycles, the rise of the magnetic flux is temporarily asymmetric when switching the excitation cycle, which affects the measurement.

Further, the noise removal calculation described above estimates and removes the magnetic induction noise component superposed on the flow rate signal in each excitation period on the assumption that the flow rate proportional component is constant, by an approximate expression. Is. However,
When noise other than magnetic induction noise is superposed in the excitation with the excitation cycle of two or more values as described above, a correct calculation result cannot be obtained because it cannot be distinguished from the magnetic induction noise component.

The main noises other than magnetic induction noise are commercial power supply induction noise and fluid noise. The commercial power supply induced noise can be reduced by setting the sampling time and the excitation period to a time that is less likely to be affected by the commercial frequency. On the other hand, since fluid noise has a 1 / f characteristic (f is a frequency), if the excitation period is large, noise that is proportional to the excitation period also becomes large. Although the influence of the fluid noise can be reduced by averaging the fluid noise, the larger the excitation period, the larger the number of samplings required in proportion to the excitation period. for that reason,
When the same number of excitation cycles of two or more values are repeatedly excited as described above, in order to reduce the influence of fluid noise, it is necessary to match the sampling number with the larger excitation cycle. You have to do sampling.

The present invention has been made in consideration of the above situation, and it is possible to eliminate a measurement error due to the asymmetrical rise of the magnetic flux when switching the excitation period, or a noise component other than the magnetic induction noise is required. It is an object of the present invention to provide an electromagnetic flow meter that can be removed without performing the above sampling and that has improved measurement accuracy.

[0020]

The invention according to claim 1 provides a measuring pipe through which a fluid to be measured flows, a plurality of electrodes arranged to face the inner wall of the measuring pipe, and the measuring pipe to the fluid to be measured. An exciting coil for giving a magnetic flux in a direction orthogonal to the tube axis direction, and an exciting means for supplying a square wave current having at least two different exciting periods to the exciting coil for excitation.
Sampling means for sampling a flow rate signal based on an electromotive force generated at each of the electrodes in synchronization with excitation in accordance with a square wave current of each excitation cycle, and the excitation cycle based on the flow rate signal fetched from the sampling means. Flow rate value calculating means for obtaining a flow rate value signal when the infinite value is set to infinity, and the exciting means supplies a square wave current to the exciting coil such that excitation in each exciting cycle is continuous for two cycles or more. . Further, the flow rate value calculation means obtains the flow rate value signal by using the flow rate signals of the second and subsequent cycles in each excitation cycle.

According to the second aspect of the present invention, a measuring pipe through which the fluid to be measured flows, a plurality of electrodes arranged to face the inner wall of the measuring pipe, and the fluid to be measured are orthogonal to the pipe axial direction of the measuring pipe. An exciting coil for giving a magnetic flux in the direction of excitation, an exciting means for supplying a square wave current having at least two different exciting periods to the exciting coil to excite it, and the electrodes according to the square wave current of each exciting period. Sampling means for sampling a flow rate signal based on the electromotive force generated respectively, and flow rate value calculating means for obtaining a flow rate value signal when the excitation period is infinite based on the flow rate signal fetched from the sampling means. The sampling means performs sampling of the flow rate signal in each excitation period by a number proportional to the length of each excitation period.

[0022]

According to the present invention corresponding to claim 1, the flow rate signal in the cycle in which the magnetic flux waveform immediately after switching the excitation cycle is asymmetric between the exciting magnetic polarities is not included in the flow rate calculation, and the magnetic flux in the second and subsequent cycles is not included. Since the flow rate signal is calculated based on the flow rate signal in a cycle in which the waveform is stable and symmetrical, it is not affected by the magnetic flux waveform asymmetry that accompanies switching of the excitation cycle, and the flow rate signal calculation by extrapolation can be performed more accurately. it can.

According to the present invention corresponding to claim 2, since the flow rate signal in each of the long and short excitation periods is sampled at a rate proportional to the excitation period, the flow rate signal due to fluid noise which increases in proportion to the excitation frequency. Can be equalized in each of the long and short excitation periods, and averaging for reducing the influence of fluid noise can be performed with a smaller number of samplings.

[0024]

Embodiments of the present invention will be described below. FIG. 1 shows the configuration of an electromagnetic flowmeter according to an embodiment. The electromagnetic flowmeter of the present embodiment has the same configuration as the electromagnetic flowmeter shown in FIG. 5 described above, except for the signal processing timing control circuit 21 of the converter 20, so the same parts are designated by the same reference numerals and described. Avoid duplication of.

The signal processing timing control circuit 21 controls opening / closing of an excitation pattern setting section 22 for storing an excitation pattern, an excitation switching section 23 for sending an excitation switching signal to the excitation switching switch 12c of the excitation circuit 12, and a sampling switch 14. Sampling timing generator 24, A /
It is composed of buffers 26a and 26b for storing the sampling signal sampled by the D converter 16 in each of the long and short cycles, an arithmetic unit 27 for calculating the flow rate value, and the like.

The excitation pattern setting section 22 stores an excitation pattern with an excitation cycle of two or more values. In the present embodiment, as the excitation pattern, the excitation is performed in the excitation cycle of two or more values, and the pattern in which the excitation in each excitation cycle is continued for two cycles or more is set. Specifically, in the example shown in FIG. 2, an excitation pattern is set in which short-period excitation is made continuous for two periods and then long-period excitation is made continuous for three periods.

The excitation changeover unit 23 includes the excitation changeover switch 1
The excitation switching signal given to 2c is applied to the excitation pattern setting unit 2
It is generated based on the excitation pattern given from 2.
Also, the switch 25 arranged between the primary processing unit (not shown) provided at the output stage of the A / D converter 16 and the buffers 26a and 26b operates so as to be switched in synchronism with the switching of the long and short excitation periods.

The sampling timing generating section 24 monitors the switching operation of the excitation switching switch 12c by the excitation switching section 23, and switches the sampling switch 14 so that the flow rate signal after the second cycle is sampled in each excitation cycle. It controls opening and closing. Further, the sampling switch 14 is set to open and close so that the number of samplings decreases in proportion to the excitation period.

The calculation unit 27 has an extrapolation calculation function for obtaining a flow rate signal when the excitation period becomes zero by linear approximation shown in FIG. 7, and subtracts an inductive noise component from the flow rate signal to calculate a true flow rate value signal. And the function to do.

Next, the operation of this embodiment configured as described above will be described with reference to the time chart of FIG. In the present embodiment, the excitation switching signal is applied from the excitation switching section 23 of the signal processing timing control circuit 21 to the excitation switching switch 12c based on the determination of the excitation pattern, so that the short cycle as shown in FIG. The exciting current is applied to the exciting coil 2 such that the exciting current waveform of (3) continues for two cycles and the long-period exciting current waveform continues for three cycles.
Since a constant exciting current is applied to the exciting coil 2 after a lapse of a predetermined time from the time of switching the polarity, FIG.
The magnetic flux shown in is generated.

FIG. 3 shows an enlarged waveform diagram of the magnetic flux waveform when the excitation period is switched from the long period to the short period.
As shown in the figure, immediately after switching the excitation period from the long period to the short period, the rising waveform of the magnetic flux is asymmetric between the exciting magnetic polarities in the first period, but it is symmetric because it stabilizes in the next period. It turns out that A similar asymmetric phenomenon occurs immediately after switching the excitation period from a short period to a long period.

On the other hand, the sampling timing generator 24
Then, the timing signal shown in FIG. 2E is sent to the sampling switch 14 to charge the capacitor 15 with the electromotive force amplified signal. That is, when the excitation cycle is switched, the electromotive force amplified signal is charged in the capacitor 15 in the closed state for a predetermined period immediately before the polarity switching from the second cycle onward in the excitation cycle. By such a switching operation, in two exciting cycles of a long cycle and a short cycle, sampling for extracting a stable region of positive polarity and a stable region of negative polarity in the electromotive force amplified signal is performed for one short period. It is performed at a rate of two cycles. Further, since the magnetic flux waveform immediately after switching the excitation period is not asymmetrical between the excitation magnetic polarities, sampling is not performed, but sampling is performed from the second period onward when the magnetic flux waveform is stable and symmetrical. The flow rate sampling is not affected by the asymmetry of the magnetic flux waveform associated with the cycle switching.

The electromotive force amplified signal charged in the capacitor 15 is sampled by the A / D converter 16 and sequentially taken into the signal processing timing control circuit 21. In the signal processing timing control circuit 21, the excitation switching unit 23 connects the switch 25 to the buffer 26a side when the exciting current has a long cycle, and connects it to the buffer 26b side when the exciting current has a short cycle. Therefore, the sampling signal from the A / D converter 16 (which has been converted into a primary processing signal by passing through a primary processing unit (not shown)) is stored in the buffers 26a and 26b for each long and short cycle.

Now, the influence of fluid noise superimposed on the flow rate signal will be described. Since fluid noise has a characteristic of (1 / f) as shown in FIG. 4, it increases in inverse proportion to the excitation frequency. This means that, of the primary processing flow rate signals corresponding to the long and short cycles, the primary processing flow rate signal corresponding to the long cycle is strongly affected by the fluid noise. In the present embodiment, the length of each of the long and short cycles is 1 for the short cycle and 2 for the long cycle. Therefore, the flow rate signal corresponding to the long cycle is affected by the fluid noise more than the flow rate signal corresponding to the short cycle. You're getting twice as much. The continuous primary processing flow rate signal (primary processing flow rate signal stored in the buffer 26a) in a long period by the influence of the fluid noise is twice as large as:
Large variations due to fluid noise.

In this embodiment, as shown in FIG. 2 (g), the primary processing flow rate signal is sampled at the ratio of the short cycle 1 to the long cycle 2, so that the variation of the signal due to the fluid noise is long and short cycle. Are evenly averaged.

The computing unit 27 separately reads the primary processing flow rate signals from each of the buffers 26a and 26b, as described above, in each of the long and short periods in which the signal variations due to the fluid noise are equal, and obtains an average value for each. The calculation unit 27 performs extrapolation by linear approximation as shown in FIG. 7 based on each average value of the primary processing flow rate signal of each long and short cycle, and the excitation frequency is zero in order to obtain a flow rate value with an infinite excitation cycle. Calculate the flow rate signal at. This flow rate signal is subtracted from the already calculated average value of the primary processing flow rate signal to calculate the induced noise component included in the flow rate signal. Next, the flow rate signal obtained by subtracting the induced noise component corresponding to each excitation cycle from the primary processed flow rate signal obtained for each excitation cycle is output to the output circuit 18.

As described above, according to this embodiment, the flow rate signal is not sampled in the period in which the magnetic flux waveform immediately after switching the excitation period becomes asymmetric between the exciting magnetic polarities, and the magnetic flux waveform in the second and subsequent periods becomes stable. Since the flow rate signal is sampled at a symmetrical period, the influence of the magnetic flux waveform asymmetry associated with the switching of the excitation period is eliminated, and the flow rate signal calculation by extrapolation can be performed more accurately.

According to this embodiment, the flow rate signal in each of the long and short excitation periods is sampled at a rate proportional to the excitation period, so that the variation of the flow rate signal due to the fluid noise can be equalized in each of the short and long excitation periods. Therefore, averaging for reducing the influence of fluid noise can be performed with a smaller number of samplings.

It should be noted that the excitation for sampling is excited at a rate of the number of times proportional to the excitation period, and immediately after switching the excitation period, one or more periods of non-sampling excitation are added to the excitation described in the above embodiment. The same effect as the method can be obtained. The present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

[0040]

As described above in detail, according to the present invention, it is possible to eliminate a measurement error due to the asymmetrical rise of the magnetic flux when switching the excitation period, or to eliminate noise components other than the magnetic induction noise more than necessary. Can be removed without performing sampling, and an electromagnetic flow meter with improved measurement accuracy can be provided.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of an electromagnetic flowmeter according to an embodiment of the present invention.

FIG. 2 is a time chart showing the operation of the electromagnetic flow meter according to the embodiment.

FIG. 3 is a diagram showing a rising waveform of magnetic flux immediately after switching of an excitation cycle.

FIG. 4 is a diagram showing a relationship between fluid noise and excitation frequency.

FIG. 5 is an overall configuration diagram of an electromagnetic flowmeter having an extrapolation function based on a flow rate signal based on a plurality of excitation periods.

FIG. 6 is a time chart showing the operation of the electromagnetic flow meter shown in FIG.

FIG. 7 is a diagram illustrating the principle of extrapolation calculation.

[Explanation of symbols] 1 ... Measuring tube, 2 ... Excitation coil, 3 ... Electrode, 4 ... Detector,
12 ... Excitation circuit, 13 ... Preamplifier, 14 ... Sampling switch, 15 ... Capacitor, 16 ... A / D converter,
18 ... Output circuit, 20 ... Converter, 21 ... Signal processing timing control circuit, 22 ... Excitation pattern setting section, 23 ... Excitation switching section, 24 ... Sampling timing generating section, 25 ...
Switches, 26a, 26b ... Buffer, 27 ... Arithmetic unit.

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[Procedure amendment]

[Submission date] June 11, 1996

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 2

[Correction method] Change

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[Fig. 2]

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 6

[Correction method] Change

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[Figure 6]

Claims (2)

[Claims] 1. A measurement tube through which a fluid to be measured flows, a plurality of electrodes arranged to face an inner wall of the measurement tube, and an excitation for giving a magnetic flux to the fluid to be measured in a direction orthogonal to a tube axis direction of the measurement tube. A coil, an exciting means for supplying a square wave current having at least two different exciting periods to the exciting coil to excite it, and an electromotive force generated in each of the electrodes in response to the square wave current in each exciting period. A sampling means for sampling the flow rate signal in synchronism with the excitation, and a flow rate value calculating means for obtaining a flow rate value signal when the excitation cycle is infinite based on the flow rate signal fetched from the sampling means. The excitation means supplies a square wave current to the excitation coil such that the excitation in each excitation cycle continues for two or more cycles, and the flow rate value calculation means sets the flow rate value calculation means in each excitation cycle. An electromagnetic flow meter characterized in that a flow rate value signal is obtained using the flow rate signals of the second and subsequent cycles. 2. A measuring tube through which a fluid to be measured flows, a plurality of electrodes arranged to face an inner wall of the measuring tube, and an excitation for giving a magnetic flux to the fluid to be measured in a direction orthogonal to a tube axis direction of the measuring tube. A coil, an exciting means for supplying a square wave current having at least two different exciting periods to the exciting coil to excite it, and an electromotive force generated in each of the electrodes in response to the square wave current in each exciting period. A sampling means for sampling the flow rate signal, and a flow rate value calculating means for obtaining a flow rate value signal when the excitation period is infinite based on the flow rate signal taken in from the sampling means. An electromagnetic flowmeter characterized in that the flow rate signal is sampled at each excitation period by a number proportional to the length of each excitation period.