JP3445521B2 - 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, and more particularly to applying a magnetic field to a fluid in a tube by an alternating magnetic excitation current having a frequency higher than a commercial power supply frequency to process a signal electromotive force of the fluid obtained from an electrode. The present invention relates to an electromagnetic flow meter that obtains a measured flow rate by doing.

[0002]

2. Description of the Related Art Generally, in this type of electromagnetic flow meter, the flow rate of
The configuration is as shown in. In the figure, 10 is a detector that applies a magnetic field to the fluid in the tube based on a predetermined alternating excitation current and detects and outputs the signal electromotive force generated in the fluid as a detection signal, and 11 is a predetermined detector for the detector 10. It is a converter that outputs an AC exciting current and calculates and outputs the flow rate in the tube by processing the detection signal from the detector 10.

The exciting unit 8 outputs an alternating exciting current of a rectangular wave having a predetermined frequency based on the exciting signal 9C from the switching unit 9. The exciting coil 10D of the detector 10 is
Excited by the alternating exciting current from the converter 11, the tube 1
A predetermined magnetic field is applied to the fluid flowing in 0C, whereby a signal electromotive force having an amplitude according to the flow velocity of the fluid is generated. This signal electromotive force is detected by the electrodes 10A and 10B provided on the inner wall of the tube 10C and at opposite positions, and is output to the converter 11 as a detection signal.

In the converter 11, a high-pass filter (hereinafter referred to as HPF) 1 attenuates low-frequency components of the detection signal obtained from the detector 10, so that pulse noise or low noise mixed in the detection signal is reduced. Attenuates frequency noise. Then, in the AC amplification unit 2, HP
The output from F1 is AC amplified and output as an AC flow rate signal 12.

In the sample hold unit 3, based on the switching signals 9A and 9B from the switching unit 9, A
The AC flow rate signal 12 from the C amplifier 2 is sampled and output as a DC flow rate signal 13. The arithmetic processing unit 6 is A-
The DC flow rate signal 13 from the sample and hold section 3 is taken in as digital information via the D conversion section 5, and a desired calculation flow rate value is calculated by executing a predetermined calculation process, and converted into a predetermined signal by the output section 7. And output.

FIG. 8 is a timing chart showing a conventional sampling operation. 9C is an excitation signal from the switching section 9 and 12 is an AC flow rate signal input to the sample hold section 3. Further, 9A and 9B are sampling signals input from the switching unit 9 to the sample hold unit 3, and define the sampling period (hatched portion) of the AC flow rate signal 12.

In this case, the sampling period is provided near the trailing edge of each pulse of the excitation signal 9C (AC flow rate signal 12) due to its waveform stability, and the sample-hold section 3 is provided.
Then, only during this sampling period, the switches 3A and 3B are short-circuited to integrate the AC flow rate signal 12 and output as a DC flow rate signal 13. When the AC flow rate signal 12 is on the positive side, only the switch 3A is short-circuited based on the switching signal 9A, and when the AC flow rate signal 12 is on the negative side, only the switch 3B is short-circuited based on the switching signal 9B. To be done.

Here, the AC flow rate signal 12 has continuous noise of a predetermined frequency, for example, a commercial power supply frequency of 50/60 Hz.
When noise or the like having a frequency equal to is mixed, fluctuations 61 occur in the DC flow rate signal 13 output from the sample hold unit 3 due to the operating characteristics of the sample hold unit 3. For example, FIG. 8 shows a state in which this type of noise is mixed in the AC flow rate signal 12 when the flow rate is kept constant.

In this case, the AC flow rate signal 12 has errors d0 to d7 due to the amplitude of noise mixed in during the sampling period of each adjacent pulse waveform. The errors d0 to d7 are sampled by the sample hold unit 3 and output as the DC flow rate signal 13 having the fluctuation 61. FIG. 9 is an explanatory diagram showing the relationship between the noise frequency and the fluctuation in the sample and hold unit. The horizontal axis represents the noise frequency as a multiple of the excitation frequency, and the vertical axis represents the fluctuation magnitude.

Here, when noise having a noise frequency equal to the excitation frequency fex is mixed, the fluctuation is greatest, and the fluctuation decreases with distance from the excitation frequency fex as the center, and the frequency is zero and the excitation frequency fex.
There is a mountain-shaped characteristic that the fluctuation is theoretically zero at twice the noise frequency. Similarly, centering on each noise frequency that is an odd multiple of the excitation frequency, for example, 3 times, 5 times,
The mountain-shaped characteristic in which the fluctuation is theoretically zero is repeatedly seen at noise frequencies of adjacent even multiples, for example, double, quadruple, ...

Therefore, conventionally, by positively utilizing the operation characteristics of the sample and hold unit 3, the noise frequency at which the fluctuation theoretically becomes zero and the frequency of the noise to be mixed, that is, the commercial power supply frequency are matched. In addition, by selecting the excitation frequency, the fluctuation of the DC flow rate signal 13 due to the noise mixed in the AC flow rate signal 12 is attenuated.

[0012]

However, in such a conventional electromagnetic flowmeter, when the noise of the commercial power source frequency is attenuated, the commercial power source frequency and the noise frequency at which the fluctuation theoretically becomes zero are matched. It is necessary to select a different excitation frequency, and in that case, the excitation frequency is inevitably a value lower than the commercial power supply frequency. Therefore, it cannot be applied when a frequency higher than the commercial power supply frequency is used as the excitation frequency, and there is a problem that the fluctuation cannot be attenuated.

Generally, when measuring the flow rate of a slurry fluid containing solid materials such as papermaking raw materials and earth and sand, noises specific to the slurry fluid (hereinafter, Slurry noise) occurs randomly. FIG. 10 is an explanatory diagram showing the frequency characteristics of slurry noise, in which the horizontal axis represents the noise frequency in multiples of the excitation frequency fex, and the vertical axis represents the magnitude of noise.

In the figure, 81 is the noise characteristic of the slurry fluid containing the papermaking raw material, which is the conventional predetermined excitation frequency fex.
The 1 / f characteristic inversely proportional to the noise frequency can be seen up to about 4 times. Further, 82 is the noise characteristic of the slurry fluid containing earth and sand, and the 1 / f characteristic can be seen up to about 6 times the excitation frequency fex. Therefore, it is considered that the influence of these slurry noises can be reduced by using a relatively high frequency as the excitation frequency.

However, in the above-mentioned conventional method, the excitation frequency inevitably has a value lower than the commercial power supply frequency, so that it cannot be applied when a relatively high excitation frequency effective for damping slurry noise is used, and the commercial method is not available. There was a problem that the fluctuation due to power frequency noise could not be attenuated. The present invention is intended to solve such a problem, and an object thereof is to provide an electromagnetic flow meter capable of attenuating the fluctuation of the measurement output caused by the commercial power source frequency noise even when a relatively high excitation frequency is used. There is.

[0016]

In order to achieve such an object, the electromagnetic flowmeter according to the present invention uses fex as an exciting frequency and fn as a frequency component contained in a DC flow rate signal obtained by sampling. When the frequency is a commercial frequency, band attenuation filter means for attenuating the component of frequency f shown in the following formula f = | mfex ± nfn | (where m and n are positive integers) is provided. Therefore, of the frequency components included in the DC flow rate signal obtained by sampling,
The frequency component of the difference between the excitation frequency and the commercial power supply frequency, that is, the fluctuation included in the DC flow rate signal is attenuated.

Further, the band attenuation filter means is an AD converter for sequentially converting the DC flow rate signal obtained by sampling into digital information, and a moving average value from a plurality of consecutive digital information among these digital information. And a moving average processing unit for calculating and outputting.
Therefore, the A / D converter sequentially converts the DC flow rate signal obtained by sampling into digital information, and the moving average processing unit sequentially calculates a moving average value from a plurality of consecutive digital information among these digital information. Is output.

The band attenuating filter means integrates the DC flow rate signal obtained by sampling and converts it into a pulse signal having a frequency corresponding to the DC potential, and outputs the voltage-frequency conversion section, and the voltage-frequency conversion section. And a counter that counts the pulse signal of (4) every predetermined period and outputs it as digital information. Therefore, the DC-flow rate signal obtained by sampling is integrated by the voltage-frequency conversion unit and converted into a pulse signal having a frequency corresponding to the DC potential, and the pulse signal from the voltage-frequency conversion unit is output by the counter. It is counted every predetermined period and output as digital information.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of an electromagnetic flow meter according to an embodiment of the present invention. In the figure, the same or equivalent parts as those described above (see FIG. 7) are designated by the same reference numerals. In FIG. 1, 10 is a detector that applies a magnetic field to a fluid in a tube based on a predetermined alternating excitation current and detects and outputs a signal electromotive force generated in the fluid as a detection signal, and 11 is a predetermined detector for the detector 10. It is a converter that outputs an AC exciting current and calculates and outputs the flow rate in the tube by processing the detection signal from the detector 10.

In the detector 10, the electrodes 10A, 10B
Is an electrode which is arranged to face the inner wall of the tube 10C through which the fluid to be measured flows, and which detects the signal electromotive force generated in the fluid, and the exciting coil 10D is excited based on the alternating exciting current from the converter 11, Is a coil that applies a magnetic field to the fluid. In the converter 11, the switching unit 9 uses sampling signals 9A and 9A,
Circuit part for generating and outputting B and excitation signal 9C, excitation part 8
Is a circuit section that outputs an alternating-current exciting current of a rectangular wave having a predetermined frequency based on the exciting signal 9C from the switching section 9.

High-pass filter (hereinafter referred to as HPF)
1 attenuates the low frequency component of the detection signal obtained from the electrodes 10A and 10B of the detector 10,
The AC amplification unit 2, which is a circuit unit that attenuates pulse-like noise and low-frequency noise mixed in this detection signal, amplifies the detection signal from the LPF 2 by AC and outputs it as an AC flow rate signal 12 whose amplitude changes according to the fluid flow velocity. The circuit unit and the sample hold unit 3 sample the AC flow rate signal 12 from the AC amplification unit 2 based on the switching signals 9A and 9B from the switching unit 9, and the DC flow rate whose DC potential changes according to the fluid flow velocity. It is a circuit unit that outputs as a signal 13.

Band attenuation filter (hereinafter referred to as BEF)
Reference numeral 4 is a circuit portion for attenuating the frequency component of the difference between the excitation frequency and the commercial power supply frequency included in the DC flow rate signal 13 from the sample and hold section 3. The AD conversion section 5 integrates the DC flow rate signal 13 from the BEF 4. Then, the arithmetic processing unit 6 calculates the desired flow rate by executing a predetermined arithmetic processing on the digital information from the AD converting unit 5, and the arithmetic processing unit 6 calculates the desired flow rate. It is a circuit unit that converts the flow rate calculated by the processing unit 6 into a predetermined signal and outputs the signal.

Next, the operation of the present invention will be described with reference to FIG. The timing of the sampling operation according to the present invention is the same as that described above (FIG. 8). Based on the excitation signal 9C from the switching unit 9,
A rectangular wave AC exciting current having a predetermined frequency fex higher than the commercial power supply frequency fn is output from the exciting unit 8 of the converter 11, and the exciting coil 10D of the detector 10 is excited.

As a result, the exciting coil 10D is excited, a predetermined magnetic field is applied to the fluid flowing in the tube 10C, and a signal electromotive force having an amplitude corresponding to the flow velocity of the fluid is generated. This signal electromotive force is detected by the electrodes 10A and 10B provided on the inner wall of the tube 10C and at opposite positions, and is output to the converter 11 as a detection signal. In the HPF 1 of the converter 11, low frequency components of the detection signal obtained from the detector 10 are attenuated, and pulsed noise and low frequency noise mixed in this detection signal are attenuated.

Then, in the AC amplifier 2, the HPF1
The output from is AC amplified and output as an AC flow rate signal 12. In the sample hold unit 3, the switching unit 9
The AC flow rate signal 12 from the AC amplification unit 2 is sampled based on the sampling period (see FIG. 8) indicated by the switching signals 9A and 9B from the AC power supply circuit 9 and is output as the DC flow rate signal 13.

The sampling period is provided near the trailing edge of each pulse of the AC flow rate signal 12 because of its waveform stability, and in the sample-hold section 3, the switches 3A and 3B are short-circuited during this sampling period. The AC flow rate signal 12 is integrated and output as a DC flow rate signal 13. Further, when the AC flow rate signal 12 is on the positive side, only the switch 3A is short-circuited based on the switching signal 9A, and when the AC flow rate signal 12 is on the negative side, only the switch 3B is short-circuited based on the switching signal 9B. To be done.

In the BEF 4, of the DC flow rate signal 13, the excitation frequency fex and the commercial power frequency fn (50/60
The frequency component of the difference with (Hz) is attenuated. Here, the noise characteristic of the DC flow rate signal 13 output from the sample hold unit 3 will be described. As described above (see FIG. 8), when noise of the commercial power supply frequency is mixed in the AC flow rate signal 12, the DC flow rate signal 13 fluctuates due to the operation characteristics of the sample hold unit 3.

FIG. 2 is an explanatory diagram showing the frequency characteristic of fluctuation included in the DC flow rate signal. The fluctuation of the DC flow rate signal 13 is m times the excitation frequency fex (21) and the commercial power supply frequency fn (22). Frequency n times (where m and n are positive integers and either m or n is other than 1), that is, mfex−nfn (23), mfex + nf
It occurs at n (24). Therefore, the BEF 4 having the frequency characteristics 25 and 26 as shown in FIG. 2 is provided in the subsequent stage of the sample and hold unit 3 to attenuate the frequency components 23 and 24 of the difference included in the DC flow rate signal 13 to reduce the commercial power supply frequency. It is possible to reduce fluctuations caused by noise.

Since the difference frequency component 24 is located at a frequency farther from the signal frequency component (DC and its vicinity) than the difference frequency component 23, a general LPF is used.
In many cases, it can be sufficiently attenuated. Therefore, the frequency component 24 of the difference is higher than the excitation frequency fex, and is attenuated to some extent by the processing of the integration type AD converter 5 or the arithmetic processor 6 in the subsequent stage.
In FIG. 4, only the frequency component 23 of the difference may be attenuated. In this way, the BEF 4 attenuates the fluctuation caused by the commercial power supply frequency noise from the DC flow rate signal 13, and outputs it to the AD conversion unit 5.

The A-D converter 5 outputs the DC flow rate signal 13 from the BEF 4 as digital information corresponding to the DC potential. In the arithmetic processing unit 6, the AD conversion unit 5
DC flow rate signal 13 from the sample and hold unit 3 via
Is taken in as digital information, and a predetermined calculation process is executed to calculate a desired measured flow rate value from the fluid flow velocity, and the output unit 7 converts it into a predetermined signal and outputs it.

As for the configuration of the BEF4, general configuration examples such as an active filter and a digital filter are conceivable. However, by utilizing the frequency characteristic of the moving average processing,
By realizing the EF4 by the AD conversion unit 5, it is not necessary to provide the BEF4 as a separate body. Figure 3
[Fig. 3] is an explanatory diagram showing a configuration example of an A-D conversion unit, (a) shows an example using a moving average processing unit, and (b) shows an example using a voltage-frequency conversion unit.

In FIG. 3A, the output from BEF4 is A
The D-converter 5A converts the digital information into digital information, and the moving average processing unit 5B sequentially calculates an average value of a plurality of consecutive data of the digital information to calculate the arithmetic processing unit 6.
Is output to. Therefore, the DC flow rate signal 13 from the sample hold unit 3 is sequentially converted into digital information by the A / D converter 5, and these digital information are averaged with a plurality of consecutive digital information before or after the digital information. The pulse noise mixed in the DC flow rate signal 13 is attenuated.

FIG. 4 is an explanatory diagram showing the frequency characteristics of the moving average processing, where the horizontal axis represents the input signal frequency f and the moving average time τ.
And fτ product, and the vertical axis shows the output level.
The moving average time τ is a fixed time section corresponding to the number of pieces of data for which moving average is performed in sequentially input digital information. The moving average process is performed at a frequency that is an integral multiple of the product fτ of the moving average time τ and the input signal frequency f.
It has the characteristic that the output level is greatly attenuated.

By utilizing this characteristic, the time interval τ for performing the moving average process is selected, and the frequency at which the output level after the moving average process is greatly attenuated is the difference frequency component 23 which is the above-mentioned fluctuation frequency component. , 24 (see FIG. 2), the fluctuation included in the DC flow rate signal 13 can be attenuated. For example, the excitation frequency fex
= 66.7 Hz (m = 1), commercial power frequency fn = 5
When a high frequency component twice as high as 0 Hz (n = 2) is generated, the frequency f (= | mfex-nfn) of the difference on the low frequency side is generated.
|) = 33.3 Hz.

Therefore, the moving average time τ = 0.03s
As a result, fτ = 1, and the difference frequency f = 3
It can significantly reduce 3.3 Hz. When the moving average time τ = 0.03 s, fτ = 5,
Frequency component f = 16 equal to the difference frequency mfex + nfn
6.7 Hz is greatly attenuated, and any difference frequency can be efficiently attenuated.

In FIG. 3B, the integration type AD converter 5 is composed of a voltage-frequency converter (hereinafter referred to as a VF converter). The VF converter integrates an input signal voltage with a predetermined time constant and outputs a frequency pulse corresponding to the integrated voltage value, and is known to have the same characteristics as the moving average processing described above. Has been.

Here, the pulse from the VF converter 5C is counted by the counter 5D every predetermined period, and the counted value is output to the arithmetic processing unit 6 as digital information. As a result, as compared with the case where the AD converter 5A and the moving average processing unit 5B described above are used, the cost of the converter 11 can be reduced and the cost of the converter 11 can be reduced by using the VF converter 5C. Can be achieved.

FIG. 5 shows an example in which the BEF 4 is composed of a passive filter. FIG. 5 is a circuit diagram showing a configuration example of the BEF. Here, the capacitive elements 41 and 4 connected in series are shown.
2 and the resistance elements 43 and 44 connected in series are connected in parallel, and the resistance element 45 is connected between the connection point of the capacitance elements 41 and 42 and the ground potential.
A BEF is configured by connecting the capacitive element 46 between the connection point of 44 and the ground potential.

By selecting the values of the capacitance elements 41, 42, 46 and the resistance elements 43, 44, 45, the BEF 4 having a desired frequency characteristic can be constructed. In addition to this, there are many BEF configuration examples, and the same effect can be obtained by using any of them.

Further, in the case where it is affected by a plurality of frequencies, the BEF 4 having a plurality of frequency characteristics corresponding to those frequencies may be constructed. For example, as shown in FIG. 6A, the excitation frequency fex = 165 Hz, the commercial frequency fn =
In the case of 50 Hz, | fex ± 3fn | = 15 Hz (m =
In addition to the characteristic of removing the component of 1, n = 3), | fex ±
4fn | = 35 Hz (m = 1, n = 4) or | 3fex ± 1
It may have a characteristic of removing a component of 0fn | = 5 Hz (m = 3, n = 10).

In the above description, the case where either m or n is other than 1 has been described as an example.
Can be applied to both cases. That is, FIG.
As shown in (b), the excitation frequency fex = 82.5 Hz,
In the case of commercial frequency fn = 50 Hz, | fex ± fn | = 3
In addition to the characteristic of removing the component of 2.5 Hz (m = 1, n = 1), | 3fex ± 5fn | = 2.5 Hz (m = 3, n
= 5) or | 5fex ± 8fn | = 12.5Hz (m = 5
n = 8) Furthermore, | 7fex ± 11fn | = 27.5Hz
A characteristic of removing a component such as (m = 7, n = 11) may be provided.

In the above description, a system in which a signal and a power source are transmitted by sharing the same line, that is, a two-wire type electromagnetic flowmeter has been described as an example, but the invention is not limited to this. The present invention can be applied even to a method in which a power source and a power source are transmitted by different lines, for example, a 4-wire type electromagnetic flowmeter,
The same effect as the above can be obtained.

[0043]

As described above, according to the present invention, among the frequency components included in the DC flow rate signal obtained by sampling, the difference between the frequency that is an integral multiple of the excitation frequency and the frequency that is an integral multiple of the commercial power supply frequency. Since the band attenuation filter means for attenuating the frequency component of is provided, the frequency component included in the DC flow rate signal is the frequency component of the difference between the frequency of the excitation frequency and the frequency of the commercial power frequency. Can be attenuated and, for example, even when an excitation frequency sufficiently higher than the commercial power supply frequency is used, the influence of harmonic components generated by distortion of the commercial power supply frequency noise can be suppressed, and the slurry noise generated by the slurry fluid can be reduced. At the same time, it is possible to attenuate the fluctuation that occurs in the DC flow rate signal after sampling due to the commercial power supply frequency noise.

Further, the band attenuation filter means is an A / D converter for sequentially converting the DC flow rate signal obtained by sampling into digital information, and a moving average value from a plurality of consecutive digital information among these digital information. And a moving average processing unit for calculating and outputting Further, the band attenuation filter means integrates the DC flow rate signal obtained by sampling and converts it into a pulse signal having a frequency corresponding to the DC potential, and outputs the pulse signal, and a pulse signal from this voltage-frequency conversion section. Is counted every predetermined period and is output as digital information. Therefore, it is possible to use the AD conversion unit which is required when digitally processing the flow rate signal, and it is not necessary to add the band attenuation filter means as a separate circuit unit.

[Brief description of drawings]

FIG. 1 is a block diagram of an electromagnetic flow meter according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a frequency characteristic of fluctuation.

FIG. 3 is an explanatory diagram showing a block configuration example of an AD conversion unit.

FIG. 4 is an explanatory diagram showing frequency characteristics of moving average processing.

FIG. 5 is a circuit diagram showing a configuration example of a BEF.

FIG. 6 is an explanatory diagram showing a relationship between a commercial frequency and an excitation frequency.

FIG. 7 is a block diagram showing a conventional electromagnetic flow meter.

FIG. 8 is a timing chart showing a conventional sampling operation.

FIG. 9 is an explanatory diagram showing the relationship between noise frequency and fluctuation in the sample hold unit.

FIG. 10 is an explanatory diagram showing frequency characteristics of slurry noise.

[Explanation of symbols]

1 ... High-pass filter, 2 ... AC amplification part (AC amplification part), 3 ... Sample hold part, 4 ... BEF (band attenuation filter means), 5 ... A-D conversion part, 5A ... A-D converter, 5B ... Moving average processing unit, 5C ... VF conversion unit (voltage-frequency conversion unit), 5D ... Counter, 6 ... Arithmetic processing unit,
7 ... Output part, 8 ... Excitation part, 9 ... Switching part, 9A,
9B ... Sampling signal, 9C ... Excitation signal, 10 ... Detector, 10A, 10B ... Electrode, 10C ... Tube, 10D ... Excitation coil, 11 ... Converter, 12 ... AC flow rate signal, 13 ... DC flow rate signal.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-6-307904 (JP, A) JP-A-7-55519 (JP, A) JP-A-60-174915 (JP, A) JP-A-11- 83575 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01F 1/00-9/02

Claims (3)

(57) [Claims] 1. A DC flow rate signal obtained by applying a magnetic field to a fluid in a tube with an AC exciting current having a predetermined exciting frequency higher than a commercial power supply frequency, amplifying a signal electromotive force detected from the fluid, and sampling the amplified signal. In an electromagnetic flow meter that converts the digital flow rate into digital information and calculates the measured flow rate, if fex is the excitation frequency and fn is the commercial frequency among the frequency components included in the DC flow rate signal obtained by sampling, the following formula f = | Mfex ± nfn | (where m and n are positive integers), an electromagnetic flowmeter comprising a band attenuation filter means for attenuating the component of the frequency f. 2. The electromagnetic flowmeter according to claim 1, wherein the band attenuation filter means is an A / D converter that sequentially converts the DC flow rate signal obtained by sampling into digital information, and a continuous one of these digital information. And a moving average processing unit that sequentially calculates and outputs a moving average value from a plurality of digital information items. 3. The electromagnetic flowmeter according to claim 1, wherein the band attenuation filter means integrates the DC flow rate signal obtained by sampling and converts it into a pulse signal having a frequency corresponding to the DC potential, and outputs the voltage-frequency conversion. An electromagnetic flowmeter, comprising: a section and a counter that counts the pulse signal from the voltage-frequency conversion section for each predetermined period and outputs it as digital information.