JPH0599719A - Electromagnetic flowmeter - Google Patents

Abstract

(57) [Summary] [Purpose] To generate a timing signal synchronized with commercial power supply noise. [Composition] The commercial power source noise detection section 8 detects commercial power source noise riding on a fluid, and the timing signal generation section 9 generates sampling timing and exciting current timing by the noise, and the excitation is performed and detected at the timing. The signal is sampled.

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 detecting the flow rate of a conductive fluid in a process control device.

[0002]

2. Description of the Related Art A detection signal corresponding to the flow rate of a fluid can be obtained by sampling the voltage generated at an electrode brought into contact with a conductive fluid flowing in a magnetic field at a predetermined timing. It is used in total.

Since electrochemical noise is superposed on the detection signal, the polarity of the exciting current is alternately changed to remove the electrochemical noise. However, if the polarity is alternately changed in this way, it takes a certain amount of time until the state stabilizes.Therefore, wait until the state stabilizes each time the exciting magnetic property changes, and perform sampling after the stable point to obtain the detection signal. I have to.

Therefore, the excitation current switching timing for switching the polarity of the excitation current and the sampling timing for sampling are required, which is provided with a timing generating means inside to generate both timings.

[0005]

However, in addition to the above-mentioned electrochemical noise, external noise (most of which is commercial power supply noise, therefore commercial power supply noise is used except when it is necessary to identify the main body of the noise). There is). If this noise is small, there is no problem, but since the commercial power supply noise and the timing signal generated internally are not synchronized, if the amplitude of the commercial power supply noise is large, output fluctuations or fluctuations may appear remarkably and sometimes A swell with a long period called a beat may occur, which may interfere with the measurement.

In the case of an apparatus which operates by a commercial power source, if the timing signal is generated from the commercial power source, the timing signal and the commercial power source noise are synchronized, so that a noise reduction effect can be expected. However, commercial power supply noise is often mixed with power supply systems having different power supply systems, and the effect as expected cannot often be obtained. Further, in the case of a device that operates by a DC power supply, since it does not have a signal that describes it as a commercial power supply, there are many cases where output fluctuations and fluctuations appear.

The present invention has been made in view of such circumstances, and an object thereof is to produce a timing signal synchronized with commercial power source noise.

[0008]

In order to solve such a problem, a first aspect of the present invention provides noise detection means for detecting noise superimposed on a fluid from electrodes, and noise detected by the noise detection means. And a timing signal generating means for generating the excitation current generating timing for generating a magnetic field and the sampling timing. A second aspect of the present invention is the first aspect of the present invention, further comprising pseudo noise generating means for outputting a pseudo signal having the same period as the noise signal to the timing signal generating means when no noise is detected by the noise detecting means.

[0009]

In the first aspect of the present invention, since the timing signal is generated based on the noise detected by the noise detecting means, the timing signal and the noise are synchronized and the noise reduction effect is obtained. In the second invention, even if there is a period in which noise is not detected for some reason, pseudo noise is generated from the pseudo noise generating means during that period, and a timing signal is generated based on the pseudo noise.

[0010]

FIG. 1 is a block diagram showing an embodiment of the first invention. In FIG. 1, a conductive fluid is flowing in the conduit 1, and a magnetic field generated by an electric current flowing through the exciting coils 2a and 2b is applied to the fluid, so that a signal corresponding to the flow rate is obtained from the electrodes 1a and 1b. It is designed to be used.

The detected signal is amplified by the amplifiers 3a and 3b, amplified by the differential amplifier 3c, sampled by the sampling circuit 4 and supplied to the A / D converter 5 through the amplifier 3d, where it is supplied as a digital signal. Is converted into a DC signal and processed by the CPU 6, and the D / A converter 7 causes the minimum value of the flow rate signal to be DC 4 mA and the maximum value to be DC 20 m.
It is adapted to be converted into an analog signal of A and output to the line L of the two-wire system. The D / A converter 7 produces a voltage V used in this device based on a power source supplied from an external device through a line.

As described above, commercial power source noise is generated on the electrodes 1a and 1b. The commercial power supply noise is amplified by the amplifiers 3a and 3b, and the commercial power supply noise detection unit 8 including, for example, the adder 8a detects the commercial power supply noise, and the commercial power supply noise is amplified by the amplifier 3e.

On the basis of the commercial power supply noise detected in this way, the timing signal generator 9 generates a signal synchronized with the commercial power supply noise. Then, based on this signal, the exciting circuit 10 generates an exciting current, and the exciting current is supplied to the exciting coils 2a and 2b. The timing signal generator 9 also generates a sampling signal in synchronization with the AC power noise, and the sampling signal causes the sampling switch 4 of the sampling circuit 4 to operate.
By controlling a and 4b, the sampled data is stored in the capacitors 4c and 4d.

FIG. 2 is a waveform diagram showing waveforms at various portions. Commercial power source noise is common mode noise, and the electrodes 1a and 1b are used.
Occurs, the phase is the same as shown in FIGS. 2 (a) and 2 (b). Therefore, when the two signals are added by the internal adder 8a in the commercial power noise detector 8, the commercial power noise shown in FIG. 2C is obtained. When this is amplified to the saturation level by the amplifier 3e, it is shown in FIG.
The signal shown in (d) is obtained.

Based on this signal, the timing signal generating circuit 9 outputs the excitation signal shown in FIG. 2 (e) and the excitation signal shown in FIG. 2 (f), (g).
Since the sampling signal shown in FIG. 3 is generated, the excitation signal drives the excitation circuit 10 to generate the excitation current by the excitation coils 2a and 2b.
To the sampling switch 4
The detection signals are sampled at the sampling timing by driving a and 4b. The sampling signal is generated at the last timing of the exciting current so that the stable detection signal can be sampled as described above.

FIG. 3 shows a signal output from the amplifier 3c, in which the flow rate signal e generated at the electrodes 1a and 1b corresponding to the flow rate is superimposed on the electrochemical noise EN, and the polarity of the exciting current is alternately changed. Therefore, the polarity also changes in synchronization. The commercial power supply noise Eac is superposed on the flow rate signal, and the exciting current is synchronized with the commercial power supply noise. Therefore, the commercial power supply noise obtained in FIG. 3 is synchronized with the polarity change of the exciting current. Note that commercial power supply noise is common mode noise and is therefore removed by the differential amplifier 3c, but it cannot be completely removed by the differential amplifier 3c alone, and for convenience of explanation, it is emphasized in FIG. is doing.

The shaded area indicates the area to be sampled, and thus the last section of the exciting current is sampled. Since this sampling signal is also generated by the commercial power supply noise, the sampling period is also synchronized with the commercial power supply noise shown in FIG. The sampled signal is stored in the capacitors 4c and 4d and subtracted by the differential amplifier 3d.

As a result, the commercial power supply noise Eac is canceled because it is in phase, and the electrochemical noise EN has a change period that is sufficiently long compared to the excitation signal period, so that the amplitude hardly changes in the adjacent excitation signal period and the cancellation is performed. To be done. On the other hand, since the flow rate signal e has a reverse phase in the adjacent excitation signal period, the flow rate signal e is output with double the amplitude.

Although commercial power supply noise cannot be obtained at the start of operation of the apparatus, it is necessary to generate an exciting current at this time as well. In this case, for example, the timing signal generator 9
If a timing signal equal to the commercial power supply frequency is generated in advance, and when the commercial power supply noise occurs, the timing signal can be synchronized with the phase of the commercial power supply noise so that a desired timing signal can be obtained. good. Further, if the commercial power supply noise detection unit 8 is provided with a filter or the like for removing components other than the commercial power supply frequency component as necessary, it is possible to prevent inconvenience due to other external noise.

FIG. 4 is a block diagram showing an embodiment of the second invention, showing only the main part, that is, the part which generates a pseudo signal when commercial power supply noise is no longer obtained.
The part shown by the dotted line is the same as the part shown in FIG.

In FIG. 4, the commercial power source noise detected by the commercial power source noise detecting section 8 is the trigger signal generating section 1
1 is supplied. The trigger signal generator 11 sends the trigger signal generated by the commercial power noise while the commercial power noise is obtained, and the pseudo signal having the cycle of the commercial power noise that has been obtained up to that point during the period when the commercial power noise is not obtained. It is designed to generate noise.

In the trigger signal generator 11, the commercial power noise detected by the commercial power noise detector 8 is supplied to the absolute value circuit 11a, where it is converted into an absolute value as shown in FIG. 5 (a). The absolute-valued signal is the comparator 11b.
5B, the pulse signal shown in FIG. 5B is generated when it becomes equal to or less than the threshold value shown by the dotted line in FIG. 5A, and the pulse signal is supplied to the timing signal generation unit 9 as a trigger signal via the OR circuit 11c. ..

The pulse signal shown in FIG.
The pulse signal is also supplied to the CPU 11d, and the CPU 11d monitors the pulse signal and stores the period. When the commercial power supply noise is no longer obtained and the pulse signal is no longer supplied, the pseudo pulse signal having the stored period Is to be output. As a result, the OR circuit 11c can continuously obtain the trigger signal even during the period when the commercial power noise is not obtained as shown in FIG. 5D. The CPU 11
Although d constitutes a pseudo noise generating means, the CPU 11d can also be used as the CPU 6.

This trigger signal is synchronized with the commercial power supply noise, and the timing signal generator to which the trigger signal output from the trigger signal generator 11 is supplied is synchronized with the commercial power supply noise as in the above description. Since the timing signal is generated, the circuit shown by the dotted line in FIG. 4 operates in synchronization with the commercial power supply noise as described above. That is, even if the commercial power supply noise cannot be obtained for some reason, the exciting current and the sampling signal synchronized with the commercial power supply noise are generated without any trouble.

FIG. 6 is an example of a flowchart showing the operation of the CPU 11d. After it is initialized in step 100, when it is detected in step 101 that an input pulse is supplied, the timing to fetch the next input pulse is determined in step 102. Also, step 10
When it is detected in 1 that the input pulse is no longer supplied, pseudo noise is generated in step 103, and then the process proceeds to step 102. Wait until the timing determined in step 102 is reached in step 104. When the timing determined in step 102 is reached, the process proceeds to step 101 and the above operation is repeated.

Although the above embodiments have been described on the assumption of commercial power supply noise, the same applies to the case where external noise has a frequency different from that of commercial power supply noise and has a large noise amplitude and is generated in a regular manner. Applicable. In this case, when the commercial power supply noise detector 8 is provided with a filter, the cutoff frequency of the filter must be determined in consideration of the frequency of this noise.

[0027]

As described above, according to the first aspect of the invention, since the excitation signal and the sampling signal are generated in synchronization with the external noise, the noise reduction action can be enhanced. The second aspect of the invention has an effect that a stable noise reduction action can be obtained because the pseudo signal having the noise period that has been obtained up to that point is generated during a period in which no external noise is obtained.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram showing waveforms at various points in the circuit of FIG.

FIG. 3 is a waveform diagram showing a flow rate signal of the circuit of FIG.

FIG. 4 is a block diagram showing an embodiment of the second invention.

5 is a waveform diagram showing waveforms at various points in the circuit of FIG.

6 is a flowchart illustrating the operation of the CPU in the circuit of FIG.

[Explanation of symbols]

 1 Pipeline 1a, 1b Electrode 2a, 2b Excitation coil 3a-3e Amplifier 4 Sampling circuit 5 A / D converter 6 CPU 7 D / A converter 8 Commercial power noise detector 8a Adder 9 Timing signal generator 10 Excitation circuit 11 Trigger signal generator 11a Absolute value conversion circuit 11b Comparator 11c OR circuit 11d CPU

Claims (2)

[Claims] 1. An electromagnetic flowmeter for obtaining a detection signal by sampling a voltage generated at an electrode brought into contact with a conductive fluid flowing in a magnetic field at a predetermined timing, to detect noise superimposed on the fluid from the electrode. An electromagnetic flowmeter, comprising: a noise detecting unit for generating a magnetic field based on the noise detected by the noise detecting unit; and a timing signal generating unit for generating the sampling timing of the exciting current for generating the magnetic field. 2. The pseudo noise generating means according to claim 1, further comprising a pseudo noise generating means for outputting to the timing signal generating circuit a pseudo signal having the same period as the noise signal when no noise is detected by the noise detecting means. And an electromagnetic flow meter.