JP3658975B2 - Electromagnetic flow meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electromagnetic flowmeter that measures the flow rate of various fluids having conductivity such as water flowing through a pipe.
[0002]
[Prior art]
In various plants, electromagnetic flow meters are widely used for measuring the flow rates of various fluids having conductivity such as water supplied through piping.
[0003]
The electromagnetic flowmeter can measure the flow rate of almost all conductive fluids regardless of the fluid composition such as viscosity and density, and has features such as a wide measurable flow range and negligible pressure loss. In particular, it is used for flow rate management and control in plants and water treatment facilities.
[0004]
FIG. 3 is a diagram showing a basic configuration of the electromagnetic flow meter.
The electromagnetic flowmeter is roughly divided into a detector 10 for measuring the flow rate and outputting the change as a voltage value change (hereinafter referred to as a flow rate signal), and amplifying the flow rate signal, A / D converting it, and outputting it as digital data. It is comprised by the converter 20 which carries out.
[0005]
The measurement principle of the electromagnetic flowmeter is based on Faraday's law of electromagnetic induction, and utilizes an electromagnetic phenomenon in which an electromotive force is generated in the conductor that cuts the magnetic flux. When a magnetic field is applied to the conductive fluid flowing in the measuring tube in a direction orthogonal to the flow, an electromotive force is generated in the fluid in a direction orthogonal to both the direction of the flow and the direction of the applied magnetic field. Since the value of this electromotive force is proportional to the velocity of the fluid, which is the moving speed of the conductor, the electromagnetic flow meter determines the flow rate by measuring this value. In the detector 10, an alternating magnetic field is applied to the fluid flowing in the measurement tube 11, and an electromotive force generated between a pair of electrodes 12 and 13 arranged in a direction orthogonal to the magnetic field flows in the measurement tube 11. The flow rate signal corresponding to the flow rate is output to the converter 20.
[0006]
In the converter 20, the electromotive force from the detector 10 is amplified by the differential amplifier 21.
The flow rate signal output from the detector 10 is very weak, about several tens of μV to several mV, and is greatly affected by ambient noise. Therefore, in order to remove noise due to the influence of the commercial power supply surrounding the surroundings, the detector 10 performs excitation in synchronization with the commercial power supply frequency, and the converter 20 performs signal sampling in synchronization with the commercial power supply frequency. ing.
[0007]
The flow rate signal amplified by the differential amplifier 21 is sampled by the synchronous sampling unit 22 at a timing synchronized with the period (50 Hz or 60 Hz) in order to remove noise caused by the commercial power source used in the plant. In the synchronous sampling unit 22, the signal input from the differential amplifier 21 is synchronized with the cycle of the commercial power source, and the GND value is set by the sample unit 25 when the signal value is positive and by the sample unit 26 when the signal value is negative. Sampling is performed as a reference voltage, which is amplified by the differential circuit 27 and then output. The output signal is smoothed by the smoothing circuit 23 at the subsequent stage, digitized by the A / D converter 24, and a flow rate value is calculated by a microprocessor (not shown) based on the value and output.
[0008]
FIG. 4 is a diagram showing response waveforms at various positions of the electromagnetic flow meter of FIG. 3 assuming an ideal time in which noise is not added to the flow signal. Note that symbols a to e attached to each signal in FIG. 4 indicate signals at the same symbol positions shown in FIG.
[0009]
In FIG. 4, the flow rate signal a1 is obtained by amplifying the output from the detector 10 by the amplifier 21, and the period T of the alternating current applied to the exciting coil (the period of the commercial power source used in the plant or the frequency division thereof). Output as a square wave with a voltage value compared to the flow rate value.
[0010]
4 is supplied to the sample unit 25 that samples a signal when the quantity flow signal a1 is positive, and the sample signal c1 is supplied to the sample unit 26 that samples a signal when negative. The sample units 25 and 26 output the input flow rate signal based on the GND value (0 V) when the sample signal is ON, that is, output as it is, and output 0 V when it is OFF. The sample signals b1 and c1 applied to the sample sections 25 and 26 are square pulse signals having a period T. The sample section 25 samples when the flow rate signal a1 is positive, and the sample section 26 samples when the flow signal a1 is negative. Turned ON. Thus, the signal sampled and input by the sample signals b1 and c1 is output from the differential circuit 27 as a square wave having a constant amplitude value such as d1, and when this is smoothed by the smoothing unit 23. A flat direct current component such as e1 is obtained, and an accurate flow rate value can be obtained by using data obtained by A / D conversion.
[0011]
[Problems to be solved by the invention]
Such an electromagnetic flow type has the following problems.
In the electromagnetic flow meter, as a means for generating a magnetic field to be applied to the fluid, a pair of exciting coils (not shown) arranged to face the outer peripheral portion of the measuring tube 10 are excited with an alternating current. When this is done, an electrochemical reaction is continuously generated between the fluid and the electrode interface due to a weak current flowing in the fluid, and the change of the interface state, weak electromotive force, or electrical resistance of the interface is caused by this reaction. Variations occur. None of these changes over time or fluctuates depending on the flow velocity and is not constant, and is therefore observed as noise with respect to the flow signal.
[0012]
The electrochemical noise resulting from the ionic nature of the fluid exhibits a characteristic of a magnitude of 1 / f, where f is the frequency. Therefore, it becomes larger as it moves to the low frequency region, and a large DC offset voltage is given. This fluid noise is known to cause saturation inside the converter circuit, and to cause fluctuations in the measured flow rate value.
[0013]
FIG. 5 shows an example of a response waveform in the converter circuit when fluid noise including electrochemical noise is taken into consideration.
In FIG. 5, the flow signal a2 is obtained by amplifying the output from the detector 10 by the amplifier 21, and corresponds to the flow signal a1 in FIG. This flow rate signal a2 carries noise having a longer wavelength (lower frequency) than the period T. Therefore, the direct current component of the flow rate signal from the detector 10 fluctuates greatly at a low frequency, and the output signal d2 of the differential circuit 27 obtained by sampling the flow rate signal is convoluted with the low frequency component waveform in the same manner as the flow rate signal a2. It becomes a rare waveform. Therefore, the output e2 of the smoothing unit 23, which is the subsequent circuit, directly reflects the swell due to the low frequency component fluid noise as shown in FIG.
[0014]
Such swell is caused by the output saturation of the differential circuit 27 and the variation in the measured value of the flow rate as seen in the waveforms of the output d2 of the differential circuit 27 and the output e2 of the smoothing unit 23 in FIG. Thus, accurate flow measurement is difficult.
[0015]
It is difficult to completely eliminate the influence of this fluid noise even if the amplifying unit 21 in FIG. 3 is configured as an HPF (high pass filter) to attenuate the low frequency component of the flow signal and amplify the high frequency component. .
[0016]
As another countermeasure method, a method of securing a dynamic range by increasing the power supply voltage of the amplifying unit 20 or a method of increasing the time constant of the smoothing unit 23 is conceivable. There is no contribution to the expansion of the dynamic range in actual use without noise, and when the time constant of the smoothing circuit 23 is increased, problems such as a delay in circuit response occur.
[0017]
It is an object of the present invention to provide an electromagnetic flow meter that solves the above problems.
[0018]
[Means for Solving the Problems]
The electromagnetic flow meter according to the present invention includes a detection means, a sampling means, and a noise separation means.
[0019]
The detection means generates an alternating magnetic field in the measurement tube and outputs a change in potential difference between at least one pair of electrodes provided in the measurement tube as a flow signal.
The noise separation unit extracts a component having a frequency equal to or lower than a certain frequency of the flow rate signal as a low frequency component.
[0020]
The sampling means samples the flow rate signal using the low frequency component extracted by the noise sorting means as a reference voltage.
The extraction of the low-frequency component of the flow rate signal by the noise separation unit is performed by, for example, extracting a frequency component that is 1/2 to 1/10 or less of the sampling frequency at which the sampling unit performs sampling.
[0021]
The extraction of the low frequency component of the flow rate signal by the noise classification means is performed by extracting the component of the flow rate signal having a frequency equal to or lower than the frequency of the fluid noise due to the fluid as the low frequency component.
[0022]
Moreover, this invention includes the converter used for not only an electromagnetic flowmeter but an electromagnetic flowmeter.
According to the present invention, since the low frequency component in the flow signal is extracted by the noise separation means and sampled by the sampling means using the low frequency component as a reference voltage, even if the flow signal contains low frequency noise. Sampling can be performed while following the movement of the noise.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a diagram showing a basic configuration of an electromagnetic flow meter in the present embodiment.
The electromagnetic flow meter shown in FIG. 1 includes a detector 30 and a converter 40. Among these, the detector 30 is basically the same as the detector 10 of FIG. 3, and an alternating magnetic field having the same frequency as that of the commercial power supply is applied to the fluid flowing in the measuring tube 31 and provided on the inner wall of the measuring tube 31. The change in the voltage generated between the electrodes 32 and 33 is output as a flow rate signal representing the change in the flow rate of the fluid.
[0024]
The converter 40 corresponds to the converter 20 of FIG. 3, and amplifies a weak signal of about several tens of μV to several mV output from the detector 30, and then generates noise caused by a commercial power source used in the plant. In order to eliminate, depending on the frequency of the commercial power supply or the installation environment of the flowmeter, sampling is performed in synchronization with the frequency divided from the commercial power supply frequency, and then smoothing and A / D conversion are performed. Based on the value, a flow rate value is calculated and output by a microprocessor (not shown).
[0025]
In the converter 40, the weak flow signal output from the detector 30 is first amplified by the amplifying unit 41. The amplifying unit 41 is an HPF, which attenuates the low frequency component of the flow signal and amplifies and outputs the high frequency component. This output signal is input to the sampling unit 42 and sampled, and also input to the noise sorting unit 50.
[0026]
The electrodes of the detector 30 are not necessarily a pair, and there may be a plurality of pairs. In this case, the converter 40 has an amplifying unit 41 for each pair of electrodes, and inputs the output of these outputs by an adder to the sampling unit 40 and the noise sorting unit 50 as an output signal. .
[0027]
The noise separation unit 50 is an LPF (low-pass filter) circuit including an operational amplifier 51, resistors R52 and 53, and a capacitor C54.
The fluid noise included in the flow rate signal has a frequency sufficiently lower than that of a commercial power source that is synchronized with the electromagnetic flow meter around 6 Hz. Therefore, when the flow rate signal is passed through the noise classification unit 50 that is an LPF, only the low-frequency fluid noise contained therein can be extracted.
[0028]
The noise classification unit 50, which is an LPF, reduces the signal level of the flow rate signal when the cutoff frequency of the filter is close to the frequency of the flow rate signal. Further, since the fluid noise has a characteristic of a strength of 1 / f with respect to the frequency f, the influence becomes larger as the frequency becomes lower. Therefore, the time constant of the filter determined by the values of the resistor R53 and the capacitor C54 does not need to be excessively large and is a value that is somewhat larger than the frequency of the flow signal, that is, about 2 to 10 times the time constant of the flow signal. As a result, the frequency component extracted by the noise separation unit 50 is one that is 1/2 to 1/10 or less of the frequency of the flow signal, that is, the frequency of the commercial power source used in the plant or the sampling frequency in the synchronous sampling unit 42 described later. It becomes.
[0029]
Note that the circuit configuration of the noise classification unit 50 in FIG. 1 is an example, and the configuration of the noise classification unit 50 in the present embodiment is not limited to this, and is an LPF that can extract the components in the frequency range. Other configurations are possible as long as they are present.
[0030]
The low frequency component of the flow rate signal extracted by the noise classification unit 50 is input to the synchronous sample unit 42.
In order to remove noise due to the commercial power source used in the plant, the synchronous sample unit 42 converts the flow rate signal output from the detection unit 30 to 1 / of the frequency of the commercial current or the installation environment of the electromagnetic flow meter. The flow rate signal is sampled in synchronization with the frequency divided by N (N = 2, 3,...).
[0031]
The synchronous sampling unit 42 includes sample units 45 and 46 that switch outputs in synchronization with a sample signal, and a differential circuit 47 that amplifies and outputs a difference between outputs of the sample units 45 and 46.
[0032]
Among these, the sample units 45 and 46 are switches for switching the output in synchronization with the input sample signal, and output an output signal based on the input signal when the sample signal is ON, and 0 V when it is OFF. The sample units 45 and 46 are supplied with the flow rate signal from the amplification unit 41 and the low frequency component of the flow rate signal extracted by the noise separation unit 50 as input signals. When the sample signal is ON, the low frequency component is used as a reference. A flow signal with voltage, that is, a signal obtained by subtracting a low frequency component from the flow signal is output as an output signal.
[0033]
The differential circuit 47 differentially amplifies the input signal. Of the two inputs, the difference between the signal input on the + side and the signal input on the − side, that is, the signal on the + side is on the − side. Invert the signal and output the superimposed signal.
[0034]
When a sampling signal with a half-cycle phase shift is applied to each of the sampling signals so that sampling is performed by the sampling unit 45 when the flow rate signal is positive and sampling by the sampling unit 46 when the flow rate signal is negative, the synchronous sampling unit 42 A sampled signal is output from the low frequency component of the flow rate signal.
[0035]
In this manner, the synchronous sampling unit 42 performs synchronous sampling while following the undulation due to the low frequency fluid noise. The output signal from the synchronous sample unit 42 is smoothed by a subsequent smoothing circuit 48 and then converted into a digital value by an A / D conversion unit 49. Based on this value, a flow rate value is calculated by a microprocessor (not shown). And output.
[0036]
As described above, in the electromagnetic flow meter of the present embodiment, only the low frequency component due to fluid noise or the like is extracted and used as a reference value for synchronous sampling, so that the undulation due to fluid noise becomes the in-phase component and the differential circuit 47. Input. Therefore, a large DC component due to fluid noise is removed from the output of the differential circuit 47. As a result, the output of the differential circuit 47 is not saturated by a large DC component, and the output value of the smoothing unit at the next stage is also stabilized.
[0037]
FIG. 2 is a diagram showing response waveforms at various positions of the electromagnetic flow meter of FIG. 1 with respect to a flow signal containing fluid noise. Note that symbols a to h attached to each signal in FIG. 2 indicate signals at the same symbol positions shown in FIG.
[0038]
In FIG. 2, the flow rate signal a3 is obtained by attenuating the low frequency component of the output from the detector 30 by the amplifying unit 41 and amplifying the high frequency component, but as shown in FIG. Even after the passage, the influence of the fluid noise cannot be completely removed, and the flow rate signal a3 swells at a period (lower frequency) larger than the period T of the commercial power source with which the electromagnetic flowmeter is synchronized.
[0039]
2 is supplied to the sample unit 45 that performs sampling when the quantity flow signal a3 is positive, and the sample signal c3 is supplied to the sample unit 46 that samples to a signal when negative. These sample signals b3 and c3 are square pulse signals having a period T, and the sample section 45 is shifted when the flow signal a3 becomes positive, and the sample section 46 performs sampling when the flow signal a3 becomes negative. Become.
[0040]
f is an output of the noise classification unit 50, and only low frequency components such as fluid noise are extracted from the flow rate signal a3 input from the amplification unit 41 and output.
In the sample units 45 and 46, the output f of the noise sorting unit 50 is used as a reference voltage for sampling, and when the sample signals b3 and c3 are ON, the signal f is used as a reference value and the flow rate signal a3 is sampled. That is, a signal obtained by subtracting the output f from the noise sorting unit 50 from the flow rate signal a3 is output, and 0 V is output when the sample signals b3 and c3 are OFF.
[0041]
g and h are input signals of the differential circuit 47, the signal g is a positive input signal of the differential circuit 47 (output signal of the sample unit 45), and a signal h is a negative input signal of the differential circuit 47 (sample) Output signal of the unit 46). As can be seen from the signals g and h, since sampling is performed using the signal f as a reference value in the sample units 45 and 46, low-frequency undulations due to fluid noise are removed.
[0042]
An output d3 of the differential circuit 47 is obtained by performing differential amplification by the differential circuit 47 as the input signals g and h. The output d3 is obtained as a square wave having a constant amplitude value as shown in FIG. 2 and can be easily smoothed by the smoothing unit 48.
[0043]
e3 indicates an output signal of the smoothing unit 48 smoothed by the smoothing unit 48. In the electromagnetic flow meter of this embodiment, even if the flow signal contains a low frequency component due to fluid noise, a flat direct current component from which a large direct current component due to fluid noise is removed, such as e3 in FIG. 2, is obtained. By using the data digitized by the A / D converter 49 and calculating the flow value by a microprocessor (not shown), an accurate flow value can be obtained.
[0044]
【The invention's effect】
According to the present invention, even when fluid noise is included in the flow rate signal, sampling is performed while following the movement of the low-frequency fluid noise, so that the influence of fluid noise can be eliminated and the accuracy is improved. This makes it possible to measure a good flow rate.
[0045]
In addition, since a large DC component due to fluid noise is eliminated, the converter is not saturated, and its output value is stabilized.
[Brief description of the drawings]
FIG. 1 is a diagram showing a basic configuration of an electromagnetic flow meter in the present embodiment.
FIG. 2 is a diagram showing each response waveform with respect to a flow rate signal including fluid noise at each position in the electromagnetic flow meter according to the present embodiment.
FIG. 3 is a diagram showing a basic configuration of an electromagnetic flow meter.
FIG. 4 is a diagram showing a response waveform in the electromagnetic flow meter when an ideal time when no noise is added to the flow signal is assumed.
FIG. 5 is a diagram showing an example of a signal waveform in the electromagnetic flow meter when fluid noise is applied.
[Explanation of symbols]
10, 30 Detector 11, 31 Measuring tube 12, 13, 32, 33 Electrode 20, 40 Conversion unit 21, 41 Amplifier 22, 42 Synchronous sample unit 23, 48 Smoothing unit 24, 49 A / D conversion unit 25, 26 , 45, 46 Sample section 27, 47 Differential circuit 50 Noise classification section

Claims (4)

Detecting means for generating an alternating magnetic field in the measurement tube and outputting a change in potential difference between at least one pair of electrodes provided in the measurement tube as a flow signal;
Noise separation means for extracting a component of the flow rate signal below a certain frequency as a low frequency component;
A sampling means for sampling the flow rate signal using the low frequency component as a reference voltage. 2. The noise separation unit extracts a frequency component of the flow rate signal that is 1/2 to 1/10 or less of a sampling frequency at which the sampling unit performs sampling as the low frequency component. Electromagnetic flow meter. 2. The electromagnetic flowmeter according to claim 1, wherein the noise separating unit extracts a component having a frequency equal to or lower than a frequency of fluid noise caused by the fluid from the flow signal as the low frequency component. A transducer used in an electromagnetic flow meter for sampling an input flow signal,
Noise separation means for extracting a component of the flow rate signal below a certain frequency as a low frequency component;
A converter for an electromagnetic flowmeter, comprising: sampling means for sampling the flow rate signal using the low frequency component as a reference voltage.

Images