JP4228765B2 - Electromagnetic flow meter - Google Patents

Description

BACKGROUND OF THE INVENTION
The present invention relates to an electromagnetic flowmeter that can use a core of a semi-hard magnetic material and can perform high-accuracy measurement with a simple configuration.
[0001]
[Prior art]
Patent Document 1 discloses an invention of an electromagnetic flow meter that can compensate for an error caused by fluctuations in excitation current and perform highly accurate measurement. The outline of the present invention will be described below with reference to FIG.
[0002]
In FIG. 5, the electromotive force generated in the electrodes 2 a and 2 b installed in the conduit 81 is amplified by the differential amplifier 82 and input to the A / D converter 83. The output of the A / D converter 83 is input to the flow rate calculation circuit 88. On the other hand, an excitation current If whose polarity is alternately changed is supplied from the excitation circuit 85 to the excitation coil 84.
[0003]
The magnitude of this exciting current is converted to a voltage Vf by the resistor Rf. This Vf is held by the sample hold circuit 86, and the hold voltage Ve is input to the A / D converter 83. 87 generates timing pulses P <b> 1 to P <b> 4 to be supplied to the excitation circuit 85, the sample hold 86, and the A / D converter 83.
[0004]
The magnetic flux density generated by the exciting coil 84 is proportional to the exciting current when a soft magnetic material is used for the core. Moreover, the electromotive force generated in the electrodes 2a and 2b is proportional to the magnetic flux density. Therefore, a double integral type A / D converter is used as the A / D converter 83, and Ve representing the magnitude of the excitation current is used as the reference voltage of the A / D converter 83, thereby providing a highly accurate electromagnetic flow rate. The total can be realized.
[0005]
In order to reduce the power consumption of the electromagnetic flow meter, the excitation current may be supplied intermittently. Patent Document 2 describes an invention of an electromagnetic flowmeter that intermittently supplies an exciting current using ordinary steel having a certain level of holding power as a core. Further, in Patent Document 3, in an electromagnetic flowmeter that intermittently supplies an excitation current, the gain of an amplifier is automatically changed according to the magnitude of the electromotive force of an electrode, so that high-precision measurement can be performed even at a small flow rate. An invention of an electromagnetic flow meter is described.
[Patent Document 1]
JP 2001-235352 A [Patent Document 2]
Japanese Patent Publication No.59-7930 [Patent Document 3]
Japanese Examined Patent Publication No. 3-81087 [0006]
[Problems to be solved by the invention]
However, such an electromagnetic flow meter has the following problems.
[0007]
Since the invention described in Patent Document 1 can automatically compensate for the influence even when the excitation current changes, high-accuracy measurement becomes possible. However, when a semi-hard magnetic material having a high holding force is used for the core, there is no proportional relationship between the exciting current and the generated magnetic flux density. Therefore, this method has a problem that it cannot be applied to an electromagnetic flow meter that uses a semi-hard magnetic material as a core and intermittently supplies an excitation current.
[0008]
The invention described in Patent Document 3 has a complicated configuration because the gain of the amplifier must be changed, and the clock frequency must be increased to increase the resolution, resulting in an increase in power consumption. There was a problem of doing it.
[0009]
Accordingly, an object of the present invention is to provide an electromagnetic flowmeter that can use a semi-hard magnetic material having a high holding force for the core and can perform highly accurate measurement even with a small flow rate with a simple configuration.
[0010]
[Means for Solving the Problems]
In order to solve such a problem, the invention according to claim 1 of the present invention is such that an exciting current is intermittently applied to an exciting coil having a core using a semi-hard magnetic material having a coercive force of a predetermined value or more. An electromagnetic flow meter that measures the flow rate of the fluid based on a flow rate signal induced due to the magnetic flux generated by the excitation coil and the flow of the fluid, and detects an excitation current flowing through the excitation coil A current detection unit; and an excitation signal generation unit that outputs a signal for controlling the excitation current when the output of the current detection unit is input. The excitation signal generation unit is configured to output the excitation current detected by the current detection unit. When the value exceeds a predetermined value, the exciting current is stopped, the flow rate is measured with a constant magnetic flux held in the core at this time, and the flow rate signal is converted into a digital value. Double product An analog-to-digital converter of the system, a comparator that compares the output of the integrator in the analog-to-digital converter with a predetermined voltage, and the output of the integrator is equal to or higher than the predetermined voltage When the comparison unit detects, the integration of the signal related to the flow rate signal is stopped and the reverse integration is started, so that the integration period of the integrator is longer when the flow rate is lower and shorter when the flow rate is higher, The predetermined voltage is determined from the count number at which the count error of the count number of the reverse integration time is not more than a desired value. It can always be measured with a constant magnetic flux density. Digital output can be obtained, cost reduction can be achieved, and a highly accurate integrated value can be obtained at a small flow rate.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings.
FIG. 1 is a block diagram showing an embodiment of an electromagnetic flow meter according to the present invention. In FIG. 1, reference numeral 1 denotes a measuring unit that outputs a signal proportional to the flow velocity of the fluid to be measured. Reference numeral 13 denotes a measuring tube through which a fluid to be measured flows. A core 11 made of a semi-hard magnetic material and an exciting coil 12 wound around the core 11 are installed outside the measuring tube 13. Reference numeral 14 denotes a measurement electrode that is installed facing the fluid and contacts the fluid to be measured, and an electromotive force proportional to the flow velocity of the fluid to be measured is generated at the measurement electrode 14. Although a normal feedback magnetic path is mounted, it is omitted in this figure.
[0024]
A buffer 15 impedance-converts the electromotive force of the measurement electrode 14. Reference numeral 6 denotes a differential amplifier, which calculates the difference in electromotive force between the two measurement electrodes 14. Reference numeral 17 denotes an inverting amplifier that outputs a signal obtained by inverting the polarity of the output of the differential amplifier 16.
[0025]
An excitation circuit 2 includes four switches Q1 to Q4 that are bridge-connected to the power sources E1 and E2 and a resistor 21 that is connected in series to the excitation coil 12. The resistor 21 constitutes a current detection unit. The switches Q1 to Q4 are controlled to be turned on / off by signals T1 to T4. Therefore, excitation currents having opposite polarities alternately flow through the two excitation coils 12. This exciting current flows through the resistor 21. Therefore, the voltage Viex across the resistor 21 is proportional to the exciting current.
[0026]
Reference numeral 3 denotes a double integration type A / D converter, an analog switch 32 for selecting the outputs of the differential amplifier 16 and the inverting amplifier 17 and the output of the integrator 33, and an analog switch 31 for selecting the reference voltages + Vref and -Vref. , An integrator 33 for integrating the outputs of the analog switches 31 and 32 and a comparator 34 to which the output of the integrator 33 is inputted.
[0027]
AD conversion is performed as follows. First, the analog switch 32 inputs the output of the measuring unit 1 to the integrator 33 and integrates it for a time T1. Then, a reference voltage Vref having a polarity opposite to the voltage integrated by the analog switch 31 is selected, and the integration is performed until the comparator 34 is inverted.
[0028]
When the reverse integration time is T2, the output Vin of the differential amplifier 16 or the inverting amplifier 17 which is the output of the measurement unit 1 is Vin = (T2 / T1) × Vref (1)
Can be obtained. T1 and T2 are measured by the CPU 4 by counting clocks having a period sufficiently shorter than these times.
[0029]
An excitation signal generator 5 generates signals for controlling the switches Q1 to Q4 and the analog switches 31 and 32. A buffer 51 receives a voltage Viex proportional to the excitation current. An inverting amplifier 52 inverts the output polarity of the buffer 51. An analog switch 53 selects the output of the buffer 51 and the inverting amplifier 52. The analog switch 53 functions as a selection unit. Reference numeral 54 denotes a reference power supply, which divides the reference power supply Vref and outputs Vi_ref.
[0030]
A comparator 55 compares the output of the analog switch 53 with the reference voltage Vi_ref and outputs the result. 56 is a D-type flip-flop, and the output of the comparator 55 is input to its clock terminal. The inverted output is input to the D terminal.
[0031]
A timing generation circuit 57 generates a timing signal from T5 to T9. Timing signals T5 and T6 drive the analog switches 31 and 32. The timing signal T7 is input to the reset terminal of the flip-flop 56. 58 is an AND gate, to which the inverted output of the flip-flop 56 and the timing signal T9 are input, and the output is output to the excitation circuit 2 as the signals T2 and T4 via the level conversion circuit 581. Reference numeral 59 denotes an AND gate, which receives the inverted output of the flip-flop 56 and the timing signal T8, and outputs the signals to the excitation circuit 2 as signals T1 and T3 via the level conversion circuit 591.
[0032]
Reference voltage circuit 6 generates reference voltages + Vref and −Vref to be supplied to the A / D converter 3. For this purpose, the power supplies E1 and E2 are stabilized by Zener diodes 61 and 62 to generate reference voltages + Vref and -Vref.
[0033]
Next, the operation of this embodiment will be described with reference to FIG. In FIG. 2, (a) is the waveform of T8. T8 is a waveform having a predetermined cycle and a duty ratio. In this high level section, the switches Q1 and Q3 are turned on, and an exciting current is supplied to the exciting coil 12. (A) is a waveform of T9, which has the same cycle and duty ratio as T8, but has a different phase. The switches Q2 and Q4 are turned on in the high level period of T9, and an exciting current is passed in the direction opposite to (a). Since a semi-hard magnetic material is used for the core 11, a period during which no exciting current flows is provided.
[0034]
(C) is a waveform of Vex. As described above, Viex represents an exciting current. Excitation current flows only during a period when T8 or T9 is at a high level, and the flow direction is reversed between T8 and T9. (D) is an output waveform of the analog switch 53. Since the analog switch 53 switches and outputs the output of the buffer 51 and the inverting amplifier 52, the output becomes positive even if the exciting current flows in the reverse direction. The exciting current gradually increases due to the inductance of the exciting coil 12.
[0035]
(E) is the output of the comparator 55. This output goes high when Viex is greater than Vi_ref. The peak value Iex of the excitation current when Viex matches Vi_ref is
Iex = Vi_ref / Ref
become. However, Ref is the resistance value of the resistor 21.
[0036]
(F) is a reset signal of the flip-flop 56. This signal becomes high level for a predetermined period when no exciting current flows, and the flip-flop 56 is reset. (K) and (K) are the output and inverted output of the flip-flop 56, respectively. Since the flip-flop 56 is set at the rising edge of the comparator 55 and reset at the high level of T7, the inverted output of the flip-flop 56 becomes low level only during this period.
[0037]
(K) is the output of the AND gate 58. The output of the AND gate 58 is at a high level only when both T8 and the inverted output of the flip-flop 56 are at a high level. As described above, the switches Q1 and Q3 are turned on during this high level period, and an exciting current flows. (K) is the output of the AND gate 59, and becomes high only when both T9 and the inverted output of the flip-flop 56 are high. During this high level period, the switches Q2 and Q4 are turned on, and an exciting current flows in the reverse direction.
[0038]
(S) is the waveform of the flow signal. Since a semi-hard magnetic material is used for the core, the magnetic flux is maintained even during a period in which no excitation current flows, and a stable flow signal is obtained. (S), (S), (C), and (S) represent the output of the integrator 33, the states of the analog switches 32 and 31, and the output of the comparator 34, respectively. Since this part is the same as the operation of a general double integration type AD converter, a description thereof will be omitted. Integration is performed at the output of the buffer 16 or the inverting amplifier 17 during a period in which the flow rate signal (s) is stable, and then back integration is performed at + Vref or -Vref.
[0039]
(Ta) represents a period for inverse integration. As can be seen from the equation (1), the digital value of the flow rate signal is proportional to the inverse integration period T2, so this inverse integration period is substantially the period of analog-digital conversion of the flow rate signal.
[0040]
Thus, in this embodiment, the exciting current is measured by the resistor 21, and the exciting current is cut off when the exciting current reaches a predetermined constant value. Since a semi-hard magnetic material is used for the core 11, the magnetic flux density is maintained at this constant value. Therefore, the flow rate can always be measured with the same magnetic flux density and is not affected by the nonlinearity of the core.
[0041]
In this embodiment, the exciting current is detected by the resistor 21, and when the exciting current reaches a predetermined value, the exciting current is cut off so that the magnetic flux density is always constant. The voltage of the power supply that supplies the power may be stabilized. Since the impedance of the exciting coil 12 is constant, the exciting current is constant even in this way, and as a result, the magnetic flux density can be made constant.
[0042]
FIG. 3 shows another embodiment of the present invention. In addition, the same code | symbol is attached | subjected to the same element as FIG. 1, and description is abbreviate | omitted. In FIG. 3, reference numeral 7 denotes a comparison circuit, which includes a comparator 71. The comparator 71 receives the output of the integrator 33 and Vad generated by dividing + Vref, and outputs a comparison result of these inputs to the CPU 4.
[0043]
In an integrated flow meter such as a water meter, the accuracy of the integrated value is important, and the accuracy of the instantaneous flow rate may be lower than that. Therefore, during the integration period, the comparator 71 compares the output of the integrator with a certain reference voltage, and if they match, the output is raised to a high level and notified to the CPU 4. With this output, the CPU 4 ends the integration period and starts the inverse integration period.
[0044]
In an ordinary double integration type A / D converter, the integration period is constant. Therefore, when the input voltage decreases, the output of the integrator also decreases and the conversion error increases. However, in this embodiment, when the flow rate is reduced and the electromotive force of the electrode 14 is reduced, the integration period by the integrator 33 is automatically lengthened, and when the flow rate is increased, the integration period is controlled to be shortened. Since the output is constant, the error does not increase even at a small flow rate.
[0045]
Although the responsiveness deteriorates as the integration period becomes longer, the instantaneous flow rate value is not so important in the integrating flow meter, so there is no practical problem. Further, since the amplifier gain is not variable as in the invention described in Patent Document 3, the configuration is simplified.
[0046]
Next, the operation of this embodiment will be described with reference to FIG. Since (a) to (su) are the same as those in FIG. In the embodiment of FIG. 2, the inverse integration is started at time t1, but in this embodiment, the output of the integrator 33 has not reached a constant value as shown in (c), so the inverse integration is not started and the integration is not performed. Continued. Since the output of the integrator 33 exceeds a certain value at time t2, reverse integration is started.
[0047]
That is, the integration period is doubled compared to FIG. As the flow rate is further reduced, the integration period is increased accordingly. Therefore, the responsiveness is deteriorated, but the error of the integrated value does not increase even at a small flow rate.
[0048]
For the clock cycle T3 for measuring the reverse integration time, the count number of T2 / T3 is set so that a predetermined resolution is obtained. In time measurement, a count error of 1 to 2 clocks inevitably occurs. Therefore, if the count number is N, a count error of 2 / N × 100 (%) occurs. In consideration of this, the count number N is set so that the error is not more than a desired value, and Vad is determined so that the count number N is obtained.
[0049]
【The invention's effect】
As is clear from the above description, the following effects can be expected according to the present invention.
According to the first aspect of the present invention, in the electromagnetic flowmeter that measures the flow rate by generating a magnetic field by passing a current through an exciting coil having a semi-rigid material core, the value of the current flowing through the exciting coil becomes a predetermined value. Then, the current was stopped.
[0050]
Since the flow rate can be measured in a state where the magnetic flux density is constant, there is no error caused by fluctuations in the excitation current. This eliminates the need for correction for fluctuations in the excitation current, and has the effect of allowing highly accurate flow measurement with a simple configuration. In addition, since the flow rate is measured using magnetism held in the core with a constant excitation current, even if a semi-hard magnetic material core having no proportional relationship between the excitation current and the generated magnetic flux density is used, There is also an effect that no error occurs. Further, since the configuration is simplified, there is an effect that the cost can be reduced.
[0051]
The flow rate value was converted into a digital value using an analog-digital converter, and a double integral type analog-digital converter was used as the analog-digital converter. Furthermore, the integration time is changed according to the magnitude of the flow rate signal, integration is performed until the output of the integrator in the analog-digital converter becomes a predetermined value, and reverse integration is started when the output exceeds the predetermined value. did.
[0052]
Since digital output is obtained, there is an effect that data processing is simplified. In addition, there is an effect that cost can be reduced by using a double integration type analog-digital converter. In addition, the integrated flow rate can be measured with high accuracy by extending the integration time with a small flow rate. By monitoring the output of the integrator and starting the reverse integration, it is optimal regardless of the flow rate. There is also an effect that digital conversion can be performed in the state.
In addition, there is an effect that the count error of the count number of the reverse integration time can be reduced to a desired value or less.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an embodiment of the present invention.
FIG. 2 is a time chart showing the operation of an embodiment of the present invention.
FIG. 3 is a configuration diagram of another embodiment of the present invention.
FIG. 4 is a time chart showing the operation of another embodiment of the present invention.
FIG. 5 is a configuration diagram of a conventional electromagnetic flow meter.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Measurement part 11 Core 12 Excitation coil 14 Electrode 2 Excitation circuit 21 Resistance 3 A / D converter 31, 32, 53 Analog switch 33 Integrator 34, 55, 71 Comparator 54 Reference power supply 56 Flip-flop 57 Timing circuit 58, 59 AND gate 6 Reference voltage circuit 7 Comparison circuit

Claims (1)

A flow rate that is induced due to a magnetic flux generated by the exciting coil and a fluid flow by intermittently passing an exciting current through the exciting coil having a core using a semi-hard magnetic material having a coercive force of a predetermined value or more. In an electromagnetic flow meter for measuring the flow rate of the fluid based on a signal,
A current detection unit for detecting an excitation current flowing in the excitation coil; and an excitation signal generation unit for outputting a signal for controlling the excitation current when an output of the current detection unit is input. When the value of the excitation current detected by the current detector exceeds a predetermined value, the excitation current is stopped, and the flow rate is measured with a constant magnetic flux held in the core at this time. And
A double integral type analog-digital converter for converting the flow rate signal into a digital value;
A comparator that compares the output of the integrator in the analog-digital converter with a predetermined voltage, and the flow rate when the comparator detects that the output of the integrator is equal to or higher than the predetermined voltage; By stopping the integration of the signal associated with the signal and starting the reverse integration, the integration period of the integrator is longer when the flow rate is lower and shorter when it is higher,
The predetermined voltage is determined from the count number at which the count error of the count number of the reverse integration time is not more than a desired value.
An electromagnetic flow meter characterized by that.

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