JP5224886B2 - Electromagnetic flow meter - Google Patents

Description

  The present invention relates to an electromagnetic flow meter that is excited at a frequency of 10 Hz or less, and more particularly, to an electromagnetic flow meter that removes fluid noise contained in a detection signal.

  An electromagnetic flow meter excited at an excitation frequency of 10H or less is affected by low-frequency noise (also referred to as fluid noise, hereinafter referred to as fluid noise) due to electrochemical action, and various improvements have been made. ing.

  For example, in the case of an electromagnetic flowmeter that measures the flow rate of a fluid using a DC magnetic field whose steady-state value changes periodically, the output voltage of the amplifier is integrated by an integration circuit, and the output voltage of the integration circuit is reversed to the input side of the amplifier. There has been disclosed a technique of providing a compensation circuit that integrates a direct-current voltage component superimposed on a signal voltage and makes this direct-current component zero by performing feedback so as to have polarity (see Patent Document 1).

  Further, in this integration circuit, the compensation voltage is tailed by spike noise (hereinafter referred to as differential noise) that occurs due to switching of the steady-state value of the DC magnetic field, and the waveform of this differential noise indicates the temperature of the fluid to be measured. Since it varies depending on the conductivity and the period including the time when the differential noise changes, a technique is disclosed in which the influence of the differential noise is removed and the zero point is stabilized by opening the input of this integration circuit (patent) Reference 2).

  However, in the case of compensation by this integrating circuit, there is a problem that the flow rate signal cannot be accurately detected due to waveform distortion and phase delay in the detection signal waveform due to this low frequency filter characteristic.

  For such a problem, a delay means for outputting the flow rate signal with a half-cycle delay from the square wave excitation signal, and a flow rate output from the electromagnetic flowmeter detector by the flow rate signal delayed by the delay means. There is an electromagnetic flow meter including an error removing unit that removes a low-frequency error component included in a signal (see Patent Document 3).

  According to this technology, the flow rate signal is delayed by a half cycle of the excitation signal, and the error contained in the flow rate signal from the electromagnetic flowmeter detector is removed by this delayed flow rate signal, so fluid noise is excessively generated. However, this noise can be surely removed.

Further, according to the noise removing means, in addition to the fluid noise, noise having a constant rate of change during one period of the excitation signal can be completely removed, and white noise that cannot be removed by the noise removing circuit It can be removed by sampling and integrating circuit just before rising and falling.
Japanese Patent No. 2514960 Japanese Patent No. 2583284 Japanese Patent Publication No. 4-77853

  However, in the case of an error removing unit that removes a low-frequency error component included in the flow rate signal by the flow rate signal delayed by the delay unit disclosed in Patent Document 3, a sample hold circuit is used for processing by an analog circuit. There is a problem that noise is unavoidably mixed into the flow rate signal due to the difference in the constants of the semiconductor switch and the integrating capacitor.

  Also, if the frequency component of fluid noise changes at a frequency that cannot be regarded as a DC component with respect to the excitation frequency, that is, if the low-frequency noise component changes within this delay time, the flow rate signal changes. Therefore, there is a problem that the measurement error due to the fluid noise component included in the flow rate signal cannot be removed.

  By the way, this fluid noise shows different levels depending on fluid components, electrode materials, surface conditions, and the like, and the level is particularly high in slurry liquid.

  If the noise level is less than the allowable value of the signal processing circuit, it is removed by the operation in the converter and does not degrade the measurement accuracy. However, if the noise level exceeds the allowable value, the signal is saturated in the signal processing circuit. As a result, there is a problem that abnormal operation is caused and the measurement accuracy is greatly lowered.

  In such a case, a negative voltage bias is applied to the flow rate signal to expand the operating range of the signal processing circuit, but a technique for increasing or decreasing the flow rate signal by adding a bias is also disclosed. There is a problem that the resolution of the flow rate signal is lowered.

  Furthermore, in combination with the removal of fluid noise as described above, there is a problem that this abnormal state cannot be detected during flow rate measurement when an empty state occurs in which the gap between the electrodes is not satisfied in the state to be measured.

  The present invention has been made in order to solve the above-mentioned problems, improves the performance of removing differential noise and fluid noise, and is difficult to cause saturation of the signal processing circuit. An object of the present invention is to provide an electromagnetic flow meter that can determine whether there is no air.

In order to achieve the above object, an electromagnetic flow meter according to the present invention generates a square wave excitation signal and is periodic from the outside of a measurement tube through which a fluid to be measured flows in an axial direction orthogonal to the tube axis of the measurement tube. A signal detected between electrodes provided in contact with the fluid to be measured on the tube wall of the measuring tube in the tube axis direction and an axial direction orthogonal to the magnetic field direction. And a differential amplifier for amplifying a signal detected between the electrodes to obtain a detection signal, and corresponding to the inversion of the magnetic field. A clamp circuit unit that clamps the detection signal to a preset reference potential at the timing of each central position of the excitation signal, and a first square wave signal in the positive direction after clamping from the output signal of the clamp circuit unit And the second flat of the negative square wave signal A calculation unit for obtaining a difference from the obtained value and obtaining a flow rate from the obtained signal; and a difference between the excitation signal, a clamp timing signal of the clamp circuit unit, and the first average value and the second average value. An electromagnetic flow meter comprising: a control unit that generates a calculation timing signal to be obtained.

In order to achieve the above object, an electromagnetic flow meter according to the present invention generates a square wave excitation signal and is periodic from the outside of a measurement tube through which a fluid to be measured flows in an axial direction orthogonal to the tube axis of the measurement tube. A signal detected between electrodes provided in contact with the fluid to be measured on the tube wall of the measuring tube in the tube axis direction and an axial direction orthogonal to the magnetic field direction. And a differential amplifier for amplifying a signal detected between the electrodes to obtain a detection signal, and corresponding to the inversion of the magnetic field. A clamp circuit unit that clamps the detection signal to a preset reference potential at the timing of each central position of the excitation signal, and a first square wave signal in the positive direction after clamping from the output signal of the clamp circuit unit And the second flat of the negative square wave signal A calculation unit for obtaining a difference from the obtained value and obtaining a flow rate from the obtained signal; and a difference between the excitation signal, a clamp timing signal of the clamp circuit unit, and the first average value and the second average value. A control unit that generates a control signal for controlling a calculation timing signal to be obtained; and a first high resistance connected between one input terminal of the differential amplifier and a ground point, and the first input terminal connected to the other input terminal. An empty detection signal generating circuit that connects a second high resistance having the same value as the high resistance and applies a minute signal having a double period synchronized with the period of the excitation signal through the second high resistance; The calculation unit obtains a difference between an average value of one half cycle signal of the output signal of the clamp circuit unit and a signal average value of the other half cycle as an empty determination detection signal, and the empty determination detection signal is preset. The measurement tube is empty. Wherein the empty judging operation and that it has to execute a calculation for obtaining the flow rate at the same time judges that.

  It is possible to provide an electromagnetic flowmeter that improves the performance of removing differential noise and fluid noise, is less likely to cause saturation of the signal processing circuit, and can determine whether the fluid to be measured is empty during flow measurement.

  Embodiments of the present invention will be described below with reference to the drawings.

  An embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a configuration diagram of a square wave excitation type electromagnetic flow meter of the present invention. A measurement tube 1 in which the inside of the fluid to be measured is insulated, two excitation coils 2a and 2b for applying a magnetic field H in a direction orthogonal to the tube axis direction of the measurement tube 1 outside the measurement tube 1, An excitation circuit 2 for supplying an excitation current to the excitation coils 2a and 2b and an axial direction orthogonal to the tube axis direction of the measurement tube 1 and the magnetic field H are in contact with the fluid to be measured at both ends of the tube wall of the measurement tube 1. It consists of a pair of electrodes 3a and 3b provided as liquids and a differential amplifier 4 that amplifies a signal detected between the electrodes and uses it as a detection signal.

  Further, the output signal of the differential amplifier 4, that is, the detection signal V0 is clamped to a reference potential at a predetermined clamp timing synchronized with an excitation signal Sh described later in detail, and the flow rate from the output of the clamp circuit 5 From the calculation unit 6, the excitation signal Sh serving as a synchronization signal for generating the excitation current, the clamp timing signal S1g for controlling the clamp timing in the clamp circuit unit 5 in synchronization with this excitation signal, and the detection signal The control unit 7 generates a calculation timing signal Sg for controlling the calculation timing in the calculation unit 6 for obtaining the flow rate.

  Next, a detailed configuration of each unit for excluding noise included in the detection signal will be described. The clamp circuit unit 5 applies the detection signal to one inverting input, and the output signal of the amplifier 5a has a reverse polarity to the other non-inverting input of the amplifier 5a, with a predetermined integration time constant, and will be described in detail later. It comprises an amplifier 5b that feeds back only during a predetermined clamp timing signal period, and a reference voltage generation circuit 5c that sets a reference potential of an output signal of the amplifier 5b.

  More specifically, the inverting input of the amplifier 5a and the output of the differential amplifier 4 are connected via a resistor R2, and the inverting input and the output are further connected to a resistor R3. The non-inverting input is grounded through a resistor R4, and further connected to the output of the integral amplifier 5b through a resistor R5.

The output of amplifier 5a is connected to one end of the switch S1 of single-pole, amplifier 5b is at one end to the common side of the other end of the single-pole switches S1, are connected at the other end to the inverting input of amplifier 5b that a resistor R1, consist capacitor C1 for connecting the output and inverting input of this, constituting an integrating circuit for integrating time constant CI × R1.

  Further, the output of the reference voltage generating circuit 5c is connected to the non-inverting input of the amplifier 5b, and the reference potential Vref is supplied to the integrating circuit.

  In the clamp circuit unit 5 configured as described above, when the switch S1 is OFF, the output V2 of the amplifier 5b is held, and the amplifier 5a operates as an amplifier having an amplification factor R3 / R2.

  When the switch S1 is on, the output V1 of the amplifier 5a approaches the reference potential Vref exponentially with a time constant Tc expressed by the following equation, and is held at the potential just before the switch S1 is turned off.

Tc = R2 / (R2 + R3). (R4 + R5) / R4. (C1.times.R1)
Here, assuming that R2 = R4 and R3 = R5, Tc = C1 × R1
It becomes.

  Next, the structure of the calculating part 6 is demonstrated. The calculation unit 6 obtains the flow rate from the ADC (Analog Digital Converter) 6a having a resolution for ensuring a predetermined accuracy and the converted digital signal with a calculation formula and calculation timing described later. CPU 6b.

  Next, with reference to FIG. 2, the setting of the timing signal to each circuit generated by the control unit 7 and the operation of the electromagnetic flowmeter of the present invention having the clamp function controlled by this timing signal will be described. To do.

  In FIG. 2, Sh indicates an excitation signal generated by the control unit 7, and a magnetic field H indicates a state generated by supplying an excitation current synchronized with the excitation signal Sh from the excitation circuit 2.

  In addition, in synchronization with the excitation signal Sh, a clamp timing signal S1g having a cycle twice the excitation signal Sh and a predetermined pulse width Ts1 at the center of the excitation signal Sh and the detection signal V0 are clamped. 5 shows an output signal V1 after clamping at 5, and a calculation timing signal Sg for obtaining a flow rate from the clamped output signal V1 by a predetermined calculation.

  Next, the excitation cycle T of the excitation signal Sh is set to a low frequency of about 5 to 10 Hz (1 / T), for example. Then, the clamp timing signal S1g is set at a frequency of 10 to 20 Hz which is twice this excitation signal.

  Then, the clamp timing signal S1g is set to a pulse width that can turn on the switch S1 and clamp the output signal V1 of the clamp circuit unit 5 to the reference potential Vref during the period of the pulse width Ts1.

  That is, the time constant Tc (= C1 × R1) of the integrating circuit constituted by the amplifier 5b is set so that the clamp pulse width Ts1> the time constant Tc. The clamp pulse width Ts1 is synchronized with the excitation signal Sn and is a positive or negative square wave. And a pulse width sufficiently smaller than ¼ of the excitation period T.

  The output signal V1 of the clamp circuit unit 5 set in this way is a signal in which the detection signal V0 is forcibly clamped to the reference potential Vref at the timing of the clamp timing signal S1g.

  Next, the calculation unit 6 inputs the output signal V1 of the clamp circuit unit 5, obtains the flow rate signal Eo based on the following calculation formula, and removes noise.

  First, the average value of the signal from when the switch S1 is turned off until the next excitation direction is reversed, and the average value of the signal after the reversal until the switch S1 is turned on next are obtained.

For example, as shown in the figure, the average value of the signal in each section of the output signal V1 of the clamp circuit unit 5 is e1, e2, e3, e4,... E6. If the signal is Eo, Eoi + 1 ...
Eoi = (e2-e1) + (e3-e4) (1)
Eoi + 1 = (e3-e4) + (e5-e6) (2)
...
(Omitted)
Here, e1 to e6 indicate average values of periods corresponding to calculation pulse widths T1 to T6 for setting valid periods of calculation data in the calculation timing signal Sg.

  The calculation pulse widths T1 and T2 for obtaining (e2-e1) are pulse width signals in the range excluding the period of the clamp pulse width Ts1 and the establishment time Tdn of differential noise, and the calculation for obtaining (e3-e4). The pulse widths T3 and T4 are signals in a range excluding the period of the clamp pulse width Ts1 and the settling period Tdn of differential noise, and are generated by the control unit 7 as signals synchronized with the excitation signal Sh.

  Further, by setting the timings such that T1 + T2 = T3 + T4, the averaging time for obtaining the difference is made equal and as long as possible.

  In the calculation for obtaining the average value from the clamp circuit output V1, the intermediate value of the input range of the ADC 6a is set to the reference potential, the calculation timing signal Sg generated by the control unit 7 is given to the ADC 6a, and the measurement is performed for an effective period (for example, It can be easily obtained by enabling the measurement only during the period of T1 to T4).

  The ADC 6a adopting a double integration method can obtain an effect of removing a noise component in which the average voltage of superimposed noise becomes zero.

  In the electromagnetic flow meter of the first embodiment set in this way, the output of the detection signal V0 is clamped to the reference potential Vref at a period that is 1/2 of the excitation period T, so that fluid noise with a large amplitude is superimposed. However, the output V1 of the clamp circuit unit 5 is not easily saturated.

  In addition, the fluid noise does not change during the excitation period T, that is, low frequency noise equal to or less than the excitation period T that is regarded as a direct current component is positive or negative of the square wave from the reference potential by the clamp output from which the differential noise is removed. It is possible to cancel completely by calculating the difference between the two.

  Next, Embodiment 2 of the present invention will be described with reference to FIGS. About each part of this Example 2, the same part as each part of the electromagnetic flowmeter of Example 1 shown in FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted.

  The difference between the second embodiment and the first embodiment is that in the first embodiment, the pulse widths of the calculation pulses T1 to T4 of the calculation timing signal Sg shown in FIG. 2 are the clamp pulse width Ts1 and the differential noise settling period Tdn, respectively. Except for the entire range of the detection signal, the averaging period is defined as an averaging period. However, in the second embodiment, the averaging period used for this calculation is narrowed and limited to the vicinity where the excitation signal Sh changes.

The reason will be described below. FIG. 3 shows a model of the noise component included in the detection signal V0. The detection signal V0 is a flow rate signal e _H synchronized with the frequency (1 / T) of the excitation signal Sh, a white noise e _AN that becomes zero when averaged, a fluid noise e _LN near the excitation frequency, and This is a signal on which the differential noise _eDN generated when the excitation current is switched is superimposed.

  For example, when the excitation frequency is 5 Hz and the generated fluid noise is 1 Hz, as shown in the figure, it does not change within the period of the clamp signal S1g and cannot be regarded as a direct current component, and the difference between the average values is obtained. The superimposed noise component cannot be canceled.

  Therefore, the pulse width of the calculation pulses (T1 to T6) for averaging the positive and negative square waves is narrowed, and the generation position of the calculation pulse is brought close to the vicinity where the excitation signal changes.

That is, in the rising portion of the excitation signal as shown in FIG. 4, operation pulses T1 and T2 (the calculation pulse T5 T6), the trailing edge of the excitation signal, calculating a pulse T3 and T4, respectively, settling of differential noise e _DN Except for the period, the averaging operation is performed at a position close to the vicinity where the excitation direction changes.

  Then, even when fluid noise close to the excitation signal frequency as shown in FIG. 3 is obtained, in order to obtain the difference between the positions where the respective calculation pulses are close, the DC potential is changed at the timing of the positive and negative square waves. The influence is reduced, the peak value of the square wave can be obtained accurately, and the measurement error of fluid noise is reduced.

  Next, Embodiment 3 of the present invention will be described with reference to FIG. About each part of this Example 3, the same part as each part of the electromagnetic flowmeter of Example 1 shown in FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted.

  The difference between the third embodiment and the first embodiment is that, in the first embodiment, the reference potential Vref for clamping the output of the clamp circuit unit 5 is fixed to an intermediate value in the input range of the ADC 6a. Then, as shown in FIG. 5, the reference potential Vref is delayed by ¼ period from the excitation signal Sh and applied in a direction in which the positive / negative input range of the ADC 6a can be expanded.

  The operation will be described with reference to FIG. The reference potential Vref is changed while the switch S1 is off. The polarity of the reference potential applied is negative and positive potentials Va and Vb in the direction opposite to the direction in which the detection signal increases, so that the output signal V1 of the clamp circuit is applied in a direction that can expand the input range of the ADC 6a.

  The switching timing of the negative and positive potentials Va and Vb of the reference potential Vref is generated by the reference voltage generation circuit 5 c based on the excitation signal Sh generated by the control unit 7.

  By doing in this way, the maximum value of the amplitude of the input signal to the calculation unit 6 can be doubled with respect to the first embodiment shown in FIG.

  Further, this switching timing is not only a phase delayed by ¼ period from the phase of the excitation signal Sh, but if there is no possibility that the signal to the calculation unit 6 is saturated, n / 2 (n is an integer) period. By setting n to a large value, it is possible to delay the clamp cycle and further reduce power consumption.

  When the flow direction of the fluid to be measured is reversed, the negative and positive potentials Va and Vb of the reference potential Vref shown in FIG.

  Next, Embodiment 4 of the present invention will be described with reference to FIGS. About each part of this Example 4, the same part as each part of the electromagnetic flowmeter of Example 1 shown in FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted.

  In the first embodiment, the configuration for removing the noise component included in the detection signal has been described. However, the fourth embodiment is different from the first embodiment in that it further includes an empty detection signal generation circuit 8 and includes a device under test. Even when a measurement abnormal state in which the fluid is not filled in the measurement tube 1 and the electrodes are in a high impedance state occurs, an empty determination calculation for detecting this abnormality is performed in parallel with the flow rate calculation. is there.

  Specifically, a high resistance R6 is connected between one input terminal of the differential amplifier 4 and the ground point, and a high resistance R7 having the same value as that of the high resistance R6 is connected to the other input terminal. A small signal V3 having a cycle twice the cycle of the excitation signal Sh is applied from the sky detection signal generation circuit 8 via the.

  Then, the calculation unit 6 obtains a difference between the average value of the first half cycle signal and the average signal value of the second half cycle of the output signal V1 of the clamp circuit unit 5 as an empty determination detection signal by the following calculation formula.

Ee = e2-e1- (e6-e5)
And when this value becomes a preset threshold value abnormality, it determines with an empty state.

  That is, according to this arithmetic expression, as shown in FIG. 7, since the (flow rate) detection signal is detected in the same phase, it is canceled out. It can be detected.

  Further, since the flow rate calculation is obtained by the calculation formula shown in the first embodiment, the superimposed minute signal V3 does not affect the measurement.

  And when this value becomes larger than the preset threshold value, it determines with the to-be-measured fluid in the measurement pipe | tube 1 having become empty.

  For example, when the fluid to be measured is flowing, the impedance between the electrodes is about 10 kΩ, but when it is empty, the impedance between the electrodes is high, for example, about 1 MΩ.

  Therefore, when a signal having a peak value of 1 mV is applied as R6, R7 = 10 MΩ, and V3, a signal of about 1 μV is generated at the input of the differential amplifier 4 when the fluid to be measured is flowing, but it is 100 when the signal is empty. Double 100μV is generated. An empty state is determined from the change in the empty determination detection signal.

  That is, according to this configuration, the sky detection minute signal V3 is superimposed on the detection signal, and the calculation unit 6 continuously performs the calculation for determining the sky detection state without affecting the calculation for determining the flow rate. And can be executed.

  The present invention is not limited to the above-described embodiment, and any method may be used as long as the detection signal is clamped at the reference potential in the middle of the positive and negative excitation waveforms and the difference between the average values of the clamped signals is obtained. The calculation timing can be appropriately adjusted according to the excitation frequency and the low frequency noise frequency component, and various modifications can be made without departing from the gist of the present invention.

The whole block diagram of the electromagnetic flowmeter of the present invention. The time chart explaining the processing operation of the noise removal of this invention. The figure explaining the noise signal component contained in the detection signal of this invention. 6 shows another embodiment of the noise removal processing operation of the present invention. FIG. 4 is an embodiment for expanding the dynamic range of the signal processing circuit of the present invention. The whole block diagram in the case of performing empty detection in the electromagnetic flowmeter of this invention. Explanatory drawing of the flow measurement and detection processing operation | movement of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Measuring tube 2 Excitation circuit 2a, 2b Excitation coil 3a, 3b Electrode 4 Differential amplifier 5 Clamp circuit part 5a, 5b Amplifier 5c Reference voltage generation circuit 6 Calculation part 6a ADC
6b CPU
7 Control unit 8 Empty detection signal generation circuit

Claims (6)

A square wave excitation signal is generated, and a magnetic field whose direction is periodically reversed is applied from the outside of the measurement tube through which the fluid to be measured flows to the axis direction orthogonal to the tube axis of the measurement tube. An electromagnetic flow rate that amplifies a signal detected between electrodes provided in contact with the fluid to be measured at the tube wall of the measuring tube in an axial direction perpendicular to the magnetic field direction, and measures the flow rate of the fluid to be measured A total of
A differential amplifier that amplifies a signal detected between the electrodes to form a detection signal;
A clamp circuit unit that clamps the detection signal to a preset reference potential at the timing of each central position of the excitation signal corresponding to the reversal of the magnetic field;
The difference between the first average value of the square wave signal in the positive direction after clamping and the second average value of the square wave signal in the negative direction is obtained from the output signal of the clamp circuit unit, and the obtained signal is A calculation unit for obtaining a flow rate;
An electromagnetic flow rate comprising: the excitation signal; a clamp timing signal of the clamp circuit unit; and a control unit that generates a calculation timing signal for obtaining a difference between the first average value and the second average value. Total.   In the calculation timing signal, the period for obtaining the first average value is equal to the period for obtaining the second average value, and the period for obtaining the first average value and the period for obtaining the second average value. The electromagnetic flow meter according to claim 1, wherein a predetermined period during which the direction of the detection signal corresponding to the reversal of the magnetic field changes is excluded. The clamp circuit unit includes an amplifier that applies the detection signal to one inverting input;
An integration circuit that feeds back the output signal of the amplifier to the other non-inverting input of the amplifier with a reverse polarity, a predetermined integration time constant, and a period of the clamp timing signal, and a reference potential of the output signal of the integration circuit are set. A reference voltage generation circuit;
With
2. The time constant of the integration circuit is set to a value smaller than the period of the clamp timing signal, and the detection signal is clamped to the reference potential within the period of the clamp timing signal. The described electromagnetic flow meter. The period for obtaining the first average value and the period for obtaining the second average value are periods equal to or less than ¼ of the period of the excitation signal,
In addition, in the period of obtaining the first average value and the second average value, the period of the clamp timing signal and the predetermined period in which the direction of the detection signal where the magnetic field is inverted change are excluded. The electromagnetic flow meter according to claim 1, wherein   The reference voltage generation circuit is configured to change the reference potential every n / 2 (n is an integer) cycle with a phase delayed by ¼ cycle from the phase of the excitation signal. The electromagnetic flow meter according to claim 3. A square wave excitation signal is generated, and a magnetic field whose direction is periodically reversed is applied from the outside of the measurement tube through which the fluid to be measured flows to the axis direction orthogonal to the tube axis of the measurement tube. An electromagnetic flow rate that amplifies a signal detected between electrodes provided in contact with the fluid to be measured at the tube wall of the measuring tube in an axial direction perpendicular to the magnetic field direction, and measures the flow rate of the fluid to be measured A total of
A differential amplifier that amplifies a signal detected between the electrodes to form a detection signal;
A clamp circuit section that clamps the detection signal to a preset reference potential at the timing of each central position of the excitation signal corresponding to the reversal of the magnetic field;
The difference between the first average value of the square wave signal in the positive direction after clamping and the second average value of the square wave signal in the negative direction is obtained from the output signal of the clamp circuit unit, and the obtained signal is A calculation unit for obtaining a flow rate;
A control unit for generating a control signal for controlling the excitation signal, a clamp timing signal of the clamp circuit unit, and a calculation timing signal for obtaining a difference between the first average value and the second average value;
A first high resistance is connected between one input terminal of the differential amplifier and a ground point, and a second high resistance having the same value as the first high resistance is connected to the other input terminal, An empty detection signal generation circuit that applies a minute signal having a double period synchronized with the period of the excitation signal via a second high resistance;
The calculation unit obtains, as an empty determination detection signal, a difference between an average value of one half cycle signal of the output signal of the clamp circuit unit and a signal average value of the other half cycle,
When the empty determination detection signal is greater than a preset threshold value, an empty determination calculation for determining that the measurement tube is empty and an operation for obtaining the flow rate are performed simultaneously. Flowmeter.

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