JPH11142199A - Electromagnetic flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a consumed current in an electromagnetic flowmeter uniform and to reduce the maximum value of the current consumption. SOLUTION: In an electromagnetic flowmeter, an exciting current is made to flow to an exciting coil 1, an alternating field is generated inside a measuring tube 3, and the flow velocity and the flow rate of a conductive fluid are found on the basis of an electromotive force which is generated across a pair of electrodes 2. In the electromagnetic flowmeter, a quiescent section in which the exciting current is not made to flow is installed. Then, the flow velocity and the flow rate are computed by a microprocessor 9 only inside the quiescent section. Thereby, a consumed current is made uniform, and the maximum value of the consumed current is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flowmeter for detecting a flow rate and a flow rate of a conductive fluid, and more particularly, it operates with a 4-20 mA current signal which is a signal line of an industrial measuring instrument.
The present invention relates to a two-wire electromagnetic flowmeter that efficiently uses electric power.

[0002]

2. Description of the Related Art 4-20 m, which is a signal line of an industrial measuring instrument.
Although a two-wire electromagnetic flowmeter operating with an A current signal is well known, it is necessary to operate all circuits, such as an excitation circuit and a flow velocity calculation circuit, with a current of 4 mA or less. This means that the consumed peak current is 4 mA or less, and it is necessary to reduce the power consumption so as to reduce the peak value of the consumed current. Since the current limit of this signal line is 4 mA and there is no restriction on the voltage, conventionally, a method of increasing the applied voltage and increasing the current consumption that can be used by using a voltage converter has been adopted. There is also a method of increasing the applied voltage and cascading an excitation circuit and a flow velocity calculation circuit to increase the current that can be used in both circuits.
The above is a method for increasing the usable power consumption. Conversely, the operation of the excitation circuit is performed intermittently,
There is also a method of using a low-pass filter to reduce the average current used in the excitation circuit and suppress the fluctuation of the current consumption.

[0003]

As described above, the operation using the 4-20 mA current signal which is the signal line of the industrial measuring instrument 2
In the linear electromagnetic flowmeter, it is necessary to operate all circuits such as the excitation circuit and the flow velocity calculation circuit with a current of 4 mA or less.
This means that the consumed peak current is 4 mA or less, and there is a problem that it is necessary to reduce the power consumption so as to reduce the peak value of the consumed current. Conventionally, measures have been taken to increase the current consumption that can be used by using a voltage converter by increasing the applied voltage, or to increase the current that can be used in both circuits by connecting the excitation circuit and flow velocity calculation circuit in cascade. However, in that case, a safety problem occurs. That is, when an industrial measuring instrument is used in an explosive atmosphere, it is necessary to suppress the possibility of ignition due to some kind of accident. In such a case, if the applied voltage is large, the content of the measure becomes large and it becomes difficult.

Further, the operation of the excitation circuit is performed intermittently,
When a low-loss low-pass filter is used to reduce the average current used in the excitation circuit and suppress fluctuations in the current consumption, a low-pass filter having a large time constant is required, and the element values (capacitor capacitance, (Inductance value, resistance value) becomes large, and realization becomes difficult. Further, in general, a low-pass filter having a large time constant (for example, a CR passive filter) has a problem that power is lost because attenuation occurs in a pass band. Therefore, an object of the present invention is to reduce the maximum value of the current consumption by making the current consumption uniform.

[0005]

In order to solve such a problem, according to the present invention, a pause section (a non-excitation section) in which no exciting current flows is provided, and the flow velocity calculating device is operated in the pause section. ing. That is, current consumption is supplied in a time-sharing manner to make power consumption uniform and reduce the maximum value of current consumption. Since the fluctuations in the current consumption are made uniform, a low-pass filter for suppressing the fluctuations in the current is not required, or a filter whose characteristics are not so severe can be used, so that the realization becomes easy, and the power of the low-pass filter is reduced. Loss can be reduced. Further, since the maximum value of the current consumption is reduced, it is not necessary to increase the applied voltage. Further, if the reduced power consumption is converted to the excitation current, the alternating magnetic field generated in the measurement tube can be increased, and the S / E of the obtained electromotive force can be increased.
N can be improved.

[0006]

FIG. 1 is a block diagram showing an embodiment of the present invention. The sensor unit is configured by providing an exciting coil 1 and a pair of electrodes 2 to the measuring tube 3. The circuit unit is configured to supply an exciting current to the exciting coil 1 to generate an alternating magnetic field in the measuring tube 3. Amplifying circuit 5 for amplifying electromotive force generated at electrode 2, rectifying circuit 6 for rectifying the amplified signal, A / D for converting the rectified signal to a digital signal
(Analog / digital) conversion circuit 7, holding circuits 81, 8 for alternately holding A / D (also referred to as AD) conversion results
2. A microprocessor 9 for calculating the flow velocity and flow rate based on the digitized signal, an output current control circuit 10 for outputting the calculation result as a 4-20 mA current, excitation and A
Timing generation circuit 11, 4-2 for / D conversion and the like
The power supply circuit 12 includes a power supply circuit 12 that generates a power supply used by the circuit unit from the 0 mA signal line. The rectifier circuit 6 has an A /
If the D conversion circuit 7 is of a type capable of A / D conversion of positive and negative inputs, it can be omitted.

FIG. 2 is a waveform chart for explaining the operation of FIG. 1, and FIG. 3 is a waveform chart when the present invention is not applied.
This is to explain in comparison with. Well, measuring tube 3
As for the excitation current timing for generating an alternating magnetic field in the motor, it takes time for the excitation current to stabilize (the time constant is 1).
0 ms) In order to remove commercial noise, sampling should be performed at a cycle of an integral multiple of 50 Hz or 60 Hz of the commercial frequency (FIG. 2 shows a case where the frequency is 50 Hz as shown in FIG.
(Example of sampling time is shown.) Because the flow velocity does not change abruptly, the sampling period can be reduced to some extent. For the reason that a non-excitation section is required to remove the offset of the circuit, as shown in Fig. 2 (b) To 20
Excitation is performed at a cycle of 0 ms. Here, there are three types of excitation states, excitation in the positive direction, excitation in the negative direction, and non-excitation. FIG. 2B shows an example in which each state is repeated every 50 ms.

The exciting current is switched between the positive direction and the negative direction by the exciting circuit 4, but the current consumed by the exciting circuit 4 is shown in FIG.
One direction is as shown in FIG. As an example, the current consumption of the excitation circuit in the two-wire electromagnetic flowmeter is 2 mA in the excitation section.
About 0 mA in the non-excitation section. On the other hand, the current consumption of the microprocessor is conventionally substantially constant, for example, as shown in FIG. 3E, but according to the present invention, as shown in FIG. To reduce the current consumption, and set the normal current consumption in the non-excitation section. This low power consumption mode is almost standard in modern microprocessors, and the necessary conditions are (a) to reduce current consumption and (b) to return to the normal mode by an external signal. (C) the return time is sufficiently shorter than 50 ms.

As shown in FIG. 2E, when the low power consumption mode is set in the excitation interval, the operation is stopped or the speed is reduced during that period. However, since the required amount of operation of the microprocessor is constant, it is necessary to improve the operation speed in the non-excitation interval. FIG. 2 illustrates the current consumption of the microprocessor assuming that the calculation speed in the non-excitation section is twice as high as that in FIG. 3 to which the present invention is not applied. The reason for the doubling is that the operation time when the present invention is applied is half that of when the present invention is not applied. In general, in order to improve the processing speed of the microprocessor, a method of increasing the supply clock is used. In this case, the current consumption increases in proportion to the clock. Based on this fact, the current consumption of the microprocessor of FIG. 2 is shown in comparison with FIG. 3 (current consumption of the microprocessor in the non-excitation section of FIG. 2 = current consumption of the microprocessor of FIG. 3). ×
(Shown as 2 ≒ 2 mA).

On the other hand, regarding the power consumption in the excitation section,
Since the crystal oscillation is continued so that the mode can be quickly returned from the low power consumption mode to the normal mode, there is some current consumption (about Ip ^* = 0.2 mA) as shown in FIG. The current consumption of the circuits other than the excitation circuit and the microprocessor is substantially constant, and is shown in FIG. 2 as about 1 mA. As described above, when the microprocessor performs an operation in the non-excitation interval and controls so as to be in the low power consumption mode in the excitation interval, the current consumption of all the circuits is as shown in FIG.
(G). In this case, it is possible not only to suppress the fluctuation of the current consumption, but also to reduce the maximum value of the current consumption, as compared with the case of FIG. 3 to which the present invention is not applied.

Here, the maximum value of the current consumption in each case of FIGS. 2 and 3 will be compared and examined. Now, in FIGS. 2 and 3, If: excitation current Ip: microprocessor current consumption in the conventional example Ir: current consumption of circuits other than the excitation circuit and microprocessor Ip ^* : microprocessor current consumption in low power consumption mode D = Ton / (Ton + Toff); defined as the excitation duty.

When defined as described above, the maximum current consumption of all the circuits in the conventional example is expressed by If + Ip + Ir (1), whereas the maximum current consumption of all the circuits in the case of the present invention is as follows. : If + Ip ^* + Ir (2) When not excited: expressed as Ip / (1−D) + Ir (3) The first term of the equation (3) indicates an operation for making the average processing current of the microprocessor the same in the conventional example and the case of the present invention. As a result, the same amount of arithmetic processing can be performed.

The condition under which the maximum current consumption according to the present invention is reduced as compared with that of the conventional example is as follows: from the expression (1)> (2), Ip> Ip ^* , but this condition is always satisfied. . Because I
This is because p ^* is a low current consumption mode for Ip. Also, from the equation (1)> (3), If> Ip · D /
(1-D). In the case of the embodiment of FIG. 2, If = 2m
Since it is assumed that A, D = 0.5 and Ip = 1 mA, it can be seen that the relationship of the above expression is satisfied.

In order to allow the microprocessor to perform the operation only during the non-excitation period as shown in FIG. 2 and to always set the low power consumption mode in the excitation period, the timing generation circuit 11 shown in FIG. At the end timing, the microprocessor is shifted from the low power consumption mode to the normal mode, the necessary flow velocity calculation is completed within the non-excitation interval, and the software is shifted to the low power consumption mode again at the end of the calculation. Can be.

Next, referring to FIG. 4, the A / D conversion result is stored.
The holding operation will be described. Excitation current and flow velocity
To calculate the flow velocity from the magnitude of the electromotive force generated by
However, when an alternating magnetic field is used, it is similar to the exciting current.
Peak-to-Peak
Peak) is generally used. Of this electromotive force
To measure peak-to-peak, align as shown in Figure 1.
When a current circuit is used, the AC
An error occurs in the amplitude of the signal. Therefore, the offset of the rectifier circuit
A method of detecting the cut-off voltage and compensating for this is also used.
This is because the electromotive force (V⁺) And
The electromotive force voltage (V⁺zero)
Difference, the electromotive force (V ^-) And Ma
The electromotive force voltage (V^-zer
o) is added as in the following equation to obtain the peak-
This is a method for calculating to-Peak (Vpp). Vpp = (V⁺-V⁺zero) + (V^--V^-zero)… (4)

FIG. 4 illustrates the operation when such offset compensation is used together with the microprocessor operation only in the non-excitation section. That is, the first holding circuit 81 V ⁺ and V ^- stores alternately as in FIG. 4 (e), the second holding circuit 82 V ⁺ zero and ^V - zer
o are alternately stored as shown in FIG. Then, at the timing when the microprocessor calculates the flow velocity, the value of the electromotive force is read from the two holding circuits 81 and 82, and the Peak-to-Peak calculation of the electromotive force is performed.
This is performed as shown in FIG. By providing the holding circuits 81 and 82 in this manner, even when the microprocessor is in the low power consumption mode during the excitation interval, one A / D conversion circuit can acquire a value required for offset compensation of the rectifier circuit. it can.

[0017]

According to the present invention, there is provided a pause section in which no exciting current flows, in which the flow velocity calculation is completed, and the power consumption for supplying the excitation current and the current consumption for the flow velocity calculation in a time-sharing manner. Can be made uniform, and the maximum value of the current consumption can be reduced. As a result, the fluctuation of the current consumption is made uniform, so that a low-pass filter for suppressing the fluctuation of the current is not required, or a filter having less severe characteristics can be used. Power loss can be reduced. Further, since the maximum value of the current consumption is reduced, it is not necessary to increase the applied voltage. By turning the current of low power consumption as described above into the exciting current, the alternating magnetic field generated in the measuring tube can be increased, and the S / N of the obtained electromotive force can be improved. It becomes possible.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is an operation explanatory diagram illustrating an overall operation of FIG. 1;

FIG. 3 is an operation explanatory diagram when the present invention is not applied.

FIG. 4 is an explanatory diagram illustrating an A / D conversion result holding operation according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Excitation coil, 2 ... Electrode, 3 ... Measurement tube, 4 ... Excitation circuit, 5 ... Amplification circuit, 6 ... Rectification circuit, 7 ... A / D conversion circuit, 81, 82 ... Holding circuit, 9 ... Microprocessor,
10: output current control circuit, 11: timing generation circuit,
12 Power supply circuit.

Claims (2)

[Claims] 1. An exciting coil disposed near a measuring tube through which a conductive fluid flows, an exciting circuit for supplying an exciting current to the exciting coil to generate an alternating magnetic field in the measuring tube, and disposed in the measuring tube. An electromagnetic flowmeter having a pair of electrodes and a flow rate detection device for obtaining a flow rate of the conductive fluid based on an electromotive force generated between the pair of electrodes, wherein a pause section in which no exciting current flows is provided, An electromagnetic flowmeter characterized in that at least a part of the flow rate detection device is stopped or operated in a low power consumption mode in a period other than the pause section. 2. The method according to claim 1, wherein the flow detecting device includes an amplifier, an A / D
9. A circuit comprising: a converter; a holding circuit capable of holding at least two or more of the A / D conversion results; and an arithmetic circuit, wherein the A / D conversion results are alternately stored in the holding circuit. 2. The electromagnetic flowmeter according to 1.