JP3659378B2 - Electromagnetic flow meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electromagnetic flowmeter that detects the flow velocity and flow rate of a conductive fluid, and more specifically, a 2-wire electromagnetic flowmeter that operates with a 4-20 mA current signal, which is a signal line of an industrial measuring instrument, and uses power efficiently. About.
[0002]
[Prior art]
A two-wire electromagnetic flowmeter that operates with a 4-20 mA current signal, which is a signal line for industrial measurement equipment, is well known, but all circuits such as an excitation circuit and a flow velocity calculation circuit must be operated with a current of 4 mA or less. is there. 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 this signal line has a current limit of 4 mA and is not limited as a voltage, conventionally, a method has been employed in which the applied voltage is increased and the current consumption that can be used with the voltage converter is increased.
Further, there is a method in which an applied voltage is increased and an excitation circuit and a flow velocity calculation circuit are connected in cascade to increase the current that can be used in both circuits.
The above is a method of increasing the power consumption that can be used, but conversely, the excitation circuit is operated intermittently to reduce the average current used in the excitation circuit and suppress fluctuations in the consumption current. Therefore, there is a method using a low-pass filter.
[0003]
[Problems to be solved by the invention]
Thus, a two-wire electromagnetic flow meter that operates with a 4-20 mA current signal, which is a signal line of an industrial measuring instrument, must operate all circuits such as an excitation circuit and a flow velocity calculation circuit with a current of 4 mA or less. This is because the peak current consumed is 4 mA or less, and there is a problem that low power consumption is required to reduce the peak value of the current consumption.
Conventional measures include increasing the applied voltage and increasing the current consumption that can be used by using a voltage converter, or increasing the current that can be used in both circuits by cascading the excitation circuit and flow velocity calculation circuit. However, in that case, a safety problem occurs. That is, when using industrial measurement equipment in an explosion atmosphere, it is necessary to suppress the possibility of ignition by some accident. In such a case, if the applied voltage is large, the countermeasure contents become large and difficult.
[0004]
In addition, when a low-loss low-pass filter is used to intermittently operate the excitation circuit to reduce the average current used in the excitation circuit and suppress fluctuations in the current consumption, a low-pass filter with a large time constant is required. It becomes necessary, and the element values (capacitor capacity value, inductance value, resistance value) to be used become large, and realization becomes difficult. Furthermore, a low-pass filter having a large time constant (for example, a CR passive filter) generally suffers from a problem that power is lost because attenuation occurs in the passband.
Accordingly, an object of the present invention is to make the current consumption uniform and reduce the maximum value of the current consumption.
[0005]
[Means for Solving the Problems]
In order to solve such a problem, in the present invention, a pause interval (no excitation interval) in which no excitation current flows is provided, and the flow velocity calculation device is operated in the pause interval.
That is, the current consumption is supplied in a time-sharing manner to make the power consumption uniform and reduce the maximum value of the current consumption. Since fluctuations in current consumption are made uniform, a low-pass filter to suppress fluctuations in current is not required, or a filter with less strict characteristics can be used, which makes it easy to realize the low-pass filter power. Loss can be reduced. Further, since the maximum value of current consumption is reduced, there is no need to increase the applied voltage. Furthermore, if the reduced current consumption is turned to the excitation current, the alternating magnetic field generated in the measurement tube can be increased, and the S / N of the electromotive force obtained can be improved. Become.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram showing an embodiment of the present invention.
The sensor unit is configured by providing an excitation coil 1 and a pair of electrodes 2 to the measurement tube 3, and the circuit unit is an excitation circuit 4 that causes an excitation current to flow through the excitation coil 1 to generate an alternating magnetic field in the measurement tube 3. An amplifying circuit 5 for amplifying the electromotive force generated in the electrode 2, a rectifying circuit 6 for rectifying the amplified signal, an A / D (analog / digital) converting circuit 7 for converting the rectified signal into a digital signal, A / D ( (Also referred to as AD) holding circuits 81 and 82 for alternately holding conversion results, a microprocessor 9 for calculating a flow velocity and a flow rate based on digitized signals, an output current control circuit 10 for outputting the calculation results as a 4-20 mA current, A timing generation circuit 11 for excitation, A / D conversion, and the like, a power supply circuit 12 for generating a power supply used by the circuit unit from a 4-20 mA signal line, and the like. The rectifier circuit 6 can be omitted if the A / D converter circuit 7 is of a type that can A / D convert positive and negative inputs.
[0007]
FIG. 2 is a waveform diagram for explaining the operation of FIG. 1, and FIG. 3 is a waveform diagram when the present invention is not applied, for comparison with FIG.
Now, as the excitation current timing for generating an alternating magnetic field in the measuring tube 3,
(1) It takes time to stabilize the excitation current (10ms in time constant)
(2) Sampling at a frequency that is an integral multiple of 50 Hz or 60 Hz of the commercial frequency in order to remove commercial noise (FIG. 2 shows an example of a sampling time of 20 ms in the case of 50 Hz as shown in FIG. 2C). Yes)
(3) Since the flow rate does not change suddenly, the sampling cycle can be delayed to some extent. (4) For the reason that a non-excitation section is necessary for removing the offset of the circuit, here, as shown in FIG. It shall be excited. Here, there are three types of excitation states: positive excitation, negative excitation, and non-excitation, and FIG. 2B shows an example in which each state is repeated every 50 ms.
[0008]
The excitation current is switched between the positive direction and the negative direction by the excitation circuit 4, but the current consumed by the excitation circuit 4 is one direction as shown in FIG. As an example, the current consumption of the excitation circuit in a two-wire electromagnetic flowmeter is about 2 mA in the excitation interval and about 0 mA in the non-excitation interval.
On the other hand, the current consumption of the microprocessor is conventionally almost constant as shown in FIG. 3 (e), for example, but according to the present invention, as shown in FIG. Is used to reduce the current consumption, and the normal current consumption in the non-excitation interval. This low power consumption mode is almost standard in recent microprocessors. The necessary conditions here are: (b) reducing the current consumption, and (b) returning to the normal mode with an external signal. (C) The return time is sufficiently shorter than 50 ms.
[0009]
As shown in FIG. 2E, when the low power consumption mode is set in the excitation interval, the calculation is stopped or slowed down during that period. However, since the required calculation amount of the microprocessor is a constant amount, it is necessary to improve the calculation speed in the non-excitation interval. In FIG. 2, the current consumption of the microprocessor is illustrated on the assumption that the calculation speed in the non-excitation interval is twice that in the case of FIG. 3 where the present invention is not applied. The reason for the double is that the computation time when the present invention is applied is half that 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 contrast to FIG. 3 (the current consumption of the microprocessor in the non-excitation section of FIG. 2 = the current consumption of the microprocessor of FIG. 3). X2≈2 mA).
[0010]
On the other hand, regarding the power consumption in the excitation interval, since the crystal oscillation is continued so that the low power consumption mode can return to the normal mode at high speed, as shown in FIG. 2, some current consumption (Ip ^* = 0. 2mA). Further, the current consumption of the circuits other than the excitation circuit and the microprocessor is substantially constant, and in FIG. 2, this is shown as about 1 mA.
As described above, when the microprocessor performs an operation in the non-excitation period and controls to enter the low power consumption mode in the excitation period, the current consumption of all the circuits is as shown in FIG. In this case, it is possible not only to suppress the fluctuation of the consumption current, but also to reduce the maximum value of the consumption current compared to the case of FIG. 3 in which the present invention is not applied.
[0011]
Here, the maximum value of the current consumption in each case of FIGS. 2 and 3 will be compared and examined.
Now, in FIG. 2 and FIG.
If: Excitation current Ip: Current consumption of microprocessor in the case of the conventional example Ir: Current consumption of circuits other than the excitation circuit and the microprocessor Ip ^* : Current consumption of the microprocessor in the low power consumption mode D = Ton / (Ton + Toff); Excitation The duty is defined as follows.
[0012]
When defined as above, the maximum current consumption of all circuits in the conventional example is
If + Ip + Ir (1)
In contrast, the maximum current consumption of all circuits in the present invention is
During excitation: If + Ip ^* + Ir (2)
No excitation: Ip / (1-D) + Ir (3)
It is expressed as Incidentally, the first term of the expression (3) indicates an operation for making the average processing current of the microprocessor the same between the conventional example and the case of the present invention. As a result, the same amount of arithmetic processing can be performed.
[0013]
Now, the conditions under which the maximum current consumption according to the present invention is reduced from that of the conventional example are as follows:
(1) From the formula> (2), Ip> Ip ^*
However, this condition is always satisfied. This is because Ip ^* is a low current consumption mode with respect to Ip.
Further, from the formula (1)> the formula (3), If> Ip · D / (1-D)
It is. In the case of the embodiment of FIG. 2, it is assumed that If = 2 mA, D = 0.5, and Ip = 1 mA, the relationship of the above equation is satisfied.
[0014]
As shown in FIG. 2, in order for the microprocessor to perform calculation only in the non-excitation period and to make sure that the low power consumption mode is set in the excitation period, the timing generation circuit 11 shown in FIG. Thus, the microprocessor can be changed from the low power consumption mode to the normal mode, the necessary flow velocity calculation is completed within the non-excitation period, and the low power consumption mode is again transferred by software when the calculation is completed.
[0015]
Next, the holding operation of the A / D conversion result will be described with reference to FIG.
The electromagnetic flowmeter obtains the flow velocity from the magnitude of the electromotive force generated by the excitation current and the flow velocity. When an alternating magnetic field is used, the peak-to-peak of the electromotive force similar to the excitation current is used. It is common to use -Peak). When a rectifier circuit is used as shown in FIG. 1 to measure the peak-to-peak of the electromotive force, an error occurs in the amplitude of the AC signal due to the offset voltage. Therefore, a method of detecting and compensating for the offset voltage of the rectifier circuit is also used.
This is because the difference between the electromotive force (V ⁺ ) during the excitation period during plus rectification and the electromotive force voltage (V ⁺ zero) during the non-excitation period during plus rectification, and the electromotive force (V ⁻ ) during the excitation period during minus rectification. This is a method for obtaining Peak-to-Peak (Vpp) of electromotive force by adding the difference of electromotive force voltage (V ^- zero) during a non-excitation period during negative rectification as shown in the following equation.
Vpp = (V ⁺ −V ⁺ zero) + (V ⁻⁻ V ⁻ zero) (4)
[0016]
FIG. 4 illustrates the operation when such offset compensation is used in combination with the microprocessor operation only in the non-excitation period.
That is, V ⁺ and V ⁻ are alternately stored in the first holding circuit 81 as shown in FIG. 4E, and V ⁺ zero and V ⁻ zero are stored in the second holding circuit 82 as shown in FIG. As shown in FIG. 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 as shown in FIG.
In this way, by providing the holding circuits 81 and 82, even if the microprocessor is set in the low power consumption mode in the excitation interval, a value necessary for offset compensation of the rectifier circuit can be acquired by one A / D conversion circuit. it can.
[0017]
【The invention's effect】
According to the present invention, a pause interval in which no excitation current is allowed to flow is provided, the flow velocity calculation is completed in this interval, and the consumption current can be equalized to supply the consumption current for the excitation current and the flow velocity calculation in a time-sharing manner, The maximum value of current consumption can be reduced. For this reason, since fluctuations in current consumption are made uniform, a low-pass filter for suppressing fluctuations in current is not required, or a filter with less severe characteristics can be used, which makes it easy to realize the low-pass filter. Power loss can be reduced. Further, since the maximum value of current consumption is reduced, there is no need to increase the applied voltage. Then, the alternating magnetic field generated in the measuring tube can be increased by turning the current consumption reduced as described above to the excitation current, and the S / N of the electromotive force obtained 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 for explaining the 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 ... Measuring 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)

An excitation coil disposed in the vicinity of the measurement tube through which the conductive fluid flows, an excitation circuit for causing an excitation current to flow through the excitation coil to generate an alternating magnetic field in the measurement tube, and a pair of electrodes disposed in the measurement tube; In the electromagnetic flow meter having a flow rate detection device for obtaining the flow rate of the conductive fluid based on the electromotive force generated between the pair of electrodes,
An electromagnetic flow meter comprising a pause section in which no excitation current flows, and operating in a state where at least a part of the flow rate detection device is stopped or in a low power consumption mode other than the pause section. The flow rate detection device includes an amplifier, an A / D converter, a holding circuit capable of holding at least two of the A / D conversion results, and an arithmetic circuit, and the A / D conversion result is stored in the holding circuit. The electromagnetic flowmeter according to claim 1, wherein the electromagnetic flowmeter is stored alternately.

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