JPS5825966B2 - Excitation circuit of electromagnetic flowmeter - Google Patents

Description

[Detailed description of the invention] The present invention relates to an excitation circuit for an electromagnetic flowmeter.

Ideally, a DC excitation method is preferable for the excitation method of an electromagnetic flowmeter. However, when a DC excitation method is adopted, a DC measurement voltage is generated at the electrodes, and the electrodes are electrolyzed by this DC voltage, which causes the zero point It has the disadvantage of fluctuations.

In order to avoid this drawback, an alternating current excitation method is inevitably adopted.

However, with the AC excitation method, unnecessary signals such as 90° noise and in-mode noise are generated.

The level of these unnecessary signals fluctuates in response to temperature changes, for example, so there is a drawback that the zero point of the flow rate measurement signal changes due to temperature changes and the like.

On the other hand, frequencies lower than the commercial power supply frequency, e.g. 1 to 10Hz
An ultra-low frequency excitation method has been proposed in which the excitation current is switched at a frequency of .

This ultra-low frequency excitation method has the advantage that it does not generate 90° noise or common mode noise, and therefore can obtain a flow rate measurement signal without zero point fluctuation.

By the way, in general, in an electromagnetic flowmeter, the detection sensitivity differs depending on the diameter of the tube constituting the flow path, so the excitation current is varied for each diameter of the tube so that the output voltage with respect to the flow rate is made the same.

For this reason, even in the case of the ultra-low frequency excitation method, conventionally a current source with a different current value is prepared for each diameter, and the output current of this current source is switched using an ultra-low frequency signal.

Therefore, the design of the excitation power source is complicated, and manufacturing management is also troublesome because many types of power sources must be manufactured.

An object of the present invention is to provide an excitation circuit for an electromagnetic flowmeter that can obtain excitation current suitable for various diameters using a power source having the same configuration.

In this invention, a switch is provided between the excitation coil and the power supply to turn on and off the excitation current supplied from the power supply to the excitation coil, and this switch is controlled on and off by the logical product of an extremely low frequency signal and a high frequency signal. By changing the duty ratio of the high frequency signal, the excitation current is controlled to match various diameters.

An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a basic embodiment of the invention.

In this figure, 1 indicates a DC power supply, and 2 indicates an excitation coil of an electromagnetic flowmeter.

In this example, a switch 3 is inserted in series between the DC power supply 1 and the excitation coil 2, and the excitation current flowing through the excitation coil 2 is controlled intermittently by on/off control of the switch 3. A diode 4 is connected in parallel so that the polarity is opposite to the voltage applied by the DC power supply 1.

Therefore, the current stored in the exciting coil 2 is released through the diode 4 while the switch 3 is off.

In this invention, the switch 3 is controlled by the logical product of a low frequency signal and a high frequency signal, and by changing the duty ratio of the high frequency signal, any desired excitation current can be obtained.

For this purpose, a low frequency generator 5 and a high frequency generator 6 are provided, and the logical product of the low frequency signal and the high frequency signal from these signal generators 5 and 6 is obtained.

In this example, the low frequency generator 5 generates a low frequency rectangular wave Pa as shown in FIG. Alternatively, the duty ratio of the high frequency generator 6 may be varied such that when the low frequency signal Pa is at the logic "1", the duty ratio is set to t/T, and when the low frequency signal Pa is at the logic "0", the duty ratio is set to 1. In this manner, the high frequency signal pb as shown in FIG. 2B is obtained as a logical product of both signals as an intermittent signal that is intermittent according to the period of the low frequency signal Pa, and the switch 3 is controlled to be turned on and off by this intermittent signal.

For example, the switch 3 is turned on when the high frequency signal Pb is a logic "1", and is turned off when the high frequency signal Pb is a logic "0".

Therefore, as shown in FIG. 2C, an exciting current iφ flows through the exciting coil 2, increasing when the switch 3 is on, decreasing when the switch 3 is off, and gradually increasing the current value (2) by repeating this process.

The maximum value of this excitation current iφ can be set to an appropriate value by the duty ratio t/T of the high frequency signal Pb.

In other words, as t/T approaches O, the maximum value of the excitation current increases because the period of logic "1" of the pulse signal Pb becomes longer, and as t/T approaches 1, the maximum value of the excitation current value increases. becomes smaller.

Therefore, any excitation current corresponding to various diameters can be set by the duty ratio t/T of the high frequency signal Pb. It can be used as an excitation device for electromagnetic flowmeters with different diameters.

Therefore, even when manufacturing electromagnetic flowmeters with various diameters, only one type of excitation power supply device needs to be manufactured, and manufacturing management can be facilitated.

In the above-described embodiment, the excitation current iφ flowing through the excitation coil 2 becomes a DC current on average.

Therefore, as explained earlier, a DC detection voltage is generated in the flow rate detection electrode, and the electrode is electrolyzed to remove it, which causes the zero point of the set value to shift, but in this example, it is different from the case of DC excitation. Unlike this, the current is controlled intermittently and there is a period when the current is always zero, so it is possible to detect the zero point fluctuation of the measured value at this point, and the zero point floating can be calibrated by the detection output. Even if electrolysis progresses, accurate measurement values can always be obtained.

As mentioned above, the method of calculating the logical product of the output of the low frequency generator 5 and the output of the high frequency generator 6 is to control the oscillation of the high frequency generator 6 intermittently using the low frequency signal Pa, or to control the duty ratio of the high frequency generator 6. In addition to switching the output to t/T and 1, for example, as shown in FIG. may be taken out through the inverter 8 and the switch 3 may be controlled by its output.

Further, as a method of varying the duty ratio of the high frequency generator 6, for example, as shown in FIG. By configuring the wave oscillator 6c to supply a sawtooth wave Sa as shown in FIG. 5A, the comparator 6a outputs a sawtooth wave S as shown in FIG. 5B.
A pulse signal sb equal to the repetition frequency of a can be obtained.

Then, by setting the set voltage Ec of the voltage setting circuit 6b to an appropriate value, the pulse signal sb obtained from the comparator 6a
The duty ratio t/T can be changed as appropriate and set to an arbitrary duty ratio.

Therefore, by setting this duty ratio, an excitation current having an arbitrary amplitude value can be supplied to the excitation coil 2.

FIG. 6 shows another embodiment of the invention.

In this example, an AC voltage supplied from a commercial power source 9 is supplied through a switch circuit 3 to a full-wave rectifier circuit 10 composed of a diode bridge, which full-wave rectifies the AC voltage and outputs the full-wave rectified output. A case is shown in which the power is supplied to the excitation coil 2.

In the switch circuit 3 and 5, an NPN transistor Q1.
A case is shown in which a bidirectional switch circuit is configured by connecting Q2 in antiparallel.

Transistor Q1. At the base of Q2, a pulse signal Pb (FIG. 7B) is made into an intermittent signal according to the period of the signal Pa (FIG. 7A) from the high-frequency pulse generator 6 to the low-frequency generator 5.
By supplying the full-wave rectifier circuit 10 with
A signal Pc sampled by the pulse signal Pb is supplied as shown in FIG.

This sampled signal Pc is full-wave rectified by a full-wave rectifier circuit 10, and a full-wave rectified signal Pd as shown in FIG. 7 is supplied to the excitation coil 2.

Therefore, a current iφ obtained by integrating the full-wave rectified signal Pd flows through the excitation coil 2, and its maximum value can be varied and set to an arbitrary value depending on the duty ratio of the pulse signal Pb.

During the period when the full-wave rectification signal Pd does not exist, the full-wave rectification circuit 10
．． Current is emitted from the coil 2 through the diode.

Therefore, in this example, there is no particular need to provide the diode 4 shown in FIG.

As explained above, according to the present invention, the value of the excitation current flowing through the excitation coil 2 can be varied by varying the duty ratio of the high-frequency pulse signal pb. It is only necessary to set the duty ratio so that the excitation power source for electromagnetic flowmeters of different diameters can be shared, and manufacturing management becomes easy.

In addition, cost reductions can be expected as parts can be standardized.

In addition, in the above, the switch 3 is inserted in series between the power supply 1 or 9 and the excitation coil 2, but if the power supply 1 or 9 is composed of a constant current source circuit, the switch 3 is connected in parallel to the power supply or the coil 2. It's okay.

Further, in the above explanation, it is assumed that rectangular waves Pa and Pb can be obtained from the low frequency generator 5 and the high frequency generator 6.
The signal does not necessarily have to be a rectangular wave, and may be a sine wave signal, for example.

Further, the low frequency generator 5 and the high frequency generator 6 do not need to constitute an oscillator themselves; for example, the signal from the reference oscillator is divided by an appropriate divider, and the high frequency signal P
It is easy to understand that it is also possible to obtain the low frequency signal Pa and the low frequency signal Pa.

[Brief explanation of drawings]

Fig. 1 is a connection diagram showing a basic embodiment of this invention, Fig. 2 is a waveform diagram for explaining its operation, and Figs. 3 and 4 are connections showing other embodiments of the main part of this invention. 5 is a waveform diagram for explaining the operation of FIG. 4, FIG. 6 is a connection diagram showing another embodiment of the present invention, and FIG. 7 is a waveform diagram for explaining the operation. 1.9: Power supply, 2: Excitation coil, 3: Switch, 5: Low frequency generator, 6: High frequency generator.

Claims (1)

[Claims] 1. An excitation coil, a power source that supplies excitation current to the excitation coil, a low frequency generator that determines the excitation frequency of the excitation coil, and a high frequency signal that generates a high frequency signal that is higher in frequency than the low frequency signal generated by the low frequency generator. a high-frequency generator, means for logically multiplying the low-frequency signal and high-frequency signal generated from the low-frequency generator and the high-frequency generator, and a switch means controlled on and off by the logical product; An excitation circuit for an electromagnetic flowmeter, characterized in that the excitation current is controlled by changing a duty ratio of a signal.