JPS6122768B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention supplies current to the excitation coil of an electromagnetic flowmeter transmitter via a switch that is controlled ON-OFF at a relatively high frequency, and when the switch is turned on, the power supply supplies current to the excitation coil. The current and switch
The current flowing through the excitation coil due to the back electromotive force of the excitation coil when it is OFF is connected to the current transformer.
The present invention relates to an improvement in an electromagnetic flowmeter in which an alternating current proportional to an excitation current is supplied to the secondary winding of a current transformer by supplying the secondary windings with opposite polarities.

FIG. 1 is a circuit configuration diagram of a conventional electromagnetic flowmeter that is the basis of the present invention, in which 1 is an electromagnetic flowmeter transmitter, 2
is a conduit through which the fluid to be measured passes, 3 and 4 are electrodes provided in the conduit, 5 is an excitation coil, 6 is a signal amplifier that amplifies the flow rate signal generated between the electrodes 3 and 4, 7 is a divider circuit, and 8 is a A current transformer for detecting electromagnetic current; 9 is a rectifier circuit; 10 is a diode for passing a current generated by the back electromotive force generated in the exciting coil 5; 11 is a switch; 12 is a switch drive circuit;
13 is a rectifier, and 14 is a commercial power source.

The configuration of this circuit is as follows.

The commercial power supply 14 is connected to a rectifier 13, and the output of the rectifier 13 is connected to a series circuit consisting of the switch 11, the first primary winding 8a of the current transformer 8, and the exciting coil 5. A series circuit of a second primary winding 8b of a current transformer 8 and a diode 10 is connected in parallel with the exciting coil 5. The electrodes 3 and 4 are connected to the input end of a signal amplifier 6, and the output end of the signal amplifier 6 is connected to a divider circuit 7. A secondary winding 8c of the current transformer 8 is connected to a rectifier circuit 9, and an output end of the rectifier circuit 9 is connected to a divider circuit 7.

The operation of this circuit is as follows.

The AC voltage of the commercial power supply 14 is full-wave rectified by the rectifier 13 . The switch 11 is driven ON-OFF by a switch drive circuit 12 at a frequency higher than the frequency of the commercial power supply 14. Therefore, current is intermittently supplied from the rectifier 13 to the excitation coil 5 via the switch 11. This current flows to the first primary winding 8a of the current transformer 8, and the secondary winding 8a
transmitted to c. Further, when the switch 11 is OFF, a back electromotive force is generated in the excitation coil 5, and the excitation coil 5 has a diode 10 and a current transformer 8.
A current flows through the second primary winding 8b. This current flows through the second primary winding 8 of the current transformer 8.
b to the secondary winding 8c.

Therefore, the excitation current flowing through the excitation coil 5 is caused by the back electromotive force generated in the excitation coil 5 both when the switch 11 is turned on as well as when it is turned off, and becomes a continuous DC excitation current. Further, in the current transformer 8, the winding directions of the first primary winding 8a and the second primary winding 8b are opposite, and the first primary winding 8a and the second primary winding 8b are wound in opposite directions.
The direction of the magnetic flux generated by the current flowing through the secondary winding 8a and the direction of the magnetic flux generated by the current flowing through the second primary winding 8b are opposite to each other. As a result, an alternating current proportional to the excitation current is generated in the secondary winding 8c of the current transformer 8 at the same frequency as the ON-OFF frequency of the switch 11.

On the other hand, the flow rate signal generated between the electrodes 3 and 4 is amplified by a signal amplifier 6 and supplied to a divider circuit 7. The alternating current proportional to the excitation current generated in the secondary winding 8c of the current transformer 8 is rectified into direct current in the rectifier circuit 9, and is used as a comparison signal by the dividing circuit 7.
is supplied to The divider circuit 7 divides the flow rate signal from the signal amplifier 6 by the comparison signal from the rectifier circuit 9, compensates for the flow rate signal due to fluctuations in the excitation current, and obtains a highly accurate flow rate signal from the output terminal OUT. be.

In addition, in the ON-OFF driving of the switch 11, if the OFF time is periodically extended, the low-frequency rectangular wave excitation method of repeating the de-energized state during the long OFF period and the energized state during the ON-OFF period can be used. It becomes an electromagnetic flowmeter.

In a low-frequency excitation type electromagnetic flowmeter, a process is generally performed to compensate for fluctuations in the flow rate signal and excitation current at the point where the excitation current is stabilized in an excitation state and a non-excitation state (or a positive excitation state and a negative excitation state). By sampling the comparison voltage and subtracting the signal in the de-energized state from the signal in the energized state, the DC component generated from the electric circuit etc. is canceled out.

This electromagnetic flowmeter, which is the basis of the present invention, is characterized in that it uses a current transformer 8 to detect a comparative voltage proportional to the excitation current, and has the following advantages.

(a) The frequency of the current flowing through the current transformer 8 is determined by the ON--
Since it is the same frequency as the OFF drive frequency and is relatively high, a highly accurate comparison signal can be easily obtained even with a small current transformer.

(b) Isolation between the commercial power supply and the detected comparison signal is provided by a current transformer.

(c) The comparison signal obtained is obtained as a relatively large value.

By the way, in such an electromagnetic flowmeter, when an attempt is made to detect a comparison signal proportional to the excitation current with higher precision, the following problem occurs near the point where the commercial power supply voltage reaches zero volts.

FIG. 2 is an operating waveform diagram to explain the problem. 1 is a voltage waveform obtained by rectifying the commercial power supply 14 with a rectifier 13, 2 is an ON-OFF driving waveform of the switch 11, and 3 is the current transformer 8's voltage waveform. 1 is the current waveform flowing through the primary winding 8a, 4 is the second primary winding 8
5 is the waveform of the current flowing through the exciting coil 5; 6 is the current waveform generated in the secondary winding 8c of the current transformer 8; 7 is the waveform obtained by rectifying the alternating current generated in the secondary winding 8c. It is. For simplicity, the current transformer is shown here as having flat frequency characteristics at least in the band above the commercial frequency.

The current supplied to the excitation coil 5 from the power supply side is
When the power supply voltage approaches zero volts, switch 11
Even in the ON state, the power supply voltage is lower than the forward voltage of the rectifier 13, so the value of the current supplied from the power supply temporarily decreases in this section, as shown in waveform 3 in FIG. On the other hand, the current flowing through the excitation coil 5 due to the back electromotive force of the excitation coil 5 flows not only when the switch 11 is OFF, but also when the power supply voltage is near zero volts even when the switch 11 is ON. As shown in waveform 4, a current corresponding to the decrease in the current supplied from the power supply side flows.

In other words, when the power supply voltage is near zero volts, switch 1
1 is in the ON state, the first primary winding 8a of the current transformer 8 also has the second primary winding 8b.
Current also flows at the same time. Then, these difference currents will be generated in the secondary winding 8c of the current transformer 8, so if the current generated in the secondary winding 8c shown in the waveform 6 in FIG. 2 is rectified, the current will be shown in the waveform 7 in FIG.
As shown in FIG. 2, when the power supply voltage is near zero volts, the value of the current flowing simultaneously through the first and second primary windings 8a and 8b decreases from the value of the excitation current shown in waveform 5 in FIG.
This reduced value becomes an error in the comparison signal.

The following method can be considered as one method to solve this problem. In other words, rectifier 1
In this method, a smoothing capacitor is inserted into the output terminal of the coil 3, and current is supplied from the smoothing capacitor to the excitation coil 5 when the AC power supply voltage becomes zero volts. However, this method requires a large-capacity capacitor for electromagnetic flowmeters that require a large excitation current of several amperes or more. Furthermore, it is difficult to obtain a highly reliable capacitor with such a large capacity.

Therefore, the present invention performs signal processing using a gate signal having a gate width that is gated for a predetermined period excluding the vicinity where the commercial power supply voltage is zero, since the comparison signal contains an error component when the commercial power supply voltage is around zero volts. This is how it was done.

FIG. 3 shows an electromagnetic flowmeter showing an embodiment of the present invention, in which 15 is a deviation amplifier, 16, 17, and 22 are semiconductor switches, 18 is a subtraction smoothing circuit, 20 is a voltage/frequency converter, and 21 is a frequency/frequency converter. current converter,
23 is an oscillator, 24 is a flip-flop, 26 is a waveform shaping circuit, 7 is a frequency dividing circuit, 28 is a timing generation circuit, and 29 is a zero voltage detection circuit as shown in FIGS. 1 and 2, for example.

The basic configuration of this electromagnetic flowmeter is the same as that shown in FIG. 1, and will be explained in detail next.

First, the configuration of the division circuit is as follows. One input terminal of the deviation amplifier 15 is connected to the output terminal of the signal amplifier 6. The output terminal of the deviation amplifier 15 is connected to the input terminal of a subtraction smoothing circuit 18 via a gate circuit consisting of semiconductor switches 16 and 17. The output end of the subtractive smoothing circuit 18 is connected to a voltage/frequency converter 20. The output end of the voltage/frequency converter 20 is connected to the input end of the frequency/current converter 21, and the output end of the frequency/current converter 21 is the output end OUT of this electromagnetic flowmeter.
The other input terminal of the deviation amplifier 15 is connected to the output terminal of the rectifier circuit 9 via a semiconductor switch 22. This semiconductor switch 22 is turned on by a frequency signal with a constant pulse width from the voltage/frequency converter 20.
- Driven OFF.

Further, the configuration of the switch drive circuit 12 is as follows. The output of oscillator 23 is connected to the input of flip-flop 24. Waveform shaping circuit 2
6 waveform-shapes the voltage of the commercial power supply 14, and its output end is connected to the input end of the frequency dividing circuit 27. Both input terminals of the AND gate 25 are connected to the output terminal of the flip-flop 24 and the output terminal of the frequency dividing circuit 27. The switch 11 is driven ON-OFF by the output of this AND gate.

The configuration of the circuit for driving the gate circuit consisting of semiconductor switches 16 and 17 is as follows. The timing generating circuit 28 receives the output of the frequency dividing circuit 27 as input. The zero voltage detection circuit 29 inputs the voltage of the commercial power supply 14, and its output terminal is connected to the input terminals of the AND gates 30 and 31. The other input terminals of AND gates 30 and 31 are connected to the output terminal of timing generation circuit 28, respectively. The outputs of the AND gates 30 and 31 drive the semiconductor switches 16 and 17 ON and OFF.

4 is a waveform diagram showing the operation of the electromagnetic flowmeter shown in FIG. 3, where 1 is the voltage waveform of the commercial power supply 14, 2
is the output waveform of the AND gate 25, 3 is the output waveform of the rectifier circuit 9 that simply rectifies the voltage of the secondary winding 8c of the current transformer 8, 4 is the output waveform of the zero voltage detection circuit 29, and 5 and 6 are the AND gate 30 , 31.

Therefore, as is clear from the waveform diagram in Figure 4, the third
In the electromagnetic flowmeter shown in the figure, when the excitation current is stable, the deviation amplifier 15 is used only during the period in which the comparison signal is proportional to the excitation current, as shown in waveform 5 in Figure 4.
Since the output voltage is gated, the flow rate signal can be obtained with high accuracy.

Note that the reason why the switch drive circuit 12 is synchronized with the frequency of the commercial power source 14 is to eliminate the influence of induced noise from the commercial power source.

Further, the semiconductor switch 22 is for dividing the flow rate signal from the signal amplifier 6 by the comparison signal. That is, if the output of the signal amplifier 6 is Vi, the comparison signal that is the output of the rectifier circuit 9 is Vr, and the output of the voltage/frequency converter 20 is F, then at the input terminal of the deviation amplifier 15, the following relational expression is obtained. To establish.

Vi=Vr・F Therefore, the output F is F=Vi/Vr, and as the output of the voltage/frequency converter 20,
A value is obtained by dividing the flow rate signal Vi by the comparison signal Vr.

Furthermore, the present invention is not limited to an electromagnetic flowmeter that repeats an excited state and a de-energized state, but also repeats a positive excitation state and a negative excitation state, or a positive excitation state, a de-excitation state, a negative excitation state, and a de-excitation state. It can also be applied to electromagnetic flowmeters that repeatedly enter and exit the excited state. It can also be applied to a DC excitation type electromagnetic flowmeter.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a conventional electromagnetic flowmeter that is the basis of the present invention, Fig. 2 is an operating waveform diagram of the electromagnetic flowmeter shown in Fig. 1, and Fig. 3 is an electromagnetic flowmeter showing an embodiment of the present invention. FIG. 4 is an operating waveform diagram of the electromagnetic flowmeter shown in FIG. 3, and FIGS. 5 1 and 2 are zero voltage detection circuit diagrams, respectively. 1: Electromagnetic flowmeter transmitter, 3, 4: Electrode, 5: Excitation coil, 6: Signal amplifier, 7: Divider circuit, 8:
Current transformer, 8a: first primary winding, 8
b: second primary winding, 8c: secondary winding, 9: rectifier circuit, 11: switch, 12: switch drive circuit, 13: rectifier, 14: commercial power supply, 15: deviation amplifier, 16, 17, 22: Semiconductor switch, 1
8: subtraction smoothing circuit, 20: voltage/frequency converter,
21: Frequency/current converter, 23: Oscillator, 2
4: Flip-flop, 26: Waveform shaping circuit, 2
7: Frequency division circuit, 28: Timing generation circuit, 2
9: Zero voltage detection circuit.

Claims (1)

[Scope of Claims] 1. A rectifying means for rectifying the voltage of the AC power source to obtain a rectified voltage; B. A switch driving signal that is controlled by a switch drive signal that includes an intermittent frequency higher than the frequency of the AC power source and determines the waveform of the excitation current. A driving switch means for turning on and off the rectified voltage; C. An excitation coil through which an excitation current is passed for applying a magnetic field to the fluid to be measured; D. First and second primary windings and a secondary winding. the second primary winding is connected in a direction that cancels the magnetic flux generated by the first primary winding, and the first primary winding is connected to the drive switch via the drive switch means. The excitation current is applied to the second primary winding, and the second primary winding is connected to both ends of the excitation coil via a diode, and a current is applied to the secondary winding due to the back electromotive force of the excitation coil. is a current transformer that obtains a comparison signal corresponding to the excitation current, E is a zero voltage detecting means for detecting a zero voltage near where the voltage of the AC power supply becomes zero, and F is a detection detected based on the zero voltage detecting means. Gate signal generating means for generating a gate signal having a gate width that allows the signal to pass for a predetermined period excluding the vicinity where the voltage of the AC power source becomes zero due to the signal; G. Amplifying the deviation between the flow rate signal output from the transmitter and the feedback signal. The output of the deviation amplifier is applied to the subtraction smoothing means through a pair of switch means opened and closed by the gate signal, and the output voltage of the subtraction smoothing means is converted into a frequency signal proportional to the voltage value. An electromagnetic current meter comprising: signal converting means that controls the comparison signal according to a signal and converts the comparison signal into the feedback voltage.