JPS6117285B2 - - 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 ON-OFF controlled 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 divided into two parts of 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 the exciting current; 9 is a rectifier circuit; 10 is a diode through which a current generated by the back electromotive force generated in the exciting coil 5 flows.
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.
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 direct current component generated from the electric circuit etc. is canceled out, but this part is not a characteristic part of the present invention, so the division method is used. It is included in the circuit 7 and its explanation will be omitted.

This electromagnetic flowmeter, which is the basis of the present invention, is characterized in that a comparison voltage proportional to the excitation current is detected by a current transformer 8, and has the following advantages.

(a) The frequency of the current flowing through the current transformer 8 is determined by the ON-OFF of the switch 11 even if the excitation current flowing through the excitation coil is DC or frequency.
Since it is the same as the driving frequency and is a relatively high frequency, 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 a current transformer 8 current waveform. 1 is the current waveform flowing through the primary winding 8a, 4 is the second primary winding 8
Current waveform flowing in b, 5 flowing in exciting coil 5〓〓〓〓
Excitation current waveform 6 is a current waveform generated in the secondary winding 8c of the current transformer 8, and 7 is a waveform obtained by rectifying the alternating current generated in the secondary winding 8c.

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, if the power supply voltage becomes lower than the forward voltage of the rectifier 13, 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 is
It flows not only when switch 11 is OFF but also when the power supply voltage is near zero volts even when switch 11 is ON, and as shown in waveform 4 in Figure 2, the current supplied from the power supply decreases. A corresponding current 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 c 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. 2. As shown, when the power supply voltage is around zero volts, the first and second
The value of the current flowing simultaneously through the 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.

As a way to solve this problem, switch 1
1 may be driven ON-OFF by synchronizing it with the commercial power supply frequency and ensuring that it is always in the OFF state when the commercial power supply voltage is near zero volts.

FIG. 3 is a circuit diagram showing a conventional example of a switch drive circuit 12 that drives the switch 11 necessary for this purpose, 15 is a zero voltage detection circuit as shown in FIG. 4 1 and 2, and 16 is a circuit with a reset terminal R. oscillator, 17
is a low frequency oscillator, and 18 is an AND gate.

FIG. 5 is a waveform diagram for explaining the operation of the electromagnetic flowmeter shown in FIG. 1 using the switch drive circuit shown in FIG.
3 is the rectified voltage waveform, 2 is the zero voltage detection circuit 15
3 is the synchronous timing signal waveform obtained from the AND gate 18 of the switch 11.
ON-OFF drive waveform, 4 is the current waveform generated in the secondary winding 8c of the current transformer 8, 5 is the waveform obtained by rectifying the alternating current generated in the secondary winding 8c, 6 is the exciting current waveform flowing in the exciting coil 5 It is.

In the switch drive circuit 12 shown in FIG.
The zero voltage detection circuit 15 detects the voltage of the commercial power supply 14 near zero volts and generates a synchronization timing signal shown in waveform 2 in FIG.

This signal is input to the reset terminal R of the oscillator 16, the output of the oscillator 16 is reset, and the switch is always turned OFF when the commercial power supply voltage is around zero volts.
A drive signal is generated from the oscillator 16. Note that the low frequency oscillator 17 oscillates a low frequency signal that regulates the excitation state and non-excitation state of the excitation coil 5.

Note that the oscillator 16 may be a timer IC having a reset terminal, or a combination of an oscillator and a counter with a reset terminal.

By the way, when the oscillation frequency of the oscillator 16 is completely stable, the oscillator 1 is activated once by the synchronous timing signal (FIG. 5, 2) near the point where the power supply voltage becomes zero.
If the switch 6 is reset, the switch 11 will be turned off without fail in the vicinity where the power supply voltage becomes zero. Therefore, the excitation current and the comparison voltage maintain a completely proportional relationship. However, the oscillation frequency of the oscillator 16 varies slightly due to temperature and other external noises, so
In reality, the duty cycle of the drive signal of the switch 11 in the vicinity synchronized by the synchronization timing signal may not be 50% as shown in Fig. 5, and the current transformer 8 contains a DC component. Current will flow. Therefore, due to the low frequency characteristics of the current transformer 8, a part of the current flows from the secondary winding of the current transformer 8 to the excitation coil 5 {Fig. 5 6} as shown in Figs. 5 4 and 5. contains a non-proportional error component.

Therefore, in an electromagnetic flowmeter that is excited using an AC power supply, the present invention provides a method for sampling flow signals generated from the electrodes 3 and 4 and a comparison signal from the rectifier circuit 9 at appropriate time intervals. The oscillation of the oscillator 16 is synchronized with the synchronization timing signal in the period where the oscillation is not performed, and the synchronization is canceled in the vicinity of the sample period in the excited state. Even in this case, considering that the amount of fluctuation in the oscillation frequency is small in a short period near the sample period, the switch 11 is turned off near the power supply voltage of the sample period to zero, and the duty cycle of the drive signal is Ensure 50%〓〓〓〓
can be maintained.

FIG. 6 is a circuit diagram showing an example of the switch driving circuit 12 necessary for carrying out the present invention, in which 19 is a waveform shaping circuit, 20 is a frequency dividing circuit, and 21 is a timing generating circuit.

The waveform shaping circuit 19 shapes the voltage waveform of the commercial power supply 14 , and its output terminal is connected to the input terminal of the frequency dividing circuit 20 . The input terminal of the timing generation circuit 21 is connected to the output terminal of the frequency dividing circuit 20.
The AND gate 22 receives the output of the timing generation circuit 21 and the synchronous timing signal from the zero voltage detection circuit 15, and its output terminal is connected to the reset terminal R of the oscillator 16. and gate 1
8 inputs the output of the frequency dividing circuit 20 and the output of the oscillator 16, and the output signal is turned on by the switch 11.
Drive OFF.

FIG. 7 is a waveform diagram for explaining the operation of the electromagnetic flowmeter shown in FIG. 1 using the switch drive circuit shown in FIG. The input output waveform of the frequency dividing circuit 20, 3 is the excitation current waveform flowing through the excitation coil 5, 4 and 5 are the outputs of the timing generation circuit 21, and the flow rate signal generated between the electrodes 3 and 4 and the output of the rectifier circuit 9. 6 is a signal waveform output from the timing generation circuit 21 to the AND gate 22, and 7 is input to the reset terminal R of the oscillator 16. This is the signal waveform.

In the switch drive circuit 12 shown in FIG.
Before and after the sample period in the excitation state, that is, in the period a shown in FIG. 7, the gate of the AND gate 22 is closed by the output of the timing generation circuit 21, as shown in FIG. 7. Therefore, in section a, the oscillator 1 is activated as shown in FIG.
The synchronous timing signal from the zero voltage detection circuit 15 is not supplied to the reset terminal R of the oscillator 16.
is not affected by the synchronization timing signal and the duty cycle of its output signal is kept at 50%.

Therefore, the obtained comparison signal does not include error components as shown in FIG. 5, and a highly accurate comparison signal can be obtained.

In the present invention, in a sample period that is not affected by the synchronization timing signal from the zero voltage detection circuit 15, the switch drive signal from the oscillator 16 remains close to zero volts of the commercial power supply voltage even without synchronization by the synchronization timing signal. De switch 11
In order to prevent the oscillator 16 from turning on, it is necessary to determine the free-running frequency of the oscillator 16 in consideration of the influence of temperature and power supply voltage fluctuations.

Furthermore, the present invention is applicable not only to an electromagnetic flowmeter using a DC excitation method or an electromagnetic flowmeter that repeats an excitation state and a non-excitation state, but also to an electromagnetic flowmeter that repeats a positive excitation state and a negative excitation state or a positive excitation state and a non-excitation state. It can also be applied to an electromagnetic flowmeter that repeats a negative excitation state and a non-excitation state. In this case, the diode 10 is converted into a series circuit of a capacitor and a resistor.

As described above in detail with the embodiments, according to the present invention, the switch 11 is turned off in the vicinity of the power supply voltage becoming zero even when the synchronization timing signal is released in the vicinity of the sample section in the excitation state, and the excitation current is Obtain a comparison signal exactly proportional to
An accurate flow rate output can be obtained from this and the flow rate signal.

[Brief explanation of the drawing]

Fig. 1 is a circuit configuration diagram of an electromagnetic flowmeter which 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 a circuit diagram showing an example of a conventional switch drive circuit. , Figures 4 1 and 2 are zero voltage detection circuit diagrams, Figure 5 is an operating waveform diagram of the electromagnetic flowmeter shown in Figure 1 using the switch drive circuit shown in Figure 3, and Figure 6 is a diagram of the switch according to the present invention. FIG. 7 is a circuit diagram showing an example of a drive circuit, and is an operating waveform diagram of the electromagnetic flowmeter shown in FIG. 1 using the switch drive circuit shown in FIG. 6. 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: zero voltage detection circuit, 16: Oscillator with reset terminal, 1
7: Low frequency oscillator, 19: Waveform shaping circuit, 20:
Frequency dividing circuit, 21: Timing generation circuit. 〓〓〓〓

Claims (1)

[Claims] 1. A rectifying means for rectifying the voltage of an AC power source to obtain a rectified voltage; B. A switch means for turning on and off the rectified voltage under the control of a drive signal; C. A magnetic field for applying a magnetic field to the fluid to be measured. an excitation coil through which an excitation current for application is passed; D first and second primary windings and a secondary winding, the second primary winding being the first primary winding; The excitation current is applied to the first primary winding through the switching means, and the excitation current is applied to the first primary winding through the diode. a current transformer connected to both ends of the excitation coil, through which a current flows due to the counter electromotive force of the excitation coil, and in the secondary winding for obtaining a comparison signal corresponding to the excitation current; E. the frequency of the AC power source; a timing generation circuit that generates a gate signal having a gate width that allows the flow rate signal to pass through for a predetermined period by dividing the frequency of F by a frequency dividing circuit; G. a reset means for generating a reset signal that is reset by the reset signal; means to generate the drive signal by synthesizing the output of the gate signal; An electromagnetic flowmeter comprising: a signal processing means for outputting a signal;