JPS588450B2 - electromagnetic flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electromagnetic flowmeter, and in particular, an object of the present invention is to improve the reliability of an electromagnetic flowmeter in which an exciting current is supplied through a switch element.

Conventional electromagnetic flowmeters use AC excitation using a commercial power source in order to separate the signal voltage from the voltage generated at the electrodes and the DC noise voltage generated due to the electrochemical properties of the electrodes.

In this case, although there is an advantage that the excitation circuit can be constructed only from passive elements, there are the following drawbacks.

■ Easily susceptible to inductive noise from commercial power sources.

■ The loop created by the electrode and the signal drawn from the electrode interlinks with the magnetic flux, generating so-called 90-degree noise that is proportional to the time change of the magnetic flux, which must be removed by a converter, making the configuration of the converter complicated. become.

For this reason, recently, a low frequency excitation method in which excitation is performed at a frequency lower than the commercial frequency is being put into practical use.

This method eliminates the above drawbacks by synchronizing the excitation frequency with an even-numbered fraction of the commercial frequency, making the excitation waveform a rectangular wave, and sampling the signal only in the part where the excitation is stable.

On the other hand, in commercial frequency excitation, the excitation circuit is composed only of passive elements, whereas in low frequency excitation, an excitation current control element, such as an active element such as a semiconductor switch element, is indispensable.

The use of active elements in the excitation circuit poses a problem of reduced reliability.

The reliability of the excitation circuit affects the reliability of the entire electromagnetic flowmeter.

Regarding reliability design of excitation current control circuit, (a) Allow sufficient delay for the elements used.

(Example) Redundancy of the control circuit section is provided by operating multiple excitation current switch elements in parallel.

The following methods have been adopted.

However, in method (a) above, there is a limit to the reliability that can be obtained by delaying, and in method (ii), when one element is destroyed, the current flowing to other switching elements increases, reducing the reliability of the remaining elements. Since it changes in the direction of decreasing, there are drawbacks such as not being able to obtain redundancy corresponding to the number of switch elements.

An object of the present invention is to improve the reliability of an electromagnetic flowmeter in which an excitation current is controlled intermittently by an active element, that is, a switching element.

In this invention, a plurality of switch elements connected in parallel are provided in an excitation current control circuit that controls an excitation current intermittently, and the excitation current control circuit is configured so that excitation is performed at a desired frequency by sequentially opening and closing each switch. At the same time, if any of the switch elements connected in parallel does not operate normally, the switch element that is not operating normally will not detect the flow signal in the section where it should be turned on, and some of the switch elements may be damaged. The system is designed to detect a normal flow rate signal even when the flow rate signal is low.

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

FIG. 1 shows an embodiment of the invention.

In the figure, 1 is a DC excitation power source.

This DC excitation power source 1 can be configured with, for example, a DC power source obtained by rectifying a commercial power source.

The DC output of the excitation power supply 1 is applied to the excitation coil 4 of the flowmeter transmitter 3 through the excitation current control circuit 2.

A resistor R constituting excitation current detection means is connected in series to the excitation coil 4, and a diode D is connected in parallel to the excitation coil 4 to release energy each time the current flowing into the coil 4 is cut off.

In this invention, a plurality of switching elements are connected in parallel in the excitation current control circuit 2, and each switching element is sequentially turned on in a time-sharing manner to supply a rectangular wave current of a predetermined frequency to the excitation coil 4. be.

This column shows a case where two switch elements Sl and S2 are connected in parallel.

As the switch elements S1 and S2, for example, transistors can be used as shown in the figure, and both switch elements S1 and S
For example, circuit breakers 5 and 6 are connected in series to 2, and when the switch elements S1 and S2 are left on for a certain period of time, or when a short-circuit accident occurs in the coil 4 or diode D, these circuit breakers are disconnected. 5
and 6 operate to switch elements S1, S2 or coil 4.
etc., to protect them.

The switching elements S1 and S2 are time-divisionally controlled on and off by a rectangular wave signal outputted from the control signal generator 7 and having a frequency lower than that of the commercial power supply.

The control signal generator 7 includes a waveform shaping circuit 9 that shapes a commercial frequency signal from a commercial power source 8 into, for example, a rectangular wave, and a waveform shaping circuit 9 that receives the output of the waveform shaping circuit 9 and converts it into a waveform of, for example, 1/2 of the commercial power frequency.
And 1/4, 1/8,! A frequency divider 10 that obtains each divided output of /16, an AND circuit 11 that AND-gates the divided outputs of 1/8 and 1/16 of this frequency divider 10, and a polarity gate of the divided output of 1/16. It can be constituted by an inverter 12 that inverts, and an AND circuit 13 that AND gates the inverted output and the frequency-divided output, and the outputs of the AND circuits 11 and 13 are supplied to the control electrodes of the switch elements S1 and S2.

By configuring the control signal generator 7 in this way, the switching elements S1, S2? means that control signals Pf and Pg as shown in FIG. 2 F and G are supplied, and these control signals Pf and Pg
The switching elements S1 and S2 are controlled to turn on and off alternately, and the excitation coil 4 receives an excitation current I as shown in FIG.
h flows.

Note that the signal Pa shown in FIG.
1/8 and 1/16 frequency divided output signals are shown.

When the excitation current Ih flows through the excitation coil 4, an electromotive force proportional to the fluid flow rate is generated between the flow rate detection electrodes 14-14 of the flowmeter transmitter 3, and this electromotive force is transmitted to the preamplifier 15 of the signal processing device K. It is amplified to a required level and supplied to the division circuit 16.

The division circuit 16 divides the flow rate signal by a signal proportional to the excitation current Ih detected from both ends of the excitation current detection resistor R, thereby eliminating fluctuations in the flow rate signal due to fluctuations in the excitation current Ih.

The output of the divider circuit 16 is supplied to a sample and hold circuit 17.

In the sample and hold circuit 17, the switches S3 and S4 are turned on and off alternately in synchronization with the current waveform Ih, sample and hold is performed together with synchronous rectification, and a DC signal proportional to the flow rate is obtained at its output.

This output signal is converted into, for example, a current signal by the output circuit 18 as required, and sent from the output terminal 19 to, for example, an indicator.

Switches 83+84 of sample and hold circuit 17 are controlled by the outputs of AND circuits 20 and 21.

The AND circuit 20 is supplied with a signal obtained by dividing the 1/2 and 1/4 frequency outputs of the frequency divider 10 through the NOR gate circuit 22 and the reduced frequency output, and the AND circuit 21 is supplied with the output of the NOR gate circuit 22. and a signal whose polarity is inverted from the 1/8 frequency divided output by an inverter 23, and the AND circuits 20 and 21 generate sampling pulses Pn and Po as shown in FIG. 2 N and O.
is obtained, and the switches S3 and S4 are controlled.

A waveform Pm shown in FIG. 2M is an output waveform of the NOR gate circuit 22.

Therefore, in the present invention, when the switching elements S, , S2 of the excitation current control circuit 2 do not operate normally, the sampling operation in that section is stopped.

Therefore, in order to detect whether or not each switch S1, S2 has operated normally, an excitation current signal is extracted from the output of the amplifier 24, and the excitation current signal is passed through the waveform shaping circuit 25 to the second waveform shaping circuit 25.
The waveform is shaped into a rectangular wave signal Pi as shown in FIG.
27 set terminal S.

Further, the reset terminal R of the flip-flop 26 is supplied with a signal obtained by inverting the polarity of the 1/i6 divided output of the frequency divider 10 by the inverter 28, and the reset terminal R of the flip-flop 27 is supplied with the 1/i6 divided output of the frequency divider 10. supply directly.

The outputs of flip-flops 26 and 27 are connected to NOR gate circuit 2.
9 to each AND circuit 20 and 21.

With this configuration, when the switch elements S1 and 82 are operating normally, the flip-flops 26 and 27 are set by the excitation current signal Pi, and the flip-flops 26 and 27 are set to 1/1 of the frequency divider 10.
Since the outputs of the flip-flops 26 and 27 are reset by mutually opposite phase signals of the 6-frequency-divided output Pe, the outputs of the flip-flops 26 and 27 are equal to the 146-frequency-divided output Pe of the frequency divider 10, as shown at J and K in FIG.
Signals Pj and Pk are obtained which are synchronized with and have mutually reversed phases.

Therefore, under normal conditions, the OR gate output always provides a logic "1" output as shown in FIG. 2L.

Therefore, the AND circuits 20 and 21 are kept in a state where they always output the sampling pulses Pn and Po.

On the other hand, one of the switch elements S1 and S2, for example S2
If it is damaged and continues to be in a conductive state, the breaker 6 is activated and the circuit of the switch element S2 is opened.

Therefore, as shown in FIG. 3A, the excitation current Ih has a cycle that is half that of normal.

Therefore, the period of the output Pi of the waveform shaping circuit 25 is also reduced to 1/2 as shown in FIG. 3B.

The output of the flip-flop 26 is normally output as shown in FIG. 3C, but since the flip-flop 27 is set and reset at the same time, there is no set period, and its output is " 0” logic.

Therefore, the output of the OR gate circuit 29 is only the output Pj of one flip-flop 26, as shown in FIG. 3E.
The AND circuit 20, 21 is closed during the period corresponding to the half cycle, that is, the period in which the switch element S2 operates, and the sampling pulses Pn and Po are prevented from passing through this period, as shown in FIG. 3F and J. The sample and hold circuit 17 does not operate during this period.

Therefore, the AND circuits 20 and 21 constitute means for stopping the sampling operation of the sample and hold circuit 17 when the excitation current detection means detects the absence of excitation current.

In this way, according to the present invention, since the sampling operation is not performed in the section where the switching element is not operating normally, the accurate flow rate signal obtained in the normal operating section is not disturbed, and the value corresponding to the flow rate is output. Obtainable.

However, since the sampling frequency falsely decreases, when the flow rate changes, the response rate at which the output signal changes decreases to 1/2.

However, even if the response speed decreases, an accurate signal corresponding to the flow rate can be obtained.

Therefore, compared to the case where the excitation current is controlled intermittently using only one switch element, the reliability is twice as high as the number of parallel switch elements.

In the above description, the two flip-flops 26 and 27 are used to control the AND circuits 20 and 21 to prevent the sampling pulse from being supplied to the sample and hold circuit 17 during the period in which a defective switch element operates. , 4th
As shown in the figure, one flip-flop 26 can perform this control.

For this purpose, the output of the waveform shaping circuit 25 and the frequency divider 10 are
The NOR gate output of /2 and 1/4 frequency-divided outputs and the polarity inverted output of 1/8 frequency-divided output are AND gated in an AND circuit 30, and the AND output is supplied to the reset terminal R of the flip-flop 26. .

Further, the configuration is such that the output of the waveform shaping circuit 25 is differentiated by a differentiating circuit 31 and then supplied to the set terminal S of the flip-flop 26.

With this configuration, for example, when the switch element S1 is damaged and becomes conductive, a constant excitation current is applied to the switch element S1.
1 to the excitation coil 4.

Therefore, the differential output is no longer output from the differentiating circuit 31, and the flip-flop 26 is not set.

Since the reset pulse is transmitted every time the NOR gate circuit 22 outputs, that is, in synchronization with the excitation period, the flip-flop 26
The output of is held at "0" logic, and the AND circuit 20.21
is held closed, and no sampling pulse is supplied to the sample hold circuit 1T.

Therefore, the output of the sample hold circuit 17 continues to hold the value before the failure.

On the other hand, when the breaker 5 connected in series with the switch element S1 detects an overcurrent and is opened, intermittent excitation is performed by the switch element S2.

At this time, since the flip-flop 26 is set only when the switch element S2 is turned on, the flip-flop 26 is set only when the switch element S2 is turned on.
In the section in which the flip-flop 26 should operate, the output of the flip-flop 26 is "
0'' logic to prevent sampling pulses from being supplied to the sample and hold circuit 17.

Therefore, sampling pulses are supplied to the sample hold circuit 17 only during the period in which the switch S2 is operated, and a normal flow rate signal can be continued to be transmitted.

Incidentally, it is also possible to use the signal of the negative phase output Q of the flip-flop 26 to immediately open the breaker 5 or 6 when the switching elements S1 and S2 become defective.

For this purpose, the output of the output terminal Q of the flip-flop 26 is supplied to one input terminal of each of two AND circuits 32 and 33 as shown in FIG. A 1/16 frequency divided output is supplied, and a signal obtained by inverting the polarity of the 1/16 frequency divided output by an inverter 34 is supplied to the other input terminal of the AND circuit 33.

With this configuration, for example, when the switch element S1 is kept in the conductive state 5, the output of the AND circuit 32 becomes logic "1" at the timing when the switch element S1 should be turned on, and this signal turns on the breaker 5. If the contact is opened, the time delay until the breaker 5 detects an overcurrent and automatically opens the contact can be reduced.

Note that the breaker 6 is also forcibly opened in the same manner by the output of the AND circuit 33.

Furthermore, in the above description, the excitation power source 1 is a DC power source, and the switching elements S, , S2 are configured to switch-control the DC, but as shown in FIG. The AC commercial power source may be turned on and off, and the turned-on and turned-off commercial voltage may be rectified by the rectifier circuit 35 and supplied to the excitation coil 4.

In this case, a current detection signal proportional to the excitation current can be obtained from both ends of a resistor inserted on the DC side, but it can also be obtained from a current transformer 36 as shown.

Moreover, although the above explanation has been given by exemplifying the case where two switch elements are connected in parallel, it will be easily understood that two or more switch elements can be connected in parallel and used.

[Brief explanation of drawings]

FIG. 1 is a connection diagram showing one embodiment of this invention, FIGS. 2 and 3 are waveform diagrams for explaining the operation of this invention, and FIGS. 4 and 5 are diagrams showing other embodiments of this invention. FIG. 1: Excitation power supply, 2: Excitation current control circuit, S1, S2: switch element.

Claims (1)

[Claims] IA. an excitation coil; B. a plurality of switch elements inserted between the excitation coil and the excitation power source and connected in parallel to each other to supply excitation current to the excitation coil through on/off operations; D. a control signal generator for controlling on and off; a sampling circuit that samples a flow rate signal obtained from the flow rate detection electrode in synchronization with the on/off operation of the switch element; E. excitation current detection means for detecting whether or not excitation current is supplied to the excitation coil; F. means for stopping the sampling operation of the sampling circuit when the excitation current detection means detects no excitation current; An electromagnetic flowmeter equipped with