JPH09325058A - Electromagnetic flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a circuit failure because of an excessive excitation current by setting a protecting circuit which controls a switching element when a reference voltage generated at a reference resistance exceeds a predetermined value, thereby to turn OFF the excitation current. SOLUTION: Electrodes 11a, 11b are fixed in touch with a measurement fluid in a conduit 10. A voltage generated to the electrodes 11a, 11b because of the mutual action of a magnetic field from an excitation coil 12 and a flow rate is processed via an amplifier 14 at a signal-processing circuit 18 and output as a flow rate signal to an output terminal 17. A protecting circuit 29 controls switches SW2 -SW5 when a reference voltage generated at a reference resistance Rs exceeds a predetermined value to shut an excitation current. In other words, if an abnormality is brought about in the excitation coil 12 and the excitation current If2 is increased, a state signal Vc is turned to a low level, bringing other input terminals of gate circuits 27, 28 to a low level. As a result of this, timing signals S20 , S40 are changed to a low level, turning OFF the switches SW2 , SW4 .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flowmeter having an excitation control circuit for inserting a switching element between a DC power source and an exciting coil to control an exciting current flowing through the exciting coil, and more particularly to this electromagnetic flowmeter. The present invention relates to an electromagnetic flow meter improved so that a circuit can be protected when an excessive current flows in an exciting coil.

[0002]

2. Description of the Related Art FIG. 4 is a block diagram showing the structure of a conventional electromagnetic flowmeter. Reference numeral 10 is a detector conduit of the electromagnetic flow meter, which has an insulating lining on its inner surface. 1
1a and 11b are electrodes for detecting a signal voltage.
Reference numeral 12 is an exciting coil, and the magnetic field generated by this is applied to the fluid to be measured. The exciting current I _f1 is supplied to the exciting coil 12 from the exciting circuit 13.

The exciting circuit 13 is constructed as follows. The series circuit of the switches SW ₂ and SW _{3 and} the series circuit of the switches SW ₄ and SW ₅ connected in parallel with the series circuit are:
In both cases, the excitation voltage E _s is applied.

The exciting coil 12 is a switch SW._Two, SW
_ThreeConnection point and switch SW_Four, SW _FiveThe reference resistance at the connection point of
Anti-R_SAre connected to each other. Timing signal S
_Two, S_Three, S_Four, S_FiveIs the switch SW_Two, SW
_Three, SW_Four, SW_FiveControl the opening and closing of. And the standard resistance
Anti-R_SThe exciting coil 12 side of is connected to the common potential point COM.
Have been.

On the other hand, the signal voltage is detected by the electrodes 11a and 11b and output to the preamplifier 14. Preamplifier 14
Then, the common mode voltage is removed and impedance conversion is performed, and the signal is output to the signal processing circuit 16 via the output terminal 15.

The signal processing circuit 16 executes a predetermined signal processing using the voltage obtained from the output end 15 and outputs it as a flow rate signal to the output end 17, but also controls the exciting current I _f1 .

The waveform of the exciting current I _{f1 in} this case is a rectangular wave-like exciting current that changes between positive and negative, and the signal processing circuit 16 controls so that its steady value is constant. Less than,
This point will be described with reference to the waveform chart shown in FIG.

The signal processing circuit 16 sends the timing signals S ₅ and S ₃ to the switches SW ₅ and SW ₃ , respectively, to complementarily open and close these switches as shown in FIGS. 5 (A) and 5 (B). To do.

Further, while the switch SW ₅ or SW ₃ is turned on by the timing signal S ₅ or S ₃ , respectively, the switches SW ₂ and SW ₄ are turned on by the timing signals S ₂ and S ₄ corresponding to the on state. (D),
On / off control is performed as shown in (E).

In this case, since the exciting current I _f1 detected by the reference resistor R _S is sent to the signal processing circuit 16 as the reference voltage V _R1 , the signal processing circuit 16 causes the reference voltage V _R1.
Is compared with a predetermined reference voltage contained therein, and the timing signal S is adjusted so that the exciting current I _f1 corresponding to this reference voltage is obtained.
Controls opening / closing of ₂ or S ₄ .

In this way, by controlling the opening and closing of the timing signals S ₂ and S ₄ , the steady value is controlled to be constant with the exciting current I _f1 which changes to positive or negative as shown in FIG. 5C. are doing.

[0012]

However, the electromagnetic flowmeter using the switching control system for the excitation circuit as described above has an advantage that it can be constructed at low cost because it can use components with low power consumption and low withstand voltage. If a situation occurs in which the inductance of the exciting coil 12 becomes zero, such as a coil short circuit, there is a problem that an excessive exciting current continues to flow due to a delay in the control circuit, causing a circuit failure.

[0013]

In order to solve the above problems, the present invention inserts a switching element between a DC power supply and an exciting coil to control the exciting current flowing through the preceding exciting coil. Related to an electromagnetic flow meter having an excitation control circuit, a reference resistance for detecting the preceding excitation current and a preceding switching element are controlled when the reference voltage generated in the reference resistance exceeds a predetermined value. It has a protection circuit for turning off.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of one embodiment of the present invention. The parts having the same functions as those of the conventional electromagnetic flow meter shown in FIG. 4 are designated by the same reference numerals and the description thereof will be appropriately omitted.

The conduit 10 of the detector of the electromagnetic flow meter has an insulating lining on its inner surface, on which the electrode 1 is placed.
1a and 11b are fixed in contact with the measurement fluid. Then, due to the interaction between the magnetic field applied from the exciting coil 12 to the measurement fluid and the flow rate of the measurement fluid, these electrodes 11
The voltages generated at a and 11b are output to the signal processing circuit 18 via the preamplifier 14, where they are signal processed and output to the output end 17 as a flow rate signal.

On the other hand, DC power source E_SIs the switch SW_TwoWhen
SW_ThreeSeries circuit and switch SW_Four And SW_FiveSeries times
Applied in parallel with the path. Excitation coil 12 is a switch
Ji SW_TwoAnd SW_ThreeConnection point and switch SW_FourAnd SW_Five
Reference resistance R at the connection point_SAre each connected via
You.

The connection point between the exciting coil 12 and the reference resistance R _S is connected to the common potential point COM. These switches SW ₂ , SW ₃ , SW ₄ , and SW
_5, the timing signal _{S 20, S 30, S 40} , and S _50, the opened and closed is controlled.

The reference resistance R _S reference voltage V _S generated at both ends of
Is input to the absolute value circuit 19, the absolute value calculation is executed, and the absolute value voltage V _A1 is output to the error amplifier 20.
The error amplifier 20 has a resistor R between the output terminal and the inverting input terminal.
₁ is connected, and its inverting input terminal (-) is connected to the output terminal of the absolute value circuit 19 via the resistor R ₂ .

Further, a reference voltage E _R1 which determines a steady value of the exciting current I _f2 is applied to the common potential point COM at the non-inverting input terminal (+) of the error amplifier 20, and the error amplifier 20 receives the reference voltage. The deviation from the absolute value voltage V _A1 with respect to E _R1 is calculated and output as the deviation voltage V _D.

In the operational amplifier 21, a resistor R ₃ is connected between the output end and the inverting input end (−), and a resistor R ₄ is connected between the output end of the error amplifier 20 and the non-inverting input end ( +) Is connected to the triangular wave circuit 22.

The duty determining circuit 2 includes the operational amplifier 21, the resistor R ₃ , the resistor R ₄ , and the triangular wave circuit 22.
3 and outputs the duty signal V _DS corresponding to the deviation voltage V _D to the output terminal of the operational amplifier 21.

The comparator 24 has its non-inverting input terminal
(+) Is connected to the common potential point COM and the inverting input terminal
Duty signal V in (-)_DSIs applied to the output terminal
Utility signal V _DSPulse voltage V with waveform shaped_POut
Power.

25 and 26 are both gate circuits,
A pulse voltage V _P is applied to one end of each of the gate circuits 25 and 26, and the gate circuits 25 and 26 receive timing signals S ₅₀ and S ₃₀ output from the signal processing circuit 18 at the other ends thereof. Gate circuit 2 by calculating the logical product of
The switches SW _{2 and} SW ₄ are opened and closed as timing signals S ₂₀ and S ₄₀ via 7 and 28, respectively.

Furthermore, from the absolute value circuit 19 the absolute value voltage V _A2 of the same absolute value voltage V _A1 is outputted to the protection circuit 29. Then, the protection circuit 29 includes a comparator 30,
It is composed of a reference power supply 31 having a reference voltage E _R2 , a DFF (D-type flip-flop) 32, gate circuits 27 and 28, and the like.

The absolute value voltage V _A2 is applied to the inverting input terminal (−) of the comparator 30 to which the reference voltage E _R2 is applied from the reference power supply 31 to the non-inverting input terminal (+), and its output terminal is the inverting output terminal. DFF32 in which <Q> and the data end D are connected
Is connected to the input terminal CK of the
_J is output.

Further, the status signal V _C generated at the output terminal Q of the DFF 32 is applied to the other ends of the inputs of the gate circuits 27 and 28, and is also output to the signal processing circuit 18.

Next, the operation of the embodiment configured as described above will be described in detail with reference to FIGS. FIG.
Shows a waveform diagram during normal operation, and FIG. 3 shows a waveform diagram during protection operation.

First, a normal excitation operation will be described.
The signal processing circuit 18 outputs timing signals S ₅₀ and S ₃₀ for determining the excitation period, and the switches SW ₅ and SW ₃ are turned on / off as shown in FIGS. 2 (A) and 2 (B). Is switched to.

For example, the timing signal S ₅₀ (see FIG. 2)
When the switch SW ₅ is turned on by (A)) and the switch SW ₂ is turned on by the timing signal S ₂₀ (FIG. 2 (D)), the exciting current I
_f2 begins to flow as shown in FIG. 2C, and a magnetic field having a waveform similar to this exciting current I _f2 is applied to the measurement fluid.

A reference voltage V _S is generated in the reference resistor R _S by the exciting current I _f2 , and this reference voltage V _S is output to the absolute value circuit 19, where the absolute value calculation is performed and the absolute value voltage V _S is obtained.
It is output to the error amplifier 20 as _A1 .

Since the reference voltage E _R1 for determining the steady value of the exciting current is applied to the non-inverting input terminal (+) of the error amplifier 20, the absolute value voltage V V is applied to the output terminal of the error amplifier 20.
_The deviation voltage V _D corresponding to the deviation between _A1 and the reference voltage E _R1 is output to the duty determining circuit 23.

The duty determining circuit 23 is compared with the triangular wave signal V _T output from the triangular wave circuit 22 and outputs the duty signal V _DS to the output terminal thereof. This duty signal V
_{The DS} is output to the comparator 24, compared with the potential of the reference potential point COM, pulsed, and output as the pulse voltage V _P to the output terminal.

This pulse voltage V _P is applied to one end of the gate circuit 25 having the timing signal S ₅₀ (FIG. 2 (A)) applied to the other end, and is output to one end of the gate circuit 27 via its output end. To be done.

The other end of the gate circuit 27 is
Since the state signal V _C is held at the high level “H”, the pulse voltage V _P passes through the gate circuit 27 as it is, and the switch S is output as the timing signal S ₂₀ (FIG. 2 (D)).
Open and close W ₂ .

As a result, the exciting current I _f2 is shown in FIG.
As shown in (C), it rises according to the time constant of the excitation circuit and changes like a sawtooth wave, but finally the deviation voltage V
It is controlled so that _D becomes zero, that is, the exciting current value corresponding to the reference voltage E _R1 .

Next, the timing signal S ₅₀ (FIG. 2A)
Causes the switch SW ₅ to turn off, and the timing signal S
_{When the} switch SW ₃ is turned on by ₃₀ (FIG. 2 (B)), the switch SW ₄ is opened / closed by the timing signal S ₄₀ (FIG. 2 (E)), and operates similarly to the operation on the switch SW ₂ side. Then, the negative-side waveform of the exciting current shown in FIG. 2C is obtained.

Next, the operation of the protection circuit 29 will be described with reference to FIG. The absolute value voltage V from the absolute value circuit 19 is applied to the inverting input terminal (−) of the comparator 30 of the protection circuit 29.
_A2 is applied.

On the other hand, at the non-inverting input terminal (+) of the comparator 30, the exciting current I _f2 (see FIG.
A reference voltage E _R2, which is a reference value for judging an abnormality when (A)) exceeds a predetermined value, is applied.

In the case of normal operation, the period T _{N in} FIG.
2, the same operation as that shown in FIG. 2 is performed. In this state, the inverting input terminal (−) of the comparator 30 is
The absolute value voltage V _A2 applied to the voltage _V.sub.A2 remains small with respect to the reference voltage _E.sub.R2 .

Therefore, the determination signal V _J (FIG. 3B) obtained at the output terminal of the comparator 30 is at the low level "L".
The state signal V _C (FIG. 3C) appearing at the output terminal Q of the DFF 32 maintains the high level “H”.

[0041] Thus, the other end of the input of the gate circuit 27 is maintained to high level "H", as the output signal as it is the timing signal S _20, S ₄₀ of the gate circuits 25 and 26, respectively It controls the opening and closing of the switches SW ₂ and SW ₄ .

However, as shown at time point Y in FIG. 3 (A), when an abnormality occurs in the exciting coil 12 and the exciting current I _f2 increases, and the absolute value voltage V _A2 exceeds the reference voltage E _R2 , The level of the determination signal V _J (FIG. 3 (B)) rises, and the DFF 32 detects this rising and changes the state signal V _C (FIG. 3 (C)) appearing at the output terminal Q thereof to the low level “L”. Let me keep this level.

Since the status signal V _C (FIG. 3C) becomes low level "L", the other ends of the inputs of the gate circuits 27 and 28 are maintained at low level "L", and as a result, the timing signal S _20, S ₄₀ becomes a low level "L", it is off the switch SW _2, SW _4. This change is also output to the signal processing circuit 18 and output as the alarm signal ALM.

[0044]

As described above in detail with the embodiments of the invention, according to the present invention, the protection circuit for detecting the excessive current flowing in the exciting circuit and turning off the exciting current is used. The advantages of the switching control method, that is, the parts with low power consumption and low withstand voltage can be used and can be constructed at low cost, can be fully utilized.

[Brief description of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention.

FIG. 2 is a waveform diagram illustrating normal operation of the embodiment shown in FIG.

FIG. 3 is a waveform diagram illustrating an abnormal operation of the embodiment shown in FIG.

FIG. 4 is a block diagram showing a configuration of a conventional electromagnetic flow meter.

5 is a waveform diagram for explaining the operation of the electromagnetic flow meter shown in FIG.

[Explanation of symbols]

10 Conduit 11a, 11b Electrode 12 Excitation Coil 13 Excitation Circuit 14 Preamplifier 16, 18 Signal Processing Circuit 19 Absolute Value Circuit 22 Triangle Wave Circuit 23 Duty Determination Circuit 24 Comparator 25, 26, 27, 28 Gate Circuit 29 Protection Circuit 30 Comparator 31 Reference power supply 32 D-type flip-flop V _A1 , V _A2 absolute value voltage V _D deviation voltage V _P pulse voltage V _DS duty signal V _J judgment signal V _C status signal

Claims (1)

[Claims] 1. An electromagnetic flowmeter having an excitation control circuit for inserting a switching element between a direct current power supply and an exciting coil to control an exciting current flowing through the exciting coil, and a reference resistor for detecting the exciting current, An electromagnetic flowmeter having a protection circuit for controlling the switching element to turn off the exciting current when a reference voltage generated in the reference resistance exceeds a predetermined value.