JPS5844314A - Electromagnetic flow meter - Google Patents

Abstract

PURPOSE:To constitute a close loop and to hold an exciting current in a constant state, by a method wherein the energization to the exciting coil is controlled to turn ON and OFF it by means of a duty cycle into which a deviation between a value of a current flowing to the exciting coil and a reference value is converted. CONSTITUTION:An output of an AC power source is rectified by a rectifying circuit 21 to input it to an exciting coil 18 through a switch 19. A current flowing to the exciting coil 18 is detected by a current detecting resistor 23, a deviation between the detecting value and a reference value 28 is found by a deviation amplifier 35 to input the result to a switch driving circuit 26. A deviation value is converted into a duty cycle by the switch driving circuit 26, and the switch 19 is controlled to turn ON and OFF it according to the duty cycle.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is characterized by repeating the intermittent supply of excitation current to the excitation coil with a cycle longer than the intermittent cycle. Related to electromagnetic flowmeters.

<Background of the Invention> Conventionally used electromagnetic flow needles have used commercial power as their excitation power source. Since the frequency of this commercial power source and the obtained measurement signal frequency are the same, it is easily influenced by induced noise from the commercial power source.

Since the signal line drawn from the transmitter electrode crosses the excitation magnetic flux, so-called 90 degree noise occurs, which is 90 degrees out of phase with the measurement signal, and a circuit to remove this noise is required, making the entire converter complex. .

From this point of view, an electromagnetic flowmeter has been proposed in which the excitation frequency is lowered than the commercial power supply frequency. This is called low-frequency excitation, and according to this low-frequency excitation C, the commercial power supply frequency and the measurement signal frequency are different, so there is less induced noise from the commercial power supply (which makes circuit design easier and shielding etc. In addition, since the magnitude of the measurement signal output may be small, the required excitation current can be reduced.・Modification C: 9
Since the 0-degree noise is reduced in proportion to the frequency, a 90-degree scribble removal circuit is not required, and the conversion circuit is simplified.

In order to perform this low frequency excitation, the excitation coil is supplied with two excitation currents intermittently with a cycle longer than the intermittent cycle, and during the intermittent period (in the second case, the current is supplied according to the time constant by the excitation coil). It has been proposed that the rectangular wave-like current is smoothed and a rectangular wave-like current flows through the excitation coil.In this way, a rectangular-wave-like low frequency current is supplied to the excitation coil, but the rise of the excitation current It takes 62 hours for the current to reach a steady state in the upper O.In other words, at the time when the excitation current flows, the current value changes, which causes the output signal between the electrodes to chimp after reaching the steady state I. In other words, it is more accurate to measure by chimpling the output at <C: near the trailing edge of the rectangular wave excitation current. In order to do (=), the chimple period becomes longer, which means that the period of the rectangular wave current must be lengthened.In other words, the repetition of intermittent excitation current becomes longer, and the flow rate fluctuation -:
The response moderation becomes slow.

On the other hand, like this 1: When the repetition frequency becomes low and the rectangular wave current period becomes long, and a current close to DC (:) flows, an unrelated voltage is generated at the electrode, and the period of this voltage is relatively long, so This period (-the period of the rectangular wave excitation current) is so close that it is impossible to separate them. That is, the influence of the DC potential change occurring at the electrode becomes an island.

The key to improving the delay in the rise of the rectangular wave excitation current is to reduce the time constant of the excitation coil, that is, the inductance value, so that the steady current is reached more quickly, but when using a rectified commercial power supply output as the excitation power source AAA: If only rectification is performed, the ripple of the excitation current will increase and the performance will deteriorate. Therefore, a P-wave device is required, and the excitation current is at most ten-odd amperes, so a P-wave device with a very large power capacity is required, which is not practical. Therefore, it is not preferable to reduce the time constant of the excitation coil, and it is desirable to increase the time constant to smooth the intermittent current in the excitation coil C.

For this reason, an electromagnetic flowmeter has been devised in which the excitation current is supplied intermittently to the excitation coil, and the excitation coil is repeatedly executed and stopped at a cycle longer than the intermittence, and a rectangular-wave excitation current is caused to flow through the excitation coil. An example of this will be explained using FIG. 1. In FIG. The signals induced between these electrodes are amplified by the amplifier 14, compensated for power fluctuations by the divider circuit 15, and supplied to the sampling and holding circuit 16c; It will be done.

The pipe 11 is provided with an excitation coil 18, and this coil C; the current 5 caused by the current 5; Right angles - two are made to occur. Are both ends of the excitation coil 18 switched? 19 through the rectifier circuit 21
For example, a commercial power source 22 is connected to the input side of the rectifier circuit 21 . A C two-excitation current detection resistor 23 is inserted in series with this excitation coil 18, the voltage across it is supplied to a comparator amplifier 24-2, and its output is supplied to a divider circuit 15. A circuit 25 is connected in parallel with the excitation coil 18 (which bypasses its reverse voltage). The switch 19 is controlled on and off by the drive circuit 26C.

In the conventional C cram school 1, the switch 19 is switched on and off at a constant period T, as shown in Figure 2, and the on/off ratio is 50-, and the cycle is longer than this on/off period T. The interruption is repeated, that is, once every T1; the interruption is repeated. This C: Therefore, a voltage as shown in FIG. 2B is generated at both ends of the excitation coil 18, and due to the smoothing action of the excitation coil 18, a rectangular wave ζ that gradually rises as shown in FIG. 2C is generated in the excitation coil 18. Current flows.

In this way, since the current flowing through the excitation coil 18 is not stable at the initial value r-, the induced signal between the electrodes 12. and 13 after this becomes a constant value is the correct signal. Therefore, in Fig. 2, Dt= As shown, T from the leading edge of the rectangular wave current
, the output of the divider circuit 1B is the sampling pulse from the sampling holding circuit 16 (:
The signal is sampled and a measurement output is obtained at terminal 17. Destination (:
As mentioned above, if the time it takes for this excitation current to reach a constant value is long, the period of this sampling pulse must be lengthened, which means that the intermittent repetition period 2Tl becomes longer. Become something.

Here, the on/off of the switch 19 is controlled so that the duty ratio is increased to approximately 100 IC at the beginning of each on/off at C:. For example, as shown in FIG. 2B, the &: switch f19 is controlled, and the switch 19 is kept on during a period T1 at the beginning i; of the intermittent repetition period T:.

Therefore, in this case, the voltage applied to the excitation coil 18C becomes C: as shown in Fig. 2, and the current flowing through the excitation coil 18 rises quickly, as shown in Fig. The time to reach it is shorter! J2 figure H1
: As shown (: The period T2 from the start of intermittent to the sampling hold pulse can be made shorter than the conventional T, and the response speed is correspondingly faster. In addition, when the switch 19 is on, the power supply 22 is The rectified output is supplied to the excitation coil 18, but when it is off, the reverse voltage suppression circuit 2 is caused by the power generated in the excitation coil.
A current is passed through 5. This current rises with a time constant determined by the inductance of the exciting coil 18 and the resistance valve. Since the on/off time of the switch 19 in each current supply lubricant is set earlier than this time constant, the excitation current rises earlier.

The specific configuration of the drive circuit 26 of the switch 19 is shown in No. **. The output of the commercial power supply 22 is a waveform shaping circuit! 7(; is used as a rectangular wave with a period of T:, and this is frequency-divided by the frequency dividing circuit 28 to become a rectangular wave with a period of 2T: as shown in FIG. When opened, the monostable multivibrator 31 is driven.
29g:: is given a duty 1 pulse from the pulse generator 32, and an intermittent output is obtained from the gate 29 every T8 as shown in FIG. 4B&2. Further, the monostable multivibrator 31 is driven at the rising edge of the rectangular wave (second drive), and a pulse with a width T is generated as shown in section C of FIG. Therefore, the output of the gate 33 has the same waveform as shown in FIG. 2, and is supplied to the output circuit 34 to drive the switch 19.

By the way, what about Suin in the above? The drive circuit No. 19 requires a pulse generator 32, a waveform shaping circuit 27, a frequency dividing circuit 28, a monostable multivibrator 31, a gate circuit 29, etc., and has a disadvantage that the structure is complicated.

Furthermore, since the period T of the high frequency wave is constant, there is a drawback that the excitation current value fluctuates when the voltage of the DC power source fluctuates. In order to eliminate this drawback (in the embodiment of the electric rate diagram 1, the excitation current is detected by the resistor 23, and the detected value - 1 is divided by the circuit 15).
6: Divide the measured flow velocity value by the excitation current value to remove the variation in the excitation current.

<Objects of the Invention> The first object of the invention is to provide an electromagnetic flow needle having a simple circuit structure C: a drive circuit for a switch 19 that performs the same operation as described above.

The purpose of In2 of this invention is to provide an electromagnetic flow needle that does not necessarily require a divider circuit 15.

<Summary of the Invention> In the present invention, the excitation coil 18 (: detects the value of the flowing current, compares the current value with a reference value to determine its deviation value, converts the deviation value into the duty cycle C; It is configured to control on/off.

Therefore, according to this invention-1, the excitation circuit and the switch drive circuit constitute a closed loop, and the fluctuation of the excitation current is caused by a swing. It is fed back in the form of a change in the density cycle of 19, and operates to keep the excitation current constant.

<Embodiment of the Invention> FIG. 5 shows an embodiment of the invention. In this embodiment, a configuration is shown in which the low frequency excitation type electromagnetic flow needle described in FIG. 1 is operated.

In 1I5WJ, parts corresponding to those in FIG. The current value generated by the amplifier 23-1 is amplified by the amplifier 24, and the amplified output is supplied to one input terminal (:) of the deviation amplifier 35.The other input terminal (:
gives a reference value from the reference value generation circuit 28. This reference value is here given as a rectangular wave having one period T: as shown in FIG.

The output of the deviation amplifier 3s is supplied to the switch drive circuit 26. In the present invention, the switch drive circuit 26 uses a voltage-duty cycle conversion circuit.

<Description of operation of the invention> The switch drive circuit 26 operates at the moment when the output rectangular wave of the reference value generation circuit 28 rises and reaches the high level C: (: indicates that no current is flowing through the excitation coil 18, and therefore the voltage at the resistor 23 is small, and at this time a larger output is generated from the deviation amplification circuit 315, the deni- ty cycle becomes 100-, and the switch 19 remains on.As a result, the current flows to the excitation coil and increases to reach the steady state -2. Near <C: Therefore, the output of the deviation amplifier 35 becomes small, and the switch 19
The data cycle that controls the
The discontinuation ratio of l becomes small.

Therefore, when the value of the excitation current fluctuates in state C, where two reference values are given to the deviation amplifier 284 from the reference value generation circuit 28, the intermittent ratio of the swing f-19 changes in accordance with the fluctuation, and the excitation current Adjust values. Therefore, the excitation current is operated at a constant value determined by the reference value. Although this embodiment shows an example in which the divider circuit 15 is provided, for the above-mentioned reason, according to the present invention 4-, the excitation current is adjusted to a constant value determined by the reference value. Since the on/off ratio of the switch 19 is controlled to be constant, the divider circuit 15 is not necessarily required.

<Effects of the Invention> As mentioned above (--According to this invention, the switch drive circuit 2
6 can be constituted by a voltage duty cycle conversion circuit C-. The structure of the voltage duty cycle conversion circuit is simple because it can be constructed from, for example, an operational amplifier and a loop connection of a Schmitts) trigger circuit.

However, the excitation circuit and the drive circuit 26 of the switch 19 constitute a closed loop, and this closed loop C: The excitation current is controlled to a constant value corresponding to the reference value. Further, when the dividing circuit 15 is used, there is an advantage that the measurement accuracy can be improved, and a highly accurate electromagnetic flow needle can be obtained.

Furthermore, when the reference value is applied to the deviation amplifier 35c as a low-frequency electric wave, its density is automatically set to 100 at the initial stage when the switch 19 starts intermittent operation.
Since it operates in one motion so that -, the excitation current can rise faster, so the excitation frequency can be increased and the response speed can be increased accordingly. Furthermore, if the excitation frequency is the same, the influence of the induced noise caused by the change in excitation current C will be reduced, and the performance will be improved accordingly. Furthermore, since the magnitude of the excitation current value can be easily adjusted by changing the reference value given to the deviation amplifier 35C, it is possible to drive oscillators of various sizes with the same circuit even with the same power supply voltage. In other words, in the past, in order to excite, for example, a low frequency excitation coil set for commercial power supply, it was necessary to adjust the excitation current by changing the power supply voltage value of the oscillator. According to 2, it is easy to change the reference value (1), so it can be done (2).In addition, excitation coils designed for commercial power supply generally have a large time constant, and by applying the electromagnetic flowmeter of this invention, it is possible to change the reference value with low frequency excitation. It is possible to use it as it is because the excitation current rises quickly.Also, there is no need to particularly reduce the time constant of this excitation coil, and in other words, it is possible to use a large coil with good smoothness as a filter for the power supply. An excitation coil with a large time constant can be used without using a power filter.

In the above-mentioned C:, there is a pause period in the supply of current to the excitation coil 18, but during the pause period, the current may be passed in the opposite direction as shown in the second diagram. Even in that case, the duty is increased in the opposite direction (at the beginning of the intermittent cycle).Also, in the above example, the commercial power supply was rectified and its output was intermittently connected to the excitation coil; It is also possible to directly utilize the intermittent supply.

[Brief Description of the Drawings] Fig. 1 is a block diagram showing an example of the electromagnetic flow needle which is the basis of this invention, and Fig. 2 shows waveforms to explain the operation of the electromagnetic flow needle shown in Fig. 1. Figure-1 Figure 3 is a block diagram showing an example of the switch drive circuit for the electromagnetic flow needle shown in Figure 1, Figure 4 is a waveform diagram for explaining the same, and Figure 5 is a waveform diagram for providing explanation-2. It is a block diagram showing one example of an electromagnetic flow meter. 11: Pipe, 12.13: Electrode, 14: Signal amplifier,
16: sample holding circuit, 18: excitation coil, 19: switch, 21: rectifier circuit, 23: resistor constituting current detection means, 26: switch drive circuit, 35: deviation amplifier. Patent Applicant: Hokushin Electric Manufacturing Co., Ltd. Agent, Takuo Kakino 1. Draft procedure amendment Commissioner of the Japan Patent Office, 1. Indication of the case, 1982 Patent Application No. 146668. 2. Name of the invention, Electromagnetic flow needle 3. Make amendments. Patent Applicant Address: 3-30-1, Shimomaruko, Ota-ku, Tokyo 146 Telephone: Tokyo 759 Bureau 4141-4 Date of amendment order: 6 Contents of amendment (1) Regarding the patent claims in the specification Correct the range column as shown in the attached sheet. Claims (1) A. Current detection means for detecting a current flowing through the excitation coil by a switch connected in series between the excitation coil and a DC power supply. C9 Calculating means for calculating the deviation between the detection side of this current detecting means and a reference value 1 D, converting the deviation value obtained from this calculating means into a duty cycle and controlling the above switch on/off according to the duty cycle tJ6X-i '-y') lI#l] Electron IIi flow needle consisting of tract and poison-. =69-

Claims (1)

[Claims] (1) Between the person, the excitation coil and the DC power supply 6: With the switch connected in series. B. Current detecting means for detecting the current flowing in the electromagnetic coil. C1 Calculating means for calculating the deviation between the detected value of the current detecting means and a reference value. D. Duplicate the deviation value obtained from this calculating means. and a switch drive circuit that converts the duty cycle into a duty cycle 1 and therefore controls the on/off of the switch.