JPS6020015Y2 - electromagnetic flow meter - Google Patents

Description

[Detailed Description of the Invention] 1. Field of Industrial Application This invention relates to an electromagnetic flowmeter, and in particular relates to the improvement of an electromagnetic flowmeter that employs a low frequency excitation method.

1. In the conventional electromagnetic flowmeter using the low frequency excitation method, as shown in FIG. A frequency lower than the commercial power supply frequency, for example, several Hz, is intermittently applied to the excitation coil 5 constituting the pump, and a flow rate signal is obtained from the detection electrode 6-6 at the period of the intermittent frequency.

By using such a low frequency excitation method, it becomes less susceptible to the effects of induced noise from commercial power supplies, and also eliminates the effects of 90° noise, eliminating the need for these noise removal circuits and reducing the flowmeter converter circuit. This has the advantage of simplifying the configuration.

By the way, in an electromagnetic flowmeter, in order to eliminate the influence of fluctuations in the excitation current, a signal proportional to the excitation current is extracted, and the flow rate signal is divided by the current detection signal.

In the example of FIG. 1, a current detection transformer 7 as a current detection means is inserted between the commercial power supply 1 and the rectifier circuit 2, and a voltage signal proportional to the excitation current is obtained at the secondary coil of this transformer 7. This voltage signal is rectified by a rectifier circuit 8, and the rectified output is supplied to a divider circuit 9.

The divider circuit 9 is supplied with the flow rate signal output from the preamplifier 10.

This flow rate signal is divided by the output of the rectifier circuit 8, the flow rate signal is sampled at the time when the excitation current is stabilized in the sampling and integration circuit 11, and the output is given to the voltage-frequency conversion circuit 12 from the voltage-frequency conversion circuit 12. For example, a signal having a frequency proportional to the flow rate is sent out.

Note that 13 is a timing signal generator that generates a signal for controlling on/off of the switch element 3 and a sampling signal.

FIG. 2 shows a waveform diagram for explaining the operation.

FIG. 2A shows the output waveform of the AC power supply 1, B shows the control signal of the switch 3, C shows the current waveform flowing through the exciting coil 5, and D shows the current detection obtained in the secondary winding of the current detection transformer 7. The signal E is a waveform obtained by rectifying the current detection signal in both waves by the rectifier circuit 8, and F and G are sampling signals.

That is, the current waveform flowing through the excitation coil 5 reaches a constant current value only after the time path from the beginning of the current as shown in C.

Therefore, the flow rate signal is obtained by sampling the state in which the excitation current reaches a certain value and the state in which the excitation current has stabilized at a value of zero, subtracting the signals in the excitation state and the non-excitation state, and then subtracting the signals in the excitation state and non-excitation state. The obtained voltage is applied to the voltage-frequency conversion circuit 12, and a signal with a frequency proportional to the voltage value is obtained from the voltage-frequency conversion circuit 12.

By subtracting the signals in the energized state and the de-energized state in this way, even if the zero point changes due to a change in the surface condition of the electrode 6-6, for example, the change in the zero point is detected and a correction operation is performed. That's what I do.

By the way, when the voltage of the commercial power supply 1 crosses the zero wire, a back electromotive force is generated by the electromagnetic energy stored in the excitation coil 5, and a current based on this back electromotive force flows through the excitation coil 5 through the diode of the rectifier circuit 2. .

Therefore, the flow rate signal obtained from the electrode 6-6 also includes a signal component obtained by this back electromotive force.

However, in the example shown in this figure, the current due to the back electromotive force flowing through the excitation coil 5 bypasses the diodes constituting the rectifier circuit 2, and the current is detected.

It doesn't flow to Lance 7.

This is why a wedge-shaped notch appears in the current detection signal shown in FIG. 2E.

By the way, if the width and depth of this wedge-shaped cut are always constant, the division ratio will always be constant and zero point fluctuations, span fluctuations, etc. will not occur, but the wedge-shaped noise that occurs in the bow current detection signal The depth and width of the cut vary depending on the inductance component of the commercial power supply circuit and fluctuations in the commercial power supply voltage.

As a result, the power supply impedance of the commercial power supply differs depending on the installation location, and the cutting depth and width of the wedge-shaped noise change, resulting in a disadvantage that the division ratio becomes a different value depending on the installation location.

In other words, the division rate must be adjusted for each installation location.

Furthermore, if the commercial power supply voltage changes during practical use, the cutting depth and width of the wedge-shaped noise will change, causing the division ratio to change, which inconveniently causes zero point fluctuations and span fluctuations.

For this reason, the present applicant previously proposed an excitation circuit for an electromagnetic flowmeter in which both the flow rate signal and the detected current value are not sampled in the vicinity where the voltage of the AC power source 1 becomes zero.
-Suggested at 6371 Toyo.

In the excitation circuit of the electromagnetic flowmeter proposed earlier, each time the voltage of the AC power source 1 becomes zero, as shown in Figure 2H, a pulse of negative polarity is generated, for example, and this pulse is combined with F and G in the same figure. The logical product with the sampling signal shown in Figure 3 is taken to obtain pulses as shown in Figures 1 and 2, and sampling is performed using these pulses.

According to the sampling pulses shown in FIG. 2 I and J, the voltage of the AC power supply 1 is held at zero logic near zero, so sampling is not performed in this zero logic section, so the flow rate signal and current detection signal are The components in the portions where the values do not match are removed from the sampling results, and an output signal with less error can be obtained.

However, it was found that the circuit applied for earlier had the following drawbacks.

In other words, the flow rate detection signal obtained from the electrode 6-6 is as shown in Fig. 3B.
Inductive noise N induced by the excitation current as shown in FIG.

This induced noise N is considered to be induced in the lead wire portion connecting the electrode 6-6 and the preamplifier 10.

Incidentally, FIG. 3A shows the excitation current waveform. The induced noise N is shown in figure C.
It is thought that the signals shown in and D were synthesized.

Since this induced noise N is originally induced by electromagnetic coupling, it is an AC signal and does not have a DC component.

Therefore, during one cycle of the noise N, the DC current is zero.

For this reason, the sampling period is generally selected to have a cycle that is exactly an integer multiple of the voltage waveform of the AC power source 1, and the AC noise component is canceled by the positive and negative components, so that no DC component due to noise is generated in the sampling period, and the influence of this induced noise I try not to get it.

However, according to the sampling pulses shown in FIG. 2 I and J, a blanking section Bt exists within the sampling section.

In this example, the sampling interval is two cycles of the induced noise shown in Figure 3C, but by providing the blanking interval Bt, the sampling interval is substantially shorter than the two cycles of the induced noise, Due to the existence of the section Bt, the positive and negative waveform areas of the induced noise N become asymmetrical, and the DC component no longer becomes zero.The generation of this DC component changes the zero point, and also causes a span error. be.

In other words, it is better not to provide a blanking section for the flow rate signal.

On the other hand, the cutting depth and width of the wedge-shaped noise of the current detection signal obtained from the current detection transformer 7 changes depending on the power supply impedance or voltage fluctuation of the commercial power supply circuit.

When the cutting depth, width, etc. of this wedge-shaped noise change, the division ratio changes, and this also causes a span error.

In addition, the cutting depth and width of the wedge-shaped noise change depending on the power supply impedance or voltage fluctuation of the commercial power circuit, so the cutting depth and width change depending on the installation location. It is troublesome to have to make adjustments.

Furthermore, even in practical use, the span may change due to fluctuations in the commercial power supply voltage, so it was not possible to improve accuracy beyond the limit.

The purpose of this invention is to eliminate these drawbacks and provide an electromagnetic flowmeter that can always obtain a normal flow rate signal regardless of the installation location.

In this invention, only the excitation current detection signal is blanked without providing a blanking section for the flow rate signal near where the voltage of the AC power source 1 becomes zero.

The width of this blanking is selected to be equal to or slightly wider than the maximum possible width of the wedge-shaped noise.

Even if a blanking section is provided for the excitation current detection signal in this way, the induction noise as shown in FIG. 3B, which is generated by the flow of the excitation current, will not be mixed into the excitation current detection signal.

Therefore, even if a blanking section is provided in the excitation current detection signal, the DC component due to noise shown in FIG. 3B will not occur.

However, according to this invention, even if the cutting depth or width of the wedge-shaped noise that occurs in the current detection signal changes, the wedge-shaped noise period is blanked, so the cutting depth or width of the wedge-shaped noise changes. There is no effect even if the voltage changes, the division ratio can always be kept constant, there is no need to make adjustments depending on the installation location, and the output does not fluctuate even if the power supply voltage fluctuates, making it stable and highly accurate. A flow signal can be obtained.

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

FIG. 4 shows an embodiment of the electromagnetic flowmeter according to this invention.

In this example, a case is shown in which gate means 14 is used as means for performing the blanking operation.

That is, in this type of electromagnetic flowmeter, the flow rate signal output from the sample/integrator circuit 11 is generally converted into a voltage-frequency converter circuit 1.
In step 2, the signal is converted into a signal having a frequency proportional to the flow rate, and a switching element 15 is provided on the output side of the voltage-frequency conversion circuit 12, and this switching element 15 is controlled intermittently according to the duty cycle of the output frequency. The output is supplied to an adder 16 intermittently at a frequency proportional to the output frequency and added to the flow rate signal supplied from the preamplifier 10, and this addition results in division.

Therefore, the adder 16, the sampling integration circuit 11, and the voltage -
The frequency conversion circuit 12 and the switch element 15 generally constitute a division circuit 9.

Therefore, in this invention, a gate means 14 is provided between the voltage-frequency conversion circuit 12 and the switch element 15, and this gate means 14 is closed near the voltage of the AC power supply 1 becoming zero, so that the voltage-frequency conversion circuit 12 The current detection signal is blanked by temporarily cutting off the frequency signal supplied from the switch element 15 to the switch element 15.

The blanking time is chosen to be equal to or wider than the maximum width of the wedge.

Also, during the blanking period, the excitation current detection signal is not supplied to the adder 16, and only the flow rate signal is given. Set.

With this configuration, the correspondence between the amount of the flow rate signal and the amount of the exciting current detection signal will deviate in the blanking section of the excitation current detection signal, but since this deviation occurs over a fixed blanking time, the amount of the deviation will be Therefore, the division ratio of the division circuit 9 can be maintained constant.

Therefore, since the output error caused by the mismatch between the excitation current detection signal and the flow rate signal becomes constant due to blanking, it is easy to eliminate the error.

Furthermore, according to this invention, even if the cutting depth and width of the wedge-shaped noise change, there is no influence from the change, and therefore, there is no need to make adjustments corresponding to the installation location, and the installation work can be simplified.

However, even if the voltage of the commercial power supply 1 fluctuates, the current detection signal supplied to the division circuit 9 changes to follow only the change in the excitation current, and the division ratio can be maintained constant.

There is no possibility that problems such as changes in span due to fluctuations in power supply voltage will occur, and a highly accurate electromagnetic flowmeter can be obtained.

In the above description, a case has been described in which a current detection transformer is used as the excitation current detection means 7, but it is easily understood that as other means, a resistor or a photocoupler between a light emitting element and a light receiving element can be used. I can do it.

[Brief explanation of drawings]

Figure 1 is a system diagram to explain a conventional electromagnetic flowmeter, Figures 2 and 3 are waveform diagrams to explain its operation, and Figure 4 is a system diagram to explain the conventional electromagnetic flowmeter.
The figure is a system diagram showing an embodiment of this invention. 1: AC power supply, 2: rectifier circuit, 3: switch, 4: electromagnetic flowmeter transmitter, 7: excitation current detection means, 9: division circuit,
14: Gate means.

Claims (1)

[Scope of Claim for Utility Model Registration] A. A rectifier circuit that rectifies the output of an AC power source; B. An excitation coil of an electromagnetic flow meter transmitter to which the rectified output of the rectifier circuit is supplied; C. The AC power source and the rectifier circuit. (D) a switch that intermittently controls the current flowing through the excitation coil at a frequency lower than the AC power frequency; (E) a flow rate signal transmitted from the electromagnetic flowmeter oscillator and the excitation current; A sample circuit that alternately samples and integrates the addition output of an adder that adds the excitation current detection signal detected by the current detection means in a state in which the excitation coil is excited and a state in which it is not excited, and an output voltage of this sample circuit. A voltage-frequency conversion circuit that converts the voltage into a frequency proportional to the voltage value, and a switch element that connects and cuts the excitation current detection signal given to the adder by the frequency signal output from the voltage-frequency conversion circuit. an electromagnetic flowmeter comprising: a calculation circuit;