JPH0291519A - Exciting method for electromagnetic flowmeter - Google Patents

Abstract

PURPOSE:To mask noises which increase as a flow velocity increases and to reduce the influence of the noises by controlling the value of an exciting current to a large value in a high flow rate range and to a small value in a low flow rate range. CONSTITUTION:A flow rate output QM is stored in a random access memory 29 and a processor 26 uses this data and sends out a signal for varying the level of an exciting current supplied from an exciting circuit 25 to an electromagnetic coil 5 in proportion to the data to the exciting circuit 25. The level of the exciting current, therefore, increases in proportion to the flow rate output QM. Consequently, even when the level of the exciting current is varied, the division between a flow rate signal and a comparison signal is carried out, so the flow rate output QM has no error. Thus, the value of the exciting current is increased in the high flow rate range and decreased in the low flow rate range to make the signal level high in the high flow rate range, thereby eliminating the influence of noises which increase as the flow rate increases.

Description

[Detailed Description of the Invention] <Industrial Field of Application> The present invention relates to an excitation method for an electromagnetic flowmeter, and is particularly concerned with a method for exciting an electromagnetic flowmeter, which should be improved so as to be less affected by noise that changes in relation to flow velocity. Concerning an excitation method for an electromagnetic flowmeter.

<Conventional technology> Although the low frequency excitation method is stable at the zero point, it has the disadvantage of slow response to changes in flow rate, and although the response is fast when excitation is performed at a commercial frequency, which is higher than the low frequency. This method has the disadvantage of causing zero point fluctuations.

Therefore, we focused on the fact that the response is not required as much in the measurement area where the flow rate is small, and in the low flow rate area, the excitation frequency is lowered to stabilize the zero point, and in the measurement area where the flow rate is large, the effect of the fluctuation in the zero point is relatively There is an electromagnetic flowmeter that focuses on reducing the flow rate by increasing the excitation frequency in the high flow rate region to ensure relatively constant accuracy.

FIG. 3 is a circuit configuration diagram disclosed in Japanese Patent Publication No. 57-56008 which realizes the excitation method of this type of conventional electromagnetic flowmeter.

Next, an overview of the contents of this disclosure will be explained.

1 is an electromagnetic flow meter detector, 2 is a conduit, 3.4 is an electrode provided on the conduit 2, 5 is an electromagnetic coil, 6 is a power source, 7 is a switch, 8 is an exciting current detection resistor, and 10 is a comparison signal reception circuit, 11 is a preamplifier, 12 is a deviation amplifier, 13.14.
15 is a semiconductor switch, 16 is a difference calculation smoothing circuit that performs difference calculation and smoothing, 17 is a voltage/frequency converter, 18 is a frequency/current converter, 19 is a frequency dividing circuit, 20 is a timing generation circuit, and 21 is an oscillator. be.

The power supply 6 is connected via a switch 7 to a series circuit of an excitation coil 5 and an excitation current detection resistor 8, and a diode 9 is connected in parallel to this series circuit. Both ends of the resistor 8 are connected to the input end of the receiving port FIj610. Electrode 3.
4 is connected to the input end of the preamplifier 11, and outputs a flow rate signal V to its output end. The flow rate signal ■I is applied to one input terminal of the deviation amplifier 12, and the comparison signal vT, which is the output of the receiving circuit 10, is applied to the other input terminal via the semiconductor switch 13. The output terminal of the deviation amplifier 12 is connected to the re-input terminal of the difference calculation smoothing circuit 16 via a sample circuit consisting of semiconductor switches 14 and 15. The output terminal of the difference calculation smoothing circuit 16 is connected to the input terminal of the voltage/frequency converter 17, and the output terminal thereof is connected to the input terminal of the voltage/frequency converter 17.
8 and one input terminal of the AND circuit 22.

The other input terminal of the AND circuit 22 is connected to the oscillator 21, and its output terminal is connected to the input terminal of the frequency divider circuit 19. The output terminal of the 0 frequency divider circuit 19 is connected to the input terminal of the timing generation circuit 20. ing. The switch 7 is turned on/off by a frequency dividing circuit 19, the semiconductor switch 13 by a voltage/frequency converter 17, and the semiconductor switches 14 and 15 by a timing generation circuit 20.

FIG. 2 is an operational waveform diagram of the electromagnetic flowmeter shown in FIG. 1.

When the switch 7 is turned on/off at the timing shown in FIG. 4(1), a rectangular-wave excitation current flows through the excitation coil 5, with a waveform shown in FIG. 4(2). A comparison signal Vr is detected by the resistor 8 and the receiving circuit 10. Further, at the output end of the preamplifier 11, a flow rate signal V as shown in FIG. 4 (5) proportional to the flow rate of the fluid flowing through the conduit 2 is obtained. The product VrF obtained by multiplying the comparison signal vr by the output frequency F of the voltage/frequency converter 17 and the flow rate signal vl are inputted to the input terminal of the deviation amplifier 12, and the flow rate signal vl is operated so that these match, so the following equation is established. do.

V, =VrF Therefore, the output frequency F is determined as F=V,/Vr, and errors due to fluctuations in the excitation current are compensated for.

The difference calculation smoothing circuit 16 operates the deviation amplifier 1 according to the timing of the semiconductor switches 14 and 15 shown in FIG. 4 (3) and (4).
Figure 4 (6) is obtained by smoothing the sampled voltage.
Set the DC voltage as shown below. This DC voltage is converted into an output frequency F by a voltage/frequency converter 17.

Here, as the on/off drive frequency of the switch 7, that is, the excitation frequency, the frequency (F+FO) obtained by adding the output frequency F and the constant frequency F0 for bias from the oscillator 21 by the AND circuit 22 is determined by the frequency dividing circuit 19. This is the frequency divided by the excitation frequency, which is proportional to the flow velocity.

Therefore, when the flow velocity is small, the response is slow but the zero point is a stable low frequency, and as the flow velocity increases, the zero point becomes less stable but changes to a high excitation frequency with a faster response. Therefore, it is possible to measure the flow rate within a certain error range for each flow rate measurement value rather than for the full scale of the flow rate.

<Problems to be Solved by the Invention> However, in reality, noise that increases as the flow velocity increases includes, for example, flow noise or slurry noise. When such noise mixes into the fluid, conventional electromagnetic flowmeters are capable of ensuring that each flow rate measurement value falls within a certain error range, but it is not possible to effectively eliminate such noise. It is difficult to reduce it.

Means for Solving the Problems> In order to solve the above problems, the present invention firstly provides a reference signal corresponding to an output signal of a detector of an electromagnetic flowmeter and an excitation current flowing through an excitation coil of this detector. In the excitation method of an electromagnetic flowmeter that outputs a flow rate signal related to the ratio to the signal, the value of the excitation current is controlled to be high in a high flow rate region and low in a low flow rate measurement range. In the excitation method of an electromagnetic flowmeter that outputs a flow rate signal related to the ratio between the output signal of a detector of one magnetic current meter and the reference signal corresponding to the excitation@flow flowing through the excitation coil of this detector, the value of the excitation current is is controlled to be high in the high flow rate range and low in the low flow rate measurement range, and the frequency of this excitation current is also controlled to be high in the high flow rate area and low in the low flow rate measurement range.

According to the first invention, the value of the excitation current is controlled to be high in the high flow rate region and low in the low flow rate measurement range, and the signal level is increased in the high flow rate region to increase the signal level as the flow velocity increases. According to the second invention, in addition to the effect of the first invention, flow noise etc. are reduced to improve accuracy, and indication accuracy for each flow rate measurement value is improved. Enables flow measurement.

<Examples> Examples 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. In FIG. 1, a so-called two-frequency excitation type electromagnetic flowmeter is explained as an example.

23 is, for example, a commercial frequency AC power supply, and the terminals and N
The power supply circuit 24 is connected to a power supply circuit 24 that generates a DC voltage for the circuit. The DC voltage generated by the power supply diagram FI@24 is supplied to the excitation circuit 25 and serves as a power supply voltage for a conversion circuit (not shown).

A wJ magnetic current is supplied from the excitation circuit 25 to the electromagnetic coil 5 via an excitation current detection resistor 8 and terminals EX+ and EX2 that separate the detector and the converter.

This excitation current is supplied to the electromagnetic coil 5 as an excitation current having two different frequencies, for example, rectangular waveform high frequency and low frequency, by switching the DC voltage of the excitation circuit 25 in response to a timing signal from the processor (CPU) 26. Ru. Therefore, a comparison signal Vr' containing a high frequency and a low frequency is generated in the excitation current detection resistor 8.

Because of this, the signal proportional to the flow rate generated at the electrode 3.4 also becomes a signal having two types of frequencies: high frequency and low frequency. The signal detected by the electrode 3.4 is input to the preamplifier 27 via terminals A, B, and C that separate the detector and the converter, and a flow rate signal Vi is obtained at its output terminal.

This flow rate signal vL- is converted into a digital signal Dh by an analog/digital converter 28 that converts the high frequency component into a digital signal.
An analog/digital converter (A/DL>29) converts the low frequency component into a digital signal.The timing of this conversion is controlled by a timing signal from the processor 26. .

Further, the comparison signals Vl- are also input to the analog/digital converters 28 and 29, respectively, and the respective digital signals Rh
, R1.

These digital signals Dh, DI, Rh, and R1 are stored in random access memory 29 under the control of processor 26, respectively. For example, digital signals Dh, Dl and Rh, Rfi are alternately taken into the random access memory 29. The procedure, initial values, initialization program 5, parameters, etc. to be operated by the processor 26 are stored in advance in the read-only memory 30.

The high frequency digital signal Dh stored in the random access memory 27 in this way is transmitted to the read only memory 3.
The high frequency wave calculation program stored in the high frequency wave calculation program 0 is executed, and the calculation result is stored in the random access memory 29. A division program is executed to divide the high frequency wave calculation result by the digital signal Rh, which is a comparison signal on the high frequency side.

Further, low frequency digital signal DJ is subjected to low frequency filtering calculation by a low frequency wave calculation program stored in read-only memory 30, and the calculation result is stored in random access memory 29. A division program is executed to divide this low-pass filter calculation result by a digital signal Rj, which is a comparison signal on the low frequency side.

The high-frequency side and low-frequency side data stored in the random access memory 29 after division has been executed in this manner are each added to form the flow rate output Qi. The obtained flow rate output QM has both a stable zero point as a result of low-frequency filter calculation and a high-speed response as a result of high-frequency wave calculation.

Next, the flow rate output QM obtained in the above manner is stored in the random access memory 29, and the processor 26 uses this data to change the excitation current change program to the excitation current change program. From electromagnetic coil 5
A signal for changing the level of the excitation current supplied to the excitation circuit 25 is sent to the excitation circuit 25. As a result, the level of the excitation current increases in proportion to the flow rate output QM. Even if the level of the excitation current is changed in this way, no error occurs in the flow rate output QM because the flow rate signal and the comparison signal are divided as described above.

Note that 31 is a man-machine interface, which is used by an operator to access parameter settings and other data from the outside. 32 is a communication interface for communicating with the outside. 33 is a flow rate signal processed by the processor 26, for example, 4 to 2.
It is used to convert into a unified current signal such as 0 mA and output it to a 2 fll transmission line. Further, the electromagnetic coil 5, the conduit 2, etc. constitute a detector, and the other parts except the AC power supply 23 constitute a converter.

Next, the effect when the level of the excitation current is increased in proportion to the increase in the flow rate output QM will be explained.

An example of noise that increases as the flow rate increases is flow noise Nf. This flow noise Nf is defined by where V is the flow velocity,
If σ is the conductivity, ν is the kinematic viscosity, and ζ is the tool potential, then
It can be expressed by the following formula.

Nt = 2Vζ/νσ As can be seen from this equation, this flow noise Nf increases as the flow velocity V increases, and tends to increase as the conductivity of the fluid to be measured decreases. Therefore, when measuring fluids with low conductivity, This is particularly problematic when targeting

On the other hand, noise that increases as the flow rate increases includes, for example, slurry noise Ns. This slurry noise NS
When the fluid contains slurry, the faster the flow rate, the higher the probability that the slurry will hit the electrode, so the noise tends to increase. Therefore, especially when measuring a slurry fluid, It becomes a problem.

As shown in FIG. 2, the spectrum shows a tendency of change of 1/f when the frequency fjl is plotted on the horizontal axis and the noise Ns is plotted on the axis, but the flowing noise Ni also shows a similar variation.

When measuring the above-mentioned fluids, noise increases as the flow velocity increases, but if the excitation method explained using Figure 1 is adopted, the excitation current increases in response to the increase in flow velocity. Therefore, the signal voltage also increases and the influence of noise is masked and does not become apparent in the flow rate signal.

Next, when the excitation method of increasing the excitation current in response to the increase in flow velocity is further combined with the excitation method of increasing the excitation frequency in proportion to the increase in flow velocity as explained using FIG. 3, Highly accurate flow measurements are made possible within a predetermined error for each flow measurement rather than for full scale. In this case, the noise spectrum shows a change of 1/f with respect to frequency as shown in FIG. 2, so the noise reduction effect is further increased.

When this excitation method is applied to the two-frequency excitation electromagnetic flowmeter shown in FIG. 1, the excitation method is such that the excitation frequency shifts simultaneously on both the low frequency side and the high frequency side in response to the flow rate output QM.

The above explanation is based on an electromagnetic flowmeter with two-frequency excitation, but it is not limited to this. For example, in the case of a conventional single excitation frequency, or in the case of an excitation waveform of a sine wave, a rectangular wave, or a triangular wave. It can be applied to various cases such as Further, the excitation current may be changed stepwise or discontinuously with respect to the flow rate output QM.

Furthermore, when changing the excitation frequency according to the flow rate,
Instead of changing the excitation frequency in proportion to the flow rate, the excitation frequency may be automatically varied, for example, to jump between a commercial frequency power source (for example, 50.60.400 Hz) and a frequency that is an integral multiple of that frequency. Eliminates the influence of power supply noise.

Note that if an insulator such as a ceramic pipe is used as the detector conduit 2, there will be no effect of iron loss, so even if the frequency is made variable, problems with span and zero point fluctuations will not occur. However, the same effect can be achieved by using amorphous metal, for example, in the core of the detector.

<Effects of the Invention> As specifically explained above in conjunction with the embodiments, according to the first invention, the value of the excitation current is controlled to be high in the high flow rate region and low in the low flow rate measurement range, thereby reducing flow noise or slurry. It is possible to reduce the influence of noise by masking noise that increases as the flow velocity increases, such as noise.
Furthermore, according to the second invention, the frequency of the excitation current is controlled to be high in the high flow rate range and low in the low flow rate measurement range, making it possible to perform highly accurate flow rate measurement within a predetermined error for each flow rate measurement value. become.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the configuration of an electromagnetic flowmeter implementing the present invention, Fig. 2 is a characteristic diagram showing the frequency distribution of slurry noise, and Fig. 3 is a diagram for explaining the excitation method of a conventional electromagnetic flowmeter. FIG. 4 is a block diagram showing the configuration, and a waveform diagram explaining the operation of the electromagnetic flowmeter shown in FIG. 3. 1... Electromagnetic flowmeter transmitter, 2... Conduit, 3.4...
・Conduit, 5... Electromagnetic coil, 6... Power supply, 8...
Excitation current detection resistor, 10... Receiving resistor, 11... Preamplifier, 12... Deviation amplifier, 13.14.15.
...Semiconductor switch, 16...Difference calculation smoothing circuit, 19
...Frequency dividing circuit, 20...Timing generation circuit, 21
...Oscillator, 23.. AC power supply, 25.. Excitation circuit, 26.. Grosser, 28.29.. Analog/digital converter. cancer 41

Claims (2)

[Claims] (1) In an electromagnetic flowmeter excitation method that outputs a flow rate signal related to a ratio between an output signal of a detector of the electromagnetic flowmeter and a reference signal corresponding to an excitation current flowing through an excitation coil of this detector, the excitation current An excitation method for an electromagnetic flowmeter characterized by controlling the value of to be high in a high flow rate region and low in a low flow rate measurement range. (2) In an electromagnetic flowmeter excitation method that outputs a flow rate signal related to a ratio between an output signal of a detector of the electromagnetic flowmeter and a reference signal corresponding to an excitation current flowing through an excitation coil of this detector, the excitation current A method for exciting an electromagnetic flowmeter, characterized in that the value of is controlled to be high in a high flow rate region and low in a low flow measurement range, and the frequency of the excitation current is also controlled to be high in a high flow rate region and low in a low flow measurement range.