JPH0422453B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applicable to various chemical processes, for example,
The present invention relates to an electromagnetic flowmeter used to measure the flow rate of water and other conductive fluids supplied through pipes.

[Conventional technology]

Generally, this type of electromagnetic flowmeter has a means for supplying an excitation signal consisting of a constant cycle alternating current to an excitation coil, and a detection electrode arranged perpendicular to the generated magnetic field and across the flow to be measured. It consists of a converter that detects the resulting potential difference, a means for sampling its output at a timing synchronized with the excitation signal, and a means for calculating the final output from the sampled value. In order to remove the noise caused by this, it is effective to excite and sample in synchronization with the commercial power supply, so conventionally the excitation frequency was set to a maximum of 25Hz when the commercial power supply frequency was 50Hz.

[Problem that the invention seeks to solve]

However, one of the major external noises to the electromagnetic flowmeter is noise caused by temporal changes in potential due to electrochemical polarization occurring between both detection electrodes. This noise is usually 0 to 2V,
It has a voltage and frequency component of about 0.1Hz, and for general fluids, it can be removed by installing an AC image width filter at the first stage input section of the converter, but for special fluids with a high tendency to ionize, this The problem is that the noise sometimes changes in a range of about 0 to 10 Hz, and the above-mentioned method cannot sufficiently cut it, especially when the excitation frequencies are close, and this appears as fluctuation in the output. FIG. 4 shows an example of this, and it can be seen that there is a special fluid with a high ionization tendency indicated by A in the figure, but the output fluctuates greatly compared to the normal fluid indicated by B in the figure.

[Means for solving problems]

In order to solve such problems, the present invention has a frequency higher than the commercial power supply frequency, and synchronizes with a commercial power supply waveform every 2n (n is an integer of 1 or more) cycles of the commercial power supply waveform, In other words, excitation is performed using an excitation signal that repeats the same waveform every 2n cycles and inverts the first half n cycles of the waveform that synchronizes every n cycles within the 2n cycles. , the same timing for the excitation signal between the first half n cycles and the second half n cycles,
In other words, sampling is performed at a timing after the same period of time has elapsed from the start of the n-cycle period, and the difference in output between the two n-cycle periods is determined from the sampled value.

[Effect]

By establishing a specific relationship with the commercial power supply waveform, an output with commercial power supply noise removed can be obtained, making high-speed excitation possible, and as a result, the effects of noise caused by electrochemical polarization can also be suppressed.

〔Example〕

FIG. 1 is a block diagram showing one embodiment of the present invention. In the figure, 1 is a pipe through which the fluid to be measured flows, 2A and 2B are detection electrodes arranged on the inner surface of the pipe 1 to face each other across the flow, and 3 is an excitation coil. By applying an excitation signal consisting of an alternating current, both detection electrodes 2
A signal electromotive force e as shown by e is obtained at A and 2B. Note that e _N equivalently represents noise caused by electrochemical polarization. That is, the excitation switching circuit 4 has four
The control circuit 5 includes switches SW1 to SW4.
By opening and closing these in accordance with control signals CNT1 and CNT2 from the constant current source 6, the current supplied from the constant current source 6 to the excitation coil 3 is invertedly controlled. On the other hand, the electromotive force obtained at both detection electrodes is amplified in the converter 7, and then the difference voltage is converted into the sampling circuit 8.
sent to. The sampling circuit 8 samples the converter output at a predetermined timing based on the control signal from the control circuit 5, converts it into digital data, and sends it to the control circuit 5. Here, the control circuit 5 is constituted by a microcomputer equipped with a processor unit such as a well-known microprocessor, and receives a high frequency clock signal outputted from a first reference clock generating circuit 9 consisting of a crystal oscillator or the like.
Create control signals CNT1 and CNT2 from the second reference clock signal and the commercial power frequency clock signal output by the second reference clock generation circuit 10, and send them to the excitation switching circuit 4.
A sampling signal is sent to the sampling circuit 9 at a specific timing in response to the control signal, and a predetermined operation is performed on the sampling data sent from the sampling circuit 8 to obtain final output data.

In the above configuration, excitation as shown in FIG. 3 is performed. In other words, Fig. 3 shows the relationship between the commercial power supply waveform (a in the figure) and the excitation signal waveform (b in the figure) consisting of positive and negative rectangular waves with rest periods. In addition, the excitation signal has a frequency twice the commercial power supply frequency, and is synchronized with the commercial power supply waveform every two cycles of the commercial power supply waveform. In other words, the same waveform is repeated every two cycles, and The waveform has a waveform that is synchronized every cycle, and has a waveform in which the polarity of the first half of the cycle is inverted from that of the second half of the cycle.

On the other hand, with respect to the excitation signal between one period that is inverted and one period that is not inverted, the polarity of t ₁ , t ₃ , t ₁ ', t ₃ ' and the excitation signal is opposite from the start of that one period. When sampling is performed at the timings t ₂ , t ₄ and t ₂ ′, t ₄ ′, the sampling potentials are s ₁ to s ₈ , and the commercial power supply noise included in them is e _r1 to e _r8 , The outputs e ₁ to _{e 4} within each period of the excitation signal are determined by the following equation.

e ₁ = (s ₁ + e _r1 ) − (s ₂ + e _r2 ) e ₂ = (s ₃ + e _r3 ) − (s ₄ + e _r4 ) e ₃ = (s ₅ + e _r3 ) − (s ₆ + e _r6 ) e ₄ = (s ₇ + e _r7 ) − (s ₈ + e _r8 ) Here, since the excitation signal is basically synchronized every cycle with the commercial power supply waveform, t _k =
When t _k ′ (k=1 to 4), the following relationship holds true for e _r1 to _{e r8} .

e _r1 = e _r5 , e _r3 = e _r7 e _r2 = e _r6 , e _r4 = e _r8 Therefore, of the two periods of the above commercial power supply waveform, the difference in output between the first half period and the second half period e _OUT Then, e _OUT = (e ₁ + e ₂ ) − (e ₃ + e ₄ ) = (s ₁ − s ₂ ) + (s ₃ −
s ₄ ) + (s ₆ - s ₅ ) + (s ₈ - s ₇ ) + e _r1 e _OUT = (e ₁ + e ₂ ) - (e ₃ + e ₄ ) = (s ₁ - s ₂ ) + (s ₃ -
s ₄ ) + (s ₆ - s ₅ ) + (s ₈ - s ₇ ) + e _r1 - e _r2 + e _r3 - e _r4 - e _r5 + e _r6 - e _r7 + e _r8 = (s ₁ - s ₂ )
+( _s3 - _s4 )+( _s6 - _s5 )+( _s8 - _s7 ), and a flow rate output that does not include commercial power supply noise e _r1 to e _r8 can be obtained.

In the embodiment described above, the excitation signal has a waveform that is synchronized every two cycles of the commercial power supply waveform and synchronized every cycle within the two cycles, with only one cycle period in the latter half being inverted from one cycle in the first half. was used, that is, n was set to 1, but n may be set to 2 or more.

In addition, in the above-mentioned embodiment, 2 of the commercial power frequency
Although an excitation signal with twice the frequency was used, there may be any number of excitation cycles within one cycle of the commercial power supply waveform.

Also, t _k = t _k ′, that is, the first half n of the commercial power supply waveform
An excitation signal having an asymmetrical waveform within n cycles of the commercial power supply waveform may be used as long as the condition that sampling is performed at the same timing in the cycle and the latter n cycles is satisfied.

〔Effect of the invention〕

As explained above, according to the present invention, the waveform has a frequency equal to or higher than the commercial power supply frequency, is synchronized with the commercial power supply waveform every 2n cycles of the commercial power supply frequency, and is synchronized every n cycles within the 2n cycles. Excitation is performed with a positive/negative square wave excitation signal having a rest period formed by inverting the first half n cycles and the second half n cycles, and the excitation signal between the first half n cycles and the second half n cycles is used. By sampling at the same timing and calculating the difference in output between both n periods, a flow rate output that is unaffected by both noise caused by electrochemical polarization and commercial power supply noise is obtained. be able to.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention;
Figure 2 is a partial detailed circuit diagram, Figure 3 is a waveform diagram to explain the excitation signal waveform and sampling timing, and Figure 4 is an output waveform to explain the influence of noise due to electrochemical polarization. It is a diagram. 1...Tube, 2A, 2B...Detection electrode, 3...Excitation coil, 4...Excitation switching circuit, 5...
Control circuit, 6... constant current source, 7... converter, 8...
...Sampling circuit, 9,10...Reference clock generation circuit.

Claims (1)

[Claims] 1. Between an excitation signal generating means that supplies an excitation signal consisting of an alternating current with a constant period to an excitation coil, and detection electrodes arranged perpendicular to the magnetic field generated by the excitation coil and facing each other across the flow. An electromagnetic flow rate comprising a converter for detecting the obtained potential difference, a sampling means for sampling the output of the converter at a predetermined timing synchronized with the excitation signal, and a signal processing means for processing the obtained sampling value. In the meter, the excitation signal generating means is
The first half of a waveform that has a frequency equal to or higher than the commercial power supply frequency and is synchronized every 2n cycles (n is an integer of 1 or more) between the commercial power supply waveform and the commercial power supply waveform, and is synchronized every n cycles within the 2n cycle. The sampling means generates a positive/negative rectangular excitation signal having a rest period having a waveform formed by inverting the n-cycle period and the n-cycle period in the latter half, and the sampling means generates an excitation signal of the first-half n period and the second-half n period. An electromagnetic flowmeter characterized in that sampling is performed at the same timing for the excitation signal of the period, and the signal processing means calculates the difference in output between the first n periods and the latter n periods from each sampling value.