JPH0554608B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to an electromagnetic flowmeter that applies a magnetic field to a fluid to be measured and measures its flow rate, and in particular improves its excitation method and accompanying signal processing method. Regarding electromagnetic flowmeter.

<Prior Art> Industrial electromagnetic flowmeters have conventionally adopted a commercial frequency excitation method using a commercial power supply. The commercial frequency excitation method is characterized by (a) fast response speed and low cost, and (b) low-frequency random noise (hereinafter referred to as flow noise) that increases with the flow velocity generated in slurry fluids and low conductivity fluids. ), but has the disadvantage that the zero point will fluctuate if left in operation for a relatively long period of time, for example, about one day.

For this reason, a low frequency excitation method that excites at a low frequency of 1/2 of the commercial frequency or lower has come to be adopted. As is well known, the low frequency excitation method has the advantage of providing an electromagnetic flowmeter with a stable zero point. However, since the carrier frequency is low, it is close to the frequency of flow noise, and is therefore susceptible to the influence of flow noise, and this influence becomes particularly noticeable as the flow velocity increases. Furthermore, when damping is applied to reduce the influence of flow noise, the response becomes slow. Furthermore, recent electromagnetic flowmeters are trending towards power saving, and power saving is particularly essential for two-wire electromagnetic flowmeters that simultaneously supply power and transmit signals through two wires. In such cases, it is necessary to reduce the electromotive force per unit flow velocity; for example, while with the conventional low frequency excitation method it was about 0.5 mV/m/s, with the two-wire method it is about 10 μV/m/s. becomes smaller. When the generated electromotive force is reduced by one order of magnitude or more compared to the conventional one, the influence of flow noise increases relatively, so there is a limit to the ability to save power using the low frequency excitation method.

<Problems to be Solved by the Invention> Excitation using a commercial frequency has the advantage of a high response speed and is less affected by flow noise, but has the disadvantage that the zero point is unstable.

On the other hand, with low frequency excitation, the zero point is stable, but it has the disadvantage of being susceptible to flow noise. Furthermore, it was not possible to obtain an electromagnetic flowmeter with a high response speed.
Another problem is that it is not possible to eliminate obstacles to power saving.

<Means for solving the problems> In order to eliminate these problems, the present invention has the following features:
excitation means for supplying magnetic fields having two different frequencies, a first frequency and a lower second frequency; and a signal voltage excited by the excitation means and generated corresponding to the flow rate, which is discriminated based on the first frequency. a first demodulating means for outputting, a high-pass filtering means for high-pass filtering the output of the first demodulating means, a second demodulating means for discriminating and demodulating the signal voltage based on a second frequency, a filtering means and a second demodulating means. and combining means for additively combining each output of the demodulating means.

<Example> Hereinafter, an example of the present invention will be described based on the drawings. FIG. 1 is a block diagram showing one embodiment of the present invention. This embodiment shows a case where a feedback loop is provided separately for a converter of an electromagnetic flowmeter.

10 is a conduit for a transmitter of an electromagnetic flowmeter, and an insulating lining is provided on the inner surface of the conduit. 11
a and 11b are electrodes for detecting a signal voltage. Reference numeral 12 denotes an excitation coil, and a magnetic field generated thereby is applied to the fluid to be measured. A constant current of commercial frequency is applied to the excitation coil 12 from a commercial constant current source 14 via a resistor 13, and at the same time, a constant current of about 50/8 Hz is applied to the excitation coil 12 from a low frequency constant current source 16 via a resistor 15. Low-frequency constant currents are superimposed on each other. As a result, magnetic fields of two different frequencies, a commercial frequency and a frequency 1/8 of the commercial frequency, are applied to the fluid to be measured.

On the other hand, the signal voltage is detected by the electrodes 11a and 11b and output to the preamplifier 17. preamplifier 1
At 7, the common mode voltage is removed and the impedance is converted, and the output is outputted to the coupling point 19 via the output terminal 18.

At the coupling point 19, the deviation between the output of the preamplifier 17 and the output of the multiplier 20 is taken, amplified by the amplifier 21, and synchronously rectified or sampled and held by the demodulator 22. The smoothed DC output is converted into a pulse frequency signal with a constant pulse width by a voltage/frequency converter 23 and fed back to a multiplier 20, and is also fed to a low pass filter 2.
4 and is smoothed, and its output V _L is output to an addition point 25 . The multiplier 20 is composed of, for example, a switch. A low-frequency comparison voltage generated across the resistor 15 is applied to one end of this switch, and the switch is opened and closed by the output pulse of the voltage/frequency converter 23, and the voltage generated at the other end is output to the connection point 19. do. Also, the demodulator 22 has a resistor 15.
A low frequency comparison voltage is applied.

The amplifier 21, the demodulator 22, the voltage/frequency converter 23, the low-pass filter 24, and the multiplier 20 constitute a low-frequency signal processing system 26 that processes a low-frequency signal voltage, and Among the flow rate signals, a signal corresponding to low frequency excitation is processed and outputted to the addition point 25 as an output V _L. This low frequency signal processing system 2
The time constant at 6 is set to be larger than that of the low-pass filter 24 to slow down the response.

On the other hand, the output terminal 18 of the preamplifier 17 and the summing point 2
5, a commercial frequency signal processing system 27 is connected in parallel with the low frequency signal processing system 26.

The deviation between the output voltage of the output 18 of the preamplifier 17 and the output voltage of the multiplier 28 is taken at a node 29 and amplified in an amplifier 30. The output of the amplifier 30 is synchronously rectified by a demodulator 31 using the commercial frequency comparison voltage generated in the resistor 13 as a reference voltage, or sampled and held to become a smoothed DC voltage. This DC voltage is converted by a voltage/frequency converter 32 into a pulse frequency signal having a constant pulse width and fed back to the multiplier 28. The output voltage of the voltage/frequency converter 32 is smoothed by a low-pass filter 33 to form a DC voltage, which is outputted to the summing point 25 as an output _VH via a high-pass filter 34. The addition point 25 adds the outputs V _L and V _H and outputs a composite output V _C to the output terminal 35 .

With such a configuration, in the case of normal operation with little flow rate fluctuation, the commercial frequency signal processing system 27 does not respond due to the presence of the high-pass filter 34, and mainly performs stable low-frequency signal processing at the zero point. Output V _L of system 26
is output as the composite output V _C. On the other hand, the influence of flow noise is reduced due to the presence of the low-pass filter 24 whose large time constant is selected, and it does not appear as a fluctuation in the output V _L. Since the excitation frequency of the commercial frequency signal processing system 27 is high, there is a large frequency difference with flow noise existing in the low frequency region, and its influence does not appear on the output _VH . In other words, in the case of normal operation with little variation in flow rate, it is possible to provide an electromagnetic flowmeter that is not easily affected by flow noise while ensuring a stable zero point.

Next, when the flow rate changes suddenly, the low frequency signal processing system 26 does not respond due to the large time constant of the low pass filter 24, but the time constant of the commercial frequency signal processing system 27 is small and the high pass filter Since the output signal is outputted through the converter 34, the output _VH is immediately responded to and outputted as the composite output _VC .

Furthermore, when the flow rate is zero, flow noise does not occur, so there is no effect, and the presence of the high-pass filter 34 in the commercial frequency signal processing system 27 causes no drift of the zero point, and the output V _H is maintained at zero. , the output V _L of the low frequency signal processing system 26 with a stable zero point is output as a composite output V _C.

Note that the overall amplification degree of the entire amplifier of the low frequency signal processing system 26 and the entire commercial frequency signal processing system 27 is made almost the same, and the time constant as a low pass filter of the entire low frequency signal processing system 26 and the commercial frequency Signal processing system 27
If the time constant as a high-pass filter is kept almost the same, the flow rate will fluctuate suddenly and the composite output will reach a steady flow rate.
The response of V _C becomes smooth.

FIG. 2 is a block diagram showing a second embodiment of the invention. This embodiment shows a case where the same feedback loop is used as the feedback loop of the converter of an electromagnetic flowmeter. Hereinafter, parts having the same functions as those shown in FIG. 1 will be denoted by the same reference numerals, and the description thereof will be omitted as appropriate.

Each constant current from the low frequency constant current source 16 and the commercial constant current source 14 is supplied to the exciting coil 12 via a resistor 36, and a comparison voltage that is a combination of low frequency and commercial frequency is generated in the resistor 36.

On the other hand, a deviation is taken between the output of the preamplifier 17 and the output of the multiplier 37 at the coupling point 19, and the output of the preamplifier 17 and the output of the multiplier 37 are
0 respectively. The output of the amplifier 21 is inputted to the demodulator 22, which separates only the low frequency component by a frequency separation circuit 38 and supplies it as a comparison voltage, and a DC flow signal generated in response to the low frequency excitation is generated at its output terminal. . This flow rate signal is passed through the low pass filter 2.
4 to the summing point 39 as an output V' _L. The output of the amplifier 30 is input to the demodulator 31, which separates only the commercial frequency component by a frequency separation circuit 38 and supplies it as a comparison voltage, and a DC flow signal generated in response to the excitation of the commercial frequency is generated at its output terminal. . This flow rate signal is output via a high-pass filter 34 to a summing point 39 as an output _V'H .

The combined output V' _C obtained by additively combining the outputs V' _L and V' _H at the summing point 39 is converted by the voltage frequency converter 40 into a pulse train having a constant pulse width duty and fed back to the multiplier 37. At the same time, an output V _O is output to the output terminal 41. The multiplier 37 is composed of a switch, etc., and this switch is connected to the voltage frequency converter 4.
It is opened and closed by a zero output pulse, and the comparison voltage of the combined frequency generated across the resistor 36 is fed back to the connection point 19.

The above configuration also operates in the same way as the case of FIG. 1 by selecting a large time constant of the low-pass filter 24.

FIG. 3 is a block diagram showing a third embodiment of the present invention. This embodiment shows a specific example of an excitation circuit that superimposes different frequencies and a specific example of signal processing by a microcomputer.

42 is an interface circuit to the microcomputer 43. 44 is an excitation power supply section that excites the excitation coil 12; 45 is a timing control section; 4
6 is an output circuit that sends a signal to the output terminal 47.

The timing control section 45 supplies a plurality of timing pulses G ₁ and G ₂ to the excitation power supply section 44 to provide excitation timing for composite excitation, and also supplies them to the microcomputer 43 as a time reference for signal processing.

In the timing control section 45, 48 is a clock generator, and 49 is a step-down counter. In compound excitation to the excitation coil 12,
Although the embodiments shown in FIGS. 1 and 2 have been described as a case of composite excitation of a commercial frequency and a low frequency having a frequency of 1/8 of the commercial frequency, the excitation is not limited to these frequencies. Here, an example will be explained in which 128 Hz is adopted as the higher frequency and 1 Hz is adopted as the lower frequency. The frequency f _c generated by the clock generator 48 is assumed to be 4.096 (KHz). This is the clock terminal of the step-down counter 49.
Applied to CL, stepped down output terminals Q ₁ , Q ₂
~ Out of the output of Q ₁₂ , 128 Hz (Fig. 4 A) from output terminal Q ₅ and 1 Hz (Fig. 4 B) from Q ₁₂ are obtained and applied to each input terminal of AND gate 52 via inverters 50 and 51, At its output end, a timing pulse _G1 shown in FIG. 4C is obtained. Also, output terminal Q ₅ , Q ₁₂
Each output is inputted to each input terminal of the NAND gate 53, and the timing pulse _G2 shown in FIG. 4D is obtained at the output terminal. Note that each logic element is connected to the positive power supply +
E, energized by negative power supply -E.

In the excitation power supply section 44, resistors 54, 55, 5
Reference numeral 6 denotes a setting unit for setting the value of the multimode excitation current based on on/off of the timing pulses _{G 1 , G 2 ;} voltage is divided by each resistor 56, and a set voltage V _T (fourth
Figure E) is generated. The resistor 57 is the excitation coil 12
It is connected in series with one end of the voltage V R to generate a comparison voltage V _R (FIG. 4).

The comparator 58 has a set voltage V _T at its non-inverting input terminal,
A comparison voltage V _R is applied to each inverting input terminal,
The switch circuit 59 is controlled to cause the comparison voltage V _R to follow the set voltage V _T . Here, ±E = ±5.5V, the value of resistors 54 and 55 is 5kΩ, and the value of resistor 56 is 500Ω.
When selected, V _T has three levels of setting values from +0.5V to 0 to -0.5V. Further, for example, if the value of the resistor 57 is selected to be 1Ω, the exciting current I _f will be 0.5A to −0.5A.

Switches 60 and 61 are formed by C-MOS transistors in the switch circuit 59, and voltages of +E and -E are applied to one end thereof, and the other end is connected to the other end of the excitation coil 12. switch 6
0 is turned on, the excitation current I _f increases, and the comparison voltage V _R
increases. When the switch 61 is turned on, the excitation current
If I _f decreases, the comparison voltage V _R decreases. By controlling this increase/decrease using the output of the comparator 58, the comparison voltage _VR follows the set voltage V _T. As a result, the excitation coil 12
The excitation voltage e _f at both ends of is the ON/OFF voltage as shown in Figure 4
It becomes an off waveform. On the other hand, the excitation current I _f has a waveform obtained by smoothing the excitation voltage e _f due to the inductance of the excitation coil 12, similar to the comparison voltage V _R shown in FIG. A magnetic field corresponding to these frequencies is therefore applied to the fluid to be measured, and a signal voltage e _s containing the corresponding frequencies appears at the output of the preamplifier 17.

The signal voltage _es has a waveform synchronized with the excitation timing with superimposed differential voltages as shown in FIG. 4 (h) when the flow velocity is zero, and as shown in FIG. 4 (ri) when there is a flow velocity.

The signal voltage e _s and the comparison voltage V _R are connected to voltage/frequency conversion circuits 62 and 6 by the interface circuit 42, respectively.
3 is converted into a frequency and input to the microcomputer 43. Note that an analog/digital converter may be used instead of the voltage/frequency conversion circuits 62, 63.

The microcomputer 43 performs signal processing as described below.

First, the commercial frequency signal processing system 27 in FIG.
The computation as a high-speed signal processing system corresponding to the following is repeatedly executed by the microcomputer 43 in response to timing pulses G ₁ and G ₂ .

H = (average signal voltage in period t ₂ ) - (average signal voltage in period t ₁ ) / (average comparison voltage in period t ₂ ) - (average comparison voltage in period t ₁ ) (1) Period t' ₂ , t ′ ₁ is also calculated in the same way by replacing each period t ₂ and t ₁ in equation (1). This calculation is to calculate the ratio between the difference in signal voltage and the difference in comparison voltage between the non-excitation period _t1 and the excitation period _t2 , and detects the signal voltage by compensating for fluctuations in the excitation current. The data update cycle in this case is 128Hz, indicating a fast response speed.

Next, the calculation as a low-speed signal processing system corresponding to the low-frequency signal processing system 26 in FIG.
The calculation shown by is repeatedly executed by the microcomputer 43 in response to timing pulses G ₁ and G ₂ .

L = (Average signal voltage of period T _A ) - (Average signal voltage of period T _B ) / (Average reference voltage of period T _A ) - (Average reference voltage of period T _B ) (2) However, period T _A is The period T B is the sum of the periods t ₁ and t ₂ , and the period T _B is the sum of the periods t' ₁ and t' ₂ . There are 64 periods t ₁ and t ₂ in the period T _A , and 64 periods t' ₁ and t' ₂ in the period T _B. The data update cycle in this case exhibits a slow response speed of 1 Hz.

In equations (1) and (2), if the excitation current does not vary, there is no need to calculate the denominator of each equation.

The microcomputer 43 performs a low-pass filtering operation on the data obtained by equation (1), performs a high-pass filtering operation on the data obtained by equation (2), and performs an addition operation on these data. It is output to the output circuit 46.

The signal processing by the microcomputer described above produces results similar to those shown in FIG. 1 or 2.

In the above explanation, excitation is based on a sine wave or a rectangular wave, but the excitation is not limited to these waveforms, and various waveforms such as a trapezoidal wave and a triangular wave can be used. Furthermore, although the configuration is such that only one excitation coil 12 is provided and excitation currents of two different frequencies are supplied to it, the excitation coil may be wound twice so that excitation currents of different frequencies are passed through each coil.

<Effects of the Invention> As specifically explained above in conjunction with the embodiments, the present invention provides an excellent electromagnetic system that has a stable zero point, is not affected by flow noise, and has good response. Flowmeter can be realized.

Furthermore, as a result of being resistant to noise, it becomes possible to reduce the electromotive force per unit flow velocity, making it possible to save power.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention;
FIG. 2 is a block diagram showing a second embodiment of the invention, and FIG. 3 is a block diagram showing a third embodiment of the invention. FIG. 4 is a waveform diagram showing waveforms of various parts in FIG. 3. . 17... Preamplifier, 22, 31... Demodulator, 2
3, 32, 40...Voltage/frequency converter, 24...Low pass filter, 26...Low frequency signal processing system, 27...Commercial frequency signal processing system, 34...High pass filter, 42...Interface circuit, 43... microcopy computer, 4
4... Excitation power supply section, 45... Timing control section, 46
...Output circuit, 48...Clock generator, 49...Step-down counter.

Claims (1)

[Scope of Claims] 1. Excitation means for supplying magnetic fields having two different frequencies, a first frequency and a second frequency lower than the first frequency, and a signal voltage excited by the excitation means and generated corresponding to the flow rate. a first demodulating means for discriminating and outputting the signal voltage based on the first frequency; a high frequency means for converting the output of the first demodulating means into a high frequency band; and a first demodulating means for discriminating and demodulating the signal voltage based on the second frequency. 2 demodulating means, a low frequency means for converting the output of the second demodulating means into a low frequency band, and a combining means for additively combining each output of the low frequency means and the high frequency means. An electromagnetic flowmeter featuring: 2. A conduit through which a fluid to be measured whose flow rate is to be measured flows; an excitation coil that passes an excitation current to generate a magnetic field perpendicular to the direction in which the fluid to be measured flows; means for generating a periodic excitation current, each period of which is generated by combining a first excitation current having a first period and a second excitation current having a second period longer than the first period; a current supply circuit, the second period of which is long enough for the corresponding signal voltage portion to become substantially flat; and a first intermediate signal having the first period and a short response time. and a second intermediate signal responsive to the voltage signal of the second period and unaffected by measurement drift, and a measurement signal having a short response time and unaffected by drift. An electromagnetic flowmeter comprising: a processing circuit having means for combining and combining first and second intermediate measurement signals;