JPH0153403B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a low frequency excitation type electromagnetic flowmeter.

Generally, an electromagnetic flowmeter is configured to apply a magnetic field perpendicular to the direction of fluid flow, simultaneously detect changes in electrical signals in the fluid flow path, and measure the fluid flow rate based on this. . Many modern electromagnetic flowmeters use low-frequency excitation methods, such as trapezoidal wave excitation and square wave excitation, which have superior zero point stability compared to AC excitation and DC excitation methods. There is. In this type of low-frequency excitation type electromagnetic flowmeter, in particular, an excitation current is supplied to the excitation coil of the electromagnetic flowmeter transmitter with a steady value that repeats in the order of negative, zero, positive, and zero. Using the signal voltages e _a1 , e _a2 , e _a3 , and e _a4 for each period in which the steady-state value of the excitation current is negative, zero, positive, and zero, the signal processing circuit essentially calculates (e _a1 +e _a2 )−( e _a3 + e _a4 ) is used to remove the effects of electrochemical DC voltage, offset voltage components based on circuits, and noise components associated with excitation current switching, and obtain outputs related to flow rate components. . By the way, even in this type of electromagnetic flowmeter, when the electrochemical direct current voltage fluctuates and the offset voltage component changes, it is affected by the fluctuations, and output fluctuations cannot be avoided. Moreover, the offset voltage reaches several hundred mV, which is extremely large compared to the span of the flow rate component (1 to 10 mV), and its transient change cannot be regarded as a straight line. Therefore, by multiplying the signal voltages e _a2 and e _a3 by coefficients and then performing the following calculation (e _a4 -3 e _a2 +3 e _a2 - e _a1 ), even the quadratic fluctuation of the offset voltage component can be removed. However, according to experiments, the offset voltage component swings in the positive and negative directions, but the swing is -1V.
It was confirmed that the potential never exceeded ~+1V, and that the potential change always had an inflection point.
Therefore, when the offset voltage component is removed by quadratic approximation, a large error occurs at the inflection point of potential change. In addition, the minimum value of the excitation current in an electromagnetic flowmeter is determined by the ratio to the input conversion noise of the signal processing circuit, and since the input conversion noise is almost vector-added, (e _a1 +e _a2 −e _a3 −e _a4 ) and those that perform the operation (e _a4 −3e _a3 +3e _a2 −e _a1 ) are √1 ² +1 ² +1 ² +1 ² :√1 ² +3 ² +3 ² +1 ² =2: 4.47, and the one that performs the latter calculation requires less excitation current to obtain the same flow rate signal.
The power consumption is 2.24 times larger, and the power consumption is 5 times larger.

In the present invention, the steady-state value of the excitation current given from the electromagnetic flowmeter transmitter is negative, zero, positive, zero, negative, zero, positive,
The signal voltages in each period of zero are e _a1 , e _a2 , e _a3 ,
When e _{a4 , e a5} , _{e a6} , _{e a7} , _{e a8}
e _a1 ) or (e _a8 −3e _a7 +2e _a6 , 2e _a5 −3e _a4 +e _a3 )
This calculation has been carried out to realize a low frequency excitation type electromagnetic flowmeter that can eliminate even third-order fluctuations in offset voltage components with low power consumption.

FIG. 1 is a connection diagram showing an embodiment of the electromagnetic flowmeter of the present invention. In the figure, reference numeral 1 denotes an excitation circuit which includes a DC constant current source 11 and switches 12 and 13 for switching the current from the constant current source 11. Reference numeral 2 denotes an electromagnetic flowmeter transmitter, which includes an exciting coil 21, a pipe 22 through which fluid flows, and electrodes 23 and 24. 3 is a signal processing circuit, which includes an AC amplifier 31 that amplifies the voltage e _a induced between the electrodes 23 and 24 of the electromagnetic flowmeter transmitter 2, and six circuits connected in series to which the output voltage e _b of the AC amplifier 31 is applied. The sample and hold circuits 32 _a to 32 _f and the outputs e _c1 to e _c6 of these sample and hold circuits 32 _a to 32 _f are respectively input to the coefficient multiplier 3.
an arithmetic circuit 34 added via _3a to _33f ;
an inverter 35 that inverts the outputs e and _d of the arithmetic circuit 34;
, a switch 36 that switches between the output e _d of the arithmetic circuit 34 and the output e _f of the inverter 35;
A sample and hold circuit 37 samples and holds e _d or e _f selected in , and pulses P _1a and P _1b that control switches 12 and 13 of the excitation circuit 1.
, pulses P _2a to P _2f that control the sampling of the sample and hold circuits 32 _a to 32 _f , a pulse P ₃ that controls the changeover switch 36, and a pulse that controls the sampling of the sample and hold circuit 37.
It has a timing pulse generation circuit 38 which generates _P4 at the timing shown in FIG. The coefficients K ₁ to _{K 6} of the coefficient units 33 _a to 33 _f are respectively selected as K ₁ = K ₆ = 1, K ₂ = K ₅ = 3, and K ₃ = K ₄ = 2. , (e _c1 −3e _c2 +2e _c3 +
2e _c4 −3e _c5 +e _c6 ).

The operation of the present invention configured in this way will be explained below with reference to the waveform diagram of FIG. 2. First, switch 1
2 and 13 are drive pulses P _1a and P _1b as shown in FIG.
During the period _T1 when _P1a is on, the current _Iw from the constant current source 11 is controlled in the positive direction by the exciting coil 2.
1, and during the period _T3 when P _1b is on, the current _Iw from the constant current source 11 is switched in the opposite direction and flows through the excitation coil 21, and during the period when both P _1a and P _1b are off. No current is applied to the excitation coil 21 at T ₂ and T ₄ . Therefore, as shown in FIG. 2, the excitation coil 21 has an excitation period _T1 in which the steady value is positive and an excitation period _T3 in which the steady value is negative.
There are rest periods T ₂ and T ₄ of zero, and an excitation current I _w whose steady value repeats in the order of positive, zero, negative, and zero is supplied. Note that the excitation current I _w is connected to the switches 12 and 13.
When the constant current source 11 can only supply a finite output voltage, the voltage actually rises due to the time constant due to the inductance and resistance of the excitation coil 21.
After a delay in the falling part, it reaches a steady value, but this is omitted in the figure. An induced voltage e a corresponding to the exciting current I _w is generated between the electrodes 23 and 24 of the electromagnetic flowmeter transmitter 2 _.
occurs. As shown in Fig. 2, the induced voltage e _a includes a flow rate component V _s proportional to the flow rate F of the fluid flowing through the pipe 22, an offset voltage component V o due to the electrochemical DC potential and the circuit, and an excitation voltage component V _s . A noise component V _e accompanying current switching is superimposed.
As a result, the steady value of the excitation current shown in Figure 2 is negative.
Signal voltages e _a1 , e _a2 , e _a3 , e _a4 , e _a5 , e obtained by sampling the induced voltage e _a generated in each period of zero, positive, zero, negative, zero, positive, zero, and negative at regular intervals Δt. _a6 , e _a7 ,
e _a8 and e _a9 are each given by the following equations. Note that the offset voltage component V _o is shown by Taylor expansion and cubic equation approximation.

e _a1 = −V _s −V _e +V _oO e _a2 = +V _e +V _oO +V _o1 +V _o2 +V _o3 e _a3 =V _s +V _e +V _oO +2V _o1 +2 ² V _o2 +2 ³ V _o3 e _a4 = −V _e +V _oO +3V _o1 +3 ² V _o2 +3 ³ V _o3 e _a5 = -V _s -V _e +V _oO +4V _o1 +4 ² V _o2 +4 ³ V _o3 e _a6 = +V _e +V _oO +5V _o1 +5 ² V _o2 +5 ³ V _o3 e _a7 =V _s +V _e +V _oO +6V _o1 +6 ² V _o2 +6 ₃ V _o3 e _a8 = -V _e +V _oO +7V _o1 +7 ² V _o2 +7 ³ V _o3 e _a9 = -V _s -V _e +V _oO +8V _o1 +8 ² V _o2 +8 ³ V _o3 (1) In the signal processing circuit 3, first the electromagnetic flowmeter transmitter 2
An amplifier 31 amplifies the induced voltage e _a from the amplifier 31, and the outputs of the amplifier 31 corresponding to signal voltages e _a1 to e _a9 are sequentially applied to the sample and hold circuit by the sampling pulse P _2a generated at the timing shown in FIG. 32
It is held at _a . Sample hold circuit 32 _a
The hold value e _c1 is held in the sample and hold circuit _32b by the sampling pulse _P2b generated at the timing shown in FIG. Similarly, the hold values _ec2 to _ec5 of the sample and hold circuits _32b to _32e are transferred to the sample and hold circuits _32c to _32f , respectively, by sampling pulses _P2c to _P2f generated at the timing shown in FIG. will be held. Therefore, sample hold circuit 3
At the time when the voltage related to the signal pressure e _a6 is held in 2 _a , the voltages related to e _a5 to e a1 are held in the sample hold circuits 32 _b to 32 _f , and the voltages related to the signal pressure e _a7 are held in the sample hold circuit 32 _{a .} At the time when the voltages related to e a6 to e a2 are held, the sample and hold circuits 32 _b to 32 _f hold the voltages related to e _a6 to e _a2 , respectively. The hold voltages e _c1 to e _c6 of these sample and hold circuits 32 _a to 32 _f are sent to the arithmetic circuit 3 via coefficient units 33 _a to 33 _f , respectively.
4, the calculation (e _c1 -3e _c2 +2e _c3 +2e _c4 -3e _c5 +e _c6 ) is performed. As a result, the output terminal of the arithmetic circuit 34 contains the arithmetic result e _d1 when the output of the amplifier 31 corresponding to the signal voltage e _a6 is held in the sample and hold circuit 32 _a , and the output of the amplifier 3 corresponding to the signal voltage e _a7 .
The calculation result e _d2 when the output of 1 is held,
A voltage with a waveform that sequentially repeats the calculation result e _d3 when the output of the amplifier 31 corresponding to the signal voltage e _a8 is held and the calculation result e _d4 when the output of the amplifier 31 corresponding to the signal voltage e _a9 is held. e _d is obtained. And e _d1 to e _d4 are respectively given by the following equations, assuming that the gain of the amplifier 31 is k.

e _d1 = k(e _a6 −3e _a5 +2e _a4 +2e _a3 −3e _a2 +e _a1 )=4
kV _s (2) e _d2 = k(e _a7 −3e _a6 +2e _a5 +2e _a4 −3e _a3 +e _a2 )=
−4k(V _s +2V _e )(3) e _d3 =k(e _a8 −3e _a7 +2e _a6 +2e _a5 3e _a4 +e _a3 )=−4
kV _s (4) e _d4 = k(e _a9 −3e _a8 +2e _a7 +2e _a6 −3e _a5 +e _a4 )=4
k (V _s + 2V _e ) (5) Therefore, the output of the amplifier 31 corresponding to the signal voltage when the steady value of the excitation current is positive or negative is held in the sample and hold circuit 32 _a as the output of the arithmetic circuit 34. At the time when the output of the amplifier 31, which corresponds to the signal voltage when the steady-state value is zero, is held, the noise component V _e due to switching of the excitation current is superimposed, but when the output of the amplifier 31 is held, the offset voltage component approximates to the cubic equation. While V _oO to V _o3 are removed, the noise component V _e accompanying the switching of the excitation current is also removed, and only the flow rate component V _s can be obtained. Therefore, the selector switch 36 is driven by the pulse _P3 generated at the timing shown in FIG. 2, and when the output of the arithmetic circuit 34 is e _d1 , the sampling pulse generated at the timing shown in FIG. 2 is taken out via the inverter 35.
At _P4 , the output e _d1 of the arithmetic circuit 34 and e _d3 taken out via the inverter 35 are sampled and held by the sample hold circuit 37.
Give alternately. As a result, at the output of the sample-and-hold circuit 37, an output voltage e _p is obtained that is related to the flow rate component V _s .

In this way, in the present invention, it is possible to remove the noise component V _e and the third-order fluctuations in the offset voltage component that occur when switching the excitation current, so even when the offset voltage component exhibits fluctuations that have an inflection point, it is possible to eliminate them. Can be effectively removed. In this case, the input equivalent noise of the signal processing circuit 3 is (1 ² +3 ² +2 ² +2 ² +3 ² +1 ² )
^1/2 = 5.29, but since the flow rate component V _s is multiplied by 4, the signal-to-noise ratio is 5.29/4 = 1.32.
On the other hand, when removing even the second-order fluctuations in the offset voltage, the input equivalent noise is 4.47 and the flow rate component is doubled, as described above, so the signal-to-noise ratio is 4.47/2 = 2.24. becomes. Therefore, it can be seen that according to the calculation of the present invention, the signal-to-noise ratio is considerably lower than when even the second-order fluctuations are removed while even the third-order fluctuations of the offset voltage are removed. Furthermore, according to the present invention, the excitation current required to obtain a flow signal of the same magnitude is 59% compared to the case where even the quadratic fluctuation of the offset voltage component is removed.
(=1.32/2.24), and power consumption can be reduced by as much as 35%. Furthermore, this method has the characteristic that no integration error occurs in response to flow rate fluctuations, and no offset occurs in response to ramp-like flow rate changes.

Note that in the above, the steady value of the excitation current is negative, zero,
The calculation results of equation (2) using signal voltages in each period of positive, zero, negative, and zero, and the calculation results of equation (4) using signal voltages in each period of positive, zero, negative, zero, positive, and zero, are Although the case in which the signals are output alternately has been illustrated, it is also possible to output either one of them. However, the case where output is performed alternately as in the embodiment has the advantage that the responsiveness can be twice as good. Furthermore, in the above description, the case where the output e _b of the amplifier ₃₁ is directly applied to the sample and hold circuit 32 _a has been illustrated, but as shown in FIG. You may also give it to _a . In this case, if the integration time T _s is selected to be an integral multiple of the commercial power supply cycle, the influence of power supply frequency noise can be removed. In Fig. 3, the integrator 39 includes a resistor RI, an operational amplifier OP, an integrating capacitor CI connected to the feedback circuit of OP, a timing switch TS that controls the input integration time _Ts , and resets the integral value. TS and RS are pulses P ₅ and P ₆ from timing pulse generator 38.
The following is an example of what is driven by. Further, the signal processing circuit 3 may be constructed using a microprocessor 40 that performs digital calculations as shown in FIG. In FIG. 4, the output e _i of the integrator 39 is converted into a digital signal by the A/D converter 41 and given to the microprocessor 40, which then converts it into equation (2) or (4). A corresponding digital calculation is performed to output a digital value corresponding to the flow rate component V _s , which is converted into an analog signal by the D/A converter 42 and then held in the sample hold circuit 37 _. An example is shown that yields an associated output voltage e _p . In FIG. 4, the reference voltage e _r is applied to the integrator 39 via the switch TS' driven by the pulse _P7 , and e _r
The A/D converter 41 converts the value integrated over a certain period of time into a digital signal, and then the microprocessor 40
It may also be possible to perform span adjustment, etc. by giving the Furthermore, although the above example uses a rectangular wave as the excitation current _Iw , it is possible to use various waveforms as necessary, such as a trapezoidal wave or a waveform obtained by converting the frequency of a commercial AC power source and then rectifying it. can.

In this way, the present invention includes an excitation circuit that supplies an excitation current whose steady value repeats in the order of zero, negative, zero, and positive to the excitation coil of the electromagnetic flowmeter transmitter, and an excitation current that is supplied from the electromagnetic flowmeter transmitter. _The signal voltages _{during each period in} which the steady _- state _{value of} Then, practically (e _a6 −3e _a5 +2e _a4 +2e _a3 −3e _a2
+e _a1 ) or (e _a8 −3e _a7 +2e _a6 +2e _a5 −3e _a4 +
Since it has a signal processing circuit that performs calculations such as e _a3 ), it is possible to eliminate even third-order fluctuations in the offset voltage component while maintaining a good signal-to-noise ratio, and it can also reduce the excitation current with low power consumption. It is possible to realize a low-frequency excitation type electromagnetic flowmeter that can eliminate noise components associated with switching.

[Brief explanation of drawings]

Fig. 1 is a connection diagram showing one embodiment of the electromagnetic flowmeter of the present invention, Fig. 2 is a waveform diagram for explaining its operation, and Fig. 3 is a waveform diagram for explaining its operation.
4 and 4 are connection diagrams showing other embodiments of the electromagnetic flowmeter of the present invention. 1... Excitation circuit, 2... Electromagnetic flow meter transmitter, 21...
Excitation coil, 23, 24... Electrode, 3... Signal processing circuit, 31... AC amplifier, 32 _a to 32 _f ... Sample hold circuit, 34... Arithmetic unit, 37... Sample hold circuit, 39... Integrator, 40... Micro programmer. 41...A/D converter, 42...D/A converter, 38...timing pulse generator.

Claims (1)

[Claims] 1. An excitation circuit that supplies an excitation current to the excitation coil of the electromagnetic flowmeter transmitter, whose steady value repeats in the order of zero, negative, zero, and positive;・Positive・Zero・Negative・Zero・
The signal voltages in each period of positive and zero are e _a1 , e _a2 , e _a3 , e _a4 ,
When e _a5 , e _a6 , e _a7 , e _{a8 ,} essentially (e _a6 −
3e _a5 +2e _a4 +2e _a3 −3e _a2 +e _a1 ) or (e _a8 −3e _a7
+2e _a6 +2e _a5 -3e _a4 +e _a3 )) and removes even third-order fluctuations in offset voltage components to obtain a flow rate signal.