JPS5815121A - Electromagnetic flowmeter - Google Patents

Abstract

PURPOSE:To obtain a device having superior stability of zero point and responsiveness by so constituting the same that the direction of the electric current to be flowed to an excitation coil is changed over periodically, a quiescent period when no current flows upon the changing-over time is reserved and the voltage induced during this period is taken in as a compensation voltage. CONSTITUTION:Switches 12a, 12b which change over the positive and negative directions of the electric current from a DC constant current source 11 to be flowed to an excitation coil 21 by means of pulses P1a, P1b periodically are provided. A signal processing circuit 4 is constituted of sample holding circuits 41a-41d which sample the output eb of an AC amplifier 3 and hold the samples, an arithmetic circuit 42 which adds and subtracts the outputs V1-V4 thereof, and a pulse generator 43. The induction voltage generated between the electrodes 23a and 23b in the period when the excitation current is flowed to the circuit 4 is taken in as a signal voltage, and the induction voltage generated in the period when no excitation current flows is taken in as a compensation voltage. The nozzle components accompanied with the changing over of the excitation current contained in the signal voltage are removed by the compensation voltage.

Description

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

Generally, an electromagnetic flow needle is configured to apply a magnetic field perpendicular to the fluid flow direction, 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 or square wave excitation, which have better zero point stability than AC or DC excitation methods. ing. In a low-frequency excitation type electromagnetic flowmeter, the current supplied to the excitation coil is periodically switched between two steady-state values, and when the 1m magnetic current becomes constant, the induced voltage generated between the electrodes is sampled, and then By taking the difference between the matched sampling signals, the influence of electrochemical direct current potential and circuit-based offset voltage is removed, and a signal corresponding to the fluid flow rate is obtained. Even in such a low-frequency excitation type electromagnetic flow needle, the zero point will triad if sufficient time is not taken to sample the edge after the excitation current reaches a certain value. This is due to the induced voltage generated between the electrodes, in addition to the signal component proportional to the fluid flow rate and the offset F component due to the electrochemical DC potential and circuit.
This includes electromagnetic coupling noise generated in the loop between the electrode and electrode lead when excitation current is switched, and first-order lag noise generated by the first-order lag circuit formed by the eddy current flowing in the fluid and the liquid resistance and the interfacial electric double layer capacitance of the electrode. The noise component associated with switching the excitation current is superimposed, and the polarity of this noise component reverses each time the excitation current is switched, so it cannot be eliminated even by taking the difference between adjacent sampling signals, and electromagnetic coupling noise can be eliminated in a short time. This is because, although it becomes zero, -th lag noise does not become zero until a sufficient amount of time has elapsed. Therefore, from the standpoint of zero point stability, the lower the excitation frequency is, the more advantageous it is, and some electromagnetic flowmeters in practical use use the commercial power frequency of 1732.

If the excitation frequency is too low, the response will be slow,
Hunting may occur when a control loop is constructed. Further, when the excitation frequency is lowered, changes in electrochemical DC potential become a problem, and a new means is required to compensate for this change.

The present invention provides a rest period in which no excitation current is passed between a period in which a positive excitation current is passed through the excitation pulley and a period in which a negative excitation current is caused to flow, and a rest period in which no excitation current is passed through the signal processing circuit is provided between the electrodes. The generated induced voltage is taken in as a signal voltage, and the induced voltage generated between the trap electrodes during the period when no excitation current is flowing is taken in as a compensation voltage, and the noise component included in the signal voltage due to switching of the excitation current is removed by the compensation voltage. In this way, a low-frequency excitation type electromagnetic flowmeter with excellent zero point stability and excellent response has been realized.

Fig. 1 is a connection diagram showing an embodiment of the electromagnetic flowmeter of the present invention. In Fig. 1, 1 is an excitation circuit, a DC constant current source 11 and a switch 12□12 for switching the current from this constant current source.
It has b. 2 is an electromagnetic flowmeter transmitter, which includes an exciting coil 21. Pipe 22 through which fluid flows and electrodes 25m, 2
31). 3 is an AC amplifier that amplifies the voltage e8 induced between the electrodes fls, , 25b of the electromagnetic flowmeter transmitter 2. 4 is a signal processing circuit, which includes sample and hold circuits u, 41b+41e, and 41d that sample and hold the output eb of the amplifier 5, an arithmetic circuit 42 that adds and subtracts the outputs v1 to v4 of the sample and hold circuits 41a to 41d, and a Vessel 4S and force) growling. Sample hold circuit 41a~41dkt Sampling switch s8maro~8sd and hold capacitor 0
The one consisting of a bell is shown. The pulse generator 45 is synchronized with a 50Hs or 60Hs commercial AC power supply (not shown) and generates pulses that control the switches 12m+12b. 1al l'1b and sampling switch 8B turtle.

88b+ Generates an I-response P2 set Pzb+ P2e+P2d that controls 88c and 88d.

The operation of the present invention configured as described above will be explained below with reference to the time chart of FIG. First, switch Ba, 1
2b is controlled by P1a+F1b as shown in FIG. , Plb are on, and the current is switched to the opposite direction to flow through the excitation coil 21, and during the T2 and 14 periods, when both P1 and Plb are off, no current is passed through the excitation coil 21. Therefore, an excitation current Iw having a rest period T2.T4 is supplied to the excitation current 21 as shown in FIG.
is selected to be an integral multiple of one commercial AC power supply period T. When the excitation current Iw is switched by the switch 12a@1tb, it rises as shown in FIG. 2(c) due to the time constant due to the inductance and resistance of the excitation fill, and reaches a steady value after a delay in the falling part. Electrodes 25m and 25b of transmitter 2
In between, as shown in FIG. 2), an induced voltage ea corresponding to the excitation current 1wK is generated. For the induced voltage e wealth, Nokuib 2
In addition to the signal component Vs proportional to the flow rate r of the fluid flowing through 2, there is a noise component Va as shown by diagonal lines in the enlarged waveform diagram of FIG.
1 to Vn4 and an offset voltage component Woo caused by an electrochemical DC potential or a circuit are superimposed. Noise components Vn1 to Vn4 are electromagnetic coupling noise generated in the loop between the electrode and electrode lead when switching the excitation current, and first-order lag caused by the eddy current flowing in the fluid formed by the liquid resistance and the interfacial electric double layer capacitance of the electrode. Contains first-order lag noise caused by the circuit. As a result, the induced voltage e1 during the period T1 during which the excitation current is flowing is a positive signal component VS proportional to the fluid flow rate, an offset component VnO, and a noise component Vn1.
(Vs+VnO+Vnl), and the rest period T
The induced voltage ea2 of 2 is the sum of the offset voltage VnO and the negative noise component Vn2 (VnQ TI12), and the induced voltage eas during the period T5 during which the negative excitation current flows is a negative signal component - proportional to the flow rate of the fluid. The sum of Vs, offset component VnO and negative noise component -VO3 (VnOVn!l -Vs
), and the induced voltage em4 during the rest period T4 is the sum of the offset voltage VnO and the positive noise voltage Vn4 (VnO+V
n4). After the induced voltage eajgem5 during periods T+ and T5 during which a positive or negative excitation current flows is amplified by the AC amplifier 3, the signal voltage v1. A rest period T, which is given to the signal processing circuit 4 as v3 and during which no excitation current flows.
After the induced voltage ea21144 of 21T4 is amplified by the AC amplifier 3, the compensation voltage V2.21T4 is amplified by the AC amplifier 3. Signal processing circuit 4 as V4
given to. In the signal processing circuit 4, sampling pulses P2a, P2b, P2c+ are generated at the timings shown in FIG.
p2d, the hold voltages v1 to v4 will be held in the sample and hold circuits 41a to 41d, respectively.These hold voltages v1 to v4 are applied to the arithmetic circuit 42, and (v1
+V2-VS-T4) is performed.

Therefore, the output V(, is) of the arithmetic circuit 42.

VO==v1+V2-T3-T4 =ek (2Vs+
Vn1-Vn7+Vn3 VO4) =----(it
However, k is the gain of the AC amplifier 3, and the influence of the offset voltage VnO is removed.

(In formula 11, Vnl -VO2+Vn3- VO4WO O...
- River... River... If the relationship (2) is satisfied, the influence of the noise components Vn1 to Vn4 can also be removed. and,
Rise of excitation current.

The period T during which the excitation current is flowing while keeping the fall time constant
1. Ts and suspension period〒2. T4 and Tj = T2
=T3 =T4, and sampling pulse P2a+
Timing time to generate P2b, P2c+ P2d Ts1s Ti2+ TsS, Ti4 to Tsl
=Ts2 ”TsS =T, if you choose 4, it will be v
r+4=VO2=Vn5=:Vn4, satisfying the relationship of equation (2), and the output voltage VD is: VC+=2 k Vs・・・・・・・・・・・・・・・
(5), the influence of the noise voltages Vn1 to Vn4 is also removed, and only the voltage component v3 proportional to the fluid flow rate is multiplied by 2. 5. In the present invention, since the noise component that causes the zero point drift is effectively removed, it is possible to stabilize the zero point without lowering the excitation side wave number (that is, without sacrificing the response Kj). Furthermore, since the present invention provides a rest period in which no excitation current flows, there is an advantage that power consumption can be reduced.

Furthermore, in the present invention, the periods Tl, T5 during which the excitation current flows and the periods T2. By selecting T4 to be an integer multiple nT of the commercial AC power supply period T, the fourth
As shown in the figure, the influence of commercial power supply frequency noise is
4 is received in the same way, and the timings Tm1 to Ts4 at which sampling pulses P2a to P2d are generated are equally selected, and (
Vl +V2-T3-Va) Since the addition/subtraction operation is performed, the influence thereof is removed. Also, even if each of T1 to T4 is not an integral multiple of 〒, (T1+T2) and (T3
+T'4) are each an integral multiple of T, they can be removed in the same way.

In the above description, the rise and fall times of the excitation current Iw are kept constant, and TI = T2, TiS = T4 and T,
1=T, 2=T, 5 == Select Ti4, Vn 1
=g Vn 2 = Vn 5- Although the case where the influence of the noise components Vn1 to V+s4 is removed as VO4 has been illustrated, if there is a difference in the rise and fall times of the excitation current Iw, Tsl, Ti2+ Ts3t Ti4
By adjusting Vnl == VO2= Vn
It is also possible to satisfy the relationship of three Vn4. In other words, for example, if Ti4 is shortened, VO4 will increase, and if Ti4 is shortened (
Then, vrI4 will decrease. Also, the noise component Vn1~V
The influence of f14 is calculated from equation (2) as (Vn1+ vns)
If = (V port 2 + VI! 4), it can be removed, so
TI+ T2* T3+ T to satisfy this relationship
4 and Ti1°Ti21 'h31 TI4 may be selected. In this case, as shown in FIG. 4, the interval t1 (-TlTsl + T2) between the sampling pulses P2a and Plc is
+ Td ) and sampling pulses P2b and P2
The distance t (xT2 Ti2 +T3 +Tsa
) are each selected as an integer multiple of the commercial AC power supply period.
The influence of commercial AC power cycle number noise can also be removed.

Furthermore, in the above description, the period T1 during which the positive excitation current is flowing,
Sampling signal V1 + V2t v3. of each period of rest period T2, period T3 where negative excitation current flows, and rest period T4. The case where the addition/subtraction operation of (Vl +V2-V!l-Va) is performed on v4 by the arithmetic circuit 12 has been exemplified, but only the T1 period and T4 period are used as Vn1.
=Vn4, and the arithmetic circuit 42 performs the calculation (va-V4) to extract the voltage component (1) 4 proportional to the flow rate. In this case, the sample and hold circuit 41bl 41c can be omitted, which simplifies the configuration, and the pause period T2 can also be made zero. Similarly, only the T2 period and T5 period are set to Vn.
2eIIvl, and the arithmetic circuit 42 may perform the calculation (V2-Vs) to extract only the voltage component vs proportional to the flow rate. In this case, the sample hold circuit 41a*41
b can be omitted, and the pause period T4 can also be made zero.

FIG. 5 is a connection diagram showing another embodiment of the electromagnetic flow needle of the present invention. The difference from the embodiment of FIG.
Instead of integrating circuit 441 inverter 45 . This is because a jFl conversion switch 46 and a sample hold circuit 47 are used. The integrating circuit 44 includes a resistor R1° and an operational amplifier QP1.
Integrating capacitor 01 connected to the +OP1 feedback circuit + timing switch T8 that controls the input integration time
It also has a quirk, tosui, and chi R8 that reset the integral value. Then, the output of the integrating circuit 44 is directly added to b
When the AC amplifier 3 is on the side, the output of the AC amplifier 3 is added via an inverting amplifier 45. By the pulse P3 generated at the timing shown in FIG.

The selector switch 46 is set to the b side during the TI and T2 periods, and is set to the T side during the T2 period.
During the 3°T4 period, it becomes the 1 side. Further, the timing switch T8 is turned on for a certain period of time every time the sampling pulse P2 shown in FIG. , compensation voltage 4 are sequentially integrated over a certain period of time, and at the output end there is an integrated voltage v with a waveform as shown in FIG.
You will get an island. Therefore, ■1 is.

v, = person k(2Va + Vnl Vn2 + Vn
A VnA) ...... (4), and since Vn1 = Vn2 = Vn3 = VnA K, Va = 2''VB, and the offset voltage and 7(!IR1 noise component Vnl~Vn4 This voltage Vi is held in the sample hold circuit 47 by the sampling pulse P4 generated at the timing shown in FIG. In this embodiment, by selecting the integration time ta to be an integer multiple of the commercial AC power supply period T, the influence of commercial power supply frequency noise can be removed. T
Adjust l31 Tea and Vnly Tl21 Vnl
Even when satisfying 1 VnA = 0, it is not affected by internal power supply frequency noise.

In the embodiment shown in FIG. 5, a sample and hold circuit 47 is used at the output stage of the signal processing circuit 4, but a microprocessor 48 may also be used as shown in FIG. In FIG. 7, the output Va of the integrating circuit 44 is converted into a digital signal by a ψ converter 49m and provided to the microprocessor 48.

After performing the desired processing with the microprocessor 46, the
If the output voltage vO of the output signal is required, it is outputted via the D/A converter 49b. In this case, the microbussessor 48
5, the results of addition and integration of v1 to v4 by the integrating circuit 44 may be converted into a digital signal by the A/D converter 49a, but as shown in the time chart of FIG. Then, the values obtained by integrating Vl, V2 * v31 and V4 by the integrating circuit 44 are respectively input to the A/D converter 4.
9a converts it into a digital signal and provides it, and the microprocessor 48 essentially converts it into a digital signal (V1+V7-Vs-va).
20rRt" may be obtained by performing a tal operation on the digital t. At this time, the reference voltage vr is applied to the integrating circuit 44 via a pulse-driven switch T8' for a certain period of time 1 as shown in FIG.
．． / The integrated value is also converted via the A/D converter 498 to
"' Vr (!IR1C1R1) If the digital calculation is performed, the influence of the characteristics of the integrating circuit 44 can be removed. In FIG. ...The case where a signal is given to S4B is shown as an example, but
A sample Holby circuit may be used instead of the integrating circuit 44, a voltage/frequency converter may be used instead of the integrating circuit 44, and a counter may be used instead of the A/D converter 49a to provide the signal to the microprocessor 48. Alternatively, a voltage pulse width conversion circuit may be used instead of the integration circuit 44, and the A/D converter 49
A reference clock and a counter may be used instead of m, and a double integration type A/D converter may be used instead of the integrating circuit 44 and A/D converter 49 to control the microprocessor 46.
may be given a signal.

In addition, in FIG. 7, the excitation circuit 1 is an excitation current circuit!
The difference between the voltage E1 and the set voltage Er related to the error increase 11
ii) The voltage Ec amplified by A and the triangular wave voltage generation circuit (
) is compared with the output Bs of
, to which the drive pulse P1= or Plb is applied to one input (G1.). Commonly applied to the other input of G2, G1. The switch 12/112b', which changes the direction of the excitation current according to the output of G2, is controlled on/off to keep the steady value of the excitation current at a constant value, thereby reducing power loss in the excitation i1 circuit and improving efficiency. It is. Note that diodes D1 to D4 are connected to switches 12m', j
When 2W is off, the energy stored in the excitation filter 21 is released to keep the excitation current IW flowing.

In the above description, the case where the excitation circuit 1 is excited with a rectangular wave is illustrated, but various waveforms may be used 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. be able to.

As explained above, according to the trap of the present invention, a rest period in which no excitation current is applied is provided between a period in which a positive excitation current is applied and a period in which a negative excitation current is applied, and the electrodes are removed during a period in which an excitation current is applied. The induced voltage generated between the electrodes is amplified by an AC amplifier and then given as a signal voltage to the signal processing circuit.The induced voltage generated between the electrodes during the rest period when no excitation current is flowing is amplified by the AC amplifier and then signal processed as a compensation voltage. Since the compensation voltage removes the noise component caused by switching the excitation current contained in the signal voltage applied to the circuit, it is possible to use a low-frequency excitation type electromagnetic flowmeter with excellent zero point stability and good response. can get. Therefore, if a control loop is constructed using the electromagnetic flowmeter of the present invention, a stable control loop that does not cause hunting or the like can be obtained.

[Brief explanation of drawings]

Fig. 1 is a connection diagram showing an embodiment of the electromagnetic flowmeter of the present invention, Figs. 2 to 4 are diagrams for explaining its operation, and Fig. 5 is a connection diagram showing another embodiment of the electromagnetic flowmeter of the invention. 6 is an explanatory diagram of its operation, FIG. 7 is a connection diagram of another embodiment of the electromagnetic flowmeter of the present invention, and FIG. 8 is an explanatory diagram of its operation. 1... Rectangular wave excitation circuit 2... Electromagnetic flow meter oscillator 3... AC amplifier 4... Signal processing circuit
/, ffi

Claims (1)

[Claims] A positive excitation current and a negative excitation current are periodically switched and passed through the excitation coil, and the period during which the positive excitation current is passed and the negative m1m
An electromagnetic current positioning pointer transmitter has a pause period in which no excitation current flows between the periods in which current flows, and a voltage signal is obtained by amplifying the induced voltage from this transmitter during the period in which the excitation current is flowing using an AC amplifier. In addition to taking in the voltage as a compensation voltage, the voltage amplified by the AC amplifier during the rest period when no excitation current is flowing is taken in as a compensation voltage, and the noise component accompanying the switching of #magnetic current included in the signal voltage is removed by the compensation voltage. - IkIflI
An electromagnetic flowmeter with a characteristic of