JP3752324B2 - Electromagnetic flow meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an improvement in an electromagnetic flow meter useful for an electromagnetic flow meter, particularly a low power consumption type such as a two-wire electromagnetic flow meter or battery drive.
[0002]
[Prior art]
An electromagnetic flow meter that operates with a direct current power supply has a built-in timing generation circuit that creates a timing between the excitation current and the sampling of the flow signal.
[0003]
FIG. 4 and FIG. 5 show two examples of excitation current and sampling timing for removing AC noise of any commercial power supply frequency of 50 Hz or 60 Hz superimposed on the flow rate signal in this type of electromagnetic flow meter.
[0004]
FIG. 4 shows a flow rate signal in the case of square wave excitation with an excitation frequency of 15 Hz. The excitation cycle is 1 / 15S, that is 66.66... MS, and the sampling period is hatched. The sampling periods Ts ₁ and Ts ₂ in the first half period T ₁ and the second half period T ₂ of the excitation cycle are both 20 mS.
[0005]
When AC noise is superimposed on such a flow rate signal, the sampling period Ts ₁ and Ts ₂ corresponds to one period of a commercial power supply of 50 Hz with a sampling period Ts ₁ and Ts _2, so the result of sampling or integrating one period of a 50 Hz sine wave is zero. And 50 Hz AC noise is removed.
[0006]
Further, the first half period T ₁ of the second half of the sampling starting time t ₁ period T ₂ (T ₁ = T ₃ to) the sampling start time t time interval T ₃ to ₃ 33.33 ... of the commercial power source of 60Hz in mS of This corresponds to twice the period 16.66... MS. Therefore, the phase of the 60 Hz sine wave at t ₁ and t ₃ always matches.
[0007]
Therefore, 60 Hz AC noise is removed by sampling in the sampling periods Ts ₁ and Ts ₂ and taking out the flow rate signal by a calculation process that subtracts the sampling output of the second half period from the sampling output of the first half period.
[0008]
In this way, both 50 Hz and 60 Hz AC noise is removed.
Figure 5 is the excitation frequency indicates the flow rate signal in the case of the square wave excitation of 6.25 Hz, the period of excitation 1 / 6.25S, that is, 160 mS, shown hatched between sampling period. The sampling periods Ts ₁ and Ts ₂ in the first half period T ₁ and the second half period T ₂ of the excitation period 160 mS are both 50 ms, corresponding to three times the period 16.66.
[0009]
Therefore, the result of sampling or integrating 60 Hz AC noise during each sampling period Ts ₁ and Ts ₂ is zero, and 60 Hz AC noise is removed.
Further, the time interval T ₃ between the sampling start times t ₁ and t _{3 in} the first half period T ₁ and the second half period T ₂ is 80 mS, which corresponds to four times the cycle 20 mS of the 50 Hz commercial power supply. Therefore, the phases of the 50 Hz sine wave at times t ₁ and t ₃ always match, and 50 Hz AC noise is also removed by the process of subtracting the sampling output of the second half period from the sampling output of the first half period accompanying the flow rate calculation.
[0010]
Noise that adversely affects flow measurement in electromagnetic flowmeters includes DC to low-frequency offset called electrochemical noise that is superimposed on the flow signal detected by the electrodes, in addition to the AC noise of the commercial power supply frequency as described above. There is voltage.
[0011]
FIG. 6 is a block diagram of a second prior art provided with an offset compensation circuit for removing such an offset voltage, which is known from Japanese Patent Laid-Open No. 2-12018. FIG. 7 is a timing chart showing the operation of the circuit of the electromagnetic flow meter of FIG.
[0012]
In FIG. 7, the output of the offset compensation circuit 3 of FIG. 6 becomes as shown in B of FIG. 7 with respect to the input signal proportional to the flow rate as shown in FIG. C in FIG. 7 is an output waveform of the sampling circuit 4 in FIG. 6, and the hatched portion is A / D converted by the A / D conversion circuit 5 in FIG. 6 by the timing signal S 5 from the MPU 6.
[0013]
The MPU 6 displays the flow rate signal converted into a digital value by the A / D converter 5 on a display (not shown) or outputs the flow rate signal to the outside as a flow rate signal.
S1 in FIG. 7 is timing when the switch S1 of the offset compensation circuit 3 is turned on. The switches S2, S3 and S4 of the sampling circuit 4 are turned on by the timing signals S2, S3 and S4 in FIG. 7, respectively, and sample or integrate the output waveform of the offset compensation circuit 3 shown in B of FIG.
[0014]
Next, the operation of the offset compensation circuit 3 of FIG. 6 will be described with reference to FIG.
In FIG. 7, A is an ideal flow signal not including an offset voltage, and the switch S1 is OFF during the period Ta ₁ , so that the peak value E ₁ of the square wave input is composed of the operational amplifier OP ₁ and the resistors Ra and Rb. Amplified by the amplifier 31 and becomes a positive voltage V ₁ of the output waveform shown in FIG.
[0015]
When the switch S1 is turned on by the sampling starting time t _1, the output of the amplifier 31 is integrated by the integrator 32. Since the output of the integrator 32 is connected to the non-inverting input of the operational amplifier OP ₁ constituting the amplifier 31, the feedback function operates in the direction in which the output of the amplifier 31 becomes zero.
[0016]
This feedback function is the basic compensation operation of the offset compensation circuit 3, by the compensation operation, the value of the output of the amplifier 31 at the end time t ₂ of the sampling period Ts ₁ becomes V ₂ (see B in FIG. 7) .
[0017]
The ratio between the voltages V ₁ and V ₂ is
V ₂ / V ₁ = ε- ^{(1 + G1) TS1 / (C1 ・ R1)}
It becomes.
[0018]
Note that G1 = Rb / Ra.
At the same time the switch S 1 at time t ₂ is turned off, the polarity of the input signal shown in A is inverted in FIG. 7, the period Ta ₁ in the case the same input signal is amplified by V ₃ becomes a period Ta ₂ This Holds the value.
[0019]
This is because in the period Ta ₂ , the switch S1 is off, the output of the integrator 32 maintains a constant value, and only the change in the input signal at time t ₂ is amplified by the amplifier 31 to −G1 times.
[0020]
When the switch S1 is turned on at time t ₃ , the compensation operation works so that the output voltage V _{3 of the} amplifier 31 becomes zero, and the output voltage becomes V ₄ at the end time t ₄ of the sampling period Ts ₂ .
[0021]
In this case, the ratio of the voltages V ₄ and V ₃ is
V ₄ / V ₃ = ε- ^{(1 + G1) TS2 / (C1 ・ R1)}
It is.
[0022]
The above is the operation of the offset compensation circuit 3 during one excitation period.
FIG. 8 is a diagram for explaining the output waveform when a sine wave is input to the offset compensation circuit 3 in comparison with the case where an ideal flow signal waveform is input.
[0023]
In the figure, A is a square wave input indicating an ideal flow signal without noise, and B is an output waveform. These A and B are the same as A and B in FIG.
FIG. 6C shows a 50 Hz sine wave input waveform and D shows its output waveform. E in the figure is a 60 Hz sine wave input waveform, and F is its output waveform.
[0024]
C is drawn with a phase intersecting with the zero level at time t ₁ as an AC sine wave of 50 Hz as an ideal sine wave. Obviously, if this waveform is sampled or integrated for 20 ms, it becomes zero.
[0025]
However, the output when the 50 Hz sine wave shown in FIG. 8C is input to the offset compensation circuit 3 in FIG. 6 is the first half period t _{0 to} t when the sine wave is deformed by the feedback function of the offset compensation circuit 3. average in ₂ changes in the positive direction, the mean value in the second half period t ₂ ~t ₄ is changed in the negative direction. Therefore, not become zero even if the integral only sampled Period Ts _1. Similarly, the sampling period Ts ₂ does not become zero, and the sampled or integrated value is different from the case of Ts ₁ .
[0026]
In the case of a 60 Hz sine wave, the output waveform indicated by F in FIG. 8 is modified in the same manner as in FIG. D, but the phases at times t ₁ and t ₃ match as shown in FIG. Therefore, the values that are not canceled by the integration in the sampling periods Ts ₁ and Ts ₂ are equal, and are canceled by the arithmetic processing for subtracting the sampling output in the period Ts ₂ from the sampling output in the period Ts ₁ , and the alternating current of 60 Hz Noise is removed.
[0027]
[Problems to be solved by the invention]
In the second prior art shown in FIG. 6, after the electrodes 2a, signals induced in the 2b is amplified by a preamplifier A _1, removing a DC-to-low-frequency offset voltage (electrochemical noise) by the offset compensation circuit 3 is doing. If this offset voltage is large, the electronic circuit may be saturated, so the offset compensation circuit 3 is indispensable.
[0028]
However, the second conventional technique still has a problem that it is impossible to remove both 50 Hz and 60 Hz AC noise based on the same principle as the conventional techniques of FIGS. 4 and 5.
[0029]
A low power consumption type electromagnetic flow meter, such as a two-wire electromagnetic flow meter or a battery-driven electromagnetic flow meter, cannot use a large excitation power, inevitably reducing the magnetic flux density and reducing the flow signal. It becomes susceptible to the noise generated due to the fluid flow. For this reason, in order to reduce the signal variation due to noise, it is advantageous to increase the sampling period because an averaging effect can be obtained.
[0030]
In view of the above, an object of the present invention is to provide an electromagnetic flow meter that can remove both 50 Hz and 60 Hz AC noise, which is suitable for a low power consumption type flow meter having an offset compensation circuit.
[0031]
[Means for Solving the Problems]
In order to achieve the object, the invention of claim 1
In an electromagnetic flowmeter that samples and outputs the signal voltage generated in response to the flow rate of the fluid to be measured in synchronization with excitation,
Sampling start time (t ₁₎ and the latter period commercial power supply time interval between the sampling start time (t ₃₎ (T ₃₎ is 50Hz and 60Hz of (T _2, T ₄₎ of the first half period of the excitation period (T ₁₎ In this electromagnetic flow meter, the excitation frequency is determined to be 100 mS , which is an integral multiple of each period.
[0032]
The invention of claim 2 is the electromagnetic flow meter of claim 1,
An offset compensation circuit (3) for removing electrochemical noise is provided.
According to a third aspect of the present invention, in the electromagnetic flow meter according to the first or second aspect, the excitation cycle is 200 mS.
[0033]
[Action]
Since the time interval T ₃ between the sampling start timing t _{1 in} the first half period of the excitation cycle and the sampling start time t _{3 in the} second half period is an integral multiple of one cycle of 50 Hz, the 50 Hz sine wave was transformed by the offset compensation circuit. Even in this case, the phase of the 50 Hz sine wave at the times t ₁ and t ₃ becomes the same, and 50 Hz AC noise is removed.
[0034]
Further, since the time interval T ₃ is an integral multiple of one cycle of 60 Hz, the phase of the 60 Hz sine wave at the times t ₁ and t ₃ is the same, and 60 Hz AC noise is also removed.
[0035]
For this reason, both 50 Hz and 60 Hz AC noise can be removed with one excitation timing.
[0036]
DETAILED DESCRIPTION OF THE INVENTION
Next, a preferred embodiment of the present invention will be described with reference to FIGS.
Reference numeral 1 denotes a cylindrical flow tube for flowing the fluid to be measured, 2a and 2b are a pair of electrodes arranged opposite to the inner peripheral wall of the flow tube 1 and take out an induced flow rate signal, and 2c is a ground electrode.
[0037]
A ₁ is a preamplifier for amplifying the signal voltage of the electrodes 2a and 2b, and 3 is an offset compensation circuit for amplifying the output of the preamplifier A ₁ and removing an offset voltage composed of electrochemical noise of DC to low frequency.
[0038]
The offset compensation circuit 3 includes an amplifier 31 in which resistors Ra and Rb and an operational amplifier OP ₁ are connected as shown in the figure, an integrator 32 in which a resistor R ₁ , a capacitor C ₁ and an operational amplifier OP ₂ are connected as shown in the figure, and will be described later. A switch S1 operated by a microprocessor (MPU) 6A is connected as shown in the figure.
[0039]
Reference numeral 4 denotes a sampling circuit, an inverter A ₂ having a gain of −1, resistors R ₂ and R _3. , Switches S2, S3, S4, are constituted by connecting, as shown capacitor C _2, and an operational amplifier OP _3. The switches S2, S3, S4 are operated by the MPU 6A.
[0040]
An A / D converter 5 converts the analog output of the sampling circuit 4 into a digital value and inputs it to the MPU 6A. S5 is a timing signal from the MPU 6A, and instructs the A / D converter 5 the time (time) at which the A / D conversion circuit 5 performs A / D conversion.
An excitation circuit 7 supplies a square wave excitation current to the excitation coil 8.
[0041]
Next, the operation of the electromagnetic flowmeter having the configuration shown in FIG. 1 will be described with reference to the timing charts shown in FIGS. 2 and 3. Since there are many similarities to the prior art shown in FIG. the same reference numerals, extra description thereof is omitted. The biggest difference between the prior art of FIG. 6 and the embodiment of FIG. 1 is that the microprocessor MPU determines the timing relationship between the signals for operating the switches S1, S2, S3 and S4 and the excitation current, and the microprocessor MPU. The excitation frequency to be determined.
[0042]
Therefore, the following explanation of the operation will mainly describe these differences.
FIG. 2 is a signal timing showing the operation of the embodiment of FIG. 1. In FIG. 2, A is an ideal square wave flow signal, and B is an output waveform of the offset compensation circuit 3 for the A flow signal. C is an output waveform of the offset compensation circuit 3 for a sine wave of 50 Hz, and D is an output waveform of the offset compensation circuit 3 for a sine wave of 60 Hz.
[0043]
In this embodiment, by setting the excitation period to 200 mS, the excitation half periods T ₁ and T ₂ are set to 100 mS, which is an integral multiple of each period of 50 Hz and 60 Hz. Thus, when the half periods T ₁ and T ₂ are set to 100 mS, the sampling start times t ₁ and t ₃ are independent of the selection of the periods Ta ₁ and Ta ₂ and the sampling periods Ts ₁ and Ts _2. The time interval T ₃ is 100 mS, which is the same as the half periods T ₁ and T ₂ .
[0044]
The time interval T ₃ of 100 mS is 5 times the 50 Hz AC frequency period 20 mS and 6 times the 60 Hz AC frequency period 16.66... MS, so the commercial power supply at times t ₁ and t ₃ The phase of the sine wave is the same in each case both at 50 Hz and 60 Hz.
[0045]
The feedback effect of the offset compensation circuit 3, since the output waveform for 50Hz sine wave is zero level of the sine wave is changed as shown in FIG. 2, C, matching the time t ₁ in the t ₃ phase, The parts that did not become zero in the integration in the sampling periods Ts ₁ and Ts ₂ are canceled and removed when the MPU 6A calculates the flow rate signal by subtracting the value in the latter half period from the value in the first half period.
[0046]
As shown in FIGS. 2 and 2, the output waveform for the 60 Hz sine wave also changes the zero level of the sine wave. In this case as well, the phase is matched at times t ₁ and t ₃ , so the 50 Hz sine wave As in the case of, the 60 Hz AC noise is eliminated by canceling out the calculation process in the MPU 6A.
[0047]
[Example 2]
FIG. 3 is a timing chart in the case of an unbalanced embodiment in which the first half period T ₁ and the second half period T ₄ of the excitation cycle are not the same time, and the hardware configuration of the electromagnetic flow meter is the same as FIG.
[0048]
Also in this case, by selecting the time interval T ₂ between the start times t ₁ and t ₃ of the sampling period Ts _{1 in the first} half period and the sampling period Ts _{2 in} the _second half period as 100 mS, the same operation as in the case of FIGS. An effect can be obtained.
[0050]
【The invention's effect】
Since the electromagnetic flowmeter of the present invention is configured as described above, in an electromagnetic flowmeter that requires an offset compensation circuit, in order to remove AC noise caused by the commercial power supply, it is necessary to synchronize with the commercial power supply. No special circuit is required, and both 50 Hz and 60 Hz AC noise can be removed with one excitation timing, so the circuit configuration is simplified, the number of parts can be reduced, and the power consumption of the circuit itself is reduced. Therefore, the present invention is particularly effective when applied to a battery-driven electromagnetic flow meter that requires low power consumption.
[0051]
In addition, since the excitation cycle can be set to a large value (low frequency) of 200 ms, a battery in which the excitation power can be significantly reduced as the excitation frequency is reduced as in the residual magnetic excitation method in which the excitation current is intermittently supplied only momentarily. The effect is effectively exhibited in the drive electromagnetic flowmeter.
[0052]
In addition, it is possible to select a longer sampling period, which is advantageous for averaging fluid noise generated in the electrodes and reducing variations in flow rate signal output. From this point also, the battery-driven electromagnetic flow rate with a low signal level is advantageous. It is especially effective for the total.
[Brief description of the drawings]
FIG. 1 is an electric circuit diagram of an embodiment of the present invention.
FIG. 2 is a diagram showing electric signal waveforms and timings in the embodiment of FIG. 1;
FIG. 3 is a diagram showing electrical signal waveforms and timing according to another embodiment of the present invention.
FIG. 4 is a diagram showing timing in the prior art.
FIG. 5 is a diagram showing the timing of another prior art.
FIG. 6 is an electric circuit diagram of still another prior art.
7 is a diagram showing electric signal waveforms and timing in the prior art of FIG. 6; FIG.
FIG. 8 is a diagram showing electric signal waveforms and timings in the prior art of FIG. 6;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Flow tube 3 Offset compensation circuit 32 Integrator as sampling circuit 8 Excitation coil 6A Microprocessor (MPU)
_First half period T ₂ of T ₁ battery excitation cycle, second half period Ts ₁ , Ts ₂ sampling period t ₁ , t ₃ sampling start time of T ₄ excitation cycle

Claims (3)

In an electromagnetic flowmeter that samples and outputs the signal voltage generated in response to the flow rate of the fluid to be measured in synchronization with excitation,
Sampling start time (t ₁₎ and the latter period commercial power supply time interval between the sampling start time (t ₃₎ (T ₃₎ is 50Hz and 60Hz of (T _2, T ₄₎ of the first half period of the excitation period (T ₁₎ An electromagnetic flow meter characterized in that an excitation frequency is determined to be 100 mS , which is an integral multiple of each period. The electromagnetic flowmeter according to claim 1, further comprising an offset compensation circuit (3) for removing electrochemical noise. 3. The electromagnetic flow meter according to claim 1, wherein an excitation cycle is 200 mS.

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