JP5817968B2 - Electromagnetic flow meter - Google Patents

Description

  The present invention relates to an electromagnetic flow meter that can reduce variations in flow rate measurement values even when noise is superimposed on the flow rate signal.

  FIG. 4 shows the configuration of the electromagnetic flow meter. In FIG. 4, electrodes 11 a and 11 b are attached to a pipe 10 through which a fluid for measuring a flow rate flows, and an alternating magnetic field is applied from an exciting coil 12. For this reason, an electromotive force proportional to the flow rate of the fluid flowing through the pipe 10 is generated in the electrodes 11a and 11b.

  The electromotive force generated in the electrodes 11 a and 11 b is amplified by the differential amplifier 13, converted into a digital value by the AD converter 14, and input to the CPU 15.

  The CPU 15 calculates the flow rate of the fluid flowing through the pipe 10 from the input digital value, and outputs the calculated flow rate value to the output unit 16 and the display unit 17. The output unit 16 converts the input flow rate value into a 4-20 mA current signal or digital signal and outputs the converted signal to the outside. The display unit 17 displays the input flow rate value.

  The CPU 15 controls the excitation circuit 18. The excitation circuit 18 generates a drive current for driving the excitation coil 12 based on a control signal input from the CPU 15. The exciting coil 12 generates an alternating magnetic field by this driving current.

  Conventionally, an integral AD converter has been frequently used as the AD converter 14. The integration type AD converter has an advantage of easily removing differential noise peculiar to an electromagnetic flow meter, but has a disadvantage that it consumes a large amount of current and is difficult to devise to improve diagnosis and signal quality. In particular, since a two-wire electromagnetic flow meter is required to reduce current consumption, there has been a problem that it is difficult to use an integral AD converter with large current consumption.

  For this reason, successive approximation type discrete sampling AD converters with low current consumption and excellent signal processing are also used. The successive approximation AD converter is an AD converter that compares the output of the DA converter with the signal to be AD converted and controls the input value of the DA converter so that they match. Since the DA converter can be composed of only a resistor and a switch element, it has a simple structure and has an advantage that it can be operated at a higher speed than the integral type AD converter.

  FIG. 5 shows the relationship between the excitation current change, the analog flow rate signal (the output signal of the differential amplifier 13), and the sampling signal that determines the timing for taking in the analog flow rate signal. In FIG. 5, (A) to (C) are waveforms of an excitation current, an analog flow rate signal, and a sampling signal, respectively.

  When the excitation current rises at time t1, the output of the analog flow signal changes greatly. When the excitation current becomes constant at time t2, the analog flow rate signal settles to a constant value proportional to the flow rate. The sampling signal transitions to a high level at time t3 when the output of the differential amplifier 13 is stabilized. At the transition timing to the high level, the AD conversion unit 14 converts the analog flow rate signal into a digital value.

  By doing so, an accurate flow rate signal can be captured. The AD converter 14 converts the analog flow rate signal into a digital value a plurality of times based on the sampling signal.

  Patent Document 1 describes an electromagnetic flow meter including a switching control type excitation circuit. In Patent Document 1, the excitation switching control frequency is selected to be a frequency other than an integral multiple of the excitation basic frequency in order to eliminate output fluctuation caused by ripple of the excitation switching control frequency component included in the excitation current. .

JP 2002-202165 A

However, such an electromagnetic flow meter has the following problems.
As shown in FIG. 5 (A), a ripple caused by the power supply is superimposed on the excitation current. Reference numeral 20 denotes a ripple superimposed on the excitation current. This ripple is superimposed on the analog flow signal as induction noise. Reference numeral 21 denotes a superimposed ripple.

  The analog flow rate signal is converted into a digital value and captured in synchronism with the rising edge of the sampling signal. However, the variation in the digital value converted due to the ripple 21 increases, and an accurate flow rate cannot be measured. was there.

When a successive approximation type discrete sampling AD converter is used as the AD converter 14, a beat occurs when the relationship of the following equation (1) exists between the sampling period T and the ripple frequency fn, and the measured value As a result, the flow rate cannot be measured accurately.
fn = (2n + 1) / T (1)
(N is an arbitrary integer)

  In particular, in an electromagnetic flow meter of a type that switches a direct current to generate an alternating magnetic field, this ripple becomes large, and there is a problem that it becomes difficult to measure an accurate flow rate due to a large variation in measured values.

  The invention described in Patent Document 1 eliminates output fluctuation caused by ripple by setting the excitation switching control frequency to a frequency other than an integral multiple of the excitation basic frequency, but the excitation switching control frequency is limited. However, there is a problem that the influence of noise other than the ripple of the excitation current cannot be removed.

  An object of the present invention is to realize an electromagnetic flow meter that can reduce variations in measured values even when noise such as ripple is superimposed on an analog flow signal.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
In an electromagnetic flowmeter that applies an alternating magnetic field to a fluid, converts an analog flow signal related to an electromotive force generated by the alternating magnetic field into a digital value, and measures the flow rate of the fluid,
An AD converter that converts the analog flow signal into a digital value in synchronization with the rising edge of the input sampling signal ;
A flow rate calculation unit that calculates and outputs a flow rate value from the output value of the AD conversion unit;
A flow rate value calculated by the flow rate calculation unit is input, a variation calculation unit that calculates and outputs a variation in the flow rate value, and
A sampling signal generation unit that receives the output of the variation calculation unit and generates the sampling signal output to the AD conversion unit , and changes the frequency of the sampling signal when the variation exceeds a predetermined threshold value When,
It is equipped with. Even if ripples are superimposed on the analog flow rate signal, the flow rate can be measured accurately.

The invention according to claim 2
In an electromagnetic flowmeter that applies an alternating magnetic field to a fluid, converts an analog flow signal related to an electromotive force generated by the alternating magnetic field into a digital value, and measures the flow rate of the fluid,
An AD converter for converting the analog flow signal into a digital value;
A thinning unit that thins out and outputs an output digital value of the AD conversion unit, and changes a thinning rate when an input variation exceeds a predetermined threshold;
An output of the thinning unit is input, and a flow rate calculation unit that calculates and outputs a flow rate value from the input value;
A flow rate value calculated by the flow rate calculation unit is input, a variation of the flow rate value is calculated, and a variation calculation unit that outputs to the thinning unit,
It is equipped with. Even if ripples are superimposed on the analog flow rate signal, the flow rate can be measured accurately.

The invention according to claim 3 is the invention according to claim 1 or 2,
The electromagnetic flow meter is a two-wire electromagnetic flow meter that outputs a current signal, and the upper limit value of the frequency of the sampling signal input to the AD converter is changed according to the value of the current signal. is there. The effect is large in a two-wire electromagnetic flow meter in which current consumption is limited.

The invention according to claim 4 is the invention according to any one of claims 1 to 3,
As the AD conversion unit, a successive approximation type discrete sampling AD conversion unit is used. The effect is particularly great in a discrete sampling AD converter that easily generates a beat between the ripple superimposed on the analog flow signal and the sampling frequency.

The present invention has the following effects.
According to the first, second, third, and fourth aspects of the present invention, in the electromagnetic flowmeter that measures the flow rate by converting the analog flow rate signal into a digital value, the variation of the flow rate value is calculated, and the variation reaches a predetermined threshold value. If exceeded, the thinning rate of the thinning part for thinning the sampling frequency or digital value of the AD conversion part is changed.

  A beat is generated between the noise such as ripple superimposed on the analog flow rate signal and the sampling frequency of the AD converter, and if the variation in the flow rate value increases, the sampling frequency or the thinning rate is changed to prevent the occurrence of the beat. There is an effect that a stable flow rate value can be obtained with little variation. The present invention is particularly effective in an electromagnetic flow meter that uses a discrete sampling AD converter that easily generates beats.

  In addition, since the consumption current of the AD converter increases as the sampling frequency increases, in the case of a two-wire electromagnetic flow meter, the upper limit value of the sampling frequency of the AD converter is changed according to the value of the output current. There is also an effect that an accurate and stable current signal can be output.

It is the block diagram which showed one Example of this invention. It is a figure for demonstrating operation | movement of the FIG. 1 Example. It is a block diagram which shows the other Example of this invention. It is a block diagram of the conventional electromagnetic flowmeter. It is a wave form diagram which shows the relationship between an exciting current, an analog flow signal, and a sampling signal.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an electromagnetic flow meter according to the present invention. The same elements as those in FIG. 4 are denoted by the same reference numerals, and description thereof is omitted.

  In FIG. 1, the drive current generated by the excitation circuit 18 is input to the excitation coil 12. The exciting coil 12 generates an alternating magnetic field by the input drive current. This alternating magnetic field is applied to the pipe 10. The electromotive force generated by the AC magnetic field is detected by the electrodes 11a and 11b, amplified by the differential amplifier 13, and converted into a digital value by the AD converter 14. The output of the differential amplifier 13 is an analog flow signal.

  A CPU 30 includes a flow rate calculation unit 31, a variation calculation unit 32, and a sampling signal generation unit 33. The flow rate calculation unit 31, the variation calculation unit 32, and the sampling signal generation unit 33 are configured by software that operates on the CPU 30.

  The digital value converted by the AD conversion unit 14 is input to the flow rate calculation unit 31. The flow rate calculation unit 31 calculates the flow rate flowing through the pipe 10 from the input digital value, and outputs the flow rate value to the output unit 16, the display unit 17, and the variation calculation unit 32.

  The output unit 16 converts the input flow rate value into a 4-20 mA current value or a digital signal and outputs it to the outside. The display unit 17 displays the input flow rate value.

The variation calculator 32 calculates a variation in the flow rate value output from the flow rate calculator 31 and outputs this variation to the sampling signal generator 33. The variation calculator 32 calculates the variance V for a plurality of continuous flow values based on the following equation (2), and outputs the calculated variance V to the sampling signal generator 33.
V = (Σ (xi−xm) ² ) / n (2)
xi: Flow rate value
xm: Average value of flow rate value xi
n: Number of flow rate values xi

  The sampling signal generation unit 33 generates a sampling signal and outputs the generated sampling signal to the AD conversion unit 14. The AD conversion unit 14 converts the analog flow rate signal into a digital value at the rising timing of the sampling signal, and outputs the digital value to the flow rate calculation unit 31.

  The sampling signal generation unit 33 changes the frequency of the sampling signal to be output when the output signal of the variation calculation unit 32 becomes larger than a preset threshold value.

  The CPU 30 also includes a control signal generation unit that generates a control signal for controlling the excitation circuit 18, but the description is omitted because it is the same as the conventional example of FIG.

  Next, the operation of this embodiment will be described with reference to FIG. FIG. 2A is a waveform diagram of the excitation current flowing through the excitation coil 12. The excitation current flows on the positive side for a certain period (positive excitation) and then flows on the negative side for a certain period of time (negative excitation) as one cycle, and this cycle is repeated.

  FIG. 5B is a waveform diagram of variation that is an output of the variation calculation unit 32. The variation calculation unit 32 calculates a variance for n consecutive output values of the flow rate calculation unit 31 based on the equation (2), and outputs this variance to the sampling signal generation unit 33.

  FIG. 6C is a waveform diagram of the sampling signal generated and output by the sampling signal generation unit 33. The AD conversion unit 14 converts the analog flow rate signal into a digital value and outputs it to the flow rate calculation unit 31 in synchronization with the rising edge of the sampling signal, and the flow rate calculation unit 31 calculates the flow rate value based on the digital value. Output.

  Until time t10, the variation output by the variation calculation unit 32 is small. For this reason, the sampling signal generator 33 outputs four sampling pulses at regular intervals in each period of positive excitation and negative excitation.

  The frequency of the ripple superimposed on the analog flow rate signal changes immediately before time t10, and a beat is generated between the sampling frequency of the AD conversion unit 14. For this reason, the variation output by the variation calculating unit 32 at time t10 increases and exceeds the threshold value Vth.

  For this reason, the sampling signal generator 33 changes the frequency of the sampling signal and outputs eight sampling pulses for each of positive excitation and negative excitation. The AD conversion unit 14 converts the output of the differential amplifier 13 into a digital value twice as often as before. If the sampling number of the AD conversion unit 14 is the same for positive excitation and negative excitation, differential noise can be easily removed.

  As a result, the beat between the ripple superimposed on the analog flow rate signal and the sampling frequency of the AD conversion unit 14 is eliminated, and the variation output by the variation calculation unit 32 at time t11 is reduced. The sampling signal generator 33 maintains the sampling frequency changed at time t10.

  As described above, the sampling signal generation unit 33 changes the frequency of the sampling signal when the variation as the output of the variation calculation unit 32 increases and exceeds the threshold value, and maintains this state even when the variation decreases. . For this reason, if a beat occurs between the ripple superimposed on the analog flow rate signal and the sampling frequency of the AD converter 14 and the variation in the flow rate signal increases, the frequency of the sampling signal automatically changes to eliminate the beat. Therefore, the flow rate can be accurately measured with little variation.

  FIG. 3 shows another embodiment of the present invention. In this embodiment, the sampling frequency of the AD conversion unit 14 is fixed, and the thinning rate is changed by the output of the variation calculation unit 32. In addition, the same code | symbol is attached | subjected to the same element as FIG. 1, and description is abbreviate | omitted.

  In FIG. 3, reference numeral 40 denotes a CPU, which includes a thinning unit 41, a flow rate calculation unit 31, a variation calculation unit 32, and a sampling signal generation unit 42. An oversampling AD converter is used as the AD converter 14.

  The digital value converted by the AD conversion unit 14 is input to the thinning unit 41. The thinning unit 41 thins the output digital value of the AD conversion unit 14 at a set thinning rate, and outputs the thinned digital value to the flow rate calculation unit 31. That is, assuming that the thinning rate is m, the thinning unit 41 outputs one digital value for m input digital values.

  The output of the thinning unit 41 is input to the flow rate calculation unit 31 and converted into a flow rate value, and the variation calculation unit 32 calculates the variation in the flow rate value and outputs it to the thinning unit 41. The sampling signal generation unit 42 generates a sampling signal having a constant frequency and outputs it to the AD conversion unit 14.

  The thinning unit 41 changes the thinning rate when the variation output by the variation calculating unit 32 becomes larger than a predetermined threshold, and thins the output of the AD conversion unit 14 with the changed thinning rate and outputs the thinned output to the flow rate calculating unit 31. In this way, the same effect as when the sampling frequency of the AD converter 14 is changed can be obtained.

  In the embodiment of FIG. 3, the AD converter 14 may autonomously convert the output of the differential amplifier 13 into a digital value at a constant cycle without using the sampling signal generator 42.

1 and 3 , the AD conversion unit 14 has been described as a successive approximation type discrete sampling AD converter. However, other types of AD converters may be used.

  In general, the current consumption of the AD converter increases as the sampling frequency increases. Since the upper limit of the consumption current is limited to the output current in the two-wire electromagnetic flow meter, the upper limit of the sampling frequency of the AD conversion unit 14 may be changed according to the value of the output current.

  For this reason, the value of the output current is input to the sampling signal generation unit 33 or 42, and the upper limit of the frequency of the sampling signal output from the sampling signal generation unit 33 or 42 to the AD conversion unit 14 can be changed according to the value of the output current. That's fine.

  In the embodiment of FIG. 2, the frequency of the sampling signal is changed to twice, but the frequency of the sampling signal output by the sampling signal generation unit 33 and the thinning rate set in the thinning unit 41 are arbitrary values. can do. In order to remove the beat, it is better to change it at a non-integer ratio. In addition, the sampling signal or the thinning interval is not necessarily constant.

  In FIG. 2, the frequency of the sampling signal is increased when the variation exceeds the threshold, but may be decreased.

  In these embodiments, the analog flow rate signal has been described as having ripples due to excitation current ripple superimposed thereon, but the same effect can be obtained even when noise other than ripples due to excitation current is superimposed. Can be obtained.

  Further, in the embodiment of FIGS. 1 and 3, the variation calculating unit 32 calculates the variance of the flow rate value, but it is not necessarily required to be the variance. In short, any value that represents the variation in the flow rate value may be used.

DESCRIPTION OF SYMBOLS 10 Piping 11a, 11b Electrode 12 Excitation coil 13 Differential amplification part 14 AD conversion part 18 Excitation circuit 30, 40 CPU
31 Flow rate calculation unit 32 Variation calculation unit 33, 42 Sampling signal generation unit 41 Decimation unit

Claims (4)

In an electromagnetic flowmeter that applies an alternating magnetic field to a fluid, converts an analog flow signal related to an electromotive force generated by the alternating magnetic field into a digital value, and measures the flow rate of the fluid,
An AD converter that converts the analog flow signal into a digital value in synchronization with the rising edge of the input sampling signal ;
A flow rate calculation unit that calculates and outputs a flow rate value from the output value of the AD conversion unit;
A flow rate value calculated by the flow rate calculation unit is input, a variation calculation unit that calculates and outputs a variation in the flow rate value, and
A sampling signal generation unit that receives the output of the variation calculation unit and generates the sampling signal output to the AD conversion unit , and changes the frequency of the sampling signal when the variation exceeds a predetermined threshold value When,
An electromagnetic flow meter comprising: In an electromagnetic flowmeter that applies an alternating magnetic field to a fluid, converts an analog flow signal related to an electromotive force generated by the alternating magnetic field into a digital value, and measures the flow rate of the fluid,
An AD converter for converting the analog flow signal into a digital value;
A thinning unit that thins out and outputs an output digital value of the AD conversion unit, and changes a thinning rate when an input variation exceeds a predetermined threshold;
An output of the thinning unit is input, and a flow rate calculation unit that calculates and outputs a flow rate value from the input value;
A flow rate value calculated by the flow rate calculation unit is input, a variation of the flow rate value is calculated, and a variation calculation unit that outputs to the thinning unit,
An electromagnetic flow meter comprising: The electromagnetic flow meter is a two-wire electromagnetic flow meter that outputs a current signal, and an upper limit value of a frequency of a sampling signal input to the AD converter is changed according to a value of the current signal. The electromagnetic flowmeter according to claim 1 or 2, characterized in that   The electromagnetic flow meter according to any one of claims 1 to 3, wherein the AD conversion unit is a successive approximation type discrete sampling AD conversion unit.

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