JP5877260B1 - How to determine the empty state of an electromagnetic flow meter - Google Patents

Abstract

An empty state of a fluid in a measurement pipe in an electromagnetic flow meter can be accurately determined. When the fluid in the measuring tube changes from the wetted state to the non-wetted state at time t, the electrode impedance between the electrode and the ground becomes infinite and is greatly affected by externally induced noise. Become. Therefore, induction noise based on a commercial AC power source as shown by the sine wave in (c) occurs after time t when the liquid contact state is reached. Measurement is performed when the signal level of the frequency component of the induced noise exceeds or falls below the threshold value s1 or threshold value s2 set during the automatic zero adjustment process when the fluid is stationary and the fluid is filled in the measurement tube. Detect that the fluid in the tube is empty. [Selection] Figure 3

Description

  The present invention relates to a method for determining an empty state of an electromagnetic flow meter that determines an empty state of a fluid in a measuring tube.

  An electromagnetic flowmeter applies an alternating magnetic field to a fluid that flows through a measuring tube and has electrical conductivity by means of an excitation coil, and in accordance with Faraday's law, an electromotive force induced in a direction perpendicular to the fluid direction and the direction of the magnetic field. The flow velocity is obtained and converted to a flow rate.

  FIG. 4 shows a configuration diagram of an electromagnetic flow meter having a conventional sky detection function disclosed in Patent Document 1. In FIG. The electromagnetic flow meter includes a detection unit 1 and a conversion unit 2. The detection unit 1 includes a measurement tube 3 through which a fluid to be measured flows, an excitation coil 4 disposed around the measurement tube 3, and a measurement. It consists of a pair of electrodes 5a and 5b arranged in the tube 3.

  The converter 2 is provided with buffer amplifiers 6a and 6b that receive two flow signals induced by the electrodes 5a and 5b. The outputs of the buffer amplifiers 6a and 6b are connected to a differential amplifier 7 for obtaining an output difference between the buffer amplifiers 6a and 6b. Further, the output of the differential amplifier 7 is supplied from the CPU 8 and the output circuit 9 for executing a flow rate calculation and the like. Connected in order.

  Further, the output of the excitation current from the excitation circuit 10 is connected to the excitation coil 4 and the CPU 8. The excitation circuit 10 supplies an excitation current, which is a constant current, to the excitation coil 4.

  A magnetic field is applied to the measuring tube 3 by the exciting coil 4 by the constant current output from the exciting circuit 10. An electromotive force Es induced in a direction orthogonal to the fluid direction and the magnetic field direction is output to the CPU 8 with respect to the conductive fluid, and the flow velocity v of the fluid is measured. The flow rate can be obtained by multiplying the area.

  In an electromagnetic flow meter, the accuracy guarantee is that the measurement tube 3 of the detection unit 1 is filled with a conductive fluid during measurement. However, in an actual plant, the fluid may be empty in the measurement tube 3, and the electrodes 5a and 5b may be exposed without the measurement tube 3 being completely filled with the fluid. Thus, if the electrodes 5a and 5b are exposed, the output of the electromagnetic flowmeter becomes indefinite, resulting in a great hindrance to plant operation.

  Therefore, as shown in FIG. 4, an empty detection unit 11 is arranged in the conversion unit 2 to determine an empty state of the fluid in the measurement tube 3 and output an alarm. The positive / negative pulse current generated in the sky detection unit 11 is passed between the electrodes 5a and 5b, and the AC voltage signal corresponding to the impedance between the electrodes 5a and 5b generated by the pulse current is differentially amplified by the differential amplifier 7. The After the output signal of the differential amplifier 7 is converted into a DC voltage by the sky detection unit 11, it is converted into a frequency signal proportional to the DC voltage and output to the CPU 8. The CPU 8 generates an empty detection output signal by comparing the input value with the stored reference frequency signal.

  In addition, a method for determining the empty state of the fluid by monitoring the output of the differential amplifier 7 without providing the conversion unit 2 with the empty detection unit 11 as shown in FIG.

JP 2002-162267 A

  Since the sky detection unit 11 shown in Patent Document 1 is arranged so as to be directly connected to the electrodes 5a and 5b, there is a problem that the impedance of the entire conversion unit 2 is affected. Further, it is necessary to supply a positive / negative pulse current between the electrodes 5a and 5b. For example, when a ground fault occurs between the contact points of the electrodes 5a and 5b and the sky detection unit 11, it is impossible to detect the fluid empty. There is also a problem.

  In the method of determining the empty state of the fluid by monitoring the output of the differential amplifier 7 without providing the above-described empty detection unit 11 in the conversion unit 2, the commercial AC power source induced by the two electrodes 5a and 5b is used. Since the noise is almost in phase and these noises are canceled at the output of the differential amplifier 7, it is difficult to detect the fluid empty state based on the noise superimposed in the common mode. Note that this noise cancellation also occurs in the conversion unit 2 of Patent Document 1.

  As described above, a technique for detecting an abnormality in the differential voltage output between the electrodes 5a and 5b and determining an empty state has been conventionally known, but the noise induced in the electrodes 5a and 5b has a large in-phase component, It is difficult to detect an accurate sky condition with differential output.

  An object of the present invention is to provide a method for determining an empty state of an electromagnetic flow meter that eliminates the above-described problems, eliminates the influence of noise, and can accurately determine the empty state of a fluid in a measurement tube.

In order to achieve the above object, the electromagnetic flow meter empty state determination method according to the present invention is a measurement tube in which an electromagnetic flow meter is intermittently energized at a predetermined cycle and the excitation and excitation pauses are repeated. In addition to the automatic zero point adjustment processing when the fluid is filled in, the A / D conversion processing is performed on the measured value of the electrode potential measured in the excitation pause section during the excitation pause , and the A / D conversion value In response to the extraction processing by the band pass filter of the AC power supply frequency, an empty determination threshold is set based on the extracted frequency component, and the measurement of the electrode potential measured in the excitation pause section during the flow measurement of the fluid A / D conversion is performed on the value, and the signal level of the AC power frequency component extracted by performing the extraction process using the bandpass filter on the A / D conversion value is compared with the empty determination threshold value. the flow And judging the empty state.

  According to the method for determining an empty state of an electromagnetic flow meter according to the present invention, the individual induced potential of each electrode is measured during automatic zero adjustment, and the measured noise is used as a determination criterion for the empty state. It is possible to determine an accurate empty state based on conditions in the actual use state regardless of the above.

It is a block diagram of the electromagnetic flowmeter of an Example. It is explanatory drawing of the external noise transmission induced | guided | derived to an electrode. It is a wave form diagram of the state which changed from the wetted state to the non-wetted state. It is a block diagram of the electromagnetic flowmeter provided with the conventional sky detection function.

The present invention will be described in detail based on the embodiment shown in FIGS.
FIG. 1 is a configuration diagram of an electromagnetic flow meter of an embodiment, and the same reference numerals as those in FIG. 4 indicate the same circuits. The detection unit 1 includes a measurement tube 3 through which a fluid flows, an excitation coil 4 disposed around the measurement tube 3, and a pair of electrodes 5 a and 5 b exposed in the measurement tube 3. The inner wall of the measuring tube 3 is coated with an insulating material, but the measuring tube 3 itself is grounded and shields induced noise from the outside.

  The conversion unit 2 is provided with buffer amplifiers 6a and 6b corresponding to the electrodes 5a and 5b, respectively, and the outputs of the buffer amplifiers 6a and 6b are connected to the differential amplifier 7 and the multiplexer 12, respectively. The multiplexer 12 switches the outputs of the buffer amplifiers 6 a and 6 b alternately and outputs the same to the CPU 8 in synchronization with the timing generated by the timing signal generating circuit 13 by the timing signal input from the timing signal generating circuit 13.

  The output of the differential amplifier 7 is sequentially connected to a CPU 8 that functions as an A / D converter and executes a flow rate calculation, and further to an output circuit 9. Further, the CPU 8 continuously samples the outputs from the buffer amplifiers 6a and 6b and performs A / D conversion processing, and then uses the commercial AC power supply frequency for band diagnosis by performing band-pass filter processing.

  On the other hand, the output of the excitation circuit 10 in the conversion unit 2 is connected to the excitation coil 4 and the CPU 8, and the output of the timing signal generation circuit 13 is connected to the multiplexer 12, the CPU 8, and the excitation circuit 10.

  In synchronization with the low frequency cycle generated by the timing signal generation circuit 13, when the excitation current Iex of the excitation cycle T having positive excitation and negative excitation is supplied from the excitation circuit 10 to the excitation coil 4, the measurement tube 3 is connected. An electromotive force Es is induced between the electrodes 5a and 5b as a flow rate signal proportional to the flow velocity of the flowing fluid.

The electromotive force Es induced between the electrodes 5a and 5b and output from the buffer amplifier 6a or 6b is given by the following equation (1).
Electromotive force Es = κ · B · v · D (1)

  Here, κ is a proportional constant, B is the magnetic flux density by the exciting coil 4, v is the flow velocity of the fluid to be measured, and D is the diameter of the measuring tube 3.

If the magnetic flux density B is proportional to the excitation current Iex, the flow velocity v can be obtained from the following equation (2) based on the equation (1). Α is a constant determined for each detection unit 1.
v = α · Es / Iex (2)

  The electromotive force Es is received by the buffer amplifier 6a or 6b, and is input to the CPU 8 through the differential amplifier 7. The CPU 8 performs synchronous rectification at the timing synchronized with the excitation current Iex based on the electromotive force Es from the electrodes 5a and 5b input from the differential amplifier 7 and the output of the timing signal generation circuit 13, and at the same time, the expression (2) Is compared with the excitation current Iex, and the obtained flow velocity v is input to the output circuit 9. In the output circuit 9, it is converted into a predetermined output signal for the process.

  Further, in order to reduce the loss in the constant current circuit in the electromagnetic flow meter, the excitation is often performed by a switching method, and the constant current continues to be supplied even when the load changes. The exciting current Iex, which is a constant current, is supplied to the exciting coil 4 by the exciting circuit 10 for the purpose of creating a stable magnetic field by exciting the exciting coil 4 of the measuring tube 3 having a different diameter.

  The magnetic field is applied to the fluid by the exciting coil 4 by the constant current output from the exciting circuit 10. For a fluid having conductivity, a flow velocity v of the fluid is obtained based on an electromotive force Es induced in a direction orthogonal to the fluid direction and the magnetic field direction, and the flow velocity v is multiplied by the cross-sectional area of the measuring tube 3 to obtain a flow rate. Can be requested.

FIG. 2 shows how external noise is transmitted to the electrodes 5a and 5b. In a normal fluid measurement state, the measurement tube 3 is filled with a conductive fluid, and when the electrodes 5a and 5b are in a liquid contact state, the signal source resistance Rs of the flow electromotive force generated between the electrodes 5a and 5b is Using the fluid conductivity σ and the diameter d of the electrodes 5a and 5b, it is expressed by the equation (3)
Rs = 1 / (σ · d) (3)

Assuming that the electromotive force of the noise source is En, the noise source impedance is Zn, and the signal source resistance Rs is a resistance between the electrodes 5a and 5b and the ground as illustrated, the magnitude of the induced noise induced in the electrodes 5a and 5b. en is expressed by the following equation (4).
en = {Rs / (Rs + Zn)} · En (4)

  As can be seen from these equations (3) and (4), the magnitude of the induction noise en depends on the fluid conductivity σ which is a use condition. If the fluid conductivity σ is within the range of the specifications of the fluid conductivity determined by the electromagnetic flow meter (Rs << Zn), the induction noise en from the outside is shielded by the ground fault of the measuring tube 3, so the flow rate There is little impact on the measurement results.

  When the measuring tube 3 is in a non-full state and the fluid is in a non-wetted state with respect to the electrodes 5a and 5b, the electrode impedance between the electrodes 5a and 5b and the ground becomes infinite, greatly affecting the influence of external induction noise. Will receive.

  The most prominent inductive noise from the outside is the inductive noise from the commercial AC power supply. Therefore, a method is conceivable in which the electrode impedance is estimated based on the frequency component of the commercial AC power source superimposed on the electrode potentials of the electrodes 5a and 5b, and whether or not the detection unit 1 is in an empty state.

  However, the size of the inductive noise source and the noise source impedance Zn by the commercial AC power supply vary greatly depending on the installation conditions, and the fluid conductivity σ also varies depending on the type of fluid in the measuring tube 3, etc. For this purpose, a criterion set for each electromagnetic flow meter is required.

  This determination criterion is based on the A / D conversion process by continuously measuring the electrode potentials of the electrodes 5a and 5b at a predetermined sampling period in addition to the adjustment process at the time of automatic zero adjustment that is adjusted and stored at the zero point of the flow rate. Is set based on the frequency component that has passed through the band-pass filter of the commercial AC power supply frequency.

  At the time of automatic zero adjustment, the above-described band-pass filter is passed through the measurement value of a predetermined sampling period every plural excitation periods T. A maximum value and a minimum value are obtained for the output component of the bandpass filter, and an average value of each of the plurality of maximum values and the plurality of minimum values is calculated. However, these average values are only a minute swing range because there is a value in a state where the measuring tube 3 is filled with fluid during automatic zero point adjustment.

Therefore, a value obtained by multiplying the average value of the plurality of maximum values and the plurality of minimum values by several tens to several hundreds is set as a threshold value, and these plus and minus threshold values are set as threshold values s1 and s2. Alternatively, a value obtained by multiplying the average value of absolute values output by the bandpass filter by several tens to several hundreds may be set as the threshold s. These magnifications are set appropriately according to the fluid, the diameter of the measuring tube 3, and the like.

  Note that automatic zero point adjustment means that even if the fluid filling the measuring tube 3 is in a state of zero flow velocity, a constant bias amount is output as an offset, so this offset is subtracted and adjusted to the zero point. This process enables the CPU 8 to calculate a more accurate flow rate.

  Next, when determining whether the fluid in the measurement tube 3 is empty using the threshold values s1 and s2 that are determination criteria, first, the multiplexer 12 outputs the outputs of the buffer amplifiers 6a and 6b based on the timing time from the timing signal generation circuit 13. The electrode potential is selectively input to the CPU 8 separately.

  Next, the CPU 8 continuously samples the input values from the buffer amplifiers 6a and 6b at predetermined time intervals and performs A / D conversion processing. The frequency of the commercial AC power supply of 50/60 Hz is obtained by performing band-pass filter processing on the commercial AC power supply frequency to the measurement values subjected to these A / D conversion processes to attenuate signal components in frequency bands other than the commercial AC power supply frequency. Extract only the components.

  And the frequency component which is an induction noise component of the commercial alternating current power supply which performed this extraction process is compared with threshold value s1, s2 set at the time of automatic zero point adjustment. When the signal level of the frequency component of the commercial AC power supply exceeds or falls below the threshold value s1 or threshold value s2, it is determined that the fluid in the measurement tube 3 is in a non-wetted state, that is, an empty state. As described above, it is possible to determine the empty state by monitoring the induction noise based on the commercial AC power input in the common mode.

  Further, when the signal level of the frequency component subjected to the band-pass filter processing of the A / D conversion value at a predetermined interval equal to or less than the excitation cycle T exceeds or falls below the threshold value s1 or s2 for a plurality of times, or A / D When the absolute value of the signal level of the frequency component subjected to the band-pass filter processing of the D conversion value exceeds the threshold value s once or a plurality of times, it can also be determined as an empty state.

  Furthermore, when the average value of the absolute values of the signal levels of a plurality of continuous frequency components exceeds the threshold value s, it may be determined as an empty state, or the signal level of the frequency component for each of a plurality of excitation periods T may be determined. The maximum value and the minimum value are obtained, and when the average value of each of the plurality of maximum values and the plurality of minimum values exceeds or falls below the threshold value s1 or the threshold value s2, the empty state is determined. Also good.

  By calculating the average value of the signal levels of the frequency components that have been subjected to the band-pass filter in this way, it is possible to avoid erroneous detection of an empty state based on sudden noise or flow noise.

  FIG. 3 shows (a) an excitation current waveform and (b) a flow rate signal waveform that is a signal waveform having an amplitude similar to the excitation current Iex when the fluid changes from a wetted state to a non-wetted state at time t. c) The waveform of the commercial AC power frequency component after the band pass filter processing.

  In the excitation current waveform of (a), an excitation pause period is provided after a positive excitation period and a negative excitation period within the excitation period T. The excitation cycle T is synchronized with an integral multiple of the cycle of the commercial AC power supply, and intermittent excitation processing is performed for power saving.

  When the fluid in the measuring tube 3 changes from the wetted state to the non-wetted state at time t, the electrode impedance between the electrodes 5a and 5b and the ground becomes infinite as described above, and the induced noise from the commercial AC power supply It will be greatly affected. For this reason, as shown in the waveform diagram of (c), the minute signal level in the liquid contact state before the time t is extremely increased after the time t in the non-wetted state.

  By comparing the signal level of the frequency component with the plus / minus threshold value s1 or threshold value s2 set in the automatic zero adjustment process when the fluid is stationary and the fluid is filled in the measurement tube, the measurement tube 3 It is possible to detect whether the fluid inside is in a wetted state or a non-wetted state, that is, in an empty state.

  In this embodiment, the empty state can also be determined using the threshold value s, which is a determination criterion stored during automatic zero adjustment, regardless of the use conditions, fluid conductivity, and grounding conditions. Therefore, it is not necessary to install the sky detection unit 11 as shown in Patent Document 1.

  Moreover, you may use the low-pass filter which extracts only the frequency component below a commercial power supply frequency instead of a band pass filter. By this low-pass filter, it is possible to eliminate the aliasing noise at the time of A / D conversion and to avoid the influence due to the sudden digital error.

  By using a low-pass filter made of hardware such as a CR filter, it is possible to reduce the load caused by the software processing of the CPU 8 during the above-described band-pass filter processing.

  Furthermore, when an excitation pause interval as shown in FIG. 3A is provided and excitation is performed intermittently, the sky determination process may be performed in the excitation pause interval. In the period in which the excitation current exists, that is, in the period in which the excitation current Iex for positive excitation and negative excitation is output, electromagnetic flow noise is generated due to the time change of the flow signal electromotive force and the excitation current.

  During the period in which the excitation current exists, electromagnetic flow noise is also superimposed and output. Therefore, A / D conversion sampled in the excitation pause period is performed in order to perform the sky determination to cut the influence of the electromagnetic flow noise. It is preferable to perform the above-described empty determination processing based on the signal level that has been subjected to the band-pass filter processing of values.

DESCRIPTION OF SYMBOLS 1 Detection part 2 Conversion part 3 Measuring tube 4 Excitation coil 5a, 5b Electrode 6a, 6b Buffer amplifier 7 Differential amplifier 8 CPU
9 Output Circuit 10 Excitation Circuit 12 Multiplexer 13 Timing Signal Generation Circuit

Claims (3)

In an electromagnetic flow meter that repeats excitation and excitation pause by energizing the excitation coil intermittently at a predetermined cycle,
Along with the automatic zero adjustment process when the fluid is stationary and the fluid is filled in the measuring tube, the measured value of the electrode potential measured in the excitation pause section at the time of the excitation pause is A / D converted, The A / D conversion value is subjected to extraction processing by a band pass filter of the AC power supply frequency, and an empty determination threshold is set based on the extracted frequency component,
An A / D conversion process is performed on the measured value of the electrode potential measured in the excitation pause interval during the flow rate measurement of the fluid , and the A / D conversion value is extracted by performing an extraction process using the band-pass filter. A method for determining an empty state of an electromagnetic flowmeter, wherein the empty state of the fluid is determined by comparing a signal level of an AC power frequency component that has been performed and the empty determination threshold value. The maximum value of the signal level of the frequency components in the plurality of the excitation pause interval, finds the minimum, compared with the respective average value of a plurality of said maximum values and a plurality of said minimum value, and the air-determination threshold Te, empty state determination method of an electromagnetic flowmeter according to claim 1, wherein the determining the empty state of the fluid in the measuring tube. The empty state determination method according to claim 2, wherein the empty determination threshold value is set to two plus or minus threshold values with respect to the average value .

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