JP2016206079A - Exciting coil short circuit determination method of electromagnetic flow meter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To determine whether or not the exciting coil of an electromagnetic flow meter is short-circuited.SOLUTION: When an exciting coil is short-circuited, a smoothing process based on the exciting coil is eliminated, and an exciting current Iex repeats an output by a triangular waveform having ripples of a large virtual fluctuation by an ON/OFF process by a FET switch. The exciting current Iex is measured for each predetermined time interval for a ripple checking of the exciting current Iex, and an average value H of these exciting currents Iex for each measurement time is calculated, and a variation h of the ripple fluctuated in a plus direction and a minus direction is calculated from the average value H. A short-circuit state of the exciting coil which is an exciting load is determined by comparing the variation h showing a magnitude of this ripple and a threshold value set beforehand.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an excitation coil short-circuit determination method for an electromagnetic flow meter that determines whether an excitation coil is short-circuited.

  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. 5 shows a configuration diagram of a general electromagnetic flow meter, which 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 pair of electrodes 5 a and 5 b disposed in the measurement tube 3.

  The conversion unit 2 is provided with a buffer amplifier 6 that differentially receives two flow rate signals induced by the electrodes 5a and 5b. The output of the buffer amplifier 6 is connected to a CPU 7 and an output circuit 8 that perform flow rate calculation and the like. Has been. On the other hand, the output of the excitation current from the excitation circuit 9 is connected to the excitation coil 4 and the CPU 7, and the output of the timing signal generation circuit 10 is connected to the excitation circuit 9 and the CPU 7.

  When a positive and negative excitation current Iex is supplied from the excitation circuit 9 to the excitation coil 4 in synchronization with the low frequency cycle generated by the timing signal generation circuit 10, the flow rate signal is proportional to the flow velocity of the fluid flowing through the measurement tube 3. The electromotive force Es is induced between the electrodes 5a and 5b.

The electromotive force Es induced between the electrodes 5a and 5b and output from the buffer amplifier 6 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).
v = α · Es / Iex (2)

  Α is a constant determined for each detection unit 1.

  The electromotive force Es from the electrodes 5 a and 5 b is received by the buffer amplifier 6 and further input to the CPU 7. In the CPU 7, based on the output of the timing signal generation circuit 10, synchronous rectification is performed at a timing synchronized with the excitation current Iex, and at the same time, a comparison calculation with the excitation current Iex according to the equation (2) is performed, and the obtained flow velocity v is obtained. Input to the output circuit 8. In the output circuit 8, it is converted into a predetermined output signal for the process.

  Moreover, in order to reduce the loss in the constant current circuit in the electromagnetic flow meter, the excitation coil 4 is often excited by a switching method, and the constant current continues to be supplied even when the load changes in this constant current circuit. To do. An exciting current Iex, which is a constant current, is supplied to the exciting coil 4 by the exciting circuit 9 for the purpose of creating a stable magnetic field by exciting the exciting coil 4 of the measuring tube 3 having a different diameter.

  A magnetic field is applied to the fluid by the exciting coil 4 by the constant current output from the exciting circuit 9. 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.

  Further, the electromagnetic flow meter is provided with a function of performing various self-judgments for ensuring the safety of the plant operation, and has a function of detecting trouble due to excitation coil open and excitation coil short circuit.

  Patent Document 1 discloses that an excitation coil short-circuit detection circuit is provided between the excitation circuit and the excitation coil as an invention for detecting an excitation coil short-circuit. When the exciting coil is short-circuited, the voltage is not lowered below the threshold by the voltage limiter in the short-circuit detecting circuit, and as a result, the current from the exciting circuit increases. And the short circuit of an exciting coil is detectable by measuring the both-ends voltage of the detection resistor connected in series with the exciting coil with time.

JP 2002-206956 A

  As described above, as an electromagnetic flow meter self-determination, an excitation coil open trouble can be easily detected because the excitation current Iex becomes zero. However, the excitation coil short circuit is controlled by constant current control. It is difficult to detect a short circuit based on the current Iex.

  In particular, as shown in FIG. 5, when there is a distance between the detection unit 1 that detects the electromotive force Es in the vicinity of the measurement tube 3 and the conversion unit 2 that includes the excitation circuit 9 and the CPU 7, the detection unit 1. The connection cable between the converter 2 and the converter 2 is easily damaged, and a short circuit often occurs at a position in the cable indicated by a dotted line S in FIG.

  Therefore, in the conventional short circuit determination method shown in the circuit configuration diagram of FIG. 6, a dedicated short circuit determination circuit P for the excitation coil 4 to be detected is provided in the excitation circuit 9. The short circuit determination circuit P measures the voltage across the exciting coil 4 at the timing when the exciting current Iex is supplied to the exciting coil 4, and the product of the coil resistance component Rc of the exciting coil 4 and the exciting current Iex flowing through the exciting coil 4. (Rc · Iex) is compared over time to determine whether the exciting coil 4 is short-circuited.

  In the conventional method of FIG. 6, a dedicated short-circuit determination circuit P is required, and a coil resistance component Rc that differs for each exciting coil 4 needs to be considered.

  Also, in the electromagnetic flow meter provided with the short circuit detection circuit of Patent Document 1, a voltage limiter or the like is provided in the short circuit detection circuit, or the limit value of the voltage limiter based on the dimensions of the measurement tube, the coil resistance component of the excitation coil, or the like. There are problems such as the need to set.

  An object of the present invention is to solve the above-described problems and provide an excitation coil short-circuit determination method for an electromagnetic flowmeter that can determine a short-circuit state of an excitation coil by switching-type excitation without considering a coil resistance component. There is.

  In order to achieve the above object, an excitation coil short-circuit determination method for an electromagnetic flow meter according to the present invention supplies the excitation current having a constant current to the excitation coil by a switching method in which excitation current excitation is intermittently turned on and off. In the excitation coil short-circuit determination method of the electromagnetic flow meter, when the excitation is turned on, an excitation current is measured every predetermined time interval, and a variation value that is a difference between an average value of these measurement values and the measurement value is calculated. It is characterized in that it is determined whether or not the exciting coil is in a short circuit state by comparing a fluctuation value with a preset threshold value.

  According to the method for determining an excitation coil short circuit of an electromagnetic flow meter according to the present invention, an excitation coil short circuit can be determined with only a basic circuit configuration, and the excitation coil is independent of the dimensions of the measurement tube, the resistance component of the excitation coil, and the inductance. It is possible to easily and reliably determine the short circuit state.

It is a circuit diagram of a switching-type excitation constant current circuit. FIG. 6 is a waveform diagram of excitation timing, duty cycle signal, and excitation current during normal operation. It is a circuit diagram of the excitation constant current circuit at the time of a short circuit of an exciting coil. It is a waveform diagram of the excitation timing, duty cycle signal, and excitation current when the excitation coil is short-circuited. It is a circuit block diagram of a general electromagnetic flowmeter. It is a circuit diagram of the excitation constant current circuit used with the conventional excitation coil short circuit determination method.

The present invention will be described in detail based on the embodiment shown in FIGS.
FIG. 1 shows a configuration diagram of a switching-type excitation constant current circuit. In addition, the conventional short circuit determination circuit P is deleted compared with the conventional example shown in FIG.

  In FIG. 1, an excitation circuit 9 has an excitation power source that applies a DC voltage, an FET switch 11 that is turned on / off to obtain a constant current, and an LC filter 12 that includes an inductance L1 and a capacitor C. The voltage switched by the FET switch 11 by the LC filter 12 is smoothed.

  Further, the excitation circuit 9 is provided with a current detection resistor R and a constant current control unit 13. By arranging the current detection resistor R in series with the excitation constant current circuit, an excitation current Iex flowing through the excitation coil 4 is provided. Can be detected.

  The constant current control unit 13 includes a deviation amplifier 14 and a duty cycle converter 15. The deviation amplifier 14 has a voltage (R · Iex) when the excitation current Iex flows through the current detection resistor R. And a constant reference voltage Vref that determines the magnitude of the excitation current Iex is applied.

  The deviation value between the voltage (R · Iex) amplified by the deviation amplifier 14 and the reference voltage Vref is input to the duty cycle converter 15. The duty ratio calculated by the duty cycle converter 15 is fed back to the FET switch 11 to perform on / off control.

  The voltage applied by turning on / off the FET switch 11 is smoothed by the LC filter 12 and becomes the voltage Vex shown in FIG. The excitation current Iex flows through the excitation coil 4 by this voltage Vex.

  Regarding the processing of the voltage (R · Iex) and the reference voltage Vref in the duty cycle converter 15, when the voltage (R · Iex) is smaller than the reference voltage Vref, the duty ratio of the output of the duty cycle converter 15 increases. On / off control of the FET switch 11 based on this duty ratio is performed. The voltage Vex increases as the duty ratio increases, and the excitation current Iex increases as the voltage Vex increases.

  When the voltage (R · Iex) is larger than the reference voltage Vref, the duty ratio of the output of the duty cycle converter 15 decreases, the voltage Vex decreases, and the excitation current Iex also decreases.

  By such feedback control, the voltage R · Iex is corrected to have the same value as the reference voltage Vref, so that the excitation current Iex is held at a constant current value determined by the reference voltage Vref.

  FIG. 2 is a waveform diagram when the excitation circuit 9 outputs a predetermined value of the excitation current Iex and normal excitation is performed with the excitation coil 4 connected. (A) is an excitation on / off timing signal, (b) is a duty cycle signal that is an output of the duty cycle converter 15 for constant current control, and (c) is an excitation that is an input of the duty cycle converter 15. Current Iex is shown.

  The excitation by the excitation current Iex is periodically turned on and off by the timing signal generation circuit 10 as shown in FIG. 4A. In the present invention, detailed explanation of the on and off timing control is provided. Omitted. The on / off control of the excitation current Iex can save power compared to the normal excitation, and it is determined whether or not the excitation coil 4 is in a short circuit state when the excitation is on.

  In the excitation-on period of (a), the excitation current Iex smooths the voltage switched by the FET switch 11 by the LC filter 12, but includes some ripple. The exciting current Iex flowing through the exciting coil 4 is further smoothed by the coil resistance component Rc and the inductance component Lc of the exciting coil 4, and the ripple becomes small as shown in FIG.

  Furthermore, by making the cycle of the duty cycle signal of (b) sufficiently smaller than the time constant τ = Lc / Rc of the exciting coil 4, the waveform of the exciting current Iex can be smoothed and the ripple component can be almost ignored. become.

  FIG. 3 is a circuit diagram of an excitation constant current circuit when the excitation coil 4 is short-circuited. FIG. 4 is a waveform diagram, in which (d) is an excitation on / off timing signal, (e) is a duty cycle signal, and (f) is an output signal of an excitation current Iex.

  When the exciting coil 4 is short-circuited as shown in FIG. 3, the exciting current Iex does not flow through the exciting coil 4, and the load of the exciting circuit 9 is only the current detection resistor R. Accordingly, the smoothing process based on the excitation coil 4 described above is eliminated, and the excitation current Iex increases rapidly as shown in (f) in proportion to the increase in the voltage across the capacitor C.

  At this time, the voltage (R · Iex) increases linearly more rapidly than the reference voltage Vref, so that the excitation current Iex also increases. When the voltage (R · Iex) exceeds the reference voltage Vref, the duty cycle converter 15 of the constant current control unit 13 immediately decreases the duty ratio. That is, the FET switch 11 is controlled to be turned off, but the FET switch 11 cannot be turned off immediately due to the delay of the control loop, and the excitation current Iex continues to increase during the delay of the control loop.

  On the other hand, when the FET switch 11 is turned off, the exciting current Iex is suddenly decreased. Since the control loop is delayed in the same manner as when the exciting current Iex increases, the exciting current Iex repeats output by a triangular waveform or a trapezoidal wave corresponding to a large up-and-down fluctuation ripple as shown in (f). At this time, the period of the duty cycle signal that becomes a triangular wave or a trapezoidal wave is larger than the period of the duty cycle signal during normal operation shown in FIG.

  The constant current control unit 13 of the excitation circuit 9 measures the excitation current Iex at predetermined time intervals for ripple check of the excitation current Iex, calculates the duty ratio, and controls the FET switch 11. At the same time, the excitation circuit 9 outputs a measured value of the excitation current Iex to the CPU 7 that also serves as a short-circuit determination processing unit.

  The CPU 7 having input the measured value of the excitation current Iex calculates the average value H from the accumulated measured values of the plurality of excitation currents Iex when the excitation is on. Next, for each measured value, a fluctuation value h that fluctuates in the positive and negative directions from the average value H, that is, the difference between the average value H and the measured value is obtained to calculate the magnitude of the ripple. And the fluctuation value h for every measurement is compared with the preset threshold value, and the short circuit state of the exciting coil 4 which is an exciting load is determined.

  Or, when the excitation is turned on excluding the time zone immediately after the excitation is turned on, a fluctuation value h ′ that is a difference between the minimum value and the maximum value of the measured excitation current Iex is calculated, and the fluctuation value h ′ is compared with a preset threshold value. Then, the short-circuit state of the exciting coil 4 may be determined. In this case, it is not necessary to calculate the above average value H.

  When the CPU 7 connected to the excitation circuit 9 detects the occurrence of a large ripple, that is, the fluctuation value h exceeds the threshold value, it is determined that the excitation coil 4 is in a short-circuit state. This determination determines that the excitation coil is in a short-circuited state when one or more of the fluctuation values h are greater than a threshold value among the fluctuation values h at predetermined time intervals. Further, when a plurality of fluctuation values h, for example, two or more fluctuation values h are larger than a threshold value within a certain time, it may be determined that a short circuit has occurred.

  Further, in order to prevent erroneous detection of a short-circuit state due to sudden noise or the like, when the fluctuation value h of the excitation current Iex is larger than the threshold value among the fluctuation values h for each predetermined time interval, You may make it determine with having short-circuited.

  As described above, the fluctuation value h of the excitation current Iex is extremely small during normal operation, whereas the fluctuation value h is extremely large during short-circuiting, which corresponds to a ripple when excitation is on. By monitoring the fluctuation value h, the short-circuit state of the exciting coil 4 can be detected easily and reliably.

4 Excitation coil 9 Excitation circuit 11 FET switch 12 LC filter 13 Constant current controller 14 Deviation amplifier 15 Duty cycle converter

In order to achieve the above object, an exciting coil short-circuit determination method for an electromagnetic flow meter according to the present invention supplies a constant-current exciting current to an exciting coil by a switching method, and periodically turns on and off excitation by the exciting current. In the method for determining an excitation coil short circuit of an electromagnetic flow meter, when the excitation is turned on, an excitation current is measured at predetermined time intervals, an average value is calculated from a plurality of accumulated measurement values of the excitation current, and the excitation current and the The fluctuation value, which is the magnitude of the ripple of the excitation current waveform, is calculated from the difference from the average value, and it is determined whether the excitation coil is in a short-circuited state by comparing the fluctuation value with a preset threshold value. It is characterized by doing.
Also, the excitation coil short-circuit determination method of the electromagnetic flow meter according to the present invention is a method of supplying an excitation current of constant current to the excitation coil by a switching method, and periodically turning on and off the excitation by the excitation current. In the excitation coil short-circuit determination method, the excitation current is measured at every predetermined time interval when excitation is turned on excluding the time zone immediately after the excitation is turned on, and a fluctuation value that is a difference between the minimum value and the maximum value of the measured excitation current is calculated. It is characterized in that it is determined whether or not the exciting coil is in a short circuit state by comparing the fluctuation value with a preset threshold value.

In the excitation-on period of (a), the excitation current Iex smooths the voltage switched by the FET switch 11 by the LC filter 12, but includes some ripple. Excitation current Iex flowing the exciting coil 4, the coil resistance component Rc of the excitation coil 4, further smoothed by the inductance component Lc, ripples as shown in FIG. 2 (c) is reduced.

Claims (4)

In the excitation coil short-circuit determination method of the electromagnetic flowmeter that supplies a constant current excitation current to the excitation coil by a switching method and periodically controls on / off excitation by the excitation current.
When the excitation is turned on, the excitation current is measured at predetermined time intervals, the fluctuation value of the excitation current is calculated based on these measurement values, and the fluctuation value is compared with a preset threshold value to thereby calculate the excitation coil. A method for determining an excitation coil short circuit in an electromagnetic flow meter, wherein:   2. The electromagnetic flow rate according to claim 1, wherein an average value is calculated from a measured value obtained by measuring an excitation current at each predetermined time interval, and the fluctuation value is calculated from a difference between the average value and the measured value. Method for determining the excitation coil short circuit of the meter.   3. The electromagnetic wave according to claim 2, wherein the excitation coil is determined to be in a short-circuited state when one or more of the fluctuation values among the fluctuation values at the predetermined time intervals are larger than the threshold value. Flow meter excitation coil short-circuit judgment method.   The said exciting coil determines with the said excitation coil being a short circuit state, when the said fluctuation value is larger than the said threshold value in multiple times among the said fluctuation values for every said predetermined time interval. How to determine the excitation coil short circuit of an electromagnetic flow meter.

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