JP6481430B2 - Electromagnetic flow meter - Google Patents

Description

  The present invention relates to an electromagnetic flow meter, and more particularly to an electromagnetic flow meter having a function of diagnosing insulation deterioration and abnormal state of a coil.

  FIG. 10 is a block diagram showing an example of a conventionally used electromagnetic flow meter. A pair of electrodes 12 and 13 are provided inside the conduit 11 through which the conductive fluid flows, and an earth ring 14 is provided below the electrodes 12 and 13, and the earth ring 14 is grounded.

  An excitation coil 15 is provided in the vicinity of the outer periphery of the conduit 11, and the excitation coil 15 is excited by an excitation circuit 16 to generate a magnetic field inside the conduit 11. When a conductive fluid flows inside the conduit 11 while a magnetic field is generated, an analog voltage proportional to the magnetic flux density inside the conduit 11 and the flow velocity of the conductive fluid is generated.

  An analog voltage proportional to the flow velocity of the conductive fluid is detected by the electrodes 12 and 13, input to the A / D converter 18 through the differential amplifier circuit 17, converted into a digital signal, and input to the CPU 19.

  The CPU 19 performs a predetermined calculation for calculating the flow velocity of the conductive fluid and outputs the calculation result to the output circuit 20.

  An excitation circuit 16 is connected to the CPU 19, and an insulation resistance detection circuit 21 that detects insulation resistance of the excitation coil 15 and detects insulation deterioration of the excitation coil 15 is also connected to the CPU 19.

  FIG. 11 is an explanatory diagram showing the connection relationship of the exciting coil 15. In FIG. 11, the current supply unit 101 is connected in parallel with a series circuit of FET 106 (Q1) and FET 108 (Q3) and a series circuit of FET 107 (Q2) and FET 109 (Q4). A series circuit of FET 106 and FET 108 is connected. A series circuit of the exciting coil 15 and the current detection resistor 110 is connected to a connection middle point a of the FET 107 and the FET 109 and a connection middle point b of the FET 109.

  A DC voltage unit (not shown) provided in the current supply unit 101 supplies an excitation current to the excitation coil 15.

  The switch SW1 (102) is connected to the gate of the FET 106, the switch SW2 (103) is connected to the gate of the FET 107, the switch SW3 (104) is connected to the gate of the FET 108, and the switch is connected to the gate of the FET 109. SW4 (105) is connected.

  The FETs 106 to 109 are turned on / off according to the on / off drive timing of the switches 102 to 105, and the direction of the excitation current flowing through the excitation coil 15 is determined. The amount of excitation current is detected by the current detection resistor 110.

  The switch 102 outputs a signal that repeatedly turns the gate on and off in a short interval to the FET 106, and the switch 103 outputs a signal that repeats the gate on and off in a short interval to the FET 107. A desired constant current can be set by the on / off time ratio / duty ratio of the gate.

  The switches 104 and 105 do not perform switching control for turning on and off the gate, but determine only the directionality of the current flowing through the series circuit of the exciting coil 15 and the current detection resistor 110.

  FIG. 12 is an explanatory diagram of the operation timing of the FETs 106 (Q1) to 109 (Q4). In the case of positive excitation, only the FET 106 (Q1) performs switching control, and in the case of negative excitation, only the FET 107 (Q2) performs switching control. In the case of no excitation, all the FETs 106 (Q1) to 109 (Q4) are turned off.

  FIG. 13 is a circuit example diagram of an insulation resistance detection circuit described in Patent Document 1. In FIG. 13, one end of the excitation coil 15 is connected to a common potential point via the coil insulation resistance 201, and a changeover switch 205 is connected to both ends of the excitation coil 15. Here, the insulation resistance 201 represents an insulation resistance between the excitation coil 15 and the ground electrode that are originally insulated.

  The movable contact of the changeover switch 205 is connected to the anode of the battery BAT, and the cathode of the battery BAT is connected to the common potential point via the detection resistor 202.

  A buffer amplifier 203 is connected to a connection point between the cathode of the battery BAT and the detection resistor 202, and an A / D converter 204 is connected to the buffer amplifier 203.

  The changeover switch 205, the battery BAT, the detection resistor 202, the buffer amplifier 203, and the A / D converter 204 form an insulation resistance detection circuit 21.

  Incidentally, when the insulation between the exciting coil 15 and the ground electrode shown in FIG. 13 deteriorates, the combined resistance value of the coil insulation resistance 201 and the detection resistance 202 decreases.

  By monitoring the change in the combined resistance value of the insulation resistance 201 and the detection resistance 202 of the coil, it is possible to accurately detect the insulation deterioration of the exciting coil 15.

  Patent Document 1 describes a specific circuit configuration for diagnosing the state of insulation deterioration of the coil portion in order to detect whether the flow rate measurement is normally performed.

JP 2003-106879 A

  However, according to the conventional configuration described in Patent Document 1, an electronic circuit for detecting insulation deterioration of the exciting coil 15 must be provided separately.

  In addition, when supplying the exciting current to the exciting coil 15, a voltage of hundreds of tens of volts is applied, and therefore components having a large rated voltage and rated power must be used.

  The present invention pays attention to such conventional problems, and its purpose is to detect insulation deterioration of the exciting coil without providing a special electronic circuit, and to accurately diagnose the presence or absence of abnormality of the exciting coil. It is to provide an electromagnetic flow meter.

The invention of claim 1 which achieves such a problem,
A DC voltage is applied to the excitation coil via a switching element driven by a pulse width signal from the excitation current supply circuit, and the direction of the excitation current flowing through the excitation coil is switched at a predetermined excitation fundamental frequency, and the value of the excitation current is In an electromagnetic flow meter of a switching control method in which the switching duty of the switching element is controlled to be on / off at an excitation switching control frequency larger than the excitation basic frequency so as to be constant,
Based on the value IEX of the exciting current and the average value VEXave of the applied voltage of the exciting coil, the resistance value Rcoil of the exciting coil is
Rcoil = VEXave / IEX
Resistance value calculating means for calculating by
Resistance value determining means for determining whether or not the resistance value Rcoil of the exciting coil calculated by the resistance value calculating means is within a predetermined allowable range set in advance;
Equipped with a,
The average value VEXave of the excitation voltage applied to the excitation coil is as follows. The output voltage of the excitation current supply circuit is VEX, and the duty ratio of the pulse width signal is Duty.
VEXave = VEX × Duty
It is represented by.

  The invention according to claim 2 determines whether or not the resistance value Rcoil of the exciting coil calculated by the resistance value calculating means is within a predetermined allowable range set in advance, and determines that the deterioration is insulation. Features.

The invention according to claim 3 is the electromagnetic flowmeter according to claim 1 or 2 ,
Means is provided for continuously monitoring a change in the resistance value Rcoil of the exciting coil.

According to a fourth aspect of the present invention, in the electromagnetic flow meter according to the first, second, or third aspect , the excitation has two frequency components.

  As a result, the resistance value of the exciting coil can be obtained without providing a special electronic circuit, and the presence or absence of an abnormality in the exciting coil can be accurately determined based on the resistance value of the exciting coil.

It is a block diagram which shows one Example of this invention. Switching excitation control and PWM acquisition timing chart It is the figure which showed the change of PWM by the increase / decrease in the resistance value of the exciting coil. It is the figure which showed the increase / decrease in the increase / decrease resistance value of the flow of the excitation current IEX at the time of abnormality of the excitation coil 15. FIG. 5 is a flowchart for explaining a flow of processing for calculating a resistance value of an exciting coil 15; It is a timing chart at the time of PWM acquisition using the timer part 19d. It is a waveform at the time of PWM acquisition using the A / D part 19e. 5 is a flowchart for explaining a flow of abnormality diagnosis processing for an excitation coil 15; Switching excitation control when excitation has two frequency components, and PWM acquisition timing chart It is a block diagram which shows an example of the electromagnetic flowmeter conventionally used. FIG. 3 is an explanatory diagram illustrating a connection relationship of exciting coils 15. It is operation | movement timing explanatory drawing of FET106 (Q1) -109 (Q4). It is a circuit example figure of the insulation resistance detection circuit provided in FIG.

  Hereinafter, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention, and the same reference numerals are given to portions common to FIG. In FIG. 1, the CPU 19 is provided with a resistance value calculation unit 19a, a resistance value comparison unit 19b, and a resistance initial value storage unit 19c, and the insulation resistance detection circuit 21 provided in FIG. 10 is omitted. A timer 19d or an A / D unit 19e may be provided for acquiring the PWM signal.

  First, switching excitation control will be described. FIG. 2 is a switching excitation control and PWM acquisition timing chart. In the switching excitation control, when the output voltage of the current supply unit 101 is VEX (V), control is performed so that VEX (V) always becomes a constant value. The output voltage of the current supply unit 101 is applied to the series circuit of the excitation coil 15 and the current detection resistor 110, and an excitation current flows through the excitation coil 15.

  At this time, the ON / OFF time of the PWM signal is controlled so that the amount of current flowing through the current detection resistor 110 is constant, and switching control is performed.

  The PWM acquisition timing is acquired in the latter half when PWM is stable for both positive excitation and negative excitation, so that the duty ratio of the PWM signal can be accurately acquired.

  FIG. 3 is a diagram showing a change in PWM due to an increase or decrease in the resistance value of the exciting coil 15. When the resistance value of the exciting coil 15 is small, the ON time of the PWM signal is shortened, and when the resistance value of the exciting coil 15 is large, the ON time of the PWM signal is lengthened. That is, the smaller the resistance value of the exciting coil 15, the shorter the voltage application time, and the larger the resistance value of the exciting coil 15, the longer the voltage application time.

  Since this is controlled so as to have a constant current value, when the resistance value of the exciting coil 15 is large, it takes a long time to reach a constant current value, and the ON time of the PWM signal also becomes long. Based.

  FIG. 4 is a diagram showing the flow of the excitation current IEX and the increase / decrease in the increase / decrease resistance value when the excitation coil 15 is abnormal. When the excitation coil is open compared to the normal time of the excitation coil, the resistance value of the excitation coil increases. When the exciting coil is short-circuited or insulated, the resistance value of the exciting coil decreases.

  FIG. 5 is a flowchart for explaining the flow of processing for calculating the resistance value of the exciting coil 15. First, the voltage value VEX of the current supply unit 101 is acquired (step S1). This voltage value is determined at the time of design and is input in advance. When it is desired to input a more accurate value, the voltage value may be directly measured and acquired.

  Subsequently, the duty ratio of the PWM signal in the switching control is acquired (step S2). When the PWM signal is output from a PWM control IC, an operational amplifier, a comparator, or the like, it is necessary to measure and acquire the PWM signal.

  FIG. 6 is a timing chart at the time of PWM acquisition using the timer unit 19d. A 19d timer unit in the CPU is used to reset the timer at the rise and fall of the PWM and read the data, thereby reading the PWM ON time or OFF time and obtaining the duty ratio.

  FIG. 7 shows waveforms at the time of PWM acquisition using the A / D unit 19e. The PWM signal is filtered and averaged. After averaging, the value is read by the 19eA / D part to determine the duty ratio.

  If the CPU is outputting a PWM signal, it is acquired directly.

Based on the voltage value VEX of the current supply unit 101 acquired in step S1 and the duty ratio of the PWM signal acquired in step S2, the average value of the voltages applied to the excitation coil 15 as expressed by the equation (1) VEXave is obtained (step S3).
VEXave = VEX × Duty (1)

Using the current value IEX acquired through the current detection resistor 110 and the average value VEXave of the voltage obtained in step S3, the resistance value Rcoil of the exciting coil 15 is obtained based on the equation (2) (step S4).
Rcoil = VEXave / IEX (2)

  FIG. 8 is a flowchart for explaining the flow of the abnormality diagnosis process for the exciting coil 15. First, the resistance value comparison unit 19b acquires the resistance value RcoilRef of the excitation coil 15 serving as a reference from the resistance initial value storage unit 19c (step S1).

  Next, the resistance value calculator 19a obtains the resistance value Rcoil of the exciting coil 15 based on the process flow shown in the flowchart of FIG. 5 (step S2).

  The resistance value comparison unit 19b compares the reference resistance value RcoilRef of the excitation coil 15 with the resistance value Rcoil of the excitation coil 15, and the resistance value Rcoil of the excitation coil 15 does not exceed the preset allowable range. Is determined (step S3).

The coil open allowable value Ropen, the resistance value Rcoil of the exciting coil 15 and the resistance value RcoilRef of the exciting coil 15 serving as a reference are used, and when Expression (3) is satisfied, it is determined that the coil is open.
Rcoil-RcoilRef> Ropen (3)

The coil short allowable value Rshort, the resistance value Rcoil of the exciting coil 15 and the resistance value RcoilRef of the exciting coil 15 serving as a reference are used, and when equation (4) is satisfied, it is determined that the coil has shorted or the insulation has deteriorated.
RcoilRef-Rcoil> Rshort (4)

  If the allowable range is not exceeded, the process returns to step S2 at predetermined time intervals, and the resistance value Rcoil of the exciting coil 15 is monitored by repeatedly executing the processes after step S2.

  If the allowable range is exceeded, the process proceeds to the next step S4, where it is determined that an abnormality has occurred in the resistance value Rcoil of the exciting coil 15 and an alarm is output (step S4).

  In the excitation circuit of the switching control system, the resistance value of the exciting coil 15 is provided without using a special electronic circuit by using a switching signal composed of a pulse width signal for controlling the exciting current and a voltage applied to the exciting coil 15. Can be requested.

  The resistance value of the exciting coil 15 thus obtained can be diagnosed by comparing the resistance value of the exciting coil 15 with the resistance value of the exciting coil at the time of installation or at the time of calibration. By continuously accumulating and monitoring, it is possible to prevent and improve the possibility of failure due to failure by grasping a sign of failure with a high probability.

  FIG. 9 is an excitation switching control and PWM acquisition timing chart when excitation has two frequency components. The excitation is not limited to having only one frequency component, and even if it has two frequency components, the PWM signal duty ratio in switching control can be acquired if PWM is appropriately acquired.

  As described above in detail, according to the present invention, the resistance value of the exciting coil can be obtained without providing a special electronic circuit, and the presence or absence of abnormality of the exciting coil can be accurately determined based on the resistance value of the exciting coil. Therefore, an electromagnetic flow meter capable of measuring in a stable state can be realized.

11 Conduit 12, 13 Electrode 14 Earth ring 15 Excitation coil 16 Excitation circuit 17 Differential amplification circuit 18 A / D converter 19 CPU
19a Resistance value calculation unit 19b Resistance value comparison unit 19c Resistance initial value storage unit 19d Timer unit 19e A / D unit 20 Output circuit

Claims (4)

A DC voltage is applied to the excitation coil via a switching element driven by a pulse width signal from the excitation current supply circuit, and the direction of the excitation current flowing through the excitation coil is switched at a predetermined excitation fundamental frequency, and the value of the excitation current is In an electromagnetic flow meter of a switching control method in which the switching duty of the switching element is controlled to be on / off at an excitation switching control frequency larger than the excitation basic frequency so as to be constant,
Based on the value IEX of the exciting current and the average value VEXave of the applied voltage of the exciting coil, the resistance value Rcoil of the exciting coil is
Rcoil = VEXave / IEX
Resistance value calculating means for calculating by
Resistance value determining means for determining whether or not the resistance value Rcoil of the exciting coil calculated by the resistance value calculating means is within a predetermined allowable range set in advance;
Equipped with a,
The average value VEXave of the excitation voltage applied to the excitation coil is as follows. The output voltage of the excitation current supply circuit is VEX, and the duty ratio of the pulse width signal is Duty.
VEXave = VEX × Duty
An electromagnetic flow meter represented by   2. The electromagnetic wave according to claim 1, wherein it is determined whether or not a resistance value Rcoil of the exciting coil calculated by the resistance value calculating means is within a predetermined allowable range set in advance, and a coil abnormality is determined. Flowmeter. Claim 1 or claim 2 electromagnetic flow meter according to characterized in that a means for continuously monitoring the change in resistance Rcoil of the exciting coil. 4. The electromagnetic flow meter according to claim 1, wherein the excitation has two frequency components.

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