Electromagnetic flowmeter and zero point measurement method thereof

An electromagnetic flowmeter for giving a magnetic field to a measurement fluid, detecting an electric signal occurring in the measurement fluid according to the magnetic field, and computing a flow quantity value based on the electric signal includes a zero point measurement section for measuring a zero point of the measurement fluid; a storage section for storing the measured zero point measurement value; a determination section for determining whether or not the difference between the preceding zero point measurement value stored in the storage section and the present zero point measurement value is beyond a predetermined value range; and at least either a transmission section for transmitting the determination result or a display section for displaying the determination result when the determination section determines that the difference is beyond the predetermined value range.

This application claims priority to Japanese Patent Application No. 2007-062942, filed Mar. 13, 2007, in the Japanese Patent Office. The priority application is incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a zero point measurement of a measurement fluid in the electromagnetic flowmeter. More specifically, the present disclosure relates to an electromagnetic flowmeter for performing zero point measurement without changing the output current of the electromagnetic flowmeter if the difference between the flow quantity value when a measurement fluid is made still and the preceding zero point measurement value is within a predetermined value range.

RELATED ART

The electromagnetic flowmeter gives a magnetic field to a measurement fluid flowing through a tube of a detector, detects an electric signal produced in the measurement fluid by the magnetic field, and computes and outputs the flow quantity of the measurement fluid based on the electric signal. The flow quantity value computed in a state in which the measurement fluid is made still is measured as a zero point measurement value and zero point correction computation of subtracting the zero point measurement value from the computed flow quantity value is performed to remove an error caused by the zero point measurement value in flow quantity measurement in a usual state. At this time, to execute the zero point measurement, the output current value, the excitation current value, and the excitation system of the electromagnetic flowmeter are changed. Such an electromagnetic flowmeter will be discussed withFIG. 11.

An electromagnetic flowmeter28is made up of a detector4, an amplification circuit8, an AD conversion section9, an insulating circuit10, a DC-DC conversion circuit12, a computation control section21, an excitation circuit22, a current output circuit23, etc.

One of a pair of output terminals of the electromagnetic flowmeter28is connected to a positive terminal (+) of a DC power supply26and the other output terminal is connected to a negative terminal (−) of the DC power supply26through a resistor25. The DC power supply26supplies a current (for example, ranging from 4 to 20 milliamperes) corresponding to the computed flow quantity value and DC voltage to the electromagnetic flowmeter28. A controller27connected to both ends of the resistor25measures a current output from the electromagnetic flowmeter28through the resistor25, converts the measurement value into a flow quantity value, and performs process control (for example, flow control).

The output terminal connected to the positive terminal (+) of the DC power supply26is connected to a first power supply line L1. Power supply terminals of the computation control section21, the excitation circuit22, the current output circuit23, and the input side (SW control circuit11) of the DC-DC conversion circuit12are connected to the first power supply line L1.

A connection part of the current output circuit23and an output current detection resistor24is connected to a first common line L2. Common (reference potential) terminals of the computation control section21, the excitation circuit22, the current output circuit23, and the input side (SW control circuit11) of the DC-DC conversion circuit12are connected to the first common line L2. The computation control section21, the excitation circuit22, the current output circuit23, and the DC-DC conversion circuit12receive power supply from the first power supply line L1.

A power supply terminal of the output side of the DC-DC conversion circuit12is connected to a second power supply line L3. Power supply terminals of the amplification circuit8, the AD conversion section9, and the insulating circuit10are connected to the second power supply line L3.

A common (reference potential) terminal of the output side of the DC-DC conversion circuit12is connected to a second common line L4. Common (reference potential) terminals of the amplification circuit8, the AD conversion section9, and the insulating circuit10are connected to the second common line L4. The amplification circuit8, the AD conversion section9, and the insulating circuit10receive power supply from the output of the DC-DC conversion circuit12(the second power supply line L3).

The excitation circuit22is connected to an excitation circuit control section20and an exciting coil1. The excitation circuit22causes an excitation current to flow into the exciting coil1based on a control signal of the excitation circuit control section20. The exciting coil1generates a magnetic field in a tube of the detector4and gives the magnetic field to a measurement fluid in the tube, whereby an electric signal (electromotive force) proportional to the magnetic flux density of the magnetic field and the flow velocity of the measurement fluid occurs in the measurement fluid flowing through the tube. The detector4is made up of the exciting coil1, electrodes2and3, the tube for allowing a measurement fluid to flow (not shown), etc., and the electric signal is detected by the electrodes2and3placed in the tube.

The amplification circuit8is made up of buffers (voltage followers)5and6and a differential amplifier7. Inputs of the buffers5and6are connected to the electrodes2and3and outputs of the buffers5and6are connected to input of the differential amplifier7. The amplification circuit8generates a difference signal between the electric signals detected by the electrodes2and3and outputs the difference signal to the AD conversion section9. The difference signal is proportional to the flow velocity of the measurement fluid.

The AD conversion section9converts the analog difference signal into digital data and outputs the digital data to a flow quantity computation section13through the isolation circuit10. The insulating circuit10has an interface function for executing signal conversion so that a signal can be transferred between circuits different in reference potential (first common line L2and second common line L4).

The DC-DC conversion circuit12is an insulation-type DC voltage conversion circuit of an inverter system. It converts DC voltage of the first power supply line L1into AC voltage by the SW control circuit11and steps up or down the voltage with a transformer and then rectifies the voltage by a diode and a capacitor and converts the voltage into DC voltage of the second power supply line L3. Accordingly, the circuits connected to the first power supply line L1and the first common line L2are electrically insulated from the circuits connected to the second power supply line L3and the second common line L4.

The computation control section21is made up of the above-mentioned flow quantity computation section13, a zero point measurement section14, a storage section15, a zero point measurement control section16, a zero point correction section17, a scaling section18, a PWM signal conversion section19, and the above-mentioned excitation circuit control section20and performs operation control and signal processing of the electromagnetic flowmeter28.

The flow quantity computation section13performs operation such as multiplying the digital data of the difference signal proportional to the flow velocity of the measurement fluid received from the isolation circuit10by the inner diameter in the tube of the detector4and computes the flow quantity value of the measurement fluid. The zero point measurement section14acquires the flow quantity value in a state in which the measurement fluid is made still (which will be hereinafter referred to as “zero point measurement value”) from the flow quantity computation section13and stores the zero point measurement value in the storage section15.

To remove an error caused by the zero point measurement value, the zero point correction section17subtracts the zero point measurement value stored in the storage section15from the flow quantity value computed in the flow quantity computation section13to compute the flow quantity value subjected to zero point correction.

The scaling section18receives the flow quantity value subjected to zero point correction and scales (normalizes) the value relative to a predetermined flow quantity value (for example, in the range of 0 to 1). The PWM signal conversion section19receives the scaled value and outputs a PWM signal (pulse width modulation signal) having a duty to output a current proportional to the value (for example, when the value is 0, 4 milliamperes; when 1, 20 milliamperes) to the current output circuit23.

Exciting of the exciting coil1is performed according to an exciting method of lessening the excitation current value when the output current value from the current output circuit23is small and is performed according to an exciting method of increasing the excitation current value when the output current value is large for providing good S/N ratio of the electric signal detected by the electrodes2and3. Thus, the excitation circuit control section20controls the excitation circuit22so as to change the excitation current value and the exciting method according to the magnitude of the output current value. At this time, the excitation circuit control section20receives the scaled value proportional to the output current value from the scaling section18and performs the control described above.

Since the zero point measurement values provided according to the different excitation current values and the different exciting methods differ, the zero point measurement section14measures the zero point measurement value according to each of the excitation current values and each of the exciting methods. To lessen the output current, the zero point measurement control section16sends data of 0, for example, to the scaling section18. The scaling section18outputs 0, whereby the output current value becomes 4 mA and exciting corresponding to the value is performed. In this state, the zero point measurement section14computes the zero point measurement value and stores the value in the storage section15. Next, to increase the output current, the zero point measurement control section16sends data of 0.5, for example, to the scaling section18. The scaling section18outputs 0.5, whereby the output current value becomes 12 mA and exciting corresponding to the value is performed. In this state, the zero point measurement section14computes the zero point measurement value and stores the value in the storage section15. In usual flow quantity computation, the zero point correction section17acquires the zero point measurement value corresponding to each of the excitation current values and the exciting methods from the storage section15and performs correction computation.

The zero point measurement value may change because of the effect of a deposit in the tube of the detector4, the electric conductivity of the measurement fluid, etc. Thus, to lessen an error of the computed flow quantity value, zero point measurement is conducted on a regular basis or as required.

In a flow control system made up of the electromagnetic flowmeter28, the controller27, and a control valve (not shown), the zero point measurement is conducted in a state in which the control valve is forcibly closed for making a measurement fluid still. At this time, the output current value of the electromagnetic flowmeter28is about 4 milliamperes. Since the output current value increases to 12 milliamperes for several minutes during which the zero point measurement is conducted, output of the electromagnetic flowmeter28and the opening of the control valve do not match. Thus, the controller27may produce an anomaly caused by the mismatch. To prevent occurrence of the anomaly, the controller27is placed in an offline and then zero point measurement is conducted and thus the flow control is temporarily interrupted meanwhile.

On the other hand, when zero point measurement is conducted with the output current value decreased to 4 milliamperes, if the difference between the zero point measurement value and the preceding zero point measurement value is within a predetermined value range, change in the zero point measurement value of the measurement fluid scarcely occurs. In such a case, the output current value need not be increased to 12 milliamperes for conducting the zero point measurement.

SUMMARY

Exemplary embodiments of the present invention provide an electromagnetic flowmeter for performing zero point measurement without changing the output current of the electromagnetic flowmeter if the difference between the present zero point measurement value and the preceding zero point measurement value is within a predetermined value range as for zero point measurement of a measurement fluid in the electromagnetic flowmeter. If the difference is beyond the predetermined value range, the zero point measurement value of the measurement fluid changes and thus the output current is changed and zero point measurement is conducted.

To the end, according to the invention of a first aspect, there is provided an electromagnetic flowmeter for supplying an excitation current to an exciting coil for giving a magnetic field to a measurement fluid, detecting an electric signal occurring in the measurement fluid according to the magnetic field, and computing a flow quantity value based on the electric signal, the electromagnetic flowmeter including:

a zero point measurement section for measuring a zero point of the measurement fluid;

a storage section for storing the measured zero point measurement value;

a determination section for determining whether or not the difference between the preceding zero point measurement value stored in the storage section and the present zero point measurement value stored in the storage section is beyond a predetermined value range; and

at least either a transmission section for transmitting the determination result or a display section for displaying the determination result when the determination section determines that the difference is beyond the predetermined value range.

The invention of a second aspect is as follows:

The electromagnetic flowmeter of the invention of the first aspect further includes an acquisition section for acquiring a request signal for further measuring a zero point of the measurement fluid based on at least either the transmitted or displayed determination result, wherein an output current, the excitation current, and an excitation method of the electromagnetic flowmeter are changed based on the acquired signal and then a zero point is measured by the zero point measurement section and the zero point measurement value is stored in the storage section.

The invention of a third aspect is as follows:

In the electromagnetic flowmeter of the invention of the first aspect, when the determination section determines that the difference is beyond the predetermined value range, further an output current, the excitation current, and an excitation method of the electromagnetic flowmeter are changed and then a zero point is measured by the zero point measurement section and the zero point measurement value is stored in the storage section.

The invention of a fourth aspect is as follows:

The electromagnetic flowmeter of the invention in any of the first aspect to third aspect is a two-wire electromagnetic flowmeter for receiving power supply from a transmission line for transmitting the output current.

According to the invention of a fifth aspect, there is provided a zero point measurement method of an electromagnetic flowmeter for supplying an excitation current to an exciting coil for giving a magnetic field to a measurement fluid, detecting an electric signal occurring in the measurement fluid according to the magnetic field, and computing a flow quantity value based on the electric signal, the zero point measurement method including the steps of:

measuring a zero point of the measurement fluid;

storing the measured zero point measurement value;

determining whether or not the difference between the stored preceding zero point measurement value and the stored present zero point measurement value is beyond a predetermined value range; and

at least either transmitting the determination result or displaying the determination result when it is determined in the determining step that the difference is beyond the predetermined value range.

According to the invention, as for zero point measurement of a measurement fluid in the electromagnetic flowmeter, if the difference between the present zero point measurement value and the preceding zero point measurement value is within the predetermined value range, zero point measurement is performed without changing the output current of the electromagnetic flowmeter. Accordingly, occurrence of an anomaly caused by the controller can be prevented and the zero point measurement can be conducted with the controller placed in an online state.

Other features and advantages may be apparent from the following detailed description, the accompanying drawings and the claims.

DETAILED DESCRIPTION

First Embodiment

A first embodiment of the invention will be discussed withFIG. 1.FIG. 1is a block diagram of an electromagnetic flowmeter incorporating the invention. Components identical with those previously described with reference toFIG. 11are denoted by the same reference numerals inFIG. 1and will not be discussed again. The embodiment is intended for determining whether or not the difference between the present zero point measurement value and the preceding zero point measurement value is within a predetermined value range and transmitting and displaying the determination result if the difference is beyond the range.

An electromagnetic flowmeter32is made up of a detector4, an amplification circuit8, an AD conversion section9, an insulating circuit10, a DC-DC conversion circuit12, a computation control section21, a transmission section30, a display section31, an excitation circuit22, a current output circuit23, etc. It may include at least either of the transmission section30and the display section31.

One of a pair of output terminals of the electromagnetic flowmeter32is connected to a positive terminal (+) of a DC power supply26and the other output terminal is connected to a negative terminal (−) of the DC power supply26through a resistor25. The DC power supply26supplies a current (for example, ranging from 4 to 20 milliamperes) corresponding to the computed flow quantity value and DC voltage to the electromagnetic flowmeter32. A controller27connected to both ends of the resistor25measures a current output from the electromagnetic flowmeter32according to a voltage occurring across the resistor25, converts the measurement value into a flow quantity value, and performs process control (for example, flow control). A flow control system is made up of the electromagnetic flowmeter32, the controller27, and a control valve (not shown), and the controller27acquires output (flow quantity value) of the electromagnetic flowmeter32and controls the opening of the control valve so as to provide a target flow quantity value.

The output terminal connected to the positive terminal (+) of the DC power supply26is connected to a first power supply line L1. Power supply terminals of the computation control section21, the excitation circuit22, the current output circuit23, the input side (SW control circuit11) of the DC-DC conversion circuit12, the transmission section30, and the display section31are connected to the first power supply line L1.

A connection part of the current output circuit23and an output current detection resistor24is connected to a first common line L2. Common (reference potential) terminals of the computation control section21, the excitation circuit22, the current output circuit23, the input side (SW control circuit11) of the DC-DC conversion circuit12, the transmission section30, and the display section31are connected to the first common line L2. The computation control section21, the excitation circuit22, the current output circuit23, the DC-DC conversion circuit12, the transmission section30, and the display section31receive power supply from the first power supply line L1.

A power supply terminal of the output side of the DC-DC conversion circuit12is connected to a second power supply line L3. Power supply terminals of the amplification circuit8, the AD conversion section9, and the insulating circuit10are connected to the second power supply line L3.

A common (reference potential) terminal of the output side of the DC-DC conversion circuit12is connected to a second common line L4. Common (reference potential) terminals of the amplification circuit8, the AD conversion section9, and the insulating circuit10are connected to the second common line L4. The amplification circuit8, the AD conversion section9, and the insulating circuit10receive power supply from the output of the DC-DC conversion circuit12(the second power supply line L3).

The excitation circuit22is connected to an excitation circuit control section20and an exciting coil1. The excitation circuit22causes an excitation current to flow into the exciting coil1based on a control signal of the excitation circuit control section20. The exciting coil1generates a magnetic field in a tube of the detector4and gives the magnetic field to a measurement fluid in the tube, whereby an electric signal (electromotive force) proportional to the magnetic flux density of the magnetic field and the flow velocity of the measurement fluid occurs in the measurement fluid flowing through the tube. The detector4is made up of the exciting coil1, electrodes2and3, the tube for allowing a measurement fluid to flow (not shown), etc., and the electric signal is detected by the electrodes2and3placed in the tube.

The amplification circuit8is made up of buffers (voltage followers)5and6and a differential amplifier7. Inputs of the buffers5and6are connected to the electrodes2and3and outputs of the buffers5and6are connected to input of the differential amplifier7. The buffers5and6execute impedance conversion of the electric signals detected by the electrodes2and3, and the differential amplifier7generates a difference signal between the outputs of the buffers5and6and outputs the difference signal to the AD conversion section9. The difference signal is proportional to the flow velocity of the measurement fluid.

The AD conversion section9converts the analog difference signal into digital data and outputs the digital data to a flow quantity computation section13through the isolation circuit10. The insulating circuit10has an interface function for executing signal conversion so that a signal can be transferred between circuits different in reference potential (first common line L2and second common line L4). The insulating circuit10uses an optical transmission device of a photocoupler, etc., for example, and a transformer.

The DC-DC conversion circuit12is an insulation-type DC voltage conversion circuit of an inverter system. The input side of the DC-DC conversion circuit12is made up of the SW (switching) control circuit and one winding of a transformer connected to the SW control circuit. The output side is made up of the other winding of the transformer, a diode connected to the winding, and a capacitor connected to the diode. The DC-DC conversion circuit12converts DC voltage of the first power supply line L1into AC voltage by the SW control circuit11and steps up or down the voltage with the transformer and then rectifies the voltage by the diode and the capacitor and converts the voltage into DC voltage of the second power supply line L3. Accordingly, the circuits connected to the first power supply line L1and the first common line L2are electrically insulated from the circuits connected to the second power supply line L3and the second common line L4.

The computation control section21is made up of the above-mentioned flow quantity computation section13, a zero point measurement section14, a storage section15, a zero point measurement control section16, a zero point correction section17, a determination section29, a scaling section18, a PWM signal conversion section19, and the above-mentioned excitation circuit control section20and performs operation control and signal processing of the electromagnetic flowmeter32.

The flow quantity computation section13performs operation such as multiplying the digital data of the difference signal proportional to the flow velocity of the measurement fluid input from the isolation circuit10by the inner diameter in the tube of the detector4and computes the flow quantity value of the measurement fluid. The zero point measurement section14acquires the flow quantity value in a state in which the measurement fluid is made still (which will be hereinafter referred to as “zero point measurement value”) from the flow quantity computation section13and stores the zero point measurement value in the storage section15.

To remove an error caused by the zero point measurement value, the zero point correction section17subtracts the zero point measurement value stored in the storage section15from the flow quantity value computed in the flow quantity computation section to compute the flow quantity value subjected to zero point correction. The flow velocity in the state in which the measurement fluid is made still may be stored as the zero point measurement value and the zero point measurement value (flow velocity) may be subtracted from the flow quantity value computed in the flow quantity computation section13and then the flow quantity value may be computed.

The scaling section18receives the flow quantity value subjected to zero point correction and scales (normalizes) the value relative to a predetermined flow quantity value (for example, in the range of 0 to 1, a predetermined flow quantity value is set to 1). The PWM signal conversion section19receives the scaled value and outputs a PWM signal (pulse width modulation signal) having a duty to output a current proportional to the value (for example, when the value is 0, 4 milliamperes; when 1, 20 milliamperes) to the current output circuit23.

Exciting of the exciting coil1is performed according to an exciting method of lessening the excitation current value when the output current value from the current output circuit23is small and is performed according to an exciting method of increasing the excitation current value when the output current value is large. The excitation current value is increased, whereby the electric signals detected by the electrodes2and3become large, so that the S/N ratio can be improved.

Thus, the excitation circuit control section20controls the excitation circuit22so as to change the excitation current value and the exciting method according to the magnitude of the output current value. The excitation circuit control section20receives the scaled value proportional to the output current value from the scaling section18and performs the control described above. In particular, inFIG. 6B, when the output current value is small (for example, less than 12 milliamperes), the excitation current value is lessened (for example, the maximum current is several milliamperes) and three-valued excitation is performed. When the output current value is large (for example, 12 milliamperes or more), the excitation current value is increased (for example, the maximum current is several ten milliamperes) and two-frequency excitation or two-valued excitation is performed.

The operation of the excitation circuit22will be discussed withFIGS. 5 and 6.FIG. 5is a circuit block diagram of the excitation circuit22andFIG. 6represents excitation control patterns. The excitation circuit control section20outputs an excitation PWM signal L10to the excitation circuit22for controlling the excitation current value and outputs timing signals L6to L9for controlling the excitation current flowing direction and time.

The excitation circuit22is made up of an excitation current direction switching circuit46, a constant current control circuit47, and a low-pass filter48. The excitation current direction switching circuit46and the constant current control circuit47are connected in series between the first power supply line L1and the first common line L2. The exciting coil1is connected at both ends to a connection part of transistors Q1and Q3of the excitation current direction switching circuit46and a connection part of transistors Q2and Q4. The transistors Q1to Q4may be FETs (field-effect transistors).

The low-pass filter48is made up of a resistor R2, a capacitor C1, and an operational amplifier A1, and the constant current control circuit47is made up of a resistor R1, a transistor Q5, and an operational amplifier A2.

The excitation PWM signal L10is input from the excitation circuit control section20to the low-pass filter48and is smoothed by the resistor R2and the capacitor C1. The smoothed voltage is buffered by the operational amplifier A1and is input to the constant current control circuit47. The constant current control circuit47operates so that detection voltage as the excitation current flowing into the exciting coil1is detected in the resistor R1and output of the operational amplifier A1match, and causes a constant excitation current to flow into the exciting coil1. Thus, the constant excitation current corresponding to the duty of the excitation PWM signal L10flows from the first power supply line L1via the transistor Q1(or Q2), the exciting coil1, the transistor Q4(or Q3), the transistor Q5, and the resistor R1into the first common line L2. InFIG. 6B, the duty of the excitation PWM signal L10is lessened, thereby allowing a small excitation current to flow; the duty is increased, thereby allowing a large excitation current to flow.

The excitation current direction switching circuit46controls the excitation current flowing direction and time according to the signal patterns shown inFIG. 6A. In signal pattern A, the transistors Q1and Q4are turned on based on timing signals L7and L9output from the excitation circuit control section20and the transistors Q2and Q3are turned off based on timing signals L6and L8output from the excitation circuit control section20, whereby the excitation current flows from the first power supply line L1via the transistor Q1, the exciting coil1, the transistor Q4, the transistor Q5, and the resistor R1into the first common line L2. The direction of the magnetic field generated by the exciting coil1according to the excitation current is the forward direction.

In signal pattern B, the transistors Q1and Q4are turned off based on timing signals L7and L9output from the excitation circuit control section20and the transistors Q2and Q3are turned on based on timing signals L6and L8output from the excitation circuit control section20, whereby the excitation current flows from the first power supply line L1via the transistor Q2, the exciting coil1, the transistor Q3, the transistor Q5, and the resistor R1into the first common line L2. The direction of the magnetic field generated by the exciting coil1according to the excitation current is the backward direction.

In signal pattern C, the transistors Q1to Q4are turned off based on timing signals L6to L9output from the excitation circuit control section20, whereby no excitation current flows and the exciting coil1does not generate a magnetic field. The timing signals L6to L9are input via insulating circuits42to45to the transistors Q1to Q4.

The three-valued excitation is performed as the signal patterns A, C, B, C, A, C are repeated. The two-valued excitation is performed as the signal patterns A, B, A, B are repeated. The two-frequency excitation is performed as two-valued excitation at high repetition frequency (for example, several ten Hz) and two-valued excitation at low repetition frequency (for example, several Hz) are combined.

The operation of the current output circuit23will be discussed withFIG. 7.FIG. 7is a block diagram of the current output circuit23. The current output circuit23is made up of a low-pass filter49, an adder50, an output current control circuit51, and an output current detection resistor24. The first power supply line L1, a resistor R9, a transistor Q6, the output current detection resistor24, and the resistor25are connected in series between the positive terminal (+) and the negative terminal (−) of the DC power supply26. The first common line L2is connected to a connection part of the transistor Q6and the output current detection resistor24.

The low-pass filter49is made up of a resistor R3, a capacitor C2, and an operational amplifier A3, the adder50is made up of resistors R4and R5and an operational amplifier A4, and the output current control circuit51is made up of resistors R6to R9, the transistor Q6, and an operational amplifier A5.

A PWM signal output from the PWM signal conversion section19is input to the low-pass filter49and is smoothed by the resistor R3and the capacitor C2. The smoothed voltage is buffered by the operational amplifier A3and is input to the adder50.

Current flowing into the inside of the electromagnetic flowmeter (circuits, etc.) toward a resistor R25flows into the output current detection resistor24via the first common line L2and current passed through the resistor R9and the transistor Q6further flows into the output current detection resistor24and an output current detection voltage L5occurs. The adder50adds voltage input from the low-pass filter49and the output current detection voltage L5and outputs the resultant voltage to the output current control circuit51. The output current control circuit51adjusts the current flowing into the transistor Q6so as to set the resultant voltage almost to zero (potential of the first common line L2). Since the absolute values of the output voltage of the low-pass filter49and the output current detection voltage L5become equal, a current proportional to the duty of the PWM signal output from the PWM signal conversion section19flows into the output current detection resistor24. Since the duty of the PWM signal is proportional to the flow quantity value subjected to zero point correction, an output current proportional to the flow quantity value subjected to zero point correction flows into the resistor25.

Next, zero point measurement will be discussed also usingFIG. 8.FIG. 8is a flowchart of zero point measurement processing performed by the computation control section21. The zero point measurement values measured in the excitation current values and the excitation methods shown inFIG. 6B differ. Thus, after a measurement fluid is made still, the zero point measurement section measures the zero point measurement value in the excitation current value and the excitation method shown inFIG. 6B. The zero point correction section17subtracts the zero point measurement value corresponding to each excitation current value and excitation method and performs correction computation.

The zero point measurement processing is started as a transmission signal having a zero point measurement execution command from the controller27is received through the resistor25, the current output circuit23, and a communication section (not shown) or as a contact input signal (not shown) to execute zero point measurement from the controller27is received.

The zero point measurement control section16sends data of 0 to the scaling section18, which then outputs the received data of 0, and the output current is set to 4 mA and becomes small. The excitation circuit22lessens the excitation current and executes three-valued excitation based on the timing signals L6to L10from the excitation circuit control section20(step S1). The zero point measurement section14acquires the computed flow quantity value from the flow quantity computation section13based on a control signal of the zero point measurement control section16(step S2). To obtain a stable zero point measurement value, zero point measurement is conducted continuously for several minutes. Thus, the acquired flow quantity values are averaged (step S3) and if the zero point measurement time (several minutes) does not expire (step S4), the process is repeated starting at S1; if the zero point measurement time expires, step S5is executed. The average flow quantity value provided at step S3is stored in the storage section15as zero point measurement value (which will be hereinafter referred to as “present zero point measurement value”) (step S5).

The determination section29determines whether or not the difference between the preceding zero point measurement value measured in the preceding zero point measurement and stored in the storage section15and the present zero point measurement value is beyond a predetermined value range (step S6). If the difference is within the range (for example, in the range of −1% to +1% of the flow quantity value corresponding to output of 100%), change in the zero point measurement value of the measurement fluid scarcely occurs. Thus, zero point measurement in a state in which the output current and the excitation current are increased and the excitation method is changed is not performed and the zero point measurement processing is terminated. On the other hand, if the difference is beyond the range, the zero point measurement value of the measurement fluid changes and thus the determination section29outputs the determination result to the transmission section30and the display section31(step S7). The transmission section30transmits a transmission signal having the determination result to the controller27through the current output circuit23and outputs a contact signal representing the determination result to the controller27. The display section31has a display function of a liquid crystal display section, etc., and displays the determination result.

According to the embodiment, as for the zero point measurement of a fluid in the electromagnetic flowmeter, if the difference between the present zero point measurement value and the preceding zero point measurement value is within the predetermined value range, zero point measurement is conducted without changing the output current of the electromagnetic flowmeter. Accordingly, occurrence of an anomaly caused by the controller can be prevented and the zero point measurement can be conducted with the controller placed in an online state. If the difference is beyond the range, the determination result is transmitted to the controller and is displayed, whereby the user can be informed of the determination result.

Second Embodiment

A second embodiment of the invention will be discussed withFIG. 2.FIG. 2is a block diagram of an electromagnetic flowmeter incorporating the invention. Components identical with those previously described with reference toFIG. 1are denoted by the same reference numerals inFIG. 2and will not be discussed again. The embodiment is intended for changing output current, etc., and conducting zero point measurement based on a zero point measurement request signal when the difference between the present zero point measurement value and the preceding zero point measurement value is beyond a predetermined value range. InFIG. 2, an acquisition section33is added to the electromagnetic flowmeter inFIG. 1and a power supply terminal of the acquisition section33is connected to a first power supply line L1and a common (reference potential) terminal is connected to a first common line L2.

The zero point measurement will be discussed also usingFIG. 9.FIG. 9is a flowchart of processing at the time of acquiring the request signal. In the zero point measurement processing inFIG. 8, when it is determined that the difference between the preceding zero point measurement value and the present zero point measurement value is beyond the predetermined value range (step S6) and the determination result is transmitted by the transmission section30and displayed by the display section31(step S7), the zero point measurement value of the measurement fluid changes and thus zero point measurement is also conducted in a state in which the output current and the excitation current are increased and the excitation method is changed.

Upon reception of the determination result, a controller27further transmits a request signal for conducting zero point measurement, and the acquisition section33receives the signal through a resistor25, a current output circuit23, and a communication section (not shown).

Upon reception of the request signal from the acquisition section33, a zero point measurement control section16sends data of 0.5 to a scaling section18, which then outputs the received data of 0.5, and the output current is set to 12 mA and becomes large. An excitation circuit22increases the excitation current and executes two-frequency excitation or two-valued excitation based on timing signals L6to L10from an excitation circuit control section20(step S8). A zero point measurement section14acquires the computed flow quantity value from a flow quantity computation section13based on a control signal of the zero point measurement control section16(step S9). To obtain a stable zero point measurement value, zero point measurement is conducted continuously for several minutes. Thus, the acquired flow quantity values are averaged (step S10) and if the zero point measurement time (several minutes) does not expire (step S11), the process is repeated starting at S1; if the zero point measurement time expires, the average flow quantity value provided at step S10is stored in a storage section15as zero point measurement value (which will be hereinafter referred to as “present zero point measurement value”) (step S12). The controller27may output the request signal to the acquisition section33as a contact signal. The user may confirm the determination result displayed on the display section31and may send a request signal through the controller27.

InFIG. 10, the zero point measurement control section16increases the output current and the excitation current and changes the excitation method and conducts zero point measurement (steps S19to S24) when it is determined that the difference between the preceding zero point measurement value and the present zero point measurement value is beyond a predetermined value range (steps S13to S18) regardless of the request signal. On the other hand, no processing is performed when it is determined that the difference is within the predetermined value range.FIG. 10is a flowchart of the zero point measurement processing. Steps S13to S19are similar to S1to S7inFIG. 8and steps S20to S24are similar to S8to S12inFIG. 9.

According to the embodiment, if the difference between the present zero point measurement value and the preceding zero point measurement value is beyond the predetermined value range, the output current of the electromagnetic flowmeter is changed and zero point measurement is conducted and an error of output of the electromagnetic flowmeter caused by change in the present zero point measurement value can be removed.

Third Embodiment

A third embodiment of the invention will be discussed withFIG. 3.FIG. 3is a block diagram of an electromagnetic flowmeter incorporating the invention. Components identical with those previously described with reference toFIG. 2are denoted by the same reference numerals inFIG. 3and will not be discussed again. The embodiment is intended for converting the computed flow quantity value into the number of pulses or output.

InFIG. 3, a number-of-pulses conversion section35and a pulse output circuit37are added to the electromagnetic flowmeter inFIG. 2. The pulse output circuit37is made up of a transistor36and a resistor and output of the number-of-pulses conversion section35is input to a base of the transistor36through the resistor. The resistor is connected between the base and an emitter of the transistor36and a first common line L2is connected to the emitter. A collector and the emitter of the transistor36are connected to a controller27through a pair of output terminals.

The number-of-pulses conversion section35receives a value scaled in a scaling section18and multiplies the value by a coefficient for converting into a predetermined number of pulses to compute the number of pulses. The number-of-pulses conversion section35outputs a pulse signal corresponding to the computed number of pulses to the pulse output circuit37. The pulse output circuit37transmits the pulse signal to the controller27by turning on and off the transistor36. A computation control section21may use a microprocessor to perform processing.

According to the embodiment, the pulse signal corresponding to the flow quantity value subjected to zero point correction is output to the controller, whereby the controller can acquire the flow quantity value as a digital value (count) and can perform flow control.

FIG. 4is a block diagram of an electromagnetic flowmeter incorporating the invention. The electromagnetic flowmeter is an electromagnetic flowmeter for receiving power from a commercial power supply (for example, 100 V) having a commercial frequency (for example, 50 Hz or 60 Hz). Components identical with those previously described with reference toFIG. 3are denoted by the same reference numerals inFIG. 4and will not be discussed again.

A commercial power supply40is connected to a power supply circuit39through a pair of terminals. The power supply circuit39converts AC voltage input from the commercial power supply40into DC voltage and supplies the DC voltage to a first power supply line L1. Accordingly, similar zero point measurement processing can also be accomplished in the electromagnetic flowmeter for receiving power from the commercial power supply.

Although output of the electromagnetic flowmeter has been described about the electric signal in the range of 4 to 20 milliamperes, the electromagnetic flowmeter may be an electromagnetic flowmeter for conducting field bus communications (foundation field bus, profi bus, etc.).