Abstrict An electromagnetic flow meter of a switching control system in
which a DC voltage is applied to an exciting coil via switching
elements, a direction of an exciting current flowing through the
exciting coil is switched over at a predetermined excitation timing,
and the switching elements are turned ON or OFF so as to hold the
exciting current to be a predetermined value, has first and second
detection resistors which are connected in series to ends of the
exciting coil via the switching elements, respectively, wherein
a midpoint of the first and second detection resistors is grounded
to set a voltage of the midpoint as a circuit reference voltage.
Claims 1. An electromagnetic flow meter of a switching control system
in which a DC voltage is applied to an exciting coil via switching
elements, a direction of an exciting current flowing through the
exciting coil is switched over by a predetermined fundamental excitation
frequency, and switching of the switching elements are controlled
by an excitation switching control frequency which is higher than
the fundamental excitation frequency so as to hold the exciting
current constant, the electromagnetic flow meter comprises: first
and second detection resistors which are connected in series to
ends of the exciting coil via switching elements, respectively,
wherein a midpoint of the first and second detection resistors is
grounded to set a voltage of the midpoint as a circuit reference
voltage.
2. The electromagnetic flow meter according to claim 1 wherein
a power source for an exciting circuit which drives ON/OFF operations
of the switching elements is used also as a power source for a controlling
circuit which produces a timing signal for controlling the ON/OFF
operations of the switching elements.
3. The electromagnetic flow meter according to claim 1 wherein
the exciting current is controlled with reference to a positive
signal based on one of voltages of the first and second detection
resistors.
4. The electromagnetic flow meter according to claim 1 wherein
the exciting current is controlled with reference to a voltage based
on one of currents of the first and second detection resistors that
are equal to the exciting current.
5. An electromagnetic flow meter of a switching control system
in which a DC voltage is applied to an exciting coil via switching
elements, a direction of an exciting current flowing through the
exciting coil is switched over at a predetermined excitation timing,
and the switching elements are turned ON or OFF so as to hold the
exciting current to be a predetermined value, the electromagnetic
flow meter comprises: first and second detection resistors which
are connected in series to ends of the exciting coil via the switching
elements, respectively, wherein a midpoint of the first and second
detection resistors is grounded to set a voltage of the midpoint
as a circuit reference voltage.
6. The electromagnetic flow meter according to claim 5 wherein
a power source for an exciting circuit which drives ON/OFF operations
of the switching elements is used also as a power source for a controlling
circuit which produces a timing signal for controlling the ON/OFF
operations of the switching elements.
7. The electromagnetic flow meter according to claim 5 wherein
the exciting current is controlled with reference to a positive
signal based on one of voltages of the first and second detection
resistors.
8. The electromagnetic flow meter according to claim 5 wherein
the exciting current is controlled with reference to a voltage based
on one of currents of the first and second detection resistors that
are equal to the exciting current.
9. An electromagnetic flow meter comprising: a first series circuit
having a power source, a first switching element, an exciting coil,
and a fourth switching element; a second series circuit having the
power source, a second switching element, the exciting coil, and
a third switching element; and a third series circuit having the
exciting coil, the fourth switching element, and the third switching
element, the electromagnetic flow meter of a switching control system
in which a direction of an exciting current flowing through the
exciting coil is switched over by a predetermined fundamental excitation
frequency, and the first switching element and the second switching
element are controlled so as to hold the exciting current to be
a predetermined value, the electromagnetic flow meter comprises:
a first detection resistor which is formed between the third switching
element and a circuit reference voltage, and detects the exciting
current; and a second detection resistor which is formed between
the fourth switching element and the circuit reference voltage,
and detects the exciting current.
10. An electromagnetic flow meter comprising: a first switching
element in which a power source is connected to one end; a second
switching element in which the power source is connected to one
end; a third switching element in which another end of the first
switching element is connected to one end, and a circuit reference
voltage is connected to another end; a fourth switching element
in which another end of the second switching element is connected
to one end, and the circuit reference voltage is connected to another
end; and an exciting coil in which a junction of the other end of
the first switching element and the one end of the third switching
element is connected to one end, and a junction of the other end
of the second switching element and the one end of the fourth switching
element is connected to another end, the electromagnetic flow meter
of a switching control system in which a direction of an exciting
current flowing through the exciting coil is switched over by a
fundamental excitation frequency, and the first switching element
and the second switching element are controlled so as to hold the
exciting current constant, a first detection resistor which is formed
between the other end of the third switching element and the circuit
reference voltage, and detects the exciting current; and a second
detection resistor which is formed between the other end of the
fourth switching element and the circuit reference voltage, and
detects the exciting current.
11. The electromagnetic flow meter according to claim 9 further
comprising: a first Schottky diode which is connected in parallel
to the third switching element; and a second Schottky diode which
is connected in parallel to the fourth switching element.
12. The electromagnetic flow meter according to claim 10 further
comprising: a first Schottky diode which is connected in parallel
to the third switching element; and a second Schottky diode which
is connected in parallel to the fourth switching element.
13. The electromagnetic flow meter according to claim 11 further
comprising: a third Schottky diode which is connected in parallel
to the first switching element; and a fourth Schottky diode which
is connected in parallel to the second switching element.
14. The electromagnetic flow meter according to claim 12 further
comprising: a third Schottky diode which is connected in parallel
to the first switching element; and a fourth Schottky diode which
is connected in parallel to the second switching element.
15. The electromagnetic flow meter according to claim 9 wherein
the first switching element is ON/OFF-controlled based on a reference
voltage generated in the second detection resistor, and the second
switching element is ON/OFF-controlled based on a reference voltage
generated in the first detection resistor.
16. The electromagnetic flow meter according to claim 10 wherein
the first switching element is ON/OFF-controlled based on a reference
voltage generated in the second detection resistor, and the second
switching element is ON/OFF-controlled based on a reference voltage
generated in the first detection resistor.
17. The electromagnetic flow meter according to claim 15 further
comprising: a switch which selects the reference voltage of the
second detection resistor when the second switching element is OFF
and the fourth switching element is ON, and selects the reference
voltage of the first detection resistor when the first switching
element is OFF and the third switching element is ON.
18. The electromagnetic flow meter according to claim 16 further
comprising: a switch which selects the reference voltage of the
second detection resistor when the second switching element is OFF
and the fourth switching element is ON, and selects the reference
voltage of the first detection resistor when the first switching
element is OFF and the third switching element is ON.
19. The electromagnetic flow meter according to claim 9 wherein
the second and third switching elements are turned ON after the
first and fourth switching elements are turned OFF, and the first
and fourth switching elements are turned ON after the second and
third switching elements are turned OFF.
20. The electromagnetic flow meter according to claim 10 wherein
the second and third switching elements are turned ON after the
first and fourth switching elements are turned OFF, and the first
and fourth switching elements are turned ON after the second and
third switching elements are turned OFF.
21. The electromagnetic flow meter according to claim 9 further
comprising: a normalizing section which normalizes a flow rate signal
which is detected from an electrode of a pipe in accordance to the
exciting current, based on a ratio of the flow rate signal to an
exciting current signal which is detected from a third detection
resistor connected in series to the exciting coil.
22. The electromagnetic flow meter according to claim 10 further
comprising: a normalizing section which normalizes a flow rate signal
which is detected from an electrode of a pipe in accordance to the
exciting current, based on a ratio of the flow rate signal to an
exciting current signal which is detected from a third detection
resistor connected in series to the exciting coil.
Description CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Applications No. 2004-235009
filed on Aug. 12 2004 No. 2004-336887 filed on Nov. 22 2004
and No. 2005-159101 filed on May 31 2005 the entire contents of
which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an electromagnetic flow
meter, and more particularly to an electromagnetic flow meter in
which the power consumption of an exciting circuit can be reduced.
[0004] 2. Description of the Related Art
[0005] As shown in FIG. 8 an exciting circuit of an electromagnetic
flow meter of the related art is configured by a DC power source
E, a capacitor C which is connected in parallel to the DC power
source E, and an exciting coil COIL. In the figure, Q1 to Q4 denote
switching elements each configured by an FET, and D1 to D4 denote
parasitic diodes which are connected in parallel to the switching
elements Q1 to Q4 and in a direction opposite to the current flow
from the DC power source E. The parasitic diodes D1 to D4 are formed
in a package during a process of producing the respective FETs,
and hence cannot be removed away.
[0006] The switching elements Q1 Q2 are FETs which operate while
being supplied with a second reference voltage and grounded to a
voltage V2 and the switching elements Q3 Q4 are FETs which operate
while being supplied with a voltage V1 grounded to a third reference
voltage. The voltage V1 is produced at the midpoint of a series
connection of a resistor R1 and a Zener diode ZD1 which are connected
in parallel to the DC power source E, and the voltage V2 is produced
at the midpoint of a series connection of a Zener diode ZD2 and
a resistor R2 which are connected in parallel to the DC power source
E.
[0007] One end (positive side) of the DC power source E is connected
to one end 13 of the exciting coil COIL via the switching element
Q1. The one end 13 is connected to the other end (negative side)
of the DC power source E via the switching element Q3.
[0008] The other end 14 of the exciting coil COIL is connected
to a first reference voltage, and also to one end of a detection
resistor R11 for an exciting current. Furthermore, the one end (positive
side) of the DC power source E is connected to the other end 15
of the detection resistor R11 via the switching element Q2. The
other end 15 is connected to the other end (negative side) of the
DC power source E via the switching element Q4.
[0009] The reference numerals T1 and T2 denote timing signals for
controlling ON/OFF operations of the switching elements Q1 and Q2
which operate while being supplied with the second reference voltage
and grounded to the voltage V2. The timing signals are supplied
to the control electrodes of the switching elements Q1 and Q2 (the
gates of the FETs) via resistors R3 and R5 isolators P1 and P2
such as photocouplers, and waveform-shaping circuits B1 and B2
respectively. The isolators P1 and P2 are insulating circuits for
converting the references of the timing signals T1 T2 which have
different reference voltages, to which the second reference voltage
is supplied via resistors R4 and R6 and which are grounded to the
voltage v2.
[0010] The reference numerals T3 and T4 denote timing signals for
controlling ON/OFF operations of the switching elements Q3 and Q4
which operate while being supplied with the voltage V1 and grounded
to the third reference voltage. The timing signals are supplied
to the control electrodes of the switching elements Q3 and Q4 (the
gates of the FETs) via resistors R7 and R9 isolators P3 and P4
such as photocouplers, and waveform-shaping circuits B3 and B4
respectively. The isolators P3 and P4 are insulating circuits for
converting the references of the timing signals T3 T4 which have
different reference voltages, to which the voltage V1 is supplied
via resistors R8 and R10 and which are grounded to the third reference
voltage.
[0011] The exciting current flows alternately in opposite directions
through the exciting current detection resistor R11 connected in
positive and negative exciting periods. Therefore, a reference voltage
VREF which is positive or negative in accordance with the positive
and negative exciting periods, and which is proportional to the
exciting current is generated between the one end 14 grounded to
the first reference voltage, and the other end 15.
[0012] The configuration and operation of a controlling circuit
which generates the timing signals T1 to T4 for ON/OFF-controlling
the switching elements Q1 to Q4 will be described with reference
to FIG. 9.
[0013] The reference numeral 11 denotes an excitation timing generating
circuit which regulates the positive and negative exciting periods,
and generates a rectangular wave of a predetermined excitation period.
A direct output is supplied as the timing signal T4 to the switching
element Q4 and an inverted output through an inverter G3 is supplied
as the timing signal T3 to the switching element Q3.
[0014] The reference numeral 12 denotes an excitation controlling
circuit. In the circuit, the reference voltage VREF which is proportional
to the exciting current, and which is positive or negative is supplied
to a negative input terminal of a hysteresis comparator CMP. An
output of the hysteresis comparator CMP is supplied to AND gates
G1 G2 and in addition fed back to a positive input terminal of
the hysteresis comparator CMP via a voltage dividing circuit of
positive feedback resistors R11 R12. The reference numeral Vs denotes
a reference DC power source (reference voltage Vs) which is connected
between the resistor R11 and the ground serving as the first reference
voltage.
[0015] The hysteresis comparator CMP operates in the following
manner. When the absolute value of the reference voltage VREF is
increased to be larger than the reference voltage Vs, and further
increased to be larger than the voltage which is determined by the
positive feedback resistors R11 R12 and which corresponds to the
hysteresis width, the output is inverted from negative to positive.
By contrast, when the absolute value of the reference voltage VREF
is decreased to be smaller than the reference voltage Vs, and further
decreased to be smaller than the voltage which is determined by
the positive feedback resistors R11 R12 and which corresponds
to the hysteresis width, the output is inverted from positive to
negative. This inverting operations are repeated. The period of
the inverting operations depends on the time constant of the control
loop including the inductance of the exciting coil COIL, and is
designed so as to be sufficiently shorter than the periods of the
excitation timing signals.
[0016] The AND gate G1 receives the output signals of the hysteresis
comparator CMP and the excitation timing generating circuit 11
and sends out the timing signal T1 in accordance with the logical
product of the output signals. Similarly, the AND gate G2 receives
the output signal of the hysteresis comparator CMP and the inverted
output of the excitation timing generating circuit 11 and sends
out the timing signal T2 in accordance with the logical product
of the output signals.
[0017] FIG. 10 illustrates the ON/OFF situations of the switching
elements Q1 to Q4 and a switching control mode in the positive and
negative exciting periods in the above described configuration.
First, by the excitation timing signals T3 T4 in the positive
exciting period, the switching element Q3 is regulated to be turned
OFF and the switching element Q4 is regulated to be turned ON, and,
in the negative exciting period, the switching element Q3 is regulated
to be turned ON and the switching element Q4 is regulated to be
turned OFF.
[0018] In the positive exciting period, the switching element Q2
is turned OFF, and the switching control is conducted by the switching
element Q1 and, in the negative exciting period, the switching
element Q1 is turned OFF, and the switching control is conducted
by the switching element Q2. As a result of the control on the switching
elements Q1 Q2 Q3 Q4 in the positive exciting period, the current
indicated by i1 in FIG. 10 flows through the switching element Q1
the exciting coil COIL, the reference resistor R11 and the switching
element Q4.
[0019] The current indicated by i2 in FIG. 10 is a current which
is caused to flow through the parasitic diode D3 connected in parallel
to the switching element Q3 by an counter electromotive force of
the exciting coil COIL when the switching element Q1 is OFF.
[0020] In the negative exciting period, a current which is similar
to the current i1 flows through the switching element Q2 the detection
resistor R11 the exciting coil COIL, and the switching element
Q3 so that a constant current control is conducted.
[0021] JP-A-2002-188945 (pages 2 and 3 FIG. 3) is referred to
as a related art.
[0022] In the exciting current of the switching system of the related
art, the circuit current flows different paths depending on the
ON/OFF states of the switching elements. When the exciting current
flowing through the exciting coil is to be controlled, therefore,
a current detection resistor for the control must be connected in
series to the exciting coil.
[0023] Therefore, since the exciting circuit which has the switching
elements, and the controlling circuit which produces the timing
signals use different signal references, it is required to have
different respective power sources. When the timing signals produced
in the controlling circuit are to be used in the exciting circuit,
consequently, insulating circuits such as isolators must be disposed.
As a result, reduction of the power consumption is limited.
SUMMARY OF THE INVENTION
[0024] An object of the invention is to provide an electromagnetic
flow meter in a small size, with a low-loss, and with a low-noise.
[0025] The invention provides an electromagnetic flow meter of
a switching control system in which a DC voltage is applied to an
exciting coil via switching elements, a direction of an exciting
current flowing through the exciting coil is switched over by a
predetermined fundamental excitation frequency, and switching of
the switching elements are controlled by an excitation switching
control frequency which is higher than the fundamental excitation
frequency so as to hold the exciting current constant, having first
and second detection resistors which are connected in series to
ends of the exciting coil via switching elements, respectively,
wherein a midpoint of the first and second detection resistors is
grounded to set a voltage of the midpoint as a circuit reference
voltage.
[0026] In the electromagnetic flow meter, a power source for an
exciting circuit which drives ON/OFF operations of the switching
elements is used also as a power source for a controlling circuit
which produces a timing signal for controlling the ON/OFF operations
of the switching elements.
[0027] In the electromagnetic flow meter, the exciting current
is controlled with reference to a positive signal based on one of
voltages of the first and second detection resistors.
[0028] In the electromagnetic flow meter, the exciting current
is controlled with reference to a voltage based on one of currents
of the first and second detection resistors that are equal to the
exciting current.
[0029] The invention also provides an electromagnetic flow meter
of a switching control system in which a DC voltage is applied to
an exciting coil via switching elements, a direction of an exciting
current flowing through the exciting coil is switched over at a
predetermined excitation timing, and the switching elements are
turned ON or OFF so as to hold the exciting current to be a predetermined
value, having first and second detection resistors which are connected
in series to ends of the exciting coil via the switching elements,
respectively, wherein a midpoint of the first and second detection
resistors is grounded to set a voltage of the midpoint as a circuit
reference voltage.
[0030] In the electromagnetic flow meter, a power source for an
exciting circuit which drives ON/OFF operations of the switching
elements is used also as a power source for a controlling circuit
which produces a timing signal for controlling the ON/OFF operations
of the switching elements.
[0031] In the electromagnetic flow meter, the exciting current
is controlled with reference to a signal based on one of voltages
of the first and second detection resistors.
[0032] In the electromagnetic flow meter, the exciting current
is controlled with reference to a voltage based on one of currents
of the first and second detection resistors that are equal to the
exciting current.
[0033] The invention also provides an electromagnetic flow meter
having: a first series circuit having a power source, a first switching
element, an exciting coil, and a fourth switching element; a second
series circuit having the power source, a second switching element,
the exciting coil, and a third switching element; and a third series
circuit having the exciting coil, the fourth switching element,
and the third switching element, the electromagnetic flow meter
of a switching control system in which a direction of an exciting
current flowing through the exciting coil is switched over by a
predetermined fundamental excitation frequency, and the first switching
element and the second switching element are controlled so as to
hold the exciting current to be a predetermined value, further having:
a first detection resistor which is formed between the third switching
element and a circuit reference voltage, and detects the exciting
current; and a second detection resistor which is formed between
the fourth switching element and the circuit reference voltage,
and detects the exciting current.
[0034] The invention also provides an electromagnetic flow meter
having: a first switching element in which a power source is connected
to one end; a second in which the power source is connected to one
end; a third switching element in which another end of the first
switching element is connected to one end, and a circuit reference
voltage is connected to another end; a fourth switching element
in which another end of the second switching element is connected
to one end, and the circuit reference voltage is connected to another
end; and an exciting coil in which a junction of the other end of
the first switching element and the one end of the third switching
element is connected to one end, and a junction of the other end
of the second switching element and the one end of the fourth switching
element is connected to another end, the electromagnetic flow meter
of a switching control system in which a direction of an exciting
current flowing through the exciting coil is switched over by a
fundamental excitation frequency, and the first switching element
and the second switching element are controlled so as to hold the
exciting current constant, further having: a first detection resistor
which is formed between the other end of the third switching element
and the circuit reference voltage, and detects the exciting current;
and a second detection resistor which is formed between the other
end of the fourth switching element and the circuit reference voltage,
and detects the exciting current.
[0035] The electromagnetic flow meter further has: a first Schottky
diode which is connected in parallel to the third switching element;
and a second Schottky diode which is connected in parallel to the
fourth switching element.
[0036] The electromagnetic flow meter further has: a third Schottky
diode which is connected in parallel to the first switching element;
and a fourth Schottky diode which is connected in parallel to the
second switching element.
[0037] In the electromagnetic flow meter, the first switching element
is ON/OFF-controlled based on a reference voltage generated in the
second detection resistor, and the second switching element is ON/OFF-controlled
based on a reference voltage generated in the second detection resistor.
[0038] The electromagnetic flow meter further has a switch which
selects the reference voltage of the second detection resistor when
the second switching element is OFF and the fourth switching element
is ON, and selects the reference voltage of the first detection
resistor when the first switching element is OFF and the third switching
element is ON.
[0039] In the electromagnetic flow meter, the second and third
switching elements are turned ON after the first and fourth switching
elements are turned OFF, and the first and fourth switching elements
are turned ON after the second and third switching elements are
turned OFF.
[0040] The electromagnetic flow meter further has a normalizing
section which normalizes a flow rate signal which is detected from
an electrode of a pipe in accordance to the exciting current, based
on a ratio of the flow rate signal to an exciting current signal
which is detected from a third detection resistor connected in series
to the exciting coil.
[0041] An electromagnetic flow meter of the invention may be configured
in the following manners.
[0042] (1) The electromagnetic flow meter may be an electromagnetic
flow meter of a switching control system in which a DC voltage is
applied to an exciting coil via switching elements, the direction
of an exciting current flowing through the exciting coil is switched
over by a predetermined fundamental excitation frequency (f1), and
switching of the switching elements are controlled by an excitation
switching control frequency (f2) which is higher than the fundamental
excitation frequency so as to hold the exciting current constant,
having first and second detection resistors which are connected
in series to ends of the exciting coil via switching elements which
are directly connected to the ends of the exciting coil, respectively,
and the midpoint of the series-connected first and second detection
resistors is grounded to set the voltage of the midpoint as a circuit
reference voltage.
[0043] (2) In the electromagnetic flow meter, a power source for
an exciting circuit which drives ON/OFF operations of the switching
elements is used also as a power source for a controlling circuit
which produces a timing signal for controlling the ON/OFF operations
of the switching elements.
[0044] (3) In the electromagnetic flow meter, the exciting current
flowing through the exciting coil is controlled with reference to
a signal by which the voltage of one of voltages of the first and
second detection resistors is positive.
[0045] (4) In the electromagnetic flow meter, the exciting current
flowing through the exciting coil is controlled with reference to
a voltage of one of the firsthand second detection resistors at
which a current identical with the exciting current flowing through
the exciting coil is obtained.
[0046] (5) The electromagnetic flow meter may be an electromagnetic
flow meter of a switching control system in which a DC voltage is
applied to an exciting coil via switching elements, the direction
of an exciting current flowing through the exciting coil is switched
over at a predetermined excitation timing, and ON/OFF operations
of the switching elements are controlled so as to hold the exciting
current constant, having first and second detection resistors which
are connected in series to ends of the exciting coil via the switching
elements, respectively, wherein a midpoint of the first and second
detection resistors is grounded to set the voltage of the midpoint
as a circuit reference voltage.
[0047] (6) In the electromagnetic flow meter, a power source for
an exciting circuit which drives ON/OFF operations of the switching
elements is used also as a power source for a controlling circuit
which produces a timing signal for controlling the ON/OFF operations
of the switching elements.
[0048] (7) In the electromagnetic flow meter, the exciting current
flowing through the exciting coil is controlled with reference to
a signal by which the voltage of one of voltages of the first and
second detection resistors is positive.
[0049] (8) In the electromagnetic flow meter, the exciting current
flowing through the exciting coil is controlled with reference to
a voltage of one of the first and second detection resistors at
which a current identical with the exciting current flowing through
the exciting coil is obtained.
[0050] In the exciting circuit of the switching system for the
electromagnetic flow meter of the invention, two detection resistors
are used, the two detection resistors are not directly connected
to the exciting coil, switching elements are interposed between
the exciting coil and the detection resistors, the two detection
resistors are connected in series to each other, and the midpoint
of the series connection is set to the circuit reference voltage
GND, whereby the circuit references of the exciting circuit and
the controlling circuit are made identical with each other. Therefore,
it is possible to realize a configuration which is not required
to have an insulating circuit that is necessary in the related art.
As a result, an exciting circuit of a low power consumption can
be realized.
[0051] As apparent from the above description, it is possible to
provide the electromagnetic flow meter in a small size, with a low-loss,
and with a low-noise.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] FIG. 1 is an explanation diagram schematically showing the
exciting circuit and the controlling circuit of the electromagnetic
flow meter of the first embodiment of the invention;
[0053] FIG. 2 is a table showing a controlling configuration due
to "on/off" of the switching element;
[0054] FIG. 3 is a circuit diagram showing a flow operation of
the exciting current flowing to the exciting coil;
[0055] FIG. 4 is an explanation diagram schematically showing the
exciting circuit and the controlling circuit of the electromagnetic
flow meter of the second embodiment of the invention;
[0056] FIG. 5 is a block diagram showing the third embodiment of
the invention;
[0057] FIG. 6 is a block diagram showing the fourth embodiment
of the invention;
[0058] FIG. 7 is a block diagram showing the fourth embodiment
of the invention;
[0059] FIG. 8 is an explanation diagram schematically showing the
exciting circuit in the related art;
[0060] FIG. 9 is an explanation diagram schematically showing the
controlling circuit in the related art; and
[0061] FIG. 10 is a circuit diagram showing the flow operation
of the exciting current in the related art.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0062] Embodiments of the electromagnetic flow meter of the invention
will be described with reference to the accompanying drawings. The
components which are identical with those of the related art are
denoted by the same references.
First Embodiment
[0063] An electromagnetic flow meter of a first embodiment of the
invention is an embodiment of a circuit of a fixed switching frequency
system. An exciting circuit formed by switching elements, and a
controlling circuit which generates ON/OFF timing signals for switching
elements are configured as shown in FIG. 1. The exciting circuit
has a DC power source E, a capacitor C which is connected in parallel
to the DC power source E, switching elements Q1 Q3 and a first
detection resistor R25 which are connected in series to one another
and in parallel to the DC power source E, and switching elements
Q2 Q4 and a second detection resistor R26 which are connected in
series to one another and in parallel to the DC power source E.
One end 13 of an exciting coil COIL is connected to the midpoint
of the series connection of the switching elements Q1 and Q3 and
the other end 15 is connected to the midpoint of the series connection
of the switching elements Q2 and Q4.
[0064] The source of the switching element Q3 is connected to one
end of the first detection resistor R25 that of the switching element
Q4 is connected to one end 20 of the second detection resistor R25
the other end 21 of the first detection resistor R25 is connected
to the other end 22 of the second detection resistor R26 and a
junction of the resistors is connected to the minus side of the
DC power source E, and grounded to a reference voltage GND.
[0065] The switching elements Q1 Q2 Q3 Q4 are FETs which operate
while being supplied with a power source voltage VEX and grounded
to the reference voltage GND, and comprise parasitic diodes D1
D2 D3 D4 which are connected in parallel to the switching elements
and in a direction opposite to the current flow from the DC power
source E, respectively.
[0066] A waveform shaping circuit B1 which receives a timing signal
T1 to apply a waveform shaping process on the signal is connected
to the gate of the switching element Q1 a waveform shaping circuit
B2 which receives a timing signal T2 to apply a waveform shaping
process on the signal is connected to the gate of the switching
element Q2 a waveform shaping circuit B3 which receives a timing
signal T3 to apply a waveform shaping process on the signal is connected
to the gate of the switching element Q3 and a waveform shaping
circuit B4 which receives a timing signal T4 to apply a waveform
shaping process on the signal is connected to the gate of the switching
element Q4.
[0067] The waveform shaping circuits B1 B2 B3 B4 are supplied
with the power source voltage VEX, and grounded to the reference
voltage GND.
[0068] In the exciting coil COIL and the first and second detection
resistors R25 R26 in the positive exciting period, the exciting
current flows through the switching element Q1-->the exciting
coil COIL-->the switching element Q4-->the second detection
resistor R26 and, in the negative exciting period, the exciting
current flows through the switching element Q2-->the exciting
coil COIL-->the switching element Q3-->the first detection
resistor R25. Namely, the exciting current flows alternately in
opposite directions. Therefore, a positive reference voltage VREF2
which is proportional to the exciting current is generated at the
one end 26 of the second detection resistor R26 in accordance with
the positive exciting period, and a negative reference voltage VREF1
which is proportional to the exciting current is generated at the
one end 20 of the first detection resistor R25 in accordance with
the negative exciting period.
[0069] The controlling circuit has an excitation timing generating
circuit 11 which regulates the positive and negative exciting periods,
a switch SW which changes over the reference voltage VREF1 from
the first detection resistor R25 and the reference voltage VREF2
from the second detection resistor R26 a pulse-width modulation
(PWM) excitation controlling circuit 25 AND gates G1 G2 and an
inverter G3. The AND gates G1 G2 and the inverter G3 are supplied
with the power source voltage VEX, and grounded to the reference
voltage GND.
[0070] The reference numeral 27 denotes an oscillator which generates
a triangular signal Vp. The triangular signal Vp of a fundamental
excitation control frequency f2 which is higher than a fundamental
excitation frequency f1 is supplied to the negative input terminal
of a comparator CMP1.
[0071] The excitation timing generating circuit 11 generates a
rectangular signal of the predetermined fundamental excitation frequency
f1. A direct output is supplied as the timing signal T4 to the switching
element Q4 of the exciting circuit, and an inverted output through
the inverter G3 is supplied as the timing signal T3 to the switching
element Q3.
[0072] In the pulse-width modulation (PWM) excitation controlling
circuit 25 the difference between the positive or negative voltages
which are proportional to the exciting current, i.e., the reference
voltage VREF1 or VREF2 from the first and second detection resistors
R25 R26 and a CD reference Vs is amplified by an error amplifier
26. An output voltage Ve of the error amplifier 26 is supplied via
a resistor R20 to a positive input terminal of the comparator CMP1
having hysteresis characteristics due to positive feedback resistors
R20 R21.
[0073] An output of the comparator CMP1 is supplied to the AND
gates G1 G2 and in addition fed back to the positive input terminal
of the comparator CMP1 via the dividing circuit of positive feedback
resistors R20 R21 to provide the comparison operation with predetermined
hysteresis.
[0074] The comparator CMP1 operates in the following manner. When
the triangular signal Vp supplied from the oscillator 17 is raised
to be higher than the output voltage Ve of the error amplifier 26
and further raised to be higher than the voltage which is determined
by the positive feedback resistors R20 R21 and which corresponds
to the hysteresis width, the output is inverted from negative to
positive. By contrast, when the triangular signal Vp is lowered
to be lower than the output voltage Ve of the error amplifier 26
and further lowered to be lower than the voltage which is determined
by the positive feedback resistors R20 R21 and which corresponds
to the hysteresis width, the output is inverted from positive to
negative. This inverting operations are repeated in each period
of the triangular signal Vp. As are result of this comparison operation,
the pulse-width modulation (PWM) is realized.
[0075] The AND gate G1 receives the output of the comparator CMP1
and the timing signal T4 and sends out the timing signal T1 in
accordance with the logical product of the inputs. Similarly, the
AND gate G2 receives the output of the comparator CMP1 and the timing
signal T4 and sends out the timing signal T2 in accordance with
the logical product of the inputs.
[0076] FIG. 2 illustrates the ON/OFF situations of the switching
elements Q1 Q2 Q3 Q4 and the switching control in the positive
and negative exciting periods in the above described configuration.
First, by the excitation timing signals T3 T4 in the positive
exciting period, the switching element Q3 is regulated to be turned
OFF and the switching element Q4 is regulated to be turned ON, and,
in the negative exciting period, the switching element Q3 is regulated
to be turned ON and the switching element Q4 is regulated to be
turned OFF. In the positive exciting period, the switching element
Q2 is turned OFF, and the switching control is conducted by the
switching element Q1 and, in the negative exciting period, the
switching element Q1 is turned OFF, and the switching control is
conducted by the switching element Q2.
[0077] FIG. 3 shows an operation in which the switching element
Q1 conducts the switching control, the switching element Q4 is turned
ON, and the switching elements Q2 Q3 are turned OFF. Current loops
in the cases where the switching element Q1 is turned ON, and where
the switching element Q1 is turned OFF are shown in FIG. 3.
[0078] In the sate where the switching element Q1 is turned ON,
a process of supplying a current from the power source E to the
exciting coil COIL is conducted, and the current flows through the
route of "power source E-->switching element Q1-->exciting
coil COIL-->switching element Q4-->second detection resistor
R26-->circuit reference voltage GND."
[0079] In the sate where the switching element Q1 is turned OFF,
a process in which the coil current attenuates with a time constant
due to the impedance of the coil is conducted, and the current flows
through the route of "exciting coil COIL-->switching element
Q4-->second detection resistor R26-->first detection resistor
R25-->switching element Q3."
[0080] In the first detection resistor R25 a current flows only
when the switching element Q1 is turned OFF. In the second detection
resistor R26 a current which is identical with a current flowing
through the exciting coil COIL flows. Under the operation conditions
where the switching element Q1 conducts the switching control, the
switching element Q4 is ON, and the switching elements Q2 Q3 are
OFF, therefore, the switching control is conducted with reference
to the reference voltage VREF2 of the second detection resistor
R26.
[0081] Similarly, under the operation conditions where the switching
element Q2 conducts the switching control, the switching element
Q3 is ON, and the switching elements Q1 Q4 are OFF, the switching
control is conducted with reference to the reference voltage VREF1
of the first detection resistor R25.
[0082] The reference voltages are changed over by the excitation
timing generating circuit connected to the first and second detection
resistors R25 R26.
[0083] The selected signal is compared with a predetermined voltage.
If the excitation current is lower, the switching element Q1 or
the switching element Q2 is turned ON, and, if the excitation current
is higher, the switching element Q1 or the switching element Q2
is turned OFF.
[0084] As a result, the circuit reference voltage GND of the exciting
circuit is identical with that of the controlling circuit, and all
the devices can be driven with using the common power source. Therefore,
it is possible to realize a low-power consumption exciting circuit
which does not require an insulating circuit and plural insulating,
power source circuits.
Second Embodiment
[0085] An electromagnetic flow meter of a second embodiment will
be described with reference to FIG. 4. The components which are
identical with those of the first embodiment are denoted by the
same reference numerals, and their description is omitted.
[0086] The electromagnetic flow meter of the second embodiment
is an embodiment of a circuit in which the switching frequency is
changed in accordance with the impedance of the load. An exciting
circuit formed by switching elements, and a controlling circuit
which generates ON/OFF timing signals for switching elements are
configured as shown in FIG. 4. The exciting circuit has a DC power
source E, a capacitor C which is connected in parallel to the DC
power source E, and an exciting coil COIL. The reference numerals
Q1 to Q4 denote switching elements configured by FETs, and D1 to
D4 denote parasitic diodes which are connected in parallel to the
switching elements Q1 to Q4 and in a direction opposite to the current
flow from the DC power source E, respectively. The parasitic diodes
D1 to D4 are formed in a package during a process of producing the
respective FETs, and hence cannot be removed away.
[0087] The switching elements Q1 Q2 are FETs which operate while
being supplied with a power source voltage VEX and grounded to a
reference voltage GND, and the switching elements Q3 Q4 are FETs
which operate while being supplied with the power source voltage
VEX and grounded to the reference voltage GND.
[0088] One end (positive side) of the DC power source E is connected
to one end 13 of the exciting coil COIL via the switching element
Q1. The one end 13 is connected to the other end (negative side)
of the DC power source E via the switching element Q3 and a first
detection resistor R25.
[0089] The other end 15 of the exciting coil COIL is connected
to the midpoint of the switching elements Q2 Q4. Furthermore, the
one end (positive side) of the DC power source E is connected to
the other end 15 of the exciting coil COIL via the switching element
Q2. The other end 15 is connected to the other end (negative side)
of the DC power source E via the switching element Q4 and a second
detection resistor R26.
[0090] The first detection resistor R25 and the second detection
resistor R26 are connected in series to each other, and the midpoint
of the series connection is grounded to the reference voltage GND.
[0091] The reference numerals T1 to T4 denote timing signals for
controlling ON/OFF operations of the switching elements Q1 to Q4
which operate while being supplied with the power source voltage
VEX and grounded to the circuit reference voltage GND. The timing
signals are supplied to the control electrodes of the switching
elements Q1 to Q4 (the gates of the FETs) via waveform-shaping circuits
B1 to B4 respectively. The waveform-shaping circuits B1 B2 B3
B4 are supplied with the power source voltage VEX, and grounded
to the circuit reference voltage GND.
[0092] In the exciting coil COIL and the first and second detection
resistors R25 R26 in the positive exciting period, the exciting
current flows through the switching element Q1-->the exciting
coil COIL-->the switching element Q4-->the second detection
resistor R26 and, in the negative exciting period, the exciting
current flows through the switching element Q2-->the exciting
coil COIL-->the switching element Q3-->the first detection
resistor R25. Namely, the exciting current flows alternately in
opposite directions. Therefore, a positive reference voltage VREF2
which is proportional to the exciting current is generated at the
one end 24 of the second detection resistor R26 in accordance with
the positive exciting period, and a negative reference voltage VREF1
which is proportional to the exciting current is generated at the
one end 20 of the first detection resistor R25 in accordance with
the negative exciting period.
[0093] The configuration and operation of a controlling circuit
which generates the timing signals T1 to T4 for ON/OFF-controlling
the switching elements Q1 to Q4 will be described with reference
to FIG. 4.
[0094] The reference numeral 11 denotes an excitation timing generating
circuit which regulates the positive and negative exciting periods,
and generates a rectangular wave of a predetermined excitation period.
A direct output is supplied as the timing signal T4 to the switching
element Q4 and an inverted output through an inverter G3 is supplied
as the timing signal T3 to the switching element Q3. The excitation
timing generating circuit controls a switch SW which changes over
the reference voltage VREF1 from the first detection resistor R25
and the reference voltage VREF2 from the second detection resistor
R26. The reference voltages are generated in proportion to the exciting
current in the controlling circuit.
[0095] The reference numeral 12 denotes an excitation controlling
circuit. In the circuit, the positive or negative reference voltage
VREF1 or VREF2 which is proportional to the exciting current is
supplied to a negative input terminal of a hysteresis comparator
CMP via the switch SW. The hysteresis comparator CMP is supplied
with the power source voltage VEX, and grounded to the circuit reference
voltage GND. An output of the hysteresis comparator CMP is supplied
to AND gates G1 G2 and in addition fed back to a positive input
terminal of the hysteresis comparator CMP via a voltage dividing
circuit of positive feedback resistors R11 R12. The reference numeral
Vs denotes a reference DC power source (reference voltage Vs) which
is connected between the resistor R11 and the ground serving as
the circuit reference voltage GND.
[0096] The hysteresis comparator CMP operates in the following
manner. When the absolute value of the reference voltage VREF1 or
VREF2 is increased to be larger than the reference voltage Vs, and
further increased to be larger than the voltage which is determined
by the positive feedback resistors R11 R12 and which corresponds
to the hysteresis width, the output is inverted from negative to
positive. By contrast, when the absolute value of the reference
voltage VREF1 or VREF2 is decreased to be smaller than the reference
voltage Vs, and further decreased to be smaller than the voltage
which is determined by the positive feedback resistors R11 R12
and which corresponds to the hysteresis width, the output is inverted
from positive to negative. This inverting operations are repeated.
The period of the inverting operations depends on the time constant
of the control loop including the inductance of the exciting coil
COIL, and is designed so as to be sufficiently shorter than the
periods of the excitation timing signals.
[0097] The AND gate G1 receives the output signals of the hysteresis
comparator CMP and the excitation timing generating circuit 11
and sends out the timing signal T1 in accordance with the logical
product of the output signals. Similarly, the AND gate G2 receives
the output signal of the hysteresis comparator CMP, the inverted
output signal of the excitation timing generating circuit 11 and
the timing signal T3 and sends out the timing signal T2 in accordance
with the logical product of the signals.
[0098] FIG. 2 illustrates the ON/OFF situations of the switching
elements Q1 to Q4 and the switching control in the positive and
negative exciting periods in the above described configuration.
First, by the excitation timing signals. T3 T4 in the positive
exciting period, the switching element Q3 is regulated to be turned
OFF and the switching element Q4 is regulated to be turned ON, and,
in the negative exciting period, the switching element Q3 is regulated
to be turned ON and the switching element Q4 is regulated to be
turned OFF.
[0099] In the same manner as the first embodiment, FIG. 3 shows
an operation in which the switching element Q1 conducts the switching
control, the switching element Q4 is turned ON, and the switching
elements Q2 Q3 are turned OFF. Current loops in the cases where
the switching element Q1 is turned ON, and where the switching element
Q1 is turned OFF are shown in FIG. 3.
[0100] In the sate where the switching element Q1 is turned ON,
a process of supplying a current from the power source to the exciting
coil COIL is conducted, and the current flows through the route
of "power source-->switching element Q1-->exciting coil
COIL-->switching element Q4-->second detection resistor R26-->circuit
reference voltage GND."
[0101] In the sate where the switching element Q1 is turned OFF,
a process in which the coil current attenuates with a time constant
due to the impedance of the coil is conducted, and the current flows
through the route of "exciting coil COIL-->switching element
Q4-->second detection resistor R26-->first detection resistor
R25-->switching element Q3."
[0102] In the first detection resistor R25 a current flows only
when the switching element Q1 is turned OFF. In the second detection
resistor R26 a current which is identical with a current flowing
through the exciting coil COIL flows. Under the operation conditions
where the switching element Q1 conducts the switching control, the
switching element Q4 is ON, and the switching elements Q2 Q3 are
OFF, therefore, the switching control is conducted with reference
to the reference voltage VREF2 of the second detection resistor
R26.
[0103] Similarly, under the operation conditions where the switching
element Q2 conducts the switching control, the switching element
Q3 is ON, and the switching elements Q1 Q4 are OFF, the switching
control is conducted with reference to the reference voltage VREF1
of the first detection resistor R25.
[0104] The reference voltages are changed over by the excitation
timing generating circuit 11 connected to the first and second detection
resistors R25 R26.
[0105] The selected signal is compared with a predetermined voltage.
If the excitation current is lower, the switching element Q1 or
the switching element Q2 is turned ON, and, if the excitation current
is higher, the switching element Q1 or the switching element Q2
is turned OFF.
[0106] As a result, the circuit reference voltage of the exciting
circuit is identical with that of the controlling circuit, and all
the devices can be driven with using the common power source. Therefore,
it is possible to realize a low-power consumption exciting circuit
which does not require an insulating circuit and plural insulating
power source circuits.
[0107] In the exciting circuit of the switching system, two detection
resistors are used, the two detection resistors are not directly
connected to the exciting coil, switching elements are interposed
between the exciting coil and the detection resistors, the two detection
resistors are connected in series to each other, and the midpoint
of the series connection is set to the circuit reference voltage
GND, whereby the circuit references of the exciting circuit and
the controlling circuit are made identical with each other. Therefore,
a low-power consumption exciting circuit which is not required to
have an insulating circuit that is necessary in the related art
is provided.
Third Embodiment
[0108] An electromagnetic flow meter of a third embodiment will
be described with reference to FIG. 5. The components which are
identical with those of the first embodiment are denoted by the
same reference numerals, and their description is omitted.
[0109] In the same manner as the first embodiment of FIG. 1 the
third embodiment of FIG. 5 is characterized in the configuration
related to the first detection resistor R25 and the second detection
resistor R26. The third embodiment of FIG. 5 is characterized also
in that a Schottky diode D11 a Schottky diode D12 a Schottky diode
D13 and a Schottky diode D14 are disposed.
[0110] The configuration of the third embodiment of FIG. 5 will
be described. The switching elements (Q1 to Q4) which are first
to fourth switching elements are formed by FETs (field effect transistors),
and comprise parasitic diodes (D1 to D4), respectively. In the third
embodiment of FIG. 5 the switching elements (Q1 to Q4) and the
parasitic diodes (D1 to D4) are integrated with each other, respectively.
[0111] The Schottky diode D11 is connected in parallel to the switching
element Q1 to be equivalently connected in parallel to the parasitic
diode D1. The Schottky diode D12 is connected in parallel to the
switching element Q2 to be equivalently connected in parallel to
the parasitic diode D2. The Schottky diode D13 is connected in parallel
to the switching element Q3 to be equivalently connected in parallel
to the parasitic diode D3. Moreover, the Schottky diode D14 is connected
in parallel to the switching element Q4 to be equivalently connected
in parallel to the parasitic diode D4.
[0112] Specifically, the anodes of the Schottky diodes (D11 to
D14) are connected to the sources of the switching elements (Q1
to Q4), and the cathodes of the Schottky diodes (D11 to D14) are
connected to the drains of the switching elements (Q1 to Q4), respectively.
[0113] The third embodiment of FIG. 5 has a first series circuit
having the power source E, the switching element. Q1 the Schottky
diode D11 the exciting coil COIL, the switching element Q4 the
Schottky diode D14 and the second detection resistor R26. The third
embodiment of FIG. 5 further has a second series circuit having
the power source E, the switching element Q2 the Schottky diode
D12 the exciting coil COIL, the switching element Q3 and the Schottky
diode D13.
[0114] The third embodiment of FIG. 5 further has a third series
circuit having the exciting coil COIL, the switching element Q4
the Schottky diode D14 the second detection resistor R26 the first
detection resistor R25 the switching element Q3 and the Schottky
diode D13. The third embodiment of FIG. 5 further has a third series
circuit having the exciting coil COIL, the switching element Q1
the Schottky diode D11 the switching element Q2 and the Schottky
diode D12.
[0115] The negative terminal of the power source E and controlling
circuit (the error amplifier 26 etc.) are connected to the circuit
reference voltage GND. The switching elements (Q1 to Q4) are switched
over by the predetermined fundamental excitation frequency (f1),
and ON/OFF-controlled by an excitation switching control frequency
(f2) so as to cause the exciting current to have a predetermined
value.
[0116] One end (drain) of the switching element Q1 serving as the
first switching element is connected to the positive terminal of
the power source E, and also one end (drain) of the switching element
Q2 serving as the second switching element is connected to the positive
terminal of the power source E.
[0117] One end (drain) of the switching element Q3 serving as the
third switching element is connected to the other end (source) of
the switching element Q1 and the other end (source) of the switching
element Q3 is connected to the circuit reference voltage GND via
the first detection resistor R25.
[0118] One end (drain) of the switching element Q4 serving as the
fourth switching element is connected to the other end (source)
of the switching element Q2 and the other end (source) of the switching
element Q4 is connected to the circuit reference voltage GND via
the second detection resistor R26.
[0119] The one end 13 of the exciting coil COIL is connected to
the junction of the other end (source) of the switching element
Q1 and the one end (drain) of the switching element Q3 and the
other end 15 of the exciting coil COIL is connected to the junction
of the other end (source) of the switching element Q2 and the one
end (drain) of the switching element Q4.
[0120] The first detection resistor R25 is formed between the other
end (source) of the switching element Q3 and the circuit reference
voltage GND, and the second detection resistor R26 is formed between
the other end (source) of the switching element Q4 and the circuit
reference voltage. GND.
[0121] The operation of the third embodiment of FIG. 5 will be
described in detail.
[0122] First, a period (state 1) in which the switching element
Q1 is ON, the switching element Q2 is OFF, the switching element
Q3 is. OFF, and the switching element Q4 is ON will be described.
At this time, the Schottky diodes (D11 to D14) are turned OFF, and
the switch SW selects the reference voltage VREF2 so that the reference
voltage VREF2 is connected to the excitation controlling circuit
12.
[0123] At this time, the power source E is applied to the exciting
coil COIL, and the coil is excited in the positive direction. In
the second detection resistor R26 the reference voltage VREF2 (VREF2>0)
which is proportional to the exciting current flowing through the
switching element Q1 the exciting coil COIL, and the switching
element Q4 is generated.
[0124] When the reference voltage VREF2 is increased to a predetermined
value, the switching element Q1 is changed from the ON state to
the OFF state, and the Schottky diode D13 is changed from the OFF
state to the ON state, so that state 1 is transferred to state 2.
[0125] Second, a period (state 2) in which the switching element
Q1 is OFF, the switching element Q2 is OFF, the switching element
Q3 is OFF, and the switching element Q4 is ON will be described.
At this time, the Schottky diode D11 is OFF, the Schottky diode
D12 is OFF, the Schottky diode D13 is ON, the Schottky diode D14
is OFF, and the switch SW selects the reference voltage VREF2 so
that the reference voltage VREF2 is connected to the excitation
controlling circuit 12.
[0126] At this time, the magnetic flux of the exciting coil COIL
is generally held, but partly reset by voltages generated in the
switching element Q4 the second detection resistor R26 the first
detection resistor R25 and the Schottky diode D13. In the second
detection resistor R26 the reference voltage VREF2 (VREF2>0)
which is proportional to the exciting current flowing through the
exciting coil COIL and the switching element Q4 is generated.
[0127] When the reference voltage VREF2 is decreased to a predetermined
value, the switching element Q1 is changed from the OFF state to
the ON state, and the Schottky diode D13 is changed from the ON
state to the OFF state, so that state 2 is transferred to state
1. At this time, the recovery current does not flow through the
Schottky diode D13 and the turn-on surge current of the switching
element Q1 does not through the second detection resistor R26.
[0128] In the third embodiment of FIG. 5 the magnetic flux of
the exciting coil in the positive direction is controlled in this
way. Specifically, the ON/OFF operation of the switching element
Q1 is controlled based on the reference voltage VREF2 generated
in the second detection resistor R26.
[0129] Third, a period (state 3) in which the switching element
Q1 is OFF, the switching element Q2 is ON, the switching element
Q3 is ON, and the switching element Q4 is OFF will be described.
At this time, the Schottky diodes (D11 to D14) are turned OFF, and
the switch SW selects the reference voltage VREF1 so that the reference
voltage VREF1 is connected to the excitation controlling circuit
12.
[0130] At this time, the power source E is applied to the exciting
coil COIL, and the coil is excited in the negative direction. In
the first detection resistor R25 the reference voltage VREF1 (VREF1>0)
which is proportional to the exciting current flowing through the
switching element Q2 the exciting coil COIL, and the switching
element Q3 is generated.
[0131] When the reference voltage VREF1 is increased to a predetermined
value, the switching element Q2 is changed from the ON state to
the OFF state, and the Schottky diode D14 is changed from the OFF
state to the ON state, so that state 3 is transferred to state 4.
[0132] Fourth, a period (state 4) in which the switching element
Q1 is OFF, the switching element Q2 is OFF, the switching element
Q3 is ON, and the switching element Q4 is OFF will be described.
At this time, the Schottky diode D11 is OFF, the Schottky diode
D12 is OFF, the Schottky diode D13 is OFF, and the Schottky diode
D14 is ON, and the switch SW selects the reference voltage VREF1
so that the reference voltage VREF1 is connected to the excitation
controlling circuit 12.
[0133] At this time, the magnetic flux of the exciting coil COIL
is generally held, but partly reset by voltages generated in the
switching element Q3 the first detection resistor R25 and the
Schottky diode D14. In the first detection resistor R25 the reference
voltage VREF1 (VREF1>0) which is proportional to the exciting
current flowing through the exciting coil COIL and the switching
element Q3 is generated.
[0134] When the reference voltage VREF1 is decreased to a predetermined
value, the switching element Q2 is changed from the OFF state to
the ON state, and the Schottky diode D14 is changed from the ON
state to the OFF state, so that state 4 is transferred to state
3. At this time, the recovery current does not flow through the
Schottky diode D14 and the turn-on surge current of the switching
element Q2 does not through the first detection resistor R25.
[0135] In the third embodiment of FIG. 5 the magnetic flux of
the exciting coil in the negative direction is controlled in this
way. Specifically, the ON/OFF operation of the switching element
Q2 is controlled based on the reference voltage VREF1 generated
in the first detection resistor R25.
[0136] The switching over from the excitation in the positive direction
to that in the negative direction will be described in detail.
[0137] In the positive excitation (state 1), a current flows from
the one end 13 to the other end 15. When all the switching elements
(Q1 to Q4) are turned OFF, the function of the exciting coil COIL
causes the Schottky diode D11 to be turned OFF, the Schottky diode
D12 to be turned ON, the Schottky diode D13 to be turned ON, and
the Schottky diode D14 to be turned OFF.
[0138] When the switching element Q1 is turned OFF, the switching
element Q2 is turned ON, the switching element Q3 is turned ON,
and the switching element Q4 is turned OFF, the excitation is conducted
in the negative direction (state 4), and a current from the other
end 15 to the one end 13.
[0139] Specifically, the switching elements Q1 Q4 are turned OFF,
and the Schottky diodes D12 D13 are turned ON. Thereafter, the
switching elements Q2 Q3 are turned ON.
[0140] At this time, the switching elements Q2 Q3 are turned ON
at zero voltage. Therefore, the loss and the noise level are low.
[0141] The switching over from the excitation in the negative direction
(state 4) to the excitation in the negative direction (step 1) will
be described in detail. The switching elements Q2 Q3 are turned
OFF, and the Schottky diodes D11 D14 are turned ON. Thereafter,
the switching elements Q1 Q4 are turned ON.
[0142] At this time, the switching elements Q1 Q4 are turned ON
at zero voltage. Therefore, the loss and the noise level are low.
[0143] The above is summarized as follows. When the switching element
Q2 is OFF and the switching element Q4 is ON, the switch SW selects
the reference voltage VREF2 of the second detection resistor R26
and, when the switching element Q1 is OFF and the switching element
Q3 is ON, the switch selects the reference voltage VREF1 of the
first detection resistor R25.
[0144] In the third embodiment of FIG. 5 the parasitic diodes
(D1 to D4) are always turned OFF by the functions of the Schottky
diodes (D11 to D14). Specifically, the forward voltage drops of
the parasitic diodes (D1 to D4) are about 0.6 V, and those of the
Schottky diodes (D11 to D14) are about 0.4 V.
[0145] In the third embodiment of FIG. 5 therefore, the recovery
current is not caused in the Schottky diodes D13 D14 the switching
elements (Q1 to Q4) are turned ON at zero voltage, and the forward
voltage drops of the Schottky diodes (D11 to D14) are small. Therefore,
the loss and the noise level are low. Since the forward voltage
drops of the Schottky diodes (D11 to D14) are small, moreover, the
excitation switching control frequency (f2) is low, and the loss
and the noise level are low.
Fourth Embodiment
[0146] An electromagnetic flow meter of, a third embodiment will
be described with reference to FIGS. 6 and 7. FIG. 6 is a diagram
showing a fourth embodiment of the invention. The components which
are identical with those of the third embodiment of FIG. 5 are denoted
by the same reference numerals, and their description is omitted.
[0147] In the same manner as the third embodiment of FIG. 5 the
fourth embodiment of FIG. 6 is characterized in the configuration
related to the first detection resistor R25 and the second detection
resistor R26. The fourth embodiment of FIG. 6 is characterized also
in a configuration related to a third detection resistor R30 and
also in a configuration related to means for normalizing a flow
rate signal.
[0148] The third detection resistor R30 is connected in series
to the exciting coil COIL. More specifically, the first, second,
third, and fourth series circuits of the third embodiment of FIG.
5 comprise the third detection resistor R30. One end 13 of a series
circuit of the exciting coil COIL and the third detection resistor
R30 is connected to the junction of the other end (source) of the
switching element Q1 and the one end (drain) of the switching element
Q3 and the other end 15 of the series circuit of the exciting coil
COIL and the third detection resistor R30 is connected to the junction
of the other end (source) of the switching element Q2 and the one
end (drain) of the switching element Q4.
[0149] The voltage of the one end of the third detection resistor
R30 is a voltage V1 and that of the other end of the third detection
resistor R30 is a voltage V2. The voltages V1 and V2 are changed
in correct proportion to the exciting current flowing through the
exciting coil COIL.
[0150] An exciting current detection circuit 31 generates an exciting
current signal S31 from the voltages V1 and V2. Specifically, an
inverting input end of a differential amplifier U33 is connected
to the voltage V1 via a buffer including a resistor R33 and an error
amplifier U31 and a high-pass filter (HPF) including a capacitor
C31 and a resistor R31. A non-inverting input end of the differential
amplifier U33 is connected to the voltage V2 via a buffer including
a resistor R35 and an error amplifier U32 and a high-pass filter
(HPF) including a capacitor C32 and a resistor R32.
[0151] An input of an AD converter U34 is connected to an output
of the differential amplifier U33. An input of a microprocessor
(CPU) U35 is connected to an output of the AD converter U34 and
that of the excitation timing generating circuit 11.
[0152] The differential amplifier U33 outputs an analog value which
is a difference between the voltages V1 and V2. The AD converter
U34 converts the analog value to a digital value to generate an
exciting current signal S31. In this way, the exciting current signal
S31 is generated from the voltages V1 and V2 produced in the third
detection resistor R30.
[0153] FIG. 7 is a diagram showing the fourth embodiment of the
invention. The components which are identical with those of the
third embodiment of FIG. 5 are denoted by the same reference numerals.
[0154] First, a detector will be described. A pipe 50 through which
fluid flows has electrodes 51 and 52. The exciting coil COIL is
placed in adjacent to the pipe 50.
[0155] Next, a converter will be described. An exciting circuit
53 which corresponds to the fourth embodiment of FIG. 6 is connected
to the exciting coil COIL. An inverting input end of a differential
amplifier U43 is connected to the electrode 51 via a buffer including
a resistor R41 and an error amplifier U41. A non-inverting input
end of the differential amplifier U43 is connected to the electrode
52 via a buffer including a resistor R43 and an error amplifier
U42.
[0156] An input of an AD converter U44 is connected to an output
of the differential amplifier U43. An input of the microprocessor
(CPU) U35 is connected to an output of the AD converter U34.
[0157] The differential amplifier U43 outputs an analog value which
is a difference between a voltage produced in the electrode 51 and
that produced in the electrode 52. The AD converter U44 converts
the analog value to a digital value to generate a flow rate signal
S41. In this way, the flow rate signal S41 is generated from the
voltages produced in the electrodes 51 and 52 of the pipe 50 in
accordance to the exciting current of the exciting coil COIL.
[0158] The microprocessor U35 calculates a ratio (S41/S31) of the
flow rate signal S41 from the AD converter U44 of FIG. 7 to the
exciting current signal S31 from the AD converter U34 of FIG. 6
and normalizes the flow rate signal. The configuration related to
the microprocessor U35 forms the means for normalizing the flow
rate signal.
[0159] The operation of the fourth embodiment of FIGS. 6 and 7
will be described.
[0160] When the exciting current of the exciting coil COIL is decreased,
the flow rate signal S41 detected in the electrodes 51 and 52 is
decreased, but the ratio of the flow rate signal S41 to the exciting
current signal S31 is little changed.
[0161] When the exciting current of the exciting coil COIL is increased,
the flow rate signal S41 detected in the electrodes 51 and 52 is
increased, but the ratio of the flow rate signal S41 to the exciting
current signal S31 is little changed.
[0162] As described above, in the fourth embodiment of FIGS. 6
and 7 the flow rate of the fluid which flows through the pipe 50
is measured, and transmitted.
[0163] Therefore, the fourth embodiment of FIGS. 6 and 7 can provide
an electromagnetic flow meter of a high measurement accuracy which
does not depend on the exciting current. In the same manner as the
third embodiment of FIG. 5 the fourth embodiment of FIGS. 6 and
7 can provide the electromagnetic flow meter in a small size, with
a low-loss, and with a low-noise.
[0164] In the fourth embodiment of FIGS. 6 and 7 it is possible
also to estimate the exciting current from the first detection resistor
R25 and the second detection resistor R26. In this case, however,
a high measurement accuracy cannot be obtained. As a result of specific
implementation experiments and studies, it has been proved that
the configuration of the fourth embodiment of FIGS. 6 and 7 provides
the highest commercial value.
[0165] In the embodiments described above, the switching elements
(Q1 to Q4) are formed by FETs (field effect transistors). Alternatively,
the switching elements (Q1 to Q4) may be formed by bipolar transistors.
Also in the alternative, a substantially identical configuration
is obtained, and equivalent effects can be attained. In the case
of bipolar transistors, there is no component corresponding to the
parasitic diodes (D1 to D4).
[0166] The invention is not restricted to the embodiments described
above, and includes many changes and modifications without departing
from the spirit of the invention.
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