Abstrict A magnetic flow meter according to this invention is composed of
a sensor for sensing a flow rate of a fluid and a converter connected
to the sensor via a signal cable. The sensor essentially consists
of a measuring tube through which the fluid flows, a pair of opposed
electrodes installed in the measuring tube so as to face each other,
an exciting coil for generating a magnetic field and applying it
to the measuring tube in a direction perpendicular to the centerline
of the measuring tube, and a current signal transmitting circuit
for converting a differential voltage across the pair of electrodes
into a corresponding current signal. The converter is made up of
a signal cable one end of which is connected to the current signal
transmitting circuit and which transmits the current signal from
the current signal transmitting circuit, and a converter to which
the other end of the signal cable is connected and which converts
the current signal transmitted over the signal cable into a flow
signal representing the flow rate through the measuring tube.
Claims What is claimed is:
1. A magnetic flow meter comprising:
flow rate sensing means which includes a measuring tube through
which a fluid flows, a pair of opposed electrodes installed in said
measuring tube, exciting means, including a magnetic coil provided
adjacent said measuring tube, for generating a magnetic field in
a direction perpendicular to a centerline of said tube and between
said electrodes and applying it to said measuring tube, and current
signal transmitting means for converting a differential voltage
across said pair of electrodes into a current signal proportional
to the differential voltage;
an operational amplifier means which has one input terminal supplied
with said differential voltage and the other input terminal supplied
with a constant voltage and produces a base signal having a voltage
corresponding to a difference between said differential voltage
and said constant voltage.
a signal cable one end of which is connected to said current signal
transmitting means and which transmits said current signal from
said current signal transmitting means; and
converting means to which the other end of said signal cable is
connected and which converts said current signal transmitted over
said signal cable into a flow signal representing a flow rate through
said measuring tube.
2. A magnetic flow meter according to claim 1 wherein said current
signal transmitting means includes a preamplifier for sensing the
differential voltage across said pair of electrodes, and a transistor
having a base and a collector-emitter path through which a current
flows when the base is supplied with the base signal, said current
being supplied to said signal cable as said current signal.
3. A magnetic flow meter according to claim 1 wherein said converting
means includes a signal sensing resistor which is connected to the
other end of said signal cable and converts said current signal
flowing through said signal cable into a voltage, a.c. signal coupling
means for coupling an a.c. voltage proportional to said differential
voltage from said voltage across said signal sensing resistor, and
converting operation means for converting into said flow signal
the a.c. voltage proportional to said differential voltage.
4. A magnetic flow meter according to claim 1 wherein said converting
means includes a current transformer having a primary-side coil
and a secondary-side coil with the primary-side coil being connected
to the other end of said signal cable, a resistor connected across
the secondary-side coil of said current transformer for generating
a voltage proportional to said differential voltage, an amplifier
to which the voltage derived across said resistor is applied, and
conversion operation means for converting an output of said amplifier
into said flow signal.
5. A magnetic flow meter according to claim 1 wherein:
said current signal transmitting means includes a preamplifier
for sensing the differential voltage across said pair of electrodes,
and a transistor having a base and a collector-emitter path through
which a current flows when the base is supplied with the base signal,
said current being supplied to said signal cable as said current
signal;
said converting means includes a power supply for applying a constant
voltage to said signal cable, a signal sensing resistor which is
connected via said power supply to the other end of said signal
cable and converts the current signal flowing through said signal
cable into a voltage, a.c. signal coupling means for coupling an
a.c. a voltage proportional to said differential voltage from said
voltage across said signal sensing resistor, and converting operation
means for converting into said flow signal, the a.c. voltage proportional
to said differential voltage; and
said signal cable includes a first signal line which has one end
connected to a collector of said transistor and the other end connected
to one end of said signal sensing resistor and a second signal line
which has one end connected to an emitter of said transistor and
the other end connected to one electrode of said power supply.
6. A magnetic flow meter according to claim 1 wherein:
said current signal transmitting means includes a preamplifier
for sensing the differential voltage across said pair of electrodes,
and a transistor having a base and a collector-emitter path through
which a current flows when the base is supplied with the base signal,
said current being supplied to said signal cable as said current
signal;
said converting means includes a power supply for applying a constant
voltage to said signal cable, a current transformer having a primary-side
coil and a secondary-side coil with the primary-side coil being
connected to the other end of said signal cable, a resistor connected
across the secondary-side coil of said current transformer for generating
a voltage proportional to said differential voltage, an amplifier
to which the voltage derived across said resistor is applied, and
conversion operation means for converting an output of said amplifier
into said flow signal; and
said signal cable includes a first signal line which has one end
connected to a collector of said transistor with the other end connected
to one end of the primary-side coil of said current transformer
and a second signal line which as one end connected to an emitter
of said transistor and the other end connected to one electrode
of said power supply.
7. A magnetic flow meter according to claim 2 wherein said current
controlling means has a series circuit of first and second Zener
diodes connected in a forward direction between a collector and
emitter of said transistor, a junction point of said first and second
Zener diodes being connected to the other input terminal of said
operational amplifier means and a voltage of the junction point
maintained at a constant voltage by said first and second Zener
diodes being applied to the other input terminal of said operational
amplifier.
8. A magnetic flow meter according to claim 2 wherein said current
signal transmitting means further comprises a feedback circuit in
which a emitter of said transistor is connected to one input terminal
of said operational amplifier means.
9. A magnetic flow meter according to claim 3 wherein said a.c.
coupling means includes a first capacitor one end of which is connected
to one end of said signal sensing resistor, a second capacitor one
end of which is connected to the other end of said signal sensing
resistor, and an amplifier having a first input terminal connected
to the other end of the first capacitor, a second input terminal
connected to the other end of said second capacitor, and an output
terminal from which a voltage obtained by amplifying a voltage across
said first and second input terminals is supplied.
10. A magnetic flow meter according to claim 5 wherein said current
signal transmitting means has a series circuit of a first and second
Zener diodes connected in a forward direction between the collector
and emitter of said transistor, a junction point of said first and
second Zener diodes being connected to the other input terminal
of said operational amplifier means.
11. A magnetic flow meter comprising:
flow rate sensing means which includes a measuring tube through
which a fluid flows, exciting means including a magnetic coil provided
adjacent said tube for generating a magnetic field in a direction
perpendicular to a centerline of said tube and applying it to said
measuring tube, detecting means for detecting a voltage in the fluid
induced by the magnetic field, and current signal transmitting means
for converting the voltage detected by said detecting means into
a current signal proportional to the voltage;
an operational amplifier means which has one input terminal supplied
with said differential voltage and the other input terminal supplied
with a constant voltage and produces a base signal having a voltage
corresponding to a difference between said differential voltage
and said constant voltage,
a signal cable means, connected to said current signal transmitting
means, for transmitting said current signal from said current signal
transmitting means; and
converting means, connected to said signal cable, for converting
said current signal transmitted through said signal cable into a
flow signal representing a flow rate of the fluid flowing through
said measuring tube.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a flow meter, and more particularly to
a magnetic flow meter composed of a sensor for producing a voltage
signal proportional to the flow rate of liquid flowing through the
measuring tube and a converter for converting the output signal
of the sensor into the measured value of flow rate, with the converter
being installed sufficiently away from the sensor.
2. Description of the Related Art
Widely known magnetic flow meters are generally made up of a sensor
and a converter with a signal cable connecting them with each other.
The sensor is provided with a measuring tube through which a fluid
to be measured flows and a exciting coil that generates a magnetic
field across the measuring tube. The measuring tube is provided
with a pair of electrodes facing each other across the tube.
With a liquid flowing through the measuring tube, the sensor produces
a differential voltage across a pair of the electrodes proportional
to the flow rate, when an electric field generated by the exciting
coil is applied to the measuring tube.
The converter is provided with an amplifier to which the differential
voltage across the electrodes of the sensor is applied via a signal
cable. It obtains a flow rate by processing the differential voltage
amplified at the amplifier. It is also provided with an exciting
circuit that supplies exciting current to the exciting coil of the
sensor.
The sensor is separated from the converter to prevent the sensor
from being installed in a poor maintenance environment such as inside
the pit. It is desirable that the sensor should be placed where
maintenance can be easily done.
For example, when the sensor is installed in an area of an explosion-proof
atmosphere, the converter is placed in a safe area. In this case,
it is difficult to secure a safe area of, for example, the order
of some 10 m.sup.2 meters to install the converter in an explosion-proof
atmosphere. This makes the separating distance between the sensor
and the converter as long as from 300 to 1000 m or more.
The cable for transmitting a voltage signal to a remote place is
made up of conducting wires wrapped with an insulating film and
then covered with a shielding tube. Because a floating capacitance
and an insulating resistance exit between the conducting wires and
shielding tube, as the signal cable to transmit the differential
voltage across the electrodes of the sensor to the converter becomes
longer, the floating capacitance of the cable increases, which makes
the cable impudance seen from the electrodes smaller. The longer
cable also decreases the insulating resistance of the signal cable.
Therefore, the length of the cable is limited by the capacitance
and resistance of the cable.
For example, with a fluid conductivity of 5 .mu./cm, the length
of the cable is limited to approximately 30 m, which is not long
enough to separate the sensor from the converter.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a magnetic flow
meter that secures a signal cable length enough to connect a sensor
to a converter with a sufficient distance between them without being
affected by the floating capacitance and insulating resistance of
the cable.
The foregoing object is accomplished by providing a magnetic flow
meter comprising a sensor that is composed of a measuring tube through
which a fluid flows, a pair of electrodes installed in the measuring
tube so as to face each other, an exciting coil for generating a
magnetic field and applying it to the measuring tube, and a current
control circuit for converting the differential voltage across the
pair of electrodes into a current signal proportional to the differential
voltage, a signal cable one end of which is connected to the current
control circuit and which transmits the current signal from the
current control circuit, and a converter to which the other end
of the signal cable is connected and which converts the current
signal transmitted over the signal cable into a flow signal representing
the flow rate through the measuring tube.
With this configuration, in the sensor, when a magnetic field generated
at the exciting coil is applied to the measuring tube through which
a fluid is flowing, a differential voltage appears across the electrodes
installed in the measuring tube so as to face each other and the
current proportional to the differential voltage is supplied. The
current flows through the signal cable and enters the converter,
which then separates from the current signal a signal proportional
to the differential voltage across the electrodes and obtains the
flow rate through the measuring tube .
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute
a part of the specification, illustrate a presently preferred embodiment
of the invention, and together with the general description given
above and the detailed description of the preferred embodiment given
below, serve to explain the principles of the invention.
FIG. 1 is a circuit diagram for a magnetic flow meter according
to an embodiment of the present invention;
FIG. 2 is a detailed circuit diagram for the magnetic flow meter
of FIG. 1; and
FIG. 3 is a circuit diagram for an important part of a modification
of the embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A magnetic flow meter according to an embodiment of the present
invention will be explained, referring to the accompanying drawings.
FIG. 1 is a circuit diagram of a magnetic flow meter according
to an embodiment of the present invention. The magnetic flow meter
is composed of a sensor 1 and a converter 2 with a signal cable
3 and an exciting cable 4 connecting the sensor 1 to the converter
2.
The sensor 1 is provided with a measuring tube 5 through which
a fluid to be measured flows. On the inside walls of the measuring
tube 5 electrodes 6 and 7 are provided so as to face each other
across the tube. An exciting coil 8 is provided so as to face the
measuring tube 5. The exciting coil 8 produces a magnetic fields
perpendicular to the direction of fluid flow and the tube diameter
on which the electrodes 6 and 7 exist.
The electrode 6 and 7 are connected to the inputs of a preamplifier
9. The output terminal of the preamplifier 9 is connected to a current
control circuit 10.
The current control circuit 10 receives a current, which is sensed
by a current sensing resistance 11 and flows through the signal
cable 3 and controls the current so that it may be proportional
to the differential voltage across the electrodes 6 and 7.
Referring to FIG. 2 which is a detailed circuit diagram of the
magnetic flow meter, the arrangements of the current control circuit
10 and converter 2 will be described.
The output terminal of the preamplifier 9 is connected to one input
terminal of an operational amplifier 13 via a resistor 12. The output
of the operational amplifier 13 is connected to the base of an n-p-n
transistor 15 via a resistor 14. The n-p-n transistor 15 has its
collector connected to one power line La, which is connected to
a first signal line L1 of the signal cable 3 and its emitter connected
to the other power line Lb, which is connected to a second signal
line L2 of the signal cable 3. The other power line Lb is connected
to one end of the second signal line L2 via the current sensing
resistor 11.
Between the power lines or between the emitter and collector of
the transistor 15 a series circuit of a first and a second Zener
diode 16 and 17 is connected. The cathode of the first Zener diode
16 (the junction point of the first and second Zener diodes 16 and
17) is connected to the other input terminal of the operational
amplifier 13 while the cathode of the second Zener diode 17 is connected
to the collector of the transistor 15 via a resistance 18 inserted
into the power line La.
The signal cable-side end of the current sensing resistor 11 is
connected to one input terminal of the operational amplifier 13
via a resistor 19 to feed back the voltage across the current sensing
resistor 11 to one input terminal of the operational amplifier 13.
In addition, one input terminal of the operational amplifier 13
is connected to one power line La via a resistor 20.
On the other hand, the converter 2 is provided with a power supply
30 and a signal sensing resistor 31. The power supply 30 and signal
sensing resistor 31 are connected to the signal cable 3. Specifically,
the other end of the first signal line L1 of the signal cable 3
is connected to one end of the signal sensing resistor 31; the other
end of the second signal line L2 is connected to the negative terminal
of the power supply 30; and the positive terminal of the power supply
30 is connected to the other end of the signal sensing resistor
31.
The signal sensing resistor 31 is connected to a signal separating
circuit 32 which separates from the voltage across the signal sensing
resistor 31 a voltage proportional to the differential voltage between
the electrodes 6 and 7.
Specifically, the signal separating circuit 32 as shown in FIG.
2 is composed of capacitors 33 and 34. The signal separating circuit
32 is connected via an amplifier 35 to an operational circuit 36
which receives the voltage separated at the signal separating circuit
32 to produce a flow signal representing the flow rate through the
measuring tube 5.
The converter 2 is also provided with an exciting circuit 37. The
exciting circuit 37 which generates a square-wave exciting current,
is connected to the exciting coil 8 of the sensor 1 via an exciting
cable 4 (not shown in FIG. 2).
The operation of the embodiment thus constructed will now be described.
The output current of the power supply 30 flows through the current
sensing resistor 31 and the signal cable 3 and then is supplied
to the sensor 1. In the sensor 1 the output current from the power
supply flows through the resistor 18 Zener diodes 16 and 17 and
resistor 11. The voltages derived across the Zener diodes 16 and
17 are then applied to the preamplifier 9 and operational amplifier
13 thereby causing them to operate. On the other hand, the exciting
circuit 37 of the converter 2 supplies an exciting current to the
exciting coil 8 via the exciting cable 4 which causes the exciting
coil 8 to generate a magnetic field across the measuring tube 5.
Under these conditions, when a fluid flows through the measuring
tube 5 an electromotive force is generated across the electrodes
6 and 7 according to Faraday's law of electromagnetic induction.
The electromotive force is proportional to the average flow rate
velocity of fluid and the intensity of magnetic field. The electromotive
force is transmitted to the preamplifier 9 which amplifies the
differential voltage V.sub.in across the electrodes 6 and 7 and
supplies it.
Because the exciting current to the exciting coil 8 is of a square
wave, the differential voltage V.sub.in is also of a square wave
with the ground potential as a reference. The differential voltage
V.sub.in is supplied to one input terminal of the operational amplifier
13 which applies via the resistor 14 to the base of the n-p-n transistor
15 the deviation voltage of the differential voltage V.sub.in from
the Zener voltage derived at the Zener diode 16. Then, the collector
current of the n-p-n transistor 15 varies with the differential
voltage V.sub.in. The collector current is supplied as the current
signal representing the differential voltage V.sub.in.
As a consequence, the signal cable 3 carries a current I.sub.o
proportional to the differential voltage V.sub.in across the electrodes
6 and 7. The current I.sub.o is represented as:
where R11 R12 R19 and R20 are the resistances of the resistors
11 12 19 and 20 respectively, and VZ18 and VZ19 are the Zener
voltages of the Zener diodes 18 and 19 respectively.
The resistances of the resistors 11 12 19 and 20 are set so
that even if the current I.sub.o decreases to a minimum, the Zener
voltages applied to the preamplifier 9 and operational amplifier
13 may be maintained at a level that allows these amplifiers 9 and
13 operate properly.
The current I.sub.o flows through the signal cable 3 and then through
the current sensing resistor 31 of the converter 2. Across the current
sensing resistor 31 a voltage appears in proportion to the current
I.sub.o. The voltage has its direct-current component blocked off
with the capacitors 33 and 34 to separate a voltage proportional
to the differential voltage V.sub.in. The separated voltage is supplied
via the capacitors 33 and 34 to the amplifier 35 which then amplifies
it. Receiving the amplified voltage, the operational circuit 36
produces the flow signal.
As noted above, in the embodiment, the differential voltage across
the electrodes 6 and 7 of the measuring tube 5 is converted into
the current I.sub.o according to the differential voltage and then
the converted current I.sub.o is transmitted via the signal cable
3 to the converter 2 which from the current signal the signal proportional
to the differential voltage across the electrodes 6 and 7 and then
obtains the rate of flow through the measuring tube 5.
As a result, the impedance of the signal cable 3 becomes nearly
equal to the resistance of the resistor 31 which reduces the impedance
significantly. The output impedance across the electrodes 6 and
7 will be as high as several megaohms when the conductivity of the
fluid is low. However, the resistance of the resistor 31 can be
set to as low as 10 .OMEGA., so that the signal cable extending
over as long as several kilometers will not be affected by the floating
capacitance and insulating resistance of the cable.
In addition, the low-impedance signal cable 3 achieves a long-distance
transmission with low noise. Therefore, a longer distance between
the sensor 1 and converter 2 has a less adverse effect on the transmission
due to the floating capacitance and insulating resistance of the
cable 3 which puts much fewer restrictions on where to install
the magnetic flow meter.
Because there are almost no restrictions on the earth capacity
and insulating resistance, inexpensive cables may be used for the
signal cable 3. The signal cable 3 has the same number of conducting
wires as that of a conventional equivalent, so that the existing
cables may be used, thereby reducing the cost.
The present invention is not limited to the abovementioned embodiment,
and may be practiced or embodied in still other ways without departing
from the spirit or essential character thereof.
For example, the separating circuit contained in the converter
2 to separate the differential voltage may be constructed as shown
FIG. 3. In this separating circuit, the signal cable 3 is connected
to a current transformer 40 whose secondary coil is connected to
a resistor 41. The resistor 41 is connected across the inputs terminal
of the amplifier 35.
With such an arrangement, the ground of the sensor 1 and that of
the converter 2 are insulated from each other and connected independently
to the earth, thereby reducing the introduction of noises into the
circuit.
Additional advantages and modifications will readily occur to those
skilled in the art. Therefore, the invention in its broader aspects
is not limited to the specific details, and representative devices,
shown and described herein. Accordingly, various modifications may
be made without departing from the spirit or scope of the general
inventive concept as defined by the appended claims and their equivalents. |