Abstrict An electromagnetic flow meter with less power consumption in which
an instantaneous pulsate current with alternately changing polarity
at given intervals is fed into an exciting coil, and a velocity
of flow of fluid is calculated by using a voltage produced between
electrodes in accordance with a residual magnetic flux of a magnetic
circuit and a flow velocity of the fluid when no exciting current
flows.
Claims What is claimed is:
1. In an electromagnetic flow meter having: a pipe of non-magnetic
and non-conductive material through which conductive fluid flows;
a pair of electrodes which are provided oppositely on said pipe
orthogonal to the flow direction of said fluid and in contact with
said fluid; a magnetic circuit for introducing magnetic flux in
the direction orthogonal to the flow direction and a straight line
connecting said electrodes, an exciting coil for producing said
magnetic flux to be introduced into said magnetic circuit; an exciting
current generating unit for generating exciting current to be supplied
to said exciting coil; and a fluid flow velocity calculating unit
for calculating fluid flow velocity on the basis of the voltage
produced between said electrodes due to said magnetic flux and the
fluid flow; said electromagnetic flow meter comprising: a control
unit for controlling said exciting current generating unit and said
fluid flow calculating unit in such a manner that said exciting
current generating unit intermittently produces said exciting current
with an alternately reversed polarity and a short time duration
and said fluid flow velocity calculating unit calculates the velocity
of the fluid flow only during a predetermined constant period from
a point in time when the residual flux retained by said magnetic
circuit, after extinguishing said exciting current, reaches a stable
state to a point in time when the subsequent exciting current is
initiated, wherein said exciting current generating unit comprises
a first fixed resistor with a large resistance value relative to
said exciting coil, connected in series with said exciting coil;
a capacitor; a normally open switch connected to said capacitor;
a polarity inverting switch, both ends of a series connection of
said capacitor and said normally open switch being connected to
both ends of a series connection of said exciting coil and said
first fixed resistor through said polarity inverting switch; a stabilizing
DC power source; and a second fixed resistor with a high resistance
value connected to said DC power source in series, said capacitor
being connected in parallel with a series connection of said second
fixed resistor and said DC power source; and said control unit controls
said normally open switch and said polarity inverting switch with
predetermined timings respectively in such a manner that a connection
polarity of said polarity inverting switch is changed at a predetermined
constant timing and said normally open switch is closed for a short
time at an intermediate timing of said predetermined constant timing.
2. In an electromagnetic flow meter having: a pipe of non-magnetic
and non-conductive material through which conductive fluid flows;
a pair of electrodes which are provided oppositely on said pipe
orthogonal to the flow direction of said fluid and in contact with
said fluid; a magnetic circuit for introducing magnetic flux in
the direction orthogonal to the flow direction and a straight line
connecting said electrodes, an exciting coil for producing said
magnetic flux to be introduced into said magnetic circuit; an exciting
current generating unit for generating exciting current to be supplied
to said exciting coil; and a fluid flow velocity calculating unit
for calculating fluid flow velocity on the basis of the voltage
produced between said electrodes due to said magnetic flux and the
fluid flow; said electromagnetic flow meter comprising: a control
unit for controlling said exciting current generating unit and said
fluid flow calculating unit in such a manner that said exciting
current generating unit intermittently produces said exciting current
with an alternately reversed polarity and a short time duration
and said fluid flow velocity calculating unit calculates the velocity
of the fluid flow only during a predetermined constant period from
a point in time when the residual flux retained by said magnetic
circuit, after extinguishing said exciting current, reaches a stable
state to a point in time when the subsequent exciting current is
initiated, wherein said exciting current generating unit 6 is comprised
of a first series connection of a first normally open switch S.sub.1a
and a first capacitor Ca; a second series connection of a second
normally open switch S.sub.1b and a second capacitor Cb, said first
and second series connections being connected to both ends of said
exciting coil 5 in parallel; series connections of DC power sources
E.sub.oa, E.sub.ob and resistors Ra, Rb being connected to said
first and second capacitors Ca, Cb respectively or in common in
such a manner that said first and second capacitors Ca, Cb are charged
with opposite polarity to obtain opposite current flows into said
exciting coil 5 when said first and second normally open switches
S.sub.1a, S.sub.1b are selectively closed; a first diode Da connected
to said first normally open switch S.sub.1a in parallel and in reverse
polarity with respect to the potential of said first capacitor Ca;
a second diode Db connected to said second normally open switch
S.sub.1b in parallel and in reverse polarity with respect to the
potential of said second capacitor Cb; and said control unit 7 controls
said first and second normally open switches S.sub.1a, S.sub.1b
so as to be alternately closed for a short time at a predetermined
constant timing.
3. In an electromagnetic flow meter having: a pipe of non-magnetic
and non-conductive material through which conductive fluid flows;
a pair of electrodes which are provided oppositely on said pipe
orthogonal to the flow direction of said fluid and in contact with
said fluid; a magnetic circuit for introducing magnetic flux in
the direction orthogonal to the flow direction and a straight line
connecting said electrodes, an exciting coil for producing said
magnetic flux to be introduced into said magnetic circuit; an exciting
current generating unit for generating exciting current to be supplied
to said exciting coil; and a fluid flow velocity calculating unit
for calculating fluid flow velocity on the basis of the voltage
produced between said electrodes due to said magnetic flux and the
fluid flow; said electromagnetic flow meter comprising: a control
unit for controlling said exciting current generating unit and said
fluid flow calculating unit in such a manner that said exciting
current generating unit intermittently produces said exciting current
with an alternately reversed polarity and a short time duration
and said fluid flow velocity calculating unit calculates the velocity
of the fluid flow only during a predetermined constant period from
a point in time when the residual flux retained by said magnetic
circuit, after extinguishing said exciting current, reaches a stable
state to a point in time when the subsequent exciting current is
initiated, wherein a reference-exceeding value detecting circuit
is provided for producing an excess signal by detecting the fact
that potential of one end of a resistor, which is proportional to
said exciting current, exceeds a predetermined reference value,
said resistor being connected in series in a circuit into which
said exciting current flows, and said control unit having the function
of immediately disconnecting the supply of said exciting current
generating unit when said control unit receives said excess signal.
4. An electromagnetic flow meter according to claim 3 wherein
said control unit comprises first and second flip-flops for controlling
first and second normally open switches respectively so as to be
closed during a period from receiving a set signal to receiving
a reset signal; a pulse circuit for supplying said set signal alternately
at a predetermined timing; said first and second flip-flops being
connected to said reference-exceeding value detecting circuit to
be received with said excess signal therefrom as said reset signal.
5. An electromagnetic flow meter according to claim 3 wherein
said reference-exceeding value detecting circuit comprises an absolute
rectifier for producing a positive potential proportional to an
absolute value in accordance with said potential of one end of said
resistor regardless of the polarity of said potential; and a comparator
having a minus input terminal connected to the output of said absolute
rectifier and a plus input terminal connected to a constant reference
voltage source, the output of said comparator being applied to said
control unit as said excess signal.
6. An electromagnetic flow meter according to claim 3 wherein
said reference-exceeding value detecting circuit comprises a Schmitt
trigger circuit having an input terminal for receiving said potential
of one end of said resistor for producing a rectangular wave form
reversing in polarity when said potential exceeds a predetermined
value; a differentiation circuit having an input terminal for receiving
the output from said Schmitt trigger circuit for producing pulses
alternately changed in polarity whenever the polarity of the output
from said Schmitt trigger circuit is changed; and an absolute rectifier
for outputting signals with a constant polarity by processing the
output from said differentiation circuit, the output of said absolute
rectifier being applied to said control unit as said excess signal.
7. An electromagnetic flow meter according to claim 3 wherein
said magnetic circuit comprises a pair of magnetic poles, and said
exciting coil and poles are substantially symmetrical with respect
to a bisector of a straight line connecting said electrodes, which
passes through the center of said pipe.
8. An electromagnetic flow meter according to claim 3 wherein
said pipe is made of synthetic resin material and is partially flattened
at least in the vicinity of a location of said electrodes.
9. In an electromagnetic flow meter having: a pipe of non-magnetic
and non-conductive material through which conductive fluid flows;
a pair of electrodes which are provided oppositely on said pipe
orthogonal to the flow direction of said fluid and in contact with
said fluid; a magnetic circuit for introducing magnetic flux in
the direction orthogonal to the flow direction and a straight line
connecting said electrodes, an exciting coil for producing said
magnetic flux to be introduced into said magnetic circuit; an exciting
current generating unit for generating exciting current to be supplied
to said exciting coil; and a fluid flow velocity calculating unit
for calculating fluid flow velocity on the basis of the voltage
produced between said electrodes due to said magnetic flux and the
fluid flow; said electromagnetic flow meter comprising: a control
unit for controlling said exciting current generating unit and said
fluid flow calculating unit in such a manner that said exciting
current generating unit intermittently produces said exciting current
with an alternately reversed polarity and a short time duration
and said fluid flow velocity calculating unit calculates the velocity
of the fluid flow only during a predetermined constant period from
a point in time when the residual flux retained by said magnetic
circuit, after extinguishing said exciting current, reaches a stable
state to a point in time when the subsequent exciting current is
initiated, wherein said magnetic circuit is a yoke made of high
permeability material with comparatively high coercive force.
10. In an electromagnetic flow meter having: a pipe of non-magnetic
and non-conductive material through which conductive fluid flows;
a pair of electrodes which are provided oppositely on said pipe
orthogonal to the flow direction of said fluid and in contact with
said fluid; a magnetic circuit for introducing magnetic flux in
the direction orthogonal to the flow direction and a straight line
connecting said electrodes, an exciting coil for producing said
magnetic flux to be introduced into said magnetic circuit; an exciting
current generating unit for generating exciting current to be supplied
to said exciting coil; and a fluid flow velocity calculating unit
for calculating fluid flow velocity on the basis of the voltage
produced between said electrodes due to said magnetic flux and the
fluid flow; said electromagnetic flow meter comprising: a control
unit for controlling said exciting current generating unit and said
fluid flow calculating unit in such a manner that said exciting
current generating unit intermittently produces said exciting current
with an alternately reversed polarity and a short time duration
and said fluid flow velocity calculating unit calculates the velocity
of the fluid flow only during a predetermined constant period from
a point in time when the residual flux retained by said magnetic
circuit, after extinguishing said exciting current, reaches a stable
state to a point in time when the subsequent exciting current is
initiated, wherein said magnetic circuit is comprised of a series
coupling of a yoke made of high permeability material with low residual
magnetic flux and a magnetic member made of permanent magnetic material.
11. In an electromagnetic flow meter having: a pipe of non-magnetic
and non-conductive material through which conductive fluid flows;
a pair of electrodes which are provided oppositely on said pipe
orthogonal to the flow direction of said fluid and in contact with
said fluid; a magnetic circuit for introducing magnetic flux in
the direction orthogonal to the flow direction and a straight line
connecting said electrodes, an exciting coil for producing said
magnetic flux to be introduced into said magnetic circuit; an exciting
current generating unit for generating exciting current to be supplied
to said exciting coil; and a fluid flow velocity calculating unit
for calculating fluid flow velocity on the basis of the voltage
produced between said electrodes due to said magnetic flux and the
fluid flow; said electromagnetic flow meter comprising: a control
unit for controlling said exciting current generating unit and said
fluid flow calculating unit in such a manner that said exciting
current generating unit intermittently produces said exciting current
with an alternately reversed polarity and a short time duration
and said fluid flow velocity calculating unit calculates the velocity
of the fluid flow only during a predetermined constant period from
a point in time when the residual flux retained by said magnetic
circuit, after extinguishing said exciting current, reaches a stable
state to a point in time when the subsequent exciting current is
initiated, wherein the cross section of said pipe is substantially
circular at least in the vicinity of a location where said electrodes
are disposed.
12. In an electromagnetic flow meter having: a pipe of non-magnetic
and non-conductive material through which conductive fluid flows;
a pair of electrodes which are provided oppositely on said pipe
orthogonal to the flow direction of said fluid and in contact with
said fluid; a magnetic circuit for introducing magnetic flux in
the direction orthogonal to the flow direction and a straight line
connecting said electrodes, an exciting coil for producing said
magnetic flux to be introduced into said magnetic circuit; an exciting
current generating unit for generating exciting current to be supplied
to said exciting coil; and a fluid flow velocity calculating unit
for calculating fluid flow velocity on the basis of the voltage
produced between said electrodes due to said magnetic flux and the
fluid flow; said electromagnetic flow meter comprising: a control
unit for controlling said exciting current generating unit and said
fluid flow calculating unit in such a manner that said exciting
current generating unit intermittently produces said exciting current
with an alternately reversed polarity and a short time duration
and said fluid flow velocity calculating unit calculates the velocity
of the fluid flow only during a predetermined constant period from
a point in time when the residual flux retained by said magnetic
circuit, after extinguishing said exciting current, reaches a stable
state to a point in time when the subsequent exciting current is
initiated, wherein the arrangement of said magnetic circuit and
said exciting coil is substantially symmetrical with respect to
a straight line connecting said pair of electrodes.
13. In an electromagnetic flow meter having: a pipe of non-magnetic
and non-conductive material through which conductive fluid flows;
a pair of electrodes which are provided oppositely on said pipe
orthogonal to the flow direction of said fluid and in contact with
said fluid; a magnetic circuit for introducing magnetic flux in
the direction orthogonal to the flow direction and a straight line
connecting said electrodes, an exciting coil for producing said
magnetic flux to be introduced into said magnetic circuit; an exciting
current generating unit for generating exciting current to be supplied
to said exciting coil; and a fluid flow velocity calculating unit
for calculating fluid flow velocity on the basis of the voltage
produced between said electrodes due to said magnetic flux and the
fluid flow; said electromagnetic flow meter comprising: a control
unit for controlling said exciting current generating unit and said
fluid flow calculating unit in such a manner that said exciting
current generating unit intermittently produces said exciting current
with an alternately reversed polarity and a short time duration
and said fluid flow velocity calculating unit calculates the velocity
of the fluid flow only during a predetermined constant period from
a point in time when the residual flux retained by said magnetic
circuit, after extinguishing said exciting current, reaches a stable
state to a point in time when the subsequent exciting current is
initiated, wherein said exciting current generating unit 6 is comprised
of a first series connection of a first normally open switch S.sub.1a
and a first capacitor Ca; a second series connection of a second
normally open switch S.sub.1b and a second capacitor Cb, said first
and second series connections being connected to both ends of said
exciting coil 5 in parallel; series connections of DC power sources
E.sub.oa, E.sub.ob and resistors Ra, Rb being connected to said
first and second capacitors Ca, Cb respectively or in common in
such a manner that said first and second capacitors Ca, Cb are charged
with opposite polarity to obtain opposite current flows into said
exciting coil 5 when said first and second normally open switches
S.sub.1a, S.sub.1b are selectively closed; a first diode Da connected
to said first normally open switch S.sub.1a in parallel and in reverse
polarity with respect to the potential of said first capacitor Ca;
a second diode Db connected to said second normally open switch
S.sub.1b in parallel and in reverse polarity with respect to the
potential of said second capacitor Cb; and said control unit 7 controls
said first and second normally open switches S.sub.1a, S.sub.1b
so as to be alternately closed for a short time at a predetermined
constant timing;
wherein said control unit comprises first and second flip-flops
for controlling said first and second normally open switches respectively
so as to be closed during a period from receiving a set signal to
receiving a reset signal; a pulse circuit for supplying said set
signal alternately at a predetermined timing; said first and second
flip-flops being connected to a reference-exceeding value detecting
circuit for detecting exciting current exceeding a threshold value
and generating a resultant signal for resetting the flip-flops.
14. In an electromagnetic flow meter having: a pipe of non-magnetic
and non-conductive material through which conductive fluid flows;
a pair of electrodes which are provided oppositely on said pipe
orthogonal to the flow direction of said fluid and in contact with
said fluid; a magnetic circuit for introducing magnetic flux in
the direction orthogonal to the flow direction and a straight line
connecting said electrodes, an exciting coil for producing said
magnetic flux to be introduced into said magnetic circuit; an exciting
current generating unit for generating exciting current to be supplied
to said exciting coil; and a fluid flow velocity calculating unit
for calculating fluid flow velocity on the basis of the voltage
produced between said electrodes due to said magnetic flux and the
fluid flow; said electromagnetic flow meter comprising: a control
unit for controlling said exciting current generating unit and said
fluid flow calculating unit in such a manner that said exciting
current generating unit intermittently produces said exciting current
with an alternately reversed polarity and a short time duration
and said fluid flow velocity calculating unit calculates the velocity
of the fluid flow only during a predetermined constant period from
a point in time when the residual flux retained by said magnetic
circuit, after extinguishing said exciting current, reaches a stable
state to a point in time when the subsequent exciting current is
initiated, wherein said exciting current generating unit comprises
a first fixed resistor with a large resistance value relative to
said exciting coil, connected in series with said exciting coil;
a capacitor; a normally open switch connected to said capacitor;
a polarity inverting switch, both ends of a series connection of
said capacitor and said normally open switch being connected to
both ends of a series connection of said exciting coil and said
first fixed resistor through said polarity inverting switch; a stabilizing
DC power source; and a second fixed resistor with a high resistance
value connected to said DC power source in series, said capacitor
being connected in parallel with a series connection of said second
fixed resistor and said DC power source; and said control unit controls
said normally open switch and said polarity inverting switch with
predetermined timings respectively in such a manner that a connection
polarity of said polarity inverting switch is changed at a predetermined
constant timing and said normally open switch is closed for a short
time at an intermediate timing of said predetermined constant timing,
wherein the cross section of said pipe is substantially rectangular
at least in the vicinity of a location where said electrodes are
disposed.
Description The present invention relates to an electromagnetic flow meter
which has a pair of electrodes facing each other for directly sandwiching
at least part of a conductive fluid and a magnetic circuit for developing
a magnetic flux crossing a straight line connecting both the electrodes
and the flow of the conductive fluid whereby a velocity of flow
of a fluid is calculated by using a voltage developed between both
the electrodes in accordance with the magnetic flux of the magnetic
circuit and the flow of the fluid.
In the electromagnetic flow meter, the use of a permanent magnet
for forming a magnetic circuit is ideal when attention is given
to only the reduction of power consumption. The permanent magnet,
however, is rarely used since the voltage polarities of both the
electrodes are invariable and contact voltage and polarized voltage
are produced by an electrochemical action, thus bringing about a
great zero shift or drift.
The use of a magnetic circuit by AC excitation solves the just-mentioned
problem. This approach, however, is accompanied by 90.degree. noise.
The summation of it with the common mode noise does not necessarily
provide a phase difference of 90.degree.. Therefore, it is difficult
to check the zero point during the course of the fluid flow.
When a magnetic circuit that is magnetically excited by a square
wave current with alternately changing polarities is used, the difficulty
in the case of using a permanent magnet is not involved and an amount
of the zero shift during the flow of the fluid may be obtained from
the average of those values obtained when there is not flux change
with respect to both the current directions. Therefore, correction
may be realized by using the zero drift obtained. This approach,
however, has a circuit design to make the exciting current constantly
flow. The result is a large power consumption.
The above-mentioned prior art is disclosed in U.S. Pat. Nos. 3783687
3802262 3894430 4010644 and 3777561.
Accordingly, an object of the present invention is to provide an
electromagnetic flow meter with less power consumption.
Another object of the present invention is to provide an electromagnetic
flow meter able to stably measure the velocity of flow of a fluid
which is free from an error due to a change of ambient temperature.
Yet another object of the present invention is to provide an electromagnetic
flow meter which is insensitive to a variation of a power source
voltage.
One of the features of the present invention produces, in a magnetic
circuit, a square wave magnetic flux with alternately changing polarities
which continue for a slight amount of time in each cycle but intermittently,
thereby ensuring the advantage of the conventional electromagnetic
flow meter. Another feature, which provides a reduction of power
consumption, causes a magnetic exciting current into a magnetic
exciting coil. The current is intermittent and instantaneous (i.e.
short time) in each cycle with alternately changing polarities,
and calculates a velocity of flow of a fluid by using a voltage
produced in accordance with a residual magnetic flux which is held
in the magnetic circuit and a flow velocity of the fluid when no
magnetic exciting current flows therethrough .
The above and other objects, features and advantages of the present
invention will be more clear from the following description with
reference to the accompanying drawings, in which:
FIG. 1 is a schematic diagram of a first embodiment of the present
invention;
FIG. 2 is a graphical representation representing a relation between
a magnetic field and a magnetic flux;
FIG. 3 is a circuit diagram representing the first embodiment of
the present invention;
FIG. 4A is a waveform illustrating the operating state of switch
S.sub.2 shown in FIG. 3.
FIG. 4B illustrates the waveform of the operating states of switch
S.sub.1 shown in FIG. 3.
FIG. 4C is a waveform illustrating the exciting current I flowing
through exciting coils 5a and 5b illustrated in FIG. 3.
FIG. 4D is a waveform illustrating residual magnetic flux produced
in magnetic poles 4a and 4b illustrated in FIG. 3.
FIG. 4E is a waveform of a voltage produced between electrodes
2a and 2b illustrated in FIG. 3.
FIG. 4F is a waveform of the sampling pulses received by sampling
circuit 32 shown in FIG. 3.
FIG. 4G illustrates the output waveform from the sampling circuit
32.
FIG. 4H is a waveform of the output from a synchronous rectifier
34 illustrated in FIG. 3.
FIG. 4I is a waveform illustrating the output from smoothing circuit
36 illustrated in FIG. 3.
FIG. 5 is a schematic diagram of a second embodiment of the present
invention;
FIG. 5A is a modification of the embodiment shown in FIG. 5.
FIG. 6 is a graph illustrating a relation between a magnetic field
and a magnetic flux in connection with the second embodiment;
FIG. 7 is a cross sectional view of a modification of the second
embodiment;
FIG. 8 is a longitudinal sectional view taken along line VIII--VIII
in FIG. 7;
FIG. 9 is a cross sectional view of a further modification of the
second embodiment;
FIG. 10 is a longitudinal sectional view taken along line X--X
in FIG. 9;
FIG. 11 is a circuit construction of a third embodiment of the
present invention, wherein electrodes and a fluid flow velocity
calculating unit are omitted because they are the same as the first
embodiment;
FIG. 12 shows a graph of an exciting current in the third embodiment;
FIG. 13 is a circuit diagram of a modification of part of the third
embodiment.
FIG. 1 is a schematic illustration of a first embodiment of the
present invention and, more particularly a cross sectional view
orthogonal to a flow direction of a fluid. A pipe 1 for fluid, made
of non-magnetic and non-conductive material, extends over an extreme
range near the cross section but along the longitudinal direction
relative to the cross section. There are provided electrodes 2a
and 2b on the inner walls of the pipe 1 which are oppositely disposed
while directly sandwiching part of the fluid flow and exposed to
the fluid. The electrodes 2a and 2b are connected to a fluid flow
velocity calculating unit 3 for calculating the velocity of a fluid
flow by using a voltage between both the electrodes 2a and 2b, by
way of conductive portions which water-tightly pass through the
side walls of the pipe 1 respectively.
A magnetic circuit 4 with magnetic poles 4a and 4b sandwiching
the pipe 1 is provided to develop between the magnetic poles 4a
and 4b, a magnetic flux crossing a straight line connecting both
the electrodes 2a and 2b and the flow of the fluid.
Preferable material of the magnetic circuit 4 has an easily magnetized
characteristic with a high permeability but with a relatively high
coercive force. The embodiment employs usual steel.
The magnetic poles 4a and 4b have respectively exciting coils 5a
and 5b wound therearound for exciting the magnetic circuit 4. The
exciting coils each have a large diameter with the total resistance
being small.
Both the exciting coils 5a and 5b are connected in series and coupled
to an exciting current generating unit 6 for feeding exciting current
to the coil which is intermittent and instantaneous (for a short
time) and is alternately changed in current direction.
When the exciting current produces magnetic fields +Hp and -Hp,
developed in the magnetic circuit 4 magnetic density B, developed
between the magnetic poles 4a and 4b, varies as indicated by a curve
in FIG. 2. When the exciting current reduces to zero, the magnetic
field passes a point of O and is settled down at a point P or P'
in the figure, because the permeance between the magnetic poles
4a and 4b is small. Under this condition the magnetic flux density
B is depicted by line segments Ob and Ob' in the figure.
To enlarge the residual magnetic flux, it is preferable to form
a rectangular cross section of pipe 1 and to narrow a gap between
the magnetic poles 4a and 4b, thereby enlarging permeance.
The exciting current generating unit 6 and the fluid flow velocity
calculating unit 3 are both responsive to pulses delivered from
a control unit 7 which produces given signals in response to pulses
generated from a single oscillator, and periodically operates with
a fixed time relation.
The control unit 7 is comprises of an oscillator 72 for producing
a signal with a given frequency, a frequency divider 74 for dividing
the frequency, and a gate circuit 76. Signals PS.sub.1 and PS.sub.2
for controlling switches S.sub.1 and S.sub.2 (discussed hereinafter)
a signal SP for controlling a sampling circuit 32 and a signal SR'
for controlling a synchronous rectifier 34 are produced with a predetermined
interrelation in timing by the divider 74. These signals are transmitted
through the gate circuit 76.
The exciting current generating unit 6 like a circuit on the upper
half of FIG. 3 has a fixed resistor r with much greater resistance
than that of coils 5a and 5b. Resistor r is inserted in series in
a series circuit of the exciting coils 5a and 5b, in order to substantially
eliminate adverse influence by a temperature change of the cooper
resistance of the exciting coils. A series connection including
a capacitor C and a normally open switch S.sub.1 with a short closed
time is further provided. Both ends of the series connection are
connected between both ends of a series connection including the
coils 5a and 5b and fixed resistor r, through a polarity inverting
switch S.sub.2. The capacitance C is connected across a series circuit
including a resistor r with a high resistance and a stabilizing
DC power source. In practical use, the switch S.sub.1 having close
duration of a short time, and the polarity inverting switch S.sub.2
may be constructed by electronic circuits having equivalent functions,
which operate under control of periodic pulses PS.sub. 1 and PS.sub.2
produced by the control unit 7. The operating states of switches
S.sub.1 and S.sub.2 are illustrated in FIGS. 4(A) and 4(B), in which
the abscissa represents time and the ordinate represents the operating
states of the switches S.sub.1 and S.sub.2. As shown, the connecting
polarity of the switch S.sub.2 is periodically inverted and the
short-time close switch S.sub.1 is closed for a very short time
in the midportion of each polarity continuing period, and then is
opened immediately.
As a result of such a switching operation, an exciting current
I flows through the exciting coils 5a and 5b, the current having
a recurring pulse waveform with alternating opposite polarities.
The result is that, as seen from the graph of FIG. 2 a residual
magnetic flux is produced in the magnetic poles 4a and 4b, the waveform
of which has a substantially fixed amplitude following a sharp leading
edge, as shown in FIG. 4(D).
Where flow velocity of the fluid in pipe 1 gradually decreases,
FIG. 4(E) illustrates that a voltage is produced between the electrodes
2a and 2b which changes in proportion to the product of the signal
shown in FIG. 4(D) and the decreasing flow velocity shown dotted
in FIG. 4(E).
The detail of the fluid flow velocity calculating unit 3 is illustrated
in block form in the lower half of FIG. 3. The voltage between the
electrodes 2a and 2b is amplified by a first amplifier A.sub.1 and
is applied to a sampling circuit 32. The sampling circuit 32 receives
sampling pulses SP as shown in FIG. 4(F), and the sampling pulses
SP define such a relation between the pulses PS.sub.1 and PS.sub.2
so that the portion of the wave form of FIG. 4(E), at which the
residual magnetism is already stable, can be sampled but the portion
of the wave form of FIG. 4(E), at which the residual magnetism is
sharply changed, can not be sampled. By such sampling pulses, the
sampling circuit 32 produces from the output of the first amplifier
A.sub.1 an output signal, the wave form of which has different
amplitudes and alternately changing polarity. The output signal
from the sampling circuit 32 is amplified by a second amplifier
A.sub.2 and is synchronously rectified by the synchronous rectifier
34 to obtain intermittent outputs changing proportionally to the
flow velocity of the fluid, as indicated by SR of FIG. 4(H). The
intermittent outputs with a fixed polarity are time-averaged by
a smoothing circuit 36 to obtain a smoothed change, as indicated
by m in FIG. 4(I), which in turn transfers the information of an
instantaneous flow speed to a meter 38. In FIGS. 4(D), 4(E), 4(G)
and 4(H), dotted lines indicate tendencies of changes of the upper
and lower limit values.
With the above-mentioned construction, by properly selecting the
frequency of the exciting current, which is determined by the output
pulse from the control unit 7 it is possible to restrict the zero
shift, which is due to electrochemical effects at the electrodes
2a and 2b when no exciting current flows. The restriction may be
made to a practical allowance, and the average flow velocity over
an entire range of the flow velocity measuring time may be made
substantially equal to the average flow velocity only during the
sampling pulses, even if there is a flow velocity variation during
the period other than the sampling pulse period.
In the electromagnetic flow meter of the first embodiment according
to the invention, period of the exciting current is a very short-time,
so that the power consumption is extremely small thereby attaining
the object as mentioned above.
Since the polarity of the voltage between the electrodes 2a and
2b is inversed each cycle, it is not feared that a great zero shift
is caused by electrochemical action. Further, the measurement of
the change of the residual magnetic flux is made only during the
period which is negligible from a practical viewpoint. Therefore,
it is easy to check the zero point even if the fluid is flowing.
The influence of a change of the resistance value of the exciting
coil, which is caused by the change of ambient temperature, upon
the magnitude of the residual magnetic flux, may be reduced to be
extremely small by connecting an exciting coil in series with a
temperature compensating resistor having a small resistance temperature
coefficient but a large resistance. Unless the material forming
the magnetic circuit is properly selected, however, the residual
magnetic flux changes from variations of ambient temperature, even
if a means to make the exciting current constant is used. This results
in an error in the measured flow velocity.
A second embodiment of the present invention is directed to overcome
the above-mentioned defect of the electromagnetic flow meter of
the intermittent (i.e. short time), inverting and exciting type.
In this embodiment, the magnetic circuit is comprised of a yoke
made of high permeability material with low residual magnetic flow
and a magnetic member of permanent magnetic material, both being
coupled in series with each other.
FIG. 5 shows a schematic diagram of the second embodiment of the
invention with the cross sectional view orthogonal to the flow of
a conductive fluid. In the figure, like numerals are used to designate
like or equivalent portions or parts in the first embodiment. The
pipe 1 for fluid, which is non-magnetic and non-conductive, extends
over a necessary long range in the longitudinal direction of the
cross section. Electrodes 2a and 2b are provided which are disposed
oppositely sandwiching at least part of the flow of the fluid and
exposed to the interior of the pipe 1. The electrodes 2a and 2b
are connected through conductive portions which water-tightly pass
through the walls of the pipe 1 to the flow velocity calculating
unit 3 which calculates the flow velocity of the fluid.
Yokes 4c and 4d, having magnetic poles 4a and 4b with the pipe
1 intervening therebetween, are made of high permeability material
with low residual magnetic flux such as electromagnetic soft iron
or silicon steel plate. The magnetic flux developed between the
magnetic poles 2a and 2b crosses a straight line connecting both
the electrodes 2a and 2b and the fluid flow. The yokes 4c and 4d
hold therebetween a magnetic member 8 for example, casting ALNICO,
for which the coercive force is not so large. The magnetic flux
steeply rises and the magnetic saturation is rapid, thereby forming
a single magnetic circuit 4.
The magnetic member 8 is wound by the exciting winding 5 to excite
the magnetic circuit 4 of which the diameter is large and the overall
resistance is small, as in the first embodiment. The exciting coil
5 is connected to the exciting current generating unit 6 which is
similar to that shown in FIG. 3.
A relation between a magnetic field and flux density, when the
permanent magnet material constituting the magnet member 8 is placed
in a magnetic field which reciprocally changes with a sufficiently
large width, is plotted by a continuous line in FIG. 6 in this
example.
In the magnetic circuit 4 of the embodiment in FIG. 5 permeability
of the yokes 4c and 4d is 8000 to 12000 times that of a gap between
the magnetic poles 4a and 4b. Therefore, a magnetomotive force developed
in the gap by the current flowing through the exciting coil 5 is
practically equal to that applied between the end surfaces of the
magnet member 8. Accordingly, a relation of the magnetic flux to
the magnetic field caused by the magnetic flux is determined by
a value which is a conversion of the permeance Pg between the magnetic
poles 4a and 4b into a permeance coefficient Pm of the magnet member
8 which is made by using a relation Pm=Pg.times.lm/Sm where 1m and
Sm are a length and a cross sectional area of the magnet member
8 respectively. The permeance between the magnetic poles 4a and
4b is geometrically determined and hence the relation of the flux
density of the magnet member 8 to the magnetic field caused by it
is fixed and may be expressed by slanted line P-P' in FIG. 6.
When a change of the magnetic field H produced in the magnet member
8 by the current flowing through the exciting coil 5 by the exciting
current generating unit 6 falls within a range from +h to -h' in
FIG. 6 the relation between the magnetic field H and the magnet
member changes along a two-dot chain line Q-Q' in FIG. 6. When it
falls out of the range but within the range +H to -H, it changes
along broken line R-P-R'-P'. Therefore, when no exciting current
flows through the exciting coil 5 points P and P', which are crosspoints
the line R-P-R'-P' and the slanted line P-P', are stable and the
residual magnetic flux at this time is B or -B'. If both the exciting
currents are equal to each other, B=B'.
As seen from the relation Pm=Pg.times.lm/Sm, the permeance Pm is
larger as the cross sectional view Sm of the magnetic member 8 is
selected smaller, and the slanted line P-P' more steeply slants
and the absolute values of the residual magnetic densities B and
B' become larger.
As described above, in the second embodiment of the magnetic flow
meter, the magnetic circuit 4 is constructed by coupling the yokes
4c and 4d which are made of high permeability material with high
residual magnetic flux in series with the magnet member 8 of permanent
magnet. Accordingly, the influence of the temperature change on
the residual magnetic flux is determined only by the magnet member
8. In the case of the permanent magnet material for the magnetic
member 8 a variation of the residual flux density is extremely
small, for example, -(0.06 to 0.02)%/.degree.C. Therefore, only
a small variation of the magnetic flux in the gap between the magnetic
poles 4a and 4b exists. As a result, the defect of the first embodiment,
namely, the ambient-temperature change which causes an error in
the measured flow velocity is removed.
In the embodiment of FIG. 5 a single magnetic circuit sandwiching
a single permanent magnet is shown, and the embodiment can be modified,
as shown in FIG. 5A, which illustrates therefor a construction including
two magnetic circuits, each of which sandwiches a respective permanent
magnet. The two magnetic circuits are symmetrically arranged with
respect to a centrally arranged pipe. Identical numerals are employed
for corresponding parts in FIGS. 5 and 5A.
A modification of the second embodiment as mentioned above is illustrated
in FIGS. 7 and 8 which are its cross sectional view and its longitudinal
view. As shown, the gap between the magnetic poles 4a and 4b is
narrowed by partially flattening the cross section of the fluid
section of the fluid path of the pipe 1 thereby increasing the
permeance of the magnetic circuit 4. Magnet members 8a and 8b and
exciting windings connected in series to each other are provided.
The yoke is comprised of portions 4c and 4d between the magnetic
poles 4a and 4b and one end of the magnetic members 8a and 8b, and
a portion 4e between the other ends of the magnet members 8a and
8b. Parts 4c, 4d and 4e as well as the lead wires of the exciting
coils 5a and 5b and the electrodes 2a and 2b, when the pipe 1 is
molded by plastic of non-magnetic and insulating material, are at
the same time held in place by an insert molding method. Provided
on the upper side of the pipe 1 is a single processing unit 9 integrally
including the exciting current generating unit 6 connected to the
exciting coils 5a and 5b, the flow velocity calculating unit 3 connected
to the electrodes 2a and 2b, a battery, and an oscillator.
The pipe 1 is hermetically screwed at both ends into a metal coupling
piece 10 and is coupled with the related tubing by the screw of
the coupling piece 10. The pipe 1 has a tensile strength enough
to withstand stretching in the longitudinal direction when it is
installed.
In this modification as mentioned above, the magnetic circuit 4
may be designed to be small as a whole keeping the satisfactory
residual flux density by properly designing the magnetic members
8a and 8b and the yokes 4e, 4c and 4d. This leads to cost reduction.
The first and second embodiments of the present invention are applicable
for an electromagnetic flow meter with an arrangement that the electrodes
2a and 2b and the magnetic electrodes 4a and 4b are disposed symmetrically
with respect to the cross section of the pipe 1 as viewed horizontally
but assymetrically disposed as viewed vertically. Such an arrangement
is illustrated in FIG. 9 of the cross section and in FIG. 10 of
the longitudinal section.
The first embodiment of FIGS. 1 and 3 and the second embodiment
of FIGS. 5 7 and 8 have a drawback. When the stabilizing DC power
source is not employed for the power source Eo of the exciting current
generating unit 6 the peak value of the exciting current (FIG.
4(D)) and thus the average value (FIG. 4(I)) varies due to the variation
of the voltage independently of an actual instantaneous flow velocity
of the fluid.
A third embodiment of the invention to be described hereinafter
aims at overcoming the just-mentioned drawback in the electromagnetic
flow meter of the intermittent, inverting, and instantaneous (short
time) exciting type. In this embodiment, in the instantaneous rise
process of the exciting current in each cycle, when the absolute
value thereof exceeds a fixed value, it is immediately reduced to
zero.
FIG. 11 of the third embodiment illustrates the circuit of the
exciting current generating unit 6 together with the control unit
7 and the exciting coil 5.
The third embodiment also has electrodes and a fluid flow velocity
calculating unit similar to that of FIG. 3 though they are omitted
in FIG. 11. Therefore, the output signals SP and SR' of the control
unit 7 are applied to the sampling circuit 32 and the synchronous
rectifier 34 respectively.
The control unit 7 is composed of a pulse generating circuit comprising
the oscillator 72 for generating pulses with a predetermined frequency,
the divider 74 for dividing the frequency and the gate circuit 76
and first and second flip-flops 78 and 80. The signal SP for controlling
the sampling circuit 32 of the calculating unit 3 and the signal
SR' for controlling the rectifier 34 are produced by the divider
74 with a predetermined timing relation therebetween as stated with
regard to FIG. 4 and transmitted to the sampling circuit 32 and
the rectifier 34 respectively through the gate circuit 76. The first
and second flip-flops 78 and 80 alternately receive a set signal
with a fixed interval of time through the divider 74 and the gate
circuit 76 in accordance with the first and second switches S.sub.1a
and S.sub.2a. The flip-flops 78 and 80 receive an excess signal
from a reference-exceeding value detecting circuit 6-2 which is
described hereinafter as a reset signal. Thus, the first or second
switch S.sub.1a or S.sub.1b receives a signal from the corresponding
flip-flop to be closed only during the period from receiving of
the set signal to receiving of the reset signal.
The exciting current generating unit 6 as shown in FIG. 11 is comprised
of a first series connection of a normally open switch S.sub.1a
and a capacitor Ca. A second series connection exists between a
normally open switch S.sub.1b and a capacitor Cb. The first and
second series connections are each connected in parallel to the
series connection of the exciting coil 5 and a resistor Rs. Respective
series connections exist for DC power source Eoa, resistor Ra, capacitor
Ca, as well as source Eob, resistor Rb and capacitor Cb. The first
and second capacitors Ca, Cb are charged with opposite polarities
to obtain opposite current flows into the exciting coil 5 when the
first and second normally open switches S.sub.1a, S.sub.1b are selectively
closed. A first diode Da is connected to the first normally open
switch S.sub.1a, in parallel, and in reverse polarity with respect
to the potential of the first capacitor Ca. A second diode Db is
connected to the second normally open switch S.sub.1b, in parallel,
and in reverse polarity with respect to the potential of the second
capacitor Cb. The control unit 7 controls the first and second normally
open switches S.sub.1a, S.sub.1b so as to be alternately closed
for a short time at a predetermined constant timing.
An electronic circuit for achieving switching operation and equivalent
to that of the switches S.sub.1a and S.sub.1b may be used for the
normally open switches S.sub.1a and S.sub.1b. Each of the diodes
Da and Db closes as surge voltage is produced by the exciting coil
5 immediately after each of the switches S.sub.1a and S.sub.1b is
opened, and current is fed back to each of the capacitors Ca and
Cb by each of the diodes Da and Db.
One end of the stabilizing resistor Rs is connected to ground.
Therefore, the potential at the other terminal Q changes in accordance
with the exciting current flow.
In the embodiment shown in FIG. 11 the potential at the point
Q is applied to an absolute rectifier AR.sub.1 which converts it
into a positive potential which is proportional to its absolute
value irrespective of the polarity thereof. The output of the rectifier
is applied to the negative input terminal of a comparator CP. A
fixed reference voltage relative to the ground potential is applied
to the input terminal of the comparator CP, from a reference voltage
supply Ec. The comparator CP produces an output signal when the
input signal at the negative input terminal exceeds that at the
positive input terminal. Every time the flip-flops of the control
unit 7 receive this output from the comparator CP, they control
the first or second switch S.sub.1a or S.sub.1b to immediately open
if the switch is closed. By the absolute rectifier AR.sub.1 the
comparator CP and the reference voltage supply E.sub.c, a reference-exceeding
value detecting circuit 6-2 for detecting the fact that the exciting
current exceeds the reference voltage is composed.
In FIG. 11 the first and second DC power sources E.sub.oa and
E.sub.ob, a connection point of which is grounded, and the resistors
R.sub.a and R.sub.b with high resistance may be replaced by a single
DC power source E and a single resistor R with high resistance to
achieve the same advantage as the circuit of FIG. 3.
With such a circuit arrangement, due mainly to the inductance of
the exciting coil 5 the absolute value of the exciting current
increases from zero through a given process and decreases toward
zero through a given process when the first or second switch S.sub.1a
or S.sub.1b is closed or opened, as shown in FIG. 12 illustrating
the waveforms with respect to an enlarged time scale. The absolute
value of the exciting current at an instant that the switch S.sub.1a
or S.sub.1b is opened is at maximum, and it is fixed every cycle
with respect to the voltage of the reference voltage supply Ec.
This reference-exceeding value detecting circuit 6-2 in FIG. 11
may be substituted by a circuit arrangement shown in FIG. 13. In
FIG. 13 with the provision of a Schmitt trigger circuit, a rectangular
waveform is produced, the polarity of which is inverted every time
that the exciting current exceeds a constant value which is determined
by resistors r1 and r2. This is processed by a differential circuit
DIF to form a pulse, the polarity of which changes every time the
polarity is changed. The formed pulse is processed by the absolute
value rectifier AR.sub.2 to form an output signal similar to that
from the comparator CP in the circuit shown in FIG. 1. The output
signal thus formed is transferred to the flip-flops of the control
unit 7.
According to the exciting current generating unit 6 as shown in
FIG. 11 or FIG. 13 in which the unit 6 of FIG. 11 is modified, the
maximum value of the exciting current is invariable even though
the voltage of the first or second DC power source Eoa or Eob varies.
As a result, the reduction of the measuring accuracy due to the
voltage variation may be avoided without using a stabilized DC power
source, whereby the above-mentioned drawback may be eliminated. |