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Claims  |
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What is claimed is:
1. An electromagnetic flowmeter comprising
a conduit (27);
a pair of electrodes (28a,28b) disposed in said conduit (27) at
diametrically opposing positions;
an electromagnet disposed adjacent said conduit (27) and having an exciting
coil (25) for generating a magnetic field within said conduit (27);
a driving means (13 etc) connected to said exciting coil (25) for exciting
said electromagnet;
control means (30) connected to said driving means for controlling the
driving means; and
detecting means (29) connected to said electrodes in said conduit (27) for
detecting a flow rate signal (Va) generated by a fluid flowing through
said conduit;
wherein said driving means comprises
a voltage regulator means (13) supplied with a DC voltage obtained by
rectifying a voltage from an AC power source (10) having a predetermined
frequency and applying a voltage control, for providing a predetermined
exciting voltage;
a first switching means (24a) connected between said exciting coil (25) and
said voltage regulator means (13) and controlled by a first signal, for
applying said exciting voltage at selected polarity to said exciting coil
(25);
a second switching means (24b) connected between said exciting coil (25)
and said voltage regulator means (13) and controlled by a second signal,
for applying said exciting voltage at another polarity to said exciting
coil (25);
wherein said control means comprises
current detection means (Rs) connected in series between said first and
second switching means and said exciting coil, for detecting exciting
current (Io) flowing through said exciting coil (25);
absolute value circuit (31) connected to said current detecting means (Rs)
for generating an absolute value of the exciting current (Io) detected by
said current detection means (Rs):
means for generating a reference voltage;
difference operating means (32) connected to said absolute value circuit
(31) for calculating the difference between said absolute value signal and
said reference voltage at a predetermined value for determining a constant
level of said exciting current (Io);
duty cycle conversion means (33) connected to said difference operating
means (32) for converting the output of said difference operating means
into a control signal (Sd) having a train of pulses having duty cycles
related to said output of said difference operating means (32);
means for switching the reference voltage between a predetermined value of
the reference voltage and a zero voltage;
means for generating a timing signal (S.sub.t); and
arithmetic operating means (34) connected to said duty cycle conversion
means (33), and responsive to said control signal (Sd) and said timing
signal (St), for switching said exciting current at a predetermined
frequency, said arithmetic operating means performing arithmetic operation
to generate said first signal (Sf.sub.1) and said second signal (Sf.sub.2)
related to said control signal and said timing signal, and for controlling
said first and second switching means, respectively, by said first and
said second signals;
and wherein said difference operating means comprises means for generating
a tristate excitation by calculating the difference between an output
corresponding to the absolute value of said exciting current and an output
generated by switching the reference voltage and the zero voltage with a
frequency twice as high as the exciting frequency.
2. An electromagnetic flowmeter comprising
a conduit (27);
a pair of electrodes (28a,28b) disposed in said conduit (27) at
diametrically opposing positions;
an electromagnet disposed adjacent said conduit (27) and having an exciting
coil (25) for generating a magnetic field within said conduit (27);
a driving means (13, etc) connected to said exciting coil (25) for exciting
said electromagnet;
control means (30) connected to said driving means for controlling the
driving means; and
detecting means (29) connected to said electrodes in said conduit (27) for
detecting a flow rate signal (Va) generated by a fluid flowing through
said conduit;
wherein said driving means comprises
a voltage regulator means (13) supplied with a DC voltage obtained by
rectifying a voltage from an AC power source (10) having a predetermined
frequency and applying a voltage control, for providing a predetermined
exciting voltage;
a first switching means (24a) connected between said exciting coil (25) and
said voltage regulator means (13) and controlled by a first signal, for
applying said exciting voltage at selected polarity to said exciting coil
(25);
a second switching means (24b) connected between said exciting coil (25)
and said voltage regulator means (13) and controlled by a second signal,
for applying said exciting voltage at another polarity to said exciting
coil (25);
wherein said control means comprises
current detection means (Rs) connected in series between said first and
second switching means and said exciting coil, for detecting exciting
current (Io) flowing through said exciting coil (25);
absolute value circuit (31) connected to said current detection means (Rs)
for generating an absolute value of the exciting current (Io) detected by
said current detection means (Rs);
means for generating a reference voltage (Es1);
difference operating means (32) connected to said absolute value circuit
(31) for calculating the difference between said absolute value signal and
said reference voltage (Es1) at a predetermined value for determining a
constant level of said exciting current (Io);
duty cycle conversion means (33) connected to said difference operating
means (32) for converting the output of said difference operating means
into a control signal (Sd) having a train of pulses having duty cycles
related to said output of said difference operating means (32);
means for generating a timing signal (St); and
arithmetic operating means (34) connected to said duty cycle conversion
means (33), and responsive to said control signal (Sd) and said timing
signal (St), for switching said exciting current at a predetermined
frequency, said arithmetic operating means performing arithmetic operation
to generate said first signal (Sf1) and said second signal (Sf2) related
to said control signal and said timing signal, and for controlling said
first and second switching means, respectively, by said first and second
signals;
and wherein said voltage regulator means comprises a transformer comprising
primary and secondary windings, a rectifier and a Zener diode, and wherein
AC voltage produced in the secondary winding of the transformer is
rectified in the rectifier and then filtered by the Zener diode.
3. An electromagnetic flowmeter comprising
a conduit (27);
a pair of electrodes (28a,28b) disposed at diametrically opposite positions
in said conduit (27);
an electromagnet disposed adjacent said conduit (27) and having an exciting
coil (25) for generating a magnetic field within said conduit (27);
a driving means (13, etc) for exciting said exciting coil (25) of said
electromagnet;
detecting means (29) connected to said electrodes in said conduit (27) for
detecting a flow rate signal (Va) generated by a fluid flowing through
said conduit; and
control means connected to said driving means for controlling the driving
means;
wherein said control means comprises
means for providing a timing signal (S.sub.t);
current detection means (Rs) for detecting current (Io) flowing through
said exciting coil (25);
absolute value circuit (35) connected to said current detection means (Rs)
for generating an absolute value signal (eA) of the exciting current;
means for generating a reference voltage (Es1);
difference operating means (32) connected to said absolute value circuit
(35) for calculating the difference between said absolute value signal
(eA) and said reference voltage (Es1) at a predetermined value for
determining a constant level of said exciting current (Io);
duty cycle conversion means (36) connected to said difference operating
means (32) for converting the output from said difference operating means
(32) into a control signal (eP) having a train of pulses having a duty
cycle proportional to the output from said difference operating means; and
arithmetic means (37) connected to said duty cycle conversion means (36)
and responsive to said control signal (eP) and said timing signal (St),
for switching said exciting current (Io) at a predetermined frequency, and
for generating a first signal (Sf1) and a second signal (Sf2) related to
said control signal (eP) and said timing signal (St) and for controlling
first (SW1) and second (SW2) switching means by said first and second
signals;
and wherein said driving means comprises
a voltage regulation means (13) for producing a predetermined exciting
voltage from a DC voltage obtained from an AC power source (10) having a
predetermined frequency and being rectified by a rectifier (11) and
subjected to voltage control;
said first switching means (SW1) connected between said exciting coil (25)
and said voltage regulation means (13) and opened and closed by said first
signal (Sf1) having a train of pulses with a duty cycle determined by the
level of the exciting current corresponding to an exciting period and
turned ON and OFF synchronous with the exciting period;
said second switching means (SW2) connected between said exciting coil (25)
and said voltage regulation means (13) and opened and closed by said
second signal (Sf2) having a phase corresponding to said exciting period
opposite to that of said first signal;
a pair of diodes (26a,26b) connected in parallel with said first (SW1) and
second (SW2) switching means, respectively, with the polarity being
opposite to the flowing direction of said exciting current;
first switching circuit (SW3) connected in parallel with said exciting coil
(25) and rendered conductive during the ON period in which said train
pulses rendered non-conductive during the OFF period of said first signal;
and
second switching circuit (SW4) connected in parallel with said exciting
coil (25) and rendered conductive during the ON period in which said train
of pulses of said second signal continues to turn ON and OFF, and being
rendered non-conductive during the OFF period of said second signal.
4. The flowmeter of claim 3, wherein said voltage regulation means
comprising means responsive to a reference voltage and a switching control
signal, providing said exciting voltage corresponding to said reference
voltage.
5. The flowmeter of claim 3, wherein said voltage regulation means
comprises means for supplying a rectified DC voltage from said rectifier
and generates said exciting voltage, and wherein said voltage regulation
means further comprises a transformer for insulating said rectifier.
6. The flowmeter of claim 3, wherein said first and second switching means
comprise field effect transistors, respectively.
7. The flowmeter of claim 5, wherein said transformer has winding used as
power supply for said flowmeter.
8. The flowmeter of claim 3, wherein said first and second switching
circuits comprise respectively serial circuits each comprising first and
second field effect transistors and paired diodes connected in a direction
inhibiting current due to said exciting voltage; and wherein said first
and second field effect transistors are controlled by timing signals.
9. The flowmeter of claim 3, wherein said first and second switching
circuits comprise first and second field effect transistors; wherein said
first field effect transistor is controlled by means for providing a third
control signal which is rendered conductive during the OFF period in the
train of pulses of said first control signal; and wherein said second
field effect transistor is controlled by means for providing a fourth
control signal which is rendered conductive during the OFF period in the
train of pulses of said second control signal.
10. An electromagnetic flowmeter comprising a conduit having a pair of
electrodes disposed at diametrically opposing positions, an electromagnet
having an exciting coil for generating a magnetic field within said
conduit, a driving means for exciting said electromagnet, and means for
outputting a flow rate signal generated by a fluid flowing through said
conduit, wherein said driving means comprises
A. a switching regulation means for stabilizing a supply voltage so that a
circuit voltage is outputted and insulated from said supply voltage, and
an exciting voltage is outputted according to a set voltage;
B. a first switching means connected between said exciting coil and said
switching regulation means for applying said exciting voltage at one of
two polarities to said exciting coil;
C. a second switching means connected between said exciting coil and said
switching regulation means for applying said exciting voltage at the other
of said polarities to said exciting coil;
D. a current detection means connected in series between said switching
regulation means and said exciting coil for detecting an exciting current
flowing through said exciting coil and for producing an output voltage;
means for supplying a reference voltage;
means for supplying a timing signal; and
E. a switching control means for controlling said first and second
switching means, said switching control means comprising means for
computing the difference between the absolute value of the output voltage
from said current detection means and the reference voltage, said
difference used for determining the magnitude of said exciting current,
means for producing a duty cycle signal corresponding to said difference,
and means for obtaining the product of said duty cycle signal and the
timing signal, said product used for switching said exciting current at a
frequency which is lower than the frequency of the power supply.
11. The flowmeter of claim 10, wherein said first and second switching
means comprise field effect transistors, respectively.
12. The flowmeter of claim 10, wherein said timing signal is prepared by
frequency dividing the commercial power supply frequency through a
frequency divider.
13. The flowmeter of claim 10, wherein further comprising means for
switching the reference voltage and a zero voltage, and wherein said
difference amplifier comprises means for computing and outputting the
difference between the output corresponding to the absolute value of said
exciting current and the output obtained by switching the reference
voltage and the zero voltage with a frequency twice as high as the
exciting frequency by using a changeover switch.
14. an electromagnetic flowmeter comprising a concuit having a pair of
electrodes disposed at diametrically opposing positions, an electromagnet
having an exciting coil for generating a magnetic field within said
conduit, and a driving means for exciting said electromagnet, and means
for outputting a flow rate signal generated by a fluid flowing through
said conduit, wherein said driving means comprises
A. a switching regulation means for stabilizing a supply voltage so that a
circuit voltage is outputted and insulated from said supply voltage, and
an exciting voltage is outputted according to a set voltage through a
capacitor connected in parallel with an output end;
B. a current detection means for detecting an exciting current flowing
through said exciting coil connected in series between said switching
regulation means and said exciting coil;
C. a difference computing means for computing the difference between the
absolute value of an output voltage detected by said current detection
means and a reference voltage for determining the magnitude of said
exciting current;
means for supplying said reference voltage;
means for supplying a duty cycle signal corresponding to said difference;
D. a computing means for receiving the duty cycle signal corresponding to
said difference and the frequency of the power supply, and for computing
and outputting timing signals corresponding to a positive exciting period
during which positive exciting current is applied and a negative exciting
period during which the negative exciting current is applied, a first
control signal containing number of said duty cycle signals within its
positive exciting period and having the OFF period corresponding to said
negative exciting period, and a second control signal containing number of
said duty cycle signals within its negative exciting period and having the
OFF period corresponding to said positive exciting period;
E. a first switching means connected in series between said exciting coil
and said switching regulation means and opened and closed by said first
control signal;
F. a second switching means connected in series between said exciting coil
and said switching regulation means and opened and closed by said second
control signal;
G. a first switching circuit connected in parallel with said exciting coil,
being rendered conductive during OFF periods contained by said positive
exciting period of said first control signal, and being rendered
non-conductive during said negative exciting period of said first control
signal; and
H. a second switching circuit connected in parallel with said exciting
coil, being rendered conductive during OFF periods contained by said
negative exciting period of said second control signal, and being rendered
non-conductive during said positive exciting period of said second control
signal.
15. The flowmeter of claim 14, wherein said first and second switching
means comprise field effect transistors, respectively.
16. The flowmeter of claim 14, wherein said first and second switching
circuits comprise respective serial circuits each composed of first and
second field effect transistors and paired diodes connected in the
direction of inhibiting the current due to said exciting voltage, and
wherein said first and second field effect transistors are controlled by
said timing signals, respectively.
17. The flowmeter of claim 14, wherein said first and second switching
circuits comprises respective first and second field effect transistors;
wherein said first field effect transistor is controlled by a third
control signal, means for causing said third control signal to flow during
the OFF periods in each train of pulses of said first control signal; and
wherein said second field effect transistor is controlled by a fourth
control signal means for causing said fourth control signal to flow during
the OFF periods in each train of pulses of said second control signal. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to an electromagnetic flowmeter which uses an
exciting current of rectangular waveform supplied to an exciting coil for
excitation and, more particularly, to an exciting circuit for such an
electromagnetic flowmeter, wherein the duty ratio of current pulses for
the ON-OFF control of the exciting current is varied to form a constant
exciting current.
2. Discussion of Prior Art
In conventional electromagnetic flowmeters, an alternating voltage at a
commercial frequency is applied as it is to an exciting coil, for
eliminating the effect of the polarization voltage produced between
electrodes, and the resulting magnetic field at the commercial frequency
is applied to a fluid being measured. Then the flow rate is detected
depending on the alternating voltage produced between the electrodes in
contact with the fluid to be measured.
However, there is a limit to the measurement of the flow rate at high
accuracy in the electromagnetic flowmeter of this type of excitation
system, since induced noises result due to the fluctuation in the magnetic
field and vary the zero point.
In view of the above situation, there is also employed an excitation system
using an AC current at a lower frequency than the above discussed
excitation current applied to the excitation coil for avoiding the effect
of the polarization, and converting the form of the excitation current
into a rectangular waveform having a period in which the level of the
current does not change, and sampling the signal voltage during this
period so that the effect of the induced noises may be eliminated.
There are also known various types of excitation systems using lower
frequency wave having a rectangular waveform for excitation. One example
is described in U.S. Pat. No. 4,462,060.
In the excitation system of U.S. Pat. No. 4,462,060, a commercial AC is
applied, after full wave rectification, to a pair of switching elements
connected in series with an excitation coil. The switching elements are
controlled by a duty cycle converter, including a comparator, that varies
the duty cycle corresponding to the value of the excitation current, to
maintain the exciting current constant. On the other hand, a gate voltage
at a low frequency and having a waveform analogous to that of a desired
exciting current is applied to the duty cycle converter to obtain an
exciting current of low frequency and having a rectangular waveform whose
flat portion is maintained constant.
However, since the level and the waveform of the excitation current are
dependent on the level and the waveform of the gate voltage in this type
of excitation system, disadvantageously, such level and waveform have to
be maintained exactly and control thereof is difficult.
Further, disadvantageously, the voltage obtained through the full wave
rectification of the commercial AC voltage is applied as it is to the
excitation coil by way of a control circuit for controlling the exciting
current. Thus, the system is strictly limited to the power source for
which it is designed. For example, the system may be limited to 110 V, and
cannot be used for 220 V, in view of the dynamic range of the control
circuit.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to overcome the aforementioned
and other disadvantages and deficiencies of the prior art.
Another object is to provide means capable of controlling the waveform of
the exciting system current in a simple circuit, without controlling the
waveform of the exciting current, by a voltage of a waveform analogous to
that of the excitation current.
A further object is to provide a general purpose excitation system of
applying a voltage obtained through rectification of a commercial voltage
to the control circuit for the exciting current by way of a switching
circuit.
A still further object is to provide an exciting circuit of rapid response
and high efficiency.
The foregoing and other objects are attained by the invention wherein a
commercial AC voltage is subjected to a full wave rectification and a
voltage regulated DC voltage obtained by way of a switching circuit is
applied to an exciting circuit including an exciting coil controlled by a
control circuit for controlling the exciting current. In this exciting
circuit, a first switching element and a detecting resistor for detecting
the exciting current are connected in series with the exciting coil. A
second switching element for supplying an exciting current in the opposite
direction is connected in parallel with the serial circuit comprising the
exciting coil and the detecting resistor. On the other hand, the control
circuit detects the value of the exciting current as a feedback voltage
generated across the detection resistor, generates an absolute value
thereof, takes a difference between the absolute value and a reference
voltage determining the level of the exciting current, converts the
difference into a train of pulses with varying duty ratio, and thereafter,
switching the train of pulses by a timing signal for switching the
exciting current, to control the first and second switching element.
BRIEF DESCRIPTION OF DRAWING
FIG. 1 is a block diagram depicting an illustrative embodiment of the
invention.
FIG. 2, comprising lines (a) through (e) is a waveform chart depicting the
waveform in each section of the FIG. 1 embodiment.
FIG. 3 is a block diagram depicting another illustrative embodiment of the
invention.
FIG. 4, comprising lines (a) through (i) is a waveform chart depicting the
waveform in each section of the FIG. 3 embodiment.
FIG. 5 is a waveform chart depicting the waveform of the exciting current
using tristate excitation.
FIG. 6 is a circuit diagram depicting a difference amplifier for obtaining
the exciting current shown in FIG. 5.
FIG. 7 is a circuit diagram depicting a circuit wherein Zener diodes are
used instead of smoothing capacitors as in FIG. 1.
FIG. 8 is a circuit diagram depicting a portion of a further illustrative
embodiment of the invention.
FIG. 9, comprising lines (a) through (g) is a waveform chart depicting
operation of the embodiment of FIG. 8.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Turning to FIG. 1, a voltage across the terminals of a commercial power
source 10, is subjected to full wave rectification in a rectifier 11. A DC
voltage, after full wave rectification, is filtered in a capacitor 12 and
then applied by way of terminals T1, T2 to a switching circuit 13.
Switching circuit 13 comprises a transformer 14, a transistor 15 as a
switching element, rectifying and smoothing circuits 16a,16b, 17, an error
amplifier 18, an oscillator 19 outputting a triagonal waveform voltage, a
comparator 20, a switching element drive circuit 21 and a reference
voltage source 22 for generating a reference voltage Er. The transformer
14 includes primary windings n1, secondary windings n2a, n2b, tertiary
windings n3 and quarternary windings n4.
The voltage from power source 10 is applied by way of smoothing capacitor
12 to a serial circuit comprising primary windings n1 and transistor 15.
When transistor 15 is turned ON, a primary current i1 flows. When
transistor 15 is turned OFF, energy stored in the core of transformer 14
by primary current i1 is discharged as secondary currents i2a, i2b and a
tertiary current i3, to the side of secondary windings n2a, n2b and
tertiary windings N3. Secondary currents i2a, i2b are smoothed through
first and second rectifying and smoothing circuits 16a,16b and applied to
exciting circuit 23 for an electromagnetic flowmeter, to supply an
exciting current Io.
On the other hand, tertiary current i3 on the side of tertiary windings n3
is rectified and filtered through rectifying and smoothing circuit 17 into
a feedback voltage Ef, which is applied to the inverted terminal (-) of
errror amplifier 18.
Error amplifier 18, having a set voltage Er applied to the non-inverted
input terminal (+), amplifies the difference between reference voltage Er
and feedback voltage Ef. The output Ea from amplifier 18 is compared with
a trigonal waveform voltage Eb from oscillator 19 in comparator 20. Based
on the results of the comparison, drive circuit 21 turns transistor 15
either to ON or OFF. In this manner, transistor turns ON and OFF,
repeatedly so as to attain the state Ef=Er, to keep constant outputs Va1,
Vb1 and the output voltage across terminals T5 and T6 of switching circuit
13.
First rectifying and smoothing circuit 16a, comprising diode Da and a
smoothing capacitor Ca, rectifies and filters the voltage produced across
secondary windings n2a of transformer 14, to obtain a positive DC voltage
Va1 between terminals T3 and T0.
Second rectifying and smoothing circuit 16b, comprising a diode Db and a
smoothing capacitor Cb, rectifies and filters the voltage produced across
secondary windings n2b of transformer 14, to obtain a negative DC voltage
Vb1 between terminals T4 and T0.
The two ends of quarternary windings n4 are connected to terminals T5,56,
respectively, and the voltage produced across the two ends, is used as a
power supply for other circuits in the electromagnetic flowmeter.
Between terminals T3 and T0, are connected in series a switch 24a, an
exciting coil 25 and a detection resistor Rs for detecting the exciting
current Io. The junction between exciting coil 25 and detection resistor
Rs is connected to a common potential point COM. Between terminals T4,T0
are connected in parallel a serial circuit comprising exciting coil 25 and
detection resistor Rs by way of switch 24b. Switches 24a, 24b are
connected in parallel with diodes 26a, 26b, respectively, by which the
energy stored in exciting coil 25 is caused to be absorbed in capacitors
Cb, Ca when the switches 24a, 24b are turned OFF, respectively. Switches
24a,24b are opened and closed by control signals Sf1, Sf2, respectively.
Switches 24a, 24b, exciting coil 25, detection resistor Rs, diodes 26a,
26b, etc, may be considered to constitute an exciting circuit 23 for the
flowmeter.
A magnetic field is produced by an exciting current Io flowing through
exciting coil 25. The magnetic field is applied to a conduit 27 in which
fluid to be measured is filled. Conduit 27 is disposed with a pair of
grounded electrodes 28a,28b and a voltage generated across the two
electrodes is outputted by way of a signal processing circuit 29 as a flow
rate signal Va.
The voltage between detection resistor Rs and common potential point COM is
inputted to a control circuit 30 for controlling exciting current Io.
Control circuit 30 comprises an absolute value circuit 31, a difference
amplifier 32, a duty cycle oscillation circuit 33 and a mathematical
operation circuit 34.
The voltage produced by exciting current Io flowing through detection
resistor Rs is inputted into absolute value circuit 31. The voltage
inputted to absolute value circuit 31 is applied in the form of its
absolute value to one input of difference amplifier 32. A DC reference
voltage Es1 is applied to the other input of the difference amplifier 32.
As a result, a voltage corresponding to the difference between the
reference voltage Es1 and the absolute value for the voltage across the
detection resistor Rs is outputted at the output terminal of difference
amplifier 32. The output terminal of the difference amplifier 32 is
connected to the input terminal of duty cycle oscillation circuit 33. The
output terminal of circuit 33 is connected to the input terminal of
mathematical operation circuit 34.
Operation circuit 34 is supplied with control signal Sd from circuit 33 and
a timing signal St, and calculates the product between signal St and
control signal Sd. Operation circuit 34 outputs control signals Sf1, Sf2,
which control the opening and closing of switches 24a,24b, respectively.
Operation of the circuit of FIG. 1 will now be described with reference to
the waveform chart of FIG. 2. Switch 24a or 24b is closed by control
signal Sf1 or Sf2, to supply exciting current Io to exciting coil 25.
Exciting current Io is detected by detection resistor Rs and then inputted
into absolute value circuit 31. The difference between the output from the
absolute value circuit 31 and a predetermined reference voltage Es1 is
amplified in and outputted from difference amplifier 32. Since the
absolute value of the exciting current is relatively small due to the
inductance L of exciting coil 25 during the transient period t1 after the
exciting current Io has been switched by timing signal St (FIG. 2, line
a), the output from difference amplifier 32 is made greater, in which
case, control signal Sd from duty cycle oscillation circuit 33 takes the
waveform of a train of pulses having a longer ON period (FIG. 2, line b).
As exciting current Io approaches a constant current value Ioc (FIG. 2,
line e), the output from difference amplifier 32 becomes smaller, in which
case, control signal Sd takes the waveform of a train of pulses having
shorter ON period (FIG. 2, line b).
Operation circuit 34 performs an arithmetic operation and obtains a logic
product between control signal Sd (FIG. 2, line b) from duty cycle
oscillation circuit 33 and timing signal St that gives the timing for the
exciting current (FIG. 2, line a) and generates control signal Sf1 or Sf2
(FIG. 2, lines c, or d). Each of switches 24a,24b is controlled by control
signal Sf1 or Sf2, respectively.
When switch 24a or 24b is opened or closed by the duty cycle, as shown in
FIG. 2, lines c or d, the average exciting voltage applied to exciting
coil 25 is increased during the transient period t1 of the exciting
current (FIG. 2, line a) to make the rise of exciting current Io faster
(FIG. 2, line e). Even when switch 24a or 24b is switched, exciting
current Io is smoothed due to the inductance of exciting coil 25 to form
exciting current Io varying at a low frequency (FIG. 2, line e). Exciting
current value Ioc, at which exciting current Io, is settled constant, can
be determined by adjusting reference voltage Es1 or difference amplifier
32.
As can be seen from the above description, since the flat portion of
exciting current Io can be determined by reference voltage Es1, which is a
constant DC voltage, and since the switching of exciting current Io can be
determined by merely applying timing signal St, the waveform of exciting
current Io can readily be determined by reference voltage Es1 and timing
signal St. Accodingly, in this embodiment, the waveform of exciting
current Io can be controlled by a simple circuit with no requirement for
preparing a control signal analogous to the level and the waveform of the
exciting current used in the prior art as described above.
Furthermore, according to this embodiment, advantageously, since the
commercial AC voltage is subjected to an efficient voltage control through
switching circuit 13 and is applied to exciting circuit 23, it is capable
of operating without requiring any substantial modification, even though
voltage ratio of the power source is changed, such as from 110 v to 220 v.
Furthermore, according to this embodiment, since the voltage resulting
between terminals T5 and T6, can be used in common with other circuit
power supplies in the flowmeter, the invention is advantageously simple
and does not require providing any exclusive power supply section for the
exciting circuit, such as required in prior art circuits as described
above.
Turning now to FIG. 3, which depicts another illustrative embodiment of the
invention, these portions having the same functions as those constituent
elements shown in FIG. 1 carry the same reference numerals. The common
components are omitted from the description for sake of clarity.
A voltage produced upon flowing of the exciting current Io through
detection resistor Rs is inputted to an absolute circuit 35. Absolute
circuit 35 calculates the absolute value for the input voltage by
conducting a transistor Q1 when the input voltage is positive and
conducting a diode D1 when it is negative.
A difference amplifier 32 amplifies the difference between the output eA
from absolute value circuit 35 and the reference voltage Es1 and outputs
the amplified difference to a duty cycle conversion circuit 36.
The duty cycle conversion circuit 36 compares the output from difference
amplifier 32 with a trigonal waveform voltage eT at a high frequency and
outputs a voltage eP of a train of pulses whose duty cycle is in
proportion to the output from difference amplifier 32 to an operation
circuit 37.
In operation circuit 37, a power source signal of frequency f0 is supplied
to frequency divider Q2, for example, which frequency divides the signal
into a lower frequency to obtain a timing signal St. The timing signal St
is inputted, by way of an inverter Q3, to a NAND gate Q4. An output
voltage eP, from duty cycle conversion circuit 36, is applied to the other
input terminal of NAND gate Q4 and a control signal SF2 is generated at
the output thereof. Timing signal St from divider Q2 and output voltage eP
from duty cycle conversion circuit 36 are inputted to a NAND gate Q5, the
output of which is outputted through an inverter Q6 as a control signal
Sf1.
Absolute value circuit 35, difference amplifier 32, duty cycle conversion
circuit 36 and operation circuit 37 may be considered to constitute a
control circuit 38, which outputs timing signal St and control signals,
Sf1, Sf2, to an exciting circuit 39 as described hereinbelow.
The exciting circuit 39 is supplied with a positive DC voltage Va1 and a
negative DC voltage Vb1 from a switching circuit 13 and supplies an
exciting current Io to an exciting coil 25 under the control of timing
signal St and control signals Sf1, Sf2 from control circuit 38. Terminal
T3 of switching circuit 13 is connected with a drain D to a switching
element SW1 comprising an N-channel field effect transistor (MOS-FET) The
source S of the FET is connected wtih one end of detection resistor Rs and
to a common potential point COM. The other end of resistor Rs is connected
to one end of exciting coil 25. A diode 26a is connected between drain D
and source S of switching element SW1 with the cathode thereof being on
the side of terminal T3. Gate G of switching element SW1 is connected with
the output terminal of inverter Q6 of operation circuit 37.
Terminal T4 of switching circuit 13 is connected with drain D of switching
element SW2 comprising a P-channel field effect transistor (MOS-FET).
Source S of the FET of SW2 is connected with the sources S of switching
elements SW4,SW3 and SW1. A diode 26b is connected between drain D and
source S of the switching element SW2 with the anode thereof being on the
side of terminal T4. Gate G of switching element SW2 is connected with the
ouput terminal of NAND gate Q4 of operation circuit 37.
Terminal T0 of switching circuit 13 is connected to another end of exciting
coil 25. Between terminal T0 and source S of element SW1, are connected in
series a switching element SW3, comprising an N-channel field effect
transistor (MOS-FET) and a diode D2 with the anode thereof being on the
side of terminal T0, to comprise a switching circuit, in which gate G of
switching element SW3 is connected with the output terminal of frequency
divider Q2 of operation circuit 37.
In the same manner, between terminal T0 and switching element SW2, are
connected in series a switching element SW4, comrprising a P-channel field
effect transistor (MOS-FET) and a diode D3 with the cathode thereof being
on the side of terminal T0, to comprise another switching circuit, in
which gate G of switching element SW4 is connected with the output
terminal of frequency divider Q2 of operation circuit 37, in the same
manner as the gate of switching element SW3.
The operation of the circuit of FIG. 3 will now be described with reference
to the waveforms of FIG. 4. Corresponding to the timing signal St at a low
frequency (FIG. 4, line a) generated by frequency dividing power source
frequency F0 in frequency divider Q2, an exciting current Io (FIG. 4, line
b) is caused to flow and is detected by detection resistor Rs. It is then
formed into an absolute value thereof (FIG. 4, line c) in absolute value
circuit 35 and then inputted, by way of difference amplifier 32, to duty
cycle conversion circuit 36. This input is compared with the trigonal
waveform voltage eT (FIG. 4, line d) in duty conversion circuit 36 and
outputted as an output voltage eP (FIG. 4, line e) in proportion to the
level of output voltage eA from absolute value circuit 35.
Switching element SW1 is opened and closed by control signal Sf1 generated
by taking a NAND operation between output voltage eP and timing signal St,
and further inverting it in inverter Q6 (FIG. 4, line f). Where the
absolute value of exciting current Io is relatively small, the ON period
of switching element SW1 is increased to make the rise of exciting current
Io faster. Furthermore, switching element SW2 is opened and closed by
control signal Sf2 which is generated by a NAND operation in NAND gate Q4,
between timing signal St which is inverted in inverter Q3 and output
voltage eP (FIG. 4, line g).
Also, in switching element SW2, where the absolute value of exciting
current Io is relatively smaller, the ON period of switching element SW2
is increased to make the rise of the exciting current Io faster.
Timing signal St (FIG. 4, line a) is applied to each of gates G of
switching elements SW3, SW4, in which switching element SW3 is turned ON
when gate G is in a positive period (logic 1) (FIG. 4, line h), while
switching element SW 4 is turned ON when gate G is in the 0 period (logic
0) (FIG. 4, line i).
Accordingly, in period T1 where timing signal St is at a positive level
(FIG. 4, line a), switching element SW1 turns ON and OFF repeatedly and
produces a train of pulses at high frequency. In each ON period, exciting
current Io is supplied to exciting coil 25 by positive DC voltage Va1. On
the other hand, if element SW1 is turned OFF, a current flows through
diode D2, switching element SW 3 is in the ON state and detection resistor
Rs, due to electromagnetic energy stored in exciting coil 25 and the
current continues to flow until the next turning ON of the switching
element SW1. Then, when element SW1 is turned ON, exciting current Io is
again supplied by positive DC voltage Va1 and continuously flows as
exciting current Io. The amplitude of exciting current Io is generated by
duty cycle determined by reference voltage Es1.
Upon switching to period T2 when timing signal St turns to zero (FIG. 4,
line a), switching element SW 3 is turned OFF in synchronism with the
switching of timing signal St. In this instance, exciting current Io,
having flowed through exciting coil 25, flows through diode 26b (FIG. 3)
to capacitor Cb in switching circuit 13, to accumulate electrical charges
therein, by which the voltage across terminal T0 and T4 is increased to
make the rise of exciting current Io faster upon switching.
Then, after the inversion of the timing signal St (period T2), switching
element SW2 turns ON and OFF repeatedly producing a train of pulses at
high frequency and switch SW4 is also turned ON. In this state, the
excitation in the negative direction is attained in the same manner as the
excitation in the positive direction as described above.
As shown by the embodiment of FIG. 3, in which switching elements SW1-SW4
of the exciting circuit comprise enhancement type of MOS transistors,
short circuiting of the power source can be prevented by using a control
signal for each of the switching elements in a simple logic circuit while
taking the common potential point COM as the reference potential.
The waveform for the exciting current Io depicted in FIG. 5 shows the case
of a tristate excitation wherein a non-excited state is present between
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