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BACKGROUND OF THE INVENTION
This invention relates generally to magnetic flowmeters, and more
particularly to a magnetic flowmeter whose electromagnet is excited by a
low-frequency wave whose frequency is less than the usual 50 or 60 Hz
frequency of a commercial electric power line.
In a conventional magnetic flowmeter arrangement, the usual practice is to
make use of a commercial AC power line as an excitation source in that
high power can easily be obtained therefrom. However, AC excitation at the
frequency of a commercial power line gives rise to eddy current problems.
These eddy currents are induced by AC excitation and flow through the
fluid to be metered, thereby introducing zero drift error.
To overcome this drawback, it has heretofore been proposed to make use of
an excitation technique in which the source is a low-frequency wave whose
frequency is less than the usual 50 or 60 Hz commercial power line
frequency. In this known technique, a low-frequency wave is generated by a
low-frequency sinusoidal oscillator energized by DC power. The output of
this oscillator is amplified by a power amplifier and then applied to the
excitation coils of the magnetic flowmeter.
This known technique has certain practical disadvantages. Thus in the case
of a magnetic flowmeter having a relatively large diameter, because its
power requirements are high, this dictates an installation including a
large-sized DC source, a large-sized amplifier and so on. It is therefore
not feasible to use this known technique in conjunction with a magnetic
flowmeter having a relatively large diameter.
Another prior art technique makes use of a low-frequency rectangular wave
that is produced by switching a constant current. However, this method has
the following drawbacks:
1. To produce a constant current, a relatively complex power source is
required.
2. Since a rectangular wave is used as an excitation current, its
high-frequency harmonic components introduce inductive noise components in
a flow signal. Thus one must employ a complicated circuit to eliminate
these unwanted electroinductive noise components.
Yet another known excitation method for a magnetic flowmeter is that
disclosed in the Mannherz et al. U.S. Pat. No. 3,783,687, wherein the
output voltage of a full-wave rectifier is applied to the electromagnet
through a switching element, thereby reversing the voltage polarity at a
low-frequency rate. However, since the excitation wave has a rectangular
wave form, this method also gives rise to unwanted harmonic noise
components.
SUMMARY OF INVENTION
In view of the foregoing, the main object of this invention is to provide a
simple and low-cost, low-frequency excitation arrangement for a magnetic
flowmeter in which harmonic wave noise component problems are obviated.
Another object of this invention is to provide a low-frequency excitation
type magnetic flowmeter in which fluctuating components included in the
excitation current are eliminated without the need for a dividing circuit
having a complex construction.
Briefly stated, in a magnetic flowmeter in accordance with the invention, a
switching element is interposed between the electromagnet and a power
source, the switching element being "on-off" controlled at a
high-frequency rate. To control the switching element, use is made of a
pulse signal whose frequency is higher than that of the usual 50 or 60 Hz
power line frequency and whose duty cycle ratio is varied at a
low-frequency rate which is less than 50 or 60 Hz.
A significant feature of the present invention is that the switching
element is on-off controlled at a high rate, thereby dispensing with the
need for a switching element of the type appropriate to high-power
applications.
Another advantage of this invention is that because the output of a
full-wave rectifier is chopped by a switching element and then applied to
the electromagnet, this makes possible the omission of a power rectifier.
Still another feature of the present invention is that since a
low-frequency sinusoidal wave is used as an excitation wave, the
undesirable effects of unwanted harmonic components are absent. Moreover,
because an error signal is derived by comparing the excitation current
with a reference and the resultant error signal acts to control the
excitation current, it becomes possible to omit an expensive and
complicated dividing circuit arranged to eliminate fluctuations included
in the excitation current.
OUTLINE OF THE DRAWINGS
For a better understanding of the present invention, as well as other
objects and further features thereof, reference is made to the following
detailed description to be read in conjunction with the accompanying
drawings, wherein:
FIG. 1 is a schematic diagram of a first preferred embodiment of a magnetic
flowmeter having an excitation system in accordance with the invention;
FIG. 2 is a schematic diagram of a second preferred embodiment of an
excitation system in accordance with the invention;
FIGS. 3A to D are wave forms illustrating the operation of the excitation
system embodiments of the invention;
FIG. 4 is a schematic diagram of a third preferred embodiment of an
excitation system for a magnetic flowmeter in accordance with the
invention;
FIGS. 5 to 7 are block diagrams showing other examples of a low-frequency
oscillator for inclusion in the invention; and
FIGS. 8 and 9 illustrate other examples of the switching means.
DESCRIPTION OF INVENTION
First Embodiment
Referring now to FIG. 1, there is shown a magnetic flowmeter having an
excitation system in accordance with this invention. In this figure,
liquid to be metered is conducted through a flow tube 1 having a pair of
electrodes 2 and 3 disposed at diametrically-opposed positions thereon.
The voltage induced between these electrodes is applied to the amplifier 4
of a signal converter 27. Serially-connected excitation coils 5 and 6 act
to generate a magnetic field whose lines of flux are perpendicular both to
the transverse axis passing through electrodes 2 and 3 and the
longitudinal axis of the flow tube, the coils being arranged at opposed
positions with respect to the flow tube.
A switching element constituted by a transistor 8 is interposed between
excitation coils 5 and 6 and an electric power source, generally
designated by reference numeral 7. The power source is formed by a
commercial power line source 9 which is connected to the input junctions
of a full-wave rectifier bridge 10 whose output junctions are connected
between the collector of switching transistor 8 and one end of a resistor
11 which serves to derive a reference signal from the excitation circuit.
The emitter of transistor 8 is connected to a fixed reference level to
which is also connected one end of excitation coil 5. The other end of
coil 5 is connected to the one end of coil 6 whose other end is connected
to resistor 11.
Switching transistor 8 is "on-off" controlled at a high-frequency rate
(i.e., 3 KHz), which is more than 10 times higher than the commercial
power line frequency. A control circuit enclosed in dotted-line block 13
acts to supply an "on-off" control signal to switching transistor 8. This
circuit is primarily composed of an operational amplifier 15 whose
noninverting (+) input terminal is connected both to an input terminal 17
through a resistor 16 and to the output terminal of the amplifier through
a positive feedback resistor 18. This output terminal is also connected to
the inverting (-) input terminal of the amplifier through negative
feedback resistors 19 and 20. The junction of these resistors is connected
to a reference level through a capacitor 21.
As is well known, the positive feedback ratio of amplifier 15 is determined
by the ratio of resistor 16 to resistor 18. On the other hand, in the
negative feedback loop of the amplifier, capacitor 21 functions as a first
order lag element. Accordingly, adjustment for the inverting frequency
and/or duty cycle thereof can be accomplished by selecting these constants
in the respective feedback loops.
In the arrangement shown in FIG. 1, the output of operational amplifier 15
is modulated by an input voltage applied to an input terminal 17. The
output of amplifier 15 is applied to a transistor 23 whose emitter is
connected to the base of switching transistor 8. As a result, transistor 8
is "on-off" controlled in conformity with the output pulses yielded by
amplifier 15, and the "on-off" duty ratio thereof is modulated gradually
by a low-frequency sinusoidal wave whose frequency is less than that of
the commercial power line frequency (i.e., 1/4 to 1/8 of the usual 50 or
60 Hz wave). To generate this low-frequency wave, a low-frequency
sinusoidal wave oscillator 24 is provided whose output is applied to input
terminal 17 of control circuit 13.
We will now explain in greater detail in connection with FIGS. 3A to D the
function of control circuit 13. The "on-off" duty ratio of the output from
operational amplifier 15 is proportional to the instantaneous sampling
value of the low-frequency sinusoidal wave produced by oscillator 24. The
control circuit 13 is pre-adjusted so that the duty ratio assumes, for
example, a 1/2 value when the low-frequency wave crosses zero level.
FIG. 3A shows the low-frequency sinusoidal wave output of oscillator 24,
while FIG. 3B shows the output of the operational amplifier 15. As will be
evident from these figures, the duty ratio assumes its maximum value at
the positive peak of the low-frequency sinusoidal wave, and it assumes its
minimum value at the negative peak of this wave. While between these
peaks, the duty ratio varies gradually in the manner of a sine wave.
FIG. 3C shows the full-wave rectified output voltage of power source 7,
which is not yet filtered. The rectified output voltage illustrated in
this figure is applied to switching transistor 8 which acts to chop this
voltage in accordance with the pulse signal shown in FIG. 3B. The
resultant chopped output is applied to excitation coils 5 and 6.
When transistor 8 is in its "on" state, a voltage proportional to the
instantaneous voltage of the full-wave rectified output is applied to
coils 5 and 6. On the other hand, when switching transistor 8 is in its
"off" state, the current is then produced by the energy stored in these
coils. This current flows through a diode 26 connected between the ends of
the serially-connected coils 5 and 6 in the same direction as that during
the "on" period so as to maintain a constant current condition.
Thus the "on-off" duty ratio of switching transistor 8 whose "on-off"
frequency is extremely high compared with the commercial power line
frequency of source 9, is modulated by a low-frequency sinusoidal wave. As
a result, a low-frequency sinusoidal current of the type shown in FIG. 3D
flows in coils 5 and 6.
Though the voltage has unfiltered ripple components, because excitation
coils 5 and 6 have a relatively high inductance, these coils function as a
filter choke. Hence the ripple components of the excitation current are
substantially eliminated, as shown in FIG. 3D, without the need for
separate filter chokes.
Diode 26 serves to block out unwanted kick-back voltages which are produced
when transistor 8 is in its "off" state and are directed toward the
switching circuit composed of transistors 8 and 23 in conjunction with
power source 7.
The signal induced between electrodes 2 and 3 is applied to a dividing
circuit 28 through the amplifier 4 provided in converter 27. A reference
signal proportional to the current flowing in the excitation coils 5 and 6
is applied to the other input terminal of the dividing circuit 28. This
reference signal is obtained from resistor 11, whereby the induced signal
output of amplifier 4 is divided by the reference signal. As a result, a
mean flow signal which is proportional to the average velocity of the
flowing liquid is yielded at the output terminal 29 of converter 27.
Second Embodiment
FIG. 2 shows a second embodiment of an excitation system in accordance with
the invention. In this figure, like reference numerals in FIG. 1 are used
to designate like components. To avoid repetition, explanations for these
components are omitted. The advantage of this embodiment is that
fluctuating components included in the excitation current can be
eliminated without the need for a dividing circuit as in FIG. 1.
In the embodiment shown in FIG. 2, a low-frequency oscillator 24, which is
constituted by a Wien bridge 30 and an operational amplifier 31 acts to
generate a sinusoidal wave having a frequency in a range from several Hz
to 20 Hz.
The output of oscillator 24 is applied to one input of a pulse-width
modulating circuit 33 (this corresponds to control circuit 13 in the first
embodiment) through a buffer 32 including an operational amplifier 34. To
the other input terminal of the pulse-width modulating circuit 33, there
is applied a pulse signal having a constant frequency which is generated
by a pulse generator 35. In the pulse-width modulating circuit 33, the
pulse width is modulated by the output of low-frequency wave oscillator 24
applied through buffer 32. In practice, the pulse-width modulating circuit
33 and the pulse generator 35 may be constituted by semiconductive
integrated circuit units which are commercially available.
We shall now explain the behavior of this embodiment, again utilizing FIGS.
3A to D. FIG. 3A shows the output sinusoidal wave of the low-frequency
oscillator 24, while FIG. 3B shows the output of the pulse-width
modulating circuit 33. The duty ratio of the output from the pulse-width
modulating circuit 33, as shown by FIGS. 3A and 3B, assumes its maximum
value at the instantaneous maximum value of the sinusoidal wave, and it
has a ratio of about 1/2 when the sinusoidal wave crosses zero level. The
duty ratio assumes its minimum value at the instantaneous minimum value of
the sinusoidal wave.
The output of pulse-width modulating circuit 33 is then applied to the base
of switching transistor 8 wherein the source current is "on-off"
controlled. A fluctuation-detecting circuit enclosed in dotted-line block
36 serves to detect fluctuation in the excitation current. A signal
proportional to the excitation current is detected by resistor 11. The
resultant signal is applied through a capacitor 37 to a full-wave
rectifier constituted by an operational amplifier 38, diodes 39 and 40,
and resistors 41, 42 and 43.
Now let us suppose that the respective resistance value of resistors 41,
42, 43, 44 and 45 are represented by values R.sub.1, R.sub.1, 2R.sub.2,
R.sub.2 and R.sub.2. Amplifier 38 produces a negative output for a
positive input. As the result, diode 40, which is connected to the output
terminal of amplifier 38, is rendered conductive and is put in its "on"
state, whereas diode 39 is put in its "off" state. Under these conditions,
a current signal U/2R.sub.2, wherein U is the input voltage signal from
resistor 11, is applied through a resistor 43 to a differential amplifier
46 which functions as a summing amplifier and also as a smoothing circuit.
On the other hand, when the polarity of the input signal voltage U turns
negative, amplifier 38 yields a positive output, so that diode 39 is then
put in the "on" state and diode 40 in the "off" state. In this case, the
current signal fed to amplifier 46 from amplifier 38 is expressed by
-U/R.sub.2, since the gain of amplifier 38 is -1. Furthermore, the current
signal U/2R.sub.2 is applied to amplifier 46 through resistor 43.
Therefore, the total input current signal given by -U/2R.sub.2 is applied
to amplifier 46.
Thus the circuit composed of operational amplifier 38, diodes 39 and 40 and
resistors 41 to 44 functions as a fullwave rectifier. Terminal 47 is a
reference voltage signal terminal. At the junction of the inverting (-)
input terminal of amplifier 46, the current signal is compared with a
current signal based upon this reference voltage. A smoothed error voltage
signal is obtained at the output terminal of amplifier 46.
In accordance with the error signal thereof, the excitation current flowing
in excitation coils 5 and 6 is continually controlled. Thus, the output of
fluctuation-detecting circuit 36 is applied to a correction circuit 48
constituted by a diode 49, a capacitor 50, and resistors 51 and 52. The
resistance value of diode 49, one end of which is grounded, the other end
being connected to resistor 51 through the capacitor 50, varies in
accordance with the applied voltage signal which is proportional to the
error voltage signal. The signal detected by diode 49 is applied to
operational amplifier 31 in the low-frequency oscillator 24 through
resistor 52.
As a consequence, the amplitude of the low-frequency wave of oscillator 24
is governed to provide a constant current, the amplitude of which is
determined by the reference voltage signal.
Thus in accordance with this embodiment, the effects of fluctuations in the
excitation current can be eliminated without the need for a complicated
dividing circuit, and a mean flow signal proportional to the volumetric
flow rate can be obtained from electrodes 2 and 3.
Third Embodiment
In the previous embodiment, the fluctuations in the excitation current
flowing in coils 5 and 6 are detected by the use of fluctuation-detecting
circuit 36, and the low-frequency wave oscillator 24 is governed by a
signal derived from the detected signal. These circuits may be replaced
with other circuits, such as those shown in FIG. 4.
In FIG. 4, the output of low-frequency oscillator 24 is applied through
buffer 32 both to a differential amplifier 53 and to the
fluctuation-detecting circuit 36, thereby detecting fluctuations in the
output of low-frequency oscillator 24. The error signal therefrom is
applied to correction circuit 48.
By means of a loop 54 which includes circuits 36 and 48, the amplitude of
low-frequency oscillator 24 is continually controlled. The resultant
low-frequency wave having a constant amplitude is then applied to a
differential amplifier 53 as a reference signal.
On the other hand, the signal voltage detected by resistor 11, which is
connected between a fixed reference level and the excitation coil 6 is
negatively fed back to the inverting input (-) terminal of differential
amplifier 53 through a capacitor 55 and a resistor 56, so as to obtain an
error signal which corresponds to the fluctuations in the excitation
current flowing in excitation coils 5 and 6. This error signal is applied
to switching transistor 8, thereby eliminating low-frequency fluctuating
components included in the excitation current in the manner explained in
connection with the second embodiment. Thus the control circuit of this
embodiment functions so that the amplitude of the excitation current is
made to conform with that of the low-frequency reference signal produced
by loop 54.
Low Frequency Oscillators
FIGS. 5 to 7 show other examples of a low-frequency oscillator suitable for
the excitation systems in accordance with the invention. In FIG. 5, a
low-frequency sinusoidal wave oscillator is constituted by a
rectangular-wave generator 57 and a band-pass filter 58. The output of
rectangular wave generator 57 is applied to band-pass filter 58 wherein
the fundamental frequency is passed, thereby producing a sinusoidal wave.
In FIG. 6, a limiter 59 is arranged between the rectangular-wave generator
57 and the band-pass filter 58 shown in FIG. 5. With this arrangement, the
output of rectangular-wave generator 57, after its amplitude is rendered
constant by this limiter, is applied to the band-pass filter 58 to yield a
sinusoidal wave having a constant amplitude. This wave is obtained by a
relatively simple construction, as compared with the loop 54 arrangement
in the third embodiment.
FIG. 7 shows still another example of the low-frequency rectangular wave
generator. In this figure, the output of a-c power source 7 of the type
employed in FIGS. 1 to 3 is applied to a wave form shaper 60, and the
usual 50 or 60 Hz wave is shaped into a rectangular wave which is applied
to a frequency divider 61, whereby its frequency is decreased to an extent
rendering it suitable for excitation systems in accordance with the
invention. Furthermore, the low-frequency oscillator 24 may be replaced
with other circuits, such as a phase-shift type oscillator.
Switching Means
FIGS. 8 and 9 show other examples of switching means in accordance with
this invention. In FIG. 8, a photo-coupler 62 is employed for input-output
isolation. In this arrangement, the output of control circuit 13 is
applied to a light-emitting diode (LED) 63 whereby the LED emits a light
signal. This light signal is received and sensed by a photo-transistor 64
so as to convert the light signal into a corresponding electrical signal.
The electrical signal is applied to switching transistor 8.
Thus, by use of this photo-coupler technique, the section of the system
including the "on-off" ratio control means an be isolated from the section
that includes the electric power source 7. Hence the reference voltage
level may be freely determined.
Though in the embodiment in FIG. 8 a photo-coupler consisting of an LED and
a photo-transistor is used, the photo-transistor may be replaced with
other devices such as a photo thyristor. Also, through switching
transistor 8 is used as a switching element, this transistor may be
replaced with other switching elements such as a thyristor, as shown in
FIG. 9, which thyristor is indicated by numeral 65.
In this case, current from AC source 9, which is not yet rectified, is
applied directly to thyristor 65. In this arrangement, the light signal
from photo-diode 63 in the photo-coupler 62 is intercepted by a
photo-thyristor 66, and the electrical signal therefrom is applied to
thyristor 65 as a gate trigger current, the firing angle thereof being
varied by the difference of the trigger position. By use of the thyristor,
a continuous current whose amplitude is modulated by the low-frequency
sinusoidal wave is obtained without a rectifier.
Pulse-width modulator 33 and pulse generator 35 shown in FIGS. 2 and 4 may
be replaced with other circuits such as the control circuit 13 shown in
FIG. 1. Furthermore, a transistor or field-effect transistor (FET), which
is generally used as a variable impedance element in an automatic gain
control (AGC) circuit and the like, may be used in place of diode 49 in
correction circuit 48 which serves to control the amplitude of the
low-frequency oscillator 24.
Though in the above-described embodiments, resistor 11 acts to detect the
excitation current flowing in excitation coils 5 and 6, such current
detection may be carried out by means of a current transformer.
Furthermore, indirect current detecting means such as a pick-up coil, a
Hall effect element or a magnet-electro converting element may be used in
place of resistor 11 which directly detects the excitation current.
Also, while in the above-described embodiments, a low-frequency sinusoidal
wave is used to vary the "on-off" ratio, a triangular wave which gradually
varies may be used in lieu thereof. By the use of a gradually varying
wave, harmonic components included in the excitation current may be
rendered negligible as compared to those produced in arrangements using a
rectangular wave.
Furthermore, if polyphase alternating current is used as an electric power
source, ripple components in the excitation current may be reduced
considerably.
It will be apparent from the foregoing that a magnetic flowmeter in
accordance with the present invention possesses the following features:
A. because the coils are excited by a low-frequency wave, the effects of
eddy currents can be obviated.
B. because the excitation current is produced by the use of a high-speed
switching technique, one may omit a power amplifier and the size of the
circuits may be reduced.
C. because a low-frequency wave whose amplitude changes gradually in sine
wave fashion is used as the excitation current, special circuits to
eliminate the effects of electromagnetic-induction caused by
high-frequency harmonics can be omitted.
D. because commercial electric power is used as a source, this invention
may be embodied effectively in a magnetic flowmeter having a relatively
large diameter.
E. fluctuating components included in the excitation current can be
eliminated without the need for a complicated dividing circuit.
While there have been shown and described preferred embodiments of a
magnetic flowmeter in accordance with the invention, it will be
appreciated that many changes and modifications may be made therein
without, however, departing from the essential spirit of the invention.
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