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BACKGROUND OF THE INVENTION
This invention relates generally to electromagnetic flowmeters, and more
particularly to a high-voltage impulse drive system for the excitation
circuit of a meter of this type serving to effect a significant reduction
in the power required to effect excitation.
In an electromagnetic flowmeter, the liquid whose flow rate is to be
measured is conducted through a flow tube provided with a pair of
diametrically-opposed electrodes, a magnetic field perpendicular to the
longitudinal axis of the tube being established by an electromagnet. When
the flowing liquid intersects this field, a voltage is induced therein
which is transferred to the electrodes. This voltage, which is
proportional to the average velocity of the liquid and hence to its
average volumetric rate, is then amplified and processed to actuate a
recorder or indicator.
The magnetic field may be either direct or alternating in nature, for in
either event the amplitude of voltage induced in the liquid passing
through the field will be a function of its flow rate. However, when
operating with direct magnetic flux, the D-C signal current flowing
through the liquid acts to polarize the electrodes, the magnitude of
polarization being proportional to the time integral of the polarization
current. With alternating magnetic flux operation, polarization is
rendered negligible, for the resultant signal current is alternating and
therefore its integral does not build up with time.
Though A-C operation is clearly advantageous in that polarization is
obviated and the A-C flow induced signal may be more easily amplified, it
has distinct drawbacks. The use of an alternating flux introduces spurious
voltages that are unrelated to flow rate and, if untreated, give rise to
inaccurate indications. The two spurious voltages that are most
troublesome are:
1. stray capacitance-coupled voltages from the coil of the electromagnet to
the electrodes, and
2. induced loop voltages in the input leads. The electrodes and leads in
combination with the liquid extending therebetween constitute a loop in
which is induced a voltage from the changing flux of the magnetic coil.
The spurious voltages from the first source may be minimized by
electrostatic shielding and by low-frequency excitation of the magnet to
cause the impedance of the stray coupling capacitance to be large. But the
spurious voltage from the second source is much more difficult to
suppress.
The spurious voltage resulting from the flux coupling into the signal leads
is usually referred to as the quadrature voltage, for it is assumed to be
90.degree. out of phase with the A-C flow-induced voltage. Actual tests
have indicated that this is not true in that a component exists in-phase
with the flow-induced voltage. Hence, that portion of the "quadrature
voltage" that is in-phase with the flow-induced voltage signal
constitutes an undesirable signal that cannot readily be distinguished
from the flow induced signal, thereby producing a changing zero shift
effect.
Pure "quadrature" voltage has heretofore been minimized by an electronic
arrangement adapted to buck out this component, but elimination of its
in-phase component has not been successful. Existing A-C operated
electromagnet flowmeters are also known to vary their calibration as a
function of temperature, fluid conductivity, pressure and other effects
which can alter the spurious voltage both with respect to phase and
magnitude.
Hence it becomes necessary periodically to manually re-zero the meter to
correct for the effects on zero by the above-described phenomena.
All of the adverse effects encountered in A-C operation of electromagnetic
flowmeters can be attributed to the rate of change of the flux field,
(d.phi.)/dt, serving to induce unwanted signals in the pick-up loop. If,
therefore, the rate of change of the flux field could be reduced to zero
value, then the magnitude of quadrature and of its in-phase component
would become non-existent. Zero drift effects would disappear.
When the magnetic flux field is a steady state field, as, for example, with
continuous d-c operation, the ideal condition d.phi./dt=0 is satisfied.
But, as previously noted, d-c operation to create a steady state field is
not acceptable, for galvanic potentials are produced and polarization is
encountered.
In the patent to Mannherz et al., U.S. Pat. No. 3,783,687, whose entire
disclosure is incorporated herein by reference, there is disclosed an
electromagnetic flowmeter in which the excitation current for the
electromagnetic coil is a low-frequency wave serving to produce a
periodically-reversed steady state flux field, whereby unwanted in-phase
and quadrature components are minimized without giving rise to
polarization and galvanic effects.
In this prior patent, the driver system for exciting the coil includes
switching means acting to periodically reverse the raw output of an
unfiltered full-wave rectifier operated from an a-c power line. Because
the electromagnet has a relatively high inductance, it functions as a
filter choke which takes out a substantial percentage of the ripple
component in the raw output of the rectifier, thereby obviating the need
for filter capacitors. In This drive system, a logic circuit or divider is
provided which is activated at the power line frequency (i.e., 50 or 60
Hz) to produce low frequency gating pulses for governing the
electromagnetic reverse switching action.
Drive systems which are presently employed to provide excitation current
for an electromagnetic flowmeter of the type disclosed in the Mannherz et
al. patent utilize a constant-voltage drive. The long L/R time constant of
the electromagnet produces a relatively slow magnet current rise time;
hence a long excitation period is required to attain a constant flux
level.
Because the total voltage and R are large, to reduce the magnet time
constant to usable values, a substantial amount of power has to be
dissipated by the drive system. As a consequence, a great amount of energy
is lost in heat and the system is inefficient in power terms.
SUMMARY OF INVENTION
In view of the foregoing, the main object of this invention is to provide
an energy-efficient drive system for the excitation coil of an
electromagnetic flowmeter, which system is adapted periodically to
generate a flow of current having a constant level through the coil.
More particularly, it is an object of this invention to provide a
high-voltage impulse drive system of the above-type which shock-excitates
the electromagnet to produce an accelerated current flow therein for a
predetermined period followed by a reduced voltage which then maintains
the current at a constant level, which system, as compared to heretofore
known constant-current drive systems, reduces the power requirements
therefor by a factor of 2 to 4.
Also an object of the invention is to provide a drive system which makes
use of an unfiltered a-c rectifier power supply and maintains a constant
current through the magnet coil of the flowmeter regardless of line
voltage variations.
Briefly stated, these objects are attained in an energy-efficient drive
system for an electromagnetic flowmeter wherein the fluid to be measured
is conducted through a flow tube to intersect a transverse magnet field
created by an electromagnet having an excitation coil, the voltage induced
in the fluid being transferred to a pair of diametrically-opposed
electrodes in contact with the fluid.
To avoid spurious voltages arising from stray couplings between the
electromagnet and the loop constituted by the electrodes and the fluid
extending therebetween, without, however, causing polarization o the
elecrodes, the drive system functions to energize the electromagnet to
periodically generate an excitation current in the coil. This current is
generated by applying the output voltage of an unfiltered full-wave
rectifier to the coil in a manner shock-exciting the coil by a high
voltage surge to produce a current flow therein having a substantially
constant current level during the excitation period.
In one preferred embodiment of the invention, the drive system is actuated
by periodic gating pulses, and the full-wave unfiltered rectifier has a
high voltage section and a relatively low voltage section, a surge of
voltage from the high-voltage section being applied to the excitation coil
at a point coincident with the leading edge of the gating pulse to
shock-excite the coil, the resultant current in the coil being maintained
for the duration of the excitation period at a constant level by the
low-voltage applied to the coil during this period. Because of the
momentary use of a high-voltage, the energy requirement for the system is
low compared to a drive system which makes use of a constant high-voltage.
OUTLINE OF DRAWINGS
For a better understanding of the 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 one preferred embodiment of a drive system
in accordance with the invention;
FIGS. 2A, 2B and 2C are waveforms illustrative of the operation of the
drive system;
FIG. 3 is a schematic diagram of a second preferred embodiment of a drive
system in accordance with the invention;
FIGS. 4A, 4B, 4C, 4D and 4E are waveforms illustrative of the behavior of
the drive system shown in FIG. 3;
FIGS. 5A and B are waveforms showing the relationship of the drive voltage
to the magnet current in the system of FIG. 3;
FIG. 6 is a schematic diagram of a third embodiment of a drive system in
accordance with the invention;
FIGS. 7A and 7B show waveforms illustrative of the FIG. 6 drive system;
FIG. 8 contains curves showing the relationship between current and time in
the magnet coils; and
FIGS. 9A and 9B show graphically how a drive voltage in accordance with the
invention serves to reduce the time it takes to attain a steady state
condition.
DESCRIPTION OF INVENTION
First Embodiment:
Referring now to FIG. 1, there is shown the drive system for an
electromagnetic flowmeter whose primary includes a flow tube 10 through
which a fluid 11 to be metered is conducted. An electromagnet having an
excitation coil 12 serves to establish a magnetic field transverse to the
direction of flow which is parallel to the longitudinal axis of the tube.
Electrodes 13 and 14 in contact with the fluid are disposed at
diametrically-opposed points on the flow tube on an axis perpendicular
both to the longitudinal direction of flow and the transverse magnetic
field.
As is well known, a voltage is induced in the fluid whose flow intersects
the magnetic field to produce a low level a-c signal at the electrode
terminals that reflect the flow rate. This flow-induced signal is applied
to a secondary 15 which may be of the type disclosed in the Mannherz et
al. patent to convert the low-level flow induced signal into a
proportional d-c output signal in a range useful for process engineering,
i.e., 4 to 20 mAdc.
The drive system for energizing excitation coil 12 includes a full-wave
rectifier power supply constituted by a transformer 16 whose primary 16A
is connected to an a-c power line, the line supplying the usual 50 or 60
Hz current. Transformer 16 is provided with three secondary sections 16B,
16C and 16D, secondaries 16B and 16C being center-tapped. Secondary 16B is
connected to the input of a full-wave bridge rectifier 17, the step-down
ratio between secondary 16B and primary 16A resulting in an unfiltered d-c
output voltage of 40 volts between the output of the rectifier 17 at
terminal T.sub.1 and the midpoint terminal T.sub.2 of secondary 16B. This,
therefore, is the high-voltage section of the power supply.
Secondary 16C is connected to the input of a full-wave bridge rectifier 18
to produce between the midpoint terminal T.sub.3 of this secondary and the
output T.sub.1 of rectifier 18 an unfiltered d-c output voltage of 10
volts. This, therefore, is the low-voltage section of the supply.
Secondary 16D is connected to the input of a full-wave bridge rectifier 19
whose output is applied to a filter capacitor 29 to yield a filtered d-c
voltage of 6 volts for energizing the electronic components of the drive
system, to be later described.
Terminal T.sub.1, which is common to both the 40 volt and 10 volt
unfiltered d-c sections, is connected to one end of excitation coil 12
whose other end is connected through a resistor 20 to the outputs of two
Darlington-type power amplifiers 21 and 22, functioning as electronic
power switches. The input of electronic switch 21 is connected to the
midpoint terminal T.sub.2 of the high-voltage section and the input of
electronic switch 22 to the midpoint terminal T.sub.3 of the low-voltage
section. Hence when switch 21 is rendered operative, 40 volts is applied
to excitation coil 12, and when switch 22 is rendered operative, the coil
has 10 volts applied thereto.
Periodic gating pulses, which are preferably generated in the manner
disclosed in the Mannherz et al. patent at a repetition rate which is low
relative to the line frequency, are applied to input terminals T.sub.4 and
T.sub.5, from which these pulses are fed to the light-emitting diode 23 of
a photo-coupler, the light emitted thereby being picked up by a photo
transistor 24 to produce output gating pulses which are applied through
transistor amplifier 25 to the gate terminal G22 of electronic switch 22.
The photo coupler acts to isolate the logic circuit producing the gating
pulses from the drive system.
The gating pulses from the output of amplifier 25 are also fed through a
second transistor amplifier 26 to the input of a one-shot 27 whose output
is coupled through a transistor amplifier 28 to the gating electrode
terminal G.sub.21 of electronic switch 21.
When a gating pulse is applied to electronic switch 22 to render it
conductive, switch 22 then acts to feed the low voltage (10 V) from the
unfiltered rectifier supply to coil 12 for the full duration of the gating
pulse. The leading edge of the same gating pulse serves to trigger
one-shot 27 which then acts to render electronic switch 21 conductive to
apply during the one-shot interval the high voltage (40 V) to excitation
coil 12, the one-shot interval being very brief, as compared to the gating
pulse period.
Electronic switch 21, during each excitation cycle, remains conductive for
a relatively short time whose start is coincident with the leading edge of
the gating pulse and whose conclusion depends on the time constant of
one-shot 27. Thus applied to excitation coil 12 is a sudden drive surge of
high voltage which functions to shock-excite the coil to produce a current
flow therein which is maintained for the remainder of the gating pulse
period at a substantially constant level by the low drive voltage. Thus
the drive voltage wave has a stepped formation constituted by a high step
followed by a low step.
Shunted across excitation coil 12 is a flyback catching network formed by a
diode 30 in series with a resistor 31. Coil 12 is shock-excited by a surge
of high voltage to initiate magnet current flow therethrough in one
direction, which flow is maintained at a constant level for the remainder
of the gating period by the low voltage applied to the coil. Upon the
termination of the gating period, the magnetic field collapses to produce
flyback flow in the reverse direction, diode 30 of the flyback catching
network being then rendered conductive. The duration of the flyback
interval is controlled by the value of resistor 31 in the network.
The waveform of the rectangular low-frequency gating pulses G is
illustrated in FIG. 2A, each pulse having a duration T. The one-shot
interval is represented by t.sub.1, and it will be seen in FIG. 2B, which
illustrates the waveform of the drive voltage for the electromagnet, that
the high voltage surge HV occurs during interval t.sub.1 which starts at
zero voltage at the leading edge of the gating pulse and continues for a
small portion of gating period T. In the remainder of this gating period,
we have the low drive voltage LV which continues to a point in time
determined by the trailing edge of the gating pulse G.
As shown in FIG. 2C, which illustrates the magnet current I.sub.c flowing
through coil 12, during the high-voltage interval t.sub.1, the current
rises from zero to a high level, which level is maintained constant for
the remainder of the gating period, the current then reversing direction
during a flyback interval t.sub.2, and falling back to zero, thereby
completing the excitation cycle which is repeated when the next gating
pulse appears.
Second Embodiment:
In the drive system shown in FIG. 1, a unidirectional drive voltage is
produced; whereas in the system shown in FIG. 3, which functions in
essentially the same way, the drive voltage yielded thereby to cause an
excitation current to flow in the electromagnet coil 12 of the flowmeter
is bi-directional in nature.
This is accomplished in a circuit arrangement which basically constitutes a
doubling of that shown in FIG. 1. The drive system in FIG. 3 produces an
alternating drive voltage, the positive half cycle of which has a stepped
formation, with a high-voltage (+50 V) step followed by a low-voltage (+10
V) step, the negative half cycle having a like stepped formation, but with
negative voltage values (see FIG. 5A).
As in the FIG. 1 arrangement, rectangular gating pulses at a low frequency
rate derived from the converter of the flowmeter or from a suitable
internal clock, are applied, as shown in FIG. 3, to terminals T.sub.4 and
T.sub.5 which are connected to the light-emitting diode 23 of the
photo-coupler having a photo-transistor 24. The output pulses from the
photo-coupler are applied through a logic circuit to electronic switches
of the Darlington-driver type, one set of switches 21 and 22 functioning
to supply one polarity of the drive voltage, and the complementary set of
switches 21' and 22' supplying the other polarity. The logic circuit,
which includes NOR gates 32 and 33, acts to effect alternate operation of
the two sets of electronic switches.
The plus and minus polarity high-voltage electronic switches 22 and 22' are
controlled by one-shot 27 and the complementary one-shot 27'. One-shot 27
is triggered by the positive-going edge of the incoming gate, and one-shot
27' by the negative-going edge thereof. The high plus and minus voltages
(50 V) are obtained from full-wave rectifier 17 coupled to center-tapped
secondary 16B. A low plus and minus operating voltage for powering the
drive circuit is obtained from full-wave rectifier 19 coupled to
center-tapped secondary 16C. For low-frequency drive operation, all of the
filter capacitors for the high and low voltage supplies, including
capacitors 34 and 35, may be omitted; for the 120 cycle ripple component
of the unfiltered full-wave rectified voltages is then effectively
filtered out by the excitation coil functioning as a filter choke.
The timing sequence is shown graphically in FIG. 4. FIG. 4A illustrates the
rectangular waveform of incoming gate pulses G of alternating polarity
applied to terminals T.sub.4 and T.sub.5. The output of one-shot 27' is
shown in FIG. 4B. This one-shot is fired at a point coincident in time
with the leading edge of negative gate pulse G, causing a +50 volt drive
to be applied to excitation coil 12 via electronic switch 22.
As shown in waveform 4C, electronic switch 21 is turned on at a point
coinciding with the trailing edge of the timing interval of one-shot 27',
causing a low voltage drive (+10 V) to be applied to coil 12. As shown in
FIG. 4D, one-shot 27 is fired at a point coinciding with the trailing edge
of the negative gating pulse G, causing a -50 volt drive to be applied to
coil 12 via electronic switch 22'. Finally, as shown in FIG. 4E, switch
21' is turned on at a point coincident with the trailing edge of the
timing interval of one-shot 27 to cause a low voltage drive (-10 V) to be
applied to the coil.
Consequently, as shown in FIG. 5A, the positive half cycle of the drive
voltage wave has a stepped formation to provide a +50 volt drive surge
followed by a +10 volt current-maintenance drive, the negative half cycle
having a like formation. The resultant magnet current is shown in FIG. 5B,
where the time interval t.sub.1, during which the drive voltage crosses
the zero line, is determined by the duration of the +50 volt surge, and
the time interval t.sub.2, during which the drive voltage crosses the line
in the reverse direction, is determined by the duration of the -50 volt
surge.
Third Embodiment:
This embodiment provides a drive voltage for excitation coil 12 which,
during the gating period, instead of being in the form of a high-voltage
surge followed by a low level current-maintenance drive voltage for the
remainder of the period, is composed of intermittent high-voltage surges
whose respective durations are such as to maintain the magnet current at a
constant level during the drive cycle. To this end, it is necessary to
sense the current flowing through coil 12.
This is accomplished, as shown in FIG. 6, by a high-gain comparator 36
which monitors the voltage drop developed across resistor 20 connected in
series with coil 12, this drop varying with magnet current flow. The
variable input voltage to the comparator is compared with a set point
voltage adjusted by a potentiometer 37 having a low d-c voltage (+5 V)
applied thereto taken from a power supply constituted by secondary 16C of
transformer 16, full-wave rectifier 18 and filter capacitor 38.
The output of comparator 36 is applied to one-shot 27, which, when fired,
actuates electronic switch 21 to apply to coil 12 a high voltage (-40 V)
obtained from secondary 16B and full-wave rectifier 17.
Comparator 36 functions to turn on one-shot 27 as frequently as is
necessary to maintain a constant current flowing in the electromagnet
coil, regardless of fluctuations in the line voltage. Thus the moment the
magnet current falls below a predetermined constant current level, this
fact is sensed by comparator 36 to fire one-shot 27 and turn on electronic
switch 21 to again shock-excite the coil and bring the magnet current up
to its desired level.
As shown in FIG. 7A, during each gating period, voltage surges are produced
intermittently to provide energy sufficient to maintain the magnet current
at a constant level. As shown in FIG. 7B, the collapse of the magnet
current at the conclusion of the gating period is again controlled by the
flyback catching network formed by diode 30 in series with resistor 31.
The advantage of the third embodiment over those disclosed in the previous
figures is that an automatic control loop is created which senses
fluctuations in magnet current and corrects therefor, thereby taking into
account line voltage variations and other variables; whereas in the other
embodiments, the drive voltages generated by the drive system are
independent of the magnet current. Net power consumption is also reduced
over that obtainable by the previous embodiments.
Power Savings:
We shall now explain how a drive system in accordance with the invention
effects significant power savings.
The power consumed by the excitation coils of the electromagnetic flowmeter
is determined by the I.sup.2 R law. Consequently, a reduction in the ohmic
value of resistance R will cut down power consumption in direct relation
to the degree of R reduction. This is best effected by using a larger
diameter wire for the excitation coils, but with the same number of turns,
so that the inductance L of the coils remains the same, whereas the
resistance R thereof is reduced.
This reduction of coil resistance R without a change in coil inductance L
results in an increase in the L/R time, thereby extending the time
required to attain a steady state condition. This is illustrated in FIG.
8, where the rise in current I in the excitation coils is plotted against
time in curve R.sub.n which shows the normal rise rate of current until a
steady state condition is attained. It will be evident by comparison with
curve R.sub.r showing the rate of rise in current resulting from a
reduction in the value of resistance R, that it takes much longer for the
current to attain a steady state condition when operating with a reduced
value of R.
In order to retain the power savings gained by reducing coil resistance
without unduly extending the time it takes to attain a steady state
current condition, a drive system in accordance with the present invention
acts initially to apply a drive voltage to the excitation coils whose
magnitude is far above the normal level, the drive voltage being abruptly
reduced once the steady state condition is achieved.
This relationship between drive voltage and excitation current is
illustrated in FIGS. 9A and B, where FIG. 9A shows the waveform of the
applied voltage, and FIG. 9B the resultant current in the excitation
coils. It will be seen from these figures that during the high voltage
surge V.sub.1, that current I in the coil rises rapidly, and that when the
current attains the desired steady state level, the voltage is sharply
reduced to level V.sub.2 to maintain the current at this level.
The only practical limitation to the present drive system in effecting a
reduction in power consumption are those imposed by copper volume and
copper cost. Once an acceptable I.sup.2 R loss is established, any ratio
of I.sup.2 and R may be selected for a given number of turns. This ratio
will determine the operating current and voltages.
The following are examples of this operating principle:
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I.sup.2 R = Constant - L = N.sup.2 - 1/2 diameter = 4 .times. R
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1000 I = 2 amp R = 10.OMEGA.
L = 1 henry
I.sup.2 R = 40 watts
turns L/R = 0.1 sec.
2000 I = 1 amp R = 40.OMEGA.
L = 4 henry
I.sup.2 R = 40 watts
turns L/R = 0.1 sec
4000 I = 0.5 amp
R = 160.OMEGA.
L = 16 henry
I.sup.2 R = 40 watts
turns L/R = 0.1 sec
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While there have been shown and described preferred embodiments of a
high-voltage impulse driver for electromagnetic 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 thereof.
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