Abstrict A flow meter includes a measuring tube for conveying a fluid to
be measured and a magnetic field system, which has at least one
field coil, through which an exciter current (I.sub.M) flows, at
least at times, for producing a magnetic field passing through the
fluid at least partly perpendicularly to a stream direction. In
the method, the voltage (U.sub.H) instantaneously driving the exciter
current (I.sub.M) is changed at a second point in time, t.sub.2
from the second voltage level, U.sub.drv, to a third voltage level,
U.sub.short, especially to a third voltage level which is constant
or controlled to be constant, in order to achieve a sinking of the
electrical current strength of the exciter current (I.sub.M) instantaneously
flowing in the at least one field coil from the maximum current
value, I.sub.m, to an electrical current end value, I.sub.czu, especially
a constant end value, predetermined for the exciter current (I.sub.M)
In such case, the third voltage level, U.sub.short, is chosen smaller
than the second voltage level, U.sub.rev. For determining a third
point in time, t.sub.3 which corresponds to the reaching of the
electrical current end value, I.sub.c, the exciter current I.sub.M
is registered, at least at times. Based on this, a first time constant,
T.sub.short, for the magnetic field system is determined, which
corresponds to a time span, t.sub.3-t.sub.2 lying between the second
point in time, t.sub.2 and the third point in time, t.sub.3 and/or
a second time constant, T.sub.rev+T.sub.short, for the magnetic
field system, which corresponds to a time span, t.sub.3-t.sub.1
lying between the first point in time, t.sub.1 and the third point
in time, t.sub.3. For determining a diagnosis value representing
an instantaneous operating state of the flow meter, the determined
first time constant, T.sub.short, is then compared with a predetermined
first reference value, T.sub.1ref, and/or the determined second
time constant, T.sub.rev+T.sub.short, is compared with a predetermined
second reference value, T.sub.2ref.
Claims 12. A method for operating a magneto-inductive flow meter having
a measuring tube for conveying a fluid to be measured and a magnetic
field system, which has at least one field coil, through which an
exciter current (I.sub.M) flows, at least at times, for producing
a magnetic field passing through the fluid at least partially perpendicularly
to a stream direction, which method comprises the following steps:
changing a voltage (U.sub.H) driving, at least at times, the exciter
current (I.sub.M), at a first point in time, t.sub.1 from an instantaneous
first voltage level, U.sub.cont, especially a first voltage level
different from zero, to a second voltage level, U.sub.drv, especially
a second voltage level which is constant or controlled to be constant,
for increasing to a maximum current value, I.sub.m, a current strength,
I, of the exciter current (I.sub.M) driven by the voltage (U.sub.H)
and flowing in the at least one field coil, wherein the second voltage
level, U.sub.drv, is selected to be greater than the first voltage
level, U.sub.cont; and changing the voltage (U.sub.H) instantaneously
driving the exciter current (I.sub.M), at a second point in time,
t.sub.2 from the second voltage level, U.sub.drv, to a third voltage
level, U.sub.short, especially a third voltage level which is constant
or controlled to be constant, for sinking the exciter current (I.sub.M)
instantaneously flowing in the at least one field coil, from the
maximum electrical current value, I.sub.m, to an electrical current
end value, I.sub.c, predetermined for the exciter current (I.sub.M),
especially to an electrical current end value which is constant,
wherein the third voltage level, U.sub.short, is selected to be
smaller than the second voltage level, U.sub.rev.
13. The method as claimed in claim 12 wherein: the third voltage
level, U.sub.short, is chosen smaller than the first voltage level,
U.sub.cont.
14. The method as claimed in claim 11 wherein: the voltage (U.sub.H)
at least at times driving the exciter current (I.sub.M) is changed
in a manner such that its second voltage level, U.sub.drv, is, at
least at times, essentially constant during a time span, t.sub.2-t.sub.1
lying between the first point in time, t.sub.1 and the second point
in time, t.sub.2 especially also immediately before the second
point in time.
15. The method as claimed in claim 11 wherein: at least one field
coil of the coil arrangement is, at least at a time, short-circuited
for the adjusting of the voltage (U.sub.H) to the third voltage
level, U.sub.short.
16. The method as claimed in claim 11 further comprising the step
of: registering the exciter current (I.sub.M) for determining a
third point in time, t.sub.3 corresponding to a reaching of the
electrical current end value, I.sub.c.
17. The method as claimed in claim 11 further comprising the step
of: determining a first time constant, T.sub.short, for the magnetic
field system, which corresponds to a time span, t.sub.3-t.sub.2
lying between the second point in time, t.sub.2 and the third point
in time, t.sub.3.
18. The method as claimed in claim 11 further comprising the step
of: determining a second time constant, T.sub.rev+T.sub.short, for
the magnetic field system, which corresponds to a time span, t.sub.3-t.sub.1
lying between the first point in time, t.sub.1 and the third point
in time, t.sub.3.
19. A method for reviewing a magneto-inductive flow meter having
a measuring tube for conveying a fluid to be measured and a magnetic
field system, which has at least one field coil, through which an
exciter current (I.sub.M) flows, at least at times, for producing
a magnetic field passing through the fluid at least partially perpendicularly
to a stream direction, which method comprises the following steps:
changing a voltage (U.sub.H) driving, at least at times, the exciter
current (I.sub.M), at a first point in time, t.sub.1 from an instantaneous
first voltage level, U.sub.cont, especially a first voltage level
different from zero, to a second voltage level, U.sub.drv, especially
a second voltage level which is constant or controlled to be constant,
for increasing to a maximum current value, I.sub.m, a current strength,
I, of the exciter current (I.sub.M) driven by the voltage (U.sub.H)
and flowing in the at least one field coil, wherein the second voltage
level, U.sub.drv, is selected to be greater than the first voltage
level, U.sub.cont; changing the voltage (U.sub.H) instantaneously
driving the exciter current (I.sub.M), at a second point in time,
t.sub.2 from the second voltage level, U.sub.drv, to a third voltage
level, U.sub.short, especially a third voltage level which is constant
or controlled to be constant, for sinking the exciter current (I.sub.M)
instantaneously flowing in the at least one field coil, from the
maximum electrical current value, I.sub.m, to an electrical current
end value, I.sub.c, predetermined for the exciter current (I.sub.M),
especially to an electrical current end value which is constant,
wherein the third voltage level, U.sub.short, is selected to be
smaller than the second voltage level, U.sub.rev; registering the
exciter current (I.sub.M), at least for a time, for determining
a third point in time, t.sub.3 which corresponds with a reaching
of the electrical current end value, I.sub.c; determining a first
time constant, T.sub.short, for the magnetic field system, which
corresponds to a time span, t.sub.3-t.sub.2 lying between the second
point in time, t.sub.2 and the third point in time, t.sub.3 and/or
determining a second time constant, T.sub.rev+T.sub.short, for the
magnetic field system, which corresponds to a time span, t.sub.3-t.sub.1
lying between the first point in time, t.sub.1 and the third point
in time, t.sub.3; and comparing the determined first time constant,
T.sub.short, with a predetermined first reference value, T.sub.1ref,
and/or comparing the determined second time constant, T.sub.rev+T.sub.short,
with a predetermined second reference value, T.sub.2ref, for determining
a diagnosis value representing an instantaneous operating state
of the flow meter.
20. The method as claimed in claim 19 wherein: the third voltage
level, U.sub.short, is chosen smaller than the first voltage level,
U.sub.cont.
21. The method as claimed in claim 19 wherein: the voltage (U.sub.H)
at least at times driving the exciter current (I.sub.M) is changed
in such a manner that its second voltage level, U.sub.drv, is essentially
constant, at least at times, during a time span, t.sub.2-t.sub.1
lying between the first point in time, t.sub.1 and the second point
in time, t.sub.2 especially also immediately before the second
point in time.
22. The method as claimed in claim 19 wherein: at least one field
coil of the coil arrangement is short-circuited, at least for a
time, for the adjustment of the voltage (U.sub.H) to the third voltage
level, U.sub.short.
Description FIELD OF THE INVENTION
[0001] The invention relates to a method for reviewing a magneto-inductive
flow meter having a measuring tube for conveying a fluid to be measured,
a magnetic field system, which has at least one field coil flowed
through, at least at times, by an exciter current for producing
a magnetic field passing through the fluid at least, in part, perpendicularly
to a stream, or flow, direction.
BACKGROUND OF THE INVENTION
[0002] For measuring electrically conductive fluids, flow meters
using a magneto-inductive flow pickup are often employed. In the
following, when necessary, for the sake of simplicity, discussion
will only be in terms of flow pickups, or flow meters, as the case
may be. As is known, magneto-inductive flow meters permit measurement
of the volume flow rate of an electrically conductive liquid flowing
in a pipeline and the reflecting of such in a corresponding, measured
value; thus, the volume of liquid flowing through a pipe cross section
per unit time is measured. Construction and operation of magneto-inductive
flow meters are known, per se, to those skilled in the art and are
described extensively and in detail in DE-A 43 26 991 EP-A 1 275
940 EP-A 1 273 892 EP-A 1 273 891 EP-A 814 324 EP-A 770 855
EP-A 521 169 U.S. Pat. No. 6031740 U.S. Pat. No. 5487310
U.S. Pat. No. 5210496 U.S. Pat. No. 4410926 US-A 2002/0117009
or WO-A 01/90702.
[0003] Flow pickups of the described kind usually each have a non-ferromagnetic
measuring tube, which is inserted fluid-tightly into the pipeline,
e.g. by means of flanges or screwed connections. An electric voltage
produced by charge separations in the flowing fluid by means of
a magnetic field directed transversely to a flow direction of the
fluid to be measured is sensed as a measured voltage by means of
at least two measuring electrodes and further processed in the measuring
device electronics to a corresponding, measured value, for example
a measured value of volume flow rate. The part of the measuring
tube contacting the fluid is, in general, electrically non-conductive,
in order that the measured voltage, induced in the fluid according
to Faraday's Law of Induction by the magnetic field passing, at
least in part, through the measuring tube, not be short circuited.
Metal measuring tubes are, therefore, usually provided internally
with an electrically non-conducting liner, e.g. of hard rubber,
polyfluoroethylene, etc., and are also, in general, non-ferromagnetic;
in contrast, in the case of measuring tubes made completely of a
plastic or a ceramic, especially aluminum oxide ceramic, the electrically
non-conducting liner is not required.
[0004] In the case of magneto-inductive flow pickups, the magnetic
field required for the measuring is produced by a corresponding
magnetic field system, which contains a coil arrangement having
an inductance L and, most often, at least two field coils, corresponding
coil cores and/or pole shoes for the field coils and, as required,
magnetically conductive, field-guiding sheets connecting the coil
cores outside of the measuring tube. However, also magnetic field
systems with a single field coil are known. The coil cores and/or
pole shoes of the magnetic system are, most often, made of a soft-magnetic
material. Magnetic systems with ferromagnetic coil cores have, however,
also already been disclosed. The magnetic field system is usually
arranged directly on the measuring tube.
[0005] For producing the magnetic field, a coil current delivered
from a corresponding measuring device electronics is caused to flow
in the coil arrangement. In order that the magnetic field produced
by the magnetic field system be as homogeneous as possible, the
field coils are, in the most frequent and simplest case, identical
to one another and connected electrically in the same sense in series,
so that the same exciter current can flow through both coils during
operation. It has, however, also already been disclosed to cause
an exciter current to flow through the coils alternately, first
with the same sense, and then with opposite sense, in order, in
this way, to be able to determine, for example, the viscosity of
liquids and/or a degree of turbulence of the flow; see, in this
connection, also EP-A 1 275 940 EP-A 770 855 DE-A 43 26 991. The
aforementioned exciter current is produced by an operating electronics;
it is regulated to a constant electrical current value of e.g. 85
mA, and its electrical current direction is periodically reversed.
The electrical current reversal is achieved by placing the coils
in a so-called T-circuit or in a so-called H-bridge circuit; on
the subject of electrical current regulation and direction reversal,
compare U.S. Pat. No. 4410926 or U.S. Pat. No. 6031740.
[0006] The coil current is, in the case of modern flow pickups,
usually a strobed, bi-polar, rectangular, alternating current, which
is positive in a first half-period of a period, with a constant,
first electrical current end value and which then, by switching,
is negative in a second half-period of the period, with a constant,
second electrical current end value essentially equal in absolute
value to that of the first electrical current end value. The coil
arrangement can be e.g. a single coil, when the magneto-inductive
flow pickup serves as a flow probe (compare U.S. Pat. No. 3529591),
or it can also be composed e.g. of two coil portions, which are
arranged diametrically opposite to one another on opposite sides
of the measuring tube. In U.S. Pat. No. 6763729 U.S. Pat. No.
6031740 or U.S. Pat. No. 4410926 a circuit arrangement for
producing such a coil current is described. This circuit arrangement
includes an energy, or power, supply driving the coil current, as
well as a bridge circuit, embodied as an H-bridge circuit, for modulating
the coil current, with the coil arrangement lying in a transverse
branch of the bridge circuit. Additionally, U.S. Pat. No. 6031740
and U.S. Pat. No. 4410926 show a circuit arrangement for producing
a coil current, which, instead of the bridge circuit embodied as
an H-bridge circuit, has a bridge circuit designed as a T-circuit
for the coil arrangement. Furthermore, U.S. Pat. No. 4204240 describes
a circuit arrangement with an internal energy supply for producing
the coil current of a magneto-inductive flow pickup, which gives
off a voltage, of which a voltage beginning value in each mentioned
half-period is higher during a rising period of the coil current--as
a first period portion of the half-period--than a voltage end value
during a second period portion--as remainder of the half-period.
[0007] The mentioned, induced voltage arises between at least two
galvanic (thus, wetted by the liquid), or between at least two capacitive
(thus, e.g., embedded inside the wall of the measuring tube), measuring
electrodes, with each of the electrodes sensing one potential. In
the most frequent case, the measuring electrodes are so arranged
lying diametrally opposite to one another that their common diameter
lies perpendicular to the direction of the magnetic field and, therefore,
perpendicular to the diameter on which the coil arrangements lie.
The induced voltage is amplified and conditioned by means of an
evaluating circuit to form a measurement signal, which is recorded,
displayed, or itself processed further. Corresponding measurement
electronics are likewise known to those skilled in the art, for
example from EP-A 814 324 EP-A 521 169 or WO-A 01/90702.
[0008] Besides the actual measuring function, modern magneto-inductive
flow meters often also include superordinated diagnosis functions,
by means of which the flow meters can be subjected to a self-test
during operation. Such diagnosis functions are described, for example,
in the already mentioned U.S. Pat. No. 6763729 or EP-A 1 217
337. These self-tests can ascertain, on the one hand, whether all
components of the measuring device are fully capable of performing
their various functions, and, on the other hand, whether the measuring
is proceeding as specified. If the result of such a self-test is
positive, thus, if no malfunctions of the measuring device, or measuring
operation, are present, then it is assured that the measurement
result issued by the measuring device corresponds to the current,
actual value, naturally, to within predetermined tolerances. Especially,
individual components and parts of the measuring device can be reviewed,
as to their ability to function, within the framework of such a
self-test, e.g. by means of impedance measurements or measurements
of ohmic resistance, or conductance, as the case may be. Furthermore,
it is also possible e.g. to review the settings data at input as
to plausibility. In such case, e.g. inputs lying outside of an allowable
range are rejected and not transferred. Independently of the testing
of the settings data at input, data calculated from the settings
data, i.e. data which directly control the measuring operation of
the measuring device, are checked again as to their allowed limits.
This reviewing occurs before starting measuring operation, so that,
in the case of an occurrence of an error with reference to the settings
data, a corresponding error report can be issued and, in keeping
with this, the measurement is not performed.
[0009] In the case of the measuring device described in EP-A 12
17 337 it is proposed to keep the exciter current variable in its
electrical current strength, to measure the measurement voltage
corresponding to the exciter current, and, on the basis of a deviation,
beyond a threshold value, of the measured voltage levels, measured
at various current strengths, from the voltage level expected for
the measurement voltage on the basis of the functional relationship
between the exciter current and the measurement voltage, a malfunctioning
of the measurement operation of the measuring device can be detected.
Thus, the functional relationship between the exciter current and
the measurement voltage forming the basis for the measurement operation
is assumed to be known and is used for the testing of the measurement
operation. In the simplest case, this functional relationship between
the exciter current and the measurement voltage can be linear, so
that a malfunctioning of the measurement operation of the measuring
device can be detected simply by a deviation of the measurement
voltage levels from one another achieved at various current strengths
by more than a threshold value from a linear curve.
[0010] In contrast, in the case of the flow meter described in
U.S. Pat. No. 6763729 the testing determines, following the change
of current direction, at least one parameter of the electrical current
increase, and this is compared with a reference value. In such case,
there is used as parameter a time span extending between two predetermined
current values. Since the increase of the current satisfies a predetermined,
physical law, which is, as a rule, an e-function, it is sufficient
to determine the rise time between two values, in order to obtain
a reliable statement concerning the electrical current increase.
Alternatively, or in supplementation thereof, there can be used
as parameter a time span extending between the switching of the
current direction and the reaching of a predetermined electrical
current value. The point in time of the switching must be determined
very accurately. For example, the switching signal can be used as
trigger signal for a time counter. The predetermined current value
can lie, for example, in the vicinity of the maximum current value,
i.e. in the vicinity of the electrical current strength, which is
assumed following the settling of the magnetic field to a constant
value suitable for the measuring. Additionally, in U.S. Pat. No.
6763729 an increased voltage is used following the switching.
As already disclosed in U.S. Pat. No. 6031740 such an extra voltage
accelerates the build-up of the magnetic field, since it increases
the electrical current rise and thus makes it possible to repeat
the actual measuring more quickly.
[0011] The threshold, or reference, value, in the case of the exceeding
of which a malfunction of the measuring operation of the measuring
device is certifiable, can be determined in the flow meter itself
at an earlier point in time. For example, the desired parameter
can be determined at the time of start-up and digitally stored as
reference value in a microcomputer provided in the measuring electronics,
so that it is available for future review procedures. In this way,
each flow meter receives an individual reference value. The reference
value for the deviation can be established in different ways, e.g.
as a constant, absolute value. Additionally, it is possible to establish
the threshold value as a percentage fraction of the measuring range
end value for the measurement voltage. In this way, it can be achieved,
that, by establishing the threshold value just once--namely as a
percentage fraction of the measuring range end value for the initial
size--a setting of the measuring device is made, which is applicable
for all settable measuring ranges of the measuring device.
[0012] Once a malfunction of the measuring operation of the measuring
device has been detected, various options are possible. For example,
measuring operation of the measuring device can be ended, since,
in such case, no further reliable measurement results can be obtained.
Alternatively, or supplementally, once a malfunction of the measuring
operation is detected, also optical and/or acoustical, warning reports
can be issued.
[0013] Despite this method already known from the state of the
art for self-test of measuring devices, there is still a need to
provide other methods for self-test, with which to establish with
yet greater accuracy, whether a malfunction of the measuring operation
or of the measuring device itself is present. Especially, the case
can arise, namely, that individual components of the measuring device
only slightly deviate from their desired values, so that each individual
component is noted within the framework of a self-test as fully
capable of functioning, yet, in the interaction of the components,
a measuring operation is generated, which is faulty, such that the
stated accuracy of the measured values cannot be assured.
[0014] Moreover, it has been found that eddy currents induced in
the magnet system, due to the switching and the rise of the coil
current, prevent, as discussed also in U.S. Pat. No. 6031740
that the rise of the magnetic field exactly follows the rise of
the coil current, such as would be the case in the absence of coil
cores and/or pole shoes. Rather, the rise of the magnetic field
is delayed and flattened, as compared with the coil current.
[0015] In the ideal case, the plot of current versus time for the
current in the coil arrangement corresponds to the plot of the magnetic
field versus time. Due to the eddy currents, which arise in the
pole shoes and cores of the coil arrangement during the switching
of the magnetic field, as a matter of fact, deviations from the
ideal case occur. The coil current measured outside of the coil
arrangement corresponds, therefore, to the sum of the electrical
current flowing in the coil and the electrical current produced
by the eddy currents. If the electrical current measured outside
of the coil arrangement is used as the controlled variable, then,
as a result, indeed, the current is held constant, but not the magnetic
field. Such is true, so long as the eddy currents have not yet decayed.
[0016] This disadvantageous effect of the eddy currents occurs
also in the case of, and despite, the mentioned increased voltage.
The effect of the eddy currents can be illustrated by imagining
that, connected in parallel with the (coil-) inductance, there is
an eddy current source, whose current adds to the current in the
inductance to make-up the total exciter current. Therefore, the
voltage drop across a resistor lying in series with the inductance
of the field coil is, indeed, a measure for the exciter current,
but not, however, for the true field current in the inductance actually
corresponding with the magnetic field. This is, however, necessary
to know for an exact control of the magnetic field and, to such
extent, also of advantage for the review of the magnetic field system.
SUMMARY OF THE INVENTION
[0017] It is, therefore, an object of the invention to provide
a method, by means of which eddy currents can be decreased in the
magnetic field system and by means of which, to such extent, a lessening
of the rise time of the magnetic field, as it undergoes its rhythmic
pole reversal and accompanying, repetitive, build-up anew. Additionally,
an object of the invention is to provide a method with which a flow
meter of the described kind can be reviewed as it is running and,
to such extent, malfunctions of the measuring device can be registered
very accurately and in simple manner, and, as required, can also
be located.
[0018] For achieving the object, the invention resides, according
to a first variant, in a method for operating a magneto-inductive
flow meter having a measuring tube for conveying a fluid to be measured
and having a magnetic field system, including at least one field
coil carrying, at least at times, an exciter current for production
of a magnetic field passing through the fluid, at least in part,
perpendicularly to a stream direction. The method of the invention
includes a step of changing, at a first point in time, a voltage,
which, at least at times, drives the exciter current, from an instantaneous
first voltage level, for example one different from zero, to a second
voltage level, for example one which is constant, or controlled
to be constant, in order to achieve an increase, to a maximum electrical
current value, of an electrical current strength of the exciter
current driven by the voltage and flowing in the at least one field
coil. In such case, the second voltage level is chosen greater than
the first voltage level. Additionally, the method of the invention
includes a step of changing, at a second point in time, the voltage
instantaneously driving the exciter current, from the second voltage
level to a third voltage level, for example one which is constant
or controlled to be constant, in order to achieve a sinking of the
electrical current strength of the exciter current instantaneous
flowing in the at least one field coil, from the maximum electrical
current value to an electrical current end value, for example a
constant electrical current end value, predetermined for the exciter
current. In such case, the third voltage level is chosen to be smaller
than the second voltage level.
[0019] Moreover, a second variant of the invention resides in a
method for reviewing a magneto-inductive flow meter having a measuring
tube for conveying a fluid to be measured and having a magnetic
field system, which has at least one field coil, through which an
exciter current flows, at least at times, for producing a magnetic
field passing through the fluid at least partly perpendicularly
to a stream direction. The method of the invention for reviewing
the magneto-inductive flow meter includes a step of changing, at
a first point in time, a voltage driving, at least at times, the
exciter current, from an instantaneous first voltage level, for
example one different from zero, to a second voltage level, for
example one which is constant or controlled to be constant, for
increasing, to a maximum electrical current value, an electrical
current strength of the exciter current driven by the voltage and
flowing in the at least one field coil. In such case, the second
voltage level is chosen greater than the first voltage level. Additionally,
the method includes a step of changing, at a second point in time,
the voltage instantaneously driving the exciter current, from the
second voltage level to a third voltage level, for example one which
is constant or controlled to be constant, for sinking the electrical
current strength of the exciter current instantaneously flowing
in the at least one field coil, from the maximum current value,
to an electrical current end value, for example a constant electrical
current end value, predetermined for the exciter current. In such
case, the third voltage level is chosen to be smaller than the second
voltage level. The exciter current is additionally registered, at
least at times, in order to determine a third point in time corresponding
with a reaching of the electrical current end value. Based on the
first, second and third points in time, a first magnetic field system
time constant corresponding with a time span lying between the second
point in time and the third point in time, and/or a second magnetic
field system time constant corresponding with a time span lying
between the first point in time and the third point in time, are/is
determined. By comparing the determined first time constant with
a predetermined, first reference value and/or by comparing the determined
second time constant with a predetermined, second reference value,
then at last one diagnosis value representing an instantaneous operating
state of the flow meter is determined.
[0020] In a first embodiment of the first variant of the invention,
the third voltage level is chosen to be smaller than the first voltage
level.
[0021] In a second embodiment of the first variant of the invention,
the voltage driving, at least at times, the exciter current is changed
in such a manner that its second voltage level is essentially constant
during a time span lying between the first point in time and the
second point in time, for example also immediately before the second
point in time.
[0022] In a third embodiment of the first variant of the invention,
at least one field coil of the coil arrangement is short-circuited,
at least for a time, for adjusting the voltage to the third voltage
level.
[0023] In a fourth embodiment of the first variant of the invention,
such further includes a step of registering the exciter current
for determining a third point in time corresponding with a reaching
of the electrical current end value.
[0024] In a fifth embodiment of the first variant of the invention,
such further includes a step of determining a first magnetic field
system time constant, which corresponds with a time span lying between
the second point in time and the third point in time.
[0025] In a sixth embodiment of the first variant of the invention,
such further includes a step of determining a second magnetic field
system time constant, which corresponds to a time span between the
first point in time and the third point in time.
[0026] A first embodiment of the second variant of the method of
the invention provides that the third voltage level is chosen to
be smaller than the first voltage level.
[0027] A second embodiment of the second variant of the method
of the invention includes that the voltage at least at times driving
the exciter current is changed in such a manner that its second
voltage level is essentially constant, at least at times, during
a time span lying between the first point in time and the second
point in time, especially also immediately before the second point
in time.
[0028] A third embodiment of the second variant of the method of
the invention provides that at least one field coil of the coil
arrangement is, at least for a time, short-circuited for the adjustment
of the voltage to the third voltage level.
[0029] A basic idea of the invention is to lessen the rise time
of the repetitively newly established magnetic field by applying
a counter-voltage for a short time after the reaching of the maximum
electrical current value for the exciter current and, thus, to lengthen,
for example, the time span with constant exciter current, at, in
comparison to conventional magneto-inductive flow meters, a half-period
which remains essentially constant in length.
[0030] An additional basic idea of the invention is to use for
the exciter current-, or magnetic field-, adjustment for the monitoring
of the flow meter, especially of its magnetic field system, the
third point in time--meant here is especially the time span measured
from a suitable reference point in time--of the above-described
method as an indicator depending very strongly on the instantaneous
operating state of the flow meter, especially, however, on the instantaneous
state of the magnetic field system, and, to such extent, reacting
very sensitively to changes in the magnetic field system. The invention
is based, in such case, on, among other things, the discovery that
the errors in the magnetic field system have a very significant
influence on the two mentioned time spans of the control for the
exciter current.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The method of the invention and further advantages will
now be explained in greater detail on the basis of the timing diagrams
and schematic circuit diagrams for a magneto-inductive flow meter
illustrated in the drawing, the figures of which show as follows:
[0032] FIGS. 1 and 2 respectively, show schematically and in the
form of a block diagram, a process measuring device--here in the
form of a magneto-inductive flow meter--suitable for performing
the method of the invention;
[0033] FIG. 3 shows schematically, a timing diagram of an exciter
current flowing during operation of the process measuring device
of FIGS. 1 and 2;
[0034] FIG. 4 is a schematic circuit diagram of a first example
of an embodiment, having an H-bridge circuit;
[0035] FIG. 5 is a schematic circuit diagram of a second example
of an embodiment, having an H-bridge circuit;
[0036] FIG. 6 is a schematic circuit diagram of a first example
of an embodiment, having a T-circuit;
[0037] FIG. 7 is a schematic circuit diagram of a second example
of an embodiment, having a T-circuit;
[0038] FIG. 8 shows waveforms of the coil current and magnetic
induction, or voltage of the voltage source;
[0039] FIG. 9 is a flow diagram of a microprocessor used for the
invention;
[0040] FIG. 10 is a graphic illustration of the waveform of the
coil current and of the magnetic field in the case of the solution
known from the state of the art;
[0041] FIG. 11 is a graphic illustration of the waveform of the
voltage lying on the coil arrangement in the case of the solution
known from the state of the art;
[0042] FIG. 12 is a graphic illustration of the waveform of the
voltage lying on the coil arrangement in the case of the apparatus
of the invention; and
[0043] FIG. 13 is a graphic illustration of the waveform of the
coil current and the waveform of the magnetic field in the case
of the apparatus of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] FIG. 1 shows schematically and partially in the form of
a block diagram a magneto-inductive flow meter suitable for carrying-out
the method of the invention. By means of this flow meter, measured
values can be produced for at least one physical variable of a medium,
especially a fluid, flowing in a pipeline (not shown). For example,
the flow meter can be used for measuring a volume flow rate and/or
a flow velocity of an electrically conducting liquid.
[0045] The flow meter shown here includes a flow pickup 1 for producing
measured potentials corresponding to the physical variable to be
measured, a measuring device electronics having a microcomputer,
especially a measuring and operating circuit 2 especially one implemented,
at least in part, by means of the microcomputer, for registering
the measuring potentials and for producing at least one measurement
signal corresponding with the physical variable, as well as an evaluating
circuit 3 especially one realized by means of the microcomputer,
serving for actuating the measuring and operating circuit 2 and,
thus, also the flow pickup 1 and for producing the measured values
representing the physical variable on the basis of the at least
one measurement signal. The measuring and operating circuit 2 and,
as required, also some components of the flow pickup 1 can be accommodated,
for example, in an electronics housing 10 of the flow meter, as
also indicated schematically in FIG. 2.
[0046] Belonging to the flow pickup 1 is a measuring tube 11 insertable
into the course of the mentioned pipeline. Measuring tube 11 has
a tube wall. The fluid to be measured is allowed to flow through
measuring tube 11 during operation, in the direction of a measuring
tube longitudinal axis.
[0047] For preventing a short-circuiting of voltages induced in
the fluid, an internal part of the measuring tube 11 contacting
the fluid is electrically non-conducting. Metal measuring tubes
are, for such purpose, usually provided internally with an electrically
non-conductive lining e.g. of hard rubber, polyfluoroethylene, etc.,
and are also, in general, non-ferromagnetic; in the case of measuring
tubes made completely of plastic or ceramic, especially aluminum
oxide ceramic, this electrically non-conducting layer is, in contrast,
not required.
[0048] A coil arrangement of a magnetic field system of the flow
meter actuated by a driving electronics 21 provided in the measuring
and operating electronics 2 includes, in the illustrated example
of an embodiment, a first field coil 12 arranged on the measuring
tube 11 and a second field coil 13 arranged on the measuring tube
11. The field coils 12 13 lie on a first diameter of the measuring
tube 11. The magnetic field system serves for producing, in cooperation
with the measuring device electronics, a magnetic field B passing
through the tube wall and the fluid flowing through the measuring
tube 11. This arises, when an exciter current I is caused by the
driving electronics 21 to flow in the field coils 12 13 connected,
in the case shown here, in series. The exciter current I, especially
a bi-polar exciter current I, can, in such case, be e.g. essentially
rectangular or at least have a waveform resembling a rectangular
waveform.
[0049] FIG. 3 shows that the field coils 12 13 do not contain
cores and are, thus, so-called air-coils. The field coils 12 13
can, however, as usual in such coil arrangements, be wound on a
core, which is, in general, magnetically soft, wherein the cores
can interact with pole-shoes; compare e.g. U.S. Pat. No. 5540103.
[0050] The coil arrangement shown in this example of an embodiment
is, here, so embodied, especially the two field coils 12 13 are
so formed and dimensioned, that the magnetic field H produced therewith
within the measuring tube 11 is, at least with reference to a second
diameter perpendicular to the first diameter, symmetrical, especially
rotationally symmetrical.
[0051] In an embodiment of the invention, a direct current, especially
one controlled to have a constant amplitude, is produced by means
of the driving electronics 21. This direct current is then periodically
reversed by means of an appropriate switching mechanism, for example
one configured as an H-, or T-, circuit, and, so, modulated into
an alternating current of controlled amplitude. As a result of this,
thus, the exciter current I is so allowed to flow through the coil
arrangement, that the coils 12 13 as schematically illustrated
in FIG. 3 have current flowing through them in a first flow direction
during a first switch phase PH11 and in an opposite direction reversed
with respect to the first current direction during a second switch
phase PH12; compare, regarding control and reversal of the electrical
current, e.g. also U.S. Pat. No. 4410926 or U.S. Pat. No. 6031740.
[0052] Following on the second switch phase PH12 is a third switch
phase PH21 during which the exciter current I flows again in the
first current direction. Following on the third switch phase is
a fourth switch phase PH22 during which the exciter current again
flows in the opposite direction. Following on this is a corresponding
switch phase PH31 etc. Regarding the reversal of direction of the
exciter current I, in each case, two switch phases, one following
the other, form a switching period P1 P2 P3 etc. Accompanying
the pole reversal of the exciter current I flowing through the coil
arrangement, aside from a possible switch-phase shifting essentially
synchronously thereto, also there is a repeated pole reversal of
the magnetic field H; compare, in this connection, FIG. 3.
[0053] For producing at least one electrical measurement signal
corresponding with the measured variable, a sensor arrangement is
additionally provided in the measurement pickup, arranged on the
measuring tube or at least in its vicinity. In an embodiment of
the invention, the sensor arrangement has electrodes applied essentially
directly on the measuring tube. A first electrode 14 arranged on
an inner side of the tube wall of the measuring tube 11 serves,
in such case, to sense a first potential e.sub.14 induced by the
magnetic field H. An electrode 15 arranged in the same manner serves
additionally for the sensing of a second potential e.sub.15 induced
by the magnetic field. The measuring electrodes 14 15 lie on the
second diameter of the measuring tube 11 with the second diameter
being perpendicular to the first diameter and to the longitudinal
axis of the measuring tube. Electrodes 14 15 can, however, also
lie on a chord of the measuring tube 11 parallel to the second diameter;
compare, in this respect, also U.S. Pat. No. 5646353.
[0054] FIG. 1 shows the measuring electrodes 14 15 to be galvanic
measuring electrodes, thus electrodes which contact the fluid. It
is possible, however, also to use two capacitive measuring electrodes,
thus electrodes arranged e.g. within the wall of the measuring tube
11. Each of the measuring electrodes 14 15 senses its electric
potential e.sub.14 e.sub.15 as such is induced, during operation,
in the fluid flowing through the measuring tube, on the basis of
Faraday's law.
[0055] As shown in FIG. 2 the measuring electrodes 14 15 are
connected during operation, at least at times, respectively with
an inverting and a non-inverting input of a differential amplifier
22. Therefore, a potential difference of the two potentials e.sub.14
e.sub.15 sensed by the measuring electrodes 14 15 is formed to
serve as measurement signal u, this potential difference corresponding
to a voltage arising in the fluid flowing through the measuring
tube and, thus, also to the physical variable to be measured. The
potentials e.sub.14 e.sub.15 lying on the measuring electrodes
14 15 are, usually, in the range of 10 mV to 100 mV. It is to be
noted here, however, that the potentials can, however, be sensed
also over separate measuring channels, and, especially, they can
also be separately digitized; compare, in this connection, also
U.S. Pat. No. 5907103. The measurement signal u present on the
output of the differential amplifier 22 in the example of an embodiment
shown here is, as schematically indicated in FIGS. 1 2 supplied
to the evaluating circuit 3 provided in the flow meter.
[0056] FIGS. 3 and 4 concern examples of embodiments for a switching
mechanism based on an H-bridge circuit. Lying in a first branch
of the bridge is the controlled current path of a first transistor
13 in a second bridge branch the controlled current path of a second
transistor 14 in a third bridge path the controlled current path
of a third transistor 15 and in a fourth bridge branch the controlled
current path of a fourth transistor 16. This structure yields four
corner points 2a, 2b, 2c, 2d of the H-bridge circuit: The transistors
13 14 are connected together by the corner point 2c, the transistors
14 16 by the corner point 2b, the transistors 15 16 by the corner
point 2d and the transistors 13 15 by the corner point 2a. A first
bridge diagonal lies between the corner points 2a, 2b and a second
bridge diagonal lies between the corner points 2c, 2d. In the second
bridge diagonal, a coil arrangement 1 of a magnetic filed system
of the flow pickup is provided. Coil arrangement 1 includes at least
one field coil. In other words, first and second terminals of the
coil arrangement are connected, respectively, with the corner points
2c and 2d.
[0057] In operation of the switching arrangement, interacting with
the coil arrangement as a rectangular modulator, either the first
and fourth transistors 13 16 or else the second and third transistors
14 15 are driven simultaneously into a conductive state. Therefore,
in the first case (transistors 13 16 conducting), an assumed-positive
current flows from corner point 2a to corner point 2b, through the
coil arrangement 1 and in the direction indicated by the solid arrow.
If, in contrast, the transistors 14 15 are conducting, then the
same current flows in the reverse direction through the coil arrangement
1 this being illustrated by the dashed arrow.
[0058] The coil arrangement 1 has an inductance L and is part of
a magnet system producing a magnetic field of a magneto-inductive
flow pickup. The entire flow pickup is not shown, since the mechanical
construction of such pickups is long known to those skilled in the
art; compare the above-cited U.S. Pat. No. 4204240. Of interest
for the invention is only that the magnet system includes a coil
core and/or a pole shoe.
[0059] As is likewise long known to those skilled in the art, the
exciter, or coil, current is so produced by the discussed, alternating
conductive states of transistors 13 16 or 14 15 that it is positive
in the first half-period of the duration of a period, with a constant
first current end value, and negative in its second half-period,
with a constant second current end value, which is equal in absolute
value to the first current end value. The current end value refers
to that constant value of the coil current, e.g. 85 mA, which is
flowing before the switching to the other electrical current direction.
[0060] The end point 2c is connected in FIG. 3 via a resistor 10
to a circuit ground, or zero point, SN. Resistor 10 is in series
with the H-bridge circuit 2 and the coil current flows through resistor
10.
[0061] Additionally shown in FIG. 3 is a controlled voltage source
7 which has a voltage output 7c and provides a voltage (here assumed
positive, as indicated by the plus sign at the output 7c) lying
across the series connection, thus between corner point 2a and the
circuit ground SN. The controlled voltage source 7 is fed by the
alternating current supply of the plant, via the two terminals 7a,
7b; additionally, output 7d of source 7 is connected to circuit
ground SN. The voltage at output 7c is applied through diode 9
in the direction from anode to cathode, to the corner point 2a.
A capacitor 12 of capacitance C leads from the cathode of diode
9 and the corner point 2a to circuit ground SN.
[0062] In FIG. 4 the sequence of coil arrangement and resistor
is reversed in the series connection: The corner point 2b of the
H-bridge circuit 2 now lies at circuit ground SN, while the resistor,
in the form of resistor 10', is, in such case, located between the
output 7c of the controlled voltage source 7 and the anode of diode
9. This yields the advantage that a possible short circuit between
the corner points 2c, 2d, thus e.g. in the coil arrangement 1 cannot
destroy the transistors 13 16 or 14 15 (depending on which pair
happens to be conducting), since the coil current is limited by
the resistor 10'.
[0063] FIGS. 5 and 6 concern examples of embodiments using T-circuits
3 and 3' respectively. A resistor 122 respectively 122', forms
with the coil arrangement 1 a series connection 4 respectively
4'. This is, in the case of the circuit of FIG. 3 so constructed,
that the coil arrangement 1 is connected through resistor 122 to
circuit ground SN, and the coil current flows through the coil arrangement
1. In contrast, in FIG. 4 the first terminal of the series connection
4' is connected to the circuit ground SN.
[0064] A first terminal of the controlled current path of a first
switching transistor 25 is connected with a second terminal of the
series connection 4 4'. A second terminal of this current path
is connected to a first output 30c of a controlled voltage source
30 which determines a positive (note the plus sign on output 30c)
voltage lying across the series connection. A first terminal of
the controlled current path of a second switching transistor 26
is connected with the second terminal of the series connection 4
4'. A second terminal of this current path is connected to a second
output 30d of the controlled voltage source 30 which determines
a negative (note the minus sign on the output 30d) voltage lying
across the series connection. The switching transistors 25 26 are
alternately driven to be conductive, so that the coil current alternately
reverses its direction, as indicated by the two arrows on the coil
arrangement 1. Also here, the coil current is positive in the first
half-period of a period, with a constant electrical current end
value and negative in its second half-period, with a constant, second
electrical current end value, which is equal in absolute value to
the first electrical current end value.
[0065] In FIGS. 5 and 6 the positive voltage at output 30c of
the voltage source 30 is applied to the second terminal of the switching
transistor 25 through a first diode 31 in its direction from-anode-to-cathode.
Additionally, a capacitor 33 of capacitance C1 leads from this terminal
and from the cathode of the diode 31 to the circuit ground SN. The
negative voltage at output 30d of the voltage source 30 is applied
to the second terminal of the switching transistor 26 through a
second diode 32 in its direction from-cathode-to-anode. Additionally,
a second capacitor 34 of capacitance C2 leads from this terminal
and from the cathode of diode 32 to the circuit ground SN.
[0066] In the examples of embodiments presented in FIGS. 3 and
4 and FIGS. 5 and 6 the mentioned inductance L of the coil arrangement
1 forms a resonance circuit with the capacitance C of the capacitor
12 or with the capacitances C1 and C2 as the case may be. This
means that the voltage across the series connection is resonance-reinforced,
and the coil current during a rise at the beginning of every half-period
has a steeper rising edge than would be the case, if the resonance
circuit were not present.
[0067] For the digitizing: A measurement signal u is preferably
digitized and at least a section thereof appropriately stored in
the form of digital data sets, so that, for determining also the
diagnosis value, information concerning the time behavior of a section
of the measurement signal u is available in digital form.
[0068] According to the invention, the evaluating circuit 3 also
serves for producing a diagnosis value serving for review of the
magneto-inductive flow meter. The diagnosis value represents an
instantaneous operating state of the flow meter, especially, however,
an instantaneous operating state of the magnetic field system. For
example, the diagnosis value can be used to generate an appropriate
alarm, when, on the basis of deviations of the diagnosis value from
a corresponding, earlier determined reference, or threshold, value,
a defective magnet system has been recognized. Such defects can
be e.g. vibration-related fractures of electrical lines and/or loosenings
of the assembled elements of the magnetic field system. Moreover,
significant, especially shock-like, fluctuations of the fluid temperature
can also lead to thermally related disturbances of the magnetic
field system or, at least, to changes regarding the calibration
of the magnetic field system.
[0069] For this purpose, the voltage source in the illustrated
example of an embodiment is, as also already described in U.S. Pat.
No. 6031740 first so controlled, that it produces in each half-period
during a first fractional period of the coil current, which is referred
to in the following as the rise time period t.sub.rev, an initial
voltage value U.sub.drv, which is higher, e.g. several times higher,
than a constantly controlled voltage end value U.sub.cont during
a second fractional period, which is referenced in the following
at the residual time period tc; compare FIGS. 5a and 5b. Additionally,
the voltage drop across resistor 10 10', or 22 22', as the case
may be, is used to compensate an effect of eddy currents, which
are induced in the coil cores and/or pole shoes during the rise
of the coil current and which delay the rising edge of the magnetic
field compared to the rising edge of the coil current. This is achieved
by influencing, or controlling, during each half-period, the rise
time period t.sub.rev of the coil current and the level of the positive,
or negative, voltage end value U.sub.cont in such a manner that,
on the one hand, following the reaching of an electrical current
maximum Im, no further increase of the coil current occurs, so that
the magnetic field already reaches a constant magnetic field end
value B.sub.m corresponding to the constant electrical current end
value of the coil current, when the coil current reaches the electrical
current maximum Im (compare FIG. 5a), and that, on the other hand,
the level of the positive, or negative, voltage end value U.sub.cont
always brings-about the constant electrical current end value Ic
of e.g. 85 mA. This is achieved by forming a correction for the
voltage across the H-bridge circuit, or T-circuit in the next half-period
from the waveform of the voltage drop across the resistor arising
during a half-period in the time span after the electrical current
maximum Im of the coil current until the reaching of the electrical
current end value, by sampling at least three times, one after the
other.
[0070] In FIGS. 3 to 6 the transistors 13 14 15 16 of the H-bridge
circuit, and the switching transistors 25 26 of the T-circuit,
are always shown with non-connected control electrodes. This is
merely a showing that the activation of these transistors is adequately
described in the state of the art, e.g. in the above-mentioned U.S.
Pat. No. 4410926 so that those skilled in the art know what needs
to be actuated and how to do that. Additionally, the controlled
current paths of the transistors 13 14 15 16 and of the transistors
25 26 are bridged with free-wheeling diodes 17 18 19 20 and
27 28. The transistors are shown as bipolar transistors; however,
naturally, also field effect transistors can be used, especially
insulating-layer field effect transistors.
[0071] FIG. 5a is a plot, over an entire period, of coil current
I and induction B of the magnetic field in the ordinate direction
versus time as abscissa. In the first half period with positive
coil current I, it is indicated that the duration of the half period
is composed of a time of rise ta and a residual time tc; the residual
time is that in which the induction B of the magnetic field is constant.
During the rise time t.sub.rev, the coil current IM, on the one
hand, rises sharply, then falls again, after reaching an electrical
current maximum Im and reaches, later than the induction B, its
constant current end value I.sub.cont; compare the only gradual
fall of the coil current after the electrical current maximum Im.
This behavior of the coil current is attributable to the above-explained
eddy currents induced in the coil cores and/or the pole shoes. The
point in time of the reaching of the electrical current maximum
Im determines the boundary line between the rise time t.sub.rev
and the residual time t.sub.cont. During the rise time t.sub.rev,
on the other hand, the induction B first rises almost with slope
identical to that of the coil current, but the rise then becomes
flatter later and reaches, already at the end of the rise time ta,
the intended, constant, induction end value B.sub.m.
[0072] The arrows drawn above the curve of the coil current during
its falling part of the residual time t.sub.cont indicate that the
course of the coil current is sampled at least three times, e.g.
every millisecond, before it reaches the electrical current end
value I.sub.m; in the example of FIG. 5a, the falling part of the
coil current is sampled four times. From the sample values, according
to the invention, the control signal for the controlled voltage
source is produced, as is yet to be explained in detail.
[0073] FIG. 5b is a plot of the effective voltage, in the ordinate
direction, on the series connection with the H-bridge circuit or
with the T-circuit versus time t as the abscissa. The first peak
Us is attributable to resonance reinforcement in the coil arrangement.
The constant value following thereon is the mentioned voltage initial
value U.sub.drv, which contributes to the steepening of the rise
of the coil current. Without voltage reinforcement, only the voltage
initial value Ua would be present during the entire rise time t.sub.rev.
During the residual time t.sub.cont the constant voltage end value
U.sub.cont comes into the picture. It can be seen from FIG. 5b that,
as was already mentioned, the voltage initial value U.sub.drv is
larger than the voltage end value U.sub.cont.
[0074] FIGS. 3 to 6 illustrate the control of the controlled voltage
source 7 or 30 by a controller 41 or 42 or 43 or 44 situated
between the resistor 10 or 10', or 122 or 122' and a control input
7e, or 30e, of the controlled voltage source 7 or 30. Additionally,
the relevant controller controls the transistors 13 14 15 16
of the H-bridge circuit, or the switching transistors 25 26 of
the T-circuit. The controller 41 or 42 or 43 or 44 is at least
partly implemented by the already mentioned, appropriately programmed
microcomputer, or microprocessor. Connected in front of this is
an analog/digital converter, which digitizes the voltage drop across
the resistor 10 or 10', or 122 or 122'. Microprocessor and analog/digital
converter are, of course, clocked, or strobed, by a clock oscillator.
[0075] In the case of the method of the invention, first, similarly
to the case with the flow meter described in U.S. Pat. No. 6763729
or in U.S. Pat. No. 6031740 the voltage U.sub.H driving, at least
at times, the exciter current I.sub.M, is, at a first point in time
t.sub.1 raised from an instantaneous first voltage level U.sub.cont
different from zero and present in the preceding half-period, to
the, especially constant, or controlled to be constant, second voltage
level U.sub.drv, for the purpose of increasing the current strength
I of the exciter current I.sub.M, driven by the voltage and flowing
in the at least one field coil, to the maximum current value I.sub.m.
[0076] As already mentioned, the coil current I.sub.M flowing during
the time span t.sub.cont is at least sectionally sampled, digitized
and stored appropriately in the already mentioned microcomputer.
The measured and digitally stored electrical current values can
now, without more, be used for calculating the voltage profile,
towards which the voltage driving the exciter current I.sub.M should
be adjusted in the future for the next half-period of the magnetic
field B. Thus, in effect, an approximation method performed interatively
or recursively over a plurality of measurement cycles is being applied.
The extra voltage U.sub.drv lying on the coil arrangement during
the time span t.sub.rev is successively so tuned from half-period
to half-period, or from measuring cycle to measuring cycle, that
the coil current I.sub.M during the remaining time span t.sub.cont
of any half-period steadily falls toward a constant electrical current
end value I.sub.c. Thus, attention is given to assuring that the
time span t.sub.rev during which the extra voltage U.sub.drv is
applied to the coil arrangement is so selected that the measured
coil current I.sub.M no longer increases during the time span t.sub.cont.
If there is a rise, then this is an indication that the time span
t.sub.rev was chosen too short. As a result of this, the measuring
and operating circuit 2 increases the time span t.sub.rev during
the following half-periods by suitable amounts, until the measured
exciter current I.sub.M exhibits the desired behavior.
[0077] The microcomputer can be programmed, for example, corresponding
to the flow diagram shown in FIG. 7 in which suitable function
and decision blocks are illustrated. Lower-case letters represent
digital signals important for the flow diagram. By means of the
already mentioned analog/digital converter, the voltage drop across
the resistor is digitized, so that a digital signal i representing
the coil current results. This is supplied to the input of a maximum
detector 61 and to the input of a gate stage, which is also fed
with a maximum signal im coming from the maximum detector 61. The
gate stage 62 forwards as current samples s only such portions of
the digital signal i representing the coil current, which occur
later than the occurrence of the signal maximum.
[0078] A first decision stage 63 reviews sequentially following,
neighboring electrical current samples s under the criterion of
whether the later sample is greater than the earlier, thus whether
the coil current has increased between two samples, and issues,
when appropriate, a control signal y at a YES-output Y and, when
not appropriate, a control signal n at a NO-output N. The control
signal y prompts a pulse length stage 64 to lengthen the rise time
ta, and the control signal n prompts the pulse length stage 64 to
lengthen the residual time t.sub.cont. An output of the pulse length
stage leads to the voltage source 7 or 30 as the case may be.
[0079] A second decision stage 65 continuously reviews the current
samples s under the criterion of whether a current sample s is greater
than, equal to, or smaller than an electrical current reference
value ir, which is proportional to the current end value I.sub.cont,
and registers such.
[0080] Depending on its decision, decision stage 65 issues at a
GREATER-output G a control signal g, at an EQUAL-output a control
signal gI, or at a SMALLER-output K a control signal k. These control
signals g, gI, k are supplied to the voltage source 7 or 30 (if
required, with interposition of a digital/analog converter) and
affect the voltage initial value Ua issued therefrom in the duration
of a period of the coil current in such a manner that the control
signal g makes it larger in the following period, the control signal
gI leaves it unchanged in the following period, or the control signal
k makes it smaller in the following period.
[0081] During the actual measurement of the volume flow rate, both
the voltage across the coil arrangement and the exciter current
I.sub.M flowing through the coil arrangement must be constant, since
the stability of the measurement signal is negatively influenced
by, among other things, also the inductive in-coupling of the coil
arrangement onto the measuring electrodes.
[0082] Ideally, the electrical current end value of the true coil
current I.sub.L is, in both half-periods, essentially constant,
opposite, and equal in absolute value. From a practical point of
view, this is, however, first the case in U.S. Pat. No. 6763729
or U.S. Pat. No. 6031740 only after a considerable length of
time has passed-by following the beginning of the time span t.sub.cont.
Therefore, the time window for possible exact measurements during
the time span T.sub.cont is essentially limited almost to just half
of a half-period. Due to the eddy currents arising in the pole shoes
and coil cores, this ideal case does not really occur. Rather, the
exciter current I.sub.M measurable outside of the coil arrangement
corresponds always to a sum of currents composed of the true coil
current I.sub.L and the eddy current I.sub.eddy. Since, however,
only the exciter current is used by the measuring and operating
circuit 2 as the controlled variable, indeed the exciter current
I.sub.M is constant, however not, as a matter of course, the magnetic
field, which is passing through the measuring tube and which really
needs to be kept constant for the measurement.
[0083] In the case of the method of the invention, the eddy currents
induced in the magnetic field system are further minimized in advantageous
manner not only by the application of an extra voltage U.sub.rev
to the coil arrangement for the initially determined reference time
span T.sub.rev=t.sub.2-t.sub.1 until the reaching of the predetermined
current maximum I.sub.m, wherein the extra voltage is so selected
that the exciter current I.sub.M flowing through the coil arrangement
falls, after expiration of the reference time span T.sub.rev, steadily
toward an essentially constant end value I.sub.c, but also by applying
to the coil arrangement 1 following the, first of all, at least
approximately determined expiration of the reference time span T.sub.rev,
for a predetermined, especially equally iteratively determined,
time span T.sub.short=t.sub.3-t.sub.2 a counter voltage U.sub.short
for sinking the exciter current I.sub.M and, therefore, also the
magnetic field back down more quickly, wherein the time span T.sub.short
is so selected that the influence of the eddy currents induced during
the switching procedure in the coil arrangement is approximately
compensated, at least, however, considerably reduced. The effect
of this method is shown in FIG. 9 wherein the continuous line indicates
the course of the measuring current, as it forms in the case of
the solution of the invention; in contrast, the dotted line shows
the time behavior of the exciter current in the case of the solution
known from the state of the art. In the case of the solution of
the invention, the constant electrical current end value I.sub.cont
is essentially reached at the expiration of the time span T.sub.short;
in the case of the known solution, the constant electrical current
end value Icont is first reached at a much later point in time within
the time span T.sub.cont. During a first switching procedure, the
measuring and operating circuit applies, as already explained above,
an appropriate counter voltage U.sub.G to the coil arrangement for
an ongoingly predetermined time span T.sub.short. Then, the measuring
and operating circuit registers a plurality of electrical current
measured values within the predetermined time span T.sub.cont. For
example, both the time span T.sub.rev and the time span T.sub.short
can be successively optimized during operation by, especially iterative,
approximation procedures. For the case in which, with expiration
of the currently valid time span T.sub.short, the electrical current
end value I.sub.c has not yet been reached, the time span T.sub.short
is correspondingly lengthened, for example by an initially determined,
suitable, step size; for the case in which, before the expiration
of the time span T.sub.short, the electrical current end value I.sub.c
has already been reached, the time span T.sub.short can, as required,
again be decreased some. Through the interaction of applied extra
voltage and subsequent, short-time application of the counter voltage
U.sub.short, the eddy currents can be effectively compensated and,
thus, their decay time significantly reduced. It is to be additionally
mentioned here that the relevant time spans T.sub.rev, T.sub.short,
T.sub.cont can, as already indicated, be determined both on the
basis of samples of digital data taken from the exciter current
and also directly by means of a sample of digital data taken from
the exciter current and also directly by means of time measuring
circuits correspondingly triggered by the exciter current I.sub.M
and/or by the voltage driving such.
[0084] A third decision stage implemented in the microcomputer
reviews--assuming that the voltage U.sub.H is momentarily held constant--continuously
the electrical current samples s on the basis of the criterion,
whether electrical current samples s following one after the other
indicate a constant exciter current I.sub.M, or not. For the case
in which the assumption of a constant voltage seems too uncertain,
its actual instantaneous waveform, for example likewise in the form
of digital samples, is appropriately brought into the decision.
The decision stage issues, as appropriate, at a CONSTANT-output
a control signal correspondingly signalizing a continuing constant
exciter current I.sub.M, for so long until a further sinking of
the exciter current I.sub.M below a change boundary value set for
the constancy of the exciter current I.sub.M is recognized.
[0085] Based on the above method optimizing the time span T.sub.cont
maximally available during each half-period for producing the actual
measured, or also diagnosis, values, additionally the following
method is provided for improving the information provided by the
diagnosis value:
[0086] The voltage U.sub.H instantaneously driving the exciter
current I.sub.M is, at a second point in time, t.sub.2 changed
from the second voltage level, U.sub.drv, to an, especially constant,
or controlled to be constant, third voltage level, U.sub.short,
in order to achieve a sinking of the electrical current strength
of the exciter current I.sub.M from the maximum current value, I.sub.m,
instantaneously flowing in the at least one field coil, down to
an, especially constant, electrical current end value, I.sub.czu,
predetermined for the exciter current I.sub.M. In such case, the
third voltage level, U.sub.short, is chosen smaller than the second
voltage level, U.sub.rev. For determining a third point in time,
t.sub.3 corresponding to the reaching of the electrical current
end value, I.sub.c, the exciter current I.sub.M is registered, at
least at times. Based on this, there is chosen for the magnetic
field system, a first time constant, T.sub.short, which corresponds
to a time span, t.sub.3-t.sub.2 lying between the second point
in time, t.sub.2 and the third point in time, t.sub.3 and/or a
second time constant, T.sub.rev+T.sub.short, which corresponds to
a time span t.sub.3-t.sub.1 lying between the first point in time,
t.sub.1 and the third point in time, t.sub.3. For determining a
diagnosis value representing an instantaneous operating state of
the flow meter, the determined first time constant, T.sub.short,
is then compared with a predetermined first reference value, T.sub.1ref,
and/or the determined second time constant, T.sub.rev+T.sub.short,
is compared with a predetermined, second reference value, T.sub.2ref.
[0087] According to an embodiment of the invention, it is provided
that the measuring and operating circuit short-circuits the coil
arrangement during the time span T.sub.short. Alternatively, it
is provided that the measuring and operating circuit again reverses
the direction of the current flowing through the coil arrangement
during the time span t.sub.short.
[0088] According to a further embodiment of the invention, it is
provided that the measuring and operating circuit so selects the
time span t.sub.short that the exciter current flowing through the
coil arrangement at the expiration of the time span t.sub.short
has at least approximately reached the constant electrical current
end value, I.sub.c.
[0089] While the invention is susceptible to various modifications
and alternative forms, exemplary embodiments thereof have been shown
by way of example in the drawings and will herein be described in
detail. It should be understood, however, that there is no intent
to limit the invention to the particular forms diclosed, but on
the contrary, the intention is to cover all modifications, equivalents,
and alternatives falling within the spirit and scope of the invention
as defined by the intended claims.
[0090] While the invention has been illustrated and described in
detail in the drawings and forgoing description, such illustration
and description is to be considered as exemplary not restrictive
in character, it being understood that only exemplary embodiments
have been shown and described and that all changes and modifications
that come within the spirit and scope of the invention as described
herein are desired to protected. |