Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND DEVICE FOR DETERMINING THE
LIMIT LEVEL OF A MEDIUM IN A VESSEL
Field of the Invention
The invention relates to a device and a method for determining a limit level
of
a medium in a vessel.
Background of the Invention
Capacitive measuring devices or vibration systems are used to detect the limit
level of liquids or granular solids in vessels. In addition to such limit
level detectors,
1o which signal that a predetermined level has been reached as soon as they
come into
contact with the medium to be measured, measuring systems exist which operate
without making contact and, as the measuring radiation, use ultrasound waves,
microwaves - or in particularly critical applications - radioactive radiation.
The known systems have been proven in practice, but cannot be used in all
possible applications. Capacitive measuring devices and vibration systems come
into
direct contact with the medium to be measured. If they are used to detect
corrosive
media, then they must be very highly resistant to corrosion. Furthermore,
deposits on
the measuring devices lead to inaccurate measurement results. Their use is
problematic if the level of different media, having different electrical
characteristics,
2o is intended to be measured.
The application options of the sensors which are known are limited by the
temperature and pressure conditions at the measurement point. For example
piezo-
ceramics which are used in vibration systems irreversibly lose their
characteristic
properties as soon as the ambient temperature exceeds a specific value, the
Curie
temperature, which depends on the respective material.
Those measuring devices which operate on the basis of ultrasound waves are
also severely dependent on the temperature. Temperature compensation is
absolutely
essential here in order to obtain reliable measurement results. Ultrasound
sensors are
also highly dependent on the composition of the gas area located above the
medium;
3o in vacuum conditions, or if the pressures in the gas area are high, they
cannot be used.
It should furthermore be mentioned that the sensitivity of ultrasound sensors
is also
influenced by the noise level in the environment.
In many fields of use, for example in the petrochemical, chemical and
foodstuffs industries, high-precision measurements are required of the level
of liquids
or granular solids in vessels (tanks, silos, etc.). For this reason sensors
are increasingly
being used in which short electromagnetic radio-frequency pulses or continuous
microwaves are coupled onto a conductive, elongated element, for example a rod
probe or a cable probe, and are introduced by means of the conductive element
into
the vessel in which the medium is stored.
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From the physical point of view, this measuring method uses the effect that a
proportion of the radio-frequency pulses or microwaves being propagated is
reflected
on the boundary surface between two different media, for example air and oil
or air
and water, as a result of the sudden change (discontinuity) in the dielectric
constants
of the two media, and this reflection is passed via the conductive element
back into
the receiving device. The reflected portion of the radio-frequency pulses or
microwaves in this case becomes greater the greater the difference in the
dielectric
constants of the two media. The time of flight of the reflected portion of the
radio-
frequency pulses or microwaves allows the distance to the boundary surface to
be
1o determined. If the height of the empty vessel is known it is possible to
calculate the
level of the medium in the vessel.
Sensors with guided radio-frequency signals (pulses or waves) are
distinguished from sensors which emit radio-frequency pulses or waves freely
(free-
field microwave systems or 'real radar systems') by having considerably less
attenuation. The reason for this is that the power flows in an entirely
controlled
manner along the rod or cable probe, or the conductive element. Furthermore,
sensors
using guided radio-frequency signals have a higher measurement quality in the
near
field than freely emitting sensors.
The advantage of sensors using guided radio-frequency signals is, furthermore,
2o the high accuracy and reliability of the level measurement. This is based
on the fact
that measurement using guided measuring signals is largely independent of the
product properties of the medium (moisture, dielectric constant, change of
medium),
the vessel structure (materials, geometry) or the other operating conditions
(dust,
deposits, and angle of the granular solids).
Until now, it has not been known for such measuring systems with guided
measuring signals to be used as limit level detectors.
The invention is based on the object of proposing a method and a device
which allow a predetermined limit level of a medium in a vessel to be detected
with
high reliability.
3o A first embodiment of the inventive method achieves the object in that a
radio-
frequency measuring signal is coupled onto a conductive element of a
predetermined
length and is guided along the conductive element, the measuring signal is
reflected as
a system-dependent echo signal in those regions of the conductive element in
which
there is a sudden change in the characteristic impedance, the measuring signal
is
reflected as a useful echo signal when the free end of the conductive element
is
located in the immediate vicinity of the surface of the medium, or in contact
with the
medium, and the fact that the medium has reached the predetermined level in
the
vessel is determined on the basis of the relative position of the useful echo
signal and
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at least one system-dependent echo signal, or on the basis of the relative
position of
two system-dependent echo signals.
According to an advantageous embodiment of the method according to the
invention, use is made of the system-dependent echo signal of the signal which
occurs
upon the transition of the coupling of the measuring signal onto the
conductive
element (~fiducial launcher). Additionally or alternatively, use is made of
the
system-dependent echo signal of the signal which is reflected at the free end
of the
conductive element ('tJend-of-line peak). Furthermore, it is proposed to make
use of
an additional impedance which is integrated in the coupling unit and produces
a
1o system-dependent echo signal. A further alternative is provided by setting
a reference
zero signal at the same moment when the measurement signal is triggered.
In the case of the method according to the invention, it is thus not
absolutely
essential for the conductive element to come into contact with the medium to
be
detected. This variant of the method described above thus also has the
abovementioned advantages of measurement methods which make no contact. At the
same time, the method according to the invention has the advantage over those
measuring methods in which the measuring signals are emitted freely into space
that
the influence of external interference variables is largely excluded. This
preferred
variant of the method according to the invention operates particularly well,
moreover,
2o when the medium has a high dielectric constant.
According to a further development of the method according to the invention,
the fact that the predetermined level has been reached is output as soon as
there occurs
inside a predetermined time window a useful echo signal which has an opposite
sign
to the peak of the fiducial launcher signal or of the end-of-line peak signal.
This type
of identification of the predetermined level is, of course, highly
advantageous since it
is based on a simple yes/no question, which can be implemented technically
without
any problems, for example by means of a threshold value detector. There is no
need
for complex evaluation methods based on the time of flight of the measuring
signals.
Furthermore, one advantageous embodiment of the method according to the
invention is the fact that the maximum or minimum level has been reached is
output
as soon as the end-of-line peak signal occurs inside a predetermined time
window.
This embodiment can be used only when the conductive element is immersed in
the
medium. As soon as this is the case, the position of the end-of-line peak
changes
(owing to the different times of flight of the measuring signals outside and
inside the
medium or to an amplitude change due to partial or complete signal absorption
in
conductive media) with respect to the constant position of the peak which
occurs
owing to the reflection of the measuring signal upon the transition from the
coupling
unit to the conductive element.
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The object is also achieved by the following alternative of the inventive
method for determining a limit level of a medium in a vessel. A radio-
frequency
measuring signal is coupled onto a conductive element of a predetermined
length and
is guided along the conductive element; the measuring signal is reflected as a
system-
s dependent echo signal in those regions of the conductive element in which
there is a
sudden change in the characteristic impedance; the measuring signal is
reflected as a
useful echo signal when the free end of the conductive element is located in
the
immediate vicinity of the surface of the medium, or in contact with the
medium, and
the fact that the medium has reached the predetermined level in the vessel is
to determined on the basis of the amplitude change of one system-dependent
echo signal,
especially on the basis of the amplitude change of the end-of-line peak. Under
specific
process conditions and depending upon the proximity of the process material to
the
sensor/probe tip, the end-of-line peak may appear as any value across a
spectrum of
various positive and negative amplitudes. Critical to the detection then, is
to monitor
15 that peak for any change of amplitude.
The object is furthermore achieved by the following alternative embodiment
of the method according to the invention. A radio-frequency measuring signal
is
coupled onto a conductive element of a predetermined length and is guided
along the
conductive element; the measuring signal is reflected in each case as an echo
signal in
2o those regions on the conductive element in which there is a sudden change
in the
characteristic impedance; a first echo signal and a second echo signal are
determined
for different levels; a differential signal is then formed from the first echo
signal and
the second echo signal, and the fact that a predetermined limit level of the
medium
has been reached in the vessel is determined with the aid of the position of
the peak or
25 of the peaks of the differential signal or with the aid of the amplitude
change of the
peak or the peaks of the differential signal. It has been found that this
variant of the
method according to the invention is highly advantageous when it is necessary
to
determine the level of a medium having a relatively low dielectric constant.
According to a development of the embodiment of the method according to
3o the invention described above, the first echo signal is preferably
determined when the
free end of the conductive element is located in the immediate vicinity of the
surface
of the medium or is in contact with the medium, while the second echo signal
is
determined when the free end of the conductive element is not located in the
immediate vicinity of the surface of the medium or is not in contact with the
medium.
35 One advantageous embodiment of the method according to the invention
envisages that the fact that the predetermined level has been reached is
output as soon
as the differential signal occurs in a time window in which the fiducial
launcher peak
and/or the end-of-line peak occur/occurs when there is no medium in the
immediate
vicinity of or in contact with the conductive element.
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One preferred (since it is simple to implement) embodiment of the method
according to the invention provides that the fact that the predetermined level
has been
reached is output when the amplitude of the useful echo signal or a peak of
the
differential signal is greater than or equal to a predetermined threshold
value.
The device according to the invention comprises a signal-generating unit, a
coupling unit, a conductive element, a receiving unit and an evaluation unit,
the
signal-generating unit generating radio-frequency measuring signals, the
coupling unit
coupling a measuring signal onto a conductive element of a predetermined
length, the
conductive element guiding the measuring signal, the measuring signal in each
case
1o experiencing reflections as a system-dependent echo signal in those regions
of the
conductive element in which there is a sudden change in the characteristic
impedance,
the measuring signal experiencing reflections as a useful echo signal when the
conductive element is in the immediate vicinity of the surface of the medium
or is in
contact with the medium, and the evaluation unit outputting the fact that a
predetermined level of the medium has been reached in the vessel when the
useful
echo signal or a first system-dependent echo signal occurs inside a
predetermined time
window with reference to the system-dependent echo signal or a second system-
dependent echo signal.
Furthermore, the object is achieved by the following variant of the device
according to the invention. The device once again comprises a signal-
generating unit,
a coupling unit, a conductive element, a receiving unit and an evaluation
unit, the
signal-generating unit generating radio-frequency measuring signals, the
coupling unit
coupling a measuring signal onto a conductive element of a predetermined
length, the
conductive element guiding the measuring signal, the measuring signal in each
case
being reflected as an echo signal in those regions of the conductive element
in which
there is a sudden change in the characteristic impedance, and the receiving
unit/evaluation unit determining a first echo signal and a second echo signal
in the
case of different levels, forming a differential signal from the first echo
signal and the
second echo signal, and determining the fact that the predetermined limit
level of the
3o medium has been reached in the vessel with the aid of the position of the
peak of the
differential signal.
According to a preferred embodiment of both device variants according to the
invention, the length of the conductive element is selected such that the peak
of the
fiducial launcher and the end-of-line peak overlap one another at least
partially. The
conductive element itself may be either a Goubau conductor or a Sommerfeld
conductor.
One advantageous development of the inventive device variants envisages that
the free end of the conductive element terminates with its front flush with
the inner
surface of a wall of the vessel or with the outer surface of the coupling
unit.
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It is advantageous and cost-effective if a comparator or discriminator circuit
is
used to identify the limit level of the medium in the vessel, which comparator
or
discriminator circuit identifies and outputs the change in sign of the echo
signal upon
the occurrence of the useful echo signal or the differential signal inside a
predetermined time window.
The sensor device of the present invention is particularly adapted for the
measurement of material levels in process vessels and storage vessels, but is
not
thereto limited. It is understood that the sensor device may be used for
measurement
of other process variables such as flow, composition, dielectric constant,
moisture
l0 content, etc. In the specification and claims, the term 'vessel' refers to
pipes, chutes,
bins, tanks, reservoirs, or any other storage vessels. Such storage vessels
may also
include fuel tanks, and a host of automotive or vehicular fluid storage
systems or
reservoirs for engine oil, hydraulic fluids, brake fluids, wiper fluids,
coolant, power
steering fluid, transmission fluid and fuel.
Brief Description of the Drawings
The invention will be explained in more detail with reference to the following
drawings, in which:
Fig. 1a shows a schematic illustration of a first embodiment of the device
according to the invention,
Fig. lb shows a schematic illustration of a second embodiment of the device
according to the invention,
Fig. lc shows a schematic illustration of a third embodiment of the device
according to the invention,
Fig. 2 is a diagram which shows the interface signal amplitude as a function
of
time for various media,
Fig. 3 is a graph which shows the change of the signal amplitude of the echo
signals for various media,
Fig. 4 is a graph which shows the change of the interface signal differential
amplitude of the echo signals for various media, and
3o Fig. 5 shows a block diagram of a preferred embodiment of the device
according to the invention.
Detailed Description of Preferred Embodiments
Fig. 1a shows a schematic illustration of a first embodiment of the device
according to the invention. The medium 9 whose limit level is intended to be
detected
is located in the vessel 8. The level measuring device, that is to say the TDR
sensor 1,
is mounted in an opening in wall 12 of the roof 13 of the vessel 8. TDR is,
moreover,
the abbreviation for _Time _Domain _Reflectometry, which is normally used for
the
measuring method in which radio-frequency measuring signals are guided along a
conductive element 6 in order to determine a limit or continuous level of a
medium 9
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in a vessel 8. The measuring signals (S) and the echo signals (R), which are
radio-
frequency pulses, are represented in stylized form in Fig. 1a. The measuring
signals
(S) are generated in the signal-generating unit 2 and are coupled onto the
conductive
element 6 via the coupling unit 3. Owing to the sudden impedance change upon
the
, transition from the coupling unit 3 (D fiducial launcher) to the conductive
element 6,
a portion of the measuring signal is reflected at the fiducial launcher 3 (See
Fig. 2).
Referring to Figs. 1a and 2, the free end 7 of the TDR sensor 1 is shown not
in
contact with the medium 9. Since the electromagnetic field which is coupled to
the
measuring signal fills a certain three-dimensional area around the conductive
element
6, a further peak (an echo signal) occurs in addition to the reflection at the
free end 7
(end of-line reflection as shown in Fig. 2). This additional echo signal is
caused by
the reflection of the measuring signal on the surface 10 of the medium 9. This
echo
signal is clearly measurable as long as the free end 7 is in contact with the
medium 9
or the distance between the free end 7 of the conductive element 6 and the
surface 10
of the medium 9 is not greater than a maximum distance which is defined
exactly in
advance. While the fiducial launcher 3 peak (signal) and the end-of-line peak
(signal)
represent system-dependent echo signals, the echo signal reflected from the
surface
10, which represents the fact that the medium 9 has reached the limit level in
the
vessel 8, is the so-called useful echo signal (See Fig. 2).
2o The useful echo signal is received by a receiving unit 4 and processed by
an
evaluation unit 5. While the signal -generating unit 2, the receiving unit 4,
and the
processing unit S could be any suitable well known units, in the preferred
embodiment
such units are those described in U.S. Patents 5,841,666 and U.S. Patents
5,884,231,
the disclosures of which are hereby incorporated by reference.
Fig. lb shows a schematic illustration of a second embodiment of the device
according to the invention. This second variant differs from the embodiment
shown in
Fig. 1a only in that, in this case, the free end 7 of the conductive element 6
terminates
with its front flush with the coupling unit 3. One consequence of this
configuration of
the TDR sensor 1 can clearly be seen in Fig. 3: the end-of-line peak and the
useful
3o echo signal occur virtually simultaneously, and interfere.
Fig. lc shows a schematic illustration of a third embodiment of the device
according to the invention. This embodiment differs from the variant shown in
Fig. lb
only in that the TDR sensor 1 is mounted in the side wall 12 of the vessel 8
in which
the medium 9 is located.
As will be discussed further the embodiments of the inventions shown in Figs.
la, lb and 1c are useful as switches. The switch is activated when the
conductive
element 6 either comes in contact with the medium 9 or the surface 10 of the
medium
9 is within a distance which is predetermined and discussed later.
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In the graph shown in Fig. 2, the signal amplitudes of echo signals (S)
without
any medium (dotted lines), with oil as the medium (dashed line) and with water
as the
medium (solid line) are shown as a function of time. The signals shown in Fig.
2 can
occur when the conductive element 6 is either in contact with the medium 9 or
within
the predetermined distance from the surface 10 of medium 9. If there is no
medium y
in the vessel 8, then a relatively strong negative peak occurs after a short
time,
reflecting the proportion of the signal which occurs at the transition from
the coupling
unit 3 (fiducial launcher) to the conductive element 6. As can be seen from
Fig. 2,
neither the position nor the amplitude of this so-called fiducial launcher
signal
changes during filling of the vessel 8. This peak thus represents a system-
dependent
echo signal for the respectively used TDR sensor 1. Owing to the fact that
this does
not vary with regard to the medium or the respective level of the medium 9 in
the
vessel 8, the fiducial launcher is used as a reference signal for the useful
echo signal,
or for a further system-dependent echo signal. The useful echo signal occurs
as soon
as a medium 9 is located in the vessel 8.
As can be seen from the amplitude profile of the echo signal, which is not
influenced by the presence of a medium 9 (i.e., dotted lines), the second
system-
dependent echo signal is the end-of-line peak (signal). The end-of-line peak
(signal)
reflects the proportion of the signal which is reflected at the free end 7 of
the
2o conductive element 6. In contrast to the fiducial launcher, the end-of-line
peak (signal)
is not a variable which is dependent exclusively on the respectively used TDR
sensor
1. In fact, when covered by the medium 9 both the position and the amplitude
of the
end-of-line peak are influenced by the medium 9 whose limit level is intended
to be
detected. The reason why the end-of-line peak occurs at different times (See
Fig. 2)
with different media is the different propagation velocity of the measuring
signal (S)
in different media 9.
One variant of the method according to the invention uses the variation in the
time at which the useful echo signal and/or the end-of-line peak (signal)
occurs. If the
useful echo signal and/or the end-of-line peak (signal) appears inside one or
more
3o defined time windows where the reference signal is in each case the system-
dependent
fiducial launcher peak (signal) then this is a clear indication that the limit
level has
been reached.
Fig. 3 shows a graph which illustrates the change of the signal amplitude of
the echo signals for different types of media 9. A TDR sensor 1 having a short
conductive element 6 is used in this case. Owing to the short length of the
conductive
element 6, the end-of-line peak (signal) for no medium and the useful echo
signal for
a medium having a high dielectric constant (e.g., water) virtually coincide in
time. As
shown in Fig. 3, if there is no medium 9 in the vicinity of the TDR sensor 1,
then
there is only one, pronounced, negative end-of-line peak (signal) in the
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time/amplitude profile of the echo signal. If a medium 9 with a high
dielectric
constant, for example water, is located in the vicinity of the TDR sensor 1,
then a
positive peak appears around the time of the end-of-line peak, as the result
of the
useful echo signal and the end-of-line peak overlapping one another.
The second variant of the method according to the invention uses the effect
described above to determine the limit level. This second variant operates
particularly
reliably, moreover, when a medium 9 having a high dielectric constant is
located in
the vessel 8. If a positive peak appears in the time window 20 in which the
negative
end-of-line peak normally occurs, then this is a clear indication that the
medium 9 in
1o the vessel 8 has reached the limit level.
As the signal profile for oil as the medium shows, this inversion effect does
not occur with a medium 9 having a relatively low dielectric constant. In
order to
allow the method according to the invention to be used in a case such as this
as well,
the echo signal is in this case not used as such, but rather the difference
between the
echo signal when the medium 9 is present, and the echo signal without any
medium 9.
Fig. 4 shows the resultant rate of change of the signal difference amplitude
of
the end-of-line peak (signal) in air and the useful echo signal for different
media 9.
The solid curve shows the profile of the difference between the signals when
the
useful echo signal is being influenced by water as the medium 9. The dashed
line
2o shows the corresponding difference between the signals in the presence of
oil as the
medium 9. With no medium present the difference would be a straight line
through
the zero voltage point (i.e., no amplitude). Both when detecting the level of
water and
when detecting the level of oil, there is a pronounced peak in a time window
20 in
which the end-of-line peak normally appears with the opposite sign (See Fig.
3).
Thus, if a positive peak occurs inside a time window 20 around the end-of-line
peak
(See Fig. 3), then this is once again a clear indication that the medium 9 in
the vessel
8 has reached the limit level. In order to improve the reliability of the
detection
results, the invention furthermore provides that the limit level is indicated
only when
the echo signal additionally exceeds a predetermined threshold value 22 inside
the
3o defined time window 20 (See Fig. 4).
Fig. 5 shows a block diagram of a preferred embodiment of the device
according to the invention. The device according to the invention can be
produced
technically very easily and cost-effectively and is included in the receiving
unit 5. It is
sufficient to confirm whether a peak or an inversion of the peak of the echo
signal
does or does not occur inside a defined time window 20. If a positive peak
appears in
the time window 20, then this is a clear indication that the medium 9 in the
vessel 8
has reached the limit level. In terms of circuitry, the echo signal is passed
to the input
of a comparator or discriminator circuit 11 or of a threshold value detector.
A signal
which appears at the output of the comparator or discriminator circuit 11 or
of the
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threshold value detector is either 1 or U. A 1 indicates that the respective
limit level to
be monitored has been reached.
