U.S. patent application number 10/327426 was filed with the patent office on 2004-06-24 for method and apparatus for radar-based level gauging.
Invention is credited to Edvardsson, Kurt Olov.
Application Number | 20040119635 10/327426 |
Document ID | / |
Family ID | 32594247 |
Filed Date | 2004-06-24 |
United States Patent
Application |
20040119635 |
Kind Code |
A1 |
Edvardsson, Kurt Olov |
June 24, 2004 |
METHOD AND APPARATUS FOR RADAR-BASED LEVEL GAUGING
Abstract
A method for radar-based gauging of the level of a substance in
a tank (13) having at least one interfering structure, e.g. a beam
(16a); an agitator (16b) or a tank side wall (16c), comprises
transmitting a microwave signal in a predetermined polarization
state (LHCP) towards the surface (14) of the substance and the at
least one interfering structure; detecting, separately in two
different polarization states (LHCP, RHCP), microwave signals (32,
33, 34) as reflected against the surf ace of the substance and
against the at least one interfering structure; distinguishing
based on signal strengths of the microwave signals detected
separately in the two different polarization states, the detected
microwave signal (32), which has been reflected against the surface
of the substance; and calculating based on a propagation time of
the distinguished microwave signal the level of the substance in
the tank.
Inventors: |
Edvardsson, Kurt Olov;
(Taby, SE) |
Correspondence
Address: |
Judson K. Champlin
WESTMAN CHAMPLIN & KELLY
Suite 1600 - International Centre
900 South Second Avenue
Minneapolis
MN
55402-3319
US
|
Family ID: |
32594247 |
Appl. No.: |
10/327426 |
Filed: |
December 20, 2002 |
Current U.S.
Class: |
342/124 ;
342/188; 73/290R |
Current CPC
Class: |
G01S 13/103 20130101;
G01S 7/026 20130101; G01S 13/88 20130101; G01S 7/024 20130101; G01F
23/284 20130101; G01S 7/025 20130101 |
Class at
Publication: |
342/124 ;
342/188; 073/290.00R |
International
Class: |
G01S 013/08 |
Claims
1. A method for radar-based gauging of the level of a substance in
a tank having at least one interfering structure, comprising the
steps of: transmitting a microwave signal in a predetermined
polarization state towards a surface of said substance and said at
least one interfering structure; detecting temporally resolved and
separately in two different polarization states, microwave signals
as reflected against the surface of said substance and as reflected
against said at least one interfering structure; distinguishing,
based on signal strengths of the microwave signals detected
temporally resolved and separately in said two different
polarization states, the detected microwave signal, which has been
reflected against the surface of said substance; and calculating
based, on a propagation time of the distinguished microwave signal
the level of said substance in said tank.
2. The method of claim 1 wherein said predetermined polarization
state is a circularly polarized polarization state.
3. The method of claim 2 wherein said two different polarization
states are two essentially orthogonal circularly polarized
polarization states.
4. The method of claim 3 wherein the detected microwave signal,
which has been reflected against the surface of said substance, is
distinguished by means having notably different signal strength in
said two different polarization states.
5. The method of claim 3 wherein a signal in two different linearly
polarized polarization states is formed from the microwave signal
in said two essentially orthogonal circularly polarized
polarization states; and said step of distinguishing is based also
on said signal in two different linearly polarized polarization
states.
6. The method of claim 5 wherein the detected microwave signal,
which has been reflected against the surface of said substance, is
distinguished by means having similar signal strength in said two
different linearly polarized polarization states.
7. The method of claim 1 wherein said predetermined polarization
state is a linearly polarized polarization state; and said two
different polarization states are two essentially orthogonal
linearly polarized polarization states.
8. The method of claim 1 wherein a signal strength as a function of
propagation time for each reflected microwave signal in each of
said two different polarization states is obtained in said step of
detecting; and the detected microwave signal, which has been
reflected against the surface of said substance, is distinguished
by means of comparing said functions.
9. The method of claim 8 wherein said signal strength as a function
of propagation time for each reflected microwave signal in each of
said two different polarization states obtained in said step of
detecting is stored in a database.
10. The method of claim 1 wherein said steps of transmitting and
detecting are repeated; and said step of distinguishing is based
also on the variation in signal strengths of the detected microwave
signals.
11. The method of claim 1 wherein said substance is a liquid.
12. The method of claim 1 wherein said substance is a granular
solid.
13. The method of claim 1 wherein said at least one interfering
structure is any of a beam; an agitator or a tank side wall.
14. The method of claim 1 wherein said step of detecting is
performed by a device including any of a power divider,
particularly a Wilkinson power divider, a directional coupler, a
ferrite circulator, or multiple antennas.
15. The method of claim 1 wherein the phase of each of said
detected microwave signals is measured.
16. The method of claim 1 wherein said temporally resolved and
separately in two different polarization states detected microwave
signals are mixed to form microwave signals, of which the microwave
signal as reflected against said at least one interfering structure
is suppressed.
17. The method of claim 16 wherein said temporally resolved and
separately in two different polarization states detected microwave
signals are mixed to form said microwave signals after having had
their respective amplitudes altered.
18. The method of claim 17 wherein said temporally resolved and
separately in two different polarization states detected microwave
signals are mixed to form said microwave signals after having had
their respective phases altered.
19. An apparatus for radar-based gauging of the level of a
substance in a tank having at least one interfering structure,
comprising: a transmitter for transmitting a microwave signal in a
predetermined polarization state towards a surface of said
substance and said at least one interfering structure; a detector
for detecting temporally resolved, separately in two different
polarization states, microwave signals as reflected against the
surface of said substance and as reflected against said at least
one interfering structure; and a processing device for
distinguishing based on signal strengths of the microwave signals
detected temporally resolved and separately in said two different
polarization states, the detected microwave signal, which has been
reflected against the surface of said substance; and for
calculating, based on a propagation time of the distinguished
microwave signal, the level of said substance in said tank.
20. The apparatus of claim 19 wherein said predetermined
polarization state is a circularly polarized polarization
state.
21. The apparatus of claim 20 wherein said two different
polarization states are two essentially orthogonal circularly
polarized polarization states.
22. The apparatus of claim 21 wherein said processing device is
adapted to distinguish the detected microwave signal, which has
been reflected against the surface of said substance, by means
having notably different signal strength in said two different
polarization states.
23. The apparatus of claim 21 wherein said processing device is
adapted to form a signal in two different linearly polarized
polarization states from the microwave signal in said two
essentially orthogonal circularly polarized polarization states;
and to base the distinguishing also on said signal in two different
linearly polarized polarization states.
24. The apparatus of claim 19 wherein said processing device is
adapted to form a signal strength as a function of propagation time
for each detected reflected microwave signal in each of said two
different polarization states; and to distinguish the detected
microwave signals which has been reflected against the surface of
said substance, by means of comparing said functions.
25. The apparatus of claim 19 wherein said transmitter and said
detector is adapted to repeatedly transmit and detect,
respectively, and said processing device is adapted to base the
distinguishing also on the variation in signal strengths of the
detected microwave signals.
26. The apparatus of claim 19 wherein said substance is a
liquid.
27. The apparatus of claim 19 wherein said substance is a granular
solid.
28. The apparatus of claim 19 wherein said at least one interfering
structure is any of a beam; an agitator or a tank side wall.
29. The apparatus of claim 19 wherein said step of detecting is
performed by a device including any of a power divider,
particularly a Wilkinson power divider, a directional coupler, a
ferrite circulator, or multiple antennas.
30. A method for radar-based gauging of the level of a substance in
a tank having at least one interfering structure comprising the
steps of: transmitting microwave signals in two different
predetermined polarization states, one after the other, towards a
surface of said substance and said at least one interfering
structure; individually for each of the transmitted microwave
signals detecting temporally resolved in a predetermined
polarization state, microwave signals as reflected against the
surface of said substance and as reflected against said at least
one interfering structure; distinguishing, based on signal
strengths of the microwave signals detected temporally resolved,
the detected microwave signal, which has been reflected against the
surface of said substance; and calculating based on a propagation
time of the distinguished microwave signal the level of said
substance in said tank.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to level gauging, and more
specifically the invention relates to a method and an apparatus for
radar-based level gauging.
BACKGROUND OF THE INVENTION AND RELATED ART
[0002] Radar-based methods are extensively used for level gauging,
i.e. measuring a distance from the top of a tank to a surface of a
liquid, or some kind of granular solid stored in the tank, by means
of transmitting microwaves towards the surface of the liquid or the
granular solid, receiving the microwaves as reflected against the
surface of the liquid or solid, i.e. the surface echo, and
calculating the level of the liquid or solid in the tank from the
propagation time of the transmitted and reflected microwaves.
[0003] One very general problem in this respect is that the tank
includes typically various structures, such as beams, support
beams, pipes, agitators, tank walls, etc. Such structures may also
reflect microwaves,. and such echoes can interfere with the
microwaves reflected from the surface of the liquid or solid, the
level of which being gauged. An interfering echo having a signal
strength considerably lower than the signal strength of the surface
echo will cause measuring errors if the structures creating the
interfering echoes is close to, i.e. within a few tenths of a meter
from, the surface. If the interfering echo is stronger than the
echo from the surface the interfering echo may falsely be selected
as the surface echo. Various echo logic methods have been applied
to decrease this risk, but nevertheless this is a large problem in
many tank environments.
[0004] The selection of correct microwaves is thus extremely
important and any possibility of distinguishing microwave signals
reflected from the surface of the liquid or solid from microwaves
reflected from other structures is very useful.
[0005] Typically, prior art radar level gauges typically select the
strongest echo.
[0006] The ideal case is to use an antenna with a rather narrow
lobe located in the tank where no disturbing echoes are close to
the antenna lobe. In this case the surface echo may be the
strongest one even after some degradations due to turbulence, foam
etc. For smaller tanks various tank structures may approach the
antenna beam, not at least since the antenna has to be smaller.
Furthermore when the echo from the surface is close to a disturbing
echo there is a risk for a large measuring error.
SUMMARY OF THE INVENTION
[0007] A main object of the invention is thus to provide a method
and an apparatus for radar-based level gauging, wherein detected
microwaves as reflected from the surface of the liquid or solid can
be distinguished from detected microwaves as reflected from other
disturbing structures.
[0008] In this respect there is a particular object of the
invention to provide such a method and such an apparatus, which are
very useful in tanks having a large number of disturbing structures
and in tanks where the radar-based level gauging equipment has to
be mounted in region where disturbing structures do occur.
[0009] A further object of the present invention is to provide such
a method and such an apparatus, which provide for level gauging
also of highly turbulent surfaces, where the reflected microwaves
are weak.
[0010] A still further object of the invention is to provide such a
method and such an apparatus, which are reliable, efficient,
accurate, and precise.
[0011] These objects, among others, are attained by methods and
apparatuses as claimed in the appended claims.
[0012] According to a first aspect of the present invention there
is provided a method for radar-based gauging of the level of a
substance, e.g. a liquid or a granular solid, in a tank having one
or several interfering structures, such as e.g. a beam, a support
beam, an agitator, or a tank side wall. The method comprises to
transmit a microwave signal in a predetermined polarization state,
e.g. left hand circular polarization, towards a surface of the
liquid or granular solid and the interfering structures(s).
Microwave signals as reflected against the surface of the liquid or
granular solid and against the interfering structure(s) are
detected temporally resolved and separately in at least two
different polarization states, e.g. left and right hand circular
polarizations. Then, the detected microwave signal, which has been
reflected against the surface of the liquid or granular solid, is
distinguished based on signal strengths of the microwave signals
detected temporally resolved and separately in the two different
polarization states. Finally, the level of the liquid or granular
solid in the tank is calculated based on a propagation time of the
distinguished microwave signal.
[0013] The inventor of the present invention has noted that the
detected microwave signal, which has been reflected against the
surface of the liquid or granular solid, is distinguishable by
means having notably different signal strength in the two different
polarization states. This is true as long as the surface is calm.
In contrast thereto, the detected microwave signal(s), which has
been reflected against the surface of the interfering structure(s),
has (have) typically similar signal strength(s) in the two
different polarization states. For the microwave signal any two
preferably orthogonal polarizations will cover all different
possible combinations. In the signal processing, however, more than
two different signals may be formed and used.
[0014] By detecting the microwave echoes in two separate
polarizations a further processing can be performed not only
limited to the detection and rejection of disturbing echoes, but
also for enabling a decrease of the influence of disturbing echoes
close to the surface echo. For each echo a linear combination of
the signals in the two received polarizations can be found, where
the echo is very weak, while other echoes are less or much less
reduced. This enables in many cases a substantial improvement of
the signal-to-disturbance ratio assuming that different linear
combinations are used for different disturbing echoes. Using prior
art equipment large measuring errors cannot be avoided if the
disturbing echo is close to the surface echo and of comparable
strength.
[0015] Further, a time variation of the signal strengths may be
recorded to distinguish a microwave signal reflected against a
turbulent surface from microwave signals reflected against fixed
interfering structures. The time variation for the echo from a
turbulent surface can in stochastic sense in most cases be
described as Raleigh distributed. This can be used as one
distinctive feature to separate the echo from the turbulent surface
from the much more steady echo from a fixed disturbing echo. In one
embodiment of the invention the polarization diversity is combined
with the time variation measurement to obtain a selection criteria,
which is usable, both for calm and turbulent surfaces.
[0016] Transceiver apparatuses capable of producing polarized
microwave radiation and of receiving reflected microwave radiation
in two different polarization states separately include preferably
any of a power divider, particularly a Wilkinson power divider, a
directional coupler, a ferrite circulator, or multiple
antennas.
[0017] According to a second aspect of the present invention there
is provided a radar-based level gauge apparatus for performing the
method according to the first aspect of the invention.
[0018] By means of the present invention a very robust routine for
distinguishing detected microwave signals, which have been
reflected against the surface of the substance gauged, may he
implemented. As compared to prior art devices the invention
provides for measurement in more disturbing environments, i.e.
where more interfering echoes do occur, with higher accuracy. For
instance, microwave signals reflected at the surface of the
substance, may be distinguished despite being weaker to much weaker
than a microwave signal as reflected against an interfering
structure.
[0019] The radar-based level gauges are used to measure levels in
tanks, which for the purpose of the present invention include not
only large containers but also processing apparatuses such as, for
example, reactors, centrifuges, mixers, hoppers, graders; or
heat-treatment furnaces and similar devices, which are used in e.g.
food chemistry, pharmaceutical chemistry, biochemistry, gene
chemistry and petrochemistry.
[0020] Further characteristics of the invention, and advantages
thereof, will be evident from the detailed description of preferred
embodiments of the present invention given hereinafter and the
accompanying FIGS. 1-4, which are given by way of illustration
only, and thus are not limitative of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1a illustrates schematically, in a side view, an
apparatus for radar-based level gauging according to a preferred
embodiment of the present invention.
[0022] FIGS. 1b-c are diagrams illustrating various polarization
properties of different kind of echoes.
[0023] FIGS. 2a-f illustrate schematically, in side views, various
implementations of a transceiver structure used in radar-based
level gauging apparatuses of the present invention.
[0024] FIG. 3 shows schematically a diagram of the amplitude of the
received reflected microwave signal as a function of propagation
time for two different polarization states as obtained by the
apparatus of FIG. 1 indicating microwave signals as reflected from
the surface of the matter gauged and from a fixed structure in the
tank.
[0025] FIG. 4 is a schematic flow diagram illustrating a method for
radar-based level gauging according to a preferred embodiment of
the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0026] With reference to FIG. 1a, which schematically illustrates,
in a side view, an apparatus aimed for radar-based level gauging, a
preferred embodiment of the present invention will be described.
The apparatus may be a frequency modulated continuous wave (FMCW)
radar apparatus, a pulsed radar apparatus, or any other type of
distance measuring radar.
[0027] The radar-based level gauge, denoted by 11 in FIG. 1a, is
mounted above an opening of a roof 12 of a tank 13 or container
filled with a liquid or a granular solid, the level 14 of which
being gauged. The tank 13, of which only portions are shown in FIG.
1a, has typically a number of structures, e.g. a support beam 16a,
an agitator 16b and a tank side wall 16c, that may create
disturbing microwave reflections within the tank. When a radar
level gauge is to be installed a typical criteria is to find a
location where the influence of such disturbances is small but the
practical possibilities to attain this criteria is generally
small,
[0028] During operation, the radar-based level gauge 11 transmits a
microwave signal towards the surface 14 of the liquid or granular
solid in the tank 13, and receives the microwave signal as
reflected against the surface, i.e. the surface echo. Further, the
radar-based level, gauge 11 comprises, or is connected to, a signal
processing device (not explicitly illustrated) for calculating from
the propagation time of the microwave signal from the radar-based
level gauge 11 to the level 14 of the liquid or granular solid, and
back to the radar-based level gauge 11.
[0029] The expression propagation time is used throughout this text
as a general expression for a result of distance calculations.
Several types of radar-based apparatuses are known for detection of
echoes and distance evaluations. Different radar-based apparatuses
use different methods and different intermediate results will
occur. In each case, however, the radar input signal can be
described as echoes of different amplitude and phase ordered after
their delay time as compared to the transmitted signal.
[0030] One example is a pulsed radar-based level gauging apparatus,
where a sampling technique is used for re-scaling or stretching the
time by a factor 1,000-1,000,000. Another example is the FMCW
radar-based level gauging apparatus, where a usually linear
frequency sweep results in a spectrum where the propagation time is
translated to intermediate frequency (IF). In this case the IF
signal corresponds to the Fourier transform of the IF signal for
the pulsed case. The very short time differences (1.5 mm distance
corresponds to 0.01 ns) will in both cases be translated
(downscaled) to signals with a much more handy time and frequency
behavior and thus the signal processing is simplified and
components having lower price, lower current consumption etc. can
be used.
[0031] According to the present invention the radar-based level
gauge 11 transmits a microwave signal in a particular or
predetermined polarization state towards the surface 14 of the
liquid or granular solid and surrounding interfering structures
16a-c; and detects temporally resolved and separately, in two
different polarization states, microwave signal echoes as reflected
against the surface 14 and the surrounding interfering structures
16a-c. By temporally resolved is here meant that the delay times as
compared to the transmitted signal, or other parameter values
depending thereon, are recorded for the echoes.
[0032] Preferably, the radar-based level gauge 11 transmits a
circularly polarized microwave signal, e.g. left hand circularly
polarized LHCP as illustrated in FIG. 1a or right hand circularly
polarized RHCP, and detects microwave signal echoes in both left
and right hand circular polarization states LHCP, RHCP. However,
other polarization states are possible, such as e.g. two orthogonal
linear polarization states XLP, YLP.
[0033] The present inventor has discovered that different
structures in the tank environment change the polarization state of
a microwave signal differently at reflection. For instance, a calm
surface 14 changes the polarization state of an incident microwave.
Given an incident left hand circularly polarized LHCP microwave
signal, the surface echo will be essentially right hand circularly
polarized RHCP, as is indicated in FIG. 1a. In contrast thereto
most interfering structures will only partly change the
polarization of the incident polarized microwave signal, and the
echoes from the interfering structures will in general have similar
signal strengths in the two different circularly polarization
states LHCP, RHCP. This is indicated for the support beam 16a in
FIG. 1a. Another way to express this is that a linearly polarized
wave will have markedly different reflection if the polarization is
parallel or if it is perpendicular to for instance a straight steel
beam.
[0034] Thus, the signal processing device of the level gauge 11 is
adapted to distinguish, based on the notably different signal
strengths of the separately detected microwave signal echoes in the
two different polarization states, the surface echo.
[0035] Depending on the type of radar-based level gauge used only
the different amplitudes may be detected or both different
amplitudes and different phase may be detected. Most pulsed systems
utilize only the amplitude while a typical FMCW-system may use both
phase and amplitude.
[0036] Finally, when the surface echo is found the level of the
liquid or granular solid is calculated from the propagation time of
the surface echo.
[0037] The received signal can be described by a diagram where the
depolarization of the received signal is shown as a function of the
degree of turbulence. Degree of turbulence can he defined in
different ways but is a measure on the time dependence and can be
defined as the standard deviation of the amplitude divided by the
average value of the amplitude. An alternative formulation is the
difference between maximum amplitude and minimum amplitude divided
by the average amplitude. In FIG. 1b the degree of turbulence is
assumed to be near 0 for the echo from a calm surface but near 1
for the echo from a turbulent surface. Likewise, depolarization can
be defined in different ways but 1 is in FIG. 1b assumed to
correspond to a smooth surface, where the transmitted LHCP signal
is received as an entirely RHCP signal (or RHCP to LHCP), while 0.5
implies that half the received power is received as LHCP and the
other half as RHCP (which is the case for a turbulent surface). A
typical property of a circularly polarized wave (LHCP or RHCP) is
that the direction of polarization is reversed by the reflection
(LHCP to RHCP etc.).
[0038] When the surface is turbulent the surface echo does not
change polarization state completely as can be seen in FIG. 1b,
wherein the relative signal strength of right hand circularly
polarized RHCP microwaves as a function of relative degree of
turbulence when left hand circularly polarized LHCP microwaves are
transmitted from the gauge is shown. A surface echo area is
indicated by 17, where echoes from a calm surface are found in the
upper portion of the area 17, i.e. a high relative RHEP signal
strength and low relative degree of turbulence, whereas echoes from
a highly turbulent surface are found in the lower portion of the
area 17, i.e. a relative RHCP signal strength closer to 0.5 and
high relative degree of turbulence. An area for typical echoes from
disturbing structures in the tank is indicated by 18, i.e. echoes
having a relative RHCP signal strength closer to 0.5 and low
relative degree of turbulence. Thus, by means of repeating the
transmission and detection a time variation of the signal strengths
of the various echoes may be measured. Then, turbulent surface
echoes may be distinguished from echoes from fixed structures by
means of searching for detected echoes with a noticeable time
variation. Thus, neither the time dependence nor the polarization
alone can distinguish fixed echoes from surface echoes, but when
combined in a suitable way a much more efficient tool for
distinguishing is obtained.
[0039] In some environments the selection procedures as discussed
above may not be sufficient for selecting the correct surface echo
by high certainty. In such circumstances a signal in two different,
preferably- orthogonal, linearly polarized polarization states XLP,
YLP can be formed from the microwave signal detected in the two
circular polarization states, and the selection procedure for
selection of the correct surface echo can be based also on the
signal in the two different linearly polarized polarization states
as formed. Surface echoes from at least calm surfaces will be of
similar strengths for two different orthogonal linear polarization
XLP, YLP states, whereas interfering echoes will have noticeably
different signal strengths in the two different linear
polarizations. This is illustrated in FIG. 1c, which shows the
relative signal strength of vertically linearly polarized YLP
microwaves as a function of the relative signal strength of
horizontally linearly polarized XLP microwaves. Most surface echoes
are found within an area denoted 19, whereas most interfering
echoes are found within any of the areas denoted by 20.
[0040] Thus, classification of the echo in order to simplify the
selection of the correct surface echo can be performed but this can
be seen as just a first step. The extraction of two signal channels
received in two polarizations enables the forming of an arbitrary
linear combination of the signals and by a suitable choice of such
a linear combination it is generally possible to improve the
signal-to-disturbance ratio. By means of the echo selection
procedure according to the present invention, surface echoes having
signal strengths which are much lower than signal strengths of
interfering echoes, are detectable and distinguishable.
[0041] Besides the signal strength the phase of the radar signal is
important. The amplitude and phase of a signal is called the
complex amplitude. Depending on details in the radar-based level
gauging apparatus the amplitude only or both amplitude and phase
are measured.
[0042] Next, with reference to FIGS. 2a-f various implementations
of the antenna structure used for sending and receiving the
microwaves in an apparatus of the present invention. Note that
while only a few examples are explicitly given below, the present
invention is not limited to those, but can be implemented by any
structure capable of sending microwaves in a particular
polarization state and receiving microwaves in two different
polarization states separately. However radar level gauges are low
cost items as compared to most other radar equipment, and thus the
antenna structure used is preferably simple and of low cost
compared.
[0043] In a typical embodiment the radar transmitter is connected
to a circularly polarized antenna, while the receiver (subsequently
named the main receiving channel) is connected to a circularly
polarized antenna with the opposite direction. By this difference
in polarization the two antenna functions can be included in one
physical antenna yet without (ideally seen) coupling between them.
By this the use of circular polarization a transmit/receive switch
is avoided in the case of a rotationally symmetric target. The
second receiver channel (subsequently named auxiliary receiving
channel) is connected via a directional coupler and receives a
reflected signal having the same polarization as the transmitted
signal. Both receiving channels include a mixer and IF
(intermediate frequency) amplifier but depending on the type of
radar different LO (local oscillator) signal can be used. A typical
FMCW system for this use have homodyne mixing with a part of the
transmitter signal with suitable delay as LO signal while a typical
pulsed signal use a separate pulsed oscillator which in this case
should be the same for both receiving channels.
[0044] In FIG. 2a is shown a transceiver structure including a
waveguide OMT (orthomode transducer) based antenna 21 connected to
a transmitter section 22 via a directional coupler 23 having a
termination 24. Two receiver channels RHCP and LHCP are connected
to the antenna via the directional coupler 23a and via a 90.degree.
hybrid 25. The microwave signals formed as described are fed to two
mixers 26 followed by two amplifiers 27 for the intermediate
frequency (IF). Reference numeral 28 indicates the local
oscillator. The exact implementation may different depending on the
type of radar (FMCW, pulse) and in some cases low noise amplifiers
will be necessary on the microwave side to get sufficient
sensitivity. Such implemental details are, however, easily
contemplated by the man skilled in the art after having read the
present text.
[0045] In FIG. 2a the directional coupler 23 acts as the
transmit/receive connection device, which functionally is a part of
most radar systems and especially those where the same antenna is
used for the transmitter and the receiver. The transmit/receive
device can be of different kinds. In FIG. 2b is shown a solution
with a ferrite circulator 23b being used instead of the directional
coupler (23a in FIG. 2a). The ferrite circulator 23b has three
connections and directs the signal entered in one output to the
next output in a given order. The ferrite circulator 23b is more
complex than directional coupler but saves some decibel in the
radar transmission power budget.
[0046] FIG. 2c shows another solution where a Wilkinson power
divider 23c is used instead of the directional coupler and still
maintaining the insulation between transmit and receive
sections--in this case with 2.times.3 dB further loss as compared
to the ideal circulator.
[0047] In FIG. 2d is shown a more general antenna structure with
one transmitter part TX and two receiving parts RX1, RX2, each
using essentially orthogonal polarizations and having a fair
insulation to the transmitting antenna. For instance many planar
antennas designs with dual polarization over the same surface are
well known and widely used (not the least in the field of mobile
phone base station antennas).
[0048] For a pulsed system also a switch can be used for the
transmit/receive device.
[0049] In FIG. 2e is illustrated how to obtain two orthogonal
linear signals XLP and YLP from the two orthogonal circular
polarized signals as obtained by the device illustrated in FIG.
2a.
[0050] The echo selection routine is implemented in the signal
processing included in, or associated with, the radar-based level
gauge 11.
[0051] The routine may include generating, separately for each
polarization state, the amplitude of the received reflected
microwave signal as a function of propagation time up to reception.
Noticeably, a calm surface of the liquid or granular solid gauged
will give echoes of noticeably different amplitude in each
polarization state, whereas many disturbing obstacles in the tank
will give echoes of similar amplitudes in the different
polarization states. Ideally, if a left hand circularly polarized
microwave signal is reflected towards a calm surface its
polarization state changes to right hand circular polarization at
reflection.
[0052] Two potential problems with disturbing echoes are incorrect
echo selection (where a disturbing echo, perhaps stronger than the
surface echo, is taken for the surface echo) and interference
errors, where the disturbing echoes cause measuring echoes (even if
they are substantially weaker than the surface echo). Errors
typically occur only when the surface echo is close to the
disturbing echo (a few tenths of a meter). The type of receiver and
procedures discussed above will decrease considerable the risk of
incorrect echo selection, whereas a reduction of the interference
errors needs other means. Still the two receiver channels can be
used and using a signal processing as shown in FIG. 2f the
disturbing echo can be suppressed considerably, which improves the
signal-to-disturbance ratio. The type of radar device has be a
device, such as for instance a commonly used FMCW radar-based level
gauge, which measures both phase and amplitude. Phase and amplitude
for the main receiver channel and the auxiliary receiver channel
(as obtained e.g. by the device of FIG. 2a) can be altered
individually by means of a respective device for amplifying and
phase delaying 29a-b under control of a control device 29c. Then,
the two channels are mixed whereby the disturbing echo can be more
or less cancelled. The factors used to obtain the cancellation are
different for different disturbing echoes but they are
approximately the same in a vicinity of a specific disturbing
echo.
[0053] FIG. 3 shows schematically an example of a diagram of the
amplitude of the received reflected microwave signals as a function
of propagation time for two orthogonal circular polarizations, e.g.
as illustrated in FIG. 1a. Left hand circularly polarized
transmitted microwaves are assumed. The dotted curve 31a indicates
received signal with left hand circular polarization, and the solid
curve 31b indicates received signal with right hand circular
polarization.
[0054] Three microwave signal echoes 32, 33, 34 are clearly
visible. By analyzing the amplitudes of the echoes it is
established that the echo 32 is reflected from a calm surface of
the liquid or granular solid gauged, whereas the echoes 33, 34 are
reflected from a fixed structure in the tank, e.g. a beam, an
agitator, or a tank side wall. With a suitable logic, selection of
the correct surface echo can be made even if the surface echo is
not the strongest one.
[0055] Thus, a method for radar-based gauging of the level of a
substance in a tank can be described with reference to the
schematic flow scheme shown in FIG. 4. A left hand circularly
polarized microwave signal is, in a step 41, transmitted. The
microwave signal as reflected towards the surface of the substance
gauged and towards any disturbing structures, is, in a step 42,
received and detected separately in left and right hand circular
polarization states. Then, for each of the two polarization states,
the amplitude of the detected reflected microwave signal as a
function of propagation time is, in a step 43, calculated. By
comparison of the two functions as being illustrated in FIG. 3 the
microwave signal 32 as reflected against the surface is, in a step
44, distinguished. Finally, in steps 45 and 46, the propagation
time for the microwave signal 32 as reflected against the surface
is determined and the level of the substance is calculated from the
propagation time.
[0056] It shall be appreciated that the amplitude of the received
reflected microwave signals as a function of propagation time for a
number of distance cells, i.e. level intervals of the surface of
the gauged matter, and for each different polarization state may be
stored in a database. Then knowledge of the localization of
disturbing structures and interfering echoes in a particular tank
may be collected with time. Once having a database it may be
consulted when a gauged level has been calculated.
[0057] It shall further be appreciated that any of the methods or
method steps identified above may be combined with the use of lobe
diversity to obtain even better capabilities of distinguishing the
surface echo under various conditions. In this respect reference is
made to our co-pending European patent application entitled "Method
and apparatus for radar-based level gauging" (inventor: Kurt-Olov
Edvardsson) and filed the very same date as the present
application.
[0058] It shall still further be appreciated that the inventive
concept of having one polarized transmit channel and two
differently polarized receive channels is fully reciprocal in the
sense that the present invention may be realized by using two
differently polarized transmit channels and one polarized receive
channel.
[0059] To implement such concept the transmit channels have to
operate one after the other in order to detect individually the
received echo signal from each transmitted channel.
[0060] A method for radar-based gauging of the level of a substance
in a tank having at least one interfering structure thus comprises
the steps of (i) transmitting microwave signals in two different
predetermined polarization states, one after the other, towards a
surface (14) of the substance and the at least one interfering
structure; individually for each of the transmitted microwave
signals detecting temporally resolved in a predetermined
polarization state, microwave signals (32, 33) as reflected against
the surface of the substance and as reflected against the at least
one interfering structure; distinguishing, based on signal
strengths of the microwave signals detected temporally resolved,
the detected microwave signal (32), which has been reflected
against the surface of the substance; and calculating based on a
propagation time of the distinguished microwave signal the level of
the substance in the tank.
[0061] If the two polarization states of the transmitted signals
are left and right hand circularly polarized the receive channel is
typically arranged to receive and detect either one of left or
right hand circularly polarized microwaves.
[0062] A surface echo will thus have very different signal
strengths for the two transmit polarizations, whereas a
depolarizing disturbing structure will give echoes with similar
signal strengths for the two transmit polarizations.
* * * * *