Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR OPTICAL
LEVEL SENSING OF AGITATED FLUID SURFACES
FIELD OF THE INVENTION
This invention generally relates to a method and apparatus for fluid-level
io measurement, and more particularly for the optical measurement of
the surface
level of an agitated fluid under degraded visibility conditions.
BACKGROUND OF THE INVENTION
Various types of instruments have been developed for applications related to
the
monitoring of tank contents, wherein the level of stored materials in either
liquid,
solid, or slurry state must be measured, either continuously or by discrete
height
steps. The expressions /eve/ measurement and tank gauging refer to the
measurement of the height of the top surface of a material stored in a tank or
the
measurement of the vertical distance that separates the surface of the
material
from a reference level and are used herein interchangeably. Traditional
levelmeter
devices include dipsticks, either in manual or electronic form, mechanical
float
levelmeters, capacitive RF levelmeters, and photoelectric levelmeters. More
recent
levelmeters based on technologies that allow measurements without contact with
the stored material have now gained wide acceptance for use in a broad variety
of
tank-gauging applications. These technologies rely mostly on the radar
principle,
that is, they take advantage of the fact that the surface of most materials
can
reflect a part of ultrasonic or electromagnetic wave energy that strikes it.
Ultrasonic
and microwave radar transceivers can be located, for instance, at the top of a
tank
to generate probe waves that propagate down to the surface of the material. A
part
of the radiated energy is then reflected off the surface material and returns
to the
transceiver for detection and measurement of the distance to the surface.
These
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tank-gauging devices operate according to various modulation schemes, among
which two of the most frequently encountered are the frequency-modulated
continuous-wave (FMCW) modulation and the pulsed time-of-flight (TOF)
modulation.
Contrary to their ultrasonic counterparts, the accuracy of level measurements
made with tank-gauging devices based on microwave radar technology is not
sensitive to changes in ambient temperature, pressure, and moisture level in
the
volume above the surface of the material. Microwave tank-gauging radars can
perform level measurements for a broad range of materials, provided that their
dielectric constants are high enough. The waves generated by both ultrasonic
and
small-size microwave tank-gauging radars diverge appreciably during their
propagation unless they are confined within a waveguiding structure in the
form of
a still pipe that extends vertically from the top of the tank down to its
bottom.
is Otherwise, the operation of both types of tank-gauging devices is often
plagued by
false return echoes due to spurious reflections of the wave energy on the
walls of
the tank or on the obstacles that could be present in it. More dramatically,
the
presence of a vertical pipe that comes in contact with the stored material is
a
major concern for several applications. Such applications include for example
the
gauging of tanks containing materials that are corrosive or that must be held
at
either elevated or cryogenic temperatures. In addition, food processing,
chemical,
and pharmaceutical industries often require that the interior of storage tanks
be
free of any structure that could contaminate the stored materials or that is
difficult
to clean.
Less intrusive tank-gauging devices that operate according to the radar
principle
can be built around optical rangefinder, laser radar, or lidar (Light
Detection And
Ranging) technologies, which rely on the emission of optical beams for
measuring
the distance of remote objects. The optical sources integrated in most lidar
devices adapted for tank-gauging applications emit at wavelengths in the
visible or
near-infrared region of the electromagnetic spectrum, so that their emission
wavelengths are shorter than those of microwave radars by orders of magnitude.
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As a consequence, the emitted light can propagate over long distances in the
form
of a narrow optical beam, without the need for being channeled by a
waveguiding
structure. lidar tank-gauging devices are then ideal for non-contact level
sensing
applications in which non-intrusiveness is of the utmost importance. This
major
asset comes from the fact that the entire device assembly can be located
outside
of the tank while the communication of the device with the interior of the
tank is
enabled by an optical window mounted in an access port of the tank or on the
front
panel of the device. lidar tank-gauging devices are then well suited for use
in
harsh environments since tightly-sealed safety windows can be made, for
instance, from a highly-resistant optical material that withstands pressures
that
exist inside pressurized tanks. In some cases, material build-up or dust
deposit on
the inner surface of an optical window can be prevented by using a wiper, as
taught in U.S. Pat. No. 5,284,105 to Wilkins, or by enclosing the window in a
compressed-air chamber that is kept opened during measurements, as disclosed
in U.S. Pat. No. 6,407,803 to Schrank. Finally, lidar tank-gauging devices are
generally less expensive than their microwave radar counterparts while they
can
perform level sensing of liquids having low dielectric constants.
In contrast with most ultrasonic and microwave radar instruments, lidar
devices
are particularly well suited for gauging of tanks that contain flammable
materials or
in which an explosive gas atmosphere may develop. This is due to the fact that
these devices can be designed in the form of an electronic/control unit
remotely
connected to an optical head unit that is free of any electronics and
electrical
wiring, so that they can be made intrinsically safe. The control unit can then
be
located at a safe distance, well away from the tank containing a flammable or
explosive material. Fiber optic cables or other types of light transmission
means
link both units. As a consequence, no electrical signal is conveyed between
the
optical head unit and the electronic/control unit. lidar tank gauges and
liquid-level
measurement devices designed in this fashion are taught in U.S. Pat.
No. 4,692,023 to Ohtomo et al., U.S. Pat. No. 4,938,590 to lshida, U.S. Pat.
No. 5,194,747 to Culpepper et al., U.S. Pat. No. 5,291,031 to MacDonald et
al.,
U.S. Pat. No. 6,339,468 to Clifford et al., and in the following paper from K.
Maatta
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et al., "High accuracy liquid level meter based on pulsed time-of-flight
principle",
(Proceedings of the SPIE, vol. 3100, pp. 268-277, 1997). Moreover, the optical
head unit may comprise only a few passive optical components and mounting
hardware, which serve to condition the emitted optical beam and to collect the
return optical signal. The optical head unit is then intrinsically safe
without any real
need for being housed in an explosion-proof enclosure since neither electrical
sparks nor local heat build-up can take place in it. Finally, lidar tank-
gauging
devices radiate light beams having optical irradiance levels well below the
ignition
threshold of even the most explosive gas atmospheres.
The surface of a liquid contained in a storage tank reflects a fraction of an
optical
beam incident on it due to the refractive-index mismatch at the interface
between
the material and the overlying atmosphere. For example, the air-water
interface
has a reflectance of about 2% at visible wavelengths and close to normal
is incidence. If desired, a highly reflecting object can be made floating
on the surface
of the liquid to increase the optical feedback well above the 2% fraction
mentioned
above. U.S. Pat. No. 4,938,590 to Ishida, U.S. Pat. No. 5,257,090 to Meinzer
et
al., U.S. Pat. No. 5,291,031 to MacDonald et al., U.S. Pat. No. 7,082,828 to
Wilkins, and Pat. application W02008/024910 to Nino et al. all disclose lidar
tank
gauges and liquid-level measurement devices that make use of a floating
reflector.
Various types of floating reflectors can be used such as stripes of reflective
tape
scotched to a floating object, a conventional plane mirror, a single corner-
cube
retroreflector or an array of retroreflecting cubes. Although a reflector
increases
substantially the optical feedback from the surface to be gauged, a lidar tank
gauge that operates with such a reflector can no longer be qualified as a non-
contact instrument. Hence, the reflector can contaminate the liquid stored in
the
tank, or the liquid itself can corrode the reflector. Above all, the need to
enclose
the floating reflector in a vertical pipe in order to prevent the reflector
from moving
outside of the surface area illuminated by the light beam is a serious
drawback.
The instrument then becomes very intrusive.
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There is therefore a clear advantage in designing a lidar tank-gauging
instrument
that provides sufficient sensitivity to operate with its light beam being
reflected off
the bare surface of a liquid, without any solid reflector floating thereon.
The
collection of the return optical signal can be maximized by optically
boresighting
5 the emission and collection channels of the instrument in such a way that
the light
reflected off the liquid surface remains within the field of view of the
collection
channel over a predetermined level interval to be covered by the instrument.
In
fact, the optical boresighting of the instrument as well as its alignment
relative to
the normal to the liquid surface must be finely tuned since the surface of a
still
lo liquid reflects light mostly in a specular manner, like a plane mirror.
This means
that the light remains highly directional after its reflection off the
surface, in
contrast with the diffuse reflection of light upon an unpolished solid
surface, which
diverts light over a broad solid angle dictated by the roughness of the solid
surface.
The performance of several lidar tank-gauging devices that operate in a quite
satisfactory manner when monitoring the surface level of liquids at rest
degrades
dramatically when the top surface of the liquid gets agitated. Let's mention
for
example the measurement of the level of liquids stored in tanks mounted in
transportation means such as railway tank cars, highway truck trailers, cargo
tanks
or other types of floating vessels. Boiling liquids have their top surface
agitated as
well. The surface of a liquid can also be agitated during both loading and
unloading of a tank. For example, the impact of the filling stream with the
liquid
surface may generate surface waves of sizeable amplitude during a loading
process. The consequences can be dramatic in these conditions since the
reading
from the tank-gauging device often serves to control both loading and
unloading
processes, based on the amount of material that is transferred. In fact, the
correct
operation of a tank-gauging instrument during both loading and unloading
processes is critical for most applications since the level measurements are
often
performed only during these events. Finally, some materials stored in the form
of
liquids or slurries must be stirred on a continuous basis, without any
shutdown of
the agitation being allowed for monitoring their top surface level.
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At any given position on the surface of an agitated liquid or slurry, the
height of the
surface will generally swing up and down, either randomly or in a roughly
periodic
manner, due to the presence of surface waves. By itself, the presence of
surface
waves or of other types of turbulent conditions in the fluid can be tackled
without
major difficulties by integrating the instantaneous reading from a lidar tank-
gauging
device over a convenient measurement period to give an accurate indication of
the
mean surface level. One primary requirement is then to select the measurement
period much longer than the period of the surface wave undulations.
Unfortunately, the presence of waves or turbulent conditions at the surface of
a
liquid also means that the local inclination or tilt angle of the limited
surface area
that is illuminated by the probe optical beam no longer remains in fixed
relationship with the vertical propagation axis of the optical beam. The
inclination
of the liquid surface fluctuates more or less rapidly according to the period
of the
surface waves. As a consequence, a fraction (or even all) of the light
specularly
reflected from this surface area can be directed out of the solid angle
subtended
by the collection channel of the tank-gauging device as seen from the liquid
surface. The collected return signal can then exhibit sizeable amplitude
fluctuations, which will often adversely affect the tracking of the useful
signal echo
coming from the reflection off the liquid surface. Note that this problem
persists
when using a solid plane reflector that floats on a liquid surface. U.S. Pat.
No.
5,648,844 to Clark discloses a liquid-level sensor that is intended to provide
better
performance for level sensing of agitated fluids. The light source enclosed in
the
disclosed apparatus emits an optical beam that passes through an optical
diffuser
before escaping from the emitter module of the apparatus. In addition to
enlarging
the optical spot size on the surface of the liquid, an optical diffuser of the
holographic type also causes a remapping of the optical irradiance
distribution to
give it a more circular shape without hot spots. Unfortunately, the use of an
optical
diffuser does not fully eliminate the amplitude fluctuations of the returned
signal,
particularly when the wavelength of the waves at the surface of the liquid
compares to the diameter of the optical spot size. In addition, the optical
losses
undergone during passage through the diffuser and the need for enlarging the
field
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of view of the optical receiver can degrade the signal-to-noise ratio of the
recorded
signals.
The challenge of remotely measuring the level of disturbed liquid surfaces
arises
when using microwave tank-gauging radars as well. For example, U.S. Pat. No.
6,759,977 to Edvardsson et al. reports that the signal strength of the radar
echo
from a turbulent surface can be reduced by about 6 to 20 dB, depending on the
size of the antenna of the microwave tank gauge. U.S. Pat. No. 5,321,408 to
Jean et al. and U.S. Pat. No. 6,107,957 to Cramer et al. disclose methods for
processing in the frequency domain the FMCW microwave radar signals to get
more accurate level measurements in presence of agitated fluid surfaces. The
methods rely on spectral averaging and/or spectral filtering of the frequency
domain signals, and they are not intended for implementation in tank-gauging
devices that operate according to the pulsed TOF principle. Likewise, U.S.
Pat.
No. 6,539,794 to Otto et al. teaches an apparatus for measuring the level of
agitated fluids or materials stored in a container, the apparatus being based
on a
microwave radar that operates in combination with a set of limit sensors
disposed
at various heights on the inner wall surface of the container, thus making the
apparatus relatively intrusive.
The level sensing of an agitated liquid surface becomes even more difficult
when
the liquid is also temporarily out of thermodynamical equilibrium with its
environment. Depending on the nature of the liquid and of its temperature,
these
non-equilibrium conditions can lead to the formation of fog in the atmosphere
above the liquid surface. For example, such conditions often prevail during
the
loading of tank carriers for maritime transportation of liquefied natural gas
(LNG).
Likewise, the loading of a tank with a high-pressure filling stream generally
causes
the formation of clouds of droplets in the atmosphere above the liquid
surface. The
difficulty to perform level measurements in both types of conditions, which
can
occur simultaneously, originates from the fact that fog and droplet clouds can
absorb and/or scatter a sizeable fraction of the optical beam energy during
the
beam propagation from the tank-gauging device to the liquid surface and then
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back to the device. The situation compares with the degraded visibility
conditions
caused by the presence of fog in the air. For the specific case of the LNG,
which
consists primarily of methane, the presence of even small amount of other
hydrocarbon species such as butane, ethane, and propane can result in some
resonant absorption of the optical beam energy, even though the methane itself
is
totally transparent at the emission wavelength of the tank-gauging device. The
absorption and scattering of the optical beam energy will manifest as a
significant
attenuation of the echo returned from the liquid surface along with the
presence of
a more or less intense false signal echo. This false signal echo comes from
the
io part of the backscattered optical energy that is captured by the optical
receiver of
the tank-gauging device, and it spatial extent covers the distance interval
from the
device down to the liquid surface. In some circumstances, the distance-
averaged
amplitude of the false signal echo can exceed the useful signal echo. Stated
otherwise, the useful signal echo can be buried within an intense background
is signal that prevents from reliably measuring the liquid surface level
with the
desired accuracy.
From the review of the prior art detailed in the preceding paragraphs, there
is a
need for methods and devices that could perform level measurements of agitated
20 fluids stored in various types of containers or tanks and under degraded
visibility
conditions, the methods being preferably suited for implementation in lidar
tank-gauging devices based on the pulsed TOE principle.
25 OBJECTS OF THE INVENTION
It is therefore a first object of the present invention to provide a lidar
apparatus
based on a pulsed time-of-flight modulation scheme in combination with
digitization of the captured signal waveforms, and which can perform accurate
and
30 reliable measurements of the level of agitated fluids stored in
containers or tanks,
without any contact with the fluids.
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Another object of the present invention is to provide a method that enables
successful retrieval of the signal echo returned from an agitated fluid
surface, the
method being intended for implementation in the processing unit of a lidar
apparatus adapted for level sensing applications wherein the distance to a
fluid
surface is determined from the position of the signal echo in the recorded
signal
waveforms.
It is another object of the present invention to provide a method that enables
successful retrieval of the signal echo returned from an agitated fluid
surface
located below an atmosphere filled with a medium that could cause a
significant
attenuation of optical beams, the method being intended for implementation in
the
processing unit of a lidar apparatus dedicated to level sensing applications.
Yet another object of the present invention is to provide a method that takes
advantage of the intrinsic fluctuations of the signal echo returned from an
agitated
fluid surface to enable better retrieval of the signal echo, the intrinsic
fluctuations
being generally of much higher amplitude than those of the signal echo
returned
from an optically scattering or absorbing medium that fills in the volume
above the
fluid surface.
Another object of the present invention is to provide a method that enables
successful retrieval of the signal echo returned from an agitated fluid
surface, and
which can be implemented in a lidar apparatus without any need for modifying
its
existing hardware or for adding new hardware components.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the present invention, there is provided
a
method for optically measuring a level of the top surface of a fluid contained
in a
storage means, comprising the steps of:
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(a) providing a lidar apparatus and adjusting a line of sight of said lidar
apparatus substantially perpendicular to said top surface, said lidar
apparatus including means for:
(al) sending a plurality of optical pulses towards said top surface,
5 (a2)
receiving a plurality of optical signals reflected by said top
surface,
(a3) converting said optical signals into digital waveforms,
(b) computing a computed waveform from the plurality of said digital
waveforms, each element of said computed waveform being given by
io the value
of a statistical estimator of the variability of the measured
signal amplitudes stored in the corresponding elements of the said
digital waveforms,
(c) locating in said computed waveform the signal echo returned from said
top surface of a fluid, and
(d) determining the vertical distance that separates said top surface of a
fluid from said lidar apparatus from the position of said signal echo
returned from said top surface.
In embodiments of the invention, there is provided a method for optically
measuring the level of fluids stored in tanks or containers, wherein the
surface of
the fluids can possibly be agitated while being located below an atmosphere
that
could cause significant absorption and/or scattering of optical beams. The
method
involves numerically processing the digitized signal waveforms generated by a
lidar apparatus that makes use of the pulsed time-of-flight modulation scheme.
The data processing relies essentially on the computation of a waveform vector
in
which each data point is given by the value of a statistical estimator of the
variability of the signal echo amplitude measured at the distance from the
lidar
apparatus associated to the rank of the data point in the waveform vector. The
statistical estimator is preferably the standard deviation of the measured
amplitude
data, but other estimators of data variability can be used as well. As
compared to
the computation of a mean waveform vector from a set of raw signal waveforms,
the computation of a statistical estimator of data variability allows for
significant
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enhancement of the signal echo returned from an unstable specular reflector
such
as the surface of an agitated fluid. This is due to the fact that the
collection of the
optical return signals from an agitated fluid surface exhibits sizeable
fluctuations.
As a consequence, the useful signal echo can be more easily identified and
then
retrieved even when buried in a stronger signal echo returned from any
optically
scattering or absorbing medium that could fill in the volume of the tank above
the
fluid surface.
In accordance with a second aspect of the invention, there is also provided a
lo method
for optically measuring a level of the top surface of a fluid contained in a
storage means, comprising the steps of:
(a) providing a lidar apparatus and adjusting a line of sight of said lidar
apparatus substantially perpendicular to said top surface, said lidar
apparatus including means for:
(al) sending a plurality of optical pulses towards said top surface,
(a2) receiving a plurality of optical signals reflected by said top
surface,
(a3) converting said optical signals into digital waveforms,
(b) computing a first computed waveform from the plurality of said digital
waveforms, each element of said first computed waveform being given
by the mean value of the measured signal amplitudes stored in the
corresponding elements of the said digital waveforms,
(c) computing a second computed waveform from the plurality of said digital
waveforms, each element of said second computed waveform being
given by the value of a statistical estimator of the variability of the
measured signal amplitudes stored in the corresponding elements of the
said digital waveforms,
(d) comparing said first and second computed waveforms and locating in
said first or second computed waveform the signal echo returned from
said top surface of a fluid,
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(e) determining the vertical distance that separates said top surface of a
fluid from said lidar apparatus from the position of said signal echo
returned from said top surface.
In accordance with another aspect of the invention, there is further provided
an
apparatus for optically measuring a level of a top surface of a fluid
contained in a
storage means, said lidar apparatus comprising:
(a) an optical emitter module for sending a plurality of optical pulses
towards said top surface,
(b) an optical receiver module for receiving a plurality of optical signals
reflected by said top surface,
(c) means for sampling analog signal waveforms at the output of the optical
receiver module and for converting said analog signal waveforms into
digital signal waveforms,
(d) a control and processing module for numerically processing said digital
signal waveforms,
wherein said numerical processing includes the computation of a waveform
formed of a statistical estimator of the variability of the signal amplitude
data
sampled for each distance value, from which the signal echo returned from
said top surface of a fluid can be retrieved for determining the vertical
distance
that separates said top surface from the apparatus.
Some embodiments of the invention provide a lidar apparatus for optically
measuring the level of the top surface of a fluid stored in a tank, the fluid
being
possibly agitated. The lidar apparatus is placed at a height above the surface
of
the fluid, generally on the rooftop of the tank, and the optical beam radiated
from
the apparatus propagates along a direction perpendicular to the surface of the
fluid. The key elements of the lidar apparatus include an optical emitter
module for
sending a plurality of optical pulses towards the surface of the fluid, an
optical
receiver module for detecting a plurality of optical signals reflected off the
surface
of the fluid and for generating analog electrical signals in response to the
input
optical signals, electronics for sampling the analog electrical signals and
for
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10c
converting them into digital signal waveforms, and a control and processing
unit
for numerically processing the digital signal waveforms. The key step of the
numerical data processing is the computation of a waveform vector in which
each
data point is given by the value of a statistical estimator of the
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variability of the signal echo amplitude measured at the distance from the
lidar
apparatus associated to the rank of the data point in the waveform vector. The
signal echo returned from the surface of the fluid stored in the tank can be
retrieved with greater reliability for further determination of the vertical
distance
that separates the surface of the fluid from the lidar apparatus.
Other features and advantages of the invention will be further appreciated by
reference to the detailed descriptions of the preferred embodiments in
conjunction
with the drawings thereof.
lo
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a set-up for a lidar tank-gauging device
for non-
contact measurement of the surface level of a material stored in a tank.
FIG. 2 shows five signal waveforms recorded by a lidar tank-gauging device
placed at a height of about 11.7 m above a bucket filled with water, wherein a
part
of the optical beam was intercepted by a set of four metallic wire meshes
located
approximately 7 m above the surface of the water. Each waveform shown in the
figure was computed from the average of 100 raw signal waveforms.
FIG. 3 is a functional block diagram that details the internal configuration
of a lidar
tank-gauging instrument according to a preferred embodiment of the invention.
FIG. 4 plots the time variations of the peak signal amplitudes returned from
an
agitated liquid surface and from the metallic wire meshes. The mean value and
standard deviation of each data set are given in the figure.
FIG. 5 shows five waveforms, wherein each of them has been obtained from the
standard deviation of a set of 100 raw signal waveforms. The raw signal
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waveforms are the same as those used to produce the averaged waveforms
shown in Fig. 2.
FIG. 6 shows five waveforms, wherein each of them has been obtained from the
mean absolute deviation of a set of 100 raw signal waveforms. The raw signal
waveforms are the same as those used to produce the waveforms shown in Figs.
2 and 5.
FIG. 7 is a flow chart diagram that shows the numerical processing steps of
the
io acquired raw signal waveforms according to a preferred embodiment of the
method of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 is a schematic illustration of a liquid-level measurement application
that
represents a preferred mode of practicing the invention with its various
embodiments. A lidar tank-gauging device 10 is shown in Fig. 1 with a
preferred
design configuration wherein the device is made up of an electronic unit 20
remote
from an optical unit 30, the units being linked to each other only through
light
transmission means 40. The light transmission means 40 consist preferably of
two
optical fibers. The optical unit 30 is mounted on the roof of a liquid storage
tank or
vessel 50, and a window 60 made of a suitable optical material provides
optical
access to the interior of the tank 50. Although the storage tank shown in Fig.
1 is
stationary, the apparatus of the present invention can be used with tanks or
containers mounted in transportation means as well. Likewise, both electronic
unit
20 and optical unit 30 can be enclosed in the same housing located on the roof
of
the tank 50 without departing from the scope of the present invention. During
operation of the lidar tank-gauging device 10, the optical unit 30 emits
pulses of
light in a repetitive manner. The emitted light propagates in the form of a
collimated optical beam that is depicted schematically in Fig. 1 by the
vertical
arrow 70 that points downward. The tank 50 contains a liquid 110 whose surface
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80 reflects some part of the incident optical beam 70 to generate a return
optical
beam depicted by the arrow 90 that points upward. The stored material can also
be in the form of slurry, but for the sake of convenience the text will refer
only to a
liquid. As seen in Fig. 1, the surface of the liquid can be agitated for
various
s
reasons. As a consequence, the amount of light reflected back towards the
optical
unit 30 can exhibit significant time fluctuations because a part of the
incident light
beam 70 would be diverted in various directions, in a random manner, as
depicted
schematically by the short-length arrows 100.
io
Instead of being mounted on the roof of the tank 50, as depicted in Fig. 1, an
optical unit 30 adapted for installation on the floor 125 of the tank can also
be
contemplated without departing from the scope of the present invention. Unless
the optical unit 30 could be located in a compartment made in the floor 125
and
closed by a sealed window 60, the optical unit 30 is otherwise always immersed
in
is the
liquid, so that it must be enclosed within a hermetically-sealed housing. The
device is oriented in such a way that the emitted light propagates upward
throughout the height of the liquid. This configuration is particulary
attractive for
use with tanks that do not allow for easy roof-mounting of an optical unit 30,
or
when the height H of the front panel of the optical unit 30 relative to the
tank floor
20 125
is subject to changes, due for example to thermal expansion of the sidewalls
of the tank. In turn, the liquid must be sufficiently transparent at the
wavelength of
the emitted light even when the tank 50 is close of being completely filled.
The invention described herein finds its best use when implemented in lidar
tank-
25
gauging devices 10 that operate according to the pulsed TOF modulation scheme.
The vertical distance D that separates the surface 80 of the stored liquid 110
from
the front panel of the optical unit 30 is then obtained by measuring the time
delay
T the optical pulses take to travel a full round trip of length 2D. The
vertical
distance D is computed with the following formula:
, cT
30 v = -' ( 1)
2n
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14
where c is the speed of light in vacuum (7-,, 3x108 m/s), while n denotes the
path-
averaged refractive index of the atmosphere above the liquid surface 80, in
which
the optical pulses propagate. The value of D can be used as a measure of the
tank
ullage, namely for determining the amount that the tank lacks of being full.
As
shown in Fig. 1, the height or level L of the liquid surface 80 can be
obtained
simply from L = H - D.
The volume between the surface 80 of the stored liquid 110 and the top of the
tank
50 is generally filled with air or with other gas mixtures. In some
circumstances,
the overlying atmosphere may also contain varying concentrations of
particulate
matter, dust, or a medium that is condensing in the form of fog or droplets,
as
depicted schematically in Fig. 1 by the cloud 120. As noted earlier, the
medium in
which the optical pulses propagate during their double travel between the
device
and the liquid surface can absorb and/or scatter a part of the pulse energy.
This
can result in an important reduction of the amplitude of the useful signal
echo
returned from the surface 80, while a more or less intense parasitic signal
echo
may arise from the backscattered pulse energy that is captured by the optical
unit
30. For the sake of better illustrating the detrimental impacts of this
parasitic signal
echo, a lidar instrument designed for ranging over short distances and
equipped
with a laser diode source emitting at a near-infrared 905-nm wavelength has
been
installed above a small bucket filled with fresh water. The depth of the water
was
80 cm. The line of sight of the instrument was pointing downward on the
bucket,
and the vertical distance between the instrument and the water surface was
about
11.7 m. The presence of an optically-scattering atmosphere above the water
surface, that would significantly degrade the visibility conditions, was
simulated by
inserting in the optical beam path a set of four metallic wire meshes, one
above
the other, at a height of about 7 m above the water surface level. The meshes
provide a strong return of the incident optical beam energy while a small part
of
the beam energy passes through them to impinge on the water surface. Fig. 2
shows a set of five typical signal waveforms recorded during operation of the
lidar
instrument under the conditions mentioned in the preceding lines. Each
waveform
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represents the measured amplitude of the collected optical signal plotted as a
function of the distance from the lidar instrument. Each curve shown in the
figure
was obtained by averaging 100 consecutive raw signal waveforms in order to
increase by a factor of 10 the signal-to-noise ratio characteristics of the
displayed
5 signal waveforms. The signature of the signal echo returned from the
water
surface is visible without difficulty as the small-amplitude peak located at
the
distance z,,-', 11-12 m from the lidar instrument. However, the salient
feature of the
waveforms shown in Fig. 2 is the strong signal echo returned from the wire
meshes, which peaks at the distance z = 3.5 m. This strong signal echo exceeds
io that of the water surface by a factor of more than 10. As a consequence,
without
any cue from the operator, the processing unit of the lidar instrument is
likely to
erroneously identify the useful signal echo as the one corresponding to the
wire
mesh return, thus resulting in this case in a largely incorrect level
measurement. In
practical situations the optically-scattering medium would completely fill the
is volume above the surface of the liquid, so that the parasitic signal
echo visible in
the recorded waveforms would extend from the distance z = 0 m up to the
distance
corresponding to the liquid surface level. In fact, the signal echo returned
from the
liquid surface level could be completely buried within the extended parasitic
signal
echo, thus making its reliable identification quite difficult, if not
possible. As will be
shown below, the invention disclosed herein relates to dedicated processing of
the
signal waveforms that would enable successful recovery of the (possibly weak)
signal echoes returned from liquid surfaces.
The functional block diagram of Fig. 3 depicts the internal configuration of a
lidar
tank-gauging device 10 that would be built according to a preferred embodiment
of
the present invention. The operation of the device can be better understood by
logically splitting the components enclosed in the electronic unit 20 in a set
of
distinct modules, each module having its specific role. For instance, the
optical
emitter module 130 receives electrical pulse trigger signals from the control
and
processing module 200 to command the repetitive emission of optical pulses.
The
duration of the optical pulses is typically in the order of a few ns
(nanoseconds)
while the pulse repetition frequency (PRF) of the module 130 can reach some
tens
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16
or even hundreds of kHz. The optical pulses are emitted from an optical source
coupled to proper drive electronics, the source being generally a laser, and
preferably a semiconductor laser diode. However, other types of optical
sources
such as light-emitting diodes (LEDs) can be envisioned without departing from
the
scope of the present invention. The selection of the optical source depends on
factors such as the peak optical output power required for successful level
sensing
of typical surfaces located at the maximum stand-off distance Dour to be
covered
by the instrument, the emission wavelength, the ease to get efficient coupling
of
the emitted light into an optical fiber, and its cost. Optical sources such as
fiber
io lasers, microchip lasers and even solid-state lasers may find their way
in this
application domain, particularly when no laser diode source exists at the
desired
emission wavelength. After exiting from the optical source, the optical pulses
pass
through appropriate optics that focuses them onto the input end of the optical
fiber
140. This fiber conveys the optical pulses with minimum attenuation and
coupling
is losses up to the emission channel 150 of the optical unit 30.
The return signal echo captured by the collection channel 160 of the optical
unit 30
is guided through another optical fiber 170 up to the input aperture of the
optical
receiver module 180 housed in the electronic unit 20. The key component of the
20 optical receiver module 180 is a photodetector, which is generally an
avalanche or
PIN photodiode having its material composition suited to the wavelength of the
optical pulses. The pre-amplified voltage signal at the output of the
photodetector
circuitry is fed to an amplifier circuit that may comprise a matched filter to
limit the
electrical bandwidth of the optical receiver module 180. It is known in the
art that
25 other amplifier configurations could be used as well, such as a
logarithmic
amplifier or a set of amplifiers mounted in parallel, each amplifier having a
fixed
gain. The invention described herein is specifically intended for
implementation in
lidar tank-gauging devices that perform digitization of the collected lidar
signal
waveforms. The electronics of these instruments thus includes an analog-to-
digital
30 (AID) converter 190 that digitizes the amplified analog output signal
from the
optical receiver module 180 to allow further numerical processing of the data
by
the control and processing module 200. The digitized lidar signal waveforms
are
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17
generated by sampling repetitively the amplified analog output signal during a
period of time whose duration relates to the maximum distance to be covered by
the instrument. The ND converter 190 has preferably a sampling rate of several
tens of MS/s (mega-samples per second) or higher. The time delay between two
consecutive digital sampling events defines the size of the so-called range
bins of
the instrument 10, when converted in units of distance with Eq. (1). For
example,
the analog signal waveforms have been sampled at a rate of 100 MS/s (10-ns
time
spacing) to give the waveforms shown in Fig. 2, wherein the range bin is then
1.5-
m wide. In addition to processing the data sent by the ND converter 190, the
io control and processing module 200 controls the operation of both emitter
130 and
receiver 180 modules while managing the input/output communications and data
logging via user-interface hardware 210 or through a data link 220 for
connection
to a network or to any other processing means.
Fig. 3 also depicts a simplified layout of the optical unit 30, which consists
basically of an emission channel 150 and of a collection channel 160 placed
side
by side, each having its own optical axis. The lidar tank-gauging device 10
with its
optical unit 30 as depicted in Fig. 3 thus has a monostatic biaxial
configuration.
However, the apparatus of the present invention could also be based on a
monostatic coaxial configuration in which both emission and collection
channels
share a common portion of their optical axes. Both optical channels 150 and
160
include, as a minimum, lenses 220 and 230. The lens 220 serves for collimation
of
the light that is radiated from the endface of the fiber 140 connected to the
emission channel 150. In turn, lens 230 focuses the optical energy incident on
its
clear aperture onto the endface of the fiber 170 that connects to the
collection
channel 160. Note that the clear apertures and focal lengths of both lenses
220
and 230 may differ. Likewise, the optical fibers 140 and 170 do not
necessarily
have the same core diameters and numerical apertures. The surfaces of both
lenses preferably have anti-reflection coatings to minimize the optical
reflection
losses at the emission wavelength. Although the optical unit 30 depicted in
Fig. 3
makes use of lenses, other optical components such as mirrors of suitable
clear
apertures and curvatures can be used as well without departing from the scope
of
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the present invention. The optical unit 30 includes hardware (not shown in the
figure) for mounting the optical components and to allow boresighting of both
emission and collection channels. A narrowband optical filter 240 can be
inserted
somewhere along the optical path in the collection channel 160 to block
parasitic
light wavelengths lying out of a narrow spectral interval centered on the
emission
wavelength.
Both emission and collection channels are enclosed in a housing 250 that
includes
specific design features to make it compliant with the requirements of the
intended
io application or of the intended end user. Since the optical unit 30 shown
in Fig. 3 is
free from any electrical component or wiring, it can be made intrinsically
safe for
use in explosive gas atmospheres. The housing 250 includes connectors that
mate to the connectorized endfaces of both optical fibers 140 and 170 as well
as
proper fittings (not shown in the figure) to secure to the access port on the
rooftop
is of the storage tank 50. A protective optical window 260 can be mounted
in front of
both lenses 220 and 230 to provide, for example, hermetical sealing of the
housing
250. Alternatively, an optical window 60 can be mounted directly in the visual
access port or tank nipple located on the rooftop of the storage tank 50, as
seen in
Fig. 1.
The liquid-level reading can be communicated to the user of the lidar tank-
gauging
device 10 in a variety of ways. For instance, the user-interface hardware 210
may
simply provide a numerical display of the liquid-level reading, updated at a
rate
that can be either selected by the user or set automatically by the control
and
processing module 200 of the device. Alternatively, or in combination, the
return
signal waveforms can be displayed in a graphical form similar to that of Fig.
2 on a
display monitor 260. In ideal conditions wherein the level of the surface of a
liquid
at rest is sensed without any significant scattering of the optical pulse
energy by
the atmosphere above the liquid surface, each return signal waveform would
exhibit a single peak that stands out from a weak noise background. The
horizontal position of the peak value in the waveform may give directly the
vertical
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19
distance D of the liquid surface after proper distance calibration of the
lidar tank-
gauging device.
As noted earlier, the amplitude of the signal echo returned from an agitated
liquid
surface and then captured by a lidar tank-gauging device 10 can exhibit
sizeable
time fluctuations as compared to the relatively stable signal echo returned
from the
optically absorbing/scattering medium that could be present above the liquid
surface. The curve 300 of Fig. 4 depicts an example of the signal amplitude
fluctuations from an agitated liquid surface. The data points in this curve
have
io been retrieved from the same set of 500 raw signal waveforms that was
used to
generate the averaged signal waveforms shown previously in Fig. 2. Note that
the
water surface was made turbulent by knocking repeatedly on the sidewalls of
the
water-filled bucket during the recording of the whole set of waveforms. For
each
raw signal waveform, the amplitude data point sampled for the specific
distance z
is = 11 m (i.e., at the peak of the signal echo returned from the liquid
surface) was
retrieved and then plotted in curve 300. The same operation was then performed
with the data points sampled for the distance z = 3.5 m, where the signal echo
returned from the metallic wire meshes gets maximum, as seen in Fig. 2. This
set
of data points then forms the curve 310 shown in Fig. 4 as well. Note that the
20 recording of the whole set of raw signal waveforms covered a time span
of about
72 s. The heavier fluctuations of the peak amplitude of the signal echo from
the
agitated water surface are clearly evidenced in Fig. 4, even though the mean
signal amplitude is weaker than that of the wire-mesh signal echo by a factor
of
about 12. As a convenient measure of the amplitude fluctuations, the standard
25 deviation of the data points of curve 300 has been computed, giving a
result that is
more than twice that obtained from the data points of curve 310 (4.8 vs 2.0).
The basis of the method of the present invention consists in making use of a
statistical estimator such as the standard deviation computed with the signal
30 amplitude data points measured at each distance z. The standard-
deviation values
are then used to generate a waveform (denoted as the standard-deviation
waveform) from which the useful signal echo returned from an agitated liquid
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surface is more likely to stand out from the more stable parasitic background
echo.
Using the symbol S(z1) for the
sampled data point comprised in the one-
dimensional vector S, that represents the ith digitized raw signal waveform of
a set
comprising a total of N waveforms, (i.e., i= 1, 2, 3,...., N), the
element Sa(zi) of
5 the standard-deviation waveform vector S, is given by:
N
S,(z) = N-11 E ks1(z1)--s(z1))2 j= 1, 2, 3, ..., Nz ,
(2)
where S(zi) stands for the
element of the mean waveform vector previously
10 computed from the same set of N raw signal waveforms:
N
(Z j) = -LE S i(2 j) j = 1, 2, 3, , Nz
(3)
N
In both formulas shown above, zi stands for the distance from the lidar tank-
15
gauging device corresponding to the signal amplitude data point, the
spacing
zto - zi between successive distance elements being given by the range bin
size
defined earlier. Each waveform vector then comprises a total of Nz data
points, Nz
being obtained from the ratio of the maximum stand-off distance DmAx at which
a
return lidar signal is to be sampled to the range bin size.
The standard deviation is a basic statistical parameter commonly used as a
useful
estimator of the variability of the data around their mean value. However,
other
types of estimators of the variability could be used as well without departing
from
the scope of the present invention. For example, a mean absolute deviation
waveform SA could be used, wherein each element SA(zf) is computed from the
expression:
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SA (zi) = 1 ¨ S1(z1)¨S(z1) j= 1, 2, 3, ,
(4)
N
where the symbol II stands for the absolute value of the expression enclosed
therein. The mean absolute deviation and the standard deviation are
statistical
parameters closely related to the first moment and second moment of a data
set,
respectively. Parameters based on higher-order moments could be used as well
for quantifying the fluctuations of the measured lidar signal amplitude about
its
mean value. However, the use of lower-order moments is preferable in many
m circumstances since the robustness of this class of estimators decreases
as the
order of the moments gets higher.
For the sake of illustration, Fig. 5 shows a set of five standard-deviation
waveforms obtained from the same 500 raw signal waveforms used for generating
the curves of both Figs. 2 and 4. Each waveform has been obtained from the
standard deviation computed with the data points included in a subset of 100
consecutive raw signal waveforms, so that each curve accounts for the
amplitude
fluctuations that occurred during a time span of about 14 s. As compared to
the
averaged signal waveforms shown in Fig. 2, the signal echoes at z = 11 m
returned from the agitated water surface get prominent in the standard-
deviation
waveforms. This means that any basic algorithm for distance evaluation that
would
be based, for example, on peak detection of the strongest echo in a waveform
would easily succeed in providing the right distance when run with standard-
deviation waveforms such as those displayed in Fig. 5. Fig. 5 also shows that
the
peak of the liquid surface signature varies appreciably from waveform to
waveform, thus indicating that the strength of the rapid amplitude
fluctuations was
not stationary over the whole 72-s acquisition period. This behavior could
have
been guessed from the curve 300 plotted in Fig. 4, which shows larger
amplitude
fluctuations during the time interval from 30 s to about 60 s.
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Another set of five waveforms has been computed from the same 500 raw signal
waveforms as discussed above, but now using Eq. (4) for the mean absolute
deviation. The mean-absolute-deviation waveforms are plotted in Fig. 6,
wherein
the same vertical scale as in Fig. 5 has been used to facilitate comparisons
between both figures. The signal echoes returned from the agitated water
surface
still stand out in the displayed waveforms, but in a slightly reduced fashion
as
compared to the standard deviation.
The amplitude fluctuations at the origin of the waveforms plotted in Figs. 5
and 6
do not come solely from the reflection of the lidar beam on the agitated
liquid
surface. Hence, additional contributions like the time fluctuations of the
radiated
lidar beam energy from pulse to pulse, beam pointing jitter, inhomogeneities
in the
atmosphere in which the lidar beam propagates, and noise either generated by
or
captured by the electronic circuitry of the optical receiver module 180 add to
the
contribution of the agitated liquid surface to give an overall time
fluctuation figure
that could be quantified using for example the waveforms such as those plotted
in
Figs. 5 and 6. The non-vanishing values of both standard-deviation and mean-
absolute-deviation waveforms for distances closer than the position z = 11 m
of
the agitated liquid surface is a consequence of the combined effects of these
additional contributions. If desired, the waveforms shown in Figs. 5 and 6 can
be
normalized to remove out some of the additional time fluctuation contributions
discussed above in order to better reveal the specific contribution due to the
agitated fluid surface.
Fig. 7 depicts a flowchart diagram from which the processing of the acquired
signal waveforms according to the method of the present invention can be
better
appreciated. After having recorded a set of N raw signal waveforms in step
350, a
previously acquired reference background signal is then subtracted from each
waveform in step 360. The purpose of the background subtraction is to remove
from each raw signal waveform a fixed pattern noise generated each time an
optical pulse is fired by the emitter module 130, a part of this noise being
picked
up by the electronics of the optical receiver module 180 of the lidar tank-
gauging
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device 10. This fixed pattern noise can be recorded as a reference waveform
prior
to the acquisition of the raw signal waveforms simply by firing an optical
pulse
while blocking the input aperture of the collection channel 160. The averaging
step
370 is then carried out to give an averaged waveform S with enhanced signal-to-
noise ratio. The waveform can be displayed on a display monitor or stored for
further processing in step 380. The same set of N raw signal waveforms is then
processed in step 390 to compute the standard-deviation waveform Sa according
to Equation (2) shown previously. Note that the averaged signal waveform S
computed in step 370 is required for completion of step 390. For the sake of
lci illustration, the standard deviation is used in the flow chart diagram
as an
estimator of the variability of the data but, as noted earlier, other
statistical
estimators could be used as well without departing from the scope of the
present
invention. Similarly to the averaged signal waveform 5, the standard-deviation
waveform So_ can be displayed on a display monitor as a function of the
distance
from the tank-gauging device or stored for further use in step 400. Both
averaged
and standard-deviation waveforms are then analyzed and compared in step 410 to
determine which of them would be best suited for further determination of the
distance to the target (fluid surface). A comparison of the waveforms, either
performed by the user or through the use of a suitable algorithm, is suggested
to
avoid, among other things, erroneous measurement results that would occur from
the blind use of the standard-deviation waveform in cases where the top
surface of
the fluid would be stable, without any agitation. The selected waveform is
then
processed in step 420 according to methods and algorithms well known in the
art
to locate the signature of the surface of the fluid and its corresponding
distance
value. As shown in step 430, the distance reading can be displayed to the user
or
stored for further use or processing.
The waveform processing detailed in the flow chart of Fig. 7 is repeated for
each
newly acquired set of N waveforms. However, it will be obvious to one skilled
in
the art that the processing flow can be modified without departing from the
spirit of
the invention. For example, the computations of both averaged waveform S and
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standard-deviation waveform So- can be updated each time a new raw signal
waveform is acquired instead of waiting for a new whole set of N signal
waveforms. In this embodiment both waveforms S and So- are computed from the
latest N signal waveforms received, but the flow of the raw signal waveforms
in the
computations now proceeds according to a first-in first-out scheme implemented
on a single raw waveform basis.
In practice, the number N of raw signal waveforms used for the computations of
_
the waveforms S and Sa is governed by factors such as the desired update rate
of
the fluid level readings and the pulse repetition frequency of the optical
emitter
module 130, which dictates the acquisition rate of the raw waveforms. Choosing
larger values of N would lead to enhanced signal-to-noise ratios for both
computed
waveforms S and Su, but at the price of a reduced time-resolved capability in
displaying the transient phenomena that could take place inside of the tank
that is
currently gauged.
While the preferred embodiments of the invention in their various aspects have
been described above, such descriptions are to be taken as illustrative of
embodiments of the invention rather than descriptions of the intended scope of
the
invention, which scope is more fully appreciated by reference to the
disclosure as
a whole and to the claims that follow.
