Note: Descriptions are shown in the official language in which they were submitted.
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VIEWING PARTICLES IN A RELATIVELY
TRANSLUCENT MEDIUM
This invention relates to viewing particles in a relatively translucent
medium. The
invention is especially useful for, but is not limited to, viewing particles
in a relatively
translucent fluid stream such as a molten polymer stream flowing in an
extruder.
BACKGROUND OF THE INVENTION
The presence of particles in a polymer melt stream during extrusion is a
common
occurrence. Some particles, such as processing additives or filler material,
are intentionally
added to the polymer melt. Other particles, such as microgels, burned
material, void spaces,
and secondary polymers or other materials carried in the primary polymer
stream, may be
undesirable contaminants which threaten product quality. The ability to
monitor in real time
the movement of particles in a melt stream, as well as to identify particle
types, sizes, shapes,
locations, concentrations and velocity profiles, provides a number of
benefits. With respect
to particles intentionally added to the melt, an effective monitoring system
ensures that the
desired dispersion ofparticles is achieved. With respect to particle
contaminants in the melt,
an effective monitoring system can be used to ensure that the polymer melt
stream meets the
required quality level for the process concerned. The use of a real time
monitoring system
can provide tighter process quality control, generate practical information on
the mixing
process during extrusion and greatly reduce off specification material waste.
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Although the benefits of a real time monitoring system for polymer melt
streams are
numerous, very few attempts to develop a viable system have succeeded. One
proposed
monitoring system is described in U.S. Patent No. 4,529,306 (Kilham et al)
which issued on
July 16, 1985 and was permitted to expire on September 23, 1997. The Kilham et
al system
proposed the use of an observation probe and illuminating probes directly
attached to the
processing equipment to obtain images of the polymer melt. The observation
probe
consisted primarily of an objective lens for focussing an image onto a focal
plane and an
image-conducting means such as a fibre optic bundle. The use of an objective
lens, as with
all conventional lenses, can result in viewing angle errors and magnification
errors which
distort both the size and the shape of the objects being imaged. In addition,
use of a
conventional lens impedes the determination of the location of particles in
the melt stream.
The use of observation and illumination probes directly attached to the
processing equipment
also causes complications because the probes must be removed from the high
temperature
and high pressure environment for maintenance purposes. Further, the Kilham et
al system
makes no provisions to reduce image blur caused by rapid movement of the
objects to be
imaged past the observation probe.
Another monitoring system is commercially available from Dynisco Polymer Test
On-
Line. The Dynisco Polymer Contaminants Analyzer (PCA) uses a sampling system
which
includes a flow cell, optical probes and fibre optic illumination and image
bundles. The
Dynisco PCA employs an on-line technique as opposed to an in-line technique.
On-line
techniques divert a sampling stream of the flow to a measurement device, such
as a flow cell.
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The sampled material is then either returned to the primary flow stream or
discarded. In-line
techniques directly monitor the primary flow stream. On-line techniques are
generally easier
to develop, since conditions in the measurement device can be tailored for
ease of
measurement. However, measurements made on a sampling stream may not
accurately
represent the primary flow stream since the sampling stream has experienced a
different flow
history. In addition, once the sampling stream has been tested, it must either
be returned to
the primary flow stream, which can introduce disturbances into the process, or
it must be
discarded, which results in material waste and disposal issues. Further, the
use of a flow cell
precludes obtaining information regarding the mixing, movement, location or
velocity
profile of the particles in the primary flow stream.
It is therefore an object of the present invention to provide an improved
method and
apparatus for viewing particles in a relatively translucent medium which
method and
apparatus are especially useful for viewing particles in a relatively
translucent fluid stream
such as a molten polymer stream flowing in an extruder.
SLITvIMARY OF THE INVENTION
According to one aspect of the invention, a method of viewing particles in a
relatively
translucent fluid stream passing through a conduit includes illuminating the
fluids and
particles therein by passing light from an external light source through a
window in the wall
of the conduit, and viewing the illuminated fluid and particles through
another window in the
wall of the conduit by means of a telecentric lens and a charge coupled device
camera both
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positioned externally of and spaced from the conduit wall to allow the
telecentric lens to
focus an image of the fluid and particles in the conduit for the charge
coupled device camera.
The telecentric lens may be mounted on a base, with adjustment mechanism being
provided for adjusting the position of the telecentric lens on the base. A
computer may be
coupled to the charge coupled device camera to manipulate images therefrom.
According to a further aspect of the invention, apparatus is provided for
viewing
particles in a relatively translucent fluid stream passing through a conduit,
the conduit having
a wall with a window for receiving light from an external light source and
transmitting the
light into the conduit to illuminate the fluid and particles, the conduit also
having a further
window through which the illuminated fluid and particles can be observed. The
apparatus
includes a telecentric lens and a charge coupled device camera positioned
externally of and
spaced from the conduit wall to enable the telecentric lens to focus an image
of the fluid and
particles in the conduit for the charge coupled device camera.
According to a still further aspect of the invention, apparatus for viewing
particles in
a relatively translucent medium includes a telecentric lens and a charge
coupled device
camera positionable adjacent the medium to enable the telecentric lens to form
an image of
the medium and particles therein for the charge coupled device camera when the
medium is
illuminated by light.
DESCRIPTION OF THE DRAWINGS
One embodiment of the invention will now be described, by way of example, with
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reference to the accompanying drawings, in which
Fig. 1 is a schematic view of the apparatus for viewing particles in a polymer
melt
stream flowing in an extruder,
Fig. 2 is a side view of the extruder and interface assembly, and
Fig. 3 is a cross-sectional view taken along the line 3-3 of Fig. 2, the
window
assemblies being shown in exploded form.
The drawings are not necessarily to scale and, in some instances, proportions
may
have been exaggerated in order to depict more clearly certain features of the
embodiment.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to the drawings, Fig. 1 shows a front view of an extruder and
interface
assembly 10. A molten polymer stream exits the extruder from a melt channel or
conduit 12.
The interface includes four window bolts 14-1 to 14-4 which will be described
in greater
detail later. A conventional band heater 16 is secured to the exterior of the
extruder and
interface assembly 10, and heater cables 18-1 and 18-2 carry electrical power
to the band
heater 16.
A light source 60 provides high intensity light, typically produced by either
a halogen
lamp for white light or a mercury lamp for ultraviolet light. In this
embodiment, light source
60 is connected to a light guide 62 which transmits the light to window bolt
14-1 to
illuminate the molten polymer contained in melt channel 12.
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An opto-mechanical assembly 100 includes a telecentric lens 102 with an extra-
long
working distance which forms images of the molten polymer contained in the
melt channel
12 through window bolt 14-3. The telecentric lens 102 focuses an image of the
molten
polymer for a charge coupled device camera 104. The lens 102 and camera 104
may be
connected by a standard optical C- mount. The camera 104 may be a color
progressive scan
charge coupled device camera with an electronic shutter. A cable 106 carries
electronic
information between the camera 104 and a computer 108 which will be described
in greater
detail later. Cable 110 carries power to the camera 104. Multiple opto-
mechanical
assemblies may also be a feature of the present invention.
A lens bracket 112 is clamped around the body of the lens 102 and is attached
to a
mounting plate 114, thereby securing the lens 102 and camera 104 to mounting
plate 114.
Mounting plate 114 attaches the lens bracket 112 to a translation stage 116.
At a minimum,
an upper plate of the translation stage 116 can move closer to or further from
the extruder
and interface assembly 10. However, additional movement in any other direction
may also
be a feature of the present invention. An actuator 118 moves the upper plate
of translation
stage 116 and may indicate the position of the stage relative to an initial
setting. By changing
the position of translation stage 116, the lens 102 and camera 104 can produce
images of the
molten polymer at varying depths in the melt channel 12. Typically, the lens
102 and camera
104 can "scan" across the melt channel 12 in thin optical sections to produce
images of the
molten polymer from one wall of the melt channel to the other. In this
embodiment, the
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actuator 118 is manually controlled. Alternatively, the actuator may be
motorized and
controlled remotely and/or automatically.
A mounting plate 120 attaches the translation stage 116 to a tripod mount 122.
In this
embodiment, the tripod mount 122 is equipped with slow motion precision
controllers 124-1
and 124-2. Precision controller 124-1 controls the up and down, or "tilting"
motion oftripod
mount 122. Precision controller 124-2 controls the side-to-side, or "panning"
motion, of
tripod mount 122. In addition, tripod mount 122 has a coarse controller 126 to
control large
scale panning motions. Tripod mount 122 is attached to a tripod 128. A typical
tripod has
adjustable legs to change the height thereof. The controllers 124-1, 124-2 and
126, in
conjunction with an adjustable tripod, are used to control the alignment of
lens 102 with
window bolt 14-3 and, more generally, are used to provide flexibility in the
positioning of
the opto-mechanical assembly 100.
Computer 108 is equipped with a color frame grabber and imaging softvvare. The
frame grabber is used to capture and record video information from camera 104.
The
1 S imaging software provides tools for processing, enhancing and
quantitatively analyzing the
images acquired by the frame grabber. The computer 108 may also be linked to a
process
control system. Power source 150 provides power to light source 60, heater
cables 18-1 and
18-2, camera cable 110 and computer 108.
Referring now to Fig. 2, molten polymer flows from the extruder 24 to the
interface
26 and exits from the melt channel 12 through a die piece 28. The interface 26
may be
constructed of steel. The extruder 24 is attached to the interface 26 by a
flange 30. Two
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band heaters 16, 20 are secured to the exterior of the interface 26 to ensure
that the molten
polymer remains at the desired temperature. Heater cables 18-1 and 22- l,
together with two
identical heater cables (not shown) carry power to band heaters 16, 20. Window
bolts 14-1
and 14-4 are also shown and will be described in more detail later.
Fig. 3 shows window bolts 14-1 to 14-4, window gaskets 40-1 to 40-4 and
windows
42-1 to 42-4 removed from the interface ports 44-1 to 44-4 for greater
clarity. The melt
channel 12 is axially concentric with the interface 26. In this embodiment,
four cylindrical
interface ports 44-1 to 44-4 are bored through the walls of the interface 26
and the melt
channel 12. Fig. 1 shows a light guides 62 positioned at port 44-1 and lens
102 positioned
at port 44-3. The lens 102 may be positioned at any one of the interface ports
44-1 to 44-4, and light guide or guides 62 may be positioned at any of the
other ports. The
interface ports 44-1 to 44-4 are positioned at various angles around the
circumference of the
melt channel. These angles are selected to provide flexibility in illumination
techniques,
whereas the optimum lighting configuration and choice of light source are
determined by the
type of molten polymer and particles to be imaged.
Each interface port 44-1 to 44-4 houses a sapphire window 42-1 to 42-4 which
sits
directly adjacent to the molten polymer in the melt channel 12, a copper
window gasket 40-1
to 40-4 which sits on top of the window 42-1 to 42-4, and a window bolt 14-1
to 14-4 which
clamps down on the gasket 40-1 to 40-4 and window 42-1 to 42-4 to hold them
securely in
place. In this embodiment, the centres of window bolts 14-1 and 14-3 have been
bored out
along the length of the bolts so that the bolts are hollow, whereas window
bolts 14-2 and
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14-4 are solid. When an interface port 44-1 to 44-4 is being used for either
illumination or
imaging purposes, a hollow bolt is used to allow access to the sapphire window
at the base
of the port. When an interface port is not is use, a solid bolt is used to
block out stray light.
The inner walls of the interface ports 44-1 to 44-4 are threaded to allow the
window bolts 14-
S 1 to 14-4 to be screwed into place.
Although the above described embodiment is based on the use of an interface 26
which fits between an extruder 24 and a die piece 28, the present invention is
not limited to
the use of such an interface. In lieu of an interface 26, two interface ports,
including
windows, gaskets and bolts, may be placed closely enough together to allow
sufficient
illumination to be delivered through one port so that an image can be acquired
at the second
port. These ports can be located anywhere along an extrusion line, provided
that there is
sufficient surrounding space to position the light source 60 and the opto-
mechanical
assembly 100 at the appropriate ports.
Unlike the Kilham et al system or the Dynisco PCA system, the optical
equipment of
the present invention is external to the polymer processing equipment. Thus,
with the present
invention, the optical equipment is not subjected to the high temperature and
high pressure
environment in the processing equipment. In addition, since the opto-
mechanical assembly
of the present invention is not attached to such processing equipment, it is
portable. The
assembly can be moved from an interface port on one processing line to an
interface port on
an entirely separate processing line or the assembly can be moved between
ports located on
the same line.
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The present invention utilizes a telecentric lens instead of an objective or
conventional
lens. Unlike an objective lens, the telecentric lens produces an image in
which objects are
dimensionally accurate, regardless of viewing angle or proximity to the lens.
This
characteristic greatly reduces distortion of the image, thereby allowing more
accurate
quantitative information with respect to particle size, shape and location to
be obtained. The
telecentric lens also has the ability to provide images of thin optical
sections of the fluid
stream. By changing the position of the lens, it is possible to select which
thin optical section
in the fluid stream is imaged. Further, by changing the aperture setting of
the lens, the
thickness of each optical section can be adjusted. With the combination of the
telecentric
lens and the moving translation stage, the present invention has the ability
to scan across the
fluid stream in thin optical sections, allowing images to be captured at any
depth in the
conduit. In addition, the actuator which is used to move the translation stage
can indicate the
position in the conduit being imaged relative to an initial zero setting. This
provides
quantitative information on the location of the particles in the conduit
which, in turn, permits
1 S the determination of information such as particle velocity profiles and
mixing behaviour of
the particles in the fluid and of the fluid itself. Further, images captured
at different depths
in the conduit may be combined to yield three-dimensional reconstructions of
the fluid
stream and the particles thereof.
Prior art imaging systems have been plagued by poor image quality, since fast
moving
particles often appear blurry or even as streaks in an image. The present
invention enables
blur to be reduced by various measures. The charge coupled device camera may
use
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progressive scan technology and an electronic shutter to greatly reduce image
blur. In
addition, as mentioned previously, the telecentric lens has the ability to
provide images of
thin optical sections of the fluid stream. This means that objects outside of
the optical
section, i.e. objects that are out of focus, are not imaged by the lens,
thereby reducing the
blurriness of the image.
In general, the type and wavelength of light used, the number of light
sources, the
number of light guides, and the configuration of the light sources) and light
guides) are
selected based on the objects to be imaged. Further, the light may not be
transmitted through
the fluid or medium but instead may be introduced from the same side as the
lens or from any
other angle as to produce a desirable image.
Other embodiments of the invention will now be readily apparent to a person
skilled
in the art from the above description, the scope of the invention being
defined in the
appended claims.
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