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Description  |
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BACKGROUND OF THE INVENTION
(1) Field of The Invention
This invention relates to camera systems and in particular to a camera
system for optically scanning moving objects to obtain optically encoded
information from the surface of the objects.
(2) Background Art
Merchandise, various component parts, letters, moving objects, containers
and a whole gamut of related items being shipped or transported,
frequently must be identified with information regarding origin, flight
number, destination, name, price, part number and numerous other kinds of
information. In other applications, the reading of encoded information
printed on labels affixed to such items permits automation of sales
figures and inventory as well as the operation of electronic cash
registers. Other applications for such encoded labels include the
automated routing and sorting of mail, parcels, baggage, and the like, and
the placing of labels bearing manufacturing instructions on raw materials
or component parts in a manufacturing process. Labels for these types of
articles are conventionally marked with bar codes, one of which is the
Universal Product Code. Numerous other bar code systems are also known in
the art.
However, certain applications require the encoding of larger amounts of
information on labels of increasingly smaller size. Commercially-available
bar code systems sometimes lack sufficient data density to accommodate
these needs. Attempts to reduce the overall size and spacing of bars in
various bar code systems in order to increase data density have not solved
the problem. Optical scanners having sufficient resolution to detect bar
codes comprising contrasting bars spaced five nils or less apart are
generally not economically feasible to manufacture because of the close
tolerances inherent in the label printing process and the sophisticated
optical apparatus required to resolve bit-encoded bars of these
dimensions. Alternatively, to accommodate increased amounts of data, very
large bar code labels have been fabricated, with the result that such
labels are not compact enough to fit on small articles. Another important
factor is the cost of the label medium, such as paper. A small label has
smaller paper costs than a large label. This cost is an important factor
in large volume operations.
Therefore, other types of codes have been investigated to overcome the
problems associated with bar codes. Some alternatives to bar codes are:
circular formats using radially disposed wedged-shaped coded elements,
such as those disclosed in U.S. Pat. No. 3,553,438, issued to Blitz, and
entitled "Mark Sensing System, or concentric black and white bit-encoded
rings, such as in U.S. Pat. Nos. 3,971,917 and 3,916,160, issued to Maddox
and Russo, respectively; grids of rows and columns of data-encoded squares
or rectangles, such as in U.S. Pat. No. 4,286,146, entitled "Coded Label
and Code Reader for the Coded Label," issued to Uno; microscopic spots
disposed in cells forming a regularly spaced grid, as disclosed in U.S.
Pat. No. 4,634,850, entitled "Quad Density Optical Data System", issued to
Pierce; and densely packed multicolored data fields of dots or elements,
such as those described in U.S. Pat. No. 4,488,679, entitled "Code and
Reading System," issued to Sockholt.
These codes were satisfactory for many applications. However, some of the
encoding systems described in the foregoing examples and other encoding
systems known in the art still did not provide the required data density.
For example the encoded circular patterns and grids of rectangular or
square boxes did not provide sufficient density. Alternatively, in the
case of the grids comprised of microscopic spots or multi-colored elements
referred to above, such systems require special orientation and transport
means, thus limiting the utility to highly controlled reading
environments. A further improvement, U.S. Pat. No. 4,874,936, entitled
"Hexagonal Information Encoding Article, Process and System, "issued to
Chandler discloses a label for storing information-encoded hexagons which
stores densely packed information and may be read at high speed in any
direction. This improvement thus solves the data density problems
associated with bar codes.
However, the newer encoding systems, including the encoding system taught
by Chandler, are of formats which are entirely different from conventional
bar codes and can not be read by conventional bar code readers. Therefore
it is difficult to use the newer encoding methods which may solve the data
density problems of bar codes in an environment in which bar codes are
also present unless separate scanning and decoding equipment is provided
for each type of code. Thus, it would be advantageous to have a single
scanning and decoding device which may detect and decode different types
of encoding systems when the different encoding systems are alternately
disposed in the range of the optical scanning and decoding device.
Additionally, when higher density codes are used higher resolution optical
scanning and therefore higher levels of illumination are required.
However, the very high levels of illumination are only required some of
the time. Thus wasted is energy and a threat of eye injury is needlessly
created during the remaining periods.
Regardless of the type of encoding system used, high quality detection is
required in many applications. Modern conveyor systems may have conveyor
belt widths of three to four feet over which the position of an
information-encoded label may be disposed and belt speeds of five hundred
feet per minute or more. They carry moving objects which may be of varying
heights upon which information-encoded labels are disposed. Thus, it can
be very difficult for optical decoding systems to locate and read the data
encoded labels disposed on these rapidly moving objects.
These problems have led to the need for providing a simple, rapid and
low-cost means of signaling the presence of a data-encoded label within
the field of view of an optical scanner mounted in a manner to permit
scanning the entire conveyor belt. It is known in the art to solve these
problems by providing easily recognizable optical acquisition targets as
part of an encoding system. For example, the system taught by Chandler
uses a concentric ring acquisition target for this purpose.
Bar code systems may also be understood to provide an acquisition target.
For example, it is conventional in the art of detecting bar codes to
pre-detect the rectangular shape formed by the bars. In this type of
system a rectangle may indicate the presence of a bar code. Conventional
bar code detectors, after acquiring the rectangle, then attempt to find
encoded data within the pt.-detected rectangle. If valid data is found
encoded within the rectangle, the bar code is thus detected. However, many
other types of rectangles within the range of the optical scanning device
may cause false pre-detects in this method.
Further data arrays having acquisition targets other than the concentric
rings and bar codes are known in the art. For example, concentric
geometric figures other than rings, such as squares, triangles, hexagons
and numerous variations thereof, are described in U.S. Pat. No. 3,513,320,
issued to Weldon, on May 19, 1970, and entitled "Article Identification
System Detecting Plurality of Colors Disposed on an Article" and U.S. Pat.
No. 3,603,728, issued to Arimura, on Sep. 7, 1979, and entitled "Position
and Direction Detecting System Using Patterns". U.S. Pat. No. 3,693,154,
issued to Kubo et al., on Sep. 19, 1972, and entitled "Method For
Detecting the Position and Direction of a Fine Object" and U.S. Pat. No.
3,801,775, issued to Acker, on Apr. 2, 1974, and entitled "Method and
Apparatus for Identifying Objects" also describe systems using symbols
comprising concentric circles as identification and position indicators,
which symbols are affixed to articles to be optically scanned.
U.S. Pat. No. 3,553,438, entitled "Mark Sensing System" issued to Melvin,
discloses a circular data array having a centrally-located acquisition
target comprising a number of concentric circles. The acquisition target
of Melvin provides an image which may be used by an optical scanning
device to locate the label. The acquisition target of Melvin also permits
determination of the geometric center of the label and the geometric
center of the data array. This is done through logic circuitry which
recognizes the pulse pattern representative of the concentric ring
configuration.
The foregoing systems are generally scanned with an optical sensor capable
of generating a video signal output. The video output signal corresponds
to the change in intensity of light reflected off the data array and is
therefore representative of the position and orientation of the scanned
symbols. The video output of such systems, after it is digitized, has a
particular bit pattern which may be matched to a predetermined bit
pattern. A common bit pattern of this type is a simple harmonic as in the
system taught by Chandler.
It is well known to detect the presence of harmonics such as those produced
by these systems in both the digital and the analog domains. However, in
high speed optical systems for acquiring digital data the recognition of
the target must take place in much less time than is available to
recognize, for example, the touch tone of a telephone. Thus, a system for
detecting any of these codes must reliably identify the harmonics caused
by an optical scan of a common optical acquisition target from a signal
which lasts only as long is the acquisition target is actually scanned.
As previously described, Chandler discloses a circular data array having a
centrally located acquisition target comprising a series of concentric
rings which produces a harmonic scan output signal. The acquisition target
of Chandler provides a means of acquiring the circular label by the
optical sensor and determining its geometric center and thereby the
geometric center of the surrounding data array. This is done through logic
circuitry which operates to recognize the pulse pattern representative of
the concentric ring configuration of the acquisition target.
This recognition method relies upon a one dimensional scan of the
concentric ring pattern. When the concentric ring acquisition target is
advanced by a conveyor belt to the scan line of the optical scanning
equipment, the scan line eventually passes through the center of the
concentric rings. At that point, the harmonic scan output signal is
provided at the output of the optical scanning device. This harmonic scan
signal is then detected by a correlation filter. Alternately it may be
detected by any other type of harmonic detection device. However, this
system is subject to some false detects since other objects scanned by the
optical scanning device may also provide an harmonic signal at
substantially the same frequency as the concentric ring acquisition
target. Another system teaching concentric ring detector of this nature is
taught by Shaw in U.S. patent application Ser. No. 07/728,219, filed Jul.
11, 1991.
The system set forth in Chandler solves many of the problems of the prior
art systems by providing very high data density as well as a reliable
system for target acquisition. However, in addition to the problem of
false detects due to the one-dimensional scan, a relatively high
resolution scanning of this label is required in order to acquire the
target as well as to decode the high density data. An optical scanning
system capable of scanning the higher density data of the codes which
solve the density problems of bar codes may therefore be more complex and
costly than a system which is adapted to merely acquire a low resolution
target.
Thus it is often necessary for optical scanning systems to acquire a target
under very difficult circumstances. The target acquired may appear at
different locations within the scanning field and may be moving rapidly.
In addition to these problems the acquisition target may be disposed at
varying distances from the optical scanning device. For example, labels on
moving objects may be scanned at varying distances from the scanning
device because of varying package sizes. This introduces magnification
into the sampled sequence acquisition target. The closer the acquisition
target is to the scanning device, the larger it appears and the lower the
frequency of the sampled sequence. Larger scanning distances produce
higher frequencies. Detection of the varying frequencies caused by varying
amounts of magnification can be difficult since digital filters with
adjustable poles and zeros may be expensive and complicated. Additionally
the varying distance introduces the need for focussing in order to
accurately scan the acquisition target.
There are two common solutions to these problems known in the prior art.
One common solution to the focusing problem known in the prior art is
using a depth of focus sufficient to permit detection of acquisition
targets at varying distances from the optical scanning device. Another
common solution to the magnification problem is fixing the distance
between the optical scanning device and the acquisition target in order to
prevent magnification.
Prior art references teaching the use of a large depth of focus in order to
avoid focusing problems include: U.S. Pat. No. 4,544,064, entitled
"Distribution Installation for Moving Piece Goods" issued to Felder; U.S.
Pat. No. 3,801,775, entitled "Method and Apparatus for Identifying
Objects", issued to Acker; U.S. Pat. No. 3,550,770, entitled "Method for
Automatic Sorting or Recording of Objects and Apparatus for Carrying Out
the Method" issued to Lund, and U.S. Pat. No 4,454,610, entitled "Methods
and Apparatus for the Automatic Classification of Patterns," issued to
Sziklai.
One example of a reference teaching a fixed distance between the
acquisition target and the optical scanning device include: U.S. Pat. No.
3,971,917, entitled "Labels and Label Readers" issued to Maddox et al
Another reference teaching this is U.S. Pat. No. 3,757,090, entitled
"Mechanical Reading and Recognition of Information Displayed on
Information Carriers" issued to Haefeli, et al.
A solution to both the focusing problem and the magnification problem is
adjusting the distance between the acquisition target and the optical
scanning device. U.S. Pat. No. 4,776,464, issued to Miller, teaches this
type of adjustment. However, this method is mechanically difficult for a
large number of quickly moving and closely spaced moving objects of widely
varying heights. Additionally, the system taught by Shaw taught in U.S.
patent application Ser. No. 07/728,219 teaches a similar solution to this
problem.
SUMMARY OF THE INVENTION
The multiple code camera system of the present invention may simultaneously
search for a number of different optical codes. Upon detecting an optical
code it decodes according to the appropriate decoding algorithm. These
codes may include information-encoded polygons, differing bar codes, and
optical character recognition codes. The multiple code camera system is
provided with a parallel decoding architecture which allows it to search
for several codes simultaneously. The system is interconnected with two
different data buses which facilitate the parallel operation. These two
buses are: (1) a system bus linking the components of the multiple code
camera system, and (2) a pixel bus connecting an analog-to-digital
convertor from the optical scanning device to a number of different code
detection boards.
Some of the different code detection boards which have already been
installed in the system include: An interface board, coupled to the pixel
bus, which contains logic for bar code predetection. A concentric ring
detector, also coupled to the pixel bus, which performs an algorithm for
detecting concentric ring targets. And for example, if optical character
recognition is to be performed, an optical character recognition device
can also be coupled to the pixel bus. Further code detectors can also be
inserted into this architecture in order to simultaneously monitor the
pixel bus and detect additional types of code. All of the processing of
the simultaneous code detectors is performed in parallel with the system
functions do to the parallel architecture of the multiple code camera
system of the present invention. For example height sensing is performed
by the system processor in parallel with the various code detection
algorithms.
Within the concentric ring acquisition target detector the data from the
optical scanning device is arranged to form two-dimensional arrays
representative of two-dimensional scanned regions through which the
acquisition target passes. The resulting two-dimensional arrays of scanned
data are correlated with selected correlation templates, wherein each
correlation template represents an image of the concentric ring
acquisition target at a predetermined height above the belt. This method
may be applied to images undergoing any type of transform in addition to
magnification provided that the transformed images may be represented as
template images for correlation and identification. For example, an image
may be identified if it is transformed by warping, by rotating or by
positioning at varying angles or rotations with respect to the scanning
device.
The optical scanning device is clocked at a rate representative of the
speed of the target to provide a constant number of scans per target
regardless of the distance of the target from the optical scanning device.
Thus the correlation templates are elliptical rather than round when they
represent magnified images because magnification occurs only along the
axis perpendicular to the direction of travel. The correct correlation
templates are determined according to amount of magnification or warping,
or the angle. The determined template is then placed into the
two-dimensional correlators.
Within the ring detector differing stages are clocked at differing rates.
However, it is necessary to provide constant throughput through the
detector. This is achieved by interleaving the data and simultaneously
performing independent processing on a current frame and a previous frame
at stages of the detector.
The camera system does not require optical calibration adjustments. This is
achieved by using extremely close tolerances in machining the housing for
all holes used for mounting mirrors and other optical elements. Thus these
elements can be secured at exactly the correct location when the camera
system is assembled. Additionally, extremely close tolerance ribbing is
provided so that when the reflector of the camera system is resiliently
secured against the ribbing it maintains its correct elliptical shape. The
illumination source of the multiple code camera system and the conveyor
belt are disposed upon respective loci of an ellipse wherein the
resiliently secured reflector above the illumination source is adapted to
follow the shape of the ellipse.
In the camera system of the present invention the scanning rate of the
optical scanning device is controlled by the belt speed. The belt speed is
applied to scanning device via the encoder output. Because the scan rate
is controlled according to the belt speed, at lower belt speeds the amount
of integration time per scan increases. Thus the illumination requirements
of the optical scanning device decreases at lower belt speeds and
increases at higher belt speeds. Compensation for the amount of
illumination provided by the illumination source as well as compensation
for the integration time is performed by adjusting the amplitude of the
entire video signal based on the amplitude of a white reference.
Two methods for performing the white reference correction are provided. One
method for performing the white reference correction is by applying the
encoder output to a frequency-to-voltage convertor and controlling the
amplitude of the video signal from the optical scanning device according
to the DC level output of this convertor.
Another method for performing the white reference integration uses light
transmitted by way of fiber optic cables. In this method the fiber optic
cables are arranged from each of the bulbs of the illumination source to
selected pixels of the optical scanning device. These selected pixels are
dedicated to the white reference integration and therefore are not
available to represent information encoded upon an optical target.
Preferably separate optical fibers are run from each bulb of the
illumination source to prevent bulbs from dominating each other due to
their relative proximity to the sensor. The output signal of the scanning
device corresponding to these dedicated pixels is then used to control the
white reference integration. The encoder may also used to control the
illumination level of the illumination source. Thus the illumination
source may be dimmed when the belt is travelling more slowly.
The dark reference is based upon the output of a blind cell within the
optical scanning device which is sampled during each scan cycle. The
problem solved by the dark reference integration is that in the output of
the optical scanning device a small information value may ride upon a
large DC offset. This offset can vary depending upon temperature and aging
of the camera system. In the present invention an iterative integration is
performed for each scan of the optical scanning device based upon the
output of the dark cell to correct for the offset. The camera system of
the present invention thus performs continuously repeated integrations to
maintain an offset correction on a scan-by-scan basis.
The multiple code camera system of the present invention is provided with a
real time focusing system. In the real time focusing system an object
height sensor constantly determines the distance from the camera to a
surface below it. The camera optics of the multiple code camera are
constantly focused according to this measured distance. In this real time
focusing system, or continuous focus system, a delay between the
measurement of a distance and the control of the camera optics according
to the measured distance is adjusted according to the speed of the
conveyor belt as determined from an encoder output.
The multiple code camera system of the present invention is provided with a
forced air convection cooling circuit for cooling, for example, the
electronics of the system. In this cooling circuit air is forced over the
electronic circuits of the camera system, through a bleed channel, over
the system power supply, through a heat exchange compartment, and back to
the electronic circuits. In the heat exchange compartment, thermal
exchange with the exterior of the camera system is permitted through the
skin of the compartment. Additionally, the bleed-through channel between
the electronics and the system power supply is adapted to permit
dissipation of some of the system heat from the air circulating from the
electronic circuits to the power system supply.
The camera of the multiple code camera system, the electronic circuits, and
the system power supply are disposed in separately sealed compartments.
The sealed electronic circuit compartment and the sealed system power
supply compartment are in fluid communication with each other by way of
the bleed-through channel. The overall air circuit is also sealed. Because
the overall circuit is sealed, the circulated air is substantially dust
free.
There are advantages to disposing a camera system such as the multiple code
camera system of the present invention horizontally rather than
vertically. One important advantage is the ability to stack conveyor belts
above each other more closely when the camera systems are disposed
horizontally. Additionally, horizontally disposed systems are less subject
to vibration. In the past these systems were always vertical and natural
convection currents could be relied upon for cooling them. Thus it is
because of the forced convection that the present system may be disposed
horizontally.
DESCRIPTION OF THE .DRAWINGS
FIG. 1 shows a side view of the multiple code camera system of the present
invention.
FIG. 2 shows a plan view of the multiple code camera system of FIG. 1.
FIG. 3 shows a side view of an alternate embodiment of the multiple code
camera system of FIG. 1 wherein the alternate embodiment is disposed
vertically and cooled by natural convection currents.
FIG. 4 shows a block diagram representation of the data processing
architecture of the multiple code camera system of FIG. 1 for scanning
moving targets simultaneously for a plurality of differing acquisition
targets and a plurality of differing codes and acquiring and decoding the
targets.
FIG. 5 shows a more detailed block diagram representation of a portion of
the concentric ring acquisition target detector of the parallel
architecture of FIG. 4.
FIG. 6 shows a more detailed block diagram representation of a portion of
the concentric ring acquisition target detector of the parallel
architecture of FIG. 4
FIG. 7 shows a more detailed representation of the analog-to-digital
converter of the parallel architecture of FIG. 4 for receiving and
adjustably processing the output of the optical scanning device according
to the speed of the conveyor belt.
FIG. 8 shows a partial view of the multiple code camera system of FIG. 1
including fiber optic bundles for transmitting light from the illumination
source to the optical scanning device for performing a white reference
integration.
FIG. 9 shows a block diagram representation of a system for controlling the
illumination of the camera system of FIG. 1.
FIG. 10 shows a system for continuously focusing the camera of the multiple
code camera system of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIG. 1, there is shown a side view of horizontally
disposed multiple code camera system 10 of the present invention. Within
multiple code camera system 10 optically readable information-encoded
label 44 is disposed upon moving package 42 which is transported by
conveyor belt 20. As information-encoded label 44 is thus transported past
camera axis 33, it is scanned by camera 50 of multiple code camera system
10 to provide electrical signals representative of light reflected off
label 44. It will be understood that the light reflected off optically
readable label 44 represents the information which is encoded in label 44.
Illumination of optically readable information-encoded label 44 within
multiple code camera system 10 is provided by adjustable illumination
system 12. Adjustable illumination system 12 includes a plurality of
illumination sources 15 or bulbs 15 each disposed within reflector box 13
and controlled by an individual power supply 16. Each individual power
supply 16 may be separately controlled in a conventional manner in order
to control the light energy provided by its corresponding illumination
source 15.
Adjustable illumination system 12 of camera system 10 is also provided with
elliptical reflector 14 within reflector box 13. Elliptical reflector 14
is conformed to the shape of a portion of illuminatio | | |
