Note: Descriptions are shown in the official language in which they were submitted.
CA 02401401 2002-09-05
A METHOD AND SYSTEM FOR DOUBLE FEED DETECTION
This application claims priority from Canadian Patent Application No.
2,361,969 filed
November 14, 2001, and incorporated herein by reference.
The present invention relates to a method and apparatus using digital imaging
and
processing for detecting overlapped flat objects in a sequence of flat
objects. More particularly,
the invention is applicable to the detection of double or multiple fed mail
pieces in a mail sorting
apparatus.
BACKGROUND OF THE INVENTION
Mechanisms for minimising multiple feeds when processing a stack of flat
objects are
well known. For example, sheet feeders, bank note readers and mail piece
sorting systems all
employ feed mechanisms for picking off work pieces sequentially and singly
from an input stack
for transport along a feed path at relatively high speed.
In a mail sorting system, the mail pieces are essentially flat rectangular
objects having a
pair of large flat surfaces and four edges, and the mail pieces are arranged
with their planar
surfaces along a common axis to form a stack.
A feeder mechanism picks off individual mail pieces from an input stack to an
OCR
(Optical Character Reader) which reads a forwarding address printed on the
mail piece and
directs the mail piece to one of several output stacks corresponding to the
destination address.
The feed rate of such sorting apparatus is typically several thousand mail
pieces per hour, so
occasionally more than one mail piece is picked off by the feeder resulting in
a multiple feed,
also referred to in the art as a double feed. Multiple feeds pose a problem in
that two or more
mail pieces may end up in the wrong destination stack with the result that the
misfed mail pieces
are not delivered on time. Furthermore, multiple fed mail pieces may cause
jamming within the
CA 02401401 2002-09-05
stacker apparatus. Both of these problems are costly. Accordingly, the
benefits of detecting
multiple fed mail pieces are evident, and particularly if the multiple feeds
are detected as early as
possible in the feed path.
A "double feed" is characterized by two or more mail pieces being stuck
together
generally along their flat sides with either one or more edges completely or
partially overlapped.
While current double feed detection systems will detect a partial or complete
overlap, few are
capable of also distinguishing a false double feed, which occurs when a
relatively thick mail
piece with a crinkled or creased edge is picked off or when the mail piece has
a dark colour, is
multicoloured or has a fold over. Of course, the detection of false double
feeds should be
avoided.
There are a number of disadvantages with current techniques for detecting
double feeds.
For example, U.S. Patent No. 4,733,226 describes a system for detecting
overlapped mail pieces
where one mail piece hides another in the feed path. A scanner is arranged
along the feed path to
detect the height of the mail piece as it moves past the scanner. Any changes
in the height of
mail pieces signals an overlap condition. As the system is limited to
detecting variations in
height, it cannot be used to detect an overlap where the mail pieces are the
same height or where
the mail pieces are fully overlapped. In United States Patent No. 4,160,546,
there is described a
system which uses changes in document translucency to trigger an overlap
indication. While this
system may be effective for detecting documents that are translucent and have
similar
characteristics, it is not as effective for mail pieces which are typically
opaque.
Also, imaging techniques for counting stacks of flat objects are known,
however these
techniques have limitations when used for double feed detection. For example,
U.S. Patent No.
5,534,690 describes a system for counting the number of bank notes in a stack
by imaging the
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CA 02401401 2002-09-05
entire side of a stack while the stack is kept stationary. The system
determines the number of
items in the stack by taking two images of the side of the stack at different
illuminations. The
number of lines in the two images is compared. The average number of lines
between the two
images indicates the number of items in the stack. A limitation of this system
is that the stacked
items must be stationary so that a meaningful comparison can be made between
the two images.
Accordingly, this technique cannot be used for determining double feeds in a
moving stream of
objects such as in a mail sorting apparatus. Other patents that describe
counting techniques are,
for example, disclosed in U.S. Patent No. 5,221,837.
A need therefore exists for the effective detection of double feeds, including
both
partially overlapped and fully overlapped mail pieces, in a mail sorting and
handling apparatus,
while the mail pieces are in motion. Furthermore, there is a need for a double
feed detection
(DFD) system that can detect double feeds where the objects have different
heights, different
colors, and different widths and with crinkled edges with minimal impact on
feed mechanisms or
existing sorters.
SUMMARY OF THE INVENTION
The invention provides a double feed detection system and method for detecting
two or
more mail pieces (e.g. envelopes), either partially or fully overlapped,
passing simultaneously
through a mail sorting and handling apparatus.
The DFD system includes a vision system with a digital camera for capturing
and
analyzing images of the bottom edges of mail pieces as they pass through the
mail sorting
apparatus, multiple photosensors for detecting and tracking the mail pieces
through the mail
sorting apparatus, and a controller for system control, system fault
monitoring, and outputting
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double feed rejection signals to the mail sorting apparatus to enable the re-
routing of detected
double feeds.
In particular, according to one aspect of the invention, a system is provided
for detecting
overlapped flat objects in a sequence of flat objects, where the flat objects
have at least one of
their edges exposed for viewing as they pass along a feed path. The system
includes: a sensor for
generating a signal in response to detecting a flat object in the feed path; a
camera responsive to
the signal for capturing a digital image of the exposed edges of the detected
flat object in the feed
path; and a vision system coupled to the camera for receiving the digital
image. The vision
system analyses at least a portion of the image to determine a pixel density
variation along a
direction perpendicular to the edges and uses the pixel density variation to
output an indication
of the number of edges in the image.
The DFD method is implemented in part by software run by the vision system.
According
to this method, an image of the bottom edges of a mail piece is captured and
an inspection is
performed on at least a portion of this image to determine if the mail piece
is of a predetermined
thickness. If the mail piece is of the predetermined thickness, low
sensitivity settings of the
expected average edge width are used by the software to count the number of
edges. If the mail
piece is less than the predetermined thickness, high sensitivity settings of
the expected average
edge width are used to count the number of edges. If the mail piece is of the
predetermined
thickness and the measured number of edges is less than two, there is no
double feed and an
output from the vision system to the controller indicates an "OK" condition
for the mail piece.
On the other hand, if the mail piece is of the predetermined thickness and the
measured number
of edges is not less than two, there is a double feed and the output to the
controller indicates a
"Double Feed" condition for the mail piece. If the mail piece is less than the
predetermined
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thickness and the measured number of edges is less than two, there is no
double feed condition
and the output to the controller indicates an "OK" condition for the mail
piece. If the mail piece
has less than the predetermined thickness and the measured number of edges is
greater than two,
there is a double feed and the output to the controller indicates a "Double
Feed" condition for the
mail piece. Finally, if the mail piece has less than the predetermined
thickness and the measured
number of edges is equal to two and the measured edge pitch is smaller than a
predetermined
threshold, there is no double feed and the output to the controller indicates
an "OK" condition for
the mail piece. On the other hand, if the mail piece has less than the
predetermined thickness, the
measured number of edges is equal to two and the measured edge pitch is
greater than a
predetermined threshold, there is a double feed and the output to the
controller indicates a
"Double Feed" condition for the mail piece.
In particular, according to another aspect of the invention, a method is
provided for
detecting overlapped flat objects in a sequence of flat objects where the flat
objects have at least
one of their edges exposed for viewing as they pass along a feed path. The
method includes the
steps of: selecting a flat object in the feed path; capturing a digital image
of the exposed edges of
the selected flat object; processing at least a portion of the captured image
encompassing the
edges to determine a pixel density variation in a direction across the edges;
analysing the pixel
density variation to identify maxima and minima in the variation, where a
start of an edge is
identified by a maximum and an end of an edge is identified by a minimum; and,
counting the
maxima and minima to output an indication of the number of edges in the image.
The method
may further include determining an edge width of the flat object. The method
of determining an
edge width of the flat object may include: computing an average pixel density
for the processed
portion; assuming a first edge width if the average density is below a
predetermined level; and,
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assuming a second edge width if the average density is above the predetermined
level. The
method of counting may further include: counting maximum and minimum pairs
that are spaced
by less than the first edge width if the average density is below the
predetermined level to output
the indication of the number of edges; and, counting maximum and minimum pairs
that are
spaced by more than the second edge width if the average density is above the
predetermined
level to output the indication of the number of edges.
The DFD method is also implemented in part by software that is run on the
controller.
The controller receives outputs from the vision system indicating that each
passing mail piece is
either "OK" or is a "Double Feed". The controller analyses these outputs from
the vision system,
information from multiple photosensors that track the progress of mail pieces
through the mail
sorting, and monitored fault information to determine when or if a double feed
rejection signal
should be sent to the mail sorting apparatus to enable the re-routing of
detected double feeds.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention may best be understood by referring to the
following
description and accompanying drawings in which:
FIG. 1 is a block diagram illustrating a mail sorting system with an
incorporated DFD
system in accordance with an embodiment of the invention;
FIG. 2 is a block diagram illustrating a double feed detection ("DFD") system
in
accordance with an embodiment of the invention;
FIG. 3 is a simplified perspective view illustrating a DFD system in
accordance with an
embodiment of the invention;
FIGS. 4(a), 4(b), and 4(c) are partial plan views of the DFD system
illustrating the
relationship between passing mail pieces and photosensors P 1 and P2;
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FIG. 5 is a partial plan view of the DFD system illustrating the relationship
between
passing mail pieces and photosensors P 1, P2, and P3;
FIG. 6(a) is a screen capture illustrating an image of mail piece bottom edges
captured by
a camera;
FIGS. 6(b) and 6(c) show a schematic diagram of the screen image of FIG. 6 (a)
and its
corresponding density variation;
FIG. 7 is a flow chart illustrating a general method for detecting a double
feed condition
using a vision system in accordance with an embodiment of the invention;
FIG. 8 is a pseudocode listing corresponding to the flow chart of FIG. 7; and
FIG. 9 is a flow chart illustrating a general method for providing a double
feed rejection
signal to a sorter using a controller in accordance with an embodiment of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
In the following description, like numerals refer to like structures and/or
processes in the
drawing. Furthermore the invention will be described in the context of a mail
sorting application,
however this is merely exemplary and not limiting of the general applicability
of the invention.
FIG. 1 is a block diagram of a mail sorting system 100 which includes a double
feed
detection (DFD) system 200 in accordance with an embodiment of the invention.
The mail
sorter 100 includes an input stacker 110 for supporting a stack of mail pieces
to be sorted, a
feeder mechanism 120 for picking off mail pieces, preferably one at a time,
and transporting
them by a conveyor belt sequentially along a feed path 132 past an OCR
(optical character
recognition) system 140 to one or more destination bins or output stacks 150.
The OCR system
140 is used to determine the destination address of the mail piece and thereby
control an
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appropriate gate to a destination bin. The arrow 130 indicates the direction
of flow of the mail
piece along the feed path 132.
The feeder mechanism 120 typically picks off mail pieces at a rate of several
thousand
per hour and may not always pick off a single mail piece from the input
stacker, but may instead
pick off two or more overlapped mail pieces.
Accordingly, the present invention addresses this problem by providing a DFD
system
200 for detecting double feeds and initiating appropriate action to the
sorter, such as providing a
signal to divert overlapped mail pieces from the feed path 132 to a rejection
bin.
The DFD system 200 is preferably located between the feeder 120 and the OCR
140 and
generally provides three functions: edge detection, overlap detection, and
mail piece tracking.
FIG. 2 is a block diagram of a DFD system 200 in accordance with an embodiment
of the
invention. The DFD system 200 includes a programmable logic controller (PLC)
270 which is
interfaced to a vision system 260 and its associated camera and lamp 210 for
edge detection;
photo sensors Pl, P2, P3 positioned along the feed path for overlap detection,
triggering the
vision system and mail piece tracking; an output device 280; and a system
panel 291. The output
device 280 provides double feed rejection signals generated by the DFD system
200 to the sorter
system 100. The output device 280 may also include a CD-ROM, a floppy disk, a
printer, a
digital output (e.g. solid state device or relay contact), or a network
connection. The panel 291
may include an input device 290 and a display 292. The input device 290 may
include a
keyboard, mouse, trackball, control switches, or similar devices. The display
292 may include a
CRT screen, LCD screen, indication lamps, or similar devices. The vision
system 260 includes
image-processing software for computing the number of edges in a passing mail
piece. The
vision system is coupled to receive inputs from the photosensor P2 230 and the
camera 250. The
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vision system 260 may also be interfaced to an input device 290 and display
292 either directly
or through a panel 291. As will be described below, the vision system 260
provides a double feed
indication to the controller 270. The vision system 260 and controller 270 may
be a single unit.
The vision system 260 and/or controller 270 may include an input device, a
central processing
unit or CPU, memory, a display, and an output device. The CPU may include
dedicated co-
processors and memory devices. The memory may include RAM, ROM, databases, or
disk
devices.
The DFD system 200 may be implemented with the following hardware components,
available from Omron Canada Inc., or equivalents: Lamp 210: Model 101K12351;
Photosensors
220, 230, 240 Model E32-T14 with amplifier E3X-F21; Camera 250 Model F150-S1A;
Vision
System 260 Model F 150-C 1 OE-3 with console F 150-KP; Controller 270 Model
CPM2C-
20CDTC-D; Output Device 280 Model CPM2C-20CDTC-D (digital outputs); Input
Device 290
Model NT2S-SF123B-E (function keys); Display Model 292 NT2S-SF123B-E (2 line
LCD
display). Persons skilled in this art will recognise that the method and
system of this invention
can be implemented with a wide variety of hardware components suitable to
perform the
disclosed functions. Each of the functions performed by the DFD system 200
will be described in
more detail below.
Edge Detection Using a Vision System. Edge detection may be better understood
by refernng
now to FIG. 3, which is a block diagram of the relative positions of the
various components in
the DFD system 200 according to an embodiment of the present invention. For
clarity, the
conveyor belts and mechanical devices for moving mail pieces 310 along the
feed path 130, 132
are well known and have been omitted. For illustrative purposes, a typical
single fed mail piece
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is indicated by the numeral 310, while a typical double feed is indicated by
the numeral 320. The
double feed 320 is shown as two overlapped envelopes 321, 322.
Mail pieces 310 typically pass along the feed path 130, 132 in an upright
orientation with
at least one of their edges 330 visible to the camera 250, which is positioned
below the feed path
130. Typically, the visible edge is at least the bottom edge of one mail piece
310 and passes
through an imaging region of the camera lens. The photocell sensor P2 230 is
positioned in the
feed path 130, 132 such that the camera 250 is triggered when a leading edge
of the mail piece
310 passes the sensor 230 and the camera 250 captures images of the bottom
edges 330 of
passing mail pieces 310. The lamp 210 is directed toward the bottom edges 330
to illuminate
them for improved image capture by the camera 250. The camera lens 250 need
not be mounted
perpendicular to the mail piece path 130, 132 but may be mounted at an angle
to the feed path
130, 132.
The imaging region of the camera is spaced a distance L, along the feed path,
from
photosensor P2 230 such that when a mail piece 310 is detected by photosensor
P2 230, a signal
is sent by the photosensor P2 230 to the vision system 260, which in turn
controls the camera
250 to capture an image of the bottom edge 330 of the passing mail piece 310.
This distance is
chosen so that the camera captures an image of the bottom edge 330 of the
shortest allowable
mail piece 310 passing along the feed path 130. For example, if the shortest
allowable bottom
edge of the mail piece 310 is 140 mm, then the camera 250 would typically be
spaced
approximately 130 mm from photosensor P2 230.
Once the image is captured, the digital image-processing software executed by
the vision
system 260 is used to analyze the image 600 to determine the number of edges
present using any
method known in the art.
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The operation of the edge determination function may be better understood by
referring
to FIGS. 6(a), 6(b), and 6(c).
FIG. 6(a) is a screen capture illustrating an image 600 of mail piece bottom
edges 330
captured by a camera 250. The image 600 is typically stored digitally in the
memory of the
vision system 260. The image 600 may also be displayed to a user on the
display 292. The
bottom edge 330 of the mail piece 310 typically appears as a line 610 against
a background 620
in the captured image 600. The image 600 contains two lines 610, 61 l,
indicating that the mail
piece 310 has two bottom edges 330. Hence, the mail piece 310 may consist of
two envelopes.
In order for the software to perform the edge detection analysis, the user
defines a
measurement region 630 in which to perform a density variation analysis within
the captured
image 600. This region will encompass the mail piece bottom edges 330 passing
along the feed
path 130, 132.
The software determines the presence of edges through an analysis of pixel
density
variations across the selected measurement region 630 of the digital image.
This may be
understood from FIGS. 6(b) and 6(c) which are, respectively, a schematic
representation of an
acquired image in the measurement region 630 and a corresponding graph of the
density
variation as a percentage of dark to light pixels over the measurement region
630. In general,
edges are detected by analysing points on the density variation graph over the
measurement
region 630 and in a direction perpendicular to the edges. This perpendicular
direction may be
inferred by the software as the orientation of the camera 250, and hence the
captured image 600,
is known relative to the feed path direction 130. The points X correspond to
maxima 604 and
minima 605 that exceed an edge level threshold value 606, 607 and are detected
as beginnings or
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ends of edges 608. The software counts the number of maxima and minima, and
depending on
the sensitivity settings (as explained below), infers the number of edges 608.
First, an average density threshold parameter for the measurement region 630
is specified
by a user. The software will perform an average density inspection on the
measurement region
630 to determine a measured average density. In general, the measured average
density is the
ratio of pixels corresponding to lines 610, 611 to pixels corresponding to
background 620 within
the measurement region 630. The software will compare the average density
threshold parameter
to the measured average density to determine if a mail piece is thick (i.e.
large) or thin (i.e.
small). If the measured average density is above the average density threshold
parameter, then
the mail piece will be considered to be thick. If the measured average density
is below the
average density threshold parameter, then the mail piece will be considered to
be thin. The
software will count the number of edges using a low sensitivity inspection for
thick mail pieces
and using a high sensitivity inspection for thin mail pieces.
Next, the user specifies a set of parameters for each of the low and high
sensitivity
inspections. These parameters are set by the user as follows: first, the user
defines an expected
average edge width parameter 640 for both thick and thin mail pieces; second,
the user defines
an expected average edge pitch parameter 650 also for both thick and thin mail
pieces. The
expected average edge pitch is the expected distance between the center of the
edges of two
double fed mail pieces 610, 611, 321, 322.
The user then defines a number of edges parameter 660 that will represent a
single feed
condition. This number will generally be "1 ". Finally, the user indicates to
the software through a
judgement parameter 670 that the entered parameters represent a single feed or
"OK" condition.
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As will be described below, the software uses these parameters to determine if
a double feed
condition exists.
The low and high sensitivity values for the expected average edge width and
edge pitch
parameters allow for differentiated handling of thick, thin, and dark colored
(e.g. red-striped
edge envelopes) mail pieces. In general, thin mail pieces usually have crisp,
well-defined edges.
On the other hand, thick mail pieces often have creases and dents in their
edges and, as such,
they may be mistakenly considered as double feeds. Consequently, inspections
are performed on
the density variation at either high or low sensitivities. As mentioned above,
the average density
inspection is performed on the measurement region 630 to determine if the mail
piece is thick or
thin, and hence, select between the results of the low and high sensitivity
inspections.
The high sensitivity inspection is performed to identify, for example, dark
colored mail
piece edges that do not generally show up well in the captured image 600
(i.e., dark colored mail
pieces may be similar in colour to the background). The high sensitivity
inspection results in a
first edge count. The low sensitivity inspection is performed to avoid false
edge counts due to
creases and dents in thick mail pieces. The low sensitivity inspection results
in a second edge
count. To choose between first and second edge counts, and consequently to
determine if a
double feed condition exists, the average density inspection is performed. If
the average density
inspection determines that the mail piece is thick, the second edge count
(i.e. at low sensitivity)
is chosen. If the average density inspection determines that the mail piece is
thin, the first edge
count (i.e. at high sensitivity) is chosen. The chosen edge count is used to
determine if a double
feed condition exists as will be described with reference to FIG. 7 below.
With respect to the first edge count (i.e. at high sensitivity), if the
measured average
density is below the average density threshold parameter (i.e. a thin mail
piece), then the
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software produces the first edge count by counting maximum and minimum pairs
604, 605 that
are spaced less than the high sensitivity expected average edge width setting
640. In this way,
maxima and minima corresponding to each thin mail piece edge are generally
included in the
first edge count. With respect to the second edge count (i.e. at low
sensitivity), if the measured
average density is above the average density threshold (i.e. a thick mail
piece), then the software
produces the second edge count by counting maximum and minimum pairs 604, 605
that are
spaced further than the low sensitivity expected average edge width setting
640. In this way,
maxima and minima corresponding to creases and dents in thick mail piece edges
are generally
excluded from the second edge count. In general, the distance or spacing
between a maximum
604 and a minimum 605 (i.e. measured edge width) or between maxima and minima
pairs 604,
605 (i.e. measured edge pitch) may be measured by the software through a count
of pixels along
the density variation as shown in FIG. 6(c).
FIG. 7 is a flow chart illustrating a general method for detecting a double
feed condition
using the vision system 260 in accordance with an embodiment of the invention.
In FIG. 7, the
flow chart is shown generally by numeral 700. At step 701, the method begins.
At step 702, an
inspection is performed to determine if the mail piece 310 is thick or thin.
At step 703, if the mail
piece is thick, low sensitivity settings are used by the software to count the
number of edges. At
step 704, if the mail piece is thin, high sensitivity settings are used to
count the number of edges.
At step 705, if the mail piece is thick and the measured number of edges 681
is less than 2, there
is no double feed and the output to the controller 270 indicates an "OK"
condition for the mail
piece. On the other hand, if the mail piece is thick and the measured number
of edges 681 is not
less than 2, there is a double feed and the output to the controller 270
indicates a "Double Feed"
condition for the mail piece. At step 706, if the mail piece is thin, and the
measured number of
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edges 681 is less than 2, there is no double feed condition and the output to
the controller 270
indicates an "OK" condition for the mail piece. At step 707, if the mail piece
is thin and the
measured number of edges 681 is greater than 2, there is a double feed and the
output to the
controller 270 indicates a "Double Feed" condition for the mail piece. At step
708, if the mail
piece is thin and the measured number of edges 681 is equal to 2 and the
measured edge pitch
682 is smaller than a predetermined threshold, there is no double feed and the
output to the
controller 270 indicates an "OK" condition for the mail piece. On the other
hand, if the mail
piece is thin and the measured number of edges 681 is equal to 2, and if the
measured edge pitch
682 is greater than a predetermined threshold (i.e., expected average edge
pitch), there is a
double feed and the output to the controller 270 indicates a "Double Feed"
condition for the mail
piece. In this way, fold-overs, for example, may be distinguished from true
double feeds. At
step 709, the method ends.
FIG. 8 is a pseudocode listing corresponding to the flow chart of FIG. 7. Note
that while
the vision system 260 need not be programmed using a logic flow sequence, for
clarity, the
method illustrated by the flowchart of FIG. 7 is presented as pseudocode in
FIG. 8.
Referring again to FIG. 6(a), where two edges 610, 611 are shown, the software
has
analyzed the measurement region 630 and has presented measured values 680,
681, 682, 683 for
the judgement 670, number of edges 660, edge pitch average 650, and edge width
average 640
parameters, respectively. While the measured number of edges 681 is "2", the
software, based on
a combination of criteria as described, provides a measured judgement 680 of
"OK". In other
words, a double feed condition does not exist even though two edges have been
detected. The
mail piece may be, for example, a "fold over" rather than two envelopes that
have stuck together.
CA 02401401 2002-09-05
If a double feed condition exists, then a signal is output from the vision
system 260 to the
controller 270. This signal indicates if the mail piece is "OK" or if it is a
"Double Feed".
Overlap Detection and Mail Piece Tracking Using Photosensors. Referring to
FIGS. 1-3,
the DFD system 200 includes three photosensors P1 220, P2 230, and P3 240. In
general, the
first two photosensors P 1 220, P2 230 are used to detect a double feed
condition in accordance
with a further embodiment of the invention.
FIGS. 4(a), 4(b), and 4(c) are partial plan views of the DFD system 200
illustrating the
relationship between passing mail pieces 310 and photosensors P1 220 and P2
230. Referring to
FIG. 4(a), the first two photosensors P1 220, P2 230 are spaced in the mail
piece path 130 at a
distance greater than the length of the largest allowable mail piece (e.g.
envelope) for the sorter
100. For example, if the maximum allowable length for a mail piece 310 is 260
mm, then the two
photosensors P 1 220, P2 230 may be spaced approximately 270 mm apart.
Referring to FIGS.
4(a) and (c), as a mail piece 310 passes through the feeder 120 and into the
OCR system 140, it
turns on the first photosensor P 1 220. If the first photosensor P 1 220
remains on until the second
photosensor P2 230 is turned on, a double feed 320 condition exists. If both
photosensors P 1
220, P2 230 are turned on simultaneously, this indicates that the mail piece
is longer than the
maximum allowable length, which may mean that two mail pieces 321, 322 are
passing through
the sorter at the same time. Refernng to FIG. 4(b), note that if two small
envelopes 410, 420 pass
through the sorter one after the other, it is possible that photosensors P1
220 and P2 230 may
both be turned on (i.e. tripped) simultaneously. This would not create a
double feed condition
320 as the first photosensor P1 220 would turn off before the second
photosensor P2 230 turns
on. That is, the gap between the two small envelopes 410, 420 is recognized by
the photosensors
P 1 220, P2 230.
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The status of photosensors P 1 220 and P2 230 is monitored by the controller
270. The
second photosensor P2 230 is also monitored by the vision system 260. A signal
is provided to
the vision system 260 indicating that an image is to captured by the camera
250. As will be
described below, the vision system 260 processes the image captured by the
camera 250 to
determine the number of edges of a passing mail piece 310 and, hence, whether
there is a double
feed. Thus, the DFD system 200 includes two means for detecting double feeds;
a first and
second photosensors means and a vision system means. Note that the DFD system
200 may
operate with one or both of these double feed detection means.
FIG. 5 is a partial plan view of the DFD system 200 illustrating the
relationship between
passing mail pieces 320 and photosensors Pl 220, P2 230, and P3 240. The third
photosensor P3
240 is required because double feed detection is typically performed by the
DFD system 200 at a
distance along the mail piece path 130 spaced before the spot 340 in the path
130 where a double
feed rejection signal is typically issued by the DFD system 200, via its
output device 280, to the
sorter system 100. The third photosensor P3 240 allows the DFD system 200 to
track the mail
piece 320 through the sorter processing apparatus 140 and to provide the
double feed rejection
signal at the appropriate time.
DFD System Fault Detection. The DFD system 200 performs several self
diagnostic
routines. In particular, the DFD system 200 monitors the number of double
feeds detected to
determine if a fault or malfunction has occurred. With respect to double feed
counts, a fault may
have occurred if the count is too low (i.e. a "too few double feeds fault") or
too high (i.e. a "too
many double feeds fault"). Normally, a number of double feeds will occur
during a routine
sorting operation. If no double feeds are detected, a malfunction may have
occurred. For
example, the camera lamp 210 may have burnt out. Typically, the DFD system 200
will generate
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a too few double feeds fault signal if a double feed has not been detected in
the last 5,000 (or
10,000) mail pieces that have passed through the sorter 100. The too few
double feeds fault may
be automatically reset when the DFD system 200 subsequently detects a double
feed.
A second type of fault that the DFD system 200 checks for is too many double
feeds.
This condition may indicate a more severe malfunction in the DFD system 200.
For example, the
lens of the camera 250 may be dirty or the DFD system 200 may have been set up
incorrectly. If
a too may double feeds fault occurs, then the DFD system 200 may generate a
fault alarm and
may shut down its output 280 to the sorter 100. The too many double feeds
fault may be
automatically reset upon a detected reduction in the number of double feeds.
Typically, the DFD system 200 will determine two types of too many double
feeds faults,
namely, a "50 in a row OR 5%" fault and a "50 in a row OR 50%" fault. Let C be
a mail piece
count, X be a count increment, and Y be an alarm level. For each passing mail
piece, if a double
feed is detected, add X to C, otherwise, if a double feed is not detected
(i.e. a single feed occurs),
subtract 1 from C. Now, for a "50 in a row OR 5%" fault, set X equal to 20 and
Y equal to 1,000.
The ratio Y/X is equal to 1000/20 or 50. Hence, if there are 50 double feeds
in a row, an alarm
will be generated and the output 280 to the sorter 100 will be shut down.
However, if there are
more than 20 (i.e. X) single feeds for each double feed (i.e. 5% double feed
occurrence rate),
then an alarm will be generated but the output 280 to the sorter 100 will not
be shut down.
For a "50 in a row OR 50%" fault, set X equal to 1 and Y equal to 50. The
ratio Y/X is
again equal to S0. Hence, if there are 50 double feeds in a row, an alarm will
be generated and
the output 280 to the sorter 100 will be shut down. However, if there are more
than 1 (i.e. X)
single feeds for each double feed (i.e. 50% double feed occurrence rate), then
an alarm will be
generated but the output 280 to the sorter 100 will not be shut down.
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CA 02401401 2002-09-05
The presence of a too few double feeds fault or too many double feeds fault
may be
reported to a user through user interface devices mounted on the panel 291, as
described above,
or to an external system through the output device 280.
Controller Operation. In general, the controller 270 monitors the photosensors
P 1 220,
P2 230, P3 240, and the vision system 270 to determine if a double feed has
occurred. If a double
feed is detected by the controller 270, it is reported to the sorter 100
through the output device
280 of the DFD system 200 and to the user locally through the devices mounted
on the system
panel 291. The controller 270 also performs self diagnostic functions for the
DFD system 200 as
described above and maintains statistics including mail piece and double feed
counts.
Referring to FIGS. 1, 2, and 3, in operation the controller 270 performs
functions
including the following:
1. Checks if a mail piece 310 passing through the DFD system 200 is too long.
In
general, a mail piece will be too long if, for example, the mail piece
consists of two overlapping
and offset envelopes. To perform this check, the controller 270 monitors
photosensors Pl 220
and P2 230 as described above. The controller 270 (a) continuously checks when
photosensor P 1
220 was last unblocked, and (b) determines that there is a double feed if
photosensor P2 230 is
blocked and photosensor P 1 220 has not become unblocked. In this case, the
mail piece 310 is
longer than the distance between photosensors P 1 220 and P2 230 and therefore
the mail piece
310 is a double feed. Alternatively, the mail piece 310 simply may be longer
than the longest
allowable length in which case it should also be rejected.
2. Checks for a double feed signal from the vision system 260. In general, the
vision
system 260 will provide a double feed indication if the mail piece 310
consists of, for example,
two fully overlapped envelopes (i.e. overlapped but not necessarily offset).
The vision system
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CA 02401401 2002-09-05
260 typically provides the controller 270 with a gate signal which indicates
that the double feed
output is ready for scanning by the controller 270. Upon receipt of the gate
signal, the controller
270 will scan the double feed output from the vision system 260 to determine
if a double feed
condition exists. The double feed output from the vision system 260 is
typically a solid state
device output or relay contact (e.g. a logical high or a normally open
contact).
3. Delays double feed signal output to the sorter 140. If a double feed is
detected by
either the photosensors P1 220, P2 230 or vision system 260 (i.e. functions 1
and 2 above), then
the mail piece 310 will be recorded or marked as a double feed by the
controller 270. Typically,
two shift registers within the controller 270 may be used. A bit in the first
shift register (i.e. the
"mail piece present shift register") is set to indicate that a mail piece 310
is passing through the
DFD system 200. A bit in the second shift register (i.e. the "double feed
shift register") is set to
indicate that the mail piece 310 is a double feed. The mail piece present and
double feed shift
registers are used to delay output of a double feed signal to the sorter 100.
4. Outputs a double feed rejection signal to the sorter processing apparatus
140 via
the output device 280 of the DFD system 200. The controller 270 determines if
a double feed
rejection signal should be output to the sorter 100, 140 by monitoring
photosensor P3 240, fault
status (as described above), and corresponding bits of the mail piece present
and double feed
shift registers. When photosensor P3 240 turns on, the controller 260 checks
for the setting of
corresponding bits for the mail piece in the mail piece present and double
feed shift registers. If
there is no fault (i.e. alarm), photosensor P3 240 is on, and the
corresponding shift register bits
are both set, then the controller provides a double feed rejection signal to
the sorter 100, 140 via
the output device 280. If photosensor P3 240 is on, but no corresponding bits
in the shift registers
CA 02401401 2002-09-05
are set, then no double feed rejection signal will be provided to the sorter
100, 140 (i.e. the mail
piece is not rejected).
5. Provides fault alarms and statistics to users through the panel 291 of the
DFD
system 200 or to external systems through the output device 280. As described
above, the
controller 270 will generate a fault alarm if too many mail pieces are double
feeds (e.g. the
vision system 260 is malfunctioning). In this event, the controller 270 will
turn the double feed
rejection signal off. In addition, the controller 270 will generate a fault
alarm if too few mail
pieces are double feeds. With respect to statistics, the controller 270
maintains and increments
counters for both mail pieces passing through the DFD system 200 and for
double feeds detected.
6. Provides a setup mode for configuring the DFD system 200. The setup of the
DFD system 200 is described in greater detail below. In the setup mode, the
controller 270
measures how long it takes for a mail piece to travel from photosensor P2 230
to photosensor P3
240. This information is used to select which bits in the mail piece present
and double feed shift
registers are to be monitored (i.e. in function 4 above).
Referring to FIG. 9, there is shown a flow chart 900 illustrating a general
method for
providing a double feed rejection signal to a sorter 100 using a controller
270 in accordance with
an embodiment of the invention. At step 901, the method begins. At step 902,
the controller 270
checks if a double feed condition exists by monitoring both the photosensors
P1 220, P2 230, and
the vision system 260. At step 903, if a double feed condition exists, then
the mail piece 310 is
marked by setting a bit in the double feed shift register. At step 904, a
corresponding bit is set in
the mail piece present shift register. At step 905, a delay is introduced to
allow the mail piece to
travel through the DFD 200 and sorter 100 systems. At step 906, the controller
270 monitors
photosensor P3 240 until the mail piece 310 passes. At step 907, when the mail
piece 310 arrives
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at photosensor P3 240, the controller 270 checks for a fault alarm and for the
setting of
corresponding bits in the mail piece present and double feed shift registers.
At step 908, if no
alarm if present and if corresponding bits in the mail piece present and
double feed shift registers
are set, then the double feed rejection signal is turned on. At step 909, the
mail piece and double
feed counters are incremented. At step 910, the method ends.
Data Carrier Product. The sequences of instructions which when executed cause
the
method described herein to be performed by the DFD system 200 of FIG. 2 can be
contained in a
data carrier product according to an embodiment of the invention. This data
carrier product can
be loaded into and run by the DFD system 200 of FIG. 2.
Computer Software Product. The sequences of instructions which when executed
cause
the method described herein to be performed by the DFD system 200 of FIG. 2
can be contained
in a computer software product according to an embodiment of the invention.
This computer
software product can be loaded into and run by the DFD system 200 of FIG. 16.
Integrated Circuit Product. The sequences of instructions which when executed
cause the
method described herein to be performed by the DFD system 200 of FIG. 2 can be
contained in
an integrated circuit product including a coprocessor or memory according to
an embodiment of
the invention. This integrated circuit product can be installed in the DFD
system 200 of FIG. 2.
In general, the invention described herein provides a double feed detection
("DFD")
system for detecting two or more mail pieces (e.g. envelopes), either
partially or fully
overlapped, passing simultaneously through a mail sorting and handling
apparatus.
While the invention is described in relation to a OCR system, it is applicable
to a wide
variety of mail sorting and handling apparatus including Multi-Line Optical
Character Readers
(MLOCR), Single Line Optical Character Readers (SLOCR); Bar Code Sorters
(BCS); Delivery
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CA 02401401 2002-09-05
Bar Code Sorters (DBCS); Enhanced Bar Code Sorters (EBCS); Input Sub System
family
devices (ISS); Advanced Facer Canceller Systems (AFCS); Advanced Facer
Canceller System
Input Sub Systems (AFCS ISS); Alcatel Flat Sorter Machines (AFSM); Flat Sorter
Machines
(FSM); and, Letter Sorter Machines (LSM).
Although preferred embodiments of the invention have been described herein, it
will be
understood by those skilled in the art that variations may be made thereto
without departing from
the spirit of the invention or the scope of the appended claims.
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