Abstract:
A system for detecting overlapped flat objects in a sequence of 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 and a camera responsive to the signal for capturing a digital image of the exposed edges of the detected flat object in the feed path. A vision system is coupled to the camera for receiving the digital image. The vision system analyzes 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.

Description:
[0001]    This application claims priority from Canadian Patent Application No. 2,361,969 filed Nov. 14, 2001, and incorporated herein by reference.  
           [0002]    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  
         [0003]    Mechanisms for minimizing 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.  
           [0004]    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.  
           [0005]    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 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.  
           [0006]    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 color, is multicolored or has a fold over. Of course, the detection of false double feeds should be avoided.  
           [0007]    There are a number of disadvantages with current techniques for detecting double feeds. For example, U.S. Pat. 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 U.S. Pat. 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.  
           [0008]    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. Pat. No. 5,534,690 describes a system for counting the number of bank notes in a stack by imaging the 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. Pat. No. 5,221,837.  
           [0009]    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  
         [0010]    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.  
           [0011]    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 double feed rejection signals to the mail sorting apparatus to enable the re-routing of detected double feeds.  
           [0012]    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 analyzes 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.  
           [0013]    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 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.  
           [0014]    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; analyzing 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, 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.  
           [0015]    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 analyzes 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  
       [0016]    Embodiments of the invention may best be understood by referring to the following description and accompanying drawings in which:  
         [0017]    [0017]FIG. 1 is a block diagram illustrating a mail sorting system with an incorporated DFD system in accordance with an embodiment of the invention;  
         [0018]    [0018]FIG. 2 is a block diagram illustrating a double feed detection (“DFD”) system in accordance with an embodiment of the invention;  
         [0019]    [0019]FIG. 3 is a simplified perspective view illustrating a DFD system in accordance with an embodiment of the invention;  
         [0020]    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 P 2 ;  
         [0021]    [0021]FIG. 5 is a partial plan view of the DFD system illustrating the relationship between passing mail pieces and photosensors P 1 , P 2 , and P 3 ;  
         [0022]    [0022]FIG. 6( a ) is a screen capture illustrating an image of mail piece bottom edges captured by a camera;  
         [0023]    FIGS.  6 ( b ) and  6 ( c ) show a schematic diagram of the screen image of FIG. 6( a ) and its corresponding density variation;  
         [0024]    [0024]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;  
         [0025]    [0025]FIG. 8 is a pseudocode listing corresponding to the flow chart of FIG. 7; and  
         [0026]    [0026]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  
       [0027]    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.  
         [0028]    [0028]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 appropriate gate to a destination bin. The arrow  130  indicates the direction of flow of the mail piece along the feed path  132 .  
         [0029]    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.  
         [0030]    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.  
         [0031]    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.  
         [0032]    [0032]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 P 1 , P 2 , P 3  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 P 2   230  and the camera  250 . The 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 coprocessors and memory devices. The memory may include RAM, ROM, databases, or disk devices.  
         [0033]    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 F150-C10E-3 with console F150-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 recognize 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.  
         [0034]    Edge Detection Using a Vision System. Edge detection may be better understood by referring 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 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 .  
         [0035]    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 P 2   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 .  
         [0036]    The imaging region of the camera is spaced a distance L, along the feed path, from photosensor P 2   230  such that when a mail piece  310  is detected by photosensor P 2   230 , a signal is sent by the photosensor P 2   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 P 2   230 .  
         [0037]    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.  
         [0038]    The operation of the edge determination function may be better understood by referring to FIGS.  6 ( a ),  6 ( b ), and  6 ( c ).  
         [0039]    [0039]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 ,  611 , indicating that the mail piece  310  has two bottom edges  330 . Hence, the mail piece  310  may consist of two envelopes.  
         [0040]    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 .  
         [0041]    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 analyzing 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 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 .  
         [0042]    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.  
         [0043]    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 .  
         [0044]    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. As will be described below, the software uses these parameters to determine if a double feed condition exists.  
         [0045]    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.  
         [0046]    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 color 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.  
         [0047]    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 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 ).  
         [0048]    [0048]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 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.  
         [0049]    [0049]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.  
         [0050]    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.  
         [0051]    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”.  
         [0052]    Overlap Detection and Mail Piece Tracking Using Photosensors. Referring to FIGS.  1 - 3 , the DFD system  200  includes three photosensors P 1   220 , P 2   230 , and P 3   240 . In general, the first two photosensors P 1   220 , P 2   230  are used to detect a double feed condition in accordance with a further embodiment of the invention.  
         [0053]    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 P 1   220  and P 2   230 . Referring to FIG. 4( a ), the first two photosensors P 1   220 , P 2   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 , P 2   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 P 2   230  is turned on, a double feed  320  condition exists. If both photosensors P 1   220 , P 2   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. Referring 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 P 1   220  and P 2   230  may both be turned on (i.e. tripped) simultaneously. This would not create a double feed condition  320  as the first photosensor P 1   220  would turn off before the second photosensor P 2   230  turns on. That is, the gap between the two small envelopes  410 ,  420  is recognized by the photosensors P 1   220 , P 2   230 .  
         [0054]    The status of photosensors P 1   220  and P 2   230  is monitored by the controller  270 . The second photosensor P 2   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.  
         [0055]    [0055]FIG. 5 is a partial plan view of the DFD system  200  illustrating the relationship between passing mail pieces  320  and photosensors P 1   220 , P 2   230 , and P 3   240 . The third photosensor P 3   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 P 3   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.  
         [0056]    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 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.  
         [0057]    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.  
         [0058]    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 “50in 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.  
         [0059]    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 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 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.  
         [0060]    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 .  
         [0061]    Controller Operation. In general, the controller  270  monitors the photosensors P 1   220 , P 2   230 , P 3   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.  
         [0062]    Referring to FIGS. 1, 2, and  3 , in operation the controller  270  performs functions including the following:  
         [0063]    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 P 1   220  and P 2   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 P 2   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 P 2   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.  
         [0064]    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  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).  
         [0065]    3. Delays double feed signal output to the sorter  140 . If a double feed is detected by either the photosensors P 1   220 , P 2   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 .  
         [0066]    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 P 3   240 , fault status (as described above), and corresponding bits of the mail piece present and double feed shift registers. When photosensor P 3   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 P 3   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 P 3   240  is on, but no corresponding bits in the shift registers are set, then no double feed rejection signal will be provided to the sorter  100 ,  140  (i.e. the mail piece is not rejected).  
         [0067]    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.  
         [0068]    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 P 2   230  to photosensor P 3   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).  
         [0069]    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 P 1   220 , P 2   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 P 3   240  until the mail piece  310  passes. At step  907 , when the mail piece  310  arrives at photosensor P 3   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.  
         [0070]    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.  
         [0071]    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.  
         [0072]    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.  
         [0073]    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.  
         [0074]    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 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).  
         [0075]    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.