Abstract:
A printing apparatus includes: a printing mechanism forming an image on a sheet transferred in a sheet transfer route; a sheet feeding mechanism feeding sheets to the sheet transfer route; a register provided between the sheet feeding mechanism and the printing mechanism on the sheet transfer route so as to position a sheet fed from the sheet feeding mechanism and adjust an obliqueness of the sheet; an edge part detector provided on the sheet transfer route having a detecting section configured between the register and the printing mechanism so as to detect edge parts of a sheet transferred to the printing mechanism by the register, wherein the edge part detector is configured to serve as a multi-feed detector to detect multi-feeding of sheets.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a printing apparatus to detect multi-feeding of sheets transferred along a sheet transfer route. 
         [0003]    2. Description of the Related Art 
         [0004]    A printing apparatus circulates a plurality of sheets separately by feeding one by one from a stack of sheets in a sheet feed rack of a feeding mechanism. Meanwhile, the plurality of sheets may be overlapped and transferred together when passing through a printing mechanism. This is so-called “multi-feeding of sheets”. Japanese Patent Laid-Open Publication No. 2001-063872 discloses a technology for detecting multi-feeding of sheets by means of a light transmission sensor as a multi-feed detection sensor provided on a sheet transfer route. The light transmission sensor measures the amount of light transmission of a sheet on the sheet transfer route so as to detect the multi-feeding of sheets since the amount of light transmission is dependent on a thickness of a sheet. 
         [0005]      FIG. 1A  shows one example of conventional arrangements of light transmission sensors used for detecting multi-feeding of sheets. In this figure, a sheet from a sheet feeding side is transferred to register rollers  750  to pause for positioning a front edge part of the sheet. Then, the sheet is guided to print heads  712  by the register rollers  750 . 
         [0006]    In the sheet transfer route, a light transmission sensor  760  composed of a light emitter  762  and a light-receiving sensor  764  is provided on the sheet feeding side of the register rollers  750 . While, a light transmission sensor  770  composed of a light emitter  772  and a light-receiving sensor  774  is provided on a sheet exit side of the register rollers  750 . The light transmission sensor  760  serves as a register sensor to detect a sheet entering the register rollers  750 . The light transmission sensor  770  serves as an edge part detection sensor to detect edge parts of the sheet further transferred to the print heads  712 . 
         [0007]    The light transmission sensor  760  provided on the sheet feeding side of the register rollers  750  also serves as a multi-feed detection sensor in order to detect multi-feeding of sheets on the feeding side as quickly as possible. 
       SUMMARY OF THE INVENTION 
       [0008]    The register rollers  750  are used for positioning a front edge part of sheets, and for adjusting oblique sheets. A sheet P enters the register rollers  750  with an excessive feed slightly toward the print head side by sheet feeding rollers not shown in the figure. Then, the sheet P pauses loosely at the register rollers  750  as shown in  FIG. 1B  in order for an adjustment of a sheet obliqueness. The light transmission sensor  760  as a multi-feed detection sensor is provided on the sheet feeding side of the register rollers  750  as mentioned above. Accordingly, the light transmission sensor  760  detects the amount of light transmission of the loose sheet. 
         [0009]    In such a case, the light transmission sensor  760  may detect the amount of light transmission variously per sheet since a loose state differs in each fed sheet. In addition, when there are several feeding mechanisms in a printing apparatus, there are also several feeding routes to transfer a sheet to the register rollers  750  per feeding mechanism. Thus, the light transmission sensor  760  may also detect the amount of light transmission variously per sheet (feeding route). As a result, the conventional multi-feed detecting method as shown in  FIG. 1A  may not be able to stabilize the accuracy of multi-feed detection. 
         [0010]    The present invention has been made to solve the above-mentioned issues. It is an object of the present invention to improve the accuracy of multi-feed detection. 
         [0011]    To achieve the above-mentioned object, according to an aspect of the present invention, a printing apparatus comprises: a printing mechanism forming an image on a sheet transferred in a sheet transfer route; a sheet feeding mechanism feeding sheets to the sheet transfer route; a register provided between the sheet feeding mechanism and the printing mechanism on the sheet transfer route so as to position a sheet fed from the sheet feeding mechanism and adjust an obliqueness of the sheet; an edge part detector provided on the sheet transfer route having a detecting section configured between the register and the printing mechanism so as to detect edge parts of a sheet transferred to the printing mechanism by the register, the edge part detector is configured to serve as a multi-feed detector to detect multi-feeding of sheets. 
         [0012]    According to the present invention, the edge part detector provided so as to detect edge parts of a sheet in a detecting section configured between the register and the printing mechanism is also used as a multi-feed detector. Therefore, it is possible to detect multi-feeding of sheets with more stable behavior in the detecting section compared with sheets transferred from the feeding mechanism to the register. Thus, the accuracy of multi-feed detection can be improved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIGS. 1A and 1B  are views showing one example of conventional arrangements of light transmission sensors used for detecting multi-feeding of sheets. 
           [0014]      FIG. 2  is a block diagram showing one example of configurations of a printing apparatus mainly including feeding mechanisms, a discharge mechanism and a printing mechanism. 
           [0015]      FIG. 3  is a view showing a positional relationship among register rollers, light transmission sensors and print heads. 
           [0016]      FIGS. 4A and 4B  are views showing the received light amount of a sheet passing through light transmission sensors. 
           [0017]      FIG. 5  is a block diagram showing a configuration of an edge part and multi-feeding sheet detection system. 
           [0018]      FIG. 6  is a view showing detection blocks and sampling points for multi-feed detection. 
           [0019]      FIG. 7  is a flow chart showing multi-feed determination processing of first sheet feeding. 
           [0020]      FIG. 8  is a flow chart showing multi-feed determination processing after first sheet feeding. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    We now describe an embodiment of the present invention with reference to the drawings.  FIG. 2  is a block diagram showing one example of configurations of a printing apparatus  10  mainly including feeding mechanisms, a discharge mechanism and a printing mechanism according to the present embodiment. As shown in the figure, the printing apparatus  10  has the sheet feeding mechanisms including a sheet feed rack  320   a , a first feed tray  320   b , a second feed tray  320   c  and a third feed tray  320   d , and the sheet discharge mechanism including a face-down discharge tray  330   a  In addition, the printing apparatus  10  may have optional discharge trays. The printing apparatus  10  may concurrently have some functions such as punching and stapling afterward, and may have a face-up discharge tray. 
         [0022]    The printing apparatus  10  has a sheet transfer route TR that includes a system of feeding routes FR for feeding a sheet, a discharging route DR for discharging the sheet, a normal transfer route PR for transferring the sheet received from the system of feeding routes FR to the discharging route DR, and an inverting route (switchback) SR branched from the normal transfer route PR, for inverting the sheet between front and back sides received from the normal transfer route PR to re-feed to the normal transfer route PR. The inverting route SR cooperates with the normal transfer route PR to constitute a looped sheet circulating transfer route CR. 
         [0023]    The printing apparatus  10  executes printing on a sheet fed from any feeding mechanism at print heads  312 , and discharges the sheet to the face-down discharge tray  330   a . The face-down discharge tray  330   a  is provided at an upper part of a side surface of the printing apparatus  10 , to which the printed sheet is discharged with a printed side down. 
         [0024]    The printing apparatus  10  has the printing mechanism including a plurality of print heads  312 , each of which is provided with multiple nozzles formed perpendicular to a sheet transfer direction. Each of the print heads  312  propels droplets of black or color ink for printing in color bylines. The printing apparatus  10  has a controller  110  composed of a substrate with a mounted CPU, memories, etc., an operation panel  150  for interfacing user operations, and other components not illustrated in the figure. 
         [0025]    A sheet is fed one by one from any feeding mechanism and transferred along the system of feeding routes FR in a casing by an associated drive mechanism such as rollers, and guided to a register R. The register R is located near a junction of the system of feeding routes FR and the normal transfer route PR. The register R is composed of a pair of register rollers for positioning a front edge of the fed sheet. The fed sheet pauses at the register R for an adjustment of a sheet obliqueness. The fed sheet enters the register R with a slightly excessive feed and pauses loosely by means of sheet feeding rollers of the feeding mechanism from which the sheet is fed in order for the adjustment of the sheet obliqueness. The adjusted sheet is transferred to the normal transfer route PR provided with the printing mechanism at a controlled timing. 
         [0026]    In the sheet transfer route TR, a first light transmission sensor  160  is provided on a sheet feeding side of the register R, and a second light transmission sensor  170  is provided on a sheet exit side of the register R. The first light transmission sensor  160  serves as a register sensor to detect a sheet being fed to the register R. The second light transmission sensor  170  serves as a sheet edge part detection sensor, and also, as a multi-feed detection sensor. 
         [0027]      FIG. 3  is a view showing a positional relationship among register rollers  250  composing the register R, the first light transmission sensor  160 , the second light transmission sensor  170  and the print heads  312 . As shown in the figure, the first light transmission sensor  160  as a register sensor includes a light emitter  162  and a light-receiving sensor  164 . Also, the second light transmission sensor  170  as a sheet edge part and multi-feed detection sensor includes a light emitter  172  and a light-receiving sensor  174 . 
         [0028]    The second light transmission sensor  170  detects edge parts of a sheet P transferred to the print heads  312 , and detects multi-feeding of sheets. According to the present embodiment, the sheet edge part detection sensor, which is provided on the sheet exit side of the register rollers  250 , is also used as a multi-feed detection sensor. The sheet P pauses at the register R, and is transferred to the print heads  312  by the register rollers  250 . Therefore, the sheet P passing through the second light transmission sensor  170  is not influenced in the multi-feed detection by sheet looseness at the register R and a distinction of feeding routes. Thus, the second light transmission sensor  170  can examine each sheet evenly with less variability in the multi-feed detection. As a result, the accuracy of multi-feed detection can be enhanced. 
         [0029]      FIG. 4A  is a view showing one example of the received light amount of the sheet P detected at the light-receiving sensor  174  of the second light transmission sensor  170  provided on the sheet exit side of the register rollers  250 .  FIG. 4B  is a view showing one example of the received light amount of the sheet P detected at the light-receiving sensor  164  of the second light transmission sensor  160  provided on the sheet feeding side of the register rollers  250 . 
         [0030]    As shown in the figures, the received light amount of the sheet P detected at the light-receiving sensor  174  of the second light transmission sensor  170  is kept at an approximately constant value throughout a part from a front edge to a back edge of the sheet. While, the received light amount of the sheet P detected at the light-receiving sensor  164  of the second light transmission sensor  160  varies precariously throughout the sheet. In particular, the received light amount of the sheet P varies in a stepwise manner when the front edge part of the sheet P passes through the sensor. Thus, the accuracy of multi-feed detection can be improved due to the light-receiving sensor  174  of the second light transmission sensor  170  provided on the sheet exit side of the register rollers  250 . 
         [0031]    As shown in  FIG. 2  again, a sheet is transferred at a controlled speed depending on printing conditions by a transfer belt  352  that is looped and provided facing an ink-droplet-propelling side of the print heads  312 . While, an image is formed on the sheet by ink droplets propelled from the print heads  312  by lines. The transfer belt  352  has a transfer belt move-down mechanism  354  capable of moving the transfer belt  352  downward. The transfer belt move-down mechanism  354  moves the transfer belt  352  downward so that sheets around the transfer belt  352  are easily removed when a transfer jam and multi-feeding of sheets is detected. 
         [0032]    For one-side printing, a sheet printed on a front side is transferred in the casing by drive mechanisms such as rollers. Then, the sheet is guided to the face-down discharge tray  330   a  to be discharged by a route selecting mechanism  386 , and stacked on the face-down discharge tray  330   a  with a printed side down. 
         [0033]    The face-down discharge tray  330   a  is formed in a shape of a tray protruding from the casing with a certain thickness. The face-down discharge tray  330   a  is inclined to a lateral wall of the casing. Thus, the discharged sheet is slid down along an inclination of the face-down discharge tray  330   a  so as to tidily pile up on the face-down discharge tray  330   a  in due course. 
         [0034]    The face-down discharge tray  330   a  has a prearranged sheet pile-up capacity. Thus, the face-down discharge tray  330   a  is provided with a tray full sensor  332   a  to detect whether sheets being piled up on the face-down discharge tray  330   a  reaches a predetermined level near the maximum pile-up capacity. Concurrently, the face-down discharge tray  330   a  is provided with a tray empty sensor  334   a  to detect whether the face-down discharge tray  330   a  is empty. 
         [0035]    For both-side printing, assuming “a front side” as the side to be printed first and “a back side” as the side to be printed next, a sheet printed on the front side is to be transferred in the casing without being guided to the face-down discharge tray  330   a  by the route selecting mechanism  386 . The sheet is transferred to the inversion route SR to be switched back for inversion between the front side and the back side. The sheet is re-fed to the register R by the drive mechanisms such as rollers. After a pause at the register R, the sheet is transferred to the printing mechanism at a controlled timing. 
         [0036]    Then, the sheet is to have an image formed on the back side in a similar manner to the front side. The sheet with images on both sides is discharged to and piled on the face-down discharge tray  330   a.    
         [0037]    In the printing apparatus  10 , an internal space of the face-down discharge tray  330   a  is used to implement a switchback for both-side printing. The space in the face-down discharge tray  330   a  is enclosed to keep sheets from being taken from outside in the course of the switchback. This prevents the sheets from being pulled out by a mistake of user in the course of the switchback. The face-down discharge tray  330   a , which is an inherent member to the printing apparatus  10 , affords to eliminate provision of an extra space for the switchback in the printing apparatus  10 . This permits the casing to be kept from being enlarged in size. The inverting route SR that is separated from the face-down discharge tray  330   a  allows for parallel operations between a sheet to be switched back and another sheet to be discharged. 
         [0038]      FIG. 5  is a block diagram showing a configuration of an edge part and multi-feeding sheet detection system, which serves as a detector to detect edge parts and multi-feeding of sheets. As shown in the figure, the edge part and multi-feed sheet detection system is composed of the second light transmission sensor  170 , an edge part detector  180 , a multi-feed determiner  190  and an encoder  196 . As described above, the second light transmission sensor  170  is provided on the sheet exit side of the register rollers  250 . The encoder  196  outputs a one-shot pulse signal when the register rollers  250  are rotated a predetermined length of a sheet sent thereby. 
         [0039]    The second light transmission sensor  170  includes the light emitter  172 , and the light-receiving sensor  174  provided so as to receive light emitted from the light emitter  172  and output an electrical signal according to the amount of the received light. The light emitter  172  can be composed of a light-emitting diode, a laser diode, a lamp, and the like. The light-receiving sensor  174  can be composed of, for instance, a photo diode. 
         [0040]    When there is no sheet between the light emitter  172  and the light-receiving sensor  174 , the light-receiving sensor  174  directly receives light emitted from the light emitter  172 . While, the light-receiving sensor  174  indirectly receives light emitted from the light emitter  172  through a sheet (or sheets) when there is any sheet therebetween. Therefore, the received light amount of the light-receiving sensor  174  is dependent on the presence or absence of sheet, the number of sheets, a sheet thickness, etc. Thus, the passage of a front edge part, the passage of a back edge part and the multi-feeding of sheets are detectable based on the electrical signal transmitted from the light-receiving sensor  174 . Note that other types of sensors, such as a light reflection sensor, may be appropriately used instead of a light transmission sensor. 
         [0041]    The edge part detector  180  includes a comparator  182 . The comparator  182  compares an output signal from the light-receiving sensor  174  with a predetermined voltage value to determine the presence or absence of sheet. Then, the comparator  182  outputs the comparative result as an edge part detection signal. Thus, the edge part detector  180  can detect front and back edge parts of a sheet passing through the second light transmission sensor  170  by monitoring a rising edge and a trailing edge of the edge part detection signal from the comparator  182 . 
         [0042]    The multi-feed detector  190  includes an analog amplifier circuit  191 , an A/D converter  192 , a sampling circuit  193 , a memory  194  and a judging circuit  195 . Those functional components are realized as hardware by means of the controller  110  composed of a CPU, RAM, ROM, integrated circuit, and the like. 
         [0043]    The analog amplifier circuit  191  amplifies an output signal based on the received light amount of the light-receiving sensor  174  so as to transmit to the A/D converter  192 . The A/D converter  192  converts the signal from the analog amplifier circuit  191  into a digital signal based on the analog value so as to transmit to the sampling circuit  193 . The sampling circuit  193  samples the digital signal from the A/D converter  192  at a controlled timing based on a one-shot pulse signal from the encoder  196  so as to store in the memory  194 . 
         [0044]    Next, we explain a method of multi-feed detection of the present embodiment. According to the present invention as shown in  FIG. 6 , a sheet P is segmented into “N” detection blocks along a sheet transfer direction. Then, each detection block is assigned with “n” sampling points. The number “N” of the detection blocks is determined depending on the length of the sheet P along the sheet transfer direction. Preferably, the sheet P is segmented so as to leave a certain margin at a mostfront edge part on the sheet P in view of a response lag of the light-receiving sensor  174 . 
         [0045]    Then, an output signal from the A/D converter  192  per sampling point in one detection block is sampled to obtain sampling data. After every sampling data of the sampling points in the detection block is obtained, an average value of the sampling data in the detection block is calculated. 
         [0046]    By comparing the calculated average value with a reference value preliminarily assigned in the detection block, the possibility that multi-feeding happens in the detection block is predicted. When there are two sequential detection blocks predicted to have a possibility from multi-feeding, a multi-feed detection signal is transmitted due to the determination of multi-feeding of sheets. The judging circuit  195  serves as a component to execute calculation processing of the average value of sampling data, prediction processing of a possibility for multi-feeding, and determination processing of multi-feeding. 
         [0047]    Next, we explain a reference value in prediction processing of a possibility for multi-feeding. After first sheet feeding, the average value of sampling data per detection block calculated in first sheet feeding is used as a reference value of each corresponding detection block of a second sheet and the following sheets. In other words, multi-feeding of sheets after first sheet feeding is determined based on sampling data of a first sheet P. For the first sheet P, it is employed a predetermined common initial value as a reference value for each detection block. Such an initial value as a reference value varies based on types of sheets. Thus, for instance, several initial values may be arranged in advance according to the type of the sheet P. 
         [0048]    Therefore, the memory  194  has a section to store “n” sampling data to be calculated per detection block, a section to store “N” reference values of respective detection blocks, and a section to preliminarily store an initial reference value. 
         [0049]    Then, we describe a flow of multi-feed determination processing in the multi-feed determiner  190 .  FIG. 7  is a flow chart showing multi-feed determination processing in first sheet feeding. 
         [0050]    When the second light transmission sensor  170  detects a front edge part of a first sheet after feeding (S 101 : Yes), a sampling point in a first detection block is determined based on a one-shot pulse signal transmitted from the encoder  196  (S 102 ). Then, an output signal from the A/D converter  192  on the sampling point is sampled so as to store in the memory  194  as sampling data (S 103 ). After “n” sampling data are stored in the memory  194  by repeating such a sampling per sampling point in the detection block (S 104 : Yes), the average value of the sampling data in the detection block is calculated (S 105 ). 
         [0051]    When the calculated average value in the detection block is less than a predetermined initial reference value (i.e. the received light amount is small) (S 106 : Yes), a possibility from multi-feeding is predicted. In such a case, the possibility is further judged whether to meet a multi-feed determining condition (S 109 ). The multi-feed determining condition is defined as a condition that when there are two sequential detection blocks predicted to have a possibility for multi-feeding, it is determined that multi-feeding of sheets occurs. Note that other multi-feed determining conditions may be applicable. For instance, multi-feeding of sheets may be determined when more than half of detection blocks are determined as ones having a possibility for multi-feeding. 
         [0052]    When the multi-feed determining condition is met as a result of the judgment (S 109 : Yes), a multi-feed detection signal is transmitted (S 110 ). According to the transmitted signal, the printing apparatus  10  executes multi-feed control processing (S 111 ). Multi-feed control processing means for instance that sheet feeding is stopped so as to move the transfer belt  253  downward by the transfer belt move-down mechanism  354 . Thus, sheets are placed on the transfer belt moved downward so that a user can easily remove the sheets from the printing apparatus  10 . 
         [0053]    When the calculated average value in the detection block is more than the predetermined initial reference value (i.e. the received light amount is large) (S 106 : No), or when the multi-feed determining condition is not met (S 109 : No), the calculated average value is stored in the memory  194  as a reference value of the detection block (S 107 ). 
         [0054]    Then, the multi-feed determiner  190  executes multi-feed determination processing repeatedly (starting from the step S 102 ) until a back edge part of the sheet is detected (S 108 ). When the second light transmission sensor  170  detects the back edge part of the sheet (S 108 : Yes), multi-feed determination processing in first sheet feeding is completed. 
         [0055]    Next, we explain multi-feed determination processing of a following sheet after first sheet feeding with reference to a flow chart in  FIG. 8 . 
         [0056]    When the second light transmission sensor  170  detects a front edge part of a following sheet after first sheet feeding (S 201 : Yes), a sampling point in a first detection block is determined based on a one-shot pulse signal transmitted from the encoder  196  (S 202 ). Then, an output signal from the A/D converter  192  on the sampling point is sampled so as to store in the memory  194  as sampling data (S 203 ). After “n” sampling data are stored in the memory  194  by repeating such a sampling per sampling point in the detection block (S 204 : Yes), the average value of the sampling data in the detection block is calculated (S 205 ). 
         [0057]    When the calculated average value in the detection block is less than the reference value of the corresponding detection block determined in first sheet feeding (i.e. the received light amount is small) (S 106 : Yes), a possibility for multi-feeding is predicted. In such a case, the possibility is further judged whether to meet a multi-feed determining condition (S 209 ). Note that the similar multi-feed determining condition to the first sheet can be employed in this feeding. 
         [0058]    When the multi-feed determining condition is met as a result of the judgment (S 209 : Yes), a multi-feed detection signal is transmitted (S 210 ). According to the transmitted signal, the printing apparatus  10  executes multi-feed control processing (S 211 ). Note that multi-feed control processing similar to the first sheet can be employed in this feeding. 
         [0059]    When the calculated average value in the detection block is more than the reference value (i.e. the received light amount is large) (S 206 : No), or when the multi-feed determining condition is not met (S 209 : No), the multi-feed determiner  190  executes multi-feed determination processing repeatedly (starting from the step S 202 ) until a back edge part of the sheet is detected (S 207 ). When the second light transmission sensor  170  detects the back edge part of the sheet (S 207 : Yes), the multi-feed determiner  190  determines whether a sheet is still following (S 208 ). When there is another following sheet (S 208 : Yes), the multi-feed determiner  190  repeats multi-feed determination processing starting from front edge part detection processing (S 201 ). While, when there is no following sheet (S 208 : No), multi-feed determination processing after first sheet feeding is completed. 
         [0060]    According to the present embodiment as described above, the printing apparatus  10  employs the second light transmission sensor  170  as a multi-feed detection sensor provided on the sheet exit side of the register rollers  250  in order to detect edge parts of a sheet. Thus, it is possible to improve the accuracy of multi-feed detection. 
         [0061]    While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims. 
         [0062]    This application is based upon the Japanese Patent Application No. 2008-226285, filed on Sep. 3, 2008, the entire content of which is incorporated by reference herein.