Patent Publication Number: US-8123220-B2

Title: Sheet-medium-conveying device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2009-018515 filed Jan. 29, 2009. The entire content the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates to a sheet-medium conveying device and an image-reading device. 
     BACKGROUND 
     A medium discrimination device for an image-forming apparatus is known in the art. The image-forming apparatus includes an accommodating unit for accommodating a plurality of overlaid sheets and conveys these sheets one at a time from the accommodating unit onto a conveying path. The medium discrimination device comprises transmitted light quantity measuring means disposed along the conveying path that measures the quantity of light transmitted in the thickness direction of the medium to detect overlapped feeds. The medium discrimination device detects the type of medium when the first sheet is conveyed and performs overlapped-feed detection on each sheet thereafter. 
     SUMMARY 
     However, when a plurality of types of media is combined in the accommodating unit of the image-forming apparatus described above, the conventional medium discrimination device may incorrectly detect overlapped feeds because the detected quantity of transmitted light may differ among the different types of media. 
     In view of the foregoing, it is an object of the invention to provide a sheet-medium-conveying device and an image-reading device capable of accurately detecting overlapped feeds of a sheet-like medium fed from a tray, even when the tray holds media of a plurality of types. 
     In order to attain the above and other objects, the invention provides a sheet-medium-conveying device. The sheet-medium-conveying device includes a tray, a conveying unit, a detecting unit, a determining unit, and a reference value changing unit. A plurality of sheets are settable on the tray. The conveying unit conveys the sheets on the tray. The measuring unit measures a quantity with respect to the sheet conveyed by the conveying unit. The detecting unit detects an overlapped feed in which at least two sheets are conveyed in a partially overlapped condition. The overlapped feed is detected by comparing the quantity measured by the measuring unit with a reference value. The determining unit is configured to determine whether a type of sheet conveyed by the conveying unit is changed. The reference value changing unit is configured to change the reference value to a value corresponding to the changed type of sheet as determined by the determining unit. 
     According to another aspect, the invention provides an image-reading device. The image-reading unit includes a tray, a conveying unit, a reading unit, a detecting unit, a determining unit, and a reference value changing unit. The plurality of sheets are settable on the tray. The conveying unit conveys the sheets on the tray. The reading unit reads an image of the sheet conveyed by the conveying unit and generates image data. The measuring unit measures a quantity with respect to the sheet conveyed by the conveying unit. The detecting unit detects an overlapped feed in which at least two sheets are conveyed in a partially overlapped condition. The overlapped feed is detected by comparing the quantity measured by the measuring unit with a reference value. The determining unit is configured to determine whether a type of sheet conveyed by the conveying unit is changed. The reference value changing unit is configured to change the reference value to a value corresponding to the changed type of sheet as determined by the determining unit. 
     According to still another aspect, the invention provides a sheet-medium-conveying device. The sheet-medium-conveying device includes a tray, a conveying unit, a counter, a length measuring unit, and a controller. The conveying unit is configured to feed a sheet from the tray. The counter is configured to count a number of sheets conveyed by the conveying unit. The length measuring unit is configured to measure a length of the sheet conveyed by the conveying unit. The memory is configured to store a reference length and a reference number. The controller is configured to control the conveying unit, the counter, the length measuring unit, and the memory. If the number of sheets counted by the counter is zero, the controller controls the length measuring unit to measures a length of a sheet conveyed by the conveying unit and stores the measured length of the sheet as a reference length in the memory. If the number of sheets counted by the counter is greater than zero, the controller controls the length measuring unit to measures the length of the sheet conveyed by the conveying unit and compares the measured length with the reference length. If the measured length is greater than the reference length, the controller determines and notifies that an overlapped feed in which at least two sheets are conveyed in a partially overlapped condition occurs whereas if the measured length is smaller than or equal to the reference length, the controller determines that the overlapped feed does not occur and the controller does not notify that the overlapped feed occurs. The controller resets the counter to zero if the number of sheets that the counter counts reaches the reference number. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a side cross-sectional view conceptually illustrating a multifunction peripheral according to a first embodiment of the invention; 
         FIG. 2  is an explanatory diagram illustrating an overlapped feed; 
         FIG. 3  is an explanatory diagram conceptually illustrating different types of sheets being conveyed along a conveying path; 
         FIG. 4  is an explanatory diagram illustrating a transmission for setting the number of sheets when transmitting a facsimile of an original document with an attached cover letter; 
         FIG. 5  is an explanatory diagram illustrating a user interface for setting the number of sheets in a cover letter group; 
         FIG. 6  is an explanatory diagram conceptually illustrating a table including the number of sheets for each group set through the user interface; 
         FIG. 7  is an explanatory diagram for illustrating how an original is copied using instruction sheets; 
         FIG. 8  is an explanatory diagram with enlarged views of the instruction sheets shown in  FIG. 7 . 
         FIG. 9  is an explanatory diagram illustrating the user interface for copying an original using instruction sheets; 
         FIG. 10  is an explanatory diagram conceptually illustrating a table including the number of sheets for each group set according to the user interface; 
         FIG. 11  conceptually illustrates a table for a normal mode; 
         FIG. 12  is a flowchart illustrating steps in an overall scanning process; 
         FIG. 13  is a flowchart illustrating steps in a process to determine a necessity for overlapped-feed detection and to set groups; 
         FIG. 14  is a flowchart illustrating steps in a process for scanning while detecting overlapped feeds for each group; and 
         FIG. 15  is a flowchart illustrating steps in a process for scanning while detecting overlapped feeds in each group according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     A first embodiment of the invention will be described with reference to  FIGS. 1 through 14 . 
     (1) Structure of a Sheet-Medium-Conveying Device and an Image-Reading Device 
       FIG. 1  is a side cross-sectional view conceptually illustrating a multifunction peripheral (MFP)  1 . The MFP  1  has a scanner function, printer function, and copier function, facsimile function. 
     The MFP  1  has a main casing  11  (only partially shown in  FIG. 1 ) that is substantially box-shaped. A first platen glass  12  and a second platen glass  13  are juxtaposed on top of the main casing  11 . A document cover  15  serving in the embodiment is coupled to the main casing  11  so as to be capable of rotating between a closed position for covering the first platen glass  12  and an open position for exposing the first platen glass  12 . The document cover  15  includes an automatic document feeder (ADF)  16 , a document tray  17  for setting paper or another sheet-like medium, and a discharge tray  18 . 
     The ADF  16  has a conveying path  19  defined therein; various types of rollers, including a pair of conveying rollers  20  and a pair of discharge rollers  21 ; and a stepper motor (not shown) for driving the rollers. The rollers convey the sheet-like medium set in the document tray  17  one sheet at a time so that each sheet passes over the second platen glass  13 . 
     Although not illustrated in  FIG. 1 , the ADF  16  is capable of conveying a sheet so that the top surface of the sheet is read and can automatically turn over the sheet after the sheet has passed over the second platen glass  13  and convey the sheet back over the second platen glass  13  so that the bottom surface of the sheet is read. Thus, the MFP  1  can automatically scan both surfaces of the sheet. 
     A reading unit  25  is disposed beneath the upper portion of the main casing  11 . The reading unit  25  employs a contact image sensor (CIS) system for scanning an original document and includes a linear image sensor  28  having a plurality of light-receiving elements arranged linearly in a direction (a width direction of the sheet) orthogonal to the surface of the drawing in  FIG. 1 , a light source  29  configured of light-emitting diodes (LEDs) in the three RGB colors, a rod lens array  30  for forming an image on the light-receiving elements of the linear image sensor  28  from light reflected off the original document, a carriage  31  in which the linear image sensor  28 , light source  29 , and rod lens array  30  are mounted, and a conveying mechanism (not shown) for conveying the carriage  31 . As a variation to this embodiment, the reading unit  25  may employ a minification optical system to scan the original documents. 
     When scanning an original document placed on the first platen glass  12 , the reading unit  25  reads the image while the carriage  31  is conveyed at a constant speed along a sub-scanning direction (indicated by A in  FIG. 1 ) parallel to the surface of the first platen glass  12 . When scanning an original document conveyed by the ADF  16 , the reading unit  25  reads data while the carriage  31  is halted directly beneath the second platen glass  13 . 
     The MFP  1  also includes an image sensor control unit  35 , an image-processing unit  36 , a CPU  37 , a ROM  38 , a RAM  39 , a facsimile transceiver  40 , a printing unit  41 , an operating unit  42 , and an interface unit  43 . 
     The image sensor control unit  35  configured of an ASIC (application-specific integrated circuit) is connected to the ADF  16  and the reading unit  25  via a flexible flat cable  34 . The image sensor control unit  35  executes processes to control the ADF  16  and the reading unit  25 , adjust the gain in image data outputted from the linear image sensor  28 , and convert the image data from analog to digital. 
     The image-processing unit  36  is also configured of an ASIC for receiving image data outputted from the image sensor control unit  35  and performing various processes on this data, such as gamma correction, shading compensation, noise filtering, color balance adjustments, enlargements/reductions, and color space conversion. 
     The CPU  37  controls each component of the MFP  1  by executing various programs stored in the ROM  38 . 
     The ROM  38  stores the various programs executed by the CPU  37 , as described above. 
     The RAM  39  serves as a primary storage unit employed by the CPU  37  when the CPU  37  executes various processes. 
     The facsimile transceiver  40  is connected to a telephone exchange via a public telephone network and transmits and receives image data to and from the telephone exchange according to a prescribed facsimile communication protocol. 
     The printing unit  41  employs a laser method or a inkjet method to form images on sheets of recording media based on image data. 
     The operating unit  42  includes a plurality of buttons and a liquid crystal display (LCD). A user can input data and instructions by operating the buttons while referencing the screen display on the LCD, for example. 
     The interface unit  43  is configured of a USB interface and a network interface. 
     The ADF  16  also includes a photoelectric sensor  44  disposed along the conveying path  19 . The photoelectric sensor  44  includes a light-emitting element  44 A and a light-receiving element  44 B opposing each other across the conveying path  19 . The light-emitting element  44 A is configured of an LED or the like. The light-receiving element  44 B receives light emitted by the light-emitting element  44 A and outputs an electric signal corresponding to the intensity of the received light. 
     The CPU  37  measures the length of a sheet based on electric signals outputted from the photoelectric sensor  44 . Specifically, when a sheet of the recording medium conveyed along the conveying path  19  reaches the photoelectric sensor  44 , the intensity of light entering the light-receiving element  44 B drops because the sheet blocks the light emitted from the light-emitting element  44 A. Consequently, the level of the electric signal outputted from the photoelectric sensor  44  also drops. The CPU  37  calculates the distance that the sheet moves from the moment at which the electric signal level changes from high (bright) to low (dark) until the moment at which the level changes from low to high based on the number of times the stepper motor of the ADF  16  rotates during this time, and sets the length of the sheet to this distance. 
     However, the invention is not limited to this method of measuring the length of a sheet. For example, the document cover  15  may include a movable member disposed in the sheet-conveying path that is dislodged from (moved out of) the path while contacted by a sheet. In this case, the CPU  37  finds the length of the sheet by calculating the distance in which the sheet moves while the movable member is dislodged. 
     The CPU  37  also counts the number of sheets conveyed by the ADF  16  based on an electric signal outputted from the photoelectric sensor  44 . Here, the electric signal outputted from the photoelectric sensor  44  changes from high to low each time a new sheet is conveyed. The CPU  37  counts the number of conveyed sheets by counting the number of times that this electric signal changes in level from high to low. 
     However, the invention is not limited to this method of counting sheets. For example, the CPU  37  may count the number of conveyed sheets by counting the number of times that the movable member described above is dislodged. 
     (2) Measured Quantities 
     The MFP  1  measures a specific quantity that differs during a normal feed in which a single sheet of recording medium is conveyed by the ADF  16 , and an overlapped feed (double feed) in which two or more sheets are conveyed simultaneously, and detects a overlapped feed by comparing the measured value with a reference value. The measured quantity in this example is the length of the sheet in the conveying direction (hereinafter simply referred to as “length”). In the embodiment, the length in the conveying direction differs according to the type of sheet. 
       FIG. 2  is an explanatory diagram conceptually illustrating a case in which two sheets  51  and  52  of the same type overlap (referred to as an “overlapped feed”). If two sheets overlap when the MFP  1  is measuring the length of sheets, the MFP  1  will measure the length from the leading edge of the first sheet  51  in the conveying direction to the trailing edge of the subsequent sheet  52 . Hence, the length measured by the MFP  1  when sheets overlap is longer than the length measured when sheets do not overlap. 
     Here, the MFP  1  assumes that the first conveyed sheet does not overlap another sheet, measures the length of the first conveyed sheet, and sets the reference value to this length. Thereafter, if the length of subsequent sheets greatly deviates from this reference value, the MFP  1  determines that an overlapped feed has occurred. However, for the first conveyed sheet, the MFP  1  only measures the length of the sheet and does not perform overlapped-feed detection. 
     (3) Overlapped-Feed Detection when a Plurality of Types of Sheets are Mixed in the Tray 
       FIG. 3  is an explanatory diagram conceptually illustrating different types of sheets being conveyed along the conveying path  19  of the ADF  16 . In this example, sheets  53  and  54  are of a first type, while sheets  55  and  56  are of a second type different from the first type. As shown in  FIG. 3 , when a plurality of types of sheets are mixed in the document tray  17 , the type of sheet being conveyed on the conveying path changes. 
     In the conceivable case where the reference value for detecting an overlapped feed is not adjusted to suit the type of sheet when the length of the sheet differs according to the type, the MFP in this case will likely detect overlapped feeds by mistake. That is, in this case, since the MFP sets the reference value to the length of the initially conveyed sheet, which is the sheet  53  of the first type in this example, the MFP will correctly detect whether an overlapped feed has occurred when the subsequent sheet  54  of the same type is conveyed. However, the measured length for the sheets  55  and  56  of the subsequently conveyed second type will deviate greatly from the reference value, regardless of whether an overlapped feed occurs. 
     Therefore, the MFP  1  according to the embodiment determines whether the type of sheet being conveyed has changed. When the MFP  1  determines that the type has changed, the MFP  1  sets the reference value to the length of the first sheet conveyed after the sheet type has changed (the sheet  55  in the example of  FIG. 3 ). In other words, the MFP  1  changes the reference value to correspond to the new type of sheet after detecting that the type has changed. Accordingly, the MFP  1  can compare the length of the next sheet  56  to the reference value set according to the second type of sheet. If an overlapped feed has not occurred, the discrepancy between the length of the sheet  56  and the reference value will be very small and the MFP  1  will not determine that an overlapped feed has occurred. Thus, this method reduces the likelihood of incorrect overlapped-feed detections. Here, the MFP  1  only measures the length of the sheet  55  and does not perform overlapped-feed detection since the sheet  55  is the first sheet conveyed after the type has changed. 
     In the embodiment, a change in the sheet type will be used to denote the beginning of sheet conveyance, as well as the timing in which the type of sheet to be conveyed next differs from the type of sheet just conveyed. 
     (4) Detecting a Change in Sheet Type 
     In the embodiment, each set of sheets of the same type that are grouped together in the document tray  17  is considered a single group, and the user specifies the number of sheets for each group except the group that is conveyed last. When the ADF  16  is conveying sheets, the MFP  1  increments a count value for conveyed sheets after each sheet is conveyed. When the count for conveyed sheets reaches the number specified for the group, the MFP  1  determines that a type change has occurred. 
     Next, the method of setting the number of sheets in each group will be described for examples in which a plurality of types of sheets is loaded in the document tray  17 . In the first example, the user wishes to transmit a facsimile that includes an original document and an attached cover letter, while in the second example, the user wishes to copy an original document using instruction sheets. 
     (4-1) Transmitting a Facsimile Including an Original Document and an Attached Cover Letter 
       FIG. 4  is an explanatory diagram for setting the number of sheets when transmitting a facsimile of an original document with an attached cover letter. In this example, the cover letter and the original document are types of sheet-like media. When transmitting a facsimile, ordinarily a cover letter  57  is added to the beginning of the original document (a three-page original  58 - 60  in this example). The type of sheet used for the cover letter  57  differs from the type of sheet used for the original  58 - 60 . Therefore, when faxing an original document with an attached cover letter, the MFP  1  prompts the user to specify the number of sheets in each group, where the cover letter  57  is the initially conveyed group (hereinafter referred to as the “cover letter group”) and the original  58 - 60  is the subsequently conveyed group (hereinafter referred to as the “original document group”). 
     In the embodiment, the MFP  1  is provided with a plurality of reading modes defining a preset number of groups and a preset number of sheets in each group. One of these reading modes is the cover letter mode. When faxing an original document with a cover letter, the user can select, by using the operation unit, the cover letter mode to facilitate setting the number of sheets for each group. 
     Specifically, values for the cover letter mode have been preset in the ROM  38 , including the value “2” for the number of groups, the value “1” for the number of sheets in the cover letter group, and a value meaning “indefinite” as the number of sheets in the original document group. When conveying the original document group, the MFP  1  need not detect a group change since the original document group is the last group to be conveyed. Accordingly, it is not necessary to set a number of sheets for the original document group. However, the MFP  1  according to the embodiment uses a prescribed value meaning “indefinite” to identify this group as the last group. Alternatively, the MFP  1  may be configured not to preset a value for the number of sheets in the original document group. 
       FIG. 5  is an explanatory diagram illustrating the user interface for setting the number of sheets in the cover letter group. By operating the operating unit  42  to select the facsimile function, the user displays an initial screen  61  shown in  FIG. 5  on the LCD of the operating unit  42  for inputting settings for a facsimile transmission. 
     If the user presses a Select Telephone Number button (not shown) in the operating unit  42  while the initial screen  61  is displayed, a telephone number selection screen  62  is displayed as shown in  FIG. 5 . With the MFP  1  of the embodiment, the user can prerecord names in association with fax numbers (telephone numbers). Thus, when the telephone number selection screen  62  is displayed, the user can scroll through the prerecorded names recorded in the telephone number selection screen  62  by operating Up and Down buttons (not shown) on the operating unit  42  and can select a desired destination for the facsimile transmission by aligning the cursor with the desired contact and pressing an OK button (not shown) in the operating unit  42 . 
     The character string “(COVER)” appended to the end of a contact name indicates a contact for which the user attaches a cover letter of a different type of sheet from the original document when transmitting a fax. Although the type of sheet used for the cover letter does not necessarily differ from the type of sheet used for the original document, for specific contacts, the size of the original document may be preset at “A4” (a document size regulated by a JIS standard), and the size of the cover letter at “A5”, for example. The character string “(COVER)” identifies such specific contacts. 
     If the user selects a contact with the character string “(COVER)” appended after the name in the telephone number selection screen  62  and subsequently presses the OK button, the MFP  1  shifts to the cover letter mode and displays a cover letter page number input screen  63  for the cover letter mode. The number of sheets in the cover letter group is initially displayed in the cover letter page number input screen  63  as “1” (“01” in  FIG. 5 ). 
     However, the user can modify this number to a desired number of sheets by operating the numerical buttons “0-9” (not shown) in the operating unit  42 . When the user subsequently presses a Start button (not shown) in the operating unit  42 , the MFP  1  sets the number of sheets for the cover letter group based on the number displayed in the cover letter page number input screen  63  at the time, and begins the scanning operation. 
       FIG. 6  is an explanatory diagram conceptually illustrating a table  71  including the number of sheets for each group set through the user interface described above. When the user presses the Start button (not shown) while the cover letter page number input screen  63  is displayed, the MFP  1  generates the table  71  associating each group number with the number of sheets in the corresponding group (a cover letter mode table) and stores the table  71  in the RAM  39 , thereby setting the number of sheets for each group. 
     The number of sheets for Group 1 in the table  71  is the number of sheets in the initially conveyed group, that is, the number of sheets in the cover letter group in this example. This number is set to the number displayed in the cover letter page number input screen  63  when the Start button is pressed. The number of sheets in Group 2 in the table  71  is the number of sheets in the group conveyed after the first group, that is, the number of sheets in the original document group in this example. This number is set to “indefinite.” 
     While the user modifies the number of sheets in the cover letter group by operating the numerical buttons “0-9” in the above example, this number of sheets may be adjusted according to the contact. For example, if the character string “(COVER)” has been appended to both Contact A and Contact B, the user may preregister the number “2” for the number of sheets in the cover letter for Contact A and “3” for the number of sheets for Contact B. Thereafter, when the user selects Contact A (COVER) in the telephone number selection screen  62 , the initial value “2” is displayed as the number of sheets in the cover letter group in the cover letter page number input screen  63  instead of the initial value of “1” in the above example. When the user selects Contact B (COVER), the value “3” is displayed as the initial value. 
     (4-2) Copying an Original Using Instruction Sheets 
     First, an overview of the process to copy an original using instruction sheets will be described.  FIG. 7  is an explanatory diagram for illustrating how an original is copied using instruction sheets.  FIG. 8  is an explanatory diagram with enlarged views of instruction sheets  64  and  65  shown in  FIG. 7 . 
     As shown in  FIG. 8 , the instruction sheets  64  and  65  provide various copy settings with a plurality of selectable values for each setting. For example, the setting “Document feed” specifies the method of feeding the original document being copied and can be set to one of the values “Flatbed,” “ADF simplex,” and “ADF duplex.” The Flatbed setting indicates that an original document placed on the first platen glass  12  is to be scanned. The setting ADF simplex indicates that the reading unit  25  is to scan one surface of the original while the ADF  16  conveys the original one sheet at a time from the document tray  17 . ADF duplex is a similar method, but the reading unit  25  scans both sides of the original. A detailed description of the other settings will not be included herein. 
     The user selects desired options by filling in the boxes positioned on the left of the desired settings in the instruction sheets  64  and  65  with a pencil or the like. After making all the desired settings, the user places the instruction sheets  64  and  65  and the original  58 - 60  in this order to be scanned on the document tray  17  and presses the Start button. 
     When the Start button is pressed, the MFP  1  initially scans the instruction sheets  64  and  65  and generates image data. Next, the MFP  1  performs image recognition on the generated image data to determine which settings have been selected, and subsequently copies the original  58 - 60  based on the selected settings. 
     When using these instruction sheets to specify settings for copying an original, one or more instruction sheets are added to the top of the original, as described above. In some cases, the instruction sheets may be formed of a different type of sheet than the original being copied. Therefore, when copying an original using instruction sheets, the user is prompted to enter the number of sheets in the group of instruction sheets that is conveyed first (hereinafter referred to as the “instruction sheet group”) and the number of sheets in the group of original documents conveyed thereafter (the “original document group”). 
     More specifically, values are prestored in the ROM  38  for an instruction sheet mode, which is the mode for copying an original using instruction sheets, and include the value “2” for the number of groups, the value “2” for the number of sheets in the instruction sheet group, and a value meaning “indefinite” for the number of sheets in the original document group. As in the previous example, the number of sheets in the original document group may also be left unspecified. 
       FIG. 9  is an explanatory diagram illustrating the user interface (the LCD of the operating unit  42 ) for copying an original using instruction sheets. A copy settings screen  66  that enables the user to choose copy settings is displayed when the user selects the copy function through an operation of the operating unit  42 . The number “01” displayed in the copy settings screen  66  indicates the number of copies. This number indicates the number of copies to be formed for each sheet of the original and does not specify the number of sheets in the group. By performing copy settings in the copy settings screen  66  and subsequently pressing the Start button, the user can copy an original without using instruction sheets. If the user presses the Start button while the copy settings screen  66  is displayed, the MFP  1  does not perform image recognition of instruction sheets. 
     If the user performs a prescribed operation for switching to the instruction sheet mode while the copy settings screen  66  is displayed, the MFP  1  enters the instruction sheet mode and displays an instruction sheet mode screen  67  shown in  FIG. 9 . When the user presses the Start button while the instruction sheet mode screen  67  is displayed, the MFP  1  sets the number of sheets for each group and begins scanning. 
       FIG. 10  is an explanatory diagram conceptually illustrating a table  72  including the number of sheets for each group set according to the user interface described above. When the user presses the Start button while the instruction sheet mode screen  67  is displayed, the MFP  1  generates the table  72  associating the number of each group with the number of sheets in that group (hereinafter referred to as the “instruction sheet mode table”) and stores this table in the RAM  39 , thereby setting the number of sheets for each group. 
     In the table  72  shown in  FIG. 10 , the number of sheets in document group  1  indicates the number of sheets in the group initially conveyed, i.e., the instruction sheet group. This number is set to “2” in the embodiment. The number of sheets in document group  2  specifies the number of sheets in the group subsequently conveyed, i.e., the original document group. This number is set to “indefinite” in the embodiment. 
     (4-3) Not Determining Changes in the Sheet Type 
     There are also cases in which the user may not be performing a facsimile transmission in the cover letter mode or a copy operation in the instruction sheet mode, even though the sheets set in the document tray  17  include a mixture of different types. In such cases, there is a danger that the MFP  1  will not detect changes in groups, leading to incorrect overlapped-feed detections. Therefore, the user can select the mixed mode through operations on the operating unit  42  for such cases. 
     For example, to transmit a fax for a mixture of sheets of various types in the document tray  17  without selecting the cover letter mode, the user selects a desired contact in the telephone number selection screen  62  to which the character string “(COVER)” has not been appended. Next, the user specifies the mixed mode through an operation on the operating unit  42  and presses the Start button. 
     Further, to copy a mixture of sheets of various types in the document tray  17  without selecting the instruction sheet mode, the user selects the mixed mode through an operation on the operating unit  42  while the copy settings screen  66  is displayed and subsequently presses the Start button. 
     When the user has selected the mixed mode and pressed the Start button, the MFP  1  begins scanning the original without detecting overlapped feed. Hence, the MFP  1  also does not determine whether groups have changed. 
     If none of the cover letter mode, instruction sheet mode, and mixed mode has been selected when the Start button is pressed, the MFP  1  assumes that only sheets of the same type are set in the document tray  17  and automatically selects a normal mode. In the normal mode, all sheets belong to a single group (original document group). For the normal mode, the value “1” is preset as the number of groups, and a value meaning “indefinite” is preset as the number of sheets in the original document group in the ROM  38 . 
       FIG. 11  conceptually illustrates a table  73  for the normal mode. The normal mode table  73  is generated and stored in the RAM  39  when the normal mode is selected, thereby setting the number of sheets for each group. 
     Since the initially conveyed group (the original document group) is also the group conveyed last in the normal mode, there is no need to determine whether groups have changed. Therefore, the MFP  1  performs overlapped-feed detection in the normal mode but does not determine whether groups have changed. 
     (5) Scanning Process 
     Next, a scanning process for scanning an original document will be described. This process will be described for cases in which scanning begins after the user performs one of the operations in 1) through 4). 
     1) The user selects a contact in the telephone number selection screen  62  having the appended character string “(COVER),” placing the MFP  1  in the cover letter mode, and presses the Start button while the cover letter page number input screen  63  is displayed (scanning in the cover letter mode). 
     2) The user performs a prescribed operation while the copy settings screen  66  is displayed, placing the MFP  1  in the instruction sheet mode, and presses the Start button while the instruction sheet mode screen  67  is displayed (scanning in the instruction sheet mode). 
     3) The user selects a contact in the telephone number selection screen  62  not having the appended character string “(COVER),” sets the mixed mode, and presses the Start button; or selects the mixed mode while the copy settings screen  66  is displayed and presses the Start button (scanning in the mixed mode). 
     4) The user presses the Start button without having selected one of the cover letter mode, instruction sheet mode, and mixed mode. That is, the MFP  1  performs the scanning in the normal mode. More specifically, the user selects a contact in the telephone number selection screen  62  not having the appended character string “(COVER)” and presses the Start button without specifying the mixed button; or presses the Start button while the copy settings screen  66  is displayed without specifying the mixed mode. 
     (5-1) Overview of the Scanning Process 
       FIG. 12  is a flowchart illustrating steps in the overall scanning process. In the following description “document” will include the cover letter or instruction sheet. 
     In S 101  the CPU  37  of the MFP  1  executes a process to determine the necessity for overlapped-feed detection and, when determining that overlapped-feed detection is necessary, to set document groups. While this process will be described later in greater detail, the MFP  1  determines that overlapped-feed detection is necessary when the MFP  1  has been set to the cover letter mode, the instruction sheet mode, or the normal mode and determines that overlapped-feed detection is unnecessary when the MFP  1  is set to the mixed mode. 
     In S 102  the CPU  37  determines whether overlapped-feed detection is required based on the results of the process performed in S 101 . The CPU  37  advances to S 103  if overlapped-feed detection is not required (the mixed mode). The CPU  37  advances to S 104  if overlapped-feed detection is required (the cover letter mode, instruction sheet mode, or normal mode). 
     In S 103  the CPU  37  scans the document without performing overlapped-feed detection. In S 104  the CPU  37  determines whether a plurality of groups are set in the process of S 101  described above. Here, the number of groups will be plural for the cover letter mode and the instruction sheet mode and singular for the normal mode. The CPU  37  advances to S 105  when determining that the number of groups is singular, i.e., when the MFP  1  is set to the normal mode (S 104 : NO). The CPU  37  advances to S 106  when determining that the number of groups is plural, i.e., when the MFP  1  is set to the cover letter mode or the instruction sheet mode (S 104 : YES). 
     In S 105  the CPU  37  scans the original while performing normal overlapped-feed detection. “Normal overlapped-feed detection” is a process for detecting overlapped feed without determining whether the document group being conveyed has changed. In S 106  the CPU  37  executes a scanning process while detecting overlapped feeds for each group. 
     Through the process described above, the CPU  37  scans the original document to generate image data. The facsimile transceiver  40  of the MFP  1  transmits this image data by fax when the facsimile function has been selected, and the printing unit  41  prints this image data when the copy function has been selected. 
     (5-2) Process to Determine the Necessity for Overlapped-Feed Detection and to Set Groups 
       FIG. 13  is a flowchart illustrating steps in the process to determine the necessity for overlapped-feed detection and to set groups. In S 201  of this process, the CPU  37  determines whether the MFP  1  is in the cover letter mode. The CPU  37  advances to S 202  when the MFP  1  is in the cover letter mode (S 201 : YES). The CPU  37  advances to S 204  when the MFP is not in the cover letter mode (S 201 : NO). 
     In S 202  the CPU  37  determines that overlapped-feed detection is necessary. In S 203  the CPU  37  sets the document group table to the cover letter mode table and sets the number of document groups to “2”, which is the number of groups in the cover letter mode. In S 204  the CPU  37  determines whether the MFP  1  is in the instruction sheet mode. The CPU  37  advances to S 205  when the MFP  1  is in the instruction sheet mode (S 204 : YES). The CPU  37  advances to S 207  when the MFP  1  is not in the instruction sheet mode (S 204 : NO). 
     In S 205  the CPU  37  determines that overlapped-feed detection is necessary. In S 206  the CPU  37  sets the document group table to the instruction sheet mode table and sets the number of document groups to “2”, which is the number of groups in the instruction sheet mode. 
     In S 207  the CPU  37  determines whether the MFP  1  is in the mixed mode. The CPU  37  advances to S 208  when the MFP  1  is not in the mixed mode (when the MFP  1  is in the normal mode; S 207 : NO). The CPU  37  advances to S 210  when the MFP  1  is in the mixed mode (S 207 : YES). 
     In S 208  the CPU  37  determines that overlapped-feed detection is necessary. In S 209  the CPU  37  sets the document group table to the normal mode table and sets the number of document groups to “1”, which is the number of groups in the normal mode. 
     In S 210  the CPU  37  determines that overlapped-feed detection is not necessary. 
     (5-3) Process for Scanning while Detecting Overlapped Feeds for Each Group 
       FIG. 14  is a flowchart illustrating steps in the process for scanning while detecting overlapped feeds for each group. In S 301  of the process in  FIG. 14 , the CPU  37  initializes a variable Document Group Number to “1”, where Document Group Number indicates the group currently targeted for processing. 
     In S 302  the CPU  37  initializes a variable Conveyed Sheet Counter to “0”, where Conveyed Sheet Counter indicates the number of sheets that have already been conveyed. In S 303  the CPU  37  acquires the number of sheets for the group having the number corresponding to the Document Group Number from the document group table. In S 304  the CPU  37  determines whether the number of sheets acquired in S 303  is plural. The CPU  37  advances to S 305  when the number of sheets is not plural (S 304 : NO), i.e., one sheet. The CPU  37  advances to S 307  when the number is plural (S 304 : YES). 
     In S 305  the CPU  37  scans the single sheet of the original without detecting overlapped feed. In S 306  the CPU  37  determines whether the group having the number set as the Document Group Number is the final group. The CPU  37  ends the current process if the group is the final group (S 306 : YES). The CPU  37  advances to S 316  if the group is not the final group (S 306 : NO). 
     In S 307  the CPU  37  controls the ADF  16  to feed the first sheet of the current group and controls the reading unit  25  to scan the sheet. At the same time, the CPU  37  controls the photoelectric sensor  44  to measure the length of the sheet. In S 308  the CPU  37  increments the Conveyed Sheet Counter by 1. 
     In S 309  the CPU  37  sets the reference value to the length measured in S 307 . Here, the CPU  37  may set the reference value to the length measured in S 307  or to a range including ±10% of the measured length, for example. Since even sheets of the same type may have slight variations in length, setting the reference value to a range can increase the reliability in detecting overlapped feeds. 
     In S 310  the CPU  37  controls the ADF  16  to convey the next sheet of the group and controls the reading unit  25  to scan the sheet. At the same time, the CPU  37  controls the photoelectric sensor  44  to measure the length of the sheet being conveyed and compares this measured length to the reference value set in S 309  to determine whether an overlapped feed has occurred. More specifically, if the reference value is set as a range, the CPU  37  determines that an overlapped feed has not occurred if the measured length falls within this range and determines that an overlapped feed has occurred if the length does not fall within this range. 
     In S 311  the CPU  37  determines whether an overlapped feed is detected in S 310 . The CPU  37  advances to S 312  if an overlapped feed is detected (S 310 : YES). The CPU  37  advances to S 313  if an overlapped feed was not detected (S 310 : NO). 
     In S 312  the CPU  37  notifies the user that an overlapped feed was detected and subsequently ends the scanning operation. The CPU  37  may notify the user of an overlapped feed by displaying a message in the LCD indicating that an overlapped feed was detected, playing an audio message, or according to another method. 
     In S 313  the CPU  37  increments the Conveyed Sheet Counter by 1. In S 314  the CPU  37  determines whether there remain any sheets of the original document to be scanned. If there are no remaining sheets to be scanned (S 314 : NO), the CPU  37  determines that all sheets of the original document have been scanned and ends the current process. However, if there remain sheets to be scanned (S 314 : YES), the CPU  37  advances to S 315 . Here, the CPU  37  may determine that there are no more sheets of the original left to be scanned if the level of the electric signal outputted from the photoelectric sensor  44  does not change from high (bright) to low (dark) within a prescribed time, for example. 
     In S 315  the CPU  37  determines whether a number of sheets of the original document equivalent to the number of sheets in the group corresponding to the Document Group Number acquired in S 303  has been scanned. If a number of sheets equivalent to the number in the current group has been scanned (S 315 : YES), the CPU  37  determines that the group has changed, i.e., that the type of sheet has changed, and advances to S 316 . If the number of sheets is not equivalent to the number in the current group (S 315 : NO), then the CPU  37  returns to S 310  and repeats the above process until a number of sheets equivalent to the number in the current group has been scanned. 
     The process in S 314  described above for determining whether there remain sheets of the original to scan is necessary for the case in which the number of sheets in a group is specified as “indefinite” because the CPU  37  may never reach a YES determination in S 315  regarding whether the number of sheets in the group has been read and, thus, will never detect the end of the group. When the number of sheets in the group is specified as “indefinite,” the CPU  37  continues until all remaining sheets of the original have been scanned. Once all sheets of the document have been scanned, the CPU  37  determines that there are no more groups to scan and ends the scanning process. 
     In S 316  the CPU  37  increments the Document Group Number by 1 and returns to S 302 . 
     In the process described above, the CPU  37  advances to S 316  upon determining in S 315  that a number of sheets equivalent to the number in the group have been scanned, after which in S 309  the CPU  37  sets the reference value to the length of the first sheet conveyed in the next group, thereby modifying the reference value to suit the type of sheet following a change in groups. 
     (6) Effects of the Embodiment 
     According to the embodiment, the MFP  1  determines whether the type of sheet being conveyed has changed and sets the reference value according to the length of the sheet conveyed after the sheet type has changed. Accordingly, the MFP  1  can detect overlapped feeds with accuracy, even when a mixture of sheets of different types has been set in the document tray  17 . 
     Further, the MFP  1  measures the length of the sheet. The length measured by the MFP  1  when sheets overlap is longer than the length measured when sheets do not overlap. Accordingly, the MFP  1  can detect overlapped feeds by measuring the length of the sheet. 
     The MFP  1  sets the reference value to the length of the first sheet conveyed after the sheet type has changed. The MFP  1  can modify the reference value to suit the type of sheet after the type has changed. 
     In the embodiment, each set of sheets of the same type that are grouped together in the document tray  17  is considered a single group, and the user specifies the number of sheets for each group except the group that is conveyed last. When the ADF  16  is conveying sheets, the MFP  1  increments a count value of the Conveyed Sheet Counter after each sheet is conveyed. The MFP  1  can determine that a type change has occurred based on the specified number of sheets and the number of conveyed sheets. 
     The MFP  1  has the ADF  16  and the reading unit  25 . The ADF  16  conveys each of mixed documents and passes each of the documents on the document tray  17  toward the position to be read. The reading unit  25  generates the image data from the document passing the position. Accordingly, the MFP  1  can detect overlapped feeds of the documents conveyed by the ADF  16 . 
     With the MFP  1  according to the embodiment, the user can select one of a plurality of scanning modes having a preset number of groups and a preset number of sheets for each group. Thus, the user can easily set the number of sheets for each group by combining (mixing) sheets of different types in the document tray  17  that conform to the number of groups and number of sheets in each group preset for a selectable scanning mode and by subsequently selecting the scanning mode. 
     For example, the MFP  1  has a cover letter mode for transmitting an original with an attached cover letter of a different type of sheet from the original that specifies the cover letter as one group and the original as a separate group. Accordingly, the user can easily set the number of sheets for each group when transmitting image data. 
     Further, with the MFP  1  according to the embodiment, the user can modify the number of sheets in the cover letter group that is set in the cover letter mode because the number of sheets in a cover letter is not necessary fixed and may vary from time to time. Thus, the MFP  1  of the embodiment is more user-friendly by allowing the user to adjust the number of sheets in the cover letter group. 
     Further, the MFP  1  according to the embodiment allows the user to select a contact, and modifies the number of sheets in the cover letter group according to the selected contact. Thus, if the number of sheets in a cover letter varies among a plurality of contacts, the MFP  1  modifies the number of sheets in the cover letter based on the selected contact, making the settings more user-friendly for transmitting image data with an attached cover letter. 
     Second Embodiment 
     Next, a second embodiment of the invention will be described with reference to  FIG. 15 . In the second embodiment, the CPU  37  updates the reference value to the length of the last sheet measured for each sheet in the group.  FIG. 15  is a flowchart illustrating steps in a process according to the second embodiment for scanning while detecting overlapped feeds in each group, where steps to those described in  FIG. 14  of the first embodiment have been designated with the same step numbers to avoid duplicating description. 
     In S 401  the CPU  37  sets the reference value to the last length measured. Specifically, if the current sheet is the first sheet of a group, the CPU  37  sets the reference value to the length measured in S 307 . When the current sheet is the second or subsequent sheet of a group, the CPU  37  sets the reference value to the length measured in S 310 . In other words, if a type of a subject sheet is the same as a type of the sheet conveyed immediately before the subject sheet, the CPU  37  changes the reference value to a quantity with respect to the sheet conveyed immediately before the subject sheet. 
     When the CPU  37  determines in S 402  that a number of sheets equivalent to the number in the current group has been scanned (S 402 : YES), the CPU  37  advances to S 316 . If a number of sheets equivalent to the number in the group has not been scanned (S 402 : NO), the CPU  37  returns to S 401  and repeats the above process until a number of sheets equivalent to the number in the current group has been scanned. 
     As described above, the MFP  1  according to the second embodiment updates the reference value to the length of the sheet last measured by the photoelectric sensor  44  after each time the photoelectric sensor  44  measures the length of a sheet. This is performed because the measuring precision of the photoelectric sensor  44  can vary over time. If the measuring precision of the photoelectric sensor  44  varies over time, the MFP  1  may calculate a larger discrepancy between the reference value and the measured value and incorrectly determine that an overlapped feed has occurred, even when that is not the case. By configuring the MFP  1  to update the reference value to the most recently measured length each time the photoelectric sensor  44  measures the length of a sheet, it is possible to reduce the amount of time that elapses between when setting the reference value and when measuring the length of the next sheet. As a result, the amount of difference between the reference value and the length of the next conveyed sheet when an overlapped feed has not occurred is less, reducing the chance of an incorrect overlapped-feed detection. 
     In all other respects, the MFP  1  according to the second embodiment is essentially identical to the MFP  1  according to the first embodiment. 
     Third Embodiment 
     Next, a third embodiment of the invention will be described. In the third embodiment, a value set as the reference value is stored in the ROM  38  for each type of sheet. When the type of sheet being conveyed changes in a scanning operation, the MFP  1  reads a value from the ROM  38  corresponding to the type of the first sheet conveyed after the change and sets the reference value to this value. 
     In the case of the instruction sheet mode, for example, if a specific type of sheet is always used as the instruction sheet, this sheet type can be set in the instruction sheet group, although the user would be able to modify the sheet type set to the instruction sheet group as needed. 
     Further, reference values associated with each sheet type are stored separately in the ROM  38 . Upon determining a change in group, the MFP  1  acquires the type of group to be conveyed after the change from the document group table, reads the value associated with this sheet type from the ROM  38 , and sets the reference value to this value. 
     According to the third embodiment described above, the MFP  1  can modify the reference value based on the type of sheet conveyed after a change, enabling the MFP  1  to detect an overlapped feed for the sheet conveyed after the sheet type has changed. 
     While values to be set as reference values are stored in the ROM  38  in the above example, these values may be stored in a nonvolatile storage device, such as Flash Memory. Further, the user may be allowed to modify the values through operations on the operating unit  42 . With this ability, the user can adjust the reference values based on the measuring accuracy of the photoelectric sensor  44  when this measuring accuracy changes over time, thereby enabling the MFP  1  to detect overlapped feeds with greater accuracy. 
     In all other respects, the MFP  1  according to the third embodiment is essentially identical to the MFP  1  according to the first embodiment. 
     Fourth Embodiment 
     Next, a fourth embodiment of the invention will be described. In the fourth embodiment, the MFP  1  measures the thickness of sheets as the specific quantity that differs between when an overlapped feed occurs and when an overlapped feed does not occur. In the event of an overlapped feed, portions of two sheets overlap each other, for example, and the overlapped portion is thicker than a single sheet. In other words, the measured thickness when sheets are overlapped is greater than the measured thickness when sheets are not overlapped. Accordingly, overlapped feeds can be detected by measuring the thickness of the conveyed sheets. 
     However, in a conceivable case where the reference value for detecting an overlapped feed is not changed according to the type of sheet when the thickness of the sheet varies according to the type, the MFP will likely detect overlapped feeds by mistake. According to the fourth embodiment, when measuring the thickness of the sheet as the measured quantity, the MFP  1  determines when the type of sheet being conveyed has changed. When the type of sheet changes, the MFP  1  modifies the reference value to suit the new sheet type. In this way, the MFP  1  can detect overlapped feeds with accuracy, even when a mixture of sheets including a plurality of types is set in the document tray  17 . 
     The MFP  1  may measure the thickness of sheets by directly measuring the distance from one surface of the sheet to the other, or may indirectly measure the thickness by measuring the intensity of transmitted light when the different types of sheets are formed of the same material. The photoelectric sensor  44  can be used for measuring the intensity of transmitted light. In this case, since there is no need to provide a separate device for measuring thickness when using the photoelectric sensor  44  for this purpose, the thickness can be measured without increasing the number of parts in the MFP  1 . 
     Further, using the photoelectric sensor  44  in combination with the second embodiment can reduce error in thickness measurements due to changes over time in the intensity of light detected by the photoelectric sensor  44 , thereby improving the precision in which the MFP  1  can detect overlapped feeds. 
     In all other respects, the MFP  1  according to the fourth embodiment is essentially identical to the MFP  1  according to the first embodiment. 
     &lt;Modifications&gt; 
     While the invention has been described in detail with reference to the embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
     (1) While the embodiments describe cases in which one or two types of sheets are set in the document tray  17 , the invention may be applied to cases in which three or more types of sheets are combined (mixed). In this case, scanning modes for handling three or more types of sheets may be prepared for the MFP  1 . Alternatively, the user may be allowed to freely set the number of groups and the number of sheets in each group through operations on the operating unit  42 , without regard for the number of types of sheets. 
     (2) The embodiments described above give examples of the length and thickness of sheets being the measured quantity, but another measured quantity may be used, provided that the measured result of this quantity differs between cases in which overlapped feeds occur and cases in which overlapped feeds do not occur. 
     (3) While a multifunction peripheral is described as an example of the image-reading device in the embodiments, the invention may be applied to an image scanner possessing only a scanning function, a facsimile device possessing only a facsimile function, and the like. 
     (4) While the document cover  15  serves as an example of the sheet-medium-conveying device in the embodiments, the sheet-medium-conveying device may be employed in a printer that forms images on a recording medium such as paper while the recording medium is conveyed one sheet at a time from a paper tray and may detect overlapped feeds occurring among the recording medium conveyed along the conveying path. The sheet-medium-conveying device of the invention may be applied to any device that conveys a plurality of types of sheets one sheet at a time, in which device it is desirable to detect overlapped feeds. 
     (5) The image sensor control unit  35 , the image-processing unit  36 , the CPU  37 , the ROM  38 , the RAM  39 , the facsimile transceiver  40 , the printing unit  41 , the operating unit  42 , and the interface unit  43  may be provided in the document cover  15 .