Patent Publication Number: US-2013234385-A1

Title: Print medium feeding apparatus with electrode in endless belt

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosures herein relate to a print medium feeding apparatus, an image forming apparatus, and a print medium feeding system. 
     2. Description of the Related Art 
     An image forming apparatus such as a printer or a copier feeds paper sheets one by one to an image forming unit by separating a paper sheet from a stack of paper sheets stored in a sheet tray, and uses the image forming unit to print an image on the surface of the paper sheet, which is then discharged to outside the apparatus. If multiple paper sheets are fed at a time from the sheet tray, a paper jam may occur along the conveyance path or in the image forming unit, or a print failure may be caused. In order to improve the productivity of the image forming apparatus, there is a need to separate paper sheets one by one from a stack of paper sheets at high speed in a stable manner. 
     An electrostatic adherence paper feed technology known in the art (see Japanese Patent Application Publication No. 2010-126317, for example) uses a plurality of electrodes embedded in an insulating paper feed belt, and applies voltage to the electrodes to polarize a paper sheet surface that is placed in contact with the paper feed belt, thereby causing the paper sheet to adhere to the paper feed belt through an electrostatic force to feed the paper sheet. 
     The electrostatic adherence paper feed technology may need to apply a large voltage to the electrodes to create sufficient paper sheet adherence when the paper sheet to be fed is larger than the paper feed belt. When the paper feed belt is the same size as the size of the largest sheet among paper sheets to be fed, voltage applied to the entirety of the paper feed belt at the time of feeding a small-sized paper sheet may result in needless consumption of electric power. 
     Accordingly, it may be desirable to provide a print medium feeding apparatus that can feed a medium by the use of an appropriate amount of electric power responsive to the size of the medium. 
     SUMMARY OF THE INVENTION 
     It is a general object of at least one embodiment of the present invention to provide a print medium feeding apparatus that substantially obviates one or more problems caused by the limitations and disadvantages of the related art. 
     In one embodiment, a print medium feeding apparatus includes an endless belt disposed to face stacked media and configured to rotate, the endless belt divided into sections having electrodes, respectively, fixed terminals configured to be in contact with two or more of the electrodes while the two or more of the electrodes are passing through a position facing the media as the electrodes are moved by rotation of the endless belt, a voltage applying unit connected to the fixed terminals and configured to apply voltage to the two or more of the electrodes that are in contact with the fixed terminals, a connection unit configured to make and break a connection between the voltage applying unit and at least one of the fixed terminals, and a switching unit configured to change a state of the connection of the connection unit in response to size of the media. 
     According to at least one embodiment, a print medium feeding apparatus is provided that can feed a medium by use of an appropriate amount of electric power responsive to the size of the medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and further features of embodiments will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a drawing illustrating an example of the appearance of an image forming apparatus that is provided with a paper feed apparatus according to a first embodiment; 
         FIG. 2  is a drawing illustrating an example of the schematic configuration of a paper feed apparatus and a paper feed cassette according to the first embodiment; 
         FIG. 3  is a drawing illustrating an example of the hardware configuration of the paper feed apparatus according to the first embodiment; 
         FIG. 4  is a drawing illustrating an example of the configuration of the paper feed apparatus according to the first embodiment; 
         FIGS. 5A and 5B  are drawings illustrating an example of the configuration of a paper feed belt of the paper feed apparatus according to the first embodiment; 
         FIG. 6  is a drawing illustrating an example of another configuration of the paper feed apparatus according to the first embodiment; 
         FIG. 7  is a drawing illustrating an example of the configuration of the paper feed apparatus according to a second embodiment; 
         FIG. 8  is a drawing illustrating an example of sections to which voltage is applied in response to the size and basis weight of a paper sheet in the paper feed apparatus according to the second embodiment; 
         FIG. 9  is a drawing illustrating an example of the configuration of a paper feed belt of the paper feed apparatus according to a third embodiment; 
         FIG. 10  is a drawing illustrating an example of the appearance of an image forming system according to a fourth embodiment; and 
         FIG. 11  is a drawing illustrating an example of the hardware configuration of the image forming system according to the fourth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments will be described by referring to the accompanying drawings. In these drawings, the same elements are referred to by the same references, and repetitive description thereof may be omitted. 
     First Embodiment 
     Configuration of Image Forming Apparatus 
       FIG. 1  is a drawing illustrating an example of the appearance of an image forming apparatus  200  that is provided with a paper feed apparatus according to a first embodiment. 
     As illustrated in  FIG. 1 , the image forming apparatus  200  includes an image forming apparatus main section  201 , an automatic document feeder (ADF)  202 , a finisher  203  having a stapler and a shift tray, a duplex-printing buffer/flipper unit  204 , an extended paper feed tray  205 , a one-bin discharge tray  207 , an insert feeder  208 , and so on. 
     The image forming apparatus main section  201  includes a scanner unit, an image forming unit, a developer unit, a fuser unit, and a paper feed apparatus, etc., accommodated therein, and prints an image on the surface of a paper sheet fed from the paper feed apparatus. 
     The image forming apparatus main section  201  may use an electrophotographic process to print an image on the surface of a paper sheet. The process of forming an image may be a different process such as an inkjet process. Further, it does not matter whether the process is a monochrome process or a full color process. 
     Configuration of Paper Feed Apparatus 
       FIG. 2  is a drawing illustrating an example of the schematic configuration of a paper feed apparatus  100  and a paper feed cassette  9  according to the first embodiment. 
     As illustrated in  FIG. 2 , the paper feed apparatus  100  includes a paper feed belt  1 , a drive roller  141 , a driven roller  142 , and a high-voltage power supply  121 . 
     The paper feed belt  1  is an endless belt having a plurality of electrodes embedded therein that are covered with an insulating layer. The paper feed belt  1  is looped around the drive roller  141  and the driven roller  142 . 
     The drive roller  141  is rotated by a drive unit (not shown) to rotate the paper feed belt  1 . The driven roller  142  is rotated by the paper feed belt  1  that is rotated by the drive roller  141 . 
     The high-voltage power supply  121  applies voltage to the electrodes in the paper feed belt  1  to polarize the surface of a paper sheet P coming in contact with the paper feed belt  1 , thereby causing the paper sheet P to adhere to the paper feed belt  1 . 
     The paper feed cassette  9  storing paper sheets P includes a sheet-width regulating guide  4 , a sheet-rear-end regulating plate  5 , a bottom plate  6 , a fan  7 , an optical sensor  8 , a conveyor sensor  10 , and a conveyor roller  11 . 
     The sheet-width regulating guide  4  and the sheet-rear-end regulating plate  5  secure the paper sheets P in their place and also detect the size of the paper sheets P upon the paper sheets P being set. 
     The bottom plate  6  elevates to such a height that the top of the stored paper sheets P can be detected by the optical sensor  8 , thereby raising the paper sheets P to the position at which paper feeding can be performed by the paper feed apparatus  100 . 
     The fan  7  blows air into a space over the stacked paper sheets P to flip a sheet, thereby causing the topmost paper sheet P to come in contact with the paper feed belt  1 . The paper sheet P may come in contact with the paper feed belt  1  while voltage is applied to the embedded electrodes. When this happens, the surface of the paper sheet P polarizes, and thus adheres to the paper feed belt  1 . Rotation of the paper feed belt  1  then causes the paper sheet P to be conveyed to outside the paper feed cassette  9 . 
     The conveyor sensor  10  detects the paper sheet P that is fed by the paper feed belt  1  to a conveyor path. Upon the conveyor sensor  10  detecting the paper sheet P, the paper feed apparatus  100  deactivates the high-voltage power supply  121  to stop the adherence and conveyance of the paper sheet P. 
     The conveyor roller  11  conveys the paper sheet P to the image forming unit or the like situated at the next stage as the paper sheet P is fed from the paper feed cassette  9  by the paper feed belt  1  of the paper feed apparatus  100 . 
       FIG. 3  is a drawing illustrating an example of the hardware configuration of the paper feed apparatus  100  according to the first embodiment. 
     As illustrated in  FIG. 3 , the paper feed apparatus  100  includes, in addition to the paper feed belt  1 , the high-voltage power supply  121 , and so on, a CPU (Central Processing Unit)  171 , an HDD (Hard Disk Drive)  172 , a ROM (Read Only Memory)  173 , a RAM (Random Access Memory)  174 , a network interface unit  175 , a recording medium interface unit  176 , etc., which are connected to each other through a bus B. 
     The CPU  171  is a computing unit that performs control operations, arithmetic operations, and data processing by executing programs stored in the ROM  173  and the RAM  174 . The CPU  171  performs overall control of the apparatus by executing programs, and also serves as a switchover unit to select the electrodes of the paper feed belt  1  to which the high-voltage power supply  121  applies voltage. 
     The HDD  172  is a nonvolatile storage device to store various programs and data. The stored programs and data include an OS (Operating System) and application programs providing various functions. 
     The ROM  173  is a nonvolatile semiconductor memory (storage device) that can retain stored data even when the power is switched off. The RAM  174  is a volatile semiconductor memory (storage device) that temporarily stores programs and data. 
     The network interface unit  175  is an interface with peripheral devices having a communication function that are connected through a network such as a LAN (Local Area Network) or WAN (Wide Area Network), which is implemented by use of wired and/or wireless data transmission lines. 
     The recording medium interface unit  176  is an interface with a computer-readable recording medium  177  such as a CD-ROM, a flexible disk (FD), a CD-R, a DVD (Digital Versatile Disk), or the like, which is connected through a data transmission line such as a USB (Universal Serial Bus). 
     A predetermined program is stored in the recording medium  177 . The program stored in the recording medium  177  is installed in the paper feed apparatus  100  via the recording medium interface unit  176 . The installed program is ready to be executed by the CPU  171 . 
     The CPU  171 , the HDD  172 , the ROM  173 , the RAM  174  and the like may be shared with one or more other units such as the image forming apparatus main section  201 . 
       FIG. 4  is a drawing illustrating an example of the configuration of the paper feed apparatus  100  according to the first embodiment. 
     As illustrated in  FIG. 4 , the paper feed apparatus  100  includes, in addition to the paper feed belt  1 , the drive roller  141  and the driven roller  142 , the high-voltage power supply  121 , a control unit  122 , etc. The paper feed apparatus  100  feeds a paper sheet taken from the paper feed cassette  9 . 
     The paper feed belt  1  is looped around the drive roller  141  and the driven roller  142 . The paper feed belt  1  is rotated in a clockwise direction by the drive roller  141  that rotates in the direction indicated by an arrow. The distance between the drive roller  141  and the driven roller  142  is equivalent to the longitudinal length of an A3 paper sheet. The distance between the drive roller  141  and the driven roller  142  may be adaptively set in response to the size of the paper sheet P that is to be fed. 
     Further, the paper feed belt  1  has four sections A through D arranged in the rotational direction thereof. The four sections A through D have four electrodes  101  through  104 , respectively, embedded therein. 
       FIGS. 5A and 5B  are drawings illustrating an example of the configuration of the paper feed belt  1  of the paper feed apparatus  100  according to the first embodiment.  FIG. 5A  is a plan view of the section A and part of the section B of the paper feed belt  1  viewed from the inner side of the paper feed belt  1 .  FIG. 5B  is a schematic view of the cross-section taken along the line a-a′ illustrated in  FIG. 5A . 
     As illustrated in  FIG. 5A , sections A and B of the paper feed belt  1  have a pair of comb-shaped electrodes  101   a  and  101   b  and a pair of comb-shaped electrodes  102   a  and  102   b , respectively, which are arranged such that the teeth of one electrode extend in the direction perpendicular to the rotational direction of the paper feed belt  1  into the spaces between the teeth of the other electrode. The same also applies in the case of the remaining sections C and D. 
     It may be noted that the electrode  101  refers to the pair of electrodes  101   a  and  101   b , and the same applies with respect to each of the remaining electrodes  102  through  104 . As illustrated in  FIG. 5A  and  FIG. 5B , the electrode  101  is covered with an insulating layer  190 , except for areas corresponding to openings  111   a  and  111   b  of the insulating layer  190  that are situated near the side ends of the paper feed belt  1 . The same also applies in the case of the remaining electrodes  102  through  104 . The side ends of electrode  101  near the side ends of the paper feed belt  1  are exposed through the openings  111   a  and  111   b , respectively, toward the inner side of the paper feed belt  1 . The same also applies in the case of the remaining electrodes  102  through  104 . The side ends of each of the electrodes  101  through  104  exposed through the openings receive a voltage applied thereto. In the section in which voltage is applied to the electrode, the surface of the paper feed belt  1  is made to exert an adhering force to attract a paper sheet P. Because the electrodes  101  through  104  are covered with the insulating layer, no electric charge moves between the electrodes. Voltage control may be performed such that different voltages are applied to the electrodes  101  through  104 . 
     On the inner side of the paper feed belt  1 , as illustrated in  FIG. 4 , roller-shaped, rotatable fixed terminals  131  through  134  are disposed that are in contact with the side ends of the electrodes  101  through  104  exposed through the openings. The fixed terminal  131  disposed at the position facing the paper feed cassette  9  (not shown) is directly connected to the high-voltage power supply  121 . The fixed terminal  132  is connected through a relay  151  to the high-voltage power supply  121 . The fixed terminals  133  and  134  are connected to ground. Alternative arrangement may be such that the fixed terminals  133  and  134  are not connected to ground. Since these terminals may be charged to an uncertain potential by paper dusts and friction at the time of feeding paper sheets P, however, the provision of a ground connection may be preferable. The fixed terminals  131  are arranged at such intervals that one electrode arranged in one section of the paper feed belt  1  does not come in contact with two fixed terminals simultaneously. 
     The relay  151  is a connecting unit situated between the high-voltage power supply  121  and the fixed terminal  132 . The relay  151  is controlled by the control unit  122  that serves as a switchover unit for changing the state of connection of the relay  151 , thereby connecting the fixed terminal  132  either to the high-voltage power supply  121  or to ground. 
     The high-voltage power supply  121  applies voltage to those electrodes among the electrodes  101  through  104  that are in contact with either the fixed terminal  131  or the fixed terminal  132  at the time of feeding a paper sheet P, thereby polarizing the surface of the paper sheet P coming in contact with the area of the paper feed belt  1  to which the voltage is applied, and thus causing the paper sheet P to adhere to the paper feed belt  1 . One of the two comb-shaped electrodes included in each of the electrodes  101  through  104  may receive a positive polarity voltage, and the other may receive a negative polarity voltage. Alternatively, one of the two comb-shaped electrodes may be connected to ground. 
     The control unit  122  serves as a switchover unit for changing the state of connection of the relay  151  based on information indicative of the size of the paper sheet P, thereby connecting the fixed terminal  132  either to the high-voltage power supply  121  or to ground. The control unit  122  obtains the information indicative of the size of the paper sheet P from information entered by a user or from the results of detections made by the sheet-width regulating guide  4  and the sheet-rear-end regulating plate  5  of the paper feed cassette  9 . 
     Operation of Paper Feed Apparatus During Feed Operation 
     A description will be given of operations performed by the paper feed apparatus  100  of the first embodiment when feeding a paper sheet P from the paper feed cassette  9 . The paper sheets P are stored in the paper feed cassette  9 , such that they are placed against the inner wall thereof positioned on the same side as the section D. 
     At the time of feeding a paper sheet P, the control unit  122  obtains information indicative of the size of the paper sheet P from information entered by a user or from the results of detections made by the sheet-width regulating guide  4  and the sheet-rear-end regulating plate  5  of the paper feed cassette  9 . 
     The paper sheet P stored in the paper feed cassette  9  may be A3 portrait. In this case, the control unit  122  controls the state of connection of the relay  151  to connect the fixed terminal  132  to the high-voltage power supply  121 . In this state, the high-voltage power supply  121  supplies voltage. When the paper feed belt  1  is in such a position as illustrated in  FIG. 4 , the voltage is applied to the electrode  104  of the section D being in contact with the fixed terminal  131  and to the electrode  103  of the section C being in contact with the fixed terminal  132 . As the paper feed belt  1  rotates, different electrodes successively come in contact with the fixed terminals  131  and  132 . Despite this, the two sections of the paper feed belt  1  that face the paper feed cassette  9  always have the electrodes thereof receiving the voltage, thereby exerting an adhering force to attract the paper sheet P. 
     The paper sheet P stored in the paper feed cassette  9  may be A4 landscape. In this case, the control unit  132  controls the state of connection of the relay  151  to connect the fixed terminal  132  to ground. In this state, the high-voltage power supply  121  supplies voltage. When the paper feed belt  1  is in such a position as illustrated in  FIG. 4 , voltage is applied only to the electrode  104  of the section D being in contact with the fixed terminal  131 . As the paper feed belt  1  rotates, different electrodes successively come in contact with the fixed terminal  131 . Despite this, the section of the paper feed belt  1  that faces the paper feed cassette  9  always has the electrode thereof receiving the voltage, thereby exerting an adhering force to attract the paper sheet P. 
     In this manner, control is performed based on the information indicative of the size of a paper sheet P, such that the fixed terminal  132  is connected to the high-voltage power supply  121  when feeding a large-size paper sheet P, and is not connected to the high-voltage power supply  121  when feeding a small-size paper sheet P. Through such control, the large-size paper sheet can be reliably fed, and, also, needless consumption of electric power is avoided by applying voltage to the electrode(s) of the fewest necessary section(s) when feeding the small-size paper sheet P. 
     Further, as illustrated in  FIG. 6 , a remaining charge removing power supply  123  may be provided and connected to the fixed terminals  133  and  134 . The remaining charge removing power supply  123  applies the voltage for removing remaining charge to an electrode that comes in contact with the fixed terminals  133  and  134  after this electrode receives the voltage of the high-voltage power supply  121  while being in contact with the fixed terminal  131  and disconnects from the fixed terminal  131  as the paper feed belt  1  rotates. An effective method to remove remaining charge is to apply an alternating voltage to the electrode while making the amplitude of the alternating voltage smaller and smaller. The remaining charge removing power supply  123  may thus be preferably an alternating current power supply and a pulse-voltage power supply capable of applying voltages of opposite polarities, and may preferably apply the voltage to the electrode until the amplitude of the voltage reaches 0 V. 
     The fixed terminals  132 ,  133  and  134  are connected either to the remaining charge removing power supply  123  or to ground through relays  151  through  154  whose connection states are controlled by the control unit  122 . 
     In the paper feed apparatus  100 , charge may remain in an electrode to which the high-voltage power supply  121  applies high voltage in order to feed a heavy basis weight paper sheet P, for example. In such a case, the control unit  122  connects the remaining charge removing power supply  123  to the fixed terminal to which the high-voltage power supply  121  does not supply voltage, so that an alternating voltage as described above is applied to each electrode to remove remaining charge. 
     Voltage applied by the high-voltage power supply  121  to an electrode may be small when feeding a light basis weight paper sheet P. In this case, remaining charge may not be in existence. The control unit  122  thus controls the connection state of the relays  151  through  154  such that the fixed terminals not connected to the high-voltage power supply  121  are connected to ground. 
     As described above, the remaining charge removing power supply  123  is provided to remove charge remaining in each electrode. This arrangement prevents the paper feed belt  1  from exerting a needless adhering force, thereby ensuring a reliable feeding operation of the paper feed apparatus  100 . 
     As described heretofore, the paper feed apparatus  100  of the first embodiment uses the control unit  122 , which changes the state of connection of the relay  151  in response to information indicative of the size of a paper sheet P to change the sections to which the high-voltage power supply  121  applies voltage, thereby attracting and feeding the paper sheet P by use of proper electric power responsive to the size of the paper sheet P. Compared to the case in which the paper sheet P is conveyed by applying voltage to all the electrodes embedded in the paper feed belt  1 , for example, power consumption is reduced to a half when conveying an A3 portrait paper sheet P, and power consumption is reduced to a quarter when conveying an A4 landscape paper sheet P. Further, the high-voltage power supply  121  may be powered off while feeding is not performed during an interval between paper sheets, which achieves further reduction in power consumption. Moreover, the provision of the remaining charge removing power supply  123  makes it possible to feed a paper sheet P in a more reliable manner. 
     The description of the first embodiment has been given with respect to an example in which the paper feed belt  1  is divided into the four sections A through D in a rotational direction. This is not a limiting example, and the number of divided sections may be two or more. The number of sections may be increased to properly cope with an increased variety of paper sheet sizes, and voltage is applied in response to the size of a paper sheet P to feed the paper sheet P by use of proper electric power while reducing needless power consumption. 
     Second Embodiment 
     In the following, a second embodiment will be described with reference to the accompanying drawings. A description will be omitted of the same elements as those of the embodiment already described. 
     &lt;Configuration of Paper Feed Apparatus&gt; 
     The paper feed apparatus  100  of the second embodiment differs from that of the first embodiment in that the paper feed belt  1  is divided into sections in a direction perpendicular to the rotational direction. 
       FIG. 7  is a drawing illustrating an example of the configuration of the paper feed apparatus  100  according to the second embodiment. 
     As illustrated in  FIG. 7 , the paper feed belt  1  is divided into sections A through C in a direction perpendicular to the rotational direction. The sections A through C have a pair of comb-shaped electrodes  101   a  and  101   b , a pair of comb-shaped electrodes  105   a  and  105   b , and a pair of comb-shaped electrodes  106   a  and  106   b , respectively, which are arranged such that the teeth of one electrode of a given pair extend in the direction perpendicular to the rotational direction of the paper feed belt  1  into the spaces between the teeth of the other electrode of the given pair. Electrodes  101   a  and  101   b  are collectively referred to as an electrode  101 . This also applies to other electrodes. 
     The paper feed belt  1  has a width (i.e., the span in the direction perpendicular to the rotational direction) equal to the longitudinal length of an A3 size paper sheet. The divided sections A through C are provided such that their widths satisfy the following relationships: a ratio of the width of the section A to the width of the section B=1:(2 0.5 −1), and a ratio of the combined width of the sections A and B to the width of the section C=1:(2 0.5 −1). The widths and numbers of divided sections may be set differently as appropriate. 
     The electrodes  101 ,  105  and  106  provided in the respective sections A through C are covered with the insulating layer  190 . The side ends of the electrodes  101 ,  105  and  106  opposite to each other in the direction perpendicular to the rotational direction are exposed through openings  111   a  and  111   b ,  112   a  and  112   b , and  113   a  and  113   b , respectively. 
     Fixed terminals that come in contact with the side ends of the electrodes  101 ,  105  and  106  through the openings in the sections A through C are provided on the inner side of the paper feed belt  1 . These fixed terminals are connected either to high-voltage power supply  121   a  or  121   b  or to ground through relays  155   a ,  155   b ,  156   a  and  156   b  whose connection states are controlled by the control unit  122 . 
     The high-voltage power supply  121   a  is connected to the electrodes  101   a ,  105   a  and  106   a  through fixed terminals. The high-voltage power supply  121   b  is connected to the electrodes  101   b ,  105   b  and  106   b  through fixed terminals. The high-voltage power supplies  121   a  and  121   b  produce voltages of opposite polarities, respectively. 
     The control unit  122  changes the state of connection of the relays  155   a ,  155   b ,  156   a  and  156   b  situated between the fixed terminals and the high-voltage power supplies  121   a  and  121   b  based on information indicative of the size of a paper sheet P fed by the paper feed apparatus  100 . 
     &lt;Operation of Paper Feed Apparatus During Feed Operation&gt; 
     The paper sheets P are stored in the paper feed cassette  9  facing the paper feed belt  1 , such that they are placed against the top right corner of the paper feed cassette  9  (wherein the term “top right” is defined with respect to the positional arrangements illustrated in  FIG. 7 ). A description will be given of an operation of feeding a paper sheet P in the paper feed apparatus  100 . 
     At the time of feeding a paper sheet P, the control unit  122  obtains information indicative of the size of the paper sheet P from information entered by a user or from the results of detections made by the sheet-width regulating guide  4  and the sheet-rear-end regulating plate  5  of the paper feed cassette  9 . 
     The paper sheet P stored in the paper feed cassette  9  may be A5 portrait. In this case, the control unit  122  controls the relays  155   a ,  155   b ,  156   a  and  156   b  to connect the electrode  105  of the section B and the electrode  106  of the section C to ground. The high-voltage power supply  121  applies voltage only to the electrode  101  of the section A of the paper feed belt  1 , so that the paper feed belt  1  attracts and conveys the A5 portrait sheet P by use of only the section A. 
     When an A4 portrait paper sheet P is to be conveyed, the control unit  122  controls the relays  156   a  and  156   b  to connect the electrode  105  of the section B to the high-voltage power supply  121 , and controls the relays  155   a  and  155   b  to connect the electrode  106  of the section C to ground. The high-voltage power supply  121  applies voltage to the electrode  101  of the section A and to the electrode  105  of the section B, so that the paper feed belt  1  attracts and conveys the A4 portrait sheet P by use of the section A and the section B. 
     When an A3 portrait paper sheet P is to be conveyed, the control unit  122  controls the state of connection of the relays to connect the electrode  105  of the section B and the electrode  106  of the section C to the high-voltage power supply  121 . In this case, the paper feed belt  1  attracts and conveys the A3 portrait sheet P by use of all the sections A, B and C. 
     Moreover, the control unit  122  may change the sections to which to apply voltage in response to the size and basis weight of a paper sheet P to be fed, based on information indicative of a sheet type entered by a user. 
       FIG. 8  is a drawing illustrating an example of sections to which voltage is applied in response to the size and basis weight of a paper sheet in the paper feed apparatus  100  according to the second embodiment. 
     As illustrated in  FIG. 8 , in the case of the A5-size paper sheet P, the control unit  122  controls the states of connection of the relays  155   a ,  155   b ,  156   a  and  156   b  in such a manner as to apply voltage to the electrode of the section A regardless of the basis weight. In the case of the A4-size paper sheet P, the control unit  122  controls the states of connection of the relays  155   a ,  155   b ,  156   a  and  156   b  in such a manner as to apply voltage to the electrodes in all the sections A through C regardless of the basis weight. 
     In the case of a B5-size paper sheet P being conveyed, the control unit  122  controls the states of connection of the relays  155   a ,  155   b ,  156   a  and  156   b  in such a manner as to apply voltage to the electrodes of the sections A and B for a heavy basis weight sheet and to apply voltage to the electrode of only the section A for a light basis weight sheet. In the case of the B4-size paper sheet P being conveyed, the control unit  122  controls the states of connection of the relays  155   a ,  155   b ,  156   a  and  156   b  in such a manner as to apply voltage to the electrodes in all the sections A through C for a heavy basis weight sheet and to apply voltage to the electrodes of only the sections A and B for a light basis weight sheet. 
     In this manner, the sections to which voltage is applied are changed in response to the size and basis weight of paper sheets P, so that the paper sheets P are properly conveyed and fed with proper power consumption even when the paper sheets P having the same size have different weights due to difference in paper type. 
     As described heretofore, the paper feed apparatus  100  of the second embodiment uses the control unit  122 , which changes the states of connection of the relays  155   a ,  155   b ,  156   a  and  156   b  in response to the size and basis weight of a paper sheet P to be fed wherein the size is measured in the direction perpendicular to the rotational direction of the paper feed belt  1 , thereby to change the sections to which the high-voltage power supply  121  applies voltage. Accordingly, the paper sheet P is attracted and fed by using a proper amount of electric power for the size and basis weight of the paper sheet P. Compared to the case in which the paper sheet P is conveyed by applying voltage to all the electrodes embedded in the paper feed belt  1 , for example, power consumption is reduced to three tenths when feeding an A5 portrait paper sheet P, and power consumption is reduced to a half when feeding an A3 portrait paper sheet P. Further, the high-voltage power supply  121  may be powered off while feeding is not performed during an interval between paper sheets, which achieves further reduction in power consumption. 
     The description of the second embodiment has been given with respect to an example in which the paper feed belt  1  is divided into the three sections A through C in the direction perpendicular to the rotational direction. This is not a limiting example, and the number of divided sections may be two or more. The number of sections may be increased to properly cope with an increased variety of paper sheet sizes and types, and voltage is applied in response to the size of a paper sheet P to feed the paper sheet P by use of proper electric power. 
     Third Embodiment 
     In the following, a third embodiment will be described with reference to the accompanying drawings. A description will be omitted of the same elements as those of the embodiments already described. 
     The paper feed apparatus  100  of the third embodiment differs from that of the first embodiment and the second embodiment in that the paper feed belt  1  is divided into sections in the rotational direction as well as in the direction perpendicular to the rotational direction. 
     The paper feed belt  1  of the third embodiment is divided into four areas in the rotational direction and further divided into three sections in the direction perpendicular to the rotational direction. In total, twelve sections A1, B1, C1, . . . , A4, B4, and C4 are created. 
       FIG. 9  is a drawing illustrating an example of the configuration of the paper feed belt  1  used by the paper feed apparatus  100  of the third embodiment.  FIG. 9  is a plan view of the paper feed belt  1  as viewed from the inner side of the loop to show the areas A1, B1, C1, A2, B2, and C2 of the paper feed belt  1 . 
     As illustrated in  FIG. 9 , the paper feed belt  1  has in each of the divided sections a pair of comb-shaped electrodes, which are arranged such that the teeth of one electrode extend in the direction perpendicular to the rotational direction into the spaces between the teeth of the other electrode. 
     The electrodes provided in the respective sections are partially covered with the insulating layer  190 , and are exposed toward the inner side of the paper feed belt  1  through openings that are formed at side ends of the electrodes opposite to each other in the direction perpendicular to the rotational direction. 
     Fixed terminals are disposed on the inner side of the paper feed belt  1  to come in contact with the side ends of the electrodes at the position facing the paper sheets P stored in the paper feed cassette  9 . Each of the fixed terminals is connected either to the high-voltage power supply or to ground through relays whose state of connection is changed by the control unit. 
     The control unit changes the state of connection of the relays situated between the high-voltage power supply and the fixed terminals in response to the size of or the size and basis weight of the paper sheet P to be fed by the paper feed apparatus  100 , thereby changing the electrodes to which the high-voltage power supply applies voltage at the time of feeding the paper sheet P. 
     In this manner, electrodes are provided in the sections into which the paper feed belt  1  is divided in the rotational direction as well as in the direction perpendicular to the rotational direction, and the electrodes to which voltage is applied are selected in response to the size, basis weight, and/or the like of the paper sheet P. This arrangement can feed paper sheets P of various sizes and types by use of proper electric power while reducing needless power consumption. 
     Fourth Embodiment 
     In the following, a fourth embodiment will be described with reference to the accompanying drawings. A description will be omitted of the same elements as those of the embodiments already described. 
       FIG. 10  is a drawing illustrating an example of the configuration of an image forming system  400  according to the fourth embodiment. 
     The image forming system  400  of the fourth embodiment is a production printing system in which an image forming apparatus  200  is provided with peripheral units having functions such as feeding, folding, stapling, cutting, etc., which may be combined at the time of use according to need. The image forming system  400  of the fourth embodiment includes a large volume paper feed unit  211 , an inserter  212 , a folding unit  213 , a finisher  214  for stapling and punching, a cutter  215 , and an information processing apparatus  300 , which are connected to the image forming apparatus  200 . 
       FIG. 11  is a drawing illustrating an example of the hardware configuration of the image forming system  400  according to the fourth embodiment. 
     The information processing apparatus  300  includes a network interface unit  301 , a ROM  302 , a RAM  303 , a CPU  304 , an HDD  305 , and an interface unit  306 , which are connected to each other through a bus B1. The information processing apparatus  300  is connected to the image forming apparatus  200  through a dedicated line  210 . 
     The CPU  300  of the information processing apparatus  300  is a computing unit that performs control operations, arithmetic operations, and data processing by executing programs stored in the ROM  302  and the RAM  303 . Further, the CPU  304  performs overall control of the apparatus by executing programs, and also serves as a control unit of a paper feed unit  403  of the image forming apparatus  200 . 
     The HDD  305  is a nonvolatile storage device to store various programs and data. The stored programs and data include an OS (Operating System) and application programs providing various functions. 
     The ROM  302  is a nonvolatile semiconductor memory (storage device) that can retain stored data even when the power is switched off. The RAM  303  is a volatile semiconductor memory (storage device) that temporarily stores programs and data. 
     The network interface unit  301  is an interface with peripheral devices such as PCs  501  through  503  having a communication function that are connected through a network  500  such as a LAN or WAN, which is implemented by use of wired and/or wireless data transmission lines. 
     The interface unit  306  is used to connect the information processing apparatus  300  to the image forming apparatus  200 , and is connected to an interface unit  403  of the image forming apparatus  200  through the dedicated line  210 . 
     The image forming apparatus  200  connected to the information processing apparatus  300  through the dedicated line  210  includes a printing unit  401 , a display unit  402 , a paper feed unit  403 , an interface unit  404 , an operation unit  405 , and a miscellaneous interface unit  406 , which are connected to each other through a bus B2. 
     The printing unit  401  includes a photosensitive unit, a fuser unit, etc., and forms an image on the surface of a paper sheet P based on image data. 
     The display unit  402  and the operation unit  405  are formed of key switches (i.e., hard keys) and an LCD (liquid crystal display) that has a touch panel function (inclusive of software keys implemented by graphical user interface). The display unit  402  and the operation unit  405  are a display and input apparatus serving as a user interface that is used to exploit the functions of the image forming apparatus  200 . Using the display unit  402  and the operation unit  405 , a user may enter information indicative of the size or the like of the paper sheet P to be printed. 
     The paper feed unit  403  is the paper feed apparatus  100  of the first embodiment, and feeds a paper sheet P stored in a paper feed cassette to the printing unit  401 . 
     The interface unit  404  is used for connection to the information processing apparatus  300 , and is connected to the interface unit  306  of the information processing apparatus  300  through the dedicated line  210 . 
     The image forming system  400  having the configuration described above uses the CPU  304  of the information processing apparatus  300  to control the state of connection of relays situated between the electrodes of the paper feed belt  1  and the high-voltage power supply in the paper feed unit  403  of the image forming apparatus  200 . This arrangement serves to feed a paper sheet P by use of optimal conditions for applying voltage. 
     It is preferable to connect the image forming apparatus  200  to the information processing apparatus  300  through the dedicated line  210  since such a connection allows the paper feed unit  403  of the image forming apparatus  200  to be controlled at high speed. As long as the control of the paper feed unit  403  is performed at high speed, the connection may be provided by use of a network or the like. 
     The paper feed unit  403  of the image forming apparatus  200  may be the paper feed apparatus  100  of the second or third embodiment. Regardless of the configuration used, a paper sheet P can be fed under proper electric power conditions without consuming needless electric power in response to the size, basis weight, and/or the like of the paper sheet P. 
     The function of the CPU  304  of the information processing apparatus  300  to serve as a control unit to control the paper feed unit  403  of the image forming apparatus  200  may be provided in the image forming apparatus  200 , or may be provided in the PC  501  or the like connected through a network or the like. 
     Although the present invention has been described heretofore by referring to one or more embodiments, the present invention is not limited to such embodiments. Various variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority application No. 2012-052869 filed on Mar. 9, 2012, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.