Patent Publication Number: US-10308451-B2

Title: Sheet feeding device, image forming apparatus, and method for controlling sheet feeding device

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2017-160900 filed Aug. 24, 2017, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure relates to a sheet feeding device that feeds paper sheets. In addition, the present disclosure relates to an image forming apparatus including the sheet feeding device. In addition, the present disclosure relates to a method for controlling the sheet feeding device. 
     There are image forming apparatuses such as a multifunction peripheral, a copier, a printer, and a facsimile machine. The image forming apparatus stores paper sheets. For example, paper sheets are housed in a sheet cassette. When performing printing, the paper sheets are fed. The image forming apparatus (sheet feeding device) may perform detection about the paper sheets. There is a known example of a device that detects remaining quantity of paper sheets by using a sensor described below. 
     Specifically, there is a known sheet feed control device, which makes pressure contact with the top surface of paper sheets stacked on a sheet placing plate so as to feed a paper sheet, identifies stack quantity of paper sheets based on an induced voltage generated by an induction coil when the induction coil and electromagnetic field generation means are disposed at corresponding positions and one of them is disposed to move according to movement of the sheet placing plate, and determines presence or absence of a sheet feed cassette based on a detection signal output from a cassette detection unit. 
     The image forming apparatus includes the sheet feeding device. The sheet feeding device may include a sheet cassette. A bundle of paper sheets is set in the sheet cassette. To replenish or change paper sheets, the sheet cassette is detachable and attachable. When paper sheets run out, the sheet cassette is pulled out from the image forming apparatus. After paper sheets are replenished, the sheet cassette is inserted into the image forming apparatus. 
     During a period while the sheet cassette is detached, paper sheets cannot be fed. In other words, during a period while the sheet cassette is not attached, printing cannot be performed. Accordingly, a sensor for detecting whether or not the sheet cassette is attached is usually provided. In addition, if the sheet cassette is insufficiently inserted, sheet jamming may occur. Therefore, a contact-type switch is used as a sensor for detecting an attached or detached state. For example, a part of the contact-type switch contacts with a case of the sheet cassette. Using the contact-type sensor, it can be checked whether or not the sheet cassette is sufficiently inserted. 
     In addition, the sheet feeding device is equipped with a plurality of sensors other than the attachment/detachment detection sensor. For example, a sensor for detecting a sheet size and a sensor for detecting remaining quantity of paper sheets are disposed. As these sensors, a sensor including an actuator, and a plurality of optical sensors are used. One or more sensors are used for one detection item. There is a problem that an increase in the number of sensors causes an increase in development time and effort and in production cost. 
     In the known technique described above, one or more sensors are used for detecting remaining quantity of paper sheets. In addition, a cassette detection unit for detecting presence or absence of the sheet feed cassette is disposed separately. There is no description about a try to reduce the number of sensors, and hence the problem described above cannot be solved. 
     SUMMARY 
     A sheet feeding device according to the present disclosure includes a cassette, a first sensor unit, a first moving mechanism, a storage unit, and a control unit. The cassette includes a sheet placing plate having an upper surface on which paper sheets are set. The cassette is detachable and attachable. The first sensor unit includes a first conductive plate and a first coil circuit board on which a coil pattern is printed. The first coil circuit board is applied with a voltage so that a magnetic field is generated. The first sensor unit outputs a first output value corresponding to a position of the first conductive plate. The first moving mechanism moves the first conductive plate so that a facing area between the first conductive plate and the first coil circuit board is increased or decreased according to remaining quantity of paper sheets in the cassette. The storage unit stores remaining quantity detection data for determining current remaining quantity of paper sheets corresponding to the first output value. The storage unit stores a reference value for determining whether or not the cassette is attached. The control unit recognizes a magnitude of the first output value. The first conductive plate is attached to the cassette. The first coil circuit board is not attached to the cassette but is disposed at a position facing the first conductive plate in a non-contact manner when the cassette is attached. The control unit determines the current remaining quantity of paper sheets based on the magnitude of the first output value and the remaining quantity detection data. The control unit determines whether or not the cassette is attached based on the magnitude of the first output value and the reference value. 
     Further features and advantages of the present disclosure will become apparent from the description of embodiments given below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating one example of a multifunction peripheral according to an embodiment. 
         FIG. 2  is a diagram illustrating one example of a sheet feeding unit according to the embodiment. 
         FIG. 3  is a diagram illustrating one example of a sheet feeding device according to the embodiment. 
         FIG. 4  is a diagram illustrating one example of a lifting mechanism according to the embodiment. 
         FIG. 5  is a diagram illustrating one example of a sensor unit according to the embodiment. 
         FIG. 6  is a diagram illustrating one example of a first coil circuit board according to the embodiment. 
         FIG. 7  is a diagram illustrating one example of a first moving mechanism according to the embodiment. 
         FIG. 8  is a diagram illustrating a first output value of a first sensor unit according to the embodiment. 
         FIG. 9  is a flowchart illustrating one example of a flow of detecting remaining quantity of paper sheets according to the embodiment. 
         FIG. 10  is a table showing one example of remaining quantity detection data according to the embodiment. 
         FIG. 11  is a flowchart illustrating one example of a flow of calculating remaining number of sheets according to the embodiment. 
         FIG. 12  is a flowchart illustrating one example of a flow of determining whether the cassette is attached or detached according to the embodiment. 
         FIG. 13  is a flowchart illustrating one example of a flow of an update process of a reference value according to the embodiment. 
         FIG. 14  is a diagram illustrating one example of a second coil circuit board and a third coil circuit board according to the embodiment. 
         FIG. 15  is a diagram illustrating one example of the output values of the sensor units corresponding to positions of a second conductive plate and a third conductive plate according to the embodiment. 
         FIG. 16  is a diagram illustrating one example of a second moving mechanism according to the embodiment. 
         FIG. 17  is a diagram illustrating one example of a third moving mechanism according to the embodiment. 
         FIG. 18  is a flowchart illustrating one example of a flow of detecting a sheet size according to the embodiment. 
         FIG. 19  is a table showing one example of first sheet size data according to the embodiment. 
         FIG. 20  is a table showing one example of second sheet size data according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is aimed at enabling a sensor for detecting remaining quantity of paper sheets to also detect whether or not a cassette is attached. An embodiment of the present disclosure is described below with reference to  FIGS. 1 to 20 . Further, the following description exemplifies a multifunction peripheral  100  (corresponding to an image forming apparatus) including a sheet feeding device  1 . However, elements such as structures and layouts described in the embodiment are merely examples for description and should not be interpreted to limit the scope of the disclosure. 
     (Outline of Image Forming Apparatus) 
     First, with reference to  FIG. 1 , the multifunction peripheral  100  according to the embodiment is described. The multifunction peripheral  100  includes a control unit  2  and a storage unit  3 . The control unit  2  integrally controls operation of the entire apparatus. The control unit  2  controls individual units of the multifunction peripheral  100 . The control unit  2  includes a CPU  21  and an image processing unit  22 . The CPU  21  performs calculation and control. The image processing unit  22  performs image processing necessary for printing on image data. The storage unit  3  includes storage devices such as a ROM, a RAM, and an HDD. The storage unit  3  stores control programs and data. 
     The control unit  2  is connected to a document feeding unit  4   a  and an image reading unit  4   b  in a communicable manner. The document feeding unit  4   a  feeds a set document (sheet) to a reading position. The image reading unit  4   b  reads the document fed by the document feeding unit  4   a  or a document set on a document table (or contact glass, not shown). The image reading unit  4   b  generates image data of the document. The control unit  2  controls operations of the document feeding unit  4   a  and the image reading unit  4   b.    
     The control unit  2  is connected to an operation panel  5  in a communicable manner. The operation panel  5  includes a display panel  51 , a touch panel  52 , and a hardware key  53 . For example, the hardware key  53  is a start key. The control unit  2  controls display of the display panel  51 . The control unit  2  controls the display panel  51  to display information. The information to be displayed is, for example, a setting screen, a status of the multifunction peripheral  100 , or a message. The control unit  2  controls the display panel  51  to display an operation image. The operation image is a software key or button. On the basis of an output of the touch panel  52 , the control unit  2  recognizes an operated operation image. In addition, the control unit  2  recognizes the hardware key  53  operated. The control unit  2  controls the display panel  51  to switch to a screen corresponding to the operated operation image or hardware key  53 . In addition, the control unit  2  controls the multifunction peripheral  100  to operate according to setting with the operation panel  5 . 
     The multifunction peripheral  100  includes a printing unit  6 . The printing unit  6  includes a sheet feeding unit  6   a , a conveying unit  6   b , an image forming unit  6   c , and a fixing unit  6   d . The control unit  2  controls the printing unit  6 . The printing unit  6  performs sheet feeding, sheet conveying, toner image forming, transferring, and fixing. In other words, the control unit  2  controls operations of the sheet feeding unit  6   a , the conveying unit  6   b , the image forming unit  6   c , and the fixing unit  6   d . Specifically, the control unit  2  controls the sheet feeding unit  6   a  to feed paper sheets one by one. The control unit  2  controls the conveying unit  6   b  to convey the fed paper sheet to a discharge tray (not shown) via the image forming unit  6   c  and the fixing unit  6   d . The control unit  2  controls the image forming unit  6   c  to form a toner image to be on the paper sheet conveyed by the conveying unit  6   b . The control unit  2  controls to transfer the toner image onto the paper sheet. The control unit  2  controls the fixing unit  6   d  to fix the toner image transferred onto the paper sheet. 
     The multifunction peripheral  100  includes a communication unit  23 . The communication unit  23  is an interface for communication. The communication unit  23  communicates with a computer  200 . The computer  200  is a PC or a server, for example. The communication unit  23  communicates with the computer  200  via a network. The communication unit  23  receives print data from the computer  200 . The print data contains image data or the like indicating print content and print setting data. The control unit  2  controls the printing unit  6  to perform printing based on the print data. 
     (Sheet Feeding Unit  6   a ) 
     Next, with reference to  FIG. 2 , the sheet feeding unit  6   a  according to the embodiment is described. The sheet feeding unit  6   a  stores a plurality of paper sheets. The sheet feeding unit  6   a  sends out paper sheets one by one. The sheet feeding unit  6   a  includes a cassette  61  and a sheet feeding mechanism  62 . The cassette  61  can be pulled out from the multifunction peripheral  100 . After the cassette  61  pulled out, paper sheets can be replenished or paper sheets can be changed. 
     The cassette  61  includes a sheet placing plate  63 , a width regulation cursor pair  64  (only one of cursors is shown in  FIG. 2 ), and a rear end regulation cursor  65 . Paper sheets (bundle of paper sheets) are set on the sheet placing plate  63 . A supporting part  66  supports an upstream end (left end in  FIG. 2 ) of the sheet placing plate  63  in a rotatable manner. The sheet placing plate  63  is rotatable in an up and down direction. A downstream end (right end in  FIG. 2 ) of the sheet placing plate  63  is a free end. 
     A lifting mechanism  67  is disposed below the downstream end of the sheet placing plate  63 . The lifting mechanism  67  moves the sheet placing plate  63  upward. The lifting mechanism  67  includes a lifting motor  67   a  (see  FIG. 3 ), a drive shaft  67   b , and a lifting member  67   c . The lifting member  67   c  has a plate-like shape. The lifting member  67   c  is secured to the drive shaft  67   b . The drive shaft  67   b  rotated by drive of the lifting motor  67   a . When rotating the lifting member  67   c , the control unit  2  controls the lifting motor  67   a  to operate. As a result, the drive shaft  67   b  rotates, and the tip end of the lifting member  67   c  move upward. When the lifting member  67   c  is rotated, the downstream end of the sheet placing plate  63  is lifted upward. 
     The width regulation cursor pair  64  can be moved to slide in a direction perpendicular to the conveying direction. Width regulation cursors  64   a  of the width regulation cursor pair  64  move together with each other. The width regulation cursors  64   a  contact with the set paper sheets so that the position of the paper sheets is regulated. The rear end regulation cursor  65  can be moved to slide in the conveying direction. The rear end regulation cursor  65  contacts with the set paper sheets. The rear end regulation cursor  65  regulates a rear end position of the paper sheets. 
     The sheet feeding mechanism  62  includes a sheet feed roller  62   a  and a handling roller pair  62   b . The sheet feed roller  62   a  is disposed above the downstream end of the sheet placing plate  63 . The handling roller pair  62   b  is disposed on the downstream side of the sheet feed roller  62   a  in the conveying direction. The handling roller pair  62   b  prevents double feeding of paper sheets. The upper roller of the handling roller pair  62   b  rotates to feed a paper sheet in the forward direction. The lower roller rotates to feed a paper sheet in the reverse direction (toward the cassette). 
     (Sheet Feeding Device  1 ) 
     With reference to  FIGS. 2 to 4 , the sheet feeding device  1  according to the embodiment is described. The sheet feeding device  1  includes the sheet feeding unit  6   a , the control unit  2 , and the storage unit  3 . The control unit  2  is also a unit that controls the sheet feeding device  1 . The storage unit  3  is also a unit that stores data related to the sheet feeding device  1 . 
     The sheet feeding unit  6   a  includes the cassette  61 , the sheet feed roller  62   a , the lifting mechanism  67 , a sensor unit  7 , and a moving mechanism  9 . The sensor unit  7  includes a first sensor unit  71 , a second sensor unit  72 , and a third sensor unit  73 . The first sensor unit  71  is a unit for detecting remaining quantity of paper sheets set in the cassette  61  (sheet placing plate  63 ). In addition, the first sensor unit  71  is also a unit for determining whether or not the cassette  61  is attached. The second sensor unit  72  and the third sensor unit  73  are units for detecting a size of paper sheets set in the sheet feeding unit  6   a  (cassette  61 ). Details of the sensor unit  7  are described later. 
     The rotation shaft of the sheet feed roller  62   a  is supported by a shaft support member  68 . The shaft support member  68  is put on the rotation shaft of the handling roller pair  62   b . With the shaft support member  68 , the sheet feed roller  62   a  swings in the up and down direction. Along with moving up and down of the sheet feed roller  62   a , the shaft support member  68  swings in the up and down direction. An upper limit sensor S 1  is provided to the sheet feeding device  1 . The upper limit sensor S 1  detects that the sheet feed roller  62   a  has reached a predetermined upper limit by movement of the sheet placing plate  63 . 
     When the downstream end of the sheet placing plate  63  moves upward, the sheet feed roller  62   a  contacts with the upper most paper sheet. When the sheet placing plate  63  is further moved upward, a position of the sheet feed roller  62   a  is also raised. The sheet placing plate  63  raises the sheet feed roller  62   a . The upper limit sensor S 1  detects that the sheet feed roller  62   a  has reached the upper limit position. Therefore, when the sheet feed roller  62   a  is at the upper limit position, the sheet placing plate  63  is also at the upper limit. The upper limit position changes depending on thickness of the bundle of paper sheets set currently. 
     The upper limit sensor S 1  is, for example, a transmission type optical sensor. The upper limit sensor S 1  changes its signal output level (high level or low level) depending on whether or not the sheet feed roller  62   a  is at the upper limit position. The sheet feed roller  62   a  or the shaft support member  68  is provided with a protrusion  69 . When the sheet feed roller  62   a  reaches the upper limit position, the protrusion  69  blocks an optical path between a light emitting part and a light receiving part of the upper limit sensor S 1  (optical sensor). The control unit  2  recognized that the sheet feed roller  62   a  has reached the upper limit based on the output of the upper limit sensor  51 . When recognizing the reaching to the upper limit, the control unit  2  stops the lifting motor  67   a.    
     With reference to  FIG. 4 , the lifting mechanism  67  is described. The lifting motor  67   a  is disposed outside the cassette  61  (on the main body side). The longitudinal direction of the drive shaft  67   b  is perpendicular to the paper sheet conveying direction. The drive shaft  67   b  is coupled to the lifting motor  67   a  via a coupling part  67   d . The coupling part  67   d  is disposed on a drive transmission path. The control unit  2  controls the lifting motor  67   a  to operate. In this case, the lifting motor  67   a  rotates the drive shaft  67   b  (lifting member  67   c ) in a direction where the sheet placing plate  63  moves upward. 
     When the cassette  61  is pulled out frontward, the coupling part  67   d  is separated. In this way, coupling between the lifting motor  67   a  and the drive shaft  67   b  is released. In other words, coupling between the coupling part  67   d  and the drive shaft  67   b  is released. As a result, the drive transmission path is disconnected. When the cassette  61  is detached (coupling is released), the sheet placing plate  63  is automatically moved downward by gravity action. The lifting mechanism  67  utilizes the gravity action to move the sheet placing plate  63  downward. Finally, the sheet placing plate  63  moves down to a lower limit position. The lifting mechanism  67  moves the sheet placing plate  63  and the lifting member  67   c  downward to the lower limit position. The sheet placing plate  63  and the lifting member  67   c  fall flat. 
     In addition, when the cassette  61  is securely and fully inserted, the drive shaft  67   b  is inserted into the coupling part  67   d . The coupling part  67   d  makes coupling between the lifting motor  67   a  and the drive shaft  67   b . On the basis of an output value (first output value V 1 ) of the first sensor unit  71 , the control unit  2  recognizes that the cassette  61  has been attached. After this recognition, or when starting sheet feeding, the control unit  2  controls the lifting motor  67   a  to operate. The control unit  2  controls the sheet feed roller  62   a  and the sheet placing plate  63  to move to the upper limit position. Note that the control unit  2  controls the lifting motor  67   a  to briefly rotate every feeding of one or more paper sheets. When the paper sheets are consumed so that the sheet feed roller  62   a  moves down a little, the sheet feed roller  62   a  is moved again up to the upper limit position. 
     When sending out the paper sheet, the control unit  2  controls a sheet feeding motor  62   c  to rotate. In this way, the sheet feed roller  62   a  and the handling roller pair  62   b  rotate. The sheet feed roller  62   a  and the handling roller pair  62   b  send the paper sheet to the downstream. The conveying unit  6   b  is provided with a plurality of conveying roller pairs  6   br . The conveying roller pairs  6   br  convey the paper sheet (see  FIG. 2 ).  FIG. 2  shows only one conveying roller pair  6   br  for convenience sake. When performing continuous printing on a plurality of paper sheets, the control unit  2  controls the sheet feed roller  62   a  to repeat rotation and temporary stop so that a constant interval between paper sheets is secured. 
     In addition, the sheet feeding unit  6   a  is provided with a set sensor S 2 . The set sensor S 2  is a sensor for detecting whether or not paper sheets are set (for example, optical sensor). An output level of the set sensor S 2  when paper sheets are set is different from that when paper sheets are not set (high level or low level). On the basis of the output of the set sensor S 2 , the control unit  2  can detect whether or not paper sheets are set in the cassette  61 . When there is no paper sheet, the control unit  2  controls the display panel  51  to display run out of paper sheets. 
     (Sensor Unit  7 ) 
     Next, with reference to  FIG. 5 , the sensor unit  7  included in the sheet feeding device  1  according to the embodiment is described. The sensor unit  7  includes the first sensor unit  71 , the second sensor unit  72 , and the third sensor unit  73 . The first sensor unit  71  includes a part for detecting remaining quantity of paper sheets set in the sheet feeding device  1  (sheet feeding unit  6   a ). The first sensor unit  71  includes a first circuit part  71   a , a first coil circuit board L 1 , a first capacitor C 1 , and a first conductive plate  81 . The first capacitor C 1  has a predetermined capacitance. The first capacitor C 1  and the first coil circuit board L 1  are connected in parallel to terminals of the first circuit part  71   a . The first coil circuit board L 1  and the first capacitor C 1  form a first resonant circuit  71   e . The first coil circuit board L 1  is a circuit board on which a coil pattern P 1  is printed (details are described later). The first conductive plate  81  is a plate having electrical conductivity. A metal plate such as a stainless steel plate or an aluminum plate can be used as the first conductive plate  81 . The first conductive plate  81  has a substantially triangular shape (shape like a part of a crescent moon). 
     When the cassette  61  is attached, the first conductive plate  81  faces the first coil circuit board L 1  in a non-contact manner. In addition, the first conductive plate  81  is moved by a first moving mechanism  9   a  along with upward movement of the sheet placing plate  63 . When the first conductive plate  81  moves, a facing area between the first coil circuit board L 1  and the first conductive plate  81  is changed. According to a position of the first conductive plate  81 , a magnitude of eddy current generated in the first conductive plate  81  or an inductance of the first coil circuit board L 1  is changed. As a result, a resonance frequency of the first resonant circuit  71   e  is changed according to a position of the first conductive plate  81 . 
     The first circuit part  71   a  includes a first input signal generating circuit  71   b , a first frequency detecting circuit  71   c , and a first output circuit  71   d . The first input signal generating circuit  71   b  supplies the first resonant circuit  71   e  (first coil circuit board L 1 ) with current (pulse signal) so that resonance occurs. The first frequency detecting circuit  71   c  counts a period of a signal waveform of the first resonant circuit  71   e . The first frequency detecting circuit  71   c  detects a resonance frequency of the first resonant circuit  71   e . The first output circuit  71   d  outputs a digital value corresponding to the resonance frequency (period count value) of the first resonant circuit  71   e , as the first output value V 1 . The first sensor unit  71  applies a voltage to the first coil circuit board L 1  so as to generate a magnetic field. The first sensor unit  71  outputs the first output value V 1  corresponding to a position of the first conductive plate  81 . The first output value V 1  is input to the control unit  2 . The control unit  2  recognizes a magnitude of the first output value V 1 . 
     The second sensor unit  72  detects a size in a direction perpendicular to the conveying direction. The second sensor unit  72  includes a second circuit part  72   a , a second coil circuit board L 2 , a second capacitor C 2 , and a second conductive plate  82 . The second capacitor C 2  has a predetermined capacitance. The second capacitor C 2  and the second coil circuit board L 2  are connected in parallel to terminals of the second circuit part  72   a . The second coil circuit board L 2  and the second capacitor C 2  form a second resonant circuit  72   e . The second coil circuit board L 2  is a circuit board on which a coil pattern P 2  is printed (details are described later). The second conductive plate  82  is a plate having electrical conductivity. A metal plate such as a stainless steel plate or an aluminum plate can be used as the second conductive plate  82 . The second conductive plate  82  has a width in the moving direction smaller than that in the longitudinal direction of the second coil circuit board L 2  (details of movement of the second conductive plate  82  are described later). 
     When the cassette  61  is attached, the second conductive plate  82  faces the second coil circuit board L 2  in a non-contact manner. The second conductive plate  82  is moved by a second moving mechanism  9   b  in the longitudinal direction of the second coil circuit board L 2 . The inductance of the second coil circuit board L 2  is changed according to a position of the second conductive plate  82 . The resonance frequency of the second resonant circuit  72   e  is changed according to a position of the second conductive plate  82 . 
     The second circuit part  72   a  includes a second input signal generating circuit  72   b , a second frequency detecting circuit  72   c , and a second output circuit  72   d . The second input signal generating circuit  72   b  supplies the second resonant circuit  72   e  (second coil circuit board L 2 ) with current (pulse signal). In this way, the second resonant circuit  72   e  is resonated. The second frequency detecting circuit  72   c  counts a period of a signal waveform of the second resonant circuit  72   e . The second frequency detecting circuit  72   c  detects a resonance frequency of the second resonant circuit  72   e . The second output circuit  72   d  outputs a second output value V 2 . The second output circuit  72   d  outputs a digital value corresponding to the resonance frequency (period count value) of the second resonant circuit  72   e , as the second output value V 2 . The second sensor unit  72  applies the second coil circuit board L 2  with a voltage so that a magnetic field is generated. The second sensor unit  72  outputs the second output value V 2  based on the resonance frequency corresponding to a position of the second conductive plate  82 . The second output value V 2  is input to the control unit  2 . The control unit  2  recognizes a magnitude of the second output value V 2 . 
     In addition, the third sensor unit  73  detects a size in a direction parallel to the conveying direction. The third sensor unit  73  includes a third circuit part  73   a , a third coil circuit board L 3 , a third capacitor C 3 , and a third conductive plate  83 . The third capacitor C 3  has a predetermined capacitance. The third capacitor C 3  and the third coil circuit board L 3  connected in parallel to terminals of the third circuit part  73   a . The third coil circuit board L 3  and the third capacitor C 3  form a third resonant circuit  73   e . The third coil circuit board L 3  is a circuit board on which a coil pattern P 3  is printed (details are described later). The third conductive plate  83  is also a plate having electrical conductivity. A metal plate such as a stainless steel plate or an aluminum plate can be used as the third conductive plate  83 . The third conductive plate  83  has a width in the moving direction smaller than that in the longitudinal direction of the third coil circuit board L 3  (details of movement of the third conductive plate  83  are described later). 
     When the cassette  61  is attached, the third conductive plate  83  faces the third coil circuit board L 3  in a non-contact manner. The third conductive plate  83  is moved by a third moving mechanism  9   c  in the longitudinal direction of the third coil circuit board L 3  (details are described later). The inductance of the third coil circuit board L 3  is changed according to a position of the third conductive plate  83 . The resonance frequency of the third resonant circuit  73   e  is changed according to a position of the third conductive plate  83 . 
     The third circuit part  73   a  includes a third input signal generating circuit  73   b , a third frequency detecting circuit  73   c , and a third output part  73   d . The third input signal generating circuit  73   b  supplies the third resonant circuit  73   e  (third coil circuit board L 3 ) with current (pulse signal). In this way, the third resonant circuit  73   e  is resonated. The third frequency detecting circuit  73   c  counts a period of a signal waveform of the third resonant circuit  73   e . The third frequency detecting circuit  73   c  detects a resonance frequency of the third resonant circuit  73   e . The third output part  73   d  outputs a third output value V 3 . The third output part  73   d  outputs a digital value corresponding to the resonance frequency (period count value) of the third resonant circuit  73   e , as the third output value V 3 . The third sensor unit  73  applies the third coil circuit board L 3  with a voltage so that a magnetic field is generated. The third sensor unit  73  outputs the third output value V 3  corresponding to the resonance frequency according to a position of the third conductive plate  83 . The third output value V 3  is input to the control unit  2 . The control unit  2  recognizes a magnitude of the third output value V 3 . 
     (Outline of Detection of Remaining Quantity of Paper Sheets) 
     Next, with reference to  FIGS. 6, 7, and 8 , detection of remaining quantity of paper sheets in the sheet feeding device  1  according to the embodiment is described. The first coil circuit board L 1  is a circuit board on which the coil pattern P 1  is printed. As illustrated in  FIG. 6 , the coil pattern P 1  of the first coil circuit board L 1  has a circular spiral shape. The first coil circuit board L 1  may be a lamination of a plurality of layers of coil patterns P 1 . 
     As illustrated in  FIG. 7 , a fan-shaped rotation plate  10  is fixed to the drive shaft  67   b , as the first moving mechanism  9   a . A rotation angle of the rotation plate  10  is changed according to the rotation angle of the drive shaft  67   b . The first conductive plate  81  is attached to the rotation plate  10 . 
     The first conductive plate  81  is provided to the cassette  61 . The first coil circuit board L 1  is disposed outside the cassette  61 . As the cassette  61  is pulling out, the first conductive plate  81  is separating from the first coil circuit board L 1 . When the cassette  61  is attached, the first coil circuit board L 1  is disposed at a position facing the first conductive plate  81  in a non-contact manner. In other words, when the cassette  61  is attached, the first coil circuit board L 1  and the first conductive plate  81  (rotation plate  10 ) face each other with a short distance (predetermined distance). The first conductive plate  81  is attached to the rotation plate  10  so that the most acute angle part (the tip part of the triangle or crescent) of the first conductive plate  81  is positioned upward. In  FIG. 7 , a broken line illustrates one example of an attachment position of the first coil circuit board L 1  viewed from the horizontal direction. The predetermined distance (between the surface of the first coil circuit board L 1  and the surface of the first conductive plate  81 ) is approximately a few millimeters to five millimeters. 
     When the drive shaft  67   b  rotates, the facing area between the first conductive plate  81  and the first coil circuit board L 1  is changed.  FIG. 7  illustrates one example of the position of the first conductive plate  81  when paper sheets are fully stored. In  FIG. 7 , the facing area between the first conductive plate  81  and the first coil circuit board L 1  is relatively small. Along with consumption of paper sheets, the drive shaft  67   b  rotates. The first conductive plate  81  (sheet placing plate  63 ) moves upward (in the direction to the first coil circuit board L 1 ). The first conductive plate  81  approaches the center of the first coil circuit board L 1 . In this way, the facing area between the first conductive plate  81  and the first coil circuit board L 1  increases. 
     A winding quantity of the coil pattern P 1  facing the first conductive plate  81  is changed according to the position of the first conductive plate  81  (height of the sheet placing plate  63 ). As illustrated in  FIG. 8 , as the sheet placing plate  63  moves upper, the winding quantity of the coil facing the first conductive plate  81  becomes more. In other words, as the sheet placing plate  63  moves upper, the facing area between the first coil circuit board L 1  and the first conductive plate  81  becomes larger. 
     A quantity of eddy current in the first conductive plate  81  (magnitude of magnetic force) is changed according to the position of the first conductive plate  81 . Strength of magnetic coupling between the first coil circuit board L 1  and the first conductive plate  81  is also changed. An inductance (impedance) of the first coil circuit board L 1  is also changed. As a result, the first output value V 1  of the first circuit part  71   a  becomes a value corresponding to the position of the first conductive plate  81 . 
       FIG. 8  illustrates one example of the output of the first circuit part  71   a . In the sheet feeding device  1 , as there are more paper sheets in the cassette  61  (as the bundle of paper sheets is thicker, or as the sheet placing plate  63  is lower, or as the distance between the sheet placing plate  63  and the sheet feed roller  62   a  is larger), the first output value V 1  is smaller. On the contrary, as there are fewer paper sheets in the cassette  61  (as the bundle of paper sheets is thinner, or as the sheet placing plate  63  is higher, or as the distance between the sheet placing plate  63  and the sheet feed roller  62   a  is smaller), the first output value V 1  is larger. In addition, the first conductive plate  81  has a substantially triangular shape. In this way, in proportion to the remaining quantity of paper sheets (height of the sheet placing plate  63 ), the first output value V 1  is changed. In other words, a ratio between a variation in height of the sheet placing plate  63  (remaining quantity of paper sheets) and a variation in the first output value V 1  is constant. 
     (Flow of Detection of Remaining Quantity of Paper Sheets) 
     Next, with reference to  FIGS. 9 and 10 , one example of the detection of remaining quantity of paper sheets according to the embodiment is described. In the sheet feeding device  1  of the multifunction peripheral  100 , when a predetermined execution condition for detecting the remaining quantity is satisfied, the detection of remaining quantity of paper sheets is performed. The execution condition can be appropriately determined. The execution condition can be power-on of the multifunction peripheral  100 , canceling of power-saving mode (restart of power supply to the sheet feeding unit  6   a  and the first sensor unit  71 ), pulling out and insertion of the cassette  61 , start of a print job (sheet feeding from the sheet feeding unit  6   a ), finish of feeding the last paper sheet for the job, an instruction to the operation panel  5  to detect remaining quantity of paper sheets, or the like. 
     The flow of  FIG. 9  starts when the execution condition for detecting the remaining quantity is satisfied. The control unit  2  controls the sheet feed roller  62   a  (sheet placing plate  63 ) to move upward to the upper limit position (Step # 11 ). In this case, the control unit  2  controls the lifting motor  67   a  to operate. Note that when it is recognized that the sheet feed roller  62   a  is at the upper limit position based on the output of the upper limit sensor S 1  at a start time point of Step # 11 , Step # 11  may be skipped. 
     The control unit  2  controls the first circuit part  71   a  to operate (Step # 12 ). In this way, resonance occurs in the first resonant circuit  71   e . The first circuit part  71   a  outputs the first output value V 1  corresponding to the resonance frequency of the first resonant circuit  71   e  (corresponding to the remaining quantity of paper sheets, or corresponding to the thickness of set paper sheets) (Step # 13 ). 
     The control unit  2  recognizes a magnitude of the first output value V 1  (Step # 14 ). On the basis of remaining quantity detection data A 1  (stored in the storage unit  3 ) and the first output value V 1 , the control unit  2  determines the remaining quantity of set paper sheets (Step # 15 ). The remaining quantity detection data A 1  is stored in the storage unit  3 . The remaining quantity detection data A 1  is data for determining current remaining quantity of paper sheets based on the first output value V 1 . 
     With reference to  FIG. 10 , the remaining quantity detection data A 1  is described. As described above, the magnitude of the first output value V 1  is a value corresponding to the position of the first conductive plate  81  (height of the sheet placing plate  63  or thickness of the bundle of paper sheets). As illustrated in  FIG. 10 , a no sheet value and a full sheet value are defined in the remaining quantity detection data A 1 . The storage unit  3  stores the no sheet value as the remaining quantity detection data A 1 . The no sheet value is the first output value V 1  when the sheet feed roller  62   a  (sheet placing plate  63 ) is moved upward to the upper limit position in the state without paper sheets. The storage unit  3  also stores the full sheet value as the remaining quantity detection data A 1 . The full sheet value is the first output value V 1  when the sheet feed roller  62   a  (sheet placing plate  63 ) is moved upward to the upper limit position in the state with full paper sheets. 
     The number of paper sheets in the cassette  61  that is full is 500 for plain paper sheets. This corresponds to the fact that plain paper sheets available in general are wrapped as a unit of 500 sheets. Note that when the cassette  61  is pulled out, the sheet placing plate  63  falls flat. After that, approximately 500 paper sheets are set. A gap is provided between the top surface of the set bundle of paper sheets and the sheet feed roller  62   a . This is to prevent the top of the bundle of paper sheets from abutting the sheet feed roller  62   a  when the cassette  61  is restored. After setting paper sheets to full, it is necessary to move the sheet placing plate  63  upward until the uppermost paper sheet contacts with the sheet feed roller  62   a.    
     The full sheet value is set to a value of a lower limit value plus a reference variation. The lower limit value is the first output value V 1  in a state where the sheet placing plate  63  is fallen flat to the lower limit position. The reference variation is determined in advance. The reference variation is a standard variation of the first output value V 1  when the sheet feed roller  62   a  is moved upward to the upper limit position in the state where paper sheets is fully set. 
     As described above with reference to  FIG. 8 , the first output value V 1  has a constant gradient. A ratio between an upward movement amount of the sheet placing plate  63  (variation in thickness of paper sheets on the sheet placing plate  63 ) and an increased amount of the first output value V 1  is constant. The first output value V 1  is linearly changed with respect to a movement amount of the first conductive plate  81 . The change of the first output value V 1  is linear. Therefore, the control unit  2  determines the current remaining quantity of paper sheets using the recognized first output value V 1 , no sheet value, and full sheet value (Step # 15 ). The control unit  2  controls the display panel  51  to display the determined remaining quantity (ratio) (Step # 16 ). In this way, detection of remaining quantity of set paper sheets is finished. This flow is finished (END). 
     The control unit  2  divides an absolute value of a difference between the recognized first output value V 1  and the no sheet value by an absolute value of a difference between the no sheet value and the full sheet value. A ratio of the current thickness of the bundle of paper sheets to the thickness of the bundle of full paper sheets is determined. As they are proportional to each other, the remaining quantity detection data A 1  can also be a linear function of Z (remaining quantity of paper sheets)=a (gradient)×X (first output value V 1 )+b (intercept). In this case, the control unit  2  determines the remaining quantity of paper sheets by calculation using the linear function. 
     (Calculation of Remaining Number of Sheets) 
     Next, with reference to  FIG. 11 , calculation of the remaining number of sheets in the sheet feeding device  1  according to the embodiment is described. The first sensor unit  71  has a high resolution. The sheet feeding device  1  can determine a variation of the first output value V 1  corresponding to a thickness of one paper sheet. The control unit  2  divides the first output value V 1  corresponding to the thickness of the set bundle of paper sheets by the first output value V 1  corresponding to a thickness of one paper sheet. In this way, the remaining number of paper sheets in the cassette  61  is determined. 
     First, the flow of  FIG. 11  starts when the remaining number of paper sheets is calculated. In the sheet feeding device  1 , the calculation time point is when the paper sheet feeding is started. The calculation time point may be other time point. The control unit  2  obtains and recognizes the first output value V 1  before the sheet feeding is started (Step # 21 ). The control unit  2  performs the paper sheet feeding (Step # 22 ). After feeding one paper sheet, the control unit  2  controls the lifting motor  67   a  to operate so as to move the sheet feed roller  62   a  upward to the upper limit position (Step # 23 ). The control unit  2  obtains and recognizes the first output value V 1  in the state where sheet feed roller  62   a  is moved upward to the upper limit position (Step # 24 ). 
     The control unit  2  determines the paper sheet thickness (Step # 25 ). The paper sheet thickness is an absolute value of a difference between the first output value V 1  obtained in Step # 21  and the first output value V 1  obtained in Step # 24 . In other words, the control unit  2  determines the variation of the first output value V 1  corresponding to one paper sheet. Next, the control unit  2  determines the current remaining number of sheets (Step # 26 ). The control unit  2  divides an absolute value of a difference between the current first output value V 1  and the no sheet value by the paper sheet thickness. An absolute value of a difference between the no sheet value and the current first output value V 1  corresponds to a thickness of the remaining bundle of paper sheets. In addition, the paper sheet thickness corresponds to one paper sheet. In other words, the control unit  2  divides a value that corresponds to a thickness of the remaining bundle of paper sheets by a value that corresponds to one paper sheet. In this way, the remaining number of paper sheets in the cassette  61  is determined. 
     Note that it is possible to configure the operation panel  5  (touch panel  52  or hardware key  53 ) to accept an input of the thickness of the set paper sheets. Then, the remaining number of sheets may be determined based on the input thickness of the paper sheets. For example, when a predetermined operation is made, the control unit  2  controls the display panel  51  to display a screen for selecting a paper sheet thickness (paper type) from a plurality of types such as thick paper, normal paper, and thin paper. Then, the touch panel  52  accepts the selection of the paper type. The storage unit  3  stores the variation of the first output value V 1  when one sheet is fed, for each paper type. On the basis of the data stored in the storage unit  3 , the control unit  2  divides the absolute value of a difference between the current first output value V 1  and the no sheet value by the variation of the first output value V 1  corresponding to one sheet of the selected paper type. In this way, the current remaining number of sheets is determined. The control unit  2  controls the display panel  51  to display the determined remaining number of sheets (Step # 27 ). In this way, the detection of remaining number of sheets is finished. This flow is finished (END). 
     (Determination Whether or not Cassette  61  is Attached) 
     Next, with reference to  FIG. 12 , determination whether or not the cassette  61  is attached according to the embodiment is described. On the basis of an output of the first sensor unit  71  (first output value V 1 ), the control unit  2  determines whether or not the cassette  61  is attached. In the sheet feeding device  1 , detection whether or not the cassette  61  is attached (mounted or unmounted) is performed using the first sensor unit  71 . A dedicated sensor for detecting whether or not the cassette  61  is attached is not disposed. 
     During the period while the control unit  2  and the first sensor unit  71  are supplied with power, the control unit  2  periodically checks the first output value V 1 . Then, the control unit  2  periodically determines whether or not the cassette  61  is attached. The period is appropriately determined. The period may be any period from one second to ten and a few seconds. In addition, the period may be less than one second. The flow of  FIG. 12  starts when the control unit  2  and the first sensor unit  71  are supplied with power so that the control unit  2  is activated. In addition, the first sensor unit  71  can also be used in this state. In addition, during the period while the control unit  2  and the first sensor unit  71  are supplied with power, the process of  FIG. 12  is continuously performed. 
     First, the control unit  2  controls the first sensor unit  71  to operate (Step # 31 ). In other words, the control unit  2  controls the first sensor unit  71  to check the resonance frequency of the first resonant circuit  71   e . Then, the control unit  2  recognizes the first output value V 1  output from the first sensor unit  71  (Step # 32 ). In addition, the control unit  2  checks whether or not the first output value V 1  is smaller than a reference value A 2  (Step # 33 ). The reference value A 2  is determined in advance. The storage unit  3  stores the reference value A 2  in a nonvolatile manner (see  FIG. 1 ). The reference value A 2  is determined based on the lower limit value. In other words, the reference value A 2  is determined based on the first output value V 1  when the sheet placing plate  63  is at the lower limit position. The reference value A 2  may be the lower limit value. In addition, the reference value A 2  may be a value of the lower limit value plus a predetermined margin value for preventing misdetection. 
     As described above, as the facing area between the first coil circuit board L 1  and the first conductive plate  81  is larger, the first output value V 1  becomes larger. As the magnetic coupling between the first coil circuit board L 1  and the first conductive plate  81  is stronger, the first output value V 1  becomes larger. Therefore the first sensor unit  71  outputs the first output value V 1  of a larger value as a distance between the first coil circuit board L 1  and the first conductive plate  81  is smaller. In addition, the first sensor unit  71  outputs the first output value V 1  of a smaller value as the distance between the first coil circuit board L 1  and the first conductive plate  81  is larger. 
     The reference value A 2  is determined based on the lower limit value. The lower limit value is the lowest value of the first output value V 1  that can be output in the state where the cassette  61  is attached. Note that the lower limit value is not zero. When the cassette  61  is detached, the sheet placing plate  63  moves downward by gravity. Further, the distance between the first coil circuit board L 1  and the first conductive plate  81  is increased. Therefore, when the cassette  61  is detached (pulled out), the first output value V 1  becomes smaller than the reference value A 2 . 
     Further, it is possible that a reference value (first reference value) for detecting that the cassette  61  is detached is different from a reference value (second reference value) for detecting that the cassette  61  is attached (mounted). When the cassette  61  is attached, the first output value V 1  may be smaller than the reference value A 2  by a slight difference. In order to prevent misdetection that the cassette  61  is not attached, the second reference value may be smaller than the first reference value. In this case, the first reference value and the second reference value are stored in the storage unit  3 . When detecting that the cassette  61  is attached, the control unit  2  switches the reference value A 2  to be used to the first reference value. When the power is turned on or when detecting that the cassette  61  is not attached, the control unit  2  switches the reference value A 2  to be used to the second reference value. 
     The control unit  2  checks whether or not the first output value V 1  is smaller than the reference value A 2  (Step # 33 ). When the first output value V 1  is larger than or equal to the reference value A 2  (No in Step # 33 ), the control unit  2  determines that the cassette  61  is attached (Step # 34 ). Then, the flow returns to Step # 31 . 
     On the other hand, when the first output value V 1  is smaller than the reference value A 2  (Yes in Step # 33 ), the control unit  2  determines that the cassette  61  is not attached (Step # 35 ). When determining that the cassette  61  is not attached, the control unit  2  controls the display panel  51  to display a message informing that the cassette  61  is not attached (Step # 36 ). 
     Further, the control unit  2  controls the sheet feeding device  1  and the printing unit  6  to be in a print job inhibiting mode (Step # 37 ). When switching to the print job inhibiting mode in a state where the print job is not being executed, the control unit  2  does not allow the sheet feeding device  1  and the printing unit  6  to operate. In other words, the control unit  2  does not allow to perform sheet feeding and image forming. When switching to the print job inhibiting mode in a state where the print job is being executed, the control unit  2  controls the printing unit  6  to print only on the paper sheet that is already fed. Then, the control unit  2  does not allow sheet feeding of a new paper sheet and toner image forming by transferring onto a new paper sheet by the printing unit  6 . Note that in a case where a plurality of sheet feeding units  6   a  are disposed and the other sheet feeding unit  6   a  can feed a paper sheet, the control unit  2  may not control the sheet feeding device  1  to be in the print job inhibiting mode. 
     Then, the control unit  2  checks again whether or not the first output value V 1  is smaller than the reference value A 2  (Step # 38 ). When the first output value V 1  is smaller than the reference value A 2  (No in Step # 38 ), the flow returns to Step # 36 . The control unit  2  maintains the determination that the cassette  61  is not attached. On the other hand, when the first output value V 1  is larger than or equal to the reference value A 2  (No in Step # 38 ), the control unit  2  determines that the cassette  61  is attached (Step # 39 ). Then, the control unit  2  cancels the print job inhibiting mode (Step # 310 ). In other words, the control unit  2  restores the sheet feeding device  1  and the image forming apparatus to a print job executable mode. Then, the flow returns to Step # 31 . 
     Note that when using the first sensor unit  71 , whose first output value V 1  becomes larger as the magnetic coupling between the first coil circuit board L 1  and the first conductive plate  81  is weaker, the reference value A 2  is determined based on the upper limit value. In this case, the upper limit value is the maximum value of the first output value V 1  that can be output in the state where the cassette  61  is attached. Note that the upper limit value is smaller than the maximum value that the first sensor unit  71  can output. When the cassette  61  is detached (pulled out), the first output value V 1  becomes larger than the reference value A 2 . In this case, when the first output value V 1  is larger than or equal to reference value A 2 , the control unit  2  determines that the cassette  61  is not attached. In addition, when the first output value V 1  is smaller than the reference value A 2 , the control unit  2  determines that the cassette  61  is attached. 
     When detecting that the cassette  61  is attached, the control unit  2  may automatically update the full sheet value. When the control unit  2  determines that the cassette  61  is attached (No in Step # 38 , and Step # 39 ), the control unit  2  recognizes a magnitude of the first output value V 1 . The control unit  2  determines a new value of the full sheet value based on the recognized first output value V 1 . Then, the control unit  2  controls the storage unit  3  to update the full sheet value to the new value. Further, the control unit  2  may automatically update the no sheet value to be associated with the full sheet value. When the control unit  2  recognizes that there are no paper sheets based on the output of the set sensor S 2 , the control unit  2  controls the sheet placing plate  63  (sheet feed roller  62   a ) to move upward to the upper limit position. Then, the control unit  2  recognizes a magnitude of the first output value V 1 . The control unit  2  controls the storage unit  3  to update the no sheet value to the recognized value. In this way, accurate remaining quantity can be detected all the time. 
     (Adjustment of Reference Value A 2 ) 
     Next, with reference to  FIG. 13 , one example of the adjustment process of the reference value A 2  according to the embodiment is described. The first output value V 1  is affected by the distance between the first coil circuit board L 1  and the first conductive plate  81 , and a position relationship between them. The position of the first coil circuit board L 1  or the position of the first conductive plate  81  may be shifted during use (the possibility is not zero). As a result, when the cassette  61  is attached, misdetermination that the cassette  61  is not attached may occur (the possibility is not zero). In addition, the position of the first coil circuit board L 1  or the position of the first conductive plate  81  may vary among the sheet feeding devices  1  (image forming apparatuses). In addition, the cassette  61  may be exchanged due to a breakdown. The set reference value A 2  is not always appropriate. 
     Therefore, in the sheet feeding device  1 , the reference value A 2  can be adjusted. The flow of  FIG. 13  starts when the operation panel  5  (the touch panel  52  or the hardware key  53 ) accepts an instruction to adjust the reference value. First, the control unit  2  controls the display panel  51  to display a message requesting to detach (pull out) the cassette  61  (Step # 41 ). In this way, the user detaches the cassette  61 . As a result, the sheet placing plate  63  moves downward to the lower limit position. 
     Next, the control unit  2  controls the display panel  51  to display a message requesting to attach (mount) the cassette  61  (Step # 42 ). The control unit  2  may control the operation panel  5  to display a detachment confirmation button. Further, when the detachment confirmation button is operated, the control unit  2  may control the display panel  51  to display an attachment request message. 
     The control unit  2  recognizes the first output value V 1  (Step # 43 ). The control unit  2  may control the operation panel  5  to display an attachment confirmation button. Further, when the attachment confirmation button is operated, the control unit  2  may recognize the first output value V 1 . The recognized first output value V 1  is the lower limit value. Further, when the cassette  61  is attached, the control unit  2  recognizes the first output value V 1  (lower limit value) before the sheet placing plate  63  moves upward. A new value of the reference value A 2  is determined based on the recognized first output value V 1 . The control unit  2  controls the storage unit  3  to update the reference value A 2  to the new value (Step # 44 ). 
     (Outline of Sheet Size Detection) 
     Next, with reference to  FIGS. 14 and 15 , sheet size detection of the sheet feeding device  1  according to the embodiment is described. The second coil circuit board L 2  is a circuit board on which the coil pattern P 2  is printed. The third coil circuit board L 3  is a circuit board on which the coil pattern P 3  is printed. As illustrated in  FIG. 14 , the coil pattern P 2  and the coil pattern P 3  have rectangular spiral shapes. As illustrated in  FIG. 14 , the center of the spiral of each of the coil pattern P 2  and the coil pattern P 3  is positioned at one end on each coil circuit board. 
     On the second coil circuit board L 2  and the third coil circuit board L 3 , a length in the longitudinal direction of the winding becomes gradually shorter toward the inside winding. For example, when n represents a length in the longitudinal direction of the most inside winding, a length in the longitudinal direction of a winding is (number of turns)×n. On the other hand, an interval between windings in the short direction is set as small as possible. The coil pattern P 2  is formed so that the winding quantity facing the second conductive plate  82  varies according to a position of the second conductive plate  82 . The coil pattern P 3  is also formed so that the winding quantity facing the third conductive plate  83  varies according to a position of the third conductive plate  83 . 
     The winding quantity facing the second conductive plate  82  varies according to a position of the second conductive plate  82 . In other words, density of the winding facing the second conductive plate  82  varies according to a position of the second conductive plate  82 . In the example illustrated in  FIG. 14 , as the second conductive plate  82  moves rightward more on the coil pattern P 2 , the winding quantity facing the second conductive plate  82  becomes larger. According to a position of the second conductive plate  82 , quantity of eddy current (magnitude of magnetic force) in the second conductive plate  82  varies. According to a position of the second conductive plate  82 , strength of magnetic coupling between the second conductive plate  82  and the second coil circuit board L 2  varies. According to a position of the second conductive plate  82 , inductance (impedance) of the second coil circuit board L 2  varies. As the resonance frequency varies, an output value of the second circuit part  72   a  varies according to a position of the second conductive plate  82 . 
     The winding quantity facing the third conductive plate  83  varies according to a position of the third conductive plate  83 . In other words, density of the winding facing the third conductive plate  83  varies according to a position of the third conductive plate  83 . In the example illustrated in  FIG. 14 , as the third conductive plate  83  moves rightward more on the coil pattern P 3 , the winding quantity of the coil facing the third conductive plate  83  becomes larger. According to a position of the third conductive plate  83 , quantity of eddy current (magnitude of magnetic force) in the third conductive plate  83  varies. According to a position of the third conductive plate  83 , strength of magnetic coupling between the third conductive plate  83  and the third coil circuit board L 3  varies. According to a position of the third conductive plate  83 , inductance (impedance) of the third coil circuit board L 3  varies. As the resonance frequency varies, an output value of the third circuit part  73   a  varies according to a position of the third conductive plate  83 . 
       FIG. 15  illustrates one example of an output of the second circuit part  72   a  (second output value V 2 ). In  FIG. 15 , when the second conductive plate  82  is on the left side, the second output value V 2  (resonance frequency) is small. As the second conductive plate  82  moves rightward more, the second output value V 2  becomes larger. An initial position is a state where the second conductive plate  82  faces the coil pattern P 2  of the second coil circuit board L 2  so that the left ends of them coincide each other.  FIG. 15  illustrates an example where the second output value V 2  becomes larger in proportion to a rightward moving distance from the initial position. Note that a relationship between a position of the third conductive plate  83  and the third output value V 3  (resonance frequency) of the third circuit part  73   a  is similar to that of the second conductive plate  82  (the relationship as illustrated in  FIG. 15 ). 
     (Second Moving Mechanism  9   b ) 
     Next, with reference to  FIGS. 2 and 16 , the second moving mechanism  9   b  according to the embodiment is described. As illustrated in  FIG. 2 , the cassette  61  has a two-layered structure with a partition plate  610  (bottom plate) as a border. The sheet placing plate  63  and the width regulation cursor pair  64  are disposed inside an upper layer  611  (above the bottom plate). The second moving mechanism  9   b  and the second conductive plate  82  for detecting a size in the direction perpendicular to the conveying direction of the set paper sheets are disposed inside a lower layer  612 . In other words, the second conductive plate  82  is disposed in the cassette  61 . On the other hand, the second coil circuit board L 2  is not disposed in the cassette  61 . The second coil circuit board L 2  is disposed on the main body side of the multifunction peripheral  100 . The second coil circuit board L 2  is disposed at a position that faces the second conductive plate  82  in a non-contact manner when the cassette  61  is attached. Note that in  FIG. 2 , the second moving mechanism  9   b  and the second conductive plate  82  are not shown for convenience sake of illustration. In addition, in  FIG. 16 , the direction of pulling out the cassette  61  is shown by a white arrow. 
       FIG. 16  is a diagram of the sheet feeding unit  6   a  (sheet feeding device  1 ) viewed from above. In addition, in  FIG. 16 , members disposed in the lower layer  612  are shown by broken lines. In addition, the sheet placing plate  63  is not shown in  FIG. 16 . The lower part of  FIG. 16  illustrates a state where paper sheets having a larger size in the direction perpendicular to the conveying direction than in the upper part are set. 
     As illustrated in  FIG. 16 , the width regulation cursor pair  64  is disposed on the upper surface of the partition plate  610 . The paper sheets are set on the upper surface of the partition plate  610 . The width regulation cursors  64   a  are parallel to the conveying direction. Each of the width regulation cursors  64   a  is a plate-like member disposed to stand vertically on the partition plate  610 . The width regulation cursors  64   a  move to slide in the direction perpendicular to the conveying direction. The inner surfaces of the width regulation cursors  64   a  contact with side faces (edges in the width direction) of the paper sheets set in the cassette  61 . The inner surfaces of the width regulation cursors  64   a  face each other. The user moves the width regulation cursor pair  64  to fit a size (width) of the set paper sheets. In this way, the position of the set paper sheets can be regulated. As illustrated in  FIG. 16 , positions of the width regulation cursors  64   a  (distance between the width regulation cursors  64   a ) varies according to a size of the set paper sheets. 
     A linkage member  64   b  is disposed below each of the width regulation cursors  64   a  (below the partition plate  610 ). The linkage member  64   b  has a longitudinal direction that is perpendicular to the conveying direction. The linkage member  64   b  is a member like a plate, a rod, or a column. The linkage member  64   b  is attached perpendicularly to each of the width regulation cursors  64   a  viewed from above. The linkage member  64   b  is positioned in the lower layer  612  of the cassette  61 . One linkage members  64   b  and the other linkage member  64   b  are disposed at different positions in the conveying direction. 
     A rotation member  64   c  is disposed at the center position between the inner surfaces of the width regulation cursors  64   a . The rotation member  64   c  is disposed between the linkage members  64   b . The rotation member  64   c  is also disposed in the lower layer  612 . The rotation member  64   c  has a rotation axis perpendicular to a paper sheet placing surface (partition plate  610 ). The outer circumferential surface of the rotation member  64   c  is provided with teeth. In addition, each of the linkage members  64   b  is also provided with a teethed surface  64   d . The teethed surface  64   d  is provided to one of sides of the linkage member  64   b  on the rotation member  64   c  side. The teethed surface  64   d  of each linkage members  64   b  is disposed so as to engage with the teeth of the rotation member  64   c.    
     The teethed surfaces  64   d  of the linkage members  64   b  engage with the rotation member  64   c . Therefore, when one of the width regulation cursors  64   a  is moved, the other width regulation cursor  64   a  is also moved (in linkage therewith). Specifically, when one of the width regulation cursors  64   a  is moved inward, the other width regulation cursor  64   a  is also moved inward. When one of the width regulation cursors  64   a  is moved outward, the other width regulation cursor  64   a  is also moved outward. The linkage movement of the width regulation cursors  64   a  enables the center of paper sheets in the direction perpendicular to the conveying direction to coincide with the center of the sheet conveying path in the width direction (center sheet feeding) even if any size of paper sheets are set. 
     The second coil circuit board L 2  is disposed outside of the cassette  61 . The longitudinal direction of the second coil circuit board L 2  is parallel to the conveying direction. The second moving mechanism  9   b  is disposed in the lower layer  612  of the cassette  61 . The second moving mechanism  9   b  moves the second conductive plate  82  in the longitudinal direction of the second coil circuit board L 2  according to positions of the cursors. The second moving mechanism  9   b  includes a first gear  91 , a first rack  92 , a second gear  93 , and a second rack  94 . The first rack  92  is connected to one of the width regulation cursors  64   a . The first rack  92  is attached to the width regulation cursor  64   a . The longitudinal direction of the first rack  92  is perpendicular to the conveying direction. The first rack  92  moves in the direction perpendicular to the conveying direction along with the movement of the width regulation cursor  64   a.    
     Teeth of the first rack  92  are engaged with the first gear  91 . In addition, the first gear  91  and the second gear  93  are engaged with each other. Teeth of the second rack  94  are engaged with the second gear  93 . In addition, the second rack  94  faces the second coil circuit board L 2 . The longitudinal direction of the second rack  94  is parallel to the conveying direction. The second rack  94  moves in the conveying direction along with rotation of the second gear  93 . Therefore, the longitudinal direction of the second rack  94  and a movement direction thereof are parallel to the longitudinal direction of the second coil circuit board L 2 . 
     The second conductive plate  82  is attached to the second rack  94 . As illustrated in  FIG. 16 , the second conductive plate  82  is attached to a surface of the second rack  94  that faces the second coil circuit board L 2 . A part of the second conductive plate  82  (the surface facing the second coil circuit board L 2 ) is exposed to outside of the case of the cassette  61 . In order to expose the second conductive plate  82 , the case of the cassette  61  is provided with a groove. In this way, the second conductive plate  82  and the second coil circuit board L 2  can be close to each other. Note that it is possible to adopt a structure in which the groove is not provided so that the second conductive plate  82  is not exposed to outside of the case. 
     The second conductive plate  82  is attached to the end of the second rack  94  on the upstream side in the conveying direction. The second moving mechanism  9   b  (the first rack  92 , the first gear  91 , the second gear  93 , and the second rack  94 ) converts the movement of the width regulation cursor  64   a  in the direction perpendicular to the conveying direction to the movement in a direction parallel to the conveying direction. According to a position of the width regulation cursor  64   a , the second moving mechanism  9   b  moves the second conductive plate  82  in the longitudinal direction of the second coil circuit board L 2 . In this way, the winding quantity facing the second conductive plate  82  varies. 
     The lower part of  FIG. 16  illustrates an example where the second conductive plate  82  moves to the downstream side in the conveying direction. When a distance between the width regulation cursor pairs  64  is increased, the first rack  92  moves. In this way, the first gear  91  and the second gear  93  rotate. Then, the second rack  94  moves to the downstream side in the conveying direction. The second conductive plate  82  moves within a range from a position when usable minimum size of paper sheets are set to a position when usable maximum size of paper sheets are set. The movement of the second conductive plate  82  is within a range between both ends of the coil pattern P 2  of the second coil circuit board L 2  in the longitudinal direction. In order that the movement is within the range between the both ends, a gear ratio between the first gear  91  and the second gear  93  is adjusted. Even when any size of paper sheets are set, the second coil circuit board L 2  and the second conductive plate  82  face each other. 
     (Third Moving Mechanism  9   c ) 
     Next, with reference to  FIGS. 2 and 17 , the third moving mechanism  9   c  according to the embodiment is described. As illustrated in  FIG. 2 , the third moving mechanism  9   c  and the third conductive plate  83  are disposed inside the lower layer  612  of the cassette  61 . The third conductive plate  83  is disposed in the cassette  61 . On the other hand, the third coil circuit board L 3  is not disposed in the cassette  61 . The third coil circuit board L 3  is disposed on the main body side of the multifunction peripheral  100 . The third coil circuit board L 3  is disposed at a position facing the third conductive plate  83  in a non-contact manner when the cassette  61  is attached. The third moving mechanism  9   c  is a mechanism for detecting a size of the set paper sheet in the conveying direction. Note that the third moving mechanism  9   c  and the third conductive plate  83  are not shown in  FIG. 2  for convenience sake of illustration. 
       FIG. 17  is a diagram of the sheet feeding unit  6   a  (sheet feeding device  1 ) viewed from above. In addition, members disposed in the lower layer  612  of the cassette  61  are shown by a broken line in  FIG. 17 . The sheet placing plate  63  is not shown in  FIG. 17 . The lower part of  FIG. 17  illustrates a state where paper sheets having a larger size than in the upper part are set. The paper sheets are set on the upper surface of the partition plate  610 . The rear end regulation cursor  65  is disposed on the upper surface of the partition plate  610 . The rear end regulation cursor  65  regulates a rear end position of the set paper sheets. 
     The rear end regulation cursor  65  moves to slide in the direction parallel to the conveying direction. The inner surface of the rear end regulation cursor  65  contacts with the rear end of the set paper sheets (edge on the upstream side in the conveying direction). As illustrated in  FIG. 17 , a position of the rear end regulation cursor  65  varies according to a size of the set paper sheets. The user moves the rear end regulation cursor  65  according to a size (length) of the set paper sheets. In this way, a position of the set paper sheets can be regulated. The third coil circuit board L 3  is disposed outside of the cassette  61 . The longitudinal direction of the third coil circuit board L 3  is parallel to the conveying direction. The third moving mechanism  9   c  is disposed inside the cassette  61 . The third moving mechanism  9   c  is disposed in the lower layer  612  of the cassette  61 . The third moving mechanism  9   c  moves the third conductive plate  83  in the longitudinal direction of the third coil circuit board L 3  according to a position of the rear end regulation cursor  65 . 
     The third moving mechanism  9   c  includes a rod member  95 . One end of the rod member  95  is connected (attached) to the rear end regulation cursor  65 . The longitudinal direction of the rod member  95  is perpendicular to the conveying direction. The other end of the rod member  95  faces the third coil circuit board L 3 . The third conductive plate  83  is attached to the other end of the rod member  95 . A part of the third conductive plate  83  (surface facing the third coil circuit board L 3 ) is exposed to outside of the case of the cassette  61 . In order to expose the third conductive plate  83 , the case of the cassette  61  is provided with a groove. In this way, the third conductive plate  83  and the third coil circuit board L 3  can be close to each other. Note that it is possible to adopt a structure in which the groove is not provided so that the third conductive plate  83  is not exposed to outside of the case. The third moving mechanism  9   c  moves the rod member  95  according to a position of the rear end regulation cursor  65 . The third conductive plate  83  moves in the longitudinal direction of the third coil circuit board L 3 . In this way, the winding quantity facing the third conductive plate  83  varies. 
     The lower part of  FIG. 17  illustrates an example where a position of the third conductive plate  83  is moved to the upstream side in the conveying direction. Together with movement of the rod member  95 , the third conductive plate  83  is moved. Note that the third conductive plate  83  moves within a range from a position when usable minimum size of paper sheets are set to a position when usable maximum size of paper sheets are set. The movement of the third conductive plate  83  is within a range between both ends of the coil pattern P 3  of the third coil circuit board L 3  in the longitudinal direction. A length of the third conductive plate  83  is set to be longer than the movement range. Even when any size of paper sheets are set, the third coil circuit board L 3  and the third conductive plate  83  face each other. 
     (Flow of Sheet Size Detection) 
     Next, with reference to  FIGS. 18 to 20 , one example of a flow of the sheet size detection in the sheet feeding device  1  according to the embodiment is described. The flow of  FIG. 18  starts when the control unit  2  determines that the cassette  61  is attached. In this case, based on the magnitude of the second output value V 2  and first sheet size data D 1 , the control unit  2  recognizes a size of the set paper sheets in the direction perpendicular to the conveying direction. When determining that the cassette  61  is attached, the control unit  2  recognizes a size of the set paper sheets in the conveying direction based on the magnitude of the third output value V 3  and second sheet size data D 2 . In this way, when a distance between the conductive plate and a corresponding coil circuit board becomes the distant in measurement (attachment), the sheet size detection is performed. Therefore a sheet size can be correctly detected. Note that the time point when the sheet size detection is performed is not limited to the above-mentioned time point. The control unit  2  may perform the sheet size detection at any time point during the period while it is determined that the cassette  61  is attached. 
     First, the control unit  2  operates the second circuit part  72   a  and the third circuit part  73   a  (Step # 51 ). The second circuit part  72   a  outputs the second output value V 2  corresponding to the resonance frequency (Step # 52 ). The third circuit part  73   a  outputs the third output value V 3  corresponding to the resonance frequency (Step # 53 ). The control unit  2  recognizes magnitudes of the second output value V 2  and the third output value V 3  (Step # 54 ). The control unit  2  recognizes a size of the set paper sheets in the direction perpendicular to the conveying direction based on the first sheet size data D 1  (stored in the storage unit  3 ) and the second output value V 2  (Step # 55 ). 
     With reference to  FIG. 19 , the first sheet size data D 1  is described. The magnitude of the second output value V 2  corresponds to a position of the second conductive plate  82  (position of the width regulation cursor pair  64 ). As illustrated in  FIG. 19 , the first sheet size data D 1  can be table data that defines paper sheet sizes corresponding to the second output values V 2 . The second sensor unit  72  (second output circuit  72   d ) has high accuracy and a resolution of approximately 16 to 24 bits. For example, a range of the second output value V 2  can be defined in steps corresponding to 1 mm of the paper sheet. It may be defined in steps corresponding to 0.1 mm. The control unit  2  can detect (recognize) a size of the paper sheet in more detail than a conventional one. As described above with reference to  FIG. 15 , a variation (gradient) of the second output value V 2  is constant with respect to a movement amount of the second conductive plate  82 . The second output value V 2  varies linearly to a movement amount of the second conductive plate  82 . Therefore, the first sheet size data D 1  may be data that defines a linear function of Z (sheet size)=a (gradient)×X (second output value V 2 )+b (intercept). In this case, the control unit  2  determines the sheet size by calculation using the linear function. 
     In addition, the control unit  2  recognizes a size of the set paper sheets in the conveying direction based on the second sheet size data D 2  (stored in the storage unit  3 ) and the third output value V 3  (Step # 56 ). With Step # 56 , the detection of the set sheet size is finished. Then, this flow is finished (END). With reference to  FIG. 20 , the second sheet size data D 2  is described. A magnitude of the third output value V 3  corresponds to a position of the third conductive plate  83  (position of the rear end regulation cursor  65 ). Therefore, as illustrated in  FIG. 20 , the second sheet size data D 2  can be table data that defines the paper sheet size corresponding to third the output value V 3 . The third sensor unit  73  (third output part  73   d ) also has a resolution of approximately 16 to 24 bits. Therefore, for example, a range of the third output value V 3  can be defined in steps corresponding to 1 mm of the paper sheet. It may be defined in steps corresponding to 0.1 mm. The control unit  2  can detect (recognize) a size of the paper sheet in the conveying direction in more detail than a conventional one. 
     In addition, as described above with reference to  FIG. 15 , a variation (gradient) of the third output value V 3  is also constant with respect to a movement amount of the third conductive plate  83 . The third output value V 3  varies linearly to a movement amount of the third conductive plate  83 . Therefore, the second sheet size data D 2  may be data that defines a linear function of Z (sheet size)=a (gradient)×Y (third output value V 3 )+b (intercept). In this case, the control unit  2  determines the sheet size in the conveying direction by calculation using the linear function. 
     In this way, the sheet feeding device  1  according to the embodiment includes the cassette  61 , the first sensor unit  71 , the first moving mechanism  9   a , the storage unit  3 , and the control unit  2 . The cassette  61  includes the sheet placing plate  63  having the upper surface on which paper sheets are set. The cassette  61  is detachable and attachable. The first sensor unit  71  includes the first conductive plate  81  and the first coil circuit board L 1  on which the coil pattern P 1  is printed. The first sensor unit  71  applies a voltage to the first coil circuit board L 1  so that a magnetic field is generated. The first sensor unit  71  outputs the first output value V 1  corresponding to a position of the first conductive plate  81 . The first moving mechanism  9   a  moves the first conductive plate  81  so that a facing area between the first coil circuit board L 1  and the first conductive plate  81  is increased or decreased according to a remaining quantity of paper sheets in the cassette  61 . The storage unit  3  stores the remaining quantity detection data A 1  for determining a current remaining quantity of paper sheets corresponding to the first output value V 1 . The storage unit  3  stores the reference value A 2  for determining whether or not the cassette  61  is attached. The control unit  2  recognizes a magnitude of the first output value V 1 . The first conductive plate  81  is disposed in the cassette  61 . The first coil circuit board L 1  is not disposed in the cassette  61 . The first coil circuit board L 1  is disposed at a position facing the first conductive plate  81  in a non-contact manner when the cassette  61  is attached. The control unit  2  determines the current remaining quantity of paper sheets based on the magnitude of the first output value V 1  and the remaining quantity detection data A 1 . The control unit  2  determines whether or not the cassette  61  is attached based on the magnitude of the first output value V 1  and the reference value A 2 . 
     In this way, the sensor for detecting the remaining quantity of paper sheets can also detect whether the cassette  61  is attached or detached. A dedicated sensor for detecting whether or not the cassette  61  is attached, which is disposed in a conventional structure, can be eliminated. Therefore, cost in manufacturing the sheet feeding device  1 , and time and effort for developing the same can be reduced. In addition, when the cassette  61  is inserted strongly (vigorously), a dedicated sensor for detecting the attachment may be damaged. However, the first coil circuit board L 1  and the first conductive plate  81  do not contact with each other. Even if the cassette  61  is inserted strongly, the sheet feeding device  1  according to the present disclosure does not cause a breakdown of the sensor. Therefore it is possible to provide the sheet feeding device  1  that is resistant to breakdown. 
     In addition, the first sensor unit  71  outputs the first output value V 1  having a larger value as the distance between the first coil circuit board L 1  and the first conductive plate  81  is smaller, and as the facing area between the first coil circuit board L 1  and the first conductive plate  81  is larger. The control unit  2  determines that the cassette  61  is not attached when the first output value V 1  is smaller than the reference value A 2 . The control unit  2  determines that the cassette  61  is attached when the first output value V 1  is larger than or equal to the reference value A 2 . In this way, it is possible to correctly determine (detect) whether or not the cassette  61  is attached based on a magnitude relationship of the first output value V 1 . 
     In addition, the sheet feeding device  1  includes the sheet feed roller  62   a , and the lifting mechanism  67 . The sheet feed roller  62   a  is disposed above the sheet placing plate  63 . The sheet feed roller  62   a  sends out paper sheets set on the sheet placing plate  63 . The lifting mechanism  67  moves the sheet placing plate  63  upward and downward. When the cassette  61  is attached, the lifting mechanism  67  moves the sheet placing plate  63  upward until the sheet feed roller  62   a  contacts with the paper sheets on the sheet placing plate  63 . The first moving mechanism  9   a  moves the first conductive plate  81  so that the facing area between the first coil circuit board L 1  and the first conductive plate  81  is increased or decreased according to a height of the sheet placing plate  63 . The lifting mechanism  67  sets the sheet placing plate  63  at the lower limit position when the cassette  61  is detached. The reference value A 2  is determined based on the first output value V 1  when the sheet placing plate  63  is at the lower limit position. In this way, when the cassette  61  is detached, the sheet placing plate  63  can be set at the lower limit position. An initial position of the sheet placing plate  63  when the cassette  61  is attached can be set at the lower limit position. The reference value A 2  can be set based on the first output value V 1  corresponding to an initial position of the sheet placing plate  63  when the cassette  61  is attached. Attachment or detachment of the cassette  61  can be correctly determined. 
     In addition, the lifting mechanism  67  includes the drive shaft  67   b , the lifting member  67   c , and the lifting motor  67   a . The lifting member  67   c  is attached to the drive shaft  67   b . The lifting member  67   c  lifts the sheet placing plate  63  to move upward. The lifting motor  67   a  rotates the drive shaft  67   b . The first moving mechanism  9   a  includes the rotation plate  10 . The rotation plate  10  is fixed to the drive shaft  67   b . The rotation plate  10  changes its rotation angle along with a rotation angle of the drive shaft  67   b . The first conductive plate  81  has a substantially triangular shape and is attached to the rotation plate  10 . The first coil circuit board L 1  is disposed at a position facing the rotation plate  10  when the cassette  61  is attached. As the sheet placing plate  63  moves upward, the rotation plate  10  moves the first conductive plate  81  to be close to the center of the first coil circuit board L 1 , viewing the first coil circuit board L 1  and the first conductive plate  81  from front, and hence the facing area between the first coil circuit board L 1  and the first conductive plate  81  is increased. In this way, the facing area between the first coil circuit board L 1  and the first conductive plate  81  can be increased in proportion to an upward moving amount of the sheet placing plate  63 . The facing area between the first coil circuit board L 1  and the first conductive plate  81  can be decreased in proportion to a downward moving amount of the sheet placing plate  63 . Therefore the first output value V 1  can be changed according to a height of the sheet placing plate  63  (remaining quantity of paper sheets). 
     In addition, the cassette  61  includes the width regulation cursors  64   a , the second moving mechanism  9   b , and the second sensor unit  72 . The width regulation cursors  64   a  regulate a position of the set paper sheets. The width regulation cursors  64   a  can move to slide in the direction perpendicular to the conveying direction. The second sensor unit  72  includes the second conductive plate  82  and the second coil circuit board L 2 . The coil pattern P 2  is printed on the second coil circuit board L 2 . The second sensor unit  72  applies a voltage to the second coil circuit board L 2  so that a magnetic field is generated. The second sensor unit  72  outputs the second output value V 2  corresponding to a position of the second conductive plate  82 . The second conductive plate  82  is disposed in the cassette  61 . The second coil circuit board L 2  is not disposed in the cassette  61 . The second coil circuit board L 2  is disposed at a position facing the second conductive plate  82  in a non-contact manner when the cassette  61  is attached. A movement range of the second conductive plate  82  is smaller than a length of the second coil circuit board L 2  in the longitudinal direction. The second moving mechanism  9   b  moves the second conductive plate  82  in the longitudinal direction of the second coil circuit board L 2  according to a position of the width regulation cursor  64   a . The storage unit  3  stores the first sheet size data D 1  for determining a size of paper sheets in the direction perpendicular to the conveying direction corresponding to a magnitude of the second output value V 2 . When determining that the cassette  61  is attached, the control unit  2  recognizes a size of the set paper sheets in the direction perpendicular to the conveying direction based on the magnitude of the second output value V 2  and the first sheet size data D 1 . 
     In this way, by supplying current to the second coil circuit board L 2 , eddy current can be generated in the second conductive plate  82 . An inductance of the second coil circuit board L 2  has a value corresponding to a degree of magnetic coupling between the second coil circuit board L 2  and the second conductive plate  82 . A sheet size in the direction perpendicular to the conveying direction can be determined based on the second output value V 2  having much higher definition (higher resolution) compared with a conventional method (a method using an optical sensor or a contact-type switch). Therefore a size of set paper sheets can be detected correctly with high accuracy. When using non-standard size paper sheets, it is not necessary to set a correct length of the paper sheets in the direction perpendicular to the conveying direction. Further, as the second coil circuit board L 2  and the second conductive plate  82  do not contact with each other, there is no abrasion between them. Therefore there is little aging deterioration. 
     In addition, the cassette  61  includes the rear end regulation cursor  65 , the third moving mechanism  9   c , and the third sensor unit  73 . The rear end regulation cursor  65  regulates a position of the set paper sheets. The rear end regulation cursor  65  can move to slide in the conveying direction. The third sensor unit  73  includes the third conductive plate  83  and the third coil circuit board L 3 . The coil pattern P 3  is printed on the third coil circuit board L 3 . The third sensor unit  73  applies a voltage to the third coil circuit board L 3  so that a magnetic field is generated. The third sensor unit  73  outputs the third output value V 3  corresponding to a position of the third conductive plate  83 . The third conductive plate  83  is disposed in the cassette  61 . The third coil circuit board L 3  is not disposed in the cassette  61 . The third coil circuit board L 3  is disposed at a position facing the third conductive plate  83  in a non-contact manner when the cassette  61  is attached. The third conductive plate  83  has a movement range smaller than a length of the third coil circuit board L 3  in the longitudinal direction. The third moving mechanism  9   c  moves the third conductive plate  83  in the longitudinal direction of the third coil circuit board L 3  according to a position of the rear end regulation cursor  65 . The storage unit  3  stores the second sheet size data D 2  for determining a paper sheet size in the conveying direction corresponding to a magnitude of the third output value V 3 . When determining that the cassette  61  is attached, the control unit  2  recognizes a size of the set paper sheets in the conveying direction based on a magnitude of the third output value V 3  and the second sheet size data D 2 . 
     In this way, by supplying the third coil circuit board L 3  with current, eddy current can be generated in the third conductive plate  83 . An inductance of the third coil circuit board L 3  has a value corresponding to a degree of magnetic coupling between the third coil circuit board L 3  and the third conductive plate  83 . A sheet size in the conveying direction can be determined based on the output value having much higher definition (higher resolution) compared with a conventional method (a method using an optical sensor or a contact-type switch). A size of set paper sheets can be correctly detected with high accuracy. When using non-standard size paper sheets, it is not necessary to set a sheet size. Further, the third coil circuit board L 3  and the third conductive plate  83  also do not contact with each other, and hence there is no abrasion. Therefore there is little aging deterioration. 
     In addition, the image forming apparatus (multifunction peripheral  100 ) includes the sheet feeding device  1 . As it includes the sheet feeding device  1  described above, cost in manufacturing the image forming apparatus, and time and effort for developing the same can be reduced. In addition, it is possible to provide the image forming apparatus including the sheet feeding device  1  that is resistant to breakdown. Although the embodiment of the present disclosure is described above, the present disclosure is not limited to this and can be variously modified for implementation without deviating from the spirit of the disclosure.