Patent Publication Number: US-2010123280-A1

Title: Sheet feeder and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 USC 119 from Japanese Patent Application No. 2008-293377 filed on Nov. 17, 2008. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a sheet feeder and an image forming apparatus. 
     2. Related Art 
     In the related art, an image forming apparatus, such as a copier and a printer, includes a sheet feeder that feeds a recording medium, on which images are recorded, to an image forming unit. 
     The recording medium which is loaded on a bottom plate that can be lifted up by a wire is stored in a tray of the sheet feeder. 
     In the image forming apparatus, when the uppermost recording medium in the tray is sequentially fed to the image forming unit, the number of recording medium loaded in the tray is reduced, and the position of the uppermost recording medium becomes lower than the lower limit of the range in which the recording medium can be fed to the image forming unit, the bottom plate is lifted up such that the uppermost recording medium is adjusted to be within the sheet feedable range. 
     SUMMARY 
     An aspect of the invention provides a sheet feeder including: 
     a lifting unit that lifts a loading portion having at least one recording medium loaded thereon; 
     a position detecting unit that detects whether the uppermost surface of the recording medium loaded on the loading portion is disposed at a predetermined first position; 
     a sheet feed mechanism that feeds the recording medium from the loading portion; 
     a thickness detecting unit that detects thickness of the recording medium fed by the sheet feeding mechanism; and 
     a control unit that, when the position detecting unit detects that the uppermost surface of the recording medium is disposed at the first position, controls the lifting unit such that the uppermost surface of the recording medium is lifted up to a second position that is higher than the first position by a distance determined based on cumulative value of the thickness of the recording medium detected by the thickness detecting unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically illustrating the structure of an image forming system according to a first exemplary embodiment; 
         FIG. 2  is a diagram schematically illustrating the structure of a sheet feeder according to the first exemplary embodiment; 
         FIG. 3  is a block diagram illustrating the structure of the image forming system according to the first exemplary embodiment; 
         FIG. 4  is a flowchart illustrating the flow of a sheet feed process according to the first exemplary embodiment; 
         FIG. 5  is a flowchart illustrating the flow of an initial operation of the sheet feeder according to the first exemplary embodiment; 
         FIG. 6  is a diagram schematically illustrating a sheet feed operation according to the first exemplary embodiment; 
         FIG. 7  is a flowchart illustrating the flow of a sheet feed process according to a second exemplary embodiment; 
         FIG. 8  is a flowchart illustrating the flow of an initial operation of a sheet feeder according to the second exemplary embodiment; 
         FIG. 9  is a diagram schematically illustrating a sheet feed operation according to the second exemplary embodiment; 
         FIG. 10  is a diagram schematically illustrating the structure of an image forming system according to a third exemplary embodiment; 
         FIG. 11  is a block diagram illustrating the structure of the image forming system according to the third exemplary embodiment; 
         FIG. 12  is a flowchart illustrating the flow of a sheet feed process according to the third exemplary embodiment; 
         FIG. 13  is a diagram schematically illustrating the structure of a sheet feeder according to a modification; and 
         FIG. 14  is a flowchart illustrating the flow of a sheet feed process according to a modification of the third exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter exemplary embodiments of the present invention will be described in detail. 
     First Exemplary Embodiment 
     As shown in  FIG. 1 , an image forming system  10  according to a first exemplary embodiment includes a sheet feeder  12  and an image forming apparatus  16 . 
     The sheet feeder  12  feeds a recording medium  14  to the image forming apparatus  16 . 
     For example, a sheet can be used as the recording medium  14 . However, the recording medium  14  is not limited to the sheet. Any medium may be used as long as the image forming apparatus  16  can form an image thereon. 
     The image forming apparatus  16  includes an image reading unit  18  and an image forming apparatus main body  20 . 
     The image reading unit  18  includes, for example, a line CCD sensor and a scanning mechanism that scans the recording medium with the line CCD sensor. The image reading unit  18  reads the image of a set document in response to, for example, instructions from a user. 
     The image forming apparatus main body  20  includes an image forming unit  22 . The image forming unit  22  forms an image on the recording medium  14  fed from the sheet feeder  12 . 
     The image forming unit  22  includes a photoconductor drum  24 , a charging unit  26 , an exposure device  28 , a developing unit  30 , a transfer roller  32 , and a fixing unit  34 . 
     The charging unit  26 , the exposure device  28 , the developing unit  30 , and the transfer roller  32  are arranged around the photoconductor drum  24 . 
     The photoconductor drum  24  is a cylindrical photoconductor, and is rotationally driven at a predetermined rotation speed. 
     The charging unit  26  uniformly charges the surface of the photoconductor drum  24 . 
     The exposure device  28  exposes the photoconductor drum  24  according to image data of an image to be formed on the recording medium  14  and forms an electrostatic latent image on the photoconductor drum  24 . The exposure device  28  may include an LED print head obtained by arranging, for example, plural LEDs. In addition, the exposure device  28  may be an optical scanning device that includes a semiconductor laser, a collimator lens, a polygon mirror, and an fθ lens, and performs scanning exposure based on image data on the photoconductor drum  24 . 
     The developing unit  30  develops the electrostatic latent image formed on the photoconductor drum  24  with toner. 
     The transfer roller  32  transfers the toner image on the photoconductor drum  24  onto the recording medium  14 . The transferred toner image is fixed when the recording medium  14  passes between a pressure roller  36  and a heating roller  38  provided in the fixing unit  34 . 
     The recording medium  14  is discharged to a discharge tray  44  through a sheet outlet  42  provided in the side surface of the image forming apparatus  16  by a discharge roller  40 . 
     Next, the structure of the sheet feeder  12  will be described with reference to  FIG. 2 . 
     The sheet feeder  12  includes a tray  46 . A lifting portion on which the recording medium  14  is loaded, for example, a bottom plate  48  is provided in the tray  46 . 
     One end of a wire  50  is connected to the bottom plate  48  while being suspended by a pulley  52 . In addition, the other end of the wire  50  is connected to a driving gear  56  that is rotationally driven by a lift-up motor  54 . When the driving gear  56  is rotated in the direction of an arrow A, the wire  50  suspended by the pulley  52  is wound to lift up the bottom plate  48 . When the driving gear  56  is rotated in the direction of an arrow B, the wire  50  suspended by the pulley  52  is unwound to drop the bottom plate  48 . 
     The sheet feeder  12  includes a sheet feed mechanism that feeds the recording medium  14 . The sheet feed mechanism includes a sheet feed unit having, for example, a sheet transport roller  58 , a sheet transport roller rotating shaft  60 , and a sheet transport roller supporting arm  62 , a position detecting unit, such as a sheet feedable position detecting sensor  64 , a feed roller  66 , a retard roller  68 , a thickness detecting sensor  70 , a takeaway pinch roller  76 , and a takeaway drive roller  78 . 
     The thickness detecting unit, for example, the thickness detecting sensor  70  is formed by mounting a thickness detecting actuator  74  to a rotary encoder  72 . 
     In the sheet feeder  12 , when the recording medium  14  is in the range in which the sheet can be fed, the sheet transport roller  58  is rotated in the direction of an arrow C, with the sheet transport roller  58  contacted with the uppermost surface of the recording medium  14  loaded on the bottom plate  48 , to feed the recording medium  14 . 
     The sheet transport roller supporting arm  62  that supports the vertical movement of the sheet transport roller  58  is mounted to the sheet transport roller  58 . 
     The sheet transport roller supporting arm  62  rotates about a sheet transport roller supporting arm rotating shaft  65 . The sheet transport roller supporting arm  62  is rotated upward when a sheet transport roller solenoid  87  (not shown) is turned on. The sheet transport roller supporting arm  62  is rotated downward by the elastic force of a spring (not shown) when the sheet transport roller solenoid  87  is turned off. In this way, the sheet transport roller  58  is moved in the vertical direction represented by an arrow D. 
     The sheet feedable position detecting sensor  64  is arranged close to the sheet transport roller supporting arm  62 . The sheet feedable position detecting sensor  64  detects the position of the uppermost surface of the recording medium  14  loaded on the bottom plate  48 . 
     The sheet feedable position detecting sensor  64  detects the uppermost surface of the recording medium  14 , without being affected by the vertical movement of the sheet transport roller  58 . 
     The fed recording medium  14  are separated one by one by the feed roller  66  and the retard roller  68  that is rotationally driven in a direction opposite to the rotation direction of the feed roller  66  by a torque limiter (not shown). 
     The separated recording medium  14  is transported to the image forming apparatus  16  by the takeaway pinch roller (driven roller)  76  and the takeaway drive roller  78 . 
     The thickness detecting sensor  70  is arranged between the feed roller  66  and the takeaway pinch roller  76 . 
     The thickness detecting sensor  70  is arranged at a position where the thickness detecting actuator  74  is lifted up whenever the recording medium  14  is fed. 
     When the recording medium  14  is fed and the thickness detecting actuator  74  is lifted up by a distance corresponding to the thickness of the recording medium  14 , the rotary encoder  72  is rotated at an angle corresponding to the amount of lift. The rotary encoder  72  outputs a signal corresponding to the rotation angle. That is, the rotary encoder  72  outputs a signal corresponding to the thickness of the recording medium  14 . 
     Next, the structure of the main components of an electrical system of the sheet feeder  12  will be described with reference to  FIG. 3 . 
     As shown in  FIG. 3 , the sheet feeder  12  includes a control unit  80 , the thickness detecting sensor  70 , a thickness cumulative value calculating unit  82 , a sheet feedable position detecting sensor  64 , a sheet transport roller motor  84 , a feed roller motor  86 , a retard roller motor  88 , a takeaway drive roller motor  90 , a lift-up motor  54 , a sheet transport roller solenoid  87 , and an I/F (interface)  92 . 
     Whenever the recording medium  14  is fed, the value of the thickness of the recording medium  14  detected by the thickness detecting sensor  70  is input to the thickness cumulative value calculating unit  82 , and the input thickness value is accumulated (added). Then, the thickness cumulative value calculating unit  82  outputs the calculated cumulative value to the control unit  80 . 
     The image forming apparatus  16  transmits an instruction signal that instructs the sheet feeder  12  to feed the recording medium  14  to the I/F  92 . The instruction signal includes, for example, information about the number of sheets to be fed. The I/F  92  outputs the instruction signal from the image forming apparatus  16  to the control unit  80 . 
     When receiving the instruction signal for feeding the recording medium  14  from the image forming apparatus  16  through the I/F  92 , the control unit  80  controls the operation of the sheet transport roller motor  84  to feed the recording medium  14  to the image forming apparatus  16 . 
     Next, a sheet feed process routine performed by the control unit  80  of the sheet feeder  12  provided in the image forming system  10  according to this exemplary embodiment will be described with reference to  FIG. 4 . 
     When the user operates the image forming apparatus  16  to select an image forming service such as copy, the routine is performed. 
     First, in Step  100 , an initial operation processing routine shown in  FIG. 5  is performed. In the initial operation processing routine, as shown in  FIG. 5 , in Step  120 , it is determined whether the sheet feedable position detecting sensor  64  is turned on. The sheet feedable position detecting sensor  64  is turned on when the uppermost surface of the recording medium  14  is in a sheet feedable range  94  shown in  FIG. 6 . 
     The sheet feedable range  94  is defined by, for example, a first position A, which is the lower limit of the range in which the medium  14  can be fed, and a predetermined second position B, which is the upper limit of the range in which the recording medium  14  can be fed and is higher than the first position A. 
     When the uppermost surface of the recording medium  14  is lower than the lower limit of the range in which the recording medium  14  can be fed, it is difficult to feed the recording medium. When the uppermost surface of the recording medium  14  is higher than the upper limit of the range in which the recording medium  14  can be fed, it is difficult to feed the recording medium. The first position A is not limited to the lower limit of the range, and any position may be used as the first position as long as it is in the range in which the recording medium  14  can be fed. 
     In addition, the second position B is not limited to the upper limit of the range, and any position may be used as the second position as long as it is higher than the first position and is in the range in which the recording medium  14  can be fed. That is, the sheet feedable range  94  is not limited to between the lower limit and the upper limit, and any range may be used as long as the recording medium can be fed therewithin. 
     The sheet feedable position detecting sensor  64  is turned off when the uppermost surface of the recording medium  14  is lower than the first position A. Therefore, when the sheet feedable position detecting sensor  64  is turned on, the current position of the uppermost surface of the recording medium  14  is in the range in which the recording medium can be fed. Therefore, it is not necessary to lift up the bottom plate  48 , and the process proceeds to Step  102  in  FIG. 4 . 
     If it is determined in Step  120  that the sheet feedable position detecting sensor is turned off, the process proceeds to Step  122 . In this state, the position of the uppermost surface of the recording medium  14  is not within the sheet feedable range. That is, since the position of the uppermost surface of the recording medium  14  is lower than the first position A, the lift-up motor  54  is driven to lift up the bottom plate  48  until the sheet feedable position detecting sensor  64  is turned on. Then, the uppermost surface of the recording medium  14  is lifted up to the first position A in the sheet feedable range  94 . 
     Then, the process proceeds to Step  124  to drive the lift-up motor  54  such that the bottom plate  48  is further lifted up by a predetermined threshold value. The threshold value corresponds to the sheet feedable range  94  in  FIG. 6 . That is, the threshold value corresponds to a thickness cumulative value when the recording medium  14  is fed up to the first position A that is lower than the second position B, which is the upper limit of the sheet feedable range. Therefore, the bottom plate  48  is lifted up such that the uppermost surface of the recording medium  14  is lifted up to the second position B in the sheet feedable range  94 . In this way, the recording medium  14  is lifted up to the second position B. 
     Then, in Step  126 , the control unit  80  outputs an instruction signal to the cumulative value calculating unit  82  to reset the thickness cumulative value calculated by the cumulative value calculating unit  82 . Then, the thickness cumulative value calculating unit  82  resets the thickness cumulative value. 
     The position of the uppermost surface of the recording medium  14  is within the sheet feedable range by the initial operation processing routine. 
     Then, the recording medium is fed in Step  102  in  FIG. 4 . That is, the sheet transport roller motor  84  is controlled such that the recording medium  14  loaded on the bottom plate  48  is fed to the image forming apparatus  16 . 
     In Step  104 , it is determined whether the sheet feedable position detecting sensor  64  is turned off, that is, whether the uppermost surface of the recording medium  14  is lower than the first position A. 
     If it is determined in Step  104  that the sheet feedable position detecting sensor  64  is not turned off, that is, if the uppermost surface of the recording medium  14  is within the sheet feedable range  94 , the process proceeds to Step  110 . 
     If it is determined in Step  104  that the sheet feedable position detecting sensor  64  is turned off, that is, sheet feeding is performed and the position of the uppermost surface of the recording medium  14  is lower than the first position A, i.e., beyond the sheet feedable range, the process proceeds to Step  106 . In Step  106 , the control unit receives the calculated thickness cumulative value output from the cumulative thickness value calculating unit  82 . 
     Then, in Step  108 , the lift-up motor  54  is driven such that the bottom plate  48  is lifted up by the thickness cumulative value input to the control unit  80 . In this way, the uppermost surface of the recording medium  14  is lifted up to the second position B. 
     Then, in Step  109 , the control unit  80  outputs an instruction signal to the thickness cumulative value calculating unit  82  to reset the thickness cumulative value calculated by the thickness cumulative value calculating unit  82 . Then, the thickness cumulative value calculating unit  82  resets the thickness cumulative value. 
     Then, in Step  110 , it is determined whether the number of sheets corresponding to the instruction signal from the image forming apparatus  16  is completely fed. 
     If it is determined in Step  110  that the number of sheets corresponding to the instruction signal is not completely fed, the process returns to Step  102 , and the same operation as described above is repeatedly performed. 
     Second Exemplary Embodiment 
     Next, a second exemplary embodiment will be described. 
     In this exemplary embodiment, the same components as those in the first exemplary embodiment are denoted by the same reference numerals, and a description thereof will be omitted. 
     The structures of an image forming system  10 , a sheet feeder  12 , and an image forming apparatus  16  are the same as those according to the first exemplary embodiment, and a description thereof will be omitted. 
     The structure of the main components of an electrical system according to the second exemplary embodiment is the same as that according to the first exemplary embodiment, and a description thereof will be omitted. 
     However, in this exemplary embodiment, a sheet feedable position detecting sensor  64  detects that the uppermost surface of the recording medium  14  is disposed at the first position A. That is, the sheet feedable position detecting sensor  64  is turned on when the recording medium  14  is disposed at the first position A, and is turned off when the position of the recording medium  14  is lower than the first position A. 
     The first position A is not limited to the upper limit of the sheet feedable range, and any position may be used as the first position as long as it is in the range in which the recording medium  14  can be fed. In addition, the second position B is not limited to the lower limit of the sheet feedable range, and any position may be used as the second position as long as it is lower than the first position and is in the range in which the recording medium  14  can be fed. That is, the first and second positions may be in the range in which the recording medium can be fed, similar to the first exemplary embodiment. 
     A sheet feed process routine performed by the control unit  80  of the sheet feeder  12  provided in the image forming system  10  according to this exemplary embodiment will be described with reference to  FIG. 7 . 
     Similar to the first exemplary embodiment, when the user operates the image forming apparatus  16  to select an image forming service such as copy, the routine is performed. 
     First, in Step  150 , an initial operation processing routine shown in  FIG. 8  is performed. A thickness cumulative value may be forcibly reset, for example, when power is supplied to the image forming apparatus at the beginning, when the main power supply is turned on or off, and when the recording medium  14  is added to the tray  46 . In this case, even when the uppermost surface of the recording medium  14  is not actually disposed at the second position B, which is the upper limit of the sheet feedable range, the thickness cumulative value is reset. Therefore, first, in Step  200 , it is determined whether the thickness cumulative value calculated by the thickness cumulative value calculating unit  82  is reset. 
     If it is determined in Step  200  that the thickness cumulative value is not reset, the process proceeds to Step  202 . On the other hand, if it is determined that the thickness cumulative value is reset, the process proceeds to Step  204 . 
     In Step  202 , it is determined whether the thickness cumulative value calculated by the thickness cumulative value calculating unit  82  is a predetermined threshold value or more. 
     The threshold value corresponds to the sheet feedable range  94  shown in  FIG. 9 . That is, the threshold value corresponds to the thickness cumulative value when the recording medium  14  is fed up to the second position B that is lower than the first position A, which is the upper limit of the sheet feedable range. 
     If the determination result in Step  202  is “YES”, the process proceeds to Step  204 . 
     In Step  204 , the lift-up motor  54  is driven to lift up the bottom plate  48  until the sheet feedable position detecting sensor  64  is turned on. Then, the uppermost surface of the recording medium  14  is lifted up to the first position A. 
     Further, the process proceeds to Step  206 , the control unit  80  outputs an instruction signal to the thickness cumulative value calculating unit  82  to reset the thickness cumulative value calculated by the thickness cumulative value calculating unit  82 . Then, the thickness cumulative value calculating unit  82  resets the thickness cumulative value. 
     On the other hand, if the determination result in Step  202  is “NO”, the current position of the position of the uppermost surface of the recording medium  14  is within the sheet feedable range. Therefore, it is not necessary to lift up the bottom plate  48 , and the process proceeds to Step  152  in  FIG. 7 . 
     The position of the uppermost surface of the recording medium  14  is within the sheet feedable range by the initial operation processing routine. 
     Then, the recording medium is fed in Step  152  in  FIG. 7 . That is, the sheet transport roller motor  84  is controlled such that the recording medium  14  loaded on the bottom plate  48  is fed to the image forming apparatus  16 . 
     In Step  154 , the control unit receives the calculated thickness cumulative value output from the cumulative value calculating unit  82 . 
     Then, in Step  156 , it is determined whether the thickness cumulative value received in Step  154  is a predetermined threshold value or more. That is, it is determined whether the position of the uppermost surface of the recording medium  14  is lower than the second position B. 
     If it is determined in Step  156  that the thickness cumulative value is less than the threshold value, i.e., if the uppermost surface of the recording medium  14  is within the sheet feedable range  94 , the process proceeds to Step  162 . 
     If it is determined in Step  156  that the thickness cumulative value is the threshold value or more, i.e., if sheet feeding is performed and the position of the uppermost surface of the recording medium  14  is lower than the second position B, that is, beyond the sheet feedable range, the process proceeds to Step  158 . In Step  158 , the lift-up motor  54  is driven to lift up the bottom plate  48  until the sheet feedable position detecting sensor  64  is turned on. In this way, the uppermost surface of the recording medium  14  is lifted up to the first position A. 
     Then, the process proceeds to Step  160 , the control unit  80  outputs an instruction signal to the thickness cumulative value calculating unit  82  to reset the thickness cumulative value calculated by the thickness cumulative value calculating unit  82 . Then, the thickness cumulative value calculating unit  82  resets the thickness cumulative value. 
     Next, the process proceeds to Step  162 , it is determined whether the number of sheets corresponding to the instruction signal from the image forming apparatus  16  is completely fed. 
     If it is determined in Step  162  that the number of sheets corresponding to the instruction signal is not completely fed, the process returns to Step  152 , and the same operation as described above is repeatedly performed. 
     Third Exemplary Embodiment 
     Next, a third exemplary embodiment will be described with reference to  FIG. 10 . 
     In this exemplary embodiment, the same components as those in the first and second exemplary embodiments are denoted by the same reference numerals, and a description thereof will be omitted. 
     An image forming system  10 A includes a sheet feeder  13 , an image forming apparatus  16 , and an image reading unit  18 . 
     As shown in  FIG. 10 , the sheet feeder  13  includes plural sheet feed units. For example, the sheet feeder  13  includes three sheet feed units  13 A- 13 C in this exemplary embodiment. 
     The sheet feeder  13  is provided in an image forming apparatus main body  20 . 
     The sheet feed units  13 A to  13 C have the same structure as the sheet feeder  12  shown in  FIG. 2 , and characters “A” to “C” are added to ends of the reference numerals of the components having the same structure. 
     The image forming apparatus  16  and the image reading unit  18  are the same as those in the first exemplary embodiment, and thus a description thereof will be omitted. 
     The sheet feed units  13 A to  13 C feed recording medium  14 A to  14 C to the image forming unit  22 , respectively. 
     The recording medium  14 A to  14 C are loaded in trays  46 A to  46 C provided in the sheet feed units  13 A to  13 C, respectively. 
     For example, the recording medium  14 A to  14 C may be different types of recording medium having different thicknesses. 
     In the following description, when the sheet feed units  13 A to  13 C are not individually specified, they are simply referred to as a “sheet feed unit  13 ”. Similarly, when the recording medium  14 A to  14 C are not individually specified, they are simply referred to as a “recording medium  14 ”. 
     Next, the structure of the main components of an electrical system of the sheet feed unit  13  will be described with reference to  FIG. 11 . 
     The same components as those according to the first exemplary embodiment are denoted by the same reference numerals, and a description thereof will be omitted. 
     In the third exemplary embodiment, sheet feedable position detecting sensors  64 A to  64 C, thickness detecting sensors  70 A to  70 C, thickness cumulative value calculating units  82 A to  82 C, lift-up motors  54 A to  54 C, sheet transport motors  84 A to  84 C, feed roller motors  86 A to  86 C, retard roller motors  88 A to  88 C, takeaway drive motors  90 A to  90 C, and sheet transport roller solenoids  87 A to  87 C are provided so as to correspond to the sheet feed units  13 A to  13 C, respectively. 
     In the third exemplary embodiment, a storage unit  81  is provided. 
     The storage unit  81  stores predetermined threshold values corresponding to the recording medium  14 A and  14 B respectively loaded in the sheet feed units  13 A and  13 B. 
     Next, a sheet feed process routine performed by the control unit  80  of the image forming system  10 A according to this exemplary embodiment will be described with reference to  FIG. 12 . 
     When the user operates the image forming apparatus  16  to select an image forming service such as copy, the routine is performed. 
     First, in Step  250 , among the threshold values stored in the storage unit  81 , a threshold value corresponding to the recording medium  14  designated by the selected image forming service is read. 
     In Steps  252  to  256 , the same process as that in Steps  150  to  156  according to the second exemplary embodiment is performed on the sheet feed unit  13  having the designated recording medium  14  loaded therein. 
     However, in Step  258 , the thickness cumulative value of the sheet feed unit  13  corresponding to the designated one of the recording medium  14 A to  14 C is read from any one of the thickness cumulative value calculating units  82 A to  82 C. 
     In Steps  260  to  264 , the same process as that in Steps  158  to  162  according to the second exemplary embodiment is performed on the sheet feed unit  13  having the designated recording medium  14  loaded therein. 
     Next, a sheet feed process routine performed by the control unit  80  of an image forming system  10 A according to a modification of the third exemplary embodiment will be described with reference to  FIG. 14 . 
     In Step  250 , predetermined threshold values corresponding to the sheet feed units  13 A to  13 C are read from the storage unit  81 . In Steps  252  to  256 , the same process as that in Steps  150  to  156  according to the second exemplary embodiment is performed on the sheet feed unit  13  having the designated recording medium  14  loaded therein. However, in Step  258 , a thickness cumulative value corresponding to the designated one of the sheet feed units  13 A to  13 C is read from any one of the thickness cumulative value calculating units  82 A to  82 C. In Steps  260  to  264 , the same process as that in Steps  158  to  162  according to the second exemplary embodiment is performed on the sheet feed unit  13  having the designated recording medium  14  loaded therein. 
     In the first to third exemplary embodiments, it is described the thickness detecting sensor  70  includes the rotary encoder  72  and the thickness detecting actuator  74 , however, the invention is not limited thereto. For example, as shown in  FIG. 13 , the thickness detecting sensor  70  may include a laser displacement meter  96  and a roller  98 . In this case, the laser displacement meter  96  measures the amount of movement of the roller  98  in the vertical direction to detect the thickness of the fed recording medium  14 . In the first to third exemplary embodiments, the sensor is provided so as to detect the position of the uppermost surface of the recording medium  14  loaded on the bottom plate  48  without being affected by the movement of the sheet transport roller  58  in the vertical direction. However, a sheet feedable position detecting sensor may be provided so as to detect the position of the sheet transport roller supporting arm  62  in the vertical direction, and the sheet feedable position detecting sensor may detect the position of the sheet transport roller supporting arm  62  when the sheet transport roller  58  comes into contact with the uppermost surface of the recording medium  14  loaded on the bottom plate  48 , thereby detecting the position of the uppermost surface of the recording medium  14  loaded on the bottom plate  48 . 
     The structures of the sheet feeders, the image forming systems, and the image forming apparatuses according to the exemplary embodiments are just examples, and various modifications and changes of the invention can be made without departing from the concept of the invention. 
     In the above-described exemplary embodiments of the invention, the image forming unit  22  of the image forming apparatus  16  forms images by electrophotography. However, the image forming unit may form images using other methods such as an ink jet recording method. 
     In addition, the flow of the processing program according to each of the exemplary embodiments is just an example, and various modifications and changes of the process flow can be made without departing from the concept of the invention. For example, the process order of the steps may be changed, unnecessary steps may be removed, and a new step may be added.