Image forming device, document feeder, and method of determining sheet size

Abstract

An image forming device includes a tray on which a sheet is to be placed and including a plurality of first electrodes arranged along a first direction, a sheet guide including a conductor and movable along the first direction to different positions relative to the first electrodes, a detection circuit electrically connected to each of the first electrodes and configured to detect an amount of electric charge in each of the first electrodes, a printer configured to form an image on the sheet supplied from the tray, and a processor configured to determine a size of the sheet placed on the tray based on amounts of electric charge detected by the detection circuit, and control the printer to form an image on the sheet using the determined size.

What is claimed is: 
     
         1 . An image forming device, comprising:
 a tray on which a sheet is to be placed and including a plurality of first electrodes arranged along a first direction;   a sheet guide including a conductor and movable along the first direction to different positions relative to the first electrodes;   a detection circuit electrically connected to each of the first electrodes and configured to detect an amount of electric charge in each of the first electrodes;   a printer configured to form an image on the sheet supplied from the tray; and   a processor configured to:
 determine a size of the sheet placed on the tray based on amounts of electric charge detected by the detection circuit, and 
 control the printer to form an image on the sheet using the determined size. 
   
     
     
         2 . The image forming device according to  claim 1 , wherein two of the first electrodes that are adjacent to each other are separated by a gap extending along a second direction that is oblique with respect to the first direction. 
     
     
         3 . The image forming device according to  claim 2 , wherein the first direction is perpendicular to a sheet conveyance direction in which the sheet is conveyed on the tray. 
     
     
         4 . The image forming device according to  claim 1 , wherein the processor is further configured to:
 determine whether the amount of electric charge in each of the first electrodes is equal to or greater than a first threshold value, and   determine the size of the sheet based on the amount of electric charge of which electrode is equal to or greater than the first threshold value.   
     
     
         5 . The image forming device according to  claim 4 , wherein
 at the different positions, the sheet guide faces one or more of the first electrodes other than one electrode located at an end of the first electrodes, and   the processor is further configured to determine that a sheet is placed on the tray when the amount of electric charge in said one electrode is equal to or greater than a second threshold value.   
     
     
         6 . The image forming device according to  claim 5 , wherein the second threshold value is smaller than the first threshold value. 
     
     
         7 . The image forming device according to  claim 1 , wherein
 the tray further includes a second electrode within an area to which the sheet guide is not movable,   the detection circuit is electrically connected to the second electrode and is further configured to an amount of electric charge in the second electrode, and   the processor is further configured to determine that a sheet is placed on the tray when the amount of electric charge in the second electrode is equal to or greater than a second threshold value.   
     
     
         8 . The image forming device according to  claim 7 , wherein
 one of the first electrodes is in the area, and the processor is further configured to determine that a sheet is placed on the tray when the amount of electric charge stored in at least one of the second electrode and said one of the first electrodes is equal to or greater than the second threshold value.   
     
     
         9 . The image forming device according to  claim 7 , wherein the area is at least partly covered by a sheet having a minimum size supported by the image forming device, when the sheet is placed on the tray. 
     
     
         10 . The image forming device according to  claim 1 , wherein the amount of electric charge in each of at least two of the first electrodes changes as the sheet guide is moved. 
     
     
         11 . A document feeder connectable to an image forming device, comprising:
 a tray on which a sheet is to be placed and including a plurality of first electrodes arranged along a first direction;   a sheet guide including a conductor and movable along the first direction to different positions relative to the first electrodes;   a detection circuit electrically connected to each of the first electrodes and configured to detect an amount of electric charge in each of the first electrodes; and   a processor configured to:
 determine a size of the sheet placed on the tray based on amounts of electric charge detected by the detection circuit, and 
 output the determined size to the image forming device. 
   
     
     
         12 . The document feeder according to  claim 11 , wherein two of the first electrodes that are adjacent to each other are separated by a gap extending along a second direction that is oblique with respect to the first direction. 
     
     
         13 . The document feeder according to  claim 12 , wherein the first direction is perpendicular to a sheet conveyance direction in which the sheet is conveyed on the tray. 
     
     
         14 . The document feeder according to  claim 11 , wherein the processor is further configured to:
 determine whether the amount of electric charge in each of the first electrodes is equal to or greater than a first threshold value, and   determine the size of the sheet based on the amount of electric charge of which electrode is equal to or greater than the first threshold value.   
     
     
         15 . The document feeder according to  claim 14 , wherein
 at the different positions, the sheet guide faces one or more of the first electrodes other than one electrode located at an end of the first electrodes, and   the processor is further configured to determine that a sheet is placed on the tray when the amount of electric charge in said one electrode is equal to or greater than a second threshold value.   
     
     
         16 . The document feeder according to  claim 15 , wherein the second threshold value is smaller than the first threshold value. 
     
     
         17 . The document feeder according to  claim 11 , wherein
 the tray further includes a second electrode within an area to which the sheet guide is not movable,   the detection circuit is electrically connected to the second electrode and is further configured to an amount of electric charge in the second electrode, and   the processor is further configured to determine that a sheet is placed on the tray when the amount of electric charge in the second electrode is equal to or greater than a second threshold value.   
     
     
         18 . A method of determining a size of a sheet placed on a tray for printing or scanning, the tray including a plurality of first electrodes arranged along a first direction and facing a sheet guide including a conductor and movable along the first direction to different positions relative to the first electrodes, the method comprising:
 detecting an amount of electric charge in each of the first electrodes; and   determining a size of the sheet placed on the tray based on amounts of electric charge detected in each of the first electrodes.   
     
     
         19 . The method according to  claim 18 , further comprising:
 determining whether the amount of electric charge in each of the first electrodes is equal to or greater than a first threshold value, wherein   the size of the sheet is determined based on the amount of electric charge of which electrode is equal to or greater than the first threshold value.   
     
     
         20 . The method according to  claim 18 , wherein
 at the different positions, the sheet guide faces one or more of the first electrodes other than one electrode located at an end of the first electrodes, and   the method further comprises determining that the sheet is placed on the tray when the amount of electric charge in said one electrode is equal to or greater than a second threshold value.