Sheet feeding device and image forming apparatus provided with the sheet feeding device

Abstract

A sheet feeding device that is suited to be employed in an image forming apparatus having a printing section for printing an image on a sheet. The sheet feeding device has feed rollers for feeding a sheet at a speed “Va” and timing rollers for feeding a sheet fed by the feed rollers to the printing section at a speed “Vb”. Sensors are provided between the feed rollers and the timing rollers, so that the gap between a foregoing sheet and a following sheet can be detected at a plurality of detection points while the foregoing sheet is being fed by the timing rollers and the following sheet is being fed by the feed rollers. When one of the sensors detects that there is a gap between the sheets, a control unit controls the feed rollers such that the speed “Va” will be higher than the speed “Vb”.

1 . A sheet feeding device that is suited to be used in an image forming apparatus comprising a printing section for printing an image on a sheet;
 a first feeder for feeding the sheet at a first speed;   a second feeder for feeding the sheet fed by the first feeder to the printing section at a second speed;   a detector, which is located between the first feeder and the second feeder, for detecting a gap between a first sheet and a second sheet at a plurality of detection points while the first sheet is being fed by the second feeder and the second sheet is being fed by the first feeder; and   a controller for controlling the first feeder or the second feeder,   wherein when the detector detects that there is a gap between the first sheet and the second sheet, the controller controls the first feeder or the second feeder such that the first speed will be higher than the second speed.   
     
     
         2 . A sheet feeding device according to  claim 1 , wherein the controller controls the first feeder or the second feeder such that the gap detected at a more downstream detection point of the plurality of detection points will be smaller than the gap detected at a more upstream detection point of the plurality of detection points. 
     
     
         3 . A sheet feeding device according to  claim 1 , wherein when the detector detects that there is no gap between the first sheet and the second sheet, the controller controls the first feeder or the second feeder such that the first speed and the second speed will be equal to each other. 
     
     
         4 . A sheet feeding device according to  claim 1 , wherein when the detector detects at the most upstream detection point that there is no gap between the first sheet and the second sheet, the controller controls the first feeder or the second feeder such that the first speed will be lower than the second speed. 
     
     
         5 . A sheet feeding device according to  claim 1 ,
 wherein each of the detection points has a specified length;   wherein the controller calculates the first speed, the second speed and a size of the gap at each of the detection points as follows:
 the controller calculates the second speed, based on a time necessary for a trailing edge of the first sheet to complete passing by the detection point and the specified length of the detection point; 
 the controller calculates the first speed, based on a time necessary for a leading edge of the second sheet to complete passing by the detection point and the specified length of the detection point; and 
 the controller calculates the size of the gap, based on the specified length of the detection point, the second speed and a time necessary for the gap to complete passing by the detection point; and 
   wherein the controller controls the first feeder or the second feeder in accordance with the size of the gap to adjust the first speed or the second speed.   
     
     
         6 . A sheet feeding device according to  claim 1 , further comprising a corrector for correcting an overlay of the first sheet and the second sheet. 
     
     
         7 . A sheet feeding device according to  claim 1 , wherein the first speed is fixed. 
     
     
         8 . An image forming apparatus comprising a printing section for printing an image on a sheet, said image forming apparatus comprising:
 a sheet feeding device comprising:
 a first feeder for feeding the sheet at a first speed; 
 a second feeder for feeding the sheet fed by the first feeder to the printing section at a second speed; 
 a detector, which is located between the first feeder and the second feeder, for detecting a gap between a first sheet and a second sheet at a plurality of detection points while the first sheet is being fed by the second feeder and the second sheet is being fed by the first feeder; and 
 a controller for controlling the first feeder or the second feeder, 
 wherein when the detector detects that there is a gap between the first sheet and the second sheet, the controller controls the first feeder or the second feeder such that the first speed will be higher than the second speed. 
   
     
     
         9 . An image forming apparatus according to  claim 8 , wherein the controller of the sheet feeding device controls the first feeder or the second feeder such that the gap detected at a more downstream detection point of the plurality of detection points will be smaller than the gap detected at a more upstream detection point of the plurality of detection points. 
     
     
         10 . An image forming apparatus according to  claim 8 , wherein when the detector of the sheet feeding device detects that there is no gap between the first sheet and the second sheet, the controller of the sheet feeding device controls the first feeder or the second feeder such that the first speed and the second speed will be equal to each other. 
     
     
         11 . An image forming apparatus according to  claim 8 , wherein when the detector of the sheet feeding device detects at the most upstream detection point that there is no gap between the first sheet and the second sheet, the controller controls the first feeder or the second feeder such that the first speed will be lower than the second speed. 
     
     
         12 . An image forming apparatus according to  claim 8 ,
 wherein each of the detection points in the sheet feeding device has a specified length;   wherein the controller of the sheet feeding device calculates the first speed, the second speed and a size of the gap at each of the detection points as follows:
 the controller calculates the first speed, based on a time necessary for a trailing edge of the first sheet to complete passing by the detection point and the specified length of the detection point: 
 the controller calculates the second speed, based on a time necessary for a leading edge of the second sheet to complete passing by the detection point and the specified length of the detection point; and 
 the controller calculates the size of the gap, based on the specified length of the detection point, the second speed and a time necessary for the gap to complete passing by the detection point; and 
   wherein the controller of the sheet feeding device controls the first feeder or the second feeder in accordance with the size of the gap to adjust the first speed or the second speed.   
     
     
         13 . An image forming apparatus according to  claim 8 , wherein the sheet feeding device further comprises a corrector for correcting an overlap of the first sheet and the second sheet. 
     
     
         14 . An image forming apparatus according to  claim 8 , wherein the first speed is fixed.