Sheet feeder and image forming apparatus having said sheet feeder

Abstract

Disclosed is a sheet feeder in which sheets are fed by a feeding member from a tray on which the sheets have been loaded. When the tray is driven by a drive unit to lift up the tray from a sheet replenishment position to a position at which sheets can be fed, a first sensor senses that the top sheet on the tray is situated at a prescribed position. Response to this sensing of the first sensor the drive unit performs a predetermined intermittent driving for lifting the tray. It is determined based upon outputs from the first and second sensors whether or not a sheet is present on the tray.

1 . A sheet feeder comprising;
 a tray on which sheets are loaded;   a feeding member configured to contact and feed a top sheet loaded on said tray;   a drive unit configured to lift said tray;   a first sensor configured to sense that the top sheet on said tray lifted by said drive unit is situated at a prescribed position;   a second sensor configured to sense that the top sheet on said tray lifted by said drive unit is situated at a feed position, at which feed is possible by said feeding member, higher than the prescribed position; and   a controller configured to control said drive unit so that said drive unit performs a predetermined intermittent driving for lifting said tray response to a sensing of said first sensor when said tray is lifted by said drive unit, and determine whether or not a sheet is present on said tray based upon the sensed states of said first and second sensors.   
     
     
         2 . The sheet feeder according to  claim 1 , wherein said controller controls so that said drive unit stops a driving response to a sensing of said second sensor. 
     
     
         3 . The sheet feeder according to  claim 1 , further comprising a first flag configured to be moved by coming into contact with the top of a sheet loaded on said tray;
 wherein in a case where a sheet is present on said tray, said first flag changes over the sensed state of said first sensor in a case that the top sheet reaches the prescribed position owing to lifting of said tray.   
     
     
         4 . The sheet feeder according to  claim 3 , further comprising a second flag configured to be moved by coming into contact with the top of a sheet loaded on said tray or with said tray per se;
 wherein said second flag changes over the sensed state of said second sensor by coming into contact with the top sheet in a case that the top sheet reaches the feed position owing to lifting of said tray.   
     
     
         5 . The sheet feeder according to  claim 4 , wherein said controller determines that there are no sheets on said tray in a case that the sensed state of said second sensor has changed, without a change in the sensed state of said first sensor, owing to lifting of said tray. 
     
     
         6 . The sheet feeder according to  claim 1 , wherein said controller drives said drive unit continuously until said first sensor senses that said tray has reached the prescribed position. 
     
     
         7 . An image forming apparatus comprising:
 the sheet feeder set forth in  claim 1 ; and   an image forming unit configured to form an image on a sheet transported from said sheet feeder.