Sheet stacking apparatus

Abstract

A sheet stacking apparatus including a first tray on which sheets discharged from an outlet are stacked, the first tray being movable between a stacking position at which sheets discharged from an outlet are stacked and a first retracted position above the staking position; a second tray on which sheets discharged from an outlet are stacked, the first tray being disposed below the first tray and being movable independently of the first tray, the second tray being movable between the stacking position and a second retracted position below the stacking position; and a controller that causes the second tray to descend when sheets are to be stacked on the first tray, the controller causing the second tray to stop descending when the second tray reaches a standby position where a distance between the outlet and a top surface of the sheets stacked on the second tray is a predetermined distance.

What is claimed is:  
     
         1 . A sheet stacking apparatus comprising: 
 a first tray on which sheets discharged from an outlet are stacked, said first tray being movable between a stacking position at which the sheets discharged from the outlet are stacked and a first retracted position above the staking position;    a second tray on which the sheets discharged from the outlet are stacked, said second tray being disposed below said first tray and being movable independently of said first tray, said second tray being movable between the stacking position and a second retracted position below the stacking position; and    a controller that causes said second tray to descend when the sheets are to be stacked on said first tray, said controller causing said second tray to stop descending when said second tray reaches a standby position where a distance between the outlet and a top surface of the sheets stacked on said second tray is a predetermined distance.    
     
     
         2 . The sheet stacking apparatus according to  claim 1 , further comprising a sensor that detects the sheets on said second tray when said second tray is at the standby position; 
 wherein said controller causes said second tray to initiate descending movement and then to stop the descending movement just before an output of said sensor changes from “sheet present” to “sheet absent”.    
     
     
         3 . The sheet stacking apparatus according to  claim 2 , wherein when said second tray is at the standby position, said controller causes said second tray to ascend in response to a change of the output of said sensor from “sheet present” to “sheet absent,” and to stop ascending in response to a change of the output of said sensor from “sheet absent” to “sheet present”.  
     
     
         4 . The sheet stacking apparatus according to  claim 2 , wherein when said second tray is descending toward the standby position, said controller causes said second tray to stop regardless of the output of said sensor.  
     
     
         5 . The sheet stacking apparatus according to  claim 2 , wherein when said second tray is descending toward the standby position, said controller causes said second tray to ascend in response to a change of the output of said sensor from “sheet present” to “sheet absent,” and to stop ascending in response to a change of the output of said sensor from “sheet absent” to “sheet present”.  
     
     
         6 . The sheet stacking apparatus according to  claim 1 , further comprising a second sensor that detects that said second tray has descended to reach a lower limit when said second tray is caused to descend to stack sheets onto said first tray, 
 wherein when said second tray is descending toward the standby position, said controller causes said second tray to stop descending in response to the detection that said second tray has reached the lower limit.    
     
     
         7 . A sheet post-handling apparatus having a plurality of stack trays disposed vertically onto which sheets discharged from an outlet are stacked, the apparatus comprising: 
 a plurality of drive devices which drive respective stack trays to ascend and descend;    a sheet surface detecting device disposed below the outlet, said sheet surface detecting device detecting a top surface of a stack tray, a top surface of the sheets, or a stack of sheets on the stack tray;    height controlling means that controls said drive device to move the stack tray to its stacking position in such a way that the top surface detected by said sheet surface detecting device is below the outlet;    retracted position detecting means which when another stack tray moves from above so that the top surface may be detected by said sheet surface detecting device, detects the stack tray or the sheet on the stack tray at a retracted position where movement of the stack tray to the stacking position is not interfered, the retracted position being lower than the stacking position;    movement distance detecting means that detects a movement distance of the stack tray driven by the drive device to the retracted position; and    retraction controlling means that causes a stack tray to stop when said movement distance detecting means has detected that the stack tray has moved from the stacking position already obtained to a position where either the stack tray or the sheets on the stack tray is detected by said retracted position detecting means.    
     
     
         8 . A sheet post-handling apparatus having a plurality of stack trays disposed vertically onto which sheets discharged from an outlet are stacked, the apparatus comprising: 
 a plurality of drive devices which drive respective stack trays to ascend and descend;    a sheet surface detecting device disposed below the outlet, said sheet surface detecting device detecting a top surface of a stack tray, a top surface of the sheets, or a stack of sheets on the stack tray;    height controlling means that controls said drive device to move the stack tray to its stacking position in such a way that the top surface detected by said sheet surface detecting device is below the outlet;    retracted position detecting means which when another stack tray moves from above so that the top surface may be detected by said sheet surface detecting device, detects the stack tray or the sheet on the stack tray at a retracted position where movement of the stack tray to the stacking position is not interfered, the retracted position being lower than the stacking position;    time measuring means that measures time; and    retraction controlling means that causes a stack tray to stop when said time measuring means has detected that the stack tray has moved for a time period calculated based on a distance from the stacking position already obtained to a position where said retracted position detecting means detects the stack tray or sheets on the stack tray, and a speed of the stack tray moving to the retracted position.    
     
     
         9 . A sheet post-handling apparatus having a plurality of stack trays disposed vertically onto which sheets discharged from an outlet are stacked, the apparatus comprising: 
 a plurality of drive devices which drive respective stack trays to ascend and descend;    a sheet surface detecting device disposed below the outlet, said sheet surface detecting device detecting a top surface of a stack tray, a top surface of the sheets, or a stack of sheets on the stack tray;    height controlling means that controls said drive device to move the stack tray to its stacking position in such a way that the top surface detected by said sheet surface detecting device is below the outlet;    retracted position detecting means which when another stack tray moves from above so that the top surface may be detected by said sheet surface detecting device, detects the stack tray or the sheet on the stack tray at a retracted position where movement of the stack tray to the stacking position is not interfered, the retracted position being lower than the stacking position;    movement distance detecting means that detects a movement distance of the stack tray driven by the drive device to the retracted position;    retraction controlling means that causes a stack tray to stop when said movement distance detecting means has detected that the stack tray has moved from the stacking position already obtained to a position where either the top surface of the stack tray or the top surface of the sheets on the stack tray is positioned below and detected by said retracted position detecting means.    
     
     
         10 . A sheet post-handling apparatus having a plurality of stack trays disposed vertically onto which sheets discharged from an outlet are stacked, the apparatus comprising: 
 a plurality of drive devices which drive respective stack trays to ascend and descend;    a sheet surface detecting device disposed below the outlet, said sheet surface detecting device detecting a top surface of a stack tray, a top surface of the sheets, or a stack of sheets on the stack tray;    height controlling means that controls said drive device to move the stack tray to its stacking position in such a way that the top surface detected by said sheet surface detecting device is below the outlet;    retracted position detecting means which when another stack tray moves from above so that the top surface may be detected by said sheet surface detecting device, detects the stack tray or the sheet on the stack tray at a retracted position where movement of the stack tray to the stacking position is not interfered, the retracted position being lower than the stacking position;    time measuring means that measures time; and    retraction controlling means that causes a stack tray to stop when said time measuring means has detected that the stack tray has moved for a time period calculated based on a distance from the stacking position already obtained to a position where either the top surface of the stack tray or the top surface of the sheets on the stack tray is positioned below and detected by said retracted position detecting means.    
     
     
         11 . The apparatus according to  claim 7 , further comprising a lower limit detecting means that detects a lower limit of a stack tray; 
 wherein when the stack tray is moving toward the retracted position, if said a lower limit detecting means detects the lower limit, the stack tray is controlled to stop.    
     
     
         12 . The apparatus according to  claim 11 , further comprising a pre-lower-limit detecting means, disposed between said lower limit detecting means and said retracted position detecting means, for detecting that the stack tray moves to a position close to the lower limit, 
 wherein when said pre-lower-limit detecting means detects that the stack tray has moved to the position close to the lower limit, if said sheet surface detecting means detects that either the top surface of the stack tray or the top surface of sheets on the stack tray has been moved to a position lower than a position that said retracted position detecting means can detect, the stack tray is controlled to stop moving.    
     
     
         13 . The apparatus according to  claim 7 , further comprising ascending movement controlling means for controlling said drive device to cause the stack tray to ascend to a position where said retracted position detecting means can detect either the top surface of the stack tray or the top surface of the sheets on the stack tray, 
 when after the stack tray has moved to the retracted position and has stopped, if said retracted position detecting means detects that either the top surface of the stack tray or the top surface of the sheets on the stack tray is below a position where said retracted position detecting means can detect.    
     
     
         14 . The apparatus according to  claim 13 , wherein if said ascending movement controlling means does not complete ascending movement of the stack tray within a predetermined time period for ascending movement, the ascending movement of the stack tray is controlled to be terminated.  
     
     
         15 . The apparatus according to  claim 7 , further comprising job-initiation controlling means that controls the stack tray to move to a predetermined stop position.