Device for aligning sheets with plural drive roller groups on a common shaft

Abstract

In a sheet feeding device, the local feed of the sheet is measured in the vicinity of respective drive rollers by means of respective sensors for the purpose of aligning rectangular sheets of a preselected set. On the basis of signals from the light sensors, a control device determines an optimum pair of rollers for aligning the sheet. The control device compares the signals of the two associated sensors and adjusts the local feed of the sheet in such a way that the two sensors establish the same local feed, before the sheet leaves one of the selected rollers.

What is claimed is:  
     
       1. A device for aligning a single sheet fed in a plane of alignment in a feed direction, comprising: 
       at least two drive groups which are controllable independently of each other, each drive group comprising a respective drive roller, and a respective pressure roller which contacts the respective drive roller generally in the plane of alignment so that the sheet can be driven between the drive roller and the respective pressure roller wherein the drive rollers of the drive groups are disposed on a common shaft mounted transversely of the feed direction;  
       a position detector operable to detect an angle of twist of the sheet; and  
       a control device responsive to the position detector to adjust a local feed speed of each drive group in dependence upon the angle of twist of the sheet.  
     
     
       2. A device according to claim  1 , wherein the position detector comprises a series of light barriers for detecting the position of the sheet upstream of the drive rollers and sensors for detecting the local feed speed of the sheet. 
     
     
       3. A device according to claim  1 , wherein a single drive motor is provided for jointly driving the drive rollers by means of the common drive shaft, and each pressure roller having a respective lifting device, the lifting devices being controllable independently by means of the control device for lifting the respective pressure rollers from the respective drive rollers. 
         4 . A device according to claim  1 , wherein each drive roller has a stepping motor which is controlled independently by the control device. 
     
     
       5. A device according to claim  2 , wherein each of said sensors comprises an angular encoder which senses rotary movement of a respective measuring wheel which is freely rotatable on a common shaft of the respective  an associated drive roller and a contact wheel being associated with each measuring wheel and rolling on the  a contact surface of the respective measuring wheel. 
     
     
       6. A device according to claim  5 , wherein each measuring wheel has a controllable braking device. 
     
     
       7. A device according to claim  2 , wherein each sensor is a light sensor which has a light transmitter on one side of the plane of alignment and a light receiver on the other side thereof, the control device being operable to measure the local feed in the region of each light sensor based on the degree of shading of the respective light receiver by the sheet. 
     
     
       8. A device according to claim  1 ,  2 ,  3 ,  4 ,   5 ,  6  or  7  wherein there are at least three such drive groups, the distance between each adjacent pair of the drive rollers of the respective drive groups being less than the length, reduced by a predetermined factor, of a smallest sheet of a preselected set of different sized sheets, and the control device being operable to determine an optimum pair of the drive groups from the signals of the position detector for aligning the sheet. 
     
     
       9. A device for aligning a single sheet fed in a plane of alignment in a feed direction, comprising: 
       
         at least two drive groups which are controllable independently of each other, each drive group comprising a respective drive roller, and a respective pressure roller which contacts the respective drive roller generally in the plane of alignment so that the sheet can be driven between the drive roller and the respective pressure roller;  
       
       
         a detector operable to detect an initial angle of twist of the sheet as the sheet becomes engaged by the drive groups and at least one subsequent angle of twist as the sheet is driven by the drive groups; and  
       
       
         a control device responsive to the detector to adjust a local feed speed of each drive group in dependence upon the initial angle of twist and the subsequent angle of twist of the sheet. 
       
     
     
       10. A device according to claim  9 , wherein the detector comprises a series of light barriers for detecting the initial angle of twist. 
     
     
       11. A device according to claim  9 , wherein a single drive motor is provided for jointly driving the drive rollers by means of the common drive shaft, and each pressure roller has a respective lifting device, the lifting devices being controllable independently by means of the control device for lifting the respective pressure rollers from the respective drive rollers. 
     
     
       12. A device according to claim  9 , wherein each drive roller has a stepping motor which is controlled independently by the control device. 
     
     
       13. A device according to claim  9 , further comprising at least two measuring wheels which are freely rotatable on a common shaft with at least two of the drive rollers, and a contact wheel associated with each measuring wheel and rolling on a contact surface of the measuring wheel, wherein the detector comprises an angular encoder which senses rotary movement of at least two of the measuring wheels. 
     
     
       14. A device according to claim  10 , further comprising at least two measuring wheels which are freely rotatable on a common shaft with at least two of the drive rollers, and a contact wheel associated with each measuring wheel and rolling on a contact surface of the measuring wheel, wherein the detector further comprises an angular encoder which senses rotary movement of at least two of the measuring wheels to detect the subsequent angle of twist. 
     
     
       15. A device according to claim  13 , wherein each measuring wheel has a controllable braking device. 
     
     
       16. A device according to claim  10 , wherein the detector further comprises a plurality of light sensors which have a light transmitter on one side of the plane of alignment and a light receiver on the other side thereof, the control device being operable to measure the local feed in the region of each light sensor based on the degree of shading of the respective light receiver by the sheet to detect the subsequent angle of twist. 
     
     
       17. A device according to claim  9 , wherein there are at least three such drive groups, the distance between each adjacent pair of the drive rollers of the respective drive groups being less than the length, reduced by a predetermined factor, of a smallest sheet of a preselected set of different sized sheets, and the control device being operable to determine an optimum pair of the drive groups from the signals of the position detector for alignment the sheet. 
     
     
       18. A device for aligning a single sheet fed in a plane of alignment in a feed direction, comprising: 
       
         at least three drive groups which are controllable independently of each other, each drive group comprising a respective drive roller, and a respective pressure roller which contacts the respective drive roller generally in the plane of alignment so that the sheet can be driven between the drive roller and the respective pressure roller;  
       
       
         a detector including a series of light barriers operable to detect an angle of twist of the sheet, wherein the light barriers detect the initial angle of twist and the detector further comprises a plurality of light sensors which have a light transmitter on one side of the plane of alignment and a light receiver on the other side thereof;  
       
       
         a control device responsive to the detector to adjust a local feed speed of each drive group in dependence upon the angle of twist, the control device being operable to measure the local feed in the region of each light sensor based on the degree of shading of the respective light receiver by the sheet to detect at least one subsequent angle of twist and to adjust a local feed speed of each drive group in dependence on the initial and subsequent angles of twist; and  
       
       
         wherein the distance between each adjacent pair of the drive rollers of the respective drive groups is less than the length, reduced by a predetermined factor, of a smallest sheet of a preselected set of different sized sheets, and the control device is operable to determine an optimum pair of the drive groups from the signals of the detector for aligning the sheet. 
       
     
     
       19. A device for aligning a single sheet fed in a plane of alignment in a feed direction, comprising: 
       
         at least two drive groups which are controllable independently of each other, each drive group comprising a respective drive roller, and a respective pressure roller which contacts the respective drive roller generally in the plane of alignment so that the sheet can be driven between the drive roller and the respective pressure roller;  
       
       
         at least two measuring wheels which are freely rotatable on a common shaft with at least two of the drive rollers, and a contact wheel associated with each measuring wheel and rolling on a contact surface of the measuring wheel;  
       
       
         a first detector comprising a series of light barriers for detecting an initial angle of twist of the sheet;  
       
       
         a second detector comprising an angular encoder which senses rotary movement of at least two of the measuring wheels to detect at least one subsequent angle of twist of the sheet; and  
       
       
         a control device responsive to the detector to adjust a local feed speed of each drive group in dependence upon the initial angle of twist and subsequent of twist of the sheet. 
       
     
     
       20. A method of aligning a single sheet fed in a plane of alignment in a feed direction by at least two drive rollers comprising: 
       
         detecting an initial angle of twist of the sheet;  
       
       
         adjusting the feed speed of each drive roller dependent upon the initial angle of the twist to deskew the sheet;  
       
       
         driving the sheet by the drive rollers;  
       
       
         detecting a subsequent angle of twist of the sheet while the sheet is being driven by the drive rollers; and  
       
       
         adjusting the feed speed of each drive roller dependent upon the subsequent angle of twist if necessary to further deskew the sheet. 
       
     
     
       21. A device for aligning a single sheet fed in a plane of alignment in a feed direction, comprising: 
       
         at least three drive groups which are controllable independently of each other, each drive group comprising a respective drive roller, and a respective pressure roller which contacts the respective drive roller generally in the plane of alignment so that the sheet can be driven between the drive roller and the respective pressure roller;  
       
       
         a detector including a series of light barriers operable to detect an angle of twist of the sheet;  
       
       
         a control device responsive to the detector to adjust a local feed speed of each drive group in dependence upon the angle of twist; and  
       
       
         at least two measuring wheels which are freely rotatable on a common shaft with at least two of the drive rollers, and a contact wheel associated with each measuring wheel and rolling on a contact surface of the measuring wheel, wherein the detector comprises an angular encoder which senses rotary movement of at least two of the measuring wheels, the light barriers detect an initial angle of twist of the sheet and the angular encoder detects a subsequent angle of twist of the sheet, and the control device adjusts the local feed speed of each drive group in dependence on the initial and subsequent angles of twist;  
       
       
         wherein the distance between each adjacent pair of the drive rollers of the respective drive groups is less than the length, reduced by a predetermined factor of a smallest sheet of a preselected set of different sized sheets, and the control device is operable to determine an optimum pair of the drive groups from the signals of the detector for aligning the sheet.