Patent Publication Number: US-2006017215-A1

Title: Vorrichtung zum vereinzeln von in einem stapel vorgehaltenen blättern

Description:
The invention concerns a device for separation of sheets held in a stack according to the preamble of claim  1 .  
      Sheet-like objects, like paper sheets, paper pages, photos, films, package wrappings, etc. are often kept stacked in order to be fed separately from this stack for further processing, for example to a printer, copier, packaging machine, etc. Such sheet-like objects are referred to subsequently for short as “sheets”.  
      Devices used for this purpose are generally based on the basic principle that in each case the uppermost sheet of the stack is raised, pulled off or pushed off from this stack in order to then be separated by a separating device from the following second sheet of the stack and fed to further processing. The uppermost sheet can be grasped by suction devices, which, however, are costly, since a vacuum system is required. It is also known to push the uppermost sheet by friction from the stack with moving friction fingers or rotating friction rollers. However, these devices only operate reliably in a specific range of sheet qualities, like sheet rigidity, sheet weight and sheet surface condition. For different sheet qualities different devices must therefore be used or the devices must be adapted to the sheet qualities.  
      Devices that reliably separate over a very broad range of sheet qualities operate with pickup devices. Such pickup devices are known for example from DE 196 41 973 A1, DE 100 16 793 A1 and DE 44 44 836 A1. One or more rollers in these pickup devices are positioned under pressure on the uppermost sheet in the stack and moved in the direction of advance over the stack. Because of this a pickup effect is exerted on the uppermost sheet of the stack, during which the sheets are arched wave-like in front of the rollers running over the stack. The upper sheets are then pushed in overlapping fashion from the stack, the uppermost sheet being pushed forward most strongly and the subsequent sheets being pushed by decreasingly smaller advance paths. If the path difference between the uppermost sheet and the following second sheet is sufficient, the uppermost sheet can be grasped by a separation device and separated from the following second sheet. Such separation devices are known in different forms. The front edge of the uppermost sheet can be introduced into the gap of a pair of transport rollers that grasp the sheet and pull it off (for example DE 196 41 973 A1). The front edge of the sheet can be grasped and pulled off by a gripper (for example DE 195 43 382 A1). It is also known to push the uppermost sheet with a front edge on an obliquely rising ramp to raise it from the following sheet (for example EP 534 245 A1). Finally, it is also known to push the uppermost sheet with its front edge against a belt running steeply upward, to entrain the front edge of the uppermost sheet under friction in order to raise it from the following second sheet (for example JP 62240237A).  
      During separation with pickup devices the overlapping fanning out of the upper sheets is due to the advance speed of the sheets, which diminishes from the top down. If the uppermost sheet is pushed forward more quickly than the following sheets, even with a short advance path a sufficient protrusion of the uppermost sheet is obtained relative to the second sheet in order to guarantee reliable separation of the uppermost sheet. If the advance speeds are only slightly different, a longer advance path is required until a sufficient protrusion of the uppermost sheet beyond the following second sheet is achieved for reliable grasping. This means that a large advance path, i.e., a large distance between the stack and the separation device is advantageous in order to achieve reliable separation. If this separation is too small, the probability of double separation increases. On the other hand, a larger distance between the stack and separation device, i.e., a greater advance path, means a slower separation period. 
    
    
      The underlying task of the invention is therefore to devise an apparatus that combines high reliability of separation with high separation speed.  
      This task is solved according to the invention by a device with the features of claim  1 .  
      Advantageous variants of the invention are mentioned in the dependent claims.  
      The essential idea of the invention consists of arranging the stack of sheets being held for separation and their receiving devices and separation not at a set spacing but making this spacing variably adjustable. Because of this it is possible to adjust the distance between the stack and separation device so that this distance is sufficiently large in order to guarantee sufficiently reliable separation of the uppermost sheet from the following second sheet but that this spacing, on the other hand, is not needlessly large and leads to an avoidable delay in the separation process on this account.  
      Adjustment of the distance between the stack and separation device can then allow for the sheet quality of the sheets being separated. Such characteristics of sheet quality include thickness, rigidity and surface condition of the sheets. The sheet advance and overlapping of the sheets during pickup are also dependent on sheet moisture content, i.e., atmospheric humidity of the surroundings. The ambient temperature also has a not insignificant effect on sheet displacement during pickup.  
      In order to adjust the distance between the sheet stack and the separation device, the sheet stack or the device carrying it can be moveable relative to a fixed separation device. It is also possible to design the separation device moveable relative to the fixed stack. Displacement of the sheet stack or separation device can be carried out manually in a simple variant. A linear drive is preferably used to displace the stack or separation device, in which a stepping motor is preferably used for the controlled linear drive. This type of linear drive can be implemented, for example, by racks and by pinions driven by the stepping motor. Otherwise, the design of such a linear drive lies within the range of expert knowledge.  
      The distance between the stack and the separation device is preferably set automatically for which purpose a computer-controlled drive of the stepping motor is preferably provided. For this purpose the humidity in the surroundings of the device can be determined by a hygrometric sensor. The humidity values determined by the sensor are fed to the computer, which calculates the optimal spacing corresponding to the humidity according to a stipulated program. In addition, the air temperature in the surroundings of the device can also be determined by a sensor in order to also feed the temperature value to the computer and allow for it during distance calculation. Measurement of the humidity and optionally at a temperature preferably occurs continuously during the entire operation of the device so that it can be automatically adjusted to any changing ambient conditions.  
      The quality of the sheets being separated is of special significance for separation by pickup and therefore also for optimal spacing of the stack and separation device. This can be considered according to the invention by the fact that a basic distance between the stack and separation device is set according to the sheet quality and this basic distance is automatically varied according to the ambient conditions determined by the sensors. It is then expedient to enter the sheet quality in the computer so that this sheet quality can already be considered in the computer program during calculation and automatic adjustment of the optimal spacing.  
      A simpler operation of the device can be implemented by automatically determining the sheet quality during startup of the device in a learning cycle, so-called teach-in-cycle in order to then be stored for further calculation of the optimal spacing as a function of the ambient conditions.  
      It is readily apparent that the invention does not depend on the special embodiment of the pickup device and the separation device. Both the pickup device and the separation device can be implemented in the context of the invention in any technique known to one skilled in the art, especially in the manner stated in the introduction concerning the prior art.