Document processing machinery should be designed to yield high speed document transport, yet there are limitations in how fast it can operate. For example, in a check sorter the electromechanical gates which open and close to direct a document into a selected pocket, can only operate so fast--so the interdocument gap becomes important. If the documents are fed too fast, a shortened gap will cause errors such as improper sorting or failure to sort.
And, if one increases document transport speed, this can increase the inter-document gap, but can result in document damage as well as processing and stacker errors.
A further problem is that components involved with the feeding of documents typically rely on mechanical friction, hence the components will wear away and change dimensions; also they are influenced by environmental factors such as temperature and humidity. One way to approach these problems is to choose an operating point which allows for contemplated wear and environmental concerns. While this can be effective, it implies some sacrifice of performance.
Another way to allow for such variable factors is to keep the gap between documents relatively constant. (e.g. see U.S. Pat. Nos. 4,451,027 and 4,331,328.) Typically, document transports are limited in performance by the inertia of the pinch-rolls they must accelerate.--These calling for a lot of power and apt to generate excessive heat. Other limitations and disadvantages of prior art systems are apparent to those skilled in the art of document processor control systems.
Other related art is the following:
U.S. Pat. No. 5,197,726, directed to sheet transportation systems that calculate a target time for sheet arrival at a downstream position and vary the transport speed so that the sheet arrives at the desired time. The sheet feeder has a control unit that receives signals from sheet detectors and controls sheet transport by controlling the speed and time of selected motors; e.g. calculated so that the sheet arrives in time at a registration roller even though it was determined by the sheet feeder.
U.S. Pat. No. 5,094,442 is directed to a sheet positioning system that performs longitudinal and lateral alignment in a sheet path without guides or gates. A sheet is skew-registered by a unit having two drive rolls driven by separate speed control stepper motors. A sheet is aligned laterally by a carriage, which is positioned by a drive system that includes a speed controlled stepper motor and a lead screw. Detectors or sensors supply sheet position signals to a controller for determining appropriate drive signals to the motors for aligning the sheet.
U.S. Pat. No. 5,121,915 is directed to a document processor that has closed loop control of the feed rate, gaps, and input station so that more documents can be processed per minute, even as the mechanism changes because of wear and the environment. A system manager and separator processor card receive input from document sensors and performs a closed loop control of drive motors. The closed loop control includes velocity feedback from the motors to the processor.
U.S. Pat. No. 5,018,716 is directed to an automatic document feeder that adjusts the transportation speed based on the operational state of the transport mechanism. Documents are fed from a roll to a separation unit and then to a feed path. Sensors on the stacker for registration, and a sensor at the discharge point supply signals to a micro-computer for controlling the separation motor, belt motor, and carrier motor. Based on the first document that passes through the system, a learning feature thereafter adjusts the speed of the belt-motor for improved operation.
U.S. Pat. No. 5,186,449 is directed to a sheet feeder unit that calculates the sheet transportation speed to prevent sheet overlap. The sheet transport mechanism feeds copy paper from a unit past sensors, one being activated when the paper hits a feed roller. A control unit analyzes the sensor inputs and selects the appropriate sheet feeder interval.
It is an object hereof to alleviate such problems and provide at least some of the here-described features and advantages. A more particular object is to provide means for increasing spacing between documents in an intermediate section of a document transport. Another object is to correct occasional small spacings that may occur due to improper feeding from a document stack or due to document slip at aligner mechanisms. A more particular object is to provide means for spacing correction, performed by changing transport speed in an intermediate section of the transport, rather than by changing transport speed at the input segment of the transport.
A further object is to avoid conventional solutions such as adjusting speeds of rollers, etc. in the initial length of a transport path because these may necessarily have large inertias because of their specific functions, such as aligning or feeding. These inertias may be impractical or difficult to decelerate and accelerate in order to increase the space between documents.
Yet another object is to provide a document transport system with means for "under-spacing" detection plus associated transport decelerate/accelerate means which are disposed at an "intermediate" transport section, not an initial or terminal sections of the transport path.