| 摘要 |
A method and apparatus for detecting double fed sheets. A sheet passes beneath a roller which is mounted on a lever so that the opposite end of the lever is deflected by an amount proportional to the thickness of the sheet. A magnet is attached to the opposite end of the lever approximate to a Hall Effect sensor fixed to the frame of the apparatus so that the hall sensor produces a signal proportional to the thickness of the sheet. The output of the hall sensor is sampled by an A/D convertor and the signals are input to a computer for processing to detect double fed sheets. Average thicknesses for subsequences of samples distributed over the sheet are computed and compared to reference levels. The length of the sheet is also compared to a reference length. If, for any of these comparisons the measured values are greater than the references a doubled detect signal is generated. In one embodiment leading and trailing edges of the sheet may be detected by detecting transitions in the sequence of signals which are greater than the design minimum sheet thickness. In another embodiment of the subject invention the reference levels are established by first measuring a selected, assured single, initial sheet. In another embodiment of the subject invention the references are updated after each sheet by combining a portion of the previous reference value, preferably 7/8th's, with a portion, preferably 1/8th, of the measure value multiplied by an appropriate scale factor.
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