Automatic document feeder

An automatic document feeder for feeding a paper, said automatic document feeder comprising:

• a paper guide casing for guiding said paper to be fed into an internal portion of said automatic document feeder;
• a photo interrupter disposed within said paper guide casing, and comprising a transmitting terminal and a receiving terminal;
• a first sensing unit disposed within said paper guide casing, and comprising a first touching part, a first transverse rod and a first sensing part, wherein said first touching part is protruded outside a first end of said paper guide casing, said first sensing part is permitted to be freely swung between said transmitting terminal and said receiving terminal, and said first sensing part has a first slot;
• a second sensing unit disposed within said paper guide casing, and comprising a second touching part, a second transverse rod and a second sensing part, wherein said second touching part is protruded outside a second end of said paper guide casing, said second sensing part is permitted to be freely swung between said transmitting terminal and said receiving terminal, and said second sensing part has a second slot;
• and
• a controller for calculating a skew angle of said paper,
• wherein during said paper is fed, said first touching part and said second touching part are respectively pressed by a first end and a second end of said paper, so that said first transverse rod and said second transverse rod are rotated to swing said first sensing part and said second sensing part between said transmitting terminal and said receiving terminal, wherein as said first sensing part and said second sensing part are swung to completely overlap said first slot with said second slot, said photo interrupter is conducted for a first duration T1, wherein as said first sensing part and said second sensing part are swung to partially overlap said first slot with said second slot, said photo interrupter is conducted for a second duration T2,
• wherein if said photo interrupter is conducted for said first duration T1, said controller judges that said paper is not skewed, wherein if said photo interrupter is conducted for said second duration T2, said controller calculates said skew angle of said paper, and
• wherein after said skew angle of said paper is obtained, said controller judges whether said skew angle is within a correctable range, if said skew angle of said paper exceeds said correctable range, said automatic document feeder is controlled by said controller to stop feeding said paper and emits an erroneous warning message, and if said skew angle of said paper is within said correctable range, said automatic document feeder is controlled by said controller to perform a skew correcting operation.