Duplex automatic sheet feeder

A duplex automatic sheet feeder, comprising:

• a supply tray for storing a sheet before the sheet is transported;
• a discharge tray for storing the sheet after the sheet is transported;
• a passageway group, which comprises a plurality of passageways, is connected to the supply tray and the discharge tray and has an image processing region;
• a sheet detector, disposed in the passageway group, for detecting the sheet and outputting a sheet detecting signal;
• a transporting mechanism for transporting the sheet from the supply tray to the discharge tray through the passageway group, wherein the sheet is transported across the image processing region at least twice and is processed in the image processing region, and the transporting mechanism has a forward-rotating state and a backward-rotating state;
• and
• a control module, connected to the transporting mechanism and the sheet detector, for controlling the transporting mechanism to transport the sheet and for making the transporting mechanism switch from the forward-rotating state to the backward-rotating state according to the sheet detecting signal and a predetermined physical quantity, wherein the transporting mechanism transports the sheet by a predetermined distance according to the predetermined physical quantity.