Patent ID: 8830494

Claim:
A duplex printing apparatus, comprising: a sheet entrance and a sheet exit; a sheet feeding channel connected to said sheet entrance; a sheet transfer channel arranged between said sheet feeding channel and said sheet exit; an inverting channel, wherein a first end of said inverting channel is connected to a junction between said sheet feeding channel and said sheet transfer channel, and a second end of said inverting channel is connected to said sheet transfer channel; an image reading module located in said sheet transfer channel for reading an image of a document; a transfer roller assembly for transferring said document when said document is located within said sheet feeding channel, said sheet transfer channel and said inverting channel; and a sheet sensing module comprising: a first sensing arm at least partially exposed to said sheet feeding channel; a second sensing arm at least partially exposed to said inverting channel; and an electronic sensor, wherein when said document is transferred through said sheet feeding channel to trigger said first sensing arm or said document is transferred through said inverting channel to trigger said second sensing arm, said electronic sensor generates a sensing signal.