Image forming apparatus eliminating influence of fluctuation in speed of a conveying belt to correction of offset in color registration

An image forming apparatus for forming a multi-color image formed by transferring and superimposing a plurality of color component images on a transfer sheet, said image forming apparatus comprising:

• an endless conveying belt conveying the transfer sheet, said conveying belt being driven by a drive roller;
• a plurality of image forming units arranged along said conveying belt, each of said image forming units transferring a color component image on the transfer sheet and also transferring a register mark on said conveying belt;
• a register mark detecting sensor located along said conveying belt for detecting the register mark on said conveying belt;
• and
• a control unit controlling said image forming units so that one of said image forming units forms a first register mark and a second register mark a first predetermined distance away from said first register mark and another one of said image forming units forms a third register mark and a fourth register mark so that said third register mark is formed a second predetermined distance away from said first register mark and said fourth register mark is formed a second predetermined distance away from said second register mark, said first predetermined distance being substantially equal to a distance corresponding to a n/2 rotation of said driving roller, n being an integer,
• wherein an amount of offset of registration of color component images transferred by said image forming units is determined based on an average value of a first amount of offset and a second amount of offset, said first amount of offset being detected based on a pair of said first register mark and said third register mark, said second amount of offset being detected based on said second register mark and said fourth register mark.

An image forming apparatus for forming a multi-color image formed by transferring and superimposing a plurality of color component images on a transfer sheet, said image forming apparatus comprising:

• an endless conveying belt conveying the transfer sheet, said conveying belt being driven by a drive roller in a first direction corresponding to a direction of conveyance of the transfer sheet;
• a plurality of image forming units arranged along said conveying belt, each of said image forming units transferring a color component image on the transfer sheet and also transferring a register mark on said conveying belt;
• a register mark detecting sensor unit located along said conveying belt for detecting the register mark on said conveying belt, said register mark detecting sensor unit comprising a first register mark detecting sensor and a second register mark detecting sensor arranged along a direction of movement of said conveying belt, said second register mark detecting sensor spaced apart from said first register mark detecting sensor by a predetermined short distance;
• and
• a control unit controlling said image forming units so that a first register mark is formed by one of said image forming units and a second register mark is formed by another one of said image forming units so that said second register mark is spaced apart from said first register mark by a distance substantially equal to said predetermined short distance,
• wherein said first register mark and said second register mark are detected by said first register mark sensor and said second register mark sensor substantially at the same time so that an amount of offset of registration of color component images transferred by said image forming units is determined based on a time difference between a detection of said first register mark and a detection of said second register mark.