Image forming apparatus having a primary transfer unit, a secondary transfer unit, and a direct transfer unit

An image forming apparatus comprising:

• a first image carrier;
• a first image forming unit that forms an image on the first image carrier;
• an intermediate transfer belt on which the image formed on the first image carrier is primarily transferred;
• a primary transfer unit that primarily transfers the image formed on the first image carrier onto the intermediate transfer belt;
• a secondary transfer unit that secondarily transfers the image transferred onto the intermediate transfer belt further onto a recording medium;
• a second image carrier that is arranged upstream or downstream of a secondary transfer position, where the image is transferred from the intermediate transfer belt onto the recording medium, in a recording sheet conveying direction;
• a second image forming unit that forms an image on the second image carrier;
• a direct transfer unit that directly transfers the image formed on the second image carrier onto the recording medium;
• a recording medium carriage belt
• that carries and conveys the recording medium to pass through a direct transfer position where the image is directly transferred from the second image carrier onto the recording medium, and carries and conveys the recording medium to the secondary transfer position, and
• that is rotatably stretched across a plurality of roller members;
• a driving roller
• that is one of the roller members that stretches the recording medium carriage belt, and
• that drives the recording medium carriage belt in rotation;
• a speed fluctuation detecting unit that detects a cyclic speed fluctuation of the recording medium carriage belt;
• and
• a drive control unit that controls driving of the driving roller so as to reduce the cyclic speed fluctuation of the recording medium carriage belt based on a detection result of the speed fluctuation detecting unit,
• wherein:
• the speed fluctuation detecting unit includes
• a pattern image detecting unit
• that detects a plurality of pattern images which are formed on the first image carrier and then transferred onto the recording medium carriage belt via the intermediate transfer belt, and
• that is arranged at a position facing an outer surface of the recording medium carriage belt,
• the drive control unit
• obtains a speed fluctuation of the recording medium carriage belt at a rotation cycle of the driving roller based on detection data detected by the pattern image detecting unit, and
• controls driving of the driving roller so as to reduce the speed fluctuation, and
• an interval, which is along a rotating direction of the recording medium carriage belt between the secondary transfer position and a detecting position of the pattern image detecting unit detecting the pattern images, is set so as not to be a positive integral multiple of a circumferential length of the driving roller.