Image forming apparatus and image forming method

An image forming apparatus comprising:

• an image forming station including a plurality of photoconductive drums and adapted to form toner images on surfaces of the respective photoconductive drums;
• and
• a drive control unit for rotating and driving the plurality of photoconductive drums;
• wherein:
• the image forming station includes a reference pulse output section for generating a reference pulse having a cycle which is equal to a rotation cycle of the photoconductive drums or an integral multiple of the rotation cycle of the photoconductive drums and outputting the reference pulse to the drive control unit;
• and
• the drive control unit includes:
• a phase peak detector for detecting phase peaks of surface speeds of the respective photoconductive drums,
• a phase difference detector for detecting phase differences between an edge of the reference pulse and the phase peaks, and
• a phase synchronization controller for synchronizing phases of the surface speeds of the respective photoconductive drums by controlling rotation speeds of the respective photoconductive drums according to the phase differences;
• and wherein,
• if the phase peak of the photoconductive drum to be synchronized lags behind the edge of the reference pulse, the phase synchronization controller controls the phase of the photoconductive drum to be synchronized by accelerating the rotation speed of the photoconductive drum at a constant acceleration rate until the phase difference is halved, then decelerating the rotation speed at a constant deceleration rate of the same magnitude as the constant acceleration rate up to an initial speed and, thereafter, setting the rotation speed at a constant speed.

An image forming method for forming toner images on surfaces of a plurality of photoconductive drums by rotating and driving the photoconductive drums, comprising the steps of:

• generating a reference pulse having a cycle which is equal to a rotation cycle of the photoconductive drums or an integral multiple of a rotation cycle of the photoconductive drums and outputting the reference pulse to a drive control unit for driving and rotating the photoconductive drums;
• detecting phase peaks of surface speeds of the respective photoconductive drums;
• detecting phase differences between an edge of the output reference pulse and the phase peaks;
• synchronizing phases of the surface speeds of the respective photoconductive drums by controlling rotation speeds of the respective photoconductive drums according to the phase differences;
• and
• if the phase peak of the photoconductive drum to be synchronized is ahead of the edge of the reference pulse, the phase synchronization controller controls the phase of the photoconductive drum to be synchronized by decelerating the rotation speed of the photoconductive drum at a constant deceleration rate until the phase difference is halved, then accelerating the rotation speed at a constant acceleration rate of the same magnitude as the constant deceleration rate up to an initial speed and, thereafter, setting the rotation speed at a constant speed.