Patent ID: 7366444

Claim:
A tandem color image forming apparatus, comprising: a monochrome photoconductive member configured to bear a monochrome image, said monochrome photoconductive member configured to be driven by a first driving motor; at least one mono color photoconductive member configured to bear a mono color image, said at least one mono color photoconductive member configured to be driven by a second driving motor; a first detection objective attached to the monochrome photoconductive member; a second detection objective attached to the at least one mono color photoconductive member; a first detecting device configured to detect the first detection objective and generate a first pulse signal having a prescribed voltage; a second detecting device configured to detect the second detection objective and generate a second pulse signal having a prescribed voltage, said second pulse signal having a same width as the first pulse signal generated by the first detecting device; a time measuring device configured to measure a time difference between a time when one of the first and second detecting devices firstly generates either said first pulse signal or said second pulse signal, respectively, and a time when the other one of the first and second detecting devices firstly generates said second pulse signal or said first pulse signal, respectively, after measuring start; a phase calculation device configured to calculate a difference in phase between the monochrome photoconductive member and the at least one mono color photoconductive member in accordance with the time difference; and a control device configured to a control driving speed of at least one of the first and second driving motors so that the phase difference decreases to a prescribed level, wherein said first and second detecting devices are configured to start detecting during image formation.