US Patent 7437100 Image forming apparatus

The color offset is reduced with a simple system by relative removal of speed variation components between drums of a tandem color image forming apparatus. Assuming that the speed variation of a magenta photosensitive drum is the largest, the speed variation components of various colors are subtracted from the speed variation component relating to magenta, and the results are added to the respective speed target values. The rotational speed differences of the photosensitive drums of various colors are eliminated by applying the same speed variation component as in the case of magenta to the rotational speed of the photosensitive drums of various colors other than magenta. Phase correction is performed with respect to the phase differences produced by the fact that the distance between drums of various colors is different from the drum peripheral length. Since the phase difference of the photosensitive drums is then zero at the transfer positions of various colors, the effect of speed variation is prevented from appearing in the image as color offset.