| 摘要 |
PROBLEM TO BE SOLVED: To prevent a developing unit from being damaged by easily recognizing the abnormality of a developer carrier based on the detected density of a patch image in the case that one developer carrier becomes abnormal when a developing action is executed by using two-component developer by a developing means having the plural developer carriers, a patch image (reference image) for controlling developer concentration is formed on an image carrier and the concentration of the developer is controlled by detecting the density of the patch image. SOLUTION: One patch image is formed by the developing action of the first and the second developing sleeves 41 and 42 of the developing unit 4 at an image forming time. Then, the concentration of the developer is controlled by comparing the density value of the patch image with an initial density value. Besides, two patch images are formed by developing the patch images one by one by the respective sleeves 41 and 42 at a post-rotating time and a rotating time dedicated for controlling after the image is formed. Then, the concentration of the developer on the sleeves 41 and 42 is controlled by comparing the density values of the respective patch images with the respective initial density values. Besides, the abnormality of the respective sleeves 41 and 42 is checked. |
