1. Field of the Invention
The present invention relates to an image quality stabilization technique for an image forming apparatus.
2. Description of the Related Art
There are a wide variety of image forming apparatuses, such as electro-photographic printers and inkjet printers. These image forming apparatuses are required to maintain a predetermined level of image quality. One of the factors that induce reduction in image quality is density unevenness (referred to as banding) in a sheet conveyance direction (i.e., a sub scanning direction). In view of the foregoing problem, a conventional technique discussed in Japanese Patent Application Laid-Open No. 2007-108246 discusses eliminating unevenness in the density occurring in the sub scanning direction.
The following contents are discussed in Japanese Patent Application Laid-Open No. 2007-108246. First, a density sensor detects a density unevenness value in the sub scanning direction, beforehand. The density unevenness is a phenomenon that is induced at a period corresponding to an outer diameter of a photosensitive drum. The detected density unevenness is associated with a phase of the photosensitive drum and stored as data of a density pattern information table in a storage unit.
The discussed technique further includes reading, during an image forming operation, density unevenness information corresponding to the phase of the photosensitive drum from the table. Then, the density unevenness occurring at the period corresponding to the outer diameter of the photosensitive drum is corrected based on the read density unevenness information.
A study on the above described banding phenomenon conducted by the applicant of this application has revealed that unevenness in rotation of a motor that drives the photosensitive drum is one of the factors that induce the density unevenness (i.e., banding) in the sub scanning direction.
More specifically, rotational unevenness occurs in the motor when it is rotating due to inherent structural features of the motor, such as the number of magnetized poles. Further, the rotational unevenness occurring in the motor induces unevenness in density. The density unevenness deteriorates an image in quality. At this point, the density unevenness induced by the rotational unevenness occurring in the motor includes relatively higher frequency components. Therefore, to eliminate the density unevenness, it is necessary to detect relatively the higher-frequency components.
However, if an optical sensor is used to read a density value of a test patch toner image in an unfixed state, for example, formed on an intermediate transfer member, detection of density unevenness including relatively higher-frequency components may not be accurately performed. More specifically, in a case where an effective diameter of a light detection area of a light receiving element of the optical sensor is not sufficiently small compared to the length of one period of the density unevenness, the detection cannot be accurately performed as understood from FIGS. 22A and 22B.
FIG. 22A illustrates an example case in which the diameter of a light detection area of the light receiving element on an intermediate transfer member is sufficiently small compared to the length of one period of the density unevenness. In this case, the density sensor can accurately read a phase of the density unevenness.
FIG. 22B illustrates another example case in which the diameter of a light detection area of the light receiving element on the intermediate transfer member is larger than the length of one period of the density unevenness. In this case, a higher density portion is always positioned in the light detection area of the light receiving element. A measured amplitude value of the density variation (intensity information of density variation) becomes smaller due to a detection result averaging effect. The phase of density variation may change. As a result, the detection deteriorates in accuracy.
In particular, the phase of density variation is very important as a parameter in performing correction of the density unevenness. If the density correction is performed based on an erroneously detected phase of density unevenness, the image quality is not so improved or may be rather deteriorated.