1. Field of the Invention
The present invention relates to image-forming apparatuses with ink-eject heads that enable to eject inks to form images on image-formed media.
2. Description of Related Art
There has been widely used an ink-jet recording apparatus as recording means for recording images on image-formed media, such as paper, based on image information. The ink-jet recording apparatus is usually applied on printers, facsimiles or copying machines, and the recording images include characters or symbols in general.
During the recording of images, however, the landing of ink droplets from respective ink-eject heads tends to deviate from predetermined positions. This will be below called the “landing misalignment”. The landing misalignment is divided into two components: the DC component that shifts with at a constant amount; and the AC component that fluctuates periodically.
The AC component of a landing misalignment is caused by unevenness in the thickness of a belt or unevenness in the radius of a roller. In order to reduce a landing misalignment with the circumferential length of a belt as one cycle due to the former unevenness or a landing misalignment with the circumferential length of a roller as one cycle due to the latter one, the following methods have been conventionally proposed. Note here that the former cycle will be below called the “belt cycle” and the latter called the “roller cycle”.
[Landing Misalignment with Belt Cycle]
(1-1) There is proposed a method to enlarge the diameter of a roller or reduce the thickness of a belt so as to make unevenness in the thickness of the belt relatively small. The former, however, makes an image forming device or its components large and heavy and the latter makes its endurance weak. This method can reduce but cannot eliminate a landing misalignment.
(1-2) There is proposed a method to measure the rotation speed of a belt by bringing a roller into contact with the surface of the belt without any influence of unevenness in the thickness of the belt.
Japanese Patent Publication Laid-open No. 2004-188921 discloses a method to measure the thickness of a belt using a laser Doppler measurement device.
The measurement of the rotation speed of a belt at a contact line of the roller and belt, however, frequently gets false detection due to slide or vibrations. Increasing the pressure of the roller to prevent the slide causes the deformation of the belt. Accordingly, the measurement receives some influence of unevenness in the thickness or elasticity of the belt. Also, the laser Doppler measurement device is quite expensive. Moreover, these methods get detection error due to paper dust or ink-mist.
(1-3) Japanese Patent Publication Laid-open No. 2000-356875 discloses a method to measure the thickness of a belt in advance and to adjust a recording timing or transfer speed based on obtained data. This method, however, requires a thickness measurement device with high-accuracy so as to accumulate minute unevenness in thickness. Moreover, when a multilayer belt is used, this method cannot deal with landing misalignments other than that due to unevenness in thickness.
(1-4) Japanese Patent Publication Laid-open No. H10-186787 discloses a method to measure and correct unevenness in the thickness of a belt by reading a registered pattern on the belt surface at a position different from recording positions. This, however, requires reading means with high-accuracy and high-speed. In addition, the measuring ability of the reading means decreases due to paper dust or ink-mist.
[Landing Misalignment with Roller Cycle]
(2-1) Japanese Patent Publication Laid-open No. H03-2067 discloses a method to eliminate a landing misalignment with a roller cycle by adjusting intervals between image-recording units to the roller cycle. This method, when combined with the method (1-1) or (1-2), enables to reduce the landing misalignment due to the unevenness of a belt and roller.
When the method (2-1) is combined with the method (1-1), an image-forming device is made large because it also requires widening an interval between ink heads. This causes to degrade the accuracy of the manufacturing dimension of head mounting units and to increase the deformation of these units. Also, when the method (2-1) is combined with the method (1-2), it cannot solve the problem of the method (1-2) itself.
Japanese Patent Publication Laid-open No. H03-2068 discloses a method to resolve unevenness with a belt cycle by using the method (1-2) and unevenness with a roller cycle by storing the roller cycle and performing correction. However, this method cannot solve the problem of the method (1-2), either.