1) Field of the Invention
The present invention relates to an endless-moving-member driving unit, an image forming apparatus, a photosensitive-element driving unit, and a degradation process of endless moving-member. More specifically, the present invention relates to an endless-moving-member driving unit that performs different controls for speed and position of an endless moving-member from regular controls when a defective portion is detected in the endless moving-member.
2) Description of the Related Art
Some image forming apparatuses such as a color copy machine include photosensitive drum belt and an intermediate transfer belt, which are endless moving-members that include endless belt.
In such a color copy machine, it is necessary to accurately control a speed or a position of the photosensitive-drum belt and the intermediate transfer belt because if a position adjustment of different color images (toner images) on the photosensitive-drum belt or the intermediate transfer belt is not accurate, it results in a color shift in an image.
Similarly, in an image forming apparatus in which a transferring material that transfers an image is carried by the endless moving-member, which includes the endless belt, it is necessary to accurately control the speed or the position of the endless moving-member because an inaccurate control of the speed or the position causes the color shift in an image.
In a conventional endless-moving-member driving unit, as it has been disclosed in Japanese Patent No. 3107259, a rotary encoder that detects an angular speed of the rotating body is coupled directly to an axis of a rotating body (the endless moving-member), and a rotational angular speed of a motor that drives the rotating body is controlled based on the angular speed detected by the encoder.
Moreover, in a conventional endless-moving-member driving unit, as it has been disclosed in, for example, Japanese Patent Application Laid-Open Publication No. H6-263281 (see FIG. 9 on page 4), a transfer belt, which is an endless moving-member, has marks on a surface of the transfer belt at regular interval along a direction of movement. The transfer belt is rotated at a constant speed, and an output pattern that is output upon detection of the marks by a sensor is stored in a memory as an output pattern relative to one of the marks. The pattern stored is a reference pattern for a first color. For an each color thereafter, the speed of the transfer belt is controlled such that an output pattern of the sensor corresponds with the reference pattern.
Similarly, in an endless-moving-member driving unit that has been disclosed in, for example, Japanese Patent Application Laid-Open Publication No. H9-114348 (see FIG. 8 on page 5), a recording-paper carrier belt, which is an endless moving-member, has marks on a surface of the recording-paper carrier belt at regular interval along the direction of movement. The movement of the recording-paper carrier belt is directly detected by detecting the marks by a mark detector, and the recording-paper carrier belt is controlled at an ideal belt speed.
However, according to the technology disclosed in the Japanese Patent No. 3107259, the speed of the rotating body is controlled based on the speed indirectly detected through the rotary encoder. Therefore, if the rotating body is formed with an elastic material such as rubber, and if the rotating body stretches or contracts while rotating, the speed cannot be controlled accurately.
The technologies disclosed in the Japanese Patent Application Laid-Open Publication No. H6-263281 and H9-114348 also have a problem. Although a method of forming the marks on the belt is not mentioned in the above patent literatures, since the transfer belt in the image forming apparatus is generally made of an elastic material such as rubber. Due to the flexibility and the deviation in the circumference of the belt, it is very difficult to provide the marks accurately at constant interval without a gap throughout the circumference.
If the marks are formed by preparing convex and concave portions in a mold with which the belt is formed, an annealing process is normally necessary for the molded belt after removing from the mold. During the annealing process, if the belt is not heated uniformly, it cannot realize the regular interval of the marks with high accuracy. Moreover, if an internal distortion that is developed in the molded belt, the coefficient of contraction becomes not even throughout the belt, it becomes difficult to arrange the marks at regular interval at high accuracy.
If the marks are provided by printing, or by sticking, on the belt, a material on which the marks are printed, a deviation occurs in the belt. For example, if a circumferential tolerance of 0.2% to 0.3%, for a 500 mm long belt, the deviation is not less than 1 mm. Therefore, it is difficult to form the marks accurately at regular interval without a gap.
In an arrangement where the speed control of the belt is performed by providing the marks to detect the speed of the belt, there is a problem in which breaks in signals, which is output from the sensor, occur not only when there is a gap in the marks regularly arranged, but also when there are dirty marks or damaged marks because the sensor cannot detect such marks.
In a typical image forming apparatus, units that use materials that cause contamination such as toner are used near the transfer belt; the transfer belt may get stained easily.
Regarding the gap of the marks, which is formed at a joint of the circumference, since presence of the gap and a position in the direction of movement on the belt are known, the gap can be detected by providing a mark for detecting the gap and a sensor to detect the gap. Therefore, the belt can be controlled to a constant speed by performing a different control from a regular control when the gap is detected.
However, the contamination and damage of the marks are not developed at an initial stage of the use, and tend to be gradually developed according to the elapse of time while using the equipment. Therefore, a position of the contamination and the damage developed in the direction of the movement on the belt is not known.
To cope with this problem, if the speed control is changed to a different control (an alternative control) from the regular control also when the signal output from the sensor is stopped due to the contamination and the damage, similarly to when the gap of the marks is detected, the speed of the belt can be controlled throughout the circumference of the belt.
However, when a faulty image is output after the speed control is changed to the alternative control (a dummy-signal control), a user cannot realize a reason of the faulty image is because the speed control is changed to the alternate speed control.
Furthermore, with the elapse of time, the contamination and the damage on the marks increase. As a result, the alternative control, which is less accurate compared to the control based on the marks, is frequently performed, and the problem above becomes more likely to occur.