1. Field of the Invention
The invention relates to a belt traveling unit, an image forming apparatus using the same, to a belt traveling unit which detects a position of a belt, an image forming apparatus including the same, and a method of forming an image.
2. Discussion of the Background
An image forming apparatus which forms not only a single color image, but also a multiple color image has been commonly used.
An image forming apparatus capable of forming a multiple color image includes a tandem-type color image forming apparatus.
The tandem-type color image forming apparatus may be equipped with a plurality of photoreceptor drums arranged along a spanned surface, for example, a belt, and directly transfers the color images formed on each of the respective color photoreceptor drums onto the belt. The photoreceptor drums may be in radial or at least partial circumferential contact with the belt. Accordingly, a multiple-color image is formed.
Alternatively, the tandem-type color image forming apparatus may sequentially overlay the color images formed on each of the respective color photoreceptor drums onto a recording sheet transported by the belt. Thereby, a multiple-color image is formed.
When using a belt, in a case where the tension balance is changed in a belt width direction, there may be such a problem that the belt may drift toward a roller around which the belt is wound or toward a shaft direction of a pulley.
When the belt is utilized as a transfer belt on which the toner image is directly transferred from the photoreceptor drum, instead of a belt used as a sheet conveyance purpose, when belt drift occurs, the positional misalignment or color misalignment of toner images of different colors may occur, thereby causing the quality of an image to deteriorate.
For this reason, ways to correct the drift of the transfer belt may be necessary.
In order to correct the drift of the transfer belt, a position detection mechanism and a drift correction mechanism have been proposed, for example.
The position detection mechanism may detect, for example, a position of the transfer belt in the width direction thereof.
Based on a detection signal from the position detection mechanism the drift correction mechanism may control a tilt angle of one of the rollers which support the transfer belt as a drift correction roller.
Accordingly, the transfer belt may be shifted in the width direction thereof, e.g., in response to the tilt angle control, so that the transfer belt returns to its reference position.
The technical difficulty of the drift correction may be to accurately detect the position of the edge of the transfer belt in the width direction without misdetection.
One example of a detection method for detecting the edge position of a belt in the width direction thereof will be described with reference to FIG. 1.
As shown in FIG. 1, the edge position detector at least includes a contact member 130, a spring 170 and a drift detector 150.
The contact member 130 is L-shaped, and the bent portion thereof has a spindle 140 which allows the contact member 130 to rotatively move around the spindle 140 along with the traveling motion of the transfer belt 100.
The spring 170 allows a vertical side 130b of the contact member 130 to abut the edge of the transfer belt 100.
The drift detector 150 is disposed facing a horizontal side 130a of the contact member 130 and serves as a detector which detects the belt position.
According to the above-described structure, when the contact member 130 moves in directions indicated by arrows C1 and C2, in accordance with a traveling motion of the transfer belt 100 in the width direction, the distance between the drift detector 150 and the portion 130a of the contact member 130 may change.
Accordingly, when the drift detector 150 detects the change in the distance, it is possible to detect the position of the transfer belt 100.
The amount of a detectable drift, that is, the traveling amount of the transfer belt 100 in the width direction may be determined by a distance Y which is a distance from the spindle 140 to the transfer belt 100, and a distance X which is a distance from the spindle 140 to the drift detector 150.
With reference to FIG. 2, there is shown an example of characteristics of the drift detector used as a position detector which detects the position of the transfer belt in the width direction.
In FIG. 2, a horizontal axis indicates a distance (mm) between the drift detector and an object to be measured. A vertical axis indicates an output voltage (V).
For example, when the drift detector 150 with the detection range of 2.0 mm is used, the detectable amount of the shift of the transfer belt 100 in the width direction may be 2.0 mm, where X equals Y (X=Y).
In this case, the ratio of X to Y is 1:1 (X:Y=1:1). Thus, the accuracy of the detection of the shift amount of the transfer belt 100 may be equal to the detection accuracy of the drift detector 150.
However, when using the drift detector with the detection range of 5.5 mm to detect the belt position located outside the range of the general use of the detection sensor, for example, the range less than 5.5 mm, the drift correction mechanism may misdetect the position of the transfer belt.
For example, when the position of the transfer belt is at 3.5 mm, the drift correction mechanism may misdetect the position to be at 5.0 mm.
Consequently, the drift correction of the transfer belt may not function properly, and thus the convergence time for recovering the transfer belt to its reference position may be extended. Furthermore, there may be a possibility that the transfer belt is damaged.
In light of the above, it is necessary to detect the amount of the shift of the belt in the width direction in a wide range. When the ratio of X to Y is 1:2, that is, X:Y=1:2, the shift amount of the transfer belt 100 may be 4.0 mm.
On the other hand, while detection in a wide range is made possible, the detection accuracy of the edge position of the transfer belt 100 may be reduced to half the detection accuracy of the drift detector 150.
Thus, the above method may not be desirable. In order to correct the drift of the transfer belt, the edge of the belt needs to be accurately detected.
Other structures for correcting the drift of the transfer belt have been proposed.
One example of such a structure allows the position of the transfer belt in the width direction to be detected within the range of the general use of the drift detector.
In addition, two drift detectors may be used to define the positional relationship of the two drift detectors so that the position of the transfer belt in the width direction may widely be detected, and thus the drift may be corrected.
However, when two drift detectors are used to detect the position of the transfer belt in the width direction, the cost may increase.
In addition, when detecting the belt position at the range less than 5.5 mm of the general use of the drift detector, there is a possibility that the detectors may misdetect the belt position due to characteristics of the drift detector.