1. Technical Field of the Invention
The present invention relates to an image forming apparatus and a driving mechanism for a latent image holding member thereof such as a photosensitive drum, on which an electrostatic latent image is formed by image exposing, which is then developed and transferred to a transfer material.
2. Description of Related Art
Image forming apparatuses which form an image in a manner described above include a facsimile machine, copying machine, printer and other various appliances. In such image forming apparatuses, a latent image is formed on the latent image holding member (photosensitive drum) by exposing it with a laser beam or a light reflected from an original. As for the driving mechanism regarding such image formation, it is desired to drive the photosensitive drum to evenly rotate as its basic function, because irregularity in rotation of the photosensitive drum when exposing the image causes unevenness in pitches of the image and adversely affects the quality of the image. Such effects are more notable in a digital exposure by a laser beam. Further, for forming a color image by superimposing a plurality of images, irregular rotation of the drum causes a severer problem as each color image cannot be accurately registered.
Conventionally, it has been devised to reduce the variation in speed of the driving mechanism in a rotational direction in order to decrease irregularity in rotating movements of the photosensitive drum.
In a general method of driving a photosensitive drum, a drive force is usually transmitted to only one end of the photosensitive drum. This causes deflection of the drum shaft by a load exerted in one direction to one end of the drum, generating a frictional force in the bearing which is the sources of irregular rotation of the drum. The vibration of the shaft is also the cause of the adverse effects to image formation on the photosensitive drum.
The problem caused by the deformation of the shaft may be countered by shortening the length of the shaft, increasing the diameter of the shaft, or reducing the load to the shaft, any of which has though only limited effects due to the restrictions in designs.
It is also assumed that the irregular rotation is ascribable to the impact when the gears come to engagement which generates vibration, as well as the vibration generated in every one cycle of rotation caused by eccentricity of the gears. Conventionally, it has been tried to reduce such components of vibration by increasing the accuracy in dimension of the gears or by providing a flywheel to the drum shaft.
Also a known method is providing a brake to the drum and controlling the torque of the brake. FIG. 1 shows one example of a conventional apparatus for such brake controlling method. A stepping motor 81 is driven by a motor drive circuit 83 according to the commands from a motor controlling circuit 82. The motor drive circuit 83 is connected to a load measuring circuit 84 by which the variation in loads to the stepping motor 81 is continuously measured. A photosensitive drum 85 is driven by the stepping motor 81 via a gear system 86, and is provided with an electromagnetic brake 87 at one extended end of a rotating shaft 85a thereof. Load measuring signals are transmitted from the load measuring circuit 84 to a brake controlling circuit 88 based on which the electromagnetic brake 87 is controlled via a brake driver 89 by feedback control. In this prior art apparatus, when the gear system 86 is made eccentric by a cleaner blade (not shown) or the like contacting with the drum 85, causing variation T.sub.1 in the load to the drum as shown in FIG. 2, the drive torque T applied by the stepping motor 81 is stabilized to be a fixed rate T.sub.0 by applying biased torque with the electromagnetic brake 87 in accordance with the amount of variation T.sub.1. More specifically, the torque T applied by the electromagnetic brake 87 is augmented as shown in FIG. 3 by increasing the electric current I supplied to the electromagnetic brake 87 when the load is decreased (T.sub.a as shown in FIG. 2). Conversely, when the load is increased (T.sub.b in FIG. 2), the electric current I is less applied to the electromagnetic brake 87 in order to reduce the torque T.
As the demand for quality of image has been sharply increased, these problems as described above cannot be any more solved by fabricating the gears more precisely in dimension, due to the limits of machining capacity. The impact generated by engagement between the gears or the vibration generated in every cycle of rotation can be reduced by providing a flywheel to the drum shaft. However, this has no effects at a low frequency area where the additional flywheel causes resonance.
As for the brake controlling method, a follow-up control of an electromagnetic brake provided to the photosensitive drum can be made at the low frequency area but not at the high frequency area. This is because the vibration caused by the engaging actions of the gears at the high frequency area cannot be eliminated with feed back control which has an inevitable time lag. Also, such a complicated control causes the increase in production cost of apparatus.