The present invention relates to an image density control method for an image forming apparatus and, more particularly, to an image density control method for an image forming apparatus which controls various image forming conditions such as toner density on the basis of a result of image density detection which is executed in a pattern detection mode.
Generally, an electrophotographic copier, facsimile machine, laser printer or similar image forming apparatus uniformly charges the surface of an image carrier in the form of a photoconductive element by a charger, then exposes the charged surface to imagewise light representative of an image to be recorded to thereby form an electrostatic latent image on the charged surface, develops the latent image by using a developer which is stored in a developing unit, and transfers the resulting toner image to a paper sheet. The density and other conditions associated with the toner image are dictated by various image forming conditions such as the density or content of toner which is contained in the developer. The image forming conditions such as toner density in the developing unit have customarily been controlled by sensing the density of the toner image in terms of reflectance or similar factor, and comparing the sensed image density with a reference value. By so controlling the image forming conditions, it is possible to maintain the density and other conditions associated with the toner image constant.
The detection of image density is extensively implemented by an optical detection system. The optical detection system is operable in a pattern detection mode in which the uniformly charged surface of the photoconductive element is exposed to an imagewise reflection from a reference pattern which has predetermined reflectance, the resulting latent image is developed by the toner to produce a toner image, and the density of the toner image is sensed and compared with a reference density. The control of image density or similar image forming condition which adopts such a pattern detection mode maintains the toner density in the developer constant by detecting a change in image density caused by a change in developing ability which is ascribable to the deterioration of the developer and the varying ambient conditions, a change in charge potential which is ascribable to the deterioration of the photoconductive element or that of the wire of the charger or the variation of a source voltage, and a change in the quantity of light for imagewise exposure. This kind of control is therefore quite effective to maintain the density of developed images constant. Typical of the image forming conditions to be controlled are the toner density mentioned above, amount of charged deposited on the toner, bias voltage for development, characteristics of the developer, gap for development, developing speed (developing time), amount of charge deposited on the photoconductive element, characteristics of the photoconductive element, amount of imagewise exposure, and sensing characteristics.
A problem with the image density detection which relies on the pattern detection mode is that when the charge or the quantity of light is irregularly distributed in a surface portion of the photoconductive element which corresponds to a reflection from the reference pattern, the image forming conditions such as toner density are apt to be overcorrected. For example, assume that the charge potential deposited on the photoconductive element is lower in the above-mentioned particular surface portion than in the other portion due to local contamination of the charger wire. Then, the sensed image density will be lower than the actual image density. In this condition, supplying the toner based on the result of detection would cause the toner density to become higher than an ordinary density and, in the worst case, smear the interior of the machine. Further, it sometimes occurs that the potential on the surface of the photoconductive element is varied over the entire surface and not in a part thereof due to the deterioration and fatigue of the element itself and the varying ambient conditions, also resulting in the above-discussed problem.
In the light of this, there has been proposed an image density control method which measures and controls a developing characteristic only, as disclosed in Japanese Patent Application No. 56-159637. The method disclosed in this Patent Application consists in providing on a photoconductive element a charged portion which is close to zero volt, or saturation potential, developing this portion under a bias potential which is opposite to a predetermined bias voltage adapted for ordinary image forming operations, sensing the density of the resulting toner image, and comparing the sensed density with a predetermined value. This kind of approach causes a toner to adhere to a photoconductive element by using a potential close to the saturation level at which the surface potential of the element is most stable. Hence, it is not necessary to take account of a change in image density due to changes in image forming conditions other than the developing ability of the developer itself, i.e., charging characteristic, developing characteristic and the like, whereby rapid and accurate image density control is achievable. However, this prior art approach has a drawback that in the image density detection mode the bias voltage for development has to be switched over to the opposite side to the bias vltage for ordinary image forming operations. Another drawback is that a complicated power supply circuit is needed.
Another approach is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 48-29488. In this Laid-Open Patent Publication, an electrode is wound around a part of a photoconductive element so that the surface of the element where the electrode is located may be held at a predetermined potential. This, however, brings about another shortcoming that the electrode itself and the arrangement for mounting it on a photoconductive element are complicated.
Although U.S. Pat. No. 4,632,537 (Imai) and U.S. Pat. No. 4,619,522 (Imai) also teach techniques relating to the image control method of the present invention, they are entirely different from the method of the present invention.