1. Technical Field
The present invention relates to an image forming apparatus for forming an image by using a toner and a control method thereof.
2. Related Art
In electrophotographic image forming apparatuses, such as printers, copiers and facsimiles, which form images by using the toner, the following procedure is adopted to ensure a predetermined image quality in a consistent manner. As required, a small test image having a predetermined image pattern (patch image) is formed while a density of the image is detected by a density sensor. Operation conditions of individual parts of the apparatus are adjusted based on the detection results.
According to an image forming apparatus set forth in Japanese Unexamined Patent Publication No. 2000-333012 previously disclosed by the present inventors, a tone correction characteristic of the apparatus is controlled based on the detected density of the patch image. Specifically, the densities of the test images of the predetermined pattern are detected by a patch sensor. Then, a gamma characteristic of the apparatus is acquired from the detection results so as to prepare a look-up table for tone correction. With reference to the look-up table thus prepared anew, an image signal is converted into data related to a pulse width to drive an exposure laser. As a result, an optimum print output may be obtained, coping with the variations of the gamma characteristic of the apparatus.
In order to avoid the consumption of a recording material each time an adjusting operation is performed, the general image forming apparatuses adopt a scheme wherein the densities of the test images carried on an internal transfer medium, such as a photosensitive member, a transfer belt or a transfer drum, are detected by means of the density sensor. However, the density of the test image temporarily carried on the transfer medium is not exactly the same as that of the test image finally transferred and fixed to the recording material.
In addition, the inventors have obtained the following finding. In an image forming apparatus of an arrangement wherein an image signal applied from an external source is subjected to a screening process selectively using one of plural types of processing screens previously prepared, the difference between the image density on the transfer medium and the image density on the recording material also varies depending upon the type of processing screen used.
A tone correction technique used in the conventional image forming apparatus involves errors resulting from such a difference between the image density on the transfer medium and the image density on the recording material. Hence, the technique leaves room for further improvement from the standpoint of ensuring the highest quality of the final image on the recording material such as recording paper.
The image forming apparatus of this type needs to keep track of the consumption or residual quantity of toner for convenience of maintenances such as toner replenishment. In some cases, the image signal may be used for this purpose. Since the image signal contains information indicative of a color and a density value of an image to be formed, how much toner is consumed by the image to be formed may be determined based on the information. A large number of proposals have heretofore been made on a technique for determining the toner consumption (hereinafter, referred to as “toner counting technique”). However, it is not easy to determine the toner consumption with high accuracies because of the errors encountered by the aforementioned tone correction technique or the variations of the characteristics of the apparatus. In this connection, the present inventors have disclosed a technique of correcting a toner count given by a toner counter, such as to deal with a case where the errors of the toner counter cannot be completely eliminated (see Japanese Unexamined Patent Publication No. 2004-354666).
According to this technique, when a residual quantity of toner in a toner cartridge, as indicated by a toner count, is not more than a half of an initial value and when a density of a toner image formed as the patch image is lower than a predetermined density, the toner count is so corrected as to be regarded as a toner end even though the toner count indicates some quantity of toner remains in the toner cartridge.
The image forming apparatus of this type may potentially encounter an event of density deficiency, which may be caused by the other factors than the toner end, which include the operation of the apparatus under severe environments such as of high temperature and high moisture, unexpected malfunction of the apparatus, and the like. However, the above conventional technique has the following problem. In a case where such an unexpected density deficiency occurs with the toner count increased to some degree, the density deficiency is necessarily attributed to the toner end irrespective of a real cause of the density deficiency. As a result, even a serviceable toner cartridge actually storing a sufficient quantity of toner may be regarded as unserviceable and may fall out of use in the subsequent image forming operation. Otherwise, a proper troubleshooting may not be performed on the malfunction of the apparatus.