Image forming apparatus

Abstract: An object of the present invention is to perform a well tonality reproduction according to a state of an image support body. In order to achieve the object, the present invention is featured by comprising the image support body for supporting an image which is to be formed on a medium, a signal generation means for generating an image signal representing the image, a detection means for detecting a value according to a membrane thickness of the image support body, and a conversion means for converting the image signal on the basis of the detected value according to the membrane thickness.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an electrophotographic image forming apparatus. Related Background Art Conventionally, it has been well known that, in an electrophotographic image forming apparatus, if a using state or using environment of a photosensitive drum or the like which acts as an image support body changes, a characteristic of the photosensitive drum also changes so that a significant change occurs in an output image. For example, as the temperature of the photosensitive drum rises, sensitivity of a photosensitive body rises, so that an output density changes. Further, as the photosensitive drum is continuously used, the membrane thickness of the photosensitive body gradually decreases. As a result, charge performance of the drum decreases and also the sensitivity deteriorates, so that the output density changes. Furthermore, due to decreasing of the membrane thickness of the photosensitive body, the scattering characteristic of an irradiation light on the photosensitive drum changes, so that reproducibility of fine or micro dots also changes. As a result, the charge performance of the photosensitive drum changes and also an output density characteristic changes to the extent which cannot be reasonably explained based on the change in sensitivity. In addition, it has been known that the change in the using environment of the apparatus affects not only photosensitive drums but also associated development units. That is, as a humidity in the using environment changes, a charge characteristic of developing powder in the development unit changes, so that the significant change occurs in the output density characteristic. In order to solve such problems as described above, various methods have been applied. For example, with respect to the change in the membrane thickness of the photosensitive drum, it has been proposed a method in which a detection means is provided to detect the membrane thickness of the photosensitive drum, and a voltage to be applied to a charge unit is changed according to a detection signal from the detection means, so as to correct the change in the charge performance of the photosensitive drum. Similarly, it has been known a method in which a voltage to be applied to a lamp for illuminating an original in accordance with the detection signal obtained by detecting the membrane thickness is changed to correct the change in the sensitivity of the photosensitive drum. Further, with respect to change in the temperature and the humidity, it has been proposed, eg in US-A-5305057, a method in which the temperature at a periphery of the photosensitive drum is detected, and the voltage to be applied to the charge unit is changed according to the detected temperature and the humidity so as to correct the change in performance of the photosensitive drum. However, in such the conventional methods, it was difficult to stabilize the output density characteristic against the changes in the sensitivity characteristic and the charge performance occurred due to the change in the membrane thickness of the medium such as the photosensitive drum or the like. Especially, a tonality characteristic including a halftone could not be well maintained against the change in the membrane thickness. The present invention was made in consideration of such circumstances, and thus a concern of the present invention is to provide an image forming apparatus which can realize good tonality reproduction irrespective of the state of an image support body. EP-A-0555102 discloses an image forming apparatus which includes a photosensitive image bearing member and a charging member contactable to the image bearing member for charging the latter. The apparatus further includes a detector for detecting electric current flowing through the charging member when it is charging a predetermined area of the image bearing member and this current detection is used to detect thickness of the photosensitive member. According to the present invention there is provided an image forming apparatus as set out in claim 1. Another concern of the present invention is to perform a well tonality characteristic control with constantly maintaining a printer characteristic. A further concern of the present invention is to perform an accurate measurement of life of a photosensitive body and also perform the tonality characteristic control based on such the measurement. The above and other features of the present invention will be apparent from the following detailed description given by way of example and in conjunction with the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective side view of an image forming apparatus according to embodiments of the present invention; Fig. 2 is a view illustrating structure of a membrane thickness detection unit; Fig. 3 is a view illustrating an image signal process circuit 4 and the like; Fig. 4 is a view illustrating a gamma (γ) conversion unit; Fig. 5 is a view illustrating a gamma (γ) conversion characteristic; Fig. 6 is a view illustrating a change in an output density characteristic for a membrane thickness of a photosensitive body of a photosensitive drum, in a case not according to the present invention; Fig. 7 is a view illustrating a change in an output density characteristic for a membrane thickness of a photosensitive body of a photosensitive drum, in a case according to a first embodiment of the present invention; Fig. 8 is a view illustrating a change in an output density characteristic for a membrane thickness of a photosensitive body of a photosensitive drum, in a case according to a second embodiment of the present invention; Fig. 9 is a view illustrating a change in an output density characteristic for a temperature and a humidity of a photosensitive drum, in a case not according to the present invention; Fig. 10 is a view illustrating a change in an output density characteristic for a temperature and a humidity of a photosensitive drum, in a case according to a third embodiment of the present invention; Fig. 11 is a view illustrating a change in an output density characteristic for a temperature and a humidity of a photosensitive drum, in a case according to a fourth embodiment of the present invention; and Fig. 12 is a flow chart showing a process procedure according to a fifth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be explained in detail with reference to the accompanying drawings. (First Embodiment) Fig. 1 is a perspective side view of an image forming apparatus according to the first embodiment of the present invention. In the apparatus, a charge member 7 is arranged to be attached to a photosensitive drum 6. By applying a predetermined voltage from a high-voltage power source 11 to the charge member 7, the photosensitive drum 6 is uniformly charged. Image information on an original 1 is converted into an image signal by introducing a light illuminated by an original illumination lamp 2 into a CCD sensor 3. The obtained image signal is processed by an image signal process circuit 4 and then output as a driven signal for driving a laser driver 5. Therefore, an image exposing according to the image information on the original 1 is performed on the photosensitive drum 6 which has been uniformly charged. Subsequently, it is performed a developing process in which a developing is performed to the photosensitive drum 6 on which an electrostatic latent image has been formed, by using a toner from a developing unit 8. Further, a toner image formed through the developing process is transferred by a transfer unit 9 onto a transfer paper P which is synchronously fed. The transfer paper P passed through such a transfer process is fed to a fixing unit 10 by a feed belt, and the toner on the transfer paper P is fixed to the paper P by the fixing unit 10. Reference numeral 12 denotes a cleaner for eliminating the toner on the photosensitive drum 6 after the transfer terminated. Fig. 6 shows an output density characteristic in a case where photosensitive bodies respectively having membrane thickness 30 µm, 20 µm and 13 µm are used as the photosensitive drum 6. In Fig. 6, a lateral axis represents an original density and a longitudinal axis represents an output density of the image forming apparatus according to the present invention. As clearly shown in Fig. 6, it can be well understood that, if the membrane thickness of the photosensitive body changes, the output density characteristic significantly changes. Therefore, in the present invention, by detecting a quantity of a current flowing in the photosensitive drum 6 in a case where a predetermined voltage is applied to the photosensitive drum 6 by the high-voltage power source 11 through the charge member 7, a detection signal corresponding to the membrane thickness of the photosensitive body of the photosensitive drum 6 is obtained, and then the output density characteristic due to the change in the membrane thickness of the photosensitive body is corrected. Hereinafter, the first embodiment of the present invention will be explained concretely. Fig. 2 is a view showing structure of a detection unit for detecting a value according to the membrane thickness of the photosensitive body. In Fig. 2, reference numeral 21 denotes an AC voltage generation source, reference numeral 22 denotes a DC voltage generation source, reference numeral 23 denotes a detection resistor, reference numeral 24 denotes an A/D converter and reference numeral 25 denotes a CPU. While the photosensitive drum 6 is rotating, the DC voltage generation source 22 generates a predetermined voltage V 0 such that the voltage V 0 overlaps a voltage V PPO , and a voltage generated across both edges of the detection resistor 23 according to a current i is converted by the A/D converter 24 into a six-bit digital signal T D . The six bit digital signal T D is a signal which is according to the membrane thickness of the photosensitive body, which thickness is utilized as a barometer in case of measuring a newness of the drum. In response to the detection signal T D , the CPU 25 classifies the membrane thickness of the photosensitive body on the basis of following standard, and selects one of a plurality of gamma (γ) conversion characteristics shown in Fig. 5. T D > A → γ1 A ≥ T D > B → γ2 B ≥ T D > C → γ3 C ≥ T D → γ4 (where A, B and C satisfy a condition A > B > C) In this case, as a value of the signal T D is large, the drum is new. Therefore, as the value of the signal T D becomes large, the membrane thickness of the photosensitive body is judged to become large. On the other hand, as the value of the signal T D becomes small, the membrane thickness of the photosensitive body is judged to become small due to deterioration of the drum. Such judgments are performed by the CPU 25, and then a control signal for selecting the gamma characteristic shown in Fig. 5 is output from the CPU 25. In Fig. 5, as the gamma conversion characteristic changes from γ1 to γ4, inclination of a curve becomes large. In a case where one of the gamma conversion characteristics γ1 to γ3 is selected, adjusting of a process parameter for image formation such as a developing bias or the like responsive to the signal T D is not adjusted. This is because it is better for easily controlling the tonality characteristic that a characteristic of a printer is made stable by maintaining an image forming condition as much as possible and the tonality characteristic including the halftone is adjusted through an image data conversion process. On the other hand, in a case where the gamma conversion characteristic γ4 is selected, the developing bias of the developing unit 8 is also adjusted according to the value of the signal T D . Concretely, with maintaining contrast, the developing bias capable of preventing a fogging phenomenon is set. According to such the developing bias, the gamma conversion characteristic γ4 becomes a gamma conversion characteristic which can be obtained with fully considering a change in the printer characteristic due to a change in this developing bias. Fig. 3 is a block diagram showing the image signal process circuit 4 and the like according to the present embodiment. An analog signal from the CCD sensor 3 is subjected to gain adjusting and shading correcting by an analog process unit 31. The obtained signal is converted into an eight-bit digital signal by an A/D conversion unit 32, and then further converted into a density signal by a logarithmic conversion unit 33. Subsequently, the obtained density signal is subjected to the above gamma converting by a gamma conversion unit 34, and then further converted into a pulse-width signal by a PWM circuit 35 for driving the laser driver 5. The gamma conversion unit 34 and other units included in the image signal process circuit 4 are controlled by the CPU 25. The CPU 25 receives the value according to the membrane thickness from the A/D converter 24 and then performs the controlling according to the received value. Reference numeral 40 denotes a printer control unit which includes a charge voltage control circuit 41 for controlling a charge voltage applied by the high-voltage power source 11, a developing bias control circuit 42 for controlling the developing bias of the developing unit 8, a laser light quantity adjustment circuit 43 for adjusting a quantity of a laser beam generated by the laser driver 5, a fixing temperature control circuit 44 for controlling a fixing temperature of the fixing unit 10, and the like. As described above, the CPU 25 controls the gamma conversion characteristic in the gamma conversion unit 34, in response to the signal T D . In a specific case, the CPU 25 also controls the developing bias through the developing bias control circuit 42, in response to the signal T D . The gamma conversion unit 34 which has the structure shown in Fig. 4, is a table conversion circuit in which one of the gamma conversion characteristics γ1 to γ4 is selected responsive to a selection signal from the CPU 25. This table conversion circuit can be realized by, e.g., causing an ROM to store input and output characteristics of γ1 to γ4. Further, if the table conversion circuit is realized by an RAM, a gamma conversion table may be formed according to the membrane thickness in such a manner that a set of table data of the characteristic to be selected is set in the RAM by the CPU 25. Relationships between the original density and the output density obtained by the above-described structure are shown in Fig. 7, with respect to 30 µm, 20 µm and 13 µm. According to the present embodiment, the constant output density characteristic can be obtained irrespective of the membrane thickness of the photosensitive body. Especially, in the present embodiment, the density characteristic of the image data representing an image for reproduction is converted based on the membrane thickness of the photosensitive body, while the parameter such as the developing bias or the like in the image forming process is not changed based on the value according to the membrane thickness, so that the image forming characteristic of the printer is fixed and the tonality characteristic including an intermediate level can be well controlled. (Second Embodiment) Fig. 8 shows a result obtained by performing a following primary conversion of a density signal, instead of a non-linear conversion by a density conversion table. D out = A × (D in + B) where D out : density signal after primary conversion D in : density signal before primary conversion A, B : coefficients In this case, it can be understood that substantially constant output density characteristics can be obtained. (Third Embodiment) Fig. 9 shows output density characteristics in a case where correcting by an image signal process circuit (or apparatus) 4 is not performed when a temperature and a humidity of a photosensitive drum is set as 15°C5%RH, 23°C60%RH and 30°C80%RH. Even in this case a significant change in the output density characteristics are observed. Fig. 10 shows output density characteristics in a case where the temperature of the photosensitive drum is detected by a temperature and humidity sensor and then density signal converting by using a density conversion table is performed in the image signal process circuit 4 on the basis of a detection output. In this case, the constant output density characteristics can be obtained irrespective of the temperature and the humidity of the photosensitive drum. (Fourth Embodiment) Fig. 11 shows a result obtained by performing correcting in an image signal process circuit (or apparatus) 4 in a case where a temperature and a humidity of a photosensitive drum is set as 15°C5%RH, 23°C60%RH and 30°C80%RH, by using a primary conversion of a density signal instead of a density conversion table. Also in such a case, it can be understood that substantially constant output density characteristics can be obtained. (Fifth Embodiment) As a timing of detecting the membrane thickness in the first embodiment, for example, there can be used a pre- and multi-rotation of the drum due to on/off of the power source of the image forming apparatus, a pre-rotation time of the image support body (before the image forming), and a post-rotation time of the image support body (after the image forming). In order to increase detection accuracy for the detection means, it is necessary to prolong a detection time. Further, as the detection is frequently performed, the detection timing increasingly follows changes in a photosensitive body, and also increasingly follows a case where the photosensitive body which has been used for a long period of time is changed to new one. However, if the detection time is unnecessarily prolonged to increase the detection accuracy a total current-carrying time for the photosensitive body is prolonged. As a result, life of the photosensitive body becomes short. Further, a throughput of the image forming apparatus comes to be decreased. In addition, if the detection is frequently performed more than needed, the life of the photosensitive body becomes short and the throughput comes to be decreased. According to the present embodiment, it can be well prevented that the life of the photosensitive body is shortened and the throughput as the image forming apparatus is decreased, and at the same time the detection timing well follows the change in the membrane thickness of the photosensitive body, especially the abrupt change caused by exchanging the photosensitive body. Such a compatibility will be explained in detail, hereinafter. Generally, if the membrane thickness of the photosensitive body reduces to the extent of 13 µm, a crack or a flaw begins to occur on the photosensitive drum and thus a faulty image is output, so that it is necessary to change the photosensitive drum 6 into new one. On the other hand, it is supposed that the detecting of the photosensitive body membrane thickness should be performed in the post-rotation of the drum after the image forming corresponding to about 500 to 1,000 sheets to prolong the life of the photosensitive drum 6 to some extent. Such the detecting of the membrane thickness at the post-rotation time is performed accurately by using an enough time of about ten seconds. In a case where the gamma conversion table is changed in the post-rotation after the image forming corresponding to about 500 to 1,000 sheets, if the photosensitive drum 6 is changed to new one, in the worst case, the image forming is performed by using the gamma conversion table which has been optimized to a state where the membrane thickness of the photosensitive drum 6 is thin, during the image forming between 500 to 1,000 sheets. For example, if the gamma conversion table for the membrane thickness of 13 µm is used for the photosensitive drum having the membrane thickness of 30 µm, inclination of the curve of the output image density for the original density becomes abrupt, so that a deviation from a linear line becomes large. In consideration of such points, in the present embodiment, a CPU 25 detects the membrane thickness of the photosensitive body and selects the gamma conversion table in accordance with a flow chart shown in Fig. 12. In this case, the fundamental structure for performing such operations is the same as that in the first embodiment. Initially, a power-on state or an open/close state of a front door for exchanging the photosensitive drum is detected (step S1). If detected, in response to a turned-on state of a copy switch (step S2), the membrane thickness of the photosensitive body is detected in the pre-rotation before the image forming (step S3). Since such the membrane thickness detecting in the pre-rotation is performed for a short period of time (1 to 2 seconds), the life of the drum is not so affected, but the detection accuracy is low. Then, an absolute value of a difference between a detection signal DT 2 which is obtained in such the detecting and a previously-stored detection signal DTM which is obtained when the previous gamma conversion table was changed (step S4). If the absolute value is larger than a predetermined threshold value dDT 2 (about 5 µm to 10 µm), the gamma conversion table is changed by using the detection signal DT 2 in the same manner as that described in the first embodiment (steps S5 to S11, and S31). The reason why such a judgment is performed is that, since the detection signal DT 2 in the pre-rotation before the image forming does not have enough accuracy, it is needed to prevent an unconditional change or switch of the gamma conversion table responsive to such the detection signal DT 2 . Because of the same reason, in a case where the gamma conversion table is changed in response to the detection signal in the pre-rotation before the image forming, a count value TDN obtained by counting the number of copies is set as "501" for the detecting of the photosensitive body membrane thickness in the post-rotation after the subsequent image forming (step S12). On the other hand, in a case (1) where the power-on state or the open/close state of the front door is not detected in the step S1, or in a case (2) where it is judged in the step S4 to satisfy |DT 2 - DTM| ≤ dDT 2 , (3) it is further judged (based on the turned-on state of the copy switch (step S13) in the above case (1)) whether or not the TDN is larger than 500 after changing the gamma conversion table based on the membrane thickness detection in the pre-rotation (step S14). If yes in the step S14, a copy job starts (step S16) and thereafter the membrane thickness detection in the post-rotation is performed (step S17) and the gamma conversion table is changed (steps S19 to S26). In the above operation, a detection signal DT 1 is set as a detected membrane thickness value in the post-rotation, into a memory DTM (step S27 to S30). As described above, by detecting the membrane thickness in the post-rotation, the suitable gamma conversion table can be selected by the less number of detection times, the short period of time and the highly accurate membrane thickness measuring. In the same manner as in the previous embodiments, a primary converting may be used instead of the gamma conversion table changing in the present embodiment. Further, in the present invention, the gamma conversion table is selected on the basis of the one-time detecting. However, in order to increase accuracy, the gamma conversion table may be selected on the basis of a result obtained through a plurality of detecting. For example, if the gamma conversion table is changed when the detection signal T D exceeds one threshold value accumulatively three times, the highly accurate optimization of the gamma table becomes possible. Furthermore, it is not needed that the value according to the membrane thickness of the photosensitive body is a value strictly representing the membrane thickness itself. That is, such the value may be a value which changes according to the change in the membrane thickness. Furthermore, the photosensitive body is not limited to the photosensitive drum, but may be a photosensitive belt. Furthermore, in the above embodiments, the developing bias is exceptionally controlled according to the signal T D . However, such exception can be eliminated, and the signal T D may be always managed independently of the developing bias. According to the above embodiments, it can be well prevented that the life of the photosensitive body is shortened and the throughput of the image forming apparatus is decreased, and at the same time the detection timing well follows the change in the membrane thickness of the photosensitive body. That is, a compatibility between such two conflicting needs can be realized. Although the present invention has been described above with respect to the preferred embodiments, the present invention is not limited to the foregoing embodiments but may modifications and variations are possible within the scope of the appended claims.

Claims: An image forming apparatus comprising: a photosensitive image support body (6) having a photosensitive membrane for supporting an image which is to be formed on a medium; signal processing means (33,34) for processing an image signal representing the image; detection means (23) for detecting a value representing the thickness of the photosensitive membrane; charging means for charging the photosensitive member; and characterised in that said charging means comprise a charge member (7) which in operation contacts said photosensitive membrane and that said detection means is adapted to detect a current flowing in said photosensitive body when it is being charged by said charging means, and that there is provided control means (25) for changing a correction table in said signal processing means so as to vary a tonality characteristic of the processed image signal in accordance with the value detected by said detection means. Apparatus according to claim 1, wherein said control means selects one from a plurality of correction tables in accordance with the detected value. Apparatus according to claim 2, wherein the correction tables are gamma conversion tables (γ 1 ,γ 2 ,γ 3 ,γ 4 ). Apparatus according to any one of claims 1 to 3, wherein said detection means is adapted to detect the value corresponding to the membrane thickness after a new image support body has been exchanged for a used image support body. Apparatus according to any preceding claim, wherein the support body (6) is a drum which is rotated during an image formation operation. Apparatus according to claim 5, wherein said detection means is adapted to detect said value either during a pre-rotation operation for an image formation or during a post-rotation operation for the image formation. Apparatus according to claim 5, wherein said detection means are adapted to use an increased detection time during a post-rotation operation in comparison with the detection time used during a pre-rotation operation. Apparatus according to claim 6 or claim 7, wherein said control means are adapted to change the correction table on the basis of plural detection results by said detection means at different timings. Apparatus according to any preceding claim, further comprising development means (8,9,10) for developing a latent image formed on the photosensitive body, and wherein said control means is adapted to change the correction table without changing a developing condition of said development means when the value detected by said detection means is more than a predetermined value and changes both the correction table and the developing condition when the value detected by said detection means is equal to or less than a predetermined value.

Inventors: ABE MITSUKA, KUNISHI TSUYOSHI, TOYOHARA YUICHIRO, WATANABE YUKIO, ABE, MITSUKA, KUNISHI, TSUYOSHI, TOYOHARA, YUICHIRO, WATANABE, YUKIO, Abe, Mitsuka, c/o Canon K.K., KUNISHI, TSUYOSHI, C/O CANON K.K., TOYOHARA, YUICHIRO, C/O CANON K.K., Watanabe, Yukio, c/o Canon K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1996
Main classification: G03G15
IPC codes: G03G15, H04N1