Image forming device and method of forming images

Abstract: An apparatus and method for inversion development control for an image forming device in which toner and carrier particles are prevented from scattering unnecessarily. Scatter prevention is performed by gradually changing both a bias voltage application to a development roller (41) and a surface voltage application to a photosensitive drum (2) of the image forming device. Scatter prevention is also performed by gradually changing both a bias voltage application to a development roller (41) and an exposure light quantity that the photosensitive drum (2) is subjected to by an exposing rod lens array (3) of the image forming device.

Description: The present invention relates to image forming devices according to the preamble part of claim 1 and claim 4, respectively, and to methods of forming images according to the preamble part of claims 10 and 13, respectively. In a copying machine with a conventional development system using a two component developer, development has been performed by first exposing the surface of a positively charged photosensitive drum to form a latent image on the drum surface. Then negatively charged toners and positively charged carriers are made to adhere onto a non-exposed region of the latent image portion on the drum surface. In a copying machine with an inversion development system using a two component developer, the surface of a photosensitive drum is negatively charged. In an inversion development system, negatively charged toners are made to adhere to an exposed portion of the negatively charged drum surface which has zero voltage. The inversion development system described above is shown in prior art Fig. 1. A negative voltage of about -700 volts to about -800 volts is applied to a portion of the surface of photosensitive drum 2 by a charger 1. This creates a negatively charged portion on the surface of photosensitive drum 2. As photosensitive drum 2 rotates, the negatively charged portion is positioned opposite exposing rod lens array 3 for exposure. Exposure creates a latent image of zero volts on the negatively charged portion of photosensitive drum 2. After exposure, photosensitive drum 2 rotates further, and the exposed negatively charged portion on the surface of photosensitive drum 2 arrives at a position opposite development roller 41. At this time, a bias voltage of about -400 volts is applied to development roller 41, causing negatively charged toners on the development roller 41 to be repulsed toward (fly) and adhere to the exposed portion of photosensitive drum 2 having zero voltage. It is desirable that the bias voltage be applied to the development roller 41 at the same time that the exposed negatively charged portion on the surface of photosensitive drum 2 reaches the position opposite development roller 41; however, it is difficult to control such timing. When the timing is off, the bias voltage may be applied either before or after the exposed negatively charged portion has reached a position opposite development roller 41. Fig. 2A shows a situation in which the bias voltage is applied before the exposed negatively charged portion reaches the position opposite development roller 41. The portion of photosensitive drum 2 which is positioned opposite development roller 41 has a surface voltage greater than the bias voltage of development roller 41. This causes toner particles to fly from development roller 41, and adhere to the portion of photosensitive drum 2 positioned opposite development roller 41. The voltage difference between development roller 41 and the portion of photosensitive drum 2 positioned opposite development roller 41 exceeds an allowable voltage difference range as shown in Fig. 2B. The allowable voltage difference range shown in Fig. 2B is the voltage difference range in which the bias voltage can differ from the surface voltage of the portion of photosensitive drum 2 opposite development roller 41 without causing toners or carrier particles to fly. Fig. 3A shows the situation in which the bias voltage applied to development roller 41 is applied after the exposed negatively charged portion of photosensitive drum 2 reaches a position opposite development roller 41. The portion of photosensitive drum 2 which is opposite development roller 41 has a voltage less than the bias voltage applied to development roller 41. When this occurs, positively charged carriers are attracted onto the surface of photosensitive drum 2. As shown in Fig. 3B, the voltage difference between the bias voltage (the voltage of development roller 41) and the surface voltage of the portion of photosensitive drum 2 opposite development roller 41 exceeds the allowable voltage difference range and carriers fly. A proposed solution to the above-mentioned problems depicted in Figs. 2 and 3 is to gradually apply the bias voltage. This solution has the disadvantage that if the timing of the bias voltage is incorrect, the resulting voltage difference between the development roller 41 and the surface of photosensitive drum 2 opposite development roller 41 exceeds the allowable voltage difference range. However, in this situation, neither toners nor carriers adhere to the photosensitive drum; instead, the toners or carriers scatter. GB-A-2 200 325 discloses an electrophotographic copier including charging means for applying a surface voltage to a portion of a photosensitive body, charging voltage supply means for supplying a voltage to said charging means, exposing means for forming a latent image on said portion of said photosensitive body, development means for developing said latent image, bias voltage application means for applying a bias voltage to said development means, sensing means for sensing a surface voltage of said photosensitive body, and control means responsive to an output signal from said sensing means, for controlling said charging voltage supply means and said bias voltage application means to change said bias voltage in accordance with said output signal of said sensing means. Moreover, said document discloses a method applicable in said copier for eliminating smears due to toner particles caused by a residual potential on the photosensive body in accordance with an increase of the number of copies. For this purpose, a potential on the photoconductive body is controlled by controlling charge, exposure light quantity and bias voltage according to the residual potential on the photosensitive body which increases in proportion to the increase of the number of copies consecutively made by use of said photosensitive body. An electrophotographic copier of a similar structure is known from DE-A-3 140 853. US-A-4 592 646 discloses an image-forming apparatus also having the basic features of an electrophotographic copier, wherein a sensor for sensing a surface voltage of a photosensitive body is disposed between exposure means and development means, and wherein outputs of a processor are modified when the length of pause in an image formation operation of the processor exceeds a predetermined value. SUMMARY OF INVENTION It is a primary object of the present invention to improve a conventional image forming device and image forming method so as to avoid toner and carrier particles of a developer adhering to the photosensitive body at the initial stage of the image forming operation. This object is solved by an image forming device according to claims 1 and 4, respectively, and by a method for forming images according to claims 10 and 13, respectively. Improvements thereof are specified in sub-claims to the aforementioned claims. Preferred embodiments of the invvention are described hereinbelow with reference to Figs. 4 to 9 of the drawings. In particular, according to a first embodiment of the invention a photosensitive body may be surrounded by of charger for applying a charge or voltage to the surface of the photosensitive body, an exposure means for forming a latent image on the surface of the photosensitive body, and a development means for developing the latent image on the photosensitive body. A surface potential control means controls the charger to gradually change the surface voltage of the photosensitive body to a first predetermined value. The exposure means then creates a latent image upon the charged portion of the photosensitive body. When the charged portion of the photosensitive body reached a position opposite the development means, a bias control means controls a bias application means to gradually change the bias voltage applied to the development means to a predetermined value. The difference between the surface voltage of the portion of the photosensitive body opposite the development means and the bias voltage of the development means falls within the allowable voltage difference range even when the timing of either the bias voltage application or surface voltage application or both is off from the norm. Therefore, toners or carriers are prevented from flying or scattering. According to a second embodiment of the present invention the elements surrounding the first embodiment are present and a light quantity control means is provided to control the exposure light quantity of the exposure means. A charger applies a predetermined voltage to the surface of the photosensitive body. When the charged photosensitive body reaches a position opposite the exposure means, the light quantity control means controls the exposure means to gradually change the exposure light quantity from a predetermined value to zero. When the exposed portion of the charged photosensitive body reaches a position opposite the development means, a bias control means controls the bias application means to change the bias voltage applied to the development means to a predetermined value. The voltage difference between the surface voltage of the portion of the photosensitive body opposite the development means and the bias voltage of the development means falls within the allowable voltage different range even when the timing of the exposure or the application of the bias voltage or both is off from the norm. Therefore, unnecessary scattering or flying of toners and carriers is prevented. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a schematic sectional view showing a main part of a copying machine with a conventional inversion development system; Figures 2A and 2B are graphs showing an example of the relationship between the surface voltage of a portion of the photosensitive drum opposite the development means and the bias voltage applied to the development means of the copying machine of Fig. 1; Figures 3A and 3B are graphs showing another example of the relationship between the surface voltage of a portion of the photosensitive drum opposite the development means and the bias voltage applied to the development means of the copying machine of Fig. 1; Fig. 4 is a schematic block diagram showing a first embodiment of the inversion development controller according to the present invention; Fig. 5 is a graph showing temporal changes of the surface voltage of a portion of a photosensitive body opposite a development means and the bias voltage applied to the development means of the embodiment of Fig. 4; Fig. 6 is a graph showing the voltage difference between the surface voltage and the bias voltage in Fig. 5; Fig. 7 is a block diagram showing a second embodiment of the inversion development controller according to the present invention; Fig. 8 is a graph showing temporal changes of the surface voltage of a portion of the photosensitive body after exposure opposite the development means and the bias voltage applied to the development means of the embodiment of Fig. 7. Fig. 9 is a graph showing the voltage difference between the surface voltage and bias voltage in Fig. 8. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment Fig. 4 is a schematic sectional view showing a main portion of a copying machine utilizing the first embodiment of the inversion development controller according to the present invention. Fig. 4 shows a charger 1 positioned at charging point P1, a surface voltage sensor 6, exposing rod lens array 3 and a developing unit 4. The developing unit 4 includes a development roller 41 positioned at development position P2. These elements and cleaning means 5 for cleaning residual toners are disposed surrounding photosensitive drum 2. A charger 1 is connected to a high voltage power supply circuit 10 which supplies a voltage of about -800 volts to a charger 1. The high voltage power supply circuit 10 is connected to control circuit 9 which controls the voltage generated by the high voltage power circuit 10. Developing unit 4 is connected to a high voltage power supply circuit 11 which supplies a voltage to developing unit 4. The high voltage power supply circuit 11 is connected to control circuit 12 which controls the voltage generated by the high voltage supply circuit 11. A CPU (central processing unit) 13, controls circuits 9 and 12 in accordance with the received output signals of the surface voltage sensor 6. CPU 13 instructs control circuit 9 to gradually change to a first predetermined value the voltage applied by a charger 1 to photosensitive drum 2. CPU 13 further instructs control circuit 12 to gradually change to a second predetermined value the bias voltage applied to development roller 41. Operation of the embodiment depicted in Fig. 4 will now be described. Photosensitive drum 2 rotates counterclockwise as shown by the arrow in Fig. 4. A charger 1 charges photosensitive drum 2 with a voltage supplied by high voltage power supply circuit 10. Surface voltage sensor 6 measures the surface voltage of the charged portion of the photosensitive drum 2 and outputs the measurements to CPU 13. Photosensitive drum 2 is then rotated until the charged portion of the photosensitive drum 2 reaches a position opposite that of exposing rod lens array 3. An original placed on contact class 8 is then exposed by light emitted from exposing lamp 7. Light emitted from exposing lens 7 which reflects from the original travels through exposing rod lens array 3, and forms a latent image on the charged portion of photosensitive drum 2. The charged portion of photosensitive drum 2 containing the latent image is then rotated to a development position P2 opposite that of developing unit 4; and development roller 41. A voltage supplied by high voltage supply circuit 11 is then applied to development roller 41. Toners then fly from development roller 41 to the charged portion of photosensitive drum 2 containing the latent image forming a development image. Thereafter the development image is transformed (not shown), and the residual toners are cleaned by cleaning means 5. A detailed description of the timing of the surface voltage and bias voltage applications will now be made. It takes a predetermined time for a portion of photosensitive drum 2 to rotate from charged position P1 to development position P2. For the purposes of illustration, assume it takes 0.4 seconds for a portion of photosensitive drum 2 to travel from charged position P1 to development position P2. CPU 13 drives control circuit 9 to cause high voltage supply circuit 10 to supply a voltage to a charger 1. This voltage is then applied by a charger 1 to photosensitive drum 2. The surface voltage applied by a charger 1 is a stepwise voltage from -100 volts to -700 volts at intervals of -100 volts as shown in Fig. 5. Fig.5 shows the surface voltage application is shifted in time by 0.4 seconds; in other words, the surface voltage of a portion of photosensitive drum 2 positioned at development point P2. Then, 0.4 seconds after the beginning of the surface voltage application by a charger 1, CPU 13 drives control circuit 12 to cause high voltage supply circuit 11 to supply a bias voltage to developing unit 4. This bias voltage applied to developing unit 4, specifically development roller 41, is a stepwise voltage from +100 volts to -400 volts at intervals at -100 volts. For purposes of illustration assume these stepwise changes are performed at intervals about 0.5 seconds. Therefore, when the portion of photosensitive drum 2 supplied with a voltage of -100 volts has reached the development point P2 the voltage of developing unit 4 is +100 volts. As shown in Fig. 5 the surface voltage on the portion of the photosensitive drum 2 at P2 and the bias voltage of developing unit 4 stepwise changes and the difference between the surface voltage and the bias voltage remains within the allowable voltage difference range as shown in Fig. 6. If the timing of the surface voltage or bias voltage application or both is off, a greater voltage difference between the surface voltage of a portion of photosensitive drum at position P2 and the bias voltage than that depicted in Fig. 6 is possible. However, both the surface and bias voltages are gradually changed, thus even if the timing of the surface voltage application or bias voltage application or both are off, the resultant voltage difference does not exceed the allowable voltage difference range. Consequently, toners and carriers are prevented from scattering. The present invention is not limited to the specific embodiment disclosed. Any means capable of gradually changing the bias voltage and surface voltage to reach a predetermined value may be used. Moreover, although the gradual change of surface and bias voltages according to the present invention is performed stepwise the gradual change may be performed continuously. Second Embodiment Fig. 7 is a schematic sectional view showing a main part of a copying machine to which a second embodiment of the inversion development controller according to the present invention is applied. In the second embodiment, elements corresponding to elements which were used in the description of the first embodiment are labeled using the same reference numerals. In the second embodiment, the exposing rod lens array 3 is connected to a control circuit 14 which controls the quantity of light output by exposing rod lens array 3; the exposure light quantity. A CPU 13 controls the control circuits 9, 12, and 14 in accordance with the received output signals of surface voltage sensor 6. CPU 13 instructs control circuit 9 to control a charger 1 to apply a high voltage to the surface of photosensitive drum 2. CPU 13 instructs control circuit 14 to control exposing rod lens array 3 to gradually change the exposure light quantity from a predetermined value to zero. CPU 13 further instructs control circuit 12 to gradually change to a predetermined value the bias voltage applied to the development roller 41. It takes a first predetermined amount of time for a portion of the photosensitive drum 2 to which a surface voltage is applied by a charger 1 at position P1 to rotate and reach the exposure position P2 opposite the exposing rod lens array 3. It takes a second predetermined amount of time for a portion of photosensitive body 2 to rotate from position P2 to position P3 opposite developing unit 4. For purposes of illustration, assume that it takes 0.2 sec for a portion of the photosensitive drum 2 to travel from position P1 to position P2, and 0.4 sec to travel from position P1 to position P3. Operation of the embodiment depicted in Fig. 7 will now be described. First CPU 13 drives control circuit 9 causing high voltage supply circuit 10 to supply a voltage to a charger 1 and charge the photosensitive drum 2 to a surface voltage of -700 volts. Next, 0.2 seconds later, CPU 13 drives control circuit 14 to cause exposing rod lens array 3 to emit an exposure light quantity which is stepwise decreased from a predetermined value to zero. As a result, the surface voltage of the charged portion of photosensitive drum 2 at position P2 is increased to -100 volts and stepwise decreases as the exposure light quantity stepwise decreases to zero. Then, 0.2 sec from the beginning of exposure, CPU 13 drives control circuit 12 causing a bias voltage to be applied to the development roller 41 from the high voltage supply circuit 11 stepwise from +100 volts toward a predetermined voltage. The surface voltage of the photosensitive body at the development position P3 and the bias voltage of the development roller 41 are changed as shown in Fig. 8. The surface voltage of the photosensitive drum 2 is changed from -100V to -700V at intervals of -100V due to the stepwise change of the exposure light quantity. The bias voltage of the development roller 41, is changed from +100V to -400V at intervals of -100V. As a result, the voltage difference between the surface voltage of the portion of the photosensitive drum 2 at position P3 and the bias voltage of the development roller 41 as shown in Fig. 9 remains within the allowable voltage difference range. Here, even if the exposure timing of the exposing rod lens array 3 or the timing of the bias voltage application or both are off, the voltage difference between the surface voltage of the portion of the photosensitive drum 2 at position P3 and the bias voltage of development roller 41 does not exceed the allowable voltage difference range because both the exposure light quantity and bias voltage change gradually. Consequently, toners and carriers are prevented from scattering. The present invention is not limited to the specific embodiment disclosed. Any means capable of gradually changing the bias voltage to a predetermined value may be used. Furthermore, any means capable of gradually changing the exposure light quantity so as to gradually change the surface voltage of the photosensitive drum may be used. Moreover, although the gradual change of exposure light quantity and bias voltage according to the present invention is performed stepwise the gradual change may be performed continuously. Moreover, although the present invention is applied to an inversion development apparatus using a two-component developer in the foregoing embodiment, the present invention may be applied to an inversion development apparatus using a one-component developer.

Claims: An image forming device, comprising charging means (1) for applying a surface voltage to a portion of a photosensitive body (2); charging voltage supply means (10) for supplying a voltage to said charging means (1); exposing means (3) for forming a latent image on said portion of said photosensitive body (2); inversion development means (4) for developing said latent image; bias voltage application means (11) for applying a bias voltage to said development means (4); sensing means (6) for sensing a surface voltage of said photosensitive body (2); and main control means (13) responsive to an output signal from said sensing means (6), for controlling said charging voltage supply means (10) and said bias voltage application means (11) to change said bias voltage in accordance with said output signal of said sensing means (6); characterized by charging control means (9) responsive to an output signal of said main control means (13), for gradually changing said surface voltage from an initial stage of operation to reach a predetermined value in accordance with lapsed time; and bias control means (12) responsive to an output signal of said main control means (13), for gradually changing said bias voltage from an initial stage of operation to reach a predetermined value in accordance with lapsed time. The image forming device as claimed in claim 1, characterized in that said charging control means (9) and said bias control means (12) are provided for changing said surface voltage and said bias voltage, respectively, so that the voltage difference between the surface voltage of the photosensitive body (2) at the position when passing the developement means (4) and the bias voltage thereof remains within an allowable voltage difference range. The image forming device as claimed in claim 1 or 2, characterized in that said charging control means (9) and said bias control means (12) are provided for changing said surface voltage and said bias voltage, respectively, stepwise. An image forming device, comprising charging means (1) for applying a surface voltage to a portion of a photosensitive body (2); charging voltage supply means (10) for supplying a voltage to said charging means (1); exposing means (3) for forming a latent image on said portion of said photosensitive body (2); inversion development means (4) for developing said latent image; bias voltage application means (11) for applying a bias voltage to said development means (4); sensing means (6) for sensing a surface voltage of said photosensitive body (2); and main control means (13) responsive to an output signal from said sensing means (6), for controlling said bias voltage application means (11) to change said bias voltage in accordance with said output signal of said sensing means (6), and to change the exposure light quantity of said exposing means (3); characterized by exposure control means (14) responsive to an output signal of said main control means (13), for gradually changing the exposure light quantity of said exposure means (3) from a predetermined value to zero; and bias control means (12) responsive to an output signal of said main control means (13), for gradually changing said bias voltage from an initial stage of operation to reach a predetermined value in accordance with lapsed time. The image forming device as claimed in claim 4, characterized in that said exposure control means (14) are provided for gradually changing said exposure light quantity in accordance with lapsed time. The image forming device as claimed in claim 4 or 5, characterized in that said exposure control means (14) and said bias control means (12) are provided for changing said exposure light quantity and said bias voltage, respectively, so that the voltage difference between the surface voltage of the photosensitive body (2) at the position when passing the developement means (4) and the bias voltage thereof remains within an allowable voltage difference range. The image forming device as claimed in any one of claims 4 to 6, characterized in that said exposure control means (14) and said bias control means (12) are provided for changing said exposure light quantity and said bias voltage, respectively, stepwise. The image forming device as claimed in any one of claims 4 to 7, characterized by additionally providing charging control means (9) responsive to an output signal of said main control means (13), for gradually changing said surface voltage from an initial stage of operation to reach a predetermined value in accordance with lapsed time. The image forming device as claimed in any one of the preceding claims, characterized in that said sensing means (6) is arranged for sensing a surface voltage of said photosensitive body (2) at a position between said charging means (1) and said exposing means (3). A method of forming images, comprising the steps of charging a portion of a photosensitive body (2) for applying a charging voltage thereto; exposing said portion of said photosensitive body (2) to light for forming a latent image on said portion of said photosensitive body (2); and developing said latent image by applying developer from a development means (4) to said portion of said photosensitive body (2); wherein a surface voltage of said photosensitive body (2) is sensed by sensing means (6) producing a sensing signal in response to said surface voltage; and wherein said charging voltage applied to said photosensitve body (2) and a bias voltage applied to said development means (4) are changed in response to the sensing signal produced by said sensing means (6); characterized in that said charging voltage is controlled for gradually changing said surface voltage from an initial stage of operation to reach a predetermined value in accordance with lapsed time; and said biasing voltage is controlled for being gradually changed from an initial stage of operation to reach a predetermined value in accordance with lapsed time. The image forming method as claimed in claim 10, characterized in that said surface voltage and said bias voltage, respectively, are changed so that the voltage difference between the surface voltage of the photosensitive body (2) at the position when passing the developement means (4) and the bias voltage thereof remains within an allowable voltage difference range. The image forming method as claimed in claim 10 or 11, characterized in that said surface voltage and said bias voltage, respectively, are changed stepwise. A method of forming images, comprising the steps of charging a portion of a photosensitive body (2) for applying a charging voltage thereto; exposing said portion of said photosensitive body (2) to light for forming a latent image on said portion of said photosensitive body (2); and developing said latent image by applying developer from a development means (4) to said portion of said photosensitive body (2); wherein a surface voltage of said photosensitive body (2) is sensed by sensing means (6) producing a sensing signal in response to said surface voltage; and wherein said charging voltage applied to said photosensitve body (2) and a bias voltage applied to said development means (4) are changed in response to the sensing signal produced by said sensing means (6); characterized in that the quantity of light to which said portion of said photosensitive body (2) is exposed is controlled for being gradually changed from a predetermined value to zero; and said biasing voltage is controlled for being gradually changed from an initial stage of operation to reach a predetermined value in accordance with lapsed time. The image forming method as claimed in claim 13, characterized in that said exposure light quantity is gradually changed in accordance with lapsed time. The image forming method as claimed in claim 13 or 14, characterized in that said exposure light quantity and said bias voltage, respectively, are changed so that the voltage difference between the surface voltage of the photosensitive body (2) at the position when passing the development means (4) and the bias voltage thereof remains within an allowable voltage difference range. The image forming method as claimed in any one of claims 13 to 15, characterized in that said eposure light quantity and said bias voltage, respectively, are changed stepwise. The image forming method as claimed in any one of claims 13 to 16, characterized by additionally controlling the charging voltage for gradually changing the surface voltage from an initial stage of operation to reach a predetermined value in accordance with lapsed time. The image forming method as claimed in any one of claims 10 to 17, characterized in that a surface voltage of said photosensitive body (2) is sensed at a position after charging and prior to exposing said portion of said photosensitive body (2).

Inventors: KONDO AKIHIRO, NAKAJIMA YOSHIHIRO, YOSHIUCHI KATSUHIRO, KONDO, AKIHIRO, NAKAJIMA, YOSHIHIRO, YOSHIUCHI, KATSUHIRO
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO. LTD.
Application year: 1991
Main classification: G03G15
IPC codes: G03G15