Image forming apparatus with voltage adjustment member

An image forming apparatus includes a voltage adjustment portion having a voltage adjustment member connected to a contact member contacting an endless belt, the voltage adjustment portion changing a current flowing from a current supply member contacting the belt at a position different from a position, at which an image bearing member contacts the belt with respect to a rotating direction of the belt, to the voltage adjustment member via the belt according to a control signal input from a control portion outputting a control signal, thereby changing a size of a transfer potential to transfer a toner image borne by the image bearing member onto the belt at a part at which the belt contacts the image bearing member.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an image forming apparatus using an electrophotographic system.

Description of the Related Art

Conventionally, there has been known an image forming apparatus such as a copier and a laser beam printer configured to use an endless belt as an intermediate transfer member. In the image forming apparatus, a toner image formed on the surface of a photosensitive drum serving as an image bearing member is transferred onto a belt as a primary transfer step when a voltage is applied from a voltage power supply to a primary transfer member arranged at a part opposing the photosensitive drum. Then, the primary transfer step is repeatedly performed with respect to toner images of a plurality of colors to form the toner images of the plurality of colors on the surface of the belt. Subsequently, the toner images of the plurality of colors formed on the surface of the belt are collectively transferred onto the surface of a recording material such as a paper as a secondary transfer step when a voltage is applied to a secondary transfer member. After that, the collectively transferred toner images are permanently fixed onto the recording material by a fixing unit. In the way described above, a color image is formed.

Japanese Patent Application Laid-open No. 2013-213990 discloses a configuration allowing a change in the surface potential of a belt while making it possible to perform the miniaturization and the cost reduction of an image forming apparatus. According to the configuration, circuits having a plurality of Zener diodes different in setting voltage are provided between the belt and ground, and the setting voltage is changed according to a use environment to change the surface potential of the belt and stabilize primary transfer efficiency.

SUMMARY OF THE INVENTION

Generally, a plurality of members such as a photosensitive drum, an intermediate transfer member, and a primary transfer member are interposed as the configuration of a primary transfer portion, and there is a case that the resistance of the primary transfer portion changes or a case that an optimum primary transfer current changes depending on a surrounding environment or the use situation of an image forming apparatus. In the configuration of Japanese Patent Application Laid-open No. 2013-213990, a surrounding environment is detected and the surface potential of a photosensitive drum is slightly adjusted with a change in a voltage maintaining unit to ensure optimum transferability. However, in the slight adjustment of the surface potential of the photosensitive drum, each of a developing potential and a primary transfer potential has a potential difference necessary for properly moving toner. Therefore, when the surface potential of the photosensitive drum is largely changed to be adjusted, a reduction in image quality is caused. That is, in order to slightly adjust various fluctuations caused by a surrounding environment or the use situation of the body of the image forming apparatus, it is necessary to further increase the number of Zener diodes serving as the voltage maintaining unit, which results in a difficulty in maintaining the miniaturization of the apparatus.

It is an object of the present invention to provide an image forming apparatus capable of making the surface of an intermediate transfer member have an optimum potential for primary transfer while maintaining the miniaturization of the image forming apparatus.

In order to achieve the above object, an embodiment of the present invention provides an image forming apparatus comprising:

an image bearing member that bears a toner image;

an endless belt that rotates in contact with the image bearing member;

a current supply member that contacts the belt at a position different from a position, at which the image bearing member contacts the belt with respect to a rotating direction of the belt, and supplies a current to the belt;

a control portion that outputs a control signal;

a contact member that contacts the belt; and

a voltage adjustment portion that has a voltage adjustment member connected to the contact member and that changes an amount of the current flowing from the current supply member to the voltage adjustment member via the belt according to the control signal input from the control portion, thereby changing a magnitude of a transfer potential at a part, at which the belt contacts the image bearing member.

According to an embodiment of the present invention, it is possible to make the surface of an intermediate transfer member have an optimum potential for primary transfer while maintaining miniaturization.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, a description will be given, with reference to the drawings, of embodiments (examples) of the present invention. However, the sizes, materials, shapes, their relative arrangements, or the like of constituents described in the embodiments may be appropriately changed according to the configurations, various conditions, or the like of apparatuses to which the invention is applied. Therefore, the sizes, materials, shapes, their relative arrangements, or the like of the constituents described in the embodiments do not intend to limit the scope of the invention to the following embodiments.

First Embodiment

FIG. 1is a schematic diagram of an image forming apparatus according to a first embodiment of the present invention. A description will be given, with reference toFIG. 1, of the configurations and the operations of the image forming apparatus of the embodiment. Examples of an image forming apparatus to which the present invention is applicable include a copier and a printer using an electrophotographic system. Here, a case in which the present invention is applied to a color laser printer will be described. Note that the image forming apparatus of the embodiment is a so-called tandem type printer having a plurality of image forming stations a to d. A first image forming station a forms an image of yellow (Y), a second image forming station b forms an image of magenta (M), a third image forming station c forms an image of cyan (C), and a fourth image forming station d forms an image of black (Bk). The configurations of the respective image forming stations are the same except for the colors of accommodated toner. Hereinafter, the first image forming station a will be described.

The first image forming station a has a drum-shaped electrophotographic photosensitive member (hereinafter called a photosensitive drum)1aserving as an image bearing member, a charging roller2aserving as a charging member, a developing device4a, and a cleaning device5a. The photosensitive drum1ais an image bearing member that is rotationally driven in an arrow direction at a prescribed peripheral speed (process speed) and bears a toner image (developer image). In addition, the developing device4ais a device that accommodates yellow toner serving as developer and develops an electrostatic latent image formed on the photosensitive drum1ausing the yellow toner. The cleaning device5ais a member that collects the toner attached onto the photosensitive drum1a. In the embodiment, the cleaning device5ahas a cleaning blade serving as a cleaning member that contacts the photosensitive drum1aand a waste toner box that accommodates the toner collected by the cleaning blade.

When an image forming operation starts with an image signal, the photosensitive drum1ais rotationally driven. In a rotation process, the photosensitive drum1ais uniformly charged by the charging roller2ato have a prescribed polarity (negative polarity in the embodiment) and a prescribed potential and then exposed by exposure unit3aaccording to the image signal. Thus, an electrostatic latent image corresponding to a yellow component image of an objective color image is formed. Then, the electrostatic latent image is developed by the developing device (yellow developing device)4aat a developing position and visualized as a yellow toner image. Here, the normal charging polarity of the toner accommodated in the developing device4ais negative.

An intermediate transfer belt10is an endless belt. The intermediate transfer belt10is extended between extending members11,12, and13serving as support members and rotationally driven at substantially the same peripheral speed while contacting the photosensitive drum1ain the same movement direction as the photosensitive drum1aat its opposing part contacting the photosensitive drum1a. The yellow toner image formed on the photosensitive drum1ais transferred onto the intermediate transfer belt10(primary transfer) when passing through the contact part (hereinafter called the primary transfer nip) between the photosensitive drum1aand the intermediate transfer belt10. The method of the primary transfer characterizing the embodiment will be described later. The untransferred toner of the primary transfer on the surface of the photosensitive drum1ais cleaned and removed by the cleaning device5aand then subjected to an image forming process following a charging process. In the same way as the above, a magenta toner image of a second color is formed by the second image forming station b (including a photosensitive drum1b, a charging roller2b, a developing device4b, and a cleaning device5b), a cyan toner image of a third color is formed by the third image forming station c (including a photosensitive drum1c, a charging roller2c, a developing device4c, and a cleaning device5c), and a black toner image of a fourth color is formed by the fourth image forming station d (including a photosensitive drum1d, a charging roller2d, a developing device4d, and a cleaning device5d), respectively, and successively transferred onto the intermediate transfer belt10in an overlapped state. Prior to transfer, the charged photosensitive drums1b-1dare exposed by exposure units3b-3d, respectively. Thus, a combined color image corresponding an objective color image is obtained.

The toner images of the four colors on the intermediate transfer belt10are collectively transferred onto the surface of a recording material P fed by paper feeding unit (secondary transfer) when passing through a secondary transfer nip formed by the intermediate transfer belt10and a secondary transfer roller20. The secondary transfer roller20serving as a secondary transfer member has an outer diameter of 18 mm in which a nickel-plated steel rod having an outer diameter of 8 mm is covered with a blowing sponge body mainly composed of NBR adjusted to have a volume resistance of 108Ω·cm and a thickness of 5 mm and epichlorohydrin rubber. In addition, the secondary transfer roller20contacts the intermediate transfer belt10with an applied pressure of 50 N and forms a secondary transfer part (hereinafter called the secondary transfer nip). The secondary transfer roller20rotates following the intermediate transfer belt10. When the toner on the intermediate transfer belt10is being secondarily transferred onto the recording material P such as a paper, a voltage of 1800 to 2300 V is applied to the secondary transfer roller20. The recording material P bearing the toner images of the four colors is introduced into a fixation unit30to be heated and pressed. Thus, the toner of the four colors is melted and mixed together and fixed onto the recording material P. Untransferred toner on the intermediate transfer belt10after the secondary transfer is cleaned and removed by a cleaning device16. By the above operations, a full-color print image is formed.

A description will be given, with reference toFIG. 2, of the configuration of a controller100that controls the body of the image forming apparatus of the embodiment. As shown inFIG. 2, the controller100has a CPU circuit portion150serving as a control portion. The CPU circuit portion150includes a ROM151and a RAM152. The CPU circuit portion150totally controls an exposure control portion101, a charging control portion102, a developing control portion103, a primary transfer control portion104, and a secondary transfer control portion105according to a control program stored in the ROM151. In addition, an environmental table and various tables for transfer control are stored in the ROM151and called and reflected by a CPU based on information on an environmental sensor106serving as a detection unit for detecting temperature and humidity in an apparatus setting environment. The RAM152temporarily retains control data and serves as a work area for calculation processing associated with control. The secondary transfer control portion105controls a secondary transfer power supply21and controls a voltage to be output from the secondary transfer power supply21based on a current value detected by a current detection circuit (not shown). In addition, the primary transfer control portion104transmits a signal to a voltage adjustment circuit15to control the potential of the primary transfer portion at a constant value. By the controller100, the secondary transfer power supply21, the voltage adjustment circuit15, and the environmental sensor106, a printer engine99of the image forming apparatus according to the embodiment is configured. When image information and a printing instruction are transmitted from a host computer97, the controller100receives respective image signals converted by a video controller98. Then, the controller100controls the respective control portions (the exposure control portion101, the charging control portion102, and the developing control portion103) to perform an image forming operation necessary for a printing operation.

Hereinafter, a description will be given of the configuration of the primary transfer portion characterizing the embodiment. The embodiment is characterized by a configuration in which a current is supplied in the peripheral direction of the intermediate transfer belt10to perform the primary transfer, i.e., a configuration in which a primary transfer current is supplied at a position different from the primary transfer nips of the photosensitive drums1a,1b,1c, and1din the peripheral direction (rotating direction) of the intermediate transfer belt10. The intermediate transfer belt10and the photosensitive drums1ato1dform the contact parts (primary transfer nips) with the extension of the intermediate transfer belt10by the extending members11and13and are connected to the voltage adjustment circuit15including a transistor serving as a voltage adjustment member connected to the extending member13. At its positions opposing the respective image forming stations a to d, the intermediate transfer belt10serving as an intermediate transfer member is arranged. The intermediate transfer belt10is an endless belt in which a conducting agent is added to a resin material to have conductivity. The intermediate transfer belt10is extended by the three shafts of the extending member (driving roller)11, the extending member (tension roller)12, and the extending member (secondary transfer opposing roller)13and extended by the tension roller12at a total pressure of 60 N. The intermediate transfer belt10is rotationally driven at substantially the same peripheral speed as those of the photosensitive drums1ato1din the same movement direction at its opposing parts contacting the photosensitive drums1ato1d.

In addition, the secondary transfer opposing roller13serving as a contact member is connected to the voltage adjustment circuit15including a transistor as a voltage adjustment unit (voltage adjustment portion). The intermediate transfer belt10used in the embodiment has a peripheral length of 700 mm and a thickness of 90 μm. The intermediate transfer belt10is made of an endless polyethylene terephthalate (PET) resin molded by mixing an ion-based conducting agent as a conducting agent. As its electrical characteristics, the intermediate transfer belt10is characterized in that the unevenness or the like of a resistance value in the peripheral direction is fine although the resistance value fluctuates with respect to temperature and humidity in an atmosphere since the intermediate transfer belt10exhibits ion conducting characteristics and electrical conductivity is obtained when ions are transmitted between high polymer chains. In the embodiment, a current is supplied in the movement direction of the intermediate transfer belt10to perform the transfer. Therefore, a voltage drop becomes large when the resistance of the intermediate transfer belt10is high. As a result, the intermediate transfer belt10preferably has a low resistance layer since there is a likelihood of its primary transferability being impaired. In the embodiment, a base layer having a volume resistivity of 1×108Ω·cm or less as a resistance was used to suppress a voltage drop in the intermediate transfer belt10. For the measurement of the volume resistivity, the type UR (MCP-HTP12) of a ring probe is used in Hiresta-UP (MCP-HT450) manufactured by Mitsubishi Chemical Corporation. In the measurement of the volume resistivity, room temperature was set at 23° C. and room humidity was set at 50%. In addition, a voltage of 100 V was applied for 10 seconds. Further, in the embodiment, the intermediate transfer belt10is configured by two layers. By the arrangement of a high resistance layer on its surface, the intermediate transfer belt10suppresses a current flowing through a non-image part to further increase its transferability. However, the intermediate transfer belt10is not limited to this configuration but may be configured by a single layer or three or more layers.

In addition, the intermediate transfer belt10is made of a polyethylene terephthalate resin in the embodiment but may be made of other materials. Examples of the other materials include polyester, polycarbonate, polyarylate, and acrylonitrile-butadiene-styrene copolymer (ABS). Besides, examples of the other materials include polyphenylene sulfide (PPS), polyvinylidene difluoride (PVdF), and polyethylene naphthalate (PEN). These materials and the mixed resins of these materials may be used as the material of the intermediate transfer belt10.

In the embodiment, the voltage adjustment circuit15having a transistor is connected as the voltage adjustment portion between the secondary transfer opposing roller13and ground. The voltage adjustment circuit15adjusts a voltage to be applied from the secondary transfer power supply21to the intermediate transfer belt10via the secondary transfer roller20to generate a primary transfer voltage for performing the primary transfer to move the toner on the respective photosensitive drum1ato1donto the intermediate transfer belt10. By the application of the primary transfer voltage adjusted by the voltage adjustment circuit15to a desired size, the surface potential of the intermediate transfer belt10becomes a desired primary transfer potential. Based on the potential differences (transfer contrast) between the surface potential of the intermediate transfer belt10and the surface potentials of the respective photosensitive drums1ato1d, the primary transfer is performed. A description will be given, with reference toFIG. 3, of the details of the voltage adjustment by the voltage adjustment circuit15.

FIG. 3is a diagram for describing the circuit configuration of the primary transfer portion in the first embodiment of the present invention. When a secondary transfer voltage Vt2(here 2100 V) is output from the secondary transfer power supply21, a current flows from the secondary transfer power supply21to the voltage adjustment circuit15via the secondary transfer roller20, the intermediate transfer belt10, and the secondary transfer opposing roller13. The voltage adjustment circuit15is electrically connected to the intermediate transfer belt10via the secondary transfer opposing roller13, while a PWM signal is input from the controller100serving as a control portion to the voltage adjustment circuit15. The voltage adjustment circuit15is configured to be capable of changing the amount of the current flowing from the secondary transfer roller20serving as a current supply member to the intermediate transfer belt10according to the size of the PWM signal input from the controller100, i.e., the size of an on-duty ratio. That is, when the controller100controls the on-duty ratio of the PWM signal, the amount of the current flowing from the secondary transfer roller20to the intermediate transfer belt10is controlled and a primary transfer voltage Vt1(potential difference between a point A and the ground inFIG. 3) formed by the current is controlled.

The primary transfer voltage Vt1indicating the potential difference between the point A and the ground inFIG. 3is the potential difference between (the surface of) the secondary transfer opposing roller13connected to the voltage adjustment circuit15and the ground and corresponds to the collector-emitter voltage of a transistor Q1in the voltage adjustment circuit15. Further, the surface potential of the intermediate transfer belt10wound on the surface of the secondary transfer opposing roller13becomes substantially the same as the surface potential of the secondary transfer opposing roller13. The collector-emitter voltage of the transistor Q1is controlled when the collector current of the transistor Q1is controlled. That is, the primary transfer voltage Vt1, i.e., the surface potential of the intermediate transfer belt10, is controlled by the control of the collector current. A current generated by the application of the secondary transfer voltage Vt2flows through the transistor Q1as the collector current when a voltage is applied to the base terminal of the transistor Q1.

The voltage input to the base terminal of the transistor Q1to control the collector current is the output voltage of an operational amplifier IC1. The PWM signal output from the controller100is smoothened by a resistor R7and a capacitor C1. A smoothened control voltage V− is input to the inversion input terminal (− terminal) of the operational amplifier IC1. A voltage output from the operational amplifier IC1is divided by resistors R9and R10and input to the base terminal of the transistor Q1. As described above, the current generated by the secondary transfer voltage Vt2flows through the transistor Q1as the collector current when the voltage is applied to the base terminal of the transistor Q1, whereby a voltage is generated between the collector and the emitter of the transistor Q1and used as the primary transfer voltage Vt1. The generated primary transfer voltage Vt1is divided by resistors R5and R6, and a resulting voltage is input to the input terminal (+ terminal) of the operational amplifier IC1as a monitor voltage V+. Accordingly, the size of the primary transfer voltage Vt1is determined according to the size of the control voltage V− by the virtual short (V+=V−) of the operational amplifier IC1. The control voltage V− is controlled by the on-duty of the PWM signal. That is, when the on-duty of the PWM signal increases, both the control voltage V− and the primary transfer voltage Vt1becomes large. Conversely, when the on-duty of the PWM signal reduces, both the control voltage V− and the primary transfer voltage Vt1becomes small.

As described above, the embodiment employs the configuration in which the voltage of the transistor Q1is controlled by the PWM signal from the controller100to determine the primary transfer voltage. Note that a resistor R8and a capacitor C2inFIG. 3are provided as elements to determine the response of the transistor Q1. The PWM signal from the controller100is used to control the control voltage V− in the embodiment, but the voltage adjustment portion may have other configurations. For example, the same effect is obtained even with a configuration using the D/A port of the controller100.

FIG. 4shows the measurement results of transfer efficiency in the primary transfer portion having the configuration of the embodiment. The value of the transfer efficiency in a vertical axis shows results obtained by measuring primary transfer residual density with a Macbeth densitometer (manufacturer: Gretag Macbeth Ltd). Since the primary transfer residual density becomes larger in proportion to the value, the transfer efficiency deteriorates. As the measurement conditions inFIG. 4, the photosensitive drum and the intermediate transfer belt10are new and the measurement is conducted at a temperature of 23° C. and a relative humidity of 50%, i.e., a so-called N/N environment (normal temperature and normal humidity environment). Under the above conditions, optimum primary transferability was obtained at a primary transfer potential of 250 V.

As shown inFIGS. 5A and 5B, the transfer efficiency changes with the environmental fluctuations or the endurance fluctuations of the resistance value of the intermediate transfer belt10in the configuration of the embodiment.

FIG. 5Ais a diagram showing the transfer efficiency affected by the environmental fluctuations of the resistance value of the intermediate transfer belt10. Optimum transfer efficiency is obtained at a low voltage under a high temperature and high humidity environment (H/H: at a temperature of 30° C. and a relative humidity of 80%) and obtained at a high voltage under a low temperature and low humidity environment (L/L: at a temperature of 15° C. and a relative humidity of 10%).

FIG. 5Bis a diagram showing the transfer efficiency affected by the endurance fluctuations of the resistance value of the intermediate transfer belt10. It appears that as the number of printed sheets increases, i.e., as the number of the times of image forming operations increases, a voltage to obtain optimum transfer efficiency increases due to an increase in resistance in the intermediate transfer belt10of the embodiment.

In view of the above circumstances, the present inventors have determined an optimum voltage for the primary transfer in the following way. First, in order to deal with the above fluctuations of the transfer efficiency, the transistor Q1variable in the range of 0 V to 600 V was used as the voltage adjustment member. The resistance value fluctuations of the intermediate transfer belt10according to a surrounding environment are predicted and a bias setting table corresponding to the output value of the environmental sensor106is generated in advance to determine an optimum primary transfer voltage. In the configuration of the present invention, the primary transfer voltage is determined with reference to the bias setting table of the following table 1.

A description will be given, with reference toFIG. 6, of a control flow in the embodiment. After receiving an image forming instruction, the controller100collects information on the environmental sensor106to calculate an absolute water amount (X) (S1). Then, a primary transfer voltage corresponding to the value of the absolute water amount (X) is selected from the bias setting table (table 1) (S2to S12). In order to obtain the value selected here, the primary transfer control portion104controls the voltage adjustment circuit15to set the primary transfer voltage. For example, when the absolute water amount is 10.64 g/m3, 210 V is set as the primary transfer voltage at a temperature of 23° C. and a relative humidity of 50%, i.e., under a so-called N/N environment. It appears from the graph ofFIG. 4that the optimum voltage is set as primary transferability, and thus an excellent image is obtained.

As described above, the resistance fluctuations of the intermediate transfer belt10according to a surrounding environment are predicted with the use of the transistor Q1as the voltage adjustment unit for the primary transfer in the embodiment, whereby an appropriate primary transfer voltage may be determined and excellent primary transferability may be ensured.

In the embodiment, the resistance fluctuations of the intermediate transfer belt10according to a surrounding environment were predicted to determine the primary transfer voltage. However, as shown inFIG. 5B, it appears that the resistance value of the intermediate transfer belt10fluctuates with an increase in the number of the times of image forming operations. Therefore, correction is performed according to the use situation of the intermediate transfer belt10shown in table 2, whereby it becomes possible to further stably ensure the primary transferability.

FIG. 7shows a control flow using information on the service life of the intermediate transfer belt10. That is, as shown in the control flow ofFIG. 7, information on the service life (Y) of the intermediate transfer belt10is first acquired with respect to the primary transfer voltage set in the control flow ofFIG. 6. (S1). Then, based on the acquired information, a primary transfer correction voltage corresponding to the service life (Y) of the intermediate transfer belt10is determined from the correction table (table 2) (S2to S10). Here, the determined primary transfer correction voltage is added to the above primary transfer voltage to determine a final primary transfer potential. Specifically, when the primary transfer voltage obtained from the bias setting table is expressed as Vt0and the correction voltage obtained from the correction table is expressed as Vtb, the finally determined primary transfer voltage Vt1is calculated as follows.
Vt1=Vt0+Vtb

That is, the controller100changes the size of the control signal output to the voltage adjustment circuit15to change the amount of the current supplied to the intermediate transfer belt10such that the size of the primary transfer potential becomes larger as the remaining service life of the intermediate transfer belt10becomes shorter. Note that in the embodiment, the above use situation of the intermediate transfer belt10is determined in such a way that the CPU circuit portion150serving not only as the control portion but also as acquisition unit collects information on the number of printed sheets accumulated in the RAM152of the image forming apparatus. However, other information may be acquired as the information on the service life. For example, the same effect is obtained even with image information such as the number of the total pixels of an image obtained by an image forming operation, the time of the rotation of the intermediate transfer belt10, and the number of the rotation times of the intermediate transfer belt10.

As described above, in the embodiment, although a primary transfer voltage is generated from a secondary transfer voltage after a current necessary for secondary transfer is ensured from a secondary transfer power supply21, it is possible to separately set the primary transfer voltage with the use of a transistor as the voltage adjustment unit for primary transfer. Further, regardless of a surrounding environment and the use situation of an intermediate transfer belt10, an appropriate primary transfer voltage may be determined and excellent primary transferability may be ensured. In addition, it is possible to select optimum settings as secondary transfer voltage settings.

In the embodiment, a transistor is used as the voltage adjustment member to adjust the voltage of the primary transfer portion. However, an element such as a digital volume (digital variable resistor) may be used so long as the same effect is obtained by the element. That is, it may be possible to use an element capable of changing the size of a current supplied from the secondary transfer roller20to the intermediate transfer belt10according to the size of a control signal such as a PWM signal variable in size.

FIG. 8is a schematic cross-sectional diagram showing the schematic configuration of the image forming apparatus according to a modified example of the embodiment. The embodiment is based on the voltage applied to the secondary transfer roller20serving as a current supply member. However, the configuration is not limited to this. As shown in the modified example ofFIG. 8, a current based on the application of a voltage to a cleaning roller17that charges toner on the intermediate transfer belt10may be used. In addition, the same effect is obtained even with the use of the overlapped currents of both the above secondary transfer roller20serving as a first current supply member and the cleaning roller17serving as a second current supply member.

Second Embodiment

A description will be given, with reference toFIGS. 9 and 10, of an image forming apparatus according to a second embodiment of the present invention. In the image forming apparatus according to the second embodiment, the same configurations as those of the first embodiment will be denoted by the same symbols, and their descriptions will be omitted.FIG. 9is a schematic diagram of the image forming apparatus according to the second embodiment of the present invention, andFIG. 10is a diagram for describing the circuit configuration of a primary transfer portion in the second embodiment of the present invention.

As shown inFIG. 9, primary transfer rollers14a,14b,14c, and14dserving not only as primary transfer members but also as contact members are arranged at positions opposing photosensitive drums1a,1b,1c, and1d, respectively, via an intermediate transfer belt10in the configuration of the embodiment. Further, a secondary transfer opposing roller13, by which the intermediate transfer belt10is extended, and the primary transfer rollers14ato14dare grounded via a Zener diode serving as a voltage stabilizing element (voltage maintaining element) connected in series to a transistor serving as a voltage adjustment member.

The primary transfer rollers14ato14dcontact the photosensitive drums1ato1d, respectively, with a prescribed pressing force in a state of sandwiching the intermediate transfer belt10and rotate following the intermediate transfer belt10. In the embodiment, the arrangement of the primary transfer rollers14ato14dresults in an increase in the number of components but allows a high degree of flexibility in selecting the intermediate transfer belt10.

A yellow toner image formed on the photosensitive drum1ais transferred onto the intermediate transfer belt10(primary transfer) when passing through the primary transfer nip between the photosensitive drum1aand the intermediate transfer belt10. A primary transfer roller serving as a primary transfer member has an outer diameter of 12 mm in which a nickel-plated steel rod having an outer diameter of 6 mm is covered with a blowing sponge body mainly composed of NBR adjusted to have a volume resistance of 107Ω·cm and a thickness of 3 mm and epichlorohydrin rubber. In addition, the primary transfer roller14acontacts the photosensitive drum1awith an applied pressure of 10 N and forms the primary transfer nip.

A description will be given, with reference toFIG. 10, of voltage adjustment unit in the embodiment. In the embodiment, the primary transfer rollers14ato14dare arranged as the primary transfer members, and a voltage necessary for the primary transfer becomes higher by an amount corresponding to the resistance of the primary transfer members compared with the first embodiment. Therefore, as shown inFIG. 10, a Zener diode ZD1serving as voltage maintaining unit is connected in series to a transistor Q1serving as a voltage adjustment member. When a secondary transfer voltage Vt2(here 2100 V) is output from a secondary transfer power supply21, a current flows through the Zener diode ZD1via a secondary transfer roller20, the intermediate transfer belt10, and a secondary transfer opposing roller13. At this time, the current sufficiently flows to generate the yield voltage of the Zener diode ZD1to maintain a yield state, and the Zener diode ZD1maintains the yield voltage as a prescribed potential. A final primary transfer voltage is a value obtained by adding together a variably adjusted voltage output from the transistor Q1and the yield voltage maintained at a prescribed size by the Zener diode ZD1. Thus, the selection of a further higher primary transfer voltage is allowed. The specific operation of the circuit is the same as that of the first embodiment.

In the embodiment, the Zener diode ZD1serving as the voltage maintaining unit for maintaining a potential of 500 V was used, and the transistor Q1serving as the voltage adjustment unit variable in the range of 0 V to 600 V like the first embodiment was used. Therefore, in the configuration of the embodiment, it becomes possible to control the potential of the primary transfer portion in the range of 500 V to 1100 V. In the configuration of the embodiment, optimum primary transferability may be ensured using a reference voltage shown in the following table 3.

The control flow of the embodiment is the same as that of the first embodiment.

As described above, the embodiment is so configured that a transistor is used as voltage adjustment unit for primary transfer and a Zener diode is used as voltage maintaining unit, the transistor and the Zener diode being connected in series to each other. Thus, even with a primary transfer member having high resistance, an appropriate primary transfer voltage may be determined and excellent primary transferability may be ensured.

Note that in the embodiment as well, primary transferability may be further improved as a matter of course in such a way as to perform correction according to the use situation (the remaining service life) of the intermediate transfer belt10described in the first embodiment. In addition, the Zener diode is used as the voltage maintaining unit in the embodiment. However, the voltage maintaining unit is not limited to such an element, and an element such as a varistor may be used so long as the same effect is obtained by the element. In addition, the roller members are used as the primary transfer members in the embodiment. However, the same effect is obtained even with, for example, conductive brushes or conductive sheet members.

Third Embodiment

FIG. 11is a schematic diagram of an image forming apparatus according to a third embodiment of the present invention. In the description of the embodiment, first to fourth image forming stations will be denoted by symbols70ato70d, photosensitive drums will be denoted by symbols700ato700d, an intermediate transfer belt will be denoted by symbol600, a driving roller will be denoted by symbol603c, a tension roller will be denoted by symbol603b, a secondary transfer opposing roller will be denoted by symbol603a, and a secondary transfer roller will be denoted by symbol601. Other than these components, the same configurations as those of the first and second embodiments will be denoted by the same symbols, and their descriptions will be omitted in the image forming apparatus according to the third embodiment. In the third embodiment, matters that will not be particularly described are the same as those of the first and second embodiments. Note that the image forming apparatus according to the embodiment is so configured that, for instance, toner (residual toner) on the intermediate transfer belt600after secondary transfer is charged by a cleaning brush602and then reversely transferred onto the photosensitive drums700ato700dto be cleaned and removed. The configuration of a controller100that controls the body of the image forming apparatus of the third embodiment is the same as those of the first and second (FIG. 2) embodiments.

Comparative Example

A description will be given, with reference toFIG. 17, of a comparative example of the third embodiment.FIG. 17schematically shows the transfer portion of the image forming apparatus configured to generate a primary transfer current Itr1with the supply of a current from the opposing member of the secondary transfer opposing roller603a. The primary transfer current Itr1is supplied from the secondary transfer roller601and the cleaning brush602. In addition, a primary transfer voltage Vtr1is generated based on a secondary transfer voltage source21and a cleaning voltage source201and configured to allow constant voltage control in a certain voltage range. The generated primary transfer voltage Vtr1forms the surface potential of the intermediate transfer belt600. Further, based on the potential differences (transfer contrast) between the surface potential of the intermediate transfer belt600and the potentials of the photosensitive drums700ato700d, toner on the photosensitive drums700ato700dmoves onto the intermediate transfer belt600to perform primary transfer.

A CPU100serving as a control portion outputs a voltage generation signal to the secondary transfer voltage source21and the cleaning voltage source201. Based on the signal, the secondary transfer voltage source21applies a direct current having a positive polarity to the secondary transfer roller601, and the cleaning voltage source201applies a direct voltage having a positive polarity to the cleaning brush602. A secondary transfer current Itr2flowing through the secondary transfer roller601and a cleaning current Iic1flowing through the cleaning brush602merge with each other via the intermediate transfer belt600and the secondary transfer opposing roller603a. Then, the current diverges into the primary transfer current Itr1necessary for the primary transfer and a control current Icon flowing through a current control circuit315. The primary transfer current Itr1flows into the ground via resistors702a,702b,702c, and702d, primary transfer brushes701a,701b,701c, and701d, the intermediate transfer belt600, and the photosensitive drums700a,700b,700c, and700d.

The image forming apparatus ofFIG. 17is a so-called tandem type image forming apparatus in which the four image forming stations70a,70b,70c, and70dare provided. The first image forming station70aforms an image of yellow (Y), the second image forming station70bforms an image of magenta (M), the third image forming station70cforms an image of cyan (C), and the fourth image forming station70dforms an image of black (Bk). The resistors702a,702b,702c, and702dare arranged to reduce the fluctuations of the primary transfer current between the image forming stations70ato70d. On the other hand, the control current Icon flowing through the current control circuit315flows into the ground via a transistor307. Note that currents flowing into the ground via resistors500and501are not taken into consideration since they are minute. The control of the primary transfer voltage Vtr1is performed by the control of the collector current of the transistor307such that a control voltage V− input to the inversion input terminal of an operational amplifier302and a monitor voltage V+ input to the non-inversion input terminal of the operational amplifier302are the same. The relationship between the primary transfer voltage Vtr1and the control voltage V− is expressed by the following formula (1).
Vtr1=(V−)=((R500+R501)/R501)  (1)

Note that R500and R501are the resistance values of the resistors500and501, respectively, and a current flowing through the input terminal of the operational amplifier302is not taken into consideration since it is minute. The monitor voltage V+ is a direct current obtained by dividing the primary transfer voltage Vtr1by the resistors500and501. On the other hand, the control voltage V− is a direct current obtained by smoothening a PWM signal serving as a current adjustment signal (control signal variable in size) output from the CPU100by a resistor300and a capacitor301. The control voltage V− changes with the on-duty (on-duty ratio) of the PWM signal. The control voltage V− becomes larger as the on-duty increases, and the primary transfer voltage Vtr1becomes larger according to the above formula (1). A resistor304and a capacitor303are connected as elements to determine the response of the operational amplifier302. The output voltage of the operational amplifier302is divided by resistors305and306and input to the base terminal of the transistor307. Thus, the collector current of the transistor307is controlled. The primary transfer voltage Vtr1is generated as the collector-emitter voltage of the transistor307.

In the configuration, an endless polyethylene terephthalate (PET) resin obtained by mixing an ion conducting agent as a conducting agent or the like is used as the intermediate transfer belt. As electrical characteristics, a resistance value fluctuates with respect to temperature and humidity in an atmosphere since the intermediate transfer belt exhibits ion conducting characteristics and electrical conductivity is obtained when ions are transmitted between high polymer chains. In addition, the resistance value of the primary transfer brush used in the configuration increases with energization deterioration due to the endurance of the image forming apparatus. In order to ensure the primary transfer current necessary to deal with the fluctuations of the resistance value of a primary transfer load like this, it is necessary to change the primary transfer voltage.

When the resistance value of a primary transfer load is predicted from a surrounding environment and an endurance sheet number and a primary transfer voltage is determined according to the resistance value of the predicted primary transfer load in the configuration of the above comparative example, there is a case that the determined applied voltage does not become optimum depending on the fluctuations of a load resistance value.

FIG. 12is a diagram for describing the transfer portion of the image forming apparatus according to the third embodiment of the present invention. The descriptions of the same functions as those of the above comparative example will be omitted, and the same components as those of the comparative example are denoted by the same symbols. The embodiment is mainly different from the comparative example in that the resistance value of a primary transfer load is calculated and acquired from a current detection result obtained by detecting the amount of a current flowing through an upstream electric load or upstream current consumer and a voltage detection result obtained by detecting the value (first voltage value) of a voltage applied to a downstream electric load or downstream current consumer.

The upstream electric load of the embodiment includes a resistance component from the secondary transfer roller601to the secondary transfer opposing roller603avia the intermediate transfer belt600(that is, an upstream electric load60a) and a resistance component from the cleaning brush602to the secondary transfer opposing roller603avia the intermediate transfer belt600(that is, an upstream electric load60b). In addition, the downstream electric load of the embodiment includes a resistance component from the secondary transfer opposing roller603ato the ground via primary transfer brushes701a,701b,701c, and701d. In addition, like the comparative example, a current control circuit315is connected in parallel to a downstream electric load70and controls a primary transfer voltage Vtr1by controlling a control current Icon flowing through itself.

The image forming apparatus ofFIG. 12is designed to control the primary transfer voltage Vtr1in the range of 0 V to 600 V. The withstand voltage of the collector-emitter voltage of a transistor307needs to be 600 V or more. In the embodiment, the withstand voltage is 800 V. In addition, the primary transfer voltage Vtr1is divided by resistors500and501of a primary transfer voltage detection circuit350serving as a voltage detection portion and input to the non-inversion input terminal of an operational amplifier302as a monitor voltage V+ and input to the AD port of a CPU100. When the primary transfer voltage Vtr1changes in the range of 0 V to 600 V, the value of a voltage divided by the resistors500and501is designed to change in the range of 0 V to 3.0 V. On the other hand, a control voltage V− input to the inversion input terminal of the operational amplifier302changes with the on-duty of a PWM signal serving as a current adjustment signal output from the CPU100. The control voltage V− becomes 0 V when the on-duty of the PWM signal is set at 0% and becomes 3.3 V when the on-duty of the PWM signal is set at 100%. The control voltage V− is designed to fall within a voltage range in which all the voltage ranges of the monitor voltage V+ may be covered.

The image forming apparatus has, as current detection portions, a secondary transfer current detection circuit400that detects a secondary transfer current Itr2flowing through the secondary transfer roller601and a cleaning current detection circuit401that detects a cleaning current Iic1flowing through the cleaning brush602. Current detection results detected by the respective current detection circuits are output to the CPU100. In general, the secondary transfer portion and the cleaning portion of an image forming apparatus often have respective current detection circuits, and these current detection circuits are applicable to the control of the embodiment. Here, the relationship between the secondary transfer current Itr2, the cleaning current Iic1, and the primary transfer current Itr1is expressed by the following formula (2).
Itr2+Iic1=Itr1+Icon  (2)

Icon is a control current flowing through the current control circuit315. In calculating the resistance value of a primary transfer load, a state in which the control current Icon flowing through the current control circuit315is known is created. In the embodiment, a condition for turning off the transistor307is set and the control current Icon is set at 0 (zero) to create the state in which the control current is known. Specifically, the on-duty of the PWM signal output from the CPU100is set at 100%, and the control voltage V− is set at 3.3 V. In addition, a secondary transfer voltage Vtr2or a cleaning voltage Vic1at which the primary transfer voltage Vtr1does not exceed 600 V regardless of the resistance values of an upstream electric load60and the downstream electric load70is set. For example, when the secondary transfer voltage Vtr2is set at 600 V and the cleaning voltage Vic1is set at 0 V, the primary transfer voltage Vtr1does not exceed 600 V. At this time, the monitor voltage V+ becomes 3.0 V or less, the control voltage V− becomes 3.3 V, the output of the operational amplifier302is fixed to its lower limit, and the transistor307is reliably turned off. However, in this case, it is necessary to design the monitor voltage and the control voltage such that the differential input voltage range of the operational amplifier302is satisfied. In this case, the following formula (3) is obtained from the above formula (2).
Itr1=Itr2+Iic1  (3)

Since the secondary transfer current Itr2and the cleaning current Iic1are detected by the current detection circuits, the primary transfer current Itr1may be calculated and acquired. In addition, since the primary transfer voltage Vtr1is detected by a voltage detection circuit350at this time, a resistance value Rtr1of the primary transfer load may be calculated and acquired by the following formula (4).
Rtr1=Vtr1/(Itr2+Iic1)  (4)

FIG. 13is a schematic diagram showing a modified example of the circuit of the primary transfer portion of the embodiment. In the above embodiment, the primary transfer voltage Vtr1is configured to be controllable in a range of 0 V to 600 V. When the primary transfer voltage Vtr1is configured to be controllable in the range of 400 V to 1000 V using a component having the withstand voltage of the same collector-emitter voltage as that of the transistor307used in the embodiment, there is a case that the current control circuit is configured as shown inFIG. 13. A Zener diode800having a Zener voltage of 400 V is arranged as a voltage drop element (also called a voltage stabilizing element or a voltage maintaining element) between the collector terminal of the transistor307and the upstream electric load60so as to be in series with the voltage adjustment circuit315. The primary transfer voltage Vtr1is generated as the sum of the collector-emitter voltage of the transistor307and the Zener voltage of the Zener diode800. In order to supply a sufficient Zener current to the Zener diode800and stably generate a Zener voltage of 400 V, a resistor308is arranged. When there is a need to create a state in which the control current Icon is known using the current control circuit shown inFIG. 13, it is only necessary to set a condition for turning off the transistor307in the same way as the above. At this time, the control current Icon may be calculated by the following formula (5) to be known.
Icon=(Vtr1−Vzd)/R308  (5)

Note that Vzd is the Zener voltage of the Zener diode800, and R308is the resistance value of the resistor308. At this time, the primary transfer current Itr1is calculated by the following formula (6) based on the above formula (2).
Itr1=Itr2+Iic1−((Vtr1−Vzd)/R308)  (6)

In addition, a resistance value Rtr1of the primary transfer load may be calculated by the following formula (7).
Rtr1=Vtr1/(Itr2+Iic1−((Vtr1−Vzd)/R308))  (7)

The CPU100has the table of the optimum primary transfer voltage Vtr1corresponding to the resistance value Rtr1of the calculated primary transfer load and performs the constant voltage control of the optimum primary transfer voltage Vtr1corresponding to the resistance value Rtr1of a primary transfer load calculated in an image forming operation.

FIG. 14shows transfer efficiency in the primary transfer portion. The value of the transfer efficiency in a vertical axis shows primary transfer residual density. Since the primary transfer residual density becomes larger in proportion to the value, the transfer efficiency deteriorates. That is, an optimum primary transfer voltage is obtained when the value of ISO status I density in the vertical axis becomes the smallest. Note that as measurement conditions shown inFIG. 14, the photosensitive drum and the intermediate transfer belt are new and the measurement is conducted at a temperature of 15° C. and a relative humidity of 10%, i.e., a so-called L/L environment.

InFIG. 15, the optimum primary transfer voltage read fromFIG. 14is shown in a vertical axis, and the resistance value of the primary transfer load is shown in a horizontal axis. The resistance value of the primary transfer load includes the fluctuations of the resistance value of the intermediate transfer belt, the fluctuations of the resistance value of the primary transfer brush, or the like and fluctuates about ±26% at maximum. By the fluctuations of the resistance value of the primary transfer load, a curve shown inFIG. 14also changes. In addition, the optimum primary transfer voltage fluctuates as shown inFIG. 15. The optimum primary transfer voltage becomes 610 V when the primary transfer load has a middle resistance value, 770 V when the primary transfer load has the highest resistance value, and 450 V when the primary transfer has the lowest resistance value.

In the control method of the comparative example, the primary transfer voltage to be applied is determined according to the primary transfer load having the middle resistance value, and a voltage of 610 V is applied as such. Therefore, the applied voltage is smaller by about 160 V than the voltage applied with respect to the primary transfer load having the highest resistance value, and larger by about 160 V than the voltage applied with respect to the primary transfer load having the lowest resistance value. On the other hand, in the control method of the embodiment, the resistance value of the primary transfer load is calculated to determine the applied voltage. Therefore, even when the primary transfer load has the highest or the lowest resistance value, the optimum primary transfer voltage may be applied.

According to the embodiment, an optimum primary transfer voltage may be applied even when the resistance value of a primary transfer load fluctuates as described above. Thus, excellent primary transferability may be ensured.

Fourth Embodiment

FIG. 16is a diagram for describing the transfer portion of an image forming apparatus according to a fourth embodiment. InFIG. 16, the same components as those of the third embodiment inFIG. 12will be denoted by the same symbols, and their descriptions will be omitted. The embodiment shown inFIG. 16is different from the third embodiment shown inFIG. 12in that each of the image forming stations of a primary transfer portion has a current control circuit, voltage detection unit, and contacting/separating unit for causing a photosensitive drum to contact or separate from an intermediate transfer belt. On the other hand, the embodiment is the same as the third embodiment in that the resistance value of a primary transfer load is calculated from a current detection result obtained by detecting a current flowing through an upstream electric load and a voltage detection result obtained by detecting a voltage applied to a downstream electric load.

In the third embodiment shown inFIG. 12, the primary transfer voltage Vtr1is supplied to the four image forming stations by one current control unit. Therefore, there is a likelihood that primary transferability fluctuates due to the fluctuations of the resistance value of a primary transfer load in each of the image forming stations. On the other hand, in the fourth embodiment shown inFIG. 16, current control circuits315a,315b,315c, and315dare provided for the respective image forming stations to adjust primary transfer voltages Vtr1a, Vtr1b, Vtr1c, and Vtr1dto eliminate the fluctuations of primary transferability for each of the image forming stations. Each of the current control circuits315a,315b,315c, and315dis the same as the current control circuit315of the third embodiment shown inFIG. 12. Resistors801a,801b,801c, and801dserving as voltage drop elements are provided to separate the primary transfer voltages for each of the stations.

In the image forming apparatus inFIG. 16, the relationship between a secondary transfer current Itr2, a cleaning current Iic1, and primary transfer currents Itr1a, Itr1b, Itr1c, and Itr1dflowing through the respective image forming stations is expressed by the following formula (8).
Itr2+Iic1=(Itr1a+Icona)+(Itr1b+Iconb)+(Itr1c+Iconc)+(Itr1d+Icond)  (8)

Icona, Iconb, Iconc, and Icond are control currents flowing through the current control circuits315a,315b,315c, and315dof the respective image forming stations.

In calculating the resistance value of a primary transfer load for each of the image forming stations, the control currents flowing through the current control circuits of all the image forming stations are set to be known, and photosensitive drums other than the photosensitive drum of an image forming station of which the resistance value of the primary transfer load is to be calculated are separated. This control is separately performed for all the four image forming stations with a time deviation for each of the image forming stations. Like the third embodiment, a condition for turning off a transistor is set in the embodiment as well. By setting the control currents Icona, Iconb, Iconc, and Icond at 0, a state in which the control currents are known is created.

In addition, the photosensitive drums of image forming stations other than an image forming station of which the resistance value is to be calculated are separated by the contacting/separating units900(900a,900b,900c, and/or900d) to set currents flowing through the image forming stations other than the image forming station of which the resistance value is to be calculated at 0. For example, when the resistance value of an image forming station70ais calculated, the photosensitive drums of image forming stations70b,70c, and70dare separated to set Iconb, Iconc, and Icond at 0. At this time, the following formula (9) is obtained from the above formula (8).
Itr1a=Itr2+Iic1  (9)

Thus, a resistance value Rtr1aof the primary transfer load of the image forming station70amay be calculated by the following formula (10).
Rtr1a=Vtr1a/(Itr2+Iic1)  (10)

The above control is performed for the image forming stations70b,70c, and70dwith a time deviation, whereby resistance values Rtr1a, Rtr1b, Rtr1c, and Rtr1dof all the image forming stations may be calculated.

A CPU100has a table for optimum primary transfer voltages corresponding to the resistance values of calculated primary transfer loads and performs the constant voltage control of optimum primary transfer voltages Vtr1a, Vtr1b, Vtr1c, and Vtr1dcorresponding to the resistance values of the primary transfer loads calculated in an image forming operation.

This application claims the benefit of Japanese Patent Application No. 2016-153769, filed on Aug. 4, 2016, and No. 2017-26151, filed on Feb. 15, 2017, which are hereby incorporated by reference herein in their entirety.