JP4590334B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that employs an electrophotographic system, an electrostatic recording system, or the like.

  FIG. 3 shows a schematic configuration of an example of a conventional image forming apparatus. As shown in FIG. 3, the image forming apparatus includes an electrophotographic photosensitive member 100 (hereinafter referred to as a photosensitive drum 100) as an image carrier. A charging device 101 and an exposure device 102 are provided around the photosensitive drum 100. A developing device 103 that develops the electrostatic latent image as a toner image, a transfer device 105, and a cleaning device 107 are included. The toner image transferred to the recording medium 104 in the transfer process is fixed by the fixing device 106. Using the above means, image formation is performed by repeating the steps of charging, exposure, development, transfer, fixing, and cleaning.

  FIG. 4 shows an operation sequence of a general image forming apparatus. The power source of the image forming apparatus main body is turned on (S401), and then the pre-multi-rotation process is performed (S402). The pre-multi-rotation process is a preparatory operation from when the power is turned on until it enters a standby state (standby). Here, the presence or absence of a process cartridge is detected. Then, the image forming apparatus enters a standby state (standby) (S403). When image information is sent from an unillustrated host computer or the like to the image forming apparatus (S404), the pre-rotation process is started (S405). The pre-rotation process is a preparation process from a standby state to image formation. In this step, preliminary charging or the like on the photosensitive drum is performed.

  When the pre-rotation process is completed, the image forming process is started (S406). In the image forming process, an image forming process such as charging, exposure, development, transfer, and fixing is performed.

  When the image forming process is completed, if there is a next print signal, the process enters the sheet interval process until the next recording medium arrives and waits for the next printing operation (S408). If there is no next print signal, the image forming apparatus performs a post-rotation process to enter a standby state (S409).

  Next, a charging bias control method will be described.

  The charging bias includes a case where only a DC voltage is used (DC charging) and a case where an AC voltage is superimposed on the DC voltage (AC + DC charging). AC + DC charging is excellent for uniformly charging the photosensitive drum 100. As an AC voltage control method in AC + DC charging, for example, the following methods are known. For example, in order to perform uniform charging, there is a control method in which a bias superimposed with an AC voltage having a peak-to-peak voltage (Vpp) of at least twice the discharge start voltage when a DC voltage is applied is used as a charging bias (Patent Document 1). reference). In addition, it is known that the impedance of the charging roller changes due to changes in the usage environment, and the thickness of the charge transport layer of the photosensitive drum varies according to the use of the image forming apparatus, so that the discharge current value optimum for charging varies. ing. Considering this, there is a method of controlling the AC voltage so that the amount of discharge current becomes a constant current (see Patent Document 2). The control method of Patent Document 2 has a great effect in terms of ensuring good chargeability and preventing excessive discharge to the photosensitive drum. However, in order to obtain a stable bias voltage, the method of Patent Document 2 requires that the AC and DC power supplies to be applied in a superimposed manner be separated, and a step-up transformer as two voltage boosting means. A step-up transformer is relatively large and expensive among power supply circuits, and is not particularly suitable for a small and low-cost image forming apparatus.

  In order to avoid this problem, a method of controlling the AC component with a substantially constant current value by using only one step-up transformer has been proposed (see Patent Document 3). At the time of non-image formation, an AC voltage (hereinafter referred to as AC peak-to-peak voltage Vpp) having different peak-to-peak voltages of a plurality of levels is applied from the power supply circuit to the charging roller. Then, a search is made for a Vpp at which the AC current value flowing through the photosensitive drum 100 is equal to or greater than the minimum necessary current value so that charging failure does not occur. Then, the Vpp having the minimum peak-to-peak voltage is selected from among the Vpps that can obtain an AC current equal to or greater than the necessary minimum current value as the peak-to-peak voltage of the AC voltage during image formation. Since this method is control for determining the peak-to-peak voltage, it will be referred to as peak-to-peak voltage selection control. Note that the peak-to-peak voltage selection control can also be referred to as pseudo-stop current control because constant current control is performed using a constant voltage circuit. By performing such charging bias voltage control, the AC current value is made substantially constant irrespective of impedance changes of the charging roller, photosensitive drum, etc., as in the case of performing constant current control with only one step-up transformer. I can keep it. This method is very effective especially in a small-sized and low-cost image forming apparatus in terms of ensuring good chargeability and preventing excessive discharge to the photosensitive drum 100.

  FIG. 5 is a conceptual diagram of a charging bias power supply circuit when using a peak-to-peak voltage selection control that controls an AC component with a substantially constant current value using only one step-up transformer. The step-up transformer is only one that creates an AC component, and the DC component is created by peak-charging the capacitor C100 by this step-up transformer.

  The engine controller applies a plurality of stages of AC peak-to-peak voltage Vpp from the power supply circuit to the charging roller, and detects the AC current Iac flowing through the photosensitive drum 100. Among AC voltages Iac, the minimum Vpp among the AC voltages that can obtain an AC current equal to or greater than the AC current value necessary for preventing charging failure (peak-to-peak voltage selection control threshold current Iac-0) is set at the time of image formation. Select the peak-to-peak voltage.

  A conventional peak-to-peak voltage selection control sequence will be described.

  FIG. 6 is a flowchart for explaining the peak-to-peak voltage selection control sequence. FIG. 10 is a conceptual configuration diagram illustrating the peak-to-peak voltage selection control circuit.

  When the power of the image forming apparatus is turned on (S301), the control unit 238 serving as a determination unit reads the peak-to-peak voltage selection control threshold value V0 from the memory 210 of the process cartridge C (S302). Here, the peak-to-peak voltage selection control threshold V0 is predetermined as a target value, and is used as a threshold for the current detection voltage V. The current detection voltage V is obtained by converting the AC current Iac flowing between the charging roller and the photosensitive drum into a voltage value when an AC voltage is applied to the charging roller. The current Iac will be seen. More specifically, the peak-to-peak voltage selection control threshold value V0 is a necessary alternating current value (peak-to-peak voltage selection control threshold current Iac-0) that flows between the charging roller and the photosensitive drum in order that charging failure does not occur during the charging process. ) Is converted to a voltage value. Since the value of Iac-0 varies depending on the process speed, charging frequency, and the like of the device, the peak-to-peak voltage selection control threshold V0 is also set as appropriate.

  Subsequently, the pre-multi-rotation process begins. In this pre-multi-rotation process, a plurality of AC voltages (detection voltage Vpp- (k)) for current detection are applied. The detection voltage Vpp- (k) is applied to the charging roller in descending order of the peak-to-peak voltage (voltage application step). Note that k in Vpp- (k) is a variable for representing a detection voltage having a different peak-to-peak voltage, and k = 2, k = 3,. It expresses.

  For example, the detection voltage Vpp is applied as Vpp− (1) = Vpp−max, Vpp− (2), Vpp− (3),... Vpp−min (S303). Here, the magnitude of the peak-to-peak voltage is Vpp− (1) = Vpp−max> Vpp− (2)> Vpp− (3)>...> Vpp−min.

  When these detection voltages Vpp- (k) are applied, the charging current value flowing through the photosensitive drum is detected. The current value is measured as the current detection voltage Vk (S304) (detection step), and the current detection voltage Vk is averaged per unit time to obtain Vk-ave (S305).

  Compared to Vk-ave and the peak-to-peak voltage selection control threshold V0, the detection voltage Vpp- (k) that minimizes the peak-to-peak voltage and Vk-ave ≧ V0 is set to the AC voltage Vpp- (n) at the time of image formation. Is determined to be applied (S306) (determination step). The minimum peak-to-peak voltage that can be charged uniformly among the AC voltages is determined by the above steps, and the process proceeds to a standby state (S307).

  Next, image information is sent from the output means such as a host computer to the image forming apparatus, and a print ON signal is transmitted from the control unit 238 (S308). Then, the AC voltage Vpp- (n) determined in (S306) is applied to the charging roller 202 during image formation (S309). Then, after the series of image forming steps, if there is no next print signal, post-rotation is performed (S310), and a transition is made to a standby state.

Conventionally, as described above, Vk-ave obtained by averaging the current detection voltage Vk is compared with the peak-to-peak voltage selection control threshold V0. Then, it has been determined to apply the detection voltage Vpp- (k) such that Vk-ave ≧ V0 and the peak-to-peak voltage is minimized as the AC voltage Vpp- (n) at the time of image formation.
JP 63-149669 A JP 06-093150 JP2004-4654

  However, when the surface of the charging roller becomes dirty due to the toner or an external additive contained in the toner adhering to the charging roller, the current detection voltage Vk may increase greatly. This is because, when a very small amount of dirt adheres to the charging roller, the gap between the charging roller and the photosensitive drum is microscopically narrowed so that the current flows easily. This is because when the amount of dirt adhering to the charging roller increases, the resistance of the charging roller surface increases and it becomes difficult for current to flow. For this reason, the fluctuation of the current detection voltage Vk becomes large due to the state of dirt on the surface of the charging roller.

  FIG. 7 shows a conceptual diagram of the current detection voltage Vk. FIG. 7A is a conceptual diagram of the current detection voltage Vk when the surface of the charging roller is less dirty, and FIG. FIG. 7 is a diagram showing the effective value of the current detection voltage Vk over time.

  In FIG. 7A, since the surface of the charging roller is less contaminated, the magnitude of the current detection voltage Vk does not change so much.

  On the other hand, in FIG. 7B, there is a part where the current detection voltage Vk is particularly small, such as part A. This portion A is a portion where the surface of the charging roller is particularly dirty.

  However, as shown in FIG. 7, even when the magnitude of the fluctuation of the current detection voltage Vk is different (that is, the maximum value and the minimum value are different), the average value of the circumference of the photosensitive drum may be almost the same. In this case, when the peak-to-peak voltage is determined based on the average value, the same peak-to-peak voltage is obtained in both the case shown in FIG. 7A and the case shown in FIG. 7B.

  However, since it is an average value, a part of the charging roller that is slightly contaminated, such as part A in FIG. 7B, is not considered, and the peak-to-peak voltage is such that the photosensitive drum cannot be sufficiently charged. May have been determined. In this case, an image defect such as a high density occurs particularly in a halftone image.

  In order to solve the above problems, the present invention has the following configuration.

  An image carrier, movable charging means for charging the image carrier, voltage applying means for applying a plurality of alternating voltages having different peak-to-peak voltages to the charging means, and flowing from the charging means to the image carrier. In the image forming apparatus having current detection means for detecting an alternating current value, an alternating current value having a minimum value that is equal to or greater than a predetermined current value among the minimum values of the alternating current values with respect to the plurality of alternating voltages. An image forming apparatus comprising: a determining unit that determines a peak-to-peak voltage of the AC voltage applied to the charging unit based on an AC voltage obtained.

  As an effect of the present invention, the charging of the image carrier by the charging means can be made uniform.

  Another effect of the present invention is that the occurrence of charging failure can be suppressed even when the charging means is locally soiled.

  Another effect of the present invention is that the image carrier can be prevented from being scraped.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

  First, an outline of the configuration and operation of the image forming apparatus will be described.

  FIG. 8 is a schematic configuration diagram of the image forming apparatus of the present embodiment. The image forming apparatus according to the present exemplary embodiment is an electrophotographic type and a process cartridge detachable laser printer.

  Reference numeral 201 denotes a rotating drum type photosensitive drum as an image carrier. The photosensitive drum 201 of this example is a negatively chargeable organic photosensitive member, and is rotationally driven in a clockwise direction indicated by an arrow at a predetermined peripheral speed by a driving motor (not shown).

The photosensitive drum 201 is uniformly charged to a predetermined negative potential by a charging device during the rotation process. In this example, the charging means is a rotatable charging roller 202, and is a contact charging method in which charging is performed by contacting the charging roller 202 to the photosensitive drum 201. The charging roller used in this example has a volume resistance of 1 × 10 ⁶ Ωcm.

  The charging roller 202 rotates following the photosensitive drum 201. A bias voltage is applied to the charging roller 202 from a charging bias power supply circuit 230 (see FIG. 10) which is a voltage applying unit. As the charging bias voltage, a method is used in which a DC voltage Vdc corresponding to a desired on-drum potential Vd is superimposed on an AC voltage having a peak-to-peak voltage (Vpp) that is twice or more the discharge start voltage. This charging method aims to eliminate local potential unevenness on the photosensitive drum by applying an alternating voltage on the direct current voltage and uniformly charge the photosensitive drum to a potential Vd equal to the direct current applied voltage Vdc. Yes. The voltage application means used in the present application has one step-up transformer as described in the section “Problems to be Solved by the Invention”. An AC component of the bias is generated by the step-up transformer, and a DC component of the bias is generated by the step-up transformer peak-charging the capacitor. Then, a charging process is performed by applying a bias in which an AC component and a DC component are superimposed on the charging roller 202. Next, the charged photosensitive drum 201 is subjected to image exposure by an exposure device 221 that is a latent image forming unit. The exposure device 221 forms an electrostatic latent image on the uniformly charged photosensitive drum 201. In this example, a semiconductor laser scanner is used. The exposure device 221 outputs a laser beam L modulated in accordance with an image signal sent from a host device (not shown). The laser beam L scans (image exposes) the uniformly charged surface of the photosensitive drum 201 via a reflecting mirror 221a and an exposure window portion a of a process cartridge C described later. On the surface of the photosensitive drum, an electrostatic latent image corresponding to image information is sequentially formed as the absolute value of the potential of the exposed portion becomes lower than the absolute value of the charging potential.

  Next, the electrostatic latent image is developed by the reversal developing device 205 to be visualized as a toner image. In this example, a jumping development method using a magnetic one-component developer was used. In this method, a developing bias voltage in which alternating current and direct current are superimposed is applied to the developing sleeve 207 from a developing bias power source (not shown). Then, the electrostatic latent image is reversely developed by applying a negatively charged toner by frictional charging to the electrostatic latent image on the surface of the photosensitive drum at the contact point between the developer layer thickness regulating member 206 and the developing sleeve 207 by the developing bias. To do.

  The toner image on the photosensitive drum surface is transferred by a transfer device to a recording medium (transfer material) such as paper fed from a paper supply unit (not shown). In this example, the contact transfer device uses a transfer roller 222. The transfer roller 222 is pressed against the photosensitive drum 201 in the central direction of the photosensitive drum by an urging means such as a pressing spring (not shown). When the transfer material is conveyed and the transfer process is started, a positive transfer bias voltage is applied to the transfer roller 222 from a transfer bias power source (not shown), and the toner on the photosensitive drum 201 is charged negatively. Is transferred onto the transfer material.

  The transfer material that has received the transfer of the toner image is separated from the surface of the photosensitive drum, introduced into the fixing device 223, subjected to a fixing process of the toner image, and discharged onto the discharge tray 225 through the sheet path 224. The fixing device 223 fixes the toner image transferred onto the transfer material to a permanent image using means such as heat or pressure.

  The photosensitive drum surface after separation of the transfer material is cleaned by scraping off the transfer residual toner by the cleaning device 204 and repeatedly used for image formation. The cleaning device 204 of this example uses a cleaning blade 203. The cleaning blade 203 collects the transfer residual toner that has not been completely transferred from the photosensitive drum 201 to the transfer material during the transfer process. The cleaning blade 203 contacts the photosensitive drum 201 with a constant pressure and collects the transfer residual toner. Clean the surface. After completion of the cleaning process, the photosensitive drum surface again enters the charging process.

  The image forming apparatus forms an image by repeating the steps of charging, exposure, development, transfer, fixing, and cleaning using the above-described means. Here, the printing operation performed after the printing preparation operation of various process devices is defined as an image forming step (at the time of image formation). That is, the image forming process in the charging process refers to the time when the charging unit is charging the area on the photosensitive drum where a latent image will be formed during the printing operation. Further, the non-image forming process (during non-image formation) in the charging process refers to a time when the charging unit charges a region other than the region on the photosensitive drum where a latent image is to be formed during the printing operation. Point to.

  The process cartridge C of this embodiment includes four process devices including a photosensitive drum 201 as an image carrier, a charging roller 202 as a contact charging unit for the photosensitive drum 201, a developing device 205, and a cleaning device 204. As a process cartridge.

  In the photosensitive drum 201 of this embodiment, a drum base such as aluminum as a conductive base layer and a photosensitive layer (photoconductive layer) as a member to be charged formed on the outer peripheral surface thereof are used as base constituent layers.

  Further, the process cartridge C is provided with a memory 210 as a storage medium. Information is read from and written to the memory 210 via a communication unit (not shown) on the image forming apparatus main body side.

  The process cartridge C is attached to and detached from the image forming apparatus main body 220 by opening and closing a main body door (cartridge door) 220 a of the image forming apparatus main body 220. The mounting is performed by opening the main body door 220a, inserting and mounting the process cartridge C in the image forming apparatus main body 220 in a predetermined manner, and closing the main body door 220a. The process cartridge C is mechanically and electrically connected to the image forming apparatus main body 220 side by being mounted on the image forming apparatus main body 220 in a predetermined manner.

  The process cartridge C is detached from the image forming apparatus main body 220 by opening the main body door 220a and pulling out the process cartridge C in the image forming apparatus main body 220 in a predetermined manner.

  FIG. 9 shows the process cartridge C in a removed state. When the process cartridge C is removed, the drum cover 208 is moved to the closed position to conceal and protect the exposed lower surface of the photosensitive drum 201. The exposure window part a is also held closed by the shutter plate 209. When the process cartridge C is mounted in the image forming apparatus main body 220, the drum cover 208 and the shutter plate 209 are each moved to an open position and held.

  Here, the process cartridge is a cartridge in which at least one of a charging unit, a developing unit, and a cleaning unit and an electrophotographic photosensitive member are integrally formed, and the cartridge can be attached to and detached from the main body of the electrophotographic image forming apparatus. It is.

  Next, an outline of an operation sequence of the image forming apparatus will be described.

  When the power supply in the image forming apparatus is turned on, the pre-multi-rotation process starts. In this process, while the main motor rotates the photosensitive drum, the presence / absence of the process cartridge is detected and the transfer roller is cleaned.

  When the front multi-rotation process ends, the image forming apparatus enters a standby (standby) state. When image information is sent to the image forming apparatus from an output unit such as a host computer (not shown), the main motor drives the image forming apparatus main body and enters a pre-rotation process. In the pre-rotation process, printing preparation operations of various process devices are performed. Mainly, preliminary charging on the photosensitive drum, startup of the laser scanner, determination of the transfer print bias, temperature adjustment of the fixing device, and the like are performed.

  When the pre-rotation process is completed, the printing process is started. In the printing process, transfer material is fed, image exposure on a photosensitive drum, development, and the like are performed at a predetermined timing. When the printing process is completed, if there is a next print signal, the process enters the sheet interval process until the next transfer material arrives and waits for the next printing operation.

  If there is no next print signal after the printing operation is completed, the image forming apparatus enters a post-rotation process. In the post-rotation process, processes such as charge removal on the surface of the photosensitive drum and discharge of toner adhering to the transfer roller to the photosensitive drum (transfer roller cleaning) are performed.

  When the post-rotation process is completed, the image forming apparatus again enters a standby (standby) state and waits for the next print signal.

  Next, the peak-to-peak voltage selection control sequence that is a feature of the first embodiment will be described. In the description, a description will be given based on a flowchart (FIG. 1) for explaining the peak-to-peak voltage selection control sequence of the first embodiment and a conceptual diagram (FIG. 10) for explaining the peak-to-peak voltage selection control circuit.

  When the power of the image forming apparatus is turned on (S101), the control unit 238 serving as a determination unit reads the peak-to-peak voltage selection control threshold V0 from the memory 210 of the process cartridge C (S102). Here, the peak-to-peak voltage selection control threshold value V0 is a threshold value of the current detection voltage V that is set in advance as a target value. The current detection voltage V is obtained by converting an alternating current Iac flowing between the charging roller and the photosensitive drum into a voltage value when an alternating voltage is applied to the charging roller. Note that the peak-to-peak voltage selection control threshold V0 is an AC current value (peak-to-peak voltage selection control threshold current Iac-0 (predetermined current) required to flow between the charging roller and the photosensitive drum so that charging failure does not occur during the charging process. Value)) is converted to a voltage value.

Subsequently, the pre-multi-rotation process begins. In the previous multi-rotation process, a plurality of stages of AC voltages Vpp- (k) (Vpp-max> Vpp- (2)> Vpp- (3)>... Vpp-min) having different peak-to-peak voltages are detected voltages. Applied to the charging roller 202. K in Vpp− (k) is a variable for representing a detection voltage having a different peak-to-peak voltage. In the first embodiment, k = 1, k = 2, k = 3 ·・ ・For example, the detection voltage Vpp is applied as Vpp− (1) = Vpp−max, Vpp− (2), Vpp− (3),... Vpp−min (S103) (voltage application step) . Here, the magnitude of the peak-to-peak voltage is Vpp-max> Vpp- (2)> Vpp- (3)>...> Vpp-min. That is, as the peak voltage level increases, the peak voltage value decreases. Here, the set number of peak-to-peak voltages of the detection voltage is 6 as shown in FIG. 10 (k = 1 to 6).
Vpp-max (ie, Vpp- (k = 1)) = 2000V
Vpp- (k = 2) = 1900v
Vpp- (k = 3) = 1800v
Vpp- (k = 4) = 1700v
Vpp- (k = 5) = 1600v
Vpp-min (ie, Vpp- (k = 6)) = 1500v,
It has become. It should be noted that Vpp-1 must be set such that the detection voltage V1 obtained when the maximum value Vpp-1 of the detection voltage that can be applied is V1> V0. This is because charging failure does not occur under any circumstances. In addition, the detection voltage Vpp-min that can be applied is set so that the relationship of Vpp-min ≧ 2 × | Vdc ′ | is established with respect to the desired DC voltage Vdc ′. This is because the image forming apparatus in the present embodiment has only one step-up transformer that creates an AC component. When there is one step-up transformer, the DC component is created by peak-charging the capacitor C100 with the AC component created by this step-up transformer. Therefore, if the peak-to-peak voltage of the AC voltage is too small, a desired DC voltage may not be obtained. Even if the AC peak-to-peak voltage is set to be the smallest (Vpp-min is set) by setting Vpp-min so that Vpp-min ≧ 2 × | Vdc ′ | A DC voltage can be obtained.

  When a charging bias is applied to Vpp-max... Vpp-min and the charging roller 202, the alternating current Iac flows through the charging roller 202 and the photosensitive drum 201 to the high-voltage power supply circuit GND. Only the alternating current having a frequency equal to the charging frequency is sampled by the alternating current detection means 236 which is a current detection means, and the alternating current flowing through the photosensitive drum 201 is detected by a filter circuit (not shown) composed of a resistor, a capacitor and the like (detection step). ). The AC current is converted into a current detection voltage Vk (S104) and input to the engine controller 237. Here, the current detection voltage Vk detects an effective value.

  The current detection voltage Vk is desirably sampled at a constant period in order to accurately detect the current. For example, in consideration of the stain unevenness for one turn of the charging roller, it is preferable to sample the current detection voltage Vk for more than one turn of the charging roller. In consideration of the film thickness unevenness of the photosensitive drum, the sampling is preferably performed for one or more times of the photosensitive drum. This is because there is a case where the photosensitive drum has uneven film thickness due to uneven shaving due to eccentric rotation or the like, and this causes the alternating current Iac to flow to change with the rotation period of the photosensitive drum. However, if the length is too long, the time of the entire process becomes longer, so it may not be too long. In the first embodiment, sampling is performed for a time corresponding to two rotations of the charging roller for each peak voltage of one magnitude. The time for which the charging roller rotates twice corresponds to the time for which the photosensitive drum rotates once. Two rotations of the charging roller are 0.6 (s), and data is sampled every 0.01 (s). That is, the data is 60 pieces.

  The minimum value of the current detection voltage Vk is selected in the engine controller 237 (S105). Here, when the peak-to-peak voltage at a certain stage is applied as the detection voltage for selecting the minimum value, the change in the effective value of the current detection voltage over time as shown in FIG. Explained. The selection of the minimum value is to select a value with the smallest value among the effective values of the obtained current detection voltage Vk as Vk-min (the value of the portion A in FIG. 7B). The portion where the effective value is small is a portion where current hardly flows at the time of charging, and a portion where charging failure is likely to occur at the time of image formation. As described above, the reason why the portion where the current during charging becomes difficult to flow is that the transfer residual toner that has passed through the cleaning blade adheres to the charging roller and the toner is solidified on the surface of the charging roller. Can be mentioned.

  The current detection voltage Vk-min selected as the minimum value is compared with a preset peak-to-peak voltage selection control threshold V0 by the comparison means in the engine controller 237. The peak-to-peak voltage selection control threshold value V0 is a threshold value of the current detection voltage V, and the necessary alternating current value Iac-0 (the peak-to-peak voltage selection control threshold current (predetermined current value)) that can perform uniform charging is photosensitive. This is the value of the current detection voltage when flowing through the drum. In other words, the current detection voltage corresponds to the AC peak-to-peak voltage without causing charging unevenness. Note that the value of Iac-0 varies depending on the process speed and charging frequency of the device, the constituent materials of the charging roller 202 and the photosensitive drum 201. For this reason, the peak-to-peak voltage selection control threshold V0 is also set appropriately. In addition, it is preferable to set the minimum value of the necessary alternating current value that can perform uniform charging as the value of Iac-0. By doing so, the peak-to-peak voltage is selected while performing uniform charging and in particular, the photosensitive drum is less likely to be scraped.

  The minimum value Vk-min of the current detection voltage is compared with the peak-to-peak voltage selection control threshold VO. Then, it is determined to apply a detection voltage Vpp- (k) that satisfies Vk−min ≧ VO and minimizes the peak-to-peak voltage as the AC voltage Vpp− (n) during the image forming process (S106). Decision process).

  The minimum peak-to-peak voltage that can be charged uniformly among the AC voltages is determined by the above steps, and the process proceeds to a standby state (S107).

  Next, when image information is sent from the output means such as a host computer to the image forming apparatus, the AC voltage Vpp- (n) determined in (S106) is applied during image formation (S108). If there is no next print signal, a post-rotation process is performed (S110), and a transition is made to a standby state.

  As described above, determining the AC voltage during the image forming process based on the minimum value of the current detection voltage Vk is sufficient even if the current hardly flows from the charging roller to the photosensitive drum. The AC voltage is set so as to be an inter-voltage.

  Therefore, even if the surface of the charging roller becomes dirty and the fluctuation of the current detection voltage Vk becomes large, the charging becomes insufficient as when the AC voltage at the time of image formation is determined based on the average value of Vk. Such things can be suppressed. And it becomes possible to make generation | occurrence | production of an image defect inconspicuous.

  The peak-to-peak voltage selection control can be effective in a situation where the charging means is likely to cause resistance fluctuation in the circumferential direction. For example, there may be a case where toner is likely to adhere to the charging unit as in a cleanerless image forming apparatus.

  In the first embodiment, as shown in FIG. 11A, an area for storing the following information is provided in the memory 210 provided in the process cartridge. By setting the information stored in the memory 210 to an appropriate value, it is possible to perform peak-to-peak voltage selection control in consideration of individual differences (lot differences) for each process cartridge. Various information is stored in the memory 210. In the first embodiment, it is assumed that at least the peak-to-peak voltage selection control threshold Vo is stored. The memory information is always in a state where it can be transmitted / received to / from the main body control unit 238, is calculated based on the information, and the control unit 238 collates the data. As in the present embodiment, an example of an experiment in which the peak-to-peak voltage selection control is performed based on the minimum value (Example 1) and in which the peak-to-peak voltage selection control is performed based on the average value (Comparative Example 1). Show.

  The experimental method will be described below. A photosensitive drum having a drum diameter of 24 mm and a photosensitive layer thickness of 22 μm was used, and the photosensitive drum process speed was 130 mm / sec. A cleaning blade is brought into contact with the photosensitive drum at a contact pressure of 40 N / mm and a set angle of 25 °. The charging roller having a roller diameter of 12 μm is brought into contact with the photosensitive drum, and is rotated following the photosensitive drum. As described above, the detection voltage applied to the charging roller for the peak-to-peak voltage selection control has six stages. Specifically, Vpp-max (i.e., Vpp- (1)) = 2000V, Vpp- (2) = 1900v, Vpp- (3) = 1800v, Vpp- (4) = 1700v, Vpp- (5) = 1600v. , Vpp-min (ie, Vpp- (6)) = 1500v. The peak-to-peak voltage selection control threshold V0 is 2.2 (V). When the peak-to-peak voltage selection control threshold value V0 is converted into an alternating current value Iac-0, this corresponds to Iac = 700 μA.

  Under such conditions, the occurrence of charging unevenness is examined by the following experiment. While performing the peak-to-peak voltage selection control of Example 1 and Comparative Example 1, 7000 halftone images of A4 size are formed and output. Based on the output halftone image, an image having a large image unevenness is determined to have a large charge unevenness. In addition, the peak-to-peak voltage selection control is performed every 1000 sheets output, and the peak-to-peak voltage value of the AC voltage is reset.

  In the above experiment, when the image unevenness from the first output image to the 7000th image was compared, Example 1 had less output image unevenness than Comparative Example 1.

  Therefore, by performing the peak-to-peak voltage selection control as in the first embodiment, the charging uniformity can be maintained throughout the use.

  When peak-to-peak voltage selection control is performed based on the average value of the current detection voltage, charging failure occurs when the current detection voltage Vk fluctuates greatly due to the unevenness of the unevenness of the charging roller as described in the problem. Cheap. In order to prevent this, in the first embodiment, the peak-to-peak voltage selection control is always performed based on the minimum value of the current detection voltage. However, since the minimum value of the current detection voltage is always smaller than the average value of the current detection voltages, it is easy to select a large value for the peak-to-peak voltage of the AC voltage selected during image formation. This is because the peak-to-peak voltage selection control is control for selecting an AC voltage having a peak-to-peak voltage at which the value of the current detection voltage Vk is equal to or greater than the peak-to-peak voltage selection control threshold VO. That is, in the case of Vk-min where the value of the current detection voltage Vk is smaller than Vk-ave, the peak-to-peak voltage of the selected AC voltage tends to increase. When an AC voltage having a large peak-to-peak voltage is applied, the photosensitive drum is easily scraped, which is disadvantageous from the viewpoint of the life of the photosensitive drum.

  Thus, in the second embodiment, when the peak-to-peak voltage selection control is performed, the difference between the minimum value Vk-min and the average value Vk-ave of the current detection voltage value is detected, and the difference is set to a predetermined value. Are going to compare. When the difference is smaller than a predetermined value (that is, when there is little dirt unevenness on the charging roller), the average value Vk-ave of the current detection voltage is detected. When the difference is large (ie when there is much dirt unevenness on the charging roller), current detection is performed. The peak-to-peak voltage selection control is performed based on the minimum voltage value Vk-min.

  By doing so, even if the charging roller is contaminated, it is possible to make the occurrence of an image defect inconspicuous and further prevent unnecessary shaving of the photosensitive drum.

  Since the configuration and operation of the image forming apparatus are the same as those in the first exemplary embodiment, description thereof is omitted here.

  A peak-to-peak voltage selection control sequence according to the second embodiment will be described. FIG. 2 is a flowchart for explaining a peak-to-peak voltage selection control sequence according to the second embodiment. The peak-to-peak voltage selection control circuit will be described with reference to FIG.

  When the power of the image forming apparatus is turned on (S201), the control unit 238 serving as a determination unit accesses the memory 210 of the process cartridge C. Then, a predetermined value X for comparing the difference between the peak-to-peak voltage selection control threshold V0, the minimum value Vk-min of the current detection voltage, and the average value Vk-ave is read (S202).

Subsequently, the pre-multi-rotation process begins. In this pre-multi-rotation process, a plurality of AC voltages (detection voltage Vpp- (k)) for current detection are applied. The detection voltage Vpp- (k) is applied to the charging roller in descending order of the peak-to-peak voltage (voltage application step). In the second embodiment, similarly to the first embodiment, the set number of peak-to-peak voltages of the detection voltage is 6 (k = 1 to 6).
Vpp-max (ie, Vpp- (k = 1)) = 2000v
Vpp- (k = 2) = 1900v
Vpp- (k = 3) = 1800v
Vpp- (k = 4) = 1700v
Vpp- (k = 5) = 1600v
Vpp-min (ie, Vpp- (k = 6)) = 1500v,
It has become.

  When a charging bias is applied to Vpp-max... Vpp-min and the charging roller 202, the alternating current Iac flows through the charging roller 202 and the photosensitive drum 201 to the high-voltage power supply circuit GND. Only the alternating current having a frequency equal to the charging frequency is sampled by the alternating current detection means 236 which is a current detection means, and the alternating current flowing through the photosensitive drum 201 is detected by a filter circuit (not shown) composed of a resistor, a capacitor and the like (detection step). ). The AC current is converted into a current detection voltage Vk (S204) and input to the engine controller 237.

  Next, the current detection voltage Vk is subjected to minimum value selection and averaging processing in the engine controller 237 (S205). Then, the difference ΔV between the current detection voltage Vk-min selected as the minimum value and the averaged current detection voltage Vk-ave is calculated, and the difference ΔV is set in advance by the comparison means in the engine controller 237. It is compared with X (S206).

  In this way, the difference ΔV between the minimum current detection voltage Vk-min and the average current detection voltage Vk-ave of the current detection voltage Vk is detected and compared with a predetermined value X, whereby the fluctuation of the current detection voltage Vk is detected. That is, it is possible to detect the presence or absence of dirt on the surface of the charging roller.

  When the difference ΔV is smaller than the predetermined value X, the average value Vk-ave of the current detection voltage is selected and compared with Vk-ave and the peak-to-peak voltage selection control threshold VO. Then, it is determined to apply the detection voltage Vpp- (k) such that Vk−ave ≧ VO and the peak-to-peak voltage is minimized as the AC voltage Vpp− (n) in the image forming process (S207).

  When the difference ΔV is larger than the predetermined value X, the minimum value Vk-min of the current detection voltage is selected and compared with the peak-to-peak voltage selection control threshold VO. Then, it is determined to apply the detection voltage Vpp- (k) such that Vk−min ≧ VO and the peak-to-peak voltage is minimized as the AC voltage Vpp− (n) during the image forming process (S208).

  The minimum peak-to-peak voltage Vpp- (n) that can be charged uniformly among the AC voltages is determined by the above steps, and the process proceeds to a standby state (S209).

  As described above, in this embodiment, the minimum value Vk-min and the average value Vk-ave are selected based on the difference between the average value and the minimum value of the current detection voltage Vk, that is, the result of detecting whether the charging roller surface is dirty. Do. By doing this, it is possible to prevent unnecessary shaving of the photosensitive drum and to make the occurrence of image defects inconspicuous.

  Next, when image information is sent from the output means such as a host computer to the image forming apparatus (S210), the AC voltage Vpp- (n) determined in (S207, S208) is applied during image formation (S211). If there is no next print signal, a post-rotation process is performed (S212), and a transition is made to a standby state.

  Here, the predetermined value X will be described in more detail.

  In the constant voltage control capable of outputting a plurality of peak-to-peak voltages, the difference between the maximum voltage and the minimum voltage and the number of stages are limited mainly from the viewpoint of cost. As described above, in the second embodiment, there are six stages from 1500 v to 2000 v, and the configuration is limited to that only 100 v can be applied. That is, unlike the constant current control that controls the AC component with a constant current, a fine output voltage cannot be set. Therefore, this point is taken into consideration when setting the predetermined value X. In other words, the AC voltage determined based on the minimum current detection voltage Vk-min is at least an AC voltage during the image forming process determined based on the average current detection voltage Vk-ave. It is preferable to set one or more steps so that the steps are lowered (high voltage). Such a predetermined value X is appropriately set depending on the resistance value of the charging roller, the applied voltage, and the like.

  In the second embodiment, at least as shown in FIG. 11- (b), the difference between the peak-to-peak voltage selection control threshold Vo, the current detection voltage minimum value Vk-min, and the current detection voltage average value Vk-ave is stored in the memory 210. It is assumed that a predetermined value X for comparison is stored. The memory information is always in a state where it can be transmitted / received to / from the main body control unit 238, is calculated based on the information, and the control unit 238 collates the data.

  As in this embodiment, when the minimum value criterion and the average value criterion are selected and the peak-to-peak voltage selection control is performed (Example 2), and when the peak-to-peak voltage selection control is performed based on the minimum value (Example) The experiment with 1) was conducted. Further, an experiment was performed in the case where the peak-to-peak voltage selection control was performed based on the average value (Comparative Example 1).

  The experimental conditions are basically the same as the experimental example in Example 1.

  In Example 2, the predetermined value X is 0.348 V (65 μA when converted to an alternating current value).

  In the above experiment, the image unevenness from the first output image to the 7000th output image was compared. In Example 2, the output image was less uneven than in Comparative Example 1. However, comparing Example 1 and Example 2, the image unevenness in Example 2 was sometimes worse than that in Example 1. This is considered to be because the peak-to-peak voltage selection control based on the average value was performed in the second embodiment in order to prevent the photosensitive drum from being scraped.

  As shown in the above results, when the peak-to-peak voltage selection control was performed by selecting the minimum value criterion and the average value criterion (Example 2), it was possible to obtain better charging uniformity compared to Comparative Example 1. .

  Further, in Example 1, Example 2, and Comparative Example 1, the film thickness of the photosensitive drum was measured after 7000 latent images were formed. The amount of film thickness reduction with respect to 1000 image forming sheets is shown below.

  In Example 1, the amount of decrease in film thickness was large, whereas in Example 2, the amount of decrease in film thickness was not so different from that of Comparative Example 1. That is, in the case of Example 2 in which the peak-to-peak voltage selection control is performed based on the minimum value only when necessary, the decrease in the film thickness of the photosensitive drum can be suppressed.

  In the second embodiment, the selection control of the peak-to-peak voltage is switched by detecting the difference ΔV between the minimum current detection voltage Vk-min and the average current detection voltage Vk-ave. is not. If the resistance unevenness in the circumferential direction of the charging roller can be measured, the switching of the peak-to-peak voltage selection control may be performed based on the measurement result. For example, when a white charging roller is used, the amount of toner adhering to the charging low may be detected by an optical sensor, and the peak-to-peak voltage selection control may be switched based on the detection result.

  In the second embodiment, when the peak-to-peak voltage selection control is performed, all the detection voltages Vpp- (k) that can be output from the voltage application unit are applied to the charging roller, and then the alternating currents for all the detection voltages are measured. It was. However, the present invention is not limited to this. For example, there is a method of applying one by one from the detection voltage having a low peak-to-peak voltage. A certain voltage for detection is applied and the alternating current at that time is detected. Then, it is determined whether the AC current is larger or smaller than the peak-to-peak voltage selection control threshold current Iac-0 (first predetermined value). If larger, the peak-to-peak voltage at that time is regarded as the peak-to-peak voltage of the AC voltage during image formation, and the peak-to-peak voltage selection control ends. If it is smaller, a detection voltage having a large one-step peak-to-peak voltage is applied, and this is repeated until a detection voltage that can obtain an AC current larger than the peak-to-peak voltage selection control threshold current Iac-0 (first predetermined value) is found. By doing so, the application time of the detection voltage can be shortened.

  A peak-to-peak voltage selection control sequence according to the third embodiment will be described with reference to the flowchart of FIG. The peak-to-peak voltage selection control circuit will be described with reference to FIG. The same parts as those in the second embodiment are omitted as much as possible.

  When the power of the image forming apparatus is turned on (S501), the control unit 238 serving as a determination unit accesses the memory 210 of the process cartridge C. Then, a predetermined value X for comparing the difference between the peak-to-peak voltage selection control threshold V0, the minimum value Vk-min of the current detection voltage, and the average value Vk-ave is read (S502).

Subsequently, the pre-multi-rotation process begins. In the pre-multi-rotation process, first, one of the detection voltages that can be applied (first AC voltage) is applied to the charging roller 202. In Example 3, as in Example 2, the set number of peak-to-peak voltages for the detection voltage is six.
Vpp-max (ie, Vpp- (k = 1)) = 2000v
Vpp- (k = 2) = 1900v
Vpp- (k = 3) = 1800v
Vpp- (k = 4) = 1700v
Vpp- (k = 5) = 1600v
Vpp-min (ie, Vpp- (k = 6)) = 1500v,
It has become.

  In this embodiment, Vpp− (k = 6) having a minimum peak-to-peak voltage is applied. Here, when Vpp-k is applied to the charging roller 202, the AC current flowing through the photosensitive drum is converted into a current detection voltage Vk (S504), and the current detection voltage Vk is input to the engine controller 237. To do.

  Next, the current detection voltage Vk is subjected to minimum value selection and averaging processing in the engine controller 237 (S505). Then, the difference ΔV between the current detection voltage Vk-min selected as the minimum value and the averaged current detection voltage Vk-ave is calculated, and the difference ΔV is set in advance by the comparison means in the engine controller 237. It is compared with X (S506).

  When the difference ΔV is smaller than the predetermined value X, the average value Vk−ave of the current detection voltage is selected (S507). When the difference ΔV is equal to or greater than the predetermined value X, the minimum value Vk-min of the current detection voltage is selected (S508).

Then, it is determined whether or not Vk−ave ≧ VO (or Vk−min ≧ VO) (S509).
If Vk−ave ≧ VO (or Vk−min ≧ VO), it is determined that Vpp− (k) is applied as the AC voltage Vpp− (n) during the image forming process (S510).

  If Vk−ave ≧ VO (or Vk−min ≧ VO) is not satisfied, the process returns to (S503). When returning to (S503), the value of k is decreased by 1 and k = 5. And (S504)-(S509) are repeated until it becomes (S510).

  The minimum peak-to-peak voltage Vpp- (n) that can be uniformly charged among the AC voltages is determined by the above steps, and the process proceeds to the standby state (S511). When a print signal is sent from the host computer or the like to the image forming apparatus (S511), the AC voltage Vpp- (n) determined in (S507, S508) is applied during image formation (S513). If there is no next print signal, a post-rotation process is performed (S514), and a transition is made to a standby state.

  In this way, it is not necessary to always apply all the detection voltages, so that the detection voltage application time can be shortened.

  In the present embodiment, Vpp− (k = 6) having the smallest peak-to-peak voltage is used as the detection voltage to be applied first, but is not limited to this.

  Further, although the present embodiment has been described based on the second embodiment, the present invention can also be applied to the first embodiment. The point of this embodiment is that the voltage for detection is applied from the smallest value, and the peak-to-peak voltage selection control is terminated when an alternating current necessary for image formation is obtained.

  The peak-to-peak voltage selection control sequence in the first to third embodiments is a control sequence in which the detection voltage Vpp- (k) is applied as it is as the AC voltage Vpp- (n) during the image forming process. That is, comparing the current detection voltage Vk with the peak-to-peak voltage selection control threshold VO, the detection voltage Vpp− (k) that minimizes the peak-to-peak voltage and Vk ≧ VO is the AC voltage Vpp− during the image forming process. It was used as (n). In the control sequence of the present invention, the AC voltage Vpp- (n) during the image forming process is not limited to this, but based on the detection voltage Vpp- (k) that Vk ≧ VO and the peak-to-peak voltage is minimized. Control to determine may be performed. For example, when Vpp− (k) is Vpp− (k) = − 1500 V, −1500 V is not used as an applied voltage in the image forming process as it is, but −50 V is added and −1550 V is added to the image. It is conceivable to use an applied voltage during the formation process.

  In this embodiment, the description has been made using the charging roller, but the present invention is not limited to this. As the shape of the charging means, the present invention can be applied to a belt type or a brush type. Further, the charging unit and the image carrier need not necessarily be in contact with each other, and the charging unit and the image carrier can also be applied to a proximity charging method in which charging is performed by placing the charging unit close to the image carrier. However, since the transfer residual toner is more likely to adhere to the charging unit in the contact charging method, it is considered that the resistance variation of the charging unit is likely to increase. Therefore, the contact charging method is more suitable for applying the present invention.

Flowchart for explaining a peak-to-peak voltage selection control sequence according to the first embodiment. Flowchart for explaining a peak-to-peak voltage selection control sequence according to the second embodiment. Schematic configuration diagram of a conventional image forming apparatus General image forming device operation sequence Conceptual diagram of a conventional charging bias power supply circuit Flowchart for explaining a conventional peak-to-peak voltage selection control sequence Conceptual diagram of current detection voltage Vk Schematic configuration diagram of image forming apparatus of embodiment Schematic configuration diagram of the process cartridge of the embodiment Conceptual diagram explaining the peak-to-peak voltage selection control circuit FIG. 3A is a detailed view of a memory mounted on the process cartridge according to the first embodiment. FIG. 7B is a detailed view of a memory mounted on the process cartridge according to the second embodiment. Flowchart for explaining the peak-to-peak voltage selection control sequence of the third embodiment

Explanation of symbols

201 Photosensitive drum 202 Charging means (charging roller)
DESCRIPTION OF SYMBOLS 203 Cleaning blade 204 Cleaning apparatus 205 Developing apparatus 206 Developer layer thickness control member 207 Developing sleeve 210 Memory 220 Image forming apparatus main body 221 Exposure apparatus 222 Transfer roller 223 Fixing apparatus 230 Charging bias power supply circuit

Claims (11)

An image carrier;
A rotatable charging means for charging the image carrier;
Voltage applying means for applying a plurality of alternating voltages having different peak-to-peak voltages to the charging means;
Current detection means for detecting an alternating current value flowing from the charging means to the image carrier;
In an image forming apparatus having
The AC voltage applied to the charging means is based on an AC voltage that provides an AC current value having a minimum value equal to or greater than a predetermined current value among the minimum values of the AC current values for the plurality of AC voltages. An image forming apparatus comprising: a determination unit that determines a peak-to-peak voltage. An image carrier;
A rotatable charging means for charging the image carrier;
Voltage applying means for applying a plurality of alternating voltages having different peak-to-peak voltages to the charging means;
Current detection means for detecting an alternating current value flowing from the charging means to the image carrier;
In an image forming apparatus having
When the difference between the average value and the minimum value of the alternating current values for the plurality of alternating voltages is smaller than a predetermined value,
The AC voltage having a minimum peak-to-peak voltage among the AC voltages from which an AC current value having an average value equal to or greater than a predetermined current value among the average values of the AC current values for the plurality of AC voltages is obtained. Based on the control, to determine the peak-to-peak voltage of the AC voltage applied to the charging means,
When the difference between the average value and the minimum value of the alternating current values for the plurality of alternating voltages is equal to or greater than the predetermined value,
The AC voltage having a minimum peak-to-peak voltage among the AC voltages from which an AC current value having a minimum value equal to or greater than a predetermined current value among the minimum values of the AC current values for the plurality of AC voltages is obtained. An image forming apparatus comprising: a determining unit that determines a peak-to-peak voltage of the AC voltage applied to the charging unit based on the above. An image carrier;
A rotatable charging means for charging the image carrier;
Voltage applying means for applying a plurality of alternating voltages having different peak-to-peak voltages to the charging means;
Current detection means for detecting an alternating current value flowing from the charging means to the image carrier;
In an image forming apparatus having
When the information on the resistance unevenness on the surface of the charging means is smaller than a predetermined value, an alternating current value having an average value that is equal to or larger than the predetermined current value among the average values of the alternating current values for the plurality of alternating voltages is obtained. Based on the AC voltage having the minimum peak-to-peak voltage in the AC voltage, control is performed to determine the peak-to-peak voltage of the AC voltage applied to the charging means,
When the information regarding the resistance unevenness on the surface of the charging unit is equal to or greater than the predetermined value, an AC current value having a minimum value that is equal to or greater than the predetermined current value among the minimum values of the AC current values for the plurality of AC voltages is obtained. An image forming apparatus comprising: a determination unit that determines a peak-to-peak voltage of the AC voltage applied to the charging unit based on the AC voltage having a minimum peak-to-peak voltage among the AC voltages .   When a plurality of AC voltages are obtained from which an AC current value having a minimum value equal to or greater than the predetermined current value is obtained, the AC voltage is applied to the charging means based on the AC voltage having the minimum peak-to-peak voltage among them. The image forming apparatus according to claim 1, further comprising a determining unit that determines a peak-to-peak voltage of the AC voltage.   5. The image forming apparatus according to claim 1, wherein the voltage applying unit includes a single step-up transformer capable of creating a bias in which an AC component and a DC component are superimposed.   The image forming apparatus according to claim 1, wherein the charging unit is a contact charging unit that performs charging by contacting the image carrier.   The image forming apparatus according to claim 6, wherein the charging unit is a charging roller provided in contact with the image carrier. Developing means for developing the electrostatic latent image into a developer image by a developer;
Transfer means for transferring the developer image to a transfer medium;
Cleaning means for cleaning the developer remaining on the image carrier after transfer;
8. The image forming apparatus according to claim 1, further comprising:   The image forming apparatus according to claim 2, further comprising a storage medium that stores the predetermined value.   The image forming apparatus according to claim 9, wherein the image carrier, the charging unit, and the storage medium are integrated into a process cartridge that can be attached to and detached from a main body of the image forming apparatus. 11. The charging device according to claim 1, wherein the charging unit is a charging roller, and the AC current values with respect to the plurality of AC voltages are detected for a time equal to or longer than a time during which the charging roller rotates once. Image forming apparatus.

Images