JP2008152089A - Image forming apparatus - Google Patents

Abstract

When charging control is performed based on an arithmetic value calculated from an average of measured values of a charging circuit, an image defect that occurs in a charging roller having a large unevenness in the rotation direction is prevented.
An amplitude value of a voltage applied to a charging roller is detected as a first detection value, an amplitude value of a differential waveform of the charging voltage is detected as a second detection value, and an alternating current flowing from the charging roller to the photosensitive member is detected. The current is detected as the third detection value. Based on the first detection value, the second detection value, and the third detection value, the minimum value of the discharge capacity in the rotation direction of the charging roller 21 is calculated, and the minimum value of the discharge capacity becomes a predetermined discharge current value. The charging voltage is controlled as follows.
[Selection] Figure 1

Description

  The present invention relates to an image forming apparatus, and is particularly suitable for application to an image forming apparatus using an electrophotographic system.

  In a printing apparatus as an image forming apparatus that prints an image by an electrophotographic method, the surface of a drum-type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) is uniformly charged to a predetermined potential by a charging unit. This charging unit is generally a non-contact corona charging in which a corona generated by applying a high voltage to a thin corona discharge wire is applied to the surface of the photosensitive drum for charging.

  In recent years, a contact charging method that is advantageous from the viewpoint of a low voltage process, a low ozone generation amount, and a low cost is becoming mainstream. In this contact charging method, a roller charging member (hereinafter referred to as a charging roller) is brought into contact with the surface of the photosensitive drum, and a voltage is applied to the charging roller to charge the photosensitive drum.

  The voltage applied to the charging roller may be only a DC voltage, but by applying an AC voltage, discharge to the plus side and the minus side is alternately generated, thereby charging uniformly.

  Specifically, an oscillating voltage obtained by superimposing a DC voltage and an AC voltage having a peak-to-peak voltage more than twice the discharge start threshold voltage (charging start voltage) to the photosensitive drum when a DC voltage is applied is applied. Thus, the charge of the member to be charged can be made uniform. When a sinusoidal voltage as an oscillating voltage is applied to the charging roller, a resistance load current flows through a resistive load between the charging roller and the photosensitive drum, and a capacitive load flows between the charging roller and the photosensitive drum. A capacitive load current and a discharge current between the charging roller and the photosensitive drum are generated. Then, a current obtained by adding these currents flows to the charging roller as the charging member. At this time, in order to obtain stable charging, it is preferable to set the discharge current amount to a predetermined value or more.

  FIG. 15 shows the characteristics of the current Ic flowing through the charging roller when the AC voltage Vc is applied to the charging roller. The alternating voltage Vc indicates the peak voltage value of the alternating voltage, and the current Ic indicates the effective value of the alternating current.

  As shown in FIG. 15, when the amplitude of the AC voltage Vc is gradually increased, a large amount of charging current flows along with this. When the voltage is less than twice the predetermined voltage Vh, the amplitude of the AC voltage and the charging current are approximately proportional. This is because the resistance load current and the capacitive load current are proportional to the voltage amplitude, and since the voltage amplitude is small, the discharge phenomenon does not occur and the discharge current does not flow. When the AC voltage is further increased, the discharge phenomenon starts at twice the predetermined voltage Vh. At this time, the charging current Ic deviates from the proportional relationship and flows by the amount corresponding to the discharge current Is. Here, in order to stably charge, the AC voltage Vc may be set so that the discharge current Is becomes a predetermined value or more.

  However, when the discharge current Is to the photosensitive drum as the electrophotographic photosensitive member increases, the deterioration of the photosensitive drum itself is promoted. This is considered to be caused by the surface of the photosensitive drum being worn by energy due to discharge. Further, when the wear of the photosensitive drum progresses, the charging performance of the photosensitive drum is lowered, and a problem such as occurrence of an abnormal image occurs.

  Therefore, in order to solve the above-mentioned problems while obtaining stable charging, it is necessary to apply the minimum necessary voltage to the charging roller to minimize the discharge that occurs alternately on the positive side and the negative side. is there.

  However, in practice, the relationship between the voltage applied to the charging roller and the amount of discharge is not always constant, and varies depending on the film thickness of the photosensitive layer and dielectric layer of the photosensitive drum, environmental changes in the charging member and air, and the like.

  In particular, in a low-temperature and low-humidity environment, the material dries and the resistance value rises, making it difficult to discharge. Therefore, in order to obtain uniform charging, a peak-to-peak voltage of a certain value or more is required. In addition, even if it is the lowest voltage value at which uniform charging can be obtained in this low temperature and low humidity environment, if the charging operation is performed in a high temperature and high humidity environment, the material absorbs moisture and the resistance value decreases. The charging member causes more discharge than necessary. As a result, the amount of discharge increases, causing problems such as occurrence of image defects, toner fusion, abrasion of the photosensitive drum due to deterioration of the surface of the photosensitive drum, and shortening of life.

  Such problems due to changes in the discharge amount are caused by the environmental fluctuations described above, as well as fluctuations in resistance values due to manufacturing variations and contamination of charging members, fluctuations in the electrostatic capacity of the photosensitive drum due to durability, and image forming apparatuses. It also occurs due to variations in the characteristics of the main body's high-pressure generator.

  Therefore, in order to suppress the change in the discharge amount, a method of controlling the discharge current value calculated from the characteristic curve of the charging voltage and the charging current has been proposed (see Patent Document 1). In this method, first, the AC voltage applied to the charging member is made variable, and at least a voltage level that is not more than twice the threshold voltage Vh at which the discharge phenomenon starts and a voltage level that is not less than twice the threshold voltage Vh. At two points, each alternating current value is detected by an alternating current value detecting means. And it is the method of calculating the voltage level of the alternating voltage applied to a charging member by calculating the alternating voltage value used as the optimal discharge amount from the detected alternating current value.

  A, B, C, and D points in FIG. 15 indicate points at which AC current values are detected. By sampling two points A and B at a voltage Vh × 2 or less at which the discharge phenomenon starts, a characteristic line LINE-A of the AC applied voltage Vc in the region where no discharge current occurs and the effective value Ic of the charging current Is obtained. Similarly, by sampling two points C and D, a characteristic line LINE-B between the AC applied voltage Vc and the charging current value Ic in the region where the discharge current is generated is obtained.

  From the relationship between the two characteristic lines obtained by this method, an AC voltage level for setting the discharge current value to a predetermined level is calculated. In response to this, control is performed to set the AC voltage Vc to set the discharge current to a desired value. At points A, B, C, and D, after voltage levels are set, average processing of measured values at each point is executed, and a discharge current value is calculated from the average value. In this method, since the number of times of sampling is large, control is usually performed in a pre-image forming process provided for warming up the fixing unit and stabilizing the rotational speed of the scanner.

  In addition, a method has been proposed in which the peak voltage of the charging voltage and the peak voltage of its differential waveform are detected and the discharge current value is calculated and controlled (see Patent Document 2).

16A and 16B show the relationship between the output voltage waveform (A) and its differential waveform (B). The differential voltage waveform in FIG. 16B is delayed in phase by 90 ° with respect to the output voltage waveform shown in FIG. Further, the output voltage waveform of FIG. 16A has a shape (Vp2) in which the level near the peak where the discharge current is generated is lowered. On the other hand, the peak level (Vp1) of the differential waveform is not affected by the discharge. Therefore, the peak level Vp1 of the differential waveform is
This corresponds to the peak level of the output voltage waveform when no discharge current is generated.

  FIG. 6 shows a characteristic curve between the charging AC voltage and the effective value of the charging current. When the discharge current value is added at 2 × Vh or more, the charging current value Ic increases. On the other hand, as described above, the peak level of the differential waveform maintains a proportional relationship with the current value. Therefore, from the graphical relationship of the characteristic curve, the discharge current value Is can be expressed by the following equation (1) using the charging AC current value Ic, the output voltage peak value Va, and the voltage peak value Vb of the differential waveform.

Is = Ic × (Vb−Va) / Vb (1)
Ic, Va, Vb can be sampled simultaneously, and the discharge current value is usually calculated from the average value of each.

Based on the detected discharge current value Is, the discharge current value Is in the image forming process can be controlled in real time.
JP 2001-201921 A JP 2004-157501 A

  When performing the discharge current control, there are the following problems when the discharge current value is calculated from the average of the measured values as in Cited Document 1 and Cited Document 2.

  In other words, when there is a difference in the discharge capability in the rotation direction of the charging roller, if the discharge current value calculated from the average value is controlled, the photosensitive drum is not sufficiently charged in the portion where the discharge capability is low. In some cases, density unevenness may occur on the halftone image at the charging roller pitch.

  It has been found that this problem occurs not only due to manufacturing irregularities (electrical resistance, surface roughness, etc.) in the rotation direction of the charging roller, but also due to deformation at the contact portion of the photosensitive drum after long-term storage. As countermeasures, it is effective to improve the charging roller manufacturing method to increase the uniformity in the rotation direction, or to select materials that are difficult to deform, but there are restrictions on the manufacturing method and the freedom of material selection is limited. As a result, productivity may decrease and costs may increase as a result.

  There is also a problem that when the main body is started from a rest state, waste toner in the cleaner slips through and adheres to the surface of the charging roller. In this case, the charging performance of the toner adhering portion is extremely lowered, so that black streaks are generated at the charging roller pitch even in the text image.

  Accordingly, an object of the present invention is to provide an image forming apparatus capable of maintaining a uniform image even when the rotation direction of the charging member is not uniform. Another object of the present invention is to provide an image forming apparatus capable of stabilizing charging control with an inexpensive configuration to prevent image defects and extending the life of a photoreceptor.

In order to achieve the above object, the first invention of the present invention provides:
Voltage applying means for applying a vibration voltage to a rotatable charging member in contact with the electrophotographic photosensitive member;
Voltage amplitude value detecting means for detecting an amplitude value of the oscillating voltage applied to the charging member;
Differential amplitude value detecting means for detecting an amplitude value of the differential waveform of the oscillating voltage;
AC current value detecting means for detecting an AC current flowing from the charging member to the electrophotographic photosensitive member when the oscillating voltage is applied, and
Prior to image formation, an oscillating voltage is applied to the charging member, and based on the amplitude value of the oscillating voltage and the amplitude value of the differential waveform, the average value and the minimum of the discharge capability in the rotation direction of the charging member Calculate the value,
When the difference between the minimum value of the discharge capacity and the average value of the discharge capacity is less than a threshold value, the discharge current value calculated from the amplitude value of the oscillating voltage, the amplitude value of the differential waveform, and the alternating current Is controlled so that the oscillating voltage applied to the charging member at the time of image formation is variable, so that is a predetermined value,
When the difference between the minimum value of the discharge capacity and the average value of the discharge capacity is greater than or equal to a threshold value, the discharge current value calculated from the amplitude value of the oscillating voltage, the amplitude value of the differential waveform, and the alternating current And a control means for performing constant current control or constant voltage control on the vibration voltage applied to the charging member at the time of image formation at a current value equal to or greater than a predetermined value.

  As described above, according to the present invention, even when the rotation direction of the rotatable charging member is non-uniform, it is possible to maintain a uniform image and stabilize charging control with an inexpensive configuration. Thus, image defects can be prevented and the life of the photoreceptor can be extended.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings of the following embodiments, the same or corresponding parts are denoted by the same reference numerals. The following embodiments are not intended to limit the invention, and all the combinations of features described in the following embodiments are not necessarily essential to the solution means of the invention.

  FIG. 1 shows an image forming apparatus according to a first embodiment of the present invention. The image forming apparatus according to the first embodiment will be described by taking an example of a laser printer of an electrophotographic system and a process cartridge detachable type, but is not necessarily limited thereto.

  The photosensitive drum 1 is a rotating drum type electrophotographic photosensitive member (photosensitive drum) as an image carrier. The photosensitive drum 1 is composed of a negatively charged organic photosensitive member. The photosensitive drum 1 is configured to be rotationally driven at a predetermined peripheral speed in a clockwise direction indicated by an arrow by a driving motor (not shown). The photosensitive drum 1 is uniformly charged to a predetermined negative potential by the charging unit during the rotation process.

  In the first embodiment, the charging unit is a contact charging unit using a charging roller 21 as a charging member. The charging roller 21 is driven to rotate with respect to the photosensitive drum 1. A charging bias voltage is applied to the charging roller 21 from a charging bias power source 2 as a voltage applying unit. As the charging bias voltage, there is a method in which a DC voltage Vdc corresponding to a desired potential Vd on the photosensitive drum 1 is superimposed and applied to an AC voltage having a peak-to-peak voltage (Vpp) that is at least twice the discharge start voltage. Adopted. According to this charging method, an AC voltage is superimposed on a DC voltage to eliminate local potential unevenness on the photosensitive drum 1, and the photosensitive drum 1 is uniformly applied with a potential Vd substantially equal to the DC applied voltage Vdc. Can be charged.

  Next, the image exposure by the exposure apparatus 30 is received. The exposure device 30 forms an electrostatic latent image corresponding to the image signal on the uniformly charged photoreceptor drum 1. In the first embodiment, a semiconductor laser scanner is used. The exposure device 30 outputs a laser beam b modulated in accordance with an image signal output from a host device (not shown) in the image forming apparatus, and an exposure window portion a of a reflection mirror 31 and a process cartridge C described later. Then, the uniformly charged surface of the photosensitive drum 1 is subjected to scanning exposure (image exposure). As a result, on the surface of the photosensitive drum 1, the absolute value of the potential at the exposed portion is lower than the absolute value of the charged potential, and electrostatic latent images corresponding to image information are sequentially formed.

  Next, the electrostatic latent image is developed by the reversal developing device 40 and visualized as a toner image. In the first embodiment, a jumping development method is adopted. In this method, a developing bias voltage in which alternating current and direct current are superimposed is applied to the developing sleeve 41 from a developing bias power source (not shown). As a result, the toner charged negatively by frictional charging is applied to the electrostatic latent image on the surface of the photosensitive drum 1 at the contact portion between the developer layer thickness regulating member 42 and the developing sleeve 41, and the electrostatic latent image. Is reversely developed.

  The toner image on the surface of the photosensitive drum 1 is transferred by a transfer device to a recording medium (transfer material) such as paper fed from a paper feeding unit. In the first embodiment, a contact transfer device using a transfer roller 5 is used. The transfer roller 5 is pressed against the photosensitive drum 1 by a biasing means such as a pressing spring (not shown) toward the center of the photosensitive drum 1. When the transfer material is conveyed and the transfer process is started, a positive transfer bias voltage is applied to the transfer roller 5 from the transfer bias power source.

  As a result, the toner on the photosensitive drum 1 charged negatively is transferred onto the transfer material. The transfer material onto which the toner image has been transferred is separated from the surface of the photosensitive drum 1 and is introduced into the fixing device 70 where the toner image is fixed and discharged onto the paper discharge tray 85. The fixing device 70 fixes the toner image transferred onto the transfer material as a permanent image using means such as heat or pressure.

  The surface of the photosensitive drum 1 after the transfer material is separated from the photosensitive drum 1 is repeatedly used for image formation after the transfer residual toner is scraped off and cleaned by the cleaning device 60. A cleaning blade 61 is used in the cleaning device 60 according to the first embodiment. The cleaning blade 61 collects transfer residual toner that could not be transferred from the photosensitive drum 1 to the transfer material during the transfer process, and contacts the photosensitive drum 1 with a constant pressure to collect the transfer residual toner. To clean the surface of the photosensitive drum 1.

  After completion of the cleaning process, the surface of the photosensitive drum 1 is charged again in the charging process. This image forming apparatus forms an image by repeating the steps of charging, exposing, developing, transferring, fixing, and cleaning using the above-described means. Reference numeral 86 denotes a main body door for attaching / detaching the process cartridge C to / from the apparatus.

  Next, the process cartridge according to the first embodiment will be described with reference to FIG. Here, the process cartridge according to the first embodiment is a cartridge in which at least one of a charging unit, a developing unit, and a cleaning unit, or at least a developing unit and an electrophotographic photosensitive member are integrally formed. This cartridge is detachable from the main body of an electrophotographic image forming apparatus.

  FIG. 2 shows a process cartridge C according to the first embodiment. As shown in FIG. 2, the cartridge C is a process cartridge that can be attached to and detached from the image forming apparatus main body L. The process cartridge C includes four process devices including a photosensitive drum 1 as a latent image carrier, a charging roller 21 as a contact charging member for the photosensitive drum 1, a reversal developing device 40, and a cleaning device 60. Cartridge. Further, the process cartridge C includes a memory 90 as a storage unit. Information is read from and written into the memory 90 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 L by opening and closing a main body door 86 (cartridge door) (see FIG. 1) of the image forming apparatus main body L. The cartridge is mounted by opening the main body door 86 and inserting the process cartridge C into the image forming apparatus main body L in a predetermined manner and then closing the main body door 86.

  Further, the process cartridge C has a positioning portion 97 and a metal contact 96 and is attached to the image forming apparatus main body L. The memory 90 is correctly adapted to the read / write means of the main body L, and electrical signals can be communicated. That is, the memory 90 is electrically connected to the image forming apparatus main body L by the positioning portion 97 and the metal contact 96.

  Further, according to the first embodiment, threshold information to be compared with the operation value of the main body L is written and stored in the memory 90. This threshold information is stored in advance in the memory 90 at the time of factory shipment or the like.

  Next, a basic image forming operation in the image forming apparatus according to the first embodiment will be described. FIG. 4 shows a flowchart of the image forming operation.

  As shown in FIG. 4, first, when the power supply in the image forming apparatus is turned on in step S101, the process proceeds to step S102 to start the initial process. In this initial process, while the main motor is rotating the photosensitive drum 1, the presence or absence of the process cartridge C is detected, and the transfer roller 5 is cleaned.

  After the initial process is completed, the process proceeds to step S103, and a process for determining whether or not there is a print input is executed. When image information (print signal) is not output from a host computer (not shown) or the like, the image forming apparatus enters a standby (standby) state in step S104. That is, the rotation of the photosensitive drum 1 is stopped and the input of image information is waited.

  When image information is received during the initial process, the process proceeds to step S105, where the main motor continues to rotate, and the pre-image formation process starts from the initial process. In this pre-image formation process, preparatory operations for various process devices are performed. Here, preliminary charging of the photosensitive drum 1, startup of the laser scanner, determination of the transfer bias, temperature adjustment of the fixing device 70, and the like are mainly performed.

  When the image formation pre-process in step S105 is completed, the process proceeds to step S106 and the image formation process is started. In this image forming process, the transfer material is fed, the image exposure to the photosensitive drum 1 and the development are performed at a predetermined timing. When the image forming process ends, the process proceeds to step S107 to determine whether or not a print signal is input.

  If a print signal is input, the next image information (print signal) is input, and the process proceeds to step S108. In step S108, an inter-sheet passing process is started until the next transfer material arrives, and the next image forming operation starts.

  On the other hand, if there is no next image information after the end of the image forming operation, the process proceeds to step S109 to enter a post-image forming process. In the post-image forming process, a process of discharging the surface of the photosensitive drum 1 and discharging the toner adhering to the transfer roller 5 to the photosensitive drum 1 (cleaning of the transfer roller) are performed. When the post-image formation process is completed, if no print signal is input, the image forming apparatus again enters a standby (standby) state, stops the rotation of the photosensitive drum 1, and waits for the next image information. Image formation is performed as described above.

  FIG. 3 shows a functional configuration for controlling the discharge current of the image forming apparatus according to the first embodiment. As shown in FIG. 3, the control unit 300 controls data writing to the memory 90 and data reading from the memory 90.

The control unit 300 includes a calculation unit 301, a storage unit 302, and an AC voltage drive signal generation unit 303.
Have The discharge current circuit 200 includes an alternating current detection unit 201, a sine wave voltage application unit 202, a control unit 203, a peak voltage detection circuit 204, and a differential waveform peak voltage detection circuit 205. The operation of each part will be described later.

  FIG. 5 shows discharge current control by the discharge current circuit 200 in the image forming apparatus according to the first embodiment. The discharge current circuit 200 includes an amplifying circuit 206, a filter circuit 207, a sine wave voltage application unit 202 including a transformer 209, an alternating current detection unit 201, and the like.

  When the CPU of the control unit 300 outputs the clock pulse 400, the clock pulse 400 is input to the amplifier circuit 206 including the transistor 213 and the pull-up resistor 214 via the pull-up resistor 211 and the base resistor 212. Here, it is amplified to an amplitude corresponding to the output of the operational amplifier 216. The amplified clock pulse is input to a filter circuit 207 including resistors 218, 219,..., 228, capacitors 229, 230,. . This sine wave output is input to the transformer 209 via a drive circuit 208 including resistors 237 to 241, a capacitor 242, a diode 243, and transistors 244 to 246. As a result, a sine wave AC high voltage is generated from the transformer 209. A DC high voltage circuit 210 is connected to the transformer 209 via a resistor 248, and an oscillating voltage obtained by superimposing a sine wave output from the transformer 209 on this DC voltage is supplied to the charging roller 21.

  The alternating current detection unit 201 serving as an alternating current value detection unit includes diodes 250 and 251 and an integration circuit including a resistor 253 and a capacitor 252 via a capacitor 249. A voltage is applied to the operational amplifier 216 via the resistor 254. Enter. An output control signal 401 is input to the operational amplifier 216 from the CPU of the control unit 300 via the resistor 255. The voltage amplitude output from the amplifier circuit 206 is adjusted according to the voltage level of the output control signal 401, and the alternating current value is controlled so as to be a constant output voltage level.

  The discharge current circuit 200 includes a peak voltage detection circuit 204 that is a voltage amplitude value detection unit and a differential waveform peak voltage detection circuit 205 that is a differential amplitude value detection unit.

  The charging AC voltage is shunted by diodes 257 and 258 via a capacitor 256 and input to an integrating circuit including an operational amplifier 259, resistors 260 and 261, and a capacitor 262 in the peak voltage detection circuit 204. The output of the integration circuit is input to the control unit 300 as a DC voltage signal 402 corresponding to the peak voltage value. Thereby, the control part 300 can detect the peak value of the alternating voltage output.

  In the differential waveform peak voltage detection circuit 205, the impedance of the capacitor 256 is set sufficiently larger than the resistance value of the resistor 265. Thereby, it is possible to generate a half waveform of the differential waveform at the input portion of the operational amplifier 266. The differentiated waveform is converted into a DC voltage signal 403 by a peak voltage detection circuit 204 including operational amplifiers 266 and 267, diodes 268 and 269, resistors 270 to 273, and a capacitor 274, and is input to the control unit 300. The

  The control unit 300 includes a CPU, a ROM 310, a RAM 311, an input / output port 312, a D / A converter 313, and an A / D converter 314. The ROM 310 stores a program executed by the CPU. The RAM 311 is used as a work area when controlled by the CPU. The D / A converter 313 is for voltage output, and the A / D converter 314 is for digitizing and inputting a voltage value.

  Next, a discharge current detection method according to the first embodiment will be described. In the image forming apparatus according to the first embodiment, the peak value of the charging AC voltage and the peak value of the differential waveform of the charging AC voltage (hereinafter, the differential waveform peak voltage value) are detected to calculate the discharge current value. FIG. 6 shows the characteristics of the peak value of the AC voltage applied to the charging roller 21 and the peak value of the differential waveform and the charging AC current value.

  As shown in FIG. 6, in a region where the charging AC voltage is not more than twice the discharge start voltage (Vh), the relational expression of the charging current value with respect to the charging AC voltage is represented by a substantially proportional straight line passing through the origin. In this region, a current corresponding to the resistive load and the capacitive load between the charging roller 21 and the photosensitive drum 1 flows. On the other hand, in a region where the charging AC voltage is twice or more the discharge start voltage (Vh), a discharge current is generated between the charging roller 21 and the photosensitive drum 1, and charging with the added discharge current value is performed. A current value Ic flows.

  In the discharge start region, the peak value of the AC voltage is represented by 500, and the peak value of the differential waveform is represented by 501. Here, the discharge current value can be calculated from the relationship between the characteristic 500 and the characteristic 501.

In the case of the peak value Va of the AC voltage, the peak value Vb of the differential waveform, and the charging current Ic, the discharge current value Is is obtained by the following equation (1).
Is = Ic × (Vb−Va) / Vb (1)

  In the image forming apparatus according to the first embodiment, the peak voltage value Va of the alternating voltage is detected by the peak voltage detection circuit 204, and the peak value Vb of the differential waveform is detected by the differential waveform peak voltage detection circuit 205. Further, the voltage amplitude output from the amplifier circuit 206 is adjusted according to the voltage level of the output control signal 401, and the charging current value Ic is adjusted based on the result to control the discharge current value.

FIG. 7 shows the relationship between the discharge capability of the charging roller and the output waveform of charging. The output waveform of the charging voltage has a shape in which the level near the peak is lowered by ΔV (= (Va′−Va)) due to the influence of the discharge. Therefore, the discharge capability of the charging roller can be measured at a ratio of the level at which the peak is reduced by the discharge (ΔV / Va ′) with respect to the peak level (Va ′) of the output voltage when not discharging. Moreover, since the phase of the waveform of the differential voltage is delayed by 90 °, the peak value is not affected by the discharge.

Therefore, the peak value (Vb) of the differential voltage corresponds to the peak level (Va ′) of the output voltage when not discharging. Therefore, the discharge performance can be expressed by the following formula (2).
σ = (Vb−Va) / Vb (2)

  Next, a control sequence in the image forming apparatus according to the first embodiment will be described with reference to the flowcharts of FIGS. 8 and 9 and the functional block diagram of FIG. FIGS. 8 and 9 are flowcharts illustrating control processing in the image forming apparatus according to the first embodiment. A program for executing this process is stored in the ROM 310 of the control unit 300 and is executed under the control of the CPU.

  First, in step S201, the image forming apparatus (laser beam printer) main body L is turned on. In step S202, the discharge capacity threshold value Δσo, the discharge current target value Iso, and the like are read from the memory 90 of the process cartridge C. The target value Iso is a reference value for performing good charging. Good charging can be performed by applying a voltage to the charging roller 21 so that a discharge current equal to or greater than the target value Iso is generated.

  Next, the process proceeds to step S203, where the variable t is initialized in order to measure the sampling time, and then the process proceeds to step S204, where Ico, which is the initial value of the charging current, is output. Subsequently, the process proceeds to step S205, where the detection value Va (402) from the peak voltage detection circuit 204 and the detection value Vb (403) from the differential waveform peak voltage detection circuit 205 are A / D converted and read.

  Next, the process proceeds to step S206, where σ, which is a discharge capacity value, is calculated from the peak detection value Va and the peak detection value Vb of the differential waveform. This calculated value is obtained by σ = (Vb−Va) / Vb according to the above-described equation (2). Next, the process proceeds to step S207, where an integrated value is calculated by adding the sampling time Δt to the time t at the start of detection.

  Subsequently, the process proceeds to step S208, where the integrated value of the detection time and the standard time to are compared. As a result of the comparison, if the detection time is long, the process proceeds to step S209. On the other hand, if the detection time is short, the process returns to step S204, and the processes of steps S204 to S208 described above are executed.

  Next, when the process proceeds to step S209, the minimum value σmin and the average value σave are obtained from the discharge capacity values calculated within the detection time. Next, the process proceeds to step S210, and it is determined whether (σave−σmin) ≧ Δσo. It can be considered that the larger the value of (σave−σmin), the greater the difference in discharge capability in the rotation direction of the charging roller. Conversely, the smaller the value of (σave−σmin), the smaller the difference in discharge capacity. In addition, about the method of calculating | requiring an average value and minimum value from several measurement data in step S209, since it is a conventionally well-known general arithmetic algorithm, description is abbreviate | omitted.

  If (σave−σmin) ≧ Δσo is satisfied in step S210, it is determined that there is a variation in discharge capacity, and constant current value control is performed, so that the process proceeds to step S211. When the process proceeds to step S211, the image forming process is started. In step S212, the constant current value control is performed at the current setting value Ic1 at which the portion having the lowest discharge capability in the rotation direction of the charging roller becomes the target discharge current value Iso. Next, the process proceeds to step S213, and the process ends after execution of the post-image formation process.

  On the other hand, if (σave−σmin) <Δσo in step S210, the process proceeds to step S214. The image forming process starts from step S214. Next, in step S215, the discharge current value control routine is called, and control is performed at the target discharge current value Iso. This discharge current value control will be described later with reference to the flowchart of FIG. Thereafter, the process proceeds to step S213, and after the post-image forming process is executed, the process ends.

  Next, the discharge current value control executed in step S215 described above will be described with reference to the flowchart of FIG.

  That is, first, in step S301, the initial current value Ico is output. Next, the process proceeds to step S302, and the charging current value Ic2 is output by the output control signal 401. Next, the process proceeds to step S303, and it is determined whether or not the image forming process is finished. If it is determined in step S303 that the image forming process has been completed, the process returns to the main routine (step S215, see FIG. 8).

On the other hand, if it is determined in step S303 that the image forming process is not completed, the process proceeds to step S304. In step S304, the detection value Va (403) of the peak voltage detection circuit 204 and the detection value Vb (402) of the differential waveform peak voltage detection circuit 205 are A / D converted and read.

  Next, the process proceeds to step S305, where the calculated value Is is calculated from the detected value Va and the detected value Vb based on the equation (2) Is = Ic2 × (Vb−Va) / Vb.

Next, the process proceeds to step S306.
(Calculated value Is−target value Iso) ≦ 0
It is determined whether or not. That is, it is determined whether or not the calculated value Is is equal to or greater than the target value Iso of the discharge current. If the calculated value Is is equal to or less than the target value Iso of the discharge current, the process proceeds to step S307. After the charging current value Ic2 is set to (Ic−ΔIc), the process returns to step S302. On the other hand, when the calculated value Is is equal to or greater than the target value Iso of the discharge current in step S306, the process proceeds to step S308, the charging current value Ic2 is set to (Ic + ΔIc), and the process returns to step S302.

  FIG. 10A and FIG. 10B show the relationship between variations in the discharge capability in the charging roller rotation direction and image defects. The horizontal axis is the elapsed time from the start of rotation of the charging roller, and the vertical axis is the measured peak voltage value (Va) and differential peak voltage value when a constant alternating current value flows from the charging roller to the photosensitive drum. The discharge capacity σ is calculated from (Vb). The vertical broken line is a line that runs through each charging roller. Since the charging roller is rotating, the discharge capacity σ varies depending on the roller cycle.

In the first embodiment, the average value of the discharge capacity is σave, and the minimum value is σmin. When the control is performed at a constant current value of Icz (= Iso / σave) based on the average value of the discharge capacity, as shown in FIG. 10B, the area of the photosensitive drum where the minimum discharge capacity value σmin is small. Charging becomes insufficient. For this reason, the surface potential on the photosensitive drum 1 is lowered, resulting in density unevenness in the halftone image. Therefore, it is necessary to control based on the minimum value σmin of the discharge performance of the charging roller so that a sufficient discharge current value is set even in the case of the minimum value.

  Moreover, when calculating and controlling a calculation value based on the detection value from each measurement means like discharge current value control, it is preferable that the time required for feedback from detection to control is short. However, a CPU with high arithmetic processing capability that requires a short time for feedback may increase costs. On the other hand, when an inexpensive CPU is used, the time required for feedback becomes long, and the control may be delayed with respect to the detection portion.

  FIG. 11 shows the transition of the control current value when there is a large variation in the discharge capability in the charging roller rotation direction. σ is a discharge capacity value in the circumferential direction of the charging roller, and a vertical line is a portion having a low discharge capacity. The control current value (Icy) is a current value flowing through the charging roller, and the magnitude of the current value is controlled by the main body circuit. When a CPU with high calculation processing capability that requires a short feedback time (dotted line) is used, the control current value (Icy) follows the position (x) in the rotation direction of the charging roller, and the discharge current value (Isy) is substantially constant.

However, when using a CPU with low arithmetic processing capability that results in a long feedback time (solid line), the control current value (Icx) is delayed with respect to the position (x) in the rotation direction of the charging roller. The discharge current value (Isx) may not be constant. Consider from the above. The current that flows from the charging roller to the photosensitive drum is better when the discharge current control is performed to control the voltage value (or current value) applied to the charging roller based on the discharging current value so that the discharging current value becomes a desired value. Charging can be performed better than the constant current control that controls the value to a desired value. This is because the amount of discharge current has an effect when the photosensitive drum is charged.

  However, when the discharge capability σ differs greatly in the rotation direction of the charging roller (when the difference between the average value and the minimum value of the discharge capability is large), the control current value follows the change in the discharge capability σ when the discharge current control is performed. And cannot change. In some cases, insufficient charging occurs in a region of the photosensitive drum having a small discharge capability σ in the rotation direction of the charging roller, and a horizontal stripe is generated in a halftone image.

  Therefore, when the discharge capability σ differs greatly in the rotation direction of the charging roller, constant current control is performed instead of discharge current control. Since the constant current control requires a shorter calculation processing time than the discharge current control, the control current value can change following the change in the discharge capability σ. Therefore, even if the discharge capability σ varies greatly in the rotation direction of the charging roller, it is difficult to cause a problem that insufficient charging occurs only in a portion where the discharge capability σ is small. However, the target current value set when performing constant current control needs to be set to a target current value that provides a discharge current amount that can be satisfactorily charged. Otherwise, good charging cannot be performed. However, as a problem of such constant current control, the target current value is set so that a good amount of discharge current can be obtained, so there is a possibility that an unnecessarily large current will flow. It is. This is because the life of the photosensitive drum is shortened if a current larger than necessary is passed. Therefore, the discharge current control is performed unless the discharge capability σ differs greatly in the rotation direction of the charging roller.

  That is, when the difference between the minimum value of the discharge capacity and the average value of the discharge capacity is less than the threshold value, the discharge current control is performed so that the oscillating voltage applied to the charging member during image formation is variable as follows. . Specifically, the discharge current value calculated from the amplitude value of the oscillating voltage, the amplitude value of the differential waveform of the oscillating voltage, and the alternating current is set to a predetermined value.

  When the difference between the minimum value of the discharge capacity and the average value of the discharge capacity is equal to or greater than the threshold value, the discharge current value calculated from the amplitude value of the vibration voltage, the amplitude value of the differential waveform of the vibration voltage, and the alternating current Constant current control is performed so that becomes equal to or greater than a predetermined value. In this case, constant voltage control may be performed.

  FIG. 12A, FIG. 12B, and FIG. 12C show the relationship between the discharge capability and resistance of the charging roller. As shown in FIG. 12, when the resistance R of the charging roller is high, the inclination of the VI characteristic decreases, and when the resistance R is low, the inclination increases. However, depending on the state of the charging roller, the discharge capability σ may be low even when the resistance R is high, the discharge capability σ indicating the ease of discharge is high, or even when the resistance R is low.

  Therefore, it is more preferable to detect the discharge capacity rather than the resistance of the charging roller as the detection value. Note that the drum pressure contact portion in FIG. 12 is the discharge capacity value of the contact portion where the photosensitive drum and the charging roller are left for a long time. Further, the slight dirt portion in FIG. 12 is a portion where a small amount of the external additive of the developer adheres to the charging roller in the initial stage of use of the cartridge, and in this state, it is known that the discharge easily occurs.

Moreover, in order to clarify the effect by this 1st Embodiment, the following evaluation was performed.
<Experiment 1>
Image evaluation is performed for the first embodiment and the comparative example.
First Embodiment: A difference (σave−σmin) in discharge capacity value is compared with a discharge capacity threshold (Δσo). When the difference between the discharge capacity values is smaller than the discharge capacity threshold, the discharge current value control is performed so that the target discharge current value Ico is constant. On the other hand, when the difference between the discharge capacity values is larger than the discharge capacity threshold value, constant constant current control is performed so that the minimum value σmin portion becomes the target discharge current value Iso.
Comparative example: discharge current value control is performed so that a constant target discharge current value Ico is obtained.
<Evaluation conditions>
Environment: temperature 25 ° C, humidity 50% Rh
Paper-passing mode A4 20,000 sheets Process speed (moving speed of the photosensitive member surface): 150 mm / second Charging AC frequency: 1600 Hz
Charging roller diameter 12mm
Charging roller length: 320mm
Target discharge current value Iso 60μA
Charging roller rotation direction threshold Δσo 1.86 × 10 ⁻³
Initial current value Ico 1600μA
ΔIc 2μA
Sampling time to 500msec

As a result of the above experiment, in the comparative example, the roller was soiled when 9000 sheets were passed, the portion corresponding to σmin became poorly charged, and density unevenness occurred in the halftone image. On the other hand, in the first embodiment, it was confirmed that there is no problem even with halftone images up to a cartridge life of 20000 sheets.

  As described above, in the first embodiment, the charge value is controlled so that the minimum value of the discharge capability in the charging roller rotation direction becomes a predetermined discharge current value. Further, the difference between the average value and the minimum value of the discharge capacity is compared with the threshold value of the cartridge memory, and when it is small, the discharge current value control is performed, and when it is large, the control is performed at the constant current value. According to such a configuration, it is possible to prevent image defects and extend the life of the photosensitive drum even in a small and inexpensive device configuration.

(Second Embodiment)
Next explained is an image forming apparatus according to the second embodiment of the invention. In the second embodiment, an example applied to an image forming apparatus that cleans the charging roller by detecting the discharging ability in the rotation direction of the charging roller will be described. In the image forming apparatus according to the second embodiment, the control means is switched according to the threshold value of the discharge capacity stored in the memory 90 provided in the process cartridge C. The hardware configuration of the image forming apparatus according to the second embodiment is the same as that in the first embodiment, and a description thereof will be omitted.

  Further, in the second embodiment, the problem of image defect of the charging roller pitch that occurs when the main body is activated is prevented. First, this problem will be described below.

  That is, in the latter half of the life of the main body having a high throughput, the toner accumulated on the blade edge portion in the cleaner tends to aggregate due to the moving speed of the drum surface. Further, when the apparatus is left unattended, the toner at the edge portion may gradually absorb moisture and adhere to the drum surface. In this case, a phenomenon occurs in which the toner at the edge portion passes through the cleaner and adheres to the charging roller when the apparatus is started after being stopped. In this case, since a lump of toner adheres to a part of the rotation direction of the charging roller, the discharge capability is extremely reduced, and black streaks due to defective charging occur in the text image.

  When this phenomenon occurs, it is preferable that the charging roller is cleaned before the image is formed, and it is confirmed that charging failure due to the adhered toner does not occur and then the image forming process is started.

FIG. 13 is a process flowchart of the image forming apparatus according to the second embodiment of the present invention. Note that the processing in steps S403 to S408 shown in FIG. 13 is the same as the processing in steps S202 to S208 in the first embodiment described above, and therefore the description thereof is omitted. Further, the discharge current value control is also the same as the flowchart shown in FIG.

  First, when the image forming apparatus (laser beam printer) main body L is turned on in step S401, the process proceeds to step S402. In step S402, the discharge capacity threshold values Δσo and Δσc and the discharge current target value Iso are read from the memory 90 of the process cartridge C.

  Next, the processing from step S403 to S408 is the same as the processing from step S202 to S208 in the first embodiment described above, the detection of the peak voltage value and the differential waveform peak voltage value, and the discharge capability value. Perform the calculation.

  Next, in step S409, σmin which is the minimum value and σave which is the average value are calculated among the discharge capacity values calculated within the detection time. Next, the process proceeds to step S410, and it is determined whether (σave−σmin) ≦ Δσc. If (σave−σmin) ≦ Δσc, the process proceeds to step S412 to start the image forming process. If (σave−σmin)> Δσc, the process proceeds to step S411, and after the charging current value Icc is output, the process proceeds to step S405.

  On the other hand, in step S413, constant current value control is performed at the current set value Ic1 where the portion with the lowest discharge capability in the rotation direction of the charging roller becomes the target discharge current value Iso.

  Next, after the image forming post-process is executed in step S415, the process is finished. On the other hand, in step S416, a discharge current value control routine is called to perform control at the target discharge current value Iso. Since the discharge current value control is the same process as the flowchart shown in FIG. 9 described above, the description thereof is omitted. Next, after the post-image formation process is performed in step S415, the process ends.

  FIG. 14 shows the relationship between the type of unevenness in the rotation direction of the charging roller and the discharge capacity. As shown in FIG. 14, dirt due to slipping of the cleaner has a great difference with respect to the discharge performance with respect to the manufacturing unevenness of the charging roller and the unevenness of the pressure contact portion with the photosensitive drum. The reason why the discharge capability is low is that the toner deposited on a part of the charging roller due to slipping through is attached and is extremely difficult to discharge. Therefore, it is possible to determine charging failure due to toner adhering to the charging roller, and it is possible to determine whether to perform charging roller cleaning based on the threshold value Δσc of the discharge capability.

  Further, in the second embodiment, the minimum value and the average value of the charging ability are calculated, but the same effect can be obtained even if the minimum value and the average value of the discharge current value in the charging roller rotation direction are adopted. Obtainable. In this case, a threshold value of the discharge current value corresponding to the difference between the toner adhesion area and the non-adhesion area in the rotation direction is stored in the memory tag. Furthermore, even during cleaning, the continuation of cleaning is determined based on the detected toner adhesion state based on the difference in discharge capability in the rotation direction, so that the cleaning time can be minimized and throughput can be reduced. It becomes possible to prevent charging failure.

  As described above, according to the second embodiment, the difference between the average value and the minimum value of the discharge capability in the rotation direction of the charging roller is compared with the threshold information of the cartridge memory, and the cleaning is performed until the difference becomes small. It is carried out. Thereby, even in a small and inexpensive apparatus configuration, it is possible to prevent image defects and extend the life of the photosensitive drum.

  In the second embodiment, the charging current value Icc is set to the maximum value in order to promote the cleaning of the toner adhering to the charging roller, but the setting value is not particularly defined.

  Further, since the charging failure due to toner adhesion depends on the type of the charging roller and the toner, it is more preferable to store them in the memory 20 of the cartridge C.

  As mentioned above, although embodiment of this invention was described concretely, this invention is not limited to embodiment mentioned above, The various deformation | transformation based on the technical idea of this invention is possible. For example, the numerical values given in the above-described embodiments are merely examples, and different numerical values may be used as necessary.

  For example, in the above-described first embodiment, the constant current value control is performed when the difference between the average value and the minimum value is larger than the threshold value, but the same effect can be obtained even with the constant voltage control.

  Further, for example, in the above-described first embodiment, the minimum value and the average value of the charging ability are calculated, but the minimum value and the average value of the discharge current value in the charging roller rotation direction are calculated. Similar effects can be obtained. In this case, a threshold value to be compared with the variation in the discharge current value in the rotation direction is stored in the memory tag. Further, although the difference between the average value and the minimum value of the charging ability is compared with the threshold value, the same effect can be obtained even if the difference between the maximum value and the minimum value is used.

  Further, for example, in the first embodiment described above, the discharge capacity in the rotation direction of the charging roller is calculated from the initial process to the image forming process. However, the present invention can be applied to any process in the initial process or the pre-image formation process, or in the inter-sheet passing process at the time of continuous output.

1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment of the present invention. It is a figure explaining the process cartridge by 1st Embodiment of this invention. FIG. 3 is a block diagram illustrating a functional configuration for realizing discharge current control by the image forming apparatus according to the first embodiment of the present invention. 3 is a flowchart illustrating a series of image forming operations by the image forming apparatus according to the first embodiment of the present invention. It is a figure explaining the discharge current control by the discharge current circuit of the image forming apparatus by the 1st Embodiment of this invention. It is a figure which shows the peak value of the alternating voltage applied to a charging roller, the peak value of a differential waveform, and the characteristic of a charging alternating current value. It is a figure which shows the relationship between an output voltage waveform (A) and its differential waveform (B). 4 is a flowchart illustrating control processing by the image forming apparatus according to the first embodiment of the present invention. 4 is a flowchart illustrating control processing by the image forming apparatus according to the first embodiment of the present invention. It is a figure explaining the relationship between the dispersion | variation in the rotation direction of a charging roller, and an image defect. It is a figure explaining the relationship between the dispersion | variation in the rotation direction of a charging roller, and a control electric current value. It is a figure explaining the dispersion | variation in the resistance value in the rotation direction of a charging roller, and the dispersion | variation in a discharge capability value. It is a flowchart explaining the process in the image forming apparatus by 2nd Embodiment of this invention. It is a figure explaining the dispersion | variation in the discharge capability value in the rotation direction of a charging roller. It is a figure showing the characteristic of the electric current Ic which flows into a charging roller, when the charging alternating voltage Vc is applied to a charging roller. It is a figure which shows the relationship between an output voltage waveform (A) and its differential waveform (B).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging bias power supply 5 Transfer roller 20 Memory 21 Charging roller 25 Temperature 30 Exposure device 31 Reflecting mirror 40 Reverse developing device 41 Developing sleeve 42 Developer layer thickness regulating member 60 Cleaning device 61 Cleaning blade 70 Fixing device 85 Paper discharge Tray 86 Main body door 90 Memory 96 Metal contact 200 Discharge current circuit 201 AC current detection unit 202 Sine wave voltage application unit 203 Control unit 204 Peak voltage detection circuit 205 Differential waveform peak voltage detection circuit 206 Amplification circuit 207 Filter circuit 208 Drive circuit 209 Transformer 210 DC high voltage circuit

Claims (2)

Voltage applying means for applying a vibration voltage to a rotatable charging member in contact with the electrophotographic photosensitive member;
Voltage amplitude value detecting means for detecting an amplitude value of the oscillating voltage applied to the charging member;
Differential amplitude value detecting means for detecting an amplitude value of the differential waveform of the oscillating voltage;
AC current value detecting means for detecting an AC current flowing from the charging member to the electrophotographic photosensitive member when the oscillating voltage is applied, and
Prior to image formation, an oscillating voltage is applied to the charging member, and based on the amplitude value of the oscillating voltage and the amplitude value of the differential waveform, the average value and the minimum of the discharge capability in the rotation direction of the charging member Calculate the value,
When the difference between the minimum value of the discharge capacity and the average value of the discharge capacity is less than a threshold value, the discharge current value calculated from the amplitude value of the oscillating voltage, the amplitude value of the differential waveform, and the alternating current Is controlled so that the oscillating voltage applied to the charging member at the time of image formation is variable, so that is a predetermined value,
When the difference between the minimum value of the discharge capacity and the average value of the discharge capacity is greater than or equal to a threshold value, the discharge current value calculated from the amplitude value of the oscillating voltage, the amplitude value of the differential waveform, and the alternating current An image forming apparatus comprising: a control unit that performs constant current control or constant voltage control on an oscillation voltage applied to the charging member at the time of image formation with a current value that is equal to or greater than a predetermined value.   2. The apparatus according to claim 1, wherein at least the electrophotographic photosensitive member and the charging member are provided in a process cartridge that can be attached to and detached from an apparatus main body, and the process cartridge includes a memory that stores the threshold value. Image forming apparatus.

Images