JP2012008448A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus being configured to make charging failure less likely to occur even when used by switching processing speed, in which a plurality of values of AC constant voltages that can be applied is provided, the value of AC current flowing when each is applied by switching among them is detected, the value of the AC constant voltage that is higher than a reference value and also the smallest is used as an electrification AC inter-peak voltage for image formation, and a voltage one-step lower than the electrification AC inter-peak voltage in image formation is applied during a pre-rotating operation for image formation to determine a voltage for image formation.SOLUTION: If the processing speed has become high, the value of the AC constant voltage applied during the pre-rotating operation in image formation is switched to the value of the AC constant voltage that has been selected or to the value of the AC constant voltage higher than that.

Description

  The present invention relates to an image forming apparatus such as a printer or a copier that employs an electrophotographic system.

  Image forming apparatuses such as laser beam printers and copiers that employ an electrophotographic method irradiate a uniformly charged electrophotographic photosensitive member with light (laser light, etc.) corresponding to image information. Form an image. Then, a developer (toner) is supplied to the electrostatic latent image by a developing means to visualize it as a developer image (toner image), and further, the image is transferred from a photoreceptor to a recording material represented by paper. An image is formed on a recording material and output.

  As a charging device for a photosensitive member, a contact charging method in which a charging member such as a roller type or a blade type is brought into contact with the surface of the photosensitive drum and a voltage is applied to the charging member to charge the surface of the photosensitive drum is widely adopted. Yes. In particular, the contact charging method using a roller-type charging member (charging roller) is excellent in that stable charging can be performed over a long period of time.

  A charging bias voltage is applied from a charging bias applying unit to a charging roller as a contact charging member. The charging bias voltage may be only a DC voltage. However, as in Patent Document 1, a peak-to-peak voltage (at least twice the discharge start voltage when a DC voltage is applied) is added to a DC voltage Vdc corresponding to the desired on-drum dark potential Vd. In many cases, a bias voltage on which an alternating voltage having Vpp) is superimposed is used. This charging method is excellent for uniformly charging the photosensitive drum, and local potential unevenness on the photosensitive drum is eliminated by superimposing an alternating voltage on the direct current voltage. Vd converges uniformly on the DC applied voltage value Vdc.

  On the other hand, if the AC peak-to-peak voltage is too large, the minimum required AC peak-to-peak voltage is applied so that charging failure does not occur because the surface of the photosensitive drum becomes rough and vertical stripes appear on the image. It is preferable to do this. In particular, the methods described in Patent Document 2 and Patent Document 3 are excellent methods that can achieve both cost reduction of the power supply circuit and more suitable charging bias application. In this method, a plurality of AC peak-to-peak voltages are applied to detect an AC current flowing through the photosensitive drum, and an AC peak-to-peak voltage that is equal to or greater than the reference current and has the smallest current value is selected as a charging bias during image formation. Is.

  On the other hand, image forming apparatuses adopting an electrophotographic system are increasingly provided with a plurality of operable process speeds from various viewpoints such as quietness of the apparatus itself and compatibility with various media. In such an image forming apparatus, when using a contact charging method in which a bias voltage superimposed with an alternating voltage is used, it is necessary to prevent charging unevenness from appearing in an image. Therefore, as in Patent Document 4, it is common to increase the frequency of the AC voltage to be applied as the process speed increases.

JP-A 63-149669 Japanese Patent No. 3902973 Japanese Patent No. 3902974 JP 2002-091097 A

  When using the methods of Patent Document 2 and Patent Document 3 and switching a plurality of operable process speeds, the following may occur. That is, when a charging bias in which an AC peak-to-peak voltage is superimposed on a direct current is applied, it is common to use a higher charging frequency as the process speed increases as described above. However, when switching from a slow process speed to a fast process speed, there is a possibility that a sufficient alternating current that does not cause a charging failure cannot be obtained if the alternating peak-to-peak voltage selected at the slow process speed remains unchanged. In order to prevent this, it is conceivable to newly provide a detection sequence when the process speed is switched. However, it takes time for the detection sequence, which is inconvenient.

  Accordingly, an object of the present invention is to provide an image forming apparatus capable of promptly applying an appropriate charging bias even when the process speed is switched.

  Another object of the present invention is to provide an image forming apparatus capable of applying an appropriate charging bias in accordance with the changed process speed.

  Still another object of the present invention is to provide an image forming apparatus capable of reducing the cost of a charging bias power supply circuit.

  In order to achieve the above object, a typical configuration of an image forming apparatus according to the present invention is an image forming apparatus capable of performing an image forming operation at a selected process speed among at least two kinds of process speeds having different speeds. And an AC peak-to-peak voltage Vpp− (1) in n (n ≧ 3) stages by a latent image carrier, a contact charging unit that contacts the latent image carrier, and a single voltage booster. A superposed bias voltage of one of Vpp- (n) (where Vpp- (1) <... <Vpp- (n)) and a DC voltage (Vdc) is applied to the contact charging means as a charging bias. A charging bias power supply circuit; and a charging alternating current detection means for detecting an alternating current flowing in the latent image carrier when a charging bias is applied to the contact charging means, and at least partly during non-image formation, the contact The charging AC current flowing through the latent image carrier is detected when the n (n.gtoreq.3) AC peak-to-peak voltage is sequentially applied to the charging means by the charging bias power supply circuit. In addition, the AC peak-to-peak voltage at which the smallest value is detected is selected as the charging AC voltage at the time of image formation, and is one step lower than the previously selected AC peak-to-peak voltage in at least part of the preparatory rotation operation before image formation. In an image forming apparatus that applies an AC peak-to-peak voltage and determines whether or not the required minimum AC current is satisfied, when the process speed is switched to a speed higher than the previous speed, at least one of the preparatory rotation operations is performed. The AC peak-to-peak voltage applied at the unit is made to be the previously selected AC peak-to-peak voltage or higher.

  According to the present invention, even when the process speed is switched, an appropriate charging bias can be quickly applied, particularly when the process speed is switched to a speed higher than the previous speed. In other words, even when the process speed of the image forming apparatus is switched from low to high, it is possible to more reliably determine whether or not the charging bias is appropriate before image formation. A possible image forming apparatus can be obtained.

In Example 1, the flowchart of charge bias application when the last process speed was Vx1 is shown. (A) is a schematic diagram of the image forming apparatus of Example 1, and (b) is an operation process diagram of the apparatus. (A) is a schematic diagram of a charging bias power supply circuit, and (b) is a correlation diagram showing the relationship between an AC peak voltage that can be applied and a DC voltage that can be output. (A) is a diagram schematically showing voltage-current characteristics and a reference current value at which charging failure does not occur, and (b) is voltage-current characteristics and charging failure does not occur due to differences in process speed and charging frequency. It is a figure explaining that a reference current value changes. It is the flowchart which showed the charging bias determination sequence performed at the time of power activation. (A) is a diagram showing a state of charging bias selection in the charging bias determination sequence, and (b) is a diagram showing a charging and driving sequence when performing an image forming operation. 6 is a flowchart of charging bias application when the previous process speed is Vx2. In Example 2, the flowchart of the charging bias application in case the last process speed was Vx1 is shown.

[Example 1]
(Outline of image forming apparatus)
FIG. 2A shows a schematic cross-sectional view of the image forming apparatus 100 in this embodiment. The apparatus 100 is a process cartridge detachable laser printer using a transfer type electrophotographic process. That is, an image can be formed on the sheet-like recording material P based on electrical image information input from the host device 200 to the engine controller (control circuit unit) 101 and output. The controller 101 comprehensively controls the operation of the apparatus 100 and executes the image forming operation of the apparatus 100 according to a predetermined image forming sequence in response to a print command from the host apparatus 200 or the operation unit 102. As will be described later, the apparatus 100 according to the present exemplary embodiment can perform an image forming operation at a selected process speed among at least two process speeds having different speeds. The host device 200 is a personal computer, an image reader, a facsimile machine, or the like connected to the controller 101 via an interface.

  A process cartridge C is detachably mounted on a cartridge mounting mechanism (not shown) in the apparatus main body 100A. The cartridge C in the present embodiment is a rotating drum type electrophotographic photosensitive member 1 as a latent image carrier with respect to the cartridge frame 6, and a contact charging device 2, a developing device 4 as process means acting thereon, The cleaning device 5 is assembled with a predetermined arrangement relationship. The contact charging device 2 is contact charging means that contacts a rotating electrophotographic photosensitive member (hereinafter referred to as a drum) 1 to uniformly charge the drum surface to a predetermined polarity / potential. A charging roller that rotates following rotation is used. The developing device 4 is a developing unit that develops (visualizes) the electrostatic latent image formed on the drum as a developer image (toner image) with a developer (toner). It has a developing roller (developer carrier) 4a that is conveyed to (developing unit) and develops the electrostatic latent image on the drum. Further, the developing device 4 is carried by the developing roller 4a, a developer containing portion 4b containing a developer (not shown), a stirring member 4c for supplying the developing roller 4a to the developing roller 4a while stirring the developer in the developer containing portion 4b. And a developing blade 4d for regulating the developer layer thickness. The cleaning device 5 is a cleaning unit that removes adhesion residues such as transfer residual toner and paper dust from the surface of the drum 1 after the toner image is transferred to the recording material P, and cleans the drum surface. In the present embodiment, a blade cleaning device using a cleaning blade 5a as a cleaning member. Transfer residual toner or the like scraped off from the drum surface by the blade 5a is stored in the waste toner storage portion 5b. In a state where the cartridge C is mounted on the apparatus main body 100A in a predetermined manner, the apparatus main body side drive output section (not shown) is coupled to the cartridge side drive input section (not shown). Further, the apparatus main body side bias output section (not shown) is coupled to the cartridge side bias input section (not shown). As a result, the apparatus 100 can perform an image forming operation.

  Based on the input of the print start signal, the controller 101 activates the drive source (main motor) M to rotate the drum 1 in a clockwise direction indicated by an arrow at a predetermined process speed (drum drive-ON). Then, at a predetermined control timing, a predetermined charging bias is applied to the charging roller 2 from a charging bias power supply circuit A (FIG. 3A) described later. As a result, the surface of the drum 1 is uniformly charged to a predetermined polarity / potential (dark portion potential Vd). Further, the controller 101 performs image exposure L by the exposure device (exposure means) 3 on the charged surface of the drum 1 at a predetermined control timing. The exposure apparatus 3 in this embodiment is a laser scanner. This apparatus 3 irradiates the surface of the drum 1 with laser light L modulated based on image information (main scanning exposure) to remove the charge of the irradiated portion, and the bright portion potential Vl of the irradiated portion and the non-irradiated portion ( An electrostatic latent image is formed on the drum surface by electrostatic contrast with the dark portion potential Vd of the non-exposed portion. Then, the electrostatic latent image is developed as a toner image by the developing device 4. As the image forming system, there are a system using a background exposure system and a regular development system, and a system using an image exposure system and a reverse development system. The former is a method in which the charged drum surface is exposed corresponding to the background portion of the image information, and a portion other than the background portion is developed. The latter is a method in which exposure is performed corresponding to the image information portion and non-exposed portions are developed. It is used taking advantage of each feature.

  The toner image formed on the drum 1 is transferred to the recording material P by a transfer roller 7 as a transfer device (transfer means). The recording material P is stacked and stored in the paper supply unit 8. When the paper feed roller 9 is driven at a predetermined control timing, the recording material P of the paper feed unit 8 is separated and fed one by one and reaches the registration roller 11 through the sheet path 10. Then, the recording material P is introduced into a transfer nip portion N which is a pressure contact portion between the drum 1 and the roller 7 by the roller 11 in synchronization with the toner image on the drum 1 and is nipped and conveyed. While the recording material P is nipped and conveyed to the roller 7, a transfer bias, which is a DC voltage having a predetermined potential opposite to the toner charging polarity, is applied to the sliding contact from a transfer bias power supply circuit (not shown). Applied. As a result, the toner images on the drum 1 are sequentially electrostatically transferred onto the recording material P. The recording material P that has exited the nip N is separated from the drum 1 and conveyed to a fixing device (fixing means) 12. Further, the drum 1 after separation of the recording material is cleaned by removing residuals such as toner and paper dust remaining on the surface by the cleaning device 5 and repeatedly used for image formation. The recording material P conveyed to the fixing device 12 is heated and pressurized at a fixing nip portion that is a pressure contact portion between the fixing roller 12a and the pressure roller 12b, and the unfixed toner image is fixed as a fixed image. The recording material Pa after fixing the toner image is discharged as an image formed product (print, copy) by a discharge roller 13 to a discharge tray 14 outside the apparatus main body.

<Operation Process of Device 100>
FIG. 2B shows an operation process diagram of the apparatus 100 performed by the controller 101.

  1) Pre-multi-rotation operation (pre-rotation): Start (start) operation (warming) of the apparatus 100. When the main power switch of the apparatus 100 is turned on, the main motor M of the apparatus 100 is activated to perform a preparatory rotation operation of the required process equipment.

  2) Standby: After completion of a predetermined start operation, the driving of the main motor M is stopped, and the apparatus 100 is held in a standby (standby) state until a print job start signal is input.

  3) Pre-rotation operation: Based on the input of the print job start signal, the main motor M is driven again, and a pre-print job operation (preparation rotation operation before image formation) of a required process device is executed. More practically, the order is: a: the apparatus 100 receives a print job start signal, b: an image is developed by the formatter (the development time varies depending on the data amount of the image and the processing speed of the formatter), and c: the pre-rotation operation starts. become.

  If the print job start signal is input during the pre-multi-rotation operation of 1), after the pre-multi-rotation operation is completed, the process proceeds to the pre-rotation operation without standby of 2).

  4) Print job execution: When a predetermined pre-rotation process is completed, the image forming process is subsequently executed, and an image-formed recording material is output. In the case of a multi-print job (continuous print job), the above-described image forming process is repeated, and a predetermined number of image-formed recording materials are sequentially output.

  5) Paper interval: In the case of a multi-print job, this is a spacing step between the trailing edge of one recording material P and the leading edge of the next recording material P, and is a non-paper passing period in the transfer unit and the fixing device.

  6) Post-rotation operation: after the recording material on which the image has been formed is output in the case of a mono print job of only one sheet, or after the recording material on which the last image has been formed is output in the case of a multi-print job The main motor is continuously driven for a predetermined time. As a result, the post-print job operation of the required process device is executed.

  7) Standby: After completion of the predetermined post-rotation operation, the driving of the main motor M is stopped, and the image forming apparatus is held in a standby state until the next print job start signal is input.

  In the above, 4) when the print job is executed is the time of image formation, 1) pre-multi-rotation operation, 3) pre-rotation operation, and 6) post-rotation operation are during non-image formation. In addition, when the print job of 4) is executed, the interval between sheets in the case of a multi-print job is also during non-image formation.

(Charging bias application method)
A charging bias application method according to this embodiment will be described. FIG. 3B is a schematic block circuit diagram of a charging bias power supply circuit (high voltage power supply circuit) A for applying a charging bias to the charging roller 2. The circuit A is composed of one voltage boosting means T1, and n (n ≧ 3) stage AC peak-to-peak voltages Vpp- (1),..., Vpp- (n) (where Vpp- (1) <. A superposed bias voltage of one of <Vpp− (n)) and the DC voltage Vdc is applied to the charging roller 2 as a charging bias. More specifically, the circuit A of the present embodiment has four types of AC peak-to-peak voltages Vpp (Vpp− (1) <Vpp− (2) <Vpp− ( 3) <Vpp- (4)) can be output. The output voltage output from the AC oscillation output unit 21 is amplified by the amplifier circuit 22. Then, after being sine-converted by a sine voltage conversion circuit 23 comprising an operational amplifier, a resistor, a capacitor, etc., the DC component is cut to zero via the capacitor C1 and input to the step-up transformer T1 as AC voltage boosting means. The voltage input to the transformer T1 is boosted to a sine voltage corresponding to the number of turns of the transformer.

  On the other hand, the capacitor C2 is peak-charged after the boosted sine voltage is rectified by the rectifier circuit D1. As a result, a certain DC voltage Vdc1 is generated. Further, the DC oscillation output unit 24 controlled by the controller 101 outputs an output voltage determined by the print density and the like, rectified by the rectifier circuit 25, and then input to the negative input terminal of the operational amplifier IC1 as a constant voltage Va. The At the same time, a voltage Vb obtained by dividing one terminal voltage of the transformer T1 by the resistors R1 and R2 is input to the plus input terminal of the operational amplifier IC1, and the transistor Q1 is set so that both values (Va and Vb) are equal. Drive. As a result, a current flows through the resistors R1 and R2, causing a voltage drop, and a DC voltage Vdc2 is generated. The desired DC voltage Vdc (Vdc1 + Vdc2) is obtained by adding the DC voltages Vdc1 and Vdc2 described above. An oscillation voltage in which the DC voltage Vdc is superimposed on the AC voltage (sine voltage) described above on the secondary side of the transformer T1 is applied to the charging roller 2 as a charging bias. In the method described here, only one transformer T1 as the voltage boosting means is required, so that the cost of the charging bias power supply circuit A can be reduced.

  Here, regarding Vpp− (4) which is the largest peak-to-peak voltage, it is necessary to set the peak-to-peak voltage at which the charging failure of the drum 1 does not occur in all cases. In general, charging failure does not occur even in consideration of variations in the charging roller 2 and applied peak-to-peak voltage in a low temperature environment where the charge transport layer of the drum 1 is thick and in the initial stage of use and in which current does not flow easily. It is necessary to use such a peak-to-peak voltage as Vpp- (4). On the other hand, when the thickness of the charge transport layer of the drum 1 decreases due to durability, a large current flows. Therefore, in the other Vpp- (1), Vpp- (2), and Vpp- (3), a large current does not continue to flow through the drum 1 as a peak-to-peak voltage lower than Vpp- (4). Yes.

  Further, in this method, since the DC voltage Vdc1 is produced using the transformer T1 which is an AC voltage boosting means, the DC voltage Vdc1 is dependent on the AC peak-to-peak voltage Vpp. That is, in order to obtain a desired DC voltage Vdc ', it is necessary to charge the capacitor C2 with a certain level of charge by the transformer T1. As shown in FIG. 3B, in order to obtain a desired DC voltage Vdc ′, the AC peak-to-peak voltage Vpp must be 2 × | Vdc ′ | or higher. In a region where the AC peak-to-peak voltage Vpp is smaller than 2 × | Vdc ′ |, the capacitor C2 cannot be fully charged, so that a desired DC voltage Vdc ′ cannot be obtained. Therefore, the drum potential (drum surface potential, dark portion potential) Vd cannot be charged to a desired value, and a good image cannot be obtained. Since this is the description of FIG. 3B, it is necessary to distinguish between the desired DC voltage Vdc ′ and the applicable DC voltage Vdc, and “′” is added to Vdc. In the general relationship between the DC voltage Vdc and Vpp, the description of Vdc ′ is not made.

  On the other hand, if the capacitance of the capacitor C2 is increased, the charge charge amount can be increased and the DC voltage Vdc can be increased. However, it takes a longer time to charge the capacitor C2 and the charge waveform is stabilized. The time will be longer. For this reason, unevenness may occur in the on-drum potential Vd.

  Therefore, in this example, the minimum value Vpp− (1) in the range in which the AC peak-to-peak voltage Vpp can be output satisfies the relationship Vpp− (1) ≧ 2 × | Vdc | with respect to the desired DC voltage Vdc. Is set. Vpp- (1) having the smallest AC peak-to-peak value satisfies the relationship of Vpp- (1) ≧ 2 × | Vdc | with respect to the superimposed DC voltage Vdc, thereby reducing the cost of the power supply circuit. An image forming apparatus can be obtained.

(Measurement of charging AC current value)
Next, a method for measuring the charging alternating current value will be described with reference to FIG. The circuit A includes an AC current detection circuit (charging AC current detection means) 26 that detects an AC current Iac that flows through the drum 1 when a charging bias is applied to the charging roller 2. The controller 101 sequentially applies an AC peak-to-peak voltage of n (n ≧ 3) stages to the charging roller 2 by the circuit A at least partially during non-image formation. At this time, the charging AC current Iac flowing in the drum 1 is detected by the circuit 26. Then, the controller 101 selects the AC peak-to-peak voltage Vpp that is equal to or more than the necessary minimum current and detects the smallest value as the charging AC voltage at the time of image formation. In at least a part of the preparatory rotation operation before image formation, an AC peak-to-peak voltage that is one step lower than the previously selected AC peak-to-peak voltage is applied to determine whether or not the necessary minimum AC current is satisfied.

  The above will be described more specifically. When a charging bias voltage (vibration voltage) is applied to the charging roller 2, the alternating current Iac flows to the GND of the circuit A through the charging roller 2 and the drum 1. At this time, the AC current detection circuit 26 extracts only the AC current having a frequency equal to the charging frequency from the AC current Iac by a filter circuit (not shown) composed of a resistor, a capacitor, etc. A voltage value is input to the controller 101. Since the AC current detection circuit 26 can be composed of a resistor, a capacitor, a diode, and the like, the influence of the increase in the cost of the circuit A and the expansion of the space is small. If the relationship between the converted voltage and the alternating current is checked in advance, the alternating current value can be detected by detecting the measured voltage.

  Here, the voltage-current characteristic as shown in FIG. 4A is examined in advance, and further, by taking correspondence between the current at which the charging failure disappears and the image, a reference current value that does not cause the charging failure occurs. (Iac-x in the figure) can be determined. In this embodiment, the charging bias is controlled based on the reference current thus determined. In addition, the current corresponding to ΔIc in the diagram deviating from the linearity in the voltage-current characteristic is considered to be a current due to discharge (hereinafter referred to as a discharge current), and it is known that a strong correlation with the charging property is observed.

(Charging bias control method)
Next, an actual charging bias control method will be described. In the apparatus 100 of this embodiment, an image forming operation can be performed at a selected process speed among at least two process speeds having different speeds. The process speed can be selected by the host device 200 or the operation unit 102, and the selection signal is input to the controller 101. The controller 101 executes an image forming operation at a process speed corresponding to the input selection signal.

  In this embodiment, it is assumed that the process speed that can be operated has two stages of a standard speed (slow speed) Vx1 and a faster speed Vx2 (Vx1 <Vx2). Since it is necessary to prevent charging unevenness from appearing in the image, the charging frequencies f at the respective process speeds Vx1 and Vx2 are set to f1 and f2 (f1 <f2), respectively. Even when the process speed is low, it is possible to use the charging frequency f2 when the charging speed is high, but generally the drum surface is likely to be roughened when the charging frequency f is increased, so that the charging frequency f depends on the process speed as shown here. Is often changed.

  In the case of this example, a reference alternating current value Iac is determined for each process speed Vx1, Vx2 so as to be equal to or higher than a current value at which charging failure does not occur as described above. Actually, FIG. 5 shows the results of taking the voltage-current characteristics at the respective process speeds Vx1, Vx2 and charging frequencies f1, f2. The process speed Vx1-charge frequency f1 is indicated by a thin line, and the process speed Vx2-charge frequency f2 is indicated by a thick line. As can be seen from FIG. 4B, it can be seen that the higher the process speed and the charging frequency (Vx2-f2), the larger the reference current value Iac-x2 at which charging failure does not occur. In addition, it can be seen that the discharge current at the reference current value increases as the process speed and the charging frequency increase, and the charging property becomes severe. The reference current value at the process speed Vx1 is defined as Iac-x1, and the reference current value at the process speed Vx2 is defined as Iac-x2.

  First, FIG. 5 shows a charging bias control flowchart in the pre-multi-rotation process (pre-rotation) which is a preparatory operation performed when the power is turned on. The process speed at this time is assumed to be Vx2 (step S1). Application is started from Vpp- (1) which is the lowest voltage, and the alternating current Iac (1) flowing at that time is measured (step S2). Similarly, Vpp- (2), Vpp- (3), and Vpp- (4) are applied to obtain AC currents Iac (2), Iac (3), and Iac (4), respectively (steps S4, S6, and S8). ). The respective Iac (1), Iac (2), Iac (3), and Iac (4) are compared with the reference current value Iac-x2 at the process speed Vx2 (steps S3, S5, and S7). Then, the minimum Vpp− (n) satisfying Iac (n) ≧ Iac−x2, n = 1, 2, 3, 4 is determined as the AC voltage (steps S11, S12, S13, S9). FIG. 6A shows an example of AC voltage selection. In this case, since Iac (3) when Vpp− (3) is applied exceeds the reference current value Iac−x2 and becomes minimum, Vpp− ( 3) is selected as the AC voltage during the image forming operation.

  Next, FIG. 6B shows a sequence when an image forming operation is performed immediately thereafter. The AC voltage selected in the previous image forming operation is defined as Vpp- (n). As a feature of this sequence, at least partly before the image forming operation (hereinafter referred to as “preparation rotation before image formation”), Vpp− (1 step lower than the previously selected AC voltage Vpp− (n). n-1) is applied. If Iac (n-1) flowing when Vpp- (n-1) is applied is greater than or equal to the reference current value Iac-x, the AC voltage during image formation is set to Vpp- (n-1), and Iac (n- If 1) does not satisfy the reference current value Iax-x, the AC voltage during image formation is set to Vpp- (n). By doing so, it is possible to gradually reduce the AC voltage during the image forming operation.

  Here, when the sequence of FIG. 5 is performed immediately before, the previous process speed is Vx2, and the alternating voltage selected at that time becomes the previous alternating voltage. On the other hand, when the previous image forming operation is performed immediately before, the process speed and AC voltage at that time become the previous process speed and AC voltage, respectively.

  FIG. 7 shows a flowchart of the current image forming operation when the process speed of the previous image forming operation is Vx2. When the current process speed is also Vx2 (Yes in step S3, step S18), if the selection voltage is Vpp- (1) (Yes in step S19), Vpp- both during the preparatory rotation before image formation and during image formation. (1) is used as an AC voltage (steps S28 to S31). If it is other than Vpp- (1) (No in step S19), Vpp- (n-1) which is one step lower than the voltage Vpp- (n) previously selected during the preparatory rotation before image formation as described above. ) And the flowing alternating current Iac (n-1) is detected (step S20). If Iac (n-1) is larger than the reference current value Iac-x1 (Yes in step S21), the AC voltage during image formation can be lowered to Vpp- (n-1) (steps S25 to S25). 27). On the other hand, if it falls below the reference current value (No in step S21), Vpp- (n) may be kept (steps S22 to S24). Even when the current process speed is Vx1 (No in step S3, S4), the operation is substantially the same as the reference current value is Iac-x2 (step S7) (steps S4 to S17).

  Next, FIG. 1 shows a flowchart of the image forming operation when the previous process speed is Vx1. If the current (next) process speed remains Vx1 (No in step S3), the operation (steps S4 to S17) is almost the same as in FIG. 7 (steps S4 to S17). On the other hand, when the process speed is changed from the slow speed Vx1 to the fast speed Vx2 (Yes in step S3, S18), the AC voltage applied during the preparatory rotation before image formation is not Vpp- (n-1) but Vpp- (N) (step S19). Thus, the AC voltages that may be applied during image formation are Vpp− (n) (step S24) and Vpp− (n + 1) (step S21).

  The reason for this will be described. In general, when the process speed is increased, the frequency is also increased in order to prevent charging unevenness from appearing on the image, and the AC current value that flows when the same AC voltage is applied increases accordingly. On the other hand, it is considered that minute charging defects are likely to occur as described in FIG. Therefore, considering that the required discharge current value becomes large, the current Iac (n) that flows when the alternating voltage Vpp- (n) selected at the process speed Vx2 is applied as it is at the process speed Vx1 is the reference The current value Iac-x1 may not be exceeded. In order to solve this problem, in this embodiment, an AC voltage Vpp- (n) larger than normal Vpp- (n-1) is applied during the preparation rotation before image formation (step S19). Thus, when Iac (n) <Iac-x1 is satisfied by applying Vpp- (n) (No in step S20), Vpp- (n + 1) can be applied during image formation (step S21).

  On the other hand, at the process speed Vx1, the current Iac (m) that flows when the alternating voltage Vpp- (m) selected at the process speed Vx2 is applied has at least a necessary discharge current value due to the decrease in the process speed. It is thought not to be large. Therefore, even if the same Vpp- (m) is maintained, the possibility of being lower than the reference current Iac-x2 is very small, and it is considered that there is no problem if the Vpp- (m) is maintained.

  From the above, it is possible to achieve a configuration in which charging defects are less likely to occur by performing control such that the AC voltage applied during the preparatory rotation before image formation is increased only when the process speed is increased. Although there is a case where it is not necessary to apply a high voltage, the voltage can be lowered by one step by performing the sequence of FIG. 1 or FIG. 7 in the next image forming operation. The possibility of continuing to apply is small, and there is no particular problem.

  On the other hand, when the process speed is switched, the application sequence shown in FIG. 5 may be performed again at the process speed. However, in the present embodiment, correction can be performed during the image forming operation. However, since the above sequence is entered separately from the image forming operation, it takes extra time to complete the image forming operation. The example is better.

  In the present embodiment, the case where the power-on sequence (FIG. 5) is performed at the process speed Vx2 is shown, but the case where the sequence is performed at Vx1 can also be applied with the configuration described so far. In the present embodiment, when the process speed is increased, a value that is one step higher than the selected AC voltage is selected. However, if the difference between the AC peak-to-peak voltages that can be applied is small, the required discharge current It is also conceivable that the current value does not increase in proportion to the increase in value. Therefore, it is possible to increase the level by two or more stages, which is preferable in this case.

[Example 2]
Up to now, there were two types of process speeds that can be operated. However, even if there are three or more types of process speeds that can be operated, it is possible to perform the same.

  Here, a case where the process speed is set in three stages of Vx1 <Vx2 <Vx3 will be described. As for the sequence when the power is turned on, as in FIG. 5, an AC voltage that can be applied at a certain process speed is applied, and the AC voltage used at the time of image formation may be selected while detecting the AC current flowing at that time. Then, explanation is omitted.

  Therefore, the case where the selection result of the charging AC voltage already exists will be described with reference to FIG. Here, the voltage selection flowchart when the previous process speed is Vx1 (step S2) is shown. In this embodiment, when the process speed is changed from Vx1 to Vx2 (step S19), the AC voltage during the preparatory rotation before image formation is increased by one step (step S20). Further, when Vx1 is changed to Vx3 (step S28), the AC voltage during the preparatory rotation before image formation is increased by two stages (step S29).

  That is, even if there are three process speeds at which the same effect as in the first embodiment can be operated by changing the change width of the constant voltage value applied during the preparation rotation before image formation according to the change in the process speed. It becomes possible to demonstrate. That is, when the process speed is switched to a speed higher than the previous speed, the amount of increase in the AC peak-to-peak voltage applied in at least a part of the preparation rotation operation is changed according to the change width. Thus, an image forming apparatus capable of applying a more appropriate charging bias according to a change in process speed can be obtained.

  Although not specifically shown here, when the process speed is changed from Vx2 to Vx3, it may be increased by one step, for example. Similarly, when the process speed increases, the number of steps to increase the AC voltage during preparatory rotation before image formation can be determined in consideration of the difference in the applicable AC peak-to-peak voltage, the process speed value, etc. The embodiment is not limited to the mode shown in this example.

  DESCRIPTION OF SYMBOLS 100..Image forming apparatus 1..Latent image carrier 2..Contact charging means T1..Voltage boosting means A..Charging bias power supply circuit 26..Charging AC current detecting means

Claims (3)

An image forming apparatus capable of performing an image forming operation at a selected process speed among at least two kinds of process speeds having different speeds,
A latent image carrier;
Contact charging means for contacting the latent image carrier;
With one voltage booster,
n (n ≧ 3) AC peak-to-peak voltage Vpp− (1),..., Vpp− (n)
(However, Vpp- (1) <... <Vpp- (n))
A charging bias power supply circuit that applies a bias bias voltage of one of the above and a DC voltage to the contact charging means as a charging bias;
Charging alternating current detection means for detecting an alternating current flowing in the latent image carrier when a charging bias is applied to the contact charging means;
And at least a portion during non-image formation, the latent image carrying when the alternating peak-to-peak voltage in the n (n ≧ 3) stage is sequentially applied to the contact charging means by the charging bias power supply circuit. The charging AC current flowing through the body is detected, the AC peak-to-peak voltage that is equal to or greater than the minimum necessary current and the smallest value is selected as the charging AC voltage during image formation, and at least one of the preparatory rotation operations before image formation In the image forming apparatus for applying the AC peak-to-peak voltage one step lower than the previously selected AC peak-to-peak voltage and determining whether or not the required minimum AC current is satisfied,
When the process speed is switched to a speed higher than the previous speed, the AC peak-to-peak voltage applied in at least part of the preparatory rotation operation is set to the previously selected AC peak-to-peak voltage or higher. An image forming apparatus.   2. The process according to claim 1, wherein when the process speed is switched to a speed higher than the previous speed, an increase amount of the AC peak-to-peak voltage applied in at least a part of the preparatory rotation operation is changed according to the change width. Image forming apparatus.   The Vpp- (1) having the smallest AC peak-to-peak value satisfies the relationship of Vpp- (1) ≧ 2 × | Vdc | with respect to the superimposed DC voltage Vdc. The image forming apparatus described in 1.