JP5258520B2 - Image forming apparatus and control method thereof - Google Patents

Description

  The present invention relates to an image forming apparatus that performs image formation at a process speed selected from among a plurality of process speeds according to image forming conditions, and a control method therefor.

  An electrophotographic image forming apparatus is provided with a photosensitive drum. When image formation is performed, a charging process for uniformly charging the surface of the photosensitive drum to a predetermined potential is performed. For this charging process, for example, a contact charging method is used in which 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 surface of the photosensitive drum. In the contact charging method, a DC voltage having a voltage value Vd + Vth is applied to the charging roller in order to charge the surface of the photosensitive drum to a desired potential Vd. Here, the voltage value Vth is a discharge start voltage to the photosensitive drum which is a member to be charged when a DC voltage is applied to the charging roller.

  In addition, a charging method called an AC charging method is used to further uniform the charging of the photosensitive drum (see Patent Document 1). In this AC charging method, a charging voltage obtained by superimposing an AC voltage having a peak-to-peak voltage value (pp voltage value) at least twice the voltage value Vth on a DC voltage having a voltage value corresponding to a desired potential Vd is charged. This is a method of applying to a roller. In the case of the AC charging method, by superimposing the AC voltage, discharge to the plus side and the minus side occurs alternately, and the surface of the photosensitive drum can be uniformly charged.

  Here, when a sinusoidal AC voltage is applied to the charging roller, a resistive load current flows through a resistive load between the charging roller and the photosensitive drum, and a capacitive load current flows through a capacitive load between the charging roller and the photosensitive drum. Each of the discharge currents between the charging roller and the photosensitive drum flows. That is, the sum of these currents flows to the charging roller. At this time, it is empirically known that the discharge current amount is set to a predetermined amount or more in order to obtain stable charging.

  However, if an excessive amount of discharge current flows, deterioration of the photosensitive drum is promoted by scraping the photosensitive drum, and abnormal images such as image flow in a high-temperature and high-humidity environment due to discharge products are generated. . In order to promote the deterioration of the photosensitive drum and the occurrence of abnormal images, it is desirable to apply an AC voltage that minimizes the discharge that alternately occurs on the positive side and the negative side.

  Here, the relationship between the voltage value Vc of the sinusoidal AC voltage and the current value Ic of the current flowing through the photosensitive drum through the photosensitive drum when the AC voltage is applied to the charging roller will be described with reference to FIG. FIG. 6 shows the relationship between the voltage value (pp voltage value) Vc of a sine wave AC voltage and the current value Ic of the AC voltage flowing through the charging roller via the photosensitive drum when the AC voltage is applied to the charging roller. FIG. The horizontal axis of FIG. 6 shows the voltage value (pp voltage value) Vc of the sinusoidal AC voltage applied to the charging roller, and the vertical axis shows the current value Ic (effective value) of the AC voltage flowing through the charging roller. Show.

  As shown in FIG. 6, when the voltage value Vc (amplitude) of the AC voltage applied to the charging roller is gradually increased, the current value Ic of the AC voltage flowing through the charging roller is increased accordingly. Here, when the voltage value Vc of the AC voltage is equal to or less than a predetermined voltage value (Vth × 2), the voltage value Vc and the current value Ic are in a substantially proportional relationship. This is because the current flowing to the resistive load and the capacitive load between the charging roller and the photosensitive drum is proportional to the voltage value Vc of the AC voltage. When the voltage value Vc of the AC voltage is increased to exceed a predetermined voltage value (Vth × 2), discharge starts between the charging roller and the photosensitive drum, and the current value Ic increases by the amount of the discharge current value Is. .

  The relationship between the voltage value Vc of the AC voltage and the current value Ic of the AC voltage flowing through the charging roller is not always constant in practice, and the film thickness of the photosensitive drum, the degree of deterioration due to long-term energization of the charging roller, the environmental temperature It changes by etc. For example, when the impedance of the charging roller increases due to the long-term use of the charging roller, the current value Ic of the AC voltage flowing through the charging roller becomes small with respect to the same voltage value Vc.

  In a low-temperature and low-humidity environment (L / L), the material dries and the resistance value increases, making it difficult to discharge. Therefore, in order to obtain uniform charging, the voltage value Vc (amplitude) of the AC voltage is increased. There is a need to. On the other hand, in a high temperature and high humidity environment (H / H), the material absorbs moisture and the resistance value decreases, so that the same voltage value Vc causes an unnecessary discharge between the charging roller and the photosensitive drum. Become.

  Therefore, in order to stably supply a high-quality image over a long period of time, the voltage value of the AC voltage applied to the charging roller and the charging roller so that uniform charging can be performed without causing excessive discharge. It is necessary to control the current value of the AC voltage flowing through the. As this control method, a discharge current control method for determining a voltage value of an AC voltage for obtaining a desired discharge current amount during image formation has been proposed (see Patent Document 2).

  In the discharge current control method, during non-image formation, each AC voltage having a different voltage value Vc less than twice the voltage value Vth is applied to the charging roller, and flows to the charging roller when each AC voltage is applied. The current value Ic of the AC voltage is measured. In addition, each AC voltage having a different voltage value Vc that is at least twice the voltage value Vth is applied to the charging roller, and the current value Ic of the AC voltage flowing through the charging roller when each AC voltage is applied is measured. Then, from the relationship between each voltage value Vc of the AC voltage applied to the charging roller and each measured current value Ic, the voltage value of the AC voltage that provides a desired amount of discharge current is determined. By using this control method, an AC voltage having a voltage value that can obtain a desired discharge current amount can be applied to the charging roller regardless of environmental changes, changes in the degree of deterioration due to long-term energization of the charging roller, manufacturing variations, and the like. it can.

  In the case of the above discharge current control method, the environment changes with the passage of time, and the degree of deterioration due to long-term energization of the charging roller changes. Therefore, in order to always maintain a desired amount of discharge current, a constant image or a constant image is required. It is necessary to perform this control for each number of sheets formed. Therefore, for example, the timing for performing the discharge current control may be reached during continuous operation. In this case, since the image forming operation is temporarily interrupted, the productivity is lowered and the convenience of the user is impaired.

  Therefore, in order to suppress the change in the discharge current amount due to the change in the relationship between the voltage value and the current value of the alternating voltage, a method of controlling the voltage value of the alternating voltage during image formation by the constant current method is preferable. . According to this method, when the resistance of the charging roller decreases due to the temperature rise in the apparatus accompanying continuous operation, the voltage value of the AC voltage is lowered, so that the amount of discharge current can be reduced as compared with the constant voltage method.

  That is, a constant current control value for controlling the voltage value of the AC voltage to the voltage value of the AC voltage that obtains a desired amount of discharge current is determined by the discharge current control, and the constant current control value is used to determine the constant current control value. A method of performing current control is preferred. By using this method, the interval at which the discharge current control is performed can be widened, and a reduction in productivity can be suppressed.

  Further, with diversifying user needs, it is required to enable image formation on various types of paper such as thick paper and OHP, and to increase resolution (high pixel density). In order to meet this demand, an apparatus has appeared that selects a process speed corresponding to a paper to be used from a plurality of process speeds and forms an image at the selected process speed.

Here, the frequency of the AC voltage applied to the charging roller is associated with each process speed. For example, when a speed PS2 that is 1/2 of the normal process speed PS1 is selected, it is required that the frequency of the AC voltage applied to the charging roller be a frequency that is 1/2 of the frequency of the process speed PS1. This is because the number of discharges corresponding to the frequency of the AC voltage is generated between the charging roller and the photosensitive drum. Therefore, when the process speed PS2 is selected, if the AC voltage frequency is held at the process speed PS1, the charging roller and the photosensitive drum are spaced from each other as compared with the case where the frequency is half the process speed PS1. The number of discharges is doubled. As a result, the degree of displacement of the photosensitive drum is increased, the deterioration of the photosensitive drum is promoted, and many abnormal images such as image flow in a high-temperature and high-humidity environment due to discharge products are generated.
JP-A 63-149668 JP 2001-201921 A

  However, in an image forming apparatus that forms an image at a process speed selected according to the paper to be used from a plurality of process speeds, an AC voltage at the time of image formation using a constant current control value obtained by discharge current control When controlling the voltage value, there are the following problems.

  For example, the relationship between the voltage value (p-p voltage value) Vc of the AC voltage and the current value Ic of the AC voltage flowing through the charging roller at the process speed PS1 and half the process speed PS2 is as shown in FIG. become. The frequency of the AC voltage is ½ of the process speed PS1 at the process speed PS2. FIG. 7 is a diagram showing a relationship between the voltage value (p-p voltage value) Vc of the AC voltage and the current value Ic of the AC voltage flowing through the charging roller at the process speed PS1 and the half of the process speed PS2. Here, the voltage value Vth × 2, which is the discharge start voltage value, is the same for each of the process speeds PS1 and PS2.

  However, since the current value Ic is composed of a capacitive load current and a resistive load current, and the capacitive load current is proportional to the frequency of the AC voltage, the current value Ic at the process speed PS2 is the current at the process speed PS1. It becomes smaller than the value Ic. Therefore, it is necessary to perform the discharge current control at each of the AC voltage frequencies corresponding to the process speeds PS1 and PS2, and the time required for the discharge current control becomes longer. As a result, when the discharge current control is performed during the image forming operation, an extra time is required for the discharge current control, and the productivity is lowered.

  Further, when the discharge current control is performed at the frequency of the corresponding AC voltage in the case of the process speed PS2, the current value Ic detected in the region below the voltage value Vth × 2 becomes very small. It is difficult to detect Ic with high detection accuracy.

  An object of the present invention is to provide an image forming apparatus capable of determining a constant current control value for each process speed by only discharging current control for a predetermined process speed, and improving the productivity, and a control method therefor. There is to do.

  In order to achieve the above object, the present invention is an image forming apparatus that forms an image at a process speed selected from among a plurality of process speeds according to image forming conditions, and is in contact with or close to an image carrier. A charging means having a charging member arranged, and applying an alternating voltage having a frequency corresponding to the selected process speed to a direct current voltage applied to the charging member; and a current value of the alternating voltage flowing through the charging member Detecting means for detecting a voltage value of the AC voltage for obtaining a predetermined amount of discharge current with respect to an AC voltage having a frequency corresponding to a predetermined process speed of the process speed applied to the charging member, and the voltage value A discharge current control means for performing a discharge current control for obtaining a current value of the AC voltage flowing through the charging member corresponding to the voltage value of the AC voltage; and Determining means for determining the determined current value as a constant current control value for the predetermined process speed, and determining a constant current control value for another process speed using the voltage value obtained by the discharge current control; When performing image formation at the selected process speed, the frequency of the alternating voltage superimposed on the direct current voltage is set to a frequency corresponding to the selected process speed, and the current value detected by the detection means is the There is provided an image forming apparatus comprising: a control unit that controls a voltage value of the AC voltage so as to coincide with a constant current control value for a selected process speed.

  The present invention also provides a method of controlling the image forming apparatus in order to achieve the above object.

  According to the present invention, the constant current control value for each process speed can be determined only by the discharge current control for a predetermined process speed, and the productivity can be improved.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a longitudinal sectional view schematically showing a configuration of an image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a block diagram illustrating a configuration of a charging voltage generation circuit that generates a charging voltage applied to the charging roller of FIG. 1 and a control unit that controls the charging generation circuit.

  As shown in FIG. 1, the image forming apparatus 100 according to the present embodiment forms an electrophotographic image that forms a color image with four color toners of yellow (Y), magenta (M), cyan (C), and black (K). The image forming apparatus of the type. The image forming apparatus 100 includes a plurality of image forming units 1Y, 1M, 1C, and 1K for forming toner images of respective colors. Each of the image forming units 1Y to 1K forms a toner image of a corresponding color among yellow (Y), magenta (M), cyan (C), and black (K) based on the image signal.

  The image forming unit 1Y includes a photosensitive drum 1 as an image carrier, a charging roller 2 as a charging member, a laser exposure device 3, a developing device 4, and a primary transfer roller 5.

  Here, the charging roller 2 is brought into contact with the photosensitive drum 1 and rotates following the rotation of the photosensitive drum 1. A charging voltage is applied to the charging roller 2, and the surface of the photosensitive drum 1 is uniformly charged to a predetermined potential (negative potential) via the charging roller 2. In this embodiment, an AC charging method is used, and a sine wave AC voltage (1300 V to 2000 V) having a frequency corresponding to a process speed described later is superimposed on a DC voltage (for example, −300 V to −900 V). The charging voltage is applied to the charging roller 2.

  In the present embodiment, a charging roller that contacts the photosensitive drum 1 is used as the charging member. However, instead, a member having another shape can be used. Further, if the discharge between the charging member and the photosensitive drum 1 is ensured, the charging member can be disposed close to the photosensitive drum 1 (for example, with a gap of about 10 μm between them). It is.

  The laser exposure device 3 modulates laser light based on the image signal. Then, the laser exposure device 3 irradiates the photosensitive drum 1 with the modulated laser beam, and exposes and scans the surface of the photosensitive drum 1. As a result, a corresponding color, that is, a yellow electrostatic latent image is formed on the surface of the photosensitive drum 1.

  The developing device 4 includes a developing sleeve 4 a that rotates so as to supply yellow toner to the photosensitive drum 1. Here, a developing voltage in which an AC voltage of 1000 V to 2000 V is superimposed on a DC voltage of −150 V to −700 V is applied to the developing sleeve 4 a. As a result, toner charged to a negative potential is supplied to the photosensitive drum 1, and the electrostatic latent image formed on the surface of the photosensitive drum 1 is developed into a yellow toner image.

  The primary transfer roller 5 is a roller for transferring the toner image formed on the surface of the photosensitive drum 1 to the intermediate transfer belt 6, and is rotated by the movement of the intermediate transfer belt 6. A DC voltage having a reverse potential to the toner is applied as a primary transfer voltage to the primary transfer roller 5, and the toner image formed on the surface of the photosensitive drum 1 is transferred to the intermediate transfer belt 6.

  Each of the image forming units 1M, 1C, and 1K includes a photosensitive drum 1, a charging roller 2, a laser exposure device 3, a developing device 4, and a primary transfer roller 5 similarly to the image forming unit 1Y. Each of the image forming units 1M, 1C, and 1K forms a toner image of a corresponding color among magenta (M), cyan (C), and black (K), and transfers the toner image to the intermediate transfer belt 6.

  The toner images respectively formed by the image forming units 1Y to 1K are sequentially superimposed on the intermediate transfer belt 6 and transferred (primary transfer). As a result, a color toner image is formed and carried on the intermediate transfer belt 6. The toner image carried on the intermediate transfer belt 6 is transferred from the transfer counter roller 7 facing the secondary transfer roller 8 via the intermediate transfer belt 6 by the secondary transfer roller 8 to which a voltage of +500 V to +7000 V is applied. It is transferred onto the paper P sent in between (secondary transfer). Here, the paper P is stored in the paper feed cassette 10 and is fed between the secondary transfer roller 8 and the transfer counter roller 7 from the paper feed cassette 10 at a predetermined timing.

  The paper P on which the toner image is transferred is sent to the fixing device 11. The fixing device 11 heats and pressurizes the toner image on the paper P, and fixes the toner image on the paper P. The paper P on which the toner image is fixed is discharged onto the paper discharge tray 14 via the transport path 12 and the paper discharge roller 13.

  Here, the photosensitive drums 1 of the image forming units 1Y to 1K are rotated at a rotational speed (linear speed of the surface of the photosensitive drum 1) called a process speed in the direction of an arrow in the drawing. In the present embodiment, a corresponding process speed is selected from the plurality of process speeds PS1 and PS2 according to the basis weight (thickness) of the paper used for image formation, which is one of the image formation conditions, The photosensitive drum 1 is rotated at the selected process speed. That is, image formation is performed at the selected process speed. The process speed PS1 is the maximum process speed and is, for example, 150 mm / sec. The process speed PS2 is a half of the process speed PS1 and is 75 mm / sec.

  For example, the process speed PS1 is selected for paper having a basis weight of 100 g or less (for example, plain paper), and for paper having a basis weight of 100 g (for example, thick paper), the fixing performance is insufficient due to the thick paper. To compensate, the process speed PS2 is selected. When the process speed PS1 is selected, a charging voltage in which an AC voltage having a frequency of 1800 Hz is superimposed on the DC voltage is applied to the charging roller 2 of each of the image forming units 1Y to 1K. When the process speed PS2 is selected, a charging voltage in which an AC voltage having a frequency of 900 Hz is superimposed on a DC voltage is applied to the charging roller 2 of each of the image forming units 1Y to 1K.

  As shown in FIG. 2, the image forming unit 1Y is provided with a charging voltage generation circuit 201. The charging voltage generation circuit 201 constitutes a charging unit that cooperates with the charging roller 2 and applies the alternating voltage having a frequency corresponding to the selected process speed to the direct current voltage to be applied to the charging roller 2.

  The charging voltage generation circuit 201 includes a DC voltage generation circuit 202 that generates a DC voltage and an AC voltage generation circuit 203 that generates a sine wave AC voltage superimposed on the DC voltage generated by the DC voltage generation circuit 202. The AC voltage generation circuit 203 superimposes the generated AC voltage on the DC voltage and outputs the voltage as a charging voltage. The AC voltage generation circuit 203 includes an AC voltage generation unit 206, a constant voltage control circuit 207, and a constant current control circuit 208. The AC voltage generation unit 206 includes a detection circuit 206a that detects the current value of the AC voltage that flows through the charging roller 2 when a charging voltage is applied to the charging roller 2, and the detection circuit 206a detects the detected current value. Is output as a detection signal Sig5. Furthermore, the AC voltage generation unit 206 includes a detection circuit 206b that detects the voltage value of the AC voltage, and the detection circuit 206b outputs a detection signal Sig6 indicating the detected voltage value.

  In the discharge current control and the like described later, the AC voltage generation circuit 203 performs a constant voltage operation so that the amplitude of the output AC voltage is constant. In the constant voltage operation, the constant voltage control circuit 207 sets the voltage so that the target voltage indicated by the constant voltage control signal Sig2 is equal to the voltage value indicated by the voltage detection signal Sig6 detected and output by the voltage detection circuit 206b. Control signal Sig7.

  During the image forming operation, the AC voltage generation circuit 203 performs a constant current operation so that the current value of the output AC voltage is constant. In the constant current operation, the constant current control circuit 208 sets the voltage so that the target current indicated by the constant current control signal Sig3 and the current value indicated by the current detection signal Sig5 detected and output by the current detection circuit 206a are the same. Control signal Sig7.

  The voltage value and frequency of the DC voltage generated by the DC voltage generation circuit 202 and the voltage value and frequency of the AC voltage generated by the AC voltage generation circuit 203 are controlled by the control unit 204.

  The control unit 204 includes a CPU 204a, a ROM 204b, and a RAM 204c. The CPU 204a controls the entire apparatus and performs a discharge current control using the RAM 204c as a work area according to a program stored in the ROM 204b. Details of this discharge current control will be described later.

  The control unit 204 (CPU 204a) outputs a control signal Sig1 for controlling the voltage value of the DC voltage generated by the DC voltage generation circuit 202 to a predetermined voltage value to the DC voltage generation circuit 202 during image formation.

  Further, the control unit 204 outputs a control signal Sig4 for setting the frequency of the AC voltage generated by the AC current generation circuit 203. Here, when the process speed PS1 is selected as the process speed at the time of image formation, the control signal Sig4 for setting the frequency of the AC voltage generated by the AC current generation circuit 203 to 1800 Hz is output. When the process speed PS2 is selected, a control signal Sig4 for setting the frequency of the AC voltage generated by the AC current generation circuit 203 to 900 Hz is output. The control signal Sig4 is used when setting a frequency corresponding to the process speed at the time of discharge current control for determining a constant current control value for the process speed PS1 to be described later or at the time of determining the constant current control value for the process speed PS2. Is output.

  In addition, the control unit 204 outputs a control signal Sig2 for controlling the voltage value (pp voltage value) Vc of the AC voltage generated by the AC voltage generation circuit 203 to a predetermined voltage value (pp voltage value). To do. This control signal S2 is output at the time of discharge current control for determining the constant current control value for the process speed PS1 or at the time of determination of the constant current control value for the process speed PS2.

  Further, the control unit 204 takes in the detection signal Sig5 output from the detection circuit 206a of the AC voltage generation circuit 203 during image formation, and calculates the average value of the current flowing through the charging roller 2 based on the detection signal Sig5. To do. Here, the average value of the current flowing through the charging roller 2 is used for the charging voltage on which the sinusoidal AC voltage is superimposed unless the waveform distortion is large. This is because the average value is almost proportional.

  An environmental sensor 205 for detecting the temperature and humidity in the image forming apparatus 100 is connected to the control unit 204. The humidity and temperature detected by the environmental sensor 205 are used to determine the voltage value of the AC voltage generated by the AC voltage generation circuit 203 in the discharge current control described later. Then, the control unit 204 controls the AC voltage generation circuit 203 so as to generate an AC voltage having the determined voltage value by the control signal S2.

  Next, the discharge current control will be described with reference to FIG. FIG. 3 is a diagram schematically showing the relationship between the voltage value of the charging voltage and the current value flowing through the charging roller 2 when the control unit 204 in FIG. 2 performs the discharge current control.

  In the discharge current control of the present embodiment, as shown in FIG. 3, first, the voltage value Vc of the AC voltage applied to the charging roller 2 is different in a region (undischarged region) less than the voltage value Vth × 2. Two voltage values V1, V2 are determined. Also, two different voltage values V3 and V4 are determined in a region (discharge region) of voltage value Vth × 2 or more. The determination of the voltage values V1 to V4 is performed by the control unit 204 based on the environment table in which the temperature and humidity are associated with the voltage value, and the temperature and humidity detected by the environment sensor 205. The environment table is held in the ROM 204b in advance. Here, for each voltage value V1 to V4, if the environment is in a low temperature and low humidity environment (L / L environment), each is determined to be a high voltage value, and if it is in a high temperature and high humidity environment (H / H environment), Each is determined to be a low voltage value.

  Next, the process speed PS1 which is the maximum process speed is selected as the process speed, the photosensitive drum 1 is rotated at the process speed PS1, and the charging roller 2 rotates following the rotation of the photosensitive drum 1.

  The AC voltage generation circuit 203 generates AC voltages having the determined voltage values V1 to V4 at a frequency of 1800 Hz corresponding to the process speed PS1, based on the control signal Sig4 and the control signal Sig2. Each of the generated AC voltages V1 to V4 is applied to the charging roller 2 in order. Then, current values I1, I2, I3, and I4 of the current flowing through the charging roller 2 are acquired based on the detection signal Sig5 from the detection circuit 203a when the AC voltages of the voltage values V1 to V4 are respectively applied.

  The voltage values V1 to V4 and the current values I1 to I4 are associated with each other and held in the RAM 204a. Here, if the points where the current value is detected are P1 to P4, the points P1 to P4 are represented by (V1, I1), (V2, I2), (V3, I3), and (V4, I4). Each will be represented.

  Next, the constant current control value for the process speed PS1 is determined based on the current values I1 to I4 of the current flowing through the charging roller 2 when the charging voltages of the voltage values V1 to V4 are respectively applied. Further, a constant current control value for the process speed PS2 that is ½ of the process speed PS1 is determined.

  In determining the constant current control value for the process speed PS1, first, a straight line L1 connecting the point P1 (V1, I1) and the point P2 (V2, I2) is obtained, and the point P3 (V3, I3) and the point P4 (V4) are obtained. , I4), a straight line L2 is obtained. And the intersection of the straight line L1 and the straight line L2 is calculated | required, and the said intersection is made into the discharge start point Pc. Further, in a voltage region higher than the discharge start point Pc, the point P5 on the straight line L2 where the current value difference Is (predetermined discharge current amount) is 60 uA relative to the straight line L1, that is, the voltage of the charging voltage at the point P5. A value V5 and a current value I5 are obtained. The obtained voltage value V5 is a voltage value in which the discharge current amount between the photosensitive drum 1 and the charging roller 2 is a predetermined discharge current amount Is, and the current value I5 is determined as a constant current control value for the process speed PS1. Current value.

  When the constant current control value for the process speed PS1 is determined, the constant current control value for the process speed PS2 is subsequently determined. In determining the constant current control value of the process speed PS2, the frequency of the AC voltage generated by the AC voltage generation circuit 203 is set to 900 Hz, which is a frequency corresponding to the process speed PS2, by the control signal Sig4. However, the rotation speed of the photosensitive drum 1 is not changed and is maintained at the normal process speed PS1.

  Next, an AC voltage having a frequency of 900 Hz and a voltage value of V5 is applied to the charging roller 2, and the current value I6 of the current flowing through the charging roller 2 at that time is acquired from the detection signal S5 from the detection circuit 203a. . This current value I6 is a current value determined as a constant current control value for the process speed PS2.

  As described above, in the present embodiment, the process speeds PS1 and PS2 are controlled based on the voltage value and current value of the AC voltage obtained for the AC voltage having the frequency corresponding to the process speed PS1 by the discharge current control. A constant current control value for each is determined. Therefore, it is not necessary to perform the discharge current control for determining each of the process speeds PS1 and PS2, and the time required for the discharge current control can be reduced.

  Further, when determining the constant current control value when performing image formation at the process speed PS2, the frequency of the alternating voltage is set to a frequency corresponding to the process speed PS2, but the voltage region (less than the voltage value Vth × 2) ( There is no need to detect the current value in the undischarged region. Therefore, it is not necessary to consider the detection accuracy of the current value Ic in the voltage region less than the voltage value Vth × 2.

  Next, control by the control unit 204 will be described with reference to FIG. FIG. 4 is a flowchart showing a control procedure including discharge current control by the control unit 204 of FIG. The procedure shown in the flowchart of FIG. 4 is executed by the CPU 204a according to a program stored in the ROM 204b.

  When the image forming apparatus 100 is turned on, as shown in FIG. 4, the control unit 204 (CPU 204a) first performs an initialization process for bringing the image forming apparatus 100 into an image formable state (steps). S1). By this initialization process, the photosensitive drum 2, the intermediate transfer belt 6, the charging voltage generation circuit 201, and the like become operable.

  Next, the control unit 204 performs discharge current control for determining the constant current control value for the process speed PS1 described above, and determines the constant current control value for the process speed PS1 based on the result obtained by the discharge current control. (Step S2). Further, the control unit 204 determines a constant current control value for the process speed PS2 using the result obtained by the discharge current control. Subsequently, the control unit 204 resets a time count t of a built-in timer (not shown) and a count value n of a counter (not shown) that counts the number of formed images to “0” (step S3). Then, the control unit 204 shifts the image forming apparatus 100 to a standby state in which an input of a job start instruction such as a document copy instruction from a user or a print instruction from an external PC (personal computer) can be received (step S4). . In the standby state, monitoring of the time count t of the timer and input of the job start instruction are monitored.

  Next, the control unit 204 determines whether or not the timer time t exceeds 60 minutes (step S5). If it is determined that the timer time t does not exceed 60 minutes, the control unit 204 determines whether or not the job start instruction is input (step S6). If it is determined that the job start instruction has not been input, the control unit 204 returns to step S4 and maintains the standby state.

  If it is determined in step S5 that the timer time t has exceeded 60 minutes without the job start instruction being input, the control unit 204 returns to step S2. This is because the impedance of the charging roller 2 may fluctuate due to environmental fluctuations with time.

  If it is determined in step S6 that the job start instruction has been input before the timer time t exceeds 60 minutes, the control unit 204 executes an image forming operation corresponding to the input job start instruction. Control is performed (step S7). Here, one of the process speed PS1 and the process speed PS2 is selected based on the basis weight (thickness) of the paper used for image formation, and image formation is performed in page units at the selected process speed. . Here, as a method for recognizing the basis weight of the sheet, for example, a table in which the type of sheet and the basis weight are associated with each other is stored in advance, and the sheet set at the time of job input with reference to the table is stored. A method of recognizing the basis weight of the paper from the type can be used. Each time the image formation for one page is completed, the count value n of the counter is incremented by +1.

  Further, the DC voltage generation circuit 202 is controlled by the control signal Sig1 so that the voltage value of the DC voltage becomes a predetermined voltage value. For the AC voltage generation circuit 203, the frequency of the AC voltage is set to a frequency corresponding to the selected process speed by the control signal Sig4. The AC voltage generation circuit 203 is instructed by the control signal S3 with the constant current control value determined for the selected process speed, and the AC voltage generation circuit 203 generates the AC voltage generated by the constant current control circuit 208. The voltage value of the voltage is controlled.

  When the image formation for one page is finished, the control unit 204 determines whether or not the image formation for the last page is finished (step S8). When it is determined that the image formation on the final page has not been completed, the control unit 204 determines whether the count value n of the counter has reached 200 sheets or more, or whether the timer time t has exceeded 60 minutes. Is determined (step S9). When it is determined that the count value n of the counter has not reached 200 sheets or more and the time count t of the timer has not exceeded 60 minutes, the control unit 204 returns to step S7 to form the next page image. Control to do.

  When it is determined in step S9 that the count value n of the page counter has reached 200 or more or the time count t of the timer has exceeded 60 minutes, the control unit 204 performs discharge current control for the image forming operation. Interrupting and performing the discharge current control (step S10). Here, as described above, the photosensitive drum 1 is rotated at the normal process speed PS1.

  Next, the control unit 204 resets the time count t of the timer and the count value n of the counter to “0” (step S11), returns to step S7, and performs an image forming operation for the next page.

  If it is determined in step S8 that the image formation of the final page has not ended before the count value n of the page counter reaches 200 sheets and before the time count t of the timer exceeds 60 minutes, the control unit 204 Returns to step S4.

  Thus, according to the present embodiment, the constant current control value for each of the process speeds PS1 and PS2 can be determined only by the discharge current control for the process speed PS1, and the productivity can be improved. .

(Second Embodiment)
Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 5 is a flowchart showing a control procedure including discharge current control of the image forming apparatus according to the second embodiment of the present invention.

  Here, the present embodiment has the same configuration as the first embodiment, and the description thereof is omitted. In the present embodiment, points different from the first embodiment will be described, and the same reference numerals as those in the first embodiment are used for the description.

  In the present embodiment, the constant current control value for the process speed PS1 is determined by the same procedure as in the first embodiment, but the constant current control value for the process speed PS2 is not determined in accordance with this. That is, in the present embodiment, when the process speed PS2 is selected according to the basis weight (thickness) of the paper during image formation, the constant current control value for the process speed PS2 is determined. In this respect, the present embodiment is different from the first embodiment.

  A procedure for determining the constant current control value for the process speed PS2 will be described with reference to FIG. 3 used in the first embodiment.

  In the present embodiment, the constant current control value for the process speed PS2 is determined when the process speed PS2 is selected according to the basis weight (thickness) of the paper at the time of image formation. Is selected. Therefore, the photosensitive drum 1 is rotated at the process speed PS2.

  Further, the frequency of the AC voltage generated by the AC voltage generation circuit 203 is set to 900 Hz, which is a frequency corresponding to the process speed PS2, by the control signal Sig4. Further, the control signal Sig2 controls the voltage value of the AC voltage to be a voltage value V5 corresponding to the current value I5 determined as the constant current control value for the process speed PS1.

  Then, as shown in FIG. 3, an alternating voltage having a frequency of 900 Hz and the voltage value V5 is applied to the charging roller 2, and a current flowing through the charging roller 2 from the detection signal Sig5 when the alternating voltage is applied. The value I6 is obtained. This current value I6 becomes a current value determined as a constant current control value when image formation is performed at the process speed PS2.

  Next, control by the control unit 204 will be described with reference to FIG. The procedure shown in the flowchart of FIG. 5 is executed by the CPU 204a in accordance with a program stored in the ROM 204b.

  When the image forming apparatus 100 is turned on, as shown in FIG. 5, the control unit 204 (CPU 204a) first performs an initialization process for putting the image forming apparatus 100 into a state where image formation is possible (step). S21). By this initialization process, the photosensitive drum 2, the intermediate transfer belt 6, the charging voltage generation circuit 201, and the like become operable.

  Next, the control unit 204 performs the discharge current control for determining the constant current control value for the process speed PS1 described above, and determines the constant current control value for the process speed PS1 (step S22). However, the constant current control value for the process speed PS2 is not determined here. Subsequently, the control unit 204 resets the time count t of the timer and the count value n of the counter to “0” (step S23). Then, the control unit 204 shifts the image forming apparatus 100 to a standby state in which an input of a job start instruction can be accepted (step S24).

  Next, the control unit 204 determines whether or not the timer time t has exceeded 60 minutes (step S25). If it is determined that the timer time t does not exceed 60 minutes, the control unit 204 determines whether the job start instruction is input (step S26). If it is determined that the job start instruction has not been input, the control unit 204 returns to step S24 and monitors the timer time t.

  If it is determined in step S25 that the timer time t has exceeded 60 minutes without the job start instruction being input, the control unit 204 returns to step S22.

  If it is determined in step S26 that the job start instruction has been input before the timer time t exceeds 60 minutes, the control unit 204 determines that the basis weight of the paper used for image formation exceeds 100 g. It is determined whether or not (step S27). When it is determined that the basis weight of the paper exceeds 100 g, the control unit 204 selects the process speed PS2 as the process speed and determines a constant current control value for the process speed PS2 (step S28). Here, as described above, the photosensitive drum is rotated at the process speed PS2, and a charging voltage having a frequency of 900 Hz and a voltage value V5 is applied to the charging roller 2. Then, the current value I6 of the current flowing through the charging roller 2 is detected, and this current value I6 is determined as a constant current control value for the process speed PS2.

  Next, the control unit 204 controls to execute an image forming operation corresponding to the input job start instruction (step S29). Here, since the basis weight (thickness) of the paper used for image formation exceeds 100 g, the process speed PS2 is already selected. Then, image formation is performed at the process speed PS2, and the voltage value of the AC voltage is controlled using the constant current control value obtained in step S28. When the image formation for one page is completed, the count value n of the counter is incremented by “+1”.

  When the image formation for one page is completed, the control unit 204 determines whether or not the image formation for the last page has been completed (step S30). When it is determined that the image formation for the last page has not ended, the control unit 204 determines whether the count value n of the page counter has reached 200 or more, or whether the timer time t has exceeded 60 minutes. Is determined (step S31). When it is determined that the count value n of the page counter has not reached 200 and the time count t of the timer has not exceeded 60 minutes, the control unit 204 returns to step S29 to perform image formation for the next page. To control.

  When it is determined in step S31 that the count value n of the counter has reached 200 sheets or the timer time t has exceeded 60 minutes, the control unit 204 interrupts the discharge current control in the image forming operation. Then, the discharge current control is performed (step S32). Here, as in step S22, the discharge current control for determining the constant current control value for the process speed PS1 and the determination of the constant current control value are performed. Then, the control unit 204 resets the time count t of the timer and the count value n of the counter to “0” (step S33), and returns to step S27.

  If it is determined in step S30 that the image formation of the last page has been completed before the count value n of the page counter reaches 200 sheets or more and before the time count t of the timer exceeds 60 minutes, the control unit 204 The process returns to step S24.

  If it is determined in step S27 that the basis weight of the paper does not exceed 100 g, the control unit 204 skips step S28 and proceeds to step S29, and the image corresponding to the input job start instruction. Control to perform the forming operation. Here, since the basis weight (thickness) of the paper used for image formation does not exceed 100 g, the process speed PS1 is selected, and image formation at the process speed PS1 is performed. Further, the voltage value of the AC voltage is controlled using the constant current control value determined in step S22 or step S32. Then, the control unit 204 proceeds to step S30.

  Thus, in the present embodiment, when the process speed PS2 is selected based on the basis weight (thickness) of the paper, the constant current control value for the process speed PS2 is determined. Thus, when image formation is performed at the process speed PS1, it is not necessary to determine a constant current control value for the process speed PS2, and the productivity can be further improved as compared with the first embodiment. .

  In this embodiment, since the image formation is performed at the process speed PS2 after the constant current control value for the process speed PS2 is determined, it is not necessary to switch the process speed, and the image forming operation is started immediately. can do.

  In the present embodiment, the constant current control value for the process speed PS2 is determined every time the process speed PS2 is selected based on the basis weight (thickness) of the paper. Instead, the constant current control value for the first process speed PS2 after the determination of the constant current control value for the first process speed PS1 is used until the constant current control value for the next process speed PS1 is determined. You may make it continue. In this case, productivity can be further improved.

1 is a longitudinal sectional view schematically showing a configuration of an image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a block diagram illustrating a configuration of a charging voltage generation circuit that generates a charging voltage applied to the charging roller of FIG. 1 and a control unit that controls the charging generation circuit. FIG. 3 is a diagram schematically illustrating a relationship between a voltage value of a charging voltage and a current value flowing through the charging roller 2 when the control unit 204 in FIG. 2 performs discharge current control. 3 is a flowchart showing a control procedure including discharge current control by a control unit 204 of FIG. 2. 6 is a flowchart showing a control procedure including discharge current control of an image forming apparatus according to a second embodiment of the present invention. FIG. 6 is a diagram illustrating a relationship between a voltage value (p-p voltage value) Vc of a sinusoidal AC voltage and a current value Ic of an AC voltage that flows through the photosensitive drum when the AC voltage is applied to the charging roller. is there. It is a figure which shows the relationship between the voltage value (pp voltage value) Vc of an alternating voltage, and the electric current value Ic of the alternating voltage which flows into a charging roller in process speed PS1 and the process speed PS2 of the half.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging roller 100 Image forming apparatus 201 Charging voltage generation circuit 202 DC voltage generation circuit 203 AC voltage generation circuit 203a Detection circuit 204 Control part

Claims (8)

An image forming apparatus that forms an image at a process speed selected according to image forming conditions from a plurality of process speeds,
A charging unit having a charging member arranged in contact with or close to the image carrier, and applying an AC voltage having a frequency corresponding to the selected process speed to the DC voltage and applying the same to the charging member;
Detecting means for detecting a current value of an AC voltage flowing through the charging member;
Corresponding to a voltage value of the AC voltage and a voltage value of the AC voltage that provides a predetermined amount of discharge current with respect to an AC voltage having a frequency corresponding to a predetermined process speed of the process speed applied to the charging member. A discharge current control means for performing a discharge current control for obtaining a current value of an alternating voltage flowing through the charging member;
The current value obtained by the discharge current control is determined as a constant current control value for the predetermined process speed, and a constant current control value for another process speed is determined using the voltage value obtained by the discharge current control. A decision means to decide;
When performing image formation at the selected process speed, the frequency of the alternating voltage superimposed on the direct current voltage is set to a frequency corresponding to the selected process speed, and the current value detected by the detection means is the selected value. An image forming apparatus comprising: a control unit that controls a voltage value of the AC voltage so as to coincide with a constant current control value corresponding to the processed speed.   The discharge current control means rotates the image carrier at the predetermined process speed by the discharge current control, and supplies each of the alternating voltages having a frequency corresponding to the predetermined process speed having different voltage values to the charging member. The charging means is controlled to be applied to the charging member, the voltage value of each AC voltage applied to the charging member, and each current detected by the detecting means when each AC voltage is applied to the charging member. 2. The image forming apparatus according to claim 1, wherein the voltage value of the alternating voltage from which the predetermined amount of discharge current is obtained and the current value corresponding to the voltage value of the alternating voltage are obtained based on the value. .   3. The image forming apparatus according to claim 1, wherein the determination unit determines a constant current control value for the other process speed as well as a constant current control value for the predetermined process speed. 4.   The determination means determines a constant current control value for the other process speed when the process speed selected according to the paper used for the image formation is the other process speed. Item 3. The image forming apparatus according to Item 1 or 2.   When determining the constant current control value for the other process speed, the determining means rotates the image carrier at the predetermined process speed, and the voltage value obtained by the discharge current control at the predetermined process speed. And a current value detected by the detecting means when an AC voltage having a frequency corresponding to the other process speed is applied to the charging member is determined as a constant current control value for the other process speed. The image forming apparatus according to claim 3.   When determining the constant current control value for the other process speed, the determining means rotates the image carrier at the other process speed, and the voltage value obtained by the discharge current control at the predetermined process speed. And a current value detected by the detecting means when an AC voltage having a frequency corresponding to the other process speed is applied to the charging member is determined as a constant current control value for the other process speed. The image forming apparatus according to claim 4.   The image forming apparatus according to claim 1, wherein the predetermined process speed is a maximum process speed among the plurality of process speeds. It has a charging member arranged in contact with or close to the image carrier, and superimposes an alternating voltage of a frequency corresponding to the process speed selected according to the image forming conditions from a plurality of process speeds on the direct current voltage. A control method of an image forming apparatus, comprising: a charging unit that applies to the charging member; and a detection unit that detects a current value of an AC voltage flowing through the charging member, and performs image formation at the selected process speed. ,
Corresponding to a voltage value of the AC voltage and a voltage value of the AC voltage that provides a predetermined amount of discharge current with respect to an AC voltage having a frequency corresponding to a predetermined process speed of the process speed applied to the charging member. Performing a discharge current control to obtain a current value of an AC voltage flowing through the charging member;
Determining the current value determined by the discharge current control as a constant current control value for the predetermined process speed;
Determining a constant current control value for another process speed using the voltage value determined by the discharge current control;
When performing image formation at the selected process speed, the frequency of the alternating voltage superimposed on the direct current voltage is set to a frequency corresponding to the selected process speed, and the current value detected by the detection means is the selected value. And a step of controlling the voltage value of the AC voltage so as to coincide with the constant current control value for the processed process speed.

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