Image forming apparatus and method for controlling image forming apparatus

Abstract

To provide an image forming apparatus and a method for controlling an image forming apparatus that can detect an abnormality occurring on the upstream side of an electrifier.SOLUTION: An overcurrent detection circuit of a printer is connected with a plurality of electrification voltage generation circuits, and when detecting an overcurrent in any one of the plurality of electrification voltage generation circuits, outputs an overcurrent detection signal Sv to an ASIC. When the overcurrent detection signal Sv is input to the ASIC from the overcurrent detection circuit, the ASIC determines that, of four corona electrifiers, a corona electrifier having the maximum wire voltage CHG is abnormal (S37).SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present application relates to a technique for detecting an overcurrent in an image forming apparatus equipped with a charger. [Background technology]

[0002] Conventionally, there are image forming apparatuses equipped with a plurality of corona chargers each having a charging wire and a grid. For example, the corona charger of Patent Document 1 applies a wire voltage from a high-voltage power supply to the charging wire to generate a corona discharge between the charging wire and the photoconductor, thereby uniformly positively charging the surface of the photoconductor. The corona charger may generate abnormal discharge due to the adhesion of silica or paper powder used as an external additive in toner. The abnormal discharge referred to here is, for example, a spark discharge or an arc discharge, which is different from a corona discharge.

[0003] For example, when silica adheres to the charging wire, the resistance between the charging wire and the grid increases. The image forming apparatus increases the wire voltage in response to the increase in resistance between the charging wire and the grid. The image forming apparatus described in Patent Document 1 is equipped with a discharge detection circuit that detects abnormal discharge, and is configured such that one discharge detection circuit is connected to multiple chargers. The image forming apparatus detects whether or not abnormal discharge has occurred based on a discharge detection signal output from the discharge detection circuit. When the image forming apparatus detects the occurrence of abnormal discharge, it determines that the charger connected to the high-voltage power supply unit with the largest output is abnormal. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2009-15168 A Summary of the Invention [Problem to be solved by the invention]

[0005] In the image forming apparatus of the above-mentioned Patent Document 1, a discharge detection circuit for detecting abnormal discharge is connected to the grid side of each charger. With this configuration, it is possible to detect abnormal discharge between the charging wire and the grid, but there is a risk that it may be difficult to detect abnormalities occurring upstream of the charging wire, such as in the connection circuit between the high-voltage power supply unit and the charging wire.

[0006] The present application has been proposed in view of the above-mentioned problems, and has an object to provide an image forming apparatus capable of detecting an abnormality occurring upstream of a charger, and a method for controlling an image forming apparatus. [Means for solving the problem]

[0007] The present application discloses an image forming apparatus comprising a plurality of photosensitive drums, a plurality of chargers for charging each of the plurality of photosensitive drums, a plurality of application circuits for applying an application voltage to each of the plurality of chargers, a control unit, and an overcurrent detection circuit connected to the plurality of application circuits and outputting a detection signal to the control unit when an overcurrent is detected in any one of the plurality of application circuits, wherein when the detection signal is input from the overcurrent detection circuit, the control unit determines that the charger among the plurality of chargers for which the application voltage applied by the application circuit was the highest is abnormal.

[0008] This makes it possible for the overcurrent detection circuit connected to the multiple application circuits to output a detection signal to the control unit via a single signal path. When the control unit receives a detection signal from the overcurrent detection circuit, it determines that the charger to which the maximum voltage was applied is abnormal. This makes it possible to use a single overcurrent detection circuit for the multiple application circuits, and also makes it possible to determine which charger has experienced an abnormality based on the detection signal from the single overcurrent detection circuit. Furthermore, the overcurrent detection circuit is connected to the application circuit that applies the voltage to the charger, which makes it possible to detect an overcurrent occurring upstream of the charger and to detect a failure in the application circuit as an abnormality.

[0009] In the image forming apparatus of the present application, the control unit may be configured to issue a notification urging cleaning of the charger determined to be abnormal.

[0010] When abnormal discharge occurs due to the adhesion of dirt such as silica or paper powder, causing an overcurrent to flow, it is preferable to clean the charging wire, etc. The control unit identifies the charger in which the overcurrent has occurred among the multiple chargers by the applied voltage, and notifies the user to clean only the identified charger. This reduces the workload of the user.

[0011] In addition, in the image forming apparatus of the present application, the control unit may be configured to determine that the charger to which the maximum applied voltage is applied, among the multiple chargers, is abnormal when it detects the occurrence of an overcurrent a predetermined number of times or more within a predetermined period based on the detection signal of the overcurrent detection circuit.

[0012] When noise occurs in the overcurrent detection circuit or the transmission path of the detection signal, there is a possibility that the occurrence of an overcurrent may be erroneously detected based on the detection signal. Therefore, when the control unit detects the occurrence of an overcurrent a predetermined number of times or more, it identifies the charger in which an abnormality has occurred. This makes it possible to eliminate erroneous detection due to temporary noise.

[0013] In addition, in the image forming apparatus of the present application, the control unit may be configured to identify, among the multiple chargers, the charger to which the maximum applied voltage has been applied, based on the applied voltage a predetermined time before the detection signal was input from the overcurrent detection circuit.

[0014] At the moment an overcurrent occurs, the applied voltage may temporarily drop. Therefore, if an abnormal charger is determined based on the applied voltage at the time when the overcurrent occurs, the wrong charger may be determined to be abnormal. Therefore, the system determines which charger had the maximum applied voltage by looking at the applied voltage a predetermined time before the detection signal was input. This makes it possible to accurately identify the charger with an abnormality.

[0015] In addition, in the image forming apparatus of the present application, the control unit may be configured to execute an alert to prompt cleaning of all of the chargers when the detection signal is input from the overcurrent detection circuit and the applied voltage of the charger at which the applied voltage applied by the application circuit was maximum is equal to or lower than a predetermined threshold value.

[0016] When abnormal discharge occurs due to factors other than the adhesion of dirt to the charging wire, such as the adhesion of large foreign matter, circuit abnormality, or grid dirt, the applied voltage at the time of occurrence may be smaller than when dirt is attached to the charging wire. In this case, the applied voltage of a specific charger does not increase significantly compared to the applied voltage of other chargers. For this reason, it becomes difficult to identify the charger in which abnormal discharge has occurred. Therefore, when the maximum applied voltage is equal to or less than a predetermined threshold, the control unit notifies the cleaning of all chargers without identifying the charger. This makes it possible to remove large foreign matter attached to any of the multiple chargers. In addition, by encouraging the cleaning of all chargers, the user can be made aware of the possibility of a circuit abnormality.

[0017] In the image forming apparatus of the present application, each of the plurality of chargers may be a corona charger having a charging wire and a grid provided between the photoconductor and the charging wire, and the control unit may be configured to control the applied voltage applied from the application circuit to the charging wire so that a grid voltage of the grid becomes a predetermined voltage.

[0018] According to this, the control unit maintains the grid voltage at a predetermined voltage regardless of dirt attached to the charging wire. Therefore, when the amount of dirt attached to the charging wire increases and the resistance between the charging wire and the grid increases, the applied voltage is increased to maintain the grid voltage, which makes it easier for abnormal discharge to occur. Therefore, in an image forming apparatus that controls the applied voltage to maintain the grid voltage of such a corona charger constant, it is extremely effective to use a single overcurrent detection circuit connected to multiple chargers, since this reduces the number of overcurrent detection circuits and the number of input ports of the control unit. Furthermore, by providing an overcurrent detection circuit on the charging wire side, even if no abnormality occurs on the grid side and no overcurrent flows, it becomes possible to detect an abnormality if an overcurrent flows on the charging wire side and returns to the overcurrent detection circuit via the ground.

[0019] Incidentally, the contents of the present application are not limited to being implemented as an image forming apparatus, but may also be implemented as, for example, a control method for an image forming apparatus. Effect of the Invention

[0020] According to the image forming apparatus and the control method for the image forming apparatus of the present application, it is possible to detect an abnormality occurring on the upstream side of the charger. [Brief description of the drawings]

[0021] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of a color laser printer according to an embodiment. [Diagram 2] 4 is a simplified diagram of a corona charger as viewed from a radial direction of a photoconductor; [Diagram 3] 2 is a schematic circuit diagram of a high-voltage power supply device of a printer. [Figure 4] FIG. 2 is a circuit diagram of a charging voltage generating circuit and an overcurrent detecting circuit. [Diagram 5] 5 is a flowchart showing the contents of an abnormality determination process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] An embodiment in which the image forming apparatus of the present application is embodied in a laser printer will be described below with reference to the drawings. The laser printer 1 according to the present application shown in FIG. 1 is a color laser printer that forms a color image on a sheet P (printing paper, OHP, etc.) by electrophotography, and is a so-called tandem type laser printer that uses four color toners. In the following description, as shown in FIG. 1, the right side of the paper in the figure is defined as the front side of the laser printer 1, and the left side of the paper is defined as the rear side. Also, the upper side of the paper in FIG. 1 is defined as the upper side of the laser printer 1, and the lower side of the paper is defined as the lower side. Also, hereinafter, the laser printer 1 will be simply referred to as printer 1.

[0023] (1. Printer 1 Configuration) As shown in FIG. 1, the printer 1 has a substantially box-shaped device body 2, and a paper feed section 10, an image forming section 20, and the like are housed inside the device body 2. An ejection tray 5 is provided on the top surface of the device body 2 for storing sheets P on which images are formed in a stacked state. The device body 2 has a housing 3 that opens forward, and a cover 7 that is rotatably connected to the housing 3 via a hinge section 4 so as to cover the opening of the housing 3. An opening / closing detection sensor 8 for detecting the opening / closing state of the cover 7 is attached to the inner wall of the housing 3. An ASIC 61 (see FIG. 3) of the printer 1 described later is capable of detecting the opening / closing state of the cover 7 based on an opening / closing signal Sg (see FIG. 3) of the opening / closing detection sensor 8. For example, when the cover 7 is open, the opening / closing detection sensor 8 outputs a high-level opening / closing signal Sg to the ASIC 61. Also, when the cover 7 is closed, the opening / closing detection sensor 8 outputs a low-level opening / closing signal Sg to the ASIC 61. This allows the ASIC 61 to determine whether the cover 7 is open or closed by determining the signal level of the open / close signal Sg.

[0024] A display unit 6 is provided on the upper front side of the housing 3. The display unit 6 is, for example, a touch panel, and includes a liquid crystal panel, a light source such as an LED that emits light from the rear side of the liquid crystal panel, and a touch-sensitive film attached to the surface of the liquid crystal panel. An ASIC 61 (see FIG. 3) of the printer 1 displays various information related to the printer 1 on the display unit 6. The ASIC 61 also displays, for example, on the display unit 6 a print color corresponding to a corona charger 41 in which abnormal discharge has occurred in an abnormality determination process shown in FIG. 5, which will be described later (S37 in FIG. 5).

[0025] The paper feed unit 10 has a paper feed tray 11 in which sheets P are stored and various rollers, and drives the various rollers to feed the sheets P to the image forming unit 20. In addition, the paper feed tray 11 is configured to be detachable from the lower part of the device main body 2.

[0026] The image forming section 20 includes a transport unit 21, four process cartridges 30C, 30M, 30Y, and 30K, an exposure section 35, and a fixing section 48. The transport unit 21 is provided between the paper feed section 10 and the process cartridges 30C and the like in the vertical direction, and includes a transport belt 23 and four transfer rollers 25 and the like. The transport belt 23 is an endless belt formed in a circular shape, and is wound around a drive roller 27 located below the rear end side of the image forming section 20 and a driven roller 29 located below the front end side. The upper surface of the transport belt 23 extends substantially horizontally directly below the process cartridges 30C and the like, and abuts against the back surface of the sheet P supplied from the paper feed section 10. The drive roller 27 rotates the transport belt 23 in a predetermined direction. In addition, the conveying belt 23 becomes negatively charged when a transfer voltage is applied to each transfer roller 25, and attracts the sheet P to its upper surface by electrostatic force, and conveys the attracted sheet P along the conveying path R toward the discharge tray 5.

[0027] Each of the process cartridges 30C, 30M, 30Y, and 30K corresponds to four colors: cyan (C), magenta (M), yellow (Y), and black (K). Each of the process cartridges 30C, etc. contains toner of the corresponding color (C, M, Y, K). The four process cartridges 30C, etc. are provided in the order of the process cartridges 30Y, 30M, 30C, and 30K from the front to the rear of the printer 1. Each of the process cartridges 30C, 30M, 30Y, and 30K is held by a drawer 30D. The drawer 30D is drawn out to the front of the housing 3 with the cover 7 open. With the drawer 30D drawn out, each of the process cartridges 30C, 30M, 30Y, and 30K is replaceable.

[0028] The process cartridge 30C has a drum-shaped photoconductor 31, a corona charger 41, a toner cartridge 33, a cleaning roller 26, etc. The other process cartridges 30M, 30Y, and 30K have different toner colors, but other configurations are similar to the process cartridge 30C. Therefore, in the following description, the process cartridge 30C will be described as a representative, and descriptions of the other process cartridges 30M, 30Y, and 30K will be omitted as appropriate.

[0029] The photoconductor 31 is located above the transfer roller 25, and the conveyor belt 23 is sandwiched between the photoconductor 31 and the transfer roller 25 in the vertical direction. The corona charger 41 is, for example, a scorotron corona charger in which a charging wire 42 and a grid 43 are housed in a shield case 45. The charging wire 42 is made of metal, for example, gold-plated tungsten, or tungsten alone. The shield case 45 is formed in a substantially rectangular tube shape that is long along the rotation axis direction of the photoconductor 31. An opening is formed in the part of the shield case 45 facing the photoconductor 31. The grid 43 is formed by stretching a conductive wire in a mesh shape in the opening of the shield case 45. The charging wire 42 is stretched along the rotation axis direction of the photoconductor 31 in the shield case 45, and is arranged at a distance from the photoconductor 31 at an upper position on the rear side. Therefore, the grid 43 is arranged between the photoconductor 31 and the charging wire 42.

[0030] When forming an image, the corona charger 41 uniformly positively charges the surface of the photoconductor 31. Specifically, when a voltage is applied to the charging wire 42, an electric field is formed between the charging wire 42 and the photoconductor 31, causing a corona discharge. When an electric field is formed between the charging wire 42 and the grid 43, the voltage of the charging wire 42 is controlled so that the voltage of the grid 43 becomes a target value, thereby controlling the strength of the electric field and the amount of charge on the photoconductor 31.

[0031] The exposure unit 35 is provided at the top of the inside of the apparatus main body 2, and is a laser scanner that forms an electrostatic latent image based on image data on the charged surface of each photoconductor 31. The toner cartridge 33 supplies the toner to the surface of the photoconductor 31 by carrying the toner contained therein on the surface of the development roller 46. The development roller 46 rotates with a development voltage applied thereto, and thereby the toner in the toner cartridge 33 is positively charged by friction caused by the rotation, and is supplied onto the photoconductor 31. The photoconductor 31 forms a toner image by supplying the toner to the electrostatic latent image formed on the surface. The transport unit 21 transports the sheet P toward the fixing unit 48, while transferring the toner image developed on the surface of the photoconductor 31 to the sheet P by applying a transfer voltage to the transfer roller 25.

[0032] The cleaning roller 26 is provided in the vicinity of the rear side of the photoconductor 31. The cleaning roller 26 collects the toner carried on the surface of the photoconductor 31 by applying a cleaning voltage thereto.

[0033] The fixing unit 48 is provided downstream of the conveying path R compared to the conveying unit 21. The fixing unit 48 has a heating roller 49 and a pressure roller 50. The heating roller 49 is disposed on the image forming surface side of the sheet P, and rotates in synchronization with the conveying belt 23 and the like, and conveys the sheet P while heating the toner transferred to the sheet P. The pressure roller 50 pinches the sheet P between itself and the heating roller 49, and rotates in response to the pressure roller 50 while pressing the sheet P against the heating roller 49. As a result, the fixing unit 48 conveys the sheet P along the conveying path R while heating and melting the toner transferred to the sheet P to fix it to the sheet P.

[0034] (2. Configuration of Corona Charger 41) Next, the configuration of the corona charger 41 will be described. Fig. 2 is a simplified diagram of the corona charger 41 as viewed from the radial direction of the photoconductor 31. The corona charger 41 is configured to include a charging wire 42 and a grid 43 (see Fig. 1) in an elongated case 51 extending in the direction of the rotation axis of the photoconductor 31. Note that in Fig. 2, the grid 43 is omitted from illustration to avoid complicating the drawing.

[0035] The case 51 is open on the side facing the photoconductor 31, and a grid 43 is provided at the opening. A charging wire 42 is arranged inside the case 51 along the longitudinal direction of the case. Both ends of the charging wire 42 are fixed by a pair of fixing members 53. A charging terminal 54 is provided at one end of the case 51. The charging terminal 54 is connected to a high-voltage side output end T1 (see FIG. 4) of the charging voltage generating circuit 70, and a wire voltage CHG is applied to the charging terminal 54.

[0036] The corona charger 41 also includes a wire cleaner 55 for cleaning the charging wire 42 and a storage section 57 for storing the wire cleaner 55. The wire cleaner 55 is wound around the charging wire 42. The storage section 57 is provided on the charge terminal 54 side of the case 51. The wire cleaner 55 can move over the entire length of the charging wire 42 and to a position where it is stored in the storage section 57. During cleaning, for example, a user opens the cover 7 (see FIG. 1) and removes one of the process cartridges 30K to 30C from the printer 1, then picks up the wire cleaner 55, removes it from the storage section 57, and moves it while sliding it against the charging wire 42. This allows the wire cleaner 55 to remove any deposits on the charging wire 42. After cleaning, the user stores the wire cleaner 55 back in the storage section 57, and mounts the process cartridges 30K to 30C with the wire cleaner 55 stored in the storage section 57 in the printer 1.

[0037] (3. Configuration of High-Voltage Power Supply Device 60) Next, the electrical configuration of the printer 1 will be described with reference to Fig. 3. Fig. 3 shows a schematic block diagram of a high-voltage power supply device 60 mounted on a circuit board (not shown) built into the printer 1, and a connection configuration related to the high-voltage power supply device 60. In the following description, when each component is to be distinguished by color, a suffix such as Y (yellow), M (magenta), C (cyan), or K (black) or a suffix such as "1 to 4" (for example, grid voltage GRID1 to GRID4) is added to the reference numeral of each part, and when no distinction is made, the suffix is ​​omitted (for example, written as grid voltage GRID).

[0038] The high-voltage power supply device 60 has an ASIC (application specific integrated circuit) 61 constituting a control unit, a high-voltage power supply circuit 62 connected to the ASIC 61, a ROM 63, and a RAM 64. The ASIC 61 controls the high-voltage power supply circuit 62 and also controls the entire printer 1. The ROM 63 is a storage medium that stores various operation programs executed by the ASIC 61. The ROM 63 in this embodiment stores a program PG for executing an abnormality determination process shown in FIG. 5, which will be described later. The RAM 64 stores temporary data in various processes and image data used in printing processes.

[0039] The high-voltage power supply circuit 62 has a plurality of charging voltage generating circuits 70 and an overcurrent detection circuit 72 corresponding to each color. The high-voltage power supply circuit 62 further has a grid voltage adjustment circuit 81 equipped with a grid current detection circuit 82 constituting a control unit. Four charging voltage generating circuits 70 (charging voltage generating circuits 70K, 70Y, 70M, 70C) are provided corresponding to each color. Each of the charging voltage generating circuits 70K to 70C is connected to each of the ports PT1 to PT4 of the ASIC 61 in this order. Each of the charging voltage generating circuits 70K to 70C inputs PWM (Pulse Width Modulation) signals Sp1 to Sp4 from the ASIC 61 via the ports PT1 to PT4. Each of the charging voltage generating circuits 70K to 70C is also connected to a corona charger 41 (41K to 41C) that charges the photoconductor 31 of each color. Each of the four charging voltage generating circuits 70 applies a wire voltage CHG1-CHG4 to each corona charger 41. Each of the charging voltage generating circuits 70 controls the voltage values ​​of the wire voltages CHG1-CHG4 according to the duty ratio of the PWM signals Sp1-Sp4. For example, as the duty ratio of the PWM signal Sp1 is increased, the voltage value of the wire voltage CHG1 gradually increases. The detailed configuration of the charging voltage generating circuit 70 will be described later.

[0040] Moreover, grid voltage adjustment circuits 81K-81C and grid current detection circuits 82K-82C are provided corresponding to the respective corona chargers 41K-41C. The grid voltages GRID1-GRID4 applied to the grid 43 are adjusted by the respective grid voltage adjustment circuits 81K-81C. The wire voltage CHG is, for example, about 6 kV. The grid voltage GRID is, for example, about 700 V.

[0041] Each of the grid voltage adjustment circuits 81 includes a voltage detection circuit 83 and an operational amplifier OP1. Since the grid voltage adjustment circuits 81K to 81C have the same circuit configuration, the grid voltage adjustment circuit 81K corresponding to the color K (black) will be described below, and the description of the other grid voltage adjustment circuits 81Y to 81C will be omitted as appropriate. The voltage detection circuit 83K includes two voltage dividing resistors R7 and R8 connected in series. A divided current Id1 of the grid current Ig1 flowing through the grid 43K flows through the voltage dividing resistors R7 and R8. The voltage detection circuit 83K outputs a detection voltage Vgr1 corresponding to the grid voltage GRID1 applied to the grid 43K from the connection point of the two voltage dividing resistors R7 and R8 to the non-inverting input terminal of the operational amplifier OP1. The capacitor C3 of the voltage detection circuit 83K is connected in parallel with the voltage dividing resistor R8 to form an RC filter.

[0042] The ASIC 61 also inputs the detection voltage Vgr1 from the connection point of the voltage-dividing resistors R7 and R8 of the voltage detection circuit 83K to the port A / D2 as a voltage-dividing detection signal Sid1. The ASIC 61 calculates the divided current Id1 from the resistance value of the voltage-dividing resistor R8 and the voltage value of the voltage-dividing detection signal Sid1. The ASIC 61 also calculates the grid voltage GRID1 from the voltage-dividing ratio between the voltage value of the voltage-dividing detection signal Sid1 and the resistance values ​​of the voltage-dividing resistors R7 and R8.

[0043] The operational amplifier OP1 performs feedback control via a voltage control line Ln1 so that the detection voltage Vgr1 input from the voltage detection circuit 83K becomes the reference voltage Vth. The reference voltage Vth is, for example, a voltage value obtained by dividing a power supply voltage Vcc of 5V by voltage dividing resistors R9 and R10, and is input to the inverting input terminal of the operational amplifier OP1. The output terminal and the inverting input terminal of the operational amplifier OP1 are connected via a resistor R6 and a capacitor C2. The capacitor C4 of the grid voltage adjustment circuit 81K is connected in parallel with the voltage dividing resistor R10 to form an RC filter.

[0044] A smoothing circuit including a voltage dividing resistor R4 and a capacitor C2 is connected to the output terminal of the operational amplifier OP1. The connection point of the voltage dividing resistor R4 on the grid 43K side (opposite to the output terminal of the operational amplifier OP1) is grounded via the capacitor C2. The base of a transistor Q1 for making the grid voltage GRID1 a constant voltage is connected to the connection point of the voltage dividing resistor R4 on the grid 43K side. The transistor Q1 is connected to a voltage control line Ln1 connected to a connection point between the voltage dividing resistor R7 and the grid 43K. The transistor Q1 is, for example, an NPN transistor, and has a collector connected to the connection point (voltage control line Ln1) on the grid 43K side and an emitter connected to the grid current detection circuit 82K (resistor R3). The transistor Q1 changes the voltage between the collector and emitter by controlling the base current by the operational amplifier OP1. This adjusts the grid voltage GRID. The transistor Q1 is not limited to a bipolar transistor, and may be, for example, a FET (field effect transistor).

[0045] For example, in feedback control, the operational amplifier OP1 varies the base voltage so that the detection voltage Vgr1 becomes the reference voltage Vth, adjusts the grid voltage GRID, and makes the grid voltage GRID1 a constant voltage at a predetermined voltage. Note that a capacitor C1 to which the grid voltage GRID is charged is connected between the voltage control line Ln1 and the grid 43K. The above-described circuit configuration of the grid voltage adjustment circuit 81K is only an example. For example, the inverting input terminal of the operational amplifier OP1 may be connected to the ASIC61, and the ASIC61 may control the reference voltage Vth of the operational amplifier OP1 to make the grid voltage GRID1 a constant voltage.

[0046] In addition, a grid current detection circuit 82K is connected to the voltage control line Ln1. The grid current detection circuit 82K detects a line current Ir1 corresponding to a grid current Ig1 flowing through the grid 43K. The grid current detection circuit 82K is configured, for example, with a resistor R3 connected between the emitter of the transistor Q1 and the ground. The grid current detection circuit 82K supplies the positive terminal voltage of the resistor R3 to the port A / D3 of the ASIC 61 as a line voltage detection signal Sir1.

[0047] For example, the ASIC 61 calculates the current value of the line current Ir1 from the resistance value of the resistor R3 and the voltage value of the line voltage detection signal Sir1. The ASIC 61 also calculates the current value of the grid current Ig1 by adding the current value of the shunt current Id1 to the current value of the line current Ir1. The ASIC 61 controls the voltage value of the wire voltage CHG1 output from the charging voltage generation circuit 70 based on the current value of the grid current Ig1. Similarly, the ASIC 61 controls the voltage values ​​of the wire voltages CHG2 to CHG4 of the charging voltage generation circuits 70Y, 70M, and 70C based on the grid currents Ig2 to Ig4 (line voltage detection signals Sir2 to Sir4) of the other colors.

[0048] The ASIC 61 controls the charging voltage generating circuits 70K to 70C based on the calculated current value of the grid current Ig, and makes the current value of the grid current Ig coincide with a desired target current value. For example, the ASIC 61 controls the charging voltage generating circuit 70 so that the grid current Ig corresponding to any one of the four corona chargers 41K to 41C becomes a predetermined current value. In this embodiment, the ASIC 61 performs constant current control of the charging voltage generating circuit 70 so that the grid current Ig corresponding to the corona charger 41 that detects the smallest line current Ir among the four corona chargers 41 becomes a predetermined current value, for example, 250 μA. The ASIC 61 changes the duty ratio of the PWM signals Sp1 to Sp4 output from the ports PT1 to PT4, and makes the current value of the grid current Ig coincide with a desired target current value. The above-mentioned method of controlling the charging voltage generating circuit 70 based on the grid current Ig is an example. For example, the ASIC 61 may appropriately select the charging voltage generating circuit 70 (corona charger 41) that performs constant current control depending on the usage status of the printer 1. Alternatively, the ASIC 61 may individually control the current value of the grid current Ig for each charging voltage generating circuit 70 so that it coincides with a desired target current value.

[0049] (4. Configuration of the Charge Voltage Generation Circuit 70) Next, the circuit configuration of the charge voltage generating circuit 70 will be described. The four charge voltage generating circuits 70K to 70C of this embodiment have the same circuit configuration, although they may differ in circuit constants (resistance value, electrostatic capacitance value, transformer winding ratio, etc.) and output capabilities. Therefore, in the following description, the charge voltage generating circuit 70K corresponding to black (K) will be mainly described, and detailed descriptions of the other charge voltage generating circuits 70Y, 70M, and 70C will be omitted.

[0050] 4, the charging voltage generating circuit 70K has a charging voltage output circuit 71K and a charging voltage detection circuit 78. The charging voltage output circuit 71K has a transformer driving circuit 73 and a boosting circuit 74. The charging voltage output circuit 71K generates a wire voltage CHG1 to be applied to the charging wire 42K of the corona charger 41K (see FIG. 3).

[0051] More specifically, the boost circuit 74 has a transformer 75, a rectifier diode 76, and a smoothing capacitor 77. One end of a primary winding 75a of the transformer 75 is connected to a DC power supply (for example, 24 V), and the other end is connected to a transformer driving circuit 73. The transformer driving circuit 73 inputs a PWM signal Sp1 from a port PT1 (see FIG. 3) of the ASIC 61, and smoothes the input PWM signal Sp1 with a smoothing circuit (not shown). The transformer driving circuit 73 supplies the smoothed PWM signal Sp1 to a driving transistor (not shown) connected to the primary winding 75a. The transformer driving circuit 73 controls an oscillation current flowing through the primary winding 75a of the transformer 75 by turning on and off the driving transistor. The transformer 75 changes the voltage value of an output voltage (wire voltage CHG1) output from a secondary winding 75b according to the duty ratio of the oscillation current. The boost circuit 74 boosts the voltage generated in the primary winding 75a of the transformer 75, outputs it from the secondary winding 75b, and rectifies and smoothes it using a rectifier diode 76 and a smoothing capacitor 77 to output it as a wire voltage CHG1. For example, the larger the duty ratio of the PWM signal Sp1, the larger the voltage value of the wire voltage CHG1 generated by the transformer 75.

[0052] The wire voltage CHG1 is applied to the charging wire 42K of the corona charger 41K from the boost circuit 74 via the high-voltage output terminal T1 of the boost circuit 74 and the power line L1. By applying the wire voltage CHG1 to the charging wire 42K, a corona discharge is generated between the charging wire 42K and the photoconductor 31K. As a result of this corona discharge, a grid voltage GRID1 is generated on the grid 43K.

[0053] Further, the four charging voltage generating circuits 70K to 70C in this embodiment are connected to a single overcurrent detection circuit 72. The "single overcurrent detection circuit 72" here means that, for example, the overcurrent detection circuit 72 is provided as a common circuit connected to the four charging voltage generating circuits 70K to 70C (corona chargers 41K to 41C), and the signal path of the overcurrent detection signal Sv output from the overcurrent detection circuit 72 to the ASIC 61 is unified into one, as described later.

[0054] The overcurrent detection circuit 72 is connected to the low-voltage side output end T2 of the boost circuit 74. The low-voltage side output end T2 is connected to the low-voltage side of the secondary winding 75b of the transformer 75. A detection resistor 92 and a capacitor 93 are connected in parallel between the low-voltage side output end T2 and ground. The detection resistor 92 and the capacitor 93 smooth the overcurrent returning via the ground. The positive terminals of the detection resistor 92 and the capacitor 93 are connected to the overcurrent detection circuit 72.

[0055] The overcurrent detection circuit 72 detects the occurrence of abnormal discharge based on the overcurrent that instantaneously flows through the grid 43 or ground due to abnormal discharge in each of the four corona chargers 41. Note that abnormal discharge is, for example, different from corona discharge and is a spark discharge or arc discharge caused by contamination of the charging wire due to the adhesion of silica or paper powder.

[0056] In normal operation, when a wire voltage CHG1 is applied to the corona charger 41K from the high-voltage side of the secondary winding 75b of the transformer 75, a grid current Ig1 (see FIG. 3) flowing through the grid 43K flows to ground, for example, via a grid current detection circuit 82K (see FIG. 3). In addition, when an abnormal discharge occurs in the corona charger 41K, a part of the overcurrent flows, for example, from the grid current detection circuit 82K to the detection resistor 92 or capacitor 93 via ground.

[0057] The overcurrent detection circuit 72 includes a Zener diode 91, a transistor Tr1, resistors 94 and 95, a capacitor 96, a detection resistor 101, a resistor 103, and a capacitor 104. The positive terminals of the detection resistor 92 and the capacitor 93 are connected to the anode of the Zener diode 91. The overcurrent detection circuit 72 receives the smoothed overcurrent from the detection resistor 92 and the capacitor 93 provided in each of the charging voltage generating circuits 70K to 70C. When the overcurrent returning through the ground reaches or exceeds a predetermined current value, the overcurrent detection circuit 72 outputs an overcurrent detection signal Sv. Therefore, when an abnormal discharge occurs in at least one of the four corona chargers 41, the overcurrent detection circuit 72 outputs an overcurrent detection signal Sv.

[0058] More specifically, the transistor Tr1 is, for example, a PNP transistor, and has an emitter connected to a DC power supply Vcc1. The voltage of the DC power supply Vcc1 is, for example, 5.0 V. The base of the transistor Tr1 is connected to the cathode of a Zener diode 91 via a resistor 94. In addition, a resistor 95 is connected between the emitter of the transistor Tr1 and the base terminal of the resistor 94. Similarly, a capacitor 96 is connected between the emitter of the transistor Tr1 and the base terminal of the resistor 94.

[0059] For example, when a spark discharge occurs between the charged wire 42K and the grid 43K, the overcurrent flowing through the grid 43K fluctuates intermittently and greatly. A part of the overcurrent flows through the detection resistor 92 via the ground, causing the voltage at the positive terminal of the detection resistor 92 to drop. When the voltage at the positive terminal of the detection resistor 92 drops to the Zener voltage (breakdown voltage) of the Zener diode 91, a base current suddenly flows through the base of the transistor Tr1. When an overcurrent of a predetermined current value or more occurs, the transistor Tr1 turns on. When the transistor Tr1 turns on, a current flows between the emitter and collector of the transistor Tr1, and an overcurrent detection signal Sv according to the voltage value of the DC power supply Vcc1 is output from the collector.

[0060] The collector of the transistor Tr1 is connected to the port A / D1 (see FIG. 3) of the ASIC 61 via the detection resistor 101. Therefore, although the charge voltage generation circuits 70K to 70C are provided corresponding to each color, the overcurrent detection circuit 72 connected to the charge voltage generation circuits 70K to 70C is shared, and the signal path for outputting the overcurrent detection signal Sv from the overcurrent detection circuit 72 is shared.

[0061] A resistor 103 and a capacitor 104 are connected to the terminal of the detection resistor 101 on the ASIC 61 side. One end of each of the resistor 103 and the capacitor 104 is connected to the detection resistor 101, and the other end is grounded. The resistor 103 and the capacitor 104 smooth the overcurrent detection signal Sv detected by the detection resistor 101 and output it to the port A / D1. Therefore, the ASIC 61 can detect the occurrence of an overcurrent based on the overcurrent detection signal Sv being input to the port A / D1. For example, when the ASIC 61 detects an overcurrent, the ASIC 61 notifies the user of the detection and performs a process such as prompting the user to clean the charging wire 42. The circuit configurations shown in FIGS. 3 and 4 are merely examples. For example, an overcurrent detection circuit 72 may be provided that is connected to at least two of the four corona chargers 41 and outputs the overcurrent detection signal Sv from one signal path. Specifically, for example, the overcurrent detection circuit 72 connected to the black charging voltage generation circuit 70K and the overcurrent detection circuit 72 connected to the remaining three color charging voltage generation circuits 70Y, 70M, and 70C may be provided separately. Also, for example, the overcurrent detection circuit 72 does not need to include the capacitor 96. Also, the transistor Tr1 is not limited to a bipolar transistor, and may be, for example, a FET (field effect transistor).

[0062] The charge voltage detection circuit 78 is a circuit that detects the wire voltage CHG1, and includes, for example, a voltage dividing resistor connected to the power line L1, and outputs a wire voltage detection signal Vs1 corresponding to the magnitude of the wire voltage CHG1 to a port PT5 of the ASIC 61. Similarly, the charge voltage detection circuit 78 provided in each charge voltage generation circuit 70 outputs wire voltage detection signals Vs2, Vs3, and Vs4 corresponding to the magnitudes of the wire voltages CHG2 to CHG4 to the ports PT6, PT7, and PT8 of the ASIC 61. The ASIC 61 detects the wire voltages CHG1 to CHG4 based on the wire voltage detection signals Vs1 to Vs4.

[0063] (5. Abnormality Judgment Processing) Next, the abnormality determination process executed by the printer 1 of this embodiment will be described with reference to FIG. 5. For example, when the ASIC 61 starts generating the wire voltage CHG by the charging voltage generating circuit 70, the ASIC 61 starts the abnormality determination process shown in FIG. 5. For example, when the ASIC 61 accepts a print instruction and starts the charging voltage generating circuit 70 to start generating the wire voltage CHG, the ASIC 61 starts the abnormality determination process. Alternatively, when the ASIC 61 starts the charging voltage generating circuit 70 to start generating the wire voltage CHG when the printer 1 is turned on and a warm-up operation is performed, the ASIC 61 starts the abnormality determination process. The ASIC 61 executes the abnormality determination process by reading and executing the program PG from the ROM 63. In the following description, as an example, a case will be described in which the ASIC 61 accepts a print instruction and generates the wire voltages CHG1 to CHG4 in all of the four charging voltage generating circuits 70K to 70C. The ASIC 61 may execute the abnormality determination process when at least two of the four charging voltage generating circuits 70K to 70C generate the wire voltages CHG. In this case, in S39 described later, the ASIC 61 may notify the cleaning of only the corona charger 41 that generates the wire voltage CHG.

[0064] For example, after receiving a print instruction and starting generation of the wire voltage CHG by the charging voltage generating circuit 70, the ASIC 61 executes the received print instruction when the grid current Ig and the grid voltage GRID reach their desired target values. Each of the wire voltages CHG1 to CHG4 varies according to the current value of each of the grid currents Ig1 to Ig4.

[0065] When the ASIC 61 starts the abnormality determination process of FIG. 5, in step (hereinafter simply referred to as "S") 11, the ASIC 61 starts storing the voltage values ​​of the fluctuating wire voltages CHG1-CHG4. The ASIC 61 calculates the wire voltages CHG1-CHG4 based on the wire voltage detection signal Vs of the charged voltage detection circuit 78. The ASIC 61 stores the calculated voltage values ​​of the wire voltages CHG1-CHG4 in, for example, the RAM 64. Note that the ASIC 61 may calculate the wire voltage CHG based on, for example, the voltage division detection signal Sid (grid voltage GRID). In this case, the printer 1 may not be provided with the charged voltage detection circuit 78.

[0066] In addition, in S11, the ASIC 61 sets a value of zero in a counter CNT. This counter CNT is a counter value for storing the number of times the overcurrent detection signal Sv has been input, i.e., the number of times an overcurrent has been detected, and is stored in, for example, the RAM 64.

[0067] After executing S11, the ASIC 61 judges whether or not to stop generating the wire voltage CHG (S13). For example, when the ASIC 61 completes printing based on the accepted print instruction, the ASIC 61 makes a positive judgment in S13 (S13: YES), stops the PWM signal Sp, and stops the charging voltage generating circuit 70 (S15). The ASIC 61 stops the operation of all the charging voltage generating circuits 70K to 70C, and stops the generation of all the wire voltages CHG. After executing S15, the ASIC 61 ends the storage process of the wire voltage CHG started in S11 (S17), and ends the process shown in FIG. 5. If the ASIC 61 wants to start generating the wire voltage CHG again, it executes the process of FIG. 5.

[0068] On the other hand, if the ASIC 61 determines in S13 that the result is negative (S13: NO), the ASIC 61 executes a process of waiting for a first time TM1 (S19). This first time TM1 is a period for determining the occurrence of an overcurrent based on the overcurrent detection signal Sv, as described later. The first time TM1 is, for example, 5 ms.

[0069] After waiting for the first time TM1 in S19, the ASIC 61 judges whether or not the second time TM2 or more has elapsed since the last count-up of the counter CNT (S21). If the second time TM2 or more has not elapsed since the last count-up (S21: NO), the ASIC 61 judges whether or not the overcurrent detection signal Sv has been input (S23). If the second time TM2 or more has elapsed since the last count-up (S21: YES), the ASIC 61 resets the counter CNT to zero (S25) and executes S23. As described later, the ASIC 61 counts up the counter CNT when the overcurrent detection signal Sv is input. Therefore, if the overcurrent detection signal Sv is not input for the second time TM2 after the last input of the overcurrent detection signal Sv, the ASIC 61 resets the counter CNT. The second time TM2 is, for example, 50 ms. In addition, in S25, together with resetting the counter CNT, the ASIC 61 may delete the data of the wire voltage CHG stored in the RAM 64 up to that point. Also, the ASIC 61 may not reset the counter CNT based on the absence of input of the overcurrent detection signal Sv.

[0070] Furthermore, in S23, the ASIC 61 judges whether or not an overcurrent detection signal Sv has been input after the previous execution of S23, i.e., whether or not an overcurrent has occurred. For example, if the ASIC 61 detects the input of the overcurrent detection signal Sv at least once between the previous execution of S23 and the next execution of S23, the ASIC 61 makes a positive judgment in S23 (S23: YES). In other words, even if the ASIC 61 detects the input of the overcurrent detection signal Sv multiple times since the previous execution of S23, it processes it as a single detection. The detection of the input of the overcurrent detection signal Sv here refers to, for example, the detection of the rising edge of the overcurrent detection signal Sv increasing from the ground level.

[0071] When the overcurrent detection signal Sv is input (S23: YES), the ASIC 61 counts up the counter CNT by one (S26) and executes S27. When the overcurrent detection signal Sv is not input (S23: NO), the ASIC 61 executes the process from S13 again. Therefore, the ASIC 61 determines whether the overcurrent detection signal Sv is input every first time TM1 until the generation of the wire voltage CHG is stopped.

[0072] In S27, the ASIC 61 judges whether the counter CNT is equal to the number threshold value TH1. If the counter CNT is equal to the number threshold value TH1 (S27: YES), the ASIC 61 stops the operation of the charging voltage generating circuit 70, stops the generation of all wire voltages CHG (S29), and executes the process from S31 onwards. In the process from S31 onwards, the ASIC 61 executes the process of identifying the corona charger 41 in which abnormal discharge has occurred. If the counter CNT is not equal to the number threshold value TH1 (S27: NO), the ASIC 61 executes the process from S13 again.

[0073] Therefore, the ASIC 61 judges the input of the overcurrent detection signal Sv every first time TM1, and judges the input of the overcurrent detection signal Sv until it detects the input of the overcurrent detection signal Sv the number of times equal to the number threshold TH1. Then, when the ASIC 61 detects the input of the overcurrent detection signal Sv the number of times equal to the number threshold TH1, it executes a process to identify the corona charger 41 in which abnormal discharge has occurred. The number threshold TH1 is, for example, 3. Note that the number threshold TH1 may be a number other than 3 times. Furthermore, the ASIC 61 may execute the process from S29 onwards when the overcurrent detection signal Sv is input once.

[0074] In S31, the ASIC 61 ends the process of storing the wire voltage CHG started in S11 (S31). In addition, in S31, the ASIC 61 executes, for example, a process of stopping the print process being executed, a process of notifying the PC that sent the print job that the print process has been stopped, and the like. After executing S31, the ASIC 61 executes S33. In S33, the ASIC 61 reads out from the RAM 64 the value of the wire voltage CHG that was stored before the overcurrent detection signal Sv was input, that is, before the abnormal discharge occurred. Here, at the moment the overcurrent occurs, the wire voltage CHG may temporarily drop. Therefore, if the wire voltage CHG at the time when the overcurrent occurs is determined, the wrong corona charger 41 may be determined to be abnormal. Therefore, the ASIC 61 executes the abnormality determination for the wire voltage CHG from the third time TM3 before the input of the overcurrent detection signal Sv.

[0075] As described above, when the overcurrent detection signal Sv is input the number of times equal to the frequency threshold TH1 (e.g., three times), the ASIC 61 executes the process from S29 onwards. For example, the ASIC 61 reads out the wire voltage CHG from the last input of the overcurrent detection signal Sv among the multiple inputs of the overcurrent detection signal Sv, that is, the third time TM3 before the time when the abnormal discharge was last detected. The third time TM3 is, for example, 120 ms. The ASIC 61 determines whether the voltage value of the wire voltage CHG read out in S33 is equal to or greater than the voltage threshold TH2 (S35).

[0076] Incidentally, the ASIC 61 may not use the time when the overcurrent detection signal Sv was last input as a reference. For example, the ASIC 61 may read the wire voltage CHG from the time when the overcurrent detection signal Sv was first inputted to the third time TM3 before. Alternatively, the ASIC 61 may read the wire voltage CHG before all three overcurrent detection signals Sv are inputted and compare it with the voltage threshold TH2. In this case, in S37 described later, the ASIC 61 may specify the corona charger 41 with the largest wire voltage CHG among the wire voltages CHG before all three overcurrent detection signals Sv are inputted as the corona charger 41 in which an abnormality has occurred. Also, the ASIC 61 may determine the voltage threshold TH2 based on the wire voltage CHG at the time when the overcurrent detection signal Sv is inputted, without going back to the third time TM3.

[0077] When dirt such as silica adheres to the charged wire 42 and an abnormal discharge occurs, the resistance between the charged wire 42 and the grid 43 increases, and the wire voltage CHG increases, causing an abnormal discharge. On the other hand, when dirt adheres to the grid 43 or when conductive foreign matter adheres to the charged wire 42, the insulation distance at a specific point between the charged wire 42 and the grid 43 temporarily shortens, causing an abnormal discharge, and there is a possibility that the resistance between the charged wire 42 and the grid 43 does not increase. Furthermore, when some kind of failure occurs in the charged voltage generating circuit 70 or the like, there is a possibility that the resistance between the charged wire 42 and the grid 43 does not increase. In the case of dirt on the grid 43, adhesion of conductive foreign matter, circuit failure, or the like, the wire voltage CHG before the occurrence of an abnormal discharge is likely to be smaller than when dirt adheres to the charged wire 42.

[0078] Therefore, in S35, the ASIC 61 judges whether or not the voltage value of the wire voltage CHG before the abnormal discharge read in S33 is equal to or greater than the voltage threshold value TH2. The voltage threshold value TH2 is a value that can distinguish between the wire voltage CHG before the abnormal discharge caused by, for example, the adhesion of dirt to the charged wire 42 and the wire voltage CHG before the abnormal discharge caused by, for example, the dirt on the grid 43. The voltage threshold value TH2 is, for example, 6600 V, which is a value that is increased by about 10% compared to the discharge voltage value (about 6000 V) of the corona discharge in a clean state in which the charged wire 42 and the grid 43 are free of dirt or conductive foreign matter. This allows the ASIC 61 to judge whether or not the abnormal discharge has occurred due to the adhesion of dirt to the grid 43. Note that the voltage threshold value TH2 may be changed according to, for example, the distance between the charged wire 42 and the grid 43.

[0079] In S35, for example, when the voltage value of at least one of the wire voltages CHG corresponding to each corona charger 41 read out in S33 is equal to or greater than the voltage threshold value TH2, the ASIC 61 makes an affirmative determination (S35: YES) and executes S37. As described above, the wire voltage CHG of a corona charger 41 in which the resistance has increased due to the adhesion of dirt to the charging wire 42 is more likely to increase than the wire voltage CHG of a corona charger 41 in which little dirt adheres to the charging wire 42. Therefore, in S37, the ASIC 61 identifies the corona charger 41 corresponding to the wire voltages CHG1 to CH4 with the maximum voltage value out of the wire voltages CHG1 to CH4 read out in S33 as the corona charger 41 in which an abnormality has occurred.

[0080] The ASIC 61, for example, displays the print color of the identified corona charger 41 as the print color of abnormal discharge on the display unit 6 (see FIG. 1) and executes a notification (S37). The ASIC 61 displays, for example, a message indicating that abnormal discharge has been detected or a message urging cleaning of the charging wire 42 of the identified print color on the display unit 6. After executing S37, the ASIC 61 ends the process shown in FIG.

[0081] The user removes the process cartridge 30 of the printing color displayed on the display unit 6 and operates the wire cleaner 55 (see FIG. 2) to clean the charged wire 42. This allows the charged wire 42 in which abnormal discharge has occurred to be cleaned. The method of notifying the identified corona charger 41 is not limited to the above-mentioned method of displaying on the display unit 6. For example, when the ASIC 61 has received a print job from a PC via a LAN interface or the like, the ASIC 61 may display a message urging the user to clean the charged wire 42 on the PC that sent the print job. The ASIC 61 may also notify the user of an abnormality by a warning lamp or buzzer without using the display unit 6.

[0082] In addition, the ASIC 61 does not have to execute the notification based on the abnormal discharge. For example, the printer 1 may be provided with a mechanism for moving the wire cleaner 55. The ASIC 61 may then automatically execute cleaning of the charging wire 42 of the corona charger 41 identified in S37 by moving the wire cleaner 55.

[0083] On the other hand, in S35, for example, when all the voltage values ​​of the wire voltages CHG corresponding to the corona chargers 41 read in S33 are less than the voltage threshold value TH2, in other words, when the voltage value of the maximum wire voltage CHG is less than the voltage threshold value TH2, the ASIC 61 makes a negative determination (S35: NO) and executes S39. In this case, as described above, abnormal discharge occurs due to a factor other than the dirt on the charging wire 42, such as dirt on the grid 43, and only the wire voltage CHG of a specific corona charger 41 does not increase significantly. Therefore, the ASIC 61 notifies the cleaning of all colors (S39). The ASIC 61 displays, for example, a message on the display unit 6 to prompt the user to clean all the process cartridges 30K to 30C of the four colors. This makes it possible to prompt the user to clean and inspect the charging wires 42 and grids 43 of all the corona chargers 41K to 41C. In addition to cleaning the charging wire 42, the user can be prompted to clean the grid 43, remove conductive foreign matter, and check for circuit failures. After executing S39, the ASIC 61 ends the processing shown in FIG.

[0084] The conditions for canceling the error process executed based on the detection of abnormal discharge, such as the notification process executed in S37 or S39 or the printing stop process executed in S31, are not particularly limited. For example, the ASIC 61 may cancel the error process on the condition that the wire voltage CHG is equal to or lower than a predetermined threshold value when the wire voltage CHG is generated next time after detecting the reattachment of the process cartridges 30K to 30C including the corona charger 41. As described above, when the user cleans the charging wire 42 or the like, the cover 7 is opened and closed or the process cartridge 30 is removed. Therefore, for example, after detecting the opening and closing of the cover 7 based on the opening and closing signal Sg of the opening and closing detection sensor 8 (see FIG. 1), the ASIC 61 performs a warm-up operation to increase the wire voltage CHG to the voltage value during printing, and if no abnormal discharge occurs and the wire voltage CHG is equal to or lower than a predetermined threshold value, the display prompting cleaning displayed on the display unit 6 is cancelled. The predetermined threshold value here is, for example, the voltage value (such as 6000V) of the corona discharge generated during normal printing.

[0085] Alternatively, when the ASIC 61 makes a positive determination in S35 (S35: YES), the ASIC 61 may use the voltage value of the wire voltage CHG determined in S35, i.e., the voltage value of the wire voltage CHG during abnormal discharge, as the threshold value. When the charged wire 42 is cleaned, the wire voltage CHG during printing operation decreases compared to when abnormal discharge occurs. Therefore, the ASIC 61 sets the voltage value of the wire voltage CHG that has increased due to contamination of the charged wire 42 as the threshold value, thereby making it possible to appropriately determine whether or not the charged wire 42 has been cleaned, i.e., whether or not error processing may be canceled, based on the decrease in the wire voltage CHG.

[0086] Furthermore, the ASIC 61 does not need to execute the determination of the wire voltage CHG or the cancellation of the error processing using the opening and closing of the cover 7 as a trigger. For example, a sensor may be provided to detect the attachment and detachment of the process cartridge 30, and when the ASIC 61 detects the removal of the process cartridge 30, it may determine the wire voltage CHG during the subsequent warming operation and cancel the error processing.

[0087] Incidentally, the printer 1 is an example of an image forming apparatus. The corona charger 41 is an example of a charger. The charging voltage generating circuit 70 is an example of an application circuit. The wire voltage CHG is an example of an application voltage. The overcurrent detection signal Sv is an example of a detection signal.

[0088] (6. Effects) According to the above-described embodiment, the following effects are achieved. (1) The overcurrent detection circuit 72 of the printer 1 in this embodiment is connected to the multiple charging voltage generation circuits 70K-70C, and when an overcurrent is detected in any one of the multiple charging voltage generation circuits 70K-70C, it outputs an overcurrent detection signal Sv to the ASIC 61. When the overcurrent detection signal Sv is input from the overcurrent detection circuit 72 to the ASIC 61 of the printer 1 (S27: YES), the ASIC 61 of the printer 1 determines that the corona charger 41 with the highest wire voltage CHG among the four corona chargers 41 is abnormal (S37).

[0089] According to this, a single overcurrent detection circuit 72 can be provided for the four charging voltage generation circuits 70K to 70C, and it is possible to determine which corona charger 41 has an abnormality by the overcurrent detection signal Sv of the single overcurrent detection circuit 72. Furthermore, by connecting the overcurrent detection circuit 72 to the charging voltage generation circuit 70 that applies the wire voltage CHG to the corona charger 41, it is possible to detect an overcurrent that has occurred upstream of the corona charger 41, and a failure or the like of the charging voltage generation circuit 70 can be detected as an abnormality.

[0090] (2) In addition, the ASIC 61 issues a notification to encourage the user to clean the corona charger 41 determined to be abnormal (S37). This identifies the corona charger 41 in which an overcurrent has occurred among the four corona chargers 41, and notifies the user to clean only the identified corona charger 41, thereby reducing the user's workload.

[0091] (3) Furthermore, when the ASIC 61 detects an occurrence of an overcurrent equal to or greater than the frequency threshold TH1 within the second time TM2 (S27: YES), it determines that the corona charger 41 to which the maximum wire voltage CHG is applied is abnormal (S37). If noise occurs in the overcurrent detection circuit 72 or the like, there is a possibility that the occurrence of an overcurrent may be erroneously detected based on the overcurrent detection signal Sv. Therefore, when the ASIC 61 detects an occurrence of an overcurrent equal to or greater than the frequency threshold TH1, it determines that the corona charger 41 is abnormal. This makes it possible to eliminate erroneous detection due to temporary noise.

[0092] (4) In addition, the ASIC 61 identifies the corona charger 41 to which the maximum wire voltage CHG is applied based on the wire voltage CHG three hours before the time when the overcurrent detection signal Sv is input from the overcurrent detection circuit 72 (S33, S37). This makes it possible to accurately detect the corona charger 41 having an abnormality.

[0093] (5) Furthermore, when the maximum wire voltage CHG is less than the voltage threshold value TH2, the ASIC 61 prompts the user to clean all of the corona chargers 41 (S39). As a result, even if an abnormal discharge occurs due to a cause other than dirt on the charging wire 42 and it is not possible to identify the corona charger 41 in which the abnormal discharge occurred, the user can be prompted to clean all of the corona chargers 41 and remove conductive foreign matter, etc.

[0094] (6) Furthermore, the ASIC 61 and the grid voltage adjustment circuit 81 control the wire voltages CHG1-CHG4 so that the grid voltages GRID1-GRID4 become predetermined voltages. In the printer 1 that controls the wire voltage CHG so as to maintain the grid voltage GRID constant, the overcurrent detection circuits 72 connected to the four corona chargers 41 can be consolidated, and the port A / D1 of the ASIC 61 can be reduced.

[0095] (7. Others) Incidentally, the present application is not limited to the above-described embodiment, and it goes without saying that various improvements and modifications are possible without departing from the spirit and scope of the present invention. For example, in the above embodiment, a single overcurrent detection circuit 72 is connected to the four charge voltage generation circuits 70K to 70C, but this is not limiting. For example, a single overcurrent detection circuit 72 may be connected to at least two charge voltage generation circuits 70, and the overcurrent detection signal Sv may be output through a single signal path. In the above embodiment, the ASIC 61 identifies only the corona charger 41 with the maximum voltage value as the corona charger 41 in which an abnormality has occurred in S37. However, the ASIC 61 may identify, for example, all corona chargers 41 with a voltage value equal to or greater than a predetermined threshold value as corona chargers 41 in which an abnormality has occurred. This predetermined threshold value is, for example, a value that serves as a reference for abnormal discharge, such as a voltage threshold value TH2. Therefore, the ASIC 61 may determine that only the corona charger 41 to which the maximum wire voltage CHG was applied is abnormal, or may determine that multiple corona chargers 41 including that corona charger 41 are abnormal. In the above embodiment, the image forming apparatus of the present application is described as an example of application to an image forming apparatus using a positively charged toner, but the present application is not limited thereto. The image forming apparatus of the present application may be an image forming apparatus using a negatively charged toner. In this case, the polarity of the charging voltage of the photoconductor 31 may be changed according to the charging polarity of the toner, and the magnitude (absolute value) may have the same relationship as in the above embodiment. In addition, the ASIC 61 may select the wire voltage CHG having the maximum voltage value on the negative side as the maximum wire voltage CHG. Therefore, the "maximum applied voltage" in the present application may be the maximum value of the wire voltage CHG in absolute value.

[0096] In the above embodiment, the scorotron corona charger 41 is used as the charger of the present application, but the present application is not limited to this. For example, the charger of the present application may be a corotron charger that does not have a grid 43. The charger of the present application may also be a charging roller that charges the photoconductor 31 without discharging. Therefore, the ASIC 61 may determine an abnormality in the charging roller based on an overcurrent that occurs due to a voltage abnormality in the charging roller, regardless of abnormal discharge. In addition, the ASIC 61 calculates the wire voltages CHG1 to CHG4 based on the wire voltage detection signals Vs1 to Vs4 of the charging voltage detection circuit 78, and identifies the corona charger 41 with the maximum wire voltage CHG, but this is not limited thereto. The ASIC 61 may detect the wire voltage CHG based on the PWM signal Sp output to the charging voltage generation circuit 70, for example. The duty ratio of the PWM signal Sp and the wire voltage CHG are in a proportional relationship. For this reason, for example, the ASIC 61 stores the PWM signal Sp, and reads out the PWM signal Sp before the third time TM3 of the abnormal discharge. Then, the ASIC 61 may identify the corona charger 41 corresponding to the PWM signal Sp with the maximum duty ratio among the read out PWM signals Sp1 to Sp4 as the corona charger 41 with the maximum wire voltage CHG.

[0097] In the above embodiment, one corona charger 41 is arranged on one photoconductor 31, but the present invention is not limited to this. The present application can also be applied to a configuration in which multiple corona chargers 41 are arranged on one photoconductor 31, for example, a printer 1 in which toner images of different colors are superimposed on one photoconductor 31 and then transferred to a sheet P at once. In addition, in the above embodiment, the photoconductor 31 is shaped like a drum, but the present invention is not limited to this. The printer 1 may include a photoconductor in the shape of a belt, for example. In the above embodiment, the ASIC 61 that executes the program PG is used as the control unit of the present application, but this is not limited to this. The control unit of the present application may be configured to execute the abnormality determination process shown in Fig. 5 by hardware processing, for example. The control unit may also be configured to use both hardware processing and software processing. In the above embodiment, the electrophotographic laser printer 1 is used as the image forming apparatus of the present application, but the present application is not limited to this. The image forming apparatus of the present application may be a multifunction machine, a copier, or the like.

[0098] The technical ideas derived from the contents of the above-described embodiment will be described below as supplementary notes. (i) a reattachment sensor (e.g., an open / close detection sensor 8) that detects reattachment of at least one of the plurality of chargers; The control unit is 3. The image forming apparatus according to claim 2, wherein after detecting reattachment of at least one of the chargers based on the reattachment sensor, when the applied voltage is applied from the multiple application circuits to each of the multiple chargers, a notification prompting cleaning of the charger determined to be abnormal is stopped on condition that the applied voltage is equal to or lower than a predetermined threshold value.

[0099] When the charger is reinstalled, it is highly likely that the user has cleaned the charger. If the applied voltage does not increase to a predetermined threshold value after the charger is reinstalled, it is highly likely that dirt on the charging wire has been removed and the image formation can be normally performed. Therefore, after detecting the reattachment of the charger, if the applied voltage is not increased to the predetermined threshold value when the control unit applies the applied voltage, it determines that the charger has returned to a normal state and stops the notification. This makes it possible to prevent the notification for cleaning from being unnecessarily continued. [Explanation of symbols]

[0100] 1 laser printer (image forming apparatus), 31 photoconductor, 41 corona charger (charger), 42 charged wire, 43 grid, 61 ASIC (control unit), 70 charged voltage generation circuit (applying circuit), 72 overcurrent detection circuit, 81 grid voltage adjustment circuit (control unit), CHG wire voltage (applied voltage), GRID grid voltage, Sv overcurrent detection signal (detection signal).

Claims

1. A plurality of photoconductors; a plurality of chargers for charging the respective photoconductors; A plurality of application circuits that apply voltages to the plurality of chargers, respectively; A control unit; an overcurrent detection circuit connected to the plurality of application circuits and configured to output a detection signal to the control unit when an overcurrent is detected in any one of the plurality of application circuits; Equipped with The control unit is When the detection signal is input from the overcurrent detection circuit, the charger to which the maximum voltage is applied by the application circuit is determined to be abnormal among the plurality of chargers; an image forming apparatus that identifies, among the plurality of chargers, the charger to which the maximum applied voltage has been applied, based on the applied voltage a predetermined time before the detection signal was input from the overcurrent detection circuit;

2. A plurality of photoconductors; a plurality of chargers for charging the respective photoconductors; A plurality of application circuits that apply voltages to the plurality of chargers, respectively; A control unit; an overcurrent detection circuit connected to the plurality of application circuits and configured to output a detection signal to the control unit when an overcurrent is detected in any one of the plurality of application circuits; Equipped with The control unit is When the detection signal is input from the overcurrent detection circuit, the charger to which the maximum voltage is applied by the application circuit is determined to be abnormal among the plurality of chargers; When the detection signal is input from the overcurrent detection circuit, if the applied voltage of the charger at which the applied voltage applied by the application circuit was maximum is less than a predetermined threshold, an image forming apparatus issues a notification urging the user to clean all of the chargers.

3. The control unit is 3. The image forming apparatus according to claim 1, further comprising: a notification that prompts the user to clean the charger that is determined to be abnormal.

4. The control unit is 4. The image forming apparatus according to claim 1, wherein, when an occurrence of an overcurrent is detected a predetermined number of times or more within a predetermined period based on the detection signal of the overcurrent detection circuit, the charger to which the maximum applied voltage is applied among the plurality of chargers is determined to be abnormal.

5. Each of the plurality of chargers includes A charged wire; a grid disposed between the photoreceptor and the charging wire; A corona charger having The control unit is 5. The image forming apparatus according to claim 1, wherein the voltage applied from the application circuit to the charging wire is controlled so that a grid voltage of the grid becomes a predetermined voltage.

6. A control method for an image forming apparatus including a plurality of photoconductors, a plurality of chargers for charging each of the plurality of photoconductors, a plurality of application circuits for applying an application voltage to each of the plurality of chargers, a control unit, and an overcurrent detection circuit connected to the plurality of application circuits and outputting a detection signal to the control unit when an overcurrent is detected in any one of the plurality of application circuits, When the detection signal is input from the overcurrent detection circuit to the control unit, the charger to which the maximum voltage is applied by the application circuit is determined to be abnormal among the plurality of chargers, A control method for an image forming apparatus, comprising: identifying, among the plurality of chargers, the charger to which the maximum applied voltage was applied, based on the applied voltage a predetermined time before the detection signal was input from the overcurrent detection circuit.

7. A control method for an image forming apparatus comprising: a plurality of photoconductors; a plurality of chargers for charging each of the plurality of photoconductors; a plurality of application circuits for applying an application voltage to each of the plurality of chargers; a control unit; and an overcurrent detection circuit connected to the plurality of application circuits and for outputting a detection signal to the control unit when an overcurrent is detected in any one of the plurality of application circuits, When the detection signal is input from the overcurrent detection circuit to the control unit, the charger to which the maximum voltage is applied by the application circuit is determined to be abnormal among the plurality of chargers, A control method for an image forming apparatus, comprising: when the detection signal is input from the overcurrent detection circuit, if the applied voltage of the charger at which the applied voltage applied by the application circuit was maximum is less than a predetermined threshold, an alert is issued to prompt cleaning of all of the chargers.

Images