JP2021092772A - Image forming apparatus - Google Patents

Abstract

To reduce the time required for setting a developing bias at which leakage does not occur.SOLUTION: An image forming apparatus has: a rotatable image carrier; an electrifying member that electrifies the image carrier; a rotatable developer carrier that is provided to be opposite to the image carrier in a non-contact state and carries a developer; an application unit that applies, to the developer carrier, a developing bias in which a DC voltage and an AC voltage are superimposed to each other; a detection unit that detects the value of a current flowing between the image carrier and the developer carrier; and a control unit that, in a state where the image carrier and the developer carrier are rotated and the developing bias is applied to the developer carrier, controls the AC voltage applied to the developer carrier from the difference between a first current value detected by the detection unit and a second current value smaller than the first current value.SELECTED DRAWING: Figure 6

Description

The present invention relates to an image forming apparatus that forms an image on a recording medium using an electrophotographic method.

In an image forming apparatus such as a printer using an electrophotographic method (electrophotographic process), various developing apparatuss are used to develop an electrostatic latent image formed on an image carrier. As an example thereof, a non-contact developing method is known in which an image carrier and a developer carrier facing the image carrier are arranged with a predetermined gap.

In the non-contact developing method, a development bias in which a DC voltage and an AC voltage are superimposed is applied to the developer carrier, so that the charged toner flies from the developer carrier to the image carrier and becomes the image carrier. A toner image is developed on the formed electrostatic latent image. The toner image developed on the image carrier is transferred and fixed on a recording medium such as paper.

By the way, in the non-contact developing method, the gap provided between the image carrier and the developer carrier may fluctuate due to the driving of the image carrier and the developer carrier. There is a problem that density unevenness occurs in the formed image because the electric field strength between the image carrier and the developer carrier fluctuates due to the fluctuation of the gap.

To solve this problem, by increasing the inter-peak voltage (peak-to-peak value) of the AC voltage in the development bias, the toner sufficiently flies from the developer carrier to the image carrier, and the occurrence of density unevenness is suppressed. It is possible. However, when the inter-peak voltage of the AC voltage in the development bias is increased, the potential difference from the surface potential of the image carrier becomes large. Therefore, there is a problem that a discharge occurs between the developer carrier and the image carrier, a current leak (hereinafter referred to as a leak) in which a discharge current flows due to the discharge occurs, and noise is generated in the formed image. It was.

The inter-peak voltage (peak-to-peak value) of the AC voltage at which the leak occurs changes with the gap, atmospheric pressure, and the like, and therefore changes with changes in the individual image forming apparatus and the usage environment.

Therefore, in Patent Document 1, the peak-to-peak voltage (peak-to-peak value) of the AC voltage in the development bias applied between the image carrier and the developer carrier is gradually increased from a value at which leakage does not occur. ing. Then, the impedance is measured based on the current value flowing between the image carrier and the developer carrier, and the occurrence of a leak is detected from the measured impedance value and the current value.

Japanese Unexamined Patent Publication No. 2005-78015

However, in Patent Document 1, it is necessary to measure the impedance in advance in order to detect the occurrence of a leak between the image carrier and the developer carrier, and the time required to set the development bias so that the leak does not occur. There was a problem that it became long.

An object of the present invention is to reduce the time required to set a development bias that does not cause leakage.

In order to achieve the above object, the present invention is provided with a rotatable image carrier, a charging member for charging the image carrier, and a developing agent so as to face the image carrier in a non-contact state. Between the image-bearing body and the developer-supporting body, a rotatable developer-supporting body that carries the image carrier, an application unit that applies a development bias in which a DC voltage and an AC voltage are superimposed on the developer-supporting body, and A first detection unit detected by the detection unit in a state where the image carrier and the developer carrier are rotated and the development bias is applied to the developer carrier. It is characterized by having a control unit that controls the AC voltage applied to the developer carrier from the difference between the current value of the above and the second current value smaller than the first current value.

According to the present invention, the time required to set the development bias that does not cause leakage can be shortened.

Cross-sectional view of the image forming apparatus Explanatory drawing which shows configuration about development bias application and discharge detection (A) Waveform diagram of the current value when Vpp is changed, (b) Relationship diagram of the difference between the maximum value and the minimum value of the AC voltage and the current value in the embodiment. (A) Relationship diagram of AC voltage and current value in Comparative Example, (b) Relationship diagram of change amount of AC voltage and current value in Example (A) Timing chart of AC voltage setting when discharge generation is detected in Comparative Example, (b) Timing chart of AC voltage setting when discharge generation is detected in Example Flow chart showing discharge detection control in Example 1 Waveform diagram of AC voltage and current value applied with development bias in Example 2 Flow chart showing discharge detection control in Example 2 Explanatory drawing for explaining the magnitude relation of each member of the image forming part in Example 3. Flow chart showing discharge detection control in Example 3

Hereinafter, preferred embodiments of the present invention will be described in detail exemplarily with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, etc. of the components described in the following examples should be appropriately changed depending on the configuration of the apparatus to which the present invention is applied and various conditions. It is not intended to limit the scope of the invention to the following examples.

[Example 1]
With reference to FIG. 1, the overall configuration of the image forming apparatus will be described together with the image forming operation. FIG. 1 is a schematic cross-sectional view showing a schematic configuration of an image forming apparatus according to a first embodiment of the present invention.

<Explanation of image forming apparatus>
The image forming apparatus is a laser printer using an electrophotographic method, and the process cartridge 20 is detachably attached to the apparatus main body M. Here, the apparatus main body M of the image forming apparatus indicates a component of the image forming apparatus excluding the process cartridge 20. Further, the image forming apparatus to which the present invention is applicable is not limited to those shown here. For example, the present invention is also applicable to a color laser printer provided with a plurality of process cartridges 20 and using an intermediate transfer belt (intermediate transfer body) to transfer a plurality of image toner images to a recording medium to form a color image. ..

The photosensitive drum 1 as an image carrier (charged body) has an OPC (organic optical semiconductor) photosensitive layer formed on the outer peripheral surface of the conductive drum, and is driven and transmitted from a drive mechanism (not shown) of the main body of the apparatus. It is rotationally driven in the direction of arrow r1 in FIG. 1 with a predetermined process speed.

A charging bias is applied to the charging roller 4 as a charging member at a predetermined timing, and the surface of the photosensitive drum 1 is uniformly charged to a predetermined polarity and potential. The laser beam scanner 6 as an exposure unit forms an electrostatic latent image on the surface of the photosensitive drum 1 by scanning and exposing (irradiating) the charged photosensitive drum 1 with a laser beam corresponding to the image information.

The developing apparatus as a developing unit develops an electrostatic latent image formed on the surface of the photosensitive drum 1 with toner as a developing agent. The developing apparatus includes a developing roller 7, a developing blade 8, and a developing container 9. The developing roller 7 is arranged so as to face the photosensitive drum 1, and is a developer carrier for supplying toner to the photosensitive drum 1. The developing blade 8 is a regulating member for regulating the layer thickness of the toner supported on the developing roller 7 and imparting an electric charge to the toner. The developing container 9 is a developing agent accommodating portion for accommodating the toner supplied to the photosensitive drum 1.

The developing roller 7 is driven and transmitted from a drive mechanism (not shown) of the main body of the apparatus, and is rotationally driven in the direction of arrow r2 in FIG. A toner layer (magnetic spike) charged by the developing blade 8 is formed on the surface of the developing roller 7. Then, when a development bias in which an AC voltage and a DC voltage are superimposed is applied to the developing roller 7, the toner carried on the developing roller 7 flies to the photosensitive drum 1 due to the electric field of the developing bias, and the surface of the photosensitive drum 1 is formed. The electrostatic latent image formed on the surface is developed as a toner image.

On the other hand, the recording medium 10 is fed by a feeding roller (not shown) or the like, and a toner image (developer image) developed on the surface of the photosensitive drum 1 is transferred at the nip portion between the photosensitive drum 1 and the transfer roller 11. Will be done. The recording medium 10 on which the toner image is transferred is separated from the surface of the photosensitive drum 1 and sent to the fixing device 12, heated and pressurized, and the transferred toner image is fixed on the recording medium 10.

The toner that is not transferred to the recording medium 10 and remains on the surface of the photosensitive drum 1 is removed by the cleaning blade 2 as a cleaning unit that comes into contact with the photosensitive drum 1 and cleans the photosensitive drum 1, and is housed in the cleaning container 5. After that, the surface of the photosensitive drum 1 is charged again by the charging roller 4, and the above steps are repeated to perform a series of image formation cycles.

In this embodiment, the photosensitive drum 1, the charging roller 4, the cleaning blade 2, the cleaning container 5, and the developing roller 7, the developing blade 8, and the developing container 9 are integrated as the process cartridge 20. The process cartridge 20 is removable from the device main body M of the image forming apparatus.

<Explanation of discharge detection configuration between photosensitive drum and developing roller>
Next, with reference to FIG. 2, a configuration relating to application of a development bias to the developing roller 7 and discharge detection between the photosensitive drum 1 and the developing roller 7 will be described. FIG. 2 is an explanatory diagram showing a configuration relating to development bias application and discharge detection.

As shown in FIG. 2, the developing roller 7 has a sleeve 7a that supports toner at the time of image formation, and circular caps 7b are fitted at both ends of the sleeve 7a in the longitudinal direction. The developing roller 7 is rotationally driven around the roller shaft 7c. Here, the outer diameter of the photosensitive drum 1 is 30 mm, the outer diameter of the developing roller 7 is 15 mm, which is smaller than the outer diameter of the photosensitive drum 1, and both the photosensitive drum 1 and the developing roller 7 are rotationally driven at a peripheral speed of 300 mm / s. ..

Further, the developing roller 7 is provided so as to face the photosensitive drum 1 in a non-contact state with a predetermined gap (SD gap). In this embodiment, the cap 7b has a larger outer diameter than the sleeve 7a, and the outer peripheral surface of the cap 7b is in contact with the surface of the photosensitive drum 1. As a result, a predetermined gap is provided between the developing roller 7 and the photosensitive drum 1, and the developing roller 7 and the photosensitive drum 1 face each other in a non-contact state. Here, a 200 μm SD gap is provided as a predetermined gap.

The configuration in which a predetermined gap is provided between the developing roller 7 and the photosensitive drum 1 is not limited to this. For example, a predetermined gap may be provided between the developing roller 7 and the photosensitive drum 1 by a frame body that rotatably supports the developing roller 7 and the photosensitive drum 1.

Further, a DC voltage application unit 30 and an AC voltage application unit 31 are connected to the roller shaft 7c of the developing roller 7 in order to supply toner to the photosensitive drum 1. The DC voltage application unit 30 and the AC voltage application unit 31 are application units for applying a development bias in which a DC voltage and an AC voltage are superimposed on the developing roller 7.

The DC voltage application unit 30 is a circuit that generates a DC component applied to the developing roller 7, and its output is input to the AC voltage application unit 31. The DC voltage application unit 30 has an output control unit 32. The output control unit 32 controls the value of the bias output by the DC voltage application unit 30 according to the instruction of the CPU 40 as the control unit.

Further, the AC voltage application unit 31 is a circuit that outputs an AC voltage having the DC voltage output by the DC voltage application unit 30 as an average value (area center value). The AC voltage application unit 31 outputs, for example, a rectangular wave-shaped (pulse-shaped) AC voltage having a frequency f = 2.5 kHz and a duty of 50%. The AC voltage application unit 31 has a Vpp control unit 33. The Vpp control unit 33 controls Vpp, which is an inter-peak voltage (peak-to-peak value) of the AC voltage, in response to an instruction from the CPU 40 as a control unit.

The detection unit 35 is a detection unit that detects the current value flowing between the photosensitive drum 1 and the developing roller 7. The detection unit 35 includes a rectifier unit 34, a detection circuit 36, and an amplifier 37. The rectifying unit 34 rectifies the current flowing between the developing roller 7 and the photosensitive drum 1 when a developing bias in which a DC voltage and an AC voltage are superimposed is applied. The detection circuit 36 converts the rectified current into a voltage. The amplifier 37 amplifies the converted voltage signal and outputs it to the CPU 40 as a discharge detection signal. The A / D converter 38 A / D converts the discharge detection signal from the amplifier 37. The CPU 40 recognizes the magnitude of the current generated between the developing roller 7 and the photosensitive drum 1 from the output of the amplifier 37 that has been A / D converted by the A / D converter 38, and has an AC voltage cycle time T. Outputs the averaged current value. Here, the CPU 40 outputs (calculates) a current value (average value) averaged at 4 ms, which is a time T of 10 cycles of AC voltage. As will be described later, the CPU 40 is a control unit that controls the development bias applied to the developing roller 7 from the current value detected by the detection unit 35.

<Explanation of leakage current detection>
The detection of the leak current (discharge detection) by the detection unit 35 will be described with reference to FIG. FIG. 3A is a waveform diagram of the current value when Vpp is changed, and FIG. 3B is a change amount of the AC voltage and the current value in the first embodiment (difference between the maximum value and the minimum value of the AC voltage). ) Is a relational diagram.

FIG. 3A plots the current value flowing between the developing roller 7 and the photosensitive drum 1 when Vpp, which is the peak-to-peak voltage of the AC voltage in the development bias, is changed. The horizontal axis is time and the vertical axis is time. It is the current value after rectification. As a measurement condition, the photosensitive drum 1 and the developing roller 7 are driven by a driving mechanism (not shown). Next, a charging bias is applied to the charging roller 4 to bring the surface potential of the photosensitive drum 1 to −500V, and the DC voltage application unit 30 applies a DC voltage of −300V to the developing roller 7. Next, the AC voltage application unit 31 applies an AC voltage peak-to-peak voltage Vpp to the developing roller 7. At this time, as shown in FIG. 3B, the peak inter-peak voltage Vpp of the AC voltage is gradually increased from 1600 V by 200 V at predetermined time intervals (here, 1 s), and the time and current at each AC voltage Vpp are increased. Plot the relationship between the output values of the values. No leak current was generated when the AC voltage Vpp was 1.8 kV and 2.0 kV, but a leak current was generated when the AC voltage Vpp was 2.2 kV. Compared with the case where the leak current does not occur, when the leak current occurs, the leak current fluctuates with the rotation cycle of the developing roller 7. This is because the region where the leak current is generated fluctuates with the rotation cycle of the developing roller 7. The distance (SD gap) between the developing roller 7 and the photosensitive drum 1 varies depending on the uneven shape of the developing roller 7, the photosensitive drum 1, and the cap 7b.

When the developing roller 7 rotates, the distance between the developing roller 7 and the photosensitive drum 1 fluctuates depending on the rotation cycle of the developing roller 7 and the photosensitive drum 1. According to Paschen's law, in the gap region of 200 μm, which is the distance between the developing roller 7 and the photosensitive drum 1 of this embodiment, the discharge start voltage decreases as the distance between the developing roller 7 and the photosensitive drum 1 approaches. I know that. Since the distance (SD gap) between the developing roller 7 and the photosensitive drum 1 is not uniform in the axial direction of the developing roller 7, the leakage current is the axial gap region between the developing roller 7 and the photosensitive drum 1. Of these, it occurs in some areas where the distance is short. When the SD gap changes in the axial direction of the developing roller 7, the region where the leak current is generated also changes, so that the current value fluctuates according to the change in the region where the leak current is generated. Therefore, as shown in FIG. 3A, in the situation where the peak voltage of the AC voltage where the leak current is generated is Vpp = 2.2 kV, the current value fluctuates depending on the rotation cycle of the photosensitive drum 1 and the developing roller 7. To do. The fluctuation of the current value fluctuates in the rotation cycle (here, 157 ms) of the developing sleeve, which is smaller than the rotation cycle of the photosensitive drum 1. Therefore, the CPU 40 can distinguish between sudden noise and current change due to leakage by outputting a current value averaged by the cycle time (time of one cycle or more) T of the AC voltage. In this embodiment, the current value averaged at 4 ms, which is the time of 10 cycles, is output as the cycle time T of the AC voltage, but the average time T is increased to several tens of ms from the viewpoint of noise removal. Is also good.

In this embodiment, the time required for the photosensitive drum 1 to rotate once is 314 ms (time T2) longer than the time required for the developing roller 7 to rotate once (time T1). Therefore, the CPU 40, which is a control unit, has a maximum value which is the first current value and a second current value of the output value (average value) of the current value at the time T2 when the photosensitive drum 1, which has a longer one rotation time, makes one rotation. Comparison with the threshold value is performed using the amount of change in the current value, which is the difference between the minimum values, which are the current values of. FIG. 3B plots the transition of the maximum value (solid line) and the transition of the minimum value (dotted line) of the output value of the current value when the inter-peak voltage Vpp of the AC voltage is changed. The relationship between the amount of change in the current value, which is the difference between the maximum value and the minimum value of the output value of the current value, and the AC voltage is shown in FIG. 4 (b). The horizontal axis of FIG. 3B is the inter-peak voltage Vpp of the AC voltage applied to the developing roller 7, and the vertical axis is the maximum value (output value) of the current value (output value) during one rotation of the photosensitive drum 1 (solid line in the figure). And the minimum value (dotted line in the figure) are shown. The difference between the maximum value and the minimum value of this current value is the amount of change in the current value. As a measurement condition, the photosensitive drum 1 and the developing roller 7 are driven by a driving mechanism (not shown). Next, a charging bias is applied to the charging roller 4 to bring the surface potential of the photosensitive drum 1 to −500V, and the DC voltage application unit 30 applies a DC voltage of −300V to the developing roller 7. Next, the AC voltage application unit 31 applies an AC voltage peak-to-peak voltage Vpp to the developing roller 7. At this time, the peak inter-peak voltage Vpp of the AC voltage is gradually increased by 200 V at a predetermined time interval (here, 1 s) from 1600 V, and the maximum value and the minimum value of the current value at each Vpp are plotted. The difference between the maximum value and the minimum value of the current value at each Vpp is the amount of change in the current value. When Vpp in the development bias is gradually increased, the amount of change in the current value sharply increases at the timing of leakage occurrence with respect to the amount of change in the current value when the leak does not occur. That is, it can be seen that when the amount of change in the current value is equal to or less than the threshold value (here, 2.0 kV or less), no leak occurs, but when the amount exceeds the threshold value, a leak occurs.

From the above, the CPU 40, which is a control unit, can detect the presence or absence of leakage at a predetermined AC voltage Vpp from the amount of change in the current value flowing between the photosensitive drum 1 and the developing roller 7.

Next, with reference to FIGS. 4 (a) and 4 (b), the relationship between the peak-to-peak voltage of the AC voltage of the comparative example and the present embodiment and the amount of change in the current value will be described. FIG. 4A is a diagram showing the relationship between the peak-to-peak voltage of the AC voltage and the amount of change in the current value in the comparative example. FIG. 4B is a diagram showing the relationship between the peak-to-peak voltage of the AC voltage and the amount of change in the current value in the embodiment. The amount of change in the current value shown in FIG. 4B represents the difference between the maximum value and the minimum value of the current value shown in FIG. 3B.

In the configuration of the comparative example, the output value is the value obtained by integrating the absolute value of the current value between the photosensitive drum 1 and the developing roller 7 for the period T of the cycle time T of the AC voltage. The horizontal axis is the inter-peak voltage of the AC voltage, and the vertical axis is the output value. As a measurement condition, the photosensitive drum 1 and the developing roller 7 are driven by a driving mechanism (not shown). Next, a charging bias is applied to the charging roller 4 to bring the surface potential of the photosensitive drum 1 to −500V, and the DC voltage application unit 30 applies a DC voltage of −300V to the developing roller 7. Next, the AC voltage application unit 31 applies an AC voltage peak-to-peak voltage Vpp to the developing roller 7. At this time, Vpp is gradually increased from 0V, and the relationship between Vpp and the output value of the current value is plotted. In FIG. 4A, it can be seen that the output value, which is the current value, increases in proportion to the voltage Vpp even when the discharge start voltage is Vpp or less, which is below the threshold value.

Since the slope of the output value at a voltage Vpp below the threshold value (below the discharge start voltage) is determined by the impedance between the photosensitive drum 1 and the developing roller 7, it changes depending on the SD gap or the like. Therefore, the output value varies due to variations in the parts of the cap 7b and wear due to durability. Therefore, it is not possible to accurately calculate the current value at which a leak occurs with respect to fluctuations in the SD gap due to wear of the cap 7b or variation in parts depending on the usage conditions. In order to accurately determine the presence or absence of leakage, it is necessary to obtain the impedance between the photosensitive drum 1 and the developing roller 7 using a voltage Vpp that does not cause leakage. Since it is necessary to measure the impedance at a voltage Vpp at which no leak occurs in order to detect the leak, it takes time to detect the occurrence of the leak.

On the other hand, the configuration of this embodiment is as described with reference to FIG. 3 (b). In the configuration of this embodiment, the presence or absence of leakage is determined by using the amount of change in the current value, which is the difference between the maximum value and the minimum value of the output value (average value) of the current value during the time T2 in which the photosensitive drum 1 rotates once. I do. As shown in FIG. 4B, it can be seen that the amount of change in the current value hardly changes at a voltage below the threshold value and below the discharge start voltage Vpp. Then, when the voltage Vpp exceeds the threshold value of the discharge start voltage Vpp, it can be seen that the amount of change in the current value increases sharply. That is, it can be seen that a leak does not occur when the amount of change in the current value is equal to or less than a threshold value (discharge start voltage Vpp or less), which is a predetermined value, but a leak occurs when the amount exceeds the threshold value. Therefore, by determining the development bias (AC voltage) applied to the developing roller 7 from the amount of change in the current value flowing between the photosensitive drum 1 and the developing roller 7, the variation in the current value due to the fluctuation of the SD gap or the like is removed. be able to. Therefore, in this embodiment, the developing roller 7 is based on the amount of change in the current value flowing between the photosensitive drum 1 and the developing roller 7 at an arbitrary applied voltage without measuring the impedance between the photosensitive drum 1 and the developing roller 7. The development bias (AC voltage) to be applied to can be determined, and the time required to set the development bias to prevent leakage can be shortened.

<Setting of AC voltage for discharge generation detection operation>
Next, with reference to FIG. 5, the timing of each applied voltage during the discharge generation detection operation of the image forming apparatus according to the first embodiment will be described in comparison with the comparative example. FIG. 5 (a) is a timing chart for the setting of the AC voltage Vpp and the leak current at the time of detecting the discharge occurrence in the comparative example, and FIG. 5 (b) is the setting of the AC voltage Vpp and the leak current at the time of detecting the discharge occurrence in the example. It is a timing chart for.

FIG. 5 (a) is the configuration of the comparative example, and FIG. 5 (b) is the configuration of the first embodiment. In the configuration of the comparative example shown in FIG. 5A, first, a measurement time for measuring the impedance between the photosensitive drum 1 and the developing roller 7 is required for the configuration of the first embodiment shown in FIG. 5B. The impedance between the photosensitive drum 1 and the developing roller 7 changes due to fluctuations in the SD gap due to the drive of the photosensitive drum 1 and the developing roller 7. Therefore, the current flowing between the photosensitive drum 1 and the developing roller 7 is measured during the period T2 until the photosensitive drum 1 makes one rotation, and the impedance is determined by using the current value that becomes the maximum value at the timing when the SD gap is the narrowest. Ask. Since the subsequent operation is the same voltage Vpp setting and timing as in this embodiment, it will be described with reference to FIG. 5 (b). The voltage Vpp during the discharge generation detection operation in FIG. 5B is determined based on the voltage Vpp during image formation. The voltage Vpp at the time of image formation is set to 1.8 kV as an initial setting. When the voltage Vpp exceeds 1.8 kV, the toner is developed on the white background of the recording medium 10, that is, the so-called background fog deteriorates. Therefore, the upper limit of the voltage Vpp is set to 1.8 kV.

As the voltage Vpp at the time of detecting the occurrence of discharge, the voltage Vpp at the time of the initial discharge generation detection operation is set in consideration of the fact that the discharge start voltage, which is the threshold value, changes due to the change in temperature and humidity during paper passing and the fluctuation of the SD gap. It is set to 2.0 kV by adding an offset value of 200 V to 1.8 kV. That is, the AC voltage Vpp applied to the developing roller 7 when comparing with the threshold value is higher than the AC voltage (here, 1.8 kV) applied to the developing roller 7 during image formation (here, 2.0 kV). ). The CPU 40 determines that the amount of change in the current value between the photosensitive drum 1 and the developing roller 7 is equal to or less than the threshold value from the output value (average value) of the current value when the voltage Vpp at the time of initial discharge generation detection is 2.0 kV. If so, the voltage Vpp during image formation is not changed. On the other hand, when the CPU 40 determines that the amount of change in the current value between the photosensitive drum 1 and the developing roller 7 exceeds the threshold value, the CPU 40 applies an AC voltage Vpp applied to the developing roller 7 as shown in FIG. 5 (b). It is gradually lowered to a voltage Vpp at which the amount of change in the current value is equal to or less than the threshold value. Then, the voltage Vpp obtained by subtracting the offset value of 200 V from the voltage Vpp at which the amount of change in the current value is equal to or less than the threshold value is reset to the applied voltage Vpp at the time of image formation. This makes it possible to prevent leaks from occurring during image formation.

Further, by adopting a configuration in which the voltage Vpp at the time of detecting the occurrence of discharge is gradually lowered as in this embodiment, a leak occurs as compared with the conventional control of increasing the voltage Vpp to obtain the discharge start voltage. The time required to set the development bias that does not occur can be shortened. The reason is that in the configuration where the voltage Vpp is gradually lowered, the detection ends when the amount of change in the current value is determined to be less than the threshold value in the initial condition voltage Vpp at the time of leak detection, so the voltage Vpp under one condition at the shortest. The detection ends with. On the other hand, in the conventional configuration in which the voltage Vpp is gradually increased, the voltage Vpp is gradually increased from the voltage Vpp at which leakage does not surely occur. Therefore, in the conventional configuration, it is necessary to compare the threshold value with the voltage Vpp at which the amount of change in the current value is surely equal to or less than the threshold value and the voltage Vpp at least two conditions of the voltage Vpp desired to be used in image formation. From the above, by adopting a configuration in which the voltage Vpp at the time of detecting the occurrence of discharge is gradually lowered, the time required to set the development bias in which leakage does not occur can be shortened.

<Flowchart of discharge generation detection operation>
Next, the flow of control of the discharge generation detection operation of the image forming apparatus according to the first embodiment will be described with reference to FIG.

FIG. 6 is a flowchart showing an example of a flow of control of the discharge generation detection operation of the image forming apparatus according to the first embodiment. In the discharge generation detection operation described below, it is determined whether or not the amount of change in the current value flowing between the photosensitive drum 1 and the developing roller 7 exceeds the threshold value, and the development bias applied to the developing roller 7 is determined from the result. To set. This discharge generation detection operation is executed by the CPU 40 (see FIG. 2), which is a control unit. This discharge generation detection operation is executed when the installation environment of the image forming apparatus such as atmospheric pressure or temperature / humidity changes. Alternatively, it is executed according to the driving time of the developing roller 7 or the photosensitive drum 1 (for example, the paper passing history of the developing device or the timing of replacing the developing device where the SD gap may change). Further, the execution timing of the discharge generation detection operation is not limited to the above-mentioned example, and can be appropriately set.

First, when the power of the image forming apparatus is turned on and the discharge generation detection operation is started (start), each rotating body such as the photosensitive drum 1 and the developing roller 7 is driven by a drive mechanism (not shown) according to the instruction of the CPU 40. Is started (step S1). The driving of each rotating body continues until the discharge generation detection operation is completed. Next, a charging bias is applied to the charging roller 4, and a DC voltage −300V is applied to the developing roller 7 by the DC voltage application unit 30 (step S2). When the time (T2) for one rotation of the photosensitive drum 1 elapses from step S2 (step S3), the surface of the photosensitive drum 1 becomes the set surface potential −500 V over the entire circumference. Next, the AC voltage Vpp applied to the developing roller 7 is set. The AC voltage Vpp applied to the developing roller 7 is set to an AC voltage Vpp higher by an offset value than the setting at the time of image formation in consideration of changes in temperature and humidity and fluctuations in the SD gap during paper passing (step S4). Here, the AC voltage applied to the developing roller 7 is set to an AC voltage Vpp that is 200 V higher than the AC voltage at the time of image formation. Next, as described with reference to FIG. 4B, when the set AC voltage Vpp is applied to the developing roller 7, the amount of change in the current value flowing between the developing roller 7 and the photosensitive drum 1 is increased. It is determined whether or not the threshold value, which is a predetermined value, has been exceeded (step S5). In this example, the threshold value is set to 1 μA. Here, the amount of change in the current value is the maximum value which is the first current value and the second current value of the output value (average value) of the current value detected by the detection unit 35 during the time T2 when the photosensitive drum 1 rotates once. It is the difference between the minimum values that are the current values.

When the amount of change in the current value exceeds the threshold value in step S5, the CPU 40 turns off the AC voltage Vpp of the development bias because a leak has occurred between the photosensitive drum 1 and the developing roller 7. Step S6). The reason why Vpp is turned off once in step S6 is that once a development leak occurs, the development leak may continue due to the continuous flow of current, and the continuously generated development leak is cut off once. When such a leak occurs, it is necessary to set the AC voltage Vpp applied to the developing roller 7 at the time of image formation smaller than the setting of the AC voltage applied to the developing roller 7 at the time of detecting the current value. Therefore, the CPU 40, which is a control unit, lowers the development bias (AC voltage Vpp) applied to the developing roller 7 to a voltage lower than that at the time of detecting the current value (step S7). Here, the voltage is lowered to 100 V lower (for example, 1.7 kV) than the AC voltage (for example, 1.8 kV) at the time of image formation. Then, the process returns to step S4, and the AC voltage applied to the developing roller 7 is set to an AC voltage Vpp that is offset value higher than the AC voltage during image formation whose setting has been changed. In this way, the AC voltage applied to the developing roller 7 at the time of leak detection is reduced from 2.0 kV to 1.9 kV. Then, in step S5 again, it is confirmed whether or not the amount of change in the current value exceeds the threshold value.

When the change amount of the current value exceeds the threshold value in step S5, the CPU 40 gradually lowers the AC voltage applied to the developing roller 7 at the time of leak detection until the change amount of the current value becomes equal to or less than the threshold value. The above operation is repeated. That is, when the CPU 40, which is the control unit, detects that the amount of change in the current value exceeds the threshold value in step S5, the AC voltage applied to the developing roller 7 is gradually lowered, and the amount of change in the current value is the threshold value again. Detects whether or not the value exceeds.

If the amount of change in the current value does not exceed the threshold value in step S5, that is, if the amount of change in the current value is equal to or less than the threshold value, step S5 is repeated for a period of time T2 in which the photosensitive drum 1 makes one rotation after applying Vpp. (Step S8). When the amount of change in the current value is equal to or less than the threshold value in the above period, the value obtained by lowering the AC voltage Vpp at the time of leak detection by the offset value (200 V) at that time is determined as the AC voltage Vpp during image formation (step S9). That is, the CPU 40 determines the development bias applied to the developing roller 7 without changing the development bias (AC voltage Vpp) applied to the developing roller 7 during image formation. Then, the development bias and the charge bias are turned off, and then the driving of the photosensitive drum 1 and the development roller 7 is stopped (step S10), and the discharge generation detection operation is terminated (end).

From the above, according to the present embodiment, the development bias (current value) in which leakage does not occur is determined by determining whether or not the amount of change in the current value flowing between the photosensitive drum 1 and the developing roller 7 exceeds the threshold value. The time required to set the development bias (development bias) in which the amount of change is equal to or less than the threshold value can be shortened.

The SD gap, charging bias, development bias, current value threshold, etc. described in this embodiment are not intended to limit the scope of the present invention to those unless otherwise specified. Absent.

Further, in this embodiment, a configuration is illustrated in which the average value obtained by averaging the current values output from the detection unit 35 in the time of 10 cycles of the AC voltage is illustrated, but the cycle time T of the AC voltage is limited to this. It is not a thing, and it may be set appropriately.

Further, in this embodiment, a configuration in which the comparison with the threshold value using the amount of change in the current value is performed during the period of one rotation of the photosensitive drum 1 is illustrated, but the period for comparison with the threshold value is limited to this. It's not something. It may be the time for the photosensitive drum 1 to rotate a plurality of times, or the time for the developing roller 7 to rotate. Further, the comparison with the threshold value may be completed before the photosensitive drum 1 or the developing roller 7 completes one rotation. However, since the distance (SD gap) between the photosensitive drum 1 and the developing roller 7 varies depending on the rotation cycle of the developing roller 7 and the photosensitive drum 1, the longer one of the developing roller 7 or the photosensitive drum 1 is rotated. It is preferable to rotate. Further, for the purpose of shortening the time required for setting the development bias that does not cause leakage, it is preferable that the time for rotating the developing roller 7 or the photosensitive drum 1 is short.

Further, in this embodiment, the detection unit 35 shown in FIG. 2 has a configuration having a rectifying unit 34 and rectifies the current flowing through the developing roller 7, but the present invention is not limited to this configuration, and for example, the detection unit 35. May not have a rectifying unit 34. In this case, the discharge generation detection operation may be performed as follows.

For example, the detection unit 35 is located on the positive side of the AC voltage applied to the developing roller 7 on the third current value flowing between the photosensitive drum 1 and the developing roller 7, or on the negative side of the AC voltage applied to the developing roller 7. The fourth current value flowing between the photosensitive drum 1 and the developing roller 7 is detected. Then, the CPU 40, which is a control unit, determines the difference between the maximum value and the minimum value of the third current value detected by the detection unit 35 during the time when the developing roller 7 or the photosensitive drum 1 rotates at least once, or the fourth current. The threshold value is compared from the difference between the maximum value and the minimum value. The CPU 40 sets the development bias to be applied to the developing roller 7 when the change amount of the third current value or the change amount of the fourth current value, which is the difference between the maximum value and the minimum value, exceeds the threshold value. It is lower than the setting of the AC voltage applied to the developing roller 7 when the current value is detected. The CPU 40 does not change the setting of the development bias applied to the developing roller 7 when the amount of change in the third current value or the amount of change in the fourth current value is equal to or less than the threshold value.

Alternatively, the CPU 40, which is a control unit, averages the first average value obtained by averaging the third current value detected by the detection unit 35 in the cycle time of the AC voltage, or the second average value obtained by averaging the fourth current value. Calculate the average value of. Then, the CPU 40 determines the difference between the maximum value and the minimum value of the first average value calculated above, or the difference between the maximum value and the minimum value of the second average value in the time during which the developing roller 7 or the photosensitive drum 1 rotates at least once. Compare with the threshold value. The CPU 40 sets the development bias to be applied to the developing roller 7 when the change amount of the first average value or the change amount of the second average value, which is the difference between the maximum value and the minimum value, exceeds the threshold value. It is lower than the setting of the AC voltage applied to the developing roller 7 when the current value is detected. The CPU 40 does not change the setting of the development bias applied to the developing roller 7 when the amount of change in the first average value or the amount of change in the second average value is equal to or less than the threshold value.

Even with this configuration, it is possible to determine whether or not the amount of change in the current value flowing between the photosensitive drum 1 and the developing roller 7 exceeds the threshold value, and development bias (change in current value) that does not cause leakage can be determined. The time required to set the development bias (development bias) in which the amount is equal to or less than the threshold value can be shortened.

In this embodiment, Vpp during image formation is controlled, but it is not limited to Vpp during image formation, and Vpp at any timing at which Vpp is applied may be used. In this embodiment, when leak is detected, Vpp higher than the setting at the time of image formation is applied to the developing roller 7 (step S4 in FIG. 6), and the amount of change in the current value flowing between the photosensitive drum and the developing roller is used as a threshold value. A comparison (step S5 in FIG. 6) has been made, but the comparison is not limited to this. For example, the Vpp set at that time may be applied to the developing roller without increasing the Vpp applied to the developing roller, and the current value flowing between the photosensitive drum and the developing roller may be compared with the threshold value. By doing so, not only Vpp during image formation but also Vpp at that time is applied to the developing roller at any timing, and the amount of change in the current value flowing between the photosensitive drum and the developing roller is compared with the threshold value. can do. In this case, the time required to set the development bias that does not cause leakage can be further shortened.

Further, in this embodiment, in FIG. 6, when the amount of change in the current value exceeds the threshold value, it is determined that a leak has occurred (step S5), and the process proceeds to step S6 in which the development Vpp is turned off, but the present invention is limited to this. It's not a thing. For example, after one rotation of the photosensitive drum 1 from the application of Vpp (step S4 in FIG. 6) (step S7 in FIG. 6), it may be determined whether the amount of change in the current value exceeds the threshold value (step S5 in FIG. 6). .. By doing so, although it takes more time to set the development bias that does not cause leakage than in Example 1 described above, the time described above can be shortened as compared with the conventional case. Further, it is possible to distinguish between the current change due to sudden noise and the current change due to the leak caused by the change in the gap between the photosensitive drum and the developing roller, and it is possible to perform more accurate detection.

Further, in the configuration of the first embodiment, the difference between the maximum value which is the first current value and the minimum value which is the second current value of the output value (average value) of the current value is changed in order to set the development Vpp. However, the comparison is made with the threshold value, but the present invention is not limited to this form. In other words, the first current value does not have to be the maximum value, and the second current value does not have to be the minimum value. Other modifications are shown below.

As described in the first embodiment, if the maximum value and the minimum value are adopted as the values to be compared with the threshold value, the detection accuracy may be lowered due to noise. Therefore, as a modification 1, for example, the maximum value of the current value in the period of one round or more of the photosensitive drum may be set as the upper n% value in the histogram, and the minimum value may be set as the lower m% value. As a specific example of the modification 1, the output value is a current value averaged every 4 ms, and sampling is performed for a period of 400 ms, which is one round or more of the photosensitive drum (data is 100 data). Assuming that n is 97% and m is 3% with respect to the sampled data, the third highest output value in 100 data is used as a substitute for the maximum value, and the third lowest output value is used as a substitute for the minimum value. When the difference between the third highest output value and the third lowest output value becomes equal to or greater than the threshold value, it can be determined that a leak has occurred. Note that m and n can be set as appropriate. Further, instead of the maximum value (minimum value), a value obtained by averaging higher n% (lower m%) or more may be used.

Further, as a modification 2, the slope when the horizontal axis is time and the vertical axis is the current value, which is the amount of change in the output value (average value) of the current value per unit time, or the absolute value of the slope is It may be determined that a leak has occurred when a certain threshold is exceeded. If no leak occurs and the SD gap fluctuates, the current value fluctuates in inverse proportion to the SD gap. When a leak occurs, the slope changes abruptly due to a sudden change in the current at the position where the leak occurs. Therefore, when the current changes more rapidly than the fluctuation of the SD gap, it can be determined that a leak has occurred. As the slope threshold, for example, assuming that the current value changes by 1 μA in one round of the photosensitive drum (314 ms) due to the influence of the SD gap fluctuation and the SD gap fluctuation with time is a sine wave, the slope threshold is 2π. It can be set to / 314 [μA / ms]. In FIG. 3A, the slope is almost unchanged at Vpp 1.8 kV and 2.0 kV, whereas the slope is greatly changed at 2.2 kV.

Further, as a modification 3, it is possible to determine the leak by looking at the standard deviation, which is the variation of the current value. When the data of the output value (average value) of the current value is sampled for one round of the photosensitive drum, for example, the output value of the current for 78 samples can be obtained. At Vpp 1.8 kV and 2.0 kV in FIG. 3A, the current value does not change with the passage of time, so that the standard deviation of the current value is small. On the other hand, the standard deviation becomes large at 2.2 kV where a leak occurs. Therefore, it is possible to determine that a leak has occurred when the standard deviation of the current value exceeds a certain threshold value. As the threshold value, for example, assuming that the current value changes by 1 μA in one round of the photosensitive drum (314 ms) due to the influence of the SD gap fluctuation and the SD gap fluctuation with time is a sine wave, the standard deviation is 0.315 μA. Become. Therefore, 0.315 μA may be adopted as the standard deviation threshold as an example of the standard deviation threshold.

Note that each of the modified examples 1 to 3 is an example, and the calculation method is not limited to the above method as long as each value is calculated.

[Example 2]
The image forming apparatus according to the second embodiment will be described with reference to FIGS. 7 and 8. In this example, the discharge detection control is different from that of the first embodiment, and the other configurations are almost the same as those of the first embodiment. Therefore, in this example, the same reference numerals are given to the same configurations as those in the first embodiment described above, so that detailed description thereof will be omitted.

<Explanation of leakage current detection>
The detection of the leak current by the detection unit 35, which is a feature of this embodiment, will be described with reference to FIG. 7. FIG. 7 is a waveform diagram of an AC voltage and a current value applied with a development bias in the second embodiment.

FIG. 7 shows the waveform of the AC voltage applied to the developing roller 7 by the AC voltage applying unit 31 forming a solid white image (dotted line in the figure) and the current value flowing between the developing roller 7 and the photosensitive drum 1. It is a plot of the waveform (solid line in the figure). Since the electric charge on the surface of the developing roller 7 moves due to the potential difference between the photosensitive drum 1 and the developing roller 7, a current flows when the potential of the AC voltage changes. In the configuration of this embodiment, since the AC voltage applied by the AC voltage application unit 31 has a rectangular wave shape, as shown in FIG. 7, at the timing of the time t = 0 when the AC voltage changes from negative to positive, the potential difference is The current value flows with the change. On the other hand, at the timing when a certain time elapses from the timing when the AC voltage changes from negative to positive, the AC voltage becomes constant, and the current value becomes a value near 0 accordingly. Here, the time T is the time of one cycle of the AC voltage, and T = 1 / frequency f. Further, the timing of t = T / 4 is illustrated as a constant time from the timing when the AC voltage changes from negative to positive.

In this embodiment, the timing for detecting the current value flowing between the photosensitive drum 1 and the developing roller 7 is set after T / 4 has elapsed, which is a certain time from the timing when the AC voltage changes from negative to positive. It is not limited to. As shown by the dotted line in FIG. 7, the AC voltage changes from a region where the AC voltage changes from negative to positive (or positive to negative) on the positive side (or negative side) to a region where the AC voltage becomes constant. As long as the timing corresponds to the region where the AC voltage becomes constant as described above, the constant time from the timing when the AC voltage changes from negative to positive is not limited to T / 4. Further, not only when a certain time elapses from the timing when it changes from negative to positive, but also when a certain time elapses from the timing when it changes from positive to negative. If the timing corresponds to the region where the AC voltage becomes constant (the timing when the voltage change becomes approximately 0), the same effect can be obtained even after an arbitrary fixed time from the timing when the AC voltage changes from negative to positive or positive to negative. can get.

In this embodiment, the CPU 40, which is a control unit, uses the current value detected by the detection unit 35 when a certain time elapses from the timing when the AC voltage Vpp applied to the developing roller 7 changes from negative to positive to a threshold value. Make a comparison with. With this configuration, when the AC voltage applied to the developing roller 7 changes, the leak current can be directly detected by the current value flowing between the photosensitive drum 1 and the developing roller 7.

<Flowchart of discharge generation detection operation>
Next, an example of the control flow of the discharge generation detection operation of the image forming apparatus according to the second embodiment will be described with reference to FIG.

FIG. 8 is a flowchart showing an example of a flow of control of the discharge generation detection operation of the image forming apparatus according to the second embodiment. In the discharge generation detection operation described below, it is determined whether or not the amount of change in the current value flowing between the photosensitive drum 1 and the developing roller 7 exceeds the threshold value, and the development bias applied to the developing roller 7 is determined from the result. To set. This discharge generation detection operation is executed by the CPU 40 (see FIG. 2), which is a control unit. This discharge generation detection operation is executed when the installation environment of the image forming apparatus such as atmospheric pressure or temperature / humidity changes. Alternatively, it is executed according to the driving time of the developing roller 7 or the photosensitive drum 1 (for example, the paper passing history of the developing device or the timing of replacing the developing device where the SD gap may change). Further, the execution timing of the discharge generation detection operation is not limited to the above-mentioned example, and can be appropriately set.

First, when the power of the image forming apparatus is turned on and the discharge generation detection operation is started (start), the rotating bodies such as the photosensitive drum 1 and the developing roller 7 are rotated by a drive mechanism (not shown) according to the instruction of the CPU 40. Is started (step S11). The driving of each rotating body continues until the discharge generation detection operation is completed. Next, a charging bias is applied to the charging roller 4, and a DC voltage −300V is applied to the developing roller 7 by the DC voltage application unit 30 (step S12). When the time (T2) for one rotation of the photosensitive drum 1 elapses from step S2 (step S13), the surface of the photosensitive drum 1 becomes the set surface potential −500 V over the entire circumference. Next, the AC voltage Vpp applied to the developing roller 7 is set. The AC voltage Vpp applied to the developing roller 7 is set to an AC voltage Vpp higher than the setting at the time of image formation in consideration of changes in temperature and humidity and fluctuations in the SD gap during paper passing (step S14). Here, the AC voltage applied to the developing roller 7 is set to an AC voltage Vpp that is 200 V higher than the AC voltage at the time of image formation. Next, as described with reference to FIG. 7, the threshold value at which the absolute value of the current value is a predetermined value when a certain time (T / 4 in this case) elapses from the timing when the AC voltage changes from negative to positive is set. It is determined whether or not the value has been exceeded (step S15). In this example, the threshold value is set to 10 μA.

When the absolute value of the current value exceeds the threshold value in step S15, the CPU 40 turns off the AC voltage Vpp of the development bias because a leak has occurred between the photosensitive drum 1 and the developing roller 7. Step S16). When such a leak occurs, it is necessary to set the AC voltage Vpp applied to the developing roller 7 during image formation to be smaller than the setting of the AC voltage applied to the developing roller 7 when the current value is detected. .. Therefore, the CPU 40, which is a control unit, lowers the development bias (AC voltage Vpp) applied to the developing roller 7 to a voltage lower than that at the time of detecting the current value (step S17). Here, the voltage is lowered to a voltage (1.7 kV) that is 100 V lower than the AC voltage (1.8 kV) during image formation. Then, returning to step S14, the AC voltage applied to the developing roller 7 is set to an AC voltage Vpp that is 200 V higher than the AC voltage (1.7 kV) during image formation whose setting has been changed. In this way, the AC voltage applied to the developing roller 7 at the time of leak detection is reduced from 2.0 kV to 1.9 kV. Then, in step S15 again, it is confirmed whether or not the absolute value of the current value exceeds the threshold value.

When the absolute value of the current value exceeds the threshold value in step S15, the CPU 40 gradually lowers the AC voltage applied to the developing roller 7 at the time of leak detection until the absolute value of the current value becomes equal to or less than the threshold value. The above operation is repeated. That is, when the CPU 40, which is the control unit, detects that the absolute value of the current value exceeds the threshold value in step S15, the AC voltage applied to the developing roller 7 is gradually lowered, and the absolute value of the current value becomes the threshold value again. Detects whether or not the value exceeds.

When the current value (absolute value) does not exceed the threshold value in step S15, that is, when the current value is equal to or less than the threshold value, step S15 is repeated for a period of time T2 in which the photosensitive drum 1 makes one rotation after applying Vpp (step S15). S18). When the current value is equal to or less than the threshold value in the above period, the value obtained by lowering the AC voltage Vpp at the time of leak detection by 200 V at that time is determined as the AC voltage Vpp during image formation (step S19). That is, the CPU 40 determines the development bias applied to the developing roller 7 without changing the development bias (AC voltage Vpp) applied to the developing roller 7 during image formation. Then, the development bias and the charge bias are turned off, and then the driving of the photosensitive drum 1 and the development roller 7 is stopped (step S20), and the discharge generation detection operation is terminated (end).

From the above, according to the present embodiment, the current value flowing between the developing roller 7 and the developing roller 7 is detected at a predetermined timing, and it is determined whether or not the absolute value of this current value exceeds the threshold value. This makes it possible to shorten the time required to set the development bias (development bias in which the absolute value of the current value is equal to or less than the threshold value) in which leakage does not occur.

The SD gap, charging bias, development bias, current value threshold, etc. described in this embodiment are not intended to limit the scope of the present invention to those unless otherwise specified. Absent.

Further, in this embodiment, a configuration is illustrated in which the average value obtained by averaging the current values output from the detection unit 35 in the time of 10 cycles of the AC voltage is illustrated, but the cycle time T of the AC voltage is limited to this. It is not a thing, and it may be set appropriately.

[Example 3]
The image forming apparatus according to the third embodiment will be described with reference to FIGS. 9 and 10. In this example, only the magnitude relationship of each member of the image forming portion and the discharge detection control are different from those in the second embodiment, and the other configurations are almost the same as those in the second embodiment. Therefore, in this example, the same reference numerals are given to the same configurations as those in the second embodiment described above, so that detailed description thereof will be omitted.

In the second embodiment, the surface potential of the photosensitive drum changes due to the discharge during the discharge generation detection operation sequence, and the toner may be unnecessarily consumed due to the development of the toner. In the third embodiment, only the current value flowing between the non-exposed region of the photosensitive drum and the developing roller facing this region is compared with the threshold value to set the development bias that does not consume toner and does not cause leakage. It is possible to reduce the time required for this.

<Relationship between the size of each member of the image forming part>
The magnitude relationship of each member of the image forming portion in the third embodiment will be described with reference to FIG. FIG. 9 is an explanatory diagram for explaining an example of the magnitude relationship of each member of the image forming portion in the third embodiment. In FIG. 9, in the axial direction of the developing roller 7, the toner coat region L1 is a developer supporting region on which the toner of the developing roller 7 is supported. The toner coat region L1 faces the developer-carrying region of the developing roller 7 and corresponds to the region of the photosensitive drum 1 to which the toner is applied from the developing roller 7. L2 is the roller width of the charging roller 4 in the axial direction. The roller width L2 of the charging roller 4 corresponds to the charging region of the photosensitive drum 1 charged by the charging roller 4. L3 is a drum coating width corresponding to the width of the photosensitive layer formed on the outer peripheral surface of the photosensitive drum 1. In the configuration of the third embodiment, the relationship between the roller width L2 of the charging roller 4 and the drum coating width L3 of the photosensitive drum 1 with respect to the toner coating region L1 of the developing roller 7 in the axial direction of the developing roller 7 ( (Size relationship) is L1 <L2, L1 <L3. That is, in the axial direction of the developing roller 7, the charging region (roller width L2) of the photosensitive drum 1 charged by the charging roller 4 is wider than the toner coat region L1 of the developing roller 7 (L1 <L2). Further, the laser beam scanner 6 can expose the photosensitive drum 1 to a region wider than the toner coat region L1 of the developing roller 7 (scanning exposure width L4 (not shown)) in the axial direction of the developing roller 7. (L1 <L4). Then, the surface potential of the charging region (roller width L2) of the photosensitive drum 1 charged by the charging roller 4 is defined as VD. Let VL be the surface potential of the exposed region of the photosensitive drum 1 exposed by the laser beam scanner 6. Here, the exposure region of the photosensitive drum 1 exposed by the laser beam scanner 6 is the region of the photosensitive drum 1 corresponding to the toner coat region L1 of the developing roller 7, and is narrower than the scan exposure possible width L4 of the laser beam scanner 6. The area. The DC voltage VDC applied to the developing roller 7 is set so that the relationship of VL <VDC is set in order to prevent the negatively charged toner 3 from being developed from the developing roller 7 to the photosensitive drum 1. When the toner is positively charged, the relationship is VL> VDC.

With the above relationship, the surface potentials of the unexposed non-exposed regions H1 and H2 of the photosensitive drum 1 shown in FIG. 9 become VD, and the surface potential of the exposed region of the photosensitive drum 1 facing the toner coat region L1 becomes VL. The relationship is VD <VL <VDC. That is, the potential VL of the region of the photosensitive drum 1 facing the toner coat region L1 of the developing roller 7 is the DC voltage VDC applied to the developing roller 7 and the laser beam scanner in the charged region (roller width L2) of the photosensitive drum 1. It is between the potential VD of the non-exposed region (non-image region) that is not exposed by 6. When the toner is positively charged, VD> VL> VDC. Therefore, the potential difference between the non-exposed regions H1 and H2 of the photosensitive drum 1 and the developing roller 7 facing the regions H1 and H2 becomes the potential difference between the photosensitive drum 1 facing the toner coat region L1 of the developing roller 7. On the other hand, it gets bigger. Therefore, leaks are likely to occur in the non-exposed regions H1 and H2 of the photosensitive drum 1. In this embodiment, it is necessary to set the potential difference so that the toner coat region L1 does not leak reliably at the time of leak detection. In this embodiment, the potential difference between VL and VDC with respect to Vpp is determined by calculating the discharge start voltage based on Paschen's law in consideration of the variation in the SD gap and the atmospheric pressure. In this embodiment, Vpp at the time of leak detection is 1.2 kV, VD is -600 V, VL is -200 V, and VDC is 0 V. With this setting, the maximum potential difference between the photosensitive drum 1 and the developing roller 7 becomes ΔVmax = Vpp / 2 + | VDC-VD | = 1200V, and ΔVmax during the initial discharge generation detection operation of Example 2 The potential difference can be the same as (Vpp = 2.0 kV, VD = −500 V, VDC = −300 V).

From the above, with the above configuration, it is possible to detect the presence or absence of a leak in the non-exposed region without causing a leak in the toner coat region during the discharge generation detection operation.

<Flowchart of discharge generation detection operation>
Next, an example of the control flow of the discharge generation detection operation of the image forming apparatus according to the second embodiment will be described with reference to FIG. FIG. 10 is a flowchart showing an example of a flow of control of the discharge generation detection operation of the image forming apparatus according to the third embodiment.

First, when the power of the image forming apparatus is turned on and the discharge generation detection operation is started (start), the rotating bodies such as the photosensitive drum 1 and the developing roller 7 are rotated by a drive mechanism (not shown) according to the instruction of the CPU 40. Is started (step S21). The driving of each rotating body continues until the discharge generation detection operation is completed. Next, a charging bias at which the surface potential of the charged region of the photosensitive drum 1 is −600 V is applied to the charging roller 4, and the laser beam scanner 6 is applied to the developer-carrying region of the photosensitive drum 1 facing the toner coat region L1. The surface potential of the toner coat region L1 of the photosensitive drum 1 is set to −200 V by exposure (step S22). When the time (T2) for one rotation of the photosensitive drum 1 elapses from step S22 (step S23), the surface of the photosensitive drum 1 is set to the set surface potential over the entire circumference. Next, the AC voltage Vpp applied to the developing roller 7 is set. The AC voltage Vpp applied to the developing roller 7 is set to an AC voltage Vpp lower than the setting at the time of image formation so as to be equivalent to ΔVmax at the initial discharge generation detection operation of the first embodiment (step S24). Here, the AC voltage applied to the developing roller 7 is set to an AC voltage Vpp which is 600 V lower than the AC voltage at the time of image formation. Next, it is determined whether or not the absolute value of the current value when a certain time T / 4 elapses from the timing when the AC voltage changes from negative to positive exceeds the threshold value which is a predetermined value (step S25). In this example, the threshold value is set to 10 μA.

When the current value exceeds the threshold value in step S25, the CPU 40 turns off the AC voltage Vpp of the development bias because a leak has occurred between the photosensitive drum 1 and the developing roller 7 (step S26). .. When such a leak occurs, it is necessary to set the AC voltage Vpp applied to the developing roller 7 during image formation to be smaller than the setting of the AC voltage applied to the developing roller 7 when the current value is detected. .. Therefore, the CPU 40, which is a control unit, lowers the development bias (AC voltage Vpp) applied to the developing roller 7 to a voltage lower than that at the time of detecting the current value (step S27). Here, the voltage is lowered to a voltage (1.7 kV) that is 100 V lower than the AC voltage (1.8 kV) during image formation. Then, returning to step S24, the AC voltage applied to the developing roller 7 is set to an AC voltage Vpp which is 600 V lower than the AC voltage (1.7 kV) during image formation whose setting has been changed. In this way, the AC voltage applied to the developing roller 7 at the time of leak detection is reduced from 1.2 kV to 1.1 kV. Then, in step S25 again, it is confirmed whether or not the current value exceeds the threshold value.

When the current value exceeds the threshold value in step S25, the CPU 40 steps down the AC voltage applied to the developing roller 7 at the time of leak detection, and repeats the above operation until the current value becomes equal to or less than the threshold value. That is, when the CPU 40, which is the control unit, detects that a leak has occurred in step S25, the AC voltage applied to the developing roller is gradually lowered to detect the leak between the photosensitive drum 1 and the developing roller 7. I do.

When the current value does not exceed the threshold value in step S25, that is, when the current value is equal to or less than the threshold value, step S25 is repeated for a period of time T2 in which the photosensitive drum 1 makes one rotation after applying Vpp (step S28). When the current value is equal to or less than the threshold value in the above period, the value increased by 600 V from the AC voltage Vpp at the time of leak detection at that time is determined as the AC voltage Vpp during image formation (step S29). That is, the CPU 40 determines the development bias applied to the developing roller 7 without changing the development bias (AC voltage Vpp) applied to the developing roller 7 during image formation. Then, the development bias and the charge bias are turned off, and then the driving of the photosensitive drum 1 and the development roller 7 is stopped (step S30), and the discharge generation detection operation is terminated (end).

From the above, according to the present embodiment, the photosensitive drum 1 facing the toner coated region is exposed during the discharge generation detection operation, and leak detection is performed in the non-exposed region of the toner coated region. As a result, it is possible to prevent wasteful consumption of toner and shorten the time required to set the development bias so that leakage does not occur.

The SD gap, charging bias, development bias, current value threshold, etc. described in this embodiment are not intended to limit the scope of the present invention to those unless otherwise specified. Absent.

[Other Examples]
In the above-described embodiment, the laser beam scanner is used as the exposure unit, but the present invention is not limited to this, and for example, an LED array or the like may be used.

Further, in the above-described embodiment, the photosensitive drum 1 and the charging unit, the developing unit, and the cleaning unit as the process means acting on the photosensitive drum 1 are integrated as a process cartridge that can be attached to and detached from the apparatus main body of the image forming apparatus. The process cartridge 20 having is illustrated. However, the process cartridge 20 is not limited to this. In addition to the photosensitive drum 1, a process cartridge having any one of a charging member, a developing unit, and a cleaning unit may be used.

Further, in the above-described embodiment, the configuration in which the process cartridge 20 including the photosensitive drum 1 is removable from the device main body of the image forming apparatus is illustrated, but the present invention is not limited to this. For example, an image forming apparatus in which the photosensitive drum 1 and each process means acting on the photosensitive drum 1 are incorporated may be used, or an image forming apparatus in which the photosensitive drum 1 and the process means acting on the photosensitive drum 1 are detachably attached to each other may be used.

Further, in the above-described embodiment, the printer is exemplified as the image forming apparatus, but the present invention is not limited thereto. For example, it may be another image forming apparatus such as a copying machine or a facsimile apparatus, or another image forming apparatus such as a multifunction device combining these functions. Alternatively, it may be an image forming apparatus that uses a recording medium carrier and sequentially superimposes and transfers toner images of each color on the recording medium supported on the recording medium carrier. Alternatively, it is an image forming apparatus that uses an intermediate transfer body, sequentially superimposes and transfers toner images of each color on the intermediate transfer body, and collectively transfers the toner images carried on the intermediate transfer body to a recording medium. May be good. Similar effects can be obtained by applying the present invention to these image forming devices.

H1, H2 ... Non-exposed area L1 ... Toner coating area L2 ... Roller width of charging roller L3 ... Drum coating width M ... Image forming apparatus main body 1 ... Photosensitive drum (image carrier)
2 ... Cleaning blade 4 ... Charging roller (charging member)
6 ... Laser beam scanner 7 ... Developing roller (developer carrier)
7a ... Sleeve 7b ... Cap 7b ... Roller shaft 8 ... Development blade 9 ... Development container 10 ... Recording medium 20 ... Process cartridge 30 ... DC voltage application unit 31 ... AC voltage application unit 32 ... Output control unit 33 ... Vpp control unit 34 ... Rectifier 35 ... Detection unit 36 ... Detection circuit 37 ... Amplifier 38 ... A / D converter 40 ... CPU (Control unit)

Claims (20)

With a rotatable image carrier,
A charging member that charges the image carrier and
A rotatable developer carrier provided so as to face the image carrier in a non-contact state and carrying a developer, and a rotatable developer carrier.
An application unit that applies a development bias in which a DC voltage and an AC voltage are superimposed on the developer carrier, and an application unit.
A detection unit that detects the current value flowing between the image carrier and the developer carrier, and
From the first current value and the first current value detected by the detection unit in a state where the image carrier and the developer carrier are rotated and the development bias is applied to the developer carrier. A control unit that controls the AC voltage applied to the developer carrier based on the difference from the second current value, which is also small.
An image forming apparatus characterized by having. The control unit applies the AC voltage applied to the developer carrier when the difference between the first current value and the second current value detected by the detection unit exceeds a threshold value. The image forming apparatus according to claim 1, wherein the current value is set to a value smaller than the AC voltage applied to the developer carrier at the time of detection. When the control unit detects that the difference between the first current value and the second current value exceeds the threshold value, the control unit gradually lowers the AC voltage applied to the developer carrier, and again. Claim 1 or 2 is characterized in that it detects whether or not the difference between the first current value and the second current value flowing between the image carrier and the developer carrier exceeds a threshold value. The image forming apparatus according to. The control unit does not change the setting of the AC voltage applied to the developer carrier when the difference between the first current value and the second current value detected by the detection unit is equal to or less than the threshold value. The image forming apparatus according to any one of claims 1 to 3, wherein the image forming apparatus is characterized. The control unit carries the developer based on the difference between the first current value and the second current value detected by the detection unit during the time during which the developer carrier or the image carrier rotates at least once. The image forming apparatus according to any one of claims 1 to 4, wherein the AC voltage applied to the body is controlled. The first current value is the maximum value of the current value detected by the detection unit, and the second current value is the minimum value of the current value detected by the detection unit. The image forming apparatus according to any one of claims 1 to 5. The detection unit has a third current value flowing between the image carrier and the developer carrier on the positive side of the AC voltage applied to the developer carrier, or an AC applied to the developer carrier. The fourth current value flowing between the image carrier and the developer carrier on the negative side of the voltage is detected.
When the difference between the maximum value and the minimum value of the third current value or the difference between the maximum value and the minimum value of the fourth current value detected by the detection unit exceeds the threshold value, the control unit said. The image according to claim 6, wherein the AC voltage applied to the developer carrier is set to a value smaller than the AC voltage applied to the developer carrier at the time of detection when the current value is detected. Forming device. The detection unit has a third current value flowing between the image carrier and the developer carrier on the positive side of the AC voltage applied to the developer carrier, or an AC applied to the developer carrier. The fourth current value flowing between the image carrier and the developer carrier on the negative side of the voltage is detected.
The control unit has a first average value obtained by averaging the third current value detected by the detection unit during the cycle time of the AC voltage, or a second average value obtained by averaging the fourth current value. When the difference between the maximum value and the minimum value of the calculated first average value or the difference between the maximum value and the minimum value of the second average value exceeds the threshold value, the developer carrier The image forming apparatus according to claim 6, wherein the AC voltage applied to the developer is set to a value smaller than the AC voltage applied to the developer carrier at the time of detecting the current value. The detection unit outputs a current value obtained by rectifying the current flowing between the image carrier and the developer carrier.
The image forming apparatus according to any one of claims 1 to 8, wherein the control unit outputs a value obtained by averaging the current values output from the detection unit in the cycle time of the AC voltage. .. With a rotatable image carrier,
A charging member that charges the image carrier and
A rotatable developer carrier provided so as to face the image carrier in a non-contact state and carrying a developer, and a rotatable developer carrier.
An application unit that applies a development bias in which a DC voltage and an AC voltage are superimposed on the developer carrier, and an application unit.
A detection unit that detects the current value flowing between the image carrier and the developer carrier, and
With the image carrier and the developer carrier rotated and the development bias applied to the developer carrier, the AC voltage applied to the developer carrier changes from negative to positive or from positive to negative. A control unit that controls the AC voltage applied to the developer carrier from the current value detected by the detection unit when a certain time elapses from the timing.
An image forming apparatus characterized by having. When a certain time elapses from the timing when the AC voltage applied to the developer carrier changes from negative to positive or positive to negative, the current value detected by the detection unit exceeds the threshold value. The ninth aspect of the present invention is characterized in that the AC voltage applied to the developer carrier is set to a value smaller than the AC voltage applied to the developer carrier at the time of detection when the current value is detected. The image forming apparatus according to the description. The image according to claim 10 or 11, wherein the control unit does not change the setting of the AC voltage applied to the developer carrier when the current value detected by the detection unit is equal to or less than the threshold value. Forming device. When the control unit detects that the current value exceeds the threshold value, the control unit gradually lowers the AC voltage applied to the developer carrier, and again between the image carrier and the developer carrier. The image forming apparatus according to claim 10 or 11, wherein it detects whether or not the flowing current value exceeds a threshold value. 10. The detection unit is characterized in that it detects a current value flowing between the image carrier and the developer carrier in a region where the AC voltage applied to the developer carrier is constant. The image forming apparatus according to any one of 13 to 13. It has an exposed portion that exposes to a charged image carrier,
In the axial direction of the developer carrier, the charged region of the image carrier charged by the charging member is wider than the developer-supporting region that supports the developer of the developer carrier.
The exposed portion can be exposed to a region wider than the developer-supporting region of the developer-supporting body with respect to the image-supporting body in the axial direction of the developing agent-supporting body.
When the control unit makes a comparison with the threshold value, the potential of the region of the image carrier facing the developer-bearing region in the axial direction of the developer-bearing body is applied to the developer-supporting body. The exposed area faces the developer-bearing area of the image-bearing body so that it is between the DC voltage and the potential of the non-exposed area that is not exposed by the exposed area in the charged area of the image-bearing body. The image forming apparatus according to any one of claims 10 to 14, wherein the image forming apparatus is exposed. The control unit is characterized in that the AC voltage applied to the developer carrier is controlled from the current value detected by the detector during a time during which the developer carrier or the image carrier rotates at least once. The image forming apparatus according to any one of claims 10 to 15. Any of claims 1 to 16, wherein the control unit performs an operation of controlling the AC voltage applied to the developer carrier according to the driving time of the developer carrier or the image carrier. The image forming apparatus according to item 1. The image forming according to any one of claims 1 to 17, wherein the control unit performs an operation of controlling the AC voltage applied to the developer carrier when the temperature / humidity or atmospheric pressure changes. apparatus. The image forming apparatus according to any one of claims 1 to 18, wherein the AC voltage applied to the developer carrier has a rectangular wavy shape. When the control unit performs an operation of controlling the AC voltage applied to the developer carrier, the control unit rotates at least one of the developer carrier and the image carrier, whichever has a longer rotation time. The image forming apparatus according to any one of claims 1 to 19.

Images