JP2021089367A - Image forming apparatus - Google Patents

Abstract

To reduce the time required for setting a developing bias at which leak 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 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 the maximum value and the minimum value of the current value detected by the detection unit and the 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 supports the image carrier, an application part that applies a development bias in which a DC voltage and an AC voltage are superimposed on the developer-supporting body, and The current value detected by the detection unit in a state where the detection unit for detecting the current value flowing in the image carrier, 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 for controlling the AC voltage applied to the developer carrier from the difference between the maximum value and the minimum value of the above and the 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 Relationship diagram of AC voltage and current value in Example 2 Relationship diagram of change amount of AC voltage and current value in Example 2 (A) Relationship diagram of voltage Vpp and current value leak determination threshold value in Example 2, (b) Relationship diagram of voltage Vpp and current value change amount leak determination threshold value in Example 2 Flow chart showing discharge detection control in Example 2 Diagram of the relationship between the potentials of the photosensitive drum and the developing roller in Example 2 Diagram of the relationship between the potentials of the photosensitive drum and the developing roller in Example 2

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 surfaced. 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 current value which is the difference between the maximum value and the minimum value of the output value (average value) of the current value at the time T2 when the photosensitive drum 1 rotates once, which is the longer time for one rotation. The amount of change is used for comparison with the threshold value. 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 in this embodiment is shown in FIG. 4 (b). The horizontal axis of FIG. 3B is the peak 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 a leak does not occur when the amount of change in the current value is equal to or less than a threshold value (here, 2.0 kV or less), which is a predetermined value, but a leak occurs when the amount exceeds the threshold value.

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 when the discharge generation is detected in the comparative example, and FIG. 5 (b) is the setting and the leak current of the AC voltage Vpp when the discharge generation is detected 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 difference between the maximum value and the minimum value of the output value (average value) of the current value detected by the detection unit 35 during the time T2 during which the photosensitive drum 1 rotates once.

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.

When the amount of change in the current value does not exceed the threshold value in step S5, that is, when 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, a value obtained by lowering the AC voltage Vpp at the time of leak detection by an 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 on the positive side of the AC voltage applied to the developing roller 7, the first 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 second 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 first 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 second 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 first current value or the change amount of the second current value, which is the difference between the maximum value and the minimum value, exceeds the threshold value. Lower than when the value was 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 current value or the amount of change in the second 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 detected by the detection unit 35 during the cycle time of the AC voltage, or the second average value obtained by averaging the second 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 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. Lower than when the value was 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 was measured, but the Vpp during image formation is not limited to Vpp, 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.

[Example 2]
In Example 1, the development bias applied to the developing roller 7 was set from the magnitude of the amount of change in the current value, which is the difference between the maximum value and the minimum value of the current value in the time T2 during which the photosensitive drum 1 makes one rotation. On the other hand, the second embodiment is characterized in that the development bias applied to the developing roller 7 is set based on the magnitude of the current value in addition to the amount of change in the current value.

The current flowing when the AC voltage and the DC voltage are applied between the developing roller 7 and the photosensitive drum 1 in this embodiment rectifies the flowing current and has a time T of 10 cycles of the AC voltage as in the first embodiment. The current value averaged at a certain 4 ms was used.

The image forming apparatus according to the second embodiment will be described with reference to FIGS. 7 to 10. 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>
FIG. 7 is a relationship diagram of the amount of change between the AC voltage and the current value (difference between the maximum value and the minimum value of the AC voltage) in the second embodiment. The current value flowing between the developing roller 7 and the photosensitive drum 1 when Vpp, which is the peak voltage of the AC voltage in the development bias, is plotted, the horizontal axis is the voltage Vpp, and the vertical axis is the current value. There is.

The solid line in FIG. 7 is the same as in Example 1, in which the current after rectification in the time T2 (here, 314 ms) until the photosensitive drum 1 makes one rotation is averaged for each period time T (here, 4 ms) of the AC voltage. The transition of the maximum value at time is represented, and the dotted line represents the transition of the minimum value. Then, FIG. 8 shows the relationship between the amount of change in the current value, which is the difference between the maximum value and the minimum value, and the AC voltage.

The position of the vertical broken line c in FIG. 8 is the leakage start voltage Vpp (discharge start voltage Vpp), and the right side of the vertical broken line c is the leak generation region where the leak occurs between the developing roller 7 and the photosensitive drum 1. Is. Looking at this leak generation region, it can be seen that when a leak occurs between the developing roller 7 and the photosensitive drum 1, the amount of change in the current value sharply increases, peaks at around Vpp = 2300V, and then decreases. It can also be seen that when Vpp = 2500V, the amount of change in the current value is the same as the leak non-occurrence region (the region on the left side of the vertical broken line c) even though the leak has occurred.

This will be described in detail with reference to FIGS. 9 (a) and 9 (b). FIG. 9 (a) shows the transition of the amount of change in the current value when the voltage Vpp is changed, and the horizontal solid line a in FIG. 9 (a) is when leak detection is performed from the magnitude of the amount of change in the current value. It represents the change amount threshold value A, which is the first threshold value. In this example, the change amount threshold value A, which is the first threshold value, is set to 1 μA, which is the same as in Example 1. However, the first threshold is not limited to this. The vertical broken line c in FIG. 9A is the voltage Vpp at the start of leakage. Further, the vertical broken line d in FIG. 9A is a voltage Vpp in which the amount of change in the current value is equal to or less than the first threshold value (the amount of change is equal to or less than the threshold value A) even though a leak has occurred. In FIG. 9A, the left side of the vertical broken line c is the area I, the area between the vertical broken line c and the vertical broken line d is the area II, and the right side of the vertical broken line d is the area III.

Region I is a region in which the amount of change in the current value is equal to or less than the change amount threshold value A and no leakage occurs. Then, as described in the first embodiment, the region II is a region in which a leak occurs and the amount of change in the current value exceeds the change amount threshold value A, and is a region in which the leak occurs.

The region III is a region in which the amount of change in the current value is equal to or less than the change amount threshold value A, even though a leak has occurred. The state of this region III occurs when the current value sticks to the upper limit when the upper limit of the voltage output capacity of the image forming apparatus is reached. Alternatively, the state of this region III is a continuous leak over the entire area by further increasing the voltage Vpp from the state of region II, which was an intermittent leak or a partial leak between the developing roller 7 and the photosensitive drum 1. It is caused by the change to.

FIG. 9B shows the transition of the current value when the voltage Vpp is changed, and here, the maximum value when the rectified current is averaged for each period time T (here, 4 ms) of the AC voltage. Shows the transition. The horizontal solid line b in FIG. 9B shows the magnitude of the current value in the region III of FIG. 9A in which the amount of change in the current value is equal to or less than the change amount threshold value A despite the occurrence of a leak. It represents the current threshold value B, which is the second threshold value when leak detection is performed. In this embodiment, the current threshold value B, which is the second threshold value, is set to 10 μA, which is larger than the change amount threshold value A, which is the first threshold value. However, the second threshold is not limited to this.

That is, in the region III in which the amount of change in the current value is equal to or less than the change amount threshold value A, the applied voltage Vpp is larger than in the region I in which the amount of change in the current value is also equal to or less than the change amount threshold value A. The current value flowing between the developing roller 7 and the photosensitive drum 1 is clearly large. For this reason, in addition to the amount of change in the current value, the magnitude of the current value is compared with the threshold value, and the development bias that does not cause leakage is set. That is, even if the amount of change in the current value is equal to or less than the first threshold value A or less, when the current value exceeds the second threshold value, the current threshold value B, the photosensitive drum 1 and the developing roller 7 are used. Since a leak has occurred between the two, the setting of the development bias applied to the developing roller 7 is lowered from the setting of the AC voltage applied to the developing roller 7 when the current value is detected. As a result, the region I where the leak does not occur and the region III where the leak occurs can be determined from the magnitude of the current value in addition to the amount of change in the current value without measuring the impedance.

<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 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 second embodiment. In the discharge generation detection operation described below, the amount of change in the current value flowing between the photosensitive drum 1 and the developing roller 7 and whether or not the current value exceeds the threshold value are determined, and the result is applied to the developing roller 7. Set the development bias to be performed. 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 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 S12 (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 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 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. 8, 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 a predetermined value. It is determined whether or not a certain first threshold value (change amount threshold value A) has been exceeded (step S15). In this embodiment, the change amount threshold value A, which is the first threshold value, is set to 1 μA. Here, the amount of change in the current value is the difference between the maximum value and the minimum value of the output value (average value) of the current value detected by the detection unit 35 during the time T2 during which the photosensitive drum 1 rotates once.

When the change amount of the current value exceeds the change amount threshold value A, which is the first threshold value, in step S15, a leak has occurred between the photosensitive drum 1 and the developing roller 7, so that the CPU 40 has a development bias. The AC voltage Vpp of the above is turned off (step S16). When such a leak occurs, the setting of the AC voltage Vpp applied to the developing roller 7 at the time of image formation is lowered to a voltage lower than the setting of the AC voltage applied to the developing roller 7 at the time of detecting the current value (step). S17). 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, returning to step S14, 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 S15 again, it is confirmed whether or not the amount of change in the current value exceeds the first threshold value.

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

When the amount of change in the current value does not exceed the first threshold value in step S15, that is, when the amount of change in the current value is equal to or less than the first threshold value, the second threshold value (current threshold value B) in which the current value is a predetermined value. Is determined (step S18). That is, a leak between the photosensitive drum 1 and the developing roller 7 is detected from the magnitude of the current value in addition to the amount of change in the current value.

When the current value exceeds the second threshold value in step S18, a leak occurs between the photosensitive drum 1 and the developing roller 7 even if the amount of change in the current value is equal to or less than the first threshold value. There is. Therefore, the CPU 40 turns off the AC voltage Vpp of the development bias (step S16). When such a leak occurs, as described above, the setting of the AC voltage Vpp applied to the developing roller 7 at the time of image formation is lower than the setting of the AC voltage applied to the developing roller 7 at the time of detecting the current value. Reduce to voltage (step S17). Then, returning to step S14, 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. Then, in step S18 again, it is confirmed whether or not the current value exceeds the second threshold value.

When the current value exceeds the second threshold value in step S18, the CPU 40 gradually lowers the AC voltage applied to the developing roller 7 at the time of leak detection until the current value becomes equal to or lower than the second threshold value. The above operation is repeated. That is, when the CPU 40, which is the control unit, detects that the current value exceeds the second threshold value in step S18, the AC voltage applied to the developing roller 7 is gradually lowered, and the current value becomes the first in step S15. If the current value exceeds the second threshold value, the detection of whether or not the current value exceeds the second threshold value is performed again (step S18).

When the current value does not exceed the second threshold value in step S18, that is, when the current value is equal to or less than the second threshold value, the period of time T2 in which the photosensitive drum 1 makes one rotation after applying Vpp, steps S15 and S18. Is repeated (step S19). When the amount of change in the current value is equal to or less than the first threshold value and the current value is equal to or less than the second threshold value in the above period, the value obtained by lowering the AC voltage Vpp at the time of leak detection at that time by an offset value (200 V) is used. The AC voltage Vpp during image formation is determined (step S20). That is, the development bias applied to the developing roller 7 is determined. 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 S21), and the discharge generation detection operation is terminated (end).

From the above, according to the present embodiment, the amount of change in the current value flowing between the photosensitive drum 1 and the developing roller 7 and the magnitude of the current value are compared with the threshold value, and the developing bias applied to the developing roller 7 is determined. Set. As a result, the time required to set the development bias that does not cause leakage can be shortened. At the same time, it becomes possible to detect the presence or absence of a leak with higher accuracy.

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. For example, the averaging time T may be increased to several tens of ms from the viewpoint of noise removal. Further, this does not apply when the peripheral speeds of the developing roller 7 and the photosensitive drum 1 and the frequencies of the AC voltage change.

Further, in this embodiment, a configuration in which the amount of change in the current value and the magnitude of the current value are compared with the threshold value during the period of one rotation of the photosensitive drum 1 is illustrated, but the comparison with the threshold value is performed. The period is not limited to this. 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. 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, similarly to the above-described first embodiment, the current flowing when the AC voltage and the DC voltage are applied between the developing roller 7 and the photosensitive drum 1 is rectified, and the time of 10 cycles as the cycle time of the AC voltage. The current value averaged at (4 ms) was used as the current value to be compared with the threshold value. However, the current value to be compared with the threshold value is not limited to this rectified current value, and the current value before rectification as shown in FIGS. 11 and 12 can also be used.

11 (a) and 12 (a) show the potential relationship between the photosensitive drum 1 and the developing roller 7 when measuring the current flowing between the developing roller 7 and the photosensitive drum 1. A development bias was applied to the developing roller 7 by superimposing an AC voltage Vpp = 2000V on a DC voltage Vdc = −300V.

FIG. 11A is an explanatory view when the surface potential Vd of the photosensitive drum 1 at the time of forming a white image is −500 V, and FIG. 12A is an explanatory diagram when the surface potential of the photosensitive drum 1 at the time of forming a black image is VL = −100 V. Is.

The currents flowing between the developing roller 7 and the photosensitive drum 1 are shown in FIGS. 11 (b) and 12 (b). If the potential difference between the surface potential of the photosensitive drum 1 and the voltage of the developing roller 7 is large, a larger current flows between the photosensitive drum 1 and the developing roller 7. That is, as shown in FIG. 11A, when the surface potential Vd of the photosensitive drum 1 is larger on the negative side than the DC voltage Vdc of the developing roller 7, the potential difference between Vd and Vmax is larger than the potential difference between Vd and Vmin. The potential difference of is larger. Therefore, as shown in FIG. 11B, the absolute value of the current flowing on the positive side is larger. Further, as shown in FIG. 12A, when the surface potential VL of the photosensitive drum 1 is larger on the positive side than the DC voltage Vdc of the developing roller 7, the potential difference between VL and Vmin is larger than the potential difference between VL and Vmax. The potential difference of is larger. Therefore, as shown in FIG. 12B, the absolute value of the current flowing on the negative side is larger.

Then, normally, a leak is likely to occur between the developing roller 7 and the photosensitive drum 1 on the side where the absolute value of this current becomes large. Therefore, it is preferable to use the average value (current value) on the side where the absolute value is large as the current value to be compared with the threshold value.

As described above, in Examples 1 and 2, the rectified current value is used for comparison with the threshold value, but the current value before rectification may be used for comparison with the threshold value. As the current value before rectification, either one or both of the maximum and minimum current values may be used, or the average value obtained by averaging the current values in a certain time unit may be used. ..

For example, the detection unit 35 is on the positive side of the AC voltage applied to the developing roller 7, the first 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 second current value flowing between the photosensitive drum 1 and the developing roller 7 is detected. Then, the CPU 40, which is the control unit, receives the difference between the maximum value and the minimum value of the first current value detected by the detection unit 35 or the second current during the time when the developing roller 7 or the photosensitive drum 1 rotates at least once. The development bias applied to the developing roller 7 is set from the difference between the maximum value and the minimum value and the magnitude of the current value. The CPU 40 determines the development bias applied to the developing roller 7 when the amount of change in the first current value or the amount of change in the second current value, which is the difference between the maximum value and the minimum value, exceeds the first threshold value. The setting is lowered from the setting of the AC voltage applied to the developing roller 7 when the current value is detected. Further, in the CPU 40, even if the amount of change in the first current value or the amount of change in the second current value is equal to or less than the first threshold value, the first current value or the second current value is second. The setting of the development bias applied to the developing roller 7 when the threshold value of 2 is exceeded is lowered from the setting of the AC voltage applied to the developing roller 7 when the current value is detected.

Further, in the CPU 40, the amount of change in the first current value or the amount of change in the second current value is equal to or less than the first threshold value, and the first current value or the second current value is the second. The setting of the development bias applied to the developing roller 7 is not changed when the value is equal to or less than the threshold value of.

Alternatively, the CPU 40, which is a control unit, has a first average value obtained by averaging the first current value detected by the detection unit in the cycle time of the AC voltage, or a second average value obtained by averaging the second current value. Calculate the average value. Then, the CPU 40 determines the difference between the maximum value and the minimum value of the first average value or the difference between the maximum value and the minimum value of the second average value calculated above during the time when the developing roller 7 or the photosensitive drum 1 rotates at least once. The development bias applied to the developing roller 7 is set from the magnitude of the current value. The CPU 40 determines the development bias 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 first threshold value. The setting is lowered from the setting of the AC voltage applied to the developing roller 7 when the current value is detected. Further, in the CPU 40, even if 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 first threshold value, the first average value or the second average value is the second. The setting of the development bias applied to the developing roller 7 when the threshold value of 2 is exceeded is lowered from the setting of the AC voltage applied to the developing roller 7 when the current value is detected.

Further, in the CPU 40, 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 first threshold value, and the first average value or the second average value is the second. The setting of the development bias applied to the developing roller 7 is not changed when the value is equal to or less than the threshold value of.

Even with this configuration, the amount of change in the current value and the current value can be used to compare with the threshold value, and the development bias applied to the developing roller 7 can be set, and the development bias is set so that leakage does not occur. The time required for this can be shortened. At the same time, it becomes possible to detect the presence or absence of a leak with higher accuracy.

[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.

a ... Change amount threshold (first threshold)
b ... Current threshold (second threshold)
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 (15)

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 difference between the maximum and minimum current values detected by the detection unit and the current. A control unit that controls the AC voltage applied to the developer carrier from the value, and a control unit that controls the AC voltage.
An image forming apparatus characterized by having. The control unit uses the AC voltage applied to the developer carrier when the difference between the maximum value and the minimum value of the current value detected by the detection unit exceeds the first threshold value. It is smaller than the AC voltage applied to the developer carrier at the time of detection.
Even if the difference between the maximum value and the minimum value of the current value detected by the detection unit is equal to or less than the first threshold value, when the current value exceeds the second threshold value, the current value is applied to the developer carrier. The image forming apparatus according to claim 1, wherein the AC voltage is made smaller than the AC voltage applied to the developer carrier at the time of detecting the current value. When the difference between the maximum value and the minimum value of the current value detected by the detection unit is equal to or less than the first threshold value and the current value is equal to or less than the second threshold value, the control unit carries the developer. The image forming apparatus according to claim 2, wherein it detects that the AC voltage applied to the device is not changed. The detection unit outputs a current value obtained by rectifying the current flowing between the image carrier and the developer carrier.
The control unit calculates an average value obtained by averaging the current values output from the detection unit during the cycle time of the AC voltage, and the difference between the maximum value and the minimum value of the calculated average value is the first threshold value. When the value exceeds, the AC voltage applied to the developer carrier is made smaller than the AC voltage applied to the developer carrier at the time of detection when the current value is detected.
Even if the difference between the maximum value and the minimum value of the calculated average value is equal to or less than the first threshold value, when the calculated average value exceeds the second threshold value, the voltage is applied to the developer carrier. The image forming apparatus according to claim 1, wherein the AC voltage is made smaller than the AC voltage applied to the developer carrier at the time of detecting the current value. The control unit is the developer carrier when the difference between the maximum value and the minimum value of the calculated average value is equal to or less than the first threshold value and the calculated average value is equal to or less than the second threshold value. The image forming apparatus according to claim 4, wherein the AC voltage applied to the image processing apparatus is not changed. The detection unit is the first 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 the AC applied to the developer carrier. The second 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 first current value or the difference between the maximum value and the minimum value of the second current value detected by the detection unit exceeds the first threshold value. In addition, the AC voltage applied to the developer carrier is made smaller than the AC voltage applied to the developer carrier at the time of detection when the current value is detected.
Even if the difference between the maximum value and the minimum value of the first current value or the difference between the maximum value and the minimum value of the second current value is equal to or less than the first threshold value, the first current value or the second current value. When the current value of the above exceeds the second threshold value, the AC voltage applied to the developer carrier is made 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 claim 1. The control unit has the first current value when the difference between the maximum value and the minimum value of the first current value or the difference between the maximum value and the minimum value of the second current value is equal to or less than the first threshold value. The image forming apparatus according to claim 6, wherein when the second current value is equal to or less than the second threshold value, it is detected that the AC voltage applied to the developer carrier is not changed. The detection unit is the first 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 the AC applied to the developer carrier. The second 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 first current value detected by the detection unit during the cycle time of the AC voltage, or a second average value obtained by averaging the second 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 calculated second average value exceeds the first threshold value. The AC voltage applied to the developer carrier is made smaller than the AC voltage applied to the developer carrier at the time of detection when the current value is detected.
Even if the difference between the maximum value and the minimum value of the first average value or the difference between the maximum value and the minimum value of the second average value is equal to or less than the first threshold value, the first average value or the second average value When the average value of the above exceeds the second threshold value, the AC voltage applied to the developer carrier shall be 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 claim 1. In the control unit, the difference between the maximum value and the minimum value of the first average value or the difference between the maximum value and the minimum value of the second average value is equal to or less than the first threshold value, and the first average value. The image forming apparatus according to claim 8, wherein when the second average value is equal to or less than the second threshold value, it is detected that the AC voltage applied to the developer carrier is not changed. Claims 2 to 9 are characterized in that, when the control unit detects that the threshold value has been exceeded, the AC voltage applied to the developer carrier is gradually lowered and compared with the threshold value again. The image forming apparatus according to any one of the above items. 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 1 to 10. Any of claims 1 to 11, 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 12, 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 13, 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 14.

Images