JP7566599B2 - Image forming device - Google Patents

Description

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

In image forming devices such as printers that use the electrophotographic method (electrophotographic process), various developing devices are used to develop the electrostatic latent image formed on the image carrier. One example is the non-contact development method in which the image carrier and the developer carrier facing it are arranged with a specified gap between them.

In the non-contact development method, a development bias consisting of a DC voltage superimposed on an AC voltage is applied to the developer carrier, causing the charged toner to fly from the developer carrier to the image carrier, and a toner image is developed onto the electrostatic latent image formed on the image carrier. The toner image developed on the image carrier is then transferred and fixed onto a recording medium such as paper.

However, in non-contact development methods, the gap between the image carrier and the developer carrier may fluctuate when the image carrier and the developer carrier are driven. Fluctuations in the gap cause fluctuations in the electric field strength between the image carrier and the developer carrier, resulting in problems such as uneven density in the formed image.

To address this issue, by increasing the peak-to-peak voltage (peak-to-peak value) of the AC voltage in the developing bias, the toner can be sufficiently dispersed from the developer carrier to the image carrier, and the occurrence of density unevenness can be suppressed. However, when the peak-to-peak voltage of the AC voltage in the developing bias is increased, the potential difference with the surface potential of the image carrier increases. This causes discharge between the developer carrier and the image carrier, resulting in a current leak (hereinafter referred to as a leak) in which a discharge current flows due to the discharge, and this causes noise in the formed image.

The peak-to-peak voltage (peak-to-peak value) of the AC voltage at which the leakage occurs varies with the gap and air pressure, and therefore varies with changes in the individual image forming device and the usage environment.

Therefore, in Patent Document 1, the peak-to-peak voltage (peak-to-peak value) of the AC voltage in the developing bias applied between the image carrier and the developer carrier is gradually increased from a value at which no leakage occurs. Then, the impedance is measured based on the value of the current flowing between the image carrier and the developer carrier, and the occurrence of leakage is detected from the measured impedance value and the current value.

JP 2005-78015 A

However, in Patent Document 1, in order to detect the occurrence of leakage between the image carrier and the developer carrier, it is necessary to measure the impedance in advance, which causes the problem that it takes a long time to set the development bias at which leakage does not occur.

The 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 provides a method for manufacturing a photovoltaic device, comprising: a rotatable image carrier; a charging member for charging the image carrier; a rotatable developer carrier arranged to face the image carrier in a non-contact state and carrying a developer; an application unit for applying a developing bias in which a DC voltage and an AC voltage are superimposed on the developer carrier; a detection unit for detecting a current value flowing between the image carrier and the developer carrier; and a control unit for controlling the AC voltage applied to the developer carrier from a difference between a first current value detected by the detection unit and a second current value smaller than the first current value, when the image carrier and the developer carrier are rotated and the developing bias is applied to the developer carrier, wherein the control unit causes one of the developer carrier or the image carrier, whichever takes a longer time to rotate once, to rotate at least once when performing an operation of controlling the AC voltage applied to the developer carrier .

The present invention can reduce the time required to set a development bias that does not cause leakage.

Cross-sectional view of an image forming apparatus FIG. 1 is an explanatory diagram showing a configuration related to application of a development bias and detection of discharge; (a) is a waveform diagram of the current value when Vpp is changed, and (b) is a relationship diagram of the difference between the maximum and minimum values of the AC voltage and the current value in the embodiment. 1A is a diagram showing the relationship between AC voltage and current value in a comparative example, and FIG. 1B is a diagram showing the relationship between AC voltage and the amount of change in current value in an embodiment. 1A is a timing chart showing the setting of an AC voltage when detecting the occurrence of a discharge in a comparative example; FIG. 1B is a timing chart showing the setting of an AC voltage when detecting the occurrence of a discharge in an embodiment; Flowchart showing discharge detection control in the first embodiment Waveform diagram of AC voltage and current value of developing bias applied in Example 2 Flowchart showing discharge detection control in the second embodiment FIG. 13 is an explanatory diagram for explaining the size relationship of each member of the image forming unit in the third embodiment; Flowchart showing discharge detection control in the third embodiment

The following describes in detail the preferred embodiments of the present invention by way of example, with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the following examples should be appropriately changed depending on the configuration and various conditions of the device to which the present invention is applied, and the scope of the present invention is not intended to be limited to the following examples.

Example 1
The overall configuration of an image forming apparatus and an image forming operation will be described with reference to Fig. 1. Fig. 1 is a schematic cross-sectional view showing the general configuration of an image forming apparatus according to a first embodiment of the present invention.

<Description of Image Forming Apparatus>
The image forming apparatus is a laser printer using an electrophotographic method, and is configured such that a process cartridge 20 can be detachably attached to an apparatus main body M. Here, the apparatus main body M of the image forming apparatus refers to the components of the image forming apparatus excluding the process cartridge 20. Furthermore, the image forming apparatus to which the present invention is applicable is not limited to the one shown here. For example, the present invention is also applicable to a color laser printer that includes multiple process cartridges 20 and transfers multiple toner images to a recording medium using an intermediate transfer belt (intermediate transfer body) to form a color image.

The photosensitive drum 1, which serves as an image carrier (chargeable body), is a conductive drum having an OPC (organic photoconductor) photosensitive layer formed on the outer peripheral surface thereof, and is driven to rotate in the direction of arrow r1 in FIG. 1 at a predetermined process speed by a drive mechanism (not shown) in the main body of the device.

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

The developing device as a developing section develops the electrostatic latent image formed on the surface of the photosensitive drum 1 with toner as a developer. The developing device is composed of a developing roller 7, a developing blade 8, and a developing container 9. The developing roller 7 is disposed opposite 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 carried by the developing roller 7 and imparting an electric charge to the toner. The developing container 9 is a developer container for containing the toner to be supplied to the photosensitive drum 1.

The developing roller 7 receives a driving force from a drive mechanism (not shown) in the main body of the device, and is driven to rotate in the direction of the arrow r2 in FIG. 1. A toner layer (magnetic brush) to which a charge has been applied by the developing blade 8 is formed on the surface of the developing roller 7. A developing bias, which is a superimposed AC voltage and DC voltage, is then applied to the developing roller 7, and the toner carried by the developing roller 7 flies to the photosensitive drum 1 due to the electric field of the developing bias, and the electrostatic latent image formed on the surface of the photosensitive drum 1 is developed into a toner image.

Meanwhile, the recording medium 10 is fed by a feed roller (not shown) or the like, and the toner image (developer image) developed on the surface of the photosensitive drum 1 is transferred to the nip portion between the photosensitive drum 1 and the transfer roller 11. The recording medium 10 with the transferred toner image is separated from the surface of the photosensitive drum 1 and sent to the fixing device 12, where it is heated and pressurized to fix the transferred toner image to the recording medium 10.

Toner that is not transferred to the recording medium 10 and remains on the surface of the photosensitive drum 1 is removed by a cleaning blade 2, which serves as a cleaning unit that contacts the photosensitive drum 1 to clean it, and is then collected in a cleaning container 5. The surface of the photosensitive drum 1 is then charged again by the charging roller 4, and the above-mentioned process is repeated to complete a series of image formation cycles.

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

<Description of Configuration for Detecting Discharge Between Photosensitive Drum and Developing Roller>
Next, a configuration relating to application of a developing bias to the developing roller 7 and detection of discharge between the photosensitive drum 1 and the developing roller 7 will be described with reference to Fig. 2. Fig. 2 is an explanatory diagram showing a configuration relating to application of a developing bias and detection of discharge.

As shown in FIG. 2, the developing roller 7 has a sleeve 7a that carries toner during image formation, and circular caps 7b are fitted into both longitudinal ends of the sleeve 7a. The developing roller 7 is driven to rotate around a 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 driven to rotate at a peripheral speed of 300 mm/s.

The developing roller 7 is also arranged to face the photosensitive drum 1 in a non-contact state with a predetermined gap (SD gap) between them. In this embodiment, the cap 7b has a larger outer diameter than the sleeve 7a, and the outer peripheral surface of the cap 7b abuts against the surface of the photosensitive drum 1. This provides a predetermined gap 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, an SD gap of 200 μm is provided as the predetermined gap.

The configuration for providing a predetermined gap 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 that rotatably supports the developing roller 7 and the photosensitive drum 1.

In addition, 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 developing bias, which is a superimposed DC voltage and AC voltage, to the developing roller 7.

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

The AC voltage application unit 31 is a circuit that outputs an AC voltage whose average value (area center value) is the DC voltage output by the DC voltage application unit 30. The AC voltage application unit 31 outputs, for example, a rectangular wave (pulse-like) AC voltage with 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 the peak-to-peak voltage (peak-to-peak value) of the AC voltage, according to instructions from the CPU 40 as a control unit.

The detection unit 35 detects the current value flowing between the photosensitive drum 1 and the developing roller 7. The detection unit 35 is composed of a rectifier 34, a detection circuit 36, and an amplifier 37. The rectifier 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 A/D converted by the A/D converter 38, and outputs a current value averaged over the cycle time T of the AC voltage. Here, the CPU 40 outputs (calculates) a current value (average value) averaged over 4 ms, which is the time T of 10 cycles of the AC voltage. As will be described later, the CPU 40 is a control unit that controls the development bias applied to the development roller 7 based on the current value detected by the detection unit 35.

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

Figure 3(a) 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 developing bias, is changed, with the horizontal axis being time and the vertical axis being 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 set the surface potential of the photosensitive drum 1 to -500V, and a DC voltage of -300V is applied to the developing roller 7 by the DC voltage application unit 30. Next, the AC voltage peak-to-peak voltage Vpp is applied to the developing roller 7 by the AC voltage application unit 31. At this time, as shown in Figure 3(b), the AC voltage peak-to-peak voltage Vpp is increased stepwise from 1600V by 200V at a predetermined time interval (here, 1s), and the relationship between time and the output value of the current value at each AC voltage Vpp is plotted. No leakage current occurred when the AC voltage Vpp was 1.8 kV and 2.0 kV, but leakage current occurred when the AC voltage Vpp was 2.2 kV. Compared to when no leakage current occurred, when leakage current occurred, the leakage current fluctuated with the rotation period of the developing roller 7. This is because the area where leakage current occurs fluctuates with the rotation period of the developing roller 7. The distance between the developing roller 7 and the photosensitive drum 1 (SD gap) fluctuates due to unevenness in the shapes 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 varies with the rotation period 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 in this embodiment, it is known that the discharge start voltage decreases as the distance between the developing roller 7 and the photosensitive drum 1 decreases. 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 occurs in a part of the gap region in the axial direction between the developing roller 7 and the photosensitive drum 1 where the distance is close. When the SD gap changes in the axial direction of the developing roller 7, the region where the leakage current occurs also changes, so the current value varies according to the change in the region where the leakage current occurs. Therefore, as shown in FIG. 3(a), in a situation where the peak-to-peak voltage Vpp of the AC voltage where the leakage current occurs is 2.2 kV, the current value varies with the rotation period of the photosensitive drum 1 and the developing roller 7. The current value fluctuates with the rotation period of the developing sleeve (157 ms in this example), which is shorter than the rotation period of the photosensitive drum 1. Therefore, the CPU 40 can distinguish between sudden noise and current changes due to leakage by outputting a current value averaged over the AC voltage cycle time (a time period of one or more cycles) T. Note that in this embodiment, the current value averaged over 4 ms, which is the time for 10 cycles, is output as the AC voltage cycle time T, but from the perspective of noise removal, the averaging time T may be increased to several tens of ms.

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), which is 157 ms. Therefore, the CPU 40, which is the control unit, compares the change in current value, which is 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 at time T2, which is the longer time required for one rotation, with the threshold value. Figure 3(b) 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 peak-to-peak voltage Vpp of the AC voltage is changed. The relationship between the change in 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 Figure 4(b). The horizontal axis of Figure 3(b) indicates the peak-to-peak voltage Vpp of the AC voltage applied to the developing roller 7, and the vertical axis indicates the maximum value (solid line in the figure) and the minimum value (dotted line in the figure) of the current value (output value) during the time required for one rotation of the photosensitive drum 1. The difference between the maximum and minimum current values 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 set the surface potential of the photosensitive drum 1 to -500 V, and a DC voltage of -300 V is applied to the developing roller 7 by the DC voltage application unit 30. Next, the peak-to-peak voltage Vpp of the AC voltage is applied to the developing roller 7 by the AC voltage application unit 31. At this time, the peak-to-peak voltage Vpp of the AC voltage is increased stepwise by 200 V at a predetermined time interval (here, 1 s) from 1600 V, and the maximum and minimum current values at each Vpp are plotted. The difference between the maximum and minimum current values at each Vpp is the amount of change in the current value. When the Vpp in the developing bias is increased stepwise, the amount of change in the current value increases rapidly at the timing when the leak occurs, compared to the amount of change in the current value when the leak does not occur. In other words, if the change in current value is below a certain threshold (here, below 2.0 kV), no leakage will occur, but if the change exceeds the threshold, leakage will occur.

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

Next, 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 and this embodiment will be described with reference to Figures 4(a) and 4(b). Figure 4(a) 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. Figure 4(b) 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 Figure 4(b) represents the difference between the maximum and minimum current values shown in Figure 3(b).

In the comparative example, the absolute value of the current between the photosensitive drum 1 and the developing roller 7 is integrated over the period of the AC voltage cycle time T, and the output value is the value. The horizontal axis is the peak-to-peak voltage of the AC voltage, and the vertical axis is the output value. As the measurement conditions, 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 set the surface potential of the photosensitive drum 1 to -500 V, and a DC voltage of -300 V is applied to the developing roller 7 by the DC voltage application unit 30. Next, the AC voltage peak-to-peak voltage Vpp is applied to the developing roller 7 by the AC voltage application unit 31. At this time, Vpp is gradually increased from 0 V, and the relationship between Vpp and the output value of the current value is plotted. In FIG. 4(a), it can be seen that the output value, which is the current value, increases in proportion to the voltage Vpp, even when the voltage is below the discharge start voltage Vpp, which is below the threshold value.

The slope of the output value at a voltage Vpp below the threshold (below the discharge start voltage) is determined by the impedance between the photosensitive drum 1 and the developing roller 7, and therefore varies depending on the SD gap, etc. Therefore, the output value varies due to variations in the components of the cap 7b and wear due to durability. Therefore, it is not possible to accurately calculate the current value at which leakage occurs when the SD gap varies due to wear of the cap 7b and variations in the components under usage conditions. In order to accurately determine whether leakage occurs, it is necessary to find the impedance between the photosensitive drum 1 and the developing roller 7 using a voltage Vpp at which no leakage occurs. Since it is necessary to measure the impedance at a voltage Vpp at which no leakage occurs to detect leakage, it takes time to detect the occurrence of leakage.

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 using the change in current value, which is the difference between the maximum and minimum values of the output value (average value) of the current value during the time T2 when the photosensitive drum 1 rotates once. As shown in FIG. 4(b), it can be seen that the change in current value is almost unchanged at a voltage below the discharge start voltage Vpp, which is below the threshold value. And, when the voltage Vpp exceeds the discharge start voltage Vpp, which is the threshold value, it can be seen that the change in current value increases rapidly. In other words, when the change in current value is below the threshold value (below the discharge start voltage Vpp), which is a predetermined value, leakage does not occur, but when it exceeds the threshold value, leakage occurs. Therefore, by determining the development bias (AC voltage) applied to the development roller 7 from the change in the current value flowing between the photosensitive drum 1 and the development roller 7, it is possible to eliminate the variation in current value due to fluctuations in the SD gap, etc. Therefore, in this embodiment, the development bias (AC voltage) applied to the development roller 7 can be determined from the change in the current value flowing between the photosensitive drum 1 and the development roller 7 at an arbitrary applied voltage without measuring the impedance between the photosensitive drum 1 and the development roller 7, and the time required to set a development bias that does not cause leakage can be shortened.

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

Figure 5(a) shows the configuration of the comparative example, and Figure 5(b) shows the configuration of the embodiment 1. In the configuration of the comparative example shown in Figure 5(a), a measurement time is required to measure the impedance between the photosensitive drum 1 and the developing roller 7, compared to the configuration of the embodiment 1 shown in Figure 5(b). The impedance between the photosensitive drum 1 and the developing roller 7 changes due to the fluctuation of the SD gap caused by the driving 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 time T2 until the photosensitive drum 1 rotates once, and the impedance is calculated using the current value that is the maximum value at the timing when the SD gap is narrowest. The subsequent operation has the same voltage Vpp setting and timing as in this embodiment, so it will be described in Figure 5(b). The voltage Vpp during the discharge generation detection operation in Figure 5(b) is determined based on the voltage Vpp during image formation. The voltage Vpp during image formation is set to 1.8 kV as the initial setting. If the voltage Vpp exceeds 1.8 kV, toner will be developed on the white background of the recording medium 10, which will worsen the so-called background fogging, so the upper limit of the voltage Vpp is set to 1.8 kV.

Considering that the discharge start voltage, which is the threshold value, changes due to changes in temperature and humidity during paper passage and fluctuations in the SD gap, the voltage Vpp during the initial discharge detection operation is set to 2.0 kV, which is 1.8 kV plus an offset value of 200 V. In other words, the AC voltage Vpp applied to the developing roller 7 when comparing with the threshold value is a higher AC voltage (here, 2.0 kV) than the AC voltage applied to the developing roller 7 during image formation (here, 1.8 kV). If the CPU 40 determines that the 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 initial discharge detection is set to 2.0 kV, 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 has exceeded the threshold, as shown in FIG. 5B, it gradually reduces the AC voltage Vpp applied to the developing roller 7 until the amount of change in the current value becomes equal to or less than the threshold. Then, it resets the voltage Vpp obtained by subtracting an offset value of 200 V from the voltage Vpp at which the amount of change in the current value becomes equal to or less than the threshold, as the applied voltage Vpp during image formation. This makes it possible to prevent leakage during image formation.

Furthermore, by adopting a configuration in which the voltage Vpp is gradually lowered when the discharge occurrence is detected as in this embodiment, the time required to set the developing bias at which leakage does not occur can be shortened compared to the conventional control in which the voltage Vpp is increased to obtain the discharge start voltage. The reason is that the configuration in which the voltage Vpp is gradually lowered ends the detection at the point when it is determined that the amount of change in the current value is equal to or less than the threshold value at the voltage Vpp of the initial condition at the time of leakage detection, so that the detection ends at the voltage Vpp of one condition at the shortest. On the other hand, the conventional configuration in which the voltage Vpp is gradually increased increases the voltage Vpp gradually from a voltage Vpp at which leakage does not occur reliably. Therefore, the conventional configuration requires comparison with the threshold value at at least two or more voltage Vpp conditions, the voltage Vpp at which the amount of change in the current value is reliably equal to or less than the threshold value and the voltage Vpp to be used in image formation. From the above, by adopting a configuration in which the voltage Vpp is gradually lowered when the discharge occurrence is detected, the time required to set the developing bias at which leakage does not occur can be shortened.

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

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

First, when the power supply of the image forming apparatus is turned on and the discharge occurrence detection operation is started (START), the drive mechanism (not shown) starts driving each rotating body such as the photosensitive drum 1 and the developing roller 7 in response to an instruction from the CPU 40 (step S1). The driving of each rotating body continues until the discharge occurrence detection operation is completed. Next, a charging bias is applied to the charging roller 4, and a DC voltage of -300V is applied to the developing roller 7 by the DC voltage application unit 30 (step S2). After the time (T2) for the photosensitive drum 1 to rotate once has elapsed from step S2 (step S3), the surface of the photosensitive drum 1 becomes the set surface potential of -500V over the entire circumference. Next, the AC voltage Vpp to be applied to the developing roller 7 is set. Taking into account the change in temperature and humidity during paper passage and the fluctuation of the SD gap, the AC voltage Vpp to be applied to the developing roller 7 is set to an AC voltage Vpp higher than the setting during image formation by an offset value (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 during image formation. Next, as described with reference to FIG. 4B, it is determined whether the amount of change in the current value flowing between the developing roller 7 and the photosensitive drum 1 exceeds a predetermined threshold value when the set AC voltage Vpp is applied to the developing roller 7 (step S5). In this embodiment, the threshold value is 1 μA. Here, the amount of change in the current value is 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 detected by the detection unit 35 during the time T2 when the photosensitive drum 1 rotates once.

If the change in the current value exceeds the threshold value in step S5, a leak occurs between the photosensitive drum 1 and the developing roller 7, so the CPU 40 turns off the AC voltage Vpp of the developing bias (step S6). The reason for turning off Vpp once in step S6 is that once a developing leak occurs, the developing leak may continue due to the continuous current flow, and the continuous developing leak is interrupted once. If a leak occurs in this way, it is necessary to set the AC voltage Vpp applied to the developing roller 7 during image formation to be lower than the AC voltage applied to the developing roller 7 when the current value was detected. Therefore, the CPU 40, which is the control unit, reduces the developing bias (AC voltage Vpp) applied to the developing roller 7 to a voltage lower than that when the current value was detected (step S7). Here, the AC voltage during image formation (e.g., 1.8 kV) is reduced to a voltage 100 V lower (e.g., 1.7 kV). Then, returning to step S4, the AC voltage applied to the developing roller 7 is set to an AC voltage Vpp that is higher than the AC voltage during image formation whose setting has been changed by the offset value. In this way, the AC voltage applied to the developing roller 7 when a leak is detected is reduced from 2.0 kV to 1.9 kV. Then, in step S5, it is again confirmed whether the amount of change in the current value exceeds the threshold value.

If the amount of change in the current value exceeds the threshold value in step S5, the CPU 40 gradually reduces the AC voltage applied to the developing roller 7 when detecting a leak, and repeats the above-mentioned operation until the amount of change in the current value becomes equal to or less than the threshold value. In other words, when the CPU 40, which is the control unit, detects in step S5 that the amount of change in the current value has exceeded the threshold value, it gradually reduces the AC voltage applied to the developing roller 7 and again detects whether the amount of change in the current value has exceeded the threshold value.

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 the period T2 during which the photosensitive drum 1 rotates once after Vpp is applied (step S8). If the amount of change in the current value during that period is equal to or less than the threshold value, the AC voltage Vpp during image formation is determined to be a value obtained by lowering the AC voltage Vpp at the time of leak detection by the offset value (200 V) (step S9). That is, the CPU 40 determines the developing bias to be applied to the developing roller 7 without changing the developing bias (AC voltage Vpp) to be applied to the developing roller 7 during image formation. Then, the developing bias and the charging bias are turned OFF, and the driving of the photosensitive drum 1 and the developing roller 7 is stopped (step S10), and the discharge occurrence detection operation is completed (END).

From the above, according to this embodiment, by determining whether the amount of change in the current value flowing between the photosensitive drum 1 and the developing roller 7 has exceeded a threshold value, it is possible to shorten the time required to set a developing bias that does not cause leakage (a developing bias where the amount of change in the current value is equal to or less than a threshold value).

The SD gap, charging bias, developing bias, current threshold value, and the like described in this embodiment are not intended to limit the scope of this invention unless otherwise specified.

In this embodiment, a configuration is illustrated in which an average value is calculated by averaging the current values output from the detection unit 35 over a period of 10 AC voltage cycles, but the AC voltage cycle time T is not limited to this and may be set as appropriate.

In this embodiment, the comparison with the threshold value is performed using the amount of change in the current value during the time period when the photosensitive drum 1 rotates once, but the period for comparing with the threshold value is not limited to this. It may be the time for the photosensitive drum 1 to rotate multiple times, or the time for the developing roller 7 to rotate. Also, 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 with the rotation period of the developing roller 7 and the photosensitive drum 1, it is preferable to rotate the developing roller 7 or the photosensitive drum 1, whichever takes the longer time to complete one rotation. In addition, in order to shorten the time required to set a developing bias that does not cause leakage, it is preferable to rotate the developing roller 7 or the photosensitive drum 1 for a shorter period.

In this embodiment, the detection unit 35 shown in FIG. 2 has a rectification unit 34 and rectifies the current flowing through the developing roller 7, but this is not limited to the configuration, and the detection unit 35 may have a configuration without the rectification unit 34, for example. In this case, the discharge occurrence detection operation may be performed as follows.

For example, the detection unit 35 detects a third current value flowing between the photosensitive drum 1 and the development roller 7 on the positive side of the AC voltage applied to the development roller 7, or a fourth current value flowing between the photosensitive drum 1 and the development roller 7 on the negative side of the AC voltage applied to the development roller 7. Then, the CPU 40, which is the control unit, compares the difference between the maximum and minimum values of the third current value detected by the detection unit 35 or the difference between the maximum and minimum values of the fourth current value with a threshold value. When the change amount of the third current value, which is the difference between the maximum and minimum values, or the change amount of the fourth current value exceeds the threshold value, the CPU 40 lowers the setting of the development bias applied to the development roller 7 below the setting of the AC voltage applied to the development roller 7 when the current value is detected. When the change amount of the third current value or the change amount of the fourth current value is equal to or less than the threshold value, the CPU 40 does not change the setting of the development bias applied to the development roller 7.

Alternatively, the CPU 40, which is the control unit, calculates a first average value obtained by averaging the third current value detected by the detection unit 35 during the cycle time of the AC voltage, or a second average value obtained by averaging the fourth current value. Then, the CPU 40 compares the difference between the maximum and minimum values of the calculated first average value, or the difference between the maximum and minimum values of the second average value, with a threshold value during the time when the developing roller 7 or the photosensitive drum 1 rotates at least once. When the change amount of the first average value, which is the difference between the maximum and minimum values, or the change amount of the second average value exceeds the threshold value, the CPU 40 lowers the setting of the developing bias applied to the developing roller 7 below the setting of the AC voltage applied to the developing roller 7 when the current value is detected. When the change amount of the first average value or the change amount of the second average value is equal to or less than the threshold value, the CPU 40 does not change the setting of the developing bias applied to the developing roller 7.

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

In this embodiment, the Vpp during image formation is controlled, but it is not limited to the Vpp during image formation, and it may be Vpp at any timing when Vpp is applied. In this embodiment, when leak detection is performed, a Vpp higher than the setting during image formation is applied to the developing roller 7 (step S4 in FIG. 6), and the change in the current value flowing between the photosensitive drum and the developing roller is compared with the threshold value (step S5 in FIG. 6), but this is not limited to this. For example, without increasing the Vpp applied to the developing roller, the Vpp set at that time may be 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. In this way, it is possible to apply the Vpp at that time to the developing roller at any timing, not limited to the Vpp during image formation, and compare the change in the current value flowing between the photosensitive drum and the developing roller with the threshold value. In this case, the time required to set the developing bias at which no leak occurs can be further shortened.

In this embodiment, in FIG. 6, if the amount of change in the current value exceeds the threshold, 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 this is not limited to this. For example, after the photosensitive drum 1 has rotated once (step S7 in FIG. 6) since the application of Vpp (step S4 in FIG. 6), it may be determined whether the amount of change in the current value has exceeded the threshold (step S5 in FIG. 6). By doing this, although it takes more time to set the development bias that does not cause a leak than in the first embodiment described above, it is possible to shorten the aforementioned time compared to the conventional method. In addition, it is possible to distinguish between a current change due to sudden noise and a current change due to a leak caused by a change in the gap between the photosensitive drum and the development roller, and more accurate detection can be performed.

In the configuration of Example 1, the difference between the maximum value, which is the first current value, of the output value (average value) of the current value in order to set the development Vpp and the minimum value, which is the second current value, is taken as the amount of change and compared with the threshold value, but this is not limited to the above. 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 modified examples are shown below.

As described in the first embodiment, if the maximum and minimum values are used as values to be compared with the threshold value, there is a risk that the detection accuracy will decrease due to noise. Therefore, as a first modification, for example, the maximum value of the current value in a period of one or more revolutions of the photosensitive drum may be the top n% value in the histogram, and the minimum value may be the bottom m% value. As a specific example of the first modification, the output value is a current value averaged every 4 ms, and is sampled for a period of 400 ms, which is one or more revolutions of the photosensitive drum (100 pieces of data). If n is 97% and m is 3% of the sampled data, the third highest output value among the 100 pieces of data is used instead of the maximum value, and the third lowest output value is used instead of the minimum value. When the difference between the third highest output value and the third lowest output value is 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 appropriately. Furthermore, instead of the maximum value (minimum value), a value obtained by averaging the top n% (bottom m%) or more of the values may be used.

In addition, as a second modification, it may be determined that a leak has occurred when the slope, which is the amount of change in the output value (average value) of the current value per unit time when the horizontal axis is time and the vertical axis is current value, or the absolute value of the slope exceeds a certain threshold. When no leak occurs, when the SD gap fluctuates, the current value fluctuates inversely proportional to the SD gap. When a leak occurs, the slope changes abruptly due to a sudden change in current at the occurrence position. Therefore, when the current changes abruptly more than the fluctuation of the SD gap, it can be determined that a leak has occurred. As the threshold value of the slope, for example, if it is assumed that the current value changes by 1 μA per rotation of the photosensitive drum (314 ms) due to the influence of the SD gap fluctuation and the fluctuation of the SD gap with respect to time is a sine wave, the threshold value of the slope can be set to 2π/314 [μA/ms]. In FIG. 3(a), the slope is almost unchanged at Vpp 1.8 kV and 2.0 kV, whereas the slope fluctuates greatly at 2.2 kV.

As a third modification, it is also possible to determine whether a leak has occurred by looking at the standard deviation, which is the variation in the current value. For example, if the data of the output value (average value) of the current value is sampled for one revolution of the photosensitive drum, 78 samples of the current output value can be obtained. At Vpp of 1.8 kV and 2.0 kV in FIG. 3A, the current value remains almost unchanged over time, so the standard deviation of the current value is small. On the other hand, at 2.2 kV where a leak occurs, the standard deviation becomes large. 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 a threshold value, for example, if the current value changes by 1 μA per revolution of the photosensitive drum (314 ms) due to the influence of the SD gap fluctuation, and the fluctuation of the SD gap with respect to time is assumed to be a sine wave, the standard deviation is 0.315 μA. Therefore, 0.315 μA may be adopted as an example of the threshold value of the standard deviation.

Note that the calculation methods for each of the modifications 1 to 3 are merely examples, and the calculation methods are not limited to the above methods as long as the respective values can be calculated using any method.

Example 2
An image forming apparatus according to the second embodiment will be described with reference to Fig. 7 and Fig. 8. In this embodiment, only the discharge detection control is different from that in the first embodiment, and the other configurations are almost the same as those in the first embodiment. Therefore, in this embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted.

<Explanation of leakage current detection>
Detection of leakage 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 the AC voltage and current value of the developing bias applied in the second embodiment.

Figure 7 plots the waveform of the AC voltage applied to the developing roller 7 by the AC voltage application unit 31 during the formation of a solid white image (dotted line in the figure) and the waveform of the current value flowing between the developing roller 7 and the photosensitive drum 1 (solid line in the figure). The 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, so that a current flows when the potential of the AC voltage changes. In the configuration of this embodiment, the AC voltage applied by the AC voltage application unit 31 has a rectangular wave shape, so as shown in Figure 7, at the timing t = 0 when the AC voltage changes from negative to positive, a current value flows with the change in the potential difference. On the other hand, at the timing when a certain time has passed since the timing when the AC voltage changes from negative to positive, the AC voltage becomes constant, and the current value is accordingly close to 0. Here, the time T is the time of one cycle of the AC voltage, and T = 1/frequency f. In addition, the timing t = T/4 is illustrated as a certain 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 to a certain time T/4 after the AC voltage becomes positive from negative, but is not limited to this. 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 from positive to negative) on the positive side (or negative side) to a region where the AC voltage is constant. In this way, if the timing corresponds to the region where the AC voltage becomes constant, the certain time from the timing when the AC voltage becomes positive from negative is not limited to T/4. In addition, it may be when a certain time has passed since the timing when the AC voltage becomes positive from negative, or when a certain time has passed since the timing when the AC voltage becomes negative from positive. If the timing corresponds to the region where the AC voltage is constant (the timing when the voltage change is approximately 0), the same effect can be obtained even after any certain time has passed since the AC voltage becomes negative to positive or positive to negative.

In this embodiment, the control unit, CPU 40, compares the current value detected by detection unit 35 with the threshold value when a certain time has elapsed since the AC voltage Vpp applied to development roller 7 changes from negative to positive. With this configuration, it is possible to directly detect the leakage current from the current value flowing between photosensitive drum 1 and development roller 7 when the AC voltage applied to development roller 7 changes.

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

Figure 8 is a flowchart showing an example of the flow of control of the discharge occurrence detection operation of the image forming apparatus according to the second embodiment. In the discharge occurrence detection operation described below, it is determined whether the amount of change in the current value flowing between the photosensitive drum 1 and the developing roller 7 exceeds a threshold value, and the development bias to be applied to the developing roller 7 is set based on the result. This discharge occurrence detection operation is performed by the CPU 40 (see Figure 2), which is the control unit. Note that this discharge occurrence detection operation is performed when the installation environment of the image forming apparatus, such as the atmospheric pressure or temperature and humidity, changes. Alternatively, it is performed in accordance with the driving time of the developing roller 7 or the photosensitive drum 1 (for example, the paper passing history of the developing device that may cause a change in the SD gap or the timing of replacing the developing device). In addition, the timing of the discharge occurrence detection operation is not limited to the above example, and can be set appropriately.

First, when the power supply of the image forming apparatus is turned on and the discharge occurrence detection operation is started (START), the rotation of each rotating body such as the photosensitive drum 1 and the developing roller 7 is started by the drive mechanism (not shown) in response to the instruction of the CPU 40 (step S11). The driving of each rotating body continues until the discharge occurrence detection operation is completed. Next, a charging bias is applied to the charging roller 4, and a DC voltage of -300V is applied to the developing roller 7 by the DC voltage application unit 30 (step S12). After the time (T2) for the photosensitive drum 1 to rotate once from step S2 has elapsed (step S13), the surface of the photosensitive drum 1 becomes the set surface potential of -500V over the entire circumference. Next, the AC voltage Vpp to be applied to the developing roller 7 is set. Taking into account the change in temperature and humidity during paper passage and the fluctuation of the SD gap, the AC voltage Vpp to be applied to the developing roller 7 is set to an AC voltage Vpp higher than the setting during image formation (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 during image formation. Next, as described with reference to FIG. 7, it is determined whether the absolute value of the current value when a certain time (here, T/4) has elapsed since the AC voltage becomes positive from negative (step S15) exceeds a predetermined threshold value. In this embodiment, the threshold value is set to 10 μA.

If the absolute value of the current value exceeds the threshold value in step S15, a leak occurs between the photosensitive drum 1 and the developing roller 7, so the CPU 40 turns off the AC voltage Vpp of the developing bias (step S16). If a leak occurs in this way, it is necessary to set the AC voltage Vpp applied to the developing roller 7 during image formation to a voltage lower than the AC voltage applied to the developing roller 7 when the current value was detected. Therefore, the CPU 40, which is the control unit, reduces the developing bias (AC voltage Vpp) applied to the developing roller 7 to a voltage lower than the voltage when the current value was detected (step S17). Here, the voltage is reduced to a voltage (1.7 kV) 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 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 when the leak is detected is reduced from 2.0 kV to 1.9 kV. Then, in step S15, it is checked again whether the absolute value of the current value exceeds the threshold value.

If the absolute value of the current value exceeds the threshold value in step S15, the CPU 40 gradually reduces the AC voltage applied to the developing roller 7 when detecting a leak, and repeats the above-mentioned operation until the absolute value of the current value becomes equal to or less than the threshold value. In other words, when the CPU 40, which is the control unit, detects in step S15 that the absolute value of the current value exceeds the threshold value, it gradually reduces the AC voltage applied to the developing roller 7 and again detects whether the absolute value of the current value has exceeded the threshold value.

If the current value (absolute value) does not exceed the threshold value in step S15, i.e., if the current value is equal to or less than the threshold value, step S15 is repeated for the period T2 during which the photosensitive drum 1 rotates once after Vpp is applied (step S18). If the current value is equal to or less than the threshold value during the period, the AC voltage Vpp during image formation is determined to be a value 200 V lower than the AC voltage Vpp at the time of leak detection at that time (step S19). In other words, the CPU 40 determines the developing bias to be applied to the developing roller 7 without changing the developing bias (AC voltage Vpp) to be applied to the developing roller 7 during image formation. Then, the developing bias and charging bias are turned OFF, and the driving of the photosensitive drum 1 and the developing roller 7 is stopped (step S20), and the discharge occurrence detection operation is completed (END).

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

The SD gap, charging bias, developing bias, current threshold value, and the like described in this embodiment are not intended to limit the scope of this invention unless otherwise specified.

In this embodiment, a configuration is illustrated in which an average value is calculated by averaging the current values output from the detection unit 35 over a period of 10 AC voltage cycles, but the AC voltage cycle time T is not limited to this and may be set as appropriate.

Example 3
An image forming apparatus according to the third embodiment will be described with reference to Figures 9 and 10. In this embodiment, only the size relationship of the members of the image forming section 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 embodiment, the same components as those in the second embodiment are denoted by the same reference numerals and detailed description thereof will be omitted.

In the second embodiment, during the discharge occurrence detection operation sequence, the surface potential of the photosensitive drum may change due to discharge, and toner may be developed and wasted. In the third embodiment, by comparing only the value of the current flowing between the non-exposed area of the photosensitive drum and the developing roller facing this area with a threshold value, it is possible to reduce the time required to set the developing bias to a level that does not consume toner and does not cause leakage.

<Size Relationship of Each Member of the Image Forming Unit>
The size relationship of each member of the image forming section in the third embodiment will be described with reference to FIG. 9. FIG. 9 is an explanatory diagram for explaining an example of the size relationship of each member of the image forming section in the third embodiment. In FIG. 9, in the axial direction of the developing roller 7, the toner coat area L1 is a developer carrying area on which the toner of the developing roller 7 is carried. The toner coat area L1 faces the developer carrying area of the developing roller 7 and corresponds to the area of the photosensitive drum 1 to which the toner is applied from the developing roller 7. L2 is the roller width in the axial direction of the charging roller 4. The roller width L2 of the charging roller 4 corresponds to the charging area 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, in the axial direction of the developing roller 7, the relationship (size relationship) of the length between the toner coat area L1 of the developing roller 7 and the roller width L2 of the charging roller 4 and the drum coating width L3 of the photosensitive drum 1 is L1<L2, L1<L3. That is, in the axial direction of the developing roller 7, the charged 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). In addition, the laser beam scanner 6 can expose an area (scanning exposure possible width L4 (not shown)) of the photosensitive drum 1 wider than the toner coat region L1 of the developing roller 7 in the axial direction of the developing roller 7 (L1<L4). The surface potential of the charged region (roller width L2) of the photosensitive drum 1 charged by the charging roller 4 is VD. The surface potential of the exposure region of the photosensitive drum 1 exposed by the laser beam scanner 6 is VL. 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 DC voltage VDC applied to the developing roller 7 is set so that the relationship VL<VDC is satisfied in order to prevent the negatively charged toner 3 from being developed from the developing roller 7 onto the photosensitive drum 1. Note that when the toner is positively charged, the relationship VL>VDC is satisfied.

By making the above relationship, the surface potential of the non-exposed areas H1, H2 of the photosensitive drum 1 that are not exposed to light as shown in FIG. 9 is VD, and the surface potential of the exposed area of the photosensitive drum 1 facing the toner coat area L1 is VL, resulting in a relationship of VD<VL<VDC. That is, the potential VL of the area of the photosensitive drum 1 facing the toner coat area L1 of the developing roller 7 is between the DC voltage VDC applied to the developing roller 7 and the potential VD of the non-exposed area (non-image area) that is not exposed by the laser beam scanner 6 in the charged area (roller width L2) of the photosensitive drum 1. Note that when the toner is positively charged, VD>VL>VDC. Therefore, the potential difference between the non-exposed areas H1, H2 of the photosensitive drum 1 and the developing roller 7 facing these areas H1, H2 is larger than the potential difference between the photosensitive drum 1 facing the toner coat area L1 of the developing roller 7. Therefore, leakage is more likely to occur in the non-exposed areas H1, H2 of the photosensitive drum 1. In this embodiment, it is necessary to set the potential difference so that no leakage occurs in the toner coat area L1 when detecting leakage. 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, taking into account the variation in the SD gap and the air pressure. In this embodiment, Vpp is set to 1.2 kV, VD is set to -600 V, VL is set to -200 V, and VDC is set to 0 V when detecting leakage. By setting in this way, the maximum potential difference between the photosensitive drum 1 and the developing roller 7 is ΔVmax = Vpp/2 + |VDC-VD| = 1200 V, which is the same potential difference as ΔVmax (Vpp = 2.0 kV, VD = -500 V, VDC = -300 V) during the initial discharge occurrence detection operation in the second embodiment.

As a result of the above, the above configuration makes it possible to detect the presence or absence of leakage in the non-exposed area without leakage occurring in the toner-coated area during discharge occurrence detection operation.

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

First, when the power supply of the image forming apparatus is turned on and the discharge occurrence detection operation is started (START), the rotation of each rotating body such as the photosensitive drum 1 and the developing roller 7 is started by the drive mechanism (not shown) in response to the instruction of the CPU 40 (step S21). The driving of each rotating body continues until the discharge occurrence detection operation is completed. Next, a charging bias is applied to the charging roller 4 so that the surface potential of the charged area of the photosensitive drum 1 is -600V, and the surface potential of the toner coat area L1 of the photosensitive drum 1 is set to -200V by exposing the developer carrying area of the photosensitive drum 1 facing the toner coat area L1 by the laser beam scanner 6 (step S22). After the time (T2) for the photosensitive drum 1 to rotate once has elapsed 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 to be 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 that set during image formation so as to be equivalent to ΔVmax during the initial discharge occurrence detection operation in Example 1 (step S24). Here, the AC voltage applied to the developing roller 7 is set to an AC voltage Vpp that is 600 V lower than the AC voltage during image formation. Next, it is determined whether the absolute value of the current value when a certain time T/4 has elapsed since the timing when the AC voltage changes from negative to positive exceeds a predetermined threshold value (step S25). In this example, the threshold value is set to 10 μA.

If the current value exceeds the threshold value in step S25, a leak occurs between the photosensitive drum 1 and the developing roller 7, so the CPU 40 turns off the AC voltage Vpp of the developing bias (step S26). If a leak occurs in this way, it is necessary to set the AC voltage Vpp applied to the developing roller 7 during image formation to a voltage lower than the AC voltage applied to the developing roller 7 when the current value was detected. Therefore, the CPU 40, which is the control unit, reduces the developing bias (AC voltage Vpp) applied to the developing roller 7 to a voltage lower than the voltage when the current value was detected (step S27). Here, the AC voltage during image formation (1.8 kV) is reduced to a voltage (1.7 kV) that is 100 V lower. Then, returning to step S24, the AC voltage applied to the developing roller 7 is set to an AC voltage Vpp that 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 when the leak is detected is reduced from 1.2 kV to 1.1 kV. Then, in step S25, it is checked again whether the current value exceeds the threshold value.

If the current value exceeds the threshold value in step S25, the CPU 40 gradually reduces the AC voltage applied to the developing roller 7 when the leak is detected, and repeats the above-mentioned operation until the current value becomes equal to or less than the threshold value. In other words, when the CPU 40, which is the control unit, detects the occurrence of a leak in step S25, it gradually reduces the AC voltage applied to the developing roller to detect a leak between the photosensitive drum 1 and the developing roller 7.

If the current value does not exceed the threshold value in step S25, i.e., if the current value is equal to or less than the threshold value, step S25 is repeated for the period T2 during which the photosensitive drum 1 rotates once after Vpp is applied (step S28). If the current value is equal to or less than the threshold value during that period, the AC voltage Vpp during image formation is determined to be a value 600 V higher than the AC voltage Vpp at the time of leak detection at that time (step S29). That is, the CPU 40 determines the developing bias to be applied to the developing roller 7 without changing the developing bias (AC voltage Vpp) to be applied to the developing roller 7 during image formation. Then, the developing bias and charging bias are turned OFF, and the driving of the photosensitive drum 1 and the developing roller 7 is stopped (step S30), and the discharge occurrence detection operation is completed (END).

As described above, according to this embodiment, the photosensitive drum 1 facing the toner-coated area is exposed during the discharge occurrence detection operation, and leakage detection is performed in the non-exposed area of the toner-coated area. This prevents the unnecessary consumption of toner and shortens the time required to set the development bias at which leakage does not occur.

The SD gap, charging bias, developing bias, current threshold value, and the like described in this embodiment are not intended to limit the scope of this invention unless otherwise specified.

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

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

Furthermore, in the above-mentioned embodiment, a configuration was exemplified in which the process cartridge 20 including the photosensitive drum 1 was detachably attached to the main body of the image forming apparatus, but this is not limited to this. For example, the image forming apparatus may incorporate the photosensitive drum 1 and each process means acting on it, or the photosensitive drum 1 and the process means acting on it may be detachably attached.

In the above-mentioned embodiment, a printer is exemplified as an image forming apparatus, but the present invention is not limited to this. For example, the present invention may be other image forming apparatuses such as a copier or facsimile machine, or other image forming apparatuses such as a multifunction machine that combines the functions of these. Alternatively, the present invention may be an image forming apparatus that uses a recording medium carrier and transfers toner images of each color in a sequentially overlapping manner onto a recording medium carried by the recording medium carrier. Alternatively, the present invention may be an image forming apparatus that uses an intermediate transfer body and transfers toner images of each color in a sequentially overlapping manner onto the intermediate transfer body, and transfers the toner images carried by the intermediate transfer body to a recording medium all at once. Similar effects can be obtained by applying the present invention to these image forming apparatuses.

H1, H2 ... non-exposed area L1 ... toner coated 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 section 31 ... AC voltage application section 32 ... output control section 33 ... Vpp control section 34 ... rectification section 35 ... detection section 36 ... detection circuit 37 ... amplifier 38 ... A/D converter 40 ... CPU (control section)

Claims (19)

A rotatable image carrier;
a charging member for charging the image carrier;
a rotatable developer carrier that is disposed opposite the image carrier in a non-contact state and that carries a developer;
an application unit that applies a developing bias in which a DC voltage and an AC voltage are superimposed on the developer carrier;
a detection unit that detects a value of a current flowing between the image carrier and the developer carrier;
a control unit that controls the AC voltage applied to the developer carrier based on a difference between a first current value detected by the detection unit and a second current value smaller than the first current value, while the image carrier and the developer carrier are rotated and the developing bias is applied to the developer carrier;
having
The image forming apparatus is characterized in that, when performing an operation of controlling the AC voltage applied to the developer carrier, the control unit rotates either the developer carrier or the image carrier, whichever takes longer to rotate once, at least once . The image forming apparatus according to claim 1, characterized in that, when the difference between the first current value and the second current value detected by the detection unit exceeds a threshold value, the control unit sets the AC voltage applied to the developer carrier to a value smaller than the AC voltage applied to the developer carrier when the current value is detected. The image forming apparatus according to claim 1 or 2, characterized in that, when the control unit detects that the difference between the first current value and the second current value exceeds a threshold value, the control unit gradually reduces the AC voltage applied to the developer carrier, and detects again whether 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 any one of claims 1 to 3, characterized in that 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 a threshold value. The image forming apparatus according to any one of claims 1 to 4, characterized in that the control unit controls the AC voltage applied to the developer carrier based on the difference between the first current value and the second current value detected by the detection unit during the time it takes for the developer carrier or the image carrier to rotate at least once. The image forming apparatus according to any one of claims 1 to 5, characterized in that the first current value is a maximum value of the current value detected by the detection unit, and the second current value is a minimum value of the current value detected by the detection unit. the detection unit detects a third current value flowing between the image carrier and the developer carrier on a positive side of an AC voltage applied to the developer carrier, or a fourth current value flowing between the image carrier and the developer carrier on a negative side of an AC voltage applied to the developer carrier,
7. The image forming apparatus according to claim 6, wherein, when a difference between a maximum value and a minimum value of the third current value detected by the detection unit or a difference between a maximum value and a minimum value of the fourth current value detected by the detection unit exceeds a threshold value, the control unit sets the AC voltage applied to the developer carrier to a value smaller than the AC voltage applied to the developer carrier at the time of detection of the current value. the detection unit detects a third current value flowing between the image carrier and the developer carrier on a positive side of an AC voltage applied to the developer carrier, or a fourth current value flowing between the image carrier and the developer carrier on a negative side of an AC voltage applied to the developer carrier,
7. The image forming apparatus according to claim 6, wherein the control unit calculates a first average value by averaging the third current values detected by the detection unit during a cycle time of the AC voltage or a second average value by averaging the fourth current values, and when a difference between a maximum value and a minimum value of the calculated first average value or a difference between a maximum value and a minimum value of the calculated second average value exceeds a threshold value, the control unit sets the AC voltage applied to the developer carrier to a value smaller than the AC voltage applied to the developer carrier at the time of detection of the current value. the detection unit outputs a current value obtained by rectifying a current flowing between the image carrier and the developer carrier;
9. The image forming apparatus according to claim 1, wherein the control unit outputs an average value of the current values output from the detection unit during a period of the AC voltage. A rotatable image carrier;
a charging member for charging the image carrier;
a rotatable developer carrier that is disposed opposite the image carrier in a non-contact state and that carries a developer;
an application unit that applies a developing bias in which a DC voltage and an AC voltage are superimposed on the developer carrier;
a detection unit that detects a value of a current flowing between the image carrier and the developer carrier;
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 has elapsed from the timing when the AC voltage applied to the developer carrier changes from negative to positive or from positive to negative while the image carrier and the developer carrier are rotated and the developing bias is applied to the developer carrier;
having
The image forming apparatus is characterized in that, when performing an operation of controlling the AC voltage applied to the developer carrier, the control unit rotates either the developer carrier or the image carrier, whichever takes longer to rotate once, at least once . 11. The image forming apparatus according to claim 10, wherein when the current value detected by the detection unit exceeds a threshold value when a certain time has elapsed since the AC voltage applied to the developer carrier changes from negative to positive or from positive to negative, the control unit sets the AC voltage applied to the developer carrier to a value smaller than the AC voltage applied to the developer carrier at the time of detection of the current value . The image forming apparatus according to claim 10 or 11, characterized in that 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 a threshold value. The image forming apparatus according to claim 10 or 11, characterized in that, when the control unit detects that the current value has exceeded the threshold value, the control unit gradually reduces the AC voltage applied to the developer carrier and detects again whether the current value flowing between the image carrier and the developer carrier has exceeded the threshold value. The image forming apparatus according to any one of claims 10 to 13, characterized in that the detection unit detects the value of the current flowing between the image carrier and the developer carrier in a region where the AC voltage applied to the developer carrier is constant. an exposure section for exposing a charged image carrier to light;
a charging area of the image carrier that is charged by the charging member in an axial direction of the developer carrier is larger than a developer carrying area of the developer carrier that carries the developer,
the exposure unit is capable of exposing a region of the image carrier that is larger than the developer carrying region of the developer carrying body in an axial direction of the developer carrying body,
15. An image forming apparatus according to claim 10, wherein when the control unit compares with the threshold value, the exposure unit exposes an area of the image carrier facing the developer carrying area so that the potential of the area facing the developer carrying area of the image carrier in the axial direction of the developer carrier is between the DC voltage applied to the developer carrier and the potential of a non-exposed area in the charged area of the image carrier that is not exposed by the exposure unit. The image forming apparatus according to any one of claims 10 to 15, characterized in that the control unit controls the AC voltage applied to the developer carrier from the current value detected by the detection unit during the time it takes for the developer carrier or the image carrier to rotate at least once. The image forming apparatus according to any one of claims 1 to 16, characterized in that the control unit controls the AC voltage applied to the developer carrier in accordance with the driving time of the developer carrier or the image carrier. The image forming apparatus according to any one of claims 1 to 17, characterized in that the control unit controls the AC voltage applied to the developer carrier when temperature, humidity, or air pressure changes. The image forming apparatus according to any one of claims 1 to 18, characterized in that the AC voltage applied to the developer carrier has a square wave shape.

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