JP2000206765A - Printing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To always charge a photoreceptor drum to have fixed potential under various environmental conditions by adjusting applied voltage to a charging roller in accordance with charging start voltage. SOLUTION: A charging start voltage measuring part 20 measures the potential at which a charging current starts to flow by applying variable voltage to the photoreceptor drum before printing is started. The measured voltage is the charging start voltage of the photoreceptor drum. The charging start voltage is compared with the charging start voltage in a reference state so as to select the appropriate applied voltage of a charging power source 3. Arithmetic processing for deciding charging voltage is executed by a CPU for a power source 14 and controlled by a charging power source control circuit 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic printing apparatus, and more particularly to a printing apparatus which stabilizes a charged potential.

[0002]

2. Description of the Related Art An electrophotographic printing apparatus has many advantages over other systems in terms of printing speed, image quality, and the like, and is widely used as a printer or a copier. In this scheme,
The photosensitive drum is charged to a certain potential, an electrostatic latent image is formed on the photosensitive drum by an exposure device, the electrostatic latent image is developed by a developing device to form a toner image, and the toner image is transferred onto paper. To perform printing. In order to charge the photosensitive drum, a charging roller is pressed against the photosensitive drum, and a constant DC voltage is applied through the charging roller.

[0003]

However, the above-mentioned prior art has the following problems to be solved. The potential of the photosensitive drum changes variously during printing. Immediately before exposing the photosensitive drum, the charging roller charges the photosensitive drum to a constant potential, but the charging potential of the photosensitive drum may fluctuate due to various environmental causes. For example, in a low-temperature and low-humidity environment, the charging potential is slightly higher,
At high temperature and high humidity, the charging potential is slightly lower. Such a variation causes variations in the density of the printed image and causes a so-called fogging phenomenon in which toner adheres to a portion that should be a white background portion.

Further, due to the charging characteristics of the toner, the potential difference between the charging roller and the photosensitive drum, and the like, the toner on the photosensitive drum may move to the charging roller side and adhere to the outer peripheral surface of the charging roller. In this case, there is a problem that the charging start voltage becomes low and the charging potential of the photosensitive drum becomes higher than a standard value.

[0005] Further, the toner adhering to the outer peripheral surface of the charging roller may be crushed by receiving a mechanical pressure to form a film on the outer peripheral surface of the charging roller, thereby causing a filming phenomenon. In this case, the charging start voltage becomes large, and the charging potential of the drum becomes lower than the standard.

At the contact point between the charging roller and the photosensitive drum, a certain peripheral speed difference is given to prevent a large amount of toner from adhering, thereby causing a slip. In this case, frictional charging occurs due to friction between the two. This also causes the charging start voltage to fluctuate. This changes variously depending on the material of the charging roller.

As described above, if the charging start voltage fluctuates due to various causes, the charging potential of the photosensitive drum cannot be kept constant, which has been a cause of deteriorating the print quality. In addition, if the material of the charging roller is improved to solve such a problem, the cost of the charging roller is increased.

[0008]

The present invention employs the following structure to solve the above problems. <Structure 1> A charging start voltage measuring unit that includes a photosensitive drum and a charging roller that charges the photosensitive drum, measures a charging start voltage of the charging roller, and the charging roller that is selected according to the charging start voltage And a charging power source for applying the applied voltage to the photosensitive drum to start charging.

<Structure 2> In the printing apparatus according to Structure 1, the charging start voltage measuring unit applies a variable voltage from the charging roller to the photosensitive drum after the charge elimination, and determines a boundary between a state where the charging current flows and a state where the charging current does not flow. A printing start voltage of the charging roller is measured by calculating the voltage of the printing roller.

<Structure 3> The printing apparatus according to Structure 2, further comprising an exposure head for exposing the photosensitive drum to remove electricity before measuring the charging start voltage.

<Structure 4> In the printing apparatus described in Structure 1, the charging power source is a charging start voltage Vt in the reference state.
h0, the charging start voltage measured by the charging start voltage measuring unit is Vth, and the applied voltage in the reference state is V0, the reference applied voltage is Vch = V0 + (Vth0
-Vth), wherein the applied voltage Vch determined by (Vth) is selected.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below using specific examples. FIG. 1 is a block diagram of a printing apparatus according to the present invention. This device charges a photosensitive drum (not shown) by a charging roller 1. A charging power supply 3 is connected to the charging roller 1, and a charging voltage is controlled by a charging power supply control circuit 12. A detection resistor 13 for measuring a charging current is provided between the charging power source 3 and the charging roller 1.
Is inserted. The arithmetic processing described later for determining the charging voltage is executed by the power supply CPU 14 and the charging power supply control circuit 12 is controlled.

The charging start voltage measuring section 20 newly provided in the present invention includes a pair of A / D converters 1.
5 and a comparator 16 are provided. Thus, the voltage at both ends of the detection resistor 13 is converted into a digital signal and supplied to the power CPU 14.
If the potential difference is obtained from the voltage across the detection resistor 13,
The charging current can be converted. This calculation is also performed by the power supply CPU 1.
4 is configured to do so.

Further, a developing power source 18 is connected to the developing roller 7, and a developing voltage is controlled by a developing power source control circuit 17. Transfer power supply 24 for transfer roller
Are connected, and the transfer voltage is controlled by the transfer power supply control circuit 19.

Further, an LED for exposing the photosensitive drum
The head 5 is configured so that an LED head control circuit 21 is connected to supply image data for printing. As will be described later, this LED head 5 is used as an exposure head that exposes the photosensitive drum to remove electricity before measuring the charging start voltage. The entire printing device
The main switch 22 is controlled by the main CPU 22.
Reference numeral 3 denotes the power switch.

FIG. 2 is a diagram showing a print engine mechanism of the printing apparatus as described above. As shown in the drawing, the print engine of this electrophotographic printing apparatus is provided with a mechanism as shown in the drawing. A photosensitive drum 4 is arranged at the center. On the outer circumference of the photosensitive drum 4, an LED head 5, a developing roller 7, a transfer roller 9, a cleaning roller 11, and a charging roller 1 are sequentially arranged.

A developing blade 6 is disposed in contact with the developing roller 7 and functions to make the toner 8 adhere to the surface of the developing roller 7 thinly. The paper 10 is transported between the photosensitive drum 4 and the transfer roller 9. Charging roller 1
Is connected to the charging power source 3 shown in FIG.

An auxiliary roller 2 is rotatably supported on the outer periphery of the charging roller 1 so as to circumscribe the auxiliary roller 2. The auxiliary roller 2 is supplied with the same voltage from the charging power supply 3. The operation of this print engine at the time of printing is the same as that of the conventional apparatus. That is, when the photosensitive drum 4 is charged to a certain potential by the charging roller 1, the photosensitive drum 4 is exposed by the LED head 5, and an electrostatic latent image corresponding to image data for printing is formed. The electrostatic latent image is developed by the developing roller 7, and the toner 8 adheres to the outer peripheral surface of the photosensitive drum 4 to form a toner image. This toner image is transferred to the sheet 10 between the transfer roller 9 and the photosensitive drum 4. The paper 10 is subjected to a fixing process by a fixing device (not shown).

The cleaning roller 11 is provided for removing the residual toner after the transfer process from the photosensitive drum 4. After such a cleaning process, the charging roller 1
Supplies a constant charging current to the photosensitive drum 4 to perform a charging process. In addition, due to the charging characteristics of the toner and other factors, the toner contacts the charging roller 1 with the toner adhered to the outer peripheral surface of the photosensitive drum even after cleaning. As a result, the toner adheres to the outer peripheral surface of the charging roller 1 and causes the above-described adverse effects.

Here, in the printing apparatus of the present invention, before the charging process by the charging roller is started, the charging start voltage of the charging roller 1 is measured by the charging start voltage measuring section 20 in FIG. The detection resistor 13 shown in FIG. 1 is used for measuring the charging start voltage.

FIG. 3 is an operation flowchart of the printing apparatus as described above. FIG. 4 is an operation timing chart of the printing apparatus. Referring to these two figures,
The operation of the above device will be described. First, in step S1, the main switch 23 shown in FIG. 1 is turned on (time t1 in FIG. 4A). In the next step S2, the main CP
U22 turns on a main motor (not shown), and starts rotation of the photosensitive drum 4 shown in FIG. 2 in step S3. Since these operations are interlocked, they start simultaneously at time t1.

Further, in conjunction with this, at time t1, (c)
1, the charging power source 3 shown in FIG. 1 is turned on, and a predetermined charging voltage is applied to the charging roller 1. This timing is also linked with the main switch. From time t1 to time t
The time up to 2 is a time required for the photosensitive drum 4 to make at least one rotation so that the entire peripheral surface of the photosensitive drum 4 is charged by the charging power supply 3. Then, at the timing of time t2, in step S5, the LED head 5 shown in FIG. 2 is turned on as shown in (b), and the process of exposing the entire surface of the photosensitive drum 4 is started.

Thus, the photosensitive drum 4 shown in FIG.
The entire surface is exposed by the D head 5 and adjusted so that the surface potential of the photosensitive drum 4 becomes 0 volt.

Next, in step S6, the developing power source 1
8 (FIG. 1) is turned on. The ON timing is shown in FIG.
As shown in (d), it is time t3. A time T5 from time t2 to time t3 is set to a time until the portion of the photosensitive drum 4 exposed by the LED head 5 reaches the developing roller 7. After time t3, the developing roller 7 is set to a positive potential. This is to prevent the toner from moving from the developing roller 7 onto the photosensitive drum 4 which has been exposed and has a surface potential of 0 volt. The transfer voltage shown in FIG. 4E is also kept at a predetermined negative voltage.

In the next step S7, the charging start voltage is measured. This measurement is performed at time T3 between times t4 and t5 in FIG. A time T2 from time t3 to time t4 is a time until a portion of the photosensitive drum 4 exposed by the LED head at time t2 reaches the charging roller 1. Time T3 is a time for detecting charging start voltage Vth. During the time T3, the voltage applied to the charging power source 3 is changed so as to be gradually reduced. Details of the measuring method will be described later.

When the measurement of the charging start voltage is completed, at time t
At the timing of 5, in step S8, the charging power source 3
Is turned off, and the LED head 5 is turned off. Then, an applied voltage according to the measured charging start voltage is calculated, and at time t6
Then, the applied voltage is supplied from the charging power source 3 to the charging roller 1 to start charging. Time T4 is the time required for this calculation and selection of the applied voltage.

That is, in step S9, the applied voltage is calculated, and in step S10, the charging power supply is turned on. And
At time t7 in step S11, the developing power is turned off, and the printing process and the like are executed following the charging. The time T6 is a time until the portion of the photosensitive drum 4 charged by the charging roller reaches the developing roller 7. Note that the main CPU 22 shown in FIG. 1 controls the transfer roller 9 shown in FIG. 2 and the cleaning roller 11 shown in FIG. ing.

FIG. 5 is an explanatory diagram of a method of measuring a charging start voltage. The vertical axis of the graph in the figure is the detection resistor 1 shown in FIG.
3 shows the product of the resistance value of No. 3 and the charging current flowing therethrough. This value is the voltage drop of the detection resistor. P is a voltage drop when the measurement of the charging start voltage is started. On the left side of the figure, the applied voltage of the charging power supply 3 is shown. The detection resistor 13 shown in FIG. 1 is set to a value of about 1 to 100 kΩ, which is sufficiently smaller than the electric resistance of the charging roller 1 so as not to affect the charging performance of the charging roller 1. The charging current flowing through the detection resistor 13 is converted into a terminal voltage at both ends thereof and sent to the power supply CPU 14 in the manner already described.

The power supply CPU 14 changes the voltage applied to the charging roller 1 from -1350 volts to gradually approach 0 volts. At this time, the voltage drop of the detection resistor 13 gradually decreases as shown in FIG. The charging current becomes 0 microamp at the timing when the applied voltage of the charging power source 3 is Vth. The applied voltage Vth at this time is a charging start voltage. In such a measurement, the voltage at the boundary between the state where the charging current flows and the state where the charging current does not flow may be obtained. Therefore,
The applied voltage may be changed from the state where the charging current does not flow, and the applied voltage when the charging current starts flowing may be measured as the charging start voltage.

Next, the operation of the power supply CPU 14 will be described. The power supply CPU 14 detects the detected charging start voltage V
The difference between th and the reference charging start voltage Vth0 in the preset reference state is calculated. This difference is caused by the adhesion of toner to the charging roller 1 and other changes in environmental conditions. The reference charging start voltage is, for example, -5
Assuming that the applied voltage is 50 volts, the applied voltage Vch to be actually supplied to the charging roller 1 can be obtained by the following equation, with the applied voltage of the charging power supply 3 in the reference state being V0. Vch = V0 + (Vth0−Vth) = − 1350 volts + (− 550−Vth)

In practice, this Vth fluctuates in a range of, for example, about -550 volts ± 100 volts. Therefore, the applied voltage Vch to be supplied to the charging roller 1 is, for example, −
A value in the range of 1250 volts to -1450 volts is selected. The applied voltage Vch corresponding to such a charging start voltage
To the charging roller 1, it is possible to always charge the photosensitive drum to the target −800 volts. The reason why the charging potential of the photosensitive drum can be controlled by the voltage applied to the charging roller is that the photosensitive drum is rotated at a constant speed while the charging roller and the photosensitive drum are in contact with each other, so that the photosensitive drum is charged. Is constant regardless of the applied voltage. Therefore, by controlling the charging current flowing per unit time, the charging potential of the photosensitive drum can be adjusted.

FIGS. 6 to 9 show graphs for explaining the effect obtained by implementing the above method. FIG.
FIG. 7 is a diagram for explaining a change in humidity of a photosensitive drum potential. The vertical axis in the figure indicates the photosensitive drum potential Vd, and the horizontal axis indicates the voltage applied to the charging roller. When charging is performed at an applied voltage V0 under reference humidity, the potential of the photosensitive drum becomes -800 volts.
However, when the environmental humidity is low, the charging start voltage Vt
hl becomes higher than the reference charging start voltage Vth0. When the environmental humidity is high, the charging start voltage Vthh becomes lower than the reference charging start voltage Vth0. Here, when charging is performed with the applied voltage V0 as in the previous case, the potential of the photosensitive drum becomes -800 volts or more. Therefore, when the environmental humidity is low, a higher applied voltage is selected, and when the environmental humidity is high, a lower applied voltage is selected.

FIG. 7 is a diagram illustrating the change in the photosensitive drum potential due to the attached toner. The vertical axis in the figure is the photosensitive drum potential Vd
, And the horizontal axis indicates the voltage applied to the charging roller. For example,
Before the toner adheres to the charging roller, the charging start voltage is V
th0. At this time, when charging is performed with the applied voltage V0, the potential of the photosensitive drum becomes -800 volts. However, when toner adheres to the charging roller, the charging start voltage drops to Vth. Here, similarly to the previous time, the applied voltage V0
, The potential of the photosensitive drum becomes -800 volts or more. Therefore, a lower applied voltage is selected as shown by the arrow in the figure.

FIG. 8 is a diagram for explaining a change in the photosensitive drum potential depending on the number of printed sheets. At the start of use of the printing apparatus, the charging start voltage was Vth0, but when printing about 20,000 sheets, the charging start voltage becomes Vth. Here, if the charging is performed with the applied voltage of the reference V0, the charging potential of the photosensitive drum becomes -800 volts or less. Therefore, V0
The photosensitive drum is charged to -800 volts by charging with the above applied voltage.

FIG. 9 is a diagram illustrating the change in the photosensitive drum potential depending on the material of the charging roller. Also in the case of this figure, the charging start voltage of Vth0 before the toner was attached,
After the toner adheres, the charging start voltage becomes Vth. Therefore,
The applied voltage is set to a voltage lower than V0, and the photosensitive drum is charged to an appropriate voltage.

The present invention is not limited to the above embodiment. The circuit for measuring the charging start voltage of the charging roller 1 can be replaced with a well-known voltage or current detection circuit other than the method using the detection resistor.

[0037]

According to the printing apparatus of the present invention described above, the charging start voltage measuring section for measuring the charging start voltage of the charging roller, and the applied voltage of the charging roller according to the charging start voltage are selected to set the photosensitive drum. Since the charging power supply for starting charging is provided, the photosensitive drum can always be charged to a constant voltage even when environmental conditions are variously changed. This makes it possible to stabilize print quality.

[Brief description of the drawings]

FIG. 1 is a block diagram of a printing apparatus according to the present invention.

FIG. 2 is a diagram illustrating a print engine mechanism.

FIG. 3 is an operation flowchart of the printing apparatus.

FIG. 4 is an operation timing chart of the printing apparatus.

FIG. 5 is an explanatory diagram of a charging start voltage measuring method.

FIG. 6 is a diagram illustrating a change in humidity of a photosensitive drum potential.

FIG. 7 is a diagram illustrating a change in photosensitive drum potential due to adhered toner.

FIG. 8 is a diagram illustrating a change in the photosensitive drum potential depending on the number of printed sheets.

FIG. 9 is a diagram illustrating a change in the photosensitive drum potential depending on the material of a charging roller.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Charging roller 3 Charging power supply 5 LED head 13 Detection resistor 14 Power supply CPU 20 Charging start voltage measuring unit 22 Main CPU

Claims (4)

[Claims] A charging roller that charges the photosensitive drum; a charging start voltage measuring unit that measures a charging start voltage of the charging roller; and the charging unit that is selected according to the charging start voltage. A charging power supply for applying a voltage applied to a roller to the photosensitive drum to start charging. 2. The printing apparatus according to claim 1, wherein the charging start voltage measuring unit applies a variable voltage from the charging roller to the photosensitive drum after the charge elimination, and determines a boundary between a state where the charging current flows and a state where the charging current does not flow. A printing apparatus, wherein a charging start voltage of the charging roller is measured by obtaining a voltage. 3. The printing apparatus according to claim 2, further comprising: an exposure head that exposes the photosensitive drum to remove electricity before measuring a charging start voltage. 4. The printing apparatus according to claim 1, wherein the charging power supply sets a charging start voltage in a reference state to Vth0, a charging start voltage measured by a charging start voltage measuring unit to Vth, and an applied voltage in the reference state. A printing apparatus comprising: selecting an applied voltage Vch obtained by an arithmetic expression Vch = V0 + (Vth0−Vth), where V0 is a reference applied voltage.