JPH0634128B2 - Image forming device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus such as an electronic copying machine or a laser beam printer which applies a developing bias between a photosensitive member and a developing device. Regarding

[Prior Art] Conventionally, electronic copying machines, laser beam printers, and other devices to which electrophotographic technology is applied are widely used. In these devices, image information is optically provided on a photoconductor such as Cds,
A method is well known in which an electrostatic latent image is formed on a photosensitive member, jumping development is then performed, and a visualized toner image is transferred onto a recording sheet.

Among the above processes, in the developing process, a technique is known in which a developing bias based on an AC signal is applied between the developing device and the photoconductor to appropriately maintain density and gradation characteristics. A sine wave is often used as the AC signal as the bias.
Further, there is also known a technique in which a direct current voltage is further superimposed on the alternating current developing bias to keep the amount of toner adhering to the photosensitive member properly, or to prevent the toner from adhering in the preparation process.

In such a conventional developing device, since the AC developing bias is a sine wave having a single spectrum, the waveform is relatively easy to manage in terms of overshoot and distortion correction, and the transformer, amplifier, etc. that handle this AC bias. On the other hand, it has the advantage that the band is narrow, but it has the following drawbacks.

That is, in the case of a sinusoidal AC bias, the difference between the average level and the peak level of the signal is large, and therefore, the transfer energy of the toner also fluctuates accordingly, and for the same reason, the developing sleeve provided facing the photoconductor. It is easy for discharge to occur between them. Further, this tendency is that the peak value is further increased when the above DC voltage is superimposed,
Being encouraged.

Further, in the conventional technique, the density of the image is variably controlled by applying a DC voltage to the developing device and changing the voltage, but the developing bias is automatically changed according to the type of the original image. There was no control.

〔Purpose〕

The present invention has been made in view of the above-mentioned drawbacks of the conventional apparatus. It is possible to stably fly the toner, the risk of discharge between the developing sleeve and the photoconductor is small, and the type of the original image, that is, a halftone image. An object of the present invention is to provide an image forming apparatus capable of automatically controlling the image quality by using a developing bias according to a character image.

[Examples] Hereinafter, the present invention will be described in detail based on examples shown in the drawings.

FIG. 1 shows an embodiment of the developing bias generating circuit of the image forming apparatus of the present invention.

In FIG. 1, the reference numeral 1 is an oscillator, the output of which is connected to the comparator 2. The comparator 2 has an input terminal 5, the output of which is connected to an amplifier 3. The amplifier 3 has an input terminal 6, and its output is connected to the step-up transformer 4. The output of the step-up transformer 4 is connected to the output terminal 7 and is supplied to the developing device from here.

In the present invention, a rectangular wave is used as the developing bias instead of the conventional sine wave, and the oscillator 1 for generating it is configured as shown in detail in FIG.

FIG. 2 shows the structures of the oscillator 1 and the comparator 2 in FIG. 1 in detail. As shown in the figure, the oscillator 1 and the comparator 2 have high gain and high impedance DP amplifiers Q1 and Q2.
It consists of The circuit of the oscillator 1 has resistors R1 and R2.
It is a known blocking oscillator circuit that is positively fed back by the above and generates a triangular wave in a cycle corresponding to the time constants C1 and R1.

Further, the comparator 2 includes the reference voltage input from the input terminal 5 via the resistor R6 and the oscillator 1 input via the resistor R4.
It is a known comparator circuit that compares the output voltages of the.

The voltage waveform of each part in FIG. 2 is shown in FIGS. 3 (A) to (E).

FIG. 3A shows the output of the OP amplifier Q1 of FIG.
FIG. 6B shows a triangular wave generated at the negative input terminal of the OP amplifier Q1. The oscillation operation of the oscillator 1 will be briefly described below. The negative input terminal of the OP amplifier Q1 has its own constant C1.
-At R1, it tries to converge to the output voltage in a periodic function. The output voltage of itself shown in FIG. 3 (A) is input to the positive input terminal after being divided by the resistors R2 and R3. When the voltage of the negative input terminal reaches the positive input terminal, the output changes from high level. The voltage is inverted to the low level or vice versa, and the convergence direction of the terminal voltage of the negative input terminal is also switched. When this negative input terminal voltage reaches the positive input terminal after a predetermined time, the output voltage is inverted again. In this way, the oscillator 1 repeatedly oscillates at the cycle of the function C1.R1 and the triangular wave shown in FIG. 3 (B) is generated.

Next, in the comparator 2, the triangular wave is compared with the DC voltage input from the input terminal 5 by the OP amplifier Q2. Third
When compared with the DC voltage of three levels shown by the symbols P, Q, and R in the figure (B), the output of the OP amplifier Q2 is inverted every time a triangular wave crosses the DC level. Rectangular waves having different duty ratios shown in FIGS. 3C to 3E are obtained. At this time, the feedback resistor R5 causes the OP amplifier Q2 to have a slight hysteresis characteristic to stabilize the output.

Return to FIG. 1 again. The rectangular wave having the duty ratio corresponding to the DC voltage of the input terminal 5 generated as described above is input to the amplifier 3, is power-amplified, and is boosted by the step-up transformer 4.
Is added to the primary side of. Step-up transformer 4 has a step-up ratio of 100
The output of 1000 to 2000 vpp is generated on the secondary side. The boosted rectangular waves having different duty ratios are input to the developing device. However, the input terminal 6 is a remote terminal for turning the output on and off, and is used to cut off the output in an emergency.

As described above, rectangular waves having different duty ratios can be used as the developing bias. In the case of a rectangular wave, when the duty ratio is 1: 1, the average DC voltage is 0V, but the average DC voltage can be changed from negative to positive by changing the duty ratio as described above.
Therefore, as shown in FIG. 4, in a copying machine or the like, a duty ratio of a rectangular wave generated by applying a power supply voltage to both ends of the image density adjusting volume VR1 and inputting this divided voltage to the comparator 2 is determined. The image density can be changed and adjusted. In FIG. 4, the resistor R8 connected to both ends of the step-up transformer 4 is a bleeder resistor for stabilizing the output, and the resistors R9 and R10 connected to the output are for protection for performing current control when the output terminal is short-circuited. Is.
An AC bypass capacitor C2 is connected to the resistor R9, and the resistor R9 blocks DC when a short circuit occurs.

Also, in the case of a copying machine or a laser beam printer in which a microcomputer is used to control image forming conditions,
As shown in the figure, the partial pressure of the image density adjusting volume VR1 is converted into a digital amount by the A / D converter 11 and input to the microcomputer 12. Microcomputer 12
Outputs a digital amount according to the duty ratio according to the process, and the digital amount is converted into an analog amount by the D / A converter 13 and applied to the comparator 2 to control the bias duty ratio.

An example of this control is shown in FIGS. 6 (A) to (C).

In a copying machine, a laser beam printer, etc., the duty ratio of the rectangular wave is controlled to be 1: 1 as shown in FIG. 6 (A) in the non-image area, that is, in the standby state where the photosensitive drum and the developing roller are stopped. To be done. As a result, the average DC voltage of the developing bias is held at 0V.

During image formation, that is, during image formation, the duty ratio is slightly changed to the negative side as shown in FIG. 6 (B), and the average DC voltage is set to about −100V. At this time, the microcomputer 12 can adjust the duty ratio according to the density adjusting volume VR1, and the duty ratio is adjusted to a value around this so that the image density becomes a density desired by the operator.

Further, in the non-image portion accompanied by rotation of the photosensitive drum and the developing roller when the optical system and the document table are reversed, the developing is performed so that the average DC voltage of the developing bias becomes about −500V as shown in FIG. 6C. The bias duty ratio is adjusted to prevent toner from adhering to the photosensitive drum.

Further, the DC voltage may be superimposed on the developing bias formed by the rectangular wave having the variable duty ratio described above. Example 7
Shown in the figure. Here, the same or similar members as those in FIG. 5 are designated by the same reference numerals, and detailed description thereof will be omitted.

In addition to the structure shown in FIG. 5, the embodiment shown in FIG. 7 is such that a high DC voltage using an inverter is superimposed on the secondary side of the step-up transformer 4. In the figure, the image density adjusting volume VR1 to D / A converter 13 is a comparator 1.
The inverter circuit 8 is controlled via 0, and the output voltage of the inverter transformer 9 is controlled. A rectifying and smoothing circuit including a diode D1 and a capacitor C3 is connected to the secondary side of the inverter transformer 9, and the generated high DC voltage is applied to the secondary side of the step-up transformer 4 from one end of the diode D1. . The DC voltage superimposed at this time is divided by resistors R11 and R12 and fed back to the comparator 10. The comparator 10 controls the switching transistor and the like that form the inverter circuit 8 so that this voltage division and the analog amount input from the D / A converter 13 become equal. Here again, the output of the step-up transformer 4 has the same bleeder resistance R8, resistances R9 and R1 as in the configuration of FIG.
A protection circuit consisting of 0 and a capacitor C2 is connected.

In this way, in addition to the above-mentioned duty ratio control, the DC density superimposed on the developing bias is controlled in accordance with the amount input from the image density adjustment volume VR1 to adjust the image density or the non-image described above. It is possible to perform the toner adhesion prevention control in the section.

Further, in this embodiment, by adjusting only the image quality adjusting volume VR2 and controlling the duty ratio of the AC component of the developing bias via the comparator 2, it is possible to adjust the γ of the superimposed DC voltage characteristic to the image density characteristic, and thereby the resolution. , Gradation can be adjusted according to preference. Both the duty ratio control of the rectangular wave and the control of the superimposed DC voltage described above may be controlled by a microcomputer as shown in FIG.

In FIG. 8, both the volume VR1 and VR2 are connected to the A / D converter 11, and the volume VR
The input amounts of 1 and VR2 are converted into digital amounts by the A / D converter 11 and input to the microcomputer 12. The control amount calculated by the microcomputer 12 according to the above-described image forming conditions is input to the D / A converter 13, converted into an analog signal, and input to the comparator 2 and the comparator 10. In this way, image density control, toner adhesion prevention in non-image areas, and image quality adjustment can be controlled via the microcomputer, and at that time, control is performed in consideration of other control conditions such as the surface potential of the photoconductor. You can also do it.

In addition to the embodiment of FIG. 8, FIG. 9 shows an example in which the surface potential of the photosensitive drum is also taken into consideration as the control condition of the duty ratio of the developing bias and the superimposed DC voltage.

In the embodiment of FIG. 9, the latent image potential formed on the photosensitive drum is measured by the known surface potential sensor 14, and the value processed by the measuring circuit 15 is input to the A / D converter 11. Has become. The output of the surface potential sensor is converted by the measuring circuit 15 into a DC voltage of about 1/300 of the surface potential, and A / D
It is input to the converter 11. This voltage is applied to the A / D converter 11
Is converted into a digital amount by, and the microcomputer 12 controls so as to correct the amount input from the image density adjusting volume VR1 and the image quality adjusting volume VR2. Although not shown here, the measured surface potential of the photosensitive drum is used by the microcomputer 12 to control the amount of light emitted from the lamp for illuminating the original, control the charger, and determine the type of the original image, as in the prior art.
In this way, the duty ratio of the developing bias and the level of the superimposed DC voltage can be controlled in consideration of the condition of the surface potential of the photosensitive drum.

As described above, the rectangular wave whose duty ratio is changed according to the image forming condition can be used as the developing bias. In this case, since the difference between the average level and the peak value is smaller than that of the conventional sinusoidal developing bias, the toner flying is stabilized and the discharge between the photoconductor and the developing sleeve is prevented. Also, the process of adjusting the average level of the developing bias by superimposing a DC voltage on a sine wave, which has been performed conventionally, can be replaced by a duty ratio adjusting process, so that similar control can be easily performed at a low level stage, and high voltage This is useful for simplifying the entire device, reducing the size, and reducing the cost, such as omitting the DC voltage generating means. There is also an advantage that the power loss of the amplifier is significantly smaller than that of the conventional developing bias by the sine wave.

Further, when the high DC voltage is superimposed on the developing bias as in the embodiment shown in FIGS. 7 and 8, the average voltage of the developing bias can be adjusted by controlling the duty ratio of the rectangular wave pulse. Compared with the example, the range of the superimposed DC voltage can be narrowed and the image quality such as gradation and resolution can be easily adjusted.

Further, in the embodiment shown in FIG. 9, since the duty ratio of the developing bias of the rectangular wave and the level of the superimposed DC voltage are controlled in consideration of the condition of the surface potential of the photosensitive drum, more accurate control can be performed, Even if fatigue due to aging of the photosensitive drum occurs, it has an excellent advantage that it can be automatically corrected. Also, by measuring the latent image potential of the photosensitive drum, especially the latent image potential of the background, whether the original image (original image in a copying machine) is a line original such as a character original or a halftone original such as a photograph. Since it can be determined by a microcomputer, the duty ratio of the developing bias or the superimposed DC voltage thereof can be controlled according to the type of the original image to control the density, the γ characteristic and the like.

Although the embodiments of the present invention have been described above, various modifications as described below are possible.

The adjustment of the duty ratio can be controlled according to the desired γ characteristic of the image, in addition to the above-described image density adjustment or the prevention of toner adhesion in the non-image portion. Also, by measuring the latent image potential of the photosensitive drum, especially the latent image potential of the background, whether the original image (original image in a copying machine) is a line original such as a character original or a halftone original such as a photograph. Since it can be determined by a microcomputer, the duty ratio of the developing bias or the superimposed DC voltage thereof is controlled according to the type of the original image, and the density, γ
It is also possible to control characteristics and the like.

Further, the generation of the rectangular wave having the variable duty ratio is not limited to the method exemplified above, and various methods such as once converting the rectangular wave into the triangular wave by using the integrator and then adjusting the duty ratio by the comparator are possible. Modifications are possible.

Further, a filter may be inserted between the comparator and the amplifier in order to prevent overshoot in the amplifier and the step-up transformer. This filter may be inserted between the amplifier and the step-up transformer in order to reduce power loss.

It is also conceivable to use a switching circuit instead of an amplifier to drive the step-up transformer to reduce power loss.

Further, the A / D converter and the D / A converter shown in FIG. 5 may be configured by hardware, but can also be configured by software of a microcomputer or a part of the hardware. is there.

[Effect] As is apparent from the above description, the present invention provides an image forming apparatus that applies a signal in which a DC voltage is superimposed on a rectangular wave pulse voltage as a developing bias between the photoconductor and the developing device. Detection means for detecting the surface potential, discrimination means for discriminating whether the original image is a halftone image or a character image according to the detected surface potential, and the duty ratio of the rectangular wave pulse voltage is variably controlled according to the discrimination result. Since a means for adjusting the γ characteristic of the image is provided, the discharge between the photoconductor and the developing device can be prevented easily and inexpensively, and the variable amount can be reduced even if the superimposed DC voltage is changed. The γ characteristic of the image can be adjusted according to the type of the automatically detected original image.

[Brief description of drawings]

FIG. 1 is a block diagram illustrating the basic configuration of the image forming apparatus of the present invention, FIG. 2 is a more detailed circuit diagram of the members in FIG. 1, and FIGS. 3 (A) to (E) are FIG. FIG. 4 is a waveform diagram showing voltage waveforms in the circuit shown in FIG. 4, FIG. 4 is a block diagram illustrating another embodiment of the image forming apparatus of the present invention, and FIG. 5 is a block diagram illustrating yet another embodiment of the present invention. FIGS. 6 (A) to 6 (C) are waveform diagrams for explaining the control in the embodiment of FIG. 5, and FIGS. 7, 8 and 9 are block diagrams for explaining further different embodiments. 1 ... Oscillator, 2, 10 ... Comparator 3 ... Amplifier, 4 ... Step-up transformer 7 ... Output terminal, 11 ... A / D converter 12 ... Microcomputer 13 ... D / A converter VR1 …… Image density adjustment volume VR2 …… Image quality adjustment volume 14 …… Surface potential sensor 15 …… Measuring circuit

Claims (1)

[Claims] 1. An image forming apparatus for applying, as a developing bias, a signal obtained by superimposing a DC voltage on a rectangular wave pulse voltage between a photoconductor and a developing device, and detecting means for detecting a surface potential on the photoconductor. A determination means for determining whether the original image is a halftone image or a character image according to the surface potential, and a means for variably controlling the duty ratio of the rectangular wave pulse voltage according to the determination result to adjust the γ characteristic of the image. An image forming apparatus characterized by being provided.