JPS61102667A - Electrophotographic copying machine - Google Patents

Abstract

PURPOSE:To obtain a good copy image by providing a means for comparing the detected output of original image density with a reference voltage and means for controlling automatically a developing bias and detecting the min. density value of an image as the ground texture density of a original. CONSTITUTION:The reflected light from a toner image is converted to a voltage by a phototransistor 17 and a variable resistor VR1 and is conducted to comparators 23 and 24. The voltage is compared with reference voltages V1, V2 preset by the variable resistor VR1 and VR2. The outputs Q1, Q2 of two RS-FF29, 30 rise both to a high level when the original image density is low and the voltage V proportional to the quantity of the refelected light from the toner image is V>V1. Q1 is the low level and Q2 is the high level at such density at which the voltages are V1>V>V2. Both Q1 and Q2 are the low level when the original image density is high and V<V2. The bias voltage for the purpose of development is changed by the means for controlling automatically the developing bias according to the above-mentioned signal, by which the density of the ground texture part is exactly detected and the control of a photosensitive body to the correct developing bias voltage is made possible.

Description

[Detailed description of the invention] Industrial applications The present invention relates to an electrophotographic copying machine.

Conventional configuration and problems Conventionally, the original image density detection method for electrophotographic copying machines, which has an automatic image density adjustment function and performs pre-exposure scanning to detect the original image density, uses a photodetector element. , detect the reflected light from the original and measure the average density of the original.

■ Measure only the concentration of the main part.

There are methods such as However, the surface potential of the photoreceptor decreases more than necessary.
This has the disadvantage that it results in an image with low contrast. In addition, in the case of (2) above, it is necessary to take time and effort to select the most important part of the document, and there is a problem that the document image density detection location must be changed each time.

OBJECTS OF THE INVENTION The present invention solves the above-mentioned conventional drawbacks, and an object of the present invention is to provide an electrophotographic copying machine that can correctly control the image density automatic adjustment function for all types of originals.

Structure of the Invention In order to achieve the above object, the electrophotographic copying machine of the present invention has the following features:
At least the area around the photoreceptor is charged and exposed.

In addition to having transfer, static neutralization, and cleaning means, a part of the shading of the toner image obtained by exposing the original onto a photoconductor and developing it during pre-exposure scanning performed prior to copying is An original image density detection means to detect, a comparison means to compare the detection output of this original image density detection means with a reference voltage, and an automatic development bias voltage to control a bias voltage for development according to the comparison result of this comparison means. The control means is configured to detect the minimum density value of the original image density as the background density of the original.

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, 2 is a document table on which the original is placed, 2 is an exposure lamp that irradiates the original onto the document on the document table, and the first
The mirror 3, the second mirror 4, the third mirror 5, the lens 6, the fourth mirror, the sixth mirror 8, and the sixth mirror 9 form an image of the reflected light from the document onto the slit 10 and the photoreceptor 11. This photoreceptor 11 is used to form an electrostatic latent image of an incident image after the complete optical system is formed. Further, 12 is a charger that charges the photoconductor 11 prior to image formation, 13 is a developing section that develops the latent image on the photoconductor 11, 14 is a paper feed section that supplies copy paper, and 1
Reference numeral 6 denotes a transfer charger that transfers the toner image on the photoreceptor 11 onto copy paper. A light emitting diode 16 and a phototransistor 1 are located near the transfer charger 15.
7 is provided.

In addition, 18 is a 5-separation charger that separates the copy paper from the photoreceptor 11, 19 is a paper transport section that transports the separated copy paper to the paper discharge section 20, and 21 is a copy paper that is transported by this paper transport section 19. A fixing section 22 is a cleaning section that cleans the photoreceptor 11. Since these sections have the same structure as a conventional electrophotographic copying machine, a detailed explanation of their operation will be omitted.

In the above configuration, during pre-exposure scanning performed prior to exposure, the original image density detecting means detects the original image density, and signal processing is performed by the circuit shown in FIG.
In the figure, the density of the original is detected, and a bias voltage for development is connected to a regulator 23 and a non-inverting input terminal of a second comparator 24 in accordance with the detected output. The inverting input terminal of the first comparator 23 is connected to a sliding terminal of a variable resistor vR2, and is connected to a first base voltage V for setting a bias voltage for development according to a preset reference document density.

is applied. One end of the variable resistor VR2 is connected to the power supply via a resistor R4, and the other end is grounded via a resistor R2. The inverting input terminal of the second comparator 24 is connected to a variable resistor V
It is connected to the sliding terminal of R3, and a second reference voltage v2 is applied thereto. Variable resistance VR.

One end is connected to the power supply via a resistor R3, and the other end is grounded via a resistor R4. The output terminal of the first comparator 23 is a timing pulse P+ synchronized with the pre-exposure scan.
(hereinafter referred to as "pre-exposure scanning pulse") is connected to the output end of an inverter 25 into which the pulse is input, and is also connected to the input end of an inverter 26. Second comparator 24
The output terminal of the inverter 27 is connected to the output terminal of the inverter 27 to which the pre-exposure scanning pulse P is input, and is also connected to the input side of the inverter 28. The output end of the inverter 26 is connected to the S terminal of an RS flip-flop 29 (hereinafter referred to as rR3-FFJ), and the output Q1 of R3-FF29 is a first developing bias voltage control signal. The output terminal of the inverter 28 is R3-FF30 S2
The output Q2 of R3-FF30 is a second developing bias voltage control signal. R8-FF
The R and R2 terminals of 29°3Q are the pre-exposure scanning pulse P.

The output terminal of the inverter 31 is connected to the output end of the inverter 31 via the inverter 32 and a differentiator circuit consisting of a capacitor C1 and a resistor R5.

Next, the operation will be explained. In the pre-exposure scan performed prior to the copying operation, the light reflected from the document by the exposure operation passes through the optical system to P3. The light body 11 is exposed to light.

Since the photoreceptor 11 is charged by the charger 12, a latent image of the original is formed on the photoreceptor 11. This latent image is visualized by the developing section 13, and the visualized toner image passes over the original image density detection means. The original image density detection means is configured to irradiate the photoreceptor 11 with the light emitting diode 16 and receive the reflected light with the phototransistor 17. Detected, the voltage is converted by the variable resistor VR. This variable resistor VR is used to change the voltage up to a certain level in accordance with the amount of reflected light from the toner image. The amount of reflected light from the voltage-converted toner image changes depending on the original image density, and is guided to a first comparator 23 and a second comparator 24, where the first reference voltage V and the second is compared with a reference voltage v2 (vl>v2). The greater the amount of light reflected from the document, the less toner adheres to the photoconductor 11, and the greater the amount of light reflected from the photoconductor 11 by the light emitting diode 16.

The larger the amount of reflected light, the higher the conversion output.

A voltage proportional to the amount of reflected light is the first reference voltage V.

If it is higher, the output of the first comparator 23 will be at a no-no level, and if it is lower, it will be at a low level. Similarly, when the voltage is higher than the second reference voltage v2, the second comparator 24 goes high, and when it is lower, it goes low. In this case, because of the relationship V, > V2, the voltage proportional to the amount of reflected light is higher than the first reference voltage V, and lower than the second reference voltage V2. It is possible to classify the voltage into three states: (1) and (2) a case where the voltage is lower than the second reference voltage v2. Normally, the pre-exposure scanning pulse P1 is at a high level, so the inverter 2
The outputs of the comparators 23 and 24 are at low level through the 6°27, but when the pre-exposure scan starts, the photo
Since the pre-exposure scanning pulse P is set to be at a low level when the reflected light from the toner image corresponding to the head position of the document on the photoreceptor 11 hits the transistor 17, The outputs of the inverters 25° and 27 change to high level, and the outputs of the comparators 23° and 24 are transmitted to the R5-FFs 29 and 30 via the inverters 2'e and 28k. In addition, the pre-exposure scanning pulse P1
When becomes a low level, a pulse of R3-FF29, 3C1)R, , R2 terminals [0-Lz-< Transmitted for a short time, output Q1
．． Q2 is in a reset state, and from this point on, the R that was set when the pre-exposure scanning pulse P1 was at a low level.
The outputs Q+ and Q2 of the 3-FFs 29 and 30 are held until the next pre-exposure scanning pulse P is input.

Next, an example of the operating state will be explained using the timing chart of FIG. 3 and the truth table of R8-F and F shown in the table below.

(Left below) When pre-exposure scanning pulse P1 is at high level, R8-F
Output Q1 of F29,30. Q2 maintains the previously set state, but the pre-exposure scanning pulse P.

When becomes low level, R5-FF29.30 R,,
A short low-level pulse generated by the differentiating circuit is applied to the R2 terminal. At this time, a voltage proportional to the amount of reflected light from the toner image on the photoreceptor 11 corresponding to the original is the first reference voltage v.
1 and the second reference voltage v2, the outputs Q+ and Q2 of R8-FF29°3o become low level from the above table. A voltage proportional to the amount of reflected light from the toner image on the photoreceptor 11 corresponding to the original is the second reference voltage v2.
When the level becomes higher, that is, when a bright part of the document is blurred, the output of the second comparator 24 becomes the NO level, and a low level is transmitted to the 82 terminal of the R3-FF 30. According to the table above, from this state R3-FF3o
It can be seen that the output Q2 changes to high level. Further, when the voltage proportional to the amount of reflected light from the toner image on the photoreceptor 11 corresponding to the original becomes higher than the first reference voltage V, the output Q1 of R8-FF29 similarly changes to a high level. If the density of the original image is low, please use two R.
The outputs Q+ and Q2 of the 3-FFs 29 and 30 are both at the NO level, and in the case of an original whose density is between the first reference voltage v1 and the second reference voltage v2, the R5-
Output Q1 of FF29 is low level, R3-FF30
The output Q2 becomes high level. Further, if the voltage proportional to the amount of reflected light from the toner image on the photoreceptor 11 is lower than the second reference voltage v2, in other words, if the original image density is high, the output Q+ of the two R3-FF29.30
, Q2 both indicate a low level. By using these signals to change the developing bias voltage in this next stage circuit, it becomes possible to change the developing bias voltage depending on the density of the original image.After this, the pre-exposure scanning pulse P, becomes high level, two R3-FF29.
Both the R terminal and the S terminal of No. 30 become high level, and from the above table, the state set as described above is maintained.

Through this operation, when there is a large amount of reflected light from the toner image on the photoreceptor 11 corresponding to the original, in other words, when the original image density is low, the developing bias voltage is increased. When there is little reflected light from the toner image on the photoreceptor 11, in other words, when the original image density is high, the developing bias voltage is controlled to be low.

In addition, in the above embodiment, the developing bias voltage is switched f
Although the number of stages is c3, the development bias voltage may be switched in more stages by increasing the number of comparators and RS flip-flops.
As described in detail, according to the present invention, the minimum density value of the original image density is detected as the background density of the original. Even if the original has a high-contrast image and a large image area, it is possible to accurately detect the density of the non-image area, that is, the background area, and control the photoreceptor to the correct developing bias voltage, resulting in a good image without background stains. A duplicate image is obtained.

[Brief explanation of drawings]

FIG. 1 is a sectional view of an electrophotographic copying machine according to an embodiment of the present invention, FIG. 2 is a circuit diagram of a developing bias voltage control circuit according to an embodiment of the present invention, and FIG. 3 is a circuit diagram of a developing bias voltage control circuit according to an embodiment of the present invention. This is a time chart. 1.°...Original table, 2..Exposure lamp, 3.
...First mirror, 4...Second mirror, S...
...Third mirror, 6...Lens, 7...
...4th mirror, 8...6th mirror, 9.
...6th mirror, 1o...slit, 11
...Photoreceptor drum, 12...Charger,
13...Developing section, 14...Paper feeding section, 1
5...Transfer charger, 16...Light emitting diode, 17...Phototransistor, 18
... Separation charger, 19 ... Paper transport section, 20 ... Paper discharge section, 21 ... Fixing section,
22...Cleaning section, 23.24...
...Comparator, 25.26, 27, 28, 31.32...
...Inba. -ta, 29.30...RS flip-flop,
C1...Capacitor, VR4~YR3...
・Variable resistance, R1-R6...Resistance.

Claims (1)

[Claims] At least charging, exposure, transfer, static neutralization, and cleaning means are provided around the photoreceptor, and the document is exposed to light on the photoreceptor during pre-exposure scanning performed prior to copying, and the image is developed. An original image density detection means for optically detecting a portion of the density of the toner image, a comparison means for comparing the detection output of the original image density detection means with a reference voltage, and a developing control means for comparing the detection output of the original image density detection means with a reference voltage. What is claimed is: 1. An electrophotographic copying machine comprising: an automatic developing bias control means for controlling a bias voltage for controlling the image density of an original;