JPH0422270B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to an electrophotographic copying machine. Conventional structure and its problems Conventionally, in an electrophotographic copying machine equipped with a variable magnification device that has an automatic image density adjustment function, a photodetector element for detecting original image density is mounted on the optical system or on an optical system branched from the optical system. However, in this case, even though the same original is used at each copying magnification of enlargement, normal magnification, and reduction, the illuminance of the incident light on the light receiving surface of the photodetecting element is different, so the original The developing bias voltage is controlled to a different developing bias voltage from the developing bias voltage of the conventional printer, and there is a problem that, for example, even if a good copied image is obtained in full-size copying, an image with background stains is produced in enlarged copying. was. Purpose of the Invention The present invention solves the above-mentioned conventional drawbacks, and provides an electrophotographic copying machine that can easily and correctly control the image density automatic adjustment function not only for full-size copying but also for enlargement and reduction copying. The purpose is to Structure of the Invention In order to achieve the above object, the electrophotographic copying machine of the present invention provides at least charging, exposing, and
A document image density detection device which has transfer, static neutralization, and cleaning means and which detects a part of the reflected light from the document to be copied using a photodetection element during pre-exposure scanning performed prior to copying; a comparison means for comparing the detection output of the original image density detection means with a reference voltage; an automatic developing bias voltage control means for controlling a bias voltage for development according to the comparison result of the comparison means; and an automatic development bias voltage control means for controlling the reference voltage. and a reference voltage control means, and by controlling the reference voltage, enlargement, normal size,
The structure is such that development is performed using the same developing bias voltage at each copying magnification of reduction. DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, reference numeral 1 denotes an original platen on which an original is placed, 2 an exposure lamp that irradiates light onto the original on the original platen, a first mirror 3, a second mirror 4,
The third mirror 5, the lens 6, the fourth mirror 7, the fifth mirror 8, and the sixth mirror 9 form an optical system for forming an image of the reflected light from the document onto the slit 10 and the photoreceptor 11. The photoreceptor 11 is for forming an electrostatic latent image of an incident image. Further, in the vicinity of the slit 10, a second
As shown in the figure, a photodetecting element 12 and a filter 13 for correcting the spectral characteristics of the photodetecting element 12 are provided.
An original image density detecting means is provided, and a portion of the reflected light from the original passes through the optical system and enters the original image density detecting means. Note that 14 is a charger that charges the photoreceptor 11 prior to forming a latent image, 15 is a developing section that develops the latent image on the photoreceptor 11, 16 is a paper feed section that supplies copy paper, and 17 is the aforementioned one. A transfer charger 18 transfers the toner image on the photoreceptor 11 onto copy paper; 18 a separation charger separates the copy paper from the photoreceptor 11; and 19 a paper conveyor to convey the separated copy paper to the paper discharge section 20. A conveyance section 21 is a fixing section that fixes the toner image on the copy paper conveyed by this paper conveyance section 19, and 22 is a cleaning section that cleans the photoreceptor 11. These have the same configuration as a conventional electrophotographic copying machine. Therefore, a detailed explanation of the operation will be omitted. Figure 3 shows an example of the relationship between each copying magnification and the illuminance of light incident on the photodetector from the same document. The amount of light used is slightly larger, and on the other hand, it is smaller when making reduced copies. FIG. 4 shows an example of a circuit for detecting the density of the original and controlling the bias voltage for _development according to the detected output. is connected to the comparator 23 and the non-inverting input terminal of the second comparator 24. The inverting input terminal of the first comparator 23 is connected to the sliding terminal of the variable resistor VR ₂ and is applied with the first reference voltage V ₁ for setting the original bias voltage according to the reference original density set in advance. . One end of the variable resistor VR ₂ is connected to the power supply via a resistor R ₁ , and the other end is grounded via a resistor R ₂ . The inverting input terminal of the second comparator 24 is connected to the sliding terminal of the variable resistor VR ₃ and the second reference voltage V ₂ is applied thereto. One end of the variable resistor VR ₃ is connected to the power supply via a resistor R ₃ , and the other end is grounded via a resistor R ₄ .
The output terminal of the first comparator 23 is connected to the output terminal of an inverter 25 to which a timing pulse P ₁ (hereinafter referred to as "pre-exposure scanning pulse") synchronized with the pre-exposure scan is input, and the output terminal of the inverter 26 is Connected to the input end. The output terminal of the second comparator 24 is connected to the output terminal of an inverter 27 to which the pre-exposure scanning pulse P ₁ is input, and is also connected to the input side of an inverter 28 . Inverter 26
The output terminal of is connected to _one terminal of the RS flip-flop 29 (hereinafter referred to as "RS-FF"),
The output Q ₁ of the RS-FF 29 is a first developing bias voltage control signal. The output end of the inverter 28 is
Connected to ₂ terminals of RS-FF30, RS-
The output Q ₂ of the FF 30 is a second developing bias voltage control signal. _{The 1} and ₂ terminals of the RS-FFs 29 and 30 are connected to the output terminal of an inverter 31 to which the output of the pre-exposure scanning pulse P ₁ is input, and a differentiator circuit consisting of a capacitor C ₁ and a resistor R ₅ and an inverter 32
connected via. Next, the operation will be explained. During pre-exposure scanning performed prior to the copying operation, when a part of the light reflected from the original by the exposure operation enters the original image density detection means consisting of the photodetector 12 and the filter 13, the photodetector 12 detects a current change. is detected and converted into voltage by variable resistor _VR1 . This variable resistor VR ₁ is used to change the voltage up to a certain level depending on the amount of reflected light. The amount of reflected light converted to voltage changes depending on the density of the original image.
It is led to a first comparator 23 and a second comparator 24, where it is compared with a first reference voltage V ₁ and a second reference voltage V ₂ (V ₁ >V ₂ ), respectively. In this case, the larger the amount of reflected light from the original, the higher the conversion output, but at this time, the copying magnification and the reflected light incident on the original image density detection means are determined as shown in FIG. 3, for example. Because of the relationship, e.g.
When copying a B5 size document to B4 size at a copying magnification of 1.414 times, the amount of light incident on the document image density detection means is approximately 1.10 times that of a full size copy. Also, it can be seen that the copy magnification of 0.707x is about 0.71x. For example, when copying at the same size, the voltage Va converted by the variable resistor VR ₁ is 08.0V, the first reference voltage V ₁ is 8.5V,
When the second reference voltage V ₂ is 6.0V, V ₁ >Va>
The relationship V ₂ holds true. However, when the copying magnification is 1.414 times, the voltage Vb converted by variable resistor VR ₁ is
It is approximately 1.10 times the voltage Va when copying at the same size, so Va
= 8.0V, Vb = 8.8V, and Vb > V ₁ > V ₂ . Therefore, if the first reference voltage V ₁ and the second reference voltage V ₂ are respectively made 1.10 times the same size,
V ₁ = 9.4V, V ₂ = 6.6V, and the magnitude relationship among V ₁ , V ₂ , and Vb is V ₁ >Vb > V ₂ , which is the same magnitude relationship as when the magnification is the same. Further, for example, at a copying magnification of 0.707 times, the first reference voltage V ₁ and the second
According to the reference voltage V ₂ of 0.71 times the same magnification, the magnitude relationship between the voltage Vc converted by the variable resistor VR ₁ at the copying magnification of 0.707 times and the above-mentioned V ₁ and V ₂ is as follows.
V ₁ > Vc > V ₂ , and the same magnitude relationship holds true as when the magnification is the same. It can be seen that this holds true regardless of the value of Va. The voltage proportional to the amount of reflected light is the first reference voltage V ₁
If it is higher, the output of the first comparator 23 will be at a high level, and if it is lower, it will be at a low level. Similarly, if it is higher than the second reference voltage V ₂ , the second comparator 24 goes high, and if it is lower, it goes low. In this case, since there is a relationship of V ₁ > V ₂ , the voltage proportional to the amount of reflected light is the first reference voltage.
It is possible to classify into three states: higher than V ₁ , lower than the first reference voltage V ₁ and higher than the second reference voltage V ₂ , and lower than the second reference voltage V ₂ . Become. Normally, pre-exposure scanning pulse P ₁
is at a high level, so inverter 2
The outputs of the comparators 23 and 24 are at a low level via the comparators 5 and 27, but when the pre-exposure scan starts,
Since the pre-exposure scanning pulse _P1 is set to be at a low level when the reflected light from the head position of the document hits the photodetector element 12, the inverter 25,
The output of 27 changes to high level, and the comparator 23,
The output of 24 is sent to RS via inverters 26 and 28.
-Can be transmitted to FF29 and FF30. Also,
When the pre-exposure scanning pulse P ₁ becomes low level, the voltage is passed through the inverter 31 , a differentiating circuit consisting of the capacitor C ₁ and the resistor R ₅ , and the inverter 32 .
A low-level pulse is transmitted to _{the 1} and ₂ terminals of RS-FF29 and 30 for a short time, and the outputs Q ₁ and Q ₂ are reset. From this time, the pre-exposure scanning pulse P ₁ is set at low level. RS-FF2
The outputs Q ₁ and Q ₂ of 9 and 30 are the next pre-exposure scanning pulse
It will be held until P ₁ is entered. Next, an example of the operating state will be explained using the timing chart in FIG. 5 and the RS-FF truth table shown in the table below.

[Table] When pre-exposure scanning pulse P ₁ is at high level, RS-FF
The outputs Q ₁ and Q ₂ of RS-FF 29 and 30 maintain the previously set state, but when the pre-exposure scanning pulse P ₁ becomes low level, the outputs Q 1 and _{Q 2} of RS-FF 29 and 30
A short low-level pulse generated by a differentiating circuit is applied to the terminal. At this time, voltages proportional to the amount of light reflected from the original are the first reference voltage V ₁ and the second reference voltage.
Since it is lower than V ₂ , from the above table RS-FF29,30
The outputs Q ₁ and Q ₂ become low level. When the voltage proportional to the amount of reflected light from the original becomes higher than the second reference voltage _V2 , that is, when a bright part appears in the original, the output of the second comparator 24 becomes high level, and RS- Low level is transmitted to ₂ terminals of FF30. From the table above, from this state RS-FF3
It can be seen that the output Q ₂ of 0 changes to high level.
Further, when the voltage proportional to the amount of reflected light from the original becomes higher than the first reference voltage _V1 , the output _Q1 of the RS-FF 29 similarly changes to a high level. In this way, when the original image density is low, the outputs Q ₁ and Q ₂ of the two RS-FFs 29 and 30 are both high level, and are intermediate between the first reference voltage V ₁ and the second reference voltage V ₂ . For an original with such a density, the output Q ₁ of the RS-FF 9 will be at a low level, and the output Q ₂ of the RS-FF 30 will be at a high level. Furthermore, if the voltage proportional to the amount of reflected light is lower than the second reference voltage V ₂ , in other words, if the original image density is high, the outputs of the two RS-FFs 29 and 30
Q ₁ and Q ₂ both indicate 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 in accordance with the density of the original image. After this, when the pre-exposure scanning pulse P ₁ becomes high level, both the terminals of the two RS-FFs 29 and 30 become high level, and from the table above, the state set as above will be maintained. . In the circuit example above, the developing bias voltage is set in three stages, but the number of developing bias voltage changes can be increased by adding a reference voltage generating circuit, a comparator, an RS-FF, and some additional circuits. . Also, the copy magnification is 1.414x and 0.707 as an example.
However, the present invention is not limited to this, and the same applies to other copying magnifications. Effects of the Invention As explained above, according to the present invention, by controlling the reference voltage, the same original is developed with the same developing bias voltage at each copying magnification of enlargement, same size, and reduction. Therefore, when copying originals with colored backgrounds at the same size, a developing bias voltage corresponding to the original image density not only provides a copy image free from background stains, but also when copying when enlarging or reducing, the background stains are also prevented. A good copy image can be 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 schematic configuration diagram of the optical system of the same electrophotographic copying machine, and FIG. 3 is a copy magnification and an original image density detection means. An explanatory diagram showing the ratio of the amount of light to
FIG. 4 is a circuit diagram of a developing bias voltage control circuit in one embodiment of the present invention, and FIG. 5 is a time chart of the circuit shown in FIG. 4. 1...Original table, 2...Exposure lamp, 3...First
Mirror, 4...second mirror, 5...third mirror,
6... Lens, 7... Fourth mirror, 8... Fifth mirror, 9... Sixth mirror, 10... Slit, 1
1...Photosensitive drum, 12...Photodetection element, 13
... Filter, 14 ... Charger, 15 ... Developing section, 16 ... Paper feeding section, 17 ... Transfer charger, 1
8...Separation charger, 19...Paper transport unit, 20...
... Paper discharge section, 21 ... Fixing section, 22 ... Cleaning section, 23, 24 ... Comparator, 25, 26, 2
7, 28, 31, 32... Inverter, 29, 3
0...RS flip-flop, C ₁ ...capacitor,
VR ₁ to VR ₃ ...variable resistance, _R1 to _R5 ...resistance.

Claims (1)

[Claims] 1 It has at least charging, exposing, transferring, neutralizing, and cleaning means around the photoreceptor, and uses a photodetector to detect a portion of the reflected light from the original to be copied during pre-exposure scanning performed prior to copying. an original image density detection means for detecting the original image density, a comparison means for comparing the detection output of the original image density detection means with a reference voltage, and an automatic developing bias for controlling the bias voltage for development according to the comparison result of the comparison means. A voltage control means and a reference voltage control means for controlling the reference voltage are provided, and by controlling the reference voltage, the same document can be enlarged, at the same size,
An electrophotographic copying machine configured to develop with the same developing bias voltage at each copying magnification of reduction.