JPH0243187B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an exposure control device for a copying machine (including an image forming apparatus such as a facsimile), which automatically controls the exposure amount, for example.

Generally, when copying originals with a dark background, such as newspapers or dark blueprints, with an electronic copying machine, the amount of exposure is increased compared to the case of normal originals, or the bias voltage of the developing device is changed. Control such as increasing the height is required. However, the conventional control method requires the operator to operate the exposure adjustment dial or exposure selection switch provided on the operation panel each time, which is very cumbersome and lacks operability. Furthermore, there were other drawbacks such as the fact that the optimum exposure amount could not always be obtained with respect to fluctuations in power supply voltage and fluctuations in document reflectance.

Therefore, recently, an exposure control device has been considered, for example, which detects reflected light from a document with a photodetector and automatically provides an optimal exposure amount in accordance with fluctuations in the reflectance of the document. However, none of these conventional methods have taken into consideration variations in the optimum exposure amount due to variations in the optical system and process system of the copying machine.
There was a big problem in guaranteeing the quality of copied images.

The present invention has been made in view of the above circumstances, and its purpose is to be able to automatically obtain the optimum exposure amount at all times with respect to fluctuations in power supply voltage and fluctuations in document reflectance, and to improve operational performance. An exposure control device for a copying machine that can further improve the stability of the device by fully coping with fluctuations in the optimum exposure amount caused by variations in the optical system and process system of the copying machine. Our goal is to provide the following.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 schematically shows an electronic copying machine according to the present invention, in which a document table 1 on which a document is placed is configured to reciprocate in the direction of arrow a as necessary. When the document table 1 moves forward, the document placed thereon is irradiated by the exposure lamp 2, and the reflected light hits the mirror 4, lens mechanism 5, and mirror 6 supported by the optical system block 3. The document is guided to the photoreceptor drum 7 through the photoreceptor drum 7, and the image of the document is formed on the surface of the photoreceptor drum 7. The photosensitive drum 7 rotates in the direction of arrow b, is first charged by a charger 8, and then the image of the document is exposed to light to form an electrostatic latent image on the surface of the photosensitive drum 7. This electrostatic latent image is made visible by applying toner by a developing device 9. On the other hand, the transfer paper in the cassette 10 is
The paper is sent out by a paper feed roller 11 that operates in accordance with the rotation of the photosensitive drum 7, and is transported by a transport roller 12. The transferred transfer paper comes into close contact with the surface of the photoreceptor drum 7 at the transfer charger 13, and the toner image on the photoreceptor drum 7 is transferred by the charger 13. After the transfer, the transfer paper is peeled off from the surface of the photoreceptor drum 7 by a peeling charger 14 and sent to a fixing device 16 by a conveying roller 15, where the transferred image is fixed. After the fixing, the transfer paper is discharged onto a tray 18 by a paper discharge roller 17. On the other hand, after the transfer, the photosensitive drum 7 is neutralized by a static eliminator 19, the image of the electrostatic latent image is erased by a fluorescent lamp 20, and finally cleaned by a cleaner 21. It is starting to return to its initial state.

A photodetector, for example, a photodiode 22 is provided in the optical path between the lens mechanism 5 and the mirror 6 in the optical system block 3, and this diode 22 is connected to the optical system by a mounting member 23. Fixed to block 3. The photodiode 22 detects a portion of the light reflected from the original by the exposure lamp 2 and converts it into a current signal.

FIG. 2 schematically shows an exposure control device according to the present invention, in which the exposure lamp 2 is connected to a commercial AC power source 31 via a bidirectional thyristor 32. In FIG. Further, a pseudo load circuit 33 is connected to the power source 31. This pseudo load circuit 33 applies a voltage corresponding to the terminal voltage of the exposure lamp 2 to the pseudo load when the thyristor 32 is turned on, and outputs the voltage across the pseudo load. This pseudo load circuit 3
The output voltage of No. 3 is supplied to the waveform shaping circuit 34.
This waveform shaping circuit 34 waveform-shapes the output voltage of the pseudo load circuit 33 so that the exposure lamp 2
It outputs a voltage corresponding to the effective value voltage of . Here, the pseudo load circuit 33 and the waveform shaping circuit 34 constitute a voltage generation circuit 35 that generates a voltage corresponding to the terminal voltage of the exposure lamp 2.

On the other hand, 36 is a manual reference voltage generation circuit which outputs a reference voltage adjusted according to variations in the optical system of the copying machine. Further, 37 is an automatic reference voltage generation circuit which subtracts a voltage proportional to the current signal output from the photodiode 22 from a preset voltage, and then adjusts the voltage to an appropriate voltage level according to variations in the optical system of the copying machine. Amplify and output. The respective output voltages of these two reference voltage generation circuits 36 and 37 are selected by a selection switch 38 and then supplied to a comparator, for example, an error amplifier 40 via a limiting circuit 39. The output voltage of the shaping circuit 34 is also supplied. The selection switch 38 is a relay contact that operates according to the selection of an operation mode between a manual mode for manually controlling the exposure amount and an automatic mode for automatically controlling the exposure amount, and is closed to the a side in the manual mode. to select the output of the manual reference voltage generation circuit 36, and in automatic mode, select b
to select the output of the automatic reference voltage generation circuit 37. Further, the limiting circuit 39 limits the output voltage of the manual reference voltage generating circuit 36 or the automatic reference voltage generating circuit 37 supplied to the error amplifier 40 so that it does not exceed a predetermined voltage level. The error amplifier 40 also compares the output voltage of the waveform shaping circuit 34 with the output voltage of the manual reference voltage generation circuit 36 or the automatic reference voltage generation circuit 37 supplied via the selection switch 38 and the limiting circuit 39, If there is a difference between the two voltages, a signal corresponding to the magnitude of the difference is output. Thus, the output signal of the error amplifier 40 is supplied to a trigger pulse generation circuit 41. This trigger pulse generation circuit 41 is
It outputs a trigger pulse synchronized with the frequency of the power supply 31, and controls the generation phase of the trigger pulse according to the output signal of the error amplifier 40.
The controlled trigger pulse is supplied to the gate of thyristor 32.

FIG. 3 specifically shows each circuit in FIG. 2, in which a primary coil of a power transformer 51 is connected to a power source 31, and a full-wave rectifier 52 is connected to a secondary coil of this transformer 51. . A series circuit of a diode 53 and a capacitor 54 is connected between the DC output ends P and N of the rectifier 52. Further, a series circuit of a resistor 55 and a Zener diode 56 is connected between the output terminals P and N, and a series circuit of a diode 57 and a capacitor 58 is connected in parallel to the diode 56. The connection point between the diode 57 and the capacitor 58 is connected to one end of a switch 59. Further, a series circuit including a resistor 60 and a Zener diode 61 is connected between the output terminals P and N, and a trapezoidal wave voltage synchronized with the power supply 31 is generated at a connection point 62 between the resistor 60 and the diode 61. ing. A series circuit of a resistor 63, a unidirectional thyristor 64, and a resistor 65, which constitutes the pseudo load circuit 33, is connected in parallel to the capacitor 54. Connection point 66 between the cathode of the thyristor 64, which serves as the output end of the pseudo load circuit 33, and the resistor 65
is connected to one end of resistor 70 via diode 67 and resistors 68 and 69 in series. A capacitor 71 and a resistor 72 are connected in parallel between the connection point of the resistors 68 and 69 and the output terminal N,
Also, the connection point between the resistors 69 and 70 and the output terminal N
A capacitor 73 is connected between the two. Here, the diode 67, resistors 68 to 70, 72
and capacitors 71 and 73 are the waveform shaping circuit 34
It consists of The other end of the resistor 70 is connected to the base of the NPN transistor 74.
The collector of this transistor 74 is connected to the other end of the switch 59 via a resistor 75. Further, a capacitor 76 and a resistor 7 for preventing oscillation are connected between the base and collector of the transistor 74.
A series circuit with 7 is connected. The emitter of the transistor 74 is commonly connected to the emitter of another NPN transistor 78, and this connection point is connected to the output terminal N via a resistor 79. Further, the collector of the transistor 78 is connected to a connection point 80 between the switch 59 and the resistor 75. Here, the transistors 74, 7
8 and the like constitute an error amplifier 40. The base of the transistor 78 is connected to the emitter of a PNP transistor 81, and the collector of the transistor 81 is connected to the output terminal N. Further, the base of the transistor 81 is
It is connected to the connection point 80 through a resistor 82 and to the output terminal N through a resistor 83. Here, the transistor 81 and the resistor 8
2 and 83 constitute a limiting circuit 39. Also,
A variable resistor 8 is connected between the connection point 80 and the output terminal N.
4 and resistors 85 and 86 are connected, and these constitute a manual reference voltage generation circuit 36.

The connection point between the collector of the transistor 74 and the resistor 75, which is the output end of the error amplifier 40, is connected to the NPN transistor 88 via the resistor 87.
The collector of this transistor 88 is connected to the connection point 62. The emitter of the transistor 88 is connected to the output terminal N via a capacitor 89 and a resistor 90.
The output terminal P is connected to the output terminal P, and further connected to the anode of a programmable union transistor (hereinafter simply referred to as PUT) 91. The cathode of the PUT 91 is connected to the output terminal N via the primary coil of a pulse transformer 92 and an NPN transistor 93 in series. The base of the transistor 93 is a resistor 94
It is connected to the connection point 80 via a resistor 95, and to the output terminal N via a resistor 95. Also
The cathode of PUT 91 is connected to the gate of the thyristor 64 through a resistor 96 and a diode 97 in series, and this connection point is connected to the connection point 66 through a resistor 98. On the other hand, the gate of PUT 91 is connected to the output terminal N via a resistor 99 and to the connection point 62 via a diode 100 and a resistor 101 in series. Further, a connection point between the diode 100 and the resistor 101 is connected to a connection point between the base of the transistor 88 and the resistor 87 via a diode 102. Therefore, the secondary coil of the pulse transformer 92 is connected to the thyristor 3 via the diode 103.
2 and the first anode.
Here, the transistor 88 and capacitor 8
9, PUT 91, pulse transformer 92, transistor 93, etc. constitute trigger pulse generation circuit 41.

On the other hand, the anode of the photodiode 22 is connected to the connection point 80 via a resistor 104 and to the output terminal N via a resistor 105. Further, the cathode of the diode 22 is connected to the non-inverting input terminal of the operational amplifier 106. The diode 22 is connected to a variable resistor 1.
07 and a capacitor 108 are each connected in parallel. Note that the resistors 104 and 105 are used to divide the power supply voltage to obtain a predetermined set voltage. Further, the variable resistor 107 is for converting the current supplied by the photodiode 22 into a voltage and subtracting it from the voltage set by the resistors 104 and 105, and varies the resistance value. This makes it possible to absorb variations in the characteristics of the photodiode 22. Further, the capacitor 108 is used to provide a predetermined time constant. The inverting input terminal of the operational amplifier 106 is connected to the output terminal N via a resistor 109 and to its own output terminal via a variable resistor 110. In addition, the above operational amplifier 1
06, a resistor 109, and a variable resistor 110 constitute a positive phase amplifier, and by changing the gain with the variable resistor 110, variations in the optical system of the copying machine are adjusted. Here, the above resistance 1
04, 105, variable resistor 107, operational amplifier 106, etc. constitute an automatic reference voltage generation circuit 37. The output end of the operational amplifier 106 is connected to the b-side fixed contact 38b of the selection switch 38 via a resistor 111. The a-side fixed contact 38a of the selection switch 38 is connected to a slider of a variable resistor 84 in the manual reference voltage generation circuit 36. Further, the movable contact 38c of the selection switch 38 is connected to the base of the transistor 78 in the error amplifier 40.

Next, the operation in the above configuration will be explained. First, we will discuss the case of manual mode, which always provides a constant optimum exposure amount despite fluctuations in power supply voltage. In this case, the selection switch 38 is closed to the a side, and therefore the automatic reference voltage generation circuit 37 becomes irrelevant to the exposure control described below. Now, when switch 59 is turned on, a voltage obtained by dividing the voltage at connection point 80 by resistors 94 and 95 is applied to the base of transistor 93, and transistor 93 is turned on. Further, the voltage at the connection point 80 is applied to the base of the transistor 88 via the resistors 75 and 87, and this transistor 88 is also turned on. As a result, capacitor 89 is charged via transistor 88 . As a result of this charging, when the anode voltage of the PUT 91 becomes equal to or higher than the gate voltage, the PUT 91 is turned on, and a pulse current flows through the primary coil of the pulse transformer 92. Therefore, a pulse is generated in the secondary coil of the pulse transformer 92, which becomes a trigger pulse and is applied to the gate of the thyristor 32. This turns on the thyristor 32,
Turn on the exposure lamp 2. Also, at this time, the thyristor 64 is connected via the resistor 96 and the diode 97.
The trigger pulse is also applied to the gate of , and the thyristor 64 is turned on in response to the pulse.
A voltage corresponding to the voltage of the exposure lamp 2 is generated across the resistor 65. This generated voltage is applied to the diode 67, resistors 68 to 70, 72, and capacitor 7.
1 and 73, the waveform is shaped by the waveform shaping circuit 34, resulting in a DC voltage corresponding to the effective value voltage of the exposure lamp 2, and this voltage is applied to the transistor 7.
Applied to the base of 4. At this time, if the base voltage of transistor 74 is lower than the base voltage of transistor 78, the collector voltage of transistor 74 becomes high, and therefore the base voltage of transistor 88 also becomes high, and the charging speed of capacitor 89 becomes faster. As a result, the PUT 91 generates pulses at a faster timing, the conduction angle of the thyristor 32 increases, the applied voltage of the exposure lamp 2 increases, and the amount of light increases. The increase in the conduction angle of thyristor 32 is fed back to thyristor 64, which causes the base voltage of transistor 74 to increase and balance when it becomes equal to the base voltage of transistor 78. A reference voltage set by a variable resistor 84 is applied to the base of the transistor 78, and since the reference voltage is held constant against voltage fluctuations of the power supply 31, the base voltage of the transistor 74 is also kept constant. In other words, the exposure lamp 2 operates so that the voltage applied to the exposure lamp 2 is constant. In this way, control is performed so that the voltage applied to the exposure lamp 2 as a whole is always constant, and as a result, the voltage set by the variable resistor 84 of the manual reference voltage generation circuit 36 is controlled regardless of fluctuations in the power supply voltage. The optimal exposure amount that has been set is always obtained.

Next, a description will be given of an automatic mode in which the exposure amount is automatically changed to the optimum amount in accordance with fluctuations in power supply voltage and fluctuations in document reflectance. In this case, the selection switch 38 is closed to the b side, and therefore the exposure lamp 2
is controlled by the output voltage of the automatic reference voltage generation circuit 37. That is, the light from the exposure lamp 2 is reflected by the original and guided to the photosensitive drum 7;
A part of the reflected light enters the photodiode 22 and is converted into current. At this time, variable resistor 10
7 converts the current supplied by the photodiode 22 into a voltage and converts the voltage into a resistor 104, 10.
After subtracting the voltage from the voltage set in step 5, the voltage is input to the operational amplifier 106. The operational amplifier 106 amplifies the voltage to a predetermined voltage level with a gain adjusted by a variable resistor 110 according to variations in the optical system of the copying machine, and outputs the amplified signal. The output voltage of this operational amplifier 106 is applied to the base of a transistor 78 constituting the error amplifier 40. At this time, pseudo load circuit 3
3 and the waveform shaping circuit 34 operate as described above, and supply a voltage corresponding to the voltage across the exposure lamp 2 to the base of the transistor 74. Here, it is assumed that the output voltage of the automatic reference voltage generation circuit 37 becomes high when the amount of light incident on the photodiode 22 is small. Operational amplifier 1 is used because the amount of light is small.
The output voltage of transistor 06, that is, the base voltage of transistor 78 becomes high. Therefore, if the base voltage of the transistor 74 is lower than the base voltage of the transistor 78 at this time, the voltage applied to the exposure lamp 2 is increased by increasing the conduction angle of the thyristor 32 as described above, and the amount of light is increased. Control is performed so that The increase in the amount of light due to this control, that is, the increase in the conduction angle of the thyristor 32, is fed back to the thyristor 64 as described above, and as a result, the base voltage of the transistor 74 increases, and when it becomes equal to the base voltage of the transistor 78, the balance is balanced. It can be taken. On the other hand, if a fluctuation in the power supply voltage occurs in this state, the base voltages of the transistors 74 and 78 will no longer be equal, so as described above, the base voltages of the transistors 74 and 78 will not be equal.
It operates so that each base voltage of 78 becomes equal again, that is, so that the voltage applied to the exposure lamp 2 becomes constant. As a result, even if the power supply voltage fluctuates, the voltage applied to the exposure lamp 2 remains constant, and the amount of light from the exposure lamp 2 automatically changes depending on the density of the original, so that the amount of light to the photoreceptor drum 7 remains constant. Control is performed to ensure that In this way, overall control is performed so that the voltage applied to the exposure lamp 2 is always constant and the amount of light reflected from the document is always constant. The optimal exposure amount can always be automatically obtained regardless of the density (changes in document reflectance). In addition, due to fluctuations in the power supply voltage, the exposure lamp 2
Since changes in the amount of light can also be detected and controlled, even more stable operation is possible even with fluctuations in power supply voltage.

On the other hand, the limiting circuit 39 performs the following operation.
Base voltage of transistor 78 (transistor 8
When the emitter voltage of transistor 1 exceeds the voltage set by resistors 82 and 83 (base voltage of transistor 81), current flows through transistor 81 and operates to lower the base voltage of transistor 78. Control the voltage so that it does not exceed the set voltage. That is, transistor 81 forcibly clamps the base voltage of transistor 78 so that it does not exceed the voltage set by resistors 82 and 83. Therefore, the base voltage of the transistor 78 does not fall below the preset voltage, which results in the voltage of the thyristor 3
The maximum conduction angle of exposure lamp 2 is suppressed, and the voltage applied to exposure lamp 2 is automatically lowered to a predetermined voltage (lower than the rated voltage).
limited to. In this way, the limiting circuit 39 limits the reference voltage from the manual reference voltage generating circuit 36 or the automatic reference voltage generating circuit 37 to the error amplifier 40, thereby preventing the voltage applied to the exposure lamp 2 from exceeding the rated voltage. This is a restriction. The reason for providing this limiting circuit 39 is to prevent the voltage applied to the exposure lamp from exceeding the rated voltage from the viewpoint of lifespan, for example, when using an exposure lamp whose rated voltage is lower than the commercial AC voltage. It's for a reason.

FIG. 4 shows a modification of the automatic reference voltage generation circuit 37. That is, photodiode 2
2, the current supplied by the operational amplifier 121,
A current-to-voltage conversion circuit consisting of a resistor 122 and a capacitor 123 converts the output voltage into a voltage, and the output voltage is connected to an operational amplifier 124, a resistor 125, and an NPN.
The voltage is supplied to a voltage-to-current conversion circuit composed of a type transistor 126 and a resistor 127. This provides a current source that can flow a current proportionally larger than the output current of the photodiode 22. Therefore, this current source is connected to resistors 128 and 12.
By connecting to one end of the voltage setting circuit formed by 9,130 series circuits and shunting a current proportional to the output current of the photodiode 22, the output terminal 131 receives a voltage proportional to the output current of the photodiode 22 from the set voltage. A voltage obtained by subtracting the voltage obtained is obtained. Even with such a circuit configuration, it is possible to achieve the same purpose as in the above embodiment.

As detailed above, according to the present invention, a voltage generation circuit is provided that generates a voltage corresponding to the terminal voltage of an exposure lamp, and a photodetector that detects reflected light from a document and converts it into an electrical signal; and an automatic reference voltage generation circuit that subtracts a voltage proportional to the output signal of this photodetector from a preset voltage and generates a corresponding voltage, and combines the output voltage of this automatic reference voltage generation circuit with the voltage generation circuit described above. The output voltage of the exposure lamp is compared with the output voltage using a comparator, and the light amount of the exposure lamp is controlled according to the comparison result, so the optimum exposure amount can always be maintained against fluctuations in power supply voltage and document reflectance. It can be obtained automatically, and thus the operability is significantly improved. Moreover, with the configuration of the present invention, even if variations in the optical system and process system of the copying machine are corrected, the contribution rate of the photodetector output to the exposure lamp voltage will always be the same regardless of the correction. Since it remains constant, the stability of the device is further improved.
Copy images of good quality can always be obtained.

[Brief explanation of drawings]

The drawings show one embodiment of the present invention, in which FIG. 1 is a side view showing a schematic configuration of an electronic copying machine, FIG. 2 is a block diagram showing a schematic configuration of an exposure control device, and FIG. FIG. 4 is a block diagram showing a modification of the automatic reference voltage generating circuit. DESCRIPTION OF SYMBOLS 1... Original table, 2... Exposure lamp, 7... Photoreceptor drum (photoreceptor), 22... Photodiode (photodetector), 31... AC power supply, 32... Thyristor, 35
...voltage generation circuit, 37...automatic reference voltage generation circuit,
40...Error amplifier (comparator), 41...Trigger pulse generation circuit.

Claims (1)

[Claims] 1. In a copying machine configured to expose an image by irradiating an original with an exposure lamp and guiding light from the original to a photoreceptor, a voltage generator that generates a voltage corresponding to the terminal voltage of the exposure lamp. A circuit, a photodetector that detects light from the document and converts it into an electrical signal, and a voltage proportional to the signal output from the photodetector is subtracted from a preset voltage to generate a voltage corresponding to the voltage. an automatic reference voltage generation circuit to generate; a comparator that compares the output voltage of this automatic reference voltage generation circuit with the output voltage of the voltage generation circuit and outputs a signal according to the comparison result; and an output signal of this comparator. 1. An exposure control device for a copying machine, comprising: a control circuit that controls the amount of light of the exposure lamp according to the amount of light emitted from the exposure lamp.