JPS6158829B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a developing device for an electrostatic copying machine, particularly having a control means for controlling a developing electrode bias potential.

Organic photoconductors have recently become relatively widely used in electrostatography, in which the surface of a photoconductor supported on a conductive support is first charged, then exposed to a light image, and then subjected to the action of a developer. There is. Although this type of photoconductor has many advantages over inorganic photoconductors, they have one significant drawback. That is, the charge dissipation of the photoconductor after exposure is not as fast as desired. Therefore, in order to operate the reproduction apparatus at a reasonable rate, the organic photoconductor retains a large amount of charge in the background or non-image areas even after normal imagewise exposure. The background level is in the range of about 100 to about 200V.

Many proposals have been made in the past to solve the problem of developer adhesion in background areas due to potentials remaining there. For example, it has been proposed to provide a bias electrode at the development location, which applies a potential that counteracts the residual potential in the background area. However, with a fixed bias potential, the background potential varies over a relatively wide range, so that an insufficient bias potential will cause development of background areas, and an excess will cause toner deposition and contamination of the bias electrode. Also, the bias potential must be applied only during the period when the latent image passes through the developing device. If the bias potential is not deenergized as the uncharged area of the drum passes through the development station, a large amount of toner will adhere to the bias electrode.

Proposals have been made to provide a device that changes the bias potential in response to variations in the potential of the background area. For example, U.S. Pat. No. 3,611,982 discloses that a charge-accumulative probe is placed outside and just in front of the development unit, and is then used to charge the edge of the photoconductor drum by means of a shutter, and then applies it to a fully exposed strip of light. The device is shown exposed to. This sensed potential is amplified and used to control a variable power source that sends a bias potential to electrodes within the developer. U.S. Pat. No. 3,788,739 discloses that a chargeable probe is placed outside the development unit and immediately in front of it;
An apparatus is shown thereby sensing the potential of the extra-image area portion of the oversized exposed area and controlling the bias potential applied to the electrodes within the development unit. Another device that uses a capacitive probe to adjust the bias potential applied to a bias electrode from a power supply is shown in US Pat. No. 3,782,818. In this device, similar to the technique described above, the bias electrode is located outside the developer application unit and immediately in front of it. U.S. Pat. No. 3,575,505 discloses an apparatus in which the bias potential is varied as a function of the number of copies to compensate for changes in photoconductor characteristics over time.

The conventional methods described above use delicate and sensitive equipment, such as an electrometer for measuring photoconductor residual charge, to detect the photoconductor voltage. Such equipment is not only expensive, but also involves critical issues such as the special geometry required for the probe and the distance between the charge measuring surface and the probe. Additionally, conventional devices require a switching device that enables the bias potential only while the image passes through the development unit. Furthermore, in order to prevent toner particles from adhering to the bias electrode, means must be provided for cleaning this electrode.

As a result of research, the present inventor has completed a developing device that eliminates the drawbacks of conventional devices. The various parameters in this device are not critical, and the device is relatively inexpensive to manufacture. The device is constructed such that the bias potential is always sufficient to prevent toner from adhering to the background areas, while no mechanical cleaning elements are required to remove toner from adhering to the bias electrode.

SUMMARY OF THE INVENTION An object of the present invention is to provide a developing device for an electrostatic copying machine having a developing electrode bias potential control means.

Another object of the present invention is to provide a device that eliminates the effects of background potential and eliminates the disadvantages of conventional devices.

Another object of the invention is to provide an apparatus in which various parameters are not critical.

Another object of the present invention is to provide an apparatus that can automatically make multiple copies from white and color background masters without requiring any adjustments by the user through automatic bias potential control.

Yet another object of the invention is to provide a device that is relatively inexpensive to manufacture.

It is a further object of the present invention to provide an apparatus that does not require a mechanical cleaning member to remove toner particles from the bias electrode.

That is, the present invention comprises (a) a photoconductive surface 16; and (b) a charging means 26 for charging the photoconductive surface.
(c) latent image forming means 32 for partially discharging the charged photoconductive surface to form a latent image thereon; (d) developing electrodes 72, 74, 76, having a certain area;
78, for developing the latent image by contacting the latent image with a conductive developer; (e) placing a gap filled with the conductive developer between the photoconductive surface; (f) a sensing electrode 64 having a small area compared to the developing electrode and exhibiting a potential substantially equal to the opposite portion of the photoconductive surface due to the conductivity of the developer; (g) storage capacitor 124; (h) (a) amplifiers 106, 114 that receive input 100 and provide an amplified output; (g) storage capacitor 124; b) connecting means 90, 96 connecting said sensing electrode to the input of said amplifier; (c) connecting means 90, 96 connected between the output of said amplifier and said storage capacitor, connecting said storage capacitor to the highest discharge area of said photoconductive surface; (d) connected between the output of the amplifier and the storage capacitor in parallel with the diode and via a switch 126; a resistor 128, which allows opening and closing of the switch to charge the storage capacitor in a negative direction and adjust the initial potential; Means 106, 11 for charging to a corresponding potential and holding that potential
4, 90, 96, 120, 128, 126; and (i) connection means 142, 144, 146 for connecting the storage capacitor to the development electrode. .

The present invention also provides the above device, in which a plurality of sensing electrodes 64, 66 are arranged on the photoconductive surface 1.
6 at different positions, and the amplifier 1
Separate connection means 90, 9 to the input 100 of 06
6, 86, and 92 are provided. This device allows a more precise detection of the highest discharge area of the photoconductive surface.

Embodiments of the developing device according to the present invention will be described in detail below with reference to the accompanying drawings.

As shown in FIGS. 1 and 2, a copying machine 10 using this apparatus includes a support, that is, a conductive cylinder 14
It is equipped with a drum 12 consisting of. The outer surface of drum 12 is provided with a layer 16 of organic photoconductive material well known in the art. drum 12
has end plates 18, 20 supporting short shafts 22, 24 which are rotatably mounted in a known manner.

Corona discharge unit 26 is connected via switch 30 to a suitable power source 28 to produce a corona discharge that applies a uniform electrostatic charge to photoconductive layer 16 as drum 12 rotates. After being charged, the photoconductive surface moves past an exposure unit 32 of known type which connects a control unit 34 when a switch 36 is closed.

After exposure to the original of the image to be reproduced, the photoconductive surface moves into cooperative relationship with development unit 38. The developing unit 38 is, for example, a saucer 42.
It may be of the type with a developer application unit or application tank 40 located within. A developer material consisting of charged toner particles in a carrier liquid having a relatively high volume resistivity, as is well known in the art, is conveyed into tank 40 via conduit 44. The liquid developer contacts the surface of the drum 12 and then reaches the tray 4.
Fill to the point where it overflows to 2. From the saucer 42 to the conduit 4
The developer is returned to the supply source (not shown) via 6.

Any device may be used to control the operation of the various components of copier 10. For reasons discussed below, it is desirable to provide a fully charged but unexposed area of photoconductive surface layer 16 after the image area. To do this, for example, the cam 48 is mounted on the shaft 22 for rotation therewith, the driven member 50 is driven, and the switch 30 is closed to sufficiently charge a predetermined area around the drum 12. A second cam 54 mounted on shaft 22 actuates a follower member 56 to close switch 36 and actuate exposure unit 32. As is clear from Fig. 1, the cam 48
The angular range of is larger than that of the cam 54 and charges a larger area when the surface layer 16 is exposed. Furthermore, this structure is such that the exposure starts at the beginning of the charged area and the fully charged unexposed area 60 is formed on the drum 1.
It is designed to follow the image in two movement directions. Furthermore, as will be apparent to those skilled in the art, such a structure can be easily adapted to a device that can be controlled to make copies of different lengths, if desired.

The developer apparatus according to the invention has a small central electrode 62 of conductive material in the developer tank 40 adjacent to its inlet and electrodes 64, 66 at each edge.
are arranged. Each detection electrode 62, 64, 66
As the image area of drum 12 moves past the development unit 38 of drum 12, the image area is exposed to electrodes 62,
64 and 66 so as to pass through them reliably.
Additionally, electrodes 62, 64, and 66 are positioned such that developer fluid flows between and contacts the electrodes and the surface of drum 12. Electrodes 62, 64, and 66 are completely insulated or floating from the ground, allowing them to assume such and such potentials. By placing the developer between and in contact with both electrodes and both surfaces of drum 12, charged toner particles are attracted to the surface of the photoconductor and are deposited on each electrode 62, 64, 66. A charge is applied such that the area of the surface of the electrode layer 16 has a potential that is a measure of the potential. Although toner has high resistance, it is not a perfect insulator. In the illustrated example, the electrode 62,
64 and 66 each have a potential corresponding to the average potential of the portion of the image area aligned with that electrode. The potential of the electrode is essentially independent of the spacing between the electrode and the photoconductor because of the conductive interconnection provided by the toner. Moreover, this potential has no relation to these if the ground electrode capacitance is small and the ground resistance is high. That is, the sensing electrodes 62, 64, 66 of the present invention operate on the principle of conduction rather than capacitance.

Electrodes 62, 64, and 66 connect to a high input impedance measurement circuit 68 that outputs the lowest detected potential. Amplifier 7 receiving input from measurement circuit 68
0 applies a bias potential to the bias electrodes 72, 74, 76, and 78 as described later. The image area average voltage detected by each electrode 62, 64, 66 is:
Equal to the residual or background potential of the transparent areas without print and higher than the residual voltage of the areas with print.

As shown in FIG.
6, 78 extend across substantially the entire width W of the copy to be obtained. detection electrode 62 in the width direction of the copy;
The dimensions and positions of 64, 66 are such that electrode 62 scans a central portion that typically corresponds to the image portion of a typed original, for example, and electrodes 64, 66 scan an edge area that typically responds to non-image portions of the original. . Among the multiple electrodes arranged in this manner, the bias voltage is selected according to the sensing electrode that shows the lowest reading. Since, as mentioned above, many originals have one or more transparent edge areas, the present apparatus ensures that the lowest bias potential is produced in most cases.
Circuit 68 also applies a lower additional bias potential to the developer electrode, resulting in a total bias potential that is slightly higher than the potential value sensed in the clear area;
It is possible to prevent development from occurring in the background areas. Through research, the inventor has learned that the resistance of the liquid developer between the sensing electrode and the drum 12 is on the order of 10 ⁹ Ω. High impedance measurement circuit 68 according to the present invention has an input impedance of greater than 10 ¹² Ω, or at least three orders of magnitude greater than the resistance between the electrodes and the drum surface. In this way a good reading of the average potential is obtained along the image area region aligned with electrode 62.

When the fully charged unexposed area 60 of drum 12 reaches the bias electrode alignment position of the development unit, the high potential in this area creates a reverse bias potential. Even if the amplifier 70 outputs the maximum value limited in advance, the potential of the developing electrode is not exposed in the area 60.
is sufficiently lower than the potential of Therefore, during the development operation, the toning agent particles adhering to the bias electrode are attracted towards the drum surface and many of them return to a suspended state in the carrier liquid. Although the areas 60 are developed to some extent by the toning agent particles, this is not a problem in many industrial applications since mechanical elements are provided to clean the surface of the photoconductor layer 16 during the copying cycle.

Instead of providing a sufficiently charged unexposed area to clean the bias electrode, a portion of the drum may be provided with a thin non-conducting plastic coating, or the unimaged area of the drum may be removed as it passes through the development unit. A switching means for applying a reverse polarity voltage to the developing electrode may be provided.

As shown in FIG. 3, the high input impedance measuring circuit 68 includes a sample and hold circuit and an amplifier 70, which will be described later. In the illustrated arrangement, shields 80, 82, and 84 are provided for conductive wires leading from sensing electrodes 66, 62, and 64, respectively. Resistors 86, 88, 90 are sensing electrodes 66, 62, 6
4, the common drain line 98 and the common power supply line 1
Insulated gate field effect transistor 9 with 00
Connect to 2, 94, 96. The power supply line 100 is connected to a terminal 104 of a potential source having a value of -600V, for example, through a resistor 102. Measuring circuit 68
The high input impedance of transistor 92,
94,96. These transistors act to shunt current from the base-emitter junction of transistor 106 in response to a sensed voltage. A common power supply line 100 connected to the base electrode of transistor 106 sends a base current to transistor 106 through resistor 102 . Transistor 108 forms a current source that delivers emitter current to transistor 106. This structure causes the emitter electrode of transistor 106 to be typically several volts higher than the input to each field effect transistor 92, 94, 96 when they are all powered from the same power source. However, in practice, as shown in FIG.
94 and 96 receive input voltages from sensing electrodes 66, 62, and 64. In the illustrated structure, circuit 68 responds to the lowest negative sensed voltage and ignores other sensed voltages. It is clear that the lowest negative voltage occurs at the probe sensing the highest discharge area of the photoconductor. This area of highest discharge is usually at the edge of the original. The parallel RC circuit 109 connects the emitter electrode of the transistor 106.
0, 82, and 84 to ensure that capacitance between the input leads and the shield does not load the sensing electrodes. The negative voltage source of the sensing circuit is a Zener diode 110 connected to a -600V power supply by a resistor 112.

Measurement circuit 68 includes a sample and hold circuit responsive to the potential at a common terminal of diode 110 and resistor 112. This signal is applied to the base electrode of transistor 114. This base electrode is connected to the emitter electrode by a diode 116. The collector electrode of transistor 114 is connected to a -300V power supply, for example. Transistor 114 connects storage capacitor 12
A low impedance exciter is formed to apply a potential to 4. The sample and hold circuit includes back-to-back diodes 118,120. Diode 118 always conducts current to resistor 1
22, an emitter follower type transistor 11
Pour into No.4 Emitsuta. This diode 118 is
Operates as a constant voltage device. Therefore, the diode 118 is not necessarily required as a sample-and-hold circuit. diode 11
The common terminal of 8 and 120 is grounded by a resistor 122 and connected to one terminal of a storage capacitor 124. A pair of micro switches 126,1
30 is closed to control charging of the capacitor 124. The resistor 128 is the switch 126
is connected to a common terminal of diodes 116 and 118. 2 switches 126, 13
The common terminal of 0 is connected to a diode 120. The other terminal of the switch 130 is connected to the capacitor 1.
It is connected to 24.

The measuring circuit 68 selects the lowest negative value of the outputs of the sensing electrodes 62, 64, 66 at any given instant. Since the latent image moves relative to this particular sensing electrode, its instantaneous output changes over time as this sensing electrode scans the latent image. As explained below, diode 120 charges storage capacitor 124 via microswitch 130 to the lowest negative value of the potentials sequentially sensed by this particular sensing electrode.

The first part of the moving duplicate image (e.g. total length 28
switch 126,1 during the first 2 to 3 cm)
30 is closed. With switch 130 closed, capacitor 124 can quickly charge in a positive direction through the low forward impedance of diode 120. With switch 126 closed, capacitor 124 is allowed to charge slowly in the negative direction through 10K resistor 128. The purpose of negative charging is to adjust the voltage of the capacitor 124 to the background potential of the copied image when the background of the copied image has a lower whiteness than the background of the preceding copied image. be. After moving this first portion of the copy, switch 126 is open and capacitor 124 is connected to diode 12.
After passing through 0, it is charged only in the positive direction.

For this purpose, for example, the first cam 132 is
2 is attached so that it rotates with this.
A driven member 134 is positioned around the shaft 22 at a location corresponding to the location where the latent image enters the development unit 38.
The cam 132 is actuated to close the switch 126 and hold the switch 126 closed over the first, e.g., about the first 2 to 3 cm, of the reproduced image. Another cam 136 mounted on shaft 22 drives a follower member 138 located at a position corresponding to the position of follower member 134 to move switch 130 over a first longer section, e.g.
It is designed to close over a cm. That is, for the first 2 to 3 cm of the copied image, transistor 1
14 can charge the capacitor 124 in either positive or negative direction. After the first few centimeters of movement of the copy, capacitor 124 can only charge in the positive direction while switch 130 is closed.

When this unidirectional charging is possible, the capacitor 124 is charged to a voltage corresponding to the lowest negative voltage among all the voltages detected by the sensing electrode. This lowest negative voltage corresponds to the part of the original that is most exposed to light and thus the most discharged photoconductor part (highest discharge area).

The resistor 140 includes a pair of transistors 142,
A storage voltage is applied to amplifier 70 consisting of 144 and a developer electrode bias voltage is applied to lead 146. The voltage on conductor 146 is all connected through a series of diodes 148, 1 between conductor 146 and ground.
50, 152 and resistor 154 to apply voltage to various development electrodes 72, 74, 76, 78. In the illustrated structure, electrode 72, the first electrode that a copy passes adjacent as it moves through development unit 38, receives the full bias potential. Second electrode 74
receives a potential at a common terminal of both diodes 148 and 150. The electrode 76 has both diodes 150,1
52, the last developing electrode 78 is connected to a diode 152 and a resistor 154.
receive a potential at their common terminal.

In order to prevent excessive adhesion of toner to each development electrode, it is desirable that no voltage be applied to the development electrodes during periods when no development is being performed. This result can be obtained in any conventional manner. For example, as shown in FIG. 3, a power source 156 that provides a -600 volt potential and a -300 volt potential to various points in the circuit is disconnected from the sensing circuit by any conventional means. For example, a switch 158 is provided on the output lead of the power source 156. The cam follower 160 is actuated and the switch 15
8 is closed to supply power to the detection circuit. Follower member 160 operates in any conventional manner. For example, the driven member 160 is positioned along with the driven members 134 and 138 at a position where the driven member 160 is driven by the exposure cam 54. Exposure cam 54 is activated by switch 158 during a period in which the latent image passes through the development device.
Close. It is clear that any other suitable means may be used to control the power supply to the sensing circuit.

In operation of the developing device according to the present invention, the copying machine 10
When set to the operating state, the drum 12 rotates in the direction of the arrow shown in FIGS. 1 and 2. The cam 48 drives the driven member 50 and the power supply 28 powers the corona discharge unit 26 so that the surface of the layer 16 receives a uniform charge over the period of time that the driven member 50 is driven by the cam 48. After drum 12 has rotated to the point where the leading edge of the charging area is adjacent exposure unit 32, cam 54 drives follower member 56 to close switch 36 and bring control unit 34 into contact with exposure unit 32 to begin the exposure process. This exposure process is carried out by the cam 54.
1, and a fully charged but unexposed area 60 follows the image area, as can be seen in FIG. When this image area enters the development unit 38,
Cam 54 closes switch 158 and powers sensing circuit 68. As the image passes past electrodes 62, 64, 66, these electrodes sense the electrical potential of the overlying image area. The high input impedance measurement circuit 68 detects the lowest negative potential among the detected potentials.
This lowest negative potential is connected to the sample-and-hold circuit described above [diode 120 and capacitor 12].
4] and retained. The signals thus obtained are amplified and applied to the developing electrodes 72, 74, 76, and 78. It is clear that this potential is equal to or somewhat higher than the actual residual potential of the background areas of the image to ensure that these background areas are not developed.

Furthermore, as is clear from the foregoing, some toner particles accumulate on the bias electrode during this development operation. However, as area 60 crosses each development electrode,
The reverse bias potential is created by the fully charged but unexposed area 60 being at a potential much higher than the highest bias potential produced by the circuitry including amplifier 70. This reverse bias potential causes the toner to migrate from each developing electrode 64, 66 toward the surface of the drum 12. During this operation, some toner particles that leave each electrode return to suspension in the developer carrier liquid. During this operation, area 60 is developed, at least to some extent. However, as mentioned above, in this case there is no problem in industrial copying machines because some members are already provided to clean the surface of the drum 12 during each copying machine operation.

It is clear that the object of the invention can be achieved in this way. The development device according to the invention eliminates the drawbacks of prior art devices of this type which are designed to suppress background development. The device produces a variable bias potential that produces automatic exposure control effects. The various parameters of the device are not critical. The device includes a portion for cleaning the bias electrode without the use of mechanical cleaning members. The device is considerably less expensive than conventional devices that use equipment such as electrometers.

[Brief explanation of the drawing]

FIG. 1 is a front view of an electrostatic copying machine equipped with a developing device of the present invention, with a portion of the machine being vertically sectioned, and FIG.
FIG. 3 is a perspective view with a portion of the figure cut away, and FIG. 3 is a wiring diagram of an electric circuit used in the developing device of the present invention. 14... Conductive cylinder, 16... Photoconductive surface (layer made of photoconductive material), 22, 24... Short axis, 26... Corona discharge unit, 32... Exposure unit, 62, 64, 66 ...Detection electrode, 72,7
4, 76, 78...Development electrode (bias electrode),
92, 94, 96...field effect transistor, 1
06,114...transistor, 120...diode, 124...storage capacitor, 142,1
44...transistor.

Claims (1)

[Claims] 1. (a) A photoconductive surface 16; (b) Charging means 26 for charging the photoconductive surface.
(c) latent image forming means 32 for partially discharging the charged photoconductive surface to form a latent image thereon; (d) developing electrodes 72, 74, 76, having a certain area;
78, for developing the latent image by contacting the latent image with a conductive developer; (e) placing a gap filled with the conductive developer between the photoconductive surface; (f) a sensing electrode 64 having a small area compared to the developing electrode and exhibiting a potential substantially equal to the opposite portion of the photoconductive surface due to the conductivity of the developer; (g) storage capacitor 124; (h) (a) amplifiers 106, 114 that receive input 100 and provide an amplified output; (g) storage capacitor 124; b) connecting means 90, 96 connecting said sensing electrode to the input of said amplifier; (c) connecting means 90, 96 connected between the output of said amplifier and said storage capacitor, connecting said storage capacitor to the highest discharge area of said photoconductive surface; (d) connected between the output of the amplifier and the storage capacitor in parallel with the diode and via a switch 126; a resistor 128, which allows opening and closing of the switch to charge the storage capacitor in a negative direction and adjust the initial potential; Means 106, 11 for charging to a corresponding potential and holding that potential
4, 90, 96, 120, 128, 126; and (i) connecting means 142, 144, 146 for connecting the storage capacitor to the developing electrode. 2. A plurality of sensing electrodes 64, 66, each facing the photoconductive surface 16 at different positions, are connected to respective connection means 90, 9 to the input 100 of the amplifier 106.
6, 86, and 92, the developing device according to item 1 above.