DE2121788A1 - Method for controlling the electric field in a development station for toner development and device for carrying out the method - Google Patents

Description

PATENTAN V / ALTE • DR.-JNG. WOLFF, H.BARTELS, DR. BRANDES, DR.-ING. HERO

STUTTGART 1. JP ^ P.fAL.l? ⁷ *

LANGE STRASSE 51 TELEPHONE (0711) 29ί310 and 297295

TELEX 0722312

Our reference: 123 029/480633 kdk

Eastman Kodak Company, Rochester, New York State, United States of America

Method for controlling the electric field in a development station for toner development: " as well as device for carrying out the method

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The invention relates to a method for controlling the Voltage of the electric field in a development station for the electrostatic application of toner particles to a photoconductive surface, which is an electrostatic Bears charge image, the background areas and image areas "has, depending on

a measured property of the image

^ Signal generated and this to control the voltage of the

electric field 'is used. Also affects the . Invention an apparatus for performing the method.

. A method of the above-mentioned type is already known in which the optical density of the image is measured by means of a photoelectric device, which is projected onto the photoconductive surface in an exposure station * in order to generate the charge image thereon. That of the photoelectric *! The signal generated by the device is compared with a reference signal, and the corresponding control of the voltage applied in the development station then takes place on the basis of this comparison. The control ψ this voltage is designed to make the toning of the charge image as the purpose that a maximum optical contrast between the surface areas and the other ... ^ _ areas of the image results.

This intended purpose is not achieved with certainty in the known methods of the type mentioned. Unsatisfactory With the known methods, development results are obtained in particular when the to-. turning photoconductors has a relatively large dark charge look the exposure shows. Particularly large irregularities in the development result when a such photoconductor exposed to flash light and in a

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relatively narrow development station (where the photoconductive Area only ir. With a limited area is exposed to the action of the electric field in the development station) is developed.

The invention is based on the object of creating a method of the type in question in which inconsistencies in the toner development are largely switched off and "the developed images are evenly a maximum exhibit optical contrast.

Based on a method of the type mentioned at the outset, this object is achieved according to the invention in that the signal is generated as a function of the electrical potential present in the background areas of the charge image.

With "background areas ¹¹ , hereinafter also" background

, ^ ^ Image areas are called ", those areas of the / are meant, which receive the lowest toner supply during toner development should. In the example of making a copy of a document with black letters

white paper are therefore those free of letters

Areas represent the background. In these areas, the background should correspond to the white in the copy Paper is reproduced white, no toner application at all. If this is achieved, a copy can be made with a Maximum contrast can be obtained.

The fact that in the method according to the invention for controlling the voltage of the electric field in the development station depending on the signal in the underground areas generated potential of the charge image the strength of the electric field can be found in

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.the .development station with very high accuracy at the value at which the desired maximum value of contrast of the developed image is given. By taking into account the charge in the background areas, the sources of error that occur in the known methods due to the dark charge loss after exposure are largely avoided, because the lowest potential in the charge image W is present in the considered background areas and thus the lowest dark charge loss occurs.

The invention is also based on the object of providing a simply constructed device for carrying out the method create.

With a device with a controllable voltage source for generating the electric field in the development station and with an electric control circuit for the According to the invention, this object is to generate a control signal for influencing the level of the voltage of the voltage source solved by the fact that the photoconductive surface traversed at one before the entry into the development station Place the trajectory of the same an electrometer to generate one of the potential of the charge of the charge image dependent electrical signal is arranged and that the signal output of the electrometer with a signal input is coupled to the control circuit.

The electrometer generates a signal, the amplitude of which depends on the strength of the charge at the measuring point areas of the charge image passing through, which have background areas and "image areas," fluctuates.

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This signal from the electrometer is the input signal to the control circuit, which in a preferred embodiment has three amplifiers connected in series. The output of the first amplifier can be connected to the input of the second amplifier can be coupled via a diode, and a capacitor can be provided at the input of the second amplifier be, which is chargeable to a predetermined voltage. As long as this is the output signal of the first amplifier is greater than it corresponds to the voltage of the capacitor, this voltage of the capacitor is decisive for the size of the input signal of the second amplifier. If the output signal of the first amplifier is lower, then the capacitor discharged through the diode, so that the input signal of the second amplifier in relation to the The minimum value of the signal supplied by the electrometer stands, i.e. it depends on the charge in the underground areas of the charge image is present.

In one embodiment, two photoelectric components can be provided, which are spaced from each other and are arranged in a specific spatial relationship to the measuring point on the path of movement of the photoconductive surface and which serve to determine the passage of the front or rear edge of the charge image, so that the individual Parts of the circuit at the appropriate times of the determined by the passage of the charge image through the device Duty cycle of the device off ^ and on can. ·

When using three amplifiers connected in series in the control circuit, a further preferred Embodiment a capacitor can be provided at the input of the third amplifier, which during certain "periods of time." maintains a signal at the input so

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the development station is supplied with a certain voltage from the controllable voltage source / which,

if the second and the third are dependent on corresponding scanning signals of the photoelectric components Amplifiers are in operation and coupled together in response to the signal generated by the first amplifier changeable

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The invention is described below with reference to the drawings individually explained.

Show it:

1 shows a schematically simplified front view of an exemplary embodiment of a device for performing of the method according to the invention and

FIG. 2 shows a schematic representation of the electrical circuit of the device according to FIG.

As is apparent from Fig. 1, _r is a band 10 which has an electroconductive support having attached this layer of photoconductive material and on which one or more, in the longitudinal direction of the strip offset to one another are electrostatic charge image, via known means or conveyed, for example via rollers 11 and 12. The tape is thereby guided through a development station 13 along a predetermined path of movement. The development station 13 has one or more magnetic brushes 14 which transport toner particles from a supply located in a container 16 to the surface of the belt carrying the charge image. The invention is described using the example of the development of charge images located on a tape. It should be understood, however, that it can also be used in conjunction with a single sheet which carries latent electrostatic charge images or charge oysters on a photoconductive surface which are on an electrically conductive substrate of the sheets. These sheets are conveyed along a predetermined movement path and guided through a development station in the same way. In other words, the invention does not involve the use of a belt or such a conveyor track for the

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material to be developed , as shown in Fi ^. 1 is shown. It is only important that a measuring head 17 is provided which can scan the surface carrying the electrostatic latent charge image, and that the surface carrying the charge image can be moved relative to the magnetic brushes 14 or other similar developing device which is below Using an electric field effective in this works.

* In a known manner there is a voltage source 18 with the magnetic brushes 14 connected so that an electric field between the magnetic brushes and the electrically conductive support which forms part of the belt 10 or an electrode to which the belt is attached when passing through the development station 13 is formed. The voltage source 18 can be of the usual commercial type available type and generates an output voltage that is variable as a function of a control signal supplied to the voltage source. The output from the voltage source Ib Voltage can be as high as that generated by a control circuit 20, as will be explained in detail below Control voltage or a multiple thereof or have a certain ratio to this.

The measuring head 17 is connected to a commercially available electrometer 19 which generates a signal, since ^{3 is} proportional to the signal received from the measuring head 17. The signal supplied by the electrometer 19 is input to the control circuit 20, which has three amplifiers 21, 22, 23 connected in series and circuit elements assigned to them. On the path of movement of the belt 10, a photoelectric component 24 is arranged immediately in front of the measuring head 17, which serves as a sensor to

Edge

the passage of the front / each located on the belt 10 To determine the charge image. The leading and trailing edges of each charge image can be defined by a line or a

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Perforations must be specified, which are in restricted spatial relationships to the pictures and at intervals. appear one after the other on the tape. Markings of other known types can also be provided for this purpose. The photoelectric component 24 is connected to a relay 25, which in turn actuates two normally closed contacts 25A and 25B, see FIG. 2. A photoelectric component 26 is arranged on the path of movement of the belt 6 adjacent to the development station 13; and generates a control signal when the same leading edge that previously caused the photoelectric component 24 to emit a signal is about to enter the development station 13. A relay 27 is connected to the photoelectric component 26 and serves to actuate a contact 27A _y provided in the control circuit 20, as will be explained below.

The way in which the development station 13 prevailing electric field is automatically controlled and changed, can best be exemplified by the sequence of a Explain the working cycle and with the aid of the schematically simplified circuit shown in FIG measured length, which is slightly less than the distance between the light-electrical components 24 and 26 corresponds. That photoelectric component 24 represents after the Vordere_Rana of the image the component 24 after exposure has taken place

has achieved the presence of this H and generates a signal by which the relay 25 is excited so that its contacts 25A and 25B open. When the charge image reaches the measuring head 17, then a capacitor 30, which is located at the input of the amplifier 22, is set to a predetermined value Charged voltage, which depends on the setting of a variable resistor 31. As can be seen from Fig. 2, is a voltage source 32 via a resistor 33, the change

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• Baren resistor 31 and the contact 25A with a connection of the Capacitor 30 \ erbunden, the other connection to earth. Therefore, the voltage of the capacitor corresponds to 30 after opening of the variable resistance setting contact 25a

31 and therefore represents a preselected voltage.

A capacitor 40 is also provided at the input of the amplifier 23, one connection of which is connected to earth and the other connection to the input of amplifier 23 and contact 25B P is connected. A variable resistor 41 and a resistor 43 are connected in series with one another and with the voltage source

32 connected. The variable resistor 41 is set so that after the opening of the contact 25B _x and if no signal is fed from the amplifier 22, the output signal generated by the amplifier 23 has such an amplitude that the magnetic brushes 14 have such a strong electric field is generated, in which by the magnetic brushes 14 essentially no deposition of clay ifpar ti can take place on the areas of the surface of the tape 10, which are image background areas. The corresponding voltage here is essentially equivalent to the potential present in the background areas, so that an image with a maximum degree of contrast is obtained through a minimum level of optical density obtained in the background areas.

When the charge image is moved past the measuring head 17, an essentially continuous "signal is generated, the amplitude of which fluctuates between the highest and lowest potential values that the charge image has. The amplitude of the signal corresponding to the potential in the background areas represents a minimum value in the Trap represents that it is a positive image _? And represents a maximum value if it is a negative image. This continuous signal is fed to the electrometer 19, whereby a corresponding proportional

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les signal is generated to the input 45 of the amplifier is given. The amplified output signal supplied by the amplifier 21 is blocked by a diode 46 as long as the The amplitude of this output signal is greater than the voltage on capacitor 30 corresponds. Is the voltage of the output signal of the amplifier 21 is less than the voltage of the capacitor 30, then this discharges through a resistor 47 and the diode 46 up to a value which corresponds to the level of the output voltage of the amplifier 21. This is the The voltage across the capacitor 30 is a function of the minimum value of the voltage supplied by the electrometer 19. This The minimum value of the voltage is assumed to be the use of a measuring head 17 of high resolution in the background areas The voltage present in the charge image. At this stage of the duty cycle, contact 27A is still open, so that no signal is supplied to the amplifier 23. When the leading edge of the charge image is the development station 13 reached, a signal is generated by the photoelectric component 26. When the photoelectric component 26 the Detects the presence of the charge image, then the relay 27 is energized and the contact 27A closes, so that the circuits the amplifiers 22 and 23 are connected to each other. The signal at the input of the amplifier 22 is the control signal for the control of the voltage generating the electric field at the development station and is, as stated above, proportional to the voltage of the background areas of the electrostatic charge image. A potentiometer 50 at the output of the Amplifier 23 is used to adjust the gain in order to determine the proportionality factor. The reinforcement will generally adjusted so that the field supplied by the voltage source 18 at the development station 13 generates Voltage is essentially equal to the voltage which prevails in the background areas of the charge image. Intended to In contrast, the voltage can be higher or lower by a certain value, this can be achieved by setting the appropriate value

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a potentiometer 51 can be achieved at the input of the amplifier 21 is provided and with a voltage source 52 is connected.

When the rear edge of a charge image (marked by a line or a hole) under the photoelectric component 24 passes through, then a signal is generated, as already mentioned above, and the relay 25 is de-energized, so that the contacts 25A and 25B return to their normal closed position. When this occurs, the last voltage value present at the input of the amplifier 23 is obtained from the capacitor 40 until the image _ through the Development station 13 is moved through. The amplifiers 22 and 23 have a high input impedance, so that the load or the voltage at the capacitors 30 and 40 at the inputs does not change within the period of time, the -, between the moment when the

front. Edge of the charge image by the photoelectric Component 24 passes through, and the moment when the trailing edge of the image leaves the development station 13 elapses.

It has been shown that by keeping the potential of the field at the development station essentially at the value which corresponds to the smallest value of the potential measured in the background areas of the charge pool, copies with highest contrast can be achieved with the lowest background density. For example, a copy with a A minimum value of the potential of 250 V is displayed on the blue background with a reflection density of 0.36. A copy, generated from this using a field at the development station of 225 V showed a low background density. However, a voltage at the development station of 25Ö V produced far better copies. With another

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Master copy, for example part of a newspaper page with a reflection density in the background of 0.19 a measured minimum value of the potential in the background areas of 170 V. A good, background-free copy was obtained with an electric field generated at the developing station by a voltage of 175V. the The above examples are merely illustrative and the invention is in no way limited to these examples.

The device described above can be used to print a positive or a negative original as well as to reproduce a copy representing a negative as well as a positive. In a known manner, the polarity of the the charge applied to the image bearing surface of the photoconductive material can be either positive or negative, and when using positive or negative charge, the polarity of the charged toner particles can be positive or negative. In all cases, the device adjusts the voltage of the field on the development button so that a Maximum value or a minimum value of background density is reached.

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Claims (7)

- 14 P at e η ta η Proverbs Method for controlling the voltage of the electric field in a development station for electrostatic application of toner particles on a photoconductive surface that bears an electrostatic charge image, the underground areas and image areas, where in Dependence on a measured property of the , Image generates a signal and this is used to control the Voltage of the electric field is used, characterized f that the signal depends on the electrical present in the underground areas of the charge image Potential is generated. 2. Device to carry out the method according to claim 1, with a controllable savings source for generating the electric field in the development stage and with a electrical control circuit for generating a control signal to influence the level of the voltage of the voltage source, characterized in that at one of the photoconductive surface prior to entry into the development station (13) (10) traversed point of the trajectory of the same an electrometer (17,19.) To generate one of the potential the charge of the charge pattern dependent electrical signal is arranged and that the signal output of the electrometer (17, 19) is coupled to a signal input (45) of the control circuit (20). - 3. Apparatus according to claim 2, characterized in that the control circuit (20) has a signal storage device (30, 40) for maintaining an essentially constant value of the control signal during the passage of the charge image to be toned through the development station (13). . 109847/1681 4. Apparatus according to claim 2 or 3, characterized. that at the input (45) of the control circuit (20) a variable resistor (51) for arbitrarily influencing the Size of the input signal of the control circuit (20) is provided. 5. Device according to one of claims 2 to 4, characterized in that that the control circuit (20) has several amplifiers (21, 22, 23) connected in series and that the control signal for the controllable voltage source (18) from the output of the last (23) of the series-connected Amplifier (21,22,23) is removed. 6. Apparatus according to claim 5, characterized in that at least one (23) of the amplifiers (21,22,23) is a potentiometer (50) for influencing the gain. 7. Apparatus according to claim 3 and claim 5 or claim 6, characterized in that as a signal storage device at least at the input having a high input impedance one of the amplifiers (21,22,23) a capacitor (30,40) is provided with a variable resistance (31 or 41) having voltage divider circuit can be connected is. 109847/1681 Blank page