AUTOMATIC BIAS CONTROL FOR ELECTROSTATIC DEVELOPMENT
United States Patent 3654893
An automatic bias control for development of a latent electrostatic image comprises an electrometer for continuously measuring the level or amplitude of the electrostatic charge in the areas comprising said image and for providing a proportional signal that is used to control the electrical bias applied to development means. The proportional signal is used to drive a circuit containing three serially connected operational amplifiers which are interconnected in accordance with and in timed relation to the location of the image relative to a probe head connected to the electrometer and to a development station. The control signal generated by the circuit is used to drive a voltage supply means whereby a bias field corresponding generally to the amplitude of the charge level of the image background areas is provided so that a minimum of toner particles will be applied to these areas.
US Patent References:
Xerographic electrometer apparatus
Allen et al. - December 1961 - 3013203
Electrostatic printing
Giaimo - October 1960 - 2956487
/3553595.html
Walsh - January 1971 - 3553595
Apparatus for developing electrostatic image on electrographic sheet
Sugarman et al. - October 1962 - 3058444
Apparatus for testing electrostatic copy material
Marguart et al. - October 1968 - 3406334
Xerographic electrometer apparatus
Allen et al. - December 1961 - 3013203
Electrostatic printing
Giaimo - October 1960 - 2956487
/3553595.html
Walsh - January 1971 - 3553595
Apparatus for developing electrostatic image on electrographic sheet
Sugarman et al. - October 1962 - 3058444
Apparatus for testing electrostatic copy material
Marguart et al. - October 1968 - 3406334
Inventors:
Piper, Douglas E. (Rochester, NY)
Jarvis, James G. (Rochester, NY)
Smith, Dale L. (Rochester, NY)
Jarvis, James G. (Rochester, NY)
Smith, Dale L. (Rochester, NY)
Application Number:
05/033571
Publication Date:
04/11/1972
Filing Date:
05/01/1970
View Patent Images:
Export Citation:
Assignee:
Eastman Kodak Company (Rochester, NY)
Primary Class:
Other Classes:
118/624, 399/270, 427/197, 327/50
International Classes:
G03G15/06; G03G15/08; G03G13/08
Field of Search:
117/17.5 118/637,7,8,9,11,2 324/32,72
Primary Examiner:
Martin, William D.
Assistant Examiner:
Sofocleous M.
Claims:
We claim
1. In an apparatus for developing with charged toner particles an electrostatic charge pattern comprising image and background areas of different charge level on a surface of a photoconductive material carried on a conducting element, including a development station arranged relative to a prescribed path of travel for said material and element and having at least one bias electrode for establishing a bias field that extends through said material to said element and in which said toner particles are distributed on said surface of said material; means fixed relative to said path and upstream of said development station for sensing the charge levels in said image and background areas as said material and element are moved in said path for generating a generally continuous signal which varies in amplitude in accordance with said sensed charge levels; and means for moving said material and element as a unit in said path which includes a traverse through said development station; wherein the improvement comprises control means for providing an electrical signal to establish and maintain said bias field between said electrode and said element at a level generally corresponding to that of said background areas, said control means comprising:
2. The apparatus in accordance with claim 1 wherein said means for generating said actuating signal includes an adjustable potentiometer for varying the amplitude of said actuating signal independent of said continuous signal.
3. The apparatus in accordance with claim 1 including means for maintaining the amplitude of said established bias field during image development.
4. The apparatus in accordance with claim 1 wherein said means for generating an actuating signal includes a plurality of serially connected operational amplifier circuits, each of which generates an output signal, the amplitude of the last generated output signal corresponding generally to said drive signal.
5. The apparatus in accordance with claim 4 wherein at least one of said amplifier circuits includes an adjustable gain control potentiometer for establishing a constant of proportionality between the amplitude of said actuating signal and that of said drive signal.
6. The apparatus in accordance with claim 1 wherein said means for generating said drive signal includes with claim high input impedance amplifier circuit having a capacitor serially connected with an adjustable potentiometer and responsive to said output signal generated by the first of said operational amplifiers, whereby the amplitude of the charge on said capacitor establishes the amplitude of said drive signal.
7. The apparatus in accordance with claim 1 wherein said means for generating said drive signal includes a high input impedance amplifier circuit having a capacitor serially connected with an adjustable potentiometer, whereby said drive signal is stored by said capacitor to control and maintain the amplitude of said bias field while said pattern is being moved through said development station.
8. The apparatus in accordance with claim 7 wherein the amplitude of said drive signal is in accordance with the charge on said capacitor while said pattern is being moved through said development station.
1. In an apparatus for developing with charged toner particles an electrostatic charge pattern comprising image and background areas of different charge level on a surface of a photoconductive material carried on a conducting element, including a development station arranged relative to a prescribed path of travel for said material and element and having at least one bias electrode for establishing a bias field that extends through said material to said element and in which said toner particles are distributed on said surface of said material; means fixed relative to said path and upstream of said development station for sensing the charge levels in said image and background areas as said material and element are moved in said path for generating a generally continuous signal which varies in amplitude in accordance with said sensed charge levels; and means for moving said material and element as a unit in said path which includes a traverse through said development station; wherein the improvement comprises control means for providing an electrical signal to establish and maintain said bias field between said electrode and said element at a level generally corresponding to that of said background areas, said control means comprising:
2. The apparatus in accordance with claim 1 wherein said means for generating said actuating signal includes an adjustable potentiometer for varying the amplitude of said actuating signal independent of said continuous signal.
3. The apparatus in accordance with claim 1 including means for maintaining the amplitude of said established bias field during image development.
4. The apparatus in accordance with claim 1 wherein said means for generating an actuating signal includes a plurality of serially connected operational amplifier circuits, each of which generates an output signal, the amplitude of the last generated output signal corresponding generally to said drive signal.
5. The apparatus in accordance with claim 4 wherein at least one of said amplifier circuits includes an adjustable gain control potentiometer for establishing a constant of proportionality between the amplitude of said actuating signal and that of said drive signal.
6. The apparatus in accordance with claim 1 wherein said means for generating said drive signal includes with claim high input impedance amplifier circuit having a capacitor serially connected with an adjustable potentiometer and responsive to said output signal generated by the first of said operational amplifiers, whereby the amplitude of the charge on said capacitor establishes the amplitude of said drive signal.
7. The apparatus in accordance with claim 1 wherein said means for generating said drive signal includes a high input impedance amplifier circuit having a capacitor serially connected with an adjustable potentiometer, whereby said drive signal is stored by said capacitor to control and maintain the amplitude of said bias field while said pattern is being moved through said development station.
8. The apparatus in accordance with claim 7 wherein the amplitude of said drive signal is in accordance with the charge on said capacitor while said pattern is being moved through said development station.
Description:
FIELD OF THE INVENTION
The invention relates to development of a latent electrostatic image and particularly to apparatus for controlling the bias field for the application of toner particles to the image area during development thereof.
DESCRIPTION OF THE PRIOR ART
In the prior art, various methods are used control the bias field in a development station, e.g., the exposure of the charged surface of a photoconductive material to an actinic image, and the level of charge applied to the photoconductive material. In each such method, a photocell is used to measure the light transmitted by a transparency in the exposure station or the density of the transparency image that is to be projected onto the photoconductive material. With any one of the signals derived from such sources, a comparison is made with a standard signal and on the basis of this comparison, the exposure, charge level or bias field is increased, decreased or maintained at a standard level.
It is also well known in the prior art to control the exposure of a photoconductive material by continuously measuring the residual charge on the surface of such material as exposure takes place. When the residual charge is at a predetermined level, circuitry responsive to the charge level provides for automatically terminating the exposure. In such a system, the control is dependent upon measuring the change in the charge level simultaneously with exposure. Consequently, the measurement of the residual charge is used only to control exposure and is in no way associated with development of the latent electrostatic image or control of the bias field for development by application of charged toner particles to the imagewise-exposed surface. In the various controls used in the prior art, the control signal is usually generated by reference to the density or light transmission quality of the image or the image carrier, respectively, and is in no way related to measuring the charge level of the background areas of an image.
SUMMARY OF THE INVENTION
A primary object of the invention is to provide an automatic bias control, the level or amplitude of the bias being determined by and in accordance with, after imagewise exposure, the charge remaining on a photoconductive material in the background areas.
Another object of the invention is to provide an automatic bias control which can be adapted to compensate for the nonuniform development of a latent electrostatic image that results from a photoconductor having a relatively high rate of dark decay.
Still another object of the invention is to provide an automatic bias control which can be adapted to compensate for the nonuniform development that results from a combination of a flash exposure, a relatively narrow development zone and a photoconductor that exhibits post-exposure "photo" decay.
Yet another object of the invention is to provide an automatic bias control that is particularly adaptable to document copying and microfilm enlarging applications.
These and other objects of the invention will be apparent to those skilled in the art by the description which follows:
The objects of the invention are attained by utilizing an electrometer in conjunction with an amplifier control circuit and a voltage supply circuit which are interconnected so as to control the bias field of a magnetic brush developing means. A probe head continuously scans a latent electrostatic image as the latter is moved past the head. A continuous signal is generated by the head, the amplitude of which varies with the charge level of the areas comprising the image per se and the background. The electrometer, in turn, generates an actuating signal that is proportional to the continuous signal and this actuating signal serves as the input to a control circuit comprising three serially connected amplifier circuits. The first and second amplifier circuits are interconnected by a diode and an input capacitor that is maintained at a predetermined amplitude corresponding to a preselected charge level. So long as the output signal from the first amplifier circuit is of greater amplitude than the charge level of the capacitor, the charge level of the capacitor controls the amplitude of an input signal to the second amplifier circuit. By means of a pair of spaced photocells, one of which is arranged relative to the probe head and the other of which is arranged relative to the development station, the leading and trailing edges of the area containing the image can be detected. The second and third amplifier circuits are interconnected and the normal input control signals are disconnected in proper timed relation relative to a print cycle. A capacitor associated with the input to the third amplifier circuit normally provides a charge level sufficient to produce a drive signal of such amplitude, as to limit the voltage output from a source of potential to which it is connected, so that a minimum bias field is applied to the magnetic brush developing means. When the second and third amplifier circuits are interconnected, the drive signal can be changed to one which will provide a bias field that is less than that which would be derived from the input capacitor associated with the first and second amplifier circuits. With such a control circuitry, nonuniform development is eliminated in that so long as the bias field can be maintained at a level proportional to the charge level of the background areas, maximum development density will be obtained.
DESCRIPTION OF THE DRAWING
FIG. 1 is a diagrammatic view showing the various control circuits and the path of movement of an image on a photoconductive member relative to a development station; and
FIG. 2 is a schematic wiring diagram of the control circuitry shown in FIG. 1 and designated as the "Control."
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to FIG. 1, a web 10 comprising an electrically conductive support or material having a layer of photoconductive material applied thereto and having a layer or more longitudinally spaced latent electrostatic images thereon is moved and guided by any well-known means, such as rolls 11 and 12, in a defined path and through a development station indicated by numeral 13. The development station 13 can comprise one or more magnetic brushes 14 that move toner particles from a supply in a container 16 onto the image bearing surface of web 10. While the invention is described with respect to a web, it is to be understood that it can be used in conjunction with a single sheet having a latent electrostatic image or charge pattern formed on the surface of a photoconductive material that is applied to or carried by an electrically conductive support or material, said sheet being moved through a predetermined path and through a development station in much the same manner. In other words, the invention does not necessitate the use of a web or a path of movement such as that disclosed in FIG. 1. It is only essential that a probe head 17 can scan the surface bearing the latent electrostatic image and that the image bearing surface can be moved relative to magnetic brushes 14 or a similar developing means in which a bias field is used.
As is well known in the art, a voltage supply indicated by numeral 18 is connected to magnetic brushes 14 so that a bias field is established between the rollers and the electrically conductive support comprising a part of the web 10 or an electrode against which the web is maintained on passing through the development station. Voltage supply means 18 can be a commercially available unit which will provide an output voltage that can be varied in accordance with a drive signal applied thereto. The bias voltage delivered by voltage supply means 18 can be the same as, proportional to, or a multiple of, the input drive signal generated by control 20 as described hereinbelow.
The probe head 17 is connected to a commercially available electrometer 19 which supplies or generates an actuating signal that is directly proportional to the signal derived from the probe head. The actuating signal generated by electrometer 19 is connected to a control means 20 which comprises three serially connected amplifier circuits designated 21, 22 and 23 and the interconnected and related parts thereof. Immediately ahead of the probe head 17, a photocell 24 is positioned relative to the path of movement of web 10 for detecting or sensing the leading edge of each image on web 10. The leading and trailing edge of each image can be indicated by a line or hole associated with the image at spaced intervals along the web or by other means well known in this art. The photocell 24 is interconnected with a relay 25 which, in turn, actuates a pair of normally closed contacts designated 25A and 25B, see FIG. 2. A photocell 26 is positioned relative to the path of movement of web 10 and adjacent development station 13 and generates a control signal from the leading edge of the same image as photocell 24 just before the image enters the development station 13. A relay 27 is interconnected with photocell 26 and serves to actuate a contact 27A in the control circuit 20 as described hereinafter.
The manner in which the bias field is automatically controlled and varied can be best described in conjunction with a cycle of operation and the schematic wiring diagram shown in FIG. 2. Assuming that an image is of a length that is slightly less than the distance between the photocells 24 and 26 as measured along the path of movement, the leading edge of such an image, following exposure, will be detected by photocell 24 and the signal generated thereby will cause relay 25 to be energized so that its contacts 25A and 25B will assume an open position. As the image approaches probe head 17, capacitor 30 on the input side of amplifier circuit 22 will have been charged to a predetermined voltage in accordance with the setting of the adjustable resistor 31. As will be noted from FIG. 2, a source of potential 32 is connected through resistor 33, resistor 31, contacts 25A and capacitor 30 to ground. As a result, when contacts 25A are open, the charge on capacitor 30 is in accordance with the adjusted setting of resistor 31 and is, in effect, a preselected charge level.
At this point, it should also be explained that capacitor 40 on the input side of amplifier circuit 23 is also connected on one side to ground and on the other side to the input of the amplifier circuit 23 at a junction point common to contacts 25B. A variable resistor 41 and a resistor 43 are serially connected to the source of potential 32. In this case, resistor 41 is adjusted so that when contacts 25B are open and no signal is being derived from amplifier circuit 22, the drive signal generated by amplifier circuit 23 will have an amplitude which will provide a bias voltage to brushes 14 so that a bias field is maintained which will generally prohibit the deposition of toner particles by brushes 14 on the surface of web 10 corresponding to the background areas. This bias voltage is generally equivalent to the potential level of the background areas so that a minimum density in the background areas will provide an image of optimum contrast.
As the image is moved relative to probe head 17, a generally continuous signal is generated which will have an amplitude that varies between the maximum and minimum charge levels comprising the image. The amplitude of the signal corresponding to the charge level in the background areas will be a minimum in the case of a positive-appearing image and a maximum in the case of a negative-appearing image. This continuous signal is transmitted to the electrometer 19 and a proportional actuating signal is generated thereby which is applied to input terminal 45 of amplifier circuit 21. The amplified output signal generated by amplifier circuit 21 is inhibited by diode 46, if its amplitude is greater than that of the voltage on capacitor 30. If such output signal from amplifier circuit 21 is less than the amplitude of the voltage on capacitor 30, then the capacitor will discharge through resistor 47 and diode 46 to a value normally equal to the output voltage of amplifier circuit 21. In this way the voltage on capacitor 30 will always be proportional to the smallest voltage being generated by electrometer 19. It is assumed that with a high resolution probe head 17, the smallest output voltage will be an indication of the background voltage of the electrostatic image. At this point, contacts 27A are still open so that there is no transmission of the generated signal to amplifier circuit 23. When the leading edge of the image approaches development station 13, a signal will be derived therefrom by photocell 26. When photocell 26 sees the image, relay 27 will be energized and contacts 27A will close so that amplifier circuits 22 and 23 will then be interconnected. The signal on the input side of amplifier circuit 22 is the signal which establishes the drive signal for controlling the bias voltage and, as stated hereinbefore, is proportional to the voltage of the background areas of the electrostatic image. An adjustable resistor 50 on the output side of amplifier circuit 23 serves as a gain control potentiometer for determining the constant of proportionality. Generally, the gain will be set so that the bias voltage from the supply means 18 is substantially equal to the voltage of the background areas of the image. If a fixed increment of voltage over and above that of the background voltage is desired, this can be obtained by adjustment of the potentiometer 51 that is on the input side of amplifier circuit 21 and connected to a source of potential 52.
As the trailing edge of an image (designated by a line or hole) passes under or relative to photocell 24, a signal is generated as described above and relay 25 is deenergized so that contacts 25A and 25B will now return to their normally closed positions. When this condition arises, the last voltage to appear at the input to amplifier circuit 23 is then stored by capacitor 40 which maintains this drive signal until the image has passed through the development station. Amplifier circuits 22 and 23 have high input impedance so that the charge or voltage on the input capacitors 30 and 40 will not change over the time initially associated with the movement of an image between sensing of the leading edge by photocell 24 and exit of the trailing edge from development station 13.
It has been found that maintaining the bias field potential at substantially the same amplitude as the minimum charge level measured in the background areas of an image results in prints of optimum contrast with minimum background density. For example, with a blue-background document having a reflective density of 0.36, a minimum potential of 250 v. was indicated. A print made with a bias field potential of 225 volts showed some background density, whereas a bias potential of 250 v. resulted in much better prints being obtained. With another type of document, for example, a part of a newspaper page having a background reflection density of 0.19, it was found that the background level of charge measured was measured at a minimum of 170 v. A good background-free print was made with a bias field potential of 175 v. The above examples are merely illustrative and are not meant to provide any limitation with respect to the invention.
The apparatus described hereinabove can be used to reproduce a positive or a negative original as either a positive or a negative-appearing print. As is well known in the art, the polarity of the charge applied to the image bearing surface of the photoconductive material can be either positive or negative and with either of such polarity, the polarity of the charged tone particles can be positive or negative. In other words, toner particles of one polarity can be used with a charged surface of either polarity. In either case, the apparatus provides a bias field to provide an optimum or minimum background density.
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
The invention relates to development of a latent electrostatic image and particularly to apparatus for controlling the bias field for the application of toner particles to the image area during development thereof.
DESCRIPTION OF THE PRIOR ART
In the prior art, various methods are used control the bias field in a development station, e.g., the exposure of the charged surface of a photoconductive material to an actinic image, and the level of charge applied to the photoconductive material. In each such method, a photocell is used to measure the light transmitted by a transparency in the exposure station or the density of the transparency image that is to be projected onto the photoconductive material. With any one of the signals derived from such sources, a comparison is made with a standard signal and on the basis of this comparison, the exposure, charge level or bias field is increased, decreased or maintained at a standard level.
It is also well known in the prior art to control the exposure of a photoconductive material by continuously measuring the residual charge on the surface of such material as exposure takes place. When the residual charge is at a predetermined level, circuitry responsive to the charge level provides for automatically terminating the exposure. In such a system, the control is dependent upon measuring the change in the charge level simultaneously with exposure. Consequently, the measurement of the residual charge is used only to control exposure and is in no way associated with development of the latent electrostatic image or control of the bias field for development by application of charged toner particles to the imagewise-exposed surface. In the various controls used in the prior art, the control signal is usually generated by reference to the density or light transmission quality of the image or the image carrier, respectively, and is in no way related to measuring the charge level of the background areas of an image.
SUMMARY OF THE INVENTION
A primary object of the invention is to provide an automatic bias control, the level or amplitude of the bias being determined by and in accordance with, after imagewise exposure, the charge remaining on a photoconductive material in the background areas.
Another object of the invention is to provide an automatic bias control which can be adapted to compensate for the nonuniform development of a latent electrostatic image that results from a photoconductor having a relatively high rate of dark decay.
Still another object of the invention is to provide an automatic bias control which can be adapted to compensate for the nonuniform development that results from a combination of a flash exposure, a relatively narrow development zone and a photoconductor that exhibits post-exposure "photo" decay.
Yet another object of the invention is to provide an automatic bias control that is particularly adaptable to document copying and microfilm enlarging applications.
These and other objects of the invention will be apparent to those skilled in the art by the description which follows:
The objects of the invention are attained by utilizing an electrometer in conjunction with an amplifier control circuit and a voltage supply circuit which are interconnected so as to control the bias field of a magnetic brush developing means. A probe head continuously scans a latent electrostatic image as the latter is moved past the head. A continuous signal is generated by the head, the amplitude of which varies with the charge level of the areas comprising the image per se and the background. The electrometer, in turn, generates an actuating signal that is proportional to the continuous signal and this actuating signal serves as the input to a control circuit comprising three serially connected amplifier circuits. The first and second amplifier circuits are interconnected by a diode and an input capacitor that is maintained at a predetermined amplitude corresponding to a preselected charge level. So long as the output signal from the first amplifier circuit is of greater amplitude than the charge level of the capacitor, the charge level of the capacitor controls the amplitude of an input signal to the second amplifier circuit. By means of a pair of spaced photocells, one of which is arranged relative to the probe head and the other of which is arranged relative to the development station, the leading and trailing edges of the area containing the image can be detected. The second and third amplifier circuits are interconnected and the normal input control signals are disconnected in proper timed relation relative to a print cycle. A capacitor associated with the input to the third amplifier circuit normally provides a charge level sufficient to produce a drive signal of such amplitude, as to limit the voltage output from a source of potential to which it is connected, so that a minimum bias field is applied to the magnetic brush developing means. When the second and third amplifier circuits are interconnected, the drive signal can be changed to one which will provide a bias field that is less than that which would be derived from the input capacitor associated with the first and second amplifier circuits. With such a control circuitry, nonuniform development is eliminated in that so long as the bias field can be maintained at a level proportional to the charge level of the background areas, maximum development density will be obtained.
DESCRIPTION OF THE DRAWING
FIG. 1 is a diagrammatic view showing the various control circuits and the path of movement of an image on a photoconductive member relative to a development station; and
FIG. 2 is a schematic wiring diagram of the control circuitry shown in FIG. 1 and designated as the "Control."
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to FIG. 1, a web 10 comprising an electrically conductive support or material having a layer of photoconductive material applied thereto and having a layer or more longitudinally spaced latent electrostatic images thereon is moved and guided by any well-known means, such as rolls 11 and 12, in a defined path and through a development station indicated by numeral 13. The development station 13 can comprise one or more magnetic brushes 14 that move toner particles from a supply in a container 16 onto the image bearing surface of web 10. While the invention is described with respect to a web, it is to be understood that it can be used in conjunction with a single sheet having a latent electrostatic image or charge pattern formed on the surface of a photoconductive material that is applied to or carried by an electrically conductive support or material, said sheet being moved through a predetermined path and through a development station in much the same manner. In other words, the invention does not necessitate the use of a web or a path of movement such as that disclosed in FIG. 1. It is only essential that a probe head 17 can scan the surface bearing the latent electrostatic image and that the image bearing surface can be moved relative to magnetic brushes 14 or a similar developing means in which a bias field is used.
As is well known in the art, a voltage supply indicated by numeral 18 is connected to magnetic brushes 14 so that a bias field is established between the rollers and the electrically conductive support comprising a part of the web 10 or an electrode against which the web is maintained on passing through the development station. Voltage supply means 18 can be a commercially available unit which will provide an output voltage that can be varied in accordance with a drive signal applied thereto. The bias voltage delivered by voltage supply means 18 can be the same as, proportional to, or a multiple of, the input drive signal generated by control 20 as described hereinbelow.
The probe head 17 is connected to a commercially available electrometer 19 which supplies or generates an actuating signal that is directly proportional to the signal derived from the probe head. The actuating signal generated by electrometer 19 is connected to a control means 20 which comprises three serially connected amplifier circuits designated 21, 22 and 23 and the interconnected and related parts thereof. Immediately ahead of the probe head 17, a photocell 24 is positioned relative to the path of movement of web 10 for detecting or sensing the leading edge of each image on web 10. The leading and trailing edge of each image can be indicated by a line or hole associated with the image at spaced intervals along the web or by other means well known in this art. The photocell 24 is interconnected with a relay 25 which, in turn, actuates a pair of normally closed contacts designated 25A and 25B, see FIG. 2. A photocell 26 is positioned relative to the path of movement of web 10 and adjacent development station 13 and generates a control signal from the leading edge of the same image as photocell 24 just before the image enters the development station 13. A relay 27 is interconnected with photocell 26 and serves to actuate a contact 27A in the control circuit 20 as described hereinafter.
The manner in which the bias field is automatically controlled and varied can be best described in conjunction with a cycle of operation and the schematic wiring diagram shown in FIG. 2. Assuming that an image is of a length that is slightly less than the distance between the photocells 24 and 26 as measured along the path of movement, the leading edge of such an image, following exposure, will be detected by photocell 24 and the signal generated thereby will cause relay 25 to be energized so that its contacts 25A and 25B will assume an open position. As the image approaches probe head 17, capacitor 30 on the input side of amplifier circuit 22 will have been charged to a predetermined voltage in accordance with the setting of the adjustable resistor 31. As will be noted from FIG. 2, a source of potential 32 is connected through resistor 33, resistor 31, contacts 25A and capacitor 30 to ground. As a result, when contacts 25A are open, the charge on capacitor 30 is in accordance with the adjusted setting of resistor 31 and is, in effect, a preselected charge level.
At this point, it should also be explained that capacitor 40 on the input side of amplifier circuit 23 is also connected on one side to ground and on the other side to the input of the amplifier circuit 23 at a junction point common to contacts 25B. A variable resistor 41 and a resistor 43 are serially connected to the source of potential 32. In this case, resistor 41 is adjusted so that when contacts 25B are open and no signal is being derived from amplifier circuit 22, the drive signal generated by amplifier circuit 23 will have an amplitude which will provide a bias voltage to brushes 14 so that a bias field is maintained which will generally prohibit the deposition of toner particles by brushes 14 on the surface of web 10 corresponding to the background areas. This bias voltage is generally equivalent to the potential level of the background areas so that a minimum density in the background areas will provide an image of optimum contrast.
As the image is moved relative to probe head 17, a generally continuous signal is generated which will have an amplitude that varies between the maximum and minimum charge levels comprising the image. The amplitude of the signal corresponding to the charge level in the background areas will be a minimum in the case of a positive-appearing image and a maximum in the case of a negative-appearing image. This continuous signal is transmitted to the electrometer 19 and a proportional actuating signal is generated thereby which is applied to input terminal 45 of amplifier circuit 21. The amplified output signal generated by amplifier circuit 21 is inhibited by diode 46, if its amplitude is greater than that of the voltage on capacitor 30. If such output signal from amplifier circuit 21 is less than the amplitude of the voltage on capacitor 30, then the capacitor will discharge through resistor 47 and diode 46 to a value normally equal to the output voltage of amplifier circuit 21. In this way the voltage on capacitor 30 will always be proportional to the smallest voltage being generated by electrometer 19. It is assumed that with a high resolution probe head 17, the smallest output voltage will be an indication of the background voltage of the electrostatic image. At this point, contacts 27A are still open so that there is no transmission of the generated signal to amplifier circuit 23. When the leading edge of the image approaches development station 13, a signal will be derived therefrom by photocell 26. When photocell 26 sees the image, relay 27 will be energized and contacts 27A will close so that amplifier circuits 22 and 23 will then be interconnected. The signal on the input side of amplifier circuit 22 is the signal which establishes the drive signal for controlling the bias voltage and, as stated hereinbefore, is proportional to the voltage of the background areas of the electrostatic image. An adjustable resistor 50 on the output side of amplifier circuit 23 serves as a gain control potentiometer for determining the constant of proportionality. Generally, the gain will be set so that the bias voltage from the supply means 18 is substantially equal to the voltage of the background areas of the image. If a fixed increment of voltage over and above that of the background voltage is desired, this can be obtained by adjustment of the potentiometer 51 that is on the input side of amplifier circuit 21 and connected to a source of potential 52.
As the trailing edge of an image (designated by a line or hole) passes under or relative to photocell 24, a signal is generated as described above and relay 25 is deenergized so that contacts 25A and 25B will now return to their normally closed positions. When this condition arises, the last voltage to appear at the input to amplifier circuit 23 is then stored by capacitor 40 which maintains this drive signal until the image has passed through the development station. Amplifier circuits 22 and 23 have high input impedance so that the charge or voltage on the input capacitors 30 and 40 will not change over the time initially associated with the movement of an image between sensing of the leading edge by photocell 24 and exit of the trailing edge from development station 13.
It has been found that maintaining the bias field potential at substantially the same amplitude as the minimum charge level measured in the background areas of an image results in prints of optimum contrast with minimum background density. For example, with a blue-background document having a reflective density of 0.36, a minimum potential of 250 v. was indicated. A print made with a bias field potential of 225 volts showed some background density, whereas a bias potential of 250 v. resulted in much better prints being obtained. With another type of document, for example, a part of a newspaper page having a background reflection density of 0.19, it was found that the background level of charge measured was measured at a minimum of 170 v. A good background-free print was made with a bias field potential of 175 v. The above examples are merely illustrative and are not meant to provide any limitation with respect to the invention.
The apparatus described hereinabove can be used to reproduce a positive or a negative original as either a positive or a negative-appearing print. As is well known in the art, the polarity of the charge applied to the image bearing surface of the photoconductive material can be either positive or negative and with either of such polarity, the polarity of the charged tone particles can be positive or negative. In other words, toner particles of one polarity can be used with a charged surface of either polarity. In either case, the apparatus provides a bias field to provide an optimum or minimum background density.
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.