CA1177873A - Electrostatic imaging apparatus and method providing stable reference potential - Google Patents
Electrostatic imaging apparatus and method providing stable reference potentialInfo
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- CA1177873A CA1177873A CA000395475A CA395475A CA1177873A CA 1177873 A CA1177873 A CA 1177873A CA 000395475 A CA000395475 A CA 000395475A CA 395475 A CA395475 A CA 395475A CA 1177873 A CA1177873 A CA 1177873A
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- conductive layer
- potential
- corona
- layer
- current
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Abstract
ELECTROSTATIC IMAGING APPARATUS AND METHOD
PROVIDING STABLE REFERENCE POTENTIAL
Abstract of the Disclosure Electrostatic imaging with an imaging member of the kind having a charge-retentive layer and a conduc-tive layer is improved by directing an alternating corona current to such conductive layer, detecting the electrical potential of the conductive layer and adjusting the net alternating corona current to maintain the conductive layer potential closely proximate a desired nominal reference potential level.
PROVIDING STABLE REFERENCE POTENTIAL
Abstract of the Disclosure Electrostatic imaging with an imaging member of the kind having a charge-retentive layer and a conduc-tive layer is improved by directing an alternating corona current to such conductive layer, detecting the electrical potential of the conductive layer and adjusting the net alternating corona current to maintain the conductive layer potential closely proximate a desired nominal reference potential level.
Description
'7~'73 ELECTROSTATIC IMAGING APPARATUS AND METHOD
PROVIDING STABLE REFERENCE POTENTIAL
BACKGROUND OF THE INVENTION
Field of the Invention . .
The present invention relates to electrostatic imaging and more particularly to improved structural and functional approaches for use in electrostatic imaging apparatus to simply yet stably maintain the potential of the imaging member's conductive layer near a desired reference potential level, e.g. ground.
Description of the Prior Art In electrostatic imaging, e.g. electrophotog-raphy or dielectric (s~ylus) recording, an electrostatic charge pattern is formed on the surface of an insulator layer (in electrophotography the surface of the photo-conductive insulator layer) of an image member and developed with marking particles which are electro-statically attracted thereto. Adjacent the charge-retentive insulator layer, such imaging members generally include an electrically conductive layer which is adap~ed to be electrically coupled to a source of reference potential, e.g. ground. If the potential on the conductive layer is maintained closely proximate a nominal reference potential level, the relative magni-tudes of the different portions of the electrostaticcharge pattern will accurately represent the imaged pattern to be reproduced and during development the marking particles will be attracted in accurate propor-tion according to the charge pattern magnitudes.
However, if the reference potential of the conductive layer varies, the variation will be reflected in the formation and development of the latent electrostatic image (e.g. in a variation in the magnitude of attracted charge and/or attracted marking particles). In many applications the image layer and conductive layer are disposed on an insulative film support. Because the conductive layer is thus sandwiched between two *
' ~ 1~;'7 ~ 3 insulators, the couplin~ of the conductive layer to the reference potential source is more complicated.
A wide variety of approaches have been developed for electrically connecting such sandwiched conductive layers to a reference potential source, which is commonly termed "grounding" the imaging member. A
non-imaged portion of the imaging member can be specially fabricated to allow the electrical coupling of a reference potential with the conductive layer. For example, a non-image portion of the imaging member can be perforated or bared, a non-image portion of a sheet or strip of the imaging member can be imbibed with a conductive coating material which penetrated to the conductive layer, or the edge of a sheet, strip or roll of the imaging member can be imbibed with a conductive coatin~ material.
Electrical coupling can be effected in various ways, e.g. by physical contact between the conductive layer and a conductive grounding member or by directing a corona discharge into the conductive layer. Physical contact has been the most commonly used technique but it requires special fabrication of the imaging member.
Corona grounding via the edge of the image member has been suggested as an approach to obviate the need for such special fabrications (see U.S. 3,650,622). How-ever, past attempts to use corona grounding have confronted difficulties in maintaining the reference potential accurately when variations occur in system parameters such as atmospheric conditions, operating velocities, operating voltage levels, imaging member composition, etc. In view of these difficulties, it has been suggested to provide, as grounding means, a corona discharge device energized by a programmable D.C. power supply and means for adjusting such energization in response to a feedback signal ~rom an electrometer positioned to sense conductive layer portions moving therepast.
~7~7t3 SUMMARY OF THE INVENTION
-A significant purpose of the present invention is to provide functional and structural corona grounding approaches that improve upon the D.C. energization-feedback control system mentioned above. Morespecifically, the present invention provides, in corona grounding, the advantages of rapid response (to varia-tion of the conductive layer from the desired reference potential) and accurate stabilization of the conductive layer potential (to levels closely proximate the reference potential level). The present invention also affords advantages in flexibility as to operability with different (e.g. bipolar) imaging operation modes. A
further significant advantage of the present invention is that it is subject to sîmple and inexpensive structural implementations.
The above and other objectives and advantages are accomplished in accordance with the present inven-tion by providing for imaging apparatus of the kind adapted to form and develop an electrostatic image on an imaging member having a charge-retentive layer and an electrically conductive layer, improved means and method for controlling the potential of such conductive layer.
In one general aspect this improvement comprises directing an alternating corona current to the conduc-tive layer of an imaging member as it moves along the operative path of such apparatus, detecting the conduc-tive layer potential of such moving imaging member and varying the net corona current in accordance with the detected potential so as to maintain the conductive layer potential closely proximate a nominal reference potential level. In another general aspect such improvement comprises A.C. corona discharge means for directing alternating corona current to the conductive layer of such imaging member; adjustable means for varying such corona current; detector means for sensing, and providing a signal representative of~ the potential '7~3 level of successive portions of such imaging member's conductive layer; and control means for receiving such signal and adjusting said current-varying means to main-tain the conductive layer potential closely proximate a nominal reference potential level.
BRIEF ~ESCRIPTION OF THE DRAWINGS
The subsequent description of preferred embodi-ments of the present invention is made with reference to the attached drawings wherein:
Figure 1 is a schematic illustration of one embodiment of electrostatic imaging apparatus in accordance with the present invention;
Figure 2 is a schematic illus~ration showing details of one preferred corona device and adjustment 15 circuit in accordance with the present invention;
Figure 3 is a circuit d;agram illustrating one preferred a~plifier/filter and phase sensitive detector embodiment in accordance with the present invention;
Figure 4 is a circuit diagram illustrating one preferred signal processing and control embodiment in accordance with the present invention; and Fi~ure 5 is a graph illustrating the variation in net alternating corona current effected, for example, by the Fig. 2 embodiment.
DETAIL.ED DESC~IPTION OF THE PREFERRED EMBODIMENTS
The subsequent description pertains to a particular electrophotographic embodiment of the present invention; however, it will be appreciated by those skilled in the art that the advantages afforded by tne present invention also can be implemented in many other electrophotographic embodiments, as well as in other electrostatic imaging systems that utilize a reference potential on their image member's conductive layer.
The electrophotographic imaging apparatus 10 shown in Fig. 1 is one particularly adapted to form images on a strip imaging member 11, e.g. a 16m~ film.
Referring briefly to Fig. 2, it can be seen that the ~ 3 film 11 comprises a photoconductive insulator layer 12 adjacent an electrically conductive layer 13 on a film support 14. As shown in Fig. 1, the imaging film 11 is moved along an operative path of the apparatus 10 from a supply roll 15 past a pr;mary charging station 16, where a uniform electrostatic charge is applied upon the exterior surface of photoconductor layer 12. Downstream along the operative path from charging station 16 are an image exposure station 17 and a liquid development station 18 which function in the usual manner to form and develop latent electrostatic images. After the development station 18, a hot air fixing station 19 is provided to dry and fuse the toner image on the film.
The film is then fed to a storage or utilization means (not shown), e.g. a take-up reel. Rollers 20 and 21 support and transport the film along the operative process path ~f the apparatus. As stated earlier, in order to obtain uniform primary-charging and development of the film 11, it is necessary that its conductive layer 13 be coupled effectively to a source of reference potential. This is accomplished by corona discharge device 25 (see Fig. 2) which can be of a conventional construction and is disposed along an edge of the film path.
In accordance with the present invention, the corona discharge device 25 is energized with alternating current continuously controlled in accordance with a signal provided from detector means for sensing the potential of the film conductive layer 13. Such detec-tor means can comprise one of various kinds of commercial electrometers and is represented in Fig. 1 by probe and housing 30, and cooperating timing driver 31, amplifier/filter 32 and phase sensitive detector 33.
The signal provided by such detector means is applied to signal processing and control circuit 35 and corona adjustment circuit 26 which, in cooperation~ provide proper ener~ization of the corona discharge electrode 25 7~'73 to effect a net corona current to the conductive layer 13 that will maintain its potential closely proximate the desired nominal reference potential.
As noted, the electrometer can be of various conventional constructions. In one preferred configura-tion, the probe and housing 30 comprises an electro-magnet enclosed in a metallic housing which shields its electrical noise. Below the electromagnet, a thin piece of electrical shielding is provided. This piece preferably is non-ferromagnetic and of relatiYely low electrical conductivity so as to provide shielding from electrical noise but not attenuate the magnetic field generated by the electromagnet. A thin reed, which is made of magnetic material (e.g. springsteel or other magnetic alloys), is sandwiched between two pieces of electrical insulator and fastened below the electro-magnet. The operative film pa~h is configured so that film 11 passes below such reed with the photoconductor side facing the reed. This minimi~es the possibility of detection error due to static charges on the film support.
The electromagnet is driven by timing driver circuit 31, preferably at the resonant frequency of the magnetic reed; and the magnetic field generated by the electromagnet forces the magnetic reed to vibrate.
Since the vibrating reed is capacitively coupled to the conducting layer of the photoconductive film below it, the electrostatic potential of the layer can be detected from the vibrating reed. Thus, the amplitude of the signal detected from the vibrating reed depends on the magnitude of the electrical potential of the conducting layer, the displacement of the vibrating reed and the film-to-reed distance. The displacement of the vibrating plate is defined by the strength of the magnetic field and also the frequency of the timing driver. Therefore the amplitude of the signal detected in the vibrating reed represents the magnitude of the ~ ~'7~ 3 electrical potential of film 11, and the phase of the signal detected with respect to the timing driver gives the sign of the voltage of the film. That is, the phase of the signal detected with positive voltage on the film is 180 out of phase with the signal detected with negative voltage on the film.
Referring to Fig. 3, the signal from the vibrating reed is amplified and filtered in circuit 32 of the electrometer, wherein an FET operational ampli-fier 70 provides high input impedance for the signal anda band-pass filter 71 has a frequency centered at the frequency of timing driver 31. The amplified and filtered signal is fed to the input of an analog FET 72 in the phase sensitive detector 33, which, in coopera-tion with a capacitor 73 at its output end, acts as asample and hold circuit. The timing position of sample and hold is provided by a retriggerable multivibrator 74 which is controlled by timing driver 31. The detected signal from the band-pass filter 71 is converted into a D.C. voltage at the output of the analog FET 72, e.g. a positive D.C. voltage will show at the output of the analog FET 72 if the voltage on the film is positive and the reed is vibrating at its resonant frequency. This D.C. voltage which is proportional to the potential of the film's conductive layer is fed to an output control amplifier 75 in the phase sensitive detector circuit 33. A bias voltage 76 can also be provided on the out-put control amplifier if it is desired that the refer-ence potential on the conductive layer be other than ground.
The signal from phase sensitive detector 33 is input to signal processing and control circuit 35, one preferred embodiment of which is shown in more detail in Fig. 4. This embodiment CQmpriSeS an inverting ampli-fier circuit 36, a differentiator circuit 37 and anintegrator circuit 38 which are coupled in parallel between the input from phase sensitive detector 33 and ~7~ 73 an output inverting amplifier 39. The output o~
amplifier 39 is applied to a voltage to current converter circuit 40 which energizes a light emissive element 41, e.g. an LED, located in cooperative relation with a photoresistor 42 in corona adjustment circu~t 26.
Details of one preferred corona adjustment circuit are shown in Fig. 2. Thus high voltage trans-former 50 provides an output voltage of amplitude sufficient to excite the corona, e.g. in the order of 7 K.V. peak. The net corona current (current averaged over a full cycle) delivered by the charger is varied by controlling the amplitude of the ne~ative half cycle of voltage applied to the corona wire via the parallel high voltage diode 51 and variable resistance 42 circuit shown in Fig. 2. The simplicity of this control circuit depends on the physical phenomenon that a symmetric (unrectified) voltage applied to the corona wire pro-vides a strongly negative net corona current. Thus, when the resistance 42 in parallel with the diode 51 is at a low value, the corona delivers relatively high net-negative current. The diode 51 is forward biased during the positive half cycle of the energizing voltage, and as resistance 42 is increased, the negative half cycle of voltage applied to the corona wire is attenuated so that the net current becomes less nega-tive, going through zero, and becoming positive. The fixed resistor, 44, in parallel with the diode 51 and variable resistance 42 is provided to limit the maximum resistance of this branch so as to insure a path to dis-charge the capacitance of the charger in a time less~han about a half cycle. Resistance 44 is much greater than ~he resistance which will bias the corona to zero net current.
One device that can be used to provide the remotely controllable variable resistance is a "Photomod"~ Model CLM-9000 made by Clairex Corporation.
It contains a light emitting diode ~e.g. such as 41 in , 7~7~3~73 Fig. 2) and a cadmium-sulfide photoresistor (e.g. such as 42 in Fig. 2) in a package that provides high voltage isolation between the diode and the photoresistor. The function of this device could also be performed by an LED and a photoresistor fabricated as separate compo-nents. The net current delivered by the corona charger as a function of current in light emitting diode 41 is shown in Fig. 5.
Now consider the overall operation of these circuits. If it is desired to maintain the conductive layer 13 at ground potential, the bias of the output control amplifier of phase sensitive circuit 33 is adjusted so that its output is a predetermined voltage which maintains the collector current in the transistor output of the voltage-to-current converter 40 at a given level (when the conducting layer is at zero volts).
This given current level, operating through LED 41, adjusts the photoresistance of element 42 so that the corona output is balanced between positive and nega-tive. This balanced corona current is injected via anair gap to the conductive layer in the photoconductor maintaining its potential at ground. If the conductive layer in the photoconductor starts to have a positive potential, for example because of the charges of the toners or main corona charger, the detecting means will sense this change and, operating through signal processing and control circuit 35, provide a signal to the corona adjustment circuit 26 which turns photo-resistor 40 more "on", i.e. decreasing its resistance.
This causes injection of a predominantly negative corona to the conductive layer, driving the conductive layer back to the preset potential, zero in this case.
Similarly, if the film conductive layer 13 goes nega-tive, the detector means senses this condition and similarly turns the photoresistor less "on", i.e.
increasing its resistance. This creates a predominantly positive corona which drives the conductive layer to the 7~73 preset potential, zero in this case. If the conductive layer is to be maintained at a potential other than zero, the bias of the output control ampli~ier can be appropriately adjusted. Thus it can be seen that after selecting a desired nominal potential for the conductive layer of the film, the present invention will actively maintain the conductive layer closely proximate such nominal potential. This operation is independent, for example, of the polarity of the main corona charger used, the polarity of the toner, the film velocity and film width.
One other structural aspect of the Fig. 1 embodiment must be noted. It is highly desirable that rollers 20 located along the operative path in position to contact the surface of the photoconductive layer 12 be conductive and coupled to ground. This provides a capacitance between ground and conductive layer 13 that is much larger than the corona wire and the conducting layer. This is important to prevent the corona from inducing a large A.C. voltage on the conducting layer.
Considering the foregoing, it will be apprecia-ted by one skilled in the art that the present invention provides a simple, yet extremely flexible and accurate means for maintaining the conductive layer potential closely proximate a desired nominal reference potential level. The provision of an A.C. corona current to control the voltage of the conductive layer enables rapid response to a wide variety of operative system requirements, and the signal processing and control circuit modulates the signal provided by the charge detector means to create high stability for the servo-loop between the conductive layer and the corona.
Examples of the utility of such flexible feedback A.C.
grounding arrangements include modes of operation where it is desired to develop (or charge and develop) differ-ent segments of the film strip with different polarity or reference levels. Also, when operating in a run-out 7~7;3 condition (i.e. where the primary charger and exposure operations are terminated but it is desired to develop existing latent images), the requirements for the grounding corona will change rapidly and in large magni-tude. The present invention is uniquely adapted tohandle these special modes, as well as more common variations in operating parameters caused by changes in film velocity, humidity, etc.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
PROVIDING STABLE REFERENCE POTENTIAL
BACKGROUND OF THE INVENTION
Field of the Invention . .
The present invention relates to electrostatic imaging and more particularly to improved structural and functional approaches for use in electrostatic imaging apparatus to simply yet stably maintain the potential of the imaging member's conductive layer near a desired reference potential level, e.g. ground.
Description of the Prior Art In electrostatic imaging, e.g. electrophotog-raphy or dielectric (s~ylus) recording, an electrostatic charge pattern is formed on the surface of an insulator layer (in electrophotography the surface of the photo-conductive insulator layer) of an image member and developed with marking particles which are electro-statically attracted thereto. Adjacent the charge-retentive insulator layer, such imaging members generally include an electrically conductive layer which is adap~ed to be electrically coupled to a source of reference potential, e.g. ground. If the potential on the conductive layer is maintained closely proximate a nominal reference potential level, the relative magni-tudes of the different portions of the electrostaticcharge pattern will accurately represent the imaged pattern to be reproduced and during development the marking particles will be attracted in accurate propor-tion according to the charge pattern magnitudes.
However, if the reference potential of the conductive layer varies, the variation will be reflected in the formation and development of the latent electrostatic image (e.g. in a variation in the magnitude of attracted charge and/or attracted marking particles). In many applications the image layer and conductive layer are disposed on an insulative film support. Because the conductive layer is thus sandwiched between two *
' ~ 1~;'7 ~ 3 insulators, the couplin~ of the conductive layer to the reference potential source is more complicated.
A wide variety of approaches have been developed for electrically connecting such sandwiched conductive layers to a reference potential source, which is commonly termed "grounding" the imaging member. A
non-imaged portion of the imaging member can be specially fabricated to allow the electrical coupling of a reference potential with the conductive layer. For example, a non-image portion of the imaging member can be perforated or bared, a non-image portion of a sheet or strip of the imaging member can be imbibed with a conductive coating material which penetrated to the conductive layer, or the edge of a sheet, strip or roll of the imaging member can be imbibed with a conductive coatin~ material.
Electrical coupling can be effected in various ways, e.g. by physical contact between the conductive layer and a conductive grounding member or by directing a corona discharge into the conductive layer. Physical contact has been the most commonly used technique but it requires special fabrication of the imaging member.
Corona grounding via the edge of the image member has been suggested as an approach to obviate the need for such special fabrications (see U.S. 3,650,622). How-ever, past attempts to use corona grounding have confronted difficulties in maintaining the reference potential accurately when variations occur in system parameters such as atmospheric conditions, operating velocities, operating voltage levels, imaging member composition, etc. In view of these difficulties, it has been suggested to provide, as grounding means, a corona discharge device energized by a programmable D.C. power supply and means for adjusting such energization in response to a feedback signal ~rom an electrometer positioned to sense conductive layer portions moving therepast.
~7~7t3 SUMMARY OF THE INVENTION
-A significant purpose of the present invention is to provide functional and structural corona grounding approaches that improve upon the D.C. energization-feedback control system mentioned above. Morespecifically, the present invention provides, in corona grounding, the advantages of rapid response (to varia-tion of the conductive layer from the desired reference potential) and accurate stabilization of the conductive layer potential (to levels closely proximate the reference potential level). The present invention also affords advantages in flexibility as to operability with different (e.g. bipolar) imaging operation modes. A
further significant advantage of the present invention is that it is subject to sîmple and inexpensive structural implementations.
The above and other objectives and advantages are accomplished in accordance with the present inven-tion by providing for imaging apparatus of the kind adapted to form and develop an electrostatic image on an imaging member having a charge-retentive layer and an electrically conductive layer, improved means and method for controlling the potential of such conductive layer.
In one general aspect this improvement comprises directing an alternating corona current to the conduc-tive layer of an imaging member as it moves along the operative path of such apparatus, detecting the conduc-tive layer potential of such moving imaging member and varying the net corona current in accordance with the detected potential so as to maintain the conductive layer potential closely proximate a nominal reference potential level. In another general aspect such improvement comprises A.C. corona discharge means for directing alternating corona current to the conductive layer of such imaging member; adjustable means for varying such corona current; detector means for sensing, and providing a signal representative of~ the potential '7~3 level of successive portions of such imaging member's conductive layer; and control means for receiving such signal and adjusting said current-varying means to main-tain the conductive layer potential closely proximate a nominal reference potential level.
BRIEF ~ESCRIPTION OF THE DRAWINGS
The subsequent description of preferred embodi-ments of the present invention is made with reference to the attached drawings wherein:
Figure 1 is a schematic illustration of one embodiment of electrostatic imaging apparatus in accordance with the present invention;
Figure 2 is a schematic illus~ration showing details of one preferred corona device and adjustment 15 circuit in accordance with the present invention;
Figure 3 is a circuit d;agram illustrating one preferred a~plifier/filter and phase sensitive detector embodiment in accordance with the present invention;
Figure 4 is a circuit diagram illustrating one preferred signal processing and control embodiment in accordance with the present invention; and Fi~ure 5 is a graph illustrating the variation in net alternating corona current effected, for example, by the Fig. 2 embodiment.
DETAIL.ED DESC~IPTION OF THE PREFERRED EMBODIMENTS
The subsequent description pertains to a particular electrophotographic embodiment of the present invention; however, it will be appreciated by those skilled in the art that the advantages afforded by tne present invention also can be implemented in many other electrophotographic embodiments, as well as in other electrostatic imaging systems that utilize a reference potential on their image member's conductive layer.
The electrophotographic imaging apparatus 10 shown in Fig. 1 is one particularly adapted to form images on a strip imaging member 11, e.g. a 16m~ film.
Referring briefly to Fig. 2, it can be seen that the ~ 3 film 11 comprises a photoconductive insulator layer 12 adjacent an electrically conductive layer 13 on a film support 14. As shown in Fig. 1, the imaging film 11 is moved along an operative path of the apparatus 10 from a supply roll 15 past a pr;mary charging station 16, where a uniform electrostatic charge is applied upon the exterior surface of photoconductor layer 12. Downstream along the operative path from charging station 16 are an image exposure station 17 and a liquid development station 18 which function in the usual manner to form and develop latent electrostatic images. After the development station 18, a hot air fixing station 19 is provided to dry and fuse the toner image on the film.
The film is then fed to a storage or utilization means (not shown), e.g. a take-up reel. Rollers 20 and 21 support and transport the film along the operative process path ~f the apparatus. As stated earlier, in order to obtain uniform primary-charging and development of the film 11, it is necessary that its conductive layer 13 be coupled effectively to a source of reference potential. This is accomplished by corona discharge device 25 (see Fig. 2) which can be of a conventional construction and is disposed along an edge of the film path.
In accordance with the present invention, the corona discharge device 25 is energized with alternating current continuously controlled in accordance with a signal provided from detector means for sensing the potential of the film conductive layer 13. Such detec-tor means can comprise one of various kinds of commercial electrometers and is represented in Fig. 1 by probe and housing 30, and cooperating timing driver 31, amplifier/filter 32 and phase sensitive detector 33.
The signal provided by such detector means is applied to signal processing and control circuit 35 and corona adjustment circuit 26 which, in cooperation~ provide proper ener~ization of the corona discharge electrode 25 7~'73 to effect a net corona current to the conductive layer 13 that will maintain its potential closely proximate the desired nominal reference potential.
As noted, the electrometer can be of various conventional constructions. In one preferred configura-tion, the probe and housing 30 comprises an electro-magnet enclosed in a metallic housing which shields its electrical noise. Below the electromagnet, a thin piece of electrical shielding is provided. This piece preferably is non-ferromagnetic and of relatiYely low electrical conductivity so as to provide shielding from electrical noise but not attenuate the magnetic field generated by the electromagnet. A thin reed, which is made of magnetic material (e.g. springsteel or other magnetic alloys), is sandwiched between two pieces of electrical insulator and fastened below the electro-magnet. The operative film pa~h is configured so that film 11 passes below such reed with the photoconductor side facing the reed. This minimi~es the possibility of detection error due to static charges on the film support.
The electromagnet is driven by timing driver circuit 31, preferably at the resonant frequency of the magnetic reed; and the magnetic field generated by the electromagnet forces the magnetic reed to vibrate.
Since the vibrating reed is capacitively coupled to the conducting layer of the photoconductive film below it, the electrostatic potential of the layer can be detected from the vibrating reed. Thus, the amplitude of the signal detected from the vibrating reed depends on the magnitude of the electrical potential of the conducting layer, the displacement of the vibrating reed and the film-to-reed distance. The displacement of the vibrating plate is defined by the strength of the magnetic field and also the frequency of the timing driver. Therefore the amplitude of the signal detected in the vibrating reed represents the magnitude of the ~ ~'7~ 3 electrical potential of film 11, and the phase of the signal detected with respect to the timing driver gives the sign of the voltage of the film. That is, the phase of the signal detected with positive voltage on the film is 180 out of phase with the signal detected with negative voltage on the film.
Referring to Fig. 3, the signal from the vibrating reed is amplified and filtered in circuit 32 of the electrometer, wherein an FET operational ampli-fier 70 provides high input impedance for the signal anda band-pass filter 71 has a frequency centered at the frequency of timing driver 31. The amplified and filtered signal is fed to the input of an analog FET 72 in the phase sensitive detector 33, which, in coopera-tion with a capacitor 73 at its output end, acts as asample and hold circuit. The timing position of sample and hold is provided by a retriggerable multivibrator 74 which is controlled by timing driver 31. The detected signal from the band-pass filter 71 is converted into a D.C. voltage at the output of the analog FET 72, e.g. a positive D.C. voltage will show at the output of the analog FET 72 if the voltage on the film is positive and the reed is vibrating at its resonant frequency. This D.C. voltage which is proportional to the potential of the film's conductive layer is fed to an output control amplifier 75 in the phase sensitive detector circuit 33. A bias voltage 76 can also be provided on the out-put control amplifier if it is desired that the refer-ence potential on the conductive layer be other than ground.
The signal from phase sensitive detector 33 is input to signal processing and control circuit 35, one preferred embodiment of which is shown in more detail in Fig. 4. This embodiment CQmpriSeS an inverting ampli-fier circuit 36, a differentiator circuit 37 and anintegrator circuit 38 which are coupled in parallel between the input from phase sensitive detector 33 and ~7~ 73 an output inverting amplifier 39. The output o~
amplifier 39 is applied to a voltage to current converter circuit 40 which energizes a light emissive element 41, e.g. an LED, located in cooperative relation with a photoresistor 42 in corona adjustment circu~t 26.
Details of one preferred corona adjustment circuit are shown in Fig. 2. Thus high voltage trans-former 50 provides an output voltage of amplitude sufficient to excite the corona, e.g. in the order of 7 K.V. peak. The net corona current (current averaged over a full cycle) delivered by the charger is varied by controlling the amplitude of the ne~ative half cycle of voltage applied to the corona wire via the parallel high voltage diode 51 and variable resistance 42 circuit shown in Fig. 2. The simplicity of this control circuit depends on the physical phenomenon that a symmetric (unrectified) voltage applied to the corona wire pro-vides a strongly negative net corona current. Thus, when the resistance 42 in parallel with the diode 51 is at a low value, the corona delivers relatively high net-negative current. The diode 51 is forward biased during the positive half cycle of the energizing voltage, and as resistance 42 is increased, the negative half cycle of voltage applied to the corona wire is attenuated so that the net current becomes less nega-tive, going through zero, and becoming positive. The fixed resistor, 44, in parallel with the diode 51 and variable resistance 42 is provided to limit the maximum resistance of this branch so as to insure a path to dis-charge the capacitance of the charger in a time less~han about a half cycle. Resistance 44 is much greater than ~he resistance which will bias the corona to zero net current.
One device that can be used to provide the remotely controllable variable resistance is a "Photomod"~ Model CLM-9000 made by Clairex Corporation.
It contains a light emitting diode ~e.g. such as 41 in , 7~7~3~73 Fig. 2) and a cadmium-sulfide photoresistor (e.g. such as 42 in Fig. 2) in a package that provides high voltage isolation between the diode and the photoresistor. The function of this device could also be performed by an LED and a photoresistor fabricated as separate compo-nents. The net current delivered by the corona charger as a function of current in light emitting diode 41 is shown in Fig. 5.
Now consider the overall operation of these circuits. If it is desired to maintain the conductive layer 13 at ground potential, the bias of the output control amplifier of phase sensitive circuit 33 is adjusted so that its output is a predetermined voltage which maintains the collector current in the transistor output of the voltage-to-current converter 40 at a given level (when the conducting layer is at zero volts).
This given current level, operating through LED 41, adjusts the photoresistance of element 42 so that the corona output is balanced between positive and nega-tive. This balanced corona current is injected via anair gap to the conductive layer in the photoconductor maintaining its potential at ground. If the conductive layer in the photoconductor starts to have a positive potential, for example because of the charges of the toners or main corona charger, the detecting means will sense this change and, operating through signal processing and control circuit 35, provide a signal to the corona adjustment circuit 26 which turns photo-resistor 40 more "on", i.e. decreasing its resistance.
This causes injection of a predominantly negative corona to the conductive layer, driving the conductive layer back to the preset potential, zero in this case.
Similarly, if the film conductive layer 13 goes nega-tive, the detector means senses this condition and similarly turns the photoresistor less "on", i.e.
increasing its resistance. This creates a predominantly positive corona which drives the conductive layer to the 7~73 preset potential, zero in this case. If the conductive layer is to be maintained at a potential other than zero, the bias of the output control ampli~ier can be appropriately adjusted. Thus it can be seen that after selecting a desired nominal potential for the conductive layer of the film, the present invention will actively maintain the conductive layer closely proximate such nominal potential. This operation is independent, for example, of the polarity of the main corona charger used, the polarity of the toner, the film velocity and film width.
One other structural aspect of the Fig. 1 embodiment must be noted. It is highly desirable that rollers 20 located along the operative path in position to contact the surface of the photoconductive layer 12 be conductive and coupled to ground. This provides a capacitance between ground and conductive layer 13 that is much larger than the corona wire and the conducting layer. This is important to prevent the corona from inducing a large A.C. voltage on the conducting layer.
Considering the foregoing, it will be apprecia-ted by one skilled in the art that the present invention provides a simple, yet extremely flexible and accurate means for maintaining the conductive layer potential closely proximate a desired nominal reference potential level. The provision of an A.C. corona current to control the voltage of the conductive layer enables rapid response to a wide variety of operative system requirements, and the signal processing and control circuit modulates the signal provided by the charge detector means to create high stability for the servo-loop between the conductive layer and the corona.
Examples of the utility of such flexible feedback A.C.
grounding arrangements include modes of operation where it is desired to develop (or charge and develop) differ-ent segments of the film strip with different polarity or reference levels. Also, when operating in a run-out 7~7;3 condition (i.e. where the primary charger and exposure operations are terminated but it is desired to develop existing latent images), the requirements for the grounding corona will change rapidly and in large magni-tude. The present invention is uniquely adapted tohandle these special modes, as well as more common variations in operating parameters caused by changes in film velocity, humidity, etc.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
Claims (7)
1. In electrostatic imaging apparatus of the kind adapted to move an imaging member, having a charge-retentive layer and an electrically conductive layer, along an operative path past apparatus process stations which form and develop an electrostatic image pattern on the charge-retentive surface of such image member, improved means for providing reference potential on such conductive layer, comprising:
(a) A.C. corona discharge means for directing an alternating corona current to the conductive layer of an imaging member moving along said operative path;
(b) adjustable means, coupled with said corona discharge means, for varying the net corona current directed to the conductive layer;
(c) detector means, located along said operative path, for sensing the potential of conduc-tive layer portions moving therepast and providing an electrical signal indicative of such potentials;
and (d) control means for receiving said electri-cal signal and adjusting said current-varying means to maintain the potential of the conductive layer closely proximate a predetermined nominal reference potential level.
(a) A.C. corona discharge means for directing an alternating corona current to the conductive layer of an imaging member moving along said operative path;
(b) adjustable means, coupled with said corona discharge means, for varying the net corona current directed to the conductive layer;
(c) detector means, located along said operative path, for sensing the potential of conduc-tive layer portions moving therepast and providing an electrical signal indicative of such potentials;
and (d) control means for receiving said electri-cal signal and adjusting said current-varying means to maintain the potential of the conductive layer closely proximate a predetermined nominal reference potential level.
2. The invention defined in Claim 1 wherein said current-varying means includes resistance means and diode means coupled in parallel between said discharge means and terminal means adapted for connection to an energizing A.C. source, said diode means being disposed so as to be forward biased by positive half cycle energizing voltage and said resistance means being adjustable to different magnitudes.
3. The invention defined in Claim 2 wherein resistance means includes a photoresistor.
4. The invention defined in Claim 3 wherein said control means comprises a variable-intensity photo-emissive device located to illuminate said photoresistor.
5. The invention defined in Claims 1, 2 or 4 wherein said control means comprises proportional-differential-integrator means for processing the electrical signal from said sensing means whereby adjustment of said current varying means provides rapidly responsive, yet highly stable, maintenance of the conductive layer potential level.
6. The invention defined in claims 1, 2 or 4 wherein said detector means includes band-pass filter means.
7. In a method of electrostatic imaging of the kind in which an electrostatic image is formed and developed on a moving imaging member having a charge-retentive layer and an electrically conductive layer, the improvement for controlling potential on said conductive layer comprising:
(a) directing an alternating corona current to the conductive layer of the moving imaging member;
(b) detecting the electrical potential of the conductive layer of the moving imaging member; and (c) varying the net alternating corona current directed to the conductive layer in accordance with the detected potential so as to maintain the conduc-tive layer potential closely proximate a nominal reference potential level.
(a) directing an alternating corona current to the conductive layer of the moving imaging member;
(b) detecting the electrical potential of the conductive layer of the moving imaging member; and (c) varying the net alternating corona current directed to the conductive layer in accordance with the detected potential so as to maintain the conduc-tive layer potential closely proximate a nominal reference potential level.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US23228281A | 1981-02-06 | 1981-02-06 | |
| US232,282 | 1981-02-06 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1177873A true CA1177873A (en) | 1984-11-13 |
Family
ID=22872525
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000395475A Expired CA1177873A (en) | 1981-02-06 | 1982-02-03 | Electrostatic imaging apparatus and method providing stable reference potential |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1177873A (en) |
-
1982
- 1982-02-03 CA CA000395475A patent/CA1177873A/en not_active Expired
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