GB2095625A - Electrographic recording - Google Patents

Abstract

An electrographic recording device for controlling the dot size of electrostatic latent charge patterns deposited upon an insulator 22 by an induced gaseous discharge across a gap between the insulator and an electrode 10 regulates the delivery of charging current to the electrode so that the potential thereof gradually rises above the discharge threshold level. <IMAGE>

Description

SPECIFICATION Gaseous discharge electrography This invention broadly relates to an electrographic recording device of the type wherein an electrostatic charge pattern is deposited by gaseous discharge upon an adjacent insulator. More particularly, it is directed to an arrangement for regulating the formation of the gaseous discharge so that the size of the latent images is controlled.

Electrographic printing encompasses processes in which an electrostatic latent image is formed directly upon the surface of an insulative material and subsequently made visible by a development process. In what may be considered classical electrography, a pattern of charge-bearing spots is produced upon an insulating film bonded to a paper base, i.e., electrographic paper, by an array of small diameter charging electrodes assembled into a print head. The latent image is developed directly upon the electrographic paper with a liquid or dry developer to produce the final hard copy.

A known variant of the foregoing process is transfer electrography. In this process, the latent image is formed upon a reusable, intermediate surface and developed thereon. The developed image is then transferred to a final substrate, such as plain paper, where it is fixed. The reusable record medium is a cycling insulating surface supported by either a conductive drum or a conductive web.

In the above-mentioned processes, the latent images are formed by charge deposition. Another modification of the electrographic process employs a charge removal method for latent image formation. In this princess, the insulating layer is uniformly precharged to a given potential, e.g. +400 volts. Thereafter, application of a negative potential, e.g. -300 volts, to the pin electrodes produces gaseous discharge and selective removal of the surface charge, yielding development sites.

For selective control of the charging electrodes, prior art electrographic recording devices typically employ coincident drive techniques. For example, U.S. Patent No. 3,483,566 discloses an electrographic printing device in which the threshold potential for electrode discharge is developed by the coincident application of two high level voltages from separately switched voltage sources. To achieve this effect, impedance elements are included between the respective voltage sources and backing electrodes. In U.S.

Patent No. 2,955,894, electrostatic images are formed upon a charge-retentive continuous web which passes between a plurality of pin electrodes and opposed anvil or backing electrode. Discharge is effected only upon establishment of a difference in potential between selected pins and the backing electrode.

A conductive drum based printer is shown in U.S. Patent No. 3,208,076. Charactercorresponding charge patterns are deposited upon an insulating web when the concurrence of voltage pulses from first and second pulse generators exceeds the discharge initiating threshold. Preferred and modified forms of the disclosed device include various resistive and capacitive coupling of the pulses to the print electrode.

U.S. Patent No. 4,030,107 shows another form of electrographic recording device in which discharge of the record electrode is occasioned by the coincident summing of voltages from two separate sources. The first voltage source is connected to the pin-shaped electrode via a resistor while the second voltage source is connected via capacitive coupling from an electrostatic induction plate which is disposed closely adjacent the record electrode or its associated lead wires. The resultant resistive capacitive combinations provide integration of current from the first voltage source and differentiation of current from the second voltage source. The sum of the voltages so provided exceeds the threshold value required for gas discharge.

These electrographic processes have enjoyed commercial success in printing devices. Factors contributing to this include amenability to digital input and the capability of providing high speed charge deposition and, hence, high speed copy output. Historically, however, devices employing electrographic techniques have experienced limitations. Notable among these is image definition. Also referred to as resolution, this parameter is most often expressed in terms of dots, or pixels, per unit length. For many printing or plotting applications, image resolution on the order of 100 or 200 dots per inch is quite satisfactory. However, in high quality imaging systems, such as a copy quality printer, it is desirable to have pixel densities in the range of 300 to 400 dots per inch.Attempts to produce dots with dimensions small enough to satisfy these high density criteria have not been totally satisfactory.

At a given air gap between a charging electrode and an insulative record medium of given dielectric thickness, the pixel size or dot diameter, is determined primarily by the air gap field intensity. It is known that dot diameter can be controlled by adjusting the various parameters of the process. For example, increasing the gap between the electrode and the recording medium will increase dot diameter. Reduced dot diameter can be obtained by increasing the field intensity through reduction of the applied voltage.

In U.S. Patent No. 3,460,156 control of the size of deposited charged spots is accomplished by a fine mesh screen grid positioned parallel to, and closely spaced from, a planar array of selectivelyenergized pin electrodes. Negative biasing of the grid relative to the electrode which backs the receptor reduces spreading of the ion stream produced by the discharge which occurs when a pulse negative with respect to the grid is applied to a selected one of the pins.

In a typical configuration, discharge is occasioned by applying an energizing pulse of high potential to a pin electrode while simultaneously applying a bias potential to a base electrode located immediately behind the insulative recording medium. Varying either pulse or bias, or both, effects a control over the size of the formed dots. Because of the charging dynamics of the discharge process, efforts to reduce dot size by lowering the applied voltages have not been entirely satisfactory. One reason for this is that, in a practical electrographic printer, to produce reliable discharge of all pins within the printhead matrix, and to provide deposition of charge sufficient for subsequent development, it is necessary to apply voltages which are in excess of the discharge threshold potential.Typically, the sum of the bias potential and of the pulse amplitude impressed across the gap is substantially higher than the discharge threshold level. Thus, the potential applied to a specific pin is, in many instances, much greater than that necessary to initiate discharge and provide deposition of the requisite charge. Consequently, oversized dots are frequently formed. Reducing the applied voltages is an apparent solution to this problem. It has been found, however, that at voltages which give the smallest dot diameter, discharge reliability is poor; i.e. there is not reliable discharge of all pins intended to be discharged.

In accordance with the present invention, there is provided an electrographic recording device of the type where a latent electrostatic charge pattern is deposited upon an insulator by a gaseous discharge produced by an electric field developed between a charging electrode positioned apart from the insulator and a biased conductor connected with the insulator on the side remote from the electrode. The device includes means for providing a gradual voltage increase across the gap formed between the electrode and insulator so that the electrostatic latent images are of controlled dimension. In one embodiment of the invention, controlled regulation of the potential delivered across the gap is provided by a parallel resistor capacitor network which couples the discharge producing potential to the charging electrode.According to another embodiment of this invention, an array of plural electrodes, or pins, are assembled into a charging head. Each pin circuit includes a parallel capacitor-resistor couple for control of discharge on a specific, per driver basis.

In the foregoing embodiments, the inclusion of the capacitor with a shunt resistor provides an internal control over the applied discharge potential which limits the development of a large overvoltage across the air gap. In response to an applied pulse, the addition of a capacitor in series with the pin provides an instantaneous voltage division between the added capacitor and capacitance of the gap. Because of this capacitive division, the initial voltage across the air gap is below the threshold level, i.e. below the level required to establish a discharge-initiating electric field. Subsequent to this initial voltage division, energy present in the applied pulse is stored, as charge, upon the inserted capacitor. The shunt resistor thereafter allows delivery of this stored energy, over time, to the gap.Consequently, the voltage across the gap increases gradually above threshold level, resulting in gas discharge and deposition of charge upon the insulator.

Preferably, the resistor-capacitor combination is selected to provide a pin drive voltage which initially spikes just below threshold level and thereafter rises slowly through the minimum breakdown voltage.

The invention will now be described by way of example with reference to the accompanying drawing, in which: Figure 1 is a schematic representation of a recording device according to the invention.

Figure 2 is a simplified equivalent circuit for a single charging electrode.

Referring to Figure 1, there is schematically shown an electrographic recording device according to the present invention in which charging electrodes, or pins, 10 are assembled into an array. For clarity and convenience, an illustrative linear array of three pins is shown. It will be understood that any number of pins may be included in the array and that schemes other than the linear matrix shown in Figure 1 may be employed. Since, as more fully developed hereinafter, the techniques of the instant invention can provide developed dot diameters on the order of 120 microns or less, print heads embodying the invention can be constructed to achieve pixel densities acceptable for high resolution systems.

Each electrode 10 is positioned spaced from a recording member 20, which comprises an insulator 22 and a backing conductor, or electrode 24. As shown, conductor 24 is held at ground reference potential. It will be appreciated that suitable electrical biasing techniques can be utilized to maintain conductor 24 at a reference potential other than ground. As well, resort may be had to the prior art coincident drive and pulsing techniques in which discharge is initiated by the summation of potentials simultaneously applied to the charging and backing electrodes. Known systems employing plural backing electrodes and multiplexing techniques could likewise be used in lieu of the single conductor 24. It will be appreciated further that resort could be had to the biasing and recording methodology of the charge removal technique mentioned hereinabove. As suggested in Figure 1 , in addition to serving as a terminal for the electric field originating from electrode 10, conductor 24 may provide physical support for insulator 22 or be integrally constructed therewith. For example, in transfer electrography, a reusable record medium would be formed with the insulator overlying a conductive web or conductive drum.

Voltage source 30 generates electric pulses which are switched by pin selector 40 to energize selected electrodes 10, producing gas discharge and deposition of charge upon the surface of insulator 22. For an operating recording device, the generated pulses must be of sufficient magnitude, relative to the reference potential applied to conductor 24, to produce discharge and deposit charge needed for the subsequent development process. The field-inducing energy within the generated pulses is regulated for delivery to electrodes 10, on a per driver basis, by individual control networks associated with each electrode. In preferred form, these networks comprise a capacitor 12 in parallel with a resistor 1 4.

Figure 2 is a simplified equivalent circuit for one of the driven electrodes of Figure 1 using lumped values to represent the various distributed capacitances. For illustrational purposes the effects of stray capacitances between adjacent electrodes and between the individual electrodes and ground are assumed to be negligible, and, hence, are eliminated from the equivalent circuit.

In Figure 2, pulse source 30, resistor 14, and capacitor 1 2 correspond to the counterparts thereof in Figure 1. The function performed by pin selector 40 is represented in Figure 2 as switch 42. Capacitor 1 6 represents the gap capacitance between electrode 10 and the surface of insulator 22 while capacitor 1 8 represents the dielectric capacitance of the insulator. During creation of the electric field, capacitor 1 8 undesirably consumes, by voltage division, a portion of the applied potential. It is thus preferable to utilize an insulator having a dielectric thickness which will minimize this effect.

Upon application of a pulse from source 30, there is a voltage division among capacitors 12, 16, and 18. Since, with proper selection of insulator 22, the impact of capacitor 1 8 can be reduced, the applied voltage substantially appears across the inserted capacitor 12 and the gap capacitance 1 6. As previously noted, this capacitive voltage division initially holds the potential across the gap formed between electrode 10 and insulator 22 below the discharge threshold level. Subsequent bleeding through resistor 14 of the charge stored on capacitor 12 allows the potential across the gap, i.e. across capacitor 1 6 in Figure 2, to rise slowly to breakdown.

A dynamic web based device was constructed in accordance with the invention. The print head comprised an array of pins, each pin being approximately 0.05 x 0.05 mm and spaced 0.05 mm apart from the adjacent pins. The insulative charge receptor was a web of 0.05 mm thick white-pigmented polyvinyl fluoride film. An air gap of approximately 25 microns was maintained between the pins and the insulative web. Gas discharge was initiated by impressing pulses of -750 volts relative to a grounded backing electrode. For dot size comparison, selected pins were first directly driven with the applied pulse. Subsequently the same pins were driven in accordance with the invention through a resistor-capacitor couple. The -750 volt pulses were 3 microseconds wide and repeated at a 10 millisecond rate.The directly-driven pins yielded liquid-developed dots having diameters equal to or greater than 200 microns. The same pins, coupled with capacitors, having values between 3.3 and 10 picofarads, which were shunted with 1 to 10 meg ohm resistors, produced developed dots between 75 microns and 1 75 microns in diameter. Further experiments with the device revealed that dot diameter is unaffected by pulse width over the range of about 25 microseconds to 0.2 microseconds. Increasing the repetition rate, i.e. the time between discharges, was found to increase the discharge reliability.

1. An electrographic recorder having a plurality of discharge-producing electrodes, means for applying to selected electrodes a voltage greater than the discharge threshold level between the selected electrodes and an adjacent conductor, and means for controlling the rate of rise of the voltage of each such electrode.

2. The recorder of Claim 1, wherein the controlling means comprises a passive linear network.

3. The recorder of Claim 2, wherein said passive linear network comprises a resistor and a capacitor connected in parallel.

4. A recorder as claimed in any preceding claim, including means for regulating the delivery of charging current to said electrode so that the potential thereof is held initially below discharge threshold and thereafter rises at a chosen rate to a value above threshold.

5. A recorder as claimed in any preceding claim, in which its electrodes are spaced uniformly from an insulator integral with, or supported by, a remote conductor held at a reference potential.

6. An electrographic method for depositing a latent electrostatic charge pattern upon an insulator comprising the steps of: (a) spacing an electrode from said insulator to form a gap therewith; (b) positioning a conductor on the side of said insulator remote from said electrode; (c) controlling the potential of said conductor; (d) generating a discharge-producing electric pulse, and (e) delivering said pulse to said electrode in a manner such that the potential across said gap is held initially below discharge threshold and thereafter rises above threshold.

7. The method of claim 6, wherein the step of delivering said pulse includes coupling a pulse generator to said electrode through a passive linear network.

8. The method of claim 7, wherein said passive linear network comprises a resistor and a capacitor connected in parallel.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. insulator 22. For an operating recording device, the generated pulses must be of sufficient magnitude, relative to the reference potential applied to conductor 24, to produce discharge and deposit charge needed for the subsequent development process. The field-inducing energy within the generated pulses is regulated for delivery to electrodes 10, on a per driver basis, by individual control networks associated with each electrode. In preferred form, these networks comprise a capacitor 12 in parallel with a resistor 1 4. Figure 2 is a simplified equivalent circuit for one of the driven electrodes of Figure 1 using lumped values to represent the various distributed capacitances. For illustrational purposes the effects of stray capacitances between adjacent electrodes and between the individual electrodes and ground are assumed to be negligible, and, hence, are eliminated from the equivalent circuit. In Figure 2, pulse source 30, resistor 14, and capacitor 1 2 correspond to the counterparts thereof in Figure 1. The function performed by pin selector 40 is represented in Figure 2 as switch 42. Capacitor 1 6 represents the gap capacitance between electrode 10 and the surface of insulator 22 while capacitor 1 8 represents the dielectric capacitance of the insulator. During creation of the electric field, capacitor 1 8 undesirably consumes, by voltage division, a portion of the applied potential. It is thus preferable to utilize an insulator having a dielectric thickness which will minimize this effect. Upon application of a pulse from source 30, there is a voltage division among capacitors 12, 16, and 18. Since, with proper selection of insulator 22, the impact of capacitor 1 8 can be reduced, the applied voltage substantially appears across the inserted capacitor 12 and the gap capacitance 1 6. As previously noted, this capacitive voltage division initially holds the potential across the gap formed between electrode 10 and insulator 22 below the discharge threshold level. Subsequent bleeding through resistor 14 of the charge stored on capacitor 12 allows the potential across the gap, i.e. across capacitor 1 6 in Figure 2, to rise slowly to breakdown. A dynamic web based device was constructed in accordance with the invention. The print head comprised an array of pins, each pin being approximately 0.05 x 0.05 mm and spaced 0.05 mm apart from the adjacent pins. The insulative charge receptor was a web of 0.05 mm thick white-pigmented polyvinyl fluoride film. An air gap of approximately 25 microns was maintained between the pins and the insulative web. Gas discharge was initiated by impressing pulses of -750 volts relative to a grounded backing electrode. For dot size comparison, selected pins were first directly driven with the applied pulse. Subsequently the same pins were driven in accordance with the invention through a resistor-capacitor couple. The -750 volt pulses were 3 microseconds wide and repeated at a 10 millisecond rate.The directly-driven pins yielded liquid-developed dots having diameters equal to or greater than 200 microns. The same pins, coupled with capacitors, having values between 3.3 and 10 picofarads, which were shunted with 1 to 10 meg ohm resistors, produced developed dots between 75 microns and 1 75 microns in diameter. Further experiments with the device revealed that dot diameter is unaffected by pulse width over the range of about 25 microseconds to 0.2 microseconds. Increasing the repetition rate, i.e. the time between discharges, was found to increase the discharge reliability.

1. An electrographic recorder having a plurality of discharge-producing electrodes, means for applying to selected electrodes a voltage greater than the discharge threshold level between the selected electrodes and an adjacent conductor, and means for controlling the rate of rise of the voltage of each such electrode.

2. The recorder of Claim 1, wherein the controlling means comprises a passive linear network.

3. The recorder of Claim 2, wherein said passive linear network comprises a resistor and a capacitor connected in parallel.

4. A recorder as claimed in any preceding claim, including means for regulating the delivery of charging current to said electrode so that the potential thereof is held initially below discharge threshold and thereafter rises at a chosen rate to a value above threshold.

5. A recorder as claimed in any preceding claim, in which its electrodes are spaced uniformly from an insulator integral with, or supported by, a remote conductor held at a reference potential.

6. An electrographic method for depositing a latent electrostatic charge pattern upon an insulator comprising the steps of: (a) spacing an electrode from said insulator to form a gap therewith; (b) positioning a conductor on the side of said insulator remote from said electrode; (c) controlling the potential of said conductor; (d) generating a discharge-producing electric pulse, and (e) delivering said pulse to said electrode in a manner such that the potential across said gap is held initially below discharge threshold and thereafter rises above threshold.

7. The method of claim 6, wherein the step of delivering said pulse includes coupling a pulse generator to said electrode through a passive linear network.

8. The method of claim 7, wherein said passive linear network comprises a resistor and a capacitor connected in parallel.