US 3631509 A
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United States Patent  Inventor Edward W. Marshall Saratoga, Calif.
 Appl. No. 838,628
 Filed July 2, 1969  Patented Dec. 28, 1971  Assignee Varian Associates Palo Alto, Calif.
 HIGH-SPEED COINCIDENT PULSE ELECTROGRAPHIC PRINTER WITH GRAY SCALE PRINTING CAPABILITY 8 Claims, 8 Drawing Figs.
 U.S.Cl 346/74 ES,
 Int. Cl ..G03g 15/00,
 Field of Search 346/74 ES;
 References Cited UNITED STATES PATENTS 3,076,968 2/1963 Schwertz 346/74 ES 3,182,333 5/1965 Amada 346/74 ES 3,188,649 6/1965 Preisinger 346/74 ES 3,208,076 9/1965 Mott 346/74 ES 3,298,030 l/l967 Lewis 346/74 ES X 3,469,028 9/1969 Yamamoto 346/74 ES X Primary Examiner-Bernard Konick Assistant Examinerl-loward W. Britton Attorney Leon F. Herbert ABSTRACT: The printer includes an array of electrographic styli electrodes disposed overlaying a backup electrode. An electrographic recording web is passed between the styli and the backup electrode. The recording web comprises a dielectric charge retention film facing the styli, such film being supported on a conductive paper backing which makes electrical contact with the backup electrode. A pulsed first writing potential is produced across the styli and the backup electrode, such first pulsed potential having a pulse amplitude and duration which is less than that which is sufficient to deposit a charge image on the recording medium but which has an amplitude and/or duration which varies in variable accordance with the density of the incremental charge image to be printed by a selective one of the styli on the recording web. A second pulsed potential is applied across the selected one of the styli and the backup electrode, such second pulsed potential being of generally invariant amplitude and/or duration and which, by itself, is insufficient to produce a charge image on the recording medium but which when combined with the first potential produces a total pulsed voltage of an amplitude and/or duration sufiicient to deposit an incremental charge image on the recording medium, such deposited charge image having an average charge density in accordance with the density of the image to be printed to produce a gray scale printing capability.
u ore DECODER SPECTRUM ANALYZER T i i i l l-l 5TYL| .liU'WaiFs TENS I HUNDREDS (I DECODER MATRIX I PATENTEU me an 3,631, 509
' SHEEI 1 BF 2 NTENSlTY CODE FREQUENCY ,NVENTOR EDWARD w. MARSHALL ATTORNEY HIGH-SPEED COINCIDENT PULSE ELECTROGRAPHIC PRINTER WITH GRAY SCALE PRINTING CAPABILITY DESCRIPTION OF THE PRIOR ART Heretofore, electrographic printers have been proposed wherein the amplitude of a pulsed potential applied across sequentially selected styli and a backup electrode is varied in amplitude for producing a gray scale printing capability. Such a printer is disclosed in US. Pat. No. 3,076,968 issued Feb. 5, 1963. Such a gray scale printing scheme is suitable when the duration of the printing pulses are long compared to the relaxation time of the recording medium. However, when an attempt is made to increase the speed of the printer such that the time available for printing an incremental charge image on the recording medium is comparable to or less than the relaxation time of the paper, which is generally on the order of 1 millsecond, the gray scale printing capability is lost. Thus, when employing electrographic recording paper of the type having a dielectric charge retentive film supported upon a conductive paper web, the bit rate at which information can be recorded on the paper, for gray scale printing capability utilizing the prior art technique, is on the order of l kilohertz. For many printer applications it is desirable to increase this recording bit rate by at least an order of magnitude while retaining gray scale printing capability.
It is also known from the prior art to superimpose two pulses across the sequentially selected styli and the backup electrode such that the total voltage of the combined pulses is sufficient to produce an incremental charge image on the recording medium, whereas neither of the two pulses is sufficient, by themselves, to deposit a charge image on the recording medium. Such a system has the advantage of reducing the voltage which must be pulsed by either one of the sources of printing potential. This system further provides the advantage of being able to employ the backup electrode as a portion of a binary data decoder circuit to reduce the complexity of the styli address circuitry. Such a printer is disclosed in US. Pat. No. 2,9l9,l7l issued Dec. 29, 1959. However, there is no teaching nor suggestion in this second reference of a printer having gray scale printing capability.
SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of a highspeed electrographic printer having gray scale printing capability.
One feature of the present invention is the provision, in a high-speed printer of pulse generators for generating sequences of first and second pulsed potentials to be combined across the sequentially addressed styli and a backup electrode, neither of pulsed potentials being of an amplitude and/or duration sufficient to deposit a charge image on the recording medium. The first potential is sequentially varied in amplitude and/or duration in accordance with the density of the incremental charge image to be printed. The combined potential of the pulses is sufficient to deposit an incremental charge image on the recording medium, such image having an average charge density in accordance with the density of incremental image to be printed, whereby gray scale printing capability is achieved.
Another feature is the same as the preceding feature wherein the generator for producing the first sequentially variable potential is capable of producing the various ones of a set of discrete pulse outputs of differing amplitude and/or duration in accordance with a sequence of different input signals to the voltage generator.
Another feature of the present invention is the same as the preceding feature wherein a binary coded signal, representative of the intensity of the incremental charge density to be deposited upon the recording medium is derived and fed to a decoder to produce the sequence of input signals to the voltage generator for generating the sequence of first pulsed potential.
Another feature of the present invention is the same as any one or more of the preceding features wherein a delay circuit is provided for delaying the application of the second pulsed potential across the selected stylus relative to the application of the first potential, whereby the first charge density establishing pulsed potential reaches equilibrium across the recording web before application of the second pulsed potential to the addressed stylus.
Another feature of the present invention is the same as any one or more of the preceding features including the provision of a bypass capacitor connected in parallel with the first pulse potential generator for bypassing the second pulsed potential around the first pulsed potential generator.
Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram, partly in block diagram form, of an electrographic printer showing the gray scale circuiting of the present invention,
FIG. 2 is a plot of signal amplitude versus time depicting the wave forms for various signals generated within the apparatus of FIG. 1,
FIG. 3 is a plot of recorded charge density D versus voltage V depicting the variation of charge density with voltage applied across the recording medium,
FIG. 4 is a schematic equivalent circuit for the electrographic recording paper,
FIG. 5 is an enlarged cross-sectional view of a stylus electrode and the backup electrode structure, as delineated by line 55 of FIG. 1,
FIG. 6 is an enlarged fragmentary cross-sectional view of a dielectrically coated recording paper,
FIG. 7 is a plot of charge image voltage e, developed across the dielectric layer of the recording paper versus time for three maximum values of voltage V,,,, V and V applied across the recording stylus and the backup member, and
FIG. 8 is a schematic diagram for an alternative embodiment of a portion of the structure of FIG. I delineated by line 8-8.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a high-speed electrographic printer incorporating gray scale printing capability of the present invention. The electrographic printer 1 includes an input analyzer 2, such as a spectrum analyzer, computer, or the like which operateson an input signal, such as a composite radiofrequency signal received from a sonobuoy or the like depicted by wavefonn A, which contains a plurality of simultaneous different Fourier components. The analyzer 2 operates upon the input signal A to separate the input signal into its various Fourier components having different amplitudes or energy intensities. The spectrum analyzer 2 repetitively time scans the input signal A and produces, for each scan, a set of sequential styli address binary codes. In a typical example, there are 500 styli across a 5 inch wide web 4 and the binary code is in terms of units, tens and hundreds. Each styli address code signal corresponds to a different frequency component if any in the signal being scanned. The array of electrographic writing styli 3 is arranged across the dielectric charge retentive surface of an electrographic recording web 4 (see FIG. 6) which is pulled past the array of styli 3 from a supply roll 5 via a motor-driven paper pulling set of drive wheels 6 which engage the recording web 4.
The binary styli address code output of the spectrum analyzer is fed to a decoder matrix 7 which decodes the styli address code and sequentially energizes the addressed stylus in the array of styli 3.
The spectrum analyzer 2 produces a second output at 8 which comprises a sequence of binary coded output signals representative of the intensity of the respective possible spectral lines. The intensity code is synchronized with the styli address code. The binary intensity code corresponds to the intensity of the charge image to be deposited upon the recording medium 4.
The entire range of gray scale to be printed is subdivided into a number of discrete levels in accordance with the gray scale resolution to be recorded. In a typical example, the gray scale range to be recorded is subdivided into eight levels of intensity. However, the gray scale range can be subdivided into a greater number or a lesser number of discrete levels, depending upon the intensity resolution desired. The binary intensity code signal is fed to a decoder matrix 11. The decoder 11 decodes the binary intensity coded input and produces an output at 12, there being one output 12 for each of the possible intensity levels to be recorded. For the case illustrated where there are 8 possible intensity levels, there are 8 output lines 12 from the decoder 11. Only one of the output lines 12 is shown in its entirety.
The output lines 12 are fed to the input of a pulsed intensity voltage driver or generator 13 for generating an output voltage pulse, as of within the range of +300 to +800 volts amplitude and 60 ,usec. duration, which varies in amplitude and/or duration (energy) in accordance with the particular gray scale intensity to be recorded. The pulse output of the intensity voltage generator 13 as applied to a channel shaped backup electrode 14 which extends across and below the length of the styli array 3 and which makes electrical contact with the conductive paper back of the recording web 4. The arrangement of the backup electrode 14 relative to the styli is shown in greater detail in FIG. 5, where a typical spacing between the upright legs of the channel is A inches. The output of the pulse generator 13 is superimposed upon a fixed bias potential, as of +100 volts derived from a fixed voltage generator 15.
The styli electrodes 3 are selectively pulsed from +300 volts to ground potential via a second pulsed voltage source 16. The second pulsed voltage source includes a source of positive DC voltage 17 as of +600 volts which is connected across a voltage divider network comprising series connected resistors 18 and 19, as of l megohm resistance each. A bus 21 is connected to the center of the voltage divider such that +300 volts is applied to the bus 21. Each stylus electrode 3 is connected to the bus 21 via a series load resistor 20, as of l megohm each. In the nonwriting condition, the impedance of the electrical path from each stylus 3 through the recording web 4, first voltage generator 13 and bias source 15 to ground is much greater than the 1 megohm impedance of resistor 19 such that the +300 volts on bus 21 appears at each stylus 3. A gating transistor 22 is connected in shunt with each stylus 3 to ground. In the nonwriting condition, the gating transistors 22 are biased in the nonconducting state via the outputs from the decoder matrix 7 applied to their control electrodes such that the gates are off and present a very high impedance to ground in shunt with each stylus 3. However, when the output from the decoder matrix is such as to select one of the styli 3 for writing, the respective gating transistor 22 is turned on" to open the respective gate to ground, thus immediately dropping the potential on the selected writing stylus 3 to ground potential. In this manner, the styli 3 are selectively and sequentially pulsed from +300 volts to volts at the bit rate of the styli address decoder in accordance with the output signal of the address decoder matrix 7.
The equivalent circuit for the paper filled gap between the stylus 3 and the backup electrode 14 is shown in FIG. 4. More particularly, even though the dielectric charge retentive surface of the recording paper 4 is in nominal contact with the styli electrodes 3, there exists a minute airgap therebetween, as of l to 11. wide, and represented by capacitor 23. The dielectric charge retentive layer of the paper can be represented by a capacitor 24 connected in series with the capacitance of the airgap 23. The conductive paper backing also has a bulk capitance, indicated by capacitor 25 and a bulk resistance take in the normal direction through the paper web 4, as indicated by resistor 26. This normal component of resistance is in shunt with the capacitance of paper. Also, because the conductive electrode 14 does not make electrical contact with the conductive side of the paper 4 in registration with the end of the styli 3, there is a horizontal component of bulk capacitance of the paper 4, as indicated by capacitor 27 and horizontal components of resistance of the paper 4, as indicated by resistors 28 and 29.
In order to deposit a charge image on the dielectric recording surface, the airgap 23 must be broken down. The airgap will break down when the voltage applied between electrodes 3 and 14 exceeds a certain gap breakdown voltage, typically on the order of 400 volts. It is found that the density of the charge image deposited upon the dielectric charge retentive surface of the recording medium, upon breakdown of the airgap, is proportional to the amount and time that the applied voltage between electrodes 3 and 14 exceeds the breakdown voltage V,,. See FIG. 3 where charge density is plotted as a function of applied voltage V for a given time which exceeds the relaxation time of the recording paper.
In the nonwriting condition, each stylus is operating at +300 volts and the backup electrode is operating at within the range of +400 to +700 volts depending upon the pulse amplitude of the output of voltage generator 13. Therefore, the combined voltage difference across the writing gap between the styli 3 and the backup electrode 14 is in the range of l00 volts to 400 volts. This voltage is insufficient to produce a discharge across the airgap for charge transfer from the stylus to the dielectric surface of the recording web 4.
Upon selection of one of the styli 3 for recording an incremental charge image, a selected one of the outputs of the decoder matrix 7 turns on the respective gate 22, thereby shunting the selected stylus to ground. This breaks down the airgap and produces a transient current flow through the circuit including the airgap and various capacitances and resistances of the recording paper 4 and through bypass capacitor 31, as of 0.001 microfarads, which bypasses the intensity voltage generator 13. The transient current flow deposits the charge image on the charge retentive surface of the recording web 4.
In this manner, the pulse gated voltage derived from voltage generator 16 is combined with the pulsed voltage derived from the intensity voltage generator 13 to produce a total voltage drop across the paper which exceeds the breakdown voltage V,, of the airgap by an amount which varies in accordance with the density of the incremental charge image to be deposited upon the recording medium. The pulses of charge density determinative voltage, as derived from the intensity voltage generator 13, are synchronized with the addressed styli such that the sequentially deposited incremental charge images have average charge densities in accordance with the intensity of the spectral lines to be recorded.
The charge image pattern deposited upon the recording web 4 pass under an inking or toner channel 30 wherein charged pigmented toner particles are attracted to the charge images for developing same. The developed charge images appear at 32 on the recording medium thereby forming permanent recording or printout of the data supplied by the analyzer 2. In the particular example shown in FIG. 1, the spectral lines are recorded across the recording medium 4 with a density corresponding to the intensity of the various spectral lines of the spectrum being analyzed. A typical spectrum corresponding to the printout is depicted as B in FIG. I. The various lines have an intensity which corresponds to the power in each of the various spectral lines of spectrum B.
Referring now to FIG. 2, the various waveforms generated within the printer I are shown. More particularly, as shown by waveform (a), each of the styli address code signals has a bit period of approximately microseconds. The intensity address code 8, as indicated by waveform (b), is synchronized with the styli address code. The charge density writing voltage output V,,of the pulsed voltage generator 13 has a waveform, as indicated at (c), and comprises a sequence of voltage pulses each of approximately 66 microseconds duration and of an amplitude in accordance with the intensity or density of the incremental charge image to be recorded. The charge density determinative voltage pulses are synchronized with the styli address code outputs by means of a selection trigger pulse having a waveform indicated by waveform (d). The trigger pulse is fed from the spectrum analyzer 2 to the intensity voltage generator 13 via lead 32 (see FIG. 1).
Due to the time constant of the recording paper 4 caused by the capacitance of capacitors 25 and 27 and resistors 26, 28 and 29, of FIG. 4, it is desirable that the selected writing stylus 3 not be energized until the charge density determinative voltage V reaches a near equilibrium potential across the recording paper 4. Therefore, energization of the selected stylus 3 via gate 22 is delayed relative to the application of the density voltage V,, by a suitable time period, as of 33 microseconds. This delay is achieved by means of a blanking or disabling pulse derived from the spectrum analyzer 2 and fed to decoder matrix 7. The blanking signal is indicated by waveform (e). The gated waveform derived from voltage generator 16 and applied to the selected stylus 3 is indicated by waveform (I).
As an alternative to varying the intensity of the charge density determinative voltage V,,, as applied to the backing electrode l4, the duration of the pulse of charge density determinative voltage V may be varied, as indicated by the dotted lines 33 in the first pulse of the charge density voltage waveform (c). It is also possible to vary not only the duration but also the amplitude of the charge density determinative voltage pulses to achieve control over the charge density of the incremental charge image which deposit upon the recording paper 4. Actually, the intensity of the charge image deposited upon the recording paper 4 is determined by the voltage e of the incremental charge image stored on capacitor 24 of the dielectric film. This stored voltage e,, is a function not only of the applied voltage V,, but also of the time during which the charge density determinative voltage V,, is applied across the paper, assuming the time is less than the relaxation time of the paper 4, as indicated in FIG. 7. Thus, a relative high applied voltage V,,, can be applied for a short time t, to achieve the given charge density voltage e or a lesser voltage V could be applied for a slightly longer time 1 or a still lower voltage V may be applied for a longer time to achieve a given incremental charge image density voltage e as indicated in FIG. 7. Thus, a certain charge density determinative voltage V may b applied for variable lengths of time to achieve charge density control in the incremental charge images deposited upon the recording paper 4, as indicated by lines 33 of waveform (c) of FIG. 2.
Referring now to FIG. 8, as an alternative to coding and decoding the charge density information for feeding to the intensity voltage generator 13, this charge density information may be derived directly from an analog type signal by means of a sample hold circuit. More specifically, the Fourier components derived from the time domain composite input signal (A) fed to the spectrum analyzer 2 may appear as an analog output signal of the spectrum analyzer 2, at 8, in the time domain, where the time base of the signal corresponds to a frequency scan. This spectrum signal is then sampled at certain time displaced intervals, as of at 100 ,usec. intervals, corresponding to certain discrete frequencies in the frequency scan by a sample and hold circuit 41. The samples are taken at suitable intervals such as once every I usec. indicated in waveform (c) and the sampled amplitude is held for a suitable interval, such as 66 usec, as indicated in waveform (c). The sampled and held signal is fed to an amplifier 42 which replaces voltage generator 13 in the circuit of FIG. 1. The amplifier 42 amplifies the sampled and held signal to a suitable voltage level and/or duration as indicated in waveform (c) to form the charge density determinative voltage pulses. These pulses are synchronized with the styli address code outputs by means of a selection trigger pulse (d) generated within the spectrum analyzer 2, as aforedescribed. Thus the sample and hold circuit 41 and the amplifier 42 form a pulse voltage generator 43 which applies a voltage to electrode 14 such voltage pulses having an energy in proportion to the charge density of the charge images to be recorded.
As an alternative to a strip printer I as shown in FIG. I, the charge density control feature of the present invention is also applicable to other types of printers such as X-Y printers, character printers, etc., wherein the recording paper is disposed in-between first and second writing electrode structures which replace electrodes 3 and 14, respectively. The recording medium 4 may be stationary or moving depending upon the particular type of printer.
The high-speed electrographic printer with gray scale printing capability and employing the wave forms of FIG. 2 produces a printing bit rate of 10 kilohertz which is approximately an order of magnitude faster than a similar printer utilizing only one pulsed source of writing potential which is variable in amplitude in accordance with the intensity of the incremental charge image to be deposited, as exemplified by the prior artprinter disclosed in the aforecited U.S. Pat. No. 3,076,968.
Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
What is claimed is:
I. A high-speed electrostatic printer responsive to an input signal to form a charge image on a charge recording medium having a charge retentive surface and a conductive backing, the charge image having a definite number of discreet charge intensities in response to the amplitude of the input signal, comprising the combination:
an array of writing electrodes;
a second electrode proximate the array;
means for retaining the recording medium proximate the array of writing electrodes with the charge retentive surface adjacent the writing electrodes and with the conductive backing in contact with the second electrode;
means for applying a series of intensity pulses to the second electrode having amplitudes selected from a distinct number of predetermined discreet levels in response to the amplitude characteristics of the input signal, the intensity pulses being insufficient to cause charge image formation on the recording medium;
means for applying a series of write pulses to selective ones of the array in response to the time characteristics of the input signal, the write pulses being insufficient to cause charge image formation on the recording medium but which are in synchronism with the intensity pulses and the summation of the intensity pulses plus the write pulses being sufficient to form a charge image on the recording medium;
means for delaying the application of the write pulses with respect to the intensity pulses to permit the intensity pulse to pass at least a portion of the transients therein caused by the relaxation characteristics of the recording medium in order to minimize the effect of these transients on the charge image; and
means for moving the charge recording medium with respect to the array so that a charge image is formed thereon as the writing electrodes are selectively pulsed in coincidence with the second electrode.
2. The high-speed electrostatic printer of claim 1, wherein the second electrode is electrically biased in favor of writing and the amplitude of the intensity pulses is correspondingly reduced to minimize the transients in the intensity pulses.
3. The high-speed electrostatic printer of claim I, wherein the intensity pulse may be varied in amplitude to adjust the relative intensities of the discreet charge image intensities.
4. The high-speed electrostatic printer of claim I, wherein the intensity pulse may be change in duration to adjust the relative intensities of the discreet charge image intensities.
5. The high-speed electrostatic printer of claim 1, wherein the means for applying the intensity pulses to the second electrode generates a binary code in response to the amplitude of the input signal and includes a decoder responsive to the binary code and having a separate output corresponding to each of the discreet intensities of charge, and further includes a voltage pulse generator responsive to the outputs of the decoder for applying the intensity pulses to the second electrode.
6. The high-speed electrostatic printer of claim 1, wherein the means for applying a series of write pulses to the writing electrodes generates a binary code in response to the time parameter of the input signal, and includes a decoder matrix having a separate output for each of the writing electrodes which selectively apply writing pulses to the writing electrodes.
7. The high-speed electrostatic printer of claim 1, wherein a bypass capacitor is connected in parallel with the means for applying the intensity pulses for bypassing the write pulses around the means for applying intensity pulses to the second electrode.
8. The high-speed electrostatic printer of claim I, wherein the means for applying intensity pulses to the second electrode includes a sample-and-hold circuit for periodically sampling the amplitude of the input signal and for holding a potential in proportion to the amplitude thereof for the pulse length of the intensity pulse.