US3818493A

US3818493A - High speed xerographic printer

Info

Publication number: US3818493A
Application number: US00255387A
Authority: US
Inventors: W Slack
Original assignee: DYK RES CORP VAN
Current assignee: DYK RES CORP VAN; VAN DYK RES CORP US
Priority date: 1972-05-22
Filing date: 1972-05-22
Publication date: 1974-06-18
Anticipated expiration: 1991-06-18

Abstract

A printing apparatus employing a rotating drum having a surface coating comprising a material which exhibits electron beam induced conductivity characteristics. The drum surface rotates through a vacuum chamber in which an electron beam scanning apparatus is situated. The electron beam surface is uniformly charged by suitable means, and is selectively discharged by the scanning electron beam in accordance with a desired pattern to form a corresponding charge pattern on the electron beam sensitive surface. This charge pattern is developed and transferred to a paper surface by xerographic techniques.

Description

United States Patent [1 1 Slack HIGH SPEED XEROGRAPHIC PRINTER [75] Inventor: William Frederick Slack, Andover,

[73] Assignee: Van Dyk Research Corporation,

Whippany, NJ. 22 Filed: May 22, 1972 [21] Appl. No.: 255,387

[52] US. Cl. 346/74 EB, 355/20, 346/74 CR [51] Int. CL... G03b 27/10, G03g 15/00, H04n 5/80 [58] Field of Search 346/74 EB, 74 ES, 74 P, 346/74 CR; 118/33, 33 X, 637, 637 X; 226/47; 355/20, 3

[56] References Cited UNITED STATES PATENTS 2,616,961 11/1952 Groak 178/5.2 2,630,484 3/1953 Groak 178/5.2 2,972,660 2/1961 Toulon l78/7.2 2,982,822 5/1961 Bacon 179/100.2 3,099,710 7/1963 Moller 178/6.6 3,205,301 9/1965 Etcheverry 178/66 ROLLER DRIVE MOTOR PAPER TAKEUP 64 DRIVE 1 June 18, 1974 3,222,678 12/1965 Jones 346/1 3,302,900 2/1967 Messenger 3,395,401 7/1968 Silverman 3,495,268 2/1970 Hurst 3,681,777 8/1972 Smura 346/74 CR Primary Examiner-Paul J. Henon Assistant Examiner-James D. Thomas [5 7] ABSTRACT 8 Claims, 4 Drawing Figures ROUGHING PUMP DIFFUSION PUMP PAIENIEDJUM 1 81914 SHEET 2 85 REE [87 /5s [59 Loop AMP SUPPLY SUPPLY SENSOR MOTOR CAPSTAN l as

L

68,69 REE sfl T BELT VAC. LOOP AMP' SENSOR TAKEUP TAKEUP MOTOR CAPSTAN '1 l 89 90 I 65 i l l 93\ TONER AMP DISPENSER DISPENSER SENSOR MOTOR WHEEL REF. 94 95 49 92 DATA 96 SOURCE INPUT BUFFER AND CONTROL cmcurr VIDEO E 3.

DEFLECTION CIRCUITRY H I01 f SAFETY INTERLOCK PRESSURE FOCUS RUN OUT SENSOR SERVO SENSOR HIGH SPEED XEROGRAPHIC PRINTER This invention relates to xerography, and more particularly to a high speed xerographic printer employing an electron beam exposure technique.

In the practice of conventional xerography, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to create and support electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby dissipate the charge in the areas where light strikes the photoconductive layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.

The latent electrostatic image is then developed with a finely divided electroscopically attractable (electroscopic) material such as a resinous powder. The powder is held in image areas by the electrostatic charge on the photoconductive layer. Where the charge is greatest, the greatest amount of material is deposited; and where the charge is least, little or no material is deposited. Thus, a visible powder image is produced in conformity with the light image of the copy being reproduced. The powder image is subsequently transferred to a sheet of paper or other surface and suitably affixed (usually by application of heat) to thereby form a permanent print of the desired image.

ln order to produce copies by means of conventional xerography, an original document must already be available.

In recent years there has been considerable interest in the possibility of producing xerographic copies directly from electrical data, i.e. without the necessity of first producing an original document by manual or impact printing techniques.

One such technique involves the use of a cathode ray tube to convert the electrical data into an optical image on the face of the tube, which image is employed to expose the photoconductive surface of the xerographic apparatus. However, the necessity for converting the electrical data into an optical image results in additional complexity and limits the printing speed obtainable.

Another technique which has been proposed employs a character drum which rotates at high speed adjacent the photoconductive surface. An array of pulsed light sources situated within the character drum exposes the photoconductive surface at the moment that the transparency of a desired character is situated between a corresponding light source and the photoconductive surface. This technique, however, is limited by the speed at which the character drum is rotated and the sensitivity of the photoconductive surface vis-a-vis the light output capability of the pulsed light sources. In addition, this technique imposes difficult system synchronization requirements.

An object of the present invention is to provide a high speed xerographic printer capable of converting electrical data directly into a printed copy.

As herein described, there is provided a high speed xerographic printer comprising a vacuum chamber and an electron beam sensitive surface, a first portion of which forms a wall of the vacuum chamber. Means is disposed within the vacuum chamber for scanning the first portion of the electron beam sensitive surface with an electron beam to establish a charge pattern on said first surface which corresponds to a desired image. Means is provided for moving the aforementioned electron beam sensitive surface portion through the vacuum chamber, so that the first surface portion leaves the chamber as another portion of the electron beam sensitive surface enters the chamber.

Means is provided for contacting the first portion of the electron beam sensitive surface with electroscopic toner particles to form a visible toner pattern corresponding to the desired image. Suitable means is provided for disposing a portion of a web adjacent the first I surface portion. Web supply drive means is provided for feeding the web towards the first surface portion, and web takeup drive means is provided for drawing the web away from the first surface portion.

Means is also provided for transferring the toner pattern to the web portion, and for fusing the transferred toner pattern to the web.

In the drawing:

FIG. 1 shows a high speed xerographic printer according to a preferred embodiment of the present invention;

FIG. 2 shows a block diagram of certain control circuits employed in conjunction with the printer of FIG.

FlG. 3 shows a block diagram of the circuitry associated with the electron beam unit employed in the printer of FIG. 1; and

H6. 4 is a cross-sectional view of the anode region of the electron beam unit employed in the printer of FIG. 1, and the adjacent portion of the electron beam sensitive surface employed in said printer.

The high speed xerographic printer 10 shown in FIG. 1 comprises a number of operating stations situated about the periphery of a generally cylindrical rotatable drum 11.

The drum 11 is rotatably mounted on an axle 12, and is continually rotated (in the direction indicated by the arrow 13) by a capstan l4 situated within the drum 11 and engaging the interior surface thereof. The capstan 14 is rotated by a suitable drive motor (not shown), which may also be situated within the drum 11.

The drum 11 may be fabricated of a suitable electrically conductive metal, such as steel or aluminum, the peripheral surface of the drum 11 being coated with a relatively hard electron beam sensitive material 15.

By the tem electron beam sensitive material as herein employed, is meant either (i) an insulating material capable of being charged or discharged by a scanning electron beam, or (ii) a material which is initially insulating or of high resistivity, and which exhibits electron beam induced conductivity characteristics such that the conductivity of the material may be substantially increased by a scanning electron beam.

Preferably, the latter mentioned type of material is employed for the coating 15 on the peripheral surface of the drum ll. Suitable materials are described on pages 33 to 51 of Electronic Image Storage by B. Kazan and M. Knoll, Academic Press, 1968, New York and London. and may be deposited pyrolytically or by any other suitable technique on the periphery on the drum 11.

For a typical high speed application where it is desired to print copy at the rate, e. g., of 6 feet per second, the outside diameter of the drum ll may be 5.7

feet corresponding to a circumference of 18 feet, and the drum may be rotated at an angular velocity of 20 rpm. 1

The operation of the high speed printer commences with the charging of the electron beam sensitive coating as the drum 11 rotates under a corona emitter or corotron 16, which is operated at a predetermined DC potential with respect to the conductive drum 11. The corotron 16 charges the electron beam sensitive coating 15 to establish a uniform electrostatic charge density thereon.

As the drum 11 continues to rotate, the charged electron beam sensitive coating 15 enters the vacuum chamber 17, the portion of the coating 15 situated within the vacuum chamber 17 forming a wall of the vacuum chamber.

The vacuum chamber 17 consists of an inner chamber portion 18 which is surrounded by an outer chamber portion 19. The wall portions of the outer chamber portion 19 adjacent the electron beam sensitive coating 15 are spaced from the drum l1, and curved adjacent the ends 20 thereof to conform closely to the shape of the sealing rollers 21. The side portions of the outer chamber portion 19 extend to form flanges along a portion of the sides of the drum 11, and are situated closely adjacent to said drum sides. These side flanges 22 are covered with a suitable sealing material, such as felt, which bears against the rotating side portions 23 of the drum 11. The sliding joints between the flanges 22 of the outer vacuum chamber portion 19 and the adjacent side portions 23 of the drum 11 are sealed by a suitable vacuum lubricant. I

A vacuum seal between the end portions 20 of the outer chamber portion 19 and the adjacent portions of the electron beam sensitive coating 15 on the drum 11 is maintained by the resilient sealing rollers 24, which are rotated by the coating 15, and which slide against the end portions 20 of the side walls of the outer chamber portion 19 of the vacuum chamber 17. The surfaces of the end portions 20 which contact the resilient rollers 24 are fabricated of highly polished resilient metal, but are sufficiently rigid so as to elastically deform the adjacent portions of the rollers 24 to maintain a tight seal therebetween.

The sealing rollers 24 may preferably comprise an outer sheath of a low surface friction material such as polytetrafluoroethylene, an underlying resilient layer of silicone rubber, and an inner core of a rigid metal such as steel. These rollers may be of the general type shown in U.S. Pat. No. 3,435,500.

The lengths of the rollers 24 are slightly greater than the length of the drum 11, the protruding ends of the rollers 24 abutting against corresponding portions of the flanges 22 to form a vacuum seal at the sides 23 of the drum 1!.

Similarly, the end portions 25 of the side walls of the inner vacuum chamber 18 contact the inner resilient sealing rollers 26 to form a vacuum seal therebetween, the rollers 26 being rotated by contact with the electron beam sensitive coating 15 on the periphery of the drum 11, the rollers 26 acting to provide a vacuum seal between the coating 15 and the side walls of the inner vacuum chamber portion 18. The sealing rollers 26 have lengths slightly greater than that of the drum 11, the flanges 27 at the ends of the side walls of the inner chamber portion 18 abutting against the protruding portions of the sealing rollers 26 and the adjacent side portions 23 of the drum 11 to form a vacuum seal therebetween. The sealing rollers 26 may be of the same general construction as the sealing rollers 21.

With the aforementioned arrangement, a portion of the electron beam sensitive coating 15 forms a wall of the vacuum chamber 17, i.e. of the inner vacuum chamber portion 18 as well as the outer vacuum chamber portion 19.

A high capacity roughing pump 28 continuously evacuates the outer vacuum chamber portion 19 to create and maintain a vacuum therein. The inner vacuum chamber portion 18 of the vacuum chamber 17 is continually evacuated by a diffusion pump 29, which is of lower volumetric capacity than the roughing pump 28, but capable of reducing the pressure within the compartment being evacuated to a lower value. The diffusion pump 29 is selected so as to maintain a pressure within the inner vacuum chamber portion 18 on the order of 10 torr or less.

A dust shield 30 extends beyond the

flanges

22 and 27, and protects them from contamination by airborne toner particles, dust, etc., which contaminants may adversely affect the vacuum seal between said flanges and the adjacent portions of the sealing rollers and the sides 23 of the drum l1.

Situated within the inner vacuum chamber portion 18 is an electron beam unit 31, which is secured to the inner walls of the chamber portion 18 by a number of vibration isolation mounts 32. The electron beam unit 31 comprises a glass supporting envelope 33 having an open end portion adjacent the electron beam sensitive coating 15.

Situated within the supporting envelope 33 of the electron beam unit 31 are an electron gun 34, electrostatic deflection plates 35, and an accelerating electrode 37. A magnetic focus coil 38 surrounds a portion of the neck of the envelope 33 of the electron beam unit 31.

The accelerating electrode 37 may comprise a metallic electrode situated within the glass envelope 33, or alternatively may be in the form of a conductive coating on the inner surface of the envelope 33 adjacent the open end thereof.

The open end of the glass envelope 33 terminates in a flange 39, the surface of the flange 39 adjacent the electron beam sensitive coating 15 being covered with a conductive coating 40 which may be utilized as a sensor to measure the distance between the envelope 33 and the coating 15 by monitoring the capacitance between the conductive coating 40 and the adjacent portion of the conductive drum 11.

By application thereto of suitable operating potentials, the electron beam unit 31 is caused to generate and scan an electron beam indicated by the dashed line 41, which beam is caused to impinge upon the adjacent portion of the electron beam sensitive coating 15 which forms a wall of the inner vacuum chamber portion 18. Those portions of the coating 15 which are exposed to the scanning electron beam 41 exhibit a substantial increase in conductivity, and are thereby discharged, i.e. lose the charge which has been placed thereon by the corotron 16. Therefore, by deflecting the scanning electron beam 41 in a suitable pattern, an electrostatic charge pattern is established on the electron beam sensitive coating 15, which charge pattern corresponds to a desired image.

As the drum ll continues to rotate, the portion of the electron beam sensitive coating containing the electrostatic charge pattern defined by the scanning electron beam 41 leaves the vacuum chamber 17, as an other portion of the coating 15 enters said chamber.

The latent electrostatic image on the electron beam sensitive surface 15 enters the developer tank 42, whose function is to convert the latent electrostatic image to a visible toner pattern.

Situated in the lower portion or sump region of the developer tank 42 is a granular developer mixture 43, which consists of resin coated steel beads and electroscopic toner powder. The developer mixture 43 is continually carried from the sump region of the developer tank 42 to a hopper 44 at or near the top of the developer tank, by means of a conveyor belt 45 having a plurality of carrier buckets 46 affixed thereto.

As the drum 11 rotates to move the portion of the electron beam sensitive coating 15 having the desired latent electrostatic image thereon through the developer tank 42, the developer mixture in the hopper 44 continually flows out the orifice at the bottom of the hopper, and cascades over the electron beam sensitive surface 15 of the drum 11. As the developer mixture cascades over the electron beam sensitive surface, toner particles are attracted away from the moving carrier beads and adhere to the discharged portions of the electron beam sensitive surface, thus converting the latent electrostatic image thereon to a corresponding visible pattern of pigmented or dyed toner particles. This technique is commonly referred to as reversal development. The developer mixture (less any toner particles which have adhered to the latent electrostatic image on the electron beam sensitive surface 15 of the drum 1 1) falls back into the sump of the developer tank 42 after the developer has cascaded over the electron beam sensitive surface 15.

The direction of rotation of the drum 11 is such that the latent electrostatic image on the electron beam sensitive surface 15 continues to encounter fresh" toner particles as the electrostatic charge pattern moves upward through the developer tank 42. This reverse" cascade development technique provides improved uniformity of image development over the conventional forward development technique, wherein the latent electrostatic image moves through the developer tank in the same direction as that in which developer mixture is cascaded over the pattern to be developed.

The proper concentration of toner in the developer mixture 43 situated in the sump of the developer tank 42 is maintained by a toner dispenser unit 47, which contains a supply of toner powder 48 and dispenses the toner powder to the sump of the developer tank 42 by means of a dispenser wheel 49, which when rotated causes toner from the reservoir 48 to be transferred to the sump of the developer tank 42 through an orifice 50.

The operation of the dispenser wheel 49 is controlled by a suitable toner concentration control circuit, which continually monitors the concentration of toner in the sump of the developer tank, and operates the dispenser wheel 49 in such a manner as to maintain the toner concentration in the developer tank sump at a desired predetermined value. The toner concentration control circuit may be of the general type described in US. patent application Ser. No. 227,965, filed Feb. 22, 1972 and assigned to the assignee of the instant application.

Upon further rotation of the drum 11, the visible pattern of toner particles (toner particles being electroscopically adherent to discharged areas of the latent electrostatic image on the electron beam sensitive surface 15) is brought into juxtaposition with a moving paper web 51, which is attracted to the electron beam sensitive coating 15 by electrostatic induction forces, and thereby caused to progress at a speed equal to the peripheral speed of the drum 11, so that there is substantially no relative motion between the paper web 51 and the adjacent portion of the periphery of the drum 11.

In order to isolate the portion of the paper web 51 in contact with the drum 11 from the remaining portions of the paper web 51, thus insuring that the speed of the portion of the paper web 51 in contact with the drum 11 is determined by the peripheral speed of the drum 1 l, in order to minimize any possibility of slippage with resultant blurring or other distortion of the toner pattern, two

slack loops

52 and 53 are formed in the web 51.

By referring to the

loops

52 and 53 in the web 51 as being slack" loops, it is not meant that there is no tension whatsoever in the portion of the web 51 within these loops. Rather, it is meant only that the tension of the portion of the web within these loops is substantially less than the tension in the portion of the web extending on one or the other side of each loop. Thus, in FIG. 1 the tension in the portion of the paper web 51 to the right of the slack loop 52 is substantially greater than the tension of the portion of the web in the loop itself, while the tension in the portion of the web 51 to the left of the slack loop 53 is substantially greater than the tension of portion of the web within that loop.

The slack loop 52 is situated within and established and maintained by a vacuum chamber 54, which has a lower orifice 55 connected to a suitable vacuum source (not shown), In order to sense the size of the slack loop 52, a light source 56 and a photodetector 57 are provided on opposite sides of the vacuum chamber 54, which is provided with transparent wall portions in the path of the light emanating from the source 56 to illuminate the detector 57.

As the size of the loop 52 increases, the bottom portion of the loop intercepts all or part of the light reaching the photodetector 57, thus causing the photodetector to provide an output signal indicative of the size of the loop 52. The output of the photodetector 57 is coupled to a suitable circuit to control the paper supply drive motor 58, which is mechanically coupled to the paper supply drive capstan 59.

The capstan 59 cooperates with the pinch roller 60 to drive the paper web 51 toward the slack loop 52 at a rate so as to maintain the slack loop 52 at a size such that the bottom of the loop just intercepts the light beam extending between the light source 56 and the photodetector 57. if desired, a more responsive and smoother operating control system can be realized by providing a number of light sources and photodetectors in vertically aligned relationship, with the outputs of the photodetectors coupled to a suitable logic circuit to provide proportional control of the paper supply drive motor 58, rather than the motor operation obtainable with a single photodetector unit.

Similarly, the slack loop 53 is situated within an inverted vacuum chamber 61, which has a lower orifice 62 therein coupled to a suitable vacuum source (not shown).

The size of the inverted loop 53 is sensed by a photodetector 63, which is illuminated by a light source 64 through aligned transparent apertures in the walls of the vacuum chamber 61.

The output of the photodetector 63 is coupled to a suitable control circuit, which operates the paper takeup drive motor 65. The paper takeup drive motor 65 is mechanically connected to the paper takeup drive capstan 66, which cooperates with the pinch roller 67 to drive the exiting portion of the paper web 51 at a speed such that the bottom of the slack loop 53 just intercepts the beam extending between light source 64 and photodetector 63.

The paper takeup drive motor 65 is also mechanically coupled to the rollers 68 which drive the fuser belt 69 In order to further minimize any external drag on the portion of the paper web 51 in contact with the drum 11, the

idler rollers

70, 71 and 72 situated along the path of the web 51 between the

slack loops

52 and 53, are driven at a peripheral speed just slightly less than that of the drum 11. The

rollers

70, 71 and 72 are driven through overrunning

clutches

73, 74 and 75 perspectively, by the roller drive motor 76. Both the roller drive motor 76 and the motor which drives the drum drive capstan 14 and therefore the drum 11, are of the synchronous type (i.e. their speed is determined by the power supply frequency and is substantially unaffected by load variations), to insure accuracy of rotational speed. The roller drive motor 76 operates to drive the

idler rollers

70, 71 and 72 at a peripheral speed equal to approximately 98 percent of the peripheral speed of the drum 11. This action insures that there is very little relative movement between the portion of the paper web 51 in proximity to the drum 11, and the

idler rollers

70, 71 and 72. The overrunning

clutches

73, 74 and 75 permit the idler rollers to which they are respectively connected to become uncoupled from the roller drive motor 76 in the event that the speed of one or more idler rollers is increased by frictional engagement with the web 51.

An additional idler roller 77 is situated at the left of the inverted loop 53 in order to guide the web 51 into the fuser oven 78.

As the toner pattern (corresponding to the desired image as scanned by the electron beam 41 of the electron beam unit 31) passes the moving paper web 51 in close proximity thereto, an image transfer corona emitter or corotron 79 attracts the toner particles away from the electron beam surface and onto the adjacent portion of the moving paper web 51. The toner pattern is thus transferred onto the moving paper web 51.

The moving paper web 51, having the pattern of toner particles adherent thereto by electrostatic attraction forces, then moves around the idler roller 72, through the slack loop 53 within the vacuum chamber 61, and around the idler roller 77 into the fuser oven 78. A brake 110 in contact with the idler roller 77 maintains proper tension in the portion of the web 51 to the left of the slack loop 53.

The purpose of providing an inverted loop 53 is to insure that the idler rollers 72 and 77, as well as the walls of the vacuum chamber 61, contact only the back surface of the paper web 51, i.e. the surface opposite that on which the toner pattern is disposed. This arrangement insures that the toner pattern will not be smeared or otherwise distorted before it is fused to the paper web 51.

The paper takeup drive motor 65 draws the paper web 51 from the slack loop 53 through the fuser oven 78, which is maintained at a temperature sufficiently high to fuse the toner pattern to the paper surface, while at the same time being sufficiently low so that the paper web 51 is not scorched or otherwise deteriorated.

The fuser oven 78 is of elongated configuration, preferably having a length on the order of 6 feet for a paper web speed of 6 feet per second.

The paper takeup drive motor 65 also rotates a plurality of fuser belt drive rollers 68, which drive the fuser belt 69 at a speed equal to that of the paper web 51. The fuser belt 69 comprises a porous material, the interior region encompassed by the fuser belt being connected to a suitable vacuum source (not shown), so that the paper web 51 is retained in contact with the fuser belt 69 by differential air pressure due to the action of the vacuum source.

The fuser belt 69 is situated above the paper web 51, so that the belt contacts the back surface of the web and does not touch the toner pattern being fused thereto.

As the drum l1 continues to rotate, it passes beneath the alternating current corona emitting device or corotron 80, which neutralizes any residual charge remaining on the electron beam sensitive surface 15, thus reducing or cancelling the electrostatic attraction between any residual toner particles and the adjacent electron beam sensitive surface.

A rotating brush 81 situated in a substantially dusttight compartment 82 mechanically removes any remaining toner particles from the electron beam sensitive surface 15 of the drum 11, the toner particles so removed being drawn out through the conduit 83 by air pressure as a result of the application of a suitable vacuum source (not shown) to the conduit 83. Before the toner-laden air is returned to the atmosphere, it is filtered by suitable means (not shown) to remove the toner particles therefrom.

After the residual toner particles have been removed from the electron beam sensitive surface 15 by the action of the brush 81 and the vacuum source associated therewith, the electron beam sensitive surface 15 passes beneath an alternating current corona emitting device or corotron 84, which substantially reduces any residual charge remaining on the surface 15, and substantially reduces any static effects introduces by triboelectric interaction between the rotating brush 81 and the electron beam sensitive surface 15.

Thereafter, the corona emitting device or corotron 16 recharges the electron beam sensitive surface 15 in a uniform manner, in preparation for the next cycle of machine operation.

The manner of operation of the vacuum loop control systems and the toner concentration circuitry is illustrated in block diagram form in FIG. 2.

The output of the supply loop sensor 57 is compared to a signal indicative of a desired loop size from a reference circuit 85, in a comparator 86, the resulting correction signal being applied to an amplifier 87, the output of which drives the supply motor 58, which is mechanically coupled to the supply capstan 59. The resulting servo loop maintains the size of the slack loop 52 at a value corresponding to that indicated by the reference circuit 85.

In similar fashion, the output of the takeup loop sensor 63 is compared to a signal indicative of a desired loop size, as generated by the reference circuit 88, in a comparator 89, the resulting correction signal being applied to an amplifier 90, the output of which drives (i) the fuser drive rollers 68 which in turn drive the fuser support belt 69, and (ii) the paper takeup drive motor 65, which in turn is coupled to the takeup capstan 66.

The concentration of toner within the developer mixture 43 in the sump of the developer tank 42 is monitored by a toner sensor 91 (not shown in FIG. 1), the output of which is compared to a signal generated by the reference circuit 92 and indicative of a desired toner concentration. The comparison is made in a comparator 93, and the resulting correction signal is applied to the amplifier 94, the output of which drives the dispenser motor 95, which in turn is mechanically coupled to the dispenser wheel 49 shown in FIG. 1.

By this arrangement, the dispenser wheel 49 is operated in such a manner as to maintain the concentration of toner within the developer mixture 43 in the sump of the developer tank 42 at a constant predetermined value corresponding to the setting of the reference circuit 92.

The circuitry provided for causing the electron beam unit 31 to scan a desired pattern so as to form a corresponding latent electrostatic image on the electron beam sensitive surface 15, is shown in FIG. 3.

The data to be printed originates from a data source 96, which may typically be a computer or data transmission terminal. This data is transferred to an input buffer and control circuit 97, which converts the input data to a line scan format, and provides buffering between the speed at which information is supplied from the data source 96, and the speed at which information is to be delivered to the electron beam unit 31.

The modified and buffered signals are coupled from the circuit 97 to the video and deflection circuitry 98, which provides video and deflection signals of proper values and at suitable power levels to drive the electron beam unit 31, the line 99 denoting the deflection signals to the deflection plates 35 of the electron beam unit 31, and the line 100 denoting the video signals delivered thereto.

A deflection monitoring signal is provided by the video and deflection circuitry 98 to the safety interlock circuit 101 on line 102. This monitoring signal is a DC level which has one value whenever deflection signals are being supplied to the electron beam unit 31, and another value when such signals are absent.

Whenever the deflection signals are absent, the safety interlock circuit 101 provides a signal to the electron gun 34 of the electron beam unit 31 on line 103, to cut off the electron beam 41, thereby preventing damage to the electron beam sensitive surface when proper deflection signals are not being applied to the electron beam unit 31.

The safety interlock circuit 101 also receives data from a pressure sensor 104, which is situated within the inner vacuum chamber portion 18 of the vacuum chamber 17. The safety interlock circuit cuts off the electron beam 41 whenever the pressure sensor 104 indicates that there is insufficient vacuum, i.e. excessive pressure within the inner vacuum chamber portion 18.

In order to obtain high imaging resolution, the electron beam 41 is focused to a small point by the action of the electron gun 34 and the focus coil 38. As a result, the depth of field of the resulting scanned electron beam pattern is limited, and small variations in the distance between the adjacent surface of the electron beam sensitive coating 15 and the electron beam unit 31 may result in loss of resolution.

In order to minimize variation of this distance, the drum drive capstan 14 is arranged to contact the inner surface of the drum 1 1 at a point opposite that at which the electron beam 41 impinges on the surface 15. Thus any runout of the drum 11 is minimized at the point where the electron beam 41 scans the surface 15.

In order to compensate for any remaining small variations in the distance between the electron beam sensitive surface 15 and the electron beam unit 31, a runout sensor 40 and focus servo 105 are provided.

The conductive coating which acts as the runout sensor 40 provides an output signal which is a measure of the distance between the conductive drum 11 and the flange 39 of the glass envelope 33 of the electron beam unit 31 (see FIG. 4), which signal is employed to vary the current to the focus coil 38 in corresponding fashion. The output of the runout sensor 40 is supplied to the focus servo 105, which in turn supplies the drive current for the focus coil 38.

As shown in FIG. 4, the runout sensor 40 comprises a conductive film disposed on the outer flange 39 of the glass envelope 33 of the electron beam unit 31, the coating 40 extending into an aperture 106 in the flange 39. A suitable connector 107 extends into the aperture 106 to make electrical contact with the coating 40.

In similar fashion, an electrical connector 108 extends into an aperture 109 in the glass envelope 33 to make electrical contact with the accelerating electrode 37.

As the distance between the electron beam unit 31 and the drum 11 varies, a corresponding variation occurs in the capacitance between the runout sensor 40 and the drum 11. This capacitance variation is utilized by the focus servo 105 to correspondingly vary the current to the focus coil 38, thereby to vary the point of focus of the electron beam 41, so that said focus point always occurs at the electron beam sensitive surface 15.

I claim: 1. A high speed xerographic printer, comprising: a vacuum chamber; an electron beam sensitive surface comprising the periphery of a generally cylindrical drum mounted for rotation about a longitudinal axis thereof, a first portion of said surface forming a wall of said chamber; means disposed within said chamber for scanning said surface portion with an electron beam to establish a charge pattern thereon corresponding to a desired image;

means for moving said surface portion through said chamber, so that said first portion leaves said chamber as another portion of said surface enters said chamber;

means for contacting said first surface portion with electroscopic toner particles to form a visible toner pattern corresponding to said desired image;

means for disposing a portion of a web adjacent said first surface portion;

web supply drive means for feeding said web toward said first surface portion;

web takeup drive means for drawing said web away from said first surface portion;

means for producing and maintaining a first slack loop in said web between said web portion and said web supply drive means;

means for producing and maintaining a second slack loop in said web between said web portion and said web takeup drive means;

means for transferring said toner pattern to said web portion; and

means for fusing said transferred toner pattern to said web.

2. The printer according to claim 1, further comprising first and second sensors responsive to the sizes of said first and second loops respectively, means for controlling said web supply drive means in response to the output of said first sensor to maintain the size of said first loop within a first desired range, and means for controlling said web takeup drive means in response to the output of said second sensor to maintain the size of said second loop within a second desired range.

3. The printer according to claim 2, further including a runout sensor for providing a signal responsive to the distance between said scanning means and the adjacent portion of said electron beam sensitive surface, and means responsive to said signal to cause said beam to focus on said adjacent surface portion irrespective of variations in said distance.

4. The printer according to claim 1, further including means for rotating said drum about said axis.

5. The printer according to claim 4, wherein said rotating means drives the periphery of said drum in a region adjacent said scanning means.

6. A high speed xerographic printer, comprising:

a vacuum chamber comprising an inner and an outer chamber portion, said inner chamber portion being sealed with respect to said outer chamber portion;

an electron beam sensitive surface, a first portion of said surface forming a wall of said chamber and being spaced from two adjacent walls of said chamher; two resilient rollers in contact with said first surface portion, each roller also contacting a corresponding one of said adjacent walls in such a manner that said rollers form seals between said first surface portion and said adjacent walls;

means disposed within said inner chamber portion for scanning said surface portion with an electron beam to establish a charge pattern thereon corresponding to a desired image;

means for moving said surface portion through said chamber, so that said first portion leaves said chamber as another portion of said surface enters said chamber; means for contacting said first surface portion with electroscopic toner particles to form a visible toner pattern corresponding to said desired image;

means for disposing a portion of a web adjacent said first surface portion;

web supply drive means for feeding said web toward said first surface portion;

means for transferring said toner pattern to said web portion; and

means for fusing said transferred toner pattern to said web.

7. The printer according to claim 6, wherein said electron beam sensitive surface comprises the periphery of a generally cylindrical drum mounted for rotation about a longitudinal axis thereof.

8. The printer according to claim 6, further including means for disabling said scanning means when the pressure within said inner chamber portion exceeds a predetermined value.

Claims

1. A high speed xerographic printer, comprising: a vacuum chamber; an electron beam sensitive surface comprising the periphery of a generally cylindrical drum mounted for rotation about a longitudinal axis thereof, a first portion of said surface forming a wall of said chamber; means disposed within said chamber for scanning said surface portion with an electron beam to establish a charge pattern thereon corresponding to a desired image; means for moving said surface portion through said chamber, so that said first portion leaves said chamber as another portion of said surface enters said chamber; means for contacting said first surface portion with electroscopic toner particles to form a visible toner pattern corresponding to said desired image; means for disposing a portion of a web adjacent said first surface portion; web supply drive means for feeding said web toward said first surface portion; web takeup drive means for drawing said web away from said first surface portion; means for producing and maintaining a first slack loop in said web between said web portion and said web supply drive means; means for producing and maintaining a second slack loop in said web between said web portion and said web takeup drive means; means for transferring said toner pattern to said web portion; and means for fusing said transferred toner pattern to said web.

6. A high speed xerographic printer, comprising: a vacuum chamber comprising an inner and an outer chamber portion, said inner chamber portion being sealed with respect to said outer chamber portion; an electron beam sensitive surface, a first portion of said surface forming a wall of said chamber and being spaced from two adjacent walls of said chamber; two resilient rollers in contact with said first surface portion, each roller also contacting a corresponding one of said adjacent walls in such a manner that said rollers form seals between said first surface portion and said adjacent walls; means disposed within said inner chamber portion for scanning said surface portion with an electron beam to establish a charge pattern thereon corresponding to a desired image; means for moving said surface portion through said chamber, so that said first portion leaves said chamber as another portion of said surface enters said chamber; means for contacting said first surface portion with electroscopic toner particles to form a visible toner pattern corresponding to said desired image; means for disposing a portion of a web adjacent said first surface portion; web supply drive means for feeding said web toward said first surface portion; web takeup drive means for drawing said web away from said first surface portion; means for transferring said toner pattern to said web portion; and means for fusing said transferred toner pattern to said web.