Toner projection printer with improved address electrode structure

Abstract: A toner projection printer is provided with a developer surface (20) which manifests a developer bias (Vd), and includes a cloud of entrained toner particles (22). A conductive platen (24) is positioned opposed to the developer surface (20) and manifests a platen voltage (Vp) that is attractive to the toner particles (22). An address plate (28) is positioned between the developer surface (20) and the conductive platen (24). The address plate (28) includes a determined thickness insulator (32) with through pixel apertures (30). Each pixel aperture (30) has at least first (52) and second (54) conductors that are electrically insulated from each other by the insulator (32). A first drive circuit (80, 82) is coupled to the first conductor (52) for controllably applying a row drive voltage which is either a reference potential that exerts a repulsive force on the toner particles (22) or a high voltage which is attractive to the toner particles (22). A second drive circuit (68, 84) is coupled to the second conductor (54) for controllably applying a column voltage drive that is either a reference voltage (repulsive to the toner particles (22)) or a high voltage (attractive to the toner particles (22)). Both the column and row drive voltages are set at levels so that only when both are high can toner particles (22) pass through the pixel aperture (30), be drawn towards the conductive platen (24) and come under influence of the platen voltage (Vp). Control circuitry (60, 62, 64) operates the first and second driver circuits (80, 82, 68, 84) to enable deposition of row and column dots of toner (22) on a media sheet (26) positioned on the platen (24), under influence of the platen potential (Vp).

Description: This invention relates to electrostatic printing devices and, more particularly, to a toner projection printer employing an electrostatic toner deposition control and an improved pixel address mechanism. The most widely used electrophotographic print apparatus employs a movable photoconductor which is selectively exposed by a source of optical energy. While such electrophotographic printers have been widely accepted and produce excellent print quality at reasonable cost, continued efforts are being directed to increase their performance and further reduce their cost. However, photoconductor-based printers will continue to exhibit certain problems which inherently arise from the use of a photoconductor. Among those are the cost of the photoconductor, photoconductor wear; and photoconductor sensitivity to light requiring continual shielding. Further, when an image is fully developed on the photoconductor, a transfer action must occur to enable removal of the toner to a media sheet. Recently, a new class of electrostatic printers has been developed which requires no photoconductor and avoids many problems inherent with the use of the photoconductor. That class of printers comprise "toner projection printers" which include a system of electrodes for controlling direct deposition of charged toner particles on a media sheet without an intervening photoreceptor or photoconductive device. Typically each electrode includes a conductive electrode ring surrounding a hole in an insulating substrate. On one side of the substrate is a developer module which includes a developer roll and a supply of charged dry toner particles. For a system employing negatively charged toner particles, when an electrode ring is driven positive with respect to the developer roll, the toner particles are attracted to the electrode ring and some pass through the hole. On the opposite side of the insulating substrate is a media sheet which rests on a conductive platen. The platen is biased to a voltage that is more positive than the electrode ring so that toner particles are attracted to the paper/platen combination. Toner that is attracted to the electrode ring but does not path through the aperture, collects around the aperture and must be removed periodically. This is accomplished by reversing the potential between the electrode ring and the developer roll to pull such toner deposits away from the insulating substrate and electrode ring and back to the developer roll. Due to the fact that each electrode ring requires an independently controllable driver circuit, a large number of driver circuits are required, with attendant complex wiring and control circuitry. U.S. Patent 5,036,341 to Larson et al. describes a toner projection printer wherein the print control matrix comprises two layers of parallel wires in each of two layers. The two layers are orthogonal and are disposed parallel to the plane of a media sheet upon which the toner is to be developed. The wires in each layer are arranged in the form of a bar pattern and each separate wire is connected to a drive circuit. A toner dot is printed when two adjacent wires in each layer are driven positively (assuming a negatively charged toner). Toner is then drawn to a hole at the intersection of the two pairs of positively driven wires, passes therebetween and is deposited upon a media sheet. The Larson system exhibits a number of disadvantages. The array of wires can only be supported by a frame structure around the edge of the print array. Very little sag in the wires can be tolerated due to the tight spacing control which must be maintained between the print wire array and the paper. The array of wires is fragile and each layer must be perfectly insulated from the other, which is difficult considering the number of cross-over points. There also may be some leakage of toner through adjacent holes between wire pairs. Lastly, the holes formed by the intersecting wires are square and may not provide optimum shaped dots for best print resolution. U.S. Patent 5,121,144 to Larson describes a multiplexing system for a toner projection printer. In lieu of employing a continuous conductive platen behind the media sheet upon which toner is to be deposited, the Larson '144 patent utilizes an insulating platen which includes many conducting wires that are inlaid across the direction of movement of the media sheet. Electrodes which control toner deposition are positioned on an insulating substrate above the media sheet and are connected together in a number of sets, so that only one electrode in each set is directly over a given wire in the conductive platen. Only one platen wire at a time is driven to a high positive voltage (for a negatively charged toner). When an electrode set is also driven positive, the single electrode which resides over the active wire in the platen causes a deposition of toner on the media sheet. The structure shown in the '144 Larson patent also exhibits a number of disadvantages. The platen structure is complex and includes many precision-inlaid conductors. The insulation between these conductors must withstand a high voltage (e.g., approximately 1000 volts) and must maintain insulating properties, even though it is subject to wear as media sheets pass over it. The drive circuits for the platen wires must also be capable of driving a high voltage -which is a much higher voltage than that required to drive the print electrodes directly (approximately 100 volts). The higher voltage drive circuits are correspondingly more expensive. Finally, the platen with its inlaid wires must be precisely aligned with the printing electrode array to achieve acceptable print quality. PCT published Application WO 90/14960 to Larson describes an improvement to the electrode structure shown in the Larson '341 patent referred to above. In the PCT published Application, Larson employs isolation electrodes to reduce cross coupling or cross talk between adjacent mesh electrodes. In PCT published Application WO 90/14959 to Larson, a procedure is described for removing deposited toner from an electrode matrix which employs a reverse voltage application during periods between address times. However, when toner particles adhere to the electrode rings, they tend to lose their charge by conduction through the electrode rings. Thus, application of a reverse voltage to remove such particles is ineffective due to their loss of charge. As can be seen from the above, while toner projection printers eliminate the need for a photoconductor belt or surface, costs and performance improvements are required before the benefits to be obtained by the elimination of the photoconductor component will be realized. US-A-5,214,451 is concerned with the toner supply leveling in a multiplexed direct electrostatic printing apparatus. A print head structure is provided which is formed by an insulative base member on top of which so called shield electrodes are provided and on the bottom of which, at an orthogonal orientation with respect to the shield electrodes, control electrodes are provided. The respective electrodes are connected to respective voltage sources. During operation, the shield electrodes are sequentially driven to their drive levels and by simultaneously applying the drive signal to the control electrodes, toner is passed through the selected apertures, however, toner is transfered from the donor roll to a row even if same is not selected, leading to a problem of blocking the apertures in the shield plate. It is the object of the present invention to provide an improved electrostatic apparatus for applying toner to a sheet preventing the blocking of the apertures in a shield plate. This object is achieved by a printer according to claim 1. Fig. 1 is a schematic sectional view of a portion of the toner projection printer including the developer surface with a cloud of entrained toner particles, an address plate and a conductive platen with a media sheet positioned thereon. Fig. 2 is a plan view of the address plate. Fig. 3 is a circuit diagram illustrating circuitry for applying row and column drive potentials to the row and column traces on the address plate of Fig. 2. Fig. 4 shows plots of field strength versus distance for the toner projection printer structure of Fig. 1, when various biases are applied to the components thereof. Fig. 5 shows plots of drive voltage versus time helpful in understanding the operation of the circuit of Fig. 3. Referring to Fig. 1, the print portion of the toner projection printer is shown in section. A developer roll surface 20 is preferably comprised of a conductive elastomer and has applied thereto a developer bias Vd. Toner 22 forms a cloud about developer roll surface 20 by virtue of charge attraction between the toner particles and developer bias Vd. In a preferred embodiment, toner particles 22 are single component dielectric particles that are negatively charged. In opposition to developer roll surface 20 is a conductive platen 24 which has applied thereto a bias voltage Vp. Voltage Vp is highly positive (e.g., 1000 volts) and creates a high electrostatic field that is attractive to toner particles 22. A media sheet 26 is positioned on conductive platen 24 and is positioned to receive toner dots configured in an image format. Positioned between developer roll surface 20 and conductive platen 24 is an address plate 28 which, in accordance with appropriate row and column drive potentials, enables toner particles 22 to selectively pass through apertures 30 to come under the influence of the electric field created by voltage Vp applied to conductive platen 24. A partial plan view of address plate 28 is shown in Fig. 2, and only a single aperture and associated electrodes are shown in Fig. 1. Aperture plate 28 comprises an insulating sheet 32 having a first surface 34 on which a plurality of column traces 36, 38, 40, etc. are positioned. On opposing surface 35, a plurality of row traces 42, 44, 46, 48, and 50 are positioned so as to intersect the respective column traces. About each aperture 30, each column trace includes a conductive electrode ring 52 and, in a similar manner, each row trace includes a conductive electrode 54 positioned on the opposite side of insulating sheet 32. Insulating layers 56 and 58 cover the respective surfaces of column electrode rings 54 and row electrode rings 52. As will become apparent from the description below, insulating layers 56 and 58 prevent toner particles from coming into contact with the conductive surfaces of the row electrode traces, column electrode traces and respective row and column electrode rings. As a result, conductive discharge of toner particle charges is prevented. Each column trace 36, 38, 40, etc. is connected to a column driver circuit (to be described below) which applies a column drive voltage Vc(t) to each of the connected column traces and connected column electrode rings. In a similar manner, each of row traces 42, 44, 46, 48, 50, etc. is connected to a row driver (to be described below) which selectively applies a row drive voltage Vr(t) thereto. Arrow 49 illustrates the direction of movement of a media sheet beneath address plate 28. In Fig. 3, circuitry for addressing the array of pixel apertures in address plate 28 is shown. A processor 60 and connected memory 62 combine to provide raster-oriented binary pixel data to an application specific integrated circuit (ASIC) 64. Within ASIC 64, the raster data is organized so that half select signals for the column traces are output on data lines 66 to a plurality of column latches 68. A clock line 70 enables operation of latches 68 in accordance with an enable signal that is impressed by ASIC 64 onto line 72. In similar fashion, ASIC 64 applies data, clock and enable signals via lines 74, 76, 78, respectively to row latches 80 which enable column drive signals to be applied to sequential column traces. The outputs from row latches 80 and column latches 68 are applied to row and column drivers 82, 84, respectively. Each row driver 82 and column driver 84 applies a drive voltage Vr(t), Vc(t) to a connected row or column trace. The drive voltage varies between a high level and a low or reference potential level. As will be understood from the description below, the potentials applied by row drivers 82 and column drivers 68 are such as to act in a "half select" mode whereby toner cannot pass through an aperture 30 unless both row and column potentials at the aperture 30 intersection are at the high level. In operation, ASIC 64 first loads column latches 68 with appropriate data signals and then provides enable signals to both a selected row latch in row latches 80 and to column latches 68 to cause a simultaneous readout of drive voltages on respectively connected row and column traces. As shown in Fig. 2, column traces 36, 38, 40, etc. are positioned on a slant so as to enable improved resolution to be obtained by closer packing of pixel apertures 30. To print a complete line, a plurality of rows of data must be printed in order to obtain the complete pixel row. ASIC 64 synchronizes the print action with the movement of media sheet 26 over platen 24. The means for moving media sheet 26 are not shown, but are well known to those skilled in the art. To better understand the operation of the invention, reference should now be had to Fig. 4. In Fig. 4, the schematic of the toner projection printer structure shown in Fig. 2 is repeated at the top. Immediately below are a plurality of plots electrostatic field strength between the printer components along the right hand axis of the plots of Fig. 4, field potentials are plotted as they are applied to the various components of the toner projection printer. As will be hereafter understood, four states, i.e., A, B, C and D occur as a result of the application of the bias and drive voltages to the printer components. Vd is the bias applied to developer roll surface 20 and Vp is the bias applied to conductive platen 24. The field strength plot of state A illustrates the variations in field strength between developer roll 20 and conductive platen 24 when both row electrode ring 52 and column electrode ring 54 are maintained at a reference potential level (e.g., ground). In this state, the negative potential gradient between developer roll surface 20 and row electrode ring 52 prevents migration of negatively charged toner particles 22. As a result, toner particles do not pass through aperture 30 and into the area affected by conductive platen voltage Vp. Under such circumstances, printing is inhibited and toner is cleaned from surface 35 of address plate 28. Those skilled in the art will understand that negatively charged toner particles will only move towards platen potential Vp if all intervening potentials are at least as high as Vd and, preferably, are more positive in potential. State B occurs when a row is not selected. Under those conditions, row electrode ring 52 is maintained at the reference potential. However, some other row has likely been selected and column electrode ring 38 has a high data voltage Vc applied thereto as a half select potential for the other row. As in state A, the negative potential gradient from developer roll surface 20 to row electrode ring 52 repels toner particles 22. Printing is inhibited and again, toner is cleaned from the surface of address plate 28 that is closest to developer roll surface 20. In state C, row electrode ring 52 is at a high voltage (the row has been selected) but a low data voltage is applied to column electrode ring 54. In this state, toner is attracted to row electrode ring 52 but the negative potential gradient between row electrode ring 52 and column electrode ring 54 prevents toner from passing through aperture 30. In state D, both row electrode ring 52 and column electrode ring 54 have high voltage applied, indicative that the respective row has been selected and that a high data level has been applied to column electrode ring 54. In this state, some of the toner reaching row electrode ring 52 passes through aperture 30 and is attracted to and deposited on a sheet 26 resting on conductive platen 14. Thus, printing occurs. State E is an alternate state wherein the high levels of the voltages applied to the row and column traces are different. In specific, the column trace voltage Vc is more positive than the high level applied to the row traces. Under these circumstances, a positive voltage gradient is created between row electrode ring 52 and column electrode ring 54 which pulls more toner particles 22 through aperture 30, giving a higher toner deposition rate. Better resolution also results because of induced the higher toner velocities. The higher toner velocities lessens the repulsive effects between the like-charged toner particles. In Fig. 5, waveforms are plotted which are employed during operation of the invention. Row drive voltages are applied to sequential row traces (e.g. 42, 44, 46, 48, 50, etc.) during succeeding clock periods. Simultaneously with application of a row drive voltage to a row trace, data signals for the particular row are applied on column traces (e.g. 36, 38, 40, etc.). When both the data and column trace drive voltage are at the high level, the printing of a dot occurs at an aperture 30 positioned at the intersection therebetween. Thus, as shown in Fig. 5, the coincident drive voltages applied at time t1 to row trace 42 and column trace 36 cause a dot to be printed at the intersection therebetween (i.e. aperture 30 at pixel position 5 in Fig. 2). Similarly, dots are printed at times t2 and t3 at row/column trace intersections at pixel positions 9 and 13, respectively. Assuming only five row traces are present on address plate 28, the sequencing of row voltages to the row traces repeats at time t6. It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. For instance, the above description has assumed the presence of a media sheet passing beneath address plate 28. By contrast, conductive platen 24 can be made movable so as to directly receive the toner deposits and then to move them to a transfer point where they are removed to a media sheet.

Claims: An electrostatic apparatus for applying toner (22) to a sheet (26), said apparatus comprising: a developer surface (20) manifesting a voltage bias Vd; toner particles (22) entrained about said developer surface (20) by charge attraction; a conductive platen (24) in opposed position to said developer surface (20) and manifesting a voltage bias Vp that exerts an attractive force on said toner particles (22); address plate means (28) disposed between said developer surface (20) and said conductive platen (24) and comprising an insulator (32) of determined thickness having plural apertures (30) therethrough, each said aperture (30) juxtaposed to at least a first conductor (52) facing said developer surface (20) and a second conductor (54) facing said conductive platen (24), said first conductor (52) and second conductor (54) electrically insulated from each other by said insulator (32); row drive means (80, 82) coupled to said first conductor (52) for controllably applying a row drive voltage which is either at a reference level or a drive level; column drive means (68, 84) coupled to the second conductor (54) for controllably applying thereto a column drive voltage which is either at a reference level or a drive level, said column drive voltage and row drive voltage manifesting drive levels such that only when both are at their respective drive levels do said toner particles (22) pass through said aperture (30) and are drawn towards said conductive platen (24) under influence of Vp; and control means (60, 62, 64) for operating said row (80, 82) and column (68, 84) drive means to concurrently output said drive level voltages when toner particles (22) are to pass through said aperture (30), and for operating at least one of said row drive means (80, 82) and column drive means (68, 84) to manifest a reference voltage if toner particles (22) are to be inhibited from passage through said aperture (33); characterized in that said row drive voltage at said reference level exerts a continuous repellant force to said toner particles repelling said toner particles towards said developer surface (20); and said row drive voltage at said drive level exerts a continuous attractive force to said toner particles attracting said toner particles from said developer surface (20). The electrostatic apparatus as recited in claim 1 wherein a media sheet (26) is positioned between said conductive platen (24) and said address plate means (28) and receives said toner particles (22) when said toner particles (22) pass through said aperture (30). The electrostatic apparatus as recited in claim 1, wherein means are provided to move said conductive platen so as to enable toner deposited thereon to be moved to a transfer station and transferred to a media sheet. The electrostatic apparatus as recited in claim 1, wherein said address plate means (28) comprises: M apertures (30) arranged in N rows (42, 44, 46, 48, 50), each of said N rows (42, 44, 46, 48, 50) having M/N apertures (30), where M and N are integer values, and a full row of toner dots on a media sheet comprises MxN dots. The electrostatic apparatus as recited in claim 4, wherein each aperture (30) in one of said N rows (42, 44, 46, 48, 50) is aligned to create one of a plurality of columns of toner dot locations on a media sheet (26). The electrostatic apparatus as recited in claim 4, wherein each said first conductor (52) and said second conductor (54) comprise a conductive annulus which surrounds an associated aperture (30). The electrostatic apparatus as recited in claim 6, wherein each said conductive annulus is covered by a further insulating layer (56, 58). The electrostatic apparatus as recited in claim 6 wherein said address plate means (28) is an insulating sheet (32) having said first and second conductors (52, 54) and associated conductive annuli disposed on opposing sides (34, 35) thereof. The electrostatic apparatus as recited in claim 1, wherein said row drive voltage and said column drive voltage are of substantially equal value. The electrostatic apparatus as recited in claim 1, wherein said column drive voltage applied to said second conductor (54) has a level that is different from said row drive voltage applied to said first conductor (52) to further accelerate said toner particles (22) towards said conductive platen (24).

Inventors: LUQUE PHILLIP R, LUQUE, PHILLIP R.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1995
Main classification: B41J2
IPC codes: B41J2