WO2001062502A1

WO2001062502A1 - Direct printing device

Info

Publication number: WO2001062502A1
Application number: PCT/EP2000/001477
Authority: WO
Inventors: Per Klockar; Johan Linder
Original assignee: Array Ab; Matsushita Electric Industrial Co. Ltd.
Priority date: 2000-02-23
Filing date: 2000-02-23
Publication date: 2001-08-30
Also published as: AU2000231594A1

Abstract

The invention relates to a direct printing apparatus in which computer generated image information is converted into a pattern of electrostatic fields, which selectively transport electrically charged particles from a particle carrier (33) toward a back electrode (12) through a printhead structure (5), whereby the charged particles are deposited in image configuration on an image receiving substrate (1) caused to move relative to the printhead structure. In order to ensure reliable and uniform print quality, means are provided for measuring an electrical property indicative of the distance between an electrode on the printhead structure and the back electrode, the particle carrier or the image receiving member, when this is a transfer belt of similar electrically conductive element. The electrical property is preferably capacitance. Any determined variation in the capacitance allows the control voltages to be modified to compensate for the distance variations and so maintain uniform high print quality.

Description

Direct Printing Device

Technical Field

The invention relates generally to direct printing apparatus . More particularly the invention is directed to a printing apparatus wherein a computer generated image is converted into a pattern of electrostatic fields, which selectively transport electrically charged particles from a particle source through a printhead structure toward a back electrode, and wherein the charged particles are deposited in image configuration on an image receiving substrate .

Background

US patent No. 5 847 733 describes a direct electrostatic printing device and a method of generating text and pictures with toner particles on an image receiving substrate directly from computer generated signals . Such a device generally includes a printhead structure through which toner particles are selectively transported in accordance with image data. The printhead structure is generally constituted by a control electrode array formed on an apertured insulating substrate. An annular control electrode is arranged around each aperture and is driven to alter the electric field around the apertures to selectively permit or block the passage of toner particles through the apertures . Each aperture is further provided with deflection electrodes which are controlled to selectively generate asymmetric electric fields around the apertures, causing toner particles to be deflected prior to their deposition on the image-receiving medium. This process is referred to as dot deflection control (DDC) . This enables each individual aperture to address several dot positions. The print addressability is thus increased without the need for densely spaced apertures.

The deflection electrodes and control electrode voltages together determine the quantity and direction of toner . particles propelled through each aperture and thus affect the position and the density or size of the resultant dots. However, the deflection, size and density of a printed dot is affected not just by the electric field profile around the aperture, but also by the distance between the aperture and the toner source and also the distance between the aperture and the image receiving medium. Variations in these distances result in non- uniform dot positions, shape and size across the surface of the image or from one image to another. In the past attempts have been made to prevent variations in these distances by using mechanical spacers. However, even with very precisely manufactured and positioned spacers, small variations in the spacing will be inevitable with the normal wear and vibrations of the apparatus during use .

Thus there is a need for a direct electrostatic image forming arrangement that alleviates the problems associated with prior art apparatus and specifically provides improved print quality.

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This object is achieved in an image forming apparatus including a particle carrier for holding a source of charged toner particles a back electrode for generating a background electric field for accelerating the transport of charged toner particles from said particle carrier towards the back electrode, a printhead structure disposed in said background electric field and having a plurality of apertures, and an electrically conductive layer including at least one electrode. Control voltage sources are connected with said electrically conductive layer to control the transport of charged toner particles through the apertures, and means are provided for transporting an image receiving member between the printhead structure and the back electrode for intercepting the transported charged particles . Moreover means associated with at least one of said electrodes are provided for monitoring variations in an electrical property between the at least one electrode and a second element that consists of the back electrode, the particle carrier and/or said image receiving member, and for determining a change in length of a gap between the electrode and the second element on the basis of the monitored variations.

The arrangement according to the invention does not seek to prevent or minimise variations in the distances between the printhead structure and the back electrode, image receiving member or particle carrier. Instead it provides a manner of accurately determining the variations in these distances. Furthermore, by measuring an electrical property, small variations in distance can be determined with accuracy and with the minimum of disruption to printing process. In addition, many of the elements already present in the apparatus can be utilised for this measurement.

Preferably the electrical property measured is capacitance . In this case the monitoring means include means for applying a constantly varying voltage in a circuit including the electrode and the second element, and means for measuring the current flowing in said circuit. The applied voltage may be a periodically varying voltage preferably with a voltage swing of between -200 Volt to 200 Volt and a frequency of at most 2 kHz. The current amplitude in the circuit is measured by measuring the voltage across a resistive element that is coupled either to the electrode or the second element.

In an alternative arrangement the phase of the current across the resistor is measured. In order to obtain a reliable measurement, the electrode used to measure the capacitance is preferably arranged on a surface of the printhead structure opposing said second element . The electrode may also include at least one group of electrode elements that are electrically coupled at least during operation of the monitoring means.

In a preferred arrangement the electrode includes at least one deflection electrode and the second element is the back electrode. The electrode preferably includes groups of deflection electrodes connected together to provide conductive areas providing a measurable capacitance.

The apparatus further includes means responsive to the monitoring means for adjusting control voltages applied to control and deflection electrodes associated with each aperture of the printhead structure to substantially mitigate the effects of a change in length of the gap.

The invention further resides in a printhead structure for use in an electrostatic printing apparatus, including a substrate layer with a plurality of apertures and at least one electrically conductive layer arranged on a first surface of said substrate layer, the electrically conductive layer including at least one electrode, and at least one resistive element having a first terminal connected to said at least one electrode.

In accordance with a further aspect, the invention resides in a method of operating an image forming apparatus having a particle carrier for holding a source of charged toner particles, a back electrode for generating a background electric field for accelerating the transport of charged toner particles from the particle carrier towards the back electrode, a printhead structure disposed in the background electric field and having a plurality of apertures and an electrically conductive layer including at least one electrode, control voltage sources connected with the electrically conductive layer for controlling the transport of charged toner particles through the apertures, and means for transporting an image receiving member between the printhead structure and the back electrode for intercepting the transported charged toner particles, the method including measuring an electrical property between said electrode and a second element of said apparatus consisting of the back electrode, the particle carrier and/or the image receiving member to determine a variation in length of a gap separating the electrode and the second element.

This method is preferably performed after predetermined time period has elapsed. It may also be performed after printing a predetermined number of pages, or on request from an operator.

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The invention will now be described in more detail for explanatory, and in no sense limiting, purposes, with reference to the following drawings, wherein like reference numerals designate like parts throughout and where the dimensions in the drawings are not to scale. In the figures

Fig.l is a schematic view of an image forming apparatus in accordance with a preferred embodiment of the present invention,

Fig.2 is a schematic section view across a print station in an image forming apparatus, such as, for example, that shown in Fig.l,

Fig.3 is a schematic section view of the print zone, illustrating the positioning of a printhead structure in relation to a particle source and an image-receiving member,

Fig.4a is a partial view of a printhead structure of a type used in an image forming apparatus, showing the surface of the printhead structure that is facing the toner delivery unit,

Fig.4b is a partial view of a printhead structure of a type used in an image forming apparatus, showing the surface of the printhead structure that faces the intermediate transfer belt,

Fig.4c is a section view across a section line I-I in the printhead structure of Fig. a and across the corresponding section line II-II of Fig.4b,

Fig. 5 is a schematic view of an arrangement for determining variations in the distance between the printhead and the back electrode in accordance with the present invention,

Fig. 6 is a schematic illustration of the second printed circuit on the printhead structure 5 in accordance with a preferred embodiment of the present invention,

Fig. 7 schematically shows an alternative arrangement for determining variations in the distance between the printhead and the back electrode in accordance with the present invention, and

Fig. 8 is a schematic illustration of the second printed circuit on the printhead structure 5 in accordance with a second preferred embodiment of the present invention.

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As shown in Fig.l, an image forming apparatus in accordance with a first embodiment of the present invention comprises at least one print station, preferably four print stations (Y, M, C, K) , an intermediate image receiving member 1, a driving roller . 10, at least one support roller 11, and preferably several adjustable holding elements 12. The four print stations_^ are arranged in relation to the intermediate image-receiving member 1. The image receiving member, preferably a transfer belt 1, is mounted over the driving roller 10. The at least one support roller 11 is provided with a mechanism for maintaining the transfer belt 1 with a constant tension, while preventing transversal movement of the transfer belt 1. The holding elements 12 are for accurately positioning the transfer belt 1 with respect to each print station.

The driving roller 10 is preferably a cylindrical metallic sleeve having a rotation axis extending perpendicular to the motion direction of the belt 1 and a rotation velocity adjusted to convey the belt 1 at a velocity of one addressable dot location per print cycle, to provide line by line scan printing. The adjustable holding elements 12 are arranged for maintaining the surface of the belt at a predetermined distance from each print station. The holding elements 12 are preferably cylindrical sleeves disposed perpendicularly to the belt motion in an arcuated configuration so as to slightly bend the belt 1 at least in the vicinity of each print station in order to create a stabilisation force component on the belt in combination with the belt tension. That stabilisation force component is opposite in direction to, and preferably larger in magnitude than, an electrostatic attraction force component acting on the belt 1 due to interaction with the different electric potentials applied on the corresponding print station.

The holding elements 12 are provided with an electrically conducting surface which is connected to a voltage source for generating a background electric field. These elements 12 thus serve as back electrodes.

The transfer belt 1 is preferably an endless band of 30 to 200 microns thick having composite material as a base. The base composite material can suitably include thermoplastic polyamide resin or any other suitable material having a high thermal resistance, such as 260°C of glass transition point and 388°C of melting point, and stable mechanical properties under temperatures in the order of 250°C. The composite material of the transfer belt has preferably a homogeneous concentration of filler material, such as carbon or the like, which provides a uniform electrical conductivity throughout the entire surface of the transfer belt 1. The outer surface of the transfer belt 1 is preferably coated with a 5 to 30 microns thick coating layer made of electrically conductive polymer material having appropriate conductivity, thermal resistance, adhesion properties, release properties and surface smoothness .

The transfer belt 1 is conveyed past the four different print stations, whereas toner particles are deposited on the outer surface of the transfer belt and superposed to form a four colour toner image . Toner images are then preferably conveyed through a fuser unit 13 comprising a fixing holder 14 arranged transversally in direct contact with the inner surface of the trans er belt . The fixing holder includes a heating element 15 preferably of a resistance type of e.g. molybdenium, maintained in contact with the inner surface of the transfer belt 1. As an electric current is passed through the heating element 15, the fixing holder 14 reaches a temperature required for melting the toner particles deposited on the outer surface of the transfer belt 1. The fusing unit 13 further includes a pressure roller 16 arranged transversally across the width of the transfer belt 1 and facing the fixing holder 14. An information carrier 2, such as a sheet of plain untreated paper or any other medium suitable for direct printing, is fed from a paper delivery unit 21 and conveyed between the pressure roller 16 and the transfer belt. The pressure roller 16 rotates with applied pressure to the heated surface of the fixing holder 14 whereby the melted toner particles are fused on the information carrier 2 to form a permanent image. After passage through the fusing unit 13, the transfer belt is brought in contact with a cleaning element 17, such as for example a replaceable scraper blade of fibrous material extending across the width of the transfer belt 1 for removing all untransferred toner particles from the outer surface.

As shown in Fig.2, a print station in an image forming apparatus in accordance with the present invention includes a particle delivery unit 3 preferably having a replaceable or refillable container 30 for holding toner particles, the container 30 having front and back walls (not shown) , a pair of side walls and a bottom wall having an elongated opening 31 extending from the front wall to the back wall and provided with a toner feeding element 32 disposed to continuously supply toner particles to a developer sleeve 33 through a particle charging member 34. The particle-charging member 34 is preferably formed of a supply brush or a roller made of, or coated with, a fibrous, resilient material. The supply brush is brought into mechanical contact with the peripheral surface of the developer sleeve 33 for charging particles by contact charge exchange due to triboelectrification of the toner particles through frictional interaction between the fibrous material on the supply brush and any suitable coating material of the developer sleeve. The developer sleeve 33 is preferably made of metal coated with a conductive material, and preferably has a substantially cylindrical shape and a rotation axis extending parallel to the elongated opening 31 of the particle container 30. Charged toner particles are held on the surface of the developer sleeve 33 by electrostatic forces essentially proportional to (Q/D)² , where Q is the particle charge and D is the distance between the particle charge center and the boundary of the developer sleeve 33. Alternatively, the charge unit may additionally include a charging voltage source (not shown) , which supplies an electric field to induce or inject charge to the toner particles. Although it is preferred to charge particles through contact charge exchange, the method can be performed using any other suitable charge unit, such as a conventional charge injection unit, a charge induction unit or a corona charging unit, without departing from the scope of the present invention.

A metering element 35 is positioned proximate to the developer sleeve 33 to adjust the concentration of toner particles on the peripheral surface of the developer sleeve 33, to form a relatively thin, uniform particle layer thereon. The metering element 35 may be formed of a flexible or rigid, insulating or metallic blade, roller or any other member suitable for providing a uniform particle layer thickness. The metering element 35 may also be connected to a metering voltage source (not shown) which influences the triboelectrification of the particle layer to ensure a uniform particle charge density on the surface of the developer sleeve.

As shown in Fig.3, the developer sleeve 33 is arranged in relation with a positioning device 40 for accurately supporting and maintaining the printhead structure 5 in a predetermined position with respect to the peripheral surface of the developer sleeve 33. The positioning device 40 is formed of a frame 41 having a front portion, a back portion and two transversally extending side rulers 42, 43 disposed on each side of the developer sleeve 33 parallel with the rotation axis thereof. The first side ruler 42, positioned at an upstream side of the developer sleeve 33 with respect to its rotation direction, is provided with fastening means 44 to secure the printhead structure 5 along a transversal fastening axis extending across the entire width of the printhead structure 5. The second side ruler 43, positioned at a downstream side of the developer sleeve 33, is provided with a support element 45, or pivot, for supporting the printhead structure 5 in a predetermined position with respect to the peripheral surface of the developer sleeve

33. The support element 45 and the f stening axis are so positioned with respect to one another that the printhead structure 5 is maintained in an arcuated shape along at least a part of its longitudinal extension. That arcuated shape has a curvature radius determined by the relative positions of the support element 45 and the fastening axis and dimensioned to maintain a part of the printhead structure 5 curved around a corresponding part of the peripheral surface of the developer sleeve 33. The support element 45 is arranged in contact with the printhead structure 5 at a fixed support location on its longitudinal axis so as to allow a slight variation of the printhead structure 5 position in both longitudinal and transversal direction about that fixed support location, in order to accommodate a possible eccentricity or any other undesired variations of the developer sleeve

33. That is, the support element 45 is arranged to make the printhead structure 5 pivotable about a fixed point to ensure that the distance between the printhead structure 5 and the peripheral surface of the developer sleeve 33 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the developer sleeve 33. The front and back portions of the positioning device 40 are provided with securing members

46 on which the toner delivery unit 3 is mounted in a fixed position to provide a constant distance between the rotation axis of the developer sleeve 33 and a transversal axis of the printhead structure 5.

Preferably, the securing members 46 are arranged at the front and back ends of the developer sleeve 33 to accurately space the developer sleeve 33 from the corresponding holding element 12 of the transfer belt 1 facing the actual print station. As shown in Fig.4a, 4b, 4c, a printhead structure 5 in an image forming apparatus in accordance with the present invention comprises a substrate 50 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the developer sleeve 33, a second surface facing the transfer belt 1, a transversal axis 51 extending parallel to the rotation axis of the developer sleeve 33 across the whole print area, and a plurality of apertures 52 arranged through the substrate 50 from the first to the second surface thereof. The first surface of the substrate is coated with a first cover layer 501 of electrically insulating material, such as for example parylene. A first printed circuit, comprising a plurality of control electrodes 53 disposed in conjunction with the apertures, and, in some embodiments, shield electrode structures (not shown) arranged in conjunction with the control electrodes 53, is arranged between the substrate 50 and the first cover layer 501. The second surface of the substrate is coated with a second cover layer 502 of electrically insulating material, such as for example parylene. A second printed circuit, including a plurality of deflection electrodes 54, is arranged between the substrate 50 and the second cover layer 502. The printhead structure 5 further includes a layer of antistatic material (not shown) , preferably a semiconducting material, such as silicon oxide or the like, arranged on at least a part of the second cover layer 502, facing the transfer belt 1. The printhead structure 5 is coupled to a control unit (not shown) comprising variable control voltage sources connected to the control electrodes 53 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 52 during each print sequence. The control unit further comprises deflection voltage sources (not shown) connected to the deflection electrodes 54 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 52. In some embodiments, the control unit may even include a shield voltage source (not shown) connected to the shield electrodes to supply a shield potential which electrostatically screens adjacent control electrodes 53 from one another.

In a preferred embodiment of the invention, the substrate 50 is a flexible sheet of polyimide having a thickness of the order of about 50 microns. The first and second printed circuits are copper circuits of approximately 8-9 microns thick deposited or otherwise positioned on the first and second surface of the substrate 50, respectively, using conventional techniques. The first and second cover layers 501, 502 are 5 to 10 microns thick parylene laminated onto the substrate 50 using vacuum deposition techniques. The apertures 52 are made through the printhead structure 5 using conventional laser micromachining methods. The apertures 52 preferably have a circular or elongated shape centered about a central axis, with a diameter in a range of 80 to 120 microns, alternatively a transversal minor diameter of about 80 microns and a longitudinal major diameter of about 120 microns.

The printhead structure 5 is preferably dimensioned to perform 600 dpi printing utilizing three deflection sequences in each print cycle, i.e. three dot locations are addressable through each aperture 52 of the printhead structure during each print cycle. Accordingly, one aperture 52 is provided for every third dot location in a transverse direction, that is, 200 equally spaced apertures per inch aligned parallel to the transversal axis 51 of the printhead structure 5. The apertures 52 are generally aligned in one or several rows, preferably in two parallel rows each comprising 100 apertures per inch. Hence, the aperture pitch, i.e. the distance between the central axes of two neighbouring apertures of a same row is 0,01 inch or about 254 microns. The aperture rows are preferably positioned on each side of the transversal axis 51 of the printhead structure 5 and transversally shifted with respect to each other such that all apertures are equally spaced in a transverse direction. The distance between the aperture rows is preferably chosen to correspond to a whole number of dot locations .

The first printed circuit comprises the control electrodes 53 each having a ring shaped structure surrounding the periphery of a corresponding aperture 52, and a connector, preferably extending in the longitudinal direction, connecting the ring shaped structure to a corresponding control voltage source. Although a ring shaped structure is preferred, the control electrodes 53 may take on various shapes for continuously or partly surrounding the apertures 52, preferably shapes having symmetry about the central axis of the apertures. In some embodiments, particularly when the apertures 52 are aligned in one single row, the control electrodes are advantageously made smaller in a transverse direction than in a longitudinal direction.

The second printed circuit comprises the plurality of deflection electrodes 54, each of which is divided into two semicircular or crescent shaped deflection segments 541, 542 spaced around a predetermined portion of the circumference of a corresponding aperture 52. The deflection segments 541, 542 are arranged symmetrically about the central axis of the aperture 52 on each side of a deflection axis 543 extending through the center of the aperture 52 at a predetermined deflection angle d to the longitudinal direction. The deflection axis 543 is dimensioned in accordance with the number of deflection sequences to be performed in each print cycle in order to neutralize the effects of the belt motion during the print cycle, to obtain transversally aligned dot positions on the transfer belt. For instance, when using three deflection sequences, an appropriate deflection angle is chosen to arctan(l/3), i.e. about 18,4°. Accordingly, the first dot is deflected slightly upstream with respect to the belt motion, the second dot is undeflected and the third dot is deflected slightly downstream with respect to the belt motion, thereby obtaining a transversal alignment of the printed dots on the transfer belt. Accordingly, each deflection electrode 54 has an upstream segment 541 and a downstream segment 542, all upstream segments 541 being connected to a first deflection voltage source Dl, and all downstream segments 542 being connected to a second deflection voltage source D2.

The deflection voltage sources Dl and D2 are controlled by a control unit (not shown) . Three deflection sequences (for instance: D1<D2; D1=D2; D1>D2) can be performed in each print cycle, whereby the difference between Dl and D2 determines the deflection trajectory of the toner stream through each aperture 52, and thus the dot position on the toner image.

The deflection experienced by charged toner particles due to the application of an asymmetric electrostatic field across an aperture 52 in the printhead structure 5 depends on a number of factors including the mass and size of the toner particles and the deflecting electric field strength. However, when a constant deflection voltage is applied across a pair of deflection electrodes 541, 542, deflected toner will describe a path that deviates from the normal by a substantially constant angle. Thus the final position of a deflected toner dot depends also on the length of the path. While the deflection voltages applied to the deflection electrodes 54 can be closely controlled, variations may still occur in the spacing of the printhead structure 5 from the image receiving surface, which in this case is the transfer belt 1, since this spacing is maintained by mechanical means, which are subject to wear and also to movement due to repeated vibrations . A further factor affecting the print quality is the amount of toner that is propelled from the developer sleeve through each aperture. This depends on the electric field generated by the control electrodes 53, which in turn is influenced by the voltages applied to the control electrodes 53 and the distance between the developer sleeve 33 and the printhead structure 5. As for the deflection electrodes 54, the applied control electrode voltages can be controlled to high precision. However, while the positioning device 40 (see Fig. 3) can maintain a relative position between the printhead to certain tolerances, the printhead structure 5 is flexible, so a small amount of relative movement is difficult to eliminate. Variations in this distance cause the variation in dot density from one page to the next, or more critically if the printhead structure 5 is no longer parallel with the developer sleeve 33, across one and the same page .

Turning now to Fig. 5 there is shown an arrangement which allows variations in distance between the printhead structure 5 and a further electrically conductive element in the printing device to be detected and quantified. Specifically, the illustrated arrangement permits changes in the spacing between the printhead structure 5 and the holding element 12, which serves as a back electrode 12 and generates the background electric field, to be measured. The arrangement in Fig. 5 includes a holding element 12 and an electrode that is mounted on a surface of the printhead structure 5 facing the holding element 12. In the present example, the electrode used is a deflection electrode 54, which is included in the second printed circuit and opposes the holding element 12. The deflection electrode 54 is separated from the holding element 12 by a spacing of length "L" . It will be appreciated that while only one electrode 54 is illustrated, the electrode used for measuring the distance L could be formed from several deflection electrodes 54 connected together. The holding element 12 is connected to a signal generator 60. The deflection electrode 54 is connected to one terminal of a resistor 62. The other terminal of the resistor is connected to ground. The deflection electrode is also connected to the input of a measuring unit 61 that measures the current flowing through the resistor. Since the value of the resistor 62 is constant, in practice the measuring unit 61 preferably measures the voltage across the resistor 62. The resistor 62 preferably has a value of lkΩ or less.

The combination of the electrically conductive holding element 12 and electrode 54 or group of electrodes form a capacitor that is connected to ground through the resistor 62. In this series RC circuit, the current I across the capacitor is defined by the expression

I = C(dV/dt) ,

where C is the capacitance of the capacitor and dV/dt is the derivative of the voltage across the capacitor against time. If a constantly varying voltage is applied to the holding element 12 by the signal generator 60, i.e. if dV/dt applied to the circuit is constant, the current flowing through the circuit will be directly proportional to the capacitance. The capacitance of the circuit element formed by the electrode 54 and the holding element 12 depends on the size and shape of both the holding element 12 and the electrode 54 or group of electrodes, the permittivity of the material contained in the gap between these two elements, in this case air, and the distance . The size and shape of the holding element 12 and electrode 54 will be constant for any single printing device. It is further assumed that the permittivity of the air gap will remain substantially unchanged. Thus, the sole factor that will influence the capacitance of the electrode 54 and the holding element 12 and so influence the current through, or the voltage across, the resistor 62 will be a change in the length L of the gap. The signal generator 60 may apply a ramp voltage to the holding unit to enable the measuring unit 61 to perform the measurement of the current or voltage of the resistor 62. However, the signal generator 60 preferably applies a periodically varying voltage, such as a periodic ramp voltage or most preferably a triangle voltage waveform with symmetrical rising and falling slopes, as illustrated in Fig. 5. The potential applied across the deflection electrodes 54 and the holding element 12 is preferably at least 150 Volt, but due to the small distance separating these elements, typically of the order of 150 μm, is preferably limited to 400 Volt to prevent the risk of discharge. A further factor limiting the magnitude of the applied voltages is the maximum stress that can be tolerated by the printhead structure 5 due to internal electric fields. The generation of substantial internal electric fields could cause the printhead structure 5 to break. The distance measurement takes place in between printing runs so the control electrodes will be . inactive during the measurement and thus generally grounded. A safe limit for the applied potential would thus be the maximum potential difference applied between the control and deflection electrodes during printing. This is of the order of 150 Volt to 200 Volt. A preferred applied voltage swing is between 200 Volt and -200 Volt. Furthermore, the frequency of the voltage variation is 2 kHz or less, and preferably of the order of 1 kHz .

Fig. 6 shows a schematic illustration of the second printed circuit that has been modified to enable the distance L measurement between the electrodes 54 on the printhead structure 5 and the holding element 12 to be made. As can be seen from Fig. 6, the deflection electrodes 54 are connected in groups . Preferably these groups are distributed across the printhead structure 5. If only two groups are used, these are preferably located at the two extreme ends of the printhead structure 5 , since this is where the greatest variation in spacing L across the printhead structure 5 will be manifest. The number of electrodes 54 included in any one group determines the capacitance value . The capacitance value typically varies from around 0.1 pF to 10 pF depending on the size of the group. The resistor 62 is also integrated in the printhead structure 5 and forms part of the second printed circuit. Specifically, two resistors 621, 622 are provided each connected to a separate group of deflection electrodes 54. In the figure, one resistor 621 is connected to a group of first electrode segments 541, while the other resistor 622 is connected to a group of second electrode segments 542. The resistors 621, 622 are each connected to ground via a switch 63 that is external to the printhead structure 5 and is controlled by the printer controller (not shown) . The deflection electrode segment groups 541, 542 are also each connected to deflection voltage sources Dl, D2 via a switch 64 that is likewise external to the printhead structure 5 and controlled by the printer controller (not shown) . The printer controller controls the switches 63 and 64 to open alternately, such that the deflection electrodes 541, 542 are either connected through the resistors 621, 622 to ground to enable measurement of the distance L, or are connected to the deflection control voltage sources Dl, D2 for normal print operation. By separating the electrodes or electrode groups, the distance measurement can be performed at all designated locations across the printhead structure 5 simultaneously.

While the switches 63 and 64 are shown external to the printhead structure 5 it will be understood that these switches could alternatively be integrated in the second printed circuit in the printhead structure 5 in the form of FETs, for example. The ground connection could likewise be provided within the printhead structure 5 either as a separate terminal on the second printed circuit or as a ground plane. The switches 64 between the deflection electrodes 54 and the deflection voltage sources Dl, D2, would be connected to an external terminal of the printhead structure 5.

When a variation in the measured current indicating a change in capacitance caused by a change in the length L is determined by the measuring unit 61, this information is relayed to the printer controller (not shown) , which controls the deflection control voltage sources Dl, D2, to compensate for this change in distance. The actual voltage correction required to compensate for a given change in capacitance value is determined on manufacture by a suitable calibration. If the measuring unit 61 determines that the distance L has increased, the deflection voltage supplied by the voltage sources Dl, D2 is reduced, while if it is determined that the distance L has become shortened, the deflection voltages are increased. The deflection electrodes are also used to control the sharpness of a printed dot by focussing the projected toner beam exiting from an aperture 52. Since the print sharpness may also be adversely affected by a change in the distance L, the printer controller may also adjust this focussing voltage accordingly.

While in Fig. 6 two groups of deflection electrodes are utilised, it will be appreciated by those skilled in the art that other numbers and sizes of deflection electrode groupings may be used depending on the measurements required.

Moreover, although the deflection electrodes 54 are utilised for the capacitance measurement in the example of Fig. 6, separate electrodes connected to resistors may be provided on the printhead structure 5 specifically for performing the capacitance measurement . This has the advantage of substantially simplifying the circuit configuration, since no switches are required. The capacitance measurement circuit will furthermore be completely separate from the deflection circuit . Fig. 7 shows an alternative arrangement for determining variations in the distance L. In Fig. 7, the deflection electrode 54, or group of deflection electrodes is again connected to a resistor 72, however they are connected to form a series RC circuit. The resistor 72 is connected to a measuring unit 71, which measures the current flowing through the series RC circuit. However, in contrast to the arrangement of Fig. 5, the monitoring unit 71 is not measuring variations in the current amplitude, but rather measures the phase of the current in the RC circuit. The resistor is also connected to a signal generator 70 which applies a sinusoidal voltage, v = V sin ώt, to the RC circuit . The current response in the RC circuit is

i = (V/ [R² + (l/toC)²]^H) sin (tot + θ)

It is apparent from this expression that the current i leads the voltage v by the angle θ = arctan (l/foCR) . As for the example described with reference to Figs . 5 and 6, any variation in C will result in a variation in this phase difference between the applied voltage and the measured current. Accordingly, a variation in the distance L can be detected by measuring this phase difference. In this case, the resistor 72 needs to have a relatively high value to enable a useful measurement to be made. A suitable value for the resistor 72 is at least 100 kΩ. Preferably this resistor 72 is chosen to ensure that a relatively low voltage is present at the input of the measuring unit 61. In the configuration of Fig. 7, other stray capacitances, such as due to cables and the like, will influence the measurement made. This is in contrast to the embodiment of Figs. 5 and 6, which measures only the capacitance between the back electrode 12 and deflection electrode 54, or group of electrodes. In order to reduce the effect of these stray capacitances on the measurement as a whole, the resistor 72 serves to "dampen" the voltage at the input of the measuring unit. While the applied voltage used in the example described with reference to Fig. 7 is a sinusoid, any periodic voltage can be used since a periodic voltage can be replaced by an equivalent combination of sinusoids . A periodic voltage with the same magnitude and frequency as that utilised for the arrangement in Fig. 5 are appropriate here also. The advantage of this arrangement is that the deflection voltage sources Dl, D2 can be used to apply a suitable periodic voltage. In this case, the printhead structure 5 can be configured as shown in Fig. 8.

Fig. 8 shows a printhead structure with the second printed circuit containing the deflection electrodes 54 modified to enable the distance measurement between the printhead structure and the back electrode 12. The configuration shown in Fig. 8 is similar to that shown in Fig. 6. In particular the deflection electrodes 54 are connected in groups. As for the embodiment shown in Fig. 6, these groups conveniently consist of the same type of electrode segments 541, 542, as these electrode segments are naturally separated and are driven by separate deflection voltages Dl and D2. Each group is further connected to a resistive element 721, 722. The resistive elements 721, 722 are selectively coupled to the voltage source Dl, D2, respectively via a switch 65. The switch may also be configured such that the deflection voltage sources Dl, D2 bypass the resistive elements altogether, for normal printing operation. This second path is connected to a terminal for the monitoring unit 61.

When the image receiving member is an electrically conductive transfer belt 1 as in the present example, it may be preferable to monitor distance variations affecting dot deflection and print sharpness utilising the capacitance between this belt 1 and the electrodes on the printhead structure 5. The configuration for performing such a measurement would be similar to that illustrated in Fig. 5. In this case the signal generator 60 would preferably be connected to the belt 1 by some form of brush contact. Utilising the belt 1 to conduct such a measurement has the advantage that the measurement of the distance L can be conducted in all print stations of a multi-colour printing device simultaneously. When the arrangement is configured to measure the phase difference between the voltage and current in the RC circuit as for the circuit shown in Fig. 7, the transfer belt 1 is connected to ground.

This form of distance measurement is not limited simply to the deflection distance. Another distance that can be measured effectively using the embodiments illustrated in Figs . 5 and 6 or 7 is the distance between developer sleeve 33 and printhead structure 5. In this case the measurement electrode is located on the upper surface of the printhead structure 5 facing the developer sleeve . As for the measurement of the distance , additional electrodes and resistors may be provided on the first printed circuit of the printhead structure 5. Alternatively, the control electrodes 53, either individually, in combination or in groups, could be utilised by providing a circuit arrangement analogous to that illustrated in Fig. 6. Again, measurements are preferably taken at electrodes or electrode groups at different locations to provide a measurement of any change in distance across the printhead structure 5. In an analogous arrangement to that shown in Fig. 5, the signal generator 60 would apply a varying voltage to the developer sleeve 33.

As mentioned above, a consequence of any variation in the distance between the printhead structure 5 and the developer sleeve is the change in the amount of toner that passes through each aperture 52 in the printhead structure 5. Thus, any variation in this distance indicated by the measured variation in capacitance, can be mitigated by changing the control voltages applied to the control electrodes . This change may concern the absolute voltage values applied to the control electrodes. Alternatively an initial "kick" voltage level may be raised or lowered depending on the measured result. The kick voltage serves to provide the initial release inertia to the charged toner particles on the developer sleeve 33.

In the examples described above, the current or voltage measurement has been performed at the printhead structure 5. In an alternative arrangement, the resistor or resistors 62, 72 and measuring unit 61, 71 are connected to the other element, be this the holding element 12, belt 1 or the developer sleeve. In the case of the measurement process illustrated in Fig. 5 the signal generator 60 is connected directly to the electrode (s) or electrode groups on the printhead structure 5; for the measurement process illustrated in Fig. 7, the signal generator 70 may be connected either to the printhead structure 5 or to the resistor 72. This has the advantage that the control and deflection voltage sources already present in the print station may be utilised for the measurement process . Switches may be provided to cut out specific groups of electrodes from the circuit to allow the distance between the printhead structure 5 and the other element to be measured at different locations on the printhead structure 5. For the measurement of the distance between the holding element 12 and the printhead structure 5 , the holding element 12 is preferably maintained at a low voltage, preferably 10 Volts or less and most preferably 1 V, above ground by a suitably selected resistor 62.. This minimises the effect of parasitic capacitances on the measurements.

In operation, the measurement of capacitance or other electrical property between one or a group of electrodes on the printhead structure 5 and a further element of the printing device as described above is controlled to be performed between printing runs . The start of the measurement process is controlled by the printer controller (not shown) . The measurement may be performed at a suitable moment once a predetermined time has elapsed. Alternatively, the measurement step may be performed after a predetermined number of pages have been printed. Measurement may also be performed on request by the operator, for example when the operator notices an anomaly in the printed pages .

Claims

What is claimed is ;

1. An image forming apparatus , including a particle carrier (33) for holding a source of charged toner particles, a back electrode (12) for generating a background electric field for accelerating the transport of charged toner particles from said particle carrier (33) towards said back electrode (12) , a printhead structure (5) disposed in said background electric field having a plurality of apertures (52) and an electrically conductive layer including at least one electrode (53, 54) ; control voltage sources connected with said electrically conductive layer to control the transport of charged toner particles through said apertures, and means (12) for transporting an image receiving member (1) between said printhead structure (5) and said back electrode (12) for intercepting the transported charged particles, the apparatus being characterised by, means (60, 61, 62; 70, 71, 72) associated with at least one of said electrodes (53, 54) for monitoring variations in an electrical property between said at least one electrode (53, 54) and a second element of said apparatus consisting of said back electrode (12) , said particle carrier (33) and/or said image receiving member (1) , and for determining a change in length (L) of a gap between said at least one electrode and said second element on the basis of said monitored variations .

2. An apparatus as claimed in claim 1, characterised in that said electrical property is capacitance.

3. An apparatus as claimed in claim 1 or 2 , characterised in that said monitoring means include means (60, 70) for applying a constantly varying voltage in a circuit including said electrode (53, 54) and said second element (12, 33, 1), and means (62, 61; 72, 71) for measuring the current flowing in said circuit .

4. An apparatus as claimed in any one of claims 1 to 3 , characterised in that said monitoring means include means (60, 70) for applying a periodically varying voltage in a circuit including said electrode and said second element and monitoring means for measuring the current flowing in said circuit.

5. An apparatus as claimed in claim 4, characterised in that said voltage applying means are adapted to apply a potential difference between said electrode and said second element of 400 V or less.

6. An apparatus as claimed in claim 4 or 5, characterised in that said voltage applying means are adapted to apply a voltage with a swing of between -200 Volt to 200 Volt.

7. An apparatus as claimed in claim 5 or 6 , characterised in that said voltage applying means are adapted to apply a periodic voltage with frequency of at most 2 kHz .

8. An apparatus as claimed in any one of claims 2 to 7, characterised in that said circuit includes a resistive element (62, 72) coupled to one of said electrode and said second element

9. An apparatus as claimed in claim 8, characterised in that said monitoring means include means (61, 71) for measuring the voltage across said resistor (62, 72).

10. An apparatus as claimed in claim 9, characterised in that said resistive element has a resistance of at most 1 kΩ.

11. An arrangement as claimed in claim 8, characterised in that said monitoring means include means (71) adapted to measure the phase of the current through the resistive element .

12. An apparatus as claimed in any previous claim, characterised in that said electrode (54, 53) is arranged on a surface of said printhead structure (5) opposing said second element .

13. An apparatus as claimed in any previous claim, characterised in that said electrode includes at least one group of electrode elements (53, 54), the electrode elements in each group being electrically coupled at least during operation of said monitoring means .

14. An apparatus as claimed in any previous claim, characterised in that said electrode includes at least one deflection electrode (54) and said second element is the back electrode (12) .

15. An apparatus as claimed in claim 14, characterised in that said electrode includes a group of interconnected deflection electrodes.

16. An apparatus as claimed in any one of claims 1 to 15, characterised by means responsive to said monitoring means for adjusting control voltages applied to control and deflection electrodes (53, 54) associated with each aperture (52) of the printhead structure (5) to substantially mitigate the effects of a change in length of said gap (L) .

17. A printhead structure for use in the electrostatic printing apparatus of any one of claims 1 to 15, said structure (5) including a substrate layer (50) having a plurality of apertures (52) and at least one electrically conductive layer arranged on a first surface of said substrate layer (50) , said electrically conductive layer including at least one electrode (53, 54), characterised in that, said electrically conductive layer further includes at least one resistive element (62, 72) having a first terminal connected to said at least one electrode (53, 54) .

18. A printhead structure as claimed in claim 17, characterised in that a plurality of electrodes (53, 54) are electrically coupled to form at least one group of electrodes, each group of electrically coupled electrodes being connected to a first terminal of a resistive element (62, 72) .

1 . A printhead structure as claimed in any one of claims 17 or 18, characterised by first switching means (64) for selectively connecting said at least one electrode to an external terminal .

20. A printhead structure as claimed in any one of claims 17 to 19, characterised by second switching means (63) for selectively connecting a second terminal of said resistive element (62) to ground.

21. A method of operating an image forming apparatus including a particle carrier (33) for holding a source of charged toner particles, a back electrode (12) for generating a background electric field for accelerating the transport of charged toner particles from said particle carrier towards said back electrode, a printhead structure (5) disposed in said background electric field and having a plurality of apertures (52) and an electrically conductive layer including at least one electrode (53, 54) ; control voltage sources connected with said electrically conductive layer for controlling the transport of charged toner particles through said apertures, and means (12) for transporting an image receiving member (1) between said printhead structure (5) and said back electrode (12) for intercepting the transported charged toner particles, the method being characterised by measuring an electrical property between said electrode (53, 54) and a second element of said apparatus consisting of said back electrode (12) , said particle carrier (33) and/or said image receiving member (1) to determine a variation in length of a gap separating said electrode and said second element .

22. A method as claimed in claim 21, characterised in that said electrical property is capacitance.

23. A method as claimed in claim 21 or 22, characterised by performing said measurement step after a predetermined time period.

24. A method as claimed in any one of claims 21 to 23, characterised by performing said determining step after printing a predetermined number of pages .

25. A method as claimed in any one of claims 21 to 24, characterised by performing said determining step on request by an operator.