Patent Publication Number: US-6209990-B1

Title: Method and apparatus for coating an intermediate image receiving member to reduce toner bouncing during direct electrostatic printing

Description:
FIELD OF THE INVENTION 
     The present invention relates to direct electrostatic printing methods in which charged toner particles are transported in accordance with an image information from a particle source to an image transfer member to form a toner image which is subsequently transferred onto an information carrier. 
     BACKGROUND OF THE INVENTION 
     According to a direct electrostatic printing method, such as that disclosed in U.S. Pat. No. 5,036,341, a background electric field if produced between a developer sleeve and a back electrode to enable the transport of charged toner particles therebetween. A printhead structure, such as an electrode matrix provided with a plurality of selectable apertures, is interposed in the background electric field and connected to a control unit which converts an image information into a pattern of electrostatic control fields which selectively open or close the apertures, thereby permitting or restricting the transport of toner particles from the developer sleeve. The modulated stream of toner particles allowed to pass through opened apertures impinges upon an information carrier, such as paper, conveyed between the printhead structure and the back electrode, to form a visible image. 
     According to such a method, each single aperture is utilized to address a specific dot position of the image in a transverse direction, i.e. perpendicular to paper motion. Thus, the transversal print addressability is limited by the density of apertures through the printhead structure. For instance, a print addressability of 300 dpi requires a printhead structure having 300 apertures per inch in a transversal direction. 
     A new concept of direct electrostatic printing, hereinafter referred to as dot deflection control (DDC), was introduced in U.S. patent application Ser. No. 80/621,074. According to the DDC method each single aperture is used to address several dot positions on an information carrier by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward a paper, and thereby the location of the obtained dot. The DDC method increases the print addressability without requiring a larger number of apertures in the printhead structure. This is achieved by providing the printhead structure with at least two sets of deflection electrodes connected to variable deflection voltages which, during each print cycle, sequentially modify the symmetry of the electrostatic control fields to deflect the modulated stream of toner particles in predetermined deflection directions. 
     For instance, a DDC method performing three deflection steps per print cycle, provides a print addressability of 600 dpi utilizing a printhead structure having 200 apertures per inch. 
     An improved DDC method, disclosed in U.S. patent application Ser. No. 08/759,481, provides a simultaneous dot size and dot position control. This later method utilizes the deflection electrodes to influence the convergence of the modulated stream of toner particles thus controlling the dot size. According to the method, each aperture is surrounded by two defection electrodes connected to a respective deflection voltage D 1 , D 2 , such that the electrode field generated by the control electrodes remains substantially symmetrical as long as both deflection voltages D 1 , D 2  have the same amplitude. The amplitudes of D 1  and D 2  are modulated to apply converging forces on toner to obtain smaller dots. The dot position is simultaneously controlled by modulating the amplitude difference between D 1  and D 2 . Utilizing this improved method enables 60 μm dots to be obtained utilizing 160 μm apertures. 
     It can be considered a drawback of current DDC methods that the properties of the information carrier, e.g. paper properties, can influence the accuracy of the dot size and dot position control. For instance, when printing directly onto paper, the deflection, and thus the dot positions become dependent on paper thickness, conductivity, triboelectric charge concentration, humidity etc. Therefore, there seems to still exist a need to improve the current DDC method. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method of and device for improving the accuracy of dot deflection control in direct electrostatic printing methods. 
     A further object of the present invention is to provide a method of direct electrostatic printing which is substantially unaffected by the physical properties of the information carrier onto which an image is to be recorded. 
     Still a further object of the present invention is to provide a method of and a device for reducing scattering in direct electrostatic printing methods. 
     Yet a further object of the present invention is to provide a method of and a device for preventing clogging of passages through which toner particles pass in direct electrostatic printing methods. 
     Another object of the present invention is to provide a method of and device for reducing or eliminating the influence of the electrostatic field used for toner particle transport on anything other than the intended toner particles in direct electrostatic printing methods. 
     Still another object of the present invention is to provide a method of and a device for trajecting toner particles to predetermined positions, in view of an image which is to be recorded, substantially unaffected by a wide range of environmental conditions and unaffected by the physical properties of the information carrier onto which an image is to be recorded. 
     Yet another object of the present invention is to provide a method of and a device for trajecting toner particles to predetermined positions, in view of an image which is due to speed variations caused for example by mechanical imperfections. 
     Said objects are achieved according to the invention by providing an intermediate image receiving medium whose properties do not alter the accuracy of the dot deflection control, in order to thereby first form a toner image on the intermediate image receiving medium and thereafter transfer that image to an information carrier. 
     Said objects are also achieved according to the invention by an image recording device and method for recording an image onto an information carrier. The image recording device comprises a pigment particle source, a voltage source, a printhead structure, and an intermediate image receiving member. The pigment particle source provides pigment particles. The intermediate image receiving member and the printhead structure move relative to each other during recording. The intermediate image receiving member has a first face and a second face. The printhead structure is placed inbetween the pigment particle source and the first face of the intermediate image receiving member. The voltage source is connected to the pigment particle source and the back electrode thereby creating an electrical field for transport of pigment particles from the pigment particle source toward the first face of the intermediate image receiving member. The printhead structure includes control electrodes to thereby be able to selectively open or close passages/apertures through the printhead structure to permit or restrict the transport of pigment particles to thereby enable the formation of a pigment image on the first face of the intermediate image receiving member, which pigment image is subsequently transferred to an information carrier. Where according to the method and device of the invention the first face of the intermediate image receiving member is substantially evenly coated with a layer of bouncing reduction agent thus providing a surface on the first face of the intermediate image receiving member that the pigment particles transported through the printhead structure substantially adhere to substantially without bouncing. 
     Suitably the bouncing reduction agent is a liquid having adhesion properties suitable for the adhesion of pigment particles to the first face of the intermediate image receiving member and also suitably the image recording device further comprises a film application means for applying the bouncing reduction agent liquid substantially evenly as a film layer onto the first face of the intermediate image receiving member. Suitably the bouncing reduction agent is a silicone oil having appropriate adhesion properties for reducing bouncing of pigment particles when pigment particles are transferred onto the first face of the intermediate image receiving means and also having appropriate release properties when a pigment image is transferred to an information carrier from the intermediate image receiving member. 
     The intermediate image receiving member can suitably include a transfer belt positioned at a predetermined distance from and being movable in reduction to the printhead structure, the transfer belt suitably being substantially of uniform thickness. The transfer belt is suitably supported by at least one holding element arranged on the side of the second face of the transfer belt adjacent to the print station. 
     In certain embodiments according to the invention the image recording device suitably further comprises a transfuser having heating means and pressurising means for transferring a pigment image on the surface of the first face of the intermediate image receiving member to an information carrier by locally applying heat and pressure to the information carrier and the pigment image by the heating means and pressurising means and thereby transferring the pigment image to the information carrier. The pigment particles can suitably be of a polyester type. The printhead structure can advantageously include deflection electrodes to thereby enable the deflection of pigment particles against predetermined locations on the first face of the image receiving member in view of the image which is to be recorded. An image recording device whose printhead structure includes deflection electrodes for controlling the deflection of pigment particles in transport, can advantageously further comprise deflection control feedback means for providing a deflection feedback signal to thereby control the deflection electrodes in such a way that pigment particles are, for formation of a pigment image on the intermediate image receiving member in view of the image which is to be recorded, trajected toward predetermined locations on the intermediate image receiving member. 
     The image recording device advantageously includes at least two pigment particle sources with corresponding control electrodes and passages/apertures on and in at least one printhead structure. The image recording device preferably includes four pigment particle sources with corresponding control electrodes and passages/apertures on and in at least one printhead structure. Image recording devices comprising at least two pigment particle source preferably comprises a corresponding printhead structure with control electrodes and passages/apertures for each pigment particle source. 
     Preferably the image recording device comprises an intermediate image receiving member bending means to enable the intermediate image receiving member to be bent a predetermined angle in the vicinity of the printhead structure in such a way that, in combination with a tension of the intermediate image receiving member, a stabilization force component is created which is equal to or greater in magnitude than a field force component created by the electrical field and acting on the intermediate image receiving member, to thereby oppose distance fluctuations between the intermediate image receiving member and the printhead structure which is caused by the field force component on the intermediate image receiving member. 
     Certain embodiments of the image recording device suitably comprises an intermediate image receiving member position measuring means for measuring the position of the intermediate image receiving member in relation to the passages/apertures to thereby be able to synchronize the selective opening and closing of the passages/apertures through the printhead structure according to the relative movement of the printhead structure and the intermediate image receiving member to thereby enable the formation of a pigment image at a predetermined position on the intermediate image receiving member in view of the image which is to be recorded. 
     Still in further embodiments according to the invention the image recording device suitably comprises a pressure changing means which can create a pressure difference on the side of the second face of the intermediate image receiving member in the vicinity of the passages/apertures of the printhead structure, and where the intermediate image receiving member comprises a cleaning area for cleaning purposes and a separate image area intended for reception of pigment particles for formation of a pigment image thereon, where the cleaning area includes at least one slot between the first face and the second face intended for transmitting the pressure difference through the intermediate image receiving member to thereby, in cooperation with the pressure changing means in the vicinity of the passages/apertures of the printhead structure, dislodge pigment agglemeration for cleaning the passages/apertures of the printhead structure. 
     Said objects are also achieved according to the invention by a method for recording an image to an information carrier. The method comprises a number of steps. In a first step pigment particles are provided from a pigment particle source. In a second step an intermediate image receiving member and a printhead structure are moved relative to each other during recording. In a third step a first face of the intermediate image receiving member is coated substantially evenly with a layer of bouncing reduction agent thus providing a surface on the first face of the intermediate image receiving member that transported pigment particles will substantially adhere to substantially without bouncing. In a fourth step an electrical field is created for transporting pigment particles from the pigment particle source toward the first face of the intermediate image receiving member. In a fifth step apertures through a printhead structure are selectively opened or closed to permit or restrict the transporting of pigment particles to thereby enable the formation of a pigment image on the first face of the intermediate image receiving member. And in a final sixth step the pigment image is subsequently transferred to an information carrier. 
     Further variations of the method according to previously described enhancements are possible in view of the application of the invention. 
     Said objects are also achieved according to the invention by an image recording device and method for recording an image onto an information carrier. The image recording device comprises an intermediate image receiving member, a pigment particle source, a printhead structure, a back electrode, and a voltage source. The pigment particle source provides pigment particles. The intermediate image receiving member is flexible under a tension and has a first face and a second face. The printhead structure is placed inbetween the pigment particle source and the first face of the intermediate image receiving member. The voltage source is connected to the pigment particle source and the back electrode thereby define an electrical field plane and create an electrical field for transport of pigment particles from the pigment particle source toward the first face of the intermediate image receiving member. The printhead structure includes control electrodes to thereby be able to selectively open or close passages/apertures through the printhead structure to permit or restrict the transport of pigment particles to thereby enable the formation of a pigment image on the first face of the intermediate image receiving member, which pigment image is subsequently transferred to an information carrier. According to the method and device of the invention the image recording device comprises an intermediate image receiving member bending means to enable the intermediate image receiving member to be bent a predetermined angle in the vicinity of the printhead structure in such a way that, in combination with the tension of the intermediate image receiving member, a stabilization force component is created which is equal to or greater in magnitude than a field force component created by the electrical field and acting on the intermediate image receiving member, to thereby oppose distance fluctuations between the intermediate image receiving member and the printhead structure which is caused by the field force component on the intermediate image receiving member. 
     The stabilization force component is preferably directed in a direction opposite and having a greater magnitude than the field force component. Suitably the intermediate image receiving member bending means comprising the back electrode. The intermediate image receiving member bending means suitably comprises one holding element supporting the intermediate image receiving member, where the holding element is arranged on the side of the second face of the intermediate image receiving member adjacent to the passages/apertures in such a way that the intermediate image receiving member is bent over the holding element creating an angle of more than 180° on the first face of the intermediate image receiving member. The intermediate image receiving member is suitably frictionally supported by the holding element. As an alternative when the holding element is rotationable then the intermediate image receiving member is suitably rotationally supported by the holding element. The holding element suitably comprises the back electrode. 
     In some embodiments according to the invention the intermediate image receiving member bending means bends the intermediate image receiving member in such a way that a first angle greater than 180° is created on the first face of the intermediate image receiving member perpendicular to the electrical field plane, and where a second angle and a third angle make up the part greater than 180°. The second angle is defined in reference to a reference plane on the first side of the electrical field plane and the third angle is defined in reference to the reference plane on the second side of the electrical field plane. The reference plane is a plane which is perpendicular to the electrical field plane and parallell to the intermediate image receiving member where the first face of the intermediate image receiving member is the closest to the passages/apertures. Preferably the second angle and the third angle are not equal in magnitude or they are substantially equal in magnitude, all depending on the specific embodiment of the invention. 
     If the particular embodiment is such that the image recording device includes at least two pigment particle sources, then each particle source has their respective corresponding, electrical field, electrical field plane, control electrodes and passages/apertures, stabilization force component, field force component, first angle, second angle, third angle, and reference plane. Which means that the intermediate image receiving member bending means bends the intermediate image receiving member for each one of the pigment particle sources in the vicinity of the respective passages/apertures. In some embodiments the image recording device is capable of recording color images and includes four pigment particle sources. In some embodiments the respective stabilization force components are not parallell. In some embodiments the respective second angles and third angles are substantially equal in magnitude. In some embodiments the respective second angles and third angles are each in the range of 0.5° to 10°. In some embodiments the magnitude of each respective stabilization force component is substantially equal. In some embodiments the magnitudes of the respective stabilization force components are not all substantially equal. In some embodiments the magnitudes of the respective stabilization force components are substantially unequal. 
     Further embodiments according to the previously described enhancements are possible in view of the application of the invention. 
     Said objects are also achieved according to the invention by a method for recording an image to an information carrier. The method comprises a number of steps. In a first step pigment particles are provided from a pigment particle source. In a second step a tension is added to a flexible intermediate image receiving member having a first face and a second face. In a third step an electrical field is created for transport of pigment particles from the pigment particle source toward the first face of the intermediate image receiving member. In a fourth step the intermediate image receiving member is bent a predetermined angle in the vicinity of a printhead structure in such a way that, in combination with the tension of the intermediate image receiving member, a stabilization force component is created which is equal to or greater in magnitude than a field force component created by the electrical field and acting on the intermediate image receiving member, to thereby oppose distance fluctuations between the intermediate image receiving member and the printhead structure which is caused by the field force component on the intermediate image receiving member. In a fifth step apertures through the printhead structure are selectively opened or closed to permit or restrict the transporting of pigment particles to thereby enable the formation of a pigment image on the first face of the intermediate image receiving member. And finally in a sixth step the pigment image is subsequently transferred to an information carrier. 
     Further variation of the method according to previously described enhancements are possible in view of the application of the invention. 
     Said objects are also achieved according to the invention by an image recording device and method for recording an image to an information carrier. The image recording device comprises a pigment particle source, a voltage source, a printhead structure, a control unit, an intermediate image receiving member. The pigment particle source provides pigment particles. The intermediate image receiving member and the printhead structure move relative to each other during recording. The intermediate image receiving member has a first face and a second face. The printhead structure is placed inbetween the pigment particle source and the first face of the intermediate image receiving member. The voltage source is connected to the pigment particle source and the back electrode to thereby create an electrical field for transport of pigment particles form the pigment particle source toward the first face of the intermediate image receiving member. The printhead structure includes control electrodes connected to the control unit to thereby selectively open or close passages/apertures through the printhead structure to permit or restrict the transport of pigment particles to thereby enable the formation of a pigment image on the first face of the intermediate image receiving member. The pigment image is subsequently transferred to an information carrier. According to the method and device of the invention the printhead structure includes deflection electrodes connected to the control unit for controlling the deflection of pigment particles in transport. The image recording device further comprises deflection control feedback means for providing a deflection feedback signal to the control unit to thereby control the deflection electrodes in such a way that pigment particles are, for formation of a pigment image on the intermediate image receiving member in view of the image which is to be recorded, trajected toward predetermined locations on the intermediate image receiving member. 
     In some embodiments of the invention the deflection control feedback means suitably comprises measuring means for measuring the density of a pattern formed on the intermediate image receiving member to thereby in dependence of the measured density create the deflection feedback signal. In other embodiments of the invention the deflection control feedback means suitably comprises measuring means for measuring the thickness of a pigment particle pattern formed on the intermediate image receiving member to thereby in dependence of the measured thickness create the deflection feedback signal. In further embodiments of the invention the deflection feedback signal is suitably created in dependence of variations of the measures thickness of a pigment particle pattern formed on the intermediate image receiving member. In still other embodiments of the invention the deflection control feedback means suitably comprises optical measuring means for optically measure the optical density of a pattern formed on the intermediate image receiving member to thereby in dependence of the measured optical density create the deflection feedback signal. In still further embodiment of the invention the deflection control feedback means suitably comprises capacitive measuring means for measuring the capacitance of a pattern formed on the intermediate image receiving member to thereby in dependence of the measures capacitance create the deflection feedback signal. In even further embodiments of the invention the deflection control feedback means suitably comprises electrical charge measuring means for measuring the electrical charge of a pattern formed on the intermediate image receiving member to thereby in dependence of the measures electrical charge create the deflection feedback signal. In even other embodiments of the invention the deflection control feedback means suitably comprises magnetic flux measuring means for measuring the density of a pattern formed on the intermediate image receiving member to thereby in dependence of the measured density create the deflection feedback signal. 
     According to one embodiment of the invention the deflection control feedback means suitably comprises measuring means for measuring the density of a predetermined test pattern formed on the intermediate image receiving member to thereby in dependence of the measured density create the deflection feedback signal. The predetermined test pattern can suitably be a line of predetermined width and length. The intermediate image receiving member can suitably comprise a test area intended for reception of pigment particles for formation of the predetermined test pattern and a separate image area intended for reception of pigment particles for formation of a pigment image thereon. 
     According to the invention at least one passage/aperture suitably comprises two deflection electrodes. The two deflection electrodes of a passage/aperture can suitably have a line of symmetry through both electrodes, which line of symmetry is not parallel or perpendicular to the direction of relative movement between the intermediate image receiving member and the printhead structure. The magnitude of the deflection of pigment particles in transport via a passage/aperture in question is suitably determined by controlling a voltage potential difference between the two deflection electrodes of the passage/aperture in question. A dot size of a dot formed on the intermediate image receiving member by transported pigment particles through a passage/aperture in question is suitably determined according to the invention by controlling an absolute voltage potential applied to both deflection electrodes of the passage/aperture in question. According to the invention the printhead structure and preferably has a first side towards the pigment particle source and a second side towards the intermediate image receiving member, and the control electrodes are preferably arranged on the first side of the printhead structure and the deflection electrodes are preferably arranged on the second side of the printhead structure. The intermediate image receiving member suitably includes a transfer belt positioned at a predetermined distance from the printhead structure, the transfer belt being substantially of uniform thickness. The image recording device suitably includes at least two, preferably four, pigment particle sources with corresponding control electrodes and passages/apertures on and in at least one printhead structure. 
     Further embodiments according to the previously described enhancements are possible in view of the application of the invention. 
     Said objects are also achieved according to the invention by a method for recording an image to an information carrier. The method comprises the a number steps. In a first step pigment particles are provided from a pigment particle source. In a second step an intermediate image receiving member and a printhead structure are moved relative to each other during recording. In a third step an electrical field is created for transporting of pigment particles from the pigment particle source toward a first face of the intermediate image receiving member. In a fourth step the deflection of pigment particles in transport is controlled. In a fifth step a deflection feedback signal is provided to thereby adjust the controlling of the deflection of pigment particles in transport in such a way that pigment particles are, for formation of a pigment image on the intermediate image receiving member in view of the image which is to be recorded, trajected toward predetermined locations on the intermediate image receiving member. In a sixth step apertures through the printhead structure are selectively opened or closed to permit or restrict the transporting of pigment particles to thereby enable the formation of a pigment image on the first face of the intermediate image receiving member. And finally in a seventh step the pigment image is substantially transferred to an information carrier. 
     Further variations of the method according to previously described enhancements are possible in view of the application of the invention. 
     Said objects are also achieved according to the invention by an image recording device and method for recording an image to an information carrier. The image recording device comprises a pigment particle source, a voltage source, a printhead structure, a control unit, and an intermediate image receiving member. The pigment particle source provides pigment particles. The intermediate image receiving member and the printhead structure move relative to each other during recording. The intermediate image receiving member have a first face and a second face. the printhead structure is placed inbetween the pigment particle source and the first face of the intermediate image receiving member. The voltage source being connected to the pigment particle source and the back electrode to thereby create an electrical field for transport of pigment particles from the pigment particle source toward the first face of the intermediate image receiving member. The printhead structure includes control electrodes connected to the control unit to thereby selectively open or close passages/apertures through the printhead structure to permit or restrict the transport of pigment particles to thereby enable the formation of a pigment image on the first face to the intermediate image receiving member. The pigment image is subsequently transferred to an information carrier. According to the method and device of the invention the image recording device comprises an intermediate image receiving member position measuring means for measuring the position of the intermediate image receiving member in relation to the passages/apertures to thereby via the control unit be able to synchronize the selective opening and closing of the passages/apertures through the printhead structure according to the relative movement of the printhead structure and the intermediate image receiving member to thereby enable the formation of a pigment image at a predetermined position on the intermediate image receiving member in view of the image which is to be recorded. 
     According to one embodiment the intermediate image receiving member position measuring means suitably comprises a capacitive movement sensor which measures the relative movement between the intermediate image receiving member and the printhead structure by means of which the position of the intermediate image receiving member in relation to the passages/apertures is determined. The capacitive movement sensor suitably comprises at least one first at least partially conductive area and at least one second at least partially conductive area arranged at a predetermined position in relation to the printhead structure. The capacitive movement sensor also suitably comprises at least one third at least partially conductive area on the intermediate image receiving member. The first, second and third areas are preferably spatially arranged in such a way that at least once in relation to the transport of pigment particles onto the intermediate image receiving member for the formation of a pigment image the first and third areas form a first capacitor, and the second and third areas form a second capacitor in order that the capacitive movement sensor can determine a transfer function of the first and second capacitor function during the relative movement of the intermediate image receiving member and the printhead structure. The transfer function of the first and second capacitor function preferably determined the relative movement between the intermediate image receiving member and the printhead structure. the capacitive movement sensor suitably comprises a plurally of second at least partially conductive areas and a plurality of third at least partially conductive areas. The distance between adjacent second areas is suitably different from the distance between adjacent third areas. The intermediate image receiving member suitably comprises at least one separate image area intended for reception of pigment particles for formation of a pigment image thereon. The at least one second at least partially conductive area is preferably arranged in relation to the at least one separate image area. 
     In one embodiment according to the invention the image recording device suitably includes at least two pigment particle sources with their respective corresponding, control electrodes and passages/apertures. Whereby the intermediate image receiving member position measuring means measures the position of the intermediate image receiving means in relation to the respective passages/apertures to thereby via the control unit be able to synchronize the selective openeing and closing of the respective passages/apertures through the at least one printhead structure according to the relative movement of the at least one printhead structure and the intermediate image receiving member to thereby enable the formation of a respective pigment image at a predetermined position on the intermediate image receiving member in view of the image which is to be recorded. The image recording device is preferably capable of recording color images and includes four pigment particle sources. 
     Further embodiments according to previously described enhancements are possible in view of the application of the invention. 
     Said objects are also achieved according to the invention by a method for recording an image to an information carrier. The method comprises a number of steps. In a first step pigment particles are provided from a pigment particle source. In a second step an intermediate image receiving member and a printhead structure are moved relative to each other during recording. In a third step an electrical field is created for transporting of pigment particles from the pigment particle source toward a first face of the intermediate image receiving member. In a fourth step the position of the intermediate image receiving member is measured in relation to apertures of the printhead structure to thereby be able to synchronize selective opening and closing of the apertures through the printhead structure according to the relative movement of the printhead structure and the intermediate image receiving member to thereby enable the formation of a pigment image at a predetermined position on the intermediate image receiving member in view of the image which is to be recorded. In a fifth step apertures through the printhead structure are selectively opened or closed to permit or restrict the transporting of pigment particles to thereby enable the formation of a pigment image on the first face of the intermediate image receiving member. An finally in a sixth step the pigment image is subsequently transferred to an information carrier. 
     Further variations of the method according to previously described enhancements are possible in view of the application of the invention. 
     Said objects are also achieved according to the invention by an image recording device and method for recording an image to an information carrier. The image recording device comprises a pigment particle source, a voltage source, a printhead structure, and an intermediate image receiving member. The pigment particle source provides pigment particles. The intermediate image receiving member and the printhead structure move relative to each other during recording. The intermediate image receiving member has a first face and a second face. The printhead structure is placed in between the pigment particle source and the first face of the intermediate image receiving member. The voltage source is connected to the pigment particle source and the back electrode to thereby create an electrical field for transport of pigment particles from the pigment particle source toward the first face of the intermediate image receiving member. The printhead structure includes control electrodes to thereby be able to selectively open or close passages/apertures through the printhead structure to permit or restrict the transport of pigment particles to thereby enable the formation of a pigment image on the first face of the intermediate image receiving member. The pigment image is subsequently transferred to an information carrier. According to the method and device of the invention the image recording device comprises a pressure changing means which can create a pressure difference on the side of the second face of the intermediate image receiving member in the vicinity of the passages/apertures. Further the intermediate image receiving member comprises a cleaning area for cleaning purposes and a separate image area intended for reception of pigment particles for formation of a pigment image thereon. The cleaning area includes at least one slot between the first face and the second face of the intermediate image receiving member. The at least one slot being intended for transferring a pressure difference through the intermediate image receiving member to thereby, in cooperation with the pressure changing means in the vicinity of the passages/apertures, be able to dislodge pigment agglomeration for the purpose of cleaning the passages/apertures. 
     The pressure changing means preferably creates a pressure difference on the second face of the intermediate image receiving member only when the cleaning area is in the vicinity of the passages/apertures. The pressure changing means suitably creates a pressure difference on the second face of the intermediate image receiving member along the cleaning area only when the cleaning area is in the vicinity of the passages/apertures. Suitably the cleaning area includes a plurality of slots between the first face and the second face, where at least a subset of the plurality of slots are intended for transferring a pressure difference through the intermediate image receiving member. Suitably the plurality of slots are arranged substantially in a row, the row being arranged substantially perpendicular to the direction of the relative movement between the intermediate image receiving member and the printhead structure. In some embodiments the row can suitably be arranged substantially at an angle diverging from 0° to the perpendicular to the direction of the relative movement between the intermediate image receiving member and the printhead structure. 
     The cleaning area preferably includes a plurality of slots between the first face and the second face, suitably at least a subset of the plurality of slots are intended for transferring a pressure difference through the intermediate image receiving member and arranged substantially in at least two rows. In some embodiments adjacent slots are suitably arranged substantially aligned in at least one row in only one dimension. 
     The at least one slot suitably has a substantially round opening on at least one of the first and second face of the intermediate image receiving member. In some embodiments the at least one slot suitably has a substantially elongated opening on at least one of the first and second face of the intermediate image receiving member, the elongation having long sides and a center line being equidistant from the long sides within the opening. In some of these embodiments the at least one slot is suitably arranged in such a way that the center line of the elongation is substantially in a direction parallell to the relative movement between the intermediate image receiving member and the printhead structure. In other of these embodiments the at least one slot is suitably arranged in such a way that the center line of the elongation is substantially in a direction perpendicular to the relative movement between the intermediate image receiving member and the printhead structure. In still other of these embodiments the at least one slot is suitably arranged in such a way that the center line of the elongation is substantially in a direction which is between a direction which is perpendicular to and a direction which is parallell to the relative movement between the intermediate image receiving member and the printhead structure. In some of all of these embodiments the long sides of the elongation are suitably not parallell thus creating a wide end and a narrow end. The long sides are suitably connected by substantially rounded end connections, connected by substantially straight end connections or a combination. 
     In some embodiments the dimension of the at least one slot is suitably greater than the magnitude of the distance between the first face of the intermediate image receiving member and the printhead structure. 
     In some embodiment the pressure changing means suitably creates an overpressure. In other embodiments the pressure changing means suitably creates a suction pressure. In still other embodiments the pressure changing means can suitably alternately create an overpressure and a suction pressure. 
     The pressure changing means can in some embodiments suitably comprises a fan for the creation of the pressure difference. In other embodiments the pressure changing means suitably comprises a volume changing means for the creation of the pressure difference, which can, for example, be a bellow or a piston. 
     At least part of the pressure changing means can suitably be arranged in a support for the intermediate image receiving member. The pressure changing means suitably creates the pressure difference on the side of the second face of the intermediate image receiving member via an opening of a controllable opening adjacent to the passages/apertures. The opening of the controllable opening can in some embodiments suitably be shifted in the relative direction of movement of the intermediate image receiving member while in other embodiments it is not shifted but substantially symmetrically arranged in view of the apertures that are to be cleaned by the opening in question. 
     Preferably the intermediate image receiving member includes a transfer belt positioned at a predetermined distance from the printhead structure, the transfer belt being substantially of uniform thickness. The thickness of the intermediate image receiving member is suitable at least partially thinner in the cleaning area compared to the thickness of the intermediate image receiving member in the image area. 
     The image recording device suitably includes at least two, and preferably four, pigment particle sources with corresponding control electrodes and passages/apertures on and in at least one printhead structure and where the pressure changing means can create a pressure difference on the side of the second face of the intermediate image receiving member for each one of the pigment particle sources in the vicinity of the corresponding respective passages/apertures. 
     Further embodiments according to previously described enhancements are possible in view of the application of the invention. 
     Said objects are also achieved according to the invention by a method for recording an image to an information carrier. The method comprises a number of. In a first step pigment particles from a pigment particle source are provided. In a second step an intermediate image receiving member and a printhead structure are moved relative to each other during recording. In a third step an electrical field is created by transport of pigment particles form the pigment particle source toward a first face of the intermediate image receiving member. In a third step apertures through the printhead structure are selectively opened or closed to permit or restrict the transport of pigment particles to thereby enable the formation of a pigment image on an image area on the first face of the intermediate image receiving member. In a fourth step the pigment image is subsequently transferred to an information carrier. In a fifth step a pressure difference is created on the side of the second face of the intermediate image receiving member in the vicinity of the apertures. And finally in a sixth step the pressure difference is transferred through the intermediate image receiving member by means of at least one slot between the first face and the second face of the intermediate image receiving member within a cleaning area for cleaning purposes which is separate from the image area, to thereby, in the vicinity of the apertures, be able to dislodge pigment agglomeration for the purpose of cleaning the apertures. 
     Further variations of the method according to previously described enhancements are possible in view of the application of the invention. 
     The present invention satisfies a need for increased accuracy of dot deflection control in direct electrostatic printing methods and apparatus by providing an intermediate image receiving medium allowing higher print uniformity and eliminating the drawbacks associated with the variations of paper and other information carrier properties. 
     The present invention relates to an image recording apparatus including an intermediate image receiving member conveyed past one or more, so called, print stations to intercept a modulated stream of toner particles from each print station. A print station includes a particle delivery unit, a particle source, such as a developer sleeve, and a printhead structure arranged between the particle source and the image receiving member. The printhead structure includes means for modulating the stream of toner particles from the particle source and means for controlling the trajectory of the modulated stream of toner particles toward the image receiving member. 
     According to a preferred embodiment of the present invention, the image recording apparatus comprises four print stations, each corresponding to a pigment colour, e.g. yellow, magenta, cyan, black (Y, M, C, K), disposed adjacent to an intermediate image receiving member formed of a seamless transfer belt made of a substantially uniformly thick, flexible material having high thermal resistance, high mechanical strength and stable electrical properties under a wide temperature range. The toner image is formed on the transfer belt and thereafter brought into contact with an information carrier, e.g. paper, in a fuser unit, where the toner image is simultaneously transferred to and made permanent on the information carrier upon heat and pressure. After image transfer, the transfer belt is brought in contact with a cleaning unit removing untransferred toner particles. 
     The present invention also relates to a direct printing method performed in consecutive print cycles, each of which includes several development periods having specific deflection modes. During each development period, control voltages are applied to control electrodes to produce electrostatic control fields which, due to control in accordance with the image information, open or close apertures through the printhead structure, thus enhancing or inhibiting the transport of toner particles from the particle source toward the intermediate image receiving member. Deflection voltages are simultaneously applied to the deflection electrodes to influence the symmetry of the electrostatic control fields to deflect the transported toner particles in predetermined directions, such that several dot locations are addressable through each aperture during each print cycle. The deflection length, i.e. the distance between a deflected dot and a central axis of the corresponding aperture, is optimized to obtain uniformly spaced dot locations across the entire width of the intermediate image receiving member. 
     Other objects, features and advantages of the present inventions will become more apparent from the following description when read in conjunction with the accompanying drawings in which preferred embodiments of the invention are shown by way of illustrative examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 sale, in which: 
     FIG. 1 is a schematic section view across an image recording apparatus according to a preferred embodiment of the invention, 
     FIG. 2 is a schematic section view across a particular print station of the image recording apparatus shown in FIG. 1, 
     FIG. 3 is an enlargement of FIG. 2 showing the print zone corresponding to a particular print station, 
     FIG. 4 is a schematic plan view of the top side of part of a transfer belt showing a cleaning area, 
     FIG. 5 a  is a schematic plan view of the top side of a printhead structure used in a print station such as that shown in FIG. 2, 
     FIG. 5 b  is a schematic section view along the section line I—I through the printhead structure shown in FIG. 5 a,    
     FIG. 5 c  is a schematic plan view of the bottom side of the printhead structure shown in FIG. 5 a,    
     FIG. 6 is a schematic view of a single aperture and its corresponding control electrode and deflection electrodes, 
     FIG. 7 a  illustrates a control voltage signal as a function of time during a print cycle having three subsequent development periods, 
     FIG. 7 b  illustrates a first deflection voltage signal as a function of time during a print cycle having three subsequent development periods 
     FIG. 7 c  illustrates a second deflection voltage signal as a function of time during a print cycle having three subsequent development periods 
     FIG. 8 a  illustrates conductive areas on the printhead structure and on the transfer belt for position determination at a first position of the transfer belt, 
     FIG. 8 b  illustrates conductive areas on the printhead structure and on the transfer belt for position determination at a second position of the transfer belt, 
     FIG. 8 c  illustrates conductive areas on the printhead structure and on the transfer belt for position determination at a third position of the transfer belt, 
     FIG. 9 a  illustrates the transport trajectory of toner particles through the printhead structure shown in FIGS. 5 a, b, c  according to a first deflection mode wherein D 1 &lt;D 2 , 
     FIG. 9 b  illustrates the transport trajectory of toner particles through the printhead structure shown in FIGS. 5 a, b, c,  ACCORDING to a second deflection mode wherein D 1 =D 2 , 
     FIG. 9 c  illustrates the transport trajectory of toner particles through the printhead structure shown in FIGS. 5 a, b, c,  according to a third deflection mode wherein D 1 &gt;D 2 , 
     FIG. 10 a  illustrates deposited toner particles and their density, trajected with a too small deflection voltage difference according to FIGS. 9 a, b, c,  from a printhead structure such as that shown in FIGS. 5 a, b, c,    
     FIG. 10 b  illustrates deposited toner particles and their density trajected with a too large deflection voltage difference according to FIGS. 9 a, b, c,  from a printhead structure such as that shown in FIGS. 5 a, b, c,    
     FIG. 10 c  illustrates deposited toner particles and their density, trajected with a substantially correct deflection voltage difference according to FIGS. 9 a, b, c,  from a printhead structure such as that shown in FIGS. 5 a, b, c.   
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In order to clarify the method and device according to the invention, some examples of its use will now be described in connection with FIGS. 1 to  10 . 
     FIG. 1 is a schematic section view of an image recording apparatus according to a first embodiment of the invention, comprising at least one print station, preferably four print stations (Y, M, C, K), an intermediate image receiving member, a driving roller  11 , at least one support roller  12 , and preferably several adjustable holding elements  13 . The four print stations (Y, M, C, K) are arranged in relation to the intermediate image receiving member. The intermediate image receiving member, preferably a transfer belt  10 , is mounted over the driving roller  11 . The at least one support roller  12  is provided with a mechanism for maintaining the transfer belt  10  with at least a constant surface tension, while preventing transversal movement of the transfer belt  10 . The preferably several adjustable holding elements  13  are for accurately positioning the transfer belt  10  at least with respect to each print station. 
     The driving roller  11  is preferably a cylindrical metallic sleeve having a rotational axis extending perpendicular to the belt motion and a rotation velocity adjusted to convey the transfer belt  10  at a velocity of one addressable dot location per print cycle, to provide line by line scan printing. The adjustable holding elements  13  are arranged for maintaining the surface of the transfer belt  10  at a predetermined distance from each print station. The holding elements  13  are preferably cylindrical sleeves disposed perpendicularly to the belt motion in an arcuated configuration for slightly bending the transfer belt  10  at least in the vicinity of each print station. The transfer belt  10  is slightly bent in order to, in combination with the belt tension, create a stabilization force component on the transfer belt  10 . The stabilization force component is opposite in direction and preferably larger in magnitude than an electrostatic attraction force component acting on the transfer belt  10 . The electrostatic attraction forces at a print station are created by different electric potentials on the transfer belt  10  and on the print station in question. 
     The transfer belt  10  is preferably an endless band of 30 to 200 μm thick 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  10  preferably has 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  10 . The outer surface of the transfer belt  10  is preferably overlaid with a 5 to 30 μm thick coating layer made of electrically conductive polymere material such as for instance PTFE (poly tethra fluoro ethylene), PFA (tetra fluoro ethylene, perfluoro alkyl vinyl ether copolymer), FEP (tetra fluoro ethylene hexafluoro, propylene copolymer), silicone, or any other suitable material having appropriate conductivity, thermal resistance, adhesion properties, release properties, and surface smoothness. To further improve for example the adhesion and release properties a layer of silicone oil can be applied to either the transfer belt vase or preferably onto a coating layer if it is applied onto the transfer belt base. The silicone oil is coated evenly onto the transfer belt  10  preferably in the order of 0.1 to 2 μm thick giving a consumption of silicone oil in the region of 1 centiliter for every 1000 pages. Silicone oil also reduces bouncing/-scattering of toner particles upon reception of toner particles and also increases the subsequent transfer of toner particles to an information carrier. Making use of silicone oil and especially coating of the transfer belt with silicone oil is made possible in an electrostatic printing method according to the present invention as there is no direct physical contact between a toner delivery and a toner recipient, i.e. the transfer belt. 
     In some embodiments the transfer belt  10  can comprise at least one separate image area and at least one of a cleaning area and/or a test area. The image area being intended for the deposition of toner particles, the cleaning area being intended for enabling the removal of unwanted toner particles from around each of the print stations, and the test area being intended for receiving test patterns of toner particles for calibration purposes. The transfer belt  10  can also in certain embodiments comprise a special registration area for use of determining the position of the transfer belt, especially an image area if available, in relation to each print station. If the transfer belt comprises a special registration area then this area is preferably at least spatially related to an image area. 
     The transfer belt  10  is conveyed past the four difference print stations (Y, M, C, K), whereby toner particles are deposited on the outer surface of the transfer belt  10  and superposed to form a toner image. Toner images are then preferably conveyed through a fuser unit  2 , comprising a fixing holder  21  arranged transversally in direct contact with the inner surface of the transfer belt. In some embodiments of the invention the fuser unit is separated from the transfer belt  10  and only acts on an information carrier. The fixing holder  21  includes a heating element preferably of a resistance type of e.g. molybdenium, maintained in contact with the inner surface of the transfer belt  10 . As an electric current is passed through the heating element, the fixing holder  21  reaches a temperature required for melting the toner particles deposited on the outer surface of the transfer belt  10 . The fuser unit  2  further comprises a pressing roller  22  arranged transversally across the width of the transfer belt  10  and facing the fixing holder  21 . An information carrier  3 , such as a sheet of plain, untreated paper or any other medium suitable for direct printing, is fed from a paper delivery unit (not shown) and conveyed between the pressing roller  22  and the transfer belt  10 . The pressing roller  22  rotates with applied pressure to the heated surface of the fixing holder  21  whereby the melted toner particles are fused on the information carrier  3  to form a permanent image. After passage through the fusing unit  2 , the transfer belt is brought in contact with a cleaning element  4 , such as for example a replaceable scraper blade of fibrous material extending across the width of the transfer belt  10  for removing all untransferred toner particles. If the transfer belt  10  is to be coated with silicone oil or the like, then after the cleaning element  4 , and before the printing stations, the transfer belt  10  is brought into contact with a coating application element  8  for evenly coating the transfer belt with silicone oil or the like. 
     FIG. 2 is a schematic section view of a print station in, for example, the image recording apparatus shown in FIG. 1. A print station includes a particle delivery unit  5  preferably having a replaceable or refillable container  50  for holding toner particles, the container  50  having front and back walls, a pair of side walls and a bottom wall having an elongated opening extending from the front wall to the back wall and provided with a toner feeding element (not shown) disposed to continuously supply toner particles to a developer sleeve  52  through a particle charging member. The particle charging member is preferably formed of a supply brush  51  or a roller made of or coated with a fibrous, resilient material. The supply brush  51  is brought into mechanical contact with the peripheral surface of the developer sleeve  52 , 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  51  and any suitable coating material of the developer sleeve  52 . The developer sleeve  52  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 of the particle container  50 . Charged toner particles are held to the surface of the developer sleeve  52  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  52 . Alternatively, the charging unit may additionally comprise a charging voltage source (not shown), which supply 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 by 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  53  is positioned proximate to the developer sleeve  52  to adjust the concentration of toner particles on the peripheral surface of the developer sleeve  52 , to form a relatively thin, uniform particle layer thereon. The metering element  53  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  53  may also be connected to a metering voltage source (not shown) which influence the triboelectrification of the particle layer to ensure a uniform particle charge density on the surface of the developer sleeve  52 . 
     The developer sleeve  52  is arranged in relation with a support device  54  for supporting and maintaining the printhead structure  6  in a predetermined position with respect to the peripheral surface of the developer sleeve  52 . The support device  54  is preferably in the form of a trough-shaped frame having two side walls, a bottom portion between the side walls, and an elongated slot arranged through the bottom portion, extending transversally across the print station, parallel to the rotation axis of the developer sleeve  52 . The support device  54  further comprises means for maintaining the printhead structure in contact with the bottom portion of the support device  54 , the printhead structure  6  thereby bridging the elongated slot in the bottom portion. 
     The transfer belt  10  is preferably slightly bent partly around each holding element  13  in order to create a stabilization force component  30 . The stabilization force component  30  is intended to counteract a field force component  31  which is acting on the transfer belt. The field force component  31  is a resultant of the electrostatic attraction forces acting on the transfer belt  10  due to difference electric potentials on the transfer belt  10  and on the print station in question. The stabilization force component  30  is directed in a direction opposite the field force component  31  and preferably also at least slightly larger in magnitude than the magnitude of the field force component  31 . If the field force component  31  is not counteracted it can cause distance fluctuations between the transfer belt  10  and the printhead structure  6  which can cause a degradation in print quality. The stabilization force component  30  is created by the slight bending of the transfer belt  10  in combination with the tension of the transfer belt  10 . The transfer belt  10  is bent around the holding element  13  in such a way that two angles  34 ,  35 , at least one of which is greater than zero (i.e. at most one angle equal to zero), are created to the transfer belt  10  from a reference plane  39 . The reference plane  39  being substantially perpendicular to an imaginary line/plane that extends between the center of the developer sleeve  52  and the holding element  13 . Preferably the imaginary line/plane also separates the two angles  34 ,  35 . The two angles  34 ,  35  are preferably both greater than zero and in the region between 0° and 10°. 
     FIG. 3 is an enlargement of the print zone in a print station of, for example, the image recording apparatus shown in FIG.  1 . The printhead structure  6  is preferably formed on an electrically insulating substrate layer  60  made of flexible, non-rigid material such as polyamide or the like. The printhead structure  6  is positioned between the peripheral surface of the developer sleeve  52  and the bottom portion of the support device  54 . The substrate layer  60  has a top surface facing the toner layer  7  on the peripheral surface of the developer sleeve  52 . The substrate layer  60  has a bottom surface facing the the bottom portion of the support device  54 . Further, the substrate layer  60  has a plurality of apertures  61  arranged through the substrate layer  60  in the part of the substrate layer  60  overlying the elongated slot in the bottom portion of the support device  54 . The printhead structure  6  further includes a first printed circuit arranged on the top surface on the substrate layer  60  and a second printed circuit arranged on the bottom surface of the substrate layer  60 . The first printed circuit includes a plurality of control electrodes  62 , each of which, at least partially, surrounds a corresponding aperture  61  in the substrate layer  60 . The second printed circuit preferably includes a first and a second set of deflection electrodes  63  spaced around first and second portions of the periphery of the apertures  61  of the substrate layer  60 . 
     The apertures  61  and their surrounding area will under some circumstances need to be cleaned from toner particles which agglomerate there. In some embodiments of the invention the transfer belt  10  advantageously comprises at least one cleaning area for the purpose of cleaning the apertures  61  and the general area of the apertures  61 . The cleaning, according to these embodiments, works by the principle of flowing air (or other gas). A pressure difference, compared to the air pressure in the vicinity of the apertures, is created on the side of the transfer belt  10  that is facing away from the apertures  61 . The pressure difference is at least created during part of the time when the cleaning area is in the vicinity of the apertures  61  of the print station in question during the transfer belt&#39;s  10  movement. The pressure difference can either be an over pressure, a suction pressure or a sequential combination of both, i.e. the cleaning is performed by either blowing, suction, blowing first then suction, suction first then blowing, or some other sequential combination of suction and blowing. The pressure difference is transferred across the transfer belt  10  by means of the cleaning area comprising at least one slot/hole  80  through the transfer belt  10 . The cleaning area preferably comprises at least one row of slots  80 , and more specifically two to eight interlaced rows of slots  80 . The slots can advantageously be in the order of 3 to 5 mm across. The pressure difference appears on the holding element  13  side of the transfer belt  10  through a transfer passage  81  through an outer sleeve  85  of the holding element  13 . The transfer passage  81  can advantageously suitably extend transversally across the printhead structure as an elongated slot with a width, in the direction of the transfer belt  10  movement, that is equal to or greater than the minimum distance between the printhead structure  6  and the transfer belt  10 . In some embodiments it can be advantageous to have a controllable passage  82  which by means of, for example, a movable inner sleeve  86  can open and close access of the pressure difference to the transfer passage  81 . Thereby a suction pressure will not increase the transfer belt&#39;s friction on the holding element  13  more than necessary. The controllable passage  82  will preferably open and close in synchronization with the movement of the transfer belt  10  to thereby coincide its openings with the passage of the cleaning area of the transfer belt  10 . The means for creating the pressure difference is not shown and can suitably be a fan, bellows, a piston, or some other suitable means for creating a pressure difference. In some embodiments according to the invention the transfer passage  81  is substantially located symmetrically in relation to the apertures. In other embodiments according to the invention the transfer passage  81  is shifted in the direction of movement of the transfer belt  10 . 
     FIG. 4 shows an example of holes  80  in a plurality of interlaced rows within a cleaning area  85  of the transfer belt  10 . The number of holes/slots, their size and design will depend on the specific application. A preferable design is elongates slots in at least two interlaced rows. 
     Although, a printhead structure  6  can take on various embodiments without departing from the scope of the present invention, a preferred embodiment of the printhead structure will be described hereinafter with reference to FIGS. 5 a ,  5   b  and  5   c . A plurality of apertures  61  are arranged through the substrate layer  60  in several aperture rows extending transversally across the width of the print zone, preferably at a substantially right angle to the motion of the transfer belt. The apertures  61  preferably have a circular cross section with a central axis  611  extending perpendicularly to the substrate layer  60  and suitably a diameter in the order of 160 μm. Each aperture  61  is surrounded by a control electrode  62  having a ring-shaped part circumscribing the periphery of the aperture  61 , with a symmetry axis coinciding with the central axis  611  of the aperture  61  and an inner diameter which is equal or sensibly larger than the aperture diameter. Each control electrode  62  is connected to a control voltage source (IC driver) through a connector  621 . As apparent in FIG. 5 a , the printhead structure further preferably includes guard electrodes  64 , preferably arranged on the top surface of the substrate layer  60  and connected to a guard potential (Vguard) aimed to electrically shields the control electrodes  62  from one another, thereby preventing undesired interaction between the electrostatic fields produced by two adjacent control electrodes  62 . Each aperture  61  is related to a first deflection electrode  631  and a second deflection electrode  632  spaced around a first and a second segment of the periphery of the aperture  61 , respectively. The deflection electrodes  631 ,  632  are preferably semicircular or crescent-shaped and disposed symmetrically on each side of a deflection axis extending diametrically across the aperture at a predetermined deflection angle to the motion of the transfer belt, such that the deflection electrodes substantially border on a first and a second half of the circumference of their corresponding aperture  61 , respectively. All first and second deflection electrodes  631 ,  632  are connected to a first and a second deflection voltage source D 1 , D 2 , respectively. 
     FIG. 6 is a schematic view of a single aperture  61  and its corresponding control electrode  62  and deflection electrodes  631 ,  632 . Toner particles are deflected in a first deflection direction R 1  when D 1 &lt;D 2 , and an opposite direction R 2  when D 1 &gt;D 2 . The deflection angle δ is chosen to compensate for the motion of the transfer belt  10  during the print cycle, in order to be able to obtain two or more transversally aligned dots. 
     A preferred embodiment of a dot deflection control function is illustrated in FIGS. 7 a ,  7   b  and  7   c  respectively showing the control voltage signal (Vcontrol), a first deflection voltage D 1  and a second deflection voltage D 2 , as a function of time during a single print cycle. According to some embodiments of the invention and as illustrated in the figure, printing is performed in print cycles having three subsequent development periods for addressing three different dot locations through each aperture. In other embodiments each print cycle can suitably have fewer or more addressable dot locations for each aperture. In still further embodiments each print cycle has a controllable number of addressable dot locations for each aperture. During the whole print cycle an electric background field is produced between a first potential on the surface of the developer sleeve and a second potential on the back electrode, to enable the transport of toner particles between the developer sleeve and the transfer belt. During each development period, control voltages are applied to the control electrodes to produce a pattern of electrostatic control fields which due to control in accordance with the image information, selectively open or close the apertures by influencing the electric background field, thereby enhancing or inhibiting the transport of toner through the printhead structure. The toner particles allowed to pass through the opened apertures are then transported toward their intended dot location along a trajectory which is determined by the deflection. 
     The examples of control function shown in FIGS. 7 a ,  7   b  and  7   c  illustrates a control function wherein the toner particles have negative polarity charge. As is apparent from FIG. 7 a , a print cycle comprises three development periods tb, each followed by a recovering period tw during which new toner is supplied to the print zone. The control voltage pulse (Vcontrol) can be amplitude and/or pulse width modulated, to allow the intended amount of toner particles to be transported through the aperture. For instance, the amplitude of the control voltage varies between a non-print level V w  of approximately −50V and a print level V b  in the order of +350V, corresponding to full density dots. Similarly, the pulse width can be varied from 0 to tb. As apparent from FIGS. 7 b  and  7   c , the amplitude difference between D 1  and D 2  is sequentially modified for providing three different toner trajectories, i.e. dot positions, during each print cycle. The amplitudes of D 1  and D 2  are modulated to apply converging forces on the toner to obtain smaller dots. Utilizing this method enables, for example, 60 μm dots to be obtained utilizing 160 μm apertures. Suitably the size of the dots are adjusted in accordance with the dot density (dpi) and thus also dynamically with the number of dot locations each aperture is to address. 
     FIGS. 8 a ,  8   b , and  8   c  illustrate conductive areas/regions/plates on the printhead structure and on or in the transfer belt for position determination. The FIGS. 8 a ,  8   b , and  8   c  illustrate the conductive areas/regions/plates at a first, second, and third position respectively of the transfer belt. A position determination means according to the invention comprises a first number of conductive areas/regions, transfer plates  703 , on or in the transfer belt. According to the figures in question the transfer belt has a direction of movement to the right as illustrated by the arrows  700 . The movements of the transfer belt between FIGS. 8 a  and  8   b , and between FIGS. 8 b  and  8   c  are substantially the same for illustrative purposes. The position determination means further comprises a large conductive area/region, an input plate  701 , intended for input of a measurement signal. The position determination means further comprises at least one conductive area/region, output plate(s)  711 ,  712 ,  713 ,  714 ,  721 ,  722 ,  723 ,  724 ,  731 ,  732 ,  733 ,  734 , intended for measurement of the transferred measurement signal. The position determination means preferably comprises at least two output plates  711 ,  712 ,  713 ,  714 ,  721 ,  722 ,  723 ,  724 ,  731 ,  732 ,  733 ,  734 . The center to center distance between two adjacent output plates  711 ,  712 ,  713 ,  714 ,  721 ,  722 ,  723 ,  724 ,  731 ,  732 ,  733 ,  734  preferably differ from the center to center distance between two adjacent transfer plates  703  of the transfer belt. The output plates  711 ,  712 ,  713 ,  714 ,  721 ,  722 ,  723 ,  724 ,  731 ,  732 ,  733 ,  734  are positioned in a fixed predetermined spatial location in relation to at least one aperture. The input plate  701  is positioned in a location so that it at least partially overlaps the transfer plates  703  that are passing by the movement of the transfer belt thereby creating a first capacitance to each transfer plate  703  that it overlaps. The output plates  711 ,  712 ,  713 ,  714 ,  721 ,  722 ,  723 ,  724 ,  731 ,  732 ,  733 ,  734  at least partially overlap the transfer plates  703  that are passing by the movement of the transfer belt thereby creating a capacitance between each output plate  711 ,  712 ,  713 ,  714 ,  721 ,  722 ,  723 ,  724 ,  731 ,  732 ,  733 ,  734  and the corresponding transfer plate  703  that is more or less overlapped. Thereby a transfer of the measurement signal can occur between the input plate  701  and a transfer plate  703  by means of a first capacitance and then between the transfer plate  703  in question and an output plate  711 ,  712 ,  713 ,  714 ,  721 ,  722 ,  723 ,  724 ,  731 ,  732 ,  733 ,  734  by means of a second capacitance with the second capacitance varying in size depending on the changing overlap due to the movement of the transfer belt. Thus the measurement signal has to have a suitable amplitude and frequency to transfer from the input plate  701  across the first and second capacitance functions created by the plates/conductive areas and to thereby be measurable at an output plate  711 ,  712 ,  713 ,  714 ,  721 ,  722 ,  723 ,  724 ,  731 ,  732 ,  733 ,  734 . As can be seen in FIG. 8 a  a first output plate  711  covers a transfer plate  703  as much as it will ever do, thus it is said to cover it to a 100%. The second output plate  712  covers only 50%, the third output plate  713  does not cover anything, i.e. 0%, and the fourth illustrated output plate  714  covers 50%. The reason for the different coverage by the output plates  711 ,  712 ,  713 ,  714  is due to the different center to center distances of the output plates  711 ,  712 ,  713 ,  714 ,  721 ,  722 ,  723 ,  724 ,  731 ,  732 ,  733 ,  734  compare with the transfer plates  703 . In FIG. 8 b , when the transfer belt has moved  700  a little, the first output plate  721  covers 75%, the second output plate  722  covers 75%, the third output plate  723  covers 25%, and the fourth output plate covers 25%. In FIG. 8 c , when the transfer belt has moved  700  a little further, the first output plate  731  covers 50%, the second output plate  732  covers 100%, the third output plate  733  covers 50%, and the fourth output plate covers 0%. Thus the first output plate  711 ,  721 ,  731  has had a decrease in coverage, the second output plate  712 ,  722 ,  732  has had an increase in coverage, the third output plate  713 ,  723 ,  733  has had an increase in coverage, and the fourth output plate  714 ,  724 ,  734  has had a decrease in coverage. The change in coverage changes the capacitance of the second capacitance function and the level of the measurement signal will thus change in accordance with the change in coverage. The relative position between the transfer plates  703  and the output plates  711 ,  712 ,  713 ,  714 ,  721 ,  722 ,  723 ,  724 ,  731 ,  732 ,  733 ,  734  can thus be measured very accurately and the absolute position can be measured by counting the number of transfer plates  703  that passes. The advantage of having a plurality of output plates  711 ,  712 ,  713 ,  714 ,  721 ,  722 ,  723 ,  724 ,  731 ,  732 ,  733 ,  734  arranged properly is that there will always be at least one that outputs the measuring signal in the advantageous linear range. A structure of the input plate  701  and output plates  711 ,  712 ,  713 ,  714 ,  721 ,  722 ,  723 ,  724 ,  731 ,  732 ,  733 ,  734  can be repeated, with a large common input plate  701  or individual input plates  701 , for each row of apertures, multiple of rows of apertures, each printhead structure, or each print station depending on the specific embodiment of the invention in a particular application. 
     FIGS. 9 a ,  9   b  and  9   c  illustrate the toner trajectories in three subsequent deflection modes. The FIGS. 9 a ,  9   b  and  9   c  illustrate a cross section of a substrate layer  60  with apertures  61  with corresponding control electrodes  62 . Also illustrated are deflection voltages D 1  and D 2  that are connected to respective deflection electrodes  631 ,  632 . During a first development period illustrated in FIG. 9 a , the modulated stream of toner particles is deflected to the left by producing a first amplitude difference (D 1 &lt;D 2 ) between both deflection voltages. The amplitude difference is adjusted to address dot locations  635  located at a deflection length L d  to the left of the central axes  611  of the apertures  61 . During a second development period illustrated in FIG. 9 b , the deflection voltages have equal amplitudes (D 1 =D 2 ) to address undeflected dot locations  636  coinciding with the central axes  611  of the apertures  61 . During a third development period illustrated in FIG. 9 c , the modulated stream of toner particles is deflected to the right by producing a second amplitude difference (D 1 &gt;D 2 ) between both deflection voltages. The amplitude difference is adjusted to address dot locations  637  located at a deflection length L d  to the right of the central axes  611  of the apertures  61 . 
     FIGS. 10 a ,  10   b , and  10   b  illustrates deposited toner particles and their density, trajected with a too small, too large and substantially correct deflection voltage difference respectively. The toner particles are trajected, for example, according to FIGS. 9 a ,  9   b ,  9   c , from a printhead structure such as that shown in FIGS. 5 a ,  5   b ,  5   c . FIG. 10 a  illustrates toner particles  635 ,  636 ,  637  trajected onto, for example, a transfer belt and their resulting density distribution when a deflection voltage difference has been to small. According to the example three addressable dots per aperture are shown, as described previously, more or fewer dots per aperture are possible. As can be seen from the density diagram there is quite a large dip  41  in the density of the recorded toner particle dots  635 ,  636 ,  637  in comparison to an average density  91 . The leftmost dot  635  leaves a gap to the rightmost dot  637  of the adjacent aperture resulting in the density dip  41 . The rightmost dot  637  overlaps the center dot  636  as does the leftmost dot  635 . It is the rightmost  637  and leftmost  635  that are in the wrong position, the center  636  dot will always be substantially along the center axis  611  where it should be. FIG. 10 b  illustrates the situation when the deflection voltage difference has been to large, thus resulting in spreading out of the rightmost dots  637  and of the leftmost dots  635  to much away from the center dots  636  and actually resulting in an overlap of the leftmost dots  635  and the rightmost dots  637  of respective adjacent apertures and thereby leaving several density dips  42 ,  43  in comparison to an average density  92 ,  93 ,  94 . FIG. 10 c  illustrates the situation when the deflection voltage difference has been at its substantially optimal value leaving all of the dots  635 ,  636 ,  637  substantially at their predetermined places which results in a substantially even density  95  across the dots. 
     According to the invention this difference in density is used to enable an automatic calibration of the deflection voltage difference. The smaller the deflection voltage difference is the greater the dip  41  according to FIG. 10 a  will become. An increase in the deflection voltage difference will decrease the size of the dip  41  until it has disappeared completely. The density across the dots will then have the appearance according to FIG. 10 c , i.e. the deflection voltage difference is, at least, substantially correct. If the deflection voltage difference is increased even further, the density across the dots will break appart again, but this time according to FIG. 10 b  with two dips  42 ,  43  per aperture. The deflection voltage difference has to be decreased again so that a density  95  across the dots will have an appearance according to FIG. 10 c  again. According to the invention a test pattern is used, either one that happens to appear during normal printing or one that is printed for the purpose of adjusting the deflection voltage difference. If a test pattern is deliberately printed then this can suitably be done on the transfer belt on the image area or if available on a specially designated test area. The test pattern is analysed by an appropriate measurement method which suitable can be optically, magnetically to thereby measure the optical density, thickness, capacitance, flux across the test pattern. Preferably the optical density of the test pattern is optically measured across the test pattern. The result of the measurement is compared with an expected optimum value to thereby create a deflection feedback signal which can adjust the deflection voltage difference to an optimum value. This can be performed at regular or irregular intervals, such as between every page, every 1000 pages, or every hour, or on demand from, for example, the user or a environmental sensor, or at every power on. 
     The invention is not limited to the embodiments described above may be varied within the scope of the appended patent claims.