WO2002053386A1

WO2002053386A1 - Method and apparatus of direct electrostatic printing

Info

Publication number: WO2002053386A1
Application number: PCT/SE2000/002696
Authority: WO
Inventors: Johan Linder
Original assignee: Array Ab
Priority date: 2000-12-29
Filing date: 2000-12-29
Publication date: 2002-07-11

Abstract

By utilizing existing drive electronics for control electrodes, and then generating specific test signals to test different aspects of a printhead structure of a direct electrostatic printing device. The specific test signals in combination with ingenious measuring methods only necessitates a limited number of measurement points for the printhead structure. By activating a varying test signal, then a coupling to electrodes on the other side of the printhead structure can be used to verify that the control electrodes and the additional electrodes are physically unbroken and that the drive electronics is capable. By activating a test signal and at the same time measuring the current and/or power consumption of the drive electronics it can be verified that there are no short circuits of the control electrodes.

Description

A51 P38PCT.SPB FPC test 2000-12-22

METHOD AND APPARATUS OF DIRECT ELECTROSTATIC PRINTING

FIELD OF THE INVENTION

The present invention relates to direct electrostatic printing apparatus in which charged toner particles are transported under control from a particle source in accordance with an image information to form a toner image used in a copier, a printer, a plotter, a facsimile, or the like. The invention especially relates to production, power up, and runtime testing of such apparatus.

BACKGROUND TO THE INVENTION

U.S Patent No. 5,036,341 discloses a direct electrostatic printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals.

In direct electrostatic printing methods a plurality of apertures, each surrounded by a control electrode, are preferably arranged in parallel rows extending transversally across the print zone, i.e. substantially perpendicular to the motion of the image receiving medium. As a pixel position on the image receiving medium passes beneath a corresponding aperture, the control electrode associated with this aperture is set on a print potential allowing the transport of toner particles through the aperture to form a toner dot at that pixel position. Accordingly, transverse image lines can be printed by simultaneously activating several apertures of the same aperture row, and longitudinal image lines can be printed by sequentially activating at least one aperture when pixel positions in question passes beneath the at least one aperture. Improvements to direct electrostatic printing methods have been made, for example dot deflection control (DDC), as is disclosed in U.S. Patent No. 5,847,733. 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.

It is very important for attaining a high print quality that all control electrodes are functioning properly, i.e. there should be proper electrical connections to all of the control electrodes and no short circuits between different control electrodes or other unintentional electrical short circuits. There is thus a need to ensure proper functioning of the control electrodes and other electrical circuitry of a printhead structure. It can be considered a drawback of current direct electrostatic printing methods that it is difficult to ensure proper electrical connectivity of control electrodes and other electrical circuitry during runtime. Further it is difficult to ensure proper electrical connectivity of electrical circuitry of a printhead structure during production and delivery/power-on of a direct electrostatic printing apparatus. Therefore, there seems to exist a need to determine proper electrical connectivity of electrical circuitry of a printhead structure in a simple and cost efficient manner.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of and a device for testing electrical connectivity of a printhead structure of a direct electrostatic printing apparatus. Another object of the present invention is to provide a method of and device for testing electrical circuitry of a printhead structure of a direct electrostatic printing apparatus.

A further object of the present invention is to provide a method of and device for testing electrical circuitry of control electrodes on a printhead structure of a direct electrostatic printing apparatus.

Still another object of the present invention is to provide a method of and device for testing electrical circuitry of control electrodes on a printhead structure of a direct electrostatic printing apparatus during production, delivery/power on, and/or runtime.

Said objects are achieved according to the invention by utilizing existing drive electronics for control electrodes, and then generating specific test signals to test different aspects of a printhead structure of a direct electrostatic printing device. The specific test signals in combination with ingenious measuring methods only necessitates a limited number of measurement points for the printhead structure. By activating a varying test signal, then a coupling to electrodes on the other side of the printhead structure can be used to verify that the control electrodes and the additional electrodes are physically unbroken and that the drive electronics is capable. By activating a test signal and at the same time measuring the current and/or power consumption of the drive electronics it can be verified that there are no short circuits of the control electrodes. The testing can be performed as soon as the drive electronics has been mounted, and can then be performed at different manufacturing stages, at delivery, during each or selected power-ups, and whenever the printhead structure is not used for printing. Said objects are also achieved according to the invention by a method of controlling the condition of a printhead structure having a first surface and a second surface. The printhead structure further comprises a carrier, a plurality of apertures arranged through the printhead structure between the first surface and the second surface, a plurality of control electrodes arranged on the first surface for selectively and electrostatically opening or closing the plurality of apertures to permit or restrict transport of toner particles in the form of toner jets, and one or more drive circuits, each drive circuit comprising one or more ouputs each connected to one of the plurality of control electrodes. According to the invention the method comprises a number of steps. In a first step activating one or more outputs in a predetermined manner to thereby subject one or more control electrodes to a test signal. In a second step measuring a response to the test signal. In a third step determining the condition of the printhead structure from the measured response.

Preferably the plurality of apertures are arranged in one or more substantially straight rows across the printhead structure.

According to some versions of the method the first step of activating one or more outputs in a predetermined manner, comprises activating one or more outputs only to such a degree that the drive circuit in question can tolerate a short circuit of the one or more outputs in question, and in that the second step of measuring a response to the test signal, comprises measuring the current and/or the power consumption of at least the drive circuit or circuits whose one or more outputs are activated. The first step of activating one or more outputs in a predetermined manner to thereby subject one or more control electrodes to a test signal, can activate the one or more outputs to generate a test signal with a predetermined level.

In some versions of the method the printhead structure further comprises additional electrodes, and the second step of measuring the response to a test signal, comprises measuring a voltage or current on the additional electrodes. The additional electrodes can preferably be arranged on the second surface of the printhead structure. Advantageously the second step of measuring the response to a test signal further comprises determining if any measured signal on the additional electrodes corresponds, in view of a transfer function between the control electrodes and the additional electrodes, to the test signal. Preferably the first step of activating one or more outputs in a predetermined manner, comprises activating one or more outputs up to as much as the drive circuit or drive circuits will allow, i.e. maximum output is preferably used to get a maximum response which will then be more easily detectable. It can, however, be preferable in the first step of activating one or more outputs in a predetermined manner, activate one or more outputs up to as much as the drive circuit or drive circuits will allow only if it has been determined that the one or more outputs in question are not short circuited (i.e. connected to other control electrodes or some other conductive material), and otherwise activate one or more outputs only to such a degree that the drive circuit in question can tolerate a short circuit of the one or more outputs in question. The first step of activating one or more outputs in a predetermined manner to thereby subject one or more control electrodes to a test signal, comprises activating the one or more outputs to generate a test signal which will propagate through a capacitor. Preferably the third step of determining the condition of the printhead structure from the measured response, comprises determining that a drive circuit is malfunctioning if there is no measured response from a majority of the drive circuit's outputs when activated. The additional electrodes can advantageously be either deflection electrodes, guard electrodes, or shield electrodes. Advantageously the first step of activating one or more outputs in a predetermined manner, comprises activating at least two outputs simultaneously to acquire a response with a higher signal level.

In some versions of the method the step of measuring a response to the test signal, comprises measuring a rise time of the respone. In some versions of the method the step of measuring a response to the test signal, comprises measuring a delay time of the respone from an activation time of the test signal.

Further variations of the method according to the invention will be described below. The enhancements can be mixed arbitrarily in view of the specific application of the invention, as long as there is no conflict.

Said objects are also achieved according to the invention by a direct electrostatic printing device. The device includes a toner particle delivery, a electrical field creator, a printhead structure, and a control unit. The toner particle delivery providing toner particles. An image receiving surface and the printhead structure are arranged for relative movement between each other during printing. The electrical field creator creating an electrical field between the toner particle delivery and the image receiving surface for transport of toner particles from the toner particle delivery toward the image receiving surface. The printhead structure being placed in the electrical field between the toner particle delivery and the image receiving surface. The printhead structure including control electrodes connected to the control unit to thereby selectively open or close toner jet passages through the printhead structure to permit or restrict the transport of toner particles during a print sequence in the form of toner jets, at least one print sequence is included in a print cycle, to thereby enable the formation of an image on the image receiving surface. According to the invention the device is arranged to perform any version of the previously described methods. Advantageously the device is arranged to perform any one of the methods only at other times than when the device is forming an image on the image receiving surface.

Said objects are also achieved according to the invention by a method for printing an image to an information carrier. The method comprises a number of steps. In a first step providing toner particles from a toner particle delivery. In a second step moving an image receiving surface and a printhead structure relative to each other during printing. In a third step creating an electrical field for transporting pigment particles from the toner particle delivery toward the image receiving surface. In a fourth step selectively opening or closing toner jet passages through the printhead structure, to thereby permit or restrict the transporting of toner particles during a print sequence in the form of toner jets, at least one print sequence is included in a print cycle, to thereby enable the formation of an image on the image receiving surface. And in a fifth step performing any one of the previously described methods. Advantageously the fifth step of performing any one of the previously described methods is only performed at other times than when the method forms an image on the image receiving surface.

The present invention relates to an image recording apparatus including an image receiving surface 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 toner particle delivery unit, a particle source, and a printhead structure arranged between the particle source and the image receiving surface. The printhead structure includes means for modulating the stream of toner particles from the particle source and possibly means for controlling the trajectory of the modulated stream of toner particles toward the image receiving surface. The image recording apparatus preferably comprises four print stations, each corresponding to a pigment color, e.g. yellow, magenta, cyan, black (Y,M,C,K), disposed adjacent to the image receiving surface for example formed of either a seamless transfer belt, of an information carrier such as paper, or of a drum. The toner image is formed on the image receiving surface according to the invention and thereafter, if it is not directly an information carrier, brought into contact with an information carrier, e.g. paper, in a fuser unit, where the toner image is transferred to and made permanent on the information carrier. After image transfer, the image receiving surface is preferably brought in contact with a cleaning unit removing untransferred toner particles.

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 scale, in which

Fig. 1 illustrates a direct electrostatic printing aparatus of a transfer belt type,

Fig. 2 illustrates a direct electrostatic printing apparatus of a drum type, Fig. 3a illustrates a first side of a printhead structure,

Fig. 3b illustrates a second side of a printhead structure,

Fig. 3c illustrates a cross-section of a printhead structure,

Fig. 4 illustrates a connectivity schematic of a first embodiment of the invention,

Fig. 5 illustrates an equivalent electrical schematic of the first embodiment according to Figure 4,

Fig. 6 illustrates a connectivity schematic of a second embodiment of the invention.

Fig. 7 illustrates an equivalent electrical schematic of the second embodiment according to Figure 6,

Fig. 8 illustrates a control unit,

Fig. 9 illustrates a high voltage control electrode driver.

DESCRIPTION OF PREFERRED EMBODIMENTS

An image recording apparatus according to the invention, comprises at least one print station, preferably four print stations (Y, M, C, K). The four print stations (Y, M, C, K) are arranged in relation to an image receiving surface, preferably an intermediate image receiving member. An intermediate image receiving member can either be a transfer belt mounted over driving rollers, or a drum. In other embodiments toner particles are deposited directly onto an information carrier without first being deposited onto an intermediate image receiving member. The image receiving surface and a print station move relative to each other at a velocity of one addressable dot location per print cycle, to provide line by line scan printing. Each print station comprises a printhead structure that has a plurality of apertures extending perpendicular to the relative motion. A transfer belt is conveyed, or a drum is rotated, past the four different print stations (Y, M, C, K), whereby toner particles are deposited on the image receiving surface and superposed to form a toner image.

In order to perform direct electrostatic printing, a background electric field is produced between a toner particle carrier of a print station and a back electrode to enable 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 between the particle carrier and the back electrode and connected to a control unit which converts the image information into a pattern of electrostatic fields which, due to control in accordance with the image information, selectively open or close passages in the electrode matrix to permit or restrict the transport of charged particles from the particle carrier by a control voltage pulse. The control voltage pulse (V_controι) 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 t_b. The modulated stream of charged particles allowed to pass through the opened apertures are thus exposed to the background electric field and propelled toward the back electrode. The charged particles are deposited on the intermediate image receiving member to provide line-by line scan printing to form a visible image.

The back electrode member or members utilized in an image forming apparatus can be of a number of different types, e.g. a stationary or rotating roll or sleeve, or a movable belt arranged in an endless loop by means of guide rolls. Depending on the application, the back electrode member can be made of different materials, e.g. a suitable metal alloy or another electrically conductive material. Furthermore, a back electrode member can be arranged behind a belt constituting an intermediate image receiving member. It is also conceivable with embodiments where a suitable information carrier, such as a printing paper, passes across the back electrode when printing so that an image is printed directly onto the information carrier, or where the information carrier also constitutes the back electrode by means of being electrically conductive. In other applications, an intermediate image is formed directly onto the surface of the back electrode member, whereafter the image is transferred to a suitable image receiving substrate such as a printing paper. It is particularly advantageous to print directly onto the back electrode in applications utilizing so-called multi-interlacing (MIC) techniques. Furthermore, it is conceivable with applications where the electrical field, by means of which the toner particles are transported, is generated by another means than a pair of electrodes, e.g. applications where the electrical field is generated by means of a suitable charge carrier which in itself is able to generate an electrostatic field.

A printhead structure for use in direct electrostatic printing may take on many designs, such as a lattice of intersecting wires arranged in rows and columns, or an apertured substrate of electrically insulating material overlaid with a printed circuit of control electrodes arranged in conjunction with the apertures. Generally, a printhead structure includes a flexible substrate of insulating material such as polyimide or the like, having a first surface facing the particle carrier, a second surface facing the back electrode and a plurality of apertures arranged through the substrate. The first surface is coated with an insulating layer and control electrodes are arranged between the first surface of the substrate and the insulating layer, in a configuration such that each control electrode surrounds a corresponding aperture. The apertures are preferably aligned in one or several rows extending transversally across the width of the substrate, i.e. perpendicularly to the motion direction of the image receiving surface. Each single aperture is utilized to address a specific dot position of the image in a transversal direction. 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 direct electrostatic printing device of the type in question can include dot deflection control (DDC). Thereby, each single aperture is used to address several dot positions on an image receiving surface by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward the image receiving surface, 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 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. According to some embodiments of the invention printing is performed in print cycles having three subsequent print sequences for addressing, i.e. three deflection steps per print cycle and thus addressing three different dot locations through each aperture. A dot location is addressed during each print sequence. This provides a print addressability of 600 dpi utilizing a printhead structure having only 200 apertures per inch. Each print sequence comprises a print period t_b followed by a recovering period t_w during which new toner is supplied to the print zone. 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

An improved DDC method provides a simultaneous dot size and dot position control. This method utilizes the deflection electrodes to influence the convergence of the modulated stream of toner particles thus controlling the dot size. Each aperture is surrounded by two deflection electrodes connected to respective deflection voltages Dl, D2, such that the electrostatic control field generated by the control electrode remains substantially symmetrical as long as both deflection voltages Dl, D2 have the same amplitude. The amplitude of Dl and D2 are modulated to apply converging forces on toner particles as they are transported toward the image receiving surface, thus providing smaller dots. The dot position is simultaneously controlled by modulating the amplitude difference between Dl and D2 to deflect the toner trajectory toward predetermined dot positions.

Also a so-called multi-pass technique can be utilized. When utilizing such a technique, the image forming apparatus is provided with moving means causing the image receiving surface and the printhead structure to move in relation to each other, i.e. the image receiving surface or the printhead structure, or both, are movable. Thereby, the relative movement is so arranged that each line on the image-receiving surface that is transverse to the direction of the relative movement passes the printhead structure at least twice in order to form an image. Accordingly, the printhead structure prints only a part of each transverse line on each pass. When utilizing multi-pass technique, the moving means further includes means to move the printhead structure and the image receiving surface relative to each other (i.e. the printhead structure or the image receiving surface, or both, are moved) between consecutive passes or during a pass, so that each time that the image receiving surface passes the printhead structure, different parts of the image receiving surface are positioned to receive charged toner particles. The multi-pass technique increases the print addressability without requiring a larger number of apertures in the printhead structure which, if desired, can eliminate the need for special deflection electrodes or the like. In many cases, the multi-pass technique results in an improved resolution and print quality in comparison to when an image is printed in one single printing pass.

Multi-interlacing (MIC) is a further developed technique where an image forming apparatus utilizing multi-pass techniques is so constructed and arranged that adjacent columns of print are not printed by the same aperture in different passes. When utilizing MIC-technique, columns of print from different passes (partial images) are "interlaced" with each order in order to form the completed image. It has been found that MIC-techniques can improve print resolution and print quality even further in comparison to conventional multi-pass techniques.

Also dot deflection techniques (DDC) can be utilized in an image forming apparatus of the types in question. By means of utilizing a combination of multipass-, MIC- and/or DDC-techniques, the print addressability/number of apertures, and the print resolution can be optimized. In some embodiments the transfer belt or drum 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 or drum can also in certain embodiments comprise a special registration area for use of determining the position of the transfer belt or drum, especially an image area if available, in relation to each print station. If the transfer belt or drum comprises a special registration area then this area is preferably at least spatially related to an image area.

Each print station comprises a particle delivery unit. The particle delivery unit preferably has a replaceable or refillable container for holding toner particles which is disposed to continuously supply toner particles to a surface of a particle carrier through a particle charging member. Toner particles are retained on the surface of the particle carrier by an adhesion force which essentially is related to the particle charge and to the distance between the particle and the surface of the particle carrier. The electrostatic field applied onto a control electrode to initiate toner transport through a selected aperture is selected to be sufficient to overcome the adhesion force in order to cause the release of an appropriate amount of toner particles from the particle carrier. The electrostatic field is applied during the time period required for these released particles to reach sufficient momentum to pass through the selected aperture, whereafter the transported toner particles are exposed to the attraction force from the back electrode and are intercepted by the image receiving surface. Properties such as charge amount, charge distribution, particle diameter etc. of the individual toner particles have been found to be of particularly great importance to the print performance in a direct printing method. Accordingly, the size and size distribution of the toner particles affect the printing result, since larger toner particles have a tendency to cause clogging of the apertures in the control electrode array. In addition, the toner particles allowed to pass through selected "opened" apertures are accelerated towards the image receiving under the influence of a uniform attraction field from the back electrode. In order to control the distribution of the transported particles onto a printing surface, the particles may be deflected by the application of a deflection pulse, resulting in an increase in the addressable area on the image receiving surface. Thereby, small particles having a low surface charge exhibit poor deflection properties.

Normally, toner particles are produced by the so-called melt-crushed method, which involves crushing and classifying colored resin, such as polyester resin or the like, with dispersed coloring agents and other additives using a compounding process. However, this method is not ideally suited for producing small-particle toner since it has a relatively low yield, and tends to produce a great variety of particle sizes and toner particles with a non-uniform composition. A non- uniform toner results in a poor charge uniformity and may impair the print quality.

Toner particles can also be produced in a chemical polymerization process, which is better suited for producing small toner particles of a uniform size. There are three basic processes, i.e. the suspension polymerization method, the dispersion polymerization method, and the emulsion polymerization method. The suspension and dispersion polymerization methods produce full-shaped spherical toner particles with a size between a few and up to 10 microns. The emulsion polymerization method produces polymer particles of sub-micron size or smaller, which particles are aggregated by means of different methods, e.g. heat- welding or coagulation, in order to form micron-order particles. The shape of the aggregated particles can vary from grape cluster to spherical, depending on the conditions prevailing in the aggregation process.

Toner particles can comprise a number of ingredients, e.g. a binding resin based on a cyclic polyolefin e.g. a copolymer of an alicyclic compound with double bonds, such as cyclohexene or norbornene, and an alpha-olefin, such as ethylene, propylene or butylene. Accordingly, the toner particles can be of 2-component or multi-component type.

Advantageously, the toner particles have an irregular surface structure and an average diameter within the range of 3-8 microns. Depending on the application in question, electrically conductive, electrically non-conductive, or magnetic toner particles can be provided and utilized.

In order to clarify the method and device according to the invention, some examples of its use will now be described in connection with Figures 1 to 9.

Figure 1 shows one type of image forming apparatus 1000 which comprises at least one print station 1003, preferably four print stations (Y, M, C, K) 1003, an intermediate member 1001 providing an image receiving surface, a driving roller 1010, at least one support roller 1011, and preferably several adjustable holding elements 1012. The four print stations 1003 are arranged in relation to the intermediate member 1001. The intermediate member, in Figure 1 a transfer belt 1001, is mounted over the driving roller 1010. The at least one support roller 1011 is provided with a mechanism for maintaining the transfer belt 1001 with a constant tension, while preventing transversal movement of the transfer belt 1001. The holding elements 1012 are for accurately positioning the transfer belt 1001 with respect to each print station.

Each print station 1003 advantageously has the form of an elongated cartridge assembly and is arranged adjacent to a printhead structure 1005, providing an electrode matrix with a plurality of selectable apertures, which is interposed in a background electric field defined between the corresponding cartridge 1003 and a back electrode, which in the image forming apparatus in Figure 1 is constituted of the holding elements 1012.

The driving roller 1010 in Figure 1 is a cylindrical metallic sleeve having a rotational axis extending perpendicularly to the motion direction of the belt 1001 and a rotation velocity adjusted to convey the belt 1001 at a velocity of one addressable dot location per print cycle, to provide line by line scan printing. The adjustable holding elements 1012 are arranged for maintaining the surface of the belt at a predetermined gap distance from each print station. The holding elements 1012 in Figure 1 are cylindrical sleeves disposed perpendicularly to the belt motion in an arcuate configuration so as to slightly bend the belt 1001 at least in the vicinity of each print station in order to, in combination with the belt tension, create a stabilization force component on the belt. That stabilization force component is opposite in direction and preferably larger in magnitude than an electrostatic attraction force component acting on the belt 1001 due to interaction with the different electric potentials applied on the corresponding print station.

The transfer belt 1001 in the apparatus shown in Figure 1 is an endless band of 30 to 200 microns thick composite material as a base. The transfer belt 1001 is conveyed past the four different print stations, whereas toner particles are deposited on the outer surface of the transfer belt and supeφosed to form a four color toner image. Toner images can then be conveyed through a fuser unit 1013 comprising a fixing holder 1014 arranged transversally in direct contact with the inner surface of the transfer belt. The fixing holder includes a heating element 1015, suitably of a resistance type of e.g. molybdenium, maintained in contact with the inner surface of the transfer belt 1001. As an electric current is passed through the heating element 1015, the fixing holder 1014 reaches a temperature required for melting the toner particles deposited on the outer surface of the transfer belt 1001. The fusing unit 1013 further includes a pressure roller 1016 arranged transversally across the width of the transfer belt 1001 and facing the fixing holder 1014. In the apparatus shown in Figure 1, an information carrier 1002, such as a sheet of plain untreated paper or any other medium suitable for direct printing, is fed from a paper delivery unit 1021 and conveyed between the pressure roller 1016 and the transfer belt. The pressure roller 1016 rotates with applied pressure to the heated surface of the fixing holder 1014 whereby the melted toner particles are fused on the information carrier 1002 to form a permanent image. After passage through the fusing unit 1013, the transfer belt is brought in contact with a cleaning element 1017, such as for example a replaceable scraper blade of fibrous material extending across the width of the transfer belt 1001 for removing all non-transferred toner particles from the outer surface.

Instead of a single unit performing a combined image transfer and fusing step, separate units for transferring the image to the information carrier and for fusing/fixating the image to the information carrier can be provided. The fusing unit is normally provided with means for feeding the paper to an out-tray, from which the paper can be collected by a user. Figure 2 is a simplified view of an image forming apparatus 2000 of another type where the image receiving surface is provided on a cylindrical drum 2001. The image forming apparatus comprises one or several print stations 2003, each adapted for printing one color. Normally, the colors being used are yellow, magenta, cyan and black. Each print station 2003 advantageously has the form of an elongated cartridge assembly and is arranged adjacent to a printhead structure 2005, providing an electrode matrix with a plurality of selectable apertures, which is inteφosed in a background electric field defined between the corresponding cartridge 2003 and a back electrode, which in the image forming apparatus in Figure 2 is constituted of the cylindrical drum 2001. The drum 2001 is arranged so as to rotate during operation of the image forming apparatus. To this end, the drum 2001 is powered by drive means (not shown). Furthermore, the drum 2001 has a circumference which is at least slightly greater than the length of the paper (or other information carrier) used during printing. The drum 2001 advantageously is made of aluminum, but can also be made from other materials with suitable properties.

Each printhead 2005 is connected to a control unit (not shown in Figure 2) which converts the image information in question into a pattern of electrostatic fields so as to selectively open or close passages in the electrode matrix to permit or restrict the transport of charged toner particles from the corresponding cartridge 2003. In this manner, charged particles are allowed to pass through the opened apertures and toward the back electrode, i.e. the drum 2001. The charged toner particles are then deposited on the surface of the drum 2001. Accordingly, in the image forming apparatus in Figure 2, the drum 2001 constitutes both back electrode and image receiving surface. Due to the fact that the drum 2001 is rotating during operation, the image being formed on the drum is then transferred onto an information carrier 2002, such as a sheet of printing paper or any other medium suitable for printing. The paper sheet 2002 is fed from a paper delivery unit 2021 and is conveyed past the underside of the drum 2001. In order to transfer the image to the paper sheet 2002, it is pressed into contact with the drum 2001 by means of belt 2017, which in turn is driven by means of two rollers 2016 around which the belt extends. In this manner, the toner particles are deposited on the outer surface of the drum 2001 and then superimposed to the paper sheet 2002 to form a four-color image. Accordingly, the operation of the belt 2017 defines a transfer step, which advantageously is positioned in the lowest section of the image receiving surface on the drum 2001. As a result, the force of gravity acting upon the toner particles will contribute to the transfer of said particles from the image receiving surface to the paper sheet 2002 during operation.

After the image has been formed on the paper sheet 2002 by said charged particles, the paper sheet 2002 is fed to a fusing unit 2013, in which the image is permanently fixed onto the paper sheet 2002. In particular, the fusing unit 2013 comprise a fixing holder (not shown) which includes a heating element, advantageously of a resistance type of e.g. molybdenium. As an electric current is passed through the heating element, the fixing holder reaches a temperature required for melting the toner particles deposited on the paper sheet 2002. The fusing unit 2013 further includes a pressure roller (not shown) arranged transversally across the width of the paper sheet 2002. Additionally, the fusing unit 2013 is provided with means for feeding the paper 2002 to an out-tray (not shown) from which the paper 2002 can be collected by a user. Furthermore, after passage through the fusing unit 2013, the paper sheet 2002 can be brought in contact with a cleaning element (not shown), such as for example a replaceable scraper blade of fibrous material extending across the width of the paper sheet 2002 (or another suitable information carrier), for removing non-transferred toner particles from the paper sheet 2002. The printstations 2003 and the printhead structures 2005 are mounted in a housing element (not shown), so that they are maintained in predetermined positions with respect to the drum 2001.

An image forming apparatus of the type shown in Figure 2 is particularly well suited for direct printing with multi-pass methods by means of which the resolution given by a printhead structure for a given number of apertures may be increased. In order to achieve a print resolution greater than the number of apertures in the printhead structure 2005, multi-pass printing takes place during two or more passes of the image receiving surface provided by the drum 2001, wherein a plurality of longitudinal columns of print are deposited in each pass. A column of print is a longitudinal line of the image receiving surface which is subject to printing of dots by an aperture or apertures even if not all the parts of the line receive dots due to the content of the image being formed requiring some parts of the columns to be left without dots. A transverse line of print is a transverse line of the image receiving surface which is subject to printing of dots from a plurality of apertures, even if not all the parts of the line receive dots due to the content of the image being formed requiring some parts to be left without dots. The closest distance between two adjacent columns of lines of print is defined as the pitch or the distance between two addressable pixel locations. After the first pass, the next passes may be in the same or opposite longitudinal directions to that of the first pass.

The transverse direction is the direction which, in case the image receiving surface is provided on a cylindrical drum, is peφendicular to a radial vector of the cylinder towards the printhead structure at the surface of the drum and parallel to the axis of rotation of the drum along the surface of the drum. In case the image receiving surface is provided on a transfer belt, the transverse direction is the direction in the plane of the belt peφendicular to the relative movement between the belt and a printhead structure in question. Thus, the transverse direction will normally be parallel to the axes of belt supporting rollers. The longitudinal direction is the direction peφendicular to the transverse direction and in the plane of the image receiving surface, i.e. transfer belt or drum. In the case of the drum, the longitudinal direction is the direction peφendicular to the transverse direction and along the surface of the drum. In the case of the transfer belt, the longitudinal direction is the direction at any point on its surface in the direction peφendicular to the axis of rotation of the belt supporting rollers and in the plane of the surface of the belt.

Figures 3a, 3b, 3c, show a printhead structure 3005 in an image forming apparatus, e.g. of the type illustrated in Figure 1 or Figure 2. The printhead structure preferably comprises a substrate 3050 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the print station (particle carrier) in question, a second surface facing an image receiving surface, a transversal axis 3051 extending parallel to the transverse direction of the image receiving surface across the whole print area, and a plurality of apertures 3052 arranged through the substrate 3050 from the first to the second surface thereof. The first surface of the substrate is coated with a first cover layer 3501 of electrically insulating material, such as for example parylene. A first printed circuit, comprising a plurality of control electrodes 3053 disposed in conjunction with the apertures, and, in some embodiments, shield electrode structures (not shown) arranged in conjunction with the control electrodes 3053, is arranged between the substrate 3050 and the first cover layer 3501. The second surface of the substrate is coated with a second cover layer 3502 of electrically insulating material, such as for example parylene. A second printed circuit, including, for example, a plurality of deflection electrodes 3054, or other electrodes, such as guard electrodes, arranged between the substrate 3050 and the second cover layer 3502. The printhead structure 3005 further includes a layer of antistatic material 3503, preferably a semiconductive material, such as silicium oxide or the like, arranged on at least a part of the second cover layer 3502, facing an image receiving surface. The antistatic material 3503 can advantageously also be arranged on at least a part of the first cover layer 3501. The printhead structure 3005 is brought in cooperation with a control unit (not shown in Figure 3) comprising variable control voltage sources connected to the control electrodes 3053 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 3052 during each print sequence. The control unit further suitably comprises either deflection voltage sources connected to the deflection electrodes 3054 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 3052, or other electrode voltage sources such as a guard electrode voltage source. In some designs, the control unit also includes a shield voltage source connected to the shield electrodes to supply a shield potential which electrostatically screens adjacent control electrodes 3053 from one another, preventing electrical interaction therebetween. The substrate 3050 is advantageously a flexible sheet of polyimide having a thickness on the order of about 50 microns. The first and second printed circuits are suitably copper circuits of approximately 8-9 microns thickness on the first and second surface of the substrate 3050, respectively. The first and second cover layers 3501, 3502 are suitably 5 to 10 microns thick parylene laminated onto the substrate 3050 using vacuum deposition techniques. The apertures 3052 can be made through the printhead structure 3005 using laser micromachining methods. The apertures 3052 have preferably a circular or elongated shape centered about a 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. Although the apertures 3052 preferably have a constant shape along their axis, for example cylindrical apertures, it may be advantageous in some embodiments to provide apertures whose shape varies continuously or stepwise along the axis, for example conical apertures.

In one advantageous design, the printhead structure 3005 is dimensioned to perform 600 dpi printing utilizing three deflection sequences in each print cycle, i.e. three dot locations are addressable through each aperture 3052 of the printhead structure during each print cycle. Accordingly, one aperture 3052 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 3051 of the printhead structure 3005. The apertures 3052 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 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 3051 of the printhead structure 3005 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 control electrodes 3053 each of which having a ring shaped structure surrounding the periphery of a corresponding aperture 3052, 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 3053 may take on various shapes for continuously or partly surrounding the apertures 3052, preferably shapes having symmetry about the axis of the apertures. In some embodiments, particularly when the apertures 3052 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 according to this example a plurality of deflection electrodes 3054, each of which is divided into two semicircular or crescent shaped deflection segments 3541, 3542 spaced around a predetermined portion of the circumference of a corresponding aperture 3052. The deflection segments 3541, 3542 are arranged symmetrically about the axis of the aperture 3052 on each side of a deflection axis 3543 extending through the center of the aperture 3052 at a predetermined deflection angle d to the longitudinal direction. The deflection axis 3543 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 relative movement between the image receiving surface and the printhead structure during the print cycle, to obtain transversally aligned dot positions on the image receiving surface. 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 3054 has an upstream segment 3541 and a downstream segment 3542. Preferably all upstream segments 3541 being connected to a first deflection voltage source Dl, and all downstream segments 3542 being connected to a second deflection voltage source D2. 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 3052, thus the dot position on the toner image.

The printhead structure can be of a number of different designs and materials. For instance, instead of being deposited onto the substrate by means of vacuum deposition techniques, the cover layers may be constituted of a 5 - 20 micron thick film laminated onto the substrate. Furthermore, the printhead structure will of course need no deflection electrodes in applications where no dot deflection control is utilized. Preferably in such applications other electrodes such as guard electrodes, which surround the apertures and which can be connected to one or more common voltage sources.

As mentioned previously it is very difficult to ensure that all the control electrodes are electrically intact. The problems that can occur are that either there is a break in the conductors to the control electrodes, there is one or more breaks on the control electrode around the aperture itself, there is a short circuit between control electrodes or conductors to the control electrodes, or there is a short circuit or unwanted coupling to other electrically conductive materials on or in the vicinity of the printhead structure. A further problem is that drive circuitry such as high voltage drive controllers mounted on the printhead structure is difficult to test. One solution would be to test each printhead structure during production by, for example, putting a printhead structure in a special automatied test equipment. This would catch any manufacturing defects of the printhead structure, but it would be more and more difficult to perform tests by this method further down in the manufacturing process, as the different layers cover any suitable connection points and after any control circuitry is added, it is doubtful if these circuits can be submitted to any active testing methods.

According to the invention the control unit is used to provide the desired test signals by utilizing the normal signals of the control unit controlled in a manner according to the invention. The printhead structure can thus be tested as soon as any necessary parts of the control unit or connections therefore are mounted on the printhead structure. The invention utilizes the ability of the control unit to individually control each control electrode. By individually controlling one or more control electrodes at a time, i.e. outputting a predetermined test signal according to the invention onto one or more control electrodes, and then measuring one or more parameters, then the status of the control electrodes can be established. According to the invention the one or more parameters are measured by only one or more measuring means per printhead structure. This is a great advantage of not having to have a separate measuring means for each individual control electrode.

Figure 4 illustrates a connectivity schematic of a first embodiment of the invention which utilizes a capacitive coupling between the control electrodes 4053 on one side of a printhead structure and corresponding electrodes 4054 on the other side of the printhead structure, such as deflection electrodes or guard electrodes. The invention utilizes the fact that deflection electrodes and guard electrodes are connected together, all or in sections. Only one measurement means 4101 per section is needed. According to the invention the control unit is utilized to generate suitable test signals. In this first embodiment of the invention the control unit controls one or more output circuits 4041 to generate an alternating signal that can be used to couple between control electrodes 4053 and corresponding electrodes 4054 on the other side, i.e. according to the invention the control electrodes 4053 and corresponding electrodes 4054 on the other side of the printhead structure are used as capacitors. The alternating signal can in its simplest form be a single change (flank) that will couple through the control electrode 4053 and corresponding electrode 4054 capacitors. This change can then be detected by the measurement means 4101. The alternating signal can take on any desireable form, such as pulses, ramps, sinus waves, digital or analogue, the important thing being that it has to be able to be detected on the onther side of a virtual capacitor by the measurement means 4101, if the circuitry is functioning correctly that is. The control unit controls the other output circuits 4040 to attain a low output, thus virtually grounding the control electrodes 4053 connected to these output circuits 4040.

Figure 5 illustrates an equivalent electrical schematic of the first embodiment according to Figure 4. The equivalent electrical schematic demontrates what a printhead structure that operates correctly should look like. The output circuit 5041 generates an alternating signal connected to an equivalent signal capacitor C_s which in turn is connected to a measuring means 5101 and also to equivalent ground capacitors C_G, which in turn are connected to ground GND. Thus, a generated alternating signal at the output circuit 5041 should couple through C_s and be voltage divided, i.e. the alternating signal multiplied by a factor of Cs/(2*C_G+C_s) will be detected at the measurement means 5101 when the circuitry is functioning correctly. If there is a break to the control electrode under measurement then there will be no signal coupled, if there is a break in the control electrode under measurement itself, then depending on how and where the break is there be anything from no coupling to substantially the same coupling as an unbroken circuit. There might also be a break in the circuitry of the corresponding electrode of a control electrode under measurement which will also result in a lower or no signal being detectable at all at the measurement means. Also if there is a short circuit of the control electrode under measurement or corresponding circuitry, or of the corresponding electrode or corresponding circuitry, will cause either no detectable signal at the measurement means or at least a degraded signal. If there is no detectable signal, this can also indicate that an output of a drive circuit is not functioning, if this is the case then normally the whole drive circuit (IC) is malfunctioning. This can be verified by activating a plurality of outputs with a test signal, if this does not produce a detectable signal, then there is a high probability that the drive circuit (IC) in question is malfunctioning. Activation of a plurality of outputs will also raise the signal level of any detectable signal and can thus be used if signal levels are not adequately high for detection. A suitable correlation with other test can determine the specific nature of the fault a printhead structure might have. If it is determined that there is a problem with the control electrode that it is only partially functional, then this is suitable compensated for by the control unit during printing.

Figure 6 illustrates a connectivity schematic of a second embodiment of the invention in which it is determined if there is a current leakage of any of the control electrodes 6053 to other control electrodes 6053 or to other electrically conducting parts. Here the control unit controls an output circuit 6041 to generate an output signal, which can be either a predetermined voltage level or an alternating voltage source of the kind described above in connection with Figure 4 and Figure 5. Preferably at least the output circuitry 6040 of the control electrodes closest to a control electrode under measurement is either grounded or put at a predetermined voltage level. According to this embodiment, the current/power consumption of at least the output circuitry 6041 used to generate a test signal is measured by measurement means 6102. Suitably the current/power consumption of several output circuitry 6040, 6041 is measured at the same time. In any given configuration of the output circuitry, the current/power consumption of the output circuits 6040, 6041 should be within certain predefined/predetermined limits for a fully functional control electrode circuitry.

Figure 7 illustrates an equivalent electrical schematic of the second embodiment according to Figure 6, where a lowered resitance R_s, such as a short circuit, to a neighboring control electrode circuit is shown. Here it is possible to measure an increase in current/power consumption of the output circuit 7041 of the control electrode under measurement by means of the measurement means 7102.

According to the invention other tests are also possible to generate by the use of the control unit controlling the ordinary control circuitry of the control electrodes. For example, pulse reflection measurements can be made to determine the condition of the control electrodes and their corresponding connections. Pulse reflection techniques utilizes the fact that pulse will reflect when there is a change of impedance, such as when there is a break or there is an end. The time it takes for the reflection to return will tell where there is this impedance change. This impedance change should only be at the end where the control electrode itself is situated and ends the signal line, there is thus a predetermined time when the reflection should return in a correctly functioning circuit.

The control functions of a printing unit according to the invention is handled by a control unit which is schematically illustrated in Figure 8. The illustration of the control unit 8900 is merely to give an example of one possible embodiment of the control unit 8900. All the different parts may be separate as illustrated or more or less integrated. The memories 8902, 8903, 8930 may be of an arbitrary type which will suit the embodiment in question. The control unit 8900 comprises a computing part which comprises a CPU 8901, program memory ROM 8902, working memory RAM 8903, a user I/O interface 8910 through which a user will communicate 8951 with the printer for downloading of commands and images to be printed, and a bus system 8950 for interconnection and communication between the different parts of the control unit 8900. The control unit 8900 also suitably comprises a bitmap 8930 for storage of the image to be printed and one or more I/O interfaces 8911, 8912 for control and monitoring of the printer. Further, if necessary, one or more output circuits such as power - high voltage drivers 8921, 8922, 8923, 8924, 8925 are connected to the hardware of the printer illustrated by an interface line 8999.

The one or more I/O interfaces 891 1, 8912 for control and monitoring of the printer can logically be divided into one simple I/O interface 8912 for on/off (digital) control and monitoring and one advanced I/O interface 891 1 for multilevel (analogue or multilevel digital) control and monitoring, speed control, and analog measurements. Typically the simple I/O interface 8912 handles keyboard input 8969 and feedback output 8968, control of simple motors and indicators, monitoring of different switches and other feedback means. Typically the advanced I/O interface 8911 will control 8954, 8955 the selection control voltages 8964 and guard voltages 8965 via high voltage drivers 8924, 8925. The advanced I/O interface 891 1 will typically also speed control 8966 one or more motors with a control loop feedback 8967.

A user, e.g. a personal computer, will download, through the user I/O interface 8910, commands and images 8951 to be printed. The CPU 8901 will inteφret the commands under control of its programs and typically load the images to be printed into the bitmap 8930. The bitmap 8930 will preferably comprise at least two logical bitmaps, one which can be printed from and one which can be used for download of the next image to be printed. The functions of the preferably at least two logical bitmaps will continuously switch when their previous function is finished.

In a preferred embodiment the bitmap 8930 will serially 8952 load a plurality of high voltage drive controllers 8921, 8922, 8923 with the image information to be printed. The number of high voltage drive controllers 8921, 8922, 8923 that are necessary will, for example, depend on the resolution and the number of apertures, i.e. master control electrodes, each controller 8921, 8922, 8923 will handle. The high voltage drive controllers 8921, 8922, 8923 will convert the image information they receive to signals 8961, 8962, 8963 with the proper voltage levels required by the master control electrodes of the printer.

According to the invention the control unit can control the condition of the printhead circuit during production phases, during power-up, and possibly also continuously during use when the printhead structre is not printing, such as between pages.

Figure 9 illustrates one possible schematic of an output circuit, a high voltage drive controller 9940. The image information is received serially via a data input 9971. The image information is clocked 9972 into a serial to parallel register 9941. When the serial to parallel register 9941 is full the image information is latched 9973 into a latch 9942 at an appropriate time, thus enabling new image information to be clocked into the serial to parallel register. The controller preferably comprises high voltage drivers 9943, 9944, 9945, 9946, 9947 for conversion of the image data in the latch to signals 9983, 9984, 9985, 9986, 9987 with the appropriate voltage levels required by the master control electrodes of the apertures. The high voltage drive controller can also suitably comprise a blanking input 9974 to enable a higher degree of control of the outputs 9983, 9984, 9985, 9986, 9987 to the master control electrodes.

According to certain embodiments according to the invention power/current consumption measurement means 9102 is connected or integrated into the output circuit 9940.

The invention is not limited to the embodiments described above but may be varied within the scope of the appended patent claims.

Claims

WHAT IS CLAIMED IS

1. A method of controlling the condition of a printhead structure having a first surface and a second surface, the printhead structure further comprising a carrier, a plurality of apertures arranged through the printhead structure between the first surface and the second surface, a plurality of control electrodes arranged on the first surface for selectively and electrostatically opening or closing the plurality of apertures to permit or restrict transport of toner particles in the form of toner jets, and one or more drive circuits, each drive circuit comprising one or more ouputs each connected to one of the plurality of control electrodes characterized in that the method comprises the steps of: - activating one or more outputs in a predetermined manner to thereby subject one or more control electrodes to a test signal; measuring a response to the test signal; determining the condition of the printhead structure from the measured response.

2. The method according to claim 1, characterized in that the plurality of apertures are arranged in one or more substantially straight rows across the printhead structure.

3 The method according to claim 1 or 2, characterized in that the step of activating one or more outputs in a predetermined manner, activates one or more outputs only to such a degree that the drive circuit in question can tolerate a short circuit of the one or more outputs in question, and in that the step of measuring a response to the test signal, measuring the current and or the power consumption of at least the drive circuit or circuits whose one or more outputs are activated.

j 4. The method according to any one of claims 1 to 3, characterized in that the step of activating one or more outputs in a predetermined manner to thereby subject one or more control electrodes to a test signal, activates the one or more outputs to generate a test signal with a predetermined level.

5. The method according to claim 1 or 2, characterized in that the printhead structure further comprises additional electrodes, and in that the step of measuring the response to a test signal, measures a voltage or current on the additional electrodes.

6. The method according to claim 5, characterized in that the additional electrodes are arranged on the second surface of the printhead structure.

7. The method according to claim 5 or 6, characterized in that the step of measuring the response to a test signal further comprises determining if any measured signal on the additional electrodes corresponds, in view of a transfer function between the control electrodes and the additional electrodes, to the test signal.

8. The method according to any one of claims 5 to 7, characterized in that the step of activating one or more outputs in a predetermined manner, activates one or more outputs up to as much as the drive circuit or drive circuits will allow.

9. The method according to any one of claims 5 to 7, characterized in that the step of activating one or more outputs in a predetermined manner, activates one or more outputs up to as much as the drive circuit or drive circuits will allow only if it has been determined that the one or more outputs in question are not short circuited, and otherwise activate one or more outputs only to such a degree that the drive circuit in question can tolerate a short circuit of the one or more outputs in question.

10. The method according to any one of claims 5 to 9, characterized in that the step of activating one or more outputs in a predetermined manner to thereby subject one or more control electrodes to a test signal, activates the one or more outputs to generate a test signal which will propagate through a capacitor.

11. The method according to any one of claims 5 to 10, characterized in that the step of determining the condition of the printhead structure from the measured response, determines that a drive circuit is malfunctioning if there is no measured response from a majority of the drive circuit's outputs when activated.

12. The method according to any one of claims 5 to 11, characterized in that the additional electrodes are deflection electrodes.

13. The method according to any one of claims 5 to 11 , characterized in that the additional electrodes are guard electrodes.

14. The method according to any one of claims 5 to 11, characterized in that the additional electrodes are shield electrodes.

15. The method according to any one of claims 5 to 14, characterized in that the step of activating one or more outputs in a predetermined manner, activates at least two outputs simultaneously to acquire a response with a higher signal level.

16. The method according to any one of claims 1 to 15, characterized in that the step of measuring a response to the test signal, comprises measuring a rise time of the respone.

17. The method according to any one of claims 1 to 16, characterized in that the step of measuring a response to the test signal, comprises measuring a delay time of the respone from an activation time of the test signal.

18. A direct electrostatic printing device including a toner particle delivery, a electrical field creator, a printhead structure, and a control unit, the toner particle delivery providing toner particles, an image receiving surface and the printhead structure are arranged for relative movement between each other during printing, the electrical field creator creating an electrical field between the toner particle delivery and the image receiving surface for transport of toner particles from the toner particle delivery toward the image receiving surface, the printhead structure being placed in the electrical field between the toner particle delivery and the image receiving surface, the printhead structure including control electrodes connected to the control unit to thereby selectively open or close toner jet passages through the printhead structure to permit or restrict the transport of toner particles during a print sequence in the form of toner jets, at least one print sequence is included in a print cycle, to thereby enable the formation of an image on the image receiving surface, characterized in that the device is arranged to perform the method according to any one of claims 1 to 17.

19. The device according to claim 18, characterized in that the device is arranged to perform the method according to any one of claims 1 to 16 only at other times than when the device is forming an image on the image receiving surface.

20. A method for printing an image to an information carrier, characterized in that the method comprises the following steps: providing toner particles from a toner particle delivery; moving an image receiving surface and a printhead structure relative to each other during printing; creating an electrical field for transporting pigment particles from the toner particle delivery toward the image receiving surface; selectively opening or closing toner jet passages through the printhead structure, to thereby permit or restrict the transporting of toner particles during a print sequence in the form of toner jets, at least one print sequence is included in a print cycle, to thereby enable the formation of an image on the image receiving surface; performing the method according to any one of claims 1 to 17.

21. The method according to claim 20, characterized in that the step of performing the method according to any one of claims 1 to 17 is only performed at other times than when the method forms an image on the image receiving surface.