Background US patent No. 5 847 733 describes a direct electrostatic printing device and a method of generating text and pictures with toner particles on an image receiving substrate directly from computer generated signals that includes the elements mentioned above. In this device a printhead structure is arranged in the electric field generated between the particle source and back electrode.

Toner particles are selectively transported through the printhead structure in accordance with image data. The printhead structure is generally constituted by a control electrode array formed on an apertured insulating substrate. A ring electrode is associated with each aperture and is driven to control the opening and closing of the apertures to toner particles. Each aperture is further provided with deflection electrodes which are controlled to selectively generate asymmetric electric fields around the apertures, causing toner particles to be deflected prior to their deposition on the image- receiving medium. This process is referred to as dot deflection control (DDC). This enables each individual aperture to address several dot positions.

A problem with such prior art devices is that while the various voltage sources supplying the ring and deflection electrodes and the back electrode can be controlled to high precision, the printing quality may nevertheless vary over time due to variations in the charge of the toner particles. This variation in charge alters the behaviour of the toner particles in the background electric field and in the asymmetric electric field generated by the deflection electrodes. This affects both the quantity of toner particles projected through each aperture and also in the degree of deflection obtained.

The resultant print can be of different resolution and also out of register with earlier print results. In some cases the variation in print quality may be so extreme that pages must be discarded.

Thus there is a need for a direct electrostatic image forming arrangement capable of alleviating the problems of prior art apparatus and providing a substantially uniform print quality.

Summary of the invention This object is achieved in an image forming apparatus including a particle carrier for holding a layer of charged toner particles, a back electrode coupled to a voltage source for generating a background electric field for accelerating the transport of charged toner particles from the particle carrier towards the back electrode, a printhead structure disposed in the background electric field having a plurality of apertures and an electrically conductive layer including at least one electrode associated with the apertures, control means coupled to electrode voltage sources connected with said electrically conductive layer to control the transport of charged toner particles through the apertures, and means for transporting an image receiving member between the printhead structure and the back electrode for intercepting the transported charged particles in a configuration defined by the control means, the apparatus being characterised by, means for determining a change in the humidity of the toner particles and means responsive to said determining means for adjusting the voltages applied to the electrodes and/or the back electrode to modify the electric field between the particle carrier and back electrode to maintain the configuration of the charged toner particles on said image receiving member substantially unchanged.

In this manner, the configuration, i. e. the positions and sizes of the toner dots printed on the image receiving member can be maintained substantially unchanged despite changes in the level of humidity in the surrounding air.

In accordance with the invention it is recognised that the charge of the toner particles will vary with the humidity within the apparatus. By measuring the humidity within the apparatus, the humidity of the toner particles and thus the charge of the toner particles can be deduced. By controlling the control voltages applied to the printhead structure and back electrode in accordance with the determined humidity, the apparatus is thus made insensitive to variations in toner charge level.

According to a preferred embodiment of the invention, the humidity is determined using means for measuring a current generated by the layer of charged toner particles on the particle carrier. The current generated by the accumulated toner will vary with the humidity of the toner particles. The current reading thus gives a direct indication of the humidity in the apparatus.

The current is measured using an electrically conductive element that may include a supply roller, a doctor blade and/or a spacer element and is disposed to contact the surface of the toner layer on the particle carrier. Means are provided for coupling the particle carrier and the electrically conductive element to ground and for measuring the current flow between the particle carrier and the element. The term ground is here intended to signify an electrical ground. This may be an electrical ground common with other electrical systems in the apparatus, or may be a separate, floating ground used only for the current flow measurements. Preferably a resistive element is used to connect the particle carrier and/or the electrically conductive element to ground. The current measurement can then be carried out by measuring the change in voltage across the resistive element.

In accordance with another preferred embodiment of the invention the humidity within the apparatus affecting the toner particles is determined using means for measuring the resistance of paper adapted to receive the toner particles in image configuration.

The paper may be used as the image receiving member and be transported between the back electrode and printhead structure by mutually opposing feed members that contact opposing sides of the paper. In this case, the humidity is determined by measuring the resistance between the feed members. The resistance is measured using a voltage source selectively coupled to one of the feed members and means for measuring the current flowing between the feed members.

Alternatively, the image is transferred from the image receiving member to paper in an image transfer station.

In this case the image transfer station includes electrically conductive elements in contact with opposing sides of the paper. These may include feed members for moving the paper, a pressure member for pressing the paper against the image receiving member and a transfer member for providing a counter force pressing the image receiving member against the paper. Means are provided for measuring the resistance between two of the electrically conductive elements. These elements are all in contact with the paper but otherwise electrically insulated from one another. Thus the sole connecting element is the paper. Preferably the resistance is measured using means for applying a voltage to one of the two electrically conductive elements, means for connecting the other of the two electrically conductive elements to ground, and means for measuring the current flowing between the elements.

In accordance with a further embodiment of the invention the humidity within the apparatus is determined by means for measuring the current flow between the back electrode and the particle carrier upon application of a voltage by the back electrode voltage source. This effectively provides a measurement of the resistance of the air gap between these two elements. The resistance will vary depending on the humidity in the air.

In accordance with a further aspect of the invention there is proposed a method of operating an image forming apparatus having a back electrode coupled to a voltage source for generating a background electric field for accelerating the transport of charged toner particles from said particle carrier towards said back electrode, a printhead structure disposed in said background electric field having a plurality of apertures and an electrically conductive layer including at least one aperture electrode associated with said apertures; control means coupled to electrode voltage sources connected with said electrically conductive layer to control the transport of charged toner particles through said apertures, and means for transporting an image receiving member between said printhead structure and said back electrode for intercepting the transported charged particles in a configuration defined by the control means, wherein the method includes the steps of halting printing operation, determining the charge of the toner particles, adjusting control voltage sources and/or the back electrode voltage source to modify the electric field between the particle carrier and back electrode in accordance with a determined change in charge.

Preferably the determined charge value is compared with a previously determined charge value to ascertain a change in charge. The adjustment of the control voltages is necessary only when a change in humidity is determined.

Accordingly an absolute value of the quantity measured, whether this be resistance, current or voltage, is not necessary.

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 the figures Fig. 1 is a schematic view of an image forming apparatus in accordance with a preferred embodiment of the present invention, Fig. 2 is a schematic view of part of an image forming apparatus in accordance with an alternative embodiment of the present invention, Fig. 3 is a schematic section view across a print station in an image forming apparatus, such as, for example, that shown in Fig. 1, Fig. 4 is a schematic section view of the print zone, illustrating the positioning of a printhead structure in relation to a particle source and an image-receiving member, Fig. Sa is a partial view of a printhead structure of a type used in an image forming apparatus, showing the surface of the printhead structure that is facing the toner delivery unit, Fig. 5b is a partial view of a printhead structure of a type used in an image forming apparatus, showing the surface of the printhead structure that faces the intermediate transfer belt, Fig. 5c is a section view across a section line I-I in the printhead structure of Fig. 5a and across the corresponding section line II-II of Fig. 5b, Fig. 6 shows a schematic view of part of the image forming apparatus of Fig. 1 illustrating an arrangement for measuring humidity according to one embodiment of the present invention, Fig. 7 shows a simplified circuit diagram representing part of the image forming apparatus of Fig. 1 illustrating an arrangement for measuring humidity according to a further embodiment of the present invention, Fig. 8 shows a simplified circuit diagram illustrating an arrangement for measuring humidity according to a still further embodiment of the present invention, Fig. 9 shows a simplified circuit diagram illustrating an arrangement for measuring humidity according to another embodiment of the present invention, and Fig. 10 shows an electrode arrangement on the printhead structure that forms part of an arrangement for measuring humidity according to a still further embodiment of the present invention.

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

The driving roller 10 is preferably a cylindrical metallic sleeve having a rotation axis extending perpendicular to the motion direction of the belt 1 and a rotation velocity adjusted to convey the belt 1 at a velocity of one addressable dot location per print cycle, to provide line by line scan printing. The adjustable holding elements 12 are arranged for maintaining the surface of the belt at a predetermined distance from each print station. The holding elements 12 are preferably cylindrical sleeves disposed perpendicularly to the belt motion in an arcuated configuration so as to slightly bend the belt 1 at least in the vicinity of each print station in order to create a stabilisation force component on the belt in combination with the belt tension. That stabilisation force component is opposite in direction to, and preferably larger in magnitude than, an electrostatic attraction force component acting on the belt 1 due to interaction with the different electric potentials applied on the corresponding print station. The holding elements 12 are provided with an electrically conducting surface which is connected to a voltage source for generating a background electric field. These elements 12 thus serve as back electrodes, which generate the electric field for propelling the toner particles towards an image receiving member as will be described in detail below.

The transfer belt 1 is preferably an endless band of 30 to 200 microns thick having composite material as a base.

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

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

An alternative arrangement is shown in Fig. 2, which illustrates a single print station of an image forming apparatus. In this embodiment, the image is printed directly onto the paper 2 or other suitable information carrier. A system of mutually opposing feed rollers 17 is provided to enable transport of the sheet of paper through the print zone of each print station and also from one print station to the next.

As shown in Fig. 3, a print station in an image forming apparatus in accordance with the present invention includes a particle delivery unit 3 preferably having a replaceable or refillable container 30 for holding toner particles, the container 30 having front and back walls (not shown), a pair of side walls and a bottom wall having an elongated opening 31 extending from the front wall to the back wall and provided with a toner feeding element 32 disposed to continuously supply toner particles to a toner sleeve or carrier 33 through a particle charging member 34. The particle-charging member 34 is preferably formed of a supply brush or a roller made of, or coated with, a fibrous, resilient material.

The supply brush is brought into mechanical contact with the peripheral surface of the toner carrier 33 for charging particles by contact charge exchange due to triboelectrification of the toner particles through frictional interaction between the fibrous material on the supply brush and any suitable coating material of the toner carrier. The toner carrier 33 is preferably made of metal coated with a conductive material, and preferably has a substantially cylindrical shape and a rotation axis extending parallel to the elongated opening 31 of the particle container 30. Charged toner particles are held on the surface of the toner carrier 33 by electrostatic forces essentially proportional to (Q/D) where Q is the particle charge and D is the distance between the particle charge center and the boundary of the toner carrier 33. Alternatively, the charge unit may additionally include a charging voltage source (not shown), which supplies an electric field to induce or inject charge to the toner particles. Although it is preferred to charge particles through contact charge exchange, the method can be performed using any other suitable charge unit, such as a conventional charge injection unit, a charge induction unit or a corona charging unit, without departing from the scope of the present invention.

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

As shown in Fig. 4, the toner carrier 33 is arranged in relation with a positioning device 40 for accurately supporting and maintaining the printhead structure 5 in a predetermined position with respect to the peripheral surface of the toner carrier 33. The positioning device 40 is formed of a frame 41 having a front portion, a back portion and two transversally extending side rulers 42, 43 disposed on each side of the toner carrier 33 parallel with the rotation axis thereof. The first side ruler 42, positioned at an upstream side of the toner carrier 33 with respect to its rotation direction, is provided with fastening means 44 to secure the printhead structure 5 along a transversal fastening axis extending across the entire width of the printhead structure 5. The second side ruler 43, positioned at a downstream side of the toner carrier 33, is provided with a support element 45, or pivot, for supporting the printhead structure 5 in a predetermined position with respect to the peripheral surface of the toner carrier 33. The spacing between the toner carrier 33 and the printhead structure 5 is defined by a spacer 36 that extends from a side of the particle container 30 into the gap between the toner carrier 33 and the printhead structure 5 in the direction of rotation of the toner carrier 33. The spacer 36 is in contact with the peripheral surface of the toner layer on the toner carrier 33 and is preferably metallic or of another suitable conductive material. The support element 45 and the fastening axis are so positioned with respect to one another, that the printhead structure 5 is maintained in an arcuated shape along at least a part of its longitudinal extension. That arcuated shape has a curvature radius determined by the relative positions of the support element 45 and the fastening axis and dimensioned to maintain a part of the printhead structure 5 curved around a corresponding part of the peripheral surface of the toner carrier 33. The support element 45 is arranged in contact with the printhead structure 5 at a fixed support location on its longitudinal axis so as to allow a slight variation of the printhead structure 5 position in both longitudinal and transversal direction about that fixed support location, in order to accommodate a possible eccentricity or any other undesired variations of the toner carrier 33. That is, the support element 45 is arranged to make the printhead structure 5 pivotable about a fixed point to ensure that the distance between the printhead structure 5 and the peripheral surface of the toner carrier 33 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the toner carrier 33. The front and back portions of the positioning device 40 are provided with securing members 46 on which the toner delivery unit 3 is mounted in a fixed position to provide a constant distance between the rotation axis of the toner carrier 33 and a transversal axis of the printhead structure 5. Preferably, the securing members 46 are arranged at the front and back ends of the toner carrier 33 to accurately space the toner carrier 33 from the corresponding holding element 12 of the transfer belt 1 facing the actual print station.

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

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

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

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

The second printed circuit comprises the plurality of deflection electrodes 54, each of which is divided into two semicircular or crescent shaped deflection segments 541,542 spaced around a predetermined portion of the circumference of a corresponding aperture 52. The deflection segments 541,542 are arranged symmetrically about the central axis of the aperture 52 on each side of a deflection axis 543 extending through the center of the aperture 52 at a predetermined deflection angle d to the longitudinal direction. The deflection axis 543 is dimensioned in accordance with the number of deflection sequences to be performed in each print cycle in order to neutralize the effects of the belt motion during the print cycle and so obtain transversally aligned dot positions on the transfer belt. For instance, when using three deflection sequences, an appropriate deflection angle is chosen to arctan (1/3), i. e. about 18,4°.

Accordingly, the first dot is deflected slightly upstream with respect to the belt motion, the second dot is undeflected and the third dot is deflected slightly downstream with respect to the belt motion, thereby obtaining a transversal alignment of the printed dots on the transfer belt. Accordingly, each deflection electrode 54 has an upstream segment 541 and a downstream segment 542, all upstream segments 541 being connected to a first deflection voltage source Dl, and all downstream segments 542 being connected to a second deflection voltage source D2.

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

The amount of toner released from the toner carrier 33 when an aperture 52 is opened upon application of a control voltage to the control electrode 53 surrounding the aperture 52 depends on the electric field strength acting on the toner. This naturally depends on the voltages applied to the holding element 12 and the control electrode, but is also influenced by the charge on the toner particles. Any variation in this charge will alter the quantity of toner particles released from the toner carrier 33, and also alter the speed at which these toner particles travel. Similarly, the degree of deflection undergone by a stream of charged toner particles under the action of the deflection electrodes 54 likewise depends on the toner particle charge. The actual deflection force applied to the toner particles will depend on the toner charge. However, the charge of the toner particles changes with the humidity of the surrounding air. A high humidity tends to discharge the toner particles resulting in a lower particle charge.

Variations in the humidity of the air in and surrounding the apparatus cause deviations in the print results obtained particularly in terms of the density and regularity of the dots produced. This non-uniformity of print quality is mitigated by the arrangement illustrated in Fig. 6.

Fig. 6 schematically depicts a portion of the image forming apparatus of Fig. 1 including an arrangement for measuring the level of humidity within the apparatus.

Fig. 6 specifically shows part of the fuser unit 13 including the fixing holder 14 and the pressure roller 16. A portion of the transfer belt 1 which passes between the fixing holder 14 and pressure roller 16 is also depicted. A sheet of paper 2 or other suitable information carrier is also shown passing between the transfer belt 1 and the pressure roller 16. The paper 2 is transported towards the fuser unit 13 by the feed rollers 17 as described with reference to Fig. 1 above. A voltage source Vt e s t 62 is connected between the pressure roller 16 and ground potential, and applies a voltage of up to 400 V to the pressure roller 16. One of the feed rollers 17 is electrically coupled to ground potential, possibly via a known series resistance. The term ground potential is here intended to signify an electrical ground. This may be an electrical ground that is common with other electrical systems in the apparatus, or may be a separate, floating ground used only for the current flow measurements. When paper or any other information carrier bridges the gap between the feed rollers 17 and the pressure roller 16, a circuit is formed and electrical current will flow from the voltage source Vtes t 62, through the pressure roller, the paper 2 or other information carrier, the feed roller 17 to ground. The amount of current flowing naturally depends on the resistance in the circuit. The resistance of the paper 2 or other information carrier is represented by the block resistance 60 designated by Paper R. This resistance will vary depending on the humidity in the surrounding air. Water vapour will be deposited on and partially absorbed by the paper, causing an increase in the electrical conductivity of the paper.

The same is true for any other type of information carrier capable of at least partially holding water vapour obtained from the surrounding air. A measurement unit 61 is connected to the circuit for measuring the current flow. The measurement unit 61 is also connected to control circuitry 63, which processes the received measurements. The control circuitry 63 is computer or microprocessor based. It may be part of a central controller (not shown) of the image forming apparatus, or be a separate unit.

The measurement unit 61 may be coupled to the circuit as illustrated in Fig. 6 between the ground potential connection and the feed roller 17, to the feed roller 17, to the pressure roller 16 or between the pressure roller and the voltage source Vt e s t 62. The measurement unit 61 includes circuitry suitable for measuring the magnitude of the current. Preferably this circuitry includes a standard current meter product, preferably in IC form, with an amplifier arrangement connected upstream to increase the sensitivity of the current meter.

However, it will be appreciated by those skilled in the art that the measurement unit 61 may alternatively include standard voltmeter circuitry for measuring the voltage across an internal resistance, or a circuit, preferably integrated, capable of giving a signal indicative of the resistance in the circuit. In the latter case, the measurement unit 61 would be connected across the paper resistance R 60, i. e. between the feed roller 17 and the pressure roller 16.

The measurement unit 61 does not need to measure an absolute value of resistance, current or voltage in the circuit. It is necessary only for the measurement unit 61 to provide the control circuitry 22 with difference values between the measured quantities in order to establish whether the humidity within the apparatus has increased or decreased relative to a predetermined standard level. A change in these values signifies a change in the conductivity of the paper 2 or other information carrier, which in turn indicates a variation in the humidity surrounding the apparatus. As the level of humidity surrounding the apparatus also has an impact on the charge of the toner particles held on the toner carrier 33, the resistance, current or voltage levels measured by the measurement unit 61 indicate whether the toner charge has increased or decreased. In order to obtain a correct correlation between the toner charge and conductivity of the paper 2 or other information carrier, the image forming apparatus is calibrated. This may be achieved by performing conductivity measurements in a controlled environment and comparing the resulting print quality.

The control circuitry 63 is further connected to the voltage sources for the control electrodes 53, the holding element 12 which serves as back electrode and the two deflection electrode segments 541,542. Accordingly, as the control circuitry 63 detects a change in paper conductivity on the basis of signal levels supplied by the measurement unit 61, the control circuitry adjusts the applied voltage levels to change the electric field between the toner carrier 33 and the holding element 12 in a manner to substantially negate the effects of any change in toner particle charge on the quantity of toner released and the trajectory of the deflected toner particles. In other words, the electric field generated between the toner particle carrier 33 and the back electrode 12 and also the field generated around the printhead structure 5 by the control electrodes 52 and deflection electrodes 54 are adjusted such that the amount of toner and also the trajectory of each toner particle is substantially unchanged.

The conductivity of paper may vary from paper type to paper type depending on thickness, density and composition, for example. In order to enable the correct adjustment of the various voltage levels, the control circuitry, or its software, should permit a calibration procedure to be carried out to enable the image forming apparatus to adjust to the properties of a new paper type. Such a procedure advantageously includes using a calibration element consisting of a sheet of material having a known conductivity at various levels of humidity. The procedure preferably takes the following form: insert the calibration element; determine the humidity level; insert the new paper type or other information carrier; measure the conductivity, correlating the measured conductivity with the determined humidity level.

A simplified calibration method can be used by assuming that all paper times have a conductivity that varies linearly with humidity. For such a method it is necessary merely to establish the level of conductivity of the paper when the paper is dry. This measurement can usefully be performed on paper exiting the fusing unit 13. The fusing unit heats the paper to elevated temperatures, typically between 100 °C to 200 °C. It can thus be assumed that paper emerging from a fusing unit 13 is substantially dry.

It will be understood by those skilled in the art that the voltage source Vt e s t 62 must be connected so as to generate a potential difference across the paper. This may be achieved by connecting the voltage source Vtes t 62 to the feed roller 17 rather than the pressure roller 16.

Fig. 7 shows a schematic circuit diagram illustrating a further embodiment for determining a variation of toner charge due to a change in humidity. This diagram includes the two feed rollers 17 of Fig. 6 arranged to substantially oppose one another. In this embodiment, a voltage source Vt e s t 62 is connected to one of the feed rollers 17 while the other feed roller is connected to ground potential, possibly via a known series resistance. The measurement unit 61 for measuring current flow in the circuit and relaying this measurement to the control circuitry 63 is connected between the lower potential feed roller 17 and ground potential.

Alternatively, the measurement unit may be connected to either of the feed rollers 17 or between the higher potential feed roller 17 and the voltage source Vtes t 62. In this arrangement the paper resistance Paper R 60 represents the transverse resistance of the paper 2 or information carrier.

Another embodiment of an arrangement for determining a variation of toner charge due to a change in humidity is illustrated by way of a schematic circuit diagram in Fig. 8 which depicts the pressure roller 16 and fixing holder 14 of the fusing unit 13. In this configuration the voltage source Vt e s t 62 is connected to the pressure roller 16 while the measuring unit is connected to the fixing holder, which is also connected to ground potential, possibly via a known series resistance. In this embodiment, as in the circuit depicted in Fig. 7, the transverse resistance Paper R 60 through the paper 2 or information carrier is measured. It will be appreciated that the voltage source Vt e s t 62 may alternatively be connected to the fixing holder. Furthermore, since the transfer belt 1 is of electrically conductive material, this belt 1 may be used in place of the fixing holder 14 for applying a potential difference across the paper.

In the three embodiments described with reference to Figs. 6 to 8, the measurement of a quantity indicative of the humidity within the apparatus is preferably performed between printing operations. The start of the measurement process is controlled by a central controller (not shown) which controls the general printing operation. The measurement may be performed at a suitable moment once a predetermined time has elapsed. Alternatively, the measurement step may be performed after a predetermined number of pages have been printed. Measurement may also be performed on request by the operator, for example when the operator notices an anomaly in the printed pages.

Turning now to Fig. 9, an alternative arrangement for measuring the humidity within an apparatus is depicted.

In this arrangement the effect of humidity on the triboelectrification of the toner layer present on the toner carrier is determined.. As explained above, the toner particles held on the toner carrier are charged by the frictional interaction of the particles with the brushes of the supply roller 34. However, the toner particles will also be charged by the other elements in contact with the toner layer. In Fig. 9, the toner carrier 33 is schematically illustrated in section with a layer 65 of charged toner particles covering its surface.

The metering element or doctor blade 35 is shown in contact with the toner layer 65. The supply roller 34 and spacer 35 may also be in contact with the toner layer during this measurement, but are not depicted in the figure for reasons of clarity. The metering element 35 is connected to ground potential, possibly via a know series resistance. The frictional interaction between this element and the toner particles on the rotating toner carrier 33 will generate a static charge on the particles. Furthermore, the movement of charge around the toner carrier 33 will, by definition, result in a current flowing between the charged toner particles and ground.

In effect the toner layer 65 on the rotating sleeve acts as a current generator for any element connected thereto.

The level of current flowing into or out of the metering element 35 will depend on the charge of the toner particles, which in turn will depend on the surrounding humidity. Accordingly, the current that may be measured flowing between the metering element 35 and ground potential will vary according to the surrounding humidity. This measurement is performed by the measuring unit 61, which, as described above, determines the current magnitude or difference value relative to a standard or previously measured value and relays this information to the control circuitry 63. It will be understood that the same current flow may be observed between any other element that is in contact with the charged toner layer 65. Indeed, the current flow may be measured between more than one contact element and ground. Furthermore, the metering element 35 or other contact element need not be connected to ground potential, but may alternatively be tied to some other potential. Although not illustrated in Fig. 8, the toner carrier 33 is preferably connected to ground potential throughout this measurement.

As for the embodiments described with reference to Figs.

5 to 7, it is not necessary to determine the absolute current flow. It is merely necessary to determine whether the current flow has changed relative to a previously measured level or relative to a predetermined standard calibrated level.

The measurement conducted with the arrangement illustrated in Fig. 9 need not be done between printing operations, but could be performed during printing.

Alternatively, a special contact element may be provided that is connected to the measuring unit 61 and can be moved against the toner layer between printing operations, for example on command by the central controller (not shown) to measure the relative current flow, and thus the degree of humidity affecting the individual charge on the toner particles.

A further embodiment for determining variations in the level of humidity consists of measuring changes in the resistance of the substrate 50 forming the printhead structure 5. As for previous embodiments, this is preferably performed using two electrodes placed on the surface of the substrate 50, applying a potential difference across the electrodes and determining the magnitude of current flow. However, since the substrate a useful measurement requires a large electrode area compared to the area of substrate 50. A schematic circuit diagram showing an example of such an electrode is shown in Fig. 10.

In Fig. 10, those elements already described in relation to previous embodiments have been given the same reference numerals. Fig. 10 depicts a portion of the printhead structure 5. Specifically, one end of the electrically insulating substrate 50 is shown. Two thin film electrodes 501 and 502 are arranged on one surface of the substrate 50. These electrodes are preferably of copper deposited to a thickness of about 8-9 microns using conventional deposition techniques. The electrodes 501,502 each have multiple arms and are arranged such that these arms interlock, so reducing the distance between the two electrodes and at the same time maximising their active lengths. The distance between any two portions of the opposing electrodes 501,502 is preferably no greater than 30 microns. One electrode 501 is connected to a voltage source (Vtest) 62. The other electrode is connected to the measurement unit 61, which determines changes in the resistance of the substrate 50 by measuring the current flow. As in the previous embodiments, the measurement unit 61 is connected to control circuitry 63, which controls the voltage applied to the control electrodes 53, the holding element 12 which serves as back electrode and the two deflection electrode segments 541,542 to compensate for any change in the measured resistivity in order to preserve both the quantity of toner released and the trajectory of the deflected toner particles.

The electrodes 501,502 are preferably arranged to be separate from the apertures and the associated control 53 and deflection electrodes 54 to minimise interference.

Preferably, the printhead structure 5 is extended slightly at one end to create an area that is free of apertures 52 and the interlocking electrodes 501,502 deposited on this extension.