WO2021242234A1

WO2021242234A1 - Electrometer calibration

Info

Publication number: WO2021242234A1
Application number: PCT/US2020/034724
Authority: WO
Inventors: Meir GRINSTEIN; Yehuda ROTH; Ran Waidman
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2020-05-27
Filing date: 2020-05-27
Publication date: 2021-12-02

Abstract

Certain examples described herein relate to a method of calibrating an electrometer in an image forming apparatus having a photoconductor engaged with a number of components to form an image. In an example a component is disengaged from the photoconductor and a series of different calibration voltages applied to the photoconductor. Respective calibration electrometer measurements are determined and used to calculate an electrometer correction. The component is engaged with the photoconductor and the engaged component and the photoconductor controlled using electrometer measurements adjusted using the electrometer correction in order to print an image.

Description

ELECTROMETER CALIBRATION

BACKGROUND

[0001] Some digital printing presses use laser elements to write pixels onto a charged photo conductive medium to create a virtual or electrostatic image. Printing fluid such as ink may then be then transferred to the selectively discharged surface of the photo conductive medium, creating an inked image. The inked image may then be transferred to a print medium, for example via a transfer medium which may be used to evaporate any liquid vehicle from the printing fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002] Examples are further described hereinafter with reference to the accompanying drawings, in which:

[0003] Figure 1 is a schematic diagram of an image forming apparatus in a printing mode according to an example;

[0004] Figure 2 is a schematic diagram of the image forming apparatus of Figure 1 in a calibration mode according to an example;

[0005] Figure 3 is a flow chart illustrating a method of calibrating an electrometer in an imaging forming apparatus according to an example; and [0006] Figure 4 is a schematic diagram of a processor and a computer readable storage medium with instructions stored thereon according to an example.

DETAILED DESCRIPTION

[0007] Figure 1 shows an image forming apparatus 100 according to an example. Certain examples described herein may be implemented within the context of this printing system. However, it should be noted that implementations may vary from the example apparatus of Figure 1 .

[0008] The image forming apparatus 100 may be a printer, for example a digital press printer. An example of a digital press printer is a digital offset press printer, for example a Liquid Electro-Photographic (LEP) printer. In examples, a digital offset printer works by using digitally controlled lasers or LED imaging modules to create a latent image on a charged surface of a photo imaging cylinder or photoconductor. The lasers are controlled according to digital instructions from a digital image file to create an electrostatic image on the charged photoconductor. Printing fluid such as ink is then transferred to the selectively discharged surface of the photoconductor, creating an inked image. The inked image is then transferred from the photoconducfor to a transfer member such as a heated blanket, where heating evaporates a liquid vehicle from the printing fluid, and finally from the transfer member to a print medium.

[0009] In the example of Figure 1 , the image forming apparatus 100 comprises a photoconductor 110. In the present example, the photoconductor 110 may be an aSi imaging plate mounted onto the exterior cylinder or drum 112 of a drum assembly also comprising an internal or bias drum 114. However other arrangements are possible including for example a single drum or cylinder onto which a photoimaging plate (PIP) is attached. In another example, a photo imaging plate may be mounted to a belt comprising a closed loop foil. In the present example, the mounted photoconductor 110 is rotatable about its axis in an anti-clockwise direction. The bias drum 114 may also be rotatable about the same axis.

[0010] The photoconductor 110 rotates past a charge roller 180 which charges the photoconductor surface at that point to a predetermined voltage. For example, the charge roller 180 may be at 1300V, the bias drum 114 at 350V, resulting in a voltage at the photoconductor of 900V.

[0011] In an example, lasers in a discharge unit 115 discharge different areas of the photoconductor 110 to different voltages, for example 400V, in order to create an electrostatic image corresponding to different intensities or gray levels of an image to be printed. The discharge unit 115 may comprise a number of optical elements each comprising a laser, and which are controllable to generate the latent or electrostatic image on the photoconductor 110. The discharge unit 115 operates in accordance a received image data, otherwise referred to as “print data”, “input data", “input image data”, “print input data”, or the like. The lasers may be arranged in an array. An array of lasers may be embodied as individual laser elements, as multiple channels of a single laser device, as a plurality of laser devices that each have multiple channels, etc. In another example, the discharge unit may employ a Light Emitting Diode (LED) array.

[0012] An electrometer 120 is locate proximate the photoconductor 110 and measures its voltage at that point. The voltage applied to the photoconductor 110 may vary over time, for example due to the buildup of dirt or dust on the photoconductor 110, charge roller 180, electrometer 120 or other components of the image forming apparatus 100, Similarly charge leakage and changes in voltage on components may increase over time due to components breakdown and/or replacement. In addition, aging or other changing conditions of other components such as laser element power or cleanliness may change the effectiveness of the discharging process which generates the electrostatic image. The voltage measurements taken by the electrometer 120 may therefore be used to compensate for some of these changes, for example by adjusting the voltage applied to the charge roller.

[0013] The electrometer may be any suitable type for measuring electric charge or electric potential or voltage. These may include vibrating reed, valve and solid- state types.

[0014] The discharge unit 115 dissipates the static charges on selected portions of the surface of the photoconductor 110 to leave an electrostatic charge pattern that represents an image to be printed. Printing fluid such as ink is then transferred onto the photoconductor 110 by at least one ink developer unit 170. The ink developer units may comprise binary ink developer (BID) units, wherein each BID unit supplies ink of a different base color. The printing fluid may contain electrically charged pigment particles, for example at 600V, which are attracted to the image areas of the photoconductor 110. The printing fluid is repelled from the non-image areas. An inked image of the print frame is thereby transferred onto the photoconductor, i.e. a representation of the image formed from printing fluid.

[0015] The ink developer unit 170 has a developer roller 175 containing charged ink at a lower bias voltage than the initial charge of the photoconductor 110. Therefore, ink is repelled from areas of the photoconductor which have not been discharged by the lasers but are attracted to the photoconductor in areas where this has been fully or partially discharged in proportion to the difference in voltage between the ink on the developer roller 175 and the photoconductor 110.

[0016] The image forming apparatus 100 also comprises a transfer member 140 such as a blanket in the form of a belt which is moved by rollers 130, 135 to engage with the photoconductor 110 at a corresponding speed to transfer the inked image from the photoconductor to the transfer member 140. In an example, the transfer member 140 may be at 0V. In other examples the transfer member 140 may be cylindrical with a blanket wrapped externally. The transfer member 140 is configured to receive the inked image from the photo imaging plate 110 and to facilitate evaporation of liquid within the transferred printing fluid. This may be achieved by heating the transfer member for example.

[0617] The image forming apparatus also comprises a media transport arrangement (not shown) which is configured to transfer the dried inked image from the transfer member 140 onto a print medium such as paper.

[6618] The image forming apparatus 100 may also comprise a discharging unit 185 which discharges the photoconductor 100 following transfer of the inked image to the transfer member 140 and prior to the photoconductor being fully charged again by the charge roller 180.

[6619] The image forming apparatus 100 comprises a controller 140 which controls the discharge unit 115, and the various voltages applied to components of the image forming apparatus including the charge roller 180, the ink developer 170, the transfer member 140 and the discharge unit 185. In different examples fewer or more components may be employed with their voltages also controlled by the controller.

[6626] The controller 140 comprises a memory 150. The memory 150 may comprise volatile and/or non-volatile memory. The memory 150 may comprise dynamic or static random-access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs) and/or flash memories. [0021] The memory 150 may store an electrometer correction which may be used to correct electrometer measurements taken during image forming. Electrometer measurements may become inaccurate over time due to build up of dirt on the electrometer, part degradation and other factors. Calibrating the electrometer measurements periodically to obtain an electrometer correction allows the electrometer measurements to be corrected and used to control operation of the image forming apparatus during production of images, resulting in improved image quality.

[0022] The controller 140 also comprises a processor 160. Processor 160 can include a microprocessor, microcontroller, processor module or subsystem, programmable integrated circuit, programmable gate array, or another control or computing device.

[0023] The processor 160 is configured to control operation of the image forming apparatus during image production. The processor is also configured to control operation of the image forming apparatus in a calibration mode to determine or update the electrometer correction for storage or updating in the memory 150. [0024] The processor 160 may be configured to adjust the voltages of various components depending on the corrected electrometer measurements; for example the voltages of one or more of the following components: charger roller; ink developer 170; transfer member 140; discharge unit 185. The components may be directly connectable to a suitable power supply providing respective voltages controlled by the processor.

[0025] The processor may also adjust the voltage applied to the bias drum 114 depending on the corrected electrometer measurements. The bias drum may be directly connectable to a suitable power supply within the image forming apparatus and controllable by the processor to apply different voltages to the bias drum 114. [0026] Figure 2 shows the image forming apparatus of Figure 1 in a disengaged state suitable for calibration of the electrometer. In this example, items depicted in Figure 2 are the same as items shown in Figure 1 but having their respective locations changed. Therefore, the same reference numerals are used. [0027] As can be seen, the various components 170, 180, 185, 140 around the photoconductor 110 have been disengaged or moved away from the photoconductor. This effectively electrically isolates the photoconductor 110 from the other components of the image forming apparatus so that they do not affect calibration voltages applied at photoconductor 110. In this disengaged state, the other components may have no voltages applied unlike in the image production mode described above. In order to prevent charge leakage from the photoconductor to the other components, or any other voltage influence, the other components are moved so that they are no longer in physical contact with the photoconductor 110. The gap or distance D between each component and the photoconductor may vary and may be for example 2 - 10mm. The distance used other modes of operation of the image forming apparatus may be used, for example the positioning of the components when replacing a drum assembly having the photoconductor may be re-used to disengage the components from the photoconductor.

[0028] In an example, one or more of the components have any external voltage source or grounding disconnected so that their voltages float. In other examples, one or more of the components may be connected to an external voltage potential such as ground (0V), calibration voltages applied to the photoconductor, or an image production voltage normally used for that component.

[0029] Whilst other components such as the ink developer 170 and charge roller 180 are moved away from the photoconductor 110, the electrometer 120 retains its proximate position.

[0030] A series of calibration voltages Vc are applied to the photoconductor 110 and respective electrometer measurements taken. In this example, the calibration voltages may be connected directly to the bias drum 114 and without the influence of the other components are reflected onto the photoconductor. In other examples, the calibration voltages may be applied to an axial member of the drum assembly 112, 114 or directly to an external drum 112 in either a multipart drum assembly or a single external cylinder. By using the bias drum 114 in a multi-cylinder drum assembly, physical connection modifications to the drum assembly may be unnecessary. [0031] Figure 3 shows a method 300 of calibrating an electrometer in an image forming device. In some examples, the method 300 is performed by a controller such as controller 140 on an image forming apparatus such as image forming apparatus 100. The controller may perform the method based on instructions retrieved from a computer-readable storage medium. The photo imaging plate may comprise photo imaging plate 110 and the developer unit may comprise developer unit 170.

[0032] At item 310, a calibration mode is entered, for example in response to a user command, detection of a substitution of a new photoconductor 110 to replace a photoconductor that has reached end of life, or periodic maintenance programs implemented by the controller, for example several times a day or following a certain number of image forming processes. At item 320, components of the image forming apparatus are disengaged from the photoconductor. In an example, at least one component which would normally be in close proximity to the photoconductor during image production is moved away from the photoconductor in the calibration mode. The components may include a charging unit such as a charge roller or corona wire, a print fluid transfer unit such as a binary ink developer, an image transfer unit such as a heated transfer member which may be formed on a cylinder or as a belt with corresponding rollers. In an example, one or more components physically contact the photoconductor when in image production mode but are distanced from the photoconductor so that there is no physical contact and there is a gap between the photoconductor and each component, for example from 2mm or more in calibration mode. This electrically isolates the photoconductor from the other components which prevents them from influencing the voltage measured at the photoconductor by the electrometer. The electrometer is maintained at the same distance from the photoconductor in calibration mode as in image production mode.

[0033] In another example, the photoconductor and electrometer may be moved laterally along the axis of a drum assembly containing the photoconductor so that they are separated from the other components without having to move the other components.

[0034] At item 330, a plurality of calibration voltages are applied to the photoconductor. For example, a series of voltages between 100V and 500V with an interval of 50V may be applied in turn. In an example this is achieved by applying the calibration voltages directly to an internal bias drum of a drum assembly having the photoconductor on an exterior surface. For example, an amorphous silicon photoconductor within such an assembly with gain the voltage applied to the bias drum when disengaged from the other components of the image forming device. The bias drum is normally directly electrically connectable to a bias drum power supply and so the application of different calibration voltages to the bias drum is readily achieved by the controlling the power supply.

[0035] At item 340, electrometer measurements are taken corresponding to each calibration voltage. At item 350, if further calibration voltages are to be measured, the method returns to item 330, otherwise the method proceeds to item 360 once all calibration voltages have been applied, and measurements taken. [0036] At item 360, an electrometer correction is calculated using the electrometer measurements of the calibration voltages. The electrometer may have a measuring error containing a constant and relative errors and a correction can be calculated and used during image production to compensate for these errors. [0037] In an example, a linear regression can be used with the electrometer measurements to determine a linear fit or equation, for example having slope and intercept.

[0038] An example using simple linear regression is shown below:

Electrometer Measurement = a*(Applied Voltage) + b In the above example, a = 1000 and b = -7V [0039] The inverse of the resulting intercept value b is used as the electrometer correction and can be applied to all subsequent electrometer readings to correct for the errors. In the example above, +7V is applied to each electrometer measurement to obtain a calibrated or corrected electrometer value.

[0040] Once a linear fit has been calculated, at item 370 the standard deviation error between the electrometer measurements and each applied calibration voltage is calculated. In the above example the standard deviation is 0. !f the standard deviation is above an error threshold (e.g. 2), the calculated electrometer correction is determined to be invalid and the method either retains the previously calculated electrometer correction or returns to recalculate a new electrometer correction.

The standard deviation is used to check whether the electrometer correction is valid. Standard deviation gives more weight to a big error in a single sample and is therefore a good indicator of a defective measurement. This may occur when certain conditions arise in the calibration mode such as a component touching the photoconductor due to a faulty movement away for example because of jamming, or a bright light contaminating the charge on the photoconductor A standard deviation above a threshold is then used to invalidate the electrometer correction and instead the previously determined electrometer correction is retained and is next updated when the next calibration is performed without an invalidation error.

[0041] In other examples, different algorithms for calculating an electrometer correction and/or validating this could be used such as different regression techniques or identifying a single sample difference between an electrometer measurement and applied voltage above a threshold.

[0042] At item 380, the components previously disengaged from the photoconductor are now engaged. For example, the charge roller and other components are moved back to physically contact the photoconductor. The image forming apparatus may then switch back into image production mode.

[0043] At item 390, the image forming apparatus prints an image by controlling the photoconductor and other components using electrometer measurements adjusted by the electrometer correction. For example, were the electrometer to indicate that the photoconductor is at 955V this may be corrected to 980V. This may in turn be used to increase the voltage on the bias drum to increase the photoconductor voltage unit it is measured by the electrometer at 975V - that is 1000V corrected. The improved accuracy of the corrected electrometer measurements allows for improved control of the image forming apparatus, including more accurate control of voltages applied to the photoconductor and other components such as charger roller and printing fluid transfer unit. This in turn improves the quality of the image printed.

[0044] Figure 4 shows a computer-readable storage medium 400, which may be arranged to implement certain examples described herein. The computer- readable storage medium 400 comprises a set of computer-readable instructions 410 stored thereon. The computer-readable instructions 410 may be executed by a processor 420 connectably coupled to the computer-readable storage medium 400. The processor 420 may be a processor of an image forming apparatus similar to image forming apparatus 100. In some examples, the processor 420 is a processor of a controller such as controller 140.

[0045] Instruction 440 instructs the processor 420 to disengage components from a photoconductor in an image forming apparatus. The components may include a charge roller, a print fluid unit and/or a transfer member. Instruction 450 instructs the processor 420 to apply a series of voltages to the photoconductor and to determine respective electrometer measurements from an electrometer proximate the photoconductor. The different voltages are calibration voltages which are measured by the electrometer. The measurements may be stored in memory for subsequent use. Instruction 460 instructs the processor 420 to calculate an electrometer correction using the electrometer measurements corresponding to the calibration voltages. The processor may calculate the correction by determining errors between each measurement and the applied calibration voltage, applying a linear fit to the measurements and determining the standard deviation of the errors from the linear fit; although alternative algorithms could be used. [0046] Instruction 470 instructs the processor 420 to re-engage the components with the photoconductor and to form images by controlling the photoconductor and components using measurements from the electrometer corrected by the calculated electrometer correction.

[0647] Processor 420 can include a microprocessor, microcontroller, processor module or subsystem, programmable integrated circuit, programmable gate array, or another control or computing device. The computer-readable storage medium 600 can be implemented as one or multiple computer-readable storage media. The computer-readable storage medium 400 includes different forms of memory including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs) and flash memories; magnetic disks such as fixed, floppy and removable disks; other magnetic media including tape; optical media such as compact disks (CDs) or digital video disks (DVDs); or other types of storage devices. The computer-readable instructions 410 can be stored on one computer-readable storage medium, or alternatively, can be stored on multiple computer-readable storage media. The computer-readable storage medium 400 or media can be located either in the image forming apparatus 100 or located at a remote site from which computer-readable instructions can be downloaded over a network for execution by the processor 420.

[6648] Certain examples described herein enable improved image quality by automatically calibrating electrometer measurements used to control a photoconductor and other components in an image forming apparatus. For example, an incorrect and uncorrected electrometer measurement of the charge or voltage on the photoconductor may result in the actual voltage on the photoconductor being controlled to be too high or low. When the voltage is too low, this results in a higher level of printing fluid applied to background areas, for example being visible as color in areas that should appear white. On the other hand, a photoconductor voltage which is too high results in reduced color or lower contrast and again is visible and undesirable, [0049] The preceding description has been presented to illustrate and describe examples of the principles described. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching.

Claims

CLAIMS What is claimed is:

1. A method of calibrating an electrometer in an image forming apparatus, the image forming apparatus having a photoconductor and a number of components which engage with the photoconductor to form an image, the method comprising: disengaging a said component from the phofoconductor; applying a series of different calibration voltages to the photoconductor and determining respective calibration electrometer measurements; calculating an electrometer correction using the calibration electrometer measurements; engaging the component with the photoconductor and controlling the engaged components and the photoconductor using electrometer measurements adjusted using the electrometer correction in order to print an image.

2. The method of claim 1, wherein disengaging the component from the photoconductor comprises electrically isolating the component from the photoconductor.

3. The method of claim 1, wherein disengaging the component from the photoconductor comprises moving the component away from the photoconductor.

4. The method of claim 3 wherein the voltage of the component is arranged to float.

5. The method of claim 1 , wherein the component comprises one or more of the following: a charging unit; a printing fluid transfer unit; an image transfer unit.

6. The method of claim 1, wherein the image forming apparatus comprises a drum assembly having an external and internal drum, where the photoconductor is formed on the external drum and the calibration voltages are connected to the internal drum.

7. The method of claim 1, wherein the electrometer correction is determined using a linear regression of the electrometer measurements and validated using a standard deviation of errors between the calibration voltages and electrometer measurements.

8. An image forming apparatus comprising: a photoconductor and a number of components which engage with the photoconductor to form an image; an electrometer to measure the voltage of the photoconductor; a voltage source to apply a plurality of different voltages to the photoconductor; a memory to store electrometer measurements and an electrometer correction; a processor to control the components to disengage from the photoconductor, to apply the plurality of voltages to the photoconductor using the voltage source and to store the respective electrometer measurements into the memory; the processor to calculate an electrometer correction using the electrometer measurements and to control the photoconductor and components when engaged using electrometer measurements corrected with the electrometer correction.

9. The apparatus of claim 8, the processor to electrically isolate the components from the photoconductor in order to disengage the components from the photoconductor.

10. The apparatus of claim 8, the processor to move the components away from the photoconductor to disengage the components from the photoconductor comprises.

11. The apparatus of claim 10, wherein the component is electrically isolated to allow a voltage thereon to float,

12. The apparatus of claim 8, wherein the component comprises one or more of the following: a charging unit; a printing fluid transfer unit; an image transfer unit.

13. The apparatus of claim 8, wherein the image forming apparatus comprises a drum assembly having an external and internal drum, where the photoconductor is formed on the external drum and the calibration voltages are connectable to the internal drum.

14. The apparatus of claim 13, wherein the photoconductor comprises amorphous silicon.

15. A non-transitory computer-readable storage medium comprising a set of computer-readable instructions that, when executed by a processor, cause the processor to: disengage components and a photoconductor in an image forming apparatus; apply a series of calibration voltages to the photoconductor and determine respective electrometer measurements from an electrometer proximate the photoconductor; calculate an electrometer correction using the electrometer measurements; engage the components with the photoconductor to form an image and control the components and photoconductor using electrometer measurements adjusted using the electrometer correction.