Patent Application
20150224763
The present invention relates to electrostatic inkjet print technologies and, more particularly, to printheads and printers of the type such as described in WO 93/11866 and related patent specifications.
Electrostatic printers of this type eject charged solid particles dispersed in a chemically inert, insulating carrier fluid by using an applied electric field to first concentrate and then eject the solid particles. Concentration occurs because the applied electric field causes electrophoresis and the charged particles move in the electric field towards the substrate until they encounter the surface of the ink. Ejection occurs when the applied electric field creates an electrophoretic force that is large enough to overcome the surface tension. The electric field is generated by creating a potential difference between the ejection location and the substrate; this is achieved by applying voltages to electrodes at and/or surrounding the ejection location.
The location from which ejection occurs is determined by the printhead geometry and the position and shape of the electrodes that create the electric field. Typically, a printhead consists of one or more protrusions from the body of the printhead and these protrusions (also known as ejection upstands) have electrodes on their surface. The polarity of the bias applied to the electrodes is the same as the polarity of the charged particle so that the direction of the electrophoretic force is towards the substrate. Further, the overall geometry of the printhead structure and the position of the electrodes are designed such that concentration and then ejection occurs at a highly localised region around the tip of the protrusions.
To operate reliably, the ink must flow past the ejection location continuously in order to replenish the particles that have been ejected. To enable this flow the ink must be of a low viscosity, typically a few centipoise. The material that is ejected is more viscous because of the concentration of particles; as a result, the technology can be used to print onto non-absorbing substrates because the material will not spread significantly upon impact.
Various printhead designs have been described in the prior art, such as those in WO 93/11866, WO 97/27058, WO 97/27056, WO 98/32609, WO 01/30576 and WO 03/101741, all of which relate to the so-called Tonejet® method described in WO 93/11866.
The solid arrow 11 shows the ejection direction and again points in the direction of the substrate. In
In operation, it is usual to hold the substrate at ground (0 V), and apply a voltage, VIE, between the intermediate electrode 10 and the substrate. A further potential difference of VB is applied between the intermediate electrode 10 and the electrodes 7 on the ejection upstand 2 and the cheeks 3, such that the potential of these electrodes is VIE+VB. The magnitude of VB is chosen such that an electric field is generated at the ejection location 6 that concentrates the particles, but does not eject the particles. Ejection spontaneously occurs at applied biases of VB above a certain threshold voltage, VS, corresponding to the electric field strength at which the electrophoretic force on the particles exactly balances the surface tension of the ink. It is therefore always the case that VB is selected to be less than VS. Upon application of VB, the ink meniscus moves forwards to cover more of the ejection upstand 2. To eject the concentrated particles, a further voltage pulse of amplitude VP is applied to the ejection upstand 2, such that the potential difference between the ejection upstand 2 and the intermediate electrode 10 is VB+VP. Ejection will continue for the duration of the voltage pulse. Typical values for these biases are VIE=500 volts, VB=1000 volts and VP=300 volts.
The voltages actually applied in use may be derived from the bit values of the individual pixels of a bit-mapped image to be printed. The bit-mapped image is created or processed using conventional design graphics software such as Adobe Photoshop and saved to memory from where the data can be output by a number of methods (parallel port, USB port, purpose-made data transfer hardware) to the printhead drive electronics, where the voltage pulses which are applied to the ejection electrodes of the printhead are generated.
One of the advantages of electrostatic printers of this type is that greyscale printing can be achieved by modulating either the duration or the amplitude of the voltage pulse. The voltage pulses may be generated such that the amplitude of individual pulses are derived from the bitmap data, or such that the pulse duration is derived from the bitmap data, or using a combination of both techniques.
The ejection characteristics of an electrostatic inkjet printhead are dependent on the geometry of the ejectors and on the positions of the electrodes at the ejector. Variation in these factors can lead to a variation in optical density or colour across a print.
The problem to be solved is to produce improved and more uniform ejection performance from an electrostatic inkjet print system whose raw performance produces a stable pattern of variation across the printhead. Prior knowledge of the characteristics of this variation enables the response of the print system to be calibrated to improve the uniformity of performance from the printhead significantly.
Electrostatic inkjet printheads can be controlled using the duration and/or amplitude of electrical pulses to the printhead ejectors to modulate the ejection from the ejectors. Unlike piezo or thermal inkjet printheads, in which the size of droplet ejected is primarily a function of the physical dimensions of the pressure chamber and nozzle, the volume of ink ejected from an electrostatic printhead ejector can be controlled by the amplitude and/or the duration of the electric field acting on the ink in the ejector, which in turn is determined by the voltage waveform applied to the electrodes of the printhead. This enables compensation for stable variations in the ejection performance across an array of ejectors to be achieved.
The ways in which the pulse duration and amplitude can be controlled are shown schematically in
The volume of ink ejected in response to an applied voltage pulse is governed by the position of the ink meniscus, the electric field acting upon the ink and the duration of the applied pulse as described above. Ideally, every ejector in the printhead will perform equally, that is, will eject the same volume of ink at the same time for the same applied pulse. However, variation in ejector geometry, electrode positions or meniscus position across the printhead will cause variations in performance of ejectors leading to variation in the optical density of print across the width of the printhead. Such variation generally manifests as a gradual bow in print density from one side of the head to the other, is stable and characteristic of an individual printhead. As such, it can be compensated by choosing a set of pulse voltages and/or durations individually for each ejector or small groups of contiguous ejectors that equalises the print performance across the printhead. The calibration process both equalises the performance across the printhead and calibrates the tone reproduction curve (optical density versus image grey level) of the printhead in a single process.
Additionally, the response of the ink to an applied voltage pulse at an ejector is dependent upon the bias electric field (i.e. the electric field created by the application of the bias voltage to the ejector between ejections). In practice, the bias voltage VB is set just below the voltage VS at which spontaneous ejection occurs. It is important that VB is held close to VS (in practice about 20V below it) for the ink to respond rapidly to an ejection pulse. However, variations described above in ejector geometry and electrode positions can give rise to variation in VS across the printhead and consequently variation in the response of an ejector dependent on its position across the array.
US2006/018561 discloses a printer which adjusts for any variation in performance across the printhead by altering the pattern of dots which are needed to make up an image, thereby creating a new image, and then carrying out a standard transformation of that new image data into standard drive pulse values and hence into uncalibrated dot sizes. The calibration is achieved by creating a series of test prints for each channel in the printhead (see
US2011/0234677 discloses a method of compensating for banding that occurs when a scanning printhead takes several interleaved passes to build up an image. Dark and light lines can result from errors in jet size and/or angle, and can result from the juxtaposition of certain nozzles on different passes, which don't have a one-to-one correspondence with individual nozzles. Hence, US2011/0234677 teaches making adjustments to the image (see
WO2012/040424 discloses colour profiling inkjet printing onto clear film. It involves printing a test pattern comprising greyscale patches, measuring the density of the greyscale patches, and adjusting output pixel values based on deviations between the expected and actual densities, all of which is well known colour profiling to achieve desired tone reproduction curves. WO2012/040424 teaches that the modification of pixel values is applied to the greyscale image before the image is then subjected to half-toning (screening to a small number of fixed dot sizes). This method does not carry out any dot size control (i.e. there is no control to the ejected volume to achieve a desired dot size) and as such, does not perform a correction of the printed dot sizes, but rather creates new image data which is then transformed into drive signals in a standard manner.
According to the invention there is provided a method of calibrating a printhead for printing two-dimensional bit-mapped images having a number of pixels per row, the printhead having a row of printing channels, wherein the volume of marking fluid ejected from each printing channel in use is independently controlled by respective control pulses determined by respective image pixel bit values, the calibration method comprising
In the types of printhead referred to in the prior art above, the control pulses are normally voltage pulses, but other possibilities exist for other printing technologies, for example, current pulses, pressure pulses, heat pulses, light pulses or the like.
The method also includes a method of printing a two-dimensional bit-mapped image having a number of pixels per row, the printhead having a row of ejection channels, each ejection channel having associated ejection electrodes to which a voltage is applied in use sufficient to cause particulate concentrations to be formed from within a body of printing fluid, and wherein, during printing, in order to cause volumes of charged particulate concentrations of one of a number of predetermined volume sizes to be ejected from selected ejection channels of the printhead to form printed pixels, voltage pulse values of respective predetermined amplitude and duration, as determined by respective image pixel bit values, are applied to the electrodes of the selected ejection channels, utilising the calibration method defined above, and
Therefore the present invention utilises control of the ejected volume for each printed pixel so that the correct printed image can be created whilst compensating for any inherent variation in the performance of the channels across the printhead. The ejected volume is, due to the application of the voltage pulse VP for a given duration at a given amplitude, ejected as a single body of fluid and particulates which may, or may not depending upon the exact volume ejected and the printing conditions at the time, break into a series of droplets prior to landing on the substrate being printed. The ejected volume is therefore variously referred to as “printed droplets”, “printed droplet”, “droplet” or “volume”.
A single test print of the image may be provided and the pulse values varied from maximum to minimum in the print direction along the test print prior to measuring the optical density.
Alternatively, the pulse values may be varied in the print direction along the test print to print a number of bands of print at different pulse values each corresponding to one of a desired set of dot sizes that are utilised by the printer in use to render images in conjunction with a suitable screening method.
In a further method, a plurality of blocks of print are provided in the test print, each block being printed by one of the ejection channels.
It is also desirable to use the in-built pulse control to supplement the effective value of the common, head-wide VB by superimposing on VB voltage pulses that are too short in duration and/or low in amplitude to cause printing, but which supplement VB by an amount which is predetermined according to the measurement of the raw performance of the printhead so that the difference between VS and the effective bias voltage is everywhere the same across the printhead. This method may further include the step of calibrating a non-ejecting, level of pulse values by extrapolating from the lowest printing level pulse values. This can be achieved by creating an effective bias level voltage for each channel, by selectively adding to the bias voltage of certain channels non-printing voltage pulses whose amplitude or duration is not sufficient to cause ejection.
Preferably, the step of recording in memory the pulse value for each of said positions across the printhead for each of said image grey levels, comprises storing said values in a memory forming part of the printhead.
The invention also includes method of printing a two-dimensional bit-mapped image having a number of pixels per row, the printhead having a row of ejection channels, each ejection channel having associated ejection electrodes to which a voltage is applied in use sufficient to cause particulate concentrations to be formed from within a body of printing fluid, and wherein, during printing, in order to cause volumes of charged particulate concentrations of one of a number of predetermined volume sizes to be ejected from selected ejection channels of the printhead to form printed pixels, voltage pulse values of respective predetermined amplitude and duration, as determined by respective image pixel bit values, are applied to the electrodes of the selected ejection channels, wherein the printhead is calibrated in accordance with any of the methods defined above.
The individual voltage pulse values determined by the respective image pixel bit values for printing the image may be modified in accordance with corresponding values stored in a look-up-table.
A calibrated scanner or scanning spectrophotometer may be used to capture the test print.
The Tonejet® method as referred to above has the feature that the ejection volume is continuously, addressably, variable through the mechanism of voltage pulse length control. In the Tonejet® method, for a given pixel level, a continuous-tone pulse value can be assigned to produce the desired dot size. Such calibrations are not possible for a conventional drop-on-demand (DOD) inkjet printhead whose drop volumes are quantised by chamber volume, nozzle size, etc.
Printheads of this type may have a single or multiple rows of ejection channels, the latter may form a two-dimensional array.
Examples of methods and apparatus according to the present invention will now be described with reference to the accompanying drawings, in which:
Before describing an example of the method according to the invention, it may be useful to describe the two methods generally usable to control the volume of fluid printed (or ejected) using the Tonejet® method.
Of these alternative techniques, in practice it is simpler to modulate the duration of the pulse, but either technique may be appropriate in given circumstances and both may be used together.
In practice of course, a printed colour image is produced by using multiple single-colour printheads, each of which is used to print one of several colour components (for example CMYK). The following description applies to each printhead, and the calibration process is repeated for each printhead. For simplicity the process is described once only.
The calibration process according to a first example of the invention, and which is illustrated in
The test print is then, preferably automatically, passed to a scanner and the image scanned (step 102).
The desired tone reproduction curve 52 (optical density versus image greyscale level) for the print process (an example of which is shown in
Prints are typically rendered from a small number of discrete dot sizes, e.g. four or eight, in a screened pattern, rather than in continuous tone. This has the advantage of reducing the bit depth of data required to define each pixel thereby allowing faster and more efficient data handling and transfer from the controlling computer to the printheads. An area of image grey level that coincides with one of these discrete dot sizes is typically rendered using that single dot size to print every pixel in the area; by contrast, image grey levels that lie between two discrete dot sizes are rendered with randomised distributions of those two dot sizes in the correct proportion to achieve the desired print density. Image grey levels lighter than the minimum dot size are rendered using randomised distributions of the minimum dot size. The screening process is applied to the image data as part of the raster image processing that is performed automatically in the controlling computer. Such screening methods are well known in the field of digital printing and are not described further here.
The curve 52 of
In step 103 seven contours 53 of constant print density corresponding to the chosen dot sizes from which to render the image are calculated, within a computer attached to the scanner, from the image scanned by the scanner and representations of these are shown in
The calibration process according to a second example of the invention is described with reference to the flow diagram of
The optical density of the test image of
If the print density uniformity is within specification no further action is taken and the calibration is complete (step 205). If it is not, then interpolation between the density measurements across the printhead is performed (step 206) to approximate individual channel densities from the area density measurements (which are typically at a lower spatial resolution than the channels of the printhead). Linear interpolation between the density measurements is generally sufficient to approximate the shape of the variation across the printhead and give a sufficient estimate of the performance of the individual channels.
To calculate the pulse values that give the desired densities, a further interpolation step (step 207) is employed in which the density error is calculated as the measured (or interpolated) channel density minus the target density for each printing level. A pulse value correction is calculated as (density error)/kL, where kL is a constant for each level chosen to be about 20% higher than the typical gradient of the curve of density versus pulse value at each level. This gives a correction value that slightly under-compensates the density error so that after two or three iterations (see below) the values are converged on the specified levels in a stable progression. kL typically ranges from 0.005ODU per increment of pulse value at the lowest level of greyscale used in the printing process to 0.011ODU per increment of pulse value at the maximum level. The computer then calculates the new pulse value as the prior pulse value minus the pulse value correction for each greyscale level for each channel.
These calculated pulse values are logged (step 208) and saved to memory (step 209), preferably within the printhead. A further test (calibration) print is printed using the pulse values so determined, and the process is repeated until the uniformity of the printed bands is within specification. Typically two iterations of this process will deliver the desired uniformity.
A calibration process according to a third example of the process is described with reference to the flow diagram of
As
In order to calibrate the printhead according to this example, a set of test prints of the type shown in
The optical density of the patches 61 of the test images of
The density measurements from these prints are used according to the flow diagram of
Any of examples 1 to 3 may include an additional step of creating a level 0 (effective bias) by extrapolating down from level 1. As explained earlier, the magnitude of the bias voltage VB is chosen such that an electric field is generated at the ejection location 6 that concentrates the particles, but does not eject the particles. Ejection spontaneously occurs at applied biases of VB above a certain threshold voltage, VS, corresponding to the electric field strength at which the electrophoretic force on the particles exactly balances the surface tension of the ink. It is therefore always the case that VB is selected to be less than VS. For of the response of ejectors to print pulses to be equal it is desirable for the difference VB−VS to be the same across the printhead; however it is common for VS to exhibit variation across the printhead for the same reasons and in the same way that the ejection strength can show variation. The variation in VB−VS can be reduced, or eliminated, by creating an effective bias level, level 0, which is created by selectively adding to the bias voltage of certain channels non-printing voltage pulses whose amplitude or duration is not sufficient to cause ejection but which raises the time-averaged value of the voltage at the ejector a small amount above VB.
Such a calibration process performs a calibration of the non-ejecting effective bias level (level 0) by extrapolating down from the lowest printing level (level 1). In the simplest case this is done by subtracting a constant number from the pulse values of level 1, that number being the minimum of the calibrated pulse values for level 1. This is illustrated by the example look-up table of
In all examples above it is noted that the calibrated pulse values are stored in memory. This memory may be contained in a so-called “smart chip” built into the printhead to hold the calibration data thus obtained, and which uploads the data in the form of a LUT to the printhead drive electronics on power up. This has the advantage of ensuring substantially identical printing in such smart chip equipped printheads in response to incoming print data.
In operation of a printhead calibrated in accordance with any of the examples described above, as shown in
In the case where multiple printheads are employed to print each colour separation, for example where printheads are joined end to end to span a substrate that is wider than the individual head width, of interleaved to provide a greater number of dots-per-inch across the substrate than the spacing of the printhead ejectors, the bitmaps 402 are separated 403 into strips to create data sets 414A, 414B, etc., corresponding to the individual printheads.
In the case where multiple passes of the printhead(s) over the substrate are used to build up the print, the bitmaps 412 are separated 413 into strips to create data sets 414A, 414B corresponding to individual passes of the printhead(s).
The bitmap data 414A (only that for the first pass ‘Head A is shown for convenience) is then transferred in step 418, according to the relative position of the print substrate and the printheads (as determined by the shaft encoder 416), to the pulse generation electronics 420. Here the LUT 54 is held in memory, having been downloaded previously to the pulse generation electronics from computer memory or smart-chip, typically on power-up of the printhead, and is used to translate the incoming bitmap data to values of pulse length and/or amplitude in accordance with the calibration values stored in the LUT for that printhead, which are utilised to determine the length and/or amplitude of the drive pulses that are generated 423 by the pulse generation electronics and applied to the individual printhead ejection channels. The data is transferred in time-dependency on the substrate position and offset 417 of the printhead from the location of the shaft encoder.
A variation to the implementation shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 12184681.0 | Sep 2012 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP13/69206 | 9/17/2013 | WO | 00 |
