Patent Application
20100214338
High precision printing relies on precise placement of ink on a substrate surface to create images. During the printing process, heat is generated by a number of components and processes within the printer. This heat can cause thermal gradients which distort the structure of the printer and result in misalignments and inaccuracies in depositing the ink on the substrate.
The accompanying drawings illustrate various embodiments of the principles described herein and are a part of the specification. The illustrated embodiments are merely examples and do not limit the scope of the claims.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.
High precision printing relies on precise placement of ink on a substrate surface to create images. During the printing process, heat is generated by a number of components and processes within the printer. This heat can cause thermal gradients which distort the structure of the printer and result in misalignments and inaccuracies in depositing the ink on the substrate.
For example, misplacements of ejected ink droplets by as little as 10 microns can cause print quality defects which are visible and unacceptable to users of the printing system. Common printing temperatures can distort the printing structure by as much as about 100 microns. Further, the printing misalignments are generally a non-linear function of temperature, which makes printing misalignments difficult to model and predict.
In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present systems and methods. It will be apparent, however, to one skilled in the art that the present apparatus, systems and methods may be practiced without these specific details. Reference in the specification to “an embodiment,” “an example” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least that one embodiment, but not necessarily in other embodiments. The various instances of the phrase “in one embodiment” or similar phrases in various places in the specification are not necessarily all referring to the same embodiment.
According to one illustrative embodiment, an automatic, real-time alignment of printhead images compensates for thermally induced structural distortion in a printing system. A small number of strategically placed temperature sensors measure the temperatures of structural components of the printing system. Mathematical “differencing” of these temperatures can be used to accurately predict the misalignment of printheads due to thermal gradients in the structural components. The mathematical differencing approach can predict printhead misalignments in position and angle. In systems where multiple printheads are used, such as page-wide-array inkjet printers, this approach provides misalignment prediction for each of the printheads. In general, the mathematical differences used are not computationally intensive, yet provide very accurate results. The printhead misalignments can then be corrected by altering the firing timing or repositioning the printhead.
As used in the specification and appended claims, the term “differencing,” “difference formula” or “mathematical differencing” refers to mathematical formulas which accept as input two or more temperature readings. The mathematical formulas then output a value which relates to one or more thermal gradients which are present in the printing system. In some cases, the differencing formula may be selected based on its ability to accurately predict printhead or other misalignments over a range of temperatures and operating conditions. The terms “differencing,” “difference formula” or “mathematical differencing” are not limited to the subtraction of one value from another value, but may include a variety of other mathematical operations such as multiplication, scaling factors, addition, division, exponents, trigonometric function, logarithmic operations, matrix operations, and others.
As used in the specification and appended claims, the term “strategically placed” or “strategically placing” refers to selecting locations for temperature sensors on structural components such that the temperatures can be correlated to misalignments of the printheads. “Strategically placing” temperature sensors can be done by accounting for a number of factors including, but not limited to, identifying locations within the printer structure that support or influence the printheads, identifying locations that will accurately measure temperature gradients that cause deformation, selecting locations that have the highest or lowest local temperatures, and other factors.
According to one illustrative embodiment, four thermistors are strategically placed throughout the printing system. The mathematical differencing of temperature measurements made by the four thermistors produces a strong correlation with printhead misalignments. The firing timing of the printheads can be adjusted in real time to compensate for the predicted misalignments. Further, these compensations may be made when printing is not occurring to ensure that even if the printing system has been idle for long periods of time, the first printed page has the desired quality.
According to one illustrative embodiment, the structural components may be formed from aluminum, such as plate aluminum or die cast aluminum. As can be seen in
The printing coordinate system is based on the outer surface of the impression cylinder (145). According to one illustrative embodiment, the impression cylinder (145) is supported by the structure (100) and spans its width. The impression cylinder (145) rotates clockwise as viewed from the front of the machine looking at the front carriage plate (110). The +X direction is the feed direction of the substrate and is tangent to the surface of the impression cylinder (145). The +Y direction is parallel to the axis of the impression cylinder as shown in
According to one illustrative embodiment, substrate is fed from the left and is captured by the impression cylinder (145) which is rotating counterclockwise. The substrate first encounters the fixer pens: Pen F1 (215) and Pen F2 (220). The fixer pens (215, 220) deposit an undercoat of fixer/crasher solution which improves the drying and stability of the subsequently deposited inks. The substrate and impression cylinder continue to rotate and encounter the black/yellow pens: Pen K3 (225) and Pen K4 (230). The black/yellow pens (225, 230) span the width of the substrate and deposit the black and yellow portions of the desired image, which may include pictures and text. For example, if the desired image is made up only of black text, the black/yellow pens (225, 230) deposit only the black ink required to form the text. However, if portions of the desired image include colors which are partially made up of the primary color yellow, the black/yellow pens (225, 230) will also deposit yellow ink in the appropriate locations.
The substrate next encounters the magenta/cyan pens: Pen M5 (235) and Pen M6 (240). Similar to the black/yellow pens, the magenta/cyan pens (235, 240) are arranged to span the width of the substrate and deposit magenta and cyan inks in the appropriate locations on the substrate to form the desired image. The substrate then rotates beneath the impression cylinder (145) and encounters the dryer (205). The dryer (205) produces convective and radiant heat which dries the ink and increases the permanence of the printed image. The printed substrate may continue to rotate with the impression cylinder (145) for multiple revolutions or may exit the impression cylinder (145) for duplexing, post processing, or collating.
The alignment of the pens (215-240) with each other and the impression cylinder (145) is critical to the correct placement of ink. As discussed above, the pens may use ink jet technology to deposit the ink. If a pen is misaligned with another pen and/or the impression cylinder, the ink droplets ejected from the pens will impact the substrate in the wrong locations. This can lead to undesirable print artifacts and loss of print quality. For example, misaligned pens can produce color variations, blurring, smear, and other undesirable artifacts.
The pens (215-240) are attached to fixed rails (210), which are connected between the front carriage plate (110) and the rear carriage plate (124). According to one illustrative embodiment, the pens can slide along the fixed rails (210) to adjust the pen location across the width of the substrate. The pens (215-240) are calibrated and adjusted as necessary to be aligned with the substrate/impression cylinder and with each other. Ideally, the pens would remain aligned and calibrated throughout the printer operation. However, due to thermal gradients in the structure (100), the plates and rails supporting the pens can become distorted. This distortion can cause changes in the location and orientation of the pens with respect to each other and the impression cylinder. Because of the cantilevered position of the pens (215-240), the distortion of the structure is amplified by a factor of about 2.4 times at the tip of the pen. As the pens become misaligned through thermal distortion of the structure, ink patterns are deposited by the pens in incorrect locations and produce artifacts which degrade the print quality.
During the printing process, a number of components generate a significant amount of heat. For example, electronics, motors, and print heads all generate heat. In particular, the dryer (205) may generate as much as 2000 watts of heat. This heat may be dissipated in a number ways, including convection, conduction, or radiation mechanisms. Additionally, the impression cylinder sucks air from the surroundings to create a vacuum which pulls the substrate against the impression cylinder. This air is exhausted out of a vent (305) on the rear print zone plate. The vacuum mechanism itself may generate significant heat as well as drawing air heated by the dryer into the impression cylinder (200) and out of the vent (305).
The combination of the vacuum mechanism, dryer (205), and other heat sources produce a large flux of heat which is introduced into the front and rear plates (110, 115, 120, 125). As heat enters a body, the body expands proportionally to its change in temperature. If a body with homogeneous material properties is uniformly heated, it expands photographically with minimal distortion. However, if a thermal gradient is present within the body, distortion occurs as one portion of the body expands while other portions of the body attempt to resist the expansion.
According to one illustrative embodiment, the rear plates (120, 125) may experience significantly more distortion than the front plates (110, 115). The rear plates (120, 125) may be exposed to a higher heat flux due to the exhaust vent (305). Additionally, the rear plates (120, 125) have thinner cross-sections through which heat can travel to other portions of the rear plates (120, 125). Further, the rear carriage plate (125) is much less rigid structure than the front carriage plate (110).
The thermal gradient produces distortion in the rear plates which is also illustrated in
As shown in
The pen layout shown in
Ideally, these misalignments would be overcome without consuming excessive computation power or requiring extensive redesign of the printing system. The solution should result in robust real time compensation which does not restrict the functionality of the device. For example, a solution which requires the printing device to be brought to a steady state condition and remain there throughout the printing process would introduce excessive delay while the printer warmed up and would waste a significant amount of energy. Further, the correction process should not disrupt the flow of print jobs to run frequent calibrations to correct for time or load dependent misalignments.
If the misalignment can be accurately predicted, the timing of the inkjet firing can be adjusted to compensate for the misalignment. Assuming that the misalignments illustrated in
The FrontPZ temperature sensor (600) and the FrontCar temperature sensor (605) rise much more slowly and have significantly less difference between the two sensors. This indicates that the Front plates have a far smaller thermal gradient than the rear plate and are much less thermally sensitive.
However, the temperature readings themselves do not correlate well with observed misalignments of the pens. Consequently the raw temperature readings may not be suitable as inputs for predicting misalignments. Rather, it was discovered that by differencing two or more temperature measurements, a strong correlation emerged which could provide the basis of predicting pen misalignments.
The dashed curve represents the difference between the RearPZ temperature sensor reading and the RearCar temperature sensor reading multiplied by a scaling constant a. Similarly, the dash-dot-dot curve represents the difference between the RearPZ temperature sensor reading and the RearCar temperature sensor reading multiplied by a scaling constant b. Superimposed on the difference curves are dot misalignment measurements for pen K4 (data points are shown as black diamonds) and pen M5 (data points are shown as open circles). The measurements of misalignment are made using pen K3 as a reference. As discussed above, pen K3 is located near the top dead center of the structure and exhibits very little temperature induced misalignment. Consequently, pen K3 can serve as a reference against which the other pens alignments can be measured.
As can be seen in
Other more complex differences of the temperature readings could be used to create stronger correlations or pin point specific misalignments within the pens. For example, to create a difference that strongly correlates to the angular misalignment, one or more temperature measurements on the front plate could be differenced with one or more temperature measurements from the back plate to give an indication of the difference in expansion of which results in an angular rotation of the fixed rails and attached pens. The differences need not be simple differences but could be a variety of mathematical combinations such as linear, polynomial, logarithmic, interpolation, regression, extrapolation, or other mathematical operations.
The horizontal axis shows differenced values which were calculated using the differencing formula:
0.55(RearPZ−RearCar)−0.4(FrontPZ−FrontCar) Eq. 1
The result of this difference is that the observed misalignments generally extend through a linear region which starts near the origin of the graph and extends into the upper right hand portion of the graph. This distribution of misalignments can be closely approximated using a linear curve fit. For example, the linear curve fit in this illustrative example is Y=0.72x-19. The “x” term is the input to the equation and indicates that the differenced value is input into the equation. The resulting “y” is the output of the equation and indicates how many dot rows of misalignment the M6 pen is predicted to be experiencing. In the illustrated example, the curve fit equation provides prediction accuracy of R=0.9703, where R is the correlation coefficient between the observe data and the curve fit.
According to one illustrative example, initial alignment of the pen can be chosen at an intermediate location in the range of measured or predicted misalignments. A solid circle indicates an intermediate location where an initial alignment could be made. For example, the pen M6 may be initially aligned in the middle of the range so that only moderate amounts of compensation are required to cover the complete range. At start up temperatures, the distortion may be minimal and the firing timing may have to be delayed to accurately place the ink droplets. As the distortion increases, less and less delay is required. At about 2.5 difference units, the pen reaches the initial calibration and no delay is required. As the temperature units continue to increase, the firing timing is then advanced to compensate for the increasing misalignment of the pen.
The relatively simple difference equation, linear relationship, and accuracy of the curve fit can make this approach very easy to implement in either hardware or software configurations. For example, if this thermal relationship is stable over time and between machines, a hardware circuit could be configured to make the difference calculation of sensed temperatures and apply the fit to predict the misalignment. This could be performed without requiring the use of a processor or memory. However, in most printing systems memory and processor time is available and the differencing functionality can be incorporated into software which already runs on the processor. For simple difference relationships, the burden on the processor and memory can be very minimal. Additionally, if the printing system is significantly altered, such as making an upgrade of the printing engine, replacement or repositioning of the pens, etc, the software can be upgraded to with a new differencing algorithm which accommodates the changes.
This compensation for pen misalignment can be performed in real time without requiring the printing process to be stopped. The direct predictive nature of the difference and correlation equations allows for pen misalignments to be calculated with only temperature measurements. The firing timing of the individual pens can then be delayed or advanced to compensate for the misalignment. Additionally, the firing timing of the individual inkjet nozzles within a pen can be delayed or advanced to compensate for angular misalignment of the pen. These adjustments to firing timing can be made in real time, perhaps as often as every printed page. By making real time adjustments to compensate for the misalignment of the pens, the printing system can be ready to print at any time and will not require a length warm up period.
Changing firing timing is only one illustrative method for compensating for pen misalignments. Other techniques could be used such as active physical realignment pens. A variety of actuators could be used to align the pens such as piezo electric actuation, motors, strategically placed heaters or other embodiments.
After the initial configuration is performed, real time compensation for thermally induced misalignments (process 1005). First, two or more temperatures of the structural components are measured using the temperature sensors. These temperature measurements are then input into the differencing equation to produce a differenced value (step 1030). The differenced value is then input into the correlation equation which outputs a predicted misalignment of the printhead (step 1035). According to one illustrative embodiment, the differencing equation and correlation equation may be mathematically combined into a mapping equation. The mapping equation accepts temperatures as inputs and outputs the predicted misalignment of a pen.
The firing timing of the printhead is then altered to compensate for the predicted misalignment such that the ink pattern deposited by the printhead is aligned with an ink pattern deposited by a separate reference printhead (step 1040). This process is periodically repeated to maintain the desired level of alignment (step 1045).
In sum, the system and method for compensating for thermal distortion in a printing system leverages existing hardware and software to produce a practical, low-cost solution, to the complex problem of printhead misalignment. The strategic placement of temperature sensors on structural components measures temperatures which can be mathematically differenced to produce a very accurate correlation to the thermally induced misalignment. The accurate prediction of printhead misalignments allows for real time compensation and allows users the freedom to print at any time or temperature.
The preceding description has been presented only to illustrate and describe embodiments and 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.
