EP1765595B1

EP1765595B1 - Method for at least partially compensating for errors in ink dot placement due to erroneous rotational displacement

Info

Publication number: EP1765595B1
Application number: EP04734974A
Authority: EP
Inventors: Simon R. W. Silverbrook Res. Pty Ltd WALMSLEY; Kia Silverbrook Res. Pty Ltd Silverbrook; M. Silverbrook Res. Pty Ltd JACKSON PULVER; John R. Silverbrook Res. Pty Ltd SHEAHAN; Richard T. Silverbrook Res. Pty Ltd PLUNKETT; Michael J. Silverbrook Res. Pty Ltd WEBB; Benjanim D. Silverbrook Res. Pty Ltd MORPHETT
Original assignee: Silverbrook Research Pty Ltd
Current assignee: Silverbrook Research Pty Ltd
Priority date: 2004-05-27
Filing date: 2004-05-27
Publication date: 2011-03-16
Anticipated expiration: 2024-05-27
Also published as: AU2009203012A1; AU2009203012B2; EP1765595A4; AU2009203027A1; AU2004320526A1; AU2008207608A1; AU2009203026A1; AU2004320526B2; AU2009203028A1; DE602004031888D1; AU2009203028B2; EP2301753A1; AU2008207608B2; CA2567724A1; AU2009203026B2; EP1765595A1; AU2009203030B2; PT2301753E; WO2005120835A1; ES2393541T3

Abstract

A printhead module includes at least one row of printhead nozzles. Each row includes at least one displaced row portion. The displacement of the row portion includes a component in a direction normal to that of a pagewidth to be printed.

Description

FIELD OF THE INVENTION

The present invention relates to a method of compensating for errors in ink dot placement due to erroneous rotational displacement of a printhead or printhead module.
The invention has primarily been developed for use in a pagewidth inkjet printer comprising a printer controller and a printhead having one or more printhead modules, and will be described with reference to this example. However, it will be appreciated that the invention is not limited to any particular type of printing technology, and is not limited to use in, for example, pagewidth and inkjet printing.

BACKGROUND

When a printhead module is being mounted to a carrier, there is the possibility that the position of the printhead will be rotationally erroneous. Such errors arise due to the tolerances in the assembly process, for example.
In cases where the printhead module is short, and particularly where it is the only module in the printhead, minor rotational errors may be acceptable. However, in the case of relatively long printheads, the amount of error introduced to dot positions due to the erroneous rotational position of the printhead module relative to the carrier may reach noticeable, and therefore unacceptable (or at least undesirable) levels.
The problem is exacerbated when multiple printhead modules are laid end to end to form a printhead, such as a pagewidth printhead, due to the fact that some forms of rotational error will cause discontinuities between rows of dots printed by adjacent modules. In general, these discontinuities are more visible and objectionable than mere consistent skew across a single printhead module.
EP-A-1375146 describes a method for compensating the effect of rotationally displaced printhead modules in a printhead comprised of overlapping printhead modules. Compensation is by means of printing alternately from adjacent printhead modules in a region of overlap, or printing from only one printhead module in the region of overlap.
It would be useful to provide a method and apparatus for at least partially compensating for errors in ink dot placement due to erroneous rotational displacement of a printhead module relative to a carrier.

SUMMARY OF THE INTENTION

The present invention provides a method of at least partially compensating for errors in ink dot placement as herein defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1.: Single SoPEC A4 Simplex system
Figure 2.: Dual SoPEC A4 Simplex system
Figure 3.: Dual SoPEC A4 Duplex system
Figure 4.: Dual SoPEC A3 simplex system
Figure 5.: Quad SoPEC A3 duplex system
Figure 6.: SoPEC A4 Simplex system with extra SoPEC used as DRAM storage
Figure 7.: SoPEC A4 Simplex system with network connection to Host PC
Figure 8.: Print construction and Nozzle position
Figure 9.: Conceptual horizontal misplacement between segments
Figure 10.: Printhead row positioning and default row firing order
Figure 11.: Firing order of fractionally misaligned segment
Figure 12.: Example of yaw in printhead IC misplacement

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Various aspects of the preferred and other embodiments will now be described.
It will be appreciated that the following description is a highly detailed exposition of the hardware and associated methods that together provide a printing system capable of relatively high resolution, high speed and low cost printing compared to prior art systems.
Much of this description is based on technical design documents, so the use of words like "must", "should" and "will", and all others that suggest limitations or positive attributes of the performance of a particular product, should not be interpreted as applying to the invention in general. These comments, unless clearly referring to the invention in general, should be considered as desirable or intended features in a particular design rather than a requirement of the invention. The intended scope of the invention is defined in the claims.
Also throughout this description, "printhead module" and "printhead" are used somewhat interchangeably. Technically, a "printhead" comprises one or more "printhead modules", but occasionally the former is used to refer to the latter. It should be clear from the context which meaning should be allocated to any use of the word "printhead".

PRINT SYSTEM OVERVIEW

1 Introduction

The SoPEC ASIC (Small office home office Print Engine Controller) is suitable for use in price sensitive SoHo printer products. The SoPEC ASIC is intended to be a relatively low cost solution for linking printhead control, replacing the multichip solutions in larger more professional systems with a single chip. The increased cost competitiveness is achieved by integrating several systems such as a modified PEC1 printing pipeline, CPU control system, peripherals and memory sub-system onto one SoC ASIC, reducing component count and simplifying board design. SoPEC contains features making it suitable for multifunction or "all-in-one" devices as well as dedicated printing systems.
This section will give a general introduction to Memjet printing systems, introduce the components that make a linking printhead system, describe a number of system architectures and show how several SoPECs can be used to achieve faster, wider and/or duplex printing. The section "SoPEC ASIC" describes the SoC SoPEC ASIC, with subsections describing the CPU, DRAM and Print Engine Pipeline subsystems. Each section gives a detailed description of the blocks used and their operation within the overall print system.
Basic features of the preferred embodiment of SoPEC include:

Continuous 30ppm operation for 1600dpi output at A4/Letter.
Linearly scalable (multiple SoPECs) for increased print speed and/or page width.
192MHz internal system clock derived from low-speed crystal input
PEP processing pipeline, supports up to 6 color channels at 1 dot per channel per clock cycle
Hardware color plane decompression, tag rendering, halftoning and compositing
Data formatting for Linking Printhead
Flexible compensation for dead nozzles, printhead misalignment etc.
Integrated 20Mbit (2.5MByte) DRAM for print data and CPU program store
LEON SPARC v8 32-bit RISC CPU
Supervisor and user modes to support multi-threaded software and security
1 kB each of I-cache and D-cache, both direct mapped, with optimized 256-bit fast cache update.
1 x USB2.0 device port and 3 x USB2.0 host ports (including integrated PHYs)
Support high speed (480Mbit/sec) and full speed (12Mbit/sec) modes of USB2.0
Provide interface to host PC, other SoPECs, and external devices e.g. digital camera
Enable alternative host PC interfaces e.g. via external USB/ethernet bridge
Glueless high-speed serial LVDS interface to multiple Linking Printhead chips
64 remappable GPIOs, selectable between combinations of integrated system control components:
2 x LSS interfaces for QA chip or serial EEPROM
LED drivers, sensor inputs, switch control outputs
Motor controllers for stepper and brushless DC motors
Microprogrammed multi-protocol media interface for scanner, external RAM/Flash, etc.
112-bit unique ID plus 112-bit random number on each device, combined for security protocol support
IBM Cu-11 0.13 micron CMOS process, 1.5V core supply, 3.3V IO.
208 pin Plastic Quad Flat Pack

2 Print Quality Considerations

The preferred embodiment linking printhead produces 1600 dpi bi-level dots. On low-diffusion paper, each ejected drop forms a 22.5µm diameter dot. Dots are easily produced in isolation, allowing dispersed-dot dithering to be exploited to its fullest. Since the preferred form of the linking printhead is pagewidth and operates with a constant paper velocity, color planes are printed in good registration, allowing dot-on-dot printing. Dot-on-dot printing minimizes 'muddying' of midtones caused by inter-color bleed.
A page layout may contain a mixture of images, graphics and text. Continuous-tone (contone) images and graphics are reproduced using a stochastic dispersed-dot dither. Unlike a clustered-dot (or amplitude-modulated) dither, a dispersed-dot (or frequency-modulated) dither reproduces high spatial frequencies (i.e. image detail) almost to the limits of the dot resolution, while simultaneously reproducing lower spatial frequencies to their full color depth, when spatially integrated by the eye. A stochastic dither matrix is carefully designed to be free of objectionable low-frequency patterns when tiled across the image. As such its size typically exceeds the minimum size required to support a particular number of intensity levels (e.g. 16×16× 8 bits for 257 intensity levels).
Human contrast sensitivity peaks at a spatial frequency of about 3 cycles per degree of visual field and then falls off logarithmically, decreasing by a factor of 100 beyond about 40 cycles per degree and becoming immeasurable beyond 60 cycles per degree. At a normal viewing distance of 12 inches (about 300mm), this translates roughly to 200-300 cycles per inch (cpi) on the printed page, or 400-600 samples per inch according to Nyquist's theorem.
In practice, contone resolution above about 300 ppi is of limited utility outside special applications such as medical imaging. Offset printing of magazines, for example, uses contone resolutions in the range 150 to 300 ppi. Higher resolutions contribute slightly to color error through the dither.
Black text and graphics are reproduced directly using bi-level black dots, and are therefore not anti-aliased (i.e. low-pass filtered) before being printed. Text should therefore be supersampled beyond the perceptual limits discussed above, to produce smoother edges when spatially integrated by the eye. Text resolution up to about 1200 dpi continues to contribute to perceived text sharpness (assuming low-diffusion paper).
A Netpage printer, for example, may use a contone resolution of 267 ppi (i.e. 1600 dpi /6), and a black text and graphics resolution of 800 dpi. A high end office or departmental printer may use a contone resolution of 320 ppi (1600 dpi / 5) and a black text and graphics resolution of 1600 dpi. Both formats are capable of exceeding the quality of commercial (offset) printing and photographic reproduction.

3 Memjet Printer Architecture

The SoPEC device can be used in several printer configurations and architectures.
In the general sense, every preferred embodiment SoPEC-based printer architecture will contain:

One or more SoPEC devices.
One or more linking printheads.
Two or more LSS busses.
Two or more QA chips.
Connection to host, directly via USB2.0 or indirectly.
Connections between SoPECs (when multiple SoPECs are used).

Some example printer configurations as outlined in Section 4.2. The various system components are outlined briefly in Section 4.1.

4.1 System Components

4.1.1 SoPEC Print Engine Controller

The SoPEC device contains several system on a chip (SoC) components, as well as the print engine pipeline control application specific logic.

4.1.1.1 Print Engine Pipeline (PEP) Logic

The PEP reads compressed page store data from the embedded memory, optionally decompresses the data and fonnats it for sending to the printhead. The print engine pipeline functionality includes expanding the page image, dithering the contone layer, compositing the black layer over the contone layer, rendering of Netpage tags, compensation for dead nozzles in the printhead, and sending the resultant image to the linking printhead.

4.1.1.2 Embedded CPU

SoPEC contains an embedded CPU for general-purpose system configuration and management. The CPU performs page and band header processing, motor control and sensor monitoring (via the GPIO) and other system control functions. The CPU can perform buffer management or report buffer status to the host. The CPU can optionally run vendor application specific code for general print control such as paper ready monitoring and LED status update.

4.1.1.3 Embedded Memory Buffer

A 2.5Mbyte embedded memory buffer is integrated onto the SoPEC device, of which approximately 2Mbytes are available for compressed page store data. A compressed page is divided into one or more bands, with a number of bands stored in memory. As a band of the page is consumed by the PEP for printing a new band can be downloaded. The new band may be for the current page or the next page.
Using banding it is possible to begin printing a page before the complete compressed page is downloaded, but care must be taken to ensure that data is always available for printing or a buffer underrun may occur. A Storage SoPEC acting as a memory buffer (Section 4.2.6) could be used to provide guaranteed data delivery.

4.1.1.4 Embedded USB2.0 Device Controller

The embedded single-port USB2.0 device controller can be used either for interface to the host PC, or for communication with another SoPEC as an ISCSlave. It accepts compressed page data and control commands from the host PC or ISCMaster SoPEC, and transfers the data to the embedded memory for printing or downstream distribution.

4.1.1.5 Embedded USB2.0 Host Controller

The embedded three-port USB2.0 host controller enables communication with other SoPEC devices as a ISCMaster, as well as interfacing with external chips (e.g. for Ethernet connection) and external USB devices, such as digital cameras.

4.1.1.6 Embedded Device/Motor Controllers

SoPEC contains embedded controllers for a variety of printer system components such as motors, LEDs etc, which are controlled via SoPEC's GPIOs. This minimizes the need for circuits external to SoPEC to build a complete printer system.

4.1.2 Linking Printhead

The printhead is constructed by abutting a number of printhead ICs together. Each SoPEC can drive up to 12 printhead ICs at data rates up to 30ppm or 6 printhead ICs at data rates up to 60ppm. For higher data rates, or wider printheads, multiple SoPECs must be used.

4.13 LSS interface bus

Each SoPEC device has 2 LSS system buses for communication with QA devices for system authentication and ink usage accounting. The number of QA devices per bus and their position in the system is unrestricted with the exception that PRINTER_QA and INK_QA devices should be on separate LSS busses.

4.1.4 QA devices

Each SoPEC system can have several QA devices. Normally each printing SoPEC will have an associated PRINTER_QA. Ink cartridges will contain an INK_QA chip. PRINTER_QA and INK_QA devices should be on separate LSS busses. All QA chips in the system are physically identical with flash memory contents defining PRINTER_QA from INK_QA chip.

4.15 Connections between SoPECs

In a multi-SoPEC system, the primary communication channel is from a USB2.0 Host port on one SoPEC (the ISCMaster), to the USB2.0 Device port of each of the other SoPECs (ISCSlaves). If there are more ISCSlave SoPECs than available USB Host ports on the ISCMaster, additional connections could be via a USB Hub chip, or daisy-chained SoPEC chips. Typically one or more of SoPEC's GPIO signals would also be used to communicate specific events between multiple SoPECs.

4.1.6 Non-USB Host PC Communication

The communication between the host PC and the ISCMaster SoPEC may involve an external chip or subsystem, to provide a non-USB host interface, such as ethernet or WiFi. This subsystem may also contain memory to provide an additional buffered band/page store, which could provide guaranteed bandwidth data deliver to SoPEC during complex page prints.

4.2 Possible SoPEC Systems

Several possible SoPEC based system architectures exist. The following sections outline some possible architectures. It is possible to have extra SoPEC devices in the system used for DRAM storage. The QA chip configurations shown are indicative of the flexibility of LSS bus architecture, but not limited to those configurations.

4.2.1 A4 Simplex at 30 ppm with 1 SoPEC device

In Figure 1, a single SoPEC device is used to control a linking printhead with 11 printhead ICs. The SoPEC receives compressed data from the host through its USB device port. The compressed data is processed and transferred to the printhead. This arrangement is limited to a speed of 30ppm. The single SoPEC also controls all printer components such as motors, LEDs, buttons etc, either directly or indirectly.

4.2.2 A4 Simplex at 60 ppm with 2 SoPEC devices

In Figure 2, two SoPECs control a single linking printhead, to provide 60ppm A4 printing. Each SoPEC drives 5 or 6 of the printheads ICs that make up the complete printhead. SoPEC #0 is the ISCMaster, SoPEC #1 is an ISCSlave. The ISCMaster receives all the compressed page data for both SoPECs and re-distributes the compressed data for the ISCSlave over a local USB bus. There is a total of 4MBytes of page store memory available if required. Note that, if each page has 2MBytes of compressed data, the USB2.0 interface to the host needs to run in high speed (not full speed) mode to sustain 60ppm printing. (In practice, many compressed pages will be much smaller than 2MBytes). The control of printer components such as motors, LEDs, buttons etc, is shared between the 2 SoPECs in this configuration.

4.23 A4 Duplex with 2 SoPEC devices

In Figure 3, two SoPEC devices are used to control two printheads. Each printhead prints to opposite sides of the same page to achieve duplex printing. SoPEC #0 is the ISCMaster, SoPEC #1 is an ISCSlave. The ISCMaster receives all the compressed page data for both SoPECs and re-distributes the compressed data for the ISCSlave over a local USB bus. This configuration could print 30 double-sided pages per minute.

4.2.4 A3 Simplex with 2 SoPEC devices

In Figure 4, two SoPEC devices are used to control one A3 linking printhead, constructed from 16 printhead ICs. Each SoPEC controls 8 printhead ICs. This system operates in a similar manner to the 60ppm A4 system in Figure 2, although the speed is limited to 30ppm at A3, since each SoPEC can only drive 6 printhead ICs at 60ppm speeds. A total of 4Mbyte of page store is available, this allows the system to use compression rates as in a single SoPEC A4 architecture, but with the increased page size of A3.

4.25 A3 Duplex with 4 SoPEC devices

In Figure 5 a four SoPEC system is shown. It contains 2 A3 linking printheads, one for each side of an A3 page. Each printhead contain 16 printhead ICs, each SoPEC controls 8 printhead ICs. SoPEC #0 is the ISCMaster with the other SoPECs as ISCSlaves. Note that all 3 USB Host ports on SoPEC #0 are used to communicate with the 3 ISCSlave SoPECs. In total, the system contains 8Mbytes of compressed page store (2Mbytes per SoPEC), so the increased page size does not degrade the system print quality, from that of an A4 simplex printer. The ISCMaster receives all the compressed page data for all SoPECs and re-distributes the compressed data over the local USB bus to the ISCSlaves. This configuration could print 30 double-sided A3 sheets per minute.

4.2.6 SoPEC DRAM storage solution: A4 Simplex with 1 printing SoPEC and 1 memory SoPEC

Extra SoPECs can be used for DRAM storage e.g. in Figure 6 an A4 simplex printer can be built with a single extra SoPEC used for DRAM storage. The DRAM SoPEC can provide guaranteed bandwidth delivery of data to the printing SoPEC. SoPEC configurations can have multiple extra SoPECs used for DRAM storage.

4.2.7 Non-USB connection to Host PC

Figure 7 shows a configuration in which the connection from the host PC to the printer is an ethernet network, rather than USB. In this case, one of the USB Host ports on SoPEC interfaces to a external device that provide ethernet-to-USB bridging. Note that some networking software support in the bridging device might be required in this configuration. A Flash RAM will be required in such a system, to provide SoPEC with driver software for the Ethernet bridging function.

5 Printhead Misplacement Types

5.1 Printhead Construction

A linking printhead is constructed from linking printhead ICs, placed on a substrate containing ink supply holes. An A4 pagewidth printer used 11 linking printhead ICs. Each printhead is placed on the substrate with reference to positioning fidicuals on the substrate.
Figure 8 shows the arrangement of the printhead ICs (also known as segments) on a printhead. The join between two ICs is shown in detail. The left-most nozzles on each row are dropped by 10 line-pitches, to allow continuous printing across the join. Figure 8 also introduces some naming and co-ordinate conventions used throughout this document.
Figure 8 shows the anticipated first generation linking printhead nozzle arrangements, with 10 nozzle rows supporting five colours. The SoPEC compensation mechanisms are general enough to cover other nozzle arrangements.

5.2 Misplacement Types

Printheads ICs may be misplaced relative to their ideal position. This misplacement may include any combination of:

3. x offset
3. y offset
3. yaw (rotation around z)
3. pitch (rotation around y)
3. roll (rotation around z)

In some cases, the best visual results are achieved by considering relative misplacement between adjacent ICs, rather than absolute misplacement from the substrate. There are some practical limits to misplacement, in that a gross misplacement will stop the ink from flowing through the substrate to the ink channels on the chip.
Correcting for misplacement obviously requires the misplacement to be measured. In general this may be achieved directly by inspection of the printhead after assembly, or indirectly by scanning or examining a printed test pattern.

6 Misplacement compensation

6.1 X offset

SoPEC can compensate for misplacement of linking chips in the X-direction, but only snapped to the nearest dot. That is, a misplacement error of less than 0.5 dot-pitches or 7.9375 microns is not compensated for, a misplacement more that 0.5 dot-pitches but less than 1.5 dot-pitches is treated as a misplacement of 1 dot-pitch, etc.
Uncompensated X misplacement can result in three effects:

3. printed dots shifted from their correct position for the entire misplaced segment
3. missing dots in the overlap region between segments.
3. duplicated dots in the overlap region between segments.

SoPEC can correct for each of these three effects.

6.1.1 Correction for overall position in X

In preparing line data to be printed, SoPEC buffers in memory the dot data for a number of lines of the image to be printed. Compensation for misplacement generally involves changing the pattern in which this dot data is passed to the printhead ICs.
SoPEC uses separate buffers for the even and odd dots of each colour on each line, since they are printed by different printhead rows. So SoPEC's view of a line at this stage is as (up to) 12 rows of dots, rather than (up to) 6 colours. Nominally, the even dots for a line are printed by the lower of the two rows for that colour on the printhead, and the odd dots are printed by the upper row (see Figure 8). For the current linking printhead IC, there are 640 nozzles in row. Each row buffer for the full printhead would contain 640x11 dots per line to be printed, plus some padding if required.
In preparing the image, SoPEC can be programmed in the DWU module to precompensate for the fact that each row on the printhead IC is shifted left with respect to the row above. In this way the leftmost dot printed by each row for a colour is the same offset from the start of a row buffer. In fact the programming can support arbitrary shapes for the printhead IC.
SoPEC has independent registers in the LLU module for each segment that determine which dot of the prepared image is sent to the left-most nozzle of that segment. Up to 12 segments are supported. With no misplacement, SoPEC could be programmed to pass dots 0 to 639 in a row to segment 0, dots 640 to 1279 in a row to segment 1, etc.
If segment 1 was misplaced by 2 dot-pitches to the right, SoPEC could be adjusted to pass to dots 641 to 1280 of each row to segment 1 (remembering that each row of data consists entirely of either odd dots or even dots from a line, and that dot 1 on a row is printed two dot positions away from dot 0). This means the dots are printed in the correct position overall. This adjustment is based on the absolute placement of each printhead IC. Dot 640 is not printed at all, since there is no nozzle in that position on the printhead (see Section 6.1.2 for more detail on compensation for missing dots).
A misplacement of an odd number of dot-pitches is more problematic, because it means that the odd dots from the line now need to be printed by the lower row of a colour pair, and the even dots by the upper row of a colour pair on the printhead segment. Further, swapping the odd and even buffers interferes with the precompensation. This results in the position of the first dot to be sent to a segment being different for odd and even rows of the segment. SoPEC addresses this by having independent registers in the LLU to specify the first dot for the odd and even rows of each segment, i.e. 2 x 12 registers. A further register bit detennines whether dot data for odd and even rows should be swapped on a segment by segment basis.

6.1.2 Correcting for duplicate and missing dots

Figure 9 shows the detailed alignment of dots at the join between two printhead ICs, for various cases of misplacement, for a single colour.
The effects at the join depend on the relative misplacement of the two segments. In the ideal case with no misplacement, the last 3 nozzles of upper row of the segment N interleave with the first three nozzles of the lower row of segment N+1, giving a single nozzle (and so a single printed dot) at each dot-pitch.
When segment N+1 is misplaced to the right relative to segment N (a positive relative offset in X), there are some dot positions without a nozzle, i.e. missing dots. For positive offsets of an odd number of dot-pitches, there may also be some dot positions with two nozzles, i.e. duplicated dots. Negative relative offsets in X of segment N+1 with respect to segment N are less likely, since they would usually result in a collision of the printhead ICs, however they are possible in combination with an offset in Y. A negative offset will always cause duplicated dots, and will cause missing dots in some cases. Note that the placement and tolerances can be deliberately skewed to the right in the manufacturing step to avoid negative offsets.
Where two nozzles occupy the same dot position, the corrections described in Section 6.1.1 will result in SoPEC reading the same dot data from the row buffer for both nozzles. To avoid printing this data twice SoPEC has two registers per segment in the LLU that specify a number (up to 3) of dots to suppress at the start of each row, one register applying to even dot rows, one to odd dot rows.
SoPEC compensates for missing dots by add the missing nozzle position to its dead nozzle map. This tells the dead nozzle compensation logic in the DNC module to distribute the data from that position into the surrounding nozzles, before preparing the row buffers to be printed.

6.2 Y Offset

SoPEC can compensate for misplacement of printhead ICs in the Y-direction, but only snapped to the nearest 0.1 of a line. Assuming a line-pitch of 15.875 microns, if an IC is misplaced in Y by 0 microns, SoPEC can print perfectly in Y. If an IC is misplaced by 1.5875 microns in Y, then we can print perfectly. If an IC is misplaced in Y by 3.175 microns, we can print perfectly. But if an IC is misplaced by 3 microns, this is recorded as a misplacement of 3.175 microns (snapping to the nearest 0.1 of a line), and resulting in a Y error of 0.175 microns (most likely an imperceptible error).
Uncompensated Y misplacement results in all the dots for the misplaced segment being printed in the wrong position on the page.
SoPEC's compensation for Y misplacement uses two mechanism, one to address whole line-pitch misplacement, and another to address fractional line-pitch misplacement. These mechanisms can be applied together, to compensate for arbitrary misplacements to the nearest 0.1 of a line.

6.2.1 Compensating for whole line-pitch misplacement

Section 6.1 described the buffers used to hold dot data to be printed for each row. These buffers contain dot data for multiple lines of the image to be printed. Due to the physical separation of nozzle rows on a printhead IC, at any time different rows are printing data from different lines of the image.
For a printhead on which all ICs are ideally placed, row 0 of each segment is printing data from the line N of the image, row 1 of each segment is printing data from row N-M of the image etc. where N is the separation of rows 0 and 1 on the printhead. Separate SoPEC registers in the LLU for each row specify the designed row separations on the printhead, so that SoPEC keeps track of the "current" image line being printed by each row.
If one segment is misplaced by one whole line-pitch, SoPEC can compensate by adjusting the line of the image being sent to each row of that segment. This is achieved by adding an extra offset on the row buffer address used for that segment, for each row buffer. This offset causes SoPEC to provide the dot data to each row of that segment from one line further ahead in the image than the dot data provided to the same row on the other segments. For example, when the correctly placed segments are printing line N of an image with row 0, line N-M of the image with row 1, etc, then the misplaced segment is printing line N+1 of the image with row 0, line N-M+1 of the image with row 1, etc.
SoPEC has one register per segment to specify this whole line-pitch offset. The offset can be multiple line-pitches, compensating for multiple lines of misplacement. Note that the offset can only be in the forward direction, corresponding to a negative Y offset. This means the initial setup of SoPEC must be based on the highest (most positive) Y-axis segment placement, and the offsets for other segments calculated from this baseline. Compensating for Y displacement requires extra lines of dot data buffering in SoPEC, equal to the maximum relative Y offset (in line-pitches) between any two segments on the printhead. For each misplaced segment, each line of misplacement requires approximately 640x10 or 6400 extra bits of memory.

6.2.2 Compensation for fractional line-pitch misplacement

Compensation for fractional line-pitch displacement of a segment is achieved by a combination of SoPEC and printhead IC fire logic.
The nozzle rows in the printhead are positioned by design with vertical spacings in line-pitches that have a integer and fractional component. The fractional components are expressed relative to row zero, and are always some multiple of 0.1 of a line-pitch. The rows are fired sequentially in a given order, and the fractional component of the row spacing matches the distance the paper will move between one row firing and the next. Figure 10 shows the row position and firing order on the current implementation of the printhead IC. Looking at the first two rows, the paper moves by 0.5 of a line-pitch between the row 0 (fired first) and row 1 (fired sixth). is supplied with dot data from a line 3 lines before the data supplied to row 0. This data ends up on the paper exactly 3 line-pitches apart, as required.
If one printhead IC is vertically misplaced by a non-integer number of line-pitches, row 0 of that segment no longer aligns to row 0 of other segments. However, to the nearest 0.1 of a line, there is one row on the misplaced segment that is an integer number of line-pitches away from row 0 of the ideally placed segments. If this row is fired at the same time as row 0 of the other segments, and it is supplied with dot data from the correct line, then its dots will line up with the dots from row 0 of the other segments, to within a 0.1 of a line-pitch. Subsequent rows on the misplaced printhead can then be fired in their usual order, wrapping back to row 0 after row 9. This firing order results in each row firing at the same time as the rows on the other printheads closest to an integer number of line-pitches away.
Figure 11 shows an example, in which the misplaced segment is offset by 0.3 of a line-pitch. In this case, row 5 of the misplaced segment is exactly 24.0 line-pitches from row 0 of the ideal segment. Therefore row 5 is fired first on the misplaced segment, followed by row 7, 9, 0 etc. as shown. Each row is fired at the same time as the a row on the ideal segment that is an integer number of lines away. This selection of the start row of the firing sequence is controlled by a register in each printhead IC.
SoPEC's role in the compensation for fractional line-pitch misplacement is to supply the correct dot data for each row. Looking at Figure 11, we can see that to print correct, row 5 on the misplaced printhead needs dot data from a line 24 lines earlier in the image than the data supplied to row 0. On the ideal printhead, row 5 needs dot data from a line 23 lines earlier in the image than the data supplied to row 0. In general, when a non-default start row is used for a segment, some rows for that segment need their data to be offset by one line, relative to the data they would receive for a default start row. SoPEC has a register in LLU for each row of each segment, that specifies whether to apply a one line offset when fetching data for that row of that segment.

6.3 Roll (rotation around X)

This kind of erroneous rotational displacement means that all the nozzles will end up pointing further up the page in Y or further down the page in Y. The effect is the same as a Y misplacement, except there is a different Y effect for each media thickness (since the amount of misplacement depends on the distance the ink has to travel).
In some cases, it may be that the media thickness makes no effective visual difference to the outcome, and this form of misplacement can simply be incorporated into the Y misplacement compensation. If the media thickness does make a difference which can be characterised, then the Y misplacement programming can be adjusted for each print, based on the media thickness.
It will be appreciated that correction for roll is particularly of interest where more than one printhead module is used to form a printhead, since it is the discontinuities between strips printed by adjacent modules that are most objectionable in this context.

6.4 Pitch (rotation around Y)

In this rotation, one end of the IC is further into the substrate than the other end. This means that the printing on the page will be dots further apart at the end that is further away from the media (i.e. less optical density), and dots will be closer together at the end that is closest to the media (more optical density) with a linear fade of the effect from one extreme to the other. Whether this produces any kind of visual artifact is unknown, but it is not compensated for in SoPEC.

6.5 Yaw (rotation around Z)

This kind of erroneous rotational displacement means that the nozzles at one end of a IC will print further down the page in Y than the other end of the IC. There may also be a slight increase in optical density depending on the rotation amount.
SoPEC can compensate for this by providing first order continuity, although not second order continuity in the preferred embodiment. First order continuity (in which the Y position of adjacent line ends is matched) is achieved using the Y offset compensation mechanism, but considering relative rather than absolute misplacement. Second order continuity (in which the slope of the lines in adjacent print modules is at least partially equalised) can be effected by applying a Y offset compensation on a per pixel basis. Whilst one skilled in the art will have little difficulty deriving the timing difference that enables such compensation, SoPEC does not compensate for it and so it is not described here in detail.
Figure 12 shows an example where printhead IC number 4 is be placed with yaw, is shown in Figure 12, while all other ICs on the printhead are perfectly placed. The effect of yaw is that the left end of segment 4 of the printhead has an apparent Y offset of -1 line-pitch relative to segment 3, while the right end of segment 4 has an apparent Y offset of 1 line-pitch relative to segment 5.
To provide first-order continuity in this example, the registers on SoPEC would be programmed such that segments 0 to 3 have a Y offset of 0, segment 4 has a Y offset of -1, and segments 5 and above have Y offset of -2. Note that the Y offsets accumulate in this example - even though segment 5 is perfect aligned to segment 3, they have different Y offsets programmed.
It will be appreciated that some compensation is better than none, and it is not necessary in all cases to perfectly correct for roll and/or yaw. Partial compensation may be adequate depending upon the particular application. As with roll, yaw correction is particularly applicable to multi-module printheads, but can also be applied in single module printheads.

Claims

A method of at least partially compensating for errors in ink dot placement by at least one of a plurality of nozzles due to erroneous rotational yaw displacement of one of a plurality of abutting printhead modules mounted on a carrier to form a printhead, the method comprising the step of:
(a) determining the rotational yaw displacement of the erroneously displaced printhead module;
and characterized by the steps of:

(b) determining an accumulative Y-offset for each printhead module in the printhead, such that a Y-position of adjacent printed line ends is matched; and

(c) altering the output of the ink dots from nozzles in each printhead module having a non-zero accumulative Y-offset to at least partially compensate for the rotational yaw displacement of the erroneously displaced printhead module.
A method according to claim 1, wherein step (c) includes altering a timing of a fire signal to at least one of the nozzles on the basis of an accumulative Y-offset for a respective printhead module containing the at least one nozzle, thereby to effect the at least partial compensation.
A method according to claim 1, wherein the accumulative Y-offsets are stored in a memory associated with the printhead.
A printer controller programmed and configured to implement the method of any one of the preceding claims.