SPACING

BACKGROUND

Inkjet printing systems typically comprise a printhead support on which are mounted multiple inkjet printheads. Each printhead comprises an array of printhead nozzles.

When multiple printheads are used in a printing system each printhead has to be accurately positioned relative to other printheads, such that nozzles on different printheads are precisely aligned relative to each other. This precision is to ensu re that printhead control signals cause each nozzle of each printhead to fire, or to eject, an ink drop at a precise location on a media. BRIEF DESCRIPTION

Examples, or embodiments, of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Figure 1 a is a block diagram showing a portion of a printing system according to one example;

Figure 1 b is a block diagram showing a portion of a printing system according to one example;

Figure 2 is a block diagram showing a portion of a printing system according to one example;

Figure 3 is a block diagram showing a portion of a printing system according to one example;

Figure 4 is a block d iagram showing a printer controller in greater detail according to one example;

Figure 5 is a flow diagram outlining an example method of operating a portion of a printing system according to one example; and

Figure 6 is a block diagram showing a portion of a printing system according to one example. DETAILED DESCRIPTION

Referring now to Figure 1 there is shown a portion of a printing system 100 according to one example. The printing system 100 comprises a printhead support 102 on which are mountable multiple printheads 104a to 104n. In one example the printheads 104 are inkjet printheads, such as thermal inkjet or piezo inkjet printheads. Each printhead may be used, for example, for printing coloured ink drops on a media.

Each printhead comprises multiple nozzles (not shown), arranged in an array configuration, th rough wh ich in k drops may be ejected in response to printhead control signals sent to each printhead 104 by a printer controller 106. The printer controller 106 may, for example, derive or receive printhead control signals from digital data representing an image or a document to be printed.

In one example, as shown in Figure 1 a, the printhead support 102 may be a printhead carriage that moves, or scans, across along a carriage bar 1 14 along scan axis 120 to print a swath on a media 1 16. The media 1 16 may then be advanced in a media advance direction 1 18 to enable a further swath to be printed.

In another example, as shown in Figure 1 b, the printhead support may be a page-wide array support on which are mountable a number of printheads 104a to 104n in an array of printheads that span the whole, or substantially the whole, width of a media 1 16 on which printing is to be performed. The media 1 16 is advanced under the printhead support 102 in media advance direction 1 18.

In use, the temperature of the printhead support 1 02 may change. For example, after periods of use the printhead support 102 may heat up, and after periods of inactivity or reduced use the printhead support 102 may cool down. The amount of heating and cool ing may vary depending on the particular printer configuration. For example, in some printing systems heaters or lamps (not shown) may be mounted on, or in proximity to, the printhead support 102 for example for use in drying or curing ink ejected from a printhead.

Changes in temperature of the printhead support 102 may result in expansion or contraction of the printhead support 1 02. Depending on the material or materials used for the printhead support 1 02, the dimensions of the printhead support 102, and the operating temperature, the printhead support 102 may expand or contract by differing amounts. Expansion or contraction of the printhead support 1 02 may affect the spacing between different ones of the printheads, referred to hereinafter as printhead-to-printhead spacing.

In a multiple printhead printing systems even a small deviation in printhead-to- printhead spacing may resu lt in noticeable image q ual ity artifacts . For example, in a printing system that prints at a resolution of 600 dots per inch (dpi) printed image qual ity may be degraded when printhead-to-printhead spacing varies as little as 80 microns (equivalent to approximately 2 dots at 600 dpi).

To compensate for any changes in printhead-to-printhead spacing, examples of printing systems described herein include a measurement system 1 08 to measure any variation in printhead-to-printhead spacing, and a system for compensating for any such variation.

In one example, the measurement system 108 enables the distance between two locations on the printhead support 1 02 to be accurately determined . This enables variations in the distance between different ones of the printheads, su ch as a first printhead 104a and a second printhead 104n, to be determined. A compensation module 1 12 of the printer controller 106 is also provided to modify operation of the printing system such that print quality issues caused by any printhead-to-printhead spacing variation are reduced or eliminated. In one example, where the printhead support 1 02 is a scann ing printhead support, the compensation module 1 12 determines timing offset data to apply to a printhead firing data controller to modify the timing of one or multiple printhead nozzle firing signals on one or multiple ones of the printheads based on the measured variations of printhead-to-printhead spacing. The timing offset data ensures that ink drops are ejected with a suitable timing delay or timing advance such that the ejected ink drops are accurately positioned on a media.

In another example, where the printhead support 1 02 is a page-wide array printhead support, the compensation module 1 1 2 may generate signals to modify printhead nozzle redundancy based on the measured variations of printhead-to-printhead spacing . Pri nthead nozzle red u nd an cy may, for example, enable nozzles on one printhead to be log ical ly switched for redundant nozzles on the same printhead or for overlapping red undant nozzles of a different printhead.

Referring now to Figure 2 there is a shown a portion of a printing system 200 according to one example.

The printing system 200 comprises a first encoder 202 and a second encoder 204. The encoders 202 and 204 are mounted on the printhead support or carriage 1 02. The encoders 202 and 204 generate electrical signals in response to reading an encoder scale 208 on an encoder strip 206.

In one example the encoders 202 and 204 are optical encoders such as HEDS9720 encoders from Avago Technologies.

In the present example the encoders 202 and 204 generate electrical signals in response to light reflected from an optical encoder strip 206 on which an encoder scale 208 is disposed. In one example the encoder scale 208 has a resolution of 600 dpi (0.0042 mm).

In one example the encoder 202 generates electrical signals that allow the printer controller 106 to determine the position of the carriage 102 along the carriage bar 1 14.

Although the encoder strip 206 and carriage bar 1 14 are shown as separate elements, in other examples the encoder strip may be integrated with the carriage bar, with the encoders 202 and 204 appropriately positioned to read the encoder strip on the carriage bar.

In one example the encoder strip 206 enables the printer controller 1 06 to determine a position of the carriage 1 02 along the carriage bar 1 14. In this example, before the controller 106 can determine the position of the carriage 1 02 along the carriage bar 1 14 the controller 106 moves the carriage 102 to abut a reference feature 210 position at a predetermined reference position towards one end of the carriage bar 1 14. Once the carriage 1 02 has been moved against the reference feature 21 0 any further movement of carriage will allow the printer controller 1 06 to determine the position of the carriage 1 02 along the carriage bar 1 1 4 relative to the reference feature 21 0. The degree of accuracy to which the position of the carriage 1 02 m ay be determined is based on the encoder scale 208. In one example the encoder 202 is used to enable the printer controller 106 to determine the position of the carriage 102 along the carriage bar 1 14 for use in general printing operations, such as controlling the timing of printhead nozzle firing data to ensure that ink drops are accurately placed on a media during a printing operation as the carriage 102 moves across the carriage bar 1 14.

In the present example, the addition of the second encoder 204, located on the carriage 102 towards the other end of the carriage 102, with one or more of the printheads 1 04 located in between the first and second encoder 202 and 204, enables the printer controller 1 06 to determine any change in the distance d between the two encoders 202 and 204. For example, if the carriage 1 02 expands in its longitudinal d irection, the distance d, and hence the distance between the two encoders 202 and 204, will increase. Based on the electrical signals generated by each encoder 202 and 204 in response to the encoder strip 206, the printer controller 1 06 can determine the amount of change in the distance d.

For example, if the carriage 1 02 is at an ambient temperature of around 20 degrees Celsius and is moved from the reference feature 21 0 to a g iven position on the carriage bar 1 14 the first encoder 202 may generate 1000 electrical impulsions, and the second encoder 204 may generate 21000 electrical impulsions. If, when the carriage 1 02 has warmed up to a h igher temperature of, for example, 40 degrees Celsius, and is moved from the reference feature 210 to a given position on the carriage bar 1 1 4 the first encoder 202 may generate 1 000 electrical im pulsions, and the second encoder 204 may generate 21004 electrical impulsions. In this example, this controller 1 06 determ ines that the distance d has increased by about 80 microns.

One advantage of this example, for printing systems that already use an encoder on a carriage to determ ine the position of the carriage along a carriage bar, is that determination of the distance d may be achieved with the addition of only a single encoder and the addition of an additional module in a printer controller 106.

Figure 3 shows a further example 300 in which the second encoder 302 reads a second encoder strip 304 having a second encoder strip scale 306. In this example the second encoder strip 304 uses the same scale as the first encoder strip 206, although in another example a different encoder strip scale may be used. A more detailed view of the printer controller 106 is shown in Figure 4.

Th e pri n ter control l er 1 06 comprises a processor 402, such as a microprocessor or microcontroller, and a memory 406 cou pl ed to th e processor 402 by a com mu n ications bus 404. The memory 406 stores processor executabl e m easu rem en t i n stru ction s 408 a nd processor executable compensation instructions 410 that, when executed by the processor 402 cause the controller 106 to determine changes in the printhead-to-printhead spacing and cause the controller 1 06 to compensate for any such changes, as described herein.

An example method performed by the controller 1 06 is outlined in the flow diagram of Figure 5.

At block 502 the controller 1 06 determ ines the d istance d between two predetermined locations on the carriage 1 02. As previously described, the distance d may be determined through use of a pair of encoders on the carriage 102.

At block 504 the controller 106 determines whether the determined distance d has changed since the previous time it was determined . If not, the distance d is determined again after a short delay. In one example the distance d may be determined once a second, once a minute, or at any suitable period.

At block 506 the controller 1 06 determines compensation to apply to the printing system to compensate for any change in the distance d.

I n one example, it may be assumed that the carriage 1 02 expands and contracts in a l inear manner in its long itud inal dimension in response to changes in temperature. If, for example there are N equally spaced (at ambient temperature) printheads mounted on the carriage, it may be assumed that changes in d istance d affects the spacing between each of the N printheads in a l inear manner. Thus, in one exam ple the control ler 1 06 determines that the spacing between each of the N printheads changes by d

N - 1 Thus, if there are four printheads and it is determined that distance d has increased by 300 microns, the controller 106 will determine that the printhead- to-printhead spacing has increased by 1 00 microns. The controller 1 06 then applies (block 508) the determined compensation to the printing system. In one example, the controller 106 applies the determined compensation by providing a timing offset to a printhead firing data controller 212 to alter the timing of nozzle firing data for one or multiple ones of the printheads by an appropriate amount. The a mou nt m ay be based , for exa m pl e , on th e determined change in printhead-to-printhead distance.

In a further example, if it is determined, for example through testing, that the carriage 1 02 does not expand and contract in a linear manner in its longitudinal dimension in response to changes in temperature, precise measurements may be made of the carriage to determine data describing the thermal expansion/contraction behavior of the carriage 102. This data may be stored in a lookup table and used by the controller 1 06 in determ ining an appropriate compensation, or correction factor, to apply to each printhead in response to a determined change in the printhead-to-printhead spacing. Although the above examples use optical encoders, in other examples other non-optical encoders, such as induction encoders, mag netic encoders, capacitive en cod ers, or the l i ke may be used. In such examples, an appropriate encoder strip or encoding arrangement may be used. Referring now to Figure 6, there is shown a block diagram of a portion of a printing system 600 according to a yet further example. In this example a measuring system comprising a pair of d istantly located elements 602 and 604 is included on the carriage 1 02. In this example the elements 602 and 604 are included in addition to an encoder 202 and an encoder strip 206.

In one example measuring element 602 comprises a wave em itter and receiving, a nd element 604 comprises a wave reflector. For example, measuring element 602 may comprise a light emitting source and a light detector for detecting light reflected by the reflector 604. In another example, measuring element 602 may comprise a microwave or sound wave generator and measuring element 604 may comprise a m icrowave or sou nd wave reflector.

In a further example the measuring element 602 may comprise a wave emitting device, and the measuring element 604 may comprise a wave receiving device.

The measuring elements 602 and 604 enable the distance d between the two measuring elements to be accurately measured by measuring the time between an emitted wave and a received wave.

In one example the measuring element 602 and 604 form a laser distance measuring system. As previously described above, the controller 106 determines (block 502) the distance d from the measuring system, and can determine a corresponding change in printhead-to-printhead spacing. The controller 106 then determines (block 506) the compensation to be appl ied , and applies (block 508) the determined compensation.

In some examples, an encoder strip may be constructed from a material or composition of materials having a low thermal expansion coefficient, such as plastic or paper. In some examples where heaters or lamps are located on the printhead support 102, the effects of thermal expansion of an encoder strip may not be considered be significant compared to the thermal expansion of the printhead support, especially in large-format printing systems having a scann ing printhead carriage. In such systems a carriage bar may be in the order of 1 m or more in length. Accordingly, due to the time taken for the carriage (with any heaters/lamps) to scan along the length of the carriage bar any heat absorbed by the encoder strip will have the time to largely dissipate prior to the carriage returning.

Although the examples above describe systems in which the distance between an array of printheads on a carriage is measured, in other examples the measuring system may not measure a d istance between all of the printheads on the carriage. For example, in one example the measuring system may be arranged to measure only a distance between a subset of the printheads on a carriage. In this case, the compensation module may be configured to infer, for example through use of a suitable look-up table, a compensation factor to apply for each of the printheads on a carriage when it is determined that expansion or contraction has occurred.

It will be appreciated that examples and embodiments of the present invention can be real ized in the form of hardware, software or a combination of hardware and software. As described above, any such software may be stored in the form of volatile or non-volatile storage such as, for example, a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM , memory ch ips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic tape. It will be appreciated that the storage devices and storage media are examples of machine-readable storage that are suitable for storing a program or programs that, when executed, implement examples of the present invention. Examples of the present invention may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection and examples suitably encompass the same.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so d isclosed , may be com bi ned i n any com bination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.