Patent Publication Number: US-2009219316-A1

Title: Printing method and apparatus

Description:
The present invention relates to the field of printing, and in particular to an improved method and apparatus for printing from a printhead with a linear array of nozzles. 
     Ink jet printheads can be used to position spots of liquids, for example biological samples, on substrates such as glass slides. An important application is the manufacture of DNA microarrays. 
     It is usually required to print one, two, three or more spots of each liquid onto each of a large number (tens to hundreds) of substrates. There is typically a very large number (hundreds to tens of thousands) of different liquids to be printed onto the substrates, so printing can be take an extremely long time. 
     It is normal practice to trigger the ejection of drops from an ink jet printhead by means of electronic pulses from an encoder that is linked to the relative motion of the printhead and substrate. When a pulse reaches the printhead, it prints from whichever nozzles are specified by the data in the printing image. 
     Patent application WO 02/11889 discloses a method involving an ink jet printhead having multiple chambers, each chamber being associated with a nozzle through which a liquid is discharged during a printing operation. The printhead can be used to print multiple different liquids at the same time without cross-contamination between the liquids, despite the fact that the chambers are connected by one or more manifolds internal to the printhead. The liquids are introduced via contiguous groups of nozzles into the associated chambers and printed before they have time to mix by diffusion via the manifolds. 
     In the manufacture of DNA microarrays, the printhead need not be oriented in the manner conventional to the ink jet printing industry. This is a consequence of the need to produce an array of spots of the liquids on the slide, rather than an image as such. It may be advantageous to orientate the printhead so that its row of nozzles is parallel to the direction of relative motion during printing to allow finely spaced spots of liquid. 
     For particular printing applications, superposition of drops may be required or desired. Accurate superposition of several drops onto the same spot on a substrate requires the printhead to eject drops from the nozzles at precisely the correct time. 
     One possibility for the triggering is that the trigger interval is equal to the nozzle spacing or the spacing divided by a small integer, to a reasonable approximation, allowing the positioning of each liquid to be achieved sufficiently accurately. The difficulty with this approach is that the spacing of the multiple spots of different liquids can be adjusted only as finely as the encoder interval. A further problem is that if the nozzle spacing is awkward, a suitable encoder may not be available. Any errors due to the awkward spacing are cumulative when more than two drops are deposited. In addition to producing imperfect positioning within each spot, this scheme suffers from the disadvantage that it permits the spots of different liquids to be positioned only to the accuracy of an encoder pulse. 
     An example of this approach might be as follows: if the printhead has nozzles at a spacing such as 137.14 μm, as is the case with the XaarJet XJ126R, the encoder interval could be 20 μm. Using the fact that 7×20=140≈137.14 μm, every seventh pulse could be used to trigger contiguous nozzles. Superposition would appear to be sufficiently accurate given that the spot produced by a single drop on a typical substrate is about 100 μm diameter. 
     It is desirable for the superposition to be more accurate, and for multiple spots of different liquids to be positioned with high precision. This can be achieved by using an encoder giving pulses at closer intervals, that is, a small fraction of the nozzle spacing (though not necessarily equal to the nozzle spacing divided by an integer). An example of this approach is the use of a 1 μm encoder. Drop superposition and spot positioning would then be superior. 
     However, the image controlling the printing, consisting of ones representing ejection of drops and zeroes where no drop is required, would have to be packed with a high proportion of zeroes to achieve the required positioning of the spots. Since the ink jet printhead can only print at a limited rate, even when “printing” zeroes or blanks, this approach limits the linear speed of the printhead and results in slow printing. If the printhead can print drops at a frequency of a few kiloHertz, the linear speed is limited to a few millimetres per second, which is far too slow for a cost-effective printing process. Furthermore, the image will often be very large, which can result in electronic and software problems. 
     It would therefore be desirable to provide a printing method and triggering scheme for a printhead that obviates or at least mitigates one or more of the drawbacks associated with the prior art. 
     It is a first aim of the invention to provide an improved method of superposing drops of liquid. 
     It is one aim of the invention to provide an improved method of printing spots of different sizes. 
     It is one aim of the invention to provide a method for triggering a printhead that improves the printing speed of a printhead. 
     It is a further aim of the invention to provide a method for triggering a printhead that allows drop superposition. 
     It is a further aim of the invention to provide spot positioning of superior accuracy. 
     It is a further aim of the invention to provide a method for triggering a printhead that reduces the amount of data sent to a printhead. 
     It is a further aim of the invention to provide a printing method and device with improved reliability. 
     Additional aims and objects of the invention will become apparent from a reading of the following description. 
     According to a first aspect of the invention, there is provided a method of printing a spot on a substrate from a printhead, the printhead having a linear array of nozzles oriented in an axis, the method comprising the steps of:
         printing a first drop of a first liquid from a first nozzle of the printhead at a first location;   displacing the printhead relative to the substrate along the axis such that the position of a second nozzle of the printhead corresponds to the first location;   printing a second drop of the first liquid from the second nozzle of the printhead at the first location.       

     The method may comprise the additional steps of:
         displacing the printhead relative to the substrate along the axis such that the position of a further nozzle of the printhead corresponds to the first location;   printing a further drop of the first liquid from the further nozzle of the printhead at the first location.       

     Preferably, the nozzles of the printhead are arranged in groups, each group comprising a plurality of nozzles, and each group corresponding to a particular liquid. More preferably, the nozzles in each group are contiguous. 
     The first, second and further nozzles may belong to a first group of nozzles. 
     The method may comprise the additional steps of:
         printing a drop of a second liquid from a nozzle belonging to a second group of nozzles at a second location;   displacing the printhead relative to the substrate along the axis such that the position of a second nozzle belonging to a second group of nozzles corresponds to the second location;   printing a second drop of the second liquid from the second nozzle of the printhead at the second location.       

     The method may include the additional step of printing repeat drops of the first liquid at a different location, the repeat drops being printed by a subset of nozzles belonging to the first group of nozzles. 
     Preferably, the subset of nozzles used for printing the repeat drops does not have identity with the nozzles used for printing drops of the first liquid at the first location. 
     A plurality of repeat drops of the first liquid may be printed at a plurality of locations. Preferably, the subsets of nozzles used for printing the repeat drops are cycled. 
     Preferably, the cycling of the subsets of nozzles used for printing the repeat drops is such that broken or disabled nozzles are avoided. 
     The ability to cycle the subset of nozzles used is a result of the linear, parallel printing method employed, and has the advantage that work can be shared between the nozzles. In addition, broken nozzles can be avoided without interrupting the printing process. 
     The method may include the step of superposing variable numbers of drops at particular locations. 
     According to a second aspect of the invention, there is provided a method of printing an array of spots on a substrate, the method comprising the steps of:
         displacing a printhead relative to the substrate along a first axis, the printhead having a linear array of nozzles oriented in the axis,   superposing drops of a first liquid at a first location on the substrate, wherein at least two drops of the first liquid are printed from distinct nozzles.       

     Preferably, the nozzles of the printhead are arranged in groups, each group comprising a plurality of nozzles, and each group corresponding to a particular liquid. More preferably, the nozzles in each group are contiguous. 
     The method may comprise the additional steps of:
         superposing drops of a second liquid at a second location on the substrate, wherein at least two drops of the second liquid are printed from distinct nozzles.       

     The method may comprise the additional steps of:
         superposing drops of a further liquid at a further location on the substrate, wherein at least two drops of the further liquid are printed from distinct nozzles.       

     The distinct nozzles may belong to a single group of nozzles. 
     The method may include the additional step of printing repeat drops of the first liquid at a different location, the repeat drops being printed by a subset of nozzles belonging to the first group of nozzles. 
     Preferably, the subset of nozzles used for printing the repeat drops does not have identity with the nozzles used for printing drops of the first liquid at the first location. 
     A plurality of repeat drops of the first liquid may be printed at a plurality of locations. Preferably, the subsets of nozzles used for printing the repeat drops are cycled. 
     Preferably, the cycling of the subsets of nozzles used for printing the repeat drops is such that broken or disabled nozzles are avoided. 
     The ability to cycle the subset of nozzles used is a result of the linear, parallel printing method used, and has the advantage that work can be shared between the nozzles. In addition, broken nozzles can be avoided without interrupting the printing process. 
     The method may include the step of superposing a variable number of drops at a particular location. 
     The methods of the first or second aspects may include a the steps of triggering the printhead by:
         transmitting a series of pulses from an encoder module to a processing module, the pulses corresponding to locations of the printhead, wherein the processing module receives pulses from the encoder module at intervals representative of a fixed unit of spatial displacement of the printhead;   transferring printing triggers from the processing module to the printhead at print intervals representative of a plurality of said fixed units of spatial displacement of the printhead.       

     According to a third aspect of the invention, there is provided apparatus for triggering a printhead, the printhead having a linear array of nozzles oriented in an axis, the apparatus comprising:
         a processing module controlling the transfer of printing triggers to the printhead;   an encoder module adapted to provide a series of pulses to the processing module, the pulses corresponding to locations of the printhead;   wherein the processing module receives pulses from the encoder module at intervals representative of a fixed unit of spatial displacement of the printhead, and the processing unit transfers printing triggers to the printhead at one or more print intervals representative of a plurality of said fixed units of spatial displacement of the printhead.       

     Preferably, the processing unit transfers printing triggers to the printhead at a plurality of print intervals. More preferably, the processing unit transfers printing triggers to the printhead at a plurality of different print intervals. 
     Preferably, the print intervals are selectable. 
     The fixed unit of spatial displacement of the printhead may be less than 20 μm. Preferably, the fixed unit of spatial displacement of the printhead is less than 5 μm. More preferably, the fixed unit of spatial displacement of the printhead is 1 μm. 
     Preferably, the print intervals are selectable within 1 μm. 
     According to a fourth aspect of the invention, there is provided a method of triggering a printhead, the printhead having a linear array of nozzles oriented in an axis, the method comprising the steps of:
         transmitting a series of pulses from an encoder module to a processing module, the pulses corresponding to locations of the printhead, wherein the processing module receives pulses from the encoder module at intervals representative of a fixed unit of spatial displacement of the printhead;   transferring printing triggers from the processing module to the printhead at print intervals representative of a plurality of said fixed units of spatial displacement of the printhead.       

     Put another way, the processing unit transfers printing triggers to the printhead at print intervals which correspond to a spatial displacement of the printhead that is equal to an integer multiple of the fixed unit of spatial displacement corresponding to an encoder pulse. That is, the processing unit receives a series of encoder pulses before transferring printing triggers to the printhead. 
     This has the advantage of reducing the amount of data processed by the printhead, and has a positive effect on printing speed. 
     Preferably, the processing unit transfers printing triggers to the printhead at a plurality of print intervals. More preferably, the processing unit transfers printing triggers to the printhead at a plurality of different print intervals. 
     Preferably, the print intervals are selectable. 
     The fixed unit of spatial displacement of the printhead may be less than 20 μm. Preferably, the fixed unit of spatial displacement of the printhead is less than 5 μm. More preferably, the fixed unit of spatial displacement of the printhead is 1 μm. 
     Preferably, the print intervals are selectable within 1 μm. 
     Preferably, the method includes the step of printing drops of liquid on a substrate in response to data received by the printhead. More preferably, the method includes the steps of printing an array of spots. 
     Preferably, the nozzles of the printhead are arranged in groups, each group comprising a plurality of nozzles, and each group corresponding to a particular liquid. More preferably, the nozzles in each group are contiguous. 
     The method may comprise the additional step of:
         transferring printing triggers to the printhead to at a series of print intervals corresponding to the spatial separation of the printhead nozzles.       

     The method may comprise the additional steps of:
         transferring printing triggers to the printhead to at a print interval corresponding to the required spatial separation of the printed drops.       

     The method may comprise the additional steps of:
         transferring printing triggers to the printhead to at a print interval corresponding to the required spatial displacement of the array of spots and a second, repeated array of spots.       

     The method may comprise the additional steps of:
         transferring printing triggers to the printhead to at a print interval corresponding to the required spatial displacement of the substrate and a second substrate;   printing drops on a second substrate.       

     The method may include the additional steps of:
         transferring print data to the printhead at said print intervals.       

     The method of the fourth aspect of the invention may include the steps of printing on a substrate by:
         displacing the printhead relative to the substrate along a first axis, the printhead having a linear array of nozzles oriented in the axis,   superposing drops of a first liquid at a first location on the substrate, wherein at least two drops of the first liquid are printed from distinct nozzles.       

     The method of the fourth aspect of the invention may include the steps of printing on a substrate by:
         printing a first drop of a first liquid from a first nozzle of the printhead at a first location;   displacing the printhead relative to the substrate along the axis such that the position of a second nozzle of the printhead corresponds to the first location;   printing a second drop of the first liquid from the second nozzle of the printhead at the first location.       

    
    
     
       There will now be described, by way of example only, various embodiments of the invention with reference to the following drawings, of which: 
         FIG. 1   a  is a representation of a printhead used in accordance with an embodiment of the invention; 
         FIG. 1   b  shows the printing action of the printhead of  FIG. 1   a;    
         FIG. 1   c  shows how the printhead of  FIG. 1   a  can be used to print a series of spots; 
         FIG. 2  is an example of a schematic representation of a system according to a conventional approach; 
         FIG. 3  is an example of a schematic representation of an image for parallel printing as seen by a single cycle printhead, using a fine encoder, according to a conventional approach; 
         FIG. 4  is an example of a schematic representation of a system according to the invention; 
         FIG. 5  is an example of a schematic representation of an image for parallel printing as seen by a single cycle printhead, using a fine encoder, according to the invention; 
         FIGS. 6   a  to  6   d  are examples of schematic representations of encoder signals between various components of the system of  FIG. 4 ; 
         FIGS. 7   a  to  7   c  are further examples of schematic representations of encoder signals between various components of the system of  FIG. 4 ; 
         FIG. 8  is an example of a schematic representation of an image for parallel printing with a single-cycle printhead, showing repeated spots, and selection of nozzles; 
         FIG. 9  is an example of a schematic representation of an image for parallel printing with a three-cycle printhead. 
     
    
    
       FIGS. 1   a  to  1   c  show a printhead  10  having a series of nozzles  12  oriented in a linear array  14 . In this example, the printhead contains 126 nozzles, labelled N 1  to N 126 , although not all of the nozzles are shown. The printhead moves over a substrate in the direction of the arrow  16 . 
     The circles  13   a  to  131  represent wells from which liquids are loaded into the printhead. The printhead loads twelve different liquids, one from each of the wells  13 . Each liquid is drawn into a group of seven contiguous nozzles, with three intermediate nozzles left unused. Groups of nozzles are labelled as  15   a  to  151  in  FIG. 1   c.    
       FIG. 1   b  shows how one group of seven nozzles is used to achieve superposition of up to seven drops onto one spot X 1  on a slide. The leading nozzle N 1  is triggered to print at the desired position, represented by the dotted line X 1 . The printhead advances in the direction of the arrow  16  until the second nozzle N 2  is at the spot position X 1 . The nozzle N 2  is triggered by a subsequent trigger from the printer electronics. The printhead advances again and the third to seventh nozzles N 3  to N 7  respectively superpose drops of liquid onto the spot X 1  on the substrate. It should be noted that the motion of the printhead is in alignment with the axis defined by the linear array of nozzles  14 , and thus displacement of the printhead by the correct distance causes an adjacent nozzle to be aligned exactly over the spot X 1 . 
     The other eleven groups of nozzles  15   b  to  151  print drops of the other liquids in a similar manner, at different locations on the substrate. The resulting spots can be of different sizes, by printing drops from different numbers of nozzles within each group. For example, it is possible to produce a row of spots X 1  to X 12  of the twelve liquids, at a selectable even spacing. The spots could be arranged in the same order as their groups of nozzles  15   a  to  151  within the printhead. Equally, a row of the twelve liquids in reverse order can be printed.  FIG. 1   c  illustrates the latter case. The first group of nozzles  15   a  prints a spot at position X 1  in the manner described above. Subsequently, the second group of nozzles  15   b  prints a spot when it has moved to a position past the original spot, shown as X 2 . Subsequent spots X 3  to X 12  are printed in the same manner resulting in a row of twelve spots of different liquids. 
       FIG. 2  shows schematically the component parts of an example system that uses a conventional triggering scheme, generally depicted at  30 . The system includes a processor  34 , a printhead  10 , and an encoder comprising an encoder readhead  31  and encoder tape  32 . 
     The encoder readhead  31  is linked to the relative movement of the printhead and the substrate via mechanical connection  39 . As the encoder readhead  31  travels along the encoder tape  32 , a series of pulses are generated to the processor along path  33 . The pulses correspond to equal intervals of motion, and thus each pulse corresponds to an exact position of the printhead in relation to the substrate. 
     Each pulse is transmitted to the printhead via printhead input line  38 , and triggers the nozzles to print the desired pattern at the corresponding position. Typically, the printhead will have a limited data handling capacity, and therefore data must be loaded to the printhead at intervals. As an example, the printhead may only be able to handle two sets of pixels simultaneously. In this case, while one pixel is printed, the next pixel is loaded to the printhead in preparation for receipt of the next triggering pulse. 
       FIG. 3  is a representation of an image for printing an array of spots of different liquids. The Figure shows an image for a printhead moving along an axis defined by the linear array of nozzles as shown in  FIG. 1 .  FIG. 3  shows a matrix having 126 rows R, and a number of columns C. Each row R corresponds to a nozzle on the printhead. Each column of the matrix corresponds to a set of commands to the printhead, with a “1” representing a command to the nozzle to print, and a “0” representing a command to the nozzle to not print. A single column represents the print commands to be carried out at one particular position of the printhead over the substrate, triggered by the corresponding encoder pulse. Adjacent columns represent print commands at different times, when the printhead is positioned over different points of the substrate. 
     As the printhead progresses over the substrate, a series of encoder pulses are sent to the printhead and cause the printhead to print in accordance with the columns of data. This results in successive drops being printed on the substrate. 
     In the context of this example, a single column of the matrix can be considered as a pixel of data within the image; the pixel consists of data that corresponds to a print/not print command to each of the nozzles of the printhead, at each encoder pulse generated by the printer electronics. 
     In the example illustrated in  FIG. 3 , the nozzle spacing is 137.14 μm, as in the case for the Xaarjet XJ126R, and the encoder pulses are 20 μm. Seven encoder pulses therefore correspond approximately to the nozzle spacing. For the superposition of drops from adjacent nozzles, as described with reference to  FIGS. 1   a  to  1   c , the nozzles will only be required to print on every seventh pulse. These “active pixels” are shown in  FIG. 3  in bold type, with all intermediate pixels being blank, that is, consisting entirely of non-print commands. 
     It becomes apparent from an examination of  FIG. 3  that the image for superposition of drops will contain a very large number of “blank pixels”, each corresponding to a command to a print nozzle to not print at the particular locations intermediate between the nozzles. In addition, the image will contain a large number of blank pixels in the region  21 , which corresponds to the time at which the second group of nozzles  15   b  (containing the second liquid) is travelling past the first printed spot X 1 . 
     Although the printhead does not print drops when a blank pixel is received, the “non-print” commands must still be processed at the printhead. This takes time, and thus has an impact on the overall printing speed. In practice, the linear speed of the printhead is determined by the number of pixels (blank or otherwise) that the printhead is required to process. The printing speed is therefore inversely proportional to the encoder pulse spacing. 
     More significant problems occur when an encoder with extremely fine encoder pulses is used. A fine encoder is desirable to enable the drop superposition to be more accurate, and for multiple spots of different liquids to be positioned with high precision. An example of this approach is the use of a 1 μm encoder. Drop superposition and spot positioning would then be superior. However, the corresponding image for a 1 μm encoder would have 136 blank pixels between each active pixel. This would place unacceptable demands on the printer electronics and reduce printing speeds to impractical levels. 
       FIG. 4  shows schematically the component parts of an example embodiment of the invention in one of its aspects, generally depicted at  40 . The system is similar to that shown in  FIG. 2 , with similar parts being referred to by the same reference numerals. However, the processing unit  34  includes additional apparatus in order to obviate the above-described problems. 
     The processing unit includes a suppression module  36  which receives the regular encoder pulse stream  33  from the encoder readhead  31 . The suppression module  36  is connected to the printhead  10  via image store  35 , into which the image file has been loaded previously, and printhead input line  38 . The processing unit in this example is an FPGA chip, although other suitable electronics may be used. 
     The suppression module includes a counter, adapted to prevent the transmission of some, but not all, of the encoder pulses to the printhead. In particular, the suppression module is programmed to suppress all encoder pulses corresponding to the blank pixels that would occur in the spaces between the nozzles. In doing so, the apparatus avoids the requirement for the printhead to process print/non-print data at each encoder pulse. 
       FIG. 5  shows an image representative of the image seen by the printhead in accordance with the present invention. As in  FIG. 3 , the matrix has a number of columns C representing pixels of print data, and a number of rows R corresponding to nozzles on the printhead. In  FIG. 5 , intermediate blank pixels do not form part of the image as seen by the printhead, because the corresponding encoder pulses are suppressed. Put another way, there are no pixels associated with the printhead positions and corresponding encoder pulses between the nozzle spacings. That is, there is no data in the gaps D between the active pixels. 
     In addition, it should be noted that there is no image data corresponding to the delay  21  shown in  FIG. 5 . 
     The pixels corresponding to pulses  7  and  8  are consecutive pixels, separated in space by the delay  21  caused by suppressing a number of encoder pulses from reaching the printhead. 
       FIGS. 6   a  to  6   d  show examples of how the encoder pulses may be suppressed by the electronics. 
       FIG. 6   a  shows the raw encoder pulse signal  50  and quadrature  51 . The raw encoder pulse is a square pulse with pulse period of 4 μm, used with quadrature to obtain an encoder pulse of 1 μm.  FIG. 6   a  is representative of the signal received by the processing unit  34  from the encoder readhead  31  via a connection  33  in  FIGS. 2 and 4 . 
     In the conventional approach, this signal would also be output from the processing unit  34  to the printhead  10  via printhead input line  38  in  FIG. 2 . This would severely hamper the printing speed of the device. However, suppression module  36 , shown in  FIG. 4 , does not allow every encoder pulse to pass through to the printhead  10 . The suppression module  36  includes a counter which counts the raw encoder pulses and allows output of an encoder pulse after a pre-determined number of input pulses.  FIG. 6   b  shows the signal transmitted to printhead  10  via printhead input line  38 . The suppression module only allows through every 137 th  pulse, a distance that corresponds to the spacing S between nozzles of the Xaarjet XJ126R printhead used. 
     The initial pulse  42   a  effects triggering of the printhead in accordance with the first pixel of data. In this example, the first pixel of data causes the first nozzle N 1  to print a drop of liquid on the substrate. 
     The second pulse  42   b  causes triggering of the printer in accordance with the next active pixel of data. In the example described, the second pixel data causes the second nozzle N 2  to print a drop of liquid after the printhead has travelled 137 μm, and therefore a second drop is superposed on the first printed drop. 
       FIG. 6   c  shows the encoder signal from the suppression unit to the printhead  10  on a larger scale. The Figure shows a series of ten pulses, generally depicted at  44 , each pulse separated by a 137 μm interval. The ten pulses cause ten triggering events at the printhead at 137 μm intervals. In the example shown seven of these triggers cause superposition of seven drops of liquid onto the same spot X 1 , with the remaining three triggers corresponding to the three unused nozzles between groups of nozzles  15   a  and  15   b.  Because the nozzles are unused, the corresponding pixels for these nozzles are always blank. 
       FIG. 6   d  shows the encoder signal from the suppression unit to the printhead on a yet larger scale. The figure shows a series of bursts of ten triggers, with the first burst corresponding to the superposition of drops onto a single spot X 1 . Each burst of triggers is separated by gap  45  which represents the distance that the printhead must travel before the second group of nozzles is in position to print drops of the second liquid at an adjacent spot X 2 . 
       FIGS. 7   a  and  7   b  show an example of how the suppression of the pulses relates to spaces between printing positions.  FIG. 7   a  shows a number of encoder pulses  61  transmitted to the printhead  10 . The first pulse,  61   a  occurs after an initial offset  62  which allows alignment of the first nozzle N 1  with the first position to be printed X 1 . Successive pulses  61   b  to  61   j  occur when successive nozzles are aligned with position X 1 . As explained above, encoder pulses intermediate to successive pulses are suppressed from reaching the printhead. The electronics suppress 136 encoder pulses to account for the nozzle pitch S of approximately 137 μm. This spacing is illustrated by space S on  FIG. 7   a.    
     When ten pulses have been received at the printhead, nozzle N 10  is aligned at position X 11 . The first nozzle N 11  of the second group  15   b  will not be required to print a drop of the second liquid until the printhead has moved so as to bring nozzle N 11  to position X 2 . The electronics initially suppress 136 encoder pulses corresponding to nozzle N 11  coming into alignment with position X 1 . Thereafter, a further suppression of pulses accounts for the space required between drops of successive liquids. This interliquid separation is shown in the Figure as P, and the distance S+P can be referred to as the interliquid offset  66 . 
     Although the distance S, and thus the inter-nozzle suppression is determined by the nozzle pitch, the distance P is a free choice. Indeed, the electronics can be configured to suppress any integer number of encoder pulses. This allows the spot separation to be determined by the programmer, and ultimately by the customer requesting the microarray. When the encoder used has a 1 μm pulse, the spacing between drops can be selected within 1 μm. This provides a highly configurable and flexible system. 
     Furthermore, the encoder pulse suppression can be used to provide an even more precisely located array by correcting any accumulated errors. For example, when using the Xaarjet XJ126R printhead, the printhead would accumulate a series of 0.14 μm errors. These errors can be accounted for by a carefully chosen interliquid offset. 
       FIG. 7   b  shows a series of bursts of triggers on a larger scale. Each burst of triggers  65  corresponds to triggering events to a group of nozzles. Intermediate spaces  66  are the interliquid offsets previously described with reference to  FIG. 6   d  and  FIG. 7   a.    
     When the printhead has printed a 1×12 array of spots of the twelve liquids on one slide, the pattern will be typically repeated immediately afterwards, a little further along the slide. The liquids may be printed in the same order as before, so that two (or more) adjacent patterns are formed on a single slide. The gap  67  between the repeated patterns need not be the same as the interliquid offset  66 . For example, the customer may desire a larger separation between repeated patterns. 
     This gap  67  may be referred to as the inter-repeat offset. During the inter-repeat offset, the encoder pulses can be suppressed while the printhead reaches correct position to print the repeated pattern. 
     When the printhead has printed a series of repeated patterns on one slide, the patterns will typically be repeated on second (and further) slides. An interslide offset is thus required for alignment of the printhead with the second slide. 
     The interslide offset is shown as space  77  on  FIG. 7   c . During the interslide offset, the encoder pulses are suppressed while the printhead travels over the space between two adjacent slides, and sufficiently into the second slide to provide the required border around the edge of the slide. 
     It will be appreciated that the interliquid, inter-repeat interslide offsets are each produced by varying the distance P for which the pulses are suppressed. In addition, the offsets can be varied independently of one another depending on the requirements of the customer. 
     The present invention can be used to suppress pulses to provide various offsets required for printing microarrays. In particular, the printhead will often be used for multiple passes over a set of slides, repeating the same patterns of spots at different locations. The electronics is configured to suppress pulses to the printhead at these “inter-pass” spaces required for alignment of the printhead for a subsequent pass. 
     The examples described above relate to the superposition of seven drops of each liquid onto each spot. However, the present invention is flexible with regard to the number of drops printed on each spot. 
       FIG. 8  is a representation of an image used in accordance with an embodiment of the invention. The Figure shows groups of nozzles whose chambers contain the same liquid, each group being used to superpose on the substrate a variable number of drops. 
     In  FIG. 8 , nozzles N 1  and N 2  are used to print drops of liquid  1 . The triggers occur at an interval of 137 μm, with intermediate encoder pulses suppressed from reaching the print head. The drop from nozzle N 2  is thus superposed on the drop from nozzle N 1 . 
     At a further interval of 137 μm, nozzle N 3  is aligned with the position X 1  at which the first two drops were printed. However, nozzle N 3  is not commanded to print, as demonstrated by the zero in the appropriate row. 
     After a further 137 μm interval, a drop is printed from nozzle N 4 . Subsequently, as the print head moves further over position X 1 , nozzles N 5  to N 7  receive triggers, but these triggers issue a non-print command. 
     The first seven columns therefore cause a spot to be printed at position X 1  consisting of three drops of liquid. The number of drops printed has a direct effect on the size of the printed spots. By controlling the number of drops printed, the user can control the size of the spot on the microarray. 
     Subsequently, following the inter liquid offset shown by the shaded region  81   a,  the second group of nozzles, nozzles N 11  to N 17 , is triggered by the burst of pulses allowed through to the printhead. It should be noted that the shaded region  81   a  does not correspond to data received by the printhead, but rather to a delay in which encoder pulses are suppressed. However, as shown in the Figure, only nozzles N 11 , N 13 , N 14  and N 15  are asked to print the drops, with nozzles N 15  to N 17  receiving non-print commands. The second group of nozzles therefore prints a spot consisting of four drops of the second liquid. 
     Following the second interliquid offset shown by shaded region  81   b,  the third group of nozzles prints six drops of the third liquid, with nozzle N 25  not being commanded to print. As above, shaded region  81   b  corresponds to a delay in which encoder pulses are suppressed. 
     In this way, the electronics can be configured to print spots of different liquids of variable sizes. In addition, if required, the interliquid offset can be adjusted to within one raw encoder pulse (in this example 1 micron) to provide different spacings for different sized spots. For example, larger spots may require a lengthened interliquid offset. 
       FIG. 8  also shows the printing of a repeated pattern of spots. This may be for example printing of the pattern of spots in a repeated pattern, and thus shaded region  82  represents an inter-repeat offset. As before, three out of the seven nozzles in the first group are printed from, resulting in a spot consisting of three drops. The second group of nozzles print a spot of four drops, and the third group of nozzles prints a spot of six drops. 
     Of significance is the fact that the nozzles used in the creation of the first and second spots of liquid need not be the same. For example, nozzles N 1 , N 2  and N 4  are used for printing the first spot of liquid one whereas nozzles N 2 , N 5  and N 7  are used for printing the second spot. In addition, the first spot of liquid two is printed from nozzles  11 ,  13 ,  14  and  15 , whereas the second spot of that liquid is printed from nozzles N 11 , N 13 , N 16  and N 17 . 
     It will be appreciated that similar cycling of the subsets of nozzles used can be carried out over further repeated patterns of spots on the same slide or subsequent slides. 
     This cycling of nozzle use offers a number of advantages as follows. 
     The cycling of the nozzles allows the workload to be shared. By using the nozzles equally, and using an approximately equal volume of liquid from each nozzle, the printhead is able to print an increased number of drops before reloading is required. In addition, since the reloading operation typically requires replacement of all the liquid in a group of nozzles, even usage of the liquids reduces the quantity of liquid wasted. Furthermore, cycling of the subsets of nozzles used places an even distribution of stress on the nozzles, and over many cycles the wear on the nozzles can be expected to be the same. This is likely to improve the working lifetime of the printhead. 
     A further advantage is that should any nozzles of the printhead become broken or temporarily disabled (eg blocked), the workload of the nozzles can be cycled such that the broken nozzles are avoided. In the example shown by  FIG. 8 , the use of nozzles N 3  and N 6  is avoided for all repeats. 
     Typically, a customer would be given a choice of drop size of one to five drops of liquid, which would allow the workload to be shared across the seven nozzles. If any nozzles became blocked, the surplus of two nozzles would enable the printhead to deliver the number of drops required by avoiding use of use of the blocked nozzles. 
     The examples described above relate to a printhead with a linear array of nozzles. The spacings and dimensions used in the examples are those that apply to the XaarJet XJ126R printhead. However, the examples relate to a single cycle printhead of the sake of simplicity. That is, the printhead has been treated as if it is possible to trigger all of the nozzles of the printhead simultaneously. 
     However, this is not how the XaarJet XJ126R operates; two adjacent nozzles cannot be triggered simultaneously. This arises because printhead shares walls between adjacent chambers, and it is the displacement of these walls which produces ejection of drops. 
     Instead the nozzles are labelled ABCABC etc, and the triggering takes place in three cycles A, B, and C. The A cycle triggers the all of the ‘A’ nozzles with a print command or a non-print command. The B cycle triggers all of the ‘B’ nozzles, and finally the C cycle triggers all of the ‘C’ nozzles. During the A cycle, the B and C nozzles cannot be printed, and vice-versa. 
     In one mode of operation, favourable for the triggering scheme of the present embodiment, a separate trigger is used at the printhead for each of the cycles. 
       FIG. 9  shows an example image for the three-cycle triggering system. As before, each column of the matrix corresponds to a set of commands to the printhead, with a “1” representing a command to the nozzle to print, and a “0” representing a command to the nozzle to not print. A single column represents one particular cycle of print commands to be carried out at one particular position of the printhead over the substrate. Each column of data is triggered by a corresponding encoder pulse. As before, encoder pulses between the pixels are suppressed from reaching the printhead. 
     The first column shows the first A cycle triggering nozzle N 1  to print. Nozzles N 4  and N 7  are also on the A cycle, and thus are triggered. However, N 4  and N 7  are not commanded to print. As explained above, the B and C nozzles are not triggered. Nozzle N 1  prints a drop at position X 1  on the substrate. 
     It should be noted that on the A cycle, all of the A nozzles (e.g. N 13 , N 16 , N 19 ) are triggered. However, in the simple example of  FIG. 9 , all of the remaining A nozzles receive non-print commands. 
     The B cycle triggers nozzles N 2  and N 5  (although only N 2  prints). The B-cycle occurs with the subsequent encoder pulse allowed through to the printhead. This occurs when the nozzle N 2  is aligned with printing position X 1 . Nozzle N 2  therefore superposes a drop of liquid onto the first printed spot X 1 . On the first C cycle, represented by the third column of the table, the nozzle N 3  is aligned with X 1 . The C cycle triggers nozzle N 3  to print, and nozzle N 6  not to print. 
     The first three printing cycles have completed, rendering a single command to each of the nozzles in the group. The three-column block can be considered as a single “pixel”, equivalent to a single column of the images shown in  FIGS. 3  or  5 . The present embodiment utilises the time shifting of the cycles necessary for the XaarJet printhead to superpose a drop onto a spot at each cycle. 
     The next A cycle triggers N 4  to print at position X 1 . The cycles continue, until N 7  is triggered to print at position X 1 . The first group of nozzles have superposed seven drops onto the position X 1 . 
     It will be appreciated that the principles outlined above in relation to the single-cycle printhead can also be applied to the 3-cycle Xaarjet case. In particular, it should be noted that the suppression of encoder pulses from reaching the printhead, as described above may be used. This is true of the inter-nozzle suppression, and the suppression that occurs at the interliquid offset, the interslide offset, and the inter-repeat offsets. 
     The present invention applies equally to the Xaarjet printhead used in two-cycle mode, that is with nozzles labelled ABABAB, triggered by A and B cycles. 
     Several modifications to the above-described embodiments are envisaged. In particular, the simple examples used for the purposes of explanation would in practice be developed to print complex patterns from the printhead. For example, microarrays would be printed on many slides, and could comprise many repeats of the same spot patterns. It is possible for many nozzles within different groups to print different liquids simultaneously at different points on the substrate. Such patterns will however need to account for constraints caused by the particular nozzle pitch on the printhead. 
     The present invention in its various aspects provides numerous advantages. 
     Firstly, the invention provides an improved method of superposing drops onto a substrate. The parallel orientation allows drops of liquid to be printed on to a single spot from different nozzles of the printhead. This provides an efficient and flexible printing process, and allows printing of spots of variable size. 
     The parallel orientation is also useful in that use of broken nozzles, or those temporarily disabled, can be avoided during printing by using others within each group instead. Furthermore, when the number of drops to be printed on a spot is less than the number of functioning nozzles in the corresponding group of nozzles, different subsets of the functioning nozzles can be used to print onto adjacent slides. 
     The present invention reduces the quantity of liquid wasted, and allows a larger number of drops to be printed before reloading the liquid is required. This results in a more efficient printing process. 
     The triggering scheme in accordance with one aspect of the invention improves the accuracy and precision of spot positioning and spot superposition. The suppression scheme reduces the amount of data sent to the printhead, thereby improving printing speed. The reduced amount of data results in a lower susceptibility to data errors. 
     Various modifications could be made to the above-described embodiments within the scope of the invention herein intended.