WO1999022942A1

WO1999022942A1 - Printer

Info

Publication number: WO1999022942A1
Application number: PCT/SE1998/001960
Authority: WO
Inventors: Joachim Kretschmer; Gerhard Beurer
Original assignee: Xaarjet Ab
Priority date: 1997-10-31
Filing date: 1998-10-29
Publication date: 1999-05-14
Also published as: AU1057699A; SE511213C2; SE9703999D0; SE9703999L

Abstract

A method of operating a droplet deposition apparatus comprising a movable carriage (60), an actuator (100) having a plurality of channels provided with electrodes being adapted to receive electric signals to cause liquid in selected channels to be ejected therefrom, where the actuator is mounted on the carriage, and means (440) for selecting a mode of operation. The method comprising the steps: receiving print information; generating a first bit map from the print information; said first bit map defining first pixel positions to be targeted in a first mode of operation; said first pixel positions having a closest spacing Sx along a straight line a first matrix direction (x); generating a second bit map from the first bit map, said second bit map defining a group of pixel positions to be targeted in a second mode of operation; activating the carriage for motion, at a speed value dependent on a selected mode of operation, and causing liquid in selected channels to be ejected such that said group of pixel positions has a first closest spacing RSx along a straight line in the first matrix direction (x), where R is a positive integer equal to or higher than 2, and a second closest spacing along a straight line inclined at an angle β to the first matrix direction (x), said second closest spacing being less than RSx.

Description

PRINTER

Technical Field of the Invention

The present invention relates to a method of achieving ink jet printing and to a droplet deposition apparatus.

Description of Related Art

Inkjet printers are commonly used for achieving printing of word processing output as well as for printing images such as drawings. Inkjet printers operate to deposit ink droplets on a substrate so as to create a printout of a desired image.

It is convenient to refer to a matrix which represents all possible positions at which ink droplets can be deposited on the substrate. Within this matrix can be defined the direction of scanning movement of the printhead relative to the substrate. The spacing of matrix points in this horizontal direction of movement is related to the frequency of droplet ejection and the carriage speed. The spacing of matrix points in the perpendicular, vertical direction is determined by the printhead configuration. A standardized drop deposition value is 360 dpi, corresponding to a little over 14 dots per millimeter.

WO 95/07185 describes a print head and signal processing for causing a print head to deposit droplets at matrix points. According to WO 95/07185 print information is received in a Print Description Language (PDL) by a PDL interpreter which generates from the PDL input a bit map of the page to be printed. The bit map is received by a geometric transform unit which transforms the bit map to take account for angling of the print head and grouping of print head channels into phases. The trans- formed bit map is supplied to a wave form generator which derives for the selected channels in each group in succession, the wave form necessary to produce a required displacement of a corresponding channel side wall in the print head.

Summary

Images to be reproduced sometimes include straight line images. Modern word processing output, for example, often includes lines such as frames for tables. Such straight lines are commonly parallel or perpendicular to the direction of scanning movement of the printhead relative to the substrate.

The present invention addresses the problem of achieving a droplet deposition apparatus capable of achieving fast printouts with a high quality of the printed image.

An aspect of the invention relates to the problem of obtaining a droplet deposition apparatus capable of selectably reproducing an image with a first resolution or with a second, lower resolution in a shorter time, while eliminating droplet positioning errors and nrinimizmg distortion of the reproduced image

Yet an aspect of the invention relates to the problem of obtaining a droplet deposi- tion apparatus enabling a quick reproduction of an image with a decreased resolution while mamtaining accurate reproduction of straight line images at least in the horizontal and in the vertical directions.

These problems are addressed by increasing the scanning speed of the printhead relative to the substrate to obtain the quicker printout, and by adapting the print head control information to eliminate some pixels in the lower resolution printout. The eliminated pixels are selected such that image distortion and pixel positioning errors are avoided. An approximately halved printout time duration is obtained at the cost of halving the number of pixels in the reproduced image while doubling the printhead velocity relative to the substrate. The purpose of mamtaining accurate reproduction of straight line images and avoiding the "loss of a line" the print head is controlled such that not more than every second pixel position is eliminated in the vertical as well as in the horizontal direction when the number of reproduced pixels is halved.

These problems are adressed by a method of operating a droplet deposition apparatus comprising a movable carriage (60), an actuator (100) having a plurality of channels provided with electrodes being adapted to receive electric signals to cause liquid in selected channels to be ejected therefrom, where the actuator is mounted on the carriage, and means (440) for selecting a mode of operation. The method comprises the steps: receiving print information; generating a first bit map from the print information; said first bit map defining first pixel positions to be targeted in a first mode of operation; said first pixel positions having a closest spacing S_x along a straight line in a first matrix direction (x); generating a second bit map from the first bit map, said second bit map defining a group of pixel positions to be tar- geted in a second mode of operation; activating the carriage for motion, at a speed value dependent on a selected mode of operation, and causing liquid in selected channels to be ejected such that said group of pixel positions has a first closest spacing RS_X along a straight line in the first matrix direction (x), where R is a positive integer equal to or higher than 2, and a second closest spacing along a straight line inclined at an angle β to the first matrix direction (x), said second closest spacing being less than RS_X.

Brief Description of the Drawings

For simple understanding of the present invention, it will be described by means of examples and with reference to the accompanying drawings, of which:

Fig. 1 A shows a schematic block diagram of an embodiment of a printer including a print head with an actuator.

Fig. IB is a block diagram illustrating the printhead of Fig. 1A in connection with the actuator and a printer data interface.

Fig. 2 is a front elevational view of the actuator as seen in the direction of arrow Z in Fig. 1A.

Fig. 3 is an exploded partly diagrammatic perspective view of a portion of the actuator of Fig. 2, including an actuator plate.

Fig. 4 is a sectional perspective view of a part of the actuator plate of Fig. 3.

Fig. 5 A is a cross section of a part of the actuator of Fig. 3, with channels defined by side walls.

Fig. 5B illustrates the cross section of Fig. 5 A when some channels are in an ex- panded state.

Fig. 6 shows, schematically, the print head of Fig. 2 against the background of a print matrix.

Fig. 7 shows, schematically, the tilted print head of Fig. 6 at a time when one group of nozzles are in a printing position. Fig. 8 illustrates the relation between shuttle velocity and horizontal resolution.

Fig. 9 illustrates a pattern of pixel positions.

Fig. 10 is a schematic block diagram of an embodiment of the printer data interface shown in Fig. 1.

Fig. 11 A and 1 IB are flow charts illustrating an embodiment of a method of opera- tion of the printer shown in Fig. 1A.

Fig. 12 illustrates an example image to be printed.

Fig. 13 illustrates a portion of a bit map corresponding to the image of Fig. 12.

Fig. 14 illustrates the bit map portion of Fig. 13 together with an indication of nozzles used for achieving the pixel positions according to Fig. 13 in a first resolution mode.

Fig. 15 illustrates a tilted matrix grid superposed on the pixel positions of Fig. 9.

Fig. 16 illustrates an altered bit map for obtaining an altered resolution printout of the image shown in Fig. 12. Detailed Description of Embodiments

Figure 1A shows a schematic block diagram of a printer 10. The printer includes a data port 20 for receiving print orders. The data port is coupled to a printer data interface 30 via a databus 40. The printer 10 comprises an ink jet printhead 50 mounted in a movable shuttle unit 60. The shuttle unit 60 is arranged for reciprocal motion in a path 70. The printer data interface 30 controls a shuttle driver 72 to the desired shuttle speed and direction. The shuttle driver 72 may include e.g. a drive motor coupled to the shuttle by a drive belt (not shown).

The printer includes means for feeding a substrate 80 in a direction y perpendicular to the reciprocal motion of the shuttle unit. During actual printing the ink jet printhead outputs ink droplets and the shuttle as well as the substrate 80 is arranged to move relative to one-another so that dots of ink are deposited in precise patterns on the surface of the substrate 80. The printhead 50 is coupled to the printer data interface 30 via a plurality of electrical conductors 90.

The ink jet printhead 50 includes an actuator 100 and an actuator control unit 110. The actuator 100 includes a plurality of channels having nozzles for ejecting ink droplets (Figure IB).

Figure 2 is a front elevational view of the actuator 100. In the Figure 2 embodiment there are a plurality of nozzles 120, arranged next to each other in an array direc- tion. The nozzles are divided in three groups, called A, B and C phase. When a maximum amount of ink is to be ejected all of the A-phase nozzles are activated, thereafter all the B-phase nozzles are activated and all C-phase nozzles are activated. In other words the A-, the B- and the C-phase nozzles are activated in sequence. In order to compensate for the time delay between these phases, the print- head is tilted. The actuator is tilted in relation to the direction (x) of reciprocating movement of the shuttle. The actuator is tilted in relation to the x-axis so that the array direction of the nozzles forms an angle α to the x-axis.

Figure 3 is an exploded partly diagrammatic perspective view of a portion of the actuator 100. The actuator comprises an actuator plate 200 made from polarized piezo-electric material. The actuator plate 200 includes grooves of a rectangular cross section forming channels 220. The channels 220 are separated by side walls 230. A plate 265 provided with nozzle openings 120 is arranged such that all channels are provided with a nozzle. A cover plate 210 is cemented onto the actuator plate 200 so as to define, together with the walls 230, channels 220 with nozzles 120.

Electrical connections Dl, D2, D3... for activating the channel side walls 230 are made to the control unit 110 e.g. by bond wires as illustrated in Figure 3.

Figure 4 is a sectional perspective view of a part of the actuator plate 200. The bond wire Dl connects to a thin metal layer 270 on a surface of the actuator plate 200. The metal layer also covers a part of the surface of the wall 230 facing towards channel Cl as illustrated by the shaded area El in Figure 3. Another bond wire D2 connects to metal layers E2 in channel C2 in the same manner. The metal layers E2 form electrodes on the wall surfaces facing channel C2.

Figure 5 A is a cross section of a part of the actuator 100, as seen from the nozzle plate 265. Reference numeral 275 indicates the joint where the cover plate is ce- mented to a top portion of each wall 230 comprised in the actuator. Thus each wall 230 is firmly attached to the cover plate. The poling direction is indicated by arrow 240 in Figs 5 A and 5B. The channels can be activated individually as described above. Figure 5B illustrates channel C2 in an expanded state. The expansion is caused by an electric field between the electrode E2 and electrodes El and E3, respectively. Since the electric field caused in a portion 300 in the wall 230 between electrode E2 and electrode El is in a direction substantially perpendicular to the direction of po- larization, the portion 300 of the wall flexes in a shear mode to the position shown in Figure 5B. When the wall part 300 flexes, it also forces the complementary part 310 of the wall to bend in the same direction.

When channel C2 expands, it draws in more ink through the ink inlet 150 (best seen in Figure 3 and 5B).

Figure IB is a block diagram of an embodiment comprising the actuator 100, the actuator control circuit 110, printer data interface 30 and a power supply 320. The actuator control unit 110 comprises a plurality of controllable actuator drive signal outputs 330. Each actuator drive signal output is coupled to the electrode E of a corresponding channel in the actuator 100, as illustrated in Figure IB and described above.

The control unit 110 includes a data conversion unit 340. The data conversion unit comprises an input 350 for receiving print data indicative of the text or picture to be printed. The input 350 is adapted to be connected to the printer data interface 30 via the bus 90.

In response to print data received on the input 350 the data conversion unit 340 converts the print data into individual electrical control signals for each actuator channel 220. Different Resolution Printing Modes

According to the invention a certain print order can be executed in different manners. According to one embodiment the print order can be executed either by print- ing the image with a first resolution, or by printing the image with second, lower resolution. The low resolution printing mode provides the advantage of being quicker to perform.

Figure 6 shows, schematically, the tilted print head 100 against the background of a print matrix. The matrix points in the Figure 6 print matrix represents all possible positions at which ink droplets can be deposited on the substrate. The spacing of matrix points in the horizontal direction corresponds to the closest spacing S_x of ink droplets in the horizontal direction. The spacing of matrix points in the perpendicular, vertical matrix direction corresponds to the closest spacing S_y of ink droplets in the vertical direction. Each matrix point may correspond to a picture element, also referred as a pixel when printed on a substrate. The print matrix according to Figure 6 is herein referred as a parallel matrix, since the rows of the matrix are parallel with the direction x of movement of the actuator. The directions of shuttle movement will herein be referred to as the horizontal directions, or the x-directions (positive or negative) for the purpose of simplifying the understanding of the invention.

The actuator is tilted by an angle α in relation to the direction x of movement of the actuator. At a tilt angle α = 31 degrees a resolution of 360 dpi is obtained. The angle 31 degrees corresponds to a tilt ratio of 5/3, as seen in Figure 6.

Figure 7 shows, schematically, the tilted print head 100 with nozzles 120 against the background of the parallel matrix at the time when nozzles of group C are in printing position. As mentioned above, the printhead is inclined in relation to the direc- tion (x) of the shuttle movement, in order to compensate for the time delay between the phases A, B and C. The relative velocity between the printhead and the substrate is adjusted to the nozzle firing frequency and the resolution in the direction (x) of shuttle movement.

From Fig. 7 it is apparent that if nozzle C is fired at this instant, there will be an ink dot deposited at pixel position x=2 , y =2. Assuming movement in the positive x- direction (left to right) in Fig 7 at a normal speed of 352,8 mm/s, the B-nozzle must be fired with a delay corresponding to the time it takes to travel one third of the distance S_x in order to reach a pixel position such as (x,y) = (4,3); and the A-nozzle must be fired with a delay corresponding to the time it takes to travel two thirds of the distance S_x in order to reach a pixel position such as (x,y) = (6,4).

The firing frequency, the shuttle velocity and the tilt angle together with the spacing p (Fig 2) of the nozzles determine the horizontal and the vertical resolution of the printer. Fig. 8 illustrates the relation between the shuttle velocity and the horizontal resolution. Each curve in Fig. 8 corresponds to a selected firing frequency, i.e. the frequency of drop ejection from one nozzle.

The actuator is inclined by an angle α = tan^"1 [( N S_y)/ (iN +j)S_x] where i is a positive integer or zero, and j equals 1,2,3...N-l;

N is the number of nozzle groups, i.e. when a three-phase print head is used N = 3.

According to one embodiment N=3, and = tan^"1 [Y/X] = tan^"1 [3/5]. The tilted matrix is inclined by an angle β = 45 degrees, rendering the altered resolution to be de- creased by a factor two as compared to the full resolution. With full resolution mode the carriage speed is 352,8 mm s and with the altered resolution mode the carriage speed is doubled to 705,6 mm/s. The frequency of droplet ejection is 5000 Hz in both modes of operation. While printing phase order is CB A in the positive shuttle movement direction under normal resolution mode, the phase order is reversed (ABC) when the horizontal resolution is decreased by a factor two (2). It is also within the scope of the invention to decrease the horizontal resolution by a factor exceeding two. At a factor 4 the phase order will be CBA in the positive shuttle movement, and the carriage speed will be quadrupled.

Among the useful actuator tilt angles are α= tan^"1 [Y/X], where Y/X is: 1/3, 2/3; 4/3; 5/3; 7/3; 8/3; 10/3; 11/3; 13/3; 14/3 or 16/3.

A 360/180 dpi Embodiment

Fig 9 illustrates a pattern of pixel positions obtained with a 360 dpi print head under normal printing conditions, using the parallel print matrix shown in Figure 6.

If the shuttle velocity is increased, while keeping the firing frequency constant, the horizontal resolution is decreased. However, since the tilt angle of the print head is adjusted to the normal shuttle speed, a mere increase of shuttle speed would lead to a distortion of the printed image and to ink dot positioning errors.

The inventors have developed a printer drive method adapted to transform the image data received on the data port 20 (Fig. 1) into printer control signals such that this drawback is avoided. The printer drive method results in coordinated control signals to the shuttle driver 72 and to the actuator 100 such that an advantageously high quality image is reproduced.

Fig 10 is a schematic block diagram of an embodiment of the printer data interface 30 shown in Fig. 1. Print information is received at the data port 20, e.g. in a Page Description Language (PDL). The PDL may for example be of the type called Postscript. The data port 20 is coupled to a PDL interpreter 432 which generates a bit map of the image to be printed, typically in a form which is printer independent. The output of the PDL interpreter 432 is coupled to an input 433 of a transform unit 434 via a selector unit 440 A, 440B.

The transform unit 434 transforms the bit map to take account to parameters such as the nozzle actuation frequency, tilt angle of the printhead and the spacing p of the nozzles. Print control signals, including a transformed bit map, generated by the transform unit are delivered to output 480.

The actuator control unit 110 (Figure IB) derives, for the selected actuator chan- nels, the wave forms necessary to produce the required displacement of piezoelectric walls in response to the transformed bit map. The print head will usually be activated to eject droplets when moving in the positive x-direction, as well as when moving in the opposite direction (negative x-direction). The firing order of the nozzles during one movement direction will be A,B,C; and in the opposite movement direction it will be C,B,A.

The printer data interface 30 includes a print mode selection input 450 for selecting one of the alternatives "full resolution print mode" or "altered resolution print mode". The selection input 450 controls the selector unit 440 A, 440B such that unit 432 delivers its output directly to transform unit 434 when the "full resolution print mode" is selected. In this mode printing is performed with 360 dpi resolution in the horizontal as well as in the vertical according to one embodiment, as described in connection with Fig 9 above.

The selection input 450 also controls a selector 440C to set a shuttle speed control signal to a normal value when the "full resolution print mode" is selected, and to "altered speed" when "altered resolution print mode" is selected. The printer data interface 30 includes an output 490 for the control signal to the shuttle driver 72 (Figure 1A and 10). A selector 440D is set to "normal phase order" when "full resolution print mode" is selected, and when "altered resolution print mode" is selected the selector 440D is set to inverted phase order. The selected phase order setting is delivered to an input of the transform unit 434.

The transform unit 434 is coupled to the shuttle driver 72 (Fig. 1A), via port 490, for activating the shuttle driver 72 to move the shuttle in a desired direction and with a selected speed when a corresponding portion of the image is to be printed.

Additionally the printer data interface 30 includes a print order adapter 470 for generating an "altered resolution print mode" bit map. The selection input 450 controls the selectors 440 such that the adapter 470 is used when altered resolution print mode is selected.

Fig. 11 is a flow chart illustrating an embodiment of a method of operation of the printer 10.

With reference to Fig 11 A a print order is received at port 20 and delivered to PDL interpreter 432 (step S10). The PDL interpreter generates a bit map corresponding to the image (S20).

Fig 12 illustrates an example image to be printed. The exemplifying image is a rectangle, six units wide and four units high. Fig 13 illustrates schematically the corre- sponding bit map in a parallel grid. Hence, Fig 13 illustrates a portion of a bit map delivered by PDL interpreter 434. The filled crossings in the grid in Fig 13 indicates pixel positions to be printed if full resolution mode is selected.

According to one embodiment such a bit map includes a matrix having a plurality of matrix elements, each element corresponding to one pixel position. For the purpose of simplicity the following description relates to a black-and-white printout and a matrix where each matrix element is a bit value, each bit corresponding to one pixel position. When the value of the bit is "1" (one) the corresponding pixel position of the image is to be provided with a dot of ink, and when the bit value is "0" (zero) the corresponding pixel position is to be left blank. The method, however is also applicable to multicolor printing. Of course, several bit map matrices may be used when a multicolor image is to be printed, e.g. one matrix per color. Alternatively multicolor printing may be achieved by defining a matrix having several bits in each matrix element, e.g. one bit the red color, one bit for green color and one bit for blue color.

After step S20 the processing differs depending on which mode of printing has been selected (S30).

Full Resolution Print Mode

If full resolution mode is selected, the selectors 440A and 440B connect the output of PDL interpreter 432 to the input of unit 434, selector 440C is set to "normal shuttle speed" (step S40) and selector 440D is set to "normal phase order" (S50).

The transform unit 434 receives the bit map on input 433, and in response thereto it generates a transformed bit map suitable for sequential feeding to the actuator control unit 110 (step S60 in Fig 11A) in portions. According to one embodiment the transformed bit map comprises a plurality of one-sweep-portions of the bit map received on input 433. Each one-sweep-portion corresponds to a part of the image to be printed in a sweep of the actuator in one direction of motion. These one-sweep- portions of the bit map are delivered sequentially to a memory in the actuator control unit 110. According to another embodiment the transformed bit map includes portions of information relating to one printhead firing sequence. These portions are delivered to the controller 110 sequentially one at a time. The output from transform unit 434 additionally includes information on phase order setting (normal or inverted) and information about which direction of shuttle movement is to be used when printing the relevant portion of the transformed bit map.

A shuttle activation signal is delivered (step S70) to shuttle driver 72 via port 490, in correlation with the delivery (S80) of a portion of the transformed bit map, as illustrated in Fig 1 IB and 10.

The order in which the nozzle phases are activated depends on whether "full" or "altered" resolution mode is selected and on the shuttle movement direction. As mentioned above the transform unit 434 delivers information on phase order setting (normal or inverted) and shuttle direction (positive or negative) to the actuator control unit 110. The actuator control unit 110 checks the phase order instruction (S90) and, when (S100) normal phase order is selected the order in which the nozzles are activated for printing the relevant transformed bit map portion is C-B-A (step SI 10) at positive shuttle motion and A-B-C at negative shuttle motion(step S120).

Fig 14 indicates nozzles (A, B or C) used for achieving the pixel positions accord- ing to Fig 13 in full resolution mode.

Altered Resolution Print Mode

With reference to Fig 11, the inventive printer drive method is described below.

Steps SIO and S20 are performed as described above. When "altered resolution print mode" is selected (S30, Fig. 11) , the selectors 440A and 440B (Fig. 10) are set to connect the output of PDL interpreter 432 to the input 435 of unit 470, selector 440C is set to "altered shuttle speed" (step S130) and selector 440D is set to "inverted phase order" (S150).

The bit map, illustrated in Fig 13, is delivered to print order adapter 470 by PDL interpreter 432. The purpose of adapter 470 is to obtain a bit map adapted such that the printer can achieve a print out of the image in short time and with optimum print quality.

By setting the shuttle speed to an increased value ( Step S 130) the printout time is reduced. According to one embodiment the altered speed is twice the normal speed.

An altered bit map is generated (S140), and delivered to transformer unit 434. Additionally the order in which to actuate the A-, B- and C-phase channels during mo- lion of the shuttle is set to be inverted (step S 150 in Fig 11A and selector 440D in Fig 10). The reason for phase order inversion will be explained later.

Thereafter steps S60, S70 and S80 are performed as illustrated in Fig 11 and described above.

When a portion of the transformed bit map is received by actuator control unit 110 it generates corresponding drive pulse signals to the relevant channel walls of the actuator 100 (See Figure IB and 1 IB).

The nozzle actuation phase order depends on shuttle motion direction. When altered resolution mode is selected the interface 30 delivers an instruction "inverted phase order" to the actuator control unit, and for each sweep the interface 30 informs the actuator control unit 110 about the shuttle motion direction. The actuator control unit 110 activates the actuator channels in the order defined by this information (Step SI 60 in Fig 11B). Hence, when the actuator moves in the positive x-direction, i.e. from left to right in Fig 2, the phase order will be A-B-C in the altered resolution mode. This is illustrated at SI 70 in Fig 1 IB. When shuttle movement is the opposite direction at the increased speed, the phase order will be C-B-A.

After completion of a sweep, steps S70, S80, S90, SI 60 SI 70 and/or SI 80 are re- peated until the complete image has been printed. As illustrated by S 190 the print method includes a test of whether the image is ready. Step S 190 is preferably performed by unit 434 in connection with the sequential feeding of portions of the transformed image bit map to controller 110.

The Function of the Print Order Adapter

As mentioned above the bit map delivered by PDL interpreter 432 is received by print order adapter 470. In response to this bit map the print order adapter 470 generates an altered bit map adapted to activate only those nozzles which, in combina- tion, will be in printing position when the actuator 100 moves at the increased speed along the surface of the substrate 80.

The inventors realized that a printing can be achieved at double shuttle velocity without any dot position errors if the bit map is altered such that every second nor- mal pixel position is targeted in the vertical as well as in the horizontal direction, as illustrated in Fig. 15. In this manner an apparently halved resolution is obtained while avoiding any loss of vertical lines. For this purpose the adapter 470 receives a matrix having a binary bit for defining each pixel position, as exemplified in Fig 14. The adapter 470 sets every second pixel position value to "0" in the first row, and every second pixel position value in the second row is set to "0" in a staggered manner such that along a vertical (direction y) line not more than every second of any two neighboring pixel position values is set to "0".

Furthermore, the inventors realized that when the nozzles are actuated in the above described three-phase manner, a suitable selection of reachable pixel positions can be identified, as illustrated in Fig 15, by a tilted matrix grid superposed on the pixel positions from Fig 9. The crossings of the tilted grid indicates the pixel positions which are targetable. The rings in Fig. 15 indicate pixel positions which are targe- table in normal print mode, but which are not targeted in the altered resolution mode. In Fig 15 the x-axis indicates the direction of movement of the actuator

(carriage motion) and the y-axis indicates the direction of movement of the substrate (paper feed) in relation to the matrix. Since the rows of the matrix form an angle β to the direction x of movement of the actuator. This matrix is referred as a tilted matrix. The angle β for the tilted matrix has a value such that the tilted matrix does not coincide with the parallel matrix. In other words, the angle β has a value deviating from 90, 180, 270 and 0 degrees. According to a preferred embodiment the angle β for the tilted matrix is 45 degrees rendering an altered resolution decreased by a factor two as compared to the full resolution.

As illustrated in Fig 15 droplets of liquid can be deposited at pixel positions with a closest spacing S_x in the first matrix direction (x) and a closest spacing S_y in the second matrix direction (y) in the altered resolution mode as well as in the full resolution mode. In the full resolution mode the droplets are ejected with a closest spacing S_x along a straight line in the first matrix direction (x), as illustrated in Fig. 9.

In the altered resolution mode, however, the droplets are ejected with a closest spacing RS_x along a straight line in the first matrix direction (x). Pixel positions along any two neighbouring straight lines in the first matrix direction (x) are mutu- ally staggered. In the example illustrated in Fig.15 the resolution is halved and accordingly R = 2. Although described by an example of halved resolution in the above, the scope of the invention includes other alterations of the resolution where R is a positive integer equal to or higher than 2.

Fig 16 illustrates the altered bit map generated by unit 470 (Fig 10) in response to the exemplifying bit map illustrated by Fig 13.

Claims

1. A method of operating a droplet deposition apparatus comprising: a movable carriage (60); an actuator (100) having a plurality of channels provided with elec- trades being adapted to receive electric signals to cause liquid in selected channels to be ejected therefrom; the actuator being mounted on the carriage; means (440) for selecting a mode of operation; the method comprising the steps: receiving print information; generating a first bit map from the print information; said first bit map defining first pixel positions to be targeted in a first mode of operation; said first pixel positions having a closest spacing S_x along a straight line in a first matrix direction (x); generating a second bit map from the first bit map, said second bit map defining a group of pixel positions to be targeted in a second mode of operation; activating the carriage for motion, at a speed value dependent on a selected mode of operation, and causing liquid in selected channels to be ejected such that said group of pixel positions has a first closest spacing RS_X along a straight line in the first matrix direction (x), where R is a positive integer equal to or higher than 2, and a second closest spacing along a straight line inclined at an angle ╬▓ to the first matrix direction (x), said second closest spacing being less than RS_X.

2. The method according to claim 1, wherein the angle ╬▓ has a value deviating from 90, 180, 270 and 0 degrees.

3. The method according to claim 1 or 2, wherein the angle ╬▓ has a value in the range 40 to 50 degrees.

4. The method according to claim 1,2 or 3, wherein the channels are arranged in N groups (A, B, C) which are successively actuated for ejection in respective print phases; the method comprising the step: setting said actuation succession (ABC; CBA) in dependence on a selected mode of operation.

5. A droplet deposition apparatus comprising: a movable carriage (60); a driver (72) for causing the carriage to move; an interface unit (30) including an input (433, 435) for receiving image information defining pixel positions upon which droplets of liquid are to be deposited; an actuator (100) having a plurality of channels provided with electrodes being adapted to receive electric signals to cause liquid in selected channels to be ejected therefrom; the actuator being mounted on the carriage; the interface unit (30) co-operating with the driver and the actuator

(100) such that in operation said actuator (100) ejects liquid droplets at pixel positions on a substrate in dependence of said image information; the apparatus further comprising: an adapter unit (470) which is devised to control the actuator (100) such that not more than every second pixel position is targeted in the vertical as well as in the horizontal direction.

6. The droplet deposition apparatus according to claim 5, wherein the adapter unit (470) is devised to control the actuator (100) such that liquid droplets ejected by neighboring channels are positioned in mutually staggered pixel positions.

7. A method of operating a multi-channel droplet deposition head (50) adapted for scanning movement in a first matrix direction (x) relative to a substrate upon which droplets of liquid are to be deposited at pixel positions in a second matrix direction (y) with a closest spacing S_x in the first matrix direction (x) and a closest spacing S_y in the second matrix direction (y); the second direction being perpendicular to the first direction; the head (50) comprising : parallel channels having respective side walls which extend in a lengthwise direction of the channels separating one from the next of the channels in an array direction; a series of nozzles which are spaced from one another and which communicate respectively with the channels for ejection of droplets of liquid from the channels; electrically actuatable means for effecting, on selection of a channel, droplet ejection therefrom; said channels being arranged in N groups (A, B, C) which are successively actuated for ejection in respective print phases; wherein the array direction of the head is inclined at an angle ╬▒ to the first ma- trix direction (x) given by: ╬▒ = tan^"1 [( N S_y)/ (iN +j)S_x] where i is a positive integer or zero, and j equals 1,2,3...N-1; the method comprising the step: ejecting droplets by any one nozzle no more frequently than once in RN print phases, where R is a positive integer equal to or higher than 2, such that the droplets are ejected at pixel positions along a straight line in the second matrix direction (y) with a closest spacing RS_y and with a closest spacing RS_X along a straight line in the first matrix direction (x).

8. The method according to claim 7, wherein two pixel positions in neighbouring straight lines in the first matrix direction (x) have a closest spacing value h; said closest spacing value h being less than said closest spacing RS_X.

9. A method of operating a droplet deposition apparatus comprising: a carriage (60) movable in a first matrix direction (x); an actuator (100) having a plurality of channels provided with electrodes being adapted to receive electric signals to cause liquid in selected channels to be ejected therefrom; the actuator being mounted on the carriage; means (440) for selecting a mode of operation; the method comprising the steps: receiving print information; generating a first bit map from the print information; said first bit map defrning first group of pixel positions to be targeted in a first mode of operation; said first pixel positions having a closest spacing S_X along a straight line in the first matrix direction (x); generating a second bit map from the first bit map, said second bit map defining a second group of pixel positions to be eliminated from the first group in a second mode of operation; activating the carriage for motion, at a speed value dependent on a selected mode of operation, and causing liquid in selected channels to be ejected in response to said second bit map such that not more than every second pixel position is eliminated in the first matrix direction (x), and such that liquid droplets ejected by neighboring channels are positioned in mutually staggered pixel positions.