Printhead attachment system

Abstract

A printhead support structure may have a receiving portion to receive a printhead, first and second portions having an adjustment mechanism therebetween for converting a translational movement of the first portion to a rotational movement of the second portion, and a coupling mechanism coupling the second portion to the receiving portion for adjusting the rotational angle of the printhead. A method for adjusting a position of a printhead coupled to a printhead support may include applying a force to a first portion of the printhead support to effect a translational movement of the first portion, converting the translational movement of the first portion into a rotational movement of a second portion of the printhead support, and applying the rotational movement of the second portion to the printhead.

Claims

1. A printhead support structure, comprising: a receiving portion for receiving a printhead; first and second portions having a flexure therebetween configured to convert a translational movement of the first portion to a rotational movement of the second portion; and a coupling mechanism for coupling the second portion to said receiving portion for adjusting the rotational angle of the printhead; wherein: the first portion is coupled to a print carriage and constrained to move substantially along a first axis; and the second portion is fixed at an edge, such that the second portion is constrained to rotate about a second axis parallel to the first axis.

2. A printhead support structure according to claim 1, wherein the second portion is fixed at the edge by the flexure.

3. A printhead support structure according to claim 2, wherein the flexure is arranged such that a translational movement of the first portion along a first axis produces a force on the second portion in a direction perpendicular to the first axis, such that said force causes the second portion to rotate about a second axis parallel to the first axis.

4. A printhead support structure according to claim 3, wherein the second portion is coupled to the printhead such that the rotational movement of the second portion about a second axis provides a rotational movement of the printhead about an axis parallel to the second axis.

5. A printhead support structure according to claim 4, wherein the flexure comprises a pair of opposed flexure points with a diagonal linkage.

6. A printhead support structure according to claim 5, wherein the printhead has an array of a plurality of nozzles and wherein the rotational movement of the printhead is in the plane of the array of nozzles.

7. A printhead support structure according to claim 1, wherein the flexure is formed within the body of the printhead support structure.

8. A printhead support structure according to claim 1, wherein the printhead support structure retains the printhead in a fixed position after adjustment without an additional locking mechanism.

9. A printhead support structure according to claim 1, wherein the second portion is fixed at a first edge, such that a second edge of the second portion, opposed to the first edge, is constrained to rotate about the first edge; and wherein the flexure is arranged to provide a reduction ratio such that the magnitude of the translational movement of the second edge of the second portion and the magnitude of the translational movement of the first portion are in a ratio of less than one.

10. A printhead support structure according to claim 1, further comprising an adjuster screw arranged such that rotation of the adjuster screw provides said translational movement of the first portion.

11. A printhead support structure according to claim 1, wherein the printhead adjustment is actuated from a direction parallel to the axis of rotation of the printhead.

12. A printhead support structure according to claim 1, further operable to provide a translational movement of the print-head.

13. A printhead support structure according to claim 1, further comprising: a motor for effecting translational movement of the first portion.

14. A print assembly comprising: an array of a plurality of printheads arranged in a plane; and a printhead support structure according to claim 1 for each of said plurality of printheads for adjusting the position of each printhead; wherein each printhead adjustment is actuated from a direction perpendicular to the plane of the printhead array.

15. A method for adjusting the position of a printhead coupled to a printhead support, comprising the steps of: applying a force to a first portion of the printhead support to effect a translational movement of the first portion, wherein the translational movement is substantially along a first axis; converting said translational movement of the first portion into a rotational movement of a second portion of the printhead support by fixing the second portion at an edge, such that the second portion is constrained to rotate about a second axis parallel to the first axis; and applying said rotational movement of the second portion to the printhead.

16. A method for adjusting the position of a printhead according to claim 15, wherein said method further comprises the step of: retaining the printhead in a fixed position after applying said rotational movement to the printhead without locking.

17. A method for adjusting the position of a printhead according to claim 15, further comprising the step of: providing a translational movement of the printhead in a cross-process direction, wherein said translational movement of the printhead in the cross-process direction is calculated to compensate for the rotational movement applied to the printhead.

18. A method for adjusting the position of a printhead according to claim 17, wherein: said compensation for the rotational movement alters the effective axis of rotation of the printhead.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments will now be described, by way of example only and with reference to the accompanying drawings, in which:

(2) FIG. 1 illustrates the spacing of lines laid down by printhead nozzles when a printhead is correctly and incorrectly rotationally aligned;

(3) FIG. 2 illustrates the lines laid down by printhead nozzles in printheads that are not aligned in the along-process (print) direction;

(4) FIG. 3 illustrates a bell-crank mechanism for converting a vertical movement into a horizontal movement;

(5) FIG. 4 illustrates a printhead adjustment mechanism according to an exemplary embodiment;

(6) FIG. 5A illustrates the printhead adjustment mechanism of FIG. 4 in context within a printhead support structure from a first direction;

(7) FIG. 5B illustrates the printhead adjustment mechanism of FIG. 5A from a second direction;

(8) FIG. 5C illustrates the printhead adjustment mechanism of FIG. 5A from a third direction;

(9) FIG. 5D illustrates the printhead adjustment mechanism of FIG. 5A from the first direction after actuation of an adjustment;

(10) FIG. 5E illustrates the printhead adjustment mechanism of FIG. 5B from the second direction after actuation of an adjustment;

(11) FIG. 5F illustrates the printhead adjustment mechanism of FIG. 5C from the third direction after actuation of an adjustment;

(12) FIG. 6 illustrates a method for aligning or adjusting printheads.

(13) FIG. 7A illustrates a test print for a printhead which is incorrectly rotationally aligned;

(14) FIG. 7B illustrates a test print for a printhead which is correctly rotationally aligned;

(15) FIG. 7C illustrates a test print for printheads which are misaligned in the cross-process direction;

(16) FIG. 7D illustrates a test print for printheads which are correctly aligned in the cross-process direction;

(17) FIG. 8A illustrates a Fourier transform created from the test print of FIG. 7A;

(18) FIG. 8B illustrates a Fourier transform created from the test print of FIG. 7B;

(19) FIG. 8C illustrates a Fourier transform created from the test print of FIG. 7C;

(20) FIG. 8D illustrates a Fourier transform created from the test print of FIG. 7D;

(21) FIG. 9 illustrates a schematic diagram of a print carriage; and

(22) FIG. 10 illustrates a section of a typical test pattern.

DETAILED DESCRIPTION OF THE INVENTION

(23) FIG. 9 shows a schematic diagram of a print carriage 210. The print carriage 210 comprises printhead supports, to secure printheads to the print carriage and enable position adjustment of the printheads. In this schematic example, there are five printheads 220(a-e) attached to the print carriage 210, but there would typically be many more printheads attached to a print carriage, typically 50, 100 or even more printheads. Each printhead 220(a-e) has an array of nozzles 10. Printhead support portions 215(a-e) are also shown for each printhead 220(a-e). A set of conventional, right-hand orthogonal axes is shown. The nozzles 10 of the printheads 220(a-e) form an array in the x-y plane. In this example, the along-process direction is parallel to the x-axis, and the cross-process direction is parallel to the y-axis. The attachment of the printheads 220(a-e) to the printhead supports 215(a-e) may be accomplished, for example, by being clamped between portions of the printhead supports 215(a-e), by being screwed or bolted to the printhead support 215(a-e) material etc. The printheads 220(a-e) are individually replaceable and can be fitted separately.

(24) One way to releasably secure printheads to the printhead support structure, so that they can be easily removed individually is to provide one or more slides in the printhead support structure for engaging each printhead, e.g. dovetail slides. The printhead support structure includes a cavity for receiving part of the printhead, and the one or more sides may be provided on one or both edges of the cavity. When the printhead is inserted into the cavity, the printhead engages with the slide. When fully inserted, the printhead may then be secured. It is advantageous to provide a mechanism for securing the printhead automatically (e.g. a clamp arrangement or a latch), without the need for actuation, once the printhead has been fully inserted. Such securing means may, for example, comprise a spring-loaded clamp or a clamp comprising a flexure arrangement formed by cutting out portions of the printhead support, which provides sufficient force against the printhead body to secure the printhead within the printhead support portion. Normally the release of the printhead would have to be actuated, for example by depressing the spring to unclamp the printhead.

(25) Once a printhead 220(a-e) has been fitted, it is advantageous to adjust its alignment. This could be, for example, to compensate for manufacturing tolerances in the printheads 220(a-e), in the print carriage 210, or in the way the print carriage 210 is aligned with an entire printer assembly. Adjustment may also be necessary to compensate for mis-alignment created when the printhead is attached to the printhead support 215(a-e). Printheads are often tightly packed, which makes it difficult to access and adjust each individual printhead, except through an axis perpendicular to the plane of the nozzle array. Adjustment can be achieved by using printhead adjustment mechanisms within the printhead supports 215(a-e), which will be described in more detail below.

(26) In one adjustment, the printhead may need to be moved translationally, e.g. to adjust the cross-process alignment of printheads, i.e. requiring an adjustment in the y-direction. Advantageously, this should be done by applying an adjustment vertically through the plane of the nozzle array (from behind the printhead).

(27) The conversion of a vertical movement into a horizontal printhead translation can be made using a wedge or a bell-crank mechanism, as illustrated in FIG. 3. The bell-crank mechanism has a first crank arm 31 of a first length L.sub.1 in the y-direction and a second crank arm 32 of a second length L.sub.2 in the z-direction, connected together at a pivot point 33. A force F.sub.1 in the z-direction applied to the first crank arm 31, causes a small movement z of the first crank arm 31 in the z-direction. This is translated into a small movement y of the second crank arm 32 in the y-direction. By adjusting the relative lengths L.sub.1, L.sub.2 of the crank arms it is possible to create a very fine translational movement in the y-direction from a less fine vertical adjustment in the z-direction. This translational adjustment may be automated, e.g. by using a motor.

(28) The conversion of a vertical movement into a rotation about the vertical axis in order to effect a rotational adjustment is harder to achieve, particularly if the space available is limited, as is often the case in print carriages, particularly in the along-process direction. There is described herein an arrangement of flexural hinges fabricated in the printhead support 215. The flexural hinges may be combined with a diagonal link between a pair of flexures; the angle of the diagonal linkage can be used to convert a coarse vertical movement into a finer horizontal movement. The horizontal movement is then used to create a rotation about a vertical pivot axis.

(29) Referring to FIG. 4, an exemplary embodiment will now be described. FIG. 4 shows part of a printhead adjustment mechanism which may be used within the printhead supports 215(a-e) shown in FIG. 9. The printhead adjustment mechanism is formed of a section of the printhead support 215(a-e) structures shown in FIG. 9. The printhead adjustment mechanism is used for converting a movement or force in the z-direction into a force in the x-direction. This can be used to convert translational movement in the z-direction to rotational movement in the x-y plane A set of conventional right-hand orthogonal axes are assumed in this example. When installed in a printer assembly, an array of printheads would lie in the x-y plane, and the z-axis would be perpendicular to the array of printheads.

(30) The section of the printhead adjustment mechanism shown in FIG. 4 has a first portion 110, which is constrained to move predominantly in the z-direction, and a second portion 120, which is constrained to move predominantly in the x-y plane. Between these portions is a pivot portion having a first flexure 130 and a second flexure 140, which are diagonally opposed in the x-z direction. A diagonal linkage between the first flexure 130 and the second flexure 140 is at an angle to the x-direction. The flexures 130, 140 are formed by machining pockets in the printhead support 215 material, leaving thin sections of metal which act as a flexural pivot mechanism. The first portion 110 is the input side of the mechanism and its movement may be actuated by, for example, a screw with an axis along the z-direction being turned. The second portion 120 is the output side of the linkage and its movement can be used to effect a rotation about an axis parallel to the z-axis as described in more detail below, and hence effect the desired rotational adjustment of the printhead 220. The printhead 220 is in communication with the second portion 120; in one example, the printhead 220 is clamped or fixed directly to the second portion 120, in another example the printhead 220 is fixed to another portion of the printhead support structure, but be in contact with the second portion 120, such that movement of the second portion 120 will cause the printhead 220 to move. The pivot portion is configured such that a force in the z-direction z on the first portion 110, which causes the first portion 110 to move translationally in the z-direction, produces a force on the second portion in the x-direction x. The second portion 120 is fixed (not shown) along an edge in the z-direction, so the x-directional force x causes the second portion 120 to rotate about the fixing in the x-y plane. The fixing of the second portion 120 may, for example, be provided in the form of another flexure strip or hinge, as described in more detail below.

(31) The rotational movement of the printhead 220 provided by this arrangement will thus effect a rotation about the point at which the second portion 120 is fixed. FIGS. 5A, 5B and 5C show the printhead adjustment mechanism of FIG. 4 in context within a printhead support 215 structure in a first position. Each of these figures shows the printhead adjustment mechanism from a different direction; a set of conventional right-hand orthogonal axes are shown on each. FIGS. 5D, 5E and 5F show the printhead support structure 215 from the different directions shown in FIGS. 5A, 5B and 5C respectively, in a second position, after an adjustment to the rotational alignment of the printhead 220 has been actuated. Like reference numerals have been used to described like components across FIGS. 5A-F.

(32) FIG. 5A is a view from the y-direction, and shows an adjuster screw 170 in communication with the printhead adjustment mechanism. The printhead adjustment mechanism has a first portion 110, which is constrained to move predominantly in the z-direction, and a second portion 120, which is constrained to move predominantly in the x-y plane. Between these portions is a pivot portion having a first flexure 130 and a second flexure 140.

(33) FIG. 5B shows the printhead adjustment mechanism from the x-direction. Adjacent to the first portion 110 in the z-direction are two segments 150,152 which constrain the first portion 110 to move predominantly in the z-direction. Due to the construction of the segments 150, 152, a force on the first portion in the z-direction will in reality cause the first portion also to move slightly in the y-direction as it moves in the z-direction, such that it moves in an arc. In this example, each constraining segment 150, 152 has a flexure 154-157, at each end to allow movement substantially along the z-direction. FIG. 5E shows how the flexures and constraining segments allow the first portion 110 to move predominantly in the z-direction. Compared to FIG. 5B, the adjuster screw 170 in FIG. 5E has been advanced in the negative z-direction. The flexures 154, 155, 156, 157 have been bent to allow the left-hand side of the constraining segments 150, 152, and hence the first portion 110, to advance predominantly in the negative z-direction, but not significantly in the x- or y-directions.

(34) FIG. 5D shows how, when the first portion 110 is caused to advance in the negative z-direction, the first and second flexures 130, 140 bend to force the end of the second portion 120 to move in the negative x-direction.

(35) FIG. 5B also shows a fixing strip 125, which secures the second portion 120 to the printhead support 215 structure along an edge in the z-direction. This fixing strip 125 may, for example, also be formed of a flexure or flexural hinge, cut into the body of the printhead support 215. The fixing strip 125 ensures that one end of the second portion 120 cannot move in the x-direction so application of the force in the x-direction by the first portion 110 causes the second portion 120 to move rotationally in the x-y plane.

(36) Since the second portion 120 is constrained by the fixing strip 125 to move rotationally in an x-y plane, when the left-hand side of the second portion 120 is advanced in the negative x-direction, the entire second portion 120 moves rotationally around the fixing strip 125 in the x-y plane. This can be seen from FIGS. 5C and 5F, which show how the fixing strip 125 bends to allow the second portion 120 to move rotationally in an x-y plane. The second portion 120 is in communication with the printhead 220, such that rotation of the second portion 120 in an x-y plane causes rotation of the printhead 220 in an x-y plane and hence allows the rotational alignment of the printhead 220 to be adjusted.

(37) The mechanism is compact, as it only requires removal of material from the existing printhead support structure. Having such a compact adjustment mechanism means it is possible to pack the printheads in a very tight array, which improves the quality of printing, and the speed of printing in multi-pass printers.

(38) The arrangement of flexures with a diagonal linkage, as shown in FIG. 4, provides a reduction ratio to match the resolution of the mechanical actuation with the required printhead rotation. The diagonal linkage converts motion in the z-direction to motion in the x-direction in the ratio of the sides of the right-angled triangle having the diagonal linkage as hypotenuse; i.e. x=z*tan(). This allows the input of a fairly large actuation movement in the z-direction, to be converted into a smaller movement in the x-direction, so that the magnitude of the rotational movement of the outer edge (i.e. the edge opposed to the fixing strip 125) of the second portion 120 is smaller than the magnitude of the actuation movement, and hence allow adjustment of the printhead to a higher degree of accuracy. The ratio between the size of the movement of the second portion 120 in the x-direction (x) and of the movement of the first portion 110 in the z-direction (z) will be less than 1 for any <45, and becomes smaller as is reduced to 0.

(39) The flexures may be formed in the body of the printhead support or clamp. Wire erosion may be used to cut the flexures. In reference to the embodiment of FIG. 4, flexures in the x-axis direction can give movement in the y-z plane. Machined pockets are used to form flexures and linkages giving translational movement in the x-z plane and rotation parallel to the z-axis.

(40) In some embodiments, the adjuster screw 170 shown in FIGS. 5A-F may be a manually adjusted screw, used to apply the input z-axis actuation, and in alternative embodiments, motors (e.g. stepper motors) may be used to drive the adjuster screw 170. This has several advantages. A motor makes it possible to adjust the positioning of the printhead automatically, under computer control, and with no manual intervention, and potentially from a distance, for example over a network connection. Computers eliminate human error and can also perform tasks quicker than a human operator and/or control multiple tasks at once. This can be particularly advantageous in print arrays with many (e.g. 100+) printheads. By using stepper motors in combination with fine pitched leadscrews, the system remains in position when power is removed. This eliminates the need for a locking device. Commonly, adjustment systems require a cycle of unlock, adjust, lock. The locking phase normally produces some unwanted movement, making precise adjustment difficult. A locking step also makes systems harder to automate. The presently described mechanism avoids a locking step because flexures do not have any backlash or slop, unlike e.g. a sliding hinge, and therefore do not require a locking or securing component.

(41) The mechanical leverage provided by the diagonal linkage means that large forces on the printhead only produce small forces at the adjustment mechanism, and in particular the actuation means, i.e. the adjustment screw. This is another reason the printhead can remain correctly aligned without the need for locking.

(42) The mechanism can be designed in such a way that any sliding part involved in positioning the printhead is decoupled from the printhead through the levered flexure components with a ratio of less than 1 (e.g. by choosing a value of of less than 45). This means that any movement between the sliding elements (e.g. screws) caused by for example vibration, changing loads or thermal cycling is divided down with regard to resulting changes in printhead position. Therefore, the adjustment is fairly stable and readjustments are not often required. In some cases, it has been found that readjustment is not needed at all during the life of the printhead.

(43) It is possible to use the flexure arrangement described above to couple the translation and rotation actuations in order to effect a composite pure rotation about an axis parallel to the z-axis but passing through any desired point in the x-y plane (normally the centre of the x-y array of nozzles is chosen). This has the advantage that the two alignments can be made with the same adjustment so that alignment can be accomplished more quickly.

(44) The rotational movement of the printhead 220 provided by the arrangement described above in relation to FIGS. 4 and 5A-F will normally effect a rotation about the fixing strip 125 along which the second portion 120 is fixed. However, in certain situations the rotational adjustment is not required about this fixing strip 125. For example, it is often preferable to provide a rotational adjustment about the centre of the nozzle array, but it is hard to provide a fixing strip 125 which corresponds with the centre of the nozzle array. Therefore, to align a printhead correctly it can be necessary to also apply a translational adjustment. This can be provided by means of a bell crank, as described above in relation to FIG. 3.

(45) The translational movement may also be actuated from the z-direction by means of another adjuster screw, and this second adjuster screw may also be controlled by a motor.

(46) The presently described adjustment mechanism allows the actuation of the rotational printhead adjustment to be accessible vertically. I.e. printhead rotation about the z-axis can be actuated by a vertical movement in the z-direction. This allows adjustment of individual printheads, even when they are tightly packed in an array (i.e. a printhead array in an x-y plane).

(47) Matrices can be used to describe rotation and translation steps, and a specific example of how matrices can be used will now be described in a system which uses stepper motors to actuate the adjustment mechanism.

(48) When both rotational and translational adjustments are each actuated by a stepper motor, the desired rotation and translation, x.sub.i, can be achieved by applying steps, n.sub.j, to the two stepper motors. There is some degree of mechanical coupling between these motions, so the general relationship is of matrix form: x.sub.i=A.sub.ij n.sub.j, where A is a square matrix. The elements of the matrix A are determined by the geometry of the mechanical system. In most systems, the matrix will be non-singular and so possess an inverse. Given a desired adjustment in position and rotation, x.sub.i, the number of stepper motor steps to be applied to the adjustment axes is simply: n.sub.j=A.sup.1.sub.ji x.sub.i.

(49) The parasitic motions in the along-process direction (and possibly other directions) may be written as: y.sub.i=B.sub.ij n.sub.j, where B is a matrix, not necessarily square. We could also write y.sub.i=C.sub.ij x.sub.j where C.sub.ij=B.sub.ik A.sup.1.sub.kj. Hence, given a desired degree of adjustment, the number of stepper motor steps can be calculated directly and the size of the parasitic along-process motions resulting from these steps can also be calculated. Once the difference in along-process translational alignment (or parasitic offset) between neighbouring printheads is determined, it is possible to calculate how firing of the nozzles on different printheads should be delayed to ensure correct distribution of ink on the substrate.

(50) Image Analysis for Printhead Alignment

(51) The adjustments required to correctly align printheads can be calculated in several ways. One way is to print a test pattern and determine the alignment by capturing and analysing an image of the test pattern. Alternatively, a camera could be mounted on the printing apparatus (e.g. on the print carriage) to measure nozzle positions.

(52) A printed image can be analysed to locate the relative positions of the centroid of printed features (i.e. the printhead nozzles), from which the degree of adjustment needed can be calculated.

(53) The printed image analysis can include finding the Fourier transform of a printed pattern of lines of ink laid down by printhead nozzles. When correctly aligned, the Fourier transform should show a perfectly periodic structure. I.e. the Fourier transform would show the primary frequency and peaks corresponding to higher harmonics, but not to sub-harmonics. Poor alignment leads to sub-harmonics of the correctly aligned pattern periodicity. Interactive adjustments can be made to minimise the magnitude of the sub-harmonics.

(54) Inspection of the local density of a print can use an imaging resolution well below that of the printing grid. By careful choice of printed pattern it is possible to discriminate between along-process and cross-process direction misalignments. This is particularly useful as printhead adjustment is normally performed to achieve prints with no artefacts visible to the eye.

(55) Image analysis for printhead alignment will now be described in relation to one example embodiment. A 1200 dpi (47.2 dpmm) single pass printhead can provide full ink coverage across a substrate in the cross-process direction if all nozzles are fired simultaneously. Therefore, in order to provide a pattern which can provide information regarding rotational and translational alignment, a special test pattern is required.

(56) Test Patterns for Visual Inspection and Manual Adjustment

(57) In general, the lines that make up a test pattern should simply be printed from every nth nozzle, where n is not a factor of the number of rows of nozzles (i.e. the number of nozzle rows is not exactly divisible by n) on a printhead. In one example, when there are 32 rows of nozzles on each printhead, a row of lines may be printed from every 7th nozzle. In this case, odd and even nozzles are on different sides of the printhead, rotational inaccuracies will show up as twinning of the lines. This is shown in the test print of FIG. 7A, in which lines of ink 20 laid down by printhead nozzles appear in closely-spaced pairs. This shows the printhead is not correctly rotationally aligned. When correctly aligned rotationally, the twinning is no longer apparent and the lines are equally spaced, as shown in FIG. 7B.

(58) A real-time Fourier transform can be used to assist manual adjustment. When incorrectly aligned, the twinning gives a repeat period at half the spatial frequency of the correctly aligned image. Therefore minimising the sub-harmonic frequency leads to better rotational alignment.

(59) FIGS. 8A and 8B show Fourier transforms created from the test print images of FIGS. 7A and 7B, respectively. FIG. 8A, which corresponds to the misaligned printheads, shows strong frequency peaks (810 and 820) at 170 in.sup.1 (6.69 mm.sup.1) and 80 in.sup.1 (3.15 mm.sup.1) and weaker peaks (830, 840 and 850) at 140 in.sup.1 (5.51 mm.sup.1), 250 in.sup.1 (9.84 mm.sup.1) and 340 in.sup.1 (13.39 mm.sup.1). In FIG. 8B, which corresponds to the printheads being better aligned, there is a strong peak (810) at 170 in.sup.1 and weaker peaks (820 and 850) at 80 in.sup.1 (3.15 mm.sup.1) and 340 in.sup.1 (13.39 mm.sup.1).

(60) Referring to FIG. 8A, the first harmonic peak (810) is at spatial frequency 170 in.sup.1 (6.69 mm.sup.1) and the peak (850) at 340 in.sup.1 (13.39 mm.sup.1) is twice the harmonic spatial frequency (i.e. the second harmonic). Whereas the peak (820) at 80 in.sup.1 (3.15 mm.sup.1) corresponds to half the harmonic spatial frequency and the peak (840) at 250 in.sup.1 (9.84 mm.sup.1) corresponds to 1.5 times the harmonic spatial frequency. It can be seen that when printheads are correctly aligned (see FIG. 8B), the sub-harmonic frequencies 820, 830, 840, 850 that occur between the first and second harmonic peaks 810, 850 are significantly reduced.

(61) The image analysis process can set certain tolerances or thresholds for sub-harmonic frequencies and determine that the printhead is correctly aligned when these sub-harmonics are below certain threshold values.

(62) Translational adjustment can also be based on this approach by imaging the overlap region between two printheads which are rotationally aligned but are not correctly aligned in the cross-process direction. FIG. 7C shows the overlap region of a test pattern for printheads which are misaligned in the cross-process direction. FIG. 7D shows the same overlap region when the printheads are correctly aligned in the cross-process direction.

(63) The mismatch in the overlap region also gives rise to a sub-harmonic peak, which is minimised when the alignment is correct. FIGS. 8C and 8D show Fourier transforms created from the test print images of FIGS. 7C and 7D, respectively. In FIG. 8C, the first and second harmonic peaks (870, 890) at 170 in.sup.1 and 340 in.sup.1 can be seen. A strong sub-harmonic peak 860 at 80 in.sup.1 and a weaker sub-harmonic peak 880 at 250 in.sup.1 can also be seen. In FIG. 8D, which corresponds to the printheads being better aligned, the first and second harmonic peaks (870, 890) at 170 in.sup.1 and 340 in.sup.1 are still relatively strong, whereas the sub-harmonic peaks (860, 880) at 80 in.sup.1 and 250 in.sup.1 are much weaker.

(64) When test patterns are analysed for automated adjustment, the requirements differ from those for manual adjustment. For example, the processing time may be longer than for a system providing real-time feedback to a human operator. Additionally, the output used to re-position the heads must not need any human interpretation, i.e. the output instructions must be suitable to be input straight into the automatic adjustment means, e.g. motors.

(65) A section of another typical test pattern is shown in FIG. 10. Each row in the pattern has a short tick mark drawn for every 16th nozzle. There are 17 rows of tick marks, with the first and last rows coming from the same set of nozzles.

(66) An image processing program can analyse the image to identify the location of every tick mark and from this deduce the relative position and rotation of each printhead. This information can be used as input to the inverted matrix equation to drive each printhead directly to the correct degree of rotation and translation. A second image can be printed and processed to confirm the adjustment has been carried out to the required degree of accuracy and to perform further refinement, if needed.

(67) Test Patterns for Adjusting Alignment Based on Colour Density

(68) Test patterns can also be used to determine how well printheads of different colours are aligned to each other. An example test pattern for comparing alignment of black and magenta printheads may comprise a series of lines drawn by the black printheads on a print carriage. In this example, black lines would be printed from the top to the bottom of the image in the along-process direction. On top of these black lines would be drawn separate blocks of magenta lines, spaced apart in the along process direction, but each magenta block covering substantially the same width in the cross-process direction as the black lines. Each magenta block would be displaced slightly in the cross-process direction with respect to the block preceding it.

(69) When the lines from the magenta block fall directly on top of those of the underlying black pattern, there is a significant change in optical density, which can be judged either by eye, or by using a low resolution digital camera.

(70) In the example just given, alignment between different colours can be set. When aligning within a colour, a similar technique can be used, but with the pitch of the lines so selected that a maximum of optical density is achieved at the point of correct alignment.

(71) In another example, sets of black and yellow lines may be overprinted. Where the alignment is good, only black is visible, but where the alignment starts to drift out yellow colour tinges will be seen as the yellow is not fully occluded by the black.

(72) Typical Alignment Procedure

(73) A method for aligning or adjusting printheads within a printhead array on a print carriage using the above-described printhead adjustment mechanism will now be described in relation to FIG. 6.

(74) At step 405, the printhead adjustment mechanisms on a print carriage are set to their nominal central positions.

(75) At step 410, one or more printheads are fitted onto printhead support portions on the print carriage in a printhead array. The printheads may all be individually replaceable.

(76) At step 415, a test pattern from all printheads is printed. The test pattern will contain features printed by a set of nozzles from each printhead.

(77) At step 420, an image of the printed test pattern is captured using a camera system (e.g. linescan camera or conventional camera) and appropriate illumination.

(78) At step 430, image analysis software is used to measure the relative positions of the features printed by the nozzles. For example, if a printhead is incorrectly rotationally aligned with respect to the movement of the print carriage in the along-process direction, the lines of ink laid down by adjacent nozzles will not be equally spaced (as is described above in relation to FIG. 1). Additionally, if adjacent printheads are not correctly translationally aligned in the cross-process direction, lines of ink laid down by the nozzles on adjacent printheads will not be equally spaced. Errors in along-process alignment can also be detected in this step.

(79) At step 435, a determination, or decision, is made as to whether the printhead is sufficiently aligned. Printers may require different degrees of alignment in different situations, so it may be possible to set different alignment tolerances.

(80) If the alignment is sufficient, the printhead alignment method will end (step 455).

(81) If the alignment is insufficient, the alignment method proceeds to step 440, in which the rotational and translational adjustments required for each printhead are calculated from the measured positions. By providing details of the design and dimensions of printhead components (i.e. the nozzle array) to image analysis software, it is possible to calculate the adjustments needed to align within and between each printhead.

(82) At step 450, the correction steps required to apply the adjustments identified in step 430 to each printhead are calculated. This could comprise, for example, the size of the actuation movement in the z-direction, which should be applied to the first portion 110 of the adjustment mechanism. When a motor is used to provide the actuation movement, this step could output the specific movement required for the motor. Calculating the correction steps can be done using the matrix equations described above.

(83) At step 450, the timing of the printhead firing is adjusted to provide suitable compensation for the along-process (or parasitic) parasitic errors in printhead alignment.

(84) The method then returns to step 415 in order to measure and analyse the printhead alignment and adjust the alignment if the accuracy is insufficient.

(85) This method will continue until the desired accuracy of alignment is attained and this is determined in step 435. If the printhead adjusters have a low degree of backlash and hysteresis, then it should be possible to achieve adequately accurate alignment with a single stage of measurement and adjustment. For example, the combination of a stepper motor to turn a screw has little backlash or hysteresis.

(86) A method for determining the adjustment required for printhead alignment, may comprise some or all of the steps of: printing a test pattern from one or more printheads; capturing an image of the printed test pattern; analysing the image of the printed test pattern to determine the alignment of said one or more printheads; calculating the required printhead rotational adjustment; and calculating the correction steps required to perform said rotational printhead adjustment.

(87) Preferably, the analysing the image comprises performing a frequency analysis, for example Fourier analysis. The frequency analysis could also comprise identifying a first harmonic frequency and identifying one or more sub-harmonic frequencies. The first harmonic frequency can be identified by calculating the expected harmonic frequency based printhead nozzle separation or resolution.

(88) Preferably, the required printhead rotational adjustment comprises the adjustment which is required to minimise the one or more subharmonic frequencies.

(89) The printed test pattern can comprise a plurality of parallel features, which would normally extend in the along-process direction. When this is the case, the frequency analysis would comprise analysing the frequency of the parallel features.

(90) Whenever a subset of one or more printheads in the array is replaced, the same method can be applied. Ideally, it should only be necessary to adjust those printheads which have been replaced. However, with the use of an automated motorised system, there is little penalty in carrying out a complete re-alignment of the system.

(91) Any system feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure.

(92) Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to system aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination.

(93) It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.