Inkjet printer

Abstract

An inkjet printer comprises a transport mechanism (120) for transporting a substrate along a first axis, a print engine (160) comprising at least two inkjet print bars (165.1 . . . 8) arranged along the first axis, and a controller for controlling the ejection of ink by the at least two inkjet print bars. It further comprises at least two first encoders for the determination of at least two first positions of substrate locations along the first axis and a compensation module for processing the at least two determined first positions to generate at least one first individual compensation parameter for each of the at least two inkjet print bars (165.1 . . . 8). The first compensation parameters are transmitted to the controller to influence the ejection of ink in such a way that effects due to variations of the at least two first positions of the substrate along the first axis are compensated. The effects of positioning errors that may affect different print bars (165.1 . . . 8) arranged along the longitudinal axis of the print engine (160) differently may be compensated using the encoders, individually for the different print bars (165.1 . . . 8).

Claims

1. Inkjet printer comprising a) a transport mechanism for transporting a substrate along a first axis; b) a print engine comprising at least two inkjet print bars arranged along the first axis; c) a controller for controlling the ejection of ink by the at least two inkjet print bars e) at least two first encoders for the determination of at least two first positions of substrate locations along the first axis; f) a compensator that processes the at least two determined first positions to generate at least one first individual compensation parameter for each of the at least two inkjet print bars; wherein the first compensation parameters are transmitted to the controller to influence the ejection of ink in such a way that effects due to variations of the at least two first positions of the substrate along the first axis are compensated, wherein a longitudinal extension of the at least two inkjet print bars of the print engine extends parallel to the first axis and wherein the inkjet printer comprises a scanning mechanism for moving the print engine along a second axis perpendicular to the first axis.

2. Inkjet printer as recited in claim 1, comprising at least three inkjet print bars arranged along the first axis.

3. Inkjet printer as recited in claim 1, the transport mechanism for transporting the substrate comprising a plurality of rollers supporting the substrate, the at least two first encoders being assigned to at least two of the rollers.

4. Inkjet printer as recited in claim 3, wherein the at least two of the rollers have a different circumference.

5. Inkjet printer as recited in claim 1, comprising at least two second encoders for the determination of at least two second positions of the at least two inkjet print bars along the second axis, wherein the compensator is configured to process the second positions to generate at least one second individual compensation parameter for each of the at least two inkjet print bars and wherein the second compensation parameters are transmitted to the controller to influence the ejection of ink in such a way that effects due to differences of the at least two second positions from predetermined reference positions are compensated.

6. Inkjet printer as recited in claim 1, wherein the first compensation parameters are spatial offset values and in that the controller selects ejection nozzles of the print bars to be used according to the spatial offset values.

7. Inkjet printer as recited in claim 1, wherein the second compensation parameters are temporal offset values and wherein the controller time shifts ejection of ink of one or several of the print bars according to the temporal offset values.

8. Inkjet printer as recited in claim 1, the transport mechanism comprising a plurality of rollers supporting the substrate.

9. Inkjet printer as recited in claim 8, herein an adhesive belt for attaching the substrate is supported on the plurality of rollers.

10. Inkjet printer as recited in claim 8, wherein the plurality of rollers are provided with a cooling mechanism.

11. Inkjet printer as recited in claim 1, wherein the transport mechanism for transporting the substrate comprises a plurality of rollers supporting the substrate, the at least two first encoders being assigned to at least two of the rollers, the at least two of the rollers having a circular cross-section and having chosen different diameters leading to a different circumference, the compensator taking into account during processing the different circumference for distinguishing between effects originating at the different rollers.

12. Inkjet printer comprising a) a transport mechanism for transporting a substrate along a first axis; b) a print engine comprising at least two inkjet print bars arranged along a longitudinal axis of the print engine; c) a scanning mechanism for moving the print engine along a second axis, the second axis being oriented perpendicular to the first axis; d) a controller for controlling the ejection of ink by the at least two inkjet print bars; wherein the longitudinal axis of the print engine runs parallel to the first axis, wherein each of the inkjet print bars comprises an ink nozzle array, a main extension of which extending along the longitudinal axis, wherein the main extension of the nozzle array has a length L, in that the inkjet print bars are arranged on the print engine in such a way that a longitudinal distance d between a start of the nozzle array of a first inkjet print bar and a start of a nozzle array of a second inkjet print bar, adjacent to the first inkjet print bar, is larger than the length L of the nozzle array, wherein the controller is programmed to control the transport mechanism to effect a main transport feed of length L between scans of subsequent bands, and wherein d L+x, where x=(m/n) L, where n is the number of inkjet print bars and m is an integer number, where 1<m<n.

13. Inkjet printer as recited in claim 12, the transport mechanism comprising a plurality of rollers supporting the substrate.

14. Inkjet printer as recited in claim 13, wherein an adhesive belt for attaching the substrate is supported on the plurality of rollers.

15. Inkjet printer as recited in claim 13, wherein the plurality of rollers are provided with a cooling mechanism.

16. Inkjet printer comprising a) a transport mechanism for transporting a substrate along a first axis; b) a print engine comprising at least two inkjet print bars arranged along the first axis; c) a controller for controlling the ejection of ink by the at least two inkjet print bars; c) at least two first encoders for the determination of at least two first positions of substrate locations along the first axis; a compensator that processes the at least two determined first positions to generate at least one first individual compensation parameter for each of the at least two inkjet print bars; wherein the first compensation parameters are transmitted to the controller to influence the ejection of ink in such a way that effects due to variations of the at least two first positions of the substrate along the first axis are compensated and wherein the transport mechanism for transporting the substrate comprises a plurality of rollers supporting the substrate, the at least two first encoders being assigned to at least two of the rollers and wherein a longitudinal extension of the at least two inkjet print bars of the print engine extends parallel to the first axis and in that the inkjet printer comprises a scanning mechanism for moving the print engine along a second axis perpendicular to the first axis, wherein the inkjet printer comprises at least two second encoders for the determination of at least two second positions of the at least two inkjet print bars along the second axis, wherein the compensator is configured to process the second positions to generate at least one second individual compensation parameter for each of the at least two inkjet print bars and wherein the second compensation parameters are transmitted to the controller to influence the ejection of ink in such a way that effects due to differences of the at least two second positions from predetermined reference positions are compensated.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The drawings used to explain the embodiments show:

(2) FIG. 1 A schematic representation of a first basic layout of an inkjet printer according to the invention;

(3) FIG. 2 a schematic representation of a second basic layout of an inkjet printer according to the invention;

(4) FIG. 3 a side view of an inkjet printer according to the invention including upstream and downstream stations;

(5) FIG. 4 a side view of the transport mechanism, the scanning mechanism and the print engine of the inkjet printer;

(6) FIG. 5 an oblique view of the inkjet printer;

(7) FIG. 6 a top view of the inkjet printer;

(8) FIG. 7 a front view of the inkjet printer;

(9) FIG. 8 a block diagram illustrating the compensation of variations of the substrate position; and

(10) FIG. 9 a schematic representation of the print engine position in scans of subsequent printing bands and of the resulting colour bands.

(11) In the figures, the same components are given the same reference symbols.

Preferred Embodiments

(12) The FIG. 1 is a schematic representation of a first basic layout of an inkjet printer according to the invention. The inkjet printer 100 comprises a transport mechanism 120 for transporting a substrate 1 along a main transport direction 2. It further comprises a scanning mechanism 140 for moving a print engine 160 along a scanning axis 3. The scanning axis 3 is perpendicular to the main transport direction 2. The print engine 160 comprises 8 print bars 165 arranged in a row, the row as well as the main extension of the print bars 165 being parallel to the main transport direction 2. In a given scan, each of the print bars 165 will act onto another region of the substrate 1.

(13) The FIG. 2 is a schematic representation of a second basic layout of an inkjet printer according to the invention. The inkjet printer 200 comprises a transport mechanism 220 for transporting a substrate 1 along a main transport direction 2. It further comprises a print engine 260 comprising four print bars 265.1, 265.2, 265.3, 265.4 extending in a direction perpendicular to the main transport direction 2 and covering the entire width of the substrate 1. Every print bar 265.1 . . . 4 comprises a number of inkjet print heads, arranged in a staggered pattern, in a manner known as such. The inkjet print heads cover the entire width of the substrate 1 to be printed on.

(14) A specific embodiment of an inkjet printer according to the invention is described in connection with FIGS. 3-7. This inkjet printer follows the first basic layout as shown in FIG. 1.

(15) The FIG. 3 is a side view of an inkjet printer according to the invention including upstream and downstream stations. The FIG. 4 is a side view of the transport mechanism, the scanning mechanism and the print engine of the inkjet printer. The FIG. 5 is an oblique view of the inkjet printer, the FIG. 6 a top view and the FIG. 7 a front view.

(16) The inkjet printer 100 is arranged in a printing line, in between a feeder 300 for unwinding and tensioning the textile substrate and a drying station 400 for drying the printed substrate. The inkjet printer 100 comprises a supporting belt 121, running around two main rollers 122, 123, one of the rollers 122 being provided with a servo drive motor 124. The supporting belt 121 is tensioned by appropriate tensioning rollers. The top surface of the transport mechanism 120, cooperating with the print engine 160, is formed by 9 support rollers 131.1 . . . 131.9. They are arranged such that they form an arc-shaped surface, a first support roller 131.1 is arranged in a position facing the leading end of a first of the print bars 165.1 of the print engine 160, a last support roller 131.9 is arranged in a position facing the trailing end of a last of the print bars 165.8, and the further support rollers 131.2 . . . 8 are arranged in positions facing a gap between adjacent print bars 165.1 . . . 8.

(17) The substrate is adhered to the supporting belt 121 by a suitable adhesive and an application of pressing force by an attaching device 125 in the region of the roller 123 on the infeed side, and it is separated again from the supporting belt 121 by a separating device 126 in the region of the roller 122 on the discharge side. The belt is cleaned, in particular freed from residual adhesive by a cleaning device 127 arranged essentially below the roller 122 on the discharge side.

(18) The roller 123 on the infeed side, adjacent to the feeder 300 as arranged at a height lower than the roller 122 on the discharge side, adjacent to the drying station 400. This allows for directly feeding the substrate to the drying station 400 at an appropriate height and feed angle.

(19) The inkjet printer 100 further comprises a scanning mechanism 140 comprising a main support 141 provided by a linear guide mechanism 142 for supporting the print engine 160 and driven by a linear drive known as such. The linear guide 142 comprises three parallel rails attached to the main support 141 cooperating with three parallel roller bearing mechanisms. This allows for moving the print engine 160 along a scanning axis 3. On both sides of the main support 141 a cable chain 143, 144 is provided comprising ink and water lines as well as power and network cables for supplying and controlling the print bars of the print engine 160.

(20) The print engine 160 comprises a main frame 161 which extends along the longitudinal direction of the print engine 160 and to which the three roller bearing mechanisms are fixedly attached. The main frame 161 includes a lattice-like flat structure defining an essentially vertical plane. To one side of the structure, the print bars 165.1 . . . 165.8 are attached (cf. FIG. 5).

(21) Each of the print bars 165.1 . . . 8 comprises 4 drop-on-demand inkjet print modules, each module comprising an inkjet print head.

(22) In a manner known as such, the bottom surfaces of the print heads as well as the respective nozzle arrays are both rhomboid shaped, the nozzle arrays being slightly slanted with respect to the longitudinal axis of the print bar. This allows for having a seamless transition between the two adjoining modules. Accordingly, a continuous uniform ink nozzle array is formed, the usable width (perpendicular to the longitudinal axis of the print bar) being 40.6 mm (2048 nozzles, 1200 dpi), the usable length being 173.4 mm (8192 nozzles, 1200 dpi). The print engine 160 has a total length of 1400 mm, a width of 100 mm and a height of 300 mm.

(23) The compensation of variations of the substrate position along the main transport direction 2 is described in the following. It is schematically shown in the block diagram of FIG. 8. The basis for the compensation are values obtained from encoders linked to the two rollers 122, 123, i. e. a first encoder value 21 obtained from roller 122 and a second encoder value 22 obtained from roller 123. First, a time series of measurements of encoder values 21, 22 is filtered by a median filter. Correction values are obtained by determination of the difference between the filtered measurements and a straight reference line (encoder step analysis 41, 42). The reference line is obtained from a least-square fit with respect to the filtered measurements. The correction values are dependent from the velocity. Therefore, in a next step, the correction values are compensated by the respective velocity to obtain velocity-independent correction values 23, 24. These values 23, 24 are stored and will be subsequently used during operation of the printer in an encoder step correction 43, 44. Each revolution of the rollers yields further correction values. They are continuously employed to refine the stored correction values 23, 24.

(24) The corrected encoder measurements 25, 26 are filtered by an adaptive notch filter. Again, the coefficients of the filter are determined revolution by revolution and continuously updated. The input of the adaptive notch filter (i. e. a cosine and a sine period) does not change for the different revolutions of the respective roller, accordingly, the adaptive notch filter may be implemented by a simple multiply-add structure. The concentricity analysis 45, 46 yields correction values 27, 28 being a sine period with a certain phase and amplitude. Averaging of the coefficients stabilizes the phase of this correction sine. The correction values 27, 28 are stored and will be subsequently used during operation of the printer in a concentricity correction 47, 48.

(25) The corrected encoder values 29, 30 are fed to the final elongation correction 50. In this stage of the process, the corrected values 30 of encoder 2 are interpolated for obtaining encoder values relating to the same points in time as the corrected values 29 of encoder 1. Furthermore, due to the fact that the roller diameters are not equal, the interpolated values of encoder 2 will be compensated by the ratio of the roller diameters.

(26) The distance between the rollers as well as between roller 1 and the individual print bars is known. For each point in time relating to a value obtained by encoder 1 and for each of the print bars, the difference between the encoder value relating to encoder 2 and the encoder value relating to encoder 1 is compensated by the ratio of the distances. The result is subtracted from the encoder value obtained from encoder 1 thus correcting encoder 1. This amounts to obtaining the dynamics of encoder 1 it would exhibit directly below the respective print bar. In other words, virtual encoders having a roller diameter 0 and being positioned directly below the print bars are calculated.

(27) The same procedure is applied to encoder 2. The virtual encoders relating to print bars that lie closer to encoder 1 are calculated on the basis of encoder 1, whereas the virtual encoders relating to print bars that lie closer to encoder 2 are calculated on the basis of encoder 2. The compensation is applied in that the region of nozzles of the respective print bar that is actually used to produce the respective part of the image is suitably shifted, according to the respective compensation parameter.

(28) In a similar way, variations of the angular position of the print engine 160 may be compensated. The encoder values are obtained from linear encoders each measuring the position of a location along the print engine 160 with respect to the second axis. The processing steps to obtain the individual compensation parameters for the print bars essentially correspond to those described before. In contrast however, for the compensation the ejection of ink by the respective print bar is delayed using a time-variable and distance-constant, velocity-dependent delay, which allows for applying the corrections into the future as well as into the past.

(29) This is also the kind of compensation that is applied in the case of a configuration as shown in FIG. 2.

(30) The FIG. 9 is a schematic representation of the print engine position in scans of subsequent printing bands and of the resulting colour bands.

(31) In order to simplify the presentation, a print engine 160 having only four print bars 165.1 . . . 165.4 is shown. The generalization to other numbers of print bars (e. g. 8) is straightforward.

(32) The left hand side of FIG. 9 shows the position of the print engine 160 with respect to the substrate 1 in scans of five subsequent printing bands 10.1 . . . 10.5. To facilitate the presentation, the position of the substrate 1 is fixed, whereas the position of the print engine 160 is moved from band to band. As a matter of course, in the context of the inkjet printer as shown in FIGS. 3-7, the change of relative position between print engine 160 and substrate 2 will be cause by moving the substrate 2 by means of the transport mechanism.

(33) The print bars 165.1 . . . 165.4 are represented by their nozzle arrays. These arrays feature an active area 166 constituting a central region along the longitudinal axis of the print bar (and thus of the nozzle array) and two inactive areas 167 constituting the end regions along the longitudinal axis of the print bar. As described in more detail below, the exact position of the active area 166 may be dynamically adapted in order to compensate for positioning errors of the respective print bar with respect to the substrate 1 along the longitudinal axis. The nozzle arrays of the print bars 165.1 . . . 4 are arranged along the longitudinal axis of the print engine 160. The length l of the active area is 173.4 mm, the distance d of the start of the active areas of two neighbouring print bars is 238.4 mm. The four print bars 165.1 . . . 4 are supplied by differently coloured ink: cyan for print bar 165.1, magenta for print bar 165.2, yellow for print bar 165.3 and black for print bar 165.4. The different colours are schematically represented in FIG. 9 by different hatchings.

(34) The substrate is shifted in between neighbouring bands by the distance l, which corresponds to the length of the active area. This means that by subsequent bands the entire area of the substrate 1 may be printed with a certain colour. The same holds true for all colours, but the transition from one band to another will be different for each of the colours, as can be seen from the right hand side of the FIG. 9 which shows the areas printed in one band for the four colours. Combining the four colours, the transitions will be evenly distributed over the substrate, and there will be no transitions between neighbouring bands that include more than one colour.

(35) The invention is not limited to the embodiments described above. In particular, the specific configuration of the print engines, print bars, transport system or scanning mechanism may substantially differ from the described embodiments. Nevertheless, the invention will still be applicable.

(36) In summary, it is to be noted that the invention creates an inkjet printer that exhibits reduced susceptibility to image defects.