PRINT HEAD FRAME STRUCTURE AND CONTROL

Abstract

The invention relates to a print system comprising at least two page-wide arrays of ink jet print heads, positioned in a frame over a conveyor belt for transporting a substrate underneath the arrays, three sensors for reading markers on the conveyor belt and a control unit that is configured to derive control signals from encoder signals of the three sensors. Two sensors are directly connected to the frame and a third sensor is connected to one of the said two sensors by an element with virtually no thermal expansion, extending in transport direction. The control signals, comprising signals for controlling a transport speed of the conveyor belt and line pulses for the at least two print heads, are derived in such a way that an amount of thermal expansion of the frame is determined and an absolute print resolution is maintained.

Claims

1., A print system comprising at least two page-wide arrays of ink jet print heads, positioned in a frame over a conveyor belt for transporting a substrate underneath the arrays in a transport direction, three sensors for reading markers on the conveyor belt and a control unit that is configured to derive control signals from encoder signals of the three sensors, wherein two sensors are directly connected to the frame and a third sensor is connected to one of the said two sensors by an element with virtually no thermal expansion, extending in the transport direction.

2. A print system according to claim 1, wherein the at least two print heads comprise at least two rows of nozzles.

3. A print system according to claim 1, wherein the element connecting the third sensor is an invar rod.

4. A print system according to claim 1, wherein the frame is made of steel.

5. A print system according to claim 1, wherein the control signals comprise signals for controlling a transport speed of the conveyor belt and line pulses for the at least two print heads.

6. A print system according to claim 5, wherein an amount of thermal expansion of the frame is derived from the encoder signals and an absolute print resolution is maintained.

7. A method for deriving control signals for printing an image in a print system comprising at least two page-wide arrays of ink jet print heads, positioned in a frame over a conveyor belt for transporting a substrate underneath the arrays in a transport direction, three sensors for reading markers on the conveyor belt and a control unit that is configured to derive control signals from encoder signals of the three sensors, wherein two sensors are directly connected to the frame and a third sensor is connected to one of the said two sensors by an element with virtually no thermal expansion, extending in the transport direction, the method comprising the steps of: starting printing for a page-wide array based on a delay derived from a distance between the two sensors directly connected to the frame and continuing printing further lines by the page-wide array based on a delay derived from a distance between the third sensor and the sensor connected to the element with virtually no thermal expansion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

[0013] FIG. 1 is a schematic sectional view of a print system according to an embodiment of the invention;

[0014] FIG. 2 is an embodiment of the frame of the print station with an indication of the sensor positions; and

[0015] FIG. 3 is a scheme for the derivation of the required control signals.

DETAILED DESCRIPTION OF EMBODIMENTS

[0016] The present invention will now be described with reference to the accompanying drawings, wherein the same or similar elements are identified with the same reference numeral.

[0017] The print system shown in FIG. 1 has a sheet supply path 10, a conveyor belt 25 and a print station 12, placed on a beam 40. The sheet supply path 10 is arranged to supply media sheets 20 successively to and past the print station 12, where an image is printed on the top side of each media sheet passing through. The conveyor belt 25 is capable of accommodating a plurality of media sheets 20 at a time and is driven by a motor roller 30. Besides a brake roller 31 and a steering roller 32, an encoder roller 33 from which a belt position signal is derived, using a line pulse multiplier (LPM). This derivation also accounts for a thermal compensation of the belt length.

[0018] The sheet supply path 10 includes a pinch 24 formed by at least two pinch rollers forming a nip through which the media sheets 20 pass through. At least one of the pinch rollers is driven for rotation so that the media sheets are advanced in a transport direction x towards the print station 12. A liftable pinch 26 is arranged in the sheet supply path 10 in a position downstream of the pinch 24. The distance between the pinches 24 and 26 in the transport direction x is smaller than the length of the media sheets 20 in that direction, so that the leading edge of the sheet 20 can be clamped in the liftable pinch 26 before the trailing edge of that sheet has left the pinch 24. Accordingly, a continuous transport of the media sheets can be assured.

[0019] The beam 40 provides support for up to seven page-wide arrays (PWA) of print heads for jetting ink drops. The shown print station 12 comprises only four of them in the colors cyan, magenta, yellow and black. These PWA's are oriented transverse to the transport direction. Each PWA receives an individual start-of-page signal (SOP) to mark the start of image lines to be printed on the media sheet 20, relative to the leading edge of the sheet. After the SOP, a start-of-line (SOL) is given for each further image line. These signals are derived from the belt position signal that is generated by the encoder roller with LPM in units of 2.64 μm, equivalent to 9600 dpi, and the sensors signals that come from the position sensors 41, 42, and 43. Whereas sensors 41 and 42 are fixed on the beam 40, sensor 43 is mounted on the beam 40 in a way that it can freely move in the transport direction, the x direction. Sensor 43 is connected to sensor 41 by an element that is made of the metal “invar”, wherein a magnetostrictive contraction counters a thermal expansion at rising temperatures. Thus, this element shows virtually no thermal expansion. In an alternative embodiment, the element is a low cost carbon fiber rod. Thus, the distance between the sensors 41 and 43 is fixed, in contrast to the distance between sensor 41 and 42, that varies with a varying temperature.

[0020] FIG. 2 shows the beam 40 for positioning the PWA's within the print station 12. This beam, comprising the sensors 41, 42, and 43, is part of the steel frame and is susceptible to thermal expansion with changing ambient temperature. The balls 45 serve as outlining elements for the various arrays that are supported and aligned by the frame. The element 46 that connects the sensors 41 and 43 runs along the full beam and has a length of 560 mm.

[0021] The sensors 41 and 42 are 235 mm space apart. From the difference between the signals given by these sensors upon monitoring the markers on a belt that is conveyed underneath these sensors, the amount of thermal expansion of the beam may be derived. The SOP signals for each array that determines the colour-to-colour registration is compensated accordingly. Once a PWA starts printing image lines, the timing between the various lines is determined by the signals stemming from the sensors 41 and 43.

[0022] FIG. 3 shows a scheme for the derivation of the various signals. Three sensors 41, 42, and 43 are shown, each monitoring markers on a the belt 25 that supports the media sheets. The encoder roller 33 gives a signal to a line pulse multiplier for deriving a time-scale that corresponds to a distance of 1/9600 inch or 2.64 micrometer on the belt. For a correct timing of subsequent lines (SOL) the sensors 41 and 43 are used, for a correct timing of the first line (SOP) the sensors 41 and 42 are used. For each PWA, up to seven in total, a separate signal 51 is derived by the calculating unit 50, comprising the line pulse multiplier, from the encoder and the sensor signals. In this way an absolute image resolution is obtained after calibration at a reference temperature, independent of the thermal expansion of the frame that supports the PWA's.

[0023] The method of operation comprises the steps of: [0024] capturing the LPM position counter T.sub.x(n) for each belt hole n passing one of the three sensors x; [0025] deriving the measured distances L.sub.21=T.sub.2(n)−T.sub.1(n) and L.sub.31=T.sub.3(n)−T.sub.1(n); [0026] controlling the belt such that L.sub.31 is kept constant; [0027] calibrating the distance L.sub.21 using a test image and saving the value CR=L.sub.21/L.sub.31; [0028] scaling the distance for starting SOP for a PWA at a temperature T in operation by a factor SF(T)=L.sub.21(T)/L.sub.31(T)*1/CR;

[0029] The skilled person will recognise that other embodiments are possible within the scope of the appended claims.