Print controller and method of printing

Abstract

The invention relates to a printing device for printing large countoured three-dimensional objects. The printing device comprises a movable robot arm mounted on a movable support, a printhead supported at a printing end of the robot arm, the print head comprising a plurality of nozzles, an ink reservoir connected to the nozzles of the print head and to a pump device for supplying ink from the reservoir to the nozzles, and a controller for moving the print head along a printing trajectory while changing the orientation of the printhead. The controller is arranged for: in a calibrating step moving the print head along a calibration trajectory and measuring ink pressures in the printing head and generating and storing ink pressure control data for the nozzles for different orientations of the print head, and in a printing step generating for varying orientations of the printhead along the printing trajectory a pressure control signal on the basis of the stored ink pressure control data, which pressure control signal is supplied to the pump device such that a pressure of the ink in the nozzles is set at a predetermined pressure value.

Claims

1. Printing device comprising: a movable robot arm mounted on a movable support, a print head supported at a printing end of the robot arm, the print head comprising a plurality of nozzles, an ink reservoir connected to the nozzles of the print head and to a pump device for supplying ink from the reservoir to the nozzles, and a controller for moving the print head along a printing trajectory while changing the orientation of the print head, wherein the controller is arranged for: in a calibrating step moving the print head along a calibration trajectory and measuring ink pressures in the printing head and generating and storing ink pressure control data for the nozzles for different orientations of the print head, and in a printing step generating for varying orientations of the print head along the printing trajectory a pressure control signal on the basis of the stored ink pressure control data, which pressure control signal is supplied to the pump device such that a pressure of the ink in the nozzles is set at a predetermined pressure value.

2. Printing device according to claim 1, wherein at least a part of the calibration trajectory corresponds with the printing trajectory.

3. Printing device according to claim 1, wherein the print head comprises a pressure sensor.

4. Printing device according to claim 3, wherein the pressure sensor comprises a inflow pressure sensor connected to an inflow end of the nozzles for sensing an inflow ink pressure at the nozzles.

5. Printing device according to claim 4, wherein the pump device supplies ink to the nozzles at the inflow pressure and is operated by inflow pressure control signals that are formed by pi=(A+K1)*f()+(B+K2)*g(), wherein f() and g() are geometry factors depending on an angle e of the print head with a horizontal direction, A is a distance from the pressor sensor to a print surface in a direction perpendicular to the print surface, B is a distance of the pressure sensor in a plane of the print surface and K1 ,K2 are constants.

6. Printing device according to claim 4, wherein the pressure sensor comprises a recirculation pressure sensor connected to an outflow end of the print head for measuring a recirculation pressure.

7. Printing device according to claim 6, wherein the pump device removes ink from an outlet of the print head at a recirculation pressure, and is operated by recirculation pressure control signals Pr that are formed by Pr=(A+K3)*f()+(B+K4)*g()+X wherein f() and g() are geometry factors depending on an angle e of the print head with a horizontal direction, A is a distance from the pressor sensor to a print surface in a direction perpendicular to the print surface, B is a distance of the pressure sensor in a plane of the print surface, K3,K4 are constants and X is a difference between the inflow pressure and the recirculation pressure measured by the pressure sensors.

8. Method of printing an object with a movable robot arm mounted on a movable support, a print head supported at a printing end of the robot arm, the print head comprising a plurality of nozzles, a pressure sensor for sensing an ink pressure in the nozzles and forming ink pressure signals, an ink reservoir connected to the nozzles of the print head and to a pump device for supplying ink from the reservoir to the nozzles, the method comprising: carrying out a calibration step by: moving the print head along a calibrating trajectory with varying orientations, measuring a pressure of the ink at the nozzles along the calibrating trajectory with the pressure sensor and deriving pressure control data from the ink pressure signals and storing the pressure control data in a memory unit of a print controller, and carrying out a printing step by: moving the print head along a printing trajectory and controlling the pump device by retrieving the pressure control data from the memory unit and generating pressure control signals at the corresponding printing head orientations along the printing trajectory such that the ink in the nozzles is at a predetermined ink pressure.

9. Method according to claim 8, wherein the calibration trajectory at least partly corresponds with the printing trajectory.

10. Method according to claim 8, wherein the pressure sensor comprises an inflow pressure sensor for sensing an ink pressure at the nozzles, wherein the pump device supplies ink to the nozzles at the inflow pressure and is operated by inflow pressure control signals that are formed by pi=(A+K1)*f()+(B+K2)*g(), wherein A is a distance from the pressor sensor to a print surface in a direction perpendicular to the print surface, and B is a distance of the pressure sensor in a plane of the print surface, K1 ,K2 are constants.

11. Method according to claim 10, wherein the pressure sensor comprises a recirculation pressure sensor connected to an outflow end of the print head, a recirculation pump device removing ink from the print head at a recirculation pressure, the method comprising the step of operating the recirculation pump device by recirculation pressure control signals Pr that are formed by Pr=(A+K3)*f()+(B+K4)*g()+X wherein A is a distance from the pressor sensor to a print surface in a direction perpendicular to the print surface, and B is a distance of the pressure sensor in a plane of the print surface, K3,K4 are constants and X is a difference between the inflow pressure and the recirculation pressure.

12. Method according to claim 8, wherein the object to be printed is part of an aeroplane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Some embodiments of a printing device and method of printing according to the invention will by way of non-limiting example be described in detail with reference to the accompanying drawings. In the drawings:

[0025] FIG. 1 shows a schematic overview of a printing device according to the invention, and

[0026] FIG. 2 shows a schematic lay-out of the printing head and the pressure control unit of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] FIG. 1 schematically shows a printing device 1 according to the invention with a robot arm 2 carrying a print head 3. The print head 3 may comprise an ink jet printer of type Fujifilm Dimatix Part Number SG1024LA-2C.The robot arm is placed on a movable support 4, for instance of the type described in US patent application Ser. No.'s 16/015,240 and 16/015,243 filed on 22 Jun. 2018. Ink is supplied to the print head 3 from a bulk ink reservoir 10 via a pump 9 and an ink duct 11.

[0028] A controller 5 is with a print control line 6 connected to pressure sensors in the print head 3 for measuring ink pressures at the nozzles in the print head. The controller 5 is with an ink supply control line 12 connected to the pump 9 for controlling of the ink supply to the print head 3. The pump 9 of ink supply system may for instance comprise a low flow recirculation supply system of the type LC-LFR as available from the company Megnajet, Northampshire, United Kingdom.

[0029] The controller 5 is via a control line 7 connected to the robot arm 2 for controlling the position of the robot arm 2 and the speed and orientation of the print head 3 along a contoured three-dimensional print surface 8, which has by way of example been shown as a circle but in practise will be of a complex geometry, such as the outer surface of an aeroplane.

[0030] The controller 5 can be made up of several dedicated and spatially distributed control units, such a meniscus pressure control unit 21, a recirculation pressure control unit 22 and a control module 25 as shown in FIG. 2, for controlling of the robot arm 2, the print head operation and the ink supply.

[0031] FIG. 2 shows a schematic overview of the print head 3 with a nozzle array 16 that is with an inlet 12 connected to a meniscus pressure sensor 13. An outlet 14 of the nozzle array 16 is connected to the a recirculation pressure sensor 15. The nozzles in the array 16 are each provided with a piezo element 17 for expelling the ink 18 that flows along the nozzles, from the nozzles in the form of small droplets.

[0032] From an ink reservoir 19, ink flows into the inlet 12 of the nozzle array 16 at the meniscus pressure Pi and is transported along all nozzles to fill each nozzle with ink. Ink is supplied to the ink reservoir 19 from the bulk ink reservoir 10 by the fill pump 9. The fill pump 9 is controlled by meniscus pressure control unit 21.

[0033] At the outlet 14 of the nozzle array 16, the recirculation pressure of the ink flowing along the filling apertures of the nozzles is smaller than the meniscus pressure by a set pressure difference, 50 mbar, so that ink flows back from the outlet 14 back to the ink reservoir 19, via a recirculation pressure control unit 22. The recirculation pressure control unit 22 comprises a recirculation pump 23 that is controlled at recirculation pressures Pr as described below.

[0034] In order to operate the nozzle array 16 at a defined meniscus pressure Pi at its inlet 12, and at a defined recirculation pressure Pr at its outlet 14, the fill pump 9 is controlled by pressure curves that are generated in controller unit 25. The pressure curves are generated based on positional data of the print head 3 and prevailing pressures at these positions, in a calibration step in which the print head 3 is moved by the robot arm 2 along a calibration printing trajectory at the required speed. During the calibration step, industry standard gradient patterns are printed and measurements are taken so that the meniscus pressures Pi and recirculation pressures Pr are tuned for consistent printed graphics across all orientations of the print head 3 for all types of ink that are used.

[0035] The result of the calibration step are pressure curves for the meniscus pressure Pi and the recirculation pressure Pr for any possible print head orientation for any type of ink that will be used in the printing step. Because the print head 3 is in motion when printing, accelerations are felt by the print head immediately prior to and possibly during printing. The pressure equations for the inlet pressure Pi and the recirculation pressure Pr are not dependant on these velocities and accelerations due to the location of the pressure sensors. If an acceleration is felt by the print head 3, the pressure sensors will detect a higher or lower pressure in the ink. This pressure change will be fed back to the inlet and recirculation pumps, which will vary their speed in order to bring the ink back to the commanded pressures Pi and Pr.

[0036] The curves that control the inlet pressure Pi and the recirculation pressure Pr are defined by:

[0037] Pi=(A+K1)*C*D*sin(90-e)+(B+K2)*C*D*Cos(90-e)

[0038] Pr=(A+K3)*C*D**sin(90-e)+(B+K4)*C*D*Cos(90-e)X

[0039] Herein is:

[0040] A: a distance from the pressure sensors 13, 15 in the print head 3 to the print surface 8 in the direction that is normal to the print surface 8, in inches

[0041] B: a distance from the pressure sensors 13,15 in the print head 3 to the print surface 8 in the direction parallel to the print surface, in inches

[0042] C: a conversion factor from inches of water to mbar

[0043] D: the density of the ink in g/cm.sup.3

[0044] : the print head angle

[0045] K1,K2,K3,K4: constants that are set for each specific ink that is used and the properties of the ink ducts. The constants account for differences in ink viscosity, pressure losses due to bends in the ink ducts and due to friction in the ducts.

[0046] X: the set difference between the inlet pressure Pi and the recirculation Pressure Pr in mbar.

[0047] The values for Pi and Pr are positive numbers that represent vacuum values, i.e. the magnitude below ambient atmospheric pressure. The print head orientation resulting in the values A and B can be calculated in the controller 5 by reading the positions of the robot arm 7 and deriving therefrom the orientation of the print surface 8. The orientation of the print head 3 may also be derived by directly reading into the controller 5, the gravity vector from an Inertial Measurement Unit (IMU) on the print head 3 or other sensors mounted near the print surface 8. The measurement rate of the print head angle and hence of the update of the calculated pressure set point values Pi and Pr should preferably at least be equal to 20 kHz.

[0048] An example of pressure curves Pi and Pr is as follows: [0049] A=3.00 inches [0050] B=2.25 inches [0051] C=0.402 mbar/inch-water [0052] D=0.800 g/cm.sup.3 [0053] =80.0 degrees (i.e. the print head will print toward a wall, but is pointed slightly down towards the floor) [0054] K1=0.250 inch [0055] K2=0.250 inch [0056] K3=0.500 inch [0057] K4=0.500 inch [0058] X=50 mbar

[0059] Pi=(3.00+0.250) *0.402*0.800* sin(90-80.0)+(2.25+0.250) *0.402*0.800* cos(90-80.0)=5.04 mbar

[0060] Pr=(3.00+0.500) *0.402*0.800* sin(90-80.0)+(2.25+0.500) *0.402*0.800* cos(90-80.0)50=50.3 mbar