Abstract
The invention describes a laser printing system (100) for illuminating an object (70) in a working plane (80). The object (70) moves relative to a print head (50) of the laser printing system (100). The print head (50) comprises a total number of laser modules (150), each laser module (150) comprises at least one laser array (110) of lasers (115). At least two of the laser modules (150) share an electrical power supply (20). The laser printing system (100) further comprises a controller (10) being adapted such that at maximum processing speed of the print head (50) only a predefined number of laser modules (150) can be driven at nominal electrical power, wherein the predefined number of laser modules (150) is smaller than the total number of laser modules. The invention further relates to a corresponding method of laser printing. The laser printing system and the method allow designing the laser printing system for, for example, only 20% of the total power which would be required to drive all lasers at nominal electrical power while having only slightly reduced processing speed.
Claims
1. A laser printing system comprising: a print head, the print head comprising a plurality of laser modules, wherein each laser module comprises at least one laser array of lasers, wherein at least two of the laser modules share an electrical power supply; and a controller circuit, wherein the controller circuit is arranged such that at maximum processing speed of the print head only a predefined number of laser modules can be driven at nominal electrical power, wherein the predefined number of laser modules is a portion of the plurality of laser modules, wherein the controller circuit is arranged to reduce a processing speed of the print head if the optical energy to be provided to the object within a predefined time period requires an electrical input power exceeding the nominal electrical power of the laser modules times the predefined number of laser modules, wherein the controller circuit is arranged to reduce an electrical input power supplied to the laser modules below the nominal electrical power of the laser modules when the processing speed is lower than the maximum processing speed such that emission of laser light by more than the predefined number of laser modules is enabled, wherein the laser printing system illuminates an object in a working plane, wherein the object is moving relative to the print head.
2. The laser printing system according to claim 1, wherein the controller circuit is arranged to control the laser modules with shifted pulse width modulation, wherein the shifted pulse width modulation has a pulse width modulation base time, a pulse width and a pulse phase.
3. The laser printing system according to claim 2, wherein the laser modules and/or the electrical power supply comprise buffer capacitors, wherein the buffer capacitors are arranged to store energy to supply electrical power to the laser modules such that more than the predefined number of laser modules can be driven at nominal electrical power for a predefined period of time.
4. The laser printing system according to claim 2, wherein the laser modules are arranged in columns, wherein one electrical power supply is arranged to supply electrical power to all laser modules of one column, wherein the controller circuit is arranged to adapt the pulse width modulation base time such that the distance of the laser pulses received on the object remains constant, wherein the controller circuit is arranged to keep the pulse width of the lasers constant, wherein the controller circuit is arranged such that a reduction of electrical power supplied to the laser modules is adapted to a reduction of the processing speed at a constant printing resolution.
5. The laser printing system according to claim 2, wherein the laser modules are arranged in columns, wherein one electrical power supply is arranged to supply electrical power to all laser modules of one column, wherein the controller circuit is arranged to to supply interleaving pulse width modulation pulses at a constant pulse width modulation base time to the lasers of the column such that the drive current supplied by the electrical power supply open is smoothed.
6. The laser printing system according to claim 5, wherein the controller circuit is arranged to start pulses with a defined pulse width, wherein the different times of starting the pulses are distributed across the pulse width modulation base time.
7. The laser printing system according to claim 6, wherein the different times of starting the pulses are randomly distributed.
8. The laser printing system according to claim 5, wherein the controller circuit is arranged to start pulses provided to the lasers of different laser modules at different times during the pulse width modulation base time, wherein the different times of starting the pulses are distributed across the pulse width modulation base time.
9. The laser printing system according to claim 3, wherein the laser modules being arranged in columns, wherein one electrical power supply is arranged to supply electrical power to all laser modules of a least two columns, wherein the at least two columns comprise a common buffer capacitance, wherein the controller circuit is arranged to supply interleaving pulse width modulation pulses at constant pulse width modulation time to the lasers of the at least two columns such that the drive current supplied by the electrical power supply is smoothed.
10. The laser printing system according to claim 2, wherein the controller circuit is arranged to adapt the pulse width modulation base time such that the distance between laser pulses received on the object remains constant, wherein the controller circuit is arranged to adapt the pulse width of the lasers such that a part of a reduction of electrical power supplied to the laser modules is caused by a shortened pulse width.
11. The laser printing system according to claim 2, wherein the controller circuit is arranged to keep the pulse width modulation base time constant, wherein the controller circuit is arranged to keep the pulse width of the lasers constant, wherein the controller circuit is arranged to skip pulses in accordance with a reduction of the processing speed.
12. The laser printing system according to claim 1, wherein all laser modules of a group of laser modules share the electrical power supply, wherein the controller circuit is arranged to switch off at least one laser module of the group of laser modules if the optical energy to be provided to the object within a predefined time period requires an electrical input power exceeding the nominal electrical power of the laser modules times the predefined number of laser modules, wherein the controller circuit is arranged to switch off the at least one laser module such that a full width of the print head can be processed within at least two passes of the print head across the object.
13. A method of laser printing, the method comprising: moving an object in a working plane relative to a print head, the print head comprising a plurality of laser modules, wherein at least two of the laser modules share an electrical power supply; emitting laser light using the laser modules, the laser modules comprising at least one laser array of lasers; controlling the laser modules such that at maximum processing speed of the print head only predefined number of laser modules can be driven at nominal electrical power, wherein the predefined number of laser modules is a portion of the plurality of laser modules; reducing a processing speed of the print head if the optical energy to be provided to the object within a predefined time period requires an electrical input power exceeding the nominal electrical power of the laser modules times the predefined number of laser modules; and reducing an electrical input power supplied to the laser modules below the nominal electrical power of the laser modules when the processing speed is lower than the maximum processing speed such that emission of laser light by means of more than the predefined number of laser modules at the same time for seamless printing is enabled.
14. The method according to claim 13, wherein the controlling uses shifted pulse width modulation.
15. The method according to claim 14, wherein the shifted pulse width modulation has a pulse width modulation base time, a pulse width and a pulse phase.
16. The method according to claim 15, wherein the laser modules are arranged in columns, wherein one electrical power supply is arranged to supply electrical power to all laser modules of one column, wherein the controlling is arranged to adapt the pulse width modulation base time such that the distance of the laser pulses received on the object remains constant, wherein the controlling is arranged to keep the pulse width of the lasers constant, wherein the controlling is arranged such that a reduction of electrical power supplied to the laser modules is adapted to a reduction of the processing speed at a constant printing resolution.
17. The method according to claim 15, wherein the laser modules are arranged in columns, wherein one electrical power supply is arranged to supply electrical power to all laser modules of one column, wherein the controlling is arranged to supply interleaving pulse width modulation pulses at a constant pulse width modulation base time to the lasers of the column such that the drive current supplied by the electrical power supply open is smoothed.
18. The method according to claim 17, wherein the controller circuit is arranged to start pulses with a defined pulse width, wherein the different times of starting the pulses are distributed across the pulse width modulation base time.
19. The method according to claim 18, wherein the different times of starting the pulses are randomly distributed.
20. The method according to claim 13, wherein all laser modules of a group of laser modules share the electrical power supply, wherein the controlling is arranged to switch off at least one laser module of the group of laser modules if the optical energy to be provided to the object within a predefined time period requires an electrical input power exceeding the nominal electrical power of the laser modules times the predefined number of laser modules, wherein the controlling is arranged to switch off the at least one laser module such that a full width of the print head can be processed within at least two passes of the print head across the object.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
(2) The invention will now be described, by way of example, based on embodiments with reference to the accompanying drawings.
(3) In the drawings:
(4) FIG. 1 shows a principal sketch of a cross-section of a laser printing system according to a first embodiment.
(5) FIG. 2 shows a principal sketch of a top view of a laser printing system according to a second embodiment.
(6) FIG. 3 shows a principal sketch of a top view of a laser printing system according to a third embodiment.
(7) FIG. 4 shows a principal sketch of a print head according to a first embodiment.
(8) FIG. 5 shows a principal sketch of a print head according to a second embodiment.
(9) FIG. 6 shows a principal sketch of a print head according to a third embodiment.
(10) FIG. 7 shows a principal sketch of a print head according to a fourth embodiment.
(11) FIG. 8 shows a principal sketch of a laser module according to a first embodiment.
(12) FIG. 9 shows a principal sketch of a group of laser modules according to a first embodiment.
(13) FIG. 10 shows a principal sketch of a group of laser modules according to a second embodiment.
(14) FIG. 11 shows a principal sketch of a first PWM driving scheme
(15) FIG. 12 shows a principal sketch of a second PWM driving scheme
(16) FIG. 13 shows a principal sketch of a third PWM driving scheme
(17) FIG. 14 shows a principal sketch of a fourth PWM driving scheme
(18) FIG. 15 shows a principal sketch of method steps of a method of laser printing.
(19) In the Figures, like numbers refer to like objects throughout. Objects in the Figures are not necessarily drawn to scale.
DETAILED DESCRIPTION OF EMBODIMENTS
(20) Various embodiments of the invention will now be described by means of the figures.
(21) FIG. 1 shows a principal sketch of a cross-section of a laser printing system according to a first embodiment. The laser printing system 100 comprises a process chamber with an object carrier 30 for carrying building material and a three-dimensional object 70 to be built thereon. On the object carrier 30 a building platform may be a provided which serves as a removable base for removing the object 70 after the building process is finished. A frame 40, such as vertical walls, may be arranged around the object carrier 30 to confine layers of the building material on the object carrier 30. The frame 40 may be removable, which may comprise a vertically movable base which is removably attached to the object carrier 30. A print head 50 is arranged above the working plane 80. The print head 50 is movable across the working plane 80 in a direction indicated by the double sided arrow in FIGS. 2 and 3. The print head 50 may be configured to be moved back in an opposite direction. The print head 50 comprises laser modules 150 (not shown) which may be adapted such that the working plane can be illuminated if the print head 50 moves in both directions indicated in FIGS. 2 and 3. The object carrier 30 is movable up and down relative to the print head 50 in a vertical direction, i.e. in a direction perpendicular to the direction of movement of the print head 50. Movement of object carrier 30 is controlled by means of a controller 10 in such a manner that an uppermost layer of the building material forms the working plane 80. The laser printing system further comprises the controller 10 for controlling various functions of the laser printing system. The controller comprises a power supply 20 which is configured to supply electrical power to all laser modules 150 of the print head 50. A recoating device (not shown) may be provided to apply layers of building material onto the building platform of the object carrier 30. Furthermore, one or more separate heating device(s) (not shown) may be provided that may be used to heat an applied layer of building material to a process temperature and/or to control the temperature of the building material within the frame 40, if necessary. The building material preferably is a powder material that is configured to transform under the influence of the laser light emitted by the lasers 115 into a coherent mass. The transformation may include, for example, melting or sintering and subsequent solidification and/or polymerization in the melt. The building material may be a plastic powder, for example, a thermoplastic powder. Examples of such plastic powders are PA 12 (polyamide 12) or other polyamides, polyaryletheretherketone, such as PEEK or other polyetherketones. The powder may also be a powder from a metal or a metal alloy with or without a plastic or metal binder, or a ceramic or composite or other kind of powder. Generally, all powder materials that have the ability to transform from powder into a coherent mass under the influence of the laser light emitted by the lasers 115 can be used. The building material may also be a paste-like material including a powder and an amount of liquid. Typical medium grain sizes of the powder lie between 10 ?m and 100 ?m. The emission wavelength of the lasers 115 is preferably in the near infrared range of the spectrum. A preferred wavelength range may be between 750 nm and 1200 nm. Examples of wavelengths which are used in present systems are, for example, 980 nm or 808 nm. The powder material may comprise laser light absorbing additives which absorbs laser light in the emission wavelengths of the lasers 115. An example of such an additive may be but not limited to Carbon Black which is suitable to enable a sufficient absorption of the preferred wavelengths described above. In principle any wavelength is possible as long as a suitable absorber material can be added to the powder material or the powder material itself is characterized by sufficient absorption at the emission wavelengths of the lasers 115.
(22) FIG. 2 shows a principal sketch of a top view of a laser printing system according to a second embodiment. A working area 82 may be defined by the frame 40. The working area 82 may have a rectangular contour. The working area 82 may have any other contour such as but not limited to a square-shape, a circular contour or the like. The print head 50 is mounted on a print carrier 52. The print head comprises one common power supply 20 configured to drive all laser modules 150 (not shown) mounted on the print head 50. A controller 10 is monolithically integrated with the print carrier 52. The print carrier 52 and controller 10 can move in the directions indicated by the double sided arrow at the left side.
(23) FIG. 3 shows a principal sketch of a top view of a laser printing system according to a third embodiment. The third embodiment is quite similar to the second embodiment. The print carrier 52 and the controller 10 are in this case separated. Only the print carrier 52 with the print head 50 is configured to move in the directions indicated by the double sided arrow. Controller 10 comprises power supply 20 which is adapted to provide electrical power to all laser modules 150 (not shown) mounted on print head 50. Controller 10 and power supply 20 provides control signals and electrical power the flexible wires to the laser modules 150 mounted on the print head 50.
(24) FIG. 4 shows a principal sketch of a print head 50 according to a first embodiment. The print head 50 comprises ten laser modules 150. All laser modules 150 are commonly supplied with electrical power by a common electrical power supply 20 as shown, for example in FIG. 3. Furthermore, all laser modules 150 are commonly controlled by controller 10 as shown, for example, in FIG. 3. The five laser modules 150 on the left of the print head 50 are switched on and the five laser modules 150 on the right are switched off (indicated by the grey shading) by means of controller 10. The print head 50 has to move two times across the working area 82 in order to process one complete layer. The controller 20 is in this case adapted such that at maximum processing speed of, for example, 300 mm/s only five of the 10 laser modules 150 can be driven at nominal electrical power such that each laser 115 comprised by the laser modules 150 can emit, for example, 1.5 W optical power.
(25) FIG. 5 shows a principal sketch of a print head 50 according to a second embodiment. All laser modules 150 are commonly supplied with electrical power by a common electrical power supply 20 as shown, for example in FIG. 2. Furthermore, all laser modules 150 are commonly controlled by controller 10 as shown, for example, in FIG. 2. The laser modules 150 are arranged in two lines. The distance between the laser modules 150 in the first line is such that a laser module 150 of the second line can fill the gap. The laser modules 150 of the second line are therefore shifted into the gaps between the laser modules 150 of the first line. The five laser modules 150 of the first line of the print head 50 are switched on and the five laser modules 150 on the second line are switched off (indicated by the grey shading) by means of controller 10. The print head 50 has to move two times across the working area 82 in order to process the gaps between the laser modules 150 of the first line. The controller 20 is in this case adapted such that at maximum processing speed of, for example, 300 mm/s only five of the 10 laser modules 150 can be driven at nominal electrical power such that each laser 115 comprised by the laser modules 150 can emit, for example, 1.5 W optical power.
(26) The print head 50 may in alternative embodiments comprise other distributions of laser modules 150 such that the print head 50 has to pass working area 82 three, four, five or more times in order to process a complete layer. The number of passes across the working area 82 is determined by the reduction of power which can be maximally supplied to the laser modules 150 of the print head 50.
(27) FIG. 6 shows a principal sketch of a print head 50 according to a third embodiment. The print head 50 comprises several diagonal columns of laser modules 150. Each diagonal column of laser modules 150 comprises 11 laser modules 150 which are commonly supplied with electrical power by one electrical power supply 20. The laser modules 150 of one diagonal column are arranged such that in the top view of the part of the print head 50 shown in FIG. 6 each laser module 150 is slightly shifted to the right starting at the upper side of the print head 50. A central controller or main controller 10 similar as shown in the embodiment of FIG. 1 controls the power supplies 20 and laser modules 150. The controller 10 is adapted to control the laser modules 150 with shifted pulse width modulation. The laser modules 150 of one diagonal column are activated at different time periods of the PWM cycle. The pulse width and starting time within the PWM modulation base time of each laser module 150 within a column are chosen such that there is no overlap between the pulses provided to the laser modules 150. The pulses are arranged next to each other essentially without overlap. The current supplied to the laser modules 150 of one column by means of the common electrical power supply 20 is thus smoothed by means of the distribution of the pulses within the PWM cycle. The distribution of the pulse shifts between the laser modules 150 is randomized in order to avoid or at least to reduce seam lines.
(28) FIG. 7 shows a principal sketch of a print head 50 according to a fourth embodiment. The print head 50 comprises several diagonal columns of laser modules 150. Each diagonal column of laser modules 150 comprises 9 laser modules 150. Three diagonal columns are commonly supplied with electrical power by one electrical power supply 20. The laser modules 150 of one diagonal column are arranged such that in the top view of the part of the print head 50 shown in FIG. 7 each laser module 150 is slightly shifted to the right starting at the upper side of the print head 50. A central controller or main controller 10 similar as shown in the embodiment of FIG. 1 controls the power supplies 20 and laser modules 150. The controller 10 is adapted to control the laser modules 150 with shifted pulse width modulation. The pulse shift is regularly increased across the diagonal columns which are supplied by the common electrical power supply 20. Pulse lengths and start time of the pulses within one period of the PWM modulation which are supplied to the different laser modules 150 are randomized within the period of the PWM modulation in order to avoid or reduce systematic printing errors.
(29) FIG. 8 shows a principal sketch of a laser module 150 according to a first embodiment. The laser module 150 comprises a laser array 110 with 32 lasers 115 (VCSEL) which are arranged in eight columns and four lines. The laser module further comprises a laser driver 120 with a DC/DC converter 122, signal isolation 124 and PWM current source 126. The laser driver 120 is configured to transfer electrical power to the lasers 115 based on data input 12 provided by controller 10 and power input 14 provided by electrical power supply 20.
(30) FIG. 9 shows a principal sketch of a group of laser modules 160 according to a first embodiment. The group of laser modules 160 receives power input 14 by means of power supply 20 which comprises a filter 25. The filter 25 comprises buffer capacitors which are arranged to supply electrical power to the laser modules 150 of the group of laser modules 160 beyond the limitations of the electrical power supply 20 (without the buffer capacitors) for small time periods. Electrical power supply 20 receives main power input 24 from a voltage source (240V/400V mains). Electrical power supply 20 further receives main data input 22 from controller 10. The electrical power supply 20 further comprises a microprocessor which is configured to adapt the main data input 22 in accordance with the capabilities of the electrical power supply 20 and to submit data input 12 to laser modules 150 of the group of laser modules 160. Control of the laser modules 150 is provided in this architecture by a distributed arrangement of controller 10, power supply 20 and laser driver 120 shown in FIG. 8.
(31) FIG. 10 shows a principal sketch of a group of laser modules 160 according to a second embodiment. The group of laser modules 160 receives power input 14 by means of power supply 20 which comprises a filter 25. The filter 25 comprises buffer capacitors which are arranged to supply electrical power to the laser modules 150 of the group of laser modules 160 beyond the limitations of the electrical power supply 20 (without the buffer capacitors) for small time periods. Electrical power supply 20 receives main power input 24 from a voltage source (240V/400V mains). The group of laser modules 160 comprises one common laser driver 120 which receives power input 14 and data input 12 provided by controller 10. Control of the laser modules 150 is provided in this architecture by a distributed arrangement of controller 10 and common laser driver 120.
(32) FIG. 11 shows a principal sketch of a first PWM driving scheme. The first PWM driving scheme is a known standard driving scheme in which a first laser pixel 171 and a second laser pixel 172 and further laser pixels (not shown) are driven synchronously. First or second laser pixel 171, 172 and so on means one or more lasers (or laser arrays) which are arranged to be imaged to a corresponding pixel on the object 70. The first line shows the pulse with modulation base time 190. The second line shows the pulse width 191 of the PWM. The pulse width 191 is 8/9 of the pulse width modulation base time 190. The pulse shape is rectangular such that a constant current is supplied to the lasers from the beginning to the end of each pulse. A rectangular pulse is only chosen as an example in order to simplify the discussion. Other pulse shapes may also be used. The third line shows the time of full pixel 192. The time of full pixel 192 comprises four pulse with modulation base times 190 and therefore four pulses with pulse width 191. The more pulses are comprised by the time of full pixel 192 the higher is the sub pixel resolution. The fourth line shows when the first laser pixel 171 is active (black rectangles) which corresponds to the pulse width 191. The fifth line shows when the second laser pixel 172 is active (black rectangles) which corresponds to the pulse width 191. The activity of the first and the second laser pixel 171, 172 are synchronized meaning that all lasers emit laser light at the same time. The example shown in FIG. 11 refers to a laser printing system 100 in which the print carrier 52 moves with a velocity of 500 mm/s. The time of full pixel 192 therefore translates in a corresponding length of full pixel 192a in the working plane 80. An energy of first pixel 171 a in the working plane 80 is shown below the length of full pixel 192a. The received energy per area element rises during the movement of the print carrier 52 or print head 50 as long as energy is received at the respective area element in the working plane 80. A linear rise of the received energy in the working plane 80 is shown during pulse width 191 followed by a small time of constant received energy at the end of pulse width 191. The rise of the received energy per area element in the working plane 80 continues until the end of the time of full pixel 192. After this moment in time no further energy is received by the corresponding area element from the lasers 115. This maximum of received energy corresponds, for example, to 200% of a predefined energy threshold level at which the material in the working plane 80 is processed. Print head 50 has passed the corresponding area element in the working plane 80. The received energy causes a rise of temperature of the area element. The rise of temperature depends on the material, particle size and other boundary conditions and is not necessarily linear as the energy received by an area element in the working plane 80. The temperature of the area element in the working plane 80 rises until a threshold temperature is reached at which the material starts melting or sintering. This is the starting point of printed full pixel 192b in the working plane 80 which corresponds to the length of full pixel 192a but shifted because of the time needed to receive sufficient energy in the area element of the working plane 80. The generated first and second pixels 181, 182 in the working plane also start as soon as the temperature reaches the threshold temperature. The generated first and second pixels 181, 182 refer to a connected area or more precise volume of sintered or melted material in the working plane 80. The generated first and second pixels 181, 182 start at the same time or at the same position in the working plane 80 and are synchronized as pulses emitted by the first and second laser pixels 171, 172. There is no phase shift between the pulses.
(33) FIG. 12 shows a principal sketch of a second PWM driving scheme. The structure of FIG. 12 is very similar as the structure of FIG. 11. The difference is that the pulses of different laser pixels 171, 172, . . . , 179 of a group of laser pixels which comprises in this case 9 laser pixels are phase shifted ? of pulse width 191 with respect to each other (or 1/9 of the pulse width modulation base time 190). The pulse width modulation base time 190 is in this case 50 ?s. And the pulse width 191 is 8/9 of the pulse width modulation base time 190. The time of full pixel 192 comprises four pulse width modulation base time is 190 and is therefore 200 ?s. Print head 50 moves with a velocity of 500 mm/s such that the length of full pixel 192a which corresponds to the time of full pixel 192 is 100 ?m. The printed full pixel 192b is also 100 ?m. The phase shifts of the starting time of the laser pulses of 1/9?50 ?s result in a corresponding shift in the generated pixels 181, 182 . . . . The maximum error 195 or the maximum shift between the first generated pixel 181 and the ninth generated pixel 189 is 8/9?25 ?m. Phase shifting of the pulses does have the effect that the electrical current or power which is needed to drive the lasers 115 or laser modules is smoothed. There is no time between two pulses in which no laser light is emitted as in the embodiment shown in FIG. 11. Eight laser pixels of the group of laser pixels are driven at the same time at nominal electrical power. Therefore on average 1/9 less electrical energy is needed to drive the group of laser pixels and the drive current is essentially constant in comparison to the embodiment shown in FIG. 11.
(34) FIG. 13 shows a principal sketch of a third PWM driving scheme. The third PWM driving scheme is adapted to a laser printing system 100 comprising a controller 10 which is adapted such that at maximum processing speed of print head 50 of 500 mm/s only two laser pixel or modules of a group of nine laser pixels or modules can be driven at nominal electrical power. Print head 50 comprises a multitude of such group of laser pixels or modules. The pulses of the different laser pixels 171, 172, . . . 179 of the group of laser pixels which comprises in this case 9 laser pixels are phase shifted ? of pulse width 191 with respect to each other (or 1/9 of the pulse width modulation base time 190). The pulse width modulation base time 190 is in this case 50 ?s. The pulse width 191 is 2/9 of the pulse width modulation base time 190 such that only two of the laser pixels 171, 172 . . . , 179 are driven at the same time. The time of full pixel 192 comprises 16 pulse width modulation base times 190 and is therefore 800 ?s. Print head 50 moves with a reduced velocity of 500/4 mm/s such that the length of full pixel 192a which corresponds to the time of full pixel 192 is again 100 ?m. The printed full pixel 192b is also 100 ?m. The phase shifts of the starting time of the laser pulses of 1/9?50 ?s result in a corresponding shift in the generated pixels 181, 182 . . . . The maximum error 195 or the maximum shift between the first generated pixel 181 and the ninth generated pixel 189 is 8/9?6.25 ?m. The third PWM driving scheme enables printing of complete layers within the working plane 80 at reduced electrical input power and printing velocity. There may be non-linear effects (e.g. caused by heat dissipation) depending on the material and the particle which is used to print the object 70. It may be necessary to adapt the driving scheme in accordance with these non-linear effects. Visibility of printing errors at the edges of an object 70 may be reduced by avoiding systematic and especially regular phase shifts between the pixels as shown in FIG. 13. Applying random phase shifts to the pulses under the boundary condition that only two laser pixels 171, 172 . . . , 179 emit laser light during printing may help to reduce visibility of such systematic printing errors
(35) FIG. 14 shows a principal sketch of a fourth PWM driving scheme. The fourth PWM driving scheme is adapted to a laser printing system 100 comprising a controller 10 which is adapted such that at maximum processing speed of print head 50 of 500 mm/s only one laser pixel or module of a group of four laser pixels or modules can be driven at nominal electrical power. Print head 50 comprises a multitude of such group of laser pixels or modules. The pulses of the different laser pixels 171, 172, 173, 174 of the group of laser pixels which comprises in this case 4 laser pixels are phase shifted ? pulse width modulation base time 190 with respect to each other. The pulse width modulation base time 190 is in this case 200 ?s. The pulse width 191 is 2/9 of the pulse width modulation base time 190 such that only one of the laser pixel 171, 172, 173, 174 is driven at one moment in time. The time of full pixel 192 comprises four pulse width modulation base times 190 and is therefore 800 ?s. Print head 50 moves with a reduced velocity of 500/4 mm/s such that the length of full pixel 192a which corresponds to the time of full pixel 192 is 100 ?m. The printed full pixel 192b is also 100 ?m. The phase shifts of the starting time of the laser pulses of ??200 ?s result in a corresponding shift in the generated pixels 181, 182, 183, 184. The maximum error 195 or the maximum shift between the first generated pixel 181 and the fourth generated pixel 189 is 18.75 ?m. The fourth PWM driving scheme enables printing of complete layers within the working plane 80 at reduced electrical input power and printing velocity.
(36) The description especially provided with respect to FIG. 13 and FIG. 14 applies to lasers 115 within a laser module or to laser modules within a group of laser modules. The driving schemes can be modified, for example, by means of buffer capacitors which enable to drive in extreme case all laser modules at the same time for a short period of time. There is a multitude of possible variations of the pulse width modulation base time 190, the pulse width 191, the phase shifts between the laser pulses, the pulse amplitude or, for example, the shape of the laser pulses which can be combined with a multitude of possible arrangements of groups of lasers or laser modules which are supplied by one electrical power supply 20.
(37) FIG. 15 shows a principal sketch of method steps of a method of laser printing. An object 70 in a working plane 80 is moved relative to a print head 50 in step 210. The print head 50 comprises a total number of laser modules 150. At least two of the laser modules 150 share an electrical power supply 20. In step 220 is laser light emitted by means of the laser modules 150. The laser module comprises at least one laser array 110 of lasers 115. The laser modules 150 are controlled in step 230 such that at maximum processing speed of the print head 50 only a predefined number of laser modules 150 can be driven at nominal electrical power wherein the predefined number of laser modules 150 is smaller than the total number of laser modules 150.
(38) It is a basic idea of the embodiments presented above to reduce the maximum electrical input power which can be provided to the lasers despite of the fact that the lasers can be driven in parallel at nominal power. This has the consequence that either a part of the lasers has to be switched off or the power emitted by the lasers is reduced if more than a predefined number of laser modules has to be driven at nominal power in order to process a given structure at maximum processing speed. Both have the consequence that the overall production time increases. This consequence is acceptable in view of the fact that usually at 80% of the production time only 10% to 20% of the lasers are driven at nominal electrical power. The reduction in processing speed is thus limited to the 20% in which more than, for example, 20% are activated in parallel. The total reduction in processing speed is small but the reduction with respect to the complexity of the print head and the power supply of laser modules is significant.
(39) To maximize the usefulness of the power reduction strategy some modifications in the electrical system may be necessary. External power supply have to be shared over sufficiently large number of laser modules. The number of laser modules depends on the configuration which is used in order to limit the electrical input power which is provided or supplied to the lasers. It may be sufficient to supply a group of laser modules as for example a diagonal column or row as described above by means of one electrical power supply. In other configurations it may be beneficial to supply all laser modules with one electrical power supply. Sharing of external power supplies has the effect that the supply current remains limited even if larger structures in some area are sintered. The reduced peak power required still keeps dimensions of current distribution acceptable. Furthermore, it may be beneficial to allow control modulation with shifted PWM. Shifted PWM may be especially implemented by control of PWM in terms of pulse length and phase. Commercially available controllers are configured to enable control by means of shifted PWM.
(40) While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive.
(41) From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the art and which may be used instead of or in addition to features already described herein.
(42) Variations to the disclosed embodiments can be understood and effected by those skilled in the art, from a study of the drawings, the disclosure and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality of elements or steps. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
(43) Any reference signs in the claims should not be construed as limiting the scope thereof.
LIST OF REFERENCE NUMERALS
(44) 10 controller 12 data input 14 power input 20 electrical power supply 22 main data input 24 main power input 25 filter (comprises buffer capacitor(s) for providing power) 30 object carrier 40 frame 50 print head 52 print carrier 70 object 80 working plane 82 working area 100 laser printing system 110 laser array 115 laser 120 laser driver 122 DC/DC converter 124 signal isolation 126 PWM current source 150 laser module 160 group of laser modules 171 first laser pixel 171a energy of first pixel on target material 172 second laser pixel 173 third laser pixel 174 fourth laser pixel 179 ninth laser pixel 181 generated first pixel 182 generated second pixel 183 generated third pixel 184 generated fourth pixel 188 generated ninth pixel 190 pulse width modulation base time 191 pulse width 192 time of full pixel 192a length of full pixel 192b printed full pixel 195 maximum error 210 step of moving 220 step of emitting laser light 230 step of controlling
