WO2021154248A1

WO2021154248A1 - Heat source calibration

Info

Publication number: WO2021154248A1
Application number: PCT/US2020/015704
Authority: WO
Inventors: Daniel Pablo ROSENBLATT; Emilio Carlos CANO ARRIBAS; Emili SAPENA MASIP
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2020-01-29
Filing date: 2020-01-29
Publication date: 2021-08-05

Abstract

A system for lamp calibration in a printer comprises a lamp, a heat sensor, a processor and a controller. The lamp is configured to apply heat over a calibration surface on a build platform of the printer and the heat sensor is configured to capture a plurality of thermal images of the calibration surface over a period of time. The processor is configured to calculate a spatial profile of incident radiation generated by the lamp based on the captured plurality of thermal images and the controller is configured to adjust a modulated power signal applied to the lamp according to the calculated spatial profile of incident radiation. A method comprises applying a modulated power signal to a lamp to apply heat over a calibration surface, capturing a plurality of thermal images of the print bed over a time period when the lamp applies heat to the print bed, calculating a spatial profile of incident radiation generated by the lamp based on the captured plurality of thermal images and adjusting a power signal applied to the lamp according to the calculated spatial profile of incident radiation.

Description

HEAT SOURCE CALIBRATION BACKGROUND

[0001]A three-dimensional printer may generate a three-dimensional object by printing a series of two-dimensional layers on top of one another. In some three-dimensional printing systems, each layer of an object may be formed by placing a uniform layer of build material on a build bed of a printer and placing liquid printing agents at the specific points at which it is desired to solidify the build material to form a layer of the object. A fusing lamp, which may be provided on a carriage, may then apply localised energy to the layer of build material, to cause the build material to solidify in accordance with where printing agents were applied. [0002] Some three-dimensional printing systems may comprise one or more heating lamps to heat the build bed of a printer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] Figure 1 is a diagram of an example system for heating lamp calibration;

[0004] Figure 2 is a flow chart of an example heating lamp calibration method; [0005] Figure 3 is an illustration of an example thermal image for heating lamp calibration;

[0006] Figure 4 is a flow chart of an example method for determining heat incident on a calibration surface;

[0007] Figure 5A is an illustration of a graphical representation of temperature of a unit area of a calibration surface; and Figure 5B is an illustration of a graphical representation an example frequency domain based on fast Fourier transform analysis. [0008] Figure 6 is a flow chart of another example method for determining heat incident on a calibration surface;

[0009] Figure 7 is another illustration of a graphical representation of temperature of a unit area of a calibration surface; and [0010] Figure 8 is a diagram of an example of a machine-readable medium in association with a processor.

DETAILED DESCRIPTION

[0011] In three-dimensional printing, a layer of a three-dimensional object may be generated by solidifying a portion of build material to which a printing agent has been applied. The build material may be a powder or powder-like material.

[0012] A first stage of a printing cycle may comprise providing a layer of powdered build material on a movable build platform, thereby forming a print bed; a subsequent stage of the printing cycle may comprise distributing a fusing agent over the layer of powdered build material in a predetermined pattern; a subsequent stage of the printing cycle may comprise applying energy over the print bed so that portions of the powder on which fusing agent is applied heat up and coalesce. In a final stage of the printing, the print bed may cool and the portions of the powder to which the fusing agent has been applied may solidify, thereby forming a layer of the object. This cycle may be repeated several times to form a plurality of successive layers, thereby generating the three- dimensional object.

[0013]The printer may comprise a plurality of heat sources, for example fusing lamps and heating lamps. One or more fusing lamps may be provided on a print carriage, wherein the print carriage may move over the print bed for the fusing lamps to heat the fusing agent.

[0014] During the printing process, it may be desirable to heat the print bed, for example to maintain the build material at a desired temperature. It may also be desirable to maintain uniformity in temperature across the print bed. The printer may comprise top heating lamps, which may be provided above the print carriage. The top heating lamps may be provided in fixed array over the heating lamps.

[0015] There may be a variability in the heating lamps, which may cause a non- uniformity in temperature across the print bed. A plurality of factors can affect the lamp heating intensity and spatial heating profile. Lamp heating intensity may vary due to hardware variability, such as power supply, filament thickness and/or optical tolerances.

[0016]The illumination profile of a heating lamp may change over time, which may affect the intensity by which the print bed is heated by the lamp and/or the location of the print bed that is heated by the lamp. A protective material, such as glass, may be provided between a heating lamp and the print bed. Over time, the glass may degrade, such that the optical transmittance of the lamp to the print bed surface may change. In other examples, condensation, dust, debris and/or burnt build material may gather on the glass over time. In another example, a lamp reflector, for example a thin gold layer sputtered onto a lamp bulb, may degrade over time. These changes may influence the intensity and spatial profile of heat from a lamp incident on the print bed.

[0017] Other conditions may affect the spatial profile of incident radiation on a print bed provided by a heating lamp. For example, the lamps may be mounted by hand and so an illumination profile of a lamp may vary due to improper mounting. In another example, components within the printer may affect the heating profile of a lamp on the build bed. For example, there may be reflective surfaces within a build chamber of the printer which may alter the illumination profile of a heating lamp. The reflective surfaces may move or degrade over time, which may affect the intensity and/or spatial profile of the incident radiation on the print bed.

[0018] Examples described herein may enable a print bed to be heated more uniformly. This may be achieved by calibrating the one or more heating lamps. [0019] Figure 1 shows an example of a system 10 for lamp calibration in a printer. The system 10 comprises a lamp 12 configured to apply heat over a calibration surface on a build platform of the printer, a thermal sensor 14 configured to capture a plurality of thermal images of the calibration surface over a period of time, a processor 16 configured to calculate a spatial profile of incident radiation generated by the lamp 12 based on the captured plurality of thermal images, and a controller 18 configured to adjust a power signal applied to the lamp 12 according to the calculated spatial profile of incident radiation.

[0020]The lamp 12 may be positioned above the build platform. The system 10 may comprise a plurality of lamps. The plurality of lamps may be arranged in an array. The array may be a regular array or an irregular array.

[0021] The at least one lamp 12 may be a top heating lamp provided in a printer. For example, the at least one lamp 12 may be provided over the print carriage, so that in use, the carriage is provided between the lamps and the build platform.

[0022]The thermal sensor 14 may be a thermal imaging camera. The thermal imaging camera may be configured to capture thermal images. A captured thermal image may be formed of a plurality of pixels, wherein a measured temperature may be determined for each pixel of the image. The thermal sensor 14 may capture images of the calibration surface on the build platform of the printer, when a print carriage is not positioned between the thermal sensor 14 and the build platform. The thermal sensor 14 may be provided in the printer adjacent the at least one lamp 12, so that the lamp 12 does not block the view of the thermal sensor 14. The thermal sensor 14 is configured to capture images of the calibration surface while the lamp 12 heats the calibration surface.

[0023] The calibration surface may be a surface of the movable build platform without any build material applied thereto. In other examples, the calibration surface may be a calibration plate, which may be provided on top of the build platform. The calibration plate may be formed of metal or ceramic or plastics material. In other examples, the calibration surface may be a surface of build material forming the print bed on the build platform. The lamp 12 may be configured to heat the calibration surface, and the thermal sensor 14 may be configured to capture a plurality of images of the calibration surface.

[0024] The thermal sensor 14 is configured to capture a plurality of images over a period of time, for example sequentially. Each of the plurality of images may correspond to the temperature of the calibration surface at a particular point in time within the period of time.

[0025] The processor 16 may be configured to calculate a spatial profile of incident radiation on the calibration surface generated by each lamp. The processor 16 may thereby be configured to calculate a map of the calibration surface, which may indicate how the calibration surface is heated by the at least one lamp 12.

[0026] The processor 16 may be configured to determine a variation in temperature over time at a unit area of the calibration surface, based on the thermal images. The unit area may correspond to a pixel in the thermal images captured by the thermal sensor 14.

[0027] The processor 16 may be configured to calculate the incident radiation generated by the lamp 12 at the unit area, based on the variation in temperature over time of the calibration surface and physical properties of the calibration surface material. The physical properties of the calibration surface material may comprise thermal conductivity and/or heat capacity and/or optical emissivity.

[0028] The processor 16 may be configured to calculate the incident radiation at each of a plurality of unit areas of the calibration surface, to generate a spatial profile of the incident radiation generated by the lamp 12. Calculating the incident radiation at each of a plurality of unit areas of the calibration surface, wherein each unit area corresponds to a pixel of the thermal images captured by the thermal sensor, may improve the accuracy of the generated spatial profile of the incident radiation generated by the lamp. [0029] Generating a spatial profile of incident radiation generated by the lamp may also be used in detecting defects in a lamp. The generated spatial profile of incident radiation may thereby be used in quality control, and may indicate whether a lamp requires replacement. [0030] The processor 16 may be configured to calculate an average lamp intensity over an area. The area may be a predetermined area or may be an area determined based on the generated spatial profile of the incident radiation.

[0031] The controller 18 is configured to adjust a power signal applied to the lamp 12 according to the calculated spatial profile of the incident radiation. The controller 18 may be configured to adjust the power signal according to the calculated average lamp intensity.

[0032] Figure 2 is a flowchart of an example method of calibrating a heating lamp. The method may be executable by the system as shown in Figure 1 .

[0033]The method comprises, in block 202, applying a modulated power signal to a lamp to apply heat over a calibration surface. The modulated power signal applied to the lamp may be frequency modulated or pulse width modulated. In other examples other modulation schemes may be applied to the power signal. The modulated signal may be any of or any combination of a pulse width modulation signal, a voltage signal, a current signal, a power signal. The power signal applied to the lamp may be a pre-determined temporally shaped power signal.

[0034] In an example, applying power to a lamp may comprise applying power to a plurality of lamps simultaneously. A modulated signal may be applied to each lamp, wherein the modulated signal applied to each lamp has a different frequency. In another example, applying power to a lamp may comprise applying power to only one lamp of a plurality of lamps in an array.

[0035] The method comprises, in block 204, capturing a plurality of thermal images of the calibration surface over a time period when the lamp applies heat to the calibration surface. Capturing thermal images of the calibration surface may comprise capturing a plurality images sequentially, such that each thermal image is taken at a particular point in time in the time period.

[0036] The method comprises, in block 206, calculating a spatial profile of incident radiation generated by the lamp based on the captured plurality of thermal images.

[0037] Calculating the spatial profile of incident radiation generated by the lamp based on the captured plurality of thermal images may comprise determining a variation in temperature over time at a unit area of the calibration surface, based on the thermal images. Calculating the spatial profile of incident radiation generated by the lamp may comprise determining a variation in temperature over time for each of a plurality of unit areas of the calibration surface, based on the thermal images. Methods for calculating the variation in temperature over time at a unit area are illustrated in figures 4 and 6.

[0038] Calculating the spatial profile of incident radiation generated by the lamp may comprise determining the incident radiant intensity generated by the lamp at each of a plurality of unit areas based on the determined variation in temperature.

[0039] Figure 3 is an illustration of an example captured thermal image 300, when one lamp of the plurality of lamps is illuminated. As indicated in figure 3, the processor may be configured to divide the captured thermal images into regions according to their temperature, for example into quartiles 302, 304, 306, 308. The central quartile 308, at which the temperature is highest, may be the region which includes the plurality of unit areas 310 for which the incident radiant intensity is determined. [0040] Returning to figure 2, the method comprises, in block 208, adjusting a power signal applied to the lamp according to the calculated spatial profile of incident radiation. Adjusting a power signal applied to the lamp may comprise modifying the modulated signal applied to the lamp. Adjusting the power signal may comprise adjusting the temporal sequence of the power signal. [0041]Adjusting the power signal applied to the lamp may comprise calculating an average lamp intensity over a predetermined area of the plurality of unit areas, based on the calculated spatial profile of incident radiation. The method may comprise comparing the average intensity with a predetermined intensity and may comprise adjusting the power based on the comparison. Adjusting the power signal applied to the lamp may allow the print bed to be heated uniformly.

[0042] Figure 4 is a flowchart of an example method 400 of determining the spatial profile of incident radiation provided by a lamp based on thermal images captured when a power signal with a predetermined frequency is applied to the lamp. The method may comprise, in block 402, generating the temperature profile of a unit area of the calibration surface over time, based on the thermal images. Figure 5A is a graphical representation of an example temperature profile of a unit area over time. As indicated by figure 5A, the temperature of the calibration surface in the unit area increases over time, but oscillates according to the oscillating power applied to the lamp.

[0043] The method may comprise, in block 404, applying a fast Fourier transform (FFT) to the temperature profile and extracting the amplitude of the FFT at the frequency F at which the power signal is applied to the lamp. Figure 5B is a graphical representation an example frequency domain based on the FFT analysis, and indicates the amplitude A at the frequency F at which the power signal is applied to the lamp. Applying a FFT for determining a temperature profile may be useful, because precision can be improved by increasing the acquisition time of the thermal images, because when using FFT, an increased number of cycles of the power signal applied to the lamp provides more precision.

[0044] The method may comprise, at block 406, calculating a peak-to-peak temperature oscillation at the applied frequency F based on the extracted amplitude. Calculating the peak-to-peak temperature oscillation at the applied frequency F may indicate how strong the influence of the lamp is at the unit area. [0045] The method may comprise, at block 408, determining the incident radiation at the unit area based on the peak-to-peak temperature oscillation and physical properties of the material of the calibration surface. The physical properties of the material of the calibration surface may be predetermined and may be pre-stored in a memory of the system. The physical properties of the material may comprise at least one of thermal conductivity, heat capacity and optical emissivity.

[0046] The method may comprise repeating blocks 402, 404, 406 and 408 for each of a plurality of unit areas of the calibration surface.

[0047] The method may be repeated for each of a plurality of lamps in a lamp array of a printer. In an example, the method may be carried out simultaneously for each of the plurality of lamps. Each of the plurality of lamps may be illuminated simultaneously when capturing the thermal images. A different power signal may be applied to each of the plurality of lamps, each power signal having a different frequency. Extracting the amplitude of the FFT applied to the temperature profile of a unit area may comprise extracting one or more amplitudes corresponding to the different frequencies of lamps which heat that unit area of the calibration surface. Calculating the peak-to-peak temperature oscillation based on each extracted amplitude for the unit area may indicate how strong the influence of each lamp is at the unit area. Carrying out the method simultaneously for each of the plurality of heating lamps may reduce a time taken in calibrating heating lamps of a printer.

[0048] In the method shown in figure 4, the lamp may begin to heat the calibration surface from the point that power is initially applied to the lamp, and thermal images may be captured from that point in time, as shown by the temperature profile in figure 5A. In another example method of determining the spatial profile of incident radiation, shown in figure 6, power may be applied to the lamp, and the lamp may illuminate the calibration surface when the lamp radiation has stabilised. This may be achieved by blocking the radiation from the lamps to the calibration surface, for example by the carriage, for a predetermined time. The carriage may be removed abruptly, and the thermal images may be captured as the lamp heats the calibration surface.

[0049]The method shown in figure 6 may comprise, in block 602, generating the temperature profile of a unit area of the calibration surface over time, based on the thermal images. Figure 7 is an example graphical representation of a temperature profile of a unit area.

[0050]As shown in figure 7, there is an initial step increase in temperature, at the time at which the carriage is removed from the position between the lamp and the calibration surface. [0051] The method may comprise, in block 604, fitting a function to the generated temperature profile. Figure 7 shows the fitted line. The function may be an exponential function.

[0052]The method may comprise, in block 606, calculating a heating rate for the unit area based on the exponential function. [0053]The method may comprise, in block 608, calculating a heat incident on the unit area by the lamp, based on the heating rate and physical properties of the calibration surface.

[0054]The method may comprise repeating the process of blocks 602, 604, 606 and 608 over a plurality of unit areas to generate the spatial profile of incident radiation generated by the lamp.

[0055]Various elements and methods described herein may be implemented through the execution of machine-readable instructions by a processor. Figure 8 shows a processing system comprising a processor 802 in association with a non-transitory machine-readable storage medium 804. The machine-readable storage medium 804 may be a tangible storage medium such as a removable storage unit or a hard disk installed in a hard disk drive.

[0056] The machine-readable storage medium 804 comprises instructions to apply a modulated power signal to a lamp to heat a calibration surface; instructions to capture thermal images of the calibration surface over a period of time; instructions to calculate a spatial profile of incident radiation generated by the lamp based on the captured plurality of thermal images; and instructions to adjust the power applied to the lamp according to the calculated spatial profile of incident radiation.

[0057]The instructions to adjust the power applied to the lamp may comprise instructions to modify a modulated power signal applied to the lamp.

[0058] The machine-readable storage medium 804 may comprise instructions to calculate an incident radiation at a plurality of areas of the calibration surface and instructions to calculate an average lamp intensity by calculating an average incident radiation over a predetermined region of the calibration surface based on the calculated incident radiation of the plurality of areas provided within the predetermined region

[0059]According to examples described herein, a heating lamp may be calibrated, so that a print bed may be heated uniformly. This may provide uniformity in printed parts. Determining a spatial profile of incident radiation generated by a lamp may improve reliability of the calibration. According to examples described herein, a heating profile generated by each lamp may be optimised, for example at a customer site and for example after replacement of a lamp.

[0060]According to examples described herein, a spatial profile of incident heat generated by a lamp may be determined. This may be used to detect defective lamps or faulty mounting of lamps in a printer.

[0061] Examples described herein comprise determining the spatial profile of heat incident on a print bed generated by a lamp, and encompasses in the determination optical effects of build material, and reflections from surfaces of components inside the printer. The accuracy of the determined spatial profile may thereby be improved, which may improve uniformity of heating of the print bed.

Claims

1. A method comprising: applying a modulated power signal to a lamp to apply heat over a calibration surface; capturing a plurality of thermal images of the calibration surface over a time period when the lamp applies heat to the print bed; calculating a spatial profile of incident radiation generated by the lamp based on the captured plurality of thermal images; and adjusting a power signal applied to the lamp according to the calculated spatial profile of incident radiation.

2. A method in accordance with the method of claim 1 , wherein determining the spatial profile of incident radiation comprises determining a temperature profile of each of a plurality of unit areas of the calibration surface.

3. A method in accordance with the method of claim 2, wherein the unit area corresponds to a pixel of a thermal image.

4. A method in accordance with the method of claim 2, wherein determining the spatial profile of incident radiation comprises calculating the incident radiation generated by the lamp at a unit area of the calibration surface, based on the determined temperature profile of the unit area and physical properties of the calibration surface material.

5. A method in accordance with the method of claim 1 , comprising calculating an average intensity of incident radiation over a predetermined area of the calibration surface and comparing the average intensity with a predetermined intensity, wherein the adjusting the power signal applied to the lamp comprises adjusting the power signal based on the comparison.

6. A method in accordance with the method of claim 1 , comprising applying a power signal to a plurality of lamps, calculating the spatial profile of incident radiation generated by each of the plurality of lamps, and adjusting the power signal applied to each of the lamps individually according to the calculated spatial profiles.

7. A method in accordance with the method of claim 6, comprising applying the power signals to the plurality of lamps simultaneously, wherein the power signals are frequency modulated and a frequency of the power signal differs for each of the plurality of lamps.

8. A method in accordance with the method of claim 6, comprising applying the power signals to the plurality of lamps sequentially.

9. A system for lamp calibration in a printer, comprising: a lamp configured to apply heat over a calibration surface on a build platform of the printer; a heat sensor configured to capture a plurality of thermal images of the calibration surface over a period of time; a processor configured to calculate a spatial profile of incident radiation generated by the lamp based on the captured plurality of thermal images; and a controller configured to adjust a modulated power signal applied to the lamp according to the calculated spatial profile of incident radiation.

10. A system in accordance with the system of claim 9, wherein the heat sensor is a thermal camera, and wherein the processor is configured to calculate a temperature profile over time of each of a plurality of pixels of an image captured by the thermal camera.

11. A system in accordance with the system of claim 10, wherein the processor is configured to calculate an incident radiation at a plurality of areas of the calibration surface corresponding to each of the plurality of pixels based on the calculated temperature profiles and physical properties of the calibration surface.

12. A system in accordance with the system of claim 11 , wherein the processor is configured to calculate an average lamp intensity by calculating an average incident radiation over a predetermined region of the calibration surface based on the calculated incident radiation of the plurality of areas provided within the predetermined region.

13. A system in accordance with the system of claim 9, comprising a plurality of lamps, wherein the processor is configured to calculate a lamp intensity of each of the plurality of lamps based on the plurality of thermal images and adjust a power applied to each of the plurality of lamps.

14. A non-transitory machine-readable storage medium encoded with instructions executable by a processor, the machine-readable storage medium comprising: instructions to apply a modulated power signal to a lamp to heat a calibration surface; instructions to capture thermal images of the calibration surface over a period of time; instructions to calculate a spatial profile of incident radiation generated by the lamp based on the captured plurality of thermal images; and instructions to adjust the modulated power signal applied to the lamp according to the calculated spatial profile of incident radiation.

15. A non-transitory machine-readable storage medium in accordance with claim 14, comprising instructions to calculate an incident radiation at a plurality of areas of the calibration surface and instructions to calculate an average lamp intensity by calculating an average incident radiation over a predetermined region of the calibration surface based on the calculated incident radiation of the plurality of areas provided within the predetermined region.