Enhanced controlling of the SLS Process during a build
Contents
1. Figure 1 Horizontal vectors with different scanning laser pattern In the encoding of the scanning pattern describing these five vectors two LS commands are used for each of the vectors The numbers following the two LS commands describe the pattern For instance the two first LS commands found in the script file are LS16383 and LS65535 They correspond to the first vector the one on the top of Figure 1 In order to derive these numbers we first choose the desired pattern For the first vector the desired pattern was vector with maximum and uniform laser intensity The maximum intensity is level 7 therefore all the ten zones are assigned level 7 In an octal form we have 7777777777 describing the desired pattern If we subdivide this octal number in two integer numbers of 16 bit each we get in binary form 11111111111111 for the first integer number ie 16383 in decimal and 1111111111111111 for the second one i e 65535 in decimal For the second vector the desired pattern is 0123456777 in octal form or 000001010011100101110111111111 in binary form We divide this binary number into two integer 16 bit numbers 00000101001110 i e 334 and 101110111111111 i e 24063 Thus the LS commands for the second vector are LS334 and LS 24063 Computer Implementation The control software of the DSP controller has two parts due to the design of the system 1 The DSP processor requires the support of a host computer for data stora2. geometry the following procedure is followed 1 We initialize all the vector zones with the laser setting specified in the input file For the sake of simplicity we have assumed that the laser setting is higher than needed Therefore the laser setting can only be decreased and the phenomenon of low sintering is not taken into consideration in the analysis rule presented here 2 We make sure to identify the vector zone we are currently in depending on the progression of the vector scanning As we saw there are ten vector zones which are equal in length for each vector and seven laser power settings 3 We call the subroutine Analyze which can be found in the module models c This subroutine will decrease the laser setting in each vector zone according to the information from the simulation data on this region of the powder bed By automatically we understand with minimum user intervention 52 4 Once a vector has been analyzed we derive the two attributes that will be specified by the two LS commands _ 5 Unless we are dealing with a scan vector we copy the same command we read in the scan file into the script file However if we are dealing with a scan vector we first write the two LS commands for the scan vector in the script file and then we copy the NX and NY command This is in order for the dynamic controller to know a priori what laser setting is required to scan the given scan vector Performance and Limi
3. Enhanced controlling of the SLS Process during a build Alexandre L Papadatos Kenneth Stanley Jr Said Ahzi and Frank W Paul Center for Advanced Manufacturing Department of Mechanical Engineering Clemson University Clemson SC 29634 Abstract Current commercial Rapid Prototyping RP systems like Stereolithography 3D Systems Corporation and Selective Laser Sintering DTM Corporation use galvanometers from General Scanning Inc GSI for the positioning of the laser beam The CSI scanners are delivered as a black box Operating Consoles which are usually Personal Computers PC have very few feedback from the performance of the GSI scanners Therefore the PC spends 90 or more of its time waiting for the GSI scanners to be over with the building of the current layer before sending the information regarding the next layer Also very little process control can be performed during the scanning of a layer using the GSI scanners This kind of setup prevents any dynamic controlling of the process that could prevent building errors like burning warping etc At Clemson University our team has developed both hardware and software components that allows a dynamic control of the building process New features like scanning one vector with laser power as a function of position and or time are now possible Both hardware and software issues will be presented Background To create objects using the Selective Laser Sintering process an X Y optica
4. LS process either with the GSI system or the newer DSP system All the signal of the DSP GSI interface board are designed on one circuit board that is sized appropriately to fit in the EURO Card Frame of the DE 2000 series controller from General Scanning A schematic for the DSP GSI interface board and further technical information can be found in Appendix A of 1 48 Software Design for the Dynamic Op tical Scanning Controller Terala 2 and Stanley 1 initially developed the control program developed for the CLEMSON SLS project using the GSI controller The new controller program incorporates some of the control software developed by Terala and Stanley Our contribution consists of further testing the software correcting some problems due to communications problem between the DSP and the PC and also enhancing the capability of the DSP system by featuring the variation of the laser intensity while scanning a vector The software developed for this research uses scan commands with the same format as the GSI controller in order to provide compatibility with previously generated scan files One new command is added called the Laser Setting The syntax is Lowonxx where xxxxx is an integer number from 0 to 65535 Each vector can now be scanned with a particular laser intensity pattern While being scanned each vector is divided into ten zones each equal in length One laser intensity can be specified for each of these ten zones Eight diffe
5. aster the layers than with the GSI system since most of the informations for the next layer have already been transferred in the available memory bank when the laser ends a layer 50 using the DSP controller Figure 2 displays th Dynamic Controller Results In order to test the ability of the new controller we took an existing scan file and added manually LS commands for each of the vectors We then proceeded to the build e result of the dynamic scanning of one layer of a dogbone sample We used paper as a medium in order to visually track the dynamic scanning Figure 2 Dynamic scanning In Figure 2 we can clearly see where the ase intensity changes The bottom dogbone used a constant laser intensity to proove that dynamic scanning can behave similarly as when using the GSI controller The three dogbones have been scanned using horizontal vectors The dynamic controller takes each of these vectors and divide them into ten equal in length zones The largest the vector the largest the zones will be and the worst the resolution will get since we can only specify one laser kiii per zone There are two ways to remedy to this problem 1 Increase the number of zones per vector This would result in a direct increase of the length of the script file since more LS commands will be necessary A major update of the software is aaa in this case including code programming 2 Scan the part in a different way For instance the use
6. erations are over for that layer It then spreads a new layer of powder and downloads to the GSI system the informations required for the processing of the next layer In the GSI system the laser controlling software that processes the scanning of a layer cannot be willingly modified It is a property of General Scanning Inc and it is engraved in a memory chip However the new controller developed by Stanley 1 uses a 47 Texas Instrument digital signal processor board 4 which is capable of implementing both digital control of the scanning system and supervisory control of the SLS process A main software is still required in order to read the commands for the layers to perform the management of the controller operation and as a user interface with the system This is mainly because the Texas Instrument board cannot access devices such as the hard drive the communication ports and the monitor Therefore an interface between the board and these devices is needed and this is the primary function of the main program in the new controller system the dynamic scanning controller The main difference with the GSI system is that the software controlling the laser position speed and intensity can now be modified to meet our needs Hardware Design for the Dynamic Optical Scanning Controller The optical scanner developed by Stanley 1 utilizes the actuators and sensors of the GSI scanning system but provides a new controller based on digital s
7. g the GSI controller 53 Figure 4 Part built using the DSP controller Conclusion The DSP system developed at Clemson University has been improved in two major ways Some problems caused by faulty communication between the DSP and PC programs have been corrected and controllin g the laser intensity defined by a new command has been implemented One single vector can be scanned using its own laser intensity pattern independently of previous or next configurations This feature allows to reduce the Sy when oan occurs Paay at the ma or to increase the intensity if the three dimensional CAD ob ject a a edi diting of the scan n file odin an oo task Our idea is to couple the results pr ided by the simulator to this new feature and automatically generate a script file t hat would correct problems like burning bonus Z or inadequate sintering The coupling ol of the simulation results and generation of the script files has been discussed i in this paper Literature cited 1 Stanley K Using Digital Signal Processing to Enhance the Optical Scanner System of Selective Laser Sintering MS Thesis Clemson University Clemson SC 1997 2 Terala K M Feed Forward Geometric Compensation for Selective Laser Sintering MS thesis Clemson University Clemson SC 1996 3 General Scanning Incorporated DE Series 2000 3000 Digital Electronics User Manual General Scanning Waterto
8. ge user interface and communication with the SLS machine Stanley named the program for the host PC PCBUILD C and for the DSP system DSPBUILD C The PC program has been written using Borland C v4 5 When the program starts a menu is displayed on the screen The user can perform from there several actions In order to start building a part the user needs to initialize the DSP system by selecting the first option of the menu Detailed instructions on how to use the PC program can be found in 6 The DSP program has been written in C using a text editor and the special compiler provided by Texas Instrument Most of the program is similar to the version written by Stanley However some communication problems between the PC and the DSP had risen as the entire system was used and tested In order to prevent communication problems we have enhanced the protocol used for the DSP and the PC by adding stages during the communication where the DSP and PC processors have acknowledge of each other s status The DSP waits on the PC to acknowledge its status before continuing Then the PC waits its turn on the DSP to acknowledge its status Due to this added checkings the time for initialization and data transfer has been slightly increased compared to Stanley s version However communication problems that caused the system to freeze have virtually disappeared Another enhancement over Stanley s version of the DSP program is the addition of the laser
9. ignal processing DSP With the DSP based controller control of the laser tracking is accomplished with software and control algorithms instead of the hardware controller from the GSI system This technique provides the user with a flexible control environment for laser tracking and also allows control of laser on off and laser intensity signals based on the galvanometer s position The sensors and actuators have been retained by Stanley in his DSP based system because they were the best technology available to move the mirrors with the required performances for rapid prototyping optical scanning The controller of the DSP optical scanner uses a C30 digital signal processor system from Spectrum Signal Processing Inc of Westborough Massachusetts 3 The C30 system board is designed to coexist in an IBM compatible PC Data transfer between the DSP and PC is accomplished through the ISA AT bus The C30 system board has 64K of 32 bit RAM memory that is accessible by both the PC and DSP These 64K of memory will be split into two memory banks of 32K each in order to speed up the process This feature will be discussed later in this chapter More detailed information on the board itself can be found in 1 3 4 since the DPS system uses the same sensors and actuators as the GSI system an interface board was required It has been successfully designed and developed by Stanley 1 The interface board hooks on the GSI system and allows the control of the S
10. intensity monitoring Since the DSP system supervises the scanning of each vector the system is also capable on setting the appropriate laser intensity during the scan of a vector The actions corresponding to the LS commands have been implemented The system monitors the scan of a vector and sets the laser intensity accordingly to the value specified after the two LS commands In order for the LS commands to be considered the commands LPO and LRO need to be previously specified This restriction was chosen to prevent confusion of the system by opposite commands LP and LR commands specify scanning of vectors with uniform laser intensity while LS commands specify vectors with variable laser intensity An optimization concerning the time required for data transfer from the PC to the DSP has been initially designed by Stanley The 64 kilobytes of memory that the DSP is equipped in order to store data have been divided into two banks of 32 kilobytes each The reason that led us to this decision is that the informations for one layer are seldom larger than 32 kilobytes This division of the DSP memory allows us to fill one memory bank with data from one layer while we process the data of the previous layer stored in the other memory bank Therefore in the ideal context where the time to process a layer is equal to the time to transfer the data into the DSP memory the data transferring time is reduced by half Practically the laser is always scanning f
11. l scanner using CAD solid model data guides or directs the laser beam to target points requiring sintering The SLS system employs the General Scanning Incorporated GSI optical scanner the DE series 2000 scanner controller and two G325 DT galvanometers with mirrors 1 3 The objective of Stanley s work 1 has been to identify the technological limitations of the GSI optical scanner as related to the scanning requirements of SLS and present a new optical scanner design based on digital signal DSP control theory One important accomplishment of the research done by Stanley is the use of information derived from the closed loop position control of the galvanometers to trigger the laser on off function This functionality has been enhanced and constitutes one of the main components for the dynamic optical scanning controller The GSI system has always been used as a black box and a very small amount of data is returned to the operating console We will describe how the build of an object is managed the software that supervises the SLS process the main software downloads the informations required to build one single layer into the GSI system When the downloading is over the main software orders the GSI system to go ahead and build scan the layer The main software then waits for the GSI system to finish When over the GSI system sends an OK command back to the main software Then the main software becomes aware of the fact that scanning op
12. lty is the main reason that led us towards the development of a new tool We will present the analysis software that uses the results from the simulation software in order to automatically generate a script file an improved scan file which will use the new features of the dynamic controller Automatic Generation of a Scan File For a machine to perform a task automatically it is necessary to give the machine certain guidelines on how to perform in different situations it will encounter The set of the guidelines is what we will be calling the analysis rules and depending on them the machine will perform accordingly in the analysis of the simulation results The analysis rule we have adopted as a testbed for our system is based on the maximum temperature reached in the powder bed We have been referring to the scan file used by the simulator and the GSI scanners as the scan file We will be referring to the new scan file that is generated by the analysis software as the script file since it also includes informations regarding the improvement of the build The scan file and the script file are describing the same geometry However the scan file uses two commands to define a vector whereas the script file describes a vector using four commands the two additional commands describe the laser power settings for a given vector The result is that the script file is close to double the size of the scan file For each scan vector describing the
13. of vertical vectors instead of horizontal will result in shorter vectors Therefore the zones will be shorter and the resolution will also be better This will lead to an increase of the length of the scan and script files since more vectors will be generated In this latter case however no update of the software is required 51 The Building Script At Clemson University a program simulating the SLS process has been developed and implemented 5 7 Once a simulation has been performed we are in possession of data representing the powder bed after the build that can be displayed on a computer screen as a visual representation of the powder bed These visual representations already provide the SLS user with important informations on how the build will actually look like Among these informations are burning thermal degradation of the material bonus z and low sintering 5 However without a tool that would modify dynamically the building parameters the user can do very little in order to prevent these unwanted phenomena The dynamic controller allows supervising the entire SLS process The dynamic controller also allows varying the laser intensity settings while scanning a vector Each vector can therefore have its individual laser power setting depending on the geometry However due to the amount of vectors required to build an entire geometry specifying manually the laser setting for each vector becomes an impossible task This difficu
14. rent levels define the laser intensity The first level level 0 represents 2 of the maximum laser intensity i e 1 Watt for a 50W C02 laser The second level level 1 represents 16 of the maximum laser intensity and so on At the end level 7 represents the maximum intensity that can be provided by the laser apparatus This configuration was chosen in order to be able to code the pattern in an octal form The octal format has been retained since the decimal format would require the use of three LS commands for each scan vector making a total of five commands required to build a single vector Choosing a smaller base would not provide such a wide range of power levers The use of the octal format requires only two LS commands to describe the way a vector should be scanned bringing the total to four commands for each scan vector Figure 1 shows five vectors scanned each with an independent scanning pattern We have scanned these vectors on paper in order to keep a visual track of the change in laser intensity The five vectors are horizontal and all are equal in length On the top of each vector we have added the laser intensity level for each of the laser zones The zones are shown at the top Zone BS Lone rd Zone 5 Lane F Zone 7 Zone 8 Ton 9 Tiea Zone ro Zone 1 Zone 3 7 7 7 7 7 7 7 7 7 7 i IOo 2 3 4 z 7 7 7 Ee SB a ik eee A j j S 0 2 4 6 7 7 6 4 2 0 Speers 7 i 7 0 7 0 7 0 7 7
15. ts The analysis rule we have used for testing the system is based on the maximum temperature reached in the powder bed The script file that is generated based on this analysis will only have an effect on the burning This is mainly due to the simplicity of the analysis that we have used In order to take into consideration other problems a more extended analysis is required and more complex analysis rules need to be implemented With a better detailed analysis the system including the simulation software the analysis software and the dynamic controller will provide better quality of the parts made using the SLS process However the goal of the research presented in this thesis has been to prove the feasibility of such a system and this is the reason we have used a simple analysis as a testbed Figure 3 shows the part built with the GSI controller using a constant laser intensity setting Figure 4 shows the same part built using the dynamic controller in conjunction with the simulation and analysis software When using the GSI controller the laser delivers too much energy into the powder bed as the length of the vector decreases at the edges We can see that the burning at the edges is eliminated when using the dynamic controller The burning was predicted by the simulation software and the analysis software created a script file in such a way that the laser intensity is decreased as the vectors become shorter Figure 3 Part built usin
16. wn MA 1991 4 Spectrum Signal Processing Incorporated TMS320 C30 System Board Technical Reference Manual Spectrum Signal Westborough MA 1992 5 Papadatos A L Ahzi S Deckard C R Paul F W On Dimensional stabilities Modeling of the Bonus Z During the SLS Process SFF Symp Proc 8 pp 709 716 1997 6 Davis A L Users Manual Clemson University Mechanical Engineering Department SLS Laboratory version 1 01 June 1998 7 Papadatos A L Computer Simulation and Dynamic Control of the SLS Process Clemson University Mechanical Engineering Department SLS Laboratory August 1998 55 9
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