Novel large-range, high-resolution 3D printing of microcomponents using dspace

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1 dspace User Conference 2010 India Sept 24 th 10 Novel large-range, high-resolution 3D printing of microcomponents using dspace Alem Baraki, Kiran Bhole Research Scholar, Department of Mechanical Engineering, IIT Bombay Prasanna Gandhi Associate Professor, Department of Mechanical Engineering, IIT Bombay ABSTRACT Microstereolithography is novel 3D micro-fabrication technique capable of producing complex high-resolution microstructures in layer-by-layer fashion. To achieve fabrication, it is crucial to have efficient slicing, scan path generating algorithm and data transfer scheme to control the scanning mechanism and laser spot. The type and capability of interfacing system is essential ingredient in the fabrication process. In this work slicing and scan path generating algorithms are developed along with data transfer scheme with an aim of fabricating large-range high- resolution components. dspace is used as real time interfacing unit to link developed algorithms with the machine hardware. Simulink model is constructed to represent data flow and data conversion in to serial compatible form and then back to double data type. Developed model is integrated with the main Simulink model constructed to control the scanning system. By building the overall integrated Simulink model, C code is generated to load the required task to DS1104 microcontroller. Laser path data for each of the layers of desired model is generated and converted in to serial compatible form in MATLAB and is directly sent to dspace DS1104 microcontroller serially. Scan path data is then dispatched to respective lines through dspace D/A interfacing board to control the optomechatronic scanning system. The flexibility of dspace has greatly helped to send data directly from computer through MATLAB thereby allowing large output buffer. Combining the potential of developed path generating algorithm and dspace serial data transfer scheme, memory limitation is alleviated and hence large components are successfully fabricated at high resolution. 1. INTRODUCTION Microstereolithography (MSL) has drawn worldwide attention due to its ability to fabricate 3D micro-structures of complex shape with few-micron resolution [1]. Like most rapid prototyping systems, MSL is computer based technique where 3D CAD model of desired part is sliced in to series of horizontal cross-sections out of which laser scan path data is extracted to control the opto-mechanical scanning system. Sliced 2D cross-sections are either beam or stage-scanned [2] or are projected into the resin using sequence of dynamic lithography masks [3, 4]. In dynamic mask method, each layer of 3D part is exposed in one irradiance step by projecting its image on surface of photosensitive resin. However, this method has inherent limitation on resolution due to diffraction of light. In laser beam scanning MSL [2, 5, 6], cross-section of 3D structure is cured line-by-line scanning of focused beam over the UVcurable photopolymer; hence, better resolution can be achieved. Once a layer is fabricated the next layer of photopolymer is allowed to flow at the focus plane for solidification of the next cross-section. These steps are then repeated, and thus 3-D freeform microstructure is fabricated by stacking these series of cured layers. There are two fundamental process used to extract the geometric information of 3D CAD objects namely 1) Slicing Process, which involves different possibilities of intersection between triangular facet of the STL model and slicing plane [7] and 2) Generation of laser scan path data, used to guide the focused laser beam during hatching process [8,9]. Though scanning MSL can achieve higher resolution components, line-by-line scanning of each layer requires large amount of data and more processing time. Hence, simultaneous need for large range and high resolution microstructures becomes a real challenge. In this study, new laser scan path generating algorithm for generating data quickly and serial communication to handle the large data which also in turn enable to eliminate the sole dependency on microcontroller memory is discussed in detail. Based on the crank scanning path pattern, developed algorithm uses path data of single x (pitch) and single y (scan width) to generate the scanning path and laser on/off control for the entire layer. Developed algorithm enables to save large amount of system memory because only initial path data, (data for two lines and laser on/off data) are stored. Moreover, simulation time is greatly reduced since further generation of path data for full layer involves only toggling and

2 concatenation of vectors without the need of point-by-point computation. In addition, the effect of acceleration and deceleration in photopolymer solidification is taken care by extending the scanning width in the direction of scanning. Furthermore, serial port interfacing is used to link the hardware components of the scanning system to computer. In the end, algorithm is experimentally verified by successful fabrication of large size microcomponents. 2. SLICING ALGORITHM The crucial step in layered micro-fabrication such as SL and MSL system is to extract the geometric information of 2D horizontal layers from 3D STL CAD model of the desired part. This process involves slicing a given 3D STL model based on the desired laminate thickness. As STL model is triangular representation of peripheral surface of CAD models, slicing algorithm involves computation of intersection points between a triangular facet and slicing plane at a given slicing height. As illustrated in Figure 1, there are five possible cases of facet/slicing plane intersection. Figure 1: Various possibilities of facet/slicing plane intersection As the next step, facet/slicing plane intersection points are calculated to form contour matrix. The last step in the slicing algorithm is reshaping and optimizing of contour points. Repeated points and if points, other than end points, lie in a straight line (Figure 2a); they are removed from the contour matrix as shown in Figure 2b. The advantages in having optimized contour matrix are as follows; Scan path can be generated in any direction Enables generating properly ordered laser on/off data Enhanced processing time and reduced system memory Ease in generating data for boundary scanning Figure 2: Reduction of contour points by removing intermediate points in a straight line

3 3 SCAN PATH GENERATING ALGORITHM Once the contour matrix of a given layer is obtained by slicing the STL model, the next step is to obtain the scanning (hatching) data and laser on/off control data. Among two raster scan hatching patterns, crank type and zigzag type, the crank pattern is used. Figure 3 shows various terminologies used in the discussion. Figure 3: Terminologies used in the laser scan path generating algorithm 3.1 Scan Path Generation Considering Effect of Machine Dynamics The phenomena of photo polymerization in MSL system is derived based on constant scanning speed. But the translational tracking system used to guide the laser spot needs to accelerate to reach the scanning speed and decelerate to rest to avoid vibrations during hatching process. This acceleration/deceleration affects the irradiation time of laser beam resulting in non-uniform layer thickness of the part. Figure 4a shows the effect of acceleration and deceleration on photopolymer solidification where heavy curing takes place near the ends. Figure 4: Effect of acceleration and deceleration on photopolymer solidification Developed path generating algorithm considers the effect of acceleration and deceleration using the concept of minimum bounding rectangle of layer (MBRL). For a given sliced layer, algorithm finds the minimum bounding rectangle for the layer. The size of the MBRL is extended in the direction of scan causing acceleration and deceleration to be outside the sliced layer. Laser power is kept off during acceleration and deceleration; hence, fabricated components have uniform layer thickness as shown in Figure 4b. 3.2 Scan Path Generation For Large Range Components It is not easy to fabricate larger structures at high resolution unless tradeoff is made. Both range and resolution are highly affected by parameters such as simulation time (processing time), system memory and fabrication speed. To achieve high resolution, scan path generating algorithm must have the ability to generate discrete data at higher sampling rate.

4 This requires huge system memory and takes more time to process. In view of this challenge, the main target of developed algorithm is to generate scanning path data for larger size components while maintaining reasonable resolution. Over all process of algorithm involves two phases: 1) slicing phase (Figure 5a) and 2) fabrication phase (Figure 5b). During slicing process, STL file is given as input from which all coordinates representing vertices of triangular facets of the solid model are extracted in to organized form to ease subsequent processes. To slice the model at the given layer thickness (Z = Z i ), slicing algorithm sorts all facets which are intercepted by the slicing plane to form matrix of intercepted facets. As the next step, algorithm calculates facet/slicing plane intersection points to form contour data matrix for the given layer. In order to form properly closed contours, intersection points are sorted and all contours in the same layer are identified. The last and crucial step in the slicing phase is to generate initial scan path data and laser on/off control data for the specified layer. Based on the required scanning speed and sampling rate, algorithm generates discrete data to describe the path of single Y scan width (scan ray) and for single X scan width (pitch). Laser on/off control data is generated by comparing position of scan ray with laser control points (intersection between scan ray and contour segment). Initial variables (Y position, X position and laser on/off control data) are saved for each layer to be used as input during fabrication phase. Figure 5: Flow chart for developed laser scan path generating algorithm The second phase, fabrication stage, of developed algorithm uses initial scan path data as input to compute the scan path and laser control data for full layer. If the size of a layer is large, it is difficult to handle variables of huge array size due to memory limitations. To overcome this difficulty, single layer is subdivided in to portions (chunks). Complete scan path data is generated for the first chunk of the layer. This data is converted in to serial compatible form and transferred to respective lines of the scanning system with the help of serial interfacing. Similar process is followed for the next

5 chunk of the layer. Once one layer is processed, algorithm loads initial laser and path data for the next layer and same process is repeated until all layers are processed. One of the hallmarks of this algorithm is its ability to generate scan data online. Once data for one chunk is transferred to microcontroller, algorithm generates scan data for the next chunk until previously sent data is dispatched. Algorithm is developed using MATLAB software, which is powerful tool in matrix manipulation. Table 1 shows comparative result in simulation time needed to generate the final executable data for fabrication (slicing and path generation). This result shows that there is tremendous reduction in simulation time (more than 3 day simulation time is reduced to less than 2 minutes). Developed algorithm is fast and stable even for large size structures. Input parameters (scanning speed = 0.8mm/s, Line spacing = 0.005mm, number of layers 1 layer each, percentage for acceleration and deceleration = 0.1) Model size Table 1: Improvements in processing time Sampling rate, Ts = seconds Elapsed simulation time, (in seconds) Sampling rate, Ts = seconds (XxYx1mm) New algorithm Algorithm used by New algorithm Algorithm used by Suhas (6) Suhas(6) 1mmx1mm mmx2mm mmx3mm mmx4mm mmx5mm mmx6mm EXPERIMENTAL VERIFICATION 4.1 Interfacing Using Serial Port Serial data transfer program is developed using MATLAB and Simulink based on RS-232 protocol. DS1104 microcontroller serial setup is used as real time interface between the software and hardware systems. Figure 6 shows the schematic diagram of data flow using serial port. Simulink model is constructed to represent data flow and data conversion in to serial compatible form and then back to double data type to be displayed in the simulation window, as shown in Figure 7. By building the simulink model, C code is generated to assign the required task to DS1104 microcontroller, which dispatches data to respective lines to control the opto-mechanical scanning system as per the desired scan path. Figure 6: Schematic diagram of serial port interfacing using DS1104 serial setup

6 Laser path data for each layer of model is generated and converted in to serial compatible form in MATLAB and is directly sent to microcontroller serially. The dspace controldesk simulation panel is used to verify that data is being transferred properly. This is shown in Figure 7, where parameters such as Y-position vs. Time, X-position vs. time, laser on/off status and XY plot of the scanning path are displayed. Figure 7: Verification of data transfer using serial port interfacing (a) Micro-channel Fabricated dimensions: Overall dimension = 3833 µm x 3983 µm, Channel dimension= 146 µm, Counter hole reservoir= diameter 740 µm and 417 µm (b)high Aspect ratio micro-tanks Fabricated dimensions: Outer dimension of large range square tank = 4066 µm x 4132 µm; Innermost high aspect ratio square tank = 866 µm x 883 µm Figure 8: SEM images of successfully fabricated 3D multi-layer and high range micro-structures a) micro-channel b) concentric tanks. Use of serial data transfer in the developed laser path generating program offers the following advantages: 1) Enables the use of computer memory providing large output buffer size hence, no need to worry about memory limitation of microcontroller. Therefore, fabrication of large size micro-components can be achieved.

7 2) Capability of online scan path generation, path and laser control data can be generated until microcontroller consumes previously transferred data. This enables efficient use of processing and fabrication time. 3) It is possible to control data and operations such as, repeating layer, skipping layer etc., can be done easily. 4.2 Fabrication of Micro-components To achieve fabrication of microstructures, algorithm is implemented using developed On-axis opto-mechanical scanning system [5]. Finally, various components were successfully fabricated employing the developed algorithm. Process parameters used for fabrication of components are: scan speed =0.8 mm/s, laser power 150 µw, and layer thickness 100 µm. Following subsection and Figure 8 highlights the dimensional characteristics obtain upon fabrication of the large range, high resolution components Micro-channel: Figure 8(a) is SEM image of microchannel. The CAD model of microstructure is prepared with overall outer square of dimensions 4 mm x 4 mm with microchannels of 150 µm width. The counter shape reservoir dimensions are diameter 750 µm on top face and diameter 425 µm at the bottom face. Structure is fabricated with 4 layers of each 100 µm thickness Large Range Micro-tanks: Large range square tank of outer size 4 mm x 4 mm alogwith high aspect ratio square tank of outer size 875 µm x 875 µm, height 1mm is fabricated in single structure. Figure 8(b) shows SEM image of the fabricated structure. Microtank structure is fabricated with number of layers varying from 2 to 10 with a step of 2 layers from outer large range tank to innermost high aspect ratio tank. 5 CONCLUSION In this study, laser scan path generating algorithm is developed for on-axis scanning MSL system. Using few initial parameters such as path data of two lines and starting and end points of the scanning span, algorithm is capable of generating laser path data for complete layer. Simulation time and memory usage are greatly reduced thereby enabling fabrication of large size and high resolution components. The effect of acceleration and deceleration on photopolymer solidification is accounted by extending the scanning width based on the rise time of the scanning mechanism. Use of serial data transfer alleviates memory limitations of microcontroller (DS1104) by sending data directly from computer. Once data for one chunk is transferred to microcontroller, algorithm generates scan data for the next chunk until previously sent data is consumed by microcontroller. This approach avoids storing huge data in memory. Developed algorithm is experimentally verified by accurate fabrication of various components. REFERENCES 1. Vardhan V.K.; Jiang X.; Vardhan V.V. (2001) Microstereolithography and Other Fabrication Techniques for 3D MEMS, John Wiley Sons, Ltd. 2. Ikuta K.; Hirowatari K. (1993) Real three dimensional microfabrication using stereolithography and metal molding, Proceedings of the 6th IEEE Workshop on Micro Electro Mechanical Systems (mems93), pp Bertsch, A.; Berhard, P.; Renaud, P. (2001) Microstereolithography: Concepts and applications, In: Proceedings of the 8 th International Conference on Emerging Technologies and Factory Automation ETFA, vol. 2, pp Bertsch A.; Jiguet, S.; Bernhard P.; Renaud P. (2003) Microstereolithography: a review, In: Mat. Res. Soc. Symp.Proc., vol. 758, pp. LL1.1.1 LL S. Deshmukh; P. Gandhi (2009) Optomechanical scanning systems for microstereolithography (MSL): Analysis and experimental verification, Journal of Materials Processing Technology, vol. 209, pp S. Deshmukh (2009) Design and Development of Microstereolithography System for 3D Fabrication, PhD Thesis, Indian Institute of Technology, Bombay, India. 7. L. Zhang; M. Han; S.H. Huang (2002) An effective error-tolerance slicing algorithm for stl files, International Journal of Advanced Manufacturing Technology, vol. 20 pp J. Zhao; W. Liu; R. Xia; and H. Bian (2009) Novel scan path generation method based on area division for sffs, Journal of Mechanical Science and Technology, vol. 23 pp Y.H.Cho; I.H.Lee; D.-W.Cho (2005) Laser scanning path generation considering photopolymer solidification in microstereolithography, Microsystem Technologies, pp CONTACT Alem Baraki, M.Tech(Design), IIT Bombay, Alex1bw@gmail.com