Assignmnt 2 Parametric Model Lining

8/8/2019 Assignmnt 2 Parametric Model Lining

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PARAMETRIC MODELLING

ASSIGNMENT NO.2(Computer Integrated Manufacturing)

SUBMITTED BY

LT CDR ABDUL RASOOL MEMON PNMS (MSE&M)SEMESTER: FALL 2008


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PARAMETRIC MODELLING

1. Many fields have witnessed the emergence of a revolutionary vision that not onlyrepresented a sharp break from the past but also reshaped their development for aperiod of many years. In the field of mechanical design software, where new productswith hundreds of innovative features are introduced every year, the development ofparametric, feature-based, fully associative, solid modelling technology has playedsuch a role. Twenty-years ago, engineers were thinking through the formulas forgeometric shapes as they positioned 2D or 3D entities, with full knowledge thatunforeseen elements in the design would be likely to force them to start over fromscratch many times before they were done.

2. Samuel P. Geisberg, a mathematics professor who left Russia to come to theUnited States, had a vision of a better way to do mechanical design. His goal was to

develop a modelling system using features and parameters, a method of linkingdimensions and variables to geometry in such a way that when the parameter valueschange, the geometry updates accordingly. With this innovation, many design conceptscould be explored and changes could be made remarkably quickly compared with theredrawing required by traditional CAD. Its not hard to make the case that thedevelopment of mechanical design software since that time has merely represented thefleshing out the original parametric vision. This article will explore the emergence ofparametric, feature-based, fully associative, solid modelling and its continueddevelopment with new features and capabilities that have kept it at the leading edge ofmechanical design to this very day.

Traditional CAD

3. The first commercial computer aided design (CAD) tools, which were introducedin the 1970s, were primarily a replacement for the drawing board. So, while theyimproved productivity and accuracy of drawing creation, they did not have a majorimpact on the mechanical design process. A considerable amount of geometrical andtrigonometric calculations were required to create even relatively simple components.The surface and solid modelling tools that followed from the late 1970s to the late1980s extended the drafting paradigm by allowing engineers and designers to drawlines in 3D space. The result was wireframe models that could later be patched withsurfaces. The extension of the drafting paradigm to 3D space had the effect of

increasing the complexity of the required calculations to the point that considerablebackground in mathematics and surface geometry was often required to effectivelyutilise the tools. Another basic problem with the earlier generation of design softwarewas that all the geometry was explicitly created in reference to a spatial coordinatesystem. This meant that changes to the design often required that major sections of thedesign, or in many cases the entire design, be re-created from scratch. If we considerthe case of a 1/4 inch thick plastic moulding with hundreds of holes, bosses, and ribs.Lets suppose that at some point in the design process the decision is made to increasethe thickness to 3/8 inch. Now the through-holes in the part have suddenly becomeblind, and bosses and ribs have become partially buried. Substantial and time-consuming changes needed to be made merely to reproduce the original design intent.This was the situation faced by every computer-aided design software user up to thepoint when Geisberg entered the scene.

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Advancement

4. Arriving in the United States in 1974, Geisberg worked for two of the pioneeringcomputer aided design firms, Computer vision and Application. He became frustratedwith the limitations of traditional CAD systems and spoke to his employers aboutdeveloping a system that was less rigid and more flexible. His employers were focusedon making incremental improvements to their existing products so Geisberg obtainedventure capital backing from Charles River Ventures and other investors and foundedParametric Technology Corporation (now PTC) in 1985 to pursue his vision. CharlesRiver brought Steven Walske in as Chief Executive Officer in 1986 and thePro/Engineer product was shipped in 1988, marking the beginning of a new era inmechanical design. One key difference between Pro/Engineer and all other CAD toolsup to that point is the way geometry is modelled. The software contains solid primitivescalled features consisting of common engineering shapes such as holes, slots, bosses,

fillets, chamfers, protrusions, shells, etc. These features know how to behave relative toeach other and are defined by a set of parameters. Rather than drawing the geometryline by line and arc by arc, the software prompts the engineer for specific constraints.For example, Pro/Engineer prompts an engineer for the surface to begin a through-holeand the diameter, which is a parameter, of the through-hole. It doesnt ask for a depthbecause the software knows that a through-hole always goes through the full depth ofthe material. This means that when the depth of the material changes, also a parameterand all the through holes referencing that material will update automatically.Pro/Engineer is parametric in that objects are positioned using parameters and matingconstraints rather than the coordinate system. This means engineers can changedimensions or move objects and the connected geometry will automatically move or

change itself to stay in sync.

Reforming the design process

5. This invention dramatically streamlined the mechanical design process, making itpractical for engineers to create many more design variations at a higher level ofintegrity in a fraction of the time. Going back to the earlier example, when the thicknessof the plastic part is increased, the through-holes will automatically adjust to go all theway through the thicker part, the bosses and ribs will move so that they remain affixedto the surface of the part, and so on. The practical impact of the parametric modellingconcept is that engineers can now change designs and modify dimensions with a few

keystrokes or mouse motions. The underlying geometry of the part automaticallyadjusts itself to accommodate the change. The initial parametric vision includedassociability, where changes ripple through the design and all related deliverables. Upto that point, CAD software was similar to the drawing board in that each drawing ormodel was independent of the others. Drafters had to wait until the design was donebefore they could make 2D manufacturing drawings. If one view of a three view set ofmanufacturing drawings changed, the others had to be manually changed as well. Theadvent of associative CAD meant that downstream tasks such as drafting could bestarted in parallel with upstream activities such as design, thereby shorteningdevelopment time. This was the birth of concurrent engineering. Whenever the designchanged, the manufacturing drawings and other downstream activities referencing thatchange would automatically update.

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Changing the Conventional and Basic Methods

6. If that was it, the concept of parametric modelling would be a milestone in thedevelopment of mechanical design software, but not something that we would betalking about today. But the parametric concept so changed the ground rules inmechanical design that it has ushered in a stream of major developments that arecontinuing to drive software development to this day. Each of these developments flowsfrom the original concept of embedding intelligence into the features that define a partthus automating the tedious aspects of mechanical design and allowing the engineer tofocus on the creative process. The expansion of the original parametric modellingconcept was driven by customer demand. One of the first improvements that customersasked for was assembly modelling. Assembly modelling was a natural extension of theparametric modelling concept that essentially provided the same advantages tocomplex assemblies that the initial concept had provided to component parts. Soon

after parametric modelling reached users, versions became available that providedhierarchically linked assembly layouts designed to simplify the conceptual design ofcomplex assemblies and relate all base components using global dimensions, relations,and common datum. Users can use either graphics or spreadsheets to adjustparameters for the entire assembly to play out what-if scenarios for differentconceptual designs and perform packaging studies to represent assembly components.

7. A solid model generally consists of a group of features, added one at a time, untilthe model is complete. Engineering solid models are built mostly with sketcher-basedfeatures; 2-D sketches that are swept along a path to become 3-D. These may be cutsor extrusions for example. Another type of modeling technique is 'surfacing' (Freeform

surface modeling). Here, surfaces are defined, trimmed and merged, and filled to makesolid. The surfaces are usually defined with datum curves in space and a variety ofcomplex commands. Surfacing is more difficult, but better applicable to somemanufacturing techniques, like injection molding. Solid models for injection moldedparts usually have both surfacing and sketcher based features. Engineering drawingsare created semi-automatically and reference the solid models.The learning curve for these software packages is steep, but a fluent machine designerwho can master these software packages is highly productive. The modeling of solids isonly the minimum requirement of a CAD systems capabilities. Parametric modelinguses parameters to define a model (dimensions, for example). The parameter may bemodified later, and the model will update to reflect the modification. Typically, there is a

relationship between parts, assemblies, and drawings. A part consists of multiplefeatures, and an assembly consists of multiple parts. Drawings can be made from eitherparts or assemblies. Example: A shaft is created by extruding a circle 100 mm. A hub isassembled to the end of the shaft. Later, the shaft is modified to be 200 mm long (clickon the shaft, select the length dimension, modify to 200). When the model is updatedthe shaft will be 200 mm long, the hub will relocate to the end of the shaft to which itwas assembled, and the engineering drawings and mass properties will reflect allchanges automatically.Examples of parameters are: dimensions used to create model features, materialdensity, formulas to describe swept features, imported data (that describe a referencesurface, for example). Related to parameters, but slightly different are Constraints.Constraints are relationships between entities that make up a particular shape. For awindow, the sides might be defined as being parallel, and of the same length.Parametric modeling is obvious and intuitive. But for the first three decades of CAD this

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http://en.wikipedia.org/wiki/Freeform_surface_modelinghttp://en.wikipedia.org/wiki/Freeform_surface_modelinghttp://en.wikipedia.org/wiki/Engineering_drawinghttp://en.wikipedia.org/wiki/Computer-aided_design#Capabilitieshttp://en.wikipedia.org/wiki/Freeform_surface_modelinghttp://en.wikipedia.org/wiki/Freeform_surface_modelinghttp://en.wikipedia.org/wiki/Engineering_drawinghttp://en.wikipedia.org/wiki/Computer-aided_design#Capabilities


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was not the case. Modification meant re-draw, or add a new cut or protrusion on top ofold ones. Dimensions on engineering drawings were created, instead of shown.Utilizing the models for Manufacturing

8. The next major logical extension was downstream into the manufacturing arena.Pro/Engineer modules were introduced that utilise the original product design definitionto delineate the process steps and operations required to build the design. Theprinciple of associability was extended to allow interactive changes to the design modelto propagate through the development of the manufacturing tool paths, which referencethe part model, and update manufacturing instructions and documentation. The finalstep was typically the creation of code that was used to drive computerised numericalcontrol (CNC) machinery such as turning centres and machining centres. Laterenhancements extended this concept to other machining operations such as sheetmetal forming and blanking. As parametric modelling tools became the standard formechanical design, their use expanded into industries with specialised requirements.For example, designers of automobile bodies and consumer electronics packages,

among other products, need to define the flowing surfaces that often distinguish cutting-edge industrial design. In the past, designers of these products typically usedsketchpads, modelling clay and foam, and specialised surface modelling software todefine the product geometry. Mechanical engineers usually wound up with thechallenge of mathematically defining the geometry to a level of precision that enabled itto be economically manufactured. These requirements were addressed with thedevelopment of parametric modelling tools with powerful surfacing capabilities neededto turn out the complex surfaces that characterise cutting edge consumer products. Theparametric surfaces produced by these tools are designed in relation to the rest of themodel so as the design changes, so do the surfaces, and vice-versa.

Integrating analysis

9. By enabling engineers to generate a vastly higher number of design alternativesin a relatively short period, parametric modelling raised the question of how engineerswould determine which designs were better than others were. Clearly the time andmoney did not exist to physically build and test all of these designs. Computer aidedengineering tools that could analyse the performance of a design from a structural,thermal, fluid flow, and other standpoints predated the original parametric modellingconcept. The critical innovation that appeared less than a decade after the originalparametric concept was linking the parametric modeller to the analysis tool so thatengineering could optimise performance, reduce manufacturing costs, and improve

quality. In this way, engineering analysis becomes an integral part of the designprocess. As competition increased, manufacturers began developing products in aglobally distributed environment, creating the need to collaborate across time zonesand between different companies. Product Data Management (PDM) software used tostore, control and provide access to design models and other engineering informationwas developed to address this need. But its usefulness was limited by the fact that itwas developed separately from design software. An important milestone in thedevelopment of mechanical design was the integration of PDM capabilities with thecore parametric modelling software so they both work as a single integral system.Using this system, companies can create highly accurate digital products, collaboratedigitally throughout their extended value chain, control all associated productinformation and processes, and communicate via dynamic technical publications thatreference the right version of the model. This product development system becomesexpandable, linking legacy applications and heterogeneous CAD systems while

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protecting past IT investments. These newer capabilities have become collectivelyknown as Product Lifecycle Management (PLM) solutions.10. Parametric modelling is very powerful, but requires more skill in model creation.A complicated model for an injection molded part may have a thousand features, andmodifying an early feature may cause later features to fail. Skillfully created parametricmodels are easier to maintain and modify. Parametric modelling also lends itself to datare-use. A whole family of cap screws can be contained in one model, for example. Eachof these developments and many others that space does not exist to include here,flowed logically from the original concept of simplifying the mechanical design processby providing a mathematical framework that maintains logical consistency between thedifferent elements of the design. Parametric, feature-based, fully associative, solidmodelling, which began by revolutionising the way that engineers define the geometryof mechanical designs, has been integrally linked with data management andcollaboration to dramatically improve the entire product development process. Theresult is that engineers have the power to model any product design from simple tocomplex, define products to a higher level of precision, create more design alternatives,

predict the performance of those alternatives without the need for a physical prototype,and manage the entire product development lifecycle. For these reasons, parametricmodelling remains in exactly the same position that it was in 20 years ago, at thecutting edge of mechanical design.

Parametric Modelling Examples

Figures 1 and 2 show views of a cylindrical dia grid generated by a lisp program, whichis used to explore different options for modelling a building's structure. Close to ahundred variations were modelled, of which two are illustrated here. In each case, asingle dia grid member, which spirals from the base of the building to the top, is created

by the program and repeated by rotation and reflection to create the entire structure. Inthe second example shown in Figure 2, as in most of the models, the member variesfrom bottom to top, in this case by splitting (one member at the base splits into fourmembers) and by tapering.

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Figure 1. Example of a cylindrical dia grid on which a building's structure could bebased. (Courtesy of Skidmore, Owings & Merrill LLP)

Figure 2. Other variations of a cylindrical dia grid generated using lisp. (Courtesy ofSkidmore, Owings & Merrill LLP)

Figures 3 and 4 illustrate another example of a lisp-based model, in which a frit patternfor glass is generated based on simple graphics that control the parameters for

generating the pattern. In each case, the red line is the centreline of the pattern, theblue line determines the spacing of the dots, and the green line determines the size ofthe dots. We have used these lisp routines to generate frit patterns for a variety ofdesign projects, where we can control the area/percentage of opacity of the glass, andvary this in a controlled and interesting way over the panel.

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Figure 3. Several examples of frit patterns created using lisp for use in glazing design.(Courtesy of Skidmore, Owings & Merrill LLP)

Figure 4. Another example showing a more complex frit pattern. (Courtesy of Skidmore,Owings & Merrill LLP)

Many of the processes described here were used in the design, analysis, anddocumentation of the building project shown in Figure 5, which is the Lotte Tower inSeoul, Korea. The building model is created both as a 3D model and as an unfoldedmodel for laser-cutting, as well as for representation (see Figure 6). The lisp programgenerates one-quarter of the structure, and uses symmetry to complete the models.Parameters in the program control the dia gridthe parameters are refined after much

iteration to optimize structural performance, program area contained within the building,and aesthetic judgments. The form of the 550+ meter tall tower transforms from asquare at the base to a circle at the top.

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Figure 5. SOM's Lotte Tower project in Seoul, Korea, on which many parametric

modelling processes have been used. (Courtesy of Skidmore, Owings & Merrill LLP)

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Figure 6. The basic structure of the Lotte Tower project derived using parametricmodelling in AutoCAD. Both the 3D model and the unfolded model are shown.(Courtesy of Skidmore, Owings & Merrill LLP)

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We also used AutoCAD-based parametric modelling to develop a tool for analyzingsolar incidence angles for the project, as shown in Figures 7 and 8.

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Figure 7. Examples of solar incidence angle analysis for the Lotte Tower project. Thetop image shows analysis results for mornings during spring, while the lower imageshows the analysis for afternoons during autumn. (Courtesy of Skidmore, Owings &

Merrill LLP)

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Figure 8. The complete set of solar incidence angle analysis for the same project,allowing solar penetration to be studied for the entire year. (Courtesy of Skidmore,Owings & Merrill LLP)

The summary image shown in Figure 9 was part of our competition entry package forthis project (which we were awarded), and represents a solar incidence angle analysisof the tower. An analysis was performed for each facet of the tower model. At each

hour from morning to evening for one day of each month, the normal vector of eachfacet was compared to the direction to the sun. If this angle is small, the facet is gettingdirect sunlight, causing poor energy performance and decreasing occupant comfort.Small angles are indicated in red and as the angles growand energy performanceimprovesthe colour changes to orange, yellow, green, and blue respectively. Whilethe analysis was done on a three-dimensional model, the results are shown on anunfolded model, allowing us to see a "report" of the analysis for the entire building in asingle image.

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Figure 9. The solar incidence angle analysis summary image for the Lotte Towerproject. (Courtesy of Skidmore, Owings & Merrill LLP)REFERENCES

1. http://en.wikipedia.org/wiki/Solid_modeling

2. http://www.ptc.com

3. http://mcadonline.com

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4. http://www.csse.monash.edu.au

5. http://www.computationaldesign.ca

6. http://www2.ivcc.edu

7. http://www.linuxjournal.com

8. http://www.aecbytes.com
http://www.csse.monash.edu.au/http://www.computationaldesign.ca/http://www2.ivcc.edu/http://www.linuxjournal.com/http://www.aecbytes.com/http://www.csse.monash.edu.au/http://www.computationaldesign.ca/http://www2.ivcc.edu/http://www.linuxjournal.com/http://www.aecbytes.com/

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Assignmnt 2 Parametric Model Lining

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