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Modern engineering designs often combine several different kinds of modeling techniques. Me-chanical designs today frequently come to life as mathematical solid models instead of as 2D draw-ings. These solid models frequently must work with other kinds of engineering software representing entities such as wiring diagrams and component layouts. Dataflow programming techniques are often the means of building such connections and expressing this information.

A solid model represents a shape as a three-dimensional object having mass properties. Solid models are useful in several ways. For example, it is easier for nontechnical personnel to understand 3D renderings than to grasp two-dimensional drawings that consist of orthographic projections, aux-iliary projections and cross sections.

Solid modeling soft-ware may use any of several methods to represent model information. Feature-based, parametric, and so-called “direct” or “explicit” tools let designers push and pull models as if they were made of clay.

Frequently, solid models are useful because their geometry can represent not only the parts being designed but the intent of the designer. An example of design intent might include keeping two part faces paral-lel no matter how other part features change, or maintain-ing the same mathematical relationship between a part’s length and width. No matter what dimensions the de-signer types in while build-ing the model, the software ensures the part definition is

Basics of Solid ModelingGeometric solid models are the preferred way of defining manufactured parts and assemblies. In recent years these techniques have taken a role in characterizing control cabinets and wiring.

Contents

Basics of Solid Modeling … 1

Surface modeling … 3

Building blocks for solids … 4

Electrical design models … 4

Presented by

Feature-based models let designers define features pertaining to geometry as well as to steps in downstream analysis and manufacturing. Parametric modeling is the term used to describe the capturing of design operations as they take place, as well as the editing that takes place on the design.

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is a constraint. It will further assume that it should preserve this relationship throughout any model changes so the part maintains its original design intent.

Another advantage of the direct-modeling approach is that the set of active constraints can change dramatically depending on events during the construction of the model. For example, the system may capture constraints to support a modification that can contradict constraints captured during an earlier design iteration. This can save time during model creation compared to the parametric approach, where it is the user who must change the constraints. The process of changing constraints can be tedious and complex.

Conversely, the benefit of using a parametric modeler is that the CAD system needn’t guess at the constraints because the user spells them out. But the predictability of the CAD model then depends on the CAD user being skillful enough to avoid difficulties that can arise because of too many or poorly defined constraints. There can also be issues when one designer must modify a parametric model that another designer created. Sometimes the complexity of the constraints can make design intent difficult to grasp.

High-level model quality problems can arise in all feature-based modelers. Typical problems include unintentional interactions among features. These interactions typically take the form of small cracks, knife edges, voids, and similar artifacts between features. These effects not only cause problems in analysis software such as FEA, but also get worse

composed of individual features that describe how the geometry is supposed to behave.

Early solid modelers were not based on features. To put a hole through a part, for ex-ample, the designer might define a simple cyl-inder having the diameter of the desired hole and which was long enough to go through the part. Designers would then tell the software to perform a Boolean difference operation between the part and the cylinder. The result: a hole in the part having the diameter of the cylinder.

The problem with this scheme comes if the part dimensions change. Suppose, for ex-ample, the designer later modifies the part and makes it thicker. If the designer didn’t happen to make the cylinder long enough to extend through the new, thicker part, the result is the model of a blind hole. In this case, the model captured the geometry the designer specified, but it did not capture the design intent of the designer.

A designer working in feature-based software, on the other hand, would approach the through-hole differently. The designer would define a feature called a through-hole such that no matter what the dimension of the part, the hole extends all the way through it. In other words, once the topology of the design has been called out in terms of features, any changes to the design always keep these features operational unless the designer specifies otherwise. Typically, the software prompts the user for inputs during the definition of the feature. These may include positional constraints, algebraic definitions, and other factors.

A related type of solid modeling scheme is parametric modeling. A parametric modeler defines the part model in terms of parameters. A simple parameter might be expressed as an equation such as, (diameter) = 3 x (depth), or (width) = 2 x sqrt(length). Parameters can also establish links between parts as in an assembly model. An example might be a part position with respect to a reference plane on another part.

Thus parametric CAD systems represent each geometrical and dimensional constraint in terms of a relationship among two or more entities. When the designer changes a parameter (dimension), the CAD system propagates the change throughout the entire solid model while maintaining the other constraint relationships.

Another type of solid modeler is a direct modeler. Here the CAD system creates constraints on the fly; the user creates no constraints. The advantage is that this approach more easily handles high-level changes to the solid model. For example, the CAD system might note that two nearby planar faces are parallel to each other. It may assume that this

Many solid modelers include primitive and boundary representations. In the primitive approach, the user combines elementary shapes in building-block fashion to create a new shape. Boolean logic commands, such as union, difference, and intersection, aid in forming new shapes. With boundary definitions, 2D surfaces get swept through space to trace out volumes. Most packages provide several types of sweeps to help create shapes.

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the designer to change a feature and then propagate it to all instances in the model. If the designer changes a bolt pattern, for example, the flexible modeler can put the new pattern in all locations where the original resided.

Surface modelingPrior to the advent of solid modeling, computerized

geometry models frequently took the form of wire frames. Wire-frame models represent 3D part shapes with inter-connected line elements. Wire frames are the simplest 3D geometric representation, though not necessarily the easiest to create. Some modeling programs still use a wire frame data structure. The benefits are that wire-frame models use little computer time and memory and provide precise information about the location of surface discontinuities on the part.

Wire frames, though, contain no information about the surfaces themselves nor do they differentiate between the inside and outside of objects. Thus, wire frames can be ambiguous in representing complex physical structures and often leave much interpretation to users.

Wire-frame models are created by specifying points and lines in space. One commonly used approach to creating a wire frame model divides the computer screen into sections showing various model views. Designers draw lines to create top, bottom, side, isometric, and other views of the model. Designers need not manually draw each line in a wire frame. Rather, the CAD package constructs the lines based on user-

specified points and commands chosen from an instruction menu.

It is important to remember, however, that not all models that look like 3D wire frames are wire-frame models. Some soft-ware lets users build isometric models that appear to have Z-axis depth, but in reality do not. This software is usually called 2½-D software.

Although wire-frame models are the simplest form of geometric model, the term is sometimes associated with both surface and solid modeling. Surface mod-els define the outside part model precisely and help produce NC machining instruc-tions where the definition of the structure boundaries is critical. However, surface models represent only an “envelope” of part geometry, even though tools such as automated hidden-line removal make the model appear as a solid.

Surface models, in turn, are created by connecting various types of surface ele-ments to user-specified lines. Typical CAD surface elements include planes, tabulated

if they pass over to another brand of CAD system where feature history is lost.

Unfortunately, parametric models imported into a different CAD system may come across without parameters, features and design intent, even if the new CAD system supports parametric modeling. Thus users of CAD systems relying on parametric feature-based approaches generally must remove and recreate different pieces of geometry that they want to change through parameters and features.

Some direct modelers can work with previously created parametric models. The resulting models and their changes remain parametric and feature-based. Another advance is the development of modelers that will associatively update imported solid models when the original model changes in a different CAD application.

Some modelers have what’s called a flexible modeling extension which works directly on geometry. It comes in handy where a designer needs to make change on imported geometry that has come in with no features, or on a model created elsewhere and there is no time to figure out how the model was built up. Flexible modelers include a facility for recognizing patterns and symmetry within the model as a means of adding intelligence. A user can pick the geometry to be modified, for example, and drag it to new place. The system then reattaches it to the model, with the automatic addition of rounds if need be.

If the designer imports a file, a flexible modeler can add intelligence to it through such measures as allowing

Surfaces available for geometric modeling range from a simple planes to complex sculptured surface. These surface usually are represented as a set of ruled lines. The computer program recognizes these lines as continuous surfaces. Users select surfaces types from a menu to model individual details or fully envelope parts.

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tation. There are many kinds of sculptured surfaces, including curve-mesh, freeform, B-spline, and cubic patch surfaces. Curves need not even be parallel. The two curve families intersect one another in crisscross fashion, creating a network of intercon-necting patches.

Building blocks for solidsSolid models can be constructed

from successive combinations of simple geometric operations with primitives or with boundary definitions. The primitive approach lets elementary shapes such as blocks and cylinders be combined in a building-block fashion. Users position the primitives and then create a new shape with the proper Boolean command. With boundary definitions, two-dimensional surfaces are swept through space to trace out volumes. A linear sweep translates the surfaces in a straight line to produce an extruded volume. A rotational sweep pro-duces a part with axial symmetry, while a compound sweep moves a surface through a specified curve to generate a more com-plex solid.

Each of these construction methods is good at handling a particular class of shapes. Most industrial parts, for example, consist of planar, cylindrical, or other sim-ple shapes and are readily modeled with primitives. But components with complex

contours such as automobile exhaust manifolds and turbine blades are more easily modeled with boundary definitions.

Electrical design modelsCAD can be used to characterize entities such as electri-

cal panels and cabinets for controls. A CAD program looks at a control cabinet as an entity described by connections between blocks representing physical functions such as relays, terminal blocks, and circuit breakers. One kind of CAD for electrical cabinets typically takes the form of stand-alone programs for drawing 2D schematics. Such programs generally also handle automation for PLC (programmable logic controller) wiring, terminal blocks, reporting, and so on.

There are facilities for bringing 2D schematic electrical design data into the 3D model. Once in the 3D model, the user can place components like motor starters, DIN rails, wiring ducts, and similar entities in enclosures. There are also routing routines that handle such tasks as proposing alternate ways to route wires in 3D, handling component spacing, and

cylinders, ruled surfaces, and surfaces of revolution along with sweep, fillet, and sculptured surfaces. Of course, the plane is the most basic surface type. The software merely cre-ates a flat surface between two user-specified straight lines. A tabulated cylinder is the projection of a free-form curve into the third dimension. A ruled surface is produced between two different edge curves. The effect is a surface generated by moving a straight line through space with the end points resting on the edge curves.

A surface of revolution is created by revolving an arbi-trary curve in a circle about an axis. This capability is useful in modeling turned parts and parts with axial symmetry. The sweep surface is an extension of the surface of revolution. Sweep curves, however, sweep an arbitrary curve through an-other arbitrary curve instead of a circle. The fillet surface is a cylindrical surface connecting to other surfaces in a smooth transition. Previously, this was a tedious and manual process. But over time CAD has solved the problem with the precise mathematical continuity required by many applications.

Sculpted surfaces are the most complex surface represen-

A typical CAD package might create a wire frame model from points the user specifies.

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Graphical data-flow gives the user access to black-box processes only through their connections. Also, blackbox processes can be viewed as reusable components that don’t know the name of other black boxes with which they communicate. Users can reconnect different black boxes endlessly to form different applications without having to change any of the blackboxes internally.

In the case of designing the contents of a control cabinet, for example, the CAD program might view each individual component mounted on a DIN rail as a blackbox process. Connections between the control cabinet compo-nents don’t affect their internal functions, only their states.

In IT lingo, the streams of data passing between black boxes are called information packets, and the connections through which they pass are bounded buffer connections. Each process identifies its related connections by port names, rather than directly. Typically, a connection engine or scheduler routine relates port names to the real network and drives the individual processes.

segregating low-voltage wires from those carrying high-voltage. Other tasks normally conducted in 3D electrical models include checking for clearances, planning wiring paths, cre-ating harnesses, and so forth. Changes typically get linked back to the 2D drawing for documentation.

Some programs for modeling such entities use a data-flow program-ming approach as a representational scheme. The key to data flow is that it is a handy way of defining networks of “blackbox” processes. These pro-cesses exchange data across predefined connections. Data-flow programming gets its name from the fact that application developers need only work with flows of data through the connections rather than having to define a sequence of commands as with conventional sequential procedure code.

The first widely used data-flow program was the spread-sheet. Each cell in the spreadsheet can be considered a black-box process. When any of those cells update, the first cell’s value automatically recalculates. One change can initiate a lengthy chain of changes when one cell depends on another cell which in turn depends on yet another, and so forth.

But data flow is not just for recalculating numeric values as in spreadsheets. The concept eventually expanded to let drawn entities represent blackbox processes. Thus it can be used to re-draw a picture as directed by mouse movements. A graphical data-flow application becomes essentially a list of connections which can be generated by a graphical tool. Among the first such graphical data flow programs to be-come commercially available was the LabView program.

Modern CAD programs let the user bring 2D schematic electrical design data into the 3D model. Once in the 3D model, the user can place components like motor starters, DIN rails, wiring ducts, and similar entities in enclosures. There are also routing routines that handle such tasks as proposing alternate ways to route wires in 3D, handling component spacing, and segregating low-voltage wires from those carrying high-voltage.