Engineering sketchers get technical

March 20, 1997
Modern sketching packages powered by Window’s object technology let engineers reshape designs into successful products with software that seems to know what the designer wants.
Tony Tanzilo
Design Automation Consulting
Patterson, N.J.


When a brainstorm strikes, the first inclination is to grab a piece of paper and start sketching. Several sheets may fill as ideas evolve and components take shape. When the dust settles and its time to sell the design to management, it would be nice to be armed with sharp looking professional drawings. That might sound like work for high-end CAD packages. But this time, they can be more hindrance than help in shaping an idea through a series of sketches, usually a process of constant change and flux.

Instead, designers and engineers might use one of several recent sketching or concept development packages such as Imagineer Technical. This type of software is geared to making sketches quickly and changing them as fast as ideas can flow.

Improved object technology in Windows, along with extensions that energize design and modeling functions, provide an interface users learn in a snap, sketching that’s quick, easy, and intuitive, an onboard variable sheet to drive drawings, and links to Visual Basic and spreadsheets. Sketching software is inexpensive (about $500) and compact enough so that a large number of designers can afford to carry it with them on portable computers. And it also generates the good looking drawings needed for presentations to those who can green-light a project.

Quick and easy
Admittedly, much of the technology in the concept development packages is not unique. Previous systems have introduced several of the capabilities mentioned here. Autodesk’s Mechanical Desktop and Microstation 95, just to mention two, have dimension- driven geometry, constraints, and parametric editing. But it is unusual to find such features together with others such as on-board variable tables and object-oriented capabilities enhanced with extensions aimed at making drawing packages smarter. While previous packages had some advanced capabilities tacked on, sketching software has the features built in, making it operate faster and smoother.

What really sets the concept-development packages apart from CAD applications that seem to have similar features is their intuitive, easily understood interface. Windows GUI means users with experience in almost any other package know enough about Windows-based sketchers to be productive from day one. Even the manuals are brief — about 150 pages.

Once users begin drawing, they find that the software seems to infer their intentions by monitoring what they are doing. For example, apply a fillet to a corner and the resulting assembly of an arc and lines remain connected and tangent even after editing. Or suppose a designer sketches a belt drive with a couple circles for pulleys and tangent lines for the belt. By attaching the belt (a line) to the pulley at a tangent, the software regards the attachment and tangency as a condition to maintain. When it’s necessary to reposition a pulley, the sketcher keeps the belt attached.

Developers have invented several other dynamic features. For example, sketchers now give users visual clues about the relationships between different drawing elements. These include small hash marks to imply parallel sides or crosses to indicate a perpendicular condition. As the cursor moves about the screen, the software displays these relationship indicators for drawing elements near the cursor. While designing the belt-drive mentioned earlier, for example, the user would construct a line and bring its end point near the pulley. When the tangency indicator pops up, clicking the left mouse button tells the software that tangency is indeed the needed condition.

Associativity between a drawing and dimensions, another benefit of objectoriented programming, allows using dimensions as an interface for parametric editing of constrained geometry. In other words, when a designer changes a dimension, the drawing updates. The opposite also works — change the geometry and the dimension updates. The variable table, as well as links to spreadsheets and word processors, provide other ways to reshape designs from values and expressions stored in other programs.

Traditional constraint-based CAD systems force users into a few more keystrokes. They must first draw a profile and then constrain it or establish a relation between drawing elements using additional commands. In contrast, the new sketching technology automatically adds constraints to geometry as it’s created, and derives inferences from the methods used to draw and edit drawing features. Sketching packages also let users easily apply or delete constraints to existing geometry at any time. There is no need to hunt for obscure commands in hidden menus.

In fact, on some occasions users may dispense with the drawing command altogether thanks to a freehand feature. It lets users sketch many frequently used elements — lines, arcs, and circles — with mouse or pen motions. As users sketch, the software recognizes and converts the movements into precise geometric shapes it recognizes from those stored in a database.

Making 1,001 variations
With the general geometry in place, one might decide it’s time to stretch it, pull it, or shrink it several times over. Sketchers give designers several ways to do so.

Associative dimensions provide one method. These can be automated in a method called driving dimensions. The advantage of the latter method comes in their use with relationships — the software’s ability to maintain some sides parallel, some perpendicular, and so on. By applying both driving dimensions and geometric relationships, users can easily modify an entire drawing to examine every possible useful variation. The same changes would require minutes or even hours with conventional CAD tools unaided by add-on dimension drivers.

A few sketching packages automatically apply dimensions for the users thereby freeing them to concentrate on a required shape rather than minutia that’s likely to change anyway. Of course, all packages let users apply dimensions to geometry afterwards. But the better sketchers speed drawing by automatically deciding what type of dimension (angular, linear, radial, and so on) a certain geometry or relationship calls for.

In some cases, relationships that govern the size, location, or orientation of drawing geometry can be so complex that simple constraints or driving dimensions cannot meaningfully change a drawing. A small assembly in which several mating parts influence the shape of other components would fall into this category. In these cases, a few sketching packages provide variable tables or abbreviated spreadsheets. These let users run a range of calculations to generate dimensions that automatically maintain functional relationships between important dimensions.

The tables hold values for dimensions and variables. For example, it can treat a length or weight as an independent variable — a value the user supplies. The value can then be used in equations to generate several other values on the drawing. For example, a table entry might use the weight value in an equation to define a support width. Equations can involve several variables generated in preceding entries. Some developers call these equations parametric expressions because they involve parameters.

The table has several functions. It defines variables which can be used in math expressions and functional relationships. Math tables provide wide flexibility for creating complex relationships among various dimensions in a design. Tables also let designers key in a stress equation or two to guide the hunt for the drawing’s optimum configuration. In one instance a manufacturer used the table as a kinematic analyzer for a dock crane. Users would supply the angle of elevation for the boom to raise or lower it in the drawing. Simultaneously, several other calculations provided preliminary stress and load values on major components.

The table’s math capability is powerful although limited to algebraic and trigonometric functions. Though tables provide a great deal of capability, they’re only four columns wide in some cases. The number of rows or variables, however, is often unlimited. Clever designers may find the limitation inhibiting.

Designers can take a step up in sophistication by applying the software object technology that links values calculated in a spreadsheet to those on the drawing. The advantage here is that spreadsheets offer more options to designers, such as unlimited columns and If-Then statements that let programs branch. When the calculated power requirement on a belt drive, for instance, falls outside defined limits, the spreadsheet could jump to a next larger belt or sprockets.

Visual Basic, another simple-to-use object-oriented program, lets ambitious users perform another level of tricks with the sketching package. For instance, rather than just calculate values and apply them to drawings, Visual Basic can search database for needed information and apply it to designs. In this way databases of structural steel might be accessed for I-beams and girders needed to support a calculated load. The sketcher could drawn them in perfect detail.

Making this link from spreadsheet to drawing has been simplified to a few mouse clicks. For example, to link a dimension in a drawing to a cell in an Excel spreadsheet, user need only select the cell in Excel and copy it to the clipboard. Then right-clicking the mouse and choosing Paste Link inserts the dimension’s cell into the variable table. Now when values in the spreadsheet change, the drawing updates to show the new information. This method is a distinct improvement over the flimsy DDE-based one-way spreadsheet links in other packages.

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