FEA is a major tool used to identify and solve design problems. Increased design efficiency provided by CAD has been augmented by the application of finite element methods to analysis, but engineers still often use a trial-and-error method for correcting the problems identified through FEA. This method inevitably increases the time and effort associated with design because it increases the time needed for interaction with the computer. Also, solution possibilities are often limited by the designer's personal experiences. As reported by many design optimizations seeks to eliminate much of this extra time by applying a logical mathematical method to facilitate modification of complex designs.

The optimization approaches were difficult to implement in the engineering environment because the process tends to be academic in nature and not viewed as easily applicable to design practices. However, if viewed as a part of the process itself, optimization techniques can be readily understood and implemented in the design process. The objectives and constraints upon the optimization must first be defined. The program then evaluates the design with respect to the objectives and constraints and makes automated adjustments in the design. Because the process is automatic, engineers should have the ability to monitor the progress of the design during optimization, stop the program if necessary, and begin again.

Target for optimization is to minimize or maximize a quality, such as weight or physical size, that is subjected to constraints on one or more parameters. Either the size, shape, or both determines the approach used to optimize a design. Optimizing the size is usually easier than optimizing the shape of a design. Optimizing the thickness of a plate does not significantly change its geometry. However, optimizing a design parameter, such as the radius of a hole, does change the geometry during shape optimization.

Preliminary design data are used to meet the desired design goals through evaluation and revision. Acceptable tolerances must then be entered along with imposed constraints on the optimization. The engineer should be able to choose from a large selection of design objectives and behavior constraints and use these with ease. Also, constraints from a variety of analytical procedures should be supported so that optimization routines can use the data from previously performed analyses.

Although designers tend to find shape optimization more difficult to perform than of size, the use of parametric modeling capabilities in some CAD software minimizes this difficulty. Shape optimization is an important tool in many industries, including shipbuilding, aerospace, and automotive manufacturing. The shape of a model can be designed using any number of parameters, but as few as possible should be used, for the sake of simplicity. If the designer cannot define the parameters, neither design nor optimization can take place. The designer's input is crucial during an optimization program.

CAD Prototyping

Creating physical models can be time-consuming and provide limited evaluation. By employing kinematic (branch of dynamics that deals with aspects of motion apart from considerations of mass and force) and dynamic analyses on a design within the computer, time is saved and often the result of the analysis is more useful than experimental results from physical prototypes. Physical prototyping often requires a great deal of manual work, not only to create the parts of the model, but to assemble them and apply the instrumentation needed as well.

CAD prototyping uses kinematic and dynamic analytical methods to perform many of the same tests on a design model. The inherent advantage of CAD prototyping is that it allows the engineer to fine-tune the design before a physical prototype is created. When the prototype is eventually fabricated, the designer is likely to have better information with which to create and test the model.

Engineers increasingly perform kinematic and dynamic analyses on a CAD prototype because a well-designed simulation leads to information that can be used to modify design parameters and characteristics that might not have otherwise been considered. Kinematic and dynamic analysis methods apply the laws of physics to a computerized model in order to analyze the motion of pans within the system and evaluate the overall interaction and performance of the system as a whole.

One advantage of kinematic/dynamic analysis software is that it allows the engineer to overload forces on the model as well as change location of the forces. Because the model can be reconstructed in an instant, the engineer can take advantage of the destructive testing data. Physical prototypes would have to be fabricated and reconstructed every time the test was repeated. There are many situations in which physical prototypes must be constructed, but those situations can often be made more efficient and informative by the application of CAD prototyping analyses.

Physical models can provide the engineer with valuable design data, but the time required to create a physical prototype is long and must be repeated often through iterations of the process. A second disadvantage is that through repeated iterations, the design is usually changed, so that time is lost in the process when parts are reconstructed as a working model. Too often, the time invested in prototype construction and testing reveals less useful data than expected.

CAD prototyping employs computer-based testing so that progressive design changes can be incorporated quickly and efficiendy into the prototype model. Also, with virtual prototyping, tests can be performed on the system or its parts in a way that might not be possible in a laboratory setting. For example, the instrumentation required to test the performance of a small part in a system might disrupt the system itself, thus denying the engineer the accurate information needed to optimize the design. It can also apply forces to the design that would be impossible to apply in the laboratory.

Prototyping and testing capabilities have been enhanced by rapid prototyping systems with the ability to convert CAD data quickly into solid, full-scale models that can be examined and tested. The major advantage of rapid prototyping is in the ability of the design to be seen and felt by the designer and less technically adept personnel, especially when esthetic considerations must come into play. There exists limitations in testing operations for rapid prototyping technology. For example, in systems with moving parts, joining rapid prototype models can be difficult and time-consuming. With a CAD prototyping system, connections between parts can be made with one or two simple inputs. Since the goal is to provide as much data in as litde time as possible, use of virtual prototyping before a prototype is fabricated can significandy benefit the design program.

Rapid Prototyping

An important application of CAD technology has been in the area of rapid prototyping. Physical models traditionally have the characteristic of being one of the best evaluative tools for influencing the design process. Unfortunately, they have also represented the most time-

consuming and cosdy stage of the design process. Rapid prototyping, such as stereolithography, addresses this problem, combining CAD data with sintering, layering, or deposidon techniques to create a solid physical model of the design or part. This industry is able to produce small-scale producdon of real products, as well as molds and dies that can then be used in subsequent traditional fabricating methods. The rapid prototyping industry is specializing in the production of highly accurate, structurally sound products to be used in the manufacturing process.

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