III. MATHEMATICAL MODELING VERSUS PHYSICAL PROTOTYPING

https://www.youtube.com/watch?v=UPVm3v5ChR4

Assuming that model preparation is complete, metrics are then identified for measuring a product's performance. We must now further the development process by developing a model to test our concepts. At this stage, an issue when considering a product model is descriptiveness, also known as detail or fidelity. How much work should we as engineers put into constructing a model that reflects the product design problem? The product development team does not have infinite resources.

This decision comes down to the comparison of model descriptiveness versus model construction difficulty:

• Should we use a lumped mass model or a finite element model?

• Should we model an idealized steady-state condition, or use a transient model?

• Should we make detailed numerical analyses of all the product options?

• Should we construct a physical model or a virtual (mathematical) model?

Determining the level of a model is a trade-off decision between model descriptiveness versus model construction and solution determination time.

https://www.youtube.com/watch?v=Ee4CKIPkIik

Example

For a fingernail clipper product, consider a scenario where we may ultimately derive a model that represents finger force .

https://www.youtube.com/watch?v=edYLW5kqXBU

We must choose between an analytical or physical model (Table 13.2). Combinations of analytical and physical prototypes are also used. An example is to embed sensors in physical prototypes to provide feedback to computer simulations to enhance the accuracy of the simulations. The other continuum determines whether the prototype has some, most, or all the attributes of the final product (Table 13.3).

The other continuum determines whether the prototype has some, most, or all of the attributes of the final product.

One way to consider the decision from these continua is depicted in Figure 13.3. As we initiate a project involving the construction of a formal model, typically we will have a project resource depletion constraint. We do not have infinite time or money to build a "perfect" description of our product, infinite in detail. Thus, we have a project management decision to make, namely, "How detailed should we model?"

To answer this question, we have data from past history or design process prediction studies. We must use our engineering judgment to decide how complex the performance metric is in terms of the design variables and how much safety margin we desire using our resources.

In general, it is smart to make detailed engineering models when proto typing is expensive and when we have reasonable expectations in obtaining an accurate model. This idea is depicted in Table 13.4, adapted from Ulrich and Eppinger.

IV. ADVANCED TOPIC: WHAT IS A PRODUCT MODEL?

As introduced above, we never analyze a real system but only an abstraction of it. The abstraction is a model it abstracts the real world, giving approximations of complex physical phenomena as part of physical systems.

http://vimeo.com/53848170

Informal Models

https://www.youtube.com/watch?v=eGkvZNRC8E0

When a typical design concept is developed, it is first conceived in informal terms. Customer needs lists use informal (English) descriptions. A function structure uses English terms with graphical structure. Morphological charts use rough pictorial sketches. Even quantities that eventually become precise start informally. Requirements that a design must satisfy, for example, are usually first described in a natural language, complete with personal connotations.

Definition. An informal model is a designer's interpretation of a description of the customers' needs, engineering requirements, manufacturing requirements, and any other product requirement, along with the designer's interpretation of the conceived solutions.

Example

Let's consider a fingernail clipper product. We have a number of concepts for the product, as depicted in Figure 13.4. An interpretation of these concepts is our informal model of each. We have a list of criteria for evaluating the concepts (Table 13.5). Our interpretation of what these criteria mean is our informal model of the objectives.

Formal Models

http://www.youtube.com/watch?v=i_Ey4rx7I_A

What is Formal?

We are seeking to construct a computable (or analytical) model of the design problem. The fundamental characteristics assumed to define a formal model are twofold. The first characteristic is that we can elucidate the alternatives to choose among them; the alternatives are assumed to have the structure of a set.

The second characteristic of defining a formal model is that when shown two different objects in the set, we can determine whether or not they are distinct. We can, with enough analysis, distinguish between the different alternatives.

V. CONSTRUCTING PRODUCT MODELS: BASIC METHOD

Early stages of product development provide us with the necessary informal description of what we need to model a design problem. Model preparation and selection (Section II) establishes the first link of this informal description to a quantified metric. Having a complete functional model and architectural layout provides us with additional structure to construct a formal model.

Date: 2016-01-14; view: 631