IV. Advanced methods: systems modeling

In addition to the general embodiment process (Figure 12.6) and embodiment checklist (Table 12.1), systems modeling techniques can be applied when embodying a concept. Systems models are representations of a product that predict the product's performance under varying input (environmental or boundary) conditions.

Systems Modeling

Functional models, as discussed in Chapter 5, represent high-level systems models of a product. Inputs of materials, energy, and signals are simultaneously converted to desired outputs. During the concept development phase of a product, these models provide a basis for systematically generating product architectures (Chapter 9) and solution concepts.

High-level model: After identifying the physical principles and assumptions for each customer need, a balance relationship is created to document a high-level physical model. Control volume methods or cause-and-effect diagrams may be used to document the balance relationship, where the "effect" is the customer need, and the "causes" are the physical principles. The ultimate goal is to refine the "causes" to the level of physical variables.

Balance relationships: The last task in formulating mathematical models for a product is to convert the balance relationships into a set of mathematical equations. Basic engineering principles may be used to choose the appropriate scaling law relationships (Miller). Variable ranges from the product's layout drawings or bill of materials should be used to augment the mathematical relationships with appropriate parameter values. If such variable values are unavailable, appropriate ranges are chosen.

Physical prototype models: In some cases, cycle-time, economic, or product-complexity considerations may prevent the development of a mathematical model. The creation of a physical prototype can be used as an alternative modeling approach. Prototype models should be designed in such circumstances. The intent here is to create a bench-top or other experiment (not an entire product prototype) for a customer need, focusing on the effected product components and variables. For instance, a customer need may exist for an electric wok lid handle to "fit comfortably in the user's hand during operation." A mathematical model of this need is not directly apparent; however, physical prototypes that vary shape, size, and texture of the handle may be designed for analysis and testing.

Mechanical Embodiment Principles

Some mechanical assemblies work better than others-we know it when we operate them. What makes a good mechanical design? Beyond proper sizing of components and tolerances, it is often the proper design of the forces through the product. Five design principles are presented in this section. As with any heuristic or principle, they are not guaranteed, and there are times to not follow them. But in most cases, they should be considered.

Alignment of Forces

Forces within an assembly are what make the parts move. The position and orientation of these forces are design choices that the designer makes, and so the reliable and carefree motions of a product are under the control of a designer. They are not the result of arbitrary effects; they are the result of how the designer applies physical principles in a product.

To properly design forces in a mechanical assembly, one should distinguish three forces that act on any moving part. The first is weight. The second type is a frictional force, which resists motion. The final type is an applied force (applied load), which a designer uses to create the motion. Whether a part moves freely or binds is completely determined by the interactions of these three forces.

As an example, consider a college design competition to deliver hockey pucks to a remote target. One design calls for a rack of pucks (mounted on a wheeled chassis) to be pushed off sequentially by driving a pusher with a worm drive, as shown in Figure 12.9. The left-hand design does not have the center of mass (weight) aligned with either the center of applied forces or with the center of friction. It will bind.

The second design has the center of applied forces aligned with the center of mass but not with the center of friction. It is less likely to bind and will avoid binding only through widening of the space between the linear bearings on the slide rails, creating a cantilever. This design is good for fast-moving applications, where the inertial force is higher than the frictional force.

The third design has the center of applied forces more aligned with the center of the friction forces. It does not have the center of mass aligned with either of the other two centers. This design is good for slower moving applications, where the inertial force is negligible. It is less likely to bind, and again, will only not bind through a wide linear bearing spacing on the slide rails.

Note that in the concept of Figure 12.9, it is impossible to align the frictional force centroid with the other two centroids unless one introduces an upper rail to frictionally impede the top surfaces of the

pucks. This results in a poor design-again, one should understand the heuristic and use wisdom in applying it.