Tuesday, June 28, 2011
8.4 Diagrams Useful in Selecting the Preferred Design
As discussed previously much of systems engineering is determining relationships between a system and its environment and among the various subsystems. Ultimately these relationships are defined in detailed drawings but understanding the relationships in order to select the preferred design is aided by examining a system with different levels of abstraction. The modern tools used for electrical and optical design and the design practices of electrical and optical design engineers develop their respective design concepts with diagrams. The diagrams start with block diagrams with a high level of abstraction, perhaps just naming the subsystems, and proceed to greater and greater detail. This process makes it easy for systems engineers and other design engineers to readily understand the electrical and optical design concepts.
Although there are excellent modern design tools for mechanical and thermal design it isn’t as easy to present the mechanical and thermal designs with ever decreasing levels of abstraction so that system engineers and other design engineers can easily understand the mechanical and thermal designs. Experienced mechanical and thermal design engineers develop the desired diagrams and tailor their diagrams to the system being developed so that others can readily understand and assess their designs. A few examples are presented here to illustrate how experienced mechanical and thermal designers examine and communicate their design concepts. Note how easy it is to think of alternative design approaches when design concepts are presented in simple block diagram form with a high degree of abstraction. This enables engineers other than expert mechanical and thermal designers to assess design concepts and suggest design alternatives.
8.4.1 Simple Mechanical and Thermal Block Diagrams - Many times a simple block diagram is useful in describing a mechanical or a combined mechanical/thermal design concept. Figure 8-7 is an example showing a system that consists of five assemblies, a frame for mounting the assemblies and a mounting plate that supports the system with a three point mount.
Figure 8-7 A simple mechanical block diagram of a system illustrates how the assemblies interact with each other and the mounting frame.
It’s easy to see that the design concept is for each assembly to be coupled to the mounting frame and uncoupled from any other assembly. Four of the assemblies are temperature controlled whereas the fifth assembly is attached to the mounting frame but not within the temperature controlled region. By abstracting all of the size, shape, material and other characteristics of the system the basic mechanical and thermal relationships are easily understood and alternative concepts are obvious. The actual models that the mechanical designer uses to conduct trade studies of alternate mounting concepts are of course much more detailed and usually include the detailed characteristics of the various assemblies, frame and mounting plate; however, this detail is not necessary to explain the concepts and results of the trades to the system engineers and other designers.
Let’s suppose that three of the assemblies of the system shown in Figure 8-7 have critical alignment requirements. A perfectly good way to record and communicate the alignment requirements is with allocation trees. However, sometimes it makes the requirements clearer and possibly less prone to misunderstandings if a simple diagram is used in place of a tree. Such a diagram with the alignment requirements for each of the three assemblies and for the attachment points of each on the system mounting frame might look like Figure 8-8.
Figure 8-8 A simple block diagram illustrating the alignment requirements for three of assemblies and their respective interfaces with the mounting frame.
Similar simple block diagrams can be used to illustrate temperatures and heat flow paths. It sometimes makes design concepts easier to understand if mechanical and thermal diagrams are developed together. Examples are shown in Figures 8-9 and 8-10 that illustrate simple block diagrams of structural interfaces and thermal interfaces on similar block diagrams.
Figure 8-9 A block diagram illustrating structural interfaces within a system and between the system and its parent platform.
Figure 8-10 A block diagram illustrating thermal interfaces within a system and between the system and its environment using the same diagram approach as used for the structural interface diagram.
Even though it takes some time and care to develop diagrams such as shown in the examples above the benefits to the team in understanding and refining design concepts to reach the preferred design are well worth the effort. Diagrams like these and related simple diagrams for electrical, optical and other design concepts are invaluable in explaining a design concept to customers and management.