Checking the Partitioning of the Physical Architecture

6.6.4 The Design Loop

As the physical design is created many alternatives should be considered. One task is to check that the grouping and sequencing of functions defined during the functional analysis task leads to an effective physical partitioning, i.e. a modular design as described in the previous section. If reasonable physical designs don’t result in subsystems being allocated to single functions or single groups of functions then recheck the grouping of functions to see if alternative grouping lead to cleaner physical partitioning and more modularity. It is important to seek clean partitioning of subsystems and their associated functions because the cleaner the partitioning the easier systems are to integrate and test, maintain and upgrade. A function that is implemented in two or more subsystems results in system designs that are more difficult to maintain and upgrade and are often more difficult to test. Envision the design loop as iteration between functional and physical design until both result in a modular physical architecture.

A second task during design synthesis is conducting trade studies, described in a later chapter, to select between design alternatives. Again when evaluating alternative architectures consider the partitioning for each design alternative and examine the possibility that modifying the functional architecture might lead to a better functional to physical allocation and partitioning.

6.6.4.1 Functional to Physical Allocation Matrices

A simple tool that is helpful in refining the functional and physical architectures is a functional to physical allocation matrix. An example matrix for the toaster functional architecture shown in Figure 6-24 and the design concept architecture shown in Figure 6-31 is shown in Figure 6-33. The functional to physical allocation matrix is particularly helpful in examining the partitioning of a design concept for modularity. Typically the more diagonal this matrix the better the modularity. However, opportunities for one physical entity to perform two or more functions are highly desirable and readily apparent in the matrix. Similarly, when the matrix shows a function spread across several physical entities the matrix provides a visual means of examining if the physical design concept is sound or if it should be changed to allow cleaner partitioning. Sometimes the nature of a function causes it to be spread across several physical entities without complicating the design in ways that cause manufacturing, testing or upgrade problems. For example, in Figure 6-32 the “apply heat” function is allocated to three entities and this is probably a reasonable design approach because it likely reduces the parts count and makes operation simpler.

Figure 6-33 A functional to physical allocation matrix for one candidate toaster design concept.

Introduction to Concept Design

6.6 Design Synthesis

Completing the initial definition of the functional architecture sets the stage for beginning design synthesis. The design synthesis task defines physical elements of hardware and software to carry out the functions in the functional architecture and to fulfill the requirements allocated to the functions. It is an allocation and partitioning task. Allocation refers to the mapping of functions to physical elements and partitioning refers to the grouping of functions and physical elements. It’s helpful if the functional architecture is defined to the second level, at least in draft form, before beginning design synthesis. Design synthesis is done in steps. Usually the steps are called concept design, preliminary design and detailed design. Each step adds more detail to the design and defines the design to lower levels of the system hierarchy. At the completion of detailed design a complete set of procurement documentation, manufacturing drawings, detailed software descriptions and integration and test (I&T) documentation is finalized and ready for procurement of parts, manufacturing, software coding and I&T.

6.6.1 Concept Design

Concept design is emphasized here as systems engineers have a greater role in the concept design than in preliminary and detailed design. The objective of concept design is to convert the functional architecture to a physical architecture. In this process the functional architecture and the allocated requirements may be refined and other supporting documentation developed. Three outputs from design synthesis during concept design are a physical architecture, a baseline design and a physical view of the system.

The physical architecture is defined by a physical block diagram or signal flow block diagram that schematically illustrates the relationships and interfaces between the physical subsystems (hardware and software) that map to the functional architecture. The physical architecture is part of the system architecture, which includes the enabling products and services needed by the system in all of its life cycle modes. An example of a simple physical block diagram of a candidate concept design for the toaster defined by the functions shown in Figure 6-24 is shown in Figure 6-31. (Again, apologies to toaster designers for any ignorance of toaster design.)

Figure 6-31 A physical block diagram for a candidate toaster concept design.

Systems that involve the collection, processing and communication of signals or similar information are often better described by a signal flow diagram. A signal flow diagram is a physical block diagram that follows the system signals from initial collection to their output from the system. Modularity is often easier to visualize in signal flow diagrams than block diagrams. Signal flow diagrams are typically more complex than simple block diagrams so it’s usually best to define alternative concepts and conduct trade studies using simple block diagrams. Once the final baseline concept is selected then constructing a signal flow diagram helps explain the selected design concept better than a simple block diagram.

The baseline design includes the functional architecture, the physical architecture, the system specification, and the ICD. The baseline design evolves with the design maturity and is the basic item under configuration management. The baseline design is a means for facilitating decision management during the three stages of design synthesis. At each stage; concept design, preliminary design and detailed design; it is good practice to force the work to a baseline design quickly and then conduct trade studies to refine the selected baseline. Otherwise too many design decisions are open at any time and control of the design work becomes difficult.

The physical view includes all of the diagrams, documents, models, etc. that describe how the system is constructed, how it interfaces with humans and other supporting systems during the life cycle modes, any customer supplied equipment, and any constraints on the design or operations.