Wednesday, January 26, 2011
System modes have a hierarchy similar to functions. Modes can be decomposed into sub modes, which are also called modes, just as functions can be decomposed into lower level functions. Modes and the transitions between modes can be shown in a hierarchy of diagrams and matrices. The top level diagram or matrix should contain all modes in the life cycle of the system from assembly to an end of life mode. An example of a simple life cycle mode diagram for a commercial product for home use is shown in Figure 6-10. In this figure the event that determines transitioning from one top level mode to another is included as a labeled arrow on the diagram. It's important to define and examine the modes for all of a systems life cycle rather than just it's "in Use" mode. Sometimes there are requirements that are necessary for integration and test, storage or end mode that are not included in the modes for the "in Use" mode and the life cycle mode diagram is useful in identifying such requirements.
Diagrams or matrices can be developed for each of the top level modes that has sub modes to define the functions required in each mode and the allowed transitions between functions. Thus the top level mode diagrams identify lower level modes that are then examined during functional analysis to assist in defining all of the functions required in every mode of the life cycle of the system. This topic is revisited after discussion of functions and functional decomposition.
Transitions between sub modes are usually more complicated than between the top level life cycle modes. A very simple example that illustrates this is shown in Figure 6-11 where the transitions among the three modes of the IN USE mode of the product having the life cycle modes shown in Figure 6-10. The transitions are not labeled in this diagram at this point because the events causing these transitions can be dependent upon the physical design. It is only necessary at this point to define the allowed and required transitions between modes.
Figure 6-10 An example top level life cycle mode diagram for a simple commercial product.
Figure 6-11 A mode transition diagram for a simple commercial product in its IN USE mode.