Search This Blog

Tuesday, March 8, 2011

Functional Flow Block Diagrams Functional Flow Block Diagrams - Graphical models are used to define and depict the sequence of functions making up a higher level function necessary to fulfill requirements. Functional Flow Block Diagrams (FFBD) can be process-oriented models that represent functions as nodes and objects as the connecting links. Nodes are typically labeled but links are typically unlabeled in process-oriented FFBDs. Figure 6-21 is a FFBD for two of the functions of a digital camera. Here only one external interface is identified, i.e. the interface with light from the scene.

Figure 6-21 A FFBD developed as a process-oriented model has named and numbered functions as nodes and objects as links.

Part of the task of functional design is to decompose high level functions into the lower level functions necessary to carry out the action implied by the high level function. For example, the function “image scene” can be decomposed as shown in the FFBD of Figure 6-22. In this example the objects linking the four lower level functions are also labeled. Note that nodes are numbered as well as named with the numbers indicating the level of the functions in the overall hierarchy of functions. The numbers are selected to provide traceability; e.g. sub functions decomposed from a function 1.1 with n sub functions are numbered 1.1.1 to 1.1.n.

Figure 6-22 The function Image Scene 1.1 from Figure 6-18 can be decomposed into four lower level functions.
Notice that functions 1.1.2 and 1.1.3 in Figure 6-22 could be interchanged in the sequence and still be a logical sequence. This is a simple example of having more than one logical sequence of lower level functions. The “best” sequence is determined by conducting design trade studies when these functions are allocated to physical elements. Note also that the functions 1.1.1 and 1.1.2 are easily grouped whereas 1.1.1 and 1.1.3 or 1.1.2 and 1.1.3 are not easily grouped. Therefore the sequence shown is likely to be the preferred sequence, at least until design trade studies are complete.
An alternative to the process-oriented model of a FFBD is an object-oriented model with the nodes and links reversed. An example of an object oriented model is shown in Figure 6-23.

  Figure 6-23 An example of a FFBD as an object-oriented model of two of the functions of a digital camera with named functions as links and named objects as nodes.

The decision to develop process-oriented or object-oriented models depends upon the experience of the systems engineer and the details of the system being designed. A comparison of the two approaches resulting from an analysis of both approaches is shown in the table.

Table 6-1 Selecting an Object-Oriented or Process-Oriented model depends on the details of the specific system design. From the paper Cognitive Fit Applied to Systems Engineering Models by Larry Doyle and Michael Pennotti, Conference on Systems Engineering Research, 2004. 

As with context or domain diagrams it is helpful to have pattern diagrams for all levels of FFBDs representing the class of systems that includes the system being designed. This saves considerable time compared to having to generate the FFBDs for all the levels of a new system under development. The objective is that the pattern diagrams contain all possible functions, sub functions and external and internal interfaces of the entire class of systems. Then the task becomes examining each top level function and interface to determine if it belongs to the new system. If a functions does not belong it is deleted along with all sub functions decomposed from the unneeded top level function. After deleting all unnecessary functions and interfaces from the top level and the sub functions and interfaces traceable to the deleted top level functions and interfaces it is necessary to examine the sub functions in each level to ensure that only necessary ones are kept. The system being designed may include a top level function from the pattern but may not include the entire set of sub functions decomposed from the top level function.   It is also necessary to examine the partitioning and grouping of functions as the best choice is likely to be system design specific and not necessarily that captured in the pattern diagrams.


  1. Deferring foг an еxpеrt iѕ amongst
    thе most imροrtаnt quаlitieѕ associateԁ with аn effeсtive manаger.
    5) Press Ctrl+1 on the kеуboard to produсe the Format Cellѕ dialog.
    Thіs ρresents an underѕtandable mаnagemеnt challеnge because successful manager muѕt еffectively
    communicate with the other three-fourths of thеir team.

    Haνe a look at my web рage; Novell Data Recovery Portal

  2. You may offer a wiԁe-aгray of numегоus servicеs, like freelanсe wгiting, childcаrе, creating, pet care, аnd in many caѕes ѕtuff liκe house-sitting.

    Johnson ωas the one coach Ηaley respected, and their relatіоnship enԁed іn Halеy ending
    his cаreеr with 5 Supегbοωl ringѕ; two wіth all
    the San Franciscο 49erѕ, three with all thе Dallas Cowboys.
    Alright, the trip hаs enԁed, and you alѕo're finally at school.

    Here is my web page ::

  3. What's up friends, good piece of writing and good arguments commented at this place, I am in fact enjoying by these.

    Also visit my web site -

  4. Thanks for the auspicious writeup. It in truth was a leisure account it.
    Glance advanced to far introduced agreeable from you!
    By the way, how could we keep up a correspondence?

    Also visit my web blog ...

  5. Very quickly this web page will be famous amid all blogging visitors, due to it's nice content

    Look at my webpage; Xength X1 Free Trial

  6. I really love your site.. Pleasant colors & theme. Did you create this website yourself?

    Please reply back as I'm looking to create my own site and would like to learn where you got this from or just what the theme is called. Many thanks!

    Here is my blog - male enhancement review

  7. wonderful issues altogether, you simply received a new reader.
    What would you suggest about your submit that you simply made
    some days in the past? Any sure?

  8. Excellent post. I was checking continuously this blog and I'm impressed! Very helpful info specifically the last part :) I care for such info a lot. I was seeking this particular information for a long time. Thank you and good luck.

    Also visit my blog post; Pur Essence Review

  9. Hi would you mind stating which blog platform you're using? I'm planning to start my own blog soon but I'm having a tough time deciding between BlogEngine/Wordpress/B2evolution and Drupal. The reason I ask is because your layout seems different then most blogs and I'm
    looking for something completely unique.
    P.S My apologies for getting off-topic but I had to ask!

    Also visit my site: Natural Weight Loss

  10. Hello there, I found your website by way of Google whilst
    looking for a similar topic, your website came up, it appears good.
    I have bookmarked it in my google bookmarks.
    Hello there, just become aware of your weblog via Google, and located that
    it's truly informative. I am gonna be careful for brussels. I will be grateful if you proceed this in future. A lot of other folks can be benefited out of your writing. Cheers!

    Also visit my page :: last longer durning sex

  11. cheap louis vuitton sale
    This is a topic which is near to my heart... Take care!
    Exactly where are your contact details though?