1.20 Teacher Toolbox - Functional Decomposition
1.20 Teacher Toolbox - Functional Decomposition
1.20 Teacher Toolbox - Functional Decomposition
Vocabulary- As a beginning engineer, there may be terms that may not be familiar. Here are some terms and
their definitions that apply to the functional decomposition process.
A subfunction is a function that has to work in order for a more general function to
take place. Remember, a subfunction may also be a general function.
What is the most general task that your design needs to accomplish? Your group
should come up with a one-sentence answer to this question and write it down. If you
find that you need more than one sentence to describe this task, it is likely that you are
incorporating some smaller functions into your answer. Try to identify the function that
reflects the overall purpose of the design.
1
Begin a functional decomposition diagram by centering a short description of
this most general function at the top of a piece of paper. Remember, all you need to
write is what the function does, not how it does it.
What are the functions that must take place immediately before the most general
function? You can place these (remember: tell us what, not how) on your functional
decomposition diagram, connecting them with lines to your description of your most
general function. Make sure that you are not choosing functions that require another
step before the most general function can be completed well use these in the next
step.
For each of the functions that you put on your chart in step 2, find their closest
sub functions and draw them on the functional decomposition diagram. Keep doing
this until the functions on your diagram are basic functions that cant be broken down
any further.
When you have completed your diagram, check to make sure that there are no
additional functions that you have not included. If you think of one, try to figure out where
it fits on the diagram and draw lines to show its place.
The most general function of the bike fender is to protect the rider from water and dirt that fly off
the wheel, but we can see here that there are subfunctions that make this happen. Protecting the rider
requires three things from the fender: shielding them from debris, directing water away from them, and
serving as the place where the splashguard attaches.
2
In the box for each function we can see that there is at least one action word (a verb) and one
object that the action is directed toward (an object, which is always a noun). Sometimes there are words
that tell us where materials are moving (for example, the fender steers water away), but other than this,
the boxes do not have to contain much information. When you are drawing your own functional
decomposition diagrams, the important thing is to write what a function does, not how it does it.
A lot of thinking goes into completing a functional decomposition diagram, but the job becomes
easier if we think of it in terms of its individual steps.
Resources:
Functional Decomposition: An Introduction for High School Engineers, EPICS Program 2009.