Identify how the solution functions in its whole and its parts.
Function structure breaks the intended overall design function into cohesive and naturally workable subfunctions that lend themselves well to error-free development. For example, you would use function structure when designing a refrigerator to move from the functional requirement (control freezer temperature at –18 degrees C) to the design parameter (turn compressor on or off when air temperature is higher or lower than the set temperature).
The function structure technique is employed when you need to create design concepts and are translating functional requirements into design parameters. But since function structure doesn't necessarily address the independence of requirements and parameters, it's most effective when applied in conjunction with Axiomatic Design (see Technique 34). Function structure is best applied with the help of a qualified engineer.
Scenario: To demonstrate function structure, we'll look at the design for an automatic hair-washing machine—a device that would take the place of the person who preps you for a haircut when you visit a salon.
What is the overall intent of the design, or the function the design must perform? Write the function in the middle box of a function structure diagram (Exhibit 35.1). The intent of our example system is to automatically wash hair.