Page 300 - Design for Six Sigma a Roadmap for Product Development

270 Chapter Eight

φ 1
Hot water φ Cold water
(HW) 2 (CW)

Figure 8.19 Water faucet baseline design.

concepts. The study objectives are many, among which, are to explain
how to use axiomatic design effectively during product development
while demonstrating the use of the its associated theorems.
For the water faucet, the customer cares about two requirements:
the water temperature and the flow. According to the Independence
Axiom, we should be able to do this with two design parameters one
for each requirement. However, there is a difficulty. Since the water
comes in two pipes (hot and cold), some may think it is easy to control
the volume of hot water and volume of cold water, but these are not the
things we want to control. This is a case study where a coupled base-
line design exists (Fig. 8.19).
The two functional requirements of the water faucets as follows:

FR1 Control the flow (Q) of water,

FR2 Control the temperature (T) of the water.
In the baseline design, there are two valves, two design parameters,
and that need to be turned to deliver the above two requirements. To
satisfy Theorem 4 (App. A)*, we know that the number of design para-
meters (DPs) have to equal or exceed the number of FRs as a pre-
requisite for the Independence Axiom. In this case there are two DPs,
namely, DP1 valve and DP2 valve .
2
1
The functional requirements are always independent by definition.
In this case, flow is definitely a physical quantity that is different
(independent) than temperature. Therefore, by the Independence
Axiom, design parameters must be chosen to maintain the indepen-
dence between the functional requirements. The baseline faucet
schematic in Fig. 8.19 does not satisfy the Independence Axiom as

*Reproduced from El-Haik (2005).