1.6 Determine Control and Noise Factors
To make products affordable, engineers need to determine how to control the CTQ characteristics at minimal cost. The fifth step in the robust design process is to develop a list of control and noise factors for each CTQ. This section covers the following:
- Definition of control factors
- Definition of noise factors
- Sources of noise
In robust design, engineering parameters related to CTQs are categorized as either control factors or noise factors (see Figure 1-7). The Engineered System, or P-Diagram (see Chapter 6), for a product or process is a diagram that shows the relationship among system (or subsystem) parts, the CTQ, and the control and noise factors.
Figure 1-7 Control and noise factors.
Brainstorming is a useful tool for developing an initial list of control and noise factors. Further investigation may be needed to research creative ideas that result or to discover additional factors. If the list of influential control and/or noise factors becomes prohibitively long, consider narrowing the scope of the study to a simpler subsystem. Then, you may need to redefine the response to establish a complete situational understanding of a wide range of data where several control factors may be interacting at once to produce an outcome.
Determining whether a factor is a noise or a control one often depends on the team's objective or the scope of the project. A factor considered control in some cases might be considered noise in others. For example, consider the material hardness factor (measured in Rockwell units). Design engineers focus on the product, so they may categorize material hardness as a control factor. However, process engineers focus on the process, so they may categorize material hardness as a noise factor; from their perspective, the process needs to be insensitive to the hardness of the material.
There are many sources of noise. Figure 1-8 shows five broad noise factor categories. Frequently, customer usage creates the most variability; but when developing a noise strategy, consider all possible sources to ensure that influential noise factors are not overlooked. Typically, control factors are obvious to the engineer because they relate directly to system design. On the other hand, it is easy to overlook some noise factors because they are often external to system design. Examining the five potential sources of noise can help engineers develop a thorough list of noise factors.
Figure 1-8 Source of noise factors.
In sum, control factors are parameters whose nominal values can be adjusted by the engineer, ideally with minimal impact on cost. A noise factor is a source of variability, either internal or external to the system. A noise factor disrupts the transfer of energy to the intended function.