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Quick reference
Taguchi DOE
Taguchi DOE is a special case Fractional Factorial DOE that is used for optimizing process performance. It separates the control factors into two categories and by simultaneously testing each category can minimize the number of test runs.
When to use
Taguchi DOE is a design for process optimization, not product design or technology characterization. Further it must be used with an existing process because it relies on expert process knowledge to separate the control factors from the noise factors. Therefore, the process must exist and someone on the design team must be knowledgeable about the process.
Instructions
The Taguchi DOE requires expert process knowledge and also often requires expert Taguchi DOE knowledge. The Taguchi DOE is a hybrid DOE. It separates the list of traditional DOE control factors into two categories the Taguchi control factor and the Taguchi noise factor.
The Taguchi control factor is a factor that the process operator of the process being studied can control as part of their normal duties. These factors can vary based upon the type of equipment used and the process management systems. For this reason, expert knowledge about the process is needed so that the correct set of factors that the operator controls – not just monitors – are identified. The other factors are referred to as noise factors. This does not mean that they are background factors with minor variations. They could be the most significant factors for the process. But since the operator cannot control them, they are treated as noise for the Taguchi DOE.
These factors are analyzed simultaneously in the Taguchi DOE. The control factors are analyzed in a test matrix known as an inner array and the noise factors are analyzed in a test matrix known as an outer array. The inner array is normally a fractional factorial array and many times these factors are multi-level factors. The outer array is normally a full factorial array and normally these are two-level factors. These arrays are quite complex and there are many books and manuals full of the arrays and their derivation. Fortunately, most of the arrays are already found in statistical software, such as Minitab, so once factor selection has been made, the software will create the test matrix.
The strength of the Taguchi DOE is that it requires very few runs to get excellent insight into process control. The disadvantage is the it requires expert process knowledge to select the correct factors and an analysis for the process at one location may be totally inadequate for another location if they have different equipment or management approach. Another caution with Taguchi DOE is that it uses its own unique terminology. In fact, sometimes it uses the same word as used in other DOE methodologies but with a different meaning. There are three terms in particular that are unique in their usage within Taguchi DOE. Control factors are the factors controlled by the operator. There may be other factors that control the process, but if the operator does not have access to them, they are not Taguchi Control Factors. Noise factors are all factors being analyzed that are not control factors. So, these are factors controlled by other factors that are part of the system, and factors in the environment. They may have a significant effect, but they are “noise” from the operator standpoint. Finally, Taguchi DOE labels a process optimization at robust if the operator is able to control it. The process may actually be very fragile, but if it is fully controlled by the operator, Taguchi calls it robust.
Hints & tips
- Only use this approach with process improvement projects.
- You must have a process expert as an advisor when setting up a Taguchi DOE; otherwise you will not know how to pick the control and noise factors.
- Be very careful with the terminology – Taguchi terms do not carry the normal meaning of the terms.
- Taguchi arrays do not use the plus 1 and minus 1 settings for high and low; they use one and two. However, the Taguchi arrays are still balanced and orthogonal. The use of the different numbers should not matter to you once you put your own values in for high, low and midpoints.
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