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1.5 Control Energy Transformation for Each CTQ Characteristic

To fulfill the intent of the system, the customer does something that initiates a transfer of energy, which produces a CTQ that might be categorized as either the intended result or the unintended result (error state). Because energy transfer creates CTQs, the system must be studied in terms of this transfer. Such a study will help the team identify a response that quantifies the system's production of intended results. It is important to consider the following:

  • Energy can neither be created nor destroyed.
  • Energy can be transformed into various states.
  • Only one energy state is intended, or ideal.

Maximizing the amount of energy used to produce an intended result will minimize the amount available to produce unintended results, or error states (Figure 1-4). Robust design shifts from examining the error states and searching for remedies, to studying the functional intent of the system and exploring ways of optimizing it.


Figure 1-4 Control energy transformation effectively.

Engineering robust products with Six Sigma requires a shift from measuring the symptoms of poor quality to measuring the transformation of energy. This philosophy requires a shift in thinking by the engineer. Robust design enhances quality through a focus on optimizing the system's intended functions—the efficient transfer of all energy.

To depict the intended function in terms of engineering metrics, study the underlying physics of a system, which should yield an engineering metric that quantifies the amount of energy used to produce a result. Use this metric as the Critical-to-Quality characteristic. Maximizing such a CTQ will optimize system functionality.

Clearly, CTQ characteristics depend on the system chosen for study. Many systems are composed of several subsystems and related processes, each with its own intended function. Therefore, what to study must be determined before the team can identify the transfer function and the related CTQs.

The following are the three types of metrics commonly used in industry:

  • Customer metrics— usually subjective and expressed in nontechnical terms
  • Management metrics— typically related to productivity or economics
  • Engineering metrics— quantitative, objective, and physics-based

All of these metrics have their place in the development of quality products and processes. In experimentation though, engineering metrics will provide more useful and reproducible information than either management or customer metrics.

Levels are the different settings a factor can have. For example, if you want to determine how the response (speed of data transmittal) is affected by the factor (connection type), you need to set the factor at different levels (e.g., modem and local area network).

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