B.3 環境特性擬訂

hlperng

B.2 環境特性擬訂 (Environmental Characterization)
了解必須知道的環境應力是設計成功的關鍵，知道環境之後才有可能採取措施防治它們不利的效應。
Knowing the environmental stresses to be withstood is critical to design success.  Measure can be taken to inhibit their detrimental effects only if they are known.  

B.2.1 目的 (Purpose)
環境特性擬訂為定義產品交給顧客使用時所遭遇的操作及環境應力的過程。
Environmental characterization is a process used to define the operational and environmental stresses that the product will experience when put into use by the customer.  
環境特性擬訂過程可以識別產品在其使用生命過程可能暴露的最終使用環境的範圍與大小，作為建立以性能為基礎的可靠度要求。
A process to identify the scope and magnitude of end-use environments to which the product will be exposed throughout its useful life.  Used to esbalish performance-based reliability requirements.

B.2.2 利益 (Benefits)

The benefits of environmental characterization include fewer failures and better product operation.  When the characteristics of the end-use environmental profile are known and udestood, the appropriate components can be selected and protective design measures can be utilized, both of which will prevent over-stressing of individual components.  
Without an understanding of the stresses to be experienced by a product, the statement of reliability objectives, explicitly or implicitly, is meaningless.  For example, a product that could have an MTBF of 500 hours in a consumer household may only experience a 200 hour MTBF in an automobile, due to a more stressful environment.  The advantage of this task is to establishes an operating envelope for design.  But it may add extra features to achieve needs (potential overdesign).  

B.2.3 Timing
The environment should be defined in gross terms every early in the Concept/Planning phase of the product life cycle, and implemented into the product design at the very first stages of product Design/Development concurrently with power, size, performance, weight and cost constraints.  Delays in environmental characterization and compensation for environmental impacts on the design are not recommended, as untimely implementation of corrective action will typically be inefficient in terms of performance, schedule and cost.   

B.2.4 Application Guidance
The ability to accurately identify the environmental and customer use characteristics increases the probability that the customer will experience reliability that approaches designed-in levels.  It also ensables design practices to be better tailored to produce a "robust" design.  
At a high level, the environment can be characterized in terms such as ground, mobile, airborne, space, etc.  This provides a rough indication of the severity ranges of stresses to be experienced.  As the design process starts, more detailed information may be necessary.  Fore more severe environments, it may be appropriate to actually use instrumentation to measure the expected stress levels.  For example, the Air Force Rome Laboratory has developed a family of Time Stress Measurement Devices (TSMDs) that can be used to measure and record such stresses as temperature, humidity, shock, vibration, and power.  Less sophisticated commercially available products are also available.  
Exposure to stress is also impacted by how the product is used operationally.  For example, is it turned on and left powered 24 hours per day or only subjected to electrical or mechanical stress when used.  Is it part of a system (like a computer system) where it is only used 10% of the system opertiong time (like a printer).  Another factor is who will use the product.  Will mature adults use the product or will children, more apt to mishandle and abuse it?
Charaterizing the environmental factors to which a product may be exposed is enssential to the design process.  The factors of importance include those experienced by a product during shipping, handling, and storage, as well as during maintenance and operation.  In some cases, the shock, vibration, and temperature extremes experienced during shipping are more severe than those experienced in operation.  Typical environmental factors inlcude:

• maximum and minimum temperature
• maximum rate of change in temperature
• vibration (magnitude and type)
• shock
• humidity and moisture
• radiation
• electromagnetic fields
• static and dynamica loading
• corrosion
  

Design for reliability requires identification of and control over the possible causes of component or product failure.  In general, component failures could be classified as follows: (1) inadquete electrical or mechanical design tolerances, (2) incorrect choise of materials, (3) insufficient quality procedures or controls, (4) deterioration of components due to environmental effects, and (5) manufacturing or component defects.  
Many approaches to environmental characterization can be used.