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design:functional_requirement [2020.03.12 13:30]
127.0.0.1 external edit
design:functional_requirement [2021.05.18 13:53] (current)
Fil Salustri [Specifying Functional Requirements]
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 A functional requirement (FR) specifies //how// product characteristics are manifested by a product; they describe what a product has //to do//. Verbs are commonly used to specify functional requirements.</WRAP> A functional requirement (FR) specifies //how// product characteristics are manifested by a product; they describe what a product has //to do//. Verbs are commonly used to specify functional requirements.</WRAP>
  
-Function are different from [[behaviour]]. Behaviour is how a product responds to a stimulus, whereas a function is how that stimulated response serves some [[purpose]] in an [[situation]].+Function is different from [[behaviour]]. Behaviour is how a product responds to a stimulus, whereas a function is how that stimulated response serves some [[purpose]] in an [[situation]].
  
 ===== What is a functional requirement? ===== ===== What is a functional requirement? =====
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 <WRAP tip> <WRAP tip>
 //**Example:**// //**Example:**//
-The behaviour of a spring is to deform elastically.  That is, when the spring is given an input of a certain physical load, the spring responds by changing shape until it'reaction force equals the applied input force.+The behaviour of a spring is to deform elastically.  That is, when the spring is given an input of a certain physical load, the spring responds by changing shape until its reaction force equals the applied input force.
  
-The //function// that this behaviour serves depends on how the spring is used.  It may be used as a vibration damper, or to increase control of a machine (such as a car), or to prevent damage to some delicate items by distributing applied forces over longer periods of time.+The //function// that this behaviour serves depends on how the spring is used.  It may be used as a vibration damper, or to increase control of a machine (such as a car), or to prevent damage to some delicate items by distributing sudden forces over longer periods of time.
 </WRAP> </WRAP>
  
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 Notwithstanding the foregoing discussion, it is generally rather silly to specify FRs with no mention at all of the physical nature of the product. This is because we generally already have some idea of what the product will be and do. [[http://www.boeing.com/|Boeing]] doesn't design cars, and [[http://www.ford.ca|Ford]] doesn't design airplanes. It is obvious that at a certain level, we will have some idea about the physical nature of the product. Planes don't look like cars; probably, they never will((Well, maybe not //never//.... {{youtube>EHXnLCIgNug}})). There are of course exceptions: consider the two telephones shown to the right.  Nonetheless, no FR should explicitly refer to a design intervention's physical characteristics. Notwithstanding the foregoing discussion, it is generally rather silly to specify FRs with no mention at all of the physical nature of the product. This is because we generally already have some idea of what the product will be and do. [[http://www.boeing.com/|Boeing]] doesn't design cars, and [[http://www.ford.ca|Ford]] doesn't design airplanes. It is obvious that at a certain level, we will have some idea about the physical nature of the product. Planes don't look like cars; probably, they never will((Well, maybe not //never//.... {{youtube>EHXnLCIgNug}})). There are of course exceptions: consider the two telephones shown to the right.  Nonetheless, no FR should explicitly refer to a design intervention's physical characteristics.
  
-Furthermore, FRs describe what a product does, but not what is done to the product. Designers should think of their products as (re)active entities rather than passive ones; that isproduct responds to stimuli by exhibiting a behaviour, which is how the product achieves its function.+So we can describe a FR as a sentence that takes the following form: "The product //does something//," where the //does something// part is a verb or verb clause describing an action or reaction of the product within its operating environment.
  
 <imgcaption f2 right|Is this still a telephone?>{{ iphone4.jpg?300}}</imgcaption> <imgcaption f2 right|Is this still a telephone?>{{ iphone4.jpg?300}}</imgcaption>
    
-So we can describe a FR as a sentence that takes the following form: "The product //does something//," where the //does something// part is a verb or verb clause describing an action or reaction of the product within its operating environment. 
- 
 All FRs must use **active verbs** - verbs that describe some kind of dynamic. In particular, the following verbs must **never** be used in FRs: //to have//, //to be//, and //to be able to//. All FRs must use **active verbs** - verbs that describe some kind of dynamic. In particular, the following verbs must **never** be used in FRs: //to have//, //to be//, and //to be able to//.
  
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   * The product must display enlarged images.   * The product must display enlarged images.
-  * The robot must orient parts on the ground at any angle. Note +  * The robot must orient parts on the ground at any angle.
     * We can mention the "ground" here as part of the [[situation]], rather than part of the product. Since designers rarely have any substantive control over the operating environment (part of the situation) of their products, it is considered acceptable (even desirable!) to mention the physical characteristics of the environment in the FR specifications.      * We can mention the "ground" here as part of the [[situation]], rather than part of the product. Since designers rarely have any substantive control over the operating environment (part of the situation) of their products, it is considered acceptable (even desirable!) to mention the physical characteristics of the environment in the FR specifications. 
-  * Resist bending. +  * The product must resist bending. 
-    * There are various equivalent English forms for expressing FRs; here, there is an implication that we are talking about "the product" during its operation. One might ask: "To what degree should the product resist bending?" But the answer to this begs the introduction of a [[constraint]].+    * There are various equivalent English forms for expressing FRs. One might ask: "To what degree should the product resist bending?" But the answer to this begs the introduction of a [[constraint]].
   * Excess heat must be dissipated to the ambient air.   * Excess heat must be dissipated to the ambient air.
   * The container must hold hot liquids.   * The container must hold hot liquids.
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   * Minimize material costs.   * Minimize material costs.
     * Cost is not a product function. And it clearly is an instruction to the designers, not about the product.     * Cost is not a product function. And it clearly is an instruction to the designers, not about the product.
 +    * Cost can be represented as a constraint.
   * It must be possible to pour hot liquids into the cup.   * It must be possible to pour hot liquids into the cup.
     * This is a passive statement, more descriptive of the cup's user (which is part of the environment) than of the cup itself.     * This is a passive statement, more descriptive of the cup's user (which is part of the environment) than of the cup itself.
design/functional_requirement.1584034210.txt.gz · Last modified: 2020.03.12 13:30 by 127.0.0.1