One NASA adage is:
Better, Faster, Cheaper…pick 2
The premise is that the third desire is mutually exclusive. This is obviously an oversimplification of the process however it quickly brings to light the interplay between goals.
I like to think of design requirements from an optimization standpoint. There are constraints and goals. Constraints are those that the design must meet otherwise it is not a viable product. Goals are parameters that you would like to improve.
Constraints usually read black and white. ‘The design shall have a fatigue life exceeding 1 million cycles as dictated by ISO##.’
Goals usually read as a desired direction with a target goal. ‘Unit cost should be minimized with a goal of $$’
Often the constraints get merged together. ‘Unit cost shall be less than $$’
Higher cost could be justifiable and a viable product as long as all constraints are met and the goals are favorable. However, most requirements are written as individual statements which must be checked off for a design to be finalized. Unfortunately, this leaves many great designs only living in the musings of the inventor thinking ‘what if’. Don’t turn away a golden square peg because your requirements are a round hole.
This can be accomplished by creating a goal function which wraps up all of the goals into a weighted global goal. This allows cost in this case to increase with increasing performance.
Design teams need to know where they should go in the vast design space. I would rephrase
Better, Faster, Cheaper…pick 2
to read
_%Better, _%Faster, _%Cheaper…fill in the blanks
Thank you.
Rob Stupplebeen
Rob@OptimalDevice.com
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