Selection

When the technical analysis  indicates  that you have  some  potentially  viable  designs,  the best one available must be selected for detailed design, prototyping, and  testing. The selection process usually  involves  a comparative  analysis  of the  available  design  solu- tions. A decision matrix sometimes helps to identify the best solution by forcing you to consider a variety of factors in a systematic way. A decision matrix for our better grass shortener is shown  in Figure  1-2.  Each  design  occupies  a row  in  the  matrix.  The  col- umns are assigned categories  in which  the  designs  are  to be judged,  such  as cost,  ease of use, efficiency, performance, reliability, and any others you  deem  appropriate  to the par- ticular problem. Each category is then assigned a weighting factor,  which  measures  its relative  importance. For  example,   reliability   may  be  a more  important   criterion   to the user  than  cost,  or vice  versa.  You as the  design  engineer  have  to exercise  your judgment as to the selection and weighting of these categories.  The body  of the matrix  is then  filled with numbers which  rank  each  design  on  a convenient  scale,  such  as  1 to  10, in each of the categories. Note that this is ultimately a subjective ranking  on  your  part.  You must examine the designs  and  decide  on  a score  for  each.  The  scores  are  then  multiplied  by the weighting  factors  (which  are  usually  chosen  so  as  to  sum  to  a convenient number such as 1) and the products summed for each design.  The  weighted  scores  then  give  a ranking of designs. Be cautious in applying these results. Remember the source and sub- jectivity  of your  scores  and the weighting  factors!   There  is a temptation   to put more faith in these results  than  is justified.  After  all, they  look  impressive!  They  can even  be taken out  to  several   decimal   places!    (But  they  shouldn't    be.)   The  real  value  of  a  decision

matrix is that it breaks the  problem  into  more  tractable  pieces  and  forces  you  to think about the relative value of each design in many categories. You can then make  a more informed   decision   as to the  "best"  design.

Detailed   Design

This  step  usually  includes  the creation  of a complete  set of assembly  and detail   drawings or computer-aided design  (CAD)  part  files,  for  each  and  every part  used  in the design. Each detail drawing must specify all the dimensions and the  material  specifications  nec- essary to make that part. From these drawings (or CAD files) a prototype  test  model (or models)  must  be  constructed  for  physical  testing.  Most  likely   the  tests  will  discover more  flaws,  requiring   further  iteration.

Prototyping   and Testing

MODELS Ultimately,   one  cannot  be  sure  of the  correctness   or  viability   of  any design until it is built and tested. This usually involves the  construction  of a prototype  physical model.  A  mathematical  model,  while  very  useful,  can  never  be  as  complete  and accu- rate  a representation  of the  actual  physical   system  as a physical   model,  due  to the   need to make  simplifying   assumptions.