Table 4: Filling out normalized values of decision matrix
Once the values are filled out, the team must decide on the importance of each parameter on a fixed scale (a 1-10 scale is probably the largest range you want to use; a 1-5 scale is the most common). These numbers will be filled into the weight section. Be aware that the weights will likely change at many points in the design process. Such as the case with the ModBot, the importance of efficiency decreased while the importance of the leadtime dramatically increased.
Table 5: Decision matrix with initial weighting values applied
Table 6: Decision matrix with heavy weighting for lead-time
With a high initial weighting for max continuous torque and efficiency in Table 5, Motor F was determined to be the best selection. However, when the weighting changed to favor short lead time in Table 6, Motor C was determined to be the best option, and in the ModBot’s case the lead-time was the major deciding factor in the motor selection. Now you have a great ability to justify why a certain motor and gear combination was selected compared to others. As long as the justification for the weighting is agreed upon, the selection is a very objective process.
Always keep in mind that system level targets may change as well as information from other subsystems. There is no such thing as the perfect motor but there is always a motor that is better given your decision critera. Being able to give clear linkage from the system level performance metrics down to the details of the selected motor is the definition of great engineering.
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