Figure 29: Small Excerpt from Interface Matrix

With Figure 29 created, you can now begin to evaluate the importance of all of these characteristics and evaluate your potential motor options. Writing down how you evaluate your options is remarkably important. Engineers commonly (but incorrectly) try to use only the judgment in their head without documenting how the selection was made. When reporting the selection to the team, often another team member will have some judgment criteria (which may even be personal or based on misconceptions) and he or she dislikes the final motor selection based on a parameter that wasn’t even considered in the initial selection.

Without having a written trail to justify your decision, your design is more at risk to be evaluated purely on the background of your reviewers, who may be far less experienced than you are in this area. With so many considerations in just motor selection, imagine how hard it would be to defend your entire system design completely off of memory. To keep the selection process as objective and civil as possible, it is best to document what characteristics were used to say Motor A is better than Motor B. One way to show the decisions that were made is to utilize a decision matrix, also known as a Pugh Matrix.²⁴ Table 2 offers a subsection of a possible decision matrix considering the seven motors whose torque-speed curves are shown below.

Decision matrices are used to create objective scores for each characteristic that are then weighted on importance and summed together to give each option a total score for comparison. Even from the decision matrix subsection shown below, you can see how this will not only help you prioritize certain characteristics but may in fact also help your team make design adjustments more quickly and confidently throughout your design process. For example, in the beginning of the process the team may decide that the efficiency is the most important feature, thus, it is given a large weighting. Then, as your system is coming together, the team may decide that the motor mass is the most critical factor because other components came in over weight. The team will increase the weight of the motor mass parameter and decrease the weight of the efficiency parameter.

Table 2: Decision Matrix Example

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