Create a Decision Matrix

Create a decision matrix

A decision matrix is a graphic organizer of the information that you have gathered about different design concepts. It should always be backed up by the analysis that you have done. This process is iterative. Initially when brainstorming, you may only have a vague rationale for each criteria. Therefore a simple list of the design criteria with pluses and minuses is a good start.

Concept #1 / Concept #2 / Concept #3
Performance / 0 / - / 0
Aesthetics / 0 / 0 / +
Total / 0 / -1 / +1

Table 1: Decision Matrix 1

Some criteria are more important than others so we need to weight them. A 1-5 scale should be sufficient to show the relative importance of each criteria to the overall design.

Weight / Concept #1 / Concept #2 / Concept #3
Performance / 3 / 0 / - / 0
Aesthetics / 4 / 0 / 0 / +
Total / 0 / -3 / +4

Table 2: Decision Matrix 2

As we analyze each concept and gather evidence, each concept can be rated against each criteria. Again, a 1-5 scale should be sufficient to show the relative performance of each concept on each criteria.

Concept #1 / Concept #2 / Concept #3
Performance / 3 / 3 / 2 / 3
Aesthetics / 4 / 3 / 3 / 5
Total / 21 / 18 / 29

Table 3: Decision Matrix 3

First, we are going to identify different qualities that can be manipulated for the mousetrap car. In this case, we can change:

Lever arm length,
Axle-Spring distance,
Axle radius,
Wheel radius,

Second, we will useour reference design as the starting point for the design and vary the parameters that we identified. A simple variation is to double or half the relevant quantities. However, some of the quantities cannot be varied in both directions. The axle-spring distance and lever arm length cannot be effectively increased in our reference design. The axle outside radius cannot be effectively decreased. So we only vary them in one direction.

The GOAL for this design is to reach 9 meters as fast as possible.

Quantity / Units / ref / base1 / base2 / lever1 / lever2 / wheel1 / wheel2 / axle1 / axle2
mass of car / kg / 0.500 / 0.500 / 0.500 / 0.500 / 0.500 / 0.500 / 0.500 / 0.500 / 0.500
mass of wheels / kg / 0.050 / 0.050 / 0.050 / 0.050 / 0.050 / 0.050 / 0.050 / 0.050 / 0.050
mass of axles / kg / 0.005 / 0.005 / 0.005 / 0.005 / 0.005 / 0.005 / 0.005 / 0.005 / 0.005
spring constant / N-m/rad / 0.020 / 0.020 / 0.020 / 0.020 / 0.020 / 0.020 / 0.020 / 0.020 / 0.020
spring torque offset / N-m / 2.000 / 2.000 / 2.000 / 2.000 / 2.000 / 2.000 / 2.000 / 2.000 / 2.000
axle-spring distance / m / 0.300 / 0.150 / 0.075 / 0.300 / 0.300 / 0.300 / 0.300 / 0.300 / 0.300
lever arm length / m / 0.300 / 0.300 / 0.300 / 0.150 / 0.075 / 0.300 / 0.300 / 0.300 / 0.300
wheel outside radius / m / 0.050 / 0.050 / 0.050 / 0.050 / 0.050 / 0.100 / 0.025 / 0.050 / 0.050
axle outside radius / m / 0.002 / 0.002 / 0.002 / 0.002 / 0.002 / 0.002 / 0.002 / 0.004 / 0.006
deceleration constant / m/s^2 / 0.100 / 0.100 / 0.100 / 0.100 / 0.100 / 0.100 / 0.100 / 0.100 / 0.100

Table 4: Design Parameters, Iteration #1

Rate each of the concepts using the data provided.

Table 5: Decision Matrix, Iteration #1

Analysis

Is acceleration or distance traveled more important? Weight the design criteria accordingly.
Rate each of the design concepts against the criteria using the graphs on the previous pages.
What observations can you make about our design concepts?
Some of the designs are not practical from a physical implementation. Which ones and why?
Engineering Design is an iterative process.
Choose eight new designs to evaluate and explain how you came up with them.