The Design Cycle

Figure 2.4 (Horeinstein)

A Design Example

Peak performance design competition: vehicle climbing a ramp, able to sustain against an opposing vehicle.

Ramp specification

Figure 2.5 (Horenstein)

Vehicle specification

-Autonomous

-Powered by: batteries of up to 9V; or rubber band (0.4cm x 10cm max size); or mouse strap (spring size 1cm x 3cm max.)

Define the overall objectives

-Speed

-Defensive/Offensive

-Adaptability

-Durability

-Simplicity

Choose a design strategy

-Power source

-Propulsion mechanism

-Stopping device

-Starting device

Figure 2.6 (Horeinstein)

Make a first cut at the design

-Battery powered

-Defensive strategy –slower moving (higher torque), wedge-shaped vehicle (destructive or helping a ride-over)

-Plan to use plastic gears (reduce speed of wheels relative to motor shaft speed providing mechanical torque) and axles

-One motor to both rear wheels: differential capability good for curved paths only; front-wheel drive a disadvantage in this application.

Figure 2.8 (Horeinstein)

Build, document, test, revise

-You and your teammates have decided, among competing possibilities, on one you think will have the best chance of winning the competition. A preliminary sketch is:

figure 2.9 (Horeinstein)

-Run your vehicle, by hand first, up the ramp. Bottom of vehicle hits ramp at top of hill:

figure 2.10 (Horeinstein)

· Larger wheels: redo calculation on force required from motor

· Shorter vehicle: steeper shape, not effective for helping a ride-over.

Revise again

· Move rear wheels forward, you claim success. Can topple backwards with a ride-over.

Figure 2.12 (Horeinstein)

· Learning from failure is a normal part of the design process.

· Decide that best option is bigger wheels. Change gear box, OK.

-Motors: no 9-V motors, lower-than-rated voltage ->torque reduced, higher-than-rated voltage -> motor damaged, but 15s OK.

-Try motors rates with 3V and 6V. Measure mechanical power delivered by these motors.

Figure 2.13 (Horeinstein)

-Records all characteristics of unused motors in case you need to reconsider one of the rejected motors.

-Records all calculations, specifications, sketches, to have ready by the time of the competition.