With less than a month before Maker Faire Bay Area, our team is getting excited as we pull together our projects. This year, in addition to bringing cool new LaunchPads like the MSP432, we have something special we’ve been working on.

We have teamed up with TX/RX Labs in Houston, Texas to sponsor a team for the Power Racing series at Maker Faire. The Power Racing Series is the culmination of all things engineering and silly wrapped up in the form of a racing competition. Teams build an electric vehicle loosely based on a plastic children’s vehicle, and then race these vehicles, competing for both speed and style. To make things interesting the race includes rules that limit the vehicles value to $500 and the power to 1440W.

Building the vehicle would require a substantial amount of mechanical design and fabrication, which we didn’t really have the expertise for, so we reached out for help. TX/RX Labs is a non-profit that excels at turning people’s engineering dreams into reality. It offers classes on wood working and welding, as well as everything in-between. In addition to the classes, it’s a full Makerspace with just about every kind of tool you could dream of.

We started out the build by coming up with a rough idea of how we would build our chassis. Because of the budget limitation we tried to recycle as much as possible, so we chose to use two bicycles that we could weld together. This would give us a chassis that was both cheap and easy to fabricate.

We reached out to some local businesses to help source parts. Bike Barn was very supportive and donated some older bicycles that we could use to fashion our frame. After stripping down the frame we found that the diameter of the crank tube was a perfect fit for 1.5” pipe, so we purchased some and welded the two bicycles together.

The chassis is only half the story though. For the motor drive, we elected to use TI’s InstaSPIN™ technology to drive two brushless DC motors – one for each rear wheel. We started with the F28069M LaunchPad and the DRV8301 BoosterPack to test out the technology. The InstaSPIN software was easy to use and worked great with our motor, but the BoosterPack could only supply 250W. Given our power limitation of 1440W, we needed to be able to deliver at least 720W to each motor. To solve this problem, Mark (a TX/RX Labs Member) took it upon himself to design and build a more powerful BoosterPack that could drive our motors. Check back next week for an update on this more powerful motor driver BoosterPack.

The motor we chose was a hobby motor designed for RC aircraft. Motors like this are great because they are low-cost, but they aren’t ideally suited for this application because they are designed to operate in the several thousand RPM range. We would need to gear this motor down in order for it to be able to drive the wheels. To figure out the reduction needed, we needed to know the speed of the wheels at a given vehicle speed. Working through the math I came up with the following formula:

X = Vehicle speed in MPH

D = Tire diameter in inches

Wheel RPM = (X * 5280) / (60 * (D * Pi / 12))

 By comparing the wheel RPM at a given vehicle speed with the target motor RPM we could figure out the reduction needed. Given a top speed of 25MPH and a tire diameter of 12 inches, the tires would be rotating at approximately 700RPM. If we wanted to achieve 25MPH at 7k RPM on the motor we would need a 10:1 reduction.

As you can see in the image above, the cog on our wheel only has 18 teeth. To get the 10:1 reduction, our motor gear would need to have 1.8 teeth, which is of course impossible. To solve this problem we need to build a two-stage reduction. We’ll swap out the cog on the wheel for a larger 22-tooth gear and couple this with a nine-tooth gear, giving us a second stage reduction of 2.444:1. If we combine this with a yet-to-be designed first stage that has a 5:1 reduction, we’ll be very close to our target 10:1 reduction.

With Maker Faire quickly approaching, it’s crunch time! Together with the TX/RX Labs team, I am working away to complete the car. Come back next week for an update on our progress and more technical details on the design of both the electrical and mechanical components of the build. And of course, don’t forget to cheer for us at the Power Racing Series at Maker Faire Bay Area.

Anonymous