2.007 Milestone 2: Motor + Drivetrain Analysis

Milestone: Time to start using our $500 budget!  We anticipated that the motor + motor controller + extra battery would take up ~$300 of our budget (after looking at the budgets of other teams.)

We did some calculations…


…and decided that we wanted as large a Kv as possible from our motor.

Motor: We wanted to accelerate from 0 to our max velocity in as little time as possible and consequently, we wanted to attain a very high acceleration (~g/4). With our current gear ratio 1:5, that meant that we needed to draw large amounts of current from our motor controller. But hey, since the Current = T / Kt (which is inversely related to Kv), a lower Kv rating on our motor means a lower current! So we decided on this motor, which also seemed to be a popular motor from previous years as well. It’s a brushless outrunner electric motor with a kv of 149. Awesome for $80. Apparently they’re used for RC helicoptors? We ordered one on Tuesday 2/24, and it “arrived” on our table on Friday, 2/28. I have a sneaking suspicion that Charles ordered a lot of these before hand, anticipating that some teams would use them. Either that or Hobbyking was surprisingly fast.

Motor Controller: Even with a good motor, for the specs we want, we need to find a motor controller that can handle high amounts of current. Or, you know, we can have a lower acceleration / lower max speed. There’s a tradeoff – I is proportional to mass, max velocity, gear ratio, Kv, and radius of the wheel, but it’s inversely proportional to t, time to accelerate. So we want to decrease all those factors except the time it takes to accelerate. I don’t want to give up any of those things :(. We set our wheel radius at 8″, and we can’t get a much better gear ratio (limited by the fact that our lab instructors hinted we’d want at least 15 teeth on our sprockets to prevent shearing) Basically, if I were to change anything, I’d decrease the max velocity we could attain with our acceleration. Since we anticipate that the racetrack will have a lot of turns, we think that a quick time to reach max acceleration is really important, and we will be able to cover 7.5 m of the max racetrack length (30m) just while accelerating. So, in our indecision, we haven’t ordered a motor controller. We should probably do so by this tuesday (3/3). We should probably talk to the lab instructors about it more. Right now, our current (haha, pun) options would vary from $119 (for 110A max current) to $149 (for 130A max current). Yay Kelly Controllers.




Physical HW Fun(another 2.007 requirement): Charles wanted us to make a box with fingerjoints and a t-nut in CAD and lasercut it. I really wanted to make a drawer because I need a place to put my pencils on my desk. I kind of have them lying around in my desk drawer…

We also had to make a fit gauge for both 1/4″ and 1/8″ sheets of MDF. This was kind of a fiasco – I made so many fit gauges and was perplexed to why they didn’t work until I realized that the MDF sheet varies in thickness for each sheet…and even within a sheet. I decided that a press-fit box wouldn’t be tight enough, so I put t-nuts into the drawers/box. So now, I have a pencil case! On the other hand…it (and my hands) smell like lasercut MDF (dangerous?).

CAD of Drawer
2014-03-01 11.07.41
I need to put a slot/fingerhole for the drawers

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